Flow Cells for Electrochemical Energy Systems: Fundamentals and Applications 3031372700, 9783031372704

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Green Energy and Technology

Liang An Rong Chen Yinshi Li   Editors

Flow Cells for Electrochemical Energy Systems Fundamentals and Applications

Green Energy and Technology

Climate change, environmental impact and the limited natural resources urge scientific research and novel technical solutions. The monograph series Green Energy and Technology serves as a publishing platform for scientific and technological approaches to “green”—i.e. environmentally friendly and sustainable—technologies. While a focus lies on energy and power supply, it also covers “green” solutions in industrial engineering and engineering design. Green Energy and Technology addresses researchers, advanced students, technical consultants as well as decision makers in industries and politics. Hence, the level of presentation spans from instructional to highly technical. **Indexed in Scopus**. **Indexed in Ei Compendex**.

Liang An · Rong Chen · Yinshi Li Editors

Flow Cells for Electrochemical Energy Systems Fundamentals and Applications

Editors Liang An Department of Mechanical Engineering The Hong Kong Polytechnic University Hong Kong SAR, China

Rong Chen School of Energy and Power Engineering Chongqing University Chongqing, China

Yinshi Li School of Energy and Power Engineering Xi’an Jiaotong University Xi’an, China

ISSN 1865-3529 ISSN 1865-3537 (electronic) Green Energy and Technology ISBN 978-3-031-37270-4 ISBN 978-3-031-37271-1 (eBook) https://doi.org/10.1007/978-3-031-37271-1 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

The irreversible environmental effects of greenhouse gas emissions, the growing demand for sustainable energy sources, and the need for energy security have forced the migration from hydrocarbon-based fossil fuels to renewable and environmentally friendly energy sources. Also, an increased awareness of the environmental issues along with a potential energy shortage has led to accelerated research efforts in energy conversion and storage. Flow cells, the configuration featuring high capacity and a long recycling life with recyclable electrolytic solutions and separately designable capacity and power, are receiving the ever-increasing attention in the recent decades. However, before making these technologies based on this configuration worldwide commercialization, the performance and lifetime of these technologies should be substantially improved, as well as the cost of the systems should be significantly reduced. To address the issues, collaboration of academia, industry, and government is required to develop advanced materials and propose innovative system and stack designs. This book provides a state-of-the-art review on recent advances in the flow cells for electrochemical energy systems, and the major features are summarized as follows: Chapter 1 focuses on the effects of ordered catalyst layer structures on multicomponent transport and the different designs of ordered catalyst layers. The designs of ordered catalyst layer are illustrated respectively, including the supporting material based one, the catalyst based one, and the proton conductor based one. Each design of the ordered catalyst layer is concluded with the fabrication process, structure characterization, and performance improvement. Chapter 2 introduces the working principle of proton exchange membrane fuel cells, electrocatalysts for oxygen reduction reaction and hydrogen/methanol/ethanol oxidation reaction, and their reaction mechanisms. Chapter 3 describes the challenges and progress of using photoelectrochemical flow cells to provide clean chemical fuels and chemicals. The development of photoelectrochemical materials with novel properties for the light absorption and the charge separation, the innovation and optimization of the photoelectrode structure, and the flow cell configuration are highlighted.

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Chapter 4 summarizes the state of the art and prospects of organic flow batteries. The key design components of organic flow batteries and their functional requirements, which distinguish them from conventional flow batteries, are summarized. The principle of design and performance analysis of different classifications of organic flow battery are discussed. The modeling approaches and numerical studies for design and performance enhancement of organic flow batteries are also presented. Chapter 5 presents state-of-the-art research and provides an up-to-date guide for the future development of aqueous organic redox flow batteries and introduces the basic information of redox flow batteries, including category, mechanism, and challenges, as well as the benefits and applications of aqueous organics. Chapter 6 reviews various modified electrodes, which can be classified into the metal or metal oxide materials modified electrodes and structure decorated or pore etched electrodes. An overview of the different strategies employed to improve electrode performance is also offered, and it provides guides for the design, development, and commercial application of electrode in all-vanadium flow battery. Chapter 7 demonstrates the development of microfluidic flow cells for energy conversion and utilization with respect to their applications in fuel production, renewable electricity storage, and electricity generation. In addition, the remaining challenges and future direction of the microfluidic flow cells are also discussed. Chapter 8 focuses on the electrochemical CO2 reduction in flow cells. First, some important components of flow cells, such as gas diffusion layer, catalyst, membrane, are introduced. Then the most representative works for CO2 reduction are discussed, emphasizing the flow cell configurations in electrochemical CO2 reduction. Finally, the remaining challenges and perspective on future development of flow cells for CO2 reduction are presented. Chapter 9 introduces the important role of ammonia as an energy storage medium in the future storage and conversion of renewable energy sources. Next, the reaction mechanism of electrochemical ammonia synthesis and typical reactors used for electrocatalytic nitrogen reduction reactions and their working principles are presented. Then, the focus is on the various components of a flow cell and their main functions, such as the membrane, the catalytic layer, the diffusion layer, and the flow field. This book is an essential reference resource for professionals, researchers, and policymakers around the globe working in academia, industry, and government. This work was fully supported by a grant from the National Natural Science Foundation of China (Project No. 52022003). We would like to express our gratitude to all the authors who submitted their contributions and shared valuable state-of-the-art knowledge and experience on associated topics for publication in this book. In addition, we are grateful to all the reviewers who helped to improve the contributions. Furthermore, we also would like to thank Dr. Zhefei Pan for his assistance in preparing this

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book. Last, but not the least, we acknowledge the professional staff from Springer for their continuous support. Hong Kong SAR, China Chongqing, China Xi’an, China

Liang An Rong Chen Yinshi Li

Contents

Ordered Catalyst Layer Design for Proton Exchange Membrane Fuel Cells: Principle and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Xiaohui Yan, Yuwei Liang, Shuiyun Shen, and Junliang Zhang

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Electrocatalysis for Proton Exchange Membrane Fuel Cells . . . . . . . . . . . Chunhui Xiao, Tianxi He, and Lu Zhang

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Photoelectrochemical Flow Cells for Solar Fuels and Chemicals . . . . . . . . He Lin and Liang An

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Design and Performance of Organic Flow Batteries . . . . . . . . . . . . . . . . . . . Oladapo Christopher Esan, Xiaoyu Huo, Xingyi Shi, and Liang An

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Aqueous Organic Redox Flow Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hao Fan, Hongyu Xu, and Jiangxuan Song

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Electrodes for All-Vanadium Redox Flow Batteries . . . . . . . . . . . . . . . . . . . 147 Rui Wang and Yinshi Li Microfluidic Flow Cells for Energy Conversion and Utilization . . . . . . . . . 173 Hao Feng, Ying Zhang, Dong Liu, and Qiang Li Flow Cells for CO2 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 Qing Xia, Mingcong Tang, and Xiao Zhang Flow Cells for Ambient Ammonia Synthesis via Electrocatalytic Nitrogen Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 Yun Liu, Zhefei Pan, and Liang An

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Ordered Catalyst Layer Design for Proton Exchange Membrane Fuel Cells: Principle and Methods Xiaohui Yan, Yuwei Liang, Shuiyun Shen, and Junliang Zhang

Abstract Proton exchange membrane fuel cell is an ideal energy conversion technology that owns unique advantages including zero emission, high efficiency, and high power density. However, the cost issue caused by the use of Pt-based catalysts limits the widespread application of PEMFC. When lowering the Pt loading in cathode catalyst layer, an increase in concentration polarization occurs that limits the cell performance. To address this issue, the improvement in catalyst layer structure is required to enhance transport process, especially for low-loading and ultralowloading PEMFCs. This chapter focuses on the effects of ordered catalyst layer structures on multicomponent transport and the different designs of ordered catalyst layers. The designs of ordered catalyst layer are illustrated respectively, including the supporting material-based one, the catalyst-based one, and the proton conductorbased one. Each design of the ordered catalyst layer is concluded with the fabrication process, structure characterization, and performance improvement. Keywords PEMFC · Multicomponent transportation · Catalyst layer · Ordered structure · Oxygen transport

X. Yan (B) · Y. Liang · S. Shen · J. Zhang (B) Institute of Fuel Cells, School of Mechanical Engineering, Shanghai Jiao Tong University, Dongchuan Rd. 800, Shanghai, China e-mail: [email protected] J. Zhang e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 L. An et al. (eds.), Flow Cells for Electrochemical Energy Systems, Green Energy and Technology, https://doi.org/10.1007/978-3-031-37271-1_1

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1 Introduction With the increasing consumption of fossil fuels and rising CO2 emissions, environmentally friendly, safe, and efficient energy and energy conversion devices are highly desired [1]. Hydrogen is regarded as promising energy with zero carbon emissions and clean products, which can be utilized by fuel cells. Proton exchange membrane fuel cells (PEMFCs), which are powered by hydrogen and oxygen, are promising energy conversion devices that have the unique advantages of fast start-up speed, quick response to load, low operating temperature, and high efficiency [2]. Although promising, the key factor limiting the commercialization of PEMFCs is the high cost. The cost of PEMFCs mainly comes from the raw materials, particularly the precious metal platinum used as catalysts, which occupies almost 55% of the total cost and will further increase with the expansion of the production scale [3]. Therefore, the reduction of Pt loading in catalyst layer (CL) is the key to decreasing cost and promote commercialization. Unfortunately, it is found that there is an unexpected increase in concentration polarization at high current density region when lowering the Pt loading, especially below 2.0 mg cm−2 which limits the cell performance [4–6]. To reduce the Pt loading without performance sacrifice, the modification of membrane electrode assembly (MEA) is required. The conventional MEAs are fabricated from a catalyst ink which is a mixture of catalyst, ionomer, and solvent. In the conventional catalyst layer, catalyst particles randomly connect with each other to form the electron transport pathway, and ionomers also randomly coat the surface of catalyst particles to form proton transport pathway. Due to the low Pt utilization and the tortuosity of mass transport channels caused by the disordered pore structures, the random distribution of catalyst nanoparticles and proton conductors, the concentration polarization is relatively large. And the design of the catalyst layer is of great significance to enhance the mass transport within MEA. Many factors of the catalyst layers are considered, such as the catalyst category, the loading method of the catalyst, the morphology of the catalyst layer, and the I/C ratio. Great efforts have been done to develop novel catalysts with high oxygen reduction reaction (ORR) activity, such as Pt alloy catalysts, catalysts with core– shell structure, and single-atom catalysts [7]. The Pt alloy catalyst is the alloy of platinum and transition metal, which shows higher performance with a low Pt loading compared to the conventional Pt/C-based MEA [8]. The core–shell catalysts with Pt skin can maximize the Pt utilization and enlarge the electrochemical active area that further improves the cell performance [9]. However, such novel catalyst-based MEA still encounters the same dilemma as conventional MEA, which is the high concentration loss at low Pt loading [10]. Apart from changing the catalysts to improve the performance, reconstructing the transport pathways of protons, electrons, gases, and water within the MEA is capable to address this issue. The anode is supplied by pure hydrogen with large diffusivity, while the cathode is supplied by air with oxygen concentration of 21% only and low diffusivity. Thus, the concentration polarization is mainly attributed to the transport process in the cathode catalyst layer. Therefore, great efforts have been paid

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Fig. 1 Schematic diagram of a conventional MEA structure, b MEA with semi-ordered structure (such as using PVA to construct pores), and c ordered MEA structure

to the modification of cathode catalyst layer structure to reduce the concentration polarization. Some researchers alter the pores in the catalyst layer to construct the semi-ordered transportation channels which are beneficial to the oxygen transport (Fig. 1b). For instance, Cheng et al. [11, 12] modified the CCLs structure by using magnesium oxide (MgO) as the pore-forming agent. They invented a dual-layer design to directly measure the bulk and local oxygen transport resistances in the cathode catalyst layer. They denoted the removal of MgO created lots of mesoscale cavities and thus reducing the bulk oxygen transport resistance. The ordered structure design of cathode catalyst layer is the well-recognized way to construct more efficient transport pathways for gases, water, and protons (Fig. 1c) that could realize the high-performance and low Pt-loading PEMFC. For example, Middleman et al. proposed a novel structure of the cathode catalyst layer with ultrathin thickness and vertically aligned support, which simultaneously enhance the transport of reactants, electrons, and protons as the reaction sites are located at the surface of the supporting materials [13]. Inspired by this CL structure, the concept of the ordered catalyst layer is proposed, which provides ordered transport channels [14] for the multicomponent transport, as shown in Fig. 1 [15, 16]. Various designs have been proposed and prepared, such as vertically aligned carbon nanotubes-based one, Pt nanorods-based one, Pt nanowires-based one, and Nafion arrays-based one [17, 18]. These nanostructures can significantly improve the Pt utilization in the catalyst layers, facilitate the transport of protons, electrons, gases, and water, as well as enlarge the electrochemical active surface area (ECSA) [5, 19–25]. Herein, this paper focuses on the previous and most recent advances in the order-structured catalyst layers for PEMFCs.

2 Effects of Ordered Catalyst Layer The effects of order-structured MEAs can be mainly attributed to the optimization in three aspects, which are Pt utilization, bulk oxygen transport, and local oxygen transport.

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The Pt utilization is directly linked to the formation of triple-phase boundaries in the catalyst layers, which is related to the formation of effective catalyst/ionomer interfaces in the catalyst layers (Fig. 2a). Therefore, the optimization of Pt utilization is also the modification of the catalyst/ionomer interfaces to construct more triplephase boundaries in the catalyst layers. In conventional MEAs, the method of Pt utilization improvement includes improving the catalyst dispersion in catalyst ink, optimizing the solvent of catalyst ink, optimizing the I/C ratio, and controlling the ink drying rate. In general, lower Pt loading would result in higher Pt utilization. However, the conventional MEAs would encounter large polarization loss at high current density region when lowering the Pt loading. This phenomenon is caused by the increased resistance for oxygen transport in the vicinity of catalyst surface as less triple-phase boundaries can be formed. In the ordered catalyst layers, the catalyst dispersion is much better compared to that in conventional catalyst layers. The Pt catalysts can either be dispersed on the ordered supporting materials or the ordered ionomer structure, and it could also become nano-scale thin films covered on the supporting materials. As a result, the catalyst/ionomer interfaces enlarge and more triple-phase boundaries can be formed, which can expose more active sites that in turn not only improves the ORR activity but also improves the oxygen transport process around the Pt surface (Fig. 2b). The bulk oxygen transport resistance is defined as the resistance for oxygen to transport through pore structures of the CL. In conventional MEAs, when the oxygen passes through the gas diffusion layer and reaches the catalyst layer, the complicated pore structures of the agglomerates are the next barriers. The pore structures mainly contain three types of pores, which are mesopores, micropores, and dead pores. The mesopores and micropores are transport channels for oxygen and water with different resistances, while the dead pores are self-closed zones which are infeasible for mass transport. For the ordered catalyst layer, the number of dead pores is minimized and it is possible to build relatively uniform pores with low tortuosity which would greatly increase the effective oxygen diffusivity and lower the proton resistance of the CL. The local oxygen transport resistance is the resistance for oxygen transport in the vicinity of catalyst surface, i.e., the resistance from the pore space to active sites

Fig. 2 a Schematic diagram of triple-phase boundaries of conventional MEAs. b Schematic diagram of triple-phase boundaries of ordered CL

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through the PFSA ionomer film. The ultrathin PFSA films with the thickness of ~ 10 nm would mostly cover the exposed surface of Pt catalysts, which hinder the paths for oxygen reaching the active sites of Pt catalysts (Fig. 2a). Lowering the Pt loading would lead to the decrease in total ECSA which requires a larger local oxygen flux (per unit area of Pt) if the MEA maintains a same current density (per unit geometric area of MEA). In conventional MEAs, besides the decrease in ECSA, the decrease in Pt loading might also result in the uneven distribution of PFSA ionomers on catalysts surface and a thicker ionomer film in some areas. This would significantly enlarge the local oxygen transport resistance and increase the polarization loss at high current density region, which is detrimental to the development of low Pt-loading PEMFC. When it comes to the ordered catalyst layers, for ionomer-based ordered CL, the ultrathin PFSA films would acts as the structure matrix rather than covering the catalysts surface that avoids the oxygen transport process through the ionomer film, which would greatly reduce the local resistance. For catalyst-based and supporting materials-based ordered CL, there is higher chance of forming some uncovered catalysts with fully exposed active sites. And for the covered catalysts, a more uniform ionomer film would be formed on catalyst surface and the ionomer agglomerates would be avoided (Fig. 2b), which could also improve the local transport process. Thus, the Pt utilization, the bulk oxygen transport process and the local oxygen transport process would be simultaneously improved with the ordered catalyst layer structure. Therefore, the ordered CL can reach much higher performance at high current density region, especially at low Pt loading.

3 Ordered Catalyst Layer Design There are three types of ordered catalyst layer design, which are supporting materialsbased one, catalyst-based one, and proton conductor-based one.

3.1 Supporting Materials-Based Ordered Catalyst Layer Supporting materials-based design refers to the catalyst supporting materials with ordered structure, and the catalyst will be uniformly distributed onto the orderstructured supporting materials. The order-structured supporting materials can lead to a synergetic effect, which results in increased conductivity, large average pore size, and high liquid permeability. The two principal methods of ordered supporting materials design are carbon nanoarrays design and oxide nanoarrays design. And carbon nanoarrays design is widely reported because the interaction between graphite lattice structure and Pt particles can also improve catalyst activity and durability. Vertically aligned carbon nanotube (VACNT) is an effective solution to construct ordered carbon support. Meng et al. [26] demonstrated that the Pt-deposited VACNTs forests significantly enhanced the cell performance at high current densities since the

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Fig. 3 Schematic diagram of fabrication of MEA with order-structured CL constructed by Pt nanoparticles onto VACNTs and commercial Pt/C as anode [26]

ordered CNT forests with better hydrophobicity provide sufficient channels for mass transport and water removal. They deposited Fe on the Al foils via the method of E-beam to catalyze the growth of VACNTs and used acetylene as a carbon resource to grow VACNTs in a PECVD system. And then Pt nanoparticles were deposited onto the VACNTs via the method of E-beam, followed by impregnating Nafion ionomers in Pt/VACNTs to ensure proton conduction and finally transferring the ordered Pt/ VACNTs from Al foils to Nafion membrane by hot-pressing (Fig. 3). They also demonstrated the existence of the optimal ratio of Pt catalyst to VACNT length for better cell performance, as the Pt catalyst-loaded regions are the reaction sites and the CNT forests without Pt catalyst are proton channels. And the optimal ratio is the Pt catalyst deposited on the top depth of around 600 nm on VACNTs with a length of 4.6 µm, with a peak power density of 1.61 (H2 /O2 , 150 kPa) and 0.79 W cm−2 (H2 /Air, 150 kPa) at an ultralow Pt loading of 50 µg cm−2 . They concluded that the hydrophilic top and hydrophobic base structure could provide sufficient water for membrane wetting, promote mass transport and reach high Pt utilization. Liang et al. [27] also prepared VACNT as ordered catalyst support, carried out three-dimensional modeling of MEA with Pt loading 1.23 eV and convert the energy into H2 and O2 . This process must generate two electron–hole pairs per hydrogen molecule or four electron–hole pairs per oxygen molecule. To carry out one or both reactions without recombination, photoinduced free charge electrons and holes in the semiconductor must travel to a liquid junction, and must react only with solution species

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directly at the semiconductor surface. This electron transfer process at semiconductor/liquid junctions produces losses due to the concentration and kinetic overpotentials needed to drive the HER and the OER. The energy required for photoelectrolysis at a semiconductor photoelectrode is therefore frequently reported as 1.6–2.4 eV per electron–hole pair generated, to account for these losses [13].

Fig. 4 Overall solar water splitting [4]

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3.2 Recent Advances A PEC cell device has to comply with several requirements for lab applications: (a) photoelectrodes configuration, (b) light penetration through the cell to reach the photoelectrodes, (c) resistance to corrosive electrolytes, (d) need for continuously electrolyte feeding and (e) need of a membrane to maintain the evolved gases separated [56]. Lopes et al. [56] reported a PFC design with two removable metallic windows screwed to a transparent acrylic part crossing a synthetic quartz window (Fig. 5). A black acrylic mask is placed next to the metallic windows, allowing an illumination area of 10 × 10 cm2 . This separator can be a commercial Nafion® membrane that allows just protons to permeate in an acid media or a stretched porous hydrophobic Teflon® membrane (diaphragm), which exhibits a high porosity of micrometer size pores that prevent hydrogen and oxygen bubbles to permeate. Three photoelectrodes were measured: WO3 applied on metal and TCO-glass substrate and undoped hematite deposited on TCO-glass substrate. The highest photocurrent density was obtained with photoelectrode WO3 -Metal, ca. 0.90 mA cm−2 at 1.45 V RHE, which corresponds to a hydrogen evolution rate of ca. 0.27 mmol H2 s−1 and an STH efficiency of 1.28%. With transparent photoelectrodes, partially due to the higher charge transport resistance imposed by the TCO layer on glass substrates, the produced photocurrent was lower: 0.30 mA cm−2 and 0.40 mA cm−2 for the WO3 and undoped-Fe2 O3 , respectively. Jeng et al. [57] reported a PFC for separate generation of hydrogen and oxygen using UV-sensitive Degussa P25 TiO2 powder as a model photoanode catalyst. The PFC was fabricated by hot pressing the photoanode and Pt dark cathode, each coated with a suitable composite catalyst layer, onto a piece of Nafion® 115 membrane (Fig. 6a). It served as a compact photocatalytic reactor for water splitting as well as an effective separator for the generated hydrogen and oxygen resulting in a safe operation. The photo conversion efficiency was found to be 15-30 % higher without the addition of water in the cathode compartment than with the addition of water. Ronge et al. [58] developed a PFC with a photoanode of carbon paper loaded with

Fig. 5 10 × 10 cm2 photoelectrochemical cell: a disassembled, b under operating conditions and c detail of the innovative feeding system [56]

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TiO2 and a cathode of Pt dispersed on carbon black, which were fixed on both sides of a Nafion film and electrically coupled by an external circuit. Anode and cathode compartments with serpentine flow fields were operated either in the liquid or vapor phase (Fig. 6b). This photocurrent corresponds with 64 μA cm−2 and overall photoconversion efficiency of ca. 0.1%. Seger et al. [59] designed a PFC based on a direct methanol fuel cell, the electrons generated by the photoanode are transferred to the platinum cathode through an external circuit. A quartz window allows direct excitation of the photocatalyst in the anode chamber. The holes are removed by methanol and eventually decomposed into carbon dioxide and protons. The protons are transported through the Nafion membrane and reduced at the cathode to form hydrogen. The membrane and electrodes are assembled in the same way as in a fuel cell, where a membrane electrode assembly (MEA) separates two graphite flow field plates (Fig. 7a). The electrolyte resistance is minimized because the two electrodes are very close. Photocurrent generation of 0.34 mA/cm2 under UV irradiation (λ > 300 nm) shows the effective operation of the fuel cell in reverse for solar hydrogen production. A polarization curve of the PFC exhibits an open-circuit voltage of 0.5 V and a maximum power of 45 μW/cm2 . The reduction of H+ at a Pt cathode under no applied bias was confirmed from the evolution of H2 with a rate of 69 μL h−1 cm−2 (Fig. 7b).

Fig. 6 Schematic diagram of a Jeng et al. [57] and b Ronge et al. [58] reported PFCs

Fig. 7 a Schematic diagram and b Hydrogen production rate of Seger et al. [59] reported PFC

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3.3 Key Findings and Challenges Currently, the research is still mainly focused on photoelectrode materials or structures, and there are not many reports on the use of photoelectrodes in PFC, and the only reports do not present the advantages of the continuous flow characteristics of PFC compared with other cell structures. The optimization of PFC assembly is also rarely involved, and the effect of assembly on mass transfer is still not fully studied. The effects of coupling photocatalytic reactions with mass transfer in PFC water splitting also deserve to be studied. The hydrogen production efficiency of the whole cell should be continued to be improved for commercialization possibility.

4 Carbon Dioxide Reduction Reaction for Solar Fuels and Chemicals The ever-increasing emission of carbon dioxide from fossil fuels (e.g., natural gas, coal, petroleum, etc.) combustion is still exacerbating the environment that significantly affects humanity [60]. The development of solar-powered carbon dioxide reduction holds the promise to mitigate greenhouse gas emissions into the atmosphere, while simultaneously converting renewable solar energy into storable value-added chemicals and fuels.

4.1 Thermodynamic and Kinetic CO2 is an extremely stable molecule generally produced by fossil fuel combustion and respiration. Returning CO2 to a useful state by activation/reduction is a scientifically challenging problem, requiring appropriate catalysts and energy input. CO2 molecule contains two C = O bonds with a linear configuration. The orbitals of C atom are sp-hybridized, forming two σ bonds with O atoms, while the other two nonhybridized p orbits in C atom form two π3 4 bonds with the p orbits in O atoms. As a result, the C = O bonds are extremely stable and have dissociation energy as high as ~ 750 kJ mol−1 , which indicates that a high activation barrier must be overcome for CO2 activation and C = O bond cleavage. Moreover, given the highest chemical state (+4) of C atom in CO2 , the reduction of CO2 can result in complex products including CO, HCOOH, methane, ethylene, methanol, ethanol, propanol, etc. [61]. The thermodynamic redox potentials for various CO2 reduction products are listed in Fig. 8 [62].

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Fig. 8 Thermodynamic redox potentials for various CO2 reduction products [62]

4.2 Recent Advances for CO2 Reduction to CO The most common product for CO2 reduction is CO, where the reduction proceeds via the *COOH-to-*CO pathway. The CO product is generated by the further desorption of *CO intermediate from the active sites. A weak adsorption ability of the *CO at the reaction sites is then the main feature of such catalysts to drive the PEC CO2 reduction to produce CO [63]. Lu et al. [64] reported a configuration for the direct reduction of gaseous carbon dioxide to carbon monoxide in a PFC using a membrane cathode based on Ag nanocubes and a WO3 /BiVO4 photoanode. Figure 9 shows the schematic illustration of the setup, in which the self-designed horizontally aligned PEC reactor, connected to a gas circulation system, was used as the flow cell to carry out the PEC conversion of CO2 . In this PFC, high selectivity of CO with the suppression of competitive H2 evolution had been observed, and an electro-to-chemical energy efficiency of 92.1% had been obtained. In most PFCs, H2 is usually evolved together with carbon monoxide on the photocathode surface, except for a few reports achieving 100% CO production. This mixture of CO and H2 , also known as syngas, is an essential and useful feedstock for industrial Fischer–Tropsch conversion [65]. Kistler et al. [66] reported a 4 cm2

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Fig. 9 Schematic illustration of the setup of the PFC for CO2 reduction to CO [64]

monolithic PFC that converts CO2 to syngas at a record STF efficiency above 10% (combined solar-to-CO and solar-to-H2 efficiency). The PV-integrated membrane provides a unique, compact structure similar to conventional membrane electrode assemblies (MEAs) used in commercial, grid-based electrolyzers and is the center of the PFC device (Fig. 10). A humidified feed of high-purity CO2 is supplied to the device cathode via a bubble humidifier, where it is electrochemically reduced to CO on an Au catalyst layer deposited on carbon paper. H2 evolution competes with the reduction of CO2 , resulting in a product mixture of CO and H2 . Both reactions produce hydroxide ions, which diffuse through the anion exchange membrane to the anode side of the cell, where they react to form oxygen over an iridium catalyst supported by carbon paper. Urbain et al. [67] reported a 10 cm2 PFC that consists of a cathode made of copper foam coated with low-cost nanosized zinc flakes as the catalyst to perform the CO2 reduction reaction to syngas, an adapted silicon heterojunction solar cell structure as photoanode with nickel foam as the catalyst to facilitate the oxygen evolution reaction, and a bipolar membrane separating the respective catholyte and anolyte compartments (Fig. 11). This stand-alone reactor device exhibited a stable and bias-free operation with a solar-to-syngas conversion efficiency of 4.3%.

4.3 Recent Advances for CO2 Reduction to C1 Products C1 products include formic acid, methanol and methane, etc. Formic acid is a common product for CO2 reduction with a 2e− -transfer reaction kinetics. It is a

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Fig. 10 Exploded view of the monolithic PFC device that converts CO2 to syngas [66]

Fig. 11 Schematic illustration of the experimental setup of 10 cm2 PFC that converts CO2 to syngas [67]

liquid product, which is not only an important feedstock for the pharmaceutical and chemical industry but also a potential hydrogen carrier for fuel cell applications [68]. Irtem et al. [40] reported a solar driven PFC consisting of TiO2 photoanode and an electrodeposited Sn on gas diffusion electrode (cathode) for the CO2 reduction to HCOO− under visible light irradiation (Fig. 12). At an applied bias potential of 1.2 V, faradaic efficiencies of 40–65% for HCOO− production were obtained on the Sn cathode, reaching energy efficiencies up to 70%.

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Fig. 12 PFC scheme using TiO2 photoanode and Sn cathode for CO2 reduction to HCOO− [40]

Kalamaras et al. [69] used the CuO-based thin films as photocathodes in a continuous flow microfluidic PEC reactor for CO2 reduction to formate. The highest photocurrent density obtained was for the α-Fe2 O3 /CuO photoelectrode yielding − 1.0 mA cm−2 at 0.3 V versus RHE and initial results indicated a solar-to-fuel (STF) efficiency of 0.48%. While the CuO, Cu2 O and CuO-Cu2 O photoelectrodes virtually only formed formate, the bilayer α-Fe2 O3 /CuO photocathode produced methanol in addition to formate. Methanol is the 6-electron transfer product of CO2 reduction, which is derived from the further hydrogenation of *CO intermediate. Due to the merits of high energy density by weight and volume and the feasibility for storage and transportation, methanol is regarded as the most promising chemical for solar energy storage, naming as the liquid sunshine [70]. Nowadays, there are few studies on the overall PFC devices for photoelectrochemical conversion of CO2 to methanol, the research keynote is mainly photoelectrodes. Some work on TiO2 -protected InP and GaP photocathodes has shown that TiO2 rich in oxygen vacancies (Ti3+ sites) has the activity to catalyze the reduction of CO2 to produce CH3 OH. However, the FEs of CH3 OH on InP/TiO2 and GaP/TiO2 photocathodes are still below 10% [63]. To further increase the activity of producing CH3OH, Lee et al. [71] introduced Cu+ species in the TiO2 overlay, which is considered to be more active than Ti3+ for CO2 activation. Using Cu2 O as a light-absorbing substrate, the Cu2 O/TiO2 -Cu+ photocathode showed a CH3 OH FE of 56.5%, which is twice as high as that of bare Cu2 O. A step further, Kang et al. [72] found that the thickness of TiO2 layer was a key factor affecting the FE toward CH3 OH during the PEC CO2 reduction process. The Cu2 O-based photocathodes with 30 nm-thick TiO2 layer only produce hydrogen, while those with a 5 nm-thick TiO2 layer showed the ability to reduce CO2 . Methane, the simplest hydrocarbon chemical with the largest heating value of 55.5 MJ Kg−1 and the lowest carbon-emission among the CO2 reduction products,

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is an important petrochemical commodity gas fuel for industrial and domestic use. It is also called natural gas, whose application has a well-established infrastructure for gas storage, distribution, and consumption [73]. The CO2 reduction to CH4 has eight electron- and proton-transfer processes, where multiple carbonaceous intermediates are involved during these processes. The PEC CO2 reduction to methane mostly uses copper-based catalysts as the cocatalyst. For example, Wang et al. [74] reported that the n+ p-Si/GaN photocathode produced only CO during PEC CO2 reduction. When copper nanoparticles were grown as cocatalysts, methane evolved on the n+ p-Si/ GaN/Cu surface. However, in this work, the FE to CH4 was quite low (below 19%) and hydrogen remained the dominant product (over 70%) on the n+ p-Si/GaN/Cu photocathode. To improve the FE to CH4 , Zhou et al. [75] introduced CuFe binary metal as a cocatalyst for PEC methane production. The FE toward methane was increased from ~ 20% on n+ p-Si/GaN/Cu to 51% on n+ p-Si/GaN/CuFe, evidencing the superior catalytic activity of CuFe for CH4 production.

4.4 Recent Advances for CO2 Reduction to C2+ Products Besides the C1 products, the CO2 reduction cells also yield C2+ chemicals, such as C2 H4 , C2 H5 OH, CH3 COOH, etc. Considering that the values of ΔG° for C2+ production are larger than those for C1 products, it is suggested that more solar energies are stored during the PEC C2+ production process. Also, these C2+ products are value-added commodities in the chemical industry. However, due to the high thermodynamic energy barrier (> 963.8 kJ mol−1 ) and the multiple electron (> 8e− ) transfer process during C2 production, the reaction kinetics for CO2 reduction to C2+ chemical is rather complicated. Acetate (CH3 COOH) is an 8e− transfer product of CO2 , which is the most common building block for biosynthesis. There are only a few reports on PEC CH3 COOH production, and among these works, CuFe oxide-based photocathodes are a common choice for the production of CH3COOH. Yang et al. [76] synthesized a CuFeO2 /CuO mixed oxide photoelectrocatalyst for PEC CO2 reduction to acetate. The highest FE toward CH3 COOH reached 80% on the mixed-phase CuO/CuFeO2 photocathode with a Fe:Cu ratio of 1.3. Especially, by varying the Fe:Cu ratio in the CuFe mixedphase oxide from 1.3 to 0.1, the CO2 reduction products on CuO/CuFeO2 changed from primarily acetate to primarily formate. Ethylene (C2 H4 ) is a 12-electron transfer product with a carbon–carbon double bond for CO2 reduction. Cu is thought to be the most effective site for electrocatalytic CO2 reduction to ethylene [54]. Kempler et al. [77] deposited highly loaded Cu cocatalysts on n+ p-Si substrates, which evolved C2+ products including C2 H4 and C2 H6 . Ba et al. [78] used Cu/Cu2 O as photocathode to reduce CO2 and detected CH4 and C2 H4 as the main products. Ethanol (C2 H5 OH) is another 12-electron transfer product of CO2 reduction. Kecsenovity et al. [79] reported that with the addition of carbon-based additives, the graphene/Cu2 O photocathodes would produce ethanol but not C2 H4 during the PEC

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test. This is because the kinetics of the CO2 reduction reaction after C–C coupling on the Cu2 O surface is modified by the carbon material. Zhou et al. [80] reported the Cu2 ZnSnS4 (CZTS)/CdS photocathode is also effective in reducing carbon dioxide to ethanol. CdS with S-vacancies favors CO production, while CdS with oxygen dopants favors ethanol production. Theoretical calculation indicated that the oxygen atom in the CdS lattice would lead to active sites for C–C coupling, resulting in the selective change from CO to ethanol in the PEC CO2 reduction process.

4.5 Key Findings and Challenges Though much effort has been devoted to PFC CO2 reduction, this technology is still in its early stage for practical applications. A deeper understanding of the reaction mechanism on cocatalyst/semiconductor photoelectrode is needed. The PFC CO2 reduction process involves surface catalysis and charge carrier dynamics. On the one hand, the catalytic activity is directly related to the photovoltaic performance of the photoelectrode, while the photovoltaic performance of the photoelectrode is sensitively affected by the interfacial energetics between the semiconductor and the cocatalyst. On the other hand, the product selectivity is affected by various factors, such as the nature of the cocatalyst, the supply of reactants, the electrolyte conditions, and the interaction of the cocatalyst and the substrate. Although much knowledge has been accumulated on the reaction mechanisms of electrocatalytic CO2 reduction and PEC water splitting, we cannot directly transfer this knowledge to PFC CO2 reduction. For example, in electrocatalysis, C2+ chemistry is the main product of Cu metal, while CH4 is produced as the main product on n+ p-Si/GaN/Cu photocathode. Therefore, it is necessary to systematically study the influence of different factors on the mechanism of PFC CRR. Many PEC CO2 reduction products, such as methanol, methane and C2+ products, are still studied at the electrode scale, and have not been put into the flow cell system to test their performance. In addition, as mentioned in PFC water splitting, there is still a lack of research on the effect of mass transport loss on PFC carbon dioxide reduction in operation conditions. The energy conversion efficiency of PFC carbon dioxide reduction also needs to be further improved.

5 Nitrogen Reduction Reaction for Solar Fuels and Chemicals Nitrogen plays an essential role in life on Earth, forming the basic building blocks of living organisms such as nucleotides and proteins. However, the natural abundance of dinitrogen in the atmosphere is chemically inert and cannot be used directly by humans unless it is converted into more reactive forms by reductive or oxidative nitrogen fixation processes [81]. Until Haber and Bosch invented an industrial process

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Fig. 13 Reduction potentials (vs. NHE at pH = 0) of typical nitrogen reactions [84]

for the reduction of N2 to NH3 via H2 converted from fossil fuels in the early twentieth century, almost all nitrogen harvested by humans came from biological nitrogen fixation by microorganisms.

5.1 Thermodynamic and Kinetic Aspects Nitrogen reduction is a natural process in which gaseous nitrogen reacts chemically with other elements and is converted into useful chemicals such as ammonia (NH3 ), hydrazine (N2 H2 ), which are the two important species in the field of both energy and the environment [82]. N2 represents one of the most inert chemical species in nature due to the high dissociation energy of N≡N bond (941 kJ mol−1 ), the absence of permanent dipole, and a high ionization potential (15.0 eV) [83]. Nitrogen reduction in the aqueous solution is a proton-coupled electron transfer reaction involving a 6-electron complete reduction process into two ammonia molecules per dinitrogen molecule or a 4-electron side reaction into hydrazine. Figure 13 shows the potential of some reactions related to photocatalytic nitrogen reduction at pH = 0 [84].

5.2 Recent Advances In recent years, several PEC NRR studies have focused on developing highperformance photoelectrode materials to further the understanding of the reaction mechanisms involved in PEC NRR. Jang et al. [85] studied PEC N2 reduction by using CuO and Cu2 O photocathodes that are known to be poorly catalytic for water reduction, the major reaction competing with N2 reduction (Fig. 14a). When tested

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with isotopically labeled 15 N2 in 0.1 M KOH solution under solar illumination, the CuO and Cu2 O photocathodes produced 15 NH3 with a Faradaic efficiency of 17% and 20% at 0.6 and 0.4 V versus reversible hydrogen electrode, respectively. Bai et al. [86] reported a p-type BiVO4 (p-BiVO4 ) photocathode for NRR under ambient conditions (Fig. 14b). Interestingly, V sites are proved as the mainly ideal active center for N2 adsorption/activation based on the density functional theory (DFT) calculations. The ammonia evolution rate (11.6 × 10–8 mol h−1 cm−2 ) and Faradaic efficiency (16.2%) were obtained at − 0.1 V versus RHE in 0.1 M Li2 SO4 solution. In addition to these metal oxides, metal sulfides can also be used as photocathode materials. Bi et al. [87] developed a Cu2 S-In2 S3 heterostructure catalyst for PECNRR possessing sufficient carriers to enable higher carrier mobility and a reasonable recombination lifetime in the PEC process. The Cu2 S-In2 S3 heterostructure catalyst achieves an NH3 production rate of 23.6721 μg h−1 cm−2 and a higher Faradaic efficiency (33.2 5% at − 0.6 V versus the reversible hydrogen electrode) in aqueous solutions under ambient conditions. The abovementioned literature refers mainly to the development of photocathodes and the reduction of nitrogen directly on the photocathode. However, the lack of suitable p-type semiconductor as well as its fair stability largely limits the application of direct photocathodic NRR [88]. A promising substitution for direct photocathodic reaction system is to employ an efficient catalyst as cathode for electrochemical NRR combined with a photoanode for light harvesting and providing photo-generated electrons. Xu et al. [89] developed a PEC system that contains the TiO2 nanorods on FTO as photoanode and B doped Bi on glassy carbon as the electrochemical cathode

Fig. 14 a CuO and Cu2 O [85] and b p-BiVO4 [86] for PEC NRR to ammonia

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Fig. 15 PEC system which contains the TiO2 nanorods on FTO as photoanode and B doped Bi on glassy carbon as the electrochemical cathode for NRR [89]

for NRR (Fig. 15). Such a PEC system offers an ammonia yield rate of 29.2 mgNH3 gcat. −1 h−1 and Faradaic efficiency of 8.3% at a bias of 0.48 V versus RHE in nitrogen fixation.

5.3 Key Findings and Challenges As a promising alternative to the Haber-Bosch process, the PFC nitrogen reduction reaction is attracting the interest of scientists, especially because it can take advantage of freely available sunlight. From the current situation, PFC nitrogen reduction technology is still far from commercialization, and research is mainly focused on the design and fabrication of suitable photoelectrode materials. Studies applied to PFC NRR systems have not been reported, let alone the effect of factors such as mass transfer in flow cell systems. In addition, NRR competes with HER and the reaction selectivity affects the energy conversion efficiency of the NRR reaction, which should also be further improved.

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6 Future Perspectives and Outlook Here, we have thoroughly examined the challenges and progress of using PFCs to provide clean chemical fuels and chemicals. The introduction of a flow cell structure into a PEC system retains the unique advantages of the flow cell architecture, while increases the flexibility of component design and system optimization, creating plenty of room for performance improvement and system cost reduction. Though there has accumulated much knowledge in electrochemical flow cell technology and PEC reactions, we could not directly transfer this knowledge to PFC for material screening or structure engineering. To make the PFC technology viable, the performance must be substantially improved. In addition to the development of photoelectrochemical materials with novel properties for the light absorption and the charge separation, the realization of performance improvement depends on the innovation and optimization of the photoelectrode structure and flow cell configuration, which require a critical understanding of mass and charge (including ions, electrons and holes) transport in this PFC system. Photocathodes, as a key component, provide sites for the light absorption, charge generation and separation, as well as water/carbon dioxide/nitrogen adsorption and reduction and offer pathways for the simultaneous transport of multiple species during the PFC operation. In addition, the PFC configuration parameters also play a very important role in transport processes, including diffusion, convection, and migration, among functional components.

References 1. Chu S, Majumdar A (2012) Nature 488:294–303 2. She ZW, Kibsgaard J, Dickens CF, Chorkendorff I, Nørskov JK, Jaramillo TF (2017) Science 355:eaad4998 3. Landman A, Dotan H, Shter GE, Wullenkord M, Houaijia A, Maljusch A, Grader GS, Rothschild A (2017) Nat Mater 16:646–651 4. Kim JH, Hansora D, Sharma P, Jang J-W, Lee JS (2019) Chem Soc Rev 48:1908–1971 5. Wang Y, Liu J, Wang Y, Wang Y, Zheng G (2018) Nat Commun 9:5003 6. Cheng W-H, Richter MH, Sullivan I, Larson DM, Xiang C, Brunschwig BS, Atwater HA (2020) ACS Energy Lett 5:470–476 7. Guo W, Zhang K, Liang Z, Zou R, Xu Q (2019) Chem Soc Rev 48:5658–5716 8. Ali M, Zhou F, Chen K, Kotzur C, Xiao C, Bourgeois L, Zhang X, MacFarlane DR (2016) Nat Commun 7:11335 9. Mousset E, Dionysiou DD (2020) Environ Chem Lett 18:1301–1318 10. Reichert R, Jusys Z, Behm RJ (2014) Phys Chem Chem Phys 16:25076–25080 11. Scialdone O, Guarisco C, Galia A, Filardo G, Silvestri G, Amatore C, Sella C, Thouin L (2010) J Electroanal Chem 638:293–296 12. Martínez-Huitle CA, Rodrigo MA, Sirés I, Scialdone O (2015) Chem Rev 115:13362–13407 13. Walter MG, Warren EL, McKone JR, Boettcher SW, Mi Q, Santori EA, Lewis NS (2010) Chem Rev 110:6446–6473 14. Tilley SD (2019) Adv Energy Mater 9:1802877 15. Pan L, Kim JH, Mayer MT, Son M-K, Ummadisingu A, Lee JS, Hagfeldt A, Luo J, Grätzel M (2018) Nat Catal 1:412–420

66

H. Lin and L. An

16. Bae D, Seger B, Vesborg PCK, Hansen O, Chorkendorff I (2017) Chem Soc Rev 46:1933–1954 17. Thalluri SM, Wei B, Welter K, Thomas R, Smirnov V, Qiao L, Wang Z, Finger F, Liu L (2019) ACS Energy Lett 4:1755–1762 18. Yao T, An X, Han H, Chen JQ, Li C (2018) Adv Energy Mater 8:1800210 19. Crespo-Quesada M, Pazos-Outón LM, Warnan J, Kuehnel MF, Friend RH, Reisner E (2016) Nat Commun 7:12555 20. Zhang H, Chen Y, Wang H, Wang H, Ma W, Zong X, Li C (2020) Adv Energy Mater 10:2002105 21. Kim TW, Choi K-S (2014) Sci 343:990 22. Sivula K, van de Krol R (2016) Nat Rev Mater 1:15010 23. Lee DK, Lee D, Lumley MA, Choi K-S (2019) Chem Soc Rev 48:2126–2157 24. Kudo A, Omori K, Kato H (1999) J Am Chem Soc 121:11459–11467 25. Kim JH, Lee JS (2019) Adv Mater 31:1806938 26. Kim JH, Han S, Jo YH, Bak Y, Lee JS (2018) J Mater Chem A 6:1266–1274 27. Ma Y, Kafizas A, Pendlebury SR, Le Formal F, Durrant JR (2016) Adv Func Mater 26:4951– 4960 28. Vincent I, Bessarabov D (2018) Renew Sustain Energy Rev 81:1690–1704 29. Gao Y, Robertson GP, Guiver MD, Jian X (2003) J Polym Sci Part A: Polym Chem 41:497–507 30. Hagesteijn KFL, Jiang S, Ladewig BP (2018) J Mater Sci 53:11131–11150 31. Gottesfeld S, Dekel DR, Page M, Bae C, Yan Y, Zelenay P, Kim YS (2018) J Power Sour 375:170–184 32. Rajabi Z, Javanbakht M, Hooshyari K, Adibi M, Badiei A (2021) Int J Hydrogen Energy 46:33241–33259 33. Jiang J, Wieckowski A (2012) Electrochem Commun 18:41–43 34. Hou H, Wang S, Jiang Q, Jin W, Jiang L, Sun G (2011) J Power Sour 196:3244–3248 35. Zeng L, Zhao TS, An L, Zhao G, Yan XH (2015) Energy Environ Sci 8:2768–2774 36. Hisatomi T, Kubota J, Domen K (2014) Chem Soc Rev 43:7520–7535 37. Pinaud BA, Benck JD, Seitz LC, Forman AJ, Chen Z, Deutsch TG, James BD, Baum KN, Baum GN, Ardo S, Wang H, Miller E, Jaramillo TF (2013) Energy Environ Sci 6:1983–2002 38. Burdyny T, Smith WA (2019) Energy Environ Sci 12:1442–1453 39. Singh AR, Rohr BA, Schwalbe JA, Cargnello M, Chan K, Jaramillo TF, Chorkendorff I, Nørskov JK (2017) ACS Catal 7:706–709 40. Irtem E, Hernández-Alonso MD, Parra A, Fàbrega C, Penelas-Pérez G, Morante JR, Andreu T (2017) Electrochim Acta 240:225–230 41. Martín de Vidales JM, Mais L, Sáez C, Cañizares P, Walsh FC, Rodrigo MA, Rodrigues CDA, Ponce de León C (2016) Chem Eng Technol 39:135–141 42. Ren Y, Yu C, Tan X, Huang H, Wei Q, Qiu J (2021) Energy Environ Sci 14:1176–1193 43. Li Z, Wang W, Liao S, Liu M, Qi Y, Ding C, Li C (2019) Energy Environ Sci 12:631–639 44. Sasaki Y, Iwase A, Kato H, Kudo A (2008) J Catal 259:133–137 45. Ma G, Chen S, Kuang Y, Akiyama S, Hisatomi T, Nakabayashi M, Shibata N, Katayama M, Minegishi T, Domen K (2016) J Phys Chem Lett 7:3892–3896 46. Miseki Y, Kusama H, Sayama K (2012) Chem Lett 41:1489–1491 47. Wang W, Chen J, Li C, Tian W (2014) Nat Commun 5:4647 48. Shehzad MA, Yasmin A, Ge X, Ge Z, Zhang K, Liang X, Zhang J, Li G, Xiao X, Jiang B, Wu L, Xu T (2021) Nat Commun 12:9 49. Yan Z, Hitt JL, Zeng Z, Hickner MA, Mallouk TE (2021) Nat Chem 13:33–40 50. Lewis NS (2016) Nat Nanotechnol 11:1010–1019 51. Wang X, Xu C, Jaroniec M, Zheng Y, Qiao S-Z (2019) Nat Commun 10:4876 52. Bamgbopa MO, Almheiri S, Sun H (2017) Renew Sustain Energy Rev 70:506–518 53. Higgins D, Hahn C, Xiang C, Jaramillo TF, Weber AZ (2019) ACS Energy Lett 4:317–324 54. García de Arquer FP, Dinh C-T, Ozden A, Wicks J, McCallum C, Kirmani AR, Nam D-H, Gabardo C, Seifitokaldani A, Wang X, Li YC, Li F, Edwards J, Richter LJ, Thorpe SJ, Sinton D, Sargent EH (2020) Science 367:661–666 55. Choi J, Suryanto BHR, Wang D, Du H-L, Hodgetts RY, Ferrero Vallana FM, MacFarlane DR (2020) Simonov AN Nat Commun 11:5546

Photoelectrochemical Flow Cells for Solar Fuels and Chemicals

67

56. Lopes T, Dias P, Andrade L, Mendes A (2014) Sol Energy Mater Sol Cells 128:399–410 57. Jeng K-T, Liu Y-C, Leu Y-F, Zeng Y-Z, Chung J-C, Wei T-Y (2010) Int J Hydrogen Energy 35:10890–10897 58. Rongé J, Nijs D, Kerkhofs S, Masschaele K, Martens JA (2013) Phys Chem Chem Phys 15:9315–9325 59. Seger B, Kamat PV (2009) J Phys Chem C 113:18946–18952 60. Wang P, Wang S, Wang H, Wu Z, Wang L (2018) Part Part Syst Charact 35:1700371 61. Zhang N, Long R, Gao C, Xiong Y (2018) Sci China Mater 61:771–805 62. Zhang W, Hu Y, Ma L, Zhu G, Wang Y, Xue X, Chen R, Yang S, Jin Z (2018) Adv Sci 5:1700275 63. Li D, Yang K, Lian J, Yan J, Liu S (2022) Adv Energy Mater n/a 2201070 64. Lu W, Zhang Y, Zhang J, Xu P (2020) Ind Eng Chem Res 59:5536–5545 65. Foit SR, Vinke IC, de Haart LGJ, Eichel R-A (2017) Angew Chem Int Ed 56:5402–5411 66. Kistler TA, Um MY, Cooper JK, Sharp ID, Agbo P (2021) Adv Energy Mater 11:2100070 67. Urbain F, Tang P, Carretero NM, Andreu T, Gerling LG, Voz C, Arbiol J, Morante JR (2017) Energy Environ Sci 10:2256–2266 68. Grigioni I, Sagar LK, Li YC, Lee G, Yan Y, Bertens K, Miao RK, Wang X, Abed J, Won DH, García de Arquer FP, Ip AH, Sinton D, Sargent EH (2021) ACS Energy Lett 6:79–84 69. Kalamaras E, Belekoukia M, Tan JZY, Xuan J, Maroto-Valer MM, John M (2019) Andresen, Faraday Discuss 215:329–344 70. Shih CF, Zhang T, Li J, Bai C (2018) Joule 2:1925–1949 71. Lee K, Lee S, Cho H, Jeong S, Kim WD, Lee S, Lee DC (2018) J Energy Chem 27:264–270 72. Kang H-Y, Nam D-H, Yang KD, Joo W, Kwak H, Kim H-H, Hong S-H, Nam KT, Joo Y-C (2018) ACS Nano 12:8187–8196 73. Nisbet EG, Dlugokencky EJ, Bousquet P (2014) Science 343:493–495 74. Wang Y, Fan S, AlOtaibi B, Wang Y, Li L, Mi Z (2016) Chem–A Eur J 22:8809–8813 75. Zhou B, Ou P, Pant N, Cheng S, Vanka S, Chu S, Rashid RT, Botton G, Song J, Mi Z (2020) Proc Natl Acad Sci 117:1330–1338 76. Yang X, Fugate EA, Mueanngern Y, Baker LR (2017) ACS Catal 7:177–180 77. Kempler PA, Richter MH, Cheng W-H, Brunschwig BS, Lewis NS (2020) ACS Energy Lett 5:2528–2534 78. Ba X, Yan L-L, Huang S, Yu J, Xia X-J, Yu Y (2014) J Phys Chem C 118:24467–24478 79. Kecsenovity E, Endrödi BZ, Tóth PTS, Zou Y, Dryfe RA, Rajeshwar K, Janáky C (2017) J Am Chem Soc 139:6682–6692 80. Zhou S, Sun K, Huang J, Lu X, Xie B, Zhang D, Hart JN, Toe CY, Hao X, Amal R (2021) Small 17:2100496 81. Rosca V, Duca M, de Groot MT, Koper MTM (2009) Chem Rev 109:2209–2244 82. Ithisuphalap K, Zhang H, Guo L, Yang Q, Yang H, Wu G (2019) Small Methods 3:1800352 83. Li M, Huang H, Low J, Gao C, Long R, Xiong Y (2019) Small Methods 3:1800388 84. Huang Y, Zhang N, Wu Z, Xie X (2020) J Mater Chem A 8:4978–4995 85. Jang YJ, Lindberg AE, Lumley MA, Choi K-S (2020) ACS Energy Lett 5:1834–1839 86. Bai Y, Lu J, Bai H, Fang Z, Wang F, Liu Y, Sun D, Luo B, Fan W, Shi W (2021) Chem Eng J 414:128773 87. Bi K, Wang Y, Zhao D-M, Wang J-Z, Bao D, Shi M-M (2021) J Mater Chem A 9:10497–10507 88. Chang X, Wang T, Zhang P, Wei Y, Zhao J, Gong J (2016) Angew Chem Int Ed 55:8840–8845 89. Xu F, Wu F, Zhu K, Fang Z, Jia D, Wang Y, Jia G, Low J, Ye W, Sun Z, Gao P, Xiong Y (2021) Appl Catal B 284:119689

Design and Performance of Organic Flow Batteries Oladapo Christopher Esan, Xiaoyu Huo, Xingyi Shi, and Liang An

Abstract Flow battery has been regarded as a promising technology for renewable energy conversion and storage on a large scale as a result of its intrinsically decoupled power output and energy storage capacity. Among its various types, organic flow battery, which employs naturally abundant organic molecules as its redox-active species, is considered as the suitable option toward achieving high performance, enhanced energy density, and reduced costs. In recent years, diverse organic materials and solvents have been employed in flow battery technology. Following this, significant research progress and development have been attained regarding the design and performance of organic flow batteries. To provide a comprehensive understanding, this chapter explores the state-of-the-art and prospects of organic flow batteries. The key design components of organic flow batteries and their functional requirements, which distinguish them from conventional flow batteries, are summarized. The principle of design and performance analysis of different classifications of organic flow battery are discussed. The modeling approaches and numerical studies for design and performance enhancement of organic flow batteries are also presented. Finally, the pressing challenges and future directions to ensure continuous design, performance, and commercialization improvement of organic flow batteries for electrochemical energy storage are highlighted. Keywords Flow batteries · Organic flow batteries · Cell design · Organic molecules · Aqueous electrolytes · Non-aqueous electrolytes · Battery performance

Oladapo Christopher Esan and Xiaoyu Huo had contributed equally to this work. O. C. Esan · X. Huo · X. Shi · L. An (B) Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 L. An et al. (eds.), Flow Cells for Electrochemical Energy Systems, Green Energy and Technology, https://doi.org/10.1007/978-3-031-37271-1_4

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1 Introduction The persistent use of fossil fuels energy has raised numerous concerns, including energy insecurity and crisis, and still remains one of the reasons for the inability to meetup with the ever-increasing energy demand around the world [1, 2]. This is as a result of the adverse environmental consequences, incessant price fluctuation, limited resources, and inefficiency of fossil fuels [3]. The quest for clean and efficient energy alternatives as well as reducing carbon emissions have therefore led to the amalgamation of different efforts toward developing and harnessing energy from renewable sources such as solar and wind [4, 5]. The increasing energy capacity from these renewable sources and their inherent variability with time and climatic conditions thus necessitated the needs for efficient energy storage devices. Following this, different types of energy storage system, such as superconducting magnetic systems, pumped hydroelectric, flywheels, compressed air, supercapacitors, and solid-state batteries, have been invented over the years [6]. However, the various lingering challenges, such as location dependence, safety issues, low storage capacity, and high operational costs, associated with these storage systems evidently limit their continuous application for energy storage, particularly for large-scale purpose [7–9]. To overcome these challenges and ensure the effective conversion, storage, and utilization of renewable energy sources, flow battery was proposed and developed [10–12]. The emergence of flow battery consequently led to a new era for electrochemical energy storage systems, particularly, battery technology. Flow batteries are rechargeable energy storage devices which reversibly engineers the conversion between chemical and electrical energy through the reduction and oxidation of electroactive species. The operation of a typical flow battery system was first demonstrated using ferric/ferrous and chromic/chromous ions as redox couples in 1976 [10]. The structural design of a flow battery majorly consists of two sides such that each side has a storage tank, a flow field design, an electrode, a pump, a current collector, and other components. The two sides of a flow battery are commonly separated by an ion-exchange membrane, also known as separator, as shown in Fig. 1. The operation of a flow battery simply involves the circulation of electrolyte from the storage tank onto the surface of the electrodes where the electrochemical reactions take place, and thereafter returns into the tank. Unlike the solid-state batteries, where energy is usually stored in the electrodes, flow batteries chemically store energy in the electrolyte. Hence, the energy capacity of a flow battery is primarily dependent on the volume and concentration of the electrolyte, while its power capacity is influenced by the properties of electrodes and the number of cells present in the battery stack [13]. The numerous and attractive advantages of flow battery systems, which include simple principles, design and operational flexibility, independent scaling of power capability and energy capacity, high round-trip efficiencies, long operating life cycle, and inherent safety, have positioned them as one of the most promising technologies suitable for large-scale electrochemical energy storage applications [14, 15]. The introduction of flow batteries, coupled with their continuous advancements, has therefore significantly revolutionized electrochemical

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Fig. 1 The structural design of a typical flow battery with its various components [21]. Reproduced with permission from Elsevier

energy storage technologies for diverse range of applications toward sustainable and clean power supply for the modern world demands. As a flexible and modular technology, different types and classifications of flow battery have been proposed based on different criteria [16]. The common ones that have received wide investigation include all-aqueous flow batteries [17], hybrid flow batteries [18], semi-solid flow batteries [19], and organic flow batteries [20]. Up until now, most studies within the flow battery community have largely focused on the all-aqueous flow battery systems using metallic ions, particularly the widely studied and developed all-vanadium flow battery [22–24]. While aqueous electrolyte systems offer some advantages, the obtainable voltage from the batteries is significantly limited due to the undesired electrolysis of water [15, 25]. In addition, candidates for their electroactive species have mostly been confined to transition metal redox species, which in turn limits the energy density of the batteries as a result of the low solubility of metal salts in water [13, 26]. Following this, research attention has therefore shifted from metal-based species to organic active materials, in recent years, to overcome the above-mentioned issues. Organic flow batteries, which employs naturally abundant organic molecules as its redox-active species, have thus been singled-out and considered as the suitable option for achieving higher cell voltage and performance, enhanced energy density, better battery efficiency, and reduced costs. This has therefore led to a growing interest in engineering organic molecules that exhibit the essential properties needed for flow battery system. Many organic materials have been reported to possess electrochemical redox characteristics, however, a few of them have been adjudged to possess the complete behavior, such as low molecular weight, stable structure, and low cost, required to be thriving redox species in a flow battery system [25].

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In this chapter, significant research progress and development on the design and performance of organic flow batteries are discussed and presented as follows. The key design components of organic flow batteries and some of their functional requirements which distinguish them from conventional flow batteries are summarized in Sect. 2. The different classifications of organic flow batteries based on either aqueous or non-aqueous electrolytes along with their design and performance analysis are discussed in Sect. 3. Modeling and simulation studies to better understand the operation and performance of organic flow batteries are presented in Sect. 4. Finally, the remaining issues to be addressed and future research directions to ensure the continuous design, performance, and commercialization improvement of organic flow batteries are highlighted.

2 Key Design Components of Organic Flow Batteries The physicochemical properties as well as various performance metrics of organic flow batteries are significantly dependent on their major materials and design components, which include electrodes, membrane, and redox-active species/electrolyte. This section thus briefly presents the functions, design principle, and recent development of these key materials used in organic flow batteries and how some of them are different from the materials employed in conventional flow battery systems.

2.1 Electrodes Electrodes used in the design and operation of flow batteries are commonly porous materials with inherent interlinked pores for the smooth passage of the electrolyte/ active species. Electrodes primarily serve as the conversion sites such that electrochemical redox reactions usually take place on the electrode surface during the operation of flow batteries. The design and structure of electrodes therefore not only influence the rate of electrochemical reactions, but also affect the transport mechanisms of mass and charge [27]. Some of the desirable requirements for electrodes in organic flow batteries include large specific surface area to enhance electrochemical reactions which in turn minimizes activation loss, high electrical conductivity to reduce ohmic loss, excellent permeability to active species to enhance mass-transfer rate and reduce the concentration loss, adequate stability, and low cost. The electrode materials that have been widely considered for application in most conventional flow batteries are graphite felt, and the various types of carbon materials such as carbon cloth, glassy carbon, carbon paper, carbon felt, and carbon-polymer composites [28]. A number of bio-sourced materials, due to abundance and high-carbon content, have also been considered as sustainable alternative for electrode materials [21]. Other than these, several organic electrode materials have also been introduced and are considered to be more environmentally friendly for energy storage devices, including

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organic flow batteries [29]. For instance, tetrachloro-p-benzoquinone (chloranil) has been used as the electrode material for the cathode in a novel organic redox flow battery [30]. The structural flexibility, diversity, and processability of organic electrode materials give room for a large number of candidates for the cathode and anode of organic flow batteries. These are some of the major advantages of organic electrode materials, in addition to their reaction kinetics, over inorganic electrode materials.

2.2 Membranes Membrane is another prominent material in the structural design of flow batteries as it significantly influences the cycling performance of the battery system. The membrane is generally positioned between the positive and negative sides to primarily separate the two compartments, thereby preventing cross-contamination of the reactants [31]. While the membrane is generally restrictive to the passage of electrons, it functionally permits the transport of charge-carrier species between both half-cells to complete the electrical circuit while preventing short circuit. Membranes to be employed in an organic flow battery are required to possess high ionic conductivity and selectivity to reduce ohmic overpotential and boost the columbic efficiency and power density of the battery. Organic redox-active species possess larger molecular size in comparison to metallic ions and could either be positively or negatively charged during cycling operation of the battery. The membrane to be employed should therefore be competent to limit the penetration and crossover of the organic redox-active species. Unlike the expensive and commonly used perfluorinated ion-exchange membranes in most conventional flow batteries, a porous composite-based membrane with flexible chemical stability, high ionic conductivity, and high selectivity was recently proposed as a promising candidate for aqueous organic flow batteries [32]. For nonaqueous organic flow battery systems, mechanical stability is highly recommended to withstand the irreversible adsorption of some organic redox-active species which stick to the membrane to reduce membrane resistance and the efficiencies of the battery. It is worth to note that some membraneless organic flow batteries have also been developed [33, 34].

2.3 Redox-Active Species/Electrolyte The redox-active specie is one of the most crucial components of a flow battery which significantly influences the electrochemical performance, among other metrics, of the battery system. In flow batteries, redox-active species are commonly dissolved in a supporting electrolyte after which the resulting solution is pumped from the electrolyte tanks into the flow cell to be distributed on the electrode surface where the redox reaction takes place. The volume and concentration of the electrolyte is therefore expected to determine the energy capacity of the organic flow battery. Over the

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years, several organic redox-active species, including quinones [35], alloxazine [36], nitroxide radicals (TEMPO) [37], and fluorenone [38], have been employed in the operation of flow batteries. Unlike the metal-based redox-active species, the physicochemical properties of organic molecules have been reported to be easily tuned through the modification of their functional groups to realize high-performance flow batteries [39]. Excellent solubility is a paramount property for an organic molecule to function well in the operation of an organic flow battery and achieve high performance. Solubility is actually an interdependent phenomenon as it involves both thermodynamic and kinetic aspects of the material and also largely depends on the interaction between the organic redox-active species and the supporting electrolyte/ solvent. Recent studies have revealed that hydrophobic and hydrophilic groups are useful to facilitate the solubility of organic molecules in organic solvents and aqueous medium, respectively [40]. Another property that should be considered in the design and selection of organic redox-active species is high electrochemical activity and notable electron-transfer rate. In addition, the electrochemical reversibility of the redox-active specie is fundamental for redox reaction to achieve smooth operation of organic flow battery. The redox potential of any material to be employed in organic flow battery is also very important and should be considered.

3 Major Classifications of Organic Flow Batteries 3.1 Aqueous Electrolyte for Organic Flow Batteries As the most popular type of the organic flow batteries, the aqueous systems using water as the solvent for the electrolytes have received ever-increasing investigations [41–43]. Compared with non-aqueous organic flow batteries, the aqueous organic flow battery systems possess several advantages. Firstly, the capital cost is reduced since the electrolyte compositions include only water and inexpensive NaCl or KOH as supporting materials. Secondly, the high ionic conductivity of aqueous electrolytes and ion-exchange membrane allow the system to operate with high efficiencies. Moreover, since the electrolytes are generally non-flammable, the systems are also safer to operate. While showing a lot of advantages, the aqueous organic flow batteries are also facing certain challenges, such as the low solubility of active species [39]. In this section, the developments of the aqueous organic flow batteries will be provided.

3.1.1

All-Organic Flow Batteries Using Aqueous Electrolyte

In the last few decades, various systems with aqueous organic electrolyte materials have been proposed and investigated, including carbonyls-based, nitroxide radicalsbased, and heterocyclic molecules-based [44]. Some organic redox couples employed in aqueous organic flow batteries are presented in Fig. 2.

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Fig. 2 Some organic redox species employed in aqueous organic flow batteries [45]. Reproduced with permission from Elsevier

Among all the developed systems, the neutral aqueous organic flow batteries, which avoid water splitting and a series of organic side reactions under extreme pH conditions, are considered to be promising [46, 47]. As the most representative neutral aqueous organic flow battery, the systems which involve the derivatives of viologen and 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) as anolyte and catholyte, respectively, have been extensively studied due to their high theoretical voltage and low capital cost. Liu et al. reported a methyl viologen (MV)/4-hydroxyTEMPO (4-HO-TEMPO) system using NaCl as supporting electrolytes, graphite felts as electrodes, and an anion-exchange membrane [48]. The battery possesses a theoretical voltage of 1.25 V and achieved energy density from ~ 83 to 45.2% when operating from 20 to 100 mA cm−2 . However, the energy density is only limited to 8.4 Wh L−1 due to the low solubility of TEMPO. In 2019, another group developed a system utilizing bis-(trimethylammonio) propyl viologen (BTMAP-Vi) and 4-[3-(trimethylammonio)propoxy]-TEMPO (TMAP-TEMPO) chloride as shown in Fig. 3a [49]. The TMAP-TEMPO with high water solubility of 4.62 M enables higher concentration of active species in the electrolytes. Consequently, the system exhibited a peak power density of 134 mW cm−2 and demonstrated stable operation for over 200 cycles at 100 mA cm−2 with high-capacity retention rate of over 99.9% per cycle due to the relatively low crossover rate. In 2021, Hu et al. focused on the enhancement of TEMPO-based catholyte and a new system utilizing 1,1' -bis[3(trimethylammonio)propyl]-4,4' -bipyridinium tetrachloride ((NPr)2 V) as anolyte and TEMPO derivative functionalized with a dual-ammonium dicationic group as catholyte as presented in Fig. 3b [50]. The system achieved an energy density of 18.1 Wh L−1 and energy efficiency of 59% at 60 mA cm−2 with 1.0 M active species. Water-soluble ferrocene derivatives have also been employed as catholyte materials in the aqueous all-organic redox flow battery system. Hu et al.

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Fig. 3 a Schematic and capacity retention capacity of an organic flow battery using TMAP-TEMPO at the positive side and BTMAP-Vi at the negative side [49]. Reproduced with permission from Elsevier. b Schematic diagram and battery reactions of N2-TEMPO/[(NPr)2 V]Cl4 aqueous organic flow battery [50]. Reproduced with permission from John Wiley and Sons. c Schematic of the cell reaction for an organic flow battery using MV as anolyte and FcNCl as catholyte [51]. Reproduced with permission from Royal Society of Chemistry. d Schematic of an organic Flow Battery using anthraquinone-2,6-disulfonic acid (AQDS) and 4,5-dihydroxy-1,3-disulfonic acid (BQDS) on the negative side and positive side, respectively [52]. Reproduced under the terms of the CC-BY license

developed a AORFB utilizing MV as anolyte and water-soluble (ferrocenylmethyl)trimethylammonium chloride FcNCl as catholyte as shown in Fig. 3c [46]. The developed system possesses high theoretical energy density 45.5 Wh L−1 , which is even comparable with the inorganic all-vanadium redox flow battery (41.8 Wh L−1 ). With low-cost AMV anion-exchange membrane and graphite felt electrodes, the system achieved a peak power density of 125 mW cm−2 and stable operation for 700 cycles at 60 mA cm−2 with average energy efficiency of 60%. Later, the group demonstrated the strategy to enhance the system performance via the optimization of ion-exchange membranes and supporting electrolytes [51]. The charge/discharge performance was found to increase when employing thinner membrane, suggesting that the effect of decreased membrane resistance outweighs the potentially higher species permeability. On the other hand, the ion conductivity of the electrolyte was increased by using KCl as supporting electrolyte. Consequently, the system achieved an enhanced energy efficiency of over 60% mA cm−2 at 100 mA cm−2 and a power density of 122.7 mW cm−2 . Furthermore, systems based on quinones derivatives such as benzoquinone (BQ) and anthraquinone (AQ) have also been proposed. The quinones derivatives possess several advantages. Firstly, they merely contain carbon, sulfur, hydrogen, and oxygen, which are abundant in earth and are renewable. Meanwhile, their manufacturing processes are generally simple, which thereby lead to the low cost. Moreover, quinones possess excellent reactivity and allow extremely fast two-electron-transfer process even on carbon-based materials, which therefore eliminate expensive noble metal catalysts and reduce capital cost [15]. However, considering the poor stability and solubility of quinones derivatives in neutral environment, the systems generally

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require acidic or alkaline electrolytes. Yang et al. reported an all-quinones system operated in acidic environment using anthraquinone-2,6-disulfonic acid (AQDS) at the negative side and concentrated 4,5-dihydroxybenzene-1,3-disulfonic acid (BQDS) at the positive side as shown in Fig. 3d [52]. To enhance the mass transport and reaction kinetic of the system, high-surface-area carbon-coated carbon-felt electrode and modified interdigitated flow field have been employed. Consequently, the enhanced flow battery achieved a peak power density of around 100 mW cm−2 , as well as a round-trip efficiency of 70% and coulombic efficiency of 100% over 100 cycles at 100 mA cm−2 at room temperature. While exhibiting improved electrochemical performance, it is worth noting that the acidic or alkaline systems generally employed Nafion series membrane as separator, which increases the capital cost of the systems compared with neutral aqueous systems. Meanwhile, due to the corrosion of the electrolytes, the systems employing quinones derivatives are less environmentally and operational friendly. Therefore, the development of neutral aqueous quinones-based organic flow batteries is also an important research direction in the future. In conclusion, though a number of advancements have been achieved for the all-organic aqueous redox flow battery with good cyclability, the development of aqueous organic electrolytes with high potential and solubility still needs more investigations. Meanwhile, how to achieve a higher operating current density should be another challenge to be resolved in the future in order to achieve further system improvement.

3.1.2

Organic/Inorganic Flow Batteries Using Aqueous Electrolyte

In order to further increase the cell voltage and the specific energy of organic flow batteries, some previous research works have combined organic and inorganic redox couples to develop hybrid systems [53]. Common approaches include using metals, such as zinc, lead, and lithium, with high electronegativity as negative materials in acidic environment. In 2009, Xu et al. first proposed organic–inorganic hybrid aqueous flow battery using cadmium and soluble tetrachloro-1,4-benzoquinone (chloranil) in aqueous intermixture of H2 SO4 —(NH4 )2 SO4 —CdSO4 [30]. The cell consists of an inert metallic negative electrode and an insoluble organic material positive electrode without a membrane to separate the two half cells. The system demonstrated an average charge and discharge voltage of 1.18 V and 0.97 V, respectively, at 10 mA cm−2 , and achieved a CE of 99% with an EE of 82% for 100 cycles as shown in Fig. 4a. However, the strongly acidic environment for the positive half cells is a major drawback of the system. Later on, the group proposed another system utilizing 2,5-dihydroxybenzenedisulfonate (DHBS) and PbSO4 /Pb in aqueous H2 SO4 solution [54]. The assembled cell achieved stable operation for 100 cycles and demonstrated the potential of soluble quinones as positive materials for hybrid aqueous RFB. In a subsequent study, another lead/4,5-dibenzoquinone1,3-benzenedisulfonate (tiron) hybride RFB with Nafion 115 membrane and graphite felt positive electrode was reported [55]. The system achieved a CE of 93% and an EE of 82% during the cycling test at 10 mA cm−2 . For more examples, Winsberg et al.

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reported a Poly(TEMPO)/Zinc RFB shown in Fig. 4b with high potential range of up to 2 V [56]. The zinc chloride is employed as the active material for the negative half-cell and supporting electrolyte for positive side simultaneously. Meanwhile, by using several TEMPO-containing polymers, the effects associated with corrosion and environmental impact were reduced compared to conventional inorganic zinc-halogen RFBs. A size-exclusion membrane made from regenerated cellulose was employed to allow the transport of small inorganic ions while preventing the permeation of polymeric active material. The cell was found to allow an operational current density as high as 20 mA cm−2 and demonstrated good cycling stability for over 500 cycles. In another study, an aqueous zinc-organic RFB using functionalized 1,4-hydroquinone bearing four (dimethylamino)methyl groups (FQH2 ) in H2 SO4 as positive electrolyte was developed [57]. The cell applied two-membrane, three-electrolyte acid–base design including an AEM and a CEM separated by a middle electrolyte to reduce cell resistance, which allow the half-cells to be operated at different pH values and thereby exhibiting a high cell voltage of 2.0 V at SOC50. The cell achieved stable operation for 50 cycles with capacity retention rate of 99.92% per cycle. The hybrid aqueous RFB utilizing halogen as negative reactive species has also been widely investigated. Huskinson et al. reported an anthraquinone-based hybrid

Fig. 4 a Coulombic and energy efficiencies of a single flow acid Cd–chloranil battery during the first 100 cycles under an operating current density of 10 mA cm−2 [30]. Reproduced with permission from Elsevier. b Schematic of Poly(TEMPO)/Zinc RFB and copolymer structures [56]. Reproduced with permission from John Wiley and Sons. c Capacity retention, current efficiency, and energy efficiency values of the alloxazine-based system coupled with ferricyanide redox couples [62]. Reproduced with permission from Springer Nature. d Cycling discharge capacities and efficiencies of a battery system based on the sodium salt of flavin mononucleotide (FMN-Na) coupled with ferrocyanide/ferricyanide in alkaline solution [63]. Reproduced with permission from Springer Nature

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aqueous flow battery with metal-free carbon electrodes and Nafion membrane, utilizing quinone/hydroquinone and Br2 /Br− as redox couples [58]. The battery performance under different SOC was studied. The system achieved a peak power density of 0.6 W cm−2 under optimized condition and demonstrated a capacity retention of over 99% per cycle at 500 mA cm−2 . Later on, Chen et al. developed a quinone/ bromide RFB based on the design proposed by Huskinson’s group [59]. The cell utilized 9,10-anthraquinone-2,7-disulfonic acid (AQDS) in sulfuric acid as positive electrolyte and hydrobromic acid and Br2 as negative electrolyte. The group studied the effects of electrolyte composition, flow rate, operating temperature, electrode and membrane materials on the battery performance. By utilizing the baked carbon papers and pre-treated Nafion 212, the optimized cell achieved performance of 1.0 Wcm−2 at 90% SOC. Subsequently, the group conducted another study to investigate the impact of the current density on polarization performance, charge capacity, and system efficiencies [60]. It was found that the major loss of current efficiency is due to the crossover of Br2 , which is directly related to the applied current density. Meanwhile, the leakage of quinone-bearing electrolyte and the destruction of redox-active species in the electrolyte can also contribute to the efficiency loss. While the organic/ halogen aqueous RFB demonstrated satisfactory performance, the utilization of Br2 / Br− , however, is a major safety concern. Lin et al. developed an alkaline quinone RFB based on 2,6-dihydroxyanthraquinone (2,6-DHAQ, Fig. 5) and ferrocyanide/ ferricyanide [60]. The system was constructed from carbon paper electrodes and a Nafion 212 membrane, utilizing 0.5 M 2,6-DHAQ dipotassium salt in 1.0 M KOH as the negative electrolyte and 0.4 M K4 Fe(CN)6 in 1 M KOH solution as the positive solution. Compared to the Br2 /Br− reaction couple, ferrocyanide/ferricyanide possesses lower toxicity and is not volatile. In subsequent work, the group proposed an alloxazine-based system coupling with ferricyanide redox couples. The 0.5 M alloxazine 7/8-carboxylic acid was used as negative electrolyte while 0.4 M ferrocyanide with 0.04 M ferricyanide was used as positive electrolyte. When assembled with carbon papers and Nafion membrane, the system achieved an average EE of 63% and a capacity retention rate of over 91% for more than 400 cycles as presented in Fig. 4c. Orita and co-workers also reported a system based on the sodium salt of flavin mononucleotide (FMN-Na) coupling with ferrocyanide/ferricyanide in alkaline solution [61]. In order to increase the solubility of FMN-Na, hydrotropic agent nicotinamide was introduced as additive and a high-capacity density of 81 Ah l−1 was achieved. The cell assembled with graphite felt electrodes and Nafion 212 membrane demonstrated a peak power density of 160 mW cm−2 . Meanwhile, the system achieved a high discharge capacity retention of 99% after 100 cycles at a current density of 80 mA cm−2 , exhibiting excellent stability which is comparable to the quinone system as shown in Fig. 4d. However, it should be noted that, while the alkaline system based on inorganic ferricyanide has demonstrated excellent performance, the strongly alkaline condition raised another concern associated with serious corrosion, which is one main challenge limiting the real application of the systems. Moreover, the chemical stability and ionic conductivity of the membrane in alkaline electrolytes, which is typically CEM, is another challenge to be resolved in the future.

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Fig. 5 Comparison of non-aqueous and aqueous flow batteries regarding their power density and current densities [75]. Reproduced with permission from Royal Society of Chemistry

3.2 Non-aqueous Electrolyte for Organic Flow Batteries As discussed in the previous section, one of the main limitations of aqueous RFBs is the limited cell voltage due to the narrow electrochemical window of water. Following this, the non-aqueous organic electrolytes with wider electrochemical window of operation have been widely studied in RFB applications in recent decades [64, 65]. Compared with aqueous RFBs, the non-aqueous systems do not only facilitate higher solubility of various metal–ligand complexes with higher energy density, but also enable extended working temperature due to the low freezing point and high boiling point of organic solvents. However, up till now, the non-aqueous RFB still possesses many uncertainties in contrast to aqueous system. In this section, the non-aqueous RFBs system using organic reactive species for both sides or single side will be introduced.

3.2.1

All-Organic Flow Batteries Using Non-aqueous Electrolyte

The theoretical concept of all-organic non-aqueous RFBs was first proposed by Singh in 1984 [66]. Later on, research works have been focused on the development of organic metal–ligand complexes dissolved in non-aqueous electrolyte, such as ruthenium complexes, vanadium complexes, chromium complexes, manganese complexes, nickel complexes, iron complexes, and uranium complexes as presented in [64]. Since the redox couples on both sides of the RFB revert to the same species, crossover problem will be less inside the system. Matsuda et al. demonstrated an experimental system using ruthenium or iron complexes in acetonitrile (CH3 CN) solution with tetraethylammonium tetrafluoroborate (TEABF4 ) as the supporting electrolyte [67]. A flow cell with carbon fiber cloth electrodes and AEM was assembled, and the charge–discharge behavior was examined. The system with [Ru(bpy)3 ](BF4 )2 /CH3 CN was found to exhibit an OCV of 2.6 V, a charge–discharge

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efficiency of 70% and 20–40% for the positive and negative half cells, respectively. Later on, more studies have been focused on the development of all-organic nonaqueous RFBs using metal–ligand complexes as a redox couple. Chakrabrati et al. studied the performances of ruthenium acetylacetonate ([Ru(acac)3 ]) and Tris(2,2' bipyridine) iron(II) perchlorate ([Fe(bpy)3 ](ClO4 )2 ) in acetonitrile as electrolytes [68]. The [Fe(bpy)3 ](ClO4 )2 was proved to possess poor charge/discharge performance with less stable cell voltage and lower energy efficiency under the same operational conditions. Vanadium acetylacetonate [69], chromium acetylacetonate [70], and manganese acetylacetonate [71] have also been employed as reactive species in RFB system using TEABF4 and CH3 CN as supporting electrolytes. The OCV of the three systems were found to be 2.2 V, 3.4 V, and 1.1 V, respectively, with the CE found to reach ~ 50% and 53–58% and 74–97%, respectively. While the organic metal–ligand complexes have been extensively studied, their limited solubility and complex preparation method greatly limit their applications. Therefore, the metal-free organic non-aqueous RFBs with the advantages of high solubility of electroactive organic compound have received increasing attentions. Currently, many organic redox molecules have been proposed for the all-organic non-aqueous RFB, such as rubrene, tetra amino anthraquinone, benzoquinone (BQ), naphthoquinone (NQ), anthraquinone (AQ), tetramethyl piperidinyloxyl (TEPMO), and phenylenediamine (PD) [72]. In 2011, Li et al. proposed a system using TEPMO/ NaClO4/acetonitrile and Methylphthalimide/NaClO4/acetonitrile as electrolyte for negative and positive half-cells, respectively [73]. The RFB was assembled with graphite felts as electrodes and cation-exchange membrane (CEM) as separator. The system was found to possess an OCV of ~ 1.6 V. Meanwhile, during the charge– discharge behavior test, the system demonstrated stable performance and achieved high coulombic efficiency of 90% for the first 20 cycles. In addition, Potash et al. studied diaminoanthraquinones (DAAQs) and its related compounds as reactive species in acetonitrile for an all-organic non-aqueous RFB system. However, the low solubility of the species remains a significant limitation. While in 2021, Sharma et al. demonstrated a system utilizing air stable diketopyrrolopyrrole (DPP)-based derivative and commercial Unisol blue (UB) dye (1,4-bis(isopropylamino) anthraquinone) or 1,4-di-tert-butyl-2,5-bis(2-methoxyethoxy)benzene (DBBB) as negative and positive electrolyte [74]. The DPP-based redox-active molecule was found to possess good solubility and thereby enable theoretical high energy density of 1.93 and 2.25 Wh L−1 , respectively. Moreover, the assembled cell was proven to be stable for up to 100 cycles with capacity retaining rate to be ∼70%. In summary, the majority of the all-organic non-aqueous RFBs were based on the single-metal–ligand complex reactive couples due to their good reversibility and moderate solubility in organic environment, while some studies have also focused on the metal-free organic system. Generally, acetonitrile has been widely used as the solvent for the non-aqueous RFBs, since many organic redox couples were proven to possess excellent electrochemical characteristics in the acetonitrile solutions. Meanwhile, supporting electrolytes such as TEABF4 and EMIPF6 have also been introduced to improve the ionic conductivity of the system. However, up till now, even

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though the all-organic non-aqueous RFBs have been considered as promising candidate for energy storage systems due to their wide operating potential window, their application is still seriously limited. Since the conductivity of organic electrolytes is generally low, to minimize the large ohmic loss associated with the organic electrolytes, the operating current density for non-aqueous systems is generally lower as shown in Fig. 5, which consequently reduce the power density of the system. Therefore, up till now, the performance of the all-organic non-aqueous RFBs is still incomparable with the aqueous RFBs. In the future, the improvement of the operating current density is considered to be an important research direction in order to achieve real application of the all-organic non-aqueous RFBs.

3.2.2

Organic/Inorganic Flow Batteries Using Non-aqueous Electrolyte

The organic/inorganic non-aqueous RFBs were first proposed by combining the features of lithium-ion battery and non-aqueous RFBs in order to obtain a high energy density of Li-ion battery while possessing the flexibility of RFBs [76]. Up till now, the most common type of hybrid non-aqueous RFBs is the hybrid lithium metal RFBs, which utilizes Li-ion electrode and non-aqueous organic reactive species. In 2012, Wang et al. first reported an organic/inorganic hybrid RFBs using anthraquinone (AQ) derivative as the positive electrolyte and lithium metal as the negative electrode [77]. The assembled static cell utilizing graphite felt disk as positive electrode, lithium metal as negative electrode, and polypropylene as separator. To improve the solubility of the AQ molecule, two triethylene glycol monomethyl ether groups were introduced into the structure to synthesis 1,5-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)anthracene-9,10dione(15D3GAQ). Consequently, the system with 0.25 M 15D3GAQ in 1.0 M LiPF6/ PC as electrolyte exhibited an EE of ∼82% with a specific discharge energy density of ∼25 Wh L−1 . However, the effect of side reactions between anthraquinone and the carbonate solvents was found to be obvious and led to a quick capacity decay of the system. In addition, Brushett et al. reported a non-aqueous lithium-ion RFB using 2,5-Di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB) and a variety of molecules derived from quinoxaline [78]. The primary system achieved CE of ∼ 70% and EE of ~ 37% with the charge and discharge plateaus to be 1.8–2.4 V and 1.7–1.3 V, respectively. In 2014, Wei et al. developed a system using ferrocene (Fc) and lithium ions [79]. The molecular structure design strategy was used to design and synthesized an ionic-derivatized ferrocene compound, which increase the solubility of the active redox species in non-aqueous electrolytes and therefore improve the energy density of the system. The designed hybrid flow battery with a lithiumgraphite hybrid negative electrode demonstrated a cell voltage of 3.49 V, EE of 75%, and the energy density of ~ 50 Wh L−1 . Meanwhile, the work also highlighted the potential of molecular structure engineering in high-performance non-aqueous electrolyte development, which could be one important direction for system performance improvement. In another work, Huang et al. utilized subtractive design approach to prepare a series of substituted 1,4-dimethoxybenzene (DDB)-based redox-active

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molecules and developed an overcharge protection material for lithium-ion RFB [80]. The designed species were found to be able to enhance the long-term cycle stability of the system while maintaining high capacities. In addition, Wei’s group studied the TEMPO-based positive electrolyte due to the fast, stable, and reversible electrochemical performance. They reported a non-aqueous hybrid Li/TEMPO flow cell, which utilized lithium foil/graphite felt hybrid negative electrode, a polyethylenebased porous separator, and graphite felt positive electrode. The TEMPO dissolved in a mixture of EC/PC/EMC was employed as positive electrolyte, while LiPF6 was utilized as negative active material. Meanwhile, fluoroethylene carbonate (FEC) was introduced as additive to protect the Li electrode. During the operation, only the positive electrolyte was circulated. The system exhibited stable cycling performance of 100 cycles with a CE of 99%, EE of ~ 86% and average capacity retention rate of 99.8% per cycle at 5 mA cm−2 . Furthermore, the energy density was found to be as high as 126 W h L−1 . In summary, the organic/inorganic hybrid RFB batteries are one of the important research directions for future system development, since it can potentially couple the advantages of higher cell voltage, increased energy density, and faster reaction kinetics. However, there are still many challenges to be solved, such as limited ion conductivity of the membrane, high ohmic resistance across the interface between membrane and electrode, as well as poor chemical compatibility. Meanwhile, the solid metal electrode needs to be prevented from reacting with the electrolyte or forming dendrites.

4 Modeling Studies on Organic Flow Batteries To better understand the operation and performance of organic flow batteries, modeling and simulation as economical tool, which allows reductions in timescales and costs, have been introduced. However, only few model-based studies have been conducted on organic flow batteries over the years as majority of the studies on the performance analysis of organic flow batteries are experiments. Following the demonstration of quinones and their derivatives as suitable candidates for low-cost organic flow batteries in 2014 [58], the first numerical study which describes a quinone–bromine flow battery using quinone/hydroquinone redox couple at the negative side and bromine Br2 /Br_ at the positive side was conducted [81]. The electrochemical reactions at both electrodes of this particular quinone–bromine flow battery is expressed as follows: Positive electrode : 2Br ⇌ Br2 + 2e−

(1)

Negative electrode : AQDS + 2e− + 2H+ ⇌ H2 AQDS

(2)

A three-dimensional model was developed to examine the impacts of electrode thickness on the operation characteristics of the quinone-bromide flow battery. The

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numerical simulations identified six layers of carbon paper as the most suitable electrode thickness for the metal-free organic–inorganic flow battery. The numerical study also considers an improvement to the existing electrolyte conductivity equation given as: σpre =

F2 ∑ 2 z Di ci RT i i

(3)

by incorporating a correction coefficient k into the equation: k=

σmeasured σpre

(4)

The resulting polarization curves from the previous and corrected electrolyte conductivity equations were compared with experimental data as presented in Fig. 6. Unlike the previous model, the introduction of the correction coefficient k makes the polarization trend calculated by the corrected equation fits with the experimental data. The scope of this modeling framework was later extended through the consideration of the effects of more operational variables on the charge and discharge behavior of the organic flow battery system [82]. The improved model lay much emphasis on the effects of graphite plates and flow channels on current distribution and the influence of temperature, different applied current densities, and time-dependent variables such as flow rate on the cell performance. From the range of applied current densities considered in the simulation as seen in Fig. 7a, lower values were found to improve the battery operation via the reduction of charging voltage and its corresponding electric energy consumption, while boosting discharge voltage and energy. At such low current density, the magnitude of flow rates has negligible effect on the battery operations, which in turn promotes the usage of low flow rates to minimize pump Fig. 6 Comparison among the results derived from the previous and corrected electrolyte conductivity equations against experimental data [81]. Reproduced with permission from Elsevier

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Fig. 7 Simulated charge–discharge curves of the metal-free quinone–bromide under various, a applied current densities and b operating temperatures [82]. Reproduced with permission from Elsevier

consumption. Nevertheless, at low current density, slow reaction and cycling rates tend to happen, which may not be beneficial to the battery operation. Hence, a tradeoff between this occurrence and operating current density is needed. As for the effect of temperature, the cell voltage and overpotential increase with the temperature to a certain level where elevating the temperature has little or no effect on the battery performance as shown in Fig. 7b. It is therefore of significant importance to monitor the thermal aspect of organic flow batteries. A two-dimensional model, with major focus on the design of porous electrode for organic flow battery, was also developed for a metal-free quinone-based flow cell [83]. The numerical study provides understanding on the influence and range of porosity, pore volume, pore size distribution, specific surface area, and morphology on the preparation and performance of electrode and concentration distribution appropriate for this particular organic flow cell. To better understand the effect of both hydrogen and oxygen evolution reactions on the performance of an aqueous organic flow battery, a zero-dimensional model was established using 2,6-dihydroanthraquinone/ ferrocyanide as the electrolytes [84]. The model was further employed to optimize the operating conditions as well as to predict the most appropriate potential of electrolytes to limit side reactions and thus enhance the battery performances. Under an alkaline condition, the standard potential for the negative electrolyte was proposed to be at − 0.8 V without competing with hydrogen evolution reaction while that of the positive electrolyte potential was mentioned to be below 0.65 V to avoid competition with oxygen evolution reaction. Similar approach is still required to be carried out under acidic and neutral conditions. Elsewhere, a zero-dimensional model was developed to assess and understand the capacity retention in an organic flow cell, using ferri/ferrocyanide as the redox couple, by simulating its capacity fade under various hypothetical combinations of cycling protocol and reactant decay mechanism [85]. This model therefore provides the basis for its use in diagnosing and designing new chemistries for application in practical organic flow batteries. A two-dimensional

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model of a membraneless zinc-quinone flow battery was also recently developed to study its performance under the influence of variations in key parameters such as flow field, electrode porosity and thickness, and applied current density [86]. All these numerical studies thus present fundamental guide to garner new insights for further development toward the rational design and performance improvement of organic flow batteries.

5 Summary and Outlook Organic flow batteries, which employ naturally abundant organic molecules as their redox-active species, have thus been singled-out and considered as the suitable option for achieving higher cell voltage and performance, enhanced energy density, better efficiency, and reduced costs. This has therefore led to a growing interest in engineering organic molecules that exhibit the essential properties needed for flow battery system. Many organic materials have been reported to possess electrochemical redox characteristics; however, a few of them have been adjudged to possess the complete behavior, such as low molecular weight, stable structure, and low cost, required to be thriving redox species in a flow battery system. Organic flow batteries are classified based on the nature of electrolyte used either aqueous or non-aqueous. The active species are classified as all-organic and organic/inorganic in both aqueous and non-aqueous-based electrolytes. Though a number of advancements have been achieved for the all-organic aqueous redox flow battery with good cyclability, the development of aqueous organic electrolytes with high potential and solubility still needs more investigations for performance improvement. Meanwhile, the operation of these organic flow batteries at high operating current density is another challenge that needs to be resolved in order to achieve further system improvement. All-organic non-aqueous RFBs have also been considered as promising candidate for energy storage systems due to their wide operating potential window; however, their application is still seriously limited. This is due to the low conductivity of organic electrolytes leading to large ohmic loss. Also, the operating current density for non-aqueous systems is generally low which consequently reduce the power density of the system. Therefore, up till now, the performance of the all-organic non-aqueous RFBs is still incomparable with the aqueous RFBs. In the future, the improvement of the operating current density is an important research direction in order to achieve real application of the all-organic non-aqueous RFBs. The organic/inorganic hybrid flow batteries, which combine organic active species with inorganic redox couples, commonly metallic ions, are one of the important research directions for future system development, since it can potentially couple the advantages of higher cell voltage, increased energy density, and faster reaction kinetics. However, there are still many challenges to be resolved, such as limited ion conductivity of the membrane, high ohmic resistance across the interface between membrane and electrode, as well as poor chemical compatibility.

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References 1. Bednar DJ, Reames TG (2020) Recognition of and response to energy poverty in the United States. Nat Energy 5(6):432–439 2. Longden T et al (2022) Energy insecurity during temperature extremes in remote Australia. Nat Energy 7(1):43–54 3. Abbasi KR, Shahbaz M, Zhang J, Irfan M, Lv K (2022) Analyze the environmental sustainability factors of China: the role of fossil fuel energy and renewable energy. Renew Energy 187:390–402 4. Duan L, Petroski R, Wood L, Caldeira K (2022) Stylized least-cost analysis of flexible nuclear power in deeply decarbonized electricity systems considering wind and solar resources worldwide. Nat Energy 1–10 5. Gür TM (2018) Review of electrical energy storage technologies, materials and systems: challenges and prospects for large-scale grid storage. Energy Environ Sci 11(10):2696–2767 6. Olabi A, Abdelkareem M (2021) Energy storage systems towards 2050. Energy 219:119634. Elsevier 7. Koohi-Fayegh S, Rosen MA (2020) A review of energy storage types, applications and recent developments. J Energy Storage 27:101047 8. Aneke M, Wang M (2016) Energy storage technologies and real life applications—a state of the art review. Appl Energy 179:350–377 9. Hameer S, van Niekerk JL (2015) A review of large-scale electrical energy storage. Int J Energy Res 39(9):1179–1195 10. Thaller L (1976) Electrically rechargeable REDOX flow cell [Fe/sup+ 3//Fe/sup+ 2/and Cr/ sup+ 2//Cr/sup+ 3/couples] 11. Bartolozzi M (1989) Development of redox flow batteries. A historical bibliography. J Power Sources 27(3):219–234. https://doi.org/10.1016/0378-7753(89)80037-0 12. Vanýsek P, Novák V (2017) Redox flow batteries as the means for energy storage. J Energy Storage 13:435–441 13. Weber AZ, Mench MM, Meyers JP, Ross PN, Gostick JT, Liu Q (2011) Redox flow batteries: a review. J Appl Electrochem 41(10):1137 14. Sánchez-Díez E et al (2021) Redox flow batteries: status and perspective towards sustainable stationary energy storage. J Power Sources 481:228804 15. Soloveichik GL (2015) Flow batteries: current status and trends. Chem Rev 115(20):11533– 11558. https://doi.org/10.1021/cr500720t 16. Esan OC, Shi X, Pan Z, Huo X, An L, Zhao T (2020) Modeling and simulation of flow batteries. Adv Energy Mater 10(31):2000758 17. Park M, Ryu J, Kim Y, Cho J (2014) Corn protein-derived nitrogen-doped carbon materials with oxygen-rich functional groups: a highly efficient electrocatalyst for all-vanadium redox flow batteries. Energy Environ Sci 7(11):3727–3735 18. Khor A et al (2018) Review of zinc-based hybrid flow batteries: from fundamentals to applications. Mater Today Energy 8:80–108 19. Ventosa E et al (2015) Non-aqueous semi-solid flow battery based on Na-ion chemistry. P2-type Na x Ni 0.22 Co 0.11 Mn 0.66 O 2–NaTi 2 (PO 4) 3. Chem Commun 51(34):7298–7301 20. Zhang H, Lu W, Li X (2019) Progress and perspectives of flow battery technologies. Electrochem Energy Rev 2(3):492–506 21. Bamgbopa MO, Fetyan A, Vagin M, Adelodun AA (2022) Towards eco-friendly redox flow batteries with all bio-sourced cell components. J Energy Storage 50:104352 22. Feng H et al. (2022) Advances and challenges in photoelectrochemical redox batteries for solar energy conversion and storage. Adv Energy Mater 12(24):2200469 23. He Z et al (2022) Electrode materials for vanadium redox flow batteries: Intrinsic treatment and introducing catalyst. Chem Eng J 427:131680 24. Huang Z, Mu A, Wu L, Wang H (2022) Vanadium redox flow batteries: flow field design and flow rate optimization. J Energy Storage 45:103526. https://doi.org/10.1016/j.est.2021.103526

88

O. C. Esan et al.

25. Wang W, Sprenkle V (2016) Redox flow batteries go organic. Nat Chem 8(3):204–206. https:/ /doi.org/10.1038/nchem.2466 26. Soloveichik GL (2014) Electrochemistry: metal-free energy storage. Nature 505(7482):163 27. Zhou XL, Zhao TS, An L, Zeng YK, Wei L (2017) Critical transport issues for improving the performance of aqueous redox flow batteries. J Power Sources 339:1–12. https://doi.org/10. 1016/j.jpowsour.2016.11.040 28. Chakrabarti MH et al (2014) Application of carbon materials in redox flow batteries. J Power Sources 253:150–166. https://doi.org/10.1016/j.jpowsour.2013.12.038 29. Song Z, Zhou H (2013) Towards sustainable and versatile energy storage devices: an overview of organic electrode materials. Energy Environ Sci 6(8):2280–2301 30. Xu Y, Wen Y, Cheng J, Cao G, Yang Y (2009) Study on a single flow acid Cd–chloranil battery. Electrochem Commun 11(7):1422–1424. https://doi.org/10.1016/j.elecom.2009.05.021 31. Shi X et al (2021) Polymer electrolyte membranes for vanadium redox flow batteries: fundamentals and applications. Prog Energy Combust Sci 85:100926 32. Tan R et al (2020) Hydrophilic microporous membranes for selective ion separation and flowbattery energy storage. Nat Mater 19(2):195–202 33. Leung P, Martin T, Shah A, Anderson M, Palma J (2016) Membrane-less organic–inorganic aqueous flow batteries with improved cell potential. Chem Commun 52(99):14270–14273 34. Leung P, Martin T, Shah AA, Mohamed MR, Anderson MA, Palma J (2017) Membrane-less hybrid flow battery based on low-cost elements. J Power Sources 341:36–45 35. Kwabi DG et al (2018) Alkaline quinone flow battery with long lifetime at pH 12. Joule 2(9):1894–1906 36. Lin K et al (2016) A redox-flow battery with an alloxazine-based organic electrolyte. Nat Energy 1(9):1–8 37. Hu B, Fan H, Li H, Ravivarma M, Song J (2021) Five-membered ring nitroxide radical: a new class of high-potential, stable catholytes for neutral aqueous organic redox flow batteries. Adv Func Mater 31(35):2102734 38. Rodriguez J, Niemet C, Pozzo LD (2019) Fluorenone based anolyte for an aqueous organic redox-flow battery. ECS Trans 89(1):49 39. Zhao Z, Zhang C, Li X (2022) Opportunities and challenges of organic flow battery for electrochemical energy storage technology. J Energy Chem 67:621–639 40. Liu W, Zhao Z, Li T, Li S, Zhang H, Li X (2021) A high potential biphenol derivative cathode: toward a highly stable air-insensitive aqueous organic flow battery. Sci Bull 66(5):457–463 41. Iwakiri I, Antunes T, Almeida H, Sousa JP, Figueira RB, Mendes A (2021) Redox flow batteries: materials, design and prospects. Energies 14(18):5643 42. Liu W, Lu W, Zhang H, Li X (2019) Aqueous flow batteries: research and development. Chem Eur J 25(7):1649–1664 43. Singh V, Kim S, Kang J, Byon HR (2019) Aqueous organic redox flow batteries. Nano Res 12(9):1988–2001 44. Chen H, Cong G, Lu Y-C (2018) Recent progress in organic redox flow batteries: active materials, electrolytes and membranes. J Energy Chem 27(5):1304–1325. https://doi.org/10.1016/ j.jechem.2018.02.009 45. Gentil S, Reynard D, Girault HH (2020) Aqueous organic and redox-mediated redox flow batteries: a review. Curr Opin Electrochem 21:7–13. https://doi.org/10.1016/j.coelec.2019. 12.006 46. Hu B, DeBruler C, Rhodes Z, Liu TL (2017) Long-cycling aqueous organic redox flow battery (AORFB) toward sustainable and safe energy storage. J Am Chem Soc 139(3):1207–1214 47. Chai J, Wang X, Lashgari A, Williams CK, Jiang J (2020) A pH-neutral, aqueous redox flow battery with a 3600-cycle lifetime: micellization-enabled high stability and crossover suppression. ChemSusChem 13(16):4069–4077 48. Liu T, Wei X, Nie Z, Sprenkle V, Wang W (2016) A total organic aqueous redox flow battery employing a low cost and sustainable methyl viologen anolyte and 4-HO-TEMPO catholyte. Adv Energy Mater 6(3):1501449

Design and Performance of Organic Flow Batteries

89

49. Liu Y et al (2019) A long-lifetime all-organic aqueous flow battery utilizing TMAP-TEMPO radical. Chem 5(7):1861–1870 50. Hu B, Hu M, Luo J, Liu TL (2022) A stable, low permeable TEMPO catholyte for aqueous total organic redox flow batteries. Adv Energy Mater 12(8):2102577 51. Hu B, Seefeldt C, DeBruler C, Liu TL (2017) Boosting the energy efficiency and power performance of neutral aqueous organic redox flow batteries. J Mater Chem A 5(42):22137– 22145 52. Yang B, Hoober-Burkhardt L, Krishnamoorthy S, Murali A, Prakash GS, Narayanan S (2016) High-performance aqueous organic flow battery with quinone-based redox couples at both electrodes. J Electrochem Soc 163(7):A1442 53. Winsberg J, Hagemann T, Janoschka T, Hager MD, Schubert US (2017) Redox-flow batteries: from metals to organic redox-active materials. Angew Chem Int Ed 56(3):686–711 54. Xu Y, Wen Y, Cheng J, Yanga Y, Xie Z, Cao G (2009) Novel organic redox flow batteries using soluble quinonoid compounds as positive materials. In: 2009 World non-grid-connected wind power and energy conference. IEEE, pp 1–4 55. Xu Y, Wen Y-H, Cheng J, Cao G-P, Yang Y-S (2010) A study of tiron in aqueous solutions for redox flow battery application. Electrochim Acta 55(3):715–720 56. Winsberg J et al (2016) Poly (TEMPO)/zinc hybrid-flow battery: a novel, “green”, high voltage, and safe energy storage system. Adv Mater 28(11):2238–2243 57. Park M et al (2019) A high voltage aqueous zinc–organic hybrid flow battery. Adv Energy Mater 9(25):1900694 58. Huskinson B et al. (2014) A metal-free organic–inorganic aqueous flow battery. Nature 505(7482):195–198. https://doi.org/10.1038/nature12909 59. Chen Q, Gerhardt MR, Hartle L, Aziz MJ (2015) A quinone-bromide flow battery with 1 W/ cm2 power density. J Electrochem Soc 163(1):A5010 60. Chen Q, Eisenach L, Aziz MJ (2015) Cycling analysis of a quinone-bromide redox flow battery. J Electrochem Soc 163(1):A5057 61. Orita A, Verde MG, Sakai M, Meng YS (2016) A biomimetic redox flow battery based on flavin mononucleotide. Nat Commun 7(1):1–8 62. Lin K et al. (2016) A redox-flow battery with an alloxazine-based organic electrolyte. Nat Energy 1(9):16102. https://doi.org/10.1038/nenergy.2016.102 63. Orita A, Verde MG, Sakai M, Meng YS (2016) A biomimetic redox flow battery based on flavin mononucleotide. Nat Commun 7(1):13230. https://doi.org/10.1038/ncomms13230 64. Gong K, Fang Q, Gu S, Li SFY, Yan Y (2015) Nonaqueous redox-flow batteries: organic solvents, supporting electrolytes, and redox pairs. Energy Environ Sci 8(12):3515–3530 65. Huang Y, Gu S, Yan Y, Li SFY (2015) Nonaqueous redox-flow batteries: features, challenges, and prospects. Curr Opin Chem Eng 8:105–113 66. Singh P (1984) Application of non-aqueous solvents to batteries. J Power Sources 11(1–2):135– 142 67. Matsuda Y, Tanaka K, Okada M, Takasu Y, Morita M, Matsumura-Inoue T (1988) A rechargeable redox battery utilizing ruthenium complexes with non-aqueous organic electrolyte. J Appl Electrochem 18(6):909–914 68. Chakrabarti M, Dryfe R, Roberts E (2007) Evaluation of electrolytes for redox flow battery applications. Electrochim Acta 52(5):2189–2195 69. Liu Q, Sleightholme AE, Shinkle AA, Li Y, Thompson LT (2009) Non-aqueous vanadium acetylacetonate electrolyte for redox flow batteries. Electrochem Commun 11(12):2312–2315 70. Liu Q, Shinkle AA, Li Y, Monroe CW, Thompson LT, Sleightholme AE (2010) Nonaqueous chromium acetylacetonate electrolyte for redox flow batteries. Electrochem Commun 12(11):1634–1637 71. Sleightholme AE, Shinkle AA, Liu Q, Li Y, Monroe CW, Thompson LT (2011) Non-aqueous manganese acetylacetonate electrolyte for redox flow batteries. J Power Sources 196(13):5742– 5745 72. Pahlevaninezhad M et al. (2020) High energy density non-aqueous organic redox flow batteries. ECS Meet Abs (2):209. IOP Publishing

90

O. C. Esan et al.

73. Li Z et al (2011) Electrochemical properties of an all-organic redox flow battery using 2, 2, 6, 6-tetramethyl-1-piperidinyloxy and N-methylphthalimide. Electrochem Solid-State Lett 14(12):A171 74. Sharma S et al. (2021) Electrochemical evaluation of diketopyrrolopyrrole derivatives for nonaqueous redox flow batteries. Chem Eur J 27(47):12172–12180 75. Shin S-H, Yun S-H, Moon S-H (2013) A review of current developments in non-aqueous redox flow batteries: characterization of their membranes for design perspective. RSC Adv 3(24):9095–9116 76. Li B, Liu J (2017) Progress and directions in low-cost redox-flow batteries for large-scale energy storage. Natl Sci Rev 4(1):91–105 77. Wang W, Xu W, Cosimbescu L, Choi D, Li L, Yang Z (2012) Anthraquinone with tailored structure for a nonaqueous metal–organic redox flow battery. Chem Commun 48(53):6669– 6671 78. Brushett FR, Vaughey JT, Jansen AN (2012) An all-organic non-aqueous lithium-ion redox flow battery. Adv Energy Mater 2(11):1390–1396 79. Wei X et al (2014) TEMPO-based catholyte for high-energy density nonaqueous redox flow batteries. Adv Mater 26(45):7649–7653 80. Huang J et al (2015) 1, 4-Bis (trimethylsilyl)-2, 5-dimethoxybenzene: a novel redox shuttle additive for overcharge protection in lithium-ion batteries that doubles as a mechanistic chemical probe. J Mater Chem A 3(14):7332–7337 81. Li X (2015) Modeling and simulation study of a metal free organic–inorganic aqueous flow battery with flow through electrode. Electrochim Acta 170:98–109. https://doi.org/10.1016/j. electacta.2015.04.075 82. Chu D, Li X, Zhang S (2016) A non-isothermal transient model for a metal-free quinone– bromide flow battery. Electrochim Acta 190:434–445. https://doi.org/10.1016/j.electacta.2015. 12.128 83. Ma C, Li X, Lin L, Chen L, Wang M, Zhou J (2018) A two-dimensional porous electrode model for designing pore structure in a quinone-based flow cell. J Energy Storage 18:16–25 84. Cacciuttolo Q, Petit M, Pasquier D (2021) Fast computing flow battery modeling to optimize the choice of electrolytes and operating conditions–application to aqueous organic electrolytes. Electrochim Acta 392:138961 85. Modak S, Kwabi DG (2021) A zero-dimensional model for electrochemical behavior and capacity retention in organic flow cells. J Electrochem Soc 168(8):080528 86. Yu F, Shah A, Leung P (2022) A numerical model of a zinc-para-benzoquinone membranefree organic flow battery. In: The 4th International conference on clean energy and electrical systems, Tokyo, Japan

Aqueous Organic Redox Flow Batteries Hao Fan, Hongyu Xu, and Jiangxuan Song

Abstract Since the 1970s, substantial research has been conducted on redox flow batteries (RFBs), which are today regarded as one of the most promising technologies for scalable energy storage. Among RFBs, the most-developed all-vanadium RFB is still not widely popularized, mainly due to its imperfect properties, such as limited solubility and relatively high scarcity. Fortunately, organic redox-active species and their derivatives are proposed as alternatives to metal-based redox species due to their tunable properties, high safety, abundance, and sustainability. This chapter aims to present state-of-the-art research and provides an up-to-date guide for the future development of aqueous organic RFBs (AORFBs). We summarize the basic information of RFBs, including category, mechanism, and challenges, as well as the benefits and applications of aqueous organics used in AORFBs. As the core components of AORFBs, the structural composition and configuration of water-soluble redoxactive organic catholyte and anolyte are mainly determining their electrochemical performances in various AORFBs chemistries. In terms of solubility, redox potential, and stability, element compositions have been primarily focused on influencing tuning characteristics. Equal consideration has been given to the practical aspects of low cost and high performance for developing next-generation AORFBs. AORFBs with redox-active organics, including quinone, ferrocene, alloxazine, active polymers, phenazine, TEMPO, viologen, phenothiazine, azobenzene, fluorenone, and their derivatives have been exclusively discussed. Lastly, the current challenges and future directions toward the practicality of AORFBs are richly proposed. Keywords Redox-active organics · Aqueous organic redox flow batteries · Electrolytes · Physicochemical properties · Cyclability

H. Fan · H. Xu · J. Song (B) State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International Research Center for Soft Matter, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 L. An et al. (eds.), Flow Cells for Electrochemical Energy Systems, Green Energy and Technology, https://doi.org/10.1007/978-3-031-37271-1_5

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1 Aqueous Organic Redox Flow Batteries With the achievement of consensus for carbon neutral, energy transition has become the world’s active response to climate change and sustainable development [1]. After two major transitions from firewood to coal and coal to oil–gas, human beings are facing the third transition from oil–gas to renewable energy in the use of energy [2]. The increasingly difficult ecological-environmental protection situations and breakthroughs in energy technology have also extensively promoted the conversion from fossil to renewable energy, and the arrival of the renewable energy era has become inevitable. In recent years, the utilization of renewable energy sources such as solar and wind has increased due to their biocompatible and environmentally friendliness. These sources also provide a mature power generation technology and the potential for scale commercialization [3]. However, their characteristics of volatility, intermittence, and uncertainty bring crucial challenges to the smooth operation of the power systems [4]. Fortunately, energy storage allows flexible conversion between various types of energy due to the high efficiency and reliability of renewable energy grid connections [5, 6]. Among energy storage technologies, electrochemical energy storage has modularization features, fast response, and a high degree of commercialization [7]. In addition, the installation of this system is flexible, and the construction period is short. With the persistent improvement of technology, its main metrics, including efficiency, power, and cycle life, have been vastly enhanced and have become a crucial direction for energy storage’s primary development [8]. The worldwide research on electrochemical energy storage technology has successfully moved from theoretical to practical commercial applications, including lithium-ion batteries, sodium-based batteries, lead–carbon batteries, flow batteries, and so on [8, 9]. Among them, redox flow battery (RFB) is a new electrochemical energy storage technology with the unique structure proposed by Thaller [10]. It can store and release energy through the change of ion valence state in the electrolyte. Furthermore, it shows the advantages of independent energy-power and long-cycle life, possessing good application potential in large-scale energy storage field [11, 12]. According to the different solvents for dissolving electrolyte, RFB can be divided into two categories: non-aqueous RFB (NARFB) and aqueous RFB (ARFB) [11, 13]. Compared with NARFB, ARFB has the lower electrolyte resistance, higher power density, higher safety, and better environmental friendliness. According to the types of redox-active materials, ARFB can be further divided into inorganic ARFB (AIRFB) and organic ARFB (AORFB) [11, 13]. As is well known, sustainable organics are readily accessible and inexpensive and can also be designed via molecular engineering. They are incredibly adaptable in redox potential, solubility, chemical and electrochemical stability, and electrode reaction kinetics [11, 14, 15]. Table 1 summarizes the main parameters for electrochemical energy storage technology of aqueous all-vanadium RFBs (VRFBs), Zn-halogen hybrid RFB, AORFBs, lithium iron phosphate (LiFePO4 ) battery, and lead–carbon (Pb/C) battery [6, 15–18]. It can be seen that AORFBs have apparent

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Table 1 Technical parameters of AORFBs, aqueous VRFBs, LiFePO4 , and Pb/C batteries Parameters

AORFBs

VRFBs

Zn-X2 (X=Br, I)

LiFePO4

Pb/C

Ed /(Wh kg−1 )

20–50

30–40

50–150

120–160

25–50

Pd /(W kg−1 )

500–3000

1000–3000

100–3000

500–15,000

150–500

EE/(%)

60–90

70–75

70–80

88–92

80–85

Life/(years)

3–20

15–20

10–15

8–12

5–10

Start Time/(s) Second level

Second level

Second level

1 M), durable stability after long-lifetime cycles, and a highly negative potential for the anolyte and positive potential for the catholyte [13, 15]. Therefore, various redox-active organics were extensively investigated, including quinone, alloxazine, TEMPO, Vi, ferrocene, phenazine, and so on. The corresponding analysis is conducted according to the sequence of acidic, neutral, and alkaline AORFBs, respectively.

3.1 Acidic AORFBs 3.1.1

Quinone-Based Organics

Quinones are a class of carbonyl compounds that contain two C=O groups in an aromatic ring [36]. As investigated, quinone-based organics can be extensively applied in AORFB fields due to their high solubility and appropriate redox potential (Fig. 7 and Table 2). Note that the first anolyte in acidic AORFB is 9,10anthraquinone-2,7-disulfonic acid (2,7-AQDS) [44], which has a high solubility of > 1 M in H2 SO4 and undergoes a reversible 2e− /2H+ reaction with a redox potential of 0.222 V. The 2,7-AQDS/Br2 battery shows a peak galvanic Pd of 0.6 W cm−2 and C rr of 99.2%/cycle. To further reduce the redox potential of AQDS anolyte, Gerhardt et al. suggested adding electron-donating groups to obtain 1,8-dihydroxy9,10-anthraquinone-2,7-disulphonic acid (DHAQDS) [45]. Subsequently, CarreteroGonzález et al. further boosted the solubility of Quinizarin-SO3 Na in water up to 1.6 M by forming organic cation replaced Quinizarin-SO3 -YKMP in dilute HClO4 [46]. Lately, to mitigate the reactivity of BQDSH2 , Hoober-Burkhardt et al. synthesized a Michael reaction-resistant hydroquinone derivative of 3,6-dihydroxy2,4-dimethylbenzenesulfonic acid (DHDMBS) [32], which reduced the chance of nucleophilic attack by water with introducing two methyl groups to open positions on the phenyl ring. The assembled battery could produce an OCV of about 0.35 V. Among the investigated AQ derivatives (AQS, DHAQDS, ARS, and 1,4dihydroxyanthraquinone-2,3-dimethylsulfonic acid (DHAQDMS)), the solubility of both DHAQDS and DHAQDMS are over 1 M in H2 SO4 , and the OCVs of the assembled DHAQDS- and DHAQDMS-based batteries can reach 0.9 and 1.1 V, respectively [45]. However, the DHAQDS/Br2 and DHAQDMS/Br2 AORFBs show poor cycling stability. The former is attributed to the irreversible chemical reactions with bromine crossing from the catholyte, while the latter may derive from a slow decomposition of the reduced form of DHAQDMS. The AQS/Br2 AORFB exhibits higher current efficiency of 96.5%, an EE of 73.5%, and better cycling stability because of

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its high stability against bromination. In 2018, Hofmann et al. proposed the modified quinone 2,3-diaza-anthracenedione (DAD) and two of its derivatives as a promising active material for AORFB [47]. A positive redox potential shift of about 300 mV is achieved by incorporating a diaza moiety into the AQ core structure. Unique aqueous polyhydroquinone (PHQ) was recently reported at a concentration of 1 M, much higher than that of the corresponding monomers hydroquinone (∼0.6 M) [48]. The redox potential for PHQ was 0.7 V in 2 M H2 SO4 . In addition, the PHQ-based AORFB displayed a discharge capacity of 4.95 Ah L−1 at 20 mA cm−2 . The average CE was 99.1% in the 300 charge–discharge cycles at 5 mA cm−2 with an EE of 58.1%, and the C rr was 70% of the initial capacity after 300 cycles. After

Fig. 7 Time-dependent molecular structures of quinone-based derivatives for the acidic aqueous system

NA

0.07, 1.07

Quinizarin-SO3 -TKMP

NA

NA

27

0.515

0.91

0.17

MHQ

TABP

NR

0.112–0.278

1.6

0.921–0.930

NA

0.796

0.70

FQH2

0.672

DHP

0.699

PHQ

1.131

4,4' -BPTS

0.538

DAD

NA

0.144–0.961

1.10

0.02

BQDSH2

NA

0.187

AQS

DHAQDMS

15.6

0.130

0.12

0.82

DHAQDS

DHDMBS

7.25

0.222

2,7-AQDS

k0 (×10−3 cm s−1 )

E (V vs. NHE)

Abbr.

17

NA

NA

5.4

1.30∼1.84

NA

0.92

3.78

NA

3.80

NA

4.12

3.19

NA

3.81

D (×10−6 cm2 s−1 )

1

NA

~1

> 0.9

140 g L−1 > 1.5 M in 2 M HCl

NA

0.905

1.99

0.65–0.81

NA

1.10

0.66–1.0

~ 0.85

1.04

0.90

1.0

~ 0.80

OCV (V)

NA

1.1

1.4 M in 2.5 M H2 SO4

~1

NA

>1

~3

>1

2

>1

~ 0.2

>1

Smax (M)

NA

NA

27.1

NA

45

~9

2–6

NA

NA

NA

~ 22.78

~ 27.87

NA

NA

50

Ed (Wh L−1 )

NA

NA

NA

NA

290–360

NA

NA

NA

NA

62.4

700

NA

NA

NA

600

Pd (mW cm−2 )

Table 2 Summary of molecular physicochemical properties for redox-active organics and their applications in acidic AORFBs

NA

NA

(continued)

94.96%/cycle

NA

99.5%/cycle

99.92%/cycle 97.78%/day

99.9948%/cycle 99.17%/day

NA

99.9%/cycle

NA

99%/cycle

99.95%/cycle

NA

NA

> 99%/cycle 97.6%/day

C rr (%/cycle or day)

Aqueous Organic Redox Flow Batteries 109

0.374–0.506 13.9–17.3

0.195–0.287

− 0.25

0.54

NDI

BB3

0.11

1.4–2.0

5.36–7.31

> 2.0

12.7–15.9

0.86

1.30

CPZ

NHPI

> 2.5

~ 1.8

3.5 M in 3.5 M H2 SO4

0.68 M in H2 O

NA

0.5

0.92

1.83–2.51

PMZ

17–32

0.45–0.57

0.76 M in 0.1 M HClO4

0.59

21.9

Smax (M)

MB

0.186

0.25

IC-H

D (×10−6 cm2 s−1 )

Azure A

k0 (×10−3 cm s−1 )

E (V vs. NHE)

Abbr.

Table 2 (continued)

0.8

NA

1.15

~ 0.8–1.1

~ 0.80

OCV (V)

34.3

NA

NA

~ 56

26.8

Ed (Wh L−1 )

NA

NA

NA

NA

NA

Pd (mW cm−2 )

> 99.991%/cycle > 98.56%/day

~ 100%/cycle

NA

99.926–99.975%/ cycle 99.24–99.48%/day

99.96%/cycle 99.54%/day

C rr (%/cycle or day)

110 H. Fan et al.

Aqueous Organic Redox Flow Batteries

111

this, Lantz et al. synthesized two sulfonated quinone derivatives, DHAQDS and 4,4' biphenol-3,3' ,5,5' -tetrasulfonic acid (4,4' -BPTS), which were utilized as the anolyte and catholyte, respectively [49]. 4,4' -BPTS has a maximum solubility of 1.1 M in H2 SO4 and redox potential of ∼1.0 V. The DHAQDS/4,4' -BPTS AORFB yielded a peak galvanic Pd of 0.29 W cm−2 and an E d of 45 Wh L−1 . However, due to nucleophilic addition (Michael reaction) occurring between 4,4' -BPTS and water, a high cycling C rr of 0.47%/cycle was observed. Additionally, Park et al. reported a catholyte with 1,4-hydroquinone modifying four (dimethylamino)methyl groups, 2,3,5,6-tetrakis((dimethylamino)methyl)hydroquinone (FQH2 ) [50]. FQH2 in 2.5 M H2 SO4 presents a redox potential of 0.7 V. The FQH2 /Zn battery can afford a high OCV of 2.0 V, a volumetric capacity of 22 Ah L−1 , a Pd of 153 mW cm−2 , and C rr of 99.92%/cycle with a CE of > 99.5%. In 2020, Hofmann et al. synthesized a novel hydroquinone of dihydroxyphthalazine (DHP) by functionalizing the parent quinone with diaza moieties as the catholyte in acidic AORFB [51]. The DHP showed much faster redox reaction kinetics and a competitive high redox potential of ∼0.8 V in H2 SO4 compared with the reported benzoquinone compounds. Schlemmer et al. conducted the synthesis of a redox-active quinone, 2-methoxy-1,4-hydroquinone (MHQ), from a bio-based feedstock and its suitability as an electrolyte in AORFB [52]. Using 0.5 M H3 PO4 as solvent, the MQ/MHQ couple gave 97–99% CE over 250 cycles in a full battery. Recently, Li et al. designed and synthesized an air-insensitive biphenol derivative catholyte, 3,3' ,5,5' -tetramethyl-aminemethylene-4,4' -biphenol (TABP), by introducing four tertiary ammonium groups to biphenol [33]. The TABP exhibited a redox potential of 0.91 V and reversible redox kinetics in 2 M HCl. Moreover, TABP showed a high level of aqueous solubility of > 1.5 M in 2 M HCl and significantly reduced the possibility of the Michael addition reaction with the solvent by steric hindrance effect in the presence of large hydrophilic tertiary ammonium groups. Benefiting from these properties, an assembled TABP-based battery demonstrated a stable cycling performance with no capacity decreases over 900 cycles in the air atmosphere.

3.1.2

Other Organics

Apart from the benzoquinone derivatives and a few functionalized AQs, many heteroaromatic organics with heteroatoms such as S, O, N, and electron-withdrawing groups could be investigated as the catholyte in acidic AORFBs (Fig. 8). For instance, a redox-active phenazine dye molecule, neutral red (NR), was reported as an anolyte in acidic AORFBs [53]. NR showed a redox potential of 0.17 V, a rapid electrochemical reaction kinetics of ~10–2 cm s−1 , and a considerable solubility of 0.34 M in 1 M H2 SO4 . The constructed battery displays a high CE of 99% and a low average voltage of ∼0.6 V. In addition, methylene blue (MB) reaches a redox potential range of 0.42–0.57 V in H2 SO4 solutions due to the redox-reversible phenothiazine (PTZ) structure [54]. To improve the solubility of PTZ-based catholyte in water, hydrophilic

112

H. Fan et al.

side chains, including amino, ammonium, or hydroxyl groups, need to be grafted. For example, after the modification of an amino group, 3-amino-7-(dimethylamino)phenothiazin-5-ium chloride (Azure A) demonstrated a solubility of ~0.6 M in a HAc-H2 SO4 mixture and a redox potential of ~0.59 V [54]. Besides, for the chlorpromazine (CPZ) and promethazine (PMZ) that were inoculated with ammonium groups, their solubility could surpass 2.5 and 2.0 M, respectively. The corresponding redox potentials can reach 0.59 and 0.92 V. These PTZ derivatives present a high redox reversibility with an extremely fast electron transfer rate constant, excellent stability in both reduced and oxidized states, and high solubility up to 1.7 M, thus resulting in the highly reversible capacity over 70 Ah L−1 . Mukhopadhyay et al. recently presented an earth-abundant and redox-active natural dye molecule, indigo carmine (IC), as an anolyte for acidic AORFBs [55].

Fig. 8 Time-dependent molecular structures of other organics for the acidic aqueous system

Aqueous Organic Redox Flow Batteries

113

The IC’s acid analog (IC-H) showed a solubility of 0.76 M in 0.1 M HClO4 due to forming an intermolecular H-bond. The IC-H/Br2 battery enabled an OCV of ∼0.85 V and a Pd of 48 mW cm−2 at 40 mA cm−2 . The battery also delivered good long-term stability with a CE of 97%, an EE of 77%, and a discharge C rr of 99.54%/ day. Benefiting from the π-conjugated aromatic structure, naphthalenediimide (NDI) derivatives exhibit stable redox reversibility. Wiberg et al. demonstrated a quaternary amine-functionalized NDI, which has a solubility of 0.68 M in water, and a redox potential of −0.13 V in H2 SO4 [56]. The modified NDI molecule showed an excellent charge–discharge stability with no degradation after the cycling lifetime in one week. Wang et al. then reported a non-persistent radical precursor, N-hydroxyphthalimide (NHPI), as a low-cost, high-potential catholyte for a semi-AORFB [57]. A highly reversible NHPI-phthalimide N-oxyl (PINO) radical demonstrates a redox couple of 1.30 V, providing a 1.15 V rechargeable battery with an attractive > 85% voltage efficiency when coupled with AQS anolyte. In 2020, Song et al. explored the viability of a phenazine-based organic dye, basic blue 3 (BB3), as a catholyte paired with vanadium anolyte [58]. BB3 received a maximum solubility of 2.5 M in a mixed solvent of H2 O/HAc with a volume ratio of 1:1 in 3.5 M H2 SO4 , corresponding to a theoretical equivalent electron concentration of 5.0 M and a charge capacity of 134 Ah L−1 . The BB3 dye showed significant stability translating to a high capacity retention of 99.991%/cycle during 1500 cycles in AORFBs. Significantly, the synthesis used readily available reagents, and the BB3 catholyte was a benign compound that was produced in good yields [59].

3.2 Neutral AORFBs The neutral AORFBs operated at pH-neutral conditions to allow the use of noncorrosive and cost-effective inorganic salts in water. The commonly observed side reaction of water electrolysis in acidic and alkaline systems can be suppressed due to a combination of the inert carbon electrode and the neutral supporting electrolyte significantly increasing the overpotentials of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [15]. Furthermore, cheaper AEMs can be used in neutral AORFBs to separate the anolyte and catholyte, replacing the expensive CEMs commonly used in acidic and alkaline systems. The redox-active organics utilized for a neutral aqueous system are summarized in Figs. 9, 10, 11, and 12; Table 3.

3.2.1

Ferrocene- and Ferrocyanide-Based Organics

Liu et al. first synthesized two redox-active Fc-based catholytes, (ferrocenylmethyl)trimethylammonium chloride (FcNCl) and N1 -ferrocenylmethylN1 ,N1 ,N2 ,N2 ,N2 -pentamethylpropane-1,2-diaminium dibromide (BTMAP-Fc) [60, 61], via a structural modification of hydrophobic Fc with a hydrophilic quaternary

114

H. Fan et al.

Fig. 9 Time-dependent molecular structures of ferrocene derivatives for the neutral aqueous system

ammonium cation group. The two organics display high solubilities of 3 M in 2 M NaCl for FcNCl and 2 M in 2 M NaCl for BTMAP-Fc, a relatively positive equilibrium potential of 0.61 V, and quasi-reversible redox reaction kinetics. The paired FcNCl-based AORFB shows an OCV of 1.05 V, and a theoretical E d can reach 45.5 Wh L−1 . The system with 0.5 M FcNCl can deliver an EE of ~ 60% at 60 mA cm−2 and a peak Pd of 125 mW cm−2 . Moreover, the discharge capacity was kept at 91% after 700 cycles at 60 mA cm−2 , indicating an unsurpassed long cycling performance. In addition, an optimal AORFB design can achieve an unprecedented average EE of 76% and capacity retention of 99.964%/cycle over 200 cycles at 60 mA cm−2 because of the synergistic effect of low area-resistance Selemion DSV membrane and high-conductivity KCl supporting electrolyte. Moreover, the paired BTMAP-Fcbased AORFB at 1.3 M reactant concentration has a significantly reduced crossover rate and a C rr as high as 99.9943%/cycle or 99.90%/day with an average CE greater than 99.95%.

Aqueous Organic Redox Flow Batteries

115

Fig. 10 Time-dependent molecular structures of TEMPO derivatives for the neutral aqueous system

Afterward, Zhu et al. reported a hydrosoluble and redox-active artificial bipolar cation, 1-(4-ferrocenyl-n-butyl)-1' -[3-(trimethylammonio)propyl]-4,4' bipyridinium (Fc-bipy3+ ), that contains a cathode-active Fc subunit and an anodeactive bipyridinium moiety, which was applied in a symmetric neutral AORFB [62]. The battery OCV was ~ 0.7 V, and its cyclability was superb with a C rr of 99.9938%/cycle over 4000 cycles and ~ 99.8% CE at 10 mA cm−2 . This result was mainly attributed to the excellent reversibility and chemical stability of Fcbipy3+ and minimum cross-contamination. In addition to Fc, potassium ferricyanide K4 [Fe(CN)6 ] was also an iron complex commonly used as the catholyte. In the same year, by using the NH4 + cation to replace K+ or Na+ as counterions for ferrocyanide

116

H. Fan et al.

Fig. 11 Time-dependent molecular structures of viologen-based derivatives complexes for the neutral aqueous system

([Fe(CN)6 ]4− ), the solubility of (NH4 )4 [Fe(CN)6 ] was improved to 1.6 M, which was more than double that of K4 [Fe(CN)6 ] (0.76 M in water) and Na4 [Fe(CN)6 ] (0.56 M in water) [63]. The strong H-bond interaction between NH4 + and water was believed to promote the dissolution. The 1.5 M half-cells of [(NH4 )3 [Fe(CN)6 ]/ (NH4 )4 [Fe(CN)6 ] confirmed the high capacity and stability of ammonium ferrocyanide catholyte at the pH-neutral condition [42]. Paired with Vi anolyte, a 0.9 M [(NH4 )4 [Fe(CN)6 ]/Vi AORFB without the supporting electrolytes exhibited excellent cycling performance, including nearly 100% C rr in 1000 cycles (~1100 h), 62.6% EE at 40 mA cm−2 and a Pd of 72.5 mW cm−2 .

Aqueous Organic Redox Flow Batteries

117

Fig. 12 Time-dependent molecular structures of other organic complexes for neutral aqueous system

Recently, an another Fc derivative bearing a negatively charged moiety, ferrocene bis(propyl sodium sulfite) (Fc-SO3 Na), was designed as the catholyte in aqueous RFBs [64]. The solubility of Fc-SO3 Na was about 2.5 M in 0.5 M Na2 SO4 , corresponding to a volumetric capacity of 67 Ah L−1 . The paired Fc-SO3 Nabased battery demonstrated superb performance with a C rr of 99.9975%/cycle after 1000 cycles, and the E d at 1.5 M catholyte could reach 27.1 Wh L−1 . At the same time, Borchers et al. reported a water-soluble and Fc-containing methacrylamide copolymers with different comonomer ratios of the solubility-promoting comonomer [2-(methacryloyloxy)-ethyl]-trimethylammonium chloride (METAC) [65]. The polymer was further tested in an RFB setup and revealed a high CE of > 99.8% and desirable apparent C rr at room temperature and 60 °C.

14

182

0.0044

10.6

8.31–37

0.39

0.39

0.72

0.33

0.5

0.28–0.39

BTMAP-Fc

(NH4 )4 [Fe(CN)6 ]

Poly-Fc

FcSO3 Na

R-Fc

Q-Fc

2.6

3.3

0.45

0.37–0.61

0.80

− 0.45

0.9

Cn -FcNCl

4-HO-TEMPO

MV

Poly-TEMPO

116

126–288

NA

0.28

0.453–0.456

Zn(Fc(SPr)2 )

NA

0.46–1.15

Zwitterionic-Fc

FeII(Dcbpy)2 (CN)2 )4−

BQ-Fc

0.0366

0.61

FcNCl

NA

k0 (× 10−3 cm s−1 )

E (V vs. NHE)

Abbr.

0.07–0.76

5.7

29.5

5.78–6.80

3.38–5.58

3.84

2.1–2.3

NA

1.87–2.22

3.17

0.76–0.98

6.31–6.84

3.1

3.64

D (×10−6 cm2 s−1 )

OCV (V)

2 μg mL−1

2.4

2.1 M in H2 O

2.3–4.0

0.66–2.01 M in H2 O

NA

1.22

3.09

0.28 M in H2 O

2.5 M in H2 O

>1

1.60–1.91 M in H2 O

1.9 M in H2 O

NA

1.25

NA

0.915

1.13

1.2

0.66–0.77

0.9

1.18

0.78

0.82

0.748

3 M in 2 M NaCl 1.06

S max (M)

8–10

8.4

11–13

NA

20–30

12.5

NA

NA

27.1

NA

9.6

13–20

7–9.9

Ed (Wh L−1 )

NA

NA

NA

NA

270.5

NA

20.24

58–69

NA

NA

76.8–99.6

60

125

Pd (mW cm−2 )

Table 3 Summary of molecular physicochemical properties for redox-active organics and their applications in neutral AORFBs

(continued)

~ 100%/cycle

99.89%/cycle 71.92%/day

99.998%/cycle 99.927%/day

~ 99%/day

~ 100%/cycle

99.992–99.9984%/ cycle 99.75–99.783%/day

99.832%/day

99.998%/cycle

~ 100%/cycle

99.9%/cycle

99.96%/cycle 97.36%/day

99.9943–99.9989%/ cycle 99.90–99.967%/day

99.99%/cycle

C rr (%/cycle or day)

118 H. Fan et al.

2.98

10.2

11.7

0.90

0.81

Zwitterionic-TEMPO

TMAP-TEMPO

NA

MI-TEMPO

21

6.42

0.80

COONa-TEMPO

0.83

MIAcNH-TEMPO

18.6

75

350

0.967

1.0

(TPABPy)Cl3

4.75–7.54

1.13, − 0.10

RIBOTEMPO

N2 -TEMPO

5.81

0.96

0.86

CPL

CPD

51.3

0.83

0.90

AcNH-TEMPO

4-NH2 -TEMPO

0.972

NA

0.80

0.91

g+ -TEMPO

1.91

58

0.81

0.87, − 0.29

S-TEMPO

VIOTEMP

2.35

3.99

0.8, − 0.4

TEMPO Phenazine Combi-Molecule

11.5

5.15

2.97

5.39–7.92

8.75

6.96

5.45

1.01

1.07

3.48

0.377

NA

NA

290

4.8

4.2

0.99

D (×10−6 cm2 s−1 )

TMA-TEMPO

k0 (× 10−3 cm s−1 )

E (V vs. NHE)

Abbr.

Table 3 (continued)

2.3

3

1.76

0.32

0.091

0.085

1.5 M in NaCl

>5

0.5

1.71

1.35

1.299

1.23

1.21

1.31

1.19

1.78

1.71

1.19

1.35

> 20 Ah L−1 4.62 M in H2 O

1.64

1.55

1.16

1.69

1.20

1.4

OCV (V)

0.4

> 0.2

~ 0.025

>1

NA

2.3

S max (M)

~ 30.9

18.1

19

7.03

~ 0.8

~ 0.9

14.7

2–5

12

5.33

8.87

NA

NA

NA

NA

38

Ed (Wh L−1 )

~ 175

114

NA

NA

NA

NA

NA

NA

99.03

NA

NA

NA

NA

NA

NA

NA

Pd (mW cm−2 )

(continued)

99.9995%/cycle 99.95%/day

99.975%/cycle

99.98%/cycle

99.95%/cycle

NA

99.96%/cycle

99.94%/cycle

99.65%/cycle

99.993%/cycle 99.376%/day

99.92%/cycle

99.45–99.58%/cycle

93.6%/cycle

NA

NA

~ 100%/cycle

99.97%/cycle 98.896%/day

C rr (%/cycle or day)

Aqueous Organic Redox Flow Batteries 119

6.11 NA

0.167

350

50

− 0.45, 1.08

−0.60, −0.93 1.7–3.3

NA

− 0.55

− 0.41

− 0.32, − 0.70

R-Vi

MVBr2

MAPVi2+

EV

Dex-Vi

3.86–4.21

1.9–2.3

5.02

6.97

3.26

NA

0.28

− 0.38

315

− 0.43

0.28

− 0.44

((NPr)2 TTz)Cl4

5.3–5.4

(SPr)2 V

0.360–0.364

− 0.39, − 0.78

((Me)(NPr)V)Cl3

3.3

2.08–3.91

3.36–5.08

2.27

4.07

55

D (×10−6 cm2 s−1 )

PV3+

2.67–3.38

22

5.29–6.24

0.805

PSS-TEMPO

− 0.801

5.94

− 0.76

− 0.35, − 0.72

1.42

0.81

Pyr-TEMPO

(PyrPV)Cl4

BTMAP-Vi

422

0.84

TMAAcNH-TEMPO

(PPBPy)Br2

k0 (× 10−3 cm s−1 )

E (V vs. NHE)

Abbr.

Table 3 (continued)

1.5

2.81 M in H2 O

0.3

2.1

1.1 M

2 M in H2 O

NA

1.3 M in H2 O

1.8 M in H2 O

2 M in H2 O

1.1 M in H2 O

2.55 M in H2 O

1.13 M in H2 O

3.35 M in H2 O

4.3

S max (M)

NA

2.81

NA

1.53

1.05

1.0

1.16

1.44

1.38

0.75

1.61

1.57

1.22

OCV (V)

NA

1.06

NA

10.2

NA

6.7

NA

53.7

8.04

13

33.1

NA

16.4

Ed (Wh L−1 )

NA

NA

NA

133

117

92.5

NA

NA

NA

60

509

317

NA

Pd (mW cm−2 )

(continued)

99.999%/day

99.69%/cycle

99.78%/cycle

99.9999%/cycle

99.979–99.993%/ cycle

99.99/cycle

NA

99.97/cycle

99.82/cycle

99.9943–99.9989%/ cycle 99.967–99.90%/day

99.988–100%/cycle

99.80–99.95%/cycle 94.27–96.53%/day

99.6544%/day

C rr (%/cycle or day)

120 H. Fan et al.

NA

NA

− 0.20, − 0.47

− 0.20

M2 -BNDI

Na2 AQDS

77

NA

− 0.20

0.55

(NH4 )2 AQDS

NA

NA

− 0.43~ − 0.52

PEGAQ

Hexasubstituted [3]radialene

0.1

− 0.47~ − 0.49

NA

NA

NA

4.55

NA

1.76 M in H2 O

0.4

0.03

0.45 M in H2 O

1.9 M in H2 O

NA

2,6-DPPEAQ

5.45–24.5 6.10

0.39–30.4

0.813

− 0.76

1

− 0.451

6.3–11.5

1.8 M in H2 O

2.69

1.23

1.1–1.31

S max (M)

(bpy-(CH2 )3 )NMe3 )I2

1.96–3.91

− 0.78, − 0.39

(SO3 )V(OH)Br

4.3

5.26

2.6–2.7

2.14–3.75

D (×10−6 cm2 s−1 )

Lawsone

11.2

350

− 0.433

− 0.41

2.2–2.9

− 0.462

BPP-Vi

BHOP-Vi

270–450

− 0.51~ − 0.68

Chalcogenophen Vi

3,4-S2 V

k0 (× 10−3 cm s−1 )

E (V vs. NHE)

Abbr.

Table 3 (continued)

0.80

1.00 1.27

0.9

0.865

1.0

1.0

1.30

1.38

1.61

0.88

1.021

0.90

1.12 ~ 1.42

OCV (V)

NA

NA

2.34

12.5

25.2

NA

NA

NA

7.1

NA

11.8

4.94

Ed (Wh L−1 )

150

~ 37

NA

91.5

170

160

NA

NA

NA

99.5

110.87

145

102

Pd (mW cm−2 )

(continued)

91.375%/cycle

99.958%/day

NA

~ 100%/cycle

99.957%/cycle 99.5%/day

99.986%/day

99.992%/cycle

99.5%/cycle

99.95%/cycle

99.955%/day

98.872%/day

99.9993%/cycle 99.984%/day

99.94–99.9994%/ cycle

C rr (%/cycle or day)

Aqueous Organic Redox Flow Batteries 121

4.08 NA

0.362

0.513

− 0.206

− 0.56

TRYP-8SO3

1,6-DPAP

2.99

3.6

3.02

69.5

− 0.42

− 0.55~ − 0.57

QAAQ

1,8-BDPAQCl2

3.63

NA

33.3

− 0.5

− 0.70

2H-NDI

1-DPAQCl

0.0698

2.2–2.4

5.9–15

− 0.25, − 0.44, − 0.63

(TPyTz)Cl6

D (×10−6 cm2 s−1 )

k0 (× 10−3 cm s−1 )

E (V vs. NHE)

Abbr.

Table 3 (continued)

1 M in H2 O

1.4 M in H2 O

1.44 M in H2 O

0.5

1.005

0.122

1.18

S max (M)

~ 1.3

1.08

1.11

0.65

1.08

0.94

0.88 1.07 1.26

OCV (V)

NA

NA

NA

NA

NA

0.046

NA

Ed (Wh L−1 )

42.2

33

134

NA

103

NA

273

Pd (mW cm−2 )

99.952%/cycle 99.12%/day

99.962%/cycle

NA

NA

~ 100%/cycle 99.9976%/day

NA

NA

C rr (%/cycle or day)

122 H. Fan et al.

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In order to figure out how the Fc structures influence the physicochemical property and battery stability. Yang et al. prepared a series of derivatives, ((6-trimethylammonio)-hexyl)-ferrocene dibromide (Q-Fc) and bis((6trimethylammonio)-hexyl)-ferrocene dibromide (BQ-Fc), with deliberately tuned molecular structures and compared them under identical operational conditions in AORFB [66]. The BQ-Fc, with a solubility of 3.1 M, exhibited the highest stability. The applied 1.5 M high-concentration battery can operate for 32 days and manifest a satisfactory C rr of 99.832%/day. Subsequently, Yang et al. also described a host–guest inclusion strategy to prepare water-soluble Fc-based electrolytes (R-Fc) by simply mixing Fc with cyclodextrins (CDs), a family of hosts with hydrophobic cavities and hydrophilic surfaces [67]. The solubility can reach 0.28 M when using a hydroxypropyl-β-CD (HP-β-CD) host. The assembled battery demonstrated an improved C rr of 99.8248%/day with a controlled state of charge of 80%. Meanwhile, Zhu et al. recently introduced a symmetry-breaking design by replacing part cyanides with bipyridine ligand 2,2' -bipyridine-4,4' -dicarboxylic acid [43]. The affording M4 [FeII(Dcbpy)2 (CN)2 ], M = Na, K) showed a significantly improved solubility of 1.22 M, as well as a 50% increase for redox potential of 0.65 V. The Na4 [FeII(Dcbpy)2 (CN)2 ]-based AORFB demonstrated a C rr of 99.99842%/cycle (99.783%/day) for 6000 cycles. Even at a concentration near the solubility limit of 1 M, the battery exhibited a C rr of 99.992%/cycle (99.75%/day) in the first 400 cycles. The AORFB, with a nearly 1:1 catholyte–anolyte electron ratio, achieved an OCV of 1.2 V and an E d of 12.5 Wh L−1 . In 2022, Zhang et al. presented two new Fc compounds, Fc3 and Fc4, with propyl and butyl zwitterionic side chains (Zwitterionic-Fc) [68]. These compounds were highly soluble in water (0.66 M for Fc3 and 2.01 M for Fc4). The paired ZwitterionicFc AORFB exhibited excellent performance under neutral aqueous conditions. EE was ca. 88%, with the CE over 99% under high-concentration conditions. Interestingly, a difference in stability between the lengths of the zwitterionic chains, with Fc4 showing higher stability than Fc3, and the capacity decreased by approximately 5% at the end of 20 cycles (approximately 1%/day). To further classify the structure-performance relationship and degradation mechanism of Fc derivatives in aqueous electrolytes, Liu et al. systemically investigated the physicochemical and electrochemical properties, battery performance, and degradation mechanisms of three Fc catholytes, (ferrocenylmethyl)trimethylammonium chloride (C1 –FcNCl), (2-ferrocenyl-ethyl)trimethylammonium chloride (C2 – FcNCl), and (3-ferrocenyl-propyl)trimethylammonium chloride (C3 –FcNCl) in pHneutral AORFBs [69]. The Cn –FcNCl electrolytes displayed cycling stability in both half-cell and full-battery RFBs in C1 –FcNCl < C2 –FcNCl < C3 – FcNCl. The electron-donating 3-(trimethylammonium)propyl group strengthened the coordination between the C3 –Cp-ligand and the Fe center and thus mitigated the ligand-dissociation degradation. To prevent the cross-contamination, Liu lately reported report a bipolar zinc-ferrocene salt compound, zinc 1,1' -bis(3sulfonatopropyl)ferrocene, Zn[Fc(SPr)2 ] (1.80 M solubility or 48.2 Ah L−1 storage capacity), as the redox-active electrolytes for AORFBs [70]. The Zn[Fc(SPr)2 ]-based batteries operated at high current densities of up to 200 mA cm−2 and delivered an

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EE of up to 81.5% and a Pd of up to 270.5 mW cm−2 . Noticeably, the battery can demonstrate an OCV of 1.13 V, an E d of 20.2 Wh L−1 , and keep nearly 100% C rr in 2000 cycles (~53.5 days).

3.2.2

Nitroxide-Based Organics

TEMPO is an N-oxyl-containing compound widely applied as an antioxidant in biology, a mediator in polymerization, and a charge storage material [26]. Wang et al. first used 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxy (4-HO-TEMPO) as the catholyte and coupled it with Vi to make an AORFB [15]. The 4-HO-TEMPO exhibited a solubility of 2.1 M in H2 O and 0.5 M in 1.5 m NaCl with one-electron transfer from a nitroxide radical to oxoammonium salts (TEMPO+ ) at the redox potential of 0.8 V. The reaction rate constant k0 is 2.6 × 10–4 cm s−1 , and the diffusion coefficient D is 2.95 × 10–5 cm2 s−1 . The AORFB composed of 0.5 M 4-HO-TEMPO and MV solutions exhibited an OCV of 1.25 V and stably operated for 100 cycles (89% capacity retention) at 60 mA cm−2 with a CE close to 100% and an EE of 62.5%. The corresponding capacity for each electrolyte was close to 13.4 Ah L−1 , and the E d was 8.4 Wh L−1 . Functionalization of hydrophilic functional groups of TEMPO that could increase the solubility to > 1.0 M, making TEMPO-based organics promising candidates for AORFBs. Janoschka et al. introduced a hydrophilic and electron-withdrawing trimethylammonium cation group (–N(CH3 )3 + ) as a 4-position substituent (i.e., N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride, TEMPTMA or NMe -TEMPO) [71], which successfully increased the solubility of 2.3 M in 1.5 M NaCl (i.e., 61 Ah L−1 ) and redox potential over 0.95 V. They demonstrated a 2.0 M system that exhibited full capacity retention over 100 cycles at 80 mA cm−2 . Winsberg et al. reported an aqueous poly(TEMPO)-based battery to increase the overall battery voltage. The OCV can reach 1.7 V, and the battery has a good long-term stability [72]. Lately, Winsberg et al. also proposed a negatively charged TEMPO-4sulfate (S-TEMPO) to adapt to the CEM used in RFBs [73]. The S-TEMPO-based system has a high voltage of 1.69 V and an E d of 20.4 Wh L−1 . The fabricated battery featured over 1100 consecutive charge–discharge cycles with constant C rr at a high current density of up to 80 mA cm−2 . A major challenge polymeric electrolytes face was to balance the solubility and viscosity. P(TEMPO-co-zwitterion), presented by Schubert et al., can reach a low viscosity of 4.59 mPa s at 10 Ah L−1 in 1.5 M NaCl [74]. Long-term stability tests over 1000 cycles indicated the good stability with a capacity loss of ca. 0.08%/cycle. The CE was ~ 99.01%, EE was ca. 93%, and initial E d was 5.33 Wh L−1 during the 125 consecutive cycles. The crossover of TEMPO led to fast capacity decay in AORFBs. Connecting bipolar redox couples by covalent bond was an effective strategy for mitigating capacity fade caused by cross-contamination. Schubert et al. reported a combined molecule of TEMPO and phenazine (TEMPO/phenazine combi-molecule) via a triethylene glycol linker [75], and TEMPO-viologen (VIOTEMP) via a benzyl

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linker (i.e., 1-(4-(((1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)oxy)carbonyl)benzyl)1-methyl-[4,4' -bipyridine]-1,1' -diiumchloride) [76], in a symmetric battery configuration. The OCV of the battery was about 1.2 V, no capacity loss was observed after 1800 cycles, and CE was as high as 98.3%. In the same year, Chang et al. reported a cationic grafting TEMPO (g+ TEMPO) by a one-step epoxide ring-opening reaction between 4-OH-TEMPO and a glycidyltrimethylammonium cation (GTMA+ ) [77]. The constructed battery showed a voltage efficiency of about 90%, a C rr of 99.954%/cycle, and a CE of 99.3% in 140 cycles. Subsequently, Chang synthesized imidazoline-grafted TEMPO (MITEMPO) and used it as a catholyte in RFBs [78]. The average CE, voltage efficiency, and EE were 95.9%, 97.7%, and 93.7%, respectively, after 30 cycles. Compared with 4-OH-TEMPO, these two molecules showed slightly enhanced battery performance in terms of efficiency and stability, probably owing to the charge and size repulsion [30]. To further increase the stability, Yang et al. introduced trimethyl butylammonium to 4-OH-TEMPO, affording 4-[3-(trimethylammonio)propoxy]-2,2,6,6tetramethylpiperidine-1-oxyl (TMAP-TEMPO) [79]. The solubility of TMAPTEMPO can reach 4.62 M in water, and its potential was about 0.81 V. The constructed system has an OCV of 1.1 V and a CE of 99.73%. Instead of being directly attached to the TEMPO core, the ammonium cation is isolated by aliphatic spacers so that the molecule could be less susceptible to ring-opening side reactions. As a result, TAMP-TEMPO manifested an unprecedented C rr of > 99.376%/day, independent of the electrolyte concentration. To classify the effects of substituent R in 4-position on redox potential and corresponding capacity fading mechanism of TEMPO, Song et al. conducted four R-TEMPO derivatives with R=–OH, –NH2 , –COOH, and –NHCOCH3 in RFBs [80]. The analyses revealed that low-radical head charge population sum and radical energy, depending on R in 4-position, played a critical role in enhancing R-TEMPO’s redox potential and cycling life. The optimal 4-NHCOCH3 -TEMPO-based battery achieved a high C rr of > 99.65%/day and an OCV of 1.71 V. The electronic effect brought along by N-acetyl could redistribute the charge and lower systematic energy, making the ring-opening joint sturdy and therefore suppressing the side reactions. In the same year, Cao et al. also showed a comprehensive study on the properties of TEMPO derivatives in aqueous electrolytes [81]. The results confirm that the redox potential, diffusion coefficient, electron transfer rate constant, and solubility were influenced by functional groups of TEMPO derivatives and supporting electrolytes. In 2021, Zhao et al. reported an electroactive 4-carboxylic-2,2,6,6tetramethylpiperidin-N-oxyl (4-CO2 Na-TEMPO) molecule for neutral AORFBs [82]. The solubility of 4-CO2 Na-TEMPO in sodium-based aqueous solution can reach 1.5 M, three times that of the original 4-OH-TEMPO. When paired with a Vi anolyte, the resulting AORFB operating through a CEM achieved an OCV of 1.19 V and a high E d of 14.7 W h L−1 . The long-term cycling study features a stable C rr of 99.94%/cycle over 400 cycles with nearly 100% CE. To further increase the redox potential, Song et al. proposed a new class of five-membered ring pyrrolidine and pyrroline motifs for AORFBs [83]. By introducing a C=C double bond

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into the pyrrolidine-based molecule, 3-carbamoyl-2,2,5,5-tetramethylpyrroline-1oxyl (CPL) with a high redox potential of 0.96 V was demonstrated, which was 160 mV higher than the common TEMPO with a six-membered ring as the core structure. The CPL-based AORFB delivered constant C rr of up to 99.96%/cycle over 500 cycles. Recently, a novel electroactive organic molecule, 1-(1-oxyl-2,2,6,6tetramethylpiperidin-4-yl)-1' -(3-(trimethylammonio)propyl)-4,4' -bipyridinium trichloride ((TPABPy)Cl3 ), was synthesized by decorating TEMPO with Vi and used as the catholyte in neutral AORFB [84]. The redox potential was elevated from + 0.745 V for TEMPO to + 0.967 V for catholyte, originating from the electron-withdrawing effect of the Vi unit. The diffusion coefficient D was 2.97 × 10–6 cm2 s−1 , and the rate constant k0 of the one-electron transfer process was 7.50 × 10–2 cm s−1 . A full battery operated with 1.50 M afforded a high E d of 19.0 Wh L−1 and a stable cycling performance with C rr of 99.98%/cycle. In the meantime, Shao et al. presented a low-cost riboflavin organic molecule coupled with a TEMPO radical to form a single Riboflavin-TEMPO (RIBOTEMPO) bifunctional redox-active material [85]. The combined molecule displayed electrochemically reversible reactions leading to a theoretical battery voltage of 1.23 V. A symmetric battery testing demonstrated over 100 consecutive charge/discharge cycles with nearly 80% CE and capacity retention of 44.7% at 2.5 mA cm−2 . In 2022, Liu et al. reported a new TEMPO derivative functionalized with a dual-ammonium dicationic group, N1 ,-N1 ,-N1 ,-N3 ,-N3 ,-2,2,6,6-nonamethyl-N-3(piperidinyloxy)propane-1,3-bis(ammonium) dichloride (N2 -TEMPO) as a stable, low permeable catholyte for AORFBs [86]. Its permeability was as low as 1.49 × 10–12 cm2 s−1 . When paired with a Vi anolyte, the AORFB has an OCV of 1.35 V and delivered a high Pd of 114 mW cm−2 and 100% capacity retention for 400 cycles at 60 mA cm−2 . At 1.0 M electrolyte concentrations, the AORFB achieved an E d of 18.1 Wh L−1 and capacity retention of 90% for 400 cycles. Recently, Song et al. adopted a molecular structure regulation strategy with “π-π” conjugated imidazolium and “p-π” conjugated acetylamino co-functionalized 2,2,6,6-tetramethylpiperidine-Noxyl (MIAcNH-TEMPO) as stable catholyte for RFBs [87]. The applied MIAcNHTEMPO-based system demonstrated the hardly time-dependent stability with a C rr of 99.95%/day over 16.7 days at 1.5 M and 50 mA cm−2 . The incorporation of double-conjugate substituents could delocalize the electron density of the N–O head and thus remarkably stabilize the radical and oxoammonium forms of TEMPO. Lately, to further reveal the decomposition mechanism of TEMPO, Song et al. designed a comparison of 4-[3-(trimethylammonium)acetylamino]2,2,6,6-tetramethylpiperidine-1-oxyl chloride (TMAAcNH-TEMPO) and 4-NH2 TEMPO [88]. Experimental characterizations and theoretical simulations revealed that TMAAcNH-TEMPO was largely stabilized by the reduced reactivity of the nitroxyl radical moiety that mitigated a ring-opening side reaction. The neutral system has an OCV of 1.22 V and exhibits a capacity decay rate as low as 0.0144%/h in the electrolyte concentration range of 0.1–0.5 M. At the same time. It showed a capacity output of 2.48 Ah L−1 (~93% of the theoretical capacity) and EE of ~ 79% at 50 mA cm−2 .

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This year, to enlarge the potential gap of the redox-active organics. Jin et al. reported a highly soluble organic redox pair based on pyrrolidinium cation functionalized TEMPO and extended Vi, namely Pyr-TEMPO and [PyrPV]Cl4 , which exhibited high OCV of 1.57 V and long cycling life over 1000 cycles in AORFBs [89]. The intrinsic hydrophilic nature of the pyrrolidinium group enabled high aqueous solubilities over 3.35 M for Pyr-TEMPO and 1.13 M for [PyrPV]Cl4 . Notably, the assembled AORFBs realized a high E d of 16.8 Wh L−1 and a peak Pd of 317 mW cm−2 . To achieve a high Pd device, Wang et al. proposed a pair of anionic organic molecules, namely PSS-TEMPO and (PPBPy)Br2 , used as catholyte and anolyte, respectively [90]. PSS-TEMPO in catholyte presented a capacity decay rate as low as 0.012%/cycle after 1000 cycles. At near-neutral conditions, the system exhibited a high voltage of 1.61 V and a high peak Pd of 509 mW cm−2 .

3.2.3

Viologen-Based Organics

Vi and its derivatives are highly soluble (3.0 M in 1.5 M NaCl, 43.2 Ah L−1 ) [34] in aqueous electrolytes. MV and ethyl Vi (EV) are the two most commonly used Vi-based compounds in AORFBs [26, 34]. The solubility and hydrophilicity of Vi organics were found to decrease with increasing size of substituent group on the pyridine N atoms. The solubility of MV with a methyl group of 3.0 M was higher than that of EV with an ethyl group of 2.0 M. The two solubilities were higher than 0.04 M of 4,4-dibenzyl bipyridinium dichloride (BV), which had the largest substituent group [26]. Vi takes two one-electron steps upon reduction. The reduction of Vi2+ to Vi+ was shown to be highly reversible at a potential of −0.5 V with fast kinetics (e.g., MV, k 0 = 2.8 × 10–4 cm s−1 ) and a high diffusion coefficient (e.g., MV, D = 2.57 × 10–5 cm2 s−1 ). Liu et al. reported a 0.5 M MV/4-OH-TEMPO AORFB with an E d of 8.4 Wh L−1 and EE of 62.1% [34]. The reduction of MV+ / MV0 occurred at −0.7 V, which was less reversible than MV2+ /MV+ due to the low solubility of < 0.01 M for MV0 . Recently, Aziz et al. synthesized a highly water-soluble Vi derivative, bis(3trimethylammonium)propyl viologen (BTMAP-Vi), with a solubility of 1.9 M in water and a similarly structured Fc derivative, BTMAP-Fc, with a solubility of 2 M in water, which were used as the anolyte and catholyte in neutral pH condition, respectively [61]. Due to the repulsion of charges and size exclusion, the permeabilities for BTMAP-Vi and BTMAP-Fc across an AEM were measured to be 6.7 × 10–10 and 6.2 × 10–10 cm2 s−1 , respectively, much lower than that of MV (3.4 × 10–9 cm2 s−1 ). Benefiting from all these merits, an AORFB with 0.75 M BTMAP-Vi as anolyte and 1 M BTMAP-Fc as catholyte provided a C rr of 99.9989%/cycle with an average CE of > 99.9% at 50 mA cm−2 over 500 cycles. To overcome the limitation of the insoluble nature of neutral-charged Vi0 in aqueous environments, Liu et al. introduced one and two ammonium groups into MV for the formation of 1-methyl-10-[3-(trimethylammonio)propyl]-4,4' -bipyridinium trichloride, ([(Me)(NPr)V]Cl3 ) and 1,10-bis[3-(trimethylammonio) propyl]4,4' bipyridinium tetrabromide ([(NPr)2 V]Br4 ) and revealed that the reversibility of

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Vi+ /Vi0 was enhanced with improved capacity utilization [35]. A neutral AORFB constructed by using 0.25 M [(Me)(NPr)V]Cl3 and 0.5 M FcNCl in 2 M NaCl demonstrated an OCV of 1.19 V, a specific capacity of 13.4 Ah L−1 , and an E d of 8.04 Wh L−1 . In addition, this battery showed a C rr of 99.82%/cycle over 50 cycles at 60 mA cm−2 with an average EE of ~ 63%. Liu et al. further synthesized a negatively charged sulfonate-grafted Vi, 1,1' -bis(3sulfonatopropyl)-4,4' -bipyridinium ((SPr)2 V), that can use as an anolyte in AORFB system with a cation transfer mechanism [91]. The battery has an EE of 71% at 100 mA cm−2 and a C rr of 99.99%/cycle at 60 mA cm−2 . In the same year, to further improve the cycling stability and oxygen insensitivity, Liu et al. reported a π-conjugated extended structure Vi compound, thiazolo[5,4-d]thiazole (TTz), 4,4' -(thiazolo[5,4-d]thiazole-2,5-diyl) bis(1-(3(trimethylammonio)propyl)pyridin1-ium) tetrachloride ([(NPr)2 TTz]Cl4 ), which successfully reduced the voltage difference between 2e− transfer steps (i.e., −0.38 and −0.5 V) [92]. The demonstrated AORFB has stable cycling for 300 cycles at 40 mA cm−2 with a C rr of 99.97%/cycle, a ~ 100% CE, and a 70% EE. Subsequently, Huang et al. demonstrated that the commonly used diquat herbicides, with solubilities of > 2 M in aqueous electrolytes, can be used as stable anolyte in AORFB [93]. When coupled with an Fc-derived catholyte, the battery with the diquat anolyte exhibited long galvanic cycling with high capacity retention. Notably, this remarkable stability was attributed to the improved π-conjugation that originated from the near-planar molecular conformations of the spatially constrained 2,2' -bipyridyl rings. Yang et al. synthesized a series of hydroxylated Vi derivatives with different alkyl spacer lengths to mitigate the crossover and investigate their structure–property relationships [94]. They selected the stable 1,1' -bis(3-hydroxypropyl) viologen dibromide (BHOP-Vi) and assembled a 2 M AORFB with FcNCl catholyte. The system gave a peak Pd of 110.87 mW cm−2 and a C rr of 98.872%/day. To better slow the membrane permeation, Aziz et al. developed more negative charges phosphonatesubstituted Vi, 1,1' -bis(3-phosphonopropyl)-[4,4' -bipyridine]-1,1' -diium (BPP-Vi), that bears two or three negative charges in the oxidized and reduced forms [95]. The battery presented a high C rr of 99.984%/day at 1 M concentration with no detectable crossover. Lately, Zhao et al. developed the first example of symmetric AORFB using a single ionic compound, the aqueous organic bipolar mono-N-alkylated bipyridinium iodide salt [(bpy-(CH2 )3 NMe3 )]I2 , to serve the three functions of anolyte, catholyte and supporting electrolyte [96]. The trifunctional behavior in the symmetric battery achieved an EE of 72% at 10 mA cm−2 within 100 cycles with a C rr of 99.5%/ cycle. In 2021, Liang et al. introduced a rotating phenyl ring between two pyridinium rings, 1,1' -bis[3-(trimethylamonium)propyl]-4,4' -(1,4-phenylene)bispyridinium tetrachloride ((APBPy)Cl4 ), to form a switchable conjugation [97]. In this design, by pairing 0.50 M (APBPy)Cl4 anolyte with a TEMPO derivative as catholyte, the battery delivered a high OCV of 1.73 V, a high specific capacity of 20.0 Ah L−1 , a high EE of 80.8% and superior cycling stability at 80 mA cm−2 . Similar work was conducted by Jin et al. with the products of [(NPr)2 PV]Cl4 and NMe -TEMPO [98]. The paired battery cycled at 20 mA cm−2 with potential holds; however, it displayed

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a capacity fade rate of 7.58%/day. This year, an unsymmetrical 2e− viologens, 3(1' -(2-hydroxyethyl)-[4,4' -bipyridin]-1,1' -diium-1-yl)propane-1-sulfonate bromide ((SO3 )V(OH)Br), with high solubility in the aqueous system was synthesized via a simple two-step reaction route [99]. Paired with a TEMPO derivative, the battery delivered an exceptionally high OCV of 1.63 V, an EE of ~ 80%, and an average C rr of 99.95%/cycle at 30 mA cm−2 . To enlarge the choice of high redox potential catholytes, Liu et al. reported methyl viologen dibromide ([MV]Br2 ) as a facile self-trapping, bipolar molecule for pHneutral AORFBs, representing the first report of MV as a highly efficient bromine complexing reagent [100]. The low-solubility [MV](Br3 )2 complex in catholyte during the battery charge process not only mitigated the crossover of charged tribromide species (Br3 − ) but also addressed the toxicity concern of volatile bromine simultaneously. Accordingly, a 1.53 V bipolar MV/Br AORFB delivered stable battery performance with a total capacity retention of ~ 100%, E d of 10.2 Wh L−1 , Pd of 133 mW cm−2 , and 56% EE at 60 mA cm−2 . Besides modifying the molecule structure, a separator-adding strategy can also be availed to protect Vi radicals. Lu et al. exemplified this concept with EV and α-CD [101]. Asymmetric AORFB coupling with a Fc derivative demonstrated stable cycling over 500 cycles during 26 days at 80% capacity utilization in the presence of α-CD. Song et al. developed a stable rod-like sulfonated Vi (R-Vi) derivative to further increase redox potential and suppress membrane permeability through a spatialstructure-adjustment strategy for neutral AORFBs [102]. The R-Vi featured four individual methyl groups on the 2,2' ,6,6' -positions of the 4,4' -bipyridine core ring. The electron-donating effect of methyls endowed R-Vi with the lowest redox potential of −0.55 V. In addition, the R-Vi with weak charge attraction and large molecular dimension displayed an ultralow membrane permeability that was only 14.7% of that of typical sigmoid Vi. The assembled AORFB exhibited an EE up to 87% and extremely high C rr of 99.993%/cycle in 3200 cycles. To enhance the solubility, Kathiresan et al. adopted an asymmetric Vi, 1-propyl-1' (3-triethylammonio) propyl-4,4' -bipyridinium tribromide (PV3+ ), in the presence of triethylammonium group enhanced the radical solubility and effectively diminished the radical dimerization [103]. The PV3+ /4-OH-TEMPO AORFB showed an OCV of 1.16 V and delivered a maximum EE of 61.25% at 8 mA cm−2 . A similar demonstration was reported by Liu et al. with an unsubstituted sulfoviologen molecule, (1-[3-sulfopropyl]-1' -4-sulfobutane]-4,4' -bipyridinium (3,4-S2 V) [104]. The redox potential of 3,4-S2 V can reach −0.41 V, and the electron transfer rate was greater than 0.35 cm s−1 . The 3,4-S2 V-based AORFB was demonstrated with a discharge capacity of 23.2 Ah L−1 for 1700 cycles or 100 days without observing chemical degradation. Furthermore, a 3,4-S2 V/(NH4 )4 [Fe(CN)6 ] AORFB with a discharge capacity of 259.9 mAh was demonstrated for 50 days of authentic energy storage for the first time with a temporal C rr of 99.955%/day. Recently, Jin et al. showed an extended Vi molecule, [PyrPV]Cl4 , which has a high water solubility of up to 1.13 M and the electron transfer rate reaches ~ 2.27 × 10–6 cm2 s−1 [89]. The OCV of [PyrPV]Cl4 /Pyr-TEMPO battery can reach 1.61 V, and the discharge capacity was 4.36 Ah L−1 . At a current density of 40 mA cm−2 , the

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CE of the battery was maintained at about 100%, while the EE could reach ~ 85%, and the C rr was 99.96/cycle of 95.44/day. Similarly, Wang et al. reported a pair of anionic organics of (PPBPy)Br2 and PSS-TEMPO in an AORFB [90]. The battery has more than 600 cycles of good cycle stability with an average CE of > 99.8% and EE of ~ 80%. Feng et al. demonstrated a robust anolyte species, dextrosil-Vi (Dex-Vi), to meet the challenges of stable and inexpensive Vi for practical applications in neutral aqueous media [105]. The solubility of Dex-Vi was 2.0 M, the first- and two-electron potentials were −0.322 and −0.699 V, and the first-electron rate constant was 8.9 × 10–2 cm s−1 . Remarkably, at a high concentration of 1.5 M (40.2 Ah L−1 theoretical anolyte volumetric capacity), Dex-Vi shows extremely stable cycling performance without observable capacity decay over one month of cycling. Moreover, they presented a high-yield hydrothermal synthetic approach for this Vi chloride salt with a low-cost precursor. In addition to changing the molecule structure, the host–guest concept can also be availed to protect Vi radicals. Liu et al. constructed a host–guest complex by introducing hydroxyethyl-β-CD (HE-β-CD) into 1-methyl1' -[3-(trimethylamino) propyl]-4,4' -bispyridine trichloride ((MAPVi)Cl3 ), which greatly improved the ability of MAPVi+ to resist proton attack [106]. The 0.30 M (MAPVi)Cl3 + HE-β-CD-based battery delivered a high capacity of 14.1 Ah L−1 with excellent stability over 100 cycles at 40 mA cm−2 . To reduce the sharp decrease of two separate electron transfer steps, Yang et al. proposed a strategy to fulfill the concurrent 2e− electrochemical reaction by designing extended bipyridinium derivatives (exBPs) with a reduced energy difference between the lowest unoccupied molecular orbital of exBPs and the β-highest occupied molecular orbital of the singly reduced form [107]. A series of exBPs were synthesized and exhibited a single peak at redox potentials of −0.75 to − 0.91 V. By prohibiting the dimerization/β-elimination side reactions. They acquired a 0.5 M (1 M e− ) exDMeBP/FcNCl battery with a high capacity of 22.35 Ah L−1 and a C rr of 99.95%/cycle. Subsequently, Liang et al. developed two resultants 1,1' -bis[3-(trimethylamonium)propyl]-4,4' -(2,5-thiophenediyl)bispyridinium tetrachloride ((ATBPy)Cl-4) and 1,1' -bis[3-(trimethylamonium)propyl]-4,4' -(2,5furandiyl)bispyridinium tetrachloride ((AFBPy)Cl-4), by inserting an electron-rich pi-bridge unit (viz. thiophene, furan) between the two pyridiniums [108]. The voltage of the AORFB system constructed was as high as 1.51 V, the volume capacity was 23.6 Ah L−1 , the EE was 77.5% at 100 mA·cm−2 , and the Pd was 302 mW cm−2 . Coincidentally, He et al. reported a series of chalcogenophene viologens([(NPr)2 FV]Cl4 , [(NPr)2 TV]Cl4 , and [(NPr)2 SeV]Cl4 ) as anolytes for neutral AORFBs via a combination of chalcogenophenes (furan, thiophene, and selenophene) and viologens [109]. The chalcogenophene viologens/FcNCl-based AORFBs endowed a higher theoretical battery voltage of 1.20 V and enhancement of stability for 1e− storage. In particular, the [(NPr)2 FV]Cl4 /FcNCl-based AORFB exhibited excellent long-cycle stability for 3000 cycles with 99.9994% C rr for 1e− storage and 300 cycles with 99.94% C rr for 2e− storage.

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Other Organics

As known, quinones were prone to side reactions in acidic solutions, such as acidcatalyzed disproportionation and nucleophilic addition [30]. It is anticipated that incorporating them in pH-neutral AORFBs will give much success. In 2018, Guo et al. successfully fabricated a high-performance biomolecule-based AORFB with lawsone anolyte and 4-HO-TEMPO catholyte [110]. The battery exhibited an OCV above 1.30 V and a C rr of 99.992%/cycle after 200 cycles at 15 mA cm−2 . Liu et al. then reported an AQ derivative, 9,10-anthraquinone-2,7-disulfonic acid diammonium salt (NH4 )2 AQDS, as an anolyte for pH-neutral AORFBs [111]. The solubility of (NH4 )2 AQDS in water was 1.9 M, more than three times that of the corresponding sodium salt. The paired AORFB has an OCV of ~ 0.87 V, a Pd of 91.5 mW cm−2 , and an E d of 12.5 Wh L−1 . At 60 mA cm−2 , the EE of the battery can reach 70.6%, and no apparent capacity loss was detected. After that, another strategy recommended by Aziz et al. was to graft trimethylene glycolic functional groups into AQ moiety for the formation of 1,8-bis(2-(2-(2-hydroxyethoxy)-ethoxy)ethoxy)anthracene9,10-dione (PEGAQ) [112]. The molecule was water-miscible, and the miscibility was independent of pH. The obtained battery delivered a relatively lower C rr of 0.5%/day over 220 cycles (18 days), reaching 95.7% of the theoretical capacity (80.4 Ah L−1 ). Subsequently, to enhance molecular stability, they established a phosphate terminated derivative, (((9,10-dioxo-9,10-dihydroanthracene2,6-diyl)bis(oxy))bis(propane-3,1-diyl))bis(phosphonic acid) (2,6-DPPEAQ) [113]. This product exhibited a significantly higher water solubility of 0.75 M at pH 9. The battery stability experiments with ferri/ferrocyanide catholyte demonstrated a record-low capacity fade rate of 0.014%/day. In 2020, Khataee et al. reported a daedal AORFB system based on anthraquinone-2,7-disulfonic acid disodium salt (Na2 AQDS) with pH = 8 and acidic bromine with pH = 2 [114]. The OCV of the battery can reach 1.3 V, and the capacity loss was about 0.05%/cycle at 40 mA cm−2 after 200 cycles. The corresponding CE and EE were about 95% and 69%. In addition, new organic redox-active couples with good properties are still desired for high-performance AORFBs. For instance, Medabalmi et al. reported a water-soluble N,N' -bis(glycinyl)naphthalenediimide (BNDI) derivatives M2 -BNDI (M=Na, K) containing carboxylic acid groups for neutral AORFBs [115]. The K2 BNDI displayed two stepwise single-electron transfers with the potential values of −0.20 and −0.47 V. An AORFB showed a peak Pd of 37 mW cm−2 , a C rr of 99.958%/day, and a EE of 86%. This year, Turner et al. investigated a negatively substituted trimethylenecyclopropane dianions, a subclass of hexasubstituted [3] radialenes as catholyte in a neutral system [116]. An AORFB provided an OCV of 0.9 V and a C rr of 99.954%/cycle over 50 cycles with a CE of 99.609%. Subsequently, Wiberg et al. reported two different naphthalene diimides (NDI) as anolyte for neutral AORFB [117]. The two molecules, one core-unsubstituted NDI (2H-NDI) and one coredimethylamino substituted NDI (2DMA-NDI), were coupled with a solubilized BTMAP-Fc at 50 mM in phosphate-buffered potassium chloride. 2H-NDI-based battery showed high EE and capacity utilization but a capacity loss of 31% in 197 cycles.

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Forming intermolecular H-bonds is an anomalous way to increase the solubility of redox-active organics. Tan et al. reported 3-((9,10-dioxo-9,10-dihydroanthracen1-yl)amino)-N,N,N-trimethylpropan-1-aminium chloride (1-DPAQCl) has high solubility at 1.44 M in water, higher than < 0.05 M of 3-((9,10-dioxo-9.10dihydroanthracen-2-yl)amino)-N,N,N-trimethylpropan-1-aminium chloride (2DPAQCl) functioned with hydrophilic groups [118]. Electrochemical investigations indicated that the peak-to-peak separation of 1-DPAQCl was very narrow in both unbuffered and buffered aqueous solutions, implying its highly electrochemical reversibility and facile reaction kinetics. The 1-DPAQCl/FcNCl system has an OCV of 1.11 V, a capacity of 4.4 Ah L−1 , a Pd of 134 mW cm−2 , and a CE of 95.6%. To satisfy the chemical stability of organic in practical implementation, Zhao et al. designed the armed-shaped quaternary ammonium groups for AQ derivative (QAAQ) and used them as anolyte for AORFBs without inert-gas protection and interference-ion elimination [119]. With the help of the synergy effect from steric hindrance and electron delocalization regulation, the paired battery displayed a stable cyclability for 500 cycles under a non-demanding atmosphere and good tolerance with the co-existence of interfering ions. Besides, phenazines are a class of N-containing heteroaromatic redox-active organics that can be used in neutral systems [13, 15, 30]. Ji et al. showed a series of amino acid-functionalized phenazines that can work in a pH-neutral environment [120]. The selected derivative, phenazine-1,6-diylbis(azanediyl))dipropionic acid (1,6-DPAP), can survive in a tautomerization process that deactivate other isomers. The 1,6-DPAP anolyte displayed a meager C rr of 99.5%/year in the 0.5 M AORFB test with ferrocyanide catholyte. Another water-soluble active material, sulfonated tryptanthrin (TRYP-8SO3 H), presented by Pinheiro et al., can work at a neutral system [121]. Single battery tests showed a high OCV of 0.94 V and reproducible charge–discharge stability at least 50 cycles. To break the limitation of one or two-electron storage, Liang et al. introduced redox-active pyridiniums on 1,3,5triazine to extend the conjugation and form molecules with multi-electrons storage [122]. The product, 2,4,6-tris[1-(trimethylamonium)-propyl-4-pyridiniumyl]-1,3,5triazine hexachloride ((TPyTz)Cl6 ), can transfer six electrons in theory (four in practice due to partial irreversibility). Galvanostatic cycling conducted at 0.3 M and 80 mA cm−2 displayed a slight capacity loss over ~ 3.46 days, but with the capacity utilization of only 62%. To further achieve low potential in the neutral condition for quinones, Tan et al. reported three low-potential quinone-based organics (1,4-BDPAQCl2 , 1,5BDPAQCl2 , and 1,8-BDPAQCl2 ) by bis-dimethylamino substitution. The redox potential of the quinones in 0.5 M KCl was approximately −0.55 to −0.57 V. Paired with Fe(glycine)2 Cl2 , the theoretical OCV of AORFBs was achieved at 1.27–1.29 V. Result of full battery at 0.4 M showed that 1,8-BDPAQCl2 displayed stability with the C rr of 99.952%/cycle or 99.12%/day during 300 charge–discharge cycles.

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3.3 Alkaline AORFBs 3.3.1

Quinone- and Ketone-Based Organics

In the alkaline aqueous system, the electroactive species involved in redox reactions are generally neutral molecules or their anions (Fig. 13), which are well blocked by the CEMs. In 2015, Aziz et al. first reported an available alkaline quinone-based AORFB with potassium ferrocyanide as catholyte and 2,6-dihydroxyanthraquinone (2,6-DHAQ) dipotassium salt as anolyte [123]. The hydroxyl groups on the AQ skeleton are highly deprotonated in alkaline solutions endowed 2,6-DHAQ with a high solubility of > 0.6 M in 1 M KOH. Furthermore, the redox potential of 2,6DHAQ in 1 M KOH can reach −0.7 V because of the substantial electron donation of -OH groups, significantly increasing the electron cloud density on the π-conjugated aromatic ring system. The 2,6-DHAQ-based battery produced an OCV of ∼1.20 V, which can be cycled 100 times at 100 mA cm−2 with an average CE of > 99%, an average EE of 84%, and a C rr of 99.9%/cycle or 92.6%/day. Lately, they showed a water solution of 2,5-dihydroxy-1,4-benzoquinone (DHBQ), rapidly receiving electrons with inexpensive carbon electrodes without any metal electrocatalyst [124]. The DHBQ-based battery delivered an OCV of 1.21 V, a peak galvanic Pd of 300 mW cm−2 , and a CE exceeding 99%. Continuous cycling at 100 mA cm−2 showed a C rr of 99.76%/cycle over 150 cycles. To further improve the performance such as voltage, rate capability, energy density, and cycling stability for quinone-based materials, Kwabi et al. functionalized 2,6-DHAQ with carboxylate terminal groups to obtain a highly alkali-soluble anodeactive substance 4,4' -((9,10-anthraquinone-2,6-diyl)dioxy) dibutyrate (2,6-DBEAQ) [125]. The formed AORFB system has an OCV of 1.12 V, showing a capacity decay rate of only 0.001%/cycle or 0.05%/day. To reveal the substituent position of hydroxyl groups on the redox potential, Cao et al. showed five isomeric DHAQs in alkaline AORFBs [126]. The molecule 1,8-dihydroxyanthraquinone (1,8-DHAQ) presented the highest redox reversibility and most rapid mass diffusion among them. The assembled battery yielded an OCV approaching 1.1 V and CE and C rr exceeding 99.3% and 99.88%/cycle, respectively. Additionally, Jin reported a carboxylated naphthoquinone derivative 2-hydroxy3-carboxyl-1,4-naphthoquinone (2,3-HCNQ) with a relatively negative redox potential of −0.526 V in an alkaline system [127]. The battery with 0.5 M 2,3-HCNQ anolyte and 0.4 M K4 Fe(CN)6 has an OCV of about 1.02 V. In 100 charge–discharge cycles, the CE of the system was about 100%, the EE was about 68.8%, and the C rr was ~ 96.6%/day at 100 mA cm−2 . Liu et al. then presented a hydrophilic functionalized alizarin derivative, alizarin-3-methyliminodiacetic acid (AMA), as the anolyte for alkaline AORFBs [128]. The AMA showed a highly reversible 2e− -transfer reaction, and the D and k 0 were 6.68 × 10–6 cm2 s−1 and 8.37 × 10–3 cm s−1 , respectively. This alkaline AORFB displayed an OCV of 1.38 V, a stable cycle performance at 100 mA cm−2 over 350 cycles with an average CE of 99.6%, an EE of 84.2%, and a C rr of 99.98%/cycle. Meanwhile, Tong et al.

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Fig. 13 Time-dependent molecular structures of quinone- and ketone-derivatives for the alkaline aqueous system

introduced a naphthoquinone dimer, bislawsone, as a negative anolyte for AORFB [129]. The battery has an OCV of 1.05 V and can be cycled at 300 mA cm−2 in an alkaline solution with 2 M electrons. In addition, other quinone-based organic molecules such as 2,5-dihydroxy-3,6-dimethyl-1,4-benzoquinone (DMBQ) and 2,5dimethoxy-3,6-dihydroxy-1,4-benzoquinone (DMOBQ) [130], were also reported as highly redox-active anolytes for the alkaline AORFBs. Recently, Aziz et al. designed two stable AQ anolytes for AORFBs, namely 3,3' (9,10-anthraquinone-diyl)bis(3-methylbutanoic acid) (DPivOHAQ) and 4,4' -(9,10anthraquinone)-diyl) dibutyric acid (DBAQ), using a new synthesis strategy [131]. The DPivOHAQ and DBAQ can achieve electron transfer up to 1.4 and 2 M, with capacity decay rates of 0.014% and 0.0084%/day, respectively. They were closely

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followed by the report of a series of AQs with water-solubilizing groups serving as redox-active anolytes in alkaline AORFBs [132]. The optimal battery demonstrated a temporal fade rate as low as 0.0128%/day when paired with a potassium ferrocyanide catholyte. To explore new suitable redox molecules for the alkaline system, Feng et al. proposed a fluorenone-based AORFB with 1.36 M 4-carboxylate-7-sulfofluorenone (4C7SFL) as anolyte and excessive ferricyanide as the catholyte [40]. The AORFB exhibited an EE of 92.1% and a C rr of 99.99761%/cycle at 20 mA cm−2 . In 2022, Wang et al. conducted a redox-reversible molecule based on the AQ motif, namely 1,3,5,7-tetrahydroxyanthraquinone (1,3,5,7-THAQ), and introduced an environmentally friendly microwave synthesis method [133]. The product has a high solubility of 1.88 M and a low redox potential of −0.68 V at pH = 14. The constructed AORFB showed an OCV of about 1.2 V, a peak output Pd of 0.36 W cm−2 , and a high C rr of ~ 99.65%/day after 1100 cycles at 100 mA cm−2 .

3.3.2

Other Organics

It has been observed that some π-conjugated heteroaromatic molecules such as riboflavin (Vitamin B2), riboflavin-5' -phosphate (FMN), and lumichrome in Fig. 14 and Table 4 could undergo the reversible two-electron redox reaction via a semiquinone radical intermediate [15]. However, the relatively low solubility of these natural organics in water limits their application in aqueous RFBs. Meng et al. improved the aqueous solubility and accessible specific capacity by adding a hydrotropic agent, nicotinamide (NA), into the anolyte composed of the sodium salt of FMN and KOH [134]. An AORFB using FMN anolyte and ferrocyanide catholyte in a strong base showed stable cycling performance, with over 99% capacity retention over 100 cycles. Another feasible way is to modify the molecular structure to yield the desirable redox potential and solubility for AORFB applications. Lin et al. introduced an alkaline-soluble carboxylic acid group to the π-conjugated alloxazine core to increase the aqueous solubility by coupling 3,4-diaminobenzoic acid with alloxan to obtain an isomeric mixture of alloxazine 7-carboxylic acid and alloxazine 8-carboxylic acid (ACA) [135]. The solubility of ACA can reach ca. 2 M in 1 M KOH. Then, a newly assembled alkaline AORFB showed an OCV of ∼1.13 V, a CE of over 99.7%, and an EE of 63%. The corresponding C rr was 99.9875%/cycle at 100 mA cm−2 over 400 cycles, indicating its outstanding long-term stability. In addition, Hollas et al. indicated that phenazine derivatives bearing carboxylate or sulfonate groups along with hydroxyl groups (e.g., 7,8-dihydroxyphenazine-2carboxylic acid (DHPC) and 7,8-dihydroxyphenazine-2-sulfonic acid (DHPS)) could be acquired by rational functionalization [136]. Both DHPC and DHPS exhibited high solubilities and highly negative redox potentials in alkaline aqueous solutions. The DHPS/K4 Fe(CN)6 AORFB provided a theoretical OCV of 1.4 V, which not only can achieve up to 94% material utilization and 86% voltage efficiency at 100 mA cm−2 but also exhibit excellent cycling stability with an average CE of 99.89% and C rr

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Fig. 14 Time-dependent molecular structures of other organic materials for alkaline aqueous system

of 99.07%/day after 1500 cycles at 50 mA cm−2 . Furthermore, this alkaline system can be operated at a near-saturation concentration of 1.4 M, equivalent to 2.8 M electron transfer, which produced a reversible capacity of 67 Ah L−1 with a material utilization of 90% and C rr of ∼99.98%/cycle over 500 cycles at 100 mA cm−2 . Similarly, Jin et al. introduced a phenyl group adjacent to the hydroxyl group on the π-conjugated phenazine core and a carboxyl group through rational molecular design to achieve a highly water-soluble and redox-active phenazine derivative, benzo[a]hydroxyphenazine-7/8-carboxylic acid (BHPC), for alkaline AORFB applications [137]. The solubility of BHPC reached 1.55 M in 1 M KOH while its redox potential was about −0.78 V. The evaluated BHPC/K4 Fe(CN)6 battery demonstrated a high OCV of 1.35 V with a C rr of about 99.9985%/cycle or 99.92%/day and CE of 99.2% over 1300 cycles at 100 mA cm−2 . The battery at a higher concentration

0.13–1.1 NA

2.12

16.9

2.07

NA

8.37

NA

1.92

2.5

NA

5.3 ± 0.5

NA

NA

1.55

− 0.574

− 0.52

− 0.515

− 0.67

− 0.551

− 0.73

− 0.48

− 0.362~ − 0.499 1.4–5.5

NA

− 0.72

− 0.68

− 0.86

− 0.517

− 0.62

− 0.79

− 0.78

1,8-DHAQ

2,3-HCNQ

2,6-DBEAQ

AMA

Bislawsone

DMBQ

DPivOHAQ

AQs

1,3,5,7-THAQ

DHPS

FMN

ACA

PPyQX

BHPC 3.52

NA

NA

1.3 ± 0.1

NA

2.4

4.75

NA

6.63

1.58

3.44

8.43

3.66

1.97

9.03

− 0.705

2,6-DHAQ

DHBQ

D (×10−6 cm2 s−1 )

k0 (×10−3 cm s−1 )

E (V vs NHE)

Abbr

1.55

NA

NA

~ 1.5

1.8

1.88 M at pH 14

0.65–1.1

0.74 M at pH 12

NA

0.56 M at pH 14

0.4

1.1 M at pH 14

1.2

3

4.31

0.6

S max (M)

NA

NA

NA

NA

17

NA

NA

NA

NA

Ed (Wh L−1 )

1.27

1.15

1.2

1.40

1.4

1.2

9.5

NA

NA

4.83

NA

13.3

~ 0.98 NA

1.0

~ 1.6

1.05

1.38

1.05

1.02

1.09

1.21

1.20

OCV (V)

(continued)

99.986%/cycle

99.975%/cycle

2700 W g−1 430

99.987%/cycle

> 99%/cycle

99.98%/cycle

99.9956%/cycle

99.9872%/cycle

99.9982%/day

99.81–99.88%/cycle

NA

99.98%/cycle

99.99%/day

99.98%/cycle

99.88%/cycle

99.9999%/cycle

95%/day

C rr (%/cycle or day)

350

160

NA

360

180

340

182.6

NA

490

240

255

152

300

NA

Pd (mW cm−2 )

Table 4 Summary of molecular physicochemical properties for redox-active organics and their applications in alkaline AORFBs

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k0 (×10−3 cm s−1 )

0.485, NA

0.192

0.49

NA

E (V vs NHE)

− 0.61, − 0.11

− 0.588

− 0.79

− 0.47

AADA

1,8-PFP

6-QCA

TBPDO

Abbr

Table 4 (continued)

NA

3.67

2.55

3.21, NA

D (×10−6 cm2 s−1 )

NA

5.5

1.5

2

S max (M)

NA

1.28

1.15

1.06

OCV (V)

NA

NA

NA

NA

Ed (Wh L−1 )

NA

199

94

NA

Pd (mW cm−2 )

NA

99.9%/cycle

99.9998%/cycle

99.95%/cycle

C rr (%/cycle or day)

138 H. Fan et al.

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of 1 M (~2 M electron transfer) can deliver a reversible anolyte capacity of ∼49 Ah L−1 and E d of ∼9.5 Wh L−1 . To develop efficient systems, Xu et al. synthesized a novel quinoxaline-bearing redox-active conjugated polymer (poly(6-(1H-pyrrol-1-yl)quinoxaline), PPyQX) via a facile bromine oxidation polymerization approach and employed as anolyte aqueous RFBs [138]. The PPyQX displayed a quasi-reversible redox reaction at −0.79 V with good stability in aqueous alkaline electrolytes. The fabricated battery revealed an OCV of 1.15 V and an average C rr of ~ 99.975%/cycle with a CE of 93.8% and EE of around 80.5%. Yu et al. first utilized an azobenzene-based compound containing hydrophilic groups, 4-amino-1,1' -azobenzene-3,4' -disulfonic acid monosodium salt (AADA), as the anolyte of the alkaline AORFBs in which an azo group (–N N–) was reported to act as a stable redox-active center [139]. The demonstrated AADA-based battery exhibited long cycling stability with a C rr of 99.95%/cycle over 500 cycles. The corresponding anolyte capacity can reach 41.5 Ah L−1 with an EE of about 88%. To explore the robust organic molecules with thermal stability for energy storage, especially in sunlight-rich areas with elevated temperature, Wang et al. demonstrated an extremely stable AORFB with high capacity based on propionic-acidfunctionalized phenazine (PFP) at both room and elevated temperatures [140]. The full battery based on 1,8-PFP with 2.0 M electron concentration in 1.0 M KOH against ferrocyanide presented an OCV of 1.15 V, and no capacity fade was observed when cycled at 45 °C for 53 days. Wang et al. recently reported a scalable and convenient synthesis of 6-quinoxalinecarboxylic acid (QCA) anolyte with high solubility of 5.5 M and low redox potential of −0.79 V [141]. The coupled QCA/K4 Fe(CN)6 AORFB showed an OCV of 1.28 V, Pd of 199 mW cm−2 , and C rr of 99.9%/ cycle. Very recently, Cao et al. adopted a highly efficient single-molecule redoxtargeting (SMRT) reaction between a solid organic-based energy storage material (tribenzo[a,c,i]phenazine-10,15-dione, TBPDO) and two water-soluble redox shuttle molecules (anthraflavic acid and lawsone) driven by Nernstian potential [142]. The aqueous battery employing TBPDO as an anodic capacity booster demonstrated a considerably enhanced volumetric capacity (E d ), high material utilization of 80.2%, and an outstanding C rr of 99.82%/cycle or ∼98.56%/day with high EE of 80.7% during long-term charge–discharge cycling.

4 Conclusions This chapter summarizes recent developments in redox-active organics and their applications in acidic, neutral, and alkaline AORFBs. Currently, AORFBs hold great promise for practical and scalable energy storage techniques in terms of overall performance and system cost. As discussed above, acidic and alkaline systems have delivered better electrochemical performance, energy efficiencies, and power densities than the neutral ones; however, the redox-active organics in the acidic and alkaline AORFBs are easily subject to long-term stability problems. Therefore, significant

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efforts should be well made to address these issues before the commercialization of AORFBs. (1) Continuous development of novel redox-active couples and organics. Most reported organics were limited by the redox-active structural units such as quinone, phenazine, thiazine, viologen, TEMPO-like nitroxyl radicals, imide, and ferrocene. Some of them, for example, quinone, are prone to self-dimerization or tautomerization, while viologen, TEMPO, and ferrocene are sensitive to the pH conditions during the charge–discharge cycle. As a result, both anolyte and catholyte are mismatched and scarce for the flow battery application. Functionalization with carefully selected motifs or groups through molecular engineering is expected to create high-concentration chemical-stable organics with high redox potential and appropriate physicochemical properties. In addition, oxygen-resistant organics are beneficial for reducing the cost and complexity of battery operation. Besides, highthroughput DFT computation is constructive for molecular designs. Ideally, the ideal redox-active organics should be highly conductive, less viscous, non-corrosive, nonflammable, low toxicity, and inexpensive. (B) In-depth understanding of battery capacity degradation mechanism. Operating parameters, such as flow rate, pH, viscosity, temperature, the degree of states of charge, and battery structure design, should be optimized since they are essential for enhancing battery efficiency and comprehending how AORFB systems function. Conversely, the crossover issue of organics flowing through or being absorbed by the membrane is a crucial hurdle that must be solved for all AORFBs. The capacity retention of AORFB is often limited by the performances of membranes, which not only have to effectively inhibit the crossover of the redox species in the anolyte and catholyte but also need to possess sufficient ionic conductivity to facilitate the transport of counterions necessary to maintain charge balance along with good chemical and mechanical stability. Intrinsically, understanding the decomposition of redox-active organics and their electrolytes is required. As discussed above, it will provide helpful information to guide further optimization of flow battery systems and feedback for improved molecular designs. Half-cell RFB tests are a direct and efficient method to evaluate the stability of a single redox molecule because of the enlarged capacity fading. The post-cycling analysis involving all kinds of semi/in-situ characterizations can be used to detect capacity fading mechanisms. In addition, DFT computation is also a powerful tool for exploring the physicochemical information on redox-active organics.

References 1. Sawin JL, Sverrisson F, Leidreiter A (2016) https://www.worldfuturecouncil.org/renewableenergy-sustainable-development/ 2. The Oil and Gas Industry in Energy Transitions, International Energy Agency (2020) https:// www.iea.org/reports/the-oil-andgas-industry-in-energy-transitions

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3. Renewable Energy, U.S. Department of Energy (2022) https://www.energy.gov/eere/renewa ble-energy 4. Rahman A, Farrok O, Haque MM (2022) Renew Sustain Energy Rev 161:112279 5. Dunn B, Kamath H, Tarascon JM (2011) Science 334:928–935 6. Wang W, Luo Q, Li B, Wei X, Li L, Yang Z (2013) Adv Func Mater 23:970–986 7. Zhang L, Hu X, Wang Z, Ruan J, Ma C, Song Z, Dorrell DG, Pecht MG (2021) Renew Sustain Energy Rev 139:110581 8. Kebede AA, Kalogiannis T, Van Mierlo J, Berecibar M (2022) Renew Sustain Energy Rev 159:112213 9. Liu W, Lu W, Zhang H, Li X (2019) Chem Eur J 25:1649–1664 10. Thaller LH (1976) US-PATENT-3,996,064|NASA-CASE-LEW-12220-1[P], Work of the US Gov., 1976-12-7. https://ntrs.nasa.gov/citations/19770007638 11. Park M, Ryu J, Wang W, Cho J (2016) Nat Rev Mater 2:16080 12. Kamat PV, Schanze KS, Buriak JM (2017) ACS Energy Lett 2:1368–1369 13. Luo J, Hu B, Hu M, Zhao Y, Liu TL (2019) ACS Energy Lett 4:2220–2240 14. Pan F, Wang Q (2015) Molecules 20:20499–20517 15. Cao J, Tian J, Xu J, Wang Y (2020) Energy Fuels 34:13384–13411 16. Viswanathan V, Crawford A, Stephenson D, Kim S, Wang W, Li B, Coffey G, Thomsen E, Graff G, Balducci P, Kintner-Meyer M, Sprenkle V (2014) J Power Sources 247:1040–1051 17. Darling RM, Gallagher KG, Kowalski JA, Ha S, Brushett FR (2014) Energy Environ Sci 7:3459–3477 18. Soloveichik GL (2015) Chem Rev 115:11533–11558 19. Kwon G, Ko Y, Kim Y, Kim K, Kang K (2021) Acc Chem Res 54:4423–4433 20. Zhang LY, Feng RZ, Wang W, Yip GH (2022) Nat Rev Chem 6:524–543 21. Singh V, Kim S, Kang J, Byon HR (2019) Nano Res 12:1988–2001 22. Sánchez-Díez E, Ventosa E, Guarnieri M, Trovò A, Flox C, Marcilla R, Soavi F, Mazur P, Aranzabe E, Ferret R (2021) J Power Sources 481:228804 23. Xiong P, Zhang L, Chen Y, Peng S, Yu G (2021) Angew Chem Int Ed 60:24770–24798 24. Yao Y, Lei J, Shi Y, Ai F, Lu Y-C (2021) Nat Energy 6:582–588 25. Lu W, Li X, Zhang H (2018) Phys Chem Chem Phys 20:23–35 26. Li Z, Lu YC (2020) Adv Mater 32:e2002132 27. Lin D, Li Y (2022) Adv Mater e2108856 28. Yuan Z, Yin Y, Xie C, Zhang H, Yao Y, Li X (2019) Adv Mater e1902025 29. Kwabi DG, Ji Y, Aziz MJ (2020) Chem Rev 120:6467–6489 30. Chen Q, Lv Y, Yuan Z, Li X, Yu G, Yang Z, Xu T (2021) Adv Func Mater 32:2108777 31. Xu Y, Wen Y-H, Cheng J, Cao G-P, Yang Y-S (2010) Electrochim Acta 55:715–720 32. Hoober-Burkhardt L, Krishnamoorthy S, Yang B, Murali A, Nirmalchandar A, Prakash GKS, Narayanan SR (2017) J Electrochem Soc 164:A600–A607 33. Liu W, Zhao Z, Li T, Li S, Zhang H, Li X (2021) Sci Bull 66:457–463 34. Liu T, Wei X, Nie Z, Sprenkle V, Wang W (2015) Adv Energy Mater 6:1501449 35. DeBruler C, Hu B, Moss J, Liu X, Luo J, Sun Y, Liu TL (2017) Chem 3:961–978 36. Son EJ, Kim JH, Kim K, Park CB (2016) J Mater Chem A 4:11179–11202 37. Wei X, Pan W, Duan W, Hollas A, Yang Z, Li B, Nie Z, Liu J, Reed D, Wang W, Sprenkle V (2017) ACS Energy Lett 2:2187–2204 38. Nutting JE, Rafiee M, Stahl SS (2018) Chem Rev 118:4834–4885 39. Nolte O, Rohland P, Ueberschaar N, Hager MD, Schubert US (2022) J Power Sources 525:230996 40. Feng R, Zhang X, Murugesan V, Hollas A, Chen Y, Shao Y, Walter E, Wellala NPN, Yan L, Rosso KM, Wang W (2021) Science 372:836 41. Rodriguez J, Niemet C, Pozzo LD (2019) ECS Trans 89:49–59 42. Luo J, Sam A, Hu B, DeBruler C, Wei X, Wang W, Liu TL (2017) Nano Energy 42:215–221 43. Li X, Gao PY, Lai YY, Bazak JD, Hollas A, Lin HY, Murugesan V, Zhang SY, Cheng CF, Tung WY, Lai YT, Feng RZ, Wang J, Wang CL, Wang W, Zhu Y (2021) Nat Energy 6:873–881

142

H. Fan et al.

44. Huskinson B, Marshak MP, Suh C, Er S, Gerhardt MR, Galvin CJ, Chen X, Aspuru-Guzik A, Gordon RG, Aziz MJ (2014) Nature 505:195–198 45. Gerhardt MR, Tong L, Gómez-Bombarelli R, Chen Q, Marshak MP, Galvin CJ, Aspuru-Guzik A, Gordon RG, Aziz MJ (2016) Adv Energy Mater 7:1601488 46. Carretero-González J, Castillo-Martínez E, Armand M (2016) Energy Environ Sci 9:3521– 3530 47. Hofmann JD, Pfanschilling FL, Krawczyk N, Geigle P, Hong LC, Schmalisch S, Wegner HA, Mollenhauer D, Janek J, Schroder D (2018) Chem Mater 30:762–774 48. Yan W, Wang CX, Tian JQ, Zhu GY, Ma LB, Wang YR, Chen RP, Hu Y, Wang L, Chen T, Ma J, Jin Z (2019) Nat Commun 10:2513 49. Lantz AW, Shavalier SA, Schroeder W, Rasmussen PG (2019) ACS Appl Energy Mater 2:7893–7902 50. Park M, Beh ES, Fell EM, Jing Y, Kerr EF, Porcellinis D, Goulet MA, Ryu J, Wong AA, Gordon RG, Cho J, Aziz MJ (2019) Adv Energy Mater 9:1900694 51. Hofmann JD, Schmalisch S, Schwan S, Hong L, Wegner HA, Mollenhauer D, Janek J, Schröder D (2020) Chem Mater 32:3427–3438 52. Schlemmer W, Nothdurft P, Petzold A, Riess G, Fruhwirt P, Schmallegger M, GescheidtDemner G, Fischer R, Freunberger SA, Kern W, Spirk S (2020) Angew Chem Int Ed 59:22943– 22946 53. Hong J, Kim K (2018) Chemsuschem 11:1866–1872 54. Zhang C, Niu Z, Peng S, Ding Y, Zhang L, Guo X, Zhao Y, Yu G (2019) Adv Mater 31:e1901052 55. Mukhopadhyay A, Zhao H, Li B, Hamel J, Yang Y, Cao D, Natan A, Zhu H (2019) ACS Appl Energy Mater 2:7425–7437 56. Wiberg C, Owusu F, Wang E, Ahlberg E (2019) Energy Technol 7:1900843 57. Tian Y, Wu KH, Cao L, Saputera WH, Amal R, Wang DW (2019) Chem Commun 55:2154– 2157 58. Li H, Fan H, Ravivarma M, Hu B, Feng Y, Song J (2020) Chem Commun 56:13824–13827 59. Deller Z, Jones LA, Maniam S (2021) Green Chem 23:4955–4979 60. Hu B, DeBruler C, Rhodes Z, Liu TL (2017) J Am Chem Soc 139:1207–1214 61. Beh ES, De Porcellinis D, Gracia RL, Xia KT, Gordon RG, Aziz MJ (2017) ACS Energy Lett 2:639–644 62. Zhu Y, Yang F, Niu Z, Wu H, He Y, Zhu H, Ye J, Zhao Y, Zhang X (2019) J Power Sources 417:83–89 63. Luo J, Hu B, Debruler C, Bi Y, Zhao Y, Yuan B, Hu M, Wu W, Liu TL (2019) Joule 3:149–163 64. Yu J, Salla M, Zhang H, Ji Y, Zhang F, Zhou M, Wang Q (2020) Energy Storage Mater 29:216–222 65. Borchers PS, Strumpf M, Friebe C, Nischang I, Hager MD, Elbert J, Schubert US (2020) Adv Energy Mater 10:2001825 66. Chen Q, Li Y, Liu Y, Sun P, Yang Z, Xu T (2021) Chemsuschem 14:1295–1301 67. Li Y, Xu Z, Liu Y, Jin S, Fell EM, Wang B, Gordon RG, Aziz MJ, Yang Z, Xu T (2021) Chemsuschem 14:745–752 68. Zhang B, Schrage BR, Frkonja-Kuczin A, Gaire S, Popov IA, Ziegler CJ, Boika A (2022) Inorg Chem 61:8117–8120 69. Luo J, Hu M, Wu W, Yuan B, Liu TL (2022) Energy Environ Sci 15:1315–1324 70. Luo J, Hu B, Hu M, Wu W, Liu TL (2022) Angew Chem Int Ed 61:e202204030 71. Janoschka T, Martin N, Hager MD, Schubert US (2016) Angew Chem Int Ed 55:14427–14430 72. Winsberg J, Janoschka T, Morgenstern S, Hagemann T, Muench S, Hauffman G, Gohy J-F, Hager MD, Schubert US (2016) Adv Mater 28:2238–2243 73. Winsberg J, Stolze C, Schwenke A, Muench S, Hager MD, Schubert US (2017) ACS Energy Lett 2:411–416 74. Hagemann T, Strumpf M, Schröter E, Stolze C, Grube M, Nischang I, Hager MD, Schubert US (2019) Chem Mater 31:7987–7999

Aqueous Organic Redox Flow Batteries

143

75. Winsberg J, Stolze C, Muench S, Liedl F, Hager MD, Schubert US (2016) ACS Energy Lett 1:976–980 76. Janoschka T, Friebe C, Hager MD, Martin N, Schubert US (2017) ChemistryOpen 6:216–220 77. Chang Z, Henkensmeier D, Chen R (2017) Chemsuschem 10:3193–3197 78. Chang Z, Henkensmeier D, Chen R (2019) J Power Sources 418:11–16 79. Liu YH, Goulet MA, Tong LC, Liu YZ, Ji YL, Wu L, Gordon RG, Aziz MJ, Yang ZJ, Xu TW (2019) Chem 5:1861–1870 80. Fan H, Zhang J, Ravivarma M, Li H, Hu B, Lei J, Feng Y, Xiong S, He C, Gong J, Gao T, Song J (2020) ACS Appl Mater Interfaces 12:43568–43575 81. Zhou W, Liu W, Qin M, Chen Z, Xu J, Cao J, Li J (2020) RSC Adv 10:21839–21844 82. Liu B, Tang CW, Jiang H, Jia G, Zhao T (2021) ACS Sustain Chem Eng 9:6258–6265 83. Hu B, Fan H, Li HB, Ravivarma M, Song JX (2021) Adv Func Mater 31:2102734 84. Hu SZ, Wang LW, Yuan XZ, Xiang ZP, Huang MB, Luo P, Liu YF, Fu ZY, Liang ZX (2021) Energy Mater Adv 2021:9795237 85. Nambafu GS, Siddharth K, Zhang C, Zhao T, Chen Q, Amine K, Shao M (2021) Nano Energy 89:106422 86. Hu B, Hu MW, Luo J, Liu TL (2022) Adv Energy Mater 12:2102577 87. Fan H, Hu B, Li H, Ravivarma M, Feng Y, Song J (2022) Angew Chem Int Ed 61:e202115908 88. Fan H, Wu W, Ravivarma M, Li H, Hu B, Lei J, Feng Y, Sun X, Song J, Liu TL (2022) Adv Func Mater 32:2203032 89. Pan MG, Gao LZ, Liang JC, Zhang PB, Lu SY, Lu Y, Ma J, Jin Z (2022) Adv Energy Mater 12:2103478 90. Gao M, Salla M, Song Y, Wang Q (2022) Angew Chem Int Ed 61:e202208223 91. DeBruler C, Hu B, Moss J, Luo J, Liu TL (2018) ACS Energy Lett 3:663–668 92. Luo J, Hu B, Debruler C, Liu TL (2018) Angew Chem Int Ed 57:231–235 93. Huang J, Yang Z, Murugesan V, Walter E, Hollas A, Pan B, Assary RS, Shkrob IA, Wei X, Zhang Z (2018) ACS Energy Lett 3:2533–2538 94. Liu Y, Li Y, Zuo P, Chen Q, Tang G, Sun P, Yang Z, Xu T (2020) Chemsuschem 13:2245–2249 95. Jin S, Fell EM, Vina-Lopez L, Jing Y, Michalak PW, Gordon RG, Aziz MJ (2020) Adv Energy Mater 10:2000100 96. Liu B, Tang CW, Jiang H, Jia G, Zhao T (2020) J Power Sources 477:228985 97. Hu S, Li T, Huang M, Huang J, Li W, Wang L, Chen Z, Fu Z, Li X, Liang Z (2021) Adv Mater 33:e2005839 98. Pan M, Lu Y, Lu S, Yu B, Wei J, Liu Y, Jin Z (2021) ACS Appl Mater Interfaces 13:44174– 44183 99. Wang H, Li D, Xu J, Wu Y, Cui Y, Chen L (2021) J Power Sources 492:229659 100. Wu W, Luo J, Wang F, Yuan B, Liu TL (2021) ACS Energy Lett 6:2891–2897 101. Liu L, Yao Y, Wang Z, Lu Y-C (2021) Nano Energy 84:105897 102. Li H, Fan H, Hu B, Hu L, Chang G, Song J (2021) Angew Chem Int Ed 60:26971–26977 103. Ambrose B, Naresh RP, Ulaganathan M, Ragupathy P, Kathiresan M (2022) Mater Lett 314:131876 104. Hu MM, Wu WD, Luo J, Liu TL (2022) Adv Energy Mater 2202085 105. Lv X-L, Sullivan P, Fu H-C, Hu X, Liu H, Jin S, Li W, Feng D (2022) ACS Energy Lett 7:2428–2434 106. Liu Y, Yuan X, Huang M, Xiang Z, Hu S, Fu Z, Guo X, Liang Z (2022) Ind Eng Chem Res 61:14508–14514 107. Tang G, Liu Y, Li Y, Peng K, Zuo P, Yang Z, Xu T (2022) JACS Au 2:1214–1222 108. Huang MB, Hu SZ, Yuan XZ, Huang JH, Li WJ, Xiang ZP, Fu ZY, Liang ZX (2022) Adv Funct Mater 2111744 109. Zhang X, Liu X, Zhang H, Wang Z, Zhang Y, Li G, Li MJ, He G (2022) ACS Appl Mater Interfaces 14:48727–48733 110. Hu P, Lan H, Wang X, Yang Y, Liu X, Wang H, Guo L (2018) Energy Storage Mater 19:62–68 111. Hu B, Luo J, Hu M, Yuan B, Liu TL (2019) Angew Chem Int Ed 58:16629–16636

144

H. Fan et al.

112. Jin S, Jing Y, Kwabi DG, Ji Y, Tong L, De Porcellinis D, Goulet M-A, Pollack DA, Gordon RG, Aziz MJ (2019) ACS Energy Lett 4:1342–1348 113. Ji Y, Goulet M-A, Pollack DA, Kwabi DG, Jin S, De Porcellinis D, Kerr EF, Gordon RG, Aziz MJ (2019) Adv Energy Mater 0:1900039 114. Khataee A, Azevedo J, Dias P, Ivanou D, Draževi´c E, Bentien A, Mendes A (2019) Nano Energy 62:832–843 115. Medabalmi V, Sundararajan M, Singh V, Baik MH, Byon HR (2020) J Mater Chem A 8:11218– 11223 116. Turner NA, Freeman MB, Pratt HD, Crockett AE, Jones DS, Anstey MR, Anderson TM, Bejger CM (2020) Chem Commun 56:2739–2742 117. Wiberg C, Evenas L, Busch M, Ahlberg E (2021) J Electroanal Chem 896:115224 118. Xia LX, Huo WB, Gao HZ, Zhang H, Chu FM, Liu H, Tan ZA (2021) J Power Sources 498:229896 119. Zhu YZ, Li YX, Qian YM, Zhang LW, Ye J, Zhang XH, Zhao Y (2021) J Power Sources 501:229984 120. Pang S, Wang X, Wang P, Ji Y (2021) Angew Chem Int Ed 60:5289–5298 121. Pinheiro D, Pineiro M, de Melo JSS (2021) Commun Chem 4:89 122. Huang J, Hu S, Yuan X, Xiang Z, Huang M, Wan K, Piao J, Fu Z, Liang Z (2021) Angew Chem Int Ed 60:20921–20925 123. Lin K, Chen Q, Gerhardt MR, Tong L, Kim SB, Eisenach L, Valle AW, Hardee D, Gordon RG, Aziz MJ, Marshak MP (2015) Science 349:1529–1532 124. Yang Z, Tong L, Tabor DP, Beh ES, Goulet M-A, Porcellinis D, Aspuru-Guzik A, Gordon RG, Aziz MJ (2017) Adv Energy Mater 8:1702056 125. Kwabi DG, Lin K, Ji Y, Kerr EF, Goulet M-A, De Porcellinis D, Tabor DP, Pollack DA, Aspuru-Guzik A, Gordon RG, Aziz MJ (2018) Joule 2:1894–1906 126. Cao J, Tao M, Chen H, Xu J, Chen Z (2018) J Power Sources 386:40–46 127. Wang CX, Yang Z, Wang YR, Zhao PY, Yan W, Zhu GY, Ma LB, Yu B, Wang L, Li GG, Liu J, Jin Z (2018) ACS Energy Lett 3:2404–2409 128. Liu YY, Lu SF, Chen S, Wang HN, Zhang J, Xiang Y (2019) ACS Appl Energy Mater 2:2469–2474 129. Tong L, Goulet M-A, Tabor DP, Kerr EF, De Porcellinis D, Fell EM, Aspuru-Guzik A, Gordon RG, Aziz MJ (2019) ACS Energy Lett 4:1880–1887 130. Sun P, Liu Y, Li Y, Shehzad MA, Liu Y, Zuo P, Chen Q, Yang Z, Xu T (2019) Ind Eng Chem Res 58:3994–3999 131. Wu M, Jing Y, Wong AA, Fell EM, Jin S, Tang Z, Gordon RG, Aziz MJ (2020) Chem 6:1432– 1442 132. Jing Y, Fell EM, Wu M, Jin S, Ji Y, Pollack DA, Tang Z, Ding D, Bahari M, Goulet M-A, Tsukamoto T, Gordon RG, Aziz MJ (2021) ACS Energy Lett 7:226–235 133. Wang C, Yang Z, Yu B, Wang H, Zhang K, Li G, Tie Z, Jin Z (2022) J Power Sources 524:231001 134. Orita A, Verde MG, Sakai M, Meng YS (2016) Nat Commun 7:13230 135. Lin KX, Gomez-Bombarelli R, Beh ES, Tong LC, Chen Q, Valle A, Aspuru-Guzik A, Aziz MJ, Gordon RG (2016) Nat Energy 1:16102 136. Hollas A, Wei X, Murugesan V, Nie Z, Li B, Reed D, Liu J, Sprenkle V, Wang W (2018) Nat Energy 3:508–514 137. Wang CX, Li X, Yu B, Wang YR, Yang Z, Wang HZ, Lin HN, Ma J, Li GG, Jin Z (2020) ACS Energy Lett 5:411–417 138. Wang B, Zhang Y, Zhu Y, Shen Y-M, Wang W, Chen Z, Cao J, Xu J (2020) J Power Sources 451:227788 139. Zu X, Zhang L, Qian Y, Zhang C, Yu G (2020) Angew Chem Int Ed Engl 59:22163–22170 140. Xu JC, Pang S, Wang XY, Wang P, Ji YL (2021) Joule 5:2437–2449

Aqueous Organic Redox Flow Batteries

145

141. Wang CX, Wang YR, Tao M, Yu B, Zhang KQ, Wei J, Liu YZ, Zhang PB, Ding GC, Tie ZX, Cao JY, Jin Z (2022) ACS Appl Energy Mater 5:10379–10384 142. Zhang H, Huang Q, Xia X, Shi Y, Shen Y-M, Xu J, Chen Z, Cao J (2022) J Mater Chem A 10:6740–6747

Electrodes for All-Vanadium Redox Flow Batteries Rui Wang and Yinshi Li

1 Introduction Flow battery is one of the most promising energy storage systems, due to their rapid response and excellent balanced capacity between demand and supply. Especially, the all-vanadium flow battery (VFB), that minimizes the adverse cross-contamination by cycling the same vanadium element for redox reactions in both negative and positive sides, exhibites long cycle and safety, suggesting large-scale application potential. In the VFB, the most crucial issues are unsatisfactory energy efficiency and operation current density, impeding its commercialization processes. The electrode, a key component for the mass transport and redox reaction in flow battery, directly determines the battery performance. Up to now, the most used materials for electrode are carbon or graphite felt (CF/GF), carbon paper (CP) and carbon cloth (CC), owing to its properties of good conductivity, excellent corrosion resistance and splendid mechanical stability. Nevertheless, these kinds of carbon-based electrodes, possessing smooth fiber with low surface area, C–C bonds with chemical inertness and high hydrophobicity, still suffer from the undesirable reaction activity and high transports resistance, lowering the efficiency and current of VFB. Developing highperformance enabling efficient redox reaction and low-resistance transport processes is in urgent needed for all-vanadium flow battery. Therefore, herein, based on deeply insight for mass transport and redox reaction processes, electrodes with various enhancing approaches for all-vanadium flow battery are summarized systematically, which can be classified into metal or metal R. Wang School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China Y. Li (B) Key Laboratory of Thermo-Fluid Science and Engineering of Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 L. An et al. (eds.), Flow Cells for Electrochemical Energy Systems, Green Energy and Technology, https://doi.org/10.1007/978-3-031-37271-1_6

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Fig. 1 Schematic diagram of various electrodes in all-vanadium flow battery

oxide materials modified electrodes and structure decorated or pore-etched electrodes shown in Fig. 1. The typical design thought, fabrication approach, advantages and recent advancement are reviewed in this chapter.

2 Metal/Metal Oxide Modified Electrode 2.1 Metal Modified Electrode Some kinds of metals, especially noble or transition metals, possess active outer electron configuration and excellent electrical conductivity, extensively adopting as electrocatalysts for vanadium redox reactions. Many works have been carried out to find a suitable electrocatalyst the vanadium redox reactions.

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149

Noble Metal Modified Electrode

Noble metals not only are active towards vanadium redox reactions but also are very inert and stable in highly acidic environment of all-vanadium RFBs. In earlier research, considering that the carbon-based material is easily to be degraded under oxidizing environment in positive side, some all-metal electrodes, such as iridium oxide and platinized titanium, were developed and proved good reversibility for both VO2+ /VO2 + and V2+ /V3+ reactions [1]. Adversely, the noble metal is expensive for all-vanadium flow batteries, limiting its large-scale application. Whereafter, the carbon-based electrode was confirmed stable in flow batteries via a suitable cut-off voltage in charge process, and various noble metals were thus used as electrochemical catalysts for electrode modification. Pt, Pd, Au, Mn, Te, In and Ir modified graphite electrodes were prepared by a wet chemical method for comparison [2]. The Ir modified electrode was observed the highest electrochemical activity towards vanadium redox reactions, decreasing 63% overpotential of flow battery [3]. Furthermore, an iridium decorated graphite was fabricated to prevent graphite aggregation, in which the iridium was used as a spacer [4]. Not only iridium, platinum, but also widely used catalysts in multifarious electrochemical devices have been found efficient for vanadium reactions. The activity of platinum toward VO2+ /VO2 + reaction was confirmed in early research [5]. A platinum/multiwalled carbon nanotubes (Pt/MWCNT) decorated graphite felt was developed and confirmed that it is suitable to VO2+ /VO2 + reaction rather than V2+ /V3+ reaction, owning to the hydrogen evolution reaction [6]. Subsequently, platinum particles with different loading value were mixed to carbon black to clarify the optimal platinum-to-carbon ratios [7]. It was found that the electrode with 15 wt% Pt/C exhibited the highest energy efficiency owning to the improved voltage efficiency. Latterly, a Pt/C is fabricated by a polyol process to compare the influence of different preparation method [8]. The Pt/ C prepared by polyol method exhibited a 34% smaller particle size than commercial Pt/C, so that improving 10% voltage efficiency in battery charge–discharge tests. Moreover, indium modified carbon paper electrode was developed and confirmed its catalytic capacity towards VO2+ /VO2 + reaction [9]. Although effectively, in view of the material cost, the noble metals are hardly adopted in the large-scale production of electrode modification in flow batteries.

2.1.2

Bismuth Modified Electrode

The transition metals, due to that their partially filled d orbital activate capacity for gain or loss of electrons, are reported efficient to vanadium redox reactions. Bismuth, a post-transition metal with nontoxic property, low corrosive and low price, has been widely used in various electrochemical devices. In previous works, the bismuth particles modified graphite felts was confirmed effective towards VO2+ / VO2 + reaction [10]. Furthermore, an interesting method that adding BiCl3 into electrolyte is proposed instead of the traditional decorating method [11]. The bismuth ions were reduced on the surface to generate nanoparticle in the charge process,

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while the generated bismuth elemental is formed ionic state in the discharge process. The role of bismuth in improving V2+ /V3+ reaction was further achieved by cyclic voltammetry testing bismuth modified graphite felt electrode [12]. This research pointed that the hydrogen ions were easily reaction on the bismuth surface to generate BiHx , preventing the adverse hydrogen evolution reaction. Moreover, bismuth was combined with other modified electrodes were studied and proved excellent performance for vanadium reactions [13– 16]. Recently, a controllable bismuth electrodeposition method was studied to clarify the effects of the electrodeposition current density, the catalyst loading and the initial Bi3+ concentration on bismuth distribution. Based on the optimal condition, the flow battery with bismuth modified electrode enabled a high voltage efficiency of 85.4% at 320 mA cm−2 [17].

2.1.3

Other Metal Modified Electrode

Not just bismuth, other nonnoble metals, including antimony, tin and cupper, were used in electrode modification to promote battery performance. Similar to the Bi modification method, SbCl3 was added into electrolyte to deposit nanoparticle on the surface of graphite felt during charging process [18]. SnCl2 and SnCl4 were added into negative and positive electrolyte, respectively, to enhance both VO2+ /VO2 + and V2+ /V3+ reactions in all-vanadium flow battery (Fig. 2a–c) [19]. Moreover, the copper nanoparticles could also deposit on to electrode surface by adding Cu2+ into electrolyte. It was found that the all-vanadium flow battery with copper ions possessed the energy efficiency of 80.1% at the current density of 300 mA cm−2 (Fig. 2d–f) [20]. These evenly distributed nanoparticles forming by the synchronously electrodepositing approach have been proved that it could enhance the electrochemical kinetics of vanadium redox reactions, contributing to improve battery performance.

2.2 Metal Oxide Modified Electrode The metal oxide, especially transition metal oxides, due to their active ion state for redox reaction and low cost, was deemed as promising candidates for electrode modification in all-vanadium flow batteries. Considering the low electronic conductivity of metal oxide, carbon-based materials, such as graphene and carbon nanotube, were combined with metal oxide to further enhance battery performance.

2.2.1

Manganese Oxide Modified Electrode

In previous work, with comparison of Mn, Ni and Co oxides, the Mn3 O4 was confirmed to possess excellent electrochemical activity towards vanadium redox reactions, in which columbic efficiency and voltage efficiency of flow battery were improved obviously [21]. To overcome the low electrical conductivity of metal

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Fig. 2 a Schematic of the electrocatalytic activity of tin. EIS spectra with the corresponding simulation fittings b Nyquist plots at 1.0 V in a solution of 1.5 M VOSO4 /3 M H2 SO4 with/without 0.01 M Sn2+ and c Nyquist plots at − 0.60 V in a solution of 1.5 M V3+ /3 M H2 SO4 with/without 0.01 M Sn2+ . d Schematic diagram of the electrodeposition process: Cu nanoparticles sourced from the electrolyte containing Cu ions decorating on the surface of graphite felt. e CV curves of the graphite electrode with and without 0.005 M Cu2+ at a scan rate of 50 mV s−1 . f Energy efficiencies employing electrolytes containing different Cu2+ concentrations as a function of cycle number at different current densities

oxide, the high-conductivity MWCNT was used to connect Mn3 O4 particles in the carbon electrode, further enhancing the activity towards VO2+ /VO2 + reactions in all-vanadium flow battery [22]. Similarly, Mn3 O4 particles were incorporated in to carbon nanofiber, exhibiting a 1% decrease with the current density from 40 mA cm−2 to 80 mA cm−2 in battery test [23]. Furthermore, the nitrogen doped graphene oxide was adopted to boost the catalytic capacity of Mn3 O4 [24]. It is found the activity of the developed N–GO–Mn3 O4 was stable after CV testing for 2000 cycles. A polydopamine (PDA)-Mn3 O4 catalyst was further fabricated by a more environmental method [25]. The flow battery assembled with PDA-Mn3 O4 modified electrode enabled an energy efficiency of 61.1% at the current density of 150 mA cm−2 , higher than that of pristine GF electrode (55.9%). As for the V2+ /V3+ reaction, the Mn3 O4 modified carbon cloth was developed and exhibited high activity [26]. The flow battery with Mn3 O4 –CC electrode exhibited an energy efficiency of 88% at 100 mA cm−2 and even up to 71.2% at a high current density of 400 mA cm−2 . Not only Mn3 O4 , the MnO2 , with advantages of low cost and environmentally friendly, has been used in all-vanadium flow battery [27]. The MnO2 modified GF electrode was fabricated by a low-pH hydrothermal method, exhibiting the energy efficiency of 77.5% at 150 mA cm−2 .

2.2.2

Tungsten Oxide Modified Electrode

The WO3 has been confirmed stable in acid media and active for some electrochemical reactions, suggesting promising application potential in flow batteries. In previous works, the WO3 was decorated into super activated carbon to enable the

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good electric conductivity and the large reaction area for vanadium flow battery [28]. The prepared electrode showed excellent electrochemical activity towards both VO2+ /VO2 + and V2+ /V3+ reactions. The flow battery with WO3 modified GF electrode was further proved stable at 70 mA cm−2 for 20 cycles [29]. Whereafter, niobium was doped into the h-WO3 nanowires to combine the advantages of transition metal oxides and metal [30]. In battery charge–discharge tests, the Nb-hWO3 electrode yielded the energy efficiency of 65.83% at 160 mA cm−2 . With a similar method, the WO3 nanowires were further decorated into the 3D graphene sheet foam to construct the W–O–C bonds for a better catalytic activity and electric conductivity [31]. Thus, the flow battery assembled with the developed electrode showed a higher energy efficiency of 67.6% at 160 mA cm−2 . Similarly, the WO3 -multiwalled carbon nanotubes were developed and modified into the graphite felt electrode, showing largely increased active area for VO2+ /VO2 + reaction [32]. Furthermore, both nitrogen and WO3 groups were used in electrode modification (HTNW electrode) [33]. In this work, the nitrogen doped surface could improve the hydrophilicity to prevented WO3 particles to agglomerate, exhibiting an enhanced battery performance (Fig. 3a and b). Recently, a W18 O48 was synthesized and evaluated for the VO2+ /VO2 + redox reactions in all-vanadium flow battery (Fig. 3c-e) [34]. The W18 O48 nanowires (W18 O49 NWs) possessed the crystalline structure with rich oxygen-containing defects, suggesting excellent catalytic activity. After treating in H2 and Ar gas, the H-W18 O48 nanowire modified electrode enabled the energy efficiency of 69% at 160 mA cm−2 .

Fig. 3 a Morphologies of HTNW modified carbon felt electrodes. b Comparison of the electrochemical performance for all as-prepared electrodes, showing the voltage profiles for charge and discharge process at 200 mA cm−2 . c Scheme of the proposed catalytic reaction mechanisms for the redox reaction toward VO2+ /VO2 + using W18 O49 NWs modified the gf surface and crystalline structure of WO3 and W18 O49 . d XPS narrow scan O 1 s of W18 O49 NW-GF. e Charge–discharge recovery test at 40 mA cm−2 rate after 160 mA cm−2 cycling for CGF and H–W18 O49 -GF samples

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Fig. 4 a Mechanism interpretation for the negative reaction of VRFBs using CF-based electrodes, b CF@TiO2 , and nitrided CF@TiO2 electrodes. c Cyclic voltammetry of the electrodes in 0.05 M V3+ and 1 M H2 SO4 solution. d Energy efficiencies for the CF@TiO2 electrodes

2.2.3

Titanium Oxide Modified Electrode

As a n-type semiconductor, the TiO2 is stable in both acidic and alkaline environments, so that has been widely used in electrochemical devices. In a previous work, a TiO2 -carbon black modified electrode was fabricated and tested in all-vanadium flow battery [35]. It was found that the adverse hydrogen evolution reaction was inhibited, and the V2+ /V3+ reaction was enhanced in the TiO2 -carbon black modified surface. The flow battery with TiO2 -carbon black modified electrode exhibited 39.57% higher energy efficiency than the pristine electrode. A similar method was used to build TiO2 modified electrode, in which the constructed flow battery enabled a high energy efficiency of 72.2% at the current density of 100 mA cm−2 [36]. Subsequently, hydrogen-treated rutile was grown on electrode surface, generating oxygen vacancies in the TiO2 lattice [34]. More than that, the nitrided GF modified with TiO2 (nitrided GF@TiO2 ) was prepared by treating TiO2 in NH3 atmosphere at a high temperature (Fig. 4) [37]. In the battery charge–discharge tests, the prepared electrode possessed a high energy efficiency of 71% at 150 mA cm−2 . The catalytic performances of different polymorphic forma of TiO2 toward the V2+ /V3+ reactions were also compared [38]. It concluded that the anatase possessed higher electrochemical activity based on the CV and EIS curves. The flow battery assembled with anatase modified GF electrode exhibited 7.5% higher energy efficiency than that of pristine GF electrode.

2.2.4

Other Single Metal Oxide Modified Electrode

Not merely manganese oxide, tungsten oxide and titanium oxide, various metal oxides, including Ta2 O5 [39], Cr2 O3 [40], RuO2 [41], Nd2 O3 [42], SnO2 [43], MoO2

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[44], NiO [45], Nb2 O5 [46], ZrO2 [47, 48], CeO2 [49, 50] and PdO2 [51], were proposed efficient towards vanadium redox reactions. For instance, the SnO2 modified CF electrode was developed by a hydrothermal method and exhibited obviously enlarged active area [43]. The flow battery with SnO2 -CF possessed an energy efficiency of 77.3% at the current density of 150 mA cm−2 . The Ta2 O5 modified GF electrode (Ta2 O5 -GF) was fabricated by a facial loading method [39]. It was found that the Ta=O and Ta–O in the Ta2 O5 could offer much more oxygen functional groups, and the nanoscale particles of Ta2 O5 could provide more active site for vanadium redox reactions. In battery charge–discharge tests, the Ta2 O5 -GF yielded a stable performance for more than 100 cycles at the current density of 150 mA cm−2 .

2.2.5

Binary Metal Oxide Modified Electrode

Recently, binary metal oxide decorated carbon electrodes were placed in focus. The catalytic performance of mixed cerium–zirconium oxides was measured in the vanadium flow batteries [52]. It was found that abundant –OH and –COOH functional groups in the CeZrO2 -GF were proved effective for vanadium ions adsorption and desorption. The titanium niobium oxide was also developed and tested for the VO2+ / VO2 + reaction [53]. In the TiNb2 O7 of monoclinic layered structure, both TiO6 and NbO6 groups possessed six oxygen atoms respectively, suggesting excellent activity and hydrophily. The flow battery assembled with TiNb2 O7 -GF electrode showed the energy efficiency of 70.32% at the current density of 150 mA cm−2 . Moreover, the TiNb2 O7 modified reduced graphene oxide was further decorated on the GF electrode towards both VO2+ /VO2 + and V2+ /V3+ reactions [54]. Thanks to the coordination of TiNb2 O7 and rGO, the aggregation of catalysts was avoided, indicating a better performance in battery tests. In addition, the NiCoO2 , that possesses higher electric conductivity than CoO and NiO, was fabricated by hydrothermal method [55]. The NiCoO2 -GF electrode yielded an improved voltage efficiency of 81.2% and an increased discharge capacity of 469.4 mAh at 150 mA cm−2 in flow battery cycling test.

2.3 Other Metal Compound Modified Electrode As mentioned above, TiN possessed better catalytic ability than TiO2 [37]. Other metal compounds, such as metal nitride, metal carbide and sulfide, were also found electrochemical activity towards vanadium redox reactions. For a simple method, the TiN [56–58] and TiC [59, 60] nanoparticles were loaded onto electrode surface directly with binders, accelerating V3+ /V2+ reaction process. TiN nanowire modified GF electrode was further prepared by hydrothermal method, exhibiting favorable charge–discharge performance in battery test. The TiC nanoparticles were also decorated on GF electrode surface without binders by a hydrothermal process, offering abundant -OH functional groups for vanadium ions redox reactions.

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3 Nonmetal Doped Electrode 3.1 Oxygen Doped Electrode Introducing oxygen functional groups onto electrode surfaces could improve hydrophilicity and activity for vanadium ions reactions, deeming as one of the most effective ways to enhance electrode performance. Since early researches, various methods have been developed to introduce oxygen functional groups, including thermal activation method, chemical activation method and electrochemical activation method.

3.1.1

Thermally Activated Oxygen Doped Electrode

The thermal activation is the most adopted method for electrode modification in allvanadium flow battery. After thermal activation process, the oxygen elements from air could been introduced into the graphite surface of electrode with the form of oxygen functional groups, so that enhance the activity and hydrophily. In an early work, the flow battery with thermally treated electrode enabled the 10% improvement of energy efficiency. The enhancing mechanism, that the generated C–O–V bond from hydroxyl and carbonyl functional groups could accelerate the transport of electrons and ions, was proposed [61]. The enhancement mechanisms of oxygen doped electrode towards vanadium redox reactions were further analyzed (Fig. 5) [62]. It was found that the V2+ /V3+ reaction depended on reaction temperature and heat treatment, suggesting the inner-sphere mechanism (Fig. 5a), and the VO2+ /VO2 + reaction was not influenced by reaction temperature and heat treatment, indicating the outer-sphere mechanism (Fig. 5b). Since then, more and more researches fucus on the effects of thermally activated electrodes. It found that the thermal activation process could increase the content of hydroxyl functional group and decrease carbonyl functional group, but the content of carboxylic functional group was unchanged [63]. More than that, due to the different electrode bases adopted in researches, the effects of thermally activated electrode towards VO2+ /VO2 + redox reactions still remained different explanations [64–67]. In order to clear the influence of electrode material, the polyacrylonitrile (PAN)-based carbon fiber and cellulose (Rayon)-based carbon fiber were studied after thermally activating [68]. It was found that the thermally treated electrodes exhibited slightly adverse influence to reaction activity and electric resistance in the positive side. The degree of graphitization and thermally treated temperature were further studied [69]. The enhanced performance of thermally activities electrode with different previous graphitization temperature was observed in CV tests. Moreover, the detailed thermally treated temperatures were adopted and measured [70]. With a comprehensive consideration, the optimal treating temperature of 475 °C was found.

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Fig. 5 Scheme of a outer and b inner-sphere mechanism on oxygen doped surface. The plot of radial distribution functions between c V2+ , d V3+ , e VO2+ and f VO2 + and H2 O molecules; the inset is a snapshot of the vanadium ions in H2 SO4 solutions after MD simulation with various vanadium oxidation states

3.1.2

Chemical Activated Oxygen Doped Electrode

The chemical activation, that treating electrode in acid or heated chemical solution, was another common method for electrode activation. In an early research similar to thermal activation, hot sulfuric and nitric acid was used to activate carbonbased electrode, exhibiting improved performance in battery test [71]. The H2 O2 solution was further used to dope hydroxy functional group on GF electrode selectively [72]. In electrochemical measurement, the H2 O2 -activitaed electrode showed excellent activity towards vanadium ion redox reactions. Moreover, after treating GF in the mixed acid solution with HNO3 and H3 PO4 , a hydroxylated electrode was prepared and tested, demonstrating advantageous efficiencies in battery charge– discharge process [73]. For a more cost-effective method, the room-temperature treating way was developed with the utilization of KMnO4 [74]. Thanks to the strong oxidized property of KMnO4 , the electrode can be oxidated in a low temperature. In the battery cycling test, the room-temperature activated electrode exhibited energy efficiencies more than 80% and 99.94% capacity retention at the current density of 200 mA cm−2 for 550 cycles.

3.1.3

Electrochemical Activated Oxygen Doped Electrode

Electrochemical activation method is also effective for the oxygen doping in carbonbased electrode in all-vanadium flow battery. The oxygen doped electrode was fabricated by an electrochemical process in acid solution, in order to study effect of

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different contents for oxygen functional groups towards vanadium redox reactions [75]. It was found that the oxygen doped electrode, with the electrochemical treating degrees from 560 mAh g−1 to 840 mAh g−1 , possessed the highest contents of hydroxy and carboxyl, indicating the best activity. Based on the electrochemical activation method, researchers found that the carboxyl functional group is the main reason of activity enhancement for vanadium redox reaction [76]. After studying the catalytic performance of electrochemically activities reduced graphene oxide, the carbonyl functional group was confirmed effective towards both VO2+ /VO2 + and V2+ /V3+ reactions [77]. The researcher also found the hydroxyl functional groups determine the VO2+ /VO2 + redox reactions while carboxyl functional groups determine the V2+ /V3+ redox reactions [78]. By a detailed investigation of various carbon materials, it was found that the anodic electrochemical treatment could enhance V2+ /V3+ reactions but inhibit VO2+ /VO2 + reactions, and the cathodic treatment was in contrast [79].

3.1.4

Other Oxygen Doped Electrode

Except for thermal activation, chemical activation and electrochemical activation method, other methods have been developed for electrode modification. For instance, the activation performance of the gamma rays, plasma treatment and mild oxidation treatments was compared in a previous work [80]. It was found that the battery energy efficiency was increased from 68% up to 75% after the mild oxidation treatment. More than that, the oxygen doped electrode treated by both corona discharge and hydrogen peroxide was studied [81]. The flow battery with the developed electrode exhibited 7% improved energy efficiency at the current density of 148 mA cm−2 . Furthermore, the dual approach, that treating electrode by O2 plasma and follow by H2 O2 solution, was developed for oxygen modification [82]. In battery charge– discharge tests, the prepared electrode showed 8.2% energy efficiency enhancement at 150 mA cm−2 . The microwaves method was also used for oxygen modification, which was confirmed effective to VO2+ /VO2 + reactions in CV tests. Moreover, a reduced graphene oxide modified GF electrode (rGO-GF) was prepared by a hydrothermal approach, in order to introduce oxygen functional groups in to electrode while enhancing its electric conductivity and stability (Fig. 6a–c) [83]. The battery assembled with the reduced graphene oxide modified GF electrode yielded a high energy efficiency of 72.0% and a large discharge capacity of 20 Ah L−1 at the current density of 200 mA cm−2 . The gradient bifunctional oxygen doped electrode (GO-rGO/GF) was further constructed by regulating the distribution of graphene oxide and reduced graphene oxide, in order to balance the requirements of high electrochemical activity and high electric conductivity (Fig. 6d–f) [84]. In the developed electrode, one side with high electric conductivity was near to bipolar plate to lower resistance, while the other side with high electrochemical activity was near to membrane for an active reactions. The battery with the developed electrode exhibited a high efficiencies and capacities in charge–discharge tests.

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Fig. 6 a Schematic diagram of the fabrication mechanism of the rGO-GF electrode. b Cyclic voltammograms obtained from an aqueous solution containing 0.05 m VOSO4 in an electrolyte of 3.0 m H2 SO4 using GF and rGO-GF electrodes at a scan rate of 10 mV s−1 . c Nyquist plots of the electrochemical impedance spectra of GF and rGO-GF electrodes. d Schematic of a gradient GO-rGO/GF hybrid as electrode in VRFBs. C1s XPS spectra of different sides of the gradient GO-rGO/GF: e the high oxygen-containing side and f the low oxygen-containing side

3.2 Nitrogen Doped Electrode Doping nitrogen is another effective approach for electrode activation. introducing nitrogen functional groups to electrode can create active sites, which could absorb vanadium ions easily and promote ion exchange during the vanadium redox reaction. According to the type of precursor, the nitrogen doped electrode can be categorized into ammonia media activated nitrogen doped electrode, organic matters derived nitrogen doped electrode and biomass derived nitrogen doped electrode.

3.2.1

Ammonia Media Activated Nitrogen Doped Electrode

Treating carbon-based electrode by the ammonia media is the earliest and most used approach for nitrogen doping. For instance, a nitrogen doped GF electrode was fabricated by the ammoniated hydrothermal method, in which the nitrogen elements were successfully introduces into the carbon matrix [85]. This nitrogen doped electrode shows enhanced discharge capacity and voltage efficiencies in battery charge– discharge tests. Combining with the thermally activated oxygen doped method, the pristine GF electrode was thermally treated in the mixed gas atmosphere with an equal content of NH3 and O2 [86]. Under the synergistic effect of both oxygen and nitrogen doped surface, the active species adsorption–desorption and electron conduction processes were further improved. Moreover, the urea and ammonia gas were always used in nitrogen doping [87–89]. Quaternary nitrogen, one nitrogen doping type in carbon matrix, was confirmed effective for the vanadium ion redox reactions [86]. Moreover, chemical vapor deposition method was used for introducing

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nitrogen defect into the graphite surface of GF electrode, which could yield 8.8% energy efficiency improvement than pristine GF electrode in battery test [90]. The ammonium sulfate was also used for nitrogen doping by a hydrothermal process, in order to enhance the hydrophilicity and activity of pristine CF electrode [91]. Flow battery assembled with ammonium sulfate activated electrode realized an energy efficiency of 87.34% at 80 mA cm−2 , 3.91% higher than that of pristine CF electrode.

3.2.2

Organic Matters Derived Nitrogen Doped Electrode

To realize a more environmental process for nitrogen doping, the nitrogen-rich organic matter was employed as a precursor in production. In recent years, various organic matters, including 1-ethyl-3-methylimidazolium dicyanamide [92], 2-methylimidazole [93], polystyrene [94], melamine [95], phenylenediamines [96], dopamine [97, 98], and polypyrrole [99], were coated and thermally treated to introduce nitrogen element into pristine carbon-based electrode. For instance, the 1-ethyl-3-methylimidazolium dicyanamide, an ionic liquid with a high nitrogen content, was coated onto pristine GF electrode for nitrogen doping after thermally treatment [92]. The flow battery with this developed nitrogen doped GF electrode exhibited a 10% higher energy efficiency than that of pristine GF electrode at the current of 150 mAcm−2 . Moreover, with the coordination of 2-methylimidazole and Zn(NO3 )2 ·6H2 O, the nitrogen doped carbon film was built between carbon fibers of GF electrode (GF@N–C), enhancing the electrochemical activity and electron conductivity (Fig. 7) [93]. In the flow battery measurements, the developed nitrogen doped film bonded GF electrode exhibited a discharge capacity of 23 Ah L−1 and energy efficiency of 74.3% at 200 mA cm−2 .

3.2.3

Biomass Derived Nitrogen Doped Electrode

Recently, some renewable, low cost and highly accessible biomass materials, due to its abundant nitrogen and oxygen content, have received more and more attentions. For example, after treating scaphium scaphigerum with hydrothermal and high temperature processes, the nitrogen doped electrocatalyst was developed for the V2+ /V3+ redox reactions [100]. It was found that the scaphium scaphigerumderived carbon materials exhibited 81.5% increased discharge capacity at the current density of 150 mA cm−2 . Similarly, the spent coffee beans, a typical biomass waste, was mixed with reduced graphene oxide to build a nitrogen doped electrochemical catalyst for vanadium redox reactions [101]. In the battery test, the spent coffee beansderived electrode yielded high energy efficiencies over 90% at low current densities, obviously higher than that of pristine graphite electrode. More than that, various biomass materials, including coin protein [102], cotton [103], coconut shell [104], orange peel [105] and wood [106, 107], were confirmed effective to construct nitrogen or oxygen doped electrode for vanadium ion redox reactions. In order to overcome the limits of commercial carbon electrode and simplify the fabrication process, a

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Fig. 7 a Schematic illustration of the preparation of the robust GF@N–C electrode and its application in the VRFB. b Energy efficiencies of the VRFB based on GF and GF@N–C electrodes at different current densities. c Charge–discharge of VRFB-based GF and GF@N–C electrodes at 200 mA cm−2

cocoon-derived monolithic carbon electrode (NO-MC) was proposed and developed via a simple hydrothermal and pyrolysis process (Fig. 8) [108]. In the developed electrode, the cross-linked silk fiber was carbonized to the carbon fiber with 3D porous structure and admirable mechanical property, due to that high-pressure alkaline-solution hydrothermal treatment removed covered sericin and enlarge pores in the twin cocoon. In battery charge–discharge tests, this all-biomass-derived electrode yielded an 83% higher discharge capacity and 20% higher energy efficiency than pristine CP electrode, indicating its promising utilization potentiality for flow batteries.

3.3 Other Heteroatom Doped Electrode Apart from nitrogen and oxygen, other nonmetal elements, such as boron, phosphorus, sulfur, fluorine, chloride and bromide, have been widely studied in recent works.

3.3.1

Boron Doped Electrode

The boron doped carbon surface could form the B-C bond with uneven electron distribution and large energy barrier, accelerating the adsorption process of vanadium ions and stabilizing surface chemical structure [105, 109]. After a simple pyrolysis process with boric acid, a boron doped carbon electrode was prepared [109]. It was

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Fig. 8 a Schematic diagram of an alternative sustainable energy ecosystem. b Cyclic voltammograms of the CP and NO-MC at a scan rate of 5 mV s−1 with a potential window of 0.3–1.4 V versus Ag/AgCl. c Nyquist plots of CP and NO-MC in the frequency range from 10−2 –105 Hz at 8.5 V

found that the boron doped electrode enabled an effective energy efficiency of 73.63% at the current density up to 400 mA cm−2 and realized stable operation for more than 2000 cycles with 0.028% capacity decay per cycle. A boron doped mesoporous graphene modified GF electrode was fabricated by a boric acid hydrothermal method [110]. The presence of BC2 O and BCO2 was confirmed to increase surface-active sites vanadium ion redox reaction in both positive and negative sides. This boron doped electrode attained an energy efficiency of 81.5% at the current density of 100 mA cm−2 , higher than that of pristine GF electrode.

3.3.2

Phosphorus Doped Electrode

Phosphorus doping has also been proved effective to vanadium redox reactions. In previous works, various phosphorus-containing materials, including ammonium hexafluorophosphate [111], 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) [95, 112], triphenylphosphine (TPP) [113, 114], phosphoric acid [115, 116] and diammonium hydrogen phosphate [117], were adopted for doping phosphorus into carbon-based electrode. Typically, the phosphorus functional groups with rich –OH were introduced into the surface of GF electrode with the utilization of ammonium hexafluorophosphate, boosting both VO2+ /VO2 + and V2+ /V3+ reactions and

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Fig. 9 a Schematic of the P-doped GF electrodes in flow battery. The charge transfer and corresponding energy stabilities of b and c P-doped graphite (0001) surface, d and e pristine graphite (0001) surface with adsorbed OH group and f and g P-doped graphite (0001) surface with adsorbed OH group. h–j SEM images of P-doped GF electrodes

suppressing adverse hydrogen evolution reaction [111]. In flow battery charge– discharge tests, this phosphorus doped electrode yielded a high energy efficiency of 83% at the current density of 32 mA cm−2 . More interestingly, the stabilized reaction interface and hierarchical porous structure were constructed in the GF electrode synchronously by a simple phosphoric acid pyrolysis method (Fig. 9) [115]. In this electrode, the phosphorus doped surface was confirmed stable in reaction process by density functional theory calculation and macro-scale and nanoscale pores was observed by SEM, suggesting the co-enhancement for active species transports and reactions. In the flow battery cycling charge–discharge measurements, this phosphorus doped hierarchical porous electrodes kept high energy efficiencies of 81% at the current density of 200 mA cm−2 , with no decay for 100 cycles.

3.3.3

Sulfur Doped Electrode

To introduce sulfur element into the carbon-based electrode, sulfur-containing materials, such as chlorosulfonic acid [118], ammonium persulfate [119], thiourea [120], ammonia sulfate, sodium thiosulfate [121] and sulfuric acid [122, 123], were used in electrode modification of all-vanadium flow batteries. For instance, with assistance of ultrasonication, the SO3 H-functionalized CP electrode was developed in chlorosulfonic acid solution, in which its carbon layer was exfoliated and surface wettability was improved [118]. The enhancing mechanism of sulfur doped electrode was further derived: (i) the hydrogen in SO3 H functional group was replaced with VO2+ in electrolyte; (ii) the oxygen atom in H2 O was transferred to VO2+ generate VO2 + in charge process and the electron was transferred through S–O–V bond; (iii) the exchange of VO2 + and H+ occur in the end of oxidation reaction. In the charge–discharge tests, the discharge capacity of flow battery with SO3 H-functionalized CP electrode achieved

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27.7 mAh at the current density of 50 mA cm−2 , 20.8 mAh higher that of pristine CP electrode. After a simple hydrothermal process with thiourea, the nitrogen and sulfur co-doped graphene was fabricated and modified in the GF electrode [120]. Thanks to the synergistic effect of nitrogen and sulfur functional groups, the co-doped electrode showed a high energy efficiency of 85.37% at 80 mA cm−2 and a high power density of 0.4925 W cm−2 .

3.3.4

Halogen Doped Electrode

In addition, the halogen family elements were also doped into the carbon-based surface of electrode for the improved vanadium redox reactions. In a previous work, the fluorine doped, chlorine doped and bromine doped graphene electrode (X-GNP (X = F, Cl, or Br)) were fabricated and measured to clear the influence of edge exfoliation degree in halogen doped electrode towards vanadium ion redox reactions (Fig. 10) [124]. Thanks to the edge exfoliation and well-remained planes in electrode, introducing halogen elements into carbon-based electrode was confirmed to increase active sits for both VO2+ /VO2 + and V2+ /V3+ reactions. The chlorine doped graphene electrodes and fluorine, oxygen and phosphorus doped electrode were also observed improved performance in all-vanadium flow batteries [125, 126]. It was found that flow battery with the fluorine, oxygen and phosphorus co-doped electrode operated stability at the current density of 250 mA cm−2 for more than 1000 cycles with only 0.003% energy efficiency decay per cycle.

4 Structure Decorated Electrode 4.1 Metal-Based Structure Decorated Electrode As mentioned above, various metal or metal oxide materials were effective towards vanadium redox reactions. Among these metal-based electrochemical catalysts, some materials, such as such as abovementioned Mn3 O4 nanoparticles [21], Nb2 O5 nanorods [46], TiN nanowire [58], possessed nanostructures for electrode modification, which would influence the electrode porous structure for mass transport after decorating them onto carbon fiber surface. Recently, the MXene, that possesses unique structure and surface chemistry with admirable mechanical stability, electric conductivity and low cost, was utilized for enhancing flow battery performance (Fig. 11a, b) [60, 127, 128]. It was found that the flow battery assembled with MXene (hollow Ti3 C2 Tx spheres) modified GF electrode yielded high energy efficiency of 81.3% and electrolyte utilization efficiency of 80.1, 41.7 and 15.7% higher than pristine GF [60]. More than that, the metal–organic frameworks (MOFs) with high electrochemical activity and excellent specific surface area were also demonstrated

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Fig. 10 Cyclic voltammograms with the pristine carbon felt (CF), X-GNP coated CF electrodes, showing a V2+ /V3+ and b VO2+ /VO2 + half-cell reactions. c Cyclic voltammetry curves of the FGNP, Cl-GNP, and Br-GNP catalyst at different scan rates ranged from 1 to 10 mV s−1 . d Proposed electrocatalytic mechanism of vanadium redox couples on the X-GNPs with the different degree of edge exfoliations

effective to catalyze vanadium redox reactions (Fig. 11c, d) [129–131]. A controllable preparation for surface chemistries and microstructures was realized with the utilization of MOFs [130]. As such, the MOFs modified decorated electrode (PCPs) exhibited high energy efficiency of 82% at 200 mA cm−2 in all-vanadium flow battery charge–discharge cycling tests.

4.2 Carbon-Based Structure Decorated Electrode Except for metal-based materials, the carbon materials with large surface area and high electric conductivity, including graphene [83, 84, 87, 132–137], carbon nanotube [6, 88, 138–140], carbon nanoparticle [141] and carbon compound [92, 107, 122–127, 142–144 145], were decorated to the pristine electrode for a favorable mass transport

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Fig. 11 a Schematic illustration of the preparation of the hollow Ti3 C2 Tx spheres. b TEM image of hollow Ti3 C2 Tx spheres. c Schematic illustration of the fabrication processes of PCPs. d SEM images of PCPs

and reaction process in all-vanadium redox flow batteries. In recent years, Bi nanodotvertically standing carbon nanosheet composites were fabricated on the carbon fibers surface, in order to enhance the pore-level mass transport and reaction processes (Fig. 12a–c) [146]. In this uniquely design electrode, the vertically standing carbon nanosheets provided a low-resistance channel for vanadium ions transport, while the uniformly distributed Bi nanodots offered abundant active sites for reactions. In the charge–discharge tests, the flow battery assembled with this developed electrode yielded an energy efficiency of 73% at a high current density of 400 mA cm−2 . In the aspect of the mass transport stability, a sandwich-like multi-scale pore-rich hydroxylated carbon (SPHC) was proposed and developed for electrode modification in all-vanadium flow batteries (Fig. 12d–f) [147]. In this sandwich-like structure, porous middle layer with abundant –OH functional groups boosted redox reactions, while compact outer layer protected inside structure against electrolyte washing out. The simulating result confirmed that the flow rate in the middle layer was onesixtieth of the outer layer, indicating an effective and stable mass transport process. In the cycling tests, the sandwich-like carbon decorated electrode exhibited stable energy efficiency with ultra-low decay of 0.0025% per cycle for more than 500,00 charge–discharge cycles.

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Fig. 12 a Preparation route of the Bi nanodot/vertically standing carbon nanosheet-decorated graphite felt. Equilibrium potential distributions for electrodes with b vertically standing and c tortuous microstructures. d Schematic diagram of the sandwich-like pore-rich hydroxylated carbon. e Schematic diagram of vanadium redox reactions in sandwich-like pore-rich hydroxylated carbon. f Velocity distribution of electrolyte in the flow channel, graphite felt electrode and SPHC with different flow forms

5 Pore-Etched Electrode 5.1 Single-Scale Pore-Etched Electrode The porous structure in electrode is a key factor affecting mass transport process of allvanadium flow batteries. For an accelerated transport, nanoscale porous was etched on the smooth surface of carbon fiber, in order to construct dual-scale porous structure cooperate with the original macro-scale pores of carbon-based electrode. In previous researches, various activating agent were used to etch pores in electrode, including copper hydroxide [148], potassium hydroxide [149–151], carbon dioxide [121, 152– 154], ferric sulfate [155], calcium carbonate [156], vanadium pentoxide [157], zinc acetate [158], ferric ammonium citrate [159], water [160, 161], iron chloride [162, 163], iron carbide [164], cobalt acetate [165] and hydroxyl oxidize iron [166]. Among various retching, KOH etching method is one of the most used approaches in recent years. In the KOH-etched electrode, abundant pores with nanoscale (2–5 nm) were generated on the electrode surface, increasing active sites for vanadium redox reaction so that enhancing battery performance [149–151]. Inspired by quenching-cracking strategy, a parallel-aligned micron flow channels were etched on the carbon fiber, realizing electrolyte smooth flow and enhanced redox reaction on electrode surface (Fig. 13) [121]. In battery cycling measurements, the as-developed electrode (S–FC– GF) yielded the energy efficiency of 80.44% and 0.01032% decay per cycle for 1000 cycles at the current density up to 500 mA cm−2 .

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Fig. 13 a Schematic diagram for the synthesis route of hierarchical fibers and reaction mechanism. b SEM images S–FC–GF at different magnifications. c Simulation of the flow trajectory for electrolyte in the flow channels

5.2 Multi-scale Pore-Etched Electrode Although etching pore on electrode surface is an effective method for electrode modification, some incidental problems were also gradually emerged in the single-scale pore-etched electrode. The large difference of scale between original macro-scale pores and the etching nanoscale pores hinders the electrolyte diffusion, substantially lowering the utilization of active sites. From this point, constructing a mesoscale pore as a “bridge” between macro-scale pore and nanoscale pore is a promising approach to realize the simultaneous enhancement of mass transports and redox reactions [167–169]. Typically, a gradient-pore-oriented graphite felt electrode that possessed pores from nanoscale to microscale was proposed and developed by a facile one-step etching method (Fig. 14) [167]. In this electrode, the microscale pores (∼20 µm) could provide pathways for electrolyte flow and the nanoscale pores (∼20 nm) render enlarge active surface for redox reactions, while the mesoscale pores (∼0.5 µm) connecting nanoscale and microscale pores could offer place for nanoscale pores generation and lower the diffusion resistance for mass transport. In the battery tests, gradient-pore-oriented electrode exhibited 19.09% higher energy efficiency than that of pristine electrode, indicating a great application potential in all-vanadium flow batteries.

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Fig. 14 a Schematic and b SEM images of the gradient-pore graphite felt electrode. c Energy efficiencies and d cycling performance of VRFBs with gradient-pore graphite felt electrodes

6 Conclusion Electrode is the integral part of all-vanadium flow batteries. The chemical surface and physical structure directly affect the mass transport and redox reaction processes of active species, determining the charge–discharge performance of VFBs. Modifying the electrode to improve its mass transport and redox reaction process is important the enhancement of all-vanadium flow batteries. In this chapter, various modified electrodes, which can be classified into the metal or metal oxide materials modified electrodes and structure decorated or pore-etched electrodes, were discussed. An overview of the different strategies employed to improve electrode performance was also offered in this chapter. This chapter will provide the guides for the design, development and commercial application of electrode in all-vanadium flow battery.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Rychcik M, Skyllas-Kazacos M (1987) J Power Sources 19:45 Sun B, Skyllas-Kazakos M (1991) Electrochim Acta 36:513 Wang WH, Wang XD (2007) Electrochim Acta 52:6755 Tsai H-M, Yang S-J, Ma C-CM, Xie X (2012) Electrochim Acta 77:232 Folkesson B, Larsson R (1989) Inorganica Chim. Acta 162:75 Huang R-H, Sun C-H, Tseng T, Chao W, Hsueh K-L, Shieu F-S (2012) J Electrochem Soc 159:A1579 Tseng T-M, Huang R-H, Huang C-Y, Hsueh K-L, Shieu F-S (2013) J Electrochem Soc 160:A690 Jeong S, Kim S, Kwon Y (2013) Electrochim Acta 114:439 Xiang Y, Daoud WA (2017) J Electrochem Soc 164:A2256 González Z, Sánchez A, Blanco C, Granda M, Menéndez R, Santamaría R (2011) Electrochem Commun 13:1379 Li B, Gu M, Nie Z, Shao Y, Luo Q, Wei X, Li X, Xiao J, Wang C, Sprenkle V, Wang W (2013) Nano Lett 13:1330

Electrodes for All-Vanadium Redox Flow Batteries

169

12. Suárez DJ, González Z, Blanco C, Granda M, Menéndez R, Santamaría R (2014) Chemsuschem 7:914 13. Yang X, Liu T, Xu C, Zhang H, Li X, Zhang H (2017) J Energy Chem 26:1 14. Liu T, Li X, Nie H, Xu C, Zhang H (2015) J Power Sources 286:73 15. Liu Y, Liang F, Zhao Y, Yu L, Liu L, Xi J (2018) J Energy Chem 27:1333 16. Jiang HR, Zeng YK, Wu MC, Shyy W, Zhao TS (2019) Appl Energy 240:226 17. Yang Z, Wei Y, Zeng Y, Yuan Y (2021) J Power Sources 506:230238 18. Shen J, Liu S, He Z, Shi L (2015) Electrochim Acta 151:297 19. Mehboob S, Mehmood A, Lee J-Y, Shin H-J, Hwang J, Abbas S, Ha HY (2017) J Mater Chem A 5:17388 20. Wei L, Zhao TS, Zeng L, Zhou XL, Zeng YK (2016) Appl Energy 180:386 21. Kim KJ, Park M-S, Kim J-H, Hwang U, Lee NJ, Jeong G, Kim Y-J (2012) Chem Commun 48:5455 22. He Z, Dai L, Liu S, Wang L, Li C (2015) Electrochim Acta 176:1434 23. Di Blasi A, Busaccaa C, Di Blasia O, Briguglioa N, Squadritoa G, Antonuccia V (2017) Appl Energy 190:165 24. Ejigu A, Edwards M, Walsh DA (2015) ACS Catal 5:7122 25. Ji Y, Li JL, Li SFY (2017) J Mater Chem A 5:15154 26. Zeng L, Zhao TS, Wei L, Zeng YK, Zhou XL (2018) Energy Technol 6:1228 27. Ma Q, Deng Q, Sheng H, Ling W, Wang H-R, Jiao H-W, Wu X-W, Zhou W-X, Zeng X-X, Yin Y-X (2018) Sci China Chem 61:732 28. Yao C, Zhang H, Liu T, Li X, Liu Z (2012) J Power Sources 218:455 29. Shen Y, Xu H, Xu P, Wu X, Dong Y, Lu L (2014) Electrochim Acta 132:37 30. Kabtamu DM, Chen J-Y, Chang Y-C, Wang C-H (2016) J Mater Chem A 4:11472 31. Kabtamu DM, Chang Y-C, Lin G-Y, Bayeh AW, Chen J-Y, Wondimu TH, Wang C-H (2017) Sustain. Energy Fuels 1:2091 32. Faraji M, Hassanzadeh A, Mohseni M (2017) Thin Solid Films 642:188 33. Hosseini MG, Mousavihashemi S, Murcia-López S, Flox C, Andreu T, Morante JR (2018) Carbon 136:444 34. Bayeh AW, Kabtamu DM, Chang Y-C, Chen G-C, Chen H-Y, Liu T-R, Wondimu TH, Wang K-C, Wang C-H, Appl ACS (2019) Energy Mater 2:2541 35. Tseng T-M, Huang R-H, Huang C-Y, Liu C-C, Hsueh K-L, Shieu F-S (2014) J Electrochem Soc 161:A1132 36. Hou B, Cui X, Chen Y (2018) Solid State Ion 325:148 37. Vázquez-Galván J, Flox C, Jervis JR, Jorge AB, Shearing PR, Morante JR (2019) Carbon 148:91 38. Cheng D, Cheng G, He Z, Dai L, Wang L (2019) Int J Energy Res 43:4473 39. Bayeh AW, Kabtamu DM, Chang Y-C, Chen G-C, Chen H-Y, Lin G-Y, Liu T-R, Wondimu TH, Wang K-C, Wang C-H, Sustain ACS (2018) Chem Eng 6:3019 40. Xiang Y, Daoud WA (2018) Electrochim Acta 290:176 41. Gobal F, Faraji M (2015) RSC Adv 5:68378 42. Fetyan A, El-Nagar GA, Derr I, Kubella P, Dau H, Roth C (2018) Electrochim Acta 268:59 43. Mehboob S, Ali G, Shin H-J, Hwang J, Abbas S, Chung KY, Ha HY (2018) Appl Energy 229:910 44. Cao L, Skyllas-Kazacos M, Wang D-W (1836) ChemElectroChem 2017:4 45. Yun N, Park JJ, Park OO, Lee KB, Yang JH (2018) Electrochim Acta 278:226 46. Li B, Gu M, Nie Z, Wei X, Wang C, Sprenkle V, Wang W (2014) Nano Lett 14:158 47. Zhou H, Shen Y, Xi J, Qiu X, Chen L, Appl ACS (2016) Mater Interf 8:15369 48. He Z, Li M, Li Y, Li C, Yi Z, Zhu J, Dai L, Meng W, Zhou H, Wang L (2019) Electrochim Acta 309:166 49. Zhou H, Xi J, Li Z, Zhang Z, Yu L, Liu L, Qiu X, Chen L (2014) Rsc Adv 4:61912 50. Jing M, Zhang X, Fan X, Zhao L, Liu J, Yan C (2016) Electrochim Acta 215:57 51. Wu X, Xu H, Lu L, Zhao H, Fu J, Shen Y, Xu P, Dong Y (2014) J Power Sources 250:274 52. Yu L, Lin F, Xiao W, Xu L, Xi J (2019) Chem Eng J 356:622

170

R. Wang and Y. Li

53. Kabtamu DM, Bayeh AW, Chiang T-C, Chang Y-C, Lin G-Y, Wondimu TH, Su S-K, Wang C-H (2018) Appl Surf Sci 462:73 54. Bayeh AW, Kabtamu DM, Chang Y-C, Chen G-C, Chen H-Y, Lin G-Y, Liu T-R, Wondimu TH, Wang K-C, Wang C-H (2018) J Mater Chem A 6:13908 55. Xiang Y, Daoud WA (2019) J Mater Chem A 7:5589 56. Wang C, Lu W, Lai Q, Xu P, Zhang H, Li X (2019) Adv Mater 31:1904690 57. Yang C, Wang H, Lu S, Wu C, Liu Y, Tan Q, Liang D, Xiang Y (2015) Electrochim Acta 182:834 58. Wei L, Zhao TS, Zeng L, Zeng YK, Jiang HR (2017) J Power Sources 341:318 59. Wei L, Zhao T, Zeng L, Zhou X, Zeng Y (2016) Energy Technol 4:990 60. Wei L, Xiong C, Jiang HR, Fan XZ, Zhao TS (2020) Energy Storage Mater 25:885 61. Sun B, Skyllas-Kazacos M (1992) Electrochim Acta 37:1253 62. Choi C, Noh H, Kim S, Kim R, Lee J, Heo J, Kim H-T (2019) J. Energy Storage 21:321 63. Li Y, Parrondo J, Sankarasubramanian S, Ramani V (2019) J Phys Chem C 123:6370 64. Roznyatovskaya N, Noack J, Pinkwart K, Tübke J (2020) Curr Opin Electrochem 19:42 65. Fink H, Friedl J, Stimming U (2016) J Phys Chem C 120:15893 66. Friedl J, Stimming U (2017) Electrochim Acta 227:235 67. Pezeshki AM, Clement JT, Veith GM, Zawodzinski TA, Mench MM (2015) J Power Sources 294:333 68. Schweiss R, Meiser C, Goh FWT (1969) ChemElectroChem 2017:4 69. Langner J, Bruns M, Dixon D, Nefedov A, Wöll C, Scheiba F, Ehrenberg H, Roth C, Melke J (2016) J Power Sources 321:210 70. Greco KV, Forner-Cuenca A, Mularczyk A, Eller J, Brushett FR, Appl ACS (2018) Mater Interfaces 10:44430 71. Sun B, Skyllas-Kazacos M (1992) Electrochim Acta 37:2459 72. Gao C, Wang N, Peng S, Liu S, Lei Y, Liang X, Zeng S, Zi H (2013) Electrochim Acta 88:193 73. Mukhopadhyay A, Yang Y, Li Y, Chen Y, Li H, Natan A, Liu Y, Cao D, Zhu H (2019) Adv Funct Mater 29:1903192 74. Jiang HR, Shyy W, Ren YX, Zhang RH, Zhao TS (2019) Appl Energy 233:544 75. Zhang W, Xi J, Li Z, Zhou H, Liu L, Wu Z, Qiu X (2013) Electrochim Acta 89:429 76. Li X, Huang K, Liu S, Tan N, Chen L (2007) Trans Nonferrous Met Soc China 17:195 77. Li W, Liu J, Yan C (2013) Carbon 55:313 78. Noack J, Roznyatovskaya N, Kunzendorf J, Skyllas-Kazacos M, Menictas C, Tübke J (2018) J Energy Chem 27:1341 79. Bourke A, Miller MA, Lynch RP, Gao X, Landon J, Wainright JS, Savinell RF, Buckley DN (2015) J Electrochem Soc 163:A5097 80. Kim KJ, Kim Y-J, Kim J-H, Park M-S (2011) Mater Chem Phys 131:547 81. Kim KJ, Lee S-W, Yim T, Kim J-G, Choi JW, Kim JH, Park M-S, Kim Y-J (2014) Sci Rep 4:1 82. Estevez L, Reed D, Nie Z, Schwarz AM, Nandasiri MI, Kizewski JP, Wang W, Thomsen E, Liu J, Zhang J-G (2016) Chemsuschem 9:1455 83. Deng Q, Huang P, Zhou W-X, Ma Q, Zhou N, Xie H, Ling W, Zhou C-J, Yin Y-X, Wu X-W (2017) Adv Energy Mater 7:1700461 84. Hu G, Jing M, Wang D-W, Sun Z, Xu C, Ren W, Cheng H-M, Yan C, Fan X, Li F (2018) Energy Storage Mater 13:66 85. Wu T, Huang K, Liu S, Zhuang S, Fang D, Li S, Lu D, Su A (2012) J Solid State Electrochem 16:579 86. Flox C, Rubio-García J, Skoumal M, Andreu T, Morante JR (2013) Carbon 60:280 87. He Z, Shi L, Shen J, He Z, Liu S (2015) Int J Energy Res 39:709 88. Jin J, Fu X, Liu Q, Liu Y, Wei Z, Niu K, Zhang J (2013) ACS Nano 7:4764 89. He Z, Zhou X, Zhang Y, Jiang F, Yu Q (2019) J Electrochem Soc 166:A2336 90. Wang S, Zhao X, Cochell T, Manthiram A (2012) J Phys Chem Lett 3:2164 91. Qiao L, Fang M, Guo J, Ma X (2022) ChemElectroChem 9:e202200292 92. Yoon SJ, Kim S, Kim DK, So S, Hong YT, Hempelmann R (2020) Carbon 166:131

Electrodes for All-Vanadium Redox Flow Batteries

171

93. Sheng H, Ma Q, Yu J-G, Zhang X-D, Zhang W, Yin Y-X, Wu X, Zeng X-X, Guo Y-G, Appl ACS (2018) Mater Interf 10:38922 94. Sun J, Zeng L, Jiang HR, Chao CYH, Zhao TS (2018) J Power Sources 405:106 95. Su J, Zhao Y, Xi J (2018) Electrochim Acta 286:22 96. Zhou Y, Liu L, Shen Y, Wu L, Yu L, Liang F, Xi J (2017) Chem Commun 53:7565 97. Wu L, Shen Y, Yu L, Xi J, Qiu X (2016) Nano Energy 28:19 98. Lee HJ, Kim H (2015) J Electrochem Soc 162:A1675 99. Park S, Kim H (2015) J Mater Chem A 3:12276 100. Jiang Y, Du M, Cheng G, Gao P, Dong T, Zhou J, Feng X, He Z, Li Y, Dai L (2021) J Energy Chem 59:706 101. Abbas A, Eng XE, Ee N, Saleem F, Wu D, Chen W, Handayani M, Tabish TA, Wai N, Lim TM (2021) J. Energy Storage 41:102848 102. Yang L, Zhang L (2019) Appl Catal B Environ 259:118053 103. Zhang ZH, Zhao TS, Bai BF, Zeng L, Wei L (2017) Electrochim Acta 248:197 104. Ulaganathan M, Jain A, Aravindan V, Jayaraman S, Ling WC, Lim TM, Srinivasan MP, Yan Q, Madhavi S (2015) J Power Sources 274:846 105. Ryu J, Park M, Cho J (2015) J Electrochem Soc 163:A5144 106. Jiao M, Liu T, Chen C, Yue M, Pastel G, Yao Y, Xie H, Gan W, Gong A, Li X, Hu L (2020) Energy Storage Mater. 27:327 107. Maharjan M, Wai N, Veksha A, Giannis A, Lim TM, Lisak G (2019) J Electroanal Chem 834:94 108. Wang R, Li Y (2019) J Power Sources 421:139 109. Jiang HR, Shyy W, Zeng L, Zhang RH, Zhao TS (2018) J Mater Chem A 6:13244 110. Opar DO, Nankya R, Raj CJ, Jung H (2021) Appl Mater Today 22:100950 111. Kim KJ, Lee HS, Kim J, Park M-S, Kim JH, Kim Y-J, Skyllas-Kazacos M (2016) Chemsuschem 9:1329 112. Yu L, Lin F, Xu L, Xi J (2019) J Energy Chem 35:55 113. Radinger H, Hartmann M, Ast M, Pfisterer J, Bron M, Ehrenberg H, Scheiba F (2022) Electrochim Acta 409:139971 114. Int J (2018) Hydrog Energy 43:189 115. Wang R, Li Y, Wang Y, Fang Z (2020) Appl Energy 261:114369 116. Gursu H, Gencten M, Sahin Y (2021) J Electrochem Soc 168:060504 117. Pasala V, Ramavath JN, He C, Ramani VK, Ramanujam K (2018) ChemistrySelect 3:8678 118. He Z, Jiang Y, Li Y, Zhu J, Zhou H, Meng W, Wang L, Dai L (2018) Carbon 127:297 119. Shah AB, Wu Y, Joo YL (2019) Electrochim Acta 297:905 120. Li Q, Bai A, Xue Z, Zheng Y, Sun H (2020) Electrochim Acta 362:137223 121. Xu Z, Zhu M, Zhang K, Zhang X, Xu L, Liu J, Liu T, Yan C (2021) Energy Storage Mater 39:166 122. Ersozoglu MG, Gursu H, Gencten M, Sarac AS, Sahin Y (2021) Int J Energy Res 45:2126 123. Ersozoglu MG, Gursu H, Gencten M, Sarac AS, Sahin Y (2022) Int J Energy Res 46:19992 124. Park M, Jeon I-Y, Ryu J, Jang H, Back J-B, Cho J (2016) Nano Energy 26:233 125. Huang P, Ling W, Sheng H, Zhou Y, Wu X, Zeng X-X, Wu X, Guo Y-G (2018) J Mater Chem A 6:41 126. Gursu H, Gençten M, Sahin Y (2018) Int J Energy Res 42:3303 127. Mizrak AV, Uzun S, Akuzum B, Agartan L, Gogotsi Y, Kumbur EC (2021) J Electrochem Soc 168:090518 128. Ferrara C, Gentile A, Marchionna S, Ruffo R (2021) Curr Opin Electrochem 29:100764 129. Jiang Y, Cheng G, Li Y, He Z, Zhu J, Meng W, Dai L, Wang L (2021) Chem Eng J 415:129014 130. Li Y, Ma L, Yi Z, Zhao Y, Mao J, Yang S, Ruan W, Xiao D, Mubarak N, Wu J, Zhao T-S, Chen Q, Kim J-K (2021) J Mater Chem A 9:5648 131. Xing F, Liu T, Yin Y, Bi R, Zhang Q, Yin L, Li X (2022) Adv Funct Mater 32:2111267 132. Jiang HR, Shyy W, Wu MC, Wei L, Zhao TS (2017) J Power Sources 365:34 133. Jing M, Zhang C, Qi X, Yang Y, Liu J, Fan X, Yan C, Fang D (2020) Int J Hydrog Energy 45:916

172

R. Wang and Y. Li

134. Jing M, Xu Z, Fang D, Fan X, Liu J, Yan C (2019) J Power Sources 425:94 135. Dai W, Shen Y, Li Z, Yu L, Xi J, Qiu X (2014) J Mater Chem A 2:12423 136. Nia PM, Abouzari-Lotf E, Woi PM, Alias Y, Ting TM, Ahmad A, Che Jusoh NW (2019) Electrochimica Acta 297:31 137. Park M, Jung Y, Kim J, il Lee H, Cho J (2013) Nano Lett 13:4833 138. González Z, Flox C, Blanco C, Granda M, Morante JR, Menéndez R, Santamaría R (2017) J Power Sources 338:155 139. Chang Y-C, Shih Y-C, Chen J-Y, Lin G-Y, Hsu N-Y, Chou Y-S, Wang C-H (2016) RSC Adv 6:107294 140. Chung Y, Noh C, Kwon Y (2019) J Power Sources 438:227063 141. Wei L, Zhao TS, Zhao G, An L, Zeng L (2016) Appl Energy 176:74 142. Aziz MdA, Hossain SI, Shanmugam S (2020) J Power Sources 445:227329 143. Jiang F, He Z, Guo D, Zhou X (2019) J Power Sources 440:227114 144. Gao Y, Wang H, Ma Q, Wu A, Zhang W, Zhang C, Chen Z, Zeng X-X, Wu X, Wu Y (2019) Carbon 148:9 145. Opar DO, Nankya R, Lee J, Jung H (2020) Electrochim Acta 330:135276 146. Zhang X, Ye X, Huang S, Zhou X, Appl ACS (2021) Mater Interf 13:37111 147. Wang R, Li Y, Liu H, He Y-L, Hao M (2021) J Mater Chem A 9:2345 148. Zhou X, Zhang X, Lv Y, Lin L, Wu Q (2019) Carbon 153:674 149. Gautam RK, Kapoor M, Verma A (2020) Energy Fuels 34:5060 150. Zhou XL, Zeng YK, Zhu XB, Wei L, Zhao TS (2016) J Power Sources 325:329 151. Zhang Z, Xi J, Zhou H, Qiu X (2016) Electrochim Acta 218:15 152. Liu T, Li X, Xu C, Zhang H, Appl ACS (2017) Mater Interf 9:4626 153. Chang Y-C, Chen J-Y, Kabtamu DM, Lin G-Y, Hsu N-Y, Chou Y-S, Wei H-J, Wang C-H (2017) J Power Sources 364:1 154. Jing M, Xu Z, Fang D, Fan X, Liu J, Yan C (2021) J Electrochem Soc 168:030539 155. Zhang L, Yue J, Deng Q, Ling W, Zhou C-J, Zeng X-X, Zhou C, Wu X-W, Wu Y (2020) RSC Adv 10:13374 156. Xu Z, Xiao W, Zhang K, Zhang D, Wei H, Zhang X, Zhang Z, Pu N, Liu J, Yan C (2020) J Power Sources 450:227686 157. Wang Y-H, Hung I-M, Wu C-Y (2021) Catalysts 11:800 158. Sun J, Jiang H, Zhao C, Fan X, Chao C, Zhao T (2021) Sci Bull 66:904 159. Cheng D, Zhu W, Gao J, Li J, Yang Y, Dai L, Liu Y, Wang L, He Z (2022) Appl Surf Sci 599:153919 160. Fang D, An X, Zhang A, Li X, Liu N, Ma X, Jing M (2022) J Electrochem Soc 169:030501 161. Chung Y, Jeong J, Pham HTT, Lee J, Kwon Y (2018) J Electrochem Soc 165:A2703 162. Oh S, Noh C, Shin M, Kwon Y (2022) Int J Energy Res 46:8803 163. Jiang HR, Shyy W, Wu MC, Zhang RH, Zhao TS (2019) Appl Energy 233–234:105 164. Zhao Y, Li Y, Ihsan-ul-haq M, Mubarak N, Xu M, Qin X, Zhao T-S, Kim J-K (2022) Nanoscale 14:5804 165. Abbas S, Mehboob S, Shin H-J, Han OH, Ha HY (2019) Chem Eng J 378:122190 166. Liu Y, Shen Y, Yu L, Liu L, Liang F, Qiu X, Xi J (2018) Nano Energy 43:55 167. Wang R, Li Y, He Y-L (2019) J Mater Chem A 7:10962 168. Wu Q, Zhang X, Lv Y, Lin L, Liu Y, Zhou X (2018) J Mater Chem A 6:20347 169. Jiang HR, Sun J, Wei L, Wu MC, Shyy W, Zhao TS (2020) Energy Storage Mater 24:529

Microfluidic Flow Cells for Energy Conversion and Utilization Hao Feng, Ying Zhang, Dong Liu, and Qiang Li

1 Introduction With the rapid development of human society, the increasingly severe energy and environmental issues result in the diversification of the global energy supply and the exploitation of advanced energy technology being of great urgency [1–3]. To achieve this vision, on the one hand, environmentally friendly renewable and sustainable energies, such as solar energy, wind energy, bioenergy, hydrogen energy, should be adopted to possibly replace fossil fuels that dominate the global energy supply [4–6]. Besides, traditional energy conversion and utilization systems need to be further progressed to save energy and reduce emissions as well as finally satisfy the requirements of sustainable development [7, 8]. Hence, in the past decades, the implementation of renewable and sustainable energy technology has attracted everincreasing attention toward the development of highly efficient energy conversion and utilization devices [9]. Microfluidics refer to the technology of controlling or operating minute amounts of fluids in microchannels with the characteristic dimension of the submillimeter scale [10]. Ascribing to the extremely large specific surface area, microfluidics inherently allow the potential in facilitating energy and mass transport. Meanwhile, microfluidics also provide other superiorities such as precise flow control, controllable distribution of selective species, optimal thermodynamic equilibrium, flexible operation. These merits of microfluidics have therefore enabled substantial opportunities for various research fields including biochemical analyses, fine chemical synthesis, and energy conversion [11–13]. Microfluidic devices usually operate in a H. Feng (B) · Y. Zhang · D. Liu · Q. Li (B) MIIT Key Laboratory of Thermal Control of Electronic Equipment, School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, China e-mail: [email protected] Q. Li e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 L. An et al. (eds.), Flow Cells for Electrochemical Energy Systems, Green Energy and Technology, https://doi.org/10.1007/978-3-031-37271-1_7

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continuous flow design and have been employed in a variety of energy technologies like electrolyzers, fuel cells, flow batteries, etc., which are also known as microfluidic flow cells. Undoubtedly, microfluidic design plays significant roles in process intensification and now become a promising candidate for realizing highly efficient energy conversion and utilization. In this chapter, the development of microfluidic flow cells for energy conversion and utilization is reviewed with respect to their applications in fuel production, renewable electricity storage, and electricity generation. In addition, the remaining challenges and future direction of the microfluidic flow cells are also discussed.

2 Microfluidic Flow Cells in Fuel Production Mainstream renewable energies still face the defects of their intermittent, dilute, stochastic, and non-uniform distributed nature [14]. Converting renewable energies into fuels has been regarded as one of the most promising routes for a sustainable future because it not only overcomes the intrinsic defects of renewable energies mentioned above but also achieves flexible storage and transportation in time and space. Renewable energies can first be converted into electricity and then generate fuels by electrochemical reactions [15–17]. Meanwhile, fuel synthesis directly driven by renewable energy is also a desired choice [18–22], e.g., photocatalytic CO2 reduction, and photoelectrochemical water splitting. In this section, microfluidic flow cells for hydrogen evolution reaction and CO2 reduction reaction (CO2 RR) are summarized and discussed. Particular attention has been focused on the design of microfluidic flow cells that are employed in both electrochemical and direct solar-powered fuel formation systems.

2.1 Water Splitting to Hydrogen Production Hydrogen is the most abundant matter in the universe, while it is extremely low on earth and the vast majority of hydrogen element is in water. Toward the global deployment as a fuel, it is necessary to design cost-effective and efficient reactors for realizing sustainable and economical hydrogen production. The electrochemical cell for water splitting is typically integrated with three main components, i.e., an anode for oxygen evolution reaction (OER), a cathode for hydrogen evolution reaction (HER), and a membrane for the passage of ions and the inhibition of anodic and cathodic products or reactants mixing. The first microfluidic electrochemical cell (MEC) for hydrogen evolution was proposed by Modestino et al. [14], where rectangular SU-8 microchannels with a width of 50 µm and height of 15 µm that were fabricated on a silicon substrate and capped with a Nafion-117 membrane composed of the microfluidic electrochemical cell (see Fig. 1a). Platinum electrodes

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were deposited on the bottom of each SU-8 channel for the anodic oxygen evolution reaction and cathodic hydrogen evolution reaction, respectively. Their results showed that the microfluidic design allows the easy transport of protons and reactants across the membrane from the anodic side to the cathodic side, resulting in the remarkable advantage in lowering the ohmic resistance and only the kinetic overpotential ascribed to the intrinsic activity of catalysts can dominate the electrochemical performance of the system (see Fig. 1b). Thus, the proposed microfluidic electrochemical cell assesses a reliable hydrogen generation performance with excellent stability during the long-term operation even at a high current density up to 175 A/m2 (see Fig. 1c). Meanwhile, it should be pointed out that, since both cathodic HER and anodic OER are gas-evolving reactions occurring at the electrode surface, the bubble behaviors then can also play significant roles in influencing the transport and kinetic processes [23]. To clarify the gas bubble behaviors in the microfluidic electrochemical cell, Shi et al. [24] performed the force analysis of a gas bubble evolved on the electrode surface and demonstrated the feasibility and superiority of microfluidic design in manipulating the surface bubble behavior, which can directly reduce the energies losses by promoting the transport of reactants and ions and releasing the active sites. The obtained results revealed that the self-generated gas– liquid two-phase flow behaviors in the microchannel significantly rely on the interaction between the electrolyte flow rate and the cell voltage (see Fig. 1d–f). Compared with conventional H-type electrochemical cells, state-of-the-art efficient and stable performance of hydrogen generation was achieved, where the increase in current density was more than five times. In addition, due to the limitations of membranes in both lifetime and price as well as manufacturing, the complete removal of membranes from the flow cells has also attracted tremendous attention. The membrane-free microfluidic electrochemical cell (MF-MEC) was then developed for the water splitting. Figure 2a schematically illustrated the MF-MEC proposed by Hashemi et al. [25], where the main body of the cell was fabricated through a 3D printed with stereolithography technology, and the electrodes were inserted into the reserved grooves. The obtained results demonstrated that the inertial fluidic forces can be used to control the gas–liquid two-phase flow in the device, where the inertial forces promoted the rapid departure of the gas bubble away from the electrodes. At the same time, the concentration gradients of the dissolved gas led to surface tension gradients at the interfaces of the bubbles. These surface tension gradients induced Marangoni forces will lead to the detached and coalesced gas bubbles returning to the electrodes. This synergism, therefore, inhibits the cross-contamination of cathodic hydrogen bubbles and anodic oxygen bubbles (see Fig. 2b). Hence, fine hydrogen purities of more than 99% were reported for water splitting through the presented MF-MEC. Furthermore, hydrogen generation using a photoelectrochemical cell (PEC) that is directly powered by solar energy is also a feasible method. To this end, one of the generalizable strategies is the integration of a microfluidic electrochemical cell with a solar cell. As shown in Fig. 2c, Oruc et al. [26] reported a microchanneled electrochemical cell with a planar design, where a silicon photovoltaic cell was integrated for providing electricity. In this system, firstly, the continuous flow of the

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Fig. 1 a Schematic diagram and optical micrograph of a MEC capped with Nafion-117 membrane, b simulation result of the potential drop pattern across two microchannels, c long-term current–time curves, adapted from [14], Copyright 2013, with permission from RSC. d Force analysis of a single bubble on the electrode surface and regional bubble behaviors at different flow rates, e current density, and f cell voltage variations with the flow rate, adapted from [24], Copyright 2022, with permission from Elsevier

electrolyte can be used as a coolant to cool the solar cell components, where the waste heat generated during the photoelectric process will play a negative role in the performance of photovoltaic cells. Meanwhile, the increase in electrolyte temperature is favorable to the reaction kinetics at both the anode and cathode sides. These synergistic effects then can promote hydrogen generation. Another strategy is using photoelectrode to capture solar light and generate electricity simultaneously [27]. As can be seen in Fig. 2d, a p-Si metal–insulator–semiconductor photoelectrode was arranged in a 3D-printed photoelectrochemical cell for the replacement of the common cathode. The limiting photocurrent densities for the hydrogen evolution under various operation parameters of electrolyte concentration and flow rate as well as light intensity were studied. The obtained results demonstrated the light absorption and mass transfer performances in influencing the hydrogen production performance in the microfluidic photoelectrochemical cell, which allows important implications for the optimization of both operating conditions and reactor design.

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Fig. 2 a Membrane-free microfluidic electrochemical cell, b snapshots of the formed hydrogen and oxygen bubbles at upstream, midstream, and downstream regions, adapted from [25], Copyright 2019, with permission from RSC. c Microchanneled electrochemical cell integrated with a photovoltaic module, adapted from [26], Copyright 2016, with permission from Elsevier. d a 3D printed microfluidic photoelectrochemical cell with a p-Si photoelectrode, adapted from [27], Copyright 2017, with permission from IOP

2.2 CO2 Reduction in Hydrocarbon Fuels The carbon dioxide emission issue ascribing to the excessive use of fossil energy is now a serious challenge facing the world [28]. Carbon sequestration technologies including photoelectrochemical and electrochemical processes driven by renewable energies then become promising candidates to realize carbon neutrality. It is worth noting that CO2 reduction is also a typical gas–liquid–solid heterogeneous reaction, where the reduction reaction significantly relies on the efficient triple-phase contact between CO2 molecule and electrolyte as well as catalyst [29]. One of the most severe challenges of CO2 reduction is the limited CO2 transfer from the bulk to the catalyst surface because the solubility of CO2 in an aqueous solution is extremely low [2]. As a result, the microfluidic design of an efficient CO2 reduction reactor has also attracted tremendous attention. A modified microchannel reactor based on the commercial H-type electrochemical cell was proposed by Zhang’s group [30]. As illustrated in Fig. 3a, a cylindrical Nafion-117 membrane was used as the microchannel and a Cu foam rod as the cathode was placed in the center of the microchannel. During operation, gas–liquid two-phase Taylor flow was pre-formed in a capillary tube outside the cell and then introduced into the above-mentioned microchannel reactor. The CO2 and proton transfer in the microchannel reactor relies on the two-phase flow behavior in the microchannel reactor. The obtained results show that benefited from the enhanced gas–liquid mass transfer, the limiting current density and CO2 reduction rate achieved a remarkable gain of 49.3% and 55.3%, respectively. Regarding the mass transfer under Taylor flow in the microchannel, as shown in Fig. 3b, the CO2 transfer contains three paths [30]: (1) from the bubble caps to the liquid slugs; (2) from the liquid slugs to the cathode through the liquid film surrounding the cathode; (3) from the gas bubble to the

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electrode through the liquid film surrounding the cathode. For those hydrophilic electrodes, since the liquid film is easily formed, CO2 first dissolves into the liquid film and then diffuses to the active surface of the electrode to participate in the reduction reaction. It can be found that the diffusion of CO2 across the liquid film is essential, which may lead to the inevitable transfer resistance. To alleviate this problem, by using the same cell design, Lin et al. [31] proposed a hydrophobic PTFE-doped cathode to substitute for the hydrophilic cathode. Because the active surface will not be fully covered by the electrolyte, the partially exposed active sites can directly capture and adsorb as well as activate CO2 molecules (see Fig. 3c). Accordingly, an optimal 96.19% CO faraday efficiency was achieved in the microchannel reactor. Another configuration of microfluidic design for CO2 reduction is based on the GDE-type electrochemical cell (see Fig. 3d). As shown, both the cathode and anode are gas diffusion electrodes, the cathode and anode are separated by a thin channel (i.e., microchannel), and the electrolytes are continuously introduced into the microchannel [32]. In this reactor, the CO2 RR occurs at the gas–solid–liquid interface, where gaseous CO2 transfers from the gas channel to the interface of the cathode catalyst/electrolyte through the gas diffusion layer. The gaseous products of

Fig. 3 Schematic illustration of a a modified microchannel reactor based on an H-type cell, b the mass transfer process, adapted from [30], Copyright 2020, with permission from Elsevier, and c reaction on the hydrophobic nanocatalyst, adapted from [31], Copyright 2022, with permission from ACS. d Microchanneled electrochemical cells integrated with GDEs, adapted from [32], Copyright 2021, with permission from RSC, e a microchanneled solid electrolyte electrochemical cell, adapted from [33], Copyright 2022, with permission from Elsevier

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CO2 reduction transfer in the opposite direction and are collected at the outlet of the gas channel, while the liquid products are directly taken away by the electrolyte and collected at the outlet of the electrolyte [34–36]. A typical GDE-type microfluidic cell was proposed by Kenis et al. [37], where a 1.5-mm-thick PMMA sheet with a 0.5 cm × 2 cm through groove was placed between the cathodic GDE and anodic GDE. This design firstly allows the precise control of the operating conditions including the electrolyte composition, pH value, flow rate, etc.; thus, high current densities for CO2 reduction were achieved [37, 38]. In addition, despite the membrane-free design, dual-side GDEs facilitate the diffusion of gaseous products in both anode and cathode, which not only avoids the cross-contamination of the reduction and oxidation products but also makes the flexible sample collection and analysis. Last but not least, the simple and convenient structure of this microfluidic design leads to the feasibility of electrode layer deposition methods, diffusion layer composition design and regulation methods, etc. This feasibility will bring more possibilities for the intensification of CO2 conversion because CO2 reduction activity significantly relies on the fabrication of GDEs [39, 40]. In addition to focusing on the performance of CO2 conversion, some researchers have also proposed the microfluidic flow cell to boost the carbon efficiency of CO2 reduction, because 70–95% of the introduced CO2 is converted to carbonates or bicarbonates [41, 42]. These (bi)carbonates can be dissolved in the electrolyte, and/ or release CO2 and are diluted in the anodic gaseous product, and/or combined with alkali metal cations to precipitate crystalline salt on the electrode, resulting in not only the extremely low CO2 utilization efficiency but also the poor long-term operation stability [43–46]. To this end, Sinton et al. [33] proposed a microchanneled solid electrolyte electrochemical cell for carbon-efficient CO2 reduction. Different from the traditional structure that uses an anion-conducting layer (i.e., anion exchange membrane) to separate anode and cathode, an integrated microchannel layer was constructed between the cathode-side and ion-conducting layer and additionally added anode-side cation-conducting layer (i.e., cation exchange membrane), as can be seen in Fig. 3e. The ion-conducting layer would realize a local environment with a high pH value at the cathode thus facilitating the CO2 RR activity, whereas the cation-conducting layer only transfers protons thus maintaining an acidic environment for internal CO2 regeneration from the carbonates and bicarbonates. The regenerated CO2 can be captured from the integrated microchannel and refed into the cathode inlet for reuse. Ascribing to the kinetic intensification and CO2 regeneration, the proposed reactor achieves a high CO2 RR product selectivity of over 90% and an extremely low CO2 loss of about 3% as well as excellent long-term operation stability over 200 h. Furthermore, a direct solar-driven photoelectrochemical flow cell is also a choice for CO2 RR [47–49]. Actually, the structures of these photoelectrochemical flow cells are similar to the electrochemical flow cell, except that photoelectrode is allocated to replace one of the electrodes (i.e., anode and cathode) [50]. A typical microfluidic photoelectrochemical cell was designed by Andreu et al. [51]; as displayed in Fig. 4a, they use a conventional TiO2 nanorods photoanode to substitute for the anode of a commercial microfluidic flow cell. Since the photoelectrochemical approach allows a

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remarkable superiority in enhancing the charge carrier separation [52], the synergism between the enhanced mass transfer attributing to the microfluidic design and the efficient charge carrier transfer then promoted the CO2 RR efficiency. Although the optimal solar-to-fuel (STF) efficiency is only 0.24%, it is worth looking forward to achieving higher STF efficiency by optimizing the photoelectrodes and cell structure design. Another type was proposed by Urbain et al. [53], where they integrated a silicon heterojunction solar cell into the microfluidic flow cells (see Fig. 4b). By realizing the optimal match between the maximum power point of the solar cell and the electrochemical characteristic of CO2 RR, this system presented a stable long-term operation with a STF efficiency of 4.3%.

Fig. 4 a Microfluidic photoelectrochemical cell using TiO2 photoanode, adapted from [51], Copyright 2017, with permission from Elsevier. b Microfluidic photoelectrochemical cell comprising the Cu–Zn cathode and Si/Ni foam photoanode, adapted from [53], Copyright 2017, with permission from RSC

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3 Microfluidic Redox Battery Employing redox couples as the medium to realize the conversion and storage of renewable electricity is another promising candidate [3], because it allows salient merits of large storage capacity, long life cycle, high efficiency in energy conversion and reverse energy production [54]. In principle, renewable energies can be first converted into electricity through photoelectric conversion, wind-electricity conversion, etc. [55–57], and then the electrical energy drives the redox reactions at the anode and cathode to store the electricity in the form of chemical energy. Besides, for solar energy, a photoelectrochemical redox battery (PRB) integrated with dualfunctional photoelectrodes that allows light capture and triggers the redox reactions is another strategy to realize the direct conversion and storage of solar energy [58– 60]. In this section, both electrochemical and photoelectrochemical redox batteries with microfluidic design are summarized and discussed.

3.1 Electrochemical Redox Battery The first microfluidic electrochemical redox battery (MERB) was proposed by Kjeang et al. [61], where the proposed MERB used the membrane-free design and was operated on commonly all-vanadium redox electrolytes. The MERB consists of two dual-pass flow through porous electrodes, two inlets, and two outlets (see Fig. 5a). In both transversal and longitudinal directions, the electrodes and fluid ports are geometrically symmetric. During the charging process, once the anolyte (VO2+ ) and catholyte (V3+ ) were introduced into the two inlets and flowed through the porous electrodes (see Sect. 1 in Fig. 5a), firstly, the anodic oxidation (VO2+ + H2 O → VO2 + + e− + 2H+ ) and cathodic reduction (V3+ + e− → V2+ ) reactions occurred at the upstream of the anode and cathode, respectively. Then, the anolyte and catholyte streams entered the central channel between the two porous electrodes and formed a symmetric and co-laminar flow with obscure mixing between the two streams. Subsequently, when this symmetric and co-laminar flow migrated downstream (i.e., Sect. 2 in Fig. 5a), the two electrolytes separated and flowed in the opposite direction through the porous electrodes, and identical redox reactions took place at the corresponding porous electrodes. Finally, the products that stored the electrical energy in the form of the highest and lowest valence states of the vanadium ions were collected from the two outlets of MERB. Whereas during the discharge process, as can be seen in the annotated image in Fig. 5a, the same flow behaviors can be observed in the proposed MERB except that the anodic oxidation and cathodic reduction reactions turned into V2+ → V3+ + e− and VO2 + + e− + 2H+ → VO2+ + H2 O, respectively. This work demonstrated the feasibility of MERB in electricity storage and release, where a peak power density of 0.3 W/cm2 that equivalent to the previously reported microfluidic fuel cell was achieved. However, it should be pointed out that the asymmetric crossover of the vanadium ions in the anolyte and

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Fig. 5 a Schematic representation and annotated image of a MERB with dual-pass flow through porous electrodes, adapted from [61], Copyright 2021, with permission from RSC. b Schematic of a microfluidic hydrogen-bromine redox flow battery, adapted from [64], Copyright 2013, with permission from Authors. c Zero-gap serpentine microchanneled vanadium–cerium redox battery, adapted from [65], Copyright 2015, with permission from Elsevier. d Stacked MPRB with all-vanadium redox couples, adapted from [66], Copyright 2017, with permission from Elsevier. e Serpentine microchanneled PRB and f the Nyquist plots, adapted from [67], Copyright 2017, with permission from Authors

catholyte still existed, which then led to not only the unbalanced state of charge (SOC) for V2+ and VO2 + [61] but also the decreased reactant concentration after each cycle [62, 63]. Another strategy for the design of MERB was proposed by Buie et al. [64], where they replaced all-vanadium redox couples with hydrogen-bromine redox couples. The assembled redox battery was shown in Fig. 5b; it consisted of a graphite electrode, a commercially available gas diffusion electrode (GDE) with 0.5 mg/cm2 platinum, two graphite current collectors, two PVDF cover plates, and a thick Viton gasket with the thick of 800 µm. During discharge, the GDE was used as the anode to trigger the hydrogen oxidation reaction and the graphite was the cathode for bromine reduction to hydrobromic acid. While during charge, contrarily, hydrogen evolution reaction and bromide oxidation reaction occurred at the gas diffusion cathode and graphite anode, respectively. The electrical energy was then stored in the form of H2 and Br2 . This design allowed multiple advantages. Firstly, although the membrane-free design was also adopted since Br2 is a liquid phase and can be dissolved in an aqueous electrolyte while insoluble gaseous H2 can be transferred through the GDE, severe cross-contamination then was avoided. Meanwhile, ascribing to the fast and reversible kinetics for both reactions during charge and discharge processes, and the

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nonexistent phase change at the liquid-electrode surface, the design limitation caused by bubble formation was eliminated. Last but not least, gaseous H2 and aqueous Br2 were used as the fuel and oxidant, respectively; this allows for an adequate supply of both fuel and oxidant at anode and cathode, which then played a positive role in expanding the mass transfer capacity of the redox battery. These characteristics then not only enabled high efficiency of both high-power density storage and discharge of energy but also decreased the cost of the system and improved the reliability issues associated with the conventional membrane-based system. As result, both the high-power density of about 0.8 mW/cm2 and round-trip energy efficiency of about 66% were enabled at room temperature and atmospheric pressure, which provide a variety of possibilities in designing smaller, inexpensive systems toward portable power systems. Meanwhile, it should be pointed out that there exists a diversity of soluble redox couples employed in redox batteries, including both all-inorganic and organic–inorganic hybrid as well as all-organic redox species [68]. Since specific applications put forward different requirements for the properties of redox species, such as redox potential, pH, and solubility [69]. Hence, to avoid the cross-contamination of the redox species in the electrolytes, membrane-based MERBs have also been investigated in the past few years. Shah et al. [65] performed a zero-gap serpentine microchanneled vanadium–cerium redox battery. As shown in Fig. 5c, the proposed architecture consisted of a Nafion-117 membrane to separate the redox couples, two PTFE gaskets with a thickness of 0.3 mm to ensure tightness, two carbon felt electrodes with the pre-compressed geometry size of 15 mm × 15 mm × 1.5 mm for the redox reactions. The electrodes were electrically connected in direct contact with the serpentine microchannel (width: 0.7 mm) in the carbon end-plates. Benefitting from this zero-gap serpentine architecture, the electroactive species were more uniformly distributed, and thus, the improved mass transfer in both carbon felt electrodes was allowed. Then, a minimized voltage drop across the redox battery can be guaranteed. These merits not only yield a high output power density of 370 mW/cm2 at a SOC of 50% but also ensure a high discharge cell voltage of 1.35 V with an energy efficiency of 70%. A similar serpentine microchanneled structure was also utilized by Gordon et al. [70], while alloxazine 7/8-carboxylic acid and ferri/ferrocyanide pairs were used as the electroactive redox species. Ascribing to the superior chemical stability and fast reaction kinetic (an order of magnitude higher than that of all-vanadium redox couples [71]), high current efficiency of over 99.7% and high round-trip energy efficiency of 74% as well as extremely excellent capacity retention (over 400 charge–discharge cycles) of about 95% were obtained. It is worth noting that the design of the microchannel structure is inherently related to the flow fields in the redox batteries, where the well-constructed flow fields not only decrease the ohmic resistance [72] but also allow the uniform distribution of the redox species over the electrode [73]. To eliminate the limitations of trial-and-error approaches and human intuition for the design of conventional flow fields, recently, machine learning-assisted design of flow fields has also been proposed by Zhao and co-authors [74]. By combining the path generation algorithm and the multiphysics simulation as well as the well-trained convolution neural network regression model, the library of

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flow fields was generated and eight promising candidates were identified through a collaborative screening process. The experimental results validated the screened flow fields yielded higher performance in both electrolyte utilization and limiting current density as well as energy efficiency compared to the above-mentioned serpentine flow field.

3.2 Photoelectrochemical Redox Battery Regarding the photoelectrochemical redox battery (PRB), the first microfluidic design was proposed by Chen et al. [66], where a stacked PRB integrating with two microchambers was used for solar energy conversion and storage. The detailed illustration was shown in Fig. 5d. As shown, firstly, different from the traditional photoanode immersing into the electrolyte, an FTO glass sprayed with TiO2 photocatalyst was used as the photoanode and cover plate, simultaneously. Besides, commercially available silicone sheets with a thickness of 0.5 mm were pre-processed through grooves and simultaneously employed as reaction microchamber and gasket for anolyte and catholyte, respectively. A Nafion membrane and a Pt-coated carbon paper were used to separate the redox species in anolyte and catholyte, and to catalyze cathodic reduction reaction, respectively. A cathode cover plate was employed to realize the fixture and seal it with the help of bolts. This compact and miniature design then emerges the following merits. On the one hand, the internal cell resistance can be significantly reduced by nearly 160% compared to the conventional H-type cell, which means that the energy loss ascribing to the overpotentials of ohmic and concentration losses is decreased [66]. On the other hand, uniform light distribution is guaranteed, resulting in a decrease in light attenuation caused by electrolyte absorption [75]. Meanwhile, benefiting from the decreased transfer path, both the photon and ion as well as reactants transfer can be remarkably intensified. Last but not least, the flexible stackable structure also makes it easier to use photocatalysts with better performance to construct photoanodes, which is particularly important for the improvement of battery performance [76–78]. As a result, using the conventional all-vanadium redox species, the developed microfluidic photoelectrochemical redox battery (MPRB) showed a superior photocatalytic activity and long-term operating stability, where the photocharging current density was increased by 122% for conventional TiO2 photocatalyst. Another structure of MPRB was developed by Liu [67], where they integrated the tortuous serpentine microchannel with a depth of 1 mm and width of 1 mm in the redox battery. As can be seen in Fig. 5e, two acrylic serpentine flow fields machining by a laser cutter were employed as the flow channels for anolyte and catholyte, and a Nafion-117 membrane was used to separate two different vanadium redox species in the electrolytes. Once the electrolytes were introduced into the anodic and cathodic flow channels, forced convective flow then formed. The formation of forced convective flow then directly improved the mass transfer performance, especially at the U-turns of the serpentine microchannels in the MPRB, which then played a significant role in enhancing distributions of

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redox species and the photocatalytic reaction. Such intensification of mass transfer also revealed a great reduction in charge transfer resistance, see the electrochemical impedance spectroscopy results in Fig. 5f. These advantages then contributed to the improvement of photocurrent density and photocharging depth, where a 5 times increase in photocurrent compared to that with stagnant electrolytes was achieved.

4 Microfluidic Fuel Cell High efficient conversion of chemical energy to electricity is an urgent requirement toward the rapid growth of modern society. Because of the excellent fuel-to-electricity conversion efficiency, fuel cell technology then emerges as one of the promising means for energy generation [79]. Especially, with the rapid development of microelectronic devices, new power sources put forward requirements of functionalization, integration, miniaturization, etc., to satisfy the practices [80, 81]. Therefore, microfluidic fuel cells (MFCs) then attract tremendous attention of researchers in the past decades, and various types of MFCs including hydrogen–oxygen MFC [82, 83], methanol MFC [84], formic acid MFC [85], biofuel MFC [86], photocatalytic MFC [87], etc., have been developed. In this part, based on the type of catalysts employed to catalyze the anodic fuel oxidation reactions, three categories of MFCs, i.e., electrochemical fuel cells and biofuel cells as well as photocatalytic fuel cells, are summarized and discussed.

4.1 Electrochemical Fuel Cells Microfluidic electrochemical fuel cells (MEFCs) typically utilize metals or metal alloys, carbon materials, etc., as catalysts [87]. In general, there is a diversity of fuels in MEFCs, commonly used including hydrogen, methanol, ethanol, formic acid, glucose, glycerol, etc., whereas the air (oxygen) is the most common oxidant. Figure 6a shows the schematic diagram of a typical MEFC with a Y-shaped microchannel [88]. The MEFC generally consists of two inlets for the feeding of anolyte dissolved with fuel and the catholyte dissolved with oxidant. The supporting electrolyte of acid or alkaline is essential in both anolyte and catholyte to guarantee ionic conductivity. Once the two electrolytes are introduced into the microchannel, ascribing to the extremely low Reynolds number of microfluidic design, two laminar flows with a stable interface are formed [89]. Because the diffusion at the interface is much slower than the advective flow of the electrolytes, the reactants crossover then is minimized, which allows the transfer of fuel and oxidant from the bulk to the respective electrodes and the normal operation of the fuel cell [90]. This configuration shows many merits including low cost, easy control, flexible fuel selection, etc., while its certainty is equally obvious, such as low fuel utilization (especially at high flow rates), low oxidant concentration, low power output, etc. Therefore, tremendous

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efforts have been made to improve the MEFC technology in the past decades and various MEFC designs were developed [91–94]. Kenis et al. [95] proposed an air-breathing MEFC to optimize the cathodic reaction. As illustrated in Fig. 6b, this design uses formic acid as the fuel and a co-flowing H2 SO4 aqueous solution, whereas the delivery of oxidant oxygen was directly from the air through an air-breathing GDE. Firstly, since the solubility and diffusivity of oxygen in the electrolyte are extremely low, the air-breathing configuration inherently solves the mass transfer limitation. Meanwhile, low oxygen concentration also means insufficient and untimely replenishment in the boundary layer of the cathode, the direct gas phase oxidant supply then avoids this issue. As a result, the cathodic oxygen reduction reaction rate was remarkably intensified. Compared to conventional MEFC without porous air-breathing GDE, the increase in current density emerges in

Fig. 6 a Schematic diagram of a typical MEFC with a Y-shaped microchannel, adapted from [88], Copyright 2021, with permission from Elsevier. b Schematic design of an air-breathing MEFC, adapted from [95], Copyright 2005, with permission from ACS. c Membrane-free MEFC with twophase flow concept, adapted from [96], Copyright 2017, with permission from Authors. d MEFC with 3D cylindrical anode array and air-breathing GDE design, adapted from [85], Copyright 2014, with permission from Elsevier. e MEFC with discrete holes film fueling anode, adapted from [97], Copyright 2021, with permission from Elsevier. f MEFC with a nanoporous membrane, adapted from [98], Copyright 2012, with permission from Elsevier. g Microchanneled fuel cell with insitu cross-linked PEG membrane, adapted from [99], Copyright 2016, with permission from RSC. h Microchanneled fuel cell with third electrolyte stream, adapted from [80], Copyright 2021, with permission from RSC

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more than one order of magnitude. Hashemi et al. [96] utilized the gas–liquid twophase flow to overcome the mass transfer limitations resulting from the low solubility of hydrogen and oxygen in the electrolyte, where controllable hydrogen-anolyte and oxygen-catholyte two-phase flows were formed in the anodic and cathodic flow channels, respectively (see Fig. 6c). Since only a thin liquid film around the center gas phase exists for annular flow, the concentration boundary layer for both fuel and oxidant over the respective electrode surface can be optimized. Hence, compared with the single phase MEFC, where the electrolytes were saturated with fuel and oxidant, the current density and power density of the proposed two-phase flow system remarkably enhanced from about 2.9 mA/cm2 and 0.55 mW/cm2 to 26.6 mA/cm2 and 3.85 mW/cm2 , respectively. Nearly, an order of magnitude intensification was achieved. In addition to those two-dimensional MEFC structures based on the planar electrodes and permeable porous electrodes, three-dimensional structures based on cylindrical electrodes have also been proposed due to the flexible increase in electrode surface area and the intensified reactants transfer [80]. Kjeang et al. [100] developed a 3D graphite rod electrode-based MEFC. This design provided a large active surface area for a given volume, resulting in a high volumetric power density of about 86.4 mW/cm3 . Zhu et al. [85, 101–103] combined the 3D cylindrical anode array and air-breathing GDE design and proposed an air-breathing direct formic acid 3D MEFC (see Fig. 6d), which not only allowed a large anode surface area but also eliminated the oxygen transfer limitation. Therefore, maximum power density of about 112.7 mW/cm3 and net power output of 48.5 mW were obtained. Moreover, to shorten the distance and restrain the fuel crossover, in the same group, Liao et al. [97] utilized a titanium capillary tube with discrete holes to furnish the fuel and replace the graphite rod anode. The anodic catalyst was prepared on the outside surface of the titanium capillary tube (see Fig. 6e). The fuel was introduced into the capillary tube and existed through the discrete holes, where an annular fuel film flow was formed around the catalytic layer and facilitated the fuel transfer. This design not only enabled a more compact cell structure but also improved the mass transfer. Hence, the volumetric power density of about 166.9 mW/cm3 was achieved, which realized a 48.1% improvement compared to that with graphite rod anode. It is worth noting that the long-term stable operation of MEFC significantly relies on the laminar flow, while the fuel crossover is still a severe challenge. Especially, due to the generation of the gaseous product (such as CO2 ), the following formation of gas bubbles at the cathode side not only prevented the fuel transfer to the cathode but also destroyed the laminar flow [85]. Hence, membrane-based MEFCs have also been investigated by researchers to eliminate the fuel crossover and guarantee efficient ion exchange between anode and cathode. Kenis et al. [98] placed a nanoporous membrane at the interface between the anolyte and catholyte to reduce the fuel crossover and ensure unrestricted ion transfer (see Fig. 6f). Combined with the operating condition of low fuel concentration, obscure mixed potential effects were observed. Ho et al. [99] developed an in situ cross-linkage method to form a leak-free polyethylene glycol (PEG) membrane in the microchannel (see Fig. 6g). The PEG membrane directly inhibited the mixing of anolyte and catholyte while

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allowing the free diffusion of glucose molecules and ions. Ye et al. [104] synthesized a Na2 SO4 -imbibed polyacrylamide (PAM) gel and posited it between the two electrodes to function as the ion exchange membrane. Ascribing to the excellent conductivity of PAM gel, both the efficient ion transfer and fuel crossover inhibition were ensured. Meanwhile, as shown in Fig. 6h, using the third electrolyte stream as a virtual membrane between the anodic fuel stream and the cathodic oxidant stream is another promising strategy [105]. This buffer stream not only ensures efficient ion transfer but also prevents the mixing between the fuel and oxidant, thus optimizes fuel utilization and improves the power output [106, 107].

4.2 Biofuel Cells The architecture of microfluidic biofuel cells (MBFCs) is actually similar to that of MEFC, except that the MBFCs make use of biocatalysts such as enzymes and microorganisms for fuel conversion and electricity generation [87]. Figure 7a schematically illustrated the working principle of MMFC. The fuel dissolved in anolyte is catalytically decomposed by the biocatalysts growing on the anode to generate electrons. The oxygen molecules from the catholyte (dissolved oxygen) or air-breathing cathode (atmospheric oxygen) are reduced to water by the electrocatalysts deposited on the cathode. Due to the low cost and high selectivity of biocatalysts, MBFCs have also obtained tremendous support in the past decades.

Fig. 7 a Working principles of a typical co-laminar MBFC, adapted from [86], Copyright 2016, with permission from Elsevier. b Schematic of a MEnzFC, adapted from [109], Copyright 2016, with permission from RSC. c Variable diameter microchanneled MMFC, adapted from [111], Copyright 2015, with permission from Elsevier. d Multiple anolyte insets MMFC, adapted from [112], Copyright 2016, with permission from Elsevier

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For the microfluidic enzymatic fuel cells (MEnzFCs), glucose or lactate is the most common fuel while atmospheric oxygen is typically employed as the oxidant. Hence, glucose oxidase and lactate oxidase extracted from bacteria Aspergillusniger and Aerococcus Williams are two predominant enzymes immobilized on anodes [108]. Shim et al. [109] immobilized glucose oxidase and horseradish peroxidase on the carboxylated polyterthiophene (poly[2,20:50,200-terthiophene-30-(p-benzoic acid)])-assembled nanoparticle layer to use as the counter electrodes and fabricated a microchannel enzymatic fuel cell (see Fig. 7b). A maximum power density of about 0.78 mW/cm2 and an open circuit voltage of about 0.48 V were achieved at a 10 mM glucose electrolyte. Arriaga et al. [110] proposed a flexible air-breathing MEnzFC fueled by glucose in the blood. A cross-linked bioanode consisting of glucose oxidase, glutaraldehyde, multi-walled carbon nanotubes, and Vulcan carbon was employed to catalyze glucose decomposition. In the real blood-feeding test, the developed MEnzFC achieved a maximum power density of about 0.2 mW/cm2 , a current density of 1.07 mA/cm2 , and an open circuit voltage of 0.52 V. Regarding microfluidic microbial fuel cells (MMFCs), electrogenic bacterias or algaes (altogether named microorganisms) are the most used biocatalysts for generating electricity from fuels [113–115]. Generally, microorganisms are living cells and they can simplify the multistep oxidation processes, making them be competent in decomposing complex fuels [108]. Ye et al. [116] fabricated an MMFC by growing biofilm on a graphite plate to be the anode and utilizing the laminar flow to separate the anolyte and catholyte streams. A maximum power density of about 618 mW/ m2 was achieved. Considering that the biofilm distribution is inherently related to the performance of MMFC, and the microchannel geometry of MMFC can play an important role in influencing the biofilm distribution. Yang et al. [111] investigated three different microchannel geometries of the converging channel, straight channel, and diverging channel and demonstrated that the diverging microchannel is favorable to achieving a good and uniform attached biofilm and lower the anode resistance (see Fig. 7c). To improve the colonization of microbial and thus intensify the power generation, Liao et al. [112] reported multiple anolyte inlets MMFC to optimize the biofilm formation by periodically replenishing the boundary anolyte near the anode (see Fig. 7d). This design resulted in a more densely packed biofilm formation and low internal resistance, maximum power density and current density of about 1810 mW/m2 and 7800 mA/m2 were obtained.

4.3 Photocatalytic Fuel Cells Microfluidic photocatalytic fuel cell (MPFC) technology combines the development of photoelectrochemical cell from microfluidic electrochemical fuel cell, where photoelectrodes with dual functions of light absorption and photocatalytic activity replace conventional electrodes [117]. The photoelectrodes in MPFCs commonly use semiconductor materials (i.e., photocatalysts) to generate photo-excited electrons and holes to trigger the oxidant reduction reaction and fuel oxidation reaction,

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respectively. MPFCs can operate with one or two electrodes under irradiation, named single-photoelectrode system (photoanode-cathode type, anode-photocathode type) and dual-photoelectrodes system (photoanode-photocathode type) [118–120]. In addition to the inherent advantages of miniaturized design, MPFCs also possess the salient merits of cost-effective and pragmatic, because MPFCs use cheap semiconductor materials instead of metal catalysts and they can degrade complex organic compounds such as wastewater [121]. Therefore, MPFCs also become one of the most promising candidates for simultaneously generating renewable electricity and treating wastewater. Guima et al. [122] proposed a 3D-printed MPFC integrated with low-content PtOx / Pt-BiVO4 photoanode and Pt/C cathode (see Fig. 8a). By introducing the anolyte containing fuel (model pollutant of rhodamine B) and catholyte containing dissolved oxygen into the MPFC, two laminar flows were formed in the microchannel. Once the light was shed on the photoanode, the MPFC allowed maximum power density and current density of 0.48 mW/cm2 and 4.09 mA/cm2 , respectively. During the longterm operation, a 73.6% rhodamine B degradation efficiency was also obtained. To improve the oxidant supply, as shown in Fig. 8b, an air-breathing MPFC was also proposed by Li et al. [123]. The MPFC employed a CdS/ZnS composite photosensitized TiO2 photoanode and a Pt-coated carbon paper gas diffusion cathode. Due to the introduction of the photosensitizer, on the one hand, the spectral response was extended from the UV region to the visible region. On the other hand, the charge carrier transfer and separation were also promoted. Employing the glucose and methylene blue (MB) solutions as the wastewater, a peak power density of about 0.58 mW/cm2 for glucose and maximum degradation efficiency of 83.9% for methylene blue were achieved. Besides the above-mentioned membrane-free MPFCs, membrane-based architecture has also been developed to satisfy diverse applications. A typical type is membrane electrode assembled microfluidic photocatalytic fuel cell (MEA-MPFC) proposed by Chen and co-authors [124]. As illustrated in Fig. 8c, the photoanode and membrane, as well as the air-breathing cathode, are sandwiched together by a hot-press method. By integrating two current collectors that pre-fabricated serpentine flow field and two PMMA cover plates, such a compact and flexible design played positive roles in enhancing the ion and oxygen transfer and eliminating the fuel crossover. An optimized power density of about 1.09 mW/cm2 was achieved when using alcohol pollutants as fuel [124–126]. Regarding the dual-photoelectrodes MPFCs, the architecture is similar to that of single-photoelectrode MPFCs and MEFCs. Li et al. [127] employed a Cu2 O photocathode and a BiVO4 photoanode to fabricate a dual-photoelectrode MPFC, where the two photoelectrodes were shoulder-to-shoulder placed in a microchamber, see Fig. 8d. However, because the gaseous oxygen was dissolved in the electrolytecontaining model organic waste of MB as fuel and Na2 SO4 as supporting electrolytes, the low oxygen concentration and oxygen transfer limitation still existed. By utilizing a 3D oriented CuS/Cu2 O/Cu nanowire photocathode and a CdS/ZnS sensitized TiO2 photoanode, He et al. [128] proposed an air-breathing dual-photoelectrodes MPFC (see Fig. 8e). Through the hydrophobic treatment, the electrolyte leakage and flooding of the photocathode can be prevented while the oxygen transfer from the atmospheric

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Fig. 8 a 3D printed membrane-free MPFC with Pt-BiVO4 /CP photoanode, adapted from [122], Copyright 2020, with permission from ACS. b Air-breathing MPFC with CdS/ZnS composite photosensitized TiO2 photoanode [123], Copyright 2014, with permission from RSC. c Typical membrane electrode assembled MPFC, adapted from [124], Copyright 2017, with permission from Elsevier. d Dual-photoelectrodes MPFC with Cu2 O photocathode and BiVO4 photoanode, adapted from [127], Copyright 2021, with permission from Elsevier. e Air-breathing dual-photoelectrodes MPFC, adapted from [128], Copyright 2021, with permission from RSC

environment to the active site in the photocathode can be optimized. Utilizing ethanol as the model organic pollutant and Na2 SO4 as the supporting electrolyte, the developed MPFC yielded an optimized open circuit voltage of 1.17 V, a maximum current density of 4.05 mA/cm2 , and a peak power density of 1.13 mW/cm2 under a simulated sunlight intensity of 100 mW/cm2 . Compared with the co-laminar flow using the dissolved oxygen supply as mentioned above, an extremely significant intensification in power generation can be achieved.

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5 Conclusion and Perspectives This chapter comprehensively discusses the development and recent progress of microfluidic flow cells for energy conversion and utilization. The applications of microfluidic flow cells in both hydrocarbon fuel production and renewable electricity storage, as well as electricity generation, are summarized. Although many effective methods and various novel architectures have been proposed to improve the performance of respective microfluidic flow cells, almost all of them were based on the design and optimization of laboratory scale. However, to date, wide market penetration and global deployment of these microfluidic flow cells still face many obstacles. Regarding some generic issues, first, the commercial scale microfluidic flow cells integrate multiple monomer laboratory scale microfluidic flow cells, the matching of current and voltage, the management of water and heat, and the long-term operation stability, among other critical aspects, should put more attention. Especially, renewable energies are directly employed as the primary energy sources in the largescale system, and the overall optimization between the large-scale renewable energy capture and conversion as well as utilization in the integrated microfluidic flow cells should be comprehensively investigated. Besides, the energy conversion and utilization efficiencies of these microfluidic flow cells necessitate further enhancement to satisfy the commercial demand, especially for those microfluidic flow cells in fuel production and solar energy conversion and storage. In this aspect, the detailed energy loss analysis and optimized system design as well as the advanced materials screen and synthesis are worth attracting more attention. Last but not least, the capital cost of the microfluidic flow cells is still too high to satisfy the requirements of commercial applications. The costs consist of not only the materials like the catalysts, the electrolytes, the membranes, etc. but also the fabrication and integrated accessories. Hence, the development of cheaper materials and more sophisticated manufacturing processes IS also urgently required. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51888103, 52006101, 51976090, 52006103), Scientific and Technological Innovation Project of Carbon Emission Peak and Carbon Neutrality of Jiangsu Province (No. BE2022024), and the Natural Science Foundation of Jiangsu Province (No. BK20200491).

References 1. Feng H, Liu D, Zhang Y, Shi X, Esan OC, Li Q, Chen R, An L (2022) Advances and challenges in photoelectrochemical redox batteries for solar energy conversion and storage. Adv Energy Mater 12(24):2200469 2. Deng K, Feng H, Liu D, Zhang Y, Li Q (2021) A high-pressure artificial photosynthetic device: pumping carbon dioxide as well as achieving selectivity. J Mater Chem A 9:3961–3967 3. Zeng Q, Lai Y, Jiang L, Liu F, Hao X, Wang L, Green MA (2020) Integrated photorechargeable energy storage system: next-generation power source driving the future. Adv Energy Mater 10(14):1903930

Microfluidic Flow Cells for Energy Conversion and Utilization

193

4. Schleussner C-F, Rogelj J, Schaeffer M, Lissner T, Licker R, Fischer EM, Knutti R, Levermann A, Frieler K, Hare W (2016) Science and policy characteristics of the Paris agreement temperature goal. Nat Clim Chang 6(9):827–835 5. Zou C, Zhao Q, Zhang G, Xiong B (2016) Energy revolution: From a fossil energy era to a new energy era. Nat Gas Ind B 3(1):1–11 6. Boutin E, Patel M, Kecsenovity E, Suter S, Janáky C, Haussener S (2022) Photoelectrochemical conversion of CO2 under concentrated sunlight enables combination of high reaction rate and efficiency. Adv Energy Mater n/a(n/a):2200585 7. Ji C, Liu D, Zhang C, Jay Guo L (2020) Ultrathin-metal-film-based transparent electrodes with relative transmittance surpassing 100%. Nat Commun 11(1):3367 8. Lin G, Almakrami H, Emran H, Ruthen A, Hu J, Wei Z, Liu F (2021) Enhanced conversion efficiency enabled by species migration in direct solar energy storage. ChemPhysChem 22(12):1193–1200 9. Chu S, Majumdar A (2012) Opportunities and challenges for a sustainable energy future. Nature 488(7411):294–303 10. Nguyen N-T, Wereley ST, Shaegh SAM (2019) Fundamentals and applications of microfluidics, 3rd edn. Artech House, Boston 11. Jiao L, Wu Y, Hu Y, Wu H, Xu Z, Han L, Guo Q, Li D, Chen R (2021) Oil/water microreactor with a core–shell wetting state on a SOB/OL-SHB/HL multilevel patterned surface. J Phys Chem C 125(50):27771–27783 12. Feng H, Zhu X, Chen R, Liao Q, Liu J, Li L (2016) High-performance gas–liquid– solid microreactor with polydopamine functionalized surface coated by Pd nanocatalyst for nitrobenzene hydrogenation. Chem Eng J 306:1017–1025 13. Feng H, Jiao X, Chen R, Zhu X, Liao Q, Ye D, Zhang B, Zhang W (2019) A microfluidic all-vanadium photoelectrochemical cell with the N-doped TiO2 photoanode for enhancing the solar energy storage. J Power Sources 419:162–170 14. Modestino MA, Diaz-Botia CA, Haussener S, Gomez-Sjoberg R, Ager JW, Segalman RA (2013) Integrated microfluidic test-bed for energy conversion devices. PCCP 15(19):7050– 7054 15. Shi T, Liu D, Feng H, Zhang Y, Li Q (2022) Evolution of triple-phase interface for enhanced electrochemical CO2 reduction. Chem Eng J 431:134348 16. Deng K, Zhang Y, Feng H, Liu N, Ma L, Duan J, Wang Y, Liu D, Li Q (2022) Efficient solar fuel production with a high-pressure CO2-captured liquid feed. Sci Bull 17. Xi W, Feng H, Liu D, Chen L, Zhang Y, Li Q (2021) Electrocatalytic generation and tuning of ultra-stable and ultra-dense nanometre bubbles: an in situ molecular dynamics study. Nanoscale 13(25):11242–11249 18. Liu M, Chen Y, Su J, Shi J, Wang X, Guo L (2016) Photocatalytic hydrogen production using twinned nanocrystals and an unanchored NiSx co-catalyst. Nat Energy 1(11):16151 19. Zhao D, Wang Y, Dong C-L, Huang Y-C, Chen J, Xue F, Shen S, Guo L (2021) Boron-doped nitrogen-deficient carbon nitride-based Z-scheme heterostructures for photocatalytic overall water splitting. Nat Energy 6:388–397 20. Fang Y, Hou Y, Fu X, Wang X (2022) Semiconducting polymers for oxygen evolution reaction under light illumination. Chem Rev 122(3):4204–4256 21. Li D, Kassymova M, Cai X, Zang S-Q, Jiang H-L (2020) Photocatalytic CO2 reduction over metal-organic framework-based materials. Coord Chem Rev 412:213262 22. Ran J, Jaroniec M, Qiao S-Z (2018) Cocatalysts in semiconductor-based Photocatalytic CO2 reduction: achievements, challenges, and opportunities. Adv Mater 30(7):1704649 23. Angulo A, van der Linde P, Gardeniers H, Modestino M, Fernández Rivas D (2020) Influence of bubbles on the energy conversion efficiency of electrochemical reactors. Joule 4(3):555–579 24. Shi T, Feng H, Liu D, Zhang Y, Li Q (2022) High-performance microfluidic electrochemical reactor for efficient hydrogen evolution. Appl Energ 325:119887 25. Hashemi SMH, Karnakov P, Hadikhani P, Chinello E, Litvinov S, Moser C, Koumoutsakos P, Psaltis D (2019) A versatile and membrane-less electrochemical reactor for the electrolysis of water and brine. Energ Environ Sci 12(5):1592–1604

194

H. Feng et al.

26. Oruc ME, Desai AV, Nuzzo RG, Kenis PJA (2016) Design, fabrication, and characterization of a proposed microchannel water electrolyzer. J Power Sources 307:122–128 27. Davis JT, Esposito DV (2017) Limiting photocurrent analysis of a wide channel photoelectrochemical flow reactor. J Phys D Appl Phys 50(8):084002 28. Tan X, Yu C, Ren Y, Cui S, Li W, Qiu J (2021) Recent advances in innovative strategies for the CO2 electroreduction reaction. Energ Environ Sci 14(2):765–780 29. Kibria MG, Edwards JP, Gabardo CM, Dinh C-T, Seifitokaldani A, Sinton D, Sargent EH (2019) Electrochemical CO2 reduction into chemical feedstocks: from mechanistic electrocatalysis models to system design. Adv Mater 31(31):1807166 30. Zhang F, Chen C, Tang Y, Cheng Z (2020) CO2 reduction in a microchannel electrochemical reactor with gas–liquid segmented flow. Chem Eng J 392:124798 31. Lin J, Yan S, Zhang C, Hu Q, Cheng Z (2022) Hydrophobic electrode design for CO2 electroreduction in a microchannel reactor. ACS Appl Mater Inter 14(6):8623–8632 32. Ma D, Jin T, Xie K, Huang H (2021) An overview of flow cell architecture design and optimization for electrochemical CO2 reduction. J Mater Chem A 9(37):20897–20918 33. Xu Y, Miao RK, Edwards JP, Liu S, O’Brien CP, Gabardo CM, Fan M, Huang JE, Robb A, Sargent EH, Sinton D (2022) A microchanneled solid electrolyte for carbon-efficient CO2 electrolysis. Joule 6(6):1333–1343 34. Yang Y, Li F (2021) Reactor design for electrochemical CO2 conversion toward large-scale applications. Curr Opin Green Sustain Chem 27:100419 35. Weekes DM, Salvatore DA, Reyes A, Huang A, Berlinguette CP (2018) Electrolytic CO2 reduction in a flow cell. Acc Chem Res 51(4):910–918 36. Endr˝odi B, Bencsik G, Darvas F, Jones R, Rajeshwar K, Janáky C (2017) Continuous-flow electroreduction of carbon dioxide. Prog Energy Combust Sci 62:133–154 37. Whipple DT, Finke EC, Kenis PJA (2010) Microfluidic reactor for the electrochemical reduction of carbon dioxide: the effect of pH. Electrochem Solid-State Lett 13(9):B109 38. Verma S, Lu X, Ma S, Masel RI, Kenis PJA (2016) The effect of electrolyte composition on the electroreduction of CO2 to CO on Ag based gas diffusion electrodes. PCCP 18(10):7075–7084 39. Jhong H-RM, Brushett FR, Kenis PJA (2013) The effects of catalyst layer deposition methodology on electrode performance. Adv Energy Mater 3(5):589–599 40. Kim B, Hillman F, Ariyoshi M, Fujikawa S, Kenis PJA (2016) Effects of composition of the micro porous layer and the substrate on performance in the electrochemical reduction of CO2 to CO. J Power Sources 312:192–198 41. Burdyny T, Smith WA (2019) CO2 reduction on gas-diffusion electrodes and why catalytic performance must be assessed at commercially-relevant conditions. Energ Environ Sci 12(5):1442–1453 42. McCallum C, Gabardo CM, O’Brien CP, Edwards JP, Wicks J, Xu Y, Sargent EH, Sinton D (2021) Reducing the crossover of carbonate and liquid products during carbon dioxide electroreduction. Cell Rep Phys Sci 2(8):100522 43. Cofell ER, Nwabara UO, Bhargava SS, Henckel DE, Kenis PJA (2021) Investigation of electrolyte-dependent carbonate formation on gas diffusion electrodes for CO2 electrolysis. ACS Appl Mater Inter 13(13):15132–15142 44. Xu Y, Edwards JP, Liu S, Miao RK, Huang JE, Gabardo CM, O’Brien CP, Li J, Sargent EH, Sinton D (2021) Self-cleaning CO2 reduction systems: unsteady electrochemical forcing enables stability. ACS Energy Lett 6(2):809–815 45. Rabinowitz JA, Kanan MW (2020) The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem. Nat Commun 11(1):5231 46. Ma M, Clark EL, Therkildsen KT, Dalsgaard S, Chorkendorff I, Seger B (2020) Insights into the carbon balance for CO2 electroreduction on Cu using gas diffusion electrode reactor designs. Energ Environ Sci 13(3):977–985 47. Castro S, Albo J, Irabien A (2018) Photoelectrochemical reactors for CO2 utilization. ACS Sustain Chem Eng 6(12):15877–15894 48. Ješi´c D, Lašiˇc Jurkovi´c D, Pohar A, Suhadolnik L, Likozar B (2021) Engineering photocatalytic and photoelectrocatalytic CO2 reduction reactions: mechanisms, intrinsic kinetics, mass transfer resistances, reactors and multi-scale modelling simulations. Chem Eng J 407:126799

Microfluidic Flow Cells for Energy Conversion and Utilization

195

49. Kan M, Wang Q, Hao S, Guan A, Chen Y, Zhang Q, Han Q, Zheng G (2022) System engineering enhances photoelectrochemical CO2 reduction. J Phys Chem C 126(4):1689–1700 50. Kalamaras E, Belekoukia M, Tan JZY, Xuan J, Maroto-Valer MM, Andresen John M (2019) A microfluidic photoelectrochemical cell for solar-driven CO2 conversion into liquid fuels with CuO-based photocathodes. Faraday Discuss 215:329–344 51. Irtem E, Hernández-Alonso MD, Parra A, Fàbrega C, Penelas-Pérez G, Morante JR, Andreu T (2017) A photoelectrochemical flow cell design for the efficient CO2 conversion to fuels. Electrochim Acta 240:225–230 52. Cheng J, Zhang M, Wu G, Wang X, Zhou J, Cen K (2014) Photoelectrocatalytic reduction of CO2 into chemicals using PT-modified reduced graphene oxide combined with Pt-modified TiO2 nanotubes. Environ Sci Technol 48(12):7076–7084 53. Urbain F, Tang P, Carretero NM, Andreu T, Gerling LG, Voz C, Arbiol J, Morante JR (2017) A prototype reactor for highly selective solar-driven CO2 reduction to synthesis gas using nanosized earth-abundant catalysts and silicon photovoltaics. Energ Environ Sci 10(10):2256– 2266 54. Soloveichik GL (2011) Battery technologies for large-scale stationary energy storage. Annu Rev Chem Biomol Eng 2:503–527 55. Yang Y, Hoang MT, Bhardwaj A, Wilhelm M, Mathur S, Wang H (2022) Perovskite solar cells based self-charging power packs: fundamentals, applications and challenges. Nano Energy 94:106910 56. Johst M, Rothstein B (2014) Reduction of cooling water consumption due to photovoltaic and wind electricity feed-in. Renew Sust Energ Rev 35:311–317 57. Hu Y, Cheng H (2013) Development and bottlenecks of renewable electricity generation in China: a critical review. Environ Sci Technol 47(7):3044–3056 58. Wei Z, Liu D, Hsu C, Liu F (2014) All-vanadium redox photoelectrochemical cell: an approach to store solar energy. Electrochem Commun 45:79–82 59. Liu D, Wei Z, Shen Y, Sajjad SD, Hao Y, Liu F (2015) Ultra-long electron lifetime induced efficient solar energy storage by an all-vanadium photoelectrochemical storage cell using methanesulfonic acid. J Mater Chem A 3(40):20322–20329 60. Almakrami H, Wei Z, Lin G, Jin X, Agar E, Liu F (2020) An integrated solar cell with built-in energy storage capability. Electrochim Acta 349:136368 61. Lee JW, Goulet M-A, Kjeang E (2013) Microfluidic redox battery. Lab Chip 13(13):2504– 2507 62. Goulet M-A, Kjeang E (2014) Reactant recirculation in electrochemical co-laminar flow cells. Electrochim Acta 140:217–224 63. Goulet M-A, Ibrahim OA, Kim WHJ, Kjeang E (2017) Maximizing the power density of aqueous electrochemical flow cells with in operando deposition. J Power Sources 339:80–85 64. Braff WA, Bazant MZ, Buie CR (2013) Membrane-less hydrogen bromine flow battery. Nat Commun 4(1):2346 65. Leung PK, Mohamed MR, Shah AA, Xu Q, Conde-Duran MB (2015) A mixed acid based vanadium–cerium redox flow battery with a zero-gap serpentine architecture. J Power Sources 274:651–658 66. Jiao X, Chen R, Zhu X, Liao Q, Ye D, Zhang B, An L, Feng H, Zhang W (2017) A microfluidic all-vanadium photoelectrochemical cell for solar energy storage. Electrochim Acta 258:842– 849 67. Wei Z, Shen Y, Liu D, Liu F (2017) An all-vanadium continuous-flow photoelectrochemical cell for extending state-of-charge in solar energy storage. Sci Rep 7(1):629 68. Winsberg J, Hagemann T, Janoschka T, Hager MD (2017) Redox-flow batteries: from metals to organic redox-active materials. Angew Chem Int Ed 56(3):686–711 69. Wedege K, Bae D, Smith WA, Al M, Bentien A (2018) Solar redox flow batteries with organic redox couples in aqueous electrolytes: a minireview. J Phys Chem C 122(45):25729–25740 70. Lin K, Gómez-Bombarelli R, Beh ES, Tong L, Chen Q, Valle A, Aspuru-Guzik A, Aziz MJ, Gordon RG (2016) A redox-flow battery with an alloxazine-based organic electrolyte. Nat Energy 1(9):16102

196

H. Feng et al.

71. Weber AZ, Mench MM, Meyers JP, Ross PN, Gostick JT, Liu Q (2011) Redox flow batteries: a review. J Appl Electrochem 41(10):1137 72. Liu QH, Grim GM, Papandrew AB, Turhan A, Zawodzinski TA, Mench MM (2012) High performance vanadium redox flow batteries with optimized electrode configuration and membrane selection. J Electrochem Soc 159(8):A1246–A1252 73. Zhou XL, Zhao TS, An L, Zeng YK, Wei L (2017) Critical transport issues for improving the performance of aqueous redox flow batteries. J Power Sources 339:1–12 74. Wan S, Jiang H, Guo Z, He C, Liang X, Djilali N, Zhao T (2022) Machine learning-assisted design of flow fields for redox flow batteries. Energ Environ Sci 75. Lin Y, Feng H, Chen R, Zhang B, An L (2020) One-dimensional TiO2 nanotube array photoanode for a microfluidic all-vanadium photoelectrochemical cell for solar energy storage. Catal Sci Technol 10(13):4352–4361 76. Lin Y, Feng H, Chen R, Ye D, Zhang B, Yu Y, Li J (2019) A microfluidic all-vanadium photoelectrochemical cell with a full-spectrum-responsive Ti2 O3 photoanode for efficient solar energy storage. Sci China Technol Sci 62(9):1628–1635 77. Li J, Lin Y, Chen R, Zhu X, Ye D, Yang Y, Yu Y, Wang D, Liao Q (2021) Solar energy storage by a microfluidic all-vanadium photoelectrochemical flow cell with self-doped TiO2 photoanode. J Energy Storage 43:103228 78. Zhang W, Lin Y, Chen R, Zhu X, Ye D, Yang Y, Li J, Yu Y, Liao Q (2021) Self-doped TiO2 nanotube array photoanode for microfluidic all-vanadium photoelectrochemical flow battery. J Electroanal Chem 897:115598 79. Kirubakaran A, Jain S, Nema RK (2009) A review on fuel cell technologies and power electronic interface. Renew Sust Energ Rev 13(9):2430–2440 80. Zhou Y, Zhu X, Yang Y, Ye D, Chen R, Liao Q (2021) Route towards high-performance microfluidic fuel cells: a review. Sustain Energ Fuels 5(11):2840–2859 81. Ito T, Kunimatsu M, Kaneko S, Ohya S, Suzuki K (2007) Microfluidic device for the detection of glucose using a micro direct methanol fuel cell as an amperometric detection power source. Anal Chem 79(4):1725–1730 82. Mitrovski SM, Nuzzo RG (2006) A passive microfluidic hydrogen–air fuel cell with exceptional stability and high performance. Lab Chip 6(3):353–361 83. Brushett FR, Thorum MS, Lioutas NS, Naughton MS, Tornow C, Jhong H-RM, Gewirth AA, Kenis PJA (2010) A carbon-supported copper complex of 3,5-diamino-1,2,4-triazole as a cathode catalyst for alkaline fuel cell applications. J Am Chem Soc 132(35):12185–12187 84. Sun F, He H, Huo W (2015) Polymer separator and low fuel concentration to minimize crossover in microfluidic direct methanol fuel cells. Int J Energy Res 39(5):643–647 85. Zhu X, Zhang B, Ye D-D, Li J, Liao Q (2014) Air-breathing direct formic acid microfluidic fuel cell with an array of cylinder anodes. J Power Sources 247:346–353 86. Yang Y, Ye D, Li J, Zhu X, Liao Q, Zhang B (2016) Microfluidic microbial fuel cells: from membrane to membrane free. J Power Sources 324:113–125 87. Safdar M, Jänis J, Sánchez S (2016) Microfluidic fuel cells for energy generation. Lab Chip 16(15):2754–2758 88. Wang Y, Luo S, Kwok HYH, Pan W, Zhang Y, Zhao X, Leung DYC (2021) Microfluidic fuel cells with different types of fuels: a prospective review. Renew Sust Energ Rev 141:110806 89. Ferrigno R, Stroock AD, Clark TD, Mayer M, Whitesides GM (2002) Membraneless vanadium redox fuel cell using laminar flow. J Am Chem Soc 124(44):12930–12931 90. Kjeang E, Djilali N, Sinton D (2009) Microfluidic fuel cells: a review. J Power Sources 186(2):353–369 91. Lim KG, Palmore GTR (2007) Microfluidic biofuel cells: The influence of electrode diffusion layer on performance. Biosens Bioelectron 22(6):941–947 92. Ward K, Fan ZH (2015) Mixing in microfluidic devices and enhancement methods. J Micromech Microeng 25(9):094001 93. Marschewski J, Jung S, Ruch P, Prasad N, Mazzotti S, Michel B, Poulikakos D (2015) Mixing with herringbone-inspired microstructures: overcoming the diffusion limit in co-laminar microfluidic devices. Lab Chip 15(8):1923–1933

Microfluidic Flow Cells for Energy Conversion and Utilization

197

94. Yoon SK, Fichtl GW, Kenis PJA (2006) Active control of the depletion boundary layers in microfluidic electrochemical reactors. Lab Chip 6(12):1516–1524 95. Jayashree RS, Gancs L, Choban ER, Primak A, Natarajan D, Markoski LJ, Kenis PJA (2005) Air-breathing laminar flow-based microfluidic fuel cell. J Am Chem Soc 127(48):16758– 16759 96. Hashemi SMH, Neuenschwander M, Hadikhani P, Modestino MA, Psaltis D (2017) Membrane-less micro fuel cell based on two-phase flow. J Power Sources 348:212–218 97. Zhu X, Zhou Y, Ye D-D, Chen R, Zhang T, Liao Q (2021) Discrete-holes film fueling anode heads for high performance air-breathing microfluidic fuel cell. J Power Sources 482:228966 98. Thorson MR, Brushett FR, Timberg CJ, Kenis PJA (2012) Design rules for electrode arrangement in an air-breathing alkaline direct methanol laminar flow fuel cell. J Power Sources 218:28–33 99. Ho WF, Lim KM, Yang KL (2016) In situ formation of leak-free polyethylene glycol (PEG) membranes in microfluidic fuel cells. Lab Chip 16(24):4725–4731 100. Kjeang E, McKechnie J, Sinton D, Djilali N (2007) Planar and three-dimensional microfluidic fuel cell architectures based on graphite rod electrodes. J Power Sources 168(2):379–390 101. Zhang B, Ye D, Li J, Zhu X, Liao Q (2012) Electrodeposition of Pd catalyst layer on graphite rod electrodes for direct formic acid oxidation. J Power Sources 214:277–284 102. Zhang B, Ye D-D, Li J, Zhu X, Liao Q (2015) Air-breathing microfluidic fuel cells with a cylinder anode operating in acidic and alkaline media. Electrochim Acta 177:264–269 103. Ye D-D, Zhang B, Zhu X, Sui P-C, Djilali N, Liao Q (2015) Computational modeling of alkaline air-breathing microfluidic fuel cells with an array of cylinder anodes. J Power Sources 288:150–159 104. Liu Z, Ye D, Zhu X, Wang S, Chen R, Yang Y, Liao Q (2021) A self-pumping microfluidic fuel cell powered by formate with Pd coated carbon cloth electrodes. J Power Sources 490:229553 105. Phirani J, Basu S (2008) Analyses of fuel utilization in microfluidic fuel cell. J Power Sources 175(1):261–265 106. Xuan J, Leung MKH, Leung DYC, Ni M, Wang H (2011) Hydrodynamic focusing in microfluidic membraneless fuel cells: breaking the trade-off between fuel utilization and current density. Int J Hydrogen Energy 36(17):11075–11084 107. Ebrahimi Khabbazi A, Richards AJ, Hoorfar M (2010) Numerical study of the effect of the channel and electrode geometry on the performance of microfluidic fuel cells. J Power Sources 195(24):8141–8151 108. Yang Y, Liu T, Tao K, Chang H (2018) Generating electricity on chips: microfluidic biofuel cells in perspective. Ind Eng Chem Res 57(8):2746–2758 109. Noh H-B, Shim Y-B (2016) Catalytic activity of polymerized self-assembled artificial enzyme nanoparticles: applications to microfluidic channel-glucose biofuel cells and sensors. J Mater Chem A 4(7):2720–2728 110. Dector A, Escalona-Villalpando RA, Dector D, Vallejo-Becerra V, Chávez-Ramírez AU, Arriaga LG, Ledesma-García J (2015) Perspective use of direct human blood as an energy source in air-breathing hybrid microfluidic fuel cells. J Power Sources 288:70–75 111. Yang Y, Ye D, Li J, Zhu X, Liao Q, Zhang B (2015) Biofilm distribution and performance of microfluidic microbial fuel cells with different microchannel geometries. Int J Hydrogen Energy 40(35):11983–11988 112. Yang Y, Ye D, Liao Q, Zhang P, Zhu X, Li J, Fu Q (2016) Enhanced biofilm distribution and cell performance of microfluidic microbial fuel cells with multiple anolyte inlets. Biosens Bioelectron 79:406–410 113. Cho H-M, Ha H, Ahn Y (2022) Co-laminar microfluidic microbial fuel cell integrated with electrophoretically deposited carbon nanotube flow-over electrode. ACS Sustain Chem Eng 10(5):1839–1846 114. Gong L, Abbaszadeh Amirdehi M, Sonawane JM, Jia N, Torres de Oliveira L, Greener J (2022) Mainstreaming microfluidic microbial fuel cells: a biocompatible membrane grown in situ improves performance and versatility. Lab Chip 22(10):1905–1916

198

H. Feng et al.

115. Yang S, Xiao N, Wang J, Zhang B, Huang JJ (2022) Development of miniature self-powered single-chamber microbial fuel cell and its response mechanism to copper ions in high and trace concentration. Sci Total Environ 834:155367 116. Ye D, Yang Y, Li J, Zhu X, Liao Q, Deng B, Chen R (2013) Performance of a microfluidic microbial fuel cell based on graphite electrodes. Int J Hydrogen Energy 38(35):15710–15715 117. Queiroz BD, Fernandes JA, Martins CA, Wender H (2022) Photocatalytic fuel cells: from batch to microfluidics. J Environ Chem Eng 10(3):107611 118. Liao Q, Li L, Chen R, Zhu X, Wang H, Ye D, Cheng X, Zhang M, Zhou Y (2015) Respective electrode potential characteristics of photocatalytic fuel cell with visible-light responsive photoanode and air-breathing cathode. Int J Hydrogen Energy 40(46):16547–16555 119. Li M, Liu Y, Dong L, Shen C, Li F, Huang M, Ma C, Yang B, An X, Sand W (2019) Recent advances on photocatalytic fuel cell for environmental applications—the marriage of photocatalysis and fuel cells. Sci Total Environ 668:966–978 120. Chen X, Chen R, Zhu X, Liao Q, Zhang Y, Ye D, Zhang B, Yu Y, Li J (2019) A solar responsive cubic nanosized CuS/Cu2 O/Cu photocathode with enhanced photoelectrochemical activity. J Catal 372:182–192 121. Li L, Xue S, Chen R, Liao Q, Zhu X, Wang Z, He X, Feng H, Cheng X (2015) Performance characteristics of a membraneless solar responsive photocatalytic fuel cell with an air-breathing cathode under different fuels and electrolytes and air conditions. Electrochim Acta 182:280–288 122. Guima K-E, Gomes LE, Alves Fernandes J, Wender H, Martins CA (2020) Harvesting energy from an organic pollutant model using a new 3D-printed microfluidic photo fuel cell. ACS Appl Mater Inter 12(49):54563–54572 123. Li L, Wang G, Chen R, Zhu X, Wang H, Liao Q, Yu Y (2014) Optofluidics based microphotocatalytic fuel cell for efficient wastewater treatment and electricity generation. Lab Chip 14(17):3368–3375 124. Chen M, Chen R, Zhu X, Liao Q, An L, Ye D, Zhou Y, He X, Zhang W (2017) A membrane electrode assembled photoelectrochemical cell with a solar-responsive cadmium sulfide-zinc sulfide-titanium dioxide/mesoporous silica photoanode. J Power Sources 371:96–105 125. Feng H, Chen M, Chen R, Zhu X, Liao Q, Ye D, Zhang B, An L, Yu Y, Zhang W (2020) Anion-exchange membrane electrode assembled photoelectrochemical cell with a visible light responsive photoanode for simultaneously treating wastewater and generating electricity. Ind Eng Chem Res 59(1):137–145 126. He X, Chen M, Chen R, Zhu X, Liao Q, Ye D, Zhang B, Zhang W, Yu Y (2018) A solar responsive photocatalytic fuel cell with the membrane electrode assembly design for simultaneous wastewater treatment and electricity generation. J Hazard Mater 358:346–354 127. He Y, Zhang Y, Li L, Shen W, Li J (2021) An efficient optofluidic photocatalytic fuel cell with dual-photoelectrode for electricity generation from wastewater treatment. J Solid State Chem 293:121780 128. He X, Chen X, Chen R, Zhu X, Liao Q, Ye D, Yu Y, Zhang W, Li J (2021) A 3D oriented CuS/Cu2 O/Cu nanowire photocathode. J Mater Chem A 9(11):6971–6980

Flow Cells for CO2 Reduction Qing Xia, Mingcong Tang, and Xiao Zhang

1 Introduction The global climate has changed continuously, and the mean temperature of Earth surface has been unprecedentedly increasing, at an average rate of 0.012 °C per year since the 1950s [1]. One of the main reasons for this unexpected phenomenon is the emission of greenhouse gases, including CO2 , CO, and CH4 et al. into the atmosphere [2]. CO2 is one of the primary greenhouse gas, mostly generated by the combustion of fossil fuels and human activities. According to the studies of the Global Carbon Project, the annual global fossil CO2 emissions have increased from 15 to 36.6 Gt between 1970 and 2019, and a drop of about 2.6 Gt emission in 2020 due to the COVID-19 pandemic (Fig. 1). To reach the goal set in the Paris Agreement, an additional 1–2 Gt should be cut off per year in the 2020s and beyond based on 2020 levels (34 Gt), keeping the global temperature upward below 2 °C above preindustrial levels by the end of century [3]. Electrochemical reduction of CO2 , powered by clean electricity, is promising in converting waste CO2 back into valuable chemicals/fuels, such as CO, HCOOH, CH4 , and CH3 CH2 OH et al., while mitigating climate change [8, 9]. Over the past few decades, tremendous efforts and remarkable advancements have been made in the electrochemical CO2 reduction reaction (CO2 RR), targeting high energy efficiency and high Faradic efficiency (FE) for desired products. Among these efforts, electrochemical reactor design and fabrication is particularly effective in influencing the catalytic performances [10, 11]. For example, early studies of CO2 RR were typically employed in H cells [12–14], where CO2 is dissolved in liquid solution as the feedstock. However, the low solubility of CO2 (30 mM CO2 in water at ambient pressures and temperature) and limited diffusion coefficient (0.0016 mm2 s−1 in water) Q. Xia · M. Tang · X. Zhang (B) Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 L. An et al. (eds.), Flow Cells for Electrochemical Energy Systems, Green Energy and Technology, https://doi.org/10.1007/978-3-031-37271-1_8

199

200

Q. Xia et al.

Fig. 1 Annual fossil CO2 emissions for 1970–2020 in GtCO2 yr−1

greatly hinder the CO2 transportation to the catalyst surfaces, leading to low conversion efficiency [15, 16] with small current densities below 50 mA cm−2 , which far stands below the industrially relevant requirements (200 mA cm−2 ) [17]. To improve the conversion efficiency in practicable current, the electrochemical CO2 RR must conquer the limitation of mass transport, which can be realized by circulating reactants and catalytic products to and away from the electrodes using continuous flow equipment. Flow cells, working as a practical and scientific device, show many advantages when compared with traditional H cells. For example, high concentration of CO2 can be delivered to the catalyst/electrolyte interface, ensuring sufficient reactant for CO2 conversion [17, 18]. Moreover, the catholyte between the membrane and the GDE in some types of flow cell (microfluidic cell and hybrid cell) enables fine-tuning of the reaction environment on the surface of the catalysts, resulting in high CO2 conversion efficiency. In addition, the flow cells show great potential for transforming from the lab-scaled cells for research into more effective and efficient stacks available for commercial utilization. In this chapter, we will focus on the electrochemical CO2 reduction in flow cells. First, some important components of flow cells, such as gas diffusion layer (GDE), catalyst, membrane, etc. will be introduced. Then the most representative works for CO2 reduction will be discussed, with emphasizing the flow cell configurations in electrochemical CO2 reduction. Finally, the remaining challenges and our perspective on future development of flow cells for CO2 reduction will be presented.

Flow Cells for CO2 Reduction

201

2 Electrochemical CO2 Reduction Reaction CO2 is known as a stable molecule that requires large activation energy to cleavage the C = O bond. The electrolytic reduction of CO2 , driven by clean electricity, offers a potentially scalable and energetically favorable way for converting CO2 into valueadded chemicals and fuels. For reduction of CO2 in traditional H cells, as shown in Fig. 2, the anode side is typically operated as oxygen evolution reaction (OER), where H2 O is oxidized at the electrocatalyst/anolyte interface to produce O2 and release H+ . At the cathode compartment, the CO2 is reduced, which involves different protoncoupled electron transfer (PCET) processes. For example, the formation of C1 products (e.g., CO, HCOOH) from CO2 only requires a 2H+ /2e− PCET process, which can have been achieved with high Faradic efficiency (FE), while the formation of C2 and C3 products invokes an over 12H+ /12e− PCET process that inherently requires large potential with well-established catalysts. In addition, the strong competition from the cathodic hydrogen evolution reaction (HER) and the similar competitive reduction potentials of different products (Table 1) make the CO2 reduction process even more complicated. Based on Web of Science, over 90% of previous studies relating to CO2 RR (before year 2020) are performed in H cells, but few of them can achieve a high current density with satisfactory Faradaic efficiency. This is because, in conventional H-type electrolyzers, the CO2 diffusion coefficient in the aqueous electrolyte solution is small, and the mass transfer is limited. Therefore, the reaction kinetics are limited with a low production rate toward a target product, which is unsuitable for large-scale applications. In comparison, the flow cell using a gas diffusion electrode (GDE) can separate the gas–liquid phases through the gas diffusion electrode, and the CO2 gas can directly participate in the reaction, which fundamentally solves the CO2 mass

Fig. 2 Schematic illustration of a traditional H cell for electrochemical CO2 reduction

202 Table 1 Redox potentials and H+ /e− numbers needed for different products of electrochemical CO2 reduction

Q. Xia et al.

Products

H+ /e− Numbers

CO

2

− 0.53

HCOOH

2

− 0.61

Potentials versus NHE (V)

HCHO

4

− 0.48

CH3 OH

6

− 0.38

CH4

8

− 0.24

C2 H4

12

− 0.34

C2 H5 OH

12

− 0.33

C3 H7 OH

18

− 0.32

H2

2

− 0.41

transfer issues in the aqueous solution and promising for practical production at high current densities.

3 Important Components in Flow Cells for CO2 RR 3.1 Gas Diffusion Electrodes The gas diffusion electrode (GDE) is the key component in flow cells, especially valuable for promoting the transportation of CO2 to the catalyst/electrolyte interface. A GDE for CO2 reduction typically includes three components: a macroporous layer (MAL), microporous layer (MPL), and a catalyst layer (Fig. 3a).

Fig. 3 Schematic illustrations of a Gas diffusion electrodes and b Membranes used in flow cells for CO2 reduction reaction

Flow Cells for CO2 Reduction

203

The GDL serves to mediate the mass transfer of reactants (CO2 ), H2 O, and catalytic products to and away from the catalyst layer and control the local environment near the electrocatalyst [20]. Commercially available GDLs that have been widely tested comprise two layers. The layer facing flow channels for introducing carbon dioxide is the macroporous layer, which usually employs carbon paper or carbon cloth. This layer allows the diffusion of CO2 and gas products such as CO, CH4 , and C2 H4 to and away from reaction regions as well as the outside environment. In addition, another layer with microspores comprises densely packed carbon nanofibers treated with hydrophobic polymers is attached to macroporous layers for preventing the water and electrolyte flood the GDL’s porous. The flooding issue is a common problem for CO2 RR because the hydrophobicity of the GDL is gradually lost under applied potentials [21, 22]. Once the GDL is flooded, pores on GDL will be saturated with water to block CO2 diffusion release of gaseous products. To solve this problem, polytetrafluoroethylene (PTFE, a hydrophobic polymer) is often loaded uniformly on GDL or catalyst layer to modulate GDE’s hydrophobicity [23, 24]. Recent studies show that adaptation of the PTFE membrane as a GDL shows promising long-term stability for CO2 RR or ORR by slowing the flooding process, but the low conductivity of this insulation polymer still causes unavoidable low energy efficiency [25, 26]. Tradeoff between the conductivity and hydrophobicity of the GDL is a crucial issue that should be urgently addressed for current stage. The electrocatalyst is typically deposited on the microporous layer. There are two factors in the catalyst layer that can affect the CO2 RR activity and selectivity: composition and thickness. The metal-based catalyst are mostly used for producing targeting products (Table 1), such as the Ag and Au based catalyst, which are known to exhibit high selectivity to CO production [27–29]. The Sn and Bi prefer to generate formate, while the Cu, always used to promote the generation of multi-carbon products for CO2 RR [30–33] In recent years, some other materials like single-atom catalysts and molecular catalysts also showed high activity and excellent stability for CO2 RR [34–36]. In addition, the morphology of catalysts should also be considered, aiming to precisely expose more active sites. Different kind of catalyst structures such as nanoparticles, nanosheets, and nanoflowers et al., with a large specific surface area were developed [37]. For the thickness of the catalyst layer, the situation is even more complex because the thickness effectively influences the pH and CO2 concentration near the catalyst [38, 39]. Therefore, before the large-scale testing for CO2 RR, with a different environment (electrolyte pH and CO2 flow rates), the thickness of the catalyst layer should be judiciously selected to achieve high CO2 RR activity and availability.

56 90

46 32

C2 H4

C2 +

C2 H5 OH

n-propanol

C2 H4

C2+

Surface-reconstructed Cu

Cu: Fe–N–C, 1:0.05

Cu5 Zn8

Defect-site-rich Cu

CuNP

Cu2 S

C2 H5 OH

C2 H4

C2 H4

C2 H4

C2 H4

Ce(OH)x modified Cu

Cu–Al

4H Au@Cu

Surface-modified Cu

Metal–organic framework derived Cu clusters

alcohol

83.2

C2 H4

Defective Cu nanosheets

45

72

44.9

80

43

52

46.6

FE (%)

Major product

Electrocatalyst

H cell

Three-electrode flow cell

262 mA cm−2 @–1.07 V versus RHE

Three-electrode flow cell

≈ 319 mA cm−2 @–0.83 V Three-electrode flow cell versus RHE

≈ 32.1 mA cm−2 @–1.11 V versus RHE

400 mA cm−2 @–1.5 V versus RHE

Three-electrode flow cell

MEA

300 mA cm−2 @–0.7 V versus RHE

Microfluidic cell

~ 400 mA cm−2

MEA

H cell

MEA

H cell

H cell

Cell type

200 mA cm−2 @–0.7 V

~ 200 mA cm−2 @3.5 V (cell voltage)

≈ 4.9 mA cm−2 @–0.8 V versus RHE

1000 mA cm−2 @ ~ 3.3 V (cell voltage)

17 mA cm−2 @–2.0 V versus Ag/AgCl

60 mA cm−2 @–1.18 V versus RHE

Current density@cathode potential

1 M KOH

1 M KHCO3

0.1 M KHCO3

1 M KOH

1 M KOH

–

1 M KOH

–

0.1 M KHCO3

–

0.05 M KHCO3

0.1 M K2 SO4

Electrolyte

(continued)

[52]

[51]

[50]

[49]

[48]

[46]

[45]

[44]

[43]

[42]

[41]

[40]

References

204 Q. Xia et al.

77.4 15.4 22

C2 H4

CH3 CH2 CH2 OH

C2 H5 OH

C2 H4

C2 H4

Formate

Activated Cu nanowires

Double-sulfur vacancy-rich CuSx

Amorphous Cu NPs

Cu4 O3 -rich catalyst

Graphite/carbon NPs/Cu/ PTFE

In-Sn alloy

93.3

Formate

Formate

Formate

Formate

Formate

Formate

Sn nanoparticles

Bi rhombic dodecahedra

Bi nanostructures

Bi dendrites

Bismuthene (0.65 nm)

Bismuthene-layered nanosheets 97.4

98

64

92

86

97

SnO2 nanoparticles (5 nm) Formate

78.6

70

43

FE (%)

Major product

Electrocatalyst

(continued)

cm−2 @-0.64

cm−2 @–0.95

H cell

H cell

H cell

Three-electrode flow cell

MEA

Three-electrode flow cell

H cell

Microfluidic cell

Three-electrode flow cell

H cell

H cell

H cell

Cell type

102.7 mA cm−2 @ − 1.0 V Three-electrode flow cell versus RHE

2.5 mA cm−2 @ − 0.58 V versus RHE

3.5 mA cm−2 @–0.78 V versus RHE

15 mA cm−2 @–0.9 V versus RHE

200 mA cm−2 @–0.68 V versus RHE

V

V

51.7 mA cm−2 @2.2 V (cell voltage)

147 mA versus RHE

–

~ 100 mA cm−2 @ − 0.55 V versus RHE

300 mA versus RHE

–

≈ 20.1 mA cm−2 @ − 1.05 V versus RHE

≈ 22.4 mA cm−2 @ − 1.01 V versus RHE

Current density@cathode potential

1 M KHCO3

0.5 M KHCO3

0.5 M KHCO3

0.5 M KHCO3

1 M KOH

–

0.1 M KHCO3

0.1 M KHCO3

7 M KOH

0.5 M Cs2 SO4

0.1 M KHCO3

0.1 M KHCO3

0.1 M KHCO3

Electrolyte

(continued)

[64]

[63]

[62]

[61]

[60]

[59]

[58]

[57]

[26]

[56]

[55]

[54]

[53]

References

Flow Cells for CO2 Reduction 205

93.6

Formate

Formate

Formate

Formate

Formate

Formate

Formate

Formate

Formate

Formate

3D Bi-ene-A/CM

Bismuthene network

BiSn bimetallic structures

Bi-doped SnO nanosheets

BiSn oxides

Sn-doped Bi2 O3 nanosheets

Eutectic BiSn nanoalloys

BiSn alloys

Sn-doped Bi/BiOx core–shell nanowires

Bi@Sn core-shells nanoparticles

94.8

Formate

Formate

BiSn bimetallic structure

RuPd/Sn 89

95.8

Bi-doped amorphous SnOx Formate nanoshells

92

100

95.8

78

93.4

80

93

96

95

FE (%)

Major product

Electrocatalyst

(continued)

cm−2 @ H cell

H cell

H cell

Three-electrode flow cell

Three-electrode flow cell

Cell type

~ 100 mA cm−2 @ − 0.55 V versus RHE

34 mA cm−2 @-1.0 V versus RHE

20.9 mA cm−2 @–0.88 V versus RHE

200 mA cm−2 @-1.1 V versus RHE

100 mA cm−2 @ − 0.7 V versus RHE

100 mA cm−2 @–0.9 V versus RHE

8.5 mA cm−2 @ − 1.1 V versus RHE

Microfluidic cell

Three-electrode flow cell

Three-electrode flow cell

Three-electrode flow cell

Three-electrode flow cell

Three-electrode flow cell

H cell

24.3 mA cm−2 @ − 0.97 V H cell versus RHE

3.5 mA cm−2 @–1.0 V versus RHE

12 mA − 1.7 V versus Ag/AgCl

–

–

–

Current density@cathode potential [65]

References

0.5 M KCl + 1 M 1 M KOH

0.1 M KHCO3

0.5 M KHCO3

2 M KHCO3

1 M KOH

1 M KHCO3 and KOH

0.1 M KHCO3

0.5 M KHCO3

0.1 M KHCO3

0.1 M KHCO3

0.5 M KHCO3

(continued)

[77]

[76]

[75]

[74]

[73]

[72]

[71]

[70]

[69]

[68]

[67]

1 M KOH/1 M KHCO3 [66]

1 M KCl /1 M KHCO3

Electrolyte

206 Q. Xia et al.

96.8

96 96.7

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

Formate

CO

CO

Tri–Ag–NPs

nanostructured e-Ag

2D Ag–NPs

3D porous Ag

PON–Ag

Mesoporous Au

Nanoporous Au film

Au/C

Porous Au film

Monodispersed Au (8 nm)

InN-C

Pd/C

Ni–SAC@NCs SAC 95

95

92.5

90

90.5

~ 95

80

75

96.3

90

FE (%)

Major product

Electrocatalyst

(continued)

~ 10 mA cm−2 @–0.6 V versus RHE

200 mA cm−2 @3 V (cell voltage)

60.1 mA cm−2 @2.96 V (cell voltage)

–

~ 11 mA cm−2 @–0.5 V versus RHE

500 mA cm−2 @3 V (cell voltage)

~ 4 mA cm−2 @ − 0.5 V versus RHE

–

4.4 mA cm−2 @ − 0.69 V versus RHE

6 mA cm−2 @ − 1.03 V versus RHE

3.89 mA cm−2 @ − 0.6 V versus RHE

312 mA cm−2 @3 V (cell voltage)

1 mA cm−2 @ − 0.86 V versus RHE

Current density@cathode potential

H cell

MEA

MEA

H cell

H cell

MEA

H cell

H cell

H cell

H cell

H cell

MEA

H cell;

Cell type

0.5 M KHCO3

–

–

0.5 M KHCO3

0.1 M KHCO3

–

0.05 M KHCO3

0.1 M KHCO3

0.5 M KHCO3

0.1 M KHCO3

0.1 M KOH

–

0.1 M KHCO3

Electrolyte

(continued)

[90]

[89]

[88]

[87]

[85]

[84]

[83]

[82]

[81]

[80]

[79]

[78]

[32]

References

Flow Cells for CO2 Reduction 207

97

95 38.6

85 97 Above95

CO

CO

CO

CO

CH4

CH4

CH4

CO

CO

CO

Ni–N4 –C–NH2

NiSA/PCFM

Zn–N4 SAC

Cu–N–C–900 SAC

CuN2 O2 SAC

Fe–N–C SAC

CoPc/CNT–MD Molecular CO catalyst

CO

FeN5 SAC

CoPc Molecular catalyst

CoPc/CB Molecular catalyst

CoPP@CNT Molecular catalyst

CoTMAPc@CNT Molecular catalyst 99

98.3

95

78

88

85

FE (%)

Major product

Electrocatalyst

(continued)

~ 20 mA cm−2 @–0.72 V versus RHE

25.1 mA cm−2 @–0.6 V versus RHE

165 mA

cm−2

150 mA cm−2

~ 35 mA cm−2 @–0.9 V versus RHE

31.8 mA cm−2 @–1.8 V versus RHE

~ 30 mA cm−2 @ − 1.44 V versus RHE

14.8 mA cm−2 @–1.6 V versus RHE

4.8 mA cm−2 @–0.43 V versus RHE

350 mA cm−2 @–1.0 V versus RHE

440 mA cm−2 @–1.0 V versus RHE

2 mA cm−2 @–0.46 V versus RHE

Current density@cathode potential

H cell

H cell

Three-electrode flow cell

MEA cell

H cell

H cell

H cell

H cell

H cell

H cell

Three-electrode flow cell

H cell

Cell type

0.5 M KHCO3

0.5 M KHCO3

1 M KOH

1 M KHCO3

0.5 M KHCO3

1 M KHCO3

0.5 M KHCO3

0.1 M KHCO3

0.5 M KHCO3

0.5 M KHCO3

1 M KOH

0.1 M KHCO3

Electrolyte

[101]

[100]

[99]

[98]

[97]

[96]

[95]

[94]

[93]

[92]

[91]

[90]

References

208 Q. Xia et al.

Flow Cells for CO2 Reduction

209

3.2 Membrane The membrane serves to mediate the ion transport between the cathode and anode by containing charged function groups as well as to separate the chemical reaction of two sides of the cell [17]. There are three types of membranes wildly used for CO2 RR: anion exchange membranes (AEMs), cation exchange membranes (CEMs), and bipolar membranes (BPMs). Each type of membrane delivers distinct ion types and the transport paths (Fig. 3b), as well as playing a role in controlling the pH environment of the cathode and the anode. AEMs are composed of polymers with covalently bonded cationic groups, which allows for the efficient and selective transportation of hydrated anions, such as OH− , SO4 2− , and Cl− . Most catalysts exhibit better CO2 RR performance in alkaline solution, making the AEMs the most commonly used in CO2 electrolyzers. However, some limitations still need to be tackled to make the commercially application of the membrane practicable. Firstly, AEMs-based flow cells suffer notable carbon crossover problems [102], especially under large current densities. A large number of OH− generated from CO2 RR will react with the CO2 stream to form carbonates and bicarbonates, which will be transported through the AEM toward the anode under the electrical field, and then react with H+ from OER causing carbon loss problem. Secondly, the AEM shows a low ion conductivity (defined as the ion mobility through the membranes) compared with CEMs (Table 2) [103]. At high pH of over 11, the main charge carried in electrolyzes is CO3 2− which just shows about half the ion mobility of the OH− , causing low CO2 RR efficiency [104]. Another unsolved challenge is the poor mechanical stability due to the high water uptakes of the AEM leading to swell during electrolysis and thus failing quickly after dozens of cycles [105]. The CEM has a much higher ion conductivity than the AEM due to the high mobility of H+ , and is considered as mechanically robust membranes that can efficiently transport water. Therefore, the commercial CEMs like Nafion™ series have been widely deployed in proton exchange membrane fuel cells (PEMFC), and water electrolyzers with high efficiency and long stability [106–108]. For CO2 RR, application of CEM is an effective solution to retard the CO2 crossover because it shows limited transportation of CO3 2− and HCO3 − to the anode. However, the CEM promotes acidic conditions at both electrodes, causing severe HER side reaction at cathode, leading to a low FE of carbon products from CO2 . At the anode, the expensive noble-metal-based catalysts are required to drive the OER. And these situations make the CEM hard to be applied to CO2 RR. Table 2 Ion mobility comparison of different ions at infinite dilution in H2 O at ambient temperature and pressure (298 K, 1 atm) [103] Ion Mobility relative to

H+

H+

OH−

CO3 2−

HCO3 −

1

0.57

0.21

0.13

210

Q. Xia et al.

BPMs comprise an anion exchange layer (AEL) and a cation exchange layer (CEL) laminated together with water dissociation electrocatalysts embedded at the AEL/ CEL interface. They exhibit a distinct mechanism with CEMs and AEMs. Under reverse bias, the water is supplied to the AEL/CEL interface and dissociated by the electrocatalyst into their ionic counterparts. Then the H+ and OH− are delivered to the cathode and the anode under the influence of electrical field, which means it can furnish an acidic condition at the cathode and a basic condition at the anode, enabling the use of some cheap catalyst materials for CO2 RR (Cu) and OER (Ni, FeNiOx ) [109, 110]. In addition, the carbon loss problem is also solved for the same reason as the CEMs-based cells. However, the biggest obstacle of the BPMs-based flow cell is the low overall energy efficiency during electrolysis due to the large membrane potential required for water dissociation (1.23 V) [111], increasing the cell voltages to drive the reactions. Hence, an urgent need for BPMs-based cells is to decrease the voltage below 3 V, which is compulsory for commercial use.

3.3 Other Components There are another three components in flow cells, including the electrodes housing, gaskets, and flow field plates of the anode and the cathode (including catholyte/ anolyte field and cathode/anode flow plate for gas diffusion). The gasket is utilized to avoid liquid and gas leakage, and the housing and the flow plates serve to deliver the aqueous electrolyte and gas stream to and away from the membrane electrodes assembly (MEA). The flow plates control the distribution of the water and gas across the MEA by changing the path length and channel numbers of the plate [112, 113].

4 Recent Advances of Flow Cells for Electrochemical CO2 Reduction Besides electrocatalysts and membranes, the flow cell assembly is the cornerstone in determining the performance of CO2 RR. Up to now, the flow cell tested in CO2 reduction electrolysis can be categorized into four different configurations: microfluidic cells, membrane electrode assemble cells, three-electrode flow cells, and solidelectrolyte flow cells. In this section, we will summarize some most recent state-of-art flow cell configurations for the efficient CO2 reduction reaction.

Flow Cells for CO2 Reduction

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4.1 Microfluidic Cells for CO2 RR The microfluidic cell was reported for the first time by Kenis in 2010 [114]. As shown in Fig. 4a, this architecture contains a very thin layer of flowing aqueous electrolyte, usually less than 3 mm, separating the anodic and catholic chemistry. The flowing electrolyte is used to control the properties of the bulk electrolyte, such as pH, reaction temperature, and flowing rate, thus affecting the chemical environment near the electrocatalysts. In addition, the gas diffusion layers at the cathodic side were used to deliver gaseous CO2 to the catalyst layer to overcome the mass diffusion limitation relative to traditional H cells. During electrolysis, the inlet CO2 diffuses from the cathode chamber to the GDL and is captured by the catalyst at the electrolyte/electrode interface. Under applied potentials, the CO2 is reduced to different products. The gas products are transported from the interface to the cathode outlet mixed with CO2 , and the liquid products are transported to outside the system mixed with electrolyte. At anode, H2 O is oxidized through OER and O2 is the only product that can be detected from the anode outlet. It should be noted, however, that a drawback of this configuration is that notable products crossover problem between the electrodes dampens the overall energy efficiency of the CO2 RR technology. The studies within microfluidic cells to date are mainly associated with electrolyte and reaction parameters optimization. Impact of electrolyte properties and parameters optimization on cell performance: The aqueous KOH, KHCO3 , K2 CO3 , and KCl solutions are commonly used in microfluidic cells to drive reactions at different electrodes. The electrolyte’s pH,

Fig. 4 Schematic diagrams of four different cell configurations of CO2 RR: a Microfluidic cell, b Membrane electrode assemble cell, c Three-electrode flow cell, and d Solid-electrolyte flow cell

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composition, and flow rate are important for controlling the overpotential of the cell reaction and the energy efficiency of the cell. As shown in Fig. 5a, an early study showed that a more acidic electrolyte could achieve lower cathode overpotentials with a stable catalyst because the large number of H+ could improve the reaction kinetics of the PCET process for CO2 RR [77]. However, this result is much different from today’s research, especially under alkaline and neutral conditions. The OH− favors a much smaller Rct, which represents the extent of the stabilization of the rate-limiting CO2 − radical (smaller Rct means a larger content) over the HCO3 − and Cl− (Fig. 5b) [115–117], thus delivers a low overpotential, and a high concentration of the anion could also decrease the solution resistance [26, 115]. And pH could also cause high anode potential when used electrolyte only favors CO2 RR but suppressing the OER [77]. To solve this problem, a dual electrolyte microfluidic reactor was proposed to provide pH-optimized anolyte/catholyte for OER/CO2 RR by constructing two thin channels between which the anolyte/catholyte is passed (Fig. 5c). After optimization the electrolyte (pH: 2/14 for anolyte/catholyte), the Faradaic and energetic efficiencies were detected to increase by 95.6% at 143 mA cm−2 and 48.5% at 62 mA cm−2 (Fig. 5d) [118]. The flow rate of the electrolyte is also a matter of cell performance. For example, the higher electrolyte flow rate can increase the energy efficiency by mediating the concentration of reactants near the catalyst, but the rate of ascent will gradually decrease (Fig. 5e) [119]. Most recently, Rosen et al. [120] employed a different electrolyte, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4 ), achieving an overpotential below 0.2 V to convert CO2 to CO with FE of 96% by a novel mechanism. As shown in Fig. 5f, the electrolyte can significantly lower the energy barriers to form CO2 − intermediates from stable CO2 , which are then in situ reduced on the Ag cathode. This research offers a fresh view for us to search for ionic liquid electrolytes to perform CO2 RR, and the relative development for microfluidic cells remains lacking, which may be a future research hotspot. In addition, the performance of microfluidic flow cell is also close to the cell configuration and operating parameters. According to several studies, the CO2 feed rate, and channel length are key factors [132–134]. For example, the CO2 conversion increased from 13 to 99% can be achieved by increasing the channel length from 1 to 3 mm, this is because a longer channel would lead to a greater active surface for CO2 RR [134]. What’s more, the CO partial current density and Faradaic efficiency appeared to improve as the input gas flow rate increase from 1 to 6 sccm due to the increasing CO2 concentration present in the catalyst layer. Although the microfluidic flow cell shows benefits to increase the current densities with a relative high Faradic efficiency for CO2 RR, the notable crossover problem is a great challenge for scale-up application.

4.2 Three-Electrode Flow Cells for CO2 RR The traditional three-electrode cell or microfluidic cell (Fig. 4a) is typically restricted by the several carbonate crossover problem during CO2 RR. One promising cell configuration to solve the problem is to use the three-electrode flow cell shown

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Fig. 5 a I–V curve of CO2 RR using a Sn cathode and Pt black anode with 0.5 M KCl as electrolyte. HCl or KOH are used to adjust the pH of the electrolyte. b Nyquist plot for different electrolytes at a cell potential of − 2.25 V; c Schematic diagram of a dual electrolyte microfluidic reactor; d Solidelectrolyte flow cell; e Peak energetic efficiencies with parametrically optimized electrolyte flow rate; f Schematic of how the free energy of the system changes during the reaction CO2 + 2H+ + 2e– ⇌ CO + H2 O in water or acetonitrile (black dots) or EMIM-BF4 (blue dots)

in Fig. 4c, where the membrane is employed to reduce product crossover and aqueous electrolytes are used to manipulate the catalytic environment at anode/ cathode during electrolysis. This configuration possesses three chambers for anolyte flowing, catholyte flowing, and CO2 flowing. During electrolysis, the CO2 was first delivered to the GDL from the inlet, then diffused to the catalyst layer to participate in the electroreduction reaction at the catholyte/electrode interface. During the reduction reaction, the gas products are typically mixed with CO2, and liquid products are typically mixed with the catholyte. The key advantage of this configuration is that it can decrease the products crossover by using membrane and turning the catholyte composition to change the reduction microenvironment which combines the feature of the microfluidic cell and H cell. In this part, we will discuss the function of the catholyte, the flow plate, and some high-performance catalytic reactions using this type of cell configurations. Catholyte affects toward the CO2 RR selectivity in the three-electrode flow cell and the flow field plate design for managing cell resistance. The first use of the catholyte was in 2007 by Delacourt et al. for the production of syngas (CO + H2 ) within a CEM-based flow cell and Ag cathode [135]. At that time, the catholyte (KHCO3 ) functions as a pH buffer layer to suppress HER, and the cell exhibit FE(H2) close to 100% without catholyte and much higher FE(CO) with catholyte (Fig. 6a). Several studies based on CEM-based flow cells were encouraged to employing the KHCO3 to shower better performance [136–138], and some other catholyte including alkali metal hydroxides/carbonates/chlorides were also investigated. For example, KOH catholyte can supply a large number of hydroxide ions to the copper surface,

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lowering the CO2 reduction and CO–CO coupling activation energy barriers to achieve high FE(C2+) [26], KCl could delay the reduction of electrodeposited cuprous oxide (Cu2 O) nanoparticles to generate a biphasic Cu2 O–Cu catalyst for the selective production of C2+ species [139, 140]. In addition, the most recent study system investigated the influence of anion and cation additives in catholyte on the FE(CO) by using an Ag cathode [141]. As shown in Fig. 6b, for the anions, the OH− furnished the best performance, which was attributed to the higher pH and conductivity of 2 M KOH (13.78, 352 mS cm−1 ) in comparison with 2 M K2 CO3 (12.89, 190 mS cm−1 ) and 2 M KHCO3 (8.84, 100 mS cm−1 ). For the cations, the Cs+ shows better performance (Fig. 6c), which is because the higher solubility of CsHCO3 can slow the carbonate deposition thus increasing the stability of the GDE and the cell, and lesser hydration of the Cs+ cation causing smaller and more well-dispersed deposits on the catalyst layer. Regarding the conductivity of the system, some studies described that the membrane and the catholyte cause the major overpotentials at a certain current density (over 80% at 200 mA cm−2 ) [111, 142], which still lack effective methods to address this problem. Until recently, a thinner flow field plate comprising a 3D-printed sparging chamber connecting the cathode gas and catholyte compartments was reported to decrease about half of the total cell resistance (Fig. 6d). This design features reducing pH gradients and concentration polarization by continuously saturating the electrolyte with CO2 as well as decreasing the electrodes distance. Catalyst design for CO2 RR in the three-electrode flow cell: Electrocatalysts, especially in powder form, are particularly valuable for CO2 RR in flow cells. In this part, we introduce some state-of-art catalysts, including metal-based, carbon-supported single-atom catalysts (C–SACs) and molecular catalysts used in three-electrode flow cell. For example, most of the Sn, Bi-based catalyst like SnO2 nanoparticles [58], Bi rhombic dodecahedra [54], bismuthate-layered nanosheets [64], BiSn alloys [66], and Bi@Sn core-shells nanoparticles [72], show a high selective for CO production. The Cu-based catalysts, such as Ce(OH)x modified Cu [48], 4H Au@Cu [50], metal– organic framework derived Cu clusters,52 and Cu4 O3 -rich catalyst [56] exhibit high selectivity for C2 product production. In addition, for the C-SACs, the single transition metal embedded in nitrogen-doped carbon supports (M–N–C) has the potential to apply at the industrial level because of the high activity as well as simple synthesis methods. Many catalysts based on M–N–C have already been employed in H cell for CO2 RR, exhibiting high FE (above 90%) but limiting to low current densities (below 50 mA cm−2 ), such as Fe–N–C [143], Zn–N–C [93], Ni–N–C [144], and Co–N–C [145]. By increasing the concentration of reactants and modifying carbon support in flow cells to enhance the adsorption capacity of CO2 is an effective strategy to solve the problem. For example, the nitride-doped aminated carbon supporting Ni single-atom (Ni–N4 –C–NH2 , Fig. 6e) shows a high FE(CO) over 85% at 1.0 V with a current density over 440 mA cm−2 , much better than Ni–N4 –C (Fig. 6f). The improvement was attributed to enhanced CO2 adsorption capacity by the −NH2 , as confirmed by Fig. 6g [91]. Some other methods, like regulating the catalyst electronic structure and catalyst surfaces, are also useful to reach industrial-relevant conditions of CO2 RR [51, 92]. In addition, the most recent studies by Hai et al.

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Fig. 6 a Faradic efficiencies of CO and H2 with/without KHCO3 catholyte at a different time; b Faradic efficiency of CO for electrolytes with different anions (OH− , HCO3 − , CO3 2− ) at a different time; c Faradic efficiency of CO for electrolytes with different cations (Cs+ , K+ , Na+ ) at a different time; d Total cell resistance at 214 mA cm−2 for three designs: reactor 1 using the thick chamber, reactor 2 using the thin chamber, reactor 3 adding a connecting chamber; e Geometric structure of the Ni–N4 –C–NH2 catalyst; f Electrocatalytic activity of Ni–N4 –C–NH2 and Ni–N4 –C in the flow cell; g CO2 adsorption isotherm of Ni–N4 –C–NH2 and Ni–N4 –C; h Structure of the molecular catalyst, CoPc; i Structure of the molecular catalyst, CoPc2 : CoPc2 bears one trimethyl ammonium group at position 1 of the isoindole subunits, and three tert-butyl groups (positions 2 or 3) of the other subunits

[146] and Chen et al. [147] developed methods to enrich the types of SACs (37) and increase the single-atom loadings (over 20 wt%), respectively, which are promising for CO2 RR toward practical application. The molecular catalysts, including metalized porphyrins, polypyridines, and cyclams have been proven to be effective for producing CO and formate from CO2 [34], but long-term stability at high current densities is still a significant obstruction for molecular catalysts. One of the best molecular catalysts to date is the cobalt phthalocyanine (CoPc) (Fig. 6h), achieving FE(CO) above 95% at a current density of 150 mA cm–2 by using a membrane electrode assemble cell [98], which remains challenging for practical application. Inspired by this, some groups try to further improve the catalytic activity of the CoPc by modification of the second coordination sphere [148–150]. For example, Wang et al. deposited the modified cobalt complex (CoPc2, Fig. 6i) with a trimethyl ammonium group appended to the phthalocyanine macrocycle on multi-walled carbon nanotubes

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(MWCNTs), which delivered a FE(CO) of 95% at 165 mA cm–2 in a three-electrode flow cell [99]. Overall, the molecular catalysts still need to be further investigated for CO2 electrolysis in terms of catalytic activity and stability.

4.3 Membrane Electrode Assemble Cells for CO2 RR Membrane electrode assemble (MEA) cells are another type of electrochemical reactors that have been widely used in CO2 reduction. The typical setup of membrane electrode assemble cell is shown in Fig. 4b. A membrane is closely sandwiched between cathode and anode to attenuate the products crossover and facilitate the transport of conductive ions. During electrolysis, the CO2 was supplied to the cathode in the gas phase with water vapor or from the decomposition of the K2 CO3 and KHCO3 through the gas diffusion layer. Small amount of water in the supplied feedstock provide protons during catalysis at the catalyst surface. Because there is no electrolyte chamber in the cell, the gas and liquid products will be delivered to the outlet in the same path. At the anode, the anolyte such as KHCO3 was used to drive the oxygen evolution reaction (OER). A membrane is used in transporting ions and decrease products crossover, which plays critical role during electrolysis. For example, the HCOO− produced at cathode could crossover the AEM and move to the anolyte, thus decreasing the production rate of HCOO− . The CEM and BPM can significantly prevent the crossover of products. Moreover, the membrane electrode assemble design can decrease the ohmic losses via a short interelectrodes distance, thus improving the overall energy efficiency of the electrolysis. Membrane electrode assemble cells for CO2 RR with gases CO2 as feedstock: The MEA cell is able to function under higher current densities by providing much excessive reactants through the flowing mode. Moreover, the compact design of MEA cells result in lower cell resistance, for which become more promising under high current densities. For example, Yin et al. [84] reported a 85% FE(CO) at 500 mA cm−2 with cell voltage of 3 V by directly depositing Au/C catalyst on AEM coupled with IrO2 anode. Some other catalysts like Pd [89], Ag [78], Sn [59], and Ir [88] based nanomaterials were also employed in MEA for CO2 RR, showing high Faradic efficiencies at large current densities of 50–400 mA cm−2 . Impressively, it was reported that the MEA can also produce C2+ products by using special design catalysts. For example, the tandem catalyst, as shown in Fig. 7a, comprising a CO-selective catalyst layer segment and Cu catalyst layer segment, features in-situ converting intermediate chemical CO to C2+ products. For example, Zhang et al. [42] synthesized an Cu/Fe– N–C catalyst with an optimized layer distance ratio of the Cu layer to Fe–N–C layer (1:0.05), achieving a FE(C2+) over 90% with a partial current density exceeding 1 A cm−2 . In addition, some other catalysts like defect-site-rich Cu and CuS2 also showed high FE(C2+) over 30% at current densities over 100 mA cm−2 in MEA cells [44, 46]. The design methods are different from each other, but the mechanism to get a high proportion of C2+ products is similar: increasing local CO2 concentration near

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Cu by improving *CO surface coverage and mediating the *CO adsorption energy [151, 152]. All the MEA reactors mentioned above employ AEM membrane for efficient CO2 RR. This is because the CEM and BPM will transport or generate the proton to/at the interface of the catalyst and membrane, accelerating the HER reaction and decrease the CO2 RR selectivity. To solve this problem, the solid-supported aqueous electrolyte layer (NaHCO3 ) is used in the MEA cell situated between the membrane such as BPM and GDL (Fig. 7b) [8], which severs to increase FE for carbon products by inhibiting H+ transportation to the catalyst layer (Fig. 7c). Today, the studies of solid-supported aqueous electrolyte layer remain in shortage, and the role of the Na+ and HCO3 − is still unclean. But there is a study claiming that the key composition of the layer is water instead of NaHCO3 , evidenced by the FE(CO) using different electrolyte compositions of the layer (Fig. 7d) [153]. In situ generation of high concentration CO2 at electrocatalyst by protonating carbonates or bicarbonates: Most CO2 RR employ gases CO2 as feedstock but ignore the high cost of producing pure CO2 , including high energy-intensive processes of capture-media regeneration and gas compressing as well as low CO2 utilization rates (< 20%) [154, 155]. To solve this problem, some studies proposed to electrochemical reduction of (bi)carbonates directly from capture media to carbonbased products, which could bypass the above energy cost processes. However, the (bi)carbonates are inactive during electrolysis, leading to very low current densities below 5 mA cm−2 [156, 157]. In recent years, directly converting (bi)carbonates to final products ((bi)carbonates → CO2 → products) have attracted considerable interest. Taking KHCO3 reduction as example, the key processes during electrolysis were shown in Fig. 7e. The HCO3 − cross the GDL to the catalyst (Ag)/BPM interface and combine with locally generated H+ from BPM to form high concentration active CO2 , then the CO2 is directly reduced to CO and OH− . Of note that the in situ generated OH− could enhance the bulk pH favoring the CO3 2− formation and dampens local CO2 concentration that leads to lower FE(CO) (40 → 25%) as the electrolysis time increases (1 → 2 h) (Fig. 7f) [158]. A high FE(HCOO−) of 60% was obtained by using a same cell with Bi/C catalyst at 100 mA cm−2 [159]. But the composition of the outlet gas of the above two works comprising dilute CO2 , CO, and H2 , requires further purification. This can be easily addressed by mediating the (bi)carbonates concentration and current densities without changing the cell configuration. As shown in Fig. 7g, by using an Ag cathode, the products of syngas at a 3:1 H2 :CO ratio can be directly used for producing ammonia at industrial-relevant current, such as at 150 mA cm−2 in 1 M K2 CO3 with 100% CO2 utilization [160]. In addition, another issue about the bicarbonate electrolysis to date is the use of BPMs, which typically induce higher resistance and more energy consumptions [158, 159, 161–163]. To overcome this limitation, Berlinguette et al. [111] design a reactor deploying CEM, and performing bicarbonate electrolysis at the cathode in tandem with hydrogen oxidation reaction (HOR) at the anode. The HOR severs as the protons’ source for CO2 RR and more thermodynamically favorable than OER. During electrolysis, the HOR|CEM|HCO3 − reactor exhibits much lower cell voltage than OER|BPM|HCO3 −

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Fig. 7 a Schematic of stacked segmented gas diffusion electrodes and cathode reaction; b Schematic diagram of the gas-fed CO2 flow cell showing the location of the solid-supported aqueous NaHCO3 layer between the BPM and cathode catalyst; c FE(CO) at current densities between 20 and 100 mA cm−2 both with and without the aforementioned NaHCO3 layer; d FEs for CO and H2 production measured by GC after 700 s of gas-phase CO2 electrolysis in a flow cell configuration containing a solid-supported aqueous NaHCO3 layer and a solid-supported water layer at different current densities; e Schematic illustration of the cathodic CO2 RR in the flow cell; f Change of pH and FE(CO) during electrolysis of 3.0 M KHCO3 at 100 mA cm−2 with a BPM within 2 h; f Change of pH and FE(CO) during electrolysis of 3.0 M KHCO3 at 100 mA cm−2 with a BPM within 2 h; g Product distribution of an Ag catalyst under different applied current density (first x-axis, mA/cm2 ) in different concentrations of KOH electrolyte (second x-axis) purged with CO2 prior to reaction, simulating the product of a CO2 capture solution; h Vcell values measured as a function of current density for the different systems

(Fig. 7h). The cell delivered a high current density of 500 mA cm−2 at merely 2.2 V, much lower than previous reports (above 3 V).

4.4 Solid-Electrolyte Flow Cells for CO2 RR For all types of flow cells mentioned in previous sections, electrons are conducted with external circuit and ions are conducted through liquid electrolyte. Even though those fuel cells have shown advantages such as high efficiency and low energy consumption, one unsolved obstacle is that liquid products from CO2 RR are always

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mixed with liquid catholyte. For practicable applications of final outcomes, further purifications are always necessary. This drawback limits the choice of catholyte and exerts demands on more energy input. To provide a circumstance for reactions involving ions and extractions of pure liquid products, a novel solid-electrolyte reactor, which was proposed by Xia et al. in 2019 [168] for the first time, was employed for CO2 RR. As shown in Fig. 4d, a middle chamber containing solid electrolyte with high ionic conductivity was inserted between cathode and anode to from a sandwiched-like cell. The porous solidstate electrolytes can be fabricated with ion-conductive polymers or inorganic ion conductors [171]. They can be cation-conductive, anion-conductive, or bifunctional depending on different functional groups existing on solid electrolytes. Moreover, depending on expected reactions in the middle chamber, ion-conductive membranes with selectivity of ions would be inserted between both electrodes and solid electrolyte. The combination of membranes can be both cation exchange membranes (CEM), anion exchange membranes (AEM), or hybrid. Because the middle chamber provides a space to separate aqueous-state electrolytes and liquid products from CO2 RR, the reactor shows great advantages in producing high purity valuable products. Typically, during electrolysis, reactants provided at the anode are hydrogen or water to evoke hydrogen oxidation reaction (HOR) or oxygen evolution reaction (OER). Generated by either of the two reactions, protons will be produced and then transported through CEM between anode and middle chamber [169]. Simultaneously, anions generated from the CO2 RR at cathode will penetrate the AEM between cathode and middle chamber, and then recombined with protons to form final products of CO2 RR. Depending on different cathodic products (HCOO− , CH3 COO− et al.), different final products such as HCOOH, CH3 COOH et al. can be produced correspondingly, without mixing with other impurity ions. For example, Wang et al. reported the production of high-purity HCOOH and CH3 COOH products by using the solid-electrolyte reactor. A styrene–divinylbenzene copolymer microspheres with sulfuric functional groups are used as the porous solid electrolyte to facilitate the transportation of H+ . During electrolysis, the protons will be conducted across the CEM and combined with HCOO− from cathode to produce HCOOH. They reported maximum selectivity with FE up to 97%, and stability of 100 h. It was also demonstrated that the purity of the final HCOOH product can be as high as nearly 100 wt%. Just as mentioned above, functional groups are not limited to acidic media for proton conductivity. In another work reported by Xia et al. [4] reported a bifunctional SE also using styrene–divinylbenzene copolymer microspheres as supporting materials for CO2 RR. The sulfonic acid functional groups is used for transporting H+ , and quaternary amino functional groups are used for HCOO− transportation. For this configuration, protons are still transported through CEM and SE with acidic functional groups, then they will be recombined in the middle chamber with HCOO− transported through AEM and SE with quaternary amino functional groups to form pure HCOOH products. As we can see in Fig. 8a, over 3.27 V the HCOO− conductor shows less than half the ion conductivity of the H+ conductor, but the FE(HCOO−) is more favorable by using HCOO− conductor

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Fig. 8 a Current densities against cell voltages using H+ and HCOO− conducting solid electrolytes. b Corresponding FEs for the HCOOH at different cell voltages using H+ and HCOO− conducting solid electrolyte. c Schematic illustration of the four-chamber CO2 reduction cell with the solid electrolyte. d Flow rate of recovery CO2 and crossover CO2 using the solid-electrolyte reactor under different currents range 0–500 mA cm−2

(Fig. 8b), which may be due to the mass transport limitation of HCOO− ions based on H+ conductor flow cells at high voltage. Although the solid-electrolyte reactor shows great advantages in producing highpurity products through CO2 RR, many challenges still remain. For both setups with single or double functional groups, the acid condition in the anode compartment is a prerequisite to provide sufficient protons to combine with the HCOO− to form HCOOH, which requires expensive precious metal-based catalysts, such as Ir and Ru, to drive this reaction (in the above two reports, the anodic catalyst is IrO2 ). Therefore, for reducing the requirement of noble-metal catalyst, constructing a cell by selecting reasonable electrolytes and PEMs for regulating electrochemical mechanism on electrolyte/electrode interfaces seems to be a good choice. As shown in Fig. 8c, a double-chamber design separated with a bipolar membrane (BPM) was also proposed by the same group [168]. This cell uses two different solid-state electrolytes, and a BPM as sources for both protons and hydroxides to mediate the anodic and cathodic chemistry, in which the alkaline/neutral solution can be used for OER. The possibility for providing alkaline environment for OER dramatically reduces the requirement of Ir and Ru. Moreover, as demonstrated in Fig. 8c, three kinds of pure

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products involving HCOOH, O2 and KOH can be extracted, which could compensate higher energy consumption because of high resistance brought by BPM. For the CO2 RR in flow cell, one of the well-known problems is the CO3 2− crossover during catalysis because of blindness of ion-conductive electrolytes, resulting in a lower efficiency of carbon utilization. The solid-state electrolyte flow cell also can help to prevent the carbon loss problem during electrolysis by rearranging locations of redox reactions. For AEM-based aqueous flow cells, the CO3 2− and HCO3 − formed by CO2 reacting with OH− would cross the AEM toward the anode and react with locally generated protons to form gaseous CO2 which mixed with anode O2 , this crossed-over CO2 cannot be reused for CO2 RR without separation. However, for the solid-state electrolyte flow cell in Fig. 8d, Kim et al. innovatively utilized the crossed CO3 2− and HCO3 − to react with H+ within the solid-state electrolyte to form pure CO2 that can be carried out with flowing water for recycling and reutilization. In Kim’s research, 90% recovery of the crossover CO2 in an ultrahigh gas purity form (> 99%) was achieved at different currents (Fig. 8d), and over 90% Fe(CO) was delivered under a 200 mA cm−2 by using the solid-state electrolyte flow cell, this result was also confirmed by Xu et al., only 3% carbon loss was found at 100 mA cm−2 with stable continuous electrolysis for > 200 h [170]. The research about the solid-state electrolyte flow cell is in the initial stage, and there is still lot of room to enhance the performance of the cell along with more functions that can be achieved with this setup. To further improve the electrochemical performance of the cell, some methods can be adopted, such as optimizing the thickness of the solid-electrolyte layer that is critical to the overall cell voltages; almost all the cells using solid electrolyte show a high cell potential (over 3 V), which bear irrational resemblance to commercial relevant conditions (< 3 V, over 200 mA cm−2 , FE over 90%), and a suitable thickness could both achieve high purity products with high FE and low ohmic losses. Moreover, the idea of recycling CO2 proposed by Kim et al. can be furtherly adopted for capturing CO2 from dilute sources like ambient air or flue glass. Once the combination and optimization of both CO2 RR and CO2 capture can be achieved, this SE configuration would provide a promising routine for closing carbon cycle.

5 Summary and Perspective Summary: In this chapter, the components of the flow cell for CO2 RR and their functions have been introduced, and some of the state-of-art works with different types of configurations have been discussed, offering readers some important insights on the properties of the different components, including electrolytes, catholyte, catalysts, flow field plates, solid-supported aqueous electrolytes, and CO2 feedstock that can control the cell performance, and will further help readers in their efforts to design and develop systems for effective reduction of CO2 .

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Fig. 9 Performance of CO2 electrolyzers by using flow cells. a Faradaic efficiency versus current density ( jpartial ) for CO, HCOOH, CH4 , C2 H4 and C2 H5 OH; b Cell voltage versus current density for CO, HCOOH, CH4 , C2 H4 ; c Faradaic efficiency versus operating time for CO, HCOOH, and C2 H4 . The data constituting these plots are based on previous reports that use flow cells for CO2 RR and can be found in the source data file [51, 77, 84, 91, 92, 98, 102, 118, 125, 128–131, 135, 139, 140, 146, 168, 169, 172–195]

Perspective: Electrocatalytic CO2 reduction in a flow cell is compelling because of the high FE at high current densities, but its overall performance needs further improvement. As shown in Fig. 9, the rectangular area represents the commercial requirement of the CO2 RR: producing products with a FE over 90% at current densities of 200 mA cm−2 for 1000 h and the cell voltage should be below 3 V. For the FE and cell voltages, some studies can achieve this goal, but the long-time cycling performance of the CO2 RR using flow cell remains a challenge. To solve this goal, several components such as catalysts with high activity and stability, membranes with high ion conductivity and physical robustness, and GDL with high hydrophobicity and conductivity should be developed and applied together.

References 1. Huang JB, Zhang XD, Zhang QY, Lin YL, Hao MJ, Luo Y, Zhao ZC, Yao Y, Chen X, Wang L, Nie SP, Yin YZ, Xu Y, Zhang JS (2017) Nat Clim Chang 7:875–879 2. Brander M (2012) Econometrica, white papers 3. Le Quéré C, Peters GP, Friedlingstein P, Andrew RM, Canadell JG, Davis SJ, Jackson RB, Jones MW (2021) Nat Clim Chang 11:197–199 4. McDonald TM, Lee WR, Mason JA, Wiers BM, Hong CS, Long JR (2012) J Am Chem Soc 134:7056–7065 5. Merkel TC, Lin HQ, Wei XT, Baker R (2010) J Membr Sci 359:126–139 6. Halliday C, Harada T, Hatton TA (2019) Chem Mater 32:348–359 7. Harada T, Halliday C, Jamal A, Hatton TA (2019) J Mater Chem A 7:21827–21834 8. Weekes DM, Salvatore DA, Reyes A, Huang A, Berlinguette CP (2018) Acc Chem Res 51:910–918 9. Niu ZZ, Chi LP, Liu R, Chen Z, Gao MR (2021) Energy Environ Sci 14:4169–4176 10. Yan Z, Hitt JL, Zeng Z, Hickner MA, Mallouk TE (2021) Nat Chem 13:33–40 11. Higgins D, Hahn C, Xiang C, Jaramillo TF, Weber AZ (2018) ACS Energy Lett 4:317–324 12. Hori Y, Wakebe H, Tsukamoto T, Koga O (1993) Electrochim Acta 39:1833–1839 13. Azuma M, Hashimoto K, Hiramoto M (1990) J Electrochem Soc 137:1772–1778

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21. 22. 23. 24. 25. 26.

27. 28. 29. 30.

31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.

42. 43. 44. 45. 46.

223

Kaneco S, Iiba K, Suzuki SK, Ohta K, Mizuno T (1999) J Phys Chem B 35:7457 P˘atru A, Binninger T, Pribyl B, Schmidt TJ (2019) J Electrochem Soc 166:F34–F43 Jähne B, Heinz G, Dietrich W (1987) J Geophys Res Atmos 92 Salvatore DA, Gabardo CM, Reyes A, O’Brien CP, Holdcroft S, Pintauro P, Bahar B, Hickner MA, Bae C, Sinton D, Sargent EH, Berlinguette CP (2021) Nat Energy 6:339–348 Liang SY, Altaf N, Huang L, Gao YS, Wang Q (2020) J CO2 Util 35:90–105 Ma D, Jin T, Xie KY, Huang HT (2021) J Mater Chem A 9:20897–20918 Wakerley D, Lamaison S, Wicks J, Clemens A, Feaster J, Corral D, Jaffer SA, Sarkar A, Fontecave M, Duoss EB, Baker S, Sargent EH, Jaramillo TF, Hahn C (2022) Nat Energy 7:130–143 Liu K, Smith WA, Burdyny T (2019) ACS Energy Lett 4:639–643 Kutana A, Giapis KP (2006) Nano Lett 6:656–661 An PD, Wei L, Li HJW, Yang BP, Liu K, Fu JW, Li HM, Liu H, Hu JH, Lu YR, Pan H, Chan TS, Zhang N, Liu M (2020) J Mater Chem A 8:15936–15941 Sikdar N, Junqueira JRC, Dieckhofer S, Quast T, Braun M, Song Y, Aiyappa HB, Seisel S, Weidner J, Ohl D, Andronescu C, Schuhmann W (2021) Angew Chem Int Ed 60:23427–23434 Moussallem I, Jörissen J, Kunz U, Pinnow S, Turek T (2008) J Appl Electrochem 38:1177– 1194 Dinh CT, Burdyny T, Kibria MG, Seifitokaldani A, Gabardo CM, Arquer F, Kiani A, Edwards JPP, Luna PD, Bushuyev OS, Zou CQ, Bermudez RQ, Pang YJ, Sinton D, Sargent EH (2018) Science 360:783–787 Shi R, Guo J, Zhang X, Waterhouse GIN, Han Z, Zhao Y, Shang L, Zhou C, Jiang L, Zhang T (2020) Nat Commun 11:3028 Kim J, Song JT, Ryoo H, Kim JG, Chung SY, Oh JH (2018) J Mater Chem A 6:5119–5128 Wu Z, Wu H, Cai W, Wen Z, Jia B, Wang L, Jin W, Ma T (2021) Angew Chem Int Ed 60:12554–12559 Zheng XL, De Luna P, García de Arquer FP, Zhang B, Becknell N, Ross MB, Li YF, Banis MN, Li YZ, Liu M, Voznyy O, Dinh CT, Zhuang TT, Stadler P, Cui Y, Du XW, Yang PD, Sargent EH (2017) Joule 1:794–805 Daiyan R, Lu XY, Ng YH, Amal R (2017) Catal. Sci Technol 7:2542–2550 Liu S, Tao H, Zeng L, Liu Q, Xu Z, Liu Q, Luo JL (2017) J Am Chem Soc 139:2160–2163 Xie H, Wang TY, Liang JS, Li Q, Sun SH (2018) Nano Today 21:41–54 Li M, Wang H, Luo W, Sherrell PC, Chen J, Yang J (2020) Adv Mater 32:e2001848 Torbensen K, Joulié D, Ren SX, Wang M, Salvatore D, Berlinguette CP, Robert M (2020) ACS Energy Lett 5:1512–1518 Saxena A, Liyanage W, Masud J, Kapila S, Nath M (2021) J Mater Chem A 9:7150–7161 Kanase RS, Lee KB, Arunachalam M, Sivasankaran RP, Oh JH, Kang SH (2022) Appl Surf Sci 584 Tan YC, Lee KB, Song H, Oh JH (2020) Joule 4:1104–1120 Zhang BA, Ozel T, Elias JS, Costentin C, Nocera DG, Cent ACS (2019) Sci 5:1097–1105 Zhang B, Zhang J, Hua M, Wan Q, Su Z, Tan X, Liu L, Zhang F, Chen G, Tan D, Cheng X, Han B, Zheng L, Mo G (2020) J Am Chem Soc 142:13606–13613 Kibria MG, Dinh CT, Seifitokaldani A, De Luna P, Burdyny T, Quintero-Bermudez R, Ross MB, Bushuyev OS, Garcia de Arquer FP, Yang P, Sinton D, Sargent EH (2018) Adv Mater 30:e1804867 Zhang TY, Bui JC, Li ZY, Bell AT, Weber AZ, Wu JJ (2022) Nat Catal 5:202–211 Su XS, Sun YM, Jin L, Zhang L, Yang Y, Kerns P, Liu B, Li SZ, He J (2020) Appl Catal BB Environ 269 Gu ZX, Shen H, Chen Z, Yang YY, Yang C, Ji YL, Wang YH, Zhu C, Liu JL, Li J, Sham TK, Xu X, Zheng GF (2021) Joule 5:429–440 Ma S, Sadakiyo M, Luo R, Heima M, Yamauchi M, Kenis PJA (2016) J Power Sources 301:219–228 Zhuang TT, Liang ZQ, Seifitokaldani A, Li Y, De Luna P, Burdyny T, Che FL, Meng F, Min YM, Quintero-Bermudez R, Dinh CT, Pang YJ, Zhong M, Zhang B, Li J, Chen PN, Zheng XL, Liang HY, Ge WN, Ye BJ, Sinton D, Yu SH, Sargent EH (2018) Nat Catal 1:421–428

224

Q. Xia et al.

47. Wakerley D, Lamaison S, Ozanam F, Menguy N, Mercier D, Marcus P, Fontecave M, Mougel V (2019) Nat Mater 18:1222–1227 48. Luo M, Wang Z, Li YC, Li J, Li F, Lum Y, Nam DH, Chen B, Wicks J, Xu A, Zhuang T, Leow WR, Wang X, Dinh CT, Wang Y, Wang Y, Sinton D, Sargent EH (2019) Nat Commun 10:5814 49. Zhong M, Tran K, Min Y, Wang C, Wang Z, Dinh CT, De Luna P, Yu Z, Rasouli AS, Brodersen P, Sun S, Voznyy O, Tan CS, Askerka M, Che F, Liu M, Seifitokaldani A, Pang Y, Lo SC, Ip A, Ulissi Z, Sargent EH (2020) Nature 581:178–183 50. Chen Y, Fan Z, Wang J, Ling C, Niu W, Huang Z, Liu G, Chen B, Lai Z, Liu X, Li B, Zong Y, Gu L, Wang J, Wang X, Zhang H (2020) J Am Chem Soc 142:12760–12766 51. Li F, Thevenon A, Rosas-Hernandez A, Wang Z, Li Y, Gabardo CM, Ozden A, Dinh CT, Li J, Wang Y, Edwards JP, Xu Y, McCallum C, Tao L, Liang ZQ, Luo M, Wang X, Li H, O’Brien CP, Tan CS, Nam DH, Quintero-Bermudez R, Zhuang TT, Li YC, Han Z, Britt RD, Sinton D, Agapie T, Peters JC, Sargent EH (2020) Nature 577:509–513 52. Nam DH, Bushuyev OS, Li J, De Luna P, Seifitokaldani A, Dinh CT, Garcia de Arquer FP, Wang Y, Liang Z, Proppe AH, Tan CS, Todorovic P, Shekhah O, Gabardo CM, Jo JW, Choi J, Choi MJ, Baek SW, Kim J, Sinton D, Kelley SO, Eddaoudi M, Sargent EH (2018) J Am Chem Soc 140:11378–11386 53. Choi C, Kwon S, Cheng T, Xu MJ, Tieu P, Lee CS, Cai J, Lee HM, Pan XQ, Duan XF, Goddard WA, Huang Y (2020) Nat Catal 3:804–812 54. Peng C, Luo G, Zhang J, Chen M, Wang Z, Sham TK, Zhang L, Li Y, Zheng G (2021) Nat Commun 12:1580 55. Duan YX, Meng FL, Liu KH, Yi SS, Li SJ, Yan JM, Jiang Q (2018) Adv Mater 30:e1706194 56. Marti´c N, Reller C, Macauley C, Löffler M, Schmid B, Reinisch D, Volkova E, Maltenberger A, Rucki A, Mayrhofer KJJ, Schmid G (2019) Adv Energy Mater 9 57. Dong WJ, Yoo CJ, Lee JL, Appl ACS (2017) Mater Interf 9:43575–43582 58. Liang CL, Kim B, Yang SZ, Liu Y, Francisco Woellner C, Li ZY, Vajtai R, Yang W, Wu JJ, Kenis PJA, Ajayan PM (2018) J Mater Chem A 6:10313–10319 59. Lee W, Kim YE, Youn MH, Jeong SK, Park KT (2018) Angew Chem Int Ed Engl 57:6883– 6887 60. Xie H, Zhang T, Xie R, Hou Z, Ji X, Pang Y, Chen S, Titirici MM, Weng H, Chai G (2021) Adv Mater 33:e2008373 61. Lu P, Gao D, He H, Wang Q, Liu Z, Dipazir S, Yuan M, Zu W, Zhang G (2019) Nanoscale 11:7805–7812 62. Wei XY, Zhang WN, Liu DP, Liu DD, Yan YD, Zhang J, Yang YD, Yan SC, Zou ZG (2022) J CO2 Util 55 63. Yang F, Elnabawy AO, Schimmenti R, Song P, Wang J, Peng Z, Yao S, Deng R, Song S, Lin Y, Mavrikakis M, Xu W (2020) Nat Commun 11:1088 64. Ma WX, Bu J, Liu ZP, Yan C, Yao Y, Chang NH, Zhang HP, Wang T, Zhang J (2020) Adv Funct Mater 31 65. He YC, Ma DD, Zhou SH, Zhang M, Tian JJ, Zhu QL (2022) Small 18:e2105246 66. Zhang M, Wei W, Zhou S, Ma D-D, Cao A, Wu X-T, Zhu Q-L (2021) Energy Environ Sci 14:4998–5008 67. Wen GB, Lee DU, Ren B, Hassan FM, Jiang GP, Cano ZP, Gostick J, Croiset E, Bai ZY, Yang L, Chen ZW (2018) Adv Energy Mater 8 68. An X, Li S, Yoshida A, Yu T, Wang Z, Hao X, Abudula A, Guan G, Appl ACS (2019) Mater Interfaces 11:42114–42122 69. Tian J, Wang R, Shen M, Ma X, Yao H, Hua Z, Zhang L (2021) Chemsuschem 14:2247–2254 70. Li X, Wu X, Li J, Huang J, Ji L, Leng Z, Qian N, Yang D, Zhang H (2021) Nanoscale 13:19610–19616 71. Tang J, Daiyan R, Ghasemian MB, Idrus-Saidi SA, Zavabeti A, Daeneke T, Yang J, Koshy P, Cheong S, Tilley RD, Kaner RB, Amal R, Kalantar-Zadeh K (2019) Nat Commun 10:4645 72. Li L, Ozden A, Guo S, Garci ADAFP, Wang C, Zhang M, Zhang J, Jiang H, Wang W, Dong H, Sinton D, Sargent EH, Zhong M (2021) Nat Commun 12:5223

Flow Cells for CO2 Reduction

225

73. Zhao Y, Liu X, Liu Z, Lin X, Lan J, Zhang Y, Lu YR, Peng M, Chan TS, Tan Y (2021) Nano Lett 21:6907–6913 74. Xing Y, Kong X, Guo X, Liu Y, Li Q, Zhang Y, Sheng Y, Yang X, Geng Z, Zeng J (2020) Adv Sci 7:1902989 75. Yang Q, Wu Q, Liu Y, Luo S, Wu X, Zhao X, Zou H, Long B, Chen W, Liao Y, Li L, Shen PK, Duan L, Quan Z (2020) Adv Mater 32:e2002822 76. Li Z, Feng YJ, Li YF, Chen XP, Li N, He WH, Liu J (2022) Chem Eng J 428 77. Whipple DT, Finke EC, Kenis PJA (2010) Electrochem Solid-State Lett 13:B109 78. Lee WH, Ko YJ, Choi YJ, Lee SY, Choi CH, Hwang YJ, Min BK, Strasser P, Oh HK (2020) Nano Energy 76 79. Qi K, Zhang Y, Li J, Charmette C, Ramonda M, Cui X, Wang Y, Zhang Y, Wu H, Wang W, Zhang X, Voiry D (2021) ACS Nano 15:7682–7693 80. Abeyweera SC, Yu J, Perdew JP, Yan Q, Sun Y (2020) Nano Lett 20:2806–2811 81. Peng X, Karakalos SG, Mustain WE, Appl ACS (2018) Mater Interfaces 10:1734–1742 82. Hall AS, Yoon Y, Wuttig A, Surendranath Y (2015) J Am Chem Soc 137:14834–14837 83. Welch AJ, DuChene JS, Tagliabue G, Davoyan A, Cheng WH, Atwater HA, Appl ACS (2018) Mater Interfaces 2:164–170 84. Yin ZL, Peng HQ, Wei X, Zhou H, Gong J, Huai MM, Xiao L, Wang GW, Lu JT, Zhuang L (2019) Energy Environ Sci 12:2455–2462 85. Chen CZ, Zhang B, Zhong JH, Cheng ZM (2017) J Mater Chem A 5:21955–21964 86. Kauffman DR, Alfonso D, Matranga C, Qian H, Jin R (2012) J Am Chem Soc 134:10237– 10243 87. Zhu W, Michalsky R, Metin O, Lv H, Guo S, Wright CJ, Sun X, Peterson AA, Sun S (2013) J Am Chem Soc 135:16833–16836 88. Hou PF, Wang XP, Kang P (2021) J CO2 Util 45 89. Lee J, Lim JK, Roh CW, Whang HS, Lee HJ (2019) J CO2 Util 31:244–250 90. Guo YB, Yao S, Xue YY, Hu X, Cui HJ, Zhou Z (2022) Appl Catal B 304 91. Chen ZP, Zhang XX, Liu W, Jiao MY, Mou KW, Zhang XP, Liu LC (2021) Energy Environ Sci 14:2349–2356 92. Yang H, Lin Q, Zhang C, Yu X, Cheng Z, Li G, Hu Q, Ren X, Zhang Q, Liu J, He C (2020) Nat Commun 11:593 93. Yang F, Song P, Liu X, Mei B, Xing W, Jiang Z, Gu L, Xu W (2018) Angew Chem Int Ed 57:12303–12307 94. Guan AX, Chen Z, Quan YL, Peng C, Wang ZQ, Sham T-K, Yang C, Ji Y, Qian L, Xu X, Zheng G (2020) ACS Energy Lett 5:1044–1053 95. Cai Y, Fu J, Zhou Y, Chang YC, Min Q, Zhu JJ, Lin Y, Zhu W (2021) Nat Commun 12:586 96. Han L, Song S, Liu M, Yao S, Liang Z, Cheng H, Ren Z, Liu W, Lin R, Qi G, Liu X, Wu Q, Luo J, Xin HL (2020) J Am Chem Soc 142:12563–12567 97. Wu X, Sun JW, Liu PF, Zhao JY, Liu Y, Guo L, Dai S, Yang HG, Zhao H (2021) Adv Funct Mater 32 98. Ren SX, Joulié D, Salvatore D, Torbensen K, Wang M, Robert M, Berlinguette CPP (2019) Science 365:367–369 99. Wang M, Torbensen K, Salvatore D, Ren S, Joulie D, Dumoulin F, Mendoza D, Lassalle-Kaiser B, Isci U, Berlinguette CP, Robert M (2019) Nat Commun 10:3602 100. Zhu M, Chen J, Huang L, Ye R, Xu J, Han YF (2019) Angew Chem Int Ed 58:6595–6599 101. Su JJ, Zhang JJ, Chen JC, Song Y, Huang LB, Zhu MH, Yakobson BI, Tang BZ, Ye RQ (2021) Energy Environ Sci 14:483–492 102. Kim JYK, Zhu P, Chen FY, Wu ZY, Cullen DA, Wang HT (2022) Nat Catal 5:288–299 103. Lees EW, Mowbray BAW, Parlane FGL, Berlinguette CP (2021) Nat Rev Mater 7:55–64 104. Lu X, Zhu C, Wu Z, Xuan J, Francisco JS, Wang H (2020) J Am Chem Soc 142:15438–15444 105. Shen CH, Wycisk R, Pintauro PN (2017) Energy Environ Sci 10:1435–1442 106. Liu YH, Yi BL, Shao ZG, Xing DM, Zhang HM (2006) Electrochem Solid-State Lett 9:A356 107. Sigwadi R, Dhlamini MS, Mokrani T, Nemavhola F, Nonjola PF, Msomi PF (2019) Heliyon 5:e02240

226

Q. Xia et al.

108. Ito H, Maeda T, Nakano A, Takenaka H (2011) Int J Hydrog Energy 36:10527–10540 109. Chen JD, Zheng F, Zhang SJ, Fisher A, Zhou Y, Wang ZY, Li YY, Xu BB, Li JT, Sun SG (2018) ACS Catal 8:11342–11351 110. Anantharaj S, Kundu S, Noda S (2021) Nano Energy 80 111. Zhang ZH, Lees EW, Ren SX, Huang AX, Berlinguette CP (2021) Chemrxiv 112. Nie JH, Chen YT (2010) Int J Hydrog Energy 35:3183–3197 113. Lobato J, Cañizares P, Rodrigo MA, Pinar FJ, Mena E, Úbeda D (2010) Int J Hydrog Energy 35:5510–5520 114. Jayashree RS, Yoon SK, Brushett FR, Lopez-Montesinos PO, Natarajan D, Markoski LJ, Kenis PJA (2010) J Power Sources 195:3569–3578 115. Verma S, Lu X, Ma S, Masel RI, Kenis PJ (2016) Phys Chem Phys 18:7075–7084 116. Ma S, Luo R, Moniri S, Lan YC, Kenis PJA (2014) J Electrochem Soc 161:F1124–F1131 117. Dufek EJ, Lister TE, McIlwain ME (2012) Electrochem Solid-State Lett 15 118. Lu X, Leung DYC, Wang HZ, Maroto-Valer MM, Xuan J (2016) Renew Energ 95:277–285 119. Lu X, Leung DYC, Wang HZ, Xuan J (2017) Appl Energy 194:549–559 120. Liu H, Ding Y, Ao Z, Zhou Y, Wang S, Jiang L (2015) Small 11:1782–1786 121. Liu M, Zhao Z, Duan X, Huang Y (2019) Adv Mater 31:e1802234 122. Chen H, Shi L, Liang X, Wang L, Asefa T, Zou X (2020) Angew Chem Int Ed 59:19654–19658 123. Khan MA, Zhao HB, Zou WW, Chen Z, Cao WJ, Fang JH, Xu JQ, Zhang L, Zhang JJ (2018) Electrochem Energy Rev 1:483–530 124. Sun J, Yu B, Yan X, Wang J, Tan F, Yang W, Cheng G, Zhang Z (2022) Materials 15 125. Ma S, Lan Y, Perez GM, Moniri S, Kenis PJ (2014) Chemsuschem 7:866–874 126. Cole EB, Lakkaraju PS, Rampulla DM, Morris AJ, Abelev E, Bocarsly AB (2010) J Am Chem Soc 132:11539–11551 127. Morris AJ, McGibbon RT, Bocarsly AB (2011) Chemsuschem 4:191–196 128. Tornow CE, Thorson MR, Ma S, Gewirth AA, Kenis PJ (2012) J Am Chem Soc 134:19520– 19523 129. Jhong HM, Tornow CE, Smid B, Gewirth AA, Lyth SM, Kenis PJ (2017) Chemsuschem 10:1094–1099 130. Ma SC, Luo R, Gold JI, Yu AZ, Kim B, Kenis PJA (2016) J Mater Chem A 4:8573–8578 131. Jhong HM, Brushett FR, Kenis PJA (2013) Adv Energy Mater 3:589–599 132. Wang HZ, Leung DYC, Xuan J (2013) Appl Energy 102:1057–1062 133. Kotb Y, Fateen SEK, Albo J, Ismail I (2017) J Electrochem Soc 164:E391–E400 134. Wu KN, Birgersson E, Kim B, Kenis PJA, Karimi IA (2015) J Electrochem Soc 162:F23–F32 135. Delacourt C, Ridgway PL, Kerr JB, Newman J (2008) J Electrochem Soc 155:B42–B49 136. Alvarez-Guerra M, Quintanilla S, Irabien A (2012) Chem Eng J 207–208:278–284 137. Alvarez-Guerra M, Del Castillo A, Irabien A (2014) Chem Eng Res Des 92:692–701 138. Del Castillo A, Alvarez-Guerra M, Irabien A (2014) AIChE J 60:3557–3564 139. Zhang X, Li J, Li YY, Jung Y, Kuang Y, Zhu G, Liang Y, Dai H (2021) J Am Chem Soc 143:3245–3255 140. Lee S, Kim D, Lee J (2015) Angew Chem Int Ed 54:14701–14705 141. Cofell ER, Nwabara UO, Bhargava SS, Henckel DE, Kenis PJA, Appl ACS (2021) Mater Interfaces 13:15132–15142 142. Salvatore DA, Berlinguette CP (2019) ACS Energy Lett 5:215–220 143. Gao S, Lin Y, Jiao X, Sun Y, Luo Q, Zhang W, Li D, Yang J, Xie Y (2016) Nature 529:68–71 144. Zhao C, Dai X, Yao T, Chen W, Wang X, Wang J, Yang J, Wei S, Wu Y, Li Y (2017) J Am Chem Soc 139:8078–8081 145. Wang X, Chen Z, Zhao X, Yao T, Chen W, You R, Zhao C, Wu G, Wang J, Huang W, Yang J, Hong X, Wei S, Wu Y, Li Y (2018) Angew Chem Int Ed 57:1944–1948 146. Hai X, Xi S, Mitchell S, Harrath K, Xu H, Akl DF, Kong D, Li J, Li Z, Sun T, Yang H, Cui Y, Su C, Zhao X, Li J, Perez-Ramirez J, Lu J (2022) Nat Nanotechnol 17:174–181 147. Ball P (2022) Nat Mater 21:610 148. Rivera Cruz KE, Liu YS, Soucy TL, Zimmerman PM, McCrory CCL (2021) ACS Catal 11:13203–13216

Flow Cells for CO2 Reduction

227

149. Lin L, Li H, Yan C, Li H, Si R, Li M, Xiao J, Wang G, Bao X (2019) Adv Mater 31:e1903470 150. Liu Y, McCrory CCL (2019) Nat Commun 10:1683 151. Liu X, Schlexer P, Xiao J, Ji Y, Wang L, Sandberg RB, Tang M, Brown KS, Peng H, Ringe S, Hahn C, Jaramillo TF, Norskov JK, Chan K (2019) Nat Commun 10:32 152. Li J, Wang ZY, McCallum C, Xu Y, Li FW, Wang YH, Gabardo CM, Dinh CT, Zhuang TT, Wang L, Howe JY, Ren Y, Sargent EH, Sinton D (2019) Nat Catal 2:1124–1131 153. Salvatore DA, Weekes DM, He JF, Dettelbach KE, Li YC, Mallouk TE, Berlinguette CP (2017) ACS Energy Lett 3:149–154 154. Dinh CT, Li YGC, Sargent EH (2019) Joule 3:13–15 155. Custelcean R (2021) Chem 7:2848–2852 156. Hori Y, Suzuki S (1983) J Electrochem Soc 130:2387 157. Min X, Kanan MW (2015) J Am Chem Soc 137:4701–4708 158. Li TF, Lees EW, Goldman M, Salvatore DA, Weekes DM, Berlinguette CP (2019) Joule 3:1487–1497 159. Li TF, Lees EW, Zhang ZH, Berlinguette CP (2020) ACS Energy Lett 5:2624–2630 160. Al-Sayari SA (2013) Open Catal J 6:17–28 161. Li YC, Lee GH, Yuan TG, Wang Y, Nam DH, Wang ZY, García de Arquer FP, Lum YW, Dinh CT, Voznyy O, Sargent EH (2019) ACS Energy Lett 4:1427–1431 162. Lees EW, Goldman M, Fink AG, Dvorak DJ, Salvatore DA, Zhang ZH, Loo NWX, Berlinguette CP (2020) ACS Energy Lett 5:2165–2173 163. Zhang ZS, Habibzadeh F, Salvatore DA, Ren SX, Berlinguette GP (2021) Chemrxiv 164. Jiang K, Sandberg RB, Akey AJ, Liu XY, Bell DC, Nørskov JK, Chan K, Wang HT (2018) Nat Catal 1:111–119 165. Chen Z, Wang T, Liu B, Cheng D, Hu C, Zhang G, Zhu W, Wang H, Zhao ZJ, Gong J (2020) J Am Chem Soc 142:6878–6883 166. Zhou Y, Che F, Liu M, Zou C, Liang Z, De Luna P, Yuan H, Li J, Wang Z, Xie H, Li H, Chen P, Bladt E, Quintero-Bermudez R, Sham TK, Bals S, Hofkens J, Sinton D, Chen G, Sargent EH (2018) Nat Chem 10:974–980 167. Xu HP, Rebollar D, He HY, Chong L, Liu YZ, Liu C, Sun CJ, Li T, Muntean JV, Winans RE, Liu DJ, Xu T (2020) Nat Energy 5:623–632 168. Xia C, Zhu P, Jiang Q, Pan Y, Liang WT, Stavitski E, Alshareef HN, Wang HT (2019) Nat Energy 4:776–785 169. Fan L, Xia C, Zhu P, Lu Y, Wang H (2020) Nat Commun 11:3633 170. Xu Y, Miao RK, Edwards JP, Liu SJ, O’Brien CP, Gabardo CM, Fan MY, Huang JE, Robb A, Sargent EH, Sinton D (2022) Joule 6:1333–1343 171. Zhu P, Wang HT (2021) Nat Catal 4:943–951 172. Rosen BA, Salehi-Khojin A, Thorson MR, Zhu W, Whipple DT, Kenis PJA, Masel RI (2011) Science 334:643–644 173. Xie Y, Ou PF, Wang X, Xu ZY, Li YC, Wang ZY, Huang JE, Wicks J, McCallum C, Wang N, Wang Y, Chen TX, Lo BTW, Sinton D, Yu JC, Wang Y, Sargent EH (2022) Nat Catal 5:564–570 174. Del Castillo A, Alvarez-Guerra M, Solla-Gullón J, Sáez A, Montiel V, Irabien A (2015) Appl Energy 157:165–173 175. Jeong HY, Balamurugan M, Choutipalli V, Jeong ES, Subramanian V, Sim U, Nam KT (2019) J Mater Chem A 7:10651–10661 176. Wang YH, Wang ZZ, Dinh CT, Li J, Ozden A, Golam Kibria M, Seifitokaldani A, Tan CS, Gabardo CM, Luo MC, Zhou H, Li FW, Lum YW, McCallum C, Xu Y, Liu MX, Proppe A, Johnston A, Todorovic P, Zhuang TT, Sinton D, Kelley SO, Sargent EH (2019) Nat Catal 3:98–106 177. Zhu WL, Kattel S, Jiao F, Chen JGG (2019) Adv Energy Mater 9 178. Endrodi B, Kecsenovity E, Samu A, Darvas F, Jones RV, Torok V, Danyi A, Janaky C (2019) ACS Energy Lett 4:1770–1777 179. Ju WB, Jiang FZ, Ma H, Pan ZY, Zhao YB, Pagani F, Rentsch D, Wang J, Battaglia C (2019) Adv Energy Mater 9

228

Q. Xia et al.

180. 181. 182. 183.

Ma C, Hou PF, Wang XP, Wang Z, Li WT, Kang P (2019) Appl Catal B 250:347–354 Hou P, Wang X, Wang Z, Kang P, Appl ACS (2018) Mater Interfaces 10:38024–38031 Aeshala LM, Uppaluri RG, Verma A (2013) J CO2 Util 3,4:49–55 Zhang TY, Lin LL, Li ZY, He XY, Xiao SD, Shanov VN, Wu JJ, Appl ACS (2020) Energy Mater 3:1617–1626 Arquer FPGD, Dinh CT, Ozden A, Wicks J, McCallum C, Kirmani AR, Nam DH, Gabardo C, Seifitokaldani A, Wang X, Li YGC, Li FW, Edwards J, Richter LJ, Thorpe SJ, Sinton D, Sargent EH (2020) Science 367:661–666 Zheng TT, Jiang K, Ta N, Hu YF, Zeng J, Liu JY, Wang HT (2019) Joule 3:265–278 Jiang K, Siahrostami S, Zheng TT, Hu YF, Hwang SY, Stavitski E, Peng YD, Dynes J, Gangisetty M, Su D, Attenkofer K, Wang H (2018) Energy Environ Sci 11:893–903 Liu ZC, Yang HZ, Kutz R, Masel RI (2018) J Electrochem Soc 165:J3371–J3377 Liu ZC, Masel RI, Chen QM, Kutz RB, Yang HZ, Lewinski K, Kaplun M, Luopa S, Lutz DR (2016) J CO2 Util 15:50–56 Reyes A, Jansonius RP, Mowbray BAW, Cao Y, Wheeler DG, Chau J, Dvorak DJ, Berlinguette CP (2020) ACS Energy Lett 5:1612–1618 Aeshala LM, Rahman SU, Verma A (2012) Sep Purif Technol 94:131–137 Narayanan SR, Haines B, Soler J, Valdez TI (2011) J Electrochem Soc 158:A167 Nishimura Y, Yoshidaf D, Mizuhata M, Asaka K, Oguro K, Takenaka H (1995) Energy Convers Manag 36:629–632 Larrazabal GO, Strom-Hansen P, Heli JP, Zeiter K, Therkildsen KT, Chorkendorff I, Seger B, Appl ACS (2019) Mater Interfaces 11:41281–41288 Gabardo CM, O’Brien CP, Edwards JP, McCallum C, Xu Y, Dinh CT, Li J, Sargent EH, Sinton D (2019) Joule 3:2777–2791 Kutz RB, Chen QM, Yang HZ, Sajjad SD, Liu ZC, Masel IR (2017) Energy Technol 5:929–936

184.

185. 186. 187. 188. 189. 190. 191. 192. 193. 194. 195.

Flow Cells for Ambient Ammonia Synthesis via Electrocatalytic Nitrogen Reduction Yun Liu, Zhefei Pan, and Liang An

Abstract Ammonia (NH3 ) is considered to be an emerging energy carrier due to its high hydrogen content (17.65 wt. %) and its ease of storage, transportation, and handling. Green ammonia can be produced via electrochemical methods, which converts the renewable electrical energy into chemical energy stored in the ammonia. The application of flow cells for electrochemical ammonia synthesis is one of the ideal paths toward large-scale green ammonia production; however, research on electrochemical ammonia synthesis via flow cells and the understanding of its reaction mechanism is limited. Therefore, in this chapter, the reaction mechanism of electrochemical ammonia synthesis and typical reactors as well as their working principles is first introduced. Subsequently, the various components of the flow cell, such as the membrane, catalyst layer, diffusion layer, and flow fields, and their corresponding functions are described in detail. Besides, the mass transfer mechanisms within the various components of the flow cell and the complex physical and chemical processes involved in the flow cell, and their impact on the electrochemical ammonia synthesis reaction are discussed. Finally, the state of the art of application of flow cells for electrochemical ammonia synthesis is reviewed and summarized. Keywords Green ammonia · Ammonia synthesis · Flow cells · Mass transport mechanism · Membrane reactors

Y. Liu · Z. Pan · L. An (B) Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 L. An et al. (eds.), Flow Cells for Electrochemical Energy Systems, Green Energy and Technology, https://doi.org/10.1007/978-3-031-37271-1_9

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1 Introduction 1.1 Renewable Energy Conversion and Storage The advancement of renewable energy technologies over the past decades has led to an increasing interest in converting renewable energy into sustainable electrical energy [1]. However, the deployment and implementation of renewable energy, such as solar and wind, is limited by its intermittent nature [2, 3]. While these renewable energy sources are ubiquitous across the globe, they also vary in intensity depending on the location and time [4]. These characteristics, thus necessitate the development of advanced energy mediators for the efficient conversion, storage, and transportation of renewable energy. Hydrogen, one of the cleanest fuels in the world, has been considered as an ideal mediator for renewable energy storage [5, 6]. On one hand, hydrogen can be produced by water electrolysis, converting the sustainable electrical energy derived from renewable energy into chemical energy, which is named hydrogen production process. On the other hand, the chemical energy stored in the hydrogen can be transferred into electrical energy by fuel cells, which is named electricity production process. Combining both energy conversion processes, it can be seen that the hydrogen serves as energy mediator, which is generated first and then consumed to achieve the process of transferring the intermittent electrical energy from renewable sources into stable chemical energy, and then converting the chemical energy into stable electrical energy for end-users. This process is also known as hydrogen cycle, which is a promising energy conversion and storage technology [7, 8]. Advantages of this technology include carbon-free characteristic, high energy conversion efficiency, and unlimited resources. Additionally, compared with battery energy storage technology such as lithium-ion battery storage, chemical energy storage technology such as the storage of chemical energy in hydrogen does not have self-discharge characteristics, thus reducing its energy loss [9]. However, the liquefaction conditions of hydrogen are harsh (− 253 °C at 1.0 atm), making its handling, storage, and transportation challenging. In this case, the commercialized cost of the hydrogen cycle is extremely high. As a result, it is necessary to explore new energy mediators.

1.2 Ammonia as Energy Mediator Ammonia, which consists of one nitrogen atom and three hydrogen atoms, has a number of favorable attributes that suggest it could be adopted as an effective energy mediator [10, 11]. Firstly, ammonia can be liquefied under mild conditions (− 33 °C at 1.0 atm), thereby drastically declining handling costs. Secondly, ammonia, on the basis of its molecular structure, has a high hydrogen storage capability of 17.65 wt.%. Thirdly, the carbon-free nature of ammonia makes it an ideal candidate for energy storage and conversion. Fourthly, the global transportation of ammonia by pipeline

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and bulk carrier is an already well-developed technology [12]. Following these considerations, using ammonia as an energy mediator is theoretically feasible. The general application of ammonia as energy mediator for renewable energy conversion and storage involves two processes, i.e., ammonia production and electricity production [13]. On one hand, ammonia can be produced by the electrochemical ammonia synthesis, driven by the sustainable electricity generated from renewable sources and realizing the energy storage process in the form of chemical energy in ammonia. On the other hand, the ammonia is used as fuel fed into the ammonia fuel cells and reacts with oxygen to generate electricity. In this process, the chemical energy stored in ammonia is directly converted into electrical energy, providing power for end-users. Combining these two processes, a nitrogen cycle is achieved, using ammonia as a mediator, to realize the energy storage process from electrical energy to chemical energy in an electrochemical cell, and subsequently the electricity production process from chemical energy to electrical energy in a fuel cell. The advantages of using ammonia as a mediator for energy conversion and storage mainly include its carbon-neutral merit, ease of transport, and storage [14]. However, the current research and development of this technology are still at the early stage of exploration. There are many challenges to overcome, such as the low ammonia production rate and faradaic efficiency of the electrochemical ammonia synthesis, the low performance of ammonia fuel cells, the exploration of high-performance catalyst materials, etc. [15].

1.3 Ammonia Production via Haber–Bosch Process Considering the necessities for the substantial use of renewable energy in the coming decades toward achieving the vision of a carbon-neutral society in 2050, continuous efforts should therefore be undertaken to promote the production, storage, and application of ammonia [16–18]. Currently, ammonia is normally produced via the catalytic reaction of nitrogen and hydrogen in the presence of the iron catalyst as follows [19]: N2 (g) + 3H2 (g) ⇌ 2NH3 (g) ΔH = − 92 kJ/mole

(1.1)

This process was developed in 1909 by German chemists Fritz Haber and Carl Bosch (Haber–Bosch process) [20]. It offers the primary advantage of being able to produce ammonia on a large scale, thereby making it economically feasible. However, as mentioned before, it is an energy-intensive process. According to a study conducted by Ghavam et al., producing one ton of ammonia via the Haber– Bosch process consumes about 30.0 GJ of energy and emits about 2.16 tons of carbon dioxide [21]. Particularly, most of the energy consumed in the process and around 90% of the carbon emissions come from the steam reforming process used to produce hydrogen (gray hydrogen) [22]. Therefore, the ammonia produced by this process is also known as gray ammonia as it is synthesized from gray hydrogen. To

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reduce carbon emissions, a new emerging technology, namely carbon capture and storage (CCS), has been integrated with the traditional Haber–Bosch process [22, 23]. Consequently, the ammonia produced has been given the name blue ammonia [24]. To further reduce the carbon emission, water electrolysis technology will be integrated with the Haber–Bosch process [25, 26]. In this perspective, hydrogen can be directly produced from the electricity transferred from renewable sources. Haber–Bosch process is therefore used to produce the green ammonia [27]. The main challenges are cost, of which about 85% is accounted for by electricity, which remains significantly more expensive than natural gas in most parts of the world [22]. This, in turn, limits the commercialization of such green ammonia production technology. In summary, the continuous improvements of the Haber–Bosch process have brought certain environmental benefits. However, it still has the disadvantage of high carbon emissions and energy consumptions, which obviously cannot meet the production demand of ammonia with low emission and low energy consumption in the future. These therefore necessitate the development and application of new ammonia synthesis technologies.

1.4 Ammonia Production via Electrolysis Process Recently, a promising strategy to synthesize ammonia via electrocatalytic nitrogen reduction has received considerable attention [28, 29]. The primary advantage of this emerging technology is its zero-carbon emission. Besides, the electrochemical ammonia synthesis technology can be directly integrated with the renewable energy, which greatly improves the energy efficiency by eliminating intermediate energy conversion processes. In addition to the above two points, the system size and device size flexibility makes this a technology that can be applied at different scales and is easy to maintain and replace, further reducing the implementation cost. Following these considerations, the ambient electrochemical ammonia synthesis has the great potential to be applied in the carbon-neutral society. In terms of the ammonia production by this technology, with electrical energy generated from renewable sources, ammonia can be directly produced in an electrochemical cell by combining water and nitrogen. Assuming the reaction environment is in an alkaline solution, the electrochemical ammonia synthesis process is described as follows. At the anode, powered by the renewable electricity, the hydroxide ions will be oxidized to produce water and oxygen, and release electrons [30]: 6OH− → 3H2 O + 6e− + 3/2O2 E a0 = + 0.4 V versus SHE

(1.2)

These electrons then transfer from the external circuit to the cathode, and combine with water and nitrogen to produce ammonia and hydroxide ions as follows [31]: N2 + 6H2 O + 6e− → 2NH3 + 6OH− E c0 = − 0.77 V versus SHE

(1.3)

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Thus, the overall reaction in alkaline aqueous media can be expressed as follows: N2 + 3H2 O → 2NH3 + 3/2O2 E 0 = 1.17 V

(1.4)

Theoretically, the above ammonia synthesis can be realized at ambient conditions through applying a voltage as the thermodynamic driving force of the reaction. However, at ambient conditions, the kinetics of nitrogen adsorption and decomposition are extremely slow, resulting in a large overpotential of the electrochemical NRR and a low rate of ammonia production. In addition, the competitive hydrogen evolution reaction will also seriously affect the synthesis rate of ammonia. The currently achieved ammonia yield rates (~ 10−10 to 10−11 mol s−1 cm−2 ) and Faradaic efficiency (FE) (generally less than 10%) of electrochemical NRR are far behind the target (NH3 yield rates of 10−6 mol s−1 cm−2 and FE of 90%) set by the U.S. Department of Energy (DOE), therefore impeding the wide range of applications for electrochemical synthesis of ammonia [32]. Following this perspective, continuous research efforts should be undertaken to promote the development of electrochemical ammonia synthesis.

2 Electrochemical Cells for Ambient Ammonia Synthesis An electrochemical cell is a device capable of generating electrical energy through chemical reactions, such as a fuel cell, or using electrical energy to facilitate chemical reactions, such as an electrolyzer. The electrochemical cells used for ammonia synthesis can be generally divided into single-chamber cells, H-type cells, and flow cells, which convert electrical energy into chemical energy.

2.1 Single-Chamber Cell The single-chamber cell is a three-electrode-based test device. The main components of the single-chamber cell are shown in Fig. 1, which include working electrode, counter electrode, and reference electrode. The working electrode is the most important component of an electrochemical cell on which the electrocatalytic nitrogen reduction reaction is occurring. The working electrode is often used in conjunction with a counter electrode and a reference electrode. The counter electrode is used to close the current circuit in the electrochemical cell. While for the reference electrode, it is used to provide a constant and defined potential, which can be determined by the electrolyte inside the electrode and the reference element used. In addition to the electrodes, the system also features two gas inlets and outlets for supplying reactant nitrogen and exhausting excess gas, respectively. In a single-chamber cell, nitrogen gas is first dissolved in the electrolyte and then transferred to the working electrode. On the working electrode, under the promotion of a catalyst, it reacts

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Fig. 1 Single-chamber cell for electrochemical ammonia synthesis

electrochemically with electrons and water molecules (in alkaline conditions, if it is acidic, it will directly combine with protons) to generate ammonia and hydroxide ions. Meanwhile, under the influence of the electric field force, the hydroxide ions migrate directly from the electrolyte to the anode, release electrons, and the oxygen evolution reaction occurs. Then, the generated oxygen and unreacted nitrogen then escape from the exhaust. The advantage of a single-chamber cell is that it is simple to install and easy to operate and use. However, since there is no membrane to separate the cathodic and anodic reactions, the two reactions may interfere with each other and affect the experimental results.

2.2 H-Type Cell An H-cell is a divided electrochemical cell that is named after its resemblance to the letter H. Essentially, it is made up of two compartments connected by a membrane, as shown in Fig. 2. Similar to the single-chamber cell, the H-cell is also a three-electrode

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test system, including a working electrode, a counter electrode and a reference electrode. In an H-cell, there are two circuits. The first circuit consists of a reference electrode and a working electrode, which is used primarily for measuring the potential of the electrochemical reaction on the working electrode. An alternative circuit is made up of a working electrode and a counter electrode which transmits electrons to form a circuit. In the H-cell, N2 (g) enters from the cathode chamber gas pipeline and dissolves in the electrolyte as N2 (aq) in the solution, and then migrates to the working electrode. On the working electrode, N2 (gas phase) forms a threephase reaction interface with the catalyst material (solid phase) and the electrolyte (liquid phase). At this interface, N2 (aq) electrochemically reacts with electrons and water molecules under the promotion of catalysts to generate ammonia and hydroxide ions. Meanwhile, the hydroxide ions are driven by the electric field through the anion exchange membrane from the cathode compartment to the anode compartment. On the counter electrode, the hydroxide ions are oxidized, releasing electrons, and generating oxygen and water. In this process, there are two mass transport processes that need to be pointed out. The first is that electrons pass from the anode to the cathode through an external circuit to form an electronic flow, and the second is that the hydroxide ions pass through the anion exchange membrane from the cathode to the anode to form an ionic flow. Macroscopically, a circuit is formed by the electronic flow and ionic flow. In comparison with single-chamber cell, the advantages of H-cell can be summarized as follows: (1) This minimizes the risk of interference caused by byproducts generated at the counter electrode. (2) The separation and/or collection of products (especially gases) is facilitated by the separation of the cathode and anode compartments. However, the structure of H-cell is not suitable for large-scale industrial production, which limits its applications.

Fig. 2 H-type cell for electrochemical ammonia synthesis

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2.3 Flow Cell In addition to the typical three-electrode test system, another electrochemical reaction cell with potential for larger scale applications is the flow cell. The basic structure of a flow cell is shown in Fig. 3 and can be broadly divided into two types: the first is a flow cell with the membrane electrode assembly (MEA) structure as the core component, which is commonly used in fuel cells. The second is a typical flow cell structure, where both the anolyte and catholyte can be circulated, thus allowing for an adequate supply of reactants and the timely removal of products from the flow cell system. In terms of the MEA-type electrolytic cell, the main structure of the flow cell consists of the cathode, the anode, and the membrane separating the cathode and the anode. From the point of view of the components, the flow cell is made up of the MEA and the flow field. As mentioned earlier, the MEA is the core component of this type of flow cell, including the ion exchange membrane, the anode and cathode catalytic layers, and the anode and cathode diffusion layers. Ion exchange membranes are used to conduct the key ions involved in a reaction and include mainly cation exchange and anion exchange membranes, which are used to conduct cations and anions, such as protons or hydroxide ions, respectively. In addition, ion exchange membranes play a role in separating the cathode from the anode, due to their inherent insulating properties. On both sides of the membrane are the cathodic catalytic layer and the anodic catalytic layer, which are the main sites for the electrochemical synthesis of ammonia, with the nitrogen reduction reaction occurring in the cathodic catalytic layer and the oxygen precipitation reaction occurring in the anodic catalytic layer. Next to the catalytic layer are the corresponding cathodic diffusion layer and anodic diffusion layer, respectively. The main functions of the diffusion layer are to provide a channel for the supply of reactants and removal of products, to conduct electrons, and to act as a backing layer to support the catalytic layer. Next to the diffusion layer are the cathode flow field and the anode flow field. The main function of the flow field is to supply the entire reaction system with reactants and to remove the products from the flow cell. In a flow cell based on the MEA structure, the nitrogen reduction reaction proceeds as follows: the nitrogen is supplied from the cathode flow field, enters through the cathode flow field inlet and then passes through the cathode diffusion layer to the cathode catalytic layer. Then, on the cathode catalyst layer, the nitrogen gas reacts with water molecules (here the electrolyte is alkaline) and electrons to produce ammonia and hydroxide ions in the presence of a catalyst. The product ammonia is then discharged from the cathode flow field outlet together with the unreacted nitrogen. The hydroxide ions pass through the anion exchange membrane under the influence of the electric field and reach the catalytic layer of the anode, where they are oxidized by the catalyst to form oxygen, water molecules, and the release of electrons. The anodic reaction products, oxygen and unreacted anodic fluid, exit the anodic flow field while the electrons migrate along the external circuit to the cathode. For a typical flow cell, the main difference to a flow cell based on the MEA structure is the structure. In a typical flow cell, the ion exchange membrane is flanked by the flowing catholyte and anolyte, respectively, rather than directly by

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the catalytic layer. The process of the nitrogen reduction reaction for this structure is therefore also different from that described previously. The nitrogen is still supplied by the cathodic flow field, entering through the cathodic flow field inlet and then passing through the cathodic diffusion layer to the cathodic catalytic layer. Then, on the cathode catalyst layer, the nitrogen gas reacts with water molecules (here the water molecules come directly from the catholyte, whereas in the MEA-type flow cell they come from the diffusion of water molecules from the anode) and electrons to form ammonia and hydroxide ions in the presence of a catalyst. The product ammonia is then incorporated into the cathode solution and discharged together with the catholyte. The unreacted nitrogen gas exits the cathode flow field. For the anode catalyst layer, the diffused hydroxide ions from the anolyte are oxidized by the catalyst to produce oxygen, water molecules, and the release of electrons. The anode reaction products, oxygen and unreacted anode solution, are discharged together at the anode outlet while the electrons migrate along the external circuit to the cathode. One of the major advantages of flow cells for electrochemical nitrogen reduction is the theoretically unlimited supply of reactants and the timely removal of products, which makes them feasible for large-scale industrial applications. The next section will focus on the materials and mass transport mechanisms of the various components of the flow cell for electrochemical nitrogen reduction.

3 Key Materials and Components The flow cell is an electrochemical device used for nitrogen reduction reactions at ambient conditions. Its main components include an ion exchange membrane, catalyst layer, diffusion layer, and flow field, as shown in Fig. 4. This section will detail the structure, and its main functions.

3.1 Membranes Membranes are a key component of flow cells, whose main functions are to separate the anode from the cathode, to conduct the charge carriers, and to transfer substances. In terms of conducting charge carriers, membranes can be divided into several types such as cation exchange membranes (H+ , K+ , and Na+ ), anion exchange membranes (OH− , HCO3 − , CO3 2− ), and bipolar membranes (H+ and OH− ) [33]. In electrocatalytic nitrogen reduction, commonly used include cation exchange membranes and anion exchange membranes. The membranes used in flow cells consist primarily of ionic groups, which are used to generate pathways for specific ions, and a polymer backbone, which is used to generate the three-dimensional structure of the membrane. The key parameters used to determine membrane properties include thickness, specific area resistance, ion exchange capacity, water uptake, swelling ratio, chemical and mechanical stability. In terms of the membrane thickness, thinner

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Fig. 3 a MEA-type electrolytic cell for electrochemical ammonia synthesis. b Typical flow cell for electrochemical ammonia synthesis

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Fig. 4 Schematic illustration of a flow cell for electrochemical ammonia synthesis

membranes typically have higher performance because they shorten the transport channels for ions, thereby reducing ohmic losses; however, thin membranes also face the challenge of mechanical damage or electrochemical degradation. For the specific area resistance, the lower the value, the lower the corresponding resistance of the membrane. In contrast, for the ion exchange capacity, the higher the value is for the ion exchange capacity of the membrane, the better performance of the membrane in delivering charge carriers. Regarding the water uptake of the membrane, this property will greatly affect the membrane conductivity. The swelling ratio of the membrane is determined by comparing the length of the membrane before and after immersion in water or solution. The chemical and mechanical stability, is another key parameter in interpreting membrane performance, especially when considering industrial applications.

3.2 Electrodes 3.2.1

Catalyst Layers

The catalyst layer is the place in a flow cell where the actual electrochemical reactions take place. The main function of the catalyst layer is to construct the reaction interfaces (such as triple-phase interfaces), conduct electrons (e− ), and transport ions (such as H+ and OH− ) and other substances such as N2 and NH3 . In terms of the structure of the catalyst layer, it is mainly consisting of three species, the catalyst powers, carbon black, and binder. The catalyst powers are the catalyst nanoparticles,

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which are evenly distributed on the surface of carbon support within the electrode. While for the carbon black (such as Vulcan XC 72R), it is mainly used to support the catalyst powders and conduct electrons. To combine the distributed, the catalyst nanoparticles along with the formation of the catalyst layer structure, the binder is used. Commonly used binders include alkaline ionomers and nafion solutions for the conduction of anions and cations, respectively. The purpose of forming the layer structure mainly includes the following perspectives: (1) It is convenient for reactants transport as well as products removal. (2) The layer structure is benefit for the construction of triple-phase reaction interfaces. (3) Such a layer structure may allow for the exits of more reaction active surfaces. Regarding the performance determination of the catalyst layer, the key parameters include the catalyst loading and hydrophilic/hydrophobic of the catalyst layer. The catalyst loading will influence the performance of the flow cell through effects on the thickness of the catalyst layer and the number of the electrochemical reaction active sties, etc. While for the hydrophilic/ hydrophobic of the catalyst layer, it is mainly modified by mixing the PTFE solution into the binders. The purpose of adjusting the hydrophilic/hydrophobic of the catalyst layer is to achieve an efficient water management in the flow cells thus to improve the performance of flow cells.

3.2.2

Diffusion Layers

Another important component in the flow cell is the diffusion layer (DL), in which main function is to transport the species within the flow cells, including reactants and products of the electrochemical reactions, as well as the electrons released or consumed by the electrochemical reactions on the electrodes. In addition, the diffusion layer serves as a support structure for the catalyst layer. The main structure of the diffusion layer is the porous structure, as shown in Fig. 5. The commonly used materials for diffusion layer include carbon-based materials such as carbon cloth and carbon paper, etc., as well as metal-based materials such as nickel and copper foams, etc. The key parameters used to measure the performance of the diffusion layer include its porosity, thickness, conductivity, strength, and hydrophilicity/ hydrophobicity. In terms of the porosity, large porosity sustains an efficient mass transport while the conductivity as well as the strength would be relatively weaker. However, the mass transport within the flow cells would be influenced when the porosity is too low. The general porosity of carbon paper and carbon cloth is about 70–80%. Regarding the thickness, which indicates the distance of the mass transport in the diffusion layer (about few hundred microns), will influence the timely supplying reactants as well as removing products. These merits therefore influence the concentration of the species on the catalyst layer, thereby affecting the electrochemical reactions behaviors, such as the concentration polarization and flooding. For conductivity and strength, highly relies on the materials used. To facilitate the water management in the flow cells, the diffusion layer can be modified to be hydrophilic/ hydrophobic. For example, the carbon paper can be treated by PTFE to enhance twophase flow in the anode and cathode. However, the addition of PTFE significantly

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Fig. 5 Microscopic view of a carbon paper; b carbon cloth

reduces the electrical conductivity of the diffusion layer. In practical applications, a microporous layer (MPL) is generally applied between the diffusion layer and catalyst layer, composed of carbon particles and fluoropolymer. The main functions of introducing the MPL include the following perspectives: (1) Improve catalyst utilization. The pore size of the MPL is around 0.05–1.0 µm, which is far smaller than that of the diffusion layer (about 20 µm). As a result, the catalyst nanoparticles smaller than 20 µm in diameter won’t penetrate into the diffusion layer, thus improving the catalyst utilization. (2) Reduce the contact resistance between catalyst layer and diffusion layer. (3) Protect the catalyst layer and membrane from mechanical damage that may result from direct contact with the DL. (4) Modulation of the hydrophobicity of the MPL by regulating the amount of fluoropolymer in the MPL to achieve better water management, for example to prevent flooding at the DL/CL interface.

3.3 Flow Fields The main function of the flow field is supplying adequate reactants to the catalyst layer while minimizing pressure drop, and removing the products. The structure of a flow field consists of two main components, the flow channels and the ribs. A parameter used to measure the typical characteristics of a flow field is the open ratio, which is the ratio of the effective area of the flow channel to the overall flow field area. In general, the commonly used open ratio in the flow field is about 50%. Common flow fields used in flow cells include serpentine, parallel, interdigitated and spot flow fields, as shown in Fig. 6. The serpentine flow field has only one flow channel from the flow field inlet to the flow field outlet, therefore the mass transport process in the flow channel is forced, which is transferred from upstream to downstream of the flow field. As a result, a pressure drop occurs in the adjacent upstream and downstream flow channels due to the presence of along-travel losses, and this pressure drop, in turn, facilitates the under-rib convection of reactants and products transport. In addition, due to the pressure drop losses along the flow path, the pressure at the inlet of the flow channel is much greater than the pressure at the outlet of the flow channel.

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For parallel flow fields, the mass transport between adjacent flow channels is almost simultaneous, as a result, there is no significant pressure drop between adjacent flow channels. Therefore, the mass transport between neighboring flow channels does not affect each other. For example, if one of the flow channels becomes blocked due to a large air bubble, the rest of the flow channels can still function normally. However, as there is no significant pressure drop, parallel flow fields are prone to local blockages. In addition, mass transport below the ribs in a parallel flow field is entirely dependent on diffusion, which is detrimental to the uniform distribution of reactants and the timely removal of products. In the case of interdigitated flow fields, the inlet and outlet of the flow field are separated by ribs. Mass transport between adjacent flow channels can only rely on under-rib convection and diffusion facilitated by the pressure difference. Regarding the spot flow field, the mass transport can be multidimensional and multidirectional from the upstream to the downstream side of the flow field. In addition, there is a tendency for localized dead zones to occur throughout the flow field, i.e. where the fluid is static or blocked by bubbles. In recent years, new flow field designs have emerged, such as the 3D fine-mesh flow field. Each flow field has its own unique structural design and corresponding mass transport mechanism, and the specific flow field to be used will depend on the specific application.

3.4 Electrolytes The main function of the electrolyte is to provide the reaction environment for the electrochemical reactions taking place in the catalytic layer in the flow cells, e.g., by providing the reactants, either in the dissolved or ionic state. Secondly, electrolytes can also be classified according to their corresponding pH as acidic (pH < 7), neutral (pH = 7), and basic (pH > 7). Electrolyte solutions generally consist of an electrolyte, which includes commonly used acids and alkaline salts, and a solvent, which is usually water or an organic solvent. In addition, due to the sluggish kinetics of the nitrogen reduction reaction, the competing hydrogen precipitation reaction is generally very reactive in aqueous-based electrolytes, resulting in very low Faraday efficiencies for the nitrogen reduction reaction. Based on the above considerations, the use of non-aqueous organic solvents is considered to be beneficial in principle to facilitate the nitrogen reduction reaction. In addition, the use of some lithium salt-based electrolytes can improve the selectivity and yield of ammonia synthesis by altering the reaction path of the nitrogen reduction reaction, details of which will be discussed in detail in Sect. 5.

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Fig. 6 Typical flow fields: a serpentine; b parallel; c interdigitated; d spot. Reproduced with permission from [34]. Copyright © 2014, The Author(s)

4 Flow and Transport 4.1 Flow Fields Mass transport in the flow field consists of two main parts, namely in plane mass transport and through plane mass transport. In an electrocatalytic nitrogen reduction reaction system, e.g., in an alkaline reaction system, mass transport in the anode flow field consists mainly of in plane mass transport from the flow channel inlet through the flow field upstream to the flow field downstream and then to the flow channel outlet, and through plane mass transport of OH− from the flow channel to the contact interface between the flow channel and the diffusion layer. In terms of the through plane direction, the mass transport behavior in the flow channel can be divided into several stages. The first is the mass transport from the solution in the

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flow channel toward the contact interface between the flow channel and the diffusion layer, the driving forces for this process include pressure differences, concentration differences, etc. The second stage is the transfer of mass from the contact interface to the interior of the diffusion layer. Secondly, a mixed gas–liquid two-phase flow can be observed in the anode flow channel because of the oxygen precipitation reaction occurring on the anode catalytic layer. However, the H2 O and O2 produced by the anodic oxidation reaction are transferred in the opposite direction, from the catalytic layer through the diffusion layer and then across the contact interface between the diffusion layer and the flow channel into the flow channel, where they are discharged from the anode outlet together with the unreacted anolyte. Similar to the anode flow field, the mass transport in the cathode flow field also consists of two parts, in plane mass transport and through plane mass transport, except that the reactants become N2 , and the mass transport rate of the gas is much higher than that of the liquid.

4.2 On the Cathode As the electrochemical synthesis of ammonia takes place at the cathode, the mass transport processes that occur at the cathode and their corresponding electrochemical reactions are described here first. The mass transport process in the cathode can be divided into several stages. The first is the mass transport from the contact interface between the flow channel and the diffusion layer to the diffusion layer and the interior of the catalytic layer. Since most of the experiments use serpentine flow channels and there is a pressure difference between the adjacent flow channels, the mass transport in the diffusion layer under the ribs is mainly based on convection due to the pressure difference, i.e. in plane mass transport. The other direction of mass transport is along the diffusion layer toward the catalyst layer, i.e. through plane mass transport, where the main mass transport behavior is the diffusion of nitrogen driven by pressure and concentration differences. Secondly, the nitrogen crosses the contact interface between the diffusion layer and the catalytic layer and then enters the catalytic layer to participate in the reduction reaction by combining water molecules and electrons to form NH3 and OH− . The ammonia produced by the reaction, together with the unreacted nitrogen, is transferred in the reverse direction from the catalytic layer through the contact interface between the catalytic layer and the diffusion layer into the diffusion layer, and then diffuses across the diffusion layer to reach the contact interface between the diffusion layer and the flow channel, before crossing the interface into the cathode flow channel and exiting at the cathode outlet. The mass transport behavior of the cathode also includes the transport of electrons through the external circuit from the cathode collector through the cathode flow field and the cathode diffusion layer to the cathode catalyst layer. It is worth noting that one of the important reactants on the cathode, the water molecules, can be obtained from two sources. Firstly, the nitrogen can be humidified directly and the water molecules enter the diffusion layer of the cathode along with the nitrogen molecules from the flow field of the cathode and then onto the catalyst

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layer of the cathode to participate in the reaction. Secondly, if the nitrogen is not humidified, then the water molecules on the cathode are sourced from the anolyte. Specifically, the water molecules in the anolyte diffuse across the membrane to the cathode side. Since the catalyst layer of the cathode is a porous layer, driven by the difference in water molecule concentration and capillary forces, water molecules near the cathode side of the membrane are transported to the cathode catalytic layer and thus participate in the electrochemical reaction. However, if there are too many water molecules on the cathode side, the water molecules will pass through the cathode catalyst layer into the cathode diffusion layer, and the water molecules that enter the porous structure inside the cathode diffusion layer will block the nitrogen transport path, thus affecting the performance and efficiency of the electrochemical reaction. In addition, too many water molecules in the cathode catalytic layer can intensify the hydrogen evolution reaction, which is a competing reaction to the main nitrogen reduction reaction. Therefore, when applying a flow cell for electrochemical ammonia synthesis, water management on the cathode side has a significant impact on the efficiency of electrochemical ammonia synthesis.

4.3 On the Anode In an electrocatalytic nitrogen reduction system, the reaction that takes place at the anode is the oxygen precipitation reaction (OER), where the reactants of this reaction, OH− , are oxidized at the anode catalytic layer to produce oxygen, water, and the release of electrons. Therefore, the mass transport at the anode consists mainly of the mass transport in the anode flow field, the mass transport in the anode diffusion layer, and the mass transport in the anode catalytic layer. Firstly, the mass transport process in the anode flow field consists of two main parts, which have already been described in Sect. 4.1. The second stage is the transfer of mass from the contact interface between the flow channel and the diffusion layer to the diffusion layer and to the interior of the catalyst layer. Similar to cathode, in the anode, there are two main types of mass transport behavior, namely in plane mass transport and through plane mass transport. As most of the serpentine flow channels are used in the experiments and there is a hydraulic pressure difference between the adjacent flow channels, the mass transport in the diffusion layer under the ribs is mainly based on convection due to the pressure difference, i.e. in plane direction. The other direction of mass transport is along the diffusion layer toward the catalyst layer, i.e. through plane direction. In the catalyst layer, OH− is oxidized to form oxygen and water, and electrons are released. The oxygen and water are transferred in the reverse direction from the anode catalyst layer to the anode flow field and then out the anode outlet. It is worth noting that due to the presence of oxygen in the anode, a two-phase flow occurs inside the anode during the reaction. In addition, the electrons released by the oxidation reaction in the anode catalyst layer will be transferred along the catalyst layer, diffusion layer, flow field plate to the collector fluid, and then across the external circuit to the cathode.

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4.4 Through the Membrane In electrochemical ammonia synthesis systems, the main role of membranes is to conduct charge carriers. In an alkaline reaction system, for example, the alkaline membrane used in a flow cell is to conduct OH− . Theoretically, the concentration of OH− ions in the anode of a flow reactor is much higher than that of the cathode, and the direction of OH− transport across the membrane should be from the anode to the cathode. However, in practice the main direction of conduction of the OH− ion concentration within the membrane is from the cathode to the anode. The main driving force for the conduction of this inverse concentration gradient is the electric field force. As the electrons released from the anode reaction reach the cathode via an external circuit, thus making the cathode side negatively charged, and the anode side, due to the loss of electrons, positively charged overall, the electric field between the anode and cathode is directed from the anode to the cathode, therefore the anions migrate from the cathode to the anode by the electric field force, and carry water molecules with them during the migration. In addition to the conduction of OH− , water molecules can also be transferred within the membrane due to the intrinsic properties of the membrane. Since the concentration of water molecules at the anode is much higher than at the cathode, the direction of conduction of water molecules within the membrane is mainly from the anode to the cathode. In addition, as the membrane is also permeable to ammonia, ammonia synthesized on the catalyst layer of the cathode may also diffuse across the membrane from the cathode to the anode in the presence of a concentration gradient.

5 Current Status and Challenges 5.1 Electrocatalysts The catalyst is one of the most important materials in the membrane electrode assembly, which is the core component of the flow cell. Its main function is to provide the active site for the nitrogen reduction reaction and to build the three-phase reaction interface on the catalyst layer. Nash et al. [35] investigated five different noble metal catalysts (Pt/C, Ir/C, Pd/C, Ru/C, and Au/C) applied in a flow cell for the electrochemical synthesis of ammonia. The results show that, in agreement with the three-electrode system, hydrogen evolution reaction remains the dominant reaction, indicating that the noble metal catalysts are not favorable for the electrochemical synthesis of ammonia under aqueous conditions. Furthermore, the ammonia yield of the flow cell with a proton exchange membrane was an order of magnitude higher than that of the flow cell with an anion exchange membrane (OH− ). In contrast, the Faraday efficiency of a flow cell with an anion exchange membrane (OH− ) is higher than that of a flow cell with a proton exchange membrane.

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Fig. 7 Schematic illustration of two-electrode configured flow-type electrochemical reactor and photographs of assembled reactor and individual cell components. Reproduced with permission from [36]. Copyright © 2021, The Author(s)

In a flow cell, a typical catalyst application is to fix the catalyst to the catalyst layer by mixing it with binder. However, the low solubility of nitrogen and the limited mass transfer efficiency make it difficult to establish three-phase interfaces for the electrocatalytic nitrogen reduction reaction on the catalyst layer, thus making the ammonia synthesis less efficient. Following these considerations, Li et al. [36] invented a novel catalyst application method in which the catalyst particles are mixed directly with the electrolyte (Fig. 7). More specifically, they homogeneously dispersed Ag nanodot in a 0.1 M Na2 SO4 solution (pH = 10.5) and could achieve an NH3 yield of 600.4 ± 23.0 µg h−1 mg Ag−1 with a Faraday efficiency (FE) of 10.1 ± 0.7% at − 0.25 V versus RHE. In addition to the above methodology for the implementation of catalysts, the application of non-precious metals and their different forms in flow cells for the electrochemical synthesis of ammonia still deserves further investigation.

5.2 Electrolytes In electrocatalytic nitrogen reduction systems, a key factor that hinders the yield and Faraday efficiency of the nitrogen reduction reaction is that the hydrogen evolution reaction on the electrode surface is excessively active. Therefore, regulating the number of water molecules on the catalyst surface by adjusting the water content of the electrolyte is one way to effectively improve the Faraday efficiency. As shown in Fig. 8a, a non-aqueous electrolyte is used to moderate H+ in the vicinity of the gas diffusion layer to retard the hydrogen precipitation reaction and improve NH3 selectivity. However, in this application, the use of conventional carbon-based gas diffusion layers (e.g., carbon cloth) for non-aqueous nitrogen reduction reactions is still hampered by the lack of hydrophobic repulsion between the electrolyte and

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Fig. 8 a Flooding in a carbon-based GDL in contact with a non-aqueous electrolyte due to absence of hydrophobic repulsion; b stainless steel cloth GDL and slight pressure gradient (ΔP) applied to prevent flooding. Reproduced with permission from [37]. Copyright © 2020, Springer Nature

the gas diffusion layer, leading to flooding. Although hydrogen precipitation reactions caused by water flooding are not a problem for non-aqueous electrolytes, the distance that the reactant nitrogen must diffuse in the water flooded section reduces the maximum current density achievable for this reaction system. To solve this problem, Lazouski et al. [37] used stainless steel cloth as a diffusion layer in the electrochemical synthesis of ammonia from lithium salts in a tetrahydrofuran electrolyte. Unlike carbon-based materials, the steel cloth does not absorb the electrolyte through capillary forces. Permeation of the electrolyte is further controlled by a slight pressure gradient across the gas diffusion layer (Fig. 8b), which establishes a clear separation of gas and liquid. In this design, the FE of NH3 is substantially increased compared with no pressure as the diffusion pathway for N2 is minimized. In addition to the use of non-aqueous electrolytes, another type of electrolyte containing lithium salts has received much attention from researchers in recent years as the lithium in it can react with nitrogen to form lithium nitride, changing the reaction path of electrochemical ammonia synthesis and thus greatly improving the Faraday efficiency of nitrogen reduction [38–41]. However, research related to the electrochemical synthesis of ammonia using lithium salts as electrolytes in applied flow cells is still limited and is one of the important directions for achieving large-scale green ammonia synthesis in the future, which deserves in-depth investigation.

5.3 Electrodes To address the problem of low nitrogen solubility, Wei et al. [42] developed a novel electrocatalytic nitrogen reduction system with a flow cell as the core for ammonia

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Fig. 9 Schematic illustration of a three-compartment flow cell, b gastight electrochemical system, c N2 diffusion through the Ru/CB CNT, and d three-phase electrocatalytic N2 reduction reaction at the gas–electrolyte–solid interface. Reproduced with permission from [42]. Copyright © 2021, American Chemical Society

production. Using ruthenium/carbon black as the catalyst, the solid–liquid–gas threephase reaction interface was constructed on the gas diffusion layer in the flow cell by hydrophobic modification of the electrode, which improved the mass transfer of nitrogen at the reaction interface as well as the local pH environment at the reaction site. Their experimental results show that the modified electrode exhibits satisfactory performance for the electrochemical synthesis of ammonia, achieving a high NH3 yield of 9.9 × 10−10 mol/cm2 s at − 0.1 V versus RHE, corresponding to a maximum Faraday efficiency of 64.8% (Fig. 9). In the electrochemical synthesis of ammonia using flow cells, as the membrane is permeable to ammonia, the ammonia synthesized on the catalytic layer of the cathode diffuses across the membrane to the anode due to the concentration gradient. This will have two negative effects: firstly, the amount of ammonia collected on the cathode side will be reduced, making the detected ammonia lower than the actual ammonia produced. Secondly, the catalyst on the anode catalyst layer may be deactivated by ammonia poisoning due to the diffusion of ammonia to the anode. To solve these problems, Chen et al. [43] devised a new membrane electrode structure by adding a diffusion layer between the cathode catalytic layer and the membrane (Fig. 10). With the introduction of this diffusion layer, the ammonia generated by the nitrogen reduction reaction on the cathode catalyst layer can be effectively retained in the cathode catalyst layer without diffusing to the anode. The results show that introducing this additional diffusion layer significantly reduces the ammonia crossover and increases the amount of ammonia accumulation on the cathode side.

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Fig. 10 Schematic view of the improved design in the three-phase reactor for electrochemical ammonia synthesis. Reproduced with permission from [43]. Copyright © 2017, American Chemical Society

5.4 Operating Conditions Water plays an important role in the application of flow cells to the reaction system for the electrochemical synthesis of ammonia. Taking the alkaline electrolyte as an example, firstly, it can be deduced from the electrochemical reaction mechanism that water molecules are consumed in the catalyst layer of the cathode due to the nitrogen reduction reaction. On the anode, water is produced due to the oxygen evolution reaction. There are two main sources of water molecules on the cathode. The first source comes directly from the humidification of the gas on the cathode side and the second comes indirectly from the diffusion of water molecules across the membrane in the anode electrolyte. However, if too much water diffuses across the membrane, then the water in the catalyst layer of the cathode may further enter the cathode diffusion layer under the influence of concentration gradients and capillary forces. As the water molecules fill the porous structure inside the diffusion layer, the nitrogen gas on the cathode side cannot be efficiently transferred to the cathode catalyst layer, thus causing a water flooding phenomenon and affecting the efficiency of electrochemical ammonia synthesis. Therefore, to investigate this phenomenon and its mechanism of mass transfer, Pan et al. [44] investigated the water flooding phenomenon in a flow cell for ammonia production via electrocatalytic nitrogen reduction by combining visualization methods and testing the performance of the electrochemical cell. In addition, the effects of the operating conditions of the reaction such as nitrogen flow rate, applied current density, and membrane thickness on the water cross-flux and ammonia production rate were comprehensively investigated.

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Fig. 11 Illustration of a setup, b working principle, c short-term operation, and d long-term operation of a flow cell for electrocatalytic nitrogen reduction reaction. Reproduced with permission from [44]. Copyright © 2022, Elsevier

The underlying mechanisms of water transport through the membrane, including diffusion and electro-osmotic resistance, are precisely revealed to provide insight into the behavior of water flooding in flow cells (Fig. 11).

6 Concluding Remarks and Perspectives Electrochemical ammonia synthesis using flow cells is a necessary way to achieve industrial green ammonia synthesis. However, research on the electrochemical synthesis of ammonia based on flow cells and their reaction mechanisms is limited. Therefore, this chapter firstly introduces the important role of ammonia as an energy storage medium in the future storage and conversion of renewable energy sources. Next, the reaction mechanism of electrochemical ammonia synthesis and typical reactors used for electrocatalytic nitrogen reduction reactions and their working principles are presented. Then, the focus is on the various components of a flow cell and their main functions, such as the membrane, the catalytic layer, the diffusion layer, and the flow field. In addition, the mechanism of mass transport within each component of the flow cell and the complex physical and chemical processes involved within the flow cell and their influence on the electrochemical ammonia synthesis reaction are introduced. Finally, key research advances in the application of flow cells for electrochemical ammonia synthesis in recent years are reviewed and summarized. The content of this chapter points the way to the future development of flow batteries for applications in electrocatalytic nitrogen reduction reactions.

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References 1. Vakulchuk R, Overland I, Scholten D (2020) Renew Sustain Energy Rev 122:109547 2. Xu X, Wei Z, Ji Q, Wang C, Gao G (2019) Resour Policy 63 3. Qazi A, Hussain F, Rahim NABD, Hardaker G, Alghazzawi D, Shaban K, Haruna K (2019) IEEE Access 7:63837 4. Guo S, Liu Q, Sun J, Jin H (2018) Renew Sustain Energy Rev 91:1121 5. Shi L, Qi S, Qu J, Che T, Yi C, Yang B (2019) Int J Hydrogen Energy 44:5345 6. Kojima Y (2019) Int J Hydrogen Energy 44:18179 7. Helen McCay M (2013) Future energy: improved, sustainable and clean options for our planet, p 495 8. D’Ovidio G, Masciovecchio C, Rotondale A (2016) IET Intell Transp Syst 10:573 9. Tan KM, Babu TS, Ramachandaramurthy VK, Kasinathan P, Solanki SG, Raveendran SK (2021) J Energy Storage 39:102591 10. Xue M, Wang Q, Le Lin B, Tsunemi K (2019) ACS Sustain Chem Eng 7:12494 11. Zendrini M, Testi M, Trini M, Daniele P, Van Herle J, Crema L (2021) Int J Hydrogen Energy 46:30112 12. Valera-Medina A, Xiao H, Owen-Jones M, David WIF, Bowen PJ (2018) Prog Energy Combust Sci 69:63 13. Juangsa FB, Irhamna AR, Aziz M (2021) Int J Hydrogen Energy 46:14455 14. Zhou M, Wang Y, Chu Y, Tang Y, Tian K, Zheng S, Chen J, Wang Z (2019) Energy Procedia 158:4986 15. Chatterjee S, Parsapur RK, Huang KW (2021) ACS Energy Lett 6:4390 16. Nayak-Luke R, Bañares-Alcántara R, Wilkinson I (2018) Ind Eng Chem Res 57:14607 17. Ye L, Nayak-Luke R, Bañares-Alcántara R, Tsang E (2017) Chem 3:712 18. Fasihi M, Weiss R, Savolainen J, Breyer C (2021) Appl Energy 294:116170 19. Kyriakou V, Garagounis I, Vourros A, Vasileiou E, Stoukides M (2020) Joule 4:142 20. Humphreys J, Lan R, Tao S (2021) Adv Energy Sustain Res 2:2000043 21. Ghavam S, Vahdati M, Wilson IAG, Styring P (2021) Front Energy Res 9:1 22. The Royal Society (2020) Ammonia: zero-carbon fertiliser, fuel and energy store. The Royal Society, London, UK 23. Yüzbasıoglu AE, Tatarhan AH, Gezerman AO (2021) Heliyon 7 24. Arnaiz del Pozo C, Cloete S (2022) Energy Convers Manag 255:115312 25. Chehade G, Dincer I (2021) Fuel 299:120845 26. Smart K (2022) Johnson Matthey Technol Rev 27. Fúnez Guerra C, Reyes-Bozo L, Vyhmeister E, Jaén Caparrós M, Salazar JL, Clemente-Jul C (2020) Renew Energy 157:404 28. Zhao R, Xie H, Chang L, Zhang X, Zhu X, Tong X, Wang T, Luo Y, Wei P, Wang Z, Sun X (2019) EnergyChem 1:100011 29. Montoya JH, Tsai C, Vojvodic A, Nørskov JK (2015) ChemSusChem 8:2180 30. Hou J, Yang M, Zhang J (2020) Nanoscale 12:6900 31. Guo W, Zhang K, Liang Z, Zou R, Xu Q (2019) Chem Soc Rev 48:5658 32. Tang C, Qiao SZ (2019) Chem Soc Rev 48:3166 33. Lees EW, Mowbray BAW, Parlane FGL, Berlinguette CP (2022) Nat Rev Mater 7:55 34. Liu H, Li P, Juarez-Robles D, Wang K, Hernandez-Guerrero A (2014) Front Energy Res 2:1 35. Nash J, Yang X, Anibal J, Wang J, Yan Y, Xu B (2017) J Electrochem Soc 164:F1712 36. Li W, Li K, Ye Y, Zhang S, Liu Y, Wang G, Liang C, Zhang H, Zhao H (2021) Commun Chem 4:1 37. Lazouski N, Chung M, Williams K, Gala ML, Manthiram K (2020) Nat Catal 3:463 38. Du HL, Chatti M, Hodgetts RY, Cherepanov PV, Nguyen CK, Matuszek K, MacFarlane DR, Simonov AN (2022) Nature 609:722 39. Li S, Zhou Y, Li K, Saccoccio M, Sažinas R, Andersen SZ, Pedersen JB, Fu X, Shadravan V, Chakraborty D, Kibsgaard J, Vesborg PCK, Nørskov JK, Chorkendorff I (2022) Joule 6:2083

Flow Cells for Ambient Ammonia Synthesis via Electrocatalytic … 40. 41. 42. 43.

253

Klein CK, Manthiram K (2022) Joule 6:1971 Chen Y, Gu S, Li W (2022) Joule 6:1973 Wei X, Pu M, Jin Y, Wessling M (2021) ACS Appl Mater Interfaces 13:21411 Chen S, Perathoner S, Ampelli C, Mebrahtu C, Su D, Centi G (2017) ACS Sustain Chem Eng 5:7393 44. Pan Z, Khalid F, Tahir A, Esan OC, Zhu J, Chen R, An L (2022) Fundam Res 2:757