Green Production of Carbon Nanomaterials in Molten Salts and Applications 981152372X, 9789811523724

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Ali Reza Kamali

Green Production of Carbon Nanomaterials in Molten Salts and Applications

Green Production of Carbon Nanomaterials in Molten Salts and Applications

Ali Reza Kamali

Green Production of Carbon Nanomaterials in Molten Salts and Applications

123

Ali Reza Kamali Energy and Environmental Materials Research Centre (E2MC), School of Metallurgy Northeastern University Shenyang, Liaoning, China

ISBN 978-981-15-2372-4 ISBN 978-981-15-2373-1 https://doi.org/10.1007/978-981-15-2373-1

(eBook)

© Springer Nature Singapore Pte Ltd. 2020 This work is subject to copyright. All rights are reserved 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 Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

The discovery of carbon black in the early twentieth-century was followed by the discovery of nanodiamonds in 1963, fullerenes in 1985, carbon nanotubes in 1991 and graphene in 2004, introducing the main members of the so-called nanostructured carbon family. This family represents the first and most commercially successful examples of nanomaterials with a current worldwide market of several billions of dollars per year, which is continuously expanding due to their unique combination of properties including tunable electronic, thermal and surface characteristics as well as biocompatibility, chemical inertness in various corrosive media, and impressive mechanical properties. Consequently, carbon nanostructures have the capability of being used in endless applications across multiple market sectors including renewable energy, environmental protection and structural/functional composites. Carbon nanostructures are currently produced in commercial scales with techniques such as vacuum arc process, chemical vapor deposition, laser ablation and detonation, and a large number of books and book chapters have been published on these technologies. Not to mention that the application potential of carbon nanostructures is far greater than their current reality, which is mainly due to the expensive and complicated methods used for their commercial production, causing an increasing demand for new methods and technologies for the green and low cost production of various kinds of high quality carbon nanostructures for the current and emerging applications. This is the first book that presents an overview of molten salt technologies for sustainable and low cost production of carbon nanostructures and their applications. Based on the author's long teaching and research experience in the field in the university of Cambridge (UK) and Northeastern University (China), this book would serve as a useful text or reference for a broad range of readers from both academia and industry, and particularly for those who wish to gain a scientific and interdisciplinary perspective of this emerging field. The book contains eight chapters that provide an integrative overview of chemical and electrochemical interactions between molten salts and carbonaceous materials for the preparation of carbon nanostructures as well as possible mechanisms involved and various applications of nanostructured carbon products. I would like to thank all those who contributed in the related experiments or the subsequent discussions, and v

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particularly professor Derek J. Fray of University of Cambridge. I would also wish to extend my gratitude to the authors whose works were cited in this book, and all those who contributed to bringing this book to its current form. Last but not least, I would like to thank my dear parents, lovely wife, Asma, and children, Bita and Amin, for standing beside me throughout my career and writing this book. Cambridge, UK

Ali Reza Kamali [email protected] [email protected]

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 Production of Advanced Materials in Molten Salts . . . . . . . . . . . . 2.1 Inert Molten Salt Synthesis Methods of Carbon Nanostructures . 2.2 Reactive Molten Salt Synthesis of Carbon Nanostructures . . . . . 2.2.1 Interactions Between Molten Salts and Solid Carbonaceous Materials . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Capture and Conversion of CO2 in Molten Salts . . . . . . 2.2.3 Molten Salt Reduction of Graphene Oxides . . . . . . . . . . 2.3 Electrochemical Exfoliation of Graphite . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 Interaction of Molten Salts with Graphite . . . . . . . . . . . . . 3.1 Thermal Analysis of Pristine Graphite Powder . . . . . . . . 3.2 Thermal Analysis of Lithium Chloride . . . . . . . . . . . . . . 3.3 Thermal Analysis of the Graphite–LiCl Mixture . . . . . . . 3.4 X-Ray Diffraction and Raman Spectroscopy Studies . . . . 3.5 Effect of Molten Salts on the Graphitization Degree of Carbon Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Effect of Molten LiCl on the Microstructure of Graphite . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes . . . . 4.1 Molten Salt Production of Carbon Nanotubes . . . . . . . . . . . . . . 4.2 Production of Graphene in Molten LiCl . . . . . . . . . . . . . . . . . . 4.3 Production of Graphene in Molten NaCl . . . . . . . . . . . . . . . . . 4.4 Molten Salt Preparation of Metal-Filled Carbon Nanostructures . 4.5 Molten Salt Preparation of Interconnected Graphene—Carbon Nanoscrolls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4.6 Molten Salt Conversion of CO2 into Li2CO3 Nanocrystals . . . . . . 4.7 Encapsulation of Li2CO3 Nanocrystals in Carbon Layers . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Mechanisms Involved in the Electrolytic Fabrication of Carbon Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Electrochemical Erosion of Graphite Under Nominally Dry Ar 5.2 Thermokinetic Characteristics of LiCl . . . . . . . . . . . . . . . . . . 5.3 Electrochemical Erosion of Graphite Under Humid Ar . . . . . . 5.4 Electrochemical Erosion of Graphite Under HydrogenContaining Atmospheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Molten Salt Formation of Metal-Filled Carbon Nanostructures References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Applications of Carbon Nanostructures Produced in Molten Salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 SnO2-Graphene Anode Materials for Li-Ion Batteries . . . . . . . 6.2 Metal–Carbon Nanocomposites as Anode Materials for Li-Ion Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Sn-Filled Carbon Nanostructures . . . . . . . . . . . . . . . . 6.2.2 Graphene-Wrapped Si Nanostructures . . . . . . . . . . . . . 6.3 Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Ceramic-Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Adsorption Performance of the Molten Salt-Produced 3D Graphene Nanosheets . . . . . . . . . . . . . . . . . . . . . . 6.5.2 The Effect of pH on the Adsorption Capacity . . . . . . . 6.5.3 Reusability and Stability of 3D Graphene Nanosheets . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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7 Molten Salt Conversion of Plastics into Highly Conductive Carbon Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Structural Characterization of PET . . . . . . . . . . . . . . . . . . . . . . 7.2 Carbonization of PET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Conversion of PET into Carbonaceous Nanomaterials . . . . . . . . 7.4 Molten Salt-Assisted Conversion of PET into Carbon Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Molten Salt Heat Treatment of PET . . . . . . . . . . . . . . . 7.5 Electrical and Electrochemical Characterization of Nanostructured Carbon Materials . . . . . . . . . . . . . . . . . . . . . . . 7.6 Molten Salt Graphitization of Amorphous Carbons . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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8 Molten Salt-Assisted Preparation of Nanodiamonds at Atmospheric Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Transformation of Graphite into Diamond . . . . . . . . . . . . 8.2 Conversion of Carbon Nanostructures into Nanodiamonds 8.3 Conversion of CO2 into Diamond Nanocrystals . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1

Introduction

Abstract The 200 years documented history of molten salt science and technology has been full of surprises, some of which brought revolutionary changes to our world. Here, a brief history of these discoveries is provided. Carbon materials, and especially the crystalline graphite, have undoubtedly played an important role in the development of molten salt-based technologies, particularly in the electrolytic production of metals as well as the construction of molten salt nuclear reactors. In these applications, carbon is a desirable material due to its moderate electrical conductivity and relatively chemical and physical stability in high-temperature molten salts. More recently, interactions between molten salts with carbonaceous materials become more interesting, since these phenomena may lead to the low cost and highly efficient fabrication advanced carbon nanostructures, including carbon nanotubes, graphene, and nanodiamonds. Keywords Molten salts · History · Graphite · Carbon nanostructures The history of molten salt technologies is almost the same as that of the extractive metallurgy of reactive metals. In 1807, the British scientist Humphry Davy delivered the Bakerian Lecture before the Royal Society and surprised the scientific community by describing the metal potassium that could be produced, for the first time, by the electrolysis of potash (potassium carbonate) in “igneous fusion.” He, in fact, had discovered that potassium carbonate becomes conductor upon melting in a platinum spoon. Potassium could then be isolated on a platinum wire negative electrode immersed in the melt, while the spoon was connected to the positive pole [1]. This discovery was the birthday of fused salt or molten salt technologies, and made a foundation for the scalable production of other alkali and also alkaline earth metals such as Mg. The latter was first produced in laboratory scale by the electrolysis of molten MgCl2 by Michael Faraday in 1833 and in larger scales by Robert Bunsen in 1852 [2]. It was not long until aluminum was produced by Charles Martin Hall in 1886 by passing an electric current through a solution of aluminum oxide in molten cryolite. In 1918, Debye and Scherrer described the ionization of symmetrically arranged atoms in a salt upon heating. Today, the molten salt production of Al is the world’s largest electrochemical industry, with an output of over 63 million tons in 2018. In a typical Al smelter, molten salt is accommodated within large carbon or © Springer Nature Singapore Pte Ltd. 2020 A. R. Kamali, Green Production of Carbon Nanomaterials in Molten Salts and Applications, https://doi.org/10.1007/978-981-15-2373-1_1

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1 Introduction

graphite-lined steel containers. In such a cell, a very high current, typically 150,000 amperes, is passed between a carbon anode, made of petroleum coke and pitch, and a cathode, formed by the thick carbon or graphite-lined steel container, in which molten Al is electrochemically deposited [3]. The chemical stability of graphite in contact with the molten salt is a key technical issue in an Al smelter. The application of molten salts in the extractive metallurgy is based on their capability of being thermally ionized to form metal cations, which can then be reduced to the corresponding metals upon electrolysis. However, molten salts could offer more than this. In the 1940s, the Oak Ridge National Laboratory began to investigate the possibility of using fluid fuels in nuclear reactors to promote its Aircraft Nuclear Propulsion plan. Molten fluoride salts appeared particularly interesting because they have high solubility for uranium, chemical stability, low vapor pressure at high temperatures and reasonably high heat transfer properties. Moreover, these salts are not damaged by radiation, do not react violently with air or water, and can be relatively inert to structural metals [4]. Today, molten salt reactors use molten fluoride salts as primary coolant at low pressure. In the fourth-generation molten salt breeder nuclear reactors, a molten fluoride salt, used as both fuel and coolant, flows between the graphite reactor core and the heat exchanger [5, 6]. In this application, the possible interaction between molten salts and graphite is of importance. In 2000, another considerable breakthrough was reported from Derek Fray’s group at University of Cambridge. They reported that the oxygen from metal oxides cathodically charged in molten CaCl2 can be ionized and dissolved in the molten salt and then discharged at the anode, leaving pure metal at the cathode [7]. Other chloride salts such as LiCl may also be used for the extraction of nuclear metals [8]. In these so-called FFC-based methods, graphite crucibles and graphite anodes are often employed for the electroreduction of metal oxide cathodes. Graphite is one of the few choices in these applications because of its relatively high physical and chemical stability and electrical conductivity. The other candidates such as platinum are very expensive to be employed. The use of graphite, particularly as the anode, however, is a main source of metal contamination in FFC processes. Therefore, there is an active research line to replace graphite with the so-called inert anodes. In all discoveries mentioned above, graphite is used as a relatively reliable structural material or electrode, and is not supposed to be included in the final product. In 1995, another discovery was reported from Hsu and his co-workers from University of Sussex, which expanded the applications of molten salts from metallurgy and nuclear industry to materials science. It was reported that a mixture of curved multiwalled carbon nanotubes and clustered carbon nanoparticles can be generated from a graphite cathode in the electrolysis of molten lithium chloride [9]. Fundamental investigations, carried out mostly in University of Cambridge during 2002–2009 [10–14], suggested that the molten salt electrolytic formation of carbon nanomaterials commences with the intercalation of alkali metals (Li or Na) from the molten LiCl or NaCl electrolyte into the graphite cathode. This is followed by the breakdown of the graphite material into a variety of mostly nanostructured carbon constituents. The scalability of the molten salt approach was demonstrated in subsequent investigations [15]. It should be mentioned that the electrochemical Li+ intercalation and

1 Introduction

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de-intercalation into/out of graphite from non-aqueous room temperature electrolytes comprising lithium salts such as LiF dissolved in organic solvents such as ethylene carbonate had already been well documented in 1990. In fact, this is the basis of graphite anode operation in modern Li-ion batteries [16, 17]. Under this condition, graphite shows an outstanding cycle life, exceeding thousands of cycles, often with no structural disintegration. Further discoveries reported from University of Cambridge by myself during 2010–2016 indicated that molten LiCl can be hydrolyzed to produce HCl, and the HCl formed can dissolve into the molten salt to create hydrogen cations. The discharge of hydrogen cations on the cathode produces hydrogen at elevated temperatures. The hydrogen produced can be considered either as fuel, as reductant to reduce metal oxides to corresponding metals [18] or as an efficient exfoliating gas causing the exfoliation of the graphite electrode into high-quality graphene nanosheets [19–21]. The graphene product was evaluated for a number of applications, including supercapacitors [22], lithium-ion batteries [23], high-performance ceramic composites [24] and water purification [25]. These series of discoveries were interesting; since graphene is a relatively new member of carbon family with a wide range of interesting properties and applications. The other valuable member of the carbon family is diamond, the hardest and the most thermally conductive material ever known. In 2015, I proposed a molten salt route for producing diamond nanocrystals in molten LiCl, at far less severe conditions than conventional processes [26]. In the above-mentioned studies, carbonaceous solid materials exposed to the molten salts are used as the carbon source. The source of carbon used in molten salt processes can also be carbon dioxide (CO2 ) which is considered to be the main greenhouse gas. Capture of CO2 and its conversion to useful carbon materials for various applications, therefore, is an interesting approach to tackle CO2 emission challenges. Carbon dioxide can be captured in a single molten salt or often a mixture of molten salts to form amorphous carbon, CNTs [27] and even nanodiamonds [28]. The accumulation of non-biodegradable plastic wastes in the earth’s environment is another emerging global problem. The molten salt conversion of plastic wastes into nanostructured carbon provides a solution for this crisis [29]. This book summarizes these findings.

References 1. F.M. Perkin, The discovery of the alkalai metals by Humphry Davy: The bearing of the discovery upon industry. Trans. Faraday Soc. 3, 205–219 (1908) 2. R. Bunsen, Darstellung des Magnesiums auf electrolutischem, Ann. d. Chem. 82, 137 (1852) 3. R. Lumle (ed.), Fundamentals of Aluminium Metallurgy, Processing and Application and Production (Woodhead Publishing, Sawston, UK, 2011) 4. M.W. Rosenthal, P.R. Kasten, R.B. Briggs, Molten-Salt Reactors—history status, and potential. Nucl. Appl. Technol. 8, 107–117 (1970) 5. J. Uhlir, Chemistry and technology of Molten Salt Reactors—history and perspectives. J. Nucl. Mater. 360, 6–11 (2007)

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6. A. Cammi, V. Di Marcello, L. Luzzi, V. Memoli, M.E. Ricotti, A multi-physics modelling approach to the dynamics of Molten Salt Reactors. Ann. Nucl. Energy 38, 1356–1372 (2011) 7. G.Z. Chen, D.J. Fray, T.W. Farthing, Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride. Nature 407, 361–364 (2000) 8. E.Y. Choi, S.M. Jeong, Electrochemical processing of spent nuclear fuels: An overview of oxide reduction in pyroprocessing technology. Prog. Nat. Sci. Mater. Int. 25, 572–582 (2015) 9. W.K. Hsu, J.P. Hare, M. Terrones, H.W. Kroto, D.R.M. Walton, P.J.F. Harris, Condensed-phase nanotubes. Nature 377, 687 (1995) 10. A.T. Dimitrov, G.Z. Chen, I.A. Kinloch, D.J. Fray, A feasibility study of scaling-up the electrolytic production of carbon nanotubes in molten salts. Electrochim. Acta 48, 91–102 (2002) 11. Q. Xu, C. Schwandt, G.Z. Chen, D.J. Fray, Electrochemical investigation of lithium intercalation into graphite from molten lithium chloride. J. Electroanal. Chem. 530, 16–22 (2002) 12. Q. Xu, C. Schwandt, D.J. Fray, Electrochemical investigation of lithium and tin reduction at a graphite cathode in molten chlorides. J. Electroanal. Chem. 562, 15–21 (2004) 13. J. Sychev, N.V. Borisenko, G. Kaptay, K.B. Kushkhov, Intercalation of sodium and lithium into graphite as a first stage in an electrochemical method for producing carbon nanotubes. Russ. J. Electrochem. 41, 956–963 (2005) 14. J. Sytchev, G. Kaptay, Influence of current density on the erosion of a graphite cathode and electrolytic formation of carbon nanotubes in molten NaCl and LiCl. Electrochim. Acta 54, 6725–6731 (2009) 15. A.R. Kamali, D.J. Fray, Towards large scale preparation of carbon nanostructures in molten LiCl. Carbon 77, 835–845 (2014) 16. M. Fujimoto, N. Yoshinaga, K. Ueno, Li-ion Secondary Batteries, Japanese Patent 3,229,635, 1991 17. R. Fong, U. von Sacken, J.R. Dahn, Studies of lithium intercalation into carbons using nonaqueous electrochemical cells. J. Electrochem. Soc. 137, 2009–2013 (1990) 18. K. Xie, A.R. Kamali, Electro-reduction of hematite using water as the redox mediator. Green Chemistry, Green Chem. 21, 198–204 (2019) 19. A.R. Kamali, D.J. Fray, Large-scale preparation of graphene by high temperature diffusion of hydrogen in graphite. Nanoscale 7, 11310–11320 (2015) 20. A.R. Kamali, Eco-friendly production of high quality low cost graphene and its application in lithium ion batteries. Green Chem. 18, 1952–1964 (2016) 21. A.R. Kamali, Scalable fabrication of highly conductive 3D graphene by electrochemical exfoliation of graphite in molten NaCl under Ar/H2 atmosphere. J. Ind. Eng. Chem. 52, 18–27 (2017) 22. H.K. Kim, A.R. Kamali, K.C. Roh, K.B. Kim, D.J. Fray, Dual coexisting interconnected graphene nanostructures for high performance supercapacitor applications. Energy Environ. Sci. 9, 2249–2256 (2016) 23. A.R. Kamali, H.K. Kim, K.B. Kim, R.V. Kumar, D.J. Fray, Large scale green production of ultra-high capacity anode consisting of graphene encapsulated silicon nanoparticles. J. Mater. Chem. A 5, 19126–19135 (2017) 24. A.R. Kamali, J. Feighan, D.J. Fray, Towards large scale preparation of graphene in molten salts and its use in the fabrication of highly toughened alumina ceramics. Faraday Discuss. 190, 451–470 (2016) 25. L. Labiadh, A.R. Kamali, Unpublished work 26. A.R. Kamali, D.J. Fray, Preparation of nanodiamonds from carbon nanoparticles at atmospheric pressure. Chem. Commun. 51, 5594–5597 (2015) 27. W. Weng, L. Tang, W. Xiao, Capture and electro-splitting of CO2 in molten salts. J. Energy Chem. 28, 128–143 (2019) 28. A.R. Kamali, Nanocatalytic conversion of CO2 into nanodiamonds. Carbon 123, 205–215 (2017) 29. A.R. Kamali, J. Yang, Q. Sun, Molten salt conversion of polyethylene terephthalate waste into graphene nanostructures with high surface area and ultra-high electrical conductivity. Appl. Surf. Sci. 476, 539–551 (2019)

Chapter 2

Production of Advanced Materials in Molten Salts

Abstract Molten salt-based methods have been prime candidates for the commercial extraction of a variety of metals, such as aluminum and lithium, which are impossible or very difficult to be produced by other techniques. In addition to these well-developed technologies, molten salt methods have also created new strategies for the preparation of advanced metallic, intermetallic and ceramic materials as well as carbon nanostructures. This chapter focuses on the latter. In contact with carbonaceous materials, molten salts can either be relatively inert or reactive. Both behaviors have been employed for the preparation of carbon nanomaterials. An inert molten salt system can provide a uniform ionically conductive heating medium for the occurrence of reactions with an enhanced reactivity, leading to a significant promotion of reaction kinetics. This promoting influence is mainly due to the enhanced values of the diffusion coefficient of ions in molten salts. In contrast, there are some molten salt methods in which the molten salt involved is reactive against solid or gaseous carbonaceous species, leading to the preparation of a variety of different carbon nanostructures. Molten salt reduction of graphene oxides and the electrochemical exfoliation of graphite are also discussed. Keywords Molten salts · Advanced materials · Reactivity · Carbon nanostructures · Graphitization · CO2 capture Molten salts, which are ionic liquids with a melting point typically above 200 °C, have historically provided the opportunity for large scale production of some of the most useful metals. The best example of this kind is aluminum which is currently the second most widely used metal in the world with an annual production of more than 60 Mt in 2018. The industrial primary production of aluminum is entirely based on the Hall–Héroult smelting process, involving the electrolysis of alumina dissolved in molten cryolite [1]. Another example is lithium; the lightest metal which is used in various applications such as an alloying element to aluminum and magnesium alloys and in the production of organolithium compounds. Lithium, which is also facing an increasing demand due to the rising interest in lithium–ion batteries, is mainly produced by the electrolysis of LiCl-based molten salts [2, 3]. The electrolytic molten salt production of magnesium [4] and calcium [5] should also be added to this list.

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In the last twenty years, molten salt-based processes have provided exciting new opportunities for the preparation of various valuable materials which are difficult and/or expensive to be fabricated by their alternative fabrication methods. These include the molten salt production of metallic materials such as Ti [6], Nb [7], Cr [8], W [9], U [10], Ni [11], Fe [12], V [13], Si [14] as well as other materials including intermetallic compounds such as TiAl3 [15], ZrSi and ZrSi2 [16], nanocomposites such as ZrC/ZrSi [17], complex alloys such as La(Ni0.7 Co0.3 )5 [18], ceramics such SnO2 nanostructures [19, 20], LiNbO3 [21] and Cr2 AlC nanostructures [22]. In addition to these, molten salt methods have created various strategies for the preparation of carbon nanostructures. The focus of this book is to discuss the possible interactions between molten salts and carbonaceous materials leading to the fabrication of valuable carbon nanostructures. In contact with carbonaceous materials, molten salts can either be relatively inert or reactive. Both behaviors have been employed for the preparation of carbon or carbide nanomaterials. In this chapter, an overview on these molten salt techniques is presented.

2.1 Inert Molten Salt Synthesis Methods of Carbon Nanostructures Molten salts can provide an inert medium for the occurrence of reactions without becoming directly involved in the chemical or electrochemical interactions. In this case, the use of molten salts is beneficial since they can provide a uniform ionically conductive heating media in which reactive species may show an enhanced reactivity, leading to a significant promotion of reaction kinetics. Cui et al. [23] used the eutectic KCl–LiCl molten salt at 800–950 °C as the medium to conduct the reaction between CVD-produced multiwall carbon nanotubes (MWCNT) and Ta powder leading to the formation of tantalum carbide (TaC) nanofibers. In a different research, molten KCl–LiCl was found to be an ideal environment for the carbonization of ZIF-8 polymers, which subsequently led to the formation of 2D N-doped amorphous carbon nanosheets [24]. Here, the salt was proposed to act as a template to form 2D carbons (Fig. 2.1). Likewise, Li et al. [25] used a template concept in which molten LiCl–KCl–KF salt system acted as the medium, supporting the reaction between transition metals with carbon nanotubes, resulting in the formation of metal carbide nanofibers. Similarly, in another study, low-graphitized nitrogen-doped carbon hollow cubes could be produced by carbonization of biomass L-lysine monohydrochloride (C6 H15 O2 N2 Cl) in molten NaCl at 1000 °C (Fig. 2.2) [26]. Nita et al. [27] produced porous carbon materials by treatment of organic carbonaceous precursors in molten KCl, NaCl and LiCl. They suggested that the molten salt can play an important role in textural properties of the carbon material produced. Ding et al. [28] produced nitrogen-doped carbon material with a large number of active sites by carbonization of PANI in molten NaCl. They found that there is a close morphological relationship between the raw materials and the products (Fig. 2.3).

2.1 Inert Molten Salt Synthesis Methods of Carbon Nanostructures

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Fig. 2.1 A schematic representation for the formation of 2D mesoporous carbon nanosheets from zeolitic imidazolate polyhedrons (ZIF-8) in an inert molten salt medium, reproduced from Ref. [24], copyright 2019, with permission from Elsevier

Fig. 2.2 A schematic representation for the formation of nitrogen-doped carbon hollow cubes. l-Lysine monohydrochloride (C6 H15 O2 N2 Cl) was processed with molten NaCl at 1000 °C to form the nanostructured carbon, reproduced from Ref. [26], copyright 2019, with permission from RSC Publishing

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Fig. 2.3 A morphological relationship was discovered between the PANI raw materials and the carbon products produced in molten NaCl, reproduced from Ref. [28], copyright 2019, with permission from with permission from American Chemical Society

In fact, the high-temperature nature and the enhanced values of the diffusion coefficient of ions in molten salts [29] can provide an ideal environment for the decomposition of carbonaceous materials into carbon nanostructures. The inert molten salt synthesis methods described above are highly attractive from an ease of deployment perspective. From an application point of view, however, the quality of carbon products and the yield factor should be further analyzed in the future studies.

2.2 Reactive Molten Salt Synthesis of Carbon Nanostructures 2.2.1 Interactions Between Molten Salts and Solid Carbonaceous Materials In contrast with the above-mentioned works, there are some preparation methods in which the molten salt involved is reactive against carbon materials. It has been shown that the direct interaction between graphite [30–33] or other carbonaceous materials such as glucose [34] and glassy carbon [35] with molten LiCl [30], LiCl/KCl [34, 35], the eutectic salt of LiF, NaF and KF [36], and KNO3 [37] can lead to the formation of carbon nanostructures [30, 31, 34] with a higher degree of graphitization than that

2.2 Reactive Molten Salt Synthesis of Carbon Nanostructures

9

of the initial carbon materials. It should be noticed that a considerable increase in the graphitization degree can often be achieved by longtime heat treatment of carbon at temperatures more than 2500 °C under protective atmospheres [38, 39]. The growth of graphite crystallites, thus the progress of graphitization observed during the molten salt heat treatment, can be attributed to the combination of various effects. These effects include the reactive dissolution of impurities from carbonaceous materials into molten salts [30], the reduction of the d-spacing fluctuation between graphite basal planes and the closure of the Mrozowski cracks in graphite crystals. The latter was suggested to be caused by compressive stresses induced by the salt network in the graphite matrix [32] contributing to ordering of the carbon structure (Fig. 2.4). Furthermore, Li et al. [40] reported that the treatment of Ni-encapsulated MWCNTs in the LiCl–KCl eutectic molten salt promotes the uncapping of the carbon capsules and simplifying the purification of nanotubes by the subsequent acid leaching. The influence of molten salts on carbonaceous materials can further be promoted by applying electric potentials, which will be discussed later.

2.2.2 Capture and Conversion of CO2 in Molten Salts In the above-mentioned studies, carbon was sourced from carbonaceous solid materials exposed to the molten salts. The source of carbon used in molten salt processes can also be carbon dioxide (CO2 ) which is considered to be the main greenhouse gas and probably the main cause of the accelerated global warming and ocean acidification with catastrophic environmental consequences. Capture of CO2 and its conversion into carbon materials for various applications, therefore, is an interesting approach to tackle CO2 emission challenges. Carbon dioxide can be captured in a single molten salt or often a mixture of molten salts to form amorphous carbons, CNTs and nanodiamonds. The molten salts used in this approach include molten carbonates, such as Li2 CO3 , Na2 CO3 , K2 CO3 [33, 39–42], Cs2 CO3 [43] and CaCO3 [44] as well as chlorides such as LiCl, NaCl, KCl and CaCl2 [45–48]. In molten carbonate salts, carbonate ions can be electrochemically reduced on the cathode electrode to form amorphous solid carbon and oxygen ions O2− , which can subsequently absorb the CO2 gas, promoting the continuous capture and electrochemical conversion of CO2 to carbon [33]. In molten chloride, the addition of alkali metal or alkaline earth metal oxides leads to the formation of oxygen ions with a strong affinity for the absorption of CO2 , resulting in the formation of carbonates [47]. Ideally, using photovoltaic (PV) energy and an inert anode, CO2 can be captured and electrochemically transferred into various carbon nanostructures (Fig. 2.5a [49]). Molten salts can also be used to promote the adsorption of CO2 on porous solid adsorbents. For instance, MgO is an attractive material for the CO2 capture application [50, 51] since MgO particles easily react with the surrounding CO2 molecules to form MgCO3 . However, MgCO3 layer formed around MgO is impervious, sharply

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Fig. 2.4 Graphite is generally produced by the graphitization of amorphous carbons at high temperatures. (Top panel) After thermal graphitization of graphite, Mrozowski defects are often generated during the cooling process induced by the tensile stress along with the c-axis of graphite crystals, resulting in various d-spacing fluctuations. (Lower panel) The infiltration of molten salts into to the graphitic structure can heal the d-spacing fluctuations under the influence of the compressive stress caused by the molten salt, reproduced from Ref. [32], copyright 2019, with permission from Elsevier

reducing the kinetics of the adsorption process [52, 53] (Fig. 2.5b [53]). The modification of MgO with molten nitrate salts [54, 55] will promote its adsorption performance since the molten salts develop structural defects in MgCO3 promoting the adsorption of CO2 . In this case, furthermore, the molten salts act as a reaction medium by dissolving both CO2 and MgO facilitating the rather slow reaction kinetics [56]. Molten carbonate salts that commonly possess higher melting temperatures than nitrides have also been employed for the promotion of CO2 capture, where double salts system are formed during the CO2 capture process (Fig. 2.5c) [56].

2.2 Reactive Molten Salt Synthesis of Carbon Nanostructures

11

Fig. 2.5 a CO2 can be captured in chloride or carbonate molten salts to form carbonate anions which can be subsequently reduced to form various carbon nanostructures on the cathode, reproduced from Ref. [49], copyright 2019, with permission from Elsevier; b MgO adsorbs CO2 to form MgCO3 . The MgCO3 formed around the unreacted MgO, considerably reduces the adsorption performance of the unreacted MgO, reproduced from Ref. [53], copyright 2019, with permission from Springer Nature; c Molten nitrides and carbonates can enhance the adsorption process by creating a reactive interface with the MgCO3 layer, reproduced from Ref. [56], copyright 2019, with permission from Elsevier

The molten salt-assisted transformation of carbon dioxide into nanodiamonds is discussed in Chap. 8. Since the environmental challenges associated with the increase of carbon dioxide is currently a global problem to be solved, the molten salt technologies for CO2 capture remain a demanding topic for years to come, with an environmental driving force. However, the practicality of these methods with an aim of producing carbon nanomaterials will depend on the quality of carbon products and the processing costs, which should further be investigated.

2.2.3 Molten Salt Reduction of Graphene Oxides Reduction of graphene oxide (GO) is considered the most commonly used method of producing graphene in large scales. The most popular technique for the production of GO is the Hummers’ method and its modified versions which imply the oxidation of graphite using chemicals such as potassium permanganate (KMnO4 ) and sulfuric

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acid [57]. Despite its popularity, however, this strategy of producing graphene suffers from several basic limitations. It should be mentioned that in GO, the extended sp2 conjugated networks of original graphene sheets are disturbed due to the presence of covalent C–O bonds on the basal planes and edges. Consequently, the existence of a high fraction of sp3 hybridized carbon atoms (~0.6), makes GO basically an insulator [58, 59]. Since conductivity is one of the most important properties of graphene for many applications such as energy storage systems [60] and conductive polymers [61], the maximum reduction of oxygen-containing groups from GO is a key step toward the production of reduced GO (rGO) with appropriate properties for those applications [62]. There are several methods reported for the reduction of GO, which are mainly based on its chemical reduction using a reducing agent such as hydrazine, thermal reduction or combination of both [63, 64]. Since the chemical reduction of GO is usually time-consuming involving the use of highly toxic reducing agents that are harmful to the environment [65, 66], the thermal reduction approach is considered relatively simpler and more eco-friendly, during which the oxygen-containing functional groups attached on the carbon backbone is converted into gaseous species [67]. However, residual functional groups and defects dramatically alter the structure of the carbon plane and also lead to the irreversible restacking or aggregation of graphene sheets due to the π–π interaction. The carbon structure can also be damaged due to the release of gaseous species [68]. Alkali and alkaline earth metals are strong reducing agents, but it is not practical to use them for the reduction of GO due to their high reactivity with the environment. Molten alkali and alkaline earth metal halides can provide an ideal medium for the reduction process, thanks to their capability to dissolve the reactive metals and oxygen. Abdelkader et al. [69] reported that GO (C/O = 2.2) can be reduced by alkali and alkaline earth metals such as Li and Ca dissolved in the corresponding molten salts, i.e., LiCl and CaCl2 , to a C/O level of 10.4 and 14.5, after 2 and 4 h of the heat treatment, respectively. Furthermore, molten salts can prevent the rGO from restacking (Fig. 2.6). Wang et al. [70] reported that the GO reduced at 600 °C for 2 h under N2 could yield a rGO product with the surface area of only 90 m2 g−1 and an electrical conductivity of about 20 S m−1 . However, when the graphene oxide was heated under the same regime but in a mixture of LiCl–KCl, the restacking phenomenon was greatly avoided, leading to a larger surface area of 392 m2 g−1 . A product with a much higher surface area of 750 m2 g−1 and enhanced electrical conductivity of 500 S m−1 was produced by adding KNO3 to the molten salt. The enhanced properties of the rGO material were attributed to the advantages of the molten salt system in preventing graphene sheets to restacked, restoring the conjugated networks and also providing a medium for KNO3 activation and nitrogen doping [70].

2.3 Electrochemical Exfoliation of Graphite

13

Fig. 2.6 Lithium dissolved in LiCl–KCl molten salt can reduce graphene oxide (GO) to reduced GO (rGO). (Right panel) XRD patterns of GO and rGO. The (002) reflection of GO appears at about 2θ = 10° because of the presence of functional groups. The removal of these groups by the molten salt process leads to the shift of the (002) reflection to greater 2θ values. The inset shows the SEM morphology of the rGO. (Left panel) the Raman spectra of GO and rGO, reproduced from Ref. [69], copyright 2019, with permission from with permission from American Chemical Society

2.3 Electrochemical Exfoliation of Graphite The electrochemical exfoliation of graphite can offer an alternative one-pot approach for the preparation of graphene-based materials with possible advantages of being more cost-effective, scalable or more environmentally friendly, in comparison with other chemical and mechanical exfoliation routes as well as chemical vapor deposition (CVD) methods [71]. The electrochemical cells used for the exfoliation of graphite normally require one or more carbon working electrodes, counter and reference electrodes, as well as an electrolyte and a source of DC electric power to operate. The carbon working electrode can be polarized either anodically or cathodically. The anodic exfoliation of graphite in room temperature electrolytes is the most common electrochemical exfoliation method since it can be conducted in aqueous solutions such as H2 SO4 [72, 73]. The anodic exfoliation of graphite has also been achieved using ionic melts such as BMPyrr BTA (Fig. 2.7a) [74]. In fact, the anodic exfoliation leads to the oxidation of the graphite anodes by allowing the intercalation of anions from the electrolyte, causing the disintegration of the hexagonal graphite lattice. However, the anodic oxidation of graphite usually has a slow kinetics and often produces a significant amount of oxygen-containing groups which cannot be avoided due to the over-oxidation of the graphite [71]. For example, the anodic electrochemical exfoliation of a high purity graphite rod of 6 mm in diameter was achieved by the anodic polarization of the rod in an ambient temperature ternary molten salt-containing acetamide, urea and ammonium nitrate with a melting point of about 8 °C. For this, two graphite electrodes were immersed in the electrolyte with a submerged electrode mass of 1.5 g. Under a cell potential of 5 V, a small electric current of 25 mA could be conducted through the cell, upon which the solvated ions NO3 2− with urea and acetamide from the electrolyte are

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Fig. 2.7 Anodic exfoliation of graphite in ionic melts. a The intercalation of BTA anions from the ionic liquid BMPyrr BTA mixed with acetonitrile leads to the exfoliation of the graphite structure, reproduced from Ref. [74], copyright 2019, with permission from Elsevier; b Graphite can be anodically exfoliated in room temperature electrolytes containing acetamide, urea and ammonium nitrate. In this case, the intercalation of NO3 − anions and electrolyte molecules into the graphite anode leads to the formation of gas species within the graphite structure and consequently the exfoliation of the graphite, reproduced from Ref. [75], copyright 2019, with permission from Elsevier

co-intercalated into the graphite anode and cause expansion and then exfoliation of the graphite into sheets of 1–5 layers with an oxygen content of around 12 at.% [75] (Fig. 2.7b). Unlike the anodic intercalation/exfoliation, organic solutions are generally used as the electrolyte in the cathodic exfoliation processes, in which cations in the electrolyte are attracted to negatively polarized graphite cathodes causing the intercalation and exfoliation. The cathodic approach, therefore, provides the advantage that no oxidation is involved thereby preventing the generation of defects in the product [70, 76]. Despite this advantage, however, the cathodic treatment of carbon materials has shown lower efficiency of intercalation and exfoliation, in comparison with the anodic process. For example, Lei et al. [77] have found that graphite cathodes can be exfoliated in AlCl3 /[EMIm] Cl ionic liquid to produce few-layer graphene. The cathode current density, however, was 10 mA g−1 . The electrochemical exfoliation of graphite in low-temperature electrolytes has been well reviewed in [71, 76, 78]. The cathodic exfoliation of graphite in high-temperature molten salt electrolytes is discussed in Chap. 4. As it can be seen later, the rate of cathodic exfoliation of graphite in molten salts can be very high, reflected by a cathode current density as high as 1 A cm−2 (~1.7 A g−1 ) [79–82].

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Chapter 3

Interaction of Molten Salts with Graphite

Abstract The evaluation of the corrosion behavior of graphite in molten salts is an emerging important issue. It is because the molten salt corrosion of graphite can either be technologically undesirable, for instance in molten salt nuclear reactors, or highly desirable, for example, in the molten salt preparation of carbon nanostructures. This chapter provides an overview of the chemical corrosion of graphite in molten salts and particularly in molten lithium chloride. For this, thermokinetic characteristics of graphite and LiCl are reviewed individually and in combination. The morphological and structural changes occurred upon the exposure of graphite to molten lithium chloride are also discussed. Keywords Graphite · Molten salts · Thermal analysis · Corrosion · Carbon nanostructures The polycrystalline synthetic graphite (so-called graphite), which consists of extended networks of graphene sheets, has a unique combination of properties, including relatively high electrical and thermal conductivity, chemical inertness, resistance to oxidation in air (which is negligible below 500 °C), excellent machinability and tribological properties as well as the low cross section for neutron absorption [1–3]. These properties make graphite the prime candidate for a variety of applications in many technological fields such as electrochemistry, melting and nuclear industry [4–6]. Among them, there are some promising technologies in which graphite is exposed to molten salts. The first example is the fourth generation molten salt breeder nuclear reactors where a mixture of molten fluoride salts, such as NaF, LiF, ZrF4 , ThF4 and UF4 are used both as fuel and coolant, flows between the graphite reactor core and the heat exchanger [7, 8]. The second example is the pyrochemical reprocessing of spent nuclear fuels using a LiCl-based molten salt, where graphite is proposed as one of the candidate materials for various equipments such as electrodes and crucibles [9]. In both applications, the inertness of graphite in the operating conditions is required in order to ensure the reliability of the processes as neither corrosion nor interaction phenomena are acceptable. Hence, it is important to evaluate the corrosion behavior of graphite in molten salts. Despite its importance, however, there are few published studies concerning the impact of molten salts on graphite. Accordingly, © Springer Nature Singapore Pte Ltd. 2020 A. R. Kamali, Green Production of Carbon Nanomaterials in Molten Salts and Applications, https://doi.org/10.1007/978-981-15-2373-1_3

19

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3 Interaction of Molten Salts with Graphite

it was demonstrated that graphite in contact with molten salts undergoes degradation and considerable attack on the surface, and therefore, protective coatings have been suggested for the prevention of corrosion [10, 11]. Bernardet et al. studied the reactivity of nuclear graphite materials toward a LiF–NaF–ZrF4 ternary molten salt at 500 °C and indicated that molten salt, to some extent, penetrates in the graphite through the surface porosity. Nevertheless, no structural disintegration of the graphite materials was observed [10]. In contrast with these examples, there are molten salt technologies in which the electrochemical reactivity of molten salts with graphite materials is desirable, leading to the preparation of carbon nanostructures. The electrochemical corrosion, reaction and/or exfoliation of graphite in molten salts [12, 13] are discussed in the Chap. 4. Since new molten salt technologies are becoming more important, the reactivity of graphite with molten salts at different conditions should be carefully investigated. From this perspective, this chapter concerns the corrosion of graphite in molten salts. Particularly, it is interesting to explore the feasibility of using molten salt corrosion of graphite as a low-cost top-down approach for the production of carbon nanomaterials, including CNTs, carbon nanorods, graphene and graphene-based nanosheets. Therefore, it would be of both fundamental and practical interests to explore whether molten salt corrosion of graphite can be considered as a method for the production of carbon nanomaterials. In the following sections, the structural and microstructural changes which might occur during non-isothermal heating of graphite in the air with and without the involvement of molten salts are discussed [14].

3.1 Thermal Analysis of Pristine Graphite Powder Figure 3.1 exhibits the SEM micrograph as well as the differential scanning calorimetry (DSC) and thermal gravimetry (TG) thermographs performed on a pristine graphite powder (EVC, Morgan USA) at varying heating rates, ranging from 20 to 80 °C min−1 under an ambient airflow of 100 mL min−1 . The DSC thermograms are plotted so that a downward peak corresponds to an exothermic event. The DSC thermogram recorded at the heating rate of 20 °C min−1 shows a wide exothermic peak with the maximum at 900 °C which is because of the oxidation of graphite. The TG curve, Fig. 3.1c, demonstrates that the oxidation begins at around 630 °C and ends at around 950 °C, resulting in a mass loss close to 98%. To characterize the final residue, the pristine graphite powder was heated in a tube furnace with similar conditions to the TG furnace, and the remaining ash was analyzed by EDX. An outline of the analytical data is shown in Table 3.1. The DSC thermogram of the pristine graphite powder acquired at the heating rate of 40 °C min−1 , Fig. 3.1b, reveals an even immense oxidation peak with the maximum at 1018 °C [14]. The analogous TG curve, Fig. 3.1c, shows that the quick oxidation of the graphite material took place at temperatures above 640 °C and was

3.1 Thermal Analysis of Pristine Graphite Powder

21

Fig. 3.1 a SEM, b DSC and c TG thermographs for 26 mg of the pristine graphite powder heated at different heating rates under an ambient airflow of 100 mL min−1 , reproduced from Ref. [14], copyright 2019, with permission from Elsevier

Table 3.1 EDX analysis of the ash formed by the burning of the pristine graphite powder, reproduced from Ref. [14], copyright 2019, with permission from Elsevier O (at. %)

Al (at. %)

Fe (at. %)

Si (at. %)

Sn (at. %)

Na (at. %)

Ti (at. %)

60

18

10

8

2

1

1

completed at about 1170 °C. Nevertheless, as it can be observed from Fig. 3.1b, the DSC exothermic peak of the oxidation reaction is not terminated at the heating rate of 60 °C min−1 . In this case, the oxidation reaction started at around 630 °C, yet about 15 mass percent of total graphite was acquirable at 1250 °C, as realized by the corresponding TG curve. Figure 3.1 shows that, the oxidation of the pristine graphite powder at the heating rate of 80 °C min−1 started at around 700 °C, yet 45 mass percent of the material endured at 1250 °C min−1 . It is clear that the intensive oxidation of graphite is evaded at this heating rate. A feasible cause for this observation will be addressed further on in this chapter. This heating rate was employed to explore the possible effects of molten LiCl on the structure and microstructure of the graphite. To this end, the LiCl powder was examined at an identical heating rate, and results acquired are discussed in the subsequent sections [14].

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3.2 Thermal Analysis of Lithium Chloride DSC and TG analyses were executed with roughly around 23 mg of LiCl powder heated at a rate of 80 °C min−1 under an airflow rate of 100 mL min−1 , and Fig. 3.2 exhibits the results. From the DSC profile, four endothermic peaks are noted, which can be related to the phase transition of hydrated lithium chloride, Eq. (3.1) [15]: H2 O

LiCl(H2 O)solid → LiClsolid → LiClliquid −−→ LiCl + Li2 O + HCl ↑→ LiCl(Li2 O)gas

(3.1)

It was confirmed [15] that nominally anhydrous LiCl particles can easily absorb moisture from the ambient atmosphere to develop a surface layer of lithium chloride monohydrate. Consequently, the foremost peak at 128 °C in the DSC curve of Fig. 3.2 complements to the dehydration of LiCl, and the following peak at 640 °C Fig. 3.2 DSC (solid line) and TG (dotted line) analyses of the pristine graphite powder, LiCl and the graphite–LiCl mixture performed at a heating rate of 80 °C min−1 under an ambient airflow of 100 mL min−1 , reproduced from Ref. [14], copyright 2019, with permission from Elsevier

3.2 Thermal Analysis of Lithium Chloride

23

is connected to the melting of the dehydrated LiCl. It was further shown that at high temperatures, molten LiCl is partly hydrolyzed by the atmospheric moisture to form LiOH which later decomposes into Li2 O. These two transitions are portrayed by a solitary endothermic reaction at high heating rates [15]. As a result, the third peak in the DSC curve of LiCl, detected at 943 °C, is credited to the creation of lithium oxides, and the fourth peak at 1212 °C is because of the augmented co-evaporation of lithium oxide and lithium chloride [15].

3.3 Thermal Analysis of the Graphite–LiCl Mixture The DSC and TG analyses were conducted on 50 mg mixtures of graphite and LiCl. The experiments were performed at the rate of 80 °C min−1 under an airflow rate of 100 mL min−1 , and the results are displayed in Fig. 3.2. The thermographs of the pristine graphite and LiCl are additionally shown in Fig. 3.2 for the aim of contrast. The peaks observed on the DSC thermograph of the graphite–LiCl mixture can be pinpointed by juxtaposing the curves shown in Fig. 3.2. Consequently, the foremost DSC peak at 135 °C is credited to dehydration of LiCl and the second peak at 641 °C to the melting of the dehydrated LiCl. It is to be noted that the area under the DSC peaks conforms to the enthalpy change for the phase transition per mass unit of the sample. Hence, the height of the peak linked to the melting of LiCl was decreased when LiCl was diluted by the introduction of graphite (see Fig. 3.2). Likewise, the third and fourth peaks, at 1047 °C and 1196 °C, can be delegated to the formation of lithium oxide, and the co-evaporation of lithium oxide and lithium chloride, respectively. As maintained by Fig. 3.2, the peak associated with the formation of lithium oxide appeared at a noticeably lower temperature in the graphite–LiCl mixture relatively with that of LiCl. It should be marked that the formation of lithium oxide is because of the interaction of the LiCl melt with atmospheric moisture, as seen in Eq. (3.1) [15]. Despite that, in the graphite–LiCl mixture, the interaction between molten LiCl and the atmospheric moisture is decreased, which is attributable to the presence of graphite particles. Consequently, the respective reaction is liable to take place at higher temperatures. In agreement with the DSC analysis, the TG thermogram of the graphite–LiCl mixture, shown in Fig. 3.2, presents a mass loss of 1.8 wt% between 100 and 140 °C, which is credited to the elimination of hydration water. The major mass loss of 45 wt% transpires between 950 and 1190 °C and is attributed to the evaporation of LiCl. From looking at the DSC and the TG measurements, it can be noted that the evaporation of lithium chloride is the sole leading event which happens during the heat treatment of the graphite–LiCl mixture at higher temperatures. That is to say that the graphite component was not considerably oxidized, and this behavior is assigned to a combination of effects, including the oxidation-protective effect of molten LiCl and the high heating rate employed. The heat-treated powders were studied by various techniques.

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For more comprehensive investigations of the phenomena taking place during the heating of the graphite–LiCl mixture, the activation energy of the respective transitions was identified based on Eq. (3.2).   EC B =− + constant In RTP TP2

(3.2)

where B is the heating rate, T p is the temperature at the specific peak, E c is the activation energy and R is the universal gas constant. Accordingly, 50 mg of the graphite–LiCl mixture was examined by heating at different rates, ranging from 75 to 90 °C min−1 , under an ambient airflow of 100 mL min−1 . The results are summarized in Fig. 3.3, and the specific peak temperatures for various events comprising surface dehydration, melting and evaporation of LiCl at different heating rates are expressed in Table 3.2. Figure 3.4 displays a graphic illustration of the data shown in Table 3.2, from which the activation energies of different transition processes occurring during heating the graphite–LiCl mixture were determined. The data obtained are presented in Table 3.3. Moreover, the activation energies of similar transitions observed during the heating of LiCl were extracted from [15] and shown in Table 3.3 for the purpose Fig. 3.3 DSC curves for 50 mg of the graphite–LiCl mixture heated at different rates under an ambient airflow of 100 mL min−1 , reproduced from Ref. [14], copyright 2019, with permission from Elsevier

3.3 Thermal Analysis of the Graphite–LiCl Mixture

25

Table 3.2 Transition temperatures for surface dehydration, melting and evaporation of LiCl observed during heating of the graphite–LiCl mixture at different heating rates under an ambient airflow of 100 mL min−1 , reproduced from Ref. [14], copyright 2019, with permission from Elsevier Heating rate (°C min−1 ) 75 Temperature of surface dehydration (°C) Temperature of melting (°C) Temperature of evaporation (°C)

133.8

80

85

135.0

90

136.2

137.3

640.1

641.2

641.6

642.1

1189.2

1196.3

1203.2

1209.4

Fig. 3.4 Graphic presentation of Eq. (3.2) for surface dehydration, melting and evaporation of LiCl observed during heating of the graphite–LiCl mixture, reproduced from Ref. [14], copyright 2019, with permission from Elsevier Table 3.3 Activation energies (kJ mol−1 ) for surface hydration, melting and evaporation of LiCl observed during heating of the graphite–LiCl mixture. Activation energies of the same transitions observed during heating of LiCl are also presented in order to comparison Surface hydration

Melting

Evaporation

LiCl [15]

68.2

739.4

64.8

LiCl in the graphite–LiCl mixture [14]

66.5

715.2

142.8

of juxtaposition. As shown, the activation energies of surface dehydration and melting of LiCl are nearly indistinguishable in both cases. Despite this, the evaporation of LiCl from the graphite–LiCl mixture is linked with a higher value of activation energy in comparison with the value acquired for LiCl. This can be explained by the interfacial adhesion energy between molten LiCl and graphite [15].

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3 Interaction of Molten Salts with Graphite

3.4 X-Ray Diffraction and Raman Spectroscopy Studies The XRD patterns of the pristine graphite, the graphite–LiCl mixture heated to 1250 °C at the heating rate of 80 °C min−1 , and commercially available LiCl are shown in Fig. 3.5. The latter signifies the high tendency of nominally pure LiCl to create a hydrated LiCl at ambient atmosphere. The XRD pattern of the pristine graphite is indexed to the hexagonal graphite structure. The diffraction peaks of hexagonal graphite can also be detected in the diffraction pattern of the heat-treated powders. On the other hand, neither peaks of LiCl nor hydrated LiCl can be observed in this profile. This is in agreement with the thermal analysis results and verifies the evaporation of LiCl during the heat treatment of the graphite–LiCl mixture. The crystallographic data linked to the sharp graphite (002) diffraction peak for the pristine graphite and the heat-treated powders as well as the average size of the crystallites in the direction perpendicular to the (002) plane, calculated by Scherrer’s Eq. (3.3) [16], are displayed in Table 3.4. L c = (0.9 × λ)/(B × cos θ )

(3.3)

In Scherrer’s equation, the crystallite size of the graphitic materials along the c-axis (L c ) was determined to employ the values of λ (the X-ray wavelength,), θ (the Bragg angle) and B (the Lorentzian full-peak width at half-maximum intensity, FWHM, expressed in radians). According to Table 3.4, the out-of-plane crystallite size of the graphite material escalated greatly after the heat treatment. The importance of this observation will be dealt with later in this chapter. Furthermore, in addition to the reflections arising from graphite, a couple of other peaks can also be noted in the XRD pattern of the heat-treated powders. Wherefore, the peaks at around 2θ = 33.92° and 38.01° can be attributed to the (111) and (-103) reflections of monoclinic lithium oxalate (Li2 C2 O4 , JCPDS 00-024-0646), Fig. 3.5 X-ray diffraction pattern of a the pristine graphite, b the mixture of graphite and LiCl heated at the rate of 80 °C min−1 to 1250 °C, and c LiCl, reproduced from Ref. [14], copyright 2019, with permission from Elsevier

3.4 X-Ray Diffraction and Raman Spectroscopy Studies

27

Table 3.4 Data extracted from the XRD and the Raman measurements for the pristine graphite and the mixture of graphite and LiCl heated at the rate of 80°C min−1 to 1250°C. The XRD data belong to the hexagonal (002) peak, reproduced from Ref. [14], copyright 2019, with permission from Elsevier XRD

Raman

The pristine graphite

The heat-treated powders

2θ(°)

26.4899

26.5646

d (nm)

0.3365

0.3356

L c (nm)

28

41

(cm−1 )

1348

1355

G line frequency (cm−1 )

1569

1575

IG /ID

7.2

16.4

D line frequency

respectively. Moreover, an unknown peak can also be detected at the 2θ value of around 51.84°. The formation of lithium oxalate is due to the possible reaction between the carbon material and lithium oxides coming from the reactions (3.1). Such a reaction may take place during either the heating or cooling operations. As mentioned, the cathodic erosion of graphite in molten lithium chloride (discussed in Chap. 4) causes carbon nanomaterials to be formed. These carbon nanostructures may contain lithium carbonate (Li2 CO3 ) nanocrystals encapsulated within their nanostructures [17, 18]. The presence of Li2 CO3 phase has scientific and technological significance as it plays a crucial role in the low-pressure transformation of carbon nanostructures into nanodiamonds. This will be explored further in the Chap. 8. The origin of the Li2 CO3 phase formed can be related to the corrosion of graphite in molten LiCl [14]. Raman spectroscopy provides practical data on the structural characteristics of graphite [19, 20]. The raw Raman spectra of the pristine graphite and the mixture of graphite and LiCl heated at the heating rate of 80 °C min−1 to 1250 °C in the Raman shift range 1200–1800 cm−1 are displayed in Fig. 3.6. Both spectra are characterized by the presence of the so-called G and D peaks, which are associated with the Raman active mode in monocrystalline graphite (sp2 bonding) and the disorder-activated zone boundary mode of microcrystalline graphite, respectively. The disorder and defects in graphitic materials can be induced by the presence of the lattice imperfections such as dislocations, crystallite boundaries, impurities and edges. Moreover, the relative intensity ratio of G band to D band (IG /ID ) shows the in-plane structural order of carbon materials [20]. The Raman data acquired for the pristine graphite along with the heat-treated powders are juxtaposed in Table 3.4. This information evidently shows an upward shift of the G and D bands, along with a significant increase of IG /ID in heat-treated powders [21–24]. These phenomena are attributed to the graphitization and growth of graphitic crystallites, caused by the heat treatment process. By considering the XRD and Raman results, it can be concluded that the heat treatment of the graphite–LiCl mixture brings on a significant rise in the average crystallite size of the graphite. Evidently, the heat-treated powder has remarkably lower density of defects in comparison with the pristine graphite.

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Fig. 3.6 Raman spectrum of a the pristine graphite and b the mixture of graphite and LiCl heated at the rate of 80 °C min−1 to 1250 °C, reproduced from Ref. [14], copyright 2019, with permission from Elsevier

3.5 Effect of Molten Salts on the Graphitization Degree of Carbon Materials It is well known that the graphite crystallites in carbon materials can grow in size and perfection as the heat treatment temperature is increased. Despite that, in order to achieve a reasonable degree of graphitization, the heating process should be conducted at extremely high temperatures under a protective atmosphere, to prevent the oxidation of graphite. As previously discussed, the oxidation of graphite was largely hindered in the current study because of the high heating rate used in addition to the protective effect of molten LiCl. Accordingly, the rise in the crystallite size of graphite transpired through the molten salt heat treatment (Table 3.4) can in part be credited to the effect of temperature on the crystalline order of graphite. This finding is interesting, considering such substantial increases of crystallinity can be obtained exclusively by the long-time heat treatment of graphite at temperatures more than 2500 °C under protective atmospheres [25, 26]. Hence, additional factors should also contribute. The fast and low-temperature crystal growth of graphite detected upon the molten salt heat treatment can be imputed to the reactive dissolution of impurities, as seen in Table 3.1, in the molten salt which contributes to ordering of the carbon structure. Other influencing phenomena include the healing of the graphite structural defects in molten salts.

3.6 Effect of Molten LiCl on the Microstructure of Graphite

29

3.6 Effect of Molten LiCl on the Microstructure of Graphite The SEM image of Fig. 3.7a displays the microstructure of the pristine graphite powder. This microstructure consists of graphite flakes with diameters between one and several micrometers. The SEM microstructure of the incompletely oxidized graphite sample heated to 900 °C in air at the heating rate of 20 °C min−1 is displayed in Fig. 3.7b. The heating condition resembles around 84% mass loss, as specified by Fig. 3.1. The image exhibits that graphite flakes are entirely disintegrated into rather fine fragments caused by the extreme oxidation. Figure 3.7c reveals the morphology of the oxidized graphite sample heated to 1250 °C at the heating rate of 80 °C min−1 . This heating causes a mass loss of about 50%, as portrayed by Fig. 3.1. As displayed, the thermal oxidation of graphite brought about the formation of shallow pits within the basal planes of graphite flakes. The corrosion pits are considered to form due to the reaction of oxygen with defects

Fig. 3.7 SEM microstructure of a the pristine graphite powder, b the partially oxidized graphite heated to 900 °C at the heating rate of 20 °C min−1 and c the partially oxidized graphite heated to 1250 °C at the heating rate of 80 °C min−1 under an ambient airflow of 100 mL min−1 , reproduced from Ref. [14], copyright 2019, with permission from Elsevier

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or impurities present on graphite surface [27, 28]. Along with this, some finer particles can additionally be seen in Fig. 3.7c, which could be residue remaining after partial oxidation of more active components like binders or less graphitized carbon phases. The inset in Fig. 3.7c is a high-magnification SEM micrograph taken from the edge of an oxidized graphite flake, showing that the edge recession took part in the degradation of graphite. It is established that while the basal plane of graphite is very difficult to oxidize with molecular oxygen, the edges of basal planes are much more active because of the presence of free bonded atoms and thus oxidize more readily [29]. Considering all the results, it is known that the oxidation of graphite is, to some degree, repressed at high heating rates. It can also be realized that the oxidation rate of carbon nanomaterials decreases when the critical oxidation temperature is traversed rapidly [18]. This conclusion can be interpreted by taking into account-independent regimes of graphite oxidation. Findings show that the oxidation of graphite in air is regulated by chemical reactions at medium temperatures below 600 °C. In contrast, the graphite oxidation is governed by in-pore diffusion at higher temperatures between 600 and 800 °C and by boundary layer diffusion at temperatures over 800 °C [30]. As maintained by the observations, heating the mixture of graphite and LiCl at the heating rate of 80 °C min−1 to 1250 °C not just induced the evaporation of LiCl, but also the microstructural modifications in graphite. Correspondingly, three microstructural features, which were largely unlike from the pristine and the partially oxidized graphite samples, were recognized in the heat-treated powders. These microstructures are portrayed in Figs. 3.8 and 3.9 and described accordingly. Figure 3.8a presents the dominant microstructure made up of bent graphite grains. These vary from the planar grains of the pristine graphite exhibited in Fig. 3.7a. The charging effect, which can take place for non-conducting materials, was not able to be detected in the SEM micrographs. This verifies the nonexistence of LiCl with the graphite. Along with the lines of this observation, no traces of chlorine were able to be found by EDX analysis. As a result, the evaporation of LiCl upon the heat treatment of the graphite–LiCl mixture is apparent. The extensions of Fig. 3.8b show the microstructure of the identical area in more details, from which it is noticed that the material maintains a layered microstructure. Using the given observation, it is apparent to suppose that this microstructure resulted from the intercalation of LiCl into the pristine graphite and the ensuing exfoliation of graphite. It should be mentioned that, derived from the thermodynamics and chemical band theory, molten LiCl is less likely to be intercalated into graphite [31]. Accordingly, it is feasible to assume that the layered structure of Fig. 3.8 can be formed by the intercalation of LiCl vapor between the graphite layers. SEM micrographs of Fig. 3.8b show the general characteristics of the exfoliated nanosheets in a higher magnification. Additionally, there is evidence present that supports the existence of individual graphene sheets in the heat-treated samples, for instance as presented in Fig. 3.8c. The figure displays a TEM micrograph of a folded graphene sheet. The inset of the image presents the selected area electron diffraction pattern taken from the edge of the sheet showing the distinctive hexagonal structure of graphene.

3.6 Effect of Molten LiCl on the Microstructure of Graphite

31

Fig. 3.8 Microstructure of the graphite–LiCl mixture heated at the rate of 80 °C min−1 to 1250 °C. a The dominant microstructure consisted of bent layered grains. b Exfoliated nanosheets which could be found in the microstructure. c TEM image of a graphene sheet. The inset is the selected area electron diffraction pattern taken from edge of the sheet showing the hexagonal structure of the (0001) basal plane, reproduced from Ref. [14], copyright 2019, with permission from Elsevier

The second microstructure feature to be considered in the heat-treated powders can be characterized by corroded graphitic sheets, which are seen in the main panel of Fig. 3.9a. The upper inset in Fig. 3.9a displays the corrosion pits with more details, in which a typical polyhedron pit is defined by the arrow. Clearly, in this case, the pits are deeper and the pit density is larger relative with those seen on the partially oxidized graphite heated at a similar heating rate (see Fig. 3.8c). Figure 3.9b presents a TEM image from a typical particle which could be observed in the heat-treated powders. The inset in Fig. 3.9b is the selected area diffraction pattern recorded on this particle. The pattern can be indexed to Li2 C2 O4 phase. The formation of these polyhedral pits is probably due to the reaction between the carbon from the graphite cathode with lithium oxides dissolved in LiCl to form polyhedral-shaped crystals containing lithium, carbon and oxygen. The subsequent physical disintegration of these crystals from the carbon substrate would leave behind holes. The thermodynamic data for Li2 C2 O4 are not available in the literature, and consequently, the values of the free energy of reactions involving the formation of Li2 C2 O4 cannot be easily calculated. Despite that, one possible reaction might be assumed as follows:

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3 Interaction of Molten Salts with Graphite

Fig. 3.9 a Second microstructure feature of the graphite–LiCl mixture heated at the rate of 80 °C min−1 to 1250 °C. The microstructure comprises of corroded graphitic sheets. b TEM micrograph from typical particles which could be observed in the heat-treated powders. The inset in (b) is the selected area diffraction pattern recorded on the particle, reproduced from Ref. [14], copyright 2019, with permission from Elsevier

2C + Li2 O + 1.5O2 = Li2 C2 O4

(3.4)

The pitting corrosion can also result in cutting of the graphitic sheets into nanosheets. The downer inset presented in Fig. 3.9 is a high-magnification micrograph taken on corroded graphite edge sites, from which carbon nanosheets can be released. The third microstructure which is able to be identified in the heat-treated powders is displayed in Fig. 3.10. The central image in this figure is a SEM micrograph displaying the carbon nanorods which are grown on the surface of graphite particles. The inset shown in the figure is a HRTEM micrograph taken from a nanorod with diameter around 10 nm grown on the graphite substrate. This examination aids the results found by Alekseev et al. [32, 33] who discovered carbon nanorods with lengths of around 2 µm in the electrolyte used in large-scale fabrication of metallic lithium (Fig. 3.11). According to those studies, the electrolysis cells were made up of graphite anodes and steel cathodes immersed in the eutectic LiCl–KCl. The electroless formation of carbon nanorods in molten LiCl (Fig. 3.10) can be interpreted as the following. The partial oxidation of graphite, taking place through the heat treatment, brings on the formation of carbon monoxide which then is catalytically decomposed to form nanorods. In essence, it has been established that alkali salts are effective catalysts to be used forth decomposition of molecules which contain carbon atoms into nanorods [34]. The formation of carbon nanorods by the catalytic decomposition of carbon monoxide has been disclosed to take effect adequately at around 600 °C [35].

3.6 Effect of Molten LiCl on the Microstructure of Graphite

33

Fig. 3.10 Third microstructure feature which could be observed in the graphite–LiCl mixture heated at the rate of 80 °C min−1 to 1250 °C, comprising of carbon nanorods grown on the surface of graphite particles, reproduced from Ref. [14], copyright 2019, with permission from Elsevier

Fig. 3.11 TEM micrographs of MWCNTs and carbon nanoparticles detected in waste from the industrial production of metallic lithium by electrolysis of the eutectic mixture of lithium and potassium chlorides with graphite anodes and steel cathodes, reproduced from Ref. [32], copyright 2019, with permission from Springer Nature

34

3 Interaction of Molten Salts with Graphite

The reason why different carbon nanostructures are able to be formed in the same sample is for the presence of local reactions in the system. It is worth noting that the reaction of molten LiCl with the atmospheric moisture occurs at the melt/gas interface. This reaction results in the formation of lithium oxides which eventually react with the neighboring graphite surfaces to create lithium-carbon-oxide components bringing about the formation of corrosion pits. The lithium-carbon-oxides then are portrayed as the catalysts to create carbon nanorods. As a result, the formation of corrosion pits and carbon nanorods, as it can be observed in Figs. 3.9 and 3.10, respectively, can be premeditated as local effects inducing contrasting microstructures. Meanwhile, the dominant microstructure incorporates layered grains such as displayed in Fig. 3.8. As mentioned previously, the microstructures shown in Figs. 3.8, 3.9 and 3.10 were brought by the attentive characterization of the graphite–LiCl mixture heated at the rate of 80 °C min−1 to 1250 °C. It should be noted that alike microstructures could be observed in the samples heated at other heating rates (10–90 °C min−1 ) as well. The interaction between molten LiCl and graphite can be considered as a straightforward and fast approach for the synthesis of carbon nanostructures, having the benefits of being inexpensive to produce and ascendable [14]. As a summary, the heating of the graphite powder at 20 °C min−1 to 900 °C brings about the intensive disintegration of graphite fakes into extremely fine fragments because of the oxidation process. The intense oxidation of graphite powder can be lowered by accelerating the heating rate to 80 °C min−1 . In the latter case, the material could be heated to a high temperature of 1250 °C in air atmosphere devoid of having undergone the entirety of the oxidation. The heating of a mixture of graphite–LiCl to 1250 °C results in the structural and microstructural modifications to the graphite. The intense oxidation of graphite could be stopped by the combination of the high heating rate employed, and the protective action brought about by molten LiCl. Additionally, the mean crystallite size of the graphite rises notably upon molten salt heat treatment. Three clearly contrasting carbon microstructures can be identified in the heat-treated graphite powder incorporating of exfoliated carbon sheets and nanosheets, pitted particles and carbon nanorods. These microstructural alterations are implied to be attributable to (a) the intercalation of LiCl vapor into the graphite structure, (b) the reaction of lithium oxides formed in the melt with graphite and (c) the catalytic decomposition of carbon monoxide, formed by the oxidation of graphite. Moreover, the evaporation of LiCl from the graphite–LiCl mixture was found to have a greater activation energy in comparison with that of LiCl. It was assigned to the interfacial adhesion energy between molten LiCl and graphite. These findings are of interest for further development of emerging molten salt-based technologies for the preparation of nanostructured carbon materials. The electrochemical erosion of graphite in molten salts is discussed in the Chap. 4.

References

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References 1. R. Taylo, K.E. Gilchrist, L.J. Poston, Thermal conductivity of polycrystalline graphite. Carbon 6, 537–544 (1968) 2. G. Cui, Q. Bi, S. Zhu, J. Yang, W. Liu, Tribological properties of bronze–graphite composites under sea water condition. Tribol. Int. 53, 76–86 (2012) 3. C. Ayache, I.L. Spain, Thermoelectric and thermomagnetic properties of graphite—I: The cylindrical band model. Carbon 17, 277–291 (1979) 4. J.H. Lee, Y.H. Kang, S.C. Hwang, J.B. Shim, E.H. Kim, S.W. Park, Application of graphite as a cathode material for electrorefining of uranium. Nucl. Technol. 162, 135–143 (2008) 5. D. Wang, X. Jina, G.Z. Chen, Solid state reactions: An electrochemical approach in molten salts. Annu. Rep. Prog. Chem. Sect. C 104, 189–234 (2008) 6. P. Hejzlar, B.T. Mattingly, N.E. Todreas, M.J. Driscoll, Advanced fuel elements for passive pressure tube light water reactors. Nucl. Eng. Des. 167, 375–392 (1997) 7. J. Uhlir, Chemistry and technology of Molten Salt Reactors—history and perspectives. J. Nucl. Mater. 360, 6–11 (2007) 8. A. Cammi, V. Di Marcello, L. Luzzi, V. Memoli, M.E. Ricotti, A multi-physics modelling approach to the dynamics of Molten Salt Reactors. Ann. Nucl. Energy 38, 1356–1372 (2011) 9. K. Nagarajan, B.P. Reddy, S. Ghosh, G. Ravisankar, K.S. Mohandas, U.K. Mudali et al., Development of pyrochemical reprocessing for spent metal fuels. Energy Procedia 7, 431–436 (2011) 10. V. Bernardet, S. Gomes, S. Delpeux, M. Dubois, K. Guérin, D. Avignant D et al., Protection of nuclear graphite toward fluoride molten salt by glassy carbon deposit. J. Nucl. Mater. 384, 292–302 (2009) 11. J. Sure, A.R. Shankar, S. Ramya, U.K. Mudali, Molten salt corrosion of high density graphite and partially stabilized zirconia coated high density graphite in molten LiCl–KCl salt. Ceram. Int. 38, 2803–2812 (2012) 12. A. Rezaei, A.R. Kamali, Green production of carbon nanomaterials in molten salts, mechanisms and applications. Diam. Relat. Mater. 83, 146–161 (2018) 13. A.R. Kamali, D.J. Fray, Electrochemical interaction between graphite and molten salts to produce nanotubes, nanoparticles, graphene and nanodiamonds. J. Mater. Sci. 51, 569–576 (2016) 14. A.R. Kamali, D.J. Fray, Molten salt corrosion of graphite as a possible way to make carbon nanostructures. Carbon 56, 121–131 (2013) 15. A.R. Kamali, D.J. Fray, C. Schwandt, Thermokinetic characteristics of lithium chloride. J. Therm. Anal. Calorim. 104, 619–626 (2011) 16. J.I. Langford, A.J.C. Wilson, Scherrer after sixty years: A survey and some new results in the determination of crystallite size. J. Appl. Crystallogr. 11, 102–113 (1978) 17. A.R. Kamali, C. Schwandt, D.J. Fray, On the oxidation of electrolytic carbon nanomaterials, Corros. Sci. 54, 307–313 (2012) 18. A.R. Kamali, G. Divitini, C. Schwandt, D.J. Fray, Correlation between microstructure and thermokinetic characteristics of electrolytic carbon nanomaterials. Corros. Sci. 64, 90–97 (2012) 19. W.W. Liu, S.P. Chai, A.R. Mohamed, U. Hashim, Synthesis and characterization of graphene and carbon nanotubes: A review on the past and recent developments. J. Ind. Eng. Chem. 20, 1171–1185 (2014) 20. M.S. Dresselhaus, A. Jorio, R. Saito R. characterizing graphene, graphite, and carbon nanotubes by Raman spectroscopy, Annu. Rev. Cond. Mat. Phys. 1, 89–108 (2010) 21. Q.Q. Dillon, J.A. Woollam, V. Katkanant, Use of Raman scattering to investigate disorder and crystallite formation in as-deposited and annealed carbon films. Phys. Rev. B 29, 3482–3489 (1984) 22. N.C. Cho, D.K. Veirs, J.W. Ager, M.D. Rubin, C.B. Hooper, D.B. Bogy, Effects of substrate temperature on chemical structure of amorphous carbon films. J. Appl. Phys. 71, 2243–2248 (1992)

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23. J.F. Freire, C.A. Achete, G. Mariotto, R. Canteri, Amorphous nitrogenated carbon films: Structural modifications induced by thermal annealing. J. Vac. Sci. Technol., A 12, 3048–3053 (1994) 24. M. Rusop, X.M. Tian, T. Kinugawa, T. Soga, T. Jimbo, M. Umeno, Preparation and characterization of boron-incorporated amorphous carbon films from a natural source of camphoric carbon as a precursor material. Appl. Surf. Sci. 252, 1693–1703 (2005) 25. H. Honda, K. Egi, S. Toyoda, Y. Sanada, T. Furuta, Electronic properties of heat treated coals. Carbon 1, 155–164 (1964) 26. D. Gonzalez, M.A. Montes-Moran, R.J. Young, A.B. Garcia, Effect of temperature on the graphitization process of a semianthracite. Fuel Process. Technol. 79, 245–250 (2002) 27. J.R. Hahn, H. Kang, S.M. Lee, Y.H. Lee, Mechanistic study of defect-induced oxidation of graphite. J. Phys. Chem. B 103, 9944–9951 (1999) 28. E.J. Hippo, N. Murdie, A. Hyjazie, The role of active sites in the inhibition of gas-carbon reactions. Carbon 27, 689–695 (1989) 29. X.W. Luo, J.C. Robin, S.Y. Yu, Effect of temperature on graphite oxidation behaviour. Nucl. Eng. Des. 227, 273–280 (2004) 30. W.M. Guo, H.N. Xiao, G.J. Zhang, Kinetics and mechanisms of non-isothermal oxidation of graphite in air. Corros. Sci. 50, 2007–2011 (2008) 31. R. Hui, K. Feiyu, J. Qing-jie, S. Wanci, Synthesis criterion for a metal chloride-graphite intercalation compound by a molten salt method. New Carbon Mater. 24, 18–22 (2009) 32. N.I. Alekseev, O.V. Arapov, I.M. Belozerov, Y.G. Osipov, K.N. Semenov, S.V. Polovtsev et al., Formation of carbon nanostructures in electrolytic production of alkali metals. Rus. J. Appl. Chem. 78, 1944–1947 (2005) 33. N.I. Alekseev, Y.G. Osipov, K.N. Semenov, S.V. Polovtsev, N.A. Charykov, O.V. Arapov, Carbon nanostructures in the industrial production of alkali metals by electrolysis. Tech. Phys. 51, 278–280 (2006) 34. Y. Zhang, X. Sun, Synthesis of carbon nanofibers and foam by catalytic chemical vapor deposition using a water-soluble alkali salt catalyst. Adv. Mater. 19, 961–964 (2007) 35. P. Chen, H.B. Zhang, G.D. Lin, Q. Hong, K.R. Tsm, Growth of carbon nanotubes by catalytic decomposition of CH4 or CO on a Ni–MgO catalyst. Carbon 35, 1495–1501 (1997)

Chapter 4

Cathodic Exfoliation of Graphite in Molten Salt Electrolytes

Abstract Scalable green production of carbon nanomaterials with enhanced physical, chemical and mechanical properties has been an interesting, yet challenging topic. Cathodic exfoliation of graphite in molten salt electrolytes has provided an efficient approach for economic and environmentally sustainable production of highquality carbon nanostructures, including carbon nanotubes, carbon nanoparticles, graphene and nanoparticles encapsulated in graphitic shells. The structure and morphology of these carbon nanostructures depend on a variety of parameters including the reactor design, the chemical composition of the molten salt, the atmosphere and the characteristics of the carbon raw material as well as the electrochemical parameters such as the voltage and the current density applied. This chapter provides an overview of the key parameters that influence the cathodic exfoliation behavior of graphite in molten salts. Keywords Graphite · Molten salts · Cathodic exfoliation · Graphene · Carbon nanotubes · Nanoparticles · Encapsulation Over the last 20 years or so, a recent family of molten salt technologies has been advanced for the large-scale preparation of high-quality carbon nanostructures for diverse applications. The fundamental features of these molten salt methods are briefed in Figs. 4.1 and 4.2, showing that varying nanostructured carbons are able to be fabricated by the treatment of nearly indistinguishable graphite raw materials, yet under dissimilar conditions, i.e., the lack or presence of an electric potential, and additionally the chemical composition of the surrounding atmosphere above the melt. Graphite is able to be exfoliated into differing carbon nanostructures in the LiCl melt. Figure 4.1 presents the SEM/TEM morphologies of the graphite feed material used as the carbon source and those of carbon products that can be manufactured under different processing conditions. Figure 4.2 displays the electrochemical cell developed for the molten salt preparation of varying nanostructured carbon materials at a scale of several hundreds of grams producible on a single run. It was discussed in Chap. 3 that the corrosion of polycrystalline synthetic graphite in LiCl melt underneath an air atmosphere can lead to the formation of carbon nanostructures which include exfoliated carbon flakes, graphene nanosheets, pitted

© Springer Nature Singapore Pte Ltd. 2020 A. R. Kamali, Green Production of Carbon Nanomaterials in Molten Salts and Applications, https://doi.org/10.1007/978-981-15-2373-1_4

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4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes

Fig. 4.1 Graphite materials with identical properties can be exfoliated into various differing carbon nanostructures in molten LiCl. a SEM morphology of the graphite feed materials. The inset shows the high-resolution TEM (HRTEM) micrograph of the graphite showing the crystalline nature of the material. b and c SEM micrographs of exfoliated graphite obtained by the chemical corrosion of graphite in molten LiCl in air (see Chap. 2) d SEM and e TEM micrographs of CNTs produced by the electrochemical exfoliation of graphite under dried Ar. f SEM micrograph of graphene– Li2 CO3 hybrid material obtained by the electrochemical exfoliation of graphite in humid Ar. g SEM morphology of the graphene materials obtained by the heat treatment of f. h SEM and i HRTEM micrographs of graphene nanosheets produced by the electrochemical exfoliation of graphite under hydrogen-containing Ar [1–5]

particles and carbon nanorods. Fig. 4.1b and c represent the morphology of the exfoliated graphite and nanosheets produced by these “chemical interactions”. The electrochemical exfoliation of graphite is disscused in following sections.

4.1 Molten Salt Production of Carbon Nanotubes The production of carbon nanostructures selectively using controllable and reproducible methods is extremely appealing. As conveyed in Chap. 3, the corrosion/erosion of graphite in molten salts is able to be evolved when a cathodic potential is applied to the graphite. Under dry argon, the cathodically charged graphite electrodes are being eroded in molten LiCl. Moreover, it was found that the erosion product in the form of CNTs and carbon nanoparticles can subsequently be retrieved

4.1 Molten Salt Production of Carbon Nanotubes

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Fig. 4.2 a Electrochemical cell developed for the fabrication of differing nanostructured carbon materials from an identical graphite raw material in the LiCl melt. b Different processing conditions, in terms of the chemical composition of the gas entering the reactor. c Photographs of a typical graphite rod which is used as the carbon source, before and after being cathodically polarized in the LiCl melt, as well as the graphene powder produced by the exfoliation of the graphite material stored in a jar [1–5], reproduced from Ref. [1], copyright 2019, with permission from Elsevier

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4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes

from the solidified salt (Figs. 4.1d and e, and 4.2 (I)). One appealing attribute of this technology is that CNTs and the spherical nanoparticles are able to be produced by adequately controlling the processing parameters. These parameters include the morphology of the graphite raw materials and the electrochemical processing parameters which are used throughout the molten salt process. The technical details of these technologies are discussed in [6–23]. The commercial graphite electrodes, which are used in the molten salt exfoliation process, are commonly manufactured by the carbonization of blended carbonaceous materials in an oxygen-free furnace at 1000–1300 °C. These mixtures may contain petroleum coke, pitch coke, carbon black, coal tar and petroleum pitch. The carbonization process is followed by the graphitization step which is usually carried out at temperatures greater than 2500 °C. This ends in the vaporization of binder residues and the crystallization of amorphous carbonaceous precursors. Depending on the processing conditions, the resulting graphite material manufactured can include graphite flakes and uneven carbon particles with diverse sizes and volume fractions. The inset in Fig. 4.3 portrays a SEM micrograph of a commercially produced graphite electrode in a powdered form. As graphically presented in the main panel of Fig. 4.3, the graphite material can be characterized by the presence of graphite flakes and particles. Keeping this in mind, it was discovered that the structure and morphology of carbon nanomaterials created in molten salts are reliant on those of the graphite cathode materials employed. In a systematic research, graphite materials of varying morphologies were polarized in molten LiCl. It was identified that a combination of

Fig. 4.3 Graphical representation showing the typical morphology of graphite electrode materials which can be used as the carbon source for the molten salt preparation of nanostructured carbon materials. The inset shows the SEM morphology of a graphite material (Morgan Advanced Materials), reproduced from Ref. [1], copyright 2019, with permission from Elsevier

4.1 Molten Salt Production of Carbon Nanotubes

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Fig. 4.4 Upper panels show the morphologies of different graphite raw materials used for the molten salt preparation of carbon nanostructures as shown in Fig. 4.2 (I). The down panels exhibit the morphologies of the molten salt-produced carbon nanostructures, formed using the specific graphite materials shown in the upper panels [1, 6], reproduced from Ref. [1], copyright 2019, with permission from Elsevier

spherical and tubular carbon nanostructures is able to be made using graphite materials with a microstructure consisting of primarily microsized flakes and a minor fraction of irregular grains. In contrast, spherical carbon nanostructures can predominantly be manufactured employing graphite materials with a microstructure of primarily nanosized grains (Fig. 4.4) [6]. Chen et al. [15] investigated the erosion of graphite electrodes with a small diameter of 1.5 mm (EC4, Graphite Technologies) in different chloride salts. The group discovered that the brief time (3 min) cathodic polarization of the graphite in molten LiCl and NaCl can prompt the creation of carbon nanoparticles and nanotubes with a maximum erosion level of 50 and 10%, respectively [15]. Figure 4.5 shows the morphology of carbon nanostructures created. On the other hand, while a complete disintegration of the graphite electrode could be noticed in molten KCl, the yield of nanoparticles in the carbon product was revealed to be considerably low. Experiments based on the EC4-grade graphite with a larger diameter of 6.5 mm were conducted by Kinloch et al. [17], at 820 °C in argon at a constant electric current of 5 A, corresponding to an initial current density of 1.1 A cm−2 in molten NaCl. In this study, the graphite cathode was shielded by a ceramic tube so that only the bottom part of the graphite rod was open to the melt (Fig. 4.5a). An identical cell design was utilized in successive studies [6, 15–18, 20–23], where commonly a graphite rod of about 6.5 mm in diameter was protected securely by an alumina tube where a length of around 1 cm at its lower end was open to the melt. Subsequently, the graphite electrode was cathodically polarized at around 770 °C underneath dry Ar at a constant potential of 2.4 V with respect to a Mo

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4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes

Fig. 4.5 TEM micrographs of the carbon nanostructures including multiwall carbon nanotubes and nanoparticles produced by the molten salt cathodic erosion of a graphite rod (d = 1.5 mm) under a cell current and voltage of 5 A and 8–10 V, respectively. The duration of the electrolysis process was 3 min at 820 °C, reproduced from Ref. [15], copyright 2019, with permission from Elsevier

pseudo-reference electrode immersed in molten salt. The counter electrode was either the graphite crucible used as the molten salt container (Fig. 4.6b) or a graphite rod immersed in the salt. The current applied (about 4.7 A) was equal to a cathode current density of about 2 A cm−2 . This was found to be the threshold current density for the high yield formation of CNTs in the conditions specified above. Despite that, the molten salt procedure depicted above could only be retained for a brief amount of time. This amount of time was commonly lower than 10 min, bringing about the fabrication of typically less than 500 mg carbon nanomaterials [23]. Essentially, it was found that alumina-shielded graphite cathodes exposed to the molten salt are fragmented into numerous pieces in the first few minutes of the process. Subsequently, this brings on the failure of the electrolysis cell. The collapse of the graphite electrode was presumed to be because of the maintenance of a high current density of 2 A cm−2 on a small area at the bottom of the electrode, lowering the scalability of the process. Afterward, it was discovered that the threshold cathode current density for the preparation of CNTs in molten LiCl can be markedly decreased to around 1 A cm−2 by using the electrolysis cell portrayed in Fig. 4.6c. Here, an entire length of a 15mm diameter graphite electrode immersed in the LiCl melt was being used as the cathode. This time, regardless of a high electric current of 33 A applied, no failure of the graphite cathode was detected even at prolonged electrolysis periods. This is due to the smaller values of the electric current density at the cathode/electrolyte interface [3].

4.1 Molten Salt Production of Carbon Nanotubes

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Fig. 4.6 Various cell designs used for the molten salt preparation of carbon nanostructures, employing a graphite crucible as the anode. a Constant current experiment (3 min) using a partially covered graphite rod (d = 6.5 mm) in molten NaCl, reproduced from Ref. [17], copyright 2019, with permission from Elsevier; b constant voltage experiment (10 min) using a partially covered graphite rod (d = 6.5 mm) in molten LiCl. The cell is equipped with a thermocouple (T) and a Mo pseudo-reference electrode (RE), reproduced from Ref. [21], copyright 2019, with permission from Elsevier; c constant current experiment (1 h) using an uncovered graphite rod (d = 15 mm). The Mo RE is used to monitor the potential of the electrodes [3]

As discussed, the morphology and structure of the graphite raw material can strongly influence the properties of the carbon products. In addition to this, the microstructure and morphology of the carbon nanostructures produced in molten salts could also be altered by the cathode current density used. This can be noticed from the SEM micrographs seen in Fig. 4.7. In this regard, the reduction of cathode current density from the value of 1 to 0.4 A cm−2 brought on the microstructural changes of the carbon product from tubular morphology to spherical nanostructures. The processes implicated in the electrochemical exfoliation of graphite will be detailed later in Chap. 5.

4.2 Production of Graphene in Molten LiCl It was discovered that under a humid Ar atmosphere, graphite cathodes placed in molten LiCl can be exfoliated into high-quality graphene nanosheets, mixed with Li2 CO3 . Furthermore, 3D graphene nanosheets could be attained by the heating of the mixture of graphene and Li2 CO3 at high temperatures. This could set in motion the evaporation of Li2 CO3 (Figs. 4.1f and g, and 4.2 (II)) [4, 9]. What’s more, it was

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4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes

Fig. 4.7 SEM micrographs of carbon nanotubes and spherical carbon nanoparticles fabricated by the cathodic exfoliation of a graphite raw material (mean grain size: 150 μm; diameter: 15 mm) at different values of the electric current density applied, reproduced from Ref. [3], copyright 2019, with permission from Elsevier

discovered that graphite cathodes can be instantly exfoliated to form mono- or fewlayer 3D graphene nanosheets, when the graphite materials are polarized in molten LiCl in an Ar–H2 atmosphere (Figs. 4.1h and i, and 4.2 (III)) [5]. Since this approach takes away the need for the further heating of the material to remove Li2 CO3 , it is a favored method for making graphene in molten LiCl. Figure 4.8 (upper panels) portrays SEM images of the graphene nanosheets made by the electrochemical exfoliation of two identical graphite rods (diameter: 1.3 cm; length: 30 cm; purity > 99.9%). The graphite rods were alternatively used as the cathode against a single graphite anode in LiCl molten salt under a flow of Ar–4% H2 and a constant direct electric current of 40 A, providing a cathode current density of around 1 A cm−2 . The SEM micrographs of Fig. 4.8 demonstrate the formation of high-yield randomly oriented graphene nanosheets having a lateral dimension up to several micrometers. The graphene nanosheets exhibit a very high quality in appearance. Figure 4.9 (down-left panel) compares the X-ray diffraction pattern of the resulting graphene nanosheets with that of the powdered graphite electrode. Furthermore, natural graphite flakes, consisted of highly oriented carbon crystallites, were also analyzed for comparison. We can observe that the intensity of the (002) XRD reflection of natural graphite

4.2 Production of Graphene in Molten LiCl

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Fig. 4.8 Upper panels show SEM micrographs of the graphene nanosheets produced by the molten salt exfoliation of graphite under a flow of Ar–H2 . The down-left panels exhibit the XRD diffraction patterns of a the graphene nanosheets, b powdered graphite material, and c natural graphite flakes. The inset in the down-left panel reveals the higher magnification of XRD patterns at the two theta values of 24–29°, around the most intense (002) peak. The down-right panel shows the Raman 2D peaks of the same materials, reproduced from Ref. [5], copyright 2019, with permission from RSC Publishing

is around 20 times more than that of the graphite electrode material. This says that the latter is made up of a much lower-dimensional graphite crystal. Moreover, this observation can confirm that the graphite flakes in the synthetic graphite material were haphazardly orientated in the bulk material, as shown in Fig. 4.3. Additionally, it can be seen that the intensity of the (002) peak in graphene nanosheets is very much lower than that of the powdered graphite raw material. This result shows that the graphite cathode was exfoliated to individual/few-layer graphene nanosheets to a high degree. Nevertheless, a small fraction of less-exfoliated flakes could still exist in the sample [5]. The down-right panel in Fig. 4.8 compares the 2D Raman band of various carbon materials: the powdered graphite electrode, graphene nanosheets and natural graphite

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4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes

Fig. 4.9 Left panel shows a SEM, and the right panel exhibits a TEM micrograph of graphene nanosheets fabricated by electrochemical exfoliation of graphite in molten NaCl. The inset in the right micrograph shows a typical selected area electron diffraction pattern recorded on the graphene material, reproduced from Ref. [26], copyright 2019, with permission from Elsevier

flakes. It is known that the 2D Raman band in bulk graphite materials is asymmetric and consists of two components. On the other hand, the 2D Raman peak of the single-layer graphene is made up of a red-shifted single peak [25]. The 2D Raman band in natural graphite flakes can be seen to consist of the well-known 2D1 and 2D2 components, which is a distinctive attribute of crystalline graphite materials. It can be observed that the 2D Raman band of the powdered graphite electrode is more symmetric and has further shifted to lower frequencies in comparison with that of the natural graphite. This confirms the low structural dimensionality of the powdered graphite. We can conclude from this 2D Raman band that the powdered graphite electrode is, in simpler terms, consisting of stacks of a limited number of graphene layers. Moreover, the Raman spectrum of the graphene nanosheets shown in Fig. 4.8 gives evidence that the product is primarily single- or few-layer graphene [5]. It was anticipated that graphene of various qualities could be produced using graphite raw materials with different morphological and structural properties.

4.3 Production of Graphene in Molten NaCl Even though we have touched on the promising attributes above, LiCl is still a pricey salt. Furthermore, it is difficult to be handled because of its particularly hygroscopic nature. In contrary to LiCl, sodium chloride is a much cheaper substance with an average world price of around US$150 per ton. It can, consequently, be thought of as an economical and sustainable raw material. NaCl occurs naturally in huge amounts. Oceans are the biggest global store of NaCl and hold around 36 thousand billion tons of dissolved NaCl. This is six times larger than the amount of identified lithium resources, with a total amount of about 40 million tons. Therefore, the sustainability and the economic output of the molten salt production of graphene are able to be enhanced using NaCl.

4.3 Production of Graphene in Molten NaCl

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Recently, the exfoliation of graphite nanosheets in molten NaCl under the influence of H2 containing inert atmospheres was evaluated. From this, it was discovered that graphite is able to be exfoliated in molten NaCl to create highly crystalline 3D graphene nanosheets with a uniform mesoporous nanostructure, high electrical conductivity of 2.1 × 10−5 S m−1 and thermal stability in air at temperatures below 500 °C. The SEM and TEM micrographs of graphene nanosheets created in molten NaCl are shown in Fig. 4.9 [26]. The comparison of these images with the SEM micrographs of Fig. 4.8 reveals the overall morphological similarity of the graphene material produced in molten NaCl to that of produced in molten LiCl. It should be noticed that the mechanisms suggested for the formation of graphene in molten NaCl are appealed to be nearly the same as that in molten LiCl. In fact, the main mechanism is based on the reduction of hydrogen cations (protons) from the melt on the graphite surface, followed by the intercalation of hydrogen into the graphite lattice structure. Later in Chap. 5, this will be discussed. On the other hand, the yield of producing graphene in molten NaCl was seen to be considerably lower than that in molten LiCl. This was blamed on the lower solubility of protons in molten NaCl in comparison with molten LiCl. Nonetheless, it was supposed that a molten salt electrochemical cell with a capacity of 1000 L molten NaCl (or 20 kg if the density of molten NaCl at 900 °C is considered to be 1.94 g cm−3 [27]) is still able to make 200 kg graphene nanosheets per day. The specific energy consumption could also be estimated to be around 50 kW h kg−1 [26]. Assuming the average price of electricity in China is around 10 US cent per kW h, and that of graphite electrodes and NaCl to be about US$2000 and US$50 per ton, respectively, we can see that the cost of the production of one kilogram of graphene in NaCl melt in China would be around US$12. What’s more, it should be said that salt can be retrieved after the washing process and reused in the process. This reduces the total cost to only about US$8 per kilogram of the graphene. Table 4.1 provides a comparison between the electrochemical exfoliations of graphite in molten LiCl and NaCl with typical state–of-the-art alternative electrochemical processes. We should take into account that the cathodic exfoliation of graphite in molten salts is much faster if compared to the low-temperature electrochemical methods, indicated by a much higher value of the current density which can be achieved. Moreover, since the graphene is produced at the cathode, a high-quality graphene is able to be produced in a short amount of time, without occurrence of oxidation. These attributes are compelling, particularly when looking at large-scale production. On the other hand, like any other molten salt-based methods, the molten salt exfoliation of graphene has its own challenges. One of these challenges is the corrosion of the equipment and the structural materials used for operating molten salt reactors [28–30]. Additionally, the level of expertise needed to conduct molten salt electrochemical experiments and initial preparations could be much higher than those in low-temperature operations. This creates a barrier for researchers to adequately employ this approach. These restrictions can be markedly expelled in largescale molten salt operations, such as the case of Hall–Héroult process for aluminum smelting.

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Table 4.1 Characteristics of the molten salt cathodic exfoliation of graphite for the preparation of graphene, in comparison with typical state–of-the-art alternative electrochemical methods, reproduced from Ref. [1], copyright 2019, with permission from Elsevier Approach

Carbon source

Electrolyte

Product/specifications

Current density (A cm−2 )

Room temperature anodic exfoliation

Graphite [61]

Acetamide, urea and ammonium nitrate

1–5-layer graphene, σ = 7.6 × 103 S m−1

0.010

CO2 capture and conversion

CO2 [31]

CaCl2 –CaO (850 °C)

Carbon sheets and nanotubes

0.2–0.6

Molten salt cathodic exfoliation

Graphite [4, 5]

LiCl under Ar–4% H2 (800 °C)

1–5-layer graphene, 450 g graphene per liter of LiCl per day, σ = 5.8 × 105 S m−1

1 A cm−2 (40 A)

Graphite [26]

NaCl under Ar–4%H2 (900 °C)

1–5-layer graphene, 200 g graphene per liter of NaCl per day, σ = 2.1 × 105 S m−1

1 A cm−2 (35 A)

4.4 Molten Salt Preparation of Metal-Filled Carbon Nanostructures In previous sections, we discussed the molten salt preparation of carbon nanostructures by the electrochemical exfoliation/erosion of graphite electrodes in molten salts. These molten salt-based technologies open up the opportunity for low-cost production of high-quality carbon nanostructures. One attractive feature of the molten salt approach is its ability of producing filled carbon nanomaterials; in which an inorganic second phase is encapsulated within graphitic shells. Earlier investigations [32–34] demonstrated the possibility of filling molten salt— produced carbon nanostructures with metals such as Sn. In a pioneering study in the field, Terrones et al. [33] produced tin-filled CNTs by the electrolysis of molten LiCl–SnCl2 at 600 °C using a graphite cathode immersed in the melt (Fig. 4.10a). The same group also reported on the preparation of CNTs filled with Sn–Pb alloy by the electrolysis of molten LiCl containing 0.5% Pb and 0.5% Sn [32] (Fig. 4.10b). However, it was realized that the electrolysis of molten LiCl containing only small amounts of SnCl2 using graphite electrodes leads to the formation of a small quantity of Sn-filled carbon nanostructures. On the other hand, any SnCl2 concentration greater than 2 wt% was found to be preventive against the formation of carbon nanostructures [24, 32–35]. For example, Fig. 4.11 shows photographs of a graphite rod before and after being used as the cathode material during the electrolysis of molten LiCl containing an initial SnCl2 concentration of 30 wt%. It can be observed that only a minor electrode erosion took place during the process. The cathodic polarization of the graphite rod immersed in molten LiCl–SnCl2 under a potential of around 7 V leads to the electrodeposition of Sn from the molten salt on the graphite cathode.

4.4 Molten Salt Preparation of Metal-Filled Carbon Nanostructures

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Fig. 4.10 TEM images showing metal (alloy)-filled carbon nanostructures produced in molten salts. a Filled tin nanotubes produced by the electrolysis of LiCl–0.5% SnCl2 at 600 °C in Ar, using a graphite crucible anode and a graphite cathode, reproduced from Ref. [33], copyright 2019, with permission from Springer Nature; b Pb–Sn alloy encapsulates CNTs produced by the electrolysis of LiCl containing 0.5% Sn and 0.5% Pb at 600 °C, reproduced from Ref. [32], copyright 2019, with permission from American Chemical Society

Since the electrodeposited Sn with a melting point of about 232 °C is in a molten state at the electrolysis temperature of around 800 °C, it can easily sink in the molten salt to the bottom of the crucible because of its higher density. This phenomenon can explain the formation of a Sn disk at the bottom of the graphite rod, as can be observed in Fig. 4.11 [36]. The dilemma recognized in Fig. 4.11 can be solved by the continuous addition of SnCl2 into the molten LiCl throughout the electrolysis process. Figure 4.12a presents the adapted setup which was developed for the large-scale preparation of Sn-containing carbon nanostructures in molten LiCl. This consists of a graphite rod immersed in LiCl melt which is used as the cathode during the electrolysis process, and a graphite crucible which is employed as the anode. In this method, SnCl2 pellets are added into the melt through an alumina tube at varying intervals during the electrolysis process. Figure 4.12b presents the potential difference between the graphite electrodes (anode and cathode) and a Mo pseudo-reference electrode, as can be seen in Fig. 4.12a. Distinct waves can be observed on the voltage–time curve of Fig. 4.12b, which is related to the addition of SnCl2 pellets into the molten salt throughout the electrolysis. The recorded voltage values contained a 1.2 V potential drop across the

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4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes

Fig. 4.11 Photographs of an industrial-grade graphite rod a before and b after being used as the cathode during the electrolysis of molten LiCl–30 wt% SnCl2 under Ar atmosphere, using the experimental setup exhibited in Fig. 4.2a. The electrolysis process was conducted in molten LiCl at 770 °C for 2 h at the constant current of 33 A, corresponding to an initial cathode current density of about 1 A cm−2 . No obvious erosion of the electrode was observed. Moreover, a Sn disk could be observed at the bottom of the cathode, reproduced from Ref. [36], copyright 2019, with permission from Elsevier

length of two stainless steel electrical connectors. The graphite electrode used as the cathode was eventually eroded, as shown in Fig. 4.12c, during which the SnCl2 pellets were being added into the molten salt at varying intervals during the electrolysis process. This figure displays the photographs of the graphite rod before and after being used as cathode in the molten salt process for varying time periods of 20 min and 2 h. At these intervals, total of 20 and 85 g SnCl2 , respectively, had been added to the melt. The mixture of solidified salt and Sn-filled carbon nanostructures inside the graphite crucible is shown in Fig. 4.12c. About 20 g Sn–C hybrid material was able to be attained after 2 h of electrolysis [34]. As depicted in Fig. 4.13a, the electrolytic product typically comprised of core– shell tubular or spherical nanostructures with a typical dimension of less than 100 nm. The central cavity of tubes and particles could be packed with Sn, as evidenced by the energy-dispersive X-ray (EDX) analysis or preferentially the electron energy loss spectroscopy (EELS) measurements, as shown in Fig. 4.13b. This figure illustrates a STEM micrograph recorded on an incompletely filled carbon nanotube. Moreover, as displayed EELS spectra were measured on the filled and empty area of the tube. The spectrum measured at the hollow cavity of the CNT shows the presence of C K-shell ionization edge at roughly 24 eV, while the spectra recorded on the filled

4.4 Molten Salt Preparation of Metal-Filled Carbon Nanostructures

51

Fig. 4.12 a Modified experimental setup employed for the large-scale production of Sn-containing carbon nanomaterials in the LiCl melt, enabling the continuous introduction of SnCl2 pellets into the melt through the alumina tube shown in the figure at different intervals during the electrolysis process. b Time variation of the potential measured between either the graphite anode and the graphite cathode, and a Mo pseudo-reference electrode immersed in the melt. The waves observed in the profiles are due to the interruption caused by the addition of SnCl2 pellets into the molten salt during the electrolysis. c The graphite electrode used as the cathode before and after being used as cathode in the process for 20 min and 2 h. (c) also shows the graphite crucible filled with Sn–C nanostructures at the end of the molten salt process, reproduced from Ref. [36], copyright 2019, with permission from Elsevier

areas exhibit the presence of the C K- and Sn M4,5-shell ionization edges at around 24 and 27 eV, respectively. The results verify the core–shell morphology of Sn–C nanostructures formed.

4.5 Molten Salt Preparation of Interconnected Graphene—Carbon Nanoscrolls Cathodic erosion of graphite cathodes can be employed as a facile and scalable approach to produce high-quality carbon nanostructures consisting of interconnected networks of graphene nanosheets and nanoscrolls with a hierarchical morphology

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4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes

Fig. 4.13 Electron micrographs of Sn-filled carbon nanostructures produced by 20 min electrolysis of the LiCl melt, during which SnCl2 was gradually introduced into the melt under nominally pure Ar. a A SEM micrograph showing the presence of CNTs and carbon nanoparticles. b STEM micrograph of a partially filled carbon nanotube, and EELS spectra recorded on the filled and empty areas of a CNT, confirming the core–shell morphology of the nanostructures, reproduced from Ref. [36], copyright 2019, with permission from Elsevier

and high electrical conductivity. The overall features of the reactor used for the preparation of this hierarchical nanostructured carbon material are similar to that observed in Fig. 4.2a. In a typical experiment, molten LiCl is electrolyzed at around 800 °C under a dry Ar flow at a constant direct current with an initial cathode current density of 1 A cm−2 for about 20 min. Then, the dry Ar gas is replaced by a moist Ar gas flow, and the electrolysis process is continued for another 60 min. Under this condition, the graphite cathode becomes exfoliated into an interconnected mesoporous mixture of carbon nanoscrolls and graphene nanosheets. In order to remove the residual salts, the carbon nanostructures produced are heated to a high temperature (greater than the evaporation temperature of the incorporated salts under a protective atmosphere) [13]. The structure of the interconnected nanostructured carbon differs from that of the as-received graphite, as can be seen in Figs. 4.14a and b. In the graphite raw material, the major diffraction peak at the 2º value of 26.35° indicates an interlayer spacing of 3.38 Å, while in the interconnected nanostructured carbon, the (002) diffraction

4.5 Molten Salt Preparation of Interconnected Graphene—Carbon …

53

Fig. 4.14 a XRD patterns, b Raman spectra and c XPS survey spectra recorded on graphite and the nanostructured carbon material, discussed in Sect. 4.5. The results indicate the formation of highquality interconnected carbon nanoscrolls–graphene nanosheets with low oxygen content. d–f TEM micrographs and an electron diffraction pattern (inset of f) of the nanostructured interconnected carbon, reproduced from Ref. [13], copyright 2019, with permission from RSC Publishing

peak appears at 26.52°, indicating an interlayer d-spacing of 3.36 Å. The Raman spectra of the carbon materials obtained with a 532-nm excitation laser indicate the presence of the three well-defined D, G and 2D peaks. The ID /IG ratio of the graphite and the interconnected nanostructured carbon could be determined to be 0.10 and 0.15, respectively. The slightly higher value of ID /IG ratio for the interconnected nanostructured carbon is attributed to its greater density of graphene edge sites. Despite this, the Raman ID /IG ratio of the interconnected nanostructured carbon is considerably smaller than that of chemically or thermally reduced graphene oxide (1.1–1.5) [13] indicating that the nanostructured carbon produced is composed of carbon crystallites with a considerably high degree of crystallinity. Moreover, the fabricated carbon has the Raman 2D band characteristics of singlelayer graphene. The chemical composition of the as-received graphite material and the fabricated hierarchical nanostructured carbon can be characterized using XPS. The oxygen content of the interconnected nanostructured carbon (around 1.36 wt%)

54

4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes

is slightly greater than that of the graphite raw material (1.14 wt%). However, the calculated C/O ratio of the nanostructured interconnected carbon (72.5) is considerably high, in comparison with other reported graphene-based hierarchical carbon nanostructures [13, 37–39]. The morphology of the nanostructured interconnected carbon produced is exhibited in TEM images of Fig. 4.14d–f. As it can be observed, the presence of rolled-up graphene species incorporated between the graphene nanosheets promotes the formation of the nanostructured interconnected porous carbon. The network structure may involve the formation of rolled-up graphene bridges between the interconnected carbon nanosheets. This nanostructured carbon material has a large specific surface area of around 470 m2 g−1 , with a high electrical conductivity of about 2 × 104 S m−1 . The nanostructured carbon material produced in the molten salt system has a much higher conductivity than that of other known hierarchical nanostructured carbon materials [13, 40–42] and also that of the graphite raw material (1.8 × 104 S m−1 ). These results indicate that the high-temperature molten salt process is capable of fabricating a high-quality well-interconnected carbon nanostructure. Since both the carbon nanoscrolls and graphene nanosheets have intrinsically high electrical conductivity and large electrochemically active sites, the observed network nanostructure with well-defined pores can serve as fast ionic and electronic conducting channels, making the nanostructured carbon material ideal to be used as electrodes for high-performance supercapacitors. It will be discussed in Chap. 6.

4.6 Molten Salt Conversion of CO2 into Li2 CO3 Nanocrystals Molten salt process can be used for the preparation of carbon nanostructures encapsulated with inorganic crystals such as Li2 CO3 . Li2 CO3 -encapsulated carbon nanostructures can be used as a precursor for the preparation of nanodiamond. The transformation of this nanostructured hybrid material into nanodiamond will be discussed in Chap. 8. This section concerns the formation of Li2 CO3 -encapsulated carbon. Figure 4.15a shows the setup used for the conversion of CO2 into Li2 CO3 nanocrystals and also the encapsulated carbon nanostructures. In order to demonstrate the transformation of CO2 to Li2 CO3 , a moist CO2 flow of 200 cm3 min−1 was bubbled into molten LiCl–2 wt% Li2 O as shown in Fig. 4.15a for 60 min, without the involvement of the graphite electrodes. Then, the furnace was allowed to be cooled down to room temperature. In order to study the possible conversion of carbon dioxide, the content of the crucible was exposed to boiling deionized distilled water. It should be considered that LiCl has a considerably high solubility of 1200 g L−1 in water at 100 °C, while the water solubility of Li2 CO3 is relatively low (7 g L−1 ) under the same condition. This contrast in water solubility allows the selective dissolution of LiCl. In order to analyze Li2 CO3 crystals, gold TEM grids were immersed in the solution and then allowed to be dried. TEM

4.6 Molten Salt Conversion of CO2 into Li2 CO3 Nanocrystals

55

Fig. 4.15 a Schematic representation of the experimental setup employed for the molten salt preparation of Li2 CO3 nanosingle crystals and carbon-encapsulated Li2 CO3 nanoparticles. For this, a moist CO2 gas stream is supplied by passing the CO2 through a U-shaped water container before getting directed into the melt through an alumina tube. b TEM micrograph and a selected area electron diffraction pattern recorded on Li2 CO3 nanocrystals formed by the reaction occurred between CO2 and Li2 O in the melt. c A high-resolution TEM micrograph and fast Fourier transformation pattern recorded on the Li2 CO3 nanocrystal shown in the micrograph, from which the presence of lattice fringes with the interplanar spacing of 0.42 nm is evident. The lattice fringes correspond to the (110) planes of monoclinic Li2 CO3 structure. d SEM and e TEM images of Li2 CO3 nanocrystals encapsulated in graphitic shells, fabricated by the molten salt electrolysis process. For this, a direct electric current was applied to the electrodes immersed in melt. The graphite cathode material is consumed together with CO2 gas in the molten salt process to fabricate the nanostructured hybrid material, reproduced from Ref. [43], copyright 2019, with permission from Elsevier

micrographs, such as shown in Fig. 4.15e, provided valuable information about the formation of Li2 CO3 . These results demonstrate that the reaction between CO2 gas and Li2 O from the molten salt occurred in molten LiCl–2 wt% Li2 O leads to the formation of nanosingle crystals of Li2 CO3 with particle sizes of less than 30 nm [43]: ◦

CO2 + Li2 O = Li2 CO3 G (at 800 ◦ C) = −66 kJ

(4.1)

This result is also remarkable from an application point of view, as Li2 CO3 nanoparticles have interesting applications in fields like drug delivery [44], gas sensing [45] and synthesis of electronic grade lithium compounds such as tantalates [46],

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4 Cathodic Exfoliation of Graphite in Molten Salt Electrolytes

niobates [47] and titanates [48]. The fabrication of Li2 CO3 in LiCl melt by the occurrence of the reaction (4.1) is generally accepted in the literature [49–51]. The Li2 CO3 nanocrystals formed, however, are very difficult to be evidenced, since the Li2 CO3 formed has a high solubility in molten LiCl at temperatures greater than 730 °C [52]. It should also be mentioned that the solubility of Li2 CO3 in LiCl decreases with the temperature and eventually becomes negligible at room temperature [52].

4.7 Encapsulation of Li2 CO3 Nanocrystals in Carbon Layers Lithium carbonate nanocrystals formed in the process explained in 4.6 can be encapsulated in graphitic layers. For this, a graphite rod was used as the carbon source. The rod had a well-defined graphitic structure with an average crystalline size of 36 nm and contained graphite flakes of several micrometers in size. After 10 min of bubbling the moist CO2 gas into the melt (Fig. 4.15a), the graphite rod and a Pt tube, which were both already immersed in the molten salt, were connected to the negative and positive terminals of a power supply, respectively, and a potential difference of 5 V was applied between the electrodes, corresponding to an electric current of 35 A. Under this condition, H+ cations, arisen from the ionization of HCl in the LiCl melt, were neutralized on the graphite cathode to form hydrogen atoms. The atomic hydrogen formed could easily intercalate into the interlayer spaces of the graphite cathode. Such an intercalation can occur, considering that the size of hydrogen atoms (0.5 A) is considerably smaller than the average interlayer space of the graphite material (3.35 A). Furthermore, the high diffusion coefficient of hydrogen in graphite at 800 °C (3.3 × 10−5 cm2 s−1 ) is five-order of magnitude greater than that at room temperature [53]. The subsequent combination of the already intercalated hydrogen atoms within the graphite lattice could lead to the formation of H2 gas. The gas generated will have enough kinetic energy to exfoliate the graphite lattice into graphene nanosheets. The details of this process will be discussed in Chap. 5. The graphene nanosheets formed are released in the bulk of LiCl melt, which already contains dissolved Li2 CO3 formed by the occurrence of the reaction (4.1). The subsequent cooling of the molten salt after the electrolysis leads to the reduction of the Li2 CO3 solubility and hence the formation of Li2 CO3 nanocrystals. Graphene nanosheets available in the melt could then easily wrap the Li2 CO3 nanoparticles in order to minimize their surface energy. This process results in the formation of carbon-encapsulated Li2 CO3 nanostructures, as shown in Fig. 4.15. SEM micrograph of this material (Fig. 4.15d ) exhibits a hierarchical-like morphology comprising 3D assemblies of nanoparticles, containing Li2 CO3 nanocrystals with a size of 5–30 nm tightly sealed by 2–5-nm-thick graphitic layers. The (002) planes of the crystalline carbon, wrapping the Li2 CO3 nanoparticles, can clearly be observed in the HRTEM micrograph of Fig. 4.15e [43]. This core–shell hybrid nanostructured material can be employed as a precursor to produce nanodiamonds, as will be discussed in Chap. 8.

4.7 Encapsulation of Li2 CO3 Nanocrystals in Carbon Layers

57

As a conclusion, the mechanism involved in this process can be explained as follows: The occurrence of the reaction (4.1) at 800 °C leads to the formation of Li2 CO3 dissolved in the LiCl melt. As the temperature decreased, the solubility of Li2 CO3 in melt reduces, causing the nucleation of Li2 CO3 crystallites. These crystallites can grow in size with further temperature reduction. The nanosingle crystals of Li2 CO3 formed are capable of being encapsulated into carbon cages to fabricate the hybrid nanostructure material observed in Fig. 4.15e. Such a hybrid material can be used for the fabrication of nanodiamonds at an ambient external pressure. It is worth mentioning that various techniques have been used in the literature in order to encapsulate nanoparticles such as Fe2 O3 [54], LiFePO4 [55], Si [56], MgO [57], W2 C [58], GdC2 [59] and Cu [60] in carbon layers for use in Li-ion batteries [54–57] and other functional applications. These methods often employ carboncontaining precursors and involve energy-intensive high-temperature steps including spray drying–carbonization–oxidation [54], aerosol processing using graphene oxide [56], combustion synthesis [57], reduction of graphite intercalation compounds [60] or arc discharge processing [59]. The molten salt method discussed in this chapter can be considered as an efficient approach for one-pot encapsulation of nanoparticles.

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55. H. Fei, Z. Peng, Y. Yang, L. Li, A.R.O. Raji, E.L.G. Samuel, J.M. Tour, LiFePO4 nanoparticles encapsulated in graphene nanoshells for high-performance lithium-ion battery cathodes. Chem. Commun. 50, 7117–7119 (2014) 56. J. Luo, X. Zhao, J. Wu, H.D. Jang, H.H. Kung, J. Huang, Crumpled graphene-encapsulated si nanoparticles for lithium ion battery anodes. J. Phys. Chem. Lett. 3, 1824–1829 (2012) 57. S. Dyjak, M. Cudziło, B. Pola´nski, J. Budner, Bystrzycki Graphitic encapsulation of MgO and Fe3 C nanoparticles in the reaction of iron pentacarbonyl with magnesium. Mater. Charact. 81, 97–104 (2013) 58. Y. Zhou, R. Ma, P. Li, Y. Chen, Q. Liu, G. Cao, J. Wang, Ditungsten carbide nanoparticles encapsulated by ultrathin graphitic layers with excellent hydrogen-evolution electrocatalytic properties. J. Mater. Chem. A 4, 8204–8210 (2016) 59. R.S. Subramoney, D.C. Ruoff, B. Lorents, R. Chan, M.J.D. Malhotra, Parvin, Magnetic separation of GdC2 encapsulated in carbon nanoparticles. Carbon 32, 507–513 (1994) 60. X. Bin, J. Chen, H. Cao, L. Chen, J. Yuan, Preparation of graphene encapsulated copper nanoparticles from CuCl2 –GIC. J. Phys. Chem. Solids 70, 1–7 (2009) 61. Y. Zhang, Y. Xu, J. Zhu, L. Li, X. Du, X. Sun, Electrochemically exfoliated highyield graphene in ambient temperature molten salts and its application for flexible solid-state supercapacitors. Carbon 127, 392–403 (2018)

Chapter 5

Mechanisms Involved in the Electrolytic Fabrication of Carbon Nanostructures

Abstract The formation of various forms of carbon nanostructures in molten salts, including spherical carbon nanoparticles, carbon nanotubes, carbon nanoscrolls, graphene and carbon encapsulated structures has been found to depend on various processing parameters, including the morphology of the graphite feed material and electrochemical conditions such as the molten salt temperature, the cathode current density and the atmosphere of the molten salt electrolysis process. The mechanisms involved in the formation of these carbon nanostructures can be speculated by the correlation between the characteristics of the products and the processing parameters. This chapter concerns various possible mechanisms by which the electrolytic formation of carbon nanostructures in molten salts can be explained. Keywords Molten salts · Graphite · Carbon nanostructures · Graphene · Intercalation · Hydrogen · Lithium · Chemical reaction · Electrochemistry

5.1 Electrochemical Erosion of Graphite Under Nominally Dry Ar As mentioned in the previous chapter, the morphological and the structural characteristics of carbon nanomaterials produced in molten salts are dependent on those of the graphite feed materials used as the cathode (Fig. 4.4). When the molten salt process shown in Fig. 4.2 is conducted under nominally dry argon using a graphite material with a microstructure including planar microsized flakes as the dominant microstructural component, a product comprised of tubular carbon nanostructures with a fraction of spherical carbon nanoparticles can be fabricated. In addition to the graphite morphology, the current density applied was also found to influence the products (Fig. 4.3). Under dry Ar, at low current densities (lower than 0.6 A cm−2 ), the “chemical reactions” occurred at the interface of the graphite cathode and the molten LiCl during the electrolysis of LiCl melt was proposed to be responsible for the formation carbon nanoparticles. One possible chemical reaction which might occur on the graphite cathode can be assumed as follows [1]: ◦

◦

2Li + 2C = Li2 C2 G800 ◦ C = −44kJ H800 ◦ C = −60 kJ © Springer Nature Singapore Pte Ltd. 2020 A. R. Kamali, Green Production of Carbon Nanomaterials in Molten Salts and Applications, https://doi.org/10.1007/978-981-15-2373-1_5

(5.1) 61

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5 Mechanisms Involved in the Electrolytic Fabrication …

Experimental results revealed that the molten salt exfoliation of graphite electrodes immersed in the LiCl melt leads to the increase of the temperature of molten salt, for example, from 770 to 800 °C. This observation can be attributed to the occurrence of exothermic reactions such as (5.1). The electrochemical formation of lithium carbide during the electrolysis of LiCl melt by graphite cathodes may be explained as reaction (5.2) [1]. ◦

◦

2LiCl + 2C = Li2 C2 + Cl2 (g) E800◦ C = −3.14V H800◦ C = 607kJ

(5.2)

The use of standard electrochemical potential (E°), which is an intensive property of the reaction, provides a tool to evaluate the possibility of occurring the reaction (5.2). During the production of carbon nanostructures in LiCl melt under a dry argon atmosphere, the typical cell potential was measured to be about 8 V, which is sufficient for the formation of Li2 C2 . Lithium carbide formed during the electrolysis can be washed away during the washing step according to the reaction (5.3) [1]: ◦

Li2 C2 + 2H2 O = 2LiOH(aq) + 2C + H2 (g) G25 ◦ C ◦

= −347 kJ H25 ◦ C = −339 kJ

(5.3)

The formation of carbon nanoparticles, therefore, can be attributed to the formation of lithium carbides on the surface of the flakes, leading to the electrochemical etching of graphite flakes, followed by removing the carbides formed either by dissolving into the LiCl melt, or by reacting with water during the washing process. This subsequently leads to the disintegration of the graphite fakes into graphitic carbon nanoparticles containing a proportion of carbon atoms with amorphous bonding structure [1]. At higher current densities (higher than 0.6 A cm−2 ), however, the intercalation of lithium electrodeposited on the graphite’s surface from the molten lithium chloride seems to play a significant role in the cathodic exfoliation. The mechanism of this process can be explained by the discharge of lithium cations on the cathode and their subsequent intercalation into the graphite between the graphene layers under the effect of the cathodic potential applied [2–4]. It is worthy to mention that the atomic size of lithium is similar to the interlamellar spacing in graphite. Therefore, the high-temperature diffusion of lithium in graphite might generate sufficient stress to extrude carbon sheets from the graphite material into the molten salt where the sheets have the chance to roll up in order to minimize their surface area exposed to the melt. The intercalation of Li into graphite cathodes can also take place in molten LiOH at 600 °C leading the exfoliation of graphite. The exfoliation rate, however, was reported to be much lower than that observed in molten LiCl [5]. Xu et al. measured the cyclic voltammograms related to the reduction of lithium from molten LiCl at 625 °C on molybdenum and graphite working electrodes, and the results are shown in Fig. 5.1a, b, respectively [6]. Table 5.1 summarizes the various cathodic and anodic events that can be observed in the cyclic voltammograms. The results suggest that lithium ions can deposit on sufficiently cathodically polarized

5.1 Electrochemical Erosion of Graphite Under Nominally Dry Ar

63

Fig. 5.1 Cyclic voltammograms recorded in molten LiCl at a potential scan rate of 50 mV s−1 at 625 °C using (a) molybdenum and (b) graphite working electrodes. A molybdenum wire was employed as the quasi-reference electrode, reproduced from Ref. [6], copyright 2019, with permission from Elsevier

Table 5.1 Various events observed in the cyclic voltammetry measurements shown in Fig. 5.1 [6] Working electrode

Current wave

Event description

Molybdenum Figure 5.1a

A/Cathodic

−2 V, metallic lithium deposition on the molybdenum

A1 /Anodic

Reverse of the cathodic reaction A

Graphite Figure 5.1b

A/Cathodic

−0.25 V, discharge of lithium ions on the graphite followed by the intercalation of Li into cavities of the graphite

B/Cathodic

−0.8 V, accelerated discharge of lithium ions followed by the intercalation of Li into the graphite structure

B1 /Anodic

Reverse of the cathodic reaction B

C/Anodic

The discharge of oxygen ions, (impurity)

D/Anodic

Chlorine evolution

graphite and then intercalate into the graphite structure. Furthermore, it is evident that the intercalation of Li occurs only with graphite and not Mo. Therefore, the potential of lithium reduction on graphite shifts to more positive values compared with that on Mo. Although these observations suggest the occurrence of Li intercalation into graphite at high temperatures in molten LiCl, complimentary experimental and theoretical investigations should be conducted in the future to prove this further. In this section, the electrochemical erosion of graphite in molten salts was assigned to the chemical or electrochemical reactions occurred between the graphite electrode and the molten salt without any direct influence from the atmosphere. Later in this chapter, it will be shown that the molten salt erosion of graphite can also be affected from the chemical composition of the atmosphere. To begin this discussion, we first need to consider the effect of moisture on the thermokinetic characteristics of LiCl.

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5.2 Thermokinetic Characteristics of LiCl LiCl can be hydrated in the presence of moisture as can be seen from the phase diagram of the LiCl-H2 O, showcased in Fig. 5.2. At the temperature range from −20 to 20 °C, the hydration of LiCl can be expressed as follows [7, 8]: H2 O

30 wt% H2 O

LiClsolid −−→ LiClsolid + LiCl · H2 Osolid −−−−−−→ LiCl · H2 Osolid 47 wt% H2 O

60 wt% H2 O

+ LiCl · 2H2 Osolid −−−−−−→ LiCl · 2H2 Osolid + Liquid −−−−−−→ Liquid (5.1) This equation can describe the general hygroscopic characteristic of LiCl, based on which the salt easily absorbs water to form LiCl · H2 O upon exposure to a moist atmosphere. At a longer exposure time, LiCl turns progressively sticky and notably wet on the surface. This signals the consecutive formation of higher hydrates and the eventual dissolution of the LiCl in its own crystal water. This demeanor has been appealing just for the applications in which LiCl is used as desiccant. Desiccants are defined as materials or systems that exhibit a strong attraction toward water molecules and hence can effectively absorb moisture from surrounding if relative humidity is above a critical limit. Desiccants should also be capable of being regenerated. The Fig. 5.2 Phase diagram of LiCl–H2 O [7, 8], reproduced from Ref. [8], copyright 2019, with permission from Springer Nature

5.2 Thermokinetic Characteristics of LiCl

65

hygroscopic properties of salts such as LiCl and CaCl2 allow them to be used in dehumidification systems. However, in molten salt-based technologies, the hygroscopic attribute of LiCl is typically considered as greatly unappealing. Thereupon, the molten salt processes using salts such as LiCl or CaCl2 are generally carried out under attentively controlled dry atmospheres, and this adds considerably to the cost at an industrial scale. Regardless, investigations carried out by the author of this book have shown that the hygroscopic property of LiCl could be used in the preparation of a variation of advanced materials. This shall be addressed later on. However, before this, the thermokinetic behavior of LiCl is described. Figure 5.3 presents the differential scanning calorimetry (DSC) analysis of LiCl at varying heating rates, under an ambient air flow of 100 mL min−1 . The critical temperatures abstracted from Fig. 5.3a are displayed in Fig. 5.3b. The curve recorded at a heating rate of 10 °C min−1 shows two endothermic peaks attributed to the surface dehydration of LiCl at the temperatures of 47 and 93 °C. The existence of these peaks

Fig. 5.3 a DSC curves for 20 mg of LiCl heated at different rates, ranging from 10 to 50 °C min−1 , under an ambient air flow of 100 mL min−1 . b Transition temperatures for surface dehydration, melting and complete evaporation of LiCl at different heating rates. c TG-MS analysis of 20 mg of LiCl heated at a rate of 10 °C min−1 under an ambient air flow of 60 mL min−1 , reproduced from Ref. [8], copyright 2019, with permission from Springer Nature

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is in qualitative agreement with the two-stage mechanism for bulk dehydration of LiCl · H2 O [7, 9]. Additionally, it is realized from Fig. 5.3 that by increasing the heating rate to values beyond 10 °C min−1 , the first endothermic peak vanishes, suggesting that the surface dehydration of LiCl proceeds in only one step at higher heating rates. The preliminary water content of the material was found to be 2.55% with 88% thereof being removed below 120 °C, by averaging the mass loss in samples heated up to 600 °C with different heating rates. Three clear endothermic peaks are apparent on the DSC curves of Fig. 5.3. The peaks seen at 93–115 °C, 608–621 °C and 1021–1195 °C are assigned to the surface dehydration, the melting and the complete evaporation of LiCl, respectively. Along with these peaks, some small endothermic peaks can also be seen at around 800–1000 °C. The source of these peaks can be indicated by using thermogravimetric analysis combined with mass spectrometry (TG-MS) analysis [7, 8]. Figure 5.3c displays the TG-MS analysis of LiCl during heating at the heating rate of 10 °C min−1 under an ambient air flow of 60 mL min−1 . The mass to charge ratios, m/z, of 18 and 44 are attributed to the H2 O+ and LiCl+ ion, consisting of the 37 Cl isotopes, respectively. The m/z values of 38 and 36 are evidence of the HCl+ ion which comprise of the 37 Cl and 35 Cl isotopes, respectively. Figure 5.3c confirms the release of water during the surface de-hydration of LiCl at low temperatures. Furthermore, there is also an indication of the water release at temperatures above 800 °C. The evaporation of LiCl is seen to initiate at a minor rate at temperatures greater than around 650 °C, and then slowly accelerates up to 800 °C where a sharp rise occurs. The release of HCl can be detected at temperatures beyond 800 °C. These observations were used to explain the small endothermic peaks detected in the DSC curve of Fig. 5.3. Accordingly, the first endothermic peak at around 801 °C at the heating rate of 10 °C min−1 is as a result of the hydrolysis of molten LiCl to LiOH, as shown by Eq. (5.2) [8]: LiCl + H2 O = LiOH + HCl(g)

(5.2)

The second endothermic event at 831 °C can be assigned to the decomposition of LiOH to form Li2 O: 2LiOH = Li2 O + H2 O(g)

(5.3)

Derived from these experimental observations, nominally anhydrous LiCl readily absorbs and retains water after exposure to air at ambient conditions, forming a surface layer of monohydrate LiCl · H2 O. The predominance of the water content is removed by heating the material to temperatures higher than 120 °C. The melting of the dehydrated LiCl takes place at temperatures very near to those indicated in the literature. This shows that the dehydration process leaves behind relatively pure LiCl. Substantial evaporation of LiCl takes place at temperatures far below its nominal boiling temperature. Upon heating, the LiCl critical temperatures do not considerably change based on the type of gas atmosphere used. However, the water content of the atmosphere plays an important role on the evaporation behavior of

5.2 Thermokinetic Characteristics of LiCl

67

LiCl. The overall thermal phase transitions of LiCl can be shown as follow [8]: [LiCl + LiCl · H2 O]solid → [LiCl]solid → [LiCl]liquid

H2 O↓ HCl↑

→

[LiCl - LiOH]liquid

H2 O↑

→ [LiCl − Li2 O]liquid → Gas (5.4)

5.3 Electrochemical Erosion of Graphite Under Humid Ar It was found that graphite cathodes immersed in molten LiCl can be exfoliated into high-quality graphene nanosheets mixed with Li2 CO3 under a humid Ar atmosphere. The heating of this mixture at high temperatures could bring about the evaporation of Li2 CO3 and the formation of 3D graphene nanosheets (Fig. 4.1f and g). The mechanism necessitated in this molten salt process was suggested as follows: the solid form of lithium chloride does not exhibit a substantial affinity for hydrolysis, because of the existence of an energy barrier. Despite that, the hydrolysis of molten LiCl, reaction (5.2) is far more consequential considering that the species formed by the hydrolysis processes are easily soluble in the melt [10, 11]. The decomposition of lithium hydroxide formed by the hydrolysis of LiCl causes the formation lithium oxide, as presented by the reaction (5.3). Hydrogen chloride is greatly soluble in molten lithium chloride-based salts, leading to the creation of protons and chlorine anions in the melt. The diffusion coefficient of protons created by the ionization of HCl in the molten salt is thought to be markedly high [11, 12]. Also, the solubility of Li2 O in LiCl melt can be more than 11 mol% [13]. As a result, ionic species including oxygen anions (O2− ) and hydrogen cations + (H ) are able to be created by the occurrence of reactions (5.2) and (5.3) in molten LiCl. Therefore, the events that take place during the cathodic polarization of graphite in LiCl melt under a humid Ar atmosphere can be viewed as: 2H+ + 2e = H2 at the cathode

(5.5a)

O2− = 0.5 O2 + 2e at the anode

(5.5b)

H2 O = H2(at the cathode) + 0.5O2(at the anode)

(5.5)

Consequently, the structural transformation of the graphite cathode materials into graphene nanosheets can be assigned to the exfoliation process caused by the intercalation of hydrogen in the graphite lattice structure. The initial stage of the cathodic disintegration process can be the formation of atomic hydrogen adsorbed on the graphite surface (Had ):

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5 Mechanisms Involved in the Electrolytic Fabrication …

H+ + e → Had

(5.6)

Because the molten salt process is conducted at the temperature of around 800 °C, the rapid diffusion of adsorbed hydrogen atoms into the graphite lattice is highly possible. The second stage of the exfoliation event can be explained by the formation of hydrogen molecules by combining the intercalated hydrogen atoms [14]: Had + H+ + e → H2

(5.7)

The diffusion of atomic and molecular hydrogen in graphite has been theoretically studied. It is known that the diffusion of hydrogen atoms chemisorbed on the surface of graphite crystallites is fairly short and direct [14, 15]. Furthermore, it entails the breaking of the C–H bond and forming another bond with a neighboring carbon atom in the same or in an adjacent sheet. This event requires an activation energy in the range 0.38–0.5 eV [14–17]. The temperature dependency of the diffusion coefficient of atomic hydrogen on graphite sheets (DH , cm2 s−1 ) can be concluded based on the Eq. (5.8) [15]:    ◦ DH = 2.0 × 10−3 exp −6.09 × 10−20 /kB T 300 − 1700 C

(5.8)

In this equation, kB is the Boltzmann constant (1.38 × 10−23 J K −1 ) and T is the temperature (K). According to (5.8), at 25 °C and 800 °C, DH has the values of 9.2 × 10−10 and 3.3 × 10−5 cm2 s−1 , respectively, expressing a five order of magnitude difference in Had diffusion speed. Such a diffusion rate allows hydrogen atoms to diffuse deeply into the bulk of graphite through either its porosity or crystalline structure [18]. In graphite materials, which have typically been used as the cathode in the molten salt exfoliation process, the graphite flakes are separated by micrometer-sized voids. Moreover, it is known that in a graphite flake, crystallites are commonly divided by nanometer-sized voids that are a few Å in size, providing chemically reacting internal surfaces on which hydrogen atoms can diffuse and integrate to make hydrogen molecules in the bulk of the graphite [18]. The penetration of atomic hydrogen in graphite porosity is greatly influenced by the temperature and the hydrogen exposure rate. The typical time taken for hydrogen atoms to penetrate several Å into graphite is found to be around 15 days at 25 °C. This time period is less than a millisecond at 600 °C and about a microsecond at 1200 °C [18]. Causey et al. described that when graphite is exposed to hydrogen atoms or ions, the retention of hydrogen significantly increases as the exposure rate surpasses 5 × 1020 atoms per cm2 , and this was credited to the fact that a higher number of atoms reach the internal porosity [19]. It was discovered that graphite cathodes are perceptibly eroded in molten LiCl underneath a humid Ar atmosphere if the cathode current density grows by around 0.5 A cm−2 . Along with that, the rate of erosion rises with the current density [14]. Therefore, we can suppose that further Had [formed on the graphite cathode based on the reaction (5.6)] can penetrate deep into the porosity of graphite material at higher current densities. Under these circumstances, the combination of

5.3 Electrochemical Erosion of Graphite Under Humid Ar

69

hydrogen atoms to create hydrogen molecules, reaction (5.7), is probable to happen in the porosity of graphite. Hence, the molecules created have a lower chance of escaping from the graphite electrode, and consequently, progressively dissolve into graphite material. Theoretical calculations explain that H2 with a size of 2.5 Å is able to diffuse in the interlayer space of graphite quicker than atomic hydrogen [14–20]. The activation energy required for the diffusion of atomic hydrogen in the interlayer space between graphene sheets in the hexagonal structure of graphite can be calculated to be extremely high (5 eV). This is because hydrogen atoms are able to bind to carbon atoms. Consequently, the probability of hydrogen intercalation in graphite is low [15]. It is known that no direct bonding can be formed between the molecular hydrogen and graphite. Moreover, the diffusion coefficient of molecular hydrogen in graphite at room temperature does not follow a single straight Arrhenius line, since the diffusion of molecular hydrogen proceeds via jumps between the nearest-neighbor adsorption sites in a random walk of the H2 molecule in the graphite interlayer space. However, at temperatures higher than around 200 °C, the hydrogen molecules jump mostly one-directional and about twice as much longer. This behavior enhances the effective diffusion length of hydrogen molecules in graphite. The diffusion coefficient of H2 in the interlayer space of graphite (DH2 ) at 800 °C (3.5 × 10−4 cm2 s−1 ) is much greater than that at room temperature (6.9 × 10−6 cm2 s−1 ). Furthermore, the activation energy required for the diffusion of H2 in graphite at 800 °C is relatively small at 0.19 eV [14, 15]. The thermal behavior of hydrogen in graphite lattice has been the subject of few studies [21]. Molecular dynamics modeling of the hydrogen movement into the graphite structure has delivered constructive information that aids in explaining the exfoliation of graphite under the effect of hydrogen intercalation. It was presented that the kinetic energy of H2 intercalated into the graphite interlayer space increases from about 23 kJ mol−1 to 33 kJ mol−1 as the temperature increases from 25 °C to 800 °C [15]. Since the molten salt process takes place at high temperatures, either the diffusion rate and the kinetic energy of H2 molecules intercalated into the graphite cathode material are substantially high which advance the exfoliation process. In highly oriented graphite, the binding energy between the individual graphene layers can be estimated to be around 19 J m−2 . Therefore, the energy needed to isolate one graphene layer from its graphite mother can be found to be in the range of 0.26– 0.32 J m−2 [21, 22]. The values of the exfoliation energy, the density and the interlayer space of typical graphite materials used in the molten salt exfoliation process can be assumed to be 0.31 J m−2 [23], 2.7 g cm−3 and 0.34 nm, respectively. Based on this information, the presence of H2 in graphite at local concentrations of greater than 2 wt% can provide adequate kinetic energy to overcome the van der Waals adhesion force which occurs between the individual layers of the graphite, leading to the exfoliation of the material into graphene nanosheets. One important thing to mention is that the solubility of hydrogen molecules in graphite can theoretically be as high as 6 wt% [24]. The cathodic polarization of the graphite electrodes in the presence of protons in the molten salt can thus offer the likelihood of surpassing this critical concentration, thus bringing about the exfoliation of graphite. Moreover, it should be noted that in the presence of a humid Ar atmosphere, the as-synthesized graphene

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5 Mechanisms Involved in the Electrolytic Fabrication …

material contained Li2 CO3 formed by the occurrence of the reaction between the anodically formed O2 (reaction 5.5b), with the anode itself and the Li2 O dissolved in the melt (reaction 5.3): O2 + C + Li2 O = Li2 CO3

(5.9)

The purification of the graphene product from the Li2 CO3 component can be achieved by heating the material at temperatures greater than the evaporation temperature of lithium carbonate.

5.4 Electrochemical Erosion of Graphite Under Hydrogen-Containing Atmospheres The likelihood of the formation of H+ in the molten LiCl resulting from the hydrolysis of melt in moist atmospheres was discussed in Sect. 5.3. This procedure is green since the only raw materials used to produce graphene nanosheets are graphite, water and electricity. Nevertheless, the disintegration of water in the melt additionally generates oxygen species in the melt which leads to the formation of Li2 CO3 mixed with graphene nanosheets. Therefore, an extra thermal treatment is needed to remove the lithium carbonate by-product from the graphene nanosheets. What’s more, it was found that graphite is able to be exfoliated into single or few-layer 3D graphene, by cathodic polarization of the material in LiCl melt in Ar atmosphere containing H2 (Figs. 4.1h and i, and 4.2 (III)). The proposed mechanism for this direct formation of graphene in molten salt is summarized in Fig. 5.5. Under a hydrogen-containing atmosphere, in the absence of protons in the melt, the electrolysis process begins with the decomposition of LiCl under the influence of the potential difference applied. It consequently leads to the evolution of Cl2 on the anode. The chlorine released can react with the H2 present in the atmosphere above the melt to form hydrogen chloride, which then dissolves in the molten salt to form H+ . The latter is the responsible for the exfoliation process, as explained in the Sect. 5.3 [25, 26]. Ideally, this cycle can repeat itself indefinitely until the complete exfoliation of the graphite cathode. Figure 5.4 illustrates the mechanism proposed for the formation of graphene in LiCl melt under H2 containing Ar atmosphere. It should be mentioned that the molten salt technology developed may provide an effective strategy for the green and economic fabrication of high-quality graphene, with an interesting combination of properties including high electronic conductivity, as high as 5.8 × 105 S m−1 , high surface area of up to 500 m2 g−1 and high oxidation resistance of up to 550 °C. The consumables of the process are mainly electrical energy and water/hydrogen, and no harmful by-product is produced, hence the process is environmentally safe. The cost and the specific energy consumption for the preparation of graphene in molten LiCl can be estimated to be about US $10–20 and 25 kWh per kilogram of the graphene product, respectively [14]. These

5.4 Electrochemical Erosion of Graphite Under Hydrogen-Containing Atmospheres

71

Fig. 5.4 Illustration of the mechanism proposed for the electrochemical exfoliation of graphite into graphene in LiCl melt. The intercalation of hydrogen atoms in the graphite lattice and their subsequent combination leads to the formation of hydrogen molecules in the interlayer space of graphite, causing its exfoliation, reproduced from Ref. [25], copyright 2019, with permission from RSC Publishing

characteristics can make the graphene product attractive for many applications. It will be discussed in Chap. 6.

5.5 Molten Salt Formation of Metal-Filled Carbon Nanostructures As discussed in Chaps. 4 and 5, the electrolysis of LiCl as the electrolyte with graphite cathodes under dry Ar can produce carbon nanostructures including CNTs. While no nanostructured carbon can be produced by the electrolysis of pure SnCl2 , the electrolysis of LiCl–SnCl2 mixture can produce Sn-filled carbon nanostructures. These experimental observations are explainable by electrochemical measurements. Figure 5.5a shows the cyclic voltammogram obtained using a molybdenum working electrode in the molten LiCl-4.5 wt%SnCl2 at 625 °C conducted under a dry Ar gas flow [6]. The anodic and cathodic events extracted from this voltammogram provide important information about the mechanisms involved in the formation of core-shell metal filled nanostructures, such as shown in Figs. 4.10 and 4.13. Accordingly, the cathodic peak A observed in Fig. 5.5a can be assigned to the deposition of metallic Sn on the molybdenum working electrode. The cathodic peaks B, C and D can be related to the formation of Li–Sn intermetallics such as Li5 Sn2 (Li13 Sn5 ), Li7 Sn2 and Li22 Sn5 . The strong cathodic current E is due to the deposition of metallic lithium. The peak E1 in the positive anodic scan is related to the dissolution of

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5 Mechanisms Involved in the Electrolytic Fabrication …

Fig. 5.5 Cyclic voltammograms recorded in molten LiCl–4.5 wt%SnCl2 at a potential scan rate of 50 mV s−1 at 625 °C using (a) molybdenum and (b) graphite working electrodes. A molybdenum wire was employed as a quasi-reference electrode, reproduced from Ref. [6], copyright 2019, with permission from Elsevier

lithium. Similarly, D1 , C1 and A1 are associated with the stripping of Li–Sn intermetallics and the dissolution of tin, respectively. The obvious difference in the size and shape of A and A1 suggests that the process of tin deposition and dissolution is rather complex, involving a chemical interaction with the molybdenum surface. Figure 5.5b shows the cyclic voltammogram obtained using a graphite working electrode in the molten LiCl-4.5 wt%SnCl2 at 625 °C conducted under a dry Ar gas flow. It can be seen that the position of A and A1 , associated with the deposition and dissolution of Sn, is almost identical with both Mo and graphite, but the peaks are more symmetric to each other with graphite electrode, demonstrating the facilitated nucleation of tin on the graphite surface and the absence of chemical reactions between tin and graphite. Features B and B1 in Fig. 5.5b can be attributed to the Li intercalation and de-intercalation, as it can also be seen in Fig. 5.1. In this case, the current peaks associated with the formation and dissolution of the various Li–Sn intermetallics are not visible, but are overshadowed by the currents due to the reaction between lithium and graphite. The waves C and C1 are attributed to the Sn2+ /Sn4+ redox couple. Finally, the anodic current increase D corresponds to chlorine evolution. Such results together with other evidences suggest that the metallic tin deposits onto cathodically polarized graphite from the mixture of LiCl–SnCl2 , and there is no evidence of tin intercalation into graphite. In contrast, lithium can intercalate into graphite. Accordingly, the reduction of tin at graphite electrodes does not lead to the formation of significant amounts of nanosized carbon materials, while the reduction of lithium does [6]. Since the electronegativity of tin is much smaller than that of lithium, the deposition of Sn on the graphite occurs before the Li intercalation event. The dissociation of SnCl2 yields molten tin, which partly coats the carbon cathode. The subsequent exfoliation of the graphite can cause the encapsulation of the deposited molten Sn to form core–shell nanostructures [6, 27].

References

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References 1. A.R. Kamali, D.J. Fray, Towards large scale preparation of carbon nanostructures in molten LiCl. Carbon 77, 835–845 (2014) 2. G.Z. Chen, X.D. Fan, A. Luget, M.S.P. Shaffer, D.J. Fray, A.H. Windle, Electrolytic conversion of graphite to carbon nanotubes in fused salts. J. Electroanal. Chem. 446, 1–6 (1998) 3. G.Z. Chen, I. Kinloch, M.S.P. Shaffer, D.J. Fray, A.H. Windle, Electrochemical investigation of the formation of carbon nanotubes in molten salts. High Temp. Mater. Process 2, 459–469 (1998) 4. I.A. Kinloch, G.Z. Chen, J. Howes, C. Boothroyd, C. Singh, D.J. Fray, A.H. Windle, Electrolytic, TEM and Raman studies on the production of carbon nanotubes in molten NaCl. Carbon 41, 1127–1141 (2003) 5. H. Huang, Y. Xia, X. Tao, J. Du, J. Fang, Y. Gan, W. Zhang, Highly efficient electrolytic exfoliation of graphite into graphene sheets based on Li ions intercalation–expansion–microexplosion mechanism. J. Mater. Chem. 22, 10452–10456 (2012) 6. Q. Xu, C. Schwandt, D.J. Fray, Electrochemical investigation of lithium intercalation into graphite from molten lithium chloride. J. Electroanal. 562, 15–21 (2004) 7. C. Monnin, M. Dubois, N. Papaiconomou, J.P. Simonin, Thermodynamics of the LiCl–H2 O system. J. Chem. Eng. Data 47, 1331–1336 (2002) 8. A.R. Kamali, D.J. Fray, C. Schwandt, Thermokinetic characteristics of lithium chloride. J. Therm. Anal. Calorim. 104, 619–626 (2011) 9. J.P. Masset, Thermogravimetric study of the dehydration reaction of LiCl–H2 O. J. Therm. Anal. Calorim. 96, 439–441 (2009) 10. V.A. Kovrov, R. Mullabaev, VYu. Shishkin, YuP Zaikov, Solubility of Li2 O in an LiCl–KCl Melt. Russian Metallurgy (Metally) 2, 169–173 (2018) 11. W.J. Burkhard, J.D. Corbett, The solubility of water in molten mixtures of LiCl and KCl. J. Am. Chem. Soc. 79(24), 6361–6363 (1957) 12. N.Q. Minh, B.J. Welch, The reduction of HCl dissolved in LiCl–KCl eutectic. Aust. J. Chem. 28, 965–973 (1975) 13. Y. Sakamura, Solubility of Li2 O in molten LiCl–MClx (M = Na, K, Cs, Ca, Sr, or Ba) binary systems. J. Electrochem. Soc. 157, E135–E139 (2010) 14. A.R. Kamali, D.J. Fray, Large-scale preparation of graphene by high temperature diffusion of hydrogen in graphite. Nanoscale 7, 11310–11320 (2015) 15. C.P. Herrero, R. Ramirez, Diffusion of hydrogen in graphite: a molecular dynamics simulation. J. Phys. D Appl. Phys. 43, 255402 (2010) 16. A. Shimizu, H. Tachikawa, Thermal behavior of hydrogen atom intercalated between two layers of C150 H30 graphite plane: MD simulationJ. Phys. Chem. Solids 64, 419–423 (2003) 17. Y. Ferro, F. Marinelli, A. Allouche, Density functional theory investigation of the diffusion and recombination of H on a graphite surface. Chem. Phys. Lett. 368, 609 (2003) 18. M. Warrier, R. Schneider, E. Salonen, K. Nordlund, Multi–scale modeling of hydrogen isotope diffusion in graphite. Contrib. Plasma Phys. 44, 307–310 (2004) 19. R.A. Causey, The interaction of tritium with graphite and its impact on tokamak operations. J. Nucl. Mater. 162, 151–161 (1989) 20. W.A. Dino, Y. Miura, H. Nakanishi, H. Kasai, T. Sugimoto, Stable hydrogen configurations between graphite layers. J. Phys. Soc. Jpn. 72, 1867 (2003) 21. L.A. Girifalco, R.A. Lad, Energy of cohesion, compressibility, and the potential energy functions of the graphite system. J. Chem. Phys. 25, 693 (1956) 22. R. Zacharia, H. Ulbricht, T. Hertel, Interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons. Phys. Rev. B: Condens. Matter 69, 155406 (2004) 23. T. Gould, S. Lebègue, J. Dobson, Dispersion corrections in graphenic systems: A simple and effective model of binding. J. Phys. Condens. Matter. 25, 445010 (2013) 24. R. Strobel, J. Garche, P.T. Moseley, L. Jorissen, G. Wolf, Hydrogen storage by carbon materials. J. Power Sources 159, 781–801 (2006)

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25. A.R. Kamali, Eco-friendly production of high quality low cost graphene and its application in lithium ion batteries. Green Chem. 18, 1952–1964 (2016) 26. A.R. Kamali, Scalable fabrication of highly conductive 3D graphene by electrochemical exfoliation of graphite in molten NaCl under Ar/H2 atmosphere. J. Ind. Eng. Chem. 52, 18–27 (2017) 27. M. Terrones, W.K. Hsu, A. Schilder, H. Terrones, N. Grobert, J.P. Hare et al., Novel nanotubes and encapsulated nanowires. Appl. Phys. A 66, 307–317 (1998)

Chapter 6

Applications of Carbon Nanostructures Produced in Molten Salts

Abstract The cathodic exfoliation of graphite in molten salts can be considered as a low-cost and efficient method for the scalable production of carbon nanostructures with various applications including anode materials for Li ion batteries, supercapacitors, ceramic-based composites and adsorbents for removal of organic pollutants. Various nanostructured carbon materials, such as molten salt-produced graphene nanosheets decorated with SnO2 nanocrystals, Sn-filled carbon nanostructures and graphene-wrapped Si nanoparticles can be fabricated for the application as active materials for lithium-ion batteries. As another application, interconnected graphene nanostructures comprising of nanosheets and nanoscrolls were found to exhibit an excellent performance in electrochemical supercapacitor studies. Moreover, slip cast alumina ceramics containing a low amount of molten salt graphene demonstrated higher values of mechanical properties in comparison with that of bare alumina. As another example, 3D graphene nanosheets produced in molten salts exhibit a high dye adsorption performance in a wide range of the solution pH from 2 to 11. The current chapter reviews these applications. Keywords Molten salts · Graphene · Supercapacitors · Li-ion batteries · Anode · Ceramic composites · Water purification

6.1 SnO2 -Graphene Anode Materials for Li-Ion Batteries As discussed in the last chapters, graphene can be efficiently produced in molten LiCl. The production rate was estimated to be more than 400 g graphene in one liter of the molten LiCl used per day. A volume of only 10 L molten LiCl can, therefore, produce more than 4 kg graphene in one day. The graphene produced exhibited an ultrahigh conductivity with a value of 5.8 × 105 S m−1 , a high surface area of up to 500 m2 g−1 and a thermal stability of up to 550 °C, as well as a high carbon purity of more than 98% [1–3]. These properties are attractive for many applications, including anode materials for lithium-ion batteries [3, 4], (b) supercapacitors [5], structural composites [6] and water purification. Molten salt graphene nanosheets decorated with SnO2 nanocrystals as anode active materials for Li-ion batteries are first discussed. © Springer Nature Singapore Pte Ltd. 2020 A. R. Kamali, Green Production of Carbon Nanomaterials in Molten Salts and Applications, https://doi.org/10.1007/978-981-15-2373-1_6

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Graphite is currently employed as the anode material in commercial Li-ion batteries for various applications. However, the theoretical capacity of graphite, based on the intercalation of Li into its lattice structure, is limited to a maximum theoretical value of 372 mAh g−1 , due to the formation of LiC6 : 6C + Li+ + e ↔ LiC6

(6.1)

Therefore, graphite has to be replaced by alternative anode materials with higher capacities, particularly for automotive applications. The new material used as the anode should be comparable with graphite in terms of the processing costs and should also be capable of being produced by sustainable methods. A number of materials with a Li storage capacity higher than graphite have been investigated as possible anode materials. Among them, SnO2 is one of the most promising candidates due to its high theoretical specific capacity of 789 mA h g−1 and low cost. It, however, suffers from large volume changes as much as 300% associated with full lithium insertion and extraction processes leading to the loss of electrical contact and therefore failure of the electrode. The other limitation of SnO2 in this application associates with its poor electronic conductivity which negatively affects the electrochemical performance of the electrode. An effective strategy to tackle these restrictions is the incorporation of graphene with SnO2 nanoparticles [7–11]. Exhibited in Fig. 6.1, a SnO2 –graphene composite material was prepared by hightemperature oxidation of SnCl2 [12–15] on graphene nanosheets produced in molten LiCl, for use as anode material in Li-ion batteries. The green and simple strategy used to prepare this SnO2 -loaded graphene nanosheets was based on heating of a mixture of graphene and SnCl2 to a temperature of 580 °C. Upon the evaporation of SnCl2, the following reaction occurs: ◦

◦

SnCl2 (g) + O2 (g) = SnO2 (s) + Cl2 (g) G = −182 KJ (at 580 C)

(6.2)

Reaction 6.2 leads to the deposition of SnO2 on the graphene nanosheets, such as shown in Figs. 6.1a and b. In this composite material, highly crystalline SnO2 nanocrystals of 5–20 nm in size are anchored on graphene nanosheets with a perfect connection as can be depicted from Fig. 6.1c. The composite material, fabricated by combining the molten salt-produced graphene with SnO2 nanocrystals, exhibited a reversible capacity of about 1000 mAh g−1 after 100 cycles at a current density of 1C in the potential range of 0.003–3 V versus Li+ /Li. This capacity is about three times more than the capacity of graphite [3]. It should be mentioned that the electrochemical performance of SnO2 nanocrystals is poor, reaching less than 200 mAh g−1 after 100 cycles [16]. This is mainly due to the poor conductivity and also the large volume changes of SnO2 during charge–discharge processes, leading to the fatigue failure and disintegration of the electrode. The high performance of the SnO2 –molten salt graphene composite is attributed to the presence of graphene nanosheets which provide excellent electronic contacts between individual SnO2 particles and clusters, overcoming the loss of the mechanical and electronic integrity of the active material over charge–discharge cycling.

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Fig. 6.1 TEM morphology and the electrochemical performance of the SnO2 anchored graphene, produced using the graphene material fabricated in molten salt. An unloaded edge of graphene is indicated in (a). The distribution of SnO2 nanoclusters on graphene nanosheets in a less loaded section of the sample can be seen in (b), in which the SnO2 clusters are still in electronic contact with each other through the graphene sheets. c A high-resolution TEM micrograph exhibiting the presence of a SnO2 nanocrystal on a graphene sheet. d Lithium charge–discharge performance of the anode material produced using the nanocomposite material in comparison with graphite, reproduced from Ref. [3], copyright 2019, with permission from RSC Publishing

High-capacity anode materials can also be fabricated based on metallic Sn and Si since these metals can be alloyed with Li up to 4.4 Li atoms per Sn or Si atom leading to the formation of Li22 Sn5 or Li22 Si5 intermetallic phases or amorphous phases of equivalent chemical compositions. Therefore, Sn and Si can provide a theoretical capacity as high as 994 and 4200 mAh g−1 , respectively, considerably greater than that of graphite. However, Sn or Si anodes cannot currently be used in commercial batteries, since these metals undergo dramatic volumetric changes up to more than 300% upon cycling, which subsequently leads to the pulverization of the metallic

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particles as well as the electrical disconnection between the particles and the current collector, and thus poor cyclability [17, 18].

6.2 Metal–Carbon Nanocomposites as Anode Materials for Li-Ion Batteries The replacement of graphite by a metallic anode can offer benefits such as a higher specific capacity, at least during the initial cycles, due to more lithium ions that can contribute to the reaction scheme (6.3) compared to that of (6.1): aMe + bLi+ + be ↔ Mea Lib

(6.3)

where Me is a metal such as Al, Sn and Al or a semi-metal such as Si and Ge. Among them, Si and Sn have a distinguished position due to the technical and/or economic advantages [17, 18]. Table 6.1 compares the theoretical capacity of selected materials with that of graphite. Despite the much greater theoretical capacity achievable, the major challenge associated with the development of metallic/semi-metallic anodes is the high volume changes involved in the reaction scheme (6.3), as indicated in Table 6.1. In the last two decades, there has been a significant deal of research worldwide, aiming to improve of the cyclability of metallic/semi-metallic anodes, including Sn and Si. The effect of volume changes on the cycling performance can be reduced by decreasing the particle sizes of Si [17] and Sn [18]. However, the decreasing of the particle sizes alone cannot completely eliminate the capacity degradation, particularly after few tens of cycles. An effective way to mitigate the failure of the metallic anodes is to incorporate (or encapsulate) Sn or Si into carbon nanostructures which can effectively buffer the volume changes of the active material and also promotes the electrical conductivity of the anode. An attractive feature of molten salt-based methods is their capability for the fabrication of carbon nanocomposites, in which a metallic second phase is encapsulated by graphitic carbon layers [4, 19–22]. Table 6.1 Lithiation–delithiation characteristics of different anode materials [18] Metal/semi-metal

Li

Si

Al

Ge

Sn

Al

Graphite

Lithiated compound

Li

Li22 Si5

Al4 Li9

Li22 Ge5

Li22 Sn5

AlLi

LiC6

Theoretical capacity (mAh g−1 )

3800

4200

2234

1600

994

993

372

Volume change (%)

Dendritic growth

323

–

370

300

97

9

6.2 Metal–Carbon Nanocomposites as Anode Materials …

79

6.2.1 Sn-Filled Carbon Nanostructures Sn-filled carbon nanostructures were produced by the electrolysis of LiCl + SnCl2 at a temperature greater than 700 °C using graphite electrodes. The reactions occurred on the graphite cathodes during the electrolysis process can be described as follows: (a) Sn2+ ions are first reduced to metallic Sn on the graphite cathode’s surface; (b) Li+ is then reduced at the graphite cathode to form a carbon intercalation compound or carbon–Li intermediate phases; and (c) sheets of graphite are extruded from the cathode which consequently encapsulate Sn to form Sn-encapsulated carbon nanostructures [21]. On the other hand, the continuous injection of SnCl2 into molten LiCl during the electrolysis process was found to be able to provide a continuous way for the preparation of Sn-filled or SnO2 -decorated carbon nanostructures, depending on the atmosphere of the electrolysis process (Fig. 6.2) [23]. The Sn-filled carbon product has been examined as anode material for Li-ion batteries, and a promising stable electrochemical performance of about 450 mAh g−1 after more than 100 cycles has been reported [18].

Fig. 6.2 Sn-core carbon sheath nanostructures or SnO2 -decorated CNTs can be scalably fabricated by continuous adding of SnCl2 into molten LiCl during the electrolysis process using graphite electrodes, reproduced from Ref. [23], copyright 2019, with permission from Elsevier

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6.2.2 Graphene-Wrapped Si Nanostructures It has been demonstrated that graphene nanosheets produced by the cathodic exfoliation of graphite in molten LiCl in an atmosphere of Ar + 4% H2 can wrap silicon nanoparticles injected into the melt in order to reduce their surface energy. This characteristic of the molten salt-produced graphene nanosheets can lead to the fabrication of silicon nanoparticles encapsulated in graphene layers. Figure 6.3 shows a process by which Si nanoparticles encapsulated in graphene nanosheets can be produced. Figure 6.3b shows photographs of the graphite reactant and the silicon starting material as well as the remaining part of the consumed graphite extracted from the molten salt, and the graphene–Si nanocomposite produced by mixing the graphene product and Si. Surprisingly, the volume of the graphene–silicon nanocomposite was considerably less than both the graphene and silicon initial materials. This volume shrinkage is related to the incorporation of Si nanoparticles in the 3D graphene nanosheets. Figure 6.4 exhibits TEM micrographs of the Si raw material and the nanocomposite produced by the molten salt method. It is evident that graphene nanosheets can wrap Si nanoparticles injected into the melt, and could also connect the individual Si nanoparticles. The formation of this architecture can be explained as follows: Si and graphene may be oppositely charged in the molten salt. The stir mixing of these oppositely charged components can lead to the partial wrapping of silicon nanoparticles by highly flexible graphene nanosheets. Further graphene wrapping of Si is difficult under the repulsive influence of identically charged graphene sheets. Therefore, Si nanoparticles can be partially covered with single or few-layer graphene as depicted from Fig. 6.4. This “tight” integration of graphene and silicon nanopowders can describe the volume shrinkage observed. The electrochemical performance of the electrodes made out of the nanocomposites fabricated in molten salts was also evaluated in the potential range 0.01–2.5 V using a 2032-type coin cell with lithium foil as the counter electrode and LiPF6 in EC/DMC/FEC (3: 5: 2 v/v) as electrolyte. For a Si–graphene nanocomposite containing 50wt% Si, the first cycle showed the discharge and charge capacities of 1764 mAh g−1 and 1355 mAh g−1 , respectively. The Coulombic efficiency at first cycle was 76.8% when tested at a constant current density of 0.5 A g−1 . The capacity loss was likely to be due to the irreversible reaction of lithium with electrode materials leading to the formation of a solid electrolyte interface (SEI) layer on the electrode surface, and also to the consumption of lithium ions trapped in nanoporosity of the electrode. Figure 6.4c shows the lithiation–delithiation specific capacity and the corresponding coulombic efficiency of the electrode containing 50 wt% Si at 0.5 A g−1 during 260 cycles. The results obtained show a highly stable performance with an impressive capacity of 981 mAh g−1 after 260 charge–discharge cycles, which is about 300% greater than that of commercially used graphite anode material. The coulombic efficiency significantly increased from 76.8% (at the first cycle) to 99.5% during further cycling; reached ~100% by the 10th cycle. It should be noticed that the active material contained only 50 wt% Si. The performance of the electrode may therefore be tailored by altering the proportional quantity of Si in the nanocomposite. The results are more significant when it

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Fig. 6.3 A green molten salt process for the large-scale fabrication of graphene-wrapped silicon nanocomposites. a A graphite electrode is negatively polarized in molten salt against another graphite electrode served as the anode under an atmosphere of Ar–4% H2 . The graphite cathode surface becomes exfoliated into graphene nanosheets. Si nanoparticles are then gradually injected into the molten salt, leading to the formation of a unique nanocomposite in which Si nanoparticles embedded on layers of 3D graphene are partially wrapped by single or few layers of graphene. b Photographs of graphite and Si starting materials as well as the eroded graphite extracted from the molten salt, and the graphene–Si product stored in a jar, reproduced from Ref. [4], copyright 2019, with permission from RSC Publishing

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(a)

(b)

(c)

Fig. 6.4 a Graphene nanosheets synthesized in molten LiCl can wrap Si nanoparticles injected into the melt. b Characteristics of the graphene encapsulated silicon nanoparticles containing 91 wt% Si. The high-resolution TEM micrograph and two FFT patterns recorded on the silicon particle and graphene layers shown in the micrograph, demonstrating the presence of lattice fringes with interplanar spacing of 0.31 and 0.35 nm, corresponding to the cubic Si structure (111) and hexagonal carbon structure (002) planes, respectively. The graphene material could also connect the individual Si nanoparticles. c Lithium charge–discharge performance of the anode material produced using the nanocomposite material with 50 and 91 wt% Si, reproduced from Ref. [4], copyright 2019, with permission from RSC Publishing

6.2 Metal–Carbon Nanocomposites as Anode Materials …

83

is considered that the nanocomposite can be produced in large scales. By increasing the amount of Si content of the nanocomposite to 91 wt%, the reversible stable capacity increased to 2217 mA h g−1 , demonstrating the capability of the molten salt method to correlate the cost and electrochemical performance of the graphene–silicon nanocomposite product (Fig. 6.4c). This finding is interesting since it proposes a sustainable approach for the large-scale preparation of high-performance anode materials using commercially available graphite and Si feed materials [4].

6.3 Supercapacitors As exhibited in Fig. 6.5a, the molten salt process was adapted to produce an interconnected graphene nanostructure comprising of nanosheets and nanoscrolls. As shown in Fig. 6.5b, the morphology of the nanostructured carbon material produced could be characterized by the presence of graphene nanosheets interconnected by carbon nanotubes, enhancing the overall conductivity and integrity of the carbon material. The electrochemical properties of the nanostructured material were investigated using a CR2032 coin cell at room temperature. The graphene electrodes were prepared using a slurry composed of 90 wt% of the prepared carbon active material and 10 wt% of polyvinylidene fluoride (PVDF; Aldrich) as the binder dissolved in N-methylpyrrolidone (NMP). Notably, no additional conducting agent was used in the electrode because the interconnected carbon nanostructure with a high surface area had sufficiently high conductivity to act as both the conductor and the active material. The slurry was uniformly cast on an etched aluminum foil using a doctor blade and then dried in a vacuum oven at 100 °C for 24 h. The 2032 coin cell, a symmetrical two-electrode unit cell, was assembled using graphene electrodes with an area of 1.13 cm2 and a microporous polyethylene film (Celgard 2400) separator in an Ar-filled glove box. The electrolyte was 1 M TEABF4 dissolved in ACN. Electrodes fabricated exhibited a reversible specific capacitance with the value of 213 F g−1 at 1 A g−1 , and excellent capacitance retention of 84.5% of the initial specific capacitance at 50 A g−1 , as well as a reasonable cyclability of about 98% after 10,000 cycles (Fig. 6.5c). In fact, the high conductivity and the reasonably high surface area of the carbon product were responsible for its performance. These properties are also highly desirable for ultracapacitor applications [5].

6.4 Ceramic-Based Composites As an important industrially used material, aluminum oxide (Al2 O3 ) serves as the raw material for a broad range of advanced ceramic products due to the combination of desirable properties including its high melting point, hardness, strength, stiffness, wear resistance, chemical inertness and biocompatibility. Therefore, alumina is one of the prime choices for a large variety of applications, including in high-temperature

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Fig. 6.5 a Setup adopted for the preparation of the interconnected carbon nanostructured material. A graphite rod is cathodically polarized in molten LiCl in a dry Ar flow, leading to the intercalation of lithium from the molten salt into the graphite structure. After 30 min of lithium intercalation, a moist Ar stream was purged into the reactor leading to the formation of H+ cations in the molten salt, which were subsequently reduced on the graphite cathode and intercalated into the graphite. Combination of the hydrogen atoms in the graphite interlayer led to the formation of H2 gas, thereby to the exfoliation of graphite. The exfoliated graphene sheets could then partially roll under the influence of intercalated lithium to form nanoscrolls. b The SEM micrograph of the resultant product demonstrating the presence of an interconnected graphene nanostructure comprising of nanosheets and nanoscrolls. c Cyclability of the electrode prepared using this carbon material in supercapacitor application, reproduced from Ref. [5], copyright 2019, with permission from RSC Publishing

electrical insulators, sealing rings, sheaths for high-voltage lamps, refractory materials, milling media and abrasives [24, 25]. Moreover, due to its biocompatibility, alumina-based ceramics have been used in the reconstruction of dental crowns [26], jaws [27] and hip joints [28]. Although more than 3.5 million alumina implants have been implemented worldwide, since 1990 [28] there have also been reports on the fracture of alumina implants, owing to the relatively low intrinsic fracture toughness of alumina which is in the range of 3–5 MPa m1/2 [29, 30]. Yttria-stabilized zirconia (YSZ) is considered as an alternative to alumina for structural applications due to its greater fracture toughness brought about by a phase transformation toughening mechanism, enhancing its resistance to crack propagation [28]. Unfortunately, YSZ ceramics suffer from limitations such as the low-temperature degradation as well as

6.4 Ceramic-Based Composites

85

the high cost limiting their practical applications [28, 31]. As a result, the toughening of alumina-based ceramics has been the subject of a large number of investigations, most of which are mainly based on the incorporation of toughening agents. In this regard, various agents and fabrication methods have been employed, leading to an increase in the value of the fracture toughness for alumina composites. For instance, the fracture toughness values of 5.4, 4.8 and 5.0 MPa m1/2 can be achieved for alumina composites containing 5 wt% glass phase [32] 13 vol.% Zr + Ag [33] and 13 vol.% Ni [34], respectively, using cold isostatic pressing and pressureless sintering. Hot pressing can lead to the fabrication of alumina composites containing 20 wt% YSZ [35] and 20 vol.% Fe [36], with fracture toughness values of 6.8 MPa m1/2 and 6.6–10.2 MPa m1/2 , respectively. Recently, graphene has been investigated as a toughening agent in alumina ceramics. The alumina–graphene ceramics were fabricated by means of spark plasma sintering and exhibited fracture toughness values in the range of 3.5–5.2 MPa m1/2 [37–39]. The graphene materials employed in these studies were prepared by various techniques comprising Hummers-based methods followed by the hydrazine reduction [37], thermal exfoliation of intercalated graphite [38] and the liquid-phase exfoliation of graphite [39]. Apart from the limitations related to the fabrication of the graphene materials employed in these studies, the fabrication methods used for the preparation of the ceramic materials, i.e., hot pressing and spark plasma sintering, are costly and involve the use of expensive equipment. Hence, more economic methods for the preparation of both graphene and alumina–graphene ceramics are needed in order to explore the future potential of graphene in the alumina ceramic industry. It is also worth mentioning that graphene materials produced by different techniques exhibit different characteristics. The effect of molten salt-produced graphene nanosheets on the morphology and the mechanical properties of alumina ceramics fabricated by means of slip casting and pressureless sintering was investigated [6]. Slip casting process was adopted to fabricate alumina and alumina–graphene green bodies. To this end, graphene nanosheets produced in molten LiCl were added to deionized water and dispersed using ultrasonication for 30 min. Then, a sufficient amount of alumina powder was added to make a slip of the desired amount of carbon. The mixture was then ultrasonicated for 30 min, and the resulting slurry was ball milled for 12 h. Then, small amounts of deionized water and dispersant were added, in order to decrease the slurry’s viscosity, enhancing its castability, followed by 30 min ball milling. The slurry made was casted into plaster molds to form rectangular specimens upon loss of the water. Figure 6.6a shows photographs of the alumina and alumina–graphene green bodies produced by the slip casting process. The darker appearance of the graphene containing alumina is clear in this figure. The casts were sintered in a resistant furnace at 1650 °C under a protective Ar atmosphere. Figure 6.6b exhibits the SEM micrographs of the sintered alumina, with no graphene addition. This sample can be characterized by the presence of coarse alumina grains with a size of 10–20 μm. The fracture toughness of this sample was measured to be 4.50 MPa m1/2 , in agreement with the fracture toughness values measured on pressureless sintered alumina ceramics [30]. By adding only 0.5wt% molten salt-produced graphene nanosheets to

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Fig. 6.6 a Photographs of alumina and alumina–graphene green bodies fabricated by the slip casting process. The SEM micrographs of sintered samples fabricated using b alumina, and c alumina +0.5 wt% graphene. The microstructure obtained is characterized by the presence of micrometer-sized alumina particles separated by alumina nanorods, reproduced from Ref. [6], copyright 2019, with permission from RSC Publishing

alumina, a nanostructured material could be produced after the sintering process. In this nanostructured material, micrometer-sized alumina particles were separated by networks of alumina nanorods. Figure 6.6c shows the SEM morphology of this sample at two different magnifications. The bulk density of this material was measured to be 3.58 g cm−3 , which is lower than that of alumina sample (3.79 g cm−3 ). This is attributed to the presence of porosity in the graphene containing sample. Porous alumina ceramics are interesting materials to be used for biomedical applications such as orthopedics implants, since the porous microstructure can promote the tissue integration of the implant. On the other hand, adding only 0.5wt% graphene could cause a sharp increase in the KIC value from about 4.50 MPa m1/2 in alumina to 6.98 MPa m1/2 in the alumina–graphene material [6]. The formation of alumina nanorods (Fig. 6.6c) is attributed to the solid-phase directional growth of α-Al2 O3 under the influence of oxygen vacancies as the driving force, occurred during the sintering. The mechanism involved in the formation of these oxygen vacancies was explained to be based on the high-temperature surface reduction of Al2 O3 to Al2.667 O4 (with lower oxygen content than that of Al2 O3 ) under the influence of graphene. The oxygen vacancies provide a chemical driving force for the surface diffusion and subsequent crystal growth of aluminum oxide into one-dimensional nanostructures. As a summary, the molten salt-produced graphene can be employed to enhance the mechanical properties of alumina ceramics manufactured by the conventional slip casting and pressureless sintering techniques. The resulting alumina articles exhibit a high porosity and mechanical properties. These properties are interesting for applications such as biomedical and dental implants, where the fracture toughness of the alumina article is critical.

6.5 Adsorption

87

6.5 Adsorption Graphene potentially possesses interesting tunable electrical [40], mechanical [41], thermal [42] and chemical [43] properties. As the result, graphene may have a combination of interesting properties such as large specific surface area [44], high conductivity and flexibility [45, 46] as well as unique optical [47, 48] and thermal [49, 50] properties. Due to these capabilities, graphene can potentially be applicable in many applications including electronics [51], energy conversion and storage devices [45, 52], medicine [53], composites [54, 55], and microfluidics [56] and also as adsorbent for removal of organic and inorganic poullants from the environment [57]. In applications such as supercapacitor or adsorbent, the interaction of graphene with its surrounding environment is of significant importance, and therefore, the graphene edge sites can be of particular interest. It is because the presence of dangling bonds makes the edge sites of graphene at least two times more reactive than the basal plane [58]. When the graphene material is exposed to the surrounding environment, the less stable dangling bonds located on the edge sites can be functionalized by oxygen-containing groups or other reactive species available in the environment [59]. This characteristic provides the graphene’s edge sites with interesting properties such as a high specific electrochemical capacitance [60, 61] and adsorption capacity [98]. As discussed in Chap. 4, 3D graphene nanosheets with a high density of edge sites can be produced in molten salts in an economic and efficient way. This graphene material can be considered as an efficient adsorbent for the removal of inorganic poullants or organic dyes. It is a demanding application, since a wide variety of organic dyes are globally used by many industries such textile, food and drink, pharmaceutical, cosmetic, leather, tannery, ink and paper sections for coloring purposes, generating large volumes of wastewater that contain synthetic dyestuffs. The increasing presence of bioactive dyes in water resources has created a global threat to the biodiversity of our planet [62, 63]. There are many different dyes, but among them, azo dyes are the most abundant which represent around 70% of the global dye production. These dyes contain one or more azo bonds (−N = N−) which are often linked to naphthalenic or benzene rings containing lateral—OH and/or −SO− 3 groups [63]. Large volumes of colored effluents, which often contain a high concentration of azo dyes, are daily discharged by various industries mentioned above into bodies of water around the world. This causes aesthetic problems and provides a barrier for the penetration of sunlight into the water. It also causes health issues for aquatic organisms due to the toxicity, carcinogenicity, mutagenicity of such dyes and also and their degradation by-products, like aromatic amines [64–68]. Moreover, azo dyes are highly resistance to biodegradation in the aquatic environment, and therefore, cannot be eliminated naturally or by physicochemical or biological treatments conducted in treatment plants. For the removal of dyes from industrial effluents, various techniques are employed including coagulation [65, 66], ion exchange [67, 68], chemical reduction [69, 70], membrane separation [71, 72], biological treatments [73, 74] and adsorption [75–79]. Among

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this, adsorption is distinctive, due to its unique combination of advantages such as simplicity, adaptability, cost-effectiveness, ease of design and insensitivity toward highly toxic substances. A large number of different adsorbents have been examined for the removal of dyestuffs, from which carbon materials, particularly activated carbon (AC), are considered as effective and low-cost adsorbents [80]. Other carbon nanostructures, such as carbon nanotubes (CNTs) [81], magnetic nanocarbons [82] and graphene oxide (GO) materials [83–85], have also shown attractive adsorbent performances, because of their favorable physical and chemical properties, such as high specific surface area and also the presence of chemically or physically active functional groups on their surfaces, which promote the adsorption of various pollutants on their surface [86–88]. During the adsorption process, the target pollutants are accumulated on the surface of the adsorbent, and therefore, the adsorption capacity of adsorbent is progressively reduced until it is saturated and finally exhausted. For the case of AC which is the most common adsorbent, the exhausted adsorbent loaded with hazardous substances is burnt or disposed in landfills, causing environmental and economic problems. Therefore, the capability of the adsorbent to be regenerated after being used in the absorption process, in an environmentally friendly way, is very important. The AC and GO adsorbents can be regenerated electrochemically [89, 90], biologically [91] and chemically [92]. However, these carbon materials are not commonly regenerated by thermal methods since their resistance against oxidation in air and/or their thermal stability is rather low; typically lower than 300 °C [93, 94]. In contrast to carbon materials, ceramic absorbents based on sodium silicates [95] and zinc ferrites [96] can effectively be thermally regenerated in air. The thermal regeneration of AC has only been studied under protective atmospheres [97], limiting the viability of the thermal process for regeneration of AC at larger scales. 3D graphene nanosheets produced in NaCl + LiCl molten salt were evaluated as the adsorbent for the removal of dyes from aqueous solutions. Furthermore, the thermal regeneration of the graphene nanosheets in air after being used in the process was studied. The molten salt-produced graphene nanosheets are curved and contain a high density of edge sites, as it can be observed from the SEM micrographs shown in Fig. 6.7a–c. This morphology provides not only porosity, but also structural stability by preventing the re-stacking of the exfoliated graphene. The specific surface area and the pore diameter of the molten salt-produced 3D graphene were calculated to be 186 m2 g−1 and 3.63 nm, respectively. A high fraction of single-, double- and few-layer graphene nanosheets together with graphitic nanolayers with a thickness of less than 10 nm could be detected in the graphene material, such as shown in the TEM micrograph of Fig. 6.7j. Furthermore, the termination of (002) planes of the graphene nanosheets can clearly be observed in this micrograph. As mentioned, the presence of dangling bonds located at the edge sides of the graphene nanosheets can enhance the adsorption performance of the material. The FFT patterns shown in Fig. 6.7 exhibit spots which can be attributed to the interlayer spacing in graphite (0.34 nm).

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89

Fig. 6.7 Electron micrographs of various carbon materials: a–c SEM images of the 3D graphene nanosheets produced by the exfoliation of graphite in molten LiCl–NaCl; d–f SEM images of 3D graphene saturated by MO; g–i SEM images of the 3D graphene after thermal regeneration. j A high-resolution TEM micrograph, exhibiting the nanostructure of the graphene material. Some graphene nanosheets with dangling bonds are marked in the micrograph. The fast Fourier transform (FFT) patterns recorded on the area shown in the TEM image are also shown. The interlayer spacing of 0.34 nm corresponds to the (002) planes in the hexagonal carbon structure, reproduced from Ref. [98], copyright 2019, with permission from Elsevier

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The adsorption capability of these 3D graphene nanosheets was evaluated and compared with that of natural graphite powder with a BET surface area of around 18 m2 g−1 . For this, the adsorbents were added into 500 ml solutions containing methyl orange (MO, 50 mg l−1 ) and shaken at 20 °C. The SEM morphology of the 3D graphene saturated with MO (Fig. 6.7d–f) demonstrates that the morphology of graphene nanosheets did not considerably change upon the dye adsorption. The morphological stability of the 3D graphene maintained even after the thermal regeneration of the 3D graphene material at 300 °C, as confirmed by the SEM micrographs shown in Fig. 6.7g–i. The stability of the 3D graphene nanosheets over the whole adsorption and thermal regeneration process is remarkable. The thermal stability of the molten salt-produced 3D graphene material can be realized from the DSC and TG thermograms recorded in an airflow rate of 100 ml min−1 , exhibited in Fig. 6.8. The TG thermogram shows that the material is stable until about 500 °C without an obvious weight loss. A major weight loss can be detected from the TG curve which is accompanied by an exothermic peak with the onset and peak temperatures at about 500 °C and 580 °C, respectively. This event is assigned to the oxidation of the graphene material. Figure 6.8 clearly confirms that the graphene material is thermally stable at temperatures below 500 °C in air. The possible structural evolution of the molten salt-produced graphene over the MO adsorption (50 mg l−1 ) and thermal regeneration at 300 °C can be realized from Raman spectra of Fig. 6.9. Three distinct bands can be identified in the Raman spectra of Fig. 6.9; D (~1360 cm−1 ), G (~1580 cm−1 ) and 2D (~2720 cm−1 ). The G-band is related to

Fig. 6.8 TG-DSC thermograms of the 3D graphene nanosheets, recorded at a heating rate of 10 °C min−1 under an air flow rate of 100 ml min−1 , reproduced from Ref. [98], copyright 2019, with permission from Elsevier

6.5 Adsorption

91

Fig. 6.9 Raman spectra of various carbon materials: a graphite, b molten salt-produced 3D graphene, c MO-saturated 3D graphene, and d 3D graphene after thermal regeneration. The Raman spectrum of MO-saturated 3D graphene in the range 1050–1500 cm−1 is shown in the inset, reproduced from Ref. [98], copyright 2019, with permission from Elsevier

the E2g vibration mode of sp2 -bonded carbon atoms in a two-dimensional hexagonal lattice, indicating the degree of graphitization. On the other hand, the D-band is associated with structural defects and partially disordered sp2 domains. The intensity ratio of the D and G bands, ID /IG , which is an index for the defects, and inversely, the crystallinity of graphitic carbons, could be measured to be 0.078 and 0.167 for the graphite and the 3D graphene nanosheets, respectively. The higher ID /IG ratio of the 3D graphene can be attributed to its higher density of carbon edges brought about by the exfoliation process. It should be mentioned that the D peak is usually absent in highly oriented crystalline graphite with low structural defects. In this case, the D peak can only appear at the edge sites of the graphitic crystals. This is because the edge locations act as crystalline defects, allowing elastic backscattering of electrons even in a defect-free graphitic sample [99]. The graphene material produced in molten salt is highly crystalline, as can be demonstrated by the TEM micrograph of Fig. 6.7j. The D peak appeared in the Raman spectrum of Fig. 6.9b, therefore, can be related to edge sites. Furthermore, Raman spectroscopy is a sensitive tool to probe the nature of graphene edges. The D peak is ideally zero for zigzag orientation and has a large value for armchair orientation [99]. The molten salt-produced graphene, therefore, possesses a high density of armchair-oriented edge sites. On the other hand, in the Raman spectrum of the MO-adsorbed graphene, shown in Fig. 6.9c, some extra peaks in the range 1050–1500 cm−1 can be observed. These peaks can be attributed to the N = N stretching vibrational modes in MO [100].

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It should be mentioned that the fast and convenient detection of dye species in the aqueous solutions is either scientifically and commercially valuable from an environmental point of view. Wu el al. [101] reported that Ag-coated SiO2 spheres can be used as a surface-enhanced Raman scattering substrate (SERS) to enhance Raman scattering signals of organic dyes, including methyl red, methyl orange, brilliant green and methylene blue. The SERS substrate coated with methyl orange exhibited several peaks including a characteristic peak at 1141 cm−1 , attributed to the deformation vibration of the aromatic C–C bonds. Figure 6.9c shows the surface-enhanced Raman spectrum of MO adsorbed on the 3D graphene produced in molten salts. On the other hand, these MO-related Raman peaks disappeared after the thermal regeneration at 300 °C, confirming the removal of MO, as can be observed from Fig. 6.9d. The Raman spectra of Fig. 6.9 can be used to evaluate the quality of the raw and regenerated graphene nanosheets, considering that the 2D band configuration in graphitic materials is sensitive to the number of carbon layers in their flakes. The 2D peak in bulk graphite is asymmetric consisting of two components, while the monolayer graphene possesses a single and sharp 2D band. The sharp and uniform 2D band of the molten salt-produced 3D graphene represents the Raman features of single- or few-layered graphene [102], with an ID /IG value of 0.105. A summary of the structural properties of the graphite and 3D graphene material, as well as those of 3D graphene after MO saturation and thermal regeneration, is presented in Table 6.2. Table 6.2 suggests that the crystalline quality of the graphene material is slightly enhanced after the thermal regeneration treatment. The structural stability of the graphene nanosheets over the MO adsorption and the thermal regeneration process could also be confirmed from the SEM micrographs of Fig. 6.7. The possible chemical bonds on the graphene surface, before and after being used in the MO adsorption, as well as the regenerated graphene could be characterized by the FTIR analysis, as shown in Fig. 6.10. The FTIR spectra of the carbon materials exhibited in this figure show a distinct peak at around 3428 cm−1 , which can be assigned to the stretching vibration of O–H groups [103, 104], suggesting the presence of hydroxyl groups on the surface of graphene materials. It can also be observed that this peak has the highest intensity in the raw graphene material. Moreover, the Table 6.2 Structural properties of graphite, molten salt-produced 3D graphene, MO-saturated 3D graphene and 3D graphene after thermal regeneration, reproduced from Ref. [98], copyright 2019, with permission from Elsevier Carbon material

d-spacing (nm)

Crystalline domain size (nm)

ID /IG

I2D /IG

Graphite

3.45

24.22

0.078

0.2824

3D graphene

3.44

18.34

0.1679

0.346

MO-saturated 3D graphene

3.44

11.41

0.1173

0.7738

3D graphene after thermal regeneration

3.44

12.22

0.1058

0.7871

6.5 Adsorption

93

Fig. 6.10 FTIR analysis of various carbon materials, reproduced from Ref. [98], copyright 2019, with permission from Elsevier

peaks at around 1632 cm−1 and 1211 cm−1 can be attributed to the skeletal vibration of aromatic C = C bonds, and the epoxy C–O–C bonds, respectively [105]. The FRIR results reveal that the saturation of the adsorbent is accompanied by a change in its surface chemistry, which can affect the subsequent heat treatment [106]. In contrast with the FTIR spectrum of the raw graphene, that of the MO-saturated graphene obtained after the adsorption process presents some new bands at 875, 1047, 1084 cm−1 ,1613 cm−1 corresponding to –S = O, CH3 −, C–N and N = N bonds, respectively [107]. The appearance of these bands confirms the substantial adsorption of MO molecules on the molten salt-produced 3D graphene, which is in agreement with the Raman results of Fig. 6.9. In addition, the FTIR band associated with the O–H group on the graphene materials has shifted from 3428 to around 3352 cm−1 , which can be attributed to the existence of a hydrogen bond between the hydroxyl groups on the graphene and the nitrogen atoms of MO [105], suggesting the contribution of hydrogen bonding interactions on the overall adsorption. Moreover, the peak related to the aromatic C = C bonds shifted from 1632 to 1652 cm−1 and became slightly widened after the adsorption, indicating the presence of π–π interaction between the benzene rings of MO and the graphene. Overall, the FTIR results provide evidence for the presence of the π–π interaction and hydrogen bonds between the MO and graphene (Fig. 6.11).

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6 Applications of Carbon Nanostructures Produced in Molten Salts

Fig. 6.11 Schematic representation of a graphite crystalline structure, b molten salt-produced exfoliated graphene nanosheets representing a high density of functionalized edge sites, and c the interaction between the graphene and MO, reproduced from Ref. [98], copyright 2019, with permission from Elsevier

6.5.1 Adsorption Performance of the Molten Salt-Produced 3D Graphene Nanosheets The mechanisms involved in adsorption of organic and mineral pollutants can often be explained using mathematical models [108], within which Langmuir, Freundlich and Temkin models are widely employed to describe the adsorption behavior of solid–liquid adsorption systems. Figure 6.12 shows the graphical representations of these models based on the experimental adsorption data obtained for the graphite and the molten salt-produced 3D graphene, and the resulting parameters are shown in Table 6.3. The analysis of the regression coefficients (R2 ) indicates that the Langmuir model provides the most accurate fit to the experimental data concerning the adsorption of MO on the 3D graphene. This suggests the formation of a continuous monolayer of MO molecules on a homogeneous graphene surface [109]. Moreover, the equilibrium parameter for the Langmuir isotherm (RL ) can be calculated using the following equation [110]: RL =

1 1 + KL C0

(6.4)

where RL is the essential characteristic of the Langmuir model. The values of RL greater than unity imply an unfavorable adsorption, while the RL value of 1 indicates

6.5 Adsorption

95

Fig. 6.12 Comparison of adsorption models for the removal of MO using graphite and the molten salt-produced graphene, reprinted from Ref. [98], copyright 2019, with permission from Elsevier Table 6.3 Parameters of adsorption models for the adsorption of MO on graphite and 3D graphene nanosheets fabricated in molten salt [98] Isotherm model

Adsorption constant

Adsorbent Graphite

3D graphene

Langmuir

Maximum adsorption capacity, qmax (mg/g)

13.020

27.932

Binding constant, KL (l mg−1 )

1.457

0.378

Separation factor, RL

0.013

0.050

Correlation coefficient, R2

0.999

0.999

Correction factor, n

18.761

10.193

Freundlich constant, KF (mg g−1 )

85.231

16.493

Correlation coefficient, R2

0.932

0.8772

Heat of adsorption constant, B1

3771.947

1156.73

Equilibrium binding constant, KT

34680.41

19044.41

Correlation coefficient, R2

0.923

0.857

Freundlich

Temkin

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6 Applications of Carbon Nanostructures Produced in Molten Salts

a linear adsorption. Finally, the values 0 < RL < 1 show a favorable adsorption, and an RL value equals to zero indicates an irreversible adsorption process. In Eq. (6.4), C0 is the highest (initial) MO concentration and KL is the Langmuir adsorption constant (L mg−1 ). It can be seen from Table 6.3 that RL has values within the range of zero and unity, indicating that the graphite and the molten salt-produced graphene are both favorable for the adsorption of MO dye under the experimental conditions used. The adsorption process was conducted using 0.15 g of the adsorbents and a solution volume of 50 ml at a pH value of 7.7. Figure 6.13a shows the effect of MO initial concentration on the adsorption capacity of the 3D graphene and graphite under the adsorption condition mentioned above. It can be observed that the adsorption of MO increases from 16.25 to 24.8 mg g−1 by an increase in the initial concentration of dye from 50 to 300 mg L−1 . This behavior could be explained by the fact that a greater amount of dye is available to be adsorbed at a higher initial dye concentration. As a result, the values of qe progressively increase reaching a maximum corresponding

Fig. 6.13 Effect of various parameters on the MO adsorption capacity and the color removal performance of graphite (red) and 3D graphene (blue): a, b The effect of dye concentration; c, d the effect of initial adsorbent dosage, reprinted from Ref. [98], copyright 2019, with permission from Elsevier

6.5 Adsorption

97

to the saturation of the adsorption sites. The initial concentration of MO has a complicated influence on the adsorption capacity of the carbon materials. To put more light on this, the effect of this parameter on the color removal was investigated. Figure 6.13b shows that graphene adsorbent outperforms the graphite, so that the color removal performance of the graphene and graphite at an initial MO concentration of 0.05 g L−1 are 98% and 70%, respectively. Moreover, the color removal performance of both carbon materials strongly depends on the initial concentration of MO. For instance, the color removal performance of the graphene drops from 98% to 25% by increasing the MO initial concentration from 0.05 to 0.3 g L−1 . The greater performance of the 3D graphene can be attributed to its higher values of the specific surface area and density of edge sites, in comparison with those of graphite. It should be considered that both the surface and wrinkles exist on graphene nanosheets may provide effective sites for the adsorption of organic pollutants consisting of one or more aromatic rings. However, the mechanisms involved in interaction between organic pollutants and these adsorption sites are complex, since they depend on the structure of graphene material used and the properties of the sorbate [111]. The flat surface of graphene can provide a unique platform for π–π interaction with aromatic molecules, and a high hydrophobic attraction toward dyes. Structural defects such as wrinkles, surface functional groups and edges of the graphene nanosheets can also contribute to the overall adsorption mechanism [112]. The electrolytic 3D graphene possesses a high mechanical and chemical stability as well as a high surface area and edge sites making it attractive for dye adsorption applications. Figures 6.13c and d present the influence of the amount of adsorbents on the MO adsorption and the color removal performance of the graphite and 3D graphene materials, repectively, from which contradictory behaviors can be observed: for both adsorbents, by increasing the adsorbent dosage, the MO adsorption capacity decreases while the color removal performance increases. To explain this, we need to consider that a greater deal of active sites becomes available for the adsorption of MO at an increased adsorbent dosage. Therefore, the color removal, consequently, increases. However, since there is a constant initial MO concentration of 50 mg L−1 , the MO adsorption per unit mass of the adsorbent decreases by the increase of the adsorbent dosage [98]. This behavior was also reported by Wang et al. [78]. As it can be observed from Fig. 6.14, by increasing the adsorption time, the absorbance of MO dye solution decreases. For instance, the absorbance intensity has values of 1.62 and 0.06 at the beginning and after 15 min of the adsorption, respectively, using the graphene material. In contrast, the absorbance remains constant at the value of 0.44 after 15 min adsorption using the graphite adsorbent. These observations indicate the efficient dye removal achieved by the use of molten salt-produced graphene.

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6 Applications of Carbon Nanostructures Produced in Molten Salts

Fig. 6.14 UV-Visible spectra of the MO dye solution during the adsorption process, using 3D graphene produced in molten salt and graphite adsorbents, reprinted from Ref. [98], copyright 2019, with permission from Elsevier

6.5.2 The Effect of pH on the Adsorption Capacity The solution pH can be regarded as a key parameter determining the adsorption properties of an adsorbent. Figure 6.15a shows the adsorption efficiency of the molten salt-produced graphene nanosheets in a solution pH range from 2 to 11. As can be observed, the adsorption capacity of the graphene material just slightly decreases from 16.6 mg g−1 to 16.2 mg g−1 with increase in the pH value from around 2 to 8, and then slightly increases to more than 16.2 by raising the pH value to 11. This indicates that the adsorption efficiency of the graphene material is largely independent from the initial pH of MO solutions. The considerably stable adsorption performance of the 3D graphene adsorbent within a wide pH range from 2 to 12 is interesting. This expands the practical applications of the molten salt-produced graphene to various wastewaters released from different industries. Figure 6.15b exhibits the UV-Vis spectra of MO solutions, after being exposed to the graphene adsorbent for 15 min at different pH values, from which the efficient adsorption of MO by graphene at all pH values is evident. The weak dependency observed between the adsorption performance of the graphene and the solution pH can be attributed to the presence of ionizable groups on the 3D graphene surface/edge sites, and the electrostatic interactions between charged species in the system [98]. At

6.5 Adsorption

99

Fig. 6.15 a The effect of solution pH on the MO adsorption performance of the molten saltproduced 3D graphene, measured after 15 min of adsorption using 0.15 g graphene added to 50 ml solution containing 50 mg L−1 MO; and the MO adsorption efficiency of the graphene at various pH values. b The UV-Vis spectra of MO solutions, after being exposed to the graphene adsorbent, recorded at different pH values, reprinted from Ref. [98], copyright 2019, with permission from Elsevier

acidic pH values, the sulfonic functional groups of MO are protonated, resulting in a drop in the negative charge of the anionic groups of MO, decreasing the electrostatic adsorption of MO by graphene. Nevertheless, the adsorption performance of the molten salt-produced 3D graphene nanosheets is, to a large extent, independent of the solution pH, in comparison with GO materials [114]. This advantageous behavior of the 3D graphene over GO materials is attributed to its much smaller amounts of ionizing functional groups.

6.5.3 Reusability and Stability of 3D Graphene Nanosheets The MO adsorption performance of the molten salt-produced 3D graphene is comparable with those of the state of the art adsorbents (Table 6.4). Furthermore, the Table 6.4 Comparison between the maximum MO adsorption capacity of the molten salt-produced 3D graphene with other carbonaceous adsorbents

Adsorbate 3D Graphene [98]

Adsorption time (min)

qmax (mg g−1 )

60

27.93

Graphene oxide aerogel [105]

360

55.56

Graphene oxide [106]

100

16.83

Carbon nanotubes [107]

120

51.74

Zeolite CuO/NaA [109]

120

79.49

100

6 Applications of Carbon Nanostructures Produced in Molten Salts

Fig. 6.16 a UV-Vis spectra of MO solutions after being exposed to the graphene adsorbent, recorded at different numbers of adsorption–regeneration cycles. b The thermal regeneration efficiency of the 3D graphene material, reprinted from Ref. [98], copyright 2019, with permission from Elsevier

graphene material exhibits a high reusability level thanks to its high structural and microstructural stability. Figure 6.16 shows the adsorption performance of the regenerated graphene. The sample was thermally regenerated in an oven at 300 °C in air for 1 h after being used as the adsorbent. It is evident that the 3D graphene material maintained a large portion of its high adsorption capacity (>73%) even after five adsorption/regeneration cycles, proposing the graphene material as a reusable adsorbent for practical applications. The adsorption capacity loss observed can be attributed to the blocking of graphene pores by the inorganic components of the MO dye during the regeneration process. Figure 6.17 shows the XRD patterns of the raw MO dye and the dye after heating at 400 °C. The pattern of the raw MO dye (Fig. 6.17a) can be indexed by the diffraction peaks of MO dye as well as those of NaCl and NH4 Cl. The presence of these salts in dyes is usually to increase their solubility. The XRD pattern of the dye after heating at 400 °C (Fig. 6.17b) could be indexed to diffractions of NaCl, leading to this conclusion that the organic part of the dye is removed and only the inorganic part of the dye (NaCl) is left during the heating process. The presence of crystalline NaCl trapped in the porosity of the regenerated graphene can explain the reduction of the

6.5 Adsorption

101

Intensity (u.a.)

5000

(b)

MO

NaCl

20

NH4Cl

30

(a)

40

50

60

70

80

90

2θ (degree) Fig. 6.17 XRD patterns of MO a before, and b after heating at 400 °C [98]

surface area of the 3D graphene from an initial value of 180 m2 g−1 to around 30 m2 g−1 after the thermal regeneration, as well as the adsorption capacity loss observed [98]. Although a complete recovery of the 3D graphene nanosheets could not be achieved by the simple heating of the material in air, the recovery value is still significant. Various techniques have been employed to regenerate carbonaceous adsorbents, including the electro-Fenton regeneration method in which the carbon adsorbent, platinum wire, Na2 SO4 and FeSO4 are employed as the cathode, anode, electrolyte and the catalyst, respectively [113]. The regeneration of carbonaceous adsorbents can also be achieved based on the soaking of the used carbon adsorbent overnight in ethanol followed by washing with copious amounts of deionized water [115]. Such processes are generally time-consuming, costly and skilled-labor-intensive, which limit the application of the methods, particularly at large scales. In contrast, the thermal regeneration can be considered an effective and fast technique for the recovery of adsorbents after being used in the adsorption process, as can be seen in Table 6.5. Activated carbon (AC) and its composites have been used for the adsorption of various organic materials including paracetamol [116], phenol [117], salicyclic acid [117], p-Nitrophenol [118], N2 [118], acetone [119] and CO2 [120], followed by thermal regeneration which is often conducted in inert atmospheres due to the poor thermal stability of AC in air [121].

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6 Applications of Carbon Nanostructures Produced in Molten Salts

Table 6.5 Thermal regeneration performance of various adsorbents Adsorbent

Adsorbate

Regeneration cycle number

Regeneration conditions

Regeneration capacity

Activated carbon

Paracetamol [116]

1–5

N2 , 400–600 °C/1 h

65–14%

Phenol [117]

1

N2 , 850 °CC

70

N2 , 850 °CC

97%

6

14%

Salicyclic acid [117]

1

p-Nitrophenol (PNP) and N2 [118]

1

Air, 360 °C/90–180 min; CO2 , 850 °C/30-90 min; Steam water, 850 °C/30–90 min

N2 70–94%, PNP 10–108%

Acetone [119]

1

Air, 80 °C

97%

N2 , 288–400 °C/3 h

Mass balance cumulative heel (%) less than 20% for all the samples

N2 , 200 °C

0.74 mmol CO2 /g

6

54%

8 Mixture of organic vapors [125]

1

95%

9 K2 CO3 /Activated carbon

CO2 [120]

1

60–90%

5 Molten salt-produced graphene

Methyl orange (MO) [98]

1 5

0.66 mmol CO2 /g Air, 300 °C/1 h

95% 74%

The 3D graphene produced by the molten salt exfoliation of graphite can be employed as adsorbent for the removal of MO dye from water, exhibiting a high adsorption performance in a wide range of the solution pH from 2 to 11, due to the combination of properties including its high surface area and density of edge sites decorated with functional groups. Furthermore, the 3D graphene could successfully be regenerated by the simple heat treating of the material in air at 300 °C, thanks to the high thermal stability of the graphene material [3, 122, 123]. This characteristic enhances the viability of 3D graphene nanosheets as an efficient adsorbent. As it previously discussed, the molten salt-produced 3D graphene can be fabricated in an economic, environmentally friendly and sustainable way [102], further enhancing the interests [124] toward its application as an efficient adsorbent.

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Chapter 7

Molten Salt Conversion of Plastics into Highly Conductive Carbon Nanostructures

Abstract The pollution caused by the increasing accumulation of plastic wastes in the environment is considered a serious emerging threat to our wildlife, habitats and to us. In fact, the efficient removal of plastic wastes from the environment is challenging in the absence of a strong economic driving force. Such a driving force can be achieved through the low-cost conversion of plastic wastes into highly valuable outputs such as high-quality graphene materials. This chapter provides an introduction into thermokinetic characterization of polyethylene terephthalate, the most commonly used plastic, and then deals with the molten salt—assisted conversion of plastic bottles into graphene nanostructures with a high surface area, degree of crystallinity and electrical conductivity. Keywords Plastic waste · Molten salt · Carbon nanotubes · Graphene · Conductivity · Pyrolysis Plastics, with an annual production of over 300 million tons, have increasingly been used for a wide variety of applications in the modern life, owing to their unique properties including low production cost, low density, durability, high chemical resistance and dimensional stability [1]. Polyethylene terephthalate ((C10 H8 O4 )n, PET) is considered to be the most commonly used plastic, often employed as containers for bottled liquids and other food products due to its affordable cost, transparency and versatile physical and mechanical properties including excellent mechanical strength, low friction coefficient, high flexural modulus and barrier properties. Its radiationresistant properties are also accountable for applications as insulator and nuclear track detector in nuclear plants and devices [2]. The global PET consumption experiences a grow rate of about 3.8% annually, with an estimated market value growth from $48.1 billion to $60 billion from 2016 to 2019 [3]. The constant increasing demand for bottled water has boosted the annual consumption of plastic bottles to somewhere around 500 billion across the world, out of which an estimated 150 million tones accumulated in the world’s oceans [4]. While only less than 10% of virgin PET plastics can practically be recycled into new products at the end of their first life, the majority of used PET plastics find themselves in landfills or in the oceans. The latter is estimated to be hundreds of millions of tones, adding up their microscopic plastic content to be ingested by © Springer Nature Singapore Pte Ltd. 2020 A. R. Kamali, Green Production of Carbon Nanomaterials in Molten Salts and Applications, https://doi.org/10.1007/978-981-15-2373-1_7

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birds, fish and other organisms, and eventually by mankind who eat these creatures, creating a serious global environmental crisis and waste management. In fact, marine plastic pollution is a key transboundary environmental crisis affecting biodiversity, marine industrial sectors, coastal communities, posing potential food security and human health risks. The current trend results in a global used plastic accumulation of about 12,000 million tons in 20 years time [3–10]. It should be mentioned that the natural degradation of PET plastics takes a long time, probably over the course of several hundreds of years [11], and therefore, their recycling or conversion is vital. Currently, the utilization of waste plastics in various applications is an active research stream. For instance, Naga et al. [12] studied the effect of PET waste plastic materials on improving the performance and properties of asphalt pavements. They found that the addition of PET has a positive impact in both reducing the penetration and increasing the softening point of asphalt binders, reducing their temperature susceptibility. This can be an advantage for asphalt pavements in hot climates. The resultant asphalt mixtures also exhibited a higher Marshall stiffness modulus, indirect tensile strength and rutting stiffness. However, the loss of stability was observed when PET is employed. Louzada et al. [13] studied the application of waste PET in geotechnical engineering and found that fine crushed PET may improve the load capacity of soil. As another example, Marques et al. [14] utilized PET waste to produce fire-resistant polyurethane boards. In these applications, PET is either directly employed with no chemical change in its structure or converted into a different polymer material. In contrast, chemical depolymerization methods have also been considered to manage PET waste through the production of new chemicals [15]. A viable approach toward the utilization of PET waste is based on the extraction of hydrogen and/or carbon content of PET. Being hydrocarbons, plastics have high caloric values [16], and therefore, are widely considered as a potential feedstock for producing fuels such as H2 and syngas [17–20]. In this regard, biodegradation of PET waste using various microorganisms is an interesting approach leading to the preparation of useful fuels such as methane [21]. PET can also be considered as a precursor material to prepare nanostructured carbon materials. In the following sections, a brief discussion on the thermochemical characteristics of PET is presented. Then, the conversion of PET into carbonaceous materials is discussed.

7.1 Structural Characterization of PET PET can possess mobile or rigid amorphous phases as well as crystalline structures [22], and as such it can be characterized by X-ray diffraction (XRD). The XRD analysis was performed on pieces of a PET water bottle, and the pattern obtained is shown in Fig. 7.1a. The broad single peak and the diffused nature of the profile indicate the presence of disorder in the amorphous PET structure. However, PET with different degrees of crystallinity can be fabricated by annealing of the material at different temperatures [23].

7.1 Structural Characterization of PET

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Fig. 7.1 a XRD analysis of the raw PET water bottle material. The diffraction pattern observed can be characterized by a broad peak cantered at the 2θ value of 25.4°, indicating the short-range (100) crystalline domains with an anorthic configuration (C10 H8 O4 , JCPDS card No. 050-2275). b The XRD analysis of PET heated at 260 °C overnight, followed by cooling down to the room temperature. The diffraction peaks are indexed to the crystalline PET with anorthic structure. c EDX analysis of the crystallized PET, reproduced from Ref. [24], copyright 2019, with permission from Elsevier

For instance, the PET material was heated in a resistance furnace at 260 °C, which is above the melting point of PET, overnight. After cooling down to the room temperature, white color large irregular-shaped crystalline particles were obtained, which were subjected to XRD analysis. The pattern obtained is shown in Fig. 7.1b. The diffraction peaks observed in the pattern are indexed according to those of PET with an anorthic crystalline structure (JCPDS: 050-2275). In this pattern, the most intense (100) reflection peak can be observed at 2θ value of 26.00°. The typical SEM morphology of the crystalline PET obtained is exhibited as the inset in Fig. 7.1b, in which a large particle with smooth surfaces and sharp edges can be observed [24]. Energy-dispersive X-ray analysis performed on the crystalline PET (Fig. 7.1c) demonstrated a C:O atomic ratio of 1.8. This value is smaller than the theoretical value of 2.5 which can be calculated for the repeat unit for PET. The C:O ratio of the virgin and plasma-treated PET was measured to be 3 and 1.7, respectively using XPS analysis [25]. In this case, the higher value of C:O measured on the virgin PET, in comparison to the theoretical value, was attributed to the presence of surface contaminations. Also, the lower value of C:O measured on the plasma-treated PET was explained by the higher concentration of C–O and C=O bonds on the PET surface, induced by the plasma treatment applied. In the above-discussed case, the lower value of C:O atomic ratio measured using the EDX analysis was attributed to the surface electron irradiation of PET occurred during the microscopy. The crystallization of PET occurred during its solidification from melt could be realized from Fig. 7.1. It

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can also be concluded that the plastic bottle was made of pure PET. The high carbon content of waste PET objects as well as the lack of inorganic impurities makes such waste materials very attractive sources for the preparation of solid carbon materials.

7.2 Carbonization of PET Thermal analysis provides valuable information about the thermal stability of polymers. The TGA and DSC thermograms recorded on amorphous PET pieces under an airflow rate of 100 mL min−1 and the heating rate of 40 °C min−1 in the temperature range 25–900 °C are shown in Fig. 7.2. From the DSC curve, three endothermic peaks can be distinguished. The first endothermic event at 254.1 °C can be attributed to the melting of PET [24]. Tanaka [26] analyzed crystallized PET under a heating rate of 5 °C min1 and observed two equilibrium melting temperatures at 262 and 276 °C, which were assigned to two forms of PET crystals. From Fig. 7.2, the second endothermic event occurred at 466.8 °C can be due to the decomposition of PET [24]. Hamidi [27] showed that the decomposition of PET occurs at a temperature in the range of 400–500 °C, depending on the heating rate used in the range of 1–50 °C min−1 . This endothermic event is accompanied by a mass loss of about 84%, as it can be depicted from the TGA curve of Fig. 7.2. The last endothermic peak observed at 791.2 °C is assigned to the minor graphitization of the residual carbon material [24].

Fig. 7.2 DSC and TGA thermograms of the PET sample, heated under a heating rate of 40 °C min−1 , and an airflow of 100 mL min−1 , reproduced from Ref. [24], copyright 2019, with permission from Elsevier

7.2 Carbonization of PET

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Fig. 7.3 a X-ray diffraction patterns, and SEM micrographs of the amorphous carbon materials produced by heating of PET in air up to different temperatures of b 620 °C and c and d 850 °C, reprinted from Ref. [24], copyright 2019, with permission from Elsevier

The XRD analysis and SEM morphology of carbon materials produced by heating in air of PET to 650 and 850 °C are shown in Fig. 6.3. The XRD patterns of the samples (Fig. 7.3a) indicate the poorly crystalline nature of the carbon materials. The origin of the endothermic peak observed at 791.2 °C was further investigated. For this, small pieces of a PET plastic were heated to different temperatures of 650 and 850 °C using a resistance furnace, and the black carbon materials obtained were characterized. The XRD pattern of the carbon material obtained at 620 °C (7.3a) shows two broad reflections centered at the 2θ values of 21.2 and 43.4°. The first peak, corresponding to an interplanar spacing 4.18 Å, can be assigned to the (002) crystalline planes in short-range ordered hexagonal arrays of turbostratic carbon [28–30]. This material, which was produced by the pyrolysis of PET at 620 °C, has also been called disordered graphite [28] or amorphous carbon [31] in the literature. The second broad XRD reflection extending from the 2θ values of 40–45° has a maximum at 43.4°. This peak corresponds to the overlapping reflections arisen from (100) and (101) short-range ordered crystalline planes. In the hexagonal structure of graphite (JCPDS card No.13-0148), the (002) reflection appears at the 2θ value of 26.6°, which indicates an interplanar spacing of 3.35 Å. In this crystalline structure, the reflection peaks arisen from (100) and (101) plans appear at the 2θ values of around 42.4 and 44.4°, respectively. The XRD patterns of amorphous carbon materials obtained by heating of PET exhibit a large deviation of

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d (002) toward higher values in comparison to that of graphite. This indicates the poorly crystalline nature of these carbon materials [29–31]. The turbostratic carbon has been identified as a variant graphite with hexagonal structure, in which the (002) carbon layers may randomly translate to each other and rotate about the normal of the layers [28]. Ramakrishnan et al. [30] observed the position of the (002) XRD reflection in a turbostratic nanocarbon at 2θ –23°. Figure 7.3a shows the XRD pattern of PET pyrolyzed at 850 °C. In this pattern, the diffraction peaks arisen from the (002) planes and the (100)/(101) overlapping can be observed at the 2θ values with the maximum of 25.2°, corresponding to an interlayer spacing value of 3.54 Å, and 43.2°, respectively. As can be observed, in comparison with the carbon material produced at 620 °C, the carbon sample obtained at 850 °C possesses a more intense (002) diffraction peak. Moreover, this peak has shifted toward a larger value. These observations indicate that the sample obtained at 850 °C has a greater degree of crystallinity. Therefore, the endothermic peak observed at 791.2 °C corresponds to the graphitization onset. Ruz et al. [29] produced a turbostratic carbon using sucrose and zeolite template at 700 °C, and detected the (002) and (101) diffraction peaks of the carbon material in the corresponding XRD pattern at 2θ values of around 20° and 44°, respectively. In their work, by increasing the processing temperature to 900 °C, the diffraction peaks became more intense and shifted toward higher angles. This was attributed to the increased translational ordering in the carbon sample produced at higher temperatures [29]. Turbostratic carbons might find interesting applications in the future such as electrode materials for sodium ion [32] or vanadium redox flow batteries [33]. However, currently, graphitized nanostructured carbon materials are candidates for a number of applications in energy and environmental fields, as will be discussed later in this chapter. As discussed, the endothermic peak observed in Fig. 7.2 at 791.2 °C can be attributed to the onset of the graphitization of the turbostratic carbon [24]. Moreover, from the TGA micrograph of Fig. 7.2, it can be realized that about 10% of the PET material, which is around 40% of the total solid carbon still remained at 900 °C in air atmosphere, where the thermal analysis was terminated. It demonstrates that the amorphous carbon material obtained by the carbonization of PET has a high thermal stability and resistance against thermal oxidation in air. It should be mentioned that usually at temperatures above 500 °C, the intensive oxidation of carbon materials in air occurs. The oxidation temperature is a function of the carbon properties including its crystallinity or degree of graphitization, flake size, porosity and purity [34]. With this in mind, the thermal oxidation of highly oriented pyrolytic graphite (HOPG) has been reported to occur at 550–950 °C [35]. At temperatures lower than 875 °C, The oxidation process mainly proceeds by the formation of pits at defect sites. At higher temperature, however, the oxidation event takes place on both defects, and also carbon basal planes. On the other hand, the oxidation rate of amorphous carbons is known to be greater than that of graphitic carbon

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materials [36]. Furthermore, the inorganic impurities can act as the oxidation catalyst, promoting the thermal degradation, and decreasing the oxidation temperature of carbon materials [37–39]. Further information can be obtained from the SEM morphology of the carbon materials shown in Fig. 7.3b–d. The carbon material produced at 620 °C (Fig. 7.3b) comprises of large particles up to several hundred micrometers with irregular shapes and sharp edges. Moreover, despite the fact that the material has been heated in air, the SEM micrograph does not show any sign of oxidation, indicating the high oxidation resistance of the carbon material. The C:O atomic ratio of the sample was recorded to be 5.2 by EDX analysis, which is considerably greater than that of crystallized PET (1.8). Nevertheless, the oxygen content of the sample is still remarkable, and this can be a barrier toward further graphitization of the carbon material. Overall, the carbon material produced at 850 °C (Fig. 7.3c, d) possesses approximately the same morphological features as that of the carbon material produced at 620 °C. The presence of irregular-shaped large particles is evident from the micrograph. Furthermore, small surface pitting holes can also be observed from the higher magnification of Fig. 7.3d, which is an indication of the initial stage of oxidation. It was observed that the carbon material is completely oxidized into gaseous species at the higher heat treatment temperature of 1100 °C. Nevertheless, the thermal analysis results presented in Fig. 7.2 and SEM micrographs of Fig. 7.3 demonstrate that the amorphous carbon driven from PET has a very high oxidation resistance up to temperatures as high as 900 °C. This characteristic can be assigned to the large particle sizes, high purity, low porosity and the low density of surface defects in PET-derived amorphous carbon materials [24]. As discussed, the endothermic peak observed at 791.2 °C in the DSC thermogram of Fig. 7.2 is attributed to the onset of the graphitization occurred in the PET-derived turbostratic carbon. Gutiérrez-Pardo et al. [40] have reported the development of graphitic structures upon pyrolysis of wood impregnated with FeCl3 at temperatures as high as 1000–1600 °C. For this, wood samples were impregnated with 3 M FeCl3 solution in isopropanol in vacuum for 2 h to ensure the complete filling of the pores. The pyrolysis process took place by heating of impregnated samples in a flow of nitrogen to a maximum temperature in the range 1000–1600 °C. After pyrolysis, the residual iron was removed by stirring in concentrated HNO3 [40]. Figures 7.4a and b show the SEM and TEM micrographs of the sample produced at 1600 °C, respectively, representing a porous carbon structure with a BET surface area of about 167 m2 g−1 . Figure 7.4c shows the heat flow versus temperature during pyrolysis of the impregnated wood, and also that without impregnation with FeCl3 (black line). The DSC curve of the impregnated sample shows a pronounced endothermic peak at 718 °C, which is related to the onset of the graphitization process. This peak could only be detected in the FeCl3 impregnated sample, demonstrating the effect of FeCl3 on the graphitization process. Figure 7.4.d shows the Raman spectra of the samples produced at different temperatures. The progress of the graphitization is evident based on the increase of the relative intensity of the G band. In this research, the graphitization occurred was related to the catalytic effect of Fe leading to the formation of FeX CY droplets from which graphitic structures

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Fig. 7.4 a SEM and b TEM micrographs of FeCl3 impregnated wood samples pyrolyzed at 1600 °C after removal of Fe by acid washing. c Heat flow versus temperature during the pyrolysis of wood, with (red line) and without (black line) impregnation with FeCl3 . d Raman spectra for samples pyrolyzed at different temperatures. e Schematic representation of the catalytic mechanism responsible for the formation of partially graphitized carbon, reproduced from Ref. [40], copyright 2019, with permission from Elsevier

could precipitate. Nevertheless, a reasonable graphitization was achieved at a temperature not less than 1600 °C. This mechanism might be based on the formation of near eutectic liquid droplets of Fex Cy. These droplets dissolve amorphous domains from the carbon matrix. More ordered graphitic structures can then precipitate from the melt, as depicted schematically in Fig. 7.4e.

7.3 Conversion of PET into Carbonaceous Nanomaterials PET has a high carbon content of about 45 at.%. The high carbon content and also the lack of inorganic components make PET a viable source of high-purity solid carbon materials. It is worth mentioning that carbon nanostructures with high surface area and conductivity are of great importance due to their increasing applications in various fields such as energy storage devices [41–43], conductive composites [44], solar energy harvesting [45], conductive inks [46] and environmental applications [47]. In order to evaluate the quality of such carbon materials, a combination of techniques are often used, including scanning electron microscopy (SEM), transmission electronic microscopy (TEM), scanning tunneling microscopy (STM), electrical and thermal conductivity measurements, X-ray diffraction, UV–Vis absorption spectroscopy, photoluminescence spectroscopy, X-ray fluorescence, surface area and pore size distribution, thermal analysis and Raman spectroscopy. These techniques have been reviewed in a number of publications [48–52].

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Among these techniques, Raman spectroscopy is widely recognized as a low cost but powerful method for characterization of carbon nanostructures, since it provides explicit insights into the layer structure, crystallinity and defects. In carbon materials, the possible presence of the main first-order Raman band formed by sp2 -carbon bonds, the G band; the disorder-induced Raman band, the D band, as well as the D band overtone, the 2D band, is readily detectable by Raman spectroscopy. Generally speaking, defects damage some attractive properties of carbon nanostructures, including their electrical and thermal conductivity. However, defects such as graphene edge sites can be engineered to be favorable in specific applications, where the reactivity of carbon nanostructures with their environment is important. An example of such application was given in 6.5. The intensity ratio of Raman bands ID /IG and I2D /IG in graphite-based materials corresponds to the density of defects and the quality of graphene flakes, respectively [53, 54]. These parameters can also provide an estimation of the material’s conductivity [55, 56]. Generally, a lower ID /IG value is measured in graphitic materials with higher values of electrical conductivity. Also, a higher I2D /IG value together with a symmetrical and sharp 2D band can be attributed to the presence of graphene with fewer layers. For example, reduced graphene oxide (rGO) with an ID /IG value of 1.55, 1.19 and 1.02 exhibited Raman I2D /IG values of 0.01, 0.07 and 0.14, and electrical conductivity values of 69, 133 and 166 S m−1 , respectively [57]. Based on the information mentioned above, the quality of PET-derived carbon materials can be extracted from the literature [58–67]. The high-temperature pyrolysis of PET under N2 and subsequent activation of the material obtained under steam, CO2 and/or KOH [58–62], and also the thermal treatment of PET under protective atmospheres in various equipment such as arc discharge [63], chemical vapor deposition (CVD) [64, 65] and autoclave [66, 67] reactors can lead to the formation of amorphous carbons, characterized by the presence of very weak Raman 2D bands as well as ID /IG values of greater than unity. For instance, the pyrolysis of PET under N2 at 400 °C for 1 h, followed by a heat treatment at 725 °C led to the formation of an amorphous carbonized char. This material was then sieved to separate particles with the size of ≤0.15 mm, and the sieved char particles were heated under N2 to 925 °C. After 1 h, N2 was replaced by CO2 and the heat treatment process was continued for another 2 h. The final product was found to be an activated carbon material with a BET surface area of around 660 m2 g−1 . The properties of the carbon product are summarized in Fig. 7.5. The X-ray diffraction pattern of the activated carbon produced exhibits a broad diffraction peak located at 2θ = 20°–30° which ascribed to reflections arisen from hexagonal carbon (002) planes. Also, the peak observed at 2θ ~43° with low intensity is attributed to the reflections of carbon (101) planes. This XRD pattern shows an intermediate structure between crystalline graphitic and amorphous carbon [42]. Likewise, the Raman spectrum of the carbon product (Fig. 7.5b) shows the presence of disordered graphitic crystallites. Particularly, the ID /IG has a high value of 1.04, and there is no obvious sign of the Raman 2D band [42]. It should be mentioned that such carbon materials with semi-crystalline structures inevitably suffer from lack of reasonable electrical conductivity. At the present time, such a carbon material doesn’t

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Fig. 7.5 Characteristics of the activated carbon produced by the pyrolysis of PET at 400 °C (1 h) and then 925 °C (2 h), followed by CO2 activation at the same temperature for another 2 h. a X-ray diffraction pattern, b Raman spectrum, c SEM and d TEM micrographs, reproduced from Ref. [58], copyright 2019, with permission from Elsevier

have a high value due to its very limited applications. Therefore, the corresponding methods don’t introduce PET as a viable source for high-value carbon materials. It is an unfortunate, since there is an increasing amount of waste plastic materials that can potentially be considered as low-cost carbon sources. Obviously, the transformation of plastic waste materials into valuable carbons can be accompanied by a highly positive environmental impact. Higher graphitized carbon nanostructures could be produced at high temperatures. For this, PET was used as the carbon source in traditional methods of producing carbon nanomaterials. Reported by Berkmans et al. [63], heating of chopped used PET mineral water bottles to 815 °C under nitrogen atmosphere, led to the formation of a black polymer char, which was then filled into a hallow carbon tube and used as the anode in a rotating cathode arc discharge equipment (Fig. 7.6a), that is traditionally used for the preparation of multiwalled carbon nanotubes (MWCNTs). Formation of nanosized carbon spheres and MWCNTs was confirmed in the soot obtained at different regions of the anode and the cathode with an approximate temperature of 1700–2600 °C [63].

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Fig. 7.6 a Schematic illustration of the arc discharge equipment used for the conversion of PETderived char into MWCNTs. b Electron micrographs of MWCNTs formed on the anode and the cathode. c Raman spectra of MWCNTs formed on the anode and the cathode, reproduced from Ref. [63], copyright 2019, with permission from Elsevier

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The internsity ratio of the D and G bands (ID /IG ) of the soot formed on the anode and the cathode was calculated to be 0.77 and 0.67, respectively. This indicates that carbon nanotubes present in the anode soot had a higher degree of structural defects, which was in agreement with the SEM and TEM results. Since the cathode soot formed at higher temperatures of the arc plume, where the arcing occurs, the graphitization level is higher as compared to the soot obtained from the hole of the anode. It should be mentioned that temperature plays an important role on determining the level of graphitization of carbon nanotubes [63]. In comparison to techniques such as arc discharge, molten salt-based technologies have the prospect of being commercially viable due to the advantages of simplicity and cost effectively.

7.4 Molten Salt-Assisted Conversion of PET into Carbon Nanomaterials Molten salts can act as diffusion-enhancing medium to promote chemical reactions [68–70]. For instance, Li et al. [69] reported that the dissolution of SrO in molten KCl enhances the diffusion of ionic species (Sr2+ and O2- ) to the surface of TiO2 particles immersed in the melt, leading to the facile synthesis of Sr3 Ti2 O7 . It was also reported that the hydrolysis of molten lithium chloride leads to the formation of O2- . The reaction of O2- and Li+ with Nb2 O5 particles added to the melt promoted the ultrafast formation of LiNbO3 , much faster than solid-state synthesis methods [70]. As discussed in the Chap. 3, the exposure to the molten salt media can enhance the crystallinity of graphitic materials [71]. These observations imply that molten salts can be able to act as the graphitization medium to improve the quality of plastic-derived carbons. NaCl is the cheapest salt and one of the most abundant natural materials. In the following sections, the molten NaCl-assisted pyrolysis of PET is introduced as a green and cost-effective method for the preparation of high-quality carbon nanostructures, with interesting properties such as a high surface area (522 m2 g−1 ) and Raman I2D /IG value (0.52), as well as a low value of Raman ID /IG (0.47) and an impressive electrical conductivity of 1150 S m−1 obtained under a compressive pressure of about 6 MPa [24].

7.4.1 Molten Salt Heat Treatment of PET A mixture of about 10 g PET and 50 g NaCl was heated in air atmosphere up to two different temperatures of 1100 and 1300 °C, which are above the melting point of NaCl (801 °C). After cooling down to room temperature, the carbon obtained was washed with water to remove the NaCl off the product [24]. In the XRD pattern of the carbon material obtained at 1100 °C (Fig. 7.7), the diffraction peaks arisen from the hexagonal carbon and also cubic NaCl can be detected. The presence of NaCl

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Fig. 7.7 (Upper panel) XRD and (down panel) Raman spectra of the carbon material obtained by heating the mixture of PET and NaCl to 1100 °C and 1300 °C, followed by cooling and washing process, reproduced from Ref. [24], copyright 2019, with permission from Elsevier

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is attributed to the residual salt, which could not be removed during the washing process. The (002) reflection of the hexagonal carbon structure appears at the 2θ value of 25.13°, representing an interplanar spacing of 3.54 Å. The broad peak observed with the maxima at the 2θ value of 43.18° is attributed to overlapping (100) and (101) reflections of hexagonal carbon. In the Raman spectrum of the carbon material obtained by heating the mixture of PET and NaCl 1100 °C (the lower panel of Fig. 7.7), the defect-induced D band and the G band which correspond to the stretching vibrations of the basal carbon layers in carbon materials [53, 54] appear at the Raman shift values of 1372 and 1599 cm−1 , respectively. Furthermore, the overtone of the D band, the 2D band, observed at 2703 cm−1 has a low intensity. The level of defects, or inversely, the graphitization degree of the carbon material can be evaluated from the intensity ratio ID /IG [53, 54], which was found to be 0.94. Moreover, the quality of the graphene sheets exists in the carbon product could be assessed from the intensity ratio I2D /IG , which was measured to be 0.23. Overall, the presence of nanocrystalline graphitized domains can be realized from the XRD and Raman results. Figure 7.8 shows the SEM morphology of the carbon material obtained at 1100 °C. The presence of irregular particles with mostly smooth surfaces is evident from the SEM micrograph of Fig. 7.8a. Overall, this morphology is similar to that observed in the sample prepared by the air heating of PET to 850 °C (see Fig. 7.3). However, a

Fig. 7.8 SEM micrographs of the carbon material produced by heating the mixture of PET with NaCl to 1100 °C, reproduced from Ref. [24], copyright 2019, with permission from Elsevier

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number of graphitized locations can be depicted on the smooth surface of the carbon particles produced in molten NaCl, particularly, on the edge sites. For example, the presence of graphite flakes on the edge sites of carbon particles can be clearly seen from the SEM micrograph shown in Fig. 7.8b. Figure 7.8c shows a lower magnification SEM image demonstrating that these graphitized zones have also occasionally developed from the edge sites toward the surface of irregular-shaped carbon particles. In some areas, the carbon material is highly crystallized into graphene nanosheets, such as shown in the SEM micrograph of Fig. 7.8d. These findings are interesting since it is demonstrated that amorphous carbon materials can be graphitized under the influence of molten NaCl at the relatively low temperature of 1100 °C. In another experiment, the mixture of PET and NaCl was heated to a higher temperature of 1300 °C, by the same heating rate of 10 °C min−1 , and the carbon material obtained was characterized after the system was cooled down to the room temperature, and the salt content was washed away. Upper panel (left) in Fig. 7.9 shows the alumina crucible and the mixture of carbon material dispersed in the solidified NaCl. In order to evaluate the distribution of the carbon phase in the solidified NaCl, the alumina crucible was broken, and the mixture of solidified salt and carbon was easily retrieved from the crucible, as can be seen from Fig. 7.9, the upper panel (right). As can be observed, the carbon material is entirely distributed into the solidified NaCl, demonstrating the high dispersibility of the carbon product in molten NaCl, which is remarkable. The high dispersibility observed greatly promotes the influence of molten NaCl on the structural evolution of the PET-derived carbon materials. The carbon–salt mixture was washed with distilled water, in order to dissolve the NaCl content of the mixture. It was observed that the carbon material floats on the surface of water due to its low density. After stirring for 20 min, the suspension was filtered, and the carbon material obtained was dried overnight. Figure 7.7, upper panel, shows the XRD analysis of the carbon material obtained, from which the presence of the (002) reflection of hexagonal carbon at the 2θ value of 25.9° is evident. This value represents an interlayer spacing of 3.44 Å. The broad peak with the maxima at the 2θ value of 42.5° corresponds to overlapping (100) and (101) reflections. Moreover, the diffraction peaks related to crystalline NaCl nearly vanished from the XRD pattern, demonstrating an effective salt removal achieved during the washing process, due to the exfoliated nature of the sample. Figure 7.7, down panel, exhibits the Raman spectrum of the carbon material, providing interesting and rather important information about the quality of the carbon material produced. First of all, the relatively small defect-induced D band observed at 1364 cm−1 , together with the sharp and distinguished graphite G band at 1590 cm−1 could produce a low ID /IG value of 0.47, revealing the presence of crystalline carbon domains with a low level of defects. Moreover, the 2D band observed at 2723 cm−1 is symmetric and sharp; leading to a high I2D /IG value of 0.52, despite the fact that G band has also a high intensity in this sample. It can be concluded from these results that the carbon product obtained consisted of carbon crystallites of few-layers graphene [24, 53, 54]. The SEM micrographs of the nanostructured carbon extracted from the salt– carbon mixture (Fig. 7.9, down panel) clearly show the presence of graphitic layers, and demonstrate the occurrence and both graphitization and the surface exfoliation of

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Fig. 7.9 Photographs and scanning electron micrographs of the carbon material obtained by heating the mixture of PET and NaCl to 1300 °C: (Upper panel) the mixture of carbon-solidified NaCl after the heat treatment process. (Down panel) SEM micrographs of the nanostructured carbon material extracted from the mixture shown in the upper panel, reproduced from Ref. [24], copyright 2019, with permission from Elsevier

the graphitic structures, leading to the formation of graphene-like nanolayers. These graphene layers had a high C:O atomic ratio of 28.4, measured by the EDX analysis (Fig. 7.10a). The surface properties of this carbon material were investigated through the nitrogen adsorption–desorption analysis, and the result is shown in Fig. 7.10b, presenting a type-II isotherm and a type-H4 hysteresis loop, according to the IUPAC classification [72]. This suggests the presence of macroporous or non-porous surfaces with narrow, slit-like pores with a total pore volume of 0.103 cm3 g−1 , and a BET specific surface area of 522 m2 g−1 .

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Fig. 7.10 a SEM and EDX analysis of the nanostructured carbon produced by the reactive molten salt treatment of PET at 1300 °C in air. The carbon material is characterized by the presence of exfoliated graphene nanolayers with a low amount of oxygen. b Nitrogen adsorption–desorption isotherms of the nanostructured carbon, reproduced from Ref. [24], copyright 2019, with permission from Elsevier

The graphitized nature of the nanostructured carbon could be further explored by TEM microscopy observations, such as shown in Fig. 6.11. The upper left panel in Fig. 7.11 shows a low magnification TEM micrograph, exhibiting the hierarchical morphology of the carbon material, which consists of a mixture of intergraded and fragmented nanosheets. The upper right panel in Fig. 7.11 shows a high magnification image taken on the fragmented nanosheets, revealing the crystalline fringes of these nanostructures. As it can be observed in the inset of Fig. 7.11, upper-right panel, the fast Fourier transform (FFT) analysis recorded on carbon fragments shows a halo ring indicating an interplanar spacing of 3.5 Å, which corresponds to the (002) crystalline planes of hexagonal carbon. The TEM observations confirm the nanocrystalline nature of the carbon material produced by the reactive molten salt treatment of PET. This nanostructured carbon has a remarkably high level of crystallinity, as can be realized from the high-resolution TEM (HRTEM) micrographs shown in Fig. 7.11, down panels. The FFT pattern recorded on a highly crystalline nanosheet, (down-left panel in Fig. 7.11), exhibits spots corresponding to the graphitic (002) planes. The crystalline nanosheets produced are very thin, typically less than 10 nm. Two nanosheets with a thickness of 5.6 and 8.5 nm are identified in the micrograph shown in the down-right panel. Further, TEM observations revealed that the majority of carbon sheets consisted of 4–20 layers [24]. These results confirm that the reactive molten salt treatment of PET leads to the formation of a nanostructured carbon, which consists of crystalline graphitic nanosheets and sheet fragments with a high surface area and a thickness of less than 10 nm.

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Fig. 7.11 TEM micrographs of the nanostructured carbon produced by the reactive molten salt treatment of PET at 1300 °C. The insets in the micrographs are the FFT analyses performed on areas indicated by black rectangles in the corresponding micrographs, reproduced from Ref. [24], copyright 2019, with permission from Elsevier

7.5 Electrical and Electrochemical Characterization of Nanostructured Carbon Materials Together with crystallinity and surface area, the electrical conductivity of carbon materials is among the most important parameters determining the material’s performance in practical applications in which the electrical conductivity plays a critical role. These applications shall include but not limited to supercapacitors [73], electromagnetic shielding [74], catalysts [75] and metal-ion batteries [76]. Generally, in carbon materials, the electrical conductivity decreases with the increase in surface area [73]. This is because the increase in the surface area is usually accompanied by the development of structural defects which inevitably distort the

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intrinsic sp2 structure of graphitized carbons, decreasing its overall electrical conductivity. The electrical conductivity of the nanostructured carbon produced by the reactive molten salt treatment of PET at 1300 °C was evaluated at various compression pressures using a four-probe method. The method is based on the DC voltage–current measurements conducted on carbon powders compressed into a cylindrical form. A representation photo of the experimental setup used for the electrical conductivity measurements is shown in Fig. 7.12. The measurement method has been described in detail elsewhere [77]. The resistivity (ρ, μm) of nanostructured carbon could be calculated by recording the applied voltage (V, mV) and the current response (I, A), as well as the crosssection area (S, mm2 ) and the height (H, mm) of the carbon cylinder compressed under the applied pressure, according to the following equation: ρ = (S · V)/(I · H)

(7.1)

For the measurement, 0.5 g nanostructured carbon was compressed using the setup shown in Fig. 7.12, and the current–voltage response at different pressures, in the range of 0.01–6.13 MPa, was measured. The results, shown in Fig. 7.13a, demonstrate a perfect Ohmic response. Moreover, the densities of the compressed

Fig. 7.12 Assembled experimental setup used for evaluation of the electrical properties of the carbon materials. The inset shows various parts of the compression unit, reproduced from Ref. [77], copyright 2019, with permission from Elsevier

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Fig. 7.13 Electrical and electrochemical properties of the nanostructured carbon produced by reactive molten salt treatment of PET at 1300 °C. a The voltage–current relationship and b the density and the electrical conductivity values recorded under various compressive applied loads. c The CV profiles recorded at various scan rates and d the potential-time profiles measured at different current densities for the electrode fabricated using the nanostructured carbon, reproduced from Ref. [24], copyright 2019, with permission from Elsevier

carbon cylinder at various pressures were calculated by measuring the height of the compressed powder. The values of density and electrical conductivity of the nanostructured carbon were plotted as the function of the pressure applied. The results are exhibited in Fig. 7.13b. It can be understood from this figure that the density of the carbon powder under a low pressure of 0.10 MPa is only 0.10 g cm−3 , exhibiting an electrical conductivity of 12.53 S m−1 . The values of both density and the conductivity sharply increased to 0.89 g cm−3 and 1071.24 S m−1 , respectively, by increasing the applied pressure to 4.14 MPa. However, as can be depicted from Fig. 7.13b, the value of density increased to 1.04 g cm−3 by a further increase of the pressure to 5.47 MPa, while the corresponding value of electrical conductivity slightly decreased to 1058.43 S m−1 . Meanwhile, the resistance of the sample indicated by the slope of the I–V curve (Fig. 7.13a) decreased from 8.76 m to 6.79 m by increasing of the applied pressure. The unexpected decrease in the electrical conductivity value occurred by increasing the applied pressure can be explained by the contribution of the reduced height of the compressed powder to the value of electrical conductivity (see Eq. 7.1). The values of density and electrical conductivity eventually increased to 1.06 g cm−3 and 1150.15 S m−1 , respectively, by the increase of the applied pressure to 6.13 MPa. The value of electrical conductivity recorded is impressive.

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The electrochemical behavior of the nanostructured carbon produced by the reactive molten salt treatment of PET was examined using a three-electrode system with 6 M KOH as the electrolyte [24]. The CV profiles were recorded at various scan rates in the range of 5–200 mV s−1 , and the results are shown in Fig. 7.13c. The CV curves exhibit nearly rectangular shapes, indicating that the nanostructured carbon stores the electric charge via the electrochemical double-layer capacitive mechanism, characterized by a reasonably high rate performance with no pseudo-capacitive effect. The high purity of the nanostructured carbon and its high electrical conductivity can also be realized. The potential-time profiles evaluated at various current densities in the range of 0.2–20 A g−1 are shown in Fig. 7.13d. The high reversibility of the electric charge storage can be realized from the quasi-symmetric feature of the charge—discharge profiles shown in this figure [24, 78–80].

7.6 Molten Salt Graphitization of Amorphous Carbons As discussed in the previous sections, the molten salt treatment of waste plastic bottles can be recognized as an effective strategy for large-scale preparation of nanostructured carbon materials with a high electrical conductivity and large specific surface area. In this strategy, the mixture of plastic and NaCl is heated in air to a critical temperature. During the heat treatment process, at about 260 °C, the plastic material melts, and then the melt decomposes at about 470 °C to form carbon particles with an amorphous structure, smooth surfaces, sharp edges and high resistance against thermal oxidation in air. This amorphous carbon material remains stable, without experiencing a considerable oxidation, until the melting point of the salt (~800 °C) is reached. The molten salt formed protects the PET-derived carbon material from oxidation at higher temperatures. Meanwhile, the molten salt causes the graphitization of the amorphous carbon. The signs of graphitization/exfoliation appear at about 1100 °C, preferentially at the edge sites of amorphous carbon particles. The promotion of this process at higher temperatures leads to the formation of exfoliated graphene-like nanosheets with interesting properties such as high crystallinity, electrical conductivity, purity and surface area. This carbon material can be extracted from its mixture with the solidified salt and the salt itself can be recycled, enhancing the economic and environmental sustainability of the process. The capability of molten salts in protecting nanostructured carbon materials against thermal oxidation has been demonstrated [81]. It was reported that multiwalled carbon nanotubes [82] and 3D graphene nanosheets [83, 84] produced by the electrochemical exfoliation of high-purity graphite in molten LiCl, and molten NaCl possess a greater degree of crystallinity in comparison with their mother graphite. The high crystallinity observed was scribed to the healing of the structural defects, including the removal of impurities from the graphite exposed to the molten salt and also the improvement of the graphite lattice stacking order [81, 85]. Altogether, the findings

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demonstrate that molten salts can cause graphitization in amorphous carbon materials produced by the thermal treatment of PET. This observation can be explained based on a combination of effects mentioned above. Investigations have been shown that the presence of oxygen atoms in amorphous carbon materials can be a considerable barrier against their graphitization. Also, it is known that the effective removal of oxygen from carbon materials requires a prolonged heat treatment at very high temperatures [86]. By comparison of the C:O ratio in the amorphous carbon material produced at 620 °C (5.2) with that of the nanostructured carbon material obtained at 1300 °C (28.4), a substantial reduction in the oxygen content of the carbon material upon the molten salt treatment can be realized. This observation can be corresponded to the dissolution of oxygen from the carbon material into the molten salt. This facilitates the graphitization of the amorphous carbon at such low temperatures. It is worth mentioning that generally amorphous carbons obtained by the pyrolysis of polymers are classified as “non-graphitizable” carbons, since it is extremely difficult to convert the disordered structure of such carbon materials into ordered graphitic structures even at temperatures around 3000 °C. This characteristic of non-graphitizable carbons is attributed to the existence of a complicated network of oxygen-containing cross-links between randomly oriented crystallites [87, 88]. The potential of molten salts to eliminate impurities from carbon materials [81] can be employed to reduce the oxygen content of non-graphitizable carbons, facilitating the graphitization of such carbons. Kim et al. [89] heat treated a non-graphitizable carbon derived from phenol resin in molten NaOH for 1 h at 900 °C under N2 . Then, the NaOH content of the resulting mixture was removed by washing the mixture by HCl solution at room temperature. Finally, the graphitization process was performed at 2800 °C. The possible function of the molten NaOH in this process was suggested to be the removal of gases and also reducing alkyl group content of the carbon material. The toxicity of NaOH as well as the acid leaching requirement, however, can limit the application of this approach. Moreover, a very high-temperature treatment was still required in order to achieve the graphitization. To reduce the graphitization temperature, Jin et al. [86, 90] die pressed an amorphous carbon black powder into cylindrical pellets and then consolidated the pellets using porous nickel foams or graphite containers. The consolidated pellets were immersed in molten CaCl2 at 820 °C, and negatively polarized at the potential of −1.7 V, based on an electrochemical deoxidation method known as the FFC-Cambridge process [91]. This process led to the graphitization of the amorphous carbon black, and this was attributed to the removal of oxygen from the amorphous carbon material under the influence of the cathodic polarization applied, facilitating the graphitization process. As noted in pervious sections of this chapter, the graphitization of the PET-derived amorphous carbons can occur by a simple molten salt heat treatment, without the involvement of high-temperature electrochemical equipment. The increase of the electrical conductivity occurred upon the heat treatment process can be an indication of the occurrence of the graphitization, caused due to the development of the hexagonal stacking structures, which would facilitate the electron transfer [92]. Considering the high diffusion rate of oxygen ions in molten salts

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[69, 70], the possible dissolution of oxygen from the PET-derived amorphous carbon into the molten NaCl may play an important role in promoting the graphitization. The exfoliation of the graphitized carbon in contact with the molten salt [81] will then form a nanostructured carbon containing exfoliated graphene nanosheets. The graphitization process initiates preferentially at the edge sites of the PET-derived amorphous carbon particles due to the higher surface energy and also a greater deal of exposure to the molten salt in these areas. Table 7.1 compares the properties of the nanostructured carbon material produced by the reactive molten salt treatment of PET with those of carbon nanostructures produced by a selected number of the state of art methods reported in the literature. These methods can be divided into two groups, depending on whether graphite or nongraphitic carbon materials have been employed as the carbon source. Methods that employ graphite as the carbon precursor include the chemical oxidation/exfoliation of graphite followed by the reduction process [93–100], as well as the liquid phase [101, 102] and molten salt exfoliation [83, 84] of graphite. Graphene-based materials have been produced using natural and synthetic graphite as the carbon source by chemical oxidation/exfoliation and the subsequent reduction of graphene oxide (GO). This is the most used method in the literature for the preparation of atomically thin sheets of carbon, in which graphite oxide is first produced by the chemical oxidation of graphite using strong oxidizing agents. The graphite oxide formed can then be exfoliated to produce GO. It should be noticed that GO is an insulating material, since it is covalently decorated with oxygen-containing functional groups formed due to the extensive use of strong oxidizing chemicals [103]. GO materials produced by this approach can be activated [104] to provide a high specific surface area, which can be as high as 3100 m2 g−1 . However, the electrical conductivity of GO is rather poor, typically from values near zero to less than 250 S m−1 [99, 104]. The reduction of GO by strong reducing agents such as hydrazine can reduce the oxygen content of the material, leading to the formation of reduced graphene oxide (rGO) with an enhanced electrical conductivity [100]. However, the improvement of the electrical conductivity of rGO brought about by the reduction process is limited, due to the substantial structural damages to carbon layers caused by both the oxidation and reduction processes. Therefore, the electrical conductivity obtained for rGO does not exceed values such as about 34 [93], 700 [94] and 1000 [98] S m−1 . By applying more extensive treatments [96] or adding highly conductive materials, such as Ag [98], a further improvement in the value of electrical conductivity, in the range of 3000–10,000 S m−1 , can be achieved. Graphite can also be exfoliated by non-oxidizing techniques, such as liquid phase [101, 102] or molten salt electrochemical exfoliation [83, 84], and the resulting graphene materials have higher values of electrical conductivity. Nevertheless, the high conductivity of the graphene materials produced by the use of graphite precursors is not a big surprise, due to the high conductivity of the graphite raw material itself. For example, the electrical conductivity (and the surface area) of natural and synthetic graphite precursors were measured to be 2.3 × 104 S m−1 (7.9 m2 g−1 ) and 1.2 × 104 S m−1 (6 m2 g−1 ), respectively [94].

KOH

GO

Polymeric molecular framework (C6 H7 N, C6 H18 O24 P6 )

Resorcinol (C6 H6 O2 )

Porous carbon

Porous graphitic carbon

Carbon xerogel

Carbon xerogel

NaCl, H2

Graphene nanosheets

NaOH, GO dopant

H2 CO, Na2 CO3 , GO scaffold

H8 N2 O8 S2 , KOH, N2

Na4P2 O7 · 10H2 O, C3 H8 O

Few-layer graphene

NaCl, Na2 O2 , HNO3 , HCl, H6 N2 O, H2 SO4 , KMnO4 , H2 O2 , (CH3 )2 NC(O)H H2 SO4 , K2 S2 O8 , P2 O5 , H2 O2 [111, 112] H2 SO4 , KMnO4 , NaNO3 , HCl

Graphite

Reduced graphene oxide

Raw materials

Graphene oxide

Source of carbon

Product

12 GPa and T > 2000 °C [16]. Graphite can alternatively be dissolved in a catalyst from which the solute carbon atoms reprecipitate as diamond at slightly less severe conditions. Various catalysts such as nickel, cobalt, iron [17], germanium [18], iron nitride [19], phosphorus [20] and sulfur [21] have been used in the HPHT synthesis of diamond crystals. By using a catalyst, the diamond formation was observed at P > 5 Pa and T > 1500 °C. During the heating at HPHT conditions, the metal catalyst is melted to form a shell covering each diamond nucleus. Then, the carbon from the graphite diffuses through the metallic melt to deposit itself onto the surface of the diamond nucleus, and the diamond particle grows. The slowest step in the growth of diamond is the diffusion of carbon through the metallic melt [22, 23]. The synthesis of diamond particles is thus a diffusion-controlled process. Nonmetallic catalysts such as inorganic salts have also been used as solvent– catalysts in the HPHT diamond growth process, although higher P–T conditions are required as well as longer reaction times [3–7]. Using alkali carbonates, it was found that the yield was related to the cation radius in the sequence Li2 CO3 > Na2 CO3 > K2 CO3 > Cs2 CO3 [4, 24]. Furthermore, it is known that carbonates are found as inclusions in natural diamonds [8, 9] and, also, diamonds occur in carbonate-bearing and carbonate-rich rocks [10]. At the pressure of 7 Gpa and temperature of 1700 °C, Pal’yanov found that the required time for diamond to nucleate and grow in a splitsphere-type high-pressure cell (Fig. 8.2a) was 2 h [4]. For this, high-purity graphite (99.99%) was used as the carbon source, and the nucleation of diamond was realized at the interface between the graphite and carbonate melt. Diamond crystals were separated from graphite by a carbonate film, demonstrating the catalytic activity of the carbonate molten salts. The diamond growth on seeds was also performed through the carbonate film. The diamond crystallization scheme is exhibited in Fig. 8.2b. The diamond crystals formed in Li2 CO3 –C and Cs2 CO3 –C systems are shown in Fig. 8.2c and d, respectively, exhibiting octahedral, cubic and trapezohedral shaped crystals. The diamond nucleation on the graphite, diamond seeds and the colder part of the Pt capsule was found to take place subsequently.

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Fig. 8.2 a HPHT setup used for the diamond nucleation and growth using molten carbonate catalysts. b The capsule after experiments. Scanning electron micrographs of diamond crystals synthesized in c Li2 CO3 –C; d Cs2 CO3 –C at P = 7 Gpa and T = 1700 °C, reproduced from Ref. [4], copyright 2019, with permission from Elsevier

Nanostructured diamond particles have been known since the 1960s, and produced by the shock wave compression of graphite and carbon black mixed with a catalyst [12]. An alternative method is to use a mixture of carbon and high-energy explosives, or to utilize carbon-containing explosives. These diamond nanomaterials are known as detonation nanodiamonds (DNDs) [13]. Other methods to produce nanodiamonds have used microwave plasma torches [14] and HPHT conditions [25]. All the methods outlined above require extreme conditions and are poorly suited for mass production, which is unfortunate as nanodiamonds have remarkable properties which could find myriad applications in various fields such as biomedicine and nanocomposites. Also, nanodiamonds can be used as seeding materials for the growth of diamond films.

8.2 Conversion of Carbon Nanostructures into Nanodiamonds Theoretical analyses have shown that sp3 diamond nucleation from sp2 carbon is preferable inside a carbon nanotube (CNT) or nanoparticle due to the effect of the

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surface tension brought about by the nanometer-sized curvature of carbon nanomaterials [11]. Furthermore, sp3 bonds have been observed in used graphitic anodes retrieved from lithium batteries in which lithium has been inserted into and extracted from the graphite over many cycles [26]. There have, therefore, been many attempts to convert CNTs into diamond using laser irradiation, shock waves, spark plasma sintering and radio frequency hydrogen plasma techniques [27–35]. Some success was achieved at 4.5 GPa and 1300 °C with a Ni–Mn–Co catalyst where it was observed that the CNTs used (Fig. 8.3a) first transformed into quasi-spherical onion-like particles (Fig. 8.3b). Diamond crystals (Fig. 8.3c) could then be nucleated from the onion-like particles with the assistance of the catalyst. Similar results were obtained with a Fe–Ni catalyst. Yang et al. [36] ultrasonically scratched polished P-type (100)-oriented silicon wafers in a solution containing diamond powder, and coated its surface with MWCNTs (Fig. 8.3d). Then, the sample was treated in a pure hydrogen plasma in a MPECVD reactor (Fig. 8.3e) under pure hydrogen and the microwave power of 1 kW. The sample was plasma-heated during the experiment at 520 °C. The formation of nanodiamond phase in the treated sample was confirmed by a combination of SEM and Raman analysis (Fig. 8.3f), where the presence of the Raman characteristic

Fig. 8.3 Transformation of carbon nanotubes to diamond. a TEM micrograph of initial CNTs. b High-resolution TEM micrograph of CNTs treated at P = 4.5 GPa and T = 300 °C, showing the presence of nested onion-like graphitic structure, as an intermediate product. c SEM micrograph of diamond crystals as the final product under the HPHT condition, reproduced from Ref. [30], copyright 2019, with permission from Elsevier; d TEM micrograph of initial MWCNTs, e schematic representation of MPECVD reactor used for the conversion of MWCNTs to nanodiamonds by the plasma treatment under pure hydrogen, and f SEM micrograph of the sample treated at 1000 W for 5 h. The inset is the Raman spectrum of the newly formed diamond particles, reproduced from Ref. [36], copyright 2019, with permission from Elsevier

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peak was confirmed at 1332 cm−1 . A solid–gas–solid mechanism was suggested to be responsible for the transformation of CNTs to nanodiamonds [36]. It should be mentioned that CNTs used in these investigations were produced by chemical vapor deposition (CVD) with the aid of catalysts. Another route of CNT and nanoparticle synthesis uses the intercalation of lithium from molten lithium chloride into graphite by electrolysis, as discussed in previous chapters. The mechanism by which this occurs is that the lithium-ions discharge on the cathode and pass into the graphite between the layers of graphite/graphene under the influence of the cathodic potential. The electrolysis reaction can be expressed as: 2Li+ + Cl− = 2Li(at the cathode) + Cl2(at the ande)

◦

G800 ◦ C = 650.8 kJ

(8.1)

Although the diameter of the lithium atoms is similar to that of interlamellar spacing in graphite, there is sufficient stress to extrude sheets of graphite into the melt where they roll up to minimize the surface area exposed to the salt. The formation of either CNTs or nanoparticles depends upon the temperature and the crystallite size of the graphite which become detached from the graphite surface and accumulate in the molten salt bath from which separation can be achieved. The presence of moisture in the atmosphere of the reactor leads to the formation of Li2 O with a solubility of more than 11 mol% in molten LiCl, according to the reaction (8.2): ◦

H2 O + 2LiCl = 2HCl + Li2 O G800 ◦ C = 182 kJ

(8.2)

Although the Gibbs free energy of reaction (8.2) is positive, yet the reaction can proceed at a finite rate as a result of dissolution of products in the molten salt. Oxygen anions formed in the molten LiCl can be oxidized on the graphite anode to produce CO2 , which subsequently reacts with the Li2 O dissolved in the molten salt to form Li2 CO3 : ◦

CO2 + Li2 O = Li2 CO3 G800 ◦ C = −66.0 kJ

(8.3)

The X-ray diffraction pattern of the post-electrolysis material taken before washing treatment (Fig. 8.4a) shows the presence of LiCl · H2 O, Li2 CO3 and carbon in graphite structure. The presence of LiCl · H2 O can be attributed to the salt which covers the carbon product. It is known that LiCl crystals easily absorb water from atmosphere to form LiCl · H2 O. In addition to the reaction (8.3), the formation of Li2 CO3 may also be related to the reaction of intercalated lithium with carbon in the presence of oxygen donor materials like impurities and binders available in the graphite cathode. The XRD analysis of the carbon product obtained after washing and drying treatments is also shown in Fig. 8.4a. It demonstrates the effective removal of the lithium chloride-based salt from the carbon product by the water washing treatment. However, a considerable amount of Li2 CO3 is left after the treatment. It can be attributed to the solubility of Li2 CO3 and LiCl in water, which is about 13 and 832 g L−1 at 20 °C, respectively. The unknown peaks in the XRD pattern may be attributed to the

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Fig. 8.4 Characterization of CNTs and nanoparticles produced by the molten salt method. a XRD analysis of the post-electrolysis material taken before washing treatment as well as the carbon product obtained after the washing and drying treatment; b SEM microstructure of the electrolytic carbon product, and c HRTEM micrograph of the carbon product showing the presence of single crystals surrounded by graphite layers. Some of the crystals are indicated by arrows, reproduced from Ref. [39], copyright 2019, with permission from RSC Publishing; d a selected area diffraction pattern and the corresponding dark-field micrograph obtained from the diffraction spots. The spots and rings are indexed as the diffraction signals of Li2 CO3 single crystals and crystalline graphitic carbon, respectively; e HRTEM micrograph of the wall of a MWCNT. The arrows point to the Li2 CO3 nanocrystals located between the graphitic layers, reproduced from Refs. [37, 38], copyright 2019, with permission from Elsevier

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lithium-containing materials which are commonly found in electrolytically produced carbon products. The presence of a sharp characteristic (002) peak in the XRD pattern shown in Fig. 8.4a demonstrates that the carbon content of the electrolytic product is mostly graphite. The crystallite size of the carbon material along the c-axis, L c , was calculated from the XRD information of the hexagonal (002) peak observed at 2θ = 26.50° according to Scherer’s approach and found to be 41 nm. The result shows the formation of nanocrystalline graphite. The SEM microstructure of the electrochemically produced carbon materials is shown in Fig. 8.4b. The inset of the figure shows a higher magnified image from a selected region. The material comprised of tubes with diameters in the wide range of 10–500 nm and spherical nanoparticles, typically less than 100 nm. Also, some graphite sheets can be seen in the microstructure. Figures 8.4c–e show the HRTEM morphology of the carbon product. These micrographs demonstrate the presence of encapsulated 1–3 nm Li2 CO3 nanosingle crystals with shells of carbon around them. These results suggest that the carbon product includes Li2 CO3 nanocrystals surrounded by graphite layers. Thermal stability of these carbon nanostructures was compared with that of CVD carbon nanotubes. DSC and TG analyses of the electrolytic carbon product produced in molten salt and CVD-MWCNTs conducted at the rate of 20 °C min−1 under ambient airflow of 100 mL min−1 are shown in Fig. 8.5. The morphological characteristics

Fig. 8.5 DSC (blue lines) and TG (red lines) of the electrolytic carbon produced in molten LiCl and CVD-MWCNTs in the range of 25–800 °C. Morphology of these nanostructured materials is also exhibited. In the electrolytic carbon, Li2 CO3 nanocrystals are encapsulated in carbon shells, reproduced from Ref. [39], copyright 2019, with permission from RSC Publishing

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of these carbon nanostructures are also shown in Fig. 8.5. The DSC thermogram of the electrolytic carbon indicates the occurrence of two exothermic events with peaks’ maximum temperature at 511 and 637 °C. The electrolytic carbon was heated to the temperatures just before and after the first exothermic peak and then cooled down to the room temperature. Heating of the electrolytic carbon up to the temperatures below the first peak (e.g., 400 °C) did not affect the morphology of the material, and CNTs and nanoparticles could still be clearly seen in the SEM image of the heated samples (Fig. 8.6a). The sample heated to 570 °C, which is beyond the first exothermic peak, experienced a massive weight loss. Moreover, no carbon nanomaterial could be identified in the SEM image of the heated sample (Fig. 8.6b). It indicates that the first and the second exothermic peaks in DSC curve of the electrolytic carbon shown in Fig. 8.5 relate to the oxidation of the nanometer-sized and the micrometer-sized fractions of the nanostructured carbon, respectively. Figure 8.5 shows the DSC and TG analyses of the CVD-MWCNTs heated at the rate of 20 °C min−1 under ambient airflow of 100 mL min−1 . The DSC curve exhibits one small exothermic peak at 454 °C, which is related to the oxidation of the amorphous fraction of the sample, followed by a large exothermic peak at 650 °C corresponding to the oxidation of the whole remaining material. It is known that the oxidation temperature of carbon nanomaterials depends on their degree of graphitization; the greater the crystallinity, the higher the oxidation temperature [40]. However, despite its higher degree of crystallinity, the electrolytic carbon exhibits a lower oxidation temperature than the CVD-MWCNTs. It can be explained by the presence of Li2 CO3 nanocrystals in the nanostructure of the electrolytic carbon, which can catalyze the oxidation of the material at lower temperatures [38]. It is known that molten carbonates of alkali and alkaline metals are able to act as a solvent–catalyst for diamond formation from graphite at typical HPHT conditions of 5–8 GPa and 1600–2150 °C [4]. Subsequently, several other inorganic melts, including metal halides such as LiCl and multicomponent systems, have also been shown to catalyze the conversion of graphite to diamond at very high temperatures and pressures. The presence of nanometer-sized catalyst crystals embedded in the graphitic nanostructure of the electrolytic carbon materials makes the molten salt-produced material an attractive precursor for the production of diamond. According to Fig. 8.5, the oxidation of CNTs and nanoparticles in the electrolytic carbon material happens at the temperature window of 420–550 °C. It was observed that if the electrolytic carbon is heated to specific temperatures within this oxidation window, the carbon nanomaterials are transformed into nanodiamonds. For example, the electrolytic carbon was heated to 515 °C and then cooled down to the room temperature. The sample, after the heat treatment process, contained octagonal nanodiamonds from 5 nm to 1 μm, as shown in Fig. 8.6c, d [39]. Figure 8.6e shows a TEM micrograph of the diamond product as well as a selected area diffraction pattern, in which the spots corresponded to the (111) plane of diamond. It was proposed that by producing carbon nanomaterials electrochemically by intercalation, lithium salts are incorporated between the graphene sheets and these are able to catalyze the transformation to diamond by simply heating of the

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Fig. 8.6 SEM morphology of the electrolytic material after heated to a 400 and b 570 °C, respectively, under ambient airflow of 100 mL min−1 . c and d SEM micrographs of the diamond crystals produced by heating of the electrolytic carbon to 515 °C in air, showing nano- and microsized diamonds. e TEM micrograph of the diamond crystals, and the electron diffraction pattern taken from a selected area of the TEM micrograph originates from the (111) plane of diamond in the cubic structure, reproduced from Ref. [39], copyright 2019, with permission from RSC Publishing

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material in air at atmospheric pressure. It should be noted that although the CNTs and nanoparticles are ignited at about 420 °C, registered in the DSC curve of Fig. 8.5, the true local temperature of the carbon nanomaterials during the oxidation is likely to exceed 4000 °C. At this temperature, the Li2 CO3 encapsulated in graphitic shells is also likely to produce a considerable amount of internal pressure. This process should be much cheaper than the HPHT approach and should be scalable to produce a more economical product. The applied voltages and current densities and the rate of production of carbon nanotubes in molten LiCl [41] are very similar to that of aluminum in the Hall–Heroult cells which produce 45 M tonnes per annum worldwide. Upscaling should not be a problem. The cost of aluminum is around $2/kg, while the present price of nanodiamonds is about $3/g. Using this novel technique should result in a substantial drop in the cost of nanodiamonds and a widening of the applications [39].

8.3 Conversion of CO2 into Diamond Nanocrystals Carbon is an interesting element due to its existence in more than ten million compounds [42] such as hydrocarbons, carbonates and CO2 as well as various elemental allotropes such as amorphous carbon, graphite and diamond. Among these, CO2 is considered to be a major greenhouse gas, contributing to global warming and climate change [43], and hence reduction in CO2 emissions is currently a global effort [44–46]. Along with this fact that the global generation of CO2 may not be significantly suppressed in the near future, the capture of CO2 has increasingly been studied, exploring the absorption of CO2 by various materials such as Zr(OH)4 [47], CaO [48], TiO2 [49], SiO2 [50], Fe2 O3 [51], carbon nanostructures [52], activated carbon [53], activated biocarbons [54], organic materials [55] and ionic liquids [56, 57]. Moreover, the conversion of CO2 emissions produced into useful materials such as hydrocarbon fuels [58], CO [59], O2 [60], acids [61, 62] and other chemicals [63] such as dimethyl carbonate [64] has been considered as a viable approach. As discussed, diamond is considered as one of the most valuable and rather expensive materials with remarkable properties including the highest known hardness, thermal conductivity and chemical resistance [65]. Mentioned in the previous section, the current technologies of diamond nucleation and growth, which are based on applying huge external pressures and heats, are very complicated and expensive requiring the use of enormously large hot presses weighing hundreds of tons [19]. Nanodiamonds can also be fabricated by massive detonation of carbonaceous explosives in closed metallic chambers, in which the pressure and temperature may rise instantaneously to more than 25 GPa and 3500 °C, respectively [66]. It should be noted that in order to accommodate an explosion arisen from only 20 kg of such explosives, a thick metallic chambers weighing over 100 tons may be required [67], indicating a high level of complexity, particularly at larger scales. The other methods of forming diamonds,

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based on the laser irradiation, electron irradiation or hydrogen plasma treating of carbon nanostructures, although interesting, are microscale techniques with very limited capability at larger scales. It is worth mentioning that with increasing shortage of natural diamonds, there is an increasing demand for simpler and more economical bulk diamond production methods. As discussed in 4.6, carbon dioxide can be converted into Li2 CO3 nanosingle crystals with particle sizes below 30 nm using a reactive LiCl–Li2 O molten salt method. The Li2 CO3 nanocrystals can then in situ encapsulated into carbon cages. The carbon-encapsulated Li2 CO3 nanoparticles could act as self-pressurized diamond nanocatalysts upon heating under ambient external pressure to nucleate diamond crystallites within encapsulated Li2 CO3 crystals. The diamond crystallites could grow to micrometer-sized octahedral crystals by further heating. It is suggested that the very high pressure required for the catalytic phase transition of graphite to diamond can be created in carbon-encapsulated Li2 CO3 nanocrystals. These “highpressure nanovessels” enable the nucleation of diamond crystallites upon heating in atmospheric pressure of air, representing a much less severe nucleation condition than those used in conventional technologies. Interestingly, these high-pressure nanovessels were fabricated in the consumption of CO2 , which is regarded as the main source of global warming and ocean acidification, with catastrophic consequences [68]. The experimental reactor used for fabrication of Li2 CO3 nanocrystals and carbonencapsulated Li2 CO3 nanoparticles is presented in Fig. 4.15a. The reactions taking place in the reactor can be explained as follows. A moist CO2 gas flow is directed into the molten LiCl–2 wt% Li2 O at 800 °C, resulting in triggering the reaction (8.3) and hence the formation of lithium carbonate. CO2(taken from atmosphere) + Li2 O(from the melt) = Li2 CO3 G◦ (at 800 ◦ C) = −66 kJ (8.3) Apart from this, the hydrolysis reaction occurs between water from the moist gas and molten LiCl, described by the reaction (8.4).  ◦ ◦ H2 O + LiCl = HCl + LiOH G at 800 C = 86 kJ

(8.4)

LiOH produced is then decomposed to form Li2 O. It should be mentioned that LiCl shows no considerable affinity for hydrolysis in the solid form due to the energy barrier involved. However, the hydrolysis of LiCl becomes much more significant in the molten state, because of the fact that the hydrolysis products (HCl [69–84] and Li2 O [75]) are readily soluble in molten LiCl. As a consequence, the hydrolysis reaction can proceed, although its standard Gibbs free energy is positive. Since LiCl is highly hygroscopic, the formation of HCl and Li2 O during the practical use of molten LiCl is common [76]. This reaction is likely to contribute to the diverse detrimental or beneficial effects associated with the corrosion of metallic equipment [77] or fabrication of high-value materials such as LiNbO3 [78]. Lithium carbonate nanocrystals formed in the process explained above were encapsulated in graphitic layers, by using a graphite cathode as the carbon source

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(Sect. 4.6). The heat treatment of this nanostructure led to the formation of nanodiamonds. The process has been summarized in Fig. 8.7. The cathodic exfoliation of a graphite electrode (Fig. 8.7a) under a moist CO2 flow led to the formation of a nanostructured hybrid material containing a high fraction of carbon-encapsulated Li2 CO3 (Fig. 8.7b). This hybrid material was heated at a heating rate of 40 °C min−1 in an airflow rate of 100 mL min−1 to 440 °C, and heated material was rapidly cooled down to the room temperature by a flow of air providing a cooling rate of

Fig. 8.7 Molten salt conversion of CO2 into diamond crystals. a SEM micrograph of the graphite cathode used. This material was consumed together with CO2 in the molten salt process to produce a hybrid nanostructure containing mostly nanoparticles of less than 50 nm as can be seen in (b-left panel). (b-right panel) A HRTEM micrograph from the hybrid nanostructure, in which Li2 CO3 nanoparticles are encapsulated into graphitic layers. These nanostructures can act as high-pressure nanovessels during the air heat treatment of the material. (c-left panel) A low magnification SEM image from the heat-treated sample (to 530 °C), indicating the presence of diamond crystals in large carbon particles. (c-middle panel) A higher magnification SEM image showing a diamond crystal. (c-right panel) A HRTEM image from the heat-treated sample, exhibiting a number of diamond nanocrystals. The inset is a fast Fourier transformation of the area indicated by rectangle on the HRTEM image, exhibiting the (111) crystal planes of diamond with a lattice spacing of 0.2 nm. d The schematic summary of the process, in which CO2 , graphite, water and LiCl are consumed and the product contains diamond crystals, reproduced from Ref. [68], copyright 2019, with permission from Elsevier

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about 50 °C min−1 . High-resolution electron micrographs of the product (Fig. 8.7c) revealed the formation of diamond crystals. As mentioned before, graphite can be transformed into diamond at very high temperatures and pressures according to the carbon phase diagram. The addition of specific transitional metals or their alloys can reduce the critical temperature and pressure required to achieve the diamond nucleation in industry. It is interesting to consider that natural diamonds are found in carbonaterich rocks or contain carbonate inclusions [85, 86]. Carbonates can catalyze the oxidation of carbon. On the other hand, graphite–carbonate systems have been studied in order to simulate the process of natural diamond formation, and their solvent–catalytic properties have been demonstrated in a number of publications [4, 87–99]. Palyanov et al. [4] treated capsules containing graphite pieces and Li2 CO3 at 7 GPa and 1700 °C, at which Li2 CO3 is in a molten state, for 2 h. After cooling down the system, it was confirmed that diamond nucleation has occurred at the interface between graphite and carbonate melt. It was further observed that diamond crystals had octahedral, cubic or trapezohedral crystalline forms, separated from graphite by carbonate films [4]. Despite the industrial and scientific importance, the mechanisms involved in the HPHT catalytic transformation of graphitic materials into diamond are still not quite clear, although in the case of metallic catalysts, it is believed to be based on the dissolution of carbon from graphite into the molten metal catalyst used [1, 100]. On the other hand, the detailed mechanisms involved in the diamond formation in carbonate–graphite systems are not available in the literature, which is mostly due to the lack of sufficient knowledge on basic issues such as behavior of alkaline carbonates at high pressures and temperatures [101], solubility and diffusion rate of carbon into carbonates [95] and possible decomposition, carbothermic decomposition [102] or reduction of carbonates into graphite or diamond [103, 104]. Li2 CO3 crystals of less than 30 nm encapsulated in graphitic shells acted as C-catalyst cells, and the diamond formation was realized within the Li2 CO3 phase. Moreover, octahedral diamond crystals could occasionally be detected in the product. The overall observations are in line with those reported in the literature concerning the diamond formation in C–Li2 CO3 system [101] with the exception of the core– shell nanospherical geometry of the cells and the fact that the diamond formation was achieved at external ambient pressure and a temperature of less than 600 °C. The formation of diamond crystallites from the core–shell nanostructures formed in molten salts can be explained based on the carbon-encapsulated morphology of the Li2 CO3 nanocrystals. This morphology facilitates the diffusion of carbon from the encapsulating graphitic layers into Li2 CO3 , leading to the saturation of carbon atoms in Li2 CO3 . This saturation was likely to occur during the processing of the material in molten LiCl [68]. In addition to the core–shell nanostructure, the presence of LiCl may play an additional catalytic role in the overall process, particularly with respect to the fact that alkali metal chlorides can be found in inclusions coated on natural diamonds [105–107], suggesting the possible involvement of alkali metal chlorides in the natural diamond formation.

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Upon heating of the nanostructured hybrid material in air, first, outer carbon layers ignite at about 400 °C, generating a considerable amount of heat. The thermal energy released from the partial oxidation process might be sufficient to cause melting and evaporation of the encapsulated Li2 CO3 crystals in nanoscale areas. In this situation, the carbon layers around Li2 CO3 nanoparticles could act as high-pressure nanovessels, providing sufficient energy for the occurrence of diamond nucleation within the saturated lithium carbonate. It should be noticed that the air oxidation of nanodiamonds with grain sizes of about 5 nm, synthesized by detonation, occurs at temperatures more than 550 °C [108]. The oxidation resistance of diamond increases with its particle size, so that micrometer-sized diamond produced by HPHT technique is oxidized at temperatures not less than 800 °C [109]. The oxidation temperature of diamond films was measured to be more than 950 °C [110]. In the core-shell nanostructure observed in Fig. 8.7, the diamond nucleation and growth was observed at the external temperature of 530 °C. At this condition, carbon nanomaterials are not thermally stable, but diamond crystallites are relatively more stable. The rapid growth of diamond crystallites observed in this study, therefore, can be attributed to the rapid diffusion of carbon atoms from unstable graphitic structures into relatively more stable diamond crystals. Figure 8.8 shows a HRTEM micrograph of the C-encapsulated Li2 CO3 nanovessels heated to 440 °C. Two core-shell nanoparticles can be recognized in the upperpanel of Fig. 8.8. The lower-panel in Fig. 8.8 exhibits a higher magnification image from one of the core-shell nanostructures observed in the upper-panel. Fast Fourier transformation patterns recorded on the encapsulated particles demonstrated that the lower nanoparticle, observed in the upper-panel, is crystalline Li2 CO3 . On the other hand, the upper core-shell particle, observed in the upper-panel of Fig. 8.8, was found to compose of Li2 CO3 and diamond crystallites. The lower panel in Fig. 8.8 shows the latter in a higher magnification. It is an interesting micrograph which shows a diamond crystallite has nucleated within Li2 CO3 nanoparticles encapsulated in graphitic carbon shells [68]. As discussed in this section, the reactive interaction of CO2 gas with molten LiCl–2 wt% Li2 O led to the formation of Li2 CO3 dissolved in the molten salt. The dissolved Li2 CO3 is crystallized upon cooling of the molten salt to form Li2 CO3 nanosingle crystals. It was also mentioned that graphite electrodes can be electrolytically exfoliated in the same system to produce graphene nanosheets. The graphene nanosheets produced had the capability of encapsulating the Li2 CO3 crystals. Overall, the process described may be used for large-scale and low-cost production of Li2 CO3 and diamond crystals using CO2 gas. However, there are few relevant issues that need further clarification: The experimental setup shown in Fig. 4.15a exhibits a batch-type process for the fabrication of Li2 CO3 and C-encapsulated Li2 CO3 nanoparticles, in which incoming CO2 gas was replaced by Ar after 60 min of processing, in order to characterize the materials produced. In a continuous process, the CO2 gas and the mixture of graphene/Li2 CO3 saturated salt can be continuously introduced into and removed from the system, respectively. As the other issue, the

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Fig. 8.8 HRTEM micrograph of the C-encapsulated Li2 CO3 nanovessels heated to 440 °C. The fast Fourier transformation patterns recorded on the encapsulated particles demonstrate that the lower nanoparticle, observed in the upper-panel, is crystalline Li2 CO3 , and the upper one composed of Li2 CO3 and diamond. The lower panel shows the nucleation of a diamond crystallite within Li2 CO3 nanoparticles encapsulated in graphitic carbon shells, reproduced from Ref. [68], copyright 2019, with permission from Elsevier

preparation of diamond proposed is based on the partial oxidation of carbon nanostructures available in the C-encapsulated Li2 CO3 precursor, leading to the nucleation and subsequent growth of diamond crystals. The final product composed of diamond crystals embedded in graphitic nanostructures. This microstructure can be of interest in some potential applications, including as anti-friction particles [111], composite materials with enhanced field emission [112–114], thermophysical [115] and mechanical [116] properties, high-performance coatings [117] and electrode materials for electrochemical destruction of organic pollutants [118, 119]. The isolation of diamond crystals from graphitic nanostructures can also be explored. Hong et al. have recently demonstrated that reactive oxygen species created by plasma jets are able to remove non-diamond carbons, including graphite and amorphous carbon, from nanodiamonds [120]. Selective thermal oxidation, therefore, can be an attractive way to produce pure diamond phase [121], and worth to be explored in future studies.

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