Molten Salts Chemistry and Technology

By Marcelle Gaune-Escard and Geir Martin Haarberg

Written to record and report on recent research progresses in the field of molten salts, Molten Salts Chemistry and Technology focuses on molten salts and ionic liquids for sustainable supply and application of materials. Including coverage of molten salt reactors, electrodeposition, aluminium electrolysis, electrochemistry, and electrowinning, the text provides researchers and postgraduate students with applications include energy conversion (solar cells and fuel cells), heat storage, green solvents, metallurgy, nuclear industry, pharmaceutics and biotechnology.

• Language: English
• Publisher: Wiley
• Release date: May 12, 2014
• ISBN: 9781118448823

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Library of Congress Cataloging-in-Publication Data

Molten salts chemistry and technology / edited by Marcelle Gaune-Escard and Geir Martin Haarberg.

pages cm

Includes index.

ISBN 978-1-118-44873-1 (cloth)

1. Fused salts. I. Gaune-Escard, Marcelle. II. Haarberg, Geir Martin.

TP230.M655 2014

546′.34—dc23

2013035011

A catalogue record for this book is available from the British Library.

ISBN: 9781118448731

List of Contributors

A.V. Abramov, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

H. Akatsuka, Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Japan

N. Akiyama, Central Research Institute of Electric Power Industry, Japan

D. E. Aleksandrov, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

I. M. Astrelin, Faculty of Chemistry and Technology, Kyiv Polytechnical Institute, National Technical University, Ukraine

O.B. Babushkina, Centre of Electrochemical Surface Technology, Austria

T. Bauer, Institute of Technical Thermodynamics, German Aerospace Center—DLR, Germany

M. Berkani, Laboratoire de Physico-Chimie des Matériaux et Catalyse, Faculté des Sciences Exactes, Université de Béjaïa, Targa ouzemmour, Algérie

C. Bessada, CNRS, CEMHTI UPR 3079, Univ. Orléans, F-45071 Orléans, France

A.-L. Bieber, Laboratoire de Génie Chimique, CNRS UMR 5503, Université de Toulouse, France

M. Boča, Institute of Inorganic Chemistry, Slovak Academy of Sciences, Slovakia; and Department of Chemistry, Faculty of Natural Sciences, Constantine The Philosopher University, Slovakia

N. P. Brevnova, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

A. Bund, Fachgebiet Elektrochemie und Galvanotechnik, Technische Universitaet Ilmenau, Germany

M. F. Butman, Ivanovo State University of Chemistry and Technology, Russia

L. Cassayre, Laboratoire de Génie Chimique, CNRS UMR 5503, Université de Toulouse, France

P. Chamelot, Laboratoire de Génie Chimique, CNRS UMR 5503, Université de Toulouse, France

M. V. Chernyshov, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

A. V. Chukin, Department of Theoretical Physics and Applied Mathematics, Ural Federal University, Russia

O. Conocar, CEA Marcoule, Nuclear Energy Division, Radiochemistry and Process Department, Laboratoires d'Elaboration des Procédés de Séparation, France

B. Davis, Kingston Process Metallurgy, Inc., Canada

S. Deki, Fuel Cell Nanomaterials Center, University of Yamanashi, Japan

R. F. Descallar-Arriesgado, Faculty of Engineering, Hokkaido University, Japan

V.S. Dolmatov, Kola Science Center RAS, Institute of Chemistry, Russia

N. Douyère, CEA Marcoule, Nuclear Energy Division, Radiochemistry and Process Department, Laboratoires d'Elaboration des Procédés de Séparation, France

P. Fellner, Slovak University of Technology in Bratislava, Slovakia

D. Fray, Department of Materials Science and Metallurgy, University of Cambridge, UK

T. Fujii, Research Reactor Institute, Kyoto University, Japan

T. Fujimori, Graduate School of Energy Science, Kyoto University, Japan

R. Fujita, Power Systems Company, Toshiba Corporation, Japan

K. Fukasawa, Graduate School of Engineering, Kyoto University, Japan

A. Fukunaga, Graduate School of Energy Science, Kyoto University, Japan; and Sumitomo Electric Industries, Ltd., Japan

T. Fukunaga, Research Reactor Institute, Kyoto University, Japan

M. Fukushima, Japan Atomic Energy Agency, Japan

T. Furuta, Permerec Electrode Ltd., Japan

A. Gab, Faculty of Chemistry and Technology, Kyiv Polytechnical Institute, National Technical University, Ukraine

I. Galasiu, Romanian Academy—Institute of Physical Chemistry Ilie Murgulescu, Romania

R. Galasiu, Romanian Academy—Institute of Physical Chemistry Ilie Murgulescu, Romania

B. Gao, School of Materials and Metallurgy, Northeastern University, China

M. Gaune-Escard, Aix-Marseille Université, CNRS IUSTI UMR 7343, Technopole de Château-Gombert, France

M. Gembický, Department of Chemistry, State University of New York, USA

M. Gibilaro, Laboratoire de Génie Chimique, CNRS UMR 5503, Université de Toulouse, France

M. Gobet, CNRS, CEMHTI UPR 3079, Univ. Orléans, F-45071 Orléans, France

A. Gray-Weale, School of Chemistry, University of Melbourne, Australia

T. R. Griffiths, Redston Trevor Consulting Ltd., UK

J. G. Gussone, German Aerospace Center (DLR), Institute of Materials Research, Germany

G. M. Haarberg, Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), Norway

R. Hagiwara, Graduate School of Energy Science, Kyoto University, Japan

J. M. Hausmann, German Aerospace Center (DLR), Institute of Materials Research, Germany

J. Híveš, Slovak University of Technology in Bratislava, Slovakia

M. Hoshi, Department of Metallurgy, Graduate School of Engineering, Tohoku University, Japan

J. Hryn, Argonne National Laboratory, USA

X. Hu, School of Materials and Metallurgy, Northeastern University, China

Y. Iida, Department of Applied Chemistry, Doshisha University, Japan

K. Ikeda, Department of Applied Chemistry, Graduate School of Engineering, Doshisha University, Japan

M. Inaba, Department of Applied Chemistry, Graduate School of Engineering, Doshisha University, Japan; and Department of Molecular Chemistry and Biochemistry, Faculty of Science and Engineering, Doshisha University, Japan

S. Inazawa, Sumitomo Electric Industries, Ltd., Japan

T. Ishibashi, Graduate School of Energy Science, Kyoto University, Japan

Y. Ishigaki, Medical Research Institute, Kanazawa Medical University, Japan

A. Ispas, Fachgebiet Elektrochemie und Galvanotechnik, Technische Universitaet Ilmenau, Germany

Y. Ito, Energy Conversion Research Center, Doshisha University, Japan

K. Itoh, Graduate School of Education, Okayama University, Japan

A. B. Ivanov, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

S. Ivanov, Fachgebiet Elektrochemie und Galvanotechnik, Technische Universitaet Ilmenau, Germany

Y. Iwadate, Graduate School of Engineering, Chiba University, Japan

A. Jacob, Centre for Innovation Competence Virtuhcon, Group Multiphase Systems, TU Bergakademie Freiberg, Germany; and Forschungszentrum Jülich, Germany

K. Jomová, Department of Chemistry, Faculty of Natural Sciences, Constantine The Philosopher University, Slovakia

A. Kajinami, Graduate School of Engineering, Kobe University, Japan

R. V. Kamalov, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

M. Keppert, Department of Materials Engineering and Chemistry, Faculty of Civil Engineering, Czech Technical University, Czech Republic

T. Kikuchi, Faculty of Engineering, Hokkaido University, Japan

K. Kinoshita, Central Research Institute of Electric Power Industry, Japan

S. Kishida, Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Japan

S. Kitawaki, Japan Atomic Energy Agency, Japan

H. Kofuji, Japan Atomic Energy Agency, Japan

T. Koketsu, Graduate School of Energy Science, Kyoto University, Japan

T. Koyama, Central Research Institute of Electric Power Industry, Japan

O. V. Kremenetskaya, Max Planck Institute for Chemical Physics of Solids, Germany

V. G. Kremenetsky, Kola Science Center RAS, Institute of Chemistry, Russia

B. Kubíková, Institute of Inorganic Chemistry, Slovak Academy of Sciences, Slovakia

L. S. Kudin, Ivanovo State University of Chemistry and Technology, Russia

M. Kurata, Central Research Institute of Electric Power Industry, Japan

S. Kuwabata, Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Japan; and CREST, Japan Science and Technology Agency, Japan

S. A. Kuznetsov, Kola Science Center RAS, Institute of Chemistry, Russia

J. Lacquement, CEA Marcoule, Nuclear Energy Division, Radiochemistry and Process Department, Laboratoires d'Elaboration des Procédés de Séparation, France

V. Laging, Department of Materials Science and Engineering, Delft University of Technology, Den Haag, The Netherlands

D. Laing, Institute of Technical Thermodynamics, German Aerospace Center—DLR, Germany

A. Laplace, CEA Marcoule, Nuclear Energy Division, Radiochemistry and Process Department, Laboratoires d'Elaboration des Procédés de Séparation, France

M. Li, Department of Metallurgy, Graduate School of Engineering, Tohoku University, Japan

F. Lisý, Department of Fluorine Chemistry, Nuclear Research Institute Řež, plc, Czech Republic

E.O. Lomako, Centre of Electrochemical Surface Technology, Austria

O.-A. Lorentsen, Primary Metal Technology, Hydro Aluminum, Norway; and Department of Materials Science and Engineering, Norwegian University of Science and Technology, Norway

P. A. Madden, Department of Materials, University of Oxford, UK

L. Maksoud, CNRS, CEMHTI UPR 3079, Univ. Orléans, F-45071 Orléans, France

D. S. Maltsev, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

V. Malyshev, V.I. Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, Ukraine

A. M. Martinez, SINTEF Materials and Chemistry, Norway

P. J. Masset, Institute Branch Sulzbach-Rosenberg, Fraunhofer UMSICHT, Germany

L. Massot, Laboratoire de Génie Chimique, CNRS UMR 5503, Université de Toulouse, France

H. Matsuura, Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Japan

R. Meirbekova, School of Science and Engineering, Reykjavik University, Iceland

E. Mendes, CEA Marcoule, Nuclear Energy Division, Radiochemistry and Process Department, Laboratoires d'Elaboration des Procédés de Séparation, France

T. Michii, Graduate School of Engineering, Chiba University, Japan

M. Miguirditchian, CEA Marcoule, Nuclear Energy Division, Radiochemistry and Process Department, Laboratoires d'Elaboration des Procédés de Séparation, France

M. Misawa, Japan Atomic Energy Agency, Japan

K. Mizuguchi, Power Systems Company, Toshiba Corporation, Japan

M. Mizuhata, Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Japan

E. Mochizuki, Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Japan

J. Moncol, Institute of Inorganic Chemistry, Slovak University of Technology, Slovakia

T. Morishige, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Japan

V. B. Motalov, Ivanovo State University of Chemistry and Technology, Russia

G. Moussaed, CNRS, CEMHTI UPR 3079, Univ. Orléans, F-45071 Orléans, France

T. Murakami, Central Research Institute of Electric Power Industry, Japan

M. Myochin, Japan Atomic Energy Agency, Japan

T. Nagai, Nuclear Fuel Cycle Engineering Laboratory, Japan Atomic Energy Agency, Japan

A. Nakayoshi, Japan Atomic Energy Agency, Japan

N. Nemoto, School of Medicine, Kitasato University, Japan; and CREST, Japan Science and Technology Agency, Japan

A. Nezu, Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Japan

Y. Nishiki, Permerec Electrode Ltd., Japan

T. Nishikiori, R&D Division, I'MSEP Co., Ltd., Japan

Y. Nishki, Permerec Electrode Ltd., Japan

K. Nitta, Sumitomo Electric Industries, Ltd., Japan

T. Nohira, Graduate School of Energy Science, Kyoto University, Japan

M. Numakura, Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Japan

K. Numata, Graduate School of Energy Science, Kyoto University, Japan

T. Ohashi, Department of Chemical Science and Engineering, Kobe University, Japan

T. Ohkubo, Graduate School of Engineering, Chiba University, Japan

N. Ohtori, Graduate School of Science and Technology, Niigata University, Japan

E. Olsen, Norwegian University of Life Sciences, Norway

S. Osaki, Faculty of Engineering, Hokkaido University, Japan

N. Osawa, Department of Applied Chemistry, Graduate School of Engineering, Doshisha University, Japan

K. S. Osen, SINTEF Materials and Chemistry, Norway

L.-E. Owe, Department of Materials Science and Engineering, Norwegian University of Science and Technology, Norway

O. Pauvert, CNRS, CEMHTI UPR 3079, Univ. Orléans, F-45071 Orléans, France; and ITU, Germany

S. Pietrzyk, Faculty of Non-Ferrous Metals, AGH—University of Science and Technology, Poland

I. B. Polovov, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

J. Qu, School of Materials and Metallurgy, Northeastern University, China

B. Qin, Department of Materials Science and Engineering, Norwegian University of Science and Technology, Norway

A. Rakhmatullin, CNRS, CEMHTI UPR 3079, Univ. Orléans, F-45071 Orléans, France

O. I. Rebrin, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

E. V. Rebrov, Laboratory of Chemical Reactor Engineering, Eindhoven University of Technology, the Netherlands

O. Rohr, EADS, Germany

A. L. Rollet, CNRS, CEMHTI UPR 3079, Univ. Orléans, F-45071 Orléans, France and Sorbonne Universités, UPMC, Univ Paris 06, UMR 8234, PHENIX, F-75005 Paris, France

S. Rolseth, SINTEF Materials and Chemistry, Norway

L. Rycerz, Chemical Metallurgy Group, Faculty of Chemistry, Wroclaw University of Technology, Poland

G. Sævarsdottir, School of Science and Engineering, Reykjavik University, Iceland

M. Saito, Department of Molecular Chemistry and Biochemistry, Faculty of Science and Engineering, Doshisha University, Japan

S. Sakai, Sumitomo Electric Industries, Ltd., Japan

Y. Sakamura, Central Research Institute of Electric Power Industry, Japan

M. Salanne, Sorbonne Universités, UPMC, Univ Paris 06, UMR 8234, PHENIX, F-75005 Paris, France

E. Sandnes, Primary Metal Technology, Hydro, Norway

V. Sarou-Kanian, CNRS, CEMHTI UPR 3079, Univ. Orléans, F-45071 Orléans, France

N. Sato, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan

Y. Sato, Department of Metallurgy, Graduate School of Engineering, Tohoku University, Japan

J.C. Schouten, Laboratory of Chemical Reactor Engineering, Eindhoven University of Technology, the Netherlands

C.A.C. Sequeira, Materials Electrochemistry Group, ICEMS, Instituto Superior Técnico, Technical University of Lisbon, Portugal

D. N. Sergeev, Ivanovo State University of Chemistry and Technology, Russia

D. Shakhnin, V.I. Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, Ukraine

Z. Shi, School of Materials and Metallurgy, Northeastern University, China

Y. Shimohara, Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Japan

T. Shiono, Department of Applied Chemistry, Doshisha University, Japan

O. Shirai, Graduate School of Agriculture, Kyoto University, Japan

A. K. Shtolts, Department of Theoretical Physics and Applied Mathematics, Ural Federal University, Russia

F. Šimko, Department of Molten Salts, Institute of Inorganic Chemistry, Slovak Academy of Sciences, Slovakia

C. Simon, Sorbonne Universités, UPMC, Univ Paris 06, UMR 8234, PHENIX, F-75005 Paris, France

J. Spangenberger, Argonne National Laboratory, USA

M. Straka, Department of Fluorine Chemistry, Nuclear Research Institute Řež, plc, Czech Republic

R. O. Suzuki, Faculty of Engineering, Hokkaido University, Japan

L. Szatmáry, Department of Fluorine Chemistry, Nuclear Research Institute Řež, plc, Czech Republic

T. Tahara, Graduate School of Engineering, Chiba University, Japan

K. Takahashi, Graduate School of Engineering, Chiba University, Japan

K. Takase, Graduate School of Science and Technology, Niigata University, Japan

O. Takeda, Department of Metallurgy, Graduate School of Engineering, Tohoku University, Japan

T. Takenaka, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Japan

R. Tamme, Institute of Technical Thermodynamics, German Aerospace Center—DLR, Germany

A. Tasaka, Department of Applied Chemistry, Graduate School of Engineering, Doshisha University, Japan; and Department of Molecular Chemistry and Biochemistry, Faculty of Science and Engineering, Doshisha University, Japan

P. Taxil, Laboratoire de Génie Chimique, CNRS UMR 5503, Université de Toulouse, France

D. Thiaudiere, SOLEIL Synchrotron, France

J. Thonstad, Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), Norway

O. Tkacheva, Argonne National Laboratory, USA

M. Tokushige, Graduate School of Engineering, Doshisha University, Kyoto, Japan

M. E. Tray, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

T. Tsuda, Frontier Research Base for Global Young Researchers, Graduate School of Engineering, Osaka University, Japan; and Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Japan

R. Tunold, Department of Materials Science and Engineering, Norwegian University of Science and Technology, Norway

T. Uda, Graduate School of Engineering, Kyoto University, Japan

A. Uehara, Research Reactor Institute, Kyoto University, Japan

M. Umehara, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Japan

N. Umesaki, Japan Synchrotron Radiation Research Institute, Japan

M. Uno, Permerec Electrode Ltd., Japan

A. van Sandwijk, Zr-Hf-Ti Metallurgie B.V., Den Haag, The Netherlands

B. D. Vasin, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

V. A. Volkovich, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

J. Wang, Department of Materials Science and Engineering, Norwegian University of Science and Technology, Norway

S. Wang, School of Materials and Metallurgy, Northeastern University, China

Z. Wang, School of Materials and Metallurgy, Northeastern University, China

J. Wehr, EADS, Germany

Y. Xiao, Department of Metallurgical Engineering, Anhui University of Technology, China; and Zr-Hf-Ti Metallurgie B.V., Den Haag, The Netherlands

S. M. Yakimov, Department of Rare Metals and Nanomaterials, Ural Federal University, Russia

D. Yamada, Faculty of Engineering, Hokkaido University, Japan

T. Yamamoto, Graduate School of Energy Science, Kyoto University, Japan

H. Yamana, Research Reactor Institute, Kyoto University, Japan

Y. Yang, Department of Materials Science and Engineering, Delft University of Technology, Den Haag, The Netherlands

Y. Yang, School of Materials and Metallurgy, Northeastern University, China

D. Zanghi, CNRS, CEMHTI UPR 3079, Univ. Orléans, F-45071 Orléans, France

Foreword

On the Occasion of His 80th Birthday

Douglas Inman was born on 5 September 1931 in London, UK and has had a long and outstanding career in electrochemistry mainly related to fundamental and technological experimental studies in molten salts at high temperatures. The topics were often related to the development of industrial processes for metal production.

He has made valuable fundamental contributions in the field of Electrochemistry in molten salts at large, including Electrometallurgy, Batteries, Fuel cells, Electrowinning, Electroplating, the Preparation of ceramics from molten salts, and Molten materials at high temperatures over many decades.

His contributions were very important for the development of the early theory for molten salt electrolytes. He made original approaches to develop the use of electrochemical techniques in new and important systems such as molten salts. His studies of the very fast kinetics of electrode reactions in molten salts were especially breakthrough achievements at the time.

His open and outgoing personality and excellent presentation skills have made him a very popular and inspirational person at international conferences, and he has made a lot of international contacts and collaborations. Furthermore, many undergraduates, graduates, postdoctoral students, and research associates have worked under his guidance.

Professor Inman has organised several international conferences and received many awards for scientific work: the Kroll Medal of the Institute of Materials for achievements in Chemical Metallurgy in 1994 and more recently a new award named the Inman Medal to mark the ‘outstanding scientific contribution’ he has made to electrochemical science and technology. Professor Inman, the first recipient of the award, was presented with the solid gold medal at an awards ceremony held at UCL on 16 June 2011.

He was the founding chairman of the still-existing biannual conference series ‘Molten Salt Discussion Group’, and he was twice the chairman of the ‘Euchem Conference on Molten Salts’. He has an impressive list of publications in international journals.

Professor Inman, in his 80s, is still keeping in touch with the scientific community by occasionally attending international conferences.

Outline of Career and Honours

Preface

Molten salts are widely used in a number of industrial applications. In connection with their exceptional properties, these fused media offer a wide panel of uses: Their thermal stability range and generally low vapor pressure are well fitted to high-temperature chemistry, enabling fast reaction rates. Their ability to dissolve many inorganic compounds such as oxides, nitrides, carbides and other salts makes them ideal solvents useful in electrometallurgy, metal coating, treatment of by-products and energy conversion. Their wide potential window between decomposition limits allows the electro-winning of highly electropositive elements or the preparation of very electronegative elements.

Molten salts play a major role in the development of energy resources. For many years, the reprocessing of nuclear wastes has been a priority for nations using nuclear energy; in that domain, different pyrochemical devices have been investigated involving molten salt solvents. Moreover, they appear as a promising route toward the emergence of safer nuclear energy (nuclear reactors, Generation IV). Quite recently, focus on thorium-based nuclear reactors aroused great expectation in terms of continuous waste molten salt processing and safety.

Now, laboratory research using fused salt is opening doors for valuable applications. Materials for energy storage devices can be successfully prepared by fused salt electrolysis: for lithium and sodium metal batteries, MFFC, lithium-hydrogen energy cycle, silicon electorefining. High-temperature molten salt batteries are also studied for high-capacity energy storage. Multicomponent alkali nitrate/nitrite melts are valuable materials for heat transport and storage in solar plants.

Historically molten salts have been and still are always widely used in industry. They remain privileged media for the surface treatment of tool steels including nitriding, nitrocarburizing, boriding and other steel surface-hardening methods. They are also well known as efficient media for heat treating not only a variety of metals from ductile iron to high-speed tool steel but also non-metals, such as glass, plastics and rubber. Indeed, this technology offers invaluable advantages that will be briefly described.

These concrete applications have induced a renewed interest for a fundamental study of the specific features of high-temperature ionic liquids and thus some chapters devoted to this description are included in the book.

The book contains 61 chapters written by authors all recognized as specialists actively working in fused salts chemistry, electrochemistry and solid state chemistry. Our purpose was to offer new aspects of Molten Salts Chemistry and Technology to readers from academia and industry. It should be useful for generating new ideas showing the interest of the molten salt route.

The present book summarizes recent advances on seven topics, namely Aluminium Electrolysis, New Processes for Electrowinning, Modeling and Thermodynamics, High-Temperature Experimental Techniques, Electrochemistry in Ionic Liquids, Nuclear Energy, Energy Technology, maintaining a link between fundamental investigations and industrial developments. It aims to present the state of the art of current research performed by the molten salt community.

Part 1

Aluminium Electrolysis

Chapter 1.1

Formation of CO2 and CO on Carbon Anodes in Molten Salts

J. Thonstad¹ and E. Sandnes²

¹Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), Norway

²Primary Metal Technology, Hydro, Norway

1.1.1 Introduction

There is a great body of information on the anode product on carbon anodes in cryolite-alumina melts, which is the electrolyte used in aluminium electrolysis. A typical electrolyte composition can be cryolite, that is Na3AlF6, with 12 wt% AlF3 + 5 wt% CaF2 + 2–6 wt% Al2O3, operating at around 960 °C. The consumable carbon anodes are baked into solid bodies from petroleum coke together with pitch, serving as binder.

For the use of carbon anodes in chloride melts containing oxides the information is much more scant. Sandnes et al. [1] presented polarisation curves on graphite anodes in NaCl-Na2O and NaCl-CaCl2-CaO melts. The content of CO and CO2 in the anode gas was determined. As shown in the following, the CO2/CO ratio varied widely. The work by Sandnes et al. [1] will be used to throw some light on the anode products as a function of composition and applied potential.

1.1.2 Results

1.1.2.1 Fluoride melts

The molten cryolite-based electrolyte (see above) is dissociated into Na+ cations and AlF6³−, AlF4−, F− and various oxyfluoride anions, like Al2O4F4⁶−. For aluminium electrolysis there are two possible cell reactions:

1.1.1 equation

1.1.2 equation

The reversible E⁰ of these reactions at 1000 °C are −1.187 and −1.065 V, respectively [2]. The difference in favour of Equation 1.1.2 reflects the fact that the so-called Boudouard reaction:

1.1.3 equation

is strongly shifted to the right at this temperature.

The question whether CO2 or CO is the primary anode product has been studied extensively [2 3] and it has been shown that, except at very low current densities, the primary product is CO2 [2]. This conclusion has been based on carbon consumption studies (Equation 1.1.2) requires twice as much carbon per Faraday as Equation 1.1.1) and careful gas analysis, by avoiding disturbing side reactions, for example by using a diaphragm to separate the anode and cathode compartments [4]. Side reactions are reactions between CO2 and carbon, either within pores in the interior of the anode, with carbon particles dispersed in the electrolyte, or with metal dissolved in the melt. In all cases the reaction product of these side reactions is normally CO. Dissolved aluminium can even reduce CO2 all the way to carbon [5]. By bubbling CO2 underneath a graphite anode it was shown [5] that, while Equation 1.1.3 did occur at zero current, the reaction ceased when the electrode was anodically polarised, even at quite low current densities (0.05–0.1 A cm−2).

Most of the studies cited above were conducted in the time period between 1950 and 1980. However, as early as in 1936 Drossbach [6] demonstrated in a very elegant way how thermodynamic arguments could be applied to clarify the problem of identification of the primary anode product. If we consider the equilibrium:

1.1.4 equation

it is seen that the CO2/CO ratio defines a partial pressure of oxygen. In principle we can consider the primary cell reaction being:

1.1.5 equation

The standard reversible potential of this reaction is E⁰ = −2.213 V at 970 °C [2]. On an inert anode the anode product would be pure oxygen, so the anode potential (provided zero overvoltage) would be 2.213 V positive to an aluminium electrode. On a carbon anode the oxygen will react with carbon, and the partial pressure of oxygen will be much reduced and so would the anode potential. For the case of unit activity of alumina, the Nernst equation simply relates anode potential and oxygen partial pressure:

1.1.6 equation

The so-called depolarisation, ΔE, that is the reduction in potential with respect to E⁰ of Equation 1.1.5, can then be written as:

1.1.7 equation

As shown by Drossbach [6], it can with good approximation be expressed as:

1.1.8 equation

Table 1.1.1 gives some data for ΔE, log c01-1-math-0009 and the partial pressures (%) of CO2, CO and O2. Drossbach's original data are used, which refer to 1161 K (= 888 °C), but the values are not far from being correct also at higher temperatures.

Table 1.1.1 Depolarisation, anode potentials and gas composition evaluated for Equation 1.1.5 at 1161 K. Reprinted from Ref. [6]. Copyright (1936) Wiley-VCH

A typical anode potential at normal current densities (0.8–1.0 A cm−2) is 1.6–1.7 V referred to aluminium. This implies that ΔE of Equation 1.1.8 is in the range 0.6–0.5 V. We see from Table 1.1.1 that the anode gas, according to equilibrium (Equation 1.1.4), then should consist of essentially pure CO2. At very low current densities, corresponding to ΔE = 0.8 (i = 0.01 A cm−2), we can expect some CO.

It can be concluded that both experimental and thermodynamic data support the conclusion that the primary anode product is CO2 at all normal current densities encountered in aluminium electrolysis. However, this statement anticipates that there are no secondary reactions converting primary O2 and CO into CO2.

1.1.2.2 Chloride melts

It is natural to compare the conditions in the cryolite system with the commonly used chloride melts. However, only limited numbers of investigations have been published on the reaction products from a carbon anode in chloride-oxide melts. A literature study by Sandnes et al. [1] is included here in the following. Gas chromatograph measurements are reported by Mohamedi et al. [7] for a CaCl2 melt at 1123 K with a CaO concentration ranging from 3.3 × 10−5 mol cm−3 CaO (0.18 mol% CaO) to 2.7 × 10−4 mol cm−3 CaO (1.4 mol% CaO). At low current densities mostly CO2 was produced with insignificant amounts of CO. At higher current densities the CO fraction increased to 10–20%. The anode outlet gas composition was measured by Cathro et al. [8] in a 65/10/25 mol% MgCl2/NdCl3/NdOCl melt. At 750 °C on a high density graphite the Cl2/(Cl2 + CO2) mole fraction decreased from 70 to 28% by varying the current density from 0.36 to 0.089 A cm−2. The effect of temperature was also studied. At constant current density 0.36 A cm−2, the chlorine mole fraction decreased from 100% at 703 °C to 72% at 750 °C to 64% at 800 °C. Three types of graphite were investigated. At 750 °C little variation in the chlorine mole fraction was found between a high density graphite (1.76 g cm−3), a lower density graphite (1.67 g cm−3) and a carbon prepared from brown coal char. CO gas was not reported to have been analysed.

Oxygen was reported by Chen and Fray [9] to be formed electrochemically, c01-1-math-0011 , on a graphite anode in a CaCl2 electrolyte. CO2 might thus be formed through a secondary reaction by combustion of O2 with the carbon anode and/or produced CO. Gas chromatography (GC) analyses performed by Chen and Fray [10] of effluent gas during electrolysis in a pure CaCl2 melt and in pure BaCl2 melt showed O2 and CO as products with the ratio 1 : 1. The exact oxide concentration in the melt was not reported. The reaction c01-1-math-0012 was suggested as the reaction for the formation of oxygen.

Molten calcium chloride has appreciable solubility of calcium oxide, and such mixtures can be used as a solvent for other oxide species. In the so-called FFC Cambridge process this electrolyte is being used in electrolytic cells having a cathode containing an oxide, for example titanium oxide or chromium oxide, from which metal is extracted, and a graphite anode, operating in the temperature range 800–1000 °C. The anode product has been reported to be a mixture of O2, CO and CO2 [11].

Sandnes et al. [1] presented gas analyses from three different melt systems, the binary NaCl-Na2O system and the ternary systems NaCl-CaCl2-CaO and NaCl-SrCl2-SrO. GC measurements of the off-gas of CO and CO2 were performed during recording of stationary polarisation curves on a graphite electrode. Oxygen was not analysed. A glassy carbon crucible served both as the melt container and counter electrode. Two reference electrodes were applied, one Ag/AgCl and one chlorine electrode. The concentration was presented as the logarithm of the mole fraction of CO and CO2 to emphasise the small gas concentration at low current densities and low oxide concentrations. A small background concentration of CO and CO2 was present before the anodic polarisation started. This is likely due to a small amount of oxygen in the argon 5.0 carrier gas, oxidising carbon material into CO and/or CO2. The concentration stays constant until CO/CO2 is formed electrochemically, as exemplified in Figure 1.1.1.

c01-1f001

Figure 1.1.1 The polarisation curve and logarithm of the CO and CO2 mole fractions in the off-gas for the NaCl-Na2O system with 0.15 mol% Na2O at 825 °C [1]. Espen Sandnes, The anode process on carbon in chloride-oxide melts, Ph.D. thesis, The Norwegian University of Technology and Science, 2008, ISBN 978-82-471-8415-8 (printed version) ISBN 978-82-471-8429-5 (electronic version) ISSN 1503-8181

The simultaneous increase in the CO and CO2 concentrations from the background concentration indicates that both gases were formed electrochemically. The CO2 concentration continues to increase throughout the whole polarisation curve until a starting anode effect occurs above Erev(Cl2/Cl−). The CO concentration curve resembles the CO2 curve until the potential approaches the chlorine potential. In the NaCl-Na2O system the CO2/CO ratio decreased with increasing oxide concentration, and CO became the dominant electrochemically formed gas at high oxide contents (0.29 and 1.2 mol% Na2O). E⁰ values for the reactions:

1.1.9

equation

1.1.10

equation

1.1.11

equation

are shown by the dashed lines in Figure 1.1.1 to illustrate the potential for O2, CO2 and CO formation relative to E⁰(Cl2/Cl−) and the polarisation curve.

E⁰ values for the corresponding Equations 1.1.12–1.1.14 in the NaCl-CaCl2-CaO system are shown in Figure 1.1.2.

1.1.12

equation

1.1.13

equation

1.1.14

equationc01-1f002

Figure 1.1.2 The polarisation curve and logarithm of the CO and CO2 mole fractions in the off-gas for the NaCl-CaCl2-CaO (40 : 60) system with 1.6 mol% CaO at 800 °C [1]. Espen Sandnes, The anode process on carbon in chloride-oxide melts, Ph.D. thesis, The Norwegian University of Technology and Science, 2008, ISBN 978-82-471-8415-8 (printed version) ISBN 978-82-471-8429-5 (electronic version) ISSN 1503-8181

In the NaCl-CaCl2-CaO system, CO became the dominant electrochemically formed gas at high oxide contents (1.6 and 6.6 mol% CaO). Measurements in the NaCl-SrCl2-SrO (40 : 60) system with 4.2 mol% SrO gave very similar results for the polarisation curve and gas concentrations. CO was the only electrochemical product that showed a concentration increase below Erev(Cl2/Cl−), as indicated by the polarisation curve. A discussion of the exit gas composition for potentials near or above Erev(Cl2/Cl−) are given by Sandnes et al. [1].

1.1.3 Discussion

It is obvious that the gaseous products in the oxide-containing cryolite-based melts and in the oxide-containing chloride melts are very different. In fluoride melts the primary anode product is CO2 at normal current densities, and the CO contained in the off-gas from aluminium cells has mainly been formed through secondary reactions, as outlined above. Only at very low current densities CO may be a primary product.

In the experimental studies by Sandnes et al. [1] that were performed in chloride-oxide melts at relatively high anodic current densities (>0.5 A cm−2), the off-gas was likely to consist mainly of primary anode products as the anode was situated near the surface, allowing the gas bubbles to escape freely, having little time to react with dissolved metal or to establish any equilibrium, C/CO/CO2/O2. Thus, the composition of the gas analysed by the gas chromatograph was probably close to the true primary gaseous product. It is unlikely that most of the CO detected has been formed through a secondary reaction with CO2.

In fluoride melts the kinetics for CO formation is believed to be slower than for CO2, and as the potential is raised above the reversible potential for CO2 formation, CO2 soon becomes the dominant product. It is likely that the primary products are mainly determined by the kinetics in both fluoride and chloride melts. It seems that none of the reactions 2 CO + O2 = 2 CO2 and CO2 + C = 2 CO are fast enough to establish equilibrium, at least at high current densities.

1.1.4 Conclusion

The presented data show that the anode reactions on carbon and graphite anodes in fluoride-oxide and in chloride-oxide mixtures are very different with respect to the composition of the gaseous anode product. This enigma might be solved by performing measurements on several different carbon materials in oxide-containing fluoride and chloride melts, as well as in mixtures thereof, focussing on the off-gas composition and minimising possible side reactions.

References

1. Sandnes,E. (2008) NTNU PhD thesis. Results under publication by Sandnes, E., Haarberg, G.M., and Tunold, R.

2. Thonstad, J. , Fellner, P. , Haarberg, G.M. , et al. (2001) Aluminium Electrolysis, Fundamentals of the Hall-Heroult Process, 3rd edn, Aluminium-Verlag, Dusseldorf, p. 159.

3. Grjotheim, K. , Krohn, C. , Malinovsky, M. , et al. (1982) Aluminium Electrolysis, Fundamentals of the Hall-Heroult Process, 2nd edn, Aluminium-Verlag, Dusseldorf, p. 228.

4. Ginsberg, H. and Wrigge, H.C. (1972) Metall, 26, 997.

5. Thonstad, J. (1964) J. Electrochem. Soc., 111, 959.

6. Drossbach, P. Z. (1936) Zur Elektrometallurgie des Aluminiums, Elektrochem.42, 65.

7. Mohamedi, M. , Børresen, B. , Haarberg, G.M. and Tunold, R. (1999) Anodic behaviour of carbon electrode in CaO-CaCl2 melts at 1123 K. J. Electrochem. Soc., 146, 1472.

8. Cathro, K.J. , Deutscher, R.L. and Sharma, R.A. (1997) Electrowinning magnesium from its oxide in a melt containing neodymium chloride, J. Appl. Electrochem., 27, 404–413.

9. Chen, G.Z. and Fray, D.J. (2004) Understanding the electro-reduction of metal oxides in molten salts. Light Met., 881–886.

10. Chen, G.Z. and Fray, D.J. (2001) Cathodic refining in molten salts: removal of oxygen, sulfur and selenium from static and flowing molten copper. J. Appl. Electrochem, 31, 155–164.

11. Gordo, E. , Chen, G.Z. and Fray, D.J. (2004) Toward optimisation of electrolytic reduction of solid chromium oxide to chromium powder in molten chloride salts. Electrochim. Acta, 49, 2195.

Chapter 1.2

Interaction of Carbon with Molten Salts-Chloride-Carbonate Melts

D. Fray

Department of Materials Science and Metallurgy, University of Cambridge, UK

1.2.1 Introduction

Carbon plays many important roles in molten salts—it can be an inert anode for the evolution of chlorine during the electrolysis of chloride salts. If oxygen is being generated at the anode, as in the Hall Heroult Process for aluminium, carbon can react to form carbon dioxide. In chloride melts containing oxygen ions, such as found in the OS and FFC processes, the carbon dioxide can dissolve in the salt as a carbonate ion which, if it becomes in contact with the cathode can result in the deposition of carbon and more oxygen ions which can further react with the carbon resulting in the transport of carbon from the anode to cathode. In high temperature carbonate fuel cells, using alkali carbonate eutectics, the carbonate ion acts as the ion that transports oxygen from the anode to the cathode. Carbonate melts can be used, as an electrolyte, to dissociate carbon dioxide to oxygen and carbon monoxide. When graphite is used as a cathode in molten salts containing alkali ions, the discharged alkali metal can intercalate into the graphite and, under well defined conditions, produce carbon nanotubes and carbon nanoparticles which can find application in lithium ion batteries, especially if the tubes and particles are filled with tin or silicon. It can be seen that, in the future, carbon will still continue to play a major part in energy generation and storage but not be consumed.

1.2.2 Carbon as an anode in molten salt cells

1.2.2.1 Inert anodes

Carbon, in the form of graphite, is used in conjunction with a steel cathode to produce lithium, sodium and magnesium from molten salts [1]. Usually, a eutectic mixture is used of the metal chloride, together with other chlorides to form a low melting point mixture. The desired anodic reaction is:

1.2.1 equation

with no attack of the anode by chlorine. If, however, the electrolytes are not thoroughly dried, some oxygen will be liberated on the anode which reacts to form carbon dioxide with some corrosion of the anode.

1.2.2.2 Reactive anodes

All of the world's aluminium is produced in the Hall Heroult cell using a carbon anode, consisting of petroleum coke and about 20% pitch as a binder [2]. Given the considerable importance of the process, an enormous amount of research has been carried out to ascertain the reaction mechanism. The overall reaction is thought to be:

1.2.2 equation

although a series of other aluminium oxyfluoride species can also participate in the reaction [2]. The formation of carbon dioxide rather than oxygen lowers the overall cell voltage by about 1 V due the fuel cell reaction:

1.2.3

equation

It is thought that an adsorption step is important in the reaction due to:

CO2 being is the primary product although CO is thermodynamically favoured.

When a current pulse is applied, the potential increase and decrease are slow.

The shape of the impedance curves.

It is also considered that COadsorbed is an intermediate adsorbed species but there is nothing confirmed about the carbon-oxygen surface species.

It is worth noting that the anodic overvoltage for carbon dioxide evolution on carbon is 0.5 V at a current density of 0.5 A cm−2 in cryolite containing dissolved alumina [2] whereas the overpotential in CaCl2-CaO melts is about 0.8 V at the same current density [3].

When the supply of oxygen containing species in the melt becomes depleted, perfluorocarbon gases are formed by the following reactions:

1.2.4 equation

1.2.5 equation

However, the net effect is considerably greater than a simple change in the gas composition as the voltage on the anode increases by several volts up to tens of volts and this is known as the anode effect. Many mechanisms have been proposed for this effect but the general consensus seems to lie between two mechanisms [2]:

Alumina depletion in the melt leads to discharge of fluoride ions, forming C-F compounds which block the surface of the anode resulting in a change in the wetting properties allowing the formation of a gas film.

The fluid-dynamic theory proposed that the anode effect occurs when the anode is completely covered by a gas film.

1.2.3 Carbon in the form of carbonate ions

One of the uses of lithium carbonate is in high temperature fuel cells where the carbonate ion transports the oxygen from the cathode to the anode. The electrolyte consists of a mixture of alkali carbonates held in a matrix of lithium aluminate [4]. The advantages of working at high temperature is that non noble metal catalysts can be used, such as lithiated nickel oxide as the cathode and a nickel chromium alloy as the anode which are not subjected to poisoning by carbon monoxide. A further advantage of this system is that external reforming is not required as fuels can be converted to hydrogen within the fuel cell.

The anodic reaction is:

1.2.6 equation

and the cathodic reaction is:

1.2.7 equation

The overall reaction is:

1.2.8 equation

In the fuel cell, the carbon dioxide that is formed at the anode is recycled back to the cathode. Although very promising, there are still problems with materials and the containment of the electrolyte.

Using a lithium carbonate electrolyte, containing some lithium oxide, Lubomirsky and his coworkers were able to convert carbon dioxide into carbon monoxide and oxygen electrochemically using a graphite anode and a titanium cathode [5]. A schematic of the cell is shown in Figure 1.2.1. The mechanism is thought to be carbon dioxide reacts with the O²− in the melt:

c01-2f001

Figure 1.2.1 Schematic representation of cell for converting CO2 to CO and O2

1.2.9 equation

At the carbon anode, the reaction was assumed to be:

1.2.10 equation

and the cathodic reaction on the titanium cathode is:

1.2.11 equation

with the overall reaction being:

1.2.12 equation

The potential for this reaction to occur is 0.9 V which corresponded very closely to the extrapolation of the linear portion of the measured cell potential to zero current. This approach has several advantages over other methods of generating carbon monoxide and oxygen from carbon dioxide, in that no precious metals are required, pure carbon monoxide is produced, which could be stored and converted into other chemicals, and the process can operate at the carbon dioxide pressures found in flue gases.

In the past decade two processes have been investigated, the OS Process [6] and the FFC Cambridge Process [7], in which metal oxides are reduced either by calcium being deposited from a chloride melt or ionisation of the oxygen from the oxide. In both cases, oxygen ions transfer from the cathode to an anode which is usually carbon. On discharge of the oxygen ions, carbon dioxide is formed, which can dissolve in the salt and this is in contrast to the cryolite melts for aluminium production which do not dissolve carbon dioxide. The carbonate ions can be transported to cathode where the ion can be discharged to form carbon and oxygen ions:

1.2.13 equation

This has two effects as it deposits carbon on the cathodic product which may cause contamination and, secondly, the newly created oxygen ions can diffuse to the anode where fresh carbonate ions can be created:

1.2.14 equation

allowing the process to start all over again, using some of the current and lowering the current efficiency. The dramatic effect of using an inert anode compared to a carbon anode is shown in Figure 1.2.2. The deposited carbon at the cathode can also be in the form of carbon nanotubes and nanoparticles which enables these novel materials to be produced by the electrolysis of carbonate melts [8 9].

c01-2f002

Figure 1.2.2 Effects of using an inert anode (a) and a carbon anode (b).

From Ref. [3], © Kamal T. Kilby, Ph.D. thesis, University of Cambridge

The substitution of the carbon anode by an inert anode, such as CaTixRu1−xO3, obviously avoids the carbon problem and raises the quality of the metallic product. It was also found that the polarisation, at a given current density, on an inert anode was far lower than that on a carbon anode so that the loss of the fuel cell effect was offset by the difference in polarisation (S. Jiao and D.J. Fray, unpublished data).

Lithium carbonate is the cheapest precursor for the production of lithium, which is predicted to play a major part in energy storage, but the only production route for lithium is by the electrolysis of lithium chloride. It would be an advantage if lithium carbonate could be electrolysed directly but electrochemical reaction of the carbonate ion is always more favourable than the deposition of lithium, assuming the melt is saturated with lithium oxide:

1.2.15

equation

1.2.16

equation

This difficulty was overcome by having a diffusion barrier between the anode and cathode so that lithium carbonate was fed to the anode and the barrier prevented the carbonate ion diffusing to the cathode [10]. Fortunately, the diffusion coefficient for the lithium ion is significantly greater than the diffusion coefficient for the carbonate ion resulting in a high current efficiency for lithium deposition.

1.2.4 Carbon in the form of carbide ions

Attempts have also been made to use the CaCl2-CaC2 electrolytes to measure the carbon content in steels at elevated temperatures using a carbon reference electrode. Unfortunately, calcium carbide is not particular stable and it dissociates in the salt to calcium and carbon:

1.2.17

equation

and the presence of this small activity calcium in the melt results in electronic conduction in the melt with the net result that carbon is transferred from the carbon reference to the alloy, gradually increasing the carbon content of the alloy up to carbon saturation [11].

At the carbon reference:

1.2.18 equation

and at the alloy:

1.2.19 equation

with the electrons passing through the electrolyte. CaCl2-CaC2 is, therefore, not a suitable electrolyte for electrochemical measurements [11].

1.2.5 Carbon as a cathode

If carbon in the form of graphite is used as a cathode, in an ionic liquid or an ionically conducting solid containing alkali ions, the alkali ions can discharge and intercalate between the planes of graphite to form intercalation compounds, such as LiC6. This forms the basis of lithium ion batteries where LiC6 forms the negative electrode of the battery and a LiMeO2 type oxide forms the positive electrode. The electrolyte can be LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate. As both compounds are intercalation compounds, lithium can be cycled between the two electrodes without forming metallic lithium which is undesirable as it can short circuit the battery, creating a lithium fire. It would be obviously advantageous to increase the amount of lithium contained in the negative electrode.

At more elevated temperatures, the lithium intercalation can cause the graphitic structure to break up to form nanotubes or nanospheres, depending on the type of graphite and temperature, which was first observed by Kroto's team at Sussex University [12] A typical experiment would use a current density of 2 A cm−2 and yields a product containing 90% nanotubes or hollow nanoparticles. This method of synthesising carbon nanotubes has significant advantages over the conventional vapour phase route as the synthesis takes place in a condensed phase so that the size of reactor is greatly reduced and, furthermore, the rate of reaction is considerably faster, by a factor of about 2000. Graphite is also a relatively cheap feedstock which should lower the cost of carbon nanotubes. However, it was found that the production of carbon nanotubes ceased when the surface of the graphite became coated with lithium metal which usually occurred after a few minutes of electrolysis. This was overcome by reversing the current, every few minutes, so that the intercalated lithium ionised, dissolved in the salt and then was intercalated into another graphite rod. In this way, virtually all the graphite could be converted into nanotubes and hollow nanoparticles [13].

This method of electrolytically producing carbon nanotubes can be compared with the electrolysis of carbonate ions to produce the same product. The production of carbon nanotubes in the latter case is entirely controlled by Faraday's laws whereas, with the intercalation approach, the production depends on the stress and strain introduced by the given current flow. The quantity and quality of nanoproducts is significantly greater for the intercalation route than for the electrolysis of carbonates.

As mentioned previously, one way of improving a lithium ion battery would be to increase the capacity of the negative electrode, and this could be achieved by using a metallic electrode and forming compounds such as Li4Sn and Li4Si. Unfortunately, the insertion and de-insertion of lithium into tin and silicon results in large volume changes which cause the alloys to decrepitate and disconnect, causing the battery to fail [14]. Kroto's team also observed that, if a less stable chloride is added to the lithium chloride, the tubes and nanoparticles are filled with metal of the less stable chloride such as tin, lead or zinc [15]. Again, by using the current reversal approach it is possible to fill all the nanotubes and nanoparticles with metal [16]. A TEM image of filled carbon nanotubes and nanoparticles is shown in Figure 1.2.3.

c01-2f003

Figure 1.2.3 Tin filled carbon nanotubes and nanoparticles.

From Ref. [16], © Rajshekar Das Gupta, Ph.D. thesis, University of Cambridge

Tin filled nanotubes have been mixed with 50% graphite and a binder and investigated as the negative electrodes in a lithium ion battery. The results showed that, after an initial decrease in capacity, the capacity remained constant for a significant number for cycles with a capacity 50% greater than a conventional anode. [16]. After use (Figure 1.2.4) it can be seen that, due to the expansion of the tin due to lithium insertion, the carbon nanotubes expanded but, after de-lithiating, the tin contracts but still remains in contact with the conducting carbon [16].

c01-2f004

Figure 1.2.4 Tin filled carbon nanotube after many charge/discharge cycles.

From Ref. [16], © Rajshekar Das Gupta, Ph.D. thesis, University of Cambridge

1.2.6 Conclusions

This paper has shown that carbon in the presence of molten salts plays a major role in the extraction of metals where energy is consumed but, perhaps, more important is that it can make a significant contribution to energy creation and storage under conditions where it is not consumed but simply acts as an intermediate allowing other reactions to take place. In the carbonate fuel cell, the carbonate ion is not consumed but is simply used as an ion to transport oxygen from one electrode to another. Carbonate melts can also be used to electrochemically dissociate carbon dioxide to carbon monoxide, which could be used as a fuel or as a precursor for other chemicals, and oxygen. Carbon nanotubes can be added to both the anode and cathode of lithium ion batteries but more importantly can be used to hold metallic elements, such as tin and silicon, which allow the capacity of the batteries to be greatly increased, thereby improving the performance of electric cars and leading to a greener future. Electric cars in the future, the role of carbon may change from being consumed to acting as a facilitator for other reactions.

References

1. Habashi, F. (ed.) (1997) Handbook of Extractive Metallurgy, Wiley-VCH Verlag GmbH, Weinham.

2. Thonstad, J. , Felner, P. , Haarberg, G.M. et al. (2001) Aluminium Electrolysis –Fundamentals of the Hall-Heroult Process, 3rd edn, Aluminium-Verlag, Dusseldorf.

3. Tripuraneni Kilby, K. (2008) The anodic testing of a tin oxide (SnO2) based material for the FFC Cambridge process. PhD thesis. University of Cambridge.

4. Steele, B.C.H. and Heinzel, A. (2001) A materials for fuel-cell technologies. Nature, 414, 345–352.

5. Kaplan, V. , Wachtel, E. , Gartsman, K. et al. (2010) Conversion of CO2 to CO by electrolysis of molten lithium carbonate. J. Electrochem. Soc, 157, B552–B556.

6. Ono, K. , Okabe, T. , Ogawa, M. and Suzuki, R. (1990) Production of titanium powders by the calciothermic reduction of TiO2. Tetsu-to-Hagane, 76, 568–575.

7. Chen, G.Z. , Farthing, T.W. and Fray, D.J. (2000) Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride. Nature, 407, 361–364.

8. Devyatkin, S.V. (2005) in Proceedings MS7 International Symposium on Molten Salt Chemistry and Technology (ed. Taxil, P. ), pp. 515–517, Inpact, Toulouse, http://inpact.Inp-toulouse.fr/MS7/.Inp-toulouse.fr/MS7/.

9. Rosenkilde, C. (2007) Method for production of carbon materials. Patent WO2007046713.

10. Kruesi, W.H. and Fray, D.J. (1993) The electrowinning of titanium from chloride-carbonate melts. Metal. Trans. B, 24, 605–615.

11. Morris, D.R. , Aksaranan, C. , Waldron, B.S. and White, S.H. (1973) Galvanic cell studies involving calcium carbide solutions. J. Electrochem. Soc., 120, 570–575.

12. Hsu, W.K. , Hare, J.P. , Terrones, M. et al. (1995) Condensed—phase nanotubes. Nature, 377, 687.

13. Fray, D.J. , Schwandt, C. , and Dimitrov, A. (2008) Electrochemical method, apparatus and carbon product. US Patent 2008/0105561.

14. Besenhard, J.O. , Yang, J. and Winter, M. (1997) Will advanced lithium-alloy anodes have a chance in lithium-ion batteries? J. Power Sources, 86, 87–90.

15. Hsu, W.K. , Terrones, M. , Terrones, H. et al. (1998) Electrochemical formation of novel nanowires and their dynamic effects. Chem. Phys. Lett., 284, 177–183.

16. Das Gupta, R. (2010) The electrochemical production of tin filled carbon nanotubes and their use as anode materials in lithium-ion batteries. PhD thesis. University of Cambridge.

Chapter 1.3

Anode Processes on Carbon in Chloride Melts with Dissolved Oxides

E. Sandnes², G. M. Haarberg¹, A. M. Martinez³, K. S. Osen³ and R. Tunold¹

¹Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), Norway

²Primary Metal Technology, Hydro, Norway

³SINTEF Materials and Chemistry, Norway

1.3.1 Introduction

Chloride melts are used as electrolytes for the electrowinning of several metals, for example Mg, Na, Li and Ca. The general reactions can be written as:

1.3.1 equation

1.3.2 equation

Also, in the last years a large research activity for the direct reduction of MO (the FFC process) in chloride-oxide melts has developed [1]. Carbon is the preferred anode material in all these processes. Oxides are present as impurities or added, and the anodic reaction can be written as:

1.3.3 equation

1.3.4 equation

The aims of the present study are to determine kinetic data and study mechanisms for CO/CO2 evolution and Cl2 evolution on carbon and to study the reciprocal effects of CO/CO2 evolution and Cl2 evolution. The following main processes for CO, CO2 and Cl2 formation are considered below.

1.3.2 Electrochemical processes in chloride-oxide melts

1.3.2.1 Cl2, CO and CO2 formation

The Cl2 formation process may be divided into a primary step and two parallel secondary steps:

The CO formation process may be formulated by the following steps:

The CO2 formation process may be divided into a primary step and two parallel secondary steps:

or CO2 may be formed through reaction between adsorbed species from two different processes:

1.3.2.2 Melt systems

Melts consisting of alkali chlorides and various fractions of alkaline earth chlorides are important for the industrial production of alkali metals like lithium, sodium and potassium as well as alkaline earth metals like magnesium, calcium, strontium and barium. As shown by Boghosian and Østvold, there exists an enhanced solubility of oxide through complex formation in NaCl-MCl2 systems, as shown by the process [2]:

1.3.5 equation

where the stoichiometric numbers vary according to the cation. The following numbers were found [2]:

equation

The solubility of oxide in the chloride melt increases by increasing the basicity of the system from the calcium system to the barium system.

1.3.3 Experimental

The working electrode was a cylindrical graphite rod (Ø = 4.55 mm, 90% graphitisation) immersed 5 mm into the melt, or a cylindrical vitreous carbon rod (Ø = 3 mm) immersed 5–10 mm into