DC Voltammetry of Electro-deoxidation of Solid Oxides A.M. Abdelkader,* ,† K. Tripuraneni Kilby, ‡ A. Cox, ‡ and D. J. Fray ‡ † School of Materials, University of Manchester, Grosvenor Street, Manchester, U.K., M1 7HS ‡ Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, U.K., CB2 3QZ CONTENTS 1. Introduction 2863 2. Theoretical Considerations 2864 3. Electro-deoxidation under Controlled Voltage or Potential 2866 3.1. Earlier Attempts to Describe the Reduction Mechanism 2866 3.2. Three-Phase Interline (3PI) Propagation Models 2867 3.3. Alkali Ternary Oxides Intermediates 2867 3.3.1. Niobium Oxide 2867 3.3.2. Tantalum Oxide 2868 3.3.3. Chromium Oxide 2868 3.3.4. Titanium Oxide 2868 3.3.5. Zirconium Oxide 2870 3.3.6. Tungsten Oxide 2870 3.3.7. Aluminum Oxide 2870 3.4. Electro-deoxidation of Mixed Oxides and Oxide Solid Solutions 2871 3.5. Attempts to Use Inert Anode 2873 4. Cyclic Voltammetry 2874 4.1. Understanding the Cathodic Reduction Reactions 2874 4.2. Determining the Optimum Operation Po- tential 2876 4.3. Exploring the Surface Phenomenon 2877 4.4. Studying the Complex Side Processes and Detecting the High Temperature Intermedi- ate Phases 2877 4.5. Providing Direct Insights into the Kinetics of Electrode Reactions 2878 5. Electro-deoxidation under Constant Current Chronopotentiometry 2879 6. Summary and Future Direction 2882 Author Information 2883 Corresponding Author 2883 Notes 2883 Biographies 2883 References 2884 1. INTRODUCTION Using electric current as a tool in metallurgy dates back to the early years of the nineteenth century. Davy and Berzelius discovered alkali metals were able to be electrolyzed from their molten salts when large number of Volta cells were connected in series in the early nineteenth century. 1 However, it was only in 1869 that the first industrial electrolytic cell was introduced near Swansea for the electrorefining of copper. 2 Heroult and Hall achieved another significant advance in 1886 when, simultaneously but independently, they patented a new process for producing aluminum by the electrolysis of a fused bath of cryolite (Na 3 AlF 6 ) containing Al 2 O 3 . Over years, considerable developments in the production and purification of metals and alloys via electrometallurgical techniques have been achieved. 2 Electrorefining is a frequently used electrometallurgical technique. Traditionally, electrorefining involves the purifica- tion of metals by anodically dissolving the impure metal, followed by plating-out of pure metals on the cathode. However, Ward and Hoar refined molten copper by making it a cathode in molten BaCl 2. A constant current was applied and nonmetallic impurities were ionised and discharged on the carbon anode, leaving pure copper on the cathode. 3 A few decades later, Okabe and his co-workers used the same principle to cathodically remove atomic oxygen from titanium and other reactive metals. 4 It was claimed that under a constant voltage of 3 V applied between the titanium cathode and carbon anode in molten CaCl 2 , the deoxidizer, calcium, is produced on the cathode according to reaction 1. Calcium is then able to deoxidize the titanium according to reaction 2, with the oxygen ion removed from the system as CO or CO 2 gas by reactions 3 and 4: 4 + = + − Ca (in CaCl ) 2e Ca(on Ti cathode) 2 2 (1) + = + + − O(in Ti) Ca Ca (in CaCl ) O (in CaCl ) 2 2 2 2 (2) + = + − − O (in CaCl ) C(anode) CO(g) 2e 2 2 (3) + = + − − 2O (in CaCl ) C(anode) CO (g) 4e 2 2 2 (4) At the end of the twentieth century, Chen et al. 5 investigated the electrochemical removal of alpha case on titanium and its alloys. Again the material to be deoxygenated was made the cathode and carbon used as an anode in molten CaCl 2 .A Received: August 4, 2011 Published: January 22, 2013 Review pubs.acs.org/CR © 2013 American Chemical Society 2863 dx.doi.org/10.1021/cr200305x | Chem. Rev. 2013, 113, 2863−2886