Influences of graphite anode area on electrolysis of solid metal oxides in molten salts

Abstract

Laboratory studies of electrochemical reduction of refractory metal oxides, e.g. TiO 2 and Ta 2 O 5 , in molten CaCl 2 often involve a graphite anode and a cell voltage of 3.0 V or higher, which deviates significantly from thermodynamic predictions. The causes considered in the past have included mechanistic, kinetic and dynamic complications of cathode reactions, but little was considered on anodic processes. This paper shows that oxidation of the O 2− ion on the graphite anode is also a significant contributor to the high cell voltages applied. Cyclic voltammetry in molten CaCl 2 containing added CaO (up to 2.51 mol%) suggested that O 2− oxidation on graphite proceeds dominantly in two steps as previously observed on glassy carbon. With increasing CaO concentration, the second step became rate-limiting over a wide range of potentials before the processes reached at diffusion controlled high current density. This understanding led to the proposal and experimental confirmation of a "low anode current density strategy" in potentiostatic reduction of thin cylindrical pellets of TiO 2 and Ta 2 O 5 in molten CaCl 2 at 850°C. It was observed that a 10-fold increase of the graphite anode area could decrease the cell voltage by about 1.0 V, which should save energy consumption by up to one third.

The Slow Professor: Challenging the Culture of Speed in the Academy, by Maggie Berg and Barbara K. SeeberThe Slow Professor: Challenging the Culture of Speed in the Academy, by Maggie Berg and Barbara K. Seeber. 2016. 115 pages, paperback. Toronto: University of Toronto Press.

“…Before a0, there is an apparent anodic current at a1, which is very stable according to the multi-cycle scans and could be attributed to the discharge of O 2À ions on the graphite anode. However, unlike the discharge of O 2À on a carbon based anode in molten CaCl 2 , which exhibited complicated voltammograms involving formation of CO 2 , CO and O 2 gases, [22][23][24] here the CVs of O 2À discharge in MNK featured a single reaction, probably the formation of O 2 gas as mentioned above. To conrm it, potentiostatic electrolysis was performed at 1.0 V vs. Ag/AgCl against a Ta 2 O 5 pellet cathode.…”

Minimising oxygen contamination through a liquid copper-aided group IV metal production process

“…The cathode potential presented a negligible increase from −1.41 to −1.35 V vs. W over 11 h at an applied current density of 500 mA/cm 2 . In contrast, the graphite anode potential increased from 0.62 to 1.29 V. This change can be explained by a decrease in the anode surface area during the electroreduction 18 . In the case of an electroreduction in which solid matter exists before and after the reaction, such as Fray-Farthing-Chen (FFC) Cambridge process and Ono-Suzuki (OS) process, the oxygen diffusion becomes difficult as the reaction progresses, and the cathode potential tends to become more negative.…”

Interactions of molten salts with cathode products in the FFC Cambridge Process

“…In the past two decades, world-wide research and development have confirmed the scientific principle and technical feasibility and flexibility of the process for the extraction of almost all metals listed in the periodic table and their alloys from their respective oxide or sulfide precursors [6][7][8][9][10][11][12][13][14]. In addition, the FFC Cambridge Process has versatile applications in other fundamental and industrial areas, such as near-net shape manufacturing of metallic artifacts of complex structures, medical implants, oxygen generation on the moon, capture and electrolytic conversion of carbon dioxide (CO 2 ) to various forms of carbon, e.g., carbon nanotubes, carbon monoxide (CO), and hydrocarbon fuels (C n H 2n+2 , n < 10), and rechargeable molten salt metal-air batteries [15][16][17][18][19][20][21][22].…”