The impact of 3-dimensional anode geometry on the electrochemical response of high temperature gas evolution reactions in molten salts

Abstract

Bulk electrodes are used in molten salt electrolysis as consumable graphite anodes for existing aluminium production (producing CO2) as well as for oxygen evolution anodes in future zero emission manufacturing processes, such as direct iron electrolysis. Here, graphite electrodes with different geometry, roughness, density, and size were assessed for their kinetic behaviour and gas evolution stability in a molten alkali-metal (Li-Na-K) carbonate salt at 600 °C. It is suggested through comparative assessment of controlled electrode sizes and geometry that radial diffusion at a small (5 mm diameter disc) electrode is likely to influence performance. Roughened electrodes and electrodes of varying porosity also impacted polarisation behaviour which was ascribed to changes in electrolyte wetting observed in separate studies, conducted under similar conditions. It was also seen that 3-dimensional cone and slant type electrodes on angles of 75° increased the electrode performance. Enhanced kinetics were observed by a decreasing Tafel slope from 254 mV.decade−1 to 188 mV.decade−1 for 45° and 75° slant electrodes, respectively. The apparent improved performance for identical electrode material is discussed to involve influence from radial diffusion of reactants, modified via electrode geometry. Additional work is required to confirm this. It was concluded that to fairly compare oxidation of graphite materials in molten carbonate salts, electrodes of 10 mm diameter equivalent should have the same geometry and consistent inclination angle (of below 45°), as well as control of bulk density and resulting porosity. This observation may well also apply to other systems and should be considered in experimental design.