On the cathodic overvoltage on aluminium

When a constant cathodic current is applied to an aluminium electrode in a Na3A1F6-Al2O3 melt at 1010°C, the potential decreases gradually and linearly with the square root of time to more negative values. Pronounced potential oscillations occurred at current densities above 2 A/cm2, accompanied by the evolution of sodium gas at very high current densities. Steady state measurements yielded straight y vs log i plots with a slope of -0.23 V/decade, indicating an overvoltage of -0.19 V at 1 A/cm2. The overvoltage decreased markedly when the melt was stirred. Potential decay measurements provided linear log time plots with slopes of 0.0145 V/decade. The charge-transfer resistance was determined by double pulse measurements to be 0.0031 ohm cm2, with AC impedance measurements yielding a similar result. Notably, the charge transfer overvoltage accounted for only about two percent of the total overvoltage, suggesting that the remainder is diffusion controlled. The findings indicate that the cathodic overvoltage in industrial aluminium cells is of similar magnitude to that observed in laboratory investigations.