ON THE ANODIC OVERVOLTAGE IN ALUMINUM ELECTROLYSIS

Abstract

The overvoltage on the carbon anode in aluminum electrolysis has been measured against a CO₂/C reference electrode placed in the cryolite melt. The behavior of the reference electrode was first investigated against the aluminum cathode. This galvanic cell showed a stable potential of 1.15 V, which was close to the reversible potential for the cell reaction in aluminum electrolysis. Similar measurements were made with CO/C and CO₂,CO/C electrodes. The potential of the CO/C electrode was in fair agreement with the reversible potential for the most probable cell reaction. The potentials obtained with CO₂–CO mixtures were intermediate between those for the pure gases, increasing with increasing CO₂ content. An explanation for the behavior of the CO₂,CO/C electrode is suggested.The anodic overvoltage depended only to a limited extent on the chemical reactivity of carbon with respect to oxygen. Graphite anodes showed higher overvoltage than all other types of carbon anodes. Catalyzing and inhibiting additives to the carbon had small but significant influences on the overvoltage. Straight Tafel plots were obtained with slopes varying between 0.26 and 0.27. On this basis it is shown that the overvoltage may be due either to slow transport of oxygen ions through the double layer or to slow reaction between chemisorbed oxygen and carbon.