INSITU ACTIVATION OF NICKEL CATHODES BY SODIUM MOLYBDATE DURING ALKALINE WATER ELECTROLYSIS AT CONSTANT CURRENT

The nickel cathode undergoes significant deactivation at 100 mA cm-2 in 30 wt% KOH at 70°C containing KOH metallic impurities. The time dependence of the potential reveals two distinct regions: the first due to hydrogen absorption, while the Tafel parameters, charge transfer resistance and double-layer capacitance remain fairly constant; the second due to the electrodeposition of metallic impurities as manifested by SEM pictures, EDX analysis and cyclic voltammograms. The resulting deactivation is due mainly to the increase in the Tafel slope, although the electrode impedance reveals a significant increase in electrode roughness. The hydrogen discharge at 100 mA cm-2 in 30 wt% KOH containing 4 mm sodium molybdate largely attenuates the deactivation process owing to hydrogen absorption and the deposition of impurities. The in situ activation is ascribed to the formation of a spongy molybdenum-base deposit on the nickel cathode during the first day of water electrolysis. © 1990 Chapman and Hall Ltd.