Time dependence of the hydrogen discharge at 70°C on nickel cathodes

The hydrogen evolution reaction on polished pure nickel cathodes in 30 w/o KOH at 70°C in the presence of 0.5 ppm dissolved iron has been investigated in constant current and potential modes combined with the potential-sweep method. The potentiostatic tests show that the cell current decreases with time, the rate of current decay being largely dependent on the applied potential. The current decay is tentatively attributed to the penetration of atomic hydrogen into the metal lattice. Iron deposits on the cathode surface, which are observed after an incubation period of several hours, are found to be detrimental to the hydrogen discharge. Two consecutive steps are envisaged for the recovery mechanism, desorption of the atomic hydrogen in the metal lattice and dissolution of the deposited iron. The significant improvement in the electrocatalytic activity of the cathode following the addition of molybdate to the electrolyte is ascribed to the deposition of molybdenum on the electrode surface by lowering significantly the Tafel slope for the HER.