KINETICS OF THE HYDROGEN EVOLUTION REACTION ON A RHODIUM ELECTRODE

Kinetics of the hydrogen evolution reaction (her) were measured at a rhodium electrode in a 1 M solution of KOH at various temperatures between 5 and 75-degrees-C. The measurements were conducted at a Rh wire electrode activated before every measurement by cycling in the range of potentials between -1.0 and 0.3 V (vs. Hg/HgO/1 M KOH) until a stationary behaviour was obtained. Experiments were also carried out on the electrode kept at 25-degrees-C for three days at -1.0 V. In this case the current was by about one order of magnitude lower than that observed for the activated electrode. For both electrodes the her could be described by the Volmer-Heyrovsky mechanism. For activated electrodes the cathodic transfer coefficient-alpha changed with temperature from 0.3 to 0.4; at deactivated electrode it was equal to 0.43. The activation resistance decreased with temperature by almost two orders of magnitude. The results were analysed according to two models of the impedance on solid electrodes. An approximation of both the Tafel plots and the logarithm of the reaction admittance vs. overpotential made the calculation of the rate constant of the slowest step possible, which was three orders of magnitudes greater than that for the nickel electrode. Total capacities of the double layer at poisoned and activated electrodes were almost the same and equal to 60-80-mu-F cm-2. We have concluded that traces of metallic impurities are responsible for poisoning of the electrode.