Kinetics of the Hydrogen Evolution Reaction on RuO2 and IrO2 Oxide Electrodes in H 2 SO 4 Solution: An AC Impedance Study

The hydrogen evolution reaction on and electrodes in 1 M at room temperature was studied by performing current‐potential and ac impedance measurements. It is shown that is more active than . Following correction of the electrode potential for the uncompensated resistance, the Tafel plot of both electrode materials shows two distinct slopes. The hydrogen evolution reaction on these metallic oxide electrodes was assumed to proceed according to three different mechanisms. The corresponding kinetic equations were derived and were used to simulate the log i = log i (η) and log A = log A (η) curves. The first and second mechanism involve three steps and two surface intermediates. Step one and two involve the electroreduction of surface species while step three occurs as a result of a chemical reaction. These two mechanisms differ only in that the first step of the overall reaction leading to hydrogen evolution is assumed to be at equilibrium at all electrode overpotentials for the second mechanism. They both fail to give satisfactory approximation of the experiment data, especially in the high overpotential range. The third mechanism proposed is based on the Volmer‐Heyrovský reaction sequence used to describe the hydrogen evolution reaction on metallic surfaces. This mechanism involves only one intermediate species. A more logical fit of the experimental data is obtained assuming this last reaction scheme.