Electrochemical kinetics of hydrogen evolution at copper-nickel alloys. Relation to electronic properties of the electrodes

Abstract

The mechanism of the cathodic hydrogen evolution reaction at a number of transition and related metals (e.g. Cu and Ni) is still undecided. Tafel slope ( b) criteria are ambiguous since b values, for example, for both of these metals are about 0·11, corresponding either to rate-determining H + discharge (I) or H + ion-radical desorption (II). However, with varying metallic properties, e.g. surface affinity for H, the direction of dependence of I on heat of adsorption of H at a series of metals will tend to be opposite from that for II. Studies have therefore been made of the electrochemical exchange currents i 0 for hydrogen evolution and corresponding apparent heats of activation Δ H ‡ at a series of CuNi alloys in order to determine the directions of dependence of i 0 and Δ H ‡ on electronic and adsorptive properties of the metals. It is found that both i 0 and the apparent experimental Δ H ‡ increase from Cu to Ni. This is an unusual and unexpected result. The same trend of heat of activation with composition is found when the “true” Δ H ‡ values are estimated by non-thermodynamic considerations. The anomaly is resolved by considering the values of i and Δ H ‡ at the potentials of zero charge φ 0 for the alloy series. At φ 0, i increases from Ni to Cu but Δ H ‡ decreases. This trend is consistent with an atom-ion desorption mechanism if the heat of adsorption of H is greater at Ni than at Cu as indicated experimentally. The dependence of the kinetic parameters i 0 and Δ H ‡ on electronic configuration in the alloys is examined.