On the kinetics of the hydrogen evolution reaction on nickel in alkaline solution - Part I. The mechanism

The mechanism and kinetics of the hydrogen evolution reaction (her) were studied in 1.0 mol dm(-3) NaOH at 393.0 K. It was found that a combination of classical steady-state voltammetry and impedance spectroscopy can help to elucidate dilemmas concerning the role of the Heyrovsky and Tafel steps in the mechanism of this reaction. Thus, within the potential region - 0.95 > E > - 1.1 V (SHE) (curvilinear part of the polarization curve) the mechanism of the her is a consecutive combination of the Volmer step, followed dominantly by a rate controlling Tafel step, while the contribution of the parallel Heyrovsky step is negligible. At potentials E < (approximately - 1.2 V) a Tafel line with a slope of - 0.121 V dec(-1) is obtained, with almost full coverage by H-ads (<theta>(H) -->1). In this potential region the mechanism of the her is a consecutive combination of a Volmer step, followed by a Heyrovsky step, while the contribution of the Tafel step is negligible. The comparison of the calculated partial rate constants for these two steps shows that the rate of the her is controlled by the Heyrovsky step. (C) 2001 Published by Elsevier Science B.V.