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 st udied 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 mechanis m of the her is a consecutive combination of the Volmer step, followed domi nantly by a rate controlling Tafel step, while the contribution of the para llel 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 f ull coverage by H-ads (<theta>(H) -->1). In this potential region the mecha nism 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. T he 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) 200 1 Published by Elsevier Science B.V.