BEHAVIOR OF A HYDRIDE PHASE FORMED IN THE HYDROGEN EVOLUTION REACTION AT A ROTATING PT ELECTRODE - ANALYSIS OF POTENTIAL RELAXATION TRANSIENTS FROM A KINETIC APPROACH

The behaviour of overpotential-deposited (OPD) H species in the hydrogen evolution reaction (HER) in acid and alkaline solutions at a rotating-disc Pt electrode (RDE) was investigated by analysing open-circuit potential relaxation curves using a kinetic approach, i.e. fitting potential-decay transients by a set of rate constants k for various reaction mechanisms. Previous indications of a hydride phase, involving a number of equivalent monolayers of OPD H in the near-surface region of the Pt interface, were confirmed in the present study for the HER on Pt in acid solution. The highest electrocatalytic activity of activated Pt in acid solution, among the three systems studied, is determined not only by large j0 (or large k values) but also by much lower Tafel slopes at low overpotentials eta because of the predominance of the recombination pathway for the HER at such surfaces. It seems that the recombination step is a preferred pathway for the HER on the active Pt surface at which a hydride phase is formed in the near surface region. However, for a deactivated Pt electrode in alkaline solution, the polarization current at eta = - 0.1 V is a factor of 10(4) smaller than for the same activated Pt in acid solution, corresponding to an OPD H yield about a factor of 10(3) less under the former conditions. The comparison was made in terms of the steady-state pseudocapacitance C(phi), calculated using best-fit rate constants and an apparent charge q1 for H accommodation, and the ''operational'' pseudocapacitance C(b)derived in previous studies.