Behavior of overpotential—deposited species in Faradaic reactions—II. ac Impedance measurements on H 2 evolution kinetics at activated and unactivated Pt cathodes

Abstract

The behavior of the “overpotential-deposited” (opd) H species in the H 2 evolution reaction (h.e.r.) proceeding with appreciable net cathodic current at activated and unactivated rotated Pt electrodes is investigated by means of impedance spectrum measurements. The experiments were designed to complement our recent study of the opd H coverage and pseudo-capacitance behavior evaluated by means of the potential-decay method. Despite many papers on theoretical aspects of involvement of adsorbed H in the kinetics and mechanisms of the h.e.r. and on characterization of upd H at noble metals, surprisingly little experimental work has been done that leads to quantitative evaluation of opd H coverage and its dependence on potential in relation to the reaction mechanism and electrocatalytic behavior. Complex-plane impedance spectrum plots are derived for the h.e.r. at Pt in acid and alkaline solutions at various potentials. The behavior is quantitatively simulated by sets of rate-constant parameters that also provide a fit of the steady-state Tafel relationships. The significance of the steady-state H adsorption pseudocapacitance is examined in relation to what is measured as the capacitance component in ac impedance determinations and to the 3-element equivalent circuit for the Faradaic impedance. The results are consistent with those derived from analysis of open-circuit potential-relaxation (potential-decay) transients at the same electrodes. The impedance behavior for active Pt in acid solution suggests the participation of a diffusion process but not involving a solution species; this process is therefore probably associated with some H sorption in the surface region of the metal as indicated by previous potential-decay studies.