Determination of adsorption of OPD H species in the cathodic hydrogen evolution reaction at Pt in relation to electrocatalysis

Abstract

The coverage and pseudocapacitance behaviour of overpotential-deposited (OPD) H species in the cathodic H 2 evolution reaction (HER) at Pt electrodes has been determined by the “potential-decay” method, coupled with prior steady-state current measurements. The Tafel slopes and mechanisms of the HER at Pt in acid and alkaline solutions are rationalized in terms of the observed OPD H coverage behaviour in relation to a parallel pathway of electrochemical and recombination-controlled desorption steps. The best electrocatalytic activity of Pt electrodes for the HER arises in acid solutions where the nominal extent of measured OPD H coverage is found to correspond to the apparent formation of ca. 8 equivalent monolayers; one possible interpretation of this result is in terms of hydride formation in the near-surface region of Pt. As a result, the OPD M-H adsorption bond can be weakened so that the recombination step with low Tafel slope becomes the favoured desorption step and characterizes the kinetics at active Pt electrodes. The OPD H coverage and the related electrocatalytic properties of Pt electrodes for the HER, depend strongly on the “activity” or “cleanliness” of the electrode surface with respect to adsorbed impurities.