State of adsorption and coverage by overpotential-deposited H in the H 2 evolution reaction at Au and Pt

Abstract

By means of potential decay measurements using a digital data acquisition and computer processing system, accurate values of overpotential decay rates, dη/d t, may be obtained from which the pseudocapacitance and coverage behaviour of overpotential-deposited (OPD) H species in the cathodic H 2 evolution reaction (HER) at Au electrodes may be quantitatively derived as a function of η. The behaviour of the HER at Au is of interest in that an unusual Tafel slope value of 2.3 RT/F is exhibited in acid solution while the value of alkaline solutions is the more familiar value of ca 2.32 RT/F. Analysis of the overpotential relaxation behaviour on open-circuit, following interruption of cathodic polarization currents, gives an almost constant and small interfacial capacitance corresponding to a double-layer capacitance. Steady-state OPD H coverage (θ) is hence quite small (θ H<0.3%). The Tafel slope value of 2.3 RT/F requires, however, a potential-dependent H coverage determined by an H electrosorption step almost at equilibrium but with θ H small. The rate-determining step is suggested to be surface diffusion to preferred desorption sites. Comparative results are presented for the HER at activated Pt where, contrary to the behaviour of Au, a large pseudocapacitance for OPD H is derived that may be associated with some surface hydride formation, the possibility of readsorption of H from a boundary layer supersaturated with H 2 having been minimized by electrode rotation at a sufficiently high rate of 3600 rpm.