Akiya Matsuda

THE MECHANISM OF THE HYDROGEN EVOLUTION REACTION

Abstract The dual mechanism of the hydrogen evolution reaction is an alternative of the catalytic or the electrochemical mechanism operative depending on the experimental conditions and electrode material, as advanced by one of the present authors. The hydrogen evolution reaction consists of a set of steps H+ + + e → H(a) and 2H(a) → H2 or 2H+ + e → H+2 (a) and H+2 (a) + e → Hs2 respectively according to the catalytic or the electrochemical mechanism, where H+ denotes a proton associated with Bronsted base H2O or OH-, e is a metal electron, H(a) or H+2(a) is a hydrogen atom or hydrogen molecule-ion respectively adsorbed on the electrode surface and A is the rate-determining step. The slow discharge mechanism is expressed by the above notations as H+ + e → H(a), 2H(a) → H2, which appeared once before quite conclusive. Experimental and theoretical investigations, to which the dual and the slow discharge mechanisms have ever since been subjected, are reviewed. The dual mechanism is thus shown to be verified with regard to the effect of overvoltage or of pH on the rate, differential capacity of electrode, electrolytic separation factor of hydrogen isotopes and alternative operation of the catalytic and the electrochemical mechanisms on the same electrode depending on experimental conditions. The slow discharge mechanism is disproved by several points raised against it, which give, on the other hand, further evidence for the dual mechanism.