Interfacial processes involving electrocatalytic evolution and oxidation of H2, and the role of chemisorbed H

States of chemisorbed H that can be involved in the mechanism of the cathodic H2 evolution reaction (HER) at metals, specially Pt, and the reverse oxidation reaction (HOR), are examined. Particular attention is paid to possible differences between underpotential-deposited (UPD) H, electrosorbed at potentials below the hydrogen reversible potential, and so-called overpotential-deposited (OPD) H that is involved, kinetically, as an intermediate in the HER. UPD H is involved in the HOR, following dissociative chemisorption of H2, at potentials positive to the RHE potential. The roles of diffusion-control involving H2, both in the HER and the HOR, are discussed in the light of experimental results for cathodic and anodic polarization. Values of exchange current-density, jo, for the HER/HOR at equilibrium depend widely on the adsorptive and electronic properties of the metal, often representable as a ‘volcano’ relation with respect to metal-to-H bond energy. The origin of such relations, especially at the catalytic noble metals, is examined in terms of UPD and OPD states of H. Mechanistic and kinetic aspects of the HER and HOR are treated and their sensitivity to presence of CO and to surface structure at Pt single-crystal faces is reviewed.