THERMODYNAMIC AND ELECTRODE KINETIC FACTORS IN CATHODIC HYDROGEN SORPTION INTO METALS AND ITS RELATIONSHIP TO HYDROGEN ADSORPTION AND POISONING

In the present paper, we examine how H sorption should depend on the c athodic H-2 evolution mechanism, through the potential dependence of t he fractional coverage by adsorbed H. The states of adsorbed and absor bed H are treated in terms of site-fraction statistics in order to det ermine the chemical potentials of the adsorbed and absorbed H. Then, t he relationships to the overpotential in the H-2 evolution reaction ar e derived and the efficiency of H sorption in relation to the coevolut ion of H-2 (important for metal hydride battery electrode charging) ca n be examined in terms of reaction mechanisms. Difficulties arising wi th application of the Nernst equation to H sorption when appreciable o verpotentials for H-2 evolution are involved, i,e. at appreciable cath odic current densities, are examined. Finally, the interesting effects of 'catalyst poisons', which promote H sorption into host metals, are examined in terms of the effects of competitive adsorption between th e poison and H at the surface. Conventional ideas about these effects are inconsistent with information on the diminution of H coverage caus ed by competitive adsorption of poisons; alternative mechanisms are di scussed and a new surface-thermodynamic basis for the enhancement of H sorption by adsorbed poisons is proposed.