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 cathodic H-2 evolution mechanism, through the potential dependence of the fractional coverage by adsorbed H. The states of adsorbed and absorbed H are treated in terms of site-fraction statistics in order to determine the chemical potentials of the adsorbed and absorbed H. Then, the relationships to the overpotential in the H-2 evolution reaction are derived and the efficiency of H sorption in relation to the coevolution of H-2 (important for metal hydride battery electrode charging) can be examined in terms of reaction mechanisms. Difficulties arising with application of the Nernst equation to H sorption when appreciable overpotentials for H-2 evolution are involved, i,e. at appreciable cathodic 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 the poison and H at the surface. Conventional ideas about these effects are inconsistent with information on the diminution of H coverage caused by competitive adsorption of poisons; alternative mechanisms are discussed and a new surface-thermodynamic basis for the enhancement of H sorption by adsorbed poisons is proposed.