Comparative effects of adsorbed S-species on H sorption into Pd from UPD and OPD H: a kinetic analysis

The effect of chemisorption of HS−, as S, on H transfer into and through Pd over the potential range for underpotential deposition (UPD) of H in aqueous NaOH solution has been studied. A kinetic-fitting model is applied to the constituent partial reactions involved at the UPD polarization side and the corresponding H permeation (exit) side of a Pd bi-electrode, including processes involving chemisorbed poison (P) species. The relations of surface coverage by H on the Pd surface in the UPD region (also extended comparatively to the overpotential-deposited (OPD) H region, involving net rates of H2 evolution) to the subsurface concentration of H, and the effect of chemisorption of sulfide species on H transfer into and through the membrane electrode have been examined. In the case of Pd, the H adsorption pseudocapacitance derived by the kinetic fitting procedure is more reliable than that evaluated by ac impedance. It is shown that the expected 1:1 ratio correspondence between the H adsorption and H permeation rates arises almost quantitatively over the UPD region for which, in both the presence and absence of sulfide species, electrodeposition of H is found to be the rate-determining step in the overall H permeation process. Over the OPD potential region, sorption of H into Pd arises both from the OPD and the UPD H.