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 aqueo us NaOH solution has been studied. A kinetic-fitting model is applied to th e constituent partial reactions involved at the UPD polarization side and t he corresponding H permeation (exit) side of a Pd bi-electrode, including p rocesses involving chemisorbed poison (P) species. The relations of surface coverage by H on the Pd surface in the UPD region (also extended comparati vely to the overpotential-deposited (OPD) H region, involving net rates of H-2 evolution) to the subsurface concentration of H, and the effect of chem isorption of sulfide species on H transfer into and through the membrane el ectrode have been examined. Tn the case of Pd, the H adsorption pseudocapac itance derived by the kinetic fitting procedure is more reliable than that evaluated by ac impedance. It is shown that the expected 1:1 ratio correspo ndence between the fl adsorption and H permeation rates arises almost quant itatively over the UPD region for which, in both the presence and absence o f sulfide species, electrodeposition of fl is found to be the rate-determin ing step in the overall H permeation process. Over the OPD potential region , sorptionof H into Pd arises both from the OPD and the UPD H. (C) 2000 Int ernational Association for Hydrogen Energy. Published by Elsevier Science L td. All rights reserved.