KINETICS OF THE ELECTRODE-REACTION AT THE H2-H2O POROUS PT STABILIZEDZIRCONIA INTERFACE

To elucidate the mechanism of the electrode reaction at the interface of H-2-H2O porous Pt/stabilized zirconia, measurements were made on th e electrode interface conductivity, sigma(E), and the steady-state pol arization current, I, as a function of H-2 partial pressure, p(H-2), a nd H2O partial pressure, p(H2O), up to 800-degrees-C. The rate of the electrode reaction at 700 to 800-degrees-C in H-2-H2O atmospheres is f aster than that in CO-CO2 and slower than that in O2 rich atmospheres by 1 to 2 orders of magnitude. The rate-determining reaction process w as the exchange of adsorbed OH radicals between the Pt- and the stabil ized zirconia-surface at the triple-phase boundary (TPB) of gas/Pt/sta bilized zirconia. When the oxygen activity, a0, (vs. O2 gas of 1.013 x 10(5) Pa) on the zirconia surface at the TPB was higher than 10(-10), the predominant rate-determining reaction and the rate equation were given by H(ad) (Pt) + OH(ad) (SZ) --> H2O(ad) (Pt) + V(ad) (SZ) and I = k1theta(H)(Pt)1/2theta(OH)(SZ)1/2 - k1'theta(H2O) (Pt)1/2theta(v) (S Z)1/2, respectively. Those in the region of a0 < 10(-12) were expresse d by H(ad) (Pt) + H2O(ad) (SZ) --> H2O(ad) (Pt) + H(ad) (SZ) and I = k 2theta(H) (Pt)1/2theta(H2O) (SZ)1/2 - k2'theta(H2O)(Pt)1/2theta(H) (SZ )1/2, respectively. Here, k1, k1', k2, and k2', are the rate constants , and theta denotes the coverage of the adsorbed species indicated by suffix on the surface of the solid shown in parentheses.