Reaction of hydrogen water mixtures on nickel-zirconia cermet electrodes I. DC polarization characteristics

The reaction of hydrogen/water mixtures on nickel-zirconia cermet electrode s has been investigated as a function of potential and temperature at three different partial pressures of the reactants. Apparent kinetic parameters, namely, reaction orders, activation enthalpies, and pre-exponential factor s, were determined as a function of potential in low-temperature (725-845 d egrees C) and high-temperature (845-950 degrees C) regions for both the hyd rogen oxidation and the hydrogen evolution reactions. In the low-temperatur e region, the charge-transfer coefficient, alpha(a), calculated according t o absolute rate theory was 0.7, independent of temperature. The apparent re action order of the hydrogen oxidation reaction is in the region of 0.5 at 725 degrees C, from which we conclude that at low temperatures, charge tran sfer involving atomically adsorbed hydrogen species determines the rate of this reaction. The results for the hydrogen oxidation reaction at temperatu res above 845 degrees C as well as for the hydrogen evolution reaction are not consistent with a charge-transfer-controlled electrode reaction. Adsorp tion and chemical reaction between adsorbed species are considered to be do minant for the hydrogen oxidation reaction at temperatures above 845 degree s C and the hydrogen evolution reaction over the entire temperature range s tudied. (C) 1999 The Electrochemical Society. S0013-4651(98)07-058-X. All r ights reserved.