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

The reaction of hydrogen/water mixtures on nickel-zirconia cermet electrodes 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 factors, were determined as a function of potential in low-temperature (725-845 °C) and high-temperature (845-950 °C) regions for both the hydrogen oxidation and the hydrogen evolution reactions. In the low-temperature region, the charge-transfer coefficient, α_a, calculated according to absolute rate theory was 0.7, independent of temperature. The apparent reaction order of the hydrogen oxidation reaction is in the region of 0.5 at 725 °C, from which we conclude that at low temperatures, charge transfer involving atomically adsorbed hydrogen species determines the rate of this reaction. The results for the hydrogen oxidation reaction at temperatures above 845 °C as well as for the hydrogen evolution reaction are not consistent with a charge-transfer-controlled electrode reaction. Adsorption and chemical reaction between adsorbed species are considered to be dominant for the hydrogen oxidation reaction at temperatures above 845 °C and the hydrogen evolution reaction over the entire temperature range studied.