Reaction of hydrogen/water mixtures on nickel-zirconia cermet electrodes. II. AC polarization characteristics

The impedance of nickel-zirconia cermet electrodes at the open-circuit potential was investigated as a function of temperature for different partial pressures of hydrogen and water. The impedance spectra for the hydrogen/water reaction show three apparent relaxation frequencies that are influenced by the temperature. The data are analyzed using an equivalent circuit representing two parallel reaction pathways which are attributed to (i) a reaction at the triple-phase boundary involving charge transfer and adsorption and (ii) a diffusion branch described by a resistor in series with a Warburg impedance. At temperatures below 890°C, a charge-transfer resistance dominates the real part of the impedance. A possible contribution of adsorption in the impedance spectra at higher temperatures is discussed. The diffusion branch is tentatively attributed to an electrochemical reaction in the area of the nickel/electrolyte interface, which is limited by diffusion of protons in the electrolyte phase. The activation enthalpy calculated for this process is between 120 and 160 kJ mol^-1.