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 . © 1999 The Electrochemical Society. All rights reserved.