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 pote ntial was investigated as a function of temperature for different partial p ressures of hydrogen and water. The impedance spectra for the hydrogen/wate r reaction show three apparent relaxation frequencies that are influenced b y the temperature. The data are analyzed using an equivalent circuit repres enting two parallel reaction pathways which are attributed to (i) a reactio n at the triple-phase boundary involving charge transfer and adsorption and (ii) a diffusion branch described by a resistor in series with a Warburg i mpedance. At temperatures below 890 degrees C, a charge-transfer resistance dominates the real part of the impedance. A possible contribution of adsor ption in the impedance spectra at higher temperatures is discussed. The dif fusion branch is tentatively attributed to an electrochemical reaction in t he 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). (C) 1999 The Electrochemic al Society. S0013-4651(98)11-075-3. All rights reserved.