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.

Modified strontium titanates have received much attention recently for their potential as anode material in solid oxide fuel cells (SOFC). Their inherent redox stability and superior tolerance to sulphur poisoning and coking as compared to Ni based cermet anodes could improve durability of SOFC systems dramatically. Various substitution strategies can be deployed to optimise materials properties in these strontium titanates, such as electronic conductivity, electrocatalytic activity, chemical stability and sinterability, and thus mechanical strength. Substitution strategies not only cover choice and amount of substituent, but also perovskite defect chemistry, distinguishing between A-site deficiency (A_1−xBO₃) and cation-stoichiometry (ABO_3+δ). Literature suggests distinct differences in the materials properties between the latter two compositional approaches. After discussing the defect chemistry of modified strontium titanates, this paper reviews three different A-site deficient donor (La, Y, Nb) substituted strontium titanates for their electrical behaviour and fuel cell performance. Promising performances in both electrolyte as well as anode supported cell designs have been obtained, when using hydrogen as fuel. Performances are retained after numerous redox cycles. Long term stability in sulphur and carbon containing fuels still needs to be explored in greater detail.

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, reactions 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, , calculated according to absolute rate theory was 0.7, independent of temperature. The apparent reaction order of the hydrogen oxidation reactions 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. 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. © 1999 The Electrochemical Society. All rights reserved.