Ni pattern electrodes with well-defined triple phase boundary ͑TPB͒ lengths were prepared in order to investigate the reaction mechanisms at solid oxide fuel cell ͑SOFC͒ anodes. The anode microstructures were stable during thermal treatment and electrochemical measurements. Electrochemical impedance spectroscopy was used to study the influence of the overpotential, of the gas atmosphere, of the temperature, and of the pattern geometry on the electrochemical behavior of SOFC anodes. It is found that the reaction kinetics are dominated by one main process. This process is thermally activated with an activation of energy of E A ϭ 0.88 Ϯ 0.04 eV. At overpotentials higher than 300 mV, a second process becomes relevant. The partial pressure of water in the fuel gas atmosphere has a catalytic effect on the anode performance, whereas variations of the patrial pressure of hydrogen in the fuel gas atmosphere have no significant influence on the electrode behavior. A model was established in order to explain the catalytic effect of water. The direct proportionality between the relaxation frequency and the TPB length suggests a TPB limitation of the anode kinetics.

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The electrochemical behavior of Ni-based SOFC anodes with different microstructures was studied. The anodes consisted either of a Ni gauze, a Ni pattern, a Ni paste, or a Ni –YSZ cermet. According to electrochemical impedance spectroscopy (EIS) measurements, the anodes are dominated by two main processes. The high frequency process decreases exponentially with an applied overpotential and is thermally activated with an activation energy of around 1 eV. The process could be attributed to the adsorption of hydrogen including charge transfer. The low frequency impedance arc arises only under a high overpotential applied between the working and the reference anode. The impedance of this process increases the higher the overpotential and is thermally activated with an activation energy of 0.5 eV. This process is assigned to the desorption of water.

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