In this study, nickel pattern electrodes were electrochemically investigated in a three-electrode setup, operating both with H 2 /H 2 O and CO/CO 2 atmospheres. Heating introduced structural differences in the nickel layer among the pattern electrodes, which appear to affect the electrode performance. Both dense and porous nickel pattern electrodes were formed by heating. Holes appeared in the nickel layer of the porous pattern electrodes, where the open cavity triple phase boundaries exhibited different limiting processes than open triple phase boundary electrodes of the dense electrode. As the temperature was raised in the experiment, the electrodes stabilized, with a degraded behavior that seemed to be strongly coupled to the structural changes in the electrode. It was possible to compare literature results with high temperature impedance measurements in H 2 /H 2 O presented here, while new results at lower temperatures in H 2 /H 2 O are also presented. Impedance spectroscopy measurements were performed, and the gas dependence of the polarization resistance was observed as the mixture ratios and temperatures were varied in both atmospheres. A positive relation between the polarization resistance and the partial pressure of CO was determined for the dense nickel pattern electrode, which agrees with previous results using nickel point electrodes.Solid oxide fuel cells ͑SOFC͒ anodes operating with H 2 /H 2 O atmospheres have been studied using several types of electrode configurations such as point, 1-5 pattern, 6-12 and cermet electrodes. 13,14 The growing interest in the use of methane and syngas has motivated new electrochemical investigations both with methane 15 and CO/CO 2 atmospheres. 4,12 Previously, porous Pt-Ce 16 and Ni cermet 17 electrodes were used to compare the electrochemistry of both H 2 /H 2 O and CO/CO 2 . In the work by Sukeshini et al. 12 and the work reported here, Ni pattern anodes were used to compare both types of fuels.Interest in patterned electrodes is growing for several reasons. The possibility to examine the structure after heating is limited when using point electrodes, but patterning provides access to the triple phase boundary ͑TPB͒ for unambiguous ex situ detection of morphological changes [6][7][8][9][10][11][12]18,19 and potential impurity effects. 20 Furthermore, patterned electrodes allow one to isolate interfacial electrochemistry from other rate processes that complicate impedance spectra when using cermet electrodes ͑e.g., reactant diffusion, homogeneous gas phase reactions, heterogeneous nonredox reactions, etc. 12 ͒. The open access afforded by the patterned structures also allows one to apply spatially resolved measurement techniques in situ for unambiguous determination of species at the interface, 18 which is not straight forward using point electrodes. As an interesting extension of the patterned electrode concept, the work by Huang and co-workers 21 has demonstrated the ability to make nanostructured electrodes on very thin ͑submicrometer͒ yttria-stabilized zirconi...

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