Single-cell solid oxide fuel cell experiments using thin-film, sputter-deposited Ni pattern anodes microfabricated on single-crystal yttria-stabilized zirconia ͑YSZ͒ electrolyte disks have been performed to examine the electrochemical oxidation of H 2 , CO, and CO/H 2 mixtures. Porous lanthanum strontium manganate ͑LSM͒/YSZ cathodes have been used and characterized in separate symmetric cell experiments such that Ni anode overpotentials and impedances can be isolated. Post-test scanning electron microscopy imaging revealed that at the high temperatures ͑735-850°C͒, the 100-nm thin Ni patterns broke up into interconnected regions resulting in three-phase boundary lengths that roughly correlated with the original coverage area of the pretested dense Ni patterns. Electrochemical characterization for H 2 , CO, and CO/H 2 oxidation under dry and wet ͑ϳ4% H 2 O͒ feeds showed that the interconnected anode overpotentials and polarization resistances correlated with the original Ni pattern area for the various pattern geometries. Higher activation overpotentials and polarization resistances observed for CO in comparison to H 2 were not observed for CO/H 2 mixtures down to 25% H 2 . Results indicated detrimental effects of H 2 O on CO oxidation power densities due to drops in open-circuit voltages without reduction in polarization resistance, and enhancement due to water-gas shift reactions was not seen. Our results provide the basis for insights into H 2 and CO electro-oxidation on Ni/YSZ anodes.High-temperature solid oxide fuel cells ͑SOFCs͒ are being developed to operate on direct feeds of methane, 1-3 larger hydrocarbons fuels, 4 and also oxygenated fuels. 5 The numerous efforts to sustain direct utilization of hydrocarbons in SOFCs have indicated that conventional Ni/yttria-stabilized zirconia ͑YSZ͒ SOFC anodes are prone to uncontrolled carbon deposition under conditions for dry hydrocarbon feeds, particularly for species with multiple carbon atoms. 6 Thus, much research has focused on alternative anode materials, such as Cu/CeO x , 7-9 and alternative operating strategies where either steam 1,3 or O 2 10,11 is added to the anode flow stream. With the addition of steam and or O 2 in a hydrocarbon feed, a porous Ni/YSZ anode matrix can provide catalytic internal reforming and thereby convert some, if not all, of the hydrocarbon to H 2 and CO either through steam reforming or partial oxidation. 8,12 Internal reforming reactions have been shown to work well under some conditions at alleviating carbon deposition in conventional Nibased anode structures. 1,3,13 Internal diffusion of products along the length of a fuel cell will promote some steam reforming even with pure hydrocarbon feeds, but it remains an outstanding question as to whether internal reforming can be sustained such that otherwise unacceptable Ni/YSZ anodes may be viable with some carbonaceous fuels. Reforming of hydrocarbons in SOFCs or even in upstream reactors 14 suggests the importance of electrochemical oxidation of H 2 and CO mixtures 15 for understan...

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