The reaction mechanism of the most commonly used anode material, Ni/yttria-stabilized zirconia ͑YSZ͒ cermets, in H 2 -H 2 O solid oxide fuel cells ͑SOFCs͒ was investigated. Because the reaction mechanism for the Ni/YSZ anode in H 2 has not been conclusively determined, we investigated the detailed dependence of dc polarization and interfacial conductivity of Ni/YSZ cermet anode on the partial pressure of hydrogen (p H 2 ). Based on our experimental results, we developed a model that links the chemical reactions on the anode with the electrical characteristics of the anode such as the dc polarization and the interfacial conductivity. In our model, we assumed competitive adsorption equilibrium of H 2 , H 2 O, and O on Ni surfaces at the three-phase boundary, and assumed the rate-determining step to be Langmuir-type reactions of H with O. In SOFCs with Ni/YSZ anodes, reported dependencies of the current and interfacial conductivity on p H and p H 2 O differ among previous studies. Both our measured dependencies and previously published dependencies were successfully reproduced by our Langmuir reaction model. Furthermore, possible reasons for these different observed dependencies include the different adsorption equilibrium constants.

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