Competitive Adsorption Reaction Mechanism of Ni/Yttria-Stabilized Zirconia Cermet Anodes in H[sub 2]-H[sub 2]O Solid Oxide Fuel Cells

Abstract

The reaction mechanism of the most commonly used anode material, Ni/yttria-stabilized zirconia (YSZ) cermets, in H2­H2OH2­H2O solid oxide fuel cells (SOFCs) was investigated. Because the reaction mechanism for the Ni/YSZ anode in H2H2 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 (pH2).(pH2). 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 H2,H2, H2O,H2O, 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 pHpH and pH2OpH2O 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. © 2001 The Electrochemical Society. All rights reserved.