Electrochemical performance and feasibility of proton-conducting solid oxide fuel cells (SOFCs) using methane (CH4) fuel were investigated. At high fuel utilization, higher protonic transport number of electrolytes enables higher open-circuit voltage (OCV) when hydrogen (H-2) is the fuel. In contrast, a high oxide ionic transport number is needed when CH4 is the fuel. Conductivities of each charge carrier of an yttrium-doped barium zirconate (BZY) electrolyte were therefore estimated by measuring the OCV under various partial pressures. A dense BZY electrolyte was obtained by the addition of 0.6 mol% nickel (Ni). This electrolyte exhibited protonic- and oxide ionic-mixed conductivity; in particular, these conductivities were similar to each other at 800-900 degrees C, indicating high efficiency. A Ni/BZY anode enabled stable operation of a proton-conducting SOFC using 3% humidified CH4 fuel. The maximum power density of this SOFC was 2.35 mW cm(-2) at 900 degrees C and 1.69 mW cm(-2) at 800 degrees C, which were higher than that using a platinum anode. The activation energy for electrode polarization of the Ni/BZY anode using 3% humidified CH4 fuel was similar to that using 3% humidified H-2 fuel, indicating that H-2 via decomposition of CH4 was the main reactant on the Ni/BZY anode. (C) 2013 The Electrochemical Society. All rights reserved.