The conversion of electricity to chemical energy is a key technology for absorbing the fluctuations of electricity and utilizing solar and wind-powered electricities as chemical fuels. An electrochemical system equipped with a phosphate-based electrolyte has been reported as a promising method to efficiently obtain CH₄ from CO₂ and H₂O in a single electrochemical cell. Electrochemical cells with 10 wt%-Ru/ZrO₂, Ru/Al₂O₃, and Ru/TiO₂ were examined for CH₄ synthesis from CO₂ and H₂O at 210–270 °C and 10 mA cm⁻². Approximately 13 and 1 nmol s⁻¹ cm⁻² of CH₄ and H₂, respectively, were formed using the Ru/ZrO₂ cell at 270 °C, and the current efficiencies for CH₄ and H₂ formation were 97 and 3%, respectively. The cells with Ru/Al₂O₃ and Ru/TiO₂ produced lower CH₄ yields. An H-permeable membrane (Pd–Ag) cathode cell, which was designed in a previous study, and a newly designed PTFE membrane filter cell was investigated with Ru/ZrO₂. Both cells demonstrated similar formation rates and current efficiencies, which means that the Pd–Ag membrane is not essential for the synthesis of CH₄ from CO₂ and H₂O. The catalysts were analyzed using transmission electron microscopy and temperature-programmed desorption of H₂.