We now report the CO2-reduction performances of Pt-Ru/C electrocatalysts and power-generation characteristic of a polymer electrolyte fuel cell driven by feeding H-2 and CO2 to the anode (Pt/C) and cathode (Pt-Ru/C), respectively. The CO2 electroreduction was evaluated by the current-voltage relationships in combination with in-line mass spectrometry. The onset potentials for the CO2 reduction are observed at 0.06-0.4 V vs. the reversible hydrogen electrode (RHE), and they depend on the Pt:Ru composition of the Pt-Ru/C cathode. Mass spectrometry revealed that a Pt-rich catalyst generates CH4, whereas this gas is not or slightly generated at a Ru-rich catalyst. Based on these results, two significant effects of a small amount of Ru doping to Pt were elucidated: (i) it allows H, which is used for H-2 evolution at the Pt/C, to be used for the CH4 generation, and (ii) it decreases the adsorption energy of the CO2-reduced intermediate (CO) on the Pt. In the fuel cell test, the fuel cell effectively generates power by combining a Pt/C anode and a Pt0.8Ru0.2/C cathode rather than a Pt/C cathode. Consequently, we demonstrated that Pt0.8Ru0.2/C exhibits greater performances than Pt/C for the CO2 reduction and power generation of the H-2-CO2 fuel cell. (C) 2019 The Electrochemical Society.