Reformed H-2 gas containing a small amount of CO and 10-30% CO2 degrades the performance of polymer electrolyte fuel cells (PEFCs). CO2 is known to be reduced at the Pt electrode at 0.06-0.30 V vs. DHE which corresponds to the anode potential of the PEFC. However, the relationship between the performance degradation of the PEFC and the reactivity of CO2 has not been clarified. In this study, the reactivity of CO2 and the adsorption of its reductant were investigated by employing Pt-Ru/C-based membrane electrode assemblies which include the Pt-Ru/C (Pt/C, Pt0.5Ru0.5/C, and Pt0.4Ru0.6/C) electrocatalyst under a pure CO2 atmosphere. As a result, it was found that the CO2 could be easily reduced at the Pt-Ru/C electrocatalyst at 0.06-0.30 V vs. DHE which was equivalent to the anode potential of the PEFC. Moreover, the reactivity of the adsorbed CO2 reductant was changed by the addition of Ru and increasing the cell temperature. Consequently, the most suitable anode catalyst among three kinds of catalysts examined was Pt0.4Ru0.6/C, which could easily desorb the CO2 reductant. (c) 2016 Elsevier Ltd. All rights reserved.