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A schematic diagram of the experimental setup used in this study is shown in Fig. 1a. Electrochemical experiments were conducted using a PEFC-operating apparatus (FCG-20S, ACE), a potentiostat/galvanostat (HA-310, Hokuto Denko), and a function generator (HB-104, Hokuto Denko). Fully humidified 100 vol% H2 and CO2 diluted with Ar (CO2 concentration: 0, 4, 7, 10, 20, 50, and 100 vol%) gas were fed to the anode and cathode at 50 cm3 min−1, respectively, in all experiments. Fully humidified 100 vol% H2 gas was supplied to the reference electrode at 10 cm3 min−1. The cell temperature was set to 40 °C because the cell humidification at least 40 °C is required to operate the MEA. The H2, CO2, and Ar gases were 99.999%, 99.995%, and 99.998% pure, respectively. Before electrochemical measurements, the cathodic potential sweep in the 0.05–0.70 V (vs. RHE) range at 50 mV s−1 was repeated until the current–potential curve of the cathode became stable. The initial cathode potential during introduction of CO2-containing gas was ~ 0.13 V (vs. RHE). The electrochemical surface area (ECSA) of the cathode electrocatalyst was obtained to be 0.294 m2 according to the hydrogen adsorption method30,42,43. The cathodic potential was scanned in the 0.08–0.70 V (vs. RHE) range at 10 mV s−1 during CV. It should be noted that that a 0.05–0.70 V (vs. RHE) potential range was used for CV with in-line product analysis. In the potential-step experiment, the cathodic potential was stepped through 14 levels in the 0.40–0.05 V (vs. RHE) range in the negative direction every 2 min at a CO2 concentration of 7 vol%. In addition, the cathodic potential was directly stepped from 0.40 to 0.20 V (vs. RHE) at 7 vol% CO2 and held there for 5 min, after which it was stepped to 0.05 V (vs. RHE). In-line mass spectrometry (MS) was carried out during the electrochemical experiments by introducing the cathode exhaust gas directly to a mass spectrometer (JMS-Q1050GC, JEOL). The ionization voltage was 23 eV. Note that the lag time for in-line MS product detection was adjusted by the H2 evolution response (7 s). A calibration curve, which was obtained using CH4 gas (purity: 99.999%) diluted with Ar, was used to calculate the amount of CH4 generated, for determining the faradaic efficiencies and CH4-yield rates. All the current densities were calculated using the apparent electrode surface area (9.0 cm2).

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