Materials. Pt0.8Ru0.2/C (42.5 wt%; TECRuE43) and Pt/C (46.2 wt%; TEC10E50E) electrocatalyst powders were obtained from Tanaka Kikinzoku Kogyo Co., Ltd. Nafion-117 membranes (0.18-mm thick) were purchased from DuPont and boiled successively in Milli-Q water, 0.5 M H2O2, 0.5 M H2SO4, and Milli-Q water (1 h each) prior to use. All chemicals (H2O2, H2SO4, acetone, 2-propanol, methanol, and 5 wt% Nafion solution) were obtained from the Fujifilm Wako Pure Chemical Corporation. Water-repellent carbon paper (TGP-H-060H) was purchased from Toray Industries, Inc, and polymer electrolyte cell components (gasket, separator with parallel flow paths, and stainless steel plate) were purchased from Miclab.
Cell fabrication. A polymer electrolyte fuel cell (PEFC) was fabricated by essentially following the same procedure reported previously23–25, with the exception that Pt0.8Ru0.2/C was used as the cathode instead of Pt/C. Briefly, a 6 × 6 cm Nafion 117 membrane and 3 × 3 cm pieces of carbon paper pretreated with acetone were used as the proton-exchange membrane and gas diffusion layers, respectively. The electrocatalyst dispersion was prepared by mixing the Pt0.8Ru0.2/C catalyst with 5 wt% Nafion (1:1 v/v) and an aqueous solution containing 1:2:1 (w/w/w) 2-propanol, methanol, and Milli-Q water, followed by spraying onto one piece carbon paper to prepare the cathode. The anode was prepared by spraying a Pt/C electrocatalyst dispersion onto another piece of carbon paper. The amount of loaded metal and the apparent electrode surface area were 1.0 mg·cm− 2 and 9.0 cm2, respectively, on both electrodes. The MEA was prepared by bringing these electrodes into contact with each side of the Nafion-117 membrane, followed by hot-pressing at 140 °C with a 4.5 kN load for 10 min. It should be noted that the Pt/C electrocatalyst dispersion was dropped onto the Nafion 117 membrane on the anode side to provide a reference reversible hydrogen electrode (RHE). Finally, the MEA, gasket, separator, and stainless steel plate were assembled to complete the PEFC used in this study, as shown in Fig. 1a.
Electrochemical CO 2 reduction and product analysis. 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. The H2, CO2, and Ar gases were 99.999%, 99.995%, and 99.998% pure, respectively. 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 faradaic efficiencies and CH4-yield rates.