Twelve carbon supports with different specific surface areas (Sc) were chosen as electroconductive carbon blacks. The carbon blacks were produced by the oil-furnace process or acetylene process. The supported Pt catalysts (Pt-C) were prepared according to a colloidal method in the literature. 4 The PFSI solution: an experimental Flemion® solution (Asahi Glass) was used and poured into butyl acetate. The mixture changed immediately to a colloidal state. Then the Pt-C made of various carbon blacks were mixed with the PFSI colloidal solution. As a result of cross-linking among the PFSI chains adsorbed on the carbon supports by ultrasonic treatment, the mixture was transformed to a paste. The paste was uniformly spread over the wet-proofed carbon paper (Toray) for a gas-diffusion layer. The electrodes layered with the paste were hot pressed on both sides of a PFSI membrane. The membrane thickness of Flemion A (experimental membrane, Asahi Glass) was 80 pm in a dry state, and the equivalent weight value was 900. Identical electrodes were used for an anode and a cathode. The Pt-loading was 0.5 mg/cnr per electrode, and the PFSI content ranged from 0.1 to 2.0 mg/ cm" per electrode. A single cell was constructed with the M&E assembly, two porous carbon plates with rib-channel patterns on one side for gas supply, two glassy carbon frames, and two copper current collectors. The cell was compressed between two stainless steel end plates. The cell temperature was held at 50 °C throughout the experiment. The geometric reaction-area of the cell was 25 cm2. The reactant gases (hydrogen/oxygen) were fed to the cell at a flow rate of 350 ml/min. The gas-outlets from the cell were opened to the atmosphere. The anode gas and the cathode gas were humidified by being flowed through water held at 60 °C in glass bottles under atmospheric pressure prior to being provided to the cell. Electrochemical measurements of the single cell were done galvanostatically. The current was increased stepwise every 15 minutes, and the potential was logged in the steady state. All of the denoted polarization curves contained IR resistance. The resistance of the cell was measured at 1 kHz under load by a digital ac milliohmmeter. The pore-size distribution and the specific pore volume were calculated from an intrusion curve obtained by a mercury pore sizer. The specific surface area (Sc) and the pore volume distribution of the carbon blacks were measured by an N2 adsorber and calculated by the Brunauer-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) equations. The Pt mean diameter and the Pt surface area of the Pt-C were measured by a CO adsorber. A particle-size distribution of the PFSI colloid made from the Flemion solution and butyl acetate was measured by a dynamic light-scattering spectrophotometer with Ar laser.

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