Meniscus Formation and Hydrogen Oxidation on Partially Immersed Pt-Carbon Electrode

Meniscus formation and hydrogen oxidation on partially immersed Nafion® coated and uncoated electrodes were investigated as a model of the reaction sites in polymer electrolyte fuel cells. Hydrogen oxidation current was measured at 0.4 V with raising the electrode, and simultaneously the optical microscope observation was carried out. The intrinsic meniscus developed from the boundary between Nafion®-coated (Pt-loaded) and the uncoated parts. A liquid thin film, which would have sub-micron order thickness, was observed above the intrinsic meniscus on the uncoated electrode under hydrogen atmosphere after development of the intrinsic meniscus was terminated. At the same time, hydrogen oxidation current was further increased. Therefore, the liquid thin film behaved as an effective supermeniscus on the uncoated electrode. On Nafion®-coated electrode, hydrogen oxidation current was small in the intrinsic meniscus region; however, it became larger with an increase in the exposed area of Nafion®-coated part to the gas phase. This fact showed that hydrogen oxidation reaction occurred mainly in the exposed Nafion®-coated part. The current distribution in the supermeniscus region was discussed, and it was found that the effective area is controlled by the ohmic drop in the Nafion® coating.