Oxygen Partial Pressure inside Gas Diffusion Layer of PEFC during Power Generation Using Micro Probe (2014)

Reaction distributions inside polymer electrolyte fuel cells (PEFCs) are inhomogeneous, which is one of the major causes of cell performance loss and catalyst degradation. Therefore, understanding the reaction distributions of a PEFC is important. In PEFCs, gas diffusion has a significant impact on the cell performance. Our group has visualized the oxygen partial pressure on the surfaces of gas channels and the gas diffusion layers of PEFCs during the power generation using an oxygen sensitive dye film 1-3) . However, the oxygen partial pressure inside the membrane electrode assembly (MEA) is still little understood. We have thus developed a new apparatus that enables to measure the oxygen partial pressure inside the GDL. In order to measure the oxygen partial pressure, we created a hole with a diameter of 100 μm through the GDL. In the hole, we inserted a glass fiber probe etched to have a diameter of 45 μm (see figure 1). The distance between the probe apex and the catalyst layer was measured by the interference of the irradiation/reflection lights of 830 nm. As an oxygen sensor, a luminescent dye compound, PtTFPP ([tetrakis(pentafluorophenyl)porphyrinato]platinum]) was used, which absorbs 380-nm blue light and emits 650-nm red light; the emission intensity lowers as the oxygen partial pressure increases. This dye film was coated on the apex of the probe. The excitation light is introduced into the probe, and the emission from dye film comes back through the same probe. Prior to the power generation, the calibration curves of the dye film were obtained under the mixed gases of air and N 2 . The gas in the cell was fully purged, and the cell operation was started at the cell temperature of 70 o C with humidified air and H 2 at 60% RH. During the operation, the emission from the dye was captured by a CCD camera, and the intensity was converted to the oxygen partial pressure by using the calibration curve. Figure 2 shows the oxygen partial pressure inside the cathode GDL during the cell operation. The apex of the optical probe was set at the distance of 20, 40, and 60 μm from the catalyst layer. The oxygen partial pressure in the middle of the flow channel was also measured. At a low utilization of oxygen, UO 2 , 8.4%, the oxygen partial pressure inside the GDL was nearly identical to that in the flow channel, but it decreased at the position 20 μm up from the catalyst layer. As the oxygen utilization increased, the oxygen partial pressure started to decrease away from the catalyst layer, several tens of micrometers up, because more oxygen needs to be consumed. We have succeeded in the measurement of the oxygen partial pressure inside the GDL for the first time. The technique should be useful for analyzing the reaction distribution inside the cell as well as for the MEA and cell designs. Acknowledgements This study was supported by JST, Japan. References 1) Junji Inukai, Kenji Miyatake, Kenji Takada, Masahiro Watanabe, Tsuyoshi Hyakutake, Hiroyuki Nishide, Yuzo Nagumo, Masayuki Watanabe, Makoto Aoki, and Hiroshi Takano, Angew. Chem. Int. Ed., 47, 2792 (2008). 2) Junji Inukai, Kenji Miyatake, Yuta Ishigami, Masahiro Watanabe, Tsuyoshi Hyakutake, Hiroyuki Nishide, Yuzo Nagumo, Masayuki Watanabe and Akira Tanaka, Chem. Commun., 1750 (2008). 3) K. Takada, Y. Ishigami, J. Inukai, Y. Nagumo, H. Takano, H. Nishide and M. Watanabe, J. Power Sources, 196, 2635 (2011).