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Mass Transport inside PEFC during Power Generation Studied By Visualization of Oxygen Partial Pressure and Current Density
We have constructed a new system, which enables to visualize the distributions of p(O2) and current density. A cathode plate of a cell is made of a transparent acrylic resin, and the current collectors are segmented to nine both at the cathode and the anode (Fig.1). The electrochemical potentials of the current collectors were controlled to be the same by the external circuit. The ribs are partially transparent, and p(O2) can be visualized even under the ribs at those locations. As an oxygen sensor, a luminescent dye compound, PtTFPP ([tetrakis(pentafluorophenyl)porphyrinato] platinum]),1) was used, which absorbs 390-nm blue light and emits 650-nm red light, and the emission intensity lowers as p(O2) increases. This dye film was coated on the GDL. During the visualization, a 407-nm laser light was diffused, spread and distributed uniformly onto the transparent cathode plate. The emission from the dye on the GDL surface was filtered (> 600 nm), and the images were captured with a CCD camera (500 x 500 pixel, 1 pixel à 0.12 mm). Meanwhile, the current density at each current collector was recorded. The MEA was prepared with a Pt/CB catalyst (TEC10E50E), a Nafion® membrane (NRE211) and a GDL (SIGRACET® 25BCH).
Fig.2 shows the distributions of p(O2) and the current density at 80 oC and 80% RH, at the oxygen utilization (Uo2) of 10 (a), 40 (b), and 80% (c). As Uo2 increased, oxygen is clearly seen to be consumed. p(O2) under ribs was seen lower than that under flow channels. At Uo2 = 10%, the current densities near the outlet were higher than that near the inlet due to the higher proton conductivity inside the membrane by the generated water existing more near the outlet. At Uo2 = 40%, the current densities became relatively uniform because of the rather uniform water distribution in the membrane. At Uo2 = 80%, the current densities near the inlet became higher than that near the outlet. The change at Uo2 = 80% can be explained by the lower oxygen diffusion to the catalyst layer and the excess formation of liquid water near the outlet. In order to understand the difference of p(O2) under flow channels and ribs, p(O2) is plotted in Fig. 3 at Uo2 = 10, 40, and 80% along the white line drawn on the cell image on the left. Large modulation is seen at any Uo2; p(O2) at the middle of the channels was the highest, and lower under the ribs. Near the inlet, the modulation was larger at higher Uo2, probably because of the increased water vapor pressure under the ribs during the power generation at a fixed air flow rate. Near the outlet, the modulation became lower than that near the inlet. This is probably because of the mixture of gasses across the flow channel and the GDL as the gas moved from the inlet to the outlet. These results indicate that the oxygen diffusion might be lowered by the increased generated water at higher Uo2 with lower p(O2). This low diffusion of oxygen could be a reason for the inhomogeneous distribution of reactions, especially under the ribs.
This technique is expected to be used for the improvement of MEAs, cell designing, and for the optimization of operating conditions.
Acknowledgement
This research was performed under the HiPer-FC project supported by NEDO, Japan.
References
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