Examination on Water Management Method in the Same Electrode in PEFC

Abstract

Recently, Polymer Electrolyte Fuel Cells (PEFCs) are beginning to be used in various fields as a clean energy source because they don’t emit carbon dioxide during operating. However, PEFCs have some problems such as a cost reduction and durability etc. Generally, PEFCs need a humidification system to avoid dryness of membrane during operating, but the amount of energy supplied to the humidifier is large, and reduces a system efficiency, and the cost of the humidification system is also necessary. And, the steam in the cathode gas becomes supersaturation in the vicinity of gas outlet, and thereafter it is condensed because the steam is generated by the cell reaction and is multiplied toward the gas outlet. Because the use of humidifier increases the amount of condensation and causes the water management issues such as the flooding phenomenon and the plugging phenomenon in the vicinity of gas outlet, these issues make the cell voltage unstable. Our laboratory has proposed the Self Water Management Separator (SWMS) with a bypass channel to solve these issues. The SWMS moves excess water from gas outlet to gas inlet through the bypass channel where filled water absorption material. Excess water was absorbed by a capillary phenomenon of water absorption material, and it moved from gas outlet to gas inlet by a concentration difference of water. The previous study clarified that the bypass position depended on the operating condition, and the optimum bypass position under non-humidifying and non-heating conditions (w/o humidifier and heater) was at gas inlet. Therefore, this study aims to select the optimum water absorption material for SWMS. The specification demanded for the water absorption material is a water supply and drainage ability and a handling ability. We prepared four water absorption material such as polyvinyl alcohol sponge (PVA), diatomite powder, cellulose fiber insulation (CFI), and EM diatomite grain. Here, cellulose insulation is a wood fiber insulation material. The SWMS was installed in only cathode side and a single serpentine was applied to both separators as a gas channel. The cell performance was evaluated by IV performance and Cole-Cole plots of AC impedance method under the standard condition and a durability test condition. Here, the standard condition and the durability test condition mean 0.3A/cm2 of the current density and Uf/Uox=70%/40% of the fuel/oxidant gas utilization, and 0.2A/cm2 and 45%/25%, respectively. The cell temperature of both conditions was 30°C. As a result, the resistance polarizations of the cell with SWMS on all water absorption material were equivalent or smaller than that of the cell w/o bypass channel as shown in Fig.1. Especially, the initial performance of the cell where PVA or CFI was filled in the bypass channel was better than that of others in a high current density region. The activation polarization of the cell with PVA was the smallest, and it has hardly changed on the durability test. Therefore, because PVA uniforms the humid condition in the cell, consequently the activation and resistance polarizations are improved by the increases of three phase interface and the ion conductivity. However, although the IV performance of the cell with CFI was almost the same as that of PVA, the resistance and activation polarizations were a little large compared with others and the diffusion polarization began to grow up. Because CFI has a high water supply and drainage ability, it deprived the membrane and three phase interface of necessary water, consequently it deteriorated the cell performance. Therefore, we decided that PVA is the most suitable as the water absorption material of the bypass channel from viewpoints of cell performance and handling ability. Figure 1