X-ray Imaging of Water Distribution in Cathode Electrode of PEFC and Control of Liquid Water Transport by Electrode Perforation

Abstract

Water flooding in cathode electrodes is a critical issue for achieving high efficiency and high durability of polymer electrolyte fuel cells (PEFCs). To alleviate flooding, it’s necessary to design the optimum electrode/channel structure for positively removing liquid water from catalyst layers (CLs) and gas diffusion layers (GDLs) toward flow channels. In the previous works, Gerteisen et al. [1] presented a concept of perforated GDL structure to secure water passages through the porous electrode. Turhan et al. [2] revealed that the hydrophilization of cathode channel wall effectively enhances the liquid suction from the under-land locations into gas channels. Nishida et al. [3] have proposed the novel electrode/channel structure combining the electrode perforation and channel hydrophilization, and have investigated its effect on the improvements of water discharge and cell performance. In this study, the liquid water distribution in the cathode electrode of an operating PEFC was directly visualized using X-ray imaging, and the effects of introduction of penetration groove on the water transport inside the GDL were examined. Furthermore, the impact of combinational structure with electrode perforation and channel hydrophilization on the performance characteristics of the cell was demonstrated during the constant-current operation test.Figs. 1(a) and 1(b) show the outline assembly drawing of the small-sized visualization cell for X-ray radiography, and the measurement region in the cross-sectional view. The membrane electrode assembly (MEA) is sandwiched between two carbon separators with a single-serpentine flow field (number of channels: 7, channel width: 1.0 mm, depth: 1.0 mm, length: 88.5 mm). The effective electrode area is 2.88cm2. The X-ray beam is emitted to the cathode GDL in the in-plane direction, and the through-plane water distributions in the GDL are observed under the 4th channel and under the land between the 3rd and 4th channel. As shown in Fig. 1(b), the penetration groove (width: 100 or 200 µm, length: 11 mm) installed into the porous media by using a thin-edged blade is positioned along the sidewall of the 4th channel. Liquid water accumulated in the CL is firstly discharged into the largely opened groove. Subsequently, water droplets gathered in the groove grow up gradually and come into contact with the channel sidewall. When the droplets are attached to the hydrophilized sidewall, they are spread out on the wall surface and liquid films are moved upward. This water suction through the penetration groove and along the hydrophilic channel wall effectively reduces water flooding near the cathode CL.Fig. 2 presents the through-plane distributions of water saturation in the cathode GDL under the channel and land. Fig. 2(a) denotes the result for the non-modified GDL/channel structure (non-perforated GDL and non-hydrophilized channel); Figs. 2(b) and 2(c) denote for the customized structures with penetration grooves (width: 100 and 200 µm) and channel hydrophilization. The current density is increased stepwise from 0.17 to 1.39 A/cm2 for 40 min. All visualization images are obtained after 40 min of operation. In the case with the penetration groove, the water saturation in the GDL is clearly decreased around the groove because liquid water accumulated in the porous media is discharged into the large groove. Furthermore, the reduction of water saturation under the channel due to the electrode perforation is more remarkable as compared to that under the land. The region of water saturation reduction in the GDL is enlarged with increasing the groove width from 100 to 200 µm.