Acceleration of Liquid Water Removal from Cathode Electrode of PEFC by Combination of Channel Hydrophilization and Diffusion Medium Perforation

Abstract

Water flooding at cathode catalyst layer (CL) is a critical issue for high power density and long-term operation of polymer electrolyte fuel cells (PEFCs). Especially, at high current densities, excessive water produced by oxygen reduction reaction (ORR) on cathode side is rapidly condensed and accumulated inside porous electrode. When open pores in CL and gas diffusion layer (GDL) are filled with liquid water, oxygen cannot be sufficiently fed to reaction sites. To alleviate this issue, it is necessary to design the optimum channel/GDL structure for accelerating through-plane water removal. Turhan et al. [1] investigated the through-plane liquid storage, transport and flooding mechanism as a function of channel wall wettability with the use of high-resolution neutron imaging. Results revealed that hydrophilization of cathode channel forms liquid film layers around channel walls, which is difficult to purge. However, hydrophilic channel effectively enhances liquid water suction from under-land locations into gas channels. Nishida et al. [2] also demonstrated that the through-plane water transport from GDL to channel is encouraged by channel hydrophilization, and the voltage drop due to flooding is reduced. On the other hand, several researchers presented a concept of perforated GDL structure to secure sufficient water passages through porous electrode. Gerteisen et al. [3] developed the customized GDL which is structured with water transport pathways by laser perforation, and revealed that this modified GDL improves the limiting current density of 8-22%. The perforated GDL structure beneficially enhances in-plane water discharge from porous media toward large penetration holes. However, the laser perforations may act as water pooling locations under high-current and high-humidity conditions because of the removal of PTFE coating, leading to performance degradation [4].This study proposes the novel channel/GDL combinational structure of channel hydrophilization and GDL perforation as shown in Fig. 1, in order to promote water removal through cathode electrode.Liquid water accumulated at the CL is discharged into the large penetration hole. Subsequently, the water droplets gathered in the hole gradually grow up and reach the hydrophilic channel. When these droplets are attached to the channel sidewall, they are immediately spread out on the hydrophilic surface and liquid films are moved upward along the sidewall. This water suction through the penetration hole effectively alleviates water flooding near the cathode CL. In this experiment, the liquid droplet behavior inside the cathode channel of an operating PEFC is firstly observed based on a cross-sectional visualization technique, and the effect of hydrophilic treatment of cathode channel on the enhancement of through-plane water transport and the performance improvement is investigated. Secondly, the impact of combinational structure of channel hydrophilization and GDL perforation on the reduction of water flooding is demonstrated under high-current and high-humidity conditions. The GDL perforation is carried by two different methods of electric discharge machining (EDM) and manual micro-drilling technique. The EDM method has the disadvantage of removing the PTFE coating of GDL, resulting in local water flooding. In contrast, the micro-drilling technique does not damage the hydrophobic treatment of GDL.Fig. 2 shows the changes of cell voltage during startup for two different channel/GDL structures. The red line denotes the result for the untreated channel/GDL structure; the blue line denotes the combinational structure with hydrophilized channel and perforated GDL. The effective electrode area of the cell is 2.88 cm2. The width, depth and total length of the gas channel are 1.0, 1.0 and 88.5 mm. The contact angle of water of the hydrophilized channel is 17 deg. In the modified cell, 35 penetration holes (Hole diameter: 0.3 mm) are installed into the cathode electrode by the micro-drilling method. The fuel cell operation is performed at 0.73 A/cm2 for 1000 s. The inlet gas temperature and humidity are 45 deg. C and 80% RH, respectively. As shown in the figure, in the case of the untreated structure, the cell voltage drastically drops after starting due to water flooding. On the other hand, the modified channel/GDL structure gradually recovers the voltage after t=150 s, because liquid water is removed from the cathode CL.