Self-Supporting Microporous Layer for Polymer Electrolyte Fuel Cells

Abstract

Introduction Improving power density is desired in research and development of polymer electrolyte fuel cells (PEFC) [1-5]. A reduction of gas diffusion resistance by down-thinning the gas diffusion layer (GDL) less than 50 μm is desirable [2]. Whilst both the electrocatalyst layers and electrolyte membrane are around 10 μm, the GDL is still relatively thick at about 150 μm [1]. The previous study investigated the possibility and technical issues in using high-strength and thin, porous metallic GDLs [3]. Based on such previous studies, this study aims to develop a self-supporting thin MPL/GDL with improved power density by creating conductive paths and reducing contact resistance with microporous layer (MPL) deposited on various GDLs. Experimental In this study, MPL was screen-printed on various mesh GDLs as substrates. Cells were fabricated by using these materials, and current-voltage characteristics and microstructural observations were performed. Stainless steel (SUS316 977mesh, 28 μm thick), titanium (200mesh, 152 μm thick), and carbon fiber (45 μm thick) were used as mesh GDL materials. The MPLs were composed of carbon black, carbon nanotubes, polytetrafluoroethylene (PTFE), and solvent (a mixture of polyethylene glycol 600 and pure water), which were mixed, screen printed, and heat treated at 300°C for 30 min to fabricate the MPL/GDL. Conventional materials such as Pt/C (TEC10E50E, Tanaka Kikinzoku, Japan) were used for preparing membrane-electrode-assemblies (MEAs). In the electrochemical characterization of this study, current-voltage characteristics were evaluated using an electrochemical impedance analyzer (SAS SP-240, Bio-Logic Science Instruments, France), and various overvoltages were separated. Results and discussion Figure 1 shows the current-voltage characteristics of cells using various MPL/GDLs. The thickness of MPL/GDLs fabricated was ranging from 62 to 180 μm. For comparison, the characteristics of a cell using a standard material, commercially available MPL/GDL (22BB, SGL Carbon, Germany), are also shown. The results indicate that the use of any kind of these MPLs improves cell performance. The electrochemical performance with metallic GDLs was considerably improved by applying MPLs. In particular, GDLs with metallic meshes made of SUS316 show a significant improvement in current-voltage characteristics by depositing MPLs. The activation overvoltage and ohmic overvoltage were comparable to those of 22BB, the commercial GDL/MPL. This is because the contact resistance between the GDL and the catalyst layer is reduced by the carbon component in the MPL, and the electrical conductivity is improved. The concentration overvoltage was also suppressed by depositing the MPL. The cell performance with MPL3/SUS316 977mesh was comparable to that of 22BB. In the future, we aim to further reduce the concentration overvoltage by optimizing e.g., the content of hydrophilic components in MPLs. Acknowledgment This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References Takahashi, T. Ikeda, K. Murata, O. Hotaka, S. Hasegawa, Y. Tachikawa, M. Nishihara, J. Matsuda, T. Kitahara,M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 169, 044523, (2022).New Energy and Industrial Technology Development Organization (NEDO), Roadmap of Fuel Cells for Heavy-Duty Vehicles, 48, (2022), (in Japanese), https://www.nedo.go.jp/content/100944011.pdf Yamamoto, M. Yasutake, Z. Noda, S. M. Lyth, J. Matsuda, M. Nishihara, A. Hayashi, and K. Sasaki, ECS Trans., 109 (9),265 (2022).Larminie and A. Dicks, Fuel Cell Systems Explained, 2nd ed., John Wiley & Sons, England, 2003.Kitahara, T. Konomi, H. Nakajima, and J. Shiraishi, Kikai B, 76 (761), 101, (2010). Figure 1