(Invited) Pulsed Neutron and Synchrotron X-Ray Imaging of Liquid Water Formation and Transport in Polymer Electrolyte Fuel Cells

Abstract

Fuel-cell electric vehicles (FCEVs) powered by hydrogen are expected to be the ultimate eco-friendly cars, and so developing low-cost, high-performance, and highly reliable FCEVs is strongly demanded. Polymer electrolyte fuel cells (PEFCs) consist of an electrolyte membrane sandwiched between an anode and a cathode, where both the anode and cathode have laminated structures composed of a catalyst-coated membrane (CCM) and gas channels in contact with a gas diffusion layer (GDL). The GDL consists of a microporous layer (MPL) and a substrate. One problem with PEFCs is that liquid water generated as a by-product of the electrochemical reaction at the catalyst accumulates in the GDL, and prevents oxygen from being transported through the gas channels to the catalyst. Therefore, to achieve a high power density, it is necessary to develop new materials and optimize the design of the gas channel to discharge water from the cathode to the exterior of the cell. We are currently constructing a multiscale analysis methodology that combines quantum beam analysis using X-rays and neutrons to clarify water behavior in a PEFC [1-9].In this presentation, we introduce three topics related to water behavior on scales ranging from several micrometers (pores in GDLs) to hundreds of millimeters (automotive fuel cells). The first topic concerns imaging the liquid water distribution in an MPL [2, 6]. The imaging analysis was performed using time-resolved synchrotron X-ray computed tomography while water vapor was supplied to the MPL side of the GDL. With increasing exposure to water vapor, the average volume of wet domains in the MPL initially increased, whereas the overall number of such domains decreased. This implies that wet domains expand by absorbing water vapor and combining with each other. As the exposure to water vapor was further increased, the volume fraction of wet domains in the MPL reached a steady state, indicating that liquid water was discharged to the outer surface of the MPL and to the substrate side. The second topic is obtaining mechanistic insights into water behavior in a subscale PEFC [8]. Time-resolved operando synchrotron X-ray radiography to determine the effects of temperature and relative humidity on the condensation and transport of water in PEFCs. The results show that condensation and transport to the channel can be classified into four categories: 1) concurrent liquid and vapor transport, 2) mainly liquid transport, 3) vapor transport, and 4) accumulation near the ribs. X-ray imaging experiments were conducted at the TOYOTA Beamline (BL33XU) at the SPring-8 facility. The third topic is operando neutron imaging of the water distribution and identifying water/ice phases in large automotive PEFCs during a cold start. We developed an experimental system using a pulsed spallation neutron beam at the RADEN instrument at BL22 of the Materials and Life Science Experimental Facility (MLF) in the Japan Proton Accelerator Research Complex (J-PARC). The results showed direct evidence of stepwise freezing inside the PEFC. The water initially accumulated in the center of the PEFC and began to freeze, and this was followed by PEFC shutdown as freezing progressed. We defined the time interval between the beginning of freezing and shutdown as the “extended time”. This is expected to be useful as an indicator for optimizing the cold-start protocol, cell design, and choice of material.Reference1) Y. Nagai, J. Eller, T. Hatanaka, S. Yamaguchi, S. Kato, A. Kato, F. Marone, H. Xu and F. N. Büchi, J. Power Sources, 435 (2019) 226809.2) S. Kato, S. Yamaguchi, W. Yoshimune, Y. Matsuoka, A. Kato, Y. Nagai and T. Suzuki, Electrochem. Commun., 111 (2020) 106644.3) S. Yamaguchi, S. Kato, A. Kato, Y. Matsuoka, Y. Nagai and T. Suzuki, Electrochem. Commun., 128 (2021) 107059.4) Y. Higuchi, D. Setoyama, K. Isegawa, Y. Tsuchikawa, Y. Matsumoto, J. D. Parker, T. Shinohara and Y. Nagai, Phys. Chem. Chem. Phys., 23 (2021) 1062-10715) S. Yamaguchi, S. Kato, W. Yoshimune, D. Setoyama, A. Kato, Y. Nagai, T. Suzuki, A. Takeuchi and K. Uesugi, J. Synchrotron Radiat., 29 (2022) 1258-1264.6) S. Kato, S. Yamaguchi, Y. Matsuoka, A. Kato, T. Suzuki and Y. Nagai, InterPore2022, Book of s, p139.7) K. Isegawa, D. Setoyama, Y. Higuchi, Y. Matsumoto, Y. Nagai and T. Shinohara, Nucl. Instrum. Methods Phys. Res. A 1040, 167260 (2022).8) A. Kato, S. Kato, S. Yamaguchi, T. Suzuki and Y. Nagai, J. Power Sources, 521 (2022), 230951.9) Y. Higuchi, W. Yoshimune, S. Kato, S. Hibi, D. Setoyama, K. Isegawa, Y. Matsumoto, H. Hayashida, H. Nozaki, M. Harada, N. Fukaya, T. Suzuki, T. Shinohara and Y. Nagai, to be submitted.