Segmentation for Preventing Ice Propagation in Operating Fuel Cells

Abstract

One of the critical requirements for automotive PEFC stack is unassisted cold-start at -30°C [1]. Sub-zero operation of polymer electrolyte fuel cells (PEFCs) can induce ice formation that causes performance failure and irreversible damages to membrane electrode assembly (MEA). It is challenging to predict the location and the moment of freezing event, because it is a stochastic process. The proposed solutions include material optimization, load control, and assisted start-up [2]. mechanisms in PEFCs needs to better understood for further developments.Our previous investigations demonstrate that water is produced in super-cooled (SC) state, and the transition from liquid to solid is initiated when a SC water cluster encounters a nucleation seed, such as broken carbon fiber. Due to the unstable state of SC water, the transition spontaneously propagates to neighboring water cluster, and a single freezing event may affect a significant portion of the active area as seen in Figure 1 (a). In this work, we investigate an operating PEFC under sub-zero temperature and introduce a assembly to prevent ice propagation.The segmentation was implemented by creating non-active lines in the 3-layer MEA, where electrodes were partially removed by laser beam. The energy level of the laser was tuned to selectively remove carbon without affecting the membrane. The pieces of GDL, matching the size of the segmented active areas, were hot-pressed on the modified MEA. A Teflon gasket was placed among GDLs to complete the MEA fabrication. The segmented 5-layer MEA was mounted on the advanced PEFC hardware with thermoelectric and heat flux sensing modules for monitoring heat release upon freezing. Furthermore, we applied time-of-flight (ToF) neutron imaging to detect freezing in the operating PEFC at ICON beamline of Swiss Spallation Neutron Source (SINQ) [4]. This technique is based on the principle that the neutron cross-sections of ice and SC water are similar at short wavelength, but different at long wavelength. Using these methods, we confirmed that the operation time of the PEFC under isothermal sub-zero condition was significantly increased with the segmentation. The improvement was attributed to localization of freezing events as in Figure 1 (b).The findings of this work will contribute to the development of cold-start technologies in PEFCs. Besides, ToF neutron imaging method will be a valuable tool for studying phase-changes of other electrochemical device materials.