Startup and Shutdown Study Applied in Polymer Electrolyte Membrane Fuel Cell System Operation

Abstract

Automotive polymer electrolyte membrane (PEM) fuel cells (FC) have been increasingly under development, due to their ability to provide extended vehicle driving ranges at zero emission. However, PEM fuel cells still face several challenges, such as durability and cost. The lifetime of the PEM fuel cell stacks is determined by the durability of materials under fuel cell operation, in which catalyst, membrane and ionomer are exposed to strongly acidic and oxidizing conditions at elevated temperatures. In addition, the stack lifetime is affected by various operational modes and hydration conditions experienced through drive cycles, idling conditions and startup (SU) / shutdown (SD) cycles.1 System operation strategy can mitigate the stress to the materials and improve the lifetime of fuel cell stacks.This study investigated several SUSD cycles with a PtCo based membrane electrode assembly (MEA) in single cell testing. The schematics of SUSD cycles are shown in Figure 1. In general, a SUSD cycle includes four parts: FC operation, shutdown, soak, and startup. Figure 1(a) is an air-SUSD cycle, which represents a long term shutdown of the vehicle, when air penetrates into the anode. During startup, when hydrogen (H2) is supplied to the anode, there will be an air-hydrogen front at the anode, which causes substantial carbon corrosion and platinum (Pt) dissolution to occur on the cathode. Thus, in accelerated air-SUSD cycles, during the shutdown, air is introduced to the anode and the cell voltage drops to zero within a second. Then, the cell is allowed to soak under air/air for anode/cathode for 60 seconds. After the soak process, during SU, air supply to the anode is stopped and hydrogen is supplied to the anode. In order to improve the SUSD durability of PEM fuel cells in FC vehicles, hydrogen protection (H2-SUSD) has been proposed.2 Figure 2(b) presents a H2-SUSD cycle. In the soak portion of the cycle, the O2 in the cathode is consumed by the H2 permeated from the anode, when the cell voltage drops to zero slowly. At higher temperature, the voltage drop is faster. The H2-SUSD process reduces the development of loop currents within a cell that are associated with high half-cell potential reactions. Because high half-cell potentials are mostly avoided, the H2-SUSD suppresses severe carbon corrosion and Pt dissolution that are observed in air-SUSD cycles. However, we still observe carbon corrosion happening in H2-SUSD process, which is confirmed with catalyst layer failure analysis results. In order to prevent this carbon corrosion, in H2-SUSD, after shutdown, a small load is drawn to consume O2 at the cathode quickly and improve the H2 protection effect as shown in Figure 1(c). These SUSD processes are studied at different temperatures as well. For diagnostics, the voltage degradation at 0.02 A/cm2 and 1.2 A/cm2 and ECSA losses are recorded either at 100 cycle intervals for air-SUSD and 500 cycle intervals for H2-SUSD. Our results show that H2-SUSD with a load preserves the cell performance greatly. The MEA degradation results in these SUSD cycles provide a reference for the FC system team to develop automotive SUSD operation strategy, for example, to determine the minimum H2-protection time for the FC system operation. Y. C. Park, K. Kakinuma, M. Uchida, D. A. Tryk, T. Kamino, H. Uchida, and M. Watanabe, Electrochimica Acta, 91, 195 (2013).Y. Yamashita, S. Itami, J. Takano, K. Kakinuma, H. Uchida, M. Watanabe, A. Iiyama, and M. Uchida, J. Electrochem. Soc., 164 (4), F181 (2017). Figure 1