In a polymer electrolyte fuel cell (PEFC) startup below freezing point, the produced water freezes, and then shutdown occurs. The authors’ research group has been conducting experimental studies on the freezing behaviors and mechanism. At -20℃, the generated water freezes at the moment when the water comes out into the catalyst layer (CL) pores, and at -10℃, the generated water moves inside the fuel cell in the supercooled state and freezes (1)-(2). It was also reported that the ice frozen at the micro porous layer (MPL) and CL interface absorbs generated water through the ionomer in the CL (3). Based on the fundamental findings of isothermal cold start characteristics and freezing behaviors, we conducted experimental investigations simulating a condition that the cell temperature is rising during cold startup due to the heat of reaction of power generation. A method to realize adiabatic condition focusing on the cell at central part in a stack mounted on automobile was developed, and the cold start characteristics and the freezing behavior during rising temperature were investigated.

A single cell with an active area of 25 cm² (5 cm × 5 cm) was used in this study. As shown in Fig. 1, a heat insulating plate and heater were inserted between the end plate and current collector plate to simulate the adiabatic condition by controlling the cell temperature. Heat release from the sides of in-plane direction was suppressed by winding a heat insulating material. After a preconditioning process to enhance the performance of a MEA, the initial conditions of water in the cell were carefully controlled by the procedure described in Ref. 1. Then the cell and the chamber were cooled to a target value in the range of -10℃ to -15℃, and cell operation was started and maintained at a constant current density 0.2 A/cm². Dry hydrogen and dry air were supplied to anode and cathode. In order to observe the ice distribution with a cryo-SEM after stopping the current load, the MEA sample was quickly removed from the cell and immersed in liquid nitrogen and the sample was set to a sample holder. The sample was cut by a cold knife in a vacuumed chamber, and the cut section area was coated with gold-palladium alloy (Au-Pd), and observed at -150℃.

As a result of the startups with the initial temperature range of -10℃ to -13℃, shutdown due to freezing was not observed, while startup failure occurred at -15℃. At the startup at -14℃, the different results (two startup successes and two failures) were confirmed. Figure 2 shows the time variation of the cell voltage, cell resistance, and cell temperature during cold startup with the initial temperature -14℃. Figure 2(a) is for the startup failure and Fig. 2(b) is for the success. In the both cases (Figs. 2 (a) and (b)), similar behaviors were observed until 70 seconds. However, after 70 seconds, the voltage decreases in Fig. 2 (a) and continues to rise in Fig. 2(b). These suggest that supercooling release of product water occurs at 70 seconds in Fig. 2(a), and does not occur in Fig. 2(b). Figure 3 shows the ice distribution on the cathode side after the shutdown in Fig. 2(a). Figure 3(a) is a cryo-SEM image around the MPL and the CL interface, and Fig. 3(b) an image of the CL at high magnification. As shown in Fig. 3(a), a layer of ice was observed at the interface between the MPL and CL, and there is, almost no ice in the CL (Fig. 3(b)). From this result, it is considered that in the operation of Fig. 2(a), the generated water moved to the MPL and CL interface, then the supercooled state was released, and the ice grew at the interface. The supply of oxygen was hindered by the formation of the ice layer, and the shutdown was induced. This indicates that adequately removing the generated water at the MPL and CL interface may be effective to improve the cold start characteristics.

Reference
(1) Y. Tabe, et al. , J. Power Sources 208 (2012), 366.

(2) Y. Tabe, et al. , J. Electrochem. Soc. 163 (10) (2016), F1139.

(3) N. Wakatake, et al. , ECS Trans. 75 (14) (2016), 623.