An Accelerated Mechanical Stress Test for Evaluating Fatigue Durability of Reinforced Fuel Cell Membranes

Abstract

For evaluating the durability of membranes in polymer electrolyte fuel cells (PEFCs) within relatively short time, accelerated stress tests (AST) are used for applying chemical and mechanical stressors by means of current density, cell voltage, temperature, and relative humidity (RH) cycling. The U.S. Department of Energy (DOE) standardized RH cycling test is considered as a routine screening test for evaluating the mechanical durability under cyclic mechanical stresses [1]. Due to limitation in diffusion kinetics of water in the membrane, each RH cycle often takes several minutes that consequently restricts the magnitude of the induced mechanical stress. Therefore, conventional mechanical ASTs become time-consuming and expensive, especially as more durable membranes are being adopted.A typical mechanical degradation mitigation strategy is to insert an insulating and chemically-inert reinforcement layer, e.g., a thin microporous expanded polytetrafluoroethylene (ePTFE) mesh in the membrane to improve dimensional stability in response to RH-induced membrane expansion and contraction. The reinforcement layer provides additional mechanical strength and enables the use of thinner and more conductive ionomer that would otherwise have insufficient strength. When tested using mechanical ASTs, reinforced membranes possessed longer lifetime [2,3].Experimental work on application of ASTs to fuel cell membranes was previously performed [4] where an accelerated membrane durability test (AMDT) was developed to characterize membrane stability when subjected to chemical and mechanical degradation relevant to field operation. In another work [5], mechanical ASTs were performed via in situ RH cycling with longer dry cycles (for maximizing the fatigue stress amplitude) and at a higher temperature than the DOE protocol. Using these conventional protocols, it is expected that tests with reinforced membranes will continue beyond 20,000 cycles (as defined by DOE [1]) without reaching failure. An ex situ blister test method was used for bi-axial fatigue-creep testing by cycling air pressure [6,7]. In this case, the inherent hygroscopic membrane properties, i.e., water uptake and dimensional changes, which are the actual source of in situ fatigue and mechanical degradation in the constrained fuel cell environment, were not considered.In this work, a custom-developed mechanical AST procedure, based in part on the DOE-standardized AST for mechanical membrane degradation [1], was developed and used for evaluating the mechanical fatigue durability of reinforced membranes in a shorter time. Here, mechanical stressors were applied to the membrane using a sustained pressure differential (ΔP) combined with RH cycling at 80oC in a bi-axial, fuel cell inspired configuration. For validation of the developed method, denoted as ΔP-AMST, ePTFE-reinforced membranes were cycled using 4 min dry (0% RH) and 2 min wet (100% RH) cycles along with a ΔP ranging from 7 to 14 kPa. When the ΔP was applied, a transparent polycarbonate spacer, placed between the cathode and anode, allowed inflation of the membrane by means of a through-thickness hole, as shown in Figure 1. In order to prevent chemical degradation, nitrogen was used as the carrier gas in the ΔP-AMST instead of air, which is used in the DOE method [1]. Depending on the applied ΔP, reinforced membranes failed within ~10 to 10,000 RH cycles, the failure criterion being the loss of ΔP, indicative of major gas leakage through the degraded membrane. Therefore, compared to conventional mechanical ASTs, this novel mechanical AST could be used as a rapid and economical in situ alternative for evaluating the mechanical durability of advanced fuel cell membranes. Acknowledgements This research was supported by Mitacs, Automotive Fuel Cell Cooperation (AFCC), Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, and Simon Fraser University. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program.