A Rapid Ex-Situ alternative for Mechanical Accelerated Stress Testing of Fuel Cells

Abstract

The durability of polymer electrolyte fuel cells (PEFCs) is one of the main challenges facing the commercialization of automotive fuel cells. The lifetime of the membrane, a critical structural component of PEFCs, is one of the principal obstacles in achieving the fuel cell industry durability targets [1]. Under dynamic automotive operating conditions and duty cycles, the membrane is subjected to chemical and mechanical degradation, which could cause hydrogen leaks and ultimate cell failure. Chemical degradation is linked to the polymer molecular decomposition caused by radical species formed during the fuel cell operation as by-products of electrochemical reactions [2]. On the other hand, mechanical degradation is attributed to the fracture caused by the induced mechanical and hygrothermal stresses in a constrained cell [2]. The US Department of Energy (DOE) introduced standardized in-situ accelerated stress test (AST) protocols [3]. Following the DOE mechanical AST protocol, the membrane mechanical durability under pure humidity cycling was investigated [4]. In this manner, two customized in-situ mechanical AST protocols were utilized by our group to evaluate the mechanical durability of PEFCs, indicating significant decay in mechanical properties, formation of microstructural cracks, and initiation of failure [5]. Despite the valuable outputs of the in-situ studies, the current protocols are time consuming and costly. As hygrothermal fatigue is expected to dominate the membrane mechanical lifetime, an ex-situ mechanical fatigue-creep based AST was recently developed by our group as a more convenient alternative [6]. The proposed ex-situ tensile fatigue-creep test demonstrated in this work is intended to evaluate the mechanical durability of catalyst coated membranes (CCMs) in a fraction of the time required for the conventional in-situ tests [7]. The proposed ex-situ tensile fatigue-creep accelerated stress test (TFC-AST) was conducted on dog bone shaped [8] CCM samples using a dynamic mechanical analyzer (DMA) equipped with environmental chamber. After equilibration at 80°C and 50% RH, as illustrated in Figure 1, a high frequency sinusoidal cyclic tensile load with stress ratio (R) of 0.2 and 6.1 MPa mean stress was applied on the CCM samples to certain fractions of fatigue lifetime (~140,000 cycles) [7]. When compared to the well-defined standard in-situ mechanical ASTs which last about 8 weeks [5], the proposed TFC-AST results in mechanical failure ~400 times faster than the in-situ tests due to higher frequency and magnitude of fatigue and creep loading. Depending on the total lifetime, TFC-ASTs were interrupted at different fractions of the CCM lifetime, i.e. 20%, 40%, 60%, and 80%, and partially fatigued samples were extracted for analysis. The obtained partially degraded CCMs were further studied through mechanical and microstructural techniques. The mechanical properties of the extracted CCMs were investigated via tensile and hygrothermal expansion experiments in the same manner as reported in [9] using DMA. Tensile tests revealed remarkable increase in the tensile strength of the partially degraded samples indicating the alignment of the polymer molecules along the TFC stress direction. CCM thermal and hygral expansions were evaluated by stepwise increase in temperature and relative humidity, respectively. Interestingly, the CCM was found to contract, which is contradictory to the typical hygrothermal expansion behaviour of these materials [5,9]. This behaviour can be attributed to the exclusively tensile loading of the TFC-AST. In addition to the mechanical testing, morphological evolution of the TFC-AST degraded CCMs was also examined and compared with the analogous in-situ mechanical AST degraded CCMs using transmission electron microscopy (TEM). The TEM micrographs provided supplementary evidence regarding the reorientation of membrane molecules along the TFC stress direction. However, the mechanical failure of the specimens was found to be dominated by fatigue, similarly to the corresponding in-situ tests. Figure 1. Schematic of the proposed tensile fatigue-creep accelerated stress test (TFC-AST) protocol for rapid mechanical durability testing of fuel cells. Acknowledgements: This research was supported by Ballard Power Systems and the Natural Sciences and Engineering Research Council of Canada through an Automotive Partnership Canada grant.