Characterization of degradation, fracture and failure in fuel cell membranes

Proton exchange membrane (PEM) fuel cells offer a promising clean energy solution for the stringent environmental challenges currently faced by the transportation sector. Ionomer membrane is a key component of this technology that enables critical functions such as protonic conduction, electronic insulation, and reactant separation. During operation, however, various chemical, mechanical and thermal stressors degrade the membrane, which compromises the performance and longevity of a fuel cell. A detailed understanding of these degradation processes and their associated membrane failure modes is, therefore, required to develop more durable and economically viable fuel cell systems. In this work, a comprehensive characterization of fuel cell membrane degradation and failure is performed, with a particular emphasis on membrane fracture, through a series of studies encompassing experimental mechanics, accelerated stress testing, microstructural characterization, and numerical modelling under scenarios relevant to the operational environment. Uniaxial tensile fatigue tests are conducted on double edge notch tension (DENT) specimens to measure fatigue crack growth rates in the ionomer membrane and its electrode-coated composite as a function of temperature, humidity and applied stress. The experimental results are further utilized to develop a Paris law based semi-empirical fracture modelling framework to simulate the crack growth rates while numerically implementing the characteristic time-, temperature- and humidity-dependent elastic-viscoplastic constitutive response of these materials. A laboratory-based X-ray computed tomography (XCT) system is utilized to introduce a novel 3D failure analysis methodology for characterizing post-mortem membrane degradation. This methodology is implemented across a series of accelerated stress tests involving both isolated and conjoint chemical and mechanical membrane degradation, respectively, to explain various membrane failure modes and their mechanisms in relation to the nature of in situ stressors, changes in material properties, and influence of neighbouring components. The non-destructive characteristics of XCT imaging are further leveraged to uniquely investigate the 3D structural evolution of a mechanically degrading membrane through periodic visualization of identical locations inside a custom-developed small-scale fuel cell. Electrode cracks and interfacial delamination are identified as critical defects influencing membrane crack initiation, and direct measurements of in situ crack propagation rates are obtained for the first time.