Failure Analysis and Defects Characterization of Polymer Electrolyte Membrane Fuel Cells after Relative Humidity Cycling

Abstract

Water content in the proton exchange membrane fuel cell (PEMFC) changes due to the spatiotemporal interplay of temperature, humidity, and electrochemical reactions. The humidity fluctuations result in membrane swelling and shrinking, and consequently, in-plane tensile stress development that causes mechanical failures [1,2]. In this study, commercial catalyst coated membranes (CCMs) were tested, under a relative humidity (RH) cycling accelerated stress test (AST), between oversaturated and fully-dry states. After the failure of the membrane, a conventional bubble jig test was performed on the membrane electrode assemblies (MEAs) to identify the defective regions. For further understanding of the morphology around the defects and the associated failure mechanism, the identified failure regions were examined using scanning electron microscopy (SEM) cross-section imaging and X-ray computed tomography (XCT) to be referenced with the beginning of life (BOL) sample [3]. The failed regions showed a wide range of defects, from small pinholes to fully-ruptured CCMs, in various forms and patches. There was good conformity between the size of the bubble obtained from the bubble jig test and the morphology of the defects in the images. Further, possible microstructural changes, i.e. porosity and ionomer distribution of the RH cycled catalyst layers were evaluated for the first time by scanning-transmission electron microscopy- energy dispersive spectroscopy (STEM-EDS) analysis along with an in-house developed quantitative method [3,4]. The STEM-EDS mapping revealed a changed ionomer distribution as well as a decrease in the catalyst layer porosity with respect to the BOL sample. The novel analysis approach revealed additional mechanisms of MEA degradation in the RH cycled MEAs, beside the membrane failure modes. To further explain the failure mechanisms, the nuclear magnetic resonance (NMR) was used to investigate the perfluorosulfonic acid (PFSA) chain and structure changes due to the RH cycling and differentiate any mechanical defect from chemical alteration of the chains in the membrane [5]. The NMR spectrum showed irregular broadening of the 19F peaks, which can be correlated to the mechanical changes in the fluorine chains of the membrane. Complementary characterization techniques such as Fourier transform infra-red spectroscopy (FTIR) and Raman spectroscopy analyses results will be shown for the catalyst layer to further explain the failure modes. Consequently, a comprehensive characterization methodology was developed for better understanding of the failure mode from the RH cycling AST.Keywords: RH cycling, Failure Analysis, Polymer Electrolyte Membrane, Fuel Cell Durability.