New Insights in Proton Exchange Membrane Fuel Cell Electrode Microstructure Changes from Accelerated Stress Testing By Microscopy Techniques

Abstract

The microstructure of the cathode catalyst layer in a proton exchange membrane fuel cell (PEMFC) plays a critical role in the performance and durability of this device towards emission-free mobility. Controlling the catalyst layer microstructure at beginning-of-life (BOL) is a viable path for achieving high fuel cell performance, whereas understanding the microstructural changes due to extended hours under fuel cell working conditions often sheds light on the degradation process and mechanism, contributing to increased knowledge of the structure-performance relationships in the fuel cell electrode. Materials characterization, as an essential tool in understanding the structure-properties-performance relationships in materials science, has been widely used to track the microstructural changes of the cathode catalyst layer. The multiple heterogeneous components in the catalyst layer including catalyst, support and ionomer, as well as various morphologies exhibited such as nanoparticles, nanopores, and polymer ionomer film, create an intrinsically complex problem for characterization and necessitate a characterization technique with sufficiently high resolution to capture the multi-scale evolutions of the catalyst layer properties. Among a wide range of characterization methods, high resolution and scanning transmission electron microscopy (HR and (S)TEM), with energy dispersive spectroscopy (EDS), supported by scanning electron microscopy (SEM) and X-ray computed tomography (XCT) have proven to be some of the best techniques for PEMFC materials characterization [1]. Here we present advanced approaches in characterization and quantification of BOL and end of life (EOL) catalyst coated membranes (CCM) that had been subjected to various accelerated stress tests (AST), revealing new insights in degradation mechanism and impact. The primary porosity (within Pt/C agglomerates with d<5 nm [2]) and secondary porosity (between Pt/C agglomerates with d>5 nm [2]), platinum particle size distribution, platinum loading, ionomer distribution, and carbon structure were characterized and quantified for a supplier CCM, and used as the BOL baseline. ASTs were designed and performed to study the effects of parameters related to startup/shutdown (SU/SD) conditions for the PEMFC [3, 4]. The post-mortem samples were analyzed and compared to the BOL sample to understand the microstructural and compositional changes. The results showed a total porosity drop, mainly stemming from a loss of secondary pores. This could be related to carbon corrosion during SU/SD AST which was reflected in the electrode thickness reduction from the scanning electron microscopy (SEM) images. The particle size distribution (PSD) analysis and ionomer imaging were indicative of further changes through platinum degradation and ionomer re-distribution, respectively. HR-TEM imaging of specially (epoxy-free) prepared samples indicated changes in carbon morphology and degree of crystallinity. The characterization approach was also utilized for the failed samples from the open circuit voltage (OCV) AST and the results revealed porosity, Pt, ionomer and carbon morphological and distribution changes. The SEM-based membrane thickness reduction from OCV AST seemed to exhibit a qualitative trend with the amount of released fluoride measured by an ion selective electrode (ISE) instrument. Multiple characterization techniques were utilized to study the relative humidity (RH) cycling effects on the membrane electrode assembly (MEA). The SEM and X-ray computed tomography (XCT) results revealed that the defects in microporous layer (MPL) could be related to the cracks in the CCM. While the STEM-EDS analysis and quantification showed no changes in Pt distribution and particle size, the porosity distribution was changed due to the stresses during the RH cycling [5, 6]. In summary, the characterization techniques discussed herein seem to be providing important information in guiding the fuel cell community towards improving performance and durability through modification of the electrode microstructure during fabrication and developing appropriate strategies to prolong life.Keywords: Failure Analysis, Polymer Exchange Membrane, Catalyst Layer Structure, Fuel Cell Durability