Novel Epoxy-Free Imaging of the Proton Exchange Membrane Fuel Cell Components and Microstructure Degradation By Transmission Electron Microscopy

Abstract

Performance, durability, and efficiency limitations of proton exchange membrane fuel cells (PEMFCs), as one of the alternative clean energy resources, has been hindering the widespread adoption of these devices. Improving those parameters is closely entangled with identifying the degradation mechanism of each component in the electrodes i.e., catalyst – commonly platinum nanoparticles, catalyst support – carbon, and the ion conductive polymer – ionomer, as well as the effects that degradation can have on the porous network of the electrodes[1]. Degradation during typical automotive operation protocols like startup/shut down (SUSD)[2,3] is of a special interest. Therefore, it is essential to observe the changes of the microstructure, as whole, and each individual component’s response to the working condition, in order to understand degradation mechanisms and devise degradation mitigation strategies. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) in conjunction with energy dispersive spectroscopy (EDS) have proven to be valuable tools in unravelling the complex changes of the microstructure and understanding of the degradation mechanisms[4]. Moreover, correlating the numerical microstructural descriptors, extracted by quantification of the TEM and EDS results, with the visual observations helped the research community to draw a connection between the degradation and microstructural changes[5]. However, the conventionally prepared TEM samples by slicing (microtomy) an epoxy-embedded piece of a catalyst coated membrane (CCM) have limitations in observing carbon and ionomer, and distinguishing them from each other and from the epoxy. Partial epoxy-embedding showed very promising results regarding distinguishing carbon from ionomer in selected regions of the CCM[6]. However, not a full CCM was observed. Therefore, we report for the first time, an epoxy-free microtomy approach for a whole CCM, successfully implemented on a fresh and degraded cathode layer of beginning-of-life (BOL) and SUSD samples, respectively. Electrochemical characterization of the samples presented here are discussed in previous publication[2]. The Pt catalyst, carbon, ionomer and porous network of the cathodes were clearly observable and distinguishable in both BOL and SUSD samples using high-resolution TEM and 3D electron tomography-TEM (ET-TEM) imaging. The microstructural comparison revealed the changes in Pt particles distribution – moving into the ionomer network rather than sitting on and inside the carbon support, disintegration of corroded carbon and dispersion of broken-off graphitic carbon pieces in the ionomer, and alteration of the ionomer network – formation of more ionomer filaments. In addition, ET-TEM of the BOL and SUSD samples confirmed the presence of Pt particles inside the amorphous core of the carbon particle and their migration during SUSD operation, respectively. Finally, the visual results were correlated with the microstructural descriptors quantified using our proprietary quantification technique. The numerical microstructure properties corroborated the observations that was made based on the TEM images observations. Therefore, implementation of whole-CCM-epoxy-free microtomy technique can be considered as a novel method in aiding the research community in understanding the degradation mechanism and microstructural properties alteration.