Multi-Modal Chemical Analysis of Fuel Cell Electrode Ionomer Binder

Abstract

Heavy-duty trucks account for most freight movement in the US and consequently contribute a large share of transportation emissions. Efforts to eliminate tailpipe emissions have focused on electrifying these vehicles through proton exchange membrane fuel cells (PEMFCs). While the scalability of PEMFCs makes them an attractive candidate for heavy-duty applications, further improvements in electrode durability are needed to achieve total cost of ownership targets to be commercially competitive. One challenge with understanding PEMFC lifetime is quantifying the extent of degradation to the the polymer electrolyte binder (commonly referred to as the ionomer) in the catalyst layers of the electrodes. The ionomer is needed for the mechanical structure and proton conductivity of the electrodes. However, the ionomer also negatively influences catalyst performance by restricting local oxygen transport at the catalyst and its catalyst adsorbtion that reduces activity. While the degradation of PEMFC membrane has been extensively studied, analysis of ionomer within the electrode is challenging due to the low total mass of material present. Typical methods for membrane degradation analysis, such as fluoride emission measurement, are frustrated by the need to distinguish degradation products of the catalyst layer ionomer from the membrane. In the present work, we assess a variety of postmortem chemical analysis techniques for their relevance to measuring catalyst layer ionomer degradation. We report methods of separating catalyst layer ionomer from the membrane in a membrane electrode assembly (MEA), and measurement conditions for a range of analytical techniques including nuclear magnetic resonance (NMR), Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS). We also describe the results of fabricating a simulated degraded MEA through ex-situ treatment of the electrode on the decal backing layer prior to transfer to the membrane and compare the cell performance with MEAs which have been left untreated, and which have undergone open-circuit voltage degradation conditions. We evaluate the effectiveness and sensitivity of each characterization approach and identify potential directions for developing a postmortem MEA ionomer degradation assessment protocol.