Quantitative Mapping of Ionomer in Catalyst Layers by Electron and X-ray Spectromicroscopy

Abstract

The perfluorosulfonic acid (PFSA) ionomer commonly used in Polymer Electrolyte Membrane Fuel Cells (PEM-FC) consists of a hydrophobic tetrafluoroethylene backbone with side chains terminated with sulfonate groups responsible for proton conduction. Optimizing the spatial distribution of ionomer in the catalyst layer maximizes the catalyst that is accessible to protons and O2, reducing the Pt loading. Thus, the methods of quantifying the distributions of ionomer relative to catalyst are important. PFSA is prone to radiation damage [1], losing fluorine which makes it challenging to analyze quantitatively with electron [2] and x-ray spectro-microscopies [3]. Scanning Transmission X-ray Microscopy (STXM) is known to cause less damage in soft materials than analytical Transmission Electron Microscopy (TEM), with either core level Electron Energy Loss (EELS) [4] or Energy Dispersive X-ray Spectroscopy (EDS). We have applied Scanning TEM – EDS (STEM-EDS) and STXM at room temperature to the same microtomed sample of a PEM-FC catalyst layer. STEM-EDS was performed using an FEI Tecnai Osiris equipped with CHEM-STEM 4 area detectors. STXM was performed using soft X-ray STXMs at the Canadian Light Source (Saskatoon) and the Advanced Light Source (Berkeley). The extent of ionomer damage (F-loss) as a function of absorbed radiation dose in the cathode due to each imaging technique was measured from F Ka maps in STEM-EDS, and by F1s stack maps (OD694 eV - OD684 eV) in STXM. Figure 1 plots the extent of fluorine signal and thus component loss, normalized to the initial amount, for a range of electron and X-ray exposures. Both the electron and X-ray beams damage the ionomer at a similar rate. However, decent quality EDS maps at room temperature (Figure 1C) with sufficient counts for statistical analysis need ~30000 e-/nm2. At that exposure, approximately 70-80% of the original fluorine signal in the ionomer material is lost. On the other hand, high quality ionomer maps using STXM require less than 10 photons/nm2 (Figure 1A), which causes negligible fluorine loss. The ‘multi’ mode of TEM-EDS acquisition does provide significantly less damage at lower exposures (Figure 1B) but by the time adequate image quality is achieved (Figure 1C), the extent of fluorine loss is unacceptably high. Whether the ionomer damage is attributed to only fluorine loss (EDS), or ionomer component loss (STXM), or significant change of ionomer chemistry is still debatable. However, this study indicates that after applying “common” imaging practices for STXM and EDS, the remaining cathode areas are significantly different. STXM performed on BL 10ID1 at CLS and on BL 5.3.2.2 at ALS. Research supported by NSERC and the Catalyst Research for Polymer Electrolyte Fuel Cells (CaRPE-FC) network. S. Yakovlev, et.al., Membranes. 3(2013) 424–439D.A. Cullen, et al., J. Electrochem. Soc. 161 (2014) F1111–F1117D. Susac, et.al. STXM ECS Trans. 41 (2011) 629–635J. Wang, et.al., J. Phys. Chem. B. 113 (2009) 1869–76. Figure 1