Advances in Structural Characterization Using Soft X-ray Scanning Transmission Microscopy (STXM): Mapping and Measuring Porosity in PEM-FC Catalyst Layers
Authors: Berejnov Viatcheslav , Saha Madhu , Susac Darija , Stumper Juergen , West Marcia , Hitchcock Adam P 
Published: February 2020

Access of fuel and oxidant gases to the active catalyst sites in dispersive electrodes of polymer electrolyte membrane fuel cells (PEM-FC) is an important characteristic defining the performance of the whole catalyst coated membrane unit (CCM). The electrode porosity – the ratio of empty space to the whole volume of the electrode – is a metric widely used to characterize the ability of a particular electrode to provide gas transport. It is common practice to present electrode porosity as a number averaged over a macroscopic volume of the electrode [1]. In compliment to already established methods, soft X-ray scanning transmission X-ray microscopy (STXM) offers the possibility to visualize the microscopic network of the pore space in the electrode with ~30 nm spatial resolution. Thus, in addition to average porosity, STXM provides porosity distributions in electrodes, with a local value averaged per voxel of ~1003 nm3. Relative to studies of the electrode porosity visualized by the focused ion beam – scanning electron microscopy method (FIB-SEM) [2], STXM provides direct visualization of the pore-space in the electrode without destroying the sample and omitting the 3D reconstruction process. Here we present STXM porosity distribution for 2D projection of the thin cathode lamella.
STXM pore space visualization relies on filling the pores with a material that STXM can discriminate spectroscopically relative to the other components (carbon support, ionomer, and metal catalyst-Pt) [3]. In these experiments the cathode catalyst decals are embedded with a custom epoxy (TTE, the product of reacting trimethylolpropane triglycidyl ether and 4,4’-methylenebis(2-methylcyclohexylamine in 1:1 wt ratio, which results in a final chemical formula (C10H10N3)n and density of 1.0 g/cm3). While the TTE epoxy is fluid it wets well the electrode inner interfaces filling most of the available space. 100 nm thick microtomed sections of the cathode embedded in TTE are measured by STXM at the C 1s edge at the Canadian Light Source (Saskatoon) or the Advanced Light Source (Berkeley). The C1s stacks, consisting of images at 50-70 energies over 280-330 eV with 100 nm pixels, are converted to optical density, and then fit using standard, quantitative reference spectra (OD1 scale) of carbon-support (Cs), ionomer (I), TTE and Pt components to construct 2D component maps of each component. The grayscale value at each pixel is the nm thickness of that component. The total volume is the sum of Cs, I, TTE, and Pt values at each pixel. The 2D porosity map is then the ratio of the TTE map to the total-volume map, where each pixel has a unit of part per volume (ppvol).
STXM performed using BL 10ID1 at CLS and BL 5.3.2.2 at ALS. Research supported by NSERC.

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