Invited: Advances in Structural Characterization of PEM Fuel Cell Catalyst Layers By Soft X-Ray Scanning Transmission Microscopy
In recent years, a novel synchrotron based technique: soft X-ray scanning transmission microscopy (STXM) has emerged and extended the number of standard available microscopic characterization tools and significantly improving our understanding of how materials are distributed from micro to nano-scales in fuel cell electrodes. STXM is able to image a fuel cell catalyst layer and differentiate each chemically specific material component due to its near edge X-ray specific absorption (NEXAFS) signature [3-11]. The spatial resolution of this method is 30 nm. We have developed several STXM applications to characterize a) materials in the dry CCM in 2D cross sections, b) in 3D, and c) while applying environmental changes in 2D.
Using two-energy mapping at F 1s and C 1s edges, a 2D material reconstruction methodology for catalyst layer micro-structural characterization and ionomer mapping was developed . Both conventional and nanostructured thin film (NSTF) based catalyst layers are being investigated [9, 10]. Applying a tomographic approach, a STXM spectro-tomography method was established and proven to provide a 3D reconstruction of the cathode catalyst layer with spatially resolved carbon and ionomer species . Due to its ability to spectroscopically differentiate gas, liquid and solid water using the O 1s absorption edge, STXM is being applied to study hydrated CCMs . An environmental wet cell for in situSTXM studies of CCM components under controllable temperature and relative humidity conditions has been developed and used to map all three phases of water in the electrode under various conditions including those relevant to cold start.
In this paper and the coming talk, we will review progress in development of STXM methodology for characterization of catalyst layer structures. Efforts in deriving parameters to aid structure-properties-performance correlations will be discussed.
STXM measurements are carried out at the Canadian Light Source and at the Advanced Light source.
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