Invited: Advances in Structural Characterization of PEM Fuel Cell Catalyst Layers By Soft X-Ray Scanning Transmission Microscopy
Authors: Susac Darija , Berejnov Viatcheslav , Saha Madhu S , Lee Vincent , West Marcia , Hitchcock Adam P , Stumper Jürgen 
Published: February 2020

Proton exchange membrane fuel cells (PEMFCs) are among the most attractive alternative energy sources for automotive application due to their high power density, high efficiency, and potential to significantly reduce adverse environmental impact. However, high material costs and durability issues remain major challenges for large scale commercialization [1, 2]. The catalyst coated membrane (CCM), as a main component in PEMFCs, that employs expensive Pt based porous electrode layers responsible for the generation of pathways for reactant transport enabling efficient electron and proton conductivity and managing product water removal. Improved understanding of the relationship between material properties, electrode structure and overall fuel cell performance is essential for material and process down-selection in order to establish the most cost effective PEMFC manufacturing process. For that reason, the development of new, non destructive characterization methods to allow evaluation of catalyst layer composition and microstructure with high spatial resolution and individual component mapping is needed.
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 [7]. 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 [8]. 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 [7]. 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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