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Imaging and Microanalysis of Thin Ionomer Layers by Scanning Transmission Electron Microscopy

by D A Cullen, R Koestner, R S Kukreja, Z Y Liu, S Minko, O Trotsenko, A Tokarev, L Guetaz, H M Meyer III, C M Parish, K L More

Abstract 

Optimized conditions for imaging and spectroscopic/elemental mapping of thin perfluorosulfonic acid (PFSA) ionomer layers in fuel cell electrodes by scanning transmission electron microscopy (STEM) have been investigated. The proper conditions were first identified using model systems of either Nafion ionomer-coated nanostructured thin film catalysts or thin films on nanoporous Si. These analysis conditions were then applied in a quantitative study of the ionomer through-layer loading for two differently-prepared electrode catalyst layers using electron energy loss (EELS) and energy dispersive X-ray spectroscopy (EDS) in the STEM. The electron-beam induced damage to the PFSA ionomer was quantified by following the fluorine mass loss with electron dose/exposure and was mitigated by several orders of magnitude using cryogenic specimen cooling and a higher incident electron voltage. Multivariate statistical analysis was applied to the analysis of both EELS and EDS spectrum images for data de-noising and unbiased separation of the independent components related to the catalyst, ionomer, and support distributions within the catalyst layers.

Figure 1. STEM images acquired from a fuel cell electrode (a) before and (b) after acquisition of an energy dispersive spectroscopy (EDS) image. (c) Principal component analysis of the EDS image showing the different components in the image (C support=blue; ionomer=green; Pt=red). (hi-res image)

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Publication Citation

Journal of the Electrochemical Society 2014 pp F111-F1117
DOI: doi:10.1149/2.1091410jes

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