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.Perfluorosulfonic acid (PFSA) ionomer is a key component within the electrode layers of polymer electrolyte fuel cells (PEFCs). The PEFC electrode layer is typically constructed at a ∼10 μm thickness and is comprised of a dispersed Pt nanoparticle catalyst supported on a highly structured carbon black support with a distributed PFSA ionomer film. This percolating solid polyelectrolyte in the electrode provides an efficient proton transport path to the active catalyst sites. The carbon and polymer occupy ∼20% volume fraction each in the electrode, which leaves ∼50-60% pore volume for transport of the reactant hydrogen/air and product water to/from the active Pt sites.Both the uniformity of the PFSA ionomer loading on a 100-nm length scale and the uniformity of the actual film thickness distribution surrounding the carbon support and catalyst nanoparticles on a 1-nm length scale are critical to electrode performance, and quantitative measurements of both these properties are highly desired. Scanning transmission electron microscopy (STEM) is an attractive tool for characterizing the distribution of ionomer within PEFC electrodes, especially when coupled with spectroscopic techniques such as electron energy loss spectroscopy (EELS) or energy dispersive X-ray spectroscopy (EDS). 1 While electron microscopy is more than capable of fulfilling the spatial resolution and chemical sensitivity requirements necessary for analysis of the PFSA ionomer, further method optimization of the STEM imaging and analysis parameters is required due to the beam-sensitive nature of the ionomer films.Fluorinated compounds, such as the PFSA ionomer, can be highly sensitive to electron beam radiation damage. 2 The high electron doses needed to acquire spectroscopic maps by either EDS or EELS can induce severe structural and chemical changes to the ionomer within the electrode, as previously demonstrated on PEF...
In this paper, we present measurements of He+ and He+2 ion‐induced sputtering of an anorthite‐like thin film at a fixed solar wind‐relevant impact energy of ~0.5 keV/amu using a quartz crystal microbalance approach (QCM) for determination of total absolute sputtering yields. He+2 ions are the most abundant multicharged ions in the solar wind, and increased sputtering by these ions in comparison to equivelocity He+ ions is expected to have the biggest effect on the overall sputtering efficiency of solar wind impact on the Moon. Our measurements indicate an almost 70% increase of the sputtering yield for doubly charged incident He ions compared to that for same velocity He+ impact (14.6 amu/ion for He+2 vs. 8.7 amu/ion for He+). Using a selective sputtering model, the new QCM results presented here, together with previously published results for Ar+q ions and SRIM results for the relevant kinetic‐sputtering yields, the effect due to multicharged‐solar‐wind‐ion impact on local near‐surface modification of lunar anorthite‐like soil is explored. It is shown that the multicharged‐solar‐wind component leads to a more pronounced and significant differentiation of depleted and enriched surface elements as well as a shortening of the timescale over which such surface‐compositional modifications might occur in astrophysical settings. In addition, to validate previous and future determinations of multicharged‐ion‐induced sputtering enhancement for those cases where the QCM approach cannot be used, relative quadrupole mass spectrometry (QMS)‐based measurements are presented for the same anorthite‐like thin film as were investigated by QCM, and their suitability and limitations for charge state‐enhanced yield measurements are discussed.
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