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...

The poly(4-vinyl pyridine) (P4VP)-brush-modified indium tin oxide (ITO) electrode was used to switch reversibly the interfacial activity by the electrochemical signal. The application of an external potential (-0.85 V vs Ag|AgCl|KCl, 3M) that electrochemically reduced O(2) resulted in the concomitant consumption of hydrogen ions at the electrode interface, thus yielding a higher pH value and triggering the restructuring of the P4VP brush on the electrode surface. The initial swollen state of the protonated P4VP brush (pH 4.4) was permeable to the anionic [Fe(CN)(6)](4-) redox species, but the electrochemically produced local pH of 9.1 resulted in the deprotonation of the polymer brush. The produced hydrophobic shrunken state of the polymer brush was impermeable to the anionic redox species, thus fully inhibiting its redox process at the electrode surface. The interface's return to the electrochemically active state was achieved by disconnecting the applied potential, followed by stirring the electrolyte solution or by slow diffusional exchange of the electrode-adjacent thin layer with the bulk solution. The developed approach allowed the electrochemically triggered inhibition ("closing") of the electrode interface. The application of this approach to different interfacial systems will allow the use of various switchable electrodes that are useful for biosensors and biofuel cells with externally controlled activity. Further use of this concept was suggested for electrochemically controlled chemical actuators (e.g. operating as electroswitchable drug releasers).

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Robust, simple, and scalable touch- and brush-spinning methods for the drawing of nanofibers, core-shell nanofibers, and their aligned 2D and 3D meshes using polymer solutions and melts are discussed.

This work provides direct experimental verification (on the level of single molecules) for the behavior of hydrophobic polyelectrolyte chains adsorbed at a solid-liquid interface in the full range of possible salt concentrations: (a) in a dilute salt solution, PE chains possess an extended coil conformation visualized as adsorbed 2D-equilibrated coils; (b) in a moderate salt concentration range, the polymer coil shrinks and approaches the dimensions of a polymer coil under θ-conditions and the chains are visualized as adsorbed 3D-projected coils; (c) at high salt concentrations, the polymer coils reexpand and the molecules are visualized as 2D-equilibrated extended coils; however, (d) reexpansion is limited in the presence of multivalent counterions, presumably due to the bridging of the polymer coils by the counterions.