Visualization of clusters in polymer electrolyte membranes by electron microscopy

Abstract

The morphology of ionic clusters that form in polyelectrolyte membranes has a strong effect on transport and electrical properties. In spite of considerable research efforts the link between morphology and properties has not been clearly established, mainly due to difficulties in assessing nanoscale morphology. Electron microscopy (EM) has the potential to visualize morphology. However success in visualization has so far been moderate. In this review we focus on the potential of EM techniques to characterize the ionic domains. We use both experimental data and models to compare the capabilities of several EM techniques: BF TEM, HAADF, core-loss EELS, and low-loss EELS in projection imaging and STEM modes. The main problems common for all these EM modes are radiation damage and overlap of features in projection. Our models show that core loss EELS with exposures that are below the typical damage threshold is incapable of resolving 2 nm diameter sulfur-rich clusters in PEMs. While low loss EELS requires lower exposure the insight it can provide is quite limited. HAADF and BF TEM present the most effective modes for imaging the sulfur clusters in PEMs. While BF TEM uses scattered electrons more efficiently, HAADF using slightly higher doses can provide unique information due to in-focus imaging and transparent interpretation of the images. Fortunately, in at least some interesting cases the clusters themselves are much more radiation resistant than the polymer and can be studied at exposures high enough to obtain clear images. Our simulations also show that tomographic 3D reconstruction provides the best approach for solving the overlap problem. In spite of the abilities of electron tomography, data obtained from all EM techniques improves if thin sections are studied. We briefly discuss methods for obtaining such sections.

High-Resolution Mapping of the PFSA Polymer Distribution in PEFC Electrode Layers

“…Although electron microscopy has the spatial resolution and chemical sensitivity necessary, chemical mapping at higher magnification is currently limited by beam damage due to the implied higher electron flux through the distributed ionomer film (2,3).…”

Evaluating focused ion beam and ultramicrotome sample preparation for analytical microscopies of the cathode layer of a polymer electrolyte membrane fuel cell

“…Paul et al [5] suggested that there may be a difference between proton conductivity of bulk PFSA membrane and the PFSA ionomer present in the catalyst layer due to the reorganization of the nanostructure of PFSA when it is in very thin (sub-10 nm) films. It is unknown exactly how the nanoscale morphology of the PFSA is related to proton and O 2 transport [6].…”

“…Transmission Electron Microscopy (TEM) in both bright field and atomic-weightsensitive, High Angular Annular Dark Field (HAADF) modes, as well as Electron Energy Loss Spectroscopy (EELS) and Energy Dispersive X-ray Spectroscopy (EDX) have been applied to image the PFSA membrane and ionomer at sub-5 nm scale [2,4,[6][7][8][9]. In order to mitigate the effects of radiation damage, the nanostructure of PFSA membranes has been imaged with electron microscopy using cryo-techniques [2,9].…”

Overcoming the Challenges of Beam-sensitivity in Fuel Cell Electrodes

“…Thin perfluorosulfonic acid (PFSA) ionomer layers serve the critical role of proton conduction within the fuel cell electrodes, but these films are very sensitive to radiolysis during examination under the electron beam [1]. Previously employed strategies for reducing beam damage in such fluorinated compounds include cryogenic cooling, sputter coating electrically conductive surface layers, increasing the accelerating voltage, and limiting the electron dose [2,3]. The different strategies were investigated for the case of thin PFSA ionomer films.…”