(694e) Exploring the Distribution of Ion Conduction Properties and Stiffness across Ionomer Thin Films and Bulk Membranes Via fluorescence Confocal Laser Scanning Microscopy

As the polymer films get thinner, their interfacial behavior becomes increasingly critical for the performance of many energy conversion and storage devices, including fuel cells, and batteries. Oftentimes, we rely on sophisticated techniques (such as neutron reflectometry, resonant soft X-ray scattering, etc.) to get zone-specific properties (e.g. composition, density, etc.) of ionomer films and membranes. In this study, we have developed a novel, everyday-accessible, fluorescence confocal laser scanning microscopy (CLSM)-based strategy that can probe the distribution of mobility, ion conduction, and other properties across polymer samples. A fluorescent photoacid probe 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt was incorporated into < 1µm thick Nafion films on an n-SiO₂ substrate and free-standing, bulk membranes (25-50 µm thick) and took several xy-plane confocal images at different depth under controlled humidity. When these images were z-stacked, it offered depth profile images representing the extent of proton conduction at different interfaces across the thickness of the sample. The depth-profile images showed interface-dependent proton conduction behavior. In sub-micron thick ionomer films, proton conduction was weak over a region next to substrate interface, then gradually increased till air interface at 88% RH. In contrast, consistently high proton conduction with no interface dependence was observed across 35-50 µm-thick bulk, free-standing Nafion membranes. A hump-like mobility/stiffness distribution was also observed across Nafion films containing mobility-sensitive probe (9-(2-carboxy-2-cyanovinyl)julolidine). The proton conduction and mobility distribution were rationalized as a combinatorial effect of interfacial interaction, ionomer chain orientation, chain density, and ionic domain characteristics. Earlier, we had to rely on an average value of ion conductivity for an entire ionomer sample (measured using electrochemical impedance spectroscopy). Using this CLSM-based strategy, we can now tell how the ion conductivity is at different depths, and how far the interfacial effects propagate deep down inside very thin films. Such information will provide valuable guidance to design ionomer-catalyst layers for electrodes of electrochemical devices.