(604i) Exploring Zone Specific Proton Transport of Ionomers Under Confinement Via Confocal Laser Scanning Microscopy

Ion conducting polymers or ionomers play critical role in the performance of energy conversion and storage devices. In these devices, ionomers are used as a bulk membrane separator (several tens of micron thick) as well as a very thin layer (sub-micron thick) at catalyst interfaces. The behavior of the bulk membrane and thin films of the same ionomers can be drastically different. Not only that, the optical, mechanical and ion conduction properties at different interfaces can be very different from those in the bulk region of the same ionomeric materials. Interfacial ion transport resistance can lead to low power density and poor energy efficiency of fuel cells and other electrochemical devices. Therefore, getting knowledge of zone-specific proton transport properties of bulk and confined, thin ionomer films is crucial. Often times, we rely on sophisticated techniques (such as neutron reflectometry, resonant soft X-ray scattering) to get zone specific properties (e.g. composition, density) of ionomer films and membranes. In our recent work, we demonstrated an innovative strategy to obtain zone specific proton transport properties in ionomer films and membranes using fluorescence confocal laser scanning microscopy. As a preliminary work, we incorporated fluorescent photoacid probe HPTS within Nafion and sulfonate polysulfone membranes (25-50 mm thick) and thin films (< 1 mm thick) and took 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 images revealed that the proton conduction can be different at different interfaces as a function of relative humidity, film thickness and ionomer structure. The data were also supported by cross-sectional morphology and other studies we performed on ionomer films and membranes. The demonstrated approach shows great promise in revealing interfacial properties reliably using an everyday accessible instrument and can make deep impact in future development of efficient ionomeric interfaces for sustainable energy and other applications.