Technological advancement of energy conversion and storage device is critical to achieve energy sustainability. A major barrier, negatively impacting efficiency of energy conversion devices like proton exchange membrane fuel cells (PEMFCs), is poor ion conductivity at ionomer-catalyst interfaces on electrodes. Exploring the routes to ion conduction limitations in nanothin ionomer films can be the key to design more efficient ionomer-catalyst interfaces and improve the oxygen reaction efficiency in PEMFCs. This work explored the local hydration and proton conduction environment within three of the most promising fluorocarbon based fuel cell ionomers, 3M PFIA, 3M PFSA and Nafion, using fluorescent photoacid probe. 3M PFIA has two different acid groups per side chain, (i) highly acidic bis(sulfonyl)imide (-SO2-NH-SO2-) and (ii) perfluorosulfonic acid (-CF2-SO3H) group; while 3M PFSA and Nafion both have single perfluoro sulfonic acid group per side chain with slight differences in side chain structure. We found that proton conductivity did not necessarily scale with water uptake in all sub-micron thick films of all three ionomers. Interestingly, the ionic domain size, measured independently using steady state fluorescence and small angle X-ray scattering (SAXS), followed the same trend as proton conductivity: 3M PFIA&gt; 3M PFSA&gt; Nafion. Moreover, the close match in ionic domain size obtained from two independent techniques indicates that fluorescence is a very powerful and reliable technique that can tell us accurately what is happening inside the ionic domains. We also performed spatial mapping using contact resonance atomic force microscopy (CR-AFM) and developed a model utilizing CR-AFM data to quantitatively obtain viscoelastic properties of ionomer ultrathin films. The spatial correlations of mechanical behavior with proton conduction properties helped a great deal to better understand the ionomer-catalyst interfaces.

Fluorocarbon-based ionomers with single- and multi-acid side chains at nanothin interfaces: What matters

2019 Spring National Meeting

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