(402e) Impact of Nanoparticle Aggregation on Ion Transport and Nanostructure in Ionomer Nanocomposite Membranes

Ionomer nanocomposites have emerged as a potential replacement for traditional polymer electrolyte membranes (PEMs) utilized in vanadium redox flow batteries, a promising grid-scale energy storage technology. The introduction of nanoparticles into PEMs such as Nafion reduces vanadium ion crossover between the electrolyte solutions during battery operation, an issue that has plagued traditional ionomer membranes. This crossover results in a decreased lifetime and efficiency of the flow battery. In this study, the surface chemistry of silica nanoparticles (SiNPs) was systematically varied to interrogate how electrostatic interactions between the SiNPs and the ionomer membrane (in this case, Nafion), dictate both the SiNP dispersion state and ion transport in the membrane. Specifically, nanoparticles were functionalized such that they interact electrostatically with the ionic domain of Nafion, either attractively (via an amine functionality) or repulsively (via a sulfonic acid functionality). The effect of nanoparticle dispersion on vanadium ion crossover and nanocomposite morphology was also explored by varying the length and rigidity of the bridging chains between the nanoparticle surface and its end functionality.

Contrast-match small-angle neutron scattering (SANS) experiments performed on these membranes indicated the presence of larger scale structures in Nafion nanocomposites, where the fractal dimension parameter, D_f, obtained from the upturn in the low-q scattering was a direct function of the SiNP surface chemistry. Sulfonic acid-functionalized SiNPs were well dispersed within the Nafion membrane and demonstrated D_f ≈ 3 (indicating rough surfaces and diffuse aggregates), while the amine-functionalized SiNPs exhibited more pronounced aggregation and demonstrated D_f ≈ 2 (indicating smooth surfaces and dense aggregates). The structure described by the fractal dimensions obtained from SANS correspond well with the dispersion state of SiNPs in the membrane observed via electron microscopy. Surprisingly, the permeability of vanadium ions through the membrane was most reduced when the ionomer membrane contained large, positively-charged nanoparticle aggregates (>300 nm). We postulate that the smooth SiNP aggregate surface, in addition to their size and surface functionality, sequester more sulfonic acid groups in the ionic domains to reduce the vanadium ion permeability without significantly affecting the proton conductivity.