Elucidating the Role of Surface Chemistry on the Dispersion of Nanoparticles in Nanophase-Segregated Ionomer Nanocomposites
Allison Domhoff, Apoorv Balwani, Tyler B. Martin, Eric M. Davis
Ionomer nanocomposites provide a promising solution to address ion crossover issues inherent to traditional ion- containing membranes used in batteries for grid-scale energy storage (e.g., vanadium redox flow batteries). Herein, we investigate the impact of surface chemistry on nanoparticle dispersion, membrane morphology, and vanadium ion permeability in Nafion membranes. Specifically, silica nanoparticles (SiNPs) were functionalized with various chemical moieties (seven in total) that electrostatically interact, either attractively or repulsively, with the sulfonic acid pendant chains that form the ionic network within Nafion. As seen from electron microscopy analysis of the nanocomposites, SiNPs with sulfonic acid end functionality were, on average, well dispersed within the ionomer membrane, though increased vanadium ion permeability, as compared to pristine (or unmodified) Nafion, was observed and attributed to changes in the Donnan potential of the system. In contrast, SiNPs with amine end functionality were, on average, observed to form large aggregates within the membrane. Surprisingly, nanocomposites containing a higher degree of nanoparticle aggregation demonstrated the lowest vanadium ion permeability. Fractal analysis of the low-Q small-angle neutron scattering data suggest that the interface between the ionomer and the SiNP surface transitions from rough to smooth as the nanoparticle surface changes from sulfonic acid-functionalized to amine-functionalized.