(649g) Vanadium Ion Dynamics of Ionomer-Nanoparticle Hybrid Membranes

Ionomer nanocomposite membranes have garnered considerable attention as proton exchange membranes (PEM) in vanadium redox flow batteries due to their chemical resistance and reduced vanadium ion crossover. Traditionally, nanoparticles (NPs) are believed to reside within hydrophilic domains, physically blocking the crossover of vanadium ions. However, recent work suggests this reduced crossover is caused by the interaction of NPs and sulfonic groups in ionic channels, leading to inaccessibility of sulfonic groups for vanadium ion transport. Herein, the dynamics of water and hydrated vanadium ions in Nafion–NP membranes was investigated via quasi-elastic neutron scattering (QENS), specifically, high-flux backscattering. The broadening of the energy spectra was attributed to the incoherent scattering from hydrogen atoms – both protons and water molecules in the vanadyl ion hydration sphere. These quasi-elastic events were deconvoluted from the scattering data through peak fitting in Data Analysis and Visualization Environment (DAVE). Vanadium ion dynamics were measured for Nafion–NP membranes prepared through both a sol-gel condensation and solution casting process. Furthermore, the silica nanoparticle (SiNP) loading in these membranes was varied from 0 mass% to 10 mass%. The q-dependent peak shape parameters, as well as the elastic incoherent structure factors were extracted from analysis of the data using a spherical diffusion model. The radius of the sphere within which dynamics occur was observed to increase with increasing temperature, while the residence time between jumps was seen to decrease. Additionally, the fraction of immobile ions (i.e., those not diffusing within the time window of the experiment) was seen to decrease with increasing temperature, indicating more vigorous ion motion at higher temperatures. To better understand the connection between water/vanadium dynamics and conductivity, the proton conductivity of these membranes was also characterized. By comparing the ion dynamics from these two types of ionomer nanocomposite membranes, at different NP concentrations, the impact of SiNPs on the ion transport in these ionomer nanocomposites was better explained, providing deeper insight into the decreased vanadium ion crossover observed for these membranes.