(651f) Lignin-Containing Sulfonated Ionomer Composite Membranes for Use in Vanadium Redox Flow Batteries

With energy obtained from renewable sources becoming more prevalent, scalable energy storage technologies, like the vanadium redox flow battery (VRFB), are needed to ensure reliable dispatch of this energy to the electrical grid. One vital component of the VRFBs is proton exchange membrane (PEM), which functions to facilitate transport of charge carriers, protons, as well as prevent cross-mixing of positive and negative electrolytes. The current benchmark material for this technology, Nafion^TM, is quite expensive and suffers from high vanadium ion crossover. Herein, we successfully fabricated SPEEK–lignin ionomer composites using lignin that has been fractionated and cleaned via a liquid-liquid extraction process. Specifically, two series of SPEEK-based membranes were fabricated, where the degree of sulfonation (DS), the lignin molecular weight, and the lignin content were varied. The SPEEK–lignin nanocomposite membranes were prepared at DS values of approximately 70% and 80%, as well as lignin loadings ranging from 0 mass % to 25 mass %. In addition to lignin concentration, two lignin molecular weights (MWs) were explored: (1) a low molecular weight (LMW) lignin fraction (5470 g/mol) and (2) a high molecular weight (HMW) lignin fraction (34500 g/mol).

To establish a better understanding of the water and ion transport properties of these nanocomposites, the diffusion of liquid water, as well as water-induced ionomer swelling kinetics were studied using in situ time-resolved attenuated total reflectance-Fourier transform infrared (tATR-FTIR) spectroscopy. A three-element viscoelastic relaxation model was applied to capture the water-induced swelling dynamics of the ionomer nanocomposites, while a combined diffusion-relaxation model was used to determine the water diffusion coefficient from the tATR-FTIR water uptake data. The relaxation time constant was seen to decrease markedly after the introduction of lignin, where this decrease was seen to be proportional to the lignin concentration, indicating a stiffening of the ionomer network. Similarly, the water diffusion coefficient was observed to decrease with the introduction of lignin, where notably, the water diffusivity was seen to decrease by over two orders of magnitude at the highest DS. However, the ionomers with a higher DS exhibited improved water diffusivity when compared to their lower DS counterparts, which was consistent with the improved proton conductivities of these membranes. In addition to transport studies, structural investigations were conducted via small-angle neutron scattering (SANS), where the periodic spacing between hydrophilic domains (d-spacing) was characterized for these nanocomposite membranes. Results from this work help elucidate the impact of sulfonic acid content, lignin concentration, and lignin MW on water diffusion and polymer swelling, as well as nanostructure of the SPEEK–lignin ionomer composite membranes.