Impact of Lignin Nanofiller on the Water Diffusion and Ionomer Swelling Kinetics of Sulfonated Ionomer Nanocomposites

Sulfonated poly(ether ether ketone) (SPEEK)-based nanocomposites have emerged as a promising class of proton exchange membranes for vanadium redox flow batteries due to their low cost and comparable proton conductivity to that of the current benchmark material Nafion. For these systems, understanding the interplay between ionomer nanostructure and water/ion transport is critical to the development of more cost-effective, better performing membranes. In this investigation, we investigated the liquid water diffusion and water-induced ionomer swelling kinetics of a series of lignin-SPEEK membranes using in situ time-resolved attenuated total reflectance-Fourier transform infrared (tATR-FTIR) spectroscopy. Specifically, SPEEK nanocomposites containing 5 mass %, 15 mass %, and 25 mass % fractionated lignin, at two lignin MWs (34500 Da and 5470 Da) were fabricated. Further, the two degrees of sulfonation (DS) were applied to investigate how sulfonic acid group density affects the microstructure and properties of the membrane. 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 and 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, though improved water diffusivity was captured at the highest DS, which is consistent with the proton conductivity of these membranes. Results from this study help elucidate the impact of lignin concentration and MW on water diffusion and polymer swelling of SPEEK–lignin ionomer nanocomposites.