(30h) Enhanced Proton Selectivity in Sulfonated Ionomer Nanocomposites Containing Fractionated, Clean Lignin

Sulfonated poly(ether ether ketone) (SPEEK) membranes have emerged as a promising proton exchange membrane (PEM) for vanadium redox flow batteries (VRFBs), due to their low cost, comparable mechanical properties and chemical resistance, as well as their similar proton conductivity to that of the current benchmark material, Nafion. While SPEEK is an encouraging alternative to Nafion, SPEEK still suffers from high rates vanadium ion crossover, leading to reduced performance and lifetimes for VRFBs. To combat this issue, nanocomposites of SPEEK and inorganic fillers, such as silica or titania nanoparticles, have been fabricated. While SPEEK-based nanocomposite membranes have been thoroughly studied for use in fuel cells, important processing-performance property relationships investigations of SPEEK for their use in VRFBs remains insufficient. In this study, the impact of lignin, an aromatic biopolymer with an abundance of hydroxyl groups, on the ion transport properties of lignin-containing SPEEK nanocomposites was investigated. Specifically, two series of SPEEK membranes were fabricated and the proton and vanadium ion transport properties of these membranes were characterized. The SPEEK-lignin nanocomposite membranes were prepared at two values of degree of sulfonation (DS), 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 were selected: (1) a low molecular weight (LMW) fraction; and (2) a high molecular weight (HMW) fraction. Prior to fabrication, the hydroxyl group content of lignin was characterized via ³¹P nuclear magnetic resonance (NMR) spectroscopy.

The DS was observed to be well controlled by the duration of the sulfonation reaction and was measured via both titration and ¹H NMR. The permeability of vanadium ions (specifically the vanadyl ion) through the ionomer nanocomposites was measured via ultraviolet-visible spectroscopy, where it was observed that the permeability of vanadium ion decreased significantly with the incorporation of both the LMW and HMW lignin, at all lignin loadings. In addition to vanadyl ion permeability, the through-plane proton conductivities of the nanocomposite membranes were measured. Notably, SPEEK membranes containing 5 mass % LMW and HMW lignin exhibited proton conductivities that were approximately 90% and 110% greater than and solution-cast, neat SPEEK and Nafion, respectively. Surprisingly, even at loadings as high as 15 mass % LMW lignin, the proton conductivity was still approximately 20% and 30% greater than and solution-cast, neat SPEEK and Nafion membranes, respectively. Furthermore, the lignin dispersion state was characterized by transmission electron microscopy (TEM). TEM images showed that the lignin was uniformly dispersed throughout the SPEEK, with the size of lignin phase ranging from 50nm to 100nm. Results from this investigation establish important processing-performance property relationships for sulfonated ionomer nanocomposites, helping to facilitate the development of novel, better performing ionomer membranes for VRFBs.