(35e) Role of Lignin Molecular Weight Dispersity and Chemical Functionality on the Structure and Transport Properties of Sulfonated Ionomer Biocomposites for Redox Flow Batteries

Sulfonated poly(ether ether ketone) (SPEEK) has emerged as a promising proton exchange membrane (PEM) for all-vanadium redox flow batteries (VRFBs) due to their high thermal, mechanical, and chemical stability, adequate proton conductivity, and low cost compared to the current benchmark material, Nafion. While SPEEK has shown to be a promising alternative to Nafion, SPEEK still suffers from high vanadium ion crossover, leading to lower columbic and energy efficiencies and an overall reduction in VRFBs lifetime and performance. To help combat this issue, various fillers have been introduced into SPEEK such as nanoparticles (e.g. silica or titania nanoparticle) where lower vanadium ion crossover was observed. Inspired by this, lignin, a naturally occurring biopolymer, has gained significant attention as a ‘green’ polymer additive. In this study, a systematic investigation on the impact of lignin on the membrane transport and performance properties of SPEEK–lignin biocomposites was conducted. Specifically, lignin fractionated to various prescribed molecular weights was introduced into SPEEK membranes at loadings of 5 and 15 mass %. Furthermore, the unfunctionalized lignins were chemically modified with cationic (amine) and anionic (sulfuric acid) groups.

The impact of chemical functionalization on the dispersion state of lignin and ionomer morphology were studied via transmission electron microscopy (TEM) and small-angle neutron scattering (SANS), respectively. Notably, TEM images indicated that the dispersion state of lignin was a strong function of the chemical functionalization and lignin loading. Hydrated SANS measurements were fit with the Teubner-Strey model where periodic spacing, or d–spacing, and correlation length values were obtained. The SANS measurements indicated minimal change in the periodic spacing of the hydrophilic domains, domains formed from the hydrated sulfonic acid groups attached to the PEEK backbone, but the correlation length between SPEEK and SPEEK–lignin biocomposites showed notable change indicating that molecular weight dispersity, chemical functionality, and concentration of lignin does have an impact on dispersity, and relatably the size of the hydrophilic domains, and is further related to its impact on transport properties. The transport properties of the SPEEK–lignin membranes, including through-plane proton conductivity and vanadium ion permeability, were also characterized. While the introduction of lignin decreased proton conductivity across the board, sulfonated lignin, at both high and low molecular weight dispersity, measured the highest proton conductivity compared to that of unfunctionalized and aminated lignin and is thought to be due to the additional sulfonic acid groups creating more interconnected hydrophilic channels when hydrated, to facilitate proton transport through the membrane. To gain further insight into the proton transport mechanism within the membranes, ion exchange capacity and equilibrium water uptake were also measured. Results from this investigation provide information regarding the fundamental processing–property relationships of SPEEK–lignin biocomposites which can be used to further elucidate the design and development of next generation ionomer composites for VRFBs.