Role of Functionalized and Unfunctionalized Lignin Nanofiller on the Nanostructure of Sulfonated Ionomer Nanocomposite Membranes

Sulfonated poly(ether ether ketone) (SPEEK) has emerged as a promising proton exchange membrane for vanadium redox flow batteries (VRFBs) and an encouraging alternative to the current benchmark material, Nafion. Compared to Nafion, SPEEK is significantly lower in cost and demonstrates similar proton conductivity, however, both membranes suffer from high vanadium ion permeability which leads to a reduction in battery lifetime and performance. In this study, lignin, a ‘green’ nanofiller, was incorporated into SPEEK to fabricate SPEEK-lignin nanocomposites to help combat this issue. The impact of unfunctionalized and various functionalized lignin on the ion transport and membrane performance properties were investigated. SPEEK was prepared via post-polymerization functionalization using concentrated sulfuric acid. Amine-functionalized and sulfuric acid-functionalized lignin were prepared by reacting lignin with epichlorohydrin and sulfuric acid, respectively. The lignin loadings introduced into SPEEK ranged from 0 to 25 mass% and were fractionated into low and high molecular weights (LMW and HMW, respectively). It was observed that vanadium ion permeability decreased with increasing lignin loading for all lignin-SPEEK nanocomposites compared to neat SPEEK. Small-angle neutron scattering study on SPEEK and functionalized lignin-SPEEK membranes can directly observe the microstructure of the membrane and elucidate the structure-property relationships of these membranes.