Superior Proton Conductivity and Selectivity in Sulfonated Ionomer Biocomposites Containing Renewably Processed and Fractionated Lignin

Abstract

Herein, we have synthesized high-performance, low-cost ionomer biocomposites comprised of sulfonated poly(ether ether ketone) (SPEEK) and softwood Kraft lignin. Specifically, SPEEK ionomer composites containing renewably processed and fractionated low and high molecular weight lignin, at concentrations ranging from 5–25 mass %, were synthesized. After fabrication, the water and ion transport properties, as well as the hydrated nanostructure, of the biocomposites were characterized. Most notably, our SPEEK–lignin composite membranes exhibited proton conductivities and proton selectivities that were two-fold and four-fold higher, respectively, than both neat SPEEK and the current benchmark ionomer for these applications, Nafion. In conjunction with ion exchange capacity and equilibrium water uptake data, as well as real-space imaging from electron microscopy, small-angle neutron scattering data suggested that the introduction of lignin increased the spacing between hydrophilic, ionic domains. As a proof of concept, the feasibility of these membranes as proton exchange membranes (PEMs) was assessed. In vanadium redox flow battery (VRFB) tests, SPEEK–lignin membranes containing 15 mass % lignin exhibited performance (power density, discharge voltage) comparable to that of neat SPEEK membranes. This comparable performance underscores the promise of SPEEK–lignin biocomposites as PEMs for next-generation VRFBs.