Effect of Lignin Molecular Weight and Chemical Functionality on the Transport Properties and Dynamics of Sulfonated Ionomer Biocomposites for All-vanadium Redox Flow Batteries

Sulfonated poly(ether ether ketone) (SPEEK) has emerged as a promising proton exchange membrane for all-vanadium redox flow batteries due to their high thermal, mechanical, and chemical stability, adequate proton conductivity, and low cost. Herein, a systematic investigation on the impact of lignin, a 'green' additive, on the membrane performance properties of SPEEK–lignin biocomposites was conducted. Specifically, lignin, both bulk and purified and fractionated to various prescribed molecular weights, were introduced into SPEEK membranes at loadings of 5 and 15 mass %. Furthermore, the unfunctionalized lignins were chemically modified with cationic (amine) and anionic (sulfonic 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, while hydrated SANS measurements indicated minimal change in nanostructure. The transport properties of the SPEEK–lignin membranes, including proton conductivity and vanadium ion permeability, were also characterized. Finally, the water dynamics and ionomer swelling kinetics were studied using quasi-elastic neutron scattering and in situ time-resolved Fourier transform infrared-attenuated total reflectance spectroscopy, respectively.