Stable and Highly Ion-Selective Membrane Made from Cellulose Nanocrystals for Aqueous Redox Flow Batteries

The design of chemically stable ion-exchange membranes with high selectivity for applications in an aqueous redox flow battery (RFB) at high acid concentrations remains a significant challenge. Herein, this study designed a stable and highly ion-selective membrane by utilizing proton conductive cellulose nanocrystals (CNCs) incorporated in a semicrystalline hydrophobic poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) matrix. The high hydrophobicity of the PVDF-HFP matrix mitigates crossover of the electrolytes, whereas the abundant and low-cost CNCs derived from wood provide high proton conductivity. The fundamental contributors for CNCs' excellent proton conductivity are the hydroxyl (-OH) functional groups, highly acidic sulfonate (-SO₃H) functional groups, and the extensive intramolecular hydrogen bonding network. In addition, CNCs exhibit a mechanically and chemically stable structure in the harsh acidic electrolyte attributed to the high crystallinity (crystalline index of ∼86%). Therefore, because of the high proton conductivity, excellent ion selectivity, high chemical stability, and structural robustness, the vanadium redox flow battery (VRFB) assembled with the homogeneous CNCs and PVDF-HFP (CNC/PVDF-HFP) membrane achieved a Coulombic efficiency (CE) of 98.2%, energy efficiency (EE) of 88.2%, and a stable cycling performance for more than 650 cycles at a current density of 100 mA cm^-2. The obtained membrane possesses excellent flexibility, high mechanical tensile strength, and superior selectivity. Meanwhile, the applied casting method is scalable for large-scale manufacturing.