Eco-friendly non-fluorinated membranes for renewable energy storage

The European Union is studying the possibility of prohibiting the use of fluorinated-based materials which could limit the use of the most studied and used Nafion and Nafion-like membranes for any applications. Therefore, this study focuses on the preparation and performance evaluation of green, non-fluorinated proton exchange membranes in a chlor-alkali-based reversible electrochemical cell system for renewable energy storage applications. The membranes were prepared by casting and cross-linking of polyvinyl alcohol (PVA) with varying chitosan (CS) concentrations (5-20 wt%), followed by sulfonation using diluted sulfuric acid at room temperature. CS concentrations significantly influenced membrane's physicochemical properties, including ion exchange capacity, ionic conductivity, mechanical strength, and water uptake. Positron annihilation lifetime spectroscopy revealed a correlation between free volume properties and other membrane characteristics. A custom-designed 3D-printed electrochemical cell was developed, capable of operating in reversible mode (both electrolysis and fuel cell modes) with a zero-gap configuration. The PVA/CS (20 wt%) membrane outperformed Nafion, exhibiting a lower voltage (4 V vs. 6 V) in electrolysis mode at 50 mA cm-2 and a higher specific power density (2.7 vs. 1.9 mW cm-2 mgPt) in fuel cell mode. The obtained results demonstrate that crosslinked PVA/CS non-fluorinated based membranes can be sulfonated using diluted sulfuric acid and work effectively in reversible chlor-alkali electrochemical cells.