Oxidative Depolymerization of Lignin to Aqueous Electrolytes for Sustainable Redox Flow Batteries

The valorization of lignin, an underutilized renewable resource with high aromaticity, is gaining significance. In this study, an aqueous electrolyte was prepared utilizing lignin from Miscanthus x giganteus, elephant grass (EGL), and employed as a redox species for redox flow batteries (RFBs). Oxidative depolymerization was utilized to alter the structure of EGL to generate electrochemical functionalities suitable for RFB applications. Central composite design (CCD)–based response surface methodology (RSM) was applied to determine the optimal operating conditions for maximizing the monomer yield. The effects of four independent parameters were investigated for the depolymerization process: temperature (160–220°C), pressure (5–15 bar O₂), residence time (30–90 min), and lignin concentration (50–150 g/L). The highest monomer yield was predicted to be 5.3 wt% under optimized process parameters of 177°C, 12 bar O₂, 85 min, and an EGL concentration of 52 g/L, which was experimentally validated with an average yield of 5.7 ± 0.45 wt%. The oxidatively depolymerized lignin (OEGL) displayed favorable electrochemical reversibility and stability for 200 cycles at a constant current of 1 mA/cm². A significant improvement in the discharge capacity of the RFB was also achieved when utilizing the OEGL compared to the case using EGL. The findings of this study establish a foundation for the development of sustainable and potentially affordable RFBs suitable for commercial production. This approach also adds a new aspect to existing pulp and paper mills and biorefinery facilities by functioning as inexpensive storage sites for variable renewable electricity.