A lignosulfonate-based negolyte for aqueous redox flow batteries

Aqueous redox flow batteries (RFBs) as grid-scale energy storage devices hold great potential to accomodate the rising demand for intermittent renewable energy sources. A significant parameter that enhances the sustainability of RFBs is selecting the appropriate redox species. In this study, we investigated the application of lignosulfonate as a redox species for the negolyte of aqueous RFBs because lignosulfonate is a low-cost, abundant, highly water-soluble material with a high phenol content. To overcome the intrinsic electrochemically irreversible nature of lignosulfonate, herein, oxidative depolymerization was employed to modify the chemical structure in a weakly alkaline media. The modified lignosulfonate exhibited improved electrochemical activity, as indicated by cyclic voltammetry, with distinct redox peaks that correspond to lignin-derived monomers, such as vanillin and 4-hydroxybenzaldehyde. The modified lignosulfonate, as negolyte, paired with ferrocyanide in the counterpart in a lab-based single RFB cell. The RFB utilizing 50 g L −1 modified lignosulfonate showed 80.6 % capacity retention over 50 cycles and nearly 1.5 times higher discharge capacity than the RFB using non-modified lignosulfonate. By increasing the concentration of modified lignosulfonate up to 200 g L −1 , the discharge capacity increased threefold; however, the capacity retention dropped to 60 %. This study presents an opportunity to utilize bio-based electrolytes in building novel, cost-effective, and sustainable RFBs. • Oxidative depolymerization was effective in modifying lignosulfonate structure. • Lignosulfonate-based negolyte showed electrochemical activity in NH 4 OH media. • The flow battery using lignosulfonate-based negolyte showed 80.6 % capacity retention. • Using 200 g L −1 lignosulfonate resulted in a significant increase in capacity.