Lignin-Based Electrolytes for Aqueous Redox Flow Batteries

Lignin is one of the most naturally occurring biopolymers on Earth and exists in a relatively large portion of the residual stream of the pulp and paper industry. Technical lignin is water-soluble, nontoxic, and rich in quinone-type groups; therefore, it could be a potential redox species for next-generation aqueous redox flow batteries (RFBs). Despite having attractive features, lignin does not show a reversible electrochemical behavior. Herein, we implemented a straightforward approach to modify the structure of soda-based lignin by oxidative depolymerization. The modified lignin showed good electrochemical activity through cyclic voltammetry with distinct redox peaks, which match lignin monomers, such as vanillin and acetovanillone. The modified lignin was used as the negolyte of the RFB setup with potassium ferrocyanide as the counterpart. The RFB was cycled for over 200 cycles with an average Coulombic efficiency of 91%. In addition, the modified lignin electrolyte maintained the (electro)chemical properties even after four months of storage, as proven by RFB tests.