Lignin-Based Electrolytes for Aqueous Redox Flow Batteries

Abstract

Lignin is one of the most naturally occurring biopolymers on Earth and exists in a relatively large portion in the residual stream of the pulp and paper industry. Technical lignins are water-soluble, non-toxic and rich in quinone-type groups; therefore, they could be a potential redox species for next generation aqueous redox flow batteries (RFBs). Despite having attractive features, lignin does not show reversible electrochemical behavior unless there is a modification in its structure. Herein, for the first time, we implemented a simple and novel approach to modify the structure of sulfur-free, soda-based lignin (50 g L-1) by oxidative depolymerization in alkaline media with KOH as a supporting electrolyte. The modified lignin showed good reversible electrochemical activity with distinct redox peaks in the cyclic voltammetry (CV) tests. The redox peaks closely match the most abundantly derived lignin monomers, vanillin, and acetovanillone, produced after oxidative depolymerization. The modified lignin was tested in the next step as a negolyte in RFB setup with potassium ferrocyanide in the counterpart. The RFB cycled for over 200 cycles with a stable average coulombic efficiency (CE) of 91%. The (electro)chemical stability of lignin-based electrolyte was tested after storing it for four months. Stored lignin exhibited nearly identical performance with the fresh electrolyte, hence proving the long-term durability of (electro)chemical properties of the lignin-based electrolyte. The depolymerization method was applied to higher concentration lignin (150 g L-1), where RFB cycling tests exhibited enhanced capacities (discharge capacity of 84.2 mAh L-1) and very consistent CE for 50 cycles.

Keywords: Lignin, Bio-based electrolyte, Organic redox flow battery, Sustainability, Energy storage