Lignin as redox-targeted catalyst for the positive vanadium electrolyte

[Display omitted] •Lignin was coated on a glassy carbon electrode and a carbon felt electrode.•Reaction kinetics and reversibility of the VO2+/VO2+ couple were greatly improved.•The discharge capacity was 1.6 times higher with lignin on the electrode.•The charge surpassed the maximum theoretical capacity for the vanadium electrolyte. Vanadium Redox Flow Batteries (VRFBs) are promising candidates for large- scale energy storage applications however, there are still some obstacles that need to be overcome such as their low energy density (25–30 Wh L−1 for a 1.8 M electrolyte), high cost and large environmental impact due to corrosiveness and the unsustainability of vanadium. To address these challenges, we introduce lignin as a low-cost electrocatalyst and capacity booster for the positive electrolyte. Coated on a glassy carbon electrode, lignin showed electrocatalytic abilities and improved the electrochemical reversibility of the VO2+/VO2+ redox couple significantly. In charge and discharge cycles volumetric capacities of 28 Ah L−1 (charge) and 23.5 Ah L−1 (discharge) were achieved with a 0.9 M concentrated vanadium electrolyte which surpassed the maximum theoretical capacity of 24.1 Ah L−1 in the charge. This demonstrates an increase of 7.5 Ah L−1 and 9 Ah L−1 for the charge and discharge cycles, respectively, compared to a pristine carbon felt electrode. This work showcases the noteworthy influence of lignin on vanadium and opens new possibilities for VRFB chemistry.