A mediated vanadium flow battery: Lignin as redox-targeting active material in the vanadium catholyte

Vanadium Redox Flow Batteries (VRFB) are promising candidates for stationary energy storage but show certain drawbacks at low energy densities (<30 Wh L−1) and a narrow operating temperature range (15–40 °C). The latter is mainly caused by the limited stability of the catholyte at elevated temperatures. Therefore, in this work we introduce a stability enhanced vanadium catholyte with redox mediating properties conferred by the highly abundant organic polymer lignin to achieve higher performance in temperature range for a penetration of the VRFB systems in thermal applications. By reducing the vanadium concentration to 0.9 M the catholyte's stability is significantly improved so that a wider operational temperature window for the flow cell can be exploited. To compensate the loss in energy density lignin as a solid capacity booster is added to the positive reservoir. Herein, the feasibility of lignin in combination with multiwalled carbon nanotubes as solid charge storage material is investigated by cyclic voltammetry and charge/discharge cycles at temperatures from 10 °C to 45 °C. Volumetric capacities >28 Ah L−1 are achieved for the capacity-boosted VRFB with 0.9 M catholyte that are comparable with the conventional 1.8 M vanadium electrolyte. The use of an abundant renewable resource like lignin in the VRFB could not only increase cell performance but also attribute to lower the high operational costs and environmental impact of the battery.