A Critical Approach to Multi-Electron Materials for High Energy Density Non-Aqueous Redox Flow Batteries

Abstract

Redox flow batteries (RFBs) are very promising storage systems in the transition towards renewable energy sources. They can be broadly classified in aqueous and non-aqueous systems. Last-named operate with organic solvents, which allow for a much broader potential window (up to three times higher) compared to water. Combination of organic solvents with organic redox active materials could pave the way for all carbon-based RFBs with superior energy densities compared to aqueous systems. In this contribution, newly developed organic electrolytes will be presented, focusing on their performance as RFB materials in acetonitrile with quaternary ammonium electrolyte salts. To push the limits of energy density, we investigated multi-electron reduction and oxidation reactions on a single molecule. As catholyte, tetrathiafulvalene (TTF) as a core structure was used. The unsubstituted TTF exhibits two reversible oxidation events (-0.04 and +0.34 V vs Fc/Fc+) but a poor solubility in acetonitrile. To optimize this and to achieve a higher oxidation potential a series of molecules with different side chains was synthesized. The resulting solubilities led to volumetric capacities of up to 71 Ah/L for the newly designed compounds. Electrochemical cycling stability was evaluated in bulk electrolysis, UV-Vis-NIR and flow cycling experiments. Current state-of-the-art anolytes based on N-methylphthalimide compounds exhibit one reversible reduction pair at a desirable low reduction potential (-1.87 V vs Fc/Fc+) and good cycling stability in bulk electrolysis experiments.[1] Expanding on this structure, new derivatives with one or two additional functional imide groups per phthalimide core were synthesized. Consequently, molecules with a single (-1.87 V vs Fc/Fc+), double (-1.26 and -1.88 V vs Fc/Fc+) and triple reduction event (-1.02, -1.65 and -2.37 V vs Fc/Fc+) can be obtained. To optimize their solubility in acetonitrile, a series of molecules with different side chains was synthesized for each of the three core structures. Determination of the solubility led to volumetric capacities of up to 66 Ah/L for the newly developed compounds. Additionally, we tested the electrochemical cycling stability in bulk electrolysis, UV-Vis-NIR, coin cell and flow cycling experiments. In the final flow battery, a high energy density of 24 Wh/L was achieved (at 1 M of transferred electrons).[2] All of this led to a critical comparison between the different molecular designs, cumulating in design rules that will influence future designs of the organic redox active compounds for organic RFBs. [1] Wei, X.; Duan, W.; Huang, J.; Zhang, L.; Li, B.; Reed, D.; Xu, W.; Sprenkle, V.; Wang, W. A High-Current, Stable Nonaqueous Organic Redox Flow Battery. ACS Energy Lett. 2016, 1, 705−711 [2] Daub, N.; Janssen, R. A. J.; Hendriks, K. H. Imide-Based Multielectron Anolytes as High-Performance Materials in Nonaqueous Redox Flow Batteries. ACS Appl. Energy Mater. 2021, 4, 9, 9248–9257