Supra-3-V Nonaqueous Redox-Flow Batteries Based on Simple Terephthalonitrile Anolytes

Organic molecules for nonaqueous redox-flow batteries tend to become increasingly complex because, for practical applications, they have to fulfill several requirements in terms of redox potential, solubility, and stability. Implementing these functionalities in the design of materials often results in an undesirable high synthetic complexity, which reduces the feasibility for large-scale applications. Considering redox potential, solubility, stability, and low synthetic complexity as important design considerations, we investigated the suitability of alkyl-substituted terephthalonitriles for use as anolytes in redox-flow batteries. These derivatives can be synthesized in two steps. With one ethyl substituent, the stability is very limited because the reduced anolyte deprotonates the solvent, which then reacts with the neutral anolyte. Experiments and density functional theory calculations show that this reaction can be slowed down by introducing two alkyl substituents. In combination with dialkoxy-substituted benzene derivatives as the catholyte, 2,5-dialkylterephthalonitriles achieve >3 V flow batteries that exhibit capacity retention of >99.8% cycle^-1 and energy efficiencies of up to 77% at a current density of 40 mA cm^-2. With these metrics, 2,5-dialkylterephthalonitriles outperform many previously reported flow batteries using benzene-based anolytes, but for future practical application, solubility and long-term stability need to be further enhanced.