Supra-3-Volt Nonaqueous Redox Flow Batteries with High Stability Based on Simple Terephthalonitrile Anolytes

Redox flow batteries (RFBs) play a crucial role in the transition towards net-zero. They are broadly categorized into inorganic and organic systems. Commercial systems predominantly use vanadium as active material, but their reliance on an expensive, health-threatening and imported resource raises concerns, especially in Europe. Consequently, there is a growing focus on organic RFBs, which can be further divided into aqueous organic and full organic flow batteries. The latter, operating in organic solvents, offer a significantly broader potential window, potentially three times higher than water-based systems. The synergy of organic solvents with redox-active materials holds the promise of establishing cost-effective, high-voltage, and power-efficient carbon-based RFBs, which can be locally produced.
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.¹ Implementing these functionalities in the design of new 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 alkyl-substituted terephthalonitriles, which can be synthesized in two steps. With only one ethyl group, the stability is limited because the reduced anolyte deprotonates the solvent, which then reacts with the anolyte. Experiments and density functional theory calculations show this reaction can be slowed down by introducing two alkyl substituents. In combination with di-alkoxy-substituted benzene derivatives as catholyte,² the 2,5-dialkylterephthalonitriles achieve >3 V flow batteries that exhibit a high capacity retention of >99.8% per cycle and energy efficiencies 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.

References:
¹ N. Daub et al ACS Appl. Energy Mater. 2021, 4, 9248–9257.
² S. R. Bheemireddy et al ACS Appl. Mater. Interfaces 2022, 14, 28834.