Insights into thiadiazole bridge in neutral aqueous organic flow batteries

Neutral aqueous organic redox flow batteries (AORFBs) are regarded as a highly promising technology for large-scale renewable energy storage, primarily owing to their use of abundant organic materials. Nevertheless, the characteristics of the redox-active materials need considerable enhancement to satisfy the performance demands of AORFB applications. Herein, we present the design and synthesis of a redox-active thiadiazole-based bridge viologens, containing enhancing conjugated thiadiazole linker framework, demonstrating solubility, low potential (−0.79 V), and excellent electrochemical reversibility in neutral AORFBs. Owing to its unique molecular architecture, the 0.1 M negolyte exhibits a capacity decay rate of 0.00014% per cycle (2.7% day−1) over 1000 cycles at a current density of 100 mA cm−2. Density Functional Theory (DFT) studies, Molecular Dynamics (MD) simulations, and mechanistic investigations reveal that thiadiazole bridge, extended p-π and π-π conjugation, and intermolecular hydrogen bonding from the lone electron pairs of nitrogen enhance viologen's redox core stability, improve solubility and redox reversibility, and reduce self-dimerization and degradation. This study demonstrates that rationally refined designed viologen derivatives exhibit significant potential for grid-scale renewable energy storage in grid-level applications based on ARFBs technology.