image: Performance and advantages based on (CBu)2NDI anolyte.
Credit: ©Science China Press
Background
With the rapid development of renewable energy, the advancement of low-cost, long-life, and high-safety large-scale energy storage technologies has become pivotal for achieving carbon neutrality. Aqueous organic redox flow batteries (AORFBs) have garnered significant attention due to their tunable molecular structures and environmental friendliness. Naphthalene diimide (NDI) derivatives stand out as promising candidates owing to their extended π-π conjugation and two-electron storage capability. However, their practical application is hindered by the susceptibility of side chains and imide rings to nucleophilic attack by OH- during cycling, as well as radical-induced molecular aggregation leading to drastic viscosity increases. Although existing NDI materials have improved solubility through hydrophilic group modifications, challenges such as insufficient molecular stability and limited cycling durability persist. Consequently, the development of NDI-based electrolytes that simultaneously exhibit high stability and low cost remains a critical step toward the commercialization of AORFBs.
Research Progress
To address these issues, a series of NDI derivatives modified with zwitterions were synthesized using the atmospheric pressure method by Gang He's group at Xi 'an Jiaotong University. The electrostatic repulsion between charges regulates the-stacking into a parallel-staggered pattern (with an angle of 42.8° between adjacent molecules and a stacking distance of 3.45 Å). Compared to (NPr)2NDI and dex-NDI, the introduction of carboxylate effectively reduces the positive charge concentration of N+. The synergistic effect between zwitterions yields multiple benefits: Firstly, the four charged centers significantly enhance the solubility of (CBu)2NDI (1.49 M). Second, the enhanced aromaticity of highly reducing species is beneficial to enhance the stability of (CBu)2NDI during electron transfer. More importantly, this effect effectively inhibits the SN2 irreversible decomposition reaction of the side chain and diimide rings caused by OH- attack, thereby further stabilizing the molecule. Additionally, single-point energy calculations confirmed the stabilizing effect of K+ on the (CBu)2NDI structure through electrostatic attraction. The total cost of the constructed battery electrolyte is as low as $6.18 Ah-1. At an electron concentration of 2 M, the (CBu)2NDI/K₄Fe(CN)6-based AORFBs shows no significant capacity decay over 220 cycles. This innovative design ensures stable capacity retention without degradation over numerous charge-discharge cycles, paving the way for high-performance and longspan neutral AORFBs.
Future Prospects
Moving forward, achieving long-term cycling stability at high concentrations remains pivotal for AORFB development. In recent years, significant breakthroughs have been made with the emergence of numerous high-performance electrolytes for neutral aqueous organic redox flow batteries, creating new opportunities for energy storage. Among these, modified naphthalene diimide (NDI) derivatives have demonstrated exceptional promise as potential anolyte candidates for AORFBs, owing to their remarkable aqueous solubility (>1.5 M) and stable two-electron transfer characteristics. These advancements lay a solid foundation for the commercialization of AORFB technology. As the low-carbon transition advances, AORFBs are poised to emerge as a critical complementary technology in large-scale energy storage, delivering sustainable solutions for the efficient utilization of renewable energy.