Aqueous organic flow batteries (AOFBs) hold significant promise for renewable energy integration and electricity grid storage due to their inherent safety, as well as the availability of naturally abundant and synthetically tunable organic redox-active molecules (ORAMs).
Despite their potential, challenges such as low energy density, poor stability at high concentrations, and high synthesis costs of ORAMs hinder their commercial viability. Among these, developing ORAMs that offer both high energy density and ultra-stable cycling performance is essential for advancing stationary energy storage solutions. Increasing the number of electron transfers in ORAMs can boost energy density and reduce electrolyte cost at the same concentration. However, multi-electron transfer ORAMs often face a "Trade-off" between stability and solubility.
In a study published in the Journal of the American Chemical Society, a research team led by Prof. LI Xianfeng and Prof. ZHANG Changkun from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) developed a high-water-soluble pyrene tetraone derivative that highly enhances the energy density of AOFBs while maintaining high-temperature stability.
The team designed an asymmetrical pyrene-4,5,9,10-tetraone-1-sulfonate (PTO-PTS) monomer via a coupling oxidation-sulfonation reaction. This innovative monomer could reversibly store four electrons, offering a high theoretical electron concentration of 4.0 M, as well as an ultra-stable intermediate semiquinone free radical. When applied to AOFBs, this monomer achieved an ultra-high volumetric capacity of approximately 90 Ah/L. Moreover, the AOFBs maintained nearly 100% capacity retention after 5,200 cycles in the air, demonstrating great potential for large-scale energy storage.
The researchers also found that the extended conjugated structure of the pyrene tetraone cores facilitates reversible four-electron transfer through enolization tautomerism. Introducing a single sulfonic acid group into the core decreased the molecular planarity, enhancing the regional charge density and hydrogen bonding with water molecules, thereby improving solubility in aqueous electrolytes.
Furthermore, the monomer stabilized the intermediate semiquinone free radical through effective delocalization of the conjugated structure and ordered π-π stacking during the redox process, contributing to excellent stability in air and high temperatures.
In this study, AOFBs incorporating the pyrene tetraone derivative achieved an energy density of 60 Wh/L. Both symmetric and full cells exhibited no obvious capacity decay after thousands of cycles at 60 °C, indicating good cycling stability (about 1500 hours) and promising performance over a broad temperature range (10 to 60 °C).