High Energy Density Anthraquinone-Based Redox Flow Batteries: From Molecular Engineering to Advanced Redox-Targeting Reaction

Anthraquinone-based redox flow batteries (AQ-RFBs) provide a tunable platform for molecularly engineered long-duration energy storage with high cell voltage and fast redox kinetics. However, high energy density and long-term stability of AQ-RFBs remain challenging. This review presents a systematic path from molecular engineering to redox-targeting flow batteries (RTFBs) for high-energy density anthraquinone-based redox flow batteries. Functional groups that regulate frontier molecular orbitals, redox potentials, and reaction kinetics are discussed for enhancing aqueous solubility and stability. Degradation pathways of anthraquinones are evaluated with spectroscopic, electrochemical, and computational methods in this work. Beyond molecular engineering strategies for anthraquinone-based redox flow batteries, novel redox-targeting (RT) flow battery systems are comprehensively reviewed. Anthraquinone-based RT flow batteries break the boundary between liquid and solid energy storage modes, greatly enhancing the energy density of AQ-based flow batteries and reducing AQ degradation at low concentrations. Redox-targeting concepts, AQ-based RTFB configurations, and advanced characterizations are well discussed. Finally, the challenges and outlooks of AQ-based flow batteries are stated for future research. In summary, this review highlights the development of anthraquinone-based flow batteries in terms of molecular engineering and redox-targeting design, providing valuable insights into high-energy-density aqueous organic redox flow batteries for wide applications.