Directional regulation on single-molecule redox-targeting reaction in neutral zinc-iron flow batteries

Abstract

Aqueous redox flow batteries (ARFBs) are promising long-duration energy storage systems but struggle with low-energy density due to the inherent properties of liquid electrolytes. Herein, we report a [Fe(CN)₆]^3−/4−-LiMn_xFe_{1 − x}PO₄/Zn flow battery utilizing redox-targeting (RT) electrochemical-chemical loop, exhibiting an outstanding energy density of 118.3 Wh L⁻¹, surpassing blank RFB by 5.6 times. Remarkably, the RT reaction between redox mediator [Fe(CN)₆]^3−/4− and solid energy booster LiMn_xFe_{1 − x}PO₄ is directionally regulated, clearly revealing the quantitative relation between capacity enhancement and potential difference. Moreover, unprecedented Coulombic efficiency (99.9%), solid booster utilization (78.4%), and capacity retention (99.8% per cycle) are achieved at 10 mA cm⁻². Intriguingly, operando synchrotron X-ray absorption spectroscopy unveils the reversible changes of the Fe–O and Fe–Fe bonds in the [Fe(CN)₆]^3−/4−-LiMn_xFe_{1 − x}PO₄ RT system during real-time monitoring. This work suggests an appealing way for capacity enhancement in ARFBs and provides profound insight into the fundamental chemistry of the RT reaction in safe, energy-dense batteries.