MIL‐53(Fe)‐Derived Oxygen Vacancy‐Regulated Iron Oxides Cathode Enabling Long‐Cycling Flexible Iron‐Ion Hybrid Supercapacitors

ABSTRACT The practical application of iron‐ion hybrid capacitors is severely hindered by inadequate energy density and insufficient cycling robustness. Herein, we present a self‐supporting electrode comprising iron‐based oxide/carbon composites, fabricated via in situ pyrolysis of MIL‐53(Fe) precursors directly on carbon cloth. A subsequent controlled oxidation treatment enables precise modulation of oxygen vacancy concentration, yielding the optimally engineered M‐CFO V composites. Notably, the binder‐free, self‐supporting configuration offers compelling advantages over traditional slurry‐coated electrodes, including accelerated electron transport, improved solid–liquid interfacial compatibility, and robust mechanical integrity, which collectively ensure outstanding capacity retention over prolonged cycling. Simultaneously, the MOF‐derived hierarchical porous structure promotes rapid electrolyte infiltration, shortens ion diffusion pathways, and exposes a large fraction of electrochemically accessible sites, thereby broadening the operational versatility of the device. When evaluated in an iron‐ion supercapacitor incorporating a choline chloride–optimized electrolyte, the M‐CFO V electrode achieves a remarkable specific capacitance of 2073.9 mF cm+ at 1 mA cm‐2, along with an energy density of 48.8 mWh g‐1. More importantly, a flexible iron‐ion hybrid supercapacitor assembled with the M‐CFO V cathode delivers high capacitance (1325.8 mF cm −2 ), energy density (35.75 mAh g −1 ), and exceptional cycling stability (92.3% retention after 10 000 cycles), offering a new strategy for advanced energy storage.