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-CFOV 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-CFOV 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-CFOV 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.