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Oxygen vacancy-engineered Fe2O3 nanoarrays as free-standing electrodes for flexible asymmetric supercapacitors.

The charge storage performance of Fe2O3 nanoarrays (NAs) as negative electrodes are limited by their poor conductivity and rate capability. Herein, we have reported the delicate interfacial engineering on carbon cloth (CC) fibers and oxygen vacancy (VO) generation on Fe2O3 nanorod arrays to boost the capacitive performance. Polydopamine-derived nitrogen-doped carbon layers were fabricated on CC fibers to govern the growth of FeOOH NAs. Rich VOs were generated in Fe2O3 NAs to construct a unique heterostructure with a crystalline core and amorphous shell via successive N2 thermal treatment and chemical reduction. Optimized by 2 h chemical reduction, the VO-rich Fe2O3 NA electrode, featuring a charged voltage of -1.10 V, exhibited a high areal specific capacitance of 2.63 F cm-2 at 0.5 mA cm-2 and 0.12 F cm-2 even at 60 mA cm-2. Impressively, 86.7% specific capacitance was retained after 10 000 cycles at 10 mA cm-2. The flexible asymmetric supercapacitor by assembling free-standing CN-Fe2O3-2 h (negative electrode) and MnO2 (positive electrode) showed an energy density of 1.33 mW h cm-3 at 15.4 mW cm-3. To the best of our knowledge, these results are the record performance for Fe2O3-based electrodes. The two-step interfacial engineering reported in this study may open a new door in the design of high energy-density electrodes for advanced energy storage.