Facile fabrication of flower-like γ-Fe₂O₃ @PPy from iron rust for high-performing asymmetric supercapacitors

Flower-like rust with porous structure can be obtained on the Fe plate by accelerated corrosion in the marine atmosphere. A core-shell structure of γ-Fe₂O₃ @PPy was developed through annealing and in situ chemical polymerization of pyrrole. The dehydroxylation after annealing enriches the inherent pore structure of rust. The conductive PPy layer with the capacitive properties were found to improve the stability of iron-based anodes while enhancing the charge storage performance. The rust-based composite exhibited superior capacitance performance over the original rust. By matching with a cathode from nickel-cobalt layered double hydroxide (NiCoLDH) based porous Ni network, the asymmetric device delivered a high volumetric energy density of 7.9 mWh cm^–3 at a power density of 14.99 mW cm^–3, and favorable cycle stability (capacitance retention of 90.1% after 8000 cycles). Most importantly, the energy storage mechanism of rust-based electrodes with flower-like structures was discussed in detail. The recycling and reuse of rust resources were successfully realized by the direct utilization of rusted steel as anode materials for supercapacitor applications.