Al₂O₃-assisted confinement synthesis of oxide/carbon hollow composite nanofibers and application in metal-ion capacitors

Abstract

Hollow micro‐/nanostructures are widely explored for energy applications due to their unique structural advantages. The synthesis of hollow structures generally involves a “top‐down” casting process based on hard or soft templates. Herein, a new and generic confinement strategy is developed to fabricate composite hollow fibers. A thin and homogeneous atomic‐layer‐deposition (ALD) Al2O3 layer is employed to confine the pyrolysis of precursor fibers, which transform into metal (or metal oxide)–carbon composite hollow fibers after removal of Al2O3. Because of the uniform coating by ALD, the resultant composite hollow fibers exhibit a hollow interior from heads to ends even if they are millimeter long. V, Fe, Co, and Ni‐based hollow nanofibers, demonstrating the versatility of this synthesis method, are successfully synthesized. Because of the carbon constituent, these composite fibers are particularly useful for energy applications. Herein, the as‐obtained hollow V2O3–C fiber membrane is employed as a freestanding and flexible electrode for lithium‐ion capacitor. The device shows an impressive energy density and a high power density.