A sequential process to synthesize Fe3O4@MnO2 hollow nanospheres for high performance supercapacitors

Designing efficient, durable, and affordable electrodes for supercapacitors is indispensable for utilizing clean and renewable energy resources. Herein, a three-step sequential process, including two hydrothermal procedures followed by an etching treatment, was developed to synthesize Fe3O4@MnO2 hollow nanospheres (Fe3O4@MnO2-HNS) using solid silica as a hard template. Fe3O4@MnO2-HNS has the largest specific surface area (121.99 m(2) g(-1)) due to a double hollow structure compared to other samples. The as-prepared Fe3O4@MnO2-HNS exhibited superior electrochemical performance as compared to pristine hollow nanospheres of either Fe3O4 or MnO2. When applied to practical asymmetric supercapacitor devices (Fe3O4@MnO2-HNS as a positive electrode and activated carbon (AC) as a negative electrode), Fe3O4@MnO2-HNS//AC exhibited prominent performance, such as a high capacity of 168.81 C g(-1) (375.14 F g(-1)) at 0.5 A g(-1), a large energy density of 15.84 W h kg(-1) at a power density of 803 W kg(-1), and an excellent stable charge-discharge durability of 70.6% over 5000 cycles at 2 A g(-1). We envision that Fe3O4@MnO2-HNS may be a promising alternative for applications in energy storage and electrocatalysis where discrete electrochemical performances are desired.