Self-branched α-MnO2/δ-MnO2 heterojunction nanowires with enhanced pseudocapacitance

Abstract

For the first time two phase MnO ₂ nanostructures are constructed into core-branch arrays which deliver high pseudocapacitance.

Abstract

Despite the extensive research on MnO ₂ as a pseudocapacitor electrode material, there has been no report on heterostructures of multiple phase MnO ₂. Here we report the combination of two high-capacitance phases of MnO ₂, namely, α-MnO ₂ nanowires and δ-MnO ₂ ultrathin nanoflakes, to form a core-branch heterostructure nanoarray. This material and structure design not only increases the mass loading of active materials (from 1.86 to 3.37 mg cm ²), but also results in evident pseudocapacitance enhancement (from 28 F g ⁻¹ for pure nanowires to 178 F g ⁻¹ for heterostructures at 5 mV s ⁻¹). The areal capacitance is up to 783 mF cm ⁻² at 1 mV s ⁻¹. Upon 20 000 cycles, the heterostructure array electrode still delivers a reversible capacitance above 100 F g ⁻¹ at 4.5 A g ⁻¹. Kinetic analysis reveals that capacitances due to both capacitive and diffusion controlled processes have been enlarged for the self-branched heterostructure array. This work presents a new route to improve the electrochemical performance of MnO ₂ as a binder-free supercapacitor electrode.