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

Despite the extensive research on MnO₂ as a pseudocapacitor electrode material, there has been no report on heterostructures of multiple phase MnO₂. Here we report the combination of two high-capacitance phases of MnO₂, namely, α-MnO₂ nanowires and δ-MnO₂ 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 cm²), 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 MnO₂ as a binder-free supercapacitor electrode.