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.