Direct Interfacial Growth of MnO₂ Nanostructure on Hierarchically Porous Carbon for High-Performance Asymmetric Supercapacitors

Developing a composite electrode composed of a carbon-based material and a transition metal oxide is an effective way to address the problems such as poor conductivity and low porosity created by transitional metal oxide electrodes for supercapacitors. In this work, the activated carbon (AC) prepared from enteromorpha prolifera (ACEP) with typical hierarchically porous structure, was used as a substrate to grow MnO₂ nanostructures via wet chemical reaction process. The morphology and crystalline phase of the MnO₂ could be controlled by facilely adjusting reaction time. For instance, δ-MnO₂ nanosheets were anchored on the ACEP in the initial stage (1-6 h), but α-MnO₂ nanowires were obtained with the extension of reaction time (7-12 h). The electrode prepared from ACEP@δ-MnO₂ nanosheets displays high specific capacitance (345.1 F g^-1 at 0.5 A g^-1) and excellent cycle stability (i.e., a capacitance retention of 92.8% after 5000 cycles). Moreover, an asymmetric supercapacitor was assembled by using as-prepared ACEP@δ-MnO₂ composite as positive electrode and AC as negative electrode. The assembled AC//δ-ACEP@MnO₂ supercapacitor is shown to work in a wide voltage range of 0-2.0 V and delivered a high energy density of 31.0 Wh kg^-1 at a power density of 500.0 W kg^-1.