Comparative study on nanostructured MnO2/carbon composites synthesized by spontaneous reduction for supercapacitor application

MnO2 has been deposited onto two types of carbon (C) substrates, including a non-porous multi-wall carbon nano-tube (CNT) and a porous carbon black (CB) powder, by a solution reduction process where MnO4− was reduced at 80 °C by the C substrate so as to give nano-crystalline MnO2 directly at the C surface. The nature of the C substrate has profound effects on polymorphicity, microstructure and electrochemical properties, in terms of supercapacitor application, of the resulting oxide. Deposition on CNT produces meso/macro-porous layer containing predominantly spinel MnO2 strongly bonded to the CNTs and having a larger surface area, while that on CB results in birnessite granules with a lower surface area. In addition to having a higher specific capacitance (309 F g−1), the MnO2/CNT electrode exhibits superior power performance (221 F g−1 at 500 mV s−1 or ca. 20 Wh kg−1at 88 kW kg−1) to MnO2/CB due to reduced electronic and ion-diffusion resistances. Furthermore, the MnO2/CNT electrode also exhibits slower self-discharging rate and greater cycling stability. The results indicate that the MnO2 spinel/CNT holds promise for supercapacitor applications.

► MnO2 is respectively deposited onto non-porous carbon nano-tube and porous carbon black. • MnO2 is synthesized by solution reduction of MnO4−. ► MnO2 on CNT has a higher specific capacitance (309 F g−1) than on CB (221 F g−1) at 500 mV s−1. ► High performance of MnO2 on CNT is due to reduced electronic and ion-diffusion resistances.