Hierarchical nanocarbon-MnO2 electrodes for enhanced electrochemical capacitor performance

High-capacity energy storage in electrochemical capacitors may benefit from the combination of electric double-layer capacitance (EDLC) and pseudocapacitance to lead to high specific energy and power beyond the current capacity of rechargeable batteries. However, commonly pursued combinations of non-conductive pseudocapacitive and conductive EDLC materials rarely achieve synergistic effects. This work addresses the issue by demonstrating unique hierarchical microstructured electrodes comprising uniformly dispersed MnO2 nanoparticles on intentionally converted “pseudocapacitive” edges of plasma-grown Vertically Oriented Graphenes (VGs), with side-walls fully open to EDLC effects, and bonded at the base to the supporting highly conducting carbon nanofibers (CNFs), without any binder. The hierarchical structure combines the benefits of good conductivity of VGs and CNFs, the unique edge nucleation behavior and small size of MnO2 nanoparticles, and the large surface areas of the exposed graphene walls. Moderate oxidation of VGs helps refine MnO2 nanostructures and improve the cycle stability. The hybrid electrode delivers a specific capacitance of 612 F g-1 (32.7 F cm-3) at scan rate of 2 mV s-1 and exhibits good stability 109% after 5000 CV cycles at the scan rate of 100 mV s-1 in three-electrode system. The asymmetric electrode configuration based on it reveals a specific energy of 30.4 Wh kg-1 (0.90 mWh cm-3) and a specific power of 27.8 kW kg-1 (824 mW cm-3) at 15 A g-1. This work suggests new ways to produce hybrid MnO2-carbon hierarchical composite materials for the improved electrochemical capacitor performance