Density effects of vertical graphene nanowalls on supercapacitor performance

Abstract

Vertical graphene (VG) nanowalls formed with controllable densities by adjusting CH4 : H2 flow ratios are achieved on silicon wafer substrates via plasma-enhanced chemical vapor deposition. The pseudocapacitive materials of MnO2 enhancing the energy storage capability are electrodeposited on VG nanowalls. The VG densities of 0.83 mg cm−2, 2.35 mg cm−2, and 3.63 mg cm−2 fabricated with the CH4 : H2 flow ratios of 1 : 5, 1 : 2.5, and 1 : 1 are carefully controlled. The supercapacitor electrode formed with a flow ratio of 1 : 2.5 exhibits the highest specific capacitance of 166 mF cm−2 at a current density of 0.5 mA cm−2 among the three electrodes. Furthermore, an asymmetric supercapacitor device with MnO2/VG/Si as the positive electrode and carbon black as the negative electrode is assembled. The supercapacitor device exhibits excellent electrochemical performance with a specific capacitance of 230.9 mF cm−2, a maximum energy density of 103.9 μW h cm−2, and the largest power density of 4.5 mW cm−2. This study presents the essential insights into the density effects on pseudocapacitive supercapacitor electrodes and the promising methods to prepare energy-storage devices with high electrochemical performance.