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.