In-situ nitrogen doping of vertical graphene via microwave plasma CVD on Cu foils for advanced electrochemical electrodes

Abstract

Three-dimensional vertical graphene (VG) possesses a high surface area and an edge-oriented architecture, making it a promising electrode material for electrochemical energy storage applications. In this study, nitrogen-doped VG (N-VG) films were synthesized via an in-situ microwave plasma process, and the effects of nitrogen doping and Cu substrate crystallinity on the structural, chemical, and electrochemical properties of VG were systematically investigated. The growth conditions for pristine VG in a CH₄–H₂ plasma were first optimized and used as a baseline for the nitrogen incorporation. The introduction of N₂ during growth preserved the vertically oriented graphene morphology while inducing distinct changes in defect characteristics and bonding configurations. Raman spectroscopy revealed progressive increases in the I_D/I_G and I_D′/I_G ratios with increasing nitrogen incorporation, indicating enhanced defect density and localized bonding perturbations within the sp² carbon lattice. X-ray photoelectron spectroscopy confirmed successful nitrogen incorporation, with graphitic (quaternary) N identified as the dominant bonding configuration. Electrochemical analyses using a PVA–H₃PO₄ gel electrolyte demonstrated that nitrogen doping, particularly when combined with a single-crystalline Cu substrate, reduced charge-transfer resistance and enhanced the charge accumulation behavior, leading to improved electric double-layer capacitor performance with excellent cycling stability. These results elucidate the role of in-situ nitrogen doping in modulating the defect characteristics and electronic structure of VG and provide insights into the structure–electrochemical property relationships of N-VG for use as an electrode material for solid-state electric double-layer capacitors (EDLCs).