Enhancing the voltage and discharge times of graphene supercapacitors depositing a CNT/V2O5 layer on their electrodes

Abstract

This work reports the electrochemical properties of graphene supercapacitors that employ flexible graphene electrodes (FGEs) coated by V2O5 (VOx) or carbon nanotubes (CNTs)/VOx layers. According to scanning electron microscopy images, the morphology of VOx consists in micro-ribbons with lengths of 20–45 μm and widths from 5 to 12 μm. The CNTs form interconnected long fibers, which were functionalized with carboxylic groups for producing defects. We demonstrated that the gel electrolyte used for the supercapacitors produces V4+-oxygen vacancies (VO) defects. Both, the CNTs and V4+-VO defects act as redox centers, which delay the current discharge in the supercapacitors. The presence of the carboxylic groups, defects in CNTs, defects in VOx, and oxidation states (V4+ and V5+), were confirmed by the UV–Vis, XPS, and FTIR techniques. The electrochemical characterization of the supercapacitors indicated that the devices made with FGEs coated by VOx or CNTs/VOx layers produced constant voltages of 0.14 V and 0.38 V during 535 and 593 min, respectively. The increase of voltage is explained by the increase of surface area and by the extra redox centers (defects) introduced by the CNTs. The results indicated that the CNTs/VOx layer increases the voltage and discharge times of the graphene supercapacitors, which can be of interest for the development of graphene systems with high charge storage capabilities.