Novel synthesis of N-doped graphene quantum dot as conductive agent for carbon based supercapacitors

Conductive agent can increase conductivity and construct pore structure in supercapacitor (SC) electrode as clean renewable energy. Carbon black (CB) is common conductive agent but its zero dimension hinders efficient charge-transfer conductive bridge. Combining CB with other carbon materials can possibly solve this issue. Graphene quantum dot (GQD) with high hydrophilicity and stable doping features is easier and cheaper to develop. Previous synthesis of GQD requires complex process and lacks large-scale production. In this study, novel one-step thermal treatment was provided to synthesize N-doped GQD. Citric acid and urea ratios were varied to synthesize N-doped GQD with different N-doping extents and electrical conductivities. CB and GQD mixtures, GQD and CB are used as conductive agent for fabricating SC electrodes. Activated carbon electrode with CB and GQD mixture shows the highest specific capacitance (C F ) of 241.4 F/g at 20 mV/s, while electrodes without conductive agent and with CB conductive agent respectively shows C F of 41.5 and 108.6 F/g, due to more functional groups, defects, hydrophilic sites and electrochemical surface area of the former case. Symmetric SC with CB and GQD shows a maximum energy density of 10.54 Wh/kg at 550 W/kg, and C F retention of 85% and Coulombic efficiency of 80% after 5000 charging/discharging cycles. • One-step thermal treatment was used to synthesize N-doped graphene quantum dot (GQD). • Citric acid and urea ratio is varied to make N-doped GQD with varied conductivities. • Carbon black and GQD mixtures are the conductive agent of activated carbon electrode. • The highest specific capacitance (C F ) of 241.4 F/g at 20 mV/s was obtained. • Symmetric supercapacitor shows maximum energy density of 10.54 Wh/kg at 550 W/kg.