Quantum capacitance of transition metal and nitrogen co-doped graphenes as supercapacitors electrodes: A DFT study

Quantum capacitance is one of the key parameters determining the energy density of graphene-based super-capacitors. The present article explored the effects of theco-doping of transition metal (Mn, Fe, Co, Ni) and N atoms (TMNx, x = 1-4) on the structural stability, quantum capacitance and surface storage charge of graphene using density functional theory calculations. We find that the formation energies and magnetic moments can be regulated by changing the type of dopant transition metal and the number of dopants N atoms. The quantum capacitance and surface storage charge of graphene are enhanced significantly with the co-doping of transition metal and N atoms. By checking the density of states, it can be found that the quantum capacitance is directly related to the density of state around the Fermi level. The enhancement of quantum capacitance is caused by the introduced localized states around the Fermi level contributed mainly from transition metal. In addition, the quantum capacitance and surface storage charge are found to increase monotonically with the increase of co-doping concentrations of transition metal and N atoms. The results can provide an effective and simple new idea for the design of graphene-based supercapacitors with high energy density.