Effect of nitrogen and transition-metal co-doping on quantum capacitance enhancement of graphene as supercapacitor electrodes: A density functional theory study

Quantum capacitance plays a crucial role in determining the energy density of graphene-based supercapacitors. In this study, density functional theory calculations were conducted to investigate the enhancement of quantum capacitance and surface storage charge density through the co-doping of transition metals (TM) such as Mn, Fe, Co, Ni, Cu, Zn, Cr, V, Ti, and N atoms in graphene (TMN3-G). At a doping concentration of 2 at% (atomic percentage), all types of metal doped graphene exhibited an increase in quantum capacitance compared to pristine graphene. The highest increase was observed in CrN3-G, where the maximum quantum capacitance increased from 19.2 μF/cm2 for pristine graphene to 122.8 μF/cm2, due to the increased density of states around the Fermi level. The maximum surface storage charge density also increased from 6.4 μC/cm2 for pristine graphene to 44.4 μC/cm2 for CrN3-G. Moreover, the study investigated the effect of different doping concentrations on quantum capacitance performance by changing the doping concentration for VN3-G. It was found that the quantum capacitance for VN3-G increased with higher doping concentration, reaching up to 282 μF/cm2 when the doping concentration was increased to 12.5 at% from 2 at%. Based on our calculation results, ZnN3-G, NiN3-G, CrN3-G, and TiN3-G are suitable for the anode of supercapacitors, while CuN3-G and CoN3-G are suitable for the cathode. FeN3-G, MnN3-G, and VN3-G are suitable for both electrodes. These findings provide valuable insights for designing high-capacity graphene-based supercapacitors.