Effect of Transition Metal and Nitrogen Co-Doping on Quantum Capacitance of Silicene-Based Electrode Materials

Exploring 2D electrode materials with high quantum capacitance (C_Q) is particularly important to improve the energy density of electrical double-layer capacitors. Generally, the structure and composition of materials determine their capacitance characteristics. In this paper, the effects of co-doping of N and transition metal (TM = Sc–Zn) atoms on the structure, stability, electronic, and capacitive properties of silicene were studied by first-principles calculation. Our results show that the co-doped TMN_x–Si systems, especially TMN₃–Si, are more stable than the silicene system doped with N or TM atoms. TMN_x–Si systems have more advantages than single-doped silicene and co-doped graphene in improving C_Q and surface charge density (Q). Among all TMN_x–Si systems studied, ScN₂–Si has the best C_Q and Q performance, with maximum values 224.88 μF/cm² and 74.41 μC/cm², respectively. Furthermore, it is observed that the C_Q and Q values of ScN₂–Si increase monotonically with the increase of doping concentration, but the bias position corresponding to the maximum C_Q does not change and remains at −0.6 V, which is obviously better than the co-doped graphene system. In the studied systems, except Sc and Ti, the C_Q and Q values of TMN₃–Si are obviously higher than those of TMN₂–Si and TMN₁–Si.