Supercapacitors due to high power density, enabling fast charging and discharging, are used to store electrical energy. However, their lower energy density in comparison with conventionalbatteries, limits the use of this energy storage device. In this regard, extensive researches in thefield of supercapacitors are performed to increase the capacity of supercapacitors [1]. Carbonnanotube (CNTs)-based material are promising materials for use in supercapacitor electrodes [2]. Despite of advantage of CNTs-based electrode, major limitation of low quantity of specificcapacity due to their low quantum capacitance remain [3]. All density functional theory (DFT) calculations were completed using QUANTUM ESPRESSO code and predew-Wang91 (GGAPW91)exchange-correlation functional. Our results indicate that quantum capacitance significantly is improved by changing in the electronic structure through dopants and defects. The obtained results from density of state (DOS) of P-doped (6, 6) CNTs indicates that impurity statesare created around the Fermi level (see figure 1a). Quantum capacitance of P-doped (6, 6) CNTs in the water stability range (for V= -0.4-0.83) are increased compared with quantum capacitance of pure (6, 6) CNTs. In these cases the quantum capacitance is almost symmetric around zero voltage and increased both the negative and the positive bias (see figure 1b). In another part of this paper, the effect of structural defects in P-doped (6, 6) CNTs on the quantum capacitance were studied. Results show that DOS of this structure is increasedcompared to DOS of pure CNTs at negative bias and Fermi Level (see figure 1a). Their calculated quantum capacitance indicate that significantly enhancement has been appeared as compared to their pure form especially at positive bias. It could be seen that the quantum capacitance is asymmetric around zero voltage (see figure 2b). As a result, these materials are proposed to operation as a positive electrode In short, in this paper, Using
DFT calculation, we investigated the effects of dopants and structural defects in carbon nanotubes electrode on the quantum capacitance. We observed that quantum capacitance has increased due to dopant and structural defects