Optimization of Quantum Capacitance of Functionalized VS₂ Monolayer Electrodes to Shrink Hybrid Supercapacitors for On-Chip Energy Sources

Quantum capacitance ( C Q ) of the electrodes plays an important role in enhancing the performance of supercapacitors by directly affecting the overall capacitance. In this study, several approaches including doping, creating vacancy, and adsorption have been used to enhance the C Q of the vanadium disulfide (VS 2 ) electrode using density functional theory calculation. The undoped VS 2 monolayer shows a maximum C Q value of 20.19 μF/cm 2 . After creating V-vacancy (Vv) in the VS 2 monolayer lattice, the C Q value increased to 35.61 μF/cm 2, which is the highest among all doped and defective VS 2 lattices at room temperature. When we use VS 2 electrodes for supercapacitors, generally ion adsorption occurs at the electrode surface, showing the necessity to investigate the adsorption of alkali/alkaline atoms (Li, Na, K, and Mg) at the VS 2 surface to know the change in different properties of the electrode. It is found that generally C Q reduces due to the adsorption of alkali/alkaline atoms at the surface, but the K-adsorption at S-vacancy (Vs) VS 2 demonstrated the increment of C Q value from 21.75 to 35.32 μF/cm 2 at room temperature. Additionally, the variation of the adsorption distance of the K atom at the Vs-VS 2 surface revealed an optimum distance of value 3.5 Å, indicating that the K atom (radius = 2.43 Å) stabilizes just above the VS 2 surface. Moreover, augmentation in C Q was seen with a decrease in temperature and attained a value of 49.96 μF/cm 2 at 100 K. The calculated C Q and open-circuit voltage (OCV) duly confirmed that the K-adsorbed Vs-VS 2 is a potential candidate for the anode of hybrid supercapacitors as it has a maximum C Q value at the positive side of the electrochemical potential and an average OCV value of +0.615 V. This study reveals that the C Q of the VS 2 electrode can be increased to minimize the size of high-performance hybrid supercapacitors for its application as an on-chip energy source.