Study of pristine and functionalized VC and MoC MXenes as novel electrode material for supercapacitors.

The quantum capacitance (QC) of pristine and functionalized molybdenum carbide and vanadium carbide MXenes is determined using density functional theory to investigate their suitability as supercapacitor electrodes. The calculations are performed using Synopsys ATK package with PBE functional under generalized gradient approximation keeping the energy cut off at 540 eV and k-point sampling at 12 × 12 × 1. The calculated QC at Fermi level in case of VC and MoC exhibit extremely high values of 3465.51 and 3243.99 μF/cm respectively. The values of QC remain high in both positive and negative bias region around Fermi level. The impact of functionalization is studied for functional groups OH, OCH, F and O. For each functional group, two layers are added (one below and one above) to the pristine structure. Band structures of functionalized MXenes show that the metallic character of the MXenes is retained, except in the case of oxygen functionalized MoC, which turns out to be a semiconductor with band gap of 0.051 eV. Although some decrease in density of state (DOS) and QC is observed in functionalized MXenes with all functional groups. However, the values are still good enough for supercapacitor electrode applications. The effect of number of layers of pristine MXene is also studied up to three layers and an increase in QC is observed with increase in number of layers.