Supercapacitors in combination with renewable energy sources can provide environment friendly and sustainable energy devices. But due to their lower energy density, they are not able to replace batteries for all commercial applications. Their energy density can be increased by using high quantum capacitance electrodes. MXenes have potential to be used for such electrodes. In present work, density of states, band structure and quantum capacitance (QC) of bare and functionalized niobium carbide and titanium carbide MXenes are studied using DFT simulations. The calculated quantum capacitances of Nb2C and Ti2C show significant values of 324.1 and 246.2 µF/cm2 respectively. Nb2C shows higher value for QC in comparison to Ti2C for both positive as well as negative electrodes. These calculations are also performed for functionalized MXenes using =O, -F and –OH as functional group. Functionalization of MXenes with these groups reduces the QC of Nb2C and Ti2C MXenes at Fermi level.

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The density of states and quantum capacitance of pure and doped Nb2N and Nb4N3 single-layer and multi-layer bulk structures are investigated using density functional theory calculations. The calculated value of quantum capacitance is quite high for pristine Nb2N and decent for Nb4N3 structures. However for cobalt-doped unpolarized structures, significant increase in quantum capacitance at Fermi level is observed in the case of Nb4N3 as compared to minor increase in case of Nb2N. These results show that pristine and doped Nb2N and Nb4N3 can be preferred over graphene as the electrode material for supercapacitors. The spin and temperature dependences of quantum capacitance for these structures are also investigated.