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.