Fig. 4. (a) Cyclic voltammogram (CV) curves for Nb₂CT_x and Ni–Nb₂CT_x in 1 M PVA–H₂SO₄ at a scan rate of 5 mV s⁻¹ (b) CV curves explicitly for Ni–Nb₂CT_x in at different scan rates (c) specific capacitance vs. scan rate for Ni–Nb₂CT_x (d) capacitance retention vs. cycle number for Ni–Nb₂CT_x.