In this article, the effect of copper (Cu) as a redox additive and dopant on the performance of a polyaniline (PANI) based supercapacitor was thoroughly investigated. The electrochemical properties of PANI in H₂SO₄ and in H₂SO₄ + CuSO₄ and Cu doped PANI in H₂SO₄ were studied using cyclic voltammetry (CV) and impedance spectroscopy (IS). The CV result indicates that the capacity of PANI in H₂SO₄ was significantly improved with the introduction of Cu²⁺ ions into the electrolyte, but it appeared unstable because of underpotential deposition of copper over the PANI surface and the relatively irreversible nature of the redox reaction. However, a stable and improved performance was obtained for Cu doped PANI due to the combined effect of an increase in conductivity and the surface modification of the PANI film. For Cu doped PANI, nearly ∼2.4 and ∼1.5 fold improved interfacial capacitance was achieved compared to that of PANI (H₂SO₄) and PANI (H₂SO₄ + CuSO₄) respectively. The obtained Nyquist spectra for all the configurations were analysed using an equivalent circuit to understand the fundamentals of capacitive and resistive response of the supercapacitor. The IS measurements lead to direct determination of parameters like series resistance, rate capability of electrodes, ion diffusion phenomena and interfacial capacitance. The experimental results and their analysis will have significant impact on understanding the effect of dopants and redox additives on the performance of PANI based supercapacitors and also lay the basis for designing a supercapacitor with an appropriate electrode and electrolyte material for numerous industrial and consumer applications.