P-GHCS-800 showed the higher specific capacitance of 321 F g -1 at 0.2 A g -1 than that of P-GHCS-750 (223 F g -1 ), P-GHCS-850 (165 F g -1 ), and HCS-800 (157 F g -1 ). P-GHCS-800 electrode retained a capacitance of about 97.1% after 2000 consecutive cycles. The Nyquist plots of P-GHCS-750, P-GHCS-800, and P-GHCS-850 displayed the Rs values of 0.53, 0.51, and 0.57 Ω, respectively, and confirmed the lowest Rs value of P-GHCS-800. Further, a symmetric two-electrode supercapacitor device was constructed using P-GHCS-800 electrode in 1 M Na2SO4 aqueous neutral electrolyte. The potential window of the symmetric cell was extended upto 2.0 V, indicating the improved oxidation stability of P-GHCS electrode by blocking the active oxidation sites of carbon materials with P-containing functional groups.