Mechanistic insights into the supercapacitive behaviour of sludge-derived activated carbon in H₂SO₄ with K₃Fe(CN)₆ redox additive

Both electrode materials and electrolyte composition strongly influence supercapacitor performance. Accordingly, the effect of incorporating a redox-active electrolyte into the pristine electrolyte on the specific capacitance of the sludge-based activated carbon (SBACK) electrode was investigated. Structural and morphological properties of SBACK were characterised by XRD, Raman, TEM, SEM-EDX, BET and FTIR. The electrochemical properties of the fabricated electrode using SBACK were investigated in a three-electrode setup with 1 M H 2 SO 4 as the pristine electrolyte and with mixtures of 1 M H 2 SO 4 and different concentrations (0.01 M, 0.03 M, and 0.05 M) of potassium ferricyanide, K 3 Fe(CN) 6 , as the redox-additive electrolyte. The results demonstrated that incorporating the redox additive significantly enhanced capacitive performance by enabling additional faradaic charge-storage reactions. The SBACK electrode exhibited a specific capacitance of 40 F g −1 in the pristine electrolyte, which increased to 679.6 F g −1 in the electrolyte containing 0.05 M redox additive at 5 A g −1 . Furthermore, the symmetric device in 1 M H 2 SO 4 + 0.01 M K 3 Fe(CN) 6 demonstrated an energy density of 91 Wh Kg −1 and a power density of 2400 W Kg −1 at 1 A g −1 , while maintaining a cycling stability of 86.4% at 3 A g −1 . The study highlights the synergistic effect between the porous activated carbon structure and the redox-active electrolyte. It provides insight into the possible reactions among the pristine electrolyte, the redox additive, and the current collector.