Influence of surface modification with nitric acid on electrochemical performance of agroindustrial waste-based activated carbon

Two activated carbons (ACs) were superficially modified with nitric acid (HNO₃). The ACs were obtained by chemical activation of palm kernel shell, the first one with KOH at a KOH/AC weight ratio of 2/1 and 700 °C (AC–KOH), and the second one with ZnCl₂ at a weight ratio of 1/1 and 735 °C (AC–ZnCl₂), both at 5 °C min⁻¹. We modified both, the AC–KOH, and the AC–ZnCl₂, taking into account the influence of temperature, impregnation time, chemical/precursor weight ratio on the morphology, textural properties, and electrochemical performance. The morphology was characterized by scanning electron microscopy, and the textural properties by N₂ adsorption/desorption at 77 K. The superficial chemistry of the samples was characterized using X-ray photoelectron spectroscopy, and the electrochemical performance by electrochemical impedance spectroscopy in 1 M H₂SO₄. The best electrochemical behavior of approximately 15 μF cm^{− 2} was obtained with an impedance of 2.9 Ω with HNO₃/AC–KOH weight ratio of 1/1 at 25 °C for 4 h. The increase in the interfacial capacitance is attributed to the high superficial nitrogen content (N_XPS: 1.8%) that involved faradic processes, as well as to an enhanced hydrophilic character (wettability) and an improved electron transfer caused by the positive charge of nitrogen species in acid electrolytes.