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 A degrees C (AC-KOH), and the second one with ZnCl2 at a weight ratio of 1/1 and 735 A degrees C (AC-ZnCl2), both at 5 A degrees 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 N-2 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 mu F cm(- 2) was obtained with an impedance of 2.9 a"broken vertical bar with HNO3/AC-KOH weight ratio of 1/1 at 25 A degrees 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.