Influence of wet oxidation of herringbone carbon nanofibers on the pseudocapacitance effect

Herringbone carbon nanofibers (CNFs) were treated with concentrated HNO3 and a mixture of diluted HNO3/H2SO4 to obtain a series of oxygen enriched CNF with different oxygen group distribution, but with a similar porous texture. Oxygen functional groups were determined by X-ray photoelectron spectroscopy. CNFs with a very high relative concentration of carbonyl and/or quinone groups and hydroxyl groups were obtained by adjusting the suitable temperature and time of oxidation with HNO3 and HNO3/H2SO4, respectively. The electrochemical behavior of the samples was studied in three- and two-electrode cells. The performance of oxidized CNFs-based supercapacitors working in 1 mol L−1 H2SO4 and 6 mol L−1 KOH was analyzed using cyclic voltammetry and galvanostatic charging/discharging. The specific capacitance of the oxidized CNFs was more than twice enhanced in acidic and alkaline media compared to the pristine CNFs due to the pseudocapacitance effect. It was revealed that not only quinone groups but also hydroxyl groups contribute into the overall capacitance through the pseudocapacitance effect. With increasing surface concentration of the CO and C-OH groups, the capacitance values increase for the capacitors operating in both media.