Surface functional groups of carbons and the effects of their chemical character, density and accessibility to ions on electrochemical performance

Wood origin activated carbon was oxidized and then treated with melamine and urea followed by carbonization at 950 °C in an inert atmosphere. The samples were characterized using elemental analysis, adsorption of nitrogen, Boehm titration, FTIR and XPS. Testing the carbons as the electrode materials in supercapacitors indicated that the electrochemical behavior of modified samples is governed mainly by the specific types of functional groups. Both surface chemistry and texture of carbons are affected by the nitrogen source and the type of oxygen functionalities preexisting on the surface. The modified carbons revealed significantly enhanced capacitances in 1 M H2SO4 reaching 300 F/g and the capacitance retention ratio is 86% at the current load of 1 A/g. Perfect correlations were found between the number of basic groups and the gravimetric capacitance and between the normalized capacitance in micropores and the distribution of quaternary and pyridinic-N-oxide nitrogen species on the surface of the micropores. The pseudocapacitance on N and O atoms is particularly dominant at higher current loads and the charge on quaternary nitrogen and pyridinic-N-oxide enhances the electron transport through the electrode improving the rate performance of treated samples. The micropores were found to be most effective in a double-layer formation.