Activated carbons modified by nitric acid treatment and their application in supercapacitor

A series of Norit activated carbons were modified by nitric acid treatment and used in electrodes of electrochemical supercapacitors. The effect of the oxidizing treatment on carbon materials was analyzed by means of nitrogen adsorption, FTIR, TPD. The electrochemical behaviour of supercapacitors using electrodes prepared with treated and untreated carbons were investigated through cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS). The supercapacitors were realised by assembling face-to-face carbon/Nafion composite electrodes and Nafion electrolyte membrane. Fourier transform infrared (FTIR) spectra evidence chemical changes of surface of carbons due to the presence of not well identified oxygenated species. Temperature programmed desorption (TPD) analysis shows very large evolution of CO2 and CO gases with increasing temperature for the modified carbons confirming the presence of functional groups on carbon surface. The strong changes of physico-chemical and electrochemical properties of carbon are also evidenced by electrochemical measurements. In fact, the voltammogramms of the supercapacitors with the modified carbons show pseudocapacitive behaviour whereas those with the untreated carbon exhibit almost ideal shape of pure capacitive behaviour. Impedance analysis evidences that the supercapacitors with the chemical treated carbons are always relatively more resistive for the presence of functional groups, which likely produce a decrease of electron conductivity in the bulk of carbon material. On the other hand, the supercapacitors with these carbons show generally higher capacitance performance, and this is found to be independent from carbon surface area. In conclusion, a higher carbon surface utilisation is caused by functional groups introduced on the carbons by the chemical treatment. This favours a higher specific double layer capacitance for larger amount of ion species distributed at the interface of the porous carbon with the electrolyte. Nevertheless, the introduced functional groups produces a lowering of electrochemical stability. In fact, a gradual loss of capacitance is recorded during endurance cyclic voltammetry tests, carried out for 20000 cycles, likely due to electrochemically unstable oxygenated groups.