Capacitance behaviour of brown coal based active carbon modified through chemical reaction with urea

The capacitive performance of activated carbons with different contents of nitrogen obtained from brown coal has been investigated. Nitrogen enrichments have been made by exposing the samples to urea without overpressure in oxidizing conditions. The effect of different temperatures of carbonization (500 and 700 °C) and the influence of the sequence of nitrogenation and activation processes have been investigated. Electrochemical study has been performed on microporous carbonaceous materials with high surface areas (BET) ranging from 2209 to 3268 m²/g, and chemical structures with different content of nitrogen (0.2-5.6 wt.%) and oxygen heteroatoms (4.5-11.1 wt.%) and different type of species formed.The presence of nitrogen heteroatoms in carbonaceous materials considerably increases their capacitance, particularly if they work as the negative electrode in alkaline capacitor. Capacitance of the same materials used as a positive electrode is characteristically reduced. Such specific influence of nitrogen has been observed even at their low content implied by the chemical composition of natural brown coal giving capacitance of negative and positive electrode of 341 and 264 F/g, respectively. The similar effect of nitrogen in the acidic medium has been much lower; capacitance of the material used for both electrodes has been of about 300 F/g. Carbonaceous materials containing nitrogen reveal different behaviour, mainly in respect of charge exchange dynamics, if they are used as capacitor electrodes of the opposite polarity.