Capacitance behaviour of the ammoxidised coal

Abstract

A possibility to obtain a new electrode material for supercapacitors by ammoxidation and activation of the hard carbon has been examined. Carbonization process of precursor material was conducted at temperature 700 °C after demineralization. Samples were activated with steam at the temperature 800 or 850 °C and controlled burn-off of ca. 50 wt%. Ammoxidation was carried out with ammonia and air mixture at 300 or 350 °C before carbonization as well as before or after activation. Active materials were characterized by porous structure, elemental and X-ray photoelectron spectroscopy analyses and electrochemical measurements in three-electrode Swagelok type capacitor. The capacitances ca. 175 F/g were obtained for activated carbon materials working as positive electrode in acidic condition (4 M H 2 SO 4 ) if ammoxidation had been made before or after carbonization. However, in alkaline condition (7 M KOH), the best performance ca. 145 F/g was demonstrated by carbon material ammoxidized after activation if used as the negative electrode. The influence of the stage when ammoxidation is applied and thermal conditions of carbon modification on the porous and chemical structure of the samples as well as the resulting outcome in capacitance characteristics are discussed. Keywords D. Electrochemical properties 1 Introduction High power supercapacitor requires the use of electrode material with strictly defined electrochemical properties, especially with excellent polarizability and charge propagation ability as well as high specific capacity [1] . Since hard coals, particularly young coals that are commonly used for the production of highly porous adsorbents, are easy available, cheap and environmentally friendly, they should be, in the first place, considered as precursors for mass production of the electrode materials for electrochemical capacitors used as power supply in electric vehicles. Carbons which are carbonised and activated in standard conditions usually reveal acidic character, particularly if additional oxidation modifications are used to intensify pseudocapacitance effects [2] . Their electron acceptor character promotes the operation of only one of the electrodes. Therefore, in order to obtain better capacity parameters of the total capacitor, improvement of operating conditions of the opposite electrode is necessary. This could be achieved with the use of nitrided carbons, which reveals electron donor characteristics owing to their alkaline nature [3] . Ammoxidation as a simultaneous process of oxidation and nitriding of the material seems to be the best way to come up to these requirements [4,5] . During ammoxidation of carbonaceous material carried out, as usual, at relatively low temperatures, nitrogen heteroatoms are located first of all at the edges of graphene planes, forming functional groups the most frequently with contribution of oxygen like imide, imine, amide or amine [5,6] and their basic influence is limited rather to a local vicinity. In order to spread out this effect, nitrogen heteroatoms must be directly embodied in graphene plane rings. Thereby, they could participate in the formation of de-localized electrons system π affecting the total carbon matrix. It needs temperature higher than 410 °C [7] , which is available during carbonization and/or activation processes. Depending on the temperature, nitrogen heteroatoms can be placed in different sites of graphene planes [8] . In the so called ‘top’ position, typical for pyridine groups N-6, electron donor effect is not observed. However, in ‘centre’ or ‘valley’ position, found in N–Q type groups, nitrogen reveals pyrolic character and its electron donor effect is subsequently increasing. Also in top position, nitrogen heteroatom can acquire pyrolic nature if additional electron is combined due to proton donation, or in N–X system, e.g. with oxygen and their electron donor character is confirmed by the shift of zero charge potential in the negative direction. The stage, at which ammoxidation is applied during production of nitrided carbon as well as the temperature of the subsequent technological operations has an essential influence on anion and cation exchange properties of the material. This work takes advantage of the earlier experience with ammoxidation of viscose carbon fibers [9] and brown coals [10] in order to define conditions for the processing and modification of electrode material which would ensure optimum capacity for each one of the opposite electrodes of supercapacitor, using hard coal as a precursor material. 2 Experimental As a precursor material for the experiments we have used a young coking coal (Sosnica mine, Poland) with selected grain size of 0.2 mm, initially demineralized by hydrochloric and hydrofluoric acid according to Radmacher method. Carbonization process was carried out in argon atmosphere with temperature increasing at the rate of 5 °C/min to the final temperature 700 °C. Carbonizated samples (K) were activated with steam at 800 °C (KA 1 ) or at 850 °C (KA 2 ) until about 50% degree of burn-off was reached. Ammoxidation was applied to demineralized precursor material (NKA), carbonization product (KNA) or active carbon (KAN), according to Table 1 . This process was performed with the mixture of ammonia and air at 1:3 ratio at the temperature of 300 °C (N 1 ) or 350 °C (N 2 ) for 5 h. Porous structure of the samples ( Table 1 ) was determined by nitrogen adsorption in 77 K (ASAP 2010) and registered isotherms are presented in Figs. 1 and 2 . Nitrogen content for elemental analysis ( Table 2 ) was determined by Kjeldahal method. Surface chemical composition ( Fig. 3 , Table 2 ) was investigated by X-ray photoelectron spectroscopy (XPS). Photoelectron spectra were obtained using photoelectron spectrometer ESCALAQB-210 from VG Scientific (England). Investigations were made using X-radiation Al...