A pyrolysis product derived from a coal-tar pitch was chemically activated using KOH in a KOH/carbon proportion of 5:1. The activated carbon was used as electrode-active material for capacitive deionization (CDI). Electrochemical parameters, such as current values, charge, specific charge and charge−discharge efficiencies were investigated using a unit cell and solutions of NaCl in different concentrations. The parent activated carbon shows an excellent behavior as electrode-active material in CDI, removing more salt than other carbons previously described in the literature. The activated carbon electrode presents an efficiency higher than 99% after 20 cycles. The parent activated carbon was modified by thermal treatment under nitrogen at different temperatures and by treatment with hydrogen and carbon dioxide. The modified activated carbons were also evaluated as electrode-active material to study the influence of the texture and surface chemistry on the CDI process. The results show the importance of both the texture and surface chemistry of the active material on the CDI process. The best behavior as electrode-active material was obtained for the materials with a high apparent specific surface area and a large quantity of oxygenated functional groups, i.e., the parent activated carbon and the sample modified by hydrogen treatment.
Abstract.-In order to understand the participation of electrical double layer and pseudocapacitance to the overall behavior of supercapacitors, a new approach to the analysis of the electrochemical data is proposed. Both the variation of the specific capacitance values and the dependence of these values with the operating voltage window (varying from 0-0.2 V to 0-1 V) were evaluated and used to quantify the contribution arising from each mechanism of energy storage to the total capacitance of the system. The suitability of the methodology here proposed was tested in various carbon materials (multiwalled carbon nanotubes, a carbon aerogel and two activated carbons), different both in nature and physicochemical characteristics. For all of the carbons studied, the capacitance with an exclusive faradic and non -faradic origin was quantified. Whereas some of the carbons studied showed a behavior close to an ideal Electrical Double Layer Capacitor (EDLC) with virtually no pseudocapacitance contribution (case of the carbon nanotubes), others presented up to a 40 % of pseudocapacitance contribution (case of KOH -activated carbon).
This work describes the chemical activation of a coke using two different activating agents in order to investigate their behaviour as electrodes in supercapacitors. A coke was chemically activated with two hydroxides (KOH and NaOH) under nitrogen flow, at a constant mass hydroxide/coke ratio of 2 and temperatures of 600, 650 and 700 ºC. All the samples were characterized in terms of porosity by N 2 sorption at 77 K, surface chemistry by temperature programmed desorption (TPD) and electrical conductivity. Their electrochemical behaviour as electric double layer capacitors was determined using galvanostatic, voltammetric and impedance spectroscopy techniques, in an aqueous medium with 1M H 2 SO 4 as electrolyte. Large differences in capacitive behaviour with the increase in current density were found between the two series of activated samples. The different trends were correlated with the results obtained from the TPD analysis of the CO-type oxygen groups. It was found that these oxygen groups make a positive contribution to capacitance finding a good correlation between the specific capacitance values and the amount of these oxygen groups was found for both series.
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