A large number of porous carbon materials with different properties in terms of porosity, surface chemistry and electrical conductivity, were prepared and systematically studied as electric double layer capacitors in aqueous medium with H 2 SO 4 as electrolyte. The precursors used are an anthracite, general purpose carbon fibres and high performance carbon fibres, which were activated by KOH, NaOH, CO 2 and steam at different conditions. Among all of them, an activated anthracite with a BET surface area close to 1500 m 2 /g, presents the best performance, reaching a value of 320 F/g, using a three-electrode system. The results obtained for all the samples, agree with the well-known relationship between capacitance and porosity, and show that the CO-type oxygen groups have a positive contribution to the capacitance. A very good correlation between the specific capacitance and this type of oxygen groups has been found.
10 Abstract 11Chemical and electrochemical techniques have been used in order to asses surface functionalities of porous carbon materials. An 12 anthracite has been chemically activated using both KOH and NaOH as activating agents. As a result, activated carbons with high 13 micropore volume (higher than 1 cm 3 /g) have been obtained. These samples were oxidized with HNO 3 and thermally treated in N 2 flow 14 at different temperatures in order to obtain porous carbon materials with different amounts of surface oxygen complexes. Thermal treat-15 ment in H 2 was also carried out. The sample treated with H 2 was subsequently treated in air flow at 450°C. Thus, materials with very 16 similar porous texture and widely different surface chemistry have been compared. The surface chemistry of the resulting materials was 17 systematically characterized by TPD experiments and XPS measurements. Galvanostatic and voltammetric techniques were used to dee-18 pen into the characterization of the surface oxygen complexes. The combination of both, chemical and electrochemical methods provide 19 unique information, regarding the key role of surface chemistry in improving carbon wettability in aqueous solution and the redox pro-20 cesses undergone by the surface oxygen groups. Both contributions are of relevance to understand the use of porous carbons as electro-21 chemical capacitors. 22
A B S T R A C TPorous carbon materials, with different porosities and surface chemistry have been prepared and characterized to obtain a better understanding of the mechanism of the electrochemical storage of hydrogen. The hydrogen storage capacity depends, not only on the porosity of the material, but also on the surface chemistry, which is a critical factor. The results show that the higher the amount of surface oxygen groups, the lower is the hydrogen uptake. Measurement of the number of active carbon sites shows the important role of the unsaturated carbon atoms in the process. In situ Raman spectroscopy has been used in order to further explore the structural changes in the carbon material during the chargedischarge processes. This technique has allowed us to observe the formation of the C(sp 2 )AH bonds during the cathodic process and its reversibility during the oxidation step.Ó 2008 Published by Elsevier Ltd. 29 Introduction 30Carbon materials have been extensively studied as electrodes 31 for energy storage devices since they present a very interest-32 ing electrochemical behaviour. These materials may have 33 high electrical conductivity and can both donate and accept 34 electrons [1]. Carbon electrodes have a good polarizability 35 and their properties are tuneable depending on the porosity, 36 thermal treatment, microtexture, hybridization, content of 37 heteroatoms, etc. Moreover, they are chemically stable in 38 most solvents and present relatively low cost and easy pro-39 cessability [2]. One important application is in rechargeable 40 batteries in which carbon materials are used as a lithium res-41 ervoir at the negative electrode [1]. Additionally, the storage of 42 energy in supercapacitors, based on the electrical double 43 layer and pseudocapacitance of carbon materials, has also 44 been deeply analysed with very promising results [2][3][4]. 45A different electrochemical application is the electrochem-46 ical storage of hydrogen by electrodecomposition of water in 47 alkaline medium at cathodic conditions. Several nanostruc-48 tured carbon materials, mainly nanotubes (CNT), have been 49 tested for this application [5][6][7][8][9][10][11]. However, the results ob-50 tained present an important scattering. In fact, taking into ac-51 count theoretical calculations and experimental results, it is 52 highly unlikely that a significant amount of hydrogen can 53 be stored in CNT [8]. The different results could be justified 54 by the different degree of purity of the samples (CNT often 55 contain amorphous and disordered carbonaceous material 56 or residual metal catalyst) [8,12]. 57 Porous carbons have also been extensively studied for this 58 application, giving higher values for hydrogen storage and 59 much better reproducibility. Probably, the most relevant re-60 search in this area has been done by Beguin, Frackowiak 61 and co-workers [13][14][15][16][17][18][19]. Similar results have been obtained 62 by other authors using ordered porous carbons with tailored 63 pore size [12], carbon blacks [...
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