Charbon actif | EPFL Graph Search

Le charbon actif, aussi nommé charbon activé ou carbone activé, est un matériau constitué essentiellement de matière carbonée à structure poreuse. On appelle charbon actif tout charbon ayant subi une préparation particulière et qui, de ce fait, possède à un haut degré la propriété de fixer et de retenir certaines molécules amenées à son contact. Il s'agit d'une structure amorphe composée principalement d'atomes de carbone, généralement obtenue après une étape de carbonisation d'un précurseur à haute température. Un charbon actif présente en général une surface spécifique élevée qui lui confère un fort pouvoir adsorbant. L'adsorption est un phénomène de surface par lequel des molécules se fixent sur la surface de l'adsorbant par des liaisons faibles : forces de van der Waals, interactions électrostatiques, liaisons hydrogène. Des preuves de l'utilisation du charbon actif remontent à l'Antiquité avec des utilisations médicinales par Hippocrate vers ou pour de la purification d'eau

Ageing of porous carbon electrodes

Manxi Zhu

Ultracapacitors, also called electrochemical double layer capacitors (EDLC) or supercapacitors, may improve the performance of conventional electrolytic capacitors in terms of specific energy, or improve the performance of rechargeable batteries in terms of specific power when combined with respective device. However, the application of ultracapacitors faces the problem of ageing – the deterioration of capacity and increase of equivalent series resistance of the system. This thesis intends to elucidate ageing mechanisms of ultracapacitors based on activated carbon electrodes with tetrafluoroborate in acetonitrile as electrolyte. Multi-method experimental investigations are presented which allow to obtain a comprehensive picture of ageing. Ultracapacitors were electrochemically aged under various conditions by applying voltages and temperatures close to the actual operation conditions. The influences of ageing on the electrodes were investigated by structural and chemical characterization. Gas volumetric measurement (porosimetry) and Raman spectroscopy were applied to follow the structural changes; elemental analysis, electron spectroscopy for chemical analysis (ESCA, XPS) and attenuated total reflection infrared spectroscopy (ATR-IR) were applied for chemical composition analysis. It was found that the changes of both structure and chemical composition, which occur on aged anodes and cathodes, are asymmetric. Aged anodes showed more significant changes such as stronger decrease of pore volume and specific area, and more complex chemical species. Chemically bound nitrogen species were found only on aged anodes, but not on fresh electrodes and aged cathodes. Based on the experimental results, several mechanisms for ageing are suggested. The functionalities in activated carbons play an important role for ageing. Various oxygen-containing functional groups can take part in the reactions. A higher amount of functionalities means that more electroactive sites can be found on the graphene sheets in activated carbons to promote electrochemical reactions. Thus one can assume that less functional groups in activate carbons would be preferable for the stability of ultracapacitors. Ageing tests were carried out on individual components of the electrodes, mainly on activated carbon powders and conductive carbon additives. Observations are consistent with those on assembled electrodes. It turned out that the activated carbons from synthetic resin precursors, which have fewer oxygen functional groups, suffer less from ageing than those from natural precursors with more oxygen functional groups. A similar phenomenon was also observed on the conductive agents.

EPFL2006

Hydrothermally Stable Sulfonated Carbons as Solid Acid Catalysts for the Hydrolysis of Lignocellulose

David Ferdinand Scholz

The recycling of homogeneous acids imposes great challenges to the feasibility of lignocellulose hydrolysis processes. Although heterogeneous catalysts would offer a perspective to circumvent these challenges, conventional solid acids are instable under the hydrothermal conditions typical of hydrolysis processes. In this regard, acid functionalized carbons appear as auspicious alternatives due to the higher perseverance of carbon frameworks. The strong Brønsted acid sites in sulfonated carbons would be particularly well suited to catalyze the hydrolysis of cellulosic biomass. The utilization of these materials has so far been impeded by the controversy regarding the stability of sulfonic acid groups in carbons. Furthermore, it remains uncertain how the constraints imposed on the reaction system by the insolubility of lignocellulose influence the ability of solid acids to function as hydrolysis catalysts. This thesis shows that hydrothermally stable sulfonated carbons can be prepared by utilizing the structure-stability dependence of sulfonic acid groups in carbonaceous materials. The sulfonation of materials featuring a low degree of graphitization resulted in active solid Brønsted acid catalysts that however deactivated through the leaching of sulfonic acid groups. An in-depth physicochemical characterization of the catalysts paired with a systematic study of the stability of model sulfonic acids, revealed that the leaching-tendency is defined by substituent effects. Activating/deactivating substituents control the hydrothermal stability of aryl sulfonic acids by decreasing/increasing the tendency of the carbon-sulfur bond to undergo solvolysis. The presence of stabilizing and destabilizing substituents was found to be tunable by the temperature dependent decomposition of oxygen containing surface groups. Sulfonation of cellulose carbonized at 350°C resulted in a catalyst with 850 µmol g-1 hydrothermally (180 °C) stable sulfonic acid groups. Additionally, a hitherto unknown mode of deactivation was identified that proceeds by the ion-exchange of cationic impurities with protons of the active sites. This mode of deactivation can be fully overcome by implementing countermeasures to reverse or suppress ion-exchange processes. To tackle the limitations of homogeneous acids, the stable sulfonated carbons were used as catalysts in a semi-batch based lignocellulose hydrolysis process. It was found that the chemical recalcitrance and the insolubility of the substrate imposed strong limitations on the reaction with the catalyst and that soluble product formation occurred predominantly via auto-hydrolysis. The propensity of cellulose to undergo auto-hydrolysis could be enhanced by amorphizing its crystal structure through mechanical pretreatment in ball-mill. A heterogeneously catalyzed hydrolysis was achieved through mechanocatalytic processes facilitated by the concerted ball-milling (CBM) of the sulfonated carbon with the substrate. Sulfonic acid groups were pinpointed as active sites during the mechanocatalytic formation of soluble oligosaccharides. The secondary hydrolysis to monosaccharides in the semi-batch reactor was no longer constrained by the insolubility of the substrate. The combination of CBM pretreatment and hydrolysis in the semi-batch reactor was successfully extrapolated to the conversion of spruce fir wood to achieve yields in C5 and C6 derived products that are competitive with state-of-the-art diluted acid processes.

EPFL2019

CROCUS Concrete Activation Study by γ-ray Spectrometry and TLD Neutron Flux Measurements

Isabelle Tanseri

The semester project was under the course Physics lab IVb at EPFL. At EPFL site, the experimental fission reactor CROCUS is shielded by concrete which becomes radioactive as a result of neutron interaction. The activation is of interest to study for validation of models and cal- culations, and to quantify the amount of radioactive waste when the reactor will be recycled. In this project, measurements on concrete ac- tivation by NaI- and HPGe-detectors were performed on an in-cavity concrete block, together with TLD measurements on neutron flux in- side the core of CROCUS and in the cavity. The concrete activation and neutron flux allows for comparison and validation of models. The in-cavity concrete block had a low activation, trace elements of 60Co and 65Zn seem to be existing. The relative flux was the highest for structures close to the core and at the ceiling. In future projects, more experimental data on both concrete activation and neutron flux would be performed to acquire uncertainty estimations and correction factors.

2017