Groupe fonctionnel | EPFL Graph Search

En chimie, les composés organiques peuvent être considérés comme constitués d'un squelette relativement non réactif appelé lalcane parent en nomenclature substitutive, et d’un ou plusieurs groupes fonctionnels. Le groupe fonctionnel est un atome, ou un groupe d'atomes, qui a des propriétés chimiques similaires chaque fois qu'il est présent dans des composés différents. Il définit les propriétés caractéristiques physiques et chimiques des familles de composés organiques. Cette manière de considérer les molécules est utilisée dans la nomenclature et dans l'estimation des propriétés physico-chimiques des composés chimiques. Si le groupe fonctionnel possède un hétéroatome, il est également possible de le considérer comme un groupe caractéristique. Les composés cycliques avec au moins un hétéroatome dans le cycle (hétérocycles) ne sont pas traités ici, car il existe une nomenclature qui leur est propre. On trouve parfois à tort le terme « groupement », utilisé

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

Fabrication and Optoelectronic Properties of Graphene Nanoribbons

Eberhard Ulrich Stützel

Graphene, a single layer of carbon atoms, exhibits excellent charge transport properties. However, due to the absence of a band gap in this two-dimensional carbon nanostructure, graphene-based field effect transistors cannot be turned off. One strategy to increase the on/off ratio relies on patterning graphene into narrow stripes, so-called graphene nanoribbons (GNRs). The present thesis aimed at developing novel fabrication and chemical functionalization methods for GNRs. Along these lines, electrical transport studies and spectroscopic investigations were envisioned as a major tool to monitor the changes brought about by the functional groups attached to the GNRs. GNRs were fabricated with the aid of V2O5 nanofibers or CdSe nanowires as an etching mask during the plasma etching of graphene. The resulting GNRs exhibited good electrical conductivity for ribbon widths as small as 10 nm. Moreover, the transport gap in the GNRs was found to scale inversely with the ribbon width. Scanning tunneling microscopy and surface enhanced Raman spectroscopy testified a good structural quality of the GNRs. Electrical characterization of GNRs revealed a pronounced hysteresis, which was exploited for the fabrication of electrically switchable GNR memory cells. Dynamic pulse response measurements demonstrated reliable switching between two conductivity states for clock frequencies of up to 1 kHz and pulse durations as short as 500 ns for >10^7 cycles. As the most likely switching mechanism, charge trapping within a water layer in the GNR surrounding could be identified. Furthermore, the optoelectronic properties of individual GNRs were studied by scanning photocurrent microscopy. The pronounced photocurrent signal close to the nanoribbon/metal contacts was found to be directly proportional to the conductance of the devices, suggesting that a local voltage source is generated at the nanoribbon/metal interface by the photo-thermoelectric Seebeck effect. The dominance of this mechanism over charge separation via built-in electrical fields was attributed to strong local heating of the GNRs by the laser spot, combined with the reduced thermal conduction capability of the nanoribbons in comparison to extended graphene sheets. Chemical functionalization of graphene and GNRs was attempted via different gas- and liquid-phase approaches. While electrical transport and Raman measurements revealed the presence of significant doping effects, it was not possible to unequivocally prove the covalent attachment of atoms at the GNR edges, neither through changes in the charge transport characteristics, nor via scanning microscopy studies.

EPFL2013

Macrocyclization of omega-Isocyanoaldehydes and Towards the Total Synthesis of Jamaicensamide A

Teerawat Songsichan

In the first chapter of this thesis, the macrocyclization of a new type of bifunctional substrates, omega-isocyanoaldehyde derivatives, is described. Ten different omega-isocyanoaldehydes in terms of different ring sizes and functional groups were prepared. They were smoothly proceeded under the simple multicomponent reactions including Passerini reaction, Ugi reaction and modified Ugi reaction. A wide range of 9- to 22-membered macrocyclic lactams were obtained in low to excellent yields with up to four bonds were formed in a single step. In the second chapter, synthetic studies towards the total synthesis of cyclic peptide jamaicensamide A are presented. We applied our strategy to cyclize the advanced intermediate containing omega-isocyanoaldehyde into the cyclic peptide in one-pot procedure. The precursors for the preparation of omega-isocyanoaldehydes with different functional groups and protecting groups were prepared. The derived 24-membered cyclic peptides were formed in good yields. The study on the deprotection of protecting groups is still on-going to complete the total synthesis of jamaicensamide A.

EPFL2021