Analyse élémentaire | EPFL Graph Search

En chimie, une analyse élémentaire est la détermination qualitative (simple liste) ou quantitative (formule brute) des éléments chimiques constituant un composé. Elle se fait au moyen d'une analyse chimique, d'une analyse organique, d'une spectroscopie, d'une spectrométrie d'absorption atomique ou d'une spectrométrie de masse, entre autres. Processus alt=Analyseur élémentaire de marque Elementar nommé le Vario El Cube introduit en 2011|vignette|Analyseur élémentaire Elementar. vignette|Analyseur élémentaire Thermo Scientific. L'analyse élémentaire est un processus dans lequel un échantillon (sols, eaux usées ou potables, fluides corporels, minéraux, composés chimiques, etc.) est analysé pour déterminer sa composition élémentaire et parfois isotopique. L'analyse élémentaire peut être qualitative (déterminer quels éléments sont présents), et elle peut être quantitative (déterminer la quantité de chacun). L'analyse élémentaire s'inscrit dans le cadre de la chimie analytique,

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

Syntheses, structures and properties of cobalt and manganese complexes based on pyridine-2,3,5,6-tetracarboxylic acid

Chi Wai Cheung, Hong-Ling Gao, Weijin Wang

Two new complexes based on pyridine-2,3,5,6-tetracarboxylic acid(H(4)pdtc) as ligand, {[Co-2(pdtc)(H2O)(4)]center dot H2O}(n) (1) and {Mn-2(pdtc)(H2O)(4)}(n) (2), have been obtained by hydrothermal syntheses and characterized by IR, elemental analysis, X-ray diffraction single crystal structure analysis, PXRD, thermal gravimetric analysis, and magnetic measurements. Complex 1 is 3D coordination polymer and complex 2 belongs to 2D grid structure by covalent bonds. In 1 and 2, the metal ions show antiferromagnetic interactions. (C) 2015 Elsevier B.V. All rights reserved.

Elsevier Science Bv2015

Syntheses, structures and properties of silver(I) complexes constructed from nitrogenous aromatic heterocyclic carboxylic acids and N-donor ligands

Chi Wai Cheung, Hong-Ling Gao

Four new coordination complexes with silver(I) ions and nitrogenous aromatic heterocyclic carboxylic ligands, namely [AB(4)(Pac)(Phen)(4)]center dot 8H(2)O (1), {[AB(4)(Pztc)(bPe)(3)] center dot 5H(2)O}(n) (2), [Ag-4(Hmptc)(2)(4,4'-bPY)(3)] center dot 4.5H(2)O (3), and [Ag-2(H(2)mPtc)(2)(2,2'-bPY)(2)] (4) (H(4)Pztc = pyrazine-2,3,5,6-tetracarboxylic acid, H(3)mptc = 6-methyl2,3,5-pyridine-tricarboxylic acid, phen = 1,10-phenanthroline, bpe = trans-1,2-bis(4-pyridyl)ethylene, 4,4'bpy = 4,4'-bipyridine, 2,2'-bpy = 2,2'-bipyridine), have been synthesized by the self-assembly reactions and characterized by X-ray diffraction single crystal structure analysis, elemental analysis, IR, PXRD, fluorescence and thermal analysis. In I, four crystallographically independent silver(l) atoms are found to coordinate with only one pztc(4-) ligand. Complex 2 is a 2D grid structure. Complexes 3 and 4 are extended from OD to 2D or 1D by the Ag center dot center dot center dot O interactions. (C) 2014 Elsevier B.V. All rights reserved.

Elsevier2014

Ageing of porous carbon electrodes

Manxi Zhu

EPFL2006