420 research outputs found
Détermination des mécanismes de dégradation d'électrodes modÚles de pile à combustible à membrane échangeuse de protons
Ce travail de thĂšse s est intĂ©ressĂ© aux mĂ©canismes de dĂ©gradation de nanoparticules de Pt supportĂ©es sur carbone utilisĂ©es pour catalyser les rĂ©actions Ă©lectrochimiques dans une pile Ă combustible Ă membrane Ă©changeuse de protons (PEMFC) et Ă leur consĂ©quences d un point de vue cinĂ©tique. Nous avons mis en Ă©vidence les diffĂ©rents mĂ©canismes (maturation d Ostwald 3D, corrosion du support carbonĂ©, migration/agrĂ©gation des cristallites mĂ©talliques) conduisant Ă une perte de surface active Ă©lectrochimiquement et avons trouvĂ© des conditions permettent d isoler chacun de ces mĂ©canismes. En premier lieu, nous avons montrĂ© que les nanoparticules de Pt supportĂ©es sur carbone ne sont pas immobiles mais agrĂšgent en conditions rĂ©actionnelles notamment en prĂ©sence de molĂ©cules rĂ©ductrices. La vitesse de ce processus varie dans l'ordre CO > CH3OH > H2 et a Ă©tĂ© reliĂ©e Ă (i) la baisse du travail d adhĂ©sion engendrĂ©e par la chimisorption de ces molĂ©cules et (ii) la rĂ©duction des groupements oxygĂ©nĂ©s prĂ©sents sur le support carbonĂ© natif.Nous nous sommes Ă©galement intĂ©ressĂ©s au mĂ©canisme d Ă©lectrooxydation Ă©lectrochimique du Vulcan XC72, un noir de carbone classiquement utilisĂ© dans les couches catalytiques de PEMFC. Des mesures par spectroscopie Raman ont montrĂ© que les domaines dĂ©sordonnĂ©s du Vulcan XC72 (non-graphitiques, hybridation sp3) sont corrodĂ©s de façon prĂ©fĂ©rentielle dans des conditions expĂ©rimentales proches de celles d une cathode de PEMFC. Les domaines ordonnĂ©s du support carbonĂ© (carbone graphitique, hybridation sp2) sont Ă©galement corrodĂ©s, la vitesse de ce processus Ă©tant largement infĂ©rieure Ă ce qui est observĂ© sur les domaines dĂ©sordonnĂ©s. En consĂ©quence, les nanoparticules de Pt se dĂ©tachent ou agglomĂšrent comme le rĂ©vĂšlent des expĂ©riences de microscopie Ă©lectronique en transmission couplĂ©es Ă l Ă©lectrochimie. L ensemble de ces mĂ©canismes de dĂ©gradation conduit Ă un abaissement de la densitĂ© du nombre de particules mĂ©talliques et augmente la distance entre ces derniĂšres. Dans le chapitre IV, nous montrons que des Ă©lectrocatalyseurs Pt/Sibunit electrocatalysts possĂ©dant (i) un faible chargement massique en Pt, et (ii) de grandes distances inter-particules prĂ©sentant une faible activitĂ© pour la rĂ©duction du dioxygĂšne de l air. Le nombre moyen d Ă©lectrons transfĂ©rĂ©s par molĂ©cule de dioxygĂšne dĂ©croĂźt bien sous la valeur thĂ©orique de 4 lorsque l Ă©paisseur de la couche catalytique ou le chargement massique diminue. Nous avons reliĂ© cela Ă un transport et Ă une rĂ©-adsorption plus difficiles des intermĂ©diaires rĂ©actionnels notamment le pĂ©roxyde d hydrogĂšne. Une diminution du nombre de sites catalytiques peut Ă©galement engendrer une limitation des cinĂ©tiques rĂ©actionnelles par l adsorption de l oxygĂšne. Au vu de l ensemble des rĂ©sultats prĂ©cĂ©dents, nous avons conclu que des cristallites de plus grande taille permettraient d amĂ©liorer la durabilitĂ© des matĂ©riaux contenus dans les couches catalytiques de PEMFC. Des nano-fils de Pt (NWs) avec une taille moyenne de cristallite de 2,1 +- 0,2 nm ont Ă©tĂ© synthĂ©tisĂ©s. Nous avons montrĂ© que la morphologie du matĂ©riau joue un rĂŽle consĂ©quent Ă la fois en termes d activitĂ© Ă©lectrocatalytique et de durabilitĂ© : les matĂ©riaux Pt NWs/C permettent une rĂ©duction de prĂȘt de 170 mV de la surtension d oxydation d une monocouche de monoxyde de carbone et possĂšdent une activitĂ© catalytique Ă©levĂ©e et stable pour l Ă©lectrooxydation du mĂ©thanol. Cette derniĂšre a Ă©tĂ© attribuĂ©e Ă (i) l augmentation de la masse des cristallites de Pt rĂ©sultant de l augmentation en taille (nanoparticules Ă nano-fils) et (ii) une surface de contact Ă©levĂ©e avec le support carbonĂ©. Ces matĂ©riaux possĂšdent un potentiel intĂ©ressant pour rĂ©soudre les problĂšmes de durabilitĂ© rencontrĂ©s avec les matĂ©riaux 0D utilisĂ©s de façon conventionnelle.State-of-the-art catalytic layers of proton-exchange membrane fuel cells (PEMFCs) utilize ionomer-bonded Pt-based nanocrystallites supported on a high surface area carbon support to accelerate the rate of the hydrogen oxidation reaction at the anode, and of the oxygen reduction reaction at the cathode. Post-mortem analysis of PEMFC catalytic layers reveal that four degradation mechanisms yield Pt surface area losses (and hence decreased PEMFC performance): (i) aggregation and/or detachment of the metal nanoparticles, (ii) corrosion of the carbon support, (iii) Ostwald ripening (dissolution/redeposition of the Pt-based crystallites) yielding the formation of ionic species and (iv) chemical reduction of the Ptz+ species in ion conductors, yielding the formation of electrically disconnected Pt crystallites. A major concern of the Ph.D. was to be able to isolate the migration of the Pt nanocrystallites. For that purpose, commercial Pt/Vulcan XC 72 electrocatalysts were aged in mild conditions, for which the corrosion of both the Pt nanoparticles and the carbon support could not be considered dominant. Evidences were provided that the Pt/C nanoparticles are not immobile but prone to agglomerate in the presence of H2, CH3OH, and CO, three molecules of interest for PEMFCs. The migration rate of the Pt crystallites was the largest in CO-containing solution and decreased in the order CO > CH3OH > H2. We postulate that the morphological changes of the Pt/C nanoparticles may be caused by (i) a change of the work of adhesion between the metal phase and the carbon support or (ii) the reduction of the oxygen-bearing surface groups strongly interacting with the Pt nanocrystallites. We also investigated the mechanism of the electrochemical oxidation of Vulcan XC72, a carbon black conventionally used in PEMFCs. Raman spectroscopy measurements evidenced that the disordered domains of the Vulcan XC72 support (non-graphitic, sp3-hybridized) are preferentially oxidized. The ordered domains (graphitic carbon, sp2-hybridized) of the Vulcan XC72 support are also oxidized but at a much smaller rate than that observed on the non-graphitic domains. A major consequence of the oxidation of the high-surface area carbon support is the aggregation and the detachment of the supported Pt nanoparticles. In Chapter V, we used model Pt/Sibunit electrocatalysts to show that aged catalytic layers with (i) low Pt to C weight fraction, (ii) large inter-particle distance, feature bad performance for the oxygen reduction reaction (ORR). The average number of transferred electrons produced during the ORR decreases below 4 with the decrease of the catalyst layer thickness or the Pt loading. This was rationalized by considering the lower probability for H2O2 molecules (the major reaction intermediate) to be re-adsorbed and further reduced into water in the catalytic layer when the inter-particle distance decreases. Finally, Pt nanowires (NWs) with 2.1 +- 0.2 nm crystallite size were synthesized by a soft template method. Evidences were provided that the morphology of the Pt material plays a pivotal role both in terms of electrocatalytic activity/stability: the 1D Pt NWs/C demonstrate a reduction by ca. 170 mV of the CO oxidation overpotential and feature high and stable MOR activity with respect to a conventional Pt/C 20 wt. % catalyst. The enhanced durability of Pt NWs/C was rationalized by considering (i) the increase in weight of the Pt nanomaterials resulting from the increase in size (from nanoparticles to nanowires) and (ii) the enhanced contact surface area between the Pt NWs and the carbon support. The development of 1D Pt nanostructures, such as Pt NWs, hold promises to solve the durability issues faced with the 0D materials currently used in PEMFCs.Keywords: proton exchange membrane fuel cell, durability of PEMFC materials, electrochemical carbon oxidation, crystallite migration, Pt nanowires.SAVOIE-SCD - Bib.Ă©lectronique (730659901) / SudocGRENOBLE1/INP-Bib.Ă©lectronique (384210012) / SudocGRENOBLE2/3-Bib.Ă©lectronique (384219901) / SudocSudocFranceF
Pt/carbon xerogel catalysts for PEM fuel cells
International audienceCarbon xerogels have been used to replace carbon black as catalyst support at the cathode of proton exchange membrane (PEM) fuel cells in order to decrease the mass transport limitations in this electrode. Carbon xerogels are very clean nanostructured carbons with well-defined pore texture, which allows forbetter reactant/product diffusion. Pt/carbon xerogel catalysts with high metal dispersion (nanoparticles ca. 2 nm in size) and high metal content (~ 25 wt.%) can be engineered via rational synthesis methods such as the 'Strong Electrostatic Adsorption' technique. The results show that choosing correctly the average pore texture of the carbon xerogel allows for minimizing the diffusion overpotential of the H2/air cell. However, the catalyst characterization indicates that the presence of chlorine, coming from H2PtCl6, induces a dramatic decrease of the Pt utilization ratio in the final PEMFC catalytic layer. To remove chlorine, a reduction of the catalyst at 450°C, at least, is necessary
Anisotropic growth of the thiophene-based layer on Si(111)-B
International audienceThe formation of large assemblies on the Si(111)-B surface is discussed with the help of STM simulations and DFT calculations. Although highly regular assemblies of DTB10B along the Si row direction are observed, the existence of two herringbone isomers introduces a lower periodicity within the 2D molecular network. The formation of herringbone units is explained by weak intermolecular interactions while the 1D assembling depends mainly on the interactions of the C10 side chains with the Si(111)-B surface
Elaboration and characterizations of platinum nanoparticles supported on cellulose-based carbon aerogel
International audienceThis work investigates the deposition of Pt nanoparticles onto carbon aerogels (CA), derived from microcrystalline cellulose. Nanoparticles are synthesised via impregnating the CA with H2PtCl6 followed by reduction either under H2 at 300 C or in a basic NaBH4 solution. H2 reduction yields uniform Pt nanoparticles (average diameter < 2nm) dispersed over the CA surface as revealed by Transmission electron microscopy (TEM). Larger agglomerates can be seen in TEM images of NaBH4 reduced samples, which is confirmed by powder X-ray diffraction (XRD). A rotating disk electrode was employed to analyse the electrochemical properties of the Pt nanoparticles. The active area of the platinum nanoparticles was evaluated using hydrogen adsorption/desorption cyclic voltammetry and CO stripping measurements. The oxygen reduction reaction was also studied to (i) obtain the kinetic parameters of oxygen reduction for the Pt/CA materials and (ii) compare them with commercial Pt/Carbon Black
Intraoperative Ketorolac and Outcomes after Ovarian Cancer Surgery
The authors want to thank Laurence Beausaert and Monique Kasa-Vubu, coordinating nurses of gynaecological oncology, Magali Alsteen and VĂ©ronique Delhaye for the recording of the data and all the nurses from the operating theatre and the hospitalisation unit (U95) of the cliniques universitaire St Luc. We also want to thank the intensive care unit team for the care of our patients.Peer reviewe
Tailoring the Oxygen Reduction Activity of Pt Nanoparticles through Surface Defects: A Simple Top-Down Approach
Breakthrough zero-emissions heat generation with hydrogen-metal systems (HERMES)
International audienc
Breakthrough zero-emissions heat generation with hydrogen-metal systems (HERMES)
International audienc
Le rĂŽle politique du Conseil souverain dâAlsace de 1658 Ă 1790.
Droit privé et sciences criminelle
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