Littérature scientifique sur le sujet « Organometallic Fuel Cells »

Production and usage of di-hydrogen, H 2 , in micro-organisms is catalysed by highly active, ‘ancient’ metalloenzymes known as hydrogenases. Based on the number and identity of metal atoms in their active sites, hydrogenases fall into three main classes, [NiFe]-, [FeFe]- and [Fe]-. All contain the unusual ligand CO (and in most cases CN − as well) making them intriguing examples of ‘organometallic’ cofactors. These ligands render the active sites superbly ‘visible’ using infrared spectroscopy, which complements the use of electron paramagnetic resonance spectroscopy in studying mechanisms and identifying intermediates. Hydrogenases are becoming a focus of attention for research into future energy technologies, not only H 2 production but also H 2 oxidation in fuel cells. Hydrogenases immobilized on electrodes exhibit high electrocatalytic activity, providing not only an important new technique for their investigation, but also a basis for novel fuel cells either using the enzyme itself, or inspired synthetic catalysts. Favourable comparisons have been made with platinum electrocatalysts, an advantage of enzymes being their specificity for H 2 and tolerance of CO. A challenge for exploiting hydrogenases is their sensitivity to O 2 , but some organisms are known to produce enzymes that overcome this problem by subtle alterations of the active site and gas access channels.

The system Y1−xSrxMnO3 (x = 0.000, 0.005, 0.010, 0.050, and 0.100) was studied as a potential cathode material for solid oxide fuel cells. Powders were prepared using an organometallic precursor; however, achieving homogeneous compositions was complicated due to the presence of intermediate, metastable phases. The desired hexagonal Y1−xSrxMnO3 phase formed from the precursor at 800 °C, while small amounts of a metastable orthorhombic (Y,Sr)MnO3 phase formed in the temperature range between 850°and 960 °C, and another orthorhombic YMn2O5 phase between 840°and 1200 °C. The metastable (Y, Sr)MnO3 phase readily transformed into the stable hexagonal phase at about 960 °C. The other metastable intermediate phase, YMn2O5, was formed as a decomposition product of a portion of the major hexagonal YMnO3 at 840 °C, and subsequently reacted with Y2O3 back to the hexagonal YMnO3 at 1200 °C. For the studied compositions, densities higher than 95% theoretical could be obtained by sintering in air at temperatures above 1400 °C for 2 h. The investigated system was comparable in electrical conductivity with the current cathode material La1−xSrxMnO3, and had an average apparent thermal expansion coefficient between 5 and 7 ppm/°C in the temperature range between 200°and 1000 °C. Unfortunately microcracking was observed in all sintered specimens, possibly caused by a high-temperature phase transition between the hexagonal and cubic polymorphs of Y1−xSrxMnO3. The microcracking presents a major obstacle to the use of this material as a cathode in solid oxide fuel cells.

The aim of the present work is the synthesis and characterization of new perfluorinated monomers bearing, similarly to Nafion®, acidic groups for proton transport for potential and future applications in proton exchange membrane (PEM) fuel cells. To this end, we focused our attention on the synthesis of various molecules with (i) sufficient volatility to be used in vacuum polymerization techniques (e.g., PECVD)), (ii) sulfonic, phosphonic, or carboxylic acid functionalities for proton transport capacity of the resulting membrane, (iii) both aliphatic and aromatic perfluorinated tags to diversify the membrane polarity with respect to Nafion®, and (iv) a double bond to facilitate the polymerization under vacuum giving a preferential way for the chain growth of the polymer. A retrosynthetic approach persuaded us to attempt three main synthetic strategies: (a) organometallic Heck-type cross-coupling, (b) nucleophilic displacement, and (c) Wittig–Horner reaction (carbanion approach). Preliminary results on the plasma deposition of a polymeric film are also presented. The variation of plasma conditions allowed us to point out that the film prepared in the mildest settings (20 W) shows the maximum monomer retention in its structure. In this condition, plasma polymerization likely occurs mainly by rupture of the π bond in the monomer molecule.

The regulation of systemic immune responses is dependent on individual cell responses that will concur to induce a coherent response against a stimulus. In turn, cell response is dependent on the processing of intracellular signals generated at the cell membrane and transmitted through successive protein modifications to the nucleus in order to activate gene transcription. This is referred to as intracellular signalling. Tight control of these mechanisms is required to generate an appropriate cell response to environmental stimulations and globally to establish an appropriate immune response. Among protein modifications used to transmit a signal to the nucleus, protein tyrosine phosphorylation represents a pivotal method used by immune cells to rapidly induce signalling. While protein tyrosine kinases (PTKs) phosphorylate proteins, protein tyrosine phosphatases (PTPs) regulate the signalling by removing the phosphate group. The goal of this study was to better characterize intracellular signalling events involved in allergic asthma, a chronic inflammatory disease involving a Th2 immune response. In a first time, we investigated the role of PTPs in the development of asthma. We show that inhibition of global PTP activity in mice, during either the allergen sensitization or the allergen challenge phase, reduces asthma development and is linked to an increased Th1 response in the spleen and lung. Secondly, we revealed that TC-PTP inhibition reduces asthma development, while PTP-1B inhibition exacerbates inflammatory cells recruitment to the lung. Inhibition of either SHP-1 or PTP-PEST activity did not significantly modulate asthma development in our model. In a third set of experiments, we got interested in the signalling pathways triggered by the pro-inflammatory molecules myeloid-related proteins (MRPs) 8 and 14. MRPs are small cytosolic proteins recently described to have extracellular functions. MRP8 expression is resistant to corticosteroid treatment, and potentially promotes inflammation in corticosteroid-treated patients. We identified that MRPs induce signal through the action of TLR-4 and trigger the activation of MEK/ERK and JNK pathways that lead to NF-kappaB translocation. Collectively, our data provide a new characterization of signalling pathways engaged in allergic asthma. This should be helpful in the elaboration of new therapeutic approaches targeting precise pathways to inhibit mechanisms of inflammation.

The world’s system of production and exploitation of fossil fuels is reaching a breaking point: finiteness, pollution and geopolitical problems mean that an energy economy based on these resources is no longer “sustainable”. The development of a new energy strategy is now a priority and renewable energies are the main candidates for replacing fossil fuels, especially for electrical energy production. Solar energy is the most abundant and easily available renewable source but it is neither constant nor distributed equally over the surface of the globe. Hydrogen and biofuels, like bioethanol, have the potential, as solar energy storage vectors, to play a fundamental role in the development of a new energy era based on renewable sources. There are two electrochemical devices, which will be fundamental for the realization of such an energy system. Firstly, fuel cells, which are devices that can convert cleanly the chemical energy stored in hydrogen or bioalcohols into electrical energy and secondly, electrolyzers, which are the best candidates for storing electric energy produced from renewable sources as hydrogen. The focus of this thesis is the development of innovative anodic electrocatalysts for three energy-related devices: a) Direct Alcohol Fuel Cells (DAFCs), b) H2/O2 fed Anion Exchange Membrane Fuel Cells (AMFCs) and c) anion exchange membrane alcohol electroreformers. Catalysts have been synthesized and characterized for their morphology and electrochemical activity, both in half-cell and in the complete cell systems. An anion exchange membrane is used in all devices because the alkaline environment is the most promising way to overcome the problems that hinder the development of fuel cells and electrolyzers that belong to the state of the art. For example, in alkaline media, alcohol oxidation kinetics are faster compared the traditional proton exchange membrane based devices. In Chapter 2 a description of the state of the art of fuel cells and electrolyzers is provided and describes the advantages in replacing the traditional proton exchange membrane electrolytes with anion exchange membranes. Chapter 3 introduces the OrganoMetallic Fuel Cell (OMFC), which employs a rhodium complex as anodic electrocatalyst. In contrast to established technologies based on metal nanoparticles, in OMFCs every single metal atom is catalytically active and thereby reduces the metal loading of fuel cell electrodes by several orders of magnitude. Because the performance of the metal complexes can be optimized based on established methods of synthetic organometallic and coordination chemistry, the approach via OMFC’s – though in its infancies – should allow to truly design electrodes which in principle can operate with earth-abundant, inexpensive metals. Organometallic fuel cells catalyse the selective electrooxidation of renewable diols, simultaneously providing high power densities and chemicals of industrial importance. It is shown that the unique organometallic complex [Rh(OTf)(trop2NH)(PPh3)], employed as molecular active site in an anode of an OMFC, selectively oxidizes a number of renewable diols, such as ethylene glycol (EG), 1,2-propanediol (1,2-P), 1,3-propanediol (1,3-P), and 1,4-butanediol (1,4-B) to their corresponding mono-carboxylates. The electrochemical performance of this molecular catalyst is discussed, with the aim to achieve cogeneration of electricity and valuable chemicals in a highly selective electrooxidation from diol precursors. Removal of platinum from polymer electrolyte membrane fuel cells is one of the most commonly cited objectives for researchers in this field. In Chapter 4 is described a platinum free anion exchange membrane fuel cell (AEM-FC) that employs nanostructured Pd anode and Fe-Co cathode electrocatalysts. AEM-FC tests run on dry hydrogen and pure air show peak power densities of more than 200 mW cm-2. Such high power output is shown to be due to a nanoparticle Pd anode catalyst with a composite Vulcan XC-72 carbon-CeO2 support that exhibits enhanced kinetics for hydrogen oxidation in alkaline media. In chapter 5, a nanostructured anodic electrocatalyst (Au@Pd supported on Vulcan XC-72) is employed in an alkaline alcohol electroreformer, which provides a net energy saving for hydrogen production compared to traditional water electrolysis. In addition, hydrogen production is coupled with the contemporaneous conversion of a bioalcohol into valuable chemicals, for example lactate and glycolate, which are industrially relevant feedstock. Traditional nanostructured palladium based anodic electrocatalysts are not selective in the oxidation of renewable polyols to carboxylic compounds. Modifying the nanoparticle architecture is a way to increase this selectivity. In addition, the energy consumption for hydrogen production by electrolysis was lowered from the 50-60 kWh kgH2¬-1 of a traditional electrolyzer to ca. 20 kWh kgH2-1. Chapter 6 summarizes the conclusions of this thesis. The results of this research clearly demonstrate that the architecture of the anodic electrocatalyst plays a fundamental role in the enhancement of the electrochemical performances of devices such as OMFCs, AMFCs and alcohol electroreformers. In addition, the anodic catalyst architecture has the important role for driving the selectivity of the alcohol oxidation reaction towards carboxylic compounds in OMFCs and electroreformers. These devices produce chemicals of industrial relevance with a contemporaneous release of energy or hydrogen at low temperature and atmospheric pressure, matching several principles of sustainable “green chemistry”.

Chemistry
The efforts described in this thesis were bifurcated along two distinct projects, but generally were directed toward the development of new materials to solve outstanding issues in contemporary electrochemical applications. The first project involved the synthesis and application of redox-switchable olefin metathesis catalysts. N-heterocyclic carbenes (NHCs) bearing ferrocene and other redox-active groups were designed, synthesized, and incorporated into model iridium complexes to evaluate their intrinsic electrochemical and steric parameters. Using these complexes, the ability to switch the electron donating ability of the ligands via redox processes was quantified using a variety of electrochemical and spectroscopic techniques. The donicity was either enhanced or attenutated upon reduction or oxidation of the redox-active group, respectively. The magnitude of the change in donicity upon reduction or oxidation did not vary significantly as a function of the proximity of the redox-active group from the metal center. Thus, other factors, including synthetic considerations, sterics, and redox potential requirements, were determined to guide ligand design. Regardless, redox-active NHCs were adapted into ruthenium-based olefin metathesis catalysts and used to gain control control over various ring-opening metathesis polymerizations and ring-closing metathesis reactions. The second project was focused on the development of new basic polymers for acid/base crosslinked proton exchange membranes intended for applications in direct methanol fuel cells. Polymers containing pendant pyridinyl and pyrimidinyl groups were obtained via the post polymerization functionalization of UDEL® poly(sulfone) and then blended with sulfonated poly(ether ether ketone) (SPEEK). Fuel cells containing these blends were found to exhibit reduced methanol crossover, higher open circuit voltages, and higher maximum power densities compared to plain SPEEK. The differences in fuel cell performance were attributed to the basicity and sterics of the pendant N-heterocycles.
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