Niobium- and antimony-doped tin dioxide aerogels as new catalyst supports for PEM fuel cells

The final publication is available at Springer via http://dx.doi.org/10.1007/s10853-016-9833-7International audienceIn order to tackle the problem of low durability, tin dioxide was studied to replace carbon black as a catalyst support in proton exchange membrane fuel cells (PEMFCs). SnO2 is a well-known n-type semi-conductor whose electronic conductivity can be improved by doping with hypervalent cations such as Nb5+ or Sb5+. In addition, as a catalyst support, this material has to develop a high specific surface area with an adequate mesoporous morphology to allow both good dispersion and activity of the catalyst (Pt). To this end, our objective was to develop doped SnO2 aerogels in order to gather in a same material both a high electronic conductivity and an adapted morphology. In this study, SnO2 xerogels and aerogels were successfully synthesized following an acid-catalyzed sol–gel route starting with metal alkoxides as precursors. Dried gels were calcined for 5 h at 600 °C in flowing air. The effect on both the structure and the morphology of the material resulting from doping with niobium or antimony was investigated by XRD, SEM, and nitrogen sorption. The electronic conductivity of pure and doped SnO2 materials was obtained from impedance spectroscopy and resistance measurements. Our materials showed a very interesting airy morphology adapted for the foreseen application: a reasonable specific surface area (80–90 m2/g) with a bimodal pore size distribution centered on around 25 and 45 nm. Moreover, all Sb-doped samples exhibited significant improvement in electronic conductivity. 5 at.% Sb-doped SnO2 even showed an electronic conductivity of 1 S/cm, very similar to that of Vulcan XC-72 (4 S/cm) and representing a 5 orders of magnitude increase compared to that of pure SnO2

Relation entre humidité relative et performances pour des Assemblages Membrane Electrodes de pile PEM à base d'aérogels de carbone - Impact de l'ajout de PTFE dans la couche catalytique

National audiencePour améliorer les performances des piles PEM (Proton Exchange Membrane Fuel Cells), il est nécessaire de mieux comprendre les phénomènes qui apparaissent lors de leur fonctionnement. L'objectif de cette étude est d'étudier différents paramètres jouant un rôle dans la gestion de l'eau en utilisant les matériaux modèles que sont les aérogels de carbone comme support de catalyseur. Pour cela, nous avons synthétisé trois aérogels de carbone d'architectures différentes que nous avons utilisés comme supports de catalyseur et mis en oeuvre en Assemblage Membrane Electrodes (AME). Dans ce travail, nous avons également étudié l'impact du taux d'humidité relative cathodique et de l'utilisation de PTFE dans la couche catalytique cathodique. Les tests des différents AME sur un banc monocellule montrent l'importance du contrôle de l'architecture du support carboné. Par ailleurs, les performances des AME peuvent être améliorées en utilisant du PTFE et en diminuant l'humidité relative à la cathod

Influence of carbon aerogel texture on PEMFC performances

International audienceTo improve Proton Exchange Membrane Fuel Cells (PEMFC), it is necessary to understand the phenomena occurring in operating conditions. The objective of this study is to determine how carbon support architecture can impact PEMFC performances, particularly diffusive limitations. In this context, as they have a controllable texture, carbon aerogels were used as catalyst supports in PEM fuel cell cathodes. Three carbon aerogels with different morphologies were synthesized. Fuel cell measurements show that the carbon support architecture has a significant impact on diffusive limitations. Moreover, they confirm that Nafion® loading must be optimized in order to preserve the catalytic layers architecture. This work finally highlights the impact of the catalytic layers architecture on the PEMFC performances

Doped TiO2 aerogels as alternative catalyst supports for proton exchange membrane fuel cells: A comparative study of Nb, V and Ta dopants

International audienceNb, Ta and V-doped TiO2 aerogels and xerogels have been synthesized as possible new alternatives to carbon blacks for Proton Exchange Membrane Fuel Cells catalyst supports. A comparative study of different dopants was realized in a single study. Nb, Ta and V showed different behaviors with respect to the final material structure and morphology, composition and electronic conductivity. They are all prone to surface segregation, to different extents. V-doped TiO2 apart, the rutile structure could only be obtained after calcination in a reducing atmosphere at 800 °C for Nb or Ta-doped TiO2. The electronic conductivity exhibited a maximum at 10 at.% for Nb and Ta, 5 at.% for V. Nb revealed to be the most appropriate dopant to increase the electronic conductivity of TiO2, followed by Ta and V. 4 to 5 orders of magnitude were gained after Nb doping for xerogels conductivity to reach almost 0.1 S cm−1. The role of point defects was discussed to account for phase transition and evolution of conductivity

Comparison of platinum deposit methods on carbon aerogels used in Proton Exchange. Membrane Fuel Cells (PEMFC)

International audienceWith the rarefaction and price increase of fossil fuels along with the consequences of greenhouse effect, many challenges have to be taken up. Consequently, a strong research effort is devoted to cleaner energy converters like fuel cells. In the car industry, Proton Exchange Membrane Fuel Cells (PEMFC) are chosen by a majority. But, remaining problems must be solved before a development at a large scale, among which: reducing the costs and increasing the power density and durability. Costs reduction mostly implies both diminishing the platinum quantity required for the oxygen reduction reaction at the cathode and increasing its activity. For these reasons, researches are devoted to the impact study of various methods of platinum deposit on the performances of new electrocatalysts and to the understanding of phenomena occurring in fuel cells

Synthesis of europium and disprosium activated strontium monoaluminate

International audienceThe formation of SrAl2O4 by solid state reaction between SrCO3 and Al2O3 has been studied. The firing conditions, namely the time, the atmosphere and the temperature, have been optimized so as to obtain pure well crystallized SrAl2O4, allowing the further study of the influence of the atmosphere on the optoelectronic properties of the phosphor. To this end, fluorescent and thermoluminescent experiments have been carried out in order to characterize the defects responsible for the phosphorescent mechanism

Low temperature synthesis of ZnGa2O4 : Mn

International audienceThe dependence of the firing conditions on the crystal structure formation of ZnGa2O4 has been investigated. The effects of temperature and atmosphere were examined and gave evidence of the instability of the zinc gallate under a reducing atmosphere at temperatures above 600 degrees C. A new preparation route has allowed to synthesize ZnGa2O4 at low temperature, right from 300 degrees C, avoiding the decomposition of the matrix during the annealing. Moreover, a different behavior under excitation has been observed for the phosphor prepared via this new route compared with the compound obtained by solid state reaction. A model, based on the optical behavior observed, is presented

Impact of three different TiO2 morphologies on hydrogen evolution by methanol assisted water splitting: Nanoparticles, nanotubes and aerogels

International audienceIncreasing the activity of a photocatalyst goes through the improvement of both its absorption (light) and adsorption (reactant) properties. For a given semiconducting material, the charge carrier separation is also a very important step. Properly combining chosen phases is one option to improve this separation (example of the commercial P25) and depositing platinum on the surface of the catalyst, another one. In some cases, coupling both may nevertheless lead to a decrease of photoactivity or at least limit the potentiality of the catalyst. A third option, consisting in modifying the morphology of the photoactive phase, has shown very promising results. In this study, we have elaborated, characterized and evaluated the hydrogen evolution potentiality (through methanol assisted water splitting) of different TiO2 morphologies: nanoparticles, nanotubes and aerogels. These materials have shown different behaviours depending on both their composition and morphology. Different types of separation processes have been claimed to account for the observed different photoactivities, with more or less pronounced synergetic effects, due to: the use of Pt as a co-catalyst, the mixture of different TiO2 phases (anatase and TiO2(B) or rutile) and the specific morphology of the samples (nanotubes or aerogels). Among all the tested samples, the TiO2 aerogel supported Pt one exhibited very promising performances, three times as active as P25 supported Pt, which is already much more active than pure P25 in our testing conditions

Pile à combustible, production d'hydrogène à partir de biomasse, stockage d'hydrogène sur carbones nanostructurés, analyse du cycle de vie : un projet du groupe des Ecoles des Mines en génie des procédés, énergie et environnement (GEM GP E2)

International audienceLes équipes de différents centres de recherche des Ecoles des Mines, impliquées dans les domaines liés à la filière hydrogène, se sont regroupées au sein du Groupe des Ecoles des Mines en Génie des Procédés, Energie et Environnement (GEM GP E2). Le projet H2-PAC mis en place, a ainsi permis de regrouper les compétences de ces équipes pour aborder de manière plus efficace les quatre problématiques suivantes : les piles à combustible à membrane échangeuse de protons, la production d'hydrogène à partir de biomasse, le stockage d'hydrogène sur matériaux carbonés nanostructurés et l'analyse du cycle de vie. Dans chacun de ces thèmes des progrès ont été enregistrés et les équipes se sont mutuellement enrichies de nouvelles connaissances. Le projet a dans l'ensemble permis de créer des liens entre les différentes équipes impliquées, qui permettront de futures collaborations et la concrétisation de nouveaux projets

Aerogels for energy applications

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