nanocomposite electrodes for proton exchange membrane fuel cell at high temperature

Ces travaux de thèse s'inscrivent dans le contexte des efforts de recherches menés pour proposer des matériaux susceptibles de lever les verrous technologiques au développement des piles à combustible à membrane. L'un de ces enjeux est l'augmentation de la température de fonctionnement (150 - 250 °C) afin d'améliorer les cinétiques réactionnelles permettant une diminution de la quantité de catalyseur ainsi qu'une simplification de la gestion de l'eau, une réduction du système de refroidissement et une meilleure résistance à l'empoisonnement au monoxyde de carbone du platine. La motivation de cette étude a été de substituer au carbone un matériau support de catalyseur avec une plus grande résistance électrochimique.Notre choix s'est porté sur le carbure de tungstène qui, en plus d'une conductivité électronique élevée, présente une activité catalytique pour l'oxydation de l'hydrogène et la réduction de l'oxygène en milieu acide. La mise au point d'une méthode de synthèse innovante par voie hydrothermale a permis l'élaboration de microsphères de carbure de tungstène (MCT) de surface spécifique élevée (68 m2.g-1 avec 4 % de carbone résiduel) et d'architecture inusuelle. Des nanoparticules de platine de taille contrôlée ont été préparées par méthode polyol afin d'être déposées en surface des MCT. Après caractérisations électrochimiques ex-situ couplées à des analyses de surface (XPS) de ces catalyseurs Pt/WC, la mise en forme d'électrodes par enduction et transfert sur la membrane a permis la réalisation d'assemblages membrane - électrode et leurs caractérisations en pile à combustible. Des membranes polybenzimidazole dopé acide phosphorique (PBI-H3PO4) ont été utilisées pour remplacer les membranes Nafion afin d'augmenter la température de fonctionnement.The objective of this work was to develop alternative suitable materials to increase operating temperature of a Proton Exchange Membrane Fuel Cell. The increase of the operating temperature (150 - 250 °C) is attractive for cost reduction and reliability in terms of reaction kinetics, catalyst tolerance, heat rejection and water management. Our work was focused on tungsten carbide which has an high electrical conductivity and exhibits a significant catalytic activity for hydrogen oxidation and oxygen reduction in acidic environment. We have reported a novel approach to produce tungsten carbide microspheres (TCM) with an high surface area (68 m2.g-1 including only 4 % of residual carbon) and an unusual architecture. Platinum nanoparticles were prepared by polyol method and were then deposited on TCM. Physical, chemical as well as electrochemical characterisations of WC supported platinum nanoparticles Pt/WC are described and discussed in comparison with a platinum electrocatalyst on a commercial carbon support (Vulcan XC-72R). Membrane Electrode Assembly was then prepared by coating - decal process, and characterised by single cell test and compared to conventional Pt/C assembly. Phosphoric acid doped polybenzimidazole PBI(H3PO4) was used as electrolyte to replace Nafion membrane in order to carry out fuel cell testing at higher temperature

High surface area tungsten carbide with novel architecture and high electrochemical stability

International audienc

Hollow microspheres with a tungsten carbide kernel for PEMFC application

International audienc

On the Effect of Non-Carbon Nanostructured Supports on the Stability of Pt Nanoparticles during Voltage Cycling: a Study of TiO2 Nanofibres

International audienceElectrospun carbon and Nb-doped TiO2 nanofibres (CNFs, TNFs) have been investigated as electrocatalyst supports for polymer electrolyte membrane fuel cells (PEMFC). The optimal Nb doping amount has been identified for TNFs, and thermal treatment of titanium oxide fibres optimised to balance the surface area and electronic conductivity requirements. The most highly conducting material is characterised by a high concentration of surface Ti3+ and Nb4+ (and oxygen vacancies). Pt nanoparticles of average diameter of 2.3 nm were loaded onto 10 %at Nb doped-TiO2, retained as the best candidate for further electrochemical analysis, and on CNFs, using a microwave-assisted polyol method. Significantly higher electrochemically active surface area was retained after voltage cycling to 1.2 V for Pt supported on TNF (73 %) than on CNFs, where only 8% of the original ECSA was conserved after 1000 voltammetric cycles. The mass activity was also slightly higher for the titanium oxide based electrodes in the oxygen reduction reaction

Catalyst Coated PBI Membrane High Temperature Assemblies

Phosphoric acid doped polybenzimidazole (PBI) membranes are generally prepared by immersing the PBI membrane in concentrated phosphoric acid, with the uptake of phosphoric acid being a function of temperature and time. Alternatively, in the so-called sol-gel method, monomer precursors to PBI are polymerised in a polyphosphoric acid solvent, and phosphoric acid formed within cast membranes from the PPA by hydrolysis. The phosphoric acid content of H3PO4-PBI membranes prepared by the latter method can be significantly higher than that which can be reached by immersion doping, and the conductivity a factor of 10 higher. We have developed a route to stabilise a polybenzimidazole gel membrane formed in polyphosphoric acid by simultaneous cross-linking. With increasing degrees of cross-linking the mechanical properties of the high acid content PBI membranes so prepared are significantly improved, while the phosphoric acid content and the membrane conductivity are not greatly affected. H 3PO4-PBI membranes are generally integrated into an assembly using gas diffusion electrodes. In the present work, the cross-linked PBI membranes have been used as substrates for catalyst coated membrane (CCM) preparation using a Hispec Pt/C catalyst transferred by decal to the membrane surfaces. The final membrane electrode assemblies have been characterized for their high temperature performance and durability, and the results compared to those obtained using an MEA prepared by spraying catalyst ink containing PBI on both sides of a PBI membrane, which was then doped by soaking in H3PO4 (1). This poster will describe the properties of the cross-linked high phosphoric acid content PBI membranes, and relate them to the degree of cross-linking and the acid content, the methodology for preparation of H3PO4-PBI-based CCMs, and the fuel cell performance of the final MEAs. We acknowledge the financial contribution under QuasiDry, contract 256821 of the European 7th Framework Programme.[no pdf