Liens transverses ENS de Lyon

Agenda de l'ENS de Lyon

FERREIRA DE MORAIS - Study of the reactivity and stability properties of Pt3Ni nanocatalysts in PEMFC : an ab-initio based multiscale modeling approach.

Soutenance de thèse

Vendredi 02 déc 2011



Description générale
The Polymer Electrolyte Membrane Fuel Cell (PEMFC) constitutes a possible solution for replacing the actual combustion engines. However the cost of the typically used catalyst and its actual low stability are restricting its economical viability. Platinum deposited on carbon is the state-of-the-art cathode catalyst but Pt-M (M= Co, Ni or Fe) alloyed nanoparticles have been proposed as cheaper, more stable and competitive alternative material, regarding the oxygen reduction reaction (ORR) activity. Up to now the fundamental reasons of such an improvement have not been elucidated from a kinetic point of view. On the other hand the standard simulation approaches of the PEMFC performance based on the empirical Butler-Volmer equations do not describe kinetics of such systems.

  In this thesis we present a multiscale theoretical methodology that scales up ab-initio calculated data into an elementary kinetic model in order to simulate the PEMFC transient behavior. Density functional theory (DFT) calculations have been performed to model and understand the formation of water and hydrogen peroxide on three different Pt<sub>3</sub>Ni(111) alloy surfaces in comparison with the Pt(111) reference catalyst. From DFT calculations, we show that the coverage of hydroxyl surface species can change the elementary reaction mechanism on pure Pt catalyst and that the reasons for the larger catalytic activity of the Pt<sub>3</sub>Ni alloys are related to the nature of the second metal, to the surface composition and to an optimal structural arrangement. By using the DFT results coming from the study of reaction pathways, we have calculated the rate constants of the main elementary steps of the reaction mechanism and we have built up a kinetic model. These parameters have then been implemented into a mean field interfacial model describing the behavior of the electric field and the charge distribution at the nanoscale, which is in turn coupled with microscale level models describing the charge and reactants transport phenomena across the cathode. The impact of different ORR mechanisms on the calculated i-V (currenty-voltage) curves is finally investigated and discussed, in comparison with experimental data.


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