Rodrigo FERREIRA DE MORAIS
– 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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