Structure, activity, and stability of platinum alloys as catalysts for the oxygen reduction reaction

Ulrik Grønbjerg Vej-Hansen

Research output: Book/ReportPh.D. thesis

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Abstract

In this thesis I present our work on theoretical modelling of platinum alloys as catalysts for the Oxygen Reduction Reaction (ORR). The losses associated with the kinetics of the ORR is the main bottleneck in low-temperature fuel cells for transport applications, and more active catalysts are essential for wide-spread use of this technology. platinum alloys have shown great promise as more active catalysts, which are still stable under reaction conditions.
We have investigated these systems on multiple scales, using either Density Functional Theory (DFT) or Effective Medium Theory (EMT), depending on the length and time scales involved.
Using DFT, we show how diffusion barriers in transition metal alloys in the L12
structure depend on the alloying energy, supporting the assumption that an intrinsically more stable alloy is also more stable towards diffusion-related degradation and dealloying due to kinetic barriers, despite the thermodynamic driving force for dissolution.
This is followed by our results on trying to decouple the strain and ligand effects
for platinum skin structures, and determining whether there is any correlation between adsorption energy and surface stability in these systems. We find that there is such a correlation for some adsorbates, indicating that there exists a limit for the stability of an overlayer for a given adsorption strength.
Finally, we introduce our work on platinum alloy nanoparticles, and our attempt to
isolate the features which result in the increased activity that has been seen experimentally. We show how the platinum-platinum distance at the surface is decreased for a variety of alloy phases in the core, with greater compression of the overlayer for core phases with lattice parameters which are either much smaller or much larger than pure platinum.
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Number of pages150
Publication statusPublished - 2015

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