Molecular simulations of the oxidation of platinum-based alloy catalysts for fuel cells
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abstract
One of the most important challenges in low-temperature fuel cell technology is improving the catalytic efficiency at the electrode-catalyst where the oxygen reduction reaction takes place. Platinum is the most popular catalyst for this reaction but its high cost and scarcity hinder the commercial implementation of fuel cells in automobiles. Pt-based alloys are promising alternatives to substitute platinum while maintaining the efficiency and life-time of the pure catalyst. However, the acid medium and the oxidation of the surface impact the activity and durability of the alloy catalyst through changes in its local composition. This study applies molecular simulation techniques to characterize the dynamic evolution of the surface composition of platinum-based alloys under reaction conditions. We investigate the complex surface dynamics occurring in the near-surface layers influenced by oxygen and alloy atoms which yields to the dissolution of the catalyst. Our simulation scheme of the surface oxidation of a supported nanoparticle combines classical molecular dynamics (MD) and density functional theory (DFT) simulations. The electrostatic interactions between the adsorbates and the two topmost layers as a function of the coverage of oxygen are modeled based on the results of the Bader charge analysis from DFT calculations. This approach is able to reproduce the main features of the oxidation phenomena observed experimentally. In addition, we implemented a coarse-grained 3-D Kinetic Monte Carlo method which utilizes thermodynamic and kinetic information obtained from DFT simulations to study the surface segregation of alloy atoms in presence of adsorbed oxygen, such phenomenon causes changes in the local composition of the catalyst affecting its performance under operating conditions.
