Browsing by Subject "fuel cells"
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Item Surface Oxidation and Dissolution of Metal Nanocatalysts in Acid Medium(2012-10-19) Callejas-Tovar, JuanOne 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 (ORR) occurs. Platinum is the best pure 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 reduce the activity and durability of the alloy catalyst through changes in its local composition and structure. Molecular simulation techniques are applied to characterize the thermodynamics and dynamic evolution of the surface of platinum-based alloy catalysts under reaction conditions.1-10 A simulation scheme of the surface oxidation is proposed which combines classical molecular dynamics (MD) and density functional theory (DFT). This approach is able to reproduce the main features of the oxidation phenomena observed experimentally, it is concluded that the dissolution mechanism of metal atoms involves: 1) Surface segregation of alloy atoms, 2) oxygen absorption into the subsurface of the catalyst, and 3) metal detachment through the interaction with ions in the solvent. Therefore, to improve the durability of platinum-based alloy catalysts, the steps of the dissolution mechanism must be prevented. A versatile 3-D kinetic Monte Carlo (KMC) code is developed to study the degradation and dealloying in nanocatalysts. The results on the degradation of Pt nanoparticles under different potential regimes demonstrate that the dissolution depends on the potential path to which the nanocatalyst is exposed. Metal atoms detach from the boundaries of (111) facets expecting a reduction in the activity of the nanoparticle. Also, the formation of Pt hollow nanoparticles by the Kirkendall effect is addressed, the role of vacancies is crucial in the removal of the non-noble core that yields to hollow nanoparticles. To investigate the reasons for the experimentally found enhanced ORR activity in porous/hollow nanoparticles, the effect of subsurface vacancies on the main ORR activity descriptors is studied with DFT. It is found that an optimum amount of vacancies may enhance the ORR activity of Pt-monolayer catalysts over certain alloy cores by changing the binding energies of O and OH.Item Theoretical studies of transition metal surfaces as electrocatalysts for oxygen electroreduction(Texas A&M University, 2007-09-17) Lamas, Eduardo J.In the last few years the quest towards a hydrogen based economy has intensified interest for effective and less expensive catalysts for fuel cell applications. Due to its slow kinetics, alternative catalysts for the oxygen electroreduction reaction are actively researched. Platinum alloys with different transition metals (for example: Ni, Co and Fe) have shown improved activity over pure Pt. The design of a Pt-free catalysts is also highly desirable, and different alternatives including metalloporphyrins and Pd-based catalysts are being researched. Pd-based catalysts constitute an attractive alternative to Pt alloys in some fuel cell applications, not only because of lower costs but also because of the lower reactivity of Pt alloys towards methanol, which is important for improved methanol crossover tolerance on direct methanol fuel cells. In this work we apply density functional theory (DFT) to the study of four catalysts for oxygen electroreduction. The electronic structure of these surfaces is characterized together with their surface reconstruction properties and their interactions with oxygen electroreduction intermediates both in presence and absence of water. The energetics obtained for the intermediates is combined with entropy data from thermodynamic tables to generate free energy profiles for two representative reaction mechanisms where the cell potential is included as a variable. The study of the barriers in these profiles points to the elementary steps in the reaction mechanisms that are likely to be rate-determining. The highest barrier in the series pathway is located at the first proton and charge transfer on all four catalytic surfaces. This is in good agreement with observed rate laws for this reaction. The instability of hydrogen peroxide on all surfaces, especially compared with the relatively higher stability of other intermediates, strongly points at this intermediate as the most likely point where the oxygen bond is broken during oxygen reduction. This adds to the argument that this path might be active during oxygen electroreduction. A better understanding behind the reaction mechanism and reactivities on these representative surfaces will help to find systematic ways of improvement of currently used catalysts in the oxygen electroreduction reaction.