New Core-shell Nanocatalyst Design for Oxygen Reduction Reaction: A Theoretical Study Based on Density Functional Theory

一個基於密度泛函的理論研究:設計用於氧還反應的核殼納米催化劑

Student thesis: Doctoral Thesis

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Award date11 Dec 2018

Abstract

High-cost of oxygen reduction reaction (ORR) catalysts is the most critical hurdle in fuel cell commercialization. Development of ORR catalysts with low noble-metal loading, but high activity and stability is the rare feat that researches currently pursue. Progress towards this goal has been made by thorough experiments with tremendous efforts. Herein, this study introduces theoretical approach for designing a desirable catalysts configuration based on density functional theory (DFT) calculations, in order to give guidance for choosing materials and better position experiments toward success. A series of proposed catalyst configurations in this thesis are low-cost, more-active compared with existing one, and also highly possible to be fabricated experimentally, which were inspired by our previous experimental observation. Particular emphasis is placed onto the unique catalyst structure design, Pt cluster decorated core-shell structure (transition metal as core, Pd as shell), and this design concept could be used to extend one transition metal to the other.

First, accurate DFT calculations of oxygen adsorption properties were conducted and the physical quantity, like lattice strain or d-band center, determining the reactivity of proposed catalysts surface were discerned, which provide opportunity to fake pure-Pt catalyst with alternatives. Modifying their electron structure by tuning the composition scaling relation of the surface to get favorable adsorption property was proved and the trend was found that decrease the Pd-shell thickness of our core-shell structure, the property of the surface is more close to pure Pt catalyst. When the Pd-shell thickness decrease to just one atomic layer, the prediction was made that the performance would be better than pure Pt catalyst, and the noble-metal content is also hugely saved.
Then, the reaction barrier of ORR on proposed catalysts was also calculated, surface composition of the catalysts were further tuned through the reaction pathway. This new atomistic perspective rooted in theoretical solid states physics opens the door for screening of catalytic materials for desired properties and provide reliable guidance for catalyst experimental synthesis.

In conclusion, this thesis reports heterogeneous catalysts design for oxygen reduction reaction (ORR) in fuel cells based on density functional theory calculations in the perspectives of atomic oxygen adsorption properties and d-band model. It note that loading small amount of Pt, i.e. Pt3 cluster, on the bimetallic model catalysts Pd/Co(111) and Pd/Ni(111) can evidently reduce oxygen binding energies to the surfaces, indicating oxidation and inactivation of catalysts can be manifestly avoided. Particularly, the optimum oxygen adsorption energy and the most uniform distribution of oxygen binding strength to the surface is found when Pt3 decorated on only monolayer of Pd(111) atop either Co or Ni(111) support among all of our models. Their ORR performance is verified by d-band model transition state calculations and the outcomes show that the ORR activity will be as high as the pure Pt catalysts.