Computational Studies of Non-Precious Catalysts for the Oxygen Reduction Reaction

Mateusz Krzysztof Reda

Research output: Book/ReportPh.D. thesis

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Proton Exchange Membrane Fuel Cells (PEMFCs) are electrochemical devices capable of direct conversion of chemical energy into electricity. They are a promising alternative power supply for future automobile applications, with advantages such as higher energy conversion e_ciency and lower emission of pollutants. A few problems stand in the way to the commercialization of the PEMFC technology. Among them, the slow kinetics of the cathodic Oxygen Reduction Reaction (ORR) is arguably the biggest. 

Currently, the best known ORR catalysts are based on rare and expensive platinum, which price contributes signi_cantly to the overall cost of the fuel cell stack. There are two main strategies to solve this problem: (i) decrease the platinum content, at the same time maximizing the catalyst activity, e.g., by alloying Pt with other metals; (ii) use a non-precious catalyst made of earth-abundant elements. In this thesis, the second possibility is investigated computationally using Density Functional Theory. 

Three main groups of non-precious ORR catalysts are studied: N-doped graphene (NG), iron carbide-supported NG (Fe3C/NG) and a catalyst containing porphyrin-like FeN4 moiety embedded in the graphene structure (Fe-N-C). First, solvation and spectator e_ects, immanently present in the catalytic environment in NG, are studied. It is shown that including explicit water solvation and bound atomic oxygen spectators is essential for the correct description of the oxygen reduction reaction on NG, and then also on Fe3C/NG catalyst.

The modeled catalytic activity of the Fe3C/NG heterostructure towards oxygen reduction reaction is found superior to that of the iron-supported NG (Fe/NG) and iron carbide-supported graphene (Fe3C/G). This is consistent with experimental evidence in the literature. Di_erences between Fe and Fe3C supports are shown to result from their electron-donating properties. Heterostructures comprising supports with electron-donating properties between Fe and Fe3C are predicted to reach or exceed the Pt(111) surface activity. 

Finally, the experimentally observed enhancement of the Fe-N-C catalyst activity in phosphoric acid solutions is shown to result from the phosphate anion adsorption on one side of the FeN4 moiety. Owing to the fact that Fe-N-C catalyst is a 2-dimensional material, the other side of the FeN4 moiety is still available for O2 adsorption. Phosphate ligand is found to slightly modify binding energies of the ORR intermediates, promoting the oxygen reduction reaction. This opens a new possibility of tuning the catalytic activity by manipulating the electrolyte composition.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages135
ISBN (Print)978-87-92986-74-0
Publication statusPublished - 2018


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