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First-Principles Modeling of Ni4M (M = Co, Fe, and Mn) Alloys as Solid Oxide Fuel Cell Anode Catalyst for Methane Reforming

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journal contribution
posted on 14.01.2016, 00:00 by Ho-Cheng Tsai, Sergey I. Morozov, Ted H. Yu, Boris V. Merinov, William A. Goddard
In this study, we used quantum mechanics (QM) to investigate steam reforming of methane on Ni-alloy catalyst surfaces and to examine the effect of anode material modifications on the catalytic processes in a solid oxide fuel cell (SOFC). The conventional Ni anode suffers from coking, coarsening, and sulfur poisoning because of the decomposition of hydrocarbon fuels, Ni particle agglomeration at high operating temperature, and impurities contained in fuels. Ni-electrode surface modification, such as alloying Ni with other metals (e.g., Fe and Cu), is probably the most practical and promising way of developing SOFC anodes tolerant to coking and sulfur poisoning. According to experimental data, Ni4Fe shows a good catalytic performance and excellent long-term stability as an SOFC anode catalyst. We have performed QM calculations of segregation energy for various surface structures of five-layer Ni4M slabs (M = Co, Fe, and Mn) and found that Ni atoms show segregation preference for the surface layer and the most favorable Ni4M surface structure has two M atoms in the 2nd layer and one M atom in the 3rd and in the 4th layer (the numbering starts from the bottom layer). This structure was used for our further QM calculations of binding energies for CHx, C, and H. We find that the Ni4M­(111) surfaces bind CHx species weaker (by 1–10 kcal/mol) than pure Ni, and the binding energy of C is always ∼10 kcal/mol lower for the Ni4M alloys compared to pure Ni. This is consistent with improved catalytic characteristics of certain Ni-based alloys compared to pure Ni obtained in experiment. Reaction energy barriers for methane decomposition on the Ni4M­(111) catalyst surfaces were calculated as well. On the basis of these results, the rate-determining step for the methane decomposition was found to be the CH → C + H reaction. Our results predict that Ni4Fe and Ni4Mn have both better activity and better coking resistance and can be considered as candidates for an SOFC anode catalyst suitable for the CH4 fuel reforming.