Graduate Thesis Or Dissertation
 

Design Principles of Transition Metal X-ide Catalysts for Electrochemical Oxygen Reduction and Evolution Reactions: In-Situ X-ray Spectroscopy Studies

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  • Hydrogen is the green fuel that can be produced from water through electrolysis and used via hydrogen fuel cells to generate electricity with zero carbon dioxide emission. The commercialization of electrolyzers and fuel cells is the critical step to achieving global carbon neutrality. However, it is severely hampered by the low efficiency in associated reactions, particularly the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) for the two devices, respectively. Highly efficient electrocatalysts are required to overcome the slow kinetics in the two reactions, but the state-of-the-art electrocatalysts are mostly based on noble metal or noble metal oxides such as Pt, IrO2, and RuO2. Therefore, many efforts have been devoted to finding cost-effective and highly active electrocatalysts for ORR and OER. The two most efficient methods are either maximizing noble metal’s activity (e.g., single atom catalysts) or replacing noble metal with low-cost transition metals. This Ph.D. thesis is focused on electrocatalysts containing transition metals, so-called transition metal X-ides (X = C, N, O, S, Se, etc.). Besides the electrochemical characterization of catalysts’ activity, selectivity and stability, we apply synchrotron X-ray absorption spectroscopy (XAS) at hard X-ray (incident X-ray energy higher than 5 keV) and soft X-ray (incident X-ray energy lower than 2 keV) regions to study the electrocatalysts’ chemical properties (oxidization state, electronic structure, and local structure), particularly in-situ/operando XAS to characterize the chemical properties change under the catalytic reaction conditions. Those findings revealed the structure-property and catalytic performance relationship. Two kinds of materials were chosen as the representative catalysts to build a systematic study. Perovskite LaCoO3, one of the transition metal oxides with moderate stability, has shown the approximate ORR and OER performance compared with standards. It was found that the electron number at the eg orbital can be the characteristic parameter to determine theactivity of this type of electrocatalyst. To verify and extend the theory, we substituted Co with Fe in LaCoO3 to tune the electronic structure, and subsequently changed not only the ORR activity but also selectivity. The soft XAS pointed out the metal-oxygen bonding hybridization and 3d transition metal electron distribution could further influence the catalytic performance in addition to metal d-band eg theory. Later, the LaCoO3 with co-substituted Fe and P was studied to confirm that the metal-oxygen bonding hybridization and electronic structure in the octahedral units would also influence the OER catalytic activity. In addition, the combination of hard and soft XAS further pointed out the hybridization between oxygen p-band and 3d transition metal eg orbit could be more critical than the general metal-oxygen hybridization for OER performance. The second material Co9S8, a representative material of transition metal X-ides that are unstable in reaction and experience restructuring throughout the reaction, has shown better OER performance than benchmark RuO2. The combination of operando hard XAS, operando Raman spectroscopy, and density of function theory suggested that the Co9S8 would completely restructure to form edge-sharing octahedral CoO6 units during OER, which is the truly active phase. The edge-sharing octahedral is hypothesized as the active center for restructuring-induced electrocatalysts, which was further verified by atomically dispersed Ir catalysts anchored on amorphous CoO for OER. The electronic structure of these catalyst materials can be altered to achieve the best OER activity by varying the composition of edge-sharing octahedral units. Through our comprehensive studies, we provide new approaches to develop high-performance ORR and OER electrocatalysts for accelerating the development of renewable energy systems.
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  • This work was financially supported by Oregon State University and National Science Foundation (CBET-2016192, CBET-1949870, and ECC-2025489). The soft X-ray absorption spectroscopy was performed at Advanced Light Source, which is an Office of Science User Facility operated for the U.S. DOE Office of Science by Lawrence Berkeley National Laboratory and supported by the DOE under Contract No. DEAC02-05CH11231. The synchrotron X-ray characterization is supported by the National Science Foundation (CBET-1949870 and DMR-1832803). The hard XAS measurements were done at Advanced Photon Source (APS) of Argonne National Laboratory (ANL) for synchrotron measurements is supported by Department of Energy under Contract No. DE-AC02-06CH11357.
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  • Pending Publication
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  • 2021-12-09 to 2023-01-09

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