Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 14, Issue 40, Pages 14010-14022Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c2cp40841d
Keywords
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Funding
- Danish Strategic Research Council's HyCycle program
- Danish Council for Technology and Innovation's FTP program
- European Commission [MRTN-CT-2006-032474]
- Danish Council for Strategic Research via SERC project [2104-06-011]
- Catalysis for Sustainable Energy (CASE) initiative
- IMI Program of the National Science Foundation [DMR 0843934]
- Center on Nanostructuring for Efficient Energy Conversion (CNEEC) at Stanford University, an Energy Frontier Research Center
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-SC0001060]
- Division Of Materials Research [0843934] Funding Source: National Science Foundation
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Progress in the field of electrocatalysis is often hampered by the difficulty in identifying the active site on an electrode surface. Herein we combine theoretical analysis and electrochemical methods to identify the active surfaces in a manganese oxide bi-functional catalyst for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). First, we electrochemically characterize the nanostructured alpha-Mn2O3 and find that it undergoes oxidation in two potential regions: initially, between 0.5 V and 0.8 V, a potential region relevant to the ORR and, subsequently, between 0.8 V and 1.0 V, a potential region between the ORR and the OER relevant conditions. Next, we perform density function theory (DFT) calculations to understand the changes in the MnOx surface as a function of potential and to elucidate reaction mechanisms that lead to high activities observed in the experiments. Using DFT, we construct surface Pourbaix and free energy diagrams of three different MnOx surfaces and identify 1/2 ML HO* covered Mn2O3 and O* covered MnO2, as the active surfaces for the ORR and the OER, respectively. Additionally, we find that the ORR occurs through an associative mechanism and that its overpotential is highly dependent on the stabilization of intermediates through hydrogen bonds with water molecules. We also determine that OER occurs through direct recombination mechanism and that its major source of overpotential is the scaling relationship between HOO* and HO* surface intermediates. Using a previously developed Sabatier model we show that the theoretical predictions of catalytic activities match the experimentally determined onset potentials for the ORR and the OER, both qualitatively and quantitatively. Consequently, the combination of first-principles theoretical analysis and experimental methods offers an understanding of manganese oxide oxygen electrocatalysis at the atomic level, achieving fundamental insight that can potentially be used to design and develop improved electrocatalysts for the ORR and the OER and other important reactions of technological interest.
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