Insights into spinel cobalt oxides may lead to efficient ammonia synthesis
image:
eNO₃RR performance of Co₃O₄ {100}, {111}, {110}, and {112} catalysts. ©Hao Li et al.
Credit: Hao Li et al.
Researchers have made a significant breakthrough in the development of catalysts for the electrochemical nitrate reduction reaction (eNO₃RR) to ammonia, a process that has broad implications for sustainable energy, agriculture, and industrial applications.
Ammonia, a critical component in global food production, also holds promise as a zero-carbon fuel due to its high energy density, clean combustion products, and established infrastructure for storage and transportation. However, the current method of producing ammonia, the Haber-Bosch process, is energy-intensive and accounts for about 1.8% of global CO₂ emissions.
In their recent study, the research team focused on spinel cobalt oxides (Co₃O₄), a promising class of catalysts for eNO₃RR due to their low cost, high activity, and selectivity. The team synthesized various Co₃O₄ nanostructures with different crystallographic facets - {100}, {111}, {110}, and {112} - to investigate how these facets influence the catalyst's performance in ammonia production. The study revealed that the {111} facet of Co₃O₄ exhibited superior performance, achieving an impressive ammonia Faradaic efficiency of 99.1% and a yield rate of 35.2 mg h−¹ cm−².
"Our findings show that the {111} facet of Co₃O₄ is effective in transforming nitrate to ammonia," said Dr. Heng Liu, the co-first author of the paper and a Specially Appointed Assistant Professor at the Advanced Institute for Materials Research (WPI-AIMR), Tohoku University. "This is due to the rapid formation of oxygen vacancies and Co(OH)₂ on this facet, which significantly enhances the catalyst's performance."
In addition, the researchers discovered that the catalyst went through a transformation process during the reaction, evolving from Co₃O₄ to a structure with oxygen vacancies, then to a Co₃O₄−x-Ov/Co(OH)₂ hybrid, and finally stabilizing as Co(OH)₂. This process was most pronounced on the {111} facet, contributing to its superior performance.
"The structural changes we observed are crucial for understanding the catalyst's activity," added Professor Hao Li, corresponding author of the paper and an associate professor at WPI-AIMR. "These insights will help us design more efficient catalysts by optimizing the exposed facets."
Ammonia's importance extends beyond agriculture, as it is a potential zero-carbon fuel and a key player in energy conversion and storage technologies. The eNO₃RR offers a sustainable alternative to the Haber-Bosch process, transforming nitrate waste into valuable ammonia while aiding environmental remediation.
"This research lays a solid foundation for the development of more efficient, sustainable catalysts," states Li. "As we move forward, our goal is to control the final phases of the catalyst's transformation to further enhance its activity, selectivity, and stability."
This breakthrough in understanding and optimizing Co₃O₄ catalysts could pave the way for cleaner and more sustainable industrial processes, contributing to the global efforts to achieve carbon neutrality by the 2050s.
About the World Premier International Research Center Initiative (WPI)
The WPI program was launched in 2007 by Japan's Ministry of Education, Culture, Sports, Science and Technology (MEXT) to foster globally visible research centers boasting the highest standards and outstanding research environments. Numbering more than a dozen and operating at institutions throughout the country, these centers are given a high degree of autonomy, allowing them to engage in innovative modes of management and research. The program is administered by the Japan Society for the Promotion of Science (JSPS).
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Advanced Institute for Materials Research (AIMR)
Tohoku University
Establishing a World-Leading Research Center for Materials Science
AIMR aims to contribute to society through its actions as a world-leading research center for materials science and push the boundaries of research frontiers. To this end, the institute gathers excellent researchers in the fields of physics, chemistry, materials science, engineering, and mathematics and provides a world-class research environment.
Structural characterization of Co₃O₄ nanostructure with different facets.
Theoretical calculations: 1D surface Pourbaix diagrams for Co₃O₄ (100), (110), and (112) surfaces, and reaction free energy diagrams for various intermediates on Co₃O₄ and Co(OH)₂ surfaces during eNO₃RR. Insets show charge density differences and N=O bond lengths.
Credit
Hao Li et al.
Journal
ACS Nano
Article Title
Facet-Dependent Evolution of Active Components on Spinel Co3O4 for Electrochemical Ammonia Synthesis
Article Publication Date
6-Aug-2024
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