Watching Catalysts Evolve in 3-D

The Science

In batteries and fuel cells, certain materials offer needed electrons. Observing and measuring chemical changes in these materials, or catalysts, at the atomic level during oxidation (electron loss) has long been a challenge. Scientists obtained this view of a nickel-cobalt catalyst. They did so by combining multiple advanced electron microscopy techniques. Their work revealed the surprising and complex ways that the tiny catalytic particles lost electrons. They watched the changes the particles undergo on both exterior and interior surfaces.

The Impact

Bimetallic (two metal) catalysts are widely employed in batteries and fuel cells. Thus, a full understanding of how they provide electrons is both fundamentally important and of broad technological relevance. Understanding the ways catalysts offer electrons is key to designing these materials for higher performance. For example, detailed images of the catalyst working in real time reveal how the particle’s shape changes as it reacts chemically under an oxygen environment. This oxidation controls how the material morphs into the resulting hollow structure.

Summary

Because bimetallic catalysts are widely employed in batteries and fuel cells, a full understanding of their oxidation (electron loss) mechanisms is both fundamentally important and of broad technological relevance. To interrogate the structural and chemical changes during oxidation, scientists imaged single Ni₂Co catalysts in three dimensions using mass-contrast and chemical sensitive electron tomography. The electron tomography technique retains 3-D information about spatial chemical segregation that is lost in 2-D projected images typically obtained in electron microscopy. Moreover, state-of-the-art environmental transmission electron microscopy allowed the scientists to track the catalyst oxidation in situ and in real time. Combining real-time imaging and electron tomography, this study revealed oxidation-induced chemical segregation on both the external and internal surfaces of the catalyst, and elucidated pathways that control and alter the morphology of hollow-structured metal oxides.

Funding

This research used resources of the Center for Functional Nanomaterials, which is a U.S. Department of Energy (DOE) Office of Science user facility, at Brookhaven National Laboratory under contract DE-SC0012704. One of the authors (L.H.) was supported by a scholarship from the China Scholarship Council. Two other authors (Q.M. and Y.Z.) were supported by the DOE, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.

Publications

L. Han, Q. Meng, D. Wang, Y. Zhu, J. Wang, X. Du, E.A. Stach, and H.L. Xin, “Interrogation of bimetallic particle oxidation in three dimensions at the nanoscale.” Nature Communications 7, 13335 (2016). [DOI: 10.1038/ncomms13335]