2021 MRS Spring Meeting

2021 MRS Communications Lecture

Monday, April 19
3:00 pm – 4:00 pm ET


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Sossina Haile

Sossina M. Haile, Northwestern University
Untangling the Surface Chemistry and Reactivity of Ceria

It is well-established that the surface of ceria (doped or undoped) is substantially more reduced than the bulk, a feature that has been identified as critical to the catalytic activity of these materials. In parallel, strong evidence has emerged that the activity of different facets of ceria can differ substantially, which has in turn, led to an explosion in efforts to control catalyst particle morphology and thus exploit the facet-dependent activity. Surprisingly, however, direct measurements of the extent of reduction at the ceria surface are lacking. We address this gap using angle-resolved X-ray Absorption Near Edge Spectroscopy (XANES) to quantify, under technologically relevant conditions, the Ce3+ concentation in the surface (2-3 nm) and bulk regions of ceria-zirconia single-crystal films. In all circumstances, we observe substantial Ce3+ enrichment at the surface. For example, in CeO2-d, at 1100 °C and » 3 ´ 10-6 atm oxygen partial pressure, at which bulk reduction is negligible, the surface Ce3+ concentration is an astonishing 62%. Even more remarkable are the trends with facet and Zr concentration. In stark contrast to computational investigations which suggest, on the basis of oxygen vacancy formation energies, that vacancy concentrations increase in the order (111) < (001) < (110), we find a negligible dependence on facet. Furthermore, in contrast to bulk behavior, which shows an Ce reduction with increasing Zr concentration, the fraction of surface Ce ions that are reduced is unchanged with cation composition. In the absence of other charge compensating defects, this implies a decrease in the surface oxygen vacancy concentration as the Zr content increases. In turn, this trend suggests that high Zr concentration, which is known to enhance thermal stability and bulk oxygen storage capacity, may be detrimental in terms of area-specific catalytic reaction rates. Preliminary kinetic data, obtained using conductivity relaxation methods, support this conclusion.

About Sossina M. Haile

Sossina M. Haile is the Walter P. Murphy Professor of Materials Science and Engineering at Northwestern University, a position she assumed in 2015 after serving 18 years on the faculty at the California Institute of Technology. She earned her PhD degree in materials science and engineering from the Massachusetts Institute of Technology in 1992 and spent two years, 1991-1993, at the Max Planck Institute for Solid State Research in Stuttgart, Germany, first as a Fulbright Fellow, then as a Humboldt Fellow. Haile’s research broadly encompasses materials, especially oxides, for sustainable electrochemical energy technologies. Her work in fuel cell science and technology has pushed the field to new insights and record performance metrics. In parallel, she has created new avenues for harnessing sunlight to meet rising energy demands. 

Haile has published approximately 200 articles and holds 14 patents on these and other topics. Amongst her many awards, in 2008 Haile received an American Competitiveness and Innovation Fellowship from the U.S. National Science Foundation in recognition of “her timely and transformative research in the energy field and her dedication to inclusive mentoring, education and outreach across many levels.” In 2010, she was the recipient of the Chemical Pioneer Award (American Institute of Chemists); in 2012, the International Ceramics Prize (World Academy of Ceramics); and in 2020, the David Turnbull Lectureship (Materials Research Society). She is a fellow of the Materials Research Society, the American Ceramics Society, the African Academy of Sciences, and the Ethiopian Academy of Sciences, and serves on the editorial boards of Materials Horizons, Annual Review of Materials Research and Joule. Her professional service includes past membership on the board of the Materials Research Society, and current membership on the board of Ethiopia Education Initiatives.

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