Characterizing and Exploiting the Remarkable Surface Redox Chemistry of Ceria and Its Derivatives

Overview: Ceria and its solid solutions play a pivotal role in a wide range of industrial processes and devices. These include solid oxide fuel and electrolyzer cells, solar-driven thermochemical fuel production, chemical looping combustion, automotive 3-way catalysts, photodegradation of organic pollutants, memristors, supercapacitors, and recently, electrostrictive devices. As a mixed ion and electron conductor, ceria not only offers the requisite functionality for these application, but is also stable in the demanding environments of interest and furthermore rather plentiful in the earth’s crust. Years of study have revealed much about ceria and its derivatives. Remarkably, however, critical aspects of the defect chemistry, particularly at interfaces, remain unresolved. For example, several studies suggest the {001} faces of ceria should be the most active for catalyzing a range of reactions due to easy vacancy formation, yet direct measurements of the extent of surface reaction suggest insensitivity to facet across the series {001}, {110} and {111}. We propose to acquire fundamental knowledge about the redox chemistry of both internal and exposed ceria surfaces using well-defined structures that support exquisite studies by sophisticated characterization techniques. Efforts will include A.C. impedance spectroscopy of thin-film materials to determine surface activity as a function of termination, cation chemistry, and strain; angle-resolved X-ray adsorption spectroscopy studies of such films to determine both Ce oxidation state and local chemical environment about cation species; impedance studies of individually defined grain boundaries prepared by fusing film-on-substrate structures; and creation and manipulation of vertically aligned nanostructures, with potential memristive properties, via substrate modification. The ambitious research scope of the project is made possible by the extensive growth and characterization tools available through the PI laboratory and Northwestern University user facilities. Students will be engaged at all levels. Graduate students will participate in all aspects of this research, and will develop mentorship skills by advising undergraduates in summer research and senior thesis research. The PI laboratory will further host high school students to participate in laboratory activities through the Northwestern Academy, a collaboration between Northwestern University and Evanston Township High. Intellectual Merit: This work will generate fundamental insights into the interfacial redox/defect chemistry of ceria and its doped derivatives, at both internal and exposed interfaces. The open questions in this field have been rigorously identified, and the approaches for addressing them clearly identified. These include innovations in methods such as X-ray adsorption spectroscopy, and the careful design of well-defined structures. Ultimately, we will establish new avenues for manipulating the structure, chemistry, and properties of this important class of materials with implications for a wide range of technological devices. Broader Impacts: Despite decades of study, important unanswered questions regarding the redox and defect chemistry of ceria and its derivatives abound. Addressing these questions are the essential first step towards designing new devices and creating new functionalities. The comprehensive characterization proposed here, embodying specific hypotheses for answering specific open questions, forms the basis for future, hypothesis-driven efforts to build new technologies. Inclusion of stude