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Charles W. Tobias Young Investigator Award Lecture


Mechanochemistry at Oxide Thin Film Interfaces

by Bilge Yildiz

Monday, October 8, 2012 | Honolulu, HI

Bilge Yildiz Improved quantitative understanding of how surface activity and charge transport kinetics are driven by the environment, including the mechanical state, is important both to fuel cell materials and to the dynamics of stress corrosion, where performance and stability depend on the state of solid state ionic films. In these systems, the mechanisms governing the interfacial activity are poorly understood, are challenging to probe due to harsh functional conditions, and sometimes require as long as years to evolve. Traditionally, electrochemical methods have been used to identify the surface reaction kinetics in fuel cell electrodes and corrosion kinetics on metals. These methods help deduce high-level kinetic parameters such as reaction constants and effective energy barriers, but involve little consideration of the underlying specific surface chemistry and atomic structure. However, it is now increasingly realized that the surface structure and chemistry govern the reaction and transport mechanisms and kinetics, and that they are not static - they dynamically respond to their surrounding harsh environments and age over extended periods. Many aspects of the bulk defect chemistry and transport properties are well-studied in solid state ionic materials, typically in the form of oxide films in fuel cells and corrosion. However, it is not fully understood how their surfaces are altered by temperature, reactive gases and mechanical stresses. The understanding and control of the surface reactivity of oxygen-electrode materials in particular is a key enabler for the efficiency and durability of solid oxide fuel and electrolysis cells at intermediate temperatures.

In this talk, Dr. Yildiz will discuss her group’s recent progress in the mechanistic understanding of the collective response of such surfaces in harsh environments on the basis of elementary processes, and of how mechanical stimuli may accelerate or suppress the governing kinetics, using in situ surface probes and computational theory. Specifically, Prof. Yildiz will present how elevated temperatures and strain states alter the electronic structure and cation chemistry on transition metal oxide surfaces, how strain state accelerates ionic diffusion, and how dissimilar oxide interfaces couple electronically to enhance surface activity.

Bilge Yildiz is an associate professor in the Nuclear Science and Engineering Department at Massachusetts Institute of Technology (MIT). The aim of Yildiz’s research is to advance the quantitative understanding of how surface activity and charge transport kinetics are driven by dynamic harsh environments, and to apply this knowledge to enable the design of novel surface chemistries for highly efficient solid oxide fuel/electrolysis cells and for corrosion-resistant materials. Yildiz’s research builds equally on experimental and computational techniques at comparable length and time scales. She and her group have developed a unique capability to probe the surface electronic state with high spatial resolution in situ at elevated temperatures, in reactive gas conditions and with induced stresses, using scanning tunneling microscopy and spectroscopy. Her research has demonstrated and explained how elevated temperatures and material strain state alter the surface cation chemistry and electronic structure on transition metal oxide surfaces. Her group has quantitatively elucidated the mechanisms by which the lattice strain facilitates oxygen ion diffusion in fluorite and perovskite oxides, and favors oxygen chemisorption and vacancy formation on perovskites. These findings are important for accelerating oxygen transport, oxygen reduction and water splitting kinetics on novel electrolyte and cathode structures made of ionic materials, as well as for suppressing corrosion kinetics. Dr. Yildiz and her group also work on capturing computationally the evolution of defect structures at the atomic level over experimental time scales, an important new capability to predict the aging of material microstructure both in high temperature fuel cells and in corrosion.

Professor Yildiz received her PhD in nuclear science and engineering at MIT (U.S., 2003), and her BSc in Nuclear Energy Engineering at Hacettepe University in Turkey (1999). After working as a postdoctoral researcher at MIT (2003-2004) and research staff at Argonne National Laboratory (ANL) (2004-2007), she returned to MIT as an assistant professor in 2007. Her teaching and research efforts have been recognized by the Outstanding Teaching (2008, 2002), the NSF CAREER (2011) and the ANL Pace Setter (2006) Awards, and the Norman C. Rasmussen Career Development Professorship (2010-2012).

 

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