Nonlinear Optical Studies of High Temperature Surface Chemistry in Energy Conversion Systems

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With support from the Chemical Measurement and Imaging Program in the Division of Chemistry, Professor Walker at Montana State University is developing new spectroscopic methods capable of examining chemical processes that occur on surfaces of fuel cell systems, which are relevant to energy storage and conversion technologies. Processes such as coal gasification, steam reforming and solar-thermal energy production require temperatures as high as 1000 C. At these high temperatures, chemical degradation can happen at an accelerated rate, leading to decreased efficiencies and, ultimately, process failure. Understanding the mechanisms responsible for sustainable operation as well as those that compromise performance is critical for designing new, durable chemical systems capable of electrochemical power generation. Professor Walker and his students are working to expand the use of state of the art spectroscopy techniques to identify and quantify the chemical reactions responsible for energy conversion processes. In addition to providing interdisciplinary research experiences to both graduate and undergraduate students involved in the project, Professor Walker actively reaches out to students from underrepresented minority groups through the Montana Apprenticeship Program (MAP) and American Indian Research Opportunities (AIRO) Office on campus and works with student volunteers to help organize and judge elementary school science fairs throughout the Bozeman Public School District.In this project, Professor Walker employs second-order nonlinear optical spectroscopy to study phase changes and surface chemistry for complex, mixed valent metal oxides commonly used in thermal solar and solid oxide fuel cell applications. Specifically, resonance enhanced second harmonic generation (SHG) is used to examine the surface properties of ceria, yttria, nickel oxide and other materials at temperatures above 600 C and under strongly oxidizing or reducing atmospheres. SHG is a well suited for stand-off, noninvasive studies of materials in chemically aggressive environments. SHG signal can arise from bulk materials that lack inversion symmetry but the surface contribution can still be extracted through analysis of the signal's phase and wavelength dependent resonance behavior. Resonance enhancement ensures the chemical (or material) selectivity necessary for answering specific questions about oxidation states, phase formation, and mass transfer. These studies could significantly expand the window of chemical systems capable of being studied with surface specific, second-order nonlinear optical spectroscopy. The goals of this work are two-fold: 1) developing and applying second-order NLO techniques to study high temperature surface chemistry, and 2) testing proposed mechanisms that describe material segregation and new phase formation with a particular emphasis on reversible and irreversible changes in electrochemically active ceramic electrodes. Professor Walker's research program actively supports aspiring scientists from underrepresented groups in science. He engages students from underrepresented minority groups in his research projects through the Montana Apprenticeship Program (MAP) and American Indian Research Opportunities (AIRO) Office on campus. His outreach activities are structured to provide a platform for his students to develop strong communication skills and mentoring abilities.