Electro-chemo-mechanically active oxides enable a wide array of energy, sensing, and electronic applications, but limitations in their charge transport, surface reactivity, and chemical expansion hinder efficiency and durability. A fundamental understanding of these point defect-mediated properties needs to be developed in order to enable rapid, rational design of optimized solid state ionics for improved device performance. Devices impacted by our current projects include: all-solid-state batteries, solid oxide fuel cells, electrolyzers for, e.g., green H2 production (PEM-based, protonic ceramic electrolysis cells, and solid oxide electrolysis cells), and chemical/electro-chemical actuators. At a fundamental level, our research explores several thematic areas:
1. Point defect states away from conventional dilute, bulk equilibrium exhibit promise to deliver unprecedented performance; therefore, we seek rational solid solution, non-equilibrium, and interface-dominated approaches to monitor, control, and design these atomic-scale states. The interplay of these point defects with dynamic crystal-, micro-, and macro-scale structure and corresponding properties is a further interest.
2. Coupled behavior underlies device performance and enables development of new techniques and applications for extreme conditions. We actively investigate and tailor coupled behavior in the opto-electro-chemo-mechanical space. For example, there is benefit in moving beyond conventional electro-chemical studies to consider the coupling between electrical, chemical, and mechanical states of materials, such as the lattice strain occurring upon non-stoichiometric composition changes during operation (chemical expansion) or the impact of strain on ionic mobility and charge carrier concentrations. We also explore and leverage the relationship between point defects and optical absorption.
3. Data-driven approaches and inverse design methods can enable rapid discovery of new materials with target properties. We develop high-throughput experimental methods, descriptor-based search strategies, and design principles to identify promising compositions. To uncover design principles, we often work with well-defined model systems and carry out in situ, simultaneous characterization with various multi-scale techniques in controlled temperature, atmosphere, and electric field environments. Our most recent work has been focusing on development of design principles for fast oxygen surface exchange kinetics, tailored grain boundary ionic/electronic transport, pure solid state cation conduction, and minimized chemical expansion in perovskite-structured ceramics that “breathe.”
Properties and Methods
Surface Exchange Kinetics
Kinetics of surface reactions, such as oxygen reduction or hydrogen oxidation, often dictate energy conversion device efficiency. Surface structures and chemistries that are highly active for these reactions but also stable for long-term operation are needed. We measure surface oxygen exchange kinetics on model thin films fabricated by pulsed laser deposition, using simultaneous in situ ac-impedance spectroscopy and a novel optical transmission relaxation technique in controlled gas and temperature environments. The optical approach is unique in offering continuous, non-contact evaluation of the native surface behavior. We have also been developing a fundamental understanding of the relationship between optical absorption, electronic structure, and point defect chemistry in non-dilute solid solutions in collaboration with the Tuller group at MIT and the Ertekin group at UIUC to provide a quantitative and rational basis for the optical approach. Regarding the surface exchange kinetics, controlled variation of overall film defect chemistry, outermost surface chemistry, orientation, and microstructure has enabled a better understanding of the relative importance of each. We have also found extremely rapid surface exchange kinetics for films fabricated by an unconventional low temperature, chemo-mechanically actuated route and are now studying the structural evolution of these materials.
Oxides that change their anion or cation stoichiometry undergo strain. During processing or operation the associated chemical stresses can cause mechanical failure. On the other hand, this coupling is also at the heart of many emerging characterization techniques. We seek to control its magnitude by developing a fundamental understanding across many length scales. We study chemical expansion behavior using in situ X-ray and neutron diffraction, thermogravimetric analysis, and dilatometry, with comparison to atomistic computational simulations by the Aluru and Ertekin groups at UIUC. Such studies are enabling identification of structural and operational factors that can be applied to tailor chemical expansion.
Ionic & Electronic Transport
Ionic conductivity impacts ohmic efficiency losses, response times, and switching speeds in solid state ionic devices (fuel cells, batteries, memristors). Electronic conductivity with optical transparency is needed for high performance photovoltaic cells and transparent electronics. Fundamental processing – defect chemistry – conductivity relationships and mechanisms need to be established. We apply ac-impedance spectroscopy, equivalent circuit analysis, and microstructure models like the “nano-Grain-Composite Model” to evaluate and separate local ionic and electronic transport and dielectric behavior in bulk ceramic, thin film, heterostructured, and nanostructured materials. We perform these measurements as a function of temperature, gas environment, and dc bias and in combination with optical, capacitive, and thermogravimetric studies of stoichiometry, to investigate bulk vs. interfacial point defect behavior.