For Bilge Yildiz, associate professor of nuclear science and engineering, and associate professor of materials science and engineering, unleashing novel properties in materials means taking a penetrating look at surfaces. Her work to understand and tailor surface chemistry sets the stage for a new generation of high-efficiency devices in both energy conversion and information processing.
“Much of what we do is basic science. We focus on fundamental problems related to surface behavior of materials in important technologies,” says Yildiz. “We are always motivated by improvements and new applications we can see coming out of the work.”
Since she joined the MIT faculty in 2007, Yildiz’s lab has produced a stream of notable discoveries. She points to one area of long-standing interest: designing surfaces for oxides that put these materials into play as active materials in highly efficient and durable fuel cells, electrolyzers, and thermochemical fuel synthesis.
The surfaces of these reactive oxides are important for the functionality of the material in thermochemical and electrochemical energy conversion. For example, these oxides are used for reducing oxygen and oxidizing hydrogen while producing electricity in solid oxide fuel cells, or for thermochemically splitting water and CO2 for conversion to synthetic liquid fuels.
But they also degrade in the presence of high temperatures and reactive gases. “If the surfaces are not ideal, more energy is required to drive reactions, making the process more costly,” Yildiz says. Yildiz’s research revealed that the surface forms an insulating layer. She got to the bottom of the problem: oxygen vacancies at the surface—a type of defect in the atomic lattice of an oxide—create an imbalance in charge and drive a chemical segregation at the surface. Based on that insight, she figured out a fix: “doping” the surface with metal cation additives that rebalance the surface charge. Ultimately, the approach could stabilize the material, and significantly reduce energy losses in surface reactions.
Her treated oxide surfaces are promising for making solid-oxide fuel cells and electrolyzers that are both more durable and substantially more efficient than the currently available devices. Although practical application is years away, Yildiz envisions installations of units slightly larger than air conditioning compressors “providing for all the electrical needs of a home.”
Drawing on advanced experimental and computational tools, in part of her own invention, Yildiz can probe and manipulate materials at the nanoscale. This has yielded a wealth of recent insights. For instance, she has discovered that nanoscale oxide thin films under mechanical strain could serve as conductivity and reactivity modulators in miniaturized fuel cells, which might one day replace lithium-ion batteries in mobile devices. Yildiz is also experimenting with using electric fields on thin films to serve as the brains for a new kind of digital memory circuitry. “These devices function at high electrochemical potentials at room temperature, and scale down in theory to the few-nanometer regime,” says Yildiz. “Because they switch state with very low voltages, they will also reduce energy consumption.”
Yildiz got her start as an undergraduate in Turkey studying nuclear energy. Today, her research also paves the way toward more durable materials for reactors, and could enhance other existing energy technologies as well, such as fossil fuel plants and offshore wind turbines facing the dual insults of salt and cold. “My dream outcome, together with colleagues at MIT, is to find materials that are corrosion- and hydrogen- and radiation-resistant, with good heat transfer properties, and that are safe,” she says.
Yildiz has a new home for her wide-ranging ambitions: the Center for Materials in Energy and Extreme Environments, one of the MIT Energy Initiative’s eight Low-Carbon Energy Centers announced in 2015 as part of MIT’s Plan for Action on Climate Change. Yildiz co-directs the center with Ju Li PhD ’00, professor of materials science and engineering.
“We want to provide industry with materials that enable new, clean energy technologies, like fuel cells, and that improve the performance and reduce the CO2 footprint of existing energy technologies,” Yildiz says. “The center brings together faculty, postdocs, and students motivated by energy questions, allowing us to pursue big problems and keep pushing boundaries forward.”