NEWS
By strategically introducing atomic flaws, researchers are transforming inefficiencies into breakthroughs in next-generation energy systems.
Written by Nicole Imeson
SURFACE DEFECTS HISTORICALLY REPRESENTED flaws or errors that required repair. But researchers have long been challenging that assumption.
Engineering specific surface flaws into renewable energy generation devices, such as fuel cells or photovoltaic cells, can dramatically increase their energy conversion efficiency (ECE), transforming material imperfections into assets and creating a new paradigm for energy efficiency.
This is according to new work being done by a collaboration between the University of Florida’s Center for Defect Engineering of Energy Materials (CDEEM) and the Australia Sustainable Energy Initiative Ltd., which together form the U.S. Australia Sustainable Energy Initiative (USASEE).
The intentional process of transforming flaws into functional sites transpires through a quasi-isolated surface structure (QISS). Researchers capitalized on a natural “inside-out” process that happened when material oxidizes, developing a surface layer with distinct properties. Rust has long suggested a one-way street where oxygen reacted with the material. Due to an imbalance of oxygen levels between the external gas and the internal material, oxygen pushes its way inward, attempting to balance the concentration. While this fast, inward flow of oxygen proceeds, a much slower, counter-directional process occurs simultaneously.
Every material harbors imperfection at the crystal level. These include oxygen vacancies (missing oxygen atoms) or iron interstitials (iron atoms in the wrong spot within the crystal structure). The difference in chemical environment between the external gas and internal material drove these defects to the surface. While the oxygen-in process takes minutes, the defect-out process takes more than 300 hours at high temperatures—1,140 K (1,590 °F)—to reach equilibrium. The slower, defect-out process ultimately defines the final properties of the material surface. Knowing how the QISS process develops allowed scientists to design and engineer the effectiveness of materials surfaces for critical applications.
A graphical illustration of defect reactions in titanium dioxide (TiO₂) leading to the formation of: (a) oxygen vacancies, (b) tetravalent Ti interstitials, and (c) Ti vacancies. (d) Electronic structure of TiO₂ showing the energy levels of intrinsic defects at different ionization states. Ec and Ev represent the conduction and valence band edges. (e) Schematic of the work function (Φ) based on the band model, where χ is the external work function, Φs is the surface-related component, Φin is the internal component, and Ef is the Fermi level. (f) Charge transfer between a donor species and a p-type semiconductor, where ΔH is the reaction enthalpy and μ_o is the ionization potential of the adsorbed species. (g) Adsorption of oxygen and segregation of extrinsic defect species in ionic solids.
Image: S.A. Sherif, University of Florida’s Center for Defect Engineering of Energy Materials
“The new surface structure drives reactivity. Using a defect related surface sensitive tool, like a high temperature electron probe, we can see and engineer what happens at the surface to produce the maximum amount of the desired effect,” explained S.A. Sherif, CDEEM director.
A High-Temperature Electron Probe (HTEP) revealed material behavior and chemical changes in-situ under extreme temperatures (above 1,000 °C), and while material remained hot and surrounded by special gases, replicating conditions in a fuel cell or solar device. The probe measured the work function, quantifying how easily electrons leave the material surface, with changes informing scientists about important surface occurrences.
“The HTEP establishes surface properties in situ conditions without cleaning or prepping the material surface,” Sherif explained.
The device also helped uncover the QISS’s defect-out component, allowing the team to observe how the defects behave when the material is heated or the surrounding gases change. This revealed precisely how hot to make the material and what gases to use to perfect the surface with the right number of defects—and ultimately how to design more efficient materials.
“Using a defect related surface sensitive tool, like a high temperature electron probe, we can see and engineer what happens at the surface to produce the maximum amount of the desired effect.”
—S.A Sherif, Director of the Center for Defect Engineering of Energy Materials at the University of Florida
Efficiency and Performance
Research showed that surface defect engineering using QISS yielded enhanced energy conversion efficiency (ECE) and stable performance. While quantifying an exact ECE increase across various materials and devices poses a challenge, studies suggested that the increase exceeded that of current materials by up to three times.
QISS improves efficiency by controlling chemical reactivity and the charge transfer process, as the surface defects determine the material’s ability to handle electrical charges efficiently. By using surface defect engineering to impose the optimal amount of disorder, researchers created a stable QISS that ensured enhanced ECE and long-term reproducible performance through fine-tuning both the overall electron activity and the specific sites where charge transfer occurs.
“We targeted a quantum leap in efficiency and performance. We could conceivably revolutionize the way we convert energy,” Sherif explained.
A solar photovoltaic panel can be used to generate electricity and electrolyze water into hydrogen and oxygen. Using a titanium dioxide surface, water molecules require specific locations, or “active sites,” on the surface to react. Titanium vacancies (missing titanium atoms) can establish crucial active sites as well. The titanium vacancy defect helps with the difficult reaction of splitting water.
By controlling the temperature and amount of available oxygen during material fabrication, researchers controlled the formation of defects. They also swapped titanium and oxygen atoms for a different element, forcing the crystal to create defects, maintain balance, and predictably tuning the material properties.
Intentional control over perceived flaws will ensure a stable, high-performance QISS, enhancing energy conversion efficiency and providing the long-term stability needed for a quantum leap forward in commercially successful, next-generation renewable energy technologies.
Nicole Imeson is an engineer and writer in Calgary, Alta.
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