Materials by Design to Overcome the Second Grand Challenge
Emily Pentzer and Andriy Zakutayev
Designing new materials for specific applications is the Holy Grail of materials science. Historically, serendipitous discoveries of important materials for energy applications have been made by trial and error or by accident. Now materials scientists focus on novel approaches that follow the model "given the property, find the material," which is the inverse of the traditional model "given the material, find the property." Development and application of these novel approaches is the essence of the second grand challenge.
Powering the Earth. For energy applications, materials by design is truly a grand challenge because of the scale of this problem as well as the scale of our energy needs. The energy demand of the world is expected to double by 2050, requiring new materials that can generate more electricity from alternative or renewable resources, store it in a small volume, transport it with little loss and convert it into other useful forms of energy. Nowadays, it takes 20 years on average to discover a new material and bring it to the market, which is not fast enough to create a sustainable energy system. Scientists are accelerating this process by developing approaches to design, instead of discover, new materials with the desired properties.
Needle in a haystack. Designing materials with tailored properties is challenging because of the astronomical number of possible compounds, combinations of elements, structures and morphologies. "Designing new materials is like playing 'Quantum Jeopardy': given the answer, find the question out of an astronomical number of possibilities," said Alex Zunger, a professor at the University of Colorado working on inverse design.
After the best possible choice has been made, the material must be synthesized in an atom- and energy-efficient way, and scientists must further understand and improve the material's properties.
Turning materials design upside down. The Center for Inverse Design, or CID, is developing an approach that can theoretically predict and experimentally realize materials with target properties. This inverse design approach combines state-of-the-art targeted synthesis and characterization with modern high-throughout methods, such as supercomputer calculations and combinatorial experiments.
The center also applies its inverse design approach to solar energy conversion materials. For example, CID researchers have improved electrical conductivity of an oxide material by a factor of 10,000, and then used this conductive oxide as a contact for a prototype organic solar cell in a collaboration with the Center for Interface Science: Solar Electric Materials. The designed oxide material is remarkably robust: atomic disorder and deviation actually improve its electrical conductivity. As another example, more than 20 new potential solar cell absorbers have been predicted, and absorbers have already been synthesized.
Hitherto unknown crystal structures and the stability of some materials are not an obstacle for inverse design: recently the structures of 335 new stable inorganic compounds have been predicted among more than 40 structure prototypes in 917 candidate materials. Hundreds of thousands of possible geometrical configurations of atoms in a solid also can be screened for the target property. For instance, the CID has predicted specific Si-Ge nanostructures that strongly absorb light despite being made of two materials that weakly absorb light. The inverse design approach is meant to apply not only to solar energy conversion but also to other areas of basic energy sciences.
Energy from plastics. The Polymers for Harvesting Solar Energy, or PHaSE, EFRC focuses on using polymers, a.k.a. plastics, to collect the sun's energy, ultimately giving rise to solar cells that can be incorporated into windows, siding or even clothing. Center researchers are synergistically addressing this goal and improving the efficiency of photovoltaic solar cells. Projects move from the design and synthesis of novel materials that efficiently absorb sunlight and readily convert it into usable energy to their assembly into new architectures that further facilitate collection of energy and improve the lifetime of the devices, and finally to the correlation of solar cell performance with the identity and architecture of the materials, giving optimized device performance.
For plastic solar cells to work efficiently, two materials need to be blended together to allow the efficient absorption and collection of solar energy, a feat that is difficult to achieve because the two materials do not want to mix. PHaSE EFRC researchers overcame this problem and prepared alternating, parallel lines of the two materials in which the width of the domains are 1/10,000th of the diameter of the human hair! This design allows for the efficient absorption of the sun's energy and the creation of an electric current that can readily be harvested. Other advances led to understanding how the initial arrangement of the two materials influences the collection of solar energy, and how this arrangement changes over time. These studies give insight into designing and arranging the two materials, and have implications in long-term stability, and hence the lifetime, of devices.
Designing safe nuclear fuel. At the Center for Materials Science of Nuclear Fuel, researchers are focused on understanding the formation of defects within the nuclear fuel, and how these defects influence performance, ultimately leading to safer fuel design. "Our collaborative effort ensures that the theoretical and experimental expertise needed to address this grand challenge is available," said Anter El-Azab, a member of the CMSNF’s Executive Committee and a professor at Purdue University.
In recent work, researchers combined theory with experiment to develop a new technique to measure thermal conductivity in metal oxides with micrometer resolution, giving researchers a valuable tool for characterizing nuclear fuels. Researchers at the center, based on previous work at the Center for Materials at Irradiation and Mechanical Extremes, also reported how defect sites that form upon fuel use interact with and influence the atomic structure of different interfaces between crystalline domains. These studies, focused on the behavior of defect sites and grain boundaries while under irradiation, revealed that the interaction between different classes of grain boundaries is similar, but has a large difference in magnitude, giving insight into safer fuel design.
Outlook. As we can no longer rely primarily on fossil fuels to meet our energy needs, the turnaround time for scientific discoveries and technologies developed by the trial-and-error approach is neither adequate nor acceptable. Scientists and engineers must design and perfect atom- and energy-efficient syntheses of new forms of matter with tailored properties. Progress in these fields is expected to lead to technologies that are scalable and cost effective, ultimately finding commercial application and widespread adoption.
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