As their name suggests, most of today’s electronic devices operate through the movement of electrons. But materials that can efficiently conduct protons—the nucleus of the hydrogen atom—could be key to a number of vital technologies to combat global climate change.
Most of the currently available inorganic proton-conducting materials require undesirably high temperatures to achieve sufficiently high conductivity. However, lower-temperature alternatives could enable a range of technologies, such as more effective and robust fuel cells for producing pristine electricity from hydrogen, electrolyzers for producing pristine fuels such as hydrogen for transportation, solid-state proton batteries, and even up-to-date types of computing devices based on ion-electronic effects.
To speed the development of proton conductors, MIT engineers have identified certain features of materials that make protons conduct quickly. Using these features quantitatively, the team has identified a half-dozen up-to-date candidates that are promising rapid proton conductors. Simulations suggest that these candidates will perform significantly better than existing materials, although they still need to be tweaked experimentally. In addition to uncovering potentially up-to-date materials, the research is also providing a deeper understanding, at the atomic level, of how such materials work.
Fresh discoveries include: Described in the journal in a paper by MIT professors Bilge Yildiz and Ju Li, postdoctoral students Pjotrs Zguns and Konstantin Klyukin, and their collaborator Sossina Haile and her students at Northwestern University. Yildiz is the Breene M. Kerr Professor in the departments of Nuclear Science and Engineering and Materials Science and Engineering.
“Proton conductors are needed in clean energy conversion applications, such as fuel cells, where we use hydrogen to produce carbon dioxide-free electricity,” Yildiz explains. “We want to do this process efficiently, so we need materials that can transport protons very quickly through these devices.”
Current methods of producing hydrogen, such as steam reforming methane, emit vast amounts of carbon dioxide. “One way to eliminate this is to produce hydrogen electrochemically from steam, and that requires very good proton conductors,” Yildiz says. Other vital industrial chemicals and potential fuels, such as ammonia, can also be produced using effective electrochemical systems that require good proton conductors.
But most inorganic materials that conduct protons can only operate at temperatures between 200 and 600 degrees Celsius (about 450 and 1,100 Fahrenheit) or higher. Such temperatures require energy to maintain and can cause the materials to degrade. “Going to higher temperatures is not desirable because it makes the whole system more difficult, and the durability of the material becomes an issue,” Yildiz says. “There is no good inorganic conductor of protons at room temperature.” Currently, the only known conductor of protons at room temperature is a polymer material, which is not practical for utilize in computing devices because it cannot be easily scaled down to the nanometer range, he says.
To solve this problem, the team first had to develop a fundamental and quantitative understanding of exactly how proton conduction works, considering a class of inorganic proton conductors called solid acids. “First, you have to understand what governs proton conduction in these inorganic compounds,” he says. By analyzing the atomic configurations of the materials, the researchers identified a couple of features that directly relate to the materials’ proton-transfer potential.
As Yildiz explains, proton conduction first involves a proton “jumping from the donor oxygen to the acceptor oxygen. Then the environment has to reorganize and take the accepted proton so it can jump to another neighboring acceptor, allowing long-range proton diffusion.” This process occurs in many inorganic solids, he says. Understanding how this last part works—how the atomic lattice gets reorganized to take the accepted proton from the original donor atom—was a key part of this research, he says.
Scientists used computer simulations to study a class of materials called solid acids, which become good conductors of protons above 200 degrees Celsius. This class of materials has a substructure called a polyanion group sublattice, and these groups have to rotate and take a proton from its original site so that it can be moved to other sites. The researchers were able to identify the phonons that contribute to the flexibility of this sublattice, which is necessary for proton conduction. They then used this information to search through extensive databases of theoretically and experimentally possible compounds in search of better proton-conducting materials.
As a result, they discovered solid acid compounds that are promising proton conductors, and that have been developed and manufactured for various applications but have never been studied as proton conductors before; it turned out that these compounds have exactly the right lattice elasticity characteristics. The team then ran computer simulations of how the specific materials they identified in their initial study would behave at the right temperatures to confirm their suitability as proton conductors for fuel cells or other applications. Sure enough, they found six promising materials, with predicted proton conduction rates faster than the best existing solid acid proton conductors.
“These simulations are uncertain,” Yildiz warns. “I don’t want to say exactly how much higher the conductivity will be, but they look very promising. I hope this will motivate the experimental community to try to synthesize them in different forms and use these compounds as proton conductors.”
It could take several years to translate these theoretical discoveries into practical devices, he says. Likely first applications will be in electrochemical cells to produce fuels and chemical feedstocks such as hydrogen and ammonia, he says.
The work was supported by the U.S. Department of Energy, the Wallenberg Foundation and the U.S. National Science Foundation.