As the name suggests, most electronic devices today work through the movement of electrons. But materials that can efficiently conduct protons – the nucleus of the hydrogen atom – could hold the key to a number of important technologies to combat global climate change.
Most proton-conducting inorganic materials now available require undesirably high temperatures to achieve sufficiently high conductivity. However, low-temperature alternatives could enable a range of technologies, such as more efficient and sustainable fuel cells to produce clean electricity from hydrogen, electrolyzers to produce clean fuels such as hydrogen for transportation, solid-state proton batteries, and even new kinds of iono-based computing devices – electronic effects.
To advance the development of proton conductors, MIT engineers have identified certain properties of materials that lead to the rapid conduction of protons. Using these properties quantitatively, the team identified half a dozen new candidates that show promise as fast proton conductors. Simulations suggest that these candidates will perform much better than existing materials, although they still need to be experimentally tailored. In addition to uncovering potential new materials, the research also provides a deeper understanding of how such materials work at the atomic level.
New findings are described in the journal Energy and environmental sciences, in a paper by MIT professors Bilge Yildiz and Ju Li, postdocs 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 electricity without carbon dioxide,” explains Yildiz. “We want to do this process efficiently, so we need materials that can transport protons very quickly through such devices.”
Current methods of hydrogen production, such as steam reforming of methane, release large amounts of carbon dioxide. “One way to remove this is to produce hydrogen electrochemically from water vapor, and that requires very good proton conductors,” Yildiz says. The production of other important industrial chemicals and potential fuels, such as ammonia, can also be done through efficient electrochemical systems that require good proton conductors.
But most inorganic materials that conduct protons can only work at temperatures of 200 to 600 degrees Celsius (roughly 450 to 1,100 Fahrenheit), or even higher. Such temperatures require energy to maintain and can cause materials to degrade. “Going to higher temperatures is not desirable because it makes the whole system more demanding and the durability of the material becomes a problem,” says Yildiz. “There is no good inorganic proton conductor at room temperature.” Today, the only known proton conductor at room temperature is a polymeric material, which is not practical for applications in computing devices because it cannot be easily scaled down to the nanometer regime, he says.
To tackle this problem, the team first needed to develop a fundamental and quantitative understanding of exactly how proton conduction works, taking a class of inorganic proton conductors called solid acids. “One must first understand what controls proton conduction in these inorganic compounds,” he says. By looking at the materials’ atomic configurations, the researchers identified a pair of characteristics that are directly related to the materials’ potential to transport protons.
As Yildiz explains, proton conduction first involves a proton “jumping from a donor oxygen atom to an acceptor oxygen. And then the environment has to reorganize and take the accepted proton away so it can jump to another neighboring acceptor, allowing protons to diffuse over long distances.” This process happens in many inorganic solids, he says. A key part of this research was figuring out how that last part works — how the atomic lattice is reorganized to take the accepted proton away from the original donor atom — he says.
The researchers used computer simulations to study a class of materials called solid acids that become good proton conductors above 200 degrees Celsius. This class of materials has a substructure called a sublattice of polyanionic groups, and these groups must rotate and take the proton away from its original location so that it can then be transferred to other locations. The scientists were able to identify the phonons that contribute to the flexibility of this sublattice, which is essential for proton conduction. They then used this information to comb through vast databases of theoretically and experimentally possible compounds in search of materials with better proton conduction.
As a result, they found solid acidic compounds that are promising proton conductors and which have been developed and produced for a number of different applications, but had never before been studied as proton conductors; these compounds turned out to have just the right lattice flexibility characteristics. The team then performed computer simulations of how the specific materials they identified in their initial screening would behave at the appropriate temperatures to confirm their suitability as proton conductors for fuel cells or other uses. Sure enough, they found six promising materials with predicted proton conduction rates faster than the best existing solid acid proton conductors.
“There are uncertainties in these simulations,” warns Yildiz. “I don’t want to say exactly how much the conductivity will be higher, but they look very promising. Hopefully this will motivate the experimental field to try to synthesize them in different forms and use these compounds as proton conductors.”
It may take several years to translate these theoretical findings into practical devices, he says. A likely first application would be electrochemical cells for the production of fuels and chemical feedstocks such as hydrogen and ammonia, he says.
The work was supported by the US Department of Energy, the Wallenberg Foundation and the US National Science Foundation.