Because the name suggests, most electronic devices today work through the movement of electrons. But materials that may efficiently conduct protons — the nucleus of the hydrogen atom — might be key to plenty of vital technologies for combating global climate change.
Most proton-conducting inorganic materials available now require undesirably high temperatures to realize sufficiently high conductivity. Nevertheless, lower-temperature alternatives could enable a wide range of technologies, reminiscent of more efficient and sturdy fuel cells to supply clean electricity from hydrogen, electrolyzers to make clean fuels reminiscent of hydrogen for transportation, solid-state proton batteries, and even recent sorts of computing devices based on iono-electronic effects.
With the intention to advance the event of proton conductors, MIT engineers have identified certain traits of materials that give rise to fast proton conduction. Using those traits quantitatively, the team identified a half-dozen recent candidates that show promise as fast proton conductors. Simulations suggest these candidates will perform much better than existing materials, although they still must be conformed experimentally. Along with uncovering potential recent materials, the research also provides a deeper understanding on the atomic level of how such materials work.
The brand new findings are described within 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 from Northwestern University. Yildiz is the Breene M. Kerr Professor within the departments of Nuclear Science and Engineering, and Materials Science and Engineering.
“Proton conductors are needed in clean energy conversion applications reminiscent of fuel cells, where we use hydrogen to supply carbon dioxide-free electricity,” Yildiz explains. “We would like to do that process efficiently, and due to this fact we’d like materials that may transport protons very fast through such devices.”
Present methods of manufacturing hydrogen, for instance steam methane reforming, emit a fantastic deal of carbon dioxide. “One method to eliminate that’s to electrochemically produce hydrogen from water vapor, and that needs superb proton conductors,” Yildiz says. Production of other vital industrial chemicals and potential fuels, reminiscent of ammonia, will also be carried out through efficient electrochemical systems that require good proton conductors.
But most inorganic materials that conduct protons can only operate at temperatures of 200 to 600 degrees Celsius (roughly 450 to 1,100 Fahrenheit), and even higher. Such temperatures require energy to keep up and may cause degradation of materials. “Going to higher temperatures will not be desirable because that makes the entire system more difficult, and the fabric durability becomes a difficulty,” Yildiz says. “There is no such thing as a good inorganic proton conductor at room temperature.” Today, the one known room-temperature proton conductor is a polymeric material that will not be practical for applications in computing devices because it may possibly’t easily be scaled right down to the nanometer regime, she says.
To tackle the issue, the team first needed to develop a basic and quantitative understanding of exactly how proton conduction works, taking a category of inorganic proton conductors, called solid acids. “One has to first understand what governs proton conduction in these inorganic compounds,” she says. While taking a look at the materials’ atomic configurations, the researchers identified a pair of characteristics that directly pertains to the materials’ proton-carrying potential.
As Yildiz explains, proton conduction first involves a proton “hopping from a donor oxygen atom to an acceptor oxygen. After which the environment has to reorganize and take the accepted proton away, in order that it may possibly hop to a different neighboring acceptor, enabling long-range proton diffusion.” This process happens in lots of inorganic solids, she says. Determining how that last part works — how the atomic lattice gets reorganized to take the accepted proton away from the unique donor atom — was a key a part of this research, she says.
The researchers used computer simulations to check a category of materials called solid acids that develop into good proton conductors above 200 degrees Celsius. This class of materials has a substructure called the polyanion group sublattice, and these groups need to rotate and take the proton away from its original site so it may possibly then transfer to other sites. The researchers were capable of discover the phonons that contribute to the pliability of this sublattice, which is crucial for proton conduction. Then they used this information to comb through vast databases of theoretically and experimentally possible compounds, looking for higher proton conducting materials.
Consequently, they found solid acid compounds which are promising proton conductors and which have been developed and produced for a wide range of different applications but never before studied as proton conductors; these compounds turned out to have just the proper characteristics of lattice flexibility. The team then carried out computer simulations of how the precise materials they identified of their initial screening would perform under relevant temperatures, to verify their suitability as proton conductors for fuel cells or other uses. Sure enough, they found six promising materials, with predicted proton conduction speeds faster than the most effective existing solid acid proton conductors.
“There are uncertainties in these simulations,” Yildiz cautions. “I don’t wish to say exactly how much higher the conductivity can be, but these look very promising. Hopefully this motivates the experimental field to attempt to synthesize them in numerous forms and make use of those compounds as proton conductors.”
Translating these theoretical findings into practical devices could take some years, she says. The likely first applications could be for electrochemical cells to supply fuels and chemical feedstocks reminiscent of hydrogen and ammonia, she says.
The work was supported by the U.S. Department of Energy, the Wallenberg Foundation, and the U.S. National Science Foundation.
