As the name suggests, most of today's electronic devices depend on the movement of electrons. However, materials that may efficiently conduct protons – the nucleus of hydrogen atoms – could hold the important thing to numerous essential technologies to combat global climate change.
Most currently available proton-conducting inorganic materials require undesirably high temperatures to realize sufficiently high conductivity. However, lower-temperature alternatives could enable a variety of technologies, equivalent to more efficient and longer-lasting fuel cells to generate clean electricity from hydrogen, electrolyzers to supply clean fuels equivalent to hydrogen for transportation, solid-state proton batteries, and even recent sorts of computing devices based on ionoelectronic effects.
To advance the event of proton conductors, MIT engineers have identified certain properties of materials that enable rapid proton conduction. Using these properties, the team was in a position to discover half a dozen recent candidates that show promise as fast proton conductors. Simulations suggest that these candidates will perform much better than existing materials, although they still have to be confirmed experimentally. In addition to discovering potential recent materials, the research also provides a deeper understanding of how such materials work on the atomic level.
The recent findings are described within the journal in a paper by MIT professors Bilge Yildiz and Ju Li, postdocs Pjotrs Zguns and Konstantin Klyukin, and their collaborator Sossina Haile and 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 for clean energy conversion applications, equivalent to fuel cells, where we use hydrogen to generate carbon dioxide-free electricity,” explains Yildiz. “We have the desire to make this process efficient and subsequently need materials that may transport protons in a short time through such devices.”
Current methods of hydrogen production, equivalent to steam reforming of methane, emit large amounts of carbon dioxide. “One strategy to avoid that is to supply hydrogen electrochemically from water vapor, and this requires excellent proton conductors,” says Yildiz. The production of other essential industrial chemicals and potential fuels equivalent to ammonia can be achieved through efficient electrochemical systems that require good proton conductors.
However, most inorganic materials that conduct protons can only operate at temperatures of 200 to 600 degrees Celsius (about 450 to 1,100 Fahrenheit) and even higher. Such temperatures require energy to take care of and might result in material degradation. “Higher temperatures usually are not desirable since it makes the entire system more demanding and the sturdiness of the fabric becomes a difficulty,” Yildiz says. “There isn’t any good inorganic proton conductor at room temperature.” The only known room-temperature proton conductor today is a polymer material, which will not be practical for applications in computing devices since it can’t be easily scaled right down to the nanometer range, she says.
To tackle the issue, the team first needed to develop a fundamental and quantitative understanding of exactly how proton conduction works. To do that, they studied a category of inorganic proton conductors called solid acids. “You first have to grasp what determines proton conduction in these inorganic compounds,” she says. By studying the atomic configurations of the materials, the researchers identified a pair of properties which can be directly related to the proton transport potential of the materials.
As Yildiz explains, proton conduction first involves a proton “jumping from an oxygen atom of the donor to an oxygen atom of the acceptor. And then the environment has to reorganize and take the accepted proton away so it may well jump to a different neighboring acceptor, allowing long-distance proton diffusion.” This process occurs in lots of inorganic solids, she says. Figuring out how that last part works – how the atomic lattice is reorganized to take the accepted proton away from the unique donor atom – was a crucial a part of this research, she says.
The researchers used computer simulations to check a category of materials called solid acids which have over 200 degrees Celsius. This class of materials has a substructure called polyanion group sublattices, and these groups must rotate and move the proton away from its original location in order that it may well then be transferred to other locations. The researchers were in a position to discover the phonons that contribute to the pliability of this sublattice, which is important for proton conduction. They then used this information to go looking through huge databases of theoretically and experimentally possible compounds, searching for materials with higher proton conduction.
As a result, they found solid acid compounds which can be promising proton conductors and which have been designed and manufactured for quite a lot of different applications, but have never been studied as proton conductors before; it seems that these compounds had exactly the suitable properties when it comes to lattice flexibility. The team then ran computer simulations to research how the particular materials they identified of their initial review would behave at relevant temperatures to substantiate their suitability as proton conductors for fuel cells or other applications. In fact, they found six promising materials with predicted proton conduction rates that were higher than those of the very best existing solid acid proton conductors.
“These simulations are subject to uncertainty,” warns Yildiz. “I don't wish to say exactly how much higher the conductivity can be, but they give the impression of being very promising. Hopefully it will motivate experimental physicists to synthesize them in several forms and use these compounds as proton conductors.”
Turning these theoretical findings into practical devices could take several years, she says. The most probably first applications are electrochemical cells for producing fuels and basic chemicals equivalent to hydrogen and ammonia, she says.
The work was supported by the US Department of Energy, the Wallenberg Foundation and the US National Science Foundation.
