Technology: Proton's "highway" can pave the way for better high-power batteries – (Feature)



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Researchers at Oregon State University have discovered that a chemical mechanism first described more than two centuries ago has the potential to revolutionize energy storage for high-power applications such as vehicles or power grids.

The research team led by Xiulei (David) Ji of OSU's Faculty of Sciences, along with collaborators from the Argonne National Laboratory, University of California Riverside and Oak Ridge National Laboratory, is the first to demonstrate that diffusion may not it is necessary to carry ionic charges within a hydrated solid state structure of a battery electrode.

"This discovery will potentially change the whole paradigm of high-power electrochemical energy storage with the new design principles for electrodes," said Xianyong Wu, a postdoctoral student at OSU and the first author of the article.

The results were published today in Energy of nature.

"The creation of Faradaic electrodes that provide battery power density and capacitor capacity with excellent life cycle has been a major challenge," said Ji, associate professor of chemistry. "So far, most of the attention has been devoted to metal ions – starting with lithium and looking down on the periodic table."

The collaborative team, however, looked up – to the only hydrogen proton – and they also looked back in time to Theodor von Grotthuss, a Lithuanian chemist born in Germany who in 1806 wrote the theory on cargo transport in electrolytes.

Von Grotthuss was only 20 years old and lived in a region ravaged by political turmoil, when he published "Memories on the decomposition of water and bodies that he holds in solution by means of galvanic electricity" in a French scientific journal.

"In the whirlwind of his time and place, he managed to make this great discovery," said Ji. "He was the first to discover how the electrolyte works and described what is now known as the Grotthuss mechanism: proton transferred by cooperative cleavage and formation of hydrogen bonds and covalent bonds OH within the hydrogen bonding network of water molecules . "

Here's how it works: Electric charge is conducted when a hydrogen atom joining two molecules of water "changes its fidelity" from one molecule to another, Wu explains.

"The switch fires one of the hydrogen atoms that was covalently bound in the second molecule, triggering a chain of similar displacements across the hydrogen bonding network," he said. "Motion is like Newton's cradle: correlated local displacements lead to long-range transport of protons, which is very different from the conduction of metal ions in liquid electrolytes, where solvated ions diffuse long distances individually in the vehicular way."

Ji adds: "The cooperative hydrogen bonding vibrations and hydrogen-oxygen covalent bonds virtually transfer a proton from one end of a chain of water molecules to the other end without mass transfer within the water chain."

The molecular relay race is the essence of a fantastically efficient charge conductor, he said.

"That's the beauty of it," Ji said. "If this mechanism is installed on battery electrodes, the proton does not need to be squeezed through narrow holes in crystal structures. If we design materials to facilitate this type of driving, that channel is so ready – we have this magical proton road built as part of the trellis. "

In their experiment, Ji Wu and his co-workers revealed the extremely high performance of a Prussian blue analogue electrode, the Turnbull Blue-known to the dye industry. The unique network of contiguous water within the electrode structure demonstrates the "greatness" promised by the Grotthuss mechanism.

"Computer scientists have made tremendous progress in understanding how proton leaps actually occur in water," said Ji. "But Grotthuss's theory was never exploited to take advantage of the storage of energy in detail, particularly in a well-defined redox reaction, which was intended to materialize the impact of that theory."

While very enthusiastic about their findings, Ji warns that there is still work to be done to get ultra-fast charge and discharge to batteries that are practical for transporting or storing network power.

"Without the proper technology involving research done by materials scientists and electrical engineers, all of this is purely theoretical," he said. "Can you have a charge under a second or discharge battery chemistry? We theoretically demonstrate this, but to realize it in consumer devices, it can be a very long engineering journey. At this time, the battery community is focused on lithium, sodium and other metal ions, but the protons are probably the most intriguing charge carriers with vast unknown potential to be perceived. "

The National Science Foundation and the US Department of Energy supported this research.

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