Commercially available lithium ion batteries operate by moving a positively charged lithium ion from a metal oxide cathode to a graphite anode, where the ions are stored during charging, and back again during discharge. In the process, the lithium ions must diffuse their way out of the cathode, through the electrolyte and intercalate into the planes of graphite that make up the anode. The rate at which this diffusion and intercalation can occur limits the speed with which the battery can be charged and discharged.
Researchers at Oregon State University are developing a battery that uses the transfer of protons described by Grotthuss in 1806 in an effort to dramatically increase charging rate. (Image source: Oregon State University)
Changing the Focus
“Right now the battery community focuses on lithium, sodium, and other metal ions, but protons are probably the most intriguing charge carriers with vast unknown potentials to realize,” said Xiulei (David) Ji of Oregon State University’s College of Science, in an OSU news release. Ji, along with collaborators at the Argonne National Laboratory, the University of California, Riverside, and the Oak Ridge National Laboratory, are the first to demonstrate that diffusion may not be necessary to transport ionic charges inside a hydrated solid-state structure of a battery electrode.
The collaborative team looked to the single proton of hydrogen and to the work of Theodor von Grotthuss, a German-born Lithuanian chemist who in 1806 developed the theory that explains charge transport in electrolytes. Ji notes, “He was the earliest to figure out how electrolyte works, and he described what’s now known as the Grotthuss mechanism: proton transferred by cooperative cleavage and formation of hydrogen bonds and O-H covalent bonds within the hydrogen-bonding network of water molecules.”
A Different Kind of Transport
Xianyong Wu, a postdoctoral scholar at OSU, adds that electrical charge is conducted when a hydrogen atom bridging two water molecules “switches its allegiance” from one molecule to the other. “The switch kicks disjointed one of the hydrogen atoms that was covalently bonded in the second molecule, triggering a chain of similar displacements throughout the hydrogen-bonding network,” Wu said. “The motion is like a Newton’s cradle: Correlated local displacements lead to the long-range transport of protons, which is very different from metal-ion conduction in liquid electrolytes, where solvated ions diffuse long distances individually in the vehicular manner,” he added in the news release.
“The cooperative vibrations of hydrogen bonding and hydrogen-oxygen covalent bonds virtually hand off a proton from one end of a chain of water molecules to the other end with no mass transfer inside the water chain,” said Ji. “That’s the beauty of it. If this mechanism is installed in battery electrodes, the proton doesn’t have to squeeze through narrow orifices in crystal structures. If we design materials with the purpose of facilitating this kind of conduction, this conduit is so ready – we have this magic proton highway built in as part of the lattice,” he said.
“Coming up with Faradaic electrodes that afford battery’s energy density and capacitor’s power with excellent cycle life has been a big challenge,” said Ji, In their experiment, the researchers revealed the extremely high power performance of an electrode of a Prussian blue analog, Turnbull’s blue – as it is known by the dye industry. The unique contiguous lattice water network inside the electrode’s lattice demonstrates the Grotthuss mechanism.
“Computational scientists have made tremendous progress on understanding how the proton hopping really occurs in water,” Ji said. “But Grotthuss’ theory was never explored to avail energy storage in detail, particularly in a well-defined redox reaction, which had the aim to materialize the impact of this theory.”
Although the concept seems promising, there is still a long way to go before practical batteries are made that take advantage of the theory. “Without the proper technology involving research by materials scientists and electrical engineers, this is all purely theoretical,” Ji said. “Can you have a sub-second charge or discharge of a battery chemistry? We theoretically demonstrated it, but to realize it in consumer devices, it could be a very long engineering journey. Right now the battery community focuses on lithium, sodium, and other metal ions, but protons are probably the most intriguing charge carriers with vast unknown potentials to realize.”
Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.
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