A team led by researchers at Oregon State University have demonstrated that diffusion may not be necessary to transport ionic charges inside a hydrated solid-state structure of a battery electrode. The discovery potentially could shift the whole paradigm of high-power electrochemical energy storage with new design principles for electrodes, said Xianyong Wu, a postdoctoral scholar at OSU and the first author of the paper published in the journal Nature Energy.
Faradaic electrodes that possess the high capacity of batteries, while simultaneously delivering the high rate capability and excellent cycle life of electrodes in electrical double-layer capacitors, represent a grand challenge for electrochemical energy storage, and a class of such materials would transform our use of electrical energy. The rate performance of Faradaic electrodes is partially governed by transportation of ion charge carriers inside electrodes, which in turn depends on the choice of ion charge carriers. So far, most attention has been devoted to devices operating on metal ions, starting with Li and looking down the periodic table. Relatively little consideration has been given to looking up the periodic table to hydrogen, despite the hydrogen ion—a single proton—being much smaller than any metal ion. In addition to its advantage of size, the presence of hydrogen as a constituent of water enables fast conduction of protons in aqueous systems by a displacive mechanism first postulated by von Grotthuss in 1806.
In this process … an H atom bridging two hydrogen-bonded water molecules switches its allegiance from one molecule to the other, kicking out one of the existing H atoms from its adopted molecule, and triggering a chain of similar displacements through the hydrogen-bonding network. The motion is akin to a Newton’s cradle, with correlated local displacements leading to long-range transport of protons—very different from conduction of metal ions, where solvated metal ions diffuse long distances individually. Grotthuss proton conduction is very fast, and it is responsible for the anomalously high conductivity of acids. Recent studies have suggested that Grotthuss conduction may also take place in hydrogen-bonding networks confined inside solids, such as hydrated metal–organic frameworks. However, it has thus far remained unknown whether Grotthuss conduction can facilitate the redox reactions in battery electrodes to enable a high rate capability for electrochemical devices.
… Herein, we report an electrode material—a defective Prussian blue analogue (PBA), more specifically a Turnbull’s blue analogue… this material explicitly plays out the benefits of Grotthuss proton conduction on the rate and cycling performance of a Faradaic electrode. At the extremely high charging rate of 4,000 C (380 A g−1, 508 mA cm−2), the CuFe-TBA electrode retains half the capacity obtained at 1 C (1 C is defined as 95 mA g−1)—rivalling the fastest electrodes in any electrochemical storage device. Furthermore, CuFe-TBA maintains 60% of its capacity after 0.73 million cycles at 500 C, the largest number of cycles ever reported for a Faradaic electrode.—Wu et al.
Theodor von Grotthuss was a German-born Lithuanian chemist who in 1806 penned the theory on charge transport in electrolytes. Von Grotthuss was just 20 when he published “Memoir on the decomposition of water and of the bodies that it holds in solution by means of galvanic electricity” in a French scientific journal.
In the turmoil of his time and place, he managed to make this big discovery. 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.—Xiulei (David) Ji, OSU, co-corresponding author
Grotthuss mechanism. Source: OSU.
In their experiment, Ji, Wu and their collaborators from Argonne National Laboratory, the University of California, Riverside, and the Oak Ridge National Laboratory revealed the extremely high power performance of an electrode of a Prussian blue analog, Turnbull’s blue. The unique contiguous lattice water network inside the electrode’s lattice demonstrates the “grandeur” promised by the Grotthuss mechanism.
Computational scientists have made tremendous progress on understanding how the proton hopping really occurs in water. 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.—Xiulei Ji
Ji cautions that there’s still work to be done to attain ultrafast charge and discharge in batteries that are practical for transportation or grid energy storage.
Without the proper technology involving research by materials scientists and electrical engineers, this is all purely theoretical. 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.—Xiulei Ji
The National Science Foundation and the U.S. Department of Energy supported this research.
Xianyong Wu, Jessica J. Hong, Woochul Shin, Lu Ma, Tongchao Liu, Xuanxuan Bi, Yifei Yuan, Yitong Qi, T. Wesley Surta, Wenxi Huang, Joerg Neuefeind, Tianpin Wu, P. Alex Greaney, Jun Lu & Xiulei Ji (2019) “Diffusion-free Grotthuss topochemistry for high-rate and long-life proton batteries” Nature Energy doi: 10.1038/s41560-018-0309-7