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In 1933, Nobel Prize winner Lev Davidovich Landau introduced the polaron theory, which describes the movement of the polaron, describing waves of electrons and their cloud of surrounding virtual phonons, within a typical covalently bonded crystal.
As these waves move through the crystal, electrons within the polaron pull positively charged ions, or cations, towards it, while pushing away the negatively charged ions, or anions. Since its discovery in 1933, active research continues to be done on the theory of polarons, as well as the role of polarons in high-temperature superconductors, magnetic fields and quasi-two dimensional systems1.
As a theory that was previously believed to be exclusively applied to electron behavior, recent work conducted by Researchers from the Swiss Federal Laboratories for Materials Science and Technology (EMPA) has determined that the movement of protons within the crystal lattice follow a similar movement as described by the polaron theory.
To study the proton dynamics present within a fuel cell, the research tem led by Artur Braun and Qianli Chen used non-conductive yttrium-doped barium ceric oxide and barium zirconium oxide crystals. When the crystals were exposed to a steam atmosphere, the formation of hydroxide (OH-) groups within the crystal led to a subsequent release of protons to move in a similar wave-like fashion as compared to that described by the polaron theory.
By utilizing a hydrated BaCe0.8Y0.2O3-d (BCY20), the Researchers employed the quasi-elastic neutron scattering (QENS) technique at both high temperature that measured up to 600 °C, as well as high pressure points2. As the interactions of neutrons with matter are typically weak, the QENS technique applies a beam of neutrons to a crystal, which are then scattered as they undergo a quantum transition to their final momentum, in which a detector measures the scattered neutrons.
In this study, Braun and Chen found that conductivity within the crystal rose to the same exact extent that was predicted by their initial mathematical calculations that were based on the lattice vibrations of the crystal. The crystals were determined to be bound to the crystal lattice, however, at temperatures within the range of 220 °C and 520 °C, thermal activation cause a delocalization of the protons to jump to their neighboring oxygen ions, thereby producing an increased conductivity.
When no pressure was applied to the hydrated BCY20 cell, the yielded activation energy (Ea) measured at 0.35 eV, whereas following an applied pressure of 0.58 GPa yielded a substantially higher Ea of -1.20 eV2. This heightened proton activity was concluded to be a direct consequent of a phonon-assisted and both temperature and pressure dependent activity, as it directly mimics the mathematical polaron model2.
The proposed theory on proton activity within the fuel cell could have a significant influence over the future of fuel cells and hydrogen storage systems. While current hydrogen storage systems typically utilize hydrogen in the form of gas or liquid tanks, metal hydrides, or chemical hydrogen storage materials, the ability to specifically target the activity of protons within a given fuel cell or storage system could drastically affect the way in which the continuously evolving energy industry supplies power around the world.
While the polaron-based model gives a greater insight on the activity of protons within the energy system, the EMPA Researchers believe that a system that is based upon the movement of protons as pairs, rather than individually, could provide much more effective results. Braun and Chen therefore encourage current Researchers studying the complex nature of fuel cells and their internal activities to focus their attention and experimental procedures on a potential system that is based on the manipulation of such coupled protons.
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References:
- “Adiabatic Theory of Nearly Small Polarons” D. Eagles. American Physical Society. (1966). DOI: 10.1103/PhysRev.145.645.
- “Experimental neutron scattering evidence for proton polaron in hydrated metal oxide proton conductors” A. Braun, Q. Chen. Nature Communications. (2017). DOI: 10.1038/ncomms15830.
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