Lattice Dynamics to Uncover Fast Proton Conductors
Bilge Yildiz a
a Massachusetts Institute of Technology Laboratory for Electrochemical Interfaces, Departments of Nuclear Science and Engineering, and Materials Science and Engineering
Proceedings of 24th International Conference on Solid State Ionics (SSI24)
Advanced characterisation techniques: fundamental and devices
London, United Kingdom, 2024 July 14th - 19th
Organizers: John Kilner and Stephen Skinner
Contributed talk, Bilge Yildiz, presentation 622
Publication date: 10th April 2024

Discovery of fast proton conductors can significantly advance a wide range of technologies, including hydrogen fuel cells, electrolyzers, electrosynthesis of fuels, batteries as well as brain-inspired computing devices. Especially the latter needs inorganic, fast proton conductors at room temperature. A quantitative understanding of the physical traits of a material that regulate proton conduction is necessary for accelerating the discovery of fast proton conductors. While electronic and structural descriptors have been found to facilitate proton conductivity, the role of lattice dynamics remain unexplored quantitatively, albeit hypothesized to be important in affecting proton conduction.

In this work, we have mapped the structural, chemical and dynamic properties of solid acids and ternary oxides to the elementary steps of the Grotthuss mechanism of proton diffusion. Our approach combines ab initio molecular dynamics simulations, analysis of phonon spectra and atomic structure calculations.

In solid acids, we have identified the donor–hydrogen bond lengths and the acidity of polyanion groups as key descriptors of local proton transfer, and the vibrational frequencies of the cation framework as the key descriptor of lattice flexibility. The latter facilitates rotations of polyanion groups and long-range proton migration in solid acid proton conductors. The calculated lattice flexibility also correlates with the experimentally reported superprotonic transition temperatures. Using these descriptors, we have identified potential solid acid proton conductors that go beyond the traditionally considered chemistries.

For ternary oxides, the oxygen sublattice presents the donor and acceptor sites for proton conduction. While the metal-oxygen polyhedra are not nearly as flexible to rotate as in solid acids, the thermal fluctuations of O…O distances is a good measure of the flexibility of this class of conductors. We hypothesize that phonon modes which induce large fluctuations in O…O distance and shorten H…O bond lengths can assist with proton hopping. Supporting this hypothesis, we found correlations between the thermal O…O fluctuations and calculated proton hopping barriers for 10 ternary oxides with varied crystal structures. These results imply that both static O…O distance and dynamic fluctuations must be considered to predict proton conductivity in ternary oxides.

These physical descriptors of proton conduction also provide paths for increasing the conductivity at low temperature. With the rapid growth of material databases, our approach lays ground for physically informed search of fast proton conductors and enlarges the chemical space of materials to power the green revolution.

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