Speaker
Description
Hydride-ion (H-) conductors attract more and more interest for their potential application as solid electrolytes in, e.g., batteries, fuel cells, and for catalysis. A particularly promising class of hydride-ion conductors are nitride-hydrides, which accommodate both nitride ions and hydride ions in the same substructure. Recently, the new nitride-hydride $\mathrm{Ca_3CrN_3H}$ was discovered and shown to be promising to be used as a catalyst for ammonia synthesis [1]. For the rational development of $\mathrm{Ca_3CrN_3H}$, or related materials, towards such an application, a fundamental understanding of its hydride ion mobility is crucial, but such an understanding is at present lacking.
In this work, we investigate the nature of hydride ion mobility in $\mathrm{Ca_3CrN_3H}$ using quasielastic neutron scattering (QENS) and machine-learning molecular dynamics (MLMD). The combined analysis of QENS and MLMD data indicates vacancy-mediated hydride ion diffusion within the channel-like structure of $\mathrm{Ca_3CrN_3H}$. This diffusion mechanism is characterized by correlated localized and long-range diffusion occurring on relatively fast timescales of approximately 10 picoseconds and 50 picoseconds, respectively, with a notably low activation energy of ~50 meV. Crucially, these findings suggest that the high catalytic performance of $\mathrm{Ca_3CrN_3H}$ for ammonia synthesis is partly due to the facile hydride ion transport from the bulk to the surface of the material, where the catalytic reaction takes place. Furthermore, the results highlight the potential for efficient hydride ion transport over macroscopic distances, positioning $\mathrm{Ca_3CrN_3H}$ as a promising fast ion conductor.
References
[1] - Cao, Y. et al. Topochemical Synthesis of $\mathrm{Ca_3CrN_3H}$ Involving a Rotational Structural Transformation for Catalytic Ammonia Synthesis. Angewandte Chemie International Edition, 2022.
