Local Distortions and Dynamics in Hydrated Y-Doped BaZrO3.

Y-doped BaZrO3 is a promising proton conductor for intermediate temperature solid oxide fuel cells. In this work, a combination of static DFT calculations and DFT based molecular dynamics (DFT-MD) was used to study proton conduction in this material. Geometry optimizations of 100 structures with a 12.5% dopant concentration allowed us to identify a clear correlation between the bending of the metal-oxygen-metal angle and the energies of the simulated cells. Depending on the type of bending, two configurations, designated as inward bending and outward bending, were defined. The results demonstrate that a larger bending decreases the energy and that the lowest energies are observed for structures combining inward bending with protons being close to the dopant atoms. These lowest energy structures are the ones with the strongest hydrogen bonds. DFT-MD simulations in cells with different yttrium distributions provide complementary microscopic information on proton diffusion as they capture the dynamic distortions of the lattice caused by thermal motion. A careful analysis of the proton jumps between different environments confirmed that the inward and outward bending states are relevant for the understanding of proton diffusion. Indeed, intra-octahedral jumps were shown to only occur starting from an outward configuration while the inward configuration seems to favor rotations around the oxygen. On average, in the DFT-MD simulations, the hydrogen bond lengths are shorter for the outward configuration which facilitates the intra-octahedral jumps. Diffusion coefficients and activation energies were also determined and compared to previous theoretical and experimental data, showing a good agreement with previous data measuring local proton motion.

Proton distribution in Sc-doped BaZrO3: a solid state NMR and first principle calculations analysis.

Perovskite-based material Sc-doped BaZrO3 is a promising protonic conductor but with substantially lower conductivities than its Y-doped counterpart. 1H solid-state NMR spectroscopy in combination with DFT modelling was used to analyze the protonic distribution in BaZr1-xScxO3-x/2-y(OH)2y and its effect on charge carrier mobility. 1H single pulse and 1H-45Sc TRAPDOR MAS NMR experiments highlighted the mobile character of the proton charge carriers at room temperature, giving rise to a single broad resonance, protons hopping between multiple sites on the NMR timescale. At low temperatures, the protonic motion was successfully slowed down allowing direct observation of the various proton environments present in the structure. For x ≤ 0.15, DFT modelling suggested a tendency for strong dopant-proton association leading to Sc-OH-Zr environments with 1H NMR shifts of 4.8 ppm. The Zr-OH-Zr environment, H-bonded to a Sc-O-Zr, lies 32 kJ mol-1 higher in energy than the Sc-OH-Zr environment, suggesting that the Sc-OH-Zr environment is trapped. However, even at these low concentrations, Sc-Sc clustering could not be ruled out as additional proton environments with stronger 1H-45Sc dipolar couplings were observed (at 4.2 and 2.8 ppm). For x = 0.25, DFT modelling on the dry material predicted that Sc-□-Sc environments were extremely stable, again highlighting the likelihood of dopant clustering. A large number of possible configurations exists in the hydrated material, giving rise to a large distribution in 1H chemical shifts and multiple conduction pathways. The 1H shift was found to be strongly related to the length of the O-H bond and, in turn, to the hydrogen bonding and OOH distances. The breadth of the NMR signal observed at low temperature for x = 0.30 indicated a large range of different OH environments, those with lower shifts being generally closer to more than one Sc dopant. Lower DFT energy structures were generally associated with weaker H-bonding environments. Both the calculations and the DFT modelling indicated that the protons tend to strongly bond to the Sc clusters, which, in conjunction with the higher energies of configurations containing Zr-OH-Zr groups, could help explain the lower conductivities recorded for the Sc-substituted BaZrO3 in comparison to its yttrium counterpart.

Dynamic Nuclear Polarization NMR of Low-γ Nuclei: Structural Insights into Hydrated Yttrium-Doped BaZrO3.

We demonstrate that solid-state NMR spectra of challenging nuclei with a low gyromagnetic ratio such as yttrium-89 can be acquired quickly with indirect dynamic nuclear polarization (DNP) methods. Proton to (89)Y cross polarization (CP) magic angle spinning (MAS) spectra of Y(3+) in a frozen aqueous solution were acquired in minutes using the AMUPol biradical as a polarizing agent. Subsequently, the detection of the (89)Y and (1)H NMR signals from technologically important hydrated yttrium-doped zirconate ceramics, in combination with DFT calculations, allows the local yttrium and proton environments present in these protonic conductors to be detected and assigned to different hydrogen-bonded environments.

Dynamic nuclear polarization enhanced natural abundance 17O spectroscopy.

We show that natural abundance oxygen-17 NMR of solids could be obtained in minutes at a moderate magnetic field strength by using dynamic nuclear polarization (DNP). Electron spin polarization could be transferred either directly to (17)O spins or indirectly via (1)H spins in inorganic oxides and hydroxides using an oxygen-free solution containing a biradical polarization agent (bTbK). The results open up a powerful method for rapidly acquiring high signal-to-noise ratio solid-state NMR spectra of (17)O nuclear spins and to probe sites on or near the surface, without the need for isotope labeling.