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In Operando Solid State Nuclear Magnetic Resonance Spectroscopy <br/>Studies of Anionic Conductors for Solid Oxide Fuel Cell

Final Report Summary - NMRSOFC (In operando solid state nuclear magnetic resonance spectroscopy studies of anionic conductors for solid oxide fuel cell)

High-temperature proton solid state nuclear magnetic resonance (NMR) of proton conductors yttrium (Y)-doped barium zirconate (BZY):

The environmental benefits of fuel cells have been increasingly appreciated in recent years. Amongst candidate electrolytes for solid oxide fuel cells (SOFCs), Y-doped BZY has garnered attention because of its high-proton conductivity, particularly in the intermediate temperature region targeted for cost- effective SOFC operation, and excellent chemical stability. However, fundamental questions surrounding the defect chemistry and macroscopic proton transport mechanism of this material remain, especially in regards to the possible role of proton trapping. Here, we contribute to the understanding of the macroscopic proton transport in BZY using high-temperature 1H NMR which shows the presence of two types of proton environments above room temperature, reflecting differences in proton-dopant configurations. This is in agreement with a combined thermogravimetric and a.c. impedance study, which reveals that macroscopic proton transport in Y-doped BZY is limited by proton-dopant association (proton trapping). Protons must overcome the association energy, 29 kJ / mol, as well as the general activation energy, 13 - 16 kJ / mol - obtained by NMR - to achieve long-range transport. This insight motivates efforts to identify suitable alternative dopants with reduced association energies and thereby higher conductivities.

This work is in press in Nature Materials: Proton trapping: macroscopic proton transport mechanism in ytttrim-doped barium zirconate. Y. Yamazaki, F. Blanc, Y. Okuyama, L. Buannic, J. C. Lucio-Vega, C. P. Grey, S. M. Haile, Nature Materials, in press.

High-temperature multinuclear solid state NMR of proton conductors caesium hydrogen phosphate:

Local dynamics and hydrogen-bonding in CsH2PO4 - a promising electrolyte for SOFC application - have been investigated by 1H, 2H and 31P solid-state NMR spectroscopy to help provide a detailed understanding of proton conduction in the paraelectric phase. Two distinct environments are observed by 1H and 2H NMR and their chemical shifts (1H) and quadrupolar coupling constants (2H) are consistent with one strong and one weaker H-bonding environment. Two different protonic motions are detected by variable temperature 1H MAS NMR and T1 spin-lattice relaxation time measurements in the paraelectric phase, which we assign to librational and long-range translational motions. An activation energy of 0.70 ± 0.07 eV is extracted for the latter motion; that of the librational motion is much lower. 31P NMR lineshapes are measured under MAS and static conditions, and spin-lattice relaxation time measurements have been performed as a function of temperature. Although the 31P lineshape is sensitive to the protonic motion, the reorientation of the phosphate ions does not lead to a significant change in the 31P CSA tensor. Rapid protonic motion and rotation of the phosphate ions is seen in the superprotonic phase, as probed by the T1 measurements along with considerable line narrowing of both the 1H and 31P NMR signals.

This work is in press in the Journal of Physical Chemistry C: Understanding the conduction mechanism of the protonic conductor CsH2PO4 by solid state NMR spectroscopy. G. Kim, F. Blanc, Y.-Y. Hu, C. P. Grey, J. Phys. Chem. C., in press. DOI: 10.1021/jp312410t

Multinuclear solid state NMR of proton conductors Y-doped barium stannate:

Hydrated BaSn1-xYxO3-x/2 is a protonic conductor that, and unlike many other related perovskites, shows high conductivity even at high substitution levels. A joint multinuclear NMR spectroscopy, and density functional theory (total energy and gauge including projector augmented waves (GIPAW) NMR calculations) investigation of BaSn1-xYxO3-x/2 (0.10 = x = 0.50) was performed to investigate cation ordering and the location of the oxygen vacancies in the dry material. The discrete Fourier transform (DFT) energetics show that Y doping on the tin (Sn) site is favoured over doping on the Ba site. The 119Sn chemical shifts are sensitive to the number of neighbouring Sn and Y cations, an experimental observation that is supported by the GIPAW calculations and that allows clustering to be monitored: Y substitution on the Sn site is close to random up to x = 0.20 while at higher substitution levels, Y-O-Y linkages are avoided leading, at x = 0.50 to strict Y-O-Sn alternation of the B-site cations. These results are confirmed by the absence of a 'Y-O-Y' 17O resonance and supported by the 17O NMR shift calculations. Although resonances due to six-coordinate Y cations were observed by 89Y NMR, the agreement between the experimental and calculated shifts was poor. Five-coordinate Sn and Y sites (i.e. sites next to the vacancy) were observed by 119Sn and 89Y NMR, respectively, these sites disappearing on hydration. More five-coordinated Sn than five-coordinated Y sites are seen, even at x = 0.50 which is ascribed to the presence of residual Sn-O-Sn defects in the cation-ordered material and their ability to accommodate oxygen vacancies. High temperature 119Sn NMR reveals that the oxygen ions are mobile above 400 degrees of Celsius, oxygen mobility being required to hydrate these materials. The high protonic mobility, even for the high Y content materials, is ascribed to the Y-O-Sn cation ordering, which prevents proton trapping on the more basic Y-O-Y sites.

This work has been published in the Journal of the American Chemical Society: Probing cation and vacancy ordering in the dry and hydrated Y substituted BaSnO3 perovskite by NMR spectroscopy and first principles calculations: implications for proton mobility. L. Buannic, F. Blanc, D. S. Middlemiss, C. P. Grey, J. Am. Chem. Soc. 2012, 134, 14483-14498.

One of the main challenges associated with NMR spectroscopy is the poor sensitivity of the technique. We have recently addressed this in the following publication which has been published in the Journal of the American Chemical Society.

Dynamic nuclear polarisation 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 polarisation (DNP). Electron spin polarisation could be transferred either directly to 17O spins or indirectly via 1H spins in inorganic oxides and hydroxides using an oxygen-free solution containing a biradical polarisation agent (bTbK). The results open up a powerful method for rapidly acquiring high signal-to-noise ratio solid state NMR spectra of 17O nuclear spins and to probe sites on or near the surface, without the need for isotope labelling.

Dynamic nuclear polarisation enhanced natural abundance 17O spectroscopy. F. Blanc, L. Sperrin, D. A. Jefferson, S. Pawsey, M. Rosay, C. P. Grey, J. Am. Chem. Soc. 2013, 135, 2975-2978.