Abstract
While solid-state NMR spectroscopic techniques have helped clarify the local structure and dynamics of ionic conductors, similar studies of mixed ionic–electronic conductors (MIECs) have been hampered by the paramagnetic behavior of these systems. Here we report high-resolution 17O (I = 5/2) solid-state NMR spectra of the mixed-conducting solid oxide fuel cell (SOFC) cathode material La2NiO4+δ, a paramagnetic transition-metal oxide. Three distinct oxygen environments (equatorial, axial, and interstitial) can be assigned on the basis of hyperfine (Fermi contact) shifts and quadrupolar nutation behavior, aided by results from periodic DFT calculations. Distinct structural distortions among the axial sites, arising from the nonstoichiometric incorporation of interstitial oxygen, can be resolved by advanced magic angle turning and phase-adjusted sideband separation (MATPASS) NMR experiments. Finally, variable-temperature spectra reveal the onset of rapid interstitial oxide motion and exchange with axial sites at ∼130 °C, associated with the reported orthorhombic-to-tetragonal phase transition of La2NiO4+δ. From the variable-temperature spectra, we develop a model of oxide-ion dynamics on the spectral time scale that accounts for motional differences of all distinct oxygen sites. Though we treat La2NiO4+δ as a model system for a combined paramagnetic 17O NMR and DFT methodology, the approach presented herein should prove applicable to MIECs and other functionally important paramagnetic oxides.
Highlights
Mixed ionic and electronic conducting (MIEC) ceramics have shown promise in recent years as oxygen-transport membranes in solid oxide fuel cells (SOFCs) and for chemical looping applications.[1−7] The use of mixed conductors, primarily perovskite-type oxides, as SOFC cathodes has been shown to improve oxygen reduction kinetics and enable device operation at lower temperatures.[8,9] Typically, the advanced functionality of these mixed-conducting systems derives from the mutual influence of metal cation mixed valency and oxygen nonstoichiometry.[10]
Owing to the wide range of oxygen nonstoichiometry reported for this system, samples have been carefully characterized by X-ray diffraction (XRD) and thermogravimetric analysis (TGA)
Our main conclusions are as follows: (1) Small compositional changes in La2NiO4+δ occur as a result of 17O-enrichment; we observe an increase in the oxygen excess and the formation of small amounts (∼3 wt %) of an impurity assigned to the higher-order La4Ni3O10 phase
Summary
Mixed ionic and electronic conducting (MIEC) ceramics have shown promise in recent years as oxygen-transport membranes in solid oxide fuel cells (SOFCs) and for chemical looping applications.[1−7] The use of mixed conductors, primarily perovskite-type oxides, as SOFC cathodes has been shown to improve oxygen reduction kinetics and enable device operation at lower temperatures.[8,9] Typically, the advanced functionality of these mixed-conducting systems derives from the mutual influence of metal cation mixed valency and oxygen nonstoichiometry.[10] While the latter property, manifesting as oxygen vacancies or interstitials, has been directly implicated in the bulk performance of MIECs, the mechanistic origins of oxide-ion conductivity often remain unclear at the atomic level. Unlike conventional diffraction-based methods sensitive to long-range order, solid-state NMR spectroscopy can provide insight into local, atomic-scale distortions in solids, with direct relevance to ionic conduction.[12−15] exchange rates and activation energies for thermally activated transport processes can be derived from NMR spectra acquired at variable temperature (VT-NMR). This quadrupolar interaction can further discriminate among different coordination environments in diamagnetic oxides.[33]
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