Oxide-Ion Dynamics in the SOFC Cathode Material La2NiO4+d by Experimental and Computational Solid-State 17 O NMR Spectroscopy
In this study, solid-state 17O MAS-NMR spectroscopy of La2NiO4+δ at temperatures up to 800°C is supported by a theoretical methodology equipped with results from periodic hybrid DFT calculations. Three distinct 17O resonances are observed and assigned to equatorial, axial, and interstitial oxygen environments in La2NiO4+δ. Moreover, with high-resolution MAT-PASS experiments, the axial feature splits into several resonances, consistent with local structural distortions due to nearby interstitials. Loss of the interstitial oxygen feature in the NMR spectra upon heating to ~150°C is attributed to onset of exchange of interstitial and axial oxygen sites. Structural rearrangements due to interstitial motion also manifest as linewidth changes in the axial and equatorial resonances. At operational SOFC temperatures (600-800°C), interstitialcy and vacancy mechanisms of oxide-ion conduction are tentatively confirmed for the first time, showing that interstitial-axial exchange continues to dominate transport at the highest temperatures. Slow vacancy diffusion (equatorial-axial exchange) is surmised to limit the overall three-dimensional ionic conduction. This work is among the first examples of dynamics in a paramagnetic oxide-ion conductor studied by 17O solid-state NMR.