Oxide-Ion Dynamics in the SOFC Cathode Material La2NiO4+d by Experimental and Computational Solid-State 17 O NMR Spectroscopy

Mixed ionic-electronic conductors (MIECs) show promise as next-generation solid oxide fuel cell (SOFC) cathode materials with improved performance at intermediate temperatures (500-800°C) as compared to materials lacking oxide-ion conductivity. The complex interplay of electronic and ionic conductivity in these strongly correlated systems often derives from the mutual influence of transition metal cation mixed valency and oxygen nonstoichiometry. Among MIECs tested as SOFC cathodes, the Ruddlesden-Popper phase La₂NiO_4+δ exhibits rapid oxygen transport at low temperatures, attributed to loosely bound interstitial oxides (0 < δ < 0.3). Key to understanding the high oxide-ion conductivity in La₂NiO_4+δ is experimental confirmation of hypothesized interstitial- and vacancy-mediated mechanisms at the atomic level.

In this study, solid-state ¹⁷O MAS-NMR spectroscopy of La₂NiO_4+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 La₂NiO_4+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.