CeNbO4+δ, a family of oxygen hyperstoichiometry materials with varying oxygen content (CeNbO4, CeNbO4.08, CeNbO4.25, CeNbO4.33) that shows mixed electronic and oxide ionic conduction, has been known for four decades. However, the oxide ionic transport mechanism has remained unclear due to the unknown atomic structures of CeNbO4.08 and CeNbO4.33. Here, we report the complex (3 + 1)D incommensurately modulated structure of CeNbO4.08, and the supercell structure of CeNbO4.33 from single nanocrystals by using a three-dimensional electron diffraction technique. Two oxide ion migration events are identified in CeNbO4.08 and CeNbO4.25 by molecular dynamics simulations, which was a synergic-cooperation knock-on mechanism involving continuous breaking and reformation of Nb2O9 units. However, the excess oxygen in CeNbO4.33 hardly migrates because of the high concentration and the ordered distribution of the excess oxide ions. The relationship between the structure and oxide ion migration for the whole series of CeNbO4+δ compounds elucidated here provides a direction for the performance optimization of these compounds.

Developing efficient Pt-based alloy as oxygen reduction reaction (ORR) electrocatalysts is critical for practical applications of fuel cells. High-temperature reduction technology can improve the alloy atoms ordering but inevitably accelerate metal sintering. Here, Pt-Ni nanoparticles (<5 nm) embedded into ordered mesoporous carbon (OMC) matrix are developed, and its confinement effect suppresses nanoparticles sintering up to 900 °C. After an elaborative dealloying process, the surface structure of Pt-Ni nanoparticles changes from a Ni-rich to Pt-rich layer (the thickness is about 0.15 nm). The optimized sample (PtNi 3 @OMC-A) displays outstanding mass and specific activities of 2.11 A mg Pt -1 and 3.23 mA cm Pt -2, more than one order of magnitude higher than commercial Pt/C (20 wt%). PtNi 3 @OMC-A also exhibits long-term stability with a negligible activity loss after 10 000 potential cycles. Both experimental results and density functional theory calculations reveal that alloying effect as well as strain effect weaken Pt-O binding strength, thus resulting in an outstanding ORR activity. Furthermore, the high long-term stability can be attributed to the confinement of OMC, inhibiting the detachment or agglomeration of the embedded alloy nanoparticles.

No abstract

Oxide ion conductors can be used as electrolytes in solid oxide fuel cells, a promising energy-conversion technology. Local structures around the defects in oxide ion conductors are key for understanding the defect stabilization and migration mechanisms. As the defect contents are generally low, it is rather difficult to characterize the defect structure and therefore elucidate how oxide ions migrate. Solid-state nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for probing the local structures. However, the interpretation of NMR signals mainly based on the empirical knowledge could lead to unprecise local structures. There is still controversy regarding the defect structures in the apatite-type interstitial oxide ion conductors containing isolated tetrahedral units. Here, we combine the experimental solid-state 29 Si NMR spectroscopy with theoretical density functional theory calculations to investigate the defect structures in La 9.33+x (SiO 4 ) 6 O 2+1.5x apatites. The results indicate that the 29 Si resonance signals on the high field side of the main peak corresponding to the Si atoms in the bulk structure are related to La vacancies and there is no steady-state SiO 5 in the defect structures. This finding provides new atomic-level understanding to the stabilization and migration of interstitial oxide ions in silicate apatites, which could guide the design and discovery of new solid oxide fuel cell electrolyte materials.

RbNaMgP 2 O 7 is a newly reported nonlinear optical (NLO) crystal that displays remarkable second harmonic generation (SHG) enhancement upon heating. In this work, we carried out in situ neutron powder diffraction and impedance spectroscopy measurements through its polar-to-polar phase-transition region and confirmed that the dynamic process for the alignment of NLO-active anionic [P 2 O 7 ] 4− groups is composed of thermoinduced incremental rotation of [P 2 O 7 ] 4− groups, which is accompanied by cationic migration enhancement. Furthermore, we elucidated a new mechanism of coupling between anionic group alignment and cationic migration for the first time in NLO materials. This mechanism could offer extra freedom for new NLO material design and anionic group alignment control for a better SHG response via tuning the cationic migration.