Characterization of Ruddlesden-Popper La2-xBaxNiO4±δ Nickelates as Potential Electrocatalysts for Solid Oxide Cells

Ruddlesden-Popper La_2-xBa_xNiO_4±δ (x = 0-1.1) nickelates were prepared by a glycine-nitrate combustion route combined with high-temperature processing and evaluated for potential application as electrocatalysts for solid oxide cells and electrochemical NO_x elimination. The characterization included structural, microstructural and dilatometric studies, determination of oxygen nonstoichiometry, measurements of electrical conductivity and oxygen permeability, and assessment of chemical compatibility with other materials. The formation range of phase-pure solid solutions was found to be limited to x = 0.5. Exceeding this limit leads to the co-existence of the main nickelate phase with low-melting Ba- and Ni-based secondary phases responsible for a strong reactivity with Pt components in experimental cells. Acceptor-type substitution of lanthanum by barium in La_2-xBa_xNiO_4+δ is charge-compensated by decreasing oxygen excess, from δ ≈ 0.1 for x = 0 to nearly oxygen-stoichiometric state for x = 0.5 at 800 °C in air, and generation of electron-holes (formation of Ni³⁺). This leads to an increase in p-type electronic conductivity (up to ~80 S/cm for highly porous La_1.5Ba_0.5NiO_4+δ ceramics at 450-900 °C) and a decline of oxygen-ionic transport. La_2-xBa_xNiO_4+δ (x = 0-0.5) ceramics exhibit moderate thermal expansion coefficients, 13.8-14.3 ppm/K at 25-1000 °C in air. These ceramic materials react with yttria-stabilized zirconia at 700 °C with the formation of an insulating La₂Zr₂O₇ phase but show good chemical compatibility with BaZr_0.85Y_0.15O_3-δ solid electrolyte.

On the stability in alkaline conditions and electrochemical performance of A2BO4-type cathodes for liquid fuel cells

Transition metal Ruddlesden–Popper oxides are active for H₂O₂ reduction in alkaline conditions but their chemical stability is questioned by experiment and Pourbaix diagrams.

Controlling Oxygen Mobility in Ruddlesden-Popper Oxides

Discovering new energy materials is a key step toward satisfying the needs for next-generation energy conversion and storage devices. Among the various types of oxides, Ruddlesden–Popper (RP) oxides (A₂BO₄) are promising candidates for electrochemical energy devices, such as solid oxide fuel cells, owing to their attractive physicochemical properties, including the anisotropic nature of oxygen migration and controllable stoichiometry from oxygen excess to oxygen deficiency. Thus, understanding and controlling the kinetics of oxygen transport are essential for designing optimized materials to use in electrochemical energy devices. In this review, we first discuss the basic mechanisms of oxygen migration in RP oxides depending on oxygen nonstoichiometry. We then focus on the effect of changes in the defect concentration, crystallographic orientation, and strain on the oxygen migration in RP oxides. We also briefly review their thermal and chemical stability. Finally, we conclude with a perspective on potential research directions for future investigation to facilitate controlling oxygen ion migration in RP oxides.

Impact of Oxygen Deficiency on the Electrochemical Performance of K2 NiF4 -Type (La1-x Srx )2 NiO4-δ Oxygen Electrodes

Perovskite-related (La_1-x Sr_x )₂ NiO_4-δ (x=0.5-0.8) phases were explored for possible use as oxygen electrodes in solid electrolyte cells with a main focus on the effect of oxygen deficiency on the electrocatalytic activity. (La_1-x Sr_x )₂ NiO_4-δ solid solutions were demonstrated to preserve the K₂ NiF₄ -type tetragonal structure under oxidizing conditions. Acceptor-type substitution by Sr is compensated by the formation of oxygen vacancies and electron holes and progressively increases high-temperature oxygen nonstoichiometry, which reaches as high as δ=0.40 for x=0.8 at 950 °C in air. The electrical conductivity of (La_1-x Sr_x )₂ NiO_4-δ ceramics at 500-1000 °C and p(O₂ )≥10^-3  atm is p-type metallic-like. The highest conductivity, 300 S cm^-1 at 800 °C in air, is observed for x=0.6. The average thermal expansion coefficients, (14.0-15.4)×10^-6  K^-1 at 25-900 °C in air, are sufficiently low to ensure the thermomechanical compatibility with common solid electrolytes. The polarization resistance of porous (La_1-x Sr_x )₂ NiO_4-δ electrodes applied on a Ce_0.9 Gd_0.1 O_2-δ solid electrolyte decreases with increasing Sr concentration in correlation with the concentration of oxygen vacancies in the nickelate lattice and the anticipated level of mixed ionic-electronic conduction. However, this is accompanied by increasing reactivity between the cell components and necessitates the microstructural optimization of the electrode materials to reduce the electrode fabrication temperature.

The determining factor for interstitial oxygen formation in Ruddlesden–Popper type La2NiO4-based oxides

In this paper, we report the electronic structure of the layered perovskite oxides and suggest the determining factor for interstitial oxygen formation.

Crystal-chemistry guidelines for noncentrosymmetric A2BO4 Ruddlesden-Popper oxides

Noncentrosymmetric (NCS) phases are seldom seen in layered A2BO4 Ruddlesden-Popper (214 RP) oxides. In this work, we uncover the underlying crystallographic symmetry restrictions that enforce the spatial parity operation of inversion and then subsequently show how to lift them to achieve NCS structures. Simple octahedral distortions alone, while impacting the electronic and magnetic properties, are insufficient. We show using group theory that the condensation of two distortion modes, which describe suitable symmetry unique octahedral distortions or a combination of a single octahedral distortion with a "compositional" A or B cation ordering mode, is able to transform the centrosymmetric aristotype into a NCS structure. With these symmetry guidelines, we formulate a data-driven model founded on Bayesian inference that allows us to rationally search for combinations of A- and B-site elements satisfying the inversion symmetry lifting criterion. We describe the general methodology and apply it to 214 iridates with A(2+) cations, identifying RP-structured Ca2IrO4 as a potential NCS oxide, which we evaluate with density functional theory. We find a strong energetic competition between two closely related polar and nonpolar low-energy crystal structures in Ca2IrO4 and suggest pathways to stabilize the NCS structure.

Structural and Physical Property Trends of the Hyperstoichiometric Series, La2Ni(1-x)CoxO4+δ

Members of the Ruddlesden-Popper system, La2Ni(1-x)CoxO4+δ (0.00 ≤ × ≤ 1.00), were synthesized and studied for their potential use as cathodes for solid-oxide fuel cells. An unusual structural trend has been noted across the series, which appears to correlate with the oxygen-hyperstoichiometry observed. Details of the structural variance by x-ray and neutron diffraction, as well as selected physical properties for this system will be presented.

Effect of Sr content on the crystal structure and electrical properties of the system La2−xSrxNiO4+δ(0 ≤ x ≤ 1)

Materials formulated as La(2-x)Sr(x)NiO(4+delta) (0 <or=x<or= 1) have been prepared and investigated by high-resolution neutron powder diffraction in order to correlate the structural variation induced by the incorporation of Sr into the crystal lattice with the electronic and thermal properties of each material. The evolution of the electrical conductivity and thermal expansion coefficients with temperature have been determined in order to study the potential use of these compounds as cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFC). These oxides show a good thermal expansion coefficient (TEC = 11-13 x 10(-6) K(-1)), and high electronic conductivity up to 273 S cm(-1). It is noticeable that a great enhancement of the electrical conductivity with the Sr content is concomitant with the shortening of the Ni-O1 bond length.