Characterization of Ruddlesden-Popper La2-xBaxNiO4 +/-delta Nickelates as Potential Electrocatalysts for Solid Oxide Cells

Ruddlesden-Popper La2-xBaxNiO4 +/-delta (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 NOx 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 La2-xBaxNiO4+delta is charge-compensated by decreasing oxygen excess, from delta approximate to 0.1 for x = 0 to nearly oxygen-stoichiometric state for x = 0.5 at 800 degrees C in air, and generation of electron-holes (formation of Ni3+). This leads to an increase in p-type electronic conductivity (up to similar to 80 S/cm for highly porous La1.5Ba0.5NiO4+delta ceramics at 450-900 degrees C) and a decline of oxygen-ionic transport. La2-xBaxNiO4+delta (x = 0-0.5) ceramics exhibit moderate thermal expansion coefficients, 13.8-14.3 ppm/K at 25-1000 degrees C in air. These ceramic materials react with yttria-stabilized zirconia at 700 degrees C with the formation of an insulating La2Zr2O7 phase but show good chemical compatibility with BaZr0.85Y0.15O3-delta solid electrolyte.