Document Type: Research Paper
School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran.
Faculty of Chemical and Materials Engineering, Shahrood University of Technology, Shahrood, Iran.
Lanthanum nickelate with the chemical formula of La2NiO4+δ has attracted research interests during recent years, due to its hA high oxygen ion diffusivity for application as cathode material in solid oxide fuel cells (SOFCs). In this research, lanthanum nickelate with Ruddlesden–Popper (RP) crystal structure was synthesized via the co-precipitation method. The effects of OH-/NO3- molar ratio, La3+/Ni2+ molar ratio, co-precipitation temperature, and calcination temperature on the phase composition, thermal behavior, morphology, and electrochemical properties of the synthesized samples were investigated. Analysis of the X-ray diffractometry (XRD) patterns revealed that the optimum OH-/NO3- and La3+/Ni2+ molar ratios in the co-precipitation stage are 1.25 and 1.7, respectively. The formation of RP lanthanum nickelate is promoted by calcination of the co-precipitated powder at 1000 °C, while the calcination at lower temperatures may lead to the formation of perovskite (P) lanthanum nickelate. Scanning electron microscopy (SEM) studies showed that the mean particle size decreases from 428 to 332 nm by increasing the OH-/NO3- molar ratio from 1 to 1.5 while it increases from 67 to 183 nm by increasing the calcination temperature from 900 to 1000 °C. Simultaneous differential thermal analysis (DTA/TG) showed that the single RP lanthanum nickelate phase starts to form at 920 °C and fully formed at 960 °C. The electrochemical impedance spectroscopy data indicated that the cell with the electrode sintered at 1050 °C has the lowest polarization resistance. The polarization resistance reached 1 Ω cm2 at the testing temperature of 800 °C, for the electrode sintered at 1050 °C. Impedance curves of the electrode were fitted and simulated with two semicircles at high and low frequencies. The activation energy of 1.14 eV was calculated for the electrode polarization resistance of the lanthanum nickelate electrode.