Structural, Electrical and Electrochemical Characteristics and Oxygen Transport Properties of the Alkaline Earth Metal Doped Ln2NiO4.
Ln2NiO4 (Ln = La, Pr) compounds with a Ruddlesden – Popper (R P) structure are promising materials for intermediate temperature solid oxide fuel cell (IT SOFC) cathodes. This work aims at clarifying effects of doping with alkaline earth cations on their transport properties and performance. Ln1.7M0.3NiO4 oxides (LMNO; M = Ca, Sr, Ba) were synthesized via the solid state reaction method and sintered at 1250 °C. All samples were characterized by XRD, electric conductivity (a four-probe dc technique) and oxygen mobility (Do estimation by temperature programmed oxygen isotope exchange of oxide powders with C18O2) measurements. The symmetrical electrochemical cell polarization resistance was studied by impedance spectroscopy.
All samples were single-phase tetragonal I4/mmm R P oxides, unit cell parameters increased with M2+ radius. Doping increased the total conductivity, the highest value being 85 S/cm at 700 °C for LCNO. For Ln2NiO4 samples Do values (~ 10-8 cm2/s at 700 °C) were close to those previously determined . For La1.7M0.3NiO4 samples a low temperature peak of exchange corresponding to a fast diffusion in rock salt layers (Do,fast ~Do in La2NiO4, minor route) and a high temperature peak due to a slow diffusion in the perovskite layers (Do,slow, main route, Do,fast/Do,slow ~102) were observed. This is due to decreasing the oxygen excess with doping and hampering oxide ion migration between layers, which results in the electrochemical performance deteriorating. Although both Do,fast and Do,slow increase with M2+ radius, the amount of oxygen involved in fast diffusion declines resulting in decreased total mobility. Electrochemical activity of the electrodes increases when using Ln1.7M0.3NiO4 -Ce0.8Sm0.2O1.9 composites.
Support by shared access center “Composition of compounds”, IHTE and the Center “Ural-M” in IMET UB RAS, the Russian Foundation for Basic Research (Project 14-03-00414), Government of the Russian Federation, agreement 02.A03.21.0006 (Act 211) and Russian Science Foundation (Project 16-13-00112) is gratefully acknowledged.
 V.A. Sadykov, N.F. Eremeev, V.V. Usol’tsev, Russ. J. Electrochem. 49 (2013) 645–651.