Synthesis and characterization of Co-doped Lanthanum Nickelate perovskites for solid oxide fuel cell cathode material

Abstract

In the perovskite structures widely investigated and used as Solid Oxide Fuel Cells (SOFC) cathodes, oxygen reduction is mainly limited to the triple phase boundary (TPB), where oxygen (air), electrode and electrolyte are in contact. It is possible via the sol-gel modified Pechini method to: 1) control the material grain size, which can increase TPBs, 2) produce a homogenous material and 3) obtain a cathode material in a faster way compared with the solid state route. LaNi$_x$Co$_{1-x}$O$_3$ ($x = 0.3$, 0.5, 0.7) were synthesized by the modified Pechini method. The perovskite phase formation began at 350$^{\circ}$C and the presence of pure LaNi$_{0.7}$Co$_{0.3}$O$_3$ LaNi$_{0.5}$Co$_{0.5}$O$_3$ and LaNi$_{0.3}$Co$_{0.7}$O$_3$ structures was evidenced by High Temperature X-ray diffraction (HT-XRD) measurements. Scanning Electron Microscopy (SEM) micrographs showed that the microstructure evolves with the amount of cobalt from a coalesced to an open structure. Electrochemical impedance spectroscopy (EIS) on symmetrical cells LaNi$_x$Co$_{1-x}$O$_3$/YSZ (yttria-stabilized zirconia)/LaNi$_x$Co$_{1-x}$O$_3$ showed that the highest ASR (area specific resistance) is obtained with $x = 0.3$, whereas ASR values are similar for $x = 0.5$ and 0.7 at temperatures higher than 600$^{\circ}$C. At temperatures lower than 600$^{\circ}$C, ASR is the lowest for LaNi$_{0.5}$Co$_{0.5}$O$_3$, showing that this composition with intermediate porosity appears as a good choice for an intermediate-temperature solid oxide fuel cell (SOFC).