Symmetry breaking and electrical conductivity of La0.7Sr0.3Cr0.4Mn0.6O3−δ perovskite as SOFC anode material

This work is focused on nanocrystalline solid oxide fuel cell synthesis and characterization (SOFC) anodes of La0.7Sr0.3Cr0.4Mn0.6O3−δ (perovskite-type) with Nickel. Perovskite-type oxide chemical reactivity, nucleation kinetics and phase composition related with La0.7Sr0.3Cr0.4Mn0.6O3−δ–NiO to La0.7Sr0.3Cr0.4Mn0.6O3−δ–Ni transformation have been analyzed. SOFC anode powders were obtained by sol–gel synthesis, using polyvinyl alcohol as an organic precursor to get a porous cermet electrode after sintering at 1365 °C and oxide reduction by hydrogen at 800 °C/1050 °C for 8 h in a horizontal tubular reactor furnace under 10% H2/N2 atmosphere. Composite powders were compressed into 10-mm diameter discs with 25–75 wt% Ni. Electrical and structural characterization by four-point probe method for conductivity, scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Rietveld method were carried out. Symmetry-breaking by phase transition from high temperature aristotype image to hettotype I4/mmm has been identified and confirmed by XRD and Rietveld method which can be produced by introducing Ni2+ cations in the perovskite solid solution. Rietveld analysis suggests that Ni contents are directly proportional to La0.7Sr0.3Cr0.4Mn0.6NiO3.95 tetragonal structure cell volume and inversely proportional to Ni cubic structure cell volume after reduction at 1050 °C. Kinetic analysis indicated that the Johnson–Mehl–Avrami equation is able to provide a good fit to phase transformation kinetics. The variation of electrical conductivity reveals the presence of two types of behavior in samples reduced at 1050 °C. First, at low Ni concentration (25%), ρ resistivity decreases when increasing the temperature; then, for Ni concentration higher than 25% ρ resistivity increases.