Symmetry breaking and electrical conductivity of La 0.7Sr 0.3Cr 0.4Mn 0.6O 3− δ perovskite as SOFC anode material

Abstract

This work is focused on nanocrystalline solid oxide fuel cell synthesis and characterization (SOFC) anodes of La 0.7Sr 0.3Cr 0.4Mn 0.6O 3− δ (perovskite-type) with Nickel. Perovskite-type oxide chemical reactivity, nucleation kinetics and phase composition related with La 0.7Sr 0.3Cr 0.4Mn 0.6O 3− δ –NiO to La 0.7Sr 0.3Cr 0.4Mn 0.6O 3− 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% H 2/N 2 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 R 3 ¯ c to hettotype I4/ mmm has been identified and confirmed by XRD and Rietveld method which can be produced by introducing Ni 2+ cations in the perovskite solid solution. Rietveld analysis suggests that Ni contents are directly proportional to La 0.7Sr 0.3Cr 0.4Mn 0.6NiO 3.95 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.