Synthesis and properties of nanocomposites with mixed ionic-electronic conductivity on the basis of oxide phases with perovskite and fluorite structures

Abstract

Nanocomposites consisting of phases with fluorite (doped CeO 2 ) and perovskite (LaMnO 3 , GdMnO 3 ) structures are synthesized using the method of ester polymeric precursors (the Pechini method) and two sources of rare-earth elements ( Ln ), such as pure cerium and gadolinium salts or a commercial mixture of rare-earth carbonates containing La, Ce, Pr, Nd, and Sm cations. The genesis of the nanocomposite structure as a function of the sintering temperature is investigated using X-ray diffraction and electron microscopy. It is revealed that the genesis of the nanocomposite structure is governed, in many respects, by the fact that the decomposition of the ester polymeric precursor leads to the formation of a metastable phase, namely, a fluoritelike solid solution based on ceria with an excess concentration of the cations Ln 3+ ( Ln 3+ = La 3+ , Pr 3+ , Nd 3+ , Sm 3+ ) as compared to the equilibrium concentration. As a result, the perovskite phase (identified by X-ray diffraction analysis) is formed only after the subsequent annealing at temperatures higher than 800 ° C, when Ln 3+ cations escape from particles of the solid solution. It is demonstrated that, at annealing temperatures of up to 1100 ° C, particles of both phases have nanometer sizes and are characterized by a uniform spatial distribution necessary for percolation. The nanocomposites possess a high total electrical conductivity and a high mobility of lattice oxygen. The reduction rate of the nanocomposites with hydrogen or methane is higher than the reduction rate of the individual phases. The characteristics of the nanocomposites prepared from the commercial mixture of rare-earth carbonates are better than those of the samples synthesized from the pure salts.

Design and Characterization of Nanocomposites Based on Complex Perovskites and Doped Ceria as Advanced Materials for Solid Oxide Fuel Cell Cathodes and Membranes

“…La 0.8 Sr 0.2 Fe 0.6 Ni 0.4 O 3-x (LSFN), La 0.8 Sr 0.2 Fe 0.8 Co 0.2 O 3-x (LSFC) and Ce 0.9 Gd 0.1 O 2-x (GDC) oxides were synthesized by Pechini route as described earlier [10] and calcined under air at 500°C (GDC) or 700°C (LSFN, LSFC). The mixtures of GDC and LSFN with mass ratio 1:1 (comp.…”

“…The oxygen mobility and reactivity were characterized by the oxygen isotope exchange using a static installation with vacuum pretreatment at 400 o C and MS control of the gas phase isotope composition (pO 2 4 Torr) [10]. O 2 -temperature programmed desorption (TPD) in He stream (heating rate 5°C/min), H 2 -temperature programmed reduction (TPR, 10% H 2 in Ar, heating rate 10°C/min) and CH 4 -TPR (1% CH 4 in He, heating rate 5 o /min) in microcatalytic installations [10] were also used. Conductivity was measured in air with a MO-10 Micro-Ohmmeter at 25 Hz frequency with four silver paste electrodes placed symmetrically at a perimeter of the pellet.…”

“…Note that the R-values did not decrease for comp. 1 despite the dilution of LSNF with GDC that has an order of magnitude lower R values [10]. This can be explained by the redistribution of the cations between the phases, thus creating new sites for oxygen activation on the GDC surface and enriching the surface of LSFN with Fe and Ni cations.…”

“…Under temperature-programmed conditions, the so-called dynamic degree of exchange (X s ) is the most straightforward parameter of the oxygen mobility in powders [9,10,12]:…”

“…Fast exchange starts at 400-500 °C, revealing higher X S values for Co-containing systems. Large values of X S (X s >100) indicate a high bulk oxygen mobility that significantly exceeds the corresponding mobility in LaMnO 3.19 (X s <0.5 at 700 o C [12]) or in LnMnO 3 -doped ceria nanocomposites (X s < 8 at 800 o C [10]). At lower temperatures of exchange, both X S (Fig.…”