Effects of Ga substitution on the high temperature properties of the n = 3 Ruddlesden-Popper system LaSr3Fe1.5-x/2Co1.5-x/2GaxO10-d(0 ≤ x ≤ 0.8).

F. PRADO; J. -H. KIM; A. MANTHIRAM

Toronto

Conferencia; SSI-17 International Conference on Solid State Ionics; 2009

The mixed conducting properties of the Ruddlesden-Popper (R-P) series of oxides An+1BnO3n+1 have been explored with an aim to assess their use in high temperature electrochemical devices such as cathodes in solid oxide fuel cells (SOFC) and oxygen separation membranes [1-4]. The crystal structure of the R-P phases consists of n ABO3 perovskite layers alternating with AO rock salt layers along the c-axis. Recently, Lee and Manthiram [3] reported the electrochemical performance of the n = 3 R-P solid solution LaSr3Fe3-yCoyO10-d as cathode material. They found that LaSr3Fe1.5Co1.5O10- exhibits electrochemical performances comparable to the perovskite phase La0.6Sr0.4CoO3-d. However, the thermal expansion coefficient a = 20.5 10-6 K-1 was too high compared to that of the conventional electrolyte materials (10 10-6 ≤ a ≤ 12 10-6 K-1) such as Ce0.9Gd0.1O1..95 (GDC) or La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM). With an objective to develop mixed conductors with lower thermal expansion coefficient, we have substituted Ga in the B sites of the R-P phase LaSr3Fe3-yCoyO10-d and studied the effects on thermal expansion, electrical conductivity, and electrochemical performance. The LaSr3Fe1.5-x/2Co1.5-x/2GaxO10-d samples with 0 ≤ x ≤ 0.8 were prepared by solid state reactions of SrCO3, Fe2O3, Co3O4, and Ga2O3. The samples were characterized by X-ray diffraction (XRD) with Cu Ka radiation. Thermal expansion and electrical conductivity were studied in the temperature range 80 ≤ T ≤ 900 ºC. The electrochemical performances were evaluated at 600 ≤ T ≤ 800 ºC with single cells constructed with LSGM electrolyte (0.5 mm thick), NiO-GDC cermet anode, GDC and La0.4Ce0.6O1.8 (LCO) interlayers, and LaSr3Fe1.5-x/2Co1.5-x/2GaxO10-d cathode [3]. Humidified H2 was supplied at the anode and air at the cathode at a rate of 100 cm3/min. The analysis of the XRD data of the samples LaSr3Fe1.5-x/2Co1.5-x/2GaxO10-d with x = 0, 0.3, 0.6 and 0.8 with the Rietveld method show that the crystal symmetry changes from tetragonal to orthorhombic with the substitution of Ga3+ for the transition metal ions [2]. For the x = 0.3 sample, the crystal structure is tetragonal, while for the x ≥ 0.6 samples, it is orthorhombic. The variations of thermal expansion (L/L0) with temperature in air are shown in Figure 1. The total expansion coefficient was calculated in the temperature range 80 ≤ T ≤ 900 ºC and its evolution with the Ga content is shown in the inset of Figure 1. The substitution of Ga reduces systematically the thermal expansion of the material. The reduction of c is more evident when the crystal structure transforms to the orthorhombic symmetry for x = 0.8. Chemical reactivity tests show that LaSr3Fe1.5-x/2Co1.5-x/2GaxO10-d compounds react at 1100 ºC with LSGM electrolyte. Therefore, the electrochemical tests were carried out with LaSr3Fe1.5-x/2Co1.5-x/2GaxO10-d /GDC/LSGM/LCO/Ni-GDC single cells, employing a GDC interlayer. The presence of Ga reduces the maximum power obtained from 0.42 W/cm2 to 0.36 W/cm2 at 800 ºC. The replacement of Fe and Co by Ga in the solid solution LaSr3Fe1.5-x/2Co1.5-x/2GaxO10-d advantageously reduces the thermal expansion coefficient of the material. However, the Ga substitution also decreases the electrochemical performance at high Ga contents.