SrZrO₃Formation at the Interlayer/Electrolyte Interface during (La_1-xSr_x)_1-δCo_1-yFe_yO₃Cathode Sintering

Abstract

This work probes the formation of SrZrO 3 at the SDC/YSZ interface (Sm doped ceria, SDC; Y stabilized zirconia, YSZ) during (La 1-x Sr x ) 1-δ Co 1-y Fe y O 3 (LSCF) cathode sintering. SEM/EDS and grazing incidence X-ray diffraction results of annealed LSCF and YSZ samples reveal that even without physical contact between LSCF and YSZ, SrZrO 3 was formed on the surface of YSZ, preferentially at the grain boundaries. It was suspected that the SrZrO 3 formation is due to the Sr-containing gas species diffused through the pores of the SDC layer and reacted with the YSZ electrolyte. Computational thermodynamics was adopted to predict the gas species formed in air during sintering by using the La-Sr-Co-Fe-O-H thermodynamic database. Sr(OH) 2 is identified as the dominant Sr-containing gas species under the experimental conditions. In addition, it was found that A-site deficiency in LSCF could effectively suppress the SrZrO 3 formation while a dense and pore-free SDC interlayer is required to totally block the SrZrO 3 formation. Cell performance was significantly improved for a cell with a dense SDC interlayer fabricated by pulsed laser deposition, due to elimination of SrZrO 3 formation and therefore reduced interfacial resistance. • C). LSCF materials exhibit electrochemical activity toward oxygen reduction that is superior to Sr-doped LaMnO 3 (LSM), an SOFC cathode material widely used at temperatures above 800• C. 1-8 However, LSCF materials also show higher chemical reactivity with the state-of-the-art electrolyte material, yttria stabilized zirconia (YSZ), than LSM. Reports have shown that LSCF reacts with YSZ to form an insulating La 2 Zr 2 O 7 (LZO) or SrZrO 3 (SZO) layer between the cathode and electrolyte at temperatures as low as 900• C, while LSM essentially does not react with YSZ until 1200• C. 9-16 Therefore, for LSCF to be used as a cathode, an interlayer is necessary between the cathode and electrolyte to prevent these detrimental reactions from occurring during cathode sintering and cell operation. Sm-or Gd-doped ceria (SDC or GDC) is usually used as the interlayer because it is chemically compatible with both the LSCF cathode and the YSZ electrolyte at high temperatures. 2,5,[9][10][11]14 An interlayer of high density is desirable to reduce the ohmic resistance in the interlayer. The sinterability of the SDC interlayer is influenced by several factors, such as the method of fabrication, initial particle size, sintering temperature, etc. Fuel cells with a dense doped ceria interlayer fabricated by either pulsed laser deposition or magnetron sputtering show a pronounced increase in power density. 17,18The significant increase in power density is attributed to decreased ohmic resistance, due to the improved density of the interlayer, better interfacial contact between the SDC and YSZ layers, and decreased insulating reaction products at this interface. Interlayers fabricated by other methods such as screen printing, dip coating, tape casting, or spray coating usually contain a certain amount of porosit...

Mechanism of SrZrO3Formation at GDC/YSZ Interface of SOFC Cathode

“…Sr can also diffuse as the gaseous species Sr(OH) 2 . 20 The amount of SrZrO 3 formation decreased if Sr diffusion was suppressed. Increase in GDC sintering temperature decreased Sr diffusivity 6 and the amount of SrZrO 3 formation was drastically decreased when the GDC sintering temperature was 1400 • C. 13 However, the SrZrO 3 tends to form near the surface because larger amounts of Zr diffused to the surface.…”

Nature and Functionality of La0.58Sr0.4Co0.2Fe0.8O3-δ/ Gd0.2Ce0.8O2-δ/ Y0.16Zr0.84O2-δInterfaces in SOFCs

“…[1][2][3][4][5] It is a commonly accepted fact that Sr depletes from the LSCF-bulk at temperatures above 600 • C, diffusing to the surface and forming SrO, or Sr(OH) 2 due to humidity and then evaporating into the gas phase. [6][7][8] Both effects are strongly adverse to surface kinetics decreasing the exchange coefficient two orders of magnitude. 9 However, there is a large scattering of both initial and final k δ values in literature, which is partly caused by differences in preparation methods and the varying chemical composition of the different model samples, and also partly by different analytical methods.…”

Achievements of NEDO Durability Projects on SOFC Stacks in the Light of Physicochemical Mechanisms

“…On the other hand, high temperature heat treatment is needed to sinter DC in dense films in a usual industrial method, and this promotes inevitably the interdiffusion between DC and YSZ , , resulting in decrease in ionic conductivity in the intermediate composition region . Furthermore, more complicated features have been clarified about the processing of the LSCF/DC/YSZ layers from the fundamental investigations on the mass transportation associated with the SrZrO 3 formation , , . These can be summarized as follows: When GDC is dense, chemical reactions between GDC and LSCF do not proceed so that no changes occur in the GDC side.…”

“…Without intentional humidification, there can be some amount of water vapors available during fabrication and operation so that the gaseous Sr transportation as the form of Sr(OH) 2 may also contribute in addition to the surface or gb diffusion to the GDC/YSZ interfaces. In many cases, formed SrZrO 3 phase contains some parts of Co and Fe , . The dense PLD film of GDC exhibits quite different behaviors in chemical reactions or diffusion. This is partly because defects in the PLD film are different from those in sintered body and partly because some cerium ions are present as Ce 3+ in the PLD film which remain even after the process of the LSCF attachment around 1,100 °C.…”

“…Without intentional humidification, there can be some amount of water vapors available during fabrication and operation so that the gaseous Sr transportation as the form of Sr(OH) 2 may also contribute in addition to the surface or gb diffusion to the GDC/YSZ interfaces. In many cases, formed SrZrO 3 phase contains some parts of Co and Fe [28,[86][87][88]. (iii) The dense PLD film of GDC exhibits quite different behaviors in chemical reactions or diffusion.…”