Mobilty of Sr in Gadolinia Doped Ceria SOFC Chemical Barrier Layers Prepared Using Spray Pyrolysis, Pulsed Laser Deposition and Magnetron Sputtering Methods
Authors: Nurk Gunnar , Möller Priit , Vestli Mihkel , Jaaniso Raivo , Kodu Margus , Mändar Hugo , Kanarbik Rait , Lust Enn 
Published: February 2020

For application of highly active La0.6Sr0.4CoO3-
d  (LSCO) and Pr0.6Sr0.4CoO3-
d (PSCO) cathodes in the systems with yttria stabilized zirconia (YSZ) membrane the ceria based chemical barrier layers have been used to prevent active reaction between Sr containing cathode and YSZ. However, even in thin chemical barrier layer the Sr has some mobility. The aim of this study was to clarify how the microstructure of chemical barrier layer influences the Sr mobility and how the electrochemical parameters of SOFC are related with these properties.

Gadolinium doped ceria Ce0.9Gd0.1O2-
d (GDC) chemical barrier layers with thickness approximately 0.7 mm were deposited to the YSZ electrolyte using pulsed laser deposition (PLD), magnetron sputtering (MS) and spray pyrolysis (SP) method. One set of chemical barrier layers were studied as prepared (PLD GDC prepared at 600 °C; MS GDC prepared at 300 °C; SP GDC pre-sintered at 950 °C) and other set of samples were sintered for 30 h at 1300 °C before printing  and sintering of the cathode paste.  PSCO and LSCO cathodes were used to compare the Sr mobility in chemical barrier layer.  Both cathodes were sintered at two different temperatures, at 950 °C and at 1100 °C, supported onto differently synthesized and thermally treated GDC layers. Concentration profiles of Zr ion in GDC, Ce in YSZ, Sr in GDC and in YSZ were recorded using TOF-SIMS analysis.  XRD, SEM and electrochemical studies were also used to establish formation of new phases, GDC layer thickness and effective electrical parameters of studied systems, respectively.

            Systematic analysis of TOF-SIMS data demonstrated that thermal treatment has a significant influence on the behavior of chemical barrier layers. If non-sintered GDC chemical barrier layer was used, then at both cathode sintering temperatures (at 950 °C and 1100 °C) mobility of Zr, Sr and Ce ions was detected, less at lower and more pronounced at higher sintering temperatures. Very slight accumulation of Sr onto the interface of YSZ and GDC was detected in the case of SP layer, which is, without correct sintering, most nonhomogeneous. When cathode sintering at 1100 °C was carried out, the mobility of ions increased which led to intensive accumulation of Sr onto the interface between YSZ and GDC layers. In addition, a slight increase of thickness of chemical barrier layer was observed when high cathode sintering temperature was used, most likely caused by the formation of SrZrO2 crystallites in GDC film. This phenomenon was also confirmed with analysis of XRD and HR-SEM data.

            When GDC barrier layer, deposited on YSZ, was sintered for 30h at 1300 °C, the mobility of Zr ions in GDC and Ce ions in YSZ was caused. When Sr containing cathode was sintered at 1100 °C on top of GDC layer contaminated with Zr ions, an intensive increase of GDC layer thickness occurred caused by the formation of SrZrO2, but no Sr accumulation at interface between GDC and YSZ has been established. When cathode sintering at 950 °C was carried out, a slight Sr mobility in highly sintered GDC layers was observed in the case of LSCO cathode, but not in the case of PSCO if PLD and MS layers were applied.  However, in SP layers some Sr mobility was observed in the case of both cathode materials studied. 

            Electrochemical tests have been carried out using LSCO | CGO | YSZ | Ni-YSZ system using impedance spectroscopy method. High frequency resistance Rex, dominantly characteristic for electrolyte and interlayers, depends significantly on synthesis method and sintering temperature of layer (microstructure of layer) and cathode. Results of electrochemical tests confirm the result obtained using TOF-SIMS analysis and show that better homogeneity of layer prevents the mobility and accumulation of Sr onto the interlayer between GDC and YSZ. The growth of SrZrO2 crystallites inside of the GDC layer seems to be less harmful compared with the accumulation of Sr onto the interlayer of GDC and YSZ initiating formation of relatively dense SrZrO2 film.