Comparative Study of BaY0.1Ce0.9O3-δ Membrane with BaY0.1Zr0.9O3-δ Protective Layers Synthesized with Spray Pyrolysis and Magnetron Sputtering Methods

Acceptor-doped perovskite-type oxides including BaY_0.1Ce_0.9O_3-d (BYCO) are known for excellent proton conductivity under fuel cell operating conditions and applicable in electrolysers, reactors and sensors.  However, chemical stability of this oxide is relatively poor. Originating from oxide’s high basicity BaCeO₃ reacts easily with H₂O and CO₂, forming BaCO₃, CeO₂ and Ba(OH)₂. At the same time zirconia based electrolyte, typically with high grain boundary resistance for protons, exhibit good chemical stability and seems to be attractive as chemical barrier layer for ceria based proton conductor.

In this study, BaY_0.1Zr_0.9O_3-d (BYZO) protective layers were synthesized onto BaY_0.1Ce_0.9O_3-d substrate using spray pyrolysis and magnetron sputtering methods, and characterized using electrochemical and microstructural analysis methods. Stability tests at carbon CO₂containing environment were conducted.

Powder for BYCO substrate was prepared by ultrasonic spray pyrolysis method. Substrates were sintered at 1500 °C in powder bed and afterward polished to obtain flat surfaces, which were coated with BYZO protective layers. Dual layer electrolytes obtained were sintered at 1350 °C between BYCO pellets, then characterized with scanning electron microscopy, x-ray diffraction and FIB-TOF-SIMS methods. Electrochemical characterization was carried out at temperature range from 550 to 800 °C.

Arrhenius plots with good linearity for four different systems (uncoated polished BYCO, uncoated unpolished BYCO, magnetron sputtered BYZO at BYCO and spray pyrolyzed BZYO at BCYO) were constructed and activation energies E_a were calculated. Lowest activation energy has been found for the polished uncoated substrate (0,35 eV). For the magnetron sputtered and spray pyrolyzed layers E_avalues were 0,37 eV and 0,38 eV, respectively. Highest activation energy was observed for unpolished substrate (0,39 eV), suggesting barium ion deficit in the near-surface layer, which can be removed by polishing. Thus, BYZO films deposited increase slightly the activation energy values compared with non-coated membranes.

 As expected, total ionic conductivity differs greatly from polished to unpolished and from coated to non-coated membrane. Lowest conductivity was characteristic for uncoated polished membranes with the smallest surface area (also the smallest length of three phase boundary between electrolyte, platinum and gas phase) and highest conductivity was characteristic for uncoated unpolished membrane with the highest surface area. BYZO layer synthesized using spray pyrolysis gave worse conductivity compared with BYZO layer synthesized using magnetron sputtering. This is very likely caused by lower homogeneity and higher grain boundary resistance of the layer prepared using spray pyrolysis.   

            In this study it has been demonstrated, that electrical conductivity depends on the microstructure of studied layers as well as on the interfacial structure of solid-gas-Pt boundary of the membrane. Protective BYZO layers significantly affected the time stability of the BYCO membrane in the CO₂ containing environment.