Elucidating the Origin of Oxide Ion Blocking Effect at GDC/Srzr(Y)O3/YSZ Heterointerfaces

Abstract

Solid oxide fuel cells (SOFCs) have attracted significant attention over recent years due to their promising use as source of clean and efficient power generation. Towards the goal of enhancing the efficiency of these devices, it is important to eludicate the mechanisms governing fundamental electrochemical processes, specifically incorporation and transport of oxygen species across heterostructures. Degradation of cell performance has been attributed to the formation of SrZrO3 (SZ), especially along the GDC/YSZ interface, resulting from cation interdiffusion across LSCF/GDC/YSZ heterostructures. In this study, using a combination of isotope oxygen tracer and secondary ion mass spectroscopy (SIMS) depth profiling, we examined oxide ion transport in dense SZ and Y-doped SrZrO3 (SZY) thin films in conjunction with other oxide materials in a multilayered heterostructure, viz., GDC/SZ(Y)/YSZ. The GDC/SZ(Y) multilayers were prepared on (100) YSZ single crystal substrates using pulsed laser deposition; X-ray diffraction results confirmed that the SZ and SZY films were highly oriented along (h00). SIMS depth profile measurements after isotope exchange revealed that ionic transport is drastically inhibited at the SZ(Y)/YSZ interface, indicating the existence of an oxide ion barrier which blocks ionic flow. A similar behavior was also observed at the GDC/SZ interface; however, this was not observed for GDC/SZY. To elucidate the mechanism for the oxide ion blocking effect, we performed detailed microstructural analyses of the interfaces using transmission electron microscopy (TEM) and spatially resolved electron energy loss spectroscopy (EELS) technique. TEM results showed that the GDC/SZ(Y) and SZ(Y)/YSZ interfaces are atomically sharp and devoid of any secondary phases. The SZ film also appears to contain a high density of microstructural defects such as stacking faults which extend across its thickness. Furthermore, evident changes in the O K-edge EELS spectra measured across the heterostructures suggest the possible existence of oxygen disorder in the YSZ region adjacent to the SZ(Y)/YSZ interface. This may indicate a possible link between oxygen disorder, which can effectively break the oxygen diffusion pathways, and blocking of oxide ion transport across the SZ(Y)/YSZ interface.