Interfaces and Durability for Different LSCF/CGO/YSZ Systems for IT-SOFC
LSCF/CGO/YSZ half-cells were consisted in a dense Ce0.9Gd0.1O2-δ buffer layer, with different thicknesses (0.11, 0.45, 1.11 to 2 mm), prepared by DC magnetron sputtering on a homemade YSZ substrate. Porous La0.6Sr0.4Co0.2Fe0.8O3-δ cathode layer, 4 mm thick, was fabricated using electrostatic spray deposition . The microstructure, thickness, crystalline state and composition of films have been characterized respectively by FEG-SEM, X-ray diffraction and microanalyses. An aging investigation of all LSCF/CGO/YSZ half cells was performed for 550 h in air at 700°C, using electrochemical impedance spectroscopy (EIS). Spectra of LSCF cathode were recorded at OCV using a Solartron (SI 1280B) potentiostat/galvanostat frequency response analyzer with frequencies ranging between 0.01 Hz and 20 kHz. For the electrolyte CGO/YSZ system, impedance measurements were carried out using a HP 4192 measuring unit within the frequency range from 5 Hz to 13 MHz.
EIS measurements have shown that the initial electrolyte series (RS) (Fig.1a) is strongly decreased from 80 to 40 Ω.cm since a 0.11mm thick CGO layer is added due to the enhancement of the ionic current collecting between YSZ and LSCF. After 550 h, a low RS variation is observed and becomes weaker with CGO layer due to a better contact. The relative Rpol variation after 550 h, is strongly reduced to reach a minimal value of 57% when the CGO layer is only 0.11 mm thick with comparison to 677% on the LSCF/YSZ system without CGO layer.
The electrode response resulted from a major contribution at high frequency (HF) (Fig. 2a) and a minor one at low frequency (LF) corresponding to cathode-electrolyte interface and oxygen-cathode exchange, respectively. These EIS data are consistent with X-ray microanalysis where Sr2+ migration was only detected at LSCF/YSZ interface.
The lowest Rpol and the lowest relative Rpol variation, has been observed for an optimized CGO thickness of only 0.11 mm, after 550 h at 700°C in air.
Durability tests under current densities by EIS of these systems will also be discussed
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