Performance and stability of doped phase-stabilized Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-Δ) as IT-SOFC cathode under different ambient conditions

摘要

Lowering the operational temperature of SOFCs will make them more suitable for practical application due to the reduction in material processing costs, fabrication and lifetime of the devices. Among different mixed ionic electronic conductors, Ba0.5Sr0.5Co0.8Fe0.2O3-Δ (BSCF) exhibits very high oxygen permeability and oxygen reduction reaction (ORR) activity. Unfortunately, decomposition of the cubic BSCF phase occurs at intermediate temperatures (IT) degrading its performance on the long-term. Recently, we reported a rational approach to stabilize the cubic phase of BSCF by B-site partial substitution with Y, Ti or Nb showing stable electrical conductivity and oxygen transport properties over time [1,2]. In the present work, we will show the feasibility of these phase-stabilized BSCF materials as cathodes of IT-SOFCs. Experiments were made in (i) anode supported cells and in (ii) symmetrical model cells using operating times up to 500 h in different O2 and CO2 atmospheres. The characterization of the cells was done by electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRTs). To gain more insight into the degradation mechanisms, the microstructure of the cells was analyzed by XRD, scanning electron microscopy (SEM) and analytical scanning transmission electron microscopy (STEM). In particular, STEM in combination with energy-dispersive X-ray spectroscopy (EDXS) depicts relevant information on the chemical composition and homogeneity of the cathodes. In agreement with the electrochemical measurements, the doped BSCF cathodes showed strong reduction or suppression of secondary phase formation and substantially diminished poisoning in the CO2 tests compared to BSCF, while the high performance is maintained.