Perovskite electrodes for intermediate temperature solid oxide fuel cells

The aim of this work was to investigate innovative „cobalt-free‟ electrode materials for the application in Solid Oxide Fuel Cells (SOFC) in the intermediate temperature (IT) range. Since the well-known La1-xSrxFe0.8Co0.2O3-δ (LSCF), Co-containing cathode material, suffers of problem of Co diffusion between electrode-electrolyte interface, the substitution with Cu can represent a solution to this problem. The study was focused on lanthanum strontium ferrite oxides (LSF) that where doped with copper or tantalum in the B site. The crystal structure of La1-xSrxFe1-yTayO3±δ (LSFT) and La1-xSrxFe0.8Cu0.2O3-δ (x= 0.4, 0.2) (LSFCu) were widely investigated by x-ray diffraction (XRD) to eventually detect the presence of secondary phases and the electrochemical properties were assessed by electrochemical impedance spectroscopy (EIS) in synthetic air. As concerning LSFT, the lowest area specific resistance (ASR), derived from the polarization resistance (Rp), 1 Ω cm2 at 750 °C, was measured for the compound doped with x= 0.40 and y= 0.05. LSFCu showed a good chemical compatibility with La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) electrolyte since a limited cation interdiffusion at the electrode-electrolyte interface was observed, whereas Co ion of the well-known La0.6Sr0.4Fe0.8Co0.2O3-δ (LSFC) could diffuse more deeply into LSGM through grain boundaries, as highlighted by energy dispersive analysis (EDX) and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis. ASR values of LSFCu were comparable to the widely spread La0.6Sr0.4Fe0.8Co0.2O3-δ (LSFC) and lower than LSFT. LSFCu showed the typical behaviour of mixed ionic and electronic conductors (MIECs), since the Cu-doping had beneficial effect both on the electronic conduction and on the formation of oxygen vacancies, promoting the ionic conduction. LSFCu conductivity in air was varying in the range of 150-200 S cm-1 between 600 and 800 °C, whereas LSFT showed low conductivity values, around 15 S cm-1. The LSFCu perovskites where finally tested in a symmetric SOFC (SSOFC). This configuration presents several advantages. The behavior in reducing atmosphere was assessed by time programmed reduction (TPR), and the XRD data of the reduced powders confirmed the formation of metallic Fe and Cu as well as the presence of secondary phases, which were forming because of the Fe and Cu depletion from the perovskite oxide. The LSFCu symmetrical cells were tested in presence of hydrogen as fuel at the anode side and static air at the cathode and promising power output was obtained in the presence of composite electrodes with gadolinia-doped ceria (GDC).