Supporting effect of silver on BSCF cathodes for SOFC

Cathode materials for solid oxide fuel cells should show good catalytic activity in the oxygen reduction reaction (ORR), high electronic and ionic conductivities at relatively low temperature, and the thermal expansion coefo cients (TECs) compatible with the electrolyte. Ba0.5Sr0.5Co0.8Fe0.2O3–�/ (BSCF) shows one of the greatest catalytic activity in ORR which occurs on the overall surface of the electrode. Oxygen vacancies in BSCF permit to transport a signio cant amount of oxygen ions. Nevertheless, BSCF revealed its high TEC and low electrical conductivity. Some properties of the electrode can be improved by adding a second phase to the BSCF electrode. Silver shows the greatest electronic conductivity among metals; it is cheap and has excellent catalytic activity in the ORR. Moreover, the presence of Ag in the composite cathode increases the electronic conductivity of the cathode and reduces the area specio c resistance (ASR). The goal of this work is to make comparison among properties of the BSCF cathode, the Ag cathode, the cathode with silver interlayer between BSCF and electrolyte (Ag|BSCF), and the Ag|BSCF cathode with the additional silver layer on the top of the cathode (Ag|BSCF|Ag). The cathodes were prepared on the Ce0.8Sm0.2O1.9 electrolyte disks and tested using scanning electron microscopy (SEM) and electrochemical impedance spectroscopy. SEM examinations of the obtained electrodes revealed consistent with the goal, well connected, porous 3D structures. The contact was signio cantly better in case of the composite electrodes. Equivalent electrical circuits were used for o tting impedance spectra. The circuits consisted of a resistor R0 and three (or two) resistors and capacitors parallel connected in pairs (Ri, Ci) and combined in series. The o tting errors were smaller than 0.7% in all cases. The m and �. coefo cients were received from Arrhenius plots of ASR and the o tting, and indicated that the charge transfer was the rate-determining step for the Ag|BSCF|Ag electrode, whereas for the BSCF and Ag|BSCF electrodes it was the oxygen ions transport through the BSCF. Activation energies of ASR for the BSCF, Ag, Ag|BSCF and Ag|BSCF|Ag cathodes were similar, yielding the values of 55.1 kJ¬ mol -1 , 47.7 kJ¬ mol -1 , 47.7 kJ¬ mol -1 and 37.9 kJ¬ mol -1