Oxygen reduction reaction at (La,Sr) (Co,Fe)O3-d electrode/La9.5Si6O26.25 apatite electrolyte interface of solid oxide fuel cells
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© 2015 Hydrogen Energy Publications, LLC. Oxygen reduction reaction (ORR) of La0.8Sr0.2Co0.5Fe0.5O3-d (LSCF) cathode on La9.5Si6O26.25 (LSO) apatite electrolyte is studied by electrochemical impedance spectroscopy over the temperature range 700-850 °C and the oxygen partial pressure range 0.01-0.21 atm. Different to the ORR on the LSCF electrode on Gd-doped ceria (GDC) electrolyte, the impedance response for the ORR on the LSCF electrode on LSO apatite electrolyte is characterized by clearly separable arcs at high, intermediate and low frequencies. The high frequency electrode process has activation energy of 231 ± 7 kJ mol-1 and is basically independent of oxygen partial pressure, which is most likely associated with the oxygen ion transfer from the LSCF electrode to LSO electrolyte at the interface. On the other hand, the electrode processes associated with the low and intermediate frequency arcs are dependent on the oxygen partial pressure with activation energies of 192 ± 12 kJ mol-1 and 124 ± 28 kJ mol-1, respectively, are related to the dissociative adsorption and diffusion of oxygen on the LSCF surface and through the bulk and/or pores of LSCF electrodes to the reaction sites. The results show that the diffusion and migration of silica from the LSO apatite electrolyte to the LSCF surface particularly at the interface region have significant adverse effect on the electrochemical activity of LSCF cathode.
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