Oxygen Reduction Properties of La0.1Sr0.9Co0.8Fe0.2O3-δ Cathode for SOFC Using Electrochemical Method

In recent years, the environmental friendly solid oxide fuel cell (SOFC) has been considered as one of the most efficient power generation systems. During the SOFC operation, the over potential is mainly given by the cathodic oxygen reduction reaction. In this respect, the mechanism of oxygen reduction on the Electrode is important to reveal. Specially, employing ac-impedance spectroscopy which provides an exceptionally is powerful tool for separating the dynamics of several electrode processes with different relaxation times. The present work considers the oxygen reduction reaction on the porous mixed conducting La_0.1Sr_0.9Co_0.8Fe_0.2O_3-δ (LSCF1982) electrode by using ac-impedance spectroscopy. Firstly, to attain a mixed conducting state of LSCF1982, the ac-impedance spectra measured at various temperatures and oxygen partial pressures were quantitatively analyses by employing the theoretical impedance model developed for oxygen reduction on the mixed conducting electrode.

All the measured impedance spectra simply consisted of two depressed arcs which overlapped slightly over the whole T and pO₂ ranges. The analysis of the ac-impedance spectra by employing discrete Fourier relaxation transformation (DFRT) allowed us to successfully differentiate the individual reaction steps. This indicates that the overall cathode reaction proceeds by the following two processes: charge transfer reaction and subsequent migration of oxygen vacancy.

It is also shown, that the magnitude of the high frequency arc decreased significantly with increasing T, but it remained nearly constant regardless of pO₂: In contrast, the low-frequency arc remained almost unchanged in magnitude irrespective of the value of T, while it was highly dependent on the value of pO₂. From the experimental results that it is thus deduced that the high frequency arc is attributed to the charge transfer reaction at the TPBs, and the low-frequency arc is associated with diffusion of adsorbed oxygen atom along the electrode/gas interface.

Figure 1 shows the steady-state current of the oxide anodes as a function of the electrode potential and a_O,int.

According to Wang and Nowick [1], the rate determining step determined to exchange current density value as a function of both temperature and oxygen partial pressure.

i₀ ∝pO₂ⁿ

The n - value in a relation gives information about the type of species involved in the electrode reaction. The n values have 1 if the rate limiting step is diffusional process of molecular oxygen, 1/2 for dissociative adsorption and 1/4 for charge transfer process depending on the conditions for Langmuir adsorption isotherm.

In this study for LSCF1982, the slopes of current exchange density were turned out to be 1/4 at 550 to 650^oC. This indicated that the rate limiting step for the electrochemical oxygen reduction in this temperature range is the charge transfer process.

In the present work, the oxygen reduction reaction on the porous LSCF1982 electrode was examined by means of analysis of the ac-impedance spectra and DC polarization. In addition, we’ll discuss the other technic like cathodic potentiostatic current transients and Gerisher impedance modeling.

Reference

[1] Da. Yu. Wang and A. S. Nowick, J. Electochem. Soc., 126, 1155 (1979) 

Acknowledgement

This work was supported by Solid Oxide Fuel Cell of New & Renewable Energy R&D Program (20123010030010) under the Korea Ministry of Knowledge Economy (MIKE)