In Operando Observation of (La, Sr)CoO3 Perovskite Oxide Solid/Gas Interface by X-ray Absorption Spectroscopy

1. Introduction

Lowering of the cathodic overvoltage is one of the paramount strategies to improve the performance of solid　oxide fuel cells (SOFCs). La1-xSrxCoO_3-δ (LSC) is considered a potential cathode material for SOFCs since it　shows the high electronic and ionic mixed conductivity. The cathodic reaction contains many elementary reaction processes such as gas diffusion, surface reaction, bulk diffusion and ion transfer. The previous studies suggested that the surface reaction is the rate-determining step of LSC electrode reaction [1]. However, the processes occurring at the electrode surface is poorly understood. In this study, we investigate the chemical structure of LSC surface during SOFC operating condition by means of in operandoX-ray absorption spectroscopy, and then surface modification is examined to improve the surface reaction kinetics.

 Some researchers indicate that LSC is exposed to Sr segregation under SOFC operating condition [2, 3]. Because this reaction is occurred at nanometer scale of the electrode surface, the true phenomena have not been clearly understood. In order to analysis surface chemistry during SOFC operation condition, in operandototal-reflection fluorescence XAS was applied. This　method is useful to observe the surface reaction at the nanometer scales[4].

 Surface modification of LaSrCoO_3-δ (LSC214) on La_0.6Sr_0.4CoO_3-δ (LSC113) is interesting strategy to improve surface reaction [5]. To understand the effect of surface modification, we examined uncoated LSC113 and LSC214 coated LSC113 electrodes with electrochemical impedance spectroscopy, in operandoXAS and fluorescence X-ray analysis (XRF).

2. Experiment

Model thin film electrodes were prepared by plasma laser deposition (PLD) method. Substrate was YSZ (110) single crystal. First we deposited Gd_0.2Ce_0.8O_1.9 buffer layer about 5 nm and then La_0.6Sr_0.4CoO_3-d was deposited for about 70 nm thickness. For the surface modification, LaSrCoO_3-δwas coated with various thicknesses.

We characterized crystal structure by XRD, thickness by a transmission electric microscope, surface roughness by atomic force microscope. Electrochemical impedance spectroscopy was measured range of 1 M-0.1 Hz.

For in operandoXAS measurement, homemade cells were used. Counter and reference electrode were porous Pt. We made these 3-electrode cells 5 mm-square as samples for XRF and XAS, and placed into small electric furnace. Co K-edge XAS was measured at BL01B1, SPring-8 in Japan. The fluorescence mode was used and incident X-ray angle was determined as a half of the count value at the critical angle in total reflection measurements. The penetration depth is estimated approximately 5 nm.

3. Results

Figure 1 shows Nyquist plots from electrochemical impedance spectroscopy for uncoated LSC113 and LSC214 coated electrodes. The LSC214 coating significantly decreases the electrode resistance. Especially LSC214 coating for 3 nm on LSC113 cathode shows highest reaction kinetics.

Figure 2 shows absorption edge energy shift from Co K-edge XAS as a function of applying voltage which is converted to effective oxygen partial pressure [1]. For the uncoated LSC113 electrode, significant energy shift is observed in cathodic condition. Combined with the result of fluorescence spectra, Sr segregation is proofed for the LSC113 electrode. On the other hand, the energy shift is prohibited by LSC 214 coating. As shown in Fig. 2, LSC214 coating suppresses the reduction of Co ion in LSC113 and Sr segregation on surface.

Fig.1. Nyquist plot of electrochemical impedance spectroscopy for the uncoated LSC113 and the LSC214 coated LSC113 at 1%-O₂atmosphere.

Fig.2. Absorption edge shift of XAS as a function of effective partial pressures of oxygen (left). Models of Sr segregation for the uncoated LSC113 and the LSC214 coated LSC113 (right).

Reference

[1] T. Kawada et. al., J. Electrochem. Soc. 149E252 (2002)

[2] E. Crumlin et. al., Energy Environ. Sci., 5, 6081(2011)

[3] E. Mutoro et al., J. Phys. Chem. Lett., 3, 40 (2012).

[4] D.Takamatsu et al., Angew. Chem. Int. Ed., 51,11597 (2012).

[5] E Crumlin et. al., J. Phys. Chem. Lett., 1, 3149 (2010)

Acknowledgement

This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Grant-in-Aid for Young Scientists (B) (25810134）