Study of the Rate Limiting Steps and Degradation of a GDC Impregnated La0.6Sr0.4Co0.8Fe0.2O3-δ Cathode

Abstract

Reducing fuel cells degradation and cathode polarization losses are currently two of the most important challenges in order to achieve more efficient and cost competitive SOFC's. One strategy has been to reduce operation temperatures to the range between 600 ºC and 800 ºC (IT-SOFC), which reduces degradation rates but increases activation over potentials as a consequence (mainly the cathode polarization resistance Rp,c, due to the oxygen reduction reaction -ORR-). Several approaches have been taken to counteract this problem and, recently, promising results [1] have been obtained by modifying the electrode surface using the impregnation technique. This technique consists of synthesizing catalyst nanoparticles on the electrode surface through a liquid solution-based process [2, 3]. The mechanisms through which the impregnated catalysts reduce the polarization resistance and even enhance durability are not completely well understood yet, especially when non conventional catalysts are impregnated (i.e.: electrolyte nanoparticles into mixed ionic and electronic conductors –MIECs- electrodes), as the performance improvement relies upon the microstructure and coupled interactions between the materials. In this work, a cathode surface modification by nanoparticle impregnation was carried out. La0.6Sr0.4Co0.8Fe0.2O3-δ –LSCF- ceramic cathodes impregnated with Ce0.8Gd0.2O3-δ –GDC- nanoparticles were characterized with electronic scanning –SEM- and transmission –TEM- microscopy. The electrochemical response as a function of temperature, oxygen partial pressure -pO2-(10-4 < pO2< 1 atm) and time was evaluated by electrochemical impedance spectroscopy –EIS. Reproducible and homogeneous impregnation were obtained, which reduce the Rp,c around 30% in the temperature range between 400ºC and 800ºC and during 500 h at 700 ºC. Fig. 1 shows a SEM image of the LSCF sample with (Fig 1b) and without impregnation (Fig 1a), the time evolution of the Rp,c for both samples (Fig 1c) and the EIS spectra of both samples at pO2 = 10-2 atm and at 700 ºC (Fig 1d). Through the pO2 measurements, the effect of the impregnation on the rate-limiting steps involved was analyzed. The EIS spectra show at least two arcs, where the pO2 dependence of the low frequency arc (µ(pO2)-n, n ~ 0.7) suggests a complex mechanism where O2-gas diffusion is not the unique driving process. The complexity of this mechanism is also verified by comparing EIS spectra collected at several pO2 by using Ar and He as gas carriers, where the observed reduction of impedance as a consequence of lower diffusion of O2 in He, was lower than expected. On the other hand, the high frequency arc, with a Gerisher-type impedance, is almost independent of pO2. Fig. 1d shows how that the surface modification by GDC-impregnation mainly affects the low frequency contribution. Although a clear effect has been observed in the Rp,c values as a consequence of impregnation, no significant difference was detected in the time evolution behavior at 700 C in air. This means that GDC impregnation reduces the cathode polarization resistance but seems not to alter significantly the origin of the degradation processes.