Study of the Electrochemical Mechanisms That Control the Electrode Reaction and Degradation of La0.6Sr0.4Co0.8Fe0.2O3-δ cathodes Impregnated with Oxide Nanoparticles

Ascolani-Yael, Julian

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 reaction overpotential, due to the high activation energy of oxygen reduction reaction -ORR-). Several approaches have been taken to counteract this problem and recently, promising results [1] have been obtained by modifying the electrodes surface using the impregnation technique. This technique consists of synthesizing catalyst nanoparticles on the electrode surface through a liquid solution-based process [2, 3].

In this work we show the results obtained when decorating La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ -LSCF- cathodes with lanthanide oxides (CeO_x, Ce_1-yGd_yO_x, PrO_x) nanoparticles after its impregnation with nitrate solutions. All systems were characterized using SEM, TEM and XRD, where spatially homogeneous and monodisperse size distributions were observed (See Figure 1a and 1b, where a cathode with and without nanoparticles is shown, respectively).

Figure 1c presents the Nyquist plot of Electrochemical Impedance Spectroscopy (EIS) measurements at 600 ºC. These measurements showed that when impregnating with lanthanide oxide nanoparticles the cathode polarization resistance at 600ºC was reduced from 0.6 Ωcm² for plain LSCF to 0.32, 0.2 and 0.08 Ωcm² for LSCF decorated with Ce_0.8Gd_0.2O₂, CeO₂ and PrO₂ oxide nanoparticles, respectively. The reduction due to the Pr oxide impregnation allows, in principle, a decrease of 100 ºC (from 700ºC to 600ºC) of the cathode's operation temperature, which could be considered as a candidate for low temperature SOFC (LT-SOFC)[4].

Figure 1. a) plain LSCF b) GDC impregnated LSCF c) Nyquist plots measured at 600 ºC of plain LSCF and LSCF impregnated with CeO₂, GDC and PrO₂.

The EIS measurements as a function of temperature and oxygen partial pressure enable the proposal of a mechanism for the oxygen reduction reaction and an explanation about the effect of lanthanide oxides nanoparticles decoration. For example, when impregnating with GDC nanoparticles the impregnation is observed to modify the surface resistance of the ORR in the cathode’s surface but leaves the O^2- ion conduction resistance in bulk LSCF unmodified [5].

Finally, the EIS measurements as a function of time during the aging of the samples exposed for periods between 300 h and 500 h to the operation temperatures was observed to produce an irreversible degradation (melting) of the impregnated nanoparticles.

References:

[1] A. Jacobson. Materials for Solid Oxide Fuel Cells. Chem. Mater. 22, 660, (2010).

[2] D. Dong. Enhancing SOFC cathode performance by surface modification through infiltration. Energy Environ. Sci. 7,552, (2014).

[3] S. P. Jiang. Nanoscale and nano-structured electrodes of solid oxide fuel cells by inﬁltration: Advances and challenges. Int J Hydrogen Energy 37, 1, (2012).

[4] S. Barnett L. Gao, L. Mogni. A perspective on low-temperature solid oxide fuel cells. Energy Environ. Sci., 9 (5):1602–1644, (2016).

[5] J. Ascolani-Yael, et. al. Study of the Rate Limiting Steps and Degradation of a GDC Impregnated La_{0. 6}Sr_{0. 4}Co_{0. 8}Fe_{0. 2}O_3-δ cathode. ECS Transactions, 78(1), 795-805, (2017).

2090 Study of the Electrochemical Mechanisms That Control the Electrode Reaction and Degradation of La0.6Sr0.4Co0.8Fe0.2O3-δ cathodes Impregnated with Oxide Nanoparticles

2090
Study of the Electrochemical Mechanisms That Control the Electrode Reaction and Degradation of La_0.6Sr_0.4Co_0.8Fe_0.2O_3-_δcathodes Impregnated with Oxide Nanoparticles