The performance of solid oxide fuel cells (SOFCs) is often limited by the sluggish oxygen reduction reaction (ORR) process at the cathode. It is also widely believed that the surface exchange kinetics and surface area of cathodes play a pivotal role in determining the overall cathodic performance. For this reason, various efforts have been made to improve these metrics through nanoscale materials engineering, most notably by the wet impregnation (infiltration) method.^[1] While numerous studies have proved the dramatically beneficial effect of infiltration, a systematic comparative study revealing the effect of infiltrated materials and their concentration is lacking.

In this study, we investigated the effect of infiltrated material and nitrate concentration used for sol-impregnation process on the resulting kinetics and rate-limiting steps of ORR. Various infiltrates were chosen based upon the specific merits of each material in terms of electronic/ionic conductivity, oxygen exchange activity and oxygen storage capability. The infiltrates of study includes La_xSr_1-xCo_yFe_1-yO_3-δ (LSCF), La_1-xSr_xMnO_3-δ (LSM), Gd_1-xCe_xO_2-δ (GDC), Sm_1-xCe_xO_3-δ (SDC), Pr_1-xCe_xO_3-δ (PDC), Pr₆O₁₁ and LaNi_1-xFe_xO_3-δ (LNF). Porous LNF was used as the cathodic backbone. For each infiltrate type, sols with different molar concentrations ranging from 0.2 M to 5.0 M were applied for the infiltration process. For every infiltration process, only one infiltration process was performed without a separate sintering/annealing step.

Results showed that an infiltration even with LNF, the same material with the backbone reduced the cathodic polarization resistance (R_p) from 2.6 (bare LNF) to 0.15 W cm² at 700 °C, indicating the critical importance of active surface area for cathodic performance in this system. Infiltration with 1.0 M of Pr₆O₁₁ rendered the best performance (R_p = 0.055 W cm² at 700 °C). With a concentration of 1.0 M or higher, the infiltrate comprises > 20 wt.%, a concentration high enough to infiltrate Pr₆O₁₁ throughout the surface of LNF backbone uniformly as observed by electron microscope images. The activation energy of cathodic polarization decreased from 1.75 eV (bare LNF) to 1.0 – 1.2 eV (1.12 eV for 1 M LNF; 1.18 eV for 1 M Pr₆O₁₁, 1.06 eV for 1 M LSCF), indicating a shift in the rate limiting step by the infiltration. There was, however, little dependency of activation energy on the molar concentration in a given infiltrate material as long as the concentration is > ~0.5 M. In addition, the dependency on oxygen partial pressure also provided critical clues on the shift in the rate-limiting steps of ORR, and relevant discussion will be provided in the presentation.

This work was supported by the NASA MIRO Program (NNX15AQ01A).

References

[1] D. Ding, X. Li, S. Y. Lai, K. Gerdes, M. Liu, Energy Environ. Sci. 2014, 7, 552.