Formation mechanism of large porosity in solid oxide cell electrode coatings fabricated using ethylene glycol-based precursor solutions and its impact on the electrochemical performance

Abstract

Recently, a novel composite solid oxide fuel cell electrode fabrication method based on ethylene glycol (EG)based polymeric precursors, which yielded long triple phase boundaries and thus, high performance, was developed. However, some of these coatings were reported to contain randomly formed, large pores. Since porosity observed in the microstructure may impact the electrochemical performance directly, the origin of its formation and ways to eliminate it were investigated in the present study. We hypothesized that the large pores detected in microstructure were related to incomplete polymerization which resulted in instant discharge of carbonaceous gases and that promoting further polymerization by adding extra nitric acid would eliminate these pores. Thermal analyses revealed that when liquid La0.8Sr0.2FeO3-Ce0.8Sm0.2O2 (LSF-SDC) precursors without additional nitric acid were heated, an abrupt weight loss took place at 200 degrees C in the case of polymeric solutions. Addition of nitric acid resulted in a weight loss that took place over a temperature range of 200-400 degrees C. Microstructural analyses showed large pores in LSF-SDC films, especially in the case of thick (ca. 4-4.5 mu m) samples. These large pores were mostly eliminated upon nitric acid addition. Electrochemical impedance spectroscopy measurements revealed ca. 60% reduction in the polarization resistance of the thick electrodes upon nitric acid addition, due to the elimination of large pores and thus enhancement of the triple phase boundary length.