High-Performance Oxygen Electrodes for Low Temperature Solid Oxide Cells

Abstract

Solid Oxide Cells (SOCs) present an efficient, environmentally friendly, and largely scalable method for converting fuel to electricity or generating fuels via electrolysis. It is generally believed that the commercial viability of SOCs can be improved by reducing the operating temperature, e.g., ≤ 650 °C. Since low temperature SOC performance is typically limited by the oxygen electrode, current research has focused on developing new oxygen electrode materials and architectures like Ba0.5Sr0.5Co0.8Fe0.2O3-δ, Sr0.5Sm0.5­CoO3-δ, and nanostructured electrodes with high surface areas in order to achieve low temperature performance. These materials, however, suffer from several degradation phenomena such as delamination, cation segregation, chemical reactivity, and coarsening. It was recently shown that PrOx infiltrated into a Gd-doped ceria scaffold provides very low polarization resistance values at low temperature.1 However, the long term stability is not known, nor is the mechanism for its good performance or its effect on other scaffold materials, either electronic conductors or mixed ionic and electronic conductors (MIECs). Here we present results on PrOx infiltrated into Sr(Ti0.3Fe0.7-xCox)O3-δ (STFC) (0 < x < 0.15), STFC-GDC (Ce0.9Gd0.1O2-δ) composite, and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) scaffolds. Figure 1 illustrates the performance enhancing effects of PrOx infiltrated into LSCF and STFC MIEC scaffolds at 550 °C. For example, it decreases the initial polarization resistance of STFC from 0.57 to 0.22 Ω·cm2. The initial resistance decrease for LSCF+PrOx was even greater, from 2.05 to 0.58 Ω·cm2. The performance improvement is seen in the low frequency response, which is attributed to the catalytic surface processes related the addition of PrOx. Similar performance improvements are seen at 650 °C. Furthermore, the resistance values of STFC+PrOx and LSCF+PrOx are reasonably stable over ~ 1000 h life tests at 550 °C. The LSCF+PrOx result is particularly interesting since the LSCF electrode degrades considerably. The composite STFC-GDC+PrOx electrodes generally provide reduced polarization resistance compared to STFC-PrOx. References Nicollet, C., Flura, A., Vibhu, V., Rougier, A., Bassat, J.-M. & Grenier, J.-C. An innovative efficient oxygen electrode for SOFC: Pr6O11 infiltrated into Gd-doped ceria backbone. Int. J. Hydrogen Energy 2–8 (2016). doi:10.1016/j.ijhydene.2016.04.024 Figure Caption Figure 1 - Evolution of Nyquist and Bode plots during a life test at 550 °C for ~ 1000 hours of (a) La0.6Sr0.4Co0.2Fe0.8O3-δ with and without nano-infiltrated PrOx and (b) SrTi0.3Fe0.55Co0.15O3-δ with and without nano-infiltrated PrOx all supported on a Ce0.9Gd0.1O2-δ electrolyte. Figure 1