Highly active oxygen electrodes for metal-supported solid oxide electrolysis cells

It has become critically important to develop highly active oxygen electrodes for metal-supported solid oxide electrolysis cells (MS-SOECs) operating at reduced temperatures. Due to their excellent mixed oxide ionic and electronic conductivities, four types of cobaltate perovskite oxides, LaBa0.5Sr0.5Co2O5+δ (LBSCO), PrBa0.5Sr0.5Co2O5+δ (PBSCO), SmBa0.5Sr0.5Co2O5+δ (SBSCO) and GdBa0.5Sr0.5Co2O5+δ (GBSCO), are selected and impregnated into the porous scandium stabilized zirconia (SSZ) scaffolds as the oxygen electrodes. Impedance measurements on symmetrical cells show that their catalytic activities toward oxygen reduction and evolution reactions decrease in the order – PBSCO > SBSCO > GBSCO > LBSCO. The polarization resistances measured for PBSCO in air are 0.03, 0.11, 0.42 and 2.25 Ω•cm2 at 650, 600, 550 and 500°C, respectively. MS-SOECs with impregnated PBSCO catalysts produce a steam electrolysis current density as high as 1.49 A•cm−2 at 650°C and 1.3 V. Analysis of impedance data at different oxygen partial pressures reveals charge transfer as the rate-limiting step. Density-functional theory (DFT) calculations show a higher oxygen p-band center for PBSCO than for SBSCO, resulting in a smaller energy barrier for electron transition between the O p-band and the Fermi level and thereby facilitating the charge transfer step during oxygen reduction and evolution reactions. The present results demonstrate a great promise of PrBa0.5Sr0.5Co2O5+δ as a highly active oxygen electrode catalyst for MS-SOECs operating at reduced temperatures.