Investigation of Sr_0.7 Ca_0.3 FeO₃ Oxygen Carriers with Variable Cobalt B-Site Substitution

A-site and B-site substitutions are effective methods towards improving well-studied oxygen carrier materials that are vital for emerging gasification technologies. Such materials include SrFeO₃ , which greatly benefits from the inclusion of calcium and/or cobalt, and Sr_0.8 Ca_0.2 Fe_0.4 Co_0.6 O₃ has been regarded as the best-performing composition. In this study, systems with higher calcium and lower cobalt contents are investigated with a view to lessening the societal and economic burdens of these dual-doped carriers. Density functional theory calculations are performed to illustrate the Fe-O bonding and relaxation contributions to the oxygen vacancy formation energy in Sr_1-x Ca_x Fe_1-y Co_y O₃ systems (x=0.1875, 0.25, 0.3125; y=0.125, 0.25, 0.375, 0.5) and determine that increased calcium A-site substitution requires the use of less cobalt B-site doping to reach the same oxygen vacancy formation. These findings are experimentally validated in situ and ex situ characterization of bulk Sr_0.7 Ca_0.3 Fe_1-y Co_y O₃ materials. Sr_0.7 Ca_0.3 Fe_0.7 Co_0.3 O₃ is found to have similar O₂ adsorption/desorption rates and storage capacity to Sr_0.8 Ca_0.2 Fe_0.4 Co_0.6 O₃ in air/N₂ cycling experiments. Additionally, both materials are outperformed by Sr_0.7 Ca_0.3 Fe_1-y Co_y O₃ systems with y=0-0.10 at 400-500 °C, which cycle 1.5 wt% O₂ in under ten minutes.