Defect chemistry and transport properties of SrCo1 − xTaxO2.5 + δ as a promising oxygen electrocatalyst for reversible solid oxide fuel cells

In this study, we report on thermodynamic and transport properties of SrCo1 − xTaxO2.5 + δ (SCT) derived from a defect chemistry model encompassing oxygen interstitials as the ionic point defect and holes/excess electrons as the electronic point defects. The results show that SCT can be reasonably modelled as a large-polaron itinerant hole-conductor with a constant mobility at high oxygen stoichiometry (2.5 + δ). At low oxygen stoichiometry, electronic carriers tend to be localized small-polarons with a trapping center at TaCo··. With the established defect model, a complete picture of electron hole concentration p = p(T, Po2), excess electron concentration n = n(T, Po2) and oxygen nonstoichiometry δ = δ(T, Po2) is mapped out, from which p = p(T) under a constant δ is derived. The latter further reveals the true activation energy for hole-conduction under a constant δ and yields thermodynamic data for the oxygen incorporation reaction creating oxygen interstitials. These fundamental data are also compared with SrCo1 − xNbxO2.5 + δ (SCN), suggesting that SCT is a better hole-conductor with higher hole concentration for oxygen electrocatalysis.