Stabilizing Coexisting n-Type Electronic and Oxide Ion Conductivities in Donor-Doped Ba–In-Based Oxides under Oxidizing Conditions: Roles of Oxygen Disorder and Electronic Structure

Surface oxygen reduction reaction (ORR) rates at n-type oxide-based mixed ionic–electronic conducting (MIEC) solid oxide fuel cell (SOFC) cathodes can be expected to be enhanced relative to that at p-type MIEC cathodes due to the greater availability of electrons at higher energies in the band structure needed for the charge transfer reaction. However, given the difficulty of achieving coexisting oxygen vacancies and electrons in the conduction band under oxidizing cathode conditions, no stable n-type MIEC cathodes have been reported to date. In this study, a predominantly n-type MIEC conductivity is confirmed in a Ba–In-based oxide (BNIM) co-doped with Nd and Mn at high temperature and high PO2 confirmed by the PO2 dependence of the electrical conductivity and negative Seebeck coefficients, combined with readily measurable oxide ion transference numbers. This coexistence of n-type electronic and oxide ion conductivities is discussed based on the electrical behavior of BNIM with different Mn levels and is attributed to the significant change in the degree of anion Frenkel ordering and the band structure associated with heavy donor doping of Ba2In2O5. This novel n-type MIEC has the potential for enhancing the ORR at SOFC cathodes at reduced temperatures and thereby identifying new potential candidate cathode materials for next-generation SOFCs.