Proton and oxide-ion conductors are attractive materials having a wide range of potential applications such as PCFC, PCEC, SOFC, and SOEC. The conventional strategy to improve the proton and oxide-ion conductivity is acceptor doping into oxides without oxygen vacancies. However, the acceptor doping results in proton and vacancy/oxide-ion trapping near dopants, leading to the high apparent activation energy and low proton conductivity at intermediate and low temperatures. Intrinsic oxygen vacancies are the oxygen vacancies in a parent material. In this keynote, I present the high proton and oxide-ion conduction via the intrinsic oxygen vacancies.

The hypothetical cubic perovskite BaScO_2.5 may have intrinsic oxygen vacancies without the acceptor doping. Herein, I report that the cubic perovskite-type BaSc_0.8Mo_0.2O_2.8 stabilized by Mo donor-doing into BaScO_2.5 exhibits high proton conductivity within the ‘Norby gap’ (e.g., 0.01 S cm⁻¹ at 320 °C) and extremely high chemical stability under oxidizing, reducing and CO₂ atmospheres [1]. The high proton conductivity of BaSc_0.8Mo_0.2O_2.8 at intermediate and low temperatures is attributable to high proton concentration, high proton mobility due to reduced proton trapping, and three-dimensional proton diffusion in the cubic perovskite stabilized by the Mo-doping into BaScO_2.5. The donor doping into the perovskite with disordered intrinsic oxygen vacancies would be a viable strategy towards high proton conductivity at intermediate and low temperatures. I also report high proton and oxide-ion conduction in hexagonal perovskite-related oxides with intrinsically deficient oxygen layers [2-7].

Many Bi-containing compounds exhibit high oxide-ion conductivity via conventional vacancy mechanism. However, interstitial oxide-ion conduction is rare in Bi-containing materials. Herein, I also report high oxide-ion conductivity through interstitial oxygen (intrinsic oxygen vacancy) sites in Sillén oxychlorides, LaBi_2-xTe_xO_4+x/2Cl [8]. Oxide-ion conductivity of LaBi_1.9Te_0.1O_4.05Cl is 20 mS cm⁻¹ at 702 °C, and higher than best oxide-ion conductors as Bi₂V_0.9Cu_0.1O_5.35 below 201 °C. Despite of the presence of Bi and Te species, LaBi_1.9Te_0.1O_4.05Cl shows extremely high chemical and electrical stability at 400 °C from oxygen partial pressure 10⁻²⁵ to 0.2 atm and high chemical stability under CO₂ flow, wet 5% H₂ in N₂ flow, and air with natural humidity. Neutron scattering length density analysis, DFT calculations, and ab initio molecular dynamics simulations indicate that the extremely high oxide-ion conduction is attributed to cooperative diffusion through interstitial oxygen sites (interstitialcy diffusion mechanism) in triple fluorite layers. The present findings demonstrate the ability of LaBi_2-xTe_xO_4+x/2Cl as superior oxide-ion conductors, which could open new horizons for oxide ion conductors.