High-temperature proton conductors based on the (110) layered perovskite BaNdScO4

Abstract

Critical to the development of solid electrolytes in clean energy applications is a new class of proton conductors. Here, we report the first example of proton conductors belonging to (110) layered perovskites. BaNdScO₄ and new materials, BaNd_1−xA_xScO_4−x/2 (x = 0.05–0.5 for A = Mg, Ca, Sr and Ba), BaSmScO₄ and BaGdScO₄, with an orthorhombic Cmcm BaNdInO₄-type structure were synthesized. BaNd_0.8Ca_0.2ScO_3.9 shows the highest electrical conductivity among the synthesized samples and the highest proton conductivity at 650–750 °C and 850–950 °C among the modular perovskites. BaNd_0.8Ca_0.2ScO_3.9 exhibits higher proton conductivity than BaNdScO₄ (ex. 27 times higher at 600 °C), which is attributable to the formation of oxygen vacancies, leading to incorporation of water. The present structure analysis using the neutron diffraction data of the BaNd_0.8Ca_0.2ScO_3.9 sample annealed in wet air (BaNd_0.8Ca_0.2ScO_3.934(2)H_0.067(3)·0.283 H₂O) and DFT calculations indicate that the protons of BaNd_0.8Ca_0.2ScO_3.9 are coordinated with O3 atoms forming OH⁻ and that oxygen vacancies exist at O2 and O3 sites near the interface between the perovskite-like slab and the A-type rare earth oxide-like layer. DFT calculations gave a slightly lower energy barrier for the proton migration along the c axis via inter- and intra-octahedral hopping compared to those along a and b axes, suggesting one-dimensional proton migration along the c axis at low temperatures and possible three-dimensional proton diffusion at high temperatures. Thus, (110) layered perovskites based on BaNdScO₄ are a new class of proton conductors, where inter- and intra-octahedral hopping plays an important role in proton conduction. This work also reports oxide-ion conduction in orthorhombic Cmcm (110) layered perovskites. These findings would offer new routes for the design of novel proton/oxide-ion conductors based on (110) layered perovskites.