Borates as a new direction in the design of oxide ion conductors

Abstract

Lowering the operating temperature of solid oxide fuel cells (SOFCs) has extensively stimulated the development of new oxide ion conductors. Here, inspired by the structural commonalities of oxide ion conductors, the inability to accommodate oxygen vacancies in the rigid, isolated, 3-fold tetrahedral rings of SrSi/GeO3-based materials, and the considerable flexibility of BOn polyhedra in terms of coordination number, rotation, deformation, and linkage, we report the first borate-base family of oxide ion conductors, (Gd/Y)1−xZnxBO3−0.5x, through combined computational prediction and experimental verification. The oxygen vacancies in (Gd/Y)BO3 can be accommodated by forming B3O8 units in isolated, 3-fold, tetrahedral rings of B3O9 and transported through a cooperative mechanism of oxygen exchange between the B3O9 and B3O8 units, which is assisted by the intermediate opening and extending of these units. This study opens a new scientific field of the borate system for designing and discovering oxide ion conductors.