Effect of Sr2+ doping on the structure and electrical properties of hexagonal perovskite Ba7-xSrxNb4MoO20-δ electrolyte: experimental and DFT modeling studies

Abstract

Ba₇Nb₄MoO₂₀ exhibits excellent oxygen ion transport properties and is a promising electrolyte material for solid oxide fuel cell (SOFC). To further enhance its oxygen ionic conductivity, element doping is an effective strategy. However, few studies have delved into the impact mechanism of doping strategies on the electrolyte's conductivity properties from the perspective of electronic structure. Here, the enhancement mechanism of oxygen ionic conductivity in Ba₇Nb₄MoO₂₀ was analyzed using the the methods of density of states (DOS) and Crystal Orbital Hamilton Populations (COHP). Since the electronic conductivities of the electrolytes are negligible, their total conductivies can essentially be regarded as the conductivies of oxygen ions. As the Sr doping amount increases, the oxygen ionic conductivities of the electrolytes also increase. The bulk conductivity shows a negative correlation with the Sr doping amount, which is due to the higher bond energy of Sr-O compared to Ba-O. On the other hand, Sr promotes grain growth and reduces the number of grain boundaries, thereby decreasing the resistance to oxygen diffusion at the grain boundaries and thus enhancing the grain boundary conductivity. In the Ba_7-xSr_xNb₄MoO_20-δ (x=0, 0.1, 0.2, 0.3, and 0.4) perovskite oxides, Ba_6.6Sr_0.4Nb₄MoO_20-δ has the highest conductivity, reaching 1.12 × 10^-4S cm^-1 at 500°C. This work not only develops a SOFC electrolyte material with promising application prospects, but also provides theoretical guidance for its doping modification.