Improved oxide-ion and lower proton conduction of hexagonal perovskite-related oxides based on Ba₇Nb₄MoO₂₀ by Cr⁶⁺ doping

Abstract

Oxide-ion conductors based on hexagonal perovskite-related oxide Ba₇Nb₄MoO₂₀ have attracted much attention due to high oxide-ion and proton conductivities and potential applications in many electrochemical devices such as solid oxide fuel cells (SOFCs). Herein, we report simultaneous improvement of oxide-ion conductivity and suppression of proton conductivity by Cr⁶⁺ doping in Ba₇Nb₄MoO₂₀. New materials Ba₇Nb_4−xCr_xMoO_20+x/2 (x = 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5) were synthesized. It was found that Ba₇Nb_3.8Cr_0.2MoO_20.1−δ (δ is the amount of oxygen deficiency in Ba₇Nb_3.8Cr_0.2MoO_20.1−δ) exhibits high oxide-ion conductivity of 1.6 × 10⁻³ S cm⁻¹ at 508 °C and 1.1 × 10⁻² S cm⁻¹ at 904 °C in static air and low proton transport number under wet conditions. Ba₇Nb_3.8Cr_0.2MoO_20.1−δ also shows wide electrolyte domain in the oxygen partial pressure P(O₂) regions from 1 to 2.2 × 10⁻²⁷ atm (304 °C) and 1 to 1.5 × 10⁻²⁶ atm (604 °C), indicating extremely high chemical and electrical stability. The structure analyses have shown that Ba₇Nb_3.8Cr_0.2MoO_20.1−δ is a hexagonal perovskite related oxide at 22 and 800 °C. The refined crystal structure of Ba₇Nb_3.8Cr_0.2MoO_20.1−δ has oxygen-deficient cubic (c′) close-packed Ba(O1)_2−y(O5)_z layer where y is the amount of oxygen vacancy at the tetrahedral O1 site and z is the amount of interstitial octahedral oxygen at the O5 site. The Cr bond valence sum indicates an oxidation number of +6: Cr⁶⁺. The Cr⁶⁺ cation is located at a crystallographic site near the c′ layer, which leads to the excess oxygen and high conductivity, and is likely to suppress the proton conduction. Maximum-entropy method (MEM) analyses have demonstrated that oxide ions two-dimensionally migrate through the lattice O1 and interstitial O5 sites in the c′ layer via the interstitialcy diffusion mechanism at 800 °C, which enables the high oxide-ion conduction. Cr⁶⁺ doping in various hexagonal perovskite-related oxides would be a new strategy for the simultaneous improvement of oxide-ion conductivity and suppression of proton conduction.