Hydration-Driven Enhancement of Interstitialcy Oxide-Ion Diffusion

Abstract

Abstract The transport properties of oxide ions (O2‒) and protons (H+) in ceramic materials are crucial to the development of fuel cells as a clean energy source. There are two main types of oxide-ion diffusion mechanism: the oxygen vacancy mechanism and the interstitialcy diffusion mechanism. Oxide-ion conduction by the oxygen vacancy mechanism is generally suppressed by hydration due to the decreased number of oxygen vacancies. Recently, the oxide-ion conduction via the interstitialcy diffusion mechanism has attracted much attention due to its high ion conductivity in both dry and wet conditions. However, the effect of hydration on the transport properties of the interstitial oxide ion conductors is not well understood compared to conventional oxygen vacancy conductors. In this study, we demonstrate that hydration enhances both the oxygen conductivity and diffusivity in the interstitial oxide ion conductor Ba7Nb4MoO20. Using both oxygen and water vapor concentration cells, we accurately determined the transport numbers of protons tH and oxide ions tO, showing that the predominant conducting species is the oxide ion, even in wet atmospheres (e.g., tO = 0.955, tH = 0.045 at 600 °C). The oxide-ion conductivity is higher in wet atmospheres than in dry conditions (e.g., twice as high at 500 oC), due to the higher oxygen diffusion coefficient in wet atmospheres. This is evidenced by measurements of both the tracer and conductivity diffusion coefficients, and by the self-diffusion coefficients from molecular dynamics (MD) simulations using a neural network potential. The MD simulations indicated that the higher oxygen diffusion coefficient in wet atmospheres is due to an increased number of (Nb/Mo)2O9 dimers. This is caused by an increased number of excess interstitial oxygen ions resulting from hydration. These findings may open up new avenues in the science and engineering of proton, oxide-ion, and dual-ion conductors.