Fluorite-doped Gd0.1Ce0.9O1.95 polycrystalline electrolytes with extraordinarily enhanced oxide ionic conductivities controlled by depletion degree of oxygen vacancies at grain boundaries

Fluorite-doped Gd0.1Ce0.9O1.95 (GDC) polycrystalline electrolytes (Gd0.1Ce0.9-xCaxF2xO1.95-2x, x = 0.00–0.10) have been synthesized for the first time, which have the same concentration of oxygen vacancies as GDC but show extraordinarily enhanced oxide ionic conductivities, along with suppressed electronic conductivities in reducing atmospheres. In particular, Gd0.1Ce0.85Ca0.05F0.1O1.85 shows its maximum ionic conductivities as high as 4.2 to 2.2 times that of GDC in the temperature range from 600 to 800 °C. On the basis of general structural characterizations and conductivity measurements of the samples, the oxide ionic conductivities of grains and grain boundaries of Gd0.1Ce0.85Ca0.05F0.1O1.85 and GDC are comparatively studied by analyzing their low-temperature AC-impedance spectra and the microstructures and compositional distributions across the grain boundaries by High Resolution Transmission Electron Microscope equipped with Energy Dispersive Spectroscopy. It has been discovered that such extraordinary enhancements in oxide ionic conductivity should arise from a reduced depletion degree of oxygen vacancies in the extended areas of grain boundaries in the fluorite-doped GDC, and the additional activation energy for oxide ionic conduction across the grain boundaries could be brought about by the local aggregation of larger dopant cations that hamper the oxide ions to transition rather than the potential barrier by space charges. •CaF2-doped GDC electrolytes have been synthesized and studied for the first time.•0.05CaF2-GDC is 2–4 times that GDC in oxide ionic conductivity at 600–800 °C.•Higher conductivity arises from less-depleted oxygen vacancies at grain boundaries.•Aggregation of larger cations limits transition of oxide ions at grain boundaries.