Recipes for superior ionic conductivities in thin-film ceria-based electrolytes

We employed Molecular Dynamics (MD) and Metropolis Monte Carlo (MMC) simulations to determine the oxide-ion mobility u_O in Ce_1-GdO_2- (y = 0.02, 0.1, 0.2) for the range of temperatures 1400 ≤ T/K ≤ 2000 and field strengths 0.6 ≤ E/MV cm^-1 ≤ 15.0. Direct, unambiguous determination of u_O(E) from MD simulations is shown to require examination of the ions' mean displacement as a function of time. MD simulations were performed for random distributions of Gd cations and equilibrium distributions obtained by MMC calculations. All u_O(E,T,y) data obtained can be described by an (empirically augmented) analytical model with four zero-field parameters, a result that allows data to be extrapolated down to the temperatures of electrolyte operation. Specifically, the oxide-ion conductivity is predicted, for example at T = 700 K, (i) to be up to 10¹ higher for a random distribution of Gd than for an equilibrium distribution; and (ii) to be a factor of 10^0.8 higher for a 6 nm thin film than for a μm-thick sample under a potential difference of 1 V. By virtue of non-equilibrium deposition and nm-thick samples, thin films thus provide two new recipes for attaining even higher oxide-ion conductivities in ceria-based electrolytes.