Characterisation of oxide-ion diffusivity and phonon vibrations in inert anode materials for molten salt electrolysis

In this study, we characterised the oxide-ion diffusivities and phonon vibrations of solid oxides to optimise inert anode materials in molten chlorides. The isotropic phonon vibration of two types of iron-based perovskite oxides (La_0.8Sr_0.2FeO_3‒δ (LSF82) and LaNi_0.6Fe_0.4O_3‒δ (LNF64)) and one fluorite oxide of (Zr_0.809Sc_0.182Ce_0.009)O_2‒δ (10SSZ) was evaluated using high-temperature X-ray diffraction measurements. The isotropic Debye temperatures were estimated to be \(\Theta\) $\mathrm{\Theta }$ _{D, LSF82} = 375 K and \(\Theta\) $\mathrm{\Theta }$ _{D, LNF64} = 345 K in air. This suggested low oxide-ion diffusivity and probable high oxide-ion blocking function of LNF64. The isotropic Debye temperature of 10SSZ ( \(\Theta\) $\mathrm{\Theta }$ _{D, 10SSZ} = 512 K) was higher than that of 8YSZ, and 10SSZ was expected to exhibit high oxide-ion permeation. The anisotropic Debye temperature revealed that oxide ions diffused preferentially along the (a, b) plane in LSF82 and the c-axis direction in LNF64. This suggested an improvement in the oxide-ion blocking function owing to the use of the layered perovskite oxide particles. The oxide-ion diffusivity in 10SSZ increased in the order a-axis > b-axis > c-axis. The oxide ion was fixed by the electrostatic three-dimensional field constructed by Zr (or Sc) in a direction parallel to the (111) face of the 10SSZ crystal lattice, and oxide ion diffusion in c-axis direction was relatively slow.