Defect chemistry, ionic and electronic conductivity of an Fe/Ni-substituted La0.49Sr0.31TiO3 exsolution material

Abstract

Solid oxide cells offer unrivalled efficiency in energy conversion and can become a key technology for the green transition of the energy system. The state-of-the-art fuel electrode of such cells, a Ni-zirconia composite, suffers from some limitations: poor durability at high polarization, sensitivity to detrimental coke formation, and limited redox stability. Electrodes made from perovskite materials may offer a solution to these challenges; they show a reduced tendency for coke formation and have the potential to enhance stability and performance. To this end, developing perovskite materials with enhanced mixed ionic and electronic conductivity (MIEC) and the capacity to exsolve nanoparticles to boost performance is important. This study introduces a defect chemistry model for a promising “exsolution” material (La_0.49Sr_0.31Ti_0.94Fe_0.03Ni_0.03O_3, LSFNT) and reports on the transport properties of the material. LSFNT retains a stable cubic perovskite structure across a wide oxygen partial pressure range (0.21 to 10⁻²¹ bar) and ex-solves Ni_1-xFe_x nanoparticles in pure hydrogen. The conductivity of LSFNT increases with decreasing oxygen partial pressure, displaying an approximate ${{\mathrm{pO}}_2}^{-1/6}$ dependence in the range of 10⁻¹⁴ to 10⁻¹⁸ bar. Below this threshold, the ${\mathrm{pO}}_2$ dependence of the conductivity deviates from this trend due to oxygen vacancy annihilation and Fe/Ni nanoparticle exsolution, consistent with the proposed defect chemistry model. This work also demonstrates the mixed ionic and electronic conductivity in LSFNT. Electron-blocking experiments reveal a high ionic conductivity of LSFNT (0.054 S/cm at 850 °C), which exceeds that of yttria-stabilized zirconia (8YSZ) and is comparable to gadolinium-doped ceria (Ce_0.9Gd_0.1O₂, CGO). Overall, these findings underscore the good stability of LSFNT alongside noteworthy electronic and ionic conductivity, rendering it a strong candidate as a fuel electrode backbone material for solid oxide cells.