Ni and Fe Doped (La,Sr)TiO3 Fuel Electrodes for Solid Oxide Cells – Electrical and Electrochemical Properties

Abstract

Development of robust and impurity tolerant solid oxide cells (SOCs) has attracted a significant R&D-effort in recent years. The shortcomings of Ni-YSZ fuel electrodes in terms of redox stability, carbon and sulfur tolerance and long-term durability under strong polarization has spurred the search for alternative fuel electrode materials. Among them, doped strontium titanate materials has shown promising performance due to their good electrical transport properties, chemical and redox stability and tolerance towards impurities.In the present study, the electrical properties of the La0.477Sr0.33Ni0.03Fe0.03Ti0.94O3- δ (LSFNT) material, which is designed with the potential for exsoluting Ni and Fe nanoparticles, is investigated in detail. In addition, the performance of LSFNT based fuel electrodes is characterized in a symmetrical cell configuration. The electrical conductivity measurements are performed on both dense and porous LSFNT layers, as a function of temperature and pO2. The pre-reduced (1100oC, 24 hours, dry 5%H2/N2) dense LSFNT sample showed an electrical conductivity of ca. 10 S/cm at 850 oC in dry 5%H2/N2. The “in-situ reduced” (850oC, 700 hours) porous LSFNT layer showed a conductivity 10 times smaller. The conductivity was tracked as a function of pO2 showing an approximate pO2 -1/6 dependence. Also the ionic conductivity of the material was determined using an electron blocking method. At 850oC at a pO2 of 10-20 bar the ionic conductivity is ca. 0.05 S/cm at which is similar to that of Y2O3 doped ZrO2 electrolytes demonstrating that LSFNT is indeed a very good mixed ion- and electron-conducting material.Motivated by the observed conductivities SOC fuel electrodes of LSFNT were prepared by incorporating Gd-doped CeO2 electrocatalyst into LSFNT porous backbones and their electrochemical performance was measured in 50% H2 / 50% H2O at OCV. An electrode polarization resistance of 0.05 Ω cm2 at 850oC and 0.26 Ω cm2 at 650 oC is measured for these electrodes. The polarization resistance at 850 oC is comparable to that of Ni-YSZ fuel electrodes; while at 650oC it is significantly lower in similar atmospheres. This promising electrochemical performance shows the potential of the LSFNT fuel electrodes, as an alternative to Ni-YSZ fuel electrodes, in solid oxide cell applications.