Strontium Free Cu-Doped La2NiO4 Oxides as Promising Oxygen Electrodes for Solid Oxide Electrolysis Cells

Abstract

In this work, strontium-free oxygen electrodes for electrode-supported SOFC/SOEC cells were evaluated. Strontium is known to react with volatile Cr from the metallic interconnect to form Sr-Cr oxides at the surface of the electrode, thus reducing the cell performance. Strontium-free cobalt- or copper-doped nickelates, La2Ni1-xBxO4 (B = Co or Co; x=0.1-0.5) were synthesized using solid state synthesis method. The crystal structures of the oxides were studied by Rietveld refinement analysis of the powder X-ray diffraction data. Similar to the parent compound, the Ruddlesden-Popper (RP) La2NiO4 oxide, the Co- and Cu-doped compositions were confirmed to crystallize with the tetragonal crystal structure with I4/mmm space group.The electrochemical performance of the La2Ni1-xBxO4 electrodes was studied using 1 inch in diameter button cells in SOEC mode and compared to that of cells with the state-of-the-art (La0.6Sr0.4)0.95Co0. 0.8O3-δ (LSCF)- ceria composite electrodes. The oxide inks were screen printed onto the pre-fired ceria barrier layers on the YSZ electrolytes. Short- and long-term tests were conducted at 650, 750, and 800oC with the Ni-YSZ hydrogen electrode exposed to 50-98% steam in hydrogen. Scanning electron microscopy with electron diffraction spectroscopy (SEM/EDS) was used to analyze the electrode microstructures before and after testing.In general, the performance of the La2Ni1-xBxO4 electrodes was slightly lower than that of LSCF. However, the electrode microstructures were not optimized, and further improvements could be made. The Co-substituted nickelate performances showed some degradation in in time. Cu-doped nickelates exhibited more performance during the same tests and the degradation rate was insignificant. The composition with 20% Cu appeared to be more active as well as more stable.