Understanding La2NiO4-La0.5Ce0.5O2 Oxygen Electrode Phase Evolution in a Solid Oxide Electrolysis Cell

Abstract

This work aims to understand the decomposition mechanism of a La2NiO4 phase in La2NiO4-La0.5Ce0.5O2 (LNO-LDC) oxygen electrodes after testing in a solid oxide electrolysis cell (SOEC) at 800oC. Scanning/transmission electron microscopy (S/TEM) examination of LNO-LDC electrode at atomic level before and after testing was undertaken. Pure LNO and LDC phases as well as a low density of lanthanum phosphate (LPO) grains were detected in the as prepared (untested) oxygen electrode. P enrichment, a potential constituent in some of the synthesis additives, was found as small, nanometer scale, deposits along all grain boundaries which is suggested to poison the LNO phase during SOEC operation. Mild to aggressive decomposition of La2NiO4-La0.5Ce0.5O2 (LNO-LDC) oxygen electrode was observed after SOEC operating at 800 °C at 1.3V for 920 hours. Detailed microstructural and microchemical characterization of the decomposed regions was performed using an aberration (CS) corrected JEM-ARM200CF transmission electron microscope operated at 200 kV. The evolution of the LNO phase decomposition was noted beginning with the expulsion of Ni into the surrounding LNO matrix and grain boundaries, forming La-rich and Ni-rich phases in the LNO correspondingly. An acicular La2O3 phase was always noted initiating at either the LNO/LDC or LNO/LNO interfaces, growing and eventually intersecting one another, especially in aggressively decomposed regions. Initial decomposition exhibited single layers of Ni atoms along (004) plane of LNO, followed by gradual formation of La3Ni2O7, LaNiO3, and fine NiO (several nanometer in size) phases during moderate to aggressive decomposition. In addition, NiO was noted at LDC/LDC grain boundaries ranging from a few to several tens of nanometers thick. This presentation will demonstrate how P contamination may affect the stability of SOEC cells.