Long-Term Tests and Advanced Post-Test Characterizations of the Oxygen Electrode in Solid Oxide Electrolysis Cells

Abstract

Solid Oxide cells (SOCs) are promising high-temperature electrochemical energy-conversion devices, which enable a fossil-free sustainable development. Nevertheless, performance degradation is still a bottleneck for the large-scale commercialization. The most commonly employed material for the air electrode is La1-xSrxCoy-1FeyO3-δ. The chemical decomposition of this material is known to considerably impact the cell performance [1,2].The studied cells are composed of a La0.6Sr0.4Co0.2Fe0.8O3 -δ (LSCF) current collecting layer (CCL) and a LSCF-Ce0.8Gd0.2O2 -δ (LSCF-GDC) composite for the air electrode, GDC for the barrier layer, (Y2O3)0.08(ZrO2)0.92 (YSZ) for the electrolyte and Ni-YSZ for the functional layer and the support layer of the fuel electrode. The present study aims to investigate the degradation occurring in the air electrode of a complete cell aged at 750°C for 2000 h in electrolysis mode. The cell degradation is assessed by electrochemical measurements, such as polarization curves and electrochemical impedance spectroscopy (EIS), and post-mortem characterizations of the air electrode using high resolution multimodal chemical imaging based on synchrotron X-ray diffraction (µ-XRD) in Paul Scherrer Institut (PSI, microXAS), complemented by X-ray photoelectron spectroscopy (XPS).The electrochemical characterizations performed at the beginning of the test and after ageing showed a decrease in the cell performance. Notably, a significant evolution at the high frequency region was observed in the impedance spectrum. However, the medium frequency region of the EIS, mainly ascribed to the air electrode using a physically-based multiscale model [3], did not show any particular evolution. Post-mortem characterizations using synchrotron µ-XRD technique showed an inter-diffusion layer (IDL) in between the GDC and YSZ layers, for both pristine and aged samples (Fig. 1). In addition, a slight increase in the LSCF unit cell volume was observed after ageing. Nevertheless, XPS results did not highlight any significant evolution in the Sr segregation after operation. According to the impedance spectra and the post-mortem analyses, the ageing of the air electrode seems to have a limited impact on the cell performance loss in these operating conditions.