Advanced Nanoscale Characterizations of Solid Oxide Cell Electrodes

The growing interest in the transition towards a fossil-free energy system has accelerated research and development on hydrogen-based energy conversion technologies. In this context, solid oxide cells (SOCs) have attracted attention as they can be operated in both electrolysis (SOEC) and fuel cell (SOFC) modes providing a clean and sustainable way of producing both green hydrogen and electricity at high efficiency. Despite their great potential, the current SOC technology faces some challenges due to the significant cell degradation upon operation. The performance loss has been attributed to several factors, such as microstructural evolution in the electrodes [1], and inter-diffusion of elements between the cell layers associated to material decomposition and reactivity [2–4]. Despite several efforts, all these degradation phenomena are still not precisely understood as they involve complex and intricate processes arising at different length scales. To address this issue, durability tests have been performed in representative operating conditions at high current density and humidity at the air side for both SOFC and SOEC modes. In addition, the tested cells were analyzed with advanced characterization techniques to study the material degradation down to the nanometer and the atomic scales. In this work, standard fuel electrode supported cells have been studied. They are composed of a La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) current collecting layer (CCL) and a LSCF/Ce 0.8 Gd 0.2 O 2-δ (LSCF/GDC) composite functional layer (FL) for the air electrode, GDC for the barrier layer, (Y 2 O 3 ) 0.08 (ZrO 2 ) 0.92 (YSZ) for the electrolyte and Ni-YSZ for the fuel electrode. The samples were aged for 2000 h in various operating conditions: temperature (750, 800, 850 °C), humidity in the air flow (0%, 3% and 8%) and operating mode (SOEC and SOFC at ±1 A cm -2 ). The electrochemical behaviour of the cells has been characterized using electrochemical impedance spectroscopy (EIS) and polarization curves. The advanced post-mortem characterizations include techniques such as synchrotron X-ray nano-diffraction and nano-fluorescence (nano-XRD and nano-XRF) spectroscopy performed at the European Synchrotron Radiation Facility (ESRF, ID16B), scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDX), and atom probe tomography (APT). A larger range of information was acquired by using complementary techniques, including focused ion beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), and high-resolution secondary ion mass spectroscopy (SIMS) imaging. The electrochemical characterizations (Fig. 1a) have shown that the degradation was aggravated by increasing the operating temperature, the presence of humidity at the air side and the operation in electrolysis mode. The impedance spectra have been interpreted using a physically-based multiscale model [5]. The analysis has revealed that the performance loss was mainly due to the fuel electrode degradation, while the air electrode degradation remained limited, except when operated under humid conditions in the SOEC mode. The post-mortem characterizations have shown that such degradation in the SOEC mode was mainly related to a strong Ni depletion, which became more pronounced with increasing operating temperature. For the cell operated under humid air in the SOEC mode, a significant amount of SrCO 3 phase was observed on top of the air electrode. On the contrary, the cell operated under humid air in the SOFC mode did not show such phase formation. Advanced characterization techniques (nano-XRD, nano-XRF, STEM-EDX, and APT) put in evidence an intermixing of elements in the air electrode (LSCF/GDC) at the atomic scale (Fig. 1b). Moreover, the presence of an inter-diffusion layer (IDL) between GDC and YSZ phases was observed (Fig. 1c) for all samples, including the freshly prepared sample. Nevertheless, neither the mixing of elements in the air electrode nor the presence of the IDL seem to have a major influence on the cell degradation. It can be speculated that these phenomena make the air electrode more stable against phase decomposition by inhibiting the formation of commonly observed insulating phases such as SrZrO 3 and SrO. References [1] F. Monaco, M. Hubert, J. Vulliet, J.P. Ouweltjes, D. Montinaro, P. Cloetens, P. Piccardo, F. Lefebvre-Joud, J. Laurencin, J. Electrochem. Soc. 166 (2019) F1229. [2] F. Wankmüller, J. Szász, J. Joos, V. Wilde, H. Störmer, D. Gerthsen, E. Ivers-Tiffée, Journal of Power Sources. 360 (2017) 399–408. [3] J. Laurencin, M. Hubert, D.F. Sanchez, S. Pylypko, M. Morales, A. Morata, B. Morel, D. Montinaro, F. Lefebvre-Joud, E. Siebert, Electrochimica Acta. 241 (2017) 459–476. [4] J. Villanova, S. Schlabach, A. Brisse, A. Léon, Journal of Power Sources. 421 (2019) 100–108. [5] G. Sassone, E. Da Rosa Silva, M. Prioux, M. Hubert, B. Morel, J. Laurencin. In preparation. Figure 1