Impact of the strontium content on the performance of Bi1.5Er0.5O3/La1‑xSrxMnO3 composite electrodes for low temperature SOFCs

Typical high temperature solid oxide fuel cells (HT-SOFC) use a La0.8Sr0.2MnO3 (LSM)/Zr1.92Y0.08O3.96 (YSZ) composite cathode on a YSZ electrolyte and a nickel/YSZ cermet anode. Due to the limited oxide conductivity of yttria stabilized zirconia, high temperatures (> 700 °C) are required in order to achieve sufficient ionic conduction. However, elevated temperatures come with significant engineering challenges motivating the long standing effort to decrease the operation temperature of SOFC. Substituting YSZ by erbium stabilized bismuth oxide (ESB) in the cathodic compartment and gadolinium doped ceria (GDC) at the anode, two much better ionic conductors, allow for a significant improvement leading to a specific power density of ~1 W cm-2 at 650 °C.1 Based on these results several groups optimized the microstructure or preparation of the cathode without changing the composition of the LSM.2 However, this stoichiometry was optimized for high temperature compatibility with YSZ and needed to be optimized as well for lower temperature. Here, we focused on the influence of the strontium content in La1-xSrxMnO3 on the area specific resistance (ASR) of composite LSM/ESB electrodes. Several compositions (x = 0.15, 0.3, 0.4, 0.5, 0.6, 0.8) were tested to investigate the influence of the structure, the electronic transport mechanism or the Mn3+/4+ ratio. Using X-ray diffraction, thermogravimetric analysis and Low Energy Ion Scattering spectroscopy (LEIS) we showed that, differently from the high temperature range, where surface segregation of SrO is the main deactivation process, at lower temperature it is the surface oxidation that inhibits the oxygen reduction efficiency of LSM. The catalytic activity of the cathodes was evaluated by Electrochemical Impedance Spectroscopy (EIS) on symmetric cells using an ESB electrolyte. We found a 2-fold decrease of the ASR at 500 °C when switching from La0.85Sr0.15MnO3, i.e. the high temperature optimized stoichiometry, to La0.6Sr0.4MnO3.3 Acknowledgements The Fonds Europeen de Developpement Regional (FEDER), CNRS, Region Hauts de France, Ministere de l'Enseignement Superieur et de la Recherche and Agence Nationale de la Recherche and BIBELOT ANR-18-CE05-0001 are acknowledged for funding. References [1]E.D. Wachsman, K.T. Lee, Science, 334 (2011), 935-939 [2]J.W. Park, B.H. Yun, D.W. Joh, K.T. Lee, Electrochemical Society, 68(1) (2015), 957-96 [3]M. Pajot, V. Duffort, E. Capoen, A.-S. Mamede, R.N. Vannier, J. Power Sources 450 (2020), 227649.