Pt-Doped Lanthanum Ferrites as Versatile Electrode Material for Solid Oxide Cells
Authors: Duranti Leonardo , Panunzi Anna Paola , Draz Umer , D'Ottavi Cadia , Licoccia Silvia , Di Bartolomeo Elisabetta 
Published: September 2023

La0.6Sr0.4Fe0.8Co0.2O3-δ (LSCF) alone or mixed with doped ceria represents the reference air electrode material for IT-SOFC/SOEC, thanks to its intrinsic MIEC nature and unrivaled catalytic activity towards oxygen exchange reactions. Nevertheless, a steep growth in cobalt demand for automotive (EVs) and storage applications is expected in the next decades, and ethical issues related to cobalt mining in politically unstable countries endanger cobalt supply chain [1]. Noble metal doping to a low extent ( ≤ 5% mol) has drawn attention as a valid strategy to provide the parent oxide structure with high catalytic activity [2][3]. Such amounts of platinum-group-metals (PGM) could be profitably harvested from secondary sources, such as spent automotive three-way catalysts (TWCs) [4], rendering their supply less dependent from localized low-grade ores. In particular, due to the tendency of PGM to be easily reduced to the metallic state, PGM-doped perovskites have found application mainly as fuel electrode materials. Noble metal exsolution endows the parent perovskite with finely dispersed nanoparticles firmly anchored to the oxide substrate, that display high catalytic activity and stability [5].

Here, platinum-doping on strontium-substituted lanthanum ferrites is explored as an innovative strategy to enhance the parent perovskite catalytic activity both in reducing and oxidizing environment. The ultimate goal is the design of a multi-functional electrode for Reversible Solid Oxide Cells (r-SOCs). La0.6Sr0.4Fe1-xPtxO3-δ, X = 0.005, 0.01, 0.05 (LSFPtX) stoichiometries were successfully synthesized and characterized by means of X-ray Powder Diffraction (XPRD) (Fig. 1), thermogravimetric analysis (TG), oxygen temperature-programmed desorption (O2-TPD) and X-ray photoelectron spectroscopy (XPS). LSFPtX were tested in symmetric cells to evaluate their activity as air electrode materials in terms of polarization resistance vs. temperature and vs. pO2. The rate-limiting steps of oxygen exchange reactions were investigated by means of electrochemical impedance spectroscopy (EIS) fitting experimental data with equivalent circuit (EC) modelling and distribution of relaxation times (DRT). Results were compared both to the undoped (LSF) and to a commercially available state-of-art 20 mol% Co-substituted LSCF. LSFPtX structural evolution is evaluated at different pO2 and in several SOCs operational environments (5%H2/Ar, 100% CO2 and 50% CO2 : 50% CO). B-site exsolution was studied by means of temperature-programmed reduction (H2-TPR) and scanning electron microscopy (FE-SEM) to assess their applicability as fuel electrode materials as well. Finally, LSGM-based symmetric cells were tested as SOFC and CO2-SOEC, evaluating the operational reversibility and the long-term stability.

[1] IEA, Total Cobalt Demand by Sector and Scenario, October 2022, pp. 2020–2040.

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[3] Marasi, M., Duranti, L., Luisetto, I., Fabbri, E., Licoccia, S., & Di Bartolomeo, E. (2023). Ru-doped lanthanum ferrite as a stable and versatile electrode for reversible symmetric solid oxide cells (r-SSOCs). Journal of Power Sources, 555, 232399.

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[5] Kothari, M., Jeon, Y., Miller, D. N., Pascui, A. E., Kilmartin, J., Wails, D., ... & Irvine, J. T. (2021). Platinum incorporation into titanate perovskites to deliver emergent active and stable platinum nanoparticles. Nature Chemistry, 13(7), 677-682.

Figure 1