Development, characterization, and performance assessment of modified Ni/GDC electrocatalysts for the reversible operation of solid oxide cells (rSOCs)

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Ζαραβέλης, Φώτιος

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The operation of solid oxide cells (SOC) at high temperatures offers several advantages, which are reflected in better performance and reduced losses due to favorable thermodynamics and kinetics of the reactions. These cells provide flexibility to switch between electrolysis and fuel oxidation processes, making them ideal for both energy production (X-to-Power) and chemical production and storage (Power-to-X).In this dissertation, cermet electrocatalysts were developed based on commercially available NiO/GDC powder (65 wt.% NiO - 35 wt.% Ce0.9Gd0.1O2-x) modified through chemical methods with transition metals such as Au, Fe, and Mo, aiming for their operation under reverse solid oxide fuel cell (rSOC) conditions. The initial efforts focused on optimizing the already studied electrode 0.4 wt.% Mo - 3 wt.% Au - Ni/GDC, with the primary goal of reducing the Au content and then finding the optimal ratio of Au to Mo. Subsequently, compositions of new electrocatalysts were created by depositing Au and Fe metals with different weight percentages (0.5 wt.% and 3 wt.% Au) to investigate the effect of metal concentration on the electrocatalyst's performance. Suitable pastes were prepared from the modified NiO/GDC powders and deposited as fuel electrodes on ceramic electrolyte 8YSZ (ZrO2 stabilized with 8 mol.% Y2O3) using screen printing. An LSCoF (La0.6Sr0.4Co0.8Fe0.2O3-δ) commercial perovskite was used as an oxygen electrode.To determine the impact of each modification on the physical-chemical and electrochemical characteristics of Ni/GDC, a series of methods were applied under different experimental conditions. Extensive physicochemical analysis of the electrodes, both in powder and half-cell forms, was conducted using techniques such as BET, SEM, XRD, H2-TPR, H2O-TPO, and in-situ H2O XPS. Electrochemical characterization was carried out over a range of temperatures from 900 to 800 °C, with the introduction of different gas mixtures (H2O/H2), switching between solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOE) operation. Data of voltage-current density (V-i) were recorded under these conditions with simultaneous analysis through Electrochemical Impedance Spectroscopy (EIS). The modified Fe-Au-Ni/GDC electrode with the best electrochemical performance was studied in a comparative stability experiment with Ni/GDC under reverse solid oxide fuel cell (rSOC) conditions. Finally, a detailed electrokinetic study of these two electrodes was performed, both in fuel cell and electrolysis cell operation, using comprehensive EIS spectra under different experimental conditions. The goal of these experiments was to calculate key kinetic parameters, such as the apparent order of the studied reaction, and to determine the effect of modifications with Fe and Au.



Energy, Hydrogen, Power-to-X, Reversible solid oxide cell operation, Ni/GDC