Abstract
Hydrogen oxidation reaction (HOR) mechanisms on a novel perovskite anode with a nominal composition of La0.3Sr0.7Fe0.7Cr0.3O3-δ (LSFCr-3) were systematically investigated for solid oxide fuel cell (SOFC) applications. In terms of electrochemical impedance spectroscopy study of LSFCr-3-based half cells in various applied DC bias, hydrogen (pH2) and water vapor partial pressure (pH2O), the HOR on the LSFCr-3 perovskite was suggested to be rate-limited by the dissociative adsorption of hydrogen. At constant pH2O and pH2, the concentration of oxygen vacancies on the LSFCr-3 surface is much higher than that in the bulk. Importantly, a positive impact of water vapor on the process of dissociative adsorption of hydrogen was demonstrated for the perovskite anodes. This finding is explained by the hydration of surface oxygen vacancies, that formed more active oxygen anions sites. Under humid reducing atmosphere, the exact composition of the bulk of the LSFCr-3 is estimated to be La0.34Sr0.67Fe3 +0.7Cr3 +0.29O2.65 at 800°C. On the surface of the LSFCr-3 anode, the average oxidation state of B-site cations (Fe and Cr) is found to be lower than that in the bulk, resulting in a presence of Cr3+, Fe2+ and Fe3+. Cr3+ on the surface contributes to holding more active oxygen anions for the dissociative adsorption of hydrogen, due to its backbone action in the perovskite lattice structure. This effect of Cr3+ is suggested to be of great benefit to the electrochemical performance of the LSFCr-3 perovskite anode.
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