Abstract
Iron incorporation is one of the key factors to generate highly active NiFe(OH)x electrocatalysts for the oxygen evolution reaction (OER). However, it remains controversial whether Fe(IV) is present during OER and how the oxygen vacancies in Fe(IV)-containing metal oxides affect their activities. In this work, we develop a facile approach to control oxygen vacancy content, via an ozone treatment under ambient pressure during cooling for the synthesis of La1-x Sr x FeO3-δ (0 ≤ x ≤ 1, 0 ≤ δ ≤ 0.5x) perovskite oxides - an important class of energy-related materials that contain a rare Fe(IV) oxidation state. We have initially synthesized a series of La1-x Sr x FeO3-δ compounds using a polymerized complex method. The concentration of oxygen vacancies and Fe(IV) are determined by redox titration, and the crystal structures are derived by analyzing X-ray diffraction patterns using Rietveld refinement. Significant amounts of oxygen vacancies are found in the as-synthesized compounds with x ≥ 0.8: La0.2Sr0.8FeO3-δ (δ = 0.066) and SrFeO3-δ (δ = 0.195). A subsequent ambient-pressure ozone treatment approach is able to substantially reduce the amount of oxygen vacancies in these compounds to achieve levels near the ideal oxygen stoichiometry of 3 for La0.2Sr0.8FeO3-δ (δ = 0.006) and SrFeO3-δ (δ = 0.021). The oxygenation/deoxygenation kinetics can be tuned by the cooling rate after annealing. As the oxygen vacancy concentration decreases, the structure of SrFeO3-δ evolves from orthorhombic to cubic, demonstrating that the crystal structures in metal oxides can be highly sensitive to the number of oxygen vacancies. We have further conducted a systematic evaluation of the OER activity and stability of La1-x Sr x FeO3-δ perovskites in 1 M KOH. Their initial OER activity first increases with increasing Sr content (from x = 0 to 0.8), and then decreases when the Sr content is increased to 1. Their stability, as evaluated by monitoring element leaching from the electrodes show that La does not leach at a detectable rate, but Sr and Fe leach substantially. The leaching of Sr occurs at similar rates under open circuit potential (OCP) and OER potential, suggesting a nonelectrochemical dissolution process. The leaching of Fe is, however, strongly dependent on the electrode potential. More Fe leaching is observed at the OER potential than OCP. Additionally, the electrode with higher initial OER activity leaches more Fe. These results indicate that OER facilitates the dissolution of Fe from the electrode. The leaching of Fe, in turn, is considered responsible for the activity loss of La1- x Sr x FeO3- δ during OER. This study brings new insight into the degradation mechanism of La1- x Sr x FeO3- δ and related metal oxides during electro-oxidation processes.
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