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- Title
Electrochemical characterization of the brownmillerite Ca<sub>2</sub>Fe<sub>2</sub>O<sub>5+δ</sub> synthesized by citrate sol–gel method. Application to photocatalytic H<sub>2</sub>-production.
- Authors
Benallal, S.; Boumaza, S.; Brahimi, R.; Trari, M.
- Abstract
The present study focuses on the preparation and electrochemical characterization of the calcium ferrite (Ca2Fe2O5+δ), a semiconductor crystallizing in the brownmillerite structure. The oxygen over-stoichiometry δ is extended to 0.16. It was synthesized by sol–gel auto combustion using nitrates as precursors and citric acid as chelating agent. The X-ray diffraction and specific surface area analysis showed a single phase with a specific surface area of ~ 6 m2 g−1. The optical gap of Ca2Fe2O5 (Eg = 2.31 eV), determined by the diffuse reflectance spectroscopy, is assigned to d-d transition of Fe3+: 3d orbital in octahedral coordination. It enables the photocatalytic exploitation of a large part of the solar spectrum. The p-type behavior of the oxide was demonstrated by photo-electrochemistry with an electro affinity of 3.69 eV (± 0.20 eV). A flat band potential (Efb = 0.93 VSCE) was evaluated from the capacitance-potential (C−2—E) characteristic while the holes density (NA = 2.63 × 1016 cm−3) was determined from the thermo-power S300K (= 1.1 mV K−1). The electrochemical oxygen intercalation is evidenced from the intensity-potential (J—E) profile. The semicircle in the Nyquist diagram is assigned to the bulk contribution whose diameter (233 kΩ cm2) decreases down to 203 kΩ cm2 under illumination, thus confirming the semi conductivity of the brownmillerite. The cathodic potential of the conduction band (CB) showed the feasibility of the water reduction. Indeed, Ca2Fe2O5 with a good chemical stability over a large pH range (5–14) was positively tested as photocatalyst for the H2 production upon visible light. The best activity was observed in alkaline medium (NaOH, 10−3 M) with an average evolution rate of 0.07 cm3 h−1 (mg catalyst)−1 under a light flux of 29 mW cm−2.
- Subjects
SOL-gel processes; CHEMICAL stability; SURFACE analysis; SOLAR spectra; NYQUIST diagram; CHELATING agents; CITRATES; REFLECTANCE spectroscopy
- Publication
Journal of Solid State Electrochemistry, 2022, Vol 26, Issue 6/7, p1541
- ISSN
1432-8488
- Publication type
Article
- DOI
10.1007/s10008-022-05183-1