Photoelectrochemical properties of the brownmillerite Sr2Fe2O5: Application to electrochemical oxygen evolution

Abstract

The brownmillerite Sr₂Fe₂O₅ is prepared by nitrate route and the physical and photo-electrochemical properties are investigated for the first time. Thermal analysis indicates the formation of the phase at 950°C. A direct optical transition at 0.94 eV, due to Fe³⁺ splitting in octahedral site, is determined from the diffuse reflectance spectra. The prepared material displays semiconducting properties, and its electrical conductivity follows an exponential type law σ = σ_oexp (–0.045/kT) with small polaron hopping. The Mott-Schottky plot at pH ~7 is characteristic of n type semiconductor with a flat band potential of 0.40 V_SCE and an electron density of 3.6 × 10¹⁸ cm^–3. The electrochemical impedance spectroscopy shows the predominance of the bulk contribution. The above data are correlated, and the energetic diagram of the junction Sr₂Fe₂O₅/KOH solution is built. The valence band of Sr₂Fe₂O₅ (1.29 V_SCE) is more anodic than the potential of the O₂/H₂O couple and oxygen is successfully evolved under visible illumination. The best performance (0.17 cm³ O₂ (g catalyst)^–1 mn^–1) occurs at pH ~ 7 with a light-to-chemical energy yield of 0.045% under a light flux of 29 mW cm^–2. No deactivation was observed during the second cycle.