Chromium-substituted hematite powder as a catalytic material for photochemical and electrochemical water oxidation

Abstract

Fe(III) and Cr(III) mixed oxide (Fe_2−xCr_xO₃) particles were examined as water oxidation catalysts under photochemical and electrochemical conditions. Photochemical water oxidation activity was evaluated under visible light irradiation (480 < λ < 500 nm) in the presence of tris(2,2′-bipyridyl)ruthenium(II) sulfate and sodium persulfate, which served as a redox photosensitizer and a sacrificial electron acceptor, respectively. Electrochemical water oxidation was conducted in a phosphate buffered aqueous solution (pH 7.5) using Fe_2−xCr_xO₃ particles, which were loaded on a transparent conductive glass support, in the presence of an electrochemical bias. While hematite (α-Fe₂O₃) is known to be a catalytic material for water oxidation, its catalytic activity dropped upon substitution of Cr(III) ions for Fe(III) sites in Fe₂O₃ at least in part due to oxidative dissolution of Cr(III) on the oxide surface, which competed with water oxidation during the reaction. On the other hand, Cr-substitution resulted in a five-fold enhancement in the activity for electrochemical water oxidation to form oxygen. Electrochemical impedance measurements revealed that the Cr-substitution decreased the resistance of charge transfer inside the Fe₂O₃ bulk, thereby leading to improved performance for electrochemical water oxidation.