Simultaneous Formation of FeO_x Electrocatalyst Coating within Hematite Photoanodes for Solar Water Splitting

Depositing an oxygen evolution electrocatalyst on the intricate pores of semiconductor light-absorbing layers of photoanodes for photoelectrochemical solar water splitting is an efficient way to improve their performance, but it adds extra costs and difficulties. In this work, we present a synthesis of hematite (α-Fe2O3) photoanodes with a self-derived conductive amorphous FeOx electrocatalyst coating. Hematite-FeOx photoanodes were prepared via FeOOH precursors modified with low levels of lactic acid additive. In the absence of lactic acid, FeOOH consisted of lepidocrocite nanorods that resulted in α-Fe2O3 particulate photoanodes with sharp crystal edges upon doctor blading and calcination. Lactic acid addition, however, resulted in goethite and amorphous FeOOH that formed α-Fe2O3 particulate photoanodes coated by a thin conductive amorphous FeOx layer. Electron microscopy studies revealed that the thickness of this layer was controlled with the addition of lactic acid in the preparation. Photoelectrochemical characterization including Tafel plots, impedance spectroscopy, and hole scavenger measurements confirmed that the FeOx layer behaved as an FeOOH electrocatalyst enhancing charge transfer efficiency and minimizing electron–hole surface recombination. Such coating and approach increased the electrochemically active surface area and amount of surface states. Photocurrent increased from 0.32 to 1.39 mA cm–2 at 1.23 VRHE under simulated sunlight, remarkable results for an auto-co-catalyzed and simple solution-process deposition.