Global warming and carbon emissions are among the most critical challenges of the 21st century. Hydrogen offers a promising solution to mitigate these issues. However, at the R&D level, the cost of hydrogen production is still three times that of gasoline. To commercialize hydrogen as a fuel, production costs must be reduced, particularly by enhancing the efficiency of hydrogen generation and by utilizing value-added products like oxygen generated at the anode.Photoelectrochemical (PEC) water splitting has emerged as a viable research avenue for harnessing solar energy. This study focuses on using hematite as a photoanode material for solar water oxidation in a PEC cell, which produces oxygen—a critical and thermodynamically challenging counterpart to clean hydrogen production. Despite the hematite’s attractive properties, such as good light absorption, suitable band positions, and robustness, experimental performance has consistently fallen short of theoretical expectations due to significant recombination processes that limit charge separation and collection. As a result, a large input voltage is required to oxidize water on hematite, leading to substantial efficiency losses. In this work, hematite thin films prepared by atomic layer deposition and electrodeposition were systematically investigated under PEC conditions to explore the mechanisms of oxygen formation during water oxidation. A combination of electrochemical, photoelectrochemical, and spectroscopic analyses was employed to better understand the fundamental mechanisms behind the performance limitations under various light conditions. It was found that the hole transfer rate is highly dependent on illumination conditions, Fermi level position, and band structure.
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