Multicomposite Nanostructured Hematite-Titania Photoanodes with Improved Oxygen Evolution: The Role of the Oxygen Evolution Catalyst.

We present a sol-gel processed hematite-titania-based photoanode, which exhibits a photocurrent of up to 2.5 mA/cm at 1.23 V under simulated AM 1.5 G illumination (100 mW/cm) thanks to the addition of an amorphous cocatalyst with the nominal composition FeCrNiO . To unveil the role of the cocatalyst interconnected to the photoanode, we performed impedance measurements. According to the one order of magnitude higher value for the capacitance associated with surface states ( ) compared to the bare photoanode, the function of the catalyst-photoanode interface resembles that of a p-n-like junction. In addition, the charge transfer resistance associated with charge transfer processes from surface states ( ) was unchanged at potentials between 0.8 and 1.1 V after adding the cocatalyst, indicating that the catalyst has a negligible effect on the hole transport to the electrolyte. The understanding of the role of oxygen evolution catalysts (OECs) in conjunction with the photoanodes is particularly important for water splitting because most OECs are studied separately at considerably higher potentials compared to the potentials at which photoanode materials are operated.