Surface Modification of Hematite Photoanodes with CeO_x Cocatalyst for Improved Photoelectrochemical Water Oxidation Kinetics

Hematite is a promising photoanode for solar water splitting by photoelectrochemical (PEC) cells, but its performance is limited by the slow kinetics of water oxidation reaction or oxygen evolution reaction (OER). Surface modification of hematite photoanodes with a suitable water oxidation cocatalyst is a key strategy for improving the kinetics of water oxidation. In this study, a CeO_x overlayer is deposited on the surface of the hematite photoanode by a water-based solution method with ceric ammonium nitrate (CAN) followed by heat treatment. The photocurrent of CeO_x-modified hematite is 3 times higher than that of pristine hematite (at 1.23 V vs. RHE) under AM 1.5G, 1 sun conditions. Through hole-scavenger measurements, Tafel plot analysis, and electrochemical impedance spectroscopy, it is concluded that CeO_x overlayer increases the hole injection efficiency, improves the surface catalytic activity, and enhances charge transfer across the photoanode/electrolyte interface. These observations are attributed to the synergistic effects of Ce³⁺/Ce⁴⁺ redox species in CeO_x and the oxygen vacancies. This work elucidates the role of CeO_x as an efficient cocatalyst overlayer to improve the OER kinetics of photoanodes.