Balancing charge recombination and hole transfer rates in hematite photoanodes by modulating the Co/Fe sites in the OER cocatalyst.

This work investigates the roles of Co and Fe sites in a composite cocatalyst on the performance of hematite photoanodes for photoelectrochemical (PEC) water splitting. The cobalt/iron-based composite (Co-Fe-O) cocatalyst, consisting of adjustable Co/Feratios, was synthesized using a one-step hydrothermal method. It reveals that Co sites with a robust capacity for low-bias hole capture, which is insignificantly affected by partial substitution by Fe, decelerate the charge recombination process. However, it also leads to a slower charge transfer, with slower oxygen-evolution kinetics on Co sites than on Fe sites. Consequently, the modulation of the Co/Fe ratio facilitates the redistribution of surface strap states, striking a delicate balance between charge recombination and charge transfer rates. This optimization led to the highest low-bias photocurrent density of 1.6 mA cm at 1.0 V vs. RHE (a 2.4-fold increase) for the cocatalyst with a Co/Fe ratio of 1:2 (CoFeO nanoparticles). Additionally, the cocatalyst with a Co/Fe ratio of 1:4 (mixture of CoFeO and FeO nanoparticles, demonstrated an impressive high-bias photocurrent density of 3.8 mA cm at 1.6 V vs. RHE (a 2.3-fold increase). This study emphasizes the promising potential of modulating active sites within a cocatalyst to achieve efficient PEC water splitting on a hematite-based photoanode.