Balancing charge recombination and hole transfer rates in hematite photoanodes by modulating the Co2+/Fe3+ 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 Co2+/Fe3+ratios, was synthesized using a one-step hydrothermal method. It reveals that Co2+ sites with a robust capacity for low-bias hole capture, which is insignificantly affected by partial substitution by Fe3+, 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 Co2+/Fe3+ 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-2 at 1.0 V vs. RHE (a 2.4-fold increase) for the cocatalyst with a Co2+/Fe3+ ratio of 1:2 (CoFe2O4 nanoparticles). Additionally, the cocatalyst with a Co2+/Fe3+ ratio of 1:4 (mixture of CoFe2O4 and Fe2O3 nanoparticles, demonstrated an impressive high-bias photocurrent density of 3.8 mA cm-2 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.