Earth-abundant iron(iii) species serves as a cocatalyst boosting the multielectron reduction of IO₃^-/I^-redox shuttle in Z-scheme photocatalytic water splitting

Z-scheme water splitting has exhibited significant potential for practical water splitting under visible light irradiation. This system comprises two photocatalysts for O2 and H2 evolution, along with a redox shuttle for transporting electrons between these photocatalysts. Therefore, the promotion of redox reaction of the shuttle play a crucial role in efficient water splitting. In particular, for multielectron redox reactions (e.g., IO3-/I-), a cocatalyst is indispensable, where only noble metals may contribute thus far. Herein, we demonstrate that FeOx catalyzes the multielectron IO3- reduction on the Bi4TaO8Cl photocatalyst. While bare Bi4TaO8Cl does not show O2 evolution from an aqueous IO3- solution because of the lack of multielectron reduction ability, FeOx loading enables O2 evolution. Based on a series of experimental investigations, such as time-resolved spectroscopy, we elucidated that the O2 evolution enhancement stems from the promotion of multielectron reduction of IO3- by FeOx. The FeIII/FeII redox couple in the loaded FeOx facilitates both electron carrier capture from Bi4TaO8Cl and reduction of the redox mediator. In addition, FeOx is effective for other photocatalysts and redox mediators. To the best of our knowledge, the developed cocatalysts is the first earth-abundant cocatalyst for multielectron redox mediators.