Synergy between Mn and Co in Mn/CoOx cocatalyst for enhanced photoelectrochemical water oxidation of hematite photoanode

Hematite (alpha-Fe2O3) photoanode is regarded widely as one of most promising photoanode candidates for photoelectrochemical (PEC) water splitting, while the poor surface reaction kinetics and severe charge recombination hinder its further application. Herein, a bimetallic Mn/CoOx film, prepared by high-temperature annealing of metal-organic frameworks (MOFs), is designed as an efficient cocatalyst to modify alpha-Fe2O3 photoanode (FTO/Sn@alpha-Fe2O3-Mn/CoOx) for PEC water oxidation. With an optimized content of Mn/CoOx cocatalyst, the FTO/Sn@alpha-Fe2O3-Mn/CoOx photoanode exhibits an excellent photocurrent density of 2.66 mA/cm2 at 1.23 VRHE , which is higher than that of CoOx (1.43 mA/cm2) or MnOx (1.86 mA/cm2) single-metal catalyst. The on-set potential achieves a remarkable cathodic shift of 0.11 V. The detailed mechanism studies demonstrate that the electron donation from Mn/CoOx cocatalyst to alpha-Fe2O3 photoanode can obtain high oxidation state of Mn and Co to enhance photogenerated carriers separation efficiency, reduce recombination and facilitate the OH- transformation, which synergically increases the catalytic activities of FTO/Sn@alpha-Fe2O3-Mn/CoOx photoanode for PEC water oxidation. Furthermore, a heterojunction, formed by Mn/CoOx doping in the surface alpha-Fe2O3, can further enhance the photogenerated carriers separation efficiency. This work provides valuable guidance for the design and construction of bimetallic cocatalyst on various photoelectrodes for outstanding PEC water splitting performance.