A Triad Photoanode for Visible Light-Driven Water Oxidation via Immobilization of Molecular Polyoxometalate on Polymeric Carbon Nitride

Due to their availability, low cost, non-toxicity and tunability, polymeric carbon nitrides (CNx) represent one of the most attractive materials classes for the development of fully sustainable photo(electro)catalytic systems for solar-driven water splitting. However, the development of CNx-based photoanodes for visible light-driven water oxidation to dioxygen is rather challenging, particularly due to issues related to photoelectrode stability and effective coupling of the light absorber with water oxidation catalysts. Herein, a triadic photoanode comprising a porous TiO2 electron collector scaffold sensitized by CNx coupled to a molecular cobalt polyoxometalate (CoPOM = [Co4(H2O)2(PW9O34)2]10─) catalyst is reported. Complete water oxidation to dioxygen under visible (λ > 420 nm) light irradiation is demonstrated, with photocurrents down to relatively low bias potentials (0.2 V vs. RHE). Furthermore, polyethyleneimine (PEI), a cationic polymer is shown to act as an effective and non-sacrificial electrostatic linker for immobilization of the anionic CoPOM onto the negatively charged surface of CNx. The optimized deposition of CoPOM using the PEI linker translates directly into improved efficiency of the transfer of photogenerated holes to water molecules and to enhanced oxygen evolution. This work thus provides important design rules for effective immobilization of POM-based catalysts into soft-matter photoelectrocatalytic architectures for light-driven water oxidation.