Molecular co-catalysis for photo-electrochemical water splitting : needs, performances, limitations and perspectives
Despite the maturity and efficiency of modern water electrolysers and the possibility to use photovoltaic panels as power source for green hydrogen production, the direct water photo-dissociation into molecular hydrogen and oxygen remains a topic of great practical interest. In conventional photo-electrochemical cells, the light absorbing semiconducting materials used to design efficient photo-electrodes play a central role. Among many other materials, titanium dioxide has been extensively used for the endergonic water photo-oxidation reaction. Various strategies such as bulk doping or surface sensitization (by organic dyes or other species) have been developed to increase the absorption of earth-incident sunlight radiations. In the quest for higher efficiencies of the photon-to-chemical-bonds reaction sequence, charge transfer enhancement via surface co-catalysis is also a topic of significant interest. Whereas metallic nano-particles are most commonly used, other nano-crystals such as Metal-Organic Frameworks (MOFs) can also be used for that purpose. Ultimately, the implementation of monolayers of electrochemically active molecular complexes deposited at the surface of appropriate semiconducting materials, also offers some interesting perspectives.
In this context, the purpose of this communication is to discuss some issues related to surface co-catalysis by functionalization of different co-catalysts, specifically active with regard to water photo-reduction into hydrogen or water photo-oxidation into oxygen. Various examples will be used to support the discussion, to analyze performances, to identify existing limitations and to discuss future perspectives. For example, p-type Rh-doped SrTiO3particles, surface modified by sorption of cobalt(II) hexachloroclathrochelate (e.g. Co(Cl2Gm)3(BCH3)2) will be used as a model system to discuss some challenges associated with water photo-reduction. Also, TiO2nanorods, surface modified by deposition of cobalt dodecahedral Zeolitic Imidazolate Framework (ZIF-67) will be used as a model system to discuss the challenges associated with water photo-oxydation.