Supported polyoxometalates as emerging nanohybrid materials for photochemical and photoelectrochemical water splitting

Sunlight and water are among the most plentiful and sustainable resources of energy. Natural photosystem II in the plants uses these resources in ecofriendly manner for the production of atmospheric oxygen and energy. Inspired by this natural process, the development of artificial catalytic system to facilitate the solar-induced water splitting for the continuous production of hydrogen is the holy grail of the chemist and energy experts to meet the future energy demand at minimal environmental cost. Despite considerable research efforts dedicated to this area in the last decade, the development of highly efficient, stable and economic photocatalysts remain a challenging task for the large scale H2 production from water. Polyoxometalates (POMs)-based materials are emerging photo/photoelectrocatalysts in this quest owing to their multi-electron redox potential and fast reversible charge transfer properties, which are the essential requirements of photo-assisted water splitting catalysis. They are generally soluble in aqueous medium and thus their inherent catalytic/co-catalytic properties can be better exploited by incorporating/immobilizing them over suitable support materials. Therefore, exploration of discrete POM units over the support materials possessing high surface area, functionalizable architecture, flexible pore size and good light harvesting ability is an attractive area of research that has resulted in the generation of a strong library of heterocatalysts. The underlying support not only offers stability and recyclability attributes to the POM units but also provides decent dispersion, easy/maximum accessibility to the active sites, enhanced absorption capability, and synergistically enhances the activity by transfer of electrons and efficient charge/carriers separation by creating POM-support junctions. This mini-review emphasizes on the strategies for the incorporation of POMs on various porous supports like metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), oxide-based semiconductors, carbonaceous materials, etc., and their applications as effective photo/photoelectrocatalysts for water splitting. In addition, the mechanistic study, comparative analysis and the future potential of these novel nanoscale materials is also highlighted. We believe that this review article will provide a new direction and scientific interest at the boundary of materials engineering, and solar-driven chemistry for the sustainable energy conversion/storage processes.