Photocatalytic Overall Water Splitting and Related Processes for Strategic Energy Storage into Hydrogen

Photocatalytic overall water splitting is defined as the direct dissociation of water into hydrogen (H 2 ) and oxygen (O 2 ), driven by the absorption of light by a material, which can in turn generate charge carriers (electrons and holes) and enable their transfer in the necessary redox steps. It implies the storage of photonic energy in the form of chemical energy, conceptually akin to natural photosynthesis. The transformation of water takes place on the surface of an irradiated photocatalyst whereby two different active sites promote H 2 and O 2 evolution. The light absorber is generally a solid semiconductor in the form of particles. Whilst the active sites may reside on the semiconductor itself, efficiency is greatly enhanced by depositing appropriate co-catalysts onto it. An overview of the development of this basic photocatalyst system by extending the range of photons that can be absorbed by the semiconductor from the UV region only (oxides such as the popular TiO 2 ) to the visible range (e.g. doped oxides, nitrides, sulfides, or organic-based counterparts) is provided. The combination of two light-absorbers with suited electronic structures to separately promote water oxidation and reduction, via a bioinspired Z-scheme, has been also adopted to assemble artificial chloroplasts with remarkable success. Artificial leaves designed by interfacing photovoltaic and catalytic materials in flat, wireless devices, spontaneously splitting water under irradiation, also deserve special attention. The final part of this chapter accounts for the potentiality of photocatalysis in alternatives to overall water splitting, namely seawater splitting, splitting to hydrogen and hydrogen peroxide, or the splitting of other binary hydrogen species, such as ammonia, hydrogen sulfides and halides, or methane.