Photoelectrochemical water splitting

Instead of storing energy, photoelectrochemical cells can also be used for producing chemicals, such as photoelectrolyzers which split water into hydrogen and oxygen. If sufficiently efficient, such a device could be of key importance for a future hydrogen economy. 

Research focuses on finding semiconductors with the correct energies to split water that are also stable when in contact with water. Photoelectrochemical water splitting is in the very early stages of research but offers long-term potential for sustainable hydrogen production with low environmental impact [59]. 

Relatively scant attention has been given to the synthesis of new polymeric materials which could photoelectrochemically split water. An obvious reason for this is the consideration that if everyday insulating polymers are subject to photodegrada-tlon in routine environmental exposures, what chance would a semiconducting or conducting polymer, with more reactive centers such c=Q, have to survive under yet more harsh chemical conditions. Such odds have not deterred polymer chemists in the past, however, and now that the attention of more chemists has been stimulated, rapid developments in this area may be anticipated. In fact even the labile polyacetylene has been found to be significantly stabilized when physically mixed with polyethylene (27c) or when Cl is available in the contacting electrolyte (27d). 

A system exemplifying photoelectrochemical synthesis to generate hydrogen is water photoelecholysis. An early demonstration of water photoelectrolysis used Ti02 (band gap 3.0 eV) and was capable of photoelecholysis at 0.1% solar to chemical energy-conversion efficiency [12]. The semiconductor SrTiOs was demonshated to successfully split water in a direct photon-driven process by Bolts and Wrighton (1976), albeit at low solar energy-conversion efficiencies [13]. 

Assemblies involving molecular catalysts are instead much more rare. In 2010, Sun and coworkers reported a photoelectrochemical device, able to efficiently split water into O2 and H2, by immobilizing a molecular ruthenium catalyst in a Nafion membrane and depositing the material onto a nanostructured Ti02 anode, sensitized with a ruthenium polypyridine dye. Visible light-driven water splitting was successfully achieved upon both illumination and application of a small bias of — 0.325 V vs. Ag/AgCl to the device. 

Andreiadis ES, Qiavarot-Kerlidou M, Fontecave M, Artero V (2011) Artificial photosynthesis from molecular catalysts for light-driven water splitting to photoelectrochemical cells. Photochem Photobiol 87 946-964... 

Andreiadis, Eugen S., Murielle Chavarot-Kerlidou, Marc Fontecave, and Vincent Artero, Artificial Photosynthesis From Molecular Catalysts for Light-Driven Water Splitting to Photoelectrochemical Cells, Photochemistry and Photobiology, 87, 946-964 (2011). 

Fig. 2.5 Water splitting in photoelectrochemical cell using (a) photoanode (b) photocathode and (c) photoanode, and photocathode in tandem configuration (Reproduced from Ref. [8] with permission Irom The Royal Society of Chemistry. Royal Society of Chemistry 2014)...

Dye-sensitized photoelectrochemical cell is used to split water under visible light. For example, an O2 evolution catalyst is formed by hydrated iridium oxide (Ir02 H20) nanoparticles attached to Ru-complex sensitizer molecules. An excited electron from LUMO is injected into the conduction band of Ti02. The outer circuit transfers this electron to a counter electrode (Pt). The water is then reduced to form hydrogen gas [41]. After water oxidation to O2, the dye returns to the ground state on accepting an electron from the I1O2 H20 nanoparticle. 

In any case, no electrode material or approach fulfills the requirements for a successful photoelectrochemical process in all respects, i.e., for routine practical use hence novel materials and approaches are constantly pursued. Note that beside the robust performance needed, the most important figure of merit for a semiconductor photoelectrode used for water splitting is the photoconversion efficiency, which is... 

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