Water Splitting and Photosynthesis

The decomposition of liquid water to form gaseous hydrogen and oxygen  

The standard potential AE° for Reaction 1 corresponding to the transfer of two electrons is given by  

F is the Faraday constant (96485 C moF1) and the negative sign denotes the thermodynamically non-spontaneous nature of the water splitting process. The actual voltage required for electrolysis will depend on the fugacities of the gaseous products in Reaction 1 as well as on the electrode reaction kinetics (overpotentials) 

Optimizing the rates of the electrochemical processes (Reactions 2 and 3) constitute much of the R D focus in electrochemical or photoelectrochemical splitting of water. Two-compartment cells are also employed to spatially separate the evolved gases with special attention being paid to the proton transport membranes (e.g., Nation1 ). Chapter 3 provides a summary of the progress made in water electrolyzer technologies. 

Water is transparent to the wavelengths constituting the solar spectrum. Therefore, photocatalytic or photoelectrochemical splitting of water requires an agent (semiconductor, dye, or chromophore) capable of first absorbing sunlight and generating electron-hole pairs. Molecular approaches are discussed in Chapter 6 and semiconductor-based approaches are described in Chapter 7. 

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