Photoelectrochemical splitting

Since the discovery of photoelectrochemical splitting of water on titanium dioxide (TiOj) electrodes (Fujishima and Honda, 1972), semiconductor-based photocatalysis has received much attention. Although TiO is superior to other semiconductors for many practical uses, two types of defects limit its photoeatalytic activity. Firstly, TiO has a high band-gap (E =3.2 eV), and it can be excited only by UV light (k < 387 nm), which is about 4-5% of the overall solar spectmm. Thus, this restricts the use of sunlight or visible light (Kormann et al., 1988). Secondly, the... 

Khan SUM, Akikusa J (1999) Photoelectrochemical splitting of water at nanociystalline n-Fe20s thin-film electrodes. J Phys Chem B 103 7184-7189... 

Aizawa and Suzuki (83,84,85,86) utilized, as an ordered system, liquid crystals in which Chi was immobilized. Electrodes were prepared by solvent-evaporating a solution consisting of Chi and a typical nematic liquid crystal, such as n-(p-methoxybenzyl-idene)-p -butylaniline, onto a platinum surface. Chl-liquid crystal electrodes in acidic buffer solutions gave cathodic photocurrents accompanied by the evolution of hydrogen gas (83). This was the first demonstration of photoelectrochemical splitting of water using in vitro Chi. Of particular interest in these studies is the effect of substituting the central metal in the Chi molecule. 

As explained earlier, photoelectrochemical splitting of water was done for the first time in 1972 (Fujishima and Honda). However, the efficiency of this cell was very low (about 1%) and hence not practical. A number of advances have brought an economical standalone, one-step solar water-splitting technology much nearer. There have been four steps in these advances. 

Photoelectrochemical splitting was discussed extensively in Chapter 10. The key point is the use of trace electrocatalysts added to the surface of both photocathode and photoanode to the appropriate extent (Kainthla, Zelenay, and Bockris, 1987 Turner, 1998). If the electrolyzer is to be entirely solar driven, both electrodes must be irradiated. It is difficult to find photoanodes with the appropriate properties. Most of them dissolve electrochemically ifused as anodes for02 evolution. This can, however, be prevented by using transparent films of nonreactive oxides (Bockris and Uosaki, 1977). 

Khan, S.U.M. and J. Akikusa (1999). Photoelectrochemical splitting of water at nanocrystalline -Fe203 thin-film electrodes. Journal of Physical Chemistry B, 103(34), 7184-7189. 

Optimizing the rates of the electrochemical processes (Reactions 2 and 3) consti tute 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., Na-fionR). 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. 

Khan, S. U. M. and Akikusa, J. 1999. "Photoelectrochemical Splitting of Water at Nanocrystalline n-Fe203 Thin-film Electrodes, Journal of Physical Chemistry B, 1999, 103 7184-7189. 

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). 

P.R. Mishra, P.K. Shukla, O.N. Srivastava, Smdy of modular PEC solar cells for photoelectrochemical splitting of water employing nanostructured Ti02 photoelectrodes , International Journal of Hydrogen Energy, 32, 1680-1685, (2007). 

Kumari S, Tripathi C, Singh AP et al (2006) Characterization of Zn doped hematite thin films for photoelectrochemical splitting of water. Curr Sci 91 1062-1064... 

Khan, S., Akikusa, J. Photoelectrochemical splitting of wato at nanocrystalline n-Fc203 thin-fihn electrodes. J. Phys. Chem. B 103, 7184 (1999)... 

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