Photoelectrochemical splitting of water

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. 

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. 

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... 

Ohashi K, McCann J, Bockris JOM (1977) Stable photoelectrochemical cell for splitting of water. Nature 266 610-611... 

Applications have been reported for photoelectrochemical experiments, for example, splitting of water [11], local generation of photoelectrodes by spatially selective laser excitation [12], and steady-state electrochemiluminescence at a band electrode array [13,14]. Band electrodes prepared from very thin films approaching molecular dimensions have been used to assess the limits of theory describing electrode kinetics at ultramicroelectrodes [9]. Spectroelectrochemical applications have been extensively reviewed [1], In an intriguing approach, thin, discontinuous metal films have been prepared on a transparent semiconductor substrate they are essentially transparent under conditions in which a continuous metal film containing the same quantity of metal would be expected to substantially absorb [15]. 

Sartoretti CJ, Ulmaim M, Alexander BD et al (2003) Photoelectrochemical oxidation of water at transparent ferric oxide film electrodes. Chem Phys Lett 376 194—200 Sartoretti CJ, Alexander BD, Solarska R et al (2005) Photoelectrochemical oxidation of water at transparent ferric oxide film electrodes. J Phys Chem B 109 13685-13692 Kumari S, Singh AP, Sonal et al (2010) Spray pyrolytically deposited nanoporous Ti " doped hematite thin films for efficient photoelectron chemical splitting of water. Int J Hydrogen Energy 35 3985-3990... 

The most desirable method for production of hydrogen, which represents a sustainable fuel of the future, is photoelectrochemical (PEC) splitting of water by visible light. Theoretically, the PEC production of hydrogen has the capacity to provide global energy security at potentially low cost (James et al., 2009). 

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