Water, photoelectrolysis

The reason for the exponential increase in the electron transfer rate with increasing electrode potential at the ZnO/electrolyte interface must be further explored. A possible explanation is provided in a recent study on water photoelectrolysis which describes the mechanism of water oxidation to molecular oxygen as one of strong molecular interaction with nonisoenergetic electron transfer subject to irreversible thermodynamics.48 Under such conditions, the rate of electron transfer will depend on the thermodynamic force in the semiconductor/electrolyte interface to... 

FIGURE 29.5 Energy diagram for water photoelectrolysis with a TiOj anode. 

Tributsch, H. Pohlmann, L. 1995. Synergetic molecular approaches towards artificial and photosynthetic water photoelectrolysis. J. Electroanal. Chem. 396 53-61. 

Apart from titanium oxide, two other carbon-modified semiconductors were studied in water photoelectrolysis due to their low band gap energy, namely iron (Fe203) and tungsten oxide (W03) [70,90]. Carbon-modified iron oxide demonstrated promising photoconversion efficiency, 4 % and 7 % for modified oxides synthesized in oven and by thermal oxidation respectively [90]. Also, carbon-modified tungsten oxide (C-W03) photocatalysts exhibited a 2 % photoconversion efficiency [70],... 

Fig. 3.4b Illustration of the operating principle of a photoelectrochemical cell producing hydrogen and oxygen during water photoelectrolysis.

Surface states can form due to abrupt distortion of the semiconductor crystal lattice. Charge transfer processes between surface states and the electrolyte have been analyzed in relation to water photoelectrolysis application [96]. Electron transfer mediated through surface states for an n-type semiconductor under dynamic equilibrium is shown in Fig. 3.13(d). 

Fig. 3.15 Energy diagram of semiconductor-metal photoelectrolysis cell, (a) No contact and no chemical potential equilibrium (b) galvanic contact in dark (c) effect of light illumination (d) effect of light illumination with bias, (e) Light illumination without bias, however in this case the semiconductor band edges straddle the redox potential for water photoelectrolysis.

Fig. 3.16 Energy diagram of semiconductor-metal photoelectrolysis cell with light illumination without bias, however in this case the semiconductor band edges straddle the redox potential for water photoelectrolysis.

Fig. 3.18 Energy diagram for p-n photochemical diode for water photoelectrolysis.

Where ria and r c are, respectively, the anodic and cathodic overpotentials. Considering all these losses an optimum bandgap of 2.0 to 2.25 eV is required for the materials used as photoelectrodes for water photoelectrolysis. In practical cases, a reasonable value of overall solar efficiency is 10% for single bandgap devices involving two photons and 16% for dual photosystem devices involving 4 photons [102,103,110,111]. 

Kainthala RC, Zelenay B, Bockris JOM (1987) Significant efficiency increase in self-driven photoelectrochemical cell for water photoelectrolysis. J Electrochem Soc 134 841-845... 

Varghese, OK Grimes, CA (2007) Appropriate Strategies For Determining The Photoconversion Efficiency Of Water Photoelectrolysis Cells A Review With Examples Using Titania Nanotuhe Array Photoanodes. Solar Energy Materials and Solar Cells, in press. 

Fig. 4.3 Illustration of the effect of bias on water photoelectrolysis at photoanode/cathode.

Crystalline Sn02 and WO3 photoanodes for water photoelectrolysis have been reported [24-26], The band gap of Sn02 (Erg = 3.5 eV) makes single crystals of this material of little interest with respect to solar induced optical properties. Wrighton and coworkers observed... 

Salvador P (1980) The influence of Niobium doping on the efficiency of n-Ti02 electrode in water photoelectrolysis. Sol Energy Mater 2 413-421... 

This chapter considers the fabrication of oxide semiconductor photoanode materials possessing tubular-form geometries and their application to water photoelectrolysis due to their demonstrated excellent photo-conversion efficiencies particular emphasis is given in this chapter to highly-ordered Ti02 nanotube arrays made by anodic oxidation of titanium in fluoride based electrolytes. Since photoconversion efficiencies are intricately tied to surface area and architectural features, the ability to fabricate nanotube arrays of different pore size, length, wall thickness, and composition are considered, with fabrication and crystallization variables discussed in relationship to a nanotube-array growth model. 

Figure 5.38 illustrates the experimental setup for water photoelectrolysis measurements with the nanotuhe arrays used as the photoanodes from which oxygen is evolved. The 1-V characteristics of 400 nm long short titania nanotuhe array electrodes, photocurrent density vs. potential, measured in IM KOH electrolyte as a function of anodization hath temperature under UV (320-400 nm, lOOmW/cm ) illumination are shown in Fig. 5.39. The samples were fabricated using a HF electrolyte. At 1.5V the photocurrent density of the 5°C anodized sample is more than three times the value for the sample anodized at 50°C. The lower anodization temperature also increases the slope of the photocurrent—potential characteristic. On seeing the photoresponse of a 10 V 5°C anodized sample to monochromatic 337 nm 2.7 mW/cm illumination, it was found that at high anodic polarization, greater than IV, the quantum efficiency is larger than 90%.

Fig. 5.38 Illustration of experimental setup for hydrogen generation by water photoelectrolysis.

Ti-Fe-O Nanotube Array Films for Solar Spectrum Water Photoelectrolysis... 

Paulose M, Mor GK, Varghese OK, Shankar K, Grimes CA (2006) Visible light photoelectrochemical and water-photoelectrolysis properties of titania nanotube arrays. J Photochem Photobiol A 178 8-15... 

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