Water, photoelectrolysis efficiency

Hydrogen Generation from Irradiated Semiconductor-Liquid Interfaces 179 Table 3. Ideal limits for water photoelectrolysis efficiencies as estimated by various authors. 

All these factors explain water photoelectrolysis efficiencies that have reached values of the order of 15-16% under 0.1 W/cm UV (320-400 nm) illumination, where limiting photocurrents exceeding 20 mA/cm are observed at 1 V vs Ag/AgCl however, under simulated sunlight (AM 1.5) the performances are obviously lower, due to intrinsic limitations in visible light absorption, and the maximum photocurrents are generally of the order of 1 mA/cm. ... 

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],... 

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. 

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%.

The principal solar water-splitting models predict dual-band gap photoelectrolysis efficiencies of 16% [40], and 10-18% [41]. 

Fig. 9. Energy conversion efficiency of solar driven water splitting to generate H2 as a function of temperature for AMI. 5 insolation, with the system minimum bandgap determined at JOH20 = 1 bar.3 The maximum photoelectrolysis efficiency is shown for various indicated values...

They further developed highly efficient, easily fabricated materials for the solar generation of hydrogen by water photoelectrolysis. Here, they modified the bandgap of Ti02 by in-situ doping or surface modifications [64, 68] so that the resulted nanotubes become photocatalytically active to visible light. 

Considerations similar to those presented above show that illumination of a semiconductor leads to a shift of both the Fermi level and the quasi-levels of holes and electrons, and both the forward and reverse reactions, proceeding according to Eq. (1), are accelerated. In other words, the result of illumination is, above all, the efficient increase of the exchange current in the redox couple but this is not the only result. If a semiconductor under illumination is an electrode in an electrochemical cell and is connected through a load resistor with an auxiliary electrode, the cathodic and anodic reactions become spatially separated, as in the case of water photoelectrolysis (Fig. 11) considered above. The reaction with the minority carriers involved proceeds on the semiconductor surface, and that with the majority carriers involved, on the auxiliary electrode. Thus, the illumination of a semiconductor electrode gives rise to an electric current in the external circuit, so that some power can be drawn from the load resistor. In other words, the energy of light is converted into electricity. This is the way a photoelectrochemical cell, called the liquid junction solar cell, operates. 

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]. 

Principal solar water-sphtting models had predicted similar dual band gap photoelectrolysis efficiencies of only 16% and 10-18% [3, 28], respectively, whereas recently dual band gap systems were calculated to be capable of attaining over 30% solar photoelectrolysis conversion efficiency [14]. The physics of the earlier models were superb, but their analysis was influenced by dated technology, and underestimated the experimental jjphoto attained by contemporary devices or underestimated the high experimental values of JJelectrolysis which Can be... 

Table 3 includes predicted maximum photoelectrolysis Using observed /(photo of various dual band gap sensitizers. It is seen that solar water spKtting efficiencies may be viable at up to double the amount of that previously predicted. Efficient, three or more multiple band gap photoelectrolysis... 

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