Photoconversion efficiency

In any case, no electrode material or approach fulfills the requirements for a successful photoelectrochemical process in all respects, i.e., for routine practical use hence novel materials and approaches are constantly pursued. Note that beside the robust performance needed, the most important figure of merit for a semiconductor photoelectrode used for water splitting is the photoconversion efficiency, which is... 

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],... 

Though equation (3.6.5) is more useful for analyzing the performance of photoelectrolysis cells, for practical purposes the photoconversion efficiency (solar conversion efficiency if sunlight is used) is calculated hy modifying (3.6.5) in the form... 

Rh2 is the rate of production (moles/s) of hydrogen in its standard state per unit area of the photoelectrode. The standard Gibbs energy AG° = 237.2 kj/mol at 25°C and 1 bar, and Pt is the power density (W/m ) of illumination. The numerator and denominator have units of power and hence, as in the case of photoelectrochemical solar cells, the photoconversion efficiency is the ratio of power output to the power input. 

Spontaneous water-splitting upon illumination needs semiconductors with appropriate electron affinity and flat band conditions. The flat band positions shift with electrolyte pH. Hence, an external bias needs to be applied between the electrodes in most cases to effect water splitting. The external bias can be either electrical or chemical. This external bias contribution should be subtracted from (3.6.11) or (3.6.12) to get the overall photoconversion efficiency. In the case of an external electrical bias, the efficiency can be defined as ... 

An issue in using this approach is that equations (3.6.14) and (3.6.15) involve overpotential losses. Hence highly catalytic metal electrodes with low overpotential are required for comparison. If a metal electrode with a low catalytic activity is used these equations yield exaggerated values for photoconversion efficiency. 

Solar photoconversion efficiency can be calculated using relation (3.6.24) in (3.6.13). 

This modified form of relation (3.6.13) is the most acceptable relation for calculating the photoconversion efficiency... 

With Vbias= 0.51V, the solar photoconversion efficiency of titania nanotube (6 pm length) array photoelectrodes was calculated as 0.6 %. 

IPCE and APCE can have values close to 100%. As discussed before, the maximum attainable photoconversion efficiency in a single bandgap photoelectrolysis cell is 30.7%. Although stable, the photoconversion efficiencies of most oxide semiconductors are low (<2% except the case of 8.35% reported for carbon modified titania [121]) due to their large band gap. 

Raja KS, Mahajan VK, Misra M (2006) Determination of photoconversion efficiency of nanotubular titanium oxide... 

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. 

A photoanode comprised of flame oxidized carbon doped n-Ti02 films have been reported to perform water splitting with high photoconversion efficiencies [65]. While chemically modified n-Ti02 can be prepared by the controlled combustion of Ti metal in a natural gas flame the authors, in investigating this technique [66], have found reproducibility to be a challenge. Various authors [67,68,69] have discussed in considerable depth issues surrounding the stated photoconversion efficiencies of [65]. 

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. 

The photoconversion efficiency of light energy to chemical energy in presence of external applied potential was calculated using the following expression [133] ... 

Fig. 5.40 Photoconversion efficiency under 320-400nm lOOmW/cm illumination as a function of measured potential [vs. Ag/AgCl] for lOV samples anodized at four temperatures 5°C, 25°C, 35°C and 50°C.

Under UV (320-400 nm, 100 mW/cm ) illumination a maximum photoconversion efficiency of 7.9% was obtained for short nanotube arrays anodized in boric acid contained electrolyte [103], with a hydrogen generation rate of 42 mL/h W. Under full spectrum illumination (AM 1.5, 100 mW/cm ), a photoconversion efficiency of 0.45% was obtained. The enhanced photoresponse of the boric acid anodized sample is not due solely to a modified nanotube array structure since the maximum nanotube array length achieved is about 600 nm. It is possible boron, which is difficult to identify by XPS, remains inside the titania matrix and affects its charge transfer properties. 

The effect of nanotube-array length on the photoresponse, with all samples annealed at 530°C was also studied both photocurrent magnitude and photoconversion efficiency are seen to increase with length [5]. On exposing 6 pm nanotube-array samples annealed at 600°C to individual wavelengths of 337 nm (3.1 mW/cm ) and 365 nm (89mW/cm ), the quantum efficiency was calculated as 81% and 80% respectively. The high quantum... 

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