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Photo 1: New high-performance photocatalyst (upper left) and an overall model of the photocatalyst-electrolysis hybrid system A photocatalytic reaction converting solar energy is used to lower the electrolysis voltage required for the hydrogen production by water electrolysis.

Abstract:
A step toward the realization of a new hydrogen production system using solar light

Development of a High-performance Photocatalyst that is Surface-treated with Cesium

Japan | Posted on May 17th, 2010

* The activity of a tungsten oxide photocatalyst was increased by surface treatment with cesium.
* The quantum yield of the new photocatalyst under visible light is larger than the previously reported values by a factor of about 50.
* The photocatalyst can reduce the voltage required for water electrolysis by almost 50%, thereby lowering the cost of hydrogen production.

Summary

Kazuhiro Sayama (Leader), Yugo Miseki (Research Scientist), et al. of Solar Light Energy Conversion Group, the Energy Technology Research Institute (Director: Yasuo Hasegawa) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Tamotsu Nomakuchi), developed a tungsten oxide (WO3) photocatalyst (Upper left in photo 1) that provides a significantly higher quantum yield under visible light than conventional photocatalysts. A photocatalyst-electrolysis hybrid system (Photo 1) using this photocatalyst is a hydrogen production system in which solar light is efficiently used. The AIST's original system employs the photocatalyst that generates oxygen by oxidizing water and reducing iron(III) ions (Fe3+) to iron(II) ions (Fe2+). The system also involves low-voltage electrolysis in which water is reduced to generate hydrogen and Fe2+ ions are oxidized to Fe3+ ions.

The high efficiency of the WO3 photocatalyst was achieved using a new method—treatment of the surface of the photocatalyst with Cesium (Cs). The activity of the treated catalyst is more than ten times that of untreated catalysts. The quantum yield of the new photocatalyst is 19% under visible light of wavelength 420 nm and is approximately 50 times the previously reported values (0.4%)*. The use of solar energy can reduce the voltage required for water electrolysis by almost 50%. Hence, the low-cost production of hydrogen is expected.

The details of this technology will be presented on March 19, 2010 at the symposium organized by the Energy and Environment Study Group at the 57th Spring Meeting, 2010, of the Japan Society of Applied Physics to be held at Tokai University.

Social Background of Research

In order to suppress the emission of carbon dioxide and create a sustainable society, it is essential to make efficient use of renewable energy. One of the technologies for the effective use of solar energy, which is the most abundant renewable energy, is a low-cost hydrogen-production technique in which water is directly decomposed by photocatalysts to obtain hydrogen and oxygen. This technology has been actively studied as a fundamental technology for a future hydrogen-energy-based society. If a photocatalyst system which is as efficient as solar cells and as simple and inexpensive as plant cultivation is being developed, it can be expected to contribute significantly to the realization of a society that is not dependent on fossil resources. However, the quantum yield and solar-energy conversion efficiency of photocatalysts are still low at present. Consequently, the development of a high-performance photocatalyst system is desired.

History of Research

AIST has studied a photocatalyst-electrolysis hybrid system (Figs. 1 and 2) that can help overcome the disadvantages of conventional photocatalytic hydrogen production. This system can possibly enhance the efficiency of the photocatalyst. Further, it has the advantages of producing pure hydrogen and not requiring a large transparent hood for hydrogen collection. Because of decreased electrolysis voltage, we can also expect to manufacture hydrogen at a lower cost compared to ordinary water electrolysis systems. This system offers the advantages of conventional photocatalytic methods as well as those of usual electrolytic processes. While certain candidate redox media are used for oxidation-reduction reactions, the technique for low-voltage hydrogen production using Fe2+ ions has already been established. Consequently, the use of iron (Fe2+ and Fe3+ ion pairs) as the redox medium is, at present, the most practical technique for the hybrid system. Thus, another major challenge that was faced in the realization of this hybrid system was the development of a high-performance photocatalyst that would reduce the redox medium (from Fe3+ to Fe2+) while generating oxygen from water.

Read the details here www.aist.go.jp/aist_e/latest_research/2010/20100517/20100517.html