Researchers at the Institute of Energy Technology (INTE) of the Universitat Politècnica de Catalunya· BarcelonaTech (UPC), the University of Auckland (New Zealand), and King Abdullah University of Science and Technology (Saudi Arabia) have developed a system to produce hydrogen from water and sunlight in a way that is clean, renewable and more cost-effective than other methods. The scientists behind the project have fused the optical properties of three-dimensional photonic crystals (inverse opals of titanium dioxide, TiO2) and 2-3 nm gold nanoparticles to develop a highly active catalyst powder. The research paper has been published in Scientific Reports, the open-access journal of Nature.
This new photocatalyst produces more hydrogen than others developed so far by harnessing the properties of both photonic crystals and nanoparticles of a metal. According to Jordi Llorca, a researcher at the UPC's Institute of Energy Technology, the process involves "tuning" the two materials to amplify the effect. "You have to choose the right photonic crystal and the right nanoparticles," he adds.
In any photocatalyst made from gold nanoparticles and titanium dioxide crystals using ultraviolet light, which is only a small part of solar radiation (less than 3%), the process is the same: light excites the TiO2 electrons and promotes them to the conduction band, leaving holes on the other side. The electrons interact with the gold nanoparticles and are captured by them. According to the scientists, the novelty in this case is the use of a 3D photonic crystal that captures the visible part of the solar spectrum, precisely at the energy level at which the gold nanoparticles "resonate". As a result, it's possible to take advantage of the visible part of the solar spectrum rather than just the ultraviolet part. This translates into a significant boost in the performance of the process.
The new catalyst has great potential for application in industrial processes. According to researcher Jordi Llorca, making the move from the laboratory to an industrial plant would mean designing a reactor to operate outdoors in the sun, and using a solar collector to capture more sunlight.
A conventional plant for the production of hydrogen from natural gas generates about 300 tons of hydrogen a day. With the new catalyst developed at the UPC, researchers have managed to produce 0.025 litres of hydrogen in one hour using one gram of catalyst. Assuming eight hours of sunlight a day, the scientists estimate that an area measuring 10 x 10 km would be needed to produce hydrogen on an industrial scale.
Energy and environmental benefits
Unlike conventional plants that run on fossil fuels at 800°C, with the new system hydrogen production is clean and renewable: the photochemical process takes place at room temperature and at no cost, since it uses solar energy and water. As a result, the energy and environmental gains are considerable.
The researchers say they have managed to pass the milestone of converting 5% of solar energy into hydrogen at room temperature, the threshold at which the technology is considered feasible. Renewable hydrogen production is essential if the gas is to become the energy carrier of the future.
- Jordi Llorca et al. Hydrogen production by Tuning the Photonic Band Gap with the Electronic Band Gap of TiO2. Scientific Reports, December 2013
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