Nanowire Forest Splits Water with Sunlight

Nanotechnology has a checkered past in improving fuel cell technology. I have cataloged some of the missteps previously. At the time, the areas in which researchers were attempting to apply nanotechnology to fuel cells—namely improved catalysts and hydrogen storage—didn’t address the real problems that have prevented fuel cells from receiving wider adoption.

One of the fundamental problems with fuel cells has been the cost of producing hydrogen. While hydrogen is, of course, the most abundant element, it attaches itself to other elements like nitrogen or fluorine, and perhaps most ubiquitously to oxygen to create the water molecule. The process used to separate hydrogen out into hydrogen gas for powering fuel cells now relies on electricity produced from fossil fuels, negating some of the potential environmental benefits. So in the last few years, a new line of research has emerged that uses nanomaterials to imitate photosynthesis and break water down into hydrogen and oxygen thereby creating a more cost-effective and environmentally-friendly method for producing hydrogen.

Angela Belcher at MIT reported on just such a method two years ago when she used man-made viruses to serve as a scaffold to attract molecules of the catalyst iridium oxide and a biological pigment (zinc porphyrins). Once these two molecules attached themselves to the scaffold, the viruses would become “wire-like,” which enabled them to split the water molecules into hydrogen and oxygen because of the precise spacing in the wire.

Now researchers at University of California, San Diego have developed a quite different approach to mimicking photosynthesis for splitting water molecules by using a 3D branched nanowire array that looks like a forest of trees.

According to Deli Wang, professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering, this tree-like structure enables both trees and the nanowire arrays to capture the maximum amount of solar energy. To illustrate what he means, Wang points to satellite imagery in which flat surfaces like oceans or deserts simply reflect the light back and forests remain dark because they are absorbing the light.

The nanowire forest that Wang and his colleagues have created uses the process of photoelectrochemical water-splitting to produce hydrogen gas. The method used by the researchers, which was published in the journal Nanoscale, found that the forest structure of the nanowires, which has a massive amount of surface area, not only captured more light than flat planar designs, but also produced more hydrogen gas.

“With this structure, we have enhanced, by at least 400,000 times, the surface area for chemical reactions,” said Ke Sun, a PhD student in electrical engineering who led the project.

While it appears from the press release that the researchers are more interested in pursuing the photosynthesis aspect of this research to expand its use into capturing carbon dioxide, it could be a cost-effective way for producing hydrogen gas.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
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