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Nanotrees harvest the sun’s energy to turn water into hydrogen fuel

University of California, San Diego electrical engineers are building a forest of tiny nanowire trees in order to cleanly capture solar energy and harvest it for hydrogen fuel generation. Reporting in the journal Nanoscale, the team said nanowires, which are made from abundant natural materials like silicon and zinc oxide, also offer a cheap way to deliver hydrogen fuel on a mass scale.

The trees’ vertical structure and branches are keys to capturing the maximum amount of solar energy, according to Deli Wang, professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering. In images of Earth from space, light reflects off of flat surfaces such as the ocean or deserts, while forests appear darker.  That’s because the vertical structure of trees grabs and adsorbs light while flat surfaces simply reflect it, Wang said, adding that it is also similar to retinal photoreceptor cells in the human eye.

Photo: Rapid hydrogen generation on the surface of nanotree electrodes that are submersed in water and illuminated by simulated sun light.

 

Wang’s team has mimicked this structure in their “3D branched nanowire array” which uses a process called photoelectrochemical water-splitting to produce hydrogen gas. Water splitting refers to the process of separating water into oxygen and hydrogen in order to extract hydrogen gas to be used as fuel. This process uses clean energy with no green-house gas by-product. By comparison, the current conventional way of producing hydrogen relies on electricity from fossil fuels

“Hydrogen is considered to be clean fuel compared to fossil fuel because there is no carbon emission, but the hydrogen currently used is not generated cleanly,” said Ke Sun, the first author of the article and graduate student in the Wang group who led the project. 

By harvesting more sun light using the vertical nanotree structure, Wang’s team has developed a way to produce more hydrogen fuel efficiently compared to planar counterparts.

“This is a clean way to generate clean fuel,” added Wang who is also affiliated with the California Institute of Telecommunications and Information Technology and the Material Science and Engineering Program at UC San Diego.  “Using solar energy for water splitting is the hottest topic in hydrogen fuel,” he said.