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 without using fossil fuels 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.

“This is a clean way to generate clean fuel,” said Deli Wang, professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering.
The trees’ vertical structure and branches are keys to capturing the maximum amount of solar energy, according to Wang. 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. In images of Earth from space, light reflects off of flat surfaces such as the ocean or deserts, while forests appear darker.
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 byproduct. By comparison, the current conventional way of producing hydrogen relies on electricity from fossil fuels.

Schematic shows the light trapping effect in nanowire arrays. Photons on are bounced between single nanowires and eventually absorbed by them (R). 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 where they are reflected off the surface (L). Image Credit: Wang Research Group, UC San Diego Jacobs School of Engineering.
“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, a PhD student in electrical engineering 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. Wang is also affiliated with the California Institute of Telecommunications and Information Technology and the Material Science and Engineering Program at UC San Diego.
The vertical branch structure also maximizes hydrogen gas output, said Sun. For example, on the flat wide surface of a pot of boiling water, bubbles must become large to come to the surface. In the nanotree structure, very small gas bubbles of hydrogen can be extracted much faster. “Moreover, with this structure, we have enhanced, by at least 400,000 times, the surface area for chemical reactions,” said Sun.

In this experiment, nanotree electrodes are submersed in water and illuminated by simulated sun light to measure electricity output of the device. Photo credit: Joshua Knoff, UC San Diego Jacobs School of Engineering.
In the long run, what Wang’s team is aiming for is even bigger: artificial photosynthesis. In photosynthesis, as plants absorb sunlight they also collect carbon dioxide (CO2) and water from the atmosphere to create carbohydrates to fuel their own growth. Wang’s team hopes to mimic this process to also capture CO2 from the atmosphere, reducing carbon emissions, and convert it into hydrocarbon fuel.
“We are trying to mimic what the plant does to convert sunlight to energy,” said Sun. “We are hoping in the near future our ‘nanotree’ structure can eventually be part of an efficient device that functions like a real tree for photosynthesis.”
The team is also studying alternatives to zinc oxide, which absorbs the sun’s ultraviolet light, but has stability issues that affect the lifetime usage of the nanotree structure.

 

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Sunflowers inspire efficient solar power


By Emily Eggleston    |  Sun, 09/30/2012 – 4:21pm

In August, UW-Madison researcher Hongrui Jiang published his design for solar panels that act like sunflowers, tracking the sun’s movement throughout the day. Jiang, a professor in computer and electrical engineering, used nanotechnology to design a system that helps the panels move by reacting to the warmth of the sun’s rays, rather than using a motor and global positioning system (GPS) as many solar tracking panels do. Read Jiang’s answers to Madison Commons’ questions about how the new solar technology works and what else he is doing with nanotechnology.

 

MC: Why are you interested in solar technology?

HJ: Renewable energy is very important right now because we are running out of fossil fuels. We have to look for other possible sources of energy. Solar energy is very promising because pretty much everywhere has sunlight, not like wind or geothermal, and it lasts forever.

MC: Describe the work you do in patterning solar panel movement after sunflowers.

HJ: The basic idea is solar-tracking. If you can have a solar panel follow the sun during the day, you’ll have more interception of light, and therefore more electricity. The idea is very simple and done by many plants in nature. Sunflower is one example, a buttercup flower is another. The idea is very simple but not easy to realizing it with solar plans is complicated because you have to mimic complex biochemical processes.

MC: Don’t some solar panels already track the sun’s movment?

HJ: In the solar tracking systems available now, most use GPS with motors. They are active mechanical systems to orient towards sun. Active systems are great but mechanics consume energy themselves. The purpose is to get as much electricity as possible. Our system is passive, it doesn’t consume electricity to drive solar tracking. Also, it is very hard for active systems to realize full range tracking, sunrise to sunset. Ours does.

MC: How does the passive system of solar tracking work?

HJ: We needed a material that would respond to natural sunlight, whole spectrum light of all wavelengths. has to be sensitive enough. Some materials are responsive to strong light like lasers, but we need the solar panel to be responsive to whatever intensity the sunlight is at. Sunlight hits a mirror which projects light onto actuator holding carbon nanotubes. When the nanotubes warm they contract, causing the panel to shift toward the contracted nanotubes.

MC: You use nanotechnology in your some of your other research. What else do you do on the super tiny nano scale?

HJ: My expertise in the microsystems and microscale optics. I’m working on making a tunable liquid contact lens that adds extra focusing power. When you are getting older the muscle in your eye starts to lose power and it becomes harder and harder for you to see up close so people wear bi- or trifocals. This contact lens autofocuses, basically like the point and shoot cameras that you use. It’s not just a lens, it’s a whole spectrum of gadgets [with] circuits and everything, but it has to be flexible. You need an energy source to provide electricity for the circuits. Right now we’re trying to harvest and store solar energy right in the lens. It’s a very challenging idea and we’re off to a good start.