Science

Solar energy-harvesting “nanotrees” could produce hydrogen fuel on a mass scale

Solar energy-harvesting “nanot...
Electronic microscopic image of a "nanoforest," with green tint added for contrast (Imge: Wang Research Group, UC San Diego Jacobs School of Engineering)
Electronic microscopic image of a "nanoforest," with green tint added for contrast (Imge: Wang Research Group, UC San Diego Jacobs School of Engineering)
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Electronic microscopic image of a "nanoforest," with green tint added for contrast (Imge: Wang Research Group, UC San Diego Jacobs School of Engineering)
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Electronic microscopic image of a "nanoforest," with green tint added for contrast (Imge: Wang Research Group, UC San Diego Jacobs School of Engineering)
The light trapping effect in nanowire arrays sees photons bounced between single nanowires and eventually absorbed by them (R), allowing hydrogen fuel to be produced more efficiently compared to planar counterparts where they are reflected off the surface (L) (Image: Wang Research Group, UC San Diego Jacobs School of Engineering)
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The light trapping effect in nanowire arrays sees photons bounced between single nanowires and eventually absorbed by them (R), allowing hydrogen fuel to be produced more efficiently compared to planar counterparts where they are reflected off the surface (L) (Image: Wang Research Group, UC San Diego Jacobs School of Engineering)

While hydrogen is considered a "clean" fuel because the only waste product it generates is water, the conventional way to produce it relies on electricity, which is usually produced through the burning of fossil fuels. Researchers at the University of California, San Diego (UCSD), have now developed a "3D branched nanowire array" that they claim could cheaply and cleanly deliver hydrogen fuel on a mass scale.

The nanowires, which are made from abundant natural materials such as silicon and zinc oxide, mimic the structure of a forest of trees, with individual vertical "trees" sprouting hundreds of nano-sized "branches." Like forests, this structure maximizes the amount of solar energy that can be captured, with the vertical structures trapping and absorbing the light, while the flat surfaces reflect it.

Using this nanotree structure, the researchers were able to maximize the amount of solar energy captured for use in producing hydrogen in a process called photoelectrochemical water-splitting. This process usually uses planar solar cells to produce hydrogen in a process similar to the electrolysis of water, but the UCSD team says their nanowire arrays produce more hydrogen fuel efficiently.

The light trapping effect in nanowire arrays sees photons bounced between single nanowires and eventually absorbed by them (R), allowing hydrogen fuel to be produced more efficiently compared to planar counterparts where they are reflected off the surface (L) (Image: Wang Research Group, UC San Diego Jacobs School of Engineering)
The light trapping effect in nanowire arrays sees photons bounced between single nanowires and eventually absorbed by them (R), allowing hydrogen fuel to be produced more efficiently compared to planar counterparts where they are reflected off the surface (L) (Image: Wang Research Group, UC San Diego Jacobs School of Engineering)

Ke Sun, a PhD student in electrical engineering who led the project, says the vertical nanotree structure also allows very small gas bubbles of hydrogen to be extracted much faster to maximize the hydrogen gas output. Additionally, the surface area for chemical reactions has been enhanced by at least 400,000 times in the nanotree structure compared to its planar counterparts.

While the team says its nanotrees provide a cheap way to produce hydrogen fuel on a mass scale, they are aiming to go further. Like other research teams, they are looking to use the nanotree structure to mimic photosynthesis in a device that not only harnesses the power of the sun to produce hydrogen fuel, but also captures CO2 from the atmosphere to reduce carbon emissions at the same time.

"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 looking at alternatives to zinc oxide which, although it absorbs the sun's ultraviolet light, has stability issues that affect the nanotree structure over time.

The team's research appears in the journal Nanoscale

Source: UCSD

18 comments
Slowburn
Hydrogen is a lousy motor fuel because it migrates through everything and has a low energy density even when liquified or highly compressed. I would prefer to use liquid ammonia as a fuel.
AussieJohn
Slowburn - Haven't you heard that graphene is completely impervious to any type of gas? so solutions are possible. If you think that this new idea to harvest H2 is not noteworthy and that you'd prefer to use liquid ammonia, just go out there and get it done yourself!
Iosif Olimpiu
The water level is rising and the forests are shrinking. Theese things...i don't know if ere good.
Russ Pinney
Slowburn. What a delicious non sequitur. You are very handsome.
Jeff Rosati
Hydrogen Fuel Cells produce electricity ... why on earth would you burn it?
billybob222
I'm always happy in advances like these but I'd be happier if any of this stuff actually made it out of the Lab- not that it never happens- the super strong, scratch resistant glass used in Iphones and the like was invented by Corning in a Lab- in the 60s
Adam Flynn
The scratch resistant screen!?! You do realize that every single component in the iPhone was in the lab at some point, a lot of it more recently than the 1960's. These projects almost always go somewhere, when they don't lead to directly marketable applications the lessons learned serve a useful purpose in another lab. This work is brilliant! Jeff- One of the main reasons you may want to burn it instead of feed it to a fuel cell is because sometimes the transition to electrical potential is just to slow for the application. There are also tech reasons. Example: You can feed an unlimited supply of H2 into a fuel cell indefinitely but even all of that electrical energy isn't going to put a rocket into space without a magnetic linear accelerator or some similar intermediary that doesn't currently exist. Burning a Hydrogen based fuel can. There are a lot of reasons to burn it.
Slowburn
re; Jeff Rosati Hydrogen Fuel Cells produce electricity by burning it in a very controlled manner. Besides an ICE engine costs less and has a longer life expectancy. ............................................................................................................................. re; AussieJohn Please demonstrate a tank that is sealed by a single sheet of graphene without joints, and a valve that seals as well for hydrogen as I can expect for ammonia.
Dave Andrews
They shouldn't sit on this until it fits their own definition of "perfect." If it can really create hydrogen as efficiently and inexpensively as they claim and this can be mass produced, they should begin releasing this for sale to companies now. Then later, once they've fixed the degradation issue and found a way to efficiently absorb carbon dioxide, release "nanotree 2.0" and continue from there. Don't sit on this "nanotree 1.0 just because you don't have the new and improved version ready yet. Bring it to market and then make it even better for future versions, just like every other product on the market.
Paul Perkins
Interesting:) I thought Hydrogen engins at the end of the day, is more about zero emissions, not how it is made or pedantic lab work. And ammonia made on a scale to power and replace the car, sounds like that idea should be on the back burner not the slow burner:)