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A Solar Powered Nanostructure Generates Hydrogen

March 8, 2012 | 1 Comment

Using silicon and zinc oxide a University of California San Diego team has built a forest of nanowires that split hydrogen free from water.

Deli Wang, professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering quoted in the University press release said, “This is a clean way to generate clean fuel.”  The study paper is at the journal Nanoscale.

The nanowires look quite like trees in the photo, becoming a forest.  The team’s press release offers Wang’s explanation – The trees’ vertical structure and branches are keys to capturing the maximum amount of solar energy because the vertical structure of trees grabs and adsorbs light while flat surfaces simply reflect it.  It’s similar to retinal photoreceptor cells in the human eye. There’s a clue in images of Earth from space, light reflects off of flat surfaces such as the ocean or deserts, while forests appear darker.

Nanotrees for Generating Hydrogen. Click image for the largest view. Image credit, Wang Research Group, UC San Diego Jacobs School of Engineering

That intuitive grasp of the opportunity to build a more three-dimensional structure suggested a “3D branched nanowire array”.  Inside the forest the process called photoelectrochemical water splitting produces hydrogen gas.  This process uses sunlight energy with no greenhouse gas byproduct. By comparison, the current conventional way of producing hydrogen relies on generated electricity or high temperature reforming using fuel sources

Ke Sun, a PhD student in electrical engineering who led the project said, “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.”  Clearly the team has a “clean” motive at least as far as most likely the funding and perhaps further research.  That’s fine if they get to a low cost hydrogen generation system.

On the technical details Wang’s team has developed a way to harvest more sunlight using the vertical nanotree structure that is said to produce more hydrogen fuel efficiently compared to planar counterparts.

Along with the vertical structure efficiency gains, the structure maximizes hydrogen gas output.  Sun explains 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.

Sun said, “Moreover, with this structure we have enhanced by at least 400,000 times the surface area for chemical reactions.”

But aside from the commentary, neither the press release nor the study abstract are detailing how much hydrogen is produced per surface area.  No information is discussed on the costs per area.  There isn’t any data on the operation.

Still, the concept is a striking and effective innovation on the quality of sunlight and its harvest.  If the numbers on building modules are reasonable and operating costs low this is an idea that suggests a little sunlit area could go a long way for making hydrogen fuel.

For the long-term Wang’s team is aiming for artificial photosynthesis.  Artificial photosynthesis is a fine idea, but generating hydrogen is a mighty fine idea on its own that’s deserves a closer look and perhaps development.

Wang’s team hopes to mimic photosynthesis to also capture CO2 from the atmosphere, reducing carbon emissions, and convert it into hydrocarbon fuel.

Sun said, “We are trying to mimic what the plant does to convert sunlight to energy. 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.”

There’s quite a bit of competition in the artificial photosynthesis effort.  Everyone remains bedeviled in capturing the CO2.  The recombining of the carbon and hydrogen remains economically elusive as well.

The team is upfront with their plans. Its also being said the team is 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.

This looks like another in a series of sunlight to hydrogen processes of which there are many.  It’s noticeable now that few ever note the actual productivity per area.  This should be casting a long shadow.  CO2 is a very small part of the atmosphere; its harvest at scale is going to be a major hurdle.  Competition with plants for getting to carbon molecules that are useful or easily reformed to fuel products is another substantial hurdle.

The test should be, can the research ideas get competitive to the common processes to free hydrogen?  Electrolysis isn’t cheap nor is steam reforming.  There is a huge market for hydrogen now.  Should a market scale low cost hydrogen fuel cell break out, a lot of researchers and funding sources are going to wonder why they chased artificial photosynthesis when they may well have a block busting revolution on the self.

This concept looks like a great idea. Let’s hope the politically correct atmosphere doesn’t derail a good idea off onto a shelf somewhere.


Comments

1 Comment so far

  1. Jagdish on March 8, 2012 6:22 AM

    Using solar energy really eliminates carbon use. How about capital cost?

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