If humans are going to mimic nature's unique way of converting sunlight into energy, we're going to need to build some very extraordinary trees.
Electrical engineers in California want to do just that. Their new ‘nanotree’ device is made from cheap, abundant materials and uses sunlight to split water molecules into oxygen and hydrogen atoms that can be used in fuel cells to produce energy.
Hydrogen fuel cells could power everything from houses to cars. But hydrogen doesn’t exist alone in nature. The atoms have to be separated from other molecules, like water.
Doing that requires energy and at present, about 90 present of hydrogen gas is created using fossil fuels, causing carbon dioxide emissions into the atmosphere, said Ke Sun, an electrical engineering PhD student at U.C. San Diego who worked with Deli Wang, a professor of electrical and computer engineering to produce the nanotree.
Their nanotrees could potentially use sunlight to ultimately generate an electrical current for a controlled reaction without greenhouse gas emissions.
"Basically it's just like a tree," said Wang, who along with Sun published an article about the nanotrees in the journal Nanoscale.
The nanotrees were several years in the making. Wang had worked on branched nanostructures before but they didn’t resemble trees, and they weren’t intended to harvest light. In August 2009, Sun approached Wang with the idea to engineer branched structures for solar cells.
“By that time we already knew that the vertical nanowire really would be very good for light harvesting,” Wang said. “We aimed for something that would be practical.”
Each nanotree is grown in a liquid. It starts when scientists put a silicon nanowire into a solution containing zinc. Silicon and zinc are abundant in the Earth and zinc oxide is best known as an ingredient in sunscreen, Wang said. In the solution, the wire undergoes a chemical reaction, growing zinc oxide branches. Altogether, a single nanotree’s length can range from a few hundred nanometers to a couple microns. That means roughly 10,000 nanotrees could fit on the cross-section of a human hair.
More than one million nanotrees forms a square centimeter-sized photoelectrochemical cell, or PEC. The nanotree array is put in another solution and exposed to sunlight. The silicon nanowire “trunk” absorbs most of the light, which transfers electrons through the zinc oxide branches to the surrounding water. That reaction generates hydrogen gas, which bubbles up through the solution.
Right now, the scientists don’t have an efficient way for collecting the hydrogen. But they’re planning on building a special two-compartment cell -- one side a cathode and the other side an anode -- which would be used to collect the hydrogen and ultimately use it to produce electricity.
Yat Li, a material chemist and assistant professor of chemistry at UC Santa Cruz, specializes in using nanowires for applications such as energy conversion and storage. He called the nanotree structure a new, novel idea in a difficult field.
“This kind of branched structure can increase the light absorption frequency and provide a large surface area for charge transfer,” Li said, adding that the point where the zinc oxide branches connect to the silicon trunks will help separate charges effectively. “The work really mimics the tree system.”
"I was thinking that if I could integrate a secondary structure on free-standing nanowires -- grow nanowire instead of using a planar substrate -- I could obtain something unique."
-- Electrical engineering PhD student Ke Sun on his A-Ha! Moment
Conventional technologies for splitting water are expensive. Compared to PEC devices, nanotrees look like a bargain. Wang said he and Sun believe their method is 100 times cheaper than current systems. But what the electrical engineers save in dollars, they lose in efficiency. Their device only has 3 percent hydrogen generation efficiency while the world record is 12.8 percent, Wang said.
Even if the engineers can improve their efficiency, Wang and his colleagues in the field face numerous challenges in creating a way to cleanly split water into hydrogen and oxygen, namely corrosion. Wang and his colleagues in the field face numerous challenges in creating a way to cleanly split water into hydrogen and oxygen. The process requires highly efficient, affordable technology that can be scaled up. Since all the reactions happen in solution, the technological components must also resist corrosion.
Currently Wang and Sun are experimenting with using different materials for the nanotree. Zinc oxide does have some drawbacks, Wang said. It only absorbs UV light, plus it’s not very stable. So the electrical engineers would like to find a more chemically robust oxide for use in a practical, scalable device.
Over the long-term, their goal is to integrate more materials, including catalysts for artificial photosynthesis. Topping nature’s photosynthesis means developing an ultra-efficient process for the clean creation of oxygen and fuel from water and sunlight.
“This is not artificial synthesis yet, but it’s a very good starting point,” Wang said.