21 April 2010 If you could use sunlight to split water into hydrogen and oxygen, you could store the two elements so that you could burn them later on. The problem is that the usual way of doing this—by generating a current with photovoltaic cells, then running it through a tank of water, triggering electrolysis—requires that you first convert light to electricity. And that itself wastes energy.
A way of avoiding that intermediate step was revealed on 11 April, when researchers at MIT reported in the journal Nature Nanotechnology that they’d gotten a virus to mimic some of the actions of photosynthesis. The virus, called M13, is a phage—that is, it preys not on animals but on bacteria. When added to a container of pigment and a catalyst, it arranges itself and the other molecules in an energy-sharing structure.
”When one pigment is struck by a photon and goes into its excited state, it can literally communicate that excitement to adjacent molecules and share the photon’s energy,” says Yoon Sung Nam, a doctoral candidate who is the leader of the project. That’s pretty much what the green pigment chlorophyll does in the leaves of a plant.
By arranging the pigment and catalyst with the proper alignment and spacing, the team found it could make them four times as efficient as their best unaided performance at carrying out the first step of the splitting reaction. That’s the part that breaks oxygen molecules off from water, leaving hydrogen nuclei, or protons, and electrons. To keep the structure stable, the researchers also added a microgel, which immobilizes the virus in a particular arrangement for roughly 10 hours. (The researchers say they haven’t focused on the second step in the reaction, in which the hydrogen proton and electron combine to create hydrogen gas, because that one’s much easier to pull off.)
According to Angela Belcher, a materials science and biological engineering professor and head of the lab where the research is taking place, the team expects to produce a prototype within the next two years that is capable of splitting water. First, though, the team must perfect a technique for assembling the virus on electrodes, thus making the structure more stable than is possible using the microgel and enabling it to conduct electrons away from the electrodes more efficiently.
Nam says that the oxygen-producing power of the first half of the reaction needs to be increased by an order of magnitude. In the lab, the team achieved 84 percent efficiency by shining 550-nanometer-wavelength light on the zinc porphyrin pigment used in the experiments. A commercial system must do the same thing not just for that wavelength but for a large swath of solar spectrum. Only then will it begin to approach the efficiency with which plants store sunlight as sugar. So far, the team has grown two pigments on the virus. Adding more is easier said than done, because different pigments have different optimal conditions for catalysis.
The researchers also want to find catalysts that are cheaper than the iridium oxide used in the first half-reaction and the platinum that’s used in the second. Nam says the team is investigating the effectiveness of materials such as cobalt oxide and manganese oxide.
Nam likens the latest breakthrough to interior design, in which the trick lies not in getting new furniture but in rearranging what you already have. Best of all, he says, when scientists and engineers do introduce ”new furniture,” viral self-assembly can be used to arrange it to its best effect.
To Probe Further
For more on this story, see our blog entry: Artificial Photosynthesis Achieved with Nanotechnology