Cheap hydrogen power gets a nanotube boost

By Robert Adler Scientists have moved us a step closer to a hydrogen-based economy by successfully “wiring up” carbon nanotubes to hydrogenase – a biological molecule that can be used to harness hydrogen as fuel. The conductive nanotubes act as tiny wires, shuttling electrons from hydrogenase molecules as they drive hydrogen-based chemical reactions. Reacting hydrogen with oxygen releases electricity and could therefore offer a greener way to power cars, for example. Splitting water into hydrogen and oxygen offers a way to store electricity generated using technology such as solar panels. But existing methods of drawing electrical power from these reactions relies on rare and expensive precious metals such as platinum, palladium, or ruthenium. Connecting nanotubes to the catalyst hydrogenase promises a much cheaper approach. “We need to be able to develop ways to generate energy that are cheap and effective,” says Paul King, who led the research effort with Michael Heben at the National Renewable Energy Laboratory, in Colorado, US. “Now we have a nice trick from biology that may be more effective in the long term.” The researchers forged an electrical connection between individual hydrogenase molecules and single-walled carbon nanotubes by first using detergent-like surfactants to create separate suspensions containing hydrogenase and nanotubes. When these two mixtures are combined, the nanotubes spontaneously attach themselves to the hydrogenase molecules, producing stable, electrically conductive bonds. Connecting the nanotubes up to a circuit could provide a way to draw electricity out of the chemical reaction for practical use. The researchers are still studying the exact location and nature of these connections, but believe that the nanotubes attach themselves to the electrically active sites on each hydrogenase molecule. In the past, getting these sensitive natural enzymes to connect with other materials has been challenging. “We were surprised, at first, at the result,” says King. “It wasn’t clear that they would interact, but they just self-assemble. And it’s a very strong and an apparently stable interaction.” The researchers were able to check that the connections successfully transmit electrons by observing changes in the optical properties of the suspended nanotubes, which occur when they absorb charged particles. Nanotubes normally absorb and re-emit light at characteristic wavelengths but, after hydrogenase is added, this photoluminescence disappears, suggesting that the enzyme is feeding electrons into the nanotubes as it catalyses the oxidation hydrogen. The team found that they could control the catalytic reaction by changing the pH balance of the solution or the amount of hydrogen in it. As expected, when they added oxygen, which inactivates hydrogenase, the nanotubes lit up again. In the absence of oxygen, the hydrogenase-nanotube connections continued to work for up to a week. “This is an exciting result,” says John Peters, at Montana State University, US. “The ability to attach hydrogenase enzymes to conductive materials paves the way for biohybrid, precious-metal-free fuel cell technologies and new hydrogen-producing materials.” This is just what King and his colleagues are aiming to do. “It’s a way to go directly from solar energy to chemical energy in the form of hydrogen,” he says. “Downstream, we would like to use the enzyme in place of a catalyst, for example platinum, in solar-hydrogen energy production.” Journal reference: Nano Letters (Vol 7, No 11, pp 3528) Energy and Fuels – Learn more about the looming energy crisis in our comprehensive special report. More on these topics:
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