Scientists Design Artificial Photosynthesis Devices that Could Make Hydrogen or Other Fuels

Scientists have long tried  to mimic photosynthesis as a way to harness the energy  in sunlight and turn it into  a usable fuel, just as plants do.  There have been big technical  challenges for just as long,  and though researchers are  far from the ultimate goal,  last month two groups of  scientists described some  ways to hurdle those obstacles.

One of the groups, led by  MIT chemistry professor  Daniel Nocera, found a new  way to reproduce part of  the photosynthesis process,  using light to split water  molecules into oxygen and  hydrogen. The gases can then  be stored and used as a fuel.

Other groups have had  some success with this process before, but there were  always stumbling blocks  that would make it hard to  scale up or commercialize,  such as extremely acidic or  basic conditions, expensive catalytic materials, or both.  However, Nocera’s group managed to get artificial photosynthesis to work using benign conditions and cheap, abundant materials as catalysts.

Specifically, the team  joined a commercially available triple-junction silicon solar cell to two catalysts: cobalt borate for splitting the water molecule and a nickel molybdenum zinc alloy to form the hydrogen gas. The water-splitting reaction achieved a sunlight-to-fuel conversion of 4.7 percent in one incarnation of the device and 2.5 percent in another. The diference between the two was that the more eicient device housed the hydrogen-generating alloy on a mesh wired to the solar cell. The less efficient version needed no wires, and the alloy was instead deposited onto the stainless-steel back of the solar cell.

It is the wireless possibility, where the entire device is self-contained, that researchers say is most exciting. “Because there are no wires, we are not limited by the size that the light-absorbing material has to be,” says Steven Reece, a research scientist with

Sun Catalytix (a company cofounded by Nocera) who worked on the discovery. “We can operate on the micro- or even nanoscale…so you can imagine micro- or nanoparticles, similar to the cells we’ve worked with here, dispersed in a solution.” The final product could be much larger, too—a leaf-size stand-alone system, for instance. Whatever the size, the researchers believe such devices could help provide power in poor areas that lack consistent sources of electricity.Sun Catalytix expects to be able to bring the device to the point where a kilogram of hydrogen could be produced for about US $3, according to its chief technology oicer, Thomas Jarvi. Given that about 3.75 liters (1 gallon) of gasoline contains about the same amount of energy as 1 kilogram of hydrogen, the cost would compare favorably to gasoline, which is currently higher than $3 per gallon in the United States.

Daniel Gamelin, a professor of chemistry at the University of Washington, who works on related topics but was not involved with the new research, says the MIT and Sun Catalytix work represents an “impressive accomplishment.” However, he says, it remains to be seen whether silicon is really the most desirable material to use. Something less susceptible to degrading by oxygen may be a better option, he says.

“For these specific devices, there remain open questions about their long-term stability,” Gamelin says. “And their eiciencies would still need to be increased substantially to be commercially viable. But there is obviously potential for improvement on both fronts. In the bigger scheme, [this research] marks important progress toward the development of truly practical solar hydrogen technologies.”

Separately, researchers in Illinois demonstrated a different part of the photo-synthesis process—a step toward using sunlight to recycle carbon dioxide. Inthe natural world, the sun’s energy extracts electrons from a water molecule. The electrons then reduce CO² into fuel (in plants, the fuel takes the form of carbohydrates). University of Illinois graduate student Brian Rosen and other scientists have invented a device that electroreduced CO² to carbon monoxide at a lower voltage than previously achieved. The high voltages usually required have been a major stumbling block in the past. Rosen’s group brought the voltage down by using a combination of a silver cathode and an ionic liquid electrolyte that presumably stabilized the CO² ion. And, according to Rich Masel, who led the research and is CEO of Dioxide Materials, a company working on CO² electroreduction with the University of Illinois, this piece of the photosynthesis process could eventually lead to a way to turn captured CO² into “syngas”—a mixture of carbon monoxide and hydrogen used in the petrochemical industry to make gasoline and other fuels.

The experiment “shows that one can make syngas efficiently from any source of electricity,” Masel says. However, large-scale versions of the device probably can’t be cooked up until 2018. “Presently we have demonstrated the process on the 1-centimeter-squared scale. We need to go to the million cm² to make significant amounts of gasoline.”

Work on artificial photosynthesis has ramped up considerably in recent years. In July 2010, the Department of Energy began funding the Joint Center for Artificial Photosynthesis to the tune of $122 million over five years. The center, with close to 200 members in universities and national laboratories across California, aims to build on nature’s photosynthetic design, bridging all the disciplines required, from chemical engineering to applied physics.

In an interview earlier this year, the center’s leader, Caltech professor Nate Lewis, told IEEE Spectrum that progress is certainly being made, but it isn’t clear yet if the right combination of catalysts and light absorbers and everything else that goes into practical artificial photosynthetic devices has been found.“We’re seeing light in the tunnel,” he said. “We don’t know where the end of the tunnel is."It’s a curved tunnel.”

  

SOURCE : IEEE SPECTRUM MAGAZINE NOVEMBER 2011