Making Water Do the Splits
It's hard to imagine a greener way to power the planet than using solar power to turn water into hydrogen gas. This clean fuel can be piped through fuel cells to produce electricity and then recombined with oxygen to yield water as a waste product. Sunlight doesn't break water molecules apart on its own, however, which is why Earth is covered with oceans. So researchers have spent decades searching for catalysts to help it along. And new work by researchers in Virginia takes a key step toward that goal.
A good solar catalyst has to be a jack of many trades. It must absorb high levels of solar energy, move the resulting electrons to a catalytic site where they can split water molecules into hydrogen and oxygen atoms, and finally stitch a new bond between two hydrogen ions to generate hydrogen gas. On top of that, the catalyst must be cheap and not generate any unwanted byproducts that would prevent the reaction from working over and over again.
No water-splitting catalyst has come close to meeting all these challenges. One major stumbling block has been that two electrons are needed to turn hydrogen ions into hydrogen gas. Previous approaches for turning water to hydrogen have created catalysts only capable of dealing with one electron at a time, largely because electrons tend to repel each other.
Now researchers led by Karen Brewer, a chemist at Virginia Polytechnic Institute and State University in Blacksburg, report in an advanced online publication in the Journal of the American Chemical Society that they've found a way to double their electron pleasure. To do so, the researchers created long, complex molecules with a pair of light-absorbing groups on both ends. These groups funnel the electrons through a pair of molecular bridges to a single atom of catalytically active rhodium in the center. The bridges turn out to be key, Brewer explains, because they keep electrons far apart from one another until they reach the metal center that can handle both at one time to carry out the necessary reactions. In tests with the catalyst dissolved in water, the researchers found that it was able to convert about 1% of the energy in sunlight into stored energy in the form of hydrogen gas, a good start but still well below the amount of energy that can be harvested by conventional solar cells.
The new catalyst isn't ready to revolutionize the energy business just yet, says Daniel Nocera, a chemist and solar fuels expert at the Massachusetts Institute of Technology in Cambridge. One limitation of Brewer's system, he points out, is that it requires organic molecules called amines, which give up their electrons to the light-absorbing complexes that in turn pass those on to the metal. Ultimately, researchers would prefer to pull these electrons from water molecules directly when they are split. Still, Nocera says, Brewer's team "is on the right track."