Technology is always looking for ways to make it easier to be green. Now, researchers in New York state report creating a new long-lived catalyst that uses the energy in sunlight to generate hydrogen gas, a carbon-free fuel. With further improvements, the advance could lead to systems that use sunlight to split water molecules, generating a fuel that can power cars and trucks without emitting any greenhouse gases.
The idea of using sunlight to convert water into a fuel may sound fanciful. But plants do it: They capture photons of sunlight and use that energy to split water molecules into their constituents of hydrogen and oxygen ions. Pairs of hydrogen ions are then knitted together with a pair of electrons (swiped from the oxygen ions) to make hydrogen molecules (H2).
Researchers have actually mimicked this same reaction for many years, but the catalysts they use to do so have been either too expensive or too quick to break down. So the search has been on for cheaper, more rugged catalysts.
To do the job, researchers usually look for two key ingredients: a good light absorber and a good catalyst. The light absorber captures photons of sunlight and then harnesses the energy to generate the energetic electrons. Those energized electrons are then passed to the catalyst, which knits the hydrogen ions into H2.
One hunting ground for good light absorbers has been semiconducting nanoparticles. But to date, when combined with catalysts, nanoparticles have managed to carry out their reactions at only modest speeds, too slow for full-scale industrial needs. One stumbling block is that semiconducting nanocrystals aren't typically soluble in water. So researchers have had to dissolve the nanocrystals into organic solvents, which prevent the crystals from interacting with the catalysts.
In the current study, reported online today in Science, researchers at the University of Rochester led by chemists Richard Eisenberg and Todd Krauss coated cadmium selenide nanoparticles with short organic chainlike molecules, abbreviated DHLA. This coating of DHLA chains allowed the nanoparticles to dissolve in water. And the individual chains were so short that they allowed the catalyst's nickel ions—also in the solution—to nuzzle close enough to the nanoparticles to grab the electrons and knit H2 molecules together. The Rochester team found that the catalysts were not only fast actors, knitting as many as 7000 H2 molecules every hour, but kept doing so for weeks on end without falling apart—a major advance over other H2-knitting catalysts.
Daniel DuBois, a chemist and hydrogen catalyst designer recently retired from the Pacific Northwest National Laboratory in Richland, Washington, calls the new work "a very nice contribution to the area" and says he's particularly impressed with the catalyst's durability.
Even so, the new light harvester-catalyst combo isn't quite ready for the real world. For their current experiment, the Rochester researchers didn't actually split water molecules to generate their hydrogen ions. Rather, they added vitamin C, which readily gives up hydrogen, to their solution. So the Rochester group still needs to show that their H2-making compounds will carry out the same reaction using water. If they do, they may give plants a run for their money in green technology.