Thanks to a new catalyst, sunlight has been converted into chemical energy with a record 13.4% efficiency.
Scientists have long dreamed of mimicking photosynthesis, by using the energy in sunlight to knit together hydrocarbon fuels from carbon dioxide (CO2) and water. Now, a cheap new chemical catalyst has carried out part of that process with record efficiency, using electricity from a solar cell to split CO2 into energy-rich carbon monoxide (CO) and oxygen. The conversion isn’t yet efficient enough to compete with fossil fuels like gasoline. But it could one day lead to methods for making essentially unlimited amounts of liquid fuels from sunlight, water, and CO2, the chief culprit in global warming.
A bright idea
A new catalyst made from copper and tin oxides uses electric current from a solar cell to split water (H2O) and carbon dioxide (CO2), creating energy-rich carbon monoxide (CO) that can be further refined into liquid fuels.
The new work is “a very nice result,” says John Turner, a renewable fuels expert at the National Renewable Energy Laboratory in Golden, Colorado.
The transformation begins when CO2 is broken down into oxygen and CO, the latter of which can be combined with hydrogen to make a variety of hydrocarbon fuels. Adding four hydrogen atoms, for example, creates methanol, a liquid fuel that can power cars. Over the last 2 decades, researchers have discovered a number of catalysts that enable that first step and split CO2 when the gas is bubbled up through water in the presence of an electric current. One of the best studied is a cheap, plentiful mix of copper and oxygen called copper oxide. The trouble is that the catalyst splits more water than it does CO2, making molecular hydrogen (H2), a less energy-rich compound, says Michael Graetzel, a chemist at the Swiss Federal Institute of Technology in Lausanne, whose group has long studied these CO2-splitting catalysts.
Last year, Marcel Schreier, one of Graetzel’s graduate students, was looking into the details of how copper oxide catalysts work. He put a layer of them on a tin oxide–based electrode, which fed electrons to a beaker containing water and dissolved CO2. Instead of splitting mostly water—like the copper oxide catalyst—the new catalyst generated almost pure CO. “It was a discovery made by serendipity,” Graetzel says.
The tin, Graetzel adds, seems to deactivate the catalytic hot spots that help split the water. As a result, almost all the electric current went into making the more desirable CO. Armed with the new insight, Graetzel’s team sought to speed up the catalyst’s work. To do so, they remade their electrode from copper oxide nanowires, which have a high surface area for carrying out the CO2-breaking reaction, and topped them with a single atom-thick layer of tin. As Graetzel’s team reports this week in Nature Energy, the strategy worked, converting 90% of the CO2 molecules into CO, with hydrogen and other byproducts making up the rest. They also hooked their setup to a solar cell and showed that a record 13.4% of the energy in the captured sunlight was converted into the CO’s chemical bonds. That’s far better than plants, which store energy with about 1% efficiency, and even tops recent hybrid approaches that combine catalysts with microbes to generate fuel.
Nate Lewis, a chemist at the California Institute of Technology in Pasadena, says the new result comes on the heels of other recent improvements that use different catalysts to turn CO2 into fuels. “Together, they show we’re making progress,” Lewis says. But he also cautions that current efforts to turn CO2into fuel remain squarely in the realm of basic research, because they can’t generate fuel at a price anywhere near to that of refining oil.
Still, exploding supplies of renewable electricity now occasionally generate more power than the grid can handle. So scientists are looking for a viable way to store the excess electricity. That’s likely to drive further progress in storing energy in chemical fuels, Graetzel says.