Fuel made from water could feature in next-generation cars
A future that runs on hydrogen looks a little more likely thanks to a device that uses common materials to make the clean fuel from sunlight and water. The proof-of-concept gadget points to a cheap and efficient way of producing hydrogen fuel to power cars or generate electricity.
Researchers want to find an easy way to make hydrogen gas because it packs as much energy per pound as a gallon of gasoline. Hydrogen fuel cells can generate electricity for power or propulsion. A fuel cell electric vehicle can operate at two to three times the efficiency of a vehicle powered by a traditional internal combustion engine. Moreover, the fuel cells produce only water vapor as exhaust. But the current method of making hydrogen from fossil fuels is energy intensive and negates some of the environmental benefits.
“A few years ago, people believed fuel cell [cars] were not a viable alternative, and that anything we saw in the next 10 years was going to be battery-powered. Clearly the culture is changing,” says David Ginley, an expert on hydrogen production at the National Renewable Energy Laboratory in Colorado. Hydrogen, he adds, is “going to become part of the mix.”
A study in the September 15 issue of the Proceedings of the National Academy of Sciences describes coupling low-cost photovoltaic cells and catalysts to efficiently generate hydrogen from water and sunlight.
Chemist Daniel Nocera of Harvard University, inspired by plants’ conversion of light energy to chemical energy, for years has worked to create an “artificial leaf” (SN: 5/26/07, p. 328). Leading a team with Tonio Buonassisi, an electrical engineer at MIT, Nocera has built a new version of a device that splits water into its constituent hydrogen and oxygen. The device relies on photovoltaic cells to produce electricity, which in turn powers a reaction using chemical catalysts.
Earlier versions of the leaf had the sunlight-collecting photovoltaics and the hydrogen-producing catalyst on the same surface. The new device separates the components. That choice gives the team flexibility to choose components that would make the device efficient and affordable, Nocera says.
The researchers used widely available crystalline silicon solar cells, which are cheap and have middle-of-the-pack efficiency, converting about 25 percent of incoming light to electricity.
The group paired the photovoltaics with previously developed catalysts. Nocera made them using combinations of nickel, boron, oxygen, hydrogen, molybdenum and zinc, all Earth-abundant elements. Overall, the researchers converted 10 percent of the solar energy that hit the panels into hydrogen fuel.
Nocera says that photosynthesis converts only around 1 percent of light into stored energy. “So using Earth-abundant materials, man-made things can go 10 times better than a plant.”
Last year, a group from Sweden also broke the 10 percent efficiency mark, but their catalysts used rarer, more expensive metals, including indium and gallium.
Generating hydrogen isn’t the only challenge — so is finding a market for the gas, Nocera says. Until recently, car and electricity manufacturers hadn’t invested heavily in hydrogen fuel cell technology, he says, because no cheap and clean way to make the gas has been available. Nocera hopes his technology will provide at least some of the needed push. “The fuel cell infrastructure will exist someday. The handwriting’s on the wall,” he says, heartened by Toyota’s plan to sell fuel cell cars starting next year, and similar plans from other manufacturers.
Now that Nocera’s team has proved the concept works, Ginley says, it must next prove the technology can perform in everyday conditions over the long term. “I'm not sure 10 percent efficiency is good enough. It's a complicated equation,” he says. “If you can do water splitting at 10 percent stably, that is a really significant result.”
C. Cox et al. Ten-percent solar-to-fuel conversion with nonprecious materials. Proceedings of the National Academy of Sciences. Published online September 15, 2014. doi: 10.1073/pnas.1414290111.
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