Stuart Licht has designed the ultimate recycling machine. The solar reactor that he and colleagues built in his lab at George Washington University in Washington, D.C., takes carbon dioxide (CO₂) from the atmosphere—a byproduct of fossil fuel combustion—and uses the energy in sunlight to turn it back into fuel. There are a few steps in between. Water is also involved in the reaction, which produces hydrogen (H₂) and carbon monoxide (CO); they in turn can be stitched into liquid hydrocarbon fuels. But Licht’s is one of the most efficient devices of its type ever constructed.

It is only one of the solar fuel technologies taking shape in labs around the world. They embody a dream: the prospect of one day bypassing fossil fuels and generating our transportation fuels from sunlight, air, and water—and in the process ridding the atmosphere of some of the CO₂ that our fossil fuel addiction has dumped into it.

These schemes are no threat to the oil industry yet. In Licht’s device, parts of the reactor run at temperatures approaching 1000°C, high enough to require specialized materials to hold the components. Other researchers are pursuing an alternative approach, developing catalysts that could carry out the same chemical reactions at or near room temperature, using electricity from sunlight or other renewables to power the chemical knitting process.

The bigger hurdle is economic. Oil is cheap, for the moment, and there is little incentive to adopt cutting-edge, costly alternatives. But the relentless march of climate change, and the elegance of the concept, have drawn researchers around the globe to the pursuit of solar fuels. “This is a very hot area right now,” says Omar Yaghi, a chemist at the University of California (UC), Berkeley. And as Licht’s reactor demonstrates, the research is making progress. “We’re not there yet, but we’re moving in the right direction,” says Andrew Bocarsly, a chemist at Princeton University who is developing low-temperature catalysts.

Enthusiasts even see a glimmer of hope for making the technology economical: the steady spread of renewable electricity sources, such as wind farms and solar plants. Already, windmills and solar cells sometimes generate more power than locals can absorb. If this oversupply could be stored in chemical fuels, experts argue, utility providers might be able to save their power for use anytime and anywhere—and make extra money on the side.

THE NEED FOR LIQUID FUELS is unlikely to go away despite concerns about climate change. The high energy density and ease of transport of gasoline and other liquid hydrocarbons have made them the mainstay of the world’s transportation infrastructure. Researchers continue to pursue the use of low-carbon gases, such as methane and hydrogen, as transportation fuels, and electric cars are proliferating. But for long-distance trucks and other heavy vehicles, as well as aviation, there is no good alternative to liquid fuels. Solar fuel proponents argue that finding a way to brew them from readily available compounds such as water and CO₂ could make a sizable dent in future CO₂ emissions.

The task essentially boils down to running combustion in reverse, injecting energy from the sun or other renewables into chemical bonds. “It’s a very challenging problem, because it’s always an uphill battle,” says John Keith, a chemist at the University of Pittsburgh in Pennsylvania. It’s what plants do, of course, to make the sugars they need to grow. But plants convert only about 1% of the energy that hits them into chemical energy. To power our industrial society, researchers need to do far better. Keith likens the challenge to putting a man on the moon.

The trouble is that CO₂ is a very stable, unreactive molecule. Chemists can force it to react by pumping in electricity, heat, or both. The first step in this process is usually ripping off one of CO₂’s oxygen atoms to make CO. That CO can then be combined with H₂ to make a combination known as syngas, which can be converted into methanol, a liquid alcohol that can be either used directly or converted into other valuable chemicals and fuels. Massive chemical plants do just that, but they make their syngas not from air, but from plentiful and cheap natural gas. So the challenge for chemists is to create syngas from renewables more cheaply than current sources can match.

Licht, who calls his solar-generated mixture of CO and H₂ “sungas,” says he’s taking aim at that challenge by using both heat and electricity from the sun. His setup, which he details in a paper accepted at Advanced Science, starts with a high-end commercially available solar cell called a concentrated photovoltaic. It focuses a broad swath of sunlight onto a semiconductor panel that converts 38% of the incoming energy into electricity at a high voltage. The electricity is shunted to electrodes in two electrochemical cells: one that splits water molecules and another that splits CO₂. Meanwhile, much of the remaining energy in the sunlight is captured as heat and used to preheat the two cells to hundreds of degrees, a step that lowers the amount of electricity needed to split water and CO₂ molecules by roughly 25%. In the end, Licht says, as much as 50% of the incoming solar energy can be converted into chemical bonds.

It’s unclear whether that process will produce syngas that’s as cheap as that made from natural gas. But Licht notes that a 2010 economic analysis of his solar water splitting setup alone, which he first described in 2002, concluded that his approach could generate a kilogram of H₂—the energy equivalent of 4 liters of gasoline—at a cost of $2.61.

Yet it may be hard for Licht’s sungas setup to lower the price further. Licht’s charge-conducting electrolyte uses lithium, a somewhat rare and costly metal whose limited supplies could prevent a massive scale-up. Licht also faces competition from other researchers who also use high temperatures to ease the splitting of water and CO₂, but rely entirely on electricity instead of solar heating. But like sungas, those schemes, called solid oxide electrolysis cells, face the longevity challenges of running at high temperatures.

GIVEN THESE HURDLES, Bocarsly and others continue to try to split CO₂ at lower temperatures. One such approach is already commercial. In Iceland, a company called Carbon Recycling International opened a plant in 2012 that uses renewable energy to create syngas. The company harnesses the island’s abundant geothermal energy to produce electricity, which drives electrolysis machines that split CO₂ and water. The resulting syngas is then turned into methanol.

Of course, most regions of the globe lack Iceland’s abundant geothermal power needed to drive the process, so researchers are hunting for new catalysts that can split CO₂ with less energy. These catalysts typically sit on the cathode, one of two electrodes in an electrolytic cell containing water. At the opposite electrode, water molecules are split into electrons, protons, and oxygen, which bubbles away. The electrons and protons pass to the cathode, where CO₂ molecules split into CO and oxygen atoms that combine with the electrons and protons to make more water.

Today, the gold standard for such catalysts is, well, gold. In the 1980s, Japanese researchers found that electrodes made from gold had the highest activity for splitting CO₂ to CO of all the low-temperature setups. Then in 2012, Matthew Kanan, a chemist at Stanford University in Palo Alto, California, and colleagues discovered something even better: Making their electrode from a thin layer of gold divided into nanosized crystallites, they reported in the Journal of the American Chemical Society, slashed the electricity needed by more than 50% and increased the catalyst’s activity 10-fold. The boundaries between the gold crystallites appear to promote the reaction.

At some $36,000 per kilogram, gold is still far too expensive for use on a massive scale. Last year, however, researchers led by Feng Jiao, a chemist at the University of Delaware (UD), Norwalk, reported in Nature Communications that catalysts made from silver nanoparticles do almost as well. And this year, they reported in ACS Catalysis that even cheaper catalysts made from tiny zinc spikes called dendrites are also proving highly effective at churning out CO.

Catalysts that could be even cheaper are in the works. Researchers at UC Berkeley, for example, reported last month that they had made a highly porous crystalline material out of organic ring-shaped compounds with a combination of cobalt and copper atoms at their core. When layered atop an electrode and dunked in a water-based solution, the porous materials split CO₂ molecules into CO at a rate of 240,000 per hour—a furious pace compared with most other room-temperature catalysts. And last year, Kanan and his colleagues reported that electrodes made of nanocrystalline copper could bypass the need for syngas, allowing them to directly synthesize a variety of more complex liquid fuels, such as ethanol and acetate, at unprecedented efficiencies.

Researchers worldwide are also pursuing another rich vein: driving the low-temperature electrolysis of CO₂ and H₂O with energy directly from sunlight. Most efforts center on using light-absorbing semiconductors, such as titanium dioxide–based nanotubes, to churn out CO, methane, or other hydrocarbons. So far, such setups aren’t very efficient; typically they convert less than 1% of the incoming solar energy into chemical bonds. Bocarsly and others have done better using the sun’s ultraviolet light, which makes up only a tiny part of the spectrum. But at the American Chemical Society meeting in Boston last month, Joel Rosenthal, a chemist at UD Newark, reported that his team has developed a bismuth-based photocatalyst that converts 6.1% of incoming visible light energy to chemical bonds in CO.

Despite progress on all these fronts, Kanan cautions that solar fuels still have a long way to go to compete directly with liquid fossil fuels, especially now that the price of oil has fallen below $50 per barrel. And barring a concerted push from governments worldwide to cap or tax carbon emissions, solar fuels may never be able beat oil-derived fuels on cost alone. “It’s a tall order,” he says.

But Paul Kenis, a solar fuels researcher at the University of Illinois, Urbana-Champaign, argues that the broad penetration of solar and wind power offers hope. Denmark, for example, already produces some 30% of its electricity from wind farms and is on pace to reach 50% by 2020. On a particularly blustery day in July, the nation’s wind turbines generated as much as 140% of the country’s electrical requirements. The excess was sent to its neighbors, Germany, Norway, and Sweden. But the oversupply added to utilities’ fears that in times of peak renewable power production, the value of electricity could fall to zero or even below, as producers would have to pay others to take it so as not to damage their grid.

That’s where solar fuel producers could stand to benefit, Kenis says: By absorbing that power and using it to make fuels and other commodities, they could essentially act as energy banks and perhaps earn some cash as well. For now, Kanan argues, it still makes the most economic sense simply to shunt excess renewable power into the grid, displacing fossil energy. But someday, if renewable power becomes widespread enough and the technology for making renewable fuels improves, we may be able to guzzle gas without guilt, knowing we are just burning sunlight.

By Robert F. Service

原文链接：http://news.sciencemag.org/chemistry/2015/09/feature-there-s-too-much-carbon-dioxide-air-why-not-turn-it-back-fuel

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