Burning fossil fuels is a handy source of energy, but it releases huge amounts of carbon dioxide into the atmosphere, warming the Earth's climate, which is a problem. But what if you could capture that carbon dioxide as it's released from a smokestack and turn it back into fuel?
Chemically, it's possible, and it's already been done on a small scale. Scientists around the world are working to make it practical on an industrial scale, where a stream of carbon dioxide could be captured and "recycled" back into fuel before ever hitting the atmosphere and contributing to climate change.
Now, Canadian and Chinese scientists say they're one step closer.
In theory, the chemical reaction in which fossil fuels are burned to generate carbon dioxide, water and energy is reversible — you can take the amount of carbon dioxide and water that was released, and combine them with an amount of energy equivalent to the amount that was produced when the fuel was burned to regenerate the fuel.
Ted Sargent, an engineering professor at the University of Toronto, who led the new research, likens it to "putting the genie back in the bottle."
Like splitting water
How do you do it? It's very similar to the electrolysis or "water splitting" process used to produce hydrogen and oxygen by running electricity through water using electrodes (the reverse of burning hydrogen in oxygen to power a rocket).
Generally, scientists dissolve carbon dioxide in a conductive liquid — Sargent uses a kind of salt water — and run electricity through it using electrodes to produce carbon monoxide. That carbon monoxide can then be combined with hydrogen to make products like synthetic diesel.
Of course, there's a catch.
To make a litre of fuel, you need to feed in at least the amount of energy in the form of electricity that would be released by burning a litre of fuel — in other words, a lot.
But the synthetic fuel could be effectively carbon neutral if both the hydrogen and carbon monoxide were produced using renewable energy, such as solar or wind.
If that were the case, not only would the process stop carbon from being spewed into the air, but it could also be a way to store renewable energy in places like Canada, where it tends not be sunny and windy at the times when we need the most electricity, Sargent says.
Fuels like gasoline conveniently store a lot of energy in a very small space, he adds. "It's amazing how far your car can go on a tank of gas."
But producing synthetic fuels from carbon dioxide would require far, far more energy than you'd use to pump or dig fossil fuels out of the ground.
"You can't afford a lot of waste," Sargent acknowledges. "Energy efficiency is really crucial."
That's the problem. Right now, scientists aren't very fast or efficient at making carbon monoxide this way. They've been trying a variety of catalysts — different electrode materials that can speed up the reaction — to try to improve the speed and efficiency.
Joel Rosenthal is an associate professor of chemistry at the University of Delaware who wasn't involved in the University of Toronto study, but does similar research. He said most catalysts require you to put in not just the amount of energy that would be released by burning the fuel, "but a lot of extra energy as well."
While scientists have gotten relatively good at generating hydrogen from electrolysis, the carbon dioxide process "needs breakthroughs," Sargent says.
He and his colleagues have come up with one way to make the process a little more efficient, they report in the journal Nature this week.
A big problem with electrolysis of carbon dioxide is that it doesn't dissolve very well in water — as anyone who has ever drunk pop knows. Because only dissolved carbon dioxide can be converted to carbon monoxide, and only when it's at an electrode, it takes a long time for the carbon dioxide in a container of water to be converted into carbon monoxide.
Tiny 'lightning rods'
While many researchers have been trying different catalyst materials, Sargent and his team used a different approach. They took an existing catalyst and re-engineered its shape in order to get carbon monoxide to pool at the electrode.
The researchers shaped a gold electrode into an array of "nanoneedles" 10,000 times finer than a human hair. Applying electricity generates an electric field that causes carbon dioxide molecules to gather at the tips of the needles — Sargent calls it a "lightning rod" effect. That concentrates the carbon dioxide at the electrode and dramatically boosts the speed of its conversion into carbon monoxide.
The team also tried making nanoneedles with another catalyst, palladium, and found a similar effect, except that the reaction produced formate — another useful fuel — instead of carbon monoxide.
Rosenthal calls the discovery "a pretty nice advance."
While gold and palladium are too expensive to use on an industrial scale, he says, it may be possible to use the same electrode shaping strategy with less-expensive catalysts. His team uses bismuth, tin and lead, for example, although they use a different conductive liquid instead of water.
Xiao-Dong Zhou, an associate professor of chemical engineering at the University of South Carolina who was also not involved in the study, but does similar research, called the nanoneedle catalyst "a very neat idea."
But he said the speed is still far from what is needed to turn carbon dioxide into fuel on an industrial scale.
"It's the first step," he says, adding that it's an idea that can be combined with other ways to improve the overall efficiency of the process.
Rosenthal has a similar view. "I don't think you're going to see an industrial reactor run tomorrow that's going to take a stream of CO2 and cleanly make carbon monoxide at an industrial feasible rate, even using these catalysts," he says. "But I think it's a very, very important step in that direction."
The new study was funded by the Ontario Research Fund, the Natural Sciences and Engineering Research Council of Canada, the CIFAR Bio-Inspired Solar Energy program, a University of Toronto Connaught grant, the Shanghai Municipal Natural Science Foundation and the National Natural Science Foundation of China.