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.
Fossil fuels such as gasoline are a great way to conveniently store a lot of energy in a very small space. Synthetic versions can theoretically be made from captured carbon dioxide. (Rich Pedroncelli/Associated Press)
"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.