JOIN USScientists are working on ways to convert carbon dioxide into a renewable, recyclable resource, writes Henry Fountain.
Think, for a moment, of carbon dioxide (CO2) as garbage, a waste product from burning fossil fuels. Like other garbage, almost all of that CO2 is thrown away — into the atmosphere, where it contributes to climate change. A small amount is captured and stored underground to keep it out of the air. But increasingly, scientists are asking, rather than throwing away or storing carbon dioxide, how about recycling some of it?
At laboratories around the world, researchers are working on ways to do just that. The X Prize Foundation has created an incentive, a $20 million prize for teams that by 2020 come up with technologies to turn carbon dioxide captured from smokestacks of coal- or gas-fired power plants into useful products.
The challenge ahead
But perhaps the ultimate goal of researchers in this field is to turn the waste product of fuel-burning into new fuel. In theory, if this could be done on a large scale using renewable energy or even sunlight, there would be no net gain of emissions — the same carbon dioxide molecules would be emitted, captured, made into new fuels and emitted again, over and over. “The grand prize is figuring out how to make CO2 be recyclable, a renewable resource,” said Harry A Atwater, a materials scientist at the California Institute of Technology and director of the Joint Center for Artificial Photosynthesis, which has laboratories at the Lawrence Berkeley National Laboratory and at Caltech. “That would be a millennial advance for society.”
Carbon dioxide is used to make some basic products like urea fertiliser and speciality plastics. But the processes are not necessarily energy efficient, and almost all use CO2 from natural underground reservoirs. Even if companies started using carbon dioxide that was captured, the amount would be less than 0.5% of the roughly 32 billion metric tonnes of CO2 emitted annually by human activity.
What Harry and others have in mind are devices that, if scaled up, could recycle a significant portion of carbon dioxide that is captured from power plants or processes like cement manufacturing, or even directly from the atmosphere. But developing devices that can efficiently and economically convert large amounts of carbon dioxide will require overcoming many hurdles, not the least of which is all the energy required to split the molecules.
“The big challenge is, how do we go from milligrams to megatonnes?” said Dick T Co, a Northwestern University professor and managing director of the Solar Fuels Institute, a group that encourages collaboration among researchers in the field. “How do we make a dent in our energy portfolio when people are working in test tubes today?”
In a research building at the Lawrence Berkeley lab, Harry leads a team of scientists that is trying to mimic what plants do through photosynthesis. They want to take carbon dioxide and water and, using only sunlight, turn it into fuel. The centre, started in 2010 with a grant from the USA’s Department of Energy, devoted its first 5 years to one aspect of photosynthesis: splitting water into its components, hydrogen and oxygen.
Harry, Frances A Houle, a deputy director, and Karl Walczak, a project scientist, showed some of the fruits of that work — a chip-size sandwich of semiconductor material, catalysts and membranes encased in a clear container with a water-based solution. When the chip was exposed to light, bubbles of gas — hydrogen on one side, oxygen on the other — formed, broke off and rose to the top. By their calculations, the chip is about 10 times more efficient than a typical plant, which uses about 1% of the sunlight that strikes it.
The centre is now working on the carbon dioxide part of the photosynthetic equation. The goal is to integrate the two processes in a device that might look a lot like a solar panel. But rather than generating electricity, it would produce fuel — perhaps methanol, which could be burned directly or converted to gasoline.
Carbon dioxide is much more difficult to split than water, however, involving 6 steps, each requiring energy and a catalyst. Nature appears to do it effortlessly, but it has had millions of years of evolution to improve the process. “The big challenge with CO2 is that you produce a whole bunch of products,” Harry said. “Nature has developed very refined mechanisms that produce precisely one product.”
Much of the work at the centre involves studying catalysis, through theoretical analysis and testing possible combinations of metal oxides to see how well they might work. The testing method is similar to one used for drug discovery, with equipment that can analyse large numbers of very small samples at a time.
Efficiency is the key
The goal is to make a device that can make just one product, as in natural photosynthesis, but more efficiently. At the same time, the device should last for years, as solar panels do. That adds to the engineering and design challenges. There are other research groups, and some startups and established companies, that are working on CO2 conversion as well. Sunfire, a company in Dresden, Germany, built a prototype to make synthetic crude oil from carbon dioxide and water. Part of the crude is diesel fuel, and in 2015, Audi used some of the Sunfire diesel to briefly power one of its cars.
The Sunfire process uses electricity, not sunlight, so the electricity would have to come from renewable sources to result in meaningful carbon reductions. Given the amount of electricity required, a big challenge is producing fuel that could compete in price with conventional fossil fuels, said Christian von Olshausen, Sunfire’s chief technology officer.
In Berkeley, at a lab building just down the hill from the Joint Center for Artificial Photosynthesis, 3 young scientists have started a small company, Opus 12, to develop their own carbon dioxide-conversion device, powered by electricity. Their idea is to exploit the fact that carbon dioxide can be converted into many different products, by coming up with catalysts tailored to produce specific ones. “Our vision is to design this reactor more as a platform,” said Nicholas Flanders, who founded Opus 12 with Kendra Kuhl and Etosha Cave.
Over the next year and a half, Nicholas said, the company plans to scale up their reactor to something the size of a dishwasher. That would be big enough to start generating revenue by making small amounts of products for niche applications that command a relatively high price. Because of the electricity needed, any device they make would need to use renewable electricity to maximise the carbon dioxide-reduction benefit.
Beyond that, Nicholas envisions larger devices that could convert tonnes of carbon dioxide a day into fuels or other products. That’s still not very much, given the billions of tonnes of CO2 pumped into the atmosphere every day. But Opus 12 is in it for the long haul, he said.
The Joint Center for Artificial Photosynthesis is, too, although Harry is realistic about the challenges of directly converting sunlight and carbon dioxide into fuel. “You can rest assured that the energy and catalysis problems of humanity will not have been resolved five years from now,” he said.
But there is growing interest in the work of Harry and his fellow researchers, particularly after the recently signed Paris climate treaty that calls for sharp emissions reductions to combat global warming. “We have some wind at our back that we haven’t had until recently,” he added.
The New York Times