WASHINGTON, D.C.—Carbon dioxide (CO2) is society’s ultimate waste product, with billions of tons of the stuff injected into the air every year. But recycling it into valuable fuels and chemicals has always required too much energy to make financial sense. Now, researchers have found two efficient ways to convert CO2 into energy-rich byproducts, they reported last week here at a meeting of the American Chemical Society (ACS). If they gain traction, they could help solve another pressing problem: Because both approaches require a steady stream of electrons from a source of electricity, they could siphon up all the “lost” solar and wind energy that can’t currently be stored in electric grids.
To recycle CO2, some researchers are mimicking photosynthesis, harnessing sunlight to convert the molecule into carbohydrates. But these solar fuel reactors often need to run at 1000°C temperatures. Other chemists favor a more traditional approach that would carry out similar reactions, but near room temperature in electrochemical cells that need electricity and special catalysts.
The first step in such an electrolytic approach is splitting CO2, a tough, stable molecule, into oxygen and carbon monoxide (CO), a slightly more energy-rich molecule that can form the basis for hydrocarbon fuels like methanol. That process starts with two catalyst-covered electrodes dunked in a beaker of water into which CO2 has been dissolved. The stream of electrons between these electrodes carry out separate reactions that split water and CO2, ultimately generating CO and more water.
Theoretically, it should take just 1.33 volts of electricity—less than that produced by a AA battery. But in practice, researchers must raise the voltage another volt or so to drive the reaction at a faster clip. This extra voltage, known as the overpotential, amounts to an energy surcharge that lowers the cell’s efficiency. Another problem is that most catalysts channel more of the available electrons into splitting water rather than converting CO2 to CO.
In 2011, researchers led by Richard Masel, a chemist and CEO of Dioxide Materials in Boca Raton, Florida, tested a setup with silver and iridium oxide catalysts and a liquid electrolyte to promote the CO2 to CO reaction. The electrolyte contained a compound called imidazolium that formed a protective layer around the silver-covered electrode. That blocked the water-splitting reaction and encouraged the catalyst to pass nearly all its electrons to converting CO2 instead. It also produced CO with an overpotential of just 0.17 volts. But ionic liquids can be expensive and corrosive. So Dioxide Materials set about making a durable and cheap plastic membrane that could serve the same function when laid atop a silver electrode.
Last year, the company reported that it had successfully made the membranes. But at the ACS meeting, Dioxide Materials chemist Richard Ni reported that devices using them produced CO with an efficiency nearly double that of the next best membrane. Ni also reported that with recent upgrades, their cells can transform CO2 to CO at double the rate of other CO2-splitting electrolyzers of a comparable size, which could help them process large volumes of CO2 when scaled up. Ni added that the company’s devices remain stable and undeteriorated after 6 months of continuous operation.
“Those are very good results,” and considered good enough for a commercial product, says Fan Shi, a chemist at the National Energy Technology Laboratory in Pittsburgh, Pennsylvania. Dioxide Materials is not alone in trying to commercialize the process: Already, chemical giant BASF has announced plans to produce liquid methanol fuel using a similar method. And a German company called Sunfire announced in May that it’s producing “blue crude,” a synthetic diesel fuel from CO2 and water using a high-temperature process.
Meanwhile, Dioxide Materials has scaled up the size of their electrodes from squares smaller than a U.S. postage stamp to ones bigger than an adult hand, enabling a larger CO flux. And the company has teamed up with industrial giant 3M to produce swaths of their imidazole membranes in a reel-to-reel process. The company is also in discussions with industrial chemical producers like Linde and Siemens, exploring places where they might be able to access pure CO2 waste streams and excess renewable power. “That could be key,” Shi says. “You can store energy during periods of low demand.”
To make a truly large-scale impact, the company may need to find cheaper electrode catalysts than silver and iridium oxide, says Haotian Wang, a chemist at Harvard University. Ni says the company is looking for cheaper options to replace iridium, a rare and expensive metal.
Another long-term prospect was raised at the ACS meeting by Paul Kenis, a chemist at the University of Illinois in Urbana. Though converting CO2 to CO is the simplest option, Kenis and others are looking transform CO2 in one fell swoop to methane, formic acid, methanol, or other complex hydrocarbons with more energy—and higher value. But the reactions are more complicated, requiring not just a source of electrons but also protons. In order to run these reactions, researchers typically use an anode to split water molecules into protons, electrons, and oxygen, and then feed the protons and electrons to a cathode, where they react with CO2 to make hydrocarbons. The water-splitting reaction also normally requires a heavy energy surcharge.
At the ACS meeting, Kenis reported that his group has created a CO2-splitting device in which they replaced the water at the anode with a liquid called glycerol, a waste product produced by the ton in biodiesel plants. By using glycerol, Kenis says that his team was able to reduce the overpotential in that part of their system by nearly two-thirds and churn out formic acid, which is widely used in chemical synthesis. Kenis confesses that the new setup may generate side products that they don’t yet know about and that it has a long way to go before becoming a commercial technology. But William Goddard, a chemist at the California Institute of Technology in Pasadena, says he was impressed with the idea. “To take what’s now considered garbage and convert it, that has real potential,” he says.