Electricity And Plasma Turn CO₂ Into Fuel And Other Useful Things

Carbon dioxide (CO_₂) can be harmful to the environment in large amounts, but can be used in the production of fuels and other useful chemicals. Doing so isn’t easy, though. Now, Yale researchers have developed a new way to combine electrocatalysis and plasma to efficiently and practically generate a number of valuable products from the gas. The results are published in Nature Catalysis.

Why it matters
Developing a scalable and practical way to convert CO_₂ would not only reduce the amount of greenhouse gases in the atmosphere, but it could also provide a means of achieving net-zero carbon emissions and the long-term storage of intermittent renewable energy.

“The sorts of multi-carbon products we generate with this approach are really useful for a lot of chemical industries,” said Professor Lea Winter, who led the study. “These include higher value products for use in pharmaceuticals, solvents, and manufacturing of a variety of chemicals—essentially alternatives and replacements for fossil fuel derivative chemicals.”

Winter also noted that it could produce the precursor chemicals needed for sustainable aviation fuel.

The problem
Converting CO_₂ is tricky even in a controlled laboratory setting. Out of the lab—in an industrial setting, for instance—it’s even more difficult. That’s because CO_₂ has such strong carbon-oxygen bonds and doesn’t break down easily. Electrocatalysis, which uses an electric current and a catalyst to speed up the chemical reactions, has been used with some success. Typically, though, using electrocatalysis alone produces a limited number of simple-carbon products.

For an extra boost, the researchers tried coupling plasma — a gas made of electrically charged particles that’s often referred to as the fourth state of matter — to electrocatalysis to start to break down CO_₂ before it reaches the electrocatalyst. In this case, the free electrons in the plasma shake up and loosen the CO_₂ bonds. The electrocatalyst, a material carrying an electrical current, then rebuilds the pre-excited species from the CO_₂ plasma into new products.

“We find that plasma unlocks new reaction pathways that lead to the generation of products like methanol and butane, which are not generated with electrocatalysis alone,” said Winter, assistant professor of chemical & environmental engineering.

But these plasma-electrochemical processes typically use a water interface, which significantly limits the method’s effectiveness. The plasma activates the CO_₂ gas, but the water quenches and neutralizes the most reactive plasma species by the time they get to the electrocatalyst.

“It's been challenging to figure out how to effectively couple plasma with catalysts in a way that allows us to actually see an effect of plasma—to get those plasma-excited species to participate in catalytic reactions,” Winter said.

The solution
To get around this problem, Winter developed what she calls a “3-phase interface.” Key to the system is a gas diffusion electrode—a membrane made of the same material used for non-stick pans - that enables the interaction of solid, liquid and gaseous components. This membrane is partly covered with a thin layer of copper that acts as a catalyst while keeping the liquid and the plasma separate. The system also includes a small amount of water, which provides the necessary hydrogen ions for the chemical reactions.

The membrane’s pores are large enough to allow the plasma particles to pass through to the electrocatalyst layer, meet the protons from the water, and then drive the chemical reactions.

Using this method, the researchers achieved some of the highest reported generation rates of valuable alcohols and products with three or four carbon atoms. These can be used for liquid fuels, pharmaceuticals and other valuable products.

Next steps
Building off this work, Winter said she plans to optimize the catalyst by experimenting with alternative materials.

“We want to figure out how to design catalysts that are specifically tuned to controlling the reaction pathways for converting plasma-activated CO₂ toward high-value products,” she said.

As for putting it to practical use, Winter said the system has the advantage of being easy to scale out because it operates at atmospheric pressure and room temperature.

“And it's a turnkey process,” she said. “You can switch it on or shut it off at will based on when electricity is available. It can also operate directly with intermittent renewable electricity, so we could see retrofitting an existing plant, and then you don't need to build an entire new factory around it.”

The work was initiated with a seed grant from the Yale Center for Natural Carbon Capture, and is now funded by Winter’s Department of Energy Early Career Award.