In a quest to find a way to convert carbon dioxide into something useful, researchers from Oak Ridge National Labs in TN stumbled across an inexpensive, room-temperature catalyst for turning CO2 into ethanol. The new catalyst is made of copper nanoparticles, electroplated onto a substrate of vapor-deposited, nitrogen-doped graphene nanospikes, all atop a slice of n-type silicon semiconductor.
“We discovered somewhat by accident that this material worked,” said lead author Adam Rondinone. “We were trying to study the first step of a proposed reaction when we realized that the catalyst was doing the entire reaction on its own.”
“By using common materials, but arranging them with nanotechnology, we figured out how to limit the side reactions and end up with the one thing that we want,” said Rondinone.
Researchers constructed the system, dropped it in a water bath, started bubbling CO2 gas through the water and then turned on the power for electrolysis. When the experiment was done, their catalyst had made the water/CO2 bath into ethanol, with a yield of 63 percent. 120 proof isn’t bad for a happy accident.
The reason it was so effective has a lot to do with how electrons behave in graphene. The molecular geometry of the edges of graphene flakes mean that electrons pull away from the edges in predictable ways, leaving folds and edges of the flake positively charged. When there are atoms of nitrogen dopant scattered through the graphene matrix, electrons tend to flee from the nitrogen atoms too, which leaves these great big positively charged pockets.
Physical chemistry is important here too. The catalyzing surface is an electrode coated in this highly crumpled graphene sheet; it’s not a neat, regular distribution of ridges, but rather a disordered aggregation of graphene edges and carbon nano-hooks sticking out every which way. “They are like 50-nanometer lightning rods that concentrate electrochemical reactivity at the tip of the spike,” Rondinone said in a statement. This distribution of spikes provides ample reactive pockets next to the copper nanoparticles, so that the catalytic surface can do its work.
But the holey distribution also limits surface area. Unlike a neatly ordered grid, the limited reaction area prevents the whole mass of CO2 from being effectively reduced into ethylene gas. While ethylene would otherwise be the end product of this reduction reaction, ethylene isn’t super helpful unless you want overripe bananas. The fact that this reaction mostly stops at ethanol means that here, getting a limited yield from an unexpected reaction is actually a good thing.
It’s not an infinite energy machine, and it’s not by any means ready to be commercialized. The reaction still requires an input of electricity, and some way of managing the hydrogen released during electrolysis. But if the power it used was functionally carbon-neutral, this would be a way to store energy as ethanol, while decreasing the amount of cropland used for ethanol fuels and scrubbing the atmosphere of CO2 in the process.
Now read: What is graphene?
Title image of ORNL’s Yang Song (seated), Dale Hensley (standing left) and Adam Rondinone via ORNL