Turning CO2 into coal at room temperature

Researchers have used liquid metals to turn carbon dioxide (CO2) back into solid coal with technology they say has the potential to revolutionise carbon capture and storage and provide a new way for removing greenhouse gases from the atmosphere.

The international team led by RMIT University in Australia developed a liquid-metal electrocatalyst that transforms gaseous carbon dioxide directly into carbon-containing solids at room temperature.

The catalyst, which is based on non-toxic gallium alloys, also prevents coking – where solid carbon sticks to the catalyst surface – which has been a problem in previous work in this area.

The key to all of this is the “room temperature” bit. Other methods to prepare carbon nanomaterials usually require working at temperatures of hundreds of degrees Celsius, making them energy-intensive and not commercially viable.

Similarly, the researchers say, technologies that focus on compressing CO2 into a liquid form and then injecting it underground have both economic and environmental drawbacks, notably possible leaks from storage sites.

And reducing CO2 to high-value products such as chemical feedstocks and fuel does not permanently trap the carbon. The fuels, for instance, are burned, releasing it all over again.

RMIT researcher Torben Daeneke says converting the gas into a solid could be a more sustainable approach.

“While we can’t literally turn back time, turning carbon dioxide back into coal and burying it back in the ground is a bit like rewinding the emissions clock,” he says.

Daeneke’s RMIT colleague Dorna Esrafilzadeh developed the electrochemical technique to capture and convert atmospheric CO2 to storable solid carbon.

With colleagues, she designed a liquid metal catalyst with specific surface properties that make it extremely efficient at conducting electricity while chemically activating the surface.

The CO2 is dissolved in a beaker filled with an electrolyte liquid and a small amount of the liquid metal, which is then charged with an electrical current.

The CO2 slowly converts into solid flakes of carbon, which are naturally detached from the liquid metal surface, allowing the continuous production of carbonaceous solid.

Esrafilzadeh says the carbon produced could also be used as an electrode.

“A side benefit of the process is that the carbon can hold electrical charge, becoming a supercapacitor, so it could potentially be used as a component in future vehicles,” she says.

“The process also produces synthetic fuel as a by-product, which could also have industrial applications.”

Daeneke stresses that more work still needs to be done, but sees the early results as “a crucial first step to delivering solid storage of carbon”.

The collaboration involved researchers from Germany, China the US and Australia. The findings are published in the journal Nature Communications.