Carbon dioxide captured from air can be used to make products including jet fuel or plastics, but the process is energy intensive and costly.
Now, a team led by Prof Ted Sargent at the University of Toronto (U of T) has developed an electrochemical method of converting CO2 that increases overall energy efficiency by avoiding some of the more energy-intensive losses.
One company investigating direct-air carbon capture is Carbon Engineering whose pilot plant in Squamish, British Columbia, captures CO2 by forcing air through an alkaline liquid solution. The CO2 dissolves in the liquid, forming a carbonate.
To be fully recycled, the dissolved carbonate is normally turned back into CO2 gas, and then into chemical building blocks that form the basis of fuels and plastics. According to U of T, one way to do this is to add chemicals that convert the carbonate into a solid salt. This salt powder is then heated at temperatures above 900oC to produce CO2 gas that can undergo further transformations. The energy required for this heating drives up the cost of the resulting products.
The U of T Engineering team's alternative method applies an electrolyser to stimulate a chemical reaction. Having used electrolysers to produce hydrogen from water, the team surmised that they could be used to convert dissolved carbonate directly back into CO2, skipping the intermediate heating step.
"We used a bipolar membrane, a new electrolyser design that is great at generating protons," said Geonhui Lee, who along with postdoctoral fellow Y. Chris Li is among the lead authors of a new paper in ACS Energy Letters describing the technique. "These protons were exactly what we needed to convert the carbonate back into CO2 gas."
Their electrolyser also contains a silver-based catalyst that converts the CO2 into a syngas, which is a common chemical feedstock for the Fischer-Tropsch process and can be turned into a variety of products, including jet fuel and plastic precursors.
"This is the first known process that can go all the way from carbonate to syngas in a single step," Sargent said in a statement.
While many types of electrolysers have been used to convert CO2 into chemical building blocks, none of them can deal effectively with carbonate. Furthermore, the fact that CO2 dissolved in liquid turns into carbonate so readily is a major problem for existing technologies.
"Once the CO2 turns into carbonate, it becomes inaccessible to traditional electrolysers," said Li. "That's part of the reason why they have low yields and low efficiencies. Our system is unique in that it achieves 100 per cent carbon utilisation: no carbon is wasted. It also generates syngas as a single product at the outlet, minimising the cost of product purification."
In the lab, the team demonstrated the ability to convert carbonate to syngas at an overall energy efficiency of 35 per cent, and the electrolyser is said to have remained stable for over six days of operation.
Sargent said that more work will be needed to scale up the process to the levels needed for industrial application, but that the proof-of-concept study demonstrates a viable alternative path for direct-air carbon capture and utilisation.