A step towards closing the carbon loop

Marit Mitchell, University of Toronto

RESEARCHERS at the University of Toronto (UoT), Canada have developed a novel electrochemical pathway to transform carbon dioxide (CO₂) into valuable products, such as jet fuel and plastics. It could significantly improve the economics of direct-air capture (DAC) of CO₂.

DAC is an emerging technology that could allow companies to produce fuels or plastics from carbon in the atmosphere, rather than using fossil fuels. Capturing CO₂ as a carbonate is a common method of capture, according to lead author Y Chris Li. For example, Canadian DAC company Carbon Engineering achieves capture using an alkali solution. The resulting carbonate solution is then processed into a pellet, which is heated to decomposition temperature to release CO₂ for recycling.

The engineering team at UoT has found a way to reduce energy requirements by eliminating the intermediate heating step, thereby reducing the costs of resulting products. The proof-of-concept research demonstrates a viable, alternative route for direct-air carbon capture and utilisation.

The process developed at UoT uses an electrolyser, a device that uses electricity to drive chemical reactions. “We utilised a bipolar membrane, a non-convectional choice in electrolyser technology,” said Li. A silver-based catalyst formed the cathode and nickel foam was used as the anode, with the bipolar membrane used as a separator between the two. CO₂ was captured as a carbonate using potassium hydroxide solution.

The bipolar membrane consists of a layer to dissociate water to generate protons and hydroxide anions. According to Li, at the membrane:catalyst interface, protons react with the carbonate salt in the surrounding electrolyte, releasing CO₂ in situ. The CO₂ released is immediately converted to syngas by the silver-based catalyst. Syngas is a common chemical feedstock which can be used to produce a wide variety of products, including jet fuel and plastic precursors.

Li said that the researchers achieved carbonate-to-syngas conversion with 100% carbon utilisation and syngas is the sole product, which minimises purification costs. The overall energy efficiency of the process was 35%, but Li said that there is a lot of room for improvement.

CO₂ readily becomes a carbonate when dissolved in a liquid, making it inaccessible to traditional electrolysers said Li. He added that this contributes to low yields and efficiencies. Ted Sargent, Professor of Electrical and Computer Engineering at UoT and study leader, said that “this is the first known process that can go all the way from carbonate to syngas in a single step.”

“Our strategy increases the overall energy efficiency by avoiding some of the more energy-intensive losses,” said Sargent, adding that: “it goes a long way toward answering the question of whether it will ever be possible to use air-captured CO₂ in a commercially compelling way” and “this is a key step toward closing the carbon loop”.

During the overall process, the capture solution is regenerated, allowing it to begin again. In experiments the electrolyser remained stable for more than six days of operation.  

According to Li, the researchers are currently working on several follow-up projects which include improving the overall energy efficiency. They are also working to design catalysts that can generate other high-value products. Sargent added that more work will be needed to scale up the process for industrial application.

The study conducted by the UoT engineers shows proof of concept, but Li said: “there are a lot of interesting chemistry and engineering problems in this process. If we can perfect the process, with help of the community, we really have a chance to achieve a carbon neutral cycle.”

Carbon Engineering has been capturing 1 t/d of CO₂ at a pilot plant in Squamish, British Columbia since 2015. Earlier this year, an investment was announced to help commercialise the company’s DAC and Air to Fuels technology.

ACS Energy Letters: http://doi.org/c7r4