New sorbent captures 3 times as much CO₂

A new sorbent can capture up to three times as much carbon dioxide from air as known materials (Science Advances 2023, DOI: 10.1126/sciadv.adg1956). When exposed to seawater, the sorbent also converts carbon dioxide into sodium bicarbonate, or baking soda, that can be safely stored in oceans.

Eighteen plants that remove carbon dioxide directly from air operate today in Canada, Europe, and United States, according to the International Energy Agency (IEA). These pilot-scale plants use amine-based liquids or solids to capture carbon dioxide. The materials are then heated to release the carbon dioxide for storage or use and free the sorbent for reuse.

But it costs a few hundred dollars to capture a ton of CO₂ with today’s technologies. Costs need to go down for direct-air capture (DAC) to be deployed widely enough to help bring the world’s carbon emissions to zero by 2050, according to the IEA.

One way to do that is to boost a sorbent’s capacity to soak up CO₂ at its 400 parts-per-million concentration in air. Arup K. SenGupta, an environmental engineer at Lehigh University and his colleagues passed a copper chloride solution through resin beads that contain polyamine groups to create a high-capacity sorbent.

Copper ions bind with the nitrogen atoms in amines, and the beads turn blue. When the researchers passed atmospheric air through a column full of the copper-modified beads, the material captured over 5 mol of CO₂/kg, three times more than both the unmodified beads and most other known solid sorbents. The positively charged copper ions play a key role, triggering the reaction of CO₂ with water vapor in air to form bicarbonates that bind to the copper-nitrogen complex.

Next, the researchers passed seawater through the column. Now, the copper ions exchange their bicarbonate ions with the chloride in the salt present in seawater, giving sodium bicarbonate which could be released into the sea. “The oceans can be a big sink for captured carbon dioxide,” says SenGupta.

The higher capacity of the new sorbent should bring down DAC costs, he says. He imagines shore-based or offshore plants where air and seawater pass through columns full of sorbent. Later, the baking soda–laden water is deposited in the ocean. “This should cost less than $100 per ton of CO₂ if we build to large capacity. If we are to use direct air capture worldwide, you need a technology that you can disseminate in poor or less developed countries.”

“The carbon-loading capacity of the material is noteworthy,” says David Keith, a physicist at Harvard University and founder of DAC company Carbon Engineering. But increasing the speed at which the material traps carbon dioxide will be crucial for bringing down cost. “Fast kinetics are required for practical air capture,” he says, “or the cost of the air contacting system dominates.”