Using inexpensive materials, researchers have made a carbon-capture device that continuously traps carbon dioxide at a rate 100 times as fast as current systems (Energy Environ. Sci. 2022, DOI: 10.1039/d1ee03018c).
The system could be made large enough to grab CO2 from smokestack exhaust or directly from the air. But given its efficiency and low cost, the technology could make carbon capture feasible even at a personal scale, says its developer Meenesh R. Singh, a chemical engineer at the University of Illinois Chicago. “A unit as small as a humidifier can capture about one kilogram of carbon dioxide per day,” he says.
Most carbon-capture systems today use amine-based solutions or solid sorbents to separate CO2 from a mix of gases. But the heat required to release the gas from the saturated materials for storage or reuse makes the technologies energy intensive and expensive, especially for the large amount of material needed to remove CO2 directly from air where its concentration is low, Singh says.
Singh’s team’s new approach uses electrodialysis, in which an electric field is used to move ions across a charged membrane. The technique is typically used for desalination, but Singh and his colleagues have now used to it to capture CO2.
In their device, a solution of potassium hydroxide (KOH) in ethylene glycol sits on one side of a positively charged membrane, with water on the other side. Electrodes sandwich this cell. When air or flue gas is injected into the KOH side, the hydroxide ions react with CO2 to form negatively charged bicarbonate ions. The researchers pass water along the outside of the porous cathode, which creates hydroxide ions to replenish the KOH.
Meanwhile, the bicarbonate ions move through the membrane towards the anode on the aqueous side of the cell. There, water triggers their conversion back into CO2 for storage or reuse without heat required by other methods to release the CO2. This is the same reaction behind the slight fizz you see when you mix baking soda in water, Singh says. “Our process is based on kitchen chemistry that people have noticed but never utilized.”
A prototype 4 cm2 device could capture 3.3 mmol of CO2/h—100 times as much per area as state-of-the-art technologies. The team calculated that a large-scale system with a capture capacity of 1,000 metric tons/h of CO2 would cost $145 per t. The DOE’s cost target for carbon removal technologies is $200/t. A direct-air-capture plant launched late last year in Iceland by start-up Climeworks currently costs around $600/t to remove CO2 from air using solid sorbents.
Tao Wang of Zhejiang University says that flue gas or air usually contains some moisture, which could affect the carbon-capture efficiency of the new device. “That may be a technical difficulty to be solved for the large-scale application of this technology,” he says. But the innovative approach, which could be easily modularized and driven by renewable energy, is still a promising alternative to mainstream carbon-capture systems that are driven by thermal energy.