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A team from Massachusetts Institute of Technology developed the technique, which could significantly boost the performance of systems that use catalytic surfaces.
“Carbon dioxide sequestration is the challenge of our times,” said mechanical engineering professor Kripa Varanasi, who worked with assistant professor Sami Khan, professor Yang Shao-Horn and recent graduate Jonathan Hwang. There are a number of approaches, including geological sequestration, ocean storage, mineralisation and chemical conversion. Electrochemical conversion is particularly promising because it can produce useful products such as fuels, but it still needs improvements to become economically viable.
In these systems, a stream of gas containing carbon dioxide is typically passed through water to deliver carbon dioxide for the electrochemical reaction. The movement through water is sluggish, which slows the rate of conversion of the carbon dioxide.
The new design ensures that the carbon dioxide stream stays concentrated in the water right next to the catalyst surface. This concentration, the researchers showed, can nearly double the performance of the system.
In electrochemical systems, the stream of carbon dioxide-containing gases is mixed with water, either under pressure or by bubbling it through a container outfitted with electrodes of a catalyst material such as copper. A voltage is then applied to promote chemical reactions, producing carbon compounds that can be transformed into fuels or other products.
Previously, the reaction has happened too quickly, and a competing reaction of water splitting has taken over. To tackle that issue, the researchers placed a gas-attracting surface in close proximity to the catalyst material. The material is a specially textured ‘gasphilic’, a superhydrophobic material that repels water but allows a smooth layer of gas to stay close along its surface. It keeps the incoming flow of carbon dioxide right up against the catalyst, so the desired carbon dioxide conversion reactions can be maximised. By using dye-based pH indicators, the researchers were able to visualise carbon dioxide concentration gradients in the test cell.
In a series of lab experiments using the set-up, the rate of the carbon conversion reaction nearly doubled. It was also sustained over time, whereas in previous experiments the reaction quickly faded out. The system produced high rates of ethylene, propanol, and ethanol, a potential automotive fuel. The process could instead be optimised for hydrogen production.
By concentrating the carbon dioxide next to the catalyst surface, the new system also produced two new potentially useful carbon compounds, acetone and acetate, which had not previously been detected in any electrochemical systems at appreciable rates.
In the initial laboratory work a single strip of the hydrophobic, gas-attracting material was placed next to a single copper electrode, but in future work a practical device might be made of a dense set of interleaved pairs of plates, Varanasi said.
The research was featured in Cell Reports Physical Science.
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