Polymer chains make membranes more selective

"We’ve demonstrated that we can vastly improve the selectivity of membranes to remove CO2 while retaining relatively high CO2 permeability.Rich Spontak, North Carolina State University

Researchers have developed a new membrane technology that allows for more efficient removal of carbon dioxide (CO₂) from mixed gases, such as emissions from power plants.

“To demonstrate the capability of our new membranes, we looked at mixtures of CO₂ and nitrogen, because CO₂/nitrogen dioxide mixtures are particularly relevant in the context of reducing greenhouse gas emissions from power plants,” says Rich Spontak, professor of chemical and biomolecular engineering and professor of materials science & engineering at North Carolina State University and co-corresponding author of a paper on this work in Science. “And we’ve demonstrated that we can vastly improve the selectivity of membranes to remove CO₂ while retaining relatively high CO₂ permeability.

“We also looked at mixtures of CO₂ and methane, which is important to the natural gas industry. In addition, these CO₂-filtering membranes can be used in any situation in which one needs to remove CO₂ from mixed gases – whether it’s a biomedical application or scrubbing CO₂ from the air in a submarine.”

Membranes are an attractive technology for removing CO₂ from mixed gases because they do not take up much physical space, can be made in a wide variety of sizes and can be easily replaced. The other technology that is often used for CO₂ removal is chemical absorption, which involves bubbling mixed gases through a column containing a liquid amine that can remove CO₂ from the gas. However, absorption technologies have a significantly larger footprint, and liquid amines tend to be toxic and corrosive.

The novel membrane filters work by allowing CO₂ to pass through the membrane more quickly than the other constituents in the mixed gas. As a result, the gas passing out the other side of the membrane has a higher proportion of CO₂ than the gas entering the membrane. By capturing this gas, the CO₂ can be separated from the other constituents.

A longstanding challenge for such membranes has been a trade-off between permeability and selectivity. The higher the permeability, the more quickly you can move gas through the membrane. But when permeability goes up, selectivity goes down – meaning that nitrogen and other constituents also pass through the membrane quickly – reducing the ratio of CO₂ to other gases in the mixture. In other words, when selectivity goes down you capture relatively less CO₂.

The researchers from the US and Norway addressed this problem by growing chemically active polymer chains that are both hydrophilic and CO₂-philic on the surface of existing membranes. This increases CO₂ selectivity and causes relatively little reduction in permeability.

“In short, with little change in permeability, we’ve demonstrated that we can increase selectivity by as much as about 150 times,” says Marius Sandru, co-corresponding author of the paper and senior research scientist at SINTEF Industry, an independent research organization in Norway. “So we’re capturing much more CO₂, relative to the other species in gas mixtures.”

Another challenge facing membrane CO₂ filters has been cost. The more effective previous membrane technologies were, the more expensive they tended to be.

“Because we wanted to create a technology that is commercially viable, our technology started with membranes that are already in widespread use,” says Spontak. “We then engineered the surface of these membranes to improve selectivity. And while this does increase the cost, we think the modified membranes will still be cost effective.”

“Our next steps are to see the extent to which the techniques we developed here could be applied to other polymers to get comparable, or even superior, results; and to upscale the nanofabrication process,” Sandru says. “Honestly, even though the results here have been nothing short of exciting, we haven’t tried to optimize this modification process yet. Our paper reports proof-of-concept results.”

The researchers are interested in exploring other applications, such as using the new membrane technology in biomedical ventilator devices or filtration devices in the aquaculture sector. They also say they are open to working with industry partners in exploring any of these questions or opportunities to help mitigate global climate change and improve device function.

This story is adapted from material from North Carolina State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.