In a corner of Kelsey Hatzell's lab sits a small jar filled with a material that has an ability far beyond what its nondescript appearance would suggest: a way to capture and release carbon dioxide from the atmosphere by simply changing the surrounding humidity.
The material could slash the energy costs associated with so-called direct air capture systems, which conventionally rely on energy-intensive temperature or pressure shifts to switch between carbon capture and release. By instead relying on humidity, the material could yield energy efficiency improvements over five times above current technologies. The researchers report their findings in Environmental Science & Technology Letters.
Direct air capture systems have been heralded as a way to combat climate change by pulling carbon dioxide out of the air to either store permanently underground or convert into a useful product.
“There's been an explosion of interest in direct air capture systems, because they're not just a way to reduce carbon emissions, but to actually remove them from the atmosphere,” said research leader Hatzell, assistant professor of mechanical and aerospace engineering and the Andlinger Center for Energy and the Environment, pointing to a recent $3.5 billion effort from the U.S. Department of Energy to develop four regional direct air capture hubs across the country.
Despite its promise, direct air capture has come under scrutiny since it requires more energy to perform than almost any other application of carbon capture. That is because the concentration of carbon dioxide in ambient air is extremely diluted, especially when compared to the waste gas from a point-source emitter such as a coal-fired power plant.
One of the process' most energy-intensive steps is regeneration. After capturing carbon dioxide from ambient air, conventional systems require heat and/or pressure changes to release the gas into storage so that the system can be prepared to capture more carbon. In one approach using a liquid solvent, the regeneration step requires heating the carbon capture material to temperatures ranging from 300° to 900°C.
By contrast, previous research has shown that regenerating carbon capture materials with humidity only requires adding or removing water vapor. Such an approach dramatically cuts the energy required to remove a ton of carbon dioxide, from up to 4.1 gigajoules using conventional techniques to just 0.7 gigajoules—an energy savings per ton greater than the energy used by the average U.S. household in a month.
To achieve the humidity-based approach, the Princeton team modified an existing type of ion-exchange resin, a material that can trade charged particles with the surrounding environment. These resins are already used for a range of commercial purposes, making them widely available and inexpensive.
Source: Princeton University