After centuries of releasing carbon dioxide into the atmosphere, humans are now looking for ways to remove it (see Physics Today, June 2022, page 26). One obstacle is the fact that carbon dioxide—despite the climate changes driven by its rising concentration—is relatively dilute in the atmosphere at about 420 ppm. Extracting carbon dioxide from the air thus requires a reversal of the natural process of diffusion, in which molecules flow from areas of high to low concentrations. Achieving that reversal can be energy intensive and slow. Ian Metcalfe, of Newcastle University in the UK, and colleagues have now found a method that overcomes some of the thermodynamic and kinetic challenges that come with pulling carbon dioxide from the air.
To reverse the typical diffusion process, biological systems such as cells can use a clever trick to move molecules “uphill” against a concentration gradient (for example, pumping out hydrogen ions to maintain a specific pH level). The trick works in a manner akin to a waterwheel: The energy released as one substance moves down its gradient (from high to low concentrations) is harnessed to move another substance against its gradient (from low to high concentrations). Metcalfe and colleagues discovered that a molten-salt membrane could engage the same mechanism to concentrate carbon dioxide.
The method’s driving force is a difference in humidity between two air masses. It’s made possible by a membrane that contains a common carrier—ions that bond exclusively with carbon dioxide or water molecules and shuttle them through the membrane to produce an equal and opposite exchange of the two substances. As shown in the figure, the addition of moisture to an air mass on one side of the membrane results in movement of water from the humid to the dry side, which provides the energy for a corresponding transfer of carbon dioxide in the opposite direction.
Both air masses started with a 400 ppm concentration of carbon dioxide, close to the concentration in today’s atmosphere. The two air masses were streamed into the chambers at equal flow rates, and the carbon dioxide concentration on the dry side dropped to 200 ppm while that on the humid side rose to 600 ppm. When the researchers dropped the flow rate on the humid side by a factor of five, the concentration of carbon dioxide on that side increased by 1000 ppm, five times as much as the increase observed at equal flow rates, for a total of 1400 ppm.
Though 1400 ppm is still much lower than the minimum concentration of carbon dioxide necessary for movement into permanent storage, the researchers note that the new process could be a valuable preconcentration step that reduces the expense of other downstream concentration methods. Clear benefits of the new approach are that it is fast and highly selective—moving only water and carbon dioxide through the membrane with no leaks. It’s unlikely that any single technology will provide a solution to looming climate change. Metcalfe and colleagues’ molten-salt membrane adds a new, efficient tool to the effort. (I. S. Metcalfe et al., Nat. Energy, 2024, doi:10.1038/s41560-024-01588-6.)
