Direct Capture of CO₂ from Ambient Air

The increase in the global atmospheric CO₂ concentration resulting from over a century of combustion of fossil fuels has been associated with significant global climate change. With the global population increase driving continued increases in fossil fuel use, humanity's primary reliance on fossil energy for the next several decades is assured. Traditional modes of carbon capture such as precombustion and postcombustion CO₂ capture from large point sources can help slow the rate of increase of the atmospheric CO₂ concentration, but only the direct removal of CO₂ from the air, or "direct air capture" (DAC), can actually reduce the global atmospheric CO₂ concentration. The past decade has seen a steep rise in the use of chemical sorbents that are cycled through sorption and desorption cycles for CO₂ removal from ultradilute gases such as air. This Review provides a historical overview of the field of DAC, along with an exhaustive description of the use of chemical sorbents targeted at this application. Solvents and solid sorbents that interact strongly with CO₂ are described, including basic solvents, supported amine and ammonium materials, and metal-organic frameworks (MOFs), as the primary classes of chemical sorbents. Hypothetical processes for the deployment of such sorbents are discussed, as well as the limited array of technoeconomic analyses published on DAC. Overall, it is concluded that there are many new materials that could play a role in emerging DAC technologies. However, these materials need to be further investigated and developed with a practical sorbent-air contacting process in mind if society is to make rapid progress in deploying DAC as a means of mitigating climate change.