As global efforts to combat climate change intensify, one of the most promising innovations in the climate solution arsenal is Direct Air Capture (DAC). This technology involves removing carbon dioxide (CO₂) directly from the atmosphere, making it possible to reduce atmospheric concentrations of CO₂—an essential step for reaching net-zero emissions. While DAC is still in its early stages, various approaches are being developed, each with unique technologies, advantages, and challenges. In this article, we’ll explore the different technologies driving DAC and assess their potential to help mitigate climate change.

At its core, DAC works by filtering atmospheric air through a system that separates out CO₂ molecules. Once captured, the CO₂ can be permanently stored underground (carbon sequestration) or used in industries like food and beverage production, synthetic fuels, or manufacturing. DAC technologies are appealing because they can be deployed anywhere in the world and do not depend on specific sources of CO₂ emissions (such as industrial plants or power stations), unlike traditional carbon capture and storage (CCS).

DAC technologies can be broadly categorized into two systems: liquid solvent-based systems and solid sorbent-based systems. Additionally, hybrid and emerging technologies offer promising avenues for improving DAC’s efficiency and scalability. Below, we’ll break down these systems and explore how each works.

Liquid-based DAC systems use chemical reactions between CO₂ and a liquid solution to capture carbon dioxide. These systems typically rely on alkaline solutions or amine-based solutions, which can absorb CO₂ from the air due to their chemical properties.

One of the most common liquid-based DAC systems involves using aqueous alkaline solutions, such as potassium hydroxide (KOH) or sodium hydroxide (NaOH), to absorb CO₂. Here’s how it works: When air passes through the alkaline solution, the CO₂ chemically reacts with the hydroxide to form carbonate compounds, such as potassium carbonate or sodium carbonate. This reaction captures the CO₂ from the air, trapping it in the form of a carbonate.

Once the CO₂ is absorbed, the carbonate solution is treated with heat and chemical agents to release the captured CO₂ gas. This process regenerates the hydroxide solution, allowing it to be reused in the system. The released CO₂ can then be compressed and transported for storage or use in other industries.

Alkaline-based DAC systems are highly efficient at capturing CO₂ from the air and are scalable for large operations. The process has been well-studied and proven effective in removing significant amounts of CO₂, especially in industrial applications. One of the major drawbacks of using alkaline solutions is the high energy demand required to regenerate the solution and release the captured CO₂. The process of heating the carbonate solution to separate CO₂ is energy-intensive, raising concerns about the net energy balance of DAC systems. Additionally, the handling of caustic chemicals like KOH or NaOH poses operational risks and requires specialized equipment.

One company leading the way with this approach is Carbon Engineering, which has developed an alkaline-based DAC process that could capture up to one million tons of CO₂ annually per large-scale plant. Their system integrates DAC with renewable energy sources, aiming to reduce the overall carbon footprint of the process.

In amine-based liquid DAC systems, air is passed through a solution containing amines, organic compounds that are highly reactive to CO₂. Amines bind to CO₂ molecules in the air, forming a weak chemical bond that allows the CO₂ to be captured. The captured CO₂ can then be released by heating the amine solution, which breaks the bond and regenerates the amine for reuse.

Amine-based systems are highly selective for CO₂, meaning they can capture it effectively even when concentrations are low, such as in ambient air. This makes them particularly suited for DAC applications where CO₂ concentrations are much lower than in industrial flue gases.

Amine-based solutions are known for their high CO₂ capture efficiency. Because the bond between CO₂ and the amine is relatively weak, the regeneration process (CO₂ release) is less energy-intensive than in alkaline systems. Despite lower energy demands for regeneration, amine solutions can degrade over time, requiring replacement or regeneration, which adds operational costs. In addition, the production and disposal of amines present environmental challenges, as these compounds may be toxic or environmentally harmful if not properly managed.

Solid-based DAC systems capture CO₂ using solid materials, known as sorbents, that attract and trap CO₂ molecules on their surface. These systems typically use porous materials, such as silica, polymers, or metal-organic frameworks (MOFs), that are treated with chemical agents like amines to enhance their CO₂-capturing properties.

A key approach in solid-based DAC systems involves using solid amine sorbents. These materials work in a similar way to liquid amines but are bonded to solid structures, such as silica gel or polymer resins. As air passes over the solid sorbent, the amines capture CO₂ by binding it to the surface.

Once the sorbent is saturated with CO₂, it is heated or exposed to a vacuum to release the CO₂, which can then be collected for storage or use. The solid sorbent is then ready for reuse in the system.

Solid amine sorbents offer several advantages over liquid systems. They generally require less energy for CO₂ release, as the heating process can be more targeted and efficient. Additionally, solid sorbents reduce the need for handling large volumes of liquid chemicals, improving safety and operational simplicity. The main challenge with solid amine systems is the degradation of the sorbents over time. Prolonged exposure to CO₂ and moisture can cause the amines to lose their effectiveness, requiring periodic replacement or regeneration of the sorbents. Additionally, while energy requirements are lower than liquid systems, heating the solid materials still demands significant energy input, especially at scale.

One company pioneering solid-based DAC is Climeworks, which uses modular units equipped with solid amine sorbent filters. Climeworks’ DAC plants, located in various parts of the world, capture CO₂ and partner with storage facilities for permanent sequestration underground.

Metal-organic frameworks (MOFs) represent an emerging class of materials being explored for DAC applications. MOFs are highly porous, crystalline structures made up of metal ions and organic molecules. Their unique composition allows them to be tailored for specific gas-capturing purposes, making them highly effective at selectively capturing CO₂ from ambient air.

MOFs work by trapping CO₂ molecules within their porous structure as air passes through. The CO₂ can then be released by applying heat or changing pressure, regenerating the MOF for further use.

MOFs are highly efficient at capturing CO₂ even at low concentrations, and they can be engineered for specific applications. Their high surface area and tunability make them promising candidates for DAC, particularly as their regeneration process can potentially be more energy-efficient than other methods. While MOFs show great promise, they are still largely in the research and development phase. Scaling up MOF-based DAC systems for commercial use poses challenges, and the long-term durability of MOFs in real-world conditions remains a concern. More research is needed to determine their practicality and cost-effectiveness for large-scale deployment.

In addition to the core liquid and solid DAC systems, several hybrid and emerging technologies are being explored to improve the efficiency, cost, and scalability of DAC. These technologies combine elements from both liquid and solid systems or incorporate novel approaches to carbon capture.

Humidity swing adsorption is an innovative approach that uses changes in humidity to capture and release CO₂ from the air. In this process, specially designed materials capture CO₂ when the air is dry and release it when the air is humid. This method takes advantage of the different chemical interactions between CO₂ and water molecules in the air.

Humidity swing adsorption has the potential to significantly reduce energy requirements, as it doesn’t rely on heat or pressure changes for CO₂ release. Instead, it uses the natural variability in air humidity, making it a potentially low-energy DAC solution. The technology is still in its early stages of development and needs further research to improve its efficiency and scalability. Current systems are small-scale, and it remains to be seen whether humidity swing adsorption can be adapted for industrial-sized operations.

Electrochemical DAC uses electricity to drive chemical reactions that capture CO₂ from the air. In this process, air is passed through an electrolyte solution, which selectively captures CO₂ molecules. The application of electrical energy forces chemical reactions that separate CO₂ from the air and store it in a concentrated form.

Electrochemical systems could potentially operate with high efficiency, especially if powered by renewable energy sources. By using electricity to drive the capture process, these systems could reduce the energy-intensive heating requirements of other DAC methods.

Electrochemical DAC is still in its infancy, and scaling it up for large-scale deployment is a significant challenge. The cost and complexity of building and operating electrochemical systems are current barriers to widespread adoption, but ongoing research could lead to breakthroughs in this area.

Direct Air Capture holds immense potential for reducing atmospheric CO₂ levels, but the technology faces several significant challenges:

• Energy Demand: Many DAC systems require large amounts of energy to capture and release CO₂, particularly in the regeneration phases. If powered by fossil fuels, DAC could become energy-intensive and negate its climate benefits. Using renewable energy sources for DAC is critical to making it a net-negative emissions technology.
• Cost: Currently, DAC is expensive, with costs ranging between $100 and $600 per ton of CO₂ captured. As technology advances, scaling up production and improving the efficiency of DAC systems will be essential to reducing costs.
• Scalability: To have a meaningful impact on global CO₂ levels, DAC needs to be deployed on a large scale. This requires not only building DAC plants but also developing infrastructure for transporting and storing the captured CO₂. Coordination between DAC companies, energy providers, and storage facilities is vital for scaling up the technology.

Despite these challenges, DAC represents one of the few available technologies capable of achieving negative emissions—essential for offsetting emissions from hard-to-decarbonize sectors such as aviation, agriculture, and heavy industry. As research progresses and costs decrease, DAC could become a crucial tool in the global effort to combat climate change.

Direct Air Capture technologies are diverse and rapidly evolving. From liquid-based systems that use alkaline or amine solutions to solid sorbents like amine filters and metal-organic frameworks, each approach offers unique advantages and challenges. As DAC technologies continue to develop, improving efficiency, reducing costs, and integrating with renewable energy sources will be critical to their success. In the long run, DAC could play an essential role in helping the world achieve net-zero emissions and avoid the worst impacts of climate change.