Efficiently removing carbon dioxide (CO₂) from the atmosphere is becoming increasingly vital in the fight against climate change. However, existing carbon capture systems face a dilemma: the chemical compounds that excel at removing CO₂ are not very good at releasing it, while those that release CO₂ efficiently struggle to capture it effectively. Optimizing one aspect often detracts from the other.
Recently, researchers at MIT have made significant strides towards solving this problem by introducing nanoscale filtering membranes. This innovative method enhances both CO₂ capture and release efficiency by up to six times and reduces costs by at least 20%, according to the findings published in ACS Energy Letters. The study was led by MIT doctoral students Simon Rufer, Tal Joseph, and Zara Aamer, alongside Professor Kripa Varanasi.
“When approaching carbon capture, it’s essential to consider scale from the outset, as meaningful impact requires processing gigatons of CO₂,” explains Varanasi. “With this mindset, we can identify critical bottlenecks and design innovative solutions that truly make a difference.”
Many current carbon-capture systems rely on hydroxides, which readily react with CO₂ to form carbonate. This carbonate is then fed into an electrochemical cell, where an acid converts it back, releasing pure CO₂ that can be utilized for fuel or other products. The challenge is that both the capture and release processes operate within the same water-based solution, but each requires different conditions for optimal performance—hydroxide ions for capture and carbonate ions for release. “These requirements create a conflict,” Varanasi notes. “Both systems are using the same liquid, but need different conditions to function effectively, making it impossible for both to operate at peak efficiency.”
The team tackled this issue by decoupling the processes and introducing a third step. Once hydroxide ions are predominantly converted to carbonate, specialized nanofiltration membranes separate the ions based on charge; carbonate ions carry a charge of 2, while hydroxide ions carry a charge of 1. “The nanofiltration does a remarkable job of separating these two,” Rufer points out.
After separation, hydroxide ions are cycled back to the absorption stage, while carbonates move on to the electrochemical release phase. In this step, protons are added to the carbonates to convert them into CO₂ and water. If hydroxides are still present, the protons might react with them instead, resulting in merely water, which is where efficiency is lost. Rufer explains, “Without separating hydroxides and carbonates, you can end up just making water rather than extracting CO₂, which undermines efficiency. Our proposal to use nanofiltration addresses this gap.”
Tests indicated that the nanofiltration could effectively separate carbonates from hydroxides with around 95% efficiency, validating the approach under realistic conditions. To assess its impact on overall efficiency and economic feasibility, the team developed a techno-economic model considering factors like electrochemical efficiency, voltage, absorption rates, capital costs, and nanofiltration efficiency.
The analysis revealed that existing carbon-capture systems cost approximately $600 per ton of captured CO₂, but with the addition of nanofiltration, this could drop to around $450 per ton. Furthermore, the new system boasts greater stability, maintaining high efficiency even amidst fluctuating ion concentrations. “The prior systems operated on thin margins,” Rufer notes, “where minor concentration changes could drastically reduce efficiency. Our nanofiltration system acts as a buffer, creating a more forgiving environment with a broader operational range, thus lowering costs significantly.”
This innovative approach may not only benefit direct air capture systems but could also apply to point-source systems connected to emission sources like power plants, or even to subsequent processes that convert captured CO₂ into valuable products such as fuels and chemical feedstocks. Those conversion processes, Varanasi asserts, are also hindered by the carbonate and hydroxide trade-off.
Additionally, this technology paves the way for safer alternatives in carbon capture chemistry. “Many absorbents used today can be toxic or harmful to the environment,” Varanasi states. “By employing our method, we enhance reaction rates, allowing for the use of safer chemistries that may not typically possess the best absorption rates.”
Another positive aspect of this breakthrough is that it utilizes commercially available materials, making it easily retrofittable to existing carbon-capture installations. If the costs are further reduced to around $200 per ton, this technology could see widespread adoption. “We’re optimistic about developing a solution that remains economically viable, producing sellable products,” Varanasi adds.
Rufer highlights the ongoing demand for carbon credits, with current prices exceeding $500 per ton. “At the projected costs, our method could already be commercially viable, appealing to a growing number of buyers.” As market demands rise, Varanasi emphasizes, “Our objective is to deliver scalable, cost-effective, and reliable technologies that support industries in achieving their decarbonization targets.”
The research received support from Shell International Exploration and Production Inc. via the MIT Energy Initiative, as well as the U.S. National Science Foundation, utilizing the facilities at MIT.nano.
Photo credit & article inspired by: Massachusetts Institute of Technology
