Capturing carbon with energy-efficient sodium carbonate−nanocarbon hybrid material

Industrial emissions are one of the main sources of climate change-inducing carbon dioxide (CO₂). While adopting renewable and clean energy alternatives is one option for mitigating these carbon emissions, carbon capture technology is another solution to control CO₂ emissions.

In big CO₂-emitting industries, such as cement, oil refineries, and thermal power plants, carbon capture technology can be easily applied to remove CO₂ emissions directly at the source at a feasible cost and with low energy consumption. Different materials have been explored for CO₂ capture in factories, including zeolites, metal−organic frameworks, natural minerals, alkalis, and alkali metal salts. Among them, alkali metal carbonates, such as sodium carbonate (Na₂CO₃), are considered effective and inexpensive materials with stable properties and easy procurement.

Theoretically, Na₂CO₃ has a decent CO₂ capture capacity and can be easily regenerated for successive uses. However, directly applying Na₂CO₃ to capture CO₂ causes crystal agglomeration, leading to poor efficiency and shorter longevity. This issue can be eliminated by using a carbon skeleton for Na₂CO₃.

Porous carbon materials with good pore connectivity provide low density, structural stability, hydrophobicity, and a large surface area that can stabilize Na₂CO₃. Previous studies report that Na₂CO₃−carbon nanocomposites have a CO₂ capture capacity of 5.2 mmol/g. However, these studies do not inspect the effect of the carbonization temperatures on the overall performance of the material.

In a study published in Energy & Fuels on June 12, 2024, Professor Hirofumi Kanoh and Bo Zhang from the Graduate School of Science, Chiba University, synthesized a hybrid CO₂ capture material consisting of Na₂CO₃ wrapped with porous nanocarbon.

They further evaluated its CO₂ capture and regeneration efficiencies at different carbonization temperatures. The Na₂CO₃−carbon hybrids (NaCH) were derived by carbonization of disodium terephthalate at temperatures ranging from 873K to 973 K in the presence of nitrogen as a protective gas.

"Reducing CO₂ emissions is an urgent issue, but research on the methods and material systems for CO₂ capture are still lacking. This Na₂CO₃−carbon hybrid system proved promising in our initial investigations, prompting us to explore it further," states Prof. Kanoh.

The team measured the hybrid materials' CO₂ capture capacity under humid conditions to mimic the conditions of factory waste exhaust gases. They found that the NaCH hybrids prepared at carbonization temperatures near 913–943 K demonstrated higher CO₂ capture capacities.

Among them, NaCH-923 had the highest CO₂ capture capacity of 6.25 mmol/g and a high carbon content of over 40%, which resulted in a larger surface area, enabling a more uniform distribution of Na₂CO₃ on the nanocarbon surface. This reduced the rate of Na₂CO₃ crystal agglomeration and led to faster reaction rates.

After NaCH-923 effectively captured CO₂, the scientists again heated the resultant NaCH-923-CO₂ in the presence of nitrogen to test its regeneration performance. They found that NaCH-923 could be regenerated and used for CO₂ capture for 10 cycles, while retaining over 95% of its initial CO₂ capture capacity. These results indicate that NaCH-923 exhibits good structural strength, durability, and regeneration, which makes it an excellent material for CO₂ capture under humid conditions.

Further experiments on the NaCH-923-CO₂ showed that the sample underwent a steep mass change at 326−373 K (around 80 °C on average). Since the temperature of the exhaust gas from thermal power plants is also typically in that range, the waste heat from factories and power plants can easily be used as a heat source for regenerating NaCH-923, thereby effectively reducing energy consumption.

These findings show that the carbonization temperature significantly influences the CO₂ capture performance and carbon content of NaCH hybrids, with NaCH-923 exhibiting the best characteristics. NaCH-923, being a solid adsorbent, can efficiently capture CO₂ at ambient temperature and pressure with high selectivity for CO₂ and without the problem of equipment corrosion that exists with liquid adsorbents currently used in industries.

Moreover, these characteristics allow for its widespread application in various configurations, environments, and diverse industrial settings.

"By transforming Na₂CO₃, which already has a good CO₂ capture capacity, into a nanocomposite, it became possible to improve the reaction rate and reduce the decomposition and regeneration temperature. This enables the use of factory waste heat for regeneration at around 80 °C, giving us an energy-cost efficient CO₂ capture system," concludes Prof. Kanoh.