Review of liquid nano-absorbents for enhanced CO₂ capture

Liquid nano-absorbents have become a topic of interest as a result of their enhanced mass-transfer performance for CO₂ capture. They are believed to have revolutionized the conventional CO₂ chemisorption process by largely improving CO₂ capture kinetics and reducing the energy requirement for solvent regeneration. Two classes of nanomaterial-based CO₂ capture absorbents, amine-based nanoparticle suspensions (nanofluids) and nanoparticle organic hybrid materials (NOHMs), have been developed, with significant progress achieved in recent decades. This review addresses two key questions for these two state-of-the-art nanomaterials: how are the physical and chemical properties of the prepared liquid nano-absorbents transformed relative to those of the base fluids? And how does the transformation of the properties affect the CO₂ capture behavior? While the current synthesis procedure for liquid nano-absorbents is quite straightforward, more advanced synthesis methods for long-term nanoparticle stability have been suggested for the future. Nanofluids have been shown to increase the CO₂ uptake by over 20% and the CO₂ capture rate by 2-93% compared with the values observed with neat amine solvents. Nanoparticles with catalytic effects on CO₂ capture can significantly increase the CO₂ desorption rate by as high as 4000%. NOHMs exhibit the interesting feature of enhanced mass transfer in CO₂ capture because of the unique pathway network that is created in them for CO₂ to reach specific functional groups. NOHMs promise an effect of combined CO₂ capture and conversion, and can be used especially as electrolytes for CO₂ electro-reduction. However, there are still some challenges for the application of these materials in real life, such as poor stability and high viscosity. Therefore, efficient CO₂ capture processes using these solvents need to be urgently developed and studied in the future.