Lower Critical Solution Temperature (LCST) Mediated Direct Air Capture of Carbon Dioxide with High Capacity and Low Regeneration Energy

Abstract

Direct air capture of carbon dioxide has been limited by the energy-intensive thermal regeneration of adsorbents imposed by the low CO2 uptake capacity, high regeneration temperature and parasitic water evaporation. Herein, we demonstrated a class of thermo-responsive CO2 adsorbents—PEI impregnated PNIPAM-grafted cellulose network (TRCNF/PEI)—achieving high CO2 uptake of 2.82 mmol g-1 from air at 25 °C and 4.33 mmol g-1 at 5 °C, and low regeneration temperature below 65 °C. The thermo-responsive adsorbent features lower critical solution temperature (LCST) phase transition that enables a swell-adsorption/collapse-desorption mechanism. At room temperature, TRCNF/PEI is at a hydrophilic swollen state, effectively exposing the PEI amine sites for efficient CO2 capture. Above LCST, it transitions to a hydrophobic collapsed state. Water is expelled in liquid phase, destabilizing bicarbonate intermediates and thereby driving rapid CO2 desorption. This obviated the latent heat penalty of water, leading to unprecedentedly low regeneration thermal energy demand of 4.36 GJ ton-1CO2 that can be satisfied solely by low-grade waste heat. Unlike zeolites activated carbons and MOFs vulnerable to high humidity where liquid water inhibits CO2 uptake due to competitive adsorption, TRCNF/PEI leverages water to enhance adsorption and exhibits exceptional cyclic stability. Notably, the CO2 uptake of TRCNF/PEI further increases at lower temperatures (4.33 mmol g-1 at 5 °C), demonstrating its carbon capture potential in cold area. The unique LCST phase transition presents a transformative strategy to couple DAC with vast low-grade industrial waste heat, significantly advancing the energy efficiency and durability of carbon removal technologies.