Cost-Effective Monolithic Hierarchical Carbon Cryogels with Nitrogen Doping and High-Performance Mechanical Properties for CO2 Capture

Cost-effective nitrogen-doped monolithic hierarchical carbon cryogels with excellent mechanical properties and carbon dioxide (CO₂) adsorption performance were prepared from phenol, melamine, and formaldehyde (PMF) by the sol-gel, freeze-drying, and then, pyrolysis processes under an inert atmosphere. The morphology, mechanical properties, pore structure, and chemical characteristics of these cryogels were investigated. The results showed that the dilution ratio played a crucial role in the preparation of nitrogen-doped PMF carbon cryogels with controlled structures. The prepared carbon cryogels were a kind of monolithic materials composed of a hierarchical pore structure and had high compression properties (0.67 and 9.4 MPa for strength and modulus), porosity (97.6%), surface area (1406 m²/g), and heteroatom nitrogen content (0.98-2.09%). CO₂ adsorption capacities up to 5.75 mmol/g at 0 °C and 4.50 mmol/g at 25 °C under 1 bar were obtained, which is at a high level among N-doped carbon materials and far better than resorcinol-based carbon gels reported. These superior CO₂ adsorption capacities, high isosteric adsorption heat (Q_st), and good CO₂/N₂ adsorption selectivity were ascribed to the synergistic effect of high surface area, appropriate pore size, and also heteroatom doping.