Abstract
Microporous materials serve as promising CO2 adsorbents owing to their close pore size (< 2 nm) with CO2 molecules, generating trapping effects via the Van der Waals interaction. However, the narrow pore size inherently imposes severe diffusion resistances, resulting in the low active sites accessibility and sluggish capture kinetics. Introducing larger pores into the microporous structures, forming a hierarchical pore structure, is proposed as an effective strategy to address this diffusion challenge via the shorten diffusion pathway and enlarged diffusivity. On the other hand, porous materials, typically powder form, are required to undergo a formulation process to construct a 3D structure for practical applications. The presence of microscale porosity on the surface of the 3D structures is crucial for effective gas exchange between internal structure and surrounding environment, mitigating pressure drop, which is challenging to achieve by traditional manufacture methods. Inspired from the optimized mass transfer systems in nature, this thesis aims at sustainably constructing an interconnected multiple porosity presenting from the internal structure to the surface in a 3D self-supporting structure. Moreover, nitrogen-doped species were introduced through a cascade reaction strategy to serve as additional active sites for CO2 capture, further increasing the capture capacity of hierarchical pore structure. The major contributions of this thesis include: 1. Sustainable and controllable methods for fabricating 3D hierarchical porous carbons (HPCs) were employed using bottom-up and top-down methods, respectively. Bidirectional freeze casting and self-templating of sodium alginate (SA) not only introduce interconnected microscale and nanopores but also establish 3D mechanically stable carbon monoliths with enhanced CO₂ capture performance. 2. This work significantly advances the pore hierarchy in porous carbons, incorporating, micro-, meso-, and macro- interconnected extra-large pores through a dual-template of ice and ZnO strategy, The resulting 3D HPC demonstrate a 27% increase in CO₂ capture capacity compared to conventional microporous carbons. This enhancement is attributed to the optimized diffusion pathways and improved the access to adsorption sites, leading to superior performance under both ideal and practical conditions with robust cycling stability in simulated flue gas streams. 3. This work offers a novel cascade reaction method that effectively addresses the trade-off between specific surface area and nitrogen doping content in N-doped carbons (NPCs) with hierarchical pore structures. This strategy enabled the achievement of an unprecedented combination of high specific surface area (1319 m² g⁻¹) and nitrogen doped content (11.84 wt%), resulting in a significant CO₂ capture capacity of 4.46 mmol g⁻¹ at 1 bar, placing it among the top 10% of reported NPCs. The material also demonstrated long-term stability of 110 cycling tests and effectiveness in practical applications. The above contributions pave the way for the exploring other N-containing precursors and new cascade reactions for N-doped carbon with interconnected hierarchical pore structure. The superior structure features of HPCs also make them promising candidates for extension to other decarbonization applications.