<p>Adsorption-based technologies have emerged as promising approaches for mitigating CO₂ emissions. However, the development of sorbents with simultaneously high adsorption capacity, fast kinetics, and long-term cyclic stability remains a critical challenge. Polyethyleneimine (PEI), a nitrogen-rich polymer, exhibits strong affinity toward CO₂, yet its practical application is limited by aggregation effects arising from intermolecular hydrogen bonding, which reduce the accessibility of active amine sites. In this work, silica-based composite aerogel sorbents (PSA-X) were successfully fabricated by immobilizing PEI within a porous SiO₂ aerogel matrix via an impregnation-freeze-drying strategy. The results show that, although PEI loading leads to a decrease in surface area and pore volume, the porous framework is partially preserved, providing accessible pathways for CO₂ diffusion. Elemental analysis reveals that the amine efficiency exhibits a non-monotonic trend with PEI loading, reaching a maximum value of 0.62 mmol CO₂/mmol N for PSA-50%, suggesting enhanced accessibility of amine functional groups at an optimal loading. The optimized PSA-50% exhibits a high CO₂ adsorption capacity of 5.32 mmol·g⁻¹ at 90 °C, along with rapid adsorption kinetics and stable performance over multiple adsorption–desorption cycles. The improved performance can be attributed to the combined effects of accessible amine sites and retained pore pathways. This work highlights the importance of optimizing the interplay between amine loading and pore accessibility for the design of high-performance composite aerogel adsorbents.</p>