Synthesis and Characterization of Amine-Modified Carbon Nanofibers for Direct Air Capture of CO2

Abstract

This work investigates the development and evaluation of aminosilane-functionalized carbon nanofibers as adsorbents for Direct Air Capture. The study focused on the impact of surface modification and textural tailoring on CO2 adsorption performance under ultradilute conditions (~400 ppm), ambient temperature, and in the presence of atmospheric humidity. A series of modifications was applied to electrospun PAN fibers, such as stabilization, carbonization, oxidation with H₂SO₄/KMnO₄, KOH activation, and silanization with (3-aminopropyl)trimethoxysilane (APTMS). Energy-Dispersive X-ray Spectroscopy (EDX), BET surface area measurements, pore size distribution, Thermogravimetric Analysis (TGA), and Scanning Electron Microscopy (SEM) were used to analyze the structural, morphological, and surface chemical properties of the fibers. H2O and CO2 adsorption isotherms, were performed to evaluate performance under conditions that are similar to direct air capture. The unmodified and oxidized carbon nanofibers showed limited microporosity and low BET surface areas of 7.8 and 77.6 m2 g-1, respectively, resulting in negligible CO2 uptake (< 0.1 mmol g-1 at 400 ppm CO2). The introduction of oxygen-containing groups during oxidation was verified by TGA, which resulted in an increase in the surface functionality. KOH activation expanded microporosity, raising the BET surface area to 436.9 m2 g-1 and micropore volume to 0.239 cm3 g-1. This resulted in a physisorptive CO2 capacity of approximately 0.45 mmol g-1 at 400 ppm. The greatest enhancement was achieved by APTMS silanization, which grafted primary amines onto carbon nanofiber surfaces, yielding a surface area of 58.9 m2 g-1 and CO2 uptake exceeding 0.7 mmol g-1 under DAC-relevant conditions. Overall, this work shows that the combination of KOH activation and APTMS functionalization creates CNFs with complementary improvements in porosity and surface chemistry, enabling enhanced CO2 capture performance under DAC scenarios. The results show that optimizing functionalization protocols such as coating time, reagent quantities, and grafting conditions, is essential to enhance adsorbents performance.