Adsorption kinetics and mechanism of hydrogen on pristine and functionalized multi-walled carbon nanotubes

Abstract

This study explores the impact of hydroxyl (–OH) and carboxyl (–COOH) functionalization on the hydrogen adsorption behavior and mechanisms of multi-walled carbon nanotubes (MWCNTs). MWCNT-OH and MWCNTCOOH samples were synthesized via oxidation reactions and characterized using BET, FTIR, Raman, TG, TEM, SEM/EDX, and AFM techniques. Functionalization reduced the BET surface area but notably increased mesopore volume. FTIR spectra confirmed the presence of hydroxyl and carbonyl groups at 3432 cm− 1 and 1755 cm− 1 , respectively. Raman analysis showed shifts in the D, G, and 2D bands, while EDX results indicated a decrease in carbon content and an increase in oxygen content after functionalization. Thermal analysis revealed that the degradation profiles of the samples were altered. TEM and SEM images illustrated improved dispersion and separation of nanotubes upon functionalization. AFM analysis indicated significant changes in surface roughness and topography, suggesting modifications in nanotube structure. Hydrogen storage capacity was measured at cryogenic temperatures under varying pressures and time intervals. MWCNT-OH demonstrated the highest storage capacity (1.023 wt% at 80 bar). Storage rates increased with pressure, and kinetic data were best fitted to the pseudo-second-order model, with equilibrium achieved in approximately 2 min, supporting a physisorption mechanism. Mechanistic evaluation using Boyd, Avrami, and Weber–Morris models revealed a two-step adsorption process: initial adsorption on the external surface followed by diffusion into mesopores. The con sistency between experimental and calculated qe values further validated the pseudo-second-order model. The qt vs t 1/2 plots produced two intersecting lines, confirming the two-stage adsorption behavior described by the Weber–Morris model. These findings highlight the potential of oxygen-functionalized MWCNTs as efficient, metal-free materials for advanced hydrogen storage applications.