The mechanism and sorption kinetic analysis of hydrogen storage at room temperature using acid functionalized carbon nanotubes

Abstract

The present work investigates the effect of acid functionalization of multiwalled carbon nanotubes (MWCNTs) on the physisorption based mechanism of hydrogen storage at room temperature. For this purpose, a suite of functionalized CNT samples is synthesized and subjected to a comprehensive range of material characterization techniques and hydrogen storage measurements. Nitric acid (HNO3) and the mixture of sulphuric acid and nitric acid (H2SO4:HNO3) are used for the synthesis at oxidation temperatures of 80 °C and 100 °C. Electron microscopy and X-ray photoelectron spectroscopy results reveal that acid functionalization causes major alternation in the physicochemical properties of the CNTs due to the varied concentration of oxygen functional groups. Particularly, the H2SO4:HNO3 functionalized sample at 100 °C is found to have the highest interlayer spacing, oxygen to carbon ratio (26.09 at. %), defect content, and specific surface area (215.3 m2/g). These features collectively contribute to substantially improved hydrogen storage properties, including a ∼150% increase in the hydrogen storage capacity at 298 K and 50 bar. Furthermore, kinetic analysis shows that the desorption follows a multiple diffusion process which is sensitive to the oxygen functional groups and structural defects, hence reducing the rate of desorption; whereas the adsorption is controlled by a more rapid, three-dimensional diffusion process.