Porosity and fractal study of functionalized carbon nanofibers: Effects of the functionalization degree on hydrogen storage capacity

Carbon nanofibers are produced by siliceous SBA-15 type materials and the casting method. The nanofibers are functionalized by HNO₃ attack in aqueous phase under microwave radiation. N₂ sorption data are treated by Non-Local Density Functional Theory and Quenched Solid Density Functional Theory to determine advanced adsorption among other textural properties. The functionalization degree of carbon nanofibers and their hydrogen storage capacity are mainly investigated by FTIR spectroscopy, capacitance studies and analysis of the fractal dimension of the surface. This latter in two ways: i) using the Neimark-Kiselev equation with N₂ sorption data and ii) using Box-counting, Information and Perimeter-area methods on TEM photomicrographs. The hydrogen storage testing reveals that functionalized carbon nanofibers adsorb hydrogen above 200% with respect to unfunctionalized carbon nanofibers. This effect is attributed to: i) the creation of extra spacing between contiguous nanofibers, as a consequence of mutual repulsion between the -COOH groups and ii) increase of volume intrawall.