Hydrogen adsorption of iron/carbon nanocomposite

Abstract

Carbon nanopowder intrinsically has the characteristics of high specific surface area and micropores, which are favorable for hydrogen adsorption. In this study, we obtained Fe/CNP nanocomposite materials by modifying the surface of the carbon nanopowder (CNP) with different iron oxides. To measure hydrogen storage behaviors of Fe/CNP nanocomposite, we used PCT apparatus at hydrogen pressure of 37atm and 300K. The specific surface areas and pore diameter were analyzed by N2/77K BET, and the surface morphologies were studied by SEM. Further, XRD analysis clearly indicated the presence of different iron oxides in these composite materials. From the experimental result, the hydrogen adsorption capacity of γ-Fe2O3 / CNP composite increased with different iron contents (7wt%、28wt%、56wt%). Especially, γ-Fe2O3 / CNP composite with 14 wt% of iron-spiked showed maximum hydrogen adsorption of 0.48wt%, which is 1.9 times of the original CNP(0.256wt%). For one reason, massive micropores appeared when iron oxide formed on the surface of CNP. For another, it was postulated that γ-Fe2O3 crystals promoted the access of hydrogen molecules by polarizing the surface, which benefited the migration of hydrogen molecules to the carbon nanopowder (CNP) surface. In this case, γ-Fe2O3 played as a promoter to attract hydrogen molecules whereas CNP skeleton performed as the principal absorbent for hydrogen storage. The hydrogen adsorption capacity of the other composite α-Fe2O3 / CNP decreased with all iron contents compared to original CNP. It was result from the decrease of micopores on surface. In order to investigate the contribution to hydrogen adsorption capacity between two different iron oxides, we transformed α-Fe2O3/CNP composite to γ-Fe2O3 / CNP composite under hydrogen atmosphere of 20atm and 633K. We found out that the hydrogen adsorption increased after phase transformation. This phenomenon proved that the presence of γ-Fe2O3 made the surface of composite more attractable for hydrogen than α-Fe2O3. Based upon our experimental results, a high hydrogen uptake could be achieved by appropriately adjusting the surface polarity of CNP with well dispersed iron oxides crystals and more micropores on surface.