Μελέτη νανοσωλήνων άνθρακα ως μέσων αποθήκευσης υδρογόνου

Abstract

In this study, hydrogen storage on carbon nanotubes was studied via two main methods: hydrogen adsorption and temperature programmed desorption. CNTs (multi-walled, thin multi-walled and single walled) of variable purity were tested for their hydrogen adsorption capacity at 298 in the pressure range of 0 to 1000 Torr. Maximum adsorption capacity per unit mass of the solid was observed over SWCNTs (0.30-0.36 wt.%), followed by thinMWCNTs (0.22 wt.%) and MWCNTs (0.12-0.17 wt.%). Temperature programmed desorption revealed that the adsorption sites on the CNTs surface are relatively uniform, due to the fact that the quantity of hydrogen desorbed is very close to the quantity of hydrogen adsorbed. The calculated values of desorption activation energy (~20 kJ/mol) revealed that adsorption on CNTs is not purely physical in nature but it also involves weak chemisorption bonds. One potential way to enhance hydrogen storage on carbon nanotubes is spillover effect. Pt was deposited on CNTs via wet impregnation (method A) or mixture of the suspension of carbon nanotubes in the solution of the precursor under sonication (method B). Both, hydrogen adsorption experiments at 298 K and temperature programmed desorption measurements revealed that hydrogen storage capacity observed over CNTs was almost double. Experimental and theoretical researches have shown that alkali doped CNTs presented higher values of hydrogen storage capacity, compared to non alkali doped CNTs. This behavior has been attributed to the creation of bipolar forces in the hydrogen molecule, due to the charge transfer in alkalis. The highest storage capacity presented in this work was 0.7 wt.%, for Li doped CNTs when Pt was deposited on them via method B. The use of CNTs as platinum support for proton exchange membrane fuel cells has been investigated as a way to reduce the cost of fuel cells through an increased utilization of platinum. This work presents results with Pt catalysts supported on CNTs and also on commonly used carbon powder, Vulcan XC-72, prepared via methods mentioned above. The results indicate electrochemical characteristics which depend strongly on the nature of the support and the Pt deposition method. Power density of 0.21 W/cm2 at 80 0C was achieved with Pt/SWCNTs fed with H2 and the activity of the anodes followed the sequence: Pt/SWCNTs > Pt/MWCNTs > Pt/Vulcan XC-72.