Published: March 18, 2011 r2011 American Chemical Society 6994 dx.doi.org/10.1021/jp108797p | J. Phys. Chem. C 2011, 115, 6994–7001 ARTICLE pubs.acs.org/JPCC Hydrogen Storage in Decorated Multiwalled Carbon Nanotubes by Ca, Co, Fe, Ni, and Pd Nanoparticles under Ambient Conditions A. Reyhani, † S. Z. Mortazavi, ‡ S. Mirershadi, † A. Z. Moshfegh,* ,§,^ P. Parvin, ‡ and A. Nozad Golikand † † Material Research School, P.O. Box 14395-836, Tehran, Iran ‡ Department of Physics, Amirkabir University of Technology, P.O. Box 15875-4413,Tehran, Iran § Department of Physics, Sharif University of Technology, P.O. Box 11155-9161, Tehran, Iran ^ Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box 14588-8969, Tehran, Iran 1. INTRODUCTION Finding alternative energy resources to substitute for mineral energy sources has been a very important issue due to serious problems including air pollution and release of greenhouse gases as well as shortage of energy with increasing the world popula- tion. Thus, to obtain a fully renewable and environmentally safe energy source, many scientists and researchers have been exten- sively investigated to ease the problem in recent years. 1À3 Hydrogen is a promising clean energy carrier and has been attracting much interest from the scientific and technological viewpoints. 4 In this context, high-capacity storage of hydrogen with a safe, effective, and cheap system at temperature ranging from near ambient to about 100 °C and at pressures below ∼100 bar is essential in practical applications. 1À5 These conditions favor storage based on the interaction of hydrogen with solid materials, rather than the compressed or liquid hydrogen, storage which requires high pressures (700 bar) or low temperatures (20 K), respectively. 4,5 Carbon-based materials, due to their low cost and weight, have long been considered as suitable adsorption substrates for the reversible storage of hydrogen; however, hydrogen storage in these materials is still at a research level and is not yet mature enough for industrial application. 6,7 Since the report of Dillon et al. 8 on a possible 5À10% hydrogen storage capacity for single- walled carbon nanotubes (SWCNTs), carbon nanotubes (CNTs) have attracted considerable interest due to the usage of CNTs as a safe hydrogen storage medium. 9,10 Significantly higher storage can be achieved in CNT by surface functionalization in two approaches: (i) via creation of nanometer-sized pores that dramatically raise both the surface area for adsorption and create deeper potential wells 7,11 and (ii) by doping with elements capable of strengthening the hydrogenÀsubstrate interaction. 7,12 Recently, many studies have revealed that hydrogen storage capacity is enhanced by added metals to carbon structures such as Ag, B, Ca, Fe, K, Li, Ni, Pd, Pt, Ru, Ti, TiO 2 , and V. 6,13À33 Gao et al. 19 also indicate that fresh CNTs could only store 0.1 wt % of hydrogen at 573 K and ambient pressure; however, oxidation treatment to produce defects and subsequent loading with a PdÀNi catalyst significantly increases the hydrogen storage capacity up to 6.6 wt % at 610 K and ambient pressure. Singh et al. 34 reported the high hydrogen storage capacity of metalla- carboranes, which the light transition metal (TM) atoms can bind up to 5 H 2 molecules via Kubas interaction. Liu et al. 35 studied a high uptake H 2 capacity of 13.45 wt % for the Li- dispersed carbon nanotubes system using density functional theory (DFT) calculations. They concluded that binding strength and uptake capacity of the carbon nanotubes can be enhanced by introducing more dopants due to electron transfer from Li atoms to carbon atoms. This finding was also reported for Ca 34,36 and transition metals 37 by the other researchers. In the present work, we explore the influence of added different metal including Ca, Co, Fe, Ni, and Pd to multiwalled Received: September 15, 2010 Revised: February 14, 2011 ABSTRACT: We report a study on hydrogen storage in Ca, Co, Fe, Ni, and Pd decorated multiwalled carbon nanotubes (MWCNTs) by using two techniques: volumetric and electro- chemical. The results showed that hydrogen molecules are adsorbed on the defect sites and transported to the spaces between adjacent carbon via diffusion through both defect sites and opened tips into the layers. Hydrogen storage capacity can be improved in the decorated MWCNT by Co, Fe, Ni, and Ca metals in two approaches: (i) H 2 adsorption via Kubas interac- tion and (ii) dissociation of H 2 molecules on the metal particles. The results reveal that Pd are more effective catalyst for hydrogen storage process. It was found that dissociation of H 2 occurs on the Pd particle, and hydrogen atoms are entered into the spaces between adjacent carbon layers. They create loosely bonds of CH x species and PdÀCÀH x complex which can be decomposed easily at lower temperatures as compared to CÀH chemical bonds.