Perspectives on an Advanced Hydrogen Storage System: Platinum-Carbon Nanotube Nanocomposite Materials
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Perspectives on an Advanced Hydrogen Storage System: Platinum-Carbon Nanotube Nanocomposite Materials Renju Zacharia1, Sami-ullah Rather1, Sang Woon Hwang1, Arul Manuel Stephan2, and Kee Suk Nahm1 1 Nanomaterials Research Center and School of Chemical Engineering and Technology, Chonbuk National University, Chonju, 561-756, Korea, Republic of 2 Electrochemical Energy Systems Division, Central Electrochemical Research Institute, Chennai, 630 006, India
ABSTRACT Transition-metal functionalized-carbon nanotubes (CNTs) represent an important genre of hydrogen storage systems that exhibit superior storage capacity and improved storage kinetics when compared with the pristine CNTs. Here, we compare the reversible gravimetric hydrogen storage capacity of platinum-functionalized CNTs with that of pristine tubes, both measured at 300 K and an equilibrium hydrogen pressure of 1.67 MPa. The maximum reversible hydrogen storage capacity exhibited by the nano-composite material is found to be 3.2 ± 0.1 wt%, which is a nearly 50 times enhancement in comparison to that of the pristine tubes. The enhanced hydrogen storage capacity of functionalized CNTs is attributed to the spillover phenomena as suggested by the estimated storage capacity of Pt phase. The hydrogen storage in Pt nanoparticles modeled using the atomic magic number calculation and Pt hydride stoichiometry of PtH4 suggests that nearly 7 closed shells of Pt atoms reversibly adsorb and spill hydrogen on to CNT binding sites. INTRODUCTION Commercial realization and success of hydrogen-based mobile energy storage systems ultimately depend on the development of solid-state hydrogen storage systems that effectively store and release hydrogen at ambient pressure and temperature conditions. Last few years have witnessed the emergence of a broad spectrum of novel nanostructured materials that reversibly adsorb hydrogen under ambience of temperature and moderate pressures [1]. They diverge from nanocrystalline intermetallic systems based on prototypical hydrogen adsorbing alloys to the state-of-the-art nanotube-metal composites [1,2]. Nanocomposites, where CNTs and fullerenes are functionalized with the transitional-metals, belong to an important class of hydrogen storage materials with exceptional hydrogen storage capacity and improved kinetics when compared with that of the pristine CNTs [3-5]. By far, the most exclusively studied nanotube-metal composites are those functionalized with transition metals, such as Sc, Ti, V, Ni, Pd and Pt [3-8]. Though, the experimental studies pertaining to above systems are limited and in their rudimentary phase, various theoretical modeling anticipates their remarkable hydrogen storage capacity [3-5]. The enhanced hydrogen storage capacity exhibited by the transition-metal doped-CNTs and fullerenes is attributed to capability of these composites to form dihydrogen complexes (η2-
complexes) via unique hybridization between metal-d, hydrogen-σ, and carbon-p orbitals [3,4,9]. The stable complexes of these types are isolated for
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