Titanium-Decorated Carbon Nanotubes as a Potential High-Capacity Hydrogen Storage Medium

Abstract

We report a first-principles study, which demonstrates that a single Ti atom coated on a singlewalled nanotube (SWNT) binds up to four hydrogen molecules. The first H2 adsorption is dissociative with no energy barrier while other three adsorptions are molecular with significantly elongated H-H bonds. At high Ti coverage we show that a SWNT can strongly adsorb up to 8-wt% hydrogen. These results advance our fundamental understanding of dissociative adsorption of hydrogen in nanostructures and suggest new routes to better storage and catalyst materials. PACS numbers: 61.46.+w,84.60.Ve,81.07.De Developing safe, cost-effective, and practical means of storing hydrogen is crucial for the advancement of hydrogen and fuel-cell technologies [1]. The current stateof-the-art is at an impasse in providing any material that meets a storage capacity of 6-wt% or more required for practical applications [1,2,3,4,5,6,7,8]. Here we report a first-principles computation of the interaction between hydrogen molecules and transition metal atoms adsorbed on carbon nanotubes. Our results are quite remarkable and unanticipated. We found that a single Ti-atom adsorbed on a SWNT can strongly bind up to four hydrogen molecules. Such an unusual and complex bonding is generated by the concerted interaction among H, Ti, and SWNT. Remarkably, this adsorption occurs with no energy barrier. At large Ti coverage we show that a (8,0) SWNT can store hydrogen molecules up to 8-wt%, exceeding the minimum requirement of 6-wt% for practical applications. Finally, we present high temperature quantum molecular dynamics simulations showing that these systems are stable and indeed exhibit associative desorption of H 2 upon heating, another requirement for reversible storage.Recent experiments [9,10] and calculations [11,12,13,14] suggest that it is possible to coat carbon nanotubes uniformly with Ti atoms without metal segregation problems [15]. Here we show that such Ti-coated carbon nanotubes exhibit remarkable hydrogen absorption properties. Below we will present our results in detail for a (8,0) nanotube and briefly for four armchair (n,n) (n=4,5,6, and 7) and five zigzag (n,0) (n=7,8,9,10,11, and 12) nanotubes.A single Ti atom on an (8,0) SWNT has a magnetic ground state with S=1 and a binding energy of 2.2 eV; this will serve as our reference system, denoted t80Ti. In order to determine different reaction paths and products between H 2 and t80Ti, we have carried out a series of single-energy calculations as H 2 molecules approaches t80Ti and when there are large enough forces acting on H 2 molecules, we let the atoms evolve according to the quantum mechanical forces obtained from density functional theory (DFT) calculations [16]. We used the conjugated-gradient (CG) minimization and optimized both the atomic positions and the c-axis of the tube.The energy calculations were performed within the plane-wave implementation [16] −1 k-point spacing resulting in 5 k-points along the tube axis. The cutoff energy of 350 eV is found to be enough for total ...

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