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Hydrogen Storage in Carbon Nanotubes Through Formation of C-H Bonds Print

Two of the major challenges for humanity in the next 20 years are the shrinking availability of fossil fuels and the global warming and potential climate changes that result from their ever-increasing use. One possible solution to these problems is to use an energy carrier such as hydrogen, and ways to produce and store hydrogen in electric power plants and vehicles is a major research focus for materials scientists and chemists. To realize hydrogen-powered transport, for example, it is necessary to find ways to store hydrogen onboard vehicles efficiently and safely. Nanotechnology in the form of single-walled carbon nanotubes provides a candidate storage medium. A U.S., German, and Swedish collaboration led by researchers from the Stanford Synchrotron Radiation Laboratory (SSRL) used ALS Beamline 11.0.2 and SSRL Beamline 5-1 to investigate the chemical interaction of hydrogen with single-walled carbon nanotubes (SWCNs). Their findings demonstrate substantial hydrogen storage is both feasible and reversible.

A Nanosolution to a

The twin specters of an energy shortage and global climate change loom over our energy-hungry modern civilization. Currently fossil hydrocarbons from coal, oil, and natural gas are the main source of energy powering today’s world, but their availability is dwindling. Moreover, their intensive use has led to increasing emission of greenhouse gases, causing an already measurable global warming. The main debate concerns how large the warming will be and how it will affect our climate. Substituting hydrogen for fossil hydrocarbons is one way to address both issues. To avoid greenhouse gases, hydrogen can be produced from the electrolysis of water using renewable energy, direct photolysis using sunlight, and thermal cracking using the heat from nuclear reactors. The hydrogen can then be consumed without any harmful emissions in fuel cells to generate electricity in power plants or vehicles. To realize hydrogen-powered transport, it is necessary to find ways to store hydrogen onboard efficiently and safely. Nikitin et al. have demonstrated with x-ray spectroscopy techniques that nanometer-sized structures known as carbon nanotubes can indeed store a useful amount of hydrogen and can also be made to release it.

Since hydrogen exists in the form of gas at ambient pressure and temperature, the most appropriate way to store hydrogen is in an adsorbed form on media capable of absorbing and releasing large quantities of this element easily and reliably. Carbon nanotubes are one of the most promising materials, and hydrogen storage through both physisorption and chemisorption mechanisms has been proposed. While most previous studies have focused on the potential of physisorption of molecular hydrogen, there is no direct reliable evidence of high hydrogen storage capacity at room temperature. It has been predicted that the chemisorption mechanism could provide hydrogen storage capacity that fulfills the technological requirement through the saturation of C–C π bonds with atomic hydrogen. However, direct experimental evidence of the feasibility of the hydrogen storage through chemisorption has not yet been demonstrated.

Carbon K-edge XAS spectra of a clean SWCN film (black) and a SWCN film after hydrogenation (blue). The decrease of the π* resonance intensity and increase of the intensity in the energy range of C–H* and σ* indicate that the hydrogenation causes the rehybridization of the carbon atoms in the SWCN film from sp2 to sp3 form along with the formation of C–H bonds.

The Stanford group studied as-grown SWCN films and SWCNs hydrogenated in situ by means of an atomic hydrogen beam. The team used x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) to observe the formation of C–H bonds through the modification of the local electronic structure around specific carbon atoms. With XPS and XAS, the researchers could also quantify the relative amount of hydrogen per carbon atom that was chemically adsorbed.

Hydrogenation leads to the breaking of C–C π bonds and C–H bond formation, as seen from the decrease in intensity of the π* resonance and the increase in the intensity of C–H* and σ* resonances in the XAS spectra. Of the two peaks in the carbon 1s XPS spectrum of hydrogenated SWCN, the higher-energy was assigned to nonhydrogenated and the lower-energy one to hydrogenated carbon atoms. The assignment is supported by the theoretical calculation for the carbon 1s chemical shifts. Based on the intensity ratio between the two peaks, the amount of hydrogenated carbon atoms was estimated to be 5.1 ± 1.2 weight % of the hydrogen capacity of SWCNs. This value is close to the 6 weight % required by U.S. Department of Energy for media to be used for an onboard hydrogen-storage system. The investigators also found that all C–H bonds in hydrogenated SWCN break at temperatures above 600° C, demonstrating the reversibility of the hydrogenation.

Carbon 1s XPS spectra of a clean SWCN film (black) and SWCN film after hydrogenation (blue). Peak 1 at higher energy (lower binding energy) corresponds to the signal from carbon atoms unaffected by hydrogenation; whereas peak 2 at lower energy is due to hydrogen-coordinated carbon atoms. The theoretical values of the carbon 1s core-level chemical shifts due to C–H bond formation for different types of SWCNs are shown as vertical lines.

The present results indicate that it is possible to store hydrogen chemically in SWCNs through hydrogenation. The group thinks that the hydrogenated SWCNs provide a storage capacity close to the technologically required values, but it is essential to find means to hydrogenate SWCN efficiently and to fine tune the energetics of the C–H bonds to allow for hydrogen release at 50° to 100° C. A hydrogenation metal catalyst can address the former, and the latter can be accomplished by using SWCNs with an appropriate diameter distribution.

C 1s XPS spectra of a SWCN film exposed to the two cycles of hydrogenation and dehydrogenation: (a) clean SWCN film, (b) hydrogenated SWCN film, (c) SWCN annealed at 600o C, (d) hydrogenated SWCN film, (e) SWCN annealed at 600o C. It is clear that the second hydrogenation led to the restoration of the shoulder that is due to the signal from C–H bonded carbon atoms.

Research conducted by A. Nikitin and H. Ogasawara (Stanford Synchrotron Radiation Laboratory); D. Mann (Stanford University); R. Denecke (Stanford Synchrotron Radiation Laboratory and Universität Erlangen-Nürnberg, Germany); Z. Zhang, H. Dai, and K Cho (Stanford University); and A. Nilsson (Stanford Synchrotron Radiation Laboratory and Stockholm University, Sweden).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES), and Global Climate Energy Project operated by Stanford University. Operation of the ALS is supported by BES. Operation of SSRL is supported by BES and the DOE Office of Biological and Environmental Research.

Publication about this research: A. Nikitin, H. Ogasawara, D. Mann, R. Denecke, Z. Zhang, H. Dai, K. Cho, and A. Nilsson, “Hydrogenation of single-walled carbon nanotubes,” Phys. Rev. Lett. 95, 225507 (2005).