Prediction of the hydrogen storage capacity of carbon nanoscrolls V. R. Coluci, 1, * S. F. Braga, 1 R. H. Baughman, 2 and D. S. Galvão 1 1 Instituto de Física “Gleb Wataghin,” Universidade Estadual de Campinas, C.P. 6165, 13083-970 Campinas, São Paulo, Brazil 2 NanoTech Institute, University of Texas, Richardson, Texas 830688, USA and Department of Chemistry, University of Texas, Richardson, Texas 830688, USA Received 28 June 2006; revised manuscript received 8 January 2007; published 6 March 2007 Classical grand-canonical Monte Carlo simulations were performed to investigate the equilibrium hydrogen storage capacity of carbon nanoscrolls. The results show that hydrogen molecules can be absorbed in the internal cavity as well as on the external surface of the scroll when the interlayer spacing is less than 4.4 Å. When the interlayer spacing is increased to 6.4 Å, by assuming spacing increase due to intercalation of other species, the hydrogen molecules can also be incorporated in the interlayer galleries, doubling the gravimetric storage capacity and reaching 5.5 wt % hydrogen per weight carbon at 150 K and 1 MPa. Our results showed that intercalated carbon nanoscrolls may be a promissing material for hydrogen storage. DOI: 10.1103/PhysRevB.75.125404 PACS numbers: 81.05.Uw, 89.30.-g, 33.15.Fm, 02.70.Uu I. INTRODUCTION Hydrogen H 2  has recently been the object of intense research as a secondarily derived energy source. It would be ideal as a synthetic fuel because it is lightweight, conve- niently produced, and its oxidant product water is environ- mentally benign. Unlike petroleum, it can be easily gener- ated from renewable energy sources. Of the problems to be solved for the utilization of hydrogen energy, the most im- portant one is how to store H 2 easily and cheaply. The target of the U.S. Department of Energy DOE for hydrogen stor- age is a gravimetric capacity weight of stored H 2 /system weight of 6.5 wt % and a volumetric density of 62 kg H 2 m -3 . 1 The development of hydrogen-fueled vehicles and portable electronics will require new materials that can store large amounts of hydrogen at ambient temperature and rela- tively low pressures, and provide small volume, low weight, and fast kinetics for recharging. 2,3 Graphite, carbon nanotubes, and nanofibers have been both theoretically and experimentally investigated as poten- tial adsorbent structures. 1,4–22 Though some reports claim very high storage capacity, these have not been supported by other investigators. 23 For instance, while the highest capacity of hydrogen storage 67.55 wt %  reported by Chambers et al. with the so-called herringbone graphite fibers 4 could not be confirmed in any laboratory up to now, 23 Ahn et al. mea- sured for comparable carbon nanofibers a storage capacity less than 0.2 wt % using the same method and under compa- rable conditions. 24 A rough estimate of about 1 wt % can be obtained from volumetric measurements for carbon nano- structures at room temperature and 10 MPa when the surface area is about 500 m 2 /g. 23 Panella et al. investigated different carbon nanostructures optimized for hydrogen storage ob- taining the highest storage capacity of 4.5 wt % at 77 K for activated carbon with a specific area of 2564 m 2 /g. 25 Alkali- doped nanotubes have also been investigated. While Chen et al. report adsorption of 20 and 14 wt % for Li-doped and K-doped carbon nanotube, 7 respectively, these extraordinar- ily high values appear to be a result of an artifact. Yang obtained adsorption of 2.5 and 1.8 wt % for the same mate- rials, respectively. 8 In those hydrogen storage experiments where the results have been confirmed by subsequent inves- tigators, the major percentage of hydrogen is stored by phy- sisorption of H 2 molecules, in contrast to chemisorption ob- served for metal hydrides. 3 The differences observed in the experimental data reflect how sensitive the reported hydro- gen storage capacity is to the details of the experimental procedures, such as nanotube synthesis and pretreatment pro- cesses, hydrogen charging techniques, and storage capacity measurement methods. Theoretical investigations on nanotubes as hydrogen stor- age materials include Monte Carlo calculations, 9–16 classical molecular-dynamics simulations, 17,18 first-principles calculations, 19,20 and geometry-based estimates. 21,22 The in- vestigations revealed that the adsorption process depends on structural parameters such as size, geometry tubular or slit pores, distance between pores, and storage temperature and pressure. Furthermore, studies involving finite-diameter ropes of single-walled carbon nanotubes have demonstrated the important role of the external surface of the rope in the hydrogen storage capacity. 11 Like the experimental results, the theoretical predictions by different groups also disagree. The range of the storage capability of carbon nanotube ad- sorption predicted by molecular simulations varies from 2% in a tube of 12.2 Å diameter 133 K, 10 MPa, and intertube distance of 3.2 Å Ref. 10 to 12% in a tube of 22 Å diameter 77 K, 10 MPa, and intertube distance of 1.1 Å Ref. 12. Recently, Viculis et al. 26 have reported that a low- temperature synthesis method produces a different type of carbon structure, named carbon nanoscroll CNS. 26,29 These structures are formed by the jelly roll-like wrapping of a graphite sheet to form a nanotube. The synthetic route for carbon nanoscrolls involves intercalation of graphite flakes with potassium metal followed by exfoliation with ethanol to make a dispersion of carbon sheets. After sonication to assist exfoliation and spiral wrapping of the graphite sheets, carbon nanoscrolls are formed. 26 Theoretical studies of the structural and dynamical properties of carbon nanoscrolls revealed that the carbon nanoscroll formation is a self-sustained curling process after a critical overlap area is reached. 27 Depending on the initial shape and dimensions of the graphite sheet, the lowest-energy activated state for self-rolling can have a chi- PHYSICAL REVIEW B 75, 125404 2007 1098-0121/2007/7512/1254046 ©2007 The American Physical Society 125404-1