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ENHANCEMENT OF HYDROGEN STORAGE WITH NANOCOMPOSITES M.C. Coelho, E. Titus, V.F. Neto, and J. Grácio Centre for Mechanical Technology and Automation (TEMA) – University of Aveiro, Aveiro, Portugal
Abstract The main objective of this research is to develop a low-cost high capacity hydrogen storage material. One of the materials used for hydrogen storage is carbon nanotubes (CNTs). The introduction of nanoparticles (dopants) to the nanotubes can enhance the hydrogen bonding at the edge and inside the tubes allowing the occurrence of hydrogen storage. The research presented in this paper was mainly focused on modifying the CNTs systems, using nickel to create nanocomposites, in an attempt to tune hydrogen storage capabilities of the nanotubes. Further, multi walled carbon nanotubes (MWCNTs) and nanocomposites were submitted to H2 adsorption using a CVD reactor. The hydrogen presence in MWCNTs was examined and compared with these nanocomposites. The analytical techniques used include CVD, SEM, Raman and FTIR spectroscopy. Keywords Carbon nanotubes, CVD, hydrogen storage, nanocomposites, nickel.
INTRODUCTION Global energy consumption is expected to increase dramatically in the 21st century. Meanwhile, the burning of fossil fuels has resulted in serious environmental concerns, such as atmospheric pollution and global warming. Therefore, worldwide effort toward renewable energies becomes urgent. Among the various alternative energy strategies, building an energy infrastructure that allows the use of hydrogen (H2) as the primary energy carrier constitutes an attractive choice. H2 has outstanding potential as an energy source with multiple desirable properties [1]: it is non-polluting and forms water as a harmless byproduct during use; the delivered energy per mass of H2 is about 3 times higher than that of conventional liquid hydrocarbons; it can be easily generated from renewable energy sources [2]. However, one of the most severe challenges of H2 use in transportation is the current lack of a safe and efficient on-board H2 storage technology [3, 4], since the minimization of the ratio “volume/energy” of the storage tank is one of the targets of efficient H2 storage. The global objective of the research being performed in the Centre for Mechanical Technology and Automation (TEMA) of the University of
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Aveiro is to develop a low-cost high capacity H2 storage material that should be compatible with the various goals of the international energy agencies [3] for a safe and efficient (in energetic terms) on-board H2 storage for transportation applications. One of the materials used for H2 storage is carbon nanotubes (CNTs). The introduction of nanoparticles (dopants) to the nanotubes can enhance the H2 bonding at the edge and inside the tubes allowing the occurrence of H2 storage [5]. More information on previous research done on hydrogen storage with carbon nanotubes and nanocomposites can be found elsewhere [6-8]. The main objective of the research presented in this paper is the development and testing of nanocomposites in order to detect the presence of adsorbed H2 in the material. This research was focused on modifying the CNTs systems in an attempt to tune H2 storage capabilities of the nanotubes, using nickel (Ni) to create nanocomposites. Further, multi-walled carbon nanotubes (MWCNTs) and nanocomposites were submitted to H2 adsorption using a Hot Filament Chemical Vapour Deposition (HFCVD) reactor. The H2 presence in MWCNTs was examined and compared with these nanocomposites. The analytical techniques used include Scanning Electron Microscopy (SEM), Raman and Fourier Transform Infra Red (FTIR) spectroscopy.
STRUCTURE OF THE EXPERIMENTAL METHOD The flow chart describing the experimental method is illustrated in Figure 1.
Figure 1. Flow chart of the experimental method of material development and H2 adsorption
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The CNTs used (reference number Nanocyl © – 7000 series) were purchased from Nanocyl S.A. (Belgium). HNO3 and H2O2 used were of analytical grade and they were purchased from PRONALAB and Panreac Quimica S.A., respectively. The functionalization of MWCNT (1 g) synthesized by CVD method was carried out in H2O2 medium (500 ml). Then, 250 ml of this solution was sonicated for 2 hours in a HNO3:H2O2 (10 ml:10 ml) medium, and subsequently, nickel powder (density = 8.909) was added to the CNTs / HNO3 / H2O2 medium and further sonicated for 3 hours. The samples were exposed to H2 using a HFCVD reactor, that was constructed in the Laboratory of TEMA (see Figure 2) and is described elsewhere [9]. H2 adsorption on functionalised samples was carried, in which the tungsten filament heated at high temperature (~ 2000 ºC) dissociated the H2 molecule, promoting the formation of atomic hydrogen.
Figure 2. HFCVD reactor (developed at TEMA Laboratory)
The process parameters used in the H2 adsorption experiments are indicated as follows: distance between the filament and the substrate, 5 cm; H2 flow rate, 200 sccm; deposition pressure, 4000 Pa; substrate temperature, 100ºC. The deposition time was 2 h. It is important to note that the substrate and filament temperatures (as measured using a thermocouple and an
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optical pyrometer, respectively), were kept constant at 100ºC and 1720ºC, respectively, during the process. Extreme care was taken to eliminate the leakage of the system. Prior to adsorption, the sample was flushed with H2 for some minutes. The high filament (tungsten) temperature (~2000ºC) initiated the H2 dissociation, allowing atomic H to interact with the CNTs subsequently. The sample was then allowed to cool down to room temperature. The SEM device used was FEG_SEM S4100 manufactured by Hitachi, while the EDS system was Rontec UHV Dewar Detektor. Room temperature Raman spectra were recorded using a Bruckner RFS 100/S FTRaman system. The excitation wavelength used was 1064 nm from an Nd:YAG laser. FT-Raman data was collected for 4000 scans with a resolution of 4 cm-1. FTIR analysis was carried out using Nicolet NEXUS bench machine with 128 scans and resolution of 1 cm-1. The IR samples were prepared by mixing the samples with KBr powder.
RESULTS AND DISCUSSION Figure 3 shows the SEM images of samples (a) commercial CNTs, (b) functionalised CNTs and (c) Ni-doped CNTs. The SEM analysis was carried out only to confirm the morphology of the samples. The SEM image of the commercial CNTs (Figure 3a) displays entangled CNTs; while the CNTs seem to be more dispersed after functionalisation (Figure 3b). The clustering of Ni-doped CNTs (Figure 3c) was expected due to the magnetic effect.
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Figure 3. SEM micrographs: (a) commercial CNTs; (b) functionalized CNTs; (c) Ni-doped CNTs
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The EDS spectrum of the Ni-doped CNT is shown in Figure 4. Only three elements, C, O and Ni, were detected; no other impurities were detected in the nanostructures.
Figure 4. EDS analysis of sample (c)
Figure 5 shows the FT-Raman spectra of (a) commercial, (b) Nidoped, (c) hydrogenated Ni-doped CNTs and (d) hydrogenated CNTs (alone, without Ni, for comparison with Ni-doped CNTs).
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Figure 5. Raman spectra: (a) commercial CNTs; (b) Ni-doped CNTs; (c) H2 adsorption in Ni-doped CNTs; (d) H2 adsorption in CNTs per se
The spectra are divided into 2 regions to facilitate the analysis: I) The first order spectra of samples shows two peaks at 1285 and 1595 cm-1, which are designated as the disorder-induced (D-band) and tangential graphitized carbon (G-band) bands, respectively. However, the integrated intensity ratio (R) between D- and G-bands, i.e., ID/IG ratio of samples (ac) varies from one to another. The relative intensity ratio calculated for each spectrum is given in the Table 1. Normally, the high quality CNTs spectrum has an intense G-band and the R-value (1.19) estimated for as-grown sample indicates the relative quantity of structural defects in the sample. These defects, most likely, occurred during the sample purification in acid solutions. The R values decrease from (c) to (a). This means that defects are introduced by nickel doping and the highest value of D band vibrations of sample (c) confirms the hydrogenation in Ni-doped CNTs. Table 1. Ratio between D- and G- bands of Raman spectra: (a) commercial CNTs; (b) Nidoped CNTs; (c) H2 adsorption in Ni-doped CNTs Sample (a) (b) (c)
D band (cm-1) 0.000077 0.000114 0.000078
G band (cm-1) 0.000065 0.000090 0.000058
Intensity D-band/G-band 1.19 1.27 1.33
II) The second order spectra consist of overtones and combination modes associated with the two dominant features of the first-order mode. The second-order Raman phonon frequencies are sufficiently sensitive to use it as a new important tool to determine order-disorder levels in materials. The spectrum of commercial CNTs (Figure 5a) exhibits a high
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intensity symmetric band in the region 2000-3500 cm-1. The spectrum of Nidoped sample was similar to that of commercial CNTs (Figure 5b). However, it is interesting to note that the spectrum of hydrogenated CNTs (Figure 5d) is very different from commercial and Ni-doped CNTs, displaying a diffuse and structureless band which is attributed to the influence of hydrogen. The spectrum intensity is much lower than that of MWCNT. It can be presumed that this change in band structure is due to the luminescence effect rising from the surface adsorbed hydrogen. The luminescence effect on doped CNT sample and the respective Raman spectra were also reported by other researchers [10]. In order to confirm the presence of hydrogen in the sample, we have performed FTIR analysis (Figure 6 (a-c)) of the samples, commercial (a), Ni-doped (b) and hydrogenated Ni-doped CNTs (c), respectively.
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Figure 6. FTIR spectra: (a) commercial CNTs; (b) Ni-doped CNTs; (c) H2 adsorption in Nidoped CNTs.
Raman-active D- and G-bands are inactive in IR spectra because they are forbidden due to the symmetry of the carbon network. The FTIR spectra of as-grown CNT usually do not show any bands. However, we have used commercial CNTs which were previously purified by acid treatment. The acid treatment introduced COOH and OH functional group onto the CNTs, inducing peaks at 1650 and 3450 cm-1. The C-H related bands, which are normally active in the region 2700-3100 cm-1, were absent in the spectrum. The FTIR spectrum of Ni-doped CNTs is reported in Figure 6b. In consequence that any C-H related peak can be identified, it is assumed the surface adsorption is the mechanism that supports the increased hydrogen in the sample – which is confirmed from Raman results. Nuclear Magnetic Resonance (NMR) would be an alternative technique for the measurement of physically adsorbed hydrogen, but the presence of magnetic nickel, unfortunately, restricts the use of this measurement technique.
CONCLUSIONS AND FUTURE WORK The main motivation of this research was to modify CNTs with the purpose of enhancing H2 adsorption. CNTs were successfully doped with nickel and the results are very promising. A novel technique was also demonstrated for the effective functionalization of CNTs with Ni in HNO3/H2O2 medium with the aid of the Ni catalyst. Another novelty of this work is a new H2 storage process using HFCVD method, while FT-Raman
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spectroscopy was identified as an effective tool for the analysis of the presence of H2. The exact mechanism of H2 adsorption could not be established using a single technique; however, this work proposes that hydrogen adsorption onto CNTs results from the catalytic activity of Ni. The nature of adsorption (surface adsorption) of H2 is confirmed by Raman and FTIR spectroscopy analyses. Further research work will be developed in order to understand the actual mechanism of H2 adsorption, in which CVD reactor parameters (such as pressure and temperature) are varied and different types of analysis (such as micro-calorimetric analysis) are employed. Eventually, it should be also examined the eventual hydrogen storage enhancement when resorting to other nanocomposites, such as zeolites.
ACKNOWLEDGEMENTS The research work of M.C. Coelho is supported by Portuguese Science and Technology Foundation (SFRH/BPD/21317/2005). This research was partially funded by MARTIFER. The authors also would like to thank Eng. Marta Ferro and Eng. Maria Celeste Azevedo (from Department of Ceramics and Department of Chemistry, respectively, of the University of Aveiro) for their help with the SEM, FTIR and Raman analysis.
NOMENCLATURE CNT – Carbon Nanotubes FTIR – Fourier Transform Infra-Red HFCVD – Hot Filament Chemical Vapour Deposition MWCNT – Multi-Walled Carbon Nanotubes SEM – Scanning Electronic Microscope
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Guay P. Modélisation Monte-Carlo de l’adsorption de l’hydrogène dans les nanostructures de carbone, Mémoire présenté pour l’obtention du grade de Maître ès sciences, INRS Énergie, Matériaux et Télécommunications, 2003. 5. Titus E., Misra D.S., Sikder A.K., Tyagi P.K., Singh M.K., Misra A., Ali N., Cabral G., Neto V.F., Grácio J. Quantitative analysis of hydrogen in chemical vapor deposited diamond films, Diamond and Related Materials; 14 (3-7): 476-481, 2005. 6. Coelho M.C., Titus E., Cabral G., Neto V., Madaleno J.C., Fan Q.H., Sousa A.C.M., Grácio, J. Hydrogen adsorption onto transition metal modified carbon nanotubes, Journal of Nanoscience and Nanotechnology; Accepted for publication, 2007. 7. Titus E., Cabral G., Shokuhfar T., Madaleno J.C., Coelho M.C., Grácio J., Ramesh Babu.P, Blau W. J., Leahy R., Misra D. S. Hydrogen in chemical vapour deposited carbon nanotubes: an active site for functionalization, Journal of Nanoscience and Nanotechnology; Accepted for publication, 2007. 8. Cabral G., Titus E., Ramesh Babu.P, Blau W.J., Misra D.S., Madaleno J.C., Coelho M.C., Grácio J. Selective growth of vertically aligned carbon nanotubes by double plasma chemical vapour deposition technique, Journal of Nanoscience and Nanotechnology; Accepted for publication, 2007. 9. Huang T.B., Tang W.Z., Lu F.X., Ali N., Grácio J. Influence of plasma power over growth rate and grain size during diamond deposition using DC arc plasma jet CVD, Thin Solid Films; 429: 108-113, 2003. 10. Curran S., Davey A.P., Coleman J., Dalton A., McCarthy B., Maier S., Drury A., Gray D., Brennan M., Ryder K., Lamy de la Chapelle M., Joumet C., Bernier P., Byrne H.J., Carroll D., Ajayan P.M., Lefrant S., Blau W. Evolution and Evaluation of the Polymer / Nanotube Composite, Synthetic Metals; 103: 2559-2562, 1999.
