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For Li-doped LA grown CNTs, the hydrogen storage capacity reached 2.4 wt%. The high hydrogen storage capacity of alkali metal-doped CNTs seems to originate from increasing the hydrogen adsorption sites of CNTs due to introducing the doped metals in nanotube bundles, and separating the tubes, but not from their chemical effects. Purified SWNTs with a large mean diameter of approximately 1.85 nm, which were synthesized by a semicontin-uous hydrogen arc-discharge method [42], were also used for electrochemical hydrogen adsorption experiments [77]. After high-temperature treatment, a reproducible maximum discharge capacity of 316 mA • h/g, corresponding to 1.2 wt% hydrogen storage, was achieved at 298 K under normal pressure. After 100 charge/discharge cycles, >81% of the maximal capacity was maintained.

Dai et al. [78] further measured the electrochemical charge/discharge hydrogen capacity of macroscopically long ropes (up to 100 mm in length) of well-aligned SWNTs (60-70 wt% purity) with a larger mean diameter of about 1.72 nm. A discharge capacity of 503 mA • h/g, corresponding to 1.84 wt% hydrogen, was achieved reproducibly at ambient temperature under normal atmosphere. A much higher charge capacity of 1157 mA • h/g, corresponding to 4.1 wt% hydrogen, was obtained using electrodes made up of MWNTs [79].

Reversible insertion of lithium into purified single-wall carbon nanotubes was achieved electrochemically [80]. Carbon nanotubes exhibited reversible capacities of approximately 460 mA • h/g and very high irreversible capacities of 1200 mA • h/g, corresponding to 4.35 wt% hydrogen, which the authors ascribe to the large specific surface area (350 m2/g). In-situ X-ray diffraction revealed an irreversible loss of crystallinity, suggesting that doping disrupts the intertube binding, analogous to exfoliation in layer hosts. In another study, lithium-inserted carbon nanotubes exhibited reversible capacities approximately 780 mA • h/g and very high irreversible capacities of 1080 mA • h/g, corresponding to 3.9 wt% hydrogen [81].

Pekker et al. [82] performed the hydrogen storage experiments with carbon nanotubes and graphite via a dissolved metal reduction method in liquid ammonia. Dried ammonia was condensed to the carbon material which had been mixed with lithium under an inert atmosphere. The suspension was stirred for 1 h at about 220 K, and then ammonia was evaporated by heating to room temperature. After a second condensation, methanol was added to the sample. The sample was heated again to room temperature to evaporate NH3, and then it was washed and dried. With thermoana-lytical and electron-microscopic studies, the authors could show that the applied hydrogenation method gives rise to the formation of strongly bound derivatives. Desorption of hydrogen takes place at around 770 K, and the content was 1.7 and 0.7 wt% for graphite and carbon nanotubes, respectively.

Generally speaking, the electrochemical loading of carbon nanostructures shows no spectacular high storage capacity. Furthermore, the uptake seems to be proportional to the specific surface area of the carbon material, and therefore comparable to activated carbon [28].

2.5. Hydrogen Storage in Carbon Nanofibers

Carbon nanotubes are not the only carbon structures which are able to retain hydrogen in their framework; graphite nanofibers also exhibit hydrogen adsorption potential. The hydrogen storage experimental results for graphite nanofibers are also listed in Table 1. Rodriguez's group prepared various graphite nanofibers, and conducted hydrogen adsorption experiments using the produced nanofibers [83]. These graphite nanofibers consist of catalytically produced graphene sheets that are oriented to form various fibrous structures. The orientation of the sheets in the fibers can be controlled by the choice of catalyst. Hydrogen gas applied at 11.35 MPa was absorbed at room temperature (298 K). The hydrogen storage capacity was found to be, for CNTs, 11.26 wt%, for herringbone carbon fibers 67.55 wt%, for platelet fibers 53.68 wt%, and finally, for graphite 4.52 wt%. Unfortunately, this surprising hydrogen adsorption capacity has not been confirmed to date, neither experimentally nor theoretically [18]. Attempts by other researchers to reproduce these high-hydrogen storage densities have failed, and typically, results of only 0.08 wt% have been achieved [84]. The extraordinarily high results were later suggested to be influenced by the presence of water vapor, which expanded the spacing between the graphite layers (typically ~3.4 A) to accept multiple layers of hydrogen [85]. Browning et al. [86] synthesized carbon nanofibers at 870 K by passing ethy-lene over a series of Fe-N-Cu catalysts. They obtained a hydrogen storage capacity in their sample at room temperature and 12 MPa of 4.18 and 6.54 wt% with a postproduction treatment. Poirier et al. [87] tested the hydrogen storage performance of carbon nanofibers, as well as intercalated and exfoliated carbon materials. At room temperature, carbon nanofibers adsorb 0.7 wt% at a hydrogen pressure of 10.5 MPa, which is comparable to activated carbon. However, the hydrogen coverage per unit surface area is found to be substantially larger on carbon nanostructures than on activated carbon. de la Casa-Lillo et al. [88] found that the highest values of hydrogen adsorption in activated carbons or activated carbon fibers was close to 1 wt% at 10 MPa and room temperature. Hwang et al. [89] reported that the carbon nanofibers from methane decomposition using an Ni-MgO catalyst exhibited a hydrogen adsorption capacity up to 1.4 wt% after heat treatment at 1470 K in N2 atmosphere. Hirscher et al. [28] also performed volumetric measurements at 11 MPa and 300 K using GNFs prepared by catalytic vapor deposition, and observed a hydrogen uptake lower than 0.1 wt%. Nevertheless, the ball-milled graphite fiber reached up to 0.5 wt% hydrogen adsorption capacity at 0.9 MPa and room temperature using the TDS technique [58]. Tibbetts et al. [32] studied the sorption of hydrogen very carefully by nine different carbon materials at pressures up to 11 MPa and temperatures from 193 to 773 K. The largest sorption observed is less than 0.1 wt% hydrogen at room temperature and 3.5 MPa. Ritschel et al. [90] observed a storage capacity of 1273 K Ar-treated GNF less than 0.2 wt% at room temperature and 4.5 MPa.

Using the thermogravimetric analysis, Strobel et al. [91] measured the hydrogen adsorption of carbon nanofibers and activated carbon at high pressure. At a pressure of 12.5 MPa and room temperature, they observed a maximum weight increase corresponding to a hydrogen uptake of 1.6 wt% in activated carbon and of 1.2 wt% for the fibrous material. Harutyunyan et al. [92] obtained consistent results on purified tubular-formed and herringbone-type GNFs. They found 1.8 wt% at 77 K and about 1 wt% at room temperature under 1.5 MPa hydrogen pressure.

However, encouraging results have been reported by Gupta and Srivastava [93, 94], who achieved hydrogen adsorption capacities of ~10 and ~15 wt% for graphite nanofibers grown by thermal cracking. The GNFs were activated by heating 1 h at 420 K under 10-7 MPa in a steel reactor. After 15-16 h, hydrogen absorption started, and the hydrogen uptake saturated at pressures higher than 9 MPa. More recently, Fan et al. reported a 10-13 wt % hydrogen adsorption capacity using vapor-grown carbon fibers [95]. In a further research paper [45], the same group reduced the storage capacity of carbon fibers by a factor of 2. As with carbon nanotubes, a large degree of variation in the results exists, and further verification is required.

The rather scattered experimental results published on hydrogen storage measurements of carbon nanofibers also listed in Table 1. The figures in the table show that the large discrepancies between claims of high hydrogen storage capacities and almost no storage at all exist up to the present day. The experimental data range from 67.55 down to 0.08 wt%.

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