Hydrogen adsorption on crystalline ropes of carbon single-walled nanotubes ͑SWNT͒ was found to exceed 8 wt. %, which is the highest capacity of any carbon material. Hydrogen is first adsorbed on the outer surfaces of the crystalline ropes. At pressures higher than about 40 bar at 80 K, however, a phase transition occurs where there is a separation of the individual SWNTs, and hydrogen is physisorbed on their exposed surfaces. The pressure of this phase transition provides a tube-tube cohesive energy for much of the material of 5 meV/C atom. This small cohesive energy is affected strongly by the quality of crystalline order in the ropes.

Graphite nanofibers were synthesized and their hydrogen desorption and adsorption properties are reported for 77 and 300 K. Catalysts were made by several different methods including chemical routes, mechanical alloying, and gas condensation. The nanofibers were grown by passing ethylene and H 2 gases over the catalysts at 600°C. Hydrogen desorption and adsorption were measured using a volumetric analysis Sieverts' apparatus, and the graphite nanofibers were characterized by transmission electron microscopy and Brunauer-Emmett-Teller surface area analysis. The absolute level of hydrogen desorption measured from these materials was typically less than the 0.01 H/C atom, comparable to other forms of carbon. © 1998 American Institute of Physics. ͓S0003-6951͑98͒04549-5͔The main impediment to the use of hydrogen as a transportation fuel is the lack of a suitable means of storage. Compressed gas storage is bulky and requires the use of high strength containers. Liquid storage of hydrogen requires temperatures of 20 K and efficient insulation. Solid state storage offers the advantage of safer and more efficient handling of hydrogen, but promises at most 7% hydrogen by weight and more typically 2%. There has therefore been much interest in recent reports 1 that certain carbon graphite nanofibers 2 can absorb and retain 67 wt % hydrogen gas at ambient temperature and moderate pressures ͑i.e., up to 23 standard liters or 2 g of hydrogen per gram of carbon at 50-120 bar͒. The lowest hydrogen adsorption reported for any graphite fiber microstructure was 11 wt %. 1 Approximately 90% of the absorbed hydrogen was claimed to be desorbed at ambient temperature by reducing the pressure, while the balance is desorbed upon heating. Such claims are especially noteworthy, given that, up to this point, the typical best value of hydrogen adsorption in carbon materials has been on the order of 4 wt %, or 0.5 H/C ͑although there is also a recent claim that up to 10 wt % was achieved for H storage in single wall nanotubes 3 ͒. Owing to the potential importance of new materials with high hydrogen storage capacity for the worldwide energy economy, transportation systems and interplanetary propulsion systems, we have synthesized graphitic structures of appropriate morphology to make our own measurements of hydrogen absorption and desorption.Several graphite nanostructured materials were prepared using Fe-Cu catalysts of different compositions, in order to generate a range of fiber sizes and morphologies. We used either chemical methods, mechanical alloying, or gas condensation to produce the catalysts. The chemical method consisted of reduction of Fe and Cu nitrate precursors using the generic conditions of Rodriguez and Baker that produce high yields of graphite nanofibers. 2,4,5 Mechanically alloyed catalysts were produced using a SPEX 8000 mixer/mill using Fe and Cu powders in appropriate proportions. 6 A variation of the gas condensation method 7 was also used to produce catalyst.Catalysts were placed in a tube furnace and their surface ...

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Performance of direct methanol fuel cells with sputter-deposited Pt-Ru anodes was investigated. The thin film catalyst layers were characterized using X-ray diffraction, energy dispersive X-ray analysis, Rutherford backscattering spectroscopy, and X-ray photoelectron spectroscopy. Different catalyst loadings and membrane electrode assembly (MEA) fabrication processes were tested. The maximum power density achieved at 90°C was 100 mW/cm 2 , and almost 75 mW/cm 2 was attained with a loading of only 0.03 mg/cm 2 . The results demonstrate that a catalyst utilization of at least 2300 mW/mg can be achieved at current densities ranging from 260 to 380 mA/cm 2 . The application of the sputter-deposition method for MEA fabrication is particularly attractive for commercialization of direct methanol fuel cell technology.

Electrochemical studies were performed on LaNi,Sn, with 0 x 0.5. We measured the effect of the Sn substituent on the kinetics of charge-transfer and diffusion during hydrogen absorption and desorption, and the cyclic lifetimes of LaNi,Sn, electrodes in 250 mAh laboratory test cells. We report beneficial effects of making small substitutions of Sn for Ni in LaNi, on the performance of the metal hydride alloy anode in terms of cyclic lifetime, capacity, and kinetics. The optimal concentration of Sn in LaNi5_Sn alloys for negative electrodes in alkaline rechargeable secondary cells was found to lie in the range 0.25 x 0.3.