Towards the mechanism of electrochemical hydrogen storage in nanostructured carbon materials

The mechanism of electrochemical hydrogen storage in a nanostructured carbon electrode using the electrodecomposition of KOH and H₂SO₄ aqueous solutions has been investigated by means of galvanostatic and voltammetry techniques. The role of charging the electrical double layer is carefully considered during the process of hydrogen insertion and deinsertion into carbon, i.e. electroreduction and electrooxidation, respectively. Once the electrode potential becomes lower than the equilibrium potential, hydrogen in the zero oxidation state is formed by the reduction of water in alkaline solution or the reduction of hydronium ions H₃O⁺ in acidic medium. In the next step, hydrogen is physically adsorbed (H_ad) onto the carbon surface and diffuses into the bulk of the carbon material with an efficiency which depends on the type of electrolyte. A higher amount of hydrogen is stored using the KOH medium, and the galvanostatic oxidation shows a well-defined plateau around -0.5 V vs. Normal Hydrogen Electrode (NHE). Due to the high overvoltage value in KOH (=0.55 V), the recombination steps of H_ad leading to molecular hydrogen evolution through the chemical (Tafel) or electrochemical (Heyrovsky) reactions are less favoured than in an H₂SO₄ medium (=0.32 V). Hence, a meaningful sorption of hydrogen is observed only in the basic electrolyte which shows a reversible capacity of 350 mAh/g (i.e. 1.3 wt.%) with a good electrical efficiency. Such performance demonstrates that nanostructured activated carbons might be a promising alternative to metallic alloys for electrochemical hydrogen storage. PACS 82.45.Yz; 81.05.Uw; 82.30.Rs; 82.45.Hk; 82.45.Fk; 81.05.Rm     

Hydrogen adsorption in microporous alkali-doped carbons (activated carbon and single wall nanotubes)

Adsorption of hydrogen gas was tested in microporous doped carbons: activated carbon (1600 m²/g) and single wall carbon nanotubes (SWNTs). The isotherms of adsorption of LiC₁₈ and KC₂₄ doped microporous activated carbons were determined in the range [0–30 bar] at room temperature and 77 K. The chemisorption ratio observed at room temperature increases with increasing the alkali/carbon rate. The isotherm profiles of doped activated carbon at 77 K show no clear enhancement of the sorption ratio compared to the raw activated carbon.The adsorption sites of potassium doped SWNTs with closed end were determined by neutron diffraction experiment using deuterium gas. The K-doped SWNTs were found only slightly intercalated by K ions so that empty cavities are preserved in between the tubes. At room temperature, the chemisorption of deuterium was not observed in doped SWNTs bundles, but only in the KC₈ graphite intercalation compound impurities. At low temperature, the isotherms analysis and neutron diffraction experiments have shown that D₂ molecules are physisorbed in the free interstitial voids in between the tubes within the bundles.     

Model for the hydrogen adsorption on carbon nanostructures

The hydrogen sorption capacity of carbon nanostructures was for several years a very controversial subject. Theoretical models have been published demonstrating a great potential for a large hydrogen sorption capacity of carbon nanostructures. Here we present a simple empirical model where condensation of hydrogen as a monolayer at the surface of nanotubes as well as bulk condensation in the cavity of the tube is assumed. The maximum potential amount of hydrogen absorbed according to the model was calculated to be 2.28×10^-3 mass%S[m²g^-1]=3.0 mass% for the adsorption of a monolayer hydrogen at the surface. The condensation of hydrogen in the cavity of the tube leads to a potential absorption for single wall nanotubes starting at 1.5 mass% and increasing with the diameter of the tubes. The experimentally measured hydrogen capacity of the nanotube samples correlates with the B.E.T. specific surface area. The slope of the linear relationship is 1.5×10^-3 mass%/m²g^-1. Therefore, the extrapolated maximum discharge capacity of a carbon sample is 2 mass%. Furthermore, it can be concluded, that the hydrogen sorption mechanism is related to the surface of the sample, i.e. a surface adsorption process. PACS 81.05.Uw; 81.07.De; 82.33.Pt     

Use of novel carbon-nanofiber-doped carbon liquid crystals as suitable adsorbents for hydrogen storage

Over the last few years a number of groups have been exploring the possibilities and limitations associated with using adsorption on newly developed materials for the storage of hydrogen. Here we report the results of hydrogen adsorption investigations on a newly synthesized form of carbon nanofiber-doped carbon liquid crystals. This material shows a marked increase in specific surface area upon activation with water vapor, leading to activated samples that are capable of adsorbing hydrogen upto 3.5% by weight at 77.3 K, at moderate pressures. The possibility of higher adsorption (6.5 wt.%) in these materials is also discussed. PACS 68.43.-h; 68.43.De; 68.60.-p     

Hydrogen storage in multi-wall carbon nanotubes using samples up to 85 g

Hydrogen storage in carbon nanotubes (CNTs) is investigated at ambient temperature and pressures of 0–12 MPa, using 35–85 g multi-wall carbon nanotube (MWNT) samples that were synthesized in a nano-agglomerate fluidized bed reactor. The volume of hydrogen gas released by the CNTs was measured by a volumetric flow meter. The capability of H₂ storage in the CNT samples of mass of up to 85 g can be obtained with a precision of 0.01 wt.%. MWNTs with average diameters ranging from 10–30 nm and were pretreated using nitric acid or a sodium hydroxide solution wash and a high temperature treatment. The influence of the hydrogen pressure, hydrogen storage time and treatment method were studied. All data show that the amount of hydrogen released by the MWNTs at room temperature is no more than 0.30 wt.%, while hydrogen released by MWNT at 77 K can reach 2.27 wt.%. PACS 61.46.+w; 68.43.-h; 84.60.Ve     

Hydrogen adsorption and desorption in carbon nanotube systems and its mechanisms

The hydrogen physisorption properties in single-walled carbon nanotube (SWNT) based materials were characterized. The SWNTs were highly purified and three useful pores for hydrogen physisorption were activated. Hydrogen was physisorbed in intra-tube pores at room temperature and the capacity was estimated to be about 0.3–0.4 wt.% at room temperature. The adsorption capacity can be explained by the Langmuir model. The intra-tube pores have large adsorption potential and this induces hydrogen physisorption at comparatively higher temperatures. This fact indicates the importance of fabricating sub-nanometer ordered pores for this phenomena. PACS 51.30.+i; 51.90.+r; 81.05.Tp; 81.07.De     

Hydrogen storage using physisorption – materials demands

A survey is presented of the storage capacities of a large number of different adsorbents for hydrogen at 77 K and 1 bar. Results are evaluated to examine the feasibility and perspectives of transportable and reversible storage systems based on physisorption of hydrogen on adsorbents. It is concluded that microporousadsorbents, e.g. zeolites and activated carbons, display appreciable sorption capacities. Based on their micropore volume (∼1 ml/g) carbon-based sorbents display the largest adsorption, viz. 238 ml (STP)/g, at the prevailing conditions. Optimization of sorbent and adsorption conditions is expected to lead to adsorption of ∼560 ml (STP)/g, close to targets set for mobile applications. Received: 9 January 2001 / Accepted: 27 January 2001 / Published online: 23 March 2001     

Reaction-Induced Transformations in Pt–Sn/SiO2 Catalysts: In Situ 119Sn Mössbauer Study

Reaction-induced separation of tin-rich surface layers and tin-depleted inner region was observed in metallic particles of Pt–Sn/SiO₂ catalysts in two reactions: (i) dechlorination of 1,2-dichloroethane at 473 K (modeling catalytic removal of chlorine from hazardous chlorocarbons) and (ii) oxidation of carbon monoxide at room temperature. In the former, a Pt:Sn catalyst (1:2 atomic ratio, 1 wt% metal content), prepared via co-impregnation, showed high selectivity (>80%) toward ethylene (at the expense of ethane), but only after a prolonged (ca. 24 h) period. In situ Mössbauer studies revealed stabilization of a homogeneous Pt–Sn alloy and SnCl₂ after activation in hydrogen; whereas tin-depleted and tin-rich components were separated after a 24-h period. Hence, inhibition of the hydrogenation activity of Pt, by surface tin enrichment and Cl deposition favors high ethylene selectivity. For the oxidation of CO at room temperature, a catalyst with a Pt:Sn atomic ratio of 3:2 (3 wt% Pt) was prepared by an organometallic (CSR) method using ¹¹⁹Sn(CH₃)₄. Platinum-rich PtSn(1) and tin-rich PtSn(2) components were separated in the Mössbauer spectra of catalyst activated at 570 K. The PtSn(2) component is primarily involved in surface reactions. Both in CO oxidation and the subsequent re-activation in hydrogen at room temperature a reversible PtSn(2) Sn⁴⁺ interconversion occurred. d ln(A ₇₇/A ₃₀₀)/dT data indicate the surface location of the involved components.     

Simulation of hydrogen adsorption in carbon nanotubes

Computer simulations are reported of hydrogen adsorption in multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs). The gas-solid interaction was modelled both as pure dispersion forces and also with a hypothetical model for chemisorption introduced in a previous paper (CRACKNELL, R., F., 2001, Phys. Chem. chem. Phys., 3, 2091). A two-centre model for hydrogen was employed and the grand canonical Monte Carlo methodology was used throughout. Uptake of hydrogen in the internal space of a carbon nanotube is predicted to be lower than in the optimal graphitic nanofibre with slitlike pores (provided the gas-solid potential is consistent). Part of the difference arises from the assumption of pore surface area used in converting the raw simulation data to gravimetric adsorption; however, the majority of the differences can be attributed to the curvature of the pore. This reduces the uptake of hydrogen (on a gravimetric basis) in spite of deepening the potential minimum inside the pore associated with dispersion forces. It is concluded that for the uptake of hydrogen in SWNTs of 5–10% reported by Heben (DILLON, A. C., JONES, K. M., BEKKEDAHL, T. A., KIANG, C. H., BETHUNE, D. S., AND HEBEN, M. J., 1997, Nature, 386, 377), gas-solid forces other than dispersion forces are required and most of the adsorption must occur in the interstices between SWNTs.     

Effects of catalysts on the internal structures of carbon nanotubes and corresponding electron field-emission properties

Using a chemical vapor deposition (CVD) method, multi-walled carbon nanotubes with uniform diameters of approximately 10 nm were synthesized on silicon substrates by the decomposition of acetylene using Fe, Co and Ni as the catalysts. Catalyst effects on the internal structures of the carbon nanotubes were evident in the Fe, Co and Ni catalyzed nanotubes. Although these nanotubes demonstrated similar morphologies, due to the variety of internal structures, the nanotubes synthesized from different catalysts demonstrated various electron field-emission characteristics including turn-on field, threshold field and field enhancement factor. Compared with carbon nanotubes from Ni catalyst, nanotubes from Fe and Co with the same diameters have better field-emission properties. Graphite layers in nanotubes from Fe and Co are much straighter and more parallel to the tube axis with fewer defects. For instance, the turn-on field and threshold field for nanotubes from Ni are 5 V/m and 9 V/m, respectively. These electric fields are much higher than those for nanotubes from Fe, which are 0.35 V/m and 2.8 V/m, respectively. This could be due to the effect of catalysts on the work function of nanotubes, since the catalyst particle usually terminates the free end of the nanotube, and the influence of internal structure on electron transportation along the nanotube axis. Therefore, this study suggests that besides a small diameter, good graphitization (crystallization) is an important prerequisite for a good carbon nanotube emitter. PACS 79.70.+q; 68.37.Lp; 81.07.De     

KrF excimer laser processing of thick diamond-like carbon films

Ablation characteristics of a commercially available diamond-like carbon (DLC) film (the EVERSCAN^TM bar code scanner window; Diamonex Division of Morgan Advanced Ceramics Inc.) in the 0–7 Jcm^-2 fluence window are reported. Glass pieces covered with 6 m thick DLC coating are ablated by 22 ns pulses of a KrF excimer laser in medium vacuum (10^-1 Pa), and in Ar and O₂ atmospheres of 10⁵ Pa. The ablation depth increases strictly linearly with an increasing number of pulses, independently of the atmosphere and the applied fluence. The threshold lies at 0.13 Jcm^-2 and is independent of the atmosphere. While above 1 Jcm^-2 the ablation rate vs. fluence plots recorded in different atmospheres coincide within experimental error, processing under vacuum between 0.5 and 1 Jcm^-2 results in significantly higher ablation rate values. Ablation rates exceeding 100 nm/pulse above 1 Jcm^-2 ensure extremely efficient machining over extended areas even at relatively low fluences. Identical characteristics in Ar and O₂ atmospheres suggest that solely physical (ablative) processes contribute to material removal. PACS 42.62.-b; 61.80.Ba; 68.35.Rh; 81.05.Uw     

Effects of carbon nanotube structures on mechanical properties

This paper describes a structural mechanics approach to modelling the mechanical properties of carbon nanotubes (CNTs). Based on a model of truss structures linked by inter-atomic potentials, a closed-form elastic solution is obtained to predict the mechanical properties of single-walled carbon nanotubes (SWNTs). Moreover, the elastic modulus of multi-walled carbon nanotubes (MWNTs) is also predicted for a group of the above mentioned SWNTs with uniform interval spacing. Following the structural mechanics approach, the elastic modulus, Poissons ratio, and the deformation behaviors of SWNTs were investigated as a function of the nanotube size and structure. Poissons ratio of SWNTs shows a chirality dependence, while the elastic modulus is insensitive to the chirality. The disposition of the strain energy of bonds shows quite a difference between the zigzag and armchair tubes subjected to axial loading. A zigzag tube is predicted to have a lower elongation property than an armchair tube. PACS 62.20-x; 62.20.Dc; 62.25+g     

Destabilization of LiBH4 by mixing with LiNH2

It was revealed that LiBH₄ is destabilized by mixing with LiNH₂ and the mixture desorbs a large amount of hydrogen. First-principles calculations predicted that the enthalpies of dehydrogenation for LiBH₄ alone and the mixture of LiBH₄+2LiNH₂ are 75 kJ/molH₂ and 23 kJ/molH₂, respectively. Motivated by this prediction, we experimentally examined the dehydrogenation properties for LiBH₄ and the mixture under hydrogen pressure. The amounts of desorbed hydrogen from LiBH₄ and the mixture at 703 K and 522 K were 10.6 mass% and 7.8 mass%, respectively. The dehydrogenation pressure of the mixture was much higher than that of LiBH₄ alone, although the mixture was measured at approximately 180 K lower temperature. This result suggests that the mixture is much unstable as compared with LiBH₄ alone.PACS 81.05.Zx; 71.20.Ps; 82.30.-b     

Production of individual suspended single-walled carbon nanotubes using the ac electrophoresis technique

We have developed a new method for aligning individual suspended single-walled carbon nanotubes (SWNTs) using a combination of the ac electrophoresis technique and electron beam lithography. A poly(methyl methacrylate) (PMMA) underpinning was used in the region between the Au electrodes to prevent the carbon nanotube (CNT) from falling down on to the substrate. O₂ plasma ashing was used to control the height of the underpinning PMMA layer so that it was same as that of the pre-defined electrodes. The biggest advantage of this method is that one can easily align the suspended nanostructure in a controllable manner. This method can also be applied to making suspended structures of organic materials that are sensitive to acid treatment. We measured the temperature-dependent I–V characteristics of the suspended SWNTs and found that most of the aligned SWNTs were metallic. PACS 61.46.+w; 73.63.Fg; 81.07.De; 81.16.Rf; 85.35.Kt     

Hydrogen storage using carbon adsorbents: past, present and future

Interest in hydrogen as a fuel has grown dramatically since 1990, and many advances in hydrogen production and utilization technologies have been made. However, hydrogen storage technologies must be significantly advanced if a hydrogen based energy system, particularly in the transportation sector, is to be established. Hydrogen can be made available on-board vehicles in containers of compressed or liquefied H₂, in metal hydrides, via chemical storage or by gas-on-solid adsorption. Although each method possesses desirable characteristics, no approach satisfies all of the efficiency, size, weight, cost and safety requirements for transportation or utility use. Gas-on-solid adsorption is an inherently safe and potentially high energy density hydrogen storage method that could be extremely energy efficient. Consequently, the hydrogen storage properties of high surface area “activated” carbons have been extensively studied. However, activated carbons are ineffective in storing hydrogen because only a small fraction of the pores in the typically wide pore-size distribution are small enough to interact strongly with hydrogen molecules at room temperatures and moderate pressures. Recently, many new carbon nanostructured absorbents have been produced including graphite nanofibers and carbon multi-wall and single-wall nanotubes. The following review provides a brief history of the hydrogen adsorption studies on activated carbons and comments on the recent experimental and theoretical investigations of the hydrogen adsorption properties of the new nanostructured carbon materials. Received: 16 October 2000 / Accepted: 15 November 2000 / Published online: 9 February 2001     

Diode-pumped passively Q-switched Nd : YAG laser at 1123 nm

A diode-pumped Nd:YAG laser at 1123 nm is passively Q-switched by using a low doping concentration Cr⁴⁺:YAG crystal as a saturable absorber. When pumped by a 1.5-W laser diode, the laser produces pulses of 50-ns duration with a pulse energy of as much as 15 J and a peak power of 300 W at a pulse-repetition rate of 10 kHz. PACS 42.60.Gd; 42.55.Rz; 42.55.Xi     

Preparation and hydrogen storage of activated rayon-based carbon fibers with high specific surface area

Activated carbon fibers were prepared from rayon-based carbon fibers by two step activations with steam and KOH treatments. Hydrogen storage properties of the activated rayon-based carbon fibers with high specific surface area and micropore volume have been investigated. SEM, XRD and Brunauer-Emmett-Teller (BET) were used to characterize the samples. The adsorption performance and porous structure were investigated by nitrogen adsorption isotherm at 77 K on the base of BET and density functional theory (DFT). The BET specific surface area and micropore volume of the activated rayon-based carbon fibers were 3144 m²/g and 0.744 m³/g, respectively. Hydrogen storage properties of the samples were measured at 77 and 298 K with pressure-composition isotherm (PCT) measuring system based on the volumetric method. The capacities of hydrogen storage of the activated rayon-based carbon fibers were 7.01 and 1.46 wt% at 77 and 298 K at 4 MPa, respectively. Possible mechanisms for hydrogen storage in the activated rayon-based carbon fibers are discussed.     

Experimental demonstration of a photonic-crystal-fiber optical diode

Two cascaded hollow-core photonic-crystal fibers with slightly shifted, but still overlapping, transmission peaks are shown to function as an optical diode for ultra-short laser pulses. Submicrojoule 100-fs Ti:sapphire laser pulses with a spectrum falling within the passband of one of the fibers, but outside the passband of the second fiber, experience spectral broadening due to self-phase modulation in the first fiber. A part of this self-phase-modulation-broadened spectrum is then transmitted through the second fiber. Identical short pulses propagating in the opposite direction are blocked by the second fiber with a shifted passband. A forward-to-backward signal ratio exceeding 40 is achieved with the created photonic-crystal fiber diode for 0.9-J, 100-fs pulses of 800-nm Ti:sapphire laser radiation. PACS 42.65.Wi; 42.81.Qb     

Iron Nanoparticles in Severe-plastic-deformed Copper

Superparamagnetic iron nanoparticles with a size of a few nanometers were produced in copper by severe plastic deformation. In a isochronal annealing experiment near a temperature of 450K, which corresponds to the temperature of structural relaxation and the first step of grain growth (from 128 to 150nm) of submicrocrystalline copper, an abrupt increase in the magnetic susceptibility is detected. This increase is shown to be due to iron nanoparticles increasing in size from 2.8 to 3.3nm. The vanishing of the ferromagnetic contribution by iron nanoparticles observed at 850K, well below the Curie temperature of iron, is due to the dissolution of nanoparticles in plastically deformed copper.     

Compact efficient intracavity optical parametric oscillator with a passively Q-switched Nd : YVO<Subscript>4</Subscript>/Cr<Superscript>4+</Superscript> : YAG laser in a hemispherical cavity

A compact eye-safe optical parametric oscillator (OPO) using a noncritically phase-matched KTP crystal intracavity pumped by a passively Q-switched Nd:YVO₄ laser is experimentally demonstrated. To enhance the performance of passive Q-switching, a Cr⁴⁺:YAG saturable absorber crystal is coated as an OPO output coupler in a nearly hemispherical cavity. With an incident pump power of 2.5 W, the compact intracavity OPO cavity, operating at 62.5 kHz, produces average powers at 1573 nm up to 255 mW and peak powers higher than 1 kW. PACS 42.60.Gd; 42.65.Yj; 42.55.Xi