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     

Correlation of hydrogen capacity in carbon material with the parameters of electrosorption

Electrochemical storage of hydrogen in activated carbon material has been investigated using different parameters of cathodic polarization. It has been proven that application of short galvanostatic pulses could be efficient for hydrogen storage in microporous carbon material. Charging current loads from 50 mA g⁻¹ to 32 A g⁻¹ have been used showing correlation between hydrogen capacity, time of charging and electrical efficiency. The anodic charge equivalent to electrooxidation of 1.0 wt% of hydrogen can be already reached after 90 s of cathodic polarization. Temperature effect has been also evaluated and a gradual increase of hydrogen capacity with a better pronounced oxidation plateau was obtained at higher temperatures. Reversible electrosorption of hydrogen is a useful reaction in supercapacitor performance and it might have a potential application for a negative electrode of supercapacitor as well as reversibly operating electrode in the secondary cell.      

Catalytic Asymmetric Epoxidation of Aldehydes with Two VANOL‐Derived Chiral Borate Catalysts

A highly diastereo‐ and enantioselective method for the epoxidation of aldehydes with α‐diazoacetamides has been developed with two different borate ester catalysts of VANOL. Both catalytic systems are general for aromatic, aliphatic, and acetylenic aldehydes, giving high yields and inductions for nearly all cases. One borate ester catalyst has two molecules of VANOL and the other only one VANOL. Catalysts generated from BINOL and VAPOL are ineffective catalysts. An application is shown for access to the side‐chain of taxol.     

On pole terms in the fixed angle approach to photoproduction of pions on nucleons

This note has two aims. The first one is to study the properties of well known pole contributions to photoproduction amplitudes but in the space of c.m. three-momentum squared and scattering angle. Some conclusions essential to the fixed angle approach are drawn. The second aim is to give some results of more general character. These are plots and numerical tables of pole contributions to multipole amplitudes.     

Colloid‐Assisted Self‐Assembly of Robust,Three‐Dimensional Networks of Carbon Nanotubes over Large Areas

Natural materials, such as bone and spider silk, possess remarkable properties as a result of sophisticated nanoscale structuring. They have inspired the design of synthetic materials whose structure at the nanoscale is carefully engineered or where nanoparticles, such as rods or wires, are self‐assembled. Although much work has been done in recent years to create ordered structures using diblock copolymers and template‐assisted assembly, no reports describe highly ordered, three‐dimensional nanotube arrays within a polymeric material. There are only reports of two‐dimensional network structures and structures on micrometer‐size scales. Here, we describe an approach that uses plasticized colloidal particles as a template for the self‐assembly of carbon nanotubes (CNTs) into ordered, three‐dimensional networks. The nanocomposites can be strained by over 200% and still retain high conductivity when relaxed. The method is potentially general and so may find applications in areas such as sensing, photonics, and functional composites.

     

Mixing study of the induction plasma reactor: Part I. Axial injection mode

A systematic study of the gas-mixing pattern in an induction plasma reactor under atmospheric and low pressure conditions is reported. Different reactor configurations were investigated in which nitrogen is injected as an auxiliary gas either axially into an Ar/H₂, discharge in the center of the induction coil region, or radially through multiple orifices, into the plasma jet at tire exit nozzle of the torch. Concentration mapping in the mixing zone was carried out, using a VG-Microniass-PC 300 D mass spectrometer at plasma power levels and reactor pressures, in the range of 13–24 k 6V and 35–93 kPa, respectively. Comparison of these results with cold-flow measurements underlined the substantial difference in the mixing pattern in each of these two cases. A considerably faster mixing of the gases is noted under cold flow conditions compared to that in the presence of the discharge. The results are discussed from the viewpoint of their use for optimum reactor design applied to tire vapor-phase synthesis of ultrafine ceramic powders, using induction plasma technology.     

A dispersion analysis of pion photoproduction from nucleons. II

The paper gives an account of numerical results forl=0 andl=1 multipole amplitudes of pion photoproduction. They have been obtained by solving integral equations which originate from fixed angle dispersion relations that have been deduced in an earlier paper. In this Part II presented are solutions of truncated integral equations for photoproduction amplitudes, which take into account coupling with the most important transitionsM ₁₊ ⁽³⁾ (in the isovector case) andE ₀₊ ⁽⁰⁾ (in the isoscalar case). Discussed are implications of such an approach and a comparison of results obtained by a different method is being made.     

A dispersion analysis of pion photoproduction from nucleons. I

The paper gives an account of numerical results forl=0 andl=1 multipole amplitudes of pion photoproduction. They have been obtained by solving integral equations which originate from fixed-angle dispersion relations that have been deduced in an earlier paper. In this Part I outlined are some difficulties usually encountered in course of solving the problem of pion photoproduction by dispersion techniques, and proposed is an approach which introduces arbitrary parameters — cutoffs in the integral equation for transition amplitudes. Numerical results show some implications of this philosophy on the rôle played by the terms representing physical singularities and terms representing left-hand singularities controlled by one-particle (Born) contributions.     

In-Flight Spheroidization of Alumina Powders in Ar–H<Subscript>2</Subscript> and Ar–N<Subscript>2</Subscript> Induction Plasmas

In-flight spheroidization of alumina powders in Ar–H₂ (H₂–7.6%, vol/vol) and Ar–N₂ (N₂–13.0%, vol/vol) RF induction plasmas was investigated numerically and experimentally. The mathematical model for the plasma flows incorporates the k– turbulence model, and that for particles is the Particle-Source-in-Cell (PSI-Cell) model. Experimental results demonstrate that spheroidized alumina particles are produced in both Ar–H₂ and Ar–N₂ RF plasmas, with different particle size distributions and crystal phases. Agreement between the predicted and measured particle size distributions is satisfactory under high particle feed rate conditions, while the results obtained for the Ar–H₂ plasma are better than those for the Ar–N₂ plasma. The discrepancy occurring in low feed rate conditions suggests that particle evaporation is an important factor affecting the plasma–particle heat transfer.     

Parametric study of the decomposition of NH3 for an induction plasma reactor design

The present paper reports a study of the gas mixing and chemical transformation in an induction plasma reactor under atmospheric pressure, and its dependence on the plasma operating conditions. For this purpose, the thermal dissociation of ammonia into nitrogen and hydrogen was chosen because of the relative simplicity of the reactions involved and its use in a number of studies on plasma synthesis of ultrafine nitride ceramic powders using ammonia as nitriding agent. A hot-wall reactor configuration is investigated in which ammonia is injected radially through multiple orifices into the gases at the exit nozzle of an induction plasma torch. Concentration mapping in the mixing zone was carried out, using a VG-Micromass-PC 300 D quadrupole mass spectrometer, for different plasma power levels, in the range 13–24 kW. A 3-point injection mode is used with the injection ports oriented upstream at 45° to the torch axis. This allows uniform mixing of the injected gas in the plasma jet. A systematic study of the effects of plate power and ammonia and plasma gas flow rates on the mixing and dissociation of NH₃ in the reactor is reported. The results are analyzed and discussed from the viewpoint of their use for optimizing the design of induction plasma reactors, to he applied to the vapor-phase synthesis of ultrafine silicon nitride powders.