Characterization of multi-wall carbon nanotubes and their applications

PhDCarbon nanotubes (CNT) and their applications is a field which has attract a lot of interest in the past two decades. Since the first invention of CNTs in 1991, and in view of utilising nanoantennas, the focus in many laboratories around the world has shifted to trying to lengthen nanotubes longer from nanometers to few centimeters. Eventually this could lead to CNTs’ use in sub-millimeter, millimiter wave and microwave antenna applications. In this thesis, fundamental properties of carbon nanotube films are investigated, and some applications such as the use of CNTs as absorbers or CNT doped liquid crystals are considered. The concept of frequency tunable patch antennas is also presented. Simulation and measurement results of the liquid crystal based antenna show that frequency tuning is possible, through the use of a liquid crystal cell as a substrate. Additionally, greater tuning can be achieved using liquid crystals with higher dielectric anisotropy at microwave frequencies. This can be achieved by using CNT doped liquid crystals. As mentioned, microwave and terahertz measurements of vertically aligned carbon nanotube arrays placed on the top surface of a rectangular silicon substrate are presented. The S-parameters are calculated allowing the extraction of the complex permittivity, permeability and conductivity of the samples. Theoretical models are being introduced delineating the behaviour of the multi-walled nanotube (MWNT) samples. The material properties of this film provide useful data for potential microwave and terahertz applications such as absorbers. Finally, finite-difference time-domain (FDTD) modelling of CNTs is introduced, verifying the measurements that have been performed, confirming that CNT arrays can be highly absorptive. A novel estimation of the permittivity and permeability of an individual carbon nanotube is presented and a periodic structure is simulated, under periodic boundary conditions, consisting of solid anisotropic cylinders. In addition, the optical properties of vertically aligned carbon nanotube (VACNT) arrays, when the periodicity is both within the sub-wavelength and wavelength iii regime are calculated. The effect of geometrical parameters of the tube such as length, diameter and inter-tube distance between two consecutive tubes are also examined

Computational-experimental study of the onset potentials for CO2 reduction on polycrystalline and oxide-derived copper electrodes

The electrocatalytic reduction of CO2 (CO2RR) is a promising yet intricate process to alleviate the alarming imbalance in the carbon cycle. One of the intricacies of CO2RR is its structural sensitivity, which is illustrated by the varying onset potentials and selectivities of the reaction products depending on the electrode morphology. Here, using electrochemical real-time mass spectrometry (EC-RTMS), we accurately determine the onset potentials for seven CO2RR products including C1, C2, and C3 species on polycrystalline and oxide-derived Cu electrodes. Density functional theory calculations affordably including solvent and cation effects produce onset potentials of C2 species matching those obtained with EC-RTMS. Our analysis leads us to conclude that the elusive active sites at oxide-derived Cu, known to enhance ethanol production, are undercoordinated square ensembles of Cu atoms

Microwave absorption and radiation from large-area multilayer CVD graphene

Here we experimentally study the microwave absorption and near-field radiation behaviour of monolayer and few-layer, large-area CVD graphene in the C and X bands. Artificial stacking of CVD graphene reduces the sheet resistance, as verified by non-contact microwave cavity measurements and four-probe DC resistivity. The proposed multilayer stacked graphene exhibits increased absorption determined by the total sheet resistance. The underlying mechanism could enable us to apply nanoscale graphene sheets as optically transparent radar absorbers. Near-field radiation measurements show that our present few-layer graphene patches with sheet resistance more than 600 /sq exhibit no distinctive microwave resonance and radiate less electromagnetic power with increasing layers; however, our theoretical prediction suggests that for samples to be significant as microwave antennas, doped multilayer graphene with sheet resistance less than 10 /sq is required.This work was funded by the Graphene Research Centre, University of Cambridge, and the Engineering and Physical Sciences Research Council (EPSRC), UK under a Program Grant (EP/K01711X/1). M.T.C thanks the Winston Churchill Trust and the International Young Scientist Research Fellowship, National Natural Science Foundation of China, for generous financial support. B.W. acknowledges the fund support from the China Scholarship Council and National Natural Science Foundation of China No. 61271017.This is the FINAL published version, also available on the publisher's website at: http://www.sciencedirect.com/science/article/pii/S000862231400540

Electrocatalytic reduction of Nitrate on Copper single crystals in acidic and alkaline solutions

Catalysis and Surface Chemistr

Selective and quantitative nitrate electroreduction to ammonium using a porous copper electrode in an electrochemical flow cell

International audienceThe aim of this work was to set up a novel electrochemical system allowing an efficient transformation of concentrated nitrate solutions to ammonium and which can be subsequently implemented on a large scale application. First, this paper describes the preparation of a porous copper modified electrode by successive electrodeposition of nickel then copper on a graphite felt of large specific surface area. Homogeneous Cu coating of all fibers in the 3D porous structure was successfully obtained using low concentrations of copper salts and high applied current intensities. The porous copper electrode was then used in a flow electrochemical process to achieve a selective and quantitative transformation of concentrated nitrate into ammonium. Different electrolytic solutions, slightly acid (acetate buffer) or neutral (phosphate buffer), and flow rates were investigated. The nitrate solution was quantitatively reduced into NH4+ with high selectivity in only one pass through the electrode. When the applied current was similar to the theoretical one, the maximum selectivity (96%) and the best current efficiency (72%) for NH4+ formation were reached at pH 7.2 with a flow rate of 2 mL min−1. The obtained ammonium solution can be subsequently used either as a potential nitrogen source during microbial culture or simply as a fertilizer

Interconversions of nitrogen-containing species on Pt(100) and Pt(111) electrodes in acidic solutions containing nitrate

This work deals with the interconversions of various nitrogen-containing compounds on Pt(111) and Pt(100) electrodes in contact with acidic solutions of nitrate. Via its reduction, nitrate acts merely as the source of adsorbed nitrogen-containing intermediates, which then undergo complex oxidative or reductive transformations depending on the electrode potential. Nitrate reduction to ammonium is structure sensitive on Pt(111) and Pt(100) because it is mediated by *NO, the adsorption and reactivity of which is also structure sensitive. Accordingly, previous knowledge from *NO electrochemistry is useful to streamline nitrate reduction and elaborate a comprehensive picture of nitrogen-cycle electrocatalysis. Our overall conclusion for nitrate reduction is that the complete conversion to ammonium under prolonged electrolysis is possible only if the reduction of nitrate to nitric oxide, and the reduction of nitric oxide to ammonium are feasible at the applied potential. Among the two surfaces studied here, this condition is fulfilled by Pt(111) in a narrow potential region. (C) 2018 Elsevier Ltd. All rights reserved.Catalysis and Surface Chemistr

Towards an efficient liquid organic hydrogen carrier fuel cell concept

The high temperature required for hydrogen release from Liquid Organic Hydrogen Carrier (LOHC) systems has been considered in the past as the main drawback of this otherwise highly attractive and fully infrastructure-compatible form of chemical hydrogen storage. According to the state-of-the art, the production of electrical energy from LOHC-bound hydrogen, e.g. from perhydro-dibenzyltoluene (H18DBT), requires provision of the dehydrogenation enthalpy (e.g. 65 kJ mol-1(H2) for H18-DBT) at a temperature level of 300 °C followed by purification of the released hydrogen for subsequent fuel cell operation. Here, we demonstrate that a combination of a heterogeneously catalysed transfer hydrogenation from H18-DBT to acetone and fuel cell operation with the resulting 2-propanol as a fuel, allows for an electrification of LOHC-bound hydrogen in high efficiency (> 50 %) and at surprisingly mild conditions (temperatures below 200 °C). Most importantly, our proposed new sequence does not require an external heat input as the transfer hydrogenation from H18-DBT to acetone is almost thermoneutral. In the PEMFC operation with 2-propanol, the endothermal proton release at the anode is compensated by the exothermic formation of water. Ideally the proposed sequence does not form and consume molecular H2 at any point which adds a very appealing safety feature to this way of producing electricity from LOHC-bound hydrogen, e.g. for applications on mobile platforms.Fil: Sievi, Gabriel. Forschungszentrum Jülich; AlemaniaFil: Geburtig, Denise. Universitat Erlangen-Nuremberg; AlemaniaFil: Skeledzic, Tanja. Forschungszentrum Jülich; AlemaniaFil: Bösmann, Andreas. Universitat Erlangen-Nuremberg; AlemaniaFil: Preuster, Patrick. Forschungszentrum Jülich; AlemaniaFil: Brummel, Olaf. Universitat Erlangen-Nuremberg; AlemaniaFil: Waidhas, Fabian. Universitat Erlangen-Nuremberg; AlemaniaFil: Montero, María de Los Angeles. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; ArgentinaFil: Khanipour, Peyman. Forschungszentrum Jülich; AlemaniaFil: Katsounaros, Ioannis. Forschungszentrum Jülich; AlemaniaFil: Libuda, Jörg. Universitat Erlangen-Nuremberg; AlemaniaFil: Mayrhofer, Karl J. J.. Forschungszentrum Jülich; AndorraFil: Wasserscheid, Peter. Universitat Erlangen-Nuremberg; Alemani