Dissertations / Theses on the topic 'Electrodes, Carbon'

Carbon materials, such as graphite and conducting diamond, are highly popular for analytical and electrochemical applications, and fundamental knowledge of heterogeneous electron transfer is required to understand and optimise applications. In this thesis, the relationship between the structure of HOPG (Highly Oriented Pyrolytic Graphite) and its electrochemical behaviour has been thoroughly studied from the macroscale to the nanoscale. With the use of data collected from a wide range of techniques, spanning voltammetry, electrochemical imaging and high resolution microscopy, on 5 different grades of basal plane HOPG whose surfaces vary in defect density, the contribution of edge plane vs. basal plane on the electrochemical activity of HOPG has been re-examined. The significant body of work presented herein shows, without doubt, that the basal plane of HOPG is a very active electrode for Ru(NH)6 3+/2+; Fe(CN)6 4-/3-; the oxidation of the neurotransmitter, dopamine (DA), and quinones in aqueous solution. This overturns a well-established (textbook) model that the basal surface is inert, which researchers have assumed for two decades, with implications that carry over to related sp2 carbon materials, such as graphene and carbon nanotubes. A second aspect has considered polycrystalline boron-doped diamond (pBDD) to study neurotransmitters, such as DA and serotonin (5-HT). The electrode surface was found to be resistive towards permanent surface blocking during the electrochemical oxidation of these neurotransmitters. The properties of the film formed by 5-HT oxidative products, was thoroughly investigated using voltammetry and high resolution microscopy. It is shown, for the first time, that electro-oxidation of 5-HT results in an electrically insulating, but charged and porous film, but procedures are demonstrated that allow the pBDD to be renewed in-situ for precise electroanalysis.

Carbon electrodes have found widespread use in electrochemistry due to its broad versatility and low cost amongst other advantages. Recent innovations in carbon materials have added new dimensions to their utility in electrochemical applications. This thesis aims to investigate aspects of carbon materials, in particular boron-doped diamond (BDD) and nanocarbon composites, mainly for electrochemical analysis and energetics studies. The electrochemical behaviour of estradiol and other endocrine disrupting compounds was examined on the BDD electrode with different surface pretreatments, as well as on a nanocarbon-modified BDD electrode. It was shown that the precise control of surface chemical termination enabled the electrode to be tuned to exhibit diffusional or adsorptive voltammetry at oxidised and hydrogenated BDD interfaces respectively. Adsorption effects were also observed on the modified electrode leading to significant pre-concentration of the analyte onto the nanocarbon and a corresponding lowering of the limit of detection by ca three orders of magnitude to nanomolar levels. Surface modification of the BDD electrodes was then explored using a simple and convenient dropcast technique to deposit microcrystalline copper phthalocyanine onto the electrode. The influence of the surface chemical termination towards the interaction with the modifier compound was demonstrated in relation to the oxygen reduction reaction. Hydrogen terminated BDD modified in such a manner was able to significantly decrease the overpotential for the cathodic reaction by ca 500 mV when compared to the unmodified electrode while modified oxidised BDD showed no such electrocatalysis, signifying greater interaction of the phthalocyanine modifier with the hydrogenated surface. The lack of a further conversion of the peroxide product was attributed to its rapid diffusion away from the triple phase boundary (comprising the phthalocyanine microcrystallite, aqueous solution and BDD electrode) at which the reaction is expected to exclusively occur. Next carbon composites were studied in the form of carbon paste electrodes (CPEs). The practicality of a nanocarbon paste was established by cyclic voltammetry with several well-characterised redox systems commonly used to test electrode activity and was found to exhibit comparable behaviour to the more typical graphitic formulation. Reversible uptake of some analytes was observed at the CPEs, giving rise to complex double peak voltammetry. This uptake phenomenon was then further examined at the nanocarbon paste electrode to monitor the transfer of species between two dissimilar liquid phases. The interfacial behaviour gave rise to voltammetric peaks which were assigned to species originating from the aqueous, binder and carbon phases respectively and this enabled the measurement of Gibbs energies of transfer between oil and aqueous phases. Finally the effect of different ionic liquids as binder for carbon-ionic liquid composite electrodes was studied. Some ionic liquids were demonstrated to offer benefits in comparison to oil in the fabrication of carbon paste type electrode due to an increased adsorption of analytes. The ionic “liquid” (with a melting point above room temperature) n-octyl-pyridinium hexafluorophosphate [C₈py][PF₆] was shown to be useful in combination with carbon nanotubes as a composite electrode or as a modifier to a screen-printed electrode to significantly enhance the sensitivity of electrochemical detection via adsorptive stripping voltammetry. Overall the carbon-based electrodes studied have demonstrated excellent utility as electrode materials in the areas of electrochemical sensing and energetics investigations.

The work presented in this thesis is concerned with electrodes modified with carbon nanotubes. Carbon nanotubes have been characterised with special emphasis on the oxygenated species generated from cutting in acid mixtures. Several different techniques have been used for the analysis, especially infrared spectroscopy (IR) in combination with X-ray spectroscopy (XPS) analysis and transmission electron microscopy (TEM) in combination with atomic force microscopy (AFM). TEM analyses were used to reveal the morphological differences between various nanotube cutting times. The lengths of the nanotube were found to decrease with increasing cutting time. Electrochemical measurements were performed on carbon nanotube modified electrodes using nanotubes of different cutting time. The peak separation of ferricyanide redox reaction was found to depend strongly on the length of nanotube and also on the orientation of nanotube at the interface. Whilst at the randomly dispersed, the peak separation showed a decrease with decreasing nanotube length, vertically aligned nanotubes showed no dependence of the peak separation on the nanotube length. Electrochemical results together with spectroscopy measurements show that the highly electroactive edge planes were located on the carbon nanotubes and the oxygenated species in the ends of the nanotubes from cutting in acid mixtures were responsible for the good electrochemical properties. Bamboo-shaped carbon nanotube is a morphological variation of multi-walled carbon nanotubes where the graphite planes are formed at an angle to the axis of the tube. Glassy carbon electrodes modified with bambootype carbon nanotubes showed greater electrochemical signal compared with electrodes modified with singlewalled carbon nanotubes due to the edge planes of graphite located at regular intervals along the walls of the bamboo-shaped carbon nanotube, thus confirming the importance of the ends of nanotube in controlling the kinetics of electron transfer events. Effect of nanotube orientation was studied using ferrocenemethylamine attached to randomly dispersed and vertically aligned nanotubes. The electron transfer kinetics was found to depend strongly on the orientation of the nanotube with the electron transfer at the randomly dispersed slower than vertically aligned. Results were addressed using the analogy that the ends of the nanotubes are like the ends of the tubes can be described as edge-plane-like whilst the tube walls are basal-plane-like. Difference in electron transfer kinetics suggested that the electron transfer in nanotubes could occur via two different pathways: through the edge plane-like opening of the nanotube or by hopping across the walls of the nanotube. Triton X-100 was used to dialyse the acid cut nanotubes. XPS analysis of dialysed nanotubes was compared with non-dialysed nanotubes. A reduced concentration of sulfate ions was found in the dialysesd sample. Nitrate ion (407 eV) was removed after dialysis. Amino groups (400 ev) and protonated amino-group (402 eV) both seemed to be removed slowly by dialysis. Theses ions could be ascribed to residual ions trapped inside nanotubes from cutting in acid mixtures. The electrochemical response of ferrocenemethylamine was also studied. The electron transfer rate constants were rate constants were higher at dialysed nanotube assembly than non-dialysed, which was attributed to doping effect incurred from cutting. Electron transfer between nanotube and gold electrode surface was studied by attaching nanotubes to linker length of 6, 8, and 11 carbons. The results were exploited to rationalise the role of the chemical structure of the nanotubes in facilitating electron transfer from the redox species to the electrode surface that was otherwise suppressed without the presence of nanotubes. The observed redox activity was a consequence of resonant electron transfer from the LUMO of the acceptor to the HOMO of the donor under the influence of an applied voltage, assuming the nanotube modified electrode behaves similarly to the metal-molecule-metal junction mode.

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2013.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 80-85).
Capacitive deionization (CDI) is a desalination method where voltage is applied across high surface area carbon, adsorbing salt ions and removing them from the water stream. CDI has the potential to be more efficient than existing desalination technologies for brackish water, and more portable due to its low power requirements. In order to optimize salt adsorption in CDI, we need a better understanding of salt adsorption and the electrode properties involved in ion removal. Current materials are highly porous, with tortuous geometeries, overlapping double layers, and subnanometer diameters. In this work, we design ordered-geometry, vertically-aligned carbon nanotube electrodes. The CNTs in this study have 2-3 walls, inner diameter of 5.6 nm and outer diameter of 7.7 nm. The capacitance and charging dynamics were investigated using three-electrode cell testing in sodium chloride solution. We found that the material capacitance was 20-40 F/g and the charging time varies linearly with CNT height. The data was matched with the Gouy-Chapman-Stern model indicating that porous effects were negligible. Charging rates of CNTs compared to microporous activated carbon fiber, show that CNTs are more efficient at charging by weight. However, densification and surface functionalization will be necessary to enhance CNT performance by planar area. Future work will involve investigating electrodes in a flow-through cell to use salt adsorption data to determine the influence on electrode thickness on salt adsorption in channel flow.
by Heena K. Mutha.
S.M.

Electrochemistry finds widespread applications in the field of chemical analysis, so-called electroanalysis, as well as in electrosynthesis. The results obtained can be highly dependent on the chemical nature of the electrode used, and chemically modified electrodes are often employed to fine tune the electrochemistry to suit particular applications. This thesis is concerned with the investigation of the use of carbon materials, both as electrode substrates and electrode modifiers, primarily for electrochemical analysis. The work has been carried out using a range of electrochemical voltammetric techniques, as well as impedance measurements. A number of physical methods, including electron microscopy and X-ray photoelectron spectroscopy, have also been used to determine the physical and chemical structures of the materials employed. The use of boron-doped diamond electrode (BDDE) for the electrochemical detection of H₂O₂ was explored. Although BDDE shows no useful electrochemical response to H₂O₂, a good electrochemical signal can be obtained if the electrode is modified with silver nanoparticles or haemoglobin. The best results are obtained using electrode interfaces fabricated by binding haemoglobin in an active form to silver nanoparticles prepared by electrodeposition on the BDDE in the presence of the surfactant CH₃(CH₂)₁₅Br, permitting state of the art performance with a limit of detection (LOD) < 0.5μM. The presence of haemoglobin at the BDDE surface is also capable of catalysing the electrochemical reduction from Ag⁺(aq) to silver particles. However it reduces their adhesion to the electrode surface, hence they are lost to solution. This observation was used to demonstrate a viable process for the electrosynthesis of Ag nanoparticles, producing particles of about 10 nm diameter at a yield of approximately 50%. The effects of modifying a glassy carbon electrode with various forms of nanocarbon material for the electrochemical detection of phenolic compounds including hydroquinone (HQ) and dihydroxybenzene (DHB) were studied. The nanocarbons considered included carbon black, graphene nanoplatelets and nanodiamond, for which the former two materials were found to show a large increase in the detection sensitivity. It was shown that the simultaneous detection of HQ and DHB was possible using these electrodes, and in 'real' samples such as river water and green tea. Additional modification of the electrodes with tyrosine also permitted detection of phenol and p-cresol. The nanodiamond and carbon black modified electrodes were also employed for the electrochemical detection of Bisphenol A (BPA), which can be severely hampered by electropolymerisation of the oxidation products of BPA, producing rapid electrode fouling. However because of the inert nature of diamond surfaces, it is shown that this fouling process can be minimised by modifying the glassy carbon electrode with nanodiamond. Alternatively, it was also observed that for the carbon black modified electrode, a strong electrochemical response could be seen associated with the quinone forms produced by BPA oxidation. The associated electrochemical signal is also found to be relatively insensitive to electrode fouling, opening up an alternative strategy for the detection of BPA. Finally the use of a carbon black modified glassy carbon electrode for the detection of dopamine in the presence of the interfering compounds, ascorbic and uric acids, was studied. The carbon black modifier is shown to increase detection sensitivity, and help separate the electrochemical signals of the differing redox active species present in the solution.

Optically transparent carbon based nanomaterials including graphene and carbon nanotubes(CNTs) are promising candidates as transparent conductive electrodes due to their high electrical conductivity coupled with high optical transparency, can be flexed several times with minimal deterioration in their electronic properties, and do not require costly high vacuum processing conditions. CNTs are easily solution processed through the use of surfactants sodium dodecyl sulfate(SDS) and sodium cholate(SC). Allowing CNTs to be deposited onto transparent substrates through vacuum filtration, ultrasonic spray coating, dip coating, spin coating, and inkjet printing. However, surfactants are electrically insulating, limit chemical doping, and increase optical absorption thereby decreasing overall performance of electrodes. Surfactants can be removed through nitric acid treatment and annealing in an inert environment (e.g. argon). In this thesis, the impact of surfactant removal on electrode performance was investigated. Nitric acid treatment has been shown to p-dope CNTs and remove the surfactant SDS. However, nitric acid p-doping is naturally dedoped with exposure to air, does not completely remove the surfactant SC, and has been shown to damage CNTs by creating defect sites. Annealing at temperatures up to 1000°C is advantageous in that it removes insulating surfactants. However, annealing may also remove surface functional groups that dope CNTs. Therefore, there are competing effects when annealing CNT electrodes. The impacts on electrode performance were investigated through the use of conductive-tip atomic force microscopy, sheet resistance, and transmittance measurements. In this thesis, the potential of graphene CNT composite electrodes as high performing transparent electrodes was investigated. As-made and annealed graphene oxide CNT composites electrodes were studied. Finally, a chemical vapor deposition grown graphene CNT composite electrode was also studied.

This thesis describes experimental work on electrochemical sensing mechanisms. Chapter 1 and Chapter 2 provide an introduction to electrochemical and surface science techniques as well as nano-carbon materials which are of interest in electroanalysis and sensing. Chapter 3 and Chapter 4 focus on electrochemical processes at liquid | liquid | electrode triple phase boundary systems. In Chapter 3 the electrochemical behaviour of CoPc (cobalt phthalocyanine) dissolved into an organic water –insoluble liquid and deposited as microdroplets on a graphite electrode is studied. Both cation and anion transfer are observed at the liquid | liquid phase boundary. Chapter 4 describes redox processes of a highly hydrophobic anthraquinone derivative where preferential transfer of protons and pH sensitivity are observed. Both systems, CoPc and anthraquinone derivative, are investigated towards CO2 sensitivity. In Chapter 3 and 4 graphite electrodes are employed, but in Chapter 5 graphitic carbon nanoaprticles are employed with a surface functionalisation to provide binding capability to DNA fragments. Layer-by-layer deposition of DNA-carbon nanoparticle composite film electrodes is demonstrated and the electrochemical properties of the films are investigated. A novel type of DNA hybridisation sensing mechanism based on a nano-gap generator – collector electrode system is proposed. Chapters 6 and 7 are dedicated to gas sensing with a novel electrochemical system based on ionomer spheres in contact to the working electrode. In Chapter 6 Dowex ionomer particles are impregnated with carbon nanoparticles which are functionalised with DOPA to provide redox activity and Faradaic current responses. The effect of ionomer type and gas composition is studied. In Chapter 7 Prussian blue nanoparticles are immobilised onto the ionomer particle surface to provide a sensing system with peroxide sensitivity. Overall, this thesis contributes to sensing of bio-molecules and of gases. By introducing new types of interfaces (triple phase boundary, ionomer contacts, carbon nanoparticle redox systems) it is shown that sensitivity and selectivity can be tailored. In future these types of sensor prototypes could be further developed for specific applications.

In this project, different molecules have been investigated with the purpose of creating anohmic contact between metals and carbon nano materials. In particular, we considered simplemolecules connecting a graphene layer and a copper-slab. In order to determine the capability of such systems, the electronic structure was computedusing Density Functional Theory (DFT). Structural relaxation was performed in order to findcandidates where the metal and the graphene binds chemically with the hypothesis that thehybridization of the states will induce more states at the Fermi level. Six different molecularchains were tested and three of them were found to chemisorb to the graphene sheet and thecopper surface simultaneously. The electronic properties for these systems were then furtherinvestigated using the density of states (DOS). An overlap density of states (ODOS) wasdefined in order to evaluate the respective contribution of the graphene, metal and molecule. From the DOS analysis, we report that these systems did not form ohmic contacts as the resultshows too few states close to the Fermi level. The most interesting case was using a hexanolchain which had some partially overlapping states seen from the ODOS of the graphenemoleculeand graphene-Cu at the Fermi level. However, these were only small contributions.Further research is crucial in order to find a more suitable molecular chain between thegraphene and the copper for an ohmic contact.

Thesis (Ph. D.)--Textile and Fiber Engineering, Georgia Institute of Technology, 2007.
Committee Chair: Satish Kumar; Committee Member: Anselm Griffin; Committee Member: John D. Muzzy; Committee Member: Ravi Bellamkonda; Committee Member: Rina Tannenbaum.

Chapter1 gives an overview of the basic principles of electrochemistry. A rigorous electrochemical study on the solution phase and solid phase cobalt phthalocyanine (CoPC) is presented in chapter2. The formof CoPC on carbon electrodes was characterized by scanning electron microscope (SEM). The use of CoPC modified edge plane pyrolytic graphite (CoPC-EPPG) for sensing nitrite (NO₂⁻) was also investigated. It was found that the claimed mediator CoPC has no influence on the process. A bare glassy carbon (GC) electrode was successfully applied for the quantitative determination of nitrite as a simple alternative to the modified electrodes reported in the literature (chapter3). Chapter4 compares the voltammetric responses of an edge plane pyrolytic graphite electrode covalently modifed with 2-anthraquinonyl groups (EPPG-AQ2) and solution phase anthraquinone monosulphonate (AQMS) in the presence of a limited concentration of protons. The solution phase and surface bound species show analogous responses resulting in split waves. Digisim™ simulation of the AQMS voltammetry have shown that the pH adjacent to the electrode may be altered by up to 5-6 pH units in low buffered solutions; this is caused by the consumption of protons during the electrochemical reaction. Chapters5 and 6 compare the electrochemical properties of 2-anthraquinonyl groups covalently attached to an edge plane pyrolytic graphite (EPPG) and to a gold electrode. In both cases simulations using newly developedMarcus-Hush-Chidsey theory for a 2e⁻ process assuming a uniform surface did not achieve a good agreement between theory and experiment. Subsequently two models of surface inhomogeneity were investigated: a distribution of formal potentials, E^Ө, and a distribution of electron tunneling distances, r₀. For both EPPG-AQ2 and Au-AQ2 modified electrodes the simulation involving E^Ө distribution turned out to be the most adequate. This is the first time that Marcus-Hush-Chidsey theory has been applied to a 2e⁻ system. Chapter7 briefly summarizes the obtained results.

This thesis reports upon the covalent attachment of 6 different types of linkers bearing the Boc protecting group to the surface of glassy carbon electrodes, highly ordered pyrolytic graphite with the edge and basal plane orientation and multi-walled carbon nanotube abrasively immobilised onto a glassy carbon substrate by electrochemical oxidation or reduction of the corresponding diazonium salt. Removal of the Boc group allows redox probes, such as anthraquinone-2-carboxyl acid, 4 nitrobenzoyl chloride and 3 and 4 dihydroxybenzoyl chloride, to be coupled to the surface using solid-phase coupling methods. The surface coverage, scan rate and pH effects, as well as the determination of the kinetic parameters, were evaluated using cyclic voltammetry. It was found that the type of carbon electrode and the choice of linkers had a significant influence on the surface coverage of redox probes and the electron transfer rate. A 4-(N-Boc aminomethyl) benzene diazonium tetrafluoroborate salt linker (C6H4CH2NH) was also spontaneously grafted onto the CNTs by refluxing in the C6H4CH2NHBoc diazonium salt at 60 oC in an acetonitrile solution. After the removal of the Boc protecting group, the anthraquinone (AQ) and nitrobenzene (NB) groups were attached to the benzyl amine linker by solid-phase amide coupling. The grafted CNTs were characterized using FTIR and cyclic voltammetry techniques; the surface coverage and the stability of the tethered functional groups were also investigated. The oxygen reduction reaction was studied on bare and anthraquinone (AQ) modified edge planes, on a basal plane with highly oriented pyrolytic graphite, and on glassy carbon (GC) and multi-walled carbon nanotube electrodes. The anthraquinone modified rotating disc GC electrode results show the two electron oxygen reduction reactions with hydrogen peroxide as the final product. The immobilization of laccase (ThL) onto GC electrodes modified with anthraquinone and anthracene through EDA and C6H4CH2NH– linkers was also achieved by employing high-throughput screening using a multichannel potentiostat for a library of 12 electrodes; the activity of these electrodes to oxygen reduction was examined. The experimental findings demonstrated a successful attachment of laccase to the modified glassy carbon, as well as successful electron transfer between substrate- and enzyme-active sites. DFT calculations were used to investigate the structural properties of the functionalized basal plane of graphene after modification and to discover why the edge site shows higher reactivity to the attachment of linkers than the basal sites and whether the type of linker has an effect on the surface coverage of AQ. On the basis of considering the relationship between binding energy and charge transfer, the root of the effect of the type of linker on surface coverage was investigated to ensure whether it is steric, electronic, or both.

A method of synthesizing silicone-based composite consisting of carbon black (CB) as a conductive ller in Polydimethylsiloxane (PDMS) was developed. The aim was to nd a cost eective and easier method to fabricate stretchable, epidermal and conductive electrodes in striving for inexpensive real-time health monitoring. In this work, instead of expensive additive materials for enhancement of PDMS conductivity, CB powder, at lower cost was used. To optimize the electrophysiological properties of the electrodes, limited amount of silver (Ag) and silver chloride (AgCl) particles were added. The electrical characteristic of the electrodes and their stretchability was studied. Since fabrication and characterization did not require clean room enviroment, the developed method was less costly and less time consuming. Samples were made of six dierent ller concentrations in three sets, which in total were 18 samples, in order to obtain better statistics. Resistance of all samples was measured and resistivity values were calculated. Tensile test were performed on all samples. The result showed that all samples had elongation of over 50 %, which is feasible for stretchable, epidermal patches. Samples with ller concentration of 10 wt% CB + 5 wt% Ag/AgCl and 10 wt% CB + 8 wt% Ag/AgCl showed resistivity of Wcm range. The electrodes were conductive, soft, stretchable and biocompatible. They fulll the requirements of epidermal patches for health monitoring.

This thesis reports the voltammetric applications and fundamental frequency-dependent properties of carbon-based electrode materials. A range of electrochemical systems hasve been investigated, and new materials have been electrochemically characterised, which will be of use to the field of electrochemistry. In Chapter 3 of this thesis, different graphenes were utilised as electrode composite materials, and their electrochemical behaviour was de-convoluted. It was found that surfactant-free graphenes were useful for the detection of guanine in terms of a reduced activation potential, which is thought to be derived from a pi-pi adsorption mechanism. The oxygen reduction reaction was also focussed upon and it was found that the type of graphene utilised did not affect the electrochemical mechanism in the respective reactions, but the peroxide yields changed. This could have dramatic ramifications for users choosing carbon materials as catalyst supports. Screen-printed electrodes were applied to novel systems including theophylline and creatinine, finding that their use as portable sensors was viable in two ways. For theophylline, a direct oxidation mechanism was useful for the detection of the medicine, while in the case of creatinine, an indirect detection method was found to be effective as creatinine is not electrochemically active. In Chapter 5, the first graphene screen-printed electrodes were developed and characterised. The result was two graphene screen-printed electrodes, with differing electrochemical properties, both of which could be used for different applications. Finally, Chapter 6 focusses upon whether electrochemical impedance spectroscopy is useful for screen-printed electrodes and carbon modifications. The work in this thesis finds that a synergy could potentially be formed, and in particularly, has found that it would be wise to operate screen-printed electrodes around +0.2 V due to this being the point where there is no net charge at the electrode surface under standard conditions.

AB5 hydrogen storage alloys have been intensively studied due to its superior ability to store hydrogen and release at ambient conditions. It is also a major component in the negative electrode of Ni-MH batteries. However, it has poor high rate capability and cycle life stability. Carbon nanotubes (CNTs) were found to store a tremendous amount of hydrogen, owing to the fact that they possess very large surface areas. It is because the hydrogen storage capacity is in general highly dependent on the surface area of the storing materials. The aim of this project has been to investigate the effect on electrochemical behaviours of Ab5 negative electrode in Ni-MH batteries by adding carbon nanotubes. The research also studied the influence of the ball milling treatments applied to both the Ab5 and CNTs. La0.59Ce0.27Nd0.08Pr0.06 (Ni0.76Mn0.08Al0.01Co0.15)5 AB5 alloy powder was used as active material in the negative electrode in the Ni-MH batteries, CNTs were used as additive, nickel powers as conductor in a three-electrode cell. Electrodes with compositions of AB5 + x wt.% CNTs (x=0, 5, 10) were studied. Activation, high rate capability and cycle life stability were investigated. The three-electrode cell in an open container with 6 M of KOH as electrolyte was connected to charge/discharge machine where galvanostatically charging and discharging took place. Hydrogenation of ball milled and as-received AB5 alloy powders were examined by conventional volumetric method. Morphology of AB5 and CNTs was examined by scanning electron microscopy (SEM) and transition electron microscopy (TEM), respectively. The phase identification and crystal lattice parameters were analysed by multi-purpose X-ray diffraction before and after ball milling treatments for both materials. The chemical composition of Ab5 alloy powders was tested using ICP chemical method. The results show the addition of CNTs in negative electrode in a Ni-MH battery enhanced the specific discharge capacity remarkably. A maximum discharge capacity of 252 mAh/g was observed for electrode with low energy ball-milled (LEBM) Ab5 with 5 wt.% of CNTs. This was due to the superior properties and great surface area of CNTs which allow more hydrogen to be stored and diffused onto the surface. Not only CNTs could act as a hydrogen reservoir in the negative electrode, it also acted as a conductor by building a conductive network between active material and nickel powders, and hence an increase in discharge capacity. However, the milling on CNTs alone will not improve the electrochemical properties of the electrode. In contrary, the activation profiles, high rate capability and cycle stability have been enhanced significantly when Ab5 alloy powders were ball-milled. The possible explanation is the smaller particle size and rough surface (and hence large surface area) obtained after ball milling induces a better hydrogen diffusion between the particles, as a result of shorter distance between particles after ball milling. Ball milling treatments on AB5 alloy powders did not improve the hydrogen absorption capacity. A highest value of 1.27 wt.% was observed for LEBM alloy powders. Ball milled samples have a slightly lower plateau pressure as compared with that of as-received alloy powders. In addition, only 4% of the maximum absorption capacity was lost after 10 repeated absorption and desorption cycles due to pulverisation of the particle over cycling. It can be concluded that LEBM Ab5 with addition of 5 wt.% CNTs, can significantly improve the electrochemical properties of negative electrode in Ni-MH batteries.

Guided by increasing legislation, the analysis of food borne toxins, including mycotoxins, seeks to address market related demands for the development of analytical systems to monitor this threat to food security and human health. This Thesis is directed at the assessment of the application of electrochemistry for direct electroanalysis and characterisation of the mycotoxin citrinin (CIT) in aqueous media as well as fundamental investigations of the surface of polished and oxidised glassy carbon electrodes (GCE). This study provides the first known account of CIT detection through electrochemical methods. Although electrochemically active, CIT current responses (Ip) were highly irreproducible at polished GCE with a coefficient of variation (C.V.) of 20.16 %. As stability of Ip across multiple electrode preparations is a key requirement in electroanalysis, investigations were directed at attaining stability in CIT Ip. Achieving stability in CIT Ip was investigated via two approaches, including: accounting for Ip variability between electrode preparations as a result of variable GCE surface conditions as a post-data-acquisition analysis and secondly, removing Ip variability through modification of GCE. Accounting for variability in Ip was investigated through the application of double layer capacitance as an indicator of the activity of an electrode, and in so doing serving as a relative mediator of Ip responses between electrodes. Application of this procedure dropped CIT C.V. to a third of starting value across polished GCE (C.V. = 7.18 %), chemically oxidised GCE (Pi-GCE, C.V = 8.47 %) and functionalised multi-walled carbon nanotube modified GCE (fMWCNT, C.V. = 25.79 %) and was effective with analysis of structurally distinct molecules, 2,4-dimethylaniline (2,4-DMA) and 1,2,4-trihydroxybenzene (Triol). Furthermore, it afforded the ability to determine discreet solution overlapping data sets of Ip. Stabilising Ip through GCE surface modification was achieved by anodic electro-oxidation of GCE and allowed for direct electroanalysis of CIT and subsequent characterisation and analysis of CIT in complex media as it reduced C.V. of CIT Ip to 0.73 %. Fundamental investigations of the electrode surface condition are described such that the source of variability could be identified and the interactions of CIT with the electrode understood. Two surface oxidation techniques were applied in modification of GCE; anodic electro-oxidation (EOx GCE) and chemical oxidation using piranha solution (Pi-GCE), analysis of which has previously not been reported. Fundamental analyses to determine surface morphology and chemistry of Pi-GCE, EOx-GCE and polished GCE were conducted using high resolution scanning electron microscopy (HRSEM), scanning electrochemical microscopy (SECM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR) and via electroanalytical methods. These studies showed that both oxidation procedures introduced a variety of oxide species at GCE surface, and further that the extent of those species was similar with total % O being 27.67 % and 33.47 % at Pi-GCE and EOx-GCE respectively. Although chemically similar, each surface was morphologically distinct. Electrochemical analyses at the surfaces revealed Pi-GCE to behave more similarly to polished GCE than EOx-GCE. As CIT responses were found to be stable at EOx-GCE (C.V. = 0.73 %) as opposed to Pi-GCE (C.V. = 22.87 %), stability of CIT Ip was likely to be as a result of a physical interaction with electrode morphology rather than interaction on a chemical basis. Morphological analyses revealed polished GCE and Pi-GCE to be highly morphologically irregular at the micro-scale. Although comparatively smooth, the surface morphology of EOx-GCE does not account for the stability of Ip. This study thus proposed a theory to describe the mechanism by which the limited conductivity and porosity of EOx-GCE allow for it to provide a relatively stable surface area within the oxide layer, adjacent to the electrode surface, and thus provided a stable platform for electroanalysis. Voltammetric characterization of CIT at EOx-GCE revealed that anodic oxidation in aqueous media involved an uneven number of electrons to protons via an ECE mechanism. This was illustrated to be nt = 2e- accompanied by the transfer of 1H⁺ per molecule oxidised. A proposed reaction scheme for the initial stages of CIT oxidation was suggested to involve both hydroxyl and carboxyl moieties of the CIT molecule. CIT oxidation was shown to arise as a result of a relatively complex mass transport regime which included both adsorptive and diffusive derived Ip₁. The LOD in buffered aqueous media was found to be 16 nM, a highly competitive result in relation to chromatographic techniques. Further application of EOx-GCE in complex media illustrated that CIT associates non-specifically with the components of food samples, primarily proteins. As a result of this, extraction of CIT from such media is mandatory. Liquid-liquid extraction illustrated a recovery in CIT Ip₁ and in so doing provided a means of accurately and sensitively detecting CIT from food samples with an LOD of 20 nM. These responses were corroborated by HPLC analyses on the same extractions and illustrate the applicability of electroanalysis as an analytical technique.

Typescript.
Thesis (PhD)--Macquarie University (Division of Environmental & Life Sciences, Dept. of Chemistry & Biomolecular Sciences), 2005.
Includes bibliographical references.
Microelectrode voltammetry -- Experimental -- Microelectrode fabrication -- Characterisation of the carbon film surface: Surface stability - X-ray photoelectron spectroscopy - Raman spectroscopy - Capacitance - Edge plane concentration - Potential window - Surface concentration of alkenes and alkynes - Outer sphere electron transfer using hexaamineruthenium (III) chloride - Reduction of potassium hexacyanoferrate (III) - Anodic oxidation: diol to dione; dopamine and ascorbic acid - Surface oxidation - Ferrocene in a non aqueous solvent -- Selectivity: Formation of carboxylic acid groups on a carbon film surface by ferrous II sulfate complex oxidation - Ethanol modified carbon film surface - Modification of carbon film microelectrode surface using aromatic amines - Modification of carbon film surfaces to form a dual functional ascorbic acid barrier -- In vivo anti fouling properties of surface modified carbon film microelectrodes -- Conclusion.
In this thesis a procedure is presented for the fabrication of a microelectrode to monitor the neurotransmitter dopamine in vivo. The microelectrodes are fabricated by in situ pyrolysis of acetylene under a nitrogen blanket onto a quartz capillary. The carbon film was then anodically oxidised in the presence of 2,4-dinitroaniline. These microelectrodes are stable, provide the physical strength to penetrate brain tissue, have a low capacitance, are resistant to fouling in vivo and selectively suppress the endogenous ascorbic acid which oxidises at the same potential as dopamine. With such properties the carbon film microelectrode appears ideally suited for fast scanning cyclic voltammetric studies of cationic neurotransmitters such as dopamine in vivo.
xxviii, 323 p. ill

This thesis describes the development and testing of a range of electrodes designed to be able to measure electrical current produced by the respiration of bacteria in direct contact with the electrode surface. The electrodes are designed to directly wire into redox processes in the cytoskeleton of the bacteria so that electron transfer can be measured in real time without the need for solution based mediator molecules. The rate of electron transfer from the bacteria is enhanced by nanostructuring the surface of graphite electrodes with vertically aligned single and multiwalled carbon nanotubes (CNTs) and covalently coupling mediator molecules to the CNT tips. A selection of the prepared electrodes are tested with the non-electrogenic bacteria Proteus vulgaris and Bacillus subtilis to demonstrate the potential of the electrode designs to be used with a wide range of microbial species.

The aim of this thesis is the development of solid contact ion selective electrodes, ISEs, where the transducer layer is made of a network of carbon nanotubes.

Potentiometric classical ion selective electrodes (ISEs) have been used for analytical applications since the beginning of 1900's. Determination of pH by a glass membrane ion selective electrode emerged at the beginning, being the first ISEs developed. pH glass electrode is still one the most useful and robust sensors for routine measurements both in laboratories and industries.

Throughout the years, new technologies, ideas and designs have been developed and incorporated successfully in the potentiometric fields so as to provide answers to the new society's needs. Therefore, the ion selective electrodes developed in this thesis are a step further in the progress of ISEs and must be considered as products of the scientific envisioning, growth, and interdisciplinary cooperation of many research teams over many years of continuous efforts.

The sensing part can be regarded nowadays as well developed, although it has been during only the last few years when considerable improvements have taken place in the development of new polymeric membranes, ionophores and lipophilic ions. Moreover, the understanding of the theoretical sensing mechanism has been a powerful solid backbone in the rise of ISEs.

Miniaturization of classical ISEs requires making all solid contact electrodes to avoid the intrinsic drawbacks of the inner solution. In this manner, the transduction layer has been the focus of attention for the two last decades. New solid contact transducers having the capacity to convert an ionic current into an electronic current have been emerging. Within them, conducting polymers have played an important role in the transduction of the potentiometric signal, being the most used in solid contact ion selective electrodes (SC-ISEs) up to now. However, the behaviour of conducting polymers can be further improved. For instance, their sensitivity to light one of main operational issues yet to be solved.

In the present context of searching for new materials able to transduce potentiometric signals we selected and tested carbon nanotubes (CNTs). CNTs, which were rediscovered by Ijima in 1991, display excellent electronic properties in terms of signal transduction. In addition, due to their chemical reactivity CNTs can be easily functionalized with receptors or other functional groups. In fact, depending on the type of functionalization the macroscopic and microscopic properties of CNTs can be drastically changed. This nanostructured material had not been used previously as a solid contact material in ISEs.

The main aim of this thesis is to demonstrate that CNTs can act as a clean and efficient transducer in SC-ISEs overcoming the drawbacks displayed by the previously assayed solid contact materials. The developed electrodes were used in different conditions to determine several ions in different sample types, demonstrating the capabilities of this nanostructured material.
The thesis has been structured in different chapters, each one containing the following information:

· Chapter 1 provides a short historical overview of potentiometric ISEs. The evolution from the "classical ISEs" to the SC-ISEs is briefly illustrated. Once the motivation for thesis is described, the general and specific objectives of the thesis are reported.
· Chapter 2 reports the scientific foundations of the developed electrodes. All components of the ISE, sensing layer, transducers and detection systems are introduced. Analytical performance characteristics of ISEs are also described.
· Chapter 3 corresponds to the experimental part. Reagents, protocols, procedures and instruments used in the thesis are reported.
· Chapter 4 provides the demonstration that CNTs can act as a transducer layer in SC-ISEs. The first SC-ISEs based on CNTs are characterized by electrochemical and optical techniques.
· Chapter 5 contains the experimental results that lead to the elucidation of the possible transduction mechanism of CNTs in SC-ISEs. Electrochemical impedance spectroscopy (EIS) is employed as the main characterization technique.
· Chapter 6 is composed of four sections reporting different analytical applications. In the first section, the common pH electrode is developed using a solid contact technology based on CNTs. In the second section, the development of SC-ISEs based on a new synthetic ionophore selective to choline, and CNTs as transducers is shown. In the third section, watertight and pressure-resistant SC-ISEs based on CNTs are developed and tested in aquatic research to obtain information about the gradient profiles along the depth of the lakes. In the fourth section, SC-ISEs based on CNTs are adapted for the on-line control of a denitrification catalytic process.
· Chapter 7 reports the possibilities of miniaturization of the SC-ISEs based on CNTs to reach a nanometric electrode. Potentiometric and optical characterizations are described in this section. Moreover, a discussion about the limitations of the real miniaturization in potentiometry is undertaken.
· Chapter 8 points out the conclusions of the thesis. In addition, future prospects are suggested.
· Finally, several appendices are added to complete the doctoral thesis.
El principal objetivo de esta tesis es el desarrollo de electrodos selectivos de iones de contacto sólido, ESIs-CS, utilizando como capa transductora una red compuesta de nanotubos de carbono.

Los electrodos potenciométricos selectivos de iones han sido utilizados en aplicaciones analíticas desde comienzos de 1900. La determinación de pH mediante electrodos de vidrio selectivo de iones fue el primer ESI desarrollado. Hoy en día, el electrodo de vidrio para la determinación de pH es todavía uno de los más útiles y robustos sensores utilizados en mediciones rutinarias tanto en laboratorios como en industrias.

A lo largo de los años, nuevas tecnologías, ideas y diseños han sido desarrollados e incorporados satisfactoriamente en el campo potenciométrico proporcionando soluciones a las necesidades en continua evolución de la sociedad. De esta manera, los electrodos selectivos de iones desarrollados en esta tesis son un paso más en el progreso de los ESIs y deben ser considerados como el producto de una sólida base científica, del crecimiento y de la cooperación interdisciplinaria de diversos grupos de investigación durante varios años.

La parte del sensor donde tiene lugar el reconocimiento químico y donde se genera el potencial dependiente de la muestra en estudio en los ESIs se puede considerar, en estos días, ampliamente desarrollada, aunque considerables mejoras han tenido lugar durante los últimos años, especialmente en el desarrollo de nuevas membranas poliméricas, ionóforos e iones lipofílicos. Sobretodo, el estudio y la comprensión del mecanismo teórico del sensor ha sido muy importante en el crecimiento y desarrollo de los ESIs.

El concepto de electrodos selectivos de iones de estado sólido surge como requisito vital para evitar las intrínsecas desventajas de la solución interna, en el proceso de miniaturización de los ESIs clásicos. De esta forma, la capa transductora ha sido el principal punto de atención durante dos décadas. Así, nuevos transductores de contacto sólido con la capacidad de convertir una corriente iónica en una corriente electrónica han sido desarrollados. Entre ellos, los polímeros conductores han jugado un importante papel en la transducción de la señal potenciométrica, siendo éstos los más empleados en los electrodos selectivos de iones de contacto sólido (ESIs-CS). Sin embargo el comportamiento de los polímeros conductores puede ser mejorado. Por ejemplo, la sensibilidad hacia la luz de estos materiales es un inconveniente todavía no resuelto.

En este contexto de investigación de nuevos materiales capaces de actuar como transductor de una señal potenciométrica, se han escogido y estudiado los nanotubos de carbono (NTCs) como transductores. Los NTCs fueros redescubiertos por Ijima en 1991, y muestran excelentes propiedades electrónicas en términos de traducción de señal. Además, debido a su reactividad química, los NTCs pueden ser fácilmente funcionalizados con receptores u otros grupos funcionales. De hecho, sus propiedades macroscópicas y microscópicas pueden ser afectadas drásticamente dependiendo del tipo y grado de funcionalización. Este material nanoestructurado no había sido previamente utilizado como transductor en ISEs.

El principal propósito de esta tesis es demostrar que los nanotubos de carbono pueden actuar de forma eficiente como transductor en electrodos selectivos de iones de estado sólido logrando vencer las desventajas de los transductores previamente mencionados. Los electrodos desarrollados fueron usados en diferentes condiciones para determinar distintos iones en diversos tipos de sistemas, demostrando las extraordinarias capacidades de este material nanoestructurado.


Esta tesis ha sido estructurada en capítulos que contienen la siguiente información:

· El Capítulo 1 proporciona una breve visión histórica de lo electrodos potenciométricos selectivos de iones. Se ilustra la evolución desde los "clásicos ESIs" hasta los actuales "ESIs-CS". Además se señalan en esta sección los objetivos generales y específicos.
· El Capitulo 2 contiene las bases científicas de los electrodos desarrollados. Se introducen todos los componentes que integran un ESI, tales como: capa reconocedora, capa transductora y sistema de detección. A continuación se describen los parámetros analíticos de calidad de los ESIs.

· El Capitulo 3 describe la parte experimental. Se recogen los reactivos, protocolos, procedimientos e instrumentos usados a lo largo de la tesis.
· El Capitulo 4 provee de la demostración de que los NTCs pueden actuar eficientemente como capa transductora en SC-ISEs. Se caracteriza el primer ESI-CS integrado por NTCs mediante técnicas ópticas y electroquímicas.
· El Capitulo 5 contiene los resultados experimentales que permiten la posible elucidación del mecanismo de transducción de los NTCs en los ESIs-CS. La Espectroscopia de Impedancia Electroquímica (ESI) es utilizada como la principal técnica de caracterización.
· El Capitulo 6 está integrado por cuatro secciones con diferentes aplicaciones analíticas. En la primera sección, se desarrolla un electrodo de pH que usa NTCs como nueva tecnología transductora en ESIs-CS. En la segunda sección se muestra el desarrollo de un ESI-CS integrado por un ionóforo sintético selectivo a colina, y NTCs como transductores. En la tercera sección, ESIs-CS basados en NTCs, resistentes a altas presiones y totalmente herméticos, se desarrollan y prueban en investigaciones acuáticas con la finalidad de obtener información sobre los gradientes de concentración de iones en función de la profundidad de un lago. En la cuarta sección ESIs-CS basados en NTCs se adaptan para el control on-line de un proceso catalítico de desnitrificación.
· El Capitulo 7 presenta la posibilidad de la miniaturización de los ESIs-CS basados en NTCs logrando obtener un electrodo nanométrico. Se muestran en esta sección la caracterización óptica y potentiométrica. Además, se discuten las limitaciones de la miniaturización real de los ESIs en potenciometría.
· El Capitulo 8 contiene las conclusiones de la tesis. Adicionalmente, se sugieren las perspectivas futuras del trabajo presentado.
· Finalmente, se añaden algunos apéndices como complemento de la tesis doctoral.

The aims of this research project were to identify and classify the binder-filler interfaces formed in carbon electrodes and to determine the effects of the interfacial quality on important electrode properties. The effects of raw materials and some fabrication process variables on interfacial characteristics and quality of laboratory produced test electrodes were also studied, and the development of binder-filler interfaces during the carbonisation process followed. Electrode quality was assessed by measurement of density, electrical resistivity and tensile strength. Pore structural data were also obtained by using a computerised image analysis system allied to an optical microscope. Interface quality data were obtained by examining etched surfaces in a scanning electron microscope and classifying the binder-filler interface observed into one of five categories. The category depending on the extent of contact between the binder and filler. Accordingly, test electrodes were produced from combinations of four filler carbons, comprising three grades of calcined petroleum coke and an electro-calcined anthracite, and four coal-tar binder pitches which varied in the type and quantity of insoluble matter content. Examination of these test electrodes showed that the nature of the filler carbon used had a dominant influence on the quality of the interface formed, as assessed by this technique. A combination of one filler carbon and one binder pitch was used to study the effects of some fabrication process variables. These were pitch content and, mixing time and temperature. Of these process variables, pitch content and mixing temperature were found to have the major effects on the binder-filler interface and electrode quality. Investigation of the development of the binder-filler interfaces during the carbonisation process showed three distinct zones of interface development and transformation. These zones were associated with three temperature dependent mechanisms; thermal stress relaxation between 200-350 degrees C, volatile gas evolution from coal-tar pitch decompositionb etween3 50-600 degrees C and stresses induced by thermal contraction of the binder phase between 600-1000 degrees C.

Biological fuel cells potentially offer efficient clean energy conversation from biomass to electricity. One of the critical issues is to develop effective electrode structures for high-capacity and stable loading of biocatalysts (e.g., bacteria and enzymes). Porous carbons are promising if the pore structures can be tailored to maximise the capacity, efficiency and durability, which form the key objectives of this project. In this project, firstly, a 2-part polyurethane foaming system and commercial carbon fibres were used to synthesise porous carbon-fibre foams with high surface area, high electrical conductivity and high mechanical strength. The carbon-fibre foams were impregnated by phenolic resin and then carbonised to improve their electrical conductivity and mechanical strength. Moreover, the foams' electron transfer capabilities were enhanced by surface modification using electrochemically reduced graphite oxide. To enrich mesopores for enzyme loading, the method of self-assembly of block copolymers was adopted to synthesise mesoporous carbons. Novolac-type phenolic resin was chosen as the carbon precursor and the block copolymers Pluronic P123 and Pluronic F127 were chosen as the structure-directing agents. The synthesised mesoporous carbons had a pore diameter range of 2 - 10 nm and 2 - 5 nm for P123 and F127 respectively. Mesoporous carbons derived from F127 were successfully incorporated into the carbon-fibre foams by solvent impregnation and carbonisation. The resulting hierarchical porous carbons presented meso-/macroporous structures with a much higher surface area (up to 209 m2 g−1) as compared to that of pure carbon-fibre foams (~ 12 m2 g−1). However, myoglobin adsorption tests revealed that these hierarchical porous carbons had low adsorption capacities (up to 0.028 µmol g−1 at room temperature) due to small mesopores (2 - 5 nm in diameter) present in the porous structures. The results are discussed with an aim for further development.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, February, 2021
Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 219-233).
Electrochemical carbon dioxide reduction (CO2R) is increasingly recognized as a viable technology for the generation of chemicals using carbon dioxide (CO₂) recovered from industrial exhaust streams or directly captured from air. If powered with low-carbon electricity, CO2R processes have the potential to reduce emissions from chemicals production. Historically, three-electrode analytical cells have been used to study catalyst activity, selectivity, and stability with a goal of incorporating proven materials into larger devices. However, it has been recognized that the limited CO₂ flux through bulk volumes of liquid electrolyte limit the effective reaction rate of CO₂ when using promising catalyst systems.
Gas-fed electrolyzers adapted from commercial water electrolyzer and fuel cell technologies have motivated researchers to explore combinations of porous electrodes, catalyst layers, and electrolytes to achieve higher areal productivity and favorable product selectivities. Present art demonstrates that high current density production (>200 mA cm₋²) of valuable chemicals at moderate cell voltages (ca. 3-4 V) is achievable at ambient conditions using electrolysis devices with catalyst-coated gas diffusion electrodes (GDEs). However, beyond short durations (1-10 h) stable performance outcomes for flowing electrolyte systems remain elusive as electrolyte often floods electrode pores, blocking diffusion pathways for CO₂, diminishing CO2R selectivity, and constraining productivity. Systematic study of the driving forces that induce electrode flooding is needed to infer reasonable operational envelopes for gas-fed electrolyzers as full-scale industrial devices are developed.
In this thesis, I investigate GDE wettability as a prominent determinant of gas-fed flowing electrolyte CO₂ electrolyzer durability. To do this, I combine experimental and computational approaches. First, I use a flow cell platform to study transient evolution of activity, selectivity, and saturation to identify failure modes, including liquid pressurization, salt precipitation, electrowetting, and liquid product enrichment. Next, I use material wettability properties and reactor mass balances to estimate how enriched liquid product streams might defy non-wetting characteristics of current GDE material sets. Finally, I construct computational electrode models and vary surface chemistry descriptors to predict transport properties in partially saturated electrodes. Specifically, I consider how saturation evolves in response to relevant scenarios (i.e., electrowetting and liquid products) that challenge CO₂ electrolyzer durability.
by McLain E. Leonard.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Chemical Engineering

Nitrogen-doped carbon materials such as carbon nanotubes and graphene have garnered much interest due to their abilities to behave as electrocatalysts for reactions important in energy production (e.g. oxygen reduction) and biosensing (e.g. hydrogen peroxide reduction). Electrocatalytic properties of these materials have been attributed to enhanced electron transfer ability exhibited by surface nitrogen atoms compared to typical carbon structures. Screen-printing has been widely employed in the production of low-cost carbon-based electrodes for sensors and biosensors. Here, we develop nitrogen-doped screen-printed carbon (N-SPCE) electrodes for detection of hydrogen peroxide - an important analyte in biosensing. Conductive ink was formulated in the lab from nitrogen-doped graphite that was produced using a simple urea-based soft nitriding technique. N-SPCEs exhibited electrocatalytic activity towards hydrogen peroxide reduction, while SPCEs prepared from unmodified carbon showed no ability to electrocatalytically reduce H2O2. Amperometric detection of H2O2 using N-SPCEs at an applied potential of -0.4 V (vs. Ag/AgCl) displayed a wide linear range of 20 µM to 5.3 mM, and a low limit of detection (2.4 µM). These performance characteristics compare favorably to other electrodes for H2O2 sensing and indicate that the low-cost, easy-to-prepare N-SPCEs described here are promising platforms for the development of biosensors.

Since the first reported use of carbon nanotubes (CNTs) as an electrode material in 1996 the use of CNTs within electrochemistry has grown rapidly. Single walled carbon nanotubes offer bio-compatibility combined with nano-scale dimensions and low background currents in the pristine state. Over the past decade the quantity of SWNTs synthesised globally has greatly increased making the material available for a variety of studies and potentially a feasible material for commercial electrodes. Despite this rise in popularity there is still an on going debate about the sites of electron transfer (ET) at a carbon nanotube. Some reports claim that the sidewall of the carbon nanotube exhibits sluggish ET rates with the majority of the ET occurring at defect sites and the end of the CNT. In contrast there is also evidence that suggests that ET at the sidewall is facile and not sluggish. The origin of ET is investigated using both theoretical and experimental data to probe the developing diffusion profiles to active ET sites. This is achieved on the timescale of a typical voltammetric experiment by significantly reducing the rate of diffusion to the electroactive sites using a NafionTM film. The reduced rate of diffusion allows the developing diffusion profiles to the individual sites to be decoupled. The use of convection and diffusion is a proven electrochemical technique to increase the sensitivity of analytical measurements and to probe reaction rates and mechanisms. The well-defined mass transport within a channel flow cell or an impinging jet electrode, combined with the continual replacement of solution, makes this geometry amenable to online studies, e.g. bedside or industrial monitoring, or a combination with chromatography. One draw back of conventional channel flow and impinging jet electrode set-ups is the need for specialist equipment or calibration steps each time the system is assembled. The use of microstereo lithography (MSL) to construct custom designed cells for use with a variety of planar electrodes is investigated. The hydrodynamics within the proposed designs are theoretically tested and verified experimentally. The devices constructed are easily assembled using a wide range of electrode materials and the computer aided manufacture provides flexibility in critical dimensions. Importantly, the devices only require a one-off determination of the height prior to assembly, removing the need for an electrochemical calibration step as the cells do not distort during assembly. Of particular interest for analytical studies is the greatly reduced background currents provided by a carbon nanotube network compared to an equivalent size carbon macroelectrode. The lower background signal allows small Faradaic currents to be observed experimentally, allowing lower concentrations to be distinguished. The enhanced sensitivity is combined with the increased mass transport of channel flow and impinging jet convective systems to determine the limit of detection for particular channel and impinging jet geometries under constant flow rates. This approach allows the successful detection of nano-molar concentrations under hydrodynamic control using standard voltammetric techniques.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.
Includes bibliographical references (p. 45-48).
The goal of this work is to make an (n,m) assignment for individual suspended single wall carbon nanotubes (SWNTs) based on the measurements of their Raman Radial Breathing Modes and electron transition energies E[sub]ii based on Raman spectroscopy. The suspended SWNTs are grown on a photolithographically defined electrode pattern, which is designed so that suspended SWNTs are grown at known locations with known directions. The suspended SWNTs are then characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), and Raman spectroscopy. Finally, the information on the diameter distribution and the energy of the electronic transitions of the resonant suspended SWNTs obtained from Raman spectroscopy is compared to other published works to make (n,m) assignments of a number of suspended SWNTs.
by Hyungbin Son.
M.Eng.

As pure “physical” devices, which do not involve any chemical or electrochemical transformations, any charge or mass transfer across the electrode-electrolyte interface, SC’s must demonstrate much faster charge/discharge operations and longer cycle life than any “chemical” batteries. Given this, SC devices can provide the key to a number of efficient power solutions that are mainly related with various backup sys-tems to compensate short-term voltage surges or drops or with load leveling the batteries in various com-bined power sources. Low internal resistance can be one of key advantages of SC’s over all other types of energy storage devices. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35522

The ability of humans to study, manipulate and understand matter at the nanoscale has enabled us to develop materials that can combine physical, chemical, optical, magnetic and mechanical properties that bulk materials do not possess. One of the materials that triggered interest in the world of Nanoscience and Nanotechnology was carbon nanotubes (CNTs). These nanostructures had already been reported more than forty years ago, but it is not until the beginning of the 90s that Dr. Sumio Iijima manages to produce them under stable conditions in his laboratory. From that time onwards, the resources devoted to the research and production of these carbon-based materials were on the rise. Although today they do not capture the same scientific interest as they did until 2010, their importance in the scientific world and especially in the market is relevant. In fact, since the technology for the production of CNTs on an industrial scale has matured, they are found in an infinite number of applications, such as reinforcing polymers, acting as scaffolds for the growth of artificial tissue, in the manufacture of conductive inks or as part of new generation battery electrodes and supercapacitors. It is precisely in this last application that scientific interest has been focused with special attention. Together with other carbon-based materials, such as graphene, they are excellent support materials for materials with high capacitance. Research groups and companies around the world are spending a lot of resources to obtain electrodes that have a three-dimensional architecture at the nanoscale and whose specific surface is high. In that sense, the objective of this work was to synthesize CNTs on the surface of a flexible and conductive material: 304 stainless steel. We focused on optimizing the growth processes by plasma enhanced chemical vapor Deposition (PECVD) and water assisted chemical vapor deposition (WACVD) with and without the contribution of external catalyst material. In addition, as will be seen in the development of this work there is an important effort to understand the effects that thermal processes, necessary for CNTs growth, produced on the properties of steel. Especially the influence on corrosion resistance, since the final use of stainless steel CNTs is the manufacture of electrodes exposed to corrosive environments.
La capacidad de los seres humanos para estudiar, manipular y comprender la materia a escala nanométrica nos ha permitido desarrollar materiales que pueden combinar propiedades físicas, químicas, ópticas, magnéticas y mecánicas que los materiales a granel no poseen. Uno de los materiales que despertó el interés en el mundo de la Nanociencia y la Nanotecnología fueron los nanotubos de carbono (CNTs por sus siglas en inglés). Estas nanoestructuras ya habían sido reportadas hace más de cuarenta años, pero no es hasta principios de los años 90 que el Dr. Sumio Iijima logra producirlas en condiciones estables en su laboratorio. A partir de ese momento, los recursos dedicados a la investigación y producción de estos materiales basados en el carbono fueron en aumento. Aunque hoy en día no captan el mismo interés científico que hasta 2010, su importancia en el mundo científico y especialmente en el mercado es relevante. De hecho, ya que la tecnología para la producción de CNTs a escala industrial ha madurado, estos se encuentran en un gran número de aplicaciones, tales como en el refuerzo de polímeros, actuando como andamiajes para el crecimiento de tejidos artificiales, en la fabricación de tintas conductoras o como parte de los electrodos para baterías y de los supercondensadores de nueva generación. Es precisamente en esta última aplicación donde el interés científico se ha centrado con especial atención. Junto con otros materiales a base de carbono, como el grafeno, son excelentes materiales de soporte para materiales con alta capacitancia. Los grupos de investigación y las empresas de todo el mundo están invirtiendo muchos recursos en la obtención de electrodos que tienen una arquitectura tridimensional a nanoescala y cuya superficie específica es elevada. En ese sentido, el objetivo de este trabajo fue sintetizar CNTs sobre la superficie de un material flexible y conductor: el acero inoxidable 304. Nos centramos en la optimización de los procesos de crecimiento mediante el depósito químico en fase de vapor asistido por plasma (PECVD por sus siglas en inglés) y el depósito químico en fase de vapor asistido por agua (WACVD por sus siglas en inglés) con y sin la contribución de material de catalizador externo. Además, como se verá en el desarrollo de este trabajo, hubo un esfuerzo importante para entender los efectos que los procesos térmicos, necesarios para el crecimiento de CNTs, producen sobre las propiedades del acero. Especialmente la influencia en la resistencia a la corrosión, ya que el uso final de los CNTs en acero inoxidable es la fabricación de electrodos expuestos a ambientes corrosivos.

The purpose of this research project was to determine the processing conditions necessary for preparing flexible carbon nanofiber electrodes by electrospinning and to explore various applications for those electrodes. It was found that by varying only the relative humidity while electrospinning a poly(acrylonitrile) precursor, fragile or flexible freestanding carbon nanofiber electrodes were prepared. The relative humidity during electrospinning controlled the fiber diameter, the bulk porosity of the material, and flexibility of the final carbon electrode. Higher porosity mats electrospun in a high relative humidity environment prevented fiber sintering, which if not minimized, resulted in non-flexible carbon electrodes. Both flexible and fragile electrodes were freestanding, binderless, and collectorless. Additionally, they required no further processing before use and were 100 wt.% active material. When cycled galvanostatically as a lithium ion battery anode, the flexible electrode exhibited a specific capacity of 379 mAH g-1 at the 100th cycle and capacity retention was 97.4% relative to the fifth cycle. When applied as an active material support electrode for lithium ion battery cathodes, the carbon support was successfully utilized with both micron and nano structured active material and cycled for 100 cycles with limited capacity loss. The same electrodes were also found to be a viable replacement for Pt electrode based actuators/artificial muscles. However, this application requires much further research to understand better the required processing and effects of the physical properties of the electrode on actuator performance. In addition to this, the flexible electrodes have a wide variety of other potential applications including, electrochemical storage and conversion devices, chemical sensing, and filtration. The focus of this work was electrochemical storage and conversion devices in the form of lithium ion battery anodes and cathodes as well as ionic polymer composite actuators.
PHD

Electrochemical capacitors, also known as supercapacitors, have attracted considerable attention over the past decades owing to their higher power density, long cycle life and moderate energy density compared. A high-performance supercapacitor integrates innovative electrode materials with desirable properties coupled with low cost and sustainability. In this thesis, a series of low cost transition metal oxide-activated carbon composite materials, lithium iron phosphate-activated carbon composite materials as well as metal oxide-graphene composite materials were prepared, characterized and evaluated as supercapacitor electrodes. Iron oxide (Fe3O4) – activated carbon (AC), zinc oxide (ZnO) – AC and titanium oxide (TiO2) – AC nanocomposites were prepared by using simple mechanical mixing method. The charge storage capabilities of these metal oxide-based composites with different loading ratios were evaluated in both mild aqueous 1 M Na2SO3 and 1 M Na2SO4 electrolytes. The incorporation of small amount of metal oxides onto AC could effectively enhance the capacitive performance of pure AC electrodes. It is believed that the presence of metal oxide nanoparticles can provide favourable surface adsorption sites for sulphite anions (SO32-). Nevertheless, bulk increasing of the metal oxide content is found to distort the capacitive performance and deteriorate the specific surface area of the electrode, mainly due to the aggregation of the metal oxide particles within the composite. On the other hand, composite materials consisting of lithium iron phosphate (LiFePO4) and AC exhibit high specific capacitance of 112.41 F/g in 1 M Na2SO3 with the incorporation of 40 wt % of LiFePO4. The synergistic effect between the faradaic battery type materials and the EDLC-based materials is greatly demonstrated. The intercalation and extraction of Li+ ions in LiFePO4 lattices are responsible for the reversible Faradaic reaction on top of the adsorption and de-adsorption of SO32- anions from Na2SO3 electrolyte. In the preparation of SnO2-graphene and MoO3-graphene nanocomposites, low-temperature solvothermal method using mild reducing agents was adopted. The preparation steps do not require high pressure or extreme synthetic condition and do not involve the usage of hazardous reactants. The electrochemical results of SnO2-graphene composite electrodes demonstrate that the composite electrodes possess a high specific energy (14 Wh/kg) with 93 % capacitive retention after 1500 cycles while MoO3-graphene composite electrodes yield an enhanced specific energy (16.3 Wh/kg) which is 28 % higher than that of pure MoO3 (11.8 Wh/kg). A maximum specific capacitance of 99 F/g was obtained from the optimized SnO2-graphene composite electrodes while a high average specific capacitance of 148 F/g was achieved for MoO3-graphene composites at a scan rate of 5mV/s in neural 1 M Na2SO3 electrolyte. The incorporation of graphene onto both SnO2 and MoO3 respectively, can promote the electrochemical utilization of metal oxides as well as the electrical conductivity of the electrodes. The graphene sheet serves as a good support in promoting effective charge transfer for redox reactions of MoO3. Additionally, deposition of metal oxides on graphene sheets prevents the graphene sheets from agglomeration, resulting in facile ion transportation pathway for electrolyte to access the surface of active material.

Synthesis and functionalization of large-area graphene and its structural, electrical and electrochemical properties has been investigated. First, the graphene films, grown by thermal chemical vapor deposition (CVD), contain three to five atomic layers of graphene, as confirmed by Raman spectroscopy and high-resolution transmission electron microscopy. Furthermore, the graphene film is treated with CF4 reactive-ion plasma to dope fluorine ions into graphene lattice as confirmed by X-ray photoelectron spectroscopy (XPS) and UV-photoemission spectroscopy (UPS). Electrochemical characterization reveals that the catalytic activity of graphene for iodine reduction enhanced with increasing plasma treatment time, which is attributed to increase in catalytic sites of graphene for charge transfer. The fluorinated graphene is characterized as a counter-electrode (CE) in a dye-sensitized solar cell (DSSC) which shows ~ 2.56% photon to electron conversion efficiency with ~11 mAcm−2 current density. Second, the large scale graphene film is covalently functionalized with HNO3 for high efficiency electro-catalytic electrode for DSSC. The XPS and UPS confirm the covalent attachment of C-OH, C(O)OH and NO3- moieties with carbon atoms through sp2-sp3 hybridization and Fermi level shift of graphene occurs under different doping concentrations, respectively. Finally, CoS-implanted graphene (G-CoS) film was prepared using CVD followed by SILAR method. The G-CoS electro-catalytic electrodes are characterized in a DSSC CE and is found to be highly electro-catalytic towards iodine reduction with low charge transfer resistance (Rct ~5.05 Wcm2) and high exchange current density (J0~2.50 mAcm-2). The improved performance compared to the pristine graphene is attributed to the increased number of active catalytic sites of G-CoS and highly conducting path of graphene. We also studied the synthesis and characterization of graphene-carbon nanotube (CNT) hybrid film consisting of graphene supported by vertical CNTs on a Si substrate. The hybrid film is inverted and transferred to flexible substrates for its application in flexible electronics, demonstrating a distinguishable variation of electrical conductivity for both tension and compression. Furthermore, both turn-on field and total emission current was found to depend strongly on the bending radius of the film and were found to vary in ranges of 0.8 – 3.1 V/μm and 4.2 – 0.4 mA, respectively.

The paper describes the electrochemical behavior of supercapacitor electrodes in both positive and negative ranges. This study has become possible due to development of a special reference electrode, which is stable in aprotic electrolytes like, e.g., 1.3 M Et3MeNBF4 in acetonitrile. Three-electrode measurements have enabled us to find the boundary potentials for various nanoporous carbon materials to be then used in the supercapacitor technology. This article describes how the electrode size can be optimized to get the maximum charge value in the double electric layer at the electrode-electrolyte interface. Besides, we illustrate how the supercapacitor rated voltage can be increased up to 2.9 V as compared with the typical value of 2.7 V. This provides the 15 % increase in energy and power. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35501