Study on electroactive and electrocatalytic surfaces of single walled carbon nanotube-modified electrodes

An investigation of the electrocatalysis of single-walled carbon nanotubes modified electrodes has been performed in this work. Nanotube-modified electrodes present a surface area much higher than the bare glassy carbon surfaces as determined by capacitance measurements. Several redox probes were selected for checking the reactivity of specific sites at the carbon nanotube surface. The presence of carbon nanotubes on the electrode improves the kinetics for all the reactions studied compared with the bare glassy carbon electrode with variations of the heterogeneous electron transfer rate constant up to 5 orders of magnitude. The most important effects are observed for the benzoquinone/hydroquinone and ferrocene/ferricinium redox couples, which show a remarkable improvement of their electron transfer kinetics on SWCNT-modified electrodes, probably due to strong π–π interaction between the organic molecules and the walls of the carbon nanotubes. For many of the reactions studied, less than 1% of the nanotube-modified electrode surface is transferring charge to species in solution. This result suggests that only nanotube tips are active sites for the electron transfer in such cases. On the contrary, the electroactive surface for the reactions of ferrocene and quinone is higher indicating that the electron transfer is produced also from the nanotube walls.     

Electrodeposition behavior of nickel in the water- and air-stable 1-ethyl-3-methylimidazolium-dicyanamide room-temperature ionic liquid

The electrodeposition behavior of nickel was investigated at glassy carbon and polycrystalline copper electrodes in the 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA) room-temperature ionic liquid. Amperometric titration experiments suggest that Ni(II) reacted with DCA⁻ anions forming [Ni(DCA)₄]²⁻ complex anion, which could be reduced to nickel metal via a single-step electron transfer process. However, the anodic dissolution of the nickel deposits was sluggish. The electrodeposition of nickel proceeds via three-dimensional progressive nucleation with diffusion-controlled growth on both glassy and copper substrates. Scanning electron microscopy images of the nickel deposits indicated that the morphology of the nickel electrodeposits is dependent on the deposition potential. Atomic force microscopy topography illustrated that the roughness of the nickel-deposited surface increased with decreasing deposition potential. The crystalline nature of the nickel deposits was revealed by powder X-ray diffraction spectroscopy results which indicated that the grains size of the nickel deposits decreased with decreasing deposition potential.     

Oxygen and chlorine electrodes on semiconductive SnO2 in molten LiCl-KCl eutectic

The oxidation of oxide (O^2–) and chloride ions were studied at tin oxide electrodes in molten LiCl-KCl eutectic at 450° C using voltammetric techniques. The polarization characteristics of the oxide semiconductor electrode were compared with that of Pt in the case of the oxygen electrode and with that of glassy carbon in the case of chlorine evolution. Both electrode reactions on tin oxide were found to be somewhat less reversible compared with the results of Pt and GC. It is suggested that the observed irreversibility is related to the electron tunnelling process through the space charge barrier formed at the semiconductor surface.     

Processing of strong and highly conductive carbon foams as electrode

Porous carbons were processed by the foaming of two-part polymer precursors with pre-loaded carbon powder (graphitic or amorphous), and then resin impregnation and carbonization to control both porosity and mechanical strength of the resulting foam. Electrical conductivity of the foams was improved by nickel-catalyzed graphitization. Different levels of graphitization were obtained for varied concentrations of nickel to the amorphous carbon foams. The presence of graphitic carbon improves the electrical conductivity by a factor of 50, compared to the amorphous counterparts. Electrochemical studies showed that graphitization of the amorphous structures increased the specific electrochemical surface area and electron transfer rate of the carbon electrodes.     

Adsorption of copper and lead species at electrochemically activated glassy carbon electrodes

The adsorption behaviors of Cu²⁺ and Pb²⁺ species at electrochemically activated glassy carbon obtained by different activation methods have been studied. Micropore structures were developed by cyclic polarization while small void space located at the bottom of the large void space was resulted from potentiostatic activation. The adsorption of the adsorbents would depend on the relative sizes of both the adsorbents and the void space created by electrochemical pretreatment. Different quinone derivatives would adsorb to different adsorption sites at the activated electrode, and consequently, affected the uptake of metal ions at the activated electrode incorporated with different quinone derivatives. Electrostatic and hydrophobic interactions between the adsorbents and the graphite oxide film might be involved.     

The underlying electrode causes the reported ‘electro-catalysis’ observed at C60-modified glassy carbon electrodes in the case of N-(4-hydroxyphenyl)ethanamide and salbutamol

The reported ‘electro-catalysis’ of C₆₀-film-modified electrodes for the electrochemical oxidation of N-(4-hydroxyphenyl)ethanamide and salbutamol has been explored at boron-doped diamond and glassy carbon electrodes. Using both C₆₀-film-modified boron-doped diamond and glassy carbon as underlying electrode substrates no electro-catalytic response is observed using the target analytes but rather the C₆₀ serves to block the electrode surface.A common experimental protocol used by researchers in this field is to electrochemically pre-treat the C₆₀-film-modified electrode. The response of employing this electrochemical pre-treatment at both bare glassy carbon and boron-doped diamond electrodes using the target analytes reveals that no effect on the electrochemical responses obtained at the boron-doped diamond electrode whereas a slight but significant effect occurs on glassy carbon which is attributed to the likely introduction of surface oxygenated species.Consequently the previously reported ‘electro-catalysis’ using C₆₀-film-modified electrode is not due to C₆₀ itself being catalytic, but rather that substrate activation through electrode pre-treatment is responsible for the observed ‘electro-catalysis’ likely through the introduction of surface oxygenated species.This work clearly shows that substrate activation is an important parameter which researchers studying C₆₀-film-modified electrodes, especially in electro-analysis needs to be considered.     

Surface functionalization of unsized carbon fiber using nitrenes derived from organic azides

The surface of both oxidized and unoxidized unsized carbon fiber was functionalized using an aziridine linking group derived from reactive nitrenes. The aziridine functionality arose from the cyclization of a reactive nitrene species onto the highly electron rich graphitic surface of the carbon fibers; the nitrene species evolved from thermal N₂ elimination from the corresponding (room temperature stable) azide. Surface functionalization using the nitrene approach was supported by X-ray Photoelectron Spectroscopy, in both oxidized and unoxidized carbon fiber. Attempts were also made to functionalize using amide chemistry, the two-step acid chloride coupling being successful for oxidized fibers by utilizing the carboxylic acid rich defect sites on the carbon fiber. None of the chemical treatment pathways had a significant impact on the tensile strength of the individual fibers, and atomic force microscopy revealed that fibers undergoing these treatment methodologies remained intact, without creating additional surface defects.     

Electrodeposition of cobalt from gluconate electrolyte

Cobalt electrodeposited onto steel substrate was carried out from solutions containing cobalt sulfate, boric acid and sodium gluconate. The study dealt with the influence of bath composition, current density, pH and temperature on the potentiodynamic cathodic polarization curves, cathodic current efficiency, and throwing power, as well as the throwing index of these baths. The microhardness of cobalt electrodeposited from gluconate baths is generally high and higher than that of cobalt deposited under similar conditions from sulfate, chloride, bromide and acetate baths. The surface morphology of the as-deposited cobalt was investigated using scanning electron microscopy (SEM) while the structure was studied using X-ray diffraction analysis. Cyclic voltammetric, as well as current-transient, techniques recorded on a glassy carbon electrode suggested that the deposition of cobalt from gluconate bath occurs via a nucleation process under charge transfer control.     

双电层电容器用多孔炭材料的研究与开发

阐述了双电层电容器的工作原理，探讨了多孔炭材料的比表面积、孔径分布、表面官能团、表面石墨微晶取向、体积密度和电导率以及电化学稳定性等微孔结构与物理化学性质对其电容特性的影响，介绍了近年来用作双电层电容器电极的几种新型多孔炭材料的研究进展。     

Effects of improved porosity and electrical conductivity on pitch-based carbon nanofibers for high-performance gas sensors

Pitch-based carbon fibers with multi-walled carbon nanotubes (MWCNTs) were fabricated via an electrospinning method and used as gas sensor electrodes. The pitch-based carbon fibers were treated at various temperatures to investigate the effect of the reaction temperature. The electrospun fibers were thermally treated to produce carbon fibers, and the resulting material was chemically activated to increase the number of active sites for efficient gas adsorption. The activation process improved the porous structure by increasing the specific surface area by approximately 86-fold. Due to the improved porosity and electrical conductivity, gas adsorption sites were enlarged and electron transfer was improved, resulting in a high-performance NO gas sensor with improved sensitivity and rapid response time. The improved porosity was attributed to the chemical activation process, and the enhanced electrical conductivity was attributed to the heat treatment and the addition of MWCNTs.     

Functionalization of multi-walled carbon nanotube for electrocatalytic oxidation of nitric oxide

The multi-walled carbon nanotube (MWCNT) was functionalized with hydroxyl, carboxyl and amido groups on the surface. Electrocatalytic oxidation of nitric oxide (NO) at the MWCNT modified electrodes was investigated. It was found that the MWCNT modified electrode could speed up greatly the electron transfer rate compared with the glass carbon electrode, and there was an adsorptive step for the oxidation of NO at the MWCNT modified electrode. The activation energy of NO electrooxidation reaction at modified electrode of MWCNT treated by alkali and mixed acids (MWCNT-OH–COOH) decreased, and current density was almost two times that of the electrode modified with alkali treated MWCNT. The modified electrodes of MWCNT amidated with the aliphatic amines decreased the activation energy of NO oxidation compared with MWCNT modified electrode, but the reaction rate of amidated MWCNT electrodes decreased because of the steric effect. The results demonstrated that MWCNT-OH–COOH modified electrode is the best for NO electrocatalytic oxidation.     

Oxygen and hydrogen peroxide reduction catalyses in neutral aqueous media using copper ion loaded glassy carbon electrode electrolyzed in ammonium carbamate solution

An aminated glassy carbon electrode (AGCE) can be obtained by the electrode oxidation of glassy carbon electrode in ammonium carbamate solution. In the cyclic voltammetric experiments, the electrode reduction of the dissolved oxygen began from −0.15 V vs. Ag/AgCl in neutral aqueous media when the aminated glassy carbon electrode was used as a working electrode although it began from −0.40 V vs. Ag/AgCl when a polished GCE was used. The nitrogen containing groups introduced by the electrode oxidation of carbamic acid must be related with the acceleration of the electron transfer rate of oxygen. Moreover, the new reduction wave of the dissolved oxygen appeared at +0.15 V vs. Ag/AgCl when copper (II) ion was coordinated to AGCE surface. This reduction potential of oxygen coincided with that of copper (II) ion and this fact suggests that the coordinated copper ion to the aminated carbon surface works as a redox mediator of oxygen. The reduction product of oxygen was monitored by rotating platinum ring - aminated glassy carbon disk electrode, and it was found that most of oxygen was reduced to water in a potential range negative than −0.4 V vs. Ag/AgCl. By using AGCE, it was recognized that the catalytic reduction of hydrogen peroxide was also taken place as well as oxygen reduction.     

Easy synthesis of highly nitrogen-enriched graphitic carbon with a high hydrogen storage capacity at room temperature

An easy method for synthesizing highly nitrogen-enriched graphitic carbon was developed and its hydrogen storage capacity was explored. The synthesis method uses a solution-based, stepwise condensation reaction between cyanuric chloride and melamine at low temperature (e.g., 0, 25, and 120 °C) and ambient pressure using conventional glassware without the need for an autoclave vessel. The physical and chemical structure of the synthesized highly nitrogen-enriched graphitic carbon was investigated by powder X-ray diffraction, scanning and transmission electron microscopy, selected area electron diffraction, energy dispersive spectroscopy, elemental analysis, Fourier transform infrared spectroscopy, X-ray photoemission spectroscopy, and electron energy loss spectroscopy. The analyzes confirmed that the product has a highly crystalline nitrogen-enriched graphitic structure (d₀₀₂ = 0.324 nm) with a carbon-to-nitrogen ratio of 1:1.12 (>50 atomic% nitrogen content). The material was determined to have an excellent hydrogen storage capacity of 0.34 wt% at room temperature under 100 bar in spite of its low BET surface area of only ∼10 m²/g.     

Electrochemical evaluation of copper deposition with gas sparging

The influence of gas sparging during copper electrolysis was studied using standard electrochemical techniques. The polarization behaviour of acid copper electrolytes was determined in the presence and absence of gas sparging on vertical electrodes. Tracer ion techniques were employed to determine the effect of gas sparging and forced circulation of the electrolyte on the mass transfer characteristics of the system. In addition to the potentiodynamic scans, 3-h copper deposits were produced for morphology and orientation studies. The effect of current density and temperature on deposition were also studied. The polarization experiments have shown that a mass transfer component becomes evident at about 40% of the limiting current density at which point the deposit becomes noticeably rougher.     

Steam activation of boron doped diamond electrodes

Boron doped diamond (BDD) electrodes were activated in steam at various temperatures, resulting in high quality BDD electrodes with a porous microstructure. Distinct columnar structures were observed by scanning electron microscopy. The electrochemically active surface area of the steam-activated BDD was up to 20 times larger than the pristine BDD electrode owing to the porous texture. In addition, a widening of the potential window was observed after steam activation, suggesting that the quality of BDD was enhanced due to oxidative removal of graphitic impurities during the activation process.     

Electrochemical reactivity at graphitic micro-domains on polycrystalline boron doped diamond thin-films electrodes

This paper deals with the electrochemical reactivity of boron doped diamond (BDD) electrodes. A comparative study has been carried out to show the influence of the presence of graphitic micro-domains upon the surface of these films. Those graphitic domains are sometimes present on as-grown boron doped diamond electrodes. The effect of doping a pure Csp³ diamond electrode is established by highly oriented pyrolytic graphite (HOPG) abrasion onto the diamond surface. In order to establish the effect of doping on a pure Csp³ diamond electrode, the amount of graphitic domains was increased by means of HOPG crystals grafted onto the BDD surface. Indeed that method allows the enrichment of the Csp² contribution of the electrode.The presence of graphitic domains can be correlatively associated with the presence of kinetically active redox sites. The electrochemical reactivity of boron doped diamond electrodes shows a distribution of kinetic constants on the whole surface of the electrode corresponding to different active sites. In this paper, we have studied by cyclic voltammetry and electrochemical impedance spectroscopy the kinetics parameters of the ferri/ferrocyanide redox couple in KCl electrolyte. A method is proposed to diagnose the presence of graphitic domains on diamond electrodes, and an electrochemical “pulse cleaning” procedure is proposed to remove them.     

Specificity of anodic processes in cyclic voltammetry to the type of carbon used in electrolysis of cryolite-alumina melts

Anodic processes associated with oxidation of carbon anodes used in electrolysis of cryolite-alumina melts, simulating the Hall-Héroult process, were studied by means of cyclic voltammetry in a comparative way at four graphitic carbon materials and at glassy carbon. Conditions were sought that give a current response function characteristic of diffusion-controlled oxidation of the anode by O^2– or oxyfluoride complex anions. Only at glassy carbon anodes are such conditions realized with a linear relation between response current in cyclic voltammetry and Al₂O₃ content in the melt. At the graphitic materials, complex mixed activation and diffusion controlled processes arise that are also relatively irreproducible from one experiment to the next, probably due to irreversible changes of the graphite surfaces. The effects of aluminium metal dissolved in the melt, to simulate practical smelter cell conditions, were also evaluated.     

Mössbauer investigation into the influence of disordering of the electronic subsystem on the electron density distribution in copper metal oxides

The disordering observed in the electronic subsystem upon superconducting phase transition in copper metal oxides is shown to be accompanied by a decrease in the electron density at copper sites. The higher the phase transition temperature, the larger the change in the electron density. The maximum decrease in the electron density is observed for the copper sites involved in the transfer of superconducting electrons.     

Structures and field emission properties of heat-treated C60 fullerene nanowhiskers

The structures and field electron emission properties of C₆₀ fullerene nanowhiskers were investigated. The single crystalline C₆₀ fullerene nanowhiskers were straight, surrounded by facets, and had a uniform submicrometer diameter along the long axis. Heat treatment of the nanowhiskers drastically transformed the inner structure from C₆₀ crystal to glassy carbon, while the outer structure kept its original morphology despite heat treatment. Field electron microscopy images of the heat-treated nanowhiskers showed striped patterns, characteristic of an agglomerate of crumpled graphitic layers.     

Formulation of a simple analytical expression for irreversible electron transfer processes in linear sweep voltammetry and its experimental verification

A simple analytical expression for the current function has been formulated for irreversible electron transfer processes in linear sweep voltammetry (LSV). The equations pertaining to the surface concentrations of the reactants and products have been obtained using the current function. The parametric dependence of the diffusion layer thickness has also been derived and its significance pointed out. The reductive cleavage of carbon tetrachloride in N,N′-dimethylformamide (DMF) at glassy carbon electrodes is investigated for verifying the theoretical expressions.