Electrochemistry of powder material studied by means of the cavity microelectrode (CME)

The kinetic aspects of powder material electrochemistry can be studied using the cavity microelectrode (CME) as it allows carrying out voltammetry at scan rates between a few millivolts per second to several hundreds of volts per second. Thus, significant voltammogram characteristics-scan rate profiles can be drawn. Theoretical models suited to each material needs to be developed for their exploitation. First, we report significant results obtained with CME on powder materials. The materials studied were chosen for their wide variety of possible applications such as battery materials (polyaniline or Bi₂O₃, which modifies the electrochemical behavior of materials in which it is included), supercapacitor (carbon black), and for the electrocatalytic hydrogenation of organic compounds (PtO₂). Secondly, we briefly describe the general action for establishing models to obtain a better understanding of the electrochemical processes.     

Synthesis, X-ray structure and Hg, Se CP MAS NMR studies on the first tris(imidazolidine-2-selone) mercury complex: {Chloro-tris[N-methyl-2(3H)-imidazolidine-2- selone]mercury(II)}chloride

A novel cationic Hg(II) complex has been synthesized with N-methyl-imidazolidine-2-selone ligand. The tris(N-Methyl-imidazolidine-2-selone) mercury(II) complex, [(MeImSe)₃HgCl]⁺Cl⁻ (1), has been characterized by single crystal X-ray analysis and CP MAS ¹⁹⁹Hg and ⁷⁷Se NMR.     

Corrosion of mild steel with composite polyaniline coatings using different formulations

The protective abilities of composite coatings based on electrochemically and chemically formed polyaniline powder against the corrosion of mild steel were investigated. A polyaniline powder has been prepared in the form of an emeraldine base and benzoate salt through chemical dedoping and doping. The composite coatings using polyaniline powders, which were obtained through different routes, and base coatings, which were not corrosion-resistant, with different formulations were prepared and applied on mild steel samples. The corrosion was investigated using an electrochemical impedance technique in 3% NaCl, and the atmospheric corrosion was assessed in a humidity chamber. Emeraldine–benzoate salts, which are a chemically synthesized polyaniline, offer the best protection with an optimal polyaniline concentration of approximately 5 wt%. The different corrosion behaviors were assessed relative to the presence of aniline oligomers in the samples after characterization using UV–vis spectrophotometry. Upon comparison between the corrosion behavior in 3% NaCl with commercial primer paint for iron and that with a paint containing 5 wt% PANI, the composite coating has superior anticorrosion characteristics. The mechanism for the protection of mild steel from corrosion through composite polyaniline coatings was also considered.     

Study on the effect of EDTA on the photocatalytic reduction of mercury onto nanocrystalline titania using quartz crystal microbalance and differential pulse voltammetry

In-situ technique quartz crystal microbalance (QCM), differential pulse voltammetry (DPV) and cyclic voltammetry (CV) were employed to investigate the effect of disodium ethylenediamine tetraacetate (EDTA) on photocatalytic reduction of mercury onto nanocrystalline titania (TiO₂). Effects of EDTA on adsorption of Hg(II) and its photocatalytic reduction process at the surface of TiO₂ in different pH solutions had been studied in detail. From the in-situ response to the adsorption of Hg(II) onto TiO₂, the reaction rate and saturation adsorption amount were estimated about 4.71 × 10⁻⁶ g mol⁻¹ min⁻¹ and 46.36 (mg Hg(II)/g TiO₂) via the model of pseudo-second-order kinetics respectively. The photocatalytic reduction of Hg at the surface of TiO₂ was influenced by pH and the mole ratio of Hg(II) to EDTA. When the ratio of Hg(II) to EDTA 1:1, it was most favorable for the photocatalytic reduction of mercury. In addition, the effects of HCOOH and EDTA on the reduction of Hg(II) was comparatively investigated and the mechanism on the photocatlytic reduction of mercury was illustrated. Therefore, it could be concluded that QCM, DPV and CV were effective methods for the investigation of photocatalytic reduction of complex heavy metal ions onto the surface of nanocrystalline TiO₂.     

Removal of mercury(II) from aqueous solution on a carbonaceous sorbent prepared from flax shive

Treatment of flax shive with sulfuric acid produced a carbonaceous material which has been used to remove mercury(II) from aqueous solution. The kinetics of sorption follows a first order reaction equation with the rate of sorption being higher for the wet material than for that which had been previously dried. Sorption of mercury depends on the pH of the aqueous solution with maximum uptake occurring in the pH range 6–7. Sorption capacity also increases with the increase of temperature. The presence of other metal ions such as K⁺, Na⁺, Mg²+ and Ca²+ decreases Hg(II) uptake capacity. A high capacity which exceeds the cation exchange capacity was observed, cumulative Hg(II) sorption exceeding 1 gg⁻¹. This arises from the reduction of mercury(II) to mercury(I) chloride and elemental mercury from chloride media and to elemental mercury from nitrate media. This was confirmed from the identification of deposits on the carbon surface by scanning electron microscopy and X‐ray diffraction. The reduction of mercury was accompanied by the oxidation of the carbon which was confirmed by the evolution of carbon dioxide. This observation was also supported by changes in the infra‐red spectrum of the carbon after reaction. The sorption mechanism is discussed. © 2000 Society of Chemical Industry     

Corrosion protection coating containing polyaniline glass flake composite for steel

Corrosion protection of steel by glass flake (GF) containing coatings is widely used in marine atmosphere. Even though, the coatings containing glass flake are highly corrosion resistant, their performance is decreased due to the presence of pinholes and coating defects. It is well established that polyaniline containing coating is able to protect the pinhole defects in the coatings due to passivating ability of polyaniline. Hence a study has been made on the corrosion protection ability of steel using polyaniline-glass flake composite containing coating with 10% loading of glass flake in epoxy binder. The polyaniline glass flake composite (PGFC) was synthesized by chemical oxidation of aniline by ammonium persulphate in presence of glass flake. The corrosion protection ability of GF and PGFC containing coating on steel was found out by salt spray test and EIS test in 3% NaCl. In both the tests, the resistance value of the PGFC containing coating has remained at 10⁸-10⁹ Ω cm² where as for the GF containing coating, the resistance values decreased to 10⁵ Ω cm². The enhanced corrosion protection ability of the PGFC containing coating is due to the passivation ability of the polyaniline present in the coating.     

Synthesis and characterization of electroactive polyamide with amine-capped aniline pentamer and ferrocene in the main chain by oxidative coupling polymerization

A novel electroactive polymer, polyamide with well-defined oligoaniline and ferrocene in the main chain, has been successfully synthesized through a novel oxidative coupling polymerization. The polymerization characteristics and structure of the copolymer were systematically studied by gel permeation chromatographic (GPC), Fourier-transform infrared (FTIR) spectra, ¹H NMR and X-ray powder diffraction (XRD). And its spectroscopic properties and electrochemical behavior were studied by UV-vis spectra and cyclic voltammetry (CV). It was found that the obtained polyamide bearing amine-capped aniline pentamer and ferrocene segments had a reversible electrochemical property in the cyclic voltammetry, but showed four distinct oxidation states, which is different from polyaniline. Moreover, its electrical conductivity is about 7.6×10⁻⁷ S cm⁻¹ at room temperature upon preliminarily protonic-doped experiment.     

Synthesis and Characterization of Polyaniline:TiO2 Nanocomposites

Thin films of polyaniline (PANi) and PANi:titanium oxide (TiO₂) composites have been synthesized by sol—gel spin coating technique. The TiO₂ powder of particle size 50–60 nm was synthesized by sol–gel technique and the polyaniline was synthesized by chemical oxidative polymerization of aniline. The composite films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) Fourier transform infrared (FTIR) and UV-vis spectroscopy, and the results were compared with polyaniline films. The intensity of the diffraction peaks for PANi:TiO₂ composites is lower than that for TiO₂. The characteristic FTIR peaks of PANi were found to shift to a higher wave number in the PANi:TiO₂ composite. These observed effects have been attributed to the interaction of TiO₂ particles with PANi molecular chains. The room temperature resistivity of polyaniline:nano-TiO₂ composite is 3.43 × 10³ Ω cm and the resistivity of pure nano-TiO₂ particles is 1.60 × 10⁶ Ω cm.     

Adsorption and Separation of Mercury: Sorption-Desorption of Hg2+ with Cross-Linked Graft Copolymer of Acrylic Acid and its Application in the Metal Ion Separation Process

A selective and reliable method has been developed for the extraction and separation of mercuric ion with cross-linked graft copolymer of acrylic acid based on sorption-desorption studies. The graft copolymer acts as an ion exchanger. The physico-chemical properties of the exchanger, and optimum pH, time, and temperature for Hg²⁺ adsorption were determined. Metal ion adsorption kinetics, isotherms, and thermodynamics have been studied. A plausible mechanism for mercury ion extraction has been suggested. Mercuric ion has been separated quantitatively from various synthetic mixtures containing metal ions (Ni²⁺, Cd²⁺, Pb²⁺, and Zn²⁺).     

Mercury species, mass flows and processes in a cement plant

The aim of the study was to evaluate the behaviour of mercury in the cement clinker production process. Simultaneous measurements of mercury in all important materials and gas streams were performed in three sampling periods on about 300 solid samples and about 80 samples taken from gas streams. Mercury species in flue gases at characteristic parts of the process were measured as total Hg(t), particulate Hg(p), elemental Hg⁰(g) and reactive Hg²⁺(g) mercury. Based on the results of measurements, mercury mass flows and mass balances of the whole and in certain parts of the process were evaluated. It was shown that the process comprises many mercury cycles which are strongly dependent on the operating conditions and technological specifics. Cycling of mercury causes a significant enrichment of mercury inside the process. In the annual mercury input of about 27 kg, raw materials accounted near by 60% and fuels about 40% (i.e. petrol coke 31%, waste tyres 10% and waste oil 0.4%). The annual emission of mercury represented 40-70% of the inputs, while cement clinker only contained about 10%. The difference between inputs and outputs (11-45%) obtained in the annual mass balance could be assigned to mercury recycling and significant enrichment inside the process, as well as variability between spot measurements. The parts of the process with the highest mercury mass flows and the lowest material/gas flows were identified. Such points represent an opportunity to remove a significant amount of mercury from the process at low material flows and to improve mercury control. Mercury was mainly emitted in gaseous form with 92% (direct mode) or 89% (combined mode) as Hg(g) on average, of which about 2/3 was as Hg²⁺(g), and about 1/3 as Hg⁰(g). Only a small part (the rest) was emitted as particulate Hg(p). Shares of individual mercury species in the last sampling period were 65.7% Hg²⁺(g), 34.0% Hg⁰(g) and 0.3% Hg(p) on average. Ratios between individual mercury forms were found to be related to operating modes. The quantities of Hg(t), Hg(g) and Hg²⁺(g) emitted were higher when operating with the raw mills off (direct mode). It was seen that the efficiency of Hg removal was strongly related to the dust removal efficiency. Bag filters very efficiently removed all mercury species.     

Electrosynthesis and capacitive performance of polyaniline–polypyrrole composite

The composite of polyaniline and polypyrrole (PPY‐PANI) was prepared by two‐step electrochemical polymerization method. Techniques of scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal gravity analysis (TG/DTG) measurements were used to characterize the morphology and structure of the composite. The electrochemical properties of the composite were investigated by cyclic voltammetry (CV), galvanostatic charge‐discharge, and electrochemical impedance spectroscopy (EIS). The results indicated that the polyaniline–polypyrrole composite showed better electrochemical capacitive performance than polypyrrole (PPY) and polyaniline (PANI). The specific capacitance of the composite electrode was 523 F/g at a current of 6 mA/cm² in 0.5 M H₂SO₄ electrolyte. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Preparation and characterization of processable electroactive polyaniline-polyvinyl alcohol composite

In this paper, the preparation and characterization of polyaniline-polyvinyl alcohol composites is described. The polyaniline composite was synthesized by chemical polymerization of aniline in media containing polyvinyl alcohol (10%, w/w). Oxidation of aniline results in a stable water based polyaniline dispersion, which can be cast to form a mechanically robust film.The electrical conductivity of the films increased with increasing amount of polyaniline to a high value of 2.5 S cm⁻¹. Cyclic voltammograms revealed that the composite materials are electroactive.     

Electrochemical behavior of dopamine in the presence of citrate: Reaction mechanism

The electrochemical behavior at a mercury electrode of dopamine (DA) in the presence of citrate was investigated in aqueous solution, using cyclic voltammetry and alternating current polarography. The influence of the composition of the supporting electrolyte, citrate concentration and pH was evaluated. At potentials more positive than 250 mV DA and Hg are oxidized yielding, in the presence of citrate, dopaminoquinone (DQ) and Hg(II). Hg(II) reacts with the DA forming DQ, leucodopaminochrome (LDC) and dopaminochrome (DC). DQ undergoes further reduction at 200 mV. Citrate, in the form of Cit³⁻, stabilizes the DC formed and this new electroactive compound suffers further reduction at −250 mV. This reaction pathway inhibits the formation of polymerized films on the surface of solid state electrodes and allows the determination of DA using the mercury electrode.     

Elimination of inorganic mercury from waste waters using crandallite‐type compounds

The removal of inorganic mercury from waste water streams arising from mines, using an artificial amorphous compound of the crandallite type synthesized in our laboratory, Ca_0.5Sr_0.5Al₃(OH)₆(HPO₄) (PO₄), has been investigated. This compound exhibits an extremely wide range of ionic substitutions: Ca²⁺ and Sr²⁺ were interchanged with Hg²⁺, so the mercury content of the waste water, ranging from 70 to 90 ppm, was reduced to less than 0.1 ppm. The process has been studied under batch conditions. The crandallite showed a high capacity for the exchange of mercury from mercuric nitrate solutions, 1.555 meq g^?1. The ion‐exchange equilibrium isotherms for Hg²⁺ were correlated by the Langmuir equation. The recovery of mercury from Hg‐crandallite using HCl solutions and thermal treatment was also studied. Optimum recuperation of mercury is achieved by chemical reaction with HCl solution (pH 2.25). At these conditions, 75% of the mercury is recovered as the HgCl₄^2? complex in a simple batch process, and the crandallite (in the protonic form) can be reused. © 2003 Society of Chemical Industry     

Phospholipid monolayer coated microfabricated electrodes to model the interaction of molecules with biomembranes

The hanging mercury (Hg) drop electrode (HMDE) has a classical application as a tool to study adsorption and desorption processes of surface organic films due to its: (a) atomically smooth surface and, (b) hydrophobicity at its potential of zero charge. In this study we report on a replacement of the HMDE for studying supported organic layers in the form of platinum (Pt) working electrodes fabricated using lithography techniques on which a thin film of Hg is electrodeposited. These wafer-based Pt/Hg electrodes are characterised and compared to the HMDE using rapid cyclic voltammetry (RCV) and show similar capacitance-potential profiles while being far more mechanically stable and consuming considerably less Hg over their lifetime of several months. The electrodes have been used to support self-assembled phospholipid monolayers which are dynamic surface coatings with unique dielectric properties. The issue of surface contamination has been solved by regenerating the electrode surface prior to phospholipid coating by application of extreme cathodic potentials more negative than −2.6 V (vs. Ag/AgCl). The phospholipid coated electrodes presented in this paper mimic one half of a phospholipid bilayer and exhibit interactions with the biomembrane active drug molecules chlorpromazine, and quinidine. The magnitudes of these interactions have been assessed by recording changes in the capacitance-potential profiles in real time using RCV at 40 V s⁻¹ over potential ranges >1 V. A method for electrode coating with phospholipids with the electrodes fitted in a flow cell device has been developed. This has enabled sequential rapid cleaning/coating/interaction cycles for the purposes of drug screening and/or on-line monitoring for molecules of interest.     

A highly selective electrochemical biosensor for Hg using hemin as a redox indicator

A highly selective electrochemical biosensor for the detection of Hg²⁺ in aqueous solution has been developed. This sensor is based on the strong and specific binding of Hg²⁺ by two DNA thymine bases (T–Hg²⁺–T). The hemin worked as a redox indicator to generate a readable electrochemical signal. Short oligonucleotide strands containing 5 thymine (T₅) were used as probe. Thiolated T₅ strands were self-assembled through Au–S bonding on gold electrode. In the presence of Hg²⁺, the specific coordination between Hg²⁺ and thymine bases resulted in more stable and porous arrangement of oligonucleotide strands, so hemin could be adsorbed on the surface of gold electrode and produced an electrochemical signal, which was monitored by differential pulse voltammetry (DPV). The DPV showed a linear correlation between the signal and the concentration of Hg²⁺ over the range 0–2 μM (R² = 0.9983) with a detection limit of 50 nM. The length of probe DNA had no significant impact on the sensor performance. This electrochemical biosensor could be widely used for selective detection of Hg²⁺.     

Three-dimensional gold micro-/nanopore arrays containing 2-mercaptobenzothiazole molecular adapters allow sensitive and selective stripping voltammetric determination of trace mercury (II)

Three-dimensional (3D) gold micro-/nanopore arrays containing 2-mercaptobenzothiazole (MBT) adapters have been prepared and employed to improve the performance of the determination of trace mercury in solution using square wave anodic stripping voltammetry (SWASV). 3D gold micro-/nanopore arrays have an active surface area which is up to 4 and 15 times higher than a two-dimensional (2D) bowl-like structured microarrays and a flat solid gold electrode characterized by cyclic voltammetry, respectively. In this study, 3D MBT molecular adapters on the array surface greatly decreased the effect of capacitive current and enhanced the sensitivity and selectivity of the electrode. A limit of detection of 0.02 nM (which is well below the guideline given by the World Health Organization) and more importantly, a sensitivity of 1.85 μA nM^?1 was obtained using this system. Furthermore, excellent linear range (0.05–10 nM) and good repeatability (relative standard deviation of 2.10%) were obtained for Hg(II). Interference experiments were also investigated, and it was determined that Pb(II), Cd(II), Zn(II), Cu(II) and Ag(I) had little or no influence on the mercury signal.     

The anodic behaviour of Ebonex® in oxalic acid solutions

A study of the oxidation of aqueous solutions of oxalic acid in various supporting electrolytes, at an Ebonex^® anode is reported. From data obtained by cyclic voltammetry, linear sweep voltammetry, chronopotentiometry, controlled potential coulometry and gas chramatography it has been shown that oxalic acid oxidation at an Ebonex^® anode, does not proceed, to any significant extent. Data collected using different electrolytes showed that sulphuric acid supported higher current densities at similar electrode potentials. Ebonex^® is essentially a selective oxygen evolver and is thus a suitable anode for the electroreduction of oxalic acid to glyoxylic acid in an undivided cell. The behaviour of a platinized titanium anode is also reported and this material has similar electrochemical characteristics to Ebonex^® in aqueous oxalic acid solutions.     

Improving the electrochemical properties of polyaniline by co-doping with titanium ions and protonic acid

Polyaniline co-doped with titanium ions and protonic acid was synthesized in aqueous H₂SO₄ solution containing Ti(SO₄)₂ (pH 2), and was characterized via Fourier-transform infrared spectra, UV–vis spectroscopy, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and cyclic voltammetry. Its conductivity was 2.8 S cm⁻¹ at room temperature (25 °C), which is the same order of magnitude as polyaniline doped with protonic acid (5 S cm⁻¹). Compared with polyaniline doped with protonic acid, it showed a better redox reversibility and cycling stability in aqueous pH 4.0 media, even in an acetate electrolyte (pH 4.0). It could be a promising material for several applications due the increase of the operating pH window of polyaniline in an aqueous environment.     

A new route to synthesize polyaniline-grafted carboxyl-functionalized graphene composite materials with excellent electrochemical performance

A new route to synthesize polyaniline (PANI)-grafted carboxyl-functionalized graphene (PGCG) composite material is established. In this paper, PGCG is first prepared through a two-step carboxyl-functionalized process. PANI can be grafted and grown on the surface of graphene due to the covalent bonding existing between the carboxyl-functionalized graphene and polyaniline. This method cannot only improve the mechanical performance and adaptive performance of polyaniline effectively, but also reduce the production costs and environmental pollution during the synthetic process. Therefore, a green and industrial synthetic process is achieved. X-ray diffraction (XRD) patterns, X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared (FTIR) all confirm that composite materials have been prepared successfully. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicate that the as-prepared PGCG has regular structure. Thermogravimetric analysis (TGA) indicates that the addition of graphene nanosheets can significantly improve the thermostability of PANI. Moreover, the as-prepared material exhibits superior electrochemical performance. As an electrode material for supercapacitors, PGCG possesses high specific capacitance of 158 F g^?1 at a scan rate of 25 mV s^?1 and 147 F g^?1 at 50 mV s^?1 in 1 M H₂SO₄. The Nyquist plot also confirms that the PGCG has low charge transfer resistance and good capacitive behavior. These great properties make PGCG a novel electrode material with potential applications in high-performance energy storage devices.