Composite Electrodes for Electrochemical Supercapacitors

Manganese dioxide nanofibers with length ranged from 0.1 to 1 μm and a diameter of about 4–6 nm were prepared by a chemical precipitation method. Composite electrodes for electrochemical supercapacitors were fabricated by impregnation of the manganese dioxide nanofibers and multiwalled carbon nanotubes (MWCNT) into porous Ni plaque current collectors. Obtained composite electrodes, containing 85% of manganese dioxide and 15 mass% of MWCNT, as a conductive additive, with total mass loading of 7–15 mg cm⁻², showed a capacitive behavior in 0.5-M Na₂SO₄ solutions. The decrease in stirring time during precipitation of the nanofibers resulted in reduced agglomeration and higher specific capacitance (SC). The highest SC of 185 F g⁻¹ was obtained at a scan rate of 2 mV s⁻¹ for mass loading of 7 mg cm⁻². The SC decreased with increasing scan rate and increasing electrode mass.     

Composite electrodes for electrochemical supercapacitors

Manganese dioxide and Ag-doped manganese dioxide powders were prepared by a chemical precipitation method using KBH₄ as a reducing agent. The powders were studied by X-ray analysis, thermogravimetry, and electron microscopy. Composite electrodes for electrochemical supercapacitors (ES) were fabricated by impregnation of slurries of the precipitated powders and carbon black into porous nickel foam current collectors. In the composite electrodes, carbon black nanoparticles formed a secondary conductivity network within the nickel foam cells. Obtained composite electrodes, containing manganese dioxide and 20 wt% carbon black with total mass loading of 50 mg cm⁻², showed a capacitive behavior in the 0.5 M Na₂SO₄ solutions. The capacitive behavior of the composite electrodes can be improved by mixing of manganese dioxide and carbon black in solutions or using Ag-doped manganese dioxide powders. The highest specific capacitance (SC) of 150 F g⁻¹ was obtained at a scan rate of 2 mV s⁻¹. The electrodes showed good cycling behavior with no loss in SC during 1,000 cycles.     

Cathodic electrosynthesis of iron oxide films for electrochemical supercapacitors

Cathodic electrosynthesis has been utilized for the fabrication of γ-Fe₂O₃ films, containing chitosan additive as a binder. The films were studied by X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, differential thermal analysis, and thermogravimetric analysis. Cyclic voltammetry and chronopotentiometry data showed that the iron oxide films exhibit electrochemical capacitance in the voltage window of −0.9 to −0.1 V vs SCE in 0.25 m Na₂SO₄ and 0.25 m Na₂S₂O₃ aqueous solutions. The highest specific capacitance (SC) of 210 F g⁻¹ was achieved using 0.25 m Na₂S₂O₃ as electrolyte, at a scan rate of 2 mV s⁻¹. The SC decreased with increasing film thickness, scan rate and cycle number. Heat treatment of the films at 140 °C resulted in increasing SC.     

Effect of electrolyte concentration on morphology,microstructure and electrochemical impedance of anodic oxide film on titanium alloy Ti-10V–2Fe–3Al

Anodic oxide films were fabricated on titanium alloy Ti-10V–2Fe–3Al in ammonium tartrate solutions at the concentrations: 1, 3, 5, 10 and 15 g L⁻¹. The morphological characteristics and microstructures of the films of the alloy were studied by optical microscopy (OM) and Raman spectroscopy (Raman), respectively. The electrochemical impedances of the films in 0.5 mol L⁻¹ H₂SO₄ solution were investigated by electrochemical impedance spectroscopy (EIS). It was showed that different electrolyte concentrations led to different change rates of anodizing forming voltage. The change rate significantly affected the morphology, microstructure and electrochemical impedance of anodic oxide film. When electrolyte concentration was 5 g L⁻¹, anodic oxide film was the most uniform, exhibited by the least and smallest breakpoints on the film. In addition, the amount of crystal phase of the film was the largest at 5 g L⁻¹, showed by the highest intensity of Raman peaks. Furthermore, the electrochemical impedance of the film of the alloy was the greatest at 5 g L⁻¹, demonstrated by the highest values of polarization resistances and lowest values of capacitances. These phenomenon were associated with the minimum value of the change rate of anodizing forming voltage at 5 g L⁻¹.     

Simultaneous determination of trace aluminum (III), copper (II) and cadmium (II) in water samples by square-wave adsorptive cathodic stripping voltammetry in the presence of oxine

A validated adsorptive cathodic stripping voltammetry method is described for simultaneous determination of Al(III), Cu(II) and Cd(II) in water samples. In acetate buffer (pH 5) containing 10 μM oxine, these metal ions were determined as oxine complexes following adsorptive accumulation onto the HMDE at −0.05 V versus Ag/AgCl/KCl_s. The best signal to noise ratio was obtained using a square wave of scan increment 10 mV, frequency 120 Hz, and pulse-amplitude 25 mV. Limits of detection as low as 0.020 μg L⁻¹ Al(III), 0.012 μg L⁻¹ Cu(II) and 0.028 μg L⁻¹ Cd(II) were achieved. Interference due to various cations (K(I), Na(I), Mg(II), Ca(II), Mn(II), Fe(III), Bi(III), Sb(III), Se(IV), Pb(II), Zn(II), Ni(II), Co(II)), anions (Cl⁻, NO³⁻, SO₄ ²⁻, PO₄ ³⁻) and ascorbic acid was minimal as the measured signals change by 4% at the maximum. The stripping voltammetry method was successfully applied for simultaneous determination of Al(III), Cu(II) and Cd(II) in tap and natural bottled water samples.     

Industrial polypyrrole electrodeposition on zinc-electroplated steel

Polypyrrole films were electrodeposited on zincated surfaces in a one-step process from aqueous solutions of sodium salicylate and pyrrole. This electrolyte passivates the substrate and makes it possible to obtain strongly adherent PPy films with controlled thickness by electrolysis at constant current. With a view to industrial application for rapid polymerization on common metals, an aqueous solution of 2 M sodium salicylate and 0.5 M pyrrole at pH 5 and room temperature provides optimum conditions. Laboratory studies and deposition with a pilot cell show that 2 m of polymer can be deposited in 1 s with a current density of 70 A dm^–2.     

Electrochemical behavior of magnesium alloys AZ91D,AZCe2, and AZLa1 in chloride and sulfate solutions

The influence of Cl⁻ and SO₄²⁻ on the electrochemical behavior of AZ91D, AZCe2, and AZLa1 was studied. For all alloys, there was a current plateau in the anodic polarization curves in Na₂SO₄ solutions. In 0.5% NaCl solution, there was a small current plateau, whereas there was none in the 3.5% and 5% NaCl solutions. This indicated that SO₄²⁻ is less aggressive than Cl⁻. The range of the current plateau decreased with increasing SO₄²⁻ concentration. For all alloys, the high frequency capacitive loop in the Nyquist plots decreased with increasing concentration consistent with the decrease in corrosion resistance with increasing Cl⁻ and SO₄²⁻ concentration.     

Fabrication and electroanalytical characterization of label-free DNA sensor based on direct electropolymerization of pyrrole on p-type porous silicon substrates

Label-free DNA sensors based on porous silicon (PS) substrate were fabricated and electrochemically characterized. p-type silicon wafer was electrochemically anodized in an ethanolic hydrofluoric (HF) solution to construct a PS layer on which polypyrrole (PPy) film was directly electropolymerized. To achieve direct electropolymerization of PPy on PS substrate without pre-deposition of any metallic thin-film underlayer, a low resistivity wafer (0.01–0.02 Ω cm) was used. The rough surface of the PS layer allowed for a strong adsorption of the PPy film. Intrinsic negative charge of the DNA backbone was exploited to electrostatically adsorb 26 base pairs of probe DNA (pDNA) into the PPy film by applying positive bias. The pDNA was designed to hybridize with the target DNA (tDNA) which is the insertion element (Iel) gene of Salmonella enterica serovar Enteritidis. Dependence of peak current (i _p ) around 0.2 V vs Ag/AgCl on tDNA concentration and incubation time were shown from the cyclic voltammograms of PS/PPy + pDNA + tDNA substrates in a 0.01 M potassium perchlorate solution. Plot of i _p vs incubation time showed a reduction in current density (J) by ca. 29 μA cm⁻² every hour. Sensitivity obtained from a plot of i _p vs tDNA concentration was −166.6 μA cm⁻² μM⁻¹. Scanning electron microscopy (SEM) image of the cross-section of a PS/PPy + pDNA + tDNA multilayered film showed successful direct electropolymerization of PPy for a nano-PS DNA biosensor.     

Properties and mechanism of solar absorber CdTe thin film synthesis by unipolar galvanic pulsed electrodeposition

Electrochemical deposition of CdTe semiconductor thin films over transparent conducting glass substrates by sequential unipolar current pulses is described. The magnitude of pulsed current and pulse periodicity affects the crystalline structure, morphology, optical absorbance and composition of CdTe films. CdTe films formed under high magnitude pulsed current density ~5–15 mA cm⁻² are crystalline with dominant cubic structure having (111) plane oriented parallel to the substrate. Stoichiometric CdTe film growth occurs with current pulses of short 25–300 ms periodicity and 3–50 ms duration. A mechanism of the CdTe growth involving in situ cathodic tellurization process step involving H₂Te formation and reaction with electrochemically deposited Cd monolayer is described. CdTe film growth in the pulsed electrodeposition occurs under mass transport conditions under strong influence of high magnitude pulsed current. This results in much higher growth rates ~5–8 μm h⁻¹ for CdTe films which is attractive for CdTe solar cells in a production environment.     

Polypyrrole electropolymerized on aluminum alloy 1100 doped with oxalate and tungstate anions

Polypyrrole films doped with oxalic acid and tungstate were potentiostatically electropolymerized on aluminum alloy 1100. Two statistical factorial designs (fractional and complete) were used to study the influence of the synthesis variables on the film performance against corrosion. Corrosion protection of the polypyrrole films doped with oxalate and tungstate anions (PPy/OXA/W) on the aluminum alloy was evaluated by potentiometric and electrochemical impedance spectroscopy (EIS) measurements in a 0.05 mol L⁻¹ NaCl solution. The results obtained showed that the best performance against corrosion was detected with the PPy/OXA/W film synthesized at 1.0 V, 1.5 C in 0.2 mol L⁻¹ pyrrole, 0.1 mol L⁻¹ oxalic acid and 0.05 mol L⁻¹ sodium tungstate solutions provide a protective effect against corrosion.     

Multicriteria optimization of mechanical and morphological properties of chromium electrodeposits under reverse pulse plating

In this work, a central composite design experiment was performed to estimate the effect of the electroplating parameters (temperature of electrolyte, cathodic and anodic pulse current densities, and cathodic and anodic pulse lengths,) on four properties of hard chromium electrodeposits. The studied responses were the hardness (Hv), the roughness quantified by the two criteria R_a (nm) and R (μm)), and the specific abrasive energy, E_s (μJ μm⁻³). Analysis of the responses using optimal path technique did not lead to a common set of experimental conditions which fulfilled the required properties. Thus, the desirability function approach has been employed in order to find the best compromise between the different experimental responses. The optimal conditions are: electrolyte temperature: 49.9 °C; cathodic pulse current density: 42.0 A dm⁻²; anodic pulse current density: 51.5 A dm⁻², cathodic pulse length: 6.23 s and anodic pulse length: 28.5 ms. Under these conditions, the estimated response values are 738 Hv, 262 nm, 2.61 μm and 0.027 μJ μm⁻³ for hardness, R_a, R and specific abrasive energy respectively, validated experimentally. The resultant coating, examined by AFM, exhibits a nodular fine-grained morphology.     

Electrochemical sensor for bisphenol A detection based on molecularly imprinted polymers and gold nanoparticles

A novel bisphenol A (BPA) sensor based on amperometric detection has been developed by using molecularly imprinted polymers (MIPs) and gold nanoparticles. The sensitive layer was prepared by electropolymerization of 2-aminothiophenol on a gold nanoparticles-modified glassy carbon electrode in the presence of BPA as a template. Cyclic voltammetry was used to monitor the process of electropolymerization. The properties of the layer were studied in the presence of Fe(CN)₆ ³⁻/Fe(CN)₆ ⁴⁻ redox couples. The template and the non-binding molecules were removed by washing with H₂SO₄ (0.65 mol L⁻¹) solution. The linear response range of the sensor was between 8.0 × 10⁻⁶–6.0 × 10⁻² mol L⁻¹, with a detection limit of 1.38 × 10⁻⁷ mol L⁻¹ (S/N = 3). The proposed MIPs sensor exhibited good selectivity for BPA. The stability and repeatability of the MIPs senor were found to be satisfactory. The results from real sample analysis confirmed the applicability of the MIPs sensor to quantitative analysis.     

Zinc hexacyanoferrate film as an effective protecting layer in two-step and one-step electropolymerization of pyrrole on zinc substrate

The two-step and one-step electrosynthesis processes of polypyrrole (PPy) films on the zinc substrate are described. The two-step process includes (i) the zinc surface pretreatment with hexacyanoferrate ion in the aqueous medium in order to form a zinc hexacyanoferrate (ZnHCF) film non-blocking passive layer on the surface and with the view to prevent its reactivity and (ii) electropolymerization of pyrrole on the ZnHCF|Zn-modified electrode in aqueous pyrrole solution. In this context, both the non-electrolytic and electrolytic procedures were adapted, and the effect of some experimental conditions such as supporting electrolyte, pH and temperature of the solution at the zinc surface pretreatment step as well as pyrrole concentration and electrochemical techniques at the polymerization step was investigated. By optimizing the experimental conditions in both steps, we have obtained a homogeneous and strongly adherent PPy films on the zinc substrate.The one-step process is based on the use of an aqueous medium containing Fe(CN)₆⁴⁻ and pyrrole. The ferrocyanide ion passivates the substrate by formation of ZnHCF film during the electropolymerization process of pyrrole and therefore makes it possible to obtain strongly adherent PPy films, with controlled thickness, either by cyclic voltammetry or by electrolysis at constant current or constant potential without any previously treatment of the zinc electrode surface. The polypyrrole films deposited on the zinc electrode were characterized by cyclic voltammetry and scanning electron microscopic (SEM) measurement.     

Investigation of N<Subscript>2</Subscript>-fixation on polyaniline electrodes in methanol by electrochemical impedance spectroscopy

The ac response of polyaniline thin films on platinum electrodes was measured at different dc potentials during the N₂-fixation in methanol + LiClO₄ electrolyte with 0.03 mol L⁻¹ H₂SO₄ for the first time. The optimum film thickness was found to be 1.5 μm, N₂-pressure 50 bar and an optimum electrolysis potential of −0.12 V (NHE). The diffusion coefficients for N₂ into the polymer film was found to be (5 ± 2)×10⁻⁹ cm² s⁻¹.     

Electrochemical synthesis,characterization and capacitive properties of novel thiophene based conjugated polymer

In this paper, a novel thiophene based monomer, 1-(pyren-1-yl)-2,5-di(thiophen-2-yl)-1H-pyrrole, PThP, was synthesized and characterized by ¹H NMR and ¹³C NMR spectroscopic methods. The electrochemical behavior and electropolymerization of this novel monomer were performed on pencil graphite electrode (PGE) by cyclic voltammetry. The effect of solvent, dopant, scan number and scan rate on the electropolymerization and properties of the conjugated polymer films were investigated. The capacitive properties of the poly(PThP) films were tested by electrochemical impedance spectroscopy (EIS). The highest specific capacitance value was calculated for the conjugated polymer modified PGE that was obtained in 0.1 M tetrabutylammonium perchlorate/dichloromethane solution for 30 cycles at 25 mV/s scan rate as 25.45 mF cm⁻². The surface morphologies of the conjugated polymer modified electrodes were determined by scanning electron microscopy (SEM).     

Passivation behavior of Type 304 stainless steel in a non-aqueous alkyl carbonate solution containing LiPF6 salt

Passivation behavior of type 304 stainless steel in a non-aqueous alkyl carbonate solution containing LiPF₆ salt was studied using electrochemical polarization, X-ray photoelectron spectroscopy (XPS) and time of flight-secondary ion mass spectroscopy (ToF-SIMS). Cathodic polarization to 0 V vs. Li/Li⁺ resulted in most but not complete reduction of the air-formed film from oxides to metal on the stainless steel, as confirmed by XPS. For complete elimination of the air-formed film, the surface of the stainless steel was scratched under anodic polarization conditions. At 3 V vs. Li/Li⁺ where an anodic current peak appeared, only an indistinct layer was recognized on the newly scratched surface, according to ToF-SIMS analysis. Above 4 V vs. Li/Li⁺, substantial passive films were formed, which were composed of oxides and fluorides of iron and chromium. The origin of oxide was due to water contained in the non-aqueous alkyl carbonate solution, and that of fluorides were the result of the decomposition of electrolytic salt, LiPF₆, especially at higher potential. The resultant passive films were stable in the non-aqueous alkyl carbonate solution containing LiPF₆ salt.     

The effect of catalyst film thickness on the electrochemical promotion of ethylene oxidation on Pt

The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion of catalysis (EPOC or NEMCA effect) was investigated for the model catalytic reaction of C₂H₄ oxidation on porous Pt paste catalyst-electrodes deposited on YSZ. It was found that the catalytic rate enhancement ρ is up to 400 for thinner (0.2 μm) Pt films (40,000% rate enhancement) and gradually decreases to 50 for thicker (1 μm) films. The results are in good qualitative agreement with model predictions describing the diffusion and reaction of the backspillover O²⁻ species which causes electrochemical promotion.     

Anodization of Al–Nd alloy films in nonaqueous electrolyte solutions for TFT-LCD application

The anodization behavior of Al–Nd alloys in nonaqueous electrolyte solutions and the electronic properties of the resultant anodic oxide films were studied for TFT-LCD application. Sputtered Al–Nd alloy films on glass substrates were anodized at 25 °C and 1 mA cm⁻² up to 100 V in ethylene glycol–water solutions containing 10 wt.% ammonium tartrate or salicylate to give uniform and flat oxide films. The incorporation of organic components into the anodic oxide films from the electrolyte solutions has lowered the relative permittivity and increased the breakdown electric field of the oxide films. This was performed by decreasing the water content in the electrolyte solutions. The tartrate solution caused higher carbon incorporation than the salicylate counterpart at the same water concentrations, giving lower relative permittivity, and higher forward breakdown electric field. The AlO stretching frequency of the oxide films decreased slightly as the amount of incorporated organic moieties increased. Nd was uniformly distributed in the oxide films and an increase in the Nd content was likely to increase both the relative permittivity and the forward breakdown electric field without any apparent change in the anodization behavior.     

Carbon nanotube-modified glassy carbon electrode for anodic stripping voltammetric detection of Uranyle

A multi-wall carbon nanotube (MWNT) modified glassy carbon electrode (GCE) is described for the measurement of trace levels of uranium by anodic stripping voltammetry. In a pH 4.4 NaAc-Hac buffer containing 0.010 mol L⁻¹ Mg(NO₃)₂, UO₂ ²⁺ was adsorbed onto the surface of a MWNT film coated glassy carbon electrode and then reduced at −0.40 V vs. Ag/AgCl. During the positive potential sweep the reduced uranium was oxidized and a well-defined stripping peak appeared at +0.20 V vs. Ag/AgCl. Low concentrations of Mg²⁺ significantly enhanced the stripping peak currents since they induced UO₂ ²⁺ to adsorb at the electrode surface. The response was linear up to 1.2 × 10⁻⁷ mol L⁻¹ and the relative standard deviation at 2.0 × 10⁻⁸ mol L⁻¹ uranium was 5.2%. Potential interferences were examined. The attractive behavior of the new “mercury-free” uranium sensor holds promise for on-site environmental and industrial monitoring of uranium.     

Pyrrole Electropolymerization on Copper and Brass in a Single-Step Process from Aqueous Solution

The electrosynthesis of polypyrrole (PPy) on copper and brass (Cu–Zn alloy) electrodes was performed by anodic oxidation of pyrrole in a sodium tartrate (C₄H₄Na₂O₆ 0.2 M) aqueous solution. The tartrate counter-ions slow the dissolution of the working electrode by leading to formation of a passivation layer on its surface, and pyrrole electropolymerization takes place. Strongly adherent and homogeneous polypyrrole films were electrodeposited on Cu and Cu–Zn alloy electrodes using different electrochemical techniques, such as potentiodynamic, galvanostatic and potentiostatic modes. The current densities of electropolymerization on brass are generally greater than those observed on copper. The corrosion behaviour of copper-coated electrodes, electrochemically modified by PPy films, was estimated by DC polarization and weight loss at different current densities in 0.1 M HCl solution. The synthesized polypyrrole films were characterised by several microscopic and spectroscopic techniques such as scanning electron microscopy, X-ray photo electron spectroscopy, Fourier transform infrared and Raman analysis. Galvanostatically deposited PPy films are shown to be an alternative to common black-nickel or black-chromium as a decorative top-coating.