Daniel Bélanger

A Hybrid Activated Carbon-Manganese Dioxide Capacitor using a Mild Aqueous Electrolyte

A hybrid electrochemical capacitor using MnO 2 and activated carbon (AC) as positive and negative electrodes, respectively, has been designed. The electrodes were individually tested in a mild aqueous electrolyte (0.65 M K 2 SO 4 ) in order to define the adequate balance of active material in the capacitor as well as the working voltage. The hybrid electrochemical capacitor was cycled between 0 and 2.2 V for over 10,000 constant current charge/discharge cycles. A real energy density of 10 Wh/kg was reproducibly measured with a real power density reaching 3600 W/kg. The hydrogen and oxygen evolution reactions on AC and MnO 2 electrodes, respectively, were investigated in 0.65 M K 2 SO 4 . Despite the good electrochemical performance of the 2.2 V capacitor, gas evolution could be a hindrance for practical use. Subsequently, a 1.5 V capacitor was tested for more than 23,000 cycles and yielded interesting electrochemical performance with negligible gas evolution.

Electrochemical Characterization of Polyaniline in Nonaqueous Electrolyte and Its Evaluation as Electrode Material for Electrochemical Supercapacitors

In this study, the performance of a polyaniline (PANI) based supercapacitor where electroactive PANI films were prepared on carbon paper electrodes from a nonaqueous solution with an organic acid (CF 3 COOH) as the proton source was investigated. The use of nonaqueous media as electrolyte led to an increase of the electroactivity window from 0.75 V in aqueous media up to 1.0 V. Low frequency capacitance, evaluated by electrochemical impedance spectroscopy, of about 150 F/g is reported. Scanning electron microscopy indicated a highly porous material for deposited charges greater than 1 C/cm 2 . Constant current charge/discharge cycling of a symmetric supercapacitor based on PANI in nonaqueous medium was performed in a two-electrode cell configuration and a loss of about 60% of the discharge capacity was demonstrated after 1000 cycles. Tetramethylammonium methanesulfonate (Me 4 NCF 3 SO 3 ) was also used instead of tetraethylammonium tetrafluoborate (Et 1 NBF 4 ) as supporting electrolyte in acetonitrile for the charge/discharge testing of the PANI-PANI capacitor. Energy and power densities of approximately 3.5 Wh/kg and 1300 W/kg, respectively, were developed by this supercapacitor for a cell voltage of I V and a discharge time of 20 s. On the other hand, an asymmetrical supercapacitor with polypyrrole and polyaniline used as positive and negative electrodes, respectively, displayed slightly improved performance. Indeed, an energy density of 5 Wh/kg and a power density of 1200 W/kg were reported for discharge time of about 20 s with 1 M Me 4 NCF 3 SO 3 /acetonitrile as electrolyte.

Electrochemistry of the polypyrrole glucose oxidase electrode

Abstract A polypyrrole-glucose oxidase electrode prepared by anodic polymerization of pyrrole and glucose oxidase with KCl aqueous electrolyte on a platinum electrode showed redox behaviour in aqueous pure supporting electrolyte. The electrochemistry of the polypyrrole-glucose oxidase electrode resembled closely that of a polypyrrole electrode prepared from an aqueous solution containing only pyrrole and KC1. The amperometric response to glucose was generated by electrochemical oxidation of enzymatically produced hydrogen peroxide and was observed only after the loss of polypyrrole electroactivity. The results presented in this paper indicate that polypyrrole served only as an immobilization matrix to glucose oxidase in the polypyrrole-glucose oxidase electrode.

Characterization and long-term performance of polyaniline-based electrochemical capacitors

The performance of polyaniline-based electrochemical capacitors was evaluated under various experimental conditions. The capacitor consisted of two platinized tantalum foils coated with polyaniline as the active material, a separator, and an appropriate aqueous electrolyte solution. The polyaniline coatings were formed galvanostatically 5 mA cm -2 from a 0.1 M aniline +1.0 M HCl aqueous solution. With a polyaniline loading formed by a deposition charge of 20 C cm -2 on each electrode and with a 4.0 M HBF 4 aqueous solution as the electrolyte for an optimized capacitor, energy and power densities of 2.7 Wh kg -1 and 1.0 kW kg -1 (of active polymer) were achieved, respectively. Cyclic voltammograms for both positive and negative polyaniline electrodes of the capacitor before and after 20,000 cycles showed only a 5% loss of polyaniline electroactivity, which was smaller than the observed 33% decrease in the discharge capacity of the capacitor. Cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the capacitors and to understand their initial performance loss upon constant-current cycling.

Electrochemical modification of a carbon electrode using aromatic diazonium salts. 2. Electrochemistry of 4-nitrophenyl modified glassy carbon electrodes in aqueous media

The electrochemical behavior of a 4-nitrophenyl modified glassy carbon electrode has been investigated in the absence and presence of electroactive species such as ferricyanide, ruthenium hexaamine and hydroquinone following its electrochemical reduction in aqueous acidic media. The blocking properties of the grafted film are significantly altered following the reduction of the 4-nitrophenyl film. The reduction yields nitrosophenyl, hydroxyaminophenyl and aminophenyl moieties. A set of redox waves, attributed to the products of the reduction of the 4-nitrophenyl, is observed at about 0.42 V versus Ag|AgCl and is related to the nitrosophenyl/hydroxyaminophenyl interconversion. XPS measurements at the N1s core level have confirmed a loss of nitro groups upon electrochemical reduction of the grafted film. The decrease of the 406 eV peak associated with nitro groups is larger than the increase of the peak at about 400 eV that would correspond to the nitro group reduction products. This can be attributed to the cleavage of a fraction of the substituted phenyl group from the glassy carbon electrode surface. The XPS data also suggest that only a fraction of the grafted 4-nitrophenyl groups are electrochemically reduced under our experimental conditions. Nonetheless, the barrier properties of the resulting layer are maintained as shown by the suppression of the Ru(NH3)63+electrochemistry.

Optimization of a polypyrrole glucose oxidase biosensor

An amperometric glucose biosensor was fabricated by the electrochemical polymerization of pyrrole onto a platinum electrode in the presence of the enzyme glucose oxidase in a KCl solution at a potential of +0.65 V versus SCE. The enzyme was entrapped into the polypyrrole film during the electropolymerization process. Glucose responses were measured by potentiostating the enzyme electrode at a potential of +0.7 V versus SCE in order to oxidize the hydrogen generated by the oxidation of glucose by the enzyme in the presence of oxygen. Experiments were performed to determine the optimal conditions of the polypyrrole glucose oxidase film preparation (pyrrole and glucose oxidase concentrations in the plating solution) and the response to glucose from such electrodes was evaluated as a function of film thickness, pH and temperature. It was found that a concentration of 0.3 M pyrrole in the presence of 65 U/ml of glucose oxidase in 0.01 M KCl were the optimal parameters for the fabrication of the biosensor. The optimal response was obtained for a film thickness of 0.17 microns (75 mC/cm2) at pH 6 and at a temperature of 313 K. The temperature dependence of the amperometric response indicated an activation energy of 41 kJ/mole. The linearity of the enzyme electrode response ranged from 1.0 mM to 7.5 mM glucose and kinetic parameters determined for the optimized biosensors were 33.4 mM for the Km and 7.2 microA for the Imax. It was demonstrated that the internal diffusion of hydrogen peroxide through the polypyrrole layer to the platinum surface was the main limiting factor controlling the magnitude of the response of the biosensor to glucose. The response was directly related to the enzyme loading in the polypyrrole film. The shelf life and the operational stability of the optimized biosensor exceed 500 days and 175 assays, respectively. The substrate specificity of the entrapped glucose oxidase was not altered by the immobilization procedure.

A Hybrid Fe3 O 4 ­ MnO2 Capacitor in Mild Aqueous Electrolyte

A hybrid electrochemical capacitor using MnO 2 and Fe 3 O 4 as active material for the positive and the negative electrode, respectively, has been designed. The electrodes have been individually tested in a mild aqueous electrolyte (0.1 M K 2 SO 4 ) to define the adequate balance of active material in the capacitor as well as the working voltage of a capacitor based on these two electrodes. The specific capacitances of MnO 2 and Fe 3 O 4 were 150 ′ 10 and 75 ′ 8 F/g, respectively whereas the specific capacitance of the Fe 3 O 4 /MnO 2 capacitor was equal to about 20 F/g of active material. The hybrid electrochemical capacitor has been cycled between 0 and 1.8 V for over 5000 constant current charge/discharge cycles. A real energy density of 7 Wh/kg was reproducibly measured with a real power density up to 820 W/kg.

Spontaneous functionalization of carbon black by reaction with 4-nitrophenyldiazonium cations.

The mechanism of the spontaneous chemical functionalization of Vulcan carbon black by reaction with 4-nitrophenyl diazonium cations was investigated by varying the reaction conditions. First, the carbon black was oxidized by nitric acid reflux to introduce oxygenated functionalities onto the surface prior to the functionalization step. Second, a reducing agent (H3PO2) was added to a solution containing 4-nitrobenzene diazonium tetrafluoroborate to generate 4-nitrophenyl radicals homogeneously in the bulk solution. The functionalized carbons were characterized by elemental analysis, X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption isotherms using the BET isotherm and DFT Monte Carlo simulations. These characterization methods were employed to determine the grafting yield as a function of the reaction conditions. Interestingly, the grafting yield was not affected by a change in the reaction conditions. An average nitrogen content of 1.4 +/- 0.1 atom % was found by elemental analysis, and XPS showed a nitrogen surface concentration of about 3.5%. XPS also indicated an important decrease in the concentration of oxygenated functionalities upon grafting 4-nitrophenyl moieties onto the oxidized carbon black. Presumably, in this case the grafting involves either the coupling of carboxylate and 4-nitrophenyl radicals or, more likely, a concerted decarboxylation where the diazonium cation, acting as an electrophile, replaces the oxygenated groups and loss of CO2. The nitrogen adsorption isotherms of the functionalized carbon blacks suggested that the grafted groups were most probably localized at the entrance of the micropores.

Stability of substituted phenyl groups electrochemically grafted at carbon electrode surface

The electrochemical reduction of an aryl diazonium tetrafluoroborate salt, dissolved in acetonitrile, at a carbon electrode surface allowed the grafting of aryl groups with the formation of a carbon-carbon bond. Groups such as 4-carboxyphenyl, 4-nitrophenyl, 4-diethylaniline (DEA), and 4-bromophenyl were grafted at a glassy carbon electrode surface. The stability of these grafted groups, present at the glassy carbon electrode surface, was studied at various electrode potentials in aqueous media. In appropriate experimental conditions, the as-grafted groups severely inhibit the cyclic voltammetry response of selected redox probes. Thus, the reappearance and/or increase of an electrochemical response, after polarization, was taken as an indication that a modification of the grafted layer occurred. Our results demonstrated that polarization at very positive (ca. 1.8 V) and negative (ca. -2 V) potentials is needed to observe an electrochemical response. Electrochemical impedance and X-ray photoelectron spectroscopies were also used to investigate the stability of the grafted layers. The impedance data usually tracks fairly well the cyclic voltammetry results, although the former appears to be more sensitive to changes that are occurring upon polarization of the modified electrode. Interestingly, the XPS data indicate clearly that the grafted layer is not always completely removed at the extreme positive and negative potentials investigated. A mechanism was proposed to explain the transformation occurring during polarization of the modified electrode and involves desorption of the substituted aryl groups during the concomitant hydrogen, oxygen, or chlorine evolution and finally leaving close to a covalently bonded monolayer of the grafted species at the electrode surface.

Nitrate removal by a paired electrolysis on copper and Ti/IrO2 coupled electrodes - Influence of the anode/cathode surface area ratio

In this study, nitrate removal in alkaline media by a paired electrolysis with copper cathode and Ti/IrO(2) anode enabled the conversion of nitrate to nitrogen. Optimum conditions for carrying out reduction of nitrate to ammonia and subsequent oxidation of the produced ammonia to nitrogen were found. At the copper cathode, electroreduction of nitrate to ammonia was optimal near -1.4 V vs Hg/HgO. At the Ti/IrO(2) anode, a pH value of 12, the presence of chloride and a potential fixed around 2.3 V vs Hg/HgO permitted the production of hypochlorite, leading to the oxidation of ammonia to nitrogen with a N(2) selectivity of 100%. Controlling the cathode/anode surface area ratio, and thus the current density, appeared to be a very efficient way of shifting electrode potentials to optimal values, consequently favoring the conversion of nitrate to nitrogen during a paired galvanostatic electrolysis. A cathode/anode surface area ratio of 2.25 was shown to be the most efficient to convert nitrate to nitrogen.