Study on the surface reaction of LaNi5 alloy during discharge process in KOH solution

A new method for studying surface reaction of LaNi₅ absorbing alloy in KOH solution (pH 12) was established. It is based on tip-substrate voltammetry of scanning electrochemical microscopy (SECM) where the tip faradic current is recorded while scanning the substrate potential. The Pt electrode is selected as tip electrode, and the Pt oxide formation-reduction is used as a pH-dependent reaction while the tip potential is held at a constant value. As substrate surface reactions proceed, the pH of solution can be changed, and then the tip faradic current is recorded. The mechanism of discharge process of LaNi₅ alloy was analyzed by comparing the tip current (I_tip) versus substrate potential (E_sub) curve, which reflects the exchange of H⁺ or OH⁻ between the alloy surface and the solution, with the substrate current (I_sub) versus substrate potential (E_sub) curve, which reflects the exchange of electron on the LaNi₅ alloy surface. The results showed that the OH⁻ adsorption process is occurred before the electron transfer process during discharge process, and the adsorptive OH⁻ helps the oxidation of adsorbed hydrogen atom on the alloy surface. A quantitative assessment for the maximum changes of pH during discharge process is also proposed, and the variation as large as 2.65 pH unit was detected.     

Characterisation of platinised Ti mesh electrodes using electrochemical methods: methanol oxidation in sodium hydroxide solutions

Characterisation of a platinised Ti mesh electrode, prepared by electrodeposition of Pt, by methanol oxidation has been studied over a range of NaOH and methanol concentrations using cyclic voltammetry (CV), quasi-steady-state polarisation, and electrochemical impedance spectroscopy (EIS). The activity of methanol oxidation in aqueous alkaline systems increased with pH or OH species coverage on the electrode surface. A reaction order of close to 0.5 was obtained for both NaOH and methanol in NaOH solutions indicating that adsorption of methanol and OH⁻ on the platinised electrode follows Termkin isotherm. In the high potential region, a poisoning effect was observed at methanol/[OH⁻] concentration ratio greater than 1, which could have arisen from an excess of methanol at the electrode surface and/or depletion of OH⁻ at the electrode surface. The methanol oxidation behaviour conforms with that on Pt electrodes suggesting a promising new method of preparing electrodes for fuel cells.     

Electrocatalytic activity of manganese oxides prepared by thermal decomposition for oxygen reduction

The oxygen reduction reaction (ORR) was studied in KOH electrolyte on manganese oxides supported on Vulcan carbon (Mn_yO_x/C). The oxides were prepared by thermal decomposition of manganese nitrate at different conditions. The oxides were characterized by X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The electrochemical studies were conducted using cyclic voltammetry (CV) and steady state polarization measurements carried out with a thin layer rotating ring/disk electrode. XRD results showed that the manganese oxide prepared at 200 °C in air is formed by a major phase of β-MnO₂ and the polarization curves indicated the highest activity for this material. In situ XANES evidenced the occurrence of a redox process involving Mn(II)/Mn(III) and Mn(III)/Mn(IV) in the range of potentials of the CV measurements. The electrocalytic activity was related to the occurrence of a mediation process involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen and by a disproportionation reaction of the HO₂⁻ species in the Mn_yO_x sites. In situ XANES results showed that the Mn(IV) species is MnO₂ and the Mn(III) is most likely MnOOH.     

Ce(III)/Ce(IV) in methanesulfonic acid as the positive half cell of a redox flow battery

The characteristics of the Ce(III)/Ce(IV) redox couple in methanesulfonic acid were studied at a platinum disk electrode (0.125 cm²) over a wide range of electrolyte compositions and temperatures: cerium (III) methanesulfonate (0.1–1.2 mol dm⁻³), methanesulfonic acid (0.1–5.0 mol dm⁻³) and electrolyte temperatures (295–333 K). The cyclic voltammetry experiments indicated that the diffusion coefficient of Ce(III) ions was 0.5 × 10⁻⁶ cm² s⁻¹ and that the electrochemical kinetics for the oxidation of Ce(III) and the reduction of Ce(IV) was slow. The reversibility of the redox reaction depended on the electrolyte composition and improved at higher electrolyte temperatures. At higher methanesulfonic acid concentrations, the degree of oxygen evolution decreased by up to 50% when the acid concentration increased from 2 to 5 mol dm⁻³. The oxidation of Ce(III) and reduction of Ce(IV) were also investigated during a constant current batch electrolysis in a parallel plate zinc–cerium flow cell with a 3-dimensional platinised titanium mesh electrode. The current efficiencies over 4.5 h of the process Ce(III) to Ce(IV) and 3.3 h electrolysis of the reverse reaction Ce(IV) to Ce(III) were 94.0 and 97.6%, respectively. With a 2-dimensional, planar platinised titanium electrode (9 cm² area), the redox reaction of the Ce(III)/Ce(IV) system was under mass-transport control, while the reaction on the 3-dimensional mesh electrode was initially under charge-transfer control but became mass-transport controlled after 2.5–3 h of electrolysis. The effect of the side reactions (hydrogen and oxygen evolution) on the current efficiencies and the conversion of Ce(III) and Ce(IV) are discussed.     

o-Phenylenediamine adsorbed onto silica gel modified with niobium oxide for electrocatalytic NADH oxidation

A study of a modified carbon paste electrode employing o-phenylenediamine (PDA) adsorbed onto silica gel modified with niobium oxide (SN) for electrocatalytic oxidation of nicotinamide adenine dinucleotide (NADH) is described. The species adsorbed on SN was used to prepare a modified carbon paste electrode to investigate its electrochemical properties. The formal potential (E^0′) of the adsorbed PDA was −140 mV vs. SCE (saturated calomel electrode). The electrochemical behavior of the adsorbed PDA, compared to that of PDA dissolved in aqueous solution, was completely different. In solution, pH between 3.0 and 8.0, E^0′ remained almost constant and the response was very stable. A linear response range for NADH between 4.0×10⁻⁵ and 8.0×10⁻⁴ mol l⁻¹, at pH 7.0, was observed for the electrode, with an applied potential of −50 mV vs. SCE. The formation of an intermediate charge transfer complex is proposed for the charge transfer reaction between NADH and adsorbed PDA. The heterogeneous electron transfer rate, k_obs, was 5480 mol⁻¹ l s⁻¹ and the apparent Michaelis-Menten constant, 1.04×10⁻⁴ mol l⁻¹ at pH 7.0, evaluated with rotating disk electrode (RDE) experiments with an electrode with a coverage of PDA of 5.7×10⁻⁹ mol cm⁻². The slight increase in the reaction rate with the solution pH was assigned to the thermodynamic driving force.     

Preparation of polypyrrole/ferrocyanide films modified carbon paste electrode and its application on the electrocatalytic determination of ascorbic acid

Functionalized polypyrrole film were prepared by incorporation of (Fe(CN)₆)⁴⁻ as doping anion, during the electropolymerization of pyrrole onto a carbon paste electrode (CPE) in aqueous solution by using potentiostatic method. The electrochemical behavior of the (Fe(CN)₆)³⁻/(Fe(CN)₆)⁴⁻ redox couple in polypyrrole was studied by cyclic voltammetry and double step potential chronoamperometry methods. In this study, an obvious surface redox reaction was observed and dependence of this reaction on the solution pH was illustrated. The electrocatalytic ability of polypyrrole/ferrocyanide films modified carbon paste electrode (Ppy/FCNMCPEs) was demonstrated by oxidation of ascorbic acid. It has been found that under optimum condition (pH 7.00), the oxidation of ascorbic acid at the surface of such electrode occurs at a potential about 540 mV less positive than unmodified carbon paste electrode. The kinetic parameters such as electron transfer coefficient, α and catalytic reaction rate constant, k_h′, were also determined by using various electrochemical approaches.The catalytic oxidation peak current showed a linear dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range of 4.5×10⁻⁴ to 9.62×10⁻³ M of ascorbic acid with a correlation coefficient of 0.9999. The detection limit (2σ) was determined as 5.82×10⁻⁵ M.     

Mechanistic study of the reduction of oxygen in air electrode with manganese oxides as electrocatalysts

Electrochemical reduction of oxygen (O₂) in air electrode with manganese oxides (MnOx) as electrocatalysts was studied with MnOx/Nafion-modified gold (Au) electrodes using cyclic voltammetry, potential-controlled amperometry and rotating ring-disk electrode (RRDE) voltammetry in alkaline aqueous solution. At Nafion-modified (MnOx free) Au electrode, O₂ reduction undergoes two successive two-electron processes with HO₂⁻ as intermediate. The presence of MnOx, including Mn₂O₃, Mn₃O₄, Mn₅O₈ and MnOOH, on Nafion-modified Au electrodes obviously increases the first reduction peak current of O₂ to hydrogen peroxide (HO₂⁻ in this case) and decreases the second one of HO₂⁻ to OH⁻, while does not shift the reduction potential. MnOx was found to show catalytic activity for the disproportionation reaction of HO₂⁻ to O₂ and OH⁻ and thus, the O₂ reduction in air electrode was considered to include an initial two-electron reduction of O₂ to HO₂⁻ followed by a disproportionation reaction of HO₂⁻ into O₂ and OH⁻ catalyzed by MnOx. The excellent activity of MnOx for the follow-up disproportionation reaction substantially results in an overall four-electron reduction of O₂ at MnOx/Nafion-modified Au electrodes in the first reduction step, depending on potential scan rate and the kind of MnOx. The present work provides a scientific significance of the mechanism of O₂ reduction in air electrode using MnOx as electrocatalysts to effect a four-electron reduction of O₂ to OH⁻.     

Oxydation anodique des ions Mn2+ et Fe2+ en milieu marin reconstitue

The anodic oxidation of Mn²⁺ and Fe²⁺ ions in a reconstituted marine solution has been studied under atmospheric pressure and 500 atm. The study concerning the Mn²⁺ ion showed that the electrode process is a monoelectronic step which is followed by a chemical reaction of disproportionation; the obtained deposit corresponds to manganese dioxide of variety γ. For the electrochemical oxidation of Fe²⁺ ion, a passivation reaction has been observed by stationnary voltammetry; a qualitative study by cyclic voltammetry showed that the slow monoelectronic step is followed by a slow chemical reaction in solution which has no influence on the transfer of electron exchange. The resulting compound is a hydrated iron (III) sulphate.     

Investigation on the electrode process of the Mn(II)/Mn(III) couple in redox flow battery

The Mn(II)/Mn(III) couple has been recognized as a potential anode for redox flow batteries to take the place of the V(IV)/V(V) in all-vanadium redox battery (VRB) and the Br₂/Br⁻ in sodium polysulfide/bromine (PSB) because it has higher standard electrode potential. In this study, the electrochemical behavior of the Mn(II)/Mn(III) couple on carbon felt and spectral pure graphite were investigated by cyclic voltammetry, steady polarization curve, electrochemical impedance spectroscopy, transient potential-step experiment, X-ray diffraction and charge-discharge experiments. Results show that the Mn(III) disproportionation reaction phenomena is obvious on the carbon felt electrode while it is weak on the graphite electrode owing to its fewer active sites. The reaction mechanism on carbon felt was discussed in detail. The reversibility of Mn(II)/Mn(III) is best when the sulfuric acid concentration is 5 M on the graphite electrode. Performance of a RFB employing Mn(II)/Mn(III) couple as anolyte active species and V(III)/V(II) as catholyte ones was evaluated with constant-current charge-discharge tests. The average columbic efficiency is 69.4% and the voltage efficiency is 90.4% at a current density of 20 mA cm⁻². The whole energy efficiency is 62.7% close to that of the all-vanadium battery and the average discharge voltage is about 14% higher than that of an all-vanadium battery. The preliminary exploration shows that the Mn(II)/Mn(III) couple is electrochemically promising for redox flow battery.     

Electrochemical behaviors of thymine on a new ionic liquid modified carbon electrode and its detection

A new electrochemical method was proposed for the determination of thymine, which relied on the oxidation of thymine at a carbon ionic liquid electrode (CILE) in a pH 5.0 Britton-Robinson buffer solution. CILE was fabricated by using ionic liquid 1-(3-chloro-2-hydroxy-propyl)-3-methylimidazole acetate as the binder, which showed strong electrocatalytic ability to promote the oxidation of thymine. A single well-defined irreversible oxidation peak appeared with adsorption-controlled process and enhanced electrochemical response on the CILE, which was due to the presence of high conductive ionic liquid on the electrode. The reaction parameters of thymine were calculated with the electron transfer coefficient (α) as 0.27, the electron transfer number (n) as 1.23, the apparent heterogeneous electron transfer rate constant (k_s) as 6.87 × 10⁻⁶ s⁻¹ and the surface coverage (Г_T) as 5.71 × 10⁻⁸ mol cm⁻². Under the selected conditions the oxidation peak current was proportional to thymine concentration in the range from 3.0 to 3000.0 μM with the detection limit as 0.54 μM (3σ) by differential pulse voltammetry. The proposed method showed good selectivity to the thymine detection without the interferences of coexisting substances.     

A novel Electro-Fenton-Like system using PW11O39Fe(III)(H2O) as an electrocatalyst for wastewater treatment

A novel Electro-Fenton-Like (EFL) system was developed using the Keggin-type iron-substituted heteropolytungstate anion PW₁₁O₃₉Fe(III)(H₂O)⁴⁻ to substitute for Fe³⁺ in the conventional Electro-Fenton (EF) system for treatment of water polluted with organic compounds. The EFL system overcomes the drawback of low pH in conventional EF approaches and can be directly applied to neutral water treatment without any pH adjustment. Experimental results for dimethylphthalate (DMP) revealed complete degradation in <80 min in pH 6.86 solution containing 0.1 mM DMP at a potential of −0.5 V and O₂ flow rate of 60 mL min⁻¹. Total organic carbon removal of ∼56% was achieved at 120 min. Comparison with conventional EF oxidation revealed better efficiency of the present system for DMP degradation, suggesting its potential in treatment of water and wastewater with a relaxed pH requirement. The cumulative H₂O₂ concentration generated in situ at the electrode was monitored and the observed degradation rate constants k_obs were determined for different initial DMP concentrations. The ligand exchange reaction of PW₁₁O₃₉Fe(III)(H₂O)⁴⁻ with H₂O₂ and the electron transfer resulting in hydroxyl radicals were examined using HPLC and electrochemical impedance spectroscopy. An electrocatalytic model involving inner-sphere electron transfer and a reaction mechanism for PW₁₁O₃₉Fe(III)(H₂O)⁴⁻ electrocatalytic reduction of H₂O₂ are proposed.     

Removal of Chromium and Organic Pollutants from Industrial Chrome Tanning Effluents by Electrocoagulation

The treatment of industrial chrome tanning effluents by electrocoagulation (EC) in a laboratory‐scale reactor was investigated. Mild‐steel (MS) electrodes have been found to outperform aluminum (Al) electrodes in reducing the Cr(III) concentration to <2 mg L^–1. The conversion of Fe(II) to Fe(III) is slow in the lower pH range (<6), and OH^– ions generated during EC are amply available for Cr(III) removal by precipitation in the case of the MS electrode. Formation of Al(OH)₃(s) in competition with Cr(OH)₃(s) while consuming the OH^– ion is a cause for lower Cr(III) removal with Al. EC with the MS electrode and chemical coagulation (CC) with addition of alkali proved to be equally efficient for removing Cr(III).     

Nile blue adsorbed onto silica gel modified with niobium oxide for electrocatalytic oxidation of NADH

A study of modified carbon paste electrode employing Nile blue (NB) adsorbed on silica gel modified with niobium oxide (SN) for electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH) is described. The adsorbed organic dye on SN was used to prepare a modified carbon paste electrode to investigate its electrochemical properties. The formal potential (E°′) of the adsorbed NB (−230 mV vs. saturated calomel electrode, SCE) showed a shift of 70 mV towards a more positive potential value, compared to NB dissolved in aqueous solution. In solutions with pH between 6.0 and 8.0 did stability and E°′ remained almost constant. However, for a solution pH lower than 6.0 the E°′ was affected by the acidity of the contacting solution, shifting the E°′ towards more positive values. For the solution pHs between 6.0 and 8.0 the electrocatalytic activity remained almost constant. A linear response range for NADH between 1.0×10⁻⁵ and 5.2×10⁻⁴ mol l⁻¹, at pH 7.0, was observed for the electrode, with an applied potential of −200 mV versus SCE. The formation of an intermediate charge transfer (CT) complex was proposed to the CT reaction between NADH and adsorbed NB. The heterogeneous electron transfer rate, k_obs, was 1400 M⁻¹ s⁻¹ and the apparent Michaelis-Menten constant, was 0.21 mM at pH 7.0 evaluated from rotating disk electrode (RDE) experiments with an electrode coverage of about 5.2×10⁻⁹ mol cm⁻². The increase in the reaction rate between NADH and the immobilized NB compared to those obtained with dissolved NB was assigned to the shift of the E°′ towards more positive values.     

First insights into the borohydride oxidation reaction mechanism on gold by electrochemical impedance spectroscopy

Direct borohydride fuel cells (DBFC) exhibit some potential regarding the powering of small portable electronic devices, thanks to their high energy density as well as the facile and safe storage of borohydride salts. However, DBFC are hindered because (i) the borohydride oxidation reaction (BOR) is complex, (ii) its mechanism imperfectly determined yet and (iii) no practical electrocatalyst exhibits both fast BOR kinetics and high faradaic efficiency. In this context, we characterized the BOR mechanism for polycrystalline bulk gold (a classical model BOR electrocatalyst) in the rotating disk electrode (RDE) setup. Modeling cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) data, we propose a simplified reaction pathway, the theoretical behavior of which agrees with the experimental findings. This pathway includes at least a first irreversible electrochemical step (E) for BH₄⁻ oxidation, which competes with the electrochemical adsorption reaction (EAR) of OH⁻ anions at high potentials.     

Towards the conception of an amperometric sensor of l-tyrosine based on Hemin/PAMAM/MWCNT modified glassy carbon electrode

A novel amperometric sensor was fabricated based on the immobilization of hemin onto the poly (amidoamine)/multi-walled carbon nanotube (PAMAM/MWCNT) nanocomposite film modified glassy carbon electrode (GCE). Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and ultraviolet visible (UV-vis) adsorption spectroscopy were used to investigate the possible state and electrochemical activity of the immobilized hemin. In the Hemin/PAMAM/MWCNT nanocomposite film, MWCNT layer possessed excellent inherent conductivity to enhance the electron transfer rate, while the layer of PAMAM greatly enlarged the surface average concentration of hemin (Γ) on the modified electrode. Therefore, the nanocomposite film showed enhanced electrocatalytical activity towards the oxidation of l-tyrosine. The kinetic parameters of the modified electrode were investigated. In pH 7.0 phosphate buffer solution (PBS), the sensor exhibits a wide linear range from 0.1 μM to 28.8 μM l-tyrosine with a detection limit of 0.01 μM and a high sensitivity of 0.31 μA μM⁻¹ cm⁻². In addition, the response time of the l-tyrosine sensor is less than 5 s. The excellent performance of the sensor is largely attributed to the electro-generated high reactive oxoiron (IV) porphyrin (O = Fe^IV-P) which effectively catalyzed the oxidation of l-tyrosine. A mechanism was herein proposed for the catalytic oxidation of l-tyrosine by oxoiron (IV) porphyrin complexes.     

Self-assembled nano-arrays of single-walled carbon nanotube-octa(hydroxyethylthio)phthalocyaninatoiron(II) on gold surfaces: Impacts of SWCNT and solution pH on electron transfer kinetics

The construction by sequential self-assembly process of reproducible, highly stable and pH-responsive redox-active nanostructured arrays of single-walled carbon nanotubes (SWCNTs) integrated with octa(hydroxyethylthio)phthalocyaninatoiron(II) (FeOHETPc) via ester bonds on a gold surface (Au-Cys-SWCNT-FeOHETPc) is investigated and discussed. The successful construction of this electrode is confirmed using atomic force microscopy and X-ray photoelectron spectroscopy as well as from the distinct cyclic voltammetric and electrochemical impedance spectroscopic profiles. The Au-Cys-SWCNT-FeOHETPc electrode exhibited strong dependence on the reaction of the head groups and the pH of the working electrolytes, the surface pK_a is estimated as 7.3. The high electron transfer capability of the Au-Cys-SWCNT-FeOHETPc electrode over other electrodes (Au-Cys-SWCNT or the Au-Cys-FeOHETPc or the Au-FeOHETPc) suggests that SWCNT greatly improves the electronic communication between FeOHETPc and the bare gold electrode. The electron transfer rate constant (k_app) of Au-Cys-SWCNT-FeOHETPc in pH 4.8 conditions (∼1.7 × 10⁻² cm⁻² s⁻¹) over that of the electrode obtained from SWCNT integrated with tetraaminophthalocyninatocobalt(II) (Au-Cys-SWCNT-CoTAPc) (5.1 × 10⁻³ cm⁻² s⁻¹) is attributed to the possible effect of the central metal on the phthalocyanine core and substituents on the peripheral positions of the phthalocyanine rings. We also prove that aligned SWCNT arrays exhibit much faster electron transfer kinetics to redox-active species in solutions compared to the randomly dispersed (drop-dried) SWCNTs.     

Impedance measurements of a chalcogenide membrane iron(III)-selective electrode in contact with aqueous electrolytes

The electrochemical characterization of a chalcogenide-based iron(III)-selective electrode [Fe(III) ISE] [i.e., Fe_2.5(Se₆₀Ge₂₈Sb₁₂)_97.5] was achieved using impedance spectroscopy. The influence of electrolyte composition (i.e., NO₃⁻, Cl⁻, and pH) on the membrane oxidation reaction has been examined, and a mechanism for its action is proposed. Equivalent circuit analysis was undertaken to determine the interfacial charge transfer resistance and corresponding double layer capacitance as a function of electrolyte composition and immersion time. Variations were detected in the charge transfer time constant, and this was attributed to changes in the dielectric/conduction properties of the surface layer. It was found that the Fe_2.5(Se₆₀Ge₂₈Sb₁₂)_97.5 oxidation kinetics depend on the pH, and the interfacial reaction is dictated by sluggish charge transfer. By contrast, chloride was shown to accelerate the rate of membrane oxidation presumably via the formation of soluble metal-chloride complexes. Electrochemical impedance spectroscopy (EIS) aging studies of the Fe_2.5(Se₆₀Ge₂₈Sb₁₂)_97.5 membrane in chloride electrolyte under alkaline conditions showed that the charge transfer resistance decreases with exposure time. However, extended aging revealed a change in the rate of oxidation, which was attributed to a combined diffusion/passivation effect. It is proposed that the development of a modified surface layer (MSL) and passive surface layer (PSL) are partly responsible for the electrochemical stability of the chalcogenide membrane. This paper attempts to clarify and address some of the misconceptions/issues reported previously in the literature on the chalcogenide iron(III)-selective electrode.     

Surface, kinetics and electrocatalytic properties of the Ti/(Ti + Ru + Ce)O2-system for the oxygen evolution reaction in alkaline medium

Ti-supported (Ti + Ru + Ce)O₂ electrodes, prepared at 450 °C, were characterised by XRD, open-circuit potential (E_oc), capacity data (C) and morphology factor (φ) determinations. XRD measurements showed mixed oxides present a low degree of crystallinity. E_oc-data and CV-spectra support surface electrochemistry of mixed oxides is governed by the Ru(III)/Ru(IV) redox couple. In situ surface characterisation revealed the active surface area increases on increasing nominal CeO₂-content. φ-Values remained in the 0.18-0.3 interval supporting the coatings have a low electrochemical porosity. Kinetics was studied recording polarisation and chronopotentiometric curves, which permitted to determine the Tafel slope and reaction order (with respect to OH⁻), in the low and high overpotential domains. Tafel slope data, b, presented a dependence on overpotential and oxide composition indicating the OER electrode mechanism depends on these variables. A unit reaction order with respect to OH⁻ was found for all electrode compositions investigated. The theoretical analysis of the electrode mechanism permitted to analyse the changes in the experimental Tafel slopes taking into account modifications in the apparent electronic transfer coefficient, α_ap. Analysis of the true and apparent electrocatalytic activities revealed the O₂-evolution reaction rate is affected by oxide composition due to morphologic effects.     

Electrochemical determination of bromide at a multiwall carbon nanotubes-chitosan modified electrode

A multiwall carbon nanotubes (MWNTs)-chitosan modified glassy carbon electrode (GCE) exhibits attractive ability for highly sensitive cathodic stripping voltammetric measurements of bromide (Br⁻). In pH 1.8 H₂SO₄ solution, a substantial increase in the stripping peak current of Br⁻ (compared to bare GCE and chitosan modified GCE) is observed using MWNTs-chitosan modified electrode. Operational parameters were optimized and the electrochemical behaviors of Br⁻ were studied by different electrochemical methods. The kinetics parameters were measured, the number of electron transfer (n) was 1 and the transfer coefficient (α) is 0.17. A wide linear calibration range (3.6 × 10⁻⁷-1.4 × 10⁻⁵ g mL⁻¹) was achieved, with a detection limit of 9.6 × 10⁻⁸ g mL⁻¹. The mechanism of electrode reaction was fully discussed.     

The oxidation of trivalent chromium at the reconstructed Au(100) electrode

The electro-oxidation of Cr(III) to Cr(VI) species at the reconstructed Au(100) electrode has been investigated in a highly alkaline solution. The subsequent comparison of the results obtained with those at Au(111) electrode points to the structural sensitivity of this oxidation process related to different adsorbability of OH⁻ anions on both electrodes. Finally, it was found that the addition of Cr(III) ions does not shift the reconstruction peak to less positive values of potential indicating that for Au(100) electrode the oxidation process under consideration has no impact on the lifting of the surface reconstruction.