Oxidative dissolution of pyrite in acidic media

The pyrite oxidative dissolution in air-saturated (AS), H₂O₂, and Fe³⁺ solutions at pH 2.5 and 25 °C was investigated by electrochemical and aqueous batch experiments. The corrosion current density (i _corr) increases from AS solution to Fe³⁺ and H₂O₂ solutions. For the same oxidant, i _corr increases when the concentration of the oxidant increases. Similar variation was observed for the corrosion potential (E _corr). Electrochemical impedance spectroscopy measurements have indicated that in AS and H₂O₂ solutions, the charge transfer is the rate determining step of pyrite oxidative dissolution. In the presence of Fe _(aq) ³⁺ , both the charge transfer process and mass transfer caused by the diffusion of oxidant or reaction products across the interface of electrode control the mineral oxidative dissolution. The corrosion current densities of oxidative dissolution measured by electrochemical methods are higher than those estimated from dissolution rates determined by aqueous bath experiments. The observed differences suggest that the mechanism of polarized electrode oxidation is different by the mechanism of pyrite oxidation under open circuit conditions.     

Study of reversible electrode processes with unsymmetrical cyclic reciprocal derivative chronopotentiometry

In this paper, the theoretical study of cyclic reciprocal derivative chronopotentiometry (CRDCP) for totally reversible electrode processes with symmetrical and unsymmetrical programmed currents is presented. The main viewpoints are: (1) for symmetrical programmed current, the amplitudes of the successive currents are the same, i.e. I(t) = (−1)ⁱ⁺¹I₀, whereas the transition times of each current step, τ_i, have different values. The properties of the dt/dE-E curves corresponding to each current cycle also differ a lot. (2) For unsymmetrical programmed current proposed in this work for the first time, the applied current successively reversed at each transition time steps to different amplitude. The use of this unique programmed current is advantageous versus symmetrical programmed current since the transition times obtained are equal to each other anticipatively, and the dt/dE-E curves also coincide with each other for the successive cycles. In this case, the results obtained in CRDCP are almost similar to those of cyclic voltammetry (CV). The characteristic parameters obtained in the dt/dE-E curves in both cases are quantitatively related to the species concentrations adjacent to the electrode surface and afford simple diagnosis criteria to characterize the reversibility of electrode processes. Properties of reversible electrode processes have also been further studied through the use of more than one current cycle.     

Determination of overall mass transport coefficients in gas diffusion electrodes

Gas diffusion electrodes are used for many purposes, for example in fuel cells, in synthesis and as anodes in electrodeposition processes. The behaviour of gas diffusion electrodes has been the subject of many studies. In this work the transport of gas in the gas diffusion electrode, characterized by the overall mass transport coefficient, has been investigated using hydrogen-nitrogen mixtures. A reactor model for the gas compartment of the gas diffusion electrode test cell is proposed to calculate the concentration of hydrogen in the gas compartment as a function of the input concentration of hydrogen and the total volumetric gas flow rate. The mass transport coefficient is found to be independent of variations in hydrogen concentration and volumetric gas flow rate. The temperature dependence of the mass transport coefficient has been determined. A maximum was found at 40°C.Notation Agd geometric electrode surface area (m²) - C _in concentration of reactive component at the inlet of the gas compartment (mol m^–3) - c _out concentration of reactive component at the outlet of the gas compartment (mol m^–3) - E potential (V) - E _e equilibrium potential (V) - E _t upper limit potential (V) - F _v volumetric flow rate (m^–3 s^–1) - F _v,H volumetric flow rate of hydrogen (m^–3 s^–1) - F _v,N volumetric flow rate of nitrogen (m^–3 s^–1) - F _vin volumetric flow rate at the inlet of the gas compartment (m^–3 s^–1) - F _v,out volumetric flow rate at the outlet of the gas compartment (in ^–3 s^–1) - F _v,reaction volumetric flow rate of reactive component into the gas diffusion electrode (m^–3 s^–1) - Faraday constant (A s mo^–1) - I _gd current for gas diffusion electrode (A) - i _gd current density for gas diffusion electrode (A m^–2) - I _gd,1 diffusion limited current for gas diffusion electrode (A) - i _gd,1 diffusion limited current density for gas diffusion electrode (A m^–2) - I _gd,1,calc calculated diffusion limited current for gas diffusion electrode (A) - i _gd,1,calc calculated diffusion limited current density for gas diffusion electrode (A m^–2) - I _hp current for hydrogen production (A) - k _s mass transport coefficient calculated from c _out (m s^–1) - n number of electrons involved in electrode reaction - T temperature (°C) - V _m molar volume of gas (m³ mol^–1) - overpotential (V)     

Automation of electrode kinetics—V. The dissolution of Al in Cl− and F− containing aqueous solutions

The dissolution of Al in Cl^? and F^? containing aqueous solutions has been investigated over a wide potential range. The steady state current-potential curves and the impedance have been measured using a new electrochemical measurement system. Using an equivalent circuit the C_dl,-E,θ-E, σ_A1-E and R_w-E curves are derived and with the primary i-E and Z(w)-E data are compared for each anion.     

AC and DC studies of the pitting corrosion of Al in perchlorate solutions

The pitting corrosion behaviour of Al in aerated neutral sodium perchlorate solutions was investigated by potentiodynamic, cyclic voltammetry, galvanostatic, potentiostatic and electrochemical impedance spectroscopy (EIS) techniques, complemented by ex situ scanning electron microscopy (SEM) examinations of the electrode surface. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al₂O₃ on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film by ClO₄⁻ ions. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allow the pitting potential (E_pit) and the repassivation potential (E_rp) to be determined. E_pit decreases with increase in ClO₄⁻ concentration, but increases with increase in potential scan rate. Potentiostatic measurements showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involve pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation (t_i⁻¹) increases with increase in ClO₄⁻ concentration and applied step anodic potential (E_s,a). EIS measurements showed that at E_s,a < E_pit, a charge-transfer semicircle is obtained. This semicircle is followed by a Warburg diffusion tail at E_s,a > E_pit. An attempt is made to compare the values of E_pit and E_rp obtained through different methods and to determine the factors influencing these values in each particular method.     

Electrochemical degradation of 2,4-dichlorophenol on a palladium modified gas-diffusion electrode

Pd/C catalyst was prepared by hydrogen reduction method and used for the Pd/C gas-diffusion electrode. It was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) techniques. The electrochemical degradation of 2,4-dichlorophenol was investigated in a diaphragm electrolysis system, feeding firstly with hydrogen gas then with air, using the Pd/C gas-diffusion electrode and the carbon/polytetrafluoroethylene (C/PTFE) gas-diffusion electrode as a cathode, respectively. The results indicated that the self-made Pd/C gas-diffusion cathode can not only reductively dechlorinate 2,4-dichlorophenols by feeding hydrogen gas, but also accelerate the two-electron reduction of O₂ to hydrogen peroxide (H₂O₂) by feeding air. Therefore, both the removal efficiency and the dechlorination degree of 2,4-dichlorophenol reached about 100% after 80 min, and the average removal efficiency of 2,4-dichlorophenol in terms of total organic carbon (TOC) exceeded 76% after 160 min by using Pd/C gas-diffusion cathode, which were better than that of the C/PTFE gas-diffusion cathode. The analysis of high-performance liquid chromatography (HPLC) identified that 4-chlorophenol, 2-chlorophenol, and phenol were the dechlorination products, and hydroquinone, benzoquinone, maleic, fumaric, crylic, malonic, oxalic, acetic and formic acids were the main oxidation intermediates. A reaction scheme involving all these intermediates was proposed.     

Tungsten carbide-based electrochemical sensors for hydrogen determination in gas mixtures

An amperometric study of gas-diffusion electrodes (GDE) catalysed by two types of tungsten carbide, WC₍₁₎ and WC₍₂₎, which differ considerably in their specific surface area (0.5 and 6 m² g^–1), was carried out. The H₂–air gas mixture (H₂ 1–4%) measurements show that for this range of hydrogen concentration the hydrogen limiting diffusion current (i _d(H2)) may be attained so that a curve of limiting current density against hydrogen concentration can be obtained. The response and stability of the electrode performance were compared to those of platinum catalysed GDEs. The most promising for use in amperometric hydrogen sensing is the WC₍₁₎ catalyst of small specific surface area. Electrodes catalysed with this catalyst show inferior response time in comparison to electrodes catalysed with the other two catalysts (WC₍₂₎ and Pt) but their overall stability is much better.     

Electrocatalytic oxidation of formic acid on ultrafine palladium particles supported on a glassy carbon

The effect of particle size of ultrafine palladium particles on the electrochemical oxidation of formic acid in a perchloric acid solution has been investigated by cyclic voltammetry (CV) and differential electrochemical mass spectroscopy (DEMS). Except in the hydrogen sorption-desorption region, only carbon dioxide was observed by the DEMS as an oxidation product, and the profile of the i_mass-E curve was in fair agreement with that of i-E curves. With a decrease in the palladium particle size, the specific activity of the palladium particles for the oxidation of formic acid gradually increased until ca. 3 nm in the mean diameter, and then it steeply decreased.     

Charge transport effects in ferrocene-streptavidin multilayers immobilized on electrode surfaces

Streptavidin (SAv) has been modified by covalent coupling of ferrocene (Fc) electron-relay groups to lysine-residues of the protein backbone. Reagent ratios were varied to obtain conjugates with three to 16 Fc groups per SAv. Biotin was covalently attached to gold electrodes for the anchoring of the conjugate monolayers. A method was devised to produce in situ bisbiotin functionalities that efficiently cross-linked Fc₁₆SAv to form multilayer electrode coatings. The electrochemical charge transport properties of the coatings were examined by cyclic voltammetry. The characteristic current density i_E measuring the rate of charge transport was 1 mA cm⁻² for one monolayer of Fc₁₆SAv. It was found that the transport of electrochemical charge through the Fc-containing SAv system is a diffusion-like process, as evidenced by the proportionality of the peak current and the square root of sufficiently high scan rate, and the inverse dependence of i_E on the number (thickness) of Fc₁₆SAv layers.     

The automation of electrode kinetics — III. The dissolution of Mg in Cl−, F− and OH− containing aqueous solutions

The dissolution of Mg in Cl^?, F^?, and OH^? containing aqueous solutions has been investigated over a large potential range, from far into H₂ evolution at ? 3000 mV (sce) to the passive state at ? 1400 mV (sce). The steady state current-potential and the impedance have been measured using a new automatic electrochemical measurement system. An equivalent circuit and a least squares fitting procedure have been used to analyse the data. The resulting C_dl-E, θ-E, σ_Mg-E, R_w,-E, and the primary i-E and Z(ω)-E data are compared for each anion, and reveal details of the electrode kinetics of these complex dissolution reactions.     

Electrochemical formation and characteristics of aluminium hydroxide on a platinum substrate from dimethylsulphoxide and water mixtures

Results obtained with Al(ClO₄)₃ in DMSO solutions with different water content at a Pt substrate are presented. Different negative potential perturbations produce the reduction of water molecules from the aluminium ion coordination sphere yielding hydrogen gas and a film of aluminium hydroxide solvated by water and DMSO molecules. The inhibiting effect of the electroformed film and its water content depend on the negative potential value reached. Potentiodynamic i/E profiles present two main cathodic current peaks which are related to [Al(DMSO)_6-iH₂O_i]³⁺ complexes with i = 1 and 2, through the current dependences with water and aluminium ion concentrations. Bulk diffusion of the complexes, charge-transfer reactions and the inhibiting nature of the film formed were thoroughly investigated. Molecular hydrogen is oxidized at positive potentials yielding hydrogen ions which in turn produce a partial recovery of the electrode activity.     

Theory for double potential step chronoamperometry for any potential values at spherical electrodes: Simultaneous determination of the diffusion coefficients of the electroactive species

We present a general and explicit analytical solution for double potential step chronoamperometry with any applied potential values (E₁, E₂) corresponding to a reversible charge transfer process at spherical electrode. This solution is essential to analyze double pulse electrochemical techniques such as RPV and DPV. We consider unequal diffusion coefficients, initial presence of both electroactive species and that the reaction product can dissolve in the electrolytic solution or in the electrode.From the analytical equation obtained it is possible to deduce interesting simplified expressions for some particular cases: both species soluble in the electrolytic solution with equal diffusion coefficients, planar electrodes, ultramicroelectrodes when both species are soluble in the electrolytic solution, and double potential step chronoamperometry with limit current potentials (E₁−E^°′→−∞, E₂−E^°′→+∞). In this last case, when reaction product is not initially present it is pointed that planar electrodes and ultramicroelectrodes cannot be used for determining both diffusion coefficients. This interesting practical consequence can be demonstrated by means of the analytical expression deduced here, which represents a notable advantage in front of numerical results.     

Oxygen transport at porous flow-by air electrodes: theoretical approach

The theory of oxygen transport at porous flow-by air electrodes is treated mathematically. The three transport mechanisms of diffusion and convection in the gas channel, pore diffusion, and diffusion in the electrolyte are considered, and combined in the derivation of an asymptotic solution to electrode performance when U_avef²/L → ∞ ¹. Together with a further asymptote for the condition of oxygen exhaustion, ieU_avef/L → 0, this effectively defines the limit of real operation of an air electrode.     

Etude de mecanismes ec sur electrode a pate de graphite

A theoretical analysis of EC mechanisms on graphite paste electrode (GPE) in linear sweep voltammetry is proposed for the following E_iC_j mechanisms. reversible electrochemical reactions E_i: Ox + ne ? Red (E₁), or: Ox + e ? S, S + e ? Red, 2S ? Ox + red (E₂); irreversible chemical reaction C_j: p Ox → A (C₁), or: p Ox → A + m Red (C₂), or: p Ox + m Red → A (C₃).Current—potential relationships are similar to those obtained by Laviron for both layer electrodes and film covered electrodes when the differential capacitance C and the circuit and electrode resistance R are negligible. For large values of the kinetic contants, it is possible to establish a set of relationships between the mechanism parameters and the experimental characteristics of the voltammograms.On a practical point of view, two types of mathematical analysis are proposed.     

Investigation of mass transport in gas diffusion layer at the air cathode of a PEMFC

In a polymer electrolyte membrane fuel cell (PEMFC), slow diffusion in the gas diffusion electrode may induce oxygen depletion when using air at the cathode. This work focuses on the behavior of a single PEMFC built with a Nafion^® based MEA and an E-TEK gas diffusion layer and fed at the cathode with nitrogen containing 5, 10 and 20% of oxygen and working at different cell temperatures and relative humidities. The purpose is to apply the experimental impedance technique to cells wherein transport limitations at the cathode are significant. In parallel, a model is proposed to interpret the polarization curves and the impedance diagrams of a single PEMFC. The model accounts for mass transport through the gas diffusion electrode. It allows us to qualitatively analyze the experimental polarization curves and the corresponding impedance spectra and highlights the intra-electrode processes and the influence of the gas diffusion layer.     

Anodic depassivation of potassium electrode in propylene carbonate medium

Depassivation of potassium electrode in a highly dried propylene carbonate-KPF₆ electrolyte (< 0.01 ppm H₂O) was studied by using galvanostatic E-t and polarization i-E curves at various scan rates. The results suggest that the passivating layer is strongly absorbed on the electrode surface even during anodic dissolution of potassium, which proceeds only on a certain depassivated fraction of the surface. A mechanism of the anodic depassivation is proposed.     

Diagnosis of specific surface area in fuel cell air electrodes

A model of the air electrode used in fuel cells was developed which accounts for the diffusion of oxygen in the gas-filled pores as well as diffusion into the liquid-filled pores, and electrochemical reaction. The model was applied to the air electrode in the phosphoric acid fuel cell (PAFC) to simulate the cathode performance under a variety of conditions. Two parameters AB and AI were introduced into the model. They reflect, respectively, the effects of the interfacial surface area between the gas and liquid phases and those of the interfacial surface area between the liquid and solid phases, on the air electrode performance. AB and AI can be determined with the aid of the operating conditions and potential–current density curves of the air electrode. The main interface parameters of the air electrode can be predicted following the determination of AB and AI. The reaction rate throughout the catalyst layer was calculated by means of the model.     

Strategies for the determination of the convective-diffusion limiting current from steady state linear sweep voltammetry

The limiting current is an important parameter for the characterization of mass transport in electrochemical systems operating under convective-diffusion control. Four methods to determine the limiting current from current (I) vs. potential (E) plots are considered. Strategies to determine the limiting current values include: (1) direct measurement from I vs. E curves, (2) estimation from the current value at E _L = ΔE/2 where ΔE is the length of the limiting current plateau, (3) evaluation of the first derivative dI/dE in the I vs. E curve and (4) from plots of E/I vs. I ⁻¹. The electrode reactions chosen to demonstrate the different strategies are: Cu(II) → Cu(I) and Cu(I) → Cu(0) in 1.5 mol dm⁻³ NaCl (pH 2) at a platinum rotating disc electrode and in 1 mol dm⁻³ NaOH at a 60 ppi reticulated vitreous carbon electrode (RVC).     

Characterization of membrane electrode assemblies in polymer electrolyte fuel cells using a.c. impedance spectroscopy

The most common methods used to characterize the electrochemical performance of fuel cells are to record current–voltage U(i) curves. However, separation of electrochemical and ohmic contributions to the U(i) characteristics requires additional experimental techniques. The application of electrochemical impedance spectra (EIS) is an approach to determine parameters which have proved to be indispensable for the development of fuel cell electrodes and membrane electrode assemblies (MEAs). This paper proves that it is possible to split the cell impedance into electrode impedances and electrolyte resistance by varying the operating conditions of the fuel cell (current load) and by simulation of the measured EIS with an equivalent circuit. Furthermore, integration in the current density domain of the individual impedance elements enables the calculation of the individual overpotentials in the fuel cell and the determination of the voltage loss fractions.     

An in situ Raman study of the intercalation of supercapacitor-type electrolyte into microcrystalline graphite

An initial Raman study on the effects of intercalation for aprotic electrolyte-based electrochemical double-layer capacitors (EDLCs) is reported. In situ Raman microscopy is employed in the study of the electrochemical intercalation of tetraethylammonium (Et₄N⁺) and tetrafluoroborate (BF₄⁻) into and out of microcrystalline graphite. During cyclic voltammetry experiments, the insertion of Et₄N⁺ into graphite for the negative electrode occurs at an onset potential of +1.0 V versus Li/Li⁺. For the positive electrode, BF₄⁻ was shown to intercalate above +4.3 V versus Li/Li⁺. The characteristic G-band doublet peak (E_2g2(i) (1578 cm⁻¹) and E_2g2(b) (1600 cm⁻¹)) showed that various staged compounds were formed in both cases and the return of the single G-band (1578 cm⁻¹) demonstrates that intercalation was fully reversible. The disappearance of the D-band (1329 cm⁻¹) in intercalated graphite is also noted and when the intercalant is removed a more intense D-band reappears, indicating possible lattice damage. For cation intercalation, such irreversible changes of the graphite structure are confirmed by scanning electron microscopy (SEM).