State of overpotential-deposited H species at electroplated platinum surfaces in comparison with bright platinum

The state of chemisorbed H at platinum electrodes, having a large roughness factor (high real-to-apparent area ratio), is of interest in relation to operation of the reversible H₂ electrode and to the performance of water electrolyser cathodes and fuel cell anodes.Results are reported of investigations of the behaviour of overpotential-deposited (OPD) H at high-area platinum surfaces generated by electroplating platinum, with and without lead as an additive, on a platinum substrate in the form of a rotating disc electrode. Comparisons are made with the behaviour of OPD H at an anodically activated, bright platinum electrode surface. The sorption behaviour of the OPD H is evaluated from digitally acquired open-circuit potential-decay transients which enable the potential dependence of OPD H coverage (the H pseudocapacitance) and the change of H coverage over a range of H₂ evolution current densities to be derived. Use of rotated platinum electrodes (4900 r.p.m.) enables possible interference from effects of H₂ supersaturation in solution to be avoided or minimized.At active platinum surfaces, the apparent extent of OPD H sorption substantially exceeds a monolayer, depending on the electroplating procedure, with or without lead as additive, or at the bright platinum surface in acid solution. The OPD H coverage is related to the activity of platinum surfaces as H₂ evolution electrocatalysts.At electroplated palladium, where H sorption is well known, the potential decay behaviour is similar to that at platinum in acid, thus supporting the role of H sorption in the near-surface region at the latter metal.     

Electrochemical characterisation and oxygen evolution at a heavily boron doped diamond electrode

Characterisation of a commercial heavily doped BDD electrode demonstrated it contains a small sp² content, which on anodic potential scanning, is oxidised to CO/CO₂. This surface modification alters the electrode activity, increasing the overpotential for the hydrogen and oxygen evolution reactions (HER and OER). Ex situ and in situ investigations indicate film morphology is mainly composed of “chain of hills”, presenting relatively high differential capacitance values and morphology factor, which is attributed to the effect of surface states and high surface roughness of the BDD film. The voltammetric behaviour depends on the applied potential; the heavily doped BDD electrode behaving as a metallic electrode at more anodic potentials. Polarisation curves (potentiostatic (1 mV s⁻¹) or galvanostatic (point-by-point)), recorded at different temperatures and H₂SO₄ concentrations, lead to the same conclusions. The high Tafel coefficients and low apparent electronic transfer coefficient (α_A) are independent of overpotential and temperature but show a dependence on H₂SO₄ concentration. The linear relationship observed between the apparent electrochemical enthalpy of activation (ΔH^#_η) and overpotential supports α_A is constant. An OER mechanism was proposed taking into account the absence of adsorption sites at the BDD surface. The OER is inhibited, explaining the high overpotentials and elevated ΔH^#_η values.     

The electrode kinetics of the evolution and dissolution of oxygen at the urania-zirconia interfaces

In order to assess the potential of urania-yttria fluorite-type solid solutions as electrodes for high-temperature electrolysis of steam, oxygen evolution and dissolution reactions have been studied at the (U_0.7Y_0.3)O_2+x/YSZ interface. A current-interruption technique was used to separate overpotential and resistive potential drop. In oxygen and air the overpotential-current curves obey the Tafel law, suggesting that a charge-transfer process is rate determining. Activation energies of 120 kJ mole^?1 and 165 kJ mole^?1 were obtained for the cathodic reaction in oxygen and air respectively. The capacitance obtained from galvanostatic transients varied with potential, temperature, and oxygen partial pressure. The average value of n, the number of electrons involved in the overall charge-transfer reaction, was determined to be 4.01 from reversible potential measurements. The overpotential losses are small for porous electrodes at high p_o2. A mechanism for the oxygen transfer reaction has been proposed and its limitations discussed.     

Anisotropic corrosion of iron in pH 1 sulphuric acid

Anisotropic corrosion behaviour of a single grain of pure iron in 0.05 mol dm⁻³ sulphuric acid (pH 1) was investigated by a simple polarization technique. Both corrosion potential and corrosion current were found to be dependent on crystallographic orientation of the iron grain. A high corrosion current flowed on a grain showing a relatively noble corrosion potential, although both cathodic and anodic Tafel slopes were independent of the orientation. It was shown that cathodic hydrogen evolution reaction (HER) governed the corrosion reaction on the iron grain. Covalent bonding of Fe and H and coverage of H on Fe seem to play important roles in the HER and anisotropic corrosion behaviour of pure iron.     

Hydrogen evolution on platinum in the presence of lead,cadmium and thallium adatoms

Hydrogen evolution on rotating platinum electrode has been measured in the presence of Pb, Tl and Cd adatoms. At low coverages of these adatoms (θ_M < 0.5) the mechanism of H₂ evolution is the same as that for Pt, giving rise to the Tafel slope of 30 mV with recombination of H adatoms as a rate controlling step. However, H₂ evolution decreases with θ_M to the fourth power, due to a simple blocking of Pt surface and a decrease of number of pairs of Pt atoms necessary for a recombination reaction. At higher coverages of foreign metal adatoms (θ_M ? 0.5), when the activation energy for the recombination of H adatoms becomes too high, the ion-plus atom reaction begins to control the rate of reaction. The Tafel slope acquires a value of 120 mV. Now, a decrease of H₂ evolution has been found proportional to the coverage of metal adatoms. A quantitative analysis based on above model agrees very well with the experiment. Some information on underpotential deposition of above metal on Pt has also been obtained.     

Hydrogen evolution reaction on anodic titanium oxide films

Oxide titanium films were prepared using linear potential sweeps up to different limiting potential values between 0 and 60 V in 0.5 M H₂SO₄. The electronic characteristics of these films were investigated through impedance measurements during their formation. The hydrogen evolution reaction (HER) in either HClO₄ or H₂SO₄ solutions in the pH range between 1 and 4 was studied on the different oxide films. The obtained experimental results could be explained assuming that the recombination of adsorbed hydrogen atoms is the rate determining step (rds), and that the Temkin type isotherm applies to the adsorbed intermediates. The log i vs E plot after correction for diffusion control in the bulk gives a slope of 2.3 RT/F.The influence of the physico-chemical properties of the oxide films on the HER has also been studied from the changes in the current—potential profiles. The oxide film thickness changes the HER overpotential, but the slope of the log i vs E relationship remains independent of it.     

Hydrogen evolution reaction on sodium-tungsten bronzes

The hydrogen evolution reaction has been investigated on sodium-tungsten bronzes (Na_xWO₃) of various stoichiometries (0·34 < × < 0·89) in sulfuric acid solutions. Steady-state current-potential relationships and capacity-potential profiles were examined. The first cathodic steady-state current-potential curve in the ascending direction indicates the presence of two distinct Tafel lines with slopes of about ?2RT/F and ?RT/2F. This behavior is interpreted in terms of alternative reactions. At low overvoltages the mechanism is fast discharge-slow electrochemical desorption, while at higher overvoltages a fast discharge-slow recombination mechanism is indicated. The change of the rate-determining step from one potential range to the other is attributed to a change in the crystallographic structure of the electrode surface, eg, a lattice dimensional change of the hydrogen tungsten bronze. The Tafel slope for an electrode pre-polarized cathodically is close to ?RT/F. In this case the rate-determining step is proposed to be the surface diffusion of adsorbed hydrogen atoms to recombination sites. The x-value in Na_xWO₃ and the platinum content of the electrode were found to have no significant effect on the rate of the hydrogen evolution reaction.     

Iodide surface coverage and its effect on the rate constants during hydrogen charging and permeation in iron

This paper demonstrates the use of the new Iyer/Pickering/Zamanzadeh/Al-Faqeer (IPZA) analysis to determine the effect of the iodide surface coverage (θ_I⁻) on the hydrogen surface coverage (θ_H) and the rate constants of the hydrogen evolution reaction (HER) and hydrogen absorption reaction (HAR). Iodide ions have an inhibiting effect on the HER but they enhance the HAR. The IPZA analysis was used to determine θ_I⁻ in addition to the usual parameters: hydrogen discharge (k₁) and recombination (k₂) rate constants, hydrogen kinetic-diffusion constant (k) and θ_H. Inclusion of the iodide coverage is important as shown by a comparison with literature values that were obtained using the original IPZ analysis, which does not include the θ_I⁻ parameter. The θ_I⁻ values from the IPZA analysis were within reasonable agreement with θ_I⁻ values measured using the electrochemical quartz crystal microbalance (EQCM) and θ_I⁻ values calculated from the measured corrosion current densities before and after adding iodide to the electrolyte.     

Iodide surface coverage and its effect on the rate constants during hydrogen charging and permeation in iron

This paper demonstrates the use of the new Iyer/Pickering/Zamanzadeh/Al-Faqeer (IPZA) analysis to determine the effect of the iodide surface coverage (θ_I⁻) on the hydrogen surface coverage (θ_H) and the rate constants of the hydrogen evolution reaction (HER) and hydrogen absorption reaction (HAR). Iodide ions have an inhibiting effect on the HER but they enhance the HAR. The IPZA analysis was used to determine θ_I⁻ in addition to the usual parameters: hydrogen discharge (k₁) and recombination (k₂) rate constants, hydrogen kinetic-diffusion constant (k) and θ_H. Inclusion of the iodide coverage is important as shown by a comparison with literature values that were obtained using the original IPZ analysis, which does not include the θ_I⁻ parameter. The θ_I⁻ values from the IPZA analysis were within reasonable agreement with θ_I⁻ values measured using the electrochemical quartz crystal microbalance (EQCM) and θ_I⁻ values calculated from the measured corrosion current densities before and after adding iodide to the electrolyte.     

Degagement anodique du chlore a une électrode de carbone vitreux dans un mélange fondu de NaCl et FeCl3

Résumé On étudie la surtension de dégagement du chlore à une électrode de carbone vitreux dans un mélange fondu de NaCl et de FeCl₃. On enregistre les courbes tension-courant à très faible vitesse de variation de la tension. La pente de Tafel estRT/F à 178° C dans un intervalle de surtension 3.9–4.2 V. Ce résultat est compatible avec des mécanismes simples à adsorption activée de type Temkin. Il est plus difficile d'expliquer cette pente de Tafel avec une adsorption de type Langmuir.

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The overpotential of anodic chlorine evolution on vitreous carbon, from a molten mixture of NaCl and FeCl₃, is studied. The current-potential curves are recorded at a very slow rate of change of potential with time. The Tafel slope isRT/F at 178°C in the overpotential region 3.9–4.2 V. This result is consistent with simple mechanisms involving Temkin-type activated adsorption. It is more difficult to explain this Tafel slope with a Langmuir-type adsorption.

     

Arsenic poisoning effects on cathodic polarization and hydrogen adsorption at platinum and steel electrodes in KF · 2HF melts

It is demonstrated how the presence of arsenic species in the KF · 2HF melt employed as the electrolyte in fluorine production cells greatly affects the electrocatalytic behaviour of the steel electrodes that are used as cathodes. Effects of the introduction of As₂O₃, AsF₃ and AsF₅ as the sources of catalyst poison species have been studied. Such species influence the electrode kinetics of the cathodic hydrogen evolution reaction (h.e.r.) and the adsorption of the overpotential-deposited (o.p.d.) H intermediate. Using Pt, as a model electrocatalyst surface, these effects were quantitatively studied by means of potential-relaxation transients, Tafel relations and cyclic voltammetry, the latter enabling changes of underpotential-deposited C (u.p.d.) H coverage due to the presence of As species to be evaluated. By means of simulation of the potential-relaxation behaviour, information on rate constants, coverages by o.p.d. H and As species were also derived. The presence of As at Pt cathodes suppresses the u.p.d. H adsorption and modifies the o.p.d. H behaviour and the associated pseudocapacitance. The presence and generation of As(–iii) species, as AsH₃, in the evolved hydrogen gas was demonstrated by means of Marsh's test. The contact-angles of hydrogen bubbles generated in the h.e.r. at mild-steel electrode surfaces were also determined comparatively in relation to observed sluggish hydrogen bubble detachment from the electrode surfaces when As species were present.Ex situ XPS analysis confirmed the presence of As species on the electrode surface.     

Surface and bulk processes at oxidized iridium electrodes—II. Conductivity-switched behaviour of thick oxide films

The electrochemical reversibility of thick oxide films that are formed on Ir electrodes by potential cycling is examined in relation to the initial monolayer surface oxidation processes. The unusual properties of thick oxide films on Ir, eg, as electrode surfaces for O₂ or Cl₂ evolution and with regard to H pseudocapacitance and double-layer capacitance being almost independent of oxide film thickness, arise because of a major increase of conductivity of the oxide as the oxidation state of Ir in the film is changed in a potential sweep between ca. 0.05 and 1.4 V E_H. In a cyclic sweep this has the effect of switching “on” or “off” the large real area of a microporous, hydrated oxide structure of the thick oxide films for electron transfer processes at their surfaces or within their bulk or pores.It is proposed that film growth which occurs on cycling is the result of accumulation of oxide produced in an underlying monolayer during each anodic sweep but which is left in an incompletely reduced state on each following cathodic sweep, due to irreversibility.     

Change of mechanism of the hydrogen electrode reaction with overpotential—III. Potential dependence of the deuterium separation factor on Pt,Au and Ni cathodes

A relationship between the overall electrolytic deuterium separation factor of the hydrogen electrode reaction and the separation factors of the steps that constitute the reaction is formulated for the discharge-combination route, without assuming an overwhelming unique rate-determining step. This is utilized for the quantitative interpretation of sharp variation of the cathodic deuterium separation factor with overpotential observed on Pt and Au in aqueous H₂SO₄ and an insignificant variation on Ni in alkaline solutions. The variation on Pt and Au is interpreted in terms of change with cathodic overpotential of the mechanism from the rapid discharge—slow combination to the slow-discharge. The variation on Ni is expected to be significant only in the anodic region.     

Electrodeposition of cobalt on gold during voltammetric cycling

Co was electrodeposited on Au(111) from solutions of CoSO₄ in 0.5 mM H₂SO₄. STM showed that the electrodeposition started at an underpotential of 0.26 V. In cathodic sweeps voltammograms exhibited a low cathodic peak C1 at an overpotential of 0.1 V and a large cathodic peak C2 at an overpotential above 0.2 V. From the cathodic charge it was estimated that at peak C1, the deposit was about one monolayer thick. AFM indicated the presence of flat deposits at peak C1, and of numerous islands at peak C2, demonstrating massive nucleation and growth at the latter peak. It is suggested that peak C1 may be associated with a hindrance of nucleation processes due to adsorption of chemical species, probably of hydrogen H_ad. During a reverse anodic sweep from C2, anodic peaks A1 and A2 were observed. Peak A1 was evidently associated with anodic oxidation of electrodeposited cobalt, whereas peak A2 at nobler potentials, probably resulted from oxidation of hydrogen, incorporated into the Co electrodeposit.     

Elementary reaction rates of the hydrogen electrode reaction on nickel analyzed by means of deuterium tracer—II: The cathodic region

The rates of the constituent steps of the hydrogen electrode reaction on nickel were determined under cathodic polarization by means of deuterium tracer. The discharge—recombination reaction route was substantiated. Logarithm of the rate of the process 2H(a) → H₂ was dependent upon the overpotential component shared by the very step with the slope of 30 mV, or the rate was proportional to the square of activity of H(a), whereas that the process H₂ → 2H(a) was independent of it. Logarithm of the rate of formation of H(a) from the solution phase was dependent upon the total overpotential with the slope of ca 100 mV. Dependences of the mechanism and the step rates upon hydrogen pressure were discussed.     

A.c.-impedance spectroscopy study of oxygen reduction at Nafion® coated gas-diffusion electrodes in sulphuric acid: Teflon loading and methanol cross-over effects

The kinetics of oxygen reduction have been studied at Nafion®-coated dual layer gas-diffusion electrodes at 60 °C in 2.5 M H₂SO₄. A.c. impedance and steady state galvanostatic measurements have been carried out on electrodes containing various PTFE loadings. A.c. impedance spectroscopy supplied information on charge transfer resistance, series resistance and double layer capacitance of electrodes during the oxygen reduction process. These parameters, together with those derived by d.c. measurements (i.e. Tafel slopes, charge transfer coefficient, etc.), allowed a better identification of the features governing the electrode efficiency and the reduction mechanism. The best electrode was found to be one containing 30% PTFE; this showed both the lowest charge transfer resistance and the lowest overpotential for oxygen reduction. This electrode also showed lower overpotential than the same uncoated electrode at low current densities. The poisoning effect by methanol has been also investigated. Upon addition of methanol to the solution, no change was observed in the reaction mechanism at low overpotentials. In contrast, a significant variation of the Tafel slope was observed at high overpotentials. This behaviour is interpreted in terms of the variation of Pt-O bond strength as a function of electrode potential.     

Investigation of the surface properties and the hydrogen evolution reaction, HER, at thermal rhodium oxide electrodes

A systematic investigation was conducted of the surface properties and the HER at electrodes of nominal composition Ti/Rh_xTi_(1−x)O_y prepared by thermal decomposition (T_cal: 500 °C; t_cal: 2 h; O₂ flux: 5 dm³ min⁻¹) from salt precursor solutions dissolved in 6.0 mol dm⁻³ HNO₃. Films were characterized ex situ by SEM, EDX, XPS and XRD and in situ by open circuit potential measurements and CV. The electrochemical behaviour was investigated by CV as function of the anodic, E_λ,a, and cathodic, E_λ,c, switching potentials showing the Rh surface oxidation states strongly depend on these experimental variables. Surface Rh-sites are reduced to metallic rhodium in the cathodic potential region while higher oxidation states (I-III) are formed at more positive potentials (E ≥ 0.5 V/RHE). Hydrogen adsorption and desorption peaks as well as a short double layer charging region are observed at intermediate potential values. The HER was investigated by Tafel coefficients and reaction order with respect to H⁺ as function of nominal Rh-content.     

Hydrogen evolution on conducting polymer electrodes in acidic media

Hydrogen evolution reaction (HER) was studied on polyaniline (PAn), polypyrrole (PPy) and on aniline/pyrrole (PAn–PPy) copolymer in acidic solutions. The cathodic Tafel slopes (b_c) and exchange current densities (j₀) were calculated from Tafel curves obtained in solutions of X M H₂SO₄ (X = 0.1, 0.2, 0.3, 0.4 and 0.5 M). Activation energies (E_a) were determined. The E_a-values were found to be ca. 26 for PAn, 36.5 for PPy, 40.6 for PAn–PPy and 20.6 kJ mol⁻¹for Pt.     

Effects of thermal treatment of activated carbon on the electrochemical behaviour in supercapacitors

This paper studies the electrochemical behaviour of activated carbons with different oxygen content and investigates the contribution of pseudocapacitance to the global behaviour of the samples. A mesophase-derived activated carbon was further heat treated to 600 or 1000 °C in nitrogen. The changes in texture and surface chemistry induced by the thermal treatment were deeply studied. The electrochemical behaviour of the samples was studied in two- and three-electrode cells. The contribution of pseudocapacitance was evaluated by cyclic voltammetry and by the differences of specific capacitance obtained from galvanostatic tests performed in acidic (H₂SO₄) and basic (KOH) media. The presence of an extra capacitance due to redox reactions has been proved both in acidic and basic media for the samples with high oxygen content, although its contribution in basic media is significantly lower. The results obtained clearly indicate that the oxygen responsible for CO-evolution participates in redox reactions, whereas the oxygen responsible for the CO₂-evolution is of minor importance.     

Hydrogen absorption by palladium electrode polarized in sulfuric acid solution containing surface active substances—II. The anodic region

The hydrogen pressure equivalent to hydrogen overpotential on Pd electrode was determined under anodic polarization by combining the data of the electric resistance of a Pd foil electrode observed at various overpotentials with the electric resistance vs hydrogen pressure relationship which was obtained in a separate, gas phase experiment. The equivalent hydrogen pressure under anodic polarization was less diminished as compared with the value calculated by the Nernst equation from the total overpotential. Results are consistent with the conclusion reported previously for the cathodic region (Part I) that the Tafel—Volmer route is followed on the Pd hydrogen electrode with comparable value of the constituent step rates. Effect of addition to the solution of tetrabutylammonium hydroxide was also examined. Hydrogen content in Pd at the equilibrium hydrogen electrode potential under 1 atm hydrogen pressure was evaluated electrochemically to be H/Pd = 0.691. Hydrogen content in Pd under anodic polarization was evaluated.