Effect of chemisorbed carbon monoxide on Pt/C electrode on the mechanism of the hydrogen oxidation reaction

The influence of poisoning of Pt catalyst by CO on the kinetics and mechanism of H₂ oxidation reaction (HOR) at Pt/C electrode in 0.5 mol dm⁻³ HClO₄, saturated with H₂ containing 100 ppm CO, was examined with rotating disc electrode (RDE) at 22 °C. Commercial carbon black, Vulcan XC-72 was used as support, while Pt/C catalyst was prepared by modified polyol synthesis method in an ethylene glycol (EG) solution. The kinetically controlled current (I_k) for the HOR at Pt/C decreases significantly at CO coverage (Θ_CO) > 0.6. For Θ_CO < 0.6 the HOR takes place through Tafel-Volmer mechanism with Tafel reaction as rate-determining step at the low CO coverage, while Volmer step controls the overall reaction rate at the medium CO coverage. When CO coverage is higher then 0.6, Heyrovsky-Volmer mechanism is operative for the HOR with Heyrovsky as the rate-determining step (rds).     

The electroreduction kinetics of silver sulfite complexes

The electroreduction kinetics of silver sulfite complexes was investigated by rotation disk electrode (RDE) voltammetry, chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS). The stability constants of the silver sulfite complexes, pβ₂ = 7.9 and pβ₃ = 8.53 were determined. For the series of isopotential solutions investigated, a reaction order of 0.67 was obtained, the diffusion coefficient of the silver complexes varies in the range of 3.36 × 10⁻⁶ to 5.54 × 10⁻⁶ cm² s⁻¹ and the silver degree of complexation (2.31-2.67) were found. The analysis of the RDE, CP data and EIS spectra indicate the existence of a slow stage of the silver electrocrystallization in the region of the equilibrium potential and at stronger polarization of the electrode at initial time moments.     

Electroreduction of oxygen on gold-supported nanostructured palladium films in acid solutions

The electrochemical reduction of oxygen on thin Pd films with a nominal thickness of 0.25-10 nm on polycrystalline Au substrate (Pd/Au) was studied. The Pd films were prepared by electron beam evaporation and oxygen reduction was studied in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions using the rotating disk electrode (RDE) method. The surface morphology of Pd overlayers was examined by scanning tunnelling microscopy (STM). O₂ reduction predominantly proceeds through 4e⁻ pathway on all Pd/Au electrodes. The specific activity (SA) of oxygen reduction was lower in H₂SO₄ solution and decreased slightly with decreasing the Pd film thickness. In HClO₄, the SA was higher and not significantly dependent on the film thickness. The Tafel slope values close to −60 mV at low current densities and −120 mV at high current densities were found for all electrodes.     

Electropolishing of NiTi shape memory alloys in methanolic H2SO4

The electropolishing of NiTi shape memory alloys was surveyed electrochemically. Anodic polarization of NiTi up to 8 V was performed in various aqueous and methanolic H₂SO₄ solutions. The passivity could be overcome in methanolic solutions with 0.1 mol dm−3≤CH₂SO₄≤7 mol dm−3. The dissolution kinetics was studied in dependence of the polarization potential, the H₂SO₄-concentration, the water concentration and the temperature. For lower concentrations of sulfuric acids (CH₂SO₄≤0.3 mol dm−3) electropolishing conditions were not observed for potentials up to 8 V. The dissolution remained under Ohmic control. In the concentration range from 1 to 7 mol dm⁻³ a potential independent limiting current was registered depending linearly on the logarithm of concentration. The best results were obtained with a 3 mol dm⁻³ methanolic sulfuric acid at 263 K which yielded an electropolishing current of 500 A m⁻² at a potential of 8 V. Surface roughness as well as current efficiency showed an optimum under these conditions.     

Direct electrochemistry and electrocatalysis of cytochrome c based on chitosan-room temperature ionic liquid-carbon nanotubes composite

A robust and effective composite film combined the benefits of room temperature ionic liquid (RTIL), chitosan (Chi) and multi-wall carbon nanotubes (MWNTs) was prepared. Cytochrome c (Cyt c) was successfully immobilized on glassy carbon electrode (GCE) surface by entrapping in the composite film. Direct electrochemistry and electrocatalysis of immobilized Cyt c were investigated in detail. A pair of well-defined and quasi-reversible redox peaks of Cyt c was obtained in 0.1 mol L⁻¹ pH 7.0 phosphate buffer solution (PBS), indicating the Chi-RTIL-MWNTs film showed an obvious promotion for the direct electron transfer between Cyt c and the underlying electrode. The immobilized Cyt c exhibited an excellent electrocatalytic activity towards the reduction of H₂O₂. The catalysis current was linear to H₂O₂ concentration in the range of 2.0 × 10⁻⁶ to 2.6 × 10⁻⁴ mol L⁻¹, with a detection limit of 8.0 × 10⁻⁷ mol L⁻¹ (S/N = 3). The apparent Michaelis-Menten constant (K_m) was calculated to be 0.45 ± 0.02 mmol L⁻¹. Moreover, the modified electrode displayed a rapid response (5 s) to H₂O₂, and possessed good stability and reproducibility. Based on the composite film, a third-generation reagentless biosensor could be constructed for the determination of H₂O₂.     

Catalytic reduction of hydrogen peroxide at metal hexacyanoferrate composite electrodes and applications in enzymatic analysis

The electrocatalytic activity of various metal hexacyanoferrates (Mhcfs) (i) immobilized on graphite electrodes, and (ii) as components of a composite electrode was investigated with respect to the reduction of hydrogen peroxide. The flow-through working electrode was a thin layer consisting of a composite of Mhcf, graphite, and polymethylmetacrylate (PMMA) as a binder, sandwiched between two Plexiglas plates. Among the pure Mhcfs immobilized on a graphite electrode, iron(III) hexacyanoferrate (Prussian blue) exhibits the highest electrocatalytic effect, whereas in the composite electrodes chromium(III) hexacyanoferrate (Crhcf) shows the highest activity and best performance and reproducibility for the electrochemical reduction of H₂O₂. The Crhcf electrode provides a linear dependence on H₂O₂ concentration in the range 2.5 × 10⁻⁶ mol L⁻¹ (LOD) to 1 × 10⁻⁴ mol L⁻¹ (phosphate buffer, pH 7). The sensor was applied for the detection of H₂O₂ enzymatically produced by glucose oxidase. The optimal conditions for the peroxide injection were 2 min after the beginning of the reaction and 25 °C with a detection limit of 7.0 × 10⁻⁶ mol L⁻¹ for glucose.     

Changes on iron electrode surface during hydrogen permeation in borate buffer solution

Hydrogen interaction with oxide films grown on iron electrodes at open circuit potential (E_oc) and in the passive region (+0.30 V_ECS) was studied by chronopotentiometry, chronoamperometry and electrochemical impedance spectroscopy techniques. The results were obtained in deaerated 0.3 mol L⁻¹ H₃BO₃ + 0.075 mol L⁻¹ Na₂B₄O₇ (BB, pH 8.4) solution before, during and after hydrogen permeation. The iron oxide film modification was also investigated by means of in situ X-ray absorption near-edge spectroscopy (XANES) and scanning electrochemical microscopy (SECM) before and during hydrogen permeation. The main conclusion was that the passive film is reduced during the hydrogen diffusion. The hydrogen permeation stabilizes the iron surface at a potential close to the thermodynamic water stability line where hydrogen evolution can occur. The stationary condition required for the determination of the permeation parameters cannot be easily attained on iron surface during hydrogen permeation. Moreover, additional attention must be paid when obtaining the transport parameters using the classical permeation cell.     

The promotion effects of Pd on Fe-Cu-Co based catalyst for higher alcohols synthesis

To increase the ability of Fe-Cu-Co based catalyst for hydrogen activation, the catalyst loaded with Pd was studied in the conversion of syngas to higher alcohols. X-ray diffraction (XRD), N₂ physisorption, H₂ temperature-programmed reduction (TPR) and H₂ temperature-programmed desorption (H₂-TPD) were applied to characterize the catalysts. The results of XRD showed that the Pd-loaded Fe-Cu-Co based samples were mainly composed of CuFe₂O₄ and CuO. After reduction, metallic Cu and Fe along with minor CuFe₂O₄ were identified, and the amount of CuFe₂O₄ decreased with the increase of the Pd content. H₂-TPR revealed that Pd facilitated the reduction of Fe-Cu-Co based catalyst. H₂-TPD confirmed that Pd enhanced the ability of Fe-Cu-Co based catalyst for H₂ activation. Therefore, the activity of the catalyst and the selectivity of alcohols were greatly improved. Over the Fe-Cu-Co based catalyst loaded with 0.5 wt.% Pd, the selectivity and the time-space yield of alcohols reached 58.7% and 1.53 g mL^− 1 h^− 1 at 350 °C, 6.0 MPa, GHSV = 10,000 h^− 1 and n(H₂)/n(CO) = 2.4.     

The reduction of l-cystine hydrochloride at stationary and rotating disc mercury electrodes

The kinetics of l-cystine hydrochloride reduction have been studied at a mercury-plated copper rotating disc electrode (RDE) and at a stationary mercury disc electrode (SMDE) in 0.1 mol dm⁻³ HCl at 298 K. The reduction of the disulphide is irreversible and hydrogen evolution is the major side reaction. In contrast to steady state electrode kinetic studies at a mercury drop electrode (which shows a well-defined limiting current), the mercury-plated Cu RDE shows overlap between disulphide reduction and hydrogen evolution. These effects are attributable to strong reactant adsorption with a calculated surface coverage close to 100%. A Tafel slope of −185 mV per decade is found with a cathodic transfer coefficient of 0.32 and a formal rate constant of 6.7 × 10⁻⁹ m s⁻¹. The relative merits of steady state voltammetry at a mercury-plated copper RDE and linear sweep voltammetry at the SMDE are discussed, as is the mechanism of l-cysteine hydrochloride formation.     

Amperometric flow injection analysis as a new approach for studying disperse systems

Fast and simple quantitative determination in dispersed systems (layered double hydroxides - LDHs - suspensions in aqueous solutions) was performed by a procedure that couples flow injection and amperometric detection (FI-AM). LDH dispersions are injected in a continuous flow (1 mL min⁻¹) of 0.05 mol L⁻¹ KNO₃ solution and [Cu(H₂O)₆]²⁺, used as a probe, is detected at a glassy carbon electrode housed in a flat electrochemical cell. The current intensity, recorded at the selected working potential (−0.25 V vs Ag/AgCl/NaCl (3 mol L⁻¹)), presents a linear relationship with [Cu(H₂O)₆]²⁺ concentration and the procedure offers high sensitivity (slope = 0.036 μA/(μmol L⁻¹)), a low detection limit (=0.7 μmol L⁻¹) and a wide quantification range (4-200 μmol L⁻¹).The method was applied to [Cu(H₂O)₆]²⁺ determination in two particular LDH-aqueous solution dispersed systems: (1) [Cu(H₂O)₆]²⁺ scavenging by etilendiammintetraacetic acid (EDTA) modified Zn-Al-LDHs, and (2) [Cu(H₂O)₆]²⁺ release from a copper doped Mg-Al-LDHs. The results obtained are comparable to those reported in previous works using different quantification techniques. FI-AM determination is applied without sample pretreatment (solid-supernatant separation) providing a high sampling rate (above 120 samples h⁻¹) that allows a better comprehension of the processes, particularly at the initial stages.     

Nucleic acid biosensor for DNA hybridization detection using rutin-Cu as an electrochemical indicator

The complex of rutin-Cu (C₈₁H₈₆Cu₂O₄₈, abbreviated by Cu₂R₃, R = rutin) was synthesized and characterized by elemental analysis and IR spectra. Cyclic voltammetry (CV) and fluorescence spectroscopy were used to investigate the interaction of Cu₂R₃ with salmon sperm DNA. It was revealed that Cu₂R₃ could interact with double-stranded DNA (dsDNA) by a major intercalation role. Using Cu₂R₃ as a novel electroactive indicator, an electrochemical DNA biosensor for the detection of specific DNA fragment was developed and its selectivity for the recognition with different target DNA was assessed by differential pulse voltammetry (DPV). The target DNA related to coliform virus gene could be quantified ranged from 1.62 × 10⁻⁸ mol L⁻¹ to 8.10 × 10⁻⁷ mol L⁻¹ with a good linearity (r = 0.9989) and a detection limit of 2.3 × 10⁻⁹ mol L⁻¹ (3σ, n = 7) was achieved by the constructed electrochemical DNA biosensor.     

The ultrasonic electropolymerization of an 5-[o-(4-bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) film electrode and its electrocatalytic properties to dopamine oxidation in aqueous solution

5-[o-(4-Bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) was electropolymerized on a glassy carbon electrode (GCE), and the electrocatalytic properties of the prepared film electrode response to dopamine (DA) oxidation were investigated. A stable o-BrPETPP film was formed on the GCE under ultrasonic irradiation through a potentiodynamic process in 0.1 M H₂SO₄ between −1.1 V and 2.2 V versus a saturated calomel electrode (SCE) at a scan rate of 0.1 V s⁻¹. The film electrode showed high selectivity for DA in the presence of ascorbic acid (AA) and uric acid (UA), and a 6-fold greater sensitivity to DA than that of the bare GCE. In the 0.05 mol L⁻¹ phosphate buffer (pH 6.0), there was a linear relationship between the oxidation current and the concentration of DA solution in the range of 5 × 10⁻⁷ mol L⁻¹ to 3 × 10⁻⁵ mol L⁻¹. The electrode had a detection limit of 6.0 × 10⁻⁸ mol L⁻¹(S/N = 3) when the differential pulse voltammetric (DPV) method was used. In addition, the charge transfer rate constant k = 0.0703 cm s⁻¹, the transfer coefficient α = 0.709, the electron number involved in the rate determining step n_α = 0.952, and the diffusion coefficient D_o = 3.54  10⁻⁵ cm² s⁻¹ were determined. The o-BrPETPP film electrode provides high stability, sensitivity, and selectivity for DA oxidation.     

Application of bismuth-film electrode for cathodic electroanalytical determination of sulfadiazine

A bismuth-film electrode for use in cathodic electrochemical detection was employed in order to quantify sulfadiazine in pharmaceutical formulations. The bismuth film was deposited ex situ onto a glassy carbon substrate. Analysis of two sulfa drugs was carried out by differential-pulse voltammetry in 0.05 mol L⁻¹ Britton-Robinson pH 4.5 solution. Sulfadiazine reduction was observed at −0.74 V vs. Ag/AgCl in one well-resolved irreversible reduction peak. The analytical curve with two slopes was obtained in the concentration range of 3.2-97.0 μmol L⁻¹. The detection limit was 2.1 μmol L⁻¹ for concentrations of 3.2-20.0 μmol L⁻¹ (r = 0.9949) and 12.2 μmol L⁻¹ for concentrations between 20.0 and 97.0 μmol L⁻¹ (r = 0.9951). Recovery studies carried out with both sulfadiazine samples gave values from 93.6 to 109.3%. The accuracy of the results supplied by the bismuth-film electrode was compared to those obtained by the standard amperometric titration method. The relative error between them was lower than 2.0%.     

Kinetics of dissociative adsorption of ethylene glycol on Pt(1 0 0) electrode surface in sulfuric acid solutions

The dissociative adsorption of ethylene glycol (EG) on Pt(1 0 0) electrode surface cooled in air after flame annealing was investigated by using programmed potential step technique and in situ FTIR spectroscopy. The stable adsorbates derived from EG dissociative adsorption on Pt(1 0 0) were determined by in situ FTIR spectroscopy as linear- and bridge-bonded CO. The quantitative results demonstrated that the average rate of dissociative adsorption of EG on Pt(1 0 0) surface varies with electrode potential, yielding a volcano-type distribution with a maximum value located near 0.10 V versus SCE. From the variation of the quantity of CO adsorbates generated in EG dissociative adsorption with the adsorption time t_ad, the initial rate (ν_i) of this surface reaction was evaluated quantitatively. The maximum value of ν_i has been determined to be 2.64 × 10⁻¹¹ mol cm⁻² s⁻¹ in a solution containing 2 × 10⁻³ mol L⁻¹ EG. The influence of the surface structure of Pt(1 0 0) electrode obtained by different pretreatment as well as of the specific adsorption of (bi)sulfate anions on the kinetics of EG dissociative adsorption has been also investigated and discussed. In comparison with a Pt(1 0 0) surface cooled in air atmosphere after flame treatment, the Pt(1 0 0) surface cooled in an Ar-H₂ stream or subjected to a treatment of fast potential cycling decreased significantly the initial rate ν_i of EG dissociative adsorption. Similar effect was also observed for the specific adsorption of (bi)sulfate anions. However, the maximum attainable coverage () of adsorbates derived from EG dissociative adsorption is not affected either by the surface structure of Pt(1 0 0) or by (bi)sulfate anions adsorption.     

Electrochemical quartz crystal microbalance studies of a palladium electrode oxidation in a basic electrolyte solution

Anodic oxidation of Pd in basic solutions (0.1 M KOH_aq and 0.1 M NaOH_aq) has been examined via cyclic voltammetry (CV) and an electrochemical quartz crystal microbalance (EQCM). Admittance tests show that Pd(II) layer behaves as a rigid one. The anodic vertex potential influences mass response during formation of the Pd(II) layer. For low anodic vertex potentials, obtained absolute mass per mole values suggest Pd(OH)₂ or PdO·H₂O to be oxidation products. At this stage of the oxidation process, contribution from adsorbed H₂O/OH⁻ in Pd(II) layer formation could explain the lower-than-expected mass gain, although the extent of H₂O/OH⁻ adsorption is unclear. The mass gain decreases with further increase in the anodic vertex potential, eventually reaching the value of ca. 8 g mol⁻¹ at about 700 mV vs. SCE. Comparing the influence of vertex potential in CV experiments on the mass and reduction potential of the Pd(II) species points to the formation of PdO at higher oxidation potentials. At this stage of the process, a fraction of the PdO species is generated during transformation of previously formed Pd(OH)₂/PdO·H₂O. A shift of the main Pd(II) reduction potential peak depends on both the anodic vertex potential and on the composition of the Pd(II) film. The order of the Pd(II) reduction process is the opposite of that observed for the oxidation process. The Pd(IV) species formed at E ≥ 500 mV vs. SCE and those reduced between 50 and 350 mV are hydrated or contain hydroxyl groups.     

Kinetics of adenine adsorption on Au(1 1 1) electrodes: An impedance study

The adsorption/desorption kinetics of adenine on Au(1 1 1) electrodes is studied by Electrochemical Impedance Spectroscopy (EIS) in 0.5 M NaF solutions at four adenine concentrations. The experimental procedure is designed in order to obtain impedance data unaffected by surface reconstruction on the entire potential region of adsorption. The frequency dispersion of the impedance at potentials of the adsorption region has been analysed according to the Frumkin-Melik-Gaykazyan adsorption theory without any “a priori” assumption about the potential dependence of the adsorption rate constant. The analysis provides the values of the adsorption capacitance, C_ad, adsorption resistance, R_ad and the Warburg coefficient, σ_ad, at every potential, and from them the relaxations times τ_H and τ_D. A mixed adsorption-diffusion control has been detected and the specific rate constant of adsorption has been obtained in a wide potential region.     

Understanding CO-stripping mechanism from NiUPD/Pt(1 1 0) in view of the measured nickel formal partial charge number upon underpotential deposition on platinum surfaces in sulphate media

We recently showed nickel-underpotential deposition (Ni-UPD) occurs on polycrystalline or single crystal platinum electrodes in acidic media. Whereas the decoupling of the nickel and hydrogen adsorption/desorption peaks is difficult for low pH, these processes can be better separated for higher pH values, typically pH > 3. However, even for platinum single crystals, high pH solutions do not enable to sufficiently separate nickel from hydrogen phenomena. As a result, electrochemistry alone cannot yield important information about Ni-UPD, such as the formal partial charge number (valency of electrosorption) and the role of the sulphate or hydrogen sulphate anions.So, we decided to couple cyclic voltammetry to electrochemical quartz crystal microbalance (EQCM). EQCM measurements enable to decorrelate the simultaneous hydrogen and nickel adsorption/desorption peaks, which we could not attempt solely with electrochemistry. The coupling between gravimetric and electrochemical measurements allows us to detect the contribution of the anions and thus to isolate that of nickel: nickel coverage can then be determined. Nearly 4/5 Ni_UPD monolayer (θ_Ni ≈ 0.8) over platinum is reached at nickel equilibrium potential for high pH solutions (5.5). The QCM and electrochemistry coupling further allows the determination of nickel formal partial charge number: ι_Ni,EQCM = 1.3 ± 0.13. Direct electrochemistry measurements (Swathirajan and Bruckenstein method) yield: ι_Ni,Pt(poly) = 1.5 ± 0.17. These two values are close, which validates the electrochemical method for the nickel/platinum system. In consequence, we used Swathirajan and Bruckenstein method for Pt(1 1 0)-(1 × 2) crystal and found: ι_{Ni,Pt(1 1 0)} ≈ 1.4 ± 0.1. Whatever the system (Ni_UPD/Pt(poly) or Ni_UPD/Pt(1 1 0)-(1 × 2)) or the experimental technique, nickel formal partial charge number is lower than nickel cation charge: ι_Ni < z_Ni = 2. In consequence, upon underpotential deposition on platinum surfaces, nickel cations discharge and then undergo additional charge exchange processes, such as anion (or water) adsorption, resulting in apparent partial nickel cation discharge. Moreover, Ni_UPD/Pt(1 1 0) surface displays high activity towards CO_ad oxidation reaction. We explain such positive effect by the possible existence of a bifunctional mechanism in which oxygenated-species-covered Ni_UPD adatoms provide the oxygen atom to CO_ad?Pt species, enabling its facile oxidation.     

Corrosion of carbon steel in sodium methanoate solutions

The behaviour of steel electrodes in sodium methanoate solutions was studied by coupling electrochemical techniques (voltammetry, OCP vs. time) with in situ micro-Raman spectroscopy analyses of the corrosion products. The polarisation curves depended strongly on the methanoate concentration. For the smallest concentration (10⁻³ mol L⁻¹), the current density increased regularly with the applied potential. So the behaviour of the electrode was typical of an active material. In contrast, for the largest concentration (10⁻¹ mol L⁻¹), the curves obtained were typical of a passive material. Methanoate ions favoured growth and stability of a passive oxide film more likely by adsorbing on its surface. The polarisation curve obtained for the intermediate concentration (10⁻² mol L⁻¹) was unusual and testified of an imperfect passivation of the steel surface. Finally, steel electrodes were left at the open circuit potential in the methanoate solutions. In any case, the passivity was rapidly lost and a general corrosion of the surface took place. In situ Raman spectroscopy analyses at the early stage of the corrosion process demonstrated that the first product to form was a green rust, GR(HCOO⁻). It was oxidised later into γ-FeOOH (lepidocrocite) by dissolved O₂. The process is then typical of what is usually observed in neutral or alkaline media, whatever the anions present and responsible of the GR formation. A new and detailed characterisation of GR(HCOO⁻) by X-ray diffraction was performed and a crystal structure is proposed.     

Kinetics and mechanism of oxygen reduction at hydrous oxide film-covered platinum electrode in alkaline solution

This study uses rotating ring-disk electrode (RRDE) and linear sweep voltammetry (LSV) to characterize oxygen reduction kinetics in alkaline solution on platinum electrodes with various thickness of hydrous oxide (oxyhydroxy) film. Oxyhydroxy films are created on Pt electrodes by pretreatment in 1.0 mol dm⁻³ KOH at a constant voltage. The pretreatment voltage ranges from −1.2 to 1.0 V and is increased stepwise before each new experimental run to produce seven discreet films. LSV plots show oxyhydroxy film thickness strongly inhibits oxygen reduction and is inversely proportional to RRDE oxygen reduction current I_D for LSV voltages E_D from −0.1 to −0.46 V, but this trend reverses at E_D more negative than −0.46 V so that the worst-performing electrode becomes the best. However, this improvement disappears at around −0.8 V, suggesting this change involves a negatively charged ion, possibly embedded into the metal in the top few atomic layers either interstitially or substitutionally. The 1.0 V-pretreated electrode in the E_D range from −0.46 to −0.9 V of highest oxygen reduction current also exhibits the lowest hydrogen peroxide production, with zero H₂O₂ produced at −0.6 V, indicating the brief presence of the oxyhydroxy film on the Pt surface has strong lingering effects. The post-oxyhydroxy Pt surface is very different than the native Pt for oxygen reduction pathway and efficiency. Reaction order with respect to oxygen is close to 1. The rate constants of the direct O₂ to H₂O electroreduction reaction are increased with decreasing the potential from −0.2 to −0.6 V, but the O₂ to H₂O₂ electroreduction is contrary to this expectation. The rate constants of H₂O₂ decomposition on the oxyhydroxy film-covered Pt electrode are near constant around 1 × 10⁻⁴ cm s⁻¹ at E_D > −0.5 V.     

A comparative study of the physicochemical and electrochemical properties of Cr and Ni-W-P amorphous electrocoatings

A comparative study of the physicochemical and electrochemical properties of Cr and amorphous Ni-W-P electrocoatings is presented here. Amorphous Ni-W-P alloys were successfully produced by electrodeposition at 70 °C on copper substrate under galvanostatic control in the range of 50-400 mA cm⁻² and constant loads of 500 and 1600 C, using a solution containing 0.20 mol L⁻¹ Na₂WO₄.2H₂O; 0.02 mol L⁻¹ NiSO₄·6H₂O; 0.02 mol L⁻¹ NaPH₂O₂; 0.02 mol L⁻¹ H₃BO₃; 0.07 mol L⁻¹ (NH₄)₂SO₄; 0.20 mol L⁻¹ Na₃C₆H₅O₇·2H₂O; 0.0001 mol L⁻¹ CH₃(CH₂)₁₀·CH₂OSO₃Na. Cr electrocoatings were obtained from an industrial plating solution. The physicochemical characterization of the as-electrodeposited and as-annealed samples was carried out by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray analysis (EDX) techniques. Corrosion tests were carried out at room temperature in 10⁻¹ mol L⁻¹ NaCl aqueous solutions, using potentiodynamic linear polarization (PLP). Among the various Ni-W-P electrocoatings studied here, the Ni₆₅W₂₀P₁₅ layer presented the best corrosion behavior and a slightly superior corrosion potential than the Cr electrocoating. Heat treatments gave rise to a cracked surface morphology in the Cr layers, while the surface morphology of the Ni₆₅W₂₀P₁₅ layers remained homogeneous and devoid of cracks. Heat treatments at 400 and 600 °C led to crystallization of the Ni-W-P layer, with precipitation of the Ni₃P, Ni and Ni-W phases and increasing hardness of the Ni-W-P layer as the heat treatment temperature rose. All the annealed Cr layers showed cracked surfaces and their hardness diminished as the annealing temperature increased. The presence of cracks impairs the mechanical and corrosion resistance properties of Cr layers. Ni₆₅W₂₀P₁₅ layer is a potential candidate to replace Cr in industrial applications, mainly at operational temperatures that exceed room temperature.