Electrochemically initiated (co)polymerization of acrylamide and acrylonitrile on a steel cathode—electrochemical and impedance study

Investigations were carried out into the processes of electrochemically initiated (co)polymerization (EIP) of acrylamide (AA) and/or acrylonitrile (AN) from acidic aqueous solutions on a steel cathode. The aim was to obtain (co)polymer films with given electroinsulating and corrosion protection properties. The kinetics of (co)polymer film formation were investigated using various electrochemical methods (constant current and constant potential electrolysis, cyclic voltammetry, double-layer capacity measurements) and ellipsometry. The electrical properties of the films formed were studied (ex situ) by measuring their impedance (Z) and phase angle () as a function of frequency. It was shown that equivalent circuit (EC), proposed for covered metal surfaces in solutions, can be satis-factorily applied. The values of the EC elements obtained by curve fitting to experimental results show the influence of the thermal treatments and/or moisture concentration on the impedance characteristics of polymer films. The mechanism of (co)polymer film formation as well as the electrical and corrosion protection properties are discussed with regard to the experimental parameters and some characteristics of the films obtained.     

Corrosion Protection of Mild Steel by Formation of Iron Oxide Polybithiophene Composite Films

Polybithiophene (PBT) was freshly electrodeposited on platinum (Pt) and steel electrodes using cyclic voltammetry from 2,2-bithiophene (BT), electrolyte and acetonitrile (AC). The electrolytes used were LiBF₄, LiClO₄, LiCF₃SO₃, Bu₄NBF₄, Bu₄NClO₄, or Bu₄NCF₃SO₃. Black to red colored PBT films with metallic lustre were electropolymerized on Pt and steel electrodes with good adherence. The electrochemical properties and the surface morphology of the deposited PBT film depend on the type of ions used during the electropolymerization. It was found that the steel surface was covered with Fe _x O _y –PBT composite. The results are discussed on the basis of the structural and the electrochemical properties of the electrolyte ions. The presence of PBT–iron oxide composite on the steel surface causes a shift of the pitting potential (E _pit) in the positive direction, indicating the inhibitive effect of PBT–Fe _x O _y composite on the chloride pitting corrosion.     

Gold Supported on Oxide Surfaces: Environmental Effects as Studied by STM

This paper begins by providing a short review of STM studies of gold particles supported on oxide surfaces. Following this, the morphology of Au particles deposited on thin FeO(111) films at elevated pressures of CO, O₂, CO+O₂, and H₂ has been examined using in situ STM at room temperature. The Au particles are found to be quite stable in oxygen and hydrogen environments at pressures up to 2 mbar. However, in CO and CO+O₂ atmospheres, the destabilization of Au particles located at the step edges occurs leading to the formation of mobile Au species, which migrate across the oxide surface. General problems encountered with high-pressure STM studies are discussed, and data clearly showing the effects of ambient gas impurities is provided. These effects may lead to erroneous conclusions, particularly about morphological changes of and CO dissociation on the gold surfaces at elevated pressures.     

A simple and novel method for preparing Ni(OH)2 Part I: Structural studies and voltammetric response

Ni/Ni(OH)₂ electrodes were prepared by in situ precipitation of nickel hydroxide into porous nickel substrates. The thermal decomposition of urea in solutions containing Ni²⁺ was used for the synthesis of the active material. The compound prepared by this method was a poorly crystallized (II)-type phase. The electrochemical behaviour of Ni/Ni(OH)₂ electrodes thus prepared was studied by cyclic voltamperommetry and the efficiency of the in situ precipitation was followed by the measurement of the cathodic charge obtained from the j-E response. The amount of incorporated material depends on the urea concentration and the electrochemical response of the electrodes was improved by increasing the number of impregnation cycles.     

Influence of surface inhomogeneities of boron doped CVD-diamond electrodes on reversible charge transfer reactions

The electrochemical behaviour of reversible charge transfer reactions on boron doped diamond (BDD) was studied by cyclic voltammetry and electrochemical impedance spectroscopy using rotating disc electrodes under defined convection. Diamond films of 5 m thickness with doping levels of 200, 3000 and 6000 ppm were prepared by hot filament chemical vapour deposition on niobium substrate. The electrochemical measurements were carried out on BDD electrodes in deaerated 0.5 M Na₂SO₄ + 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] solution at rotation frequencies 0 < f _rot < 4000 rpm. Comparative measurements were carried out on an active Pt electrode. The BDD electrodes exhibit distinct irreversibilities indicated by a larger peak potential difference in the cyclic voltammograms, lower diffusion limiting current densities and an additional impedance element at high frequencies. Mechanical polishing with carbon fleece and SiC paper strongly affects the irreversible behaviour of the BDD electrodes. The experimental results are explained by a partial blocking of the diamond surface with reversible charge transfer at active sites. The impedance spectra are analysed quantitatively using a transport impedance model for reversible reactions on partially blocked electrode surfaces.     

Membranes from polyelectrolyte complexes

Poly(1-butene) (PB-1) films drawn from the melt have been deposited on highly oriented pyrolytic graphite (HOPG) substrates for investigation with the scanning tunneling microscope (STM). The STM investigations showed images of PB-1 flakes extending over some hundred nanometers. Their thickness was determined to be much larger than the normal tunneling distance established between the tip and a good conducting (metallic) sample surface. Close to monomolecular film steps, our STM measurements simultaneously revealed both the atomic resolution of the HOPG substrate and a superlattice showing an ordered structure pseudomorphic to the helical nature of the PB-1 macromolecule.     

Nucleation effects on structural and optical properties of electrodeposited zinc oxide on tin oxide

Zinc oxide was electrodeposited from oxygenated aqueous solutions of zinc chloride at 80 C on tin oxide covered glass substrates. A new activation treatment for the substrate is established. This consists in the initial formation, in the deposition solution, of a thin metallic zinc layer (5–50 nm) converted to ZnO by in situ reoxidation. Variable densities of nucleation centers (with values approaching 10¹⁰ cm^–2) are formed by this treatment. This allows control of the formation of a zinc oxide layer ranging from open deposits of isolated crystallites to compact and homogeneous layers. Compact layers have high specular transmission below the band gap value (3.5 eV), whereas open films exhibit extensive light scattering. The shapes of the current–time curves during deposition are discussed in terms of nucleation and structural effects. A possible influence of the semiconducting properties of the films is pointed out.     

Fabrication and functionalization of carbon nanotube films for high-performance flexible supercapacitors

1. Download : Download full-size image

Carbon nanotube (CNT) films have shown many promising advantages in the development of high-performance flexible supercapacitors in terms of electrode specific area, mechanical reliability under bending and stretching, electron and ionic mobility tailored for high-rate performance etc. In this review, the recent progress in the design, preparation and functionalization of CNT film based electrodes for the fabrication of high-performance flexible supercapacitors are introduced in details, including the synthesis of conductive CNT films for the electrodes of supercapacitors, and the functionalizations of CNT film with other high-capacitance materials by both mixing and in situ growth strategies for high-performance composite electrodes. Furthermore, we also discussed the assembly strategies, prototypes and electrochemical performance of flexible supercapacitors based on CNT film composite electrodes. At last, the challenges and trends of the CNT film based flexible supercapacitors are prospected as well.     

Corrosion of metals in molten lithium sulphate-potassium sulphate eutectic

In conjunction with the development of a new electrochemical flue gas desulphurization process, a study is made of the corrosion of several electrode materials in molten lithium sulphate-potassium sulphate eutectic at 600°C. Measurements of the open-circuit potentials are made in air, oxygen and nitrogen to determine the existence of a stable oxide layer on the electrode surface. Voltammetric measurements are also made to determine the corrosion current densities in O₂/SO₂ mixtures at various compositions. The study shows that the valve metals studies (zirconium, tantalum and titanium) develop stable oxide layers on their surfaces which are highly corrosion-resistant. The corrosion currents decrease in the following order NiFe>CuAg>Zr.     

Effects of γ-radiation versus H2O2 on carbon steel corrosion

The effect of ionizing radiation on steel corrosion is an important materials issue in nuclear reactors. In the presence of ionizing radiation water decomposes into both oxidizing and reducing species (e.g., OH, H₂O₂, O₂⁻) whose net interactions with steels are not fully understood. The effect of radiation on the corrosion kinetics of carbon steel has been studied at pH 10.6 and room temperature, using electrochemical and chemical speciation analyses. The present study investigates the effect of γ-radiation on carbon steel corrosion and compares it with that of chemically added H₂O₂, which is considered to be the key radiolytically produced oxidant at room temperature. Various oxide films were pre-grown potentiostatically on carbon steel electrodes, and then exposed to either γ-radiation at a dose rate of ∼6.8 kGy h⁻¹ or to H₂O₂ in a concentration range of 10⁻⁶ to 10⁻² M. The corrosion kinetics were studied by monitoring the corrosion potential (E_CORR), and periodically performing linear polarization (LP) and electrochemical impedance spectroscopy (EIS) measurements.     

Electrodeposited oxotungstate films: Towards the molecular nature of recharging processes

In situ Raman spectroscopy is applied to supplement voltammetric, spectroelectrochemical, and XRD data on redox transformations of electrodeposited oxotungstate films. These films undergo electrochromic transition at rather positive potentials, as compared to usual sputtered tungsten oxides. The depth of electroreduction for the films conditioned in acidic solutions under open circuit is about 0.11 e⁻ per W atom. Coloration of the films correlates with the decrease of Raman band, corresponding to the terminal W(VI)O vibration in the hydrated phase of highly defective tungstic acid (hydrated tungsten oxide). Our data allow to state the absence of oxotungstate octahedra rearrangement in the course of reduction at positive RHE potentials, and to assume that slightly deeper reduction up to 0.15 W(V)/[W(V) + W(VI)] ratio is possible at more negative potentials. We also demonstrate that the gas phase reduction is less reversible as compared to electrochemical reduction in solution. The most possible nature of films degradation in the gas phase is their partial dehydration in the course of reduction.     

Active surface area in oxide electrodes by overpotential deposited oxygen species for the oxygen evolution reaction

The electrochemical active surface area at oxide electrodes of Pt and electrodeposited Ni, Co and Ni₂₀Co₈₀ alloys was evaluated in 5m KOH solutions based on the charge for electrochemical desorption of a monolayer of overpotential deposited oxygen (OPD O) species. Thein situ technique empllyed for the charge measurement involves galvanostatic charging (OPD O) adsorption), followed by simple discharging (OPD O desorption) experiments. It is observed that surface area estimated by this new technique for the oxidized surfaces of the metals studied here are consistent with those from a.c. impedance spectroscopy. The activity of the metal towards the oxygen evolution reaction (OER) is also discussed in terms of their active surface area estimated in this study.     

Electrochemical behaviour of Ni + Al alloy as an alternative material for molten carbonate fuel cell cathodes

This paper presents an investigation of the performance and stability for oxygen reduction on in situ oxidized Ni alloys, specially focused on 95 at % Ni + 5 at % Al and 85 at % Ni + 15 at % Al alloy electrodes in Li/Na carbonate eutectic. Test specimens of the alloys were prepared as thin film electrodes sputtered onto Au substrates. In situ oxidation of alloy electrodes and electrochemical measurements for oxygen reduction on the electrodes were performed in the free-volume melt at 923 K. It was found that the in situ oxidized Ni + Al alloys exhibit higher performance for the oxygen reduction than the NiO without Al. Electrochemical fractal analysis (EFA) revealed higher oxide film stability of the Ni + Al alloys in comparison to NiO electrodes. The surface morphology of the alloy specimen after oxidation was investigated with SEM and AFM.     

In vitro corrosion and mineralization of novel Ti–Si–C alloy

The in vitro electrochemical behaviour of a new titanium based α-alloy (Ti–0.5 wt% Si–0.65 wt% C), fabricated via casting and rapid cooling route, was determined using linear, Tafel, potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS), complemented with ex situ SEM-EDS analysis to evaluate the corrosion mechanism. The experimental results revealed that silicon and carbon, in addition to titanium, resulted in the enhancement of mechanical properties. The polarization tests confirmed that Ti–Si–C alloy possessed excellent corrosion resistance (a low corrosion current density of 0.033 μA cm⁻²), comparable to cp Ti and better than Ti6Al4V in phosphate buffered saline (PBS). The mechanism of corrosion was identified as selective dissolution of titanium solid solution matrix. EIS studies indicated the formation of a stable, passive oxide film on the alloy. Further, in vitro bioactivity was evaluated using mineralization tests i.e. by immersing the pre-treated alloy in a concentrated simulated body fluid (10× SBF). Chemical and microstructural characterization of the mineral layer, formed during immersion, revealed the deposition of fine, porous micron-sized globules of a phase rich in calcium-phosphate (Ca-P). In summary, the bulk properties and excellent in vitro electrochemical and mineralization behaviour of the as-cast Ti–Si–C alloy reveal a high potential for its application as load bearing metallic implants.     

Abnormal infrared effects and electrocatalytic properties of nanometer scale thin film of PtRu alloys for CO adsorption and oxidation

Nanometer scale thin films of PtRu alloy supported on glassy carbon (nm-PtRu/GC) were prepared using electrochemical codeposition under cyclic voltammetric conditions. The composition of the PtRu alloy was altered by varying the concentration of Pt⁴⁺ and Ru³⁺ ions in the deposition solution. STM results demonstrated that the nm-PtRu film was composed of crystallites appearing in a layered hexagonal form. The nm-PtRu/GC exhibited a high catalytic activity for CO oxidation, consisting mainly in a negative shift of potential for CO_ad oxidation. In situ FTIR spectroscopic studies revealed that the nm-PtRu/GC alloy electrodes of different surface composition exhibit abnormal infrared effects (AIREs) as observed on electrodes of nanometer scale thin films of transition metals. The AIREs observed on nm-PtRu/GC electrodes consist in the inversion of CO_ad bands, the significant enhancement of IR absorption and an increase in the FWHM. Following the increase in Ru component in the nm-PtRu thin film the FWHM was increased progressively from 20 cm^–1 on a nm-Pt/GC to 55 cm^–1 on a nm-Ru/GC electrode, and the enhancement factor of IR absorption by CO_L was between 10.5 to 13.1. The present study has provided new understanding of the structure and properties of nanometer scale thin film PtRu alloy material, and highlights its potential for fuel cell applications.     

Spectroelectrochemical behaviour of poly (3-octylthiophene): application to electrochromic windows with polyaniline and iridium oxide

The spectroelectrochemical behaviour of cast poly(3-octylthiophene) (POT) films (0.2 and 0.5 m) on indium-tin-oxide (ITO) glass electrodes has been investigated in organic media. These thin films exhibit interesting electrochromic properties and their application in electrochromic devices has been examined in liquid (CH₃CN + LiClO₄ 0.3 M) and viscous electrolyte (PEO + CH₃CH + LiClO₄). Polyaniline (PANI) film appears to be a convenient complementary counter electrode since its transmission maximum corresponds to its oxidized state and that of POT film to its reduced state. Thin films of iridium oxide (IrO₂) are also possible counter electrodes, even in acetonitrile, the electrochemical behaviour being mostly capacitive with a low transmission change. However, the best contrast is obtained with the POT/PANI system.     

Monitoring surfaces on the molecular level during catalytic reactions at high pressure by sum frequency generation vibrational spectroscopy and scanning tunneling microscopy

Sum frequency generation (SFG), using non-linear laser optics, detects vibrational spectra of submonolayer amounts of adsorbates with excellent energy and time resolution. Scanning tunneling spectroscopy (STM) is sensitive to the atomic surface structure; readily imaging defects, steps and kinks as well as stationary adsorbed species. Both of these techniques can be used during reactions at high pressures and temperatures to obtain molecular information in situ. We report studies of propylene hydrogenation over Pt(111) crystal surfaces at atmospheric pressures and 300 K using SFG and STM. Four surface species (2-propyl, -bonded propylene, di -bonded propylene, and propylidyne) were identified; the first two being implicated as reaction intermediates. The platinum surface structure remains unchanged during the reaction, consistent with the structure insensitive nature of olefin hydrogénation. Propylene decomposition induced substantial surface reconstruction.     

Electrochemical oxidation of ammonia (NH4/NH3) on thermally and electrochemically prepared IrO2 electrodes

The electrochemical oxidation of ammonia (NH₄⁺/NH₃) in sodium perchlorate was investigated on IrO₂ electrodes prepared by two techniques: the thermal decomposition of H₂IrCl₆ precursor and the anodic oxidation of metallic iridium. The electrochemical behaviour of Ir(IV)/Ir(III) surface redox couple differs between the electrodes indicating that on the anodic iridium oxide film (AIROF) both, the surface and the interior of the electrode are electrochemically active whereas on the thermally decomposed iridium oxide films (TDIROF), mainly the electrode surface participates in the electrochemical processes.On both electrodes, ammonia is oxidized in the potential region of Ir(V)/Ir(IV) surface redox couple activity, thus, may involve Ir(V). During ammonia oxidation, TDIROF is deactivated, probably by adsorbed products of ammonia oxidation. To regenerate TDIROF, it is necessary to polarize the electrode in the hydrogen evolution region. On the contrary, AIROF seems not to be blocked during ammonia oxidation indicating its fast regeneration during the potential scan. The difference between both electrodes results from the difference in the activity of the iridium oxide surface redox couples.     

Understanding corrosion of ancient metals for the conservation of cultural heritage

Corrosion studies on ancient metallic objects are performed with the aim of evaluating their stability for a very long term. They have crossed centuries and should be transmitted in good shape to future generations. They are generally protected from aggressive conditions by a layer formed by corrosion products and this layer stability has to be checked. This work intends to show that coupling an in situ technique like Raman spectroscopy to electrochemical measurements allows a mechanism of corrosion layer formation and stability to be proposed. For bronze the beneficial presence of tin IV oxide to slow down copper corrosion is evidenced. For steel, the hypothesis of lepidocrocite reduction to be coupled with iron corrosion is infirmed.     

Investigation of structure and ORR reactivity of fuel cell catalysts by in-situ STM

In order to provide insight into the interfacial relationships at fuel cell electrodes, a measuring technique based on a scanning tunneling microscope working in an electrochemical cell has been developed. The structure of model electrodes consisting of carbon supported Pt and ionomer mixtures as well as MEA electrode surfaces is imaged by the electrochemical scanning tunneling microscope (EC-STM) from microscale to nanoscale. Images with subnanometer resolution are obtained indicating that some ordering of the particles on the carbon support is present in the model system and in the MEA. It is demonstrated that in both cases nanometer structures can be imaged reliably and that the interface is rather clean and active under reaction conditions. In addition, a technique has been developed to measure the local reactivity by using the STM tip as a sensor electrode for the ORR.