Atomic emission spectroelectrochemistry applied to dealloying phenomena II. Selective dissolution of iron and chromium during active-passive cycles of an austenitic stainless steel

Atomic emission spectroelectrochemistry was used to investigate selective dissolution of a 304 austenitic stainless steel sample in 2 M H₂SO₄. The partial dissolution rates of Fe, Cr, Ni, Mn, Mo, and Cu were measured as function of time during a series of potentiostatic triggered activation/passivation cycles. When first exposed to sulfuric acid solution, the steel sample was in a passive state with a total steady state ionic dissolution rate expressed as an equivalent current density of 10 μA cm⁻². A transition into the active and passive state could be triggered by cathodic (−700 mV vs. Ag/AgCl) and anodic (+400 to +700 mV vs. Ag/AgCl) potentiostatic pulses respectively of variable time. Excess Cr dissolution was observed during the activation cycle as compared to Fe and a depletion of Cr dissolution was observed during the passivation cycle. These results are interpreted in terms of the dissolution of a Cr rich passive layer during activation and selective dissolution of Fe, Mn, Ni and other elements to form a Cr rich passive layer during passivation. Quantitative analysis of the excess Cr showed that the residual film contained approximately 0.38 μg Cr/cm². Fe does not appear to be incorporated into the film at this early stage of passive film growth. Residual films of metallic nickel and copper were formed on the surface during the active period that subsequently dissolved during passivation.     

A SNIFTIRS study of the adsorption of ethylene carbonate at a Au(110) electrode from aqueous solutions

An in situ infrared spectroelectrochemical technique based upon the thin layer ATR optical configuration was used to examine the potential induced reorientation of ethylene carbonate (EC) at a Au(110) electrode. The behavior of EC was studied in aqueous solutions containing HClO₄, NaClO₄ and tetramethylammonium perchlorate electrolytes. Intense absorption bands due to the carbonyl and ring vibrational modes of EC were observed in the 1900-925 cm⁻¹ region. The carbonyl and ring vibrational modes are clearly influenced by the Au electrode and subsequently were frequency shifted; however, they did not show any potential dependent frequency behavior. Other bands did exhibit potential dependency. In addition, the magnitudes of some EC bands shift depending on the nature of the electrolyte cation. The spectral bands associated with the electrolyte provided evidence of ion migration in the diffuse double layer under the applied electric field.     

Optimization of electrochemical infrared reflection absorption spectroscopy using Fresnel equations

To enhance the signal-to-noise ratio of electrochemical in situ IRRAS techniques, experimental conditions such as the angle of incidence and the thin cavity thickness should be optimized. Software which utilizes the Fresnel equations in matrix form was used to determine the experimental conditions at which the maximum in the mean square electric field strength (MSEFS) at the electrode surface is obtained. Calculations were done for Au in the mid infrared range from 4000 to 800 cm⁻¹ for six systems combined of three common IR windows (CaF₂, BaF₂ and ZnSe) and two electrolyte solvents (H₂O and D₂O). It was determined that the angle of incidence strongly depends on the refractive index of the electrolyte material, and that approximate values of the optimum angle of incidence can be found using a simple model and Snell's law. It was also shown that the thin cavity thickness affects the performance of the experimental setup via the phase shift of the radiation due to transmission through the solvent. The effect of the window materials and collimation on the experimental conditions is also discussed. The paper is designed to help researchers select the experimental conditions that optimize the performance of their spectroelectrochemical setup.     

Spectroelectrochemistry of Quantum Dots

Spectroelectrochemistry (SEC) is a set of techniques with many advantages in the study and characterization of materials. Although SEC has not yet been widely used to study quantum dots (QDs), the information extracted from SEC experiments about these nanostructures is very useful. Most of the works that use SEC to study QDs are high‐quality pieces of research. This review intends to show how to perform SEC in an easy way and what information can be obtained using these techniques. Most of the examples shown in this review are related to semiconductor and carbon QDs. After a brief introduction, some optoelectronic properties of QDs and the main SEC techniques are described. The capabilities of SEC for the study of QDs are illustrated with examples extracted from literature. Finally, the needs of SEC to become a user‐friendly technique and its evolution to become more powerful are commented in the last section of the review.     

Copolymers—A refined way to tailor intrinsically conducting polymers

An overview is given on electrochemically prepared intrinsically conducting copolymers, their preparation, their properties, potential applications and significant differences from the respective homopolymers. Particular attention is paid to verification of the formation of true copolymers and their characteristic features in comparison to mixtures of homopolymers.     

In situ FTIR study of the decomposition of N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ionic liquid during cathodic polarization of lithium and graphite electrodes

In this work we analyzed the cathodic reactions of an important ionic liquid (IL) based electrolyte solution, namely lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)/N-methyl-N-methylpyrrolidinium (BMP) TFSI. In situ FTIR spectroscopy was used for the analysis of gaseous products of the electrochemical decomposition of this IL solution during cathodic polarization of lithium metal and graphite electrodes. The main volatile product of the reductive decomposition of the anion in these BMPTFSI solutions is trifluoromethane. BMP cations decompose to mixtures of tertiary amines and hydrocarbons. The composition of the products is influenced by the nature of the anode material. Graphite possesses a catalytic activity in the electroreduction process of BMP cations which occurs along with their intercalation into the graphite structure. The liquid phase after cathodic polarization of graphite electrodes was analyzed by multinuclear NMR spectroscopy coupled with FTIR spectroscopy. ¹⁵N NMR and FTIR spectra revealed an increase in the Li cations content in the electrolyte solution, as a result of BMP cations decomposition during repeated cycling of graphite electrodes.     

Electrochemical synthesis and characterization of mixed molybdenum-rhenium oxides

The electrodeposition of Mo_xRe_1−xO_y films (0.6 ≤ x ≤ 1) on indium-tin oxide (ITO) coated glass substrates from acidic peroxo-polymolybdo-perrhenate solutions is described. Trends in film growth were established as a function of potential from +0.4 V to −0.7 V vs Ag/AgCl by analyzing the composition and stoichiometry of the deposit using inductively coupled plasma mass spectrometry (ICPMS) and X-ray photoelectron spectroscopy (XPS). These experiments show that the concentration of rhenium increases linearly with the deposition potential and that the deposits are mixed-valent containing up to five different metal oxidation states (i.e., Mo^IV, Mo^V, Mo^VI, Re⁰, Re^IV). Electroanalytical techniques were used to explore the deposition mechanism, including chronocoulometry, cyclic voltammetry, spectroelectrochemistry, and electrochemical quartz crystal nanogravimetry (EQCN). At potentials positive to −0.26 V, perrhenate (Re^VIIO₄⁻) behaves as a redox mediator to accelerate the deposition of a mixed-valent molybdenum oxide, but at more negative potentials mixed molybdenum-rhenium oxides are produced.     

Synthesis, electrochemistry and in situ spectroelectrochemistry of soluble lead phthalocyanines

The synthesis, characterization and voltammetric and spectroelectrochemical properties of newly synthesized lead phthalocyanines bearing tetra-(1,1-(dicarbpenthoxy)-2-(4-biphenyl)-ethyl), tetra-(1,1-(dicarbpenthoxy)-2-(1-naphthyl)-ethyl and tetra-((1,1,2-(tricarbopentoxyethyl)) substituents have been presented in this work for the first time. The characterization of the complexes was made by elemental analysis, ¹H NMR, FT-IR, UV-vis and Maldi-TOF. The solution redox properties and spectroelectrochemical investigation of the complexes are studied using various electrochemical techniques in DCM on a platinum electrode. Cyclic voltammetry and differential pulse voltammetry studies show that the complexes give three one-electron ligand-based reductions and two one-electron oxidation couples having diffusion-controlled mass transfer character. Assignments of the redox couples were confirmed by spectroelectrochemical measurements. Spectroelectrochemical studies reveal that all complexes are demetallized during the spectroelectrochemical measurement under the applied potentials at the first reduction and oxidation potentials of the complexes. Different ring substituents of the complexes affect the easy demetallization processes of the complexes.     

Mechanistic comparison between oxidative and reductive charge transport by [(bpy)2(H2O)RuORu(H2O)(bpy)2] confined in a Nafion membrane as studied by potential-step chronocoulospectrometry

Charge transport (CT) in a Nafion membrane containing μ-oxobis[aquabis(2,2′-bipyridine)ruthenium(III)] complex, [(bpy)₂(H₂O)RuORu(H₂O)(bpy)₂]⁴⁺ (bpy=2,2′-bipyridine, abbreviated to Ru^IIIORu^III) was investigated by potential-step chronocoulospectrometry (PSCCS). Electrochemical reduction of Ru^IIIORu^III in the membrane occurred irreversibly to form [Ru(bpy)₂(OH₂)₂]²⁺ monomer. The CT by reduction of Ru^IIIORu^III in the membrane was suggested to take place by physical displacement of the complex, which is quite different from the mechanism in the CT by oxidation of Ru^IIIORu^III in the same membrane in which charge is transported by charge hopping based on reversible redox reaction between Ru^IIIORu^III and Ru^IIIORu^IV. The fractions of the electrochemically reacted complex in the membrane for the oxidative CT was dependent on the complex concentration, and the yield was low (maximum fraction=0.42 at 0.87 M) relative to the reductive CT. By contrast, the fraction for the reductive CT was independent of the concentration over 0.12 M and close to unity. The different concentration dependence of the fraction was discussed related to the difference in the CT mechanism.     

Electrochemical and spectroelectrochemical characterization of the phthalocyanines with pentafluorobenzyloxy substituents

The solution redox properties and spectroelectrochemical investigation of the novel metal-free, zinc, nickel and cobalt phthalocyanines with tetra-pentafluorobenzyloxy substituents at the periphery were studied using various electrochemical and spectroelectrochemical measurements. Cyclic voltammetry and differential pulse voltammetry studies show that while Ni(II), Zn(II) and free-phthalocyanines give up to two reduction and two oxidation processes having ligand-based diffusion controlled reversible one-electron electron transfer characters, Co(II) phthalocyanine represents one ligand-based oxidation, one metal-based reduction and one ligand-based reduction processes having diffusion controlled reversible one-electron transfer characters. Assignments of the redox couples are also confirmed by spectroelectrochemical measurements. Reduction potentials of all complexes shift to positive potentials due to the electron withdrawing tetra-pentafluorobenzyloxy substituents compared with those of the phthalocyanines bearing phenoxy derivatives. A linear variation of the first reduction and oxidation potentials versus ze/r has been obtained for zinc and nickel phthalocyanines.