Spectroelectrochemical study of perchloroethylene reduction at copper electrodes in neutral aqueous medium

The potential-dependent chemical reaction of perchloroethylene (PCE) on copper in neutral noncomplexing aqueous media is explored by means of surface-enhanced Raman spectroscopy (SERS), linear sweep voltammetry and preparative electrolysis at controlled potential. Voltammetric peaks associated with copper oxide reduction in Na₂SO₄ solution in the presence and the absence of Cl⁻ are correlated with simultaneously acquired SER spectra. Perchloroethylene undergoes a dechlorination process at potentials at E ≤ −0.3 V vs. Ag/AgCl/KCl (3 M), as shown by the emergence of an intense CuCl stretching band at 290 cm⁻¹ and a CH stretching band together with the presence of Cl⁻ in the catholyte. In the potential region between 0 and −0.9 V vs. Ag/AgCl/KCl (3 M) a broad band assigned to CC structures is observed in the triple-bond region (∼1900 cm⁻¹, FWHM = 180 cm⁻¹). In addition, dichloroethylene (DCE) is detected (but not trichloroethylene (TCE)) in this potential region during preparative electrolysis. At potentials lower than −1 V vs. Ag/AgCl/KCl (3 M) carbon residues are the main product, detected on the copper surface by SERS (and confirmed by XPS), whereas in solution higher levels of dichloroethylene and trichloroethylene are detected with a DCE/TCE ratio below 1.     

Evolution of the local structure and electrochemical properties of spinel LiNixMn2−xO4 (0 ≤ x ≤ 0.5)

A series of Ni substituted spinel LiNi_xMn_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized to study the evolution of the local structure and their electrochemical properties. X-ray diffraction showed a few Ni cations moved to the 8a sites in heavily substituted LiNi_xMn_2−xO₄ (x ≥ 0.3). X-ray photoelectron spectroscopy confirmed Ni²⁺ cations were partially oxidized to Ni³⁺. The local structures of LiNi_xMn_2−xO₄ were studied by analyzing the and A_1g Raman bands. The most compact [Mn(Ni)O₆] octahedron with the highest bond energy of Mn(Ni)O was found for LiNi_0.2Mn_1.8O₄, which showed a Mn(Ni)O average bond length of 1.790 Å, and a force constant of 2.966 N cm⁻¹. Electrolyte decomposition during the electrochemical charging processes increased with Ni substitution. The discharge capacities at the 4.1 and 4.7 V plateaus obeyed the linear relationships with respect to the Ni substitution with the slopes of −1.9 and +1.9, which were smaller than the theoretical values of −2 and +2, respectively. The smaller slopes could be attributed to the electrochemical hysteresis and the presence of Ni³⁺ in the materials.     

Electron transfer reaction and mechanism of oxidation of Inosine

Electrochemical oxidation of Inosine has been studied in the phosphate buffers of pH range 3.3-10.9 at pyrolytic graphite electrode. In the entire pH range a single well-defined oxidation peak (I_a) was observed, when the sweep was initiated in the positive direction. In the reverse sweep no cathodic peak was obtained. The peak potential of the oxidation peak was dependent on pH and shifted to less positive potential with increase in pH. The kinetics of the UV absorbing intermediate was followed spectrophotometrically and the decay occurred in a pseudo first order reaction having k values in the range 0.50-0.92 × 10⁻³ s⁻¹ in the entire pH range studied. The value of n was found to be 2.95 ± 0.3. The products of oxidation were silylated and characterized by using GC-Mass. Two tetramers having CC, CN, NN, CON and COOC linkages were identified. A plausible mechanism for the electrooxidation of Inosine has been suggested.     

Di-ureasil xerogels containing lithium bis(trifluoromethanesulfonyl)imide for application in solid-state electrochromic devices

In this study we report the characterization of a prototype solid-state electrochromic device based on poly(ethylene oxide) (PEO)/siloxane hybrid networks doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The polymer networks prepared, designated as di-ureasils and represented as d-U(2000), were produced by a sol-gel procedure and are composed of a siliceous framework to which both ends of polyether chains containing about 40 CH₂CH₂O units are covalently bonded through urea linkages. Samples with compositions of 200 ≥ n ≥ 0.5 (where n is the molar ratio of CH₂CH₂O to Li⁺) were characterized by thermal analysis, complex impedance measurements and cyclic voltammetry at a gold microelectrode. Electrolyte samples were obtained as self-supporting, transparent, amorphous films and at room temperature the highest conductivity was observed with the d-U(2000)₃₅LiTFSI composition (3.2 × 10⁻⁵ Ω⁻¹ cm⁻¹). We report the results of preliminary evaluation of these polymer electrolytes as multi-functional components in prototype electrochromic displays. Device performance parameters such as coloration efficiency, optical contrast and image stability were also evaluated. The electrolytes with n > 8 presented an optical density above 0.56 and display assemblies exhibited good open-circuit memory and stable electrochromic performances.     

Synthesis and high electrochemical activity of poly(aniline-co-2-amino-4-hydroxybenzenesulfonic acid)

Poly(aniline-co-2-amino-4-hydroxybenzenesulfonic acid) (PAAHB) was synthesized using chemical oxidative copolymerization of aniline and 2-amino-4-hydroxybenzenesulfonic acid (AHB) in the presence of an ionic liquid at 50 °C. The conductivity of the PAAHB copolymer synthesized at the optimum conditions is 0.47 S cm⁻¹ that is lower than that of polyaniline, but is slightly affected by water. The cyclic voltammograms demonstrate that the PAAHB copolymer has excellent redox activity from highly acidic solution to pH 12.0 in a wider potential range. This is attributed to the synergistic effect of the SO₃⁻ and OH functional groups in the copolymer chain and the ionic liquid incorporated into the PAAHB film. It is evident that the pH dependence of the redox activity and conductivity of the PAAHB copolymer prepared chemically is much better than that of polyaniline, and is further improved, compared to the PAAHB copolymer prepared electrochemically. The proton NMR spectrum of the PAAHB copolymer demonstrates that the SO₃⁻ group exists in the copolymer chain instead of the SO₃H group. The ESR spectra show that the ESR signal intensity is a function of the monomer concentration ratio of AHB to aniline in the mixture. The morphology of the PAAHB copolymer is also dependent on the monomer concentration ratio in the mixture.     

Enhancement of ionic conductivity by mixing lithium borate with lithium aluminate

Several lithium borates (Salt A and Salt B) and a lithium aluminate (Salt C) with electron-withdrawing groups, OC₆F₅, OCOCF₃ or N(SO₂CF₃)₂, and oligoether chains (O(CH₂CH₂O)_nCH₃) directly bonded to the ate complex center, B or Al, were prepared. Lithium borate and lithium aluminate were mixed to get mix-salt electrolytes. Higher ionic conductivity was observed for the mix-salt than for the pure-salt. Conductivity as high as 1.1 × 10⁻⁴ S/cm at ambient temperature (25 °C) was achieved for the electrolyte in the optimized composition. The reason for such mixing effect on enhancement of ionic conductivity was discussed. Other electrochemical properties including electrochemical stability, compatibility with lithium anode and cyclic performance were also investigated for the mix-salt electrolytes.     

Two new siloxanic proton-conducting membranes: Part I. Synthesis and structural characterization

The development of stable polymer electrolytes having good proton conductivity, low cost and operating at medium temperatures represent a crucial step in the evolution of polymer electrolyte fuel cells. We describe two new siloxanic proton-conducting membranes that were synthesized through a two-stage protocol. In the first stage, a poly(methyl hydrosiloxane) precursor (P) bearing siloxane side chains with sulfonic acid groups was prepared. In the second step, the hydrolysis of pristine precursor or its derivative obtained by grafting siloxane chains on P yielded two types of membranes with the formulas {Si(CH₃)₃O[Si(CH₃)HO]_21.26[Si(CH₃)((CH₂)₃SO₃H)O]_1.8[Si(CH₃)((CH₂)₃Si(CH₃)₂O)O]₁₄Si(CH₃)₃}_n (A) and {Si(CH₃)₃O[Si(CH₃)HO]_21.26[Si(CH₃)((CH₂)₃SO₃H)O]_1.8[Si(CH₃)((CH₂)₃(Si(CH₃)₂O)_w)O]_v[Si(CH₃)((CH₂)₃Si(CH₃)₂O-)O]_14 − vSi(CH₃)₃}_n (B), with w = 20.31. Polymer membranes of A and B were prepared by means of a hot-pressing process at 80 °C and 10 t/cm². Scanning electron microscopy showed that A and B are rubbery materials with rough and transparent surfaces. Thermogravimetric investigations performed under air atmosphere disclosed that A and B are thermally stable up to at least 198 °C. DSC measurements yielded T_g(s) of −44 and −60 °C for A and B, respectively. The polymers exhibit ionic exchange capacities of 0.33 (A) and 0.15 meq/g (B). FT-IR and FT-Raman investigations revealed that the polymers consist of reticulated siloxane networks with pendant silicone chains having sulfonic acid groups.     

Oxidation of single-walled carbon nanotubes in dilute aqueous solutions by ozone as affected by ultrasound

A clean and simple wet chemical process using dilute aqueous ozone (O₃) solution with or without ultrasound (US) was used to functionalize single-walled carbon nanotubes (SWCNTs). Both O₃ and O₃/US treatments greatly increased the stability of SWCNTs in water. Results of X-ray photoelectron spectroscopy (XPS) showed that the surface oxygen to carbon atomic ratio increased by more than 600% after 72 h of O₃ treatment. Moreover, the effective particle size of SWCNTs was reduced from the initial 4400 to ∼300 and ∼150 nm, after 24 h of O₃ and O₃/US treatment, respectively. The zeta potential of treated SWCNTs decreased from 3.0 to −35.0 mV (at pH 4) after 2 h of treatment with both O₃ and O₃/US. Based on the XPS results, the oxidation pathway was proposed: at the onset of the oxidation reaction, the CC double bond was first converted to COH which was then oxidized to CO and OCOH concurrently. Oxidation reactions could be described well with first order expressions. Treatment time controlled the extent of surface oxidation and subsequently the stability and dispersion of SWCNTs in water.     

Covalent layer-by-layer type modification of electrodes using ferrocene derivatives and crosslinkers

A series of mono-substituted ferrocenes (Fecp₂R, R = CONHCH₂N(C₂H₄NH₂)₂ (4), R = CH₂NHCH₂N(C₂H₄NH₂)₂ (5)) and di-substituted ferrocenes (Fe(cpR)₂, R = COF (3)) have been prepared, with 4 and 5 representing new compounds. The ferrocenes were grown layer-by-layer on metal oxide electrodes and crosslinked. The underlying principle of synthesis is based on repetitive and fast amide forming reactions between trimesic acid chloride (2) or Fe(cpCOF)₂ (3) and tris(2-aminoethyl)amine (6), or one of the amine-substituted ferrocenes 4 or 5 on a 3-aminopropyl triethoxysilane (APS) (1) modified ITO, FTO or ATO electrode. Linear growth of the films electroactivity with the number of cascade steps is observed for up to 12 cascade steps yielding 1.1 × 10⁻⁸ ferrocene centers/cm² on a flat ITO electrode, whereas 3 cascade steps yield 2.2 × 10⁻⁷ ferrocene centers/cm² on a mesoporeous ATO electrode. In case of the ATO electrode, the inner walls of the mesopores exhibiting a very large surface are completely modified in the procedure. Beside this large coulometry fast electron transfer kinetics, high stability and low optical density in both oxidation states is observed.The electrodes were checked for their potential use as optically transparent counter electrodes in electrochromic devices.     

Electrochemical impedance spectroscopy of electrodes modified with thin films of MgMnCO3 layered double hydroxides

The electrochemical impedance spectra of MgMnCO₃ LDH films oxidized at different dc potentials were recorded. The results were fitted to a Randles type cell by replacing the Warburg impedance with a mass transfer resistance in parallel with a constant phase element. The films charge transfer resistances decreased dramatically at the onset of manganese oxidation. In thin films, R_ct decreased from 10⁴ Ω for an un-oxidized film to a minimum of 40 Ω in a film oxidized at 0.32 V, before increasing back to 10⁴ Ω in a film oxidized at 0.5 V. Iodometry measurements show these changes correspond to increases in the manganese average valence in the films from 3.09+ prior to oxidation, to 3.80+ at 0.32 V and 3.95+ at 0.5 V. In thicker films, however, a much higher dc potential, 1.0 V, was required to return R_ct to 10⁴ Ω. There was also less change in the manganese average valence in the thicker films. Oxidation at 1.0 V only increased the manganese valence to 3.33+. For the partially oxidized films, the Nyquist plots consisted of depressed semicircles at high frequency, followed by linear regions at lower frequency where the impedance was controlled by mass transport. The effective diffusion coefficient estimated from the low frequency impedance was 1 × 10⁻⁹ cm² s⁻¹, consistent with proton diffusion in solid electrodes. The impedance spectrum of a partially oxidized film reduced at −0.2 V was similar to that of the un-oxidized film.     

Cathodic deposition of molybdenum and vanadium mixed oxyhydroxide films from V-substituted polymolybdophosphate

We report a new electrochemical route for fabricating molybdenum and vanadium mixed oxyhydroxide films on Au electrode from Keggin-type vanadium-substituted polymolybdophosphate. The process involves a potentiodynamic reduction of aqueous 9-molybdo(VI)-3-vanadophosphosphate(V) ([PMo₉V₃O₄₀]⁶⁻) in the potential region between 0 and −0.7 V versus Ag/AgCl. The resulting MoV oxyhydroxide film electrode gave a stable redox behavior in Na₂SO₄ electrolyte of pH 3. X-ray photoelectron spectroscopy revealed that this results from oxidation/reduction of Mo⁵⁺/Mo⁶⁺ in the film which accompanies extraction/insertion of protons for charge compensation. The deposited V ions remained in the film upon repetitive potential cycling without affecting their oxidation state. Voltammetric data in the presence of sodium nitrite showed electrocatalytic activity of the MoV oxyhydroxide toward the electroreduction of nitrite.     

Raman spectroelectrochemical study of self-doped copolymers of aniline and selected aminonaphthalenesulfonates

Novel self-doped polyaniline-like copolymers have been prepared by electrochemical copolymerization of aniline with four aminonaphthalenesulfonates. All copolymer films prepared show their electrochemical redox activity even in pH-neutral solutions at a midpoint potential around 0.0 V versus Ag/AgCl. Raman spectroelectrochemical study of the copolymers prepared has been done with a red laser excitation (632.8 nm) within a broad electrochemical potential window of 0.0-1.0 V, and specific Raman features have been identified. Raman bands within the range of 1300-1400 cm⁻¹ have been discussed regarding localized or delocalized polaronic ν_s(CN⁺) vibrations. The influence of sulfonate group position in aminonaphthalenesulfonates on the parameters of polaronic bands has been demonstrated.     

Theoretical study of the structural effects of polymethylene amines on corrosion inhibition of iron in acid solutions

Quantum chemical calculations were performed on azacyclo C5 to C14 amines, open chain C6 to C14 amines and phenylazacyclo C5 to C14 amines. Inspection of the calculated parameters and corrosion inhibition efficiencies were made to observe any clear links, which might exist between the two. Possible correlations between experimental inhibition efficiencies and parameters such as dipole moment (μ), highest occupied (E_HOMO) and lowest unoccupied (E_LUMO) molecular orbitals and the differences between them, HOMO-LUMO gap (Δ), as well as some structural characteristics were investigated. The models of the inhibitors were optimized with the Modified Neglect of Diatomic Overlap (MNDO) method. The Quantitative Structure Activity Relationship (QSAR) approach has been used and a composite index of some quantum chemical parameters were constructed in order to characterize the inhibition performance of the tested molecules. The inhibition effect of polymethylene amines is closely related to orbital energies and/or energy gap and dipole moment.     

FTIR investigation of the influence of diisocyanate symmetry on the morphology development in model segmented polyurethanes

Novel segmented polyurethanes with hard segments based on a single diisocyanate molecule with no chain extenders were prepared by the stoichiometric reactions of poly(tetramethylene oxide)glycol (M_n=1000 g/mol) (PTMO-1000) and 1,4-phenylene diisocyanate (PPDI), trans-1,4-cyclohexyl diisocyanate (CHDI), bis(4-isocyanatocyclohexyl)methane (HMDI) and bis(4-isocyanatophenyl)methane (MDI). Time dependent microphase separation and morphology development in these polyurethanes were studied at room temperature using transmission FTIR spectroscopy. Solvent cast films on KBr discs were annealed at 100 °C for 15 s and microphase separation due to self organization of urethane hard segments was followed by FTIR spectroscopy, monitoring the change in the relative intensities of free and hydrogen-bonded carbonyl (CO) peaks. Depending on the structure of the diisocyanate used, while the intensity of free CO peaks around 1720-1730 cm⁻¹ decreased, the intensity of H-bonded CO peaks around 1670-1690 cm⁻¹, which were not present in the original samples, increased with time and reached saturation in periods ranging up to 5 days. Structure of the diisocyanate had a dramatic effect on the kinetics of the process and the amount of hard segment phase separation. While PPDI and CHDI based polyurethanes showed self-organization and formation of well ordered hard segments, interestingly no change in the carbonyl region or no phase separation was observed for MDI and HMDI based polyurethanes. Quantitative information regarding the relative amounts of non-hydrogen bonded, loosely hydrogen bonded and strongly hydrogen bonded and ordered urethane hard segments were obtained by the deconvolution of CO region and analysis of the relative absorbances in CO region.