On the influence of the nucleation overpotential on island growth in electrodeposition

Electrochemical deposition of a metal onto a foreign substrate usually occurs by an island growth mechanism. A key feature of island growth for a material M on a foreign substrate S is that the onset potential for deposition is shifted negative from the equilibrium potential for the metal ion couple. The nucleation overpotential, defined as η_n(M⁺/S) = |U_n(M⁺/S) − U_eq(M⁺/M)|, influences key aspects of deposition of a metal on a foreign substrate. Here we discuss how the nucleation overpotential influences the kinetics of island growth, the implications of the nucleation overpotential on island shape and orientation, and the consequences of the coupling between the island density (applied potential) and the island size at coalescence (grain size). We then discuss the kinetics of island growth in terms of the contributions to vertical and lateral growth. Finally, we present examples of experimental methods to manipulate the nucleation overpotential and overcome some of the limitations imposed by the nucleation overpotential.     

An efficient catalyst Co(salophen) for synthesis of diethyl carbonate by oxidative carbonylation of ethanol

The oxidative carbonylation of ethanol to diethyl carbonate (DEC) was investigated by an efficient catalyst system comprising of Co-Schiff base complexes. Effects of Schiff base ligands, reaction time, catalyst concentration, temperature and pressure on the catalytic activity were studied. Co(salophen) [N,N′-bis(salicylidene) o-phenylenediamine cobalt] catalyst exhibited better catalytic activity compared with other Co complexes. When the oxidative carbonylation was carried out at the reaction conditions: 0.12 mol/L Co(salophen), P(CO)/P(O₂) = 2:1, 3.0 MPa, 140 °C, 2.5 h, the conversion of ethanol is 15.8%, the selectivity to DEC is 99.5% and the turnover number (TON) is 22.2. The corrosion behavior of Co(salophen) catalyst to the stainless steel reactor was also examined. The corrosion rate to the stainless steel by Co(salophen) catalyst is below 0.005 mm/a. SEM images demonstrated that the pitting corrosion was not observed on the surface of the stainless steel.     

The nucleation and crystallization of MgO-B2O3-SiO2 glass

The nucleation and crystallization of MgO-B₂O₃-SiO₂ (MBS) glass were studied by means of a non-isothermal, thermal analysis technique, X-ray diffraction and scanning electron microscopy. The temperature range of the nucleation and the temperature of the maximum nucleation rate for MBS glass were determined from the dependences of the inverse temperature at the DSC peak (1/T_p) and the maximum intensity of the exothermic DSC crystallization peak ((δT)_p) on the nucleation temperature (T_n). For MBS glass the nucleation occurred at 600-750 °C, with the maximum nucleation rate at 700 °C, whereas the nucleation and crystal growth processes overlapped at 700 °C < T ≤ 750 °C. The analyses of the non-isothermal data for the bulk MBS glass using the most common models (Ozawa, Kissinger, modified Kissinger, Ozawa-Chen, etc.) revealed that the crystallization of Mg₂B₂O₅ was three-dimensional bulk with a diffusion-controlled crystal growth rate, that n = m = 1.5 and that the activation energy for the crystallization was 410-440 kJ/mol.     

Corrosion inhibition of mild steel by alkylimidazolium ionic liquids in hydrochloric acid

The acid corrosion inhibition process of mild steel in 1 M HCl by 1-butyl-3-methylimidazolium chlorides (BMIC) and 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM]HSO₄) has been investigated using electrochemical impedance, potentiodynamic polarization and weight loss measurements. Potentiodynamic polarization studies indicate the studied inhibitors are mixed type inhibitors. For both inhibitors, the inhibition efficiency increased with increase in the concentration of the inhibitor and the effectiveness of the two inhibitors are in the order [BMIM]HSO₄ > BMIC. The adsorption of the inhibitors on mild steel surface obeyed the Langmuir's adsorption isotherm. The effect of temperature on the corrosion behavior in the presence of 5 × 10⁻³ M of inhibitors was studied in the temperature range of 303-333 K. The associated activation energy of corrosion and other thermodynamic parameters such as enthalpy of activation (ΔH), entropy of activation (ΔS), adsorption equilibrium constant (K_ads) and standard free energy of adsorption (ΔG_ads) were calculated to elaborate the mechanism of corrosion inhibition.     

RAFT polymerization and self-assembly of thermoresponsive poly(N-decylacrylamide-b-N,N-diethylacrylamide) block copolymers bearing a phenanthrene fluorescent α-end group

Phenanthrene α-end-labeled poly(N-decylacrylamide-b-N,N-diethylacrylamide) (PDcA_n-b-PDEA_m) block copolymers consisting in a highly hydrophobic block (n = 11) and a thermoresponsive block with variable length (79 ≤ m ≤ 468) were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. A new phenanthrene-labeled chain transfer agent (CTA) was synthesized and used to control the RAFT polymerization of a hydrophobic acrylamide derivative, N-decylacrylamide (DcA). This first block was further used as macroCTA to polymerize N,N-diethylacrylamide (DEA) in order to prepare diblock copolymers with the same hydrophobic block of PDcA (number average molecular weight: M_n = 2720 g mol⁻¹, polydispersity index: M_w/M_n = 1.13) and various PDEA blocks of several lengths (M_n = 10,000-60,000 g mol⁻¹) with a very high blocking efficiency. The resulting copolymers self-assemble in water forming thermoresponsive micelles. The critical micelle concentration (CMC) was determined using Förster resonance energy transfer (FRET) between phenanthrene linked at the end of the PDcA block and anthracene added to the solution at a low concentration (10⁻⁵ M), based on the fact that energy transfer only occurs when phenanthrene and anthracene are located in the core of the micelle. The CMC (∼2 μM) was obtained at the polymer concentration where the anthracene fluorescence intensity starts to increase. The size of the polymer micelles decreases with temperature increase around the lower critical solution temperature of PDEA in water (LCST ∼ 32 °C) owing to the thermoresponsiveness of the PDEA shell.     

Surface, kinetics and electrocatalytic properties of the Ti/(Ti + Ru + Ce)O2-system for the oxygen evolution reaction in alkaline medium

Ti-supported (Ti + Ru + Ce)O₂ electrodes, prepared at 450 °C, were characterised by XRD, open-circuit potential (E_oc), capacity data (C) and morphology factor (φ) determinations. XRD measurements showed mixed oxides present a low degree of crystallinity. E_oc-data and CV-spectra support surface electrochemistry of mixed oxides is governed by the Ru(III)/Ru(IV) redox couple. In situ surface characterisation revealed the active surface area increases on increasing nominal CeO₂-content. φ-Values remained in the 0.18-0.3 interval supporting the coatings have a low electrochemical porosity. Kinetics was studied recording polarisation and chronopotentiometric curves, which permitted to determine the Tafel slope and reaction order (with respect to OH⁻), in the low and high overpotential domains. Tafel slope data, b, presented a dependence on overpotential and oxide composition indicating the OER electrode mechanism depends on these variables. A unit reaction order with respect to OH⁻ was found for all electrode compositions investigated. The theoretical analysis of the electrode mechanism permitted to analyse the changes in the experimental Tafel slopes taking into account modifications in the apparent electronic transfer coefficient, α_ap. Analysis of the true and apparent electrocatalytic activities revealed the O₂-evolution reaction rate is affected by oxide composition due to morphologic effects.     

In situ analysis of oxygen partial pressure at the cathode catalyst layer/membrane interface during PEFC operation

To understand the concentration overpotential in the polymer electrolyte fuel cell (PEFC), we have performed an in situ analysis of the oxygen partial pressure (p[O₂]_CL/PEM) at the interface between the cathode catalyst layer (CL) and the polymer electrolyte membrane (PEM). Diffusion-limited oxygen reduction current was measured, with Pt probes inserted into the PEM, during cell operation by supplying H₂ to the anode and O₂ + N₂ to the cathode at 80 °C. It was found that the p[O₂]_CL/PEM decreased by ca. 20% when the current density was stepped from 0 to 2.0 A cm⁻² at p[O₂]_gas = 54 kPa and 100% RH at the cathode inlet, irrespective of the oxygen utilization UO₂ (from 10% to 50%). Such a change in p[O₂]_CL/PEM might result in a concentration overpotential of ca. 10 mV, based on the Tafel slope of 120 mV decade⁻¹ in the high current density region. It was also found that ohmic losses in the ionomer phase of the CL increased with decreasing humidity, from 100% to 80% RH, and became a dominant factor in the increased total overpotential, while the corresponding concentration overpotential was unchanged. The present results provide new insight into the transport of oxygen and water at the CL/PEM interface, especially at the high current densities required for the electric vehicle application.     

Preparation of glass-ceramics from red mud in the aluminium industries

The feasibility of recycling red mud and fly ash in the aluminium industries by producing glasses and glass-ceramics has been investigated. The crystallization behavior of glass-ceramics mostly produced from red mud and fly ash was studied by DTA, XRD, optical microscopy techniques. According to DTA curve, nucleation experiments were carried out at various nucleation temperatures at the same crystallization temperature of 900 °C for 2 h, and crystallization experiments were performed at the same nucleation temperature of 697 °C for 2 h followed by crystallization at various temperatures. The nucleation results show that optimum nucleation temperature is near 697 °C, and the crystallization experiments show that the crystallization at a high temperature of over 900 °C results in denser grain size. The major crystallized phases were gehlenite (Ca₂Al₂SiO₇), and augite (Ca(Fe,Mg)Si₂O₆). The XRD results show that with the increase of crystallization temperature, the amount of gehlenite increases, and augite decreases, which is the result of augite transformation into gehlenite.     

Double thermoresponsive polybetaine-based ABA triblock copolymers with capability to condense DNA

ABA type MPDSAH_y-b-PMEO₂MA_x-b-MPDSAH_y (A = N-(3-(methacryloylamino)propyl)-N,N-dimethyl-N-(3-sulfopropyl) ammonium hydroxide (MPDSAH), B = 2-(2-methoxyethoxy)ethyl methacrylate (MEO₂MA)) triblock copolymers with narrow polydispersity index were prepared by atomic transfer radical polymerization (ATRP) in the mixture of water/methanol with addition of sodium chloride. The copolymer solution was shown to exhibit UCST and LCST behaviors. The dual temperature sensitiveness was investigated via turbidity measurement and steady-state fluorescence spectroscopy. The UCST was found to be dependent upon the solution concentration, and UCST shifted towards LCST with the increment in the block length of MPDSAH block. In the selected low temperature region, the micropolarity of pyrene slightly increased due to the weak positive-negative interaction in diluted solution; while above LCST, pyrene experienced more hydrophobic milieu owing to the noticeable dehydration of PMEO₂MA. The analysis of ethidium bromide displacement suggested the strong capability of MPDSAH homopolymer to bind DNA; MEO₂MA moieties in copolymers weakened the binding ability of PMPDSAH to DNA, but 54-60% EB was still replaced by copolymers at complexing ratio of 10/1. AFM confirmed that PMPDSAH and copolymers were capable of condensing DNA to nanoparticles at an appropriate complexing ratio. Complexing with DNA, UCST of solution vanished, but LCST was slightly increased due to the enhanced hydrophilicity caused by liberation of negative charges.     

Inhibitory effect of non-ionic surfactants of the TRITON-X series on the corrosion of carbon steel in sulphuric acid

The corrosion inhibition characteristics of non-ionic surfactants of the TRITON-X series, known as TRITON-X-100 and TRITON-X-405, on stainless steel (SS) type X4Cr13 in sulphuric acid were investigated by potentiodynamic polarisation measurements. It was found that these surfactants act as good inhibitors of the corrosion of stainless steel in 2 mol L⁻¹ H₂SO₄ solution, but the inhibition efficiency strongly depends on the electrode potential. The polarisation data showed that the non-ionic surfactants used in this study acted as mixed-type inhibitors and adsorb on the stainless steel surface, in agreement with the Flory-Huggins adsorption isotherm. Calculated ΔG_ads values are −57.79 kJ mol⁻¹ for TRITON-X-100, and −87.5 kJ mol⁻¹ for TRITON-X-405. From the molecular structure it can be supposed that these surfactants adsorb on the metal surface through two lone pairs of electrons on the oxygen atoms of the hydrophilic head group, suggesting a chemisorption mechanism.     

Electrochemical reduction of high pressure CO2 at a Cu electrode in cold methanol

The electrochemical reduction of high pressure CO₂ with a Cu electrode in cold methanol was investigated. A high pressure stainless steel vessel, with a divided H-type glass cell, was employed. The main products from CO₂ by the electrochemical reduction were methane, ethylene, carbon monoxide and formic acid. In the electrolysis of high pressure CO₂ at low temperature, the reduction products were formed in the order of carbon monoxide, methane, formic acid and ethylene. The best current efficiency of methane was of 20% at −3.0 V. The maximum partial current density for CO₂ reduction was approximately 15 mA cm⁻². The partial current density ratio of CO₂ reduction and hydrogen evolution, i(CO₂)/i(H₂), was more than 2.6 at potentials more positive than −3.0 V. This work can contribute to the large-scale manufacturing of fuel gases from readily available and inexpensive raw materials, CO₂-saturated methanol from industrial absorbers (the Rectisol process).     

RF MEMS capacitive switch with leaky nanodiamond dielectric film

RF MEMS capacitive switches using leaky nanodiamond as a dielectric film are studied and compared with those using Si₃N₄. Characteristics of dielectric charging and discharging are analyzed at temperature ranging from − 196 °C to 150 °C. Electrical resistivity of leaky nanodiamond is measured to be lower than that of Si₃N₄ by 3 to 6 orders of magnitude at room temperature. Trapped charges in leaky nanodiamond dielectric discharge much more quickly than those in Si₃N₄ while the power dissipation of nanodiamond based switches remains low. As a result, charge trapping induced shift in electrostatic actuation voltage is greatly reduced compared to that with Si₃N₄ and becomes non-detectable under the reported conditions. RF MEMS capacitive switches based on leaky nanodiamond dielectric are, therefore, more reliable than those with Si₃N₄.     

Effect of ethylenediamine tetraacetic acid disodium on the corrosion of cold rolled steel in the presence of benzotriazole in hydrochloric acid

The inhibition behavior of cold rolled steel in 0.1 M hydrochloric acid (HCl) by ethylenediamine tetraacetic acid disodium (EDTA) in the absence and presence of benzotriazole (BTA) was investigated with Tafel polarization curve and electrochemical impedance spectroscopy (EIS). The polarization curve results show that the single EDTA acts as an anodic type inhibitor while the combination of EDTA and BTA acts as mixed type inhibitor and mainly inhibits anodic reaction. All impedance spectra in EIS tests exhibit one capacitive loop which indicates that the corrosion reaction is controlled by charge transfer process. Inhibition efficiencies obtained from Tafel polarization, charge transfer resistance (R_t) are consistent. The corrosion of cold rolled steel in 0.1 M HCl is obviously reduced by EDTA in combination with lower concentrations of BTA. Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) were used to characterize the corrosion surface of cold rolled steel. Probable mechanisms are present to explain the experimental results.     

Effect of zwitterionic salt on the electrochemical properties of a solid polymer electrolyte with high temperature stability for lithium ion batteries

In this study, we prepare a kind of solid polymer electrolyte (SPE) based on N-ethyl-N′-methyl imidazolium tetrafluoroborate (EMIBF₄), LiBF₄ and poly(vinylidene difluoride-co-hexafluoropropylene) [P(VdF-HFP)] copolymer. The resultant SPE displays high thermal stability above 300 °C and high room temperature ionic conductivity near to 10⁻³ S cm⁻¹. Its electrochemical properties are improved with incorporation of a zwitterionic salt 1-(1-methyl-3-imidazolium)propane-3-sulfonate (MIm3S). When the SPE contains 1.0 wt% of the MIm3S, it has a high ionic conductivity of 1.57 × 10⁻³ S cm⁻¹ at room temperature, the maximum lithium ions transference number of 0.36 and the minimum apparent activation energy for ions transportation of 30.9 kJ mol⁻¹. The charge-discharge performance of a Li₄Ti₅O₁₂/SPE/LiCoO₂ cell indicates the potential application of the as-prepared SPE in lithium ion batteries.     

High performance nanoporous carbon membranes for air separation

The preparation of porous stainless steel supports was found to have a significant impact on the properties of nanoporous carbon membranes fabricated upon them. Nanofillers were incorporated into porous stainless steel supports to modify the pore structure by reducing the average pore size and porosity. Carbon membrane properties were examined as a function of support variables such as filler content, shape, size and nature of the particles. Optimum performances, in terms of the ideal selectivity ratio for oxygen to nitrogen permeances (S_O2/N2∼3-6) and the oxygen permeance (10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹), were obtained when the filler completely saturated the support. This represents about a two order of magnitude improvement in oxygen permeance when compared to carbon membranes prepared on unmodified porous stainless steel supports. The origin of the improvement in the permeance is due to the formation of carbon membranes which are on average two orders of magnitude thinner than those formed on unmodified supports, i.e., the carbon membranes exists as very thin layers around and between the silica nanoparticles. A simple geometric model based on the packing of silica particles inside the porous stainless steel support is proposed to visualize and quantify this effect. The generality of the support modification concept is also demonstrated by the ability to employ different types of nanofillers and support geometries to obtain carbon membranes with high flux. Air separation experiments show that these membranes can produce both oxygen rich streams enriched to as much as 48% by volume and nitrogen rich streams enriched to over 90% by volume at reasonable operating conditions.     

Diffusion phenomena in the α-Si3N4-solid solution and 304L stainless steel static interaction couple

Compositional varieties of α-SiAlON a yttrium stabilized solid solution of α-Si₃N₄, were studied against 304L stainless steel through interacting couples prepared under hot pressing (1100-1300 °C, 1.8 MPa, 5 h) with respect to their chemical interactions and interface bonding formation. Interdiffusion of elements in both ceramic and steel side followed by the series of reactions those produced the interfaces between the joints were seen. Chemical behaviour of α′ vis-à-vis its compositional nitrogen content has been discussed.     

Tantalum (oxy)nitrides prepared using reactive sputtering for new nonplatinum cathodes of polymer electrolyte fuel cell

Tantalum (oxy)nitrides (TaO_xN_y) have been investigated as new cathodes for polymer electrolyte fuel cells without platinum. TaO_xN_y films were prepared using a radio frequency magnetron sputtering under Ar + O₂ + N₂ atmosphere at substrate temperatures from 50 to 800 °C. The effect of the substrate temperature on the catalytic activity for the oxygen reduction reaction (ORR) and properties of the TaO_xN_y films were examined. The catalytic activity of the TaO_xN_y for the ORR increased with the increasing substrate temperature. The ORR current density at 0.4 V vs. RHE on TaO_xN_y prepared at 800 °C was approximately 20 times larger than that on TaO_xN_y prepared at 50 °C. The onset potential of the TaO_xN_y for the ORR was obtained at the ORR current density of −0.2 μA cm⁻². The onset potential of the TaO_xN_y prepared at 800 °C was ca. 0.75 V vs. RHE. The X-ray diffraction patterns revealed that Ta₃N₅ structure grew as the substrate temperature increased. While, the ionization potentials of all specimens were lower than that of Ta₃N₅, and decreased with the increasing substrate temperature. The TaO_xN_y which had Ta₃N₅ structure and lower ionization potential might have a definite catalytic activity for the ORR.     

Adiabatic versus non-adiabatic approach to the inner sphere vibrational effects on electrochemical reduction rates

This work is a theoretical comparative study on the adiabatic and non-adiabatic mechanisms of electrochemical reductions. In our previous paper [A. Ignaczak, W. Schmickler, Electrochim. Acta 52 (2007) 5621] several aspects of the reaction kinetics were discussed for the adiabatic electron transfer from the electrode to the reactant, taking aquocomplexes as a testing case. In the present work the same reactions are studied in the non-adiabatic approach. First order perturbation theory is applied to analyze the rate dependence on quantum effects associated with inner sphere nuclear vibrations using the same model Hamiltonian. It describes independently the solvent reorganization and the intramolecular modification of the reactant. In case of complexes the ligand shell in the oxidized and reduced states is characterized by two different frequencies, ω₁ and ω₂, respectively, so the ratio θ = ω₂/ω₁ describes asymmetry of the potential energy surface. An influence of different θ values combined with high and low ligand frequencies on the rate enhancement due to the nuclear tunnelling is tested.The quantum correction, calculated as k_q/k_s (k_q, the non-adiabatic rate; k_s, the adiabatic rate), is found to depend strongly on θ and ω₁ parameters as well as on the overpotential applied. The rate enhancement is especially sensitive to the initial state frequency ω₁ for systems characterized by larger θ values. For overpotential η = 0 V, when ω₁ = 250 cm⁻¹ the ratio k_q/k_s is equal to 1.35 (θ = 2/3), 0.65 (θ = 1) and 0.38 (θ = 3/2), while for ω₁ = 1000 cm⁻¹ the corresponding values are 4.05, 3.94 and 4.77. These values increase significantly when the overpotential is applied, up to 14 for the largest values of ω₁ and θ. The transfer coefficient is also presented for the two reaction mechanisms as a function of all parameters tested. An effect of the reactant-electrode interaction and the friction parameter on the rate constants is also analyzed for all cases.     

The production of a homogeneous and well-attached layer of carbon nanofibers on metal foils

Carbon nanofibers (CNFs) were deposited on metal foils including nickel (Ni), iron (Fe), cobalt (Co), stainless steel (Fe:Ni; 70:11 wt.%) and mumetal (Ni:Fe; 77:14 wt.%) by the decomposition of C₂H₄ at 600 °C. The effect of pretreatment and the addition of H₂ on the rate of carbon formation, as well the morphology and attachment of the resulting carbon layer were explored. Ni and mumetal show higher carbon deposition rates than the other metals, with stainless steel and Fe the least active. Pretreatment including an oxidation step normally leads to higher deposition rates, especially for Ni and mumetal. Enhanced formation of small Ni particles by in situ reduction of NiO, compared to formation using a Ni carbide, is probably responsible for higher carbon deposition rates after oxidation pretreatment. The addition of H₂ during the CNF growth leads to higher carbon deposition rates, especially for oxidized Ni and mumetal, thus enhancing the effect of the reduction of NiO. The diameters of CNFs grown on metal alloys are generally larger compared to those grown on pure metals. Homogenously deposited and well-attached layers of nanotubes are formed when the carbon deposition rate is as low as 0.1-1 mg cm⁻² h⁻¹, as mainly occurs on stainless steel.     

Pulsed electrodeposition of Zn in the presence of surfactants

The preparation of Zn deposits has been performed by galvanostatic pulsed electrolysis, from acidic zinc sulphate solutions, on a stainless steel substrate. The influence of the surfactants (cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulphate (SDS) and octylphenolpoly(ethyleneglycolether)_n, n = 10, Triton X-100) on the voltammetric behaviour, structural and morphological characteristics of the deposits have been investigated. The characterization of the samples was made by X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS). The experimental data shows that the presence of surfactants affects the zinc deposition process. The electrodeposits are mainly composed by Zn with different texture, crystal shape and size (grain size ranging from 40 to 20 nm). The obtained results led us to conclude that the Zn deposits prepared in the absence of surfactants and in the presence of SDS are more crystalline and with a higher grain size than the ones obtained in the presence of CTAB and Triton X-100. These facts may be justified by the increase on the overpotential deposition as the electrochemical studies confirm.The XRD results show that the deposits prepared, in the absence of surfactant and in the presence of SDS, contain ZnSO₄ and Zn₄SO₄(OH)₆ as oxidation products. ZnO is also detected on the deposits obtained in the presence of CTAB and Triton X-100.