Blocking Of Electrode Surface Research Articles

Passivation of the positive electrode during the discharge of Li-O2 and O2-assisted Li-CO2 batteries: A comparative study

The effect of passivation of the positive electrode by solid products under discharge is assessed for the first time as one of the factors determining the discharge characteristics of Li-O2 (LOB) and oxygen-assisted Li-CO2 (LCOB) batteries. In order to estimate the passivation, the electrode surface coverage θ by the reaction product (Li2O2 or Li2CO3) is determined on the basis of a decrease in the double-layer capacitance value in the course of the discharge. It is shown that for a given amount of electricity, the electrode surface coverage by Li2CO3 in LCOB is 1.5–2 times that of the coverage by Li2O2 under discharge in LOB. Furthermore, for each of the batteries, achievement of the maximum discharge capacity does not result in the full electrode surface blocking by the discharge products. LCOB manifests higher discharge characteristics than LOB under the kinetic reaction control (in the 100–500 mA g−1 current density range), owing to the accelerated conversion of intermediates and slower solid reaction product deposition on a larger electrode surface area. As the current density increases from 500 to 2000 mA g−1, the discharge capacity of LOB and LCOB decreases, which is mainly due to the effect of diffusion limitations. Moreover, while in the case of LOB the parameter θ decreases, in the case of LCOB it is practically independent of the current density in the studied range. The latter effect may be due to increased passivating properties of Li2CO3 deposits formed under high current density. The results of the work allow considering θ as a new criterion for estimation of the discharge depth and for predicting the characteristics of LCOB and LOB.

Synthesis of ethylene bis [(2-hydroxy-5,1,3-phenylene) bis methylene] tetraphosphonic acid and their anticorrosive effect on carbon steel in 3%NaCl solution

The inhibition performance of the newly synthesized Ethylene bis [(2-hydroxy-5,1,3-phenylene) bis methylene] tetraphosphonic acid (ETPA) toward carbon steel in 3% NaCl was investigated at different concentrations using potentiodynamic polarization (PDP) and impedance spectroscopy (EIS) methods. It was found that the inhibition capability was increased with increasing inhibitor dose and reach 92% at 10−3 mol/L. Also, Polarization curves showed that ETPA acts as a mixed type inhibitor with predominantly control of anodic reaction. The new inhibitor was investigated by different spectroscopic methods such as 1H, 13C and 31PNMR. The quantum parameters such as absolute electronegativity (χ), energy gap ΔE (EHOMO-ELUMO), global softness (σ), global hardness (η), electrophilicity index (ω) and the number of transfer electrons (ΔN) are calculated by density functional theory (DFT). The experimental also correlated with density functional theory results. The calculations show that ETPA has high density of negative charge located on the oxygen atoms of the phosphonate group facilitating the adsorption of ETPA on the surface of carbon steel. The inhibition efficiency of ETPA was discussed in terms of blocking of electrode surface by adsorption of ETPA molecules through active centers. The adsorption of ETPA on the surface of carbon steel obeyed the Langmuir isotherm paradigm.

Hydrothermal synthesis of FeVO4 and the use of composites on their basis as the cathode materials for lithium power sources

The synthesis of finely divided FeVO 4 from aqueous solutions was carried out, iron vanadate particles were incorporated in the polyaniline matrix through the process of oxidative condensation of aniline in a suspension of FeVO 4 in the presence of graphite. In the first case, the content of sulfanilic acid in the initial mixture for the preparation of the composite was 2.0 g, in the second – 1.0 g. The obtained materials on the basis of FeVO 4 are used for making positive electrodes of lithium current sources. Two kinds of lithium batteries were constructed and investigated. The discharge characteristics of the first element are relatively low. This may be due to the relatively higher content of sulfanilic acid on FeVO 4 surface in an electrically active material, which makes it difficult for vanadate particles to participate in the electrochemical process. Probably, sulfanilic acid on FeVO 4 surface can interact with lithium ions with producing insoluble salt. In addition to this case, nitrate and sulfate ions can be ion-dopants of polyaniline. The latter also form with lithium ions insoluble salts in propylene carbonate-ethylene carbonate electrolyte. That leads to the electrode surface blocking and leads to a sharp increase in the internal resistance of element 1. In the case of element 2, a smaller amount of sulfanilic acid was used during the synthesis of the composite. The discharge characteristics of the second element are much higher. There is a small plateau on discharge voltage and current curves. It should be noted that the recharge of element 2 was difficult, as evidenced by small charging currents (1–1.5 mA). That is, the lithium ions slowly diffused from the electrically active material. After reloading the characteristics of element 2 significantly deteriorated. This indicates that not all lithium ions are capable of reversibly intercalating into synthesized electrically active material. Keywords: iron (III) vanadate, lithium power sources, charge-discharge properties.

The importance of the active complexes of 6 - mercaptopurine with Bi(III) with regards to kinetics and electrode mechanism changes in the presence of non-ionic surfactants

The presence of mixed adsorption layers affects the mechanism and kinetics of irreversible Bi(III) ions electroreduction process in 2 mol·dm−3 chlorates(VII). Triton X-100 and Tween 80 change the dynamics of the catalytic activity of 6-mercaptopurine (6MP) on Bi(III) ions electroreduction with the tendency of inhibition. Above the critical micelle concentration of Triton X-100 and Tween 80, the irreversibility of Bi(III) ions electroreduction decrease to the size comparable to the irreversible reduction of Bi(III) in 2 mol·dm−3 chlorates(VII) which can be associated with the electrode surface blocking by formed hemispherical surface micelles. It was proved that the unstable complexes of Bi – 6MP or mixed Bi – 6MP – TritonX-100 and Bi – 6MP – Tween 80 intermediating in electrons transfer, play a significant role in the multi-stage mechanism of the electrode process. The hydration shell and composition of active complexes change as well. The obtained regularities indicate that in this case confirmed changes in entropy are a decisive factor.

Mentha pulegium extract as a natural product for the inhibition of corrosion. Part I: electrochemical studies

The inhibitory effect of Mentha pulegium extract (MPE) on steel corrosion in 1 M HCl solution was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy. The inhibition efficiency of MPE was found to increase with the concentration and reached 88% at 33% (v/v). Polarisation measurements show that the natural extract acted as a mixed inhibitor. The remarkable inhibition efficiency of MPE was discussed in terms of blocking of electrode surface by adsorption of inhibitor molecules through active centres. The adsorption of MPE was found to accord with the Temkin isotherm.

Electrocatalysis and pH (a review)

The factors determining pH effects on principal catalytic reactions in low-temperature fuel cells (oxygen reduction, hydrogen oxidation, and primary alcohols oxidation) are analyzed. The decreasing of hydrogen oxidation rate when passing from acidic electrolytes to basic ones was shown to be due to the electrode surface blocking by oxygen-containing species and changes in the adsorbed hydrogen energy state. In the case of oxygen reduction, the key factors determining the process’ kinetics and mechanism are: the O2 adsorption energy, the adsorbed molecule protonation, and the oxygen reaction thermodynamics. The process’ high selectivity in acidic electrolytes at platinum electrodes is caused by rather high Pt-O2 bond energy and its protonation. The passing from acidic electrolytes to basic ones involves a decrease in the oxygen adsorption energy, both at platinum and nonplatinum catalysts, hence, in the selectivity of the oxygen-to-water reduction reaction. The increase in the methanol and ethanol oxidation rate in basic media, as compared with acidic ones, is due to changes in the reacting species’ structure (because of the alcohol molecules dissociation) on the one hand, and active OHads species inflow to the reaction zone, on the other hand. In the case of ethanol, the above-listed factors determine the process’ increased selectivity with respect to CO2 at higher pHs. Based on the survey and valuation, priority guidelines in the electrocatalysis of commercially important reactions are formulated, in particular, concepts of electrocatalysis at nonplatinum electrode materials that are stable in basic electrolytes, and approaches to the practical control of the rate and selectivity of oxygen reduction and primary alcohols oxidation over wide pH range.

Rapid Mineralization of Azo-Dye Wastewater by Microwave Synergistic Electro-Fenton Oxidation Process

A method for in situ improvement of the electro-Fenton (EF) oxidation process using microwave radiation was proposed. The microwave enhanced electro-Fenton (MW-EF) degradation of azo dye wastewater with boron-doped diamond (BDD) anode was carried out in a continuous flow system under atmospheric pressure. The activation effects of microwave were studied through determining the accumulated hydroxyl radicals, electrogenerated H2O2, and the evolution of iron ions and intermediates. The results showed that, under microwave irradiation, both cathode and anode surfaces were activated efficiently. For cathode, the presence of microwave in electro-Fenton oxidation process accelerated the Fe(III)/Fe(II) redox cycles, leading to relatively steady Fe(II) recovery. Moreover, the transduction of electrons was promoted and the electrosynthesis of H2O2 from O2 was greatly accelerated on the cathode. For BDD anode, under microwave, the blocking of electrode surface by intermediates was enormously alleviated and more acti...

Benzene Oxidation at Diamond Electrodes: Comparison of Microcrystalline and Nanocrystalline Diamonds

A comparative study of benzene oxidation at boron-doped diamond (BDD) and nitrogenated nanocrystalline diamond (NCD) anodes in 0.5 M K(2)SO(4) aqueous solution is conducted by using cyclic voltammetry and electrochemical impedance spectroscopy. It is shown by measurements of differential capacitance and anodic current that during the benzene oxidation at the BDD electrode, adsorption of a reaction intermediate occurs, which partially blocks the electrode surface and lowers the anodic current. At the NCD electrode, benzene is oxidized concurrently with oxygen evolution, a (quinoid) intermediate being adsorbed at the electrode. The adsorption and the electrode surface blocking are reflected in the impedance-frequency and impedance-potential complex-plane plots.

Benzene Oxidation on Boron-Doped Diamond Electrode: Electrochemical-Impedance Study of Adsorption Effects

Benzene oxidation at a boron-doped diamond anode in 0.5 M K2SO4aqueous solution is studied by cyclic voltammetry and electrochemical impedance spectroscopy. It is shown by measurements of differential capacitance and anodic current that in the ideal-polarizability potential region benzene either is not adsorbed at the diamond electrode or the benzene adsorption does not affect its capacitance. At more positive potentials, the adsorption of some intermediate of the benzene oxidation occurs at the electrode. The intermediate partially blocks the electrode surface and lowers the anodic current. The very fact of the electrode surface blocking is reflected in the complex-plane presentation of the impedance-potential plots.

Adsorption and corrosion inhibition effect of 2-((5-mercapto-1,3,4-thiadiazol-2-ylimino)methyl)phenol Schiff base on mild steel

In this study, the inhibition effect of 2-((5-mercapto-1,3,4-thiadiazol-2-ylimino)methyl)phenol Schiff base (MTMP) on mild steel corrosion in 0.5 M HCl solution was studied. For this aim, electrochemical techniques such as potentiodynamic polarization curves, weight loss (WL), electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) were used. It was shown that, the MTMP Schiff base has remarkable inhibition efficiency on the corrosion of mild steel in 0.5 M HCl solution. Polarization measurements indicated that, the studied inhibitor acts as mixed type corrosion inhibitor with predominantly control of cathodic reaction. The inhibition efficiency depends on the concentration of inhibitor and reaches 97% at 1.0 mM MTMP. The remarkable inhibition efficiency of MTMP was discussed in terms of blocking of electrode surface by adsorption of inhibitor molecules through active centers. The adsorption of MTMP molecules on the mild steel surface obeys Langmuir adsorption isotherm.

Identification of electrode surface blocking by means of thin-layer cell

A new method for the identification of electrode surface blocking, based on the measured differences of the diffusion limiting current densities in a thin-layer cell and in a cell with rotating-disk electrode is presented. An equation describing the diffusion limiting current density dependence on the inter-electrode distance in a thin-layer cell was derived. Experimental results were obtained from measurements with model electrodes to demonstrate the possibilities for unambiguous identification of surface blocking phenomena. A model electrode, covered with a partially conductive blocking layer was also discussed and the appearance of two diffusion limiting current density levels on the voltammograms was experimentally verified.

Electrochemical study on screen-printed carbon electrodes with modification by iron nanoparticles in Fe(CN)6 4−/3− redox system

The remarkable enhancement of electron transfer on screen-printed carbon electrodes (SPCEs) with modification by iron nanoparticles (Fe(nano)), coupled with Fe(CN)(6) (4-/3-) redox species, was characterized with an increase of electroactive area (A (ea)) at electrode surface together with a decrease of heterogeneous electron transfer rate constant (k degrees ) in the system. Hence, Fe(nano)-Fe(CN)(6) (3-) SPCEs with deposition of glucose oxidase (GOD) demonstrated a higher sensitivity to various glucose concentrations than Fe(CN)(6) (3-)/GOD-deposited SPCEs. In addition, an inhibited diffusion current from cyclic voltammograms was also observed with an increase in redox concentration and complicated the estimation of A (ea). Further analysis by the electrochemical impedance method, it was shown that this effect might be resulted from the electrode surface blocking by the products of activated complex decomposition.

Pseudopolarography of lead (II) in sediment and in interstitial water measured with a solid microelectrode

Pseudopolarograms of lead (II) constructed from the voltammograms measured in situ in the sediment and in the interstitial water by using an Ir solid microelectrode with a thin mercury film have shown as a kind of fingerprints of the sample. Despite shortcomings when compared to measurements with the mercury drop electrode and in model solutions, the measurement procedure was adapted for enough signal repeatability, avoiding to a reasonable extent the memory effect and electrode surface blocking. To make the best use of the information available, besides the classical pseudopolarograms, i.e. besides the dependence of the peak-height on the deposition potential, it is necessary to analyze the peak-area, the peak-position and the half-peak width versus deposition potential, and combine them with the knowledge from various theoretical and model situations. They have shown to contain interesting information about speciation. This information is not always unambiguous, it is often semi-quantitative, and cannot be reached by other methods, however, in combination with other methods it could be useful for the characterization of the sample solution. Pseudopolarograms of lead (II) in different liquid fractions of the sediment were measured and compared, the electrode sensitivity varying from 4 to 20 nA/μmol L −1 of lead (II). The differences in half-wave potentials recorded were ranging up to 0.6 V and those in the slopes of pseudopolarograms were three-fold, having interesting relationships with the peak potentials of single voltammetric curves.

Kinetics of metal hydride electrodes when increasing the internal cell pressure

An external argon gas pressure was applied to an electrochemical system where metal hydride electrodes made of LaNi 5 and MmNi 5 were investigated at different pressures using various electrochemical techniques. One could expect that the distinctive properties of the same alloy were unchanged regardless of the external pressure, but the differences in various properties for LaNi 5 at different applied external pressures were notable. It is assumed that this is a result of an improved charge efficiency because of a reduced hydrogen evolution. An increase of the discharge capacity with increasing external pressure was observed. The results indicate that the overpotential for the discharge reaction increases as the hydrogen evolution rate during charging increases. This increase might either be a result of mechanical rearrangement of the surface during hydrogen evolution or blocking of electrode surface because of remaining hydrogen gas bubbles in the electrode structure after charging.

The kinetics of electron transfer in redox system on platinum standard-size and ultramicroelectrodes

Abstract The heterogeneous electron transfer rate constant for Fe(CN) 6 4− 3− redox system was measured with platinum standard-size and ultramicroelectrodes in NaCl solutions using fast cyclic voltammetry, steady-state voltammetry and FFT-square wave voltammetry. A ks of about 0.1 cm s−1 was obtained with a Pt (0.2 mm radius) electrode. At a standard-size electrode, the electron transfer is also strongly inhibited by the reactant decomposition products. A significant increase in the standard rate constant up to 0.2 cm s−1 was observed with ultramicroelectrodes. Additionally, the effect of partial blocking of electrode surface by the ferro- and ferricyanide ion decomposition products is significantly reduced at platinum ultramicroelectrodes.