Chronopotentiometric Curves Research Articles

Comparison of bipolar membranes by means of chronopotentiometry

Chronopotentiometry is used as a tool to investigate the transport processes in and the energy requirements of different bipolar membranes under water splitting conditions. The bipolar membranes studied are the BP-1 from Tokuyama Corporation, Japan, the MB-3, the AQ-6 from Aqualytics, and a modified sample of the WSI bipolar membrane. In accordance with the predictions from phenomenological transport equations, the switch-on transition is faster for highly selective membranes, however, also the interface structure (roughness, effective contact area) has to be considered to interpret the results unambiguously. In general, the switch-off behaviour shows a faster relaxation for the less selective membranes. Some bipolar membranes show a remarkable phenomenon: a second charging and discharging time is observed, indicating asymmetric transport behaviour of the membrane layers. For bipolar membranes with firmly attached layers, the chronopotentiometric curves allow to separate the theoretically necessary electrical energy (the reversible contribution) from the energy lost by dissipation (the irreversible contribution). The reversible part is recorded at current switch-off after steady state operation. The ratio of the two contributions indicates that in the series of MB-3, AQ-6 and BP-1 the membranes show an increasing energy efficiency. This is also the order of increasing selectivity, as indicated by the limiting current density. For the WSI membrane, the transport processes and also the chronopotentiometric response curves are different because the layers are not permanently attached. Our results indicate that chronopotentiometric series can be used for detailed comparison of bipolar membranes.

Pore size characterization of cation-exchange membranes by chronopotentiometry using homologous amine ions

Pore size of Neosepta CMX (Tokuyama Soda, Japan), Selemion CMV (Asahi Glass, Japan) and heterogeneous HJC cation-exchange membrane (Hanguk Jungsoo, Korea) was characterized by chronopotentiometry in NH 4Cl and in other amine chloride solutions — simple homologous amine chloride, R–NH 3Cl, where R=CH 3, CH 2CH 3, (CH 2) 2CH 3, (CH 2) 3CH 3, (CH 2) 4CH 3, and (CH 2) 5CH 3. The chronopotentiometric curves obtained in the CMX and CMV membranes showed a steep inflection while diffusive in the heterogeneous HJC membrane. In all membranes, the inflections of chronopotentiometric curves became diffusive with the enlargement in molecular size of transporting ions. Assuming that the ion-exchange membrane consisted of conducting and non-conducting phases, Sand’s equation was modified in this study. Using the modified equation, the fraction of inert material contained in the membrane and the pore size were determined by comparing the theoretical and experimental transition times. The pore size of the membranes, determined by the chronopotentiometry method, correlated with membrane conductivity. It was considered that the diffusive inflection in the chronopotentiometric curve of the heterogeneous membrane was caused by the delay in the formation of the concentration gradient. This delay, in turn, was due to a high fraction of non-conducting region.

Electrochemical performance of cathodes based on LiMn 2O 4 spinel obtained by combustion synthesis

Composite cathodes have been fabricated based on LiMn 2O 4 spinel powders, which were synthesized by combustion process and characterized by X-ray diffraction as a pure phase. The electrochemical response of the electrode was analyzed by cyclic voltammetry, chronopotentiometric curves and electrochemical impedance spectroscopy (EIS). Cyclic voltammograms showed two reversible processes, which is the typical response of LiMn 2O 4. The charge–discharge curves showed two plateaux with the charge density of 107 mAh g −1. From the EIS data the Li + diffusion coefficient was determined.

Theory of Second- and Third-Order Reciprocal Derivative Chronopotentiometry for a Reversible Reaction

Equations describing the second- and third-order reciprocal derivative chronopotentiometric curves, dt/dE vs E are deduced for a reversible reaction. Theoretical calculated values are verified experimentally.

Nanoparticles produced by borohydride reduction as precursors for metal hydride electrodes

The performance of electrodes, prepared from amorphous CoxByHz nanoparticles without additives, in 20% KOH solution was tested by means of cyclic voltammetry and chronopotentiometry. Peaks, assigned to hydrogen adsorption and desorption, are observed in the cyclic voltammograms. After charging, hydrogen atoms occupy different types of sites in the substrate, from which electrochemical desorption occurs. An increase in hydrogen content as a result of repeated cycling was established. Discharge capacity of the electrodes, estimated from the chronopotentiometric discharge curves obtained, is about 250mAhg−1. The observed changes in hydrogen and boron content, due to electrochemical treatment, indicate that the electrode material is an active participant in the whole electrochemical process. Hypotheses for the reaction mechanism are proposed.

Chronopotentiometric studies of electroporation of bilayer lipid membranes

The constant-intensity current chronopotentiometric measurements of egg yolk phosphatidylcholine bilayer membranes (BLM) are presented. It is demonstrated that a constant-intensity current flowing through the bilayer membranes generates the pores in their structures. For the current intensity from 0.1 to 2.0 nA, the generated pores open and close cyclically. The frequency of oscillations depends on the current intensity: the higher current intensity, the higher frequency of pore oscillations. It is suggested that the presented method may allow to create one pore in BLM and to observe its dynamical behaviour. Based on chronopotentiometric curves, a method of pore conductance calculations is presented. It is demonstrated that the value of obtained conductance can be applied for pore diameter estimation. The hypothetical application of constant-current method as a biotechnological tool for selective and controlled incorporation of molecules into microorganisms is discussed.

In Situ Conductivity Study of the Corrosion Layers on Lead‐Tin Alloys in Sulfuric Acid

Cyclic polarization of lead and lead‐tin alloys in 4.5 M with and without the ceric‐cerous redox couple, from −1500 to sat. , showed that during polarization in the anodic direction up to −200 mV, the film was electronically conducting. When PbO was formed under the sulfate layer, the duplex passive film was nonconducting and an anodic passive current was observed up to the formation potential of . During reverse polarization from +1500 mV, after reduction, the passive film on lead was shown to be nonconducting. When the tin content in the alloy was higher than 1.0 weight percent (w/o), the conductivity of this passive film was largely increased. It was shown, by electrochemical impedance spectroscopy that alloying with tin decreases the polarization resistance and reduces the thickness of the PbO layer. After alloy oxidation at +1500 mV, chronopotentiometric curves revealed the presence of , and PbO. can be gradually reduced at decreasing potentials below +800 mV because of its non‐stoichiometric nature. Two steps were demonstrated in the reduction of . Alloying tin at more than 1.5 w/o appears to favor the growth of and .

An Investigation of Lithium Ion Insertion into Spinel Structure Li‐Mn‐O Compounds

Two kinds of spinel structure lithium manganese oxides obtained by a melt‐impregnation method were examined in a lithium nonaqueous cell. The first type shows a voltage profile of a typical spinel electrode with the characteristic two‐step process, which delivers a larger capacity, but fading capacity on cycling (type I). The other type delivers a slightly lower capacity and has ideal rechargeability with a quasi‐one‐step process (type II). The mechanism of the lithium ion insertion into the spinel compound was studied by several physical and chemical methods (differential chronopotentiometric curves, cyclic voltammetry, and open‐circuit voltage measurement). X‐ray diffraction was also used to investigate the structural changes for both types of compounds at different oxidation depths at first charge. The results show that a two‐phase reaction (cubic a0 = 8.154 Å and a0 = 8.072 Å) occurred in the range of 0.1 < x <0.45, and a one‐phase reaction [cubic a0 = 8.163 to 8.247 Å] occurred in the range of 0.45 < x < 1.0 for the first charge for the type I spinel. A one‐phase reaction (cubic a0 = 8.105 to 8.223 Å) was observed over the entire intercalated range of 0.25 < x < 1.04 for the type II spinel compound. The capacity fading on cycling for the type I spinel occurs only in the range of 0.1 < x < 0.45, where the shape of the charge and discharge curves in this range was gradually changed to an S‐shape from and L‐shape during cycling. This is attributed to the fact that the two‐phase structure is transformed to more stable one‐phase structure.

Digital simulation of chronopotentiometric and steady-state voltammetric curves at microelectrodes in the presence of a low concentration of supporting electrolyte

A simulation scheme for the calculation of theoretical chronopotentiograms at microelectrodes in solutions containing low amounts of supporting electrolyte is presented. The scheme allows computation of the changes in the concentration profiles of the substrates, products and the supporting electrolyte ions with time. The electrode potentials that are established after reaching the steady-state, together with the appropriate current intensities, can be used for constructing the steady-state voltammograms. The simulation of the mixed diffusional and migrational transport is based on the Crank-Nicolson method with an exponentially expanding time and space grids. The scheme does not impose any limitations on diffusion coefficients and it can be applied both to simple electrode reactions (one reactant-one product) and more complicated reactions under the assumption that the double-layer thickness is small in comparison to the diffusion layer. Five simple types of electrode reactions and an example of a more complicated scheme were considered. The results obtained demonstrate that the dependence of the steady-state limiting current on the support ratio (csupp.el.csubst) depends not only on the charge of the reactant and the product, but also on the diffusion coefficient ratio of the substrate and product. If the difference between diffusion coefficients is large, the predictions based on simpler theories available in literature can become invalid.

Rechargeable γ-MnO 2 for lithium batteries using a sulfone-based electrolyte at 150 °C

The electrochemical lithium intercalation in γ-MnO 2 as well as in the ramsdellite and pyrolusite phases has been studied in molten dimethylsulfone at 150 °C. Two redox steps are evidenced: in the composition range 0 < x < 0.35–0.4, a slow decrease of the potential (3.8-3V) is observed while for 0.35–0.4 < x < 0.9 a voltage plateau appears at 2.95 V. Comparison of the chronopotentiometric curves for γ-MnO 2, ramsdellite and rutile forms prove that these two insertion processes cannot be assigned to lithium accommodation into ramsdellite and pyrolusite units. At this temperature, the lithium insertion process into γ-MnO 2 is found to be highly reversible on the whole lithium concentration range (0 < x < 0.95) and characterized by a high kinetics of lithium transport. D Li evaluated from potentiometric and voltammetric measurements is in the range 5 × 10 −9 cm 2 s −1. During extended galvanostatic experiments performed at a discharge/charge rate of C, a high efficiency is recovered since, from an initial capacity of 300 A h kg −1, about 180 A h kg −1 are still available after the 20th cycle.

Some aspects of the existence of trivalent copper in YBa 2Cu 3O 7− x superconductors and related corrosion phenomena

Direct evidence was obtained for the presence of trivalent copper (3d 8) in the YBa 2Cu 3O 7− x (0⪕ x⪕0.5) lattice through extraction of the copper components of the superconductor in aqueous KOH solutions in the presence of (IO 6) 5- anions, with formation of the stable bis-periodato cuprate anion, [Cu III(IO 6) 2] 7-. Solutions of this substance have a characteristic intense yellow colour (with an absorbance maximum at a wavelength of 415 nm and a molar decadic extinction coefficient of ϵ 415=1.3×10 4), permitting sensitive, specific detection of Cu III in solution. The analogous Cu II complex has a light blue- green colour (λ max=621 nm, ϵ 621=45) and is poorly soluble (5̃×10 -4M). Bis-tellurato cuprate, [Cu III(TeO 6) 2] 9-, behaves similarly. The complexes of Cu III and Cu II also yield characteristic polarographic reduction waves in alkaline media. Both the spectrophotometric and the polarographic method can be used for the quantitative determination of both forms of copper in solution. The superconductor was also studied in the solid phase by the electrochemical method of chronopotentiometric curves. The results obtained also indicated the presence of trivalent copper in the superconductor. Some aspects of corrosion are discussed in connection with the high oxidation potential of Cu III.

Electrochemical Study of Iodine in the LiAlCl4 · 3 SO 2 Electrolyte

Cyclic voltammograms and chronopotentiometric curves recorded at microelectrodes in solvate containing iodine reagents are discussed. Iodine reduction was found to be a diffusion‐limited process. The product of this reduction reacts with the solvate allowing a catalytic reduction of sulfur dioxide.

Electrochemical Study of Halides and Oxyhalides in LiAlCl4, 3 SO 2 Electrolyte: I . Electrochemical Studies of

The solvate has been used to investigate discharge limiting factors of lithium/sulfuryl chloride cells. solvate to ratio was varied over a wide range of current densities in order to simulate composition changes occurring during the discharge of actual liquid cathode cells. The effect of varying the concentrations in solvate and the electrode material on chronopotentiometric curves and diffusion processes are discussed.

Chronopotentiometric Potential-Time Curves of facilitated Transfer of Cation Across the Interface between two Immiscible Electrolyte Solutions

Chronopotentiometric Potential-Time Curves of facilitated Transfer of Cation Across the Interface between two Immiscible Electrolyte Solutions

Chronopotentiometric Potential-Time Curves of facilitated Transfer of Cation Across the Interface between two Immiscible Electrolyte Solutions

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Kinetics of metal-oxygen reaction by solid oxide electrolyte cells

Chronopotentiometric curves, generated by galvanostatic single steps, applied to solid oxide electrolyte cells, have been analysed on the basis of a dimensionless equation derived on the assumption that a scale of oxide grows at one of the electrode-electrolyte interfaces. This process is rate limiting for developing the charge transfer-diffusion overvoltage, and Wagner's theory on tarnishing under retarding electric field conditions, has been assumed for treating the kinetics of the growing scale. At constant temperature, the oxidation rate and oxidation rate constant have been measured as a function of the oxygen partial pressure in the range of pressures near the equilibrium pressure of the metal-oxide system.

Chronopotentiometric potential-time curves of electron transfer across the interface between two immiscible electrolyte solutions

Theoretical potential-time curves were derived that are mathematical expressions of the electron transfer reaction O1 ( w) + R2( non) ⇌ R1( w) + O2( non) proceeding at the interface of two immiscible phases between the redox couple O1/R1 in the aqueous phase ( w) and the redox couple O2/R2 in the non-aqueous phase ( non) under galvanostatic conditions. Procedures for the determination of the diffusion coefficients of the reactants in the corresponding phase and for the evaluation of thermodynamic and kinetic parameters of the electron transfer reaction from the potential-time curves were suggested.

Chronopotentiometric potential-time curves of electron transfer across the interface between two immiscible electrolyte solutions

Chronopotentiometric potential-time curves of electron transfer across the interface between two immiscible electrolyte solutions

Chronopotentiometric potential-time curves of electron transfer across the interface between two immiscible electrolyte solutions

Theoretical potential-time curves were derived that are mathematical expressions of the electrotransfer reaction O1(w)+R2(non)(w)+O2(non) proceeding at the interface of two immiscible phases between the redox couple 01/R1 in the aqueous phase (w) and the redox couple 02/R2 in the non-aqueous phase (non) under galvanostatic conditions. Procedures for determination of the diffusion coefficients of the reactants in the corresponding phase and for the evaluation of thermodynamic and kinetic parameters of the electron-transfer reaction from the potential-time curves were suggested.

High temperature kinetics of metal-oxygen reactions by solid oxide electrolyte cells: An application to nickel

Chronopotentiometric curves, generated by galvanostatic single steps, applied to a solid electrolyte cell, have been analysed on the basis of a dimensionless equation derived on the assumption that a scale of oxide grows at one of the electrode-electrolyte interfaces. This process is rate limiting for developing the charge transfer-diffusion overvoltage; Wagner's theory on tarnishing, under retarding electric field conditions, has been assumed for treating the kinetics of the growing scale. The kinetic model developed has been tested for the reaction of solid nickel (positive electrode) at 1268 K with oxygen partial pressures ranging from 3.9 × 10 −11 Atm [equilibrium pressure of Ni(s)-NiO(s) system] to 1.7 × 10 −10 Atm. Both the rate and the rate constant of the scale formation go through the maximum but at different times, while the rate constant dependence of the oxygen partial pressure agrees with the theory. The maximum rate constant value was found to be 9.8 × 10 8 molecules cm −1 s −1. The calculated scale thicknesses were between 6 and 24 nm.