Counterion Transfer Research Articles

Counterion Transference Numbers in the Cell Model of a Charged Membrane

The paper suggests exact formulae for calculating the electromigration, diffusion and convective numbers of counterion transport in the cell model of a charged membrane depending on the physicochemical parameters and the equilibrium concentration of the electrolyte. The cell model was previously developed to calculate all the kinetic coefficients of the Onsager matrix and the asymmetry of the cross coefficients was established. The limiting case of an ideally selective membrane is studied in detail, for which approximate formulae for transference numbers are obtained. The obtained dependences are illustrated by graphs using the example of the MK-40 cation-exchange membrane after conditioning at room temperature. The proposed method for calculating the transference numbers is applicable to any single-layer membranes in binary electrolyte solutions.

Aggregation behavior and thermodynamic properties of the mixture of sodium carboxymethyl cellulose and cetyltrimethylammonium bromide in numerous temperatures and mixed solvents

Abstract Interaction of sodium carboxymethylcellulose (SCMC) with cetyltrimethylammonium bromide (CTAB; cationic in nature) in H2O and additives (alcohols and diols) media has been investigated using conductivity technique. The micellar parameters such as critical micelle concentration (cmc), fraction of counter ion binding (β), thermodynamic parameters, transfer properties, and enthalpy-entropy compensation parameters of CTAB + SCMC mixture have been assessed in water and aq. alcohols/diols media. One cmc value was achieved for CTAB + SCMC mixtures in the entire circumstances and the attendance of SCMC disfavors the CTAB micellization. The cmc values were obtained to be greater in alcohols and diols media compared to H2O medium. The cmc values also exhibit a dependency on the solvent composition and temperature variation. In all the cases, the ΔG 0 m values were achieved to be negative which signifying the spontaneous formation of micelles while the extent of spontaneity is decreased in alcohols and diols media. Both the ΔH 0 m and ΔS 0 m reveal that hydrophobic, ion-dipole as well as electrostatic interactions are the proposed binding forces between CTAB and SCMC. The compensation parameters (ΔH 0* m and T c ) are in decent agreement with the biological fluid.

Selective ion transport through hydrated cation and anion exchange membranes I. The effect of specific interactions

Ion partitioning between membranes with fixed ionic groups and aqueous solutions containing different simple salts and acids (LiCl, CsCl, HCl) is studied experimentally for dilute and concentrated solutions and cation and anion exchange membranes including Nafion®, Aquivion®, sulfonated polystyrene, sulfonated polyether ketone, sulfonated poly(phenylene sulfone), poly(acrylic acid) and a poly(phenylene oxide) functionalized with quaternized ammonium groups. For the given membranes and ions, ion partitioning is fully described by the degree of ion pairing which is controlled by specific interactions between counter- and fixed-ions (site-binding), ion hydration and membrane swelling. In the case of cation exchange membranes, specific interaction is highest for the most acidic fixed ionic groups and cations with the highest polarizability and lowest electronegativity (here Cs+). Data on ion partitioning combined with data on ion diffusion (obtained by 7Li, 133Cs NMR) and direct electrochemical transference number measurements show that selective ion uptake does not significantly change counter-ion transference numbers (compared to transference numbers of the corresponding aqueous solution), but it does reduce the total conductivity. Especially for membranes in highly concentrated salt solutions, where Donnan-exclusion does not apply, all ionic transference numbers approach these of the corresponding aqueous solutions. Under these conditions, the transport properties of cation and anion exchange membranes are similar and close to the transport characteristics of simple diaphragms. With appropriate implementation of counter-ion association/dissociation equilibria and ion hydration numbers, the Donnan-equation describes co- and counter-ion partitioning in a quantitative way. Apart from counter-ion/co-ion discrimination in dilute solution, no selective transport of specific ions significantly beyond selectivity characteristics of homogeneous aqueous solutions is expected. For larger ions or other dissolved species within ionomers with narrow aqueous ionic domains, however, there are additional steric effects (ion-sieving) which will be discussed in the second part of this series while the third part will present results of atomistic modelling.

(Invited) Electrochemistry of Solid-State Inorganic Polymer Electrolyte

Electrochemistry explores the charge transfer reaction across the interfaces, such as solid/liquid interface, liquid/liquid interface and also solid/solid interface. Most reported charge transfer behaviours across the “solid/solid” interface are involved liquid electrolyte. For example, in the lithium-ion batteries, the redox of cobalt-based active material is coupled by the heterogeneous intercalation/deintercalation of Li+ ions, which is existed in the electrolyte solution. In another words, the electron transfer occurs at the solid/solid interface while the ion transfer occurs at the solid/electrolyte solution. We employ SECM to construct the solid-solid interface and to study the electron transfer phenomena therein. That means there is no liquid electrolyte environment. When the electron transfer occurs at the solid/solid interface, the counterion transfer will occur in the crystal due to the crystalline defects. For example, we cultured the NaCl microcrystals, which are doped with iron oxides, between the pair electrodes on a microchip. The multi-step electron transfer processes of iron from Fe(0) to Fe(VI) through Fe(II) and Fe(III) were observed. The solid-state electrolyte solution just like an inorganic polymer. The electron transfer is wired through hopping between the neighboring redox lattice sites, coupling with the counterion transfer in the crystalline defects. The electrochemical behavior in the solid-state inorganic “polymer” electrolyte will be discussed in this presentation.

Enhancement of counter-ion transport through ion-exchange membranes in electrodialytic processes

Ion-exchange membranes (IEMs) are ionic conducting materials. They have various applications such as: fuel cell (PEMFC), electrochemical synthesis (Cl2/NaOH), desalination, purification, separation, and environment. Despite these applications, several aspects are still unknown, such as: the membrane structure, the conduction mechanisms, and concentration polarization. The main obstacle in electro-membrane processes such as electrodialysis is the concentration polarization phenomenon, which remains one of the incomprehensible phenomena in IEM transport. This phenomenon is common to all systems operating a selective ionic transfer through an interface; it arises from the difference in ions mobility in the solution and in the membrane. A better understanding of concentration polarization can help to improve the membrane performance, the process efficiency, and in the reduction the process operation cost. In this research, we studied the effect of the ammonia buffer (NH3/NH4+) on the counter-ion transfer through the anion- and the cation-exchange membranes AMX and CMX, respectively. The results show that the ammonia addition facilitates the counter-ion transfer in both cases and gives a total elimination of the system polarization, but with different behaviors of CMX and AMX membranes. The classical concentration polarization theory remains insufficient to explain the obtained results.

Theory of membrane capacitive deionization including the effect of the electrode pore space

Membrane capacitive deionization (MCDI) is a technology for water desalination based on applying an electrical field between two oppositely placed porous electrodes. Ions are removed from the water flowing through a channel in between the electrodes and are stored inside the electrodes. Ion-exchange membranes are placed in front of the electrodes allowing for counterion transfer from the channel into the electrode, while retaining the coions inside the electrode structure. We set up an extended theory for MCDI which includes in the description for the porous electrodes not only the electrostatic double layers (EDLs) formed inside the porous (carbon) particles, but also incorporates the role of the transport pathways in the electrode, i.e., the interparticle pore space. Because in MCDI the coions are inhibited from leaving the electrode region, the interparticle porosity becomes available as a reservoir to store salt, thereby increasing the total salt storage capacity of the porous electrode. A second advantage of MCDI is that during ion desorption (ion release) the voltage can be reversed. In that case the interparticle porosity can be depleted of counterions, thereby increasing the salt uptake capacity and rate in the next cycle. In this work, we compare both experimentally and theoretically adsorption/desorption cycles of MCDI for desorption at zero voltage as well as for reversed voltage, and compare with results for CDI. To describe the EDL-structure a novel modified Donnan model is proposed valid for small pores relative to the Debye length.

Biphasic Voltammetry of N,N,N′,N′‐Tetraphenyl‐para‐phenylenediamine Microdroplets, Microparticles and Microparticle Suspensions

AbstractThe aqueous biphasic voltammetry of the title compound is examined when it is immobilised on an electrode surface either when dissolved in an array of nitrobenzene droplets, in the form of solid microparticles, or when an electrode is immersed into a three‐dimensional aqueous suspension of the solid. In each case, a different electrode reaction is observed to occur, even though the supporting electrolyte is unchanged, with either counter ion transfer across a liquid | liquid interface, or with electrochemical dissolution of the solid particles, or a suggested diffusive‐type electron hopping between individual redox centres embedded within the solid crystals.

Electrodeposition of gold nanoparticles at a solid|ionic liquid|aqueous electrolyte three-phase junction

Gold nanoparticles have been electrodeposited on an electrode through electrogeneration at an ITO|AuCl 4 − solution in an ionic liquid|aqueous electrolyte three-phase junction. The electrodeposition was carried out by inverted double-pulse potential chronoamperometry. The direct reduction of AuCl 4 − ions at the electrode is followed by a counterion transfer through the liquid|liquid interface. Contrary to the electrodeposition from a single ionic liquid phase, scanning electron microscopy reveals that the shape of the resulting nanoparticles is highly angular and well-developed with a diameter of 110 ± 30 nm. Catalytic oxidation of glucose on the modified electrode is demonstrated.

Lower rate of H+(OH-) ions generation at an anion-exchange membrane in electrodialysis

A way of improvement of electromembrane technology for water production required in heat-and-power engineering by applying modified anion exchange membranes is proposed. The novel membranes are produced by treatment of commercial Russian heterogeneous MA-40 membranes with a polyelectrolyte bearing quaternary ammonium groups. It is found that this modification results in lowering the rate of H+ and OH− ions generation (water splitting) at overlimiting currents and in a considerable increase in current efficiency due to the decrease in the OH− ions transfer across the anion-exchange membrane. Moreover, we observed an increment in the salt counterion transfer through the membrane. The decrease in water splitting rate at the interface ‘anion-exchange membrane/depleted solution’ leads to increasing pH in the desalting compartment and decreasing pH in the concentrating one. As a consequence, the risk of the salt precipitation on the membrane surface in the concentrating compartment is reduced, and the removal of weak acids from the desalting water is found enhanced.

Transfer of counterions and diffusion of electrolyte in inorganic membranes modified by ion exchanger nanoparticles

Using the method of impedance spectroscopy we have investigated charge selectivity of macroporous inorganic membranes modified with nanoparticles of the inorganic ion exchanger—hydrated zirconium dioxide, which displays anion-exchange properties in an acid area. It has been found that transfer numbers of the SO42− reach 0.81–0.9 at the H2SO4 concentration 10 mol · m−3 and decrease as the acid concentration in solution goes down. It has also been found that modified membranes demonstrate a two-fold decrease of the diffusion coefficient of H2SO4 compared with a free solution. It has been shown that charge selectivity of the membrane improves as the amount of the ion exchanger in the matrix increases.

Regeneration of nano-ZnO photocatalyst by the means of soft-mechanochemical ion exchange method

Nano-crystalline ZnO particles synthesized via an ion exchange method were used in a methyl-orange photocatalytic degradation process. An ion exchange resin mixed bed system was used in the suspension for separation of nano-ZnO and methyl-orange after photocatalysis reaction. Ion exchange resin, photocatalyst and water were mixed evenly by airflow. Nano-ZnO photocatalyst and soluble carboxyl zinc were gathered in the soft-mechanochemical process simultaneously. Ion exchange reaction ZnO photocatalyst and counterion transfer process on resin were all improved. Byproducts and other inorganic ions were transfered into resin phase. Nano-ZnO photocatalyst can be regenerated and reused.

Ion Limited Charging of Nanoparticle Thin Films

This article reports on the charging behavior of thin films of alkanethiol protected gold nanoparticles, so-called monolayer protected clusters (MPCs), immersed in aqueous solution. We demonstrate that the oxidation of the MPCs is controlled by the transfer of counter-ions across the aqueous/MPC film interface. A model is developed to describe this and its predictions compared with experiment. Langmuir-Schaefer MPC thin films were transferred to a glassy carbon electrode and film charging behavior investigated in a series of aqueous solutions comprised of different base electrolyte anions. The dependence of peak current on film thickness and peak position on anion lipophilicity for MPC oxidative charging can all be accounted for with the ion-transfer limited model. The impact of ion partitioning into the film at equilibrium is also discussed and the effect of the aqueous phase cation is theoretically considered. In addition, the effect of co-transported water with hydrophilic ions transferring into the film on film charging is rationalized.

Structural and electrokinetic characteristics of ultra-and nanoporous glasses in tetraethylammonium chloride solutions

The structural and electrokinetic characteristics of ultra-and nanoporous glasses produced from basic sodium borosilicate glass have been investigated in tetraethylammonium chloride solutions. Colloido-chemical characteristics of the membranes have been compared with those determined in sodium and potassium chloride solutions. It has been found that the regularities of variations in counterion transference numbers and efficiency coefficients are related to a decrease in tetraethylammonium cation mobility in the membranes.

Decoupling of the Nernst−Planck and Poisson Equations. Application to a Membrane System at Overlimiting Currents

This paper deals with one-dimensional stationary Nernst-Planck and Poisson (NPP) equations describing ion electrodiffusion in multicomponent solution/electrode or ion-conductive membrane systems. A general method for resolving ordinary and singularly perturbed problems with these equations is developed. This method is based on the decoupling of NPP equations that results in deduction of an equation containing only the terms with different powers of the electrical field and its derivatives. Then, the solution of this equation, analytical in several cases or numerical, is substituted into the Nernst-Planck equations for calculating the concentration profile for each ion present in the system. Different ionic species are grouped in valency classes that allows one to reduce the dimension of the original set of equations and leads to a relatively easy treatment of multi-ion systems. When applying the method developed, the main attention is paid to ion transfer at limiting and overlimiting currents, where a significant deviation from local electroneutrality occurs. The boundary conditions and different approximations are examined: the local electroneutrality (LEN) assumption and the original assumption of quasi-uniform distribution of the space charge density (QCD). The relations between the ion fluxes at limiting and overlimiting currents are discussed. In particular, attention is paid to the "exaltation" of counterion transfer toward an ion-exchange membrane by co-ion flux leaking through the membrane or generated at the membrane/solution interface. The structure of the multi-ion concentration field in a depleted diffusion boundary layer (DBL) near an ion-exchange membrane at overlimiting currents is analyzed. The presence of salt ions and hydrogen and hydroxyl ions generated in the course of the water "splitting" reaction is considered. The thickness of the DBL and its different zones, as functions of applied current density, are found by fitting experimental current-voltage curves.

Coupled convection of solution near the surface of ion-exchange membranes in intensive current regimes

Mechanisms responsible for the overlimiting ion transfer in membranes systems are discussed. The overlimiting transfer is shown to be due largely to the action of four effects coupled with the concentration polarization of the system. Two of these are connected with the water dissociation near the membrane/solution interface: the emergence of additional charge carriers (ions H+ or OH−) in the depleted solution layer and the exaltation of transfer of salt counterions. The latter effect is connected with the perturbation of electric field caused by the water dissociation products. The other two effects are two versions of coupled convection, which leads to partial destruction of the depleted diffusion layer. These include gravitational convection and electroconvection. The former is caused by the emergence of the solution’s density gradient. The latter develops via a mechanism of electroosmotic slip. In this work, methods of voltammetry and chronopotentiometry and pH measurements are used to study the transfer of ions through homogeneous membranes Nafion-117 and AMX as a function of the concentration of sodium chloride solutions in the underlimiting and overlimiting current regimes. In a 0.1 M NaCl solution, gravitational convection makes a considerable contribution to the transfer of salt ions near the membrane surface in intensive current regimes. The influence of this effect on the electrochemical behavior of membrane systems weakens with the solution dilution and with increasing relative transfer of the H+ and OH− ions that are generated at the membrane/solution interface. In conditions where gravitational convection is suppressed and the water dissociation near the membrane/solution interface is not great, the major contribution to the overlimiting growth of current is made by electroconvection. Topics for discussion in the paper include the mutual influence of effects on one another, in particular, the effect the rate of generation of the H+ and OH− ions exerts on the gravitational convection and electroconvection and the reasons for the different behavior of cation-and anion-exchange membranes in intensive current regimes.

Electron Transport Dynamics in a Room-Temperature Au Nanoparticle Molten Salt

A room-temperature Au38 nanoparticle polyether melt has been prepared by exchanging poly(ethylene glycol) (PEG) thiolate ligands, HS-C6-PEG163, into the organic protecting monolayer of Au38(PhC2)24 nanoparticles. Spectral and electrochemical properties verify that the Au38 core size is preserved during the exchange. Adding LiClO4 electrolyte, free PEG plasticizer, and/or partitioned CO2 leads to an ionically conductive nanoparticle melt, on which voltammetric, chronoamperometric, and impedance measurements have been made, respectively, of the rates of electron and ion transport in the melt. Electron transport occurs by electron self-exchange reactions, or electron hopping, between diffusively relatively immobile Au38(0) and Au38(1+) nanoparticles. The rates of physical diffusion of electrolyte ions (diffusion coefficients DCION) are obtained from ionic conductivities. The measured rates of electron and of electrolyte ion transport are very similar, as are their thermal activation energy barriers, observations that are consistent with a recently introduced ion atmosphere relaxation model describing control of electron transfer in semisolid ion and electron-conductive media. The model has been previously demonstrated using a variety of metal complex polyether melts; the present results extend it to electron transfers between Au nanoparticles. In ion atmosphere relaxation control, measured rates and energy barriers for electron transfer are not intrinsic values but are instead characteristic of competition between back-electron transfer caused by a Coulombic disequilibrium resulting from an electron transfer and relaxation of counterions around donor-acceptor reaction partners so as to reachieve local electroneutrality.

Mixed ion transfer in redox processes of poly(3,4-ethylenedioxythiophene)

A model considering the transfer of counter ions and solvent molecules has been presented to explain the complicated ion transfer in polymer films during electrochemical redox processes. The developed mixed ion transfer model is used to analyze the ion transfer behavior of poly(3,4-ethylenedioxythiophene) during doping and dedoping processes. The results indicate that both anions and cations are involved in n-doping as well as in p-doping processes. The behavior of different electrolyte ions is also studied. The developed model shows that solvent molecules are also transferred in conjunction with the ion transfer.

Redox history effects accompanying the electrochemical cycling of poly(vinylferrocene)

Cyclic voltammetric studies of poly(vinylferrocene), PVF, in aqueous sodium perchlorate, sodium p-toluenesulfonate, and sodium naphthalenesulfonate solutions revealed unexpected history effects. The films contain redox sites in two local environments. One type of ferrocene site is in a compact, unsolvated environment; it undergoes redox switching in NaClO4 with only the transfer of counter ions. The other type of ferrocene site is in a more diffuse environment; it undergoes redox switching with the transfer of perchlorate counter ion and water. Only the latter sites can accommodate large anions (toluenesulfonate or naphthalenesulfonate), and therefore undergo redox switching in such media. Upon multiple redox cycling, these large anions get progressively locked into the structure, thereby blocking the diffuse sites. Upon transfer back to NaClO4 solution, only the “compact” sites are able to undergo redox switching.

Transport Properties of Selective Membranes Reversible to Nitrogen-Containing Organic Base Cations: Permeability and Ion Flow

Main transport properties were studied for selective membranes with low dielectric constants based on liquid ion exchangers involving nitrogen-containing organic base cations. Permeabilities and ion flows through a membrane were calculated for major and interfering ions. Dependences of the transport properties of membranes on the concentrations of the ion exchanger and near-membrane solution and their potentiometric characteristics are presented. It was demonstrated that the transport properties of liquid membranes are determined by two main factors: the transfer of counterions through the phase boundary by the extraction–exchange mechanism and the leaching of the ion exchanger from the membrane.

Kinetics of redox switching of electroactive polymers using the electrochemical quartz crystal microbalance. Part I. Identifying the rate limiting step in the presence of coupled electron/ion and solvent transfer

The oxidation of poly(vinylferrocene) (PVF) films in aqueous bathing solutions proceeds ia the oxidation of the uncharged PVF followed by entry of water into the film. The change in mechanism between rate controlling coupled electron/ion and solvent transfer was studied in 0.1 M sodium perchlorate bathing solution using the electrochemical quartz crystal microbalance (EQCM). The film's redox switching was investigated using cyclic voltammetry at voltage scan rates between 0.004 and 0.080 V s−1. The controlling kinetic step depends on the instantaneous oxidation state of the film, the voltage scan rate and its direction. These quantities determine the relative fluxes of water and counter ion during the redox cycle; the flux ratio at any charge level is diagnostic of the instantaneous rate determining step. We give these diagnostic relations, and show how to use EQCM data to obtain these flux ratios. The flux ratio that maintains the (pseudo) equilibrium population ratio in the PVF film is the dividing line between kinetic control by water transfer or counter ion transfer. High charge fluxes are likely to be associated with kinetic control by solvent transfer. Low charge fluxes lead to kinetic control by coupled electron/counter ion transfers. Both kinetic situations are observed with PVF under the conditions that were used here.