Surface Of Ion-exchange Membranes Research Articles

Investigating the Impact of Electrolyte Flow Velocity on the Resistivity of Vanadium Redox Batteries: A Theoretical Analysis and Experimental Data Comparison.

The influence of electrolyte velocity over the ion-exchange membrane surface on ion and vanadium redox batteries' conductivity was formalized and quantified. The increase in electrolyte velocity dramatically improves proton conductivity, resulting in improved battery efficiency. An analysis of conductivity was carried out using a math model considering diffusion and drift ion motion together with their mass transport. The model is represented by the system of partial differential together with algebraic equations describing the steady-state mode of dynamic behavior. The theoretical solution obtained was compared qualitatively with the experimental results that prove the correctness of the submitted math model describing the influence of the electrolyte flow on the resistance of the vanadium redox battery. The presented theoretical approach was employed to conduct a parametric analysis of flow batteries, aiming to estimate the impact of electrolyte velocity on the output characteristics of these batteries.

Fabrication of Nonconductive Microscale Patterns on Ion Exchange Membrane by Laser Process

The electroconvection generated on the surface of an ion exchange membrane (IEM) is closely related to the electrical/chemical characteristics or topology of the IEM. In particular, when non-conductive regions are mixed on the surface of the IEM, it can have a great influence on the transfer of ions and the formation of nonlinear electroconvective vortices, so more theoretical and experimental studies are necessary. Here, we present a novel method for creating microscale non-conductive patterns on the IEM surface by laser ablation, and successfully visualize microscale vortices on the surface modified IEM. Microscale (~300 µm) patterns were fabricated by applying UV nanosecond laser processing to the non-conductive film, and were transferred to the surface of the IEM. In addition, UV nanosecond laser process parameters were investigated for obvious micro-pattern production, and operating conditions were optimized, such as minimizing the heat-affected zone. Through this study, we found that non-conductive patterns on the IEM surface could affect the generation and growth of electroconvective vortices. The experimental results provided in our study are expected to be a good reference for research related to the surface modification of IEMs, and are expected to be helpful for new engineering applications of electroconvective vortices using a non-conductive patterned IEM.

Sessile Drop Method: Critical Analysis and Optimization for Measuring the Contact Angle of an Ion-Exchange Membrane Surface.

The contact angle between a membrane surface and a waterdrop lying on its surface provides important information about the hydrophilicity/hydrophobicity of the membrane. This method is well-developed for solid non-swelling materials. However, ion-exchange membranes (IEMs) are gel-like solids that swell in liquids. When an IEM is exposed to air, its degree of swelling changes rapidly, making it difficult to measure the contact angle. In this paper, we examine the known experience of measuring contact angles and suggest a simple equipment that allows the membrane to remain swollen during measurements. An optimized protocol makes it possible to obtain reliable and reproducible results. Measuring parameters such as drop size, water dosing speed and others are optimized. Contact angle measurements are shown for a large number of commercial membranes. These data are supplemented with values from other surface characteristics from optical and profilometric measurements.

Numerical modeling for 3D vortices patterns of electroconvective flow developing in shear flow

In this study, using direct numerical modeling, we investigate the electroconvective flow developing on the surface of an ion-exchange membrane surface in the high applied voltage condition. The modeling is obtained by solving the system of Poisson–Nernst–Planck–Navier–Stokes equations in a direct and coupled manner on the OpenFOAM platform. We report simulation results proving the dependence of the flow's pattern on the applied voltage and the mechanism behind the formation of vortices at high electric fields. For the first time, different types of vortices and the concurrent appearance of helical and unidirectional vortices are studied. The role of the vortices on the distribution of ions and the electric current is clarified to explain the over-limiting current phenomenon. This work contributes a useful OpenFOAM solver integration tool for modeling electrochemical problems.

White carbon black wastewater treatment by electrodialysis: Salt separation, silicon sol transporting and wastewater recycling

White carbon black (WCB) as an important silica product has been widely used in many industrial processes. At present, precipitation method is commonly adopted for WCB production, in which a large amount of wastewater containing sodium sulfate and silica is produced. Therefore, it is important and urgent to develop an efficient, low-cost wastewater treatment process, which could extract sodium sulfate from wastewater as value-added byproducts and then recycle this wastewater back to WCB process. In this study, electrodialysis (ED) method, as a proven salt-separation technology, was used to treat WCB wastewater. The existence of silica in the wastewater brings more difficulties for its ED treatment. Firstly, to evaluate ED performance on treating WCB wastewater, the effects of applied voltage, flow rate and initial volume ratio on the overall current efficiency (η), recovery ratio (R), residual rate (γ) and concentration ratio (ω) were investigated. The results show that under the optimized conditions, the highest recovery ratio of Na2SO4 could reach 86.45%, the lowest overall residual rate of SiO2 could go down to 2.69%, and the dilute solution could achieve the reuse standard of waste water. Secondly, the influence of silica on ED performance was investigated. It is found that only about 5.91% of silica in wastewater was transporting from dilute solution to concentrated solution under the optimized conditions. The SEM images of ED membrane indicate that after ED experiments some SiO2 particles adhered to the surface of ion exchange membrane, which would not affect ED normal performance in a short term. These results illustrate that ED is an effective, low energy-cost and reliable process to treat WCB wastewater.

Surface and internal modification of composite ion exchange membranes for removal of molybdate, phosphate, and nitrate from polluted groundwater

Al 2 O 3 /chitosan-multiwall carbon nanotubes (MWCNTs) were created to increase the exchange capacity of polyvinylidene fluoride (PVDF) ion-exchange membranes. The composite membranes were made by mixing Al 2 O 3 nanoparticles into the PVDF cast solution, then applying a thin coating of chitosan functionalized carbon nano tubes (Cs-MWCNTs) to the PVDF membrane surface. The structure and characteristics of the hybrid membranes were described using XRD, SEM, IR, and TG-DTA. The Al 2 O 3 -PVDF/Cs-MWCNTs membrane beat the other Al 2 O 3 -PVDF/Cs, Al 2 O 3 -PVDF, and PVDF membranes in terms of molybdate, phosphate, and nitrate adsorption. The removal efficiency, pH solution, adsorption capacity, and desorption process of molybdate, phosphate, and nitrate anions by Al 2 O 3 -PVDF and PVDF membranes were investigated. The removal effectiveness of molybdate, phosphate, and nitrate, according to the testing findings, was 94.3, 65.6, and 85.78 %, respectively. The adsorption of MoO 4 2− , PO 4 3− , and NO 3 − increased as the pH increased initially until the best adsorption was achieved, and then decreased significantly as the pH increased further. The total adsorption capabilities of MoO 4 2− , PO 4 3− , and NO 3 − for the Al 2 O 3 -PVDF/Cs-MWCNTs membrane were 65.50, 61.22, and 59.77 mg/g, respectively. Using regeneration and reuse experiments for the simultaneous adsorption of molybdate, phosphate, and nitrate during three consecutive cycles, the adsorption/desorption of Al 2 O 3 -PVDF/Cs-MWCNTs was assessed. Al 2 O 3 -PVDF/Cs-MWCNTs offer a lot of promise when it comes to eliminating MoO 4 2− , PO 4 3− , and NO 3 − from actual wastewater samples.

Multi-length-scale heterogeneous structured ion exchange membranes for cost-effective electrolysis and hydrogen production

The design of an ultrabubble-repellent ion exchange membrane (IEM) with high ion transport efficiency for the electrolysis process remains an ongoing challenge. Herein, a multi-length-scaled heterogeneous ZrO2 coating was successfully introduced onto a sodium perfluorinated ion exchange membrane (SPIEM) surface by a facile coating method. Surface morphology optimization technology is developed to control the mesopore size, density and surface coverage of particles, thus forming a “popcorn-like” hierarchical micro/nanostructured ZrO2 coating (M-Ⅱ), which is conducive to bubble repellency in time and efficient ion transport. By using this membrane in chlor-alkali electrolysis, the power consumption can reach 2037.8 kWh t−1, representing reductions of 166.2 kWh and 113.2 kWh, respectively, for producing one ton of 100% NaOH compared with pristine SPIEM (2204 kWh t−1) and commercial Nafion TM N2030 (2151 kWh t−1) at 6 kA cm−2. As a result, M-Ⅱ can realize the production of sodium hydroxide and hydrogen at low cost. The M-Ⅱ also showed remarkable stability with negligible fluctuation for 360 h. Furthermore, after simple purification, the hydrogen from chlor-alkali electrolysis shows excellent performance in fuel cell tests. This study demonstrates the effectiveness of micro/nanoscale engineering on chlor-alkali electrolysis performance, providing a simple and efficient method for solving the gas adhesion problem in the electrolysis process.

Enhancing mechanistic models with neural differential equations to predict electrodialysis fouling

Fouling of the ion-exchange membranes by colloidal substances present in bio-based process streams is an important hurdle for electrodialysis. The development of a model is challenged by the limited availability of experimental data and the complexity of the underlying physics. This research addresses this challenge by combining a mechanistic description of the transport processes with a machine learning model to describe the complex phenomena of colloidal aggregation and attachment to the surface of ion-exchange membranes. After validation with fouling experiments using acrylamide as colloidal foulant, it was found that this hybrid model improves the predictive power of the model while reducing the need for experimental data. An analysis of both mechanistic and machine learning models showed that the attachment probability of anion polyacrylamide (APAM) is influenced by the current density and the size of the fouling layer in a non-linear manner. An increase in current density leads to an increase in the attachment probability while the opposite holds for the size of the fouling layer. This research shows that machine learning can complement mechanistic models where fundamental knowledge is lacking or computational resources are limiting. The combination maintains the interpretability and generalisability of mechanistic models while harnessing the accuracy of machine learning.

How Electrical Heterogeneity Parameters of Ion-Exchange Membrane Surface Affect the Mass Transfer and Water Splitting Rate in Electrodialysis.

Electrodialysis (ED) has been demonstrated as an effective membrane method for desalination, concentration, and separation. Electroconvection (EC) is a phenomenon which can essentially increase the mass transfer rate and reduce the undesirable water splitting effect. Efforts by a number of researchers are ongoing to create conditions for developing EC, in particular, through the formation of electrical heterogeneity on the membrane surface. We attempt, for the first time, to optimize the parameters of surface electrical heterogeneity for ion-exchange membranes used in a laboratory ED cell. Thirteen different patterns on the surface of two Neosepta anion-exchange membranes, AMX and AMX-Sb, were tested. Low-conductive fluoropolymer spots were formed on the membrane surface using the electrospinning technique. Spots in the form of squares, rectangles, and circles with different sizes and distances between them were applied. We found that the spots’ shape did not have a visible effect. The best effect, i.e., the maximum mass transfer rate and the minimum water splitting rate, was found when the spots’ size was close to that of the diffusion layer thickness, δ (about 250 μm in the experimental conditions), and the distance between the spots was slightly larger than δ, such that the fraction of the screened surface was about 20%.

Confining Photoacidity

Confining Photoacidity

The nature of two transition times on chronopotentiograms of heterogeneous ion exchange membranes: 2D modelling

Chronopotentiometry is largely used for electrochemical characterization of ion-exchange membranes. Nevertheless, the theory is poorly developed in the range of overlimiting currents. This paper proposes a two dimensional convective-diffusion model of ion transport in electromembrane system in galvanostatic mode. We use direct numerical simulation taking into account electroconvection to develop a comprehensive analysis of mechanisms of reaching transition states, which depend on the parameters of electrically heterogeneous surface of ion exchange membrane. We find that the occurrence of two transition times is typical for the simulated electrodialysis systems with heterogeneous membranes. The value of the first transition time, τ1, is conditioned by the fact that the normal and tangential diffusion delivery of ions from the solution bulk to the conductive membrane areas reaches its limiting value. When τ1 is attained, electroconvective vortices appear at the conductive-nonconductive boundaries. They mix the near-surface solution in the vicinity of these boundaries and increase the delivery of the “fresh” solution to the conductive regions. The second transition time, τ2, is reached when electroconvection-diffusion ion delivery to the overall membrane surface attains its limiting value. At t = τ2, electroconvective vortices whose size is comparable with the diffusion layer thickness arise. Both τ1 and τ2 depend on the size and fraction of conductive regions. In the case of conventional commercial ion-exchange membranes τ1 < τSand and τ2 ≥ τSand.

1D Mathematical Modelling of Non-Stationary Ion Transfer in the Diffusion Layer Adjacent to an Ion-Exchange Membrane in Galvanostatic Mode.

The use of the Nernst–Planck and Poisson (NPP) equations allows computation of the space charge density near solution/electrode or solution/ion-exchange membrane interface. This is important in modelling ion transfer, especially when taking into account electroconvective transport. The most solutions in literature use the condition setting a potential difference in the system (potentiostatic or potentiodynamic mode). However, very often in practice and experiment (such as chronopotentiometry and voltammetry), the galvanostatic/galvanodynamic mode is applied. In this study, a depleted stagnant diffusion layer adjacent to an ion-exchange membrane is considered. In this article, a new boundary condition is proposed, which sets a total current density, i, via an equation expressing the potential gradient as an explicit function of i. The numerical solution of the problem is compared with an approximate solution, which is obtained by a combination of numerical solution in one part of the diffusion layer (including the electroneutral region and the extended space charge region, zone (I) with an analytical solution in the other part (the quasi-equilibrium electric double layer (EDL), zone (II). It is shown that this approach (called the “zonal” model) allows reducing the computational complexity of the problem tens of times without significant loss of accuracy. An additional simplification is introduced by neglecting the thickness of the quasi-equilibrium EDL in comparison to the diffusion layer thickness (the “simplified” model). For the first time, the distributions of concentrations, space charge density and current density along the distance to an ion-exchange membrane surface are computed as functions of time in galvanostatic mode. The calculation of the transition time, τ, for an ion-exchange membrane agree with an experiment from literature. It is suggested that rapid changes of space charge density, and current density with time and distance, could lead to lateral electroosmotic flows delaying depletion of near-surface solution and increasing τ.

Phenomenon of two transition times in chronopotentiometry of electrically inhomogeneous ion exchange membranes

Transition time in chronopotentiometry is an important parameter, which is widely used for electrochemical characterization of various systems. Occurrence of multiple transition times is typical for multicomponent or multilayer electrode and membrane systems. In this paper we show that there may be another cause of multiple transition times. It is electrical heterogeneity of ion exchange membrane surface. It is found that there are two transition times on the chronopotentiograms of a commercial anion-exchange MA-41 (Shchekinoazot) and two specially prepared cation-exchange heterogeneous membranes in dilute electrolyte solutions (0.02 M NaCl in the experiment). It is found that both transition times are determined by diffusion limitations of ion delivery to the membrane surface in the depleted diffusion layer. The value of the first transition time depends on the dimensions of the conductive regions and their surface fraction; this transition time is determined as the time necessary for the depletion of electrolyte concentration near the conductive regions of the surface. The rate of concentration depletion depends on the electromigration through the conductive regions, on the one hand, and on the normal the tangential ion diffusion to the conductive surface regions, which mitigate the concentration decrease, on the other hand. The experimental value of the first transition time for the laboratory-made membranes is in a good agreement with simulation using a 3D electrodiffusion model. The value of the second transition time is in a good agreement with the Sand theory, hence it is conditioned by the normal diffusion delivery of electrolyte from the solution bulk to the entire membrane surface. The tangential diffusion plays a secondary role in this stage of concentration polarization since current-induced convection levels off the concentrations along the heterogeneous membrane surface.

Effect of electroconvection and its use in intensifying the mass transfer in electrodialysis (Review)

The modern concepts on the origin of electroconvection (EC) are surveyed briefly and the known mechanisms of this phenomenon are classified. Factors that influence the EC character and intensity at the surface of ion-exchange membranes are analyzed, such as electrical and geometrical heterogeneity of the membrane surface, its degree of hydrophobicity, and the surface charge. The EC mechanism is shown also to depend on the applied potential difference and the rate of solution flow between membranes. The mechanism of the EC-induced gain in the mass transfer is elucidated, the possible gain in the mass transfer is estimated, and the prospects for using the EC for reducing the membrane fouling caused by sedimentation and formation of organic deposits are assessed.

Electroconvection in systems with heterogeneous ion-exchange membranes after thermal modification

It is found that the variations in the structure (morphology and microrelief) and chemical composition of surface of heterogeneous ion-exchange membranes as a result of thermal modification have different effects on the current—voltage characteristics and conditions for the generation of electroconvective instability at the membrane/solution interface under intense current modes. After thermal treatment of strongly acidic sulfocation-exchange membrane, which is characterized by a low catalytic activity in the reaction of water dissociation and a high thermal stability of fixed groups, a fraction of conducting surface area increases and the membrane microrelief develops. As a result, the diffusion limiting current density increases and the length of plateau of the current—voltage curve decreases. Therewith, the thickness of the region of electroconvective instability of solution in the near-membrane region increases and the polarization of electromembrane system, at which the mode of unstable electroconvection is reached, decreases. The thermodestruction of strongly basic anion-exchange membranes, conversely, leads to suppression of electroconvection and an increase in the length of plateau of the current—voltage curve due to the formation of fixed weakly basic amino groups, which are catalytically active in the reaction of water dissociation. A linear correlation is found between the dimensions of the region of electroconvective instability and a fraction of weakly basic functional amino groups in the composition of strongly basic membranes.

In situ investigation of electrical inhomogeneity of ion exchange membrane surface using scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) is a relatively new, but well-developed method to study the electrochemical characteristics of the surface of heterogeneous materials, such as microelectrodes and biological cells. This paper presents the adaptation of this method to studying the distribution of electric potential near the surface of a heterogeneous ion exchange membrane. A special electrodialysis flow-through cell whose design allows for in situ 3D SECM measurements has been developed. The cell provides a laminar flow of liquid along the membrane surface, thereby making it possible to control the thickness of the diffusion layer. The SECM method has been verified by examining a model heterogeneous ion exchange membrane with preset surface and bulk properties. It has been found that the potential distribution is axisymmetric in the presence of cylindrical pores and the method allows the unambiguous determination of the position of the centers of conductive and nonconductive areas. An extremely nonuniform distribution of the potential with no signs of symmetry has been revealed in the case of MK-40 membrane. Unlike other methods for investigating inhomogeneous surfaces, SECM enable the determination of the distribution and evaluation of the size of conductive and nonconductive areas of heterogeneous ion exchange membranes in the swollen state.

Impact of ion exchange membrane surface charge and hydrophobicity on electroconvection at underlimiting and overlimiting currents

The mechanism of electroconvection at a permselective surface presents a high interest for electrodialysis separation processes as well as for microfluidics and other applications. We have studied a commercial Neosepta AMX-Sb anion-exchange membrane and its three modifications differing in the surface charge and, as a consequence, in the degree of hydrophobicity. The zeta-potential and the contact angle were measured; the membranes were characterized by chronopotentiometry and voltammetry. It is shown that at the current densities slightly lower or equal to the limiting current density, the mass transfer rate is mainly affected by the membrane surface charge. However, at the higher current densities, the main factor is the degree of hydrophobicity: the samples with a weakly charged highly hydrophobic surface show lower voltage under the same current density. This peculiarity is explained by the fact that the mechanism of electroconvection (EC) depends on the current density. At underlimiting currents and low voltages, EC occurs as electroosmosis of the first kind; the surface charge determines the parameters of the (quasi)equilibrium electric double layer (EDL), playing the main role in the phenomenon. At overlimiting currents and high voltages, it is the extended space charge region (much thicker than the EDL), which controls EC occurring apparently as electroosmosis of the second kind (nonequilibium EC). Then the contribution of the EDL is less important, while the impact of hydrophobicity increases. It is shown that the equilibrium EC may be quite strong at the AMX-Sb membrane having a highly developed surface roughness of different scales. In the range of 0.03–0.06V there is an “anomaly”: with increasing current density the potential drop over the AMX-Sb is decreasing instead of increasing.

Chronopotentiometry and impedancemetry of homogeneous and heterogeneous ion-exchange membranes

It is known that the behavior of a membrane with homogeneous and heterogeneous surface under the same current density is different. In order to find out this difference, a two-dimensional non-stationary model of mass transfer through electrically inhomogeneous surface of ion-exchange membrane under direct current is proposed. In this paper, the general formulation of the boundary conditions at the conductive sections has the form of the integral function of the local current density. The problem was solved by applying an electric current stream function, which is quite similar to the stream function in hydrodynamics. For the first time, it is proved that the distribution of current lines over the conductive sections of the membrane is not uniform. It is shown experimentally and theoretically that the shape of chronopotentiograms and electrochemical impedance spectra changes with changing conductive portion of the ion-exchange membrane surface.

Layer-by-Layer Modification of Cation Exchange Membranes Controls Ion Selectivity and Water Splitting

The present study investigates the possibility of inducing monovalent ion permselectivity on standard cation exchange membranes, by the layer-by-layer (LbL) assembly of poly(ethyleneimine) (PEI)/poly(styrenesulfonate) (PSS) polyelectrolyte multilayers. Coating of the (PEI/PSS)N LbL multilayers on the CMX membrane caused only moderate variation of the ohmic resistance of the membrane systems. Nonetheless, the polyelectrolyte multilayers had a substantial influence on the monovalent ion permselectivity of the membranes. Permselectivity comparable to that of a commercial monovalent-ion-permselective membrane was obtained with only six bilayers of polyelectrolytes, yet with significantly lower energy consumption per mole of Na(+) ions transported through the membranes. The monovalent ion permselectivity stems from an increased Donnan exclusion for divalent ions and hydrophobization of the surface of the membranes concomitant to their modification. Double-layer capacitance obtained from impedance measurements shows a qualitative indication of the divalent ion repulsion of the membranes. At overlimiting current densities, water dissociation occurred at membranes with PEI-terminated layers and increased with the number of layers, while it was nearly absent for the PSS-terminated layers. Hence, LbL layers allow switching on and turning off water splitting at the surface of ion exchange membranes.

SEM diagnostics of the surface of MK-40 and MA-40 heterogeneous ion-exchange membranes in the swollen state after thermal treatment

The morphology of the surface of MK-40 and MA-40 ion-exchange membranes in the swollen state after chemical and thermal treatment is studied using scanning electron microscopy. The distribution of portions of the ion exchanger on the surface of heterogeneous membranes is found to be of a complex stochastic nature; their sizes for the MK-40 and MA-40 membranes are 5–30 and 8–40 μm, respectively, while the sizes typical for the secondary porous structure are 2–8 μm. An increase in the share of the ion-exchange component of the surface (by 20–40%), the sizes of the ion-exchange portions (by 20%), and the surface porosity (by two-three times) in the samples subjected to thermal treatment is revealed.