Ion transport and electrochemical stability of strongly basic anion-exchange membranes under high current electrodialysis conditions

Author’s Accepted Manuscript ION TRANSPORT AND ELECTROCHEMICAL STABILITY OF STRONGLY BASIC ANIONEXCHANGE MEMBRANES UNDER HIGH CURRENT ELECTRODIALYSIS CONDITIONS V.I. Zabolotskiy, A. Yu. But, V.I. Vasil'eva, E.M. Akberova, S.S. Melnikov www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(16)31313-8 http://dx.doi.org/10.1016/j.memsci.2016.12.028 MEMSCI14953

To appear in: Journal of Membrane Science Received date: 15 August 2016 Revised date: 22 November 2016 Accepted date: 13 December 2016 Cite this article as: V.I. Zabolotskiy, A. Yu. But, V.I. Vasil'eva, E.M. Akberova and S.S. Melnikov, ION TRANSPORT AND ELECTROCHEMICAL STABILITY OF STRONGLY BASIC ANION-EXCHANGE MEMBRANES UNDER HIGH CURRENT ELECTRODIALYSIS CONDITIONS, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.12.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ION TRANSPORT AND ELECTROCHEMICAL STABILITY OF STRONGLY BASIC ANION-EXCHANGE MEMBRANES UNDER HIGH CURRENT ELECTRODIALYSIS CONDITIONS V. I. Zabolotskiy1, A. Yu. But1, V. I. Vasil’eva2, E. M. Akberova2, S. S. Melnikov1 1

Federal State Budget Educational Establishment of Higher Education «Kuban State University», 149 Stavropolskaya st., Krasnodar, 350040, Russia 2

Federal State Budget Educational Establishment of Higher Education «Voronezh State University», 1 Universitetskaya pl., Voronezh, 394018, Russia

Abstract Rotating membrane disk and laser interferometry methods were used to study the mechanism of ion transport, hydrodynamic instability at the solution/membrane interface and electrochemical destruction of strongly basic homogeneous (AMX), heterogeneous (MA-41) and modified heterogeneous (MA-41M) membranes in sodium chloride solutions under intensive current regimes. Electrochemical destruction of strongly basic membranes AMX and MA-41 at current densities above limiting diffusion current is accompanied by conversion of fixed quaternary ammonium groups into weakly basic secondary and tertiary amines. The corresponding increase in the water splitting reaction leads to an increase in the transport number of hydroxide ions, a decrease in the convective instability region thickness and a reduction in the transfer of salt ions. The properties of MA-41M membranes (in which ammonium bases in the surface layer are replaced by quaternary amines, bidentate bonded to the polymer matrix) during testing at overlimiting current regimes remain stable: the partial currents of hydroxide ions are close to zero, and the mass transfer through the modified membrane under intensive current regimes is close in magnitude to the mass transfer through the homogeneous membrane AMX. At the same time, electroconvection becomes the dominant mechanism for the transfer of salt ions through the modified heterogeneous membrane.

Keywords: anion-exchange membrane; water splitting; electroconvective instability; rotating membrane disk; laser interferometry.

1 Introduction The transition to electrodialysis at current densities exceeding the limiting electrodiffusion current (so-called high-intensity electrodialysis) is one of the priorities of

membrane electrochemistry today. Use of overlimiting current conditions and high temperatures has led to new requirements of the membranes used in these processes. These requirements include high chemical and thermal stability, low water dissociation reaction rate and the ability to generate powerful electroconvective fluxes. In moderately dilute electrolyte solutions, the primary mechanisms of ion transport through the ion exchange membranes are electromigration and diffusion. The current passing through the electro-membrane systems (EMS) is limited by the diffusion limiting current (ilim). Under this current boundary the concentration of electrolyte solution at the membrane surface decreases to zero. The numerical value of the limiting current value can be found using the Pierce equation [1]:

ilim 

FDc Ti  ti 

(1)

where D is electrolyte diffusion coefficient, С is solution concentration, Ti and ti are electrodiffusion transport number of counterions in the membrane and electromigration transport number of counterions in the solution, and δ is diffusion boundary layer thickness. In most cases the diffusion boundary layer thickness depends on the longitudinal coordinate of the membrane channel. In the case of a membrane channel without a spacer and with a length of L<0.02V0h2/D [2], δ can be calculated as [3] (Lévêque equation):

1/ 3

 LDh     1.02 V  0 

(2)

where L is channel length, h is channel height, and V0 is mean linear velocity of the solution inside the channel. The dependence of the diffusion boundary layer thickness on the longitudinal coordinate (channel length) is one of the reasons for the distortion of the classical form of the currentvoltage characteristics (CVC) of the membrane and membrane channel [2]. This gives rise to some inconveniences in the theoretical analysis and interpretation of experimental CVC data [4]. EMS with a rotating membrane disk (RMD) is convenient for experimental and theoretical studies. Levich showed [5] that the smooth surface of the rotating electrode disk is equiaccessible in diffusion and electrical relation while the solution viscosity (ν) and the angular 2

velocity of the disk rotation (ω) determine diffusion boundary layer thickness. The latter can be calculated as:

  1.61 1/ 2 1/ 6 D1/ 3

(3)

By substituting δ into the Pierce equation (eq. 1) we can calculate the limiting current density for an EMS with an RMD [6]:

zi FCD 2 / 3 1 / 2 ilim  0.62 Ti  ti  1/ 6

(4)

In the case of classical electrodiffusion, a limiting diffusion current across the membrane is linearly dependent on the square root of the angular velocity. The angular coefficients of this dependence for the cation- and anion-exchange membranes are different. Verification of Eq. (4) and resolution of some problems of membrane electrochemistry using an RMD was carried out in [7-13]. The influence of surface inhomogeneity on ion transfer in the EMS has been investigated previously in several studies. Using the RMD method the authors of [11] showed that in moderately dilute solutions, when the primary mechanism of ion transport is classical electrodiffusion, the limiting current for homogeneous membranes is always higher than that of heterogeneous membranes. In [14-19], it was shown that the decrease of the limiting current in EMS with heterogeneous membranes is due to the presence of inert parts on the membrane surface. For swollen samples of Russian industrial heterogeneous membranes MA-40, MA-41 and MK-40, manufactured by Shchekinoazot (www.n-azot.ru) by hot pressing, the proportion of inert surface Θ reaches values of 0.62 - 0.85. For industrial heterogeneous membranes Ralex AMH and Ralex CMH, produced by hot rolling at Mega a.s. (Czech Republic) (www.mega.cz), Θ = 0.3 [12]. The morphology of a heterogeneous membrane surface is very complex and its mathematical description is confusing. One possible solution to this problem is simulation of a membrane surface with simplified geometry. Such equivalent substitution was introduced in [20] using a surface geometry in which round conductive particles with the same effective radius R were placed equidistant from each other chequerwise, and the remainder of the surface was covered with a non-conductive surface with Ѳ fraction. The ratio of inert and active fractions of 3

the surface and the effective radius R of conductive particles of the model and the real surface remained the same. The problem of theoretical description of electrodiffusive transport of electrolyte ions through a heterogeneous surface was resolved using a metal electrode in [21]. Concerning heterogeneous ion-exchange membranes the Baltrunas equation derived in [21] can be rewritten as follows [20]:

0.27   ln 1   1 Ti  ti  R 1      ilim zi FCD zi FCD 21   

(5)

The first term in Eq. (5) corresponds to the Pierce equation (Eq. (1)) and the second term is a correction of the electrodiffusive transport due to partial screening of the membrane surface. Eq. (5) takes into account redistribution of current lines and tangential diffusion of ions along the membrane surface. Despite the significant simplification of the geometry of the heterogeneous membrane made when deriving the eq. (5), it correctly describes the dependence of ilim on the heterogeneity of the membrane surface and diffusion layer thickness. In dilute electrolyte solutions, conjugate concentration polarization effects start playing an essential role in the EMS. The most significant of these effects are electroconvection and dissociation of the water [22-25]. Theoretical [4, 26-30] and experimental [19, 20, 31-33] studies developing the idea of electroconvection show that it is one of the core mechanisms of intensification of mass transfer in electrodialysis of dilute solutions and can significantly exceed the classical electrodiffusion transfer. Following traditional notions outlined in the works of Dukhin and Mishchuk [34, 35] and Rubinstein and Zaltsmann [27, 28, 36], electroconvection in membrane systems is the result of the interaction of the electric field and space charge at the membrane/solution interface induced by this field. More intensive electroconvection takes place in a threshold regime in heterogeneous membrane systems. The presence of areas with high electrical resistance in these membranes leads to an increase in the local current density on the active sites with high electrical conductivity. In the same conditions, the surface concentration around the active sites becomes smaller than on the surface of homogeneous membranes. Therefore, for the same current density potential drop and the thickness of the space charge region in those areas of the heterogeneous membrane is higher than in homogeneous membranes. In addition, alternating conductive and non-conductive regions give rise to the tangential component of the electric power [4, 27, 32, 37, 38] which leads to the appearance of the Dukhin4

Mishchuk electroconvective mechanism [34, 35]. Therefore, the decisive role in the behavior of an EMS with a heterogeneous membrane is that of the morphology and chemical composition of the surface. Currently, various studies are being conducted on the development of heterogeneous membranes with a surface morphology that provides generation of electroconvective flows and improvements in mass transport in high-intensity electrodialysis conditions [19, 20, 27, 32, 38, 39]. Water dissociation reduces the intensity of electroconvection at the membrane/solution interface due to the simultaneous effect of two factors. Firstly, the water splitting reaction produces new charge carriers, leading to a decrease in the magnitude of the space charge. Secondly, substitution of some part of counterions with large Stokes radius (salt ions), transported by the hydrodynamic mechanism, by hydrogen or hydroxyl ions carried by relay mechanism [20, 31, 32, 40]. On the other hand, the water dissociation can provide some increase in the mass transfer of salt ions by conjugation with streams of water dissociation products. The existence of this effect in EMS was predicted by Kharkats [41]. The equation (6) describes the partial current for the salt ions at overlimiting currents (i> ilim):

i  ilim 

where

D iW DH / OH

(6)

D , DH , DOH are salt ion (cation or anion), hydrogen, and hydroxyl diffusion

coefficients, and iW is water dissociation product partial current. To date, the regularities of the water dissociation reaction in an EMS have been studied in detail. It has already been established that the rate of water dissociation at the anion-exchange membrane interface is more intensive compared to the boundary of the cation exchange membrane/solution of [43-46]. Cation-exchange membranes ionic groups are chemically and thermally more resistant than fixed groups of anion-exchange membranes and cation-exchange membranes are hardly subjected to thermochemical degradation in the process of high-intensity electrodialysis [46-48]. In [46, 49-53] it was found that water dissociation takes place faster in the process of electrodialysis with strongly basic anion-exchange membranes. Quaternary amines do not participate in protonation reaction and water dissociation reaction kinetics with their 5

participation should be close to that of pure water or an aqueous solution (kd = 2.5·10-5 s-1). However, experimental studies of strongly basic membranes containing quaternary ammonium bases have shown that the water dissociation process of these membranes proceeds at a sufficiently high speed [54]. The high intensity of water dissociation reaction observed for strongly basic membranes is usually ascribed to transformation of quaternary bases into secondary and tertiary amines, which has high catalytic activity towards water splitting reaction. Water splitting reaction rate constant in the presence of weakly basic amines is five orders of magnitude higher, compared with pure water. In [49] using FTIR spectroscopy, the authors were able to fix the appearance of tertiary amines on the strong base homogeneous anion-exchange membrane AMX, which worked for a long time at a current density higher than the limiting one. Transformation of quaternary ammonium bases in the surface layer of the membrane into tertiary amino groups due to partial chemical degradation has been demonstrated in [56]. The phenomenon of hydrolysis of ammonium bases after direct current polarization of heterogeneous anion-exchange membranes MA-41 and Ralex AMH in 0.01 M NaCl at overlimiting current modes was shown in [12, 13]. Shaposhnik et al. carried out ab initio quantum mechanical calculations in [46] and demonstrated the possibility of transforming trimethyl ammonium into a tertiary amine by reaction with hydroxyl ions. Local heating of the ion-exchange membrane surface and pH shift towards an alkaline solution at the membrane/solution interface was shown in [12, 49-51]. These two effects leads to deamination of the membrane matrix: t° C6 H5CH2 N(CH3 )3 OH + H2O  C6H5CH2OH + NH(CH3 )3 OH

(7)

and transformation of quaternary ammonium bases into tertiary amines: t° C6 H5CH2 N(CH3 )3 OH + H2O  C6H5CH2NH(CH3 )2 OH +CH3OH

(8)

which can be further transformed into secondary or primary amines. Thus, weakly basic amines which are catalytically active in relation to water dissociation reactions are formed by the hydrolysis reaction. Their presence promotes the water splitting reaction even further, leading to increased concentration of hydroxyl ions at the membrane/solution interface. Such an increase in the concentration of hydroxide ions leads to an

6

even more substantial hydrolysis of quaternary ammonium bases with all the attendant negative consequences. Recently, several papers have appeared dealing with obtaining anion-exchange membranes with high chemical and thermal stability, primarily for fuel cell applications. The authors [57-62] have achieved some success in development and characterization of anionexchange membranes with improved thermal and chemical stability in alkaline solutions. Merle, Wessling and Nijmeijer [63] have recently published a review of these studies about the problem of creating alkaline fuel cell. Surface-modified membranes with suppressed water dissociation reaction were obtained in [64] by substitution of the secondary and tertiary amines groups with quaternary ammonium bases bonded to the membrane polymer matrix by two C-N bonds [65]. Such membranes were stable under overlimiting current modes for more than 300 hours [66] whereas homogeneous membrane AMX was subject to degradation of its quaternary groups under similar conditions [49]. These membranes provide a significant increase in mass transfer due to the development of electroconvection without changing the pH of the solution. The absence of stable, strong base anion-exchange membranes hinders the development of high-intensity electrodialysis. The influence of hydrogen and hydroxide ions resulting from water dissociation and Joule heating of the surface layer on the properties of the membranes themselves and their structure are not well understood. The task of the present work is to study the mechanism of ion transport, water dissociation and electrochemical stability of homogeneous and heterogeneous strong base anion-exchange membranes, and search for methods to improve their mass transfer characteristics at high current densities.

2 Experimental 2.1 Physicochemical characteristics of the anion-exchange membranes The objects of research are industrial strongly basic anion-exchange membranes: heterogeneous MA-41 (JSC Shchekinoazot, Russia), homogeneous AMX (Tokuyama Soda, Japan) and surface-modified MA-41M membranes (designed and manufactured according to [67]). Table 1 shows physicochemical characteristics of the studied membranes.

Table 1. Physicochemical characteristics of the membranes studied. 7

Membrane Fixed groups

АМХ -N+(CH3)3

МА-41 -N+(CH3)3;

Ion exchanger Inert binder Ion-exchange capacity*, mmol/g-wet Water uptake* W, %

1.32±0.08

AV-17-8 PE 0.98±0.07

МА-41М -N+(CH3)3, Surface layer: =N+(CH3)2 AV-17-8 PE 1.06±0.07

27±3

36±2

33±2

Thickness in wet state, microns Density*, g/cm3-wet

180±10

540±10

530±10

1.14

1.15

1.16

*Measured according to the standard procedures described in

[68].

All membrane samples were subjected to a standard pretreatment procedure [69]: the surface was wiped with CCl4, and then treated sequentially for 24 hours in ethanol, saturated NaCl solution, NaCl solution at a concentration of 100 g/L, and NaCl solution at a concentration of 30 g/L. Then the samples were washed with distilled water. Modified strongly basic membranes were prepared using the following procedure [67]: initial membranes MA-41 were subjected to controlled hydrolysis in 0.2 M NaOH solution at 50°C. After hydrolysis, fixed weakly basic groups were protonated in a solution of hydrochloric acid. Then the membrane was washed until neutral pH of the rinsing solution was achieved. After that, modification of the membrane surface was carried out with a copolymer of acrylic acid and dimethyldiallylammonium chloride in anhydrous dimethylacetamide solution. Figure 1 shows the FTIR spectra of the initial strongly basic anion resin AV-17-8, and the same resin that was hydrolyzed and modified as described in [67]. For the FTIR study the ion-exchange resin was crushed into dust and pressed together with potassium bromide into tablets. As can be seen from a comparison of the spectra of the modified ion-exchanger, a new absorption band appears at 985 cm-1. This band is characteristic of the vibrations of symmetric and asymmetric-type bidentate C-N-C [70] bonds. In addition, an intense narrow band at 1630 cm-1 corresponds to the stretching vibrations of the C-N bond, which may indicate the formation of bidentate nitrogen bound in the modifying layer.

8

Figure 1. FTIR spectra of the initial strong base anion resin AV-17-8 (1) and membrane modified as described in [67] (2)

2.2 Method of membranes surface study In this work, the surface of the studied membranes was investigated in a wet state, as opposed to the work presented in [71]. The morphology of the membrane surface was studied using rasterizing electron microscopy under low vacuum [72]. The surface of heterogeneous ion-exchange membranes consists of conducting parts, which are particles of ion-exchange resin, and a non-conducting inert binder, polyethylene. Due to the complicated nature of the surface, some parameters are required to describe it. Digital processing of the images obtained made it possible to calculate portions of conducting area Sc (ion-exchanger plus gaps between it and the inert binder), sole ion-exchanger S, total surface porosity P, effective radii of conducting site and weighted average of the radii of the pores. The last three parameters are represented, as a radius of a circular shape section with an area equivalent to the actual area of the arbitrary inhomogeneity shape. Ionexchanger portion was calculated using Eq. (9):

  Si   S  i  S   A 

(9)

where ΣSi is the sum of all ion-exchanger surfaces, and SA is the total area of the part studied. The digital processing of the images was done using in-house software described in [73]. Microrelief of the membrane surface was investigated using an atomic force microscopy (AFM) on a SolverP47 Pro microscope (Moscow, Russia) in tapping mode on dry samples [74]. Scanning was performed using cantilever type NSG20 with a length of 90 ± 5 microns with a 9

resonance frequency of 260-630 kHz and a radius of curvature of the probe tip - 10 nm. Scanning area was 40x40 microns. Experiments were carried out in air at a temperature of 25±1°C. Processing of the AFM images was carried out on AFM Solver P47 Pro Nova RC1 software. Surface roughness was analyzed in accordance with international standards ISO 4287/1 and ANSI B. 46.1. 2.3 Method of direct measurement of thickness of hydrolyzed layer of anionexchange membranes and of distribution of weakly basic amino groups in the membrane phase. The ability of weakly basic amino groups to form a chemically stable complex compound with copper (II) ions [75] lies at the core of a method to identify the thickness of the hydrolyzed layer of an anion-exchange membrane. Complexation allows the quantification of the proportion of weakly basic functional groups and their distribution across the thickness of the membrane. Determination of the percentage of the functional groups exposed to chemical or thermal hydrolysis with a formation of secondary and tertiary amino groups was performed using the following method. Samples of the anion-exchange membranes studied were kept for 48 hours in 1 M copper sulphate solution at pH = 4 so that weakly basic amino groups can form chemically stable complex compounds with the Cu2+ ions. This complexation reaction is similar to ammonia complexes with copper. After this procedure, the samples studied were washed with deionized water and then dried. Then cross-section cuts of the membrane were made, and X-Ray diffraction was used to examine the distribution of copper along the thickness of the membrane. Electron micrographs of the membranes were obtained using a Jeol JSM 7500F scanning electron microscope with an elemental analysis attachment. As an example, Fig. 2 shows the contrasted SEM image and copper elemental analysis for the MA-41 membrane, which worked for 40 hours in high-intensity electrodialysis.

10

b

25 20 15 10 5

0 0

40

80

120 L, μm

160

200

Figure 2. Contrasted SEM image with elemental analysis (a) and the corresponding distribution of secondary and tertiary amino groups in membrane cross section (b). The data shown was obtained for a MA-41 membrane which had worked for 40 hours at an electric current density i = 3 ilim in 0.01M solution of NaCl. Half of the cross section is shown. The left side of the membrane was facing the desalted solution

Each of the white dots corresponds to detection signals for copper ions in the local area of the membrane of about 5 microns in diameter. The area occupied by bright points (S), was normalised to the area of the scanned strip (So). The ratio of these regions was then calculated as a percentage α = S/Sо 100%. To minimize the experimental error at least five samples of membrane were studied using this technique. The resulting curves represent the mean values of α for this samples. 2.4 Method of study of the electrical transport properties of the membranes using a rotating membrane disk Study of the mass transfer and polarization characteristics of the membranes was carried out on an installation with a rotating membrane disk. The main difference from the previously used [7-11] RMD systems is the ability to study the current-voltage characteristic of the membrane and measure the effectiveness of ion transport through it simultaneously. Schematically the experimental setup is shown in Fig. 3.

11

Figure 3. Scheme of the RMD cell. 1 - anion exchange membrane under investigation, 2- top half-cell with a NaCl solution (anode chamber), 3- bottom half-cell with a NaCl solution (cathode chamber), 4 - inlet solution capillary, 5 - outlet solution capillary, 6 - Pt polarizing electrodes, 7 - Luggina-Haber capillaries, 8 - galvanostat, 9 - millivoltmeter, 10 - Ag/AgCl reference electrodes, 11 - pulley

The central element of the experimental setup is a rotating glass tube (2) with a diameter of 11 mm. At the end portion of the tube an ion exchange membrane (1) with a working diameter of8.5 mm is attached (using waterproof adhesive SC-1). Inside the tube there are inlet (4) and outlet (5) solution capillaries and a polarizing platinum electrode (6), as well as one of the Luggina-Haber capillaries (7). The rotating glass tube with its polarizing platinum electrode and membrane forms the anode chamber. The bottom half-cell (3), which is the cathodic chamber contains a second platinum polarizing electrode (6) and the second Luggina-Haber capillary (7). The Luggina-Haber capillaries are located on opposite sides of the membrane on an axis passing through the centre of the membrane disc. Measurement of the current-voltage characteristics of the membrane system was carried out in galvanostatic mode, gradually increasing the current density with the galvanostat (8). The potential drop across the membrane was recorded on a millivoltmeter (9) with silver chloride electrodes (10). The rotation speed was varied from 100 to 500 rpm and measured using an optical-mechanical transducer coupled to the digital display unit. In this range of speeds a laminar fluid flow regime is observed. The transmission mechanism consists of a pulley (11), a belt and a motor (not shown in the figure). The feed rate of the 12

solution inside the glass tube (2) (anode half-cell) was 7.50 ± 0.05 ml/min, temperature – 25.0 ± 0.1°C. For each sample a few current-voltage curves were recorded, until the difference between the last two did not exceed 10 mV. In this case, it was considered that the membrane has reached equilibrium. At the same time as recording the general CVC of the membrane, the solution of electrolyte directed to the anode compartment (2) was withdrawn via the capillary (2) for chemical analysis. The composition of the solution was determined by measuring pH and electrical conductivity and independently confirmed by direct chemical analysis. Transport numbers of ions were measured by modified Hittorf method. 2.5 Methods of visualizing the hydrodynamic state in the solution at the membrane/solution interface For

direct

experiments

on

visualizing

electroconvective

instability

at

the

membrane/solution interface, an interferometric installation based on the Mach-Zehnder scheme was used. For visualization an LGN-503 helium-neon laser (λ = 632.8 nm) was used. The receiving and decoding of interferograms was carried out using the method described in [76-78]. Experiments were conducted in an electrodialysis cell with seven flow-through chambers, the center section of which was made of optical glass. The height of the membrane channel L was 4.1 cm, width 2.4 cm, intermembrane distance h = 2 mm. Electrodialysis was carried out in a galvanostatic mode. With a horizontal orientation of the electrodialysis cell in the gravitational field, the depleted diffusion layer was beneath the membrane being investigated, i.e. gravitational convection currents do not occur for any value of concentration and/or temperature gradients. The interference pattern was recorded at the longitudinal coordinate y = 2.6 cm by a camcorder with a sampling frequency of 15 Hz, in the direction of the solution flow and then was presented in digital form. Visualization of the interference pattern in the solution at the interface made it possible to determine the characteristic size of the convective instability. The convective instability arises directly at the membrane/solution interface and manifests itself in an irregular oscillation and changes in the position and width of the interference fringes [78]. In the present work, we define the thickness of the convective instability region as the distance d from the surface of the membrane to the point in the solution in which the interference fringe and, accordingly, the concentration profile, are of a non-stationary oscillatory character. As an example, Fig. 4 shows interferograms for the initial MA-41 and surface-modified MA-41M membrane at a potential drop value of Δφ = 4.0 V. 13

(a)

(b)

Figure 4. Interferograms of the solution on the boundary with the MA-41 (a) and MA-41M (b) membranes at Δφ=4,0 V, C0(NaCl) = 2.0·10–2 М, V = 1.3·10–3 m/s (Re = 2.6), h = 2.0·10–3 m, y = 2.6·10–2 m (0.64L); nonstationary (0–1) and stationary (1–2) parts of the concentration profile

Assessing the impact of random errors in the measurement of the thickness of the area of the convective instability at high intensity current modes showed that the relative standard deviation was 0.10-0.15.

3 Results and Discussion 3.1 Heterogeneity and microrelief of the surface of strongly basic membranes The SEM images of the strongly basic homogeneous membrane AMX and heterogeneous membranes MA-41 and MA-41M in a swollen state are shown in Fig. 5.

14

Figure 5. SEM images of the surfaces of the anion-exchange membranes: homogeneous AMX (a), heterogeneous MA-41 (b), modified heterogeneous MA-41M (c) and a schematic surface model of heterogeneous membranes (d). The black circles are conductive areas and the white area is the non-conductive portion. The effective (average) radius of the conductive regions for MA-41 and MA-41M is respectively 5.2 microns and 7.5 microns; inert section surface is Θ = 1 Sc – 0.84 and 0.85. (The designations are given in Table 2) The surface of the homogeneous membrane AMX is uniform in all directions, with only a slight distortions in the places where the reinforcing mesh is presented (Fig. 5a). In heterogeneous membranes, the size of the regions that conduct electric current is by 1-2 orders of magnitude higher. Profilograms of MA-41 and MA-41M membranes, as well as a histogram of values for the heights of the density distribution of these membranes are shown in Figure 6.

15

a

b

Figure 6. Profilograms (a) and height distribution histograms (b) for heterogeneous membranes: 1 - MA-41; 2 - MA-41M

Table 2 shows the characteristics of surface roughness and inhomogeneity of the initial heterogeneous membrane MA-41 and modified membrane MA-41M.

Table 2. The characteristics of the surface roughness and inhomogeneity of the heterogeneous membrane. Membrane

S, %

P, %

Sc, %

R , µm

r , µm

Rc ,

Ra, nm

Ry, nm

µm MA-41

13±1

3.4±0.1

16.0±0.8

4.6±0.3

2.5±0.2

5.2±0.4

86

1340

MA-41M

12±2

3.4±0.6

15±2

6.4±0.7

2.7±0.2

7.5±0.8

138

1662

By processing the SEM images (Fig. 5) the following parameters of the membrane surface can be found: S is the proportion of ion exchanger; Sc is the proportion of conductive surface (ion-exchange material and space between the particles of the ion exchanger and inert binder); P is the proportion of macropores; R is the average radius of the ion exchanger; r is the average radius of the macropores. Parameters which are derived from analysis of AFM images are Ry, the scope of microprofile heights on the surface of the membrane, and Ra, the arithmetic average roughness. From the data presented in the table it can be seen that the characteristics of surface irregularities for initial and modified membranes are similar. The proportion of the active surface area Sc is 15-16%. The rest of the surface is occupied by an inert binder Θ 1  Sc and totals 84-85%. After modification the average radius of the conductive area increases from 5.2 microns to 7.5 microns, with a simultaneous increase in the membrane surface roughness Ra from 86 nm 16

to 138 nm. The height distribution Ry on the surface of the membrane at the same time increased from 1340 nm to 1662 nm.

3.2 Current-voltage characteristics and limiting current Figure 7 shows the current-voltage characteristics of an electromembrane system containing the strongly basic homogeneous membrane AMX in 0.1 M NaCl solution at different RMD rotation speeds.

Figure 7. Total current-voltage characteristics of the EMS containing an AMX membrane in 0.01M NaCl solution at different disk rotation speeds (ω, RPM): 1 - 100; 2 - 200; 3 - 300; 4 - 500

Fig. 8 (a, b) shows the CVC in the same electrolyte solution for all anion exchange membranes investigated, respectively, at the minimum (ω = 100 rpm) and maximum (ω = 500 rpm) membrane disk rotation speeds.

17

Figure 8. General current-voltage characteristics of EMS comprising AMX (1), MA-41 (2) and MA-41M (3) in 0.01 M NaCl solution at membrane disk rotation speeds of ω = 100 rpm (a) and ω = 500 rpm (b)

It should be noted that all the experimental data shown in Fig. 7 and Fig. 8 were obtained for membranes not subjected to prolonged high current density polarization. For voltage drops across the membrane up to and including some threshold value (approx. 1.5 – 3 V) water dissociation in an EMS with these membranes does not occur (Fig. 9). For all studied membranes this voltage drop is higher than the voltage drop which corresponds to the limiting current value (Fig. 8), therefore, near the limiting current, there is no contribution of hydroxide ion in the total current value, and for the same reason there is no exaltation effect of the limiting diffusion current (Eq. 6) [41, 42]. The design of the RMD installation (Figure 3) also eliminates the possibility of gravitational convection. At the same time in dilute solutions near the limiting current space charge occurs and the conditions for the development of electroconvection are created [26-28, 31, 34-36]. Thus, in dilute electrolytes solutions limiting currents are not purely electrodiffusion: electroconvection influences their formation.

6 2

i, mA/cm2

5 4

1

3

3

2 1 0

0

2

Δφ, V

4

6

Figure 9. Partial by hydroxyl ions CVCs of the EMS with 0.01 M NaCl solution at disk rotation speed -100 rpm, for studied membranes: 1 – АМХ, 2 – МА-41, 3 – МА-41М

At current densities higher than the limiting current water dissociation starts and the total mass transfer and the shape of the CVC are defined by the values of electrodiffusion and electroosmotic fluxes of salt ions, water dissociation and exaltation effect. 18

The data presented show that even for a homogeneous AMX membrane, considering that its surface is equally accessible for electrodiffusion of salt ions, the CVC differs from the classic: there is no horizontal plateau of the limiting current. After the linear ohmic section of the CVC a subsequent rise in the current is observed as a result of the electroconvection. The length of the plateau section represented the ability of the system to develop electroconvection and is interpreted as a transition from the electrodiffusion of ions to the electroconvective transport mechanism [15, 28, 29, 32, 37]. Dissociation of water in these membranes begins at an electrical potential drop across the membrane Δφ > 1.5 V (Fig. 9) and causes a subsequent current rise in the current-voltage characteristics. Data of the limiting currents at different RMD rotation speeds derived from experimental CVC is summarized in the Levich coordinates [5] (ilim -  ) in Fig. 10.

20

ilim , mA/cm2

1

2

3

15 10 5 0 0

2

4 6 w0,5 (rad/s)0.5

8

Figure 10. The dependence of the limiting current density on the square root of the angular speed of RMD. The solid line shows the values of limiting current for the AMX membrane, calculated using Eq. (1). The dashed and dash-dotted lines are the theoretical values of limiting current for heterogeneous membranes MA-41 and MA-41M respectively, calculated according to Eq. (3) and (5) taking into account the screening of membrane surface by an inert binder. The points are the experimental values of the limiting current for the membranes: 1 – АМХ; 2 – MA-41; 3 – MA-41M. Parameters for MA-41 membrane Θ =0.84; Rc =5.2 microns; MA-41M Θ =0.85; Rc = 7.5 microns.

The contribution of the electroconvective component in the limiting current value can be estimated by comparing the experimental limiting current (ilim.e) with the calculated electrodiffusion limiting currents for AMX (calculated with use of eq. (1)), MA-41 and MA-41M 19

(calculated with use of eq. (3)) shown in Fig. 10. Eq. (3) is required to take into account the heterogeneous nature of MA-41 and MA-41M. Table 3 shows the relative contributions of electroconvection to electrodiffusion transport of ions through the membranes at the limiting current density (i = ilim) at various membrane disk rotation speeds. Equation (9) determines the electroconvection contribution to the limiting current (β):



i

lim.e

 ilim 

ilim.e

 100%

(9)

Table 3. The relative fraction of electroconvection component of the limiting current β for the membranes studied for various membrane disc rotation speeds (ω). Membrane

ω = 100 rpm

ω = 300 rpm

ω = 500 rpm

АМХ

23.1

18.7

19.9

MA-41

44.3

22.7

18.6

MA-41M

77.9

76.4

77.0

Comparison of the experimental and calculated limiting currents (fig. 10, Table 3) shows that electroconvective ion transport in dilute solutions is observed at i = ilim. The excess of experimental ilim over the calculated values is more pronounced for heterogeneous membranes than for homogeneous in the whole range of disc rotation speeds. For the modified heterogeneous membrane with suppressed water splitting ability, i = ilim electroconvection ion transport is more than three times greater than the electrodiffusion transport of ions. In the EMS with heterogeneous membranes, the electroconvection mechanism is an equilibrium electroconvection according to Dukhin-Mishchuk [34, 35]. Such an electroconvective mode is more pronounced for heterogeneous membranes than for an EMS with homogeneous membranes. Comparison of the values of the limiting currents for the modified heterogeneous membrane MA-41M and the homogeneous membrane AMX suggests that the contribution of electroconvection on heterogeneous membranes MA-41M virtually compensates the reduction in mass transfer due to the effect of surface screening with non-conductive portions of polyethylene. For overlimiting current modes, the contribution of electroconvection and exaltation effects was carried out by comparing the calculated limiting electrodiffusion current using the 20

Eq. (3) and (5) with experimental ones given in Figures 8 and 10. The contribution of exaltation effect in the total fluxes is calculated using Harkats equation (Eq. (6)). Table 4 shows the contributions of water dissociation, exaltation effect and electroconvection to the total mass transfer when the voltage drop across the membranes is Δφ = 3 V. Table 4. Relative proportions of electroconvective current in the limiting current β, water splitting rate and exaltation effect for the membranes studied under potential drop of Δφ = 3 V and RMD rotation speed ω = 100 rpm Membrane

β, %

АМХ

61.0

МА-41 МА-41М

Water splitting

Exaltation effect, %

Electrodiffusion, %

3.6

0.7

34.7

33.0

34.0

7.5

25.5

93.0

0.0

0.0

7.0

ration, %

As seen from Table 4, in overlimiting state when the electrical potential drop on the membranes reaches 3 V the water dissociation on homogeneous membrane AMX is 3.6%. In a heterogeneous membrane MA-41 the dissociation of water, due to higher local current density on the conductive portions of its surface, is almost one order of magnitude higher than in homogeneous. A strong increase in the water dissociation rate on heterogeneous membrane MA41 leads to suppression of electroconvection and it becomes less pronounced. The exaltation effect caused by the conjugation of salt ions and water dissociation product fluxes on both AMX and MA-41 membranes is not large and does not exceed 0.7%. On the modified membrane MA41M when Δφ = 3 V dissociation of water does not occur and the exaltation effect is not evident. The maximum contribution to mass transfer of electroconvective fluxes of salt ions is observed on the membrane MA-41M with suppressed water splitting ability and reaches a value of β = 93%. Thus, electroconvection in EMS with modified membrane becomes the dominant process. Experimental research on visualization of solution convective instability near the surface of the anion-exchange membranes conducted using the laser interferometry method confirm these conclusions. Fig. 11 shows the dependence of the thickness of the region of electroconvective instability near the surface of the MA-41 and MA-41M membranes studied on potential drop across membrane.

21

Figure 11. Size of electroconvective instability regions in the sodium chloride solution at the interface of anion-exchange membranes: MA-41 (1) and MA-41M (2).

Analysis of the interferograms (Fig. 4) and the results of decoding them (Fig. 11) indicates that when the current reaches the limiting value a convective instability zone occurs near the membrane surface. This instability is the result of the electroconvection effect. The length of this hydrodynamically unstable region, characterizing the intensity of electroconvection increases with a rise in the potential drop (current density). For the modified membrane MA41M where the water dissociation reaction is suppressed [67] electroconvection intensity and mass transfer of salt ions are higher than the salt flux in the EMS with the initial membrane MA41. The potential drop, in which there is the electroconvective instability in the membrane MA41M (Δφ = 3.0 V), is smaller than the original membrane MA-41 (Δφ = 4.3 V). 3.3 Electrochemical stability of strongly basic membranes To study the electrochemical stability of the homogeneous membrane AMX and heterogeneous membranes MA-41 and MA-41M they were polarized with a high current density directly in the RMD cell. Fig. 12 shows partial current-voltage characteristics for Cl- and OHions for all the membranes studied after polarization in 0.01 M NaCl solution at a current density of i = 3ilim.

22

25

a

1'

20 2' 15

10 2 5 1

0 0

2

4

6

∆φ, V

14

b

1'

12 10

2'

8

2

6 1

4 2

0 0

1

2

3

∆φ, V c

20 1'

2' 15

10

5

2 1

0 0

1

2

3

4

5

∆φ, V

Figure 12. Partial CVCs by hydroxide (1 and 2) and chloride (1 'and 2') ions for membranes: AMX (a), MA-41 (b) and MA-41M (c) in 0.01 M NaCl solution at a rotation speed of 100 rpm. 1 - initial membrane; 2 - after 10 h of operation. The partial CVCs show that the properties of AMX (Figure 12 a) and MA-41 (Figure 12 b) membranes are unstable. After just 10 hours of high current density polarization, the water dissociation rate significantly increases which leads to an electroconvection suppression due to the reduction of salt ions transport and changes the threshold value of the potential drop at which 23

water dissociation reaction begins in the EMS. For the initial homogeneous membrane AMX in the field of under limiting currents (i< ilim) when the membrane potential drop is lower than 1 V there is practically no water dissociation. When potential drop reaches 4.5 V the contribution of water dissociation reaches 15% of the total transport (Fig. 12 a). From partial CVC of Cl- ions it can be seen that on the original (non-polarized membrane), in the current range i > ilim, mass transfer increase of salt ions occurs due to electroconvection. For AMX membranes, pre-polarized by overlimiting current, i = 3ilim for 10 hours, the water dissociation process begins at the potential drop value Δφ = 1 V, and at Δφ = 4 V the transport number of hydroxide ions values reach 0.21. With an increasing proportion of current carried by hydroxide ions a decrease in the mass transfer is observed (Fig. 12 a). The same patterns are seen in the analysis of the partial current-voltage characteristics for the heterogeneous membranes MA-41 (Figure 12 b). The only difference is that the increase of water dissociation rate and the decrease of mass transfer occurs to a much greater extent. After polarized for 10 hours of polarization the transport of hydroxide ions on an MA-41 membrane reaches ТОН- = 0.4 already at Δφ = 2.7V, and the partial current for Cl- ions are thus reduced from 11 mA/cm2 to 6 mA/cm2. The causes of more intensive water dissociation in EMS with heterogeneous membranes, as compared with homogeneous membranes in the same conditions was discussed in [71]. These reasons are associated with increased local current density and concentrated polarization on the active areas of the conducting heterogeneous membranes. The modified strongly basic membrane MA-41M remains stable after 10 hours of polarization in 0.01M NaCl solution at i = 3ilim (Fig. 12c). Significant dissociation of water on both the initial and the polarized modified membrane begins only at Δφ = 4V. Lifetime tests of the MA-41M membrane in an electrodialyzer have also shown that it is electrochemically stable and suppresses the water dissociation reaction at overlimiting current regimes [67]. 3.4 Changing the chemical composition of the strongly based membrane at intense current regimes and thermochemical treatment Distribution of weakly basic groups was determined along the MA-41 membrane thickness by the method described in section 2.2. Fig. 13 shows the change in concentration of weakly basic amino groups and the thickness of the hydrolyzed layer depending on the polarization time at a current density i = 3ilim in a 0.01M NaCl solution.

24

Figure 13. The portion of weakly basic amines coordinated with copper ions in the MA-41 membrane, which had worked for different periods of time under current polarization i = 3ilim: 1 - initial membrane; 2 - 3 hours; 3 - 10 hours; 4 - 30 hours; 5 - 60 hours

In the original membrane the proportion of weakly basic, functional amine groups is 34%. Similar values for the content of tertiary and secondary amine groups in the industrial membrane MA-41 are shown in [12]. After 10 hours of membrane polarization the proportion of weakly basic amine groups in the surface layer of the membrane reaches a value of α = 12% and the thickness of the hydrolyzed layer is about 70 microns. With further increases in polarization time, there is an increase in α and the thickness of the hydrolyzed layer x x. After 60 hours of polarization, the proportion of weakly basic groups in the surface layer of the membranes reaches 20%, and the thickness of the hydrolyzed layer reaches 120 microns (about ¼ of the thickness of the membrane). Fig. 14 shows similar data during processing of membrane MA-41 in 0.2 M NaOH solution at 50°C. Chemical hydrolysis of the membranes occurs on both sides, and after just 15 minutes the thickness of the hydrolyzed layers reaches ¼ of the total membrane thickness, wherein α = 28%.

25

Figure 14. The thickness of the hydrolyzed layers of heterogeneous membrane MA-41 for different periods of hydrolysis in 0.2M NaOH solution at 50°C: 1 - initial membrane; 2 - 1 minute; 3 - 5 minutes; 4 - 15 minutes; 5 - 30 minutes

On boiling a sample of the MA-41 membrane in distilled water the deamination processes (reaction (8)) and degradation (reaction (9)) proceed simultaneously [12, 49-51]. The reactions take place uniformly throughout the volume of the membrane. Table 5 shows the values of the total exchange capacity Q0, exchange capacity for strongly basic (Qs) and weakly basic (Qw) groups for the initial MA-41 membrane and after heat treatment in distilled water at 100°C for 50 hours. The same table shows the proportion of weakly basic groups α.

Table 5. Total ion-exchange capacity Q0, exchange capacity for strongly basic (Qs) and weakly basic (Qw) groups for the initial MA-41 membrane and after heat treatment in distilled water at 100°C for 50 hours. Conditions Initial Heat-treated

Q0, mmol/g 0.98 0.76

Qs, mmol/g 0.94 0.63

Qw, mmol/g 0.04 0.13

α=(Qw/Q0)∙100 % 4 17

The partial current-voltage characteristics of the heat-treated membranes were obtained using the RMD. After heat treatment the rate of water dissociation on the membranes increases and the mass transfer of salt ions decreases. The values for the partial densities of hydroxide ions for current the original (α = 4%) and heat-treated (α = 17%) strongly basic membranes MA-41 are shown in Fig. 14. Thus, for a strongly based membrane MA-41 polarized by overlimiting current, chemical and thermal treatment in distilled water at 100°C, a change in the chemical composition of the membrane surface occurs due to the formation of tertiary and secondary amines on the 26

membranes` surface. In accordance with a range of catalytic activity by ion groups in the water dissociation reaction [50]: –N+(CH3)3 < –SO3H < –PO3H- < =NH, – NH2 < ≡ N < –COO- < –PO32weakly basic groups exhibit very high activity in the water splitting reaction (the effective rate constant of water dissociation reaction for these groups is k = 0.1 s-1 [55]). Fixed quaternary ammonium groups do not participate in dissociation reactions, and the rate of water dissociation is negligibly small by the conventional non-catalytic mechanism (k = 2.5·10-5 s-1). Therefore, it can be assumed that the rate of water dissociation is affected only by the concentration of weakly basic amino groups. Catalytic water dissociation reaction of water with weakly basic groups takes place in two stages [4], with the rate-limiting step being the transfer of a proton to the amino group [54]. Therefore, the reaction rate of water dissociation in a strong base membrane must increase in proportion to the growth in concentration of weakly basic amino groups. Fig. 15 shows the correlation between the partial current of hydroxide ions and the proportion of weakly basic amino groups in the surface layer of the MA-41 membrane after electrochemical polarization, during alkaline hydrolysis and after boiling in distilled water.

Figure 15. The dependence of the partial current of hydroxide ions on the fraction of functional weak base amino groups in the surface layer of membranes MA-41 that had operated for different time intervals in 0.01M sodium chloride solution at a current density of i = 3ilim (

)

and those that were subjected to thermal hydrolysis in distilled water at 100°C ( ) and alkaline hydrolysis in 0.2M NaOH solution at 50°C ( ). Figures on the experimental points (

) indicate

the duration of polarization in hours. 27

The resulting correlation shows that regardless of the conditions under which degradation reaction (9) occurs (in intensive electrochemical polarization of the membranes, during alkaline hydrolysis or by boiling in distilled water) the cause of the reduction in speed of mass transfer through strongly basic membranes is the appearance in the surface layer of tertiary and secondary amines which are catalytically active in relation to the dissociation of water molecules. 3.5 Visualization of convective instability in the electrodialysis cell with strongly basic and modified membranes The results of research on dynamic visualization of hydrodynamic instability in the solution at the surface of the membranes MA-41 and MA-41M were carried out in an electrodialysis cell with optically transparent walls by laser interferometry according to the procedure set out in Section 2.5. (Fig. 16).

a b

28

Figure 16. Dependence of the thickness of the convective instability region in a 0.02 М NaCl solution (a) and fluxes of Clˉ ions through anion exchange membranes MA-41 and MA-41M (b) obtained under polarization at a constant current density i = 3ilim in an electrodialysis cell. The feed rate of the solution in the membrane channel is V = 1.3 mm/s, intermembrane distance is h = 2.0 mm

As can be seen from Fig. 16, the thickness of the convective instability region is virtually unchanged for MA-41M (not more than 2-3%) over 30 hours of operation, whereas for the original membrane MA-41 it suddenly decreased after 3-4 hours. After 10 hours of operation, the dimensions of this region decreased 3-fold. Intense convective mixing of the solution on the border with membrane MA-41M, which removes part of the diffusion limitation due to the delivery of additional quantities of the substance to the boundary, is the primary cause of the effective participation of electrolyte ions in mass transport (Fig. 15b). After 30 hours of operation, selective transport of chloride ions at an effective transport number of about 0.87 continues on the modified membrane MA-41M. At the same time, degradation of quaternary ammonium bases in the membrane MA-41 leads to an increase in the reaction rate of water dissociation and a decrease in the effective number of counterion transfer to 0.30. 4 Conclusion In dilute electrolyte solutions, electroconvection is the dominant mechanism of transport of salt ions. As the result of the increase in local current density on the conductive areas of the heterogeneous membranes and the appearance of a tangential component of electric force at overlimiting currents (i > ilim), electroconvection on membranes with a microheterogeneous surface proceeds more rapidly than on membranes with a homogeneous surface. Thus, optimization of heterogeneous membrane surface morphology can provide partial or full compensation for the screening effect. The dissociation of water at overlimiting current modes is an undesirable process: it reduces the density of electric charge in a solution at the membrane/solution interface and causes the destruction of the anion-exchange membrane. Increased mass transfer of salt ions as a result of the coupling of those streams with streams of water dissociation products (exaltation effect) is insignificant. The rate of water dissociation on the membrane is proportionally dependent on the proportion of weakly basic amino groups in the strongly basic membrane. This process is one of the main causes of the degradation of strongly basic membranes under intense current regimes. 29

Electrochemical destruction of these membranes under intense current conditions, thermal hydrolysis in distilled water and alkaline hydrolysis proceeds using a common mechanism to form weakly basic amino groups which are catalytically active in the water dissociation reaction. Surface modification of strongly basic anion exchange membrane MA-41 with formation of their surface layer of quaternary ammonium bases, bidentately associated with the matrix of the membrane, can suppress the water dissociation reaction, greatly improving the electrochemical stability of the modified membranes during the process of high intensity electrodialysis and significantly increasing the useful mass transfer in the system, thereby bringing the properties of the modified heterogeneous membrane MA-41M closer to the properties of the homogeneous AMX membrane. A study of the high-intensity electrodialysis process with the surface-modified anionexchange membranes MA-41M developed has shown that their electrochemical properties remain stable over a long time in the conditions of high density electric current. The use of these membranes in electrodialysis processes for desalination and deionization of natural waters and technological solutions creates the preconditions for a significant increase in process efficiency.

Acknowledgements This work was supported by the Russian Science Foundation (Project № 14-13-00882). The results of the study of the chemical and thermal stability of strongly basic membranes using the methods of laser interferometry and rotating membrane disk were obtained with the financial support of RFBR (project № 15-08-05031, 16-38-00572mol_a, 16-48-23364r_a).

References [1] A.M. Peers, Membrane phenomena, Discussions Faraday Society 21 (1956) 124–125. [2] N.P. Gnusin, V.I. Zabolotsky, V.V. Nikonenko, M.K. Urtenov, Convective-diffusion model of the electrodialytic desalination. Limiting current and diffusion layer, Soviet Electrochemistry 22 (1986) 298-302. [3] M.A. Lévêque, Les lois de la transmission de chaleur par conection, Annales des Mines 13 (1928) 201-239. [4] M.K. Urtenov, A.M. Uzdenova, A.V. Kovalenko, V.V. Nikonenko, N.D. Pismenskaya, V.V. Vasil’eva, P. Sistat, G. Pourcelly, Basic mathematical model of overlimiting transfer enhanced by electroconvection in flow-through electrodialysis membrane cells, J. Membr. Sci. 447 (2013) 190–202. 30

[5] V.G. Levich, Physicochemical hydrodynamics, Prentice Hall, Englewood Cliffs, NJ, 1962. [6] A.J. Makai, J.C.R. Turner, Polarization in electrodialysis, J. Chem. Soc. Faraday Trans. Part 1. 74 (1978) 2851–2856. [7] Yu.V. Pleskov, V.Yu. Filinovskii, Rotating disc electrode, Nauka, Moscow, 1972. [8] N.I. Isaev, R.I. Zolotaryova, E.M. Ivanov, Study of polarization on the rotating ion-exchange membrane, Zh. Fiz. Khim. 41 (1967) 849–852. [9] D.A. Gough, J.K. Leypoldt, Membrane-covered rotated disc electrode, J. Analytical Chemistry 51(1979) 439–444. [10] L.A. Zagorodnykh, I.V. Aristov, O.V. Bobreshova, P.I. Kulintsov, Limiting current densities in a system with cation-exchange membrane MK-100 rotating in a glycine–HCl solution, Russ. J. Electrochem. 37 (2001) 413–415. [11] E.M. Balavadze, O.V. Bobreshova, P.I. Kulintsov, The concentration polarization in the process of electrodialysis and ion-selective membrane polarization characteristics,

Uspekhi

Khimii [Russ. Chem. Rev.] 57 (1988) 1031–1041. [12] V.I. Zabolotsky, V.V. Bugakov, M.V. Sharafan, R. Kh. Chermit, Transfer of electrolyte ions and water dissociation in anion-exchange membranes under intense current conditions, Rus. J. Electrochem. 48 (2012) 650–659. [13] V.I. Zabolotsky, R. Kh. Chermit, M. V. Sharafan, Mass transfer mechanism and chemical stability of strongly basic anion-exchange membranes under overlimiting current conditions, Rus. J. Electrochem. 50 (2014) 38–45. [14] J.H. Choi, S.H. Moon, Pore size characterization of cation-exchange membranes by chronopotentiometry using homologous amine ions, J. Membr. Sci. 191 (2001) 225–236. [15] N.Ya. Pivovarov, Heterogeneous ion-exchange membranes in electrodialysis processes, Dal’nauka, Vladivostok, 2001. [16] J.J. Krol, M. Wessling, H. Strathmann, Chronopotentiometry and overlimiting ion transport through monopolar ion exchange membranes, J. Membr. Sci. 162 (1999) 155–164. [17] J.-H. Choi, S.H. Kim, S.-H. Moon, Heterogeneity of Ion-Exchange Membranes: The Effects of Membrane Heterogeneity on Transport Properties, J. Colloid Interface Sci. 241 (2001) 120– 126. [18]

N.D.

Pismenskaya,

Ph.

Sistat,

P.

Huguet,

V.V.

Nikonenko,

G.

Pourcelly,

Chronopotentiometry applied to the study of ion transfer through anion exchange membranes, J. Membr. Sci. 228 (2004) 65-76.

31

[19] R. Ibanez, D.F. Stamatialis, M. Wessling, Role of membrane surface in concentration polarization at cation exchange membranes, J. Membr. Sci. 239 (2004) 119–128. [20] V.I. Zabolotsky, V.V. Nikonenko, M. H. Urtenov, K.A. Lebedev, V.V. Bugakov, Electroconvection in systems with heterogeneous ion-exchange membranes, Rus. J. Electrochem. 48 (2012) 692–703. [21] G. Baltrunas, R. Valiunas, G. Popkirov, Identitication of electrode surface blocking by means of thin-layer cell. 1. The model, Electrochimica Acta 52 (2007) 7091–7096. [22] V.I. Zabolotsky, V.V. Nikonenko, Ion transport in membranes, Nauka, Moscow, 1996. [23] V.I. Zabolotsky, V.V. Nikonenko, N.D. Pismenskaya, E.V. Laktionov, M.Kh. Urtenov, H. Strathman, M. Wessling, C.H. Koops, Coupled transport phenomena in overlimiting current electrodialysis, Sep. Purif. Technology 14 (1998) 255–267. [24] H. Strathman, Electrodialysis, a mature technology with multitude of new application, Desalination, 264 (2010) 268–288. [25] H. Strathman, Ion-exchange membrane separation processes, Membrane Science and Technology Series, 9, Elsevier, Amsterdam, 2004. [26] V.I. Zabolotskii, K.A. Lebedev, M.Kh. Urtenov, V.V. Nikonenko, P.A. Vasilenko, V.A. Shaposhnik, V.I. Vasil’eva, A mathematical model describing voltammograms and transport numbers under intensive electrodialysis modes, Russ. J. Electrochem. 49 (2013) 369–380. [27] I. Rubinstein, F. Maletzki, Electroconvection at an electrically inhomoheneous permselective membrane surface, J. Chem. Soc., Faraday Trans. II. 87 (1991) 2079–2087. [28] I. Rubinstein, B. Zaltzman, O. Kedem, Electric fields in and around ion-exchange membranes, J. Memb. Sci. 125 (1997) 17–21. [29] A.V. Kovalenko, V.I. Zabolotskiy, V.V. Nikonenko, M.Kh. Urtenov, Electrochemical stability of strong basic anion exchange membranes in conditions of high intensive electrodialysis process, Politematicheskiy setevoy elektronnyy nauchnyy zhurnal Kubanskogo gosudarstvennogo agrarnogo universiteta 14 (2014). [30] A.M. Uzdenova, A.V. Kovalenko, M.Kh. Urtenov. Mathematical

models

of

electroconvection in electro-membrane systems, KChGU, Karachaevsk, 2011. [31] V.I. Vasil'eva, A.V.Zhiltsova, M.D. Malykhin, V.I. Zabolotskii, K.A. Lebedev, R. Kh. Chermit, M.V. Sharafan, Effect of the chemical nature of the ionogenic groups of ion-exchange membranes on the size of the electroconvective instability region in high-current modes, Russ. J. Electrochem. 50 (2014) 120–128.

32

[32] N.D. Pis’menskaya, V.V. Nikonenko, N.A. Mel’nik, G. Pourcelli, G. Larchet, Effect of the ion-exchange-membrane/solution interfacial characteristics on the mass transfer at severe current regimes, Russ. J. Electrochem. 48 (2012) 610–628. [33] J.-H. Choi, H.-J. Lee, S.-H. Moon, Effects of Electrolytes on the Transport Phenomena in a Cation-Exchange Membrane, J. Colloid Interface Sci. 38 (2001) 188–195. [34] S. S. Dukhin, N.A. Mishchuk, Intensification of electrodialysis based on electro-osmosis of the second kind, J. Membr. Sci. 79 (1993) 199–210. [35] N.A. Mishchuk, Electro-osmosis of second kind near the heterogeneous ionexchange membrane, Colloids and Surfaces A. Physicochemical and Engineerin Aspects 40 (1998) 75–89. [36] I. Rubinshtein, B. Zaltzman, J. Pretz, C. Linder, Experimental verification of the electroosmotic mechanism

of overlimiting conductance through

a cation exchange

electrodialysis membrane, Russ. J. Electrochem. 38 (2002) 853–863. [37] V. I. Vasil’eva, A.V. Zhiltsova, M.D. Malykhyn, N.D. Pismenskaya, N.A. Melnik, Influence

hydrophobic

surface

of

the

sulfocation

exchange

membrane

todevelopmentofinstabilityelectroconvective in stratified electrosystems, Vestnik VGU, Seriya: Khimiya, Biologiya, Farmatsiya 2 (2013) 35–38. [38] E.M. Akberova, V.I. Vasil'eva, M.D. Malykhin, Influence of temperature modification of sulfocation-exchange membrane on electroconvective instability development at overlimiting current regimes, Kondensirovannyye sredy i mezhfaznyye granitsy 17 (2015) 273–280. [39] V.I. Vasil’eva, A.V. Zhil’tsova, E.M. Akberova, A.I. Fataeva, Influence of surface inhomogeneity on current-voltage characteristics of heterogeneous ion exchange membranes, Kondensirovannye sredy i mezhfaznye granitsy 16 (2014) 257–261. [40] V.I. Zabolotsky, S.A. Loza, M.V. Sharafan, Physicochemical properties of profiled heterogeneous ion-exchange membranes, Rus. J. Electrochem. 41 (2005) 1053–1060. [41] Yu.I. Harkats, About the mechanism of occurrence of limiting current at the membrane / electrolyte boundary, Elektrokhimiya, 21 (1985) 974–977. [42] V.V. Nikonenko, N.D. Pismenskaya, V.I. Zabolotskii, Not hydrodynamic intensification of electrodialysis of dilute electrolyte solutions, Elektrokhimiya 27 (1991) 1236–1244. [43] Y.A. Kononov, B.M. Vrevskii, Role of water dissociation products in electric current transfer through ionite membranes, Zh. Prikl. Khim. 44 (1971) 929–932. [44] V.K. Varentsov, M.V. Pevnitskaya, Ion transfer across ion-exchange membranes at electrodialysis, Izv.Sib. Otd. AN SSSR (Ser. Khim. Nauk) 4 (1973) 134–138.

33

[45] Y. Tanaka, M. Seno, The concentration polarization and dissociation of water in ionexchange membrane electrodialysis, Denki kagaku 51 (1983) 267–270 [46] V.A. Shaposhnik, A.S. Kastyuchik, O.A. Kozaderova, Irreversible dissociation of water molecules on the ion-exchange membrane–electrolyte solution interface in electrodialysis, Rus. J. Electrochem 44 (2008) 1073–1078. [47] Y. Iwai, T. Yamanishi, Thermal stability of ion-exchange Nafion N117CS membranes, Polym. Degrad. Stab. 94 (2009) 679–687. [48] Ch. Chen, T. F. Fuller, The effect of humidity on the degradation of Nafion membrane, Polym. Degrad. Stab. 94 (2009) 1436–1447. [49] J.H. Choi, S.H. Moon, Structural change of ion-exchange membrane surfaces under high electric fields and its effects on membrane properties, J. Colloid Interface Sci. 265 (2003) 93–100. [50]

T. Sata, M. Tsujimoto, T. Yamaguchi, K. Matsusaki, Change of anion exchange

membranes in an aqueous sodium hydroxide solution at high temperature, J. Memb. Sci. 112 (1996) 161–170. [51] U. Hwang, J.H. Choi, Changes in the electrochemical characteristics of a bipolar membrane immersed in high concentration of alkaline solutions, Sep. Purif. Technology 48 (2006) 16–23. [52] R. Simons, Electric field effects on proton transfer between ionizable groups and water in ion exchange membranes, Electrochimica Acta 29 (1984) 151–158. [53] B. Bauer, H. Strathmann, F. Effenberger, Anion-exchange membranes with improved alkaline stability, Desalination 79 (1990) 125–144. [54] V.I. Zabolotsky, N.V. Sheldeshov, N.P. Gnusin, Dissociation of water molecules in systems with ion-exchange membranes, Uspekhi Khimii 57 (1988) 1403–1414. [55] R. Simons, Electric field effects on proton transfer between ionizable groups and water in ion exchange membranes, Electrochimica Acta 29 (1984) 151–158. [56] U. Hwang, J.H. Choi, Changes in the electrochemical characteristics of a bipolar membrane immersed in high concentration of alkaline solutions, Sep. Purif. Technology 48 (2006) 16–23. [57] E.N. Komkova, D.F. Stamatialis, H. Strathmann, M. Wessling, Anion-exchange membranes containing diamines: preparation and stability in alkaline solution, J. Memb. Sci. 244 (2004) 25– 34.

34

[58] Q. Zhang, Q. Zhang, J. Wang, S. Zhang, Sh. Li, Synthesis and alkaline stability of novel cardo poly(aryl ether sulfone)s with pendent quaternary ammonium aliphatic side chains for anion exchange membranes, Polymer 51 (2010) 5407–5416. [59] Y. Wang, J. Qiu, J. Peng, L. Xu, J. Li, M. Zhai, Study on the chemical stability of the anion exchange membrane of grafting dimethylaminoethyl methacrylate, J. Memb. Sci. 378 (2011) 70– 77. [60] G. Couture, A. Alaaeddine, F. Boschet, B. Ameduri, Polymeric materials as anion-exchange membranes for alkaline fuel cells, Progress in Polymer Science 36 (2011) 1521–1557. [61] H. Xu, J. Fang , M. Guo, X. Lu, X. Wei, S. Tu, Novel anion exchange membrane based on copolymer of methyl methacrylate, vinylbenzyl chloride and ethyl acrylate for alkaline fuel cells, J. Memb. Sci. 354 (2010) 206–211. [62] M. Guo, J. Fang, H. Xu, W. Li, X. Lu, Ch. Lan, K. Li, Synthesis and characterization of novel anion exchange membranes based on imidazolium-type ionic liquid for alkaline fuel cells, J. Memb. Sci. 362 (2010) 97–104. [63] G. Merle, M. Wessling, K. Nijmeijer, Anion exchange membranes for alkaline fuel cell: A review, J. Memb. Sci. 377 (2011) 1–35. [64] N.D. Pismenskaya, E.I. Belova, V.V. Nikonenko, V.I. Zabolotsky, G.I. Lopatkova, Y.N. Karzhavin, C. Larchet, Lower rate of H+(OHˉ) ions generation at an anion-exchange membrane in electrodialysis, Desalin. Water Treat. 21 (2010) 109–114. [65] N.D. Pismenskaya, Y.A. Fedotov, V.V. Nikonenko, E.I. Belova, G.Yu. Lopatkova, V.I. Zabolotsky, Method of modifying anion-exchange membranes, RF Patent No 2410147 dated 22.10.2008. [66] E.I. Belova, G.Y. Lopatkova, N.D. Pismenskaya, V.V. Nikonenko, C. Larchet, G. Pourcelly, Effect of anion-exchange membrane surface properties on mechanisms of overlimiting mass transfer, J. Phys. Chem. 110 (2006) 13458–13469. [67] V.I. Zabolotsky, M.V. Sharafan, R. Kh. Chermit, Multi-layer composite polymer strongly basic membrane and method of its preparation, RF Patent No 2013133028 dated 2015. [68] N.P. Gnusin, N.P. Berezina, O.A. Dyomina, N.A. Kononenko, Physicochemical Principles of Testing Ion-Exchange Membranes, Russ, J. Electrochem. 32 (1996) 154–163. [69] V.I. Vasil’eva, E.M. Akberova, A.V. Zhiltsova, E.I. Chernykh, E.A. Sirota, B.L. Agapov, J. Surf. Investigation. X-ray, Synchrotron and Neutron Techniques 7 (2013) 833–840. [70] E.A. Sirota, N.A. Kranina, V.I. Vasil'eva, M.D. Malykhyn, V.F. Selemenev, Development and experimental approbation of program complex for definition of a share ion-spending 35

surfaces of heterogeneous membranes according to raster electronic microscopy, Vestnik VGU, Seriya: Khimiya. Biologiya. Farmatsiya 2 (2011) 53–59. [71] E. Volodina, N.D. Pismenskaya, V.V. Nikonenko, C. Larchet, G. Pourcelly, Ion transfer across ion-exchange membranes with homogeneous and heterogeneous surfaces, J. Colloid Interface Sci. 285 (2005) 247–258. [72] V.I. Vasil’eva, N.A. Kranina, M.D. Malykhin, E.M. Akberova, A.V. Zhiltsova, The surface inhomogeneity of ion-exchange membranes by SEM and AFM data, J. Surf. Investigation. Xray, Synchrotron and Neutron Techniques 7 (2013) 144–153. [73] V.I. Zabolotsky, V.V. Ganych, N.V. Sheldeshov, Transport numbers of salt ions and water dissociation products in cation and anion-exchange membranes. Elektrokhimiya 27 (1991) 15– 19. [74] V.A. Shaposhnik, V. I. Vasil’eva, O.V. Grigorchuk, The interferometric investigations of electromembrane processes, Adv. Colloid Interface Sci. 139 (2008) 74–82. [75] V.I. Vasil’eva, V.A. Shaposhnik, O.V. Grigorchuk, I.P. Petrunya, The membrane-solution interface under high-performance current regimes of electrodialysis by means of laserinterferometry, Desalination 192 (2006) 408–414. [76] V.I. Vasil’eva, V.A. Shaposhnik, O.V. Grigorchuk, M.D.

Malykhin, Electrodialysis

kinetics by laser interferometry, Russ. J. Electrochem. 38 (2002) 846–852. [77] V.A. Shaposhnik, V.I. Vasil’eva, O.V. Grigorchuk, Diffusion boundary layers during electrodialysis, Russ. J. Electroch. 42 (2006). 1202–1207. [78] V.I. Vasil’eva, V.A. Shaposhnik, V.I. Zabolotsky, K.A. Lebedev, I.P. Petrunya, Diffusion boundary layers at the ion exchange membrane/solution interface under high intensity electrodialysis, Sorbtsionnye i khromatograficheskie protsessy. 5 (2005) 545–560.

Highlights 

Effects of electrochemical destruction of strong base membranes in intense current modes were investigated.



Electrochemical destruction, thermal hydrolysis in water and alkaline hydrolysis of strong base membranes have a common mechanism.



A method is developed for obtaining stable surface-modified anion exchange membranes.

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