Electrochemical Properties of Homogeneous and Heterogeneous Anion Exchange Membranes Coated with Cation Exchange Polyelectrolyte - MDPI
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membranes
Article
Electrochemical Properties of Homogeneous and
Heterogeneous Anion Exchange Membranes Coated
with Cation Exchange Polyelectrolyte
Xenia Nebavskaya 1, * , Veronika Sarapulova 1 , Dmitrii Butylskii 1 , Christian Larchet 2
and Natalia Pismenskaya 1
1 Kuban State University, 149 Stavropolskaya st., 350040 Krasnodar, Russia; vsarapulova@gmail.com (V.S.);
dmitrybutylsky@mail.ru (D.B.); n_pismen@mail.ru (N.P.)
2 Institut de Chimie et des Matériaux Paris-Est, UMR7182 CNRS—Université Paris-Est, 2 rue Henri Dunant,
94320 Thiais, France; larchet@u-pec.fr
* Correspondence: k.a.nebav@gmail.com; Tel.: +7-918-32-32-996
Received: 3 December 2018; Accepted: 6 January 2019; Published: 11 January 2019
Abstract: Coating ion exchange membranes with polyelectrolyte has been proven to be a cheap way
to reduce concentration polarization and increase limiting current (for polyelectrolytes carrying fixed
groups of the same sign of charge with respect to the membrane bulk), to create high monovalent
selectivity, and to add the function of H+ /OH− ions generation (for polyelectrolytes bearing fixed
groups of the opposite sign of charge with respect to the membrane bulk). In the latter case, the balance
between the counterion transport and the H+ /OH− ions generation is affected by parameters of the
substrate and the modifying layer. In this study we investigated the electrochemical characteristics
of homogeneous Neosepta AMX-Sb and heterogeneous MA-41P membranes coated with one, two,
or three layers of oppositely charged polyelectrolyte (the maximum thickness of each layer was
5 µm). It was found that the limiting current decreased earlier and the generation of H+ /OH−
ions was stronger in the case of the heterogeneous membrane. The shift in the pH of the solution
depended more on the generation of H+ /OH− ions at the modifying layer/solution interface than on
the generation at the membrane/modifying layer interface, and in all cases water splitting started in
the same range of potential drops over the membrane.
Keywords: ion exchange membrane; polymer; membrane modification; current–voltage curve;
limiting current
1. Introduction
The modification of ion exchange membrane (IEM) surfaces with a polyelectrolyte layer was first
proposed by Sata et al. [1], who aimed to improve monovalent selectivity through the introduction of
a selective layer carrying fixed groups oppositely charged with respect to the fixed groups of membrane
bulk. In practice, the samples with functional groups of modifying layers being oppositely charged
to the groups in membrane bulk became bipolar membranes with two functions: desalination and
H+ /OH− ions generation [2]. The great role in the electrochemical properties of such IEM is played
by the bipolar junction between the membrane and the modifying layer, at which the H+ /OH− ions
generation occurs.
Further studies broadened the range of application of layered electrochemical materials.
Membrane-coated electrodes attracted interest for various electrochemical applications [3–5]. In the
case of IEM, for redox flow batteries, layered polyelectrolyte composite membranes showed good
conductivity and low permeability to vanadium ions [6], and a thorough review of methods to achieve
Membranes 2019, 9, 13; doi:10.3390/membranes9010013 www.mdpi.com/journal/membranesMembranes 2019, 9, 13 2 of 13
high monovalent selectivity of IEM in the separation processes (including several techniques for
coating surfaces with charged and uncharged layers) was recently published [7]. Surface modification
of ion exchange membranes with polyelectrolytes was used for pH correction [8], for suppression
of fouling [9,10] or scaling [11], and for facilitation of the electro-osmotic slip of water near the
surface [12]. It was shown that excellent monovalent selectivity can be achieved for IEM using
layer-by-layer coating with polyelectrolytes carrying functional groups of alternating charges [13,14],
or even for multilayers applied at uncharged porous supports [15]. For these systems, very high
separation factors and adequate electric resistance were reported. However, several possible additional
properties of layered IEM, such as the ability to generate H+ and OH− ions, should be taken into
account for possible applications.
The balance between the transport of salt ions and the generation of H+ /OH− ions depends on
the thickness of the applied layer. For asymmetric bipolar membranes produced by the pouring and
spreading of relatively thick modifying layers, this dependence was studied by Zabolotsky et al. [8],
who found that the water splitting increases (and salt ion transport decreases) with the layer thickness.
For thinner layers, the details of such dependency are not yet known, hence one of the tasks of our
study was to determine the effect of modifying-layer thickness on the electrochemical properties
(limiting current density of salt counterion and ability for H+ /OH− ions generation) of resulting
membranes in the cases of applied layers with low thickness.
Another question not yet answered was the role of the electrical heterogeneity of membrane
support. Heterogeneous membranes are cheaper than homogeneous ones, hence they can be
used for the design of cheaper novel materials with improved monovalent selectivity and
reasonable electrochemical properties. What makes the difference between the homogeneous and
the heterogeneous membranes in this case is that the appearance of nonconductive zones at the
surface of heterogeneous membranes make the current lines concentrated on conductive zones [16].
As a result, the local current density is higher on conductive zones, which causes stronger concentration
polarization and water splitting. Modified homogeneous membranes [13] and modified heterogeneous
membranes [8] were studied separately, but the differing methods of modification and testing protocols
do not allow direct comparison of results. To study the effect of this difference, we compared the
properties of two series of membranes based on supporting membranes differing in homogeneity.
The homogeneous supporting membrane was produced by the paste method (Neosepta AMX-Sb) and
the heterogeneous supporting membrane was produced by hot rolling (MA-41P).
2. Materials and Methods
2.1. Membranes and Materials
All samples in this study were prepared from the stock of commercial MA-41P (Shchekinoazot,
Russia) and Neosepta AMX-Sb (Astom, Tokyo, Japan) membranes purchased from their respective
companies. Both membranes contained an ion exchange matrix of the same composition,
poly(styrene-divinylbenzene) copolymer, which carried quaternary ammonium bases (Table 1).
They differed in their degrees of heterogeneity.
Data from manufacturers and from previous studies described the procedures of the commercial
production of MA-41P and AMX-Sb as follows: During the production of the MA-41P, a mixture of
powdered ion exchanger and powdered polyethylene was hot rolled between two layers of Nylon
6 reinforcing cloth. As determined from analysis of the surface fraction of the ion exchanger (based
on scanning electron microscopy visualizations of Russian heterogeneous membranes in top view),
only about 28% of the resulting membranes can conduct electrical current, even in a swollen state [17].
During the production of AMX-Sb, a mixture of finely powdered polyvinyl chloride and functionalized
monomer was applied to reinforced polyvinyl chloride cloth, then heated for polymerization [18],
so that the dimensions of heterogeneities were far smaller than in the case of the MA-41P [19] and,
before coating, almost 100% of the commercial AMX-Sb membrane was electrically conductive [20].PVC grains up to 100 nm in continuous phase of PE;
Reinforcing material
diameter [21] *; PVC cloth Nylon 6 cloth [22]
Ion exchange capacity, mM/g (swollen sample) 1.28 ± 0.05 1.25 ± 0.05
Thickness, µm (swollen sample) 134 ± 3 545 ± 3
Resistance in 0.02 M NaCl solution (calculated
46 52
from current–voltage curves), Ohm
Membranes 2019, 9, 13 3 of 13
* From visualization of the surface of cation exchange membranes prepared by the same method.
Commercial membranes used in this study were chosen based on their broad use for
electrodialysis. 1. Properties
Table The AMX-Sb of AMX-Sb
homogeneous and MA-41P anioncommercial
exchangesubstrate
membrane membranes.
(AEM) operating in
electrodialyzers [23]
Property is recommended by its manufacturer
AMX-Sb for a number of applications
MA-41P including
separation of monovalent ions. In Reference poly(styrene-divinylbenzene) copolymer which carries quaternaryin ED for
[24], this type of AEM is cited as widely used
Ion exchanger
food industry applications. The MA-41P belongs to the MA-41bases
ammonium series of membranes, which is
Diameter
currently of ion
the exchange
standard grains heterogeneous
Russian continuous
AEM phase
routinely used both for 5–40studies
µm [17]and in industry
PVC grains up to 100 nm in continuous phase of PE; Nylon 6
[25–27]. Reinforcing
The MA-41P membrane is known
material for its mechanical stability and durability.
diameter [21] *; PVC cloth cloth [22]
Coupled with
its low cost, these properties
Ion exchange capacity, mM/g make the MA-41P membrane promising for the production of modified
1.28 ± 0.05 1.25 ± 0.05
membranes (swollen
[28]. sample)
Thickness, µm (swollen sample) 134 ± 3 545 ± 3
Prior to modification, the membranes were equilibrated with isopropyl alcohol. The dispersion
Resistance in 0.02 M NaCl solution
of (calculated
Nafion was fromused for coating. Nafion used46in this study was perfluorinated
current–voltage 52 ionomer, which
contains sulfonic
curves), groups.
Ohm To produce the modifier, a 5% (wt.) commercial dispersion of ionomer in
water and * Fromalcohols bought
visualization of thefrom
surfaceSigma-Aldrich
of cation exchange was diluted prepared
membranes fivefoldbywith isopropyl
the same method.alcohol. The
resulting dispersion was spread to produce the modifying layer as thin as possible (differing here
from the approach
Commercial used toused
membranes produce in this the study
asymmetric were bipolar
chosen membranes
based on their with relatively
broad usethick for
modifying layers [8]) onto the 5 cm × 5 cm samples
electrodialysis. The AMX-Sb homogeneous anion exchange membrane (AEM) operating in fixed to glass; the membranes were then left to
dry in air for 24 h. For spreading, the selected volume (0.25 mL
electrodialyzers [23] is recommended by its manufacturer for a number of applications including in this case) was applied at the center
of the samples
separation and fixed
of monovalent to glass
ions. (which rests
In Reference [24],at this
the level
type surface),
of AEM is then theas
cited dispersion
widely used was inmanually
ED for
spread onto the membrane with a spatula (Figure 1) and left to
food industry applications. The MA-41P belongs to the MA-41 series of membranes, which is currentlydry.
Previous
the standard Russian preparation
heterogeneous of the modified
AEM routinely membranesused bothfollowing this protocol
for studies showed that
and in industry the
[25–27].
thickness of the single layer applied to homogeneous membranes
The MA-41P membrane is known for its mechanical stability and durability. Coupled with its does not exceed 5 µm [29]. After
that, the membranes were soaked in concentrated (6.15 M, or 36%) NaCl solution, and every 2 h the
low cost, these properties make the MA-41P membrane promising for the production of modified
solution was diluted two-fold. After four such dilutions, the membranes were transferred into 0.02
membranes [28].
M NaCl solution and left for 16 h.
Prior to modification, the membranes were equilibrated with isopropyl alcohol. The dispersion of
It should be noted that, according to the row of catalytic activity of functional groups [30], neither
Nafion was used for coating. Nafion used in this study was perfluorinated ionomer, which contains
sulfonic groups of Nafion nor quaternary ammonium bases of AMX-Sb/MA-41P boost the H+/OH−
sulfonic groups. To produce the modifier, a 5% (wt.) commercial dispersion of ionomer in water and
ions generation reaction.
alcohols bought from Sigma-Aldrich was diluted fivefold with isopropyl alcohol. The resulting
The thicknesses were measured for swollen commercial membranes and swollen modified
dispersion
membranes was withspread to produce However,
a micrometer. the modifying it waslayer
found asthat
thintheas modification
possible (differingdid nothere leadfrom
to a
the approach used to produce the asymmetric bipolar membranes
detectable change in thickness, and that the thickness of commercial AMX-Sb and all composite with relatively thick modifying
layers [8]) onto the
membranes based 5 cmon×it 5were
cm samples
in range fixedof 134to± glass;
3 µm, the whilemembranes
thickness were then left to
of commercial dry in and
MA-41P air for
all
24 h.composite
For spreading, the selected volume (0.25 mL in this case) was applied
membranes based on it were in the range of 545 ± 3 µm (Table 1). Visualizations of cross at the center of the samples
and sections
fixed to glass
of MA-41P(whichmembranes
rests at themodified
level surface),with athen thelayer
single dispersion was manually
of polyelectrolyte spread
(Figure 1e)onto the
showed
membrane
that thewith a spatula
thickness of dry (Figure
layers 1) and left
applied to to dry.
heterogeneous membranes was about 1 µm.
Nafion chain
spatula
solvent
reinforcing
cloth
inert binder
ion exchanger
Membranes 2019, 9, x FOR PEER REVIEW 4 of 13
(a) (b) (c)
(d) (e) (f)
Figure1.1. Sketch
Figure Sketch of
of the
the process
process of modification
modification (a–d)
(a–d)and
andfragments
fragmentsofofSEM SEMimages
images of of
resulting
resulting
membranes. (a) The commercial membrane to be subjected to modification (the
membranes. (a) The commercial membrane to be subjected to modification (the sketch illustratessketch illustrates thethe
caseofofAMX-Sb);
case AMX-Sb);(b)(b)the
the modifying
modifying dispersion
dispersion isisapplied;
applied;(c)
(c)ititisismanually
manually spread
spreadwith
witha spatula;
a spatula;(d)(d)
thesolvent
the solventevaporates,
evaporates, leaving
leaving the relatively
relatively thin
thinmodifying
modifyinglayer.
layer.(e)
(e)AAfragment
fragment ofof
ananSEMSEM image
image
portrayingaacross
portraying crosssection
section of
of MA-41P
MA-41P membrane
membrane modified
modifiedwith
withone onelayer
layerofofpolyelectrolyte;
polyelectrolyte; (f) (f)
AA
fragmentofofan
fragment anSEM
SEMimage
imageshowing
showing aa top
top view
view ofof the
the same
samemembrane.
membrane.
2.2. Current–Voltage (I–V) Curves
Current–voltage (I–V) curves of studied samples were obtained using a flow-through 4-chamber
cell (Figure 2). In all cases tested, the AEM formed a desalination chamber with a Neosepta CMX
cation exchange membrane. On the other side, the concentration chamber was separated from theMembranes 2019, 9, 13 4 of 13
Previous preparation of the modified membranes following this protocol showed that the
thickness of the single layer applied to homogeneous membranes does not exceed 5 µm [29]. After that,
the membranes were soaked in concentrated (6.15 M, or 36%) NaCl solution, and every 2 h the solution
was diluted two-fold. After four such dilutions, the membranes were transferred into 0.02 M NaCl
solution and left for 16 h.
It should be noted that, according to the row of catalytic activity of functional groups [30], neither
sulfonic groups of Nafion nor quaternary ammonium bases of AMX-Sb/MA-41P boost the H+ /OH−
Membranes 2019, 9, x FOR PEER REVIEW 4 of 13
ions generation reaction.
The thicknesses were measured for swollen commercial membranes and swollen modified
membranes with a micrometer. However, it was found that the modification did not lead to a detectable
change in thickness, and that the thickness of commercial AMX-Sb and all composite membranes
based on it were in range of 134 ± 3 µm, while thickness of commercial MA-41P and all composite
(d) (e) (f)
membranes based on it were in the range of 545 ± 3 µm (Table 1). Visualizations of cross sections
Figure 1. Sketch of the process of modification (a–d) and fragments of SEM images of resulting
of MA-41P membranes modified with a single layer of polyelectrolyte (Figure 1e) showed that the
membranes. (a) The commercial membrane to be subjected to modification (the sketch illustrates the
thickness of dry
case of AMX-Sb); (b) theto
layers applied heterogeneous
modifying dispersion ismembranes was about
applied; (c) it is manually 1 with
spread µm.a spatula; (d)
the solvent evaporates, leaving the relatively thin modifying layer. (e) A fragment of an SEM image
2.2. Current–Voltage (I–V)
portraying Curves
a cross section of MA-41P membrane modified with one layer of polyelectrolyte; (f) A
fragment of an SEM image showing a top view of the same membrane.
Current–voltage (I–V) curves of studied samples were obtained using a flow-through 4-chamber
cell (Figure2.2.2).
Current–Voltage
In all cases(I–V) Curvesthe AEM formed a desalination chamber with a Neosepta CMX
tested,
cation exchange membrane.(I–V)
Current–voltage Oncurves
the other side,
of studied the concentration
samples chamber
were obtained using was separated
a flow-through 4-chamber from the
cell (Figure 2). In all cases tested, the AEM formed a desalination chamber
electrode chamber by the MA-41P membrane. The operating area was 2 cm × 2 cm, the intermembrane with a Neosepta CMX
cation exchange membrane. On the other side, the concentration chamber was separated from the
distance was 0.65 cm, and the operating solution was 0.02 M NaCl pumped at a flow rate of 0.46 cm/s.
electrode chamber by the MA-41P membrane. The operating area was 2 cm × 2 cm, the
The polarizing electrodes
intermembrane were was
distance made 0.65ofcm,
polished platinum
and the operating 2 cm ×
solution was2 cm
0.02 and thepumped
M NaCl measurementat a flow electrodes
were closed rateAg/AgCl
of 0.46 cm/s.electrodes
The polarizing connected
electrodesto Luggin
were made capillaries, the tips
of polished platinum of which
2 cm × 2 cm and were the located at
measurement electrodes were closed Ag/AgCl electrodes connected to Luggin
both sides of the studied membrane against its geometrical center. The distance from the surface capillaries, the tips of was
which were located at both sides of the studied membrane against its geometrical center. The distance
around 0.5frommm.theThe setup was laboratory-assembled and based on the setup described
surface was around 0.5 mm. The setup was laboratory-assembled and based on the setup
in more detail
in [31,32], described
with severalin morechanges
detail in in equipment.
[31,32], with several changes in equipment.
Figure 2. Principal scheme of the flow-through four-chamber electrodialysis cell (1) with an anion
Figure 2. Principal scheme of the flow-through four-chamber electrodialysis cell (1) with an anion
exchange membrane (AEM)(AEM)
exchange membrane under study
under study(A*);
(A*);auxiliary cation
auxiliary cation exchange
exchange (C) and(C) andexchange
anion anion exchange
(A) (A)
membranes; tanks with
membranes; 0.02
tanks M0.02
with NaCl solutions
M NaCl (2,3);
solutions Luggin
(2,3); Luggin capillaries
capillaries (4)(4) with
with Ag/AgCl
Ag/AgCl electrodes (5);
electrodes
an Autolab(5);PGSTAT
an Autolab PGSTAT
N100 N100source/voltmeter
power power source/voltmeter (6);and
(6); and aa pH-sensitive
pH-sensitive electrode (7) connected
electrode (7) connected to
to a Mettler Toledo SevenCompact S220 pH meter (8). Adapted from [32].
a Mettler Toledo SevenCompact S220 pH meter (8). Adapted from [32].
I–V curves were obtained in galvanodynamic mode. The current density was swept from 0 to
I–V curves were
6.0 mA/cm obtained
2, increasing in galvanodynamic
stepwise mode.Autolab
at 2.5 µA/cm2 per second. The current
PGSTATdensity
N100 waswas
used swept
both to from 0 to
2 2
6.0 mA/cm , increasing stepwise at 2.5 µA/cm per second. Autolab PGSTAT N100 was used both toMembranes 2019, 9, 13 5 of 13
create the current sweep and to register the potential drop over the membrane. The theoretical limiting
current density was calculated by the Lévêque equation [33]:
Membranes 2019, 9, x FOR PEER REVIEW 5 of 13
" 2 1/3 #
z C FD h drop
V0 over the membrane. The theoretical
create the current sweep and
jlim = 1 1 the potential
theorto register
1.47 − 0.2 ,
h ( T
limiting current density was calculated by − t
1 the ) LD
1 Lévêque equation [33]:
where z1 and C1 are the charge and molar concentration ℎ of counterion in solution bulk, respectively;
= 1.47 − 0.2 ,
ℎ −
F is the Faraday constant; D is the diffusion coefficient of salt in solution; h is the intermembrane
distance; T1 and t1 are the counterion transport number in the membrane and solution, respectively;
where z1 and C1 are the charge and molar concentration of counterion in solution bulk, respectively;
V 0 is the linear solution pumping rate; and L is the length of the desalination path. The D value was
F is the Faraday constant; D is the diffusion coefficient of salt in solution; h is the intermembrane
takendistance;
equal toT1that at infinite dilution and 25 ◦ C, which is 1.61 × 10−9 m2 /s. For the experimental
and t1 are the counterion transport number in the membrane and solution, respectively;
V0 is theT1linear
conditions, was solution
assumedpumping
to be equalrate; to 1 (since
and L is thethe membranes
length are highlypath.
of the desalination selective
The Dfor counterions
value was
in such
takendilute solutions,
equal and
to that at at currents
infinite dilution close
andto25the °C,limiting
which iscurrent
1.61 × density
10−9 m2/s. the
Forinput of water splitting
the experimental
is found to be negligible),
conditions, Т1 was assumed t1 toto0.603,
be equal h to
to 16.3 mm,
(since theand L to 2.0 are
membranes cm.highly
The calculated
selective forlimiting current
counterions
in such dilute
density was 3.12 mA/cm . 2
solutions, and at currents close to the limiting current density the input of water
splitting
To is found
determine to be
such negligible),
parameters ast1experimental
to 0.603, h to 6.3 mm, and
limiting L to 2.0
current cm. The
density and calculated limiting
the current at which
current density was 3.12 mA/cm 2.
the coupled effects of concentration polarization arise, the following procedure was employed:
To determine such parameters as experimental limiting current density and the current at which
The I–V curves were built in ∆φ vs. j coordinates, where ∆φ was the experimental potential
the coupled effects of concentration polarization arise, the following procedure was employed:
difference registered between Luggin capillaries and j was the experimental current density (Figure 3).
The I–V curves were built in Δϕ vs. j coordinates, where Δϕ was the experimental potential
Then,difference
the tangents were drawn
registered betweentoLuggin
the initial sectionand
capillaries of the I–V
j was thecurve (the so-called
experimental currentohmic
densityregion),
(Figureto the
initial3).section of the plateau region and to the end of the overlimiting currents
Then, the tangents were drawn to the initial section of the I–V curve (the so-called ohmic region), region. The slope of
the first
to the initial section of the plateau region and to the end of the overlimiting currents region. The slope due
tangent depends on the resistance of the membrane. It shows if the resistance changed
to modification. The intersection
of the first tangent depends on the ofresistance
tangentsofdrawn to the ohmic
the membrane. It shows region
if the and to thechanged
resistance plateau region
due
givestothe modification.
experimental Thelimiting
intersection of tangents
current. drawn to the
The intersection of ohmic
tangents region
drawn andto to the
the plateau
plateau region
region and
to thegives the experimental
overlimiting currentslimiting
regioncurrent.
givesThetheintersection
point where of tangents drawn to
the transition tothe
theplateau region and
overlimiting current
to the
range occurs. overlimiting currents region gives the point where the transition to the overlimiting current
range occurs.
6
5
4
j (mA/cm2)
3
2
1
0
0 0.5 1 1.5 2 2.5
∆φ (V)
Figure 3. Determination
Figure 3. Determination of of
thetheexperimental
experimental limiting currentand
limiting current and the
the current
current at which
at which the coupled
the coupled
effects of concentration
effects polarization
of concentration polarizationarise. TheThe
arise. resistance
resistanceof of
thethe
membrane
membranecan
canalso
alsobe
bedetermined
determined from
the slope
from of
thethe tangent
slope of the drawn
tangent to the initial
drawn part ofpart
to the initial theofcurve.
the curve.
In allIncases, the the
all cases, difference
differenceininpHpHbetween
between the outletand
the outlet andthe
theinlet
inletof of
thethe desalination
desalination chamber
chamber
was registered
was registered together
togetherwith
withthe
theI–V
I–Vcurves
curves with
with aa Mettler
MettlerToledo
ToledoSevenCompact
SevenCompact S220S220 pH meter
pH meter
(Shanghai,
(Shanghai, China).
China). Before
Before thethe theoreticallimiting
theoretical limiting current
currentwas
wasreached,
reached, pHpH value waswas
value recorded each each
recorded
5 min (starting at the beginning of the experiment at 0 min). When the limiting current
5 min (starting at the beginning of the experiment at 0 min). When the limiting current was reached was reached
and the water dissociation reaction was expected to intensify, the lap between the measurements was
and the water dissociation reaction was expected to intensify, the lap between the measurements
reduced to 2 min. Since the cation exchange membrane in all tests was the same (strongly acidicMembranes 2019, 9, 13 6 of 13
was reduced to 2 min. Since the cation exchange membrane in all tests was the same (strongly acidic
Membranes 2019, 9, x FOR PEER REVIEW 6 of 13
Neosepta CMX), the pH-metry data were used to evaluate the relative rate of generation of H+ and
OH−Neosepta
ions on theCMX),AEM. the pH-metry data were used to evaluate the relative rate of generation of H+ and
OH− ions on the AEM.
3. Results and Discussion
3. Results
Figure and Discussion
4 gives the I–V curves of studied membranes. It should be noted that even the experimental
limiting current
Figure of the commercial
4 gives MA-41P
the I–V curves heterogeneous
of studied membranes.membrane
It should wasbelower
notedthan
that the theoretical
even the
experimental
limiting limiting current
current calculated by theofLévêque
the commercial MA-41P
equation. Thisheterogeneous
can be explainedmembrane was lower
by a higher than
local current
the theoretical limiting current calculated by the Lévêque equation. This can be explained
density at the conductive zones due to the surface being partially occupied by nonconductive material,by a higher
localtocurrent
leading higherdensity at the conductive
concentration polarization zones
anddue to thedecrease
a faster surface being partially
of salt occupiednear
concentration by the
nonconductive material, leading to higher concentration polarization and a faster decrease of salt
membrane surface.
concentration near the membrane surface.
Comparison of the I–V curves that were measured for homogeneous and heterogeneous
Comparison of the I–V curves that were measured for homogeneous and heterogeneous
membranes showed
membranes showed that in in
that general
generalthe
thechanges
changes that occurredwith
that occurred withelectrochemical
electrochemical properties
properties (due (due
to their
to their modification with polymer carrying the functional groups oppositely charged to groups in in
modification with polymer carrying the functional groups oppositely charged to groups
membrane
membranebulk) were
bulk) similar.
were However,
similar. However,there
therewere
were also peculiarities.
also peculiarities.
6
5
increasing nb. 0 1
4 of layers 2
j (mA/cm2)
3
3
j lim theor
2 no coating
1 layer
1 2 layers
3 layers
0
0 0.5 1 1.5 2 2.5 3
∆φ (V)
(a)
6
3 2
1
5
4 0
j (mA/cm2)
3 0 j lim theor
1
no coating
2 2
3 1 layer
increasing nb.
1 2 layers
of layers
3 layers
0
0 0.5 1 1.5 2 2.5 3
∆φ (V)
(b)
Figure
Figure 4. I–V
4. I–V curves
curves of of
thethehomogeneous
homogeneous AMX-Sb
AMX-Sb membrane
membraneand andits its
modifications (a) and
modifications the the
(a) and
heterogeneous MA-41P membrane and its modifications (b). Arrows show the
heterogeneous MA-41P membrane and its modifications (b). Arrows show the increased number increased number of of
changes occurring with the additional layers of Nafion applied. The dashed line shows the theoretical
changes occurring with the additional layers of Nafion applied. The dashed line shows the theoretical
limiting current calculated by the Lévêque equation. Numerals denote the number of applied layers
limiting current calculated by the Lévêque equation. Numerals denote the number of applied layers
ranging from zero (no coating, i.e., the commercial membrane) to three.
ranging from zero (no coating, i.e., the commercial membrane) to three.Membranes 2019, 9, 13 7 of 13
Membranes 2019, 9, x FOR PEER REVIEW 7 of 13
Firstly, it can be seen that in all cases the experimental limiting current density decreased with
Firstly, it can be seen that in all cases the experimental limiting current density decreased with
the increasing number of layers. Such changes are typical for monopolar ion exchange membranes
the increasing number of layers. Such changes are typical for monopolar ion exchange membranes
transforming into bipolar membranes due to the increasing thickness of the oppositely charged layer [8]
transforming into bipolar membranes due to the increasing thickness of the oppositely charged layer
when [8]
thewhen
bipolar
the junction blocksblocks
bipolar junction the transport of salt
the transport ions.ions.
of salt
Secondly,
Secondly, in all cases, the differential resistance ofmembranes,
in all cases, the differential resistance of whichcould
membranes, which couldbe be determined
determined fromfrom
the slope of theof I–V
the slope curve
the I–V in the
curve underlimiting
in the underlimitingcurrents
currents region, didnot
region, did notstrongly
strongly change
change withwith
the the
application of layers.
application However,
of layers. However, thethecharacter
character of
of changes thatoccurred
changes that occurredwithwith modifications
modifications of the
of the
AMX-Sb membrane differed from the character of changes that occurred with
AMX-Sb membrane differed from the character of changes that occurred with modifications of the modifications of the
MA-41PMA-41P membrane.
membrane. Properties
Properties of the
of the MA-41Pchanged
MA-41P changed gradually
gradually (the
(theexperimental
experimental limiting currents
limiting currents
and currents at which the overlimiting current range starts to decrease step by
and currents at which the overlimiting current range starts to decrease step by step with each step with each newlynewly
applied layer), while properties of the commercial AMX-Sb and samples modified with one or two
applied layer), while properties of the commercial AMX-Sb and samples modified with one or two
layers of polymer only slightly differed, and the significant decline occurred only after three layers
layers of polymer only slightly differed, and the significant decline occurred only after three layers
were applied (Figure 5).
were applied (Figure 5).
5
4
j (mA/cm2)
3
2
1
0
0 1 2 3
Number of layers
(a)
5
4
j (mA/cm2)
3
2
1
0
0 1 2 3
Number of layers
(b)
exp
FigureFigure 5. Dependence
5. Dependence of experimental
of experimental limitingcurrent
limiting current densities
densities ( (i , ,circles) and current densities
lim circles) and current densities
of transition to the overlimiting current range (i , squares) on
of transition to the overlimiting current range (icoupled , squares) on the number
coupled the number of of
applied layers
applied of of
layers
polyelectrolyte for AMX-Sb (a) and MA-41P
polyelectrolyte for AMX-Sb (a) and MA-41P (b). (b).
Data
Data of of pH-metry
pH-metry showedshowed how water
how water splitting
splitting was balanced
was balanced in theindesalination
the desalination chamber
chamber between
between a cation and an anion exchange membrane. The H+ ions produced at the AEM and the OH −
a cation and an anion exchange membrane. The H ions produced at the AEM and the OH− ions
+
produced at the cation exchange membrane entered the desalination chamber, and acidification
of desalted solution provided evidence that the reaction occurred more strongly at anion exchangeMembranes 2019, 9, x FOR PEER REVIEW 8 of 13
ions produced at the cation exchange membrane entered the desalination chamber, and acidification
Membranes 2019, 9, 13 8 of 13
of desalted solution provided evidence that the reaction occurred more strongly at anion exchange
membranes, while alkalification of desalted solution showed that the reaction occurred more strongly
membranes, while alkalification
at cation exchange membranesof[34]. desalted
Sincesolution
the cationshowed that the
exchange reaction occurred
membrane (Neosepta more
CMX)strongly
was the at
cation exchange membranes [34]. Since the cation exchange membrane (Neosepta
same in all measurements, the difference in pH between the exit and the entrance of the desalination CMX) was the same
inchamber
all measurements,
could be the useddifference
to comparein pHthe
between the exit
intensity and thesplitting
of water entranceatofdifferent
the desalination chamber
anion exchange
could be used to compare the intensity of water splitting at different anion exchange membranes.
membranes.
−−
In
Inthe
thecase
caseofofcommercial
commercialmonopolar
monopolar membranes,
membranes, it is known
it is known [34] that
[34] thethe
that generation
generation of+H
of H /OH+/OH
ions
ionsoccurs
occursonly
onlyatatthe
themembrane/desalted
membrane/desalted solution solution boundary.
boundary.In Inthe
thecase
caseof ofmonopolar
monopolarmembranes
membranes
modified
modifiedwithwithpolyelectrolyte
polyelectrolytecarrying
carryingthethefixed
fixedgroups
groupswith withthethecharge
chargeopposite
oppositeto tomembrane
membranebulk, bulk,
the
thereaction
reactioncancanoccur
occurboth
bothatatthe
thesubstrate
substratemembrane/modifying
membrane/modifying layer layer (bipolar
(bipolarjunction)
junction)ororatatthe
the
modifying
modifyinglayer/desalinated
layer/desalinated solution
solution boundaries
boundaries (Figure
(Figure6). 6). The
Theelectrolyte
electrolyteconcentration
concentrationdecreases
decreases
faster
fasteratatthe
thebipolar
bipolarjunction
junctionsince
sincethe
thedelivery
deliveryof ofsalt
saltions
ionstotothis
thisinterface
interfaceisishindered
hinderedfromfrombothboth
directions
directionsby byelectrostatic
electrostaticrepulsion
repulsionforces,
forces,meaning
meaningthatthatthe thelimiting
limitingstatestateisisreached
reachedfaster
fasteratatthis
this
boundary.
boundary. ThisThis first
firstlimiting
limitingstate
statecorresponds
correspondstotothe theexperimental
experimentallimitinglimitingcurrent.
current. With
Withfurther
further
increase
increaseofofpotential
potentialdrop,drop,twotwosituations
situationsare arepossible:
possible: either
either growing
growing potential
potential drop
drop will
willcause
cause
increased
increased transport
transport of of salt
salt ions
ionsbybythe
theelectromigration
electromigrationmechanism
mechanism(despite (despite counterions
counterions for
for thethe
substrate
substratemembrane
membranebeing beingco-ions
co-ionsfor
forthe
themodifying
modifyinglayer),
layer),or orgrowing
growingpotential
potentialdrop
dropwill
willcause
cause
intensive /OH−−ions
intensiveHH++/OH ionsgeneration
generationatatthethebipolar
bipolarjunction.
junction.
DBL DBL
Na+ Cl-
Na+ Cl- Na+ Cl-
ML
(a) (b)
DBL
Na+ Cl-
ML
(c)
Figure6.6.Limiting
Figure Limitingstates
statesreached
reachedatatthe
thecommercial
commercialmembrane
membrane(a), (a),atatthe
themodifying
modifyinglayer/substrate
layer/substrate
membrane
membraneboundary
boundaryofofthethemodified
modifiedmembrane
membrane(b) (b)and
andatatthethemodifying
modifyinglayer/desalted
layer/desaltedsolution
solution
boundary
boundaryofofthe
themodified
modifiedmembrane
membrane(c). (c).Pink
Pinkzone
zonedenotes
denotesthethesubstrate
substratemembrane,
membrane,DBL DBLdenotes
denotes
the
the(depleted)
(depleted)diffusion
diffusionboundary
boundarylayer
layerandandMLMLdenotes
denotesthe themodifying
modifyinglayer. layer.Black
Blacklines
linesshow
showthe
the
concentration
concentrationprofile
profileofofsalt
saltand
andgrey
greylines
linesshow
showthe
thefluxes
fluxesofofsalt
saltions.
ions.
The
Theexperimental
experimentalpH-metry
pH-metrydata
dataare
aregiven
givenininFigure
Figure7.7.Dependences
Dependencesof ofpH
pHdifference
differenceare
aregiven
given
in Figure 7a,b together with experimental limiting current values, and it can be seen that
in Figure 7a,b together with experimental limiting current values, and it can be seen that intensive intensive
water
watersplitting
splittingstarted
startedmuch
muchlater
laterthan
thanthe
thelimiting
limitingcurrents
currentswerewerereached.
reached.In Inthese
thesecases,
cases,therefore,
therefore,
the + −
the bipolar junction did not determine the beginning of H /OH ions generation. In all caseswater
bipolar junction did not determine the beginning of H /OH
+ − ions generation. In all cases water
splitting
splittingstarted
startedininthe
thesame
samerelatively
relativelysmall
smallwindow
windowof ofpotential
potentialdrops
dropsover
overthethemembrane
membrane(denoted
(denoted
by empty black boxes in Figure 7c,d), and the corresponding current value decreased only due to
increasing resistance from the system. This dependence of critical potential drop on the number ofMembranes 2019, 9, x FOR PEER REVIEW 9 of 13
by empty2019,
Membranes black
9, 13boxes in Figure 7c,d), and the corresponding current value decreased only due
9 of to
13
increasing resistance from the system. This dependence of critical potential drop on the number of
layers shows that the desalted solution/modifying layer boundary plays a more important role in
layers shows of
development thatH+the
/OHdesalted solution/modifying layer boundary plays a more important role in
− ions generation.
development of H+ /OH− ions generation.
j (mA/cm2)
0
0 2 4 6
∆pH
-1
no coating
1 layer
2 layers
3 layers
-2
(a)
j (mA/cm2)
0
0 2 4 6
∆pH
-1
no coating
1 layer
2 layers
3 layers
-2
(b)
∆φ (V)
0
0 1 2 3 4
∆pH
-1
no coating
1 layer
2 layers
3 layers
-2
(c)
Figure 7. Cont.Membranes 2019, 9, x FOR PEER REVIEW 10 of 13
Membranes 2019, 9, 13 10 of 13
Membranes 2019, 9, x FOR PEER REVIEW 10 of 13
∆φ (V)
0
0 1 2 3 ∆φ (V)
4
∆pH∆pH
0
0 1 2 3 4
-1
-1 no coating
1 layer
2no
layers
coating
31layers
layer
2 layers
-2 3 layers
-2 (d)
Figure 7. Dependence of the difference in pH between (d) the exit and entrance of the desalination
chamber formed by CMX cation exchange membranes and AEMs based on AMX-Sb (a,c) and MA-
Figure7.7.Dependence
Figure Dependence of of
thethedifference in pH
difference inbetween
pH betweenthe exit
theand entrance
exit of the desalination
and entrance chamber
of the desalination
41P (b,d) on current density (a,b) or potential drop over the membrane (c,d). Crosses show the
formed by CMX cation exchange membranes and AEMs based on AMX-Sb
chamber formed by CMX cation exchange membranes and AEMs based on AMX-Sb (a,c) and (a,c) and MA-41P (b,d)MA-
on
approximations of pH made for experimental limiting current densities based on pH values registered
current density
41P (b,d) (a,b) ordensity
on current potential drop
(a,b) orover the membrane
potential drop over (c,d).
the Crosses
membrane show(c,d).
the approximations
Crosses show the of
for the two closest current densities. Squares denote the commercial membranes, diamonds represent
pH made for experimental
approximations of pH made limiting current densities
for experimental based
limiting on pH
current valuesbased
densities registered
on pH for the two
values closest
registered
the membranes with one layer of coating, triangles represent the membranes with two layers of
current densities.
for the two closestSquares denote theSquares
current densities. commercial
denote membranes,
the commercialdiamonds represent
membranes, the membranes
diamonds represent
coating, and circles represent the membranes with three layers of coating.
with one layer of coating, triangles represent the membranes with two layers
the membranes with one layer of coating, triangles represent the membranes with two layers of coating, and circles
of
represent
coating, the
and membranes
circles with
represent three
the layers
membranes of coating.
with three layers of coating.
The registered pH values (Figure 7) show the common trend between the homogeneous and
heterogeneous
The membranes: the H+/OH− 7) ions generation increased with the numberhomogeneous
of applied layers,
The registered
registered pH pH values
values (Figure
(Figure 7) show
show thethe common
common trend trend between
between the the homogeneous and and
while water splitting wasthe
heterogeneous much + stronger
− ions in the caseincreased
of modified membranes of based
appliedon the
heterogeneousmembranes:
membranes: theHH+/OH /OH− ions generation
generation increased withwith the
the number of
number applied layers,
layers,
heterogeneous
while MA-41P membrane than modified membranes based on the homogeneous AMX-Sb
whilewater
watersplitting waswas
splitting muchmuch stronger in the case
stronger of modified
in the case ofmembranes
modified based
membraneson the heterogeneous
based on the
membrane.
MA-41P membrane than modified membranes based on the homogeneous AMX-Sb membrane.
heterogeneous MA-41P membrane than modified membranes based on the homogeneous AMX-Sb
A
A hypothesis
hypothesis can
can be
be suggested
suggested regarding
regarding the
the differences
differences in
in behavior
behavior of
of AMX-Sb
AMX-Sb and
and MA-41P.
MA-41P.
membrane.
It
It was previously concluded
wasA previously concluded from the peculiarities
from the peculiarities of electrochemical properties
of electrochemical properties of
of andthe AMX-Sb
the AMX-Sb
hypothesis can be suggested regarding the differences in behavior of AMX-Sb MA-41P.
membrane
membrane modified
modified with
withNafion
Nafion that the singular
singularapplication ofof
modifier leads to to
formation notnotof
It was previously concluded fromthat
thethepeculiarities application
of electrochemicalmodifier leads
properties offormation
the AMX-Sb
aofcontinuous
a continuous and even
and even layer, but of island-like structures [29]. This is further supported by the
membrane modified with layer,
Nafionbut of the
that island-like
singularstructures
application [29]. This is further
of modifier leads tosupported
formationby nottheof
geometrical
geometrical heterogeneity
heterogeneity of
of the
the homogeneous
homogeneous membrane
membrane produced
produced by Astom,
by Astom, which
which does
does not
not have
have
a continuous and even layer, but of island-like structures [29]. This is further supported by the
aa flat
flat surface but
but instead
instead possesses
surfaceheterogeneity possesses repeating
repeating hills
hills and
and valleys
valleys [20,35]
[20,35]by(Figure
(Figure 8), where
8),which
wheredoes the
the modifier
modifier
geometrical of the homogeneous membrane produced Astom, not have
might
might be
be accumulated
accumulated in
in the
the valleys.
valleys. Since
Since the
the application
application of
of a
a second
second layer
layer does
does not
not lead
lead to
to such
such big
big
a flat surface but instead possesses repeating hills and valleys [20,35] (Figure 8), where the modifier
changes
changes in
in the
the I–V
I–V curve,
curve, it
it might
might be
be suggested
suggested that,
that, like
like coating
coating with
with the
the first
first layer,
layer, the
the second-layer
second-layer
might be accumulated in the valleys. Since the application of a second layer does not lead to such big
coating
coating does not
not lead
lead to
tocomplete
completecontinuous
continuouscoverage,
coverage,whilewhilecoating
coatingwith
with a third layer might lead
changesdoesin the I–V curve, it might be suggested that, like coating a third
with the first layer
layer, themight lead
second-layer to
to
fullfull coverage.
coverage.
coating does not lead to complete continuous coverage, while coating with a third layer might lead
to full coverage.
Figure 8. Cross section of swollen AMX-Sb membrane. Adapted from [20].
Figure 8. Cross section of swollen AMX-Sb membrane. Adapted from [20].
Smaller limiting currents and stronger water splitting found for modified membranes based on
Figure 8. Cross section of swollen AMX-Sb membrane. Adapted from [20].
Smaller
MA-41P mightlimiting currents
be explained byand stronger waterofsplitting
the heterogeneity found for modified
MA-41P—according to [17], membranes based
only about 28% on
of the
MA-41P
surface ofmight
MA-41 beseries
explained by theisheterogeneity
membranes represented by ofion
MA-41P—according to [17],
exchanger. This causes onlycurrent
higher about density
28% of
Smaller limiting currents and stronger water splitting found for modified membranes based on
the surface of areas
at conductive MA-41 series
and, as amembranes
result, higheris represented
concentration bypolarization
ion exchanger.
and This
morecauses highercoupled
pronounced current
MA-41P might be explained by the heterogeneity of MA-41P—according to [17], only about 28% of
density at conductive
effects—which areasjudging
in this case, and, asbya result,
Figurehigher
7, wereconcentration polarization
represented by and more
water splitting. pronounced
Additional factors
the surface of MA-41 series membranes is represented by ion exchanger. This causes higher current
coupled effects—which in this case, judging by Figure 7, were represented by water
that may play a role in the difference are the possible preferable sorption of polyelectrolyte on the splitting.
density at conductive areas and, as a result, higher concentration polarization and more pronounced
coupled effects—which in this case, judging by Figure 7, were represented by water splitting.Membranes 2019, 9, 13 11 of 13
grains of ion exchanger (due to electrostatic interaction between the oppositely charged fixed groups)
and lessened sorption on (uncharged) polyethylene. This would cause the thickening of the layer on
the conductive zones in comparison with a homogeneous surface, meaning that such materials might
benefit from a decrease in the amount of added modifier.
4. Conclusions
It was shown that spreading one or two thin layers of polymer, with functional groups oppositely
charged with respect to the ones in membrane bulk, onto the surface of commercial homogeneous
AMX-Sb membrane resulted in the creation of new membranes with resistance and limiting current
comparable with the original one. As such, this method is suitable for the creation of materials that
combine good transport properties of supporting membrane with monovalent selectivity, due to the
oppositely charged layer that was applied. The application of a larger number of layers or the use
of heterogeneous membrane led to a decrease of the limiting current and hastening of the onset of
overlimiting mode, accompanied by growing H+ /OH− ions generation. In all cases, the H+ /OH−
ions generation started in the same range of potential drops and this reaction was more active at the
external (modifying layer/desalted solution) boundary than at the internal (modifying layer/substrate
membrane) boundary. It was found that H+ /OH− ions generation was stronger, and that the limiting
current decreased more readily, in the case of a heterogeneous membrane. Such peculiarities would
require special attention if the heterogeneous membranes were to be used for the production of novel
modified membranes.
Author Contributions: Conceptualization, X.N.; Methodology, X.N.; Validation, V.S., D.B., and N.P.; Formal
Analysis, X.N.; Investigation, V.S. and D.B.; Resources, C.L. and N.P.; Data Curation, X.N.; Writing—Original
Draft Preparation, X.N.; Writing—Review & Editing, C.L. and N.P.; Visualization, X.N.; Supervision, C.L. and N.P.;
Project Administration, X.N.; Funding Acquisition, X.N. and C.L. All authors have approved the final article.
Funding: This research was funded by CNRS and RFBR grant number 18-58-16008_NCNIL_a.
Conflicts of Interest: The authors declare no conflict of interest.
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