Anion Exchange Membranes with Dynamic Redox-Responsive

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Anion exchange membranes with dynamic redox responsive properties Clara Capparelli, Carlos Rolando Fernandez Pulido, Raymond Lopez-Hallman, Geoffrey M. Geise, and Michael A. Hickner ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04622 • Publication Date (Web): 04 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019

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ACS Applied Materials & Interfaces

Anion exchange membranes with dynamic redox responsive properties Clara Capparelli1, Carlos R. Fernandez Pulido2, Raymond Lopez-Hallman2, Geoffrey M. Geise3, and Michael A. Hickner1,2*

Deparment of Chemical Engineering, 2Department of Material Science and Engineering, The Pennsylvania

1

State University, University Park, Pennsylvania 16802 Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904

3

*Corresponding author: Michael Hickner, 405 Steidle Building, University Park, PA 16802. E-mail: [email protected]

Key words: anion exchange membranes, redox responsive membranes, viologen, transport properties, stimuli responsive membranes

ABSTRACT Redox responsive anion exchange membranes were developed using photoinitiated free radical polymerization and reversible oxidation and reduction of viologen. The membranes were formulated using poly(ethylene glycol diacrylate) and diurethane dimethacrylate oligomers, dipentaerythritol penta-/hexaacrylate crosslinker, photoinitiators, and 4-vinylbenzyl chloride as a precursor for functionalization. In the membrane, 4,4’-bipyridinereacted with the 4-vinylbenzyl chloride residues and, subsequently, unreacted amines were methylated with iodomethane to obtain viologen as both the ion carrier and redox responsive group. Upon oxidation, viologen supports two cations, where the reduced form only contains one cation. Thus, the redox responsiveness changed the membrane ionicity by a factor of two. The area specific resistance of the membranes in the oxidized, +2, state was lower than in the reduced, +1, state. The

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resistance increased between 40.6±0.1% and 111.6±0.1%, depending on membrane thickness, with the most significant increment being a resistance change from 4.88x10-4 Ω m2 in the oxidized state to 1.03x10-3 Ω m2 in the reduced state. Membrane permselectivity in the reduced, +1, state was between 15.9±0.1% to 26.5±0.01% lower than in the oxidized, +2, state, with no change in water uptake, spanning an average of 0.87±0.02 in the oxidized state to an average of 0.7±0.01 in the reduced state. Upon reduction, membrane ion exchange capacity decreases, increasing ionic resistance and decreasing membrane permselectivity due to a reduction in fixed charge concentration without a measurable change in water uptake. This trend is not generally observed for ion exchange membranes and explains that the changes in transport properties result from changes in ionicity, not water uptake or domain size. The reversibility and stability of the stimuli responsiveness was confirmed by the absence of transport property changes after redox cycling.

1. Introduction Stimuli responsive membranes have been developed as sensors,1,2 actuators,3 and for drug delivery systems4,5. In particular, polymers with reversible properties or “switchable” responses to changes in their environment are sought for advanced applications where the transport properties of membranes can be modulated in real time in response to external conditions or triggers. With these capabilities, new polymer membranes can increase the functionality afforded by the current technology where transport properties are generally static with time or age in undesirable ways during use. Various stimuli responsive membranes have been developed, especially those that respond to pH,6 temperature,7 and light.8 In most cases, porous, commercially available membranes are modified by either “grafting from” or “grafting to” techniques, by attaching the functional or responsive moiety to the surface of a pre-formed membrane. Consequently, the responsiveness of the film is due to changes in the size of the pores that arise from the swelling or shrinkage of the grafts. For example, quadra-stimuli responsive membranes were developed by surface initiated atom transfer radical polymerization of commercial Nylon6 porous membranes.9 The responsive gates were designed as poly(N-isopropylacrylamide)-block-


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poly(methacrylic acid) (PNM) block copolymers. Because poly(N-isopropylacrylamide) presents a lower critical solution temperature (LCST) of 32 ºC, its chains undergo a reversible conformational transition from swollen above its LCST to coiled below its LCST. In the case of poly(methacrylic acid), the same type of phenomenon occurs upon protonation/deprotonation that yields similar swelling and shrinking. As a consequence, the effective pore size and permeability of the membrane change reversibly at different pH values and temperatures. Dense, non-porous ion containing membranes are also interesting candidates as platforms that present changes in their transport properties in response to changes in their environment. For example, an actuator was developed by plating both sides of a perfluorocarboxylic acid film with gold as electrode layers.10 The actuator bent to a 90º angle when a pulse voltage of 2.0 V was applied between the gold electrodes. It was proposed that the bending motion was driven by differences in water content in the membrane carried by the ions being polarized by the applied field. Additionally, potentiometric humidity sensors have been prepared from anion exchange membranes and poly(pyrrole) composite membranes.3 The test strip prepared from the composite membrane generated an electrical potential dependent on the relative humidity and the type of anion exchanged. Redox responsive membranes have been proposed as candidates for drug and gene delivery.11 A redox sensitive microporous membrane was prepared by grafting viologen-containing side chains to poly(vinylidene fluoride) porous membranes.12 Viologen moieties are chemically stable and reversibly convert from a dicationic (V2+) state to a radical cationic (V+•) state in the presence of a reducing agent, Figure 1.13,14 These porous membranes displayed higher permeation rates of 4-styrenesulfonic acid sodium salt in the radical cationic state compared to the dicationic state. It was hypothesized that the increased solubility of viologen in water in the dicationic state may cause the side chains to extend into the pores, reducing the free volume, and thus, decreasing the permeation rate.



H 2C

H 2C

N

N e

N

N

CH2

CH2

Figure 1. Viologen in its dicationic state (oxidized, V2+) and radical cationic state (reduced, V+•).

Anion exchange membranes (AEMs) have gained increased attention in the scientific community in the last 10 years, mainly due to technological advances in some areas of electrochemical separations. Accordingly, it is of interest to develop strategies to manipulate AEM transport properties in real time for advanced applications. Our group has previously investigated a strategy to influence the ionic resistance of AEMs through the modification of the membrane’s surface morphology.15 Here, we explore a chemical modification of our photopolymerizable AEMs to impart reversible redox responsive properties. This article describes the development of viologen-containing AEMs with reversible changes in ionic resistance and permselectivity in the reduced and oxidized states. Our results demonstrate that ion-containing membranes can present reversible changes in their transport properties upon changes in their environment. As explained above, most stimuli responsive membranes that have been developed were modified porous commercial membranes in which changes in pore size in response to an external stimulus resulted in changes in permeation rates of species in solution, or water flux. In this work, we have elucidated that ion-selective membranes containing viologen groups present changes in their ionicity or ion exchange capacity at the different redox states, which entails changes in both the selective transport of counter-ions and exclusion of co-ions.


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2. Experimental Materials and preparation of AEMs. The anion exchange membrane samples were prepared by photoinitiated free radical polymerization of poly(ethylene glycol) diacrylate (PEGDA, Mn 700 g/mol, 30 wt %), diurethane dimethacrylate (DUDMA, Mw 470.56 g/mol, 50 wt %), and vinyl benzyl chloride (VBC, 15 wt %) as a precursor for functionalization. This quantity of VBC yielded samples with an IEC that is representative of ion exchange membranes in the literature. Less than 15 wt % VBC gave samples that were not conductive enough and more VBC gave samples that did not have good dimensional stability. Dipentaerythritol penta-/hexa-acrylate was used as crosslinker (5 wt %). To this base resin, 1 wt % of phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide and 1 wt % of 1-hydroxycyclohexyl phenyl ketone were added as initiators. PEGDA, DUDMA, VBC, dipentaerythritol penta-/hexa-acrylate, phenylbis(2,4,6trimethylbenzoyl) phosphine oxide, 1-hydroxycyclohexyl phenyl ketone, and 4,4’-bipyridine were purchased from Sigma-Aldrich and used without further purification. Iodomethane was purchased from Fisher Scientific. The liquid resin was spread on a flat glass plate and light was projected onto the resin to initiate polymerization and crosslinking. The resin was photocured by exposing it to white light from the light source (~2800 lm brightness, Epson EX5210 projector) for 10 min. The thickness of the membranes was adjusted by using a doctor blade for spreading the resin before illumination. Samples were produced with thicknesses ranging from 80 to 200 μm. Once the membranes were obtained, they were reacted with a 7.8 wt % solution of 4,4’-bipyridine in ethanol at 60 ºC for 72 h. To methylate unreacted amine groups, the membranes were immersed in a 2 M solution of iodomethane in ethanol at 60 ºC for 48 h. The procedure for incorporating viologen in the photopolymerized membrane is depicted in Figure 2. The membranes were then washed with ethanol and equilibrated in a 0.5 M solution of NaCl for 24 h, replacing the solution three times in the 24 h period. The final membranes were stored in 0.5 M NaCl solution. Once hydrated, the thickness of the samples was measured using a digital outside micrometer (Mitutoyo IP65). For the measurement, each sample was removed from its storage solution, wiped gently to remove surface water,



and its thickness was measured at 8 different points. The thickness is reported as an average of the 8 measurements and its standard deviation was calculated.

Figure 2. Incorporation of viologen in the PEGDA-co-DUDMA-co-VBC base membrane. In order to change the membrane from the oxidized state to the reduced state, a solution of 0.1 M sodium hydrosulfite (Na2S2O4) was prepared as a reducing agent. To oxidized the samples back to their dicationic state, a solution of 0.01 M ammonium cerium nitrate ((NH4)2Ce(NO3)6) was prepared as an oxidizing agent. Infrared Spectroscopy. IR spectra of the membranes were obtained to confirm the presence of the viologen moieties in the samples and to quantify the reaction described in Figure 2. For the measurement, a portion of the viologen-containing membrane of approximately 1 cm x 1 cm was placed on a ZnSe crystal and its IR spectra was recorded on a Bruker Vertex 70 FTIR spectrometer (Bruker, Billerica, MA) using a nitrogen cooled mercury cadmium telluride detector. The spectra were collected on an attenuated total reflection (ATR) accessory at a spectral resolution of 2 cm-1 and 500 scans. An unfunctionalized membrane (PEGDA-co-DUDMA-co-VBC) was also measured to confirm the reaction between VBC and 4,4’bipyridine by comparison of the reacted and unreacted sample. Ionic resistance. The ionic resistance of the membranes was measured using a direct current (DC) 4point method as reported in our previous work.15 In order to analyze differences in ionic resistance, the measurement was performed in the oxidized state and repeated in the reduced state of the sample. Briefly, offset and membrane potential drop values were recorded at currents between 1 and 6 mA. The data was regressed to Ohm’s law after subtracing the offset from the measured values with the membrane in place. The membrane in its oxidized state was measured in 0.5 M NaCl. To reduce the viologen, the sample was immersed in the reducing solution and the ionic resistance measurement was performed in a mixed solution 6 ACS Paragon Plus Environment

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of 0.5 M NaCl and 0.1 M Na2S2O4. The offset for the membrane in the oxidized state was obtained from the measurement of the ionic resistance of the 0.5 M NaCl solution without the membrane, while the offset for the membrane in the reduced state was obtained from the ionic reasistance of the mixed solution of 0.5 M NaCl and 0.1 M Na2S2O4 without the membrane. Permselectivity. The membrane potential was measured using a previously reported method,16,17 and the membrane permselectivity was calculated from the potential measurement.18 The experimental cell used to measured permselectivity was similar to the one used for ionic resistance, as described in our previous work.15 Similar to ionic resistance, the membranes were measured in both their oxidized and reduced states. The oxidized membranes were measured using a low and a high concentration NaCl solution (0.1 M and 0.5 M, respectively), while the reduced membranes were measured using the low and high concentration NaCl solution mixed with the reducing agent (0.1 M Na2S2O4), in order to maintain the membrane in its reduced state. As explained before, the offset for the measurement in the oxidized and reduced state was measured in 0.5 M NaCl and in the mixed solution, respectively. Permselectivity of the oxidized membrane was calculated according to equation 1:

𝛼𝛼 =

⎡ ⎤ ⎢𝐸𝐸𝑚𝑚 ⎥ ⎢ � 𝑅𝑅𝑅𝑅 𝑎𝑎𝐿𝐿 ⎥+1−2𝑡𝑡𝑀𝑀 ± ⎢ � 𝐹𝐹 𝑙𝑙𝑙𝑙 0 �⎥ 𝑎𝑎± ⎦ ⎣ 2𝑡𝑡𝑋𝑋

(1)

where 𝛼𝛼 is apparent permselectivity, Em is the measured membrane potential without the offset, R is the gas

𝐿𝐿 constant, T is the absolute temperature, F is Farady’s constant, 𝑎𝑎± is the mean electrolyte activity of the 0 is the mean electrolyte activity of the high concentration solution, 𝑡𝑡𝑀𝑀 is the low concentration solution, 𝑎𝑎±

transport number of the counter-ion in the solution phase, and 𝑡𝑡𝑋𝑋 is the transport number of the co-ion in

the solution phase.18 The transport numbers and activity coefficients used for the calculation can be found in Table 1. Table 1. Transport numbers and activity coefficients used for equation 1. Salt NaCl19

Transport number at infinite dilution 𝑡𝑡𝑀𝑀 𝑡𝑡𝑋𝑋 0.397 0.603 7 ACS Paragon Plus Environment

Thermodynamic activity (mol/L) 𝐿𝐿 0 𝑎𝑎± 𝑎𝑎± 0.0778 0.341


Water uptake and swelling ratio. The water uptake of the membranes was measured in both the oxidized and the reduced states for all the samples, and calculated as the difference between the hydrated mass of the membrane in equilibrium with its storage solution (0.5 M NaCl) and the dry mass, according to equation 2: 𝑊𝑊𝑊𝑊(%) =

𝑀𝑀𝑤𝑤𝑤𝑤𝑤𝑤 −𝑀𝑀𝑑𝑑𝑑𝑑𝑑𝑑 𝑀𝑀𝑑𝑑𝑑𝑑𝑑𝑑

× 100

(2)

where WU is water uptake, Mwet is the hydrated mass and Mdry is the dry mass of the membrane. First, each sample was removed from its storage solution and cut in two pieces. Excess water was gently removed from the membrane’s surface using a Kimwipe. One of the hydrated pieces was weighed in the oxidized state (no further modification) and then placed in a vacuum oven at 50 ºC overnight. The dry mass of the oxidized sample was recorded and the water uptake was calculated. Each measurement was performed three times and the value was reported as the average with a standard deviation. The second hydrated sample piece was placed in a vial with a solution consisting of 0.5 M NaCl and 0.1 M Na2S2O4 to obtain the membrane in the reduced state. The membrane was left in the presence of the reducing agent for 20 min to ensure full reduction. The membrane was removed from the solution, excess water was gently wiped off the membrane, and the hydrated mass was quickly measured. The membrane was placed in a vacuum oven at 50 ºC overnight to dry. The dry mass of the reduced sample was recorded and the water uptake was calculated according to equation 2. Each measurement was performed three times and the value was reported as the average and standard deviation. Three 1 cm x 1 cm membrane samples were used to measure the swelling ratio of the material in both the oxidized and reduced states. Their lengths in the hydrated and dry states were measured for the samples in the oxidized state and then in the reduced state after being reduced by the mixed solution. The swelling ratio was obtained as follows: 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 =

𝐿𝐿𝑤𝑤𝑤𝑤𝑤𝑤 −𝐿𝐿𝑑𝑑𝑑𝑑𝑑𝑑 𝐿𝐿𝑑𝑑𝑑𝑑𝑑𝑑

𝑥𝑥 100 (3)

where 𝐿𝐿𝑤𝑤𝑤𝑤𝑤𝑤 is the length in the hydrated state and 𝐿𝐿𝑑𝑑𝑑𝑑𝑑𝑑 is the length in the dry state.20 8

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3. Results and discussion Redox responsive AEMs. Anion exchange membranes were obtained by free radical photopolymerization. Post-curing functionalization of the membranes with 4,4’-bipyridine and iodomethane was achieved, as depicted in Figure 2. IR spectra of an unfunctionalized and a functionalized membrane were compared to confirm the success of reaction, Figure 3. The peak at 1638 cm-1 was attributed to quaternized 4-vinylpyridine groups, probably due to C=C and C=N stretching vibrations in the pyridine ring.21 Additionally, the peak at 670 cm-1 is likely due to C-Cl stretching vibrations from VBC in the unfunctionalized membrane.22 These two peaks confirm the introduction of the viologen moiety in the membrane.

0.7

Before Reaction With Viologen

0.6 0.5 Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.3 0.2

670 cm

1638 cm

-1

-1

0.1 0.0

800

1000

1200

1400

1600

1800

Wavenumber (cm-1) Figure 3. IR spectra of unfunctionalized (black) and functionalized (red) membrane.

In order to quantitatively analyze the reaction conversion in Figure 2, the reaction was monitored by following the peak corresponding to C-Cl symmetric stretching (νs) vibrations (670 cm-1) before and after 9 ACS Paragon Plus Environment


the reaction. When succesful incorporation of 4,4’-bipyridine occurs, the halide is dettached from the membrane matrix. The total absorbance of this peak after normalization was compared at the beginning and at the end of the reaction, and it was confirmed that 93.4±1.6% of the vinylbenzylchloride groups succesfully reacted with 4,4’-bipyridine. We assume that nearly all remaining tertiary amines were methylated since iodomethane was incorporated in excess and quaternization of tertiary amines by halogenated hydrocarbons (Menshutkin reaction) has been reported to be fast and complete.23–25 The incorporation of viologen into the membrane structure was also confirmed by observing a colorimetric change between the membrane in the two different redox states.12 In the oxidized state, the membrane presented an orange color; upon reduction by the reducing solution, the membrane changed to a deep blue color, as demonstrated by Figure 4.

Figure 4 . Colorimetric change between a membrane sample in its oxidized (left) and reduced (right) state.

The color was maintained in open air for about 2 min before starting to fade due to oxidation caused by the presence of oxygen. Similarly, the reduced membrane submerged in 0.5 M NaCl maintained its color, and thus, its reduced state, for approximately 2.5 min before beginning to oxidize back to the oxidized state, due to the presence of dissolved oxygen in water. For this reason, the measurement of the transport properties in the reduced state was performed in the presence of the reducing agent. To analyze the extent of ionization in the samples, the FTIR spectra of a membrane sample in its oxidized and reduced states were compared. First, the IR spectrum of the sample was measured in its oxidized state, then reduced with the reducing agent and the spectrum was recorded again. The pyridinium ring vibration (ν8b) peak at 1638 cm1

for each spectrum was compared after normalization.26,27 It was observed that the absorbance of this peak 10 ACS Paragon Plus Environment

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in the oxidized state of the membrane was 1.92±0.09 greater than the absorbance of the peak in the reduced state, confirming that the oxidized state contains approximately double the ions as the reduced state, Figure S1. The ionic resistance of an anion exchange membrane depends on the nature and concentration of the fixed ions, the interaction between the mobile ions and the fixed ions, and the water content in the membrane.28 As the concentration of the fixed ions increases, the ionic resistance generally decreases, due to a greater number of charge carriers present in the membrane. More charge carriers leads to an increase in the number of sorbed counter-ions in order to maintain electroneutrality, reducing the ionic resistance of the membrane. Similarly, high values of water uptake favor the sorption of ions from solution, which increase charge carrier density and decreases membrane resistance. When viologen-containing AEMs are switched from the oxidized state to the reduced state as described in Figure 1, one charged quaternary ammonium group is transformed into a radical by a one-electron reduction.13,29 Thus, the viologen residue changes from a dicationic pendant with two charges to a radical cation moiety with one charge. This change in ionicity which essentially arises from changing the IEC of the membrane, should be associated with a change in the material’s ionic resistance due to a reduction in the fixed charge concentration. In order to explore this hypothesis, the ionic resistance of the membranes was measured in the oxidized and reduced states as explained in the Materials and Methods section. Each sample was first measured in its oxidized form (dicationic) using a solution of 0.5 M NaCl in the experimental cell. Afterwards, the membrane was reduced in the mixed solution (0.5 M NaCl and 0.1 M Na2S2O4) for approximately 15 minutes and then placed in the experimental cell. The measurement in the reduced form was performed using the same mixed solution used for the reduction to ensure that the membrane was in its reduced state throughout the duration of the measurement.



2.0x10-3 Area specific resistance (Ω m2)

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Reduced Oxidized

1.5x10-3 less ionic

1.0x10-3

5.0x10-4 more ionic

0.0

60

80

100

120

140

160

180

200

220

Thickness (µm)

Figure 5. Resistance as a function of thickness for the different samples in the oxidized (blue squares) and reduced (red triangles) states.

Figure 5 shows the resistance as a function of thickness of the different samples in the reduced and oxidized states. As stated in the hypothesis, the resistance of the membranes in the oxidized state was lower than the resistance of the membranes in the reduced state, for all the samples. As explained previously, the membrane in its oxidized, more ionic form exhibits lower ionic resistance due to a higher concentration of fixed charges in the membrane (i.e., higher IEC). Oppositely, when the membrane’s ionicity is decreased by changing from a dicationic to a radical cationic form, the resistance of the membrane increases. The theoretical ion exchange capacity (IEC) of the membrane in its oxidized state is 0.64 meq/g (dry polymer), while the IEC in its reduced form is 0.32 meq/g (dry polymer). The percentage change in resistance was between 40.6±0.1% and 111.6±0.1%. No specific trend between membrane thickness and percentage change in resistance could be identified. It would be expected that the area specific resistance increases


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with thickness. We hypothesize that this could be due to interfacial resistance due to the relative low concentration of NaCl or the low water uptake of the membranes (see Supporting Information).30 In order to characterize the kinetic response of the materials during switching, a membrane sample with a thickness of 105±11 µm was placed in the ionic resistance cell and its resistance was recorded in the 0.5 M NaCl solution. The electrolyte was then quickly replaced with the mixed electrolyte/reduction solution (0.5 M NaCl and 0.1 M Na2S2O4) and the ionic resistance of the membrane was recorded as a function of time while the reduction of the sample was taking place. After observing a consistently increasing resistance approaching steady-state, the mixed solution was again replaced with 0.5 M NaCl and the membrane was oxidized by dissolved oxygen from the atmosphere, as can be observed in Figure 6.

Figure 6. Ionic resistance as a function of time for a membrane sample in its oxidized and reduced states. The dashed lines represent the incorporation or removal of the reducing agent (mixed solution), respectively.

By analyzing the increase in ionic resistance after contact with reducing agent, we estimated that the membrane reduces in approximately 1.5 min where some noise can be observed in the moments after the 13 ACS Paragon Plus Environment


replacement of the solutions. Additionally, a visual observation of the colorimetric change after the addition of the reducing agent was performed. Three 1 cm x 1 cm pieces of membrane were transferred from the 0.5 M NaCl solution to the mixed solution and the time lapsed for the colorimetric change was recorded. The samples changed after 52±16 s upon the addition of the reducing agent. This value is consistent with the observations in the kinetic resistance study, taking into account that between 0.3 and 0.6 min are required to perform the resistance measurement. The membranes changed from the reduced to oxidized states at a similar rate (48±12 s) after the reducing solution was replaced with NaCl. Membrane permselectivity is another important parameter that defines its performance. Permselectivity refers to the selective exclusion of co-ions from the membrane phase, as dictated by Donnan exclusion.16 A high activity of the counter-ions in the membrane phase due to the electrostatic interactions between the fixed charges and the mobile ions, results in a low activity of co-ions in the membrane phase as quantified by the Donnan equilibrium. The permselectivity of the samples was measured in an experimental cell containing a high concentration electrolyte solution on one side (0.5 M NaCl) and a low concentration electrolyte solution on the other side (0.1 M NaCl).15 The membrane potential was recorded as the voltage drop between each side of the membrane. In order to measure the permselectivity of the membrane in the reduced form, the membrane was first reduced in the mixed solution of NaCl and the reducing agent for 15 min, and then placed in the cell for measurement. Both compartments were then filled with mixed solutions of high and low concentrations (0.5 M NaCl and 0.1 M Na2S2O4, and 0.1 M NaCl and 0.1 M Na2S2O4, respectively). Adding equal concentrations of the reducing agent in each side of the cell ensured that the mean activity of this solution was equivalent in the high and low concentration chambers, and thus, the difference in the Donnan potential between each side was only attributed to the differences in activity between the NaCl solutions. The permselectivity of the samples was calculated using equation 1 in both the oxidized and reduced states by neglecting the presence of the reducing agent for the measurement in the reduced state. Although this assumption is not strictly accurate, due to the higher affinity of anion exchange groups to monovalent ions, we hypothesize that most of the quaternary ammonium groups will be


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exchanged to their chloride form.31 The apparent permselectivity of the samples in their oxidized and reduced states is compared in Figure 7.

more ionic

1.0 Apparent permselectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6

less ionic

0.4 0.2 0.0

Oxidized Reduced

60

80

100

120

140

160

180

200

220

Thickness (µm)

Figure 7. Apparent permselectivity of the samples as a function of thickness in the oxidized (blue) and reduced (red) state.

The apparent permselectivity of the redox responsive membranes in the oxidized state was consistent between all the samples, as expected, since the fixed charge concentration did not change between samples. The average permselectivity was 0.87±0.03. When the membranes were reduced, the permselectivity decreased for all the samples. This was expected, since the IEC and ionicity of the membrane was reduced, and thus, the effectiveness of Donnan exclusion. As the IEC is lowered, the activity of the counter-ion in the membrane phase decreases due to less electrostatic interactions between counter-ions and fixed charges, and the activity of the co-ion in the membrane phase increases, which translates in a lower permselectivity. The percentage decrease in permselectivity between the oxidized and the reduced state was between 15.9±0.1% to 26.5±0.01%.



In general, the permselectivity of a membrane decreases when the conductivity increases (i.e., ionic resistance decreases).32 However, the changes in ionicity in these samples predominate and drive the changes in transport properties. As ionicity is decreased when going from the oxidized (dicationic, +2) to the reduced (radical cationic, +1) state, ionic resistance increases and permselectivity decreases. This trend can be observed in Figure 8. Essentially, by lowering the IEC when going from the oxidized to the reduced state, the ionic resistance increases due to a lower fixed charge concentration and the permselectivity decreases due to a higher activity of the co-ions in the membrane phase.

1.0 Oxidized Reduced

Apparent permselectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.9 more ionic less ionic

0.8

0.7

0.6

5.0x10-4

1.0x10-3

1.5x10-3

2.0x10-3

Area specific resistance (Ω m2)

Figure 8. Apparent permselectivity as a function of resistance for each sample in the oxidized and reduced state.

Except for the thickest sample, the higher the percentage increase in ionic resistance in the samples, the higher the percentage decrease in permselectivity. In order words, as the difference between the resistance in the oxidized and reduced state increases, so does the difference in permselectivity. Thus, it can be


Page 17 of 27

hypothesized that the degree of change in ionicity is responsible for the change in ionic resistance and permselectivity between the oxidized and reduced states. In order to further analyze the factors behind the changes in the transport properties of redox responsive membranes, the water uptake of the membranes was measured in both the oxidized and reduced states, following the procedure described in the Materials and Methods sections. The water uptake of the samples in their oxidized and reduced states can be analyzed in Figure 9.

40 Oxidized Reduced

35 Water uptake (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30 25 20 15

60

80

100

120

140

160

180

200

220

Thickness (µm)

Figure 9. Water uptake as a function of thickness for the samples in the oxidized and reduced states.

As explained above, high values of water uptake translate into higher conductivity (i.e., lower ionic resistance) due to increased ion sorption. However, no changes in water uptake between the membranes in their oxidized and reduced states were observed. While in porous, stimuli responsive membranes changes in pore size are responsible for the changes in transport properties, these results reveal that dense, redox responsive ion exchange membranes exhibit changes in their transport properties due to changes in their ionicity. 17 ACS Paragon Plus Environment


Additionally, the swelling ratio of the membranes was calculated as described in the Materials and Methods section. The average swelling ratio was 0.03±0.14 in the oxidized state (dry membrane compared to hydrated membrane) and no differences were observed for the swelling ratio of the dry to hydrated sample in the reduced state compared to the oxidized state, indicating no large dimensional changes occurred for these materials, which are consistent with no observable changes in water uptake. Reversibility of membrane properties. Viologen undergoes a reversible conversion between the dicationic and radical cationic states, as depicted in Figure 1. Both the dicationic and radical cationic species (in the absence of O2) are highly stable, independently of pH.33 The viologen-containing membranes can be reduced and oxidized reversibly with suitable reducing and oxidizing solutions. In order to analyze the extent of the reversibility of the process, a membrane sample was subjected to a cycling experiment. In this experiment, the membrane in its oxidized state was reduced by submerging it in the appropriate reducing solution (0.1 M Na2S2O4), and then oxidized back using an oxidizing solution (0.01 M (NH4)2Ce(NO3)6). The redox change was performed for a number of times (n = 18) and the resistance in the oxidized and reduced states was measured at certain intervals (n = 1, n = 6, n = 12, and n = 18), Figure 10.

1.5x10-3 Area specific resistance (Ω m2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Reduced Oxidized

1.0x10-3

5.0x10-4

0.0

0

2

4

6

8

10

12

14

16

Cycle number


18

20

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Figure 10. Resistance of one sample in the oxidized and reduced state during a cycling experiment.

The sample chosen for the cycling experiment had an initial ionic resistance in the oxidized state of 5.16x10-4 ± 3.54x10-5 Ω m2, and 9.4x10-4 ± 6.36x10-5 Ω m2 in the reduced state. The resistance of the sample in its oxidized state remained lower than the resistance in its reduced state throughout the 18 redox cycles. The final resistance of the sample in the oxidized state was of 7.56x10-4 ± 3.55x10-5 Ω m2, while in its reduced state the final ionic resistance was of 1.14x10-3 ± 7.78x10-5 Ω m2. A slight increase in resistance at increasing cycling number can be observed, and may be related to membrane aging upon repeated exposure to the oxidizing and reducing agents. The permselectivity of the sample was also measured at the beginning and end of the cycling experiment. The initial permselectivity was 0.87±0.01 in the oxidized state and 0.69±0.01 in the reduced state. The final permselectivity was 0.89±0.01 in the oxidized state and 0.71±0.01 in the reduced state, exhibiting no changes in permselectivity at the end of the cycling experiment. These results demonstrate the good stability of viologen and the extent of the reversibility of the process, and is relevant for the possible applications of this technology. Redox responsive ion exchange membranes can be used as integrated sensors, actuators, and part of drug delivery systems. Ensuring good stability and extended reversibility of the redox responsive properties is key for the development of appropriate applications. The Young’s modulus of the membranes was 345.3±45.4 MPa and other mechanical properties are detailed in the Supporting Information, Table S2.



Figure 11. Membrane sample during a cycling experiment: a-b) initial, oxidized sample and first reduction, respectively (n = 1), c-d) n = 5, e-f) n = 7, g-h) n = 10.

Figure 11 shows the progression of a membrane sample during a cycling experiment with n = 10 cycles. It can be observed that the membrane in its oxidized state (Figure 11 a, c, e, and g) presents an even color throughout the cycling experiment; however, the dark color of the membrane in its reduced state (Figure 11 b, d, f, and h) fades throughout the cycles. The area specific resistance, however, does not present such changes as stated in Figure 10. We hypothesize that repeated reduction and oxidation may cause some loss in electroactivity of the viologen as reported previously, which could be improved by using an oxygen-free atmosphere during cycling.34

4. Conclusions Non-porous, redox responsive anion exchange membranes were developed by functionalizing a membrane prepared by light-induced free radical polymerization with 4,4’-bipyridine and methylating unreacted amines with iodomethane. Through this synthesis and fabrication process, viologen moieties were incorporated in the membrane. Viologen undergoes a change in its ionicity from +2 in its oxidized


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state to +1 in its reduced state, in the presence of an appropriate reducing agent (sodium hydrosulfite). Thus, viologen acts as both the ion carrier and the stimuli responsive group.

The ionic resistance and

permselectivity of the samples were measured in the oxidized state (+2, high ionicity) and reduced state (+1, low ionicity). The resistance of the membranes in the oxidized state was lower (i.e., higher conductivity) than for the membranes in the reduced state, with a resistance change ranging from 40.6±0.1% up to 111.6±0.1%. This was attributed to the higher IEC of the membranes in their oxidized state (0.64 meq/g) compared to the reduced state (0.32 meq/g). When the membranes were reduced in the presence of a reducing agent, the fixed charge density decreased, lowering the IEC, and thus, increasing the resistance. Similarly, the permselectivity of the membranes in the oxidized state was an average of 20.1±0.1% higher due to a higher concentration of fixed charges (higher IEC) which translates into a low activity of the coion in the membrane phase. When the ionicity was lowered by reducing the membrane, the activity of the co-ion in the membrane phase increases and the permselectivity is reduced. No changes in water uptake for the membrane between its oxidized and reduced state also supports the hypothesis that changes in IEC govern the changes in the transport properties of these dense, redox responsive ion exchange membranes. The process showed good stability and reversibility as demonstrated by the cycling experiment. These membranes have potential applications as integrated sensors in separation processes, actuators, and as drug delivery systems. By understanding the fundamentals behind the manipulation of the transport properties of ion exchange membranes, the spectrum of new membrane materials can be broadened to meet the scientific challenges of the 21st century.

Supporting information: extent of ionization, intrinsic resistance as a function of thickness, membrane potentials as a function of thickness, mechanical properties, SAXS data of an oxidized and a reduced sample.

Acknowledgements



The authors would like to thank Dr. T. J. Zimudzi for the fruitful discussions regarding FTIR measurements and Yifan Deng for his help on mechanical testing. M.A.H acknowledges the Corning Foundation for fellowship support and the Penn State Materials Research Institute and Penn State Institutes for Energy and the Environment for infrastructure support.


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For Table of Contents only:

Anion exchange membranes with dynamic redox responsive properties Clara Capparelli1, Carlos R. Fernandez Pulido2, Raymond Lopez-Hallman2, Geoffrey M. Geise3, and Michael A. Hickner1,2*

Deparment of Chemical Engineering, 2Department of Material Science and Engineering, The Pennsylvania

1

State University, University Park, Pennsylvania 16802 Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904

3

*Corresponding author: Michael Hickner, 405 Steidle Building, University Park, PA 16802. E-mail: [email protected]

Apparent permselectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

Oxidized Reduced

0.9

more ionic 0.8

less ionic

0.7 0.6

5.0x10-4

1.0x10-3

1.5x10-3

2.0x10-3

Area specific resistance (Ω m2)


Anion Exchange Membranes with Dynamic Redox-Responsive

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