Control of Ion Transport in Sulfonated Mesoporous Polymer Membranes

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Control of Ion Transport in Sulfonated Mesoporous Polymer Membranes Choongseop Jeon, Jungjin Han, and Myungeun Seo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14712 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 5, 2018

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ACS Applied Materials & Interfaces

Control of Ion Transport in Sulfonated Mesoporous Polymer Membranes

Choongseop Jeon,1 Joong Jin Han,2 Myungeun Seo1,3,4*

1Graduate

School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea 2LG 3Department

4Advanced

Chem, Daejeon 34122, Republic of Korea

of Chemistry, KAIST, Daejeon 34141, Republic of Korea

Battery Center, KI for Nanocentury, KAIST, Daejeon 34141, Republic of Korea

*To whom correspondence should be addressed: [email protected]

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Abstract We investigated proton conductivity and the permeability of monovalent cations across sulfonated mesoporous membranes (SMMs) prepared with well-defined pore sizes and adjustable sulfonic acid content. Mesoporous membranes with three-dimensionally continuous pore structure were produced by the polymerization-induced microphase separation (PIMS) process

involving

the

reversible

addition-fragmentation

chain

transfer

(RAFT)

copolymerization of styrene and divinylbenzene in the presence of polylactide (PLA) macrochain transfer agent and subsequent PLA etching. This allowed us to control pore size by varying PLA molar mass. Postsulfonation of the mesoporous membranes yielded SMMs whose pore structure was retained. The sulfonic acid content was adjusted by reaction time. While proton conductivity increased with increasing ion exchange capacity (IEC) without noticeable dependence on the pore size, ion permeability was strongly influenced by the pore size and IEC values. Decreasing pore size and increasing IEC resulted in a decrease in ion permeability, suggesting that ions traverse across the membrane via the vehicular mechanism, through the mesoporous spaces filled with water. We further observed that the permeability of vanadium oxide ion was dramatically suppressed by reducing the pore size below 4 nm, which was consistent with a preliminary vanadium redox flow battery data. Our approach suggests a route to developing permselective membranes by decoupling proton conductivity and ion permeability, which could be useful for designing separator materials for next-generation battery systems.

Keywords: Mesoporous polymer, polymerization-induced microphase separation, postsulfonation, ion transport, permselectivity, vanadium redox flow battery 2 ACS Paragon Plus Environment

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Introduction Secondary batteries such as lithium-ion batteries (LIBs), lithium–sulfur (Li–S) batteries and redox flow batteries (RFBs) are key components in modern electronic devices ranging from mobile phones to electric vehicles, and are also important in energy storage systems, because they can reversibly convert chemical energy to electricity.1,2 Redox flow battery systems in particular, like zinc-bromine flow batteries and vanadium redox flow batteries (VRFBs), are promising because they provide high efficiency, long lifetime and safety.3,4 In a typical RFB configuration, a membrane separates two electrolytes at the positive and negative electrodes to confine the reduction and oxidation reactions within each half-cell during charging/discharging cycles, and prevent self-discharging. The permselectivity of the membrane to different ions is critical to cell performance, by promoting the transfer of charge carriers and impeding undesired cross-mixing of electrolytes through the membrane. For example, a separator in VRFBs should easily transfer protons (and HSO4- or SO42- as their counter ions) to complete the electrical circuit, but prevent the cross-over of vanadium ions. This allows separate VO+/VO2+ and V2+/V3+ redox couples in the positive and negative halfcells, respectively.2,3,4 Proton exchange membranes (PEMs) such as Nafion, which contain pendent sulfonate groups along the hydrophobic perfluorinated backbone, were originally developed for the chlor-alkali process,5 and have been widely used proton exchange membrane fuel cells and VRFBs.6 In the presence of water, the hydrated sulfonate groups in Nafion form threedimensionally (3D) continuous ionic clusters in the nanometer length scale which are well segregated from the perfluorinated matrix.7,8 The ionic clusters provide high conductivity for cations by acting as ion conducting channels. However, there have been increasing efforts to replace Nafion in VRFB applications for several reasons, including high cost, dehydration at high temperature causing decreasing conductivity, and most importantly, the cross-over of 3 ACS Paragon Plus Environment


vanadium ions.9 The cross-over occurs because of the low permselectivity of the Nafion PEMs to protons over vanadium ions, resulting in the continuous loss of cell voltage potential. The cross-over issue is also common in hydrocarbon-based PEMs such as sulfonated polyarylenes, since the increased degree of sulfonation needed for high proton conductivity often results in low permselectivity between cations.10,11 Recently, porous membranes containing micropores (< 2 nm) or mesopores (2 – 50 nm) have been demonstrated to be efficient and less expensive alternatives to PEMs for VRFBs.12,13 In porous membranes, the continuous porous spaces can be filled with water and utilized as ion conducting channels. As a result, ion permeability is strongly affected by the pore characteristics, and this suggests it may be possible to achieve the selective transport of protons over vanadium ions, based on differences in the hydrated radii. Indeed, Zhou et al. theoretically proposed that vanadium ion cross-over can be effectively minimized by reducing pore size below 4 nm based on a 2D transient model, if the convection mechanism is dominant for the transport.14 In 2011, Zhang et al. introduced a porous polyacrylonitrile (PAN) nanofiltration (NF) membrane for VRFB fabricated by a phase inversion method, and suggested that selectivity between protons and vanadium ions increased with decreasing pore size distribution.13 Lu et al. also fabricated NF membranes composed of poly(vinyl pyrrolidone)(PVP)/polyethersulfone (PES) and controlled the pore size by adjusting the PVP/PES ratio and the evaporation rate of isopropanol during the phase inversion. They observed a decrease in the vanadium ion cross-over when the pore size was decreased from 3.9 nm to 1.5 nm.15 Chae et al. achieved high selectivity for protons over vanadium ions by integrating a thin film polymer of intrinsic microporosity (PIM-1) into a PAN ultrafiltration (UF) membrane support.16 While both proton conductivity and VO2+ permeability through the micropores of 0.4 – 0.8 nm in PIM-1 were lower than with conventional PEMs, the larger vanadium ions were more selectively rejected by the small micropores and the cross-over was 4 ACS Paragon Plus Environment

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suppressed. Nonetheless, a systematic investigation of the relationship between pore size and the permeability of protons and vanadium ions through pores acting as ionic conducting channels has not been conducted. In addition, an experimental determination of the critical pore size for the effective suppression of vanadium ion cross-over has yet to be reported. We propose that this challenge might be addressed by using well-defined porous membranes derived from block polymer precursors as a model system. This approach would allow precise control of pore size by adjusting the molar mass of the sacrificial block in the precursor. To that end, we investigated proton conductivity and ion permeability in well-defined mesoporous membranes consisting of a 3D continuous pore structure, using robust control over pore size in the sub-10 nm regime. We fabricated such mesoporous membranes with different pore sizes, based on the polymerization-induced microphase separation (PIMS) process. The process includes the bulk copolymerization of styrene (S) and divinylbenzene (DVB) in the presence of a polylactide macro-chain transfer agent (PLA-CTA) via reversible additionfragmentation chain transfer (RAFT) mechanism. Use of RAFT polymerization17,18,19 can be beneficial because PLA-CTA with different molar masses can be obtained by straightforward synthesis, and target PLA-b-P(S-co-DVB) cross-linked block polymer can be readily obtained by neat polymerization. During polymerization, the emergent PLA-b-P(S-co-DVB) crosslinked block polymer is also simultaneously microphase-separated to form a bicontinuous disordered nanostructure composed of PLA and P(S-co-DVB) microdomains.20 Well-defined and 3D continuous mesopores can be derived by the selective removal of the PLA, and the pore size is directly controlled by varying the PLA molar mass,21,22,23 which can be clearly evidenced by small angle X-ray scattering (SAXS), scanning electron microscopy (SEM), and nitrogen sorption isotherm analysis. To improve proton conductivity, we introduced sulfonic acid groups into the cross-linked polystyrenic framework using the conducting postsulfonation 5 ACS Paragon Plus Environment


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reaction. The sulfonic acid group density was evaluated by elemental analysis (EA) and ion exchange capacity (IEC) measurements, and adjusted by controlling the reaction time. We determined that proton conductivity increased as more sulfonic acid groups were incorporated in the membrane, while the permeability of monovalent cations (H+, Li+, Na+, and K+) in the aqueous solutions became lower at higher sulfonic acid content. Permeability of the membranes estimated by using U-shaped glass bi-cell setup also strongly depended on pore size. Our data suggests that protons are likely to traverse the membrane by hopping along the sulfonic acid groups under an external electrical potential, while ions mainly travel through the mesoporous space via vehicular mechanism, without electrical potential. The transport rate appears to be strongly influenced by the size of the hydrated ions in relation to the mesopores, and the intensity of the electrostatic interaction between the ions and the sulfonated surface. We further demonstrate that permeation of the VO2+ can be almost exclusively inhibited when the pore size becomes smaller than 4 nm while preserving reasonable proton conductivity. A preliminary VRFB cell data suggests that cross-over of the vanadium ion can be indeed effectively prevented under the electrical potential gradient. This provides a guideline for the size of the ion conducting channels in the VRFB membranes needed to prevent the cross-over of vanadium ions.

Experimental Section Materials. Styrene (99%) and DVB (tech., 80%) were purchased from Sigma-Aldrich (St. Louis, MO) and purified by passing through a basic alumina column before polymerization. Azobisisobutyronitrile (AIBN, 98%) was purchased from Junsei (Tokyo, Japan), and purified by recrystallization in methanol stored at -20 C. PLA-CTAs with different number-average molar masses (Mn) were synthesized by ring opening polymerization of d,l-lactide in the 6 ACS Paragon Plus Environment

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presence of 1,8-diaza-biscyclo[5.4.0]undec-7-ene (DBU) as a catalyst and 2-hydroxyethyl 2(((dodecylthio)carbonothioyl)-thio)-2-methylpropanoate as an initiator, following the literature procedure.24 PLA-CTAs were designated PLA-CTA-XX, where XX represents the Mn values in kg mol-1, and their characterization detail is summarized in Table S1. d,l-Lactide was kindly provided by Corbion-Purac (Amsterdam, Netherlands), and stored under nitrogen after recrystallization from toluene. Sodium chloride (NaCl) (99.5%), potassium chloride (KCl) (99.0%), lithium chloride (>99.0%), and magnesium sulfate (MgSO4) (99.5%) were purchased from Sigma-Aldrich and used without purification. Vanadium (IV) oxide sulfate was purchased from Alfa Aesar (Haverhill, MA) and used without purification. Benzoic acid (99.5%), ethanol (90%), sodium hydroxide (97%), hydrochloric acid (37%), and sulfuric acid (95%) were purchased from Duksan (Daejeon, Korea) and used without purification. Oriented polypropylene (OPP) film spacer with a thickness of 50 μm was purchased from Jagang Industry (Seoul, Korea). Methods. 1H nuclear magnetic resonance (NMR) spectra of the synthesized PLA-CTAs were obtained using a Bruker Advance 300 MHz spectrometer. The molar mass and dispersity (Ð) of the PLA-CTAs were measured using an Agilent Infinity 1260 series (Santa Clara, CA) size exclusion chromatography (SEC) system equipped with an Optilab UT-rEX refractive index detector (Santa Barbara, CA) and PLgel 10 m MIXED-B columns. Chloroform was used as an eluent at 35 C, and the number- and weight-average molar masses (Mn,SEC and Mw,SEC) of the polymers were calculated relative to linear polystyrene standards (EasiCal) purchased from Agilent Technologies. Nitrogen sorption isotherms of the mesoporous membranes were obtained on a Mirae SI (Gwangju, Korea) nanoPOROSITY-XQ analyzer or a Quantachrome Autosorb iQ2-MP at the temperature of liquid nitrogen (77.3 K). Samples were loaded in 6 mm stems and degassed 7 ACS Paragon Plus Environment


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at room temperature for 4-20 h before measurement. Specific surface area was determined by multipoint Brunauer–Emmett–Teller (BET) analysis.25 Mode pore diameter (DN2) was estimated by Barrett-Joyner-Halenda (BJH) analysis of the desorption branch of the nitrogen sorption isotherms. Scanning electron microscope (SEM) images were obtained using a Hitachi S-4700 field-emission SEM (Schaumburg, IL) with 5 kV and 10 μA. Samples were coated with Os before collecting images. Energy dispersive X-ray spectroscopy (EDS) measurement was carried out using an FEI Magellan 400 FE-SEM after coating with Os. Small Angle X-ray scattering (SAXS) experiments were performed at the 9A U-SAXS beamline at Pohang Accelerator Laboratory (PAL) using an energy density of 19.95 keV and a sample to-detector distance of 1.5 m. Samples were loaded into a cell sandwiched with Kapton tape and exposed to X rays at RT. Scattering intensity was monitored with a Mar 165 mm diameter CCD detector with 20482048 pixels. The two-dimensional scattering patterns were azimuthally integrated to produce one-dimensional profiles presented as scattering vector (q) versus scattering intensity, where the magnitude of the scattering vector was calculated with q = (4/)sin(/2). Domain spacing (d) was obtained from the position of the principal scattering (q*) using the relationship d = 2π/q*. The elemental composition of the membranes was estimated using elemental analysis (EA) using the FLASH 2000 series model from Thermo Scientific (Waltham, MA) to detect C, H, N, S, and the FlashEA 1112 model from Thermo Finnigan to quantify O. A small size universal testing machine (QRS-S11H, QURO Corp., Korea) was used for tensile tests of SMMs. Samples with dimensions of 50 × 10 × 0.07 mm (length × width × thickness) were prepared, and stretched with a displacement rate of 5 mm min-1. Synthesis of mesoporous membranes. The literature procedure for the synthesis of monolithic mesoporous polymer via the PIMS process20,23 was modified to produce mesoporous membranes with designated thicknesses. Synthesis of a mesoporous membrane 8 ACS Paragon Plus Environment

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using PLA-CTA-11 is described here as an example. A polymerization mixture containing PLA-CTA-11 (1.00 g, 0.091 mmol), styrene (2.20 g, 21.1 mmol), DVB (0.50 g, 3.8 mmol), and AIBN (0.0040 g, 20 μmol, 0.25 eq to PLA-CTA-11) was poured on a glass plate which had the OPP film spacer (50 μm thick) placed around the edge. The solution was sandwiched with another glass plate, and the assembly was heated to 120 C for 24 h and then 200 C for 30 min. After cooling to RT, the glass sandwich was disassembled and the resulting crosslinked block polymer PLA-b-P(S-co-DVB) membrane was detached from the bottom plate. The free-standing membrane was immersed in 0.5 M NaOH solution in methanol/H2O = 1/2 (v/v) at RT for 1 day to selectively remove PLA. The resulting mesoporous P(S-co-DVB) membrane was dried in nitrogen atmosphere for 1 day. Synthesis of sulfonated mesoporous membranes (SMMs). The literature procedure for the postsulfonation of P(S-co-DVB) resin26 was modified to produce SMMs from the mesoporous membranes as precursors. Use of the mesoporous membrane derived from PLA-CTA-11 is given as a representative case. 0.05 g of the mesoporous membrane prewetted with ethanol (50 mL) for 2 h at RT was placed in a pressure vessel equipped with a magnetic stirbar, and concentrated sulfuric acid (25 mL) was added. After heating the reaction mixture to 70 C for 6 h, the membrane was taken out of the reaction mixture and thoroughly washed with deionized (DI) water until the water after washing became neutral. The resulting SMM was stored in DI water prior to use. Water uptake. The water uptake (WU) of the SMMs was determined by comparing mass changes between the wet and dried SMMs. A dried SMM with a predetermined mass was immersed in DI water at RT for 12 h, and then its weight was measured after wiping excess water off the surface, to estimate WU as follows: WU = (wwet – wdry)/wdry  100%

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Ion exchange capacity (IEC). The ion exchange capacity (IEC) values of the SMMs were determined by back titration measurements. SMM (0.05 g) was soaked in 5 mM NaCl aqueous solution (300 mL) containing phenolphthalein (0.005 g) and gently stirred. After 3 days, a 5 mM NaOH aqueous solution was gradually added until a red color appeared. The IEC values of the SMMs were calculated from the volume of NaOH solution added. Proton conductivity. The measurements were conducted in DI water after 1 cm  5 cm of SMM was installed between electrodes. A BioLogic SP-200 system (Grenoble, France) was used obtain Nyquist plots for frequencies ranging from 0.1 Hz to 1 MHz. The resistance of the SMM was measured by collecting impedance spectroscopy (EIS) spectra. Proton conductivity was calculated using the following equation: R = L/(σ  A)

(2)

R, σ and A are the resistance, proton conductivity, and effective area of the SMM, respectively. L denotes the distance between electrodes. Ion permeability. Ion permeability across the SMMs was measured following the method previously reported in the literature,27 using a U-shaped glass bi-cell composed of two half cells separated by the membrane. The effective area was 3.14 cm2 and the volume of each half cell was 50 mL. The SMM was fixed between the two half cells using silicon gaskets and the assembly was clamped. 25 mL of 0.1 M NaCl, KCl, HCl or LiCl(aq) and DI water were placed in each cell, respectively. Then the assembly was placed in a temperature-controlled water bath preset at 26 C. In case of SMM(5.4, 2.14) with HCl(aq) for proton permeability measurements, the experiments were performed at 25, 40, 60, 80 °C to evaluate temperature dependence. Conductivity in the receiving cell was measured using a ET902 model single probe from EDAQ with a cell constant (k) of 1 and range of 100 mS cm-1 connected to a Eutech Cond6+ model ion conductivity meter (Singapore) in the conductivity range of 0 – 19.99 μS cm-1. 10 ACS Paragon Plus Environment

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Conductivity was recorded at 15 min intervals for 120 min, and converted into ion concentration using the calibration curves shown in Figure S7. Finally, ion permeability (P) was calculated according to the following equation: V  dCt/dt = AP(C0-Ct)/d

(3)

where V represents the volume of the solution in both cells, A and d are the effective area and the thickness of the membrane, and C0 and Ct are the ion concentration in the receiving cell at the initial state and the time t, respectively. VOSO4 cross-over. Using the bi-cell described above, VOSO4 cross-over measurements were performed with 2 M VOSO4 in a 2 M H2SO4 solution and 2 M MgSO4 in a 2 M H2SO4 solution.28 VOSO4 concentration in the MgSO4-containing cell was monitored by UV-vis spectroscopy using a Mecasys Optizen Pop UV/vis spectrometer (Daejeon, Korea). Absorbance at 765 nm, where the VO2+ exhibits maximum absorption intensity, was determined at 15-min intervals over 120 min. The absorbance value was converted into VO2+ concentration based on a calibration curve (Figure S11b), and then the ion permeability of the VOSO4 was calculated according to equation (3). VRFB cell test. SMM with xS of 6.15 mmol g-1 derived from SMM(3.6, 0.04) was used for VRFB cell fabrication. We estimated the IEC value as 1.99 and denoted the membrane as SMM(3.6, 1.99). A VRFB cell was fabricated with SMM(3.6, 1.99) having dimensions of approximately 60 × 70 mm (length × width). 50 mL of 1.6 M vanadium in 2 M H2SO4 was used as electrolytes, and carbon felts were used for electrodes. The effective cross-sectional area of the membrane was 25 cm2. Voltage efficiency, current efficiency, and their discharge capacity retention were measured at 50 mA cm-2 of current density, voltage range from 0.8 to 1.7 V, and 1 mL min-1 cm-2 of flow rate for 6 cycles as a first cycle as a dummy.



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Results and Discussion Synthesis of sulfonated mesoporous membranes (SMMs). The synthetic route to SMMs is schematically depicted in Figure 1 along with photos of each step.

Figure 1. Schematic description of SMM synthesis.

Following our previous report on the synthesis of mesoporous P(S-co-DVB)s from PLA-b-P(S-co-DVB) precursors obtained by the PIMS process,20,23 we fabricated PLA-b-P(Sco-DVB) precursors in the form of free-standing films by sandwiching the polymerization mixture of PLA-CTA, styrene, DVB, and AIBN as a thermal radical initiator (0.25 eq to PLACTA) between two glass plates, and then heating the assembly at 120 C for 24 h. The assembly was further heated to 200 C for 30 min to promote the consumption of pendent double bonds generated by the polymerization of the DVB.29 An almost quantitative conversion of the monomers was achieved with the heating process, resulting in free-standing films. The film thickness was roughly controlled by using a film spacer (50 m thick), and ca. 70 m thick films were obtained by using one layer of the spacer. The weight fraction of the PLA-CTA in the polymerization mixture was set to 27%, and the molar ratio between styrene and DVB was maintained at 4:1 for all reactions. The characterization data of the PLA-CTAs used in this study are shown in Figure S1, and their Mn and Đ values are summarized in Table S1.

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Mesoporous membranes were derived from the PLA-b-P(S-co-DVB) precursors by etching the PLA in a basic condition. The complete removal of PLA was confirmed by FTIR, where strong vibrational bands at 1752 and 1050 cm-1 corresponding to C=O and C-O stretching in PLA completely disappeared after the basic treatment (Figure S2). C-H vibrations at 3100 - 2900 cm-1 were intact during the basic treatment, suggesting stability of the crosslinked polystyrenic framework. Visualization of the pore structure by scanning electron microscopy (SEM) confirmed that mesoporous membranes containing 3D continuous mesopores throughout the film thickness were successfully produced (Figure 2a). Small angle X-ray scattering (SAXS) data of the mesoporous membranes as a function of the scattering vector q is shown in Figure 2e along with the corresponding PLA-b-P(S-coDVB) precursors, synthesized with PLA-CTAs with different Mns. The characteristic scattering pattern of the precursors, including a broad principal peak at q* followed by a second-order shoulder, was retained in the SAXS data of all the mesoporous membranes, but a huge increase in scattering intensities was observed compared with the precursors. This is consistent with the presence of a 3D continuous mesoporous space percolating the framework, which was templated by the disordered bicontinuous morphology of the PLA and P(S-co-DVB) microdomains. In the PIMS process, controlled growth of the P(S-co-DVB) block from the end of the PLA-CTA by the RAFT polymerization process induces microphase separation at a critical conversion during polymerization. The resulting bicontinuous morphology is arrested by simultaneous cross-linking, due to the incorporation of DVB.20,21 The position of q* was shifted to a smaller q when the molar mass of the PLA-CTA was increased from 11 to 50 kg mol-1, which corresponds to a gradual increase in domain spacing d = 2/q* from 19 nm to 31 nm. This also confirms the reliable control of pore size, as indicated in our previous report.20,23 13 ACS Paragon Plus Environment


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Nitrogen sorption isotherms of the mesoporous membranes obtained at 77 K are presented in Figure 2f. While a partial pore collapse seems to have resulted in a relatively small pore volume in the case of the mesoporous membrane synthesized with PLA-CTA-11, a large hysteresis shift to higher relative pressure with the increasing molar mass of the PLA-CTA indicates the successful formation of mesopores, with controlled pore size. The pore size distributions of the mesoporous membranes estimated by BJH analysis from the desorption branches of the isotherms indicates that mesopores with narrow pore size distribution, ranging from 3.6 nm to 6.1 nm in mode pore diameter (D), were produced by controlling the PLA molar mass (Figure 2g). The specific surface area (SA) of the mesoporous membranes, estimated by multipoint Brunauer–Emmett–Teller (BET) analysis, decreased as the pore size increased except for PLA-CTA-11, which was also consistent with our previous report.20,23 Table 1 summarizes the pore characteristics of the mesoporous membranes.


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Figure 2. Characterization of the mesoporous membranes synthesized with PLA-CTAs of different Mns. (a)-(d) SEM images of the mesoporous membranes. Samples were coated with Os prior to imaging (scale bar: 200 nm). (e) SAXS data of the PLA-b-P(S-co-DVB) precursors (dashed line) and corresponding mesoporous membranes (solid line). The data is vertically shifted for clarity. (f) Nitrogen sorption isotherms of the mesoporous membranes obtained at 77 K. (g) BJH pore size distributions of the mesoporous membranes estimated from the desorption branch of the nitrogen sorption isotherms shown in (f).



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Table 1. Pore characteristics of the mesoporous membranes Mn,PLA-CTA

dPLA-b-P(S-co-DVB)

dmesoporous membrane

D

SA

(kg mol-1)

(nm)a

(nm)a

(nm)b

(m2 g-1)c

11

19.3

19.6

3.6

85

20

20.4

20.7

4.7

193

30

26.4

27.5

5.4

106

50

30.9

31.2

6.1

99

adetermined bmode

by SAXS analysis

diameter determined by BJH analysis of the desorption branch of nitrogen sorption

isotherms cdetermined

by multipoint BET analysis of nitrogen sorption isotherms at P/P0 = 0.2 - 0.35

The SMMs were derived from the mesoporous membranes by the postsulfonation reaction of aromatic rings in the P(S-co-DVB) framework with concentrated sulfuric acid, which occurs via the electrophilic aromatic substitution mechanism. The FTIR spectra of the SMM showed the appearance of new vibrational bands at 1000 and 1040 cm-1 corresponding to S=O stretching.26 Elemental analysis (EA) data also indicated the presence of sulfur in the membrane and further indicated an increase in the sulfur content (xS (mmol g-1)) as reaction time increased (Figure 3a). The xS values did not vary significantly after 8 h of sulfonation, suggesting saturation of the sulfonic acid groups incorporated to the polystyrenic framework under this condition. Ion exchange capacity (IEC) values, as the effective amount of sulfonic acid groups, were determined by back titration with NaOH, which were roughly proportional to xS determined by EA, but approximately 2-3 times smaller (Figure 3b). This is presumably because the sulfonic acid groups formed inside of the P(S-co-DVB) framework may not 16 ACS Paragon Plus Environment

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function effectively as acid sites due to limited access, or sulfur-containing groups other than sulfonic acid might be generated by the reaction with concentrated sulfuric acid. The representative xS and IEC values of SMMs synthesized with PLA-CTA-30 are shown in Figure 3.

Figure 3. Control of sulfonic acid contents by sulfonation time determined for SMMs synthesized with PLA-CTA-30. (a) Relationship between sulfonation time and xS determined by EA. (b) A plot between the xS and IEC of the SMMs.

By using the D of the mesoporous membranes determined by BJH analysis with the IEC values determined by back titration, we designated the SMM samples as SMM(D (nm), IEC (meq g-1)) and investigated the effects of the pore size and sulfonic acid content of the SMMs on the transport of ions across the membrane. The series of SMMs were derived from 17 ACS Paragon Plus Environment


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an identical mesoporous membrane but with different sulfonic acid contents, and the resulting pore size of the SMM is indicated in the parentheses. SMMs used for the ion permeation studies are listed in Table 2. Unsulfonated pristine mesoporous membranes (i.e., sulfonation time = 0 h) are also included in Table 2. We note that they also exhibited a small but nonzero xS and IEC values, presumably due to the presence of trithiocarbonate moieties from the CTA, and the formation of carboxylic acid groups on the pore surface by PLA etching.


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Table 2. SMM information Sample namea

Sulfonation time

xS

IEC

(h)

(mmol g-1)b

(meq g-1)c

SMM(3.6, 0.04)

0

0.12

0.04

SMM(3.6, 1.74)

4

4.45

1.74

SMM(3.6, 2.14)

8

6.61

2.14

SMM(4.7, 0.03)

0

0.11

0.03

SMM(4.7, 1.89)

4

5.92

1.89

SMM(4.7, 2.33)

8

6.78

2.33

SMM(5.4, 0.04)

0

0.11

0.04

SMM(5.4, 1.77)

4

4.36

1.77

SMM(5.4, 2.14)

8

5.81

2.14

SMM(6.1, 0.03)

0

0.12

0.03

SMM(6.1, 1.13)

3

1.92

1.13

SMM(6.1, 1.61)

6

3.12

1.61

aeach

sample was coded in the format (D (nm), IEC (meq g-1))

bdetermined

by EA analysis

cdetermined

by back titration

SEM images of the SMMs revealed no particular morphological changes during the postsulfonation reaction (Figure S3). However, results from energy dispersive X-ray spectroscopy (EDS) in the SEM imaging indicated a uniform distribution of sulfur throughout the membrane thickness, indicating that sulfonic acids were fully introduced in the mesoporous membranes (Figure S4 and Table S2). SAXS data of the SMMs also showed that the mesopore 19 ACS Paragon Plus Environment


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structure was well preserved without changes to the pore structure during the postsulfonation reaction (Figure 4). Particularly, the q* position remained constant regardless of the sulfonation time for all SMMs suggesting that the pore size was retained. Attempts to obtain nitrogen sorption isotherms of SMMs were not successful. We posit that water strongly adsorbed onto the pore surface may not have been completely evaporated under vacuum at ambient temperature and interfered with the measurements. We also examined mechanical properties of SMMs by taking SMM(5.4, 2.14) as an example by a tensile test (Figure S5). Compared with pristine unsulfonated SMM(5.4, 0.04), SMM(5.4, 2.14) exhibited similar Young’s modulus (~ 250 MPa) but reduced ultimate stress and elongation at break by 20% and 36%. Nonetheless, the sulfonated membranes well retained the structural integrity.


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Figure 4. SMM SAXS data. (a) SMMs derived from PLA-CTA-11 (D = 3.6 nm). (b) SMMs derived from PLA-CTA-20 (D = 4.7 nm). (c) SMMs derived from PLA-CTA-30 (D = 5.4 nm). (d) SMMs derived from PLA-CTA-50 (D = 6.1 nm).

Proton conductivity. The proton conductivity of SMMs filled with DI water was determined at RT and is shown in Figure 5. Water easily infiltrated into the mesopores of the SMMs, presumably due to the presence of highly hydrophilic sulfonic acid groups on the pore surface. The water uptake (WU) of the SMMs measured after immersing the membrane in deionized (DI) water suggested that most of the pores in the SMM were filled with water (Figure S6). As 21 ACS Paragon Plus Environment


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relatively hydrophobic mesoporous membranes, the membranes were prewetted with ethanol and then immersed in water to ensure complete filling of the pore. It is apparent that proton conductivity increased as xS increased. Mesoporous membranes without sulfonic acid groups exhibited very low proton conductivity, but above xS = 1.5 conductivity exceeded 0.1 S cm-1, which is the conductivity of conventional Nafion. The highest proton conductivity value of 0.16 S cm-1 was obtained from the SMM derived from PLA-CTA-11 after 12 h of sulfonation. Sulfonic acid content influenced proton conductivity much more than pore size, and we did not see a noticeable relationship between the pore size and the proton conductivity. Given that the pore size should influence the ion transport in the vehicular mode,14 we suggest that protons traverse the SMMs mainly via hopping mechanism, utilizing the sulfonic acid groups on the pore surface for conduction under an external electrical potential.31

Figure 5. Proton conductivity of SMMs.

Ion permeability. We explored the permeability of ions across the SMMs using a glass bi-cell setup separated by the SMM, where one half-cell contained DI water and the other half-cell contained an electrolyte aqueous solution. To investigate the effect of cation size, we used HCl, 22 ACS Paragon Plus Environment

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LiCl, NaCl, and KCl as the electrolyte. We observed that conductivity in the DI watercontaining cell gradually increased over time (t) as ions traversed through the SMM driven by chemical potential difference. The conductivity value was converted into ion concentration (C) based on the calibration curves shown in Figure S7, and ion permeability was taken as a slope of C vs. t plot. Figure 6a shows the increase in ion concentration as a function of time when unsulfonated mesoporous membranes were used. Unlike proton conductivity, which is attributed to proton hopping, it is evident that NaCl and KCl diffuse faster as the SMM pore size of increases. We also observed that the permeability of KCl was higher than NaCl for all the membranes investigated presumably because the hydrated radius of Na+ is larger than that of K+.26 This suggests that Na+ and K+ prefer to follow the vehicular mechanism and primarily diffuse through the water filling the mesopores.30,31 Figure 6b shows representative data for NaCl and KCl permeability (p) through the SMM (D = 5.4 nm) with different IEC values (see all the data for other SMMs in Figure S8). As IEC value increased, p decreased for both NaCl and KCl. We posit that coulombic attraction between the sulfonate anion and the cation reduces the mobility of the cation in the aqueous solution, and therefore decreases p. Thus, control of pore size and sulfonic acid content was very effective for adjusting ion permeability over a range of 1 order of magnitude, from 5.7810-5 cm2 min-1 for SMM(6.1, 0.03) to 1.8710-6 cm2 min-1 for SMM(3.6, 2.14) in the case of NaCl, as shown in Figure 6c.



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Figure 6. Ion permeability through SMMs. Filled and open squares represent data obtained with NaCl and KCl, respectively. (a) Concentration changes through unsulfonated mesoporous membranes. (b) Concentration changes through SMM(D = 5.4 nm) with varying sulfonic acid content. (c) Ion permeability (p) through the SMMs.

For SMM(D = 5.4 nm), we also measured the permeability of HCl and LiCl to confirm the effect of hydrated ion size on permeability through SMMs (Figure S9). At a constant IEC value, ion permeability increased in the order of Li+ < Na+ < K+ < H+, which is exactly the opposite order of the sizes of the hydrated alkali metal ions (K+ < Na+ < Li+).33 This data corroborates that transport of ions driven by chemical potential difference occurs via diffusion through the water channel provided by the water-filled mesopores via vehicular mechanism,31,32 and the resistance to the transport can be controlled by the relative pore size to the hydrated ion and the ionic group density on the pore surface. Proton permeability through SMM(5.4, 2.14) increased with increasing temperature from 25 to 80 °C, presumably due to increased diffusion (Figure S10). Given that the vehicular mechanism is dominant for the diffusion of ions, we considered that the permeation of large cations such as VO2+ through the membrane may be almost 24 ACS Paragon Plus Environment

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completely suppressed if the pore size was sufficiently small. Figures 7 and S12 depict the permeability of VOSO4 across SMMs, measured using the identical bi-cell setup and 2 M H2SO4 solution. VOSO4 concentration was monitored by determining the absorbance of the solution at 765 nm, where VO2+ shows the highest absorption intensity (Figure S11a). Consistent with the NaCl and KCl data, the p of VOSO4 also increased as the pore size of the SMM increased and decreased with increasing IEC values. More importantly, SMM(D = 3.6 nm) was very effective at preventing VOSO4 crossover across the membrane, presumably due to its small pore size. This is consistent with the theoretical prediction made by Zhou et al. where the authors suggested 4 nm as the critical pore size to prevent VOSO4 cross-over.14 Because the introduction of sulfonic acid on the pore surface further decreases p, SMM(3.6, 2.14) exhibited the lowest VOSO4 permeability of p = 3.7310-8 cm2 min-1, which is 100 times lower than that of Nafion 117.28 Our data clearly demonstrates that it is possible to orthogonally control proton conduction and ion permeation in SMMs by adjusting pore size and sulfonic acid content, and further, that it is possible to target a specific cation and inhibit its permeation. A preliminary data of a VRFB cell fabricated with SMM(3.6, 1.99) showed higher current efficiency and superior discharge capacity retention than Nafion 212, suggesting that migration of the vanadium ion can be also effectively prevent under the electrical potential gradient and cross-over of the vanadium ion can be indeed suppressed (Figure S13). We not that voltage efficiency of SMM(3.6, 1.99) was lower than Nafion 212 presumably because of surface roughness and fluctuation in thickness of SMM leading to higher resistance, which needs to be improved for VRFB applications.



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Figure 7. Permeability of VOSO4 through SMMs. (a) Concentration changes through unsulfonated mesoporous membranes. (b) Concentration changes through SMM(D = 5.4 nm) with varying sulfonic acid content. (c) VOSO4 permeability (p) through the SMMs. Conclusion We have shown that the permeation of ions across a sulfonated mesoporous membrane (SMM) can be selectively controlled by pore size and sulfonic acid content, which can decouple proton conduction under an external electrical potential and ion permeation driven by chemical potential difference. The postsulfonation of mesoporous P(S-co-DVB) free-standing membranes containing three-dimensionally continuous mesopores, fabricated using the polymerization-induced microphase separation (PIMS) process, allowed us to produce SMMs with well-defined pore size and adjustable ion exchange capacity (IEC). While the proton conductivity of the SMMs increased with increasing IEC values due to proton hopping, the permeability of monovalent cations (having Cl as the counter anion) in water decreased as the IEC increased, presumably because of higher electrostatic attraction to the pore surface. More importantly, ion permeability decreased with decreasing pore size and increasing hydrated cation size, indicating that a cation with a larger hydrated radius experiences higher resistance against diffusion in the mesopores. We further demonstrated that the permeability of VOSO4 26 ACS Paragon Plus Environment

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can be dramatically reduced with a membrane containing < 4 nm pores, while retaining the proton conductivity originating from the high IEC, leading to higher current efficiency and superior discharge capacity retention than Nafion 212 in the preliminary VRFB cell data. This finding provides useful information for designing porous separators for battery systems where permselectivity is important.

Supporting information 1H

NMR spectra and SEC traces of PLA-CTA, FT-IR spectra of the membrane precursor after

PLA etching and postsulfonation, SEM images of SMMs, representative EDS data, strainstress curves of SMMs, water uptake of SMMs, calibration curves for conductivity determination, additional ion permeability graphs, and VRFB cell performance data.

Acknowledgements This research was supported by LG Chem. SEM and EDS measurements conducted in National NanoFab Center were supported by Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (MSICT) (2009-0082580). SAXS experiments at Pohang Accelerator Laboratory (PAL) were supported in part by MSICT and POSTECH. M.S. thanks prof. Hee-Tak Kim for helpful input.

Conflict of Interest The authors are declared to be inventors on the patent filed by KAIST and LG Chem related to the work presented here.



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for Table of Contents use only


Control of Ion Transport in Sulfonated Mesoporous Polymer Membranes

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