Ionic Liquid Ion Gel

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Phosphonium-Based Poly(ionic liquid)/Ionic Liquid Ion Gel Membranes: Influence of Structure and Ionic Liquid Loading on Ion Conductivity and Light Gas Separation Performance Alexander M. Lopez,† Matthew G. Cowan,†,‡ Douglas L. Gin,*,†,‡ and Richard D. Noble*,† †

Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309 United States Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309 United States

‡

S Supporting Information *

ABSTRACT: The influence of free IL structure and loading level in phosphoniumbased poly(ionic liquid)/ionic liquid (PIL/IL) ion gel composite membranes was studied with regards to their ion conductivity and light gas separation performance. The resulting series of ion gels demonstrated ion conductivity values of 10−7−10−5 S/cm at 25 °C and 10−5−10−3 S/cm at 110 °C. It was found that shorter alkyl chain lengths on the phosphonium cation of the IL resulted in increased thermal stability of the ionic conductivity for the resulting PIL/IL membrane materials with negligible changes in ionic conductivity performance at temperatures up to 110 °C. Further, an increase of free IL loading resulted in an increase in overall ion conductivity and membrane mechanical stability. Similar structure/composition/ function relationships were observed for the light gas separation performance of these phosphonium-based PIL/IL ion gel membranes. However, the resulting performance is lower than PIL/IL ion gel membranes based on imidazolium cations reported in the literature.

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INTRODUCTION Ionic liquids (ILs) are molten salts below 100 °C that have been used for a variety of applications, from electrochemistry and energy storage to catalysis and reaction solvation.1−4 Although ILs have a unique combination of materials properties (e.g., negligible vapor pressure, intrinsic ion conductivity, and intrinsic thermal conductivity), their liquid morphology is disadvantageous in many applications that require a material with a fixed form factor and good mechanical stability. In order to overcome some of these limitations, polymerized ILs (PILs) have been prepared and studied over about the last 15 years. These charged polymers are based on the polymerization of IL monomers and retain not only many of the desirable properties of ILs (intrinsic ion conductivity, selective CO2 uptake versus other light gases, and so forth) but also the mechanical stability and processability of traditional solid polymers.5−8 For example, Elabd et al. developed PIL-based membranes for use as ion conductors in fuel cells and batteries.9,10 Their membranes were synthesized using methacrylate-based block copolymers and imidazolium-based PILs to develop membranes with high ion conductivity (10−3−10−2 S/cm at 110 °C). However, PILs have less fractional free volume compared to ILs, so diffusion and transport within PILs is substantially lower than in ILs. Recent efforts have been made to incorporate ILs into polymers to combine the benefits and materials properties of ILs with the benefits of polymeric materials (e.g., mechanical stability and processability).11,12 These polymer/IL compo© XXXX American Chemical Society

sites (often called polymer-based ion gels) have been explored as electrochemical membranes in the development of fuel cells and batteries10,13,14 as well as dense membranes for gas separations and pervaporation.15 The major limitations associated with polymer-based ion gels are exclusion of IL from the polymer matrix during gas permeation and degradation of the polymer network at high temperatures.16 However, these issues can be addressed to a large degree by the use of physical or chemical cross-linking in the polymer matrix used to hold the entrained IL, thereby reinforcing the surrounding polymer structure.17 Coupled with the tunability of ILs, these ion gels can become task-specific for the desired application (e.g., for use in fuel cells and batteries).18,19 For example, Lodge et al. developed ion gels based on uncharged triblock copolymer (e.g., poly(styrene-b-ethylene oxide-bstyrene), poly(styrene-b-methyl methacrylate-b-styrene), and imidazolium-based ILs for the treatment of light gases and successfully demonstrated ion gel assembly at free-IL loadings of about 90 wt % through chemical cross-linking.19−21 More recently, polymer-based ion gels have been prepared by incorporating ILs within PILs (i.e., charged polymer matrices made from IL monomers) instead of within conventional uncharged polymers to yield PIL/IL ion gels.22 These PIL/IL ion gel membranes have demonstrated Received: June 13, 2017 Accepted: March 6, 2018

A

DOI: 10.1021/acs.jced.7b00541 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Synthesis schemes for preparing (A) the phosphonium ILs and (B) the phosphonium IL monomer used in this study.

enhanced performance in gas separations,22−24 as well as conductive materials for ion transport.25 McDanel et al. recently developed PIL/IL ion gels for CO2/light gas separations via use of epoxide-amine-functionalized PIL resins and imidazolium-based ILs that afford CO2/CH4 permeability close to the 2008 Robeson plot upper bound.26 Similarly, Cowan et al. developed free-standing charged imidazoliumbased PIL/IL membranes with loadings of about 50−80 wt % IL that exhibit ion conductivities close to that of the pure free ILs.25 However, the majority of PIL/IL ion gels developed previously have focused on using an imidazolium cation in the PIL and IL structures, due to the high CO2 solubility27,28 and relative ease of synthesis of imidazolium cations.29 Phosphonium-based PILs have been found to have improved thermal stability, chemical stability, and lower viscosity than their imidazolium-based counterparts,30,31 yet little research has been done on developing phosphonium-based PIL/IL ion gels. The limited examples of work in the latter area are as follows: Ye et al. developed phosphonium-based anionexchange membranes based on phosphonium carbonate blended with poly(styrene-random-trimethylvinylbenzyl phosphonium chloride) that demonstrated ion conductivities of about 10−8 to 10−3 S/cm for a relative humidity range of 20− 90%. However, the developed membranes exhibited significant water swelling (between 16.6−41.3 wt %).32 Previous research from our groups reported the development of neat, phosphonium-based PIL materials and their ion conductive properties for electrochemical applications and their light gas transport properties for membrane separations.33 Developing phosphonium-based PIL/IL ion gels may further improve the ion-conductive properties and gas solubility of the developed PILs. However, to our knowledge no research on the development of phosphonium-based ion gels has been reported in literature to date. Herein, we report the fabrication of free-standing phosphonium-based PIL/IL ion gel membranes based on the radical cross-linking of the phosphonium IL monomer

[P444VB][Tf2N] with divinylbenzene (DVB) and containing different loadings of several free phosphonium ILs. We also report the influence of phosphonium cation alkyl chain length and IL loading level on the mechanical properties and variable-temperature ionic conductivity performance of the resulting PIL/IL composite membranes. These phosphoniumbased PIL/IL ion gel membranes exhibit ion conductivities of up to 10−5 S/cm at room temperature and up to about 10−3 S/cm at 110 °C. Furthermore, free-standing composite membranes of these materials were synthesized at a variety of thicknesses (10−120 μm) and areas (>90 cm2) with no loss in ion conductivity performance. Changes in the phosphonium IL alkyl chain length were found to have a significant impact on the ionic conductivity behavior of the ion gel as a function of increasing temperature with longer alkyl chains demonstrating decreased ion conductivity at temperatures above 80 °C. Increasing the IL loading in these phosphonium-based PIL/IL ion gel membranes resulted in an increase in light gas permeability. However, the light gas separation performance of these membranes was only found to be marginal compared to other PIL/IL ion gel membranes in the literature based on imidazolium cations.

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EXPERIMENTAL SECTION Chemical, Material, and Analysis Information. All commercial chemicals and solvents were purchased from TCI America, Sigma-Aldrich, Alfa Aesar, or 3M and used as received. Thermogravimetric analysis (TGA) was performed using a Mettler-Toledo TGA/DSC 1 STAR system. Fouriertransform infrared (FTIR) spectra were obtained using a Thermo-Scientific Nicolet 6700 FTIR spectrometer in attenuated total reflectance (ATR) mode. The degree of polymerization for cross-linking the reactive vinyl side chain on the IL monomer was determined using ATR-FTIR spectroscopy as previously reported.25 A digital micrometer was used to measure ion gel membrane thicknesses. Preparation of the Phosphonium-Based PIL/IL Ion Gels. The synthesis of the phosphonium-based ILs and IL B



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Figure 2. ILs, IL monomer, and cross-linking agent used for the production of ion-gel membranes.

permeation testing. The process of producing these ion-gel films is summarized in Figure 2. Variable-Temperature Ion Conductivity Behavior of the Phosphonium-Based PIL/IL Ion Gels. The ion conductivity of the prepared PIL/IL ion gels as a function of temperature was determined via electrical impedance spectroscopy (EIS) using a Gamry Reference 600 potentiostat/galvanostat operating in potentiostat mode, a custommade electrochemical test cell containing two stainless-steel electrodes and a Tenney TJR programmable environmental chamber to regulate sample temperature. The applied voltage during testing was 10 mV with frequency range between 1 MHz and 1 Hz. The material resistance was obtained from locating the x-intercept of the generated Nyquist plot.35 The ion conductivity was then calculated using eq 1 below.

monomer followed the reaction schemes shown in Figure 1. The phosphonium-based ILs were synthesized as reported in the literature, and their characterization and purity data matched reported values.34 Similarly, the phosphonium-based IL monomer, [P444VB][Tf2N], was synthesized according to the literature, and its characterization and purity data matched those reported.33 PIL/IL ion gels were prepared by preparing a mixture of [P444VB][Tf2N] with 20 mol % of DVB. The prepolymer mixture was combined with the desired amount of chosen free IL to produce mixtures with 10, 20, 30, or 40 wt % free IL. Finally, 1 wt % of the commercial radical photoinitiator, 2-hydroxy-2-methylpropiophenone was added, slightly distorting the prepolymer/free IL ratio. The mixture was then vortexed until a homogeneous solution was obtained. In some instances, acetonitrile was added as a solvent to the mixture to ensure homogeneity. For mixtures that required acetonitrile, removal of the acetonitrile was necessary prior to radical photopolymerization. Dynamic vacuum (0.1 Torr) was used to remove the acetonitrile and ensure a neat prepolymer/IL mixture. The prepolymer/IL mixture was then sandwiched between two quartz plates and irradiated with 365 nm light (6.1 mW cm−2 at the sample surface) for 4 h at ambient temperature (22−24 °C). The resulting cross-linked PIL/IL ion gel films were then removed from the quartz plates for ion conductivity and light gas

σ=

L RA

(1)

The Gamry system was calibrated by measuring the conductivity of stock aq. KCl and NaCl solutions and comparing the collected data to reported literature values for these solutions and through measurement of a test cell. The results of these calibrations are available in the Supporting Information (see Section 1.3). Each PIL/IL ion gel sample was tested in triplicate to ensure accuracy. C



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in units of (moli·L−1polymer·atm−1) were converted to (cm3 of i at STP·cm−3 of polymer·atm−1) using IUPAC defined STP.

Single-Gas Permeability, Diffusivity, Solubility, and Selectivity Performance of the Phosphonium-Based PIL/IL Ion Gels. Single-gas CO 2 , CH 4 , N 2 , and H 2 permeability measurements were performed using a time-lag apparatus similar to those reported previously.5,26 Experiments were performed at room temperature (22−24 °C), and each gas was tested in triplicate for each membrane sample. Between experiments, the apparatus and membrane were evacuated for 6 h at room temperature using an Edwards RV8 vacuum pump. Data from the steady-state region was used to calculate the flux (J in cm3 (STP)·cm−2·s−1), permeability (P in barrers; 1 barrer = 10−10·cm3 (STP)·cm·cm−2·s−1·cmHg−1), and gas diffusivity (Di in cm2·s−1) using eqs 2, 3, and 4 below36,37 ⎛ Δp Δpleak ⎞ V 273.15 ΔVi (STP) · = Ji = ⎜ i − ⎟· Δt ⎠ A ·T 14.504 A ·Δt ⎝ Δt

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RESULTS AND DISCUSSION Characterization of the Phosphonium-Based PIL/IL Ion Gels. The phosphonium-based PIL/IL ion gels were prepared as described in the Experimental Section. The ion gel membranes were measured for area size and thickness, and their degree of photopolymerization was confirmed using ATR-FTIR spectroscopy22 (see the Supporting Information, Section 1.1). The thicknesses of the ion gel membranes varied between 80−120 μm with areas of between 30−94 cm2. Figure 3 shows a sample of a large area PIL/IL ion gel

(2)

The steady-state flux (Ji) (cm3 (STP)·cm−2·s−1) was determined using eq 2, where Δpi is the change in permeate pressure (cm Hg), Δt is the change in time (s), Δpleak (cm Hg) is the change in the permeate pressure when system is evacuated and then sealed (i.e., the ‘leak rate’), V is the permeate volume (cm3); A is the membrane area (cm2), T is the absolute temperature (K), and ΔVi is the volume of gas accumulated in the permeate volume at standard temperature and pressure (cm3·STP). Pi =

Ji ΔPi

·l

(3)

The permeability (Pi) (barrers) was determined using eq 3 above, where Ji is the flux, (cm3 (STP)·cm−2·s−1), l is the membrane thickness (cm), and ΔPi is the trans-membrane pressure difference (psi). Di =

l2 6θ

Figure 3. Image of a 94 cm2 PIL/IL membrane made using 30 wt % [P4448][Tf2N] in 66.7 wt % [P444VB][Tf2N] and 3.3 wt % DVB. Note: a circular outline around the membrane sample is drawn to aid viewing.

(4)

membrane. The resulting membranes were qualitatively mechanically stable and capable of being handled as freestanding films. These ion gels also exhibited a brittle glassy nature at low IL loadings (10 wt % free IL) but became more flexible with increasing IL content (20−40 wt % free IL). Incorporation of >40 wt % IL into these materials resulted in tacky ion gels that were incapable of being handled as freestanding membranes; thus, the PIL/IL ion gels developed for this study were limited to maximum free IL loading of 40 wt %. Ionic Conductivity Behavior of the PhosphoniumBased PIL/IL Ion Gels. The variable-temperature ionic conductivity behavior of the PIL/IL ion gels was determined using electrical impedance spectroscopy with the temperature of the samples regulated by a programmable environmental chamber. In general, the PIL/IL ion gel materials showed improved ionic conductivity in comparison to the neat phosphonium-based PIL material (Figure 4). The ionic conductivity of the phosphonium-based PIL/IL ion gels increased with increasing free IL loading. For the [P4448][Tf2N]-containing PIL/IL ion gels (Figure 5), the addition of 20 wt % free IL into the material resulted in a significant increase (1000×) in the observed ambient-temperature conductivity (from 10−8 to 10−5 S/cm) with this effect diminishing when the samples are heated to elevated temperatures (100×). This effect was exhibited similarly by

2 −1

The diffusion coefficient (Di) (cm ·s ) was calculated using eq 4, where l is the membrane thickness (cm), and θ is the time-lag (s). The time lag (θ) was determined from the x-axis intercept from a plot of the steady-state flow rate (dVi (STP)) against time (t). Pi = Di ·Si

(5) 3

The solubility coefficient (Si) for a given gas i (in (cm of i at STP)·(cm−3 of polymer)·atm−1) was extracted from the permeability and diffusion coefficient data using eq 5, where Pi is the permeability of the selected gas i (barrers), and Di is the diffusion coefficient of that gas (cm2·s−1).38 For these materials, the solubility of the light gases tested (i.e., H2, CH4, N2) was inconsistent, probably due to being too low to measure with our experimental equipment. αi / j =

Pi Pj

(6)

The ideal permeability selectivity (αi/j) was calculated using eq 6, where Pi (barrers) is the permeability of gas i, and Pj (barrers) is the permeability of gas j (barrers). The membranes used for gas testing were masked, freestanding films, so no tortuosity or porosity corrections were applied. Note that solubility values reported in the literature D



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the phosphonium cation for the free IL within the ion gels resulted in a slight decrease in ion conductivity (Figure 6). This behavior suggests that the effect of IL cation alkyl chain length may be slight compared to other factors affecting overall ionic conductivity (e.g., free IL viscosity).

Figure 4. Ionic conductivity versus inverse temperature behavior of a neat PIL membrane made with [P444VB][Tf2N] and PIL/IL ion gel membranes prepared with 76.2 wt % [P444VB][Tf2N], 3.8 wt % DVB, and 20 wt % of two different phosphonium-based free ILs. Figure 6. Ionic conductivity versus inverse temperature behavior of PIL/IL membranes made using [P444VB][Tf2N], DVB, and different loading levels of [P4448][Tf2N] free IL.

Thermal Stability of the Ionic Conductivity Behavior of the Phosphonium-Based PIL/IL Ion Gels. In order to determine the long-term efficacy of the synthesized ion gels, thermal stability experiments were performed on the prepared ion gels materials to determine how their ionic conductivity changed with increasing temperature and repeated thermal cycling. For these experiments, the samples were subjected to heating and cooling cycles while electrical impedance spectroscopy was performed at regular intervals during cycling. During ac impedance testing, the sample temperatures were sustained to ensure stable measurements. It was observed during repeated thermal cycling from ambient conditions to 80 °C that these ion gels exhibited no noticeable loss in ion conductivity when the materials returned to room temperature upon completion of each cycle (Figures 7A and 8A). However, for repeated heating cycling up to 110 °C, the materials exhibited significant loss in ion conductivity, especially for the PIL/IL ion gels containing [P4448][Tf2N] (Figures 7B and 8B). This loss of ionic conductivity at elevated temperatures was attributed to some sort of thermal change/degradation associated with the n-octyl chain on the free IL phosphonium cation, because a control PIL/IL sample containing [P4444][Tf2N] (an analogous IL with no n-octyl chain) showed no loss in ion conductivity during thermal cycling up to 110 °C (Figure 9). Single-Gas Permeation Results. The single-gas permeability data for CO2, CH4, N2, and H2 through the phosphonium-based PIL/IL ion gel membranes are shown in Tables 1 and 2, with additional details available in the Supporting Information (see Section 1.4). It should be noted that these ion gel membrane materials include 20 mol % relative to the [P444VB][Tf2N] cross-linked DVB, which is not used in the “neat” PIL membranes previously reported but likely has a small impact on overall gas transport properties.33

Figure 5. Ionic conductivity versus inverse temperature behavior of PIL/IL membranes made using [P444VB][Tf2N], DVB, and different loading levels of [P4448][Tf2N] free IL.

ion gels containing [P4444][Tf2N], [P4446][Tf2N], and [P4448][Tf2N] as well (see the Supporting Information, Section 1.3). For all of the ion gel materials tested, their ion conductivity at elevated temperatures increased as expected due to a decrease in free IL viscosity at higher temperatures. Similar effects due to temperature have been reported for ILs and PIL materials.39,40 The phosphonium-based PIL/ILs ion gels show lower (10×) ionic conductivities than analogous imidazolium-based PIL/IL ion gels19 because of differences in free IL loading of the ion gels (10−40 wt %) for phosphonium-based and (70− 90 wt % free IL) for imidazolium-based. As noted above, we were unable to study phosphonium-based ion gels containing higher free IL loadings due to the physical/mechanical properties and resulting handling difficulties. Comparison of the phosphonium PIL/IL materials produced reveals pertinent information concerning the effect of IL cation alkyl chain length on ion conductivity. Increasing alkyl chain length on E



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Figure 7. Ionic conductivity versus inverse temperature behavior of PIL/IL membranes made using 66.7 wt % [P444VB][Tf2N], 3.3 wt % DVB, and 30 wt % [P4448][Tf2N] free IL upon repeated temperature cycling to (A) 80 °C and (B) 110 °C.

Figure 8. Ionic conductivity versus inverse temperature behavior of PIL/IL membranes made using 66.7 wt % [P444VB][Tf2N], 3.3 wt % DVB, and 30 wt % [P8888][Tf2N] free IL upon repeated temperature cycling to (A) 80 °C and (B) 110 °C.

As observed previously in other types of PIL/IL ion gel membrane materials,20,28 incorporation of free IL in the crosslinked poly([P4448VB][Tf2N]) matrix produced ion gels with increased gas permeability that is proportional to the amount of free IL entrained in the membrane. Similar to previous materials,41,42 ion gels with low free IL loadings (10 wt %) showed only a minor increase in gas permeability compared to the neat PIL (58−74 versus 51 barrers).33 At these low IL loadings, it is likely that the free IL is ‘filling up’ the void space within the polymer matrix, and the free IL domains are small enough to not display the liquid-like diffusion that is observed for neat ILs28 and PIL/IL composites containing a large amount of free IL.20,43−45 Increasing the free IL loading to 40 wt % produced PIL/IL materials with significantly higher CO2 permeability (111−162 barrers, a 300% increase in base CO2 permeability). However, the CO2 permeability values of these cross-linked, free-standing phosphonium-based PIL/IL ion gel membranes are much lower than those reported for previously reported supported imidazolium-based PIL/IL ion gel membranes (containing 75 wt % imidazolium IL and reaching CO2 permeabilities of >500 barrers) that rely on entrainment in a porous support to provide the mechanical stability required for testing a membrane material.

Figure 9. Thermal stability behavior of an ion gel film prepared with 66.7 wt % [P444VB][Tf2N], 3.3 wt % DVB, and 30 wt % [P4444][Tf2N] measured via ion conductivity and repeated temperature cycling to 110 °C. F



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Table 1. Single-Gas Permeability Data for Masked, Free-Standing, Neat Poly([P444VB][Tf2N]) and DVB-Cross-Linked Poly([P444VB][Tf2N])/[P4448][Tf2N] Ion Gel Membranes CO2 permeability (barrers)

membrane Neat poly([P444VB][Tf2N])4 85.7 wt % poly([P444VB][Tf2N]) + 4.3 wt % DVB + 10 wt % [P4448][Tf2N] 76.2 wt % poly([P444VB][Tf2N]) + 3.8 wt % DVB + 20 wt % [P4448][Tf2N] 57.1 wt % poly([P444VB][Tf2N]) + 2.9 wt % DVB + 40 wt % [P4448][Tf2N]

± ± ± ±

51 58 95 162

CH4 permeability (barrers)

1 1 3 9

4.5 4.7 8.6 16.8

± ± ± ±

0.2 0.2 0.4 0.2

N2 permeability (barrers) 2.7 2.5 4.2 7.4

± ± ± ±

H2 permeability (barrers)

0.2 0.1 0.2 0.2

31 28 37 45

± ± ± ±

1 1 1 1

Table 2. Single-Gas Permeability Data for Masked, Free-Standing Poly([P444VB][Tf2N]) and DVB-Cross-Linked Poly([P444VB][Tf2N])/[P8888][Tf2N] Ion Gel Membranes membrane

CO2

CH4

N2

H2

neat poly([P444VB][Tf2N]) 76.2 wt % poly([P444VB][Tf2N]) + 3.8 wt % DVB + 20 wt % [P8888][Tf2N] 57.1 wt % poly([P444VB][Tf2N]) + 2.9 wt % DVB + 40 wt % [P8888][Tf2N]

51 ± 1 83 ± 1 111 ± 6

4.5 ± 0.2 8.1 ± 0.1 16 ± 3

2.7 ± 0.2 4.0 ± 0.1 5± 1

31 ± 1 34 ± 1 27 ± 4

4

Table 3. Single-Gas Permeability Data for Masked, Free-Standing, Neat Poly([P444VB][Tf2N]) Membranes and DVB-CrossLinked Poly([P444VB][Tf2N])/[EMIM][Tf2N] Ion Gel Membranes CO2 permeability (barrers)

membrane neat 85.7 66.7 57.1

poly([P444VB][Tf2N])4 wt % poly([P444VB][Tf2N]) + 4.3 wt % DVB + 10 wt % [EMIM][Tf2N] wt % poly([P444VB][Tf2N]) + 3.3 wt % DVB + 30 wt % [EMIM][Tf2N] wt % poly([P444VB][Tf2N]) + 2.9 wt % DVB + 40 wt % [EMIM][Tf2N]

51 74.0 131 139

± ± ± ±

CH4 permeability (barrers)

1 0.5 2 2

4.5 5.0 7.7 9.5

± ± ± ±

0.2 0.2 0.9 0.3

N2 permeability (barrers) 2.7 3.2 4.6 5.1

± ± ± ±

H2 permeability (barrers)

0.2 0.1 0.4 0.1

31 26 25 21

± ± ± ±

1 1 1 1

Table 4. Comparison of Ideal Selectivity for DVB-Cross-Linked Poly([PnnnVB][Tf2N]) Ion Gel Membranes Containing the Free ILs [P4448][Tf2N], [P8888][Tf2N], and [EMIM][Tf2N] membrane neat 85.7 76.2 57.1 85.7 76.2 57.1 85.7 66.7 57.1

poly([P444VB][Tf2N])4 wt % poly([P444VB][Tf2N]) wt % poly([P444VB][Tf2N]) wt % poly([P444VB][Tf2N]) wt % poly([P444VB][Tf2N]) wt % poly([P444VB][Tf2N]) wt % poly([P444VB][Tf2N]) wt % poly([P444VB][Tf2N]) wt % poly([P444VB][Tf2N]) wt % poly([P444VB][Tf2N])

+ + + + + + + + +

4.3 3.8 2.9 4.3 3.8 2.9 4.3 3.3 2.9

wt wt wt wt wt wt wt wt wt

% % % % % % % % %

DVB DVB DVB DVB DVB DVB DVB DVB DVB

+ + + + + + + + +

10 20 40 10 20 40 10 30 40

wt wt wt wt wt wt wt wt wt

% % % % % % % % %

CO2/N2 selectivity

CO2/CH4 selectivity

H2/CO2 selectivity

H2/CH4 selectivity

N2/CH4 selectivity

H2/N2 selectivity

19.5 23.2 22.6 21.9

11.5 12.3 11.0 3.5

0.60 0.5 0.4 0.3

6.8 6.0 4.3 2.7

0.5 0.5 0.5 0.4

11.2 8.8 6.1

20.8 22.2 23.1 28.5 27.3

10.2 6.9 14.8 17.0 14.6

0.4 0.2 0.4 0.2 0.2

0.5 1.7 5.2 3.2 2.2

0.5 0.3 0.6 0.6 0.5

8.5 5.4 8.1 5.4 4.1

[P4448][Tf2N] [P4448][Tf2N] [P4448][Tf2N] [P8888][Tf2N] [P8888][Tf2N] [P8888][Tf2N] [EMIM][Tf2N] [EMIM][Tf2N] [EMIM][Tf2N]

ILs and an increase in the fractional free volume in the resulting PIL/IL composite resulting from having three alkyl substituents on the phosphonium IL. Interestingly, for all types of IL used in this work, increasing the free IL loading level led to significant decreases in H2/N2 selectivity, as would be expected from transitioning between a “glassy” to a “soft” membrane material, a transition observed and validated by DSC in other studies of polymer/IL composite materials.18,46

At a constant free IL loading level of 40 wt %, the CO2 permeability of the phosphonium-based PIL/IL membranes decreased with the type of free IL incorporated in the order: [P4448][Tf2N] (162 barrers) > [EMIM][Tf2N] (139 barrers) > [P8888][Tf2N] (111 barrers). Even with the relatively high IL loading level used, the permeability of these PIL/IL ion gel materials is still lower than that of the neat poly([P888VB][Tf2N] membrane material.33 As observed for the neat poly([PnnnVB][Tf2N] membrane materials we reported on previously,33 increasing the alkyl chain length of the phosphonium-based IL increases CH4 gas permeability (Tables 3 and 4), which leads to lower light-gas/CH4 selectivity values. Inclusion of the imidazolium IL [EMIM][Tf2N] produced higher CO2/N2 selectivity values than the neat poly([P4448VB][Tf2N]) (max. 28.5 vs 19); however, the addition of the phosphonium-containing ILs appeared to have little effect on light gas selectivity (CO2/N2 selectivity values ranging between 20.8 and 23.2). This is in contrast to results previously reported for other (e.g., imidazolium-based) PIL/IL ion gel membranes and is likely a combination of the lack of CO2/light gas solubility selectivity for the phosphonium-based

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CONCLUSIONS In summary, phosphonium-based PIL/IL ion gel membranes were prepared and tested for their ion conductivity and light gas transport performance. We established that the alkyl chain length on the cation of the phosphonium ILs has a significant impact on the resulting PIL/IL ion gels in terms of the stability of their ionic conductivity as a function of thermal cycling, with shorter IL chain lengths retaining ion conductivity performance better at relatively high temperature (110 °C). As observed for similar imidazolium-base systems, increasing the free IL into these ion gel materials resulted in significant improvements in ion conductivity (1000×) for all G



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temperatures tested with further improved ion conductivities at high IL loadings. However, relatively low IL loadings resulted in less than desirable ionic conductivity for the phosphonium-based PIL/IL composites when compared to imidazolium-based counterparts. Similarly, increasing the free IL content within the phosphonium-based PIL/IL ion gels afforded increased gas permeabilities, similar to previously reported trends for imidazolium-based PIL/IL ion gels.41,43 Significantly higher gas permeabilities were observed for ion gel membranes containing the [P8888] cation versus their [EMIM]-containing analogs (162 barrers versus 139 barrers). This emphasizes the importance of modulating the free IL structure to tailor material properties toward specific applications. Improvements in IL structure may advance the free IL loading capabilities, thermal stability, ion conductivity, and gas solubilities for phosphonium-based ion gels, making them more competitive for the development of electrochemical membranes for fuel cells and batteries as well as serving as membranes for CO2/light gas separations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00541. Additional data on the phosphonium-based PIL/IL ion gels including FTIR, TGA, supplemental ion conductivity, and supplemental gas permeation data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected].. *E-mail: [email protected] ORCID

Douglas L. Gin: 0000-0002-6215-668X Funding

The authors would like to acknowledge financial support for this work from the U.S. Department of Energy ARPA-E program (Grant DE-AR0000343). Notes

The authors declare no competing financial interest.

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REFERENCES

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