PEDOT-PSS embedded comb copolymer membranes with improved CO2 capture

PEDOT-PSS embedded comb copolymer membranes with improved CO2 capture

Author’s Accepted Manuscript PEDOT-PSS Embedded Comb Copolymer Membranes with Improved CO2 Capture Jae Hun Lee, Jung Pyo Jung, Eunji Jang, Ki Bong Lee...

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Author’s Accepted Manuscript PEDOT-PSS Embedded Comb Copolymer Membranes with Improved CO2 Capture Jae Hun Lee, Jung Pyo Jung, Eunji Jang, Ki Bong Lee, Yun Jeong Hwang, Byoung Koun Min, Jong Hak Kim www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(16)30717-7 http://dx.doi.org/10.1016/j.memsci.2016.06.025 MEMSCI14564

To appear in: Journal of Membrane Science Received date: 23 April 2016 Revised date: 14 June 2016 Accepted date: 17 June 2016 Cite this article as: Jae Hun Lee, Jung Pyo Jung, Eunji Jang, Ki Bong Lee, Yun Jeong Hwang, Byoung Koun Min and Jong Hak Kim, PEDOT-PSS Embedded Comb Copolymer Membranes with Improved CO2 Capture, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.06.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

PEDOT-PSS Embedded Comb Copolymer Membranes with Improved CO2 Capture Jae Hun Leea, Jung Pyo Junga, Eunji Jangb, Ki Bong Leeb, Yun Jeong Hwangc, Byoung Koun Minc,d*, Jong Hak Kima* a

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro,

Seodaemun-gu, Seoul, 03722, South Korea b

Department of Chemical and Biological Engineering, Korea University, Anam-ro 145, Seongbuk-

gu, Seoul 02841, South Korea c

Clean Energy Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5,

Seongbuk-gu, Seoul 02792, South Korea d

Green School, Korea University, Anam-ro 145, Seongbuk-gu, Seoul 02841, South Korea

[email protected] [email protected] *

Correspondence should be addressed.

Abstract Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT-PSS) is a widely used conductive polymer in various electronic devices. Here we report the first use of PEDOT-PSS to enhance CO2 capture performance of all-polymeric membranes. Specifically, an amphiphilic comb copolymer, i.e. poly(2-[3-(2H-benzotriazol-2-yl)-4-hydroxyphenyl] ethyl methacrylate)-poly(oxyethylene methacrylate) (PBEM-POEM or PBE), was synthesized to disperse PEDOT-PSS chains. Isolated and

aggregated PEDOT-PSS transformed into an interconnected network structure upon combination with PBE, due to specific interactions. Incorporation of PEDOT-PSS generated a facile pathway for enhanced diffusive transport, resulting in improved CO2 and N2 permeability. However, CO2 permeability increased more significantly due to enhanced CO2 solubility, resulting in slight increase in CO2/N2 selectivity. The PBE membrane containing PEDOT-PSS 5 wt% showed the highest performance with a CO2 permeability of 59.6 Barrer and CO2/N2 selectivity of 77.4. The performance of PBE/PEDOT-PSS membranes was very close to the 2008 Robeson upper bound and much higher than those of PBE/PEDOT, PBE/PSS and commercial PEBAX membranes. Keywords: CO2 capture; gas separation; membrane; conducting polymer; graft copolymer.

Introduction Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, otherwise known as PEDOT-PSS, is a mixture of two ionomers, PEDOT and PSS, which carry positive and negative charges, respectively. Because of its high electrical conductivity (up to 1000 S cm−1), PEDOT-PSS has been extensively used as the conducting material in electrochemical devices such as light-emitting diodes, solar cells, and sensors [1-3]. This conductivity is comparable to that of indium tin oxide, which is a transparent conducting material that is widely used in electronic devices. However, there have been no reports on the use of PEDOT-PSS in CO2 capture membranes, which is the focus of the present study.

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Recently, CO2 capture technology has attracted considerable attention because of increased awareness of CO2 emissions generated from fossil fuel combustion and industrial processes, which result in global warming [4-6]. Among the various CO2 capture technologies, such as cryogenic distillation, chemical absorption and adsorption, membrane separation has many advantages including scale-up simplicity, high energy efficiency, and environmental sustainability [7-10]. However, conventional polymer membranes typically suffer from a trade-off between permeability and selectivity [11-13]. To overcome this problem, extensive research has been conducted on the introduction of small inorganic fillers into the polymer matrix to form mixed matrix membranes. For example, metal oxide particles (e.g., TiO2 [14], SiO2 [15], and MgO [16]) and multi-dimensional porous materials (e.g., graphene [17], zeolites [18], and metal-organic frameworks [19-21]) have been used as inorganic fillers to enhance gas transport properties. Despite there being many studies on the preparation of inorganic porous materials, the use of such materials in gas separation membranes as the filler poses its own challenges in terms of optimizing selectivity, size, compatibility, and stability [22]. Thus, the development of alternative organic fillers, which might be more compatible with the organic polymer matrix, is critical for the commercialization of CO2 capture membranes. The separation of gases by polymer membranes occurs via a solution–diffusion mechanism [22]. Gas permeability is the product of the solubility and diffusivity of gas molecules. Thus, improving CO2 solubility (or CO2 uptake) is a possible approach for enhancing the performance of 2

membranes for CO2/N2 separation, especially in post-combustion of fossil fuels. Poly(ethylene oxide) (PEO) is a good candidate for achieving this goal because the Lewis acid–base interaction between the ether oxygen of PEO (Lewis base) and CO2 (Lewis acid) leads to improved CO2 solubility [23-25]. However, application of PEO membranes to CO2 capture has been limited because of their high degree of crystallinity, which results from the regular helical structure of PEO. Poly(oxyethylene methacrylate) (POEM), a PEO analogue, is completely amorphous and thus has been suggested as an alternative to crystalline PEO; however, its poor dimensional stability and sticky properties have prevented its use in CO2 capture membranes [25]. Copolymerization of PEO with other polymer chains is considered one of the possible methods for simultaneously addressing the problems of high crystallinity and poor mechanical strength [26-33]. In particular, poly(ether-block-amide), a commercially available block copolymer known as PEBAX, has been shown to exhibit good CO2 capture performance and thus is one of the most widely used materials for CO2 separation membranes [31-34]. However, the synthesis of these block copolymers is very sensitive to experimental conditions and impurities; therefore, it requires a complex and expensive process. Recently, we reported gas separation membranes based on CO2philic comb copolymers composed of poly(2-[3-(2H-benzotriazol-2-yl)-4-hydroxyphenyl] ethyl methacrylate) (PBEM) and POEM, synthesized via one-step free radical polymerization, which is a simple synthetic method and is more economical than block copolymerization [35].

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Here we report the first use of PEDOT-PSS as a polymeric filler for high-performance CO2 capture membranes. The PEDOT-PSS was introduced to a low-cost PBEM-POEM (named PBE) comb copolymer to form dual-phase, all-polymeric membranes. The interactional, structural, and morphological properties of PBE/PEDOT-PSS membranes were characterized using Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), energydispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), differential scanning calorimetry (DSC), and X-ray diffractometry (XRD). The polymers were directly coated onto microporous polysulfone support layers to form composite membranes and their CO2/N2 separation performance at 25 °C was evaluated.

Experimental Materials 2-[3-(2H-Benzotriazol-2-yl)-4-hydroxyphenyl] ethyl methacrylate (BEM, Mw = 323.35 g mol−1), poly(oxyethylene methacrylate) (POEM, poly(ethylene glycol) methyl ether methacrylate, Mn = 500 g mol−1), azobisisobutyronitrile (AIBN, Mw = 164.21 g mol−1), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT-PSS, 1.3 wt% dispersion in H2O), poly(3,4-ethylenedioxythiophene) (PEDOT), and poly(4-styrenesulfonic acid) (PSSA, Mw = 75,000, 18 wt% in H2O) were purchased from Aldrich. N,N-dimethylformamide (DMF), isopropyl alcohol (IPA), and methanol were obtained from J. T. Baker. All solvents and chemicals were reagent grade and were used as received. 4

Synthesis of PBE comb copolymer BEM (2 g) was dissolved in DMF (30 mL) by stirring in a 40 °C oil bath. After homogeneous dissolution was achieved, POEM (8 mL) and AIBN (0.002 g) were added to the solution. The mixed solution was purged with nitrogen gas for 1 h and placed in a 90 °C oil bath for 24 h. After polymerization, the polymer was precipitated in IPA. To purify the polymer, the polymer was dissolved in methanol and re-precipitated in IPA three times. The polymer was then dried completely in a vacuum oven at 50 °C. The color of the synthesized comb copolymer was light yellow. The yield of the reaction was approximately 60%.

Preparation of PBE/PEDOT-PSS membranes First, PBE (0.2 g) was dissolved in methanol (1 mL) with vigorous stirring to prepare a polymer solution. Then, PEDOT-PSS solution (0.15, 0.77, or 1.55 mL) was added to the polymer solution. The resulting membranes were labeled as PBE/PEDOT-PSS 1%, PBE/PEDOT-PSS 5%, and PBE/PEDOT-PSS 10%, respectively. The mixed solutions were then coated onto a microporous polysulfone support layer (average pore size of 0.5 μm, Toray Chemical Inc.) using an RK Control coater (Model 101, Control RK Print-Coat Instruments Ltd., UK). The prepared composite membranes were then dried in a vacuum oven at 80 °C overnight.

5

Gas permeation measurements Gas permeation measurements were conducted using a gas separation apparatus provided by Airrane Co. Ltd (Korea). The measurements were performed using a constant pressure/variable volume method according to our previous works [34-36]. The permeability of each pure gas was calculated using the gas permeance and the membrane thickness, and the membrane selectivity was defined as the ratio of the permeabilities of the gases.

Characterization Intermolecular interactions were investigated using an Excalibur series FT-IR instrument (DIGLAB CO., Hannover, Germany) in the frequency range 4000–500 cm−1. For FT-IR measurements, the PEDOT-PSS solution was cast onto a glass dish and dried in a vacuum oven overnight. The dried PEDOT-PSS film was scraped from the glass dish and used for the measurements. The morphology of the membranes was examined using a field-emission scanning electron microscope (FE-SEM, SUPAR 55VP, Carl Zeiss, Germany). TEM images were obtained using a Zeiss Libra 120 operating at 120 kV. To prepare TEM samples, polymeric solutions were drop-cast onto a standard copper grid and measured without staining. DSC measurements (DSC8000, Perkin Elmer, USA) were performed to analyze the structure and thermal properties of the polymers at a heating rate of 10 °C/min under a nitrogen atmosphere. WAXS measurements were conducted using a Rigaku RINT2000 wide-angle goniometer with a Cu cathode operated at 40 kV and 300 mA. The equilibrium CO2 uptake of the 6

polymers was investigated using thermogravimetric analysis (TGA, Q50, TA Instruments). Before adsorption of CO2, samples were pretreated at 30 °C for 1 h in a nitrogen atmosphere. The weight change of the samples was recorded under a pure CO2 gas flow at a pressure of ~1 atm at 25 °C.

Result and discussion Structure, morphology and interaction of PBE/PEDOT-PSS membranes High-performance CO2 capture membranes were designed and prepared by incorporating PEDOTPSS conducting polymer as the polymeric filler in a PBE comb copolymer matrix. It resulted in the formation of dual-phase all-polymeric membranes consisting of the PEDOT-PSS filler phase dispersed in the PBE matrix phase, as illustrated in Scheme 1. PBE is a comb copolymer synthesized via a low-cost, one-step free radical polymerization method, which is a commercially attractive method that is feasible for scale-up [35]. More importantly, since PBE is soluble in many common solvents such as alcohol, it could be directly coated onto cheap microporous polymer (e.g., polysulfone) supports, without dissolving the support layer. PBE was designed to have a nanostructured property, in which the rigid hydrophobic PBEM chains and the flexible hydrophilic POEM chains are separated. The PBEM chains, which contain three tertiary amine groups in their triazole units, not only provide a high capacity for CO2 uptake [36-38] but also compensate for the liquid-like, poor dimensional stability of POEM chains [35]. The chemical interaction between the PEDOT-PSS filler and the PBE matrix should lead to good interfacial contact of the membranes, as 7

will be characterized later. Thus, the PEDOT-PSS filler should function as a CO2 transport facilitator to boost CO2 permeability without deteriorating the dimensional stability of the membranes.

Scheme 1. Schematic of interactions and nanostructures in PBE/PEDOT-PSS membranes.

The secondary bonding interaction between the PEDOT-PSS filler and the PBE matrix was characterized using FT-IR spectroscopy, as shown in Figure 1. PBE exhibits strong absorption bands at 1726 and 1100 cm−1, which are assigned to the stretching vibration modes of carbonyl (C=O) and ether (C–O–C) groups, respectively. No strong absorption bands are observed for PEDOT-PSS because of its very low concentration (typical PEDOT-PSS is commercially available as a 1.3 wt% dispersion in water). However, when a small amount of PEDOT-PSS (5 wt%) is added to PBE, an additional carbonyl absorption band appears at 1652 cm−1 and the ether absorption band shifts from 1100 to 1080 cm−1. The shift to lower wavenumber indicates that the carbonyl and ether bonds of PBE interact with the functional groups (i.e., SO3−H+) of PEDOT-PSS through hydrogen bonding, as illustrated in Scheme 1. Specific interaction between the PEDOT-PSS filler and the PBE matrix 8

should lead to good miscibility of the two components, which is pivotal in preventing structural defects that otherwise could result in significant loss of separation properties [27,34].

1080

Absorbance

1726

1652 1100

1726

PBE/PEDOT-PSS

PBE PEDOT-PSS 4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Figure 1. FT-IR spectra of neat PBE, neat PEDOT-PSS, and PBE/PEDOT-PSS 5% membrane.

The thermal properties and interactions of PBE/PEDOT-PSS membranes were characterized by DSC analysis, as shown in Figure 2. PBE appears to be in the amorphous state without an endothermic melting temperature (Tm). The glass transition temperature (Tg) of neat PBE is observed at −52.4 °C, implying a rubbery nature and flexible properties. Upon addition of 1 wt% PEDOT-PSS to the PBE, Tg slightly increases to −50.4 °C. This might be because of hydrogen bonding interactions between the PBE matrix and the PEDOT-PSS filler leading to reduced chain mobility, as indicated by FT-IR spectroscopy. However, there is no significant difference in Tg values above 1 wt% PEDOT-PSS loading. This implies that only a small amount (~1 wt%) of PEDOT-PSS

9

participates in the hydrogen bonding interaction, regardless of the amount added. All PBE/PEDOTPSS membranes studied were completely amorphous and of rubbery nature, which is desirable for the transport of gas molecules [35-36].

Heat Flow (W/g)

exo

o

-52.4 C

PBE

o

-50.4 C

PBE/PEDOT-PSS 1%

o

-50.4 C

PBE/PEDOT-PSS 5%

o

-50.6 C

PBE/PEDOT-PSS 10%

-50

0

50

o

100

150

Temperature ( C)

Figure 2. DSC curves of neat PBE, neat PEDOT-PSS, and PBE/PEDOT-PSS 5% membrane.

XRD patterns for neat PBE, neat PEDOT-PSS, and PBE/PEDOT-PSS membranes are shown in Figure 3. No sharp or strong peaks are observed for all of the polymers, indicating their completely amorphous nature, which is consistent with the above DSC results. A broad amorphous peak centered at 2θ = 19.7° can be observed for neat PBE. Upon addition of PEDOT-PSS to PBE, the peak slightly shifts to a higher 2θ value. Using Bragg’s law, the d-spacing is determined to be approximately 4.50 Å for PBE, which is slightly reduced to 4.43, 4.41, 4.37 Å for PBE/PEDOT-PSS membranes with 1, 5 and 10 wt% loading, respectively. The reduced interchain d-spacing might

10

result from the hydrogen bonding interactions between the PEDOT-PSS filler and the PBE matrix, as revealed by the above FT-IR and DSC analysis. The d-spacing values do not significantly change above 1 wt% PEDOT-PSS loading, indicating that only a small amount (~1 wt%) of PEDOT-PSS participates in the hydrogen bonding interaction, which is consistent with the DSC results. Neat PEDOT-PSS exhibits two weak peaks at 18.4° and 25.8°, which are in good agreement with previously reported values [39-40]. However, the XRD peaks of PEDOT-PSS are scarcely observed in PBE/PEDOT-PSS membranes, regardless of the added amount, indicating that the overall structure significantly depends on the PBE matrix, rather than the PEDOT-PSS filler.

19.7 20.0

PBE

Intensity (a.u.)

20.1

PBE/PEDOT-PSS 1% PBE/PEDOT-PSS 5%

20.3

PBE/PEDOT-PSS 10% 25.8

18.4

PEDOT-PSS

5

10

15

20

25

30

35

40

Two theta (degree)

Figure 3. XRD patterns of neat PBE, neat PEDOT-PSS, and PBE/PEDOT-PSS 5% membrane.

11

Figure 4. TEM images of (a) neat PBE, (b, c) neat PEDOT-PSS, (d) PBE/PEDOT-PSS 1%, (e) PBE/PEDOT-PSS 5%, and (f) PBE/PEDOT-PSS 10%.

The morphology of neat PBE, PEDOT-PSS, and PBE/PEDOT-PSS membranes was characterized using TEM analysis, as shown in Figure 4. Neat PBE exhibits worm-like, nanostructured morphology because of the amphiphilic properties of the rigid hydrophobic PBEM and the flexible hydrophilic POEM chains. The PBEM domains appear dark and the POEM domains appear bright because of the higher electron density in the former. Neat PEDOT-PSS irregularly aggregates to form approximately 1–2 μm-sized micelles comprising a hydrophobic PEDOT core and a hydrophilic PSS shell, which might arise from a strong ionic bonding interaction between the cationic sulfur atoms in PEDOT and the anionic sulfonic acid groups in PSS [39-41]. According to a 12

previous study, the thickness of the PSS shell was approximately 5–10 nm [41], which is not clearly observed here. Interestingly, when PBE is combined with PEDOT-PSS, the isolated and agglomerated PEDOT-PSS micelles are disrupted and start to break up into small, fiber-like, threedimensional network chains. Each PEDOT-PSS chain is well interconnected and homogeneously dispersed throughout the PBE matrix. This structure could provide effective pathways for gas transport, resulting in enhanced gas permeability through the membranes. At high loading (i.e., 10 wt%), the PEDOT-PSS chains slightly re-aggregate to form darker agglomerates, which might obstruct gas transport pathways. The morphology of the PEDOT-PSS core–shell structure in the PBE matrix is illustrated in Scheme 1.

Figure 5. EDS mapping images of PBE and PBE/PEDOT-PSS membranes.

13

The elemental distribution of the PEDOT-PSS filler in the PBE matrix was confirmed by EDS-SEM analysis, as shown in Figure 5. Uniform distributions of C, O, and N atoms are observed in all membranes. For PBE/PEDOT-PSS membranes with 1 wt% and 5 wt% PEDOT-PSS loading, homogeneous distributions of S atoms are detected, whereas partial aggregation of S atoms occurs at 10 wt% loading. This indicates that aggregation occurs above a critical PEDOT-PSS loading, which is consistent with the above TEM result. The concentration of each atom was determined using the EDS mapping analysis and summarized in Table S1. The atomic concentration of S increases continuously with the amount of PEDOT-PSS in the membranes.

Figure 6. AFM images of (a) PBE, (b) PBE/PEDOT-PSS 1%, (c) PBE/PEDOT-PSS 5%, and (d) PBE/PEDOT-PSS 10% after O2 etching for 100s. 14

The morphology and microstructure of neat PBE and PBE/PEDOT-PSS membranes were characterized using AFM measurements after O2 plasma etching, as shown in Figure 6. The O2 plasma etching method was employed to obtain the microstructure of a copolymer because two (or sometimes more) polymer segments degrade at different rates because of their different reactivity with the feed gas plasma [34,42]. The rubbery hydrophilic ethylene oxide segments are found to be more reactive with O2 plasma than the rigid hydrophobic segments [35-36]. When PBE is exposed to O2 plasma, the POEM chains containing ethylene oxide groups are cleaved and degraded, whereas the rigid hydrophobic PBEM chains containing benzene rings and triazole groups remain in the structure. Therefore, the dark regions of neat PBE represent the etched POEM domains, and the bright regions represent the unetched PBEM domains. Neat PBE exhibits a self-assembled, dualphase nanostructure. Upon addition of PEDOT-PSS to the PBE matrix, the size of each region increases and each region becomes interconnected to form a valley-like structure. This might be because the POEM chains selectively assemble along the PEDOT-PSS chains because of hydrogen bonding interactions between the sulfonic groups in PEDOT-PSS and the ether oxygens or the carbonyl groups in the PBE, as indicated by FT-IR spectroscopy. The interconnected hydrophilic channels might reduce gas transport resistance, resulting in increased membrane permeability. At high PEDOT-PSS loading (10 wt%), the valley-like structure collapses and becomes isolated, which might be responsible for an increase in gas transport resistance, as will be characterized later. 15

Membrane preparation and separation performance For the commercialization of gas separation membranes, a composite membrane consisting of a top gas-selective layer (which is usually rather expensive) supported on a bottom microporous layer (cheap) is highly preferred to save on the cost of materials. Despite extensive research into highperformance CO2 capture membranes, their application in composite membranes has been limited, mostly because of problems associated with the solubility of the support layer [43-45], i.e., solvents that dissolve the top selective polymer layer also tend to dissolve the support layer. However, the PBE/PEDOT-PSS mixtures presented here could be directly coated onto a microporous polysulfone (one of the cheapest membrane materials) support layer without the solubility problem. PBE is soluble in a variety of alcohols and PEDOT-PSS is well dispersed in water/alcohol mixtures, as PSS acts as a dispersant.

16

Figure 7. Cross-sectional SEM images of membranes comprising (a) neat PBE, (b) PBE/PEDOTPSS 1%, (c) PBE/PEDOT-PSS 5%, and (d) PBE/PEDOT-PSS 10%.

Cross-sectional SEM images of PBE/PEDOT-PSS layers on polysulfone supports are shown in Figure 7. Neat PBE without PEDOT-PSS slightly penetrates into the microporous polysulfone layer, which makes the boundary between the two layers rather vague. However, when PEDOT-PSS is added to the PBE solution (even at very low concentration, such as 1 wt%), penetration of coating layer is inhibited and the two layers become clearly distinguishable. This might result from the increased hydrophilicity and viscosity of the solution due to the PEDOT-PSS chains being dispersed in water (note that the polysulfone support is hydrophobic). All of the membranes are homogeneously coated and defect-free, which is pivotal for their application to gas separation.

70

CO2 permeability (Barrer)

(b) 2.0

80

60

PBE PBE/PEDOT-PSS 1% PBE/PEDOT-PSS 5% PBE/PEDOT-PSS 10%

50 40 30 20 10

N2 permeability (Barrer)

(a)

1.5

PBE PBE/PEDOT-PSS 1% PBE/PEDOT-PSS 5% PBE/PEDOT-PSS 10%

1.0

0.5

0.0

0

Sample

Sample

17

(c)

120

CO2/N2 selectivity

100

PBE PBE/PEDOT-PSS 1% PBE/PEDOT-PSS 5% PBE/PEDOT-PSS 10%

80 60 40 20 0

Sample

Figure 8. (a) CO2 permeability, (b) N2 permeability, (c) CO2/N2 selectivity for neat PBE and PBE/PEDOT-PSS membranes.

A large amount of CO2 is emitted by fossil fuel combustion in power plants, which causes global warming. We were, therefore, interested in testing the ability of our membranes to separate CO2/N2 mixtures, which are the main components of flue gas. The gas permeability and CO2/N2 selectivity of the neat PBE and PBE/PEDOT-PSS membranes at 25 °C are shown in Figures 8a–c and Table 1. The neat PBE membrane exhibits relatively low CO2 permeability (6.1 Barrer, where 1 Barrer = 1 × 10−10 cm3 (STP) cm cm−2 s−1 cm Hg−1) but high selectivity (75.8) due to very low N2 permeability. This might be because the PBE contains CO2-philic ether and triazole groups, which enhance the CO2 permeability, and the rigid PBEM plays a key role in minimizing the N2 permeability [35]. Both the CO2 and N2 permeabilities are significantly enhanced by increasing the amount of PEDOT-PSS in the membranes. The increased N2 permeability probably results from its increased diffusivity in the membranes. As confirmed by the above TEM and AFM images, the core–shell structure of PEDOT-PSS gradually changes into a fiber-like, three-dimensional network 18

structure with good interconnectivity. Such an organized structure could be responsible for enhancing gas diffusivity through the membranes, resulting in reduced resistance to the transport of small gas molecules.

0.16

CO2 uptake (mmol/kg)

0.14 0.12

PBE PBE/PEDOT-PSS 1% PBE/PEDOT-PSS 5% PBE/PEDOT-PSS 10%

0.10 0.08 0.06 0.04 0.02 0.00

Sample

Figure 9. CO2 uptake for neat PBE and PBE/PEDOT-PSS membranes.

Table 1. Pure gas permeability and CO2/N2 selectivity of various membranes at 25 °C. Permeability (Barrer)

Selectivity

Membrane

CO2

N2

(CO2/N2)

PBE only

6.1

0.08

75.8

PBE/PEDOT-PSS 1%

22.6

0.3

76.8

PBE/PEDOT-PSS 5%

59.6

0.8

77.4

PBE/PEDOT-PSS 10%

67.8

1.4

51.0

PBE/PEDOT 5%

51.7

7.9

6.5

19

PBE/PSS 5%

67.9

8.7

7.8

PEBAX only

48.1

2.2

21.9

Furthermore, the CO2 uptake of the membranes increases from 0.052 to 0.127 mol kg−1 as the PEDOT-PSS loading is increased to 5 wt%, above which the CO2 uptake decreases (Figure 9). The enhanced capacity for CO2 uptake might arise from increased interaction between quadrupolar CO2 and polar PSS chains, leading to increased CO2 solubility and permeability in the PBE/PEDOTPSS membranes. The reduced CO2 uptake at high PEDOT-PSS loading (10 wt%) might be because of aggregation and agglomeration of the PEDOT-PSS filler in the PBE matrix, as supported by the above TEM and AFM images, which also leads to an increase in N2 permeability and a loss of CO2/N2 selectivity. Thus, the highest separation performance (i.e., CO2 permeability of 59.6 Barrer and CO2/N2 selectivity of 77.4) is obtained from the PBE/PEDOT-PSS 5 wt% membrane. To more clearly investigate which of the two components, PEDOT and PSS, is more critical, PBE membranes containing only PEDOT or only PSS were prepared and tested in the same way. As shown in Table 1, both membranes exhibit very low CO2/N2 selectivity (approximately 6–7), despite their uniform and homogeneous coating on the polysulfone support layer (Figure S1). Thus, the loss of selectivity arises from the poor interfacial interaction between the PBE matrix and the PEDOT or PSS filler, which results in increased N2 permeability. This result demonstrates that the combination of PEDOT and PSS is pivotal in enhancing the CO2 capture performance of the membranes, which is

20

due to the fiber-like, three-dimensional network structure of the PEDOT-PSS filler and its good interconnectivity in the PBE matrix.

10000 PBE PBE/PEDOT-PSS 1% PBE/PEDOT-PSS 5% PBE/PEDOT-PSS 10% PEBAX

P(CO2)/P(N2)

1000 Upper

100

Bound

(2008)

10

1 1

10

100

1000

10000

P(CO2) (Barrers)

Figure 10. Plot of CO2 permeability vs. CO2/N2 selectivity for neat PBE, neat PEBAX, and PBE/PEDOT-PSS membranes.

A Robeson plot for CO2/N2 separation at different PEDOT-PSS loadings is shown in Figure 10, together with the upper bound limit [46]. Also, a plot of CO2 permeability vs. CO2/N2 selectivity for PBE/PEDOT-PSS membranes at different temperatures (25, 35, 45 and 55 oC) is shown in Figure S3. The separation performance at 25 oC was not significantly different from that at 35 oC. Upon addition of PEDOT-PSS filler to the PBE matrix, the CO2 permeability significantly increases and the selectivity is maintained. The separation performance of the PBE/PEDOT-PSS 5 wt% membrane is very close to the 2008 Robeson upper bound line and considerably higher than that of commercial PEBAX membrane (CO2 permeability of 48.1 Barrer and CO2/N2 selectivity of 21.9). It should also 21

be noted that upon addition of PEDOT-PSS filler to PEBAX solution, the solution forms a heterogeneous gel (Figure S2). This indicates that PBE is a more effective matrix for PEDOT-PSS filler in terms of interfacial properties and miscibility. Furthermore, PBE is easily synthesized via one-step free radical polymerization, making it feasible for scale-up and commercialization.

Conclusions We report the first use of PEDOT-PSS conducting polymer as an organic filler to form all-polymeric membranes with enhanced CO2 capture performance. In particular, the PBE comb copolymer is an effective matrix for the dispersal of the PEDOT-PSS filler to form homogeneous membranes, which is due to hydrogen bonding interactions between PSS and PBE, as confirmed by FT-IR. DSC results show that Tg slightly increases with the addition of PEDOT-PSS, which supports there being interactions between the PBE matrix and the PEDOT-PSS filler. All PBE/PEDOT-PSS membranes studied were of a rubbery and amorphous nature. The isolated, agglomerated PEDOT-PSS chains changed into fiber-like, three-dimensional network structures with good interconnectivity when combined with the PBE matrix. This structure is believed to be responsible for enhanced gas diffusivity through the membranes containing PEDOT-PSS. Furthermore, the CO2 uptake increased with PEDOT-PSS loading up to 5 wt%. Thus, the enhanced CO2 permeability and CO2/N2 selectivity of PBE/PEDOT-PSS membranes result from the simultaneous increase in CO2 solubility and diffusivity. The separation performance of PBE/PEDOT-PSS 5 wt% membrane was the highest with 22

a CO2 permeability of 59.6 Barrer and CO2/N2 selectivity of 77.4, which is considerably higher than that of commercial PEBAX membranes and very close to the 2008 Robeson upper bound.

Acknowledgements We acknowledge financial support from the National Research Foundation (NRF) through the Korea Center for Artificial Photosynthesis (KCAP) (2009-0093883).

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Highlights 

The first use of PEDOT-PSS to enhance CO2 capture performance was demonstrated.



PBE played an effective role as the matrix to disperse PEDOT-PSS.



Isolated and aggregated PEDOT-PSS changed to interconnected network structure.



CO2 permeability of 59.6 Barrer and CO2/N2 selectivity of 77.4 were obtained.



This performance is much better than commercially available PEBAX membrane.

Graphical Abstract

PEDOT-PSS

PBE comb copolymer

High CO2 capture PBE/PEDOT-PSS membranes

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