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Sulfonated polymers containing polyhedral oligomeric silsesquioxane (POSS) core for high performance proton exchange membranes
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 7 1 3 5 e7 1 4 3
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Sulfonated polymers containing polyhedral oligomeric silsesquioxane (POSS) core for high performance proton exchange membranes Jie Zhang a, Fang Chen a,*, Xiaoyan Ma a,*, Xinghua Guan a, Dongyang Chen b, Michael A. Hickner b,** a
Department of Applied Chemistry, School of Natural and Applied Science, Northwestern Polytechnical University, 710129, Xi'an, Shaanxi, PR China b Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, 16802, United States
article info
abstract
Article history:
A series of POSS containing star-shaped block copolymers were synthesized by atom
Received 15 December 2014
transfer radical polymerization (ATRP), with POSS-(Cl)8 as initiator, polymethyl methac-
Received in revised form
rylate as the first building block, and polystyrene as the second building block. Sulfonation
11 February 2015
of the polystyrene block yielded star-shaped ionic polymers POSS-(PMMA-b-SPS)8 that
Accepted 21 February 2015
were evaluated as proton exchange membranes (PEMs) subsequently. Under low relative
Available online 24 April 2015
humidities (RHs), the PEM with longer SPS block exhibited higher proton conductivity than the PEM with shorter SPS block when compared at same hydration number (l) conditions,
Keywords:
which was attributed to the better connected hydrophilic domains of the former as evi-
Proton exchange membrane
denced by electron microscopes. However, this conductivity trend for the two PEMs was
POSS containing block copolymer
reversed at 100% RHs. Low field nuclear magnetic resonance analysis revealed that the PEM
Sulfonation
with shorter SPS block had more loosely bonded water than the PEM with longer SPS block
Morphology
at 100% RH, giving an explanation of why the conductivity trend was reversed. This study
Proton conductivity
suggested that both ionic domain structure and waterepolymer interaction are important parameters for achieving high proton conductivities. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Proton exchange membrane fuel cells (PEMFCs) have attracted a great amount of attention as clean power sources for stationary and portable applications due to their low CO2
emission, low noise and high efficiency [1e6]. One of the key challenge in the design of novel PEM materials for PEMFCs is to retain high proton conductivity at low relative humidity (RH, <50%) and high temperature (>100 C) [7e11]. Accordingly, desirable PEMs should possess a good balance of waterepolymer interactions to retain water at high temperature,
* Corresponding authors. Tel.: þ86 29 88431676. ** Corresponding author. Tel.: þ1 814 867 1847. E-mail addresses: [email protected] (F. Chen), [email protected] (X. Ma), [email protected] (M.A. Hickner). http://dx.doi.org/10.1016/j.ijhydene.2015.02.090 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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yet facilitate the dynamical motion of water that leads to high proton conductivity [12]. The connectivity, morphology, and size of the water-absorbing ionic domains have been recognized as the main factors that determines the physical properties and proton conductivity of the PEMs [13e15]. Ion-containing block copolymers have been studied as potential candidates for PEM applications, because they have higher proton conductivity than random copolymers at similar sulfonation level. It is believed that the higher proton conductivity of block copolymers is due to their well-defined hydrophilic-hydrophobic phase separation [16,17]. Zhang [18] studied the PEM performance of multiblock copolymers based on sulfonated copolyimides which was highly rely on the distribution of sulfonic acid and the length of the blocks. Miyatake [19] reported that Poly(arylene ether sulfone ketone) (SPESK) multiblock copolymer membranes shows high local concentration of sulfonic acid groups within the hydrophilic blocks enhanced phase separation between the hydrophobic and hydrophilic blocks. The hydrophobic domains contribute to the mechanical properties of PEMs while the hydrophilic domains are responsible for proton conduction. Takinoto [20] reported that some hydrocarbon type PEMs such as PEEKs preserve high water uptake, however they only show very poor proton conductivity under low RH, because of the lack of phase continuity from the irregular distribution of the sulfonic acid groups. McGrath [13] reported that there is a distinct difference in morphology between block copolymer and random copolymer with same components. In details, a random copolymer with a statistical distribution of sulfonic acid groups had very small domain sizes, whereas an alternating polymer with sulfonic acid groups spaced evenly along the polymer chain was found to have larger, but quite isolated, domains. A multiblock copolymer showed highly phaseseparated hydrophilic and hydrophobic domains, with good long-range connectivity. Consequently, the RH dependence of proton conductivity for the multiblock copolymer is significantly reduced compared to non-block samples. Therefore, the distribution of sulfonic acid groups along polymer chains has a strong impact on proton conductivity. It is still challenging to tune the connectivity of hydrophilic domains for fast proton transport. Multiblock or graft copolymers with controlled segmental length [13], side-chain type sulfonated polymers with controlled sulfonation sites [21], and non-linear block copolymers with complex structures [10,22e24], have been reported to possess promising morphologies for PEMs. Polyhedral oligomeric silsesquioxane (POSS) containing block copolymers have great potential in designing novel ordered nanostructures due to its easy functionalization and well-defined self-assembly [25]. POSS filler has been incorporated into PEMs to achieve enhanced proton conductivity, physical and chemical durability and mechanical properties [26]. The main motivation of this study is to design sulfonated star-shaped POSS containing block copolymers and investigate their morphology-property relationship for PEM applications. We used a POSS core as macroinitiator to polymerize methyl methacrylate as the inner block and styrene as the outside block. The polystyrene block was sulfonated subsequently to afford novel PEMs. The water sorption, proton conductivity and morphology of the samples were fully characterized. Also, the segmental
dynamics under low hydrated states and water distributions under fully hydration were investigated by low field nuclear magnetic resonance to give insight into the protonconduction mechanisms under low and high RHs.
Experimental Materials POSS-(Cl)8 was synthesized and used as initiator, as described in a previous report [27]. All chemicals were of reagent grade and used as received, unless specified otherwise. Toluene, anhydrous methanol, 1,2-dichloroethane (DCE), tetrahydrofuran (THF), neutral alumina, Methyl methacrylate (MMA), Styrene (St), and N,N-dimethylformamide (DMF) were purchased from Tianjin Fuyu Fine Chemicals Co. Ltd. Pentamethyl divinyl amine (PMDETA) was purchased from Shanghai Jingchu Reagent Co. Ltd. Concentrated sulfuric acid (H2SO4) was purchased from Beijing Chemical Co. MMA and St were washed using an aqueous solution of sodium hydroxide (5 wt %) for three times and then with deionized water until neutralization to remove presented inhibitors and further purified by reduced pressure distillation. Copper (I) chloride (CuCl) was purchased from BASF Co. Ltd. and treated with an appropriate amount of glacial acetic acid under dark stirring for 10 h, washed by anhydrous ethanol and acetone sequentially, dried in a vacuum oven at 40 C for 24 h, and finally stored it in a brown bottle prior to use.
Synthesis of POSS-(PMMAm-b-SPSn)8 block copolymers The synthesis route is depicted in Scheme 1. A series of POSS containing block copolymers POSS-(PMMAm-b-PSn)8 with different block length ratios were synthesized via atom transfer radical polymerization (ATRP) using a two-step approach, and then subjected to post-sulfonation. At the first step, POSS-(Cl)8 (0.2 g), PMDETA (0.15 ml), CuCl (0.015 g), MMA (20 ml) and toluene (20 ml) were added in sequence to a three-necked flask with continuous stirring at 70 C and then the temperature was gradually increased to 110 C and maintained at this temperature until the viscosity of the system increased significantly. The product was dissolved with a large amount of tetrahydrofuran, passed through a neutral alumina column to remove the catalyst, and precipitated out by a large amount of anhydrous methanol and then dried in a vacuum oven at 50 C to constant weight, yielding white solid polymer POSS-(PMMAm-Cl)8 in 48.3% yield. 1H NMR (CDCl3, 500 MHz): 0.99e1.39 ppm (6H, m),1.7e1.86 (2H, d), 1.5e1.57 ppm (3H, s), 3.44 ppm (3H, s). At the second step, the obtained POSS-(PMMAm-Cl)8 was used as macroinitiator. POSS-(PMMAm-Cl)8 (2 g), PMDETA (0.15 ml), CuCl (0.015 g), St (10 ml or 20 ml) and toluene (20 ml) were added to a nitrogen-flushed three-necked flask with continues stirring. The mixture was gradually increased to 110 C and maintained at this temperature for 48 h. Same purification procedure as POSS-(PMMAm-Cl)8 was applied and white solid polymer POSSe(PMMAm-b-PSn)8 was obtained in 37.6% yield. 1H NMR (CDCl3, 500 MHz): 6.44e7.25 ppm (6H, t), 3.67 ppm (3H, s), 1.29e1.5 ppm(6H, m), 1.3e1.9 ppm(5H, t).
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Scheme 1 e Synthesis scheme of POSS-(PMMAm-b-SPSn)8.
Post-sulfonation: POSSe(PMMAm-b-PSn)8 (2.5 g) was dissolved in DCE (25 ml) in a three-necked flask, concentrated H2SO4 (2 ml) was dropwise added into the flask at 50 C under vigorous stirring. The mixture were stirred for additional 24 h. The product was washed by deionized water until seeing brown precipitation from the solution. The solid product was washed by D.I. water until the PH value of waste water reached 7, then dried at 50 C in a vacuum oven to constant weight. Yellowish polymer, POSSe(PMMAm-b-SPSn)8, was obtained in 58.3% yield.
Membrane preparation POSSe(PMMAm-b-SPSn)8 (0.5 g) was dissolved in DMF (10 ml) at 60 C, and then poured into a Petri dish and dried in an oven at 65 C for 8 h. Membranes were formed on the dish surface during this process, and were dried in a vacuum oven at 80 C for another 24 h.
Instruments The Fourier transform infrared spectrometer of block copolymers were carried out by WQF - 520, which DMF as solvent dissolved coating in potassium bromide tabletting, scan wave number range of 400e4000 cme1.1H nuclear magnetic
resonance (1H NMR)was tested by Bruker-AV400 nuclear magnetic resonance spectroscopy, with a scanning frequency of 400 MHz CDCl3,DMSO and TMS was selected as solvent and internal standard, respectively. The molecular weight and polydispersity of the block copolymers were characterized by WATERS 1500-C GPC, with Shodex OHpak SB-803 HQ(300 8 mm) as chromatographic column and THF as eluent. X-ray photoelectron spectroscopies were recorded on a Ka ALTHA spectrometer (VG company, UK). Higheresolution spectra were acquired from 0 to 1400 eV. All binding energies were referenced to C 1s peak for neutral carbon, which is 284.6 eV. Thermo-gravimetric analysis (TGA) was characterized by Q500(American TA company) under nitrogen atmosphere, the temperature will increased from room temperature to 600 with heating rate of 10 C/min.The surface morphology of POSS-(PMMAm-b-SPSn)8 membranes were measured by an AFM (MFP-3D-SA, Asylum based PEM Research Inc., USA) with ARC2TM scanning probe using tapping mode. The nanostructure morphology was also captured by HITACHI H-7650 TEM with an accelerating voltage of 80 KV. PEMs were stained by soaking in 0.5 M Lead acetate solution at room temperature for a week, then rinsed with deionized water and dried under vacuum at room temperature overnight. Then the stained PEMs were embedded in epoxy resin, cured under frozen and cut into 100 nm thick slices. The
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mechanical property of PEMs were carried out by universal testing machine RGT-1(Shenzhen REGER instrument Co.,Ltd.)
where V1 and V2 are the volumes (mL) of the NaOH solution and HCl solution respectively, and m (g) is the weight of the dry membranes.
Proton conductivity(s) measurement The proton conductivity (s) of POSSe(PMMAm-b-SPSn)8 was measured using the AC impedance method, and calculated by equation (1): s¼
L RWT
(1)
where L means the distance between the two platinum electrodes, R represents the impedance of the sample, W and T are the width and thickness of the sample, respectively. The measurements were carried out by Solartron 1260 a frequency response analyzer, with scanning frequency ranging from 100 Hz to 1 MHz. The proton conductivity under different relative humidities was carried out in a constant temperature and humidity chamber Espec SH - 241. Samples were conditioned at specific relative humidities for 1 h before testing.
Water uptake and hydration number Water uptake as function of temperature was determined by immersing dry membranes in deionized water at different temperature points for 1 h. The membranes were taken out and excess water on the surface of the sample was removed by wiping gently with filter papers. The membranes were immediately weighed on a microbalance. The water uptake was calculated according to the following equation: h¼
mwet mdry 100% mdry
(2)
where mwet is the weight of the wet film and mdry is the weight of the dry film. Water uptake as a function of relative humidity at 30 C was measured using a TA instruments Q5000SA dynamic vapor sorption analyzer. The relative humidity steps and equilibration times were the same as used in conductivity measurements. Hydration number was calculated through equation (3), l¼
mwet mdry 1000 mdry IEC 18:01
(3)
where mwet is the weight of the wet film under under different relative humidities, and mdry is the weight of the dry film.
Ion exchange capacity (IEC) The experimental IECs of POSS-(PMMAm-b-SPSn)8 ionomers were determined using a titration method. Membranes were immersed in an excess amount of sodium hydroxide (0.01 mol L1) solution for 12 h before titration. Then the aqueous solution were titrated with a standard HCl (0.01 mol L1) solution. The experimental IEC values of the ionomer membranes were calculated according to the equation below: IEC ¼
0:01ðV1 V2 Þ 103 meqg1 m
(4)
Low-filed NMR measurements Spinspin relaxation time T2 and water 1H nuclei measurements of the block copolymers were performed on a Niumag Desktop Pulsed NMR Analyzer (Shanghai Niumag Electronics Technology Co. Ltd.) using the CarrPurcellMeiboomGill (CPMG) pulse sequence. A spacing of 2 ms between the 90 and 180 pulse was used, and a recycle delay of at least 5 times of the spinlattice relaxation time between consecutive scans was used to ensure full recovery of the magnetization between acquisitions. All relaxation measurements were performed at 25 C. The spinspin relaxation time, T2r and T2f of different block copolymers, were determined by fitting each free-induction decay FID curve via nonlinear least-squares algorithm: t t þ M0f exp þ M0 yðtÞ ¼ M0r exp T2r T2f
(5)
in which M0r represents magnetization at the beginning of the pulse sequence of rigid block, and M0f represents the magnetization at the beginning of the pulse sequence of flexible block. Proton Spin diffusion coefficient calculation method was the same as in Ref. [28]. The T2 distribution of different water components of wet PEMs were analyzed by the distributed exponential fitting analysis using the Multi Exp Inv Analysis Software developed by Niumag Co., Ltd., China. A continuous exponentials distribution of the CPMG experiment was defined by the following equation: Z∞ gi ¼
AðTÞeti =T dT
(6)
0
where gi is the intensity of the decay at time ti and A(T) is the amplitude of the component with transverse relaxation time T. This analysis resulted in a plot of relaxation amplitude for individual relaxation processes versus relaxation time. The time constant for each peak was calculated from the peak position.
Results and discussion ATRP was successfully used to synthesize two star-shape block copolymers, with PMMA as the first block and PS as the second block. The basic physicochemical properties of the two copolymers are listed in Table 1. It can be seen that both samples have high molecular weight. After determining the
Table 1 e Molecular weight information of the samples. Sample
Mn Mw Mw/Mn m n X(Mw) Y(Mw)
POSS-(PMMA26-b-PS156)8 62 k 151 k POSS-(PMMA16-b-PS200)8 99 k 188 k
2.4 1.9
26 156 2730 16 200 1664
16,380 21,000
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Fig. 1 e (a) Proton Conductivity of PEMs as a fuction of RH at 30 C and 80 C; (b) Proton conductivity of PEMs as a fuction of hydration number (l) at 30 C.
molar ratio of PMMA block and PS block by 1H NMR, the numbers of the repeating units can be derived, which are used to describe each sample, namely POSS- (PMMA26-b-PS156)8 and POSS-(PMMA16-b-PS200)8. These samples are sulfonated to yield POSS- (PMMA26-b-SPS156)8 and POSS-(PMMA16-b-SPS200)8, respectively. Their ion exchange capacities (IECs) are titrated to be 2.77 mmol g1 and 3.85 mmol g1. The proton conductivities under different RHs were measured at 30 and 80 C, Fig. 1. It can be seen that the proton conductivity of POSS-(PMMA16-b-SPS200)8 is higher than that of POSS-(PMMA26-b-SPS156)8 at all the RH/temperature conditions. The proton conductivity of POSS-(PMMA26-b-SPS156)8 shows higher dependence on RH compared to that of POSS(PMMA16-b-SPS200)8, while both the productivities are not sensitive to the change in temperature at these low RH conditions. At 30% RH, the proton conductivity of POSS-(PMMA16b-SPS200)8 is ~5 times higher than that of POSS-(PMMA26-bSPS156)8. Fig. 1(b) is a plot of proton conductivity against the hydration number (l). It is obvious that the proton conductivity of two PEMs increases with the increasing of hydration number. POSS-(PMMA16-b-SPS200)8 exhibits higher proton conductivity than POSS-(PMMA26-b-SPS156)8 at similar hydration numbers.
The hydration number and proton conductivity of two PEMs as a function of temperature under fully hydration states are shown in Fig. 2. (Water uptake as a function of temperature of two PEMs was plotted in figure S6) POSS(PMMA26-b-SPS156)8 has larger hydration number and higher proton conductivity than POSS-(PMMA16-b-SPS200)8 at all the temperatures. The consistent relationship of proton conductivity and hydration number suggests that the proton conductivity for these two PEMs under fully hydration condition is ruled by vehicle mechanism. Morphologies of the two PEMs were firstly determined by TEM and shown in Fig. 3. In the TEM images, ionic block SPS appears as dark areas while the PMMA block appears as light areas. Nanoscale phase separation can be clearly seen in both images. Comparing the two images, a cylinder-like morphology with domain size of around 100 nm is observed for POSS-(PMMA26-b-SPS156)8, while a long and narrow fiberliked morphology with a excellent connectivity of ionic domains (around 30e40 nm) is observed for POSS-(PMMA16-bSPS200)8. In general, the formation of hydrophilic and hydrophobic domains depends on several factors, such as acidity, concentration and distribution of sulfonated groups, and difference in backbone/side chain mobility [29]. Star-shape
Fig. 2 e (a) l and (b) proton conductivity of the membranes as a function of temperature at 100% RH.
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Fig. 3 e TEM images of (a) POSS-(PMMA26-b-SPS156)8 and (b) POSS-(PMMA16-b-SPS200)8.
copolymers are expected to behave differently with linear polymers because of the interactions between the blocks. The different morphologies of two copolymers can be attributed to the different POSS core attractions to the ionic SPS block, since they have different lengths. An inter-connected columnar morphology with domain size around 100 nm is observed for
POSS-(PMMA26-b-SPS156)8 due to severe mobility restriction to outer SPS block from POSS sore. By replacing hydrophilic block with a longer SPS block, higher mobility will be performed because of the reduction in mobility restriction to SPS block from POSS core. Meanwhile, a fiber-liked morphology with excellent connectivity of ionic domains is formed to achieving
Fig. 4 e AFM images of (a) POSS-(PMMA26-b-SPS156)8 and (b) POSS-(PMMA16-b-SPS200)8.
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Fig. 5 e The dotted line represents FIDs of two PEMs and solid line represent two-exponentially decaying fitting.
Fig. 6 e T2 distribution of hydrated POSS-(PMMA26-bSPS156)8 and POSS-(PMMA16-b- SPS200)8 PEMs.
equilibrium. It is been found a well connected with hydrophobic and hydrophilic domain as well. The well-designed microscopic morphology, which can connect the proton path with a small amount of absorbed water, is necessary to improve proton conductivity at low RH [20]. Tapping-mode atomic force microscopy (AFM) was also employed to investigate the morphologies of block copolymers under ambient conditions, Fig. 4. The bright and dark regions are corresponding to the hydrophobic and hydrophilic units, respectively. Comparing two AFM images, the corresponding periodic structure with well connected hydrophobic and hydrophilic domains was observed for copolymer with longer SPS block, which is well consistent with TEM analysis results. This morphology investigation will attract a great interest because it showed that various phase separated structure might be achieved by a proper molecular design of star-shaped copolymers. Combined the TEM and AFM analysis, we could illustrate that the well-connected ionic domains morphology caused by longer SPS block could play a positive role in proton conductivity under low RH. Low-field nuclear magnetic resonance (LF-NMR) has been applied by others to determine the segmental and chain dynamic parameters such as spinespin relaxation time (T2) and spin-diffusion coefficients for block copolymers [30]. In this study, we studied T2 and proton spin-diffusion processes through free-induction decay (FID) of dry PEM to analyze segmental dynamics difference induced by the two blocks to understand proton diffusion under low RH. Typical FID curves
of dry PEMs are shown in Fig. 5, and colored lines showed the well-fitted discrete bi-exponential fitting procedure of the dry PEMs. The related relaxation constants are listed in Table 2. It is worth mentioning that a larger spinespin diffusion efficient of rigid phase is detected for POSS-(PMMA16-bSPS200)8 after comparing D(T2r) of the two PEMs. Since POSS(PMMA16-b-SPS200)8 have higher sulfonation density, we can conclude that the sulfonation plays a major role in influencing proton conductivity under low RH. T2 distribution of different water components shows the spinespin relaxation process of protons, explaining the different chemical environments of water components and reflecting the speed of energy equilibrium recovery process. In general, T2 and its amplitude values represent the speed and strength of spinespin relaxation, respectively [31]. Both the T2 distribution of two fully hydrated PEMs were measured and displayed in Fig. 6. Two peaks corresponding to two water components are observed, in which the difference of T2 and amplitude of two PEMs is clearly presented. The T2s and T2l represent strongly bonded water and loose bonded water to copolymer matrix, respectively. It is clearly shown that the loosely bonded water component in POSS-(PMMA26-b-SPS156)8 is weaker than that in POSS-(PMMA16-b-SPS200)8. The diffusivity of H3Oþ will increase with the increasing of loosely bonded water and facilitate proton conductivity on the basis of the vehicle mechanism. This is consistent with the proton conductivity behavior difference between two PEMs under fully hydration states.
Table 2 e Relaxation time constants (T2) of two different blocks and corresponding concentrations (M0) obtained by discrete bi-exponential fitting of the FID curves of dry PEMs obtained at room temperature and spin-diffusion coefficients (D(T2r)) for rigid phase(SPS). Sample
Rigid phase (SPS) T2r/ms
POSS-(PMMA26-b-SPS156)8 POSS-(PMMA16-b-SPS200)8
0.9 0.7
2
D(T2r)/nm ms 0.29 0.33
1
Flexible phase(PMMA) M0r
T2f/ms
M0f
M0
612.8 553.4
5.2 3.7
194.4 261.5
56.1 64.4
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Conclusions Two POSS containing block copolymers (POSS-PMMA-b-PS) with different block ratios were synthesized by ATRP. Subsequent sulfonation yielded two novel star-polymers for high performance proton exchange membranes, namely POSS(PMMA26-b-SPS156)8 and POSS-(PMMA16-b-SPS200)8. The POSS(PMMA16-b-SPS200)8 showed higher proton conductivity than POSS-(PMMA26-b-SPS156)8 under low RHs at same hydration number (l) conditions, but lower proton conductivity under full hydration state. It is believed that the excellent phase connectivity of POSS-(PMMA16-b-SPS200)8, as visualized by TEM and AFM, provides it higher proton conductivity at low RH and the proton conductivity is less dependent on RH compared to that of POSS-(PMMA26-b-SPS156)8. Higher amount of loosely bonded water was found for the PEM with shorter SPS block at 100% RH by low-field NMR, which is consistent with its higher proton conductivity at such condition compared to that of the PEM with longer SPS block. This study suggested that both ionic domain structure and waterepolymer interaction are important parameters for achieving high proton conductivities.
Acknowledgments This research is supported by National Natural Science Foundation of China (No. 51103117) and The Natural Science Foundation of Shaanxi Province (2013JQ2010 and 2013JM2012); NPU Foundations for Fundamental Research (No. 3102014JCQ01089).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.02.090.
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Sulfonated polymers containing polyhedral oligomeric silsesquioxane (POSS) core for high performance proton exchange membranes Jie Zhang a, Fang Chen a,*, Xiaoyan Ma a,*, Xinghua Guan a, Dongyang Chen b, Michael A. Hickner b,** a
Department of Applied Chemistry, School of Natural and Applied Science, Northwestern Polytechnical University, 710129, Xi'an, Shaanxi, PR China b Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, 16802, United States
article info
abstract
Article history:
A series of POSS containing star-shaped block copolymers were synthesized by atom
Received 15 December 2014
transfer radical polymerization (ATRP), with POSS-(Cl)8 as initiator, polymethyl methac-
Received in revised form
rylate as the first building block, and polystyrene as the second building block. Sulfonation
11 February 2015
of the polystyrene block yielded star-shaped ionic polymers POSS-(PMMA-b-SPS)8 that
Accepted 21 February 2015
were evaluated as proton exchange membranes (PEMs) subsequently. Under low relative
Available online 24 April 2015
humidities (RHs), the PEM with longer SPS block exhibited higher proton conductivity than the PEM with shorter SPS block when compared at same hydration number (l) conditions,
Keywords:
which was attributed to the better connected hydrophilic domains of the former as evi-
Proton exchange membrane
denced by electron microscopes. However, this conductivity trend for the two PEMs was
POSS containing block copolymer
reversed at 100% RHs. Low field nuclear magnetic resonance analysis revealed that the PEM
Sulfonation
with shorter SPS block had more loosely bonded water than the PEM with longer SPS block
Morphology
at 100% RH, giving an explanation of why the conductivity trend was reversed. This study
Proton conductivity
suggested that both ionic domain structure and waterepolymer interaction are important parameters for achieving high proton conductivities. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Proton exchange membrane fuel cells (PEMFCs) have attracted a great amount of attention as clean power sources for stationary and portable applications due to their low CO2
emission, low noise and high efficiency [1e6]. One of the key challenge in the design of novel PEM materials for PEMFCs is to retain high proton conductivity at low relative humidity (RH, <50%) and high temperature (>100 C) [7e11]. Accordingly, desirable PEMs should possess a good balance of waterepolymer interactions to retain water at high temperature,
* Corresponding authors. Tel.: þ86 29 88431676. ** Corresponding author. Tel.: þ1 814 867 1847. E-mail addresses: [email protected] (F. Chen), [email protected] (X. Ma), [email protected] (M.A. Hickner). http://dx.doi.org/10.1016/j.ijhydene.2015.02.090 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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yet facilitate the dynamical motion of water that leads to high proton conductivity [12]. The connectivity, morphology, and size of the water-absorbing ionic domains have been recognized as the main factors that determines the physical properties and proton conductivity of the PEMs [13e15]. Ion-containing block copolymers have been studied as potential candidates for PEM applications, because they have higher proton conductivity than random copolymers at similar sulfonation level. It is believed that the higher proton conductivity of block copolymers is due to their well-defined hydrophilic-hydrophobic phase separation [16,17]. Zhang [18] studied the PEM performance of multiblock copolymers based on sulfonated copolyimides which was highly rely on the distribution of sulfonic acid and the length of the blocks. Miyatake [19] reported that Poly(arylene ether sulfone ketone) (SPESK) multiblock copolymer membranes shows high local concentration of sulfonic acid groups within the hydrophilic blocks enhanced phase separation between the hydrophobic and hydrophilic blocks. The hydrophobic domains contribute to the mechanical properties of PEMs while the hydrophilic domains are responsible for proton conduction. Takinoto [20] reported that some hydrocarbon type PEMs such as PEEKs preserve high water uptake, however they only show very poor proton conductivity under low RH, because of the lack of phase continuity from the irregular distribution of the sulfonic acid groups. McGrath [13] reported that there is a distinct difference in morphology between block copolymer and random copolymer with same components. In details, a random copolymer with a statistical distribution of sulfonic acid groups had very small domain sizes, whereas an alternating polymer with sulfonic acid groups spaced evenly along the polymer chain was found to have larger, but quite isolated, domains. A multiblock copolymer showed highly phaseseparated hydrophilic and hydrophobic domains, with good long-range connectivity. Consequently, the RH dependence of proton conductivity for the multiblock copolymer is significantly reduced compared to non-block samples. Therefore, the distribution of sulfonic acid groups along polymer chains has a strong impact on proton conductivity. It is still challenging to tune the connectivity of hydrophilic domains for fast proton transport. Multiblock or graft copolymers with controlled segmental length [13], side-chain type sulfonated polymers with controlled sulfonation sites [21], and non-linear block copolymers with complex structures [10,22e24], have been reported to possess promising morphologies for PEMs. Polyhedral oligomeric silsesquioxane (POSS) containing block copolymers have great potential in designing novel ordered nanostructures due to its easy functionalization and well-defined self-assembly [25]. POSS filler has been incorporated into PEMs to achieve enhanced proton conductivity, physical and chemical durability and mechanical properties [26]. The main motivation of this study is to design sulfonated star-shaped POSS containing block copolymers and investigate their morphology-property relationship for PEM applications. We used a POSS core as macroinitiator to polymerize methyl methacrylate as the inner block and styrene as the outside block. The polystyrene block was sulfonated subsequently to afford novel PEMs. The water sorption, proton conductivity and morphology of the samples were fully characterized. Also, the segmental
dynamics under low hydrated states and water distributions under fully hydration were investigated by low field nuclear magnetic resonance to give insight into the protonconduction mechanisms under low and high RHs.
Experimental Materials POSS-(Cl)8 was synthesized and used as initiator, as described in a previous report [27]. All chemicals were of reagent grade and used as received, unless specified otherwise. Toluene, anhydrous methanol, 1,2-dichloroethane (DCE), tetrahydrofuran (THF), neutral alumina, Methyl methacrylate (MMA), Styrene (St), and N,N-dimethylformamide (DMF) were purchased from Tianjin Fuyu Fine Chemicals Co. Ltd. Pentamethyl divinyl amine (PMDETA) was purchased from Shanghai Jingchu Reagent Co. Ltd. Concentrated sulfuric acid (H2SO4) was purchased from Beijing Chemical Co. MMA and St were washed using an aqueous solution of sodium hydroxide (5 wt %) for three times and then with deionized water until neutralization to remove presented inhibitors and further purified by reduced pressure distillation. Copper (I) chloride (CuCl) was purchased from BASF Co. Ltd. and treated with an appropriate amount of glacial acetic acid under dark stirring for 10 h, washed by anhydrous ethanol and acetone sequentially, dried in a vacuum oven at 40 C for 24 h, and finally stored it in a brown bottle prior to use.
Synthesis of POSS-(PMMAm-b-SPSn)8 block copolymers The synthesis route is depicted in Scheme 1. A series of POSS containing block copolymers POSS-(PMMAm-b-PSn)8 with different block length ratios were synthesized via atom transfer radical polymerization (ATRP) using a two-step approach, and then subjected to post-sulfonation. At the first step, POSS-(Cl)8 (0.2 g), PMDETA (0.15 ml), CuCl (0.015 g), MMA (20 ml) and toluene (20 ml) were added in sequence to a three-necked flask with continuous stirring at 70 C and then the temperature was gradually increased to 110 C and maintained at this temperature until the viscosity of the system increased significantly. The product was dissolved with a large amount of tetrahydrofuran, passed through a neutral alumina column to remove the catalyst, and precipitated out by a large amount of anhydrous methanol and then dried in a vacuum oven at 50 C to constant weight, yielding white solid polymer POSS-(PMMAm-Cl)8 in 48.3% yield. 1H NMR (CDCl3, 500 MHz): 0.99e1.39 ppm (6H, m),1.7e1.86 (2H, d), 1.5e1.57 ppm (3H, s), 3.44 ppm (3H, s). At the second step, the obtained POSS-(PMMAm-Cl)8 was used as macroinitiator. POSS-(PMMAm-Cl)8 (2 g), PMDETA (0.15 ml), CuCl (0.015 g), St (10 ml or 20 ml) and toluene (20 ml) were added to a nitrogen-flushed three-necked flask with continues stirring. The mixture was gradually increased to 110 C and maintained at this temperature for 48 h. Same purification procedure as POSS-(PMMAm-Cl)8 was applied and white solid polymer POSSe(PMMAm-b-PSn)8 was obtained in 37.6% yield. 1H NMR (CDCl3, 500 MHz): 6.44e7.25 ppm (6H, t), 3.67 ppm (3H, s), 1.29e1.5 ppm(6H, m), 1.3e1.9 ppm(5H, t).
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Scheme 1 e Synthesis scheme of POSS-(PMMAm-b-SPSn)8.
Post-sulfonation: POSSe(PMMAm-b-PSn)8 (2.5 g) was dissolved in DCE (25 ml) in a three-necked flask, concentrated H2SO4 (2 ml) was dropwise added into the flask at 50 C under vigorous stirring. The mixture were stirred for additional 24 h. The product was washed by deionized water until seeing brown precipitation from the solution. The solid product was washed by D.I. water until the PH value of waste water reached 7, then dried at 50 C in a vacuum oven to constant weight. Yellowish polymer, POSSe(PMMAm-b-SPSn)8, was obtained in 58.3% yield.
Membrane preparation POSSe(PMMAm-b-SPSn)8 (0.5 g) was dissolved in DMF (10 ml) at 60 C, and then poured into a Petri dish and dried in an oven at 65 C for 8 h. Membranes were formed on the dish surface during this process, and were dried in a vacuum oven at 80 C for another 24 h.
Instruments The Fourier transform infrared spectrometer of block copolymers were carried out by WQF - 520, which DMF as solvent dissolved coating in potassium bromide tabletting, scan wave number range of 400e4000 cme1.1H nuclear magnetic
resonance (1H NMR)was tested by Bruker-AV400 nuclear magnetic resonance spectroscopy, with a scanning frequency of 400 MHz CDCl3,DMSO and TMS was selected as solvent and internal standard, respectively. The molecular weight and polydispersity of the block copolymers were characterized by WATERS 1500-C GPC, with Shodex OHpak SB-803 HQ(300 8 mm) as chromatographic column and THF as eluent. X-ray photoelectron spectroscopies were recorded on a Ka ALTHA spectrometer (VG company, UK). Higheresolution spectra were acquired from 0 to 1400 eV. All binding energies were referenced to C 1s peak for neutral carbon, which is 284.6 eV. Thermo-gravimetric analysis (TGA) was characterized by Q500(American TA company) under nitrogen atmosphere, the temperature will increased from room temperature to 600 with heating rate of 10 C/min.The surface morphology of POSS-(PMMAm-b-SPSn)8 membranes were measured by an AFM (MFP-3D-SA, Asylum based PEM Research Inc., USA) with ARC2TM scanning probe using tapping mode. The nanostructure morphology was also captured by HITACHI H-7650 TEM with an accelerating voltage of 80 KV. PEMs were stained by soaking in 0.5 M Lead acetate solution at room temperature for a week, then rinsed with deionized water and dried under vacuum at room temperature overnight. Then the stained PEMs were embedded in epoxy resin, cured under frozen and cut into 100 nm thick slices. The
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mechanical property of PEMs were carried out by universal testing machine RGT-1(Shenzhen REGER instrument Co.,Ltd.)
where V1 and V2 are the volumes (mL) of the NaOH solution and HCl solution respectively, and m (g) is the weight of the dry membranes.
Proton conductivity(s) measurement The proton conductivity (s) of POSSe(PMMAm-b-SPSn)8 was measured using the AC impedance method, and calculated by equation (1): s¼
L RWT
(1)
where L means the distance between the two platinum electrodes, R represents the impedance of the sample, W and T are the width and thickness of the sample, respectively. The measurements were carried out by Solartron 1260 a frequency response analyzer, with scanning frequency ranging from 100 Hz to 1 MHz. The proton conductivity under different relative humidities was carried out in a constant temperature and humidity chamber Espec SH - 241. Samples were conditioned at specific relative humidities for 1 h before testing.
Water uptake and hydration number Water uptake as function of temperature was determined by immersing dry membranes in deionized water at different temperature points for 1 h. The membranes were taken out and excess water on the surface of the sample was removed by wiping gently with filter papers. The membranes were immediately weighed on a microbalance. The water uptake was calculated according to the following equation: h¼
mwet mdry 100% mdry
(2)
where mwet is the weight of the wet film and mdry is the weight of the dry film. Water uptake as a function of relative humidity at 30 C was measured using a TA instruments Q5000SA dynamic vapor sorption analyzer. The relative humidity steps and equilibration times were the same as used in conductivity measurements. Hydration number was calculated through equation (3), l¼
mwet mdry 1000 mdry IEC 18:01
(3)
where mwet is the weight of the wet film under under different relative humidities, and mdry is the weight of the dry film.
Ion exchange capacity (IEC) The experimental IECs of POSS-(PMMAm-b-SPSn)8 ionomers were determined using a titration method. Membranes were immersed in an excess amount of sodium hydroxide (0.01 mol L1) solution for 12 h before titration. Then the aqueous solution were titrated with a standard HCl (0.01 mol L1) solution. The experimental IEC values of the ionomer membranes were calculated according to the equation below: IEC ¼
0:01ðV1 V2 Þ 103 meqg1 m
(4)
Low-filed NMR measurements Spinspin relaxation time T2 and water 1H nuclei measurements of the block copolymers were performed on a Niumag Desktop Pulsed NMR Analyzer (Shanghai Niumag Electronics Technology Co. Ltd.) using the CarrPurcellMeiboomGill (CPMG) pulse sequence. A spacing of 2 ms between the 90 and 180 pulse was used, and a recycle delay of at least 5 times of the spinlattice relaxation time between consecutive scans was used to ensure full recovery of the magnetization between acquisitions. All relaxation measurements were performed at 25 C. The spinspin relaxation time, T2r and T2f of different block copolymers, were determined by fitting each free-induction decay FID curve via nonlinear least-squares algorithm: t t þ M0f exp þ M0 yðtÞ ¼ M0r exp T2r T2f
(5)
in which M0r represents magnetization at the beginning of the pulse sequence of rigid block, and M0f represents the magnetization at the beginning of the pulse sequence of flexible block. Proton Spin diffusion coefficient calculation method was the same as in Ref. [28]. The T2 distribution of different water components of wet PEMs were analyzed by the distributed exponential fitting analysis using the Multi Exp Inv Analysis Software developed by Niumag Co., Ltd., China. A continuous exponentials distribution of the CPMG experiment was defined by the following equation: Z∞ gi ¼
AðTÞeti =T dT
(6)
0
where gi is the intensity of the decay at time ti and A(T) is the amplitude of the component with transverse relaxation time T. This analysis resulted in a plot of relaxation amplitude for individual relaxation processes versus relaxation time. The time constant for each peak was calculated from the peak position.
Results and discussion ATRP was successfully used to synthesize two star-shape block copolymers, with PMMA as the first block and PS as the second block. The basic physicochemical properties of the two copolymers are listed in Table 1. It can be seen that both samples have high molecular weight. After determining the
Table 1 e Molecular weight information of the samples. Sample
Mn Mw Mw/Mn m n X(Mw) Y(Mw)
POSS-(PMMA26-b-PS156)8 62 k 151 k POSS-(PMMA16-b-PS200)8 99 k 188 k
2.4 1.9
26 156 2730 16 200 1664
16,380 21,000
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Fig. 1 e (a) Proton Conductivity of PEMs as a fuction of RH at 30 C and 80 C; (b) Proton conductivity of PEMs as a fuction of hydration number (l) at 30 C.
molar ratio of PMMA block and PS block by 1H NMR, the numbers of the repeating units can be derived, which are used to describe each sample, namely POSS- (PMMA26-b-PS156)8 and POSS-(PMMA16-b-PS200)8. These samples are sulfonated to yield POSS- (PMMA26-b-SPS156)8 and POSS-(PMMA16-b-SPS200)8, respectively. Their ion exchange capacities (IECs) are titrated to be 2.77 mmol g1 and 3.85 mmol g1. The proton conductivities under different RHs were measured at 30 and 80 C, Fig. 1. It can be seen that the proton conductivity of POSS-(PMMA16-b-SPS200)8 is higher than that of POSS-(PMMA26-b-SPS156)8 at all the RH/temperature conditions. The proton conductivity of POSS-(PMMA26-b-SPS156)8 shows higher dependence on RH compared to that of POSS(PMMA16-b-SPS200)8, while both the productivities are not sensitive to the change in temperature at these low RH conditions. At 30% RH, the proton conductivity of POSS-(PMMA16b-SPS200)8 is ~5 times higher than that of POSS-(PMMA26-bSPS156)8. Fig. 1(b) is a plot of proton conductivity against the hydration number (l). It is obvious that the proton conductivity of two PEMs increases with the increasing of hydration number. POSS-(PMMA16-b-SPS200)8 exhibits higher proton conductivity than POSS-(PMMA26-b-SPS156)8 at similar hydration numbers.
The hydration number and proton conductivity of two PEMs as a function of temperature under fully hydration states are shown in Fig. 2. (Water uptake as a function of temperature of two PEMs was plotted in figure S6) POSS(PMMA26-b-SPS156)8 has larger hydration number and higher proton conductivity than POSS-(PMMA16-b-SPS200)8 at all the temperatures. The consistent relationship of proton conductivity and hydration number suggests that the proton conductivity for these two PEMs under fully hydration condition is ruled by vehicle mechanism. Morphologies of the two PEMs were firstly determined by TEM and shown in Fig. 3. In the TEM images, ionic block SPS appears as dark areas while the PMMA block appears as light areas. Nanoscale phase separation can be clearly seen in both images. Comparing the two images, a cylinder-like morphology with domain size of around 100 nm is observed for POSS-(PMMA26-b-SPS156)8, while a long and narrow fiberliked morphology with a excellent connectivity of ionic domains (around 30e40 nm) is observed for POSS-(PMMA16-bSPS200)8. In general, the formation of hydrophilic and hydrophobic domains depends on several factors, such as acidity, concentration and distribution of sulfonated groups, and difference in backbone/side chain mobility [29]. Star-shape
Fig. 2 e (a) l and (b) proton conductivity of the membranes as a function of temperature at 100% RH.
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Fig. 3 e TEM images of (a) POSS-(PMMA26-b-SPS156)8 and (b) POSS-(PMMA16-b-SPS200)8.
copolymers are expected to behave differently with linear polymers because of the interactions between the blocks. The different morphologies of two copolymers can be attributed to the different POSS core attractions to the ionic SPS block, since they have different lengths. An inter-connected columnar morphology with domain size around 100 nm is observed for
POSS-(PMMA26-b-SPS156)8 due to severe mobility restriction to outer SPS block from POSS sore. By replacing hydrophilic block with a longer SPS block, higher mobility will be performed because of the reduction in mobility restriction to SPS block from POSS core. Meanwhile, a fiber-liked morphology with excellent connectivity of ionic domains is formed to achieving
Fig. 4 e AFM images of (a) POSS-(PMMA26-b-SPS156)8 and (b) POSS-(PMMA16-b-SPS200)8.
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Fig. 5 e The dotted line represents FIDs of two PEMs and solid line represent two-exponentially decaying fitting.
Fig. 6 e T2 distribution of hydrated POSS-(PMMA26-bSPS156)8 and POSS-(PMMA16-b- SPS200)8 PEMs.
equilibrium. It is been found a well connected with hydrophobic and hydrophilic domain as well. The well-designed microscopic morphology, which can connect the proton path with a small amount of absorbed water, is necessary to improve proton conductivity at low RH [20]. Tapping-mode atomic force microscopy (AFM) was also employed to investigate the morphologies of block copolymers under ambient conditions, Fig. 4. The bright and dark regions are corresponding to the hydrophobic and hydrophilic units, respectively. Comparing two AFM images, the corresponding periodic structure with well connected hydrophobic and hydrophilic domains was observed for copolymer with longer SPS block, which is well consistent with TEM analysis results. This morphology investigation will attract a great interest because it showed that various phase separated structure might be achieved by a proper molecular design of star-shaped copolymers. Combined the TEM and AFM analysis, we could illustrate that the well-connected ionic domains morphology caused by longer SPS block could play a positive role in proton conductivity under low RH. Low-field nuclear magnetic resonance (LF-NMR) has been applied by others to determine the segmental and chain dynamic parameters such as spinespin relaxation time (T2) and spin-diffusion coefficients for block copolymers [30]. In this study, we studied T2 and proton spin-diffusion processes through free-induction decay (FID) of dry PEM to analyze segmental dynamics difference induced by the two blocks to understand proton diffusion under low RH. Typical FID curves
of dry PEMs are shown in Fig. 5, and colored lines showed the well-fitted discrete bi-exponential fitting procedure of the dry PEMs. The related relaxation constants are listed in Table 2. It is worth mentioning that a larger spinespin diffusion efficient of rigid phase is detected for POSS-(PMMA16-bSPS200)8 after comparing D(T2r) of the two PEMs. Since POSS(PMMA16-b-SPS200)8 have higher sulfonation density, we can conclude that the sulfonation plays a major role in influencing proton conductivity under low RH. T2 distribution of different water components shows the spinespin relaxation process of protons, explaining the different chemical environments of water components and reflecting the speed of energy equilibrium recovery process. In general, T2 and its amplitude values represent the speed and strength of spinespin relaxation, respectively [31]. Both the T2 distribution of two fully hydrated PEMs were measured and displayed in Fig. 6. Two peaks corresponding to two water components are observed, in which the difference of T2 and amplitude of two PEMs is clearly presented. The T2s and T2l represent strongly bonded water and loose bonded water to copolymer matrix, respectively. It is clearly shown that the loosely bonded water component in POSS-(PMMA26-b-SPS156)8 is weaker than that in POSS-(PMMA16-b-SPS200)8. The diffusivity of H3Oþ will increase with the increasing of loosely bonded water and facilitate proton conductivity on the basis of the vehicle mechanism. This is consistent with the proton conductivity behavior difference between two PEMs under fully hydration states.
Table 2 e Relaxation time constants (T2) of two different blocks and corresponding concentrations (M0) obtained by discrete bi-exponential fitting of the FID curves of dry PEMs obtained at room temperature and spin-diffusion coefficients (D(T2r)) for rigid phase(SPS). Sample
Rigid phase (SPS) T2r/ms
POSS-(PMMA26-b-SPS156)8 POSS-(PMMA16-b-SPS200)8
0.9 0.7
2
D(T2r)/nm ms 0.29 0.33
1
Flexible phase(PMMA) M0r
T2f/ms
M0f
M0
612.8 553.4
5.2 3.7
194.4 261.5
56.1 64.4
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Conclusions Two POSS containing block copolymers (POSS-PMMA-b-PS) with different block ratios were synthesized by ATRP. Subsequent sulfonation yielded two novel star-polymers for high performance proton exchange membranes, namely POSS(PMMA26-b-SPS156)8 and POSS-(PMMA16-b-SPS200)8. The POSS(PMMA16-b-SPS200)8 showed higher proton conductivity than POSS-(PMMA26-b-SPS156)8 under low RHs at same hydration number (l) conditions, but lower proton conductivity under full hydration state. It is believed that the excellent phase connectivity of POSS-(PMMA16-b-SPS200)8, as visualized by TEM and AFM, provides it higher proton conductivity at low RH and the proton conductivity is less dependent on RH compared to that of POSS-(PMMA26-b-SPS156)8. Higher amount of loosely bonded water was found for the PEM with shorter SPS block at 100% RH by low-field NMR, which is consistent with its higher proton conductivity at such condition compared to that of the PEM with longer SPS block. This study suggested that both ionic domain structure and waterepolymer interaction are important parameters for achieving high proton conductivities.
Acknowledgments This research is supported by National Natural Science Foundation of China (No. 51103117) and The Natural Science Foundation of Shaanxi Province (2013JQ2010 and 2013JM2012); NPU Foundations for Fundamental Research (No. 3102014JCQ01089).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.02.090.
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