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Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics
international journal of hydrogen energy xxx (xxxx) xxx
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Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics Prem P. Sharma a, Hariom Gupta a,b,*, Vaibhav Kulshrestha a,** a
CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), G.B.Marg, Bhavnagar, 364002, Gujarat, India b CSIR-Central Institute of Medicinal and Aromatic Plants (CSIR-CIMAP), Near Kukrail Picnic Spot, Lucknow, 226015, India
highlights Enhanced proton conductivity in phosphorylated hybrid silica-SPES compare to SPES. T1, T2 relate fall in Proton-channel interaction, better hydrated protons interlinking. Better proton hopping interlinking pathway causing to enhance proton conductivity.
article info
abstract
Article history:
The understating of hydrated proton activity and its dynamics inside the ion exchange
Received 3 March 2019
channel of proton exchange membranes (PEMs) are one of the basic need to develop quality
Received in revised form
membranes for desalination and energy applications. In this work, sulfonated poly-
9 September 2019
ethersulfone (SPES), hybrid Silica-SPES and phosphorylated hybrid Silica-SPES composite
Accepted 12 September 2019
PEMs are prepared; and membrane properties are characterized with appropriate analyt-
Available online xxx
ical tools. The enhanced conductivity is described effectively with 1H spin-lattice (T1), spin-
Keywords:
mined by magnetic resonance imaging (MRI) technique. This study gives useful insight on
Magnetic resonance imaging (MRI)
variation in the interactions and dynamics of hydrated protons confining inside the
Proton exchange membrane (PEM)
membrane channel with the incorporation of silica and its further phosphorylation. Ob-
Spin-lattice relaxation
tained spin-lattice (T1), spin-spin (T2) relaxation and diffusion results relate that silica and
Spin-spin relaxation
phosphorylation have facilitated the formation more hurdle free organized pathway for
Diffusion coefficients
proton conduction.
spin (T2) relaxation and diffusion weighted imaging and their corresponding value deter-
Hybrid silica-SPES and
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
phosphorylated hybrid silica-SPES
* Corresponding author. CSIR-Central Salt and Marine Chemical Research Institute, G.B. Road, Bhavnagar, Gujarat, India. ** Corresponding author. E-mail addresses: [email protected], [email protected] (H. Gupta), [email protected] (V. Kulshrestha). https://doi.org/10.1016/j.ijhydene.2019.09.106 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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Introduction Proton exchange membranes (PEMs) have established an excellent capability in the field of energy (fuel cells, water electrolysis, redox-flow battery etc.) and separation applications [1e6]. Nafion is widely used PEM but it is high in cost. Various non-nafion based cost effective membranes, such as sulfonated polysulfone (SPS), sulfonated polyethersulfone (SPES) and sulfonated poly(ether ether ketone) (SPEEK) with silica hybrid have been reported to achieve high conductivity equivalent to nafion and overcome the limitation of nafion [7e10]. Silica improves the conductivity and mechanical stability but excessive amount makes the membrane brittle in nature causing to reduce in mechanical stability. Proton conductivity of such membranes unambiguously depends on characteristics of membrane channels, exchange sites and dynamics of confined hydrated protons in the membrane channels [11e13]. Shahi has reported sulfonated poly(ether sulfone)-silica polyelectrolyte composite membranes for fuel cell, explained the improved membrane property with frictional coefficient between proton and membrane matrix calculated using conductivity data and found lowering in its value on incorporation of silica [14]. Roelofs et al. have prepared sulfonatedpoly(ether ether ketone)-based silica nanocomposite membranes for direct ethanol fuel cells and found the enhance conductivity and improved water uptake characteristics [15]. Some other researchers have suggested and explained the enhanced conductivity for hybrid membranes based on improve water uptake and reduction in frictional coefficient [16e19]. But these studies are almost untouched with direct in-situ measurement of parameters associated with behavior of hydrated proton present in membrane channel and its dynamics causing to enhance the conductivity on incorporation of silica. Nuclear magnetic resonance (NMR) is a convenient and noninvasive means to measure the parameters directly co-related with motion of hydrated protons confining inside the membrane channel, its interaction with channel walls and ordered hydrogen bonded connectivity with other protons. Self diffusion coefficient measured using pulse field gradient (PFG) diffusion NMR technique has been reported in literature and used to depict hydrated proton activity and its dynamics in various PEMs [20e23]. Spin-lattice (T1) and spin-spin (T2) NMR relaxations times of 1H nuclei (i.e. proton) of hydrated proton provides the information about its local condition and interactions, including its interactions with membrane channel walls and ordered hydrogen bonded linkage connecting hydrated protons. These parameters are quite sensitive with the variation of small structural and functional modification in membrane matrix. Magnetic resonance imaging (MRI) is another face of NMR introduces the feature of spatial cross section imaging and spectroscopic parameters measurement including diffusion coefficient and relaxation times in noninvasive manner [3,24]. It utilizes the behavior of protons of confined water to disclose the feature and local functional variation in the sample and generate contrast in images directly correlating with its features [25e27]. MRI based diffusion analysis for PEMs have been reported to disseminate the dynamics of hydrated proton in literature and our previous published work [3,24,28e30]. Lui et
al. have studied the diffusion of water molecule inside the carbon nanotubes (CNTs) of diameter 2.3 ± 0.3 nm and found its value one order higher in magnitude than the mesoporous silica material of similar pore size [31]. They have concluded that fast diffusion is caused due to weak interaction of confined water molecules with CNT walls and formation of ordered hydrogen bonding between the water molecules. Therefore, relaxation and diffusion measurement can jointly enumerate the behavior and dynamics of hydrated proton confining inside the channel of proton exchange membrane but no such direct studies are reported to explain the cause of enhanced conductivity of silica hybrid proton exchange membranes. In this work, we have described the detailed behavior and dynamics of hydrated protons in three different sulfonated polyether sulfone (SPES), hybrid Silica-SPES (SPES-Si) and phosphorylated hybrid Silica-SPES (SPES-Si-P) composite proton exchange membranes (PEMs) of same polymeric backbone. The cause for enhanced proton conductivity in membrane channels on incorporation of silica and its further phosphorylation is explained with spin-lattice (T1), spin-spin (T2) relaxation and diffusion MRI investigations.
Experiment section Material and membrane synthesis Polyethersulfone (PES), obtained from Solvay Chemicals Pvt Ltd., India, is used after drying under vacuum for 24 h. Sulfonation of PES was carried out using conc. H2SO4 (95e98%) under vigorous stirring at 60 C for 6 h [32]. The membranes ware synthesized by reported method performed by our group in earlier paper [14].
Characterization FT-IR micro-ATR spectra to characterize functional groups present on membranes were acquired in transmittance modeusing Agilent Cary 600 series FTIR Spectrometer. Mechanical properties were analysed using Universal Testing Machine (UTM) on ZwickRoell, Z2.5 instrument. Thermal stability of membranes was characterized by thermogravimetric analyser (TGA) instrument of Mettler Toledo TGA/ SDTA851 under N2 gas atmosphere for the temperature range of 30e700 C with heating rate of 5 C/min. SEM images and elemental mapping by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis of membrane were obtained with Field Emission Scanning Electron Microscope (FE-SEM-Jeol JSM 7100F). For SEM imaging small pieces of membrane are mounted on SEM stub and stub was coated with Au to make the sample conductive for SEM images of better quality. Ion exchange capacities (IEC) of prepared membranes were estimated using acid-base titration method. Briefly a piece of examining membrane (area: 1 cm2) is dipped in 0.1M HCl solution for 24 h; dipped membrane piece is wash properly with water to remove excess amount of acid and then transferred into 0.1 M NaCl solution for same time. During this process protons are exchanged with sodium ions and come out into
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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the NaCl solution. This resulted NaCl solution was titrated with 0.01M solution of the NaOH using phenolphthalein as an indicator. The IEC values of examined membranes were estimated using following co-relation equation IECðmeq = gmÞ ¼
CNa VNaOH WDry
Where CNa is the concentration of Naþ ion in titrating NaOH solution, VNaOH is volume consumed NaOH solution in titration and Wdry is dry weight of membrane piece. Ionic conductivity of the membranes was measured using potentiostat/galvanostat (CHI 608) at room temperature. Membrane resistance was obtained from Nyquist plot by fitting. Ionic conductivity was calculated using the following equation: km U1 cm1 ¼
Where I0 is maximum signal intensity; I'0 and I}0 are the maximum signal intensity at fix value of TE and TR. For pure T1and T2weighted images, intensity of MRI image of corresponding weighting is purely depend on T1and T2 relaxation time respectively. Pure T1and T2 weighting can be achieved by eliminating or minimizing the T2 term (T2 term /1) for T1 weighting and T1 term (T1 term /1) for T2 with the selection of appropriate combination of MRI acquisition parameter TE (time of echo) and TR (time of repetition) value. For Diffusion weighted MRI, Image intesity for pulse field gadient spin echo (PFGSE) based MRI pulse sequence can be mathematically represented as [38]. eBD |ffl{zffl}
(2)
Diffusionterm
Where, L is the distance between the electrodes used to measure the potential, R is the resistance of the membrane, and A is the surface area of the membrane. Magnetic resonance images, relaxometric (T1 and T2) and ¨ KER diffusion analysis of membranes were acquired with BRU AVANCE 500 MHz (11.7 T) NMR instrument using microimaging probe and Paravision imaging software. T1 and T2 weighted MRI images of membranes were acquired using the spin echo pulse sequences (RAREVTR and MSME) with acquisition parameters for TR (time of repetition) ¼ 350 ms, TE (time of echo) ¼ 8 ms, 128 128 matrix and TR ¼ 1500 ms, TE ¼ 20, 256 256 matrix respectively. T1 and T2 relaxation times of membranes were measured using of Bruker RAREVTR (TE ¼ 8 ms, TR ¼ 250e3500 ms and 128 128 matrix) and MSME (TE ¼ 10e80 ms, TR ¼ 1500 ms and 256 256 matrix) MRI pulse sequences respectively. T1 and T2 relaxation times for each membranes were calculated with fitting of T1 and T2 experiment data to Eq. (1b) and Eq. (1c) using Paravision software [26,33]. A series of diffusion weighted MRI images were acquired using PFGSE (pulse field gradient spin echo) based MRI imaging method with different B value, TR ¼ 500ms, TE ¼ 15.3 ms and 128 128 matrix size. The selfdiffusion coefficient (D) of protons in hydrated membranes is calculated by fitting of Stejskal Equation (2) using Paravision imaging software [34e36]. The intensity of MRI image of a sample depends on number of 1H nuclei (proton) present in sample and their relaxation rate. Spin-echo (SE) MRI pulse sequence is widely used for T1 and T2 weighted imaging and the image intensity for SE pulse sequence is defined as [37].
T1 term
(1c)
T2 term
I ¼ I0
LðcmÞ RðUÞ Aðcm2 Þ
TE=T 2 I ¼ I0 1 eTR=T1 e|fflfflffl{zfflfflffl} |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl}
TE=T2 I ¼ I}0 e|fflfflffl{zfflfflffl}
3
Where B ¼ g2g2d2 D 3d , D ¼ diffusivity or diffusion coefficient, g ¼ gyromagnetic ratio, g ¼ strength of gradient pulse, d ¼ length of gradient pulse and D ¼ diffusion time.
Results and discussion Structural characterization of membranes Fourier transform infrared (FTIR) spectra of SPES, SPES-Si and SPES-Si-P are presented in Fig. 1. IR bands observed at 1575, 1485, 1469 1407 cm1 are assigned to C]C stretching of aromatic rings of poly (ether sulfone). A characteristic symmetric and asymmetric stretching of O]S]O of sulfone group (eSO2e) present in all membranes are found to be at 1147 and a doublet bands at 1293 and 1319 cm1. An IR band at 1025 cm1 is attributed to symmetric stretching of O]S]O of sulfonate group (-SO3‾) and found to be present in all membranes. IR bands at 1072 and 1180 cm1 are correspond to the aromatic ring vibration of the poly(ether sulfone), a peak at
(1a)
T2 term
For T1 experiments T2 term /1 or constant for fix value of TE, i.e. I ¼ I'0 1 eTR=T1 |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl}
(1b)
T1 term
For T2 experiments T1 term /1 or constant for fix value of TR, i.e.
Fig. 1 e FTIR spectra of SPES, SPES-Si and SPES-Si-P membrane representing characteristic peaks correspond to various functional group present on these membranes.
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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869 cm1 is due to out-of-plane CeH deformation vibration. A strong band observed at 1236 cm1 is assigned to asymmetric CeOeC stretching of aryl ether group of poly(ether sulfone). A slight relative intensity enhancement for bands at 835 cm1 for SPES-Si and SPES-Si-P compare to SPES is attributed to the contribution of -Si-O- stretching and out-of-plane bending vibration of eN-H of APTES incorporated in these membranes [39]. A weak band at 1652 cm1 observed significantly in SPESSi and SPES-Si-P is attributed to in-plane bending of -N-H group of APTES present in SPES-Si and SPES-Si-P membrane [40]. It characterizes and suggests the formation of silica-SPES composite membrane. A band correspond to P]O stretching (1139 cm1) is possibly overlapped with band (1147 cm1) of sulfone group and not significantly distinguishable in this spectrum [40]. A spectral region containing few other bands below 800 are corresponding to the bending mode vibration of above mentioned functional groups present in polymer membranes.
Ion exchange capacity (IEC) and ionic conductivity (IC) Ion exchange capacity (IEC) and ionic conductivity are important parameters to evaluate the membrane property for electro chemical applications. The IEC values as shown in Table 1a are found to be 0.92, 1.34, 1.61 meq g1 for SPES, SPESSi and SPES-Si-P membranes. An enhanced IEC values for SPES-Si and SPES-Si-P are caused due to the incorporation of Si and phosporalization into the membrane; both these membranes also shows an enhanced water uptake. A moderate amount of water uptake into ion exchange membrane is
necessary for ion transportation; it provides ion transportation and ion exchange sites connecting hydrogen bonded water molecule pathway in the membrane matrix which results an increase in the ion conductivity in membrane. Water uptake and proton conductivity for examined membranes are presented in Table 1a and compared with Nafion 117. The trend of proton conductivity and water uptake is found to increase and well consistent with IEC trend. These results attribute a significant enhancement in electro and physicochemical properties of SPES membrane on incorporation of silica and further phosphorylation. Membrane properties of prepared hybrid composite are well comparative with previously reported hybrid membranes and comparison is represented in Table 1b.
Mechanical and thermal property The appearance and transparency of all these membranes (SPES, SPES-Si and SPES-Si-P) are identical as shown in Fig. 2 however last two membranes are comparably hard and brittle structure compare to SPES. Mechanical properties of membranes measured by universal testing machine (UTM) are given in Table 2 and obtained stress-strain curves are shown in Fig. 2. The young modulus and elongation are found to decrease with the incorporation of silica and phosphorylation into SPES matrix. A drastic decrease in stress value is observed for both SPES-Si (4.94 MPa) and SPES-Si-P (6.30 MPa) compare to SPES (13.18 MPa) membrane. These results suggest a decrease in mechanical stability of these modified membranes in comparison to SPES but it is well comparable to
Table 1a e Ion exchange capacity (IEC), % water uptake and Ionic Conductivity (IC). Membrane type SPES SPES-Si SPES-Si-P Nafion
IEC (meq g1) 0.92 ± 1.34 ± 1.61 ± 0.93 ±
IC 102 (Scm1)
% Water Uptake
0.01 0.01 0.01 0.01
17.87 19.20 22.13 19.00
± 0.01 ± 0.01 ± 0.01 ± 0.01
2.48 ± 3.67 ± 5.41 ± 9.10 ±
0.01 0.01 0.01 0.01
Table 1b e Comparison of characteristics of present membrane with other reported hybrid membranes. Membrane type Si-PWA/PVA SM-10 MGOeSCH-5 SG-10 SPEEK/[email protected] SPES-Si-P
IEC (meq g1)
Water Content (%)
IC (102 S cm1)
Reference
0.90 1.44 2.56 1.27 1.35 1.61
35.00 20.26 43.00 15.00 20.93 22.13
0.70 5.72 6.77 6.40 3.55 5.41
[41] [18] [19] [42] [43] This study
Si-PWA/PVA: silica immobilized phosphotungstic acid-poly(vinyl alcohol) (PVA), SM: SPES-sulfonated mesoporous silica, MGO-SCH: Sulfinated chitosan-silica modified Graphine oxide; SG: SPES-graphine oxide, SPEEK/HPW@MSNs: SPEEK-phosphotungstic acid in mesoporous silica nanospheres.
Table 2 e Mechanical properties of membranes. Membrane type SPES SPES-Si SPES-Si-P
Young's Modulus (MPa)
Stress (MPa)
% Elongation
3.5 ± 0.1 3.0 ± 0.1 2.0 ± 0.1
13.18 ± 0.2 4.94 ± 0.2 6.30 ± 0.2
29.87 ± 0.2 21.74 ± 0.2 14.25 ± 0.2
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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Fig. 2 e Optical appearance of membranes and their mechanical property. other reported PEMs [44,45]. SPES-Si and SPES-Si-P can be useful for electro-chemical applications as these membranes have shown significant enhancement in conductivity with some sacrifice of mechanical stability. The thermal stability of prepared SPES and SPES-Si-P membranes were studied by thermo-gravimetric (TG) analysis and obtained TG and DTG results are shown in Fig. 3. Weight loss in range of 100e400 C is corresponding to desorption of water from the membrane and disintegration of eS]O groups present in the membrane [42]. A sharp weight loss in TG and its corresponding DTG peak in the range of 450e 550 is attributed to the decomposition of poly(ether sulfone) backbone of the membranes [3]. A slight larger weight loss in SPES-Si-P above 500 C is possibly contributed by the deoxygenation of silica present in this membrane. The thermogravimetric results for both examined membrane are almost similar which suggests that incorporation of silica and phosphorylation into SPES membrane does not affect its thermal stability significantly. The structural morphology of the membrane matrix is key feature of membrane that are intended to use for separation application. Fig. 4 shows the surface and cross sectional SEM images of SPES and SPES-Si-P membrane which indicates the formation of a uniform matrix of dense morphology for both the membranes. Formation of silica hybrid, phosphorylation and sulfonation in membrane are further established through SEM-EDS elemental mapping and EDX spectrum analysis of SPES-Si-P membrane and results are shown in Fig. 5. It shows an even distribution of Si, P and S elements throughout the membrane which reveals the formation of uniformity phosphorylated hybrid silica SPES membrane.
exchangeable proton and associated water spectroscopically, and deliver information of its interaction with polymer channels and neighboring hydrated protons. 1H spin-lattice (T1), 1H spin-spin (T2) relaxation times and diffusion value
Magnetic resonance imaging (MRI) analysis MRI is a noninvasive technique and most commonly used for biomedical and biological investigations of behavior (interaction and dynamics) of proton (i.e. 1H nuclei) proton associated with water molecule present in biological sample. It could also be more suitable for the determinations proton exchange mechanism in the membrane as this technique exploits directly hydrogen nuclei (i.e. 1H nuclei) of hydrated
Fig. 3 e Thermogravimetric (TG) and its derivative (DTG) results of SPES and SPES-Si-P membranes.
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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Fig. 4 e Surface and cross sectional SEM images of SPES (a, b) and SPES-Si-P (c, d) representing the formation of membrane of dense morphology.
Fig. 5 e SEM-EDS elemental maps of selected portion of SPES-Si-P membrane: a) mix image of elemental map of Si (red), P (green) and S (blue); b) Individual map of Si; c) Individual map of P; d) EDX spectrum representing the presence of Si and P in membrane. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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Fig. 6 e T1 weighted MRI image at TR ¼ 350 ms and TE ¼ 8 ms; Normalized intensity recovery plot of spin-lattice (T1) relaxation MRI experiment, representing the decrease in T1 relaxation times with modification of SPES.
Table 3 e T1, T2 relaxation times and diffusion coefficients. Membrane type SPES SPES-Si SPES-Si-P Nafion
T1 (ms)
T2 (ms)
389.545 ± 17.100 614.056 ± 24.242 778.556 ± 20.543 e
8.465 ± 1.033 18.993 ± 0.370 15.113 ± 0.827 e
(D) of 1H spin in the membrane are important and key parameters for the deep understanding of protons behavior and their dynamics in the matrix of membrane. Spin-lattice relaxation (T1) mainly reveals the interaction of proton with
DiffusionCoeff. (1010 m2/S) 1.061 2.699 5.532 5.394
± 0.014 ± 0.103 ± 0.552 ± 0.619
membrane matrix however spin-spin relaxation dominantly disclose the interaction between two protons and their hydrogen bond connectivity between ion exchange sites in the membrane. The diffusion value provides direct information of
Fig. 7 e T2 weighted MRI images at TR ¼ 1500 ms and TE ¼ 20 ms; Normalized intensity attenuation plot of spin-lattice (T2) relaxation MRI experiment, representing the decrease in T2 relaxation times with modification of SPES. Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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Fig. 8 e Diffusion weighted MRI images at TR ¼ 500 ms and TE ¼ 14.3 ms; Normalized intensity attenuation plot of diffusion map MRI experiment, representing enhancement in diffusion coefficient with modification of SPES.
dynamic movement of the protons which is well correlated and equivalent to the proton conductivity information of membrane. So these parameters are well capable to explore the exact behavior and dynamics of protons in the matrix of membrane. T1, T2 and diffusion MRI analysis for SPES, hybrid Silica-SPES and phosphorylated hybrid Silica-SPES composite membrane as per the MRI acquisition parameters and methods given in experimental section. T1 weighted MRI images and corresponding intensity recovery plot of T1 MRI experiment are represented in Fig. 6. T1 relaxation times (Table 3) are found to increase for silicahybrid and phosphorylated silica-hybrid which relates to attenuation in spin-lattice correlation time of transportable proton resulting to a decrease in its interaction with the lattice of membrane channels. It suggests that incorporation of silica and further phosphorylation encounter the impediment and deliver relatively less unperturbed channels for proton transportation.
Fig. 7 shows T2 weighted MRI images and their corresponding normalize signal attenuation curve, and obtained T2 relaxation times are listed in Table 3. T2 values are found to be 08.46, 18.99, 15.11 ms for SPES, SPES-Si and SPES-Si-P respectively. Increase in T2 value implies that decrease in spin-spin correlation time resulting to reduce in mobility constrains between transportable protons due to improvement in ordered hydrogen bonded linkage connecting other hydrated protons and provides an easy pathway for proton conduction in the membrane channels. The diffusion weighted MRI images and Stejskal attenuation plot is shown in Fig. 8, estimated diffusion coefficients are listed in Table 3. Diffusion coefficients have significantly improved and found to be about 2.5 times for SPES-Si and 5.2 times for SPES-Si-P in comparison to SPES. SPES-Si-P has effectively achieved the diffusion coefficient of Nafion117. Above results suggest a formation of well-connected and relatively more hurdle-free proton hopping organized pathway causing to enhance proton conductivity. It is schematically represented and shown in Fig. 9.
Conclusion
Fig. 9 e Schematic diagram of possible hydrogen bonded proton hopping pathway.
The incorporation of silica into SPES and its further phosphorylation have shown significant improvement in proton conductivity with minor decline in mechanical stability. Water uptake is found to increase with the incorporation of silica and further phosphorylation. Improved self diffusion coefficients of hydrated protons present inside these membraneshave suggested reduction in conduction constrains. Spin-lattice (T1), spin-spin (T2) relaxation times of hydratedprotons attributes that incorporation of silica improves the ordered hydrogen bonded linkage connectivity among the hydrated protons and reduces their interaction with membrane channel. It can be concluded that incorporated silica facilitate a well-connected and relatively more hurdle-free
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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proton hopping organized pathway causing to enhance proton conductivity in the membrane. [15]
Acknowledgement [16]
Authors are thankful of Mr. Jayesh Chaudhary, Mr. Vinod Agrawal and Mr. Satyaveer of Centralized Instrument Facility of CSMCRI for helpful assistance in characterization. Authors are thankful to SERB, DST New Delhi for providing financial support.
references
[1] Xie H, Tao D, Xiang X, Ou Y, Bai X, Wang L. Synthesis and properties of highly branched star-shaped sulfonated block poly(arylene ether)s as proton exchange membranes. J Membr Sci 2015;473:226. [2] Ganeshkumar A, Bera D, Ali EM, Banerjee S. Triphenyl amine containing sulfonated aromatic polyimide proton exchange membranes. Eur Polym J 2014;60:235. [3] Gahlot S, Gupta H, Jha PK, Kulshrestha V. Enhanced electrochemical performance of stable SPES/SPANI composite polymer electrolyte membranes by enriched ionic nanochannels. ACS Omega 2017;2:5831e9. [4] Zhang H, Shen PK. Recent development of polymer electrolyte membranes for fuel cells. Chem Rev 2012;112:2780e832. A, Carbone A, Gatto I, Pedicini R, Freni A, Patti A, [5] Sacca Passalacqua E. Composites nafion-titania membranes for polymer electrolyte fuel cell (PEFC) applications at low relative humidity. Polym Test 2016;56:10e8. [6] David O, Percin K, Luo T, Gendel Y, Wessling M. Proton exchange membranes based on sulfonated poly(ether ether ketone)/polyaniline blends for all- and air-vanadium redox flow battery applications. J. Energy Storage 2015;1:65e71. [7] Fu Y-Z, Manthiram A. Synthesis and characterization of sulfonatedpolysulfone membranes for direct methanol fuel cells. J Power Sources 2006;157:222e5. [8] Jiang Z, Zhao X, Manthiram A. Sulfonatedpoly(ether ether ketone) membranes with sulfonatedgraphene oxide fillers for direct methanol fuel cells. Int J Hydrogen Energy 2013;38(14):5875e84. [9] Gahlot Swati, Sharma Prem P, Gupta Hariom, Prafulla K, Jha VaibhavKulshrestha. Synthesis of SPEEK/TiO2 nanocomposites and their characterization. Adv Electrochem 2015;2:1e6. [10] Lim Y, Lee S, Seo D, Jang H, Hossain M, Ju H, Hong T, Kim W. Sulfonated poly(ether sulfone) electrolytes structured with mesonaphthobifluorenegraphene moiety for PEMFC. Int J Hydrogen Energy 2014;39:1532. [11] Sone Y, Ekdunge P, Simonsson D. Proton conductivity of nafion 117 as measured by a four -electrode AC impedance method. J Electrochem Soc 1996;143(4):1254e9. [12] Pili Simona, et al. Proton conduction in a phosphonate-based MetalOrganic framework mediated by intrinsic “free diffusion inside a sphere”. J Am Chem Soc 2016;138:6352e5. [13] Liang X, Zhang F, Feng W, Zou X, Zhao C, Na H, Liu C, Sun F, Zhu G. From metaleorganic framework (MOF) to MOFepolymer composite membrane: enhancement of lowhumidity proton conductivity. Chem Sci 2013;4:983. [14] Shahi Vinod K. Highly charged proton-exchange membrane: sulfonatedpoly(ether sulfone)-silica polyelectrolyte
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
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composite membranes for fuel cells. Solid State Ion 2007;177:3395e404. Roelofs Kimball S, Hirth Thomas, Schiestel Thomas. Sulfonated poly(ether ether ketone)-based silica nanocomposite membranes for direct ethanol fuel cells. J Membr Sci 2010;346:215e26. Park Junghwa, Kim Dukjoon. Sulfonatedmesoporous benzene-silica-embedded sulfonatedpoly(ether ether ketone) membranes for enhanced proton conduction and anti-dehydration. Int J Hydrogen Energy 2014;39:1063e70. Park Ki Tae, et al. Composite membranes based on a sulfonatedpoly(arylene ether sulfone) and protonconducting hybrid silica particles for high temperature PEMFCs. Int J Hydrogen Energy 2011;36:10891e900. Gahlot Swati, Prem P, Sharma VikrantYadav, Prafulla K Jha, VaibhavKulshrestha. Nanoporous composite proton exchange membranes: high conductivity and thermal stability. Colloids Surf, A 2018;542:8e14. Prem P, Sharma, VaibhavKulshrestha. Synthesis of highly stable and high water retentive functionalized biopolymergraphene oxide modified cation exchange membranes. RSC Adv 2015;5:56498e506. Jayakody JRP, Stallworth PE, Mananga ES, FarringtonZapata J, Greenbaum SG. High pressure NMR study of water self-diffusion in NAFION-117 membrane. J Phys Chem B 2004;108:4260e2. Li J, Wilmsmeyer KG, Madsen LA. Anisotropic diffusion and morphology in PerfluorosulfonateIonomers investigated by NMR. Macromolecules 2009;42:255e62. Ma Z, Jiang R, Myers Jr ME, Thompsona EL, Gittleman CS. NMR studies of proton transport in fuel cell membranes at sub-freezing conditions. J Mater Chem 2011;21:9302e11. Ma Zhiru, Jiang Ruichun, Mark E, Myers Jr, Eric L, Thompson Craig S Gittleman. NMR studies of proton transport in fuel cell membranes at sub-freezing conditions. J Mater Chem 2011;21:9302. Gahlot S, Gupta H, Jha PK, Kulshrestha V. Hydrated proton self-diffusion study in ion-exchange membranes by MRI and impedance spectroscopy. Polym Bull 2017;74:4437. Wu Y, Dai G, Ackerman JL, Hrovat MI, Glimcher MJ, Snyder BD, Nazarian A, DA Chesler. Water- and fatsuppressed proton projection MRI (WASPI) of rat femur bone. Magn Reson Med 2007;57:554e67. Gupta H, Paul P, Kumar N, Baxi S, Das DP. One pot synthesis of water dispersible dehydroascorbic acid coated Fe3O4 nanoparticles under atmospheric air: blood cell compatibility and enhanced magnetic resonance imaging. J Colloid Interface Sci 2014;430:221e8. Vinogradov E, Soesbe TC, Balschi JA, Sherry AD, Lenkinski RE. pCEST: positive contrast using chemical exchange saturation transfer. J MagnReson 2012;215:64e73. Feindel Kirk W, Bergens Steven H, Wasylishen Roderick E. Insights into the distribution of water in a self-humidifying H2/O2 proton-exchange membrane fuel cell using 1H NMR microscopy. J Am Chem Soc 2006;128(43):14192e9. Feindel Kirk W, Bergens Steven H, Wasylishen Roderick E. The use of 1 H NMR microscopy to study ProtonExchange membrane fuel cells. Chem Phys Chem 2006;7:67e75. Sahu AK, Selvarani G, Bhat SD, Pitchumani S, Sridhar P, Shukla AK, Narayanan N, Banerjee A, Chandrakumar N. Effect of varying poly(styrene sulfonic acid) content in poly(vinyl alcohol)epoly(styrene sulfonic acid) blend membrane and its ramification in hydrogeneoxygen polymer electrolyte fuel cells. J Membrane Sci 2008;319:298e305. Liu | Xin, Pan Xiulian, Zhang Shanmin, Han Xiuwen, XinheBao. Diffusion of water inside carbon nanotubes studied by pulsed field gradient NMR spectroscopy. Langmuir 2014;30:8036e45.
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
10
international journal of hydrogen energy xxx (xxxx) xxx
[32] Thakur AK, Gahlot S, Kulshrestha V, Shahi VK. Highly stable Acidebase complex membrane for ethanol dehydration by pervaporation separation. RSC Adv 2013;3:22014e22. [33] Haque MI, Rathore MS, Gupta H, Jha B. Inorganic solutes contribute more than organic solutes to the osmotic adjustment in Salicorniabrachiata (Roxb.) under natural saline conditions. Aquat Bot 2017;142:78e86. [34] Gupta H, Paul P, Kumar N. Synthesis and characterization of DHA/ZnO/ZnFe2O4 nanostructures for biomedical imaging application. Procedia Mater Sci 2014;5:198e203. [35] Stejskal E-O, Tanner J-E. Spin diffusion measurements: spin echoes in the presence of a time dependent field gradient. J Chem Phys 1965;42:288e92. [36] Sharma Prem P, Gahlot S, Gupta H, Kulshrestha V. Hybrid anion conducting membranes (ACM) for industrial applications: excellent salt removal efficiency and electro-chemical properties. Separ Purif Technol 2017;189:449e58. € der L, Faber C. Basic contrast mechanisms. In: [37] Schro € der L, Faber C, editors. In-Vivo NMR Imaging methods Schro and protocols. Humana Press-Springer; 2011. p. 45e67. [38] Mori S, Zhang J. Principles of diffusion tensor imaging and its applications to basic neuroscience research. Neuron 2006;51:527e39. [39] Kim J, Seidler P, Fill C, Wan LS. Investigations of the effect of curing conditions on the structure and stability of aminofunctionalized organic films on silicon substrates by Fourier
[40]
[41]
[42]
[43]
[44]
[45]
transform infrared spectroscopy, ellipsometry, and fluorescence microscopy. Surf Sci 2008;602:3323e30. Yang W, et al. Synthesis of phosphorus-containing silane coupling agent for surface modification of glass fibers: effective reinforcement and flame retardancy in poly(1,4butylene terephthalate). Chem Eng J 2017;321:257e67. Synthesis of silica immobilized phosphotungstic acid (SiPWA)-poly(vinyl alcohol) (PVA) composite ion-exchange membrane for direct methanol fuel cell. Energy 2014;39(17):9473e81. Gahlot Swati, Sharma Prem P, Gupta Hariom, Kulshrestha Vaibhav, Prafulla K Jha. Preparation of graphene oxide nano-composite ion-exchange membranes for desalination application. RSC Adv 2014;4:24662e70. Gao Haixia, et al. Improving the proton conductivity of protonexchange membranes via incorporation of HPWfunctionalized mesoporoussilicananospheres into SPEEK. Int J Hydrogen Energy 2018;43(48):21940e8. Sharma Prem P, Gahlot Swati, Bhil Batuk M, Gupta Hariom, VaibhavKulshrestha. Synthesis and characterization of partial SulfonatedPVDF-co-HFP ion exchange MembranesMacromol. Symp 2015;357:154e8. Prem P. Sharma, Swati Gahlot, VaibhavKulshrestha, One pot synthesis of PVDF based copolymer proton conducting membrane by free radical polymerization for electrochemical energy applications, Colloid Surf Physicochem Eng Asp, 520, 239-245.
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Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics Prem P. Sharma a, Hariom Gupta a,b,*, Vaibhav Kulshrestha a,** a
CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), G.B.Marg, Bhavnagar, 364002, Gujarat, India b CSIR-Central Institute of Medicinal and Aromatic Plants (CSIR-CIMAP), Near Kukrail Picnic Spot, Lucknow, 226015, India
highlights Enhanced proton conductivity in phosphorylated hybrid silica-SPES compare to SPES. T1, T2 relate fall in Proton-channel interaction, better hydrated protons interlinking. Better proton hopping interlinking pathway causing to enhance proton conductivity.
article info
abstract
Article history:
The understating of hydrated proton activity and its dynamics inside the ion exchange
Received 3 March 2019
channel of proton exchange membranes (PEMs) are one of the basic need to develop quality
Received in revised form
membranes for desalination and energy applications. In this work, sulfonated poly-
9 September 2019
ethersulfone (SPES), hybrid Silica-SPES and phosphorylated hybrid Silica-SPES composite
Accepted 12 September 2019
PEMs are prepared; and membrane properties are characterized with appropriate analyt-
Available online xxx
ical tools. The enhanced conductivity is described effectively with 1H spin-lattice (T1), spin-
Keywords:
mined by magnetic resonance imaging (MRI) technique. This study gives useful insight on
Magnetic resonance imaging (MRI)
variation in the interactions and dynamics of hydrated protons confining inside the
Proton exchange membrane (PEM)
membrane channel with the incorporation of silica and its further phosphorylation. Ob-
Spin-lattice relaxation
tained spin-lattice (T1), spin-spin (T2) relaxation and diffusion results relate that silica and
Spin-spin relaxation
phosphorylation have facilitated the formation more hurdle free organized pathway for
Diffusion coefficients
proton conduction.
spin (T2) relaxation and diffusion weighted imaging and their corresponding value deter-
Hybrid silica-SPES and
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
phosphorylated hybrid silica-SPES
* Corresponding author. CSIR-Central Salt and Marine Chemical Research Institute, G.B. Road, Bhavnagar, Gujarat, India. ** Corresponding author. E-mail addresses: [email protected], [email protected] (H. Gupta), [email protected] (V. Kulshrestha). https://doi.org/10.1016/j.ijhydene.2019.09.106 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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Introduction Proton exchange membranes (PEMs) have established an excellent capability in the field of energy (fuel cells, water electrolysis, redox-flow battery etc.) and separation applications [1e6]. Nafion is widely used PEM but it is high in cost. Various non-nafion based cost effective membranes, such as sulfonated polysulfone (SPS), sulfonated polyethersulfone (SPES) and sulfonated poly(ether ether ketone) (SPEEK) with silica hybrid have been reported to achieve high conductivity equivalent to nafion and overcome the limitation of nafion [7e10]. Silica improves the conductivity and mechanical stability but excessive amount makes the membrane brittle in nature causing to reduce in mechanical stability. Proton conductivity of such membranes unambiguously depends on characteristics of membrane channels, exchange sites and dynamics of confined hydrated protons in the membrane channels [11e13]. Shahi has reported sulfonated poly(ether sulfone)-silica polyelectrolyte composite membranes for fuel cell, explained the improved membrane property with frictional coefficient between proton and membrane matrix calculated using conductivity data and found lowering in its value on incorporation of silica [14]. Roelofs et al. have prepared sulfonatedpoly(ether ether ketone)-based silica nanocomposite membranes for direct ethanol fuel cells and found the enhance conductivity and improved water uptake characteristics [15]. Some other researchers have suggested and explained the enhanced conductivity for hybrid membranes based on improve water uptake and reduction in frictional coefficient [16e19]. But these studies are almost untouched with direct in-situ measurement of parameters associated with behavior of hydrated proton present in membrane channel and its dynamics causing to enhance the conductivity on incorporation of silica. Nuclear magnetic resonance (NMR) is a convenient and noninvasive means to measure the parameters directly co-related with motion of hydrated protons confining inside the membrane channel, its interaction with channel walls and ordered hydrogen bonded connectivity with other protons. Self diffusion coefficient measured using pulse field gradient (PFG) diffusion NMR technique has been reported in literature and used to depict hydrated proton activity and its dynamics in various PEMs [20e23]. Spin-lattice (T1) and spin-spin (T2) NMR relaxations times of 1H nuclei (i.e. proton) of hydrated proton provides the information about its local condition and interactions, including its interactions with membrane channel walls and ordered hydrogen bonded linkage connecting hydrated protons. These parameters are quite sensitive with the variation of small structural and functional modification in membrane matrix. Magnetic resonance imaging (MRI) is another face of NMR introduces the feature of spatial cross section imaging and spectroscopic parameters measurement including diffusion coefficient and relaxation times in noninvasive manner [3,24]. It utilizes the behavior of protons of confined water to disclose the feature and local functional variation in the sample and generate contrast in images directly correlating with its features [25e27]. MRI based diffusion analysis for PEMs have been reported to disseminate the dynamics of hydrated proton in literature and our previous published work [3,24,28e30]. Lui et
al. have studied the diffusion of water molecule inside the carbon nanotubes (CNTs) of diameter 2.3 ± 0.3 nm and found its value one order higher in magnitude than the mesoporous silica material of similar pore size [31]. They have concluded that fast diffusion is caused due to weak interaction of confined water molecules with CNT walls and formation of ordered hydrogen bonding between the water molecules. Therefore, relaxation and diffusion measurement can jointly enumerate the behavior and dynamics of hydrated proton confining inside the channel of proton exchange membrane but no such direct studies are reported to explain the cause of enhanced conductivity of silica hybrid proton exchange membranes. In this work, we have described the detailed behavior and dynamics of hydrated protons in three different sulfonated polyether sulfone (SPES), hybrid Silica-SPES (SPES-Si) and phosphorylated hybrid Silica-SPES (SPES-Si-P) composite proton exchange membranes (PEMs) of same polymeric backbone. The cause for enhanced proton conductivity in membrane channels on incorporation of silica and its further phosphorylation is explained with spin-lattice (T1), spin-spin (T2) relaxation and diffusion MRI investigations.
Experiment section Material and membrane synthesis Polyethersulfone (PES), obtained from Solvay Chemicals Pvt Ltd., India, is used after drying under vacuum for 24 h. Sulfonation of PES was carried out using conc. H2SO4 (95e98%) under vigorous stirring at 60 C for 6 h [32]. The membranes ware synthesized by reported method performed by our group in earlier paper [14].
Characterization FT-IR micro-ATR spectra to characterize functional groups present on membranes were acquired in transmittance modeusing Agilent Cary 600 series FTIR Spectrometer. Mechanical properties were analysed using Universal Testing Machine (UTM) on ZwickRoell, Z2.5 instrument. Thermal stability of membranes was characterized by thermogravimetric analyser (TGA) instrument of Mettler Toledo TGA/ SDTA851 under N2 gas atmosphere for the temperature range of 30e700 C with heating rate of 5 C/min. SEM images and elemental mapping by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis of membrane were obtained with Field Emission Scanning Electron Microscope (FE-SEM-Jeol JSM 7100F). For SEM imaging small pieces of membrane are mounted on SEM stub and stub was coated with Au to make the sample conductive for SEM images of better quality. Ion exchange capacities (IEC) of prepared membranes were estimated using acid-base titration method. Briefly a piece of examining membrane (area: 1 cm2) is dipped in 0.1M HCl solution for 24 h; dipped membrane piece is wash properly with water to remove excess amount of acid and then transferred into 0.1 M NaCl solution for same time. During this process protons are exchanged with sodium ions and come out into
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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the NaCl solution. This resulted NaCl solution was titrated with 0.01M solution of the NaOH using phenolphthalein as an indicator. The IEC values of examined membranes were estimated using following co-relation equation IECðmeq = gmÞ ¼
CNa VNaOH WDry
Where CNa is the concentration of Naþ ion in titrating NaOH solution, VNaOH is volume consumed NaOH solution in titration and Wdry is dry weight of membrane piece. Ionic conductivity of the membranes was measured using potentiostat/galvanostat (CHI 608) at room temperature. Membrane resistance was obtained from Nyquist plot by fitting. Ionic conductivity was calculated using the following equation: km U1 cm1 ¼
Where I0 is maximum signal intensity; I'0 and I}0 are the maximum signal intensity at fix value of TE and TR. For pure T1and T2weighted images, intensity of MRI image of corresponding weighting is purely depend on T1and T2 relaxation time respectively. Pure T1and T2 weighting can be achieved by eliminating or minimizing the T2 term (T2 term /1) for T1 weighting and T1 term (T1 term /1) for T2 with the selection of appropriate combination of MRI acquisition parameter TE (time of echo) and TR (time of repetition) value. For Diffusion weighted MRI, Image intesity for pulse field gadient spin echo (PFGSE) based MRI pulse sequence can be mathematically represented as [38]. eBD |ffl{zffl}
(2)
Diffusionterm
Where, L is the distance between the electrodes used to measure the potential, R is the resistance of the membrane, and A is the surface area of the membrane. Magnetic resonance images, relaxometric (T1 and T2) and ¨ KER diffusion analysis of membranes were acquired with BRU AVANCE 500 MHz (11.7 T) NMR instrument using microimaging probe and Paravision imaging software. T1 and T2 weighted MRI images of membranes were acquired using the spin echo pulse sequences (RAREVTR and MSME) with acquisition parameters for TR (time of repetition) ¼ 350 ms, TE (time of echo) ¼ 8 ms, 128 128 matrix and TR ¼ 1500 ms, TE ¼ 20, 256 256 matrix respectively. T1 and T2 relaxation times of membranes were measured using of Bruker RAREVTR (TE ¼ 8 ms, TR ¼ 250e3500 ms and 128 128 matrix) and MSME (TE ¼ 10e80 ms, TR ¼ 1500 ms and 256 256 matrix) MRI pulse sequences respectively. T1 and T2 relaxation times for each membranes were calculated with fitting of T1 and T2 experiment data to Eq. (1b) and Eq. (1c) using Paravision software [26,33]. A series of diffusion weighted MRI images were acquired using PFGSE (pulse field gradient spin echo) based MRI imaging method with different B value, TR ¼ 500ms, TE ¼ 15.3 ms and 128 128 matrix size. The selfdiffusion coefficient (D) of protons in hydrated membranes is calculated by fitting of Stejskal Equation (2) using Paravision imaging software [34e36]. The intensity of MRI image of a sample depends on number of 1H nuclei (proton) present in sample and their relaxation rate. Spin-echo (SE) MRI pulse sequence is widely used for T1 and T2 weighted imaging and the image intensity for SE pulse sequence is defined as [37].
T1 term
(1c)
T2 term
I ¼ I0
LðcmÞ RðUÞ Aðcm2 Þ
TE=T 2 I ¼ I0 1 eTR=T1 e|fflfflffl{zfflfflffl} |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl}
TE=T2 I ¼ I}0 e|fflfflffl{zfflfflffl}
3
Where B ¼ g2g2d2 D 3d , D ¼ diffusivity or diffusion coefficient, g ¼ gyromagnetic ratio, g ¼ strength of gradient pulse, d ¼ length of gradient pulse and D ¼ diffusion time.
Results and discussion Structural characterization of membranes Fourier transform infrared (FTIR) spectra of SPES, SPES-Si and SPES-Si-P are presented in Fig. 1. IR bands observed at 1575, 1485, 1469 1407 cm1 are assigned to C]C stretching of aromatic rings of poly (ether sulfone). A characteristic symmetric and asymmetric stretching of O]S]O of sulfone group (eSO2e) present in all membranes are found to be at 1147 and a doublet bands at 1293 and 1319 cm1. An IR band at 1025 cm1 is attributed to symmetric stretching of O]S]O of sulfonate group (-SO3‾) and found to be present in all membranes. IR bands at 1072 and 1180 cm1 are correspond to the aromatic ring vibration of the poly(ether sulfone), a peak at
(1a)
T2 term
For T1 experiments T2 term /1 or constant for fix value of TE, i.e. I ¼ I'0 1 eTR=T1 |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl}
(1b)
T1 term
For T2 experiments T1 term /1 or constant for fix value of TR, i.e.
Fig. 1 e FTIR spectra of SPES, SPES-Si and SPES-Si-P membrane representing characteristic peaks correspond to various functional group present on these membranes.
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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869 cm1 is due to out-of-plane CeH deformation vibration. A strong band observed at 1236 cm1 is assigned to asymmetric CeOeC stretching of aryl ether group of poly(ether sulfone). A slight relative intensity enhancement for bands at 835 cm1 for SPES-Si and SPES-Si-P compare to SPES is attributed to the contribution of -Si-O- stretching and out-of-plane bending vibration of eN-H of APTES incorporated in these membranes [39]. A weak band at 1652 cm1 observed significantly in SPESSi and SPES-Si-P is attributed to in-plane bending of -N-H group of APTES present in SPES-Si and SPES-Si-P membrane [40]. It characterizes and suggests the formation of silica-SPES composite membrane. A band correspond to P]O stretching (1139 cm1) is possibly overlapped with band (1147 cm1) of sulfone group and not significantly distinguishable in this spectrum [40]. A spectral region containing few other bands below 800 are corresponding to the bending mode vibration of above mentioned functional groups present in polymer membranes.
Ion exchange capacity (IEC) and ionic conductivity (IC) Ion exchange capacity (IEC) and ionic conductivity are important parameters to evaluate the membrane property for electro chemical applications. The IEC values as shown in Table 1a are found to be 0.92, 1.34, 1.61 meq g1 for SPES, SPESSi and SPES-Si-P membranes. An enhanced IEC values for SPES-Si and SPES-Si-P are caused due to the incorporation of Si and phosporalization into the membrane; both these membranes also shows an enhanced water uptake. A moderate amount of water uptake into ion exchange membrane is
necessary for ion transportation; it provides ion transportation and ion exchange sites connecting hydrogen bonded water molecule pathway in the membrane matrix which results an increase in the ion conductivity in membrane. Water uptake and proton conductivity for examined membranes are presented in Table 1a and compared with Nafion 117. The trend of proton conductivity and water uptake is found to increase and well consistent with IEC trend. These results attribute a significant enhancement in electro and physicochemical properties of SPES membrane on incorporation of silica and further phosphorylation. Membrane properties of prepared hybrid composite are well comparative with previously reported hybrid membranes and comparison is represented in Table 1b.
Mechanical and thermal property The appearance and transparency of all these membranes (SPES, SPES-Si and SPES-Si-P) are identical as shown in Fig. 2 however last two membranes are comparably hard and brittle structure compare to SPES. Mechanical properties of membranes measured by universal testing machine (UTM) are given in Table 2 and obtained stress-strain curves are shown in Fig. 2. The young modulus and elongation are found to decrease with the incorporation of silica and phosphorylation into SPES matrix. A drastic decrease in stress value is observed for both SPES-Si (4.94 MPa) and SPES-Si-P (6.30 MPa) compare to SPES (13.18 MPa) membrane. These results suggest a decrease in mechanical stability of these modified membranes in comparison to SPES but it is well comparable to
Table 1a e Ion exchange capacity (IEC), % water uptake and Ionic Conductivity (IC). Membrane type SPES SPES-Si SPES-Si-P Nafion
IEC (meq g1) 0.92 ± 1.34 ± 1.61 ± 0.93 ±
IC 102 (Scm1)
% Water Uptake
0.01 0.01 0.01 0.01
17.87 19.20 22.13 19.00
± 0.01 ± 0.01 ± 0.01 ± 0.01
2.48 ± 3.67 ± 5.41 ± 9.10 ±
0.01 0.01 0.01 0.01
Table 1b e Comparison of characteristics of present membrane with other reported hybrid membranes. Membrane type Si-PWA/PVA SM-10 MGOeSCH-5 SG-10 SPEEK/[email protected] SPES-Si-P
IEC (meq g1)
Water Content (%)
IC (102 S cm1)
Reference
0.90 1.44 2.56 1.27 1.35 1.61
35.00 20.26 43.00 15.00 20.93 22.13
0.70 5.72 6.77 6.40 3.55 5.41
[41] [18] [19] [42] [43] This study
Si-PWA/PVA: silica immobilized phosphotungstic acid-poly(vinyl alcohol) (PVA), SM: SPES-sulfonated mesoporous silica, MGO-SCH: Sulfinated chitosan-silica modified Graphine oxide; SG: SPES-graphine oxide, SPEEK/HPW@MSNs: SPEEK-phosphotungstic acid in mesoporous silica nanospheres.
Table 2 e Mechanical properties of membranes. Membrane type SPES SPES-Si SPES-Si-P
Young's Modulus (MPa)
Stress (MPa)
% Elongation
3.5 ± 0.1 3.0 ± 0.1 2.0 ± 0.1
13.18 ± 0.2 4.94 ± 0.2 6.30 ± 0.2
29.87 ± 0.2 21.74 ± 0.2 14.25 ± 0.2
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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Fig. 2 e Optical appearance of membranes and their mechanical property. other reported PEMs [44,45]. SPES-Si and SPES-Si-P can be useful for electro-chemical applications as these membranes have shown significant enhancement in conductivity with some sacrifice of mechanical stability. The thermal stability of prepared SPES and SPES-Si-P membranes were studied by thermo-gravimetric (TG) analysis and obtained TG and DTG results are shown in Fig. 3. Weight loss in range of 100e400 C is corresponding to desorption of water from the membrane and disintegration of eS]O groups present in the membrane [42]. A sharp weight loss in TG and its corresponding DTG peak in the range of 450e 550 is attributed to the decomposition of poly(ether sulfone) backbone of the membranes [3]. A slight larger weight loss in SPES-Si-P above 500 C is possibly contributed by the deoxygenation of silica present in this membrane. The thermogravimetric results for both examined membrane are almost similar which suggests that incorporation of silica and phosphorylation into SPES membrane does not affect its thermal stability significantly. The structural morphology of the membrane matrix is key feature of membrane that are intended to use for separation application. Fig. 4 shows the surface and cross sectional SEM images of SPES and SPES-Si-P membrane which indicates the formation of a uniform matrix of dense morphology for both the membranes. Formation of silica hybrid, phosphorylation and sulfonation in membrane are further established through SEM-EDS elemental mapping and EDX spectrum analysis of SPES-Si-P membrane and results are shown in Fig. 5. It shows an even distribution of Si, P and S elements throughout the membrane which reveals the formation of uniformity phosphorylated hybrid silica SPES membrane.
exchangeable proton and associated water spectroscopically, and deliver information of its interaction with polymer channels and neighboring hydrated protons. 1H spin-lattice (T1), 1H spin-spin (T2) relaxation times and diffusion value
Magnetic resonance imaging (MRI) analysis MRI is a noninvasive technique and most commonly used for biomedical and biological investigations of behavior (interaction and dynamics) of proton (i.e. 1H nuclei) proton associated with water molecule present in biological sample. It could also be more suitable for the determinations proton exchange mechanism in the membrane as this technique exploits directly hydrogen nuclei (i.e. 1H nuclei) of hydrated
Fig. 3 e Thermogravimetric (TG) and its derivative (DTG) results of SPES and SPES-Si-P membranes.
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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Fig. 4 e Surface and cross sectional SEM images of SPES (a, b) and SPES-Si-P (c, d) representing the formation of membrane of dense morphology.
Fig. 5 e SEM-EDS elemental maps of selected portion of SPES-Si-P membrane: a) mix image of elemental map of Si (red), P (green) and S (blue); b) Individual map of Si; c) Individual map of P; d) EDX spectrum representing the presence of Si and P in membrane. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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Fig. 6 e T1 weighted MRI image at TR ¼ 350 ms and TE ¼ 8 ms; Normalized intensity recovery plot of spin-lattice (T1) relaxation MRI experiment, representing the decrease in T1 relaxation times with modification of SPES.
Table 3 e T1, T2 relaxation times and diffusion coefficients. Membrane type SPES SPES-Si SPES-Si-P Nafion
T1 (ms)
T2 (ms)
389.545 ± 17.100 614.056 ± 24.242 778.556 ± 20.543 e
8.465 ± 1.033 18.993 ± 0.370 15.113 ± 0.827 e
(D) of 1H spin in the membrane are important and key parameters for the deep understanding of protons behavior and their dynamics in the matrix of membrane. Spin-lattice relaxation (T1) mainly reveals the interaction of proton with
DiffusionCoeff. (1010 m2/S) 1.061 2.699 5.532 5.394
± 0.014 ± 0.103 ± 0.552 ± 0.619
membrane matrix however spin-spin relaxation dominantly disclose the interaction between two protons and their hydrogen bond connectivity between ion exchange sites in the membrane. The diffusion value provides direct information of
Fig. 7 e T2 weighted MRI images at TR ¼ 1500 ms and TE ¼ 20 ms; Normalized intensity attenuation plot of spin-lattice (T2) relaxation MRI experiment, representing the decrease in T2 relaxation times with modification of SPES. Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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Fig. 8 e Diffusion weighted MRI images at TR ¼ 500 ms and TE ¼ 14.3 ms; Normalized intensity attenuation plot of diffusion map MRI experiment, representing enhancement in diffusion coefficient with modification of SPES.
dynamic movement of the protons which is well correlated and equivalent to the proton conductivity information of membrane. So these parameters are well capable to explore the exact behavior and dynamics of protons in the matrix of membrane. T1, T2 and diffusion MRI analysis for SPES, hybrid Silica-SPES and phosphorylated hybrid Silica-SPES composite membrane as per the MRI acquisition parameters and methods given in experimental section. T1 weighted MRI images and corresponding intensity recovery plot of T1 MRI experiment are represented in Fig. 6. T1 relaxation times (Table 3) are found to increase for silicahybrid and phosphorylated silica-hybrid which relates to attenuation in spin-lattice correlation time of transportable proton resulting to a decrease in its interaction with the lattice of membrane channels. It suggests that incorporation of silica and further phosphorylation encounter the impediment and deliver relatively less unperturbed channels for proton transportation.
Fig. 7 shows T2 weighted MRI images and their corresponding normalize signal attenuation curve, and obtained T2 relaxation times are listed in Table 3. T2 values are found to be 08.46, 18.99, 15.11 ms for SPES, SPES-Si and SPES-Si-P respectively. Increase in T2 value implies that decrease in spin-spin correlation time resulting to reduce in mobility constrains between transportable protons due to improvement in ordered hydrogen bonded linkage connecting other hydrated protons and provides an easy pathway for proton conduction in the membrane channels. The diffusion weighted MRI images and Stejskal attenuation plot is shown in Fig. 8, estimated diffusion coefficients are listed in Table 3. Diffusion coefficients have significantly improved and found to be about 2.5 times for SPES-Si and 5.2 times for SPES-Si-P in comparison to SPES. SPES-Si-P has effectively achieved the diffusion coefficient of Nafion117. Above results suggest a formation of well-connected and relatively more hurdle-free proton hopping organized pathway causing to enhance proton conductivity. It is schematically represented and shown in Fig. 9.
Conclusion
Fig. 9 e Schematic diagram of possible hydrogen bonded proton hopping pathway.
The incorporation of silica into SPES and its further phosphorylation have shown significant improvement in proton conductivity with minor decline in mechanical stability. Water uptake is found to increase with the incorporation of silica and further phosphorylation. Improved self diffusion coefficients of hydrated protons present inside these membraneshave suggested reduction in conduction constrains. Spin-lattice (T1), spin-spin (T2) relaxation times of hydratedprotons attributes that incorporation of silica improves the ordered hydrogen bonded linkage connectivity among the hydrated protons and reduces their interaction with membrane channel. It can be concluded that incorporated silica facilitate a well-connected and relatively more hurdle-free
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
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proton hopping organized pathway causing to enhance proton conductivity in the membrane. [15]
Acknowledgement [16]
Authors are thankful of Mr. Jayesh Chaudhary, Mr. Vinod Agrawal and Mr. Satyaveer of Centralized Instrument Facility of CSMCRI for helpful assistance in characterization. Authors are thankful to SERB, DST New Delhi for providing financial support.
references
[1] Xie H, Tao D, Xiang X, Ou Y, Bai X, Wang L. Synthesis and properties of highly branched star-shaped sulfonated block poly(arylene ether)s as proton exchange membranes. J Membr Sci 2015;473:226. [2] Ganeshkumar A, Bera D, Ali EM, Banerjee S. Triphenyl amine containing sulfonated aromatic polyimide proton exchange membranes. Eur Polym J 2014;60:235. [3] Gahlot S, Gupta H, Jha PK, Kulshrestha V. Enhanced electrochemical performance of stable SPES/SPANI composite polymer electrolyte membranes by enriched ionic nanochannels. ACS Omega 2017;2:5831e9. [4] Zhang H, Shen PK. Recent development of polymer electrolyte membranes for fuel cells. Chem Rev 2012;112:2780e832. A, Carbone A, Gatto I, Pedicini R, Freni A, Patti A, [5] Sacca Passalacqua E. Composites nafion-titania membranes for polymer electrolyte fuel cell (PEFC) applications at low relative humidity. Polym Test 2016;56:10e8. [6] David O, Percin K, Luo T, Gendel Y, Wessling M. Proton exchange membranes based on sulfonated poly(ether ether ketone)/polyaniline blends for all- and air-vanadium redox flow battery applications. J. Energy Storage 2015;1:65e71. [7] Fu Y-Z, Manthiram A. Synthesis and characterization of sulfonatedpolysulfone membranes for direct methanol fuel cells. J Power Sources 2006;157:222e5. [8] Jiang Z, Zhao X, Manthiram A. Sulfonatedpoly(ether ether ketone) membranes with sulfonatedgraphene oxide fillers for direct methanol fuel cells. Int J Hydrogen Energy 2013;38(14):5875e84. [9] Gahlot Swati, Sharma Prem P, Gupta Hariom, Prafulla K, Jha VaibhavKulshrestha. Synthesis of SPEEK/TiO2 nanocomposites and their characterization. Adv Electrochem 2015;2:1e6. [10] Lim Y, Lee S, Seo D, Jang H, Hossain M, Ju H, Hong T, Kim W. Sulfonated poly(ether sulfone) electrolytes structured with mesonaphthobifluorenegraphene moiety for PEMFC. Int J Hydrogen Energy 2014;39:1532. [11] Sone Y, Ekdunge P, Simonsson D. Proton conductivity of nafion 117 as measured by a four -electrode AC impedance method. J Electrochem Soc 1996;143(4):1254e9. [12] Pili Simona, et al. Proton conduction in a phosphonate-based MetalOrganic framework mediated by intrinsic “free diffusion inside a sphere”. J Am Chem Soc 2016;138:6352e5. [13] Liang X, Zhang F, Feng W, Zou X, Zhao C, Na H, Liu C, Sun F, Zhu G. From metaleorganic framework (MOF) to MOFepolymer composite membrane: enhancement of lowhumidity proton conductivity. Chem Sci 2013;4:983. [14] Shahi Vinod K. Highly charged proton-exchange membrane: sulfonatedpoly(ether sulfone)-silica polyelectrolyte
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
9
composite membranes for fuel cells. Solid State Ion 2007;177:3395e404. Roelofs Kimball S, Hirth Thomas, Schiestel Thomas. Sulfonated poly(ether ether ketone)-based silica nanocomposite membranes for direct ethanol fuel cells. J Membr Sci 2010;346:215e26. Park Junghwa, Kim Dukjoon. Sulfonatedmesoporous benzene-silica-embedded sulfonatedpoly(ether ether ketone) membranes for enhanced proton conduction and anti-dehydration. Int J Hydrogen Energy 2014;39:1063e70. Park Ki Tae, et al. Composite membranes based on a sulfonatedpoly(arylene ether sulfone) and protonconducting hybrid silica particles for high temperature PEMFCs. Int J Hydrogen Energy 2011;36:10891e900. Gahlot Swati, Prem P, Sharma VikrantYadav, Prafulla K Jha, VaibhavKulshrestha. Nanoporous composite proton exchange membranes: high conductivity and thermal stability. Colloids Surf, A 2018;542:8e14. Prem P, Sharma, VaibhavKulshrestha. Synthesis of highly stable and high water retentive functionalized biopolymergraphene oxide modified cation exchange membranes. RSC Adv 2015;5:56498e506. Jayakody JRP, Stallworth PE, Mananga ES, FarringtonZapata J, Greenbaum SG. High pressure NMR study of water self-diffusion in NAFION-117 membrane. J Phys Chem B 2004;108:4260e2. Li J, Wilmsmeyer KG, Madsen LA. Anisotropic diffusion and morphology in PerfluorosulfonateIonomers investigated by NMR. Macromolecules 2009;42:255e62. Ma Z, Jiang R, Myers Jr ME, Thompsona EL, Gittleman CS. NMR studies of proton transport in fuel cell membranes at sub-freezing conditions. J Mater Chem 2011;21:9302e11. Ma Zhiru, Jiang Ruichun, Mark E, Myers Jr, Eric L, Thompson Craig S Gittleman. NMR studies of proton transport in fuel cell membranes at sub-freezing conditions. J Mater Chem 2011;21:9302. Gahlot S, Gupta H, Jha PK, Kulshrestha V. Hydrated proton self-diffusion study in ion-exchange membranes by MRI and impedance spectroscopy. Polym Bull 2017;74:4437. Wu Y, Dai G, Ackerman JL, Hrovat MI, Glimcher MJ, Snyder BD, Nazarian A, DA Chesler. Water- and fatsuppressed proton projection MRI (WASPI) of rat femur bone. Magn Reson Med 2007;57:554e67. Gupta H, Paul P, Kumar N, Baxi S, Das DP. One pot synthesis of water dispersible dehydroascorbic acid coated Fe3O4 nanoparticles under atmospheric air: blood cell compatibility and enhanced magnetic resonance imaging. J Colloid Interface Sci 2014;430:221e8. Vinogradov E, Soesbe TC, Balschi JA, Sherry AD, Lenkinski RE. pCEST: positive contrast using chemical exchange saturation transfer. J MagnReson 2012;215:64e73. Feindel Kirk W, Bergens Steven H, Wasylishen Roderick E. Insights into the distribution of water in a self-humidifying H2/O2 proton-exchange membrane fuel cell using 1H NMR microscopy. J Am Chem Soc 2006;128(43):14192e9. Feindel Kirk W, Bergens Steven H, Wasylishen Roderick E. The use of 1 H NMR microscopy to study ProtonExchange membrane fuel cells. Chem Phys Chem 2006;7:67e75. Sahu AK, Selvarani G, Bhat SD, Pitchumani S, Sridhar P, Shukla AK, Narayanan N, Banerjee A, Chandrakumar N. Effect of varying poly(styrene sulfonic acid) content in poly(vinyl alcohol)epoly(styrene sulfonic acid) blend membrane and its ramification in hydrogeneoxygen polymer electrolyte fuel cells. J Membrane Sci 2008;319:298e305. Liu | Xin, Pan Xiulian, Zhang Shanmin, Han Xiuwen, XinheBao. Diffusion of water inside carbon nanotubes studied by pulsed field gradient NMR spectroscopy. Langmuir 2014;30:8036e45.
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106
10
international journal of hydrogen energy xxx (xxxx) xxx
[32] Thakur AK, Gahlot S, Kulshrestha V, Shahi VK. Highly stable Acidebase complex membrane for ethanol dehydration by pervaporation separation. RSC Adv 2013;3:22014e22. [33] Haque MI, Rathore MS, Gupta H, Jha B. Inorganic solutes contribute more than organic solutes to the osmotic adjustment in Salicorniabrachiata (Roxb.) under natural saline conditions. Aquat Bot 2017;142:78e86. [34] Gupta H, Paul P, Kumar N. Synthesis and characterization of DHA/ZnO/ZnFe2O4 nanostructures for biomedical imaging application. Procedia Mater Sci 2014;5:198e203. [35] Stejskal E-O, Tanner J-E. Spin diffusion measurements: spin echoes in the presence of a time dependent field gradient. J Chem Phys 1965;42:288e92. [36] Sharma Prem P, Gahlot S, Gupta H, Kulshrestha V. Hybrid anion conducting membranes (ACM) for industrial applications: excellent salt removal efficiency and electro-chemical properties. Separ Purif Technol 2017;189:449e58. € der L, Faber C. Basic contrast mechanisms. In: [37] Schro € der L, Faber C, editors. In-Vivo NMR Imaging methods Schro and protocols. Humana Press-Springer; 2011. p. 45e67. [38] Mori S, Zhang J. Principles of diffusion tensor imaging and its applications to basic neuroscience research. Neuron 2006;51:527e39. [39] Kim J, Seidler P, Fill C, Wan LS. Investigations of the effect of curing conditions on the structure and stability of aminofunctionalized organic films on silicon substrates by Fourier
[40]
[41]
[42]
[43]
[44]
[45]
transform infrared spectroscopy, ellipsometry, and fluorescence microscopy. Surf Sci 2008;602:3323e30. Yang W, et al. Synthesis of phosphorus-containing silane coupling agent for surface modification of glass fibers: effective reinforcement and flame retardancy in poly(1,4butylene terephthalate). Chem Eng J 2017;321:257e67. Synthesis of silica immobilized phosphotungstic acid (SiPWA)-poly(vinyl alcohol) (PVA) composite ion-exchange membrane for direct methanol fuel cell. Energy 2014;39(17):9473e81. Gahlot Swati, Sharma Prem P, Gupta Hariom, Kulshrestha Vaibhav, Prafulla K Jha. Preparation of graphene oxide nano-composite ion-exchange membranes for desalination application. RSC Adv 2014;4:24662e70. Gao Haixia, et al. Improving the proton conductivity of protonexchange membranes via incorporation of HPWfunctionalized mesoporoussilicananospheres into SPEEK. Int J Hydrogen Energy 2018;43(48):21940e8. Sharma Prem P, Gahlot Swati, Bhil Batuk M, Gupta Hariom, VaibhavKulshrestha. Synthesis and characterization of partial SulfonatedPVDF-co-HFP ion exchange MembranesMacromol. Symp 2015;357:154e8. Prem P. Sharma, Swati Gahlot, VaibhavKulshrestha, One pot synthesis of PVDF based copolymer proton conducting membrane by free radical polymerization for electrochemical energy applications, Colloid Surf Physicochem Eng Asp, 520, 239-245.
Please cite this article as: Sharma PP et al., Phosphorylated hybrid silica-sulfonated polyethersulfone composite proton-exchange membranes: Magnetic resonance investigation for enhanced proton-exchange dynamics, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.106