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Crosslinked carbon nanodots with highly sulfonated polyphenylsulfone as proton exchange membrane for fuel cell applications
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Crosslinked carbon nanodots with highly sulfonated polyphenylsulfone as proton exchange membrane for fuel cell applications Nor Azureen Mohamad Nor a,b,d, Hidenobu Nakao c, Juhana Jaafar d,*, Je-Deok Kim a,b,** a
Hydrogen Production Materials Group, Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan b Polymer Electrolyte Fuel Cell Group, Global Research Center for Environmental and Energy Based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan c Photon and Ion Beam Physics Group, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan d Advanced Membrane Technology Research Centre (AMTEC), School of Chemical & Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor, Malaysia
highlights Highly
sulfonated
graphical abstract poly-
phenylsulfone as polymer based for proton exchange membrane. Carbon nanodots facilitate more channel for proton transfer in the membrane. Thermal crosslinking applied to crosslink the sulfonic acid and amine group. Carbon nanodots improve proton conductivity under low and high RH conditions. High membrane flexibility ensuing good interface between membrane and electrodes.
article info
abstract
Article history:
The crosslinked highly sulfonated polyphenylsulfone (SPPSU) membranes that consists of
Received 22 July 2019
carbon nanodots (CNDs) was prepared as a proton exchange membrane for fuel cell ap-
Received in revised form
plications. The crosslinked membranes were developed by annealing at 180 C, in which
3 October 2019
the crosslinking occurred between the SPPSU and CNDs. The CNDs potential was explored
* Corresponding author. ** Corresponding author. Hydrogen Production Materials Group, Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan. E-mail addresses: [email protected] (J. Jaafar), [email protected] (J.-D. Kim). https://doi.org/10.1016/j.ijhydene.2020.01.142 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Accepted 21 January 2020
in detail under various loadings (0 wt %, 1 wt %, 2 wt %, and 3 wt %). Upon annealing at
Available online 26 February 2020
180 C, the flexibilities and strength of the SPPSU-CNDs membranes improved. The proton conductivity of the crosslinked membrane was enhanced than that of pristine SPPSU
Keywords:
membrane due to the crosslinking effect between SPPSU and CNDs. The highest conduc-
Highly sulfonated
tivity, which was at 56.3 mS/cm was obtained when 3 wt % of CNDs was incorporated at
polyphenylsulfone
80 C and 90% relative humidity (RH). The results indicated that the incorporation of CNDs
Carbon nanodots
in the SPPSU membrane by annealing at 180 C, exhibited a proton conductive membrane
Proton exchange membrane
in combination with superior dimensional stability, and proton conductivity suitable for fuel cell applications. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cell is a promising technology as a heat and electricity source for buildings, and an electrical power source for electric motor propelling vehicles. Fuel cells cleanly and efficiently convert chemical energy from hydrogen-rich fuels into electrical power and useable high quality heat in an electrochemical process while crude oil prices surge [1]. Proton exchange membrane fuel cell (PEMFC) is a type of fuel cell that is developed mainly for transport as well as stationary and portable applications. PEMFCs work with a proton exchange membrane (PEM), which is a thin (~20e250 mm) permeable sheet that is generally made from ionomers and designed to conduct protons, while it acts as a barrier to reactants and electron insulator. The essential function of PEM in a fuel cell system is as reactants separator and protons transportation medium, while blocking an electronic pathway through the membrane. Generally, a good PEM requires high proton conductivity (>102 S/cm) that enables proton transport between the electrodes. PEM also needs excellent chemical stability under fuel cell operating conditions, low operating cost, and highly flexible membrane that is compatible with other cell components for efficient cell performance [1e3]. PEM materials based on perfluorinated ionomer membrane such as Nafion are widely practiced in both industry and research [2,3]. This kind of ion exchange membrane has exceptional proton conductivity (~0.01e0.1 S/cm), excellent thermal and chemical stabilities, and high mechanical strength that enable Nafion to sustain in fuel cell markets for more than 30 years. However, Nafion tends to dehydrate at above 80 C, causing the membrane proton conductivity, chemical and mechanical stabilities to be drastically reduced and limit its operation at over 100 C due to water vapour pressure [4]. Furthermore, high fuel permeability and expensive material of perfluorosulfonic acid polymer contribute to high development cost that limit the commercialisation of Nafion in fuel cell applications. There are growing number of research that emphasise on sulfonated hydrocarbon aromatic polymer-based PEM development, such as sulfonated poly (ether ketone) (SPEEK) [5e7], sulfonated Poly (p-phenylene) [8], sulfonated poly (arylene ether sulfone) (SPAES) [9], sulfonated polysulfone (SPSf) [10], sulfonated Poly (phenylene oxide) (SPPO) [11], sulfonated Polybenzimidazole (SPBI) [12,13], sulfonated polyethersulfone (SPES) [14,15], and sulfonated polyphenylsulfone (SPPSU) [16e18] as the polymer backbone. Other
than applying the sulfonated hydrocarbon polymers as polymer electrolyte membrane, the polymer were also explored as polymer material for redox flow battery [19], sensors [20], protein separation [21] and water purification [22,23]. State-of-the-art of PEM is that sulfonated membrane needs to achieve high proton conductivity at high and low relative humidity conditions, while maintaining its mechanical and chemical stability under the operating temperature of a fuel cell. It has been proven that the degree of sulfonation (DS) plays an important role enhancing the proton conductivity of PEM [24,25]. However, typically at high DS of above 80%, the mechanical and chemical properties of hydrocarbon polymers become unstable due to excess membrane swelling in water [26]. Recently, crosslinking process has been studied to improve the poor dimensional stability and mechanical properties of sulfonated PEM without proton conductivity deterioration. The polymer chain was modified by applying the crosslinking technique to link polymer chains in order to control the swelling degree of a high degree sulfonated polymer [27]. A sulfonated polymer can be crosslinked between itself under thermal treatment at certain temperature through bonding between sulfonic acid groups. Unfortunately, the cross linking that occurs between the sulfonic acid groups will reduced the acid function for proton transfer through the membrane. To overcome the shortcoming of having only pristine polymer as PEM, some fillers were studied on interpenetrating network structures with a base polymer, such as polyhedral silicon oxide [28,29], zirconium oxide [30], carbonaceous nanomaterials [31], and metal oxide [32]. This nanocomposite membranes is expected to show better properties as compared to pristine polymeric membranes. Carbonaceous nanomaterials such as carbon nanotubes, graphenes, graphene oxides, and fullerenes, are attractive fillers for preparing a variety of nanocomposite for PEM. Carbon nanodots are in a new class of carbonaceous nanomaterials with size of below 5 nm. These carbon nanoparticles have emerged as a new class of nanomaterial additives in SPPSU polymer matrix due to high stability, good conductivity, low toxicity, environmental friendliness, excellent water solubility and chemical inertness [33]. Nanomaterials with hydrophilic oxygencontaining groups (-OH, eCOOH, etc.) can promote water retention ability of the composite membrane and facilitate more channel for proton transfer in the SPPSU polymer matrix.
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As far as the subjects are concerned, none of the studies investigated on the composite have used carbon nanodots and highly sulfonated SPPSU as PEM. Therefore, the aim of this study is to improve the function of carbon nanodots by incorporating this material into the highly sulfonated PPSU to enhance the conductivity and membrane flexibility. In addition, thermal crosslinking was applied to crosslink the SPPSUCNDs membrane between the SPPSU sulfonic acid groups and CNDs amine group. The effect of the weight% loading of CNDs on physic-chemical properties of SPPSU membrane was critically discussed.
Experimental Preparation of highly sulfonated polyphenylsulfone (SPPSU) The commercial polyphenylsulfone (PPSU, Solvay Radel R5000 NT, MW~50,000) used was purchased from Solvay Specialty Polymers Japan. PPSU polymers were first oven dried at 80 C for about 48 h to remove moisture content. Dried PPSU polymers were added into 1 L of sulphuric acid (H2SO4, ~98%) at 80 C in oil bath. The ratio of the PPSU to the sulphuric acid content was 1 M of PPSU to 225 M of H2SO4. The solution was continuously stirred for 48 h and maintained at 60 C. The sulfonated polymers were recovered by precipitating the sulfonic acid solution into great excess of ice. Then the resulted precipitate of sulfonated PPSU was then filtered by using a mild vacuum filtration and then subjected to neutralisation by excess amount of deionised water in a dialysis-tubing cellulose membrane until pH7 was achieved. The resulting SPPSU polymers were dried at 80 C until it were completely dried and ready for use. The 1H nuclear magnetic resonance (1HNMR) spectrum was recorded at room temperature with a JEOL JNM-ECA400 spectrometer operating at 400 MHz through deuterated dimethyl sulfoxide (DMSO‑d6) solutions usage and a chemical shift (PPM) by using tetramethylsilane (TMS) as reference. 1 HNMR spectra showed that one repeating unit carried about two sulfonic acid groups. It was proven by determining the degree of sulfonation of the SPPSU polymer. The degree of sulfonation was determined by comparative integration of distinct aromatic signals. Alternatively, the degree of sulfonation was determined by using titration method to acquire IEC values. The ion exchange capacity (IEC) was referred to milliequivalents of the sulfonic group per gram of dried polymer sample. The IEC value of the prepared SPPSU polymer was estimated by using titration technique. The evaluated IEC value by titration for SPPSU polymer was about 3.2 meq/g correspondents about 2 for degree of sulfonation (DS). It means that, there are two sulfonic acid groups attached to one repeating unit of the SPPSU polymer matrix. The structural formula and 1HNMR spectra of SPPSU having DS~2 is shown in Fig. 1.
Preparation of carbon nanodots
Fig. 1 e Chemical structure and 1H NMR of SPPSU at DS~2. dissolved in 1 mL of water. The solution was then put into a glass tube oven and heated at 240 C for 30 min. A red-brown and foamy solid was formed. 10 wt % of CNDs was suspended in the DMSO solution for further used.
SPPSU-CNDs crosslink membrane preparation The SPPSU-CNDs crosslink membranes were prepared under single casting step by using slow evaporations technique to develop a dense membrane structure. The dope solution for membrane preparation was carried out by varying the CNDs wt. % loading. Firstly, 10 wt % of SPPSU was dissolved in the 10 mL DMSO and the dope solution was continuously stirred until it became homogeneous. In the meantime, several wt. % of CNDs was dispersed in the DMSO to develop 10 wt % of CNDs in DMSO. The pre-dispersed CNDs were then mixed inside the polymer solution and continued to stir until homogenous. The membrane dope preparation at different loading is tabulated in Table 1. The dope solution was then casted into a Petri dish and heated to remove the solvent by oven drying at 80 C for 24 h. The SPPSU-CNDs membranes were then subjected under crosslinking step at different level of temperature and time. The membranes were further heated in air at 120 C (24 h), 160 C (24 h), and 180 C (24 h). After the thermal crosslinking process, the membranes were then activated in different solutions. The residual acid and water extractable molecules were removed by a post activation step. The crosslinking membranes were then immersed in boiling water (2 h), 1 M of H2SO4 (80 C, 2 h), and boiling water (2 h). Then, the activated
Table 1 e Designation for SPPSU-CNDs dope solution. Varied parameter CNDs loading (wt. %)
In the preparation of carbon nanodots, citric acid was used as the carbon precursor. 1.0 g of citric acid was mixed with 384 mL of 1, 2-ethylenediamine (EDA). This mixture was then
Variable parameter 0 1 2 3
Membrane designation SPPSU-0% SPPSU-1% SPPSU-2% SPPSU-3%
CNDs CNDs CNDs CNDs
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membranes were dried at room temperature for further characterizations.
Measurements Structure characterisation The vibration properties of the molecular structure were characterised by using attenuated total reflection (ATR) with infrared (IR) spectrophotometer (Nicolet-6700, Thermo Scientific) at the frequency range of 550 cm1 e 4000 cm1.
Mechanical properties The mechanical properties were measured by determining the tensile strength and elongation at the break of crosslink membranes. The membrane thickness of samples were as follows: SPPSU-0% CNDs (t ¼ 0.066 mm), SPPSU-1% CNDs (t ¼ 0.051 mm), SPPSU-2% CNDs (t ¼ 0.076 mm), and SPPSU-3% CNDs (t ¼ 0.081 mm), respectively. The samples were cut by using a super dumbbell cutter SDMP-1000 (Dumbbell Co.). The measurement was performed by using a Tension Test Machine (Shimadzu, EZ-S) at room temperature with a constant crosshead speed of 5 mm/min.
Ion exchange capacity (IEC), water uptake (W.U.), and swelling ratio (S.R.) The IEC values of the crosslink membranes were determined by using back titration technique. Dried polymer or membrane was soaked in 20 mL of 2 M NaCl for about 24 h under continuous stirring to replace the protons with sodium ions. The solution was then titrated with 0.01 M NaOH solution until the pH turned to 7. The IEC value of the test sample was calculated by using the following equation: IEC (meq/g) ¼ cv/ wdry, where c (molar) of titration solution (0.01 M), v (mL) is the volume of the neutralised NaOH and wdry (g) is the dry weight of the sample tested. The degree of sulfonation (D.S.) referred to the amount of sulfonic acid group per repeating unit (R.U.) of the tested sample, was calculated by using the subsequent equation: D.S ¼ [IEC/1000 Fw (R.U.)]/[1 - (IEC/1000 Fw (SO3))], where Fw (R.U.) for PPSU ¼ 400.45 and Fw (SO3) ¼ 80.06. Meanwhile, the water uptake of the prepared membrane was determined by the membrane weight variation before and after hydration. The membrane was first oven dried at 80 C for 24 h. The dry membrane was weighed and denoted as Wd. Then, the dry membrane was immersed in boiling water for 1 h. After that, the wet membrane was removed from boiling water and the membrane surface was mopped with a blotting paper to remove excess water. The wet membrane was weighed and denoted as Ww. The water uptake of the prepared sample was calculated as following equation: W.U. (%) ¼ [(Ww Wd)/Wd] 100. The swelling ratio of the prepared sample was determined based on the membrane variation before and after hydration on the membrane thickness and lengths. The swelling ratio was determined based on the subsequent equation: Thickness, S. R. (%) ¼ [(Tw Td)/Td] 100 and Length, S. R. (%) ¼ [(lw ld)/ld] 100, where Tw and lw are the membrane thickness and length after being immersed in boiling water for 1 h while Td and ld are the membrane thickness and length in dry condition.
Proton conductivity The proton conductivity of the SPPSU-CNDs crosslinks membrane was measured by using four-point probe impedance spectroscopy via MTS 740 test system (Scribner Associates, Inc.) with a phase sensitive multimeter (model PSM1735, Newtons4th Ltd.) combined with an impedance analysis interface. The conductivity was measured at temperature of 80 C and 120 C, at different RH % (40, 60, 80, and 90). A frequency range of 1 Hz to 1 MHz and a peak-to-peak voltage of 10 mV were used for the impedance measurements. The membrane resistance (R), was obtained from the intercept of the impedance curve with the real axis at the high frequency end. The proton conductivity was calculated by using the following equation: Proton conductivity (S/cm) ¼ L/RS, where L are the thickness of the membrane and S is the area of the electrode.
Single cell performance characterization The thickness of the tested membrane was approximately 53 mm. 20 wt % Pt/C was used as a single cell electrode catalyst. The electrode catalyst slurry for anode and cathode was prepared by mixing Pt/C powder with 40 wt % of Nafion solution. The slurry was brushed on the carbon paper for the electrode substrate. The Pt loadings was 0.3 mg/cm2 and effective electrode area are 4.84 cm2. The membrane electrode assembly (MEA) was prepared by hot pressing the membrane between the anode and cathode layer at 1 ton and 130 C for 20 min. The IeV polarization curves were recorded by single cell test at 80 C under 60% RH and 100% RH. The flow rate of hydrogen was fixed at 50 mL/min and 100 mL/min for the pure oxygen. The back pressure was atmospheric pressure.
Results and discussion ATR-IR spectra of CNDs and SPPSU-CNDs crosslink membranes Carbon dots (CNDs) are a type of nanomaterials with substantial fractions of oxygen and hydrogen. The method to synthesise the CNDs can be classified into top-down and bottom-up technique [34]. In this study, bottom-up method by hydrothermal was applied to develop the CNDs due to the simple and inexpensive technique. Citric acid and 1, 2ethylenediamine (EDA) were used as starting precursor materials. The chemical structure and functional group distributions of CNDs were greatly affected by the reactions precursor and method of preparation conditions. The structure of CNDs and crosslink membrane was analysed by using ATR-IR spectroscopy to study the changes of functional group in the SPPSU-CNDs crosslink membrane. The proposed chemical structure of CNDs is shown in Fig. 2. (a). CNDs are primarily composed of approximately 70%e90% of carbon and oxygen, while the remaining ascribed to hydrogen, nitrogen and any other elements depends on the precursor materials use for synthesis [35]. Elemental analysis of CNDs was further illustrated in IR spectrum in Fig. 2. (b). In Fig. 2. (b), IR spectra displays characteristic peaks between 2800 cm1 and 3000 cm1 shows the CeH stretching, the broad peak around 3300 cm1 shows OeH stretching and 1000 cm1e1200 cm1 of
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Fig. 2 e a) Chemical structure of CNDs [42], b) IR spectra of CNDs, c) IR spectra of CNDs, SPPSU-0% CNDs and SPPSU-1% CNDs at 4000-2000 cm¡1, and d) IR spectra of SPPSU-0% CNDs, SPPSU-1% CNDs, SPPSU-2% CNDs, SPPSU-3% CNDs at 18001000 cm¡1.
OeH bending, indicating an abundance of hydroxyl groups. In addition, a very strong intensity peak that appeared at 1534 cm1 was assigned to NeH asymmetric vibration donated from EDA structure. The presence of the amine group was further verified by the presence of C]O stretching vibration in primary/tertiary amides around 1650 cm1. Two bands at 1390 cm1 and 1337 cm1 were assigned to COO- and eOH groups in carboxylic acids respectively. These IR spectra indicated that various functional groups were present in the CNDs particles which made the CNDs highly hydrophilic [36]. These data were consistent with those of other studies [37e39]. The IR spectra of the crosslink membrane at 1 wt % CNDs in Fig. 2. (c), NeH peak is observed at 1510 cm1. It proved the presence of the CNDs inside the SPPSU polymer matrix. This phenomenon indicated that there was an existence of hydrogen bonding between the CNDs and SPPSU polymer matrix through crosslinking process. This NeH absorption peak in crosslink membrane was shifted to a lower wavenumber as compared to CNDs spectra, which was at 1534 cm1. It resulted from the reorganisation or change of the microstructure inside the crosslink membrane matrix [40]. In addition, the thermal treatment applied on the crosslink membrane formed a network between CNDs and SPPSU matrix in which resulted the microstructure to be more compact and denser [41]. Water absorption effect on the crosslink membranes could be seen clearly in Fig. 2. (d), whereas the OeH stretching band was detected around wavenumber of 3200 cm1 to 3600 cm1. This vibration was assigned from OeH vibration from sulfonic
acid group or CNDs that interacted with water molecule. As can be seen in Fig. 2. (d), the intensity of the OeH stretching is increased upon CNDs loading increased. This might be due to the hydrophilic behaviour of the CNDs particles and sulfonate group which do not take part in crosslinking process and move freely to react with water. The hydrophilic characteristics of CNDs were proven by the stretching vibration bands assigned to CeOH at around 3200 cm1 to 3600 cm1 and consistent with other study [43]. The possible crosslinking mechanism of SPPSU-CNDs membrane is illustrated in Fig. 3. The unique structure of CNDs due to its small size and rich surface functional groups, can easily from chemical bonds with other compounds [44]. The crosslinking reaction occurs mainly between the sulfonic acid group in the SPPSU and amine group in the CNDs.
Mechanical properties of SPPSU-CNDs crosslink membranes The properties of the crosslink membranes were further characterised based on the membrane strength and flexibilities. The crosslink membrane mechanical properties were analysed by using tensile test at room temperature. Fig. 4 shows the stress-strain curve of the crosslink membranes. The membrane tensile strength was lied between 47.8 MPa and 57.2 MPa. Meanwhile, elongation at break of the crosslink membranes was in the range of 66%e121.5%. As can be seen in Fig. 4. (b), incorporation of CNDs particles inside SPPSU polymer matrix increased the mechanical strength of the hydrocarbon polymer membrane. The crosslink membranes showed characteristics of a tough and ductile polymer
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Fig. 3 e Possible crosslinking mechanism of crosslink membrane during thermal treatment.
Fig. 4 e a) Stress-strain curve, b) tensile strength and elongation at break of the crosslink membranes.
membrane. The crosslinking will restrict the motion of the polymer chain and increase the strength of the polymer. SPPSU-0% CNDs membrane had the lowest tensile strength values as compared to the crosslink membrane. This was due to the crosslinking phenomenon that took place between the SPPSU sulfonic group and CNDs amine group that might lead to a more compact and rigid membrane structure [45]. The tensile strength of the crosslink membrane significantly enhanced up to 57.2 MPa from 47.8 MPa with the incorporation of 1 wt % CNDs. However, further increase in the CNDs loading caused the reduction of the mechanical strength but still higher than pristine SPPSU membrane. Further increase in the CNDs loading in the SPPSU polymer matrix led to excess CNDs particles, which did not participate in crosslinking process. This phenomenon could not contribute to the improvement of membrane mechanical strength [46]. Different polymer molecular structure effects the mechanical behaviour of the crosslink membrane. Although, the crosslink membrane prepared with 1 wt % CNDs exhibited a maximum tensile strength as compared to other membranes, it showed the lowest elongation at maximum stress. The heat treatment crosslinked the CNDs particle with SPPSU matrix, making the membrane become a more rigid and denser
structure. Membrane prepared with 2 wt % CNDs has the highest membrane flexibilities of about 121.5%. Higher membrane flexibility subsequent to good interface between the membrane and electrodes and can also prevent membrane cracking. High membrane flexibility means that the membrane can be used with low thickness. The membrane thickness is one of the hot issues to acquire high performance as thin film can be possible for water back diffusion and reduce membrane resistance [47].
IEC, W.U., and S. R. of the SPPSU-CNDs crosslink membranes IEC and water uptake of the crosslink membranes were measured to confirm the effect of CNDs loading towards crosslink membrane. The IEC, WU and swelling ratio value of the membranes are summarised in Table 2. Ion exchange capacity (IEC) refers to the total active site of functional groups on transporting ion in polymer electrolyte membrane. It is suggested that increase in the number of sulfonic acid group within the polymer matrix will only increase the IEC values of the membrane [45]. The IEC and DS of SPPSU polymer that was used to fabricate SPPSU-CNDs crosslink membranes were
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Table 2 e IEC, Water uptake, swelling ratio, and conductivity of the crosslink membranes. CNDs Loading IEC, meq/g W.U., %
0% 1% 2% 3%
2.00 1.92 1.96 1.67
43.0 58.6 82.0 134.0
S.R. (t/l), % Conductivity, S/cm at 80 C (t ¼ Thickness, l ¼ length) 40% RH 90% RH 16.4/11.0 16.4/10.0 20.0/10.0 29.4/20.0
3.2 meq/g and 2 respectively. The IEC values of the crosslink membranes were within the range of 1.67 meq/g to 2.00 meq/ g. It can be seen in Table 2 that the IEC values of crosslink membranes consisted of CNDs is lower than without CNDs. The different in IEC values might be due to the difference in the reactive number of crosslinkable site as the crosslinking interactions may take place between the SPPSU sulfonic acid groups or SPPSU sulfonic acid groups and CNDs amine groups during annealing process. This provide a good agreement with the lower IEC values of the crosslink membrane due to plummeting number of exchangeable eSO3Na groups in the SPPSU polymer chains as the measurement of IEC by titration only recognised the exchangeable eSO3Na groups [48,49]. Polymer electrolyte membrane requires water as a medium for proton transportation across the membrane. It is transported by diffusion, electro-osmotic effects which refer to migration of hydrogen ions, pressure gradients and electrochemical reactions [50]. Insufficient of hydration causes difficulties in proton transportation, resulting in poor proton conductivity. Higher water uptake also disturbs the intermolecular force between polymer chains and results in decreased mechanical strength [51,52]. It is crucial for polymer electrolyte membrane to have an adequate water retention capacity for proton transfer. The water uptake and swelling ratio of the crosslink membrane are tabulated in Table 2. Incorporation of CNDs in the SPPSU polymer matrix affected the membrane water uptake. As the wt. % CNDs was increased, the membrane water uptake also increased. This was due to the presence of CeOH functional group in CNDs matrix which have been proven by the FTIR spectra that contributed to enhance the water retention capacity of the crosslink membrane [53,54]. Nevertheless, the water uptake of the crosslink membrane was very high, the membrane swelling ratio was not much different. Crosslinking process reduce the hydrophilic character of the SPPSU, enhanced the membrane strength, and rigidity that affected in reducing membrane swelling [55,56]. As can be seen in Table 2, although the water uptake of the crosslinked membrane achieved more than 100%, the membrane thickness swells only about 30% from its original membrane thickness in water. Generally, moderate membrane swelling ratio will provide larger space and more continuous path for proton transportation [57]. The membrane swell in thickness direction is important in order to ensure the better contact of PEM with the electrodes.
Proton conductivity of SPPSU-CNDs crosslink membrane The crosslink membrane was further characterised in terms of its proton conducting properties by measuring the impedance spectroscopic under different temperatures and relative
0.0011 0.0038 0.0050 0.0078
0.0112 0.0342 0.0364 0.0563
Conductivity, S/cm at 120 C 40% RH
90% RH
0.0006 0.0024 0.0038 0.0059
0.0131 0.0424 0.0516 0.0818
humidity (RH %). Proton conductivity referred to the conductive properties of the polymer electrolyte membrane to transport proton. The sulfonate group that was present in the crosslink membrane will act as an active site to transport proton within the membrane. The membrane properties that affected the proton conductivity efficiency were the membrane water uptake, number of available acid groups and their dissociation capability [48]. The proton conductivity of the polymer electrolyte membrane was greatly influenced by the presence of water [58,59]. The proton conductivity trends based on relative humidity dependence at 80 C and 120 C on different CNDs contents are illustrated in Fig. 5. The proton conductivity of the crosslink membrane was lied in the range of 103 to 102 S/cm. The exact value of the proton conductivity is tabulated in Table 2. As can be seen from the trend line, incorporating the CNDs in the SPPSU polymer increased the proton conductivity of the polymer electrolyte membrane at both temperatures along the increasing RH %. Apparently, the proton conductivity of the crosslink membranes seemed to be very dependant towards the amount of CNDs. Increasing the amount of CNDs also increased the value of proton conductivity. CNDs substantially improved the transport of proton inside the crosslink membrane [60]. The SPPSU-3% CNDs had the highest proton conductivity value as compared to other sample at any RH % conditions. However, it is worth to be reminded that SPPSU-3% CND membrane had shown deterioration in mechanical stabilities that was discussed in section Mechanical properties of SPPSU-CNDs crosslink membranes. Based on the obtained results, it can be suggested that CNDs had indeed provide a medium for better conducting path in proton transportation within the SPPSU polymer matrix.
Single cell performance Based on the physicochemical characterizations, SPPSU-2% CNDs crosslinked membrane was yielded a significant improvement towards the properties of proton exchange membrane compared to SPPSU-1% CNDs and SPPSU-3% CNDs. Therefore, a single cell performance using SPPSU-2% CNDs crosslinked membrane was further characterized in the conditions of 100% RH and 60% RH under 80 C. Fig. 6(a) shows the IeV performance in iR free. The IeV performance of 60% RH showed lower cell performance compared to 100% RH conditions. These behaviours are correlated to the lower proton conductivity values of SPPSU-2% CND membrane measured at low RH% conditions (Fig. 5(a)). The open current voltage (OCV) of the SPPSU-2% CND crosslinked membrane under 60% RH and 100% RH was measured as 1.0069 V and 1.0224 V, respectively. The value might be due to the high durability
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Fig. 5 e Proton conductivity of the crosslink membranes at a) 80 C and b) 120 C at different RH %.
Fig. 6 e a) Polarization curves at 80 C, 60% RH and 100% RH, and b) Durability at 80 C, 100% RH, 1A of SPPSU-2% CND crosslink membrane.
towards the crossover of hydrogen and oxygen gases across the membrane [61e67]. The long-term durability of the MEAs was measured to predict the feasibility of the prepared membrane towards fuel cell applications (Fig. 6(b)). The MEA using SPPSU-2% CND crosslink membrane shows good stability although small voltage drop about 6.9% was observed after 350 h operations.
Conclusions The high proton conductive proton exchange membranes consisted of highly sulfonated polyphenylsulfone (SPPSU)carbon nanodots (CNDs) crosslink membranes were successfully prepared. In this study, the membranes were selfcrosslinked under 180 C and it had greatly altered the chemical structure of crosslink membrane. The presence of amine group that was proven by FTIR resulted in the CNDs structure enabled this group to be crosslinked with the PPSU sulfonic group. The crosslinking treatment also improved membrane flexibilities by ensuring the good interface between the membrane and electrodes that can prevent membrane cracking. CNDs were useful to enhance the proton conductivity under low and high RH conditions and also improved the mechanical flexibility. Based on this findings, SPPSU-2% CNDs crosslink membrane yielded a significant improvement, which had high membrane flexibilities and higher proton conductivity as compared to SPPSU-0% CNDs.
Acknowledgements The authors would like to express gratitude to the Ministry of Higher Education (MOHE), Universiti Teknologi Malaysia (UTM) and Research Management Centre (RMC), UTM for supporting the research management activities specifically for the UTM Transdisciplinary Research Grant (Q.J130000.3552.06G88). The authors would also like to acknowledge supported by the International Cooperative Graduate School (ICGS) Fellowship under the “Universiti Teknologi Malaysia-NIMS Cooperative Graduate School Program” to conduct research in National Institute of Materials Science (NIMS), Tsukuba, Japan. This work also was supported by the MEXT Program for Development of Environmental Technology using Nanotechnology.
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Available online at www.sciencedirect.com
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Crosslinked carbon nanodots with highly sulfonated polyphenylsulfone as proton exchange membrane for fuel cell applications Nor Azureen Mohamad Nor a,b,d, Hidenobu Nakao c, Juhana Jaafar d,*, Je-Deok Kim a,b,** a
Hydrogen Production Materials Group, Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan b Polymer Electrolyte Fuel Cell Group, Global Research Center for Environmental and Energy Based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan c Photon and Ion Beam Physics Group, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan d Advanced Membrane Technology Research Centre (AMTEC), School of Chemical & Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor, Malaysia
highlights Highly
sulfonated
graphical abstract poly-
phenylsulfone as polymer based for proton exchange membrane. Carbon nanodots facilitate more channel for proton transfer in the membrane. Thermal crosslinking applied to crosslink the sulfonic acid and amine group. Carbon nanodots improve proton conductivity under low and high RH conditions. High membrane flexibility ensuing good interface between membrane and electrodes.
article info
abstract
Article history:
The crosslinked highly sulfonated polyphenylsulfone (SPPSU) membranes that consists of
Received 22 July 2019
carbon nanodots (CNDs) was prepared as a proton exchange membrane for fuel cell ap-
Received in revised form
plications. The crosslinked membranes were developed by annealing at 180 C, in which
3 October 2019
the crosslinking occurred between the SPPSU and CNDs. The CNDs potential was explored
* Corresponding author. ** Corresponding author. Hydrogen Production Materials Group, Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan. E-mail addresses: [email protected] (J. Jaafar), [email protected] (J.-D. Kim). https://doi.org/10.1016/j.ijhydene.2020.01.142 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Accepted 21 January 2020
in detail under various loadings (0 wt %, 1 wt %, 2 wt %, and 3 wt %). Upon annealing at
Available online 26 February 2020
180 C, the flexibilities and strength of the SPPSU-CNDs membranes improved. The proton conductivity of the crosslinked membrane was enhanced than that of pristine SPPSU
Keywords:
membrane due to the crosslinking effect between SPPSU and CNDs. The highest conduc-
Highly sulfonated
tivity, which was at 56.3 mS/cm was obtained when 3 wt % of CNDs was incorporated at
polyphenylsulfone
80 C and 90% relative humidity (RH). The results indicated that the incorporation of CNDs
Carbon nanodots
in the SPPSU membrane by annealing at 180 C, exhibited a proton conductive membrane
Proton exchange membrane
in combination with superior dimensional stability, and proton conductivity suitable for fuel cell applications. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cell is a promising technology as a heat and electricity source for buildings, and an electrical power source for electric motor propelling vehicles. Fuel cells cleanly and efficiently convert chemical energy from hydrogen-rich fuels into electrical power and useable high quality heat in an electrochemical process while crude oil prices surge [1]. Proton exchange membrane fuel cell (PEMFC) is a type of fuel cell that is developed mainly for transport as well as stationary and portable applications. PEMFCs work with a proton exchange membrane (PEM), which is a thin (~20e250 mm) permeable sheet that is generally made from ionomers and designed to conduct protons, while it acts as a barrier to reactants and electron insulator. The essential function of PEM in a fuel cell system is as reactants separator and protons transportation medium, while blocking an electronic pathway through the membrane. Generally, a good PEM requires high proton conductivity (>102 S/cm) that enables proton transport between the electrodes. PEM also needs excellent chemical stability under fuel cell operating conditions, low operating cost, and highly flexible membrane that is compatible with other cell components for efficient cell performance [1e3]. PEM materials based on perfluorinated ionomer membrane such as Nafion are widely practiced in both industry and research [2,3]. This kind of ion exchange membrane has exceptional proton conductivity (~0.01e0.1 S/cm), excellent thermal and chemical stabilities, and high mechanical strength that enable Nafion to sustain in fuel cell markets for more than 30 years. However, Nafion tends to dehydrate at above 80 C, causing the membrane proton conductivity, chemical and mechanical stabilities to be drastically reduced and limit its operation at over 100 C due to water vapour pressure [4]. Furthermore, high fuel permeability and expensive material of perfluorosulfonic acid polymer contribute to high development cost that limit the commercialisation of Nafion in fuel cell applications. There are growing number of research that emphasise on sulfonated hydrocarbon aromatic polymer-based PEM development, such as sulfonated poly (ether ketone) (SPEEK) [5e7], sulfonated Poly (p-phenylene) [8], sulfonated poly (arylene ether sulfone) (SPAES) [9], sulfonated polysulfone (SPSf) [10], sulfonated Poly (phenylene oxide) (SPPO) [11], sulfonated Polybenzimidazole (SPBI) [12,13], sulfonated polyethersulfone (SPES) [14,15], and sulfonated polyphenylsulfone (SPPSU) [16e18] as the polymer backbone. Other
than applying the sulfonated hydrocarbon polymers as polymer electrolyte membrane, the polymer were also explored as polymer material for redox flow battery [19], sensors [20], protein separation [21] and water purification [22,23]. State-of-the-art of PEM is that sulfonated membrane needs to achieve high proton conductivity at high and low relative humidity conditions, while maintaining its mechanical and chemical stability under the operating temperature of a fuel cell. It has been proven that the degree of sulfonation (DS) plays an important role enhancing the proton conductivity of PEM [24,25]. However, typically at high DS of above 80%, the mechanical and chemical properties of hydrocarbon polymers become unstable due to excess membrane swelling in water [26]. Recently, crosslinking process has been studied to improve the poor dimensional stability and mechanical properties of sulfonated PEM without proton conductivity deterioration. The polymer chain was modified by applying the crosslinking technique to link polymer chains in order to control the swelling degree of a high degree sulfonated polymer [27]. A sulfonated polymer can be crosslinked between itself under thermal treatment at certain temperature through bonding between sulfonic acid groups. Unfortunately, the cross linking that occurs between the sulfonic acid groups will reduced the acid function for proton transfer through the membrane. To overcome the shortcoming of having only pristine polymer as PEM, some fillers were studied on interpenetrating network structures with a base polymer, such as polyhedral silicon oxide [28,29], zirconium oxide [30], carbonaceous nanomaterials [31], and metal oxide [32]. This nanocomposite membranes is expected to show better properties as compared to pristine polymeric membranes. Carbonaceous nanomaterials such as carbon nanotubes, graphenes, graphene oxides, and fullerenes, are attractive fillers for preparing a variety of nanocomposite for PEM. Carbon nanodots are in a new class of carbonaceous nanomaterials with size of below 5 nm. These carbon nanoparticles have emerged as a new class of nanomaterial additives in SPPSU polymer matrix due to high stability, good conductivity, low toxicity, environmental friendliness, excellent water solubility and chemical inertness [33]. Nanomaterials with hydrophilic oxygencontaining groups (-OH, eCOOH, etc.) can promote water retention ability of the composite membrane and facilitate more channel for proton transfer in the SPPSU polymer matrix.
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As far as the subjects are concerned, none of the studies investigated on the composite have used carbon nanodots and highly sulfonated SPPSU as PEM. Therefore, the aim of this study is to improve the function of carbon nanodots by incorporating this material into the highly sulfonated PPSU to enhance the conductivity and membrane flexibility. In addition, thermal crosslinking was applied to crosslink the SPPSUCNDs membrane between the SPPSU sulfonic acid groups and CNDs amine group. The effect of the weight% loading of CNDs on physic-chemical properties of SPPSU membrane was critically discussed.
Experimental Preparation of highly sulfonated polyphenylsulfone (SPPSU) The commercial polyphenylsulfone (PPSU, Solvay Radel R5000 NT, MW~50,000) used was purchased from Solvay Specialty Polymers Japan. PPSU polymers were first oven dried at 80 C for about 48 h to remove moisture content. Dried PPSU polymers were added into 1 L of sulphuric acid (H2SO4, ~98%) at 80 C in oil bath. The ratio of the PPSU to the sulphuric acid content was 1 M of PPSU to 225 M of H2SO4. The solution was continuously stirred for 48 h and maintained at 60 C. The sulfonated polymers were recovered by precipitating the sulfonic acid solution into great excess of ice. Then the resulted precipitate of sulfonated PPSU was then filtered by using a mild vacuum filtration and then subjected to neutralisation by excess amount of deionised water in a dialysis-tubing cellulose membrane until pH7 was achieved. The resulting SPPSU polymers were dried at 80 C until it were completely dried and ready for use. The 1H nuclear magnetic resonance (1HNMR) spectrum was recorded at room temperature with a JEOL JNM-ECA400 spectrometer operating at 400 MHz through deuterated dimethyl sulfoxide (DMSO‑d6) solutions usage and a chemical shift (PPM) by using tetramethylsilane (TMS) as reference. 1 HNMR spectra showed that one repeating unit carried about two sulfonic acid groups. It was proven by determining the degree of sulfonation of the SPPSU polymer. The degree of sulfonation was determined by comparative integration of distinct aromatic signals. Alternatively, the degree of sulfonation was determined by using titration method to acquire IEC values. The ion exchange capacity (IEC) was referred to milliequivalents of the sulfonic group per gram of dried polymer sample. The IEC value of the prepared SPPSU polymer was estimated by using titration technique. The evaluated IEC value by titration for SPPSU polymer was about 3.2 meq/g correspondents about 2 for degree of sulfonation (DS). It means that, there are two sulfonic acid groups attached to one repeating unit of the SPPSU polymer matrix. The structural formula and 1HNMR spectra of SPPSU having DS~2 is shown in Fig. 1.
Preparation of carbon nanodots
Fig. 1 e Chemical structure and 1H NMR of SPPSU at DS~2. dissolved in 1 mL of water. The solution was then put into a glass tube oven and heated at 240 C for 30 min. A red-brown and foamy solid was formed. 10 wt % of CNDs was suspended in the DMSO solution for further used.
SPPSU-CNDs crosslink membrane preparation The SPPSU-CNDs crosslink membranes were prepared under single casting step by using slow evaporations technique to develop a dense membrane structure. The dope solution for membrane preparation was carried out by varying the CNDs wt. % loading. Firstly, 10 wt % of SPPSU was dissolved in the 10 mL DMSO and the dope solution was continuously stirred until it became homogeneous. In the meantime, several wt. % of CNDs was dispersed in the DMSO to develop 10 wt % of CNDs in DMSO. The pre-dispersed CNDs were then mixed inside the polymer solution and continued to stir until homogenous. The membrane dope preparation at different loading is tabulated in Table 1. The dope solution was then casted into a Petri dish and heated to remove the solvent by oven drying at 80 C for 24 h. The SPPSU-CNDs membranes were then subjected under crosslinking step at different level of temperature and time. The membranes were further heated in air at 120 C (24 h), 160 C (24 h), and 180 C (24 h). After the thermal crosslinking process, the membranes were then activated in different solutions. The residual acid and water extractable molecules were removed by a post activation step. The crosslinking membranes were then immersed in boiling water (2 h), 1 M of H2SO4 (80 C, 2 h), and boiling water (2 h). Then, the activated
Table 1 e Designation for SPPSU-CNDs dope solution. Varied parameter CNDs loading (wt. %)
In the preparation of carbon nanodots, citric acid was used as the carbon precursor. 1.0 g of citric acid was mixed with 384 mL of 1, 2-ethylenediamine (EDA). This mixture was then
Variable parameter 0 1 2 3
Membrane designation SPPSU-0% SPPSU-1% SPPSU-2% SPPSU-3%
CNDs CNDs CNDs CNDs
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membranes were dried at room temperature for further characterizations.
Measurements Structure characterisation The vibration properties of the molecular structure were characterised by using attenuated total reflection (ATR) with infrared (IR) spectrophotometer (Nicolet-6700, Thermo Scientific) at the frequency range of 550 cm1 e 4000 cm1.
Mechanical properties The mechanical properties were measured by determining the tensile strength and elongation at the break of crosslink membranes. The membrane thickness of samples were as follows: SPPSU-0% CNDs (t ¼ 0.066 mm), SPPSU-1% CNDs (t ¼ 0.051 mm), SPPSU-2% CNDs (t ¼ 0.076 mm), and SPPSU-3% CNDs (t ¼ 0.081 mm), respectively. The samples were cut by using a super dumbbell cutter SDMP-1000 (Dumbbell Co.). The measurement was performed by using a Tension Test Machine (Shimadzu, EZ-S) at room temperature with a constant crosshead speed of 5 mm/min.
Ion exchange capacity (IEC), water uptake (W.U.), and swelling ratio (S.R.) The IEC values of the crosslink membranes were determined by using back titration technique. Dried polymer or membrane was soaked in 20 mL of 2 M NaCl for about 24 h under continuous stirring to replace the protons with sodium ions. The solution was then titrated with 0.01 M NaOH solution until the pH turned to 7. The IEC value of the test sample was calculated by using the following equation: IEC (meq/g) ¼ cv/ wdry, where c (molar) of titration solution (0.01 M), v (mL) is the volume of the neutralised NaOH and wdry (g) is the dry weight of the sample tested. The degree of sulfonation (D.S.) referred to the amount of sulfonic acid group per repeating unit (R.U.) of the tested sample, was calculated by using the subsequent equation: D.S ¼ [IEC/1000 Fw (R.U.)]/[1 - (IEC/1000 Fw (SO3))], where Fw (R.U.) for PPSU ¼ 400.45 and Fw (SO3) ¼ 80.06. Meanwhile, the water uptake of the prepared membrane was determined by the membrane weight variation before and after hydration. The membrane was first oven dried at 80 C for 24 h. The dry membrane was weighed and denoted as Wd. Then, the dry membrane was immersed in boiling water for 1 h. After that, the wet membrane was removed from boiling water and the membrane surface was mopped with a blotting paper to remove excess water. The wet membrane was weighed and denoted as Ww. The water uptake of the prepared sample was calculated as following equation: W.U. (%) ¼ [(Ww Wd)/Wd] 100. The swelling ratio of the prepared sample was determined based on the membrane variation before and after hydration on the membrane thickness and lengths. The swelling ratio was determined based on the subsequent equation: Thickness, S. R. (%) ¼ [(Tw Td)/Td] 100 and Length, S. R. (%) ¼ [(lw ld)/ld] 100, where Tw and lw are the membrane thickness and length after being immersed in boiling water for 1 h while Td and ld are the membrane thickness and length in dry condition.
Proton conductivity The proton conductivity of the SPPSU-CNDs crosslinks membrane was measured by using four-point probe impedance spectroscopy via MTS 740 test system (Scribner Associates, Inc.) with a phase sensitive multimeter (model PSM1735, Newtons4th Ltd.) combined with an impedance analysis interface. The conductivity was measured at temperature of 80 C and 120 C, at different RH % (40, 60, 80, and 90). A frequency range of 1 Hz to 1 MHz and a peak-to-peak voltage of 10 mV were used for the impedance measurements. The membrane resistance (R), was obtained from the intercept of the impedance curve with the real axis at the high frequency end. The proton conductivity was calculated by using the following equation: Proton conductivity (S/cm) ¼ L/RS, where L are the thickness of the membrane and S is the area of the electrode.
Single cell performance characterization The thickness of the tested membrane was approximately 53 mm. 20 wt % Pt/C was used as a single cell electrode catalyst. The electrode catalyst slurry for anode and cathode was prepared by mixing Pt/C powder with 40 wt % of Nafion solution. The slurry was brushed on the carbon paper for the electrode substrate. The Pt loadings was 0.3 mg/cm2 and effective electrode area are 4.84 cm2. The membrane electrode assembly (MEA) was prepared by hot pressing the membrane between the anode and cathode layer at 1 ton and 130 C for 20 min. The IeV polarization curves were recorded by single cell test at 80 C under 60% RH and 100% RH. The flow rate of hydrogen was fixed at 50 mL/min and 100 mL/min for the pure oxygen. The back pressure was atmospheric pressure.
Results and discussion ATR-IR spectra of CNDs and SPPSU-CNDs crosslink membranes Carbon dots (CNDs) are a type of nanomaterials with substantial fractions of oxygen and hydrogen. The method to synthesise the CNDs can be classified into top-down and bottom-up technique [34]. In this study, bottom-up method by hydrothermal was applied to develop the CNDs due to the simple and inexpensive technique. Citric acid and 1, 2ethylenediamine (EDA) were used as starting precursor materials. The chemical structure and functional group distributions of CNDs were greatly affected by the reactions precursor and method of preparation conditions. The structure of CNDs and crosslink membrane was analysed by using ATR-IR spectroscopy to study the changes of functional group in the SPPSU-CNDs crosslink membrane. The proposed chemical structure of CNDs is shown in Fig. 2. (a). CNDs are primarily composed of approximately 70%e90% of carbon and oxygen, while the remaining ascribed to hydrogen, nitrogen and any other elements depends on the precursor materials use for synthesis [35]. Elemental analysis of CNDs was further illustrated in IR spectrum in Fig. 2. (b). In Fig. 2. (b), IR spectra displays characteristic peaks between 2800 cm1 and 3000 cm1 shows the CeH stretching, the broad peak around 3300 cm1 shows OeH stretching and 1000 cm1e1200 cm1 of
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Fig. 2 e a) Chemical structure of CNDs [42], b) IR spectra of CNDs, c) IR spectra of CNDs, SPPSU-0% CNDs and SPPSU-1% CNDs at 4000-2000 cm¡1, and d) IR spectra of SPPSU-0% CNDs, SPPSU-1% CNDs, SPPSU-2% CNDs, SPPSU-3% CNDs at 18001000 cm¡1.
OeH bending, indicating an abundance of hydroxyl groups. In addition, a very strong intensity peak that appeared at 1534 cm1 was assigned to NeH asymmetric vibration donated from EDA structure. The presence of the amine group was further verified by the presence of C]O stretching vibration in primary/tertiary amides around 1650 cm1. Two bands at 1390 cm1 and 1337 cm1 were assigned to COO- and eOH groups in carboxylic acids respectively. These IR spectra indicated that various functional groups were present in the CNDs particles which made the CNDs highly hydrophilic [36]. These data were consistent with those of other studies [37e39]. The IR spectra of the crosslink membrane at 1 wt % CNDs in Fig. 2. (c), NeH peak is observed at 1510 cm1. It proved the presence of the CNDs inside the SPPSU polymer matrix. This phenomenon indicated that there was an existence of hydrogen bonding between the CNDs and SPPSU polymer matrix through crosslinking process. This NeH absorption peak in crosslink membrane was shifted to a lower wavenumber as compared to CNDs spectra, which was at 1534 cm1. It resulted from the reorganisation or change of the microstructure inside the crosslink membrane matrix [40]. In addition, the thermal treatment applied on the crosslink membrane formed a network between CNDs and SPPSU matrix in which resulted the microstructure to be more compact and denser [41]. Water absorption effect on the crosslink membranes could be seen clearly in Fig. 2. (d), whereas the OeH stretching band was detected around wavenumber of 3200 cm1 to 3600 cm1. This vibration was assigned from OeH vibration from sulfonic
acid group or CNDs that interacted with water molecule. As can be seen in Fig. 2. (d), the intensity of the OeH stretching is increased upon CNDs loading increased. This might be due to the hydrophilic behaviour of the CNDs particles and sulfonate group which do not take part in crosslinking process and move freely to react with water. The hydrophilic characteristics of CNDs were proven by the stretching vibration bands assigned to CeOH at around 3200 cm1 to 3600 cm1 and consistent with other study [43]. The possible crosslinking mechanism of SPPSU-CNDs membrane is illustrated in Fig. 3. The unique structure of CNDs due to its small size and rich surface functional groups, can easily from chemical bonds with other compounds [44]. The crosslinking reaction occurs mainly between the sulfonic acid group in the SPPSU and amine group in the CNDs.
Mechanical properties of SPPSU-CNDs crosslink membranes The properties of the crosslink membranes were further characterised based on the membrane strength and flexibilities. The crosslink membrane mechanical properties were analysed by using tensile test at room temperature. Fig. 4 shows the stress-strain curve of the crosslink membranes. The membrane tensile strength was lied between 47.8 MPa and 57.2 MPa. Meanwhile, elongation at break of the crosslink membranes was in the range of 66%e121.5%. As can be seen in Fig. 4. (b), incorporation of CNDs particles inside SPPSU polymer matrix increased the mechanical strength of the hydrocarbon polymer membrane. The crosslink membranes showed characteristics of a tough and ductile polymer
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Fig. 3 e Possible crosslinking mechanism of crosslink membrane during thermal treatment.
Fig. 4 e a) Stress-strain curve, b) tensile strength and elongation at break of the crosslink membranes.
membrane. The crosslinking will restrict the motion of the polymer chain and increase the strength of the polymer. SPPSU-0% CNDs membrane had the lowest tensile strength values as compared to the crosslink membrane. This was due to the crosslinking phenomenon that took place between the SPPSU sulfonic group and CNDs amine group that might lead to a more compact and rigid membrane structure [45]. The tensile strength of the crosslink membrane significantly enhanced up to 57.2 MPa from 47.8 MPa with the incorporation of 1 wt % CNDs. However, further increase in the CNDs loading caused the reduction of the mechanical strength but still higher than pristine SPPSU membrane. Further increase in the CNDs loading in the SPPSU polymer matrix led to excess CNDs particles, which did not participate in crosslinking process. This phenomenon could not contribute to the improvement of membrane mechanical strength [46]. Different polymer molecular structure effects the mechanical behaviour of the crosslink membrane. Although, the crosslink membrane prepared with 1 wt % CNDs exhibited a maximum tensile strength as compared to other membranes, it showed the lowest elongation at maximum stress. The heat treatment crosslinked the CNDs particle with SPPSU matrix, making the membrane become a more rigid and denser
structure. Membrane prepared with 2 wt % CNDs has the highest membrane flexibilities of about 121.5%. Higher membrane flexibility subsequent to good interface between the membrane and electrodes and can also prevent membrane cracking. High membrane flexibility means that the membrane can be used with low thickness. The membrane thickness is one of the hot issues to acquire high performance as thin film can be possible for water back diffusion and reduce membrane resistance [47].
IEC, W.U., and S. R. of the SPPSU-CNDs crosslink membranes IEC and water uptake of the crosslink membranes were measured to confirm the effect of CNDs loading towards crosslink membrane. The IEC, WU and swelling ratio value of the membranes are summarised in Table 2. Ion exchange capacity (IEC) refers to the total active site of functional groups on transporting ion in polymer electrolyte membrane. It is suggested that increase in the number of sulfonic acid group within the polymer matrix will only increase the IEC values of the membrane [45]. The IEC and DS of SPPSU polymer that was used to fabricate SPPSU-CNDs crosslink membranes were
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Table 2 e IEC, Water uptake, swelling ratio, and conductivity of the crosslink membranes. CNDs Loading IEC, meq/g W.U., %
0% 1% 2% 3%
2.00 1.92 1.96 1.67
43.0 58.6 82.0 134.0
S.R. (t/l), % Conductivity, S/cm at 80 C (t ¼ Thickness, l ¼ length) 40% RH 90% RH 16.4/11.0 16.4/10.0 20.0/10.0 29.4/20.0
3.2 meq/g and 2 respectively. The IEC values of the crosslink membranes were within the range of 1.67 meq/g to 2.00 meq/ g. It can be seen in Table 2 that the IEC values of crosslink membranes consisted of CNDs is lower than without CNDs. The different in IEC values might be due to the difference in the reactive number of crosslinkable site as the crosslinking interactions may take place between the SPPSU sulfonic acid groups or SPPSU sulfonic acid groups and CNDs amine groups during annealing process. This provide a good agreement with the lower IEC values of the crosslink membrane due to plummeting number of exchangeable eSO3Na groups in the SPPSU polymer chains as the measurement of IEC by titration only recognised the exchangeable eSO3Na groups [48,49]. Polymer electrolyte membrane requires water as a medium for proton transportation across the membrane. It is transported by diffusion, electro-osmotic effects which refer to migration of hydrogen ions, pressure gradients and electrochemical reactions [50]. Insufficient of hydration causes difficulties in proton transportation, resulting in poor proton conductivity. Higher water uptake also disturbs the intermolecular force between polymer chains and results in decreased mechanical strength [51,52]. It is crucial for polymer electrolyte membrane to have an adequate water retention capacity for proton transfer. The water uptake and swelling ratio of the crosslink membrane are tabulated in Table 2. Incorporation of CNDs in the SPPSU polymer matrix affected the membrane water uptake. As the wt. % CNDs was increased, the membrane water uptake also increased. This was due to the presence of CeOH functional group in CNDs matrix which have been proven by the FTIR spectra that contributed to enhance the water retention capacity of the crosslink membrane [53,54]. Nevertheless, the water uptake of the crosslink membrane was very high, the membrane swelling ratio was not much different. Crosslinking process reduce the hydrophilic character of the SPPSU, enhanced the membrane strength, and rigidity that affected in reducing membrane swelling [55,56]. As can be seen in Table 2, although the water uptake of the crosslinked membrane achieved more than 100%, the membrane thickness swells only about 30% from its original membrane thickness in water. Generally, moderate membrane swelling ratio will provide larger space and more continuous path for proton transportation [57]. The membrane swell in thickness direction is important in order to ensure the better contact of PEM with the electrodes.
Proton conductivity of SPPSU-CNDs crosslink membrane The crosslink membrane was further characterised in terms of its proton conducting properties by measuring the impedance spectroscopic under different temperatures and relative
0.0011 0.0038 0.0050 0.0078
0.0112 0.0342 0.0364 0.0563
Conductivity, S/cm at 120 C 40% RH
90% RH
0.0006 0.0024 0.0038 0.0059
0.0131 0.0424 0.0516 0.0818
humidity (RH %). Proton conductivity referred to the conductive properties of the polymer electrolyte membrane to transport proton. The sulfonate group that was present in the crosslink membrane will act as an active site to transport proton within the membrane. The membrane properties that affected the proton conductivity efficiency were the membrane water uptake, number of available acid groups and their dissociation capability [48]. The proton conductivity of the polymer electrolyte membrane was greatly influenced by the presence of water [58,59]. The proton conductivity trends based on relative humidity dependence at 80 C and 120 C on different CNDs contents are illustrated in Fig. 5. The proton conductivity of the crosslink membrane was lied in the range of 103 to 102 S/cm. The exact value of the proton conductivity is tabulated in Table 2. As can be seen from the trend line, incorporating the CNDs in the SPPSU polymer increased the proton conductivity of the polymer electrolyte membrane at both temperatures along the increasing RH %. Apparently, the proton conductivity of the crosslink membranes seemed to be very dependant towards the amount of CNDs. Increasing the amount of CNDs also increased the value of proton conductivity. CNDs substantially improved the transport of proton inside the crosslink membrane [60]. The SPPSU-3% CNDs had the highest proton conductivity value as compared to other sample at any RH % conditions. However, it is worth to be reminded that SPPSU-3% CND membrane had shown deterioration in mechanical stabilities that was discussed in section Mechanical properties of SPPSU-CNDs crosslink membranes. Based on the obtained results, it can be suggested that CNDs had indeed provide a medium for better conducting path in proton transportation within the SPPSU polymer matrix.
Single cell performance Based on the physicochemical characterizations, SPPSU-2% CNDs crosslinked membrane was yielded a significant improvement towards the properties of proton exchange membrane compared to SPPSU-1% CNDs and SPPSU-3% CNDs. Therefore, a single cell performance using SPPSU-2% CNDs crosslinked membrane was further characterized in the conditions of 100% RH and 60% RH under 80 C. Fig. 6(a) shows the IeV performance in iR free. The IeV performance of 60% RH showed lower cell performance compared to 100% RH conditions. These behaviours are correlated to the lower proton conductivity values of SPPSU-2% CND membrane measured at low RH% conditions (Fig. 5(a)). The open current voltage (OCV) of the SPPSU-2% CND crosslinked membrane under 60% RH and 100% RH was measured as 1.0069 V and 1.0224 V, respectively. The value might be due to the high durability
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Fig. 5 e Proton conductivity of the crosslink membranes at a) 80 C and b) 120 C at different RH %.
Fig. 6 e a) Polarization curves at 80 C, 60% RH and 100% RH, and b) Durability at 80 C, 100% RH, 1A of SPPSU-2% CND crosslink membrane.
towards the crossover of hydrogen and oxygen gases across the membrane [61e67]. The long-term durability of the MEAs was measured to predict the feasibility of the prepared membrane towards fuel cell applications (Fig. 6(b)). The MEA using SPPSU-2% CND crosslink membrane shows good stability although small voltage drop about 6.9% was observed after 350 h operations.
Conclusions The high proton conductive proton exchange membranes consisted of highly sulfonated polyphenylsulfone (SPPSU)carbon nanodots (CNDs) crosslink membranes were successfully prepared. In this study, the membranes were selfcrosslinked under 180 C and it had greatly altered the chemical structure of crosslink membrane. The presence of amine group that was proven by FTIR resulted in the CNDs structure enabled this group to be crosslinked with the PPSU sulfonic group. The crosslinking treatment also improved membrane flexibilities by ensuring the good interface between the membrane and electrodes that can prevent membrane cracking. CNDs were useful to enhance the proton conductivity under low and high RH conditions and also improved the mechanical flexibility. Based on this findings, SPPSU-2% CNDs crosslink membrane yielded a significant improvement, which had high membrane flexibilities and higher proton conductivity as compared to SPPSU-0% CNDs.
Acknowledgements The authors would like to express gratitude to the Ministry of Higher Education (MOHE), Universiti Teknologi Malaysia (UTM) and Research Management Centre (RMC), UTM for supporting the research management activities specifically for the UTM Transdisciplinary Research Grant (Q.J130000.3552.06G88). The authors would also like to acknowledge supported by the International Cooperative Graduate School (ICGS) Fellowship under the “Universiti Teknologi Malaysia-NIMS Cooperative Graduate School Program” to conduct research in National Institute of Materials Science (NIMS), Tsukuba, Japan. This work also was supported by the MEXT Program for Development of Environmental Technology using Nanotechnology.
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