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lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties
international journal of hydrogen energy xxx (xxxx) xxx
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High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties Sara Barati a, Mahdi Abdollahi b,*, Mohsen Mehdipourghazi a,**, Behnam Khoshandam a a
Faculty of Chemical, Petroleum, and Gas Engineering, Semnan University, Semnan, 35131-19111, Iran Polymer Reaction Engineering Department, Faculty of Chemical Engineering, Tarbiat Modares University, P.O. Box: 14115-114, Tehran, Iran
b
highlights
graphical abstract
Phosphoric acid doped PBI/lignosulfonate (LS) porous membranes were prepared. Proton conductivity of the PBI/LS membranes
were
evaluated
in
high temperature. The LS and pores in the membranes had a significant effect on the proton transfer. Proton conductivity of 187 mS/cm (160
C) was obtained for PBI/LS
(20 wt%) membrane.
article info
abstract
Article history:
Porous polybenzimidazole (PBI) based blend membranes were prepared by adding different
Received 30 June 2019
amounts of lignosulfonate (LS) in the presence of LiCl salt. The morphology characteristics
Received in revised form
of the PBI/LS blends were investigated by FT-IR, atomic force microscopy (AFM) and
25 September 2019
scanning electron microscopy (SEM) analyses. The relation between the membrane
Accepted 27 September 2019
morphology and membrane proton conductivity was studied. Results showed that LS
Available online xxx
content has a significant influence on the membrane morphology. High amount of LS in the blend created micro-pores within the membrane where increase in the LS content up to
Keywords:
20 wt% resulted in membranes containing pores with a mean diameter of about 0.8 mm.
Lignosulfonate
The resulting PBI/LS (0e20 wt%) membranes indicated high PA doping levels, ranging from
Polybenzimidazole
3 to 16 mol of PA per mole of PBI repeat units, which contributed to their unprecedented
Porous blend membrane
high proton conductivities of 4e96 mS cm1, respectively, at 25 C. The effect of temperature on the proton conductivity of blends was also investigated. The results showed that
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Abdollahi), [email protected] (M. Mehdipourghazi). https://doi.org/10.1016/j.ijhydene.2019.09.216 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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High temperature proton exchange
by rising the temperature, the proton conductivity increases in PBI/LS blends. In the blend
membrane
containing 20 wt% LS, proton conductivity increased from 98 mS cm1 at 25
Fuel cell
1
C to
187 mS.cm at 160 C which can be considered as an excellent candidate for use in both high and low temperature proton exchange membrane fuel cells. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells, due to their ability to work without environmental side effects, are an attractive alternative for energy generation. Among the various types of fuel cells, polymer electrolyte membrane fuel cells (PEMFC) are one of the most attractive types of fuel cells [1,2]. Due to high physicochemical and physicothermal characteristics of Nafion, it is used in proton exchange membrane fuel cells [3]. However, it is a high-price solid electrolyte and at the operation temperatures higher than 80 C dehydrates and the proton conductivity decrease [4]. High-temperature PEMFCs, (e.g. PBI membrane which works higher 100 C [5]) are developed to break those limitations [5e8]. The proton conductivity of PBI membrane needs improvement by strong acid, such as phosphoric acid (PA), sulfuric acid, perchloric acid, nitric acid or hydrochloric acid. Special interests have been focused on the PA because of its high boiling point, thermal stability and proton conductivity [7,9,10]. The proton conductivity of PBI after doping with PA has been investigated frequently [11,12] at different temperatures [13e15], humidity [9,16] and the acid doping level [17]. The results have shown that the PA-PBI membrane has good proton conductivity at different temperatures even in an anhydrous state [2,18,19]. However, the system has disadvantages such as leaching of PA, low loading of PA and mechanical stability by the oxoacid at high temperatures [8]. To overcome these drawbacks, blend of PBI with triazole [20] and fluorine [21] based polymers and nanocomposite of PBI with different fillers such as TiO2 [22], SiO2 [23] and nanoclay [24] have been studied. PBI blended with Nafion [25,26], polysulfone [27], polystyrene [28] and poly(vinylidene fluoride) [29] have been investigated in order to use PEMFC at high temperature. Another method for increasing the acid doping level of the PBI membrane for use in fuel cells, which has been developed in recent years, is the preparation of porous membranes [14,30e33]. Preparation of PBI porous membrane using glucose and saccharose as porogens has been investigated through a solvent casting/porogen leaching technique. As a consequence of increased free space within the membrane, the acid doping level, and accordingly, the proton productivity of membrane is increased. Mecerreyes et al. [14] reported the preparation of porous PBI membranes using different phthalates and phosphates as porogen. A maximum conductivity of 50 mS.cm-1 was reported for the doped porous membrane containing 70% porogen. Zhang et al. [33] have utilized Polysulfone (PSf) polymer for use in fuel cells and investigated the effect of adding LS polymer on the morphology and proton conductivity of the membrane. In this
method, the prepared PSf/LS wet film was placed in a water bath and the PSf/LS porous solid membrane was produced. The results indicated that higher LS concentration caused a decrease in macrovoid formation and induced larger pores, but since LS is an anionic polymer and has a high dispersing property in N,N-dimethylformamide (DMF), by increasing LS content in the membrane, the structure of the membrane becomes more symmetrical. As a result, in this method, the prepared porous membrane has a lower resistance to proton transfer. The results showed that in addition to porosity formation, the sulfonic acid group and also the proton conductivity of the membrane have increased substantially, with increasing LS concentration. Thus, using the sulfonated polymer, in addition to porosity formation within the membrane, could reduce the resistance of the membrane to proton conductivity through creating ion transfer channels, and also could be a good way to increase the proton productivity of the PBI membrane for use in fuel cells. In the previous work, we have used lignin polymer to increase the absorption of acid PBI membrane resulting of micro-pores forming in the membrane for improvement of proton conductivity [30]. It has been seen that by increasing lignin content in the membrane, the porosity of the membrane increases and the membrane proton conductivity reaches 162 mS cm1 at the highest elevated temperature [30]. It is expected that if lignin polymer has sulfonated groups which provides ionic sites for ion transportation, it can significantly help to improve the proton conductivity. Furthermore, the strong interaction between sulfonated groups with the PBI nitrogen group scan improved the membrane mechanical properties. LS is an amorphous and contains phenolic hydroxyl, aliphatic hydroxyl and carboxylic and sulfonic acid groups. Its molecular structure is shown in Fig. 1. When degrees of sulfonation (DS) of LS is above 0.4, it is soluble in water over the entire pH range [34,35]. Therefore, the use of LS in PBI membrane can increase the membrane hydrophilic property. Also LS is an anionic polymer where presence of sulfonic acid group of LS in the membrane results in a reduction in its electrical resistance. Moreover, LS is a natural polymer with high production in all around the world as a waste of mostly paper making industry. Therefore, LS is an attractive polymer for use in electrochemical devices and fuel cells from both the economic and environmental points of view. In this work, anionic polymer of LS is blended with PBI to prepare porous PBI/LS membrane. Presence of sulfonic acid group in the LS structure and the pore formation in membrane is expected to cause an improvement in hydrophilic property and acid absorption capability of the membrane, resulting in
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Fig. 1 e Schematic representation of the chemical structures of LS (a) and mPBI (b).
an increase in the proton conductivity. The main aim of this research is to evaluate simultaneous effect of the LS polymer and LiCl salt as porogen on the membrane morphology and proton conductivity of the PBI/LS blend for use in high temperature PEM fuel cells. Chemical structure of the PBI/LS blend is investigated by ATR-FTIR. Cross-section morphology and the spatial ionic site distribution of the membranes are investigated by SEM and AFM, respectively. The proton conductivity of blends is measured as a function of temperature in the range of ambient temperature to 160 C by AC impedance spectroscopy (IS). By combining the results of AFM, SEM and IS, the relationship between the proton conductivity of membrane and the microstructure of membrane is predicted. Water and methanol uptake, PA doping level, methanol permeability, mechanical property and ion exchange capacity properties of the membranes are also investigated.
in oven at 100 C for 8 h. Finally, the wet film was washed in boiling water for 1 day to porogen leaching (LiCl salt) and remove the remaining solvent in membrane. The PBI/LS membrane was dried at 110 C in vacuum oven to remove water. The thickness of resultant membranes was about 100 ± 20 mm.
Characterization ATR-FTIR spectroscopy The ATR-FTIR spectra of membranes were recorded on a Perkin Elmer spectrophotometer in the wave number range of 650e4000 cm1 at ambient temperature.
Atomic force microscopy
Experimental Materials 0
0
Polybenzimidazole (poly[2, 2 -m-phenylene-5, 5 -benzimidazole]) (mPBI) (Fig. 1), with a glass transition temperature of 425e435 C, molecular weight (62000e59000 g/mol) was purchased from FumaTech company. PA, methanol and dimethyl sulfoxide (DMSO) were purchased from Merck. LS was provided by Aldrich. Distilled de-ionized was water used in all experiments.
Preparation of PBI/LS blend membranes The PBI/LS (0e20 wt%) blend membrane was prepared at the required ratio by solution-casting method. Both LS and PBI were dissolved in DMSO separately. In order for preparation of the PBI/LiCl/DMSO solution, PBI/LiCl (1 wt%/1 wt%) was dissolved in DMSO at 150 C for 24 h. LiCl salt has been used for two reasons, that is, to increase stability of solution and as a porogen to create pore in the structure of the membrane. LS polymer was easily dissolved in DMSO at the ambient temperature. Then the LS/DMSO (1 wt%) solution was added to the PBI/LiCl/DMSO (1 wt%) solution slowly and mixed by magnetic stirrer at room temperature for 24 h. PBI/LiCl/LS/DMSO solution was casted within a petri dish and the petri dish was put
Atomic force microscopy (AFM) was performed with a commercial instrument (AFM, Nanotech Electronica S.L., Madrid, Spain) operating in tracing mode. The samples were dried at 80 C for 24 h in vacuum oven, then imaged immediately at ambient condition.
Scanning electron microscopy (SEM) Scanning electron microscopy analyses were done for morphological characterization of membranes. Cross-section image of membranes was carried out by SEM. For samples preparation the membrane was immersed into liquid nitrogen. This action allows membrane to be broken, but internal porous structure is not affected. By using image J® software, geometrical information such as size and distribution of pores was obtained.
Water and methanol uptake of membranes To determine water uptake (WU) of membranes, the membrane was immersed in distilled water at ambient temperature for 24 h, then the excess water on the membrane surface was removed by tissue paper and weighted as the wet state (Wwet). For dry state, the membrane was put in oven at 100 C for 24 h, then weighted immediately (Wdry). Consequently, the water uptake of membrane sample was calculated by equation (1).
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Water uptakeð%Þ ¼
wwet wdry 100 wdry
(1)
Calculation of methanol uptake (MU) of membranes was the same as WU. Except that membranes were immersed in methanol. MU of the PBI/LS membranes was calculated by equation (2). Methanol uptakeð%Þ ¼
wwet wdry 100 wdry
(2)
PA doping level In order to determine PA doping level of blend membranes, samples were first dried in a vacuum oven at 110 C, then the weight of membrane was measured. The membrane immersed in aqueous concentrated PA (85 wt%) for 8 h. The PA doping level (PAdop ) of membrane was defined as PA mole amount adsorbed per 1 mol of PBI repeat unit, and was calculated using the following equation: PAdop ¼
½ðW1 W0 Þ=98 ½W0 =308
(3)
Where W0, W1 are the weight of a dry membrane and the weight of a PA doped PBI membrane, respectively. The value 98 and 308 are the molecular weights of PA and PBI repeat unit, respectively.
Results and discussion Structural and morphological characterization Formation of the PBI/LS blend was according to acidebase interaction between the sulfonic acid group of LS and the imidazole nitrogen of PBI. The structure of the PBI/LS blends was investigated by AT-FTIR spectroscopy (Fig. 2). The peak at 3100-3451 cm1 and 1700-1600 cm1 were attributed to OeH and NH for PBI and LS polymer and C]O stretching for LS, respectively. These peaks are related to the phenolic hydroxyl and carbonyl groups of LS. Two distinct absorption peaks at 1230 and 1160 cm1 are observed in all blend membranes. Previous works have reported that those bands are highly sensitive to the sulfonate group [36,37]. The altered appearance of the adsorption groups is most likely due to the formation of ionic sulfonate groups as a result of the acid-base reaction between lignosulfonate and PBI. The absorb peak in the 3400 cm1 range indicates non-bonded NH stretching. It has been reported in former researches that when PBI is mixed with sulfonated compounds, there is very little change to lower frequencies due to more pronounced hydrogen
Ion exchange capacity (IEC) The IEC of PBI/LS blend membrane was determined by acidbase titration method. In this method, in order to obtain the amount of Hþ ion exchanged, the membrane was immersed in a 1M NaCl solution for 24 h. Then membrane was removed and the solution was titrated with 0.05M NaOH solution. Subsequently, the IEC was calculated with equation (4): IEC ¼
CNaOH VNaOH WMembrane
ðmequiv=gÞ
(4)
in which IEC, CNaOH, VNaOH and WMembrane are ion exchange capacity (meq/g), molar concentration of NaOH solution (M), volume of NaOH solution used in titration (mL) and weight of the membrane specimen (g) used, respectively.
Proton conductivity measurements The proton conductivity of membrane was measured using a precision LCR meter (GW INSTEK LCR-8101G; Good Will Instrument Co., Ltd., New Taipei City, Taiwan) from 25 C to 160 C where a programmable oven was used to measure and control temperature in dry condition. For thermal equilibrium, the membranes were held for 30 min at each temperature. Proton conductivity (s) of membrane was calculated based on the equation (5): s¼
L RA
(5)
Where L is the membrane thickness (cm), A is the area (cm2) of the membrane used in the electrochemical impedance spectroscopy, s is proton conductivity (S:cm1 ) and R ðUÞ is the resistance of the membrane obtained from Nyquist plots.
Fig. 2 e ATR- FTIR of PBI, LS and PBI/LS blend membranes.
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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bonding present. In PBI/LS membranes, as the amount of LS in the blend increased, the non-bonded hydrogen adsorption peak decreased and the bonded hydrogen peak raised sharply with enhancing lignosulfonate content [36]. This indicates the presence of a hydrogenic acid-base bond between the two polymers. Also, this shows that the non-hydrogen bonded NeH protons are lower than to the bonded hydrogen because they have participated in the inter-ionic bonding of the two polymers. In the PBL/LS (5 wt %) membrane, the interaction between the carboxyl group and sulfonic acid, with the NH group has led to the elimination of the peak at 3400 cm-1 in the PBI/LS (5 and 10 wt%) membrane blends. And a peak has appeared in the range of 1000e1250 cm1 for PBI/LS blend, which is related to the CeN group. The peak at 2800-3000 cm1 due to the Nþ-H stretching mode can be referred to the protonation of the nitrogen in the imidazole ring of PBI, which by combining the nitrogen atom and the proton of the sulfonic groups could be formed. The intensity of peak appeared in 1150 cm1 rises by increasing the amount of LS from 0 to 20 wt% in PBI/LS membrane blend. This peak can be attributed to the sulfonic acid group of LS as can be seen in the FTIR spectrum of LS. The glass transition temperature of PBI and LS polymers are 398 C and 70 C, respectively [36,37]. Adding more lignosulfonate to the membrane causes glass temperature reduction, so that by adding 5 and 20 wt% lignosulfonate, the glass temperature of membrane was 289 and 163 C, respectively. The microstructures can influence the electrochemical behavior of PBI/LS blend membrane, especially the spatial distribution of ionic site and ionic clusters. Extensive researches have been carried out in order to investigate the aggregation of ionic polymers by analyzing AFM, TEM, SAXS, etc [38e40]. The results show that the sulfonic groups may aggregate into hydrophilic clusters that can provide the cation transport pathway or ionic transport channel [41]. The connectivity, distribution and size of hydrophilic regions can influence transport properties of membranes (i.e. proton conductivity of membrane) [42]. To study ionic clusters and distribution of ionic site in the PBI/LS blend, tracing mode phase images of AFM were used at ambient conditions. Fig. 3 shows the AFM tracing phase and topographic trace image for PBI, PBI/LS (10 wt%) and PBI/LS (20 wt%) membranes. The presence of LS in PBI, because of interaction of sulfonic groups with amine groups, can lead to the changes in the size of hydrophilic domains. Fig. 3b and c shows the phase images of the PBI/LS blends, the light regions were related to a softer region which represents the hydrophilic sulfonic acid groups of LS and dark regions were assigned to a hydrophobic polymer backbone [43]. The results showed an increasing tendency of the domain size and continuous of clusters with increase in the content of LS. Therefore, the presence of high content of LS may create more ionic transport channel, resulting in the proton conductivity of membrane suitable for use in PEMFCs. Fig. 4 shows the SEM cross-section micrographs for PBI, PBI/LS (10 wt%) and PBI/LS (20 wt%) membranes. Successful preparation of PBI porous membrane using LiCl as porogen is shown in Fig. 4. As seen in the figure, the internal morphology of the membrane was changed after adding LS, and more and
5
larger voids have been created within the membrane as a result of increasing LS content. Void size distribution for PBI/LS (0e20 wt%) membranes is shown in Fig. 6, which has been determined using the image J® software. The average sizes of the voids within these membranes are 0.2, 0.6 and 0.9 mm, respectively. Furthermore, the total volume and average diameter of pores is determined using BET test. The characteristics of inside membrane pores obtained by means of BET test is listed in Table 1. Mean diameter of inside membrane pores taken by imagJ software has major different with that of BET test. This can be inability of imagJ to measure pore diameters in nanoscale as well inability to determine the all pores. To better understand the internal structure of the membrane, the FESEM analysis for the sample of PBI/LS(20 wt%) membrane is attached in the Appendix section, Fig. 1. The based PBI membrane with LS content of 30 wt% was prepared. The SEM analysis results are shown in Fig. 5. The large micro-pores in the membrane’s structure was created which leads to mechanical resistance reduction of the membrane. Addition, it is causes the ability of storing acid or water decreases as well as the fuel passability increases. Hence, the performance of the membrane including Lignosulfonate content up to 20 wt% was studied. The formation of larger voids can be considered as a reservoir for retaining PA in the membrane, and thus, the proton conductivity of the membrane increase with increasing porosity of the membrane. The PBI is dissolved in the DMSO under high temperature and high pressure, while LS is dissolved in the DMSO at the ambient temperature. Therefore, the interaction between LS and DMSO would delay the DMSO/water exchange process, after preparing the wet film and immersing the membrane in the water bath. That is why by increasing the content of LS in the PBI/LS blend the structure of the membrane becomes much more regular structure and the number of the voids in the membrane increases, due to the good dispersing property of LS. The presence of void in the membrane morphology can influence transport properties and increases the membrane ability to trap PA.
IEC, WU, MU and proton conductivity of the hydrated blend membranes The IEC of the membranes are presented in Fig. 7. By increasing the content of LS in PBI, the sulfonate groups in PBI increase, resulting in the increased IEC (Fig. 7b). The PBI membrane has a relatively high water uptake in comparison with other polymers such as polyimide (1.2 wt%), polyetherketone (0.5 wt%) and polycarbonate (0.3 wt%) [45]. The water uptake of PBI membrane was measured to be 14 wt% at room temperature. It can be attributed to hydrogen bonding between H2O, N and NeH groups [45]. As shown in Fig .1, LS is highly complex aromatic polymers with multiple functionalities, including phenolic and aliphatic hydroxyls, carboxyl and sulfonic acid groups. Due to the strong hydrophilicity of the sulfonic acid groups, the PBI/LS blend showed WU increment (Fig. 7a). By increasing the LS content in the membrane, the sulfonate (eSO3H) and carboxylic (eCOOH) groups increases in the membrane; therefore the hydrophilic property and
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Fig. 3 e AFM tracing phase image for PBI, (a) PBI/LS (10 wt%), (b) PBI/LS (20 wt%), and AFM topographic trace image (c) PBI/LS (10 wt%) and (d) PBI/LS (20 wt%).
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Fig. 4 e SEM image from cross-section of (a) PBI, (b) PBI/LS (10 wt%) and (c) PBI/LS (20 wt%) blends.
Table 1 e BET test for characteristics determination of porosity of membrane containing 20 wt% lignosulfonate. Special area
2.8894 [m2 g1]
Void volume Mean pore diameter
0.6639 [cm3 g1] 54.034 [44]
Fig. 5 e SEM image from surface of PBI/LS (30 wt%) blend.
thereby WU of the membranes increases. In addition, by increasing the content of LS leads to formation of voids in the membrane which is, in fact, considered as a reservoir for retaining water. Furthermore, high WU of the membranes can increase the proton conductivity of the membranes. As is shown in Fig. 7, the proton conductivity increased with the increase of LS content in the PBI, due to increase in the hydrophilic properties as well as presence of SO3H and COOH groups in the membrane. Proton conductivity of membrane has water uptake dependency. Also, the proton conductivity is directly proportional to IEC. An increase in the amount of LS (from 5 to 20 wt%) leads to a significant rise in both proton conductivity and IEC of PBI/LS blends. Despite of high water uptake, all PBI/LS blends have low proton conductivity in comparison with Nafion® 117 (13.4 mS cm1) at room temperature [46]. Therefore, hydrated blend membranes are not suitable candidates for use in PEMFC at ambient temperatures. Methanol uptake (MU) of the PBI/LS blend membranes as a function of LS content is presented in Fig. 8. The blend membrane showed an increasing tendency in methanol uptake with increase in the content of LS and reached a value of 33 wt% for PBI/LS (20 wt%). The MU is related to the methanol permeability while water uptake is associated with the proton conductivity. It can be seen from Fig. 8 that by an increase in the amount of LS from 15 to 20 wt%, MU remains almost constant. On the other hand, the WU increased significantly
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Fig. 6 e Pore size distribution of the PBI (a), PBI/LS (10 wt%) (b) and PBI/LS (20 wt%) (c) membranes.
with addition of LS in PBI (Fig. 7a). Consequently, the membranes with high LS content can be more effective on the proton conductivity in comparison to with the methanol permeability.
PA doping levels and proton conductivities of the PA-doped blend membranes The proton conductivity of Nafion is depend on the membrane’s humidity. Because the water evaporates at temperatures above 100 C, the conductivity have been rarely reported for Nafion at high temperature. When PBI is doped with PA, it gains high proton conductivity in dry conditions at the assessed temperature, as the acid acts as a proton conducting carrier in the membrane. Fig. 9 illustrates the PAdop and the proton conductivity of PA-PBI/LS blends at 25 C. The proton conductivity of PBI and PA-PBI porous membranes was obtained to be 2.23 109 and 2.4 mS cm1 at ambient temperature in dry condition, respectively.
By increasing LS content in the membrane, the PAdop of membranes increases from 4.8 to 15.3 (mol H3PO4/mol PBI repeat unit). The interaction between H3PO4 and imidazole C] N group leads to retention of H3PO4 by the PBI membrane. It has been suggested that the maximum degree of H3PO4 interacting with nitrogen in the imidazole ring of PBI are two PA molecules per PBI repeat unit [47,48]. Therefore, when PAdop >2, excess free PA is not retained by imidazole- N]C exists in the membranes. All the PA-PBI/LS membranes with PA > 2 result in excess free PA, facilitating proton transport via vehicle mechanism. The key functional groups controlling the proton transport in PA-PBI/LS membranes are the excess free PA, the PA interacted with imidazole eNH groups and the free imidazole eNH group. It is suggested that when PA>3, there is much excess free H3PO4. Result in H2PO4…H3PO4 distance is less than the imidazole NH…-N]C- distance [49]. In this case, the proton conductivity in membrane is resulted from a motion of proton along the mixed H2PO4…H3PO4 and the
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Fig. 7 e WU (left) IEC and proton conductivity (right) of PBI/LS blend membrane at 25 C.
Fig. 8 e Methanol uptake of the PBI/LS blends membranes at 25 C.
polymer-anionic chains. A maximum conductivity of 98 mS/ cm of porous PA-PBI/LS (20 wt%) was achieved at 25 C, much higher than the conductivity of Nafion® 117 membrane (13.4 mS/cm) at same temperature. It is due to the fact that larger and more voids are formed in the PBI/LS (20 wt%) membrane which act as reservoirs for retaining PA. The minimum value for proton conductivity in the fuel cell application is equal to 10 mS cm1. When the LS polymer weight percentage was more than 5, the conductivity of the blends was higher than 10 mS cm1 at ambient temperature.
Proton conductivity of membrane as a function of temperature The enhancement in the proton conductivity of PEM is one of the most important factor for its application in PEMFCs.
Fig. 9 e Proton conductivity and PAdop plots of PA-PBI/LS membranes at 25 C.
Design and development of PEMFCs with high proton conductivity in anhydrous state have been studied by many groups [50e53]. The proton conductivity of membranes were obtained from resistance measurement using Nyquist plot. Fig. 10 shows the Nyquist plot of the PA-PBI/LS (5 wt%) porous membrane at three temperatures of 40, 80,100 and 160 C in the ambient conditions. According to the figure, the resistance of the membrane decreases with increasing temperature and, according to equation (5), the proton conductivity of the membrane increases. The proton conductivity of PA-PBI, PAPBI/LS (5 wt%), PA-PBI/LS (10 wt%) and PA- PBI/LS (20 wt%) membranes were measured at different temperatures. Fig. 11 shows the proton conductivity of PA-PBI/lignin (20 wt%), PA-PBI/LS (5,10 and 20 wt%) and PA-PBI membranes as a function of temperature, in the temperature range of 25e160 C. As seen in the Fig. 10, by increasing temperature,
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Fig. 11 e Proton conductivity of PA-doped PBI/LS blend at different temperature.
lignosulfonate in the membrane. In fact, the existence of sulfonate groups of LS causes reduction of the proton transfer resistance for membrane. Moreover, in accordance with the results of SEM, increase in proton conductivity with increasing the content of LS in the PBI/LS blend membrane is a result of formation of the larger and more voids, by virtue of which the ability for retaining acid increases. The temperature dependent proton conductivity of PA-PBI/ LS membranes follows the Arrhenius relationship: Fig. 10 e The Nyquist plot of the PA-PBI/LS (5 wt %) porous membrane at temperatures of 40, 80, 100 (top) and 160 C (bottom).
the membranes proton conductivity increases with temperature. Moreover, the proton conductivity increased by increasing the LS content in the PBI at constant temperature. A significant increase of PBI/LS membranes proton conductivity is for two reasons: 1) an increase in acid group (sulfonic acid) and 2) increase of the porosity and free space within the membrane that acid absorption ability of membrane could be increased by increasing of porosity. Additionally, by increasing of sulfonic acid and carboxylic acid groups of lignin polymer the proton transfer resistance is reduced and the protonation degree is increased. It can be seen that the PA-PBI/LS blend membrane shows the highest proton conductivity for PA-PBI/LS (20 wt%) at high temperature (120 Ce160 C). The highest proton conductivity was obtained to be 187.66 mS cm1 at 160 C for PA- PBI/LS (20 wt%) blend membrane, much higher than the maximum 34 mS cm1 for the porous PA-doped PBI membranes. It can be observed in Fig. 11 that the proton conductivity of PBI/LS (20 wt%) membrane is higher than the PBI/Lignin (20 wt %) membrane for the same content of lignin and
s ¼ s0 exp
Ea RT
(6)
in which s, s0 , Ea, R, and T are proton conductivity (S/cm), preexponential parameter, activation energy of proton conduction (kJ/mol), the gas constant (8.314 J/mol.K) and the temperature (K), respectively. Arrhenius plots of conductivity as a function of temperature are shown in Fig. 12. The activation energies of membrane (Ea) can be obtained by using the slope of Arrhenius plots. It can be mentioned that by activation energy, the proton transport mechanism in membranes can be estimated. From Fig. 12, the activation energies of PA-PBI, PA-PBI/LS (5 wt%), PA-PBI/LS (10 wt%) and PA-PBI/LS (20 wt%) membranes were obtained to be 32.03,28.51, 14.13 and 5.9 kJ/ mol, respectively. It can be concluded that the proton transport dominant mechanism in the PA-PBI/LS and PA-PBI blend membranes is Grotthus, because the activation energies are in the range of 14e40 kJ/mol [54] for Grotthus mechanism. The minimum activation energy of 5.9 kJ mol1 for the PA-PBI/LS (20 wt%) membrane is due to formation of large void in the membrane structure, which also serves as a reservoir for retain more PA in the membrane. However, the ability of the membrane to retain the acid is low, and, accordingly the proton conductivity of the PA-PBI/LS (20 wt%) membrane has not been increased significantly with increasing the temperature, compared to PA-PBI/LS (0e15 wt%) and the gradient of
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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analysis, the internal structure of the membrane based on its size distribution and number of porous are uniform. Thus, growth of the size and number of porous in the membrane can reduce the membrane mechanical resistance. On the other hand, according to FTIR analysis with increase of the lignosulfonate content, the peak is appeared in the range of 3000e3451 cm1 which indicates the extra lignosulfonate polymer in the membrane. This may change the membrane structure and consequently the membrane mechanical resistance. Thus, these two reasons could reduce the membrane mechanical resistance by increase of the lignosulfonate.
Methanol permeability
Fig. 12 e Arrhenius plot of proton conductivity for LS/PBIPA.
changes in proton conductivity with the temperature has been lower. In fact, numerous open-surface macro-voids result in the evaporation of PA at high temperatures. However, this membrane still has a higher proton conductivity at the assessed temperature compared to others membranes.
Table 3 shows methanol permeability of membranes as well as proton conductivity of water-absorbed membranes (those shown in Fig. 6) at room temperature. According to the results, fuel permeability increased by adding lignosulfonate polymer to PBI which the PBI/LS (20 wt%) membrane has the maximum fuel permeability. Since, the numbers and the size of pores are increased by increasing of lignosulfonate content. In PBI/LS (5 wt%) and PBI/LS (10 wt%) membranes the methanol permeability is near to the porous PBI membrane which indicates existed pores resulting of adding lignosulfonate polymer into membrane are mostly closed pores and lead to a bit increase for the fuel permeability through the membrane. As mentioned in the previous sections, a useful membrane for fuel sells should have low fuel permeability and high proton conductivity. Also, it should have the high ratio of proton conductivity to fuel permeability which is known as membrane selectivity.
Mechanical properties f¼ Table 2 shows mechanical properties of the PBI/LS (5 wt%), PBI/LS (20 wt%) and PBI membranes. As clearly seen, by adding 5 wt% lignosulfonate polymer to PBI has been caused the significant increase of membrane tensile strength (from 72.4 to 105.23 MPa). According to the survey, the increasing of the membrane mechanical resistance could be due to strong interaction between the two polymers. Since the lignosulfonate polymer has the sulfonic acid group, which is a strong acid, it makes a strong interaction with amine group of PBI polymer. As it observed from FTIR analysis, the peak of OH and NH in PBI/LS (5 wt%) membrane has been eliminated which reflects the strong bond between the two polymers. But, by adding of 20 wt% lignosulfonate polymer in to membrane, its mechanical resistance has been declined. This reduction maybe due to two reasons; on the one hand according to SEM
Table 2 e Mechanical properties of the prepared membranes. Blend Tensile Maximum membrane strength(MPa) elongation (%) PBI PBI/LS (5 wt %) PBI/LS (20 wt %)
Young modulus (MPa)
72.42 105.23
14.07 15.6
5.147 6.74
79.3
9.2
8.6
s P
(7)
Selectivity of membranes has been reported in the Table 3. Although the fuel permeability has been raised to somewhat by increasing of the hydrophilic lignosulfonate content, however; membrane selectivity is comparable with that of Nafion because of the significant high proton conductivity of PBI/LS membranes. Fuel cell simulation method is used to investigate the performance of proton exchange membrane fuel cell. The
Table 3 e Methanol permeability and selectivity of waterabsorbed membranes at room temperature. Methanol permeability ðcm2 s1 Þ Nafion [46] PBI PBI/LS (5 wt %) PBI/LS (10 wt %) PBI/LS (20 wt %)
Proton conductivity ðmS:cm1 Þ
Selectivity ðmS:s:cm3 Þ
14:9 107
13.4
9:0 10þ6
4:24 109 4:8 109
2:6 108 1:45 104
6:1 101 3:02 10þ4
6:5 109
3:06 104
4:7 10þ4
4:32 108
26:3 102
6:1 10þ6
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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defined model in our previous article [55] was simulated by means of COMSOL Multiphysics software and was verified by experimental data. According to the verification results, the model accurately predicts the experimental data. Therefore, in this study, the properties results of PBI/LS (20 wt%) membrane were putted in simulating process to assess the fuel cell performance. Fig. 13 indicates the fuel cell performance at the temperatures of 25, 120, and 160 C. As shown in the figure, the fuel cell performance is improved by increasing the temperature, because the membrane proton conductivity increases by temperature (Fig. 13).
a selectivity of the proton conductivity over the methanol permeability comparable with that of Nafion. Due to its high proton conductivity, PA-PBI/LS (20 wt%) porous membrane can be considered as a good candidate for fuel cells applicable for both low and high temperatures. Furthermore, environmentally friendly LS is a low cost by-product in the various industries and is found in the environment abundantly, the PBI/LS membrane has environmental and economic advantages over other polymers.
Appendix A. Supplementary data Conclusion Novel porous PBI/LS membranes were prepared for use in low and high temperature PEMFCs. LiCl salt was used for two reasons, that is, to increase stability of solution and as a porogen to create porosity in the membrane. LS polymer was used as a sulfonated polymer with the aim of increasing proton conductivity through increasing the sulfonic acid group and formation of pores in the PBI membrane structure. SEM images revealed the successful preparation of PBI/LS and PBI porous membranes. However, the results showed that by increasing lignosulphonate content up to 20 wt%, the prepared membrane would be not structurally and mechanically suitable to be used in fuel cell. As a consequence of the porous structure of the membrane, the amount of PA trapped in the membranes increased. Therefore, by increasing the content of LS from 0 to 20 wt%, the proton conductivity of PA-PBI/LS membranes at ambient temperature increased from 1.98 mS cm1 for PA-PBI to 98 mS/cm for PA-PBI/LS (20 wt%). The proton conductivity of the PBI/LS (20 wt%) porous membrane at room temperature was close to that of the Nafion at 80 C, and by increasing temperature to 160 C, the proton conductivity of this membrane reached to 187 mS/cm. The formation of pores within the PBI/LS (20 wt%) membrane led to a significant increase in the proton conductivity. Results showed that the membrane with high LS content (20 wt%) has
Fig. 13 e Performance of proton exchange membrane fuel cell for PBI/LS (20 wt%) membrane.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.216.
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Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
Available online at www.sciencedirect.com
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High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties Sara Barati a, Mahdi Abdollahi b,*, Mohsen Mehdipourghazi a,**, Behnam Khoshandam a a
Faculty of Chemical, Petroleum, and Gas Engineering, Semnan University, Semnan, 35131-19111, Iran Polymer Reaction Engineering Department, Faculty of Chemical Engineering, Tarbiat Modares University, P.O. Box: 14115-114, Tehran, Iran
b
highlights
graphical abstract
Phosphoric acid doped PBI/lignosulfonate (LS) porous membranes were prepared. Proton conductivity of the PBI/LS membranes
were
evaluated
in
high temperature. The LS and pores in the membranes had a significant effect on the proton transfer. Proton conductivity of 187 mS/cm (160
C) was obtained for PBI/LS
(20 wt%) membrane.
article info
abstract
Article history:
Porous polybenzimidazole (PBI) based blend membranes were prepared by adding different
Received 30 June 2019
amounts of lignosulfonate (LS) in the presence of LiCl salt. The morphology characteristics
Received in revised form
of the PBI/LS blends were investigated by FT-IR, atomic force microscopy (AFM) and
25 September 2019
scanning electron microscopy (SEM) analyses. The relation between the membrane
Accepted 27 September 2019
morphology and membrane proton conductivity was studied. Results showed that LS
Available online xxx
content has a significant influence on the membrane morphology. High amount of LS in the blend created micro-pores within the membrane where increase in the LS content up to
Keywords:
20 wt% resulted in membranes containing pores with a mean diameter of about 0.8 mm.
Lignosulfonate
The resulting PBI/LS (0e20 wt%) membranes indicated high PA doping levels, ranging from
Polybenzimidazole
3 to 16 mol of PA per mole of PBI repeat units, which contributed to their unprecedented
Porous blend membrane
high proton conductivities of 4e96 mS cm1, respectively, at 25 C. The effect of temperature on the proton conductivity of blends was also investigated. The results showed that
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Abdollahi), [email protected] (M. Mehdipourghazi). https://doi.org/10.1016/j.ijhydene.2019.09.216 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
2
international journal of hydrogen energy xxx (xxxx) xxx
High temperature proton exchange
by rising the temperature, the proton conductivity increases in PBI/LS blends. In the blend
membrane
containing 20 wt% LS, proton conductivity increased from 98 mS cm1 at 25
Fuel cell
1
C to
187 mS.cm at 160 C which can be considered as an excellent candidate for use in both high and low temperature proton exchange membrane fuel cells. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells, due to their ability to work without environmental side effects, are an attractive alternative for energy generation. Among the various types of fuel cells, polymer electrolyte membrane fuel cells (PEMFC) are one of the most attractive types of fuel cells [1,2]. Due to high physicochemical and physicothermal characteristics of Nafion, it is used in proton exchange membrane fuel cells [3]. However, it is a high-price solid electrolyte and at the operation temperatures higher than 80 C dehydrates and the proton conductivity decrease [4]. High-temperature PEMFCs, (e.g. PBI membrane which works higher 100 C [5]) are developed to break those limitations [5e8]. The proton conductivity of PBI membrane needs improvement by strong acid, such as phosphoric acid (PA), sulfuric acid, perchloric acid, nitric acid or hydrochloric acid. Special interests have been focused on the PA because of its high boiling point, thermal stability and proton conductivity [7,9,10]. The proton conductivity of PBI after doping with PA has been investigated frequently [11,12] at different temperatures [13e15], humidity [9,16] and the acid doping level [17]. The results have shown that the PA-PBI membrane has good proton conductivity at different temperatures even in an anhydrous state [2,18,19]. However, the system has disadvantages such as leaching of PA, low loading of PA and mechanical stability by the oxoacid at high temperatures [8]. To overcome these drawbacks, blend of PBI with triazole [20] and fluorine [21] based polymers and nanocomposite of PBI with different fillers such as TiO2 [22], SiO2 [23] and nanoclay [24] have been studied. PBI blended with Nafion [25,26], polysulfone [27], polystyrene [28] and poly(vinylidene fluoride) [29] have been investigated in order to use PEMFC at high temperature. Another method for increasing the acid doping level of the PBI membrane for use in fuel cells, which has been developed in recent years, is the preparation of porous membranes [14,30e33]. Preparation of PBI porous membrane using glucose and saccharose as porogens has been investigated through a solvent casting/porogen leaching technique. As a consequence of increased free space within the membrane, the acid doping level, and accordingly, the proton productivity of membrane is increased. Mecerreyes et al. [14] reported the preparation of porous PBI membranes using different phthalates and phosphates as porogen. A maximum conductivity of 50 mS.cm-1 was reported for the doped porous membrane containing 70% porogen. Zhang et al. [33] have utilized Polysulfone (PSf) polymer for use in fuel cells and investigated the effect of adding LS polymer on the morphology and proton conductivity of the membrane. In this
method, the prepared PSf/LS wet film was placed in a water bath and the PSf/LS porous solid membrane was produced. The results indicated that higher LS concentration caused a decrease in macrovoid formation and induced larger pores, but since LS is an anionic polymer and has a high dispersing property in N,N-dimethylformamide (DMF), by increasing LS content in the membrane, the structure of the membrane becomes more symmetrical. As a result, in this method, the prepared porous membrane has a lower resistance to proton transfer. The results showed that in addition to porosity formation, the sulfonic acid group and also the proton conductivity of the membrane have increased substantially, with increasing LS concentration. Thus, using the sulfonated polymer, in addition to porosity formation within the membrane, could reduce the resistance of the membrane to proton conductivity through creating ion transfer channels, and also could be a good way to increase the proton productivity of the PBI membrane for use in fuel cells. In the previous work, we have used lignin polymer to increase the absorption of acid PBI membrane resulting of micro-pores forming in the membrane for improvement of proton conductivity [30]. It has been seen that by increasing lignin content in the membrane, the porosity of the membrane increases and the membrane proton conductivity reaches 162 mS cm1 at the highest elevated temperature [30]. It is expected that if lignin polymer has sulfonated groups which provides ionic sites for ion transportation, it can significantly help to improve the proton conductivity. Furthermore, the strong interaction between sulfonated groups with the PBI nitrogen group scan improved the membrane mechanical properties. LS is an amorphous and contains phenolic hydroxyl, aliphatic hydroxyl and carboxylic and sulfonic acid groups. Its molecular structure is shown in Fig. 1. When degrees of sulfonation (DS) of LS is above 0.4, it is soluble in water over the entire pH range [34,35]. Therefore, the use of LS in PBI membrane can increase the membrane hydrophilic property. Also LS is an anionic polymer where presence of sulfonic acid group of LS in the membrane results in a reduction in its electrical resistance. Moreover, LS is a natural polymer with high production in all around the world as a waste of mostly paper making industry. Therefore, LS is an attractive polymer for use in electrochemical devices and fuel cells from both the economic and environmental points of view. In this work, anionic polymer of LS is blended with PBI to prepare porous PBI/LS membrane. Presence of sulfonic acid group in the LS structure and the pore formation in membrane is expected to cause an improvement in hydrophilic property and acid absorption capability of the membrane, resulting in
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Fig. 1 e Schematic representation of the chemical structures of LS (a) and mPBI (b).
an increase in the proton conductivity. The main aim of this research is to evaluate simultaneous effect of the LS polymer and LiCl salt as porogen on the membrane morphology and proton conductivity of the PBI/LS blend for use in high temperature PEM fuel cells. Chemical structure of the PBI/LS blend is investigated by ATR-FTIR. Cross-section morphology and the spatial ionic site distribution of the membranes are investigated by SEM and AFM, respectively. The proton conductivity of blends is measured as a function of temperature in the range of ambient temperature to 160 C by AC impedance spectroscopy (IS). By combining the results of AFM, SEM and IS, the relationship between the proton conductivity of membrane and the microstructure of membrane is predicted. Water and methanol uptake, PA doping level, methanol permeability, mechanical property and ion exchange capacity properties of the membranes are also investigated.
in oven at 100 C for 8 h. Finally, the wet film was washed in boiling water for 1 day to porogen leaching (LiCl salt) and remove the remaining solvent in membrane. The PBI/LS membrane was dried at 110 C in vacuum oven to remove water. The thickness of resultant membranes was about 100 ± 20 mm.
Characterization ATR-FTIR spectroscopy The ATR-FTIR spectra of membranes were recorded on a Perkin Elmer spectrophotometer in the wave number range of 650e4000 cm1 at ambient temperature.
Atomic force microscopy
Experimental Materials 0
0
Polybenzimidazole (poly[2, 2 -m-phenylene-5, 5 -benzimidazole]) (mPBI) (Fig. 1), with a glass transition temperature of 425e435 C, molecular weight (62000e59000 g/mol) was purchased from FumaTech company. PA, methanol and dimethyl sulfoxide (DMSO) were purchased from Merck. LS was provided by Aldrich. Distilled de-ionized was water used in all experiments.
Preparation of PBI/LS blend membranes The PBI/LS (0e20 wt%) blend membrane was prepared at the required ratio by solution-casting method. Both LS and PBI were dissolved in DMSO separately. In order for preparation of the PBI/LiCl/DMSO solution, PBI/LiCl (1 wt%/1 wt%) was dissolved in DMSO at 150 C for 24 h. LiCl salt has been used for two reasons, that is, to increase stability of solution and as a porogen to create pore in the structure of the membrane. LS polymer was easily dissolved in DMSO at the ambient temperature. Then the LS/DMSO (1 wt%) solution was added to the PBI/LiCl/DMSO (1 wt%) solution slowly and mixed by magnetic stirrer at room temperature for 24 h. PBI/LiCl/LS/DMSO solution was casted within a petri dish and the petri dish was put
Atomic force microscopy (AFM) was performed with a commercial instrument (AFM, Nanotech Electronica S.L., Madrid, Spain) operating in tracing mode. The samples were dried at 80 C for 24 h in vacuum oven, then imaged immediately at ambient condition.
Scanning electron microscopy (SEM) Scanning electron microscopy analyses were done for morphological characterization of membranes. Cross-section image of membranes was carried out by SEM. For samples preparation the membrane was immersed into liquid nitrogen. This action allows membrane to be broken, but internal porous structure is not affected. By using image J® software, geometrical information such as size and distribution of pores was obtained.
Water and methanol uptake of membranes To determine water uptake (WU) of membranes, the membrane was immersed in distilled water at ambient temperature for 24 h, then the excess water on the membrane surface was removed by tissue paper and weighted as the wet state (Wwet). For dry state, the membrane was put in oven at 100 C for 24 h, then weighted immediately (Wdry). Consequently, the water uptake of membrane sample was calculated by equation (1).
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Water uptakeð%Þ ¼
wwet wdry 100 wdry
(1)
Calculation of methanol uptake (MU) of membranes was the same as WU. Except that membranes were immersed in methanol. MU of the PBI/LS membranes was calculated by equation (2). Methanol uptakeð%Þ ¼
wwet wdry 100 wdry
(2)
PA doping level In order to determine PA doping level of blend membranes, samples were first dried in a vacuum oven at 110 C, then the weight of membrane was measured. The membrane immersed in aqueous concentrated PA (85 wt%) for 8 h. The PA doping level (PAdop ) of membrane was defined as PA mole amount adsorbed per 1 mol of PBI repeat unit, and was calculated using the following equation: PAdop ¼
½ðW1 W0 Þ=98 ½W0 =308
(3)
Where W0, W1 are the weight of a dry membrane and the weight of a PA doped PBI membrane, respectively. The value 98 and 308 are the molecular weights of PA and PBI repeat unit, respectively.
Results and discussion Structural and morphological characterization Formation of the PBI/LS blend was according to acidebase interaction between the sulfonic acid group of LS and the imidazole nitrogen of PBI. The structure of the PBI/LS blends was investigated by AT-FTIR spectroscopy (Fig. 2). The peak at 3100-3451 cm1 and 1700-1600 cm1 were attributed to OeH and NH for PBI and LS polymer and C]O stretching for LS, respectively. These peaks are related to the phenolic hydroxyl and carbonyl groups of LS. Two distinct absorption peaks at 1230 and 1160 cm1 are observed in all blend membranes. Previous works have reported that those bands are highly sensitive to the sulfonate group [36,37]. The altered appearance of the adsorption groups is most likely due to the formation of ionic sulfonate groups as a result of the acid-base reaction between lignosulfonate and PBI. The absorb peak in the 3400 cm1 range indicates non-bonded NH stretching. It has been reported in former researches that when PBI is mixed with sulfonated compounds, there is very little change to lower frequencies due to more pronounced hydrogen
Ion exchange capacity (IEC) The IEC of PBI/LS blend membrane was determined by acidbase titration method. In this method, in order to obtain the amount of Hþ ion exchanged, the membrane was immersed in a 1M NaCl solution for 24 h. Then membrane was removed and the solution was titrated with 0.05M NaOH solution. Subsequently, the IEC was calculated with equation (4): IEC ¼
CNaOH VNaOH WMembrane
ðmequiv=gÞ
(4)
in which IEC, CNaOH, VNaOH and WMembrane are ion exchange capacity (meq/g), molar concentration of NaOH solution (M), volume of NaOH solution used in titration (mL) and weight of the membrane specimen (g) used, respectively.
Proton conductivity measurements The proton conductivity of membrane was measured using a precision LCR meter (GW INSTEK LCR-8101G; Good Will Instrument Co., Ltd., New Taipei City, Taiwan) from 25 C to 160 C where a programmable oven was used to measure and control temperature in dry condition. For thermal equilibrium, the membranes were held for 30 min at each temperature. Proton conductivity (s) of membrane was calculated based on the equation (5): s¼
L RA
(5)
Where L is the membrane thickness (cm), A is the area (cm2) of the membrane used in the electrochemical impedance spectroscopy, s is proton conductivity (S:cm1 ) and R ðUÞ is the resistance of the membrane obtained from Nyquist plots.
Fig. 2 e ATR- FTIR of PBI, LS and PBI/LS blend membranes.
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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bonding present. In PBI/LS membranes, as the amount of LS in the blend increased, the non-bonded hydrogen adsorption peak decreased and the bonded hydrogen peak raised sharply with enhancing lignosulfonate content [36]. This indicates the presence of a hydrogenic acid-base bond between the two polymers. Also, this shows that the non-hydrogen bonded NeH protons are lower than to the bonded hydrogen because they have participated in the inter-ionic bonding of the two polymers. In the PBL/LS (5 wt %) membrane, the interaction between the carboxyl group and sulfonic acid, with the NH group has led to the elimination of the peak at 3400 cm-1 in the PBI/LS (5 and 10 wt%) membrane blends. And a peak has appeared in the range of 1000e1250 cm1 for PBI/LS blend, which is related to the CeN group. The peak at 2800-3000 cm1 due to the Nþ-H stretching mode can be referred to the protonation of the nitrogen in the imidazole ring of PBI, which by combining the nitrogen atom and the proton of the sulfonic groups could be formed. The intensity of peak appeared in 1150 cm1 rises by increasing the amount of LS from 0 to 20 wt% in PBI/LS membrane blend. This peak can be attributed to the sulfonic acid group of LS as can be seen in the FTIR spectrum of LS. The glass transition temperature of PBI and LS polymers are 398 C and 70 C, respectively [36,37]. Adding more lignosulfonate to the membrane causes glass temperature reduction, so that by adding 5 and 20 wt% lignosulfonate, the glass temperature of membrane was 289 and 163 C, respectively. The microstructures can influence the electrochemical behavior of PBI/LS blend membrane, especially the spatial distribution of ionic site and ionic clusters. Extensive researches have been carried out in order to investigate the aggregation of ionic polymers by analyzing AFM, TEM, SAXS, etc [38e40]. The results show that the sulfonic groups may aggregate into hydrophilic clusters that can provide the cation transport pathway or ionic transport channel [41]. The connectivity, distribution and size of hydrophilic regions can influence transport properties of membranes (i.e. proton conductivity of membrane) [42]. To study ionic clusters and distribution of ionic site in the PBI/LS blend, tracing mode phase images of AFM were used at ambient conditions. Fig. 3 shows the AFM tracing phase and topographic trace image for PBI, PBI/LS (10 wt%) and PBI/LS (20 wt%) membranes. The presence of LS in PBI, because of interaction of sulfonic groups with amine groups, can lead to the changes in the size of hydrophilic domains. Fig. 3b and c shows the phase images of the PBI/LS blends, the light regions were related to a softer region which represents the hydrophilic sulfonic acid groups of LS and dark regions were assigned to a hydrophobic polymer backbone [43]. The results showed an increasing tendency of the domain size and continuous of clusters with increase in the content of LS. Therefore, the presence of high content of LS may create more ionic transport channel, resulting in the proton conductivity of membrane suitable for use in PEMFCs. Fig. 4 shows the SEM cross-section micrographs for PBI, PBI/LS (10 wt%) and PBI/LS (20 wt%) membranes. Successful preparation of PBI porous membrane using LiCl as porogen is shown in Fig. 4. As seen in the figure, the internal morphology of the membrane was changed after adding LS, and more and
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larger voids have been created within the membrane as a result of increasing LS content. Void size distribution for PBI/LS (0e20 wt%) membranes is shown in Fig. 6, which has been determined using the image J® software. The average sizes of the voids within these membranes are 0.2, 0.6 and 0.9 mm, respectively. Furthermore, the total volume and average diameter of pores is determined using BET test. The characteristics of inside membrane pores obtained by means of BET test is listed in Table 1. Mean diameter of inside membrane pores taken by imagJ software has major different with that of BET test. This can be inability of imagJ to measure pore diameters in nanoscale as well inability to determine the all pores. To better understand the internal structure of the membrane, the FESEM analysis for the sample of PBI/LS(20 wt%) membrane is attached in the Appendix section, Fig. 1. The based PBI membrane with LS content of 30 wt% was prepared. The SEM analysis results are shown in Fig. 5. The large micro-pores in the membrane’s structure was created which leads to mechanical resistance reduction of the membrane. Addition, it is causes the ability of storing acid or water decreases as well as the fuel passability increases. Hence, the performance of the membrane including Lignosulfonate content up to 20 wt% was studied. The formation of larger voids can be considered as a reservoir for retaining PA in the membrane, and thus, the proton conductivity of the membrane increase with increasing porosity of the membrane. The PBI is dissolved in the DMSO under high temperature and high pressure, while LS is dissolved in the DMSO at the ambient temperature. Therefore, the interaction between LS and DMSO would delay the DMSO/water exchange process, after preparing the wet film and immersing the membrane in the water bath. That is why by increasing the content of LS in the PBI/LS blend the structure of the membrane becomes much more regular structure and the number of the voids in the membrane increases, due to the good dispersing property of LS. The presence of void in the membrane morphology can influence transport properties and increases the membrane ability to trap PA.
IEC, WU, MU and proton conductivity of the hydrated blend membranes The IEC of the membranes are presented in Fig. 7. By increasing the content of LS in PBI, the sulfonate groups in PBI increase, resulting in the increased IEC (Fig. 7b). The PBI membrane has a relatively high water uptake in comparison with other polymers such as polyimide (1.2 wt%), polyetherketone (0.5 wt%) and polycarbonate (0.3 wt%) [45]. The water uptake of PBI membrane was measured to be 14 wt% at room temperature. It can be attributed to hydrogen bonding between H2O, N and NeH groups [45]. As shown in Fig .1, LS is highly complex aromatic polymers with multiple functionalities, including phenolic and aliphatic hydroxyls, carboxyl and sulfonic acid groups. Due to the strong hydrophilicity of the sulfonic acid groups, the PBI/LS blend showed WU increment (Fig. 7a). By increasing the LS content in the membrane, the sulfonate (eSO3H) and carboxylic (eCOOH) groups increases in the membrane; therefore the hydrophilic property and
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Fig. 3 e AFM tracing phase image for PBI, (a) PBI/LS (10 wt%), (b) PBI/LS (20 wt%), and AFM topographic trace image (c) PBI/LS (10 wt%) and (d) PBI/LS (20 wt%).
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Fig. 4 e SEM image from cross-section of (a) PBI, (b) PBI/LS (10 wt%) and (c) PBI/LS (20 wt%) blends.
Table 1 e BET test for characteristics determination of porosity of membrane containing 20 wt% lignosulfonate. Special area
2.8894 [m2 g1]
Void volume Mean pore diameter
0.6639 [cm3 g1] 54.034 [44]
Fig. 5 e SEM image from surface of PBI/LS (30 wt%) blend.
thereby WU of the membranes increases. In addition, by increasing the content of LS leads to formation of voids in the membrane which is, in fact, considered as a reservoir for retaining water. Furthermore, high WU of the membranes can increase the proton conductivity of the membranes. As is shown in Fig. 7, the proton conductivity increased with the increase of LS content in the PBI, due to increase in the hydrophilic properties as well as presence of SO3H and COOH groups in the membrane. Proton conductivity of membrane has water uptake dependency. Also, the proton conductivity is directly proportional to IEC. An increase in the amount of LS (from 5 to 20 wt%) leads to a significant rise in both proton conductivity and IEC of PBI/LS blends. Despite of high water uptake, all PBI/LS blends have low proton conductivity in comparison with Nafion® 117 (13.4 mS cm1) at room temperature [46]. Therefore, hydrated blend membranes are not suitable candidates for use in PEMFC at ambient temperatures. Methanol uptake (MU) of the PBI/LS blend membranes as a function of LS content is presented in Fig. 8. The blend membrane showed an increasing tendency in methanol uptake with increase in the content of LS and reached a value of 33 wt% for PBI/LS (20 wt%). The MU is related to the methanol permeability while water uptake is associated with the proton conductivity. It can be seen from Fig. 8 that by an increase in the amount of LS from 15 to 20 wt%, MU remains almost constant. On the other hand, the WU increased significantly
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Fig. 6 e Pore size distribution of the PBI (a), PBI/LS (10 wt%) (b) and PBI/LS (20 wt%) (c) membranes.
with addition of LS in PBI (Fig. 7a). Consequently, the membranes with high LS content can be more effective on the proton conductivity in comparison to with the methanol permeability.
PA doping levels and proton conductivities of the PA-doped blend membranes The proton conductivity of Nafion is depend on the membrane’s humidity. Because the water evaporates at temperatures above 100 C, the conductivity have been rarely reported for Nafion at high temperature. When PBI is doped with PA, it gains high proton conductivity in dry conditions at the assessed temperature, as the acid acts as a proton conducting carrier in the membrane. Fig. 9 illustrates the PAdop and the proton conductivity of PA-PBI/LS blends at 25 C. The proton conductivity of PBI and PA-PBI porous membranes was obtained to be 2.23 109 and 2.4 mS cm1 at ambient temperature in dry condition, respectively.
By increasing LS content in the membrane, the PAdop of membranes increases from 4.8 to 15.3 (mol H3PO4/mol PBI repeat unit). The interaction between H3PO4 and imidazole C] N group leads to retention of H3PO4 by the PBI membrane. It has been suggested that the maximum degree of H3PO4 interacting with nitrogen in the imidazole ring of PBI are two PA molecules per PBI repeat unit [47,48]. Therefore, when PAdop >2, excess free PA is not retained by imidazole- N]C exists in the membranes. All the PA-PBI/LS membranes with PA > 2 result in excess free PA, facilitating proton transport via vehicle mechanism. The key functional groups controlling the proton transport in PA-PBI/LS membranes are the excess free PA, the PA interacted with imidazole eNH groups and the free imidazole eNH group. It is suggested that when PA>3, there is much excess free H3PO4. Result in H2PO4…H3PO4 distance is less than the imidazole NH…-N]C- distance [49]. In this case, the proton conductivity in membrane is resulted from a motion of proton along the mixed H2PO4…H3PO4 and the
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Fig. 7 e WU (left) IEC and proton conductivity (right) of PBI/LS blend membrane at 25 C.
Fig. 8 e Methanol uptake of the PBI/LS blends membranes at 25 C.
polymer-anionic chains. A maximum conductivity of 98 mS/ cm of porous PA-PBI/LS (20 wt%) was achieved at 25 C, much higher than the conductivity of Nafion® 117 membrane (13.4 mS/cm) at same temperature. It is due to the fact that larger and more voids are formed in the PBI/LS (20 wt%) membrane which act as reservoirs for retaining PA. The minimum value for proton conductivity in the fuel cell application is equal to 10 mS cm1. When the LS polymer weight percentage was more than 5, the conductivity of the blends was higher than 10 mS cm1 at ambient temperature.
Proton conductivity of membrane as a function of temperature The enhancement in the proton conductivity of PEM is one of the most important factor for its application in PEMFCs.
Fig. 9 e Proton conductivity and PAdop plots of PA-PBI/LS membranes at 25 C.
Design and development of PEMFCs with high proton conductivity in anhydrous state have been studied by many groups [50e53]. The proton conductivity of membranes were obtained from resistance measurement using Nyquist plot. Fig. 10 shows the Nyquist plot of the PA-PBI/LS (5 wt%) porous membrane at three temperatures of 40, 80,100 and 160 C in the ambient conditions. According to the figure, the resistance of the membrane decreases with increasing temperature and, according to equation (5), the proton conductivity of the membrane increases. The proton conductivity of PA-PBI, PAPBI/LS (5 wt%), PA-PBI/LS (10 wt%) and PA- PBI/LS (20 wt%) membranes were measured at different temperatures. Fig. 11 shows the proton conductivity of PA-PBI/lignin (20 wt%), PA-PBI/LS (5,10 and 20 wt%) and PA-PBI membranes as a function of temperature, in the temperature range of 25e160 C. As seen in the Fig. 10, by increasing temperature,
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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Fig. 11 e Proton conductivity of PA-doped PBI/LS blend at different temperature.
lignosulfonate in the membrane. In fact, the existence of sulfonate groups of LS causes reduction of the proton transfer resistance for membrane. Moreover, in accordance with the results of SEM, increase in proton conductivity with increasing the content of LS in the PBI/LS blend membrane is a result of formation of the larger and more voids, by virtue of which the ability for retaining acid increases. The temperature dependent proton conductivity of PA-PBI/ LS membranes follows the Arrhenius relationship: Fig. 10 e The Nyquist plot of the PA-PBI/LS (5 wt %) porous membrane at temperatures of 40, 80, 100 (top) and 160 C (bottom).
the membranes proton conductivity increases with temperature. Moreover, the proton conductivity increased by increasing the LS content in the PBI at constant temperature. A significant increase of PBI/LS membranes proton conductivity is for two reasons: 1) an increase in acid group (sulfonic acid) and 2) increase of the porosity and free space within the membrane that acid absorption ability of membrane could be increased by increasing of porosity. Additionally, by increasing of sulfonic acid and carboxylic acid groups of lignin polymer the proton transfer resistance is reduced and the protonation degree is increased. It can be seen that the PA-PBI/LS blend membrane shows the highest proton conductivity for PA-PBI/LS (20 wt%) at high temperature (120 Ce160 C). The highest proton conductivity was obtained to be 187.66 mS cm1 at 160 C for PA- PBI/LS (20 wt%) blend membrane, much higher than the maximum 34 mS cm1 for the porous PA-doped PBI membranes. It can be observed in Fig. 11 that the proton conductivity of PBI/LS (20 wt%) membrane is higher than the PBI/Lignin (20 wt %) membrane for the same content of lignin and
s ¼ s0 exp
Ea RT
(6)
in which s, s0 , Ea, R, and T are proton conductivity (S/cm), preexponential parameter, activation energy of proton conduction (kJ/mol), the gas constant (8.314 J/mol.K) and the temperature (K), respectively. Arrhenius plots of conductivity as a function of temperature are shown in Fig. 12. The activation energies of membrane (Ea) can be obtained by using the slope of Arrhenius plots. It can be mentioned that by activation energy, the proton transport mechanism in membranes can be estimated. From Fig. 12, the activation energies of PA-PBI, PA-PBI/LS (5 wt%), PA-PBI/LS (10 wt%) and PA-PBI/LS (20 wt%) membranes were obtained to be 32.03,28.51, 14.13 and 5.9 kJ/ mol, respectively. It can be concluded that the proton transport dominant mechanism in the PA-PBI/LS and PA-PBI blend membranes is Grotthus, because the activation energies are in the range of 14e40 kJ/mol [54] for Grotthus mechanism. The minimum activation energy of 5.9 kJ mol1 for the PA-PBI/LS (20 wt%) membrane is due to formation of large void in the membrane structure, which also serves as a reservoir for retain more PA in the membrane. However, the ability of the membrane to retain the acid is low, and, accordingly the proton conductivity of the PA-PBI/LS (20 wt%) membrane has not been increased significantly with increasing the temperature, compared to PA-PBI/LS (0e15 wt%) and the gradient of
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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analysis, the internal structure of the membrane based on its size distribution and number of porous are uniform. Thus, growth of the size and number of porous in the membrane can reduce the membrane mechanical resistance. On the other hand, according to FTIR analysis with increase of the lignosulfonate content, the peak is appeared in the range of 3000e3451 cm1 which indicates the extra lignosulfonate polymer in the membrane. This may change the membrane structure and consequently the membrane mechanical resistance. Thus, these two reasons could reduce the membrane mechanical resistance by increase of the lignosulfonate.
Methanol permeability
Fig. 12 e Arrhenius plot of proton conductivity for LS/PBIPA.
changes in proton conductivity with the temperature has been lower. In fact, numerous open-surface macro-voids result in the evaporation of PA at high temperatures. However, this membrane still has a higher proton conductivity at the assessed temperature compared to others membranes.
Table 3 shows methanol permeability of membranes as well as proton conductivity of water-absorbed membranes (those shown in Fig. 6) at room temperature. According to the results, fuel permeability increased by adding lignosulfonate polymer to PBI which the PBI/LS (20 wt%) membrane has the maximum fuel permeability. Since, the numbers and the size of pores are increased by increasing of lignosulfonate content. In PBI/LS (5 wt%) and PBI/LS (10 wt%) membranes the methanol permeability is near to the porous PBI membrane which indicates existed pores resulting of adding lignosulfonate polymer into membrane are mostly closed pores and lead to a bit increase for the fuel permeability through the membrane. As mentioned in the previous sections, a useful membrane for fuel sells should have low fuel permeability and high proton conductivity. Also, it should have the high ratio of proton conductivity to fuel permeability which is known as membrane selectivity.
Mechanical properties f¼ Table 2 shows mechanical properties of the PBI/LS (5 wt%), PBI/LS (20 wt%) and PBI membranes. As clearly seen, by adding 5 wt% lignosulfonate polymer to PBI has been caused the significant increase of membrane tensile strength (from 72.4 to 105.23 MPa). According to the survey, the increasing of the membrane mechanical resistance could be due to strong interaction between the two polymers. Since the lignosulfonate polymer has the sulfonic acid group, which is a strong acid, it makes a strong interaction with amine group of PBI polymer. As it observed from FTIR analysis, the peak of OH and NH in PBI/LS (5 wt%) membrane has been eliminated which reflects the strong bond between the two polymers. But, by adding of 20 wt% lignosulfonate polymer in to membrane, its mechanical resistance has been declined. This reduction maybe due to two reasons; on the one hand according to SEM
Table 2 e Mechanical properties of the prepared membranes. Blend Tensile Maximum membrane strength(MPa) elongation (%) PBI PBI/LS (5 wt %) PBI/LS (20 wt %)
Young modulus (MPa)
72.42 105.23
14.07 15.6
5.147 6.74
79.3
9.2
8.6
s P
(7)
Selectivity of membranes has been reported in the Table 3. Although the fuel permeability has been raised to somewhat by increasing of the hydrophilic lignosulfonate content, however; membrane selectivity is comparable with that of Nafion because of the significant high proton conductivity of PBI/LS membranes. Fuel cell simulation method is used to investigate the performance of proton exchange membrane fuel cell. The
Table 3 e Methanol permeability and selectivity of waterabsorbed membranes at room temperature. Methanol permeability ðcm2 s1 Þ Nafion [46] PBI PBI/LS (5 wt %) PBI/LS (10 wt %) PBI/LS (20 wt %)
Proton conductivity ðmS:cm1 Þ
Selectivity ðmS:s:cm3 Þ
14:9 107
13.4
9:0 10þ6
4:24 109 4:8 109
2:6 108 1:45 104
6:1 101 3:02 10þ4
6:5 109
3:06 104
4:7 10þ4
4:32 108
26:3 102
6:1 10þ6
Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216
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international journal of hydrogen energy xxx (xxxx) xxx
defined model in our previous article [55] was simulated by means of COMSOL Multiphysics software and was verified by experimental data. According to the verification results, the model accurately predicts the experimental data. Therefore, in this study, the properties results of PBI/LS (20 wt%) membrane were putted in simulating process to assess the fuel cell performance. Fig. 13 indicates the fuel cell performance at the temperatures of 25, 120, and 160 C. As shown in the figure, the fuel cell performance is improved by increasing the temperature, because the membrane proton conductivity increases by temperature (Fig. 13).
a selectivity of the proton conductivity over the methanol permeability comparable with that of Nafion. Due to its high proton conductivity, PA-PBI/LS (20 wt%) porous membrane can be considered as a good candidate for fuel cells applicable for both low and high temperatures. Furthermore, environmentally friendly LS is a low cost by-product in the various industries and is found in the environment abundantly, the PBI/LS membrane has environmental and economic advantages over other polymers.
Appendix A. Supplementary data Conclusion Novel porous PBI/LS membranes were prepared for use in low and high temperature PEMFCs. LiCl salt was used for two reasons, that is, to increase stability of solution and as a porogen to create porosity in the membrane. LS polymer was used as a sulfonated polymer with the aim of increasing proton conductivity through increasing the sulfonic acid group and formation of pores in the PBI membrane structure. SEM images revealed the successful preparation of PBI/LS and PBI porous membranes. However, the results showed that by increasing lignosulphonate content up to 20 wt%, the prepared membrane would be not structurally and mechanically suitable to be used in fuel cell. As a consequence of the porous structure of the membrane, the amount of PA trapped in the membranes increased. Therefore, by increasing the content of LS from 0 to 20 wt%, the proton conductivity of PA-PBI/LS membranes at ambient temperature increased from 1.98 mS cm1 for PA-PBI to 98 mS/cm for PA-PBI/LS (20 wt%). The proton conductivity of the PBI/LS (20 wt%) porous membrane at room temperature was close to that of the Nafion at 80 C, and by increasing temperature to 160 C, the proton conductivity of this membrane reached to 187 mS/cm. The formation of pores within the PBI/LS (20 wt%) membrane led to a significant increase in the proton conductivity. Results showed that the membrane with high LS content (20 wt%) has
Fig. 13 e Performance of proton exchange membrane fuel cell for PBI/LS (20 wt%) membrane.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.216.
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Please cite this article as: Barati S et al., High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation, morphology and physical and proton conductivity properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.216