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Novel composite Nafion membranes modified with copper phthalocyanine tetrasulfonic acid tetrasodium salt for fuel cell application
Journal of Materiomics 5 (2019) 252e257
Contents lists available at ScienceDirect
Journal of Materiomics journal homepage: www.journals.elsevier.com/journal-of-materiomics/
Novel composite Nafion membranes modified with copper phthalocyanine tetrasulfonic acid tetrasodium salt for fuel cell application Yanan Wei a, b, Tianhua Qian b, Jiawen Liu c, Xaojing Guo b, Qiaojuan Gong a, *, Zhaorong Liu a, Binglun Tian d, Jinli Qiao a, b, ** a
Department of Applied Chemistry, Yuncheng University, 1155 Fudan West Street, Yun Cheng, 04400, China State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Environmental Science and Engineering, Donghua University, 2999 Ren'min North Road, Shanghai, 201620, China c Shanghai Jinyuan Senior High School, Shanghai, 200333, China d Shanghai Boxuan Energy Technology Company, Shanghai, 201803, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2018 Received in revised form 29 November 2018 Accepted 16 January 2019 Available online 23 January 2019
In this paper, Nafion membrane was firstly modified by copper phthalocyanine tetrasulfonic acid tetrasodium salt (CuTSPc) to prepare the Nafion/CuTSPc-x composite membranes. FTIR, XRD and SEM results revealed the successful incorporation of CuTSPc into Nafion and good compatibility between the two composites. The proton conductivities of the Nafion/CuTSPc-x composite membranes were evidently higher than pure cast Nafion membrane, and increased with CuTSPc contents. Among them, the Nafion/ CuTSPc-6% membrane with the highest ion exchange capacity (1.14 mequivg1) exhibited the highest proton conductivity of 0.084 S cm1 at 30 C and 0.131 S cm1 at 80 C, respectively. When fabricated of a membrane electrode assembly (MEA), the Nafion/CuTSPc-4.5% membrane displayed an initial fuel cell performance with a power density of 43.3 mW cm2 at room temperature, close to that for pure cast Nafion membrane. Benefiting from the compact structure, high proton conductivity and outstanding stability, the Nafion/CuTSPc-x composite membranes show promising potentials for fuel cell applications. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Proton exchange membrane Proton conductivity Stability Fuel cell
1. Introduction Fuel cells are considered to be popular energy conversion systems that relies on renewable energy to produce power and convert chemical energy into electrical energy [1]. Proton exchange membrane (PEM) fuel cells have attracted considerable attention owing to their high efficiency, environmental compatibility, potentially lower cost for stationary and portable power applications [2,3]. Proton exchange membrane, which separates two electrodes and transports protons, is one of key materials in fuel cell system. Presently, perfluorinated sulfonic acid membranes have been widely used and investigated because of their superior mechanical
* Corresponding author. ** Corresponding author. College of Environmental Science and Engineering, Donghua University, 2999 Ren’min North Road, Shanghai 201620, China. E-mail addresses: [email protected] (Q. Gong), [email protected] (B. Tian), [email protected] (J. Qiao). Peer review under responsibility of The Chinese Ceramic Society.
strength, high proton conductivity and superior stability [4e7]. However, it also exhibits many drawbacks that restricted practical applications in a large-scale, such as high cost, high fuel cross over and significant loss of proton conductivity at high temperature [8e10]. Two different pathways exist to overcome these troubles: the first is the development of suitable proton-exchange membranes; the second is the modification of the perfluorinated sulfonic membrane. Developing modified Nafion membranes that containing multiple components is an efficient strategy to enhance the proton conductivity at high temperature and other properties. One or more species have to be employed for the Nafion modification via coating, blending and grafting with selected polymers [11e15]. In addition, several polymers have also been successfully introduced, such as polysilsesquioxane (POSS) [16], polyphenyl sulfone (PPSU) [17] and poly ether sulfone (PES) [18]. Transition metalphthalocyanine (MPc) is an interesting kind of compound with a conjugated ligand that includes eight N atoms in a unit structure. A delocalized conjugated p-bond in the molecule makes MPc liable to form a chelate structure with charged groups
https://doi.org/10.1016/j.jmat.2019.01.006 2352-8478/© 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
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(sulfonic acid) tailed on Nafion backbone. Therefore, it is expected that both the thermal stability and mechanical strength could be improved. Specifically, the copper phthalocyanine tetrasulfonic acid tetrasodium salt (CuTSPc), with tetrasulfonic acid in its structure, is a good hydrophilic agent, thereby making the composite membrane more increased ion transmigration rates. The more charged groups can contribute to the ion-exchange capacities and further enhance the proton conductivity. In our previous work, CuTs and CuTsPc have been successfully used as efficient catalysts for oxygen reduction reaction [19,20], but what used for Nafion membrane modifications have not been reported. In this study, as a proof-of-concept application, through a simple casting method with Nafion solution, the (CuTSPc) firstly used as a promoter is demonstrated to be an effective strategy for fabrication of high-performing Nafion composite membranes, Nafion/CuTSPc-x. We used several instrumentation methods including X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM) and thermal gravimetric analysis (TGA) to characterize the structure and composition of the composite membranes, and thereby obtain insight into their proton conductivity and its dependence on the mass ratio of Nafion/CuTSPc in casting solution. The proton conduction mechanisms of the as-prepared membranes were analyzed on the basis of proton conducting dependence on temperature. The water uptake, thermal stability, oxidation stability and tensile strength were thoroughly studied. The membrane electrode assemblies (MEAs) with different compositions of Nafion/ CuTSPc-x were also fabricated and compared with pure cast Nafion membrane to evaluate the applicability of Nafion/CuTSPc-x in proton-exchange membrane fuel cells. 2. Experimental 2.1. Membrane preparation The membrans were prepared by a simple solution-casting method. A certain amount of D2020 Nafion solution was diluted with alcohol to form a 5 wt% solution. Then appropriate amounts of copper phthalocyanine tetrasulfonic acid tetrasodium salt (CuTSPc) were dissolved in the obtained Nafion solution under stirring condition. After stirring at room temperature for 12 h, the solution was cast on a glass plate and dried at 100 C for 3 h. The resulting composite membrane was peeled from the glass, and immersed in 1 M H2SO4 at room temperature for 24 h to thoroughly convert into acid form. The eSO-3 groups on the skeleton of CuTsPc were then combined with the eSO3H groups of Nafion to form a network structure (Fig. 1). All composite membranes were denoted as Nafion/CuTSPc-x (x ¼ 0%, 1.5%, 3%, 4.5%, 6%), where x indicates the mass content of CuTSPc in Nafion/CuTSPc-x composite membranes. 3. Results and discussion 3.1. Characterizations of pure cast Nafion and Nafion/CuTSPc-x composite membranes Fig. 2(a) shows the FTIR spectra, which was carried out to identify the changes of structure of the composite membranes compared with pure cast Nafon membrane. It is clearly that the peak at 970 cm1 for pure cast Nafion is due to the CeOeC stretching. The peak at 1057 cm1 is attributed to the SeO stretching vibration of eSO3H groups, while two strong peaks appeared at 1149 cm1 and 1205 cm1 are due to Nafion backbone's CeF symmetric and antisymmetric stretching vibration [21]. On the other hand, the synthesized membranes also have these characteristic peaks. However, the intensities of peak at 1057 cm1 were all increased due to the increase in concentration of eSO3H groups, which confirms the
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successful incorporation of CuTSPc in Nafion. The X-ray diffraction measurements were conducted to examine the degree of crystallinity of the membranes. Fig. 2(b) shows the XRD spectra of Nafion and Nafion/CuTSPc-3% membrane. From the figure, it can be clearly observed that for the pure cast Nafion membrane, a sharp 2q peak appeared at 17, which is assigned to the amorphous scattering at 16 and the crystalline scattering at 17.5 from the polyfluorocarbon chains, and at the same time there is a wide peak at 38 e39 [22e24]. The XRD pattern of the Nafion/CuTSPc-3% composite membrane matches well with Nafion, indicating that Nafion/ CuTSPc-x composite membranes still maintained the basic framework of Nafion. In addition, the peak at 2q ¼ 10e20 of Nafion/ CuTSPc-3% membrane become narrower, implying that composite membranes have a higher crystallinity than pure cast Nafion membrane, which leads to an improvement on proton conductivity [25]. The SEM images of pure cast Nafion membrane and Nafion/ CuTSPc-3% composite membrane cross section are shown in Fig. 2(c) and (d) and Figure S1. It can be observed that pure cast Nafon membrane exhibits a smooth and a dense characteristic on the interface. After incorporation of CuTSPc, the composite membrane results in surface roughness (Fig. 2(d)), but no obvious surface degradation or phase separation phenomena was observed when compared to pristine Nafion membrane. It indicates that there is an excellent mutual compatibility between CuTSPc and Nafion matrix. 3.2. Properties of pure cast Nafion and Nafion/CuTSPc-x composite membranes In a polymer structure, the concentration of the fixed charged (SO 3 ) sites are critical to ensure high proton conductivity [26]. From Fig. 3(a), it can be clearly observed that the proton conductivity increase with the CuTSPc contents in the composite membranes. Moreover, the proton conductivities of all the investigated composite membranes were superior to the pure cast Nafion membrane, where the Nafion/CuTSPc-6% membrane exhibited the highest proton conductivity reaching to 0.084 S cm1. In addition, as carriers in proton transport, the water molecules forming and hydrogen bonds breaking through vehicular mechanism [27] are remarkably affect the proton transport. However, excessive water uptake would make the dimensional stability variation, thus leads to the loss of mechanical strength of PEMs [28]. Therefore, PEMs with appropriate water uptake is one of the critical factors for the application of fuel cell. As shown in Figure S2(a), when temperature increased from 25 C to 80 C, the water uptake of pure cast Nafion increases from 25.85% at 25 C to 48% at 80 C. For Nafion/CuTSPc-x composite membranes, a similar tendency of water uptake with the temperature were observed, nevertheless, the water uptake values are particularly higher than that of pure cast Nafion at 60 C and 80 C. This result was possibly due to the fact that the formation degree of phase separation becomes clear at higher hydrophilic sulfonic acid contents and high temperature. As a key parameter to proton exchange membrane, the proton conductivities of Nafion/CuTSPc-x composite membranes were measured as a function of temperature and the results are displayed in Fig. 3(b). As shown in Fig. 3(b), the proton conductivities of all these membranes showed an increasing tendency with temperature increased. As listed in Table 1, with the ion exchange capacity (IEC) values increased from 0.81 to 1.14 mequiv$g1, the proton conductivity increases from 0.066 to 0.084 S cm1 at 30 C and from 0.096 to 0.131 S cm1 at 80 C. All these values are higher than those of pure cast Nafion membrane, which are suitable for practical applications in fuel cells. The activation energy (Ea) was calculated using the Arrhenius equation and listed in Table 1. Interestingly, the Nafion/CuTSPc-x composite membrane showed the lower Ea than pure cast Nafion memrbane, indicating that the
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Fig. 1. Schematic diagram of the Nafion/CuTSPc-x membranes.
Fig. 2. (a) FTIR spectras of pure cast Nafion and Nafion/CuTSPc-3% membrane. (b) XRD photographs of pure cast Nafion and Nafion/CuTSPc-3% membrane. (c) Cross-section SEM image of Nafion. (d) Cross-section SEM image of Nafion/CuTSPc-3% membrane.
proton mobility of Nafion/CuTSPc-x composite membranes were higher than Nafion [29], which is mainly through a hopping mechanism for proton transportation [30,31]. The TGA measurements of pure cast Nafion and Nafion/CuTSPcx composite membranes are needed to evaluate the potential to be used for fuel cell. As shown in Fig. 3(c), all membranes display excellent thermal stability as the temperature is reduced below 300 C. The slight degradation from 100 to 300 C is attributed to the loss of water in the membrane. Moreover, both the pure cast Nafion memrbrane and Nafion/CuTSPc-x composite membranes display two decomposition stages. The first stage from 300 to 400 C is due to the decomposition of side-chain sulfonic acid
groups. The second stage from 400 to 500 C is assigned to the decomposition of the polymer main chain. It is noteworthy that composite membranes showed lower weight drops than pristine Nafion, particular to temperature beyond 500 C, demonstrating that the addition of CuTSPc effectively improved the thermal stability of Nafion. Fig. 3(d) shows the oxidative durability in 30% H2O2 solution of the Nafion/CuTSPc-x composite membranes. It was observed that the weight of Nafion/CuTSPc-3% membrane tends to maintain 90% and with no further weight losses again up to 168 h. As a reference, the pure cast Nafion was tested under the same measuring conditions. Encouragingly, the Nafion/CuTSPc-x composite membranes
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Fig. 3. (a) Proton conductivity and water uptake of pure cast Nafion and Nafion/CuTSPc-x membranes. (b) Temperature dependence of proton conductivity of pure cast Nafion and Nafion/CuTSPc-x membranes. (c) The TGA curves of pure cast Nafion and Nafion/CuTSPc-x membranes. (d) Weight loss of pure cast Nafion and Nafion/CuTSPc-3% membrane in 30% H2O2 solution.
Table 1 IECs, Proton Conductivity (s), and Activation Energy (Ea) of pure cast Nafion and Nafion/CuTSPc-x membranes. Samples
Pure cast Nafion Nafion/CuTSPc-1.5% Nafion/CuTSPc-3% Nafion/CuTSPc-4.5% Nafion/CuTSPc-6%
IEC (mequiv$g1)
0.78 0.81 1.01 1.08 1.14
s (S cm1) 30 C
80 C
0.055 0.066 0.078 0.075 0.084
0.104 0.096 0.117 0.121 0.131
Ea (kJ mol1)
Nafion/CuTSPc-x composite membranes are presented in Fig. 4(b) and Figure S3. The Nafion/CuTSPc-4.5% membrane possessed an open circuit voltage of 0.55 V, and an initial power density around 43.3 mW cm2 at room temperature, which approach to that of pure
11.33 6.33 6.10 8.8 6.94
show superior oxidative stability than cast Nafion membrane. Therefore, the Nafion/CuTSPc-x composite membranes are promising oxidation stability for fuel cell applications. Further as shown in Table S1, the Nafion/CuTSPc-x composite membranes exhibited excellent tensile strength, which are much higher than that of pure cast Nafion. Specifically, the Nafion/CuTSPc-1.5% and Nafion/ CuTSPc-3% membranes still possessed the higher elongation at break 10.83% and 28.62%, respectively, indicating there exists a premium mass ratio range for Nafion/CuTSPc-x composition membrane preparation. All these results indicated that the introduction of CuTSPc could enhance the mechanical properties of Nafion. 3.3. Single cell performance Membrane electrode assembly (MEA) is the central core of the fuel cell. In this part, electrochemical reaction will be done. Hydrogen in anode electrode changes into a hydrogen ion. The ions move towards the cathode through the proton-exchange membrane and react with the reduced oxygen at the cathode surface to generate water. A schematic representation of the PEM fuel cell is shown in Fig. 4(a). To evaluate the practical application of Nafion/ CuTSPc-x composite membranes in fuel cell systems, the polarization and power density curves of pure cast Nafion membrane and
Fig. 4. (a) Schematic design of the PEM fuel cell. (b) Polarization and power density curves of pure cast Nafion and Nafion/CuTSPc-4.5% membrane at room temperature.
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cast Nafion membrane with an open circuit voltage of 0.68 V, and a maximum power density of 61.3 mW cm2. The initial performance is still low, it may be due to the high contact resistances caused by the poor compatibility between the composite membrane and the electrode. The MEA fabrication is a very complex process, and it will be influenced by many factors. At this stage, the MEA preparation conditions are not optimal. However, these results demonstrated that Nafion/CuTSPc-x composite membranes have the potential to be proton-exchange membrane materials for fuel cell applications. 4. Conclusions A series of novel sulfonated composite membranes were successfully fabricated by facile solution casting method. We characterized it with FTIR, XRD. The obtained results confirmed the successful incorporation of CuTSPc within the Nafion. SEM images showed that the CuTSPc have excellent mutual compatibility with Nafion. Moreover, because of the increase contents of hydrophilic sulfonic acid groups, the water uptake and proton conductivity of the composite membranes significantly increased. Especially, Nafion/CuTSPc-6% membrane exhibited the highest proton conductivities with 0.084 S cm1. Meanwhile, Nafion/CuTSPc-x composite membranes exhibited the proton conductivities with from 0.066 to 0.084 S cm1 at 30 C and from 0.096 to 0.131 S cm1 at 80 C, which were higher than those of pure cast Nafion. The single cell performance test showed that the Nafion/CuTSPc-4.5% membrane possessed an open circuit voltage of 0.55 V, and power density around 43.3 mW cm2 at room temperature. In short, the membrane exhibited high proton conductivity, excellent stability and could be the promising materials for fuel cell applications. Acknowledgements This work is financially supported by the National Key R&D Program of China (2017YFB0102900) and National Natural Science Foundation of China (U1510120). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmat.2019.01.006. References [1] Lemmon JP. Energy: reimagine fuel cells. Nature 2015;525:447. [2] Park CH, Lee CH, Guiver MD, Lee YM. Sulfonated hydrocarbon membranes for medium-temperature and low-humidity proton exchange membrane fuel cells (PEMFCs). Prog Polym Sci 2011;36:1443e98. [3] Wu L, Zhang Z, Ran J, Zhou D, Li C, Xu T. Advances in proton-exchange membranes for fuel cells: an overview on proton conductive channels (PCCs). Phys Chem Chem Phys 2013;15:4870e87. [4] Schmidt-Rohr K, Chen Q. Parallel cylindrical water nanochannels in Nafion fuel-cell membranes. Nat Mater 2008;7:75e83. [5] Kreuer K-D, Portale G. A critical revision of the nano-morphology of proton conducting ionomers and polyelectrolytes for fuel cell applications. Adv Funct Mater 2013;23:5390e7. [6] Xiao L, Mischa Dr B, Dr PSH, Dr DKF. Nanoscale distribution of sulfonic acid groups determines structure and binding of water in nafion membranes. Angew Chem 2016;55:4011. [7] Yang L, Li H, Ai F, Chen X, Tang J, Zhu Y, Wang C, Yuan WZ, Zhang Y. A new method to prepare high performance perfluorinated sulfonic acid ionomer/ porous expanded polytetrafluoroethylene composite membranes based on perfluorinated sulfonyl fluoride polymer solution. J Power Sources 2013;243: 392e5. [8] Yoshimura K, Iwasaki K. Aromatic polymer with pendant perfluoroalkyl sulfonic acid for fuel cell applications. Macromolecules 2009;42:9302e6. [9] Wang L, Li K, Zhu G, Li J. Preparation and properties of highly branched sulfonated poly(ether ether ketone)s doped with antioxidant 1010 as proton exchange membranes. J Membr Sci 2011;379:440e8. [10] Xie H, Tao D, Ni J, Xiang X, Gao C, Wang L. Synthesis and properties of highly branched star-shaped sulfonated block polymers with sulfoalkyl pendant
groups for use as proton exchange membranes. J Membr Sci 2016;497:55e66. [11] Zheng J, He Q, Liu C, Yuan T, Zhang S, Yang H. Nafion-microporous organic polymer networks composite membranes. J Membr Sci 2015;476:571e9. [12] Abouzari-lotf E, Nasef MM, Ghassemi H, Zakeri M, Ahmad A, Abdollahi Y. Improved methanol barrier property of nafion hybrid membrane by incorporating nanofibrous interlayer self-immobilized with high level of phosphotungstic acid. ACS Appl Mater Interfaces 2015;7:17008e15. [13] Li J, Xiong J, Ma L, Xu G, Jiang Y, Luo X, Cai W. An in-situ organic-inorganic hybrid methanol-permeation resistant PEM with great mechanical stability retention capacity. ChemistrySelect 2017;2:1525e9. [14] Zhang Y, Cai W, Si F, Ge J, Liang L, Liu C, Xing W. A modified Nafion membrane with extremely low methanol permeability via surface coating of sulfonated organic silica. Chem Commun 2012;48:2870e2. [15] Yan XH, Wu R, Xu JB, Luo Z, Zhao TS. A monolayer graphene e Nafion sandwich membrane for direct methanol fuel cells. J Power Sources 2016;311: 188e94. [16] Geng Z, Huo M, Mu J, Zhang S, Lu Y, Luan J, Huo P, Du Y, Wang G. Ultra low dielectric constant soluble polyhedral oligomeric silsesquioxane (POSS)e poly(aryl ether ketone) nanocomposites with excellent thermal and mechanical properties. J Mater Chem C 2014;2:1094e103. [17] Devrim Y. Fabrication and performance evaluation of hybrid membrane based on a sulfonated polyphenyl sulfone/phosphotungstic acid/silica for proton exchange membrane fuel cell at low humidity conditions. Electrochim Acta 2014;146:741e51. [18] Nikouei MA, Oroujzadeh M, Mehdipour-Ataei S. The PROMETHEE multiple criteria decision making analysis for selecting the best membrane prepared from sulfonated poly(ether ketone)s and poly(ether sulfone)s for proton exchange membrane fuel cell. Energy 2017;119:77e85. [19] Ding L, Qiao J, Dai X, Zhang J, Zhang J, Tian B. Highly active electrocatalysts for oxygen reduction from carbon-supported copper-phthalocyanine synthesized by high temperature treatment. Int J Hydrogen Energy 2012;37:14103e13. [20] Qing X, Shi J, Ma C, Fan M, Bai Z, Chen Z, Qiao J, Zhang J. Simultaneous formation of nitrogen and sulfur-doped transition metal catalysts for oxygen reduction reaction through pyrolyzing carbon-supported copper phthalocyanine tetrasulfonic acid tetrasodium salt. J Power Sources 2014;266:88e98. [21] Wei Y, Shen L, Wang F, Yang W-D, Zhu H, Wang Z, Han K. Synthesis and characterization of novel nanocomposite membrane of sodium titanate/ Nafion®. Mater Lett 2011;65:1684e7. AS, Cretı P, Ramunni E, Antonucci V. Investigation of a [22] Antonucci PL, Arico direct methanol fuel cell based on a composite Nafion ®; -silica electrolyte for high temperature operation. Solid State Ionics 1999;125:431e7. [23] Dimitrova P, Friedrich KA, Stimming U, Vogt B. Modified Nafion®;-based membranes for use in direct methanol fuel cells. Solid State Ionics 2002;150: 115e22. [24] Shao Z-G, Joghee P, Hsing IM. Preparation and characterization of hybrid Nafionesilica membrane doped with phosphotungstic acid for high temperature operation of proton exchange membrane fuel cells. J Membr Sci 2004;229:43e51. [25] Byun S, Jeong Y, Park J, Kim S, Ha H, Kim W. Effect of solvent and crystal size on the selectivity of ZSM-5/Nafion composite membranes fabricated by solution-casting method. Solid State Ionics 2006;177:3233e43. [26] Barbora L, Acharya S, Singh R, Scott K, Verma A. A novel composite Nafion membrane for direct alcohol fuel cells. J Membr Sci 2009;326:721e6. [27] Shahi V. Highly charged proton-exchange membrane: sulfonated poly(ether sulfone)-silica polyelectrolyte composite membranes for fuel cells. Solid State Ionics 2007;177:3395e404. [28] Smitha B. Synthesis and characterization of proton conducting polymer membranes for fuel cells. J Membr Sci 2003;225:63e76. [29] Wang B, Hong L, Li Y, Zhao L, Zhao C, Na H. Property enhancement effects of side-chain-type naphthalene-based sulfonated poly(arylene ether ketone) on nafion composite membranes for direct methanol fuel cells. ACS Appl Mater Interfaces 2017;9:32227e36. [30] Zhang J, Zhou T, Qiao J, Liu Y, Zhang J. Hydroxyl anion conducting membranes poly(vinyl alcohol)/poly(diallyldimethylammonium chloride) for alkaline fuel cell applications: effect of ̀molecular weight. Electrochim Acta 2013;111: 351e8. [31] Zhang J, Qiao J, Jiang G, Liu L, Liu Y. Cross-linked poly(vinyl alcohol)/poly (diallyldimethylammonium chloride) as anion-exchange membrane for fuel cell applications. J Power Sources 2013;240:359e67.
Yanan Wei received her B.S. degree from Henan University of Urban Construction in 2016. She is currently pursuing her M.S. degree in the School of Donghua University of Environmental Science and Engineering, China, under the supervision of Prof. Jinli Qiao. Her research focuses on the alkaline-exchange polymer membrane electrolyte and their applications on all-solid-state electrochemical devices.
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Jiawen Liu is a high school student in Shanghai Jinyuan Senior High School. She is currently pursuing her research subject under the supervision of Prof. Qiaojuan Gong and Jinli Qiao in Donghua University. Her research focuses on the membrane/catalyst preparation and their applications on energy conversion devices.
Binglun Tian received his master degree from Chinese acad of science, Dalian Institute of Chemical Physics in 2000. He focus on High-performing air-cooling selfhumidified PEM fuel cell stacks for clean power applications about 12 years and 6 years experience on liquid cooled fuel cell systems which used for heavy truck applications. He has applied for about 60 patents, including 2 PCT international patents during this years. As a science and senior engineer, he has developed high performance fuel cell catalyst, membrane, carbon paper and bipolar plate to medium scale production.
Qiaojuan Gong received her Ph.D. degree in materialogy from Northwestern Polytechnical University in 2007. She is a professor of chemistry in the Department of Chemistry, Yuncheng University. Her research interest is functionalization nanocarbon materials and electrochemical biosensors.
Jinli Qiao received her Ph.D. degree in Electrochemistry from Yamaguchi University, Japan in 2004. Then she joined the National Institute of Advanced Industrial Science and Technology (AIST) as a research fellow. From 2008, she is a professor of the Clean Energy Automotive Engineering Center, Tongji University and College of Environmental Science and Engineering, Donghua University, Shanghai, China. Her research focuses on PEM fuel cells, metaleair batteries and CO2 electroreduction.
Contents lists available at ScienceDirect
Journal of Materiomics journal homepage: www.journals.elsevier.com/journal-of-materiomics/
Novel composite Nafion membranes modified with copper phthalocyanine tetrasulfonic acid tetrasodium salt for fuel cell application Yanan Wei a, b, Tianhua Qian b, Jiawen Liu c, Xaojing Guo b, Qiaojuan Gong a, *, Zhaorong Liu a, Binglun Tian d, Jinli Qiao a, b, ** a
Department of Applied Chemistry, Yuncheng University, 1155 Fudan West Street, Yun Cheng, 04400, China State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Environmental Science and Engineering, Donghua University, 2999 Ren'min North Road, Shanghai, 201620, China c Shanghai Jinyuan Senior High School, Shanghai, 200333, China d Shanghai Boxuan Energy Technology Company, Shanghai, 201803, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2018 Received in revised form 29 November 2018 Accepted 16 January 2019 Available online 23 January 2019
In this paper, Nafion membrane was firstly modified by copper phthalocyanine tetrasulfonic acid tetrasodium salt (CuTSPc) to prepare the Nafion/CuTSPc-x composite membranes. FTIR, XRD and SEM results revealed the successful incorporation of CuTSPc into Nafion and good compatibility between the two composites. The proton conductivities of the Nafion/CuTSPc-x composite membranes were evidently higher than pure cast Nafion membrane, and increased with CuTSPc contents. Among them, the Nafion/ CuTSPc-6% membrane with the highest ion exchange capacity (1.14 mequivg1) exhibited the highest proton conductivity of 0.084 S cm1 at 30 C and 0.131 S cm1 at 80 C, respectively. When fabricated of a membrane electrode assembly (MEA), the Nafion/CuTSPc-4.5% membrane displayed an initial fuel cell performance with a power density of 43.3 mW cm2 at room temperature, close to that for pure cast Nafion membrane. Benefiting from the compact structure, high proton conductivity and outstanding stability, the Nafion/CuTSPc-x composite membranes show promising potentials for fuel cell applications. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Proton exchange membrane Proton conductivity Stability Fuel cell
1. Introduction Fuel cells are considered to be popular energy conversion systems that relies on renewable energy to produce power and convert chemical energy into electrical energy [1]. Proton exchange membrane (PEM) fuel cells have attracted considerable attention owing to their high efficiency, environmental compatibility, potentially lower cost for stationary and portable power applications [2,3]. Proton exchange membrane, which separates two electrodes and transports protons, is one of key materials in fuel cell system. Presently, perfluorinated sulfonic acid membranes have been widely used and investigated because of their superior mechanical
* Corresponding author. ** Corresponding author. College of Environmental Science and Engineering, Donghua University, 2999 Ren’min North Road, Shanghai 201620, China. E-mail addresses: [email protected] (Q. Gong), [email protected] (B. Tian), [email protected] (J. Qiao). Peer review under responsibility of The Chinese Ceramic Society.
strength, high proton conductivity and superior stability [4e7]. However, it also exhibits many drawbacks that restricted practical applications in a large-scale, such as high cost, high fuel cross over and significant loss of proton conductivity at high temperature [8e10]. Two different pathways exist to overcome these troubles: the first is the development of suitable proton-exchange membranes; the second is the modification of the perfluorinated sulfonic membrane. Developing modified Nafion membranes that containing multiple components is an efficient strategy to enhance the proton conductivity at high temperature and other properties. One or more species have to be employed for the Nafion modification via coating, blending and grafting with selected polymers [11e15]. In addition, several polymers have also been successfully introduced, such as polysilsesquioxane (POSS) [16], polyphenyl sulfone (PPSU) [17] and poly ether sulfone (PES) [18]. Transition metalphthalocyanine (MPc) is an interesting kind of compound with a conjugated ligand that includes eight N atoms in a unit structure. A delocalized conjugated p-bond in the molecule makes MPc liable to form a chelate structure with charged groups
https://doi.org/10.1016/j.jmat.2019.01.006 2352-8478/© 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
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(sulfonic acid) tailed on Nafion backbone. Therefore, it is expected that both the thermal stability and mechanical strength could be improved. Specifically, the copper phthalocyanine tetrasulfonic acid tetrasodium salt (CuTSPc), with tetrasulfonic acid in its structure, is a good hydrophilic agent, thereby making the composite membrane more increased ion transmigration rates. The more charged groups can contribute to the ion-exchange capacities and further enhance the proton conductivity. In our previous work, CuTs and CuTsPc have been successfully used as efficient catalysts for oxygen reduction reaction [19,20], but what used for Nafion membrane modifications have not been reported. In this study, as a proof-of-concept application, through a simple casting method with Nafion solution, the (CuTSPc) firstly used as a promoter is demonstrated to be an effective strategy for fabrication of high-performing Nafion composite membranes, Nafion/CuTSPc-x. We used several instrumentation methods including X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM) and thermal gravimetric analysis (TGA) to characterize the structure and composition of the composite membranes, and thereby obtain insight into their proton conductivity and its dependence on the mass ratio of Nafion/CuTSPc in casting solution. The proton conduction mechanisms of the as-prepared membranes were analyzed on the basis of proton conducting dependence on temperature. The water uptake, thermal stability, oxidation stability and tensile strength were thoroughly studied. The membrane electrode assemblies (MEAs) with different compositions of Nafion/ CuTSPc-x were also fabricated and compared with pure cast Nafion membrane to evaluate the applicability of Nafion/CuTSPc-x in proton-exchange membrane fuel cells. 2. Experimental 2.1. Membrane preparation The membrans were prepared by a simple solution-casting method. A certain amount of D2020 Nafion solution was diluted with alcohol to form a 5 wt% solution. Then appropriate amounts of copper phthalocyanine tetrasulfonic acid tetrasodium salt (CuTSPc) were dissolved in the obtained Nafion solution under stirring condition. After stirring at room temperature for 12 h, the solution was cast on a glass plate and dried at 100 C for 3 h. The resulting composite membrane was peeled from the glass, and immersed in 1 M H2SO4 at room temperature for 24 h to thoroughly convert into acid form. The eSO-3 groups on the skeleton of CuTsPc were then combined with the eSO3H groups of Nafion to form a network structure (Fig. 1). All composite membranes were denoted as Nafion/CuTSPc-x (x ¼ 0%, 1.5%, 3%, 4.5%, 6%), where x indicates the mass content of CuTSPc in Nafion/CuTSPc-x composite membranes. 3. Results and discussion 3.1. Characterizations of pure cast Nafion and Nafion/CuTSPc-x composite membranes Fig. 2(a) shows the FTIR spectra, which was carried out to identify the changes of structure of the composite membranes compared with pure cast Nafon membrane. It is clearly that the peak at 970 cm1 for pure cast Nafion is due to the CeOeC stretching. The peak at 1057 cm1 is attributed to the SeO stretching vibration of eSO3H groups, while two strong peaks appeared at 1149 cm1 and 1205 cm1 are due to Nafion backbone's CeF symmetric and antisymmetric stretching vibration [21]. On the other hand, the synthesized membranes also have these characteristic peaks. However, the intensities of peak at 1057 cm1 were all increased due to the increase in concentration of eSO3H groups, which confirms the
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successful incorporation of CuTSPc in Nafion. The X-ray diffraction measurements were conducted to examine the degree of crystallinity of the membranes. Fig. 2(b) shows the XRD spectra of Nafion and Nafion/CuTSPc-3% membrane. From the figure, it can be clearly observed that for the pure cast Nafion membrane, a sharp 2q peak appeared at 17, which is assigned to the amorphous scattering at 16 and the crystalline scattering at 17.5 from the polyfluorocarbon chains, and at the same time there is a wide peak at 38 e39 [22e24]. The XRD pattern of the Nafion/CuTSPc-3% composite membrane matches well with Nafion, indicating that Nafion/ CuTSPc-x composite membranes still maintained the basic framework of Nafion. In addition, the peak at 2q ¼ 10e20 of Nafion/ CuTSPc-3% membrane become narrower, implying that composite membranes have a higher crystallinity than pure cast Nafion membrane, which leads to an improvement on proton conductivity [25]. The SEM images of pure cast Nafion membrane and Nafion/ CuTSPc-3% composite membrane cross section are shown in Fig. 2(c) and (d) and Figure S1. It can be observed that pure cast Nafon membrane exhibits a smooth and a dense characteristic on the interface. After incorporation of CuTSPc, the composite membrane results in surface roughness (Fig. 2(d)), but no obvious surface degradation or phase separation phenomena was observed when compared to pristine Nafion membrane. It indicates that there is an excellent mutual compatibility between CuTSPc and Nafion matrix. 3.2. Properties of pure cast Nafion and Nafion/CuTSPc-x composite membranes In a polymer structure, the concentration of the fixed charged (SO 3 ) sites are critical to ensure high proton conductivity [26]. From Fig. 3(a), it can be clearly observed that the proton conductivity increase with the CuTSPc contents in the composite membranes. Moreover, the proton conductivities of all the investigated composite membranes were superior to the pure cast Nafion membrane, where the Nafion/CuTSPc-6% membrane exhibited the highest proton conductivity reaching to 0.084 S cm1. In addition, as carriers in proton transport, the water molecules forming and hydrogen bonds breaking through vehicular mechanism [27] are remarkably affect the proton transport. However, excessive water uptake would make the dimensional stability variation, thus leads to the loss of mechanical strength of PEMs [28]. Therefore, PEMs with appropriate water uptake is one of the critical factors for the application of fuel cell. As shown in Figure S2(a), when temperature increased from 25 C to 80 C, the water uptake of pure cast Nafion increases from 25.85% at 25 C to 48% at 80 C. For Nafion/CuTSPc-x composite membranes, a similar tendency of water uptake with the temperature were observed, nevertheless, the water uptake values are particularly higher than that of pure cast Nafion at 60 C and 80 C. This result was possibly due to the fact that the formation degree of phase separation becomes clear at higher hydrophilic sulfonic acid contents and high temperature. As a key parameter to proton exchange membrane, the proton conductivities of Nafion/CuTSPc-x composite membranes were measured as a function of temperature and the results are displayed in Fig. 3(b). As shown in Fig. 3(b), the proton conductivities of all these membranes showed an increasing tendency with temperature increased. As listed in Table 1, with the ion exchange capacity (IEC) values increased from 0.81 to 1.14 mequiv$g1, the proton conductivity increases from 0.066 to 0.084 S cm1 at 30 C and from 0.096 to 0.131 S cm1 at 80 C. All these values are higher than those of pure cast Nafion membrane, which are suitable for practical applications in fuel cells. The activation energy (Ea) was calculated using the Arrhenius equation and listed in Table 1. Interestingly, the Nafion/CuTSPc-x composite membrane showed the lower Ea than pure cast Nafion memrbane, indicating that the
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Fig. 1. Schematic diagram of the Nafion/CuTSPc-x membranes.
Fig. 2. (a) FTIR spectras of pure cast Nafion and Nafion/CuTSPc-3% membrane. (b) XRD photographs of pure cast Nafion and Nafion/CuTSPc-3% membrane. (c) Cross-section SEM image of Nafion. (d) Cross-section SEM image of Nafion/CuTSPc-3% membrane.
proton mobility of Nafion/CuTSPc-x composite membranes were higher than Nafion [29], which is mainly through a hopping mechanism for proton transportation [30,31]. The TGA measurements of pure cast Nafion and Nafion/CuTSPcx composite membranes are needed to evaluate the potential to be used for fuel cell. As shown in Fig. 3(c), all membranes display excellent thermal stability as the temperature is reduced below 300 C. The slight degradation from 100 to 300 C is attributed to the loss of water in the membrane. Moreover, both the pure cast Nafion memrbrane and Nafion/CuTSPc-x composite membranes display two decomposition stages. The first stage from 300 to 400 C is due to the decomposition of side-chain sulfonic acid
groups. The second stage from 400 to 500 C is assigned to the decomposition of the polymer main chain. It is noteworthy that composite membranes showed lower weight drops than pristine Nafion, particular to temperature beyond 500 C, demonstrating that the addition of CuTSPc effectively improved the thermal stability of Nafion. Fig. 3(d) shows the oxidative durability in 30% H2O2 solution of the Nafion/CuTSPc-x composite membranes. It was observed that the weight of Nafion/CuTSPc-3% membrane tends to maintain 90% and with no further weight losses again up to 168 h. As a reference, the pure cast Nafion was tested under the same measuring conditions. Encouragingly, the Nafion/CuTSPc-x composite membranes
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Fig. 3. (a) Proton conductivity and water uptake of pure cast Nafion and Nafion/CuTSPc-x membranes. (b) Temperature dependence of proton conductivity of pure cast Nafion and Nafion/CuTSPc-x membranes. (c) The TGA curves of pure cast Nafion and Nafion/CuTSPc-x membranes. (d) Weight loss of pure cast Nafion and Nafion/CuTSPc-3% membrane in 30% H2O2 solution.
Table 1 IECs, Proton Conductivity (s), and Activation Energy (Ea) of pure cast Nafion and Nafion/CuTSPc-x membranes. Samples
Pure cast Nafion Nafion/CuTSPc-1.5% Nafion/CuTSPc-3% Nafion/CuTSPc-4.5% Nafion/CuTSPc-6%
IEC (mequiv$g1)
0.78 0.81 1.01 1.08 1.14
s (S cm1) 30 C
80 C
0.055 0.066 0.078 0.075 0.084
0.104 0.096 0.117 0.121 0.131
Ea (kJ mol1)
Nafion/CuTSPc-x composite membranes are presented in Fig. 4(b) and Figure S3. The Nafion/CuTSPc-4.5% membrane possessed an open circuit voltage of 0.55 V, and an initial power density around 43.3 mW cm2 at room temperature, which approach to that of pure
11.33 6.33 6.10 8.8 6.94
show superior oxidative stability than cast Nafion membrane. Therefore, the Nafion/CuTSPc-x composite membranes are promising oxidation stability for fuel cell applications. Further as shown in Table S1, the Nafion/CuTSPc-x composite membranes exhibited excellent tensile strength, which are much higher than that of pure cast Nafion. Specifically, the Nafion/CuTSPc-1.5% and Nafion/ CuTSPc-3% membranes still possessed the higher elongation at break 10.83% and 28.62%, respectively, indicating there exists a premium mass ratio range for Nafion/CuTSPc-x composition membrane preparation. All these results indicated that the introduction of CuTSPc could enhance the mechanical properties of Nafion. 3.3. Single cell performance Membrane electrode assembly (MEA) is the central core of the fuel cell. In this part, electrochemical reaction will be done. Hydrogen in anode electrode changes into a hydrogen ion. The ions move towards the cathode through the proton-exchange membrane and react with the reduced oxygen at the cathode surface to generate water. A schematic representation of the PEM fuel cell is shown in Fig. 4(a). To evaluate the practical application of Nafion/ CuTSPc-x composite membranes in fuel cell systems, the polarization and power density curves of pure cast Nafion membrane and
Fig. 4. (a) Schematic design of the PEM fuel cell. (b) Polarization and power density curves of pure cast Nafion and Nafion/CuTSPc-4.5% membrane at room temperature.
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cast Nafion membrane with an open circuit voltage of 0.68 V, and a maximum power density of 61.3 mW cm2. The initial performance is still low, it may be due to the high contact resistances caused by the poor compatibility between the composite membrane and the electrode. The MEA fabrication is a very complex process, and it will be influenced by many factors. At this stage, the MEA preparation conditions are not optimal. However, these results demonstrated that Nafion/CuTSPc-x composite membranes have the potential to be proton-exchange membrane materials for fuel cell applications. 4. Conclusions A series of novel sulfonated composite membranes were successfully fabricated by facile solution casting method. We characterized it with FTIR, XRD. The obtained results confirmed the successful incorporation of CuTSPc within the Nafion. SEM images showed that the CuTSPc have excellent mutual compatibility with Nafion. Moreover, because of the increase contents of hydrophilic sulfonic acid groups, the water uptake and proton conductivity of the composite membranes significantly increased. Especially, Nafion/CuTSPc-6% membrane exhibited the highest proton conductivities with 0.084 S cm1. Meanwhile, Nafion/CuTSPc-x composite membranes exhibited the proton conductivities with from 0.066 to 0.084 S cm1 at 30 C and from 0.096 to 0.131 S cm1 at 80 C, which were higher than those of pure cast Nafion. The single cell performance test showed that the Nafion/CuTSPc-4.5% membrane possessed an open circuit voltage of 0.55 V, and power density around 43.3 mW cm2 at room temperature. In short, the membrane exhibited high proton conductivity, excellent stability and could be the promising materials for fuel cell applications. Acknowledgements This work is financially supported by the National Key R&D Program of China (2017YFB0102900) and National Natural Science Foundation of China (U1510120). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmat.2019.01.006. References [1] Lemmon JP. Energy: reimagine fuel cells. Nature 2015;525:447. [2] Park CH, Lee CH, Guiver MD, Lee YM. Sulfonated hydrocarbon membranes for medium-temperature and low-humidity proton exchange membrane fuel cells (PEMFCs). Prog Polym Sci 2011;36:1443e98. [3] Wu L, Zhang Z, Ran J, Zhou D, Li C, Xu T. Advances in proton-exchange membranes for fuel cells: an overview on proton conductive channels (PCCs). Phys Chem Chem Phys 2013;15:4870e87. [4] Schmidt-Rohr K, Chen Q. Parallel cylindrical water nanochannels in Nafion fuel-cell membranes. Nat Mater 2008;7:75e83. [5] Kreuer K-D, Portale G. A critical revision of the nano-morphology of proton conducting ionomers and polyelectrolytes for fuel cell applications. Adv Funct Mater 2013;23:5390e7. [6] Xiao L, Mischa Dr B, Dr PSH, Dr DKF. Nanoscale distribution of sulfonic acid groups determines structure and binding of water in nafion membranes. Angew Chem 2016;55:4011. [7] Yang L, Li H, Ai F, Chen X, Tang J, Zhu Y, Wang C, Yuan WZ, Zhang Y. A new method to prepare high performance perfluorinated sulfonic acid ionomer/ porous expanded polytetrafluoroethylene composite membranes based on perfluorinated sulfonyl fluoride polymer solution. J Power Sources 2013;243: 392e5. [8] Yoshimura K, Iwasaki K. Aromatic polymer with pendant perfluoroalkyl sulfonic acid for fuel cell applications. Macromolecules 2009;42:9302e6. [9] Wang L, Li K, Zhu G, Li J. Preparation and properties of highly branched sulfonated poly(ether ether ketone)s doped with antioxidant 1010 as proton exchange membranes. J Membr Sci 2011;379:440e8. [10] Xie H, Tao D, Ni J, Xiang X, Gao C, Wang L. Synthesis and properties of highly branched star-shaped sulfonated block polymers with sulfoalkyl pendant
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Yanan Wei received her B.S. degree from Henan University of Urban Construction in 2016. She is currently pursuing her M.S. degree in the School of Donghua University of Environmental Science and Engineering, China, under the supervision of Prof. Jinli Qiao. Her research focuses on the alkaline-exchange polymer membrane electrolyte and their applications on all-solid-state electrochemical devices.
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Jiawen Liu is a high school student in Shanghai Jinyuan Senior High School. She is currently pursuing her research subject under the supervision of Prof. Qiaojuan Gong and Jinli Qiao in Donghua University. Her research focuses on the membrane/catalyst preparation and their applications on energy conversion devices.
Binglun Tian received his master degree from Chinese acad of science, Dalian Institute of Chemical Physics in 2000. He focus on High-performing air-cooling selfhumidified PEM fuel cell stacks for clean power applications about 12 years and 6 years experience on liquid cooled fuel cell systems which used for heavy truck applications. He has applied for about 60 patents, including 2 PCT international patents during this years. As a science and senior engineer, he has developed high performance fuel cell catalyst, membrane, carbon paper and bipolar plate to medium scale production.
Qiaojuan Gong received her Ph.D. degree in materialogy from Northwestern Polytechnical University in 2007. She is a professor of chemistry in the Department of Chemistry, Yuncheng University. Her research interest is functionalization nanocarbon materials and electrochemical biosensors.
Jinli Qiao received her Ph.D. degree in Electrochemistry from Yamaguchi University, Japan in 2004. Then she joined the National Institute of Advanced Industrial Science and Technology (AIST) as a research fellow. From 2008, she is a professor of the Clean Energy Automotive Engineering Center, Tongji University and College of Environmental Science and Engineering, Donghua University, Shanghai, China. Her research focuses on PEM fuel cells, metaleair batteries and CO2 electroreduction.