- Email: [email protected]
metal oxide nanoparticles hybrid proton exchange membranes
Journal of Membrane Science 347 (2010) 26–31
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Self-assembled Nafion® /metal oxide nanoparticles hybrid proton exchange membranes Ke Li a , Gongbo Ye a , Jingjing Pan a , Haining Zhang a,b,∗ , Mu Pan a,b a b
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China Key Laboratory of Fuel Cell Technology of Hubei Province, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 20 April 2009 Received in revised form 31 August 2009 Accepted 1 October 2009 Available online 9 October 2009 Keywords: Proton exchange membrane Self-assembly Metal oxide nanoparticles Sol–gel process Ionic conductivity
a b s t r a c t One of the very important barriers for proton exchange membrane fuel cells (PEMFCs) is the decrease in proton conductivity of membrane at elevated temperature and low relative humidity. In this study, inorganic–organic hybrid membranes have been developed in order to improve water retention and proton conductivity at elevated temperature. Using in situ self-assembly technique, the well-dispersed metal oxide nanoparticles (SiO2 , ZrO2 ) with diameters of ∼5 nm can be formed through sol–gel process in Nafion® dispersion. It was found that the doped metal oxide nanoparticles did not affect the crystallinity and structure of Nafion® in the membrane significantly. Compared to Nafion® membrane, hybrid membranes show better water retention properties and higher proton conductivity at relatively low relative humidity. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Proton exchange membrane fuel cells (PEMFCs) operating at elevated temperature have attracted considerable attention in recent years because they could benefit from enhanced tolerance to impurity of the fuel gas, simplified water and heat management, and increased reaction rates at both cathode and anode compared with PEMFCs operating below 80 ◦ C [1–6]. One of the great technical challenges in elevated temperature PEMFC systems is proton exchange membranes, which should maintain reasonable proton conductivity and good mechanical stability at relatively low relative humidity. The often used membranes for current PEMFC technology are perfluorosulfonic acid polymeric membranes, for example Nafion® membrane, because of its robust structure and excellent proton conductivity in the hydrated state [7,8]. However, the drastic decrease in the proton conductivity at low relative humidity limits the operating temperature of fuel cells using such membranes to below 80 ◦ C as water evaporates rapidly at higher temperature and ambient pressures. If the membrane keeps in hydrate state during the operation of PEMFCs at elevated temperature, the increased pressure required by the system could offset the benefits arisen from the high tolerance to impurity of fuel gas.
∗ Corresponding author at: State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China. Fax: + 86 27 8787 9468. E-mail address: [email protected] (H. Zhang). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.10.002
Hence, development of membranes working at elevated temperature and low relative humidity becomes an active area of research on PEMFCs. A number of efforts have been made on the search for membranes with reasonable proton conductivity at elevated temperature and low relative humidity. It has been demonstrated that modified Nafion® membrane by incorporating hygroscopic inorganic nanoparticles such as SiO2 , TiO2 , and ZrO2 can improve water retention ability and enhance proton conductivity [9–20]. For example, Adjeman et al. [11] investigated Nafion® 115/SiO2 composite membranes in hydrogen-oxygen PEMFCs from 80 ◦ C to 140 ◦ C with silicon oxide content less than 10% in weight. The silicon oxide improved the water retention of the composite membranes, leading to an increase in proton conductivity at elevated temperatures under ambient pressure. More recently, Pereira et al. [21] synthesized hybrid membranes of Nafion® and mesoporous silica containing sulfuric acid groups using sol–gel process. Compared to standard Nafion® 112 membrane, the hybrid membranes shows improved proton conductivity at 95 ◦ C and 120 ◦ C and over the whole range of relative humidity under ambient pressure. Although most of the work has concentrated on silica-based hybrid membranes, zirconia doped hybrid Nafion® membranes have shown more promising results based on accomplished work on high temperature membranes [13–15,22]. Jalani et al. [13] synthesized composite Nafion® membranes containing different inorganic nanoparticle fillers (ZrO2 , SiO2 , and TiO2 ) using in situ sol–gel process. It was found that composite membranes showed higher water uptake and proton conductivity than unmodified
K. Li et al. / Journal of Membrane Science 347 (2010) 26–31
27
2. Experimental
Fig. 1. Schematic protocol of the preparation of Nafion® –metal oxide nanocomposite membranes.
Nafion® membrane and other composite membranes at both 90 ◦ C and 120 ◦ C. Sacca et al. [15] re-casted composite Nafion® membranes by addition of commercial ZrO2 with different percentages and tested cell performance using such membranes in the temperature range of 80–130 ◦ C. The formed composite membranes showed higher water retention capacity and better single cell performance compared to referenced Nafion® 112 membrane. The membrane containing 10% ZrO2 in weight has supplied a power density of about 400 mW cm−2 at 130 ◦ C (85 RH%) with absolute pressure of 3.0 bar. It is generally believed that the improved proton conductivity of composite membranes is attributed to the surface hydroxide groups of additives which retain water and increase surface acidity at elevated high temperature [23,24]. The well developed hybrid membranes require the additives uniformly and well distributed in the membrane to minimize the interfacial resistance of resin and dopants. However, Nafion® /metal oxide composite membranes are usually prepared either by recasting of mixture of commercial available metal oxide nanoparticles and Nafion® ionomer [14,15], or by impregnation of Nafion® membranes with metal oxide precursor solution followed by in situ sol–gel reaction [13,22]. In the first approach, the doped large nanoparticles make the particle/Nafion® interface unsatisfactory as the mechanical properties of the membrane could be affected. In the second approach, hydrolysis and condensation reactions of metal alkoxides drive the distribution of metal oxide nanoparticles inside the membrane unevenly as described in the literature [22]. In addition, silica doped Nafion® membranes have also been fabricated using in situ formation of silica nanoparticles in Nafion® solution, followed by a solution-casting process in literature [19,21] where the authors focused on the performance of assembled cells with hybrid membranes. In the present work, Nafion® /metal oxide (ZrO2 , SiO2 ) composite membranes were formed by in situ hydrolysis of precursors in Nafion® solution through sol–gel process. The synthetic process was schematically described in Fig. 1. Nafion® molecules were expected to self-assemble onto the surface of formed metal oxide nanoparticles through electrostatic interactions and stabilize the initial formed metal oxide nanoparticles.
Nafion® solution (DE-520, EW1100) was purchased from DuPont Ind. Co., which contains 5 wt% of perfluorosulfonate resin (H+ form) and 95 wt% of isopropanol/water mixture (10:9 weight ratios). Tetrabutylzirconate and tetraethyl orthosilicate were received from Shanghai Reagent Co. Ltd. (China). Water was de-ionized through a Milli-Q system (Barnsted Nanopore, resistivity = 18.0 M cm−1 ). N-methyl-2-pyrrolidone (NMP) was dried over CaH2 and distilled under vacuum. All the other solvents and chemicals were of reagent grade and were used as received. Nafion® solutions used in the study were prepared by dissolving Nafion® resin in NMP, which was obtained by solvent evaporation of the purchased Nafion® solution under vacuum at 60 ◦ C. The preparation process of Nafion® –metal oxide hybrid membranes is schematically shown in Fig. 1. The desired quantity of precursor solutions was added dropwise to the Nafion® solution under vigorous stirring in an inert nitrogen atmosphere at 80 ◦ C. After desired amount of 2 M HCl solution was added, the mixture was continuously stirred for 1 h at 80 ◦ C and allowed to cool down to room temperature. After addition of desired amount of de-ionized water, the mixture was then continuously stirred for another 8 h to complete condensation of precursors and a clear sol containing hybrid Nafion® –metal oxide nanoparticles was obtained. Unless otherwise stated, the final concentration of Nafion® is about 2% in weight based on our previous results [25,26] and the content of metal oxide nanoparticles is about 5% in weight regarding to Nafion® . Hybrid membranes were prepared using a recasting process: the hybrid sol was first placed in a Petri dish, followed by the solvent evaporation at 100 ◦ C for 8 h and then heat-treated at 150 ◦ C under vacuum for 3 h. The formed membranes were than treated using a standard procedure at 80 ◦ C for 30 min in 5% H2 O2 solution, in deionized water, in 0.5 M H2 SO4 solution, and finally in de-ionized water again. For comparison, pure Nafion® membrane was prepared and treated using the same procedure without addition of precursors. Thickness of prepared membranes is about 78 ± 5 m. Zeta potential measurements were carried out on Zetasizer Nano-ZS (Malvern, UK) using laser Doppler velocimetry and phase analysis light scattering. The temperature of the scattering cell was 25 ◦ C and the data were analyzed with the software from supplier. The size of formed nanoparticles was examined using high resolution transmission electron microscopy (TEM, JEM-2010FEF equipped with Energy Dispersive Spectrum Analyzer). Samples for TEM measurements were prepared by directly placing a drop of the solution on a thin carbon film supported by a copper grid. Selective area electron diffraction measurement was carried out on the same instrument as well. X-ray diffraction (XRD) patterns for membranes were obtained on a D/MaxRB X-ray Diffractometer (Rigaku, Japan) using Cu K␣ radiation operating at 40 kV and 30 mA. The water uptake of membranes at different relative humidity was calculated as the ratio of the difference between the swollen and the dry weight of the membrane. The weight of swollen membrane was measured after keeping the membrane at 100 ◦ C under desired relative humidity for 10 h using a temperature and relative humidity-controllable oven (ZTH, Zuoke Equip. Co. Shanghai). For the weight of dry membrane, the measurement was made directly after drying the sample at 100 ◦ C for 2 h. The weight of the completely hydrated membrane was measured after boiling the membrane in de-ionized water for 2 h and wiping out the surface adsorbed water using tissue paper. Mechanical strength of membranes at dry state and fully hydrated state was measured using an Electromechanical Universal Testing Machine (WDW-1C) based on Chinese Standard QB-1302291. Samples were measured at a strain rate of 50 mm/min.
28
K. Li et al. / Journal of Membrane Science 347 (2010) 26–31
Fig. 2. Pictures of proton conductivity setup: the testing cell (left) and the whole setup (right).
Proton conductivity measurements were carried out in-plane using an impedance analyzer (Autolab PG30/FRA, Eco Chemie, Netherland) at different temperatures without external humidification using homemade conductivity testing cell (Fig. 2). Two copper sheets were placed onto the as-formed membranes (2.2 cm × 2.2 cm) in the same face. The distance between two copper electrodes was 2 cm. Membranes with two copper electrodes were sandwiched between two polytetrafluoroethylene sheets. Electrochemical impedance spectra were recorded in the frequency range of 1 Hz and 105 KHz and the signal amplitude of 10 mV. Membranes were first hydrated before assembling into the testing cell. The measurements were carried out from lower temperature to higher temperature. Unless otherwise stated, membranes were stabilized under each testing condition for 1 h before data were recorded. It should also be noted that Nafion® membranes are really isotropic according to the literature [27] and the through-plane conductivity of membrane is similar to in-plane conductivity.
3. Results and discussion Metal oxide nanoparticles were formed through in situ hydrolysis of precursors in Nafion® solution, as illustrated in Fig. 1. Since hydrolysis and polymerization of precursor occur rapidly in the presence of water, the used Nafion® solution for the in situ growth of metal oxide nanoparticles is required to be anhydrous before hydrolysis of precursors in order to have a better control of size distribution and morphology of the formed metal oxide nanoparticles. To minimize undesired water content in the system, Nafion® solution was heated up to 80 ◦ C under nitrogen atmosphere and the addition and hydrolysis of precursors were then performed under the same condition. Fig. 3 shows transmission electron microscopy images for Nafion® –silica and Nafion® –zirconia hybrid dispersions formed in 2% (weight percent) Nafion® dispersion in NMP. The final contents of doped metal oxides in designed membrane are 5% in weight. Samples for TEM measurement were prepared by directly plac-
Fig. 3. TEM micrographs of Nafion® –SiO2 (a) and Nafion® –ZrO2 (b) hybrid dispersions and energy dispersive spectra for one particles in TEM micrographs of Nafion® –SiO2 (c) and Nafion® –ZrO2 (d). The insert picture in (b) is selected area electron diffraction pattern.
K. Li et al. / Journal of Membrane Science 347 (2010) 26–31
29
Fig. 4. XRD patterns for nanocomposite membranes: Nafion® –zirconia (a), Nafion® –silica (b) and recast Nafion® membrane (c).
Fig. 5. Water uptake of formed membranes as a function of relative humidity at 100 ◦ C: Nafion® –zirconia membrane (triangles), Nafion® –silica membrane (circles), and recast Nafion® membrane (squares). Solid lines are guide to eyes.
ing a drop of the solution on a thin carbon film supported by a copper grid. It is evident that the in situ formed nanoparticles for both silica and zirconia were uniformly distributed in the dispersion and the average diameters of the formed nanoparticles were about 4.2 ± 0.5 nm and 6.3 ± 0.5 nm, respectively. Since pH value of the mixture is around 1.5, surfaces of formed such nanoparticles are positively charged as indicated by zeta potential values of 10.5 mV for silica and 16.0 mV for zirconia. Once metal oxide nanoparticles formed from the sol, Nafion® molecules adsorbed onto the surface through electrostatic interaction, leading to a reversed charge sign as indicated by zeta potential (−44 mV for silica system and −37 mV for zirconia system). Thus, the coverage of Nafion® molecules onto the surface prevents further growth of the particles. Energy dispersive spectrometry was measured based on one nanoparticle in TEM images and the recorded spectra were shown in Fig. 3c and d. The coexistence of the atom peaks for Si or Zr, C, F, and S suggests the coverage of Nafion® molecules on the formed metal oxide nanoparticles. In addition, a clear diffraction ring obtained from selected area electron diffraction was observed and shown in the insert figure in Fig. 3b, indicating that the formed zirconia nanoparticles are crystallized. The Nafion® –metal oxide nanocomposite membranes were formed using a recasting process and were treated using a standard process as described in Section 2. To determine whether the formed nanoparticles are in crystallized state in the formed hybrid membranes, X-ray diffraction (XRD) measurements of recast Nafion® membrane and hybrid membranes were performed and diffraction patterns were shown in Fig. 4. The Nafion® –silica membrane has similar XRD pattern (Fig. 4b) to the recast Nafion® membrane (Fig. 4c), indicating no crystallized silica particles formed. For Nafion® –zirconia composite membrane (Fig. 4a), crystallized zirconia nanoparticles were clearly observed. According to Ludvigsson et al. [28], diffraction maxima for the hybrid membrane at 2 of 17.48◦ and 39.33◦ , corresponding to d-spacing of 5.5 Å and 2.4 Å, were attributed to Teflon-like domains of Nafion® . Diffraction maxima at 28.9◦ (d-spacing of 3.3 Å) was attributed to crystallized ZrO2 nanoparticles in hybrid membrane. By comparison with standard XRD database (JCPDS data no. 37-1484), it was found that the crystallized ZrO2 nanoparticles in the hybrid membrane were in the monoclinic phase. The full width at half-maximum of the diffraction peak was used to estimate the particle size of ZrO2 using Debye–Sherrer formula. The ZrO2 particles are estimated to be 6.7 nm, which agrees well with the TEM investigation of the Nafion® –zirconia dispersion, indicating no further aggregation of zirconia occurred during the membrane formation process. It can also be clearly seen from XRD patterns that the diffraction maxima
of Teflon-like domains for three membranes are at the same scattering angles, indicating that the addition of metal oxide nanoparticles did not affect the crystallinity and structure of Nafion® in the membrane significantly. Because water is proton transporting medium for proton exchange membranes, the water retention ability is one of the important parameters for fuel cell applications. At room temperature, the fully hydrated Nafion® –silica and Nafion® –zirconia hybrid membranes contained 26.6% and 34.2% water in weight, which is much higher than that of fully hydrated recast Nafion® membrane (13.7% in weight). As the main idea of this work is to seek for membranes working at elevated temperature and low relative humidity, we investigated the water retention ability of hybrid membranes under different relative humidity at 100 ◦ C, shown in Fig. 5. As comparison, the water uptake of recast reference Nafion® membrane as a function of relative humidity at 100 ◦ C was plotted in the same figure. It can be clearly seen that the formed composite membranes take up more water than recast Nafion® membrane in the measured range of relative humidity. The composite membranes just show a slightly higher water uptake than reference Nafion® membrane at low relative humidity (less than 20%) at 100 ◦ C. However, when the relative humidity is higher than 40%, the composite membrane takes up about 2–3 times more water than that in the recast Nafion® membrane at 100 ◦ C. The enhancement of water uptake of Nafion® –metal oxide nanocomposite membranes at 100 ◦ C shows potentials of the hybrid membrane for elevated temperature PEMFC applications. Since the mechanical strength strongly affects the durability of the membrane, the mechanical strengths of the formed composite membranes at different states were measured. Fig. 6 shows the tensile strength of formed hybrid membranes with 5% elongation at both fully hydrated state and dry state. As comparison, the tensile strength of recast Nafion® membrane was recorded in the same figure. It can be clearly seen that the tensile strength for composite membranes with 5% elongation is lower than that of recast Nafion® membrane at both dry and fully hydrated states. This behavior is probably caused by the increased interfacial interaction of Nafion® and doped nanoparticles, as discussed in the literature as well [29]. However, for Nafion® –ZrO2 composite membrane, the mechanical strength is just slightly lower than that of recast membrane and the difference in tensile strength at 5% elongation for fully hydrated state and dry state is even smaller than that of recast Nafion® membrane, indicating the similar durability of the composite membrane with Nafion® membrane. Another very important parameter for proton exchange membranes is the proton conductivity, which decides the performance
30
K. Li et al. / Journal of Membrane Science 347 (2010) 26–31
Fig. 6. Tensile strength of formed membranes with 5% elongation at different humidification states: fully hydrated (black) and dry state (gray).
Fig. 8 shows the proton conductivities of formed membranes at different temperatures without external humidification, as described in Section 2. It is evident, that the proton conductivity of Nafion® –metal oxide nanocomposite membranes is higher than that of recast Nafion® membrane in the measured temperature range, attributed to the enhanced water retention property for metal oxide nanoparticles doped membranes. With the increase in temperature (higher than 80 ◦ C), the proton conductivities for all the measured membranes decrease dramatically. However, the composite membranes, especially ZrO2 doped membrane, still show better proton conductivity. This behavior can be attributed to the water entrapped into the membrane that dehydrates with the increase of temperature, indicating higher water retention ability of composite membranes. Even when the temperature reaches 100 ◦ C, the proton conductivity of ZrO2 doped membranes is close to 0.01 S cm−1 without external humidification. Jalani et al. [13] reported similar result that the proton conductivity of zirconia doped Nafion® membrane is close to 0.01 S cm−1 at 20 RH% under 90 ◦ C, attributed to the higher water uptake and enhanced acidity compared to Nafion® membrane. The composite membranes were formed in their work by impregnation of Nafion® membranes with metal oxide precursor solution followed by in situ sol–gel reaction. However, the distribution of zirconia nanoparticles formed using our technique seems more homogenous than the literature work [13], leading to similar proton conductivity at higher testing temperature. Thus, the ZrO2 doped Nafion® membranes can be potentially used in the elevated temperature PEMFCs. The fuel cells performance using metal oxide doped membranes is still under investigation. 4. Conclusion
®
◦
Fig. 7. Proton conductivity of Nafion –zirconia membrane at 100 C without external humidification as a function of time. The zirconia content is 5% in weight.
of the assembled fuel cells. In order to determine whether the 1 h stabilization time is enough or not, the hydrated Nafion® –zirconia membrane with zirconia content of 5% in weight was assembled into the testing cell. The testing cell was heated to 100 ◦ C and impedance spectra were recorded every 15 min. The calculated proton conductivity of the membrane as a function of stabilization time is shown in Fig. 7. It can be clearly seen that the proton conductivity decreases dramatically in first 30 min, attributed to the rapid loss of water inside the membrane. It, however, almost remains constant 30 min later, indicating that the stabilization time of 1 h should be enough.
In this communication, Nafion® –metal oxide (SiO2 , ZrO2 ) nanocomposite membranes were formed through in situ sol–gel process. Metal oxide nanoparticles were in situ synthesized in Nafion® solution by hydrolysis and condensation of precursors. The existing Nafion® molecules can be self-assembled onto metal oxide particles through electrostatic interactions and prevent the further growth of the initial formed metal oxide nanoparticles. Using this technique, the synthesized metal oxide nanoparticles were evenly distributed inside the membrane with diameters of ∼5 nm and the well-dispersed metal oxide nanoparticles did not affect the crystallinity and structure of Nafion® in the membrane significantly. The formed Nafion® –metal oxide nanocomposite membranes show enhanced water retention ability and higher proton conductivity compared to recast pure Nafion® membrane at all measured temperature range. Although the proton conductivity of the composite membrane decreases with increasing temperature, the proton conductivity for composite membrane, especially for Nafion® –ZrO2 composite membrane is close to 0.01 S cm−1 at 100 ◦ C without external humidification, which is important for fuel cell applications. Thus, the hybrid membrane developed here has the potential for PEMFC applications at elevated temperatures. Acknowledgements This work was supported by the Foundation of National Natural Science of China (50632050 and 20806061) and the National High Technology Research and Development Program (“863” Program) of China (2008AA050403). The authors thank Mr. Yueqing Li for the technical help on conductivity testing cells. References
Fig. 8. Proton conductivities of the formed membranes as a function of temperature without external humidification: Nafion® –zirconia membrane (triangles), Nafion® –silica membrane (circles), and recast Nafion® membrane (squares). Solid lines are guide to eyes.
[1] Q. Li, R. He, J.O. Jensen, N.J. Bjerrum, Approaches and recent development of polymer electrolyte membranes for fuel cells operating above 100 ◦ C, Chem. Mater. 15 (2003) 4896–4915.
K. Li et al. / Journal of Membrane Science 347 (2010) 26–31 [2] G. Alberti, M. Casciola, Composite membranes for medium-temperature PEM fuel cells, Annu. Rev. Mater. Res. 33 (2003) 129–154. [3] P. Jannasch, Recent developments in high-temperature proton conducting polymer electrolyte membranes, Curr. Opin. Coll. Interf. Sci. 8 (2003) 96–102. [4] W.H.J. Hogarth, J.C. Diniz da Costa, G.Q. Liu, Solid acid membranes for high temperature (>140 ◦ C) proton exchange membrane fuel cells, J. Power Sources 142 (2005) 223–237. [5] J. Zhang, Z. Xie, J. Zhang, Y. Tang, C. Song, T. Navessin, Z. Shi, D. Song, H. Wang, D.P. Wilkinson, Z. Liu, S. Holdcroft, High temperature PEM fuel cells, J. Power Sources 160 (2006) 872–891. [6] N.L. Garland, J.P. Kopasz, The United States Department of Energy’s high temperature and low relative humidity membrane program, J. Power Sources 172 (2007) 94–99. [7] Fuel Cell Handbook, 7th ed., EG&G Technical Services Inc., West Virginia, US Department of Energy, 2004. [8] V.M. Vishnyakov, Proton exchange membrane fuel cells, Vacuum 80 (2006) 1053–1065. [9] P.L. Antonucci, A.S. Arico, P. Creti, E. Ramunni, V. Antonucci, Investigation of a direct methanol fuel cell based on a composite Nafion® –silica electrolyte for high temperature operation, Solid State Ionics 125 (1999) 431–437. [10] N. Miyake, J.S. Wainright, R.F. Savinell, valuation of a sol–gel derived Nafion® /silica hybrid membrane for proton electrolyte membrane fuel cell applications—I. Proton conductivity and water content, J. Electrochem. Soc. 148 (2001) A898–A904. [11] K.T. Adjeman, S.J. Lee, S. Srinivasan, J. Benziger, A.B. Bocarsly, Silicon oxide Nafion® composite membranes for proton exchange membrane fuel cell operation at 80–140 ◦ C, J. Electrochem. Soc. 149 (2002) A256–A261. [12] M. Ramani, H.R. Kunz, J.M. Fenton, Investigation of Nafion® /HPA composite membranes for high temperature/low relative humidity PEMFC operation, J. Membr. Sci. 232 (2004) 31–44. [13] N.H. Jalani, K. Dunn, R. Datta, Synthesis and characterization of Nafion® –MO2 (M = Zr, Si, Ti) nanocomposite membranes for high temperature PEM fuel cells, Electrochim. Acta 51 (2005) 553–560. [14] K.T. Adjemian, R. Dominey, L. Krishnan, H. Ota, P. Majsztrik, T. Zhang, J. Mann, B. Kirby, L. Gatto, M. Velo-Simpson, J. Leahy, S. Srinivasan, J.B. Benziger, A.B. Bocarsly, Function and characterization of metal oxide–Nafion® composite membranes for elevated temperature H2 /O2 PEM fuel cells, Chem. Mater. 18 (2006) 2238–2248. [15] A. Sacca, I. Gatto, A. Carbone, R. Pedicini, E. Passalacqua, ZrO2 –Nafion® composite membranes for polymer electrolyte fuel cells (PEFCs) at intermediate temperature, J. Power Sources 163 (2006) 47–51.
31
[16] V. Di Noto, R. Gliubizzi, E. Negro, G. Pace, Effect of SiO2 on relaxation phenomena and mechanism of in conductivity of [Nafion® /(SiO2 )x ] composite membranes, J. Phys. Chem. B 110 (2006) 24972–24986. [17] Z.G. Shao, H.F. Xu, M.Q. Li, I.M. Hsing, Nafion® /inorganic oxides membrane doped with heteropolyacids for high temperature operation of proton exchange membrane fuel cell, Solid State Ionics 177 (2006) 779–785. [18] Y.F. Zhai, H.M. Zhang, J.W. Hu, B.L. Yi, Preparation and characterization of sulfated zirconia (SO4 2− /ZrO2 )/Nafion® composite membranes for PEMFC operation at high temperature/low humidity, J. Membr. Sci. 280 (2006) 148–155. [19] H.L. Tang, Z.H. Wan, M. Pan, S.P. Jiang, Self-assembled Nafion® –silica nanoparticles for elevated high-temperature polymer electrolyte membrane fuel cells, Electrochem. Commun. 9 (2007) 2003–2008. [20] K.T. Park, U.H. Jung, D.W. Choi, K. Chun, H.M. Lee, S.H. Kim, ZrO2 /SiO2 //Nafion composite membrane for polymer electrolyte membrane fuel cells operation at high temperature and low humidity, J. Power Sources 177 (2008) 247–253. [21] F. Pereira, K. Valle, P. Belleville, A. Morin, S. Lambert, C. Sanchez, Advanced mesostructured hybrid silica–Nafion® membranes for high-performance PEM fuel cell, Chem. Mater. 20 (2008) 1710–1718. [22] W. Apichatachutapan, R.B. Moore, K.A. Mauritz, Asymmetric Nafion® /(zirconium oxide) hybrid membranes via in situ sol–gel chemistry, J. Appl. Polym. Sci. 62 (1996) 417–426. [23] K.A. Mauritz, J.T. Payne, Perfluorosulfonate ionomer/silicate hybrid membranes via base-catalyzed in situ sol–gel processes for tetraethylorthosilicate, J. Membr. Sci. 168 (2000) 39–51. [24] G. Alberti, M. Casciola, Solid state protonic conductors, present main applications and future prospects, Solid State Ionics 145 (2001) 3–16. [25] J.J Pan, H.N. Zhang, M. Pan, Self-assembly of Nafion® molecules onto silica nanoparticles formed in situ through sol–gel process, J. Colloid Interf. Sci. 326 (2008) 55–60. [26] H.N. Zhang, J.J. Pan, X.C. He, M. Pan, Zeta potential of Nafion® molecules in isopropanol–water mixture solvent, J. Appl. Polym. Sci. 107 (2008) 3306–3309. [27] R.F. Silva, M. De Francesco, A. Pozio, Tangential and normal conductivities of Nafion® membranes used in polymer electrolyte fuel cells, J. Power Sources 134 (2004) 18–26. [28] M. Ludvigsson, J. Lindgren, J. Tegenfeldt, Crystallinity in cast Nafion® , J. Electrochem. Soc. 147 (2000) 1303–1305. [29] H.L. Tang, M. Pan, Synthesis and characterization of a self-assembled Nafion® /silica nanocomposite membrane for polymer electrolyte fuel cells, J. Phys. Chem. C 112 (2008) 11556–11568.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Self-assembled Nafion® /metal oxide nanoparticles hybrid proton exchange membranes Ke Li a , Gongbo Ye a , Jingjing Pan a , Haining Zhang a,b,∗ , Mu Pan a,b a b
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China Key Laboratory of Fuel Cell Technology of Hubei Province, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 20 April 2009 Received in revised form 31 August 2009 Accepted 1 October 2009 Available online 9 October 2009 Keywords: Proton exchange membrane Self-assembly Metal oxide nanoparticles Sol–gel process Ionic conductivity
a b s t r a c t One of the very important barriers for proton exchange membrane fuel cells (PEMFCs) is the decrease in proton conductivity of membrane at elevated temperature and low relative humidity. In this study, inorganic–organic hybrid membranes have been developed in order to improve water retention and proton conductivity at elevated temperature. Using in situ self-assembly technique, the well-dispersed metal oxide nanoparticles (SiO2 , ZrO2 ) with diameters of ∼5 nm can be formed through sol–gel process in Nafion® dispersion. It was found that the doped metal oxide nanoparticles did not affect the crystallinity and structure of Nafion® in the membrane significantly. Compared to Nafion® membrane, hybrid membranes show better water retention properties and higher proton conductivity at relatively low relative humidity. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Proton exchange membrane fuel cells (PEMFCs) operating at elevated temperature have attracted considerable attention in recent years because they could benefit from enhanced tolerance to impurity of the fuel gas, simplified water and heat management, and increased reaction rates at both cathode and anode compared with PEMFCs operating below 80 ◦ C [1–6]. One of the great technical challenges in elevated temperature PEMFC systems is proton exchange membranes, which should maintain reasonable proton conductivity and good mechanical stability at relatively low relative humidity. The often used membranes for current PEMFC technology are perfluorosulfonic acid polymeric membranes, for example Nafion® membrane, because of its robust structure and excellent proton conductivity in the hydrated state [7,8]. However, the drastic decrease in the proton conductivity at low relative humidity limits the operating temperature of fuel cells using such membranes to below 80 ◦ C as water evaporates rapidly at higher temperature and ambient pressures. If the membrane keeps in hydrate state during the operation of PEMFCs at elevated temperature, the increased pressure required by the system could offset the benefits arisen from the high tolerance to impurity of fuel gas.
∗ Corresponding author at: State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China. Fax: + 86 27 8787 9468. E-mail address: [email protected] (H. Zhang). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.10.002
Hence, development of membranes working at elevated temperature and low relative humidity becomes an active area of research on PEMFCs. A number of efforts have been made on the search for membranes with reasonable proton conductivity at elevated temperature and low relative humidity. It has been demonstrated that modified Nafion® membrane by incorporating hygroscopic inorganic nanoparticles such as SiO2 , TiO2 , and ZrO2 can improve water retention ability and enhance proton conductivity [9–20]. For example, Adjeman et al. [11] investigated Nafion® 115/SiO2 composite membranes in hydrogen-oxygen PEMFCs from 80 ◦ C to 140 ◦ C with silicon oxide content less than 10% in weight. The silicon oxide improved the water retention of the composite membranes, leading to an increase in proton conductivity at elevated temperatures under ambient pressure. More recently, Pereira et al. [21] synthesized hybrid membranes of Nafion® and mesoporous silica containing sulfuric acid groups using sol–gel process. Compared to standard Nafion® 112 membrane, the hybrid membranes shows improved proton conductivity at 95 ◦ C and 120 ◦ C and over the whole range of relative humidity under ambient pressure. Although most of the work has concentrated on silica-based hybrid membranes, zirconia doped hybrid Nafion® membranes have shown more promising results based on accomplished work on high temperature membranes [13–15,22]. Jalani et al. [13] synthesized composite Nafion® membranes containing different inorganic nanoparticle fillers (ZrO2 , SiO2 , and TiO2 ) using in situ sol–gel process. It was found that composite membranes showed higher water uptake and proton conductivity than unmodified
K. Li et al. / Journal of Membrane Science 347 (2010) 26–31
27
2. Experimental
Fig. 1. Schematic protocol of the preparation of Nafion® –metal oxide nanocomposite membranes.
Nafion® membrane and other composite membranes at both 90 ◦ C and 120 ◦ C. Sacca et al. [15] re-casted composite Nafion® membranes by addition of commercial ZrO2 with different percentages and tested cell performance using such membranes in the temperature range of 80–130 ◦ C. The formed composite membranes showed higher water retention capacity and better single cell performance compared to referenced Nafion® 112 membrane. The membrane containing 10% ZrO2 in weight has supplied a power density of about 400 mW cm−2 at 130 ◦ C (85 RH%) with absolute pressure of 3.0 bar. It is generally believed that the improved proton conductivity of composite membranes is attributed to the surface hydroxide groups of additives which retain water and increase surface acidity at elevated high temperature [23,24]. The well developed hybrid membranes require the additives uniformly and well distributed in the membrane to minimize the interfacial resistance of resin and dopants. However, Nafion® /metal oxide composite membranes are usually prepared either by recasting of mixture of commercial available metal oxide nanoparticles and Nafion® ionomer [14,15], or by impregnation of Nafion® membranes with metal oxide precursor solution followed by in situ sol–gel reaction [13,22]. In the first approach, the doped large nanoparticles make the particle/Nafion® interface unsatisfactory as the mechanical properties of the membrane could be affected. In the second approach, hydrolysis and condensation reactions of metal alkoxides drive the distribution of metal oxide nanoparticles inside the membrane unevenly as described in the literature [22]. In addition, silica doped Nafion® membranes have also been fabricated using in situ formation of silica nanoparticles in Nafion® solution, followed by a solution-casting process in literature [19,21] where the authors focused on the performance of assembled cells with hybrid membranes. In the present work, Nafion® /metal oxide (ZrO2 , SiO2 ) composite membranes were formed by in situ hydrolysis of precursors in Nafion® solution through sol–gel process. The synthetic process was schematically described in Fig. 1. Nafion® molecules were expected to self-assemble onto the surface of formed metal oxide nanoparticles through electrostatic interactions and stabilize the initial formed metal oxide nanoparticles.
Nafion® solution (DE-520, EW1100) was purchased from DuPont Ind. Co., which contains 5 wt% of perfluorosulfonate resin (H+ form) and 95 wt% of isopropanol/water mixture (10:9 weight ratios). Tetrabutylzirconate and tetraethyl orthosilicate were received from Shanghai Reagent Co. Ltd. (China). Water was de-ionized through a Milli-Q system (Barnsted Nanopore, resistivity = 18.0 M cm−1 ). N-methyl-2-pyrrolidone (NMP) was dried over CaH2 and distilled under vacuum. All the other solvents and chemicals were of reagent grade and were used as received. Nafion® solutions used in the study were prepared by dissolving Nafion® resin in NMP, which was obtained by solvent evaporation of the purchased Nafion® solution under vacuum at 60 ◦ C. The preparation process of Nafion® –metal oxide hybrid membranes is schematically shown in Fig. 1. The desired quantity of precursor solutions was added dropwise to the Nafion® solution under vigorous stirring in an inert nitrogen atmosphere at 80 ◦ C. After desired amount of 2 M HCl solution was added, the mixture was continuously stirred for 1 h at 80 ◦ C and allowed to cool down to room temperature. After addition of desired amount of de-ionized water, the mixture was then continuously stirred for another 8 h to complete condensation of precursors and a clear sol containing hybrid Nafion® –metal oxide nanoparticles was obtained. Unless otherwise stated, the final concentration of Nafion® is about 2% in weight based on our previous results [25,26] and the content of metal oxide nanoparticles is about 5% in weight regarding to Nafion® . Hybrid membranes were prepared using a recasting process: the hybrid sol was first placed in a Petri dish, followed by the solvent evaporation at 100 ◦ C for 8 h and then heat-treated at 150 ◦ C under vacuum for 3 h. The formed membranes were than treated using a standard procedure at 80 ◦ C for 30 min in 5% H2 O2 solution, in deionized water, in 0.5 M H2 SO4 solution, and finally in de-ionized water again. For comparison, pure Nafion® membrane was prepared and treated using the same procedure without addition of precursors. Thickness of prepared membranes is about 78 ± 5 m. Zeta potential measurements were carried out on Zetasizer Nano-ZS (Malvern, UK) using laser Doppler velocimetry and phase analysis light scattering. The temperature of the scattering cell was 25 ◦ C and the data were analyzed with the software from supplier. The size of formed nanoparticles was examined using high resolution transmission electron microscopy (TEM, JEM-2010FEF equipped with Energy Dispersive Spectrum Analyzer). Samples for TEM measurements were prepared by directly placing a drop of the solution on a thin carbon film supported by a copper grid. Selective area electron diffraction measurement was carried out on the same instrument as well. X-ray diffraction (XRD) patterns for membranes were obtained on a D/MaxRB X-ray Diffractometer (Rigaku, Japan) using Cu K␣ radiation operating at 40 kV and 30 mA. The water uptake of membranes at different relative humidity was calculated as the ratio of the difference between the swollen and the dry weight of the membrane. The weight of swollen membrane was measured after keeping the membrane at 100 ◦ C under desired relative humidity for 10 h using a temperature and relative humidity-controllable oven (ZTH, Zuoke Equip. Co. Shanghai). For the weight of dry membrane, the measurement was made directly after drying the sample at 100 ◦ C for 2 h. The weight of the completely hydrated membrane was measured after boiling the membrane in de-ionized water for 2 h and wiping out the surface adsorbed water using tissue paper. Mechanical strength of membranes at dry state and fully hydrated state was measured using an Electromechanical Universal Testing Machine (WDW-1C) based on Chinese Standard QB-1302291. Samples were measured at a strain rate of 50 mm/min.
28
K. Li et al. / Journal of Membrane Science 347 (2010) 26–31
Fig. 2. Pictures of proton conductivity setup: the testing cell (left) and the whole setup (right).
Proton conductivity measurements were carried out in-plane using an impedance analyzer (Autolab PG30/FRA, Eco Chemie, Netherland) at different temperatures without external humidification using homemade conductivity testing cell (Fig. 2). Two copper sheets were placed onto the as-formed membranes (2.2 cm × 2.2 cm) in the same face. The distance between two copper electrodes was 2 cm. Membranes with two copper electrodes were sandwiched between two polytetrafluoroethylene sheets. Electrochemical impedance spectra were recorded in the frequency range of 1 Hz and 105 KHz and the signal amplitude of 10 mV. Membranes were first hydrated before assembling into the testing cell. The measurements were carried out from lower temperature to higher temperature. Unless otherwise stated, membranes were stabilized under each testing condition for 1 h before data were recorded. It should also be noted that Nafion® membranes are really isotropic according to the literature [27] and the through-plane conductivity of membrane is similar to in-plane conductivity.
3. Results and discussion Metal oxide nanoparticles were formed through in situ hydrolysis of precursors in Nafion® solution, as illustrated in Fig. 1. Since hydrolysis and polymerization of precursor occur rapidly in the presence of water, the used Nafion® solution for the in situ growth of metal oxide nanoparticles is required to be anhydrous before hydrolysis of precursors in order to have a better control of size distribution and morphology of the formed metal oxide nanoparticles. To minimize undesired water content in the system, Nafion® solution was heated up to 80 ◦ C under nitrogen atmosphere and the addition and hydrolysis of precursors were then performed under the same condition. Fig. 3 shows transmission electron microscopy images for Nafion® –silica and Nafion® –zirconia hybrid dispersions formed in 2% (weight percent) Nafion® dispersion in NMP. The final contents of doped metal oxides in designed membrane are 5% in weight. Samples for TEM measurement were prepared by directly plac-
Fig. 3. TEM micrographs of Nafion® –SiO2 (a) and Nafion® –ZrO2 (b) hybrid dispersions and energy dispersive spectra for one particles in TEM micrographs of Nafion® –SiO2 (c) and Nafion® –ZrO2 (d). The insert picture in (b) is selected area electron diffraction pattern.
K. Li et al. / Journal of Membrane Science 347 (2010) 26–31
29
Fig. 4. XRD patterns for nanocomposite membranes: Nafion® –zirconia (a), Nafion® –silica (b) and recast Nafion® membrane (c).
Fig. 5. Water uptake of formed membranes as a function of relative humidity at 100 ◦ C: Nafion® –zirconia membrane (triangles), Nafion® –silica membrane (circles), and recast Nafion® membrane (squares). Solid lines are guide to eyes.
ing a drop of the solution on a thin carbon film supported by a copper grid. It is evident that the in situ formed nanoparticles for both silica and zirconia were uniformly distributed in the dispersion and the average diameters of the formed nanoparticles were about 4.2 ± 0.5 nm and 6.3 ± 0.5 nm, respectively. Since pH value of the mixture is around 1.5, surfaces of formed such nanoparticles are positively charged as indicated by zeta potential values of 10.5 mV for silica and 16.0 mV for zirconia. Once metal oxide nanoparticles formed from the sol, Nafion® molecules adsorbed onto the surface through electrostatic interaction, leading to a reversed charge sign as indicated by zeta potential (−44 mV for silica system and −37 mV for zirconia system). Thus, the coverage of Nafion® molecules onto the surface prevents further growth of the particles. Energy dispersive spectrometry was measured based on one nanoparticle in TEM images and the recorded spectra were shown in Fig. 3c and d. The coexistence of the atom peaks for Si or Zr, C, F, and S suggests the coverage of Nafion® molecules on the formed metal oxide nanoparticles. In addition, a clear diffraction ring obtained from selected area electron diffraction was observed and shown in the insert figure in Fig. 3b, indicating that the formed zirconia nanoparticles are crystallized. The Nafion® –metal oxide nanocomposite membranes were formed using a recasting process and were treated using a standard process as described in Section 2. To determine whether the formed nanoparticles are in crystallized state in the formed hybrid membranes, X-ray diffraction (XRD) measurements of recast Nafion® membrane and hybrid membranes were performed and diffraction patterns were shown in Fig. 4. The Nafion® –silica membrane has similar XRD pattern (Fig. 4b) to the recast Nafion® membrane (Fig. 4c), indicating no crystallized silica particles formed. For Nafion® –zirconia composite membrane (Fig. 4a), crystallized zirconia nanoparticles were clearly observed. According to Ludvigsson et al. [28], diffraction maxima for the hybrid membrane at 2 of 17.48◦ and 39.33◦ , corresponding to d-spacing of 5.5 Å and 2.4 Å, were attributed to Teflon-like domains of Nafion® . Diffraction maxima at 28.9◦ (d-spacing of 3.3 Å) was attributed to crystallized ZrO2 nanoparticles in hybrid membrane. By comparison with standard XRD database (JCPDS data no. 37-1484), it was found that the crystallized ZrO2 nanoparticles in the hybrid membrane were in the monoclinic phase. The full width at half-maximum of the diffraction peak was used to estimate the particle size of ZrO2 using Debye–Sherrer formula. The ZrO2 particles are estimated to be 6.7 nm, which agrees well with the TEM investigation of the Nafion® –zirconia dispersion, indicating no further aggregation of zirconia occurred during the membrane formation process. It can also be clearly seen from XRD patterns that the diffraction maxima
of Teflon-like domains for three membranes are at the same scattering angles, indicating that the addition of metal oxide nanoparticles did not affect the crystallinity and structure of Nafion® in the membrane significantly. Because water is proton transporting medium for proton exchange membranes, the water retention ability is one of the important parameters for fuel cell applications. At room temperature, the fully hydrated Nafion® –silica and Nafion® –zirconia hybrid membranes contained 26.6% and 34.2% water in weight, which is much higher than that of fully hydrated recast Nafion® membrane (13.7% in weight). As the main idea of this work is to seek for membranes working at elevated temperature and low relative humidity, we investigated the water retention ability of hybrid membranes under different relative humidity at 100 ◦ C, shown in Fig. 5. As comparison, the water uptake of recast reference Nafion® membrane as a function of relative humidity at 100 ◦ C was plotted in the same figure. It can be clearly seen that the formed composite membranes take up more water than recast Nafion® membrane in the measured range of relative humidity. The composite membranes just show a slightly higher water uptake than reference Nafion® membrane at low relative humidity (less than 20%) at 100 ◦ C. However, when the relative humidity is higher than 40%, the composite membrane takes up about 2–3 times more water than that in the recast Nafion® membrane at 100 ◦ C. The enhancement of water uptake of Nafion® –metal oxide nanocomposite membranes at 100 ◦ C shows potentials of the hybrid membrane for elevated temperature PEMFC applications. Since the mechanical strength strongly affects the durability of the membrane, the mechanical strengths of the formed composite membranes at different states were measured. Fig. 6 shows the tensile strength of formed hybrid membranes with 5% elongation at both fully hydrated state and dry state. As comparison, the tensile strength of recast Nafion® membrane was recorded in the same figure. It can be clearly seen that the tensile strength for composite membranes with 5% elongation is lower than that of recast Nafion® membrane at both dry and fully hydrated states. This behavior is probably caused by the increased interfacial interaction of Nafion® and doped nanoparticles, as discussed in the literature as well [29]. However, for Nafion® –ZrO2 composite membrane, the mechanical strength is just slightly lower than that of recast membrane and the difference in tensile strength at 5% elongation for fully hydrated state and dry state is even smaller than that of recast Nafion® membrane, indicating the similar durability of the composite membrane with Nafion® membrane. Another very important parameter for proton exchange membranes is the proton conductivity, which decides the performance
30
K. Li et al. / Journal of Membrane Science 347 (2010) 26–31
Fig. 6. Tensile strength of formed membranes with 5% elongation at different humidification states: fully hydrated (black) and dry state (gray).
Fig. 8 shows the proton conductivities of formed membranes at different temperatures without external humidification, as described in Section 2. It is evident, that the proton conductivity of Nafion® –metal oxide nanocomposite membranes is higher than that of recast Nafion® membrane in the measured temperature range, attributed to the enhanced water retention property for metal oxide nanoparticles doped membranes. With the increase in temperature (higher than 80 ◦ C), the proton conductivities for all the measured membranes decrease dramatically. However, the composite membranes, especially ZrO2 doped membrane, still show better proton conductivity. This behavior can be attributed to the water entrapped into the membrane that dehydrates with the increase of temperature, indicating higher water retention ability of composite membranes. Even when the temperature reaches 100 ◦ C, the proton conductivity of ZrO2 doped membranes is close to 0.01 S cm−1 without external humidification. Jalani et al. [13] reported similar result that the proton conductivity of zirconia doped Nafion® membrane is close to 0.01 S cm−1 at 20 RH% under 90 ◦ C, attributed to the higher water uptake and enhanced acidity compared to Nafion® membrane. The composite membranes were formed in their work by impregnation of Nafion® membranes with metal oxide precursor solution followed by in situ sol–gel reaction. However, the distribution of zirconia nanoparticles formed using our technique seems more homogenous than the literature work [13], leading to similar proton conductivity at higher testing temperature. Thus, the ZrO2 doped Nafion® membranes can be potentially used in the elevated temperature PEMFCs. The fuel cells performance using metal oxide doped membranes is still under investigation. 4. Conclusion
®
◦
Fig. 7. Proton conductivity of Nafion –zirconia membrane at 100 C without external humidification as a function of time. The zirconia content is 5% in weight.
of the assembled fuel cells. In order to determine whether the 1 h stabilization time is enough or not, the hydrated Nafion® –zirconia membrane with zirconia content of 5% in weight was assembled into the testing cell. The testing cell was heated to 100 ◦ C and impedance spectra were recorded every 15 min. The calculated proton conductivity of the membrane as a function of stabilization time is shown in Fig. 7. It can be clearly seen that the proton conductivity decreases dramatically in first 30 min, attributed to the rapid loss of water inside the membrane. It, however, almost remains constant 30 min later, indicating that the stabilization time of 1 h should be enough.
In this communication, Nafion® –metal oxide (SiO2 , ZrO2 ) nanocomposite membranes were formed through in situ sol–gel process. Metal oxide nanoparticles were in situ synthesized in Nafion® solution by hydrolysis and condensation of precursors. The existing Nafion® molecules can be self-assembled onto metal oxide particles through electrostatic interactions and prevent the further growth of the initial formed metal oxide nanoparticles. Using this technique, the synthesized metal oxide nanoparticles were evenly distributed inside the membrane with diameters of ∼5 nm and the well-dispersed metal oxide nanoparticles did not affect the crystallinity and structure of Nafion® in the membrane significantly. The formed Nafion® –metal oxide nanocomposite membranes show enhanced water retention ability and higher proton conductivity compared to recast pure Nafion® membrane at all measured temperature range. Although the proton conductivity of the composite membrane decreases with increasing temperature, the proton conductivity for composite membrane, especially for Nafion® –ZrO2 composite membrane is close to 0.01 S cm−1 at 100 ◦ C without external humidification, which is important for fuel cell applications. Thus, the hybrid membrane developed here has the potential for PEMFC applications at elevated temperatures. Acknowledgements This work was supported by the Foundation of National Natural Science of China (50632050 and 20806061) and the National High Technology Research and Development Program (“863” Program) of China (2008AA050403). The authors thank Mr. Yueqing Li for the technical help on conductivity testing cells. References
Fig. 8. Proton conductivities of the formed membranes as a function of temperature without external humidification: Nafion® –zirconia membrane (triangles), Nafion® –silica membrane (circles), and recast Nafion® membrane (squares). Solid lines are guide to eyes.
[1] Q. Li, R. He, J.O. Jensen, N.J. Bjerrum, Approaches and recent development of polymer electrolyte membranes for fuel cells operating above 100 ◦ C, Chem. Mater. 15 (2003) 4896–4915.
K. Li et al. / Journal of Membrane Science 347 (2010) 26–31 [2] G. Alberti, M. Casciola, Composite membranes for medium-temperature PEM fuel cells, Annu. Rev. Mater. Res. 33 (2003) 129–154. [3] P. Jannasch, Recent developments in high-temperature proton conducting polymer electrolyte membranes, Curr. Opin. Coll. Interf. Sci. 8 (2003) 96–102. [4] W.H.J. Hogarth, J.C. Diniz da Costa, G.Q. Liu, Solid acid membranes for high temperature (>140 ◦ C) proton exchange membrane fuel cells, J. Power Sources 142 (2005) 223–237. [5] J. Zhang, Z. Xie, J. Zhang, Y. Tang, C. Song, T. Navessin, Z. Shi, D. Song, H. Wang, D.P. Wilkinson, Z. Liu, S. Holdcroft, High temperature PEM fuel cells, J. Power Sources 160 (2006) 872–891. [6] N.L. Garland, J.P. Kopasz, The United States Department of Energy’s high temperature and low relative humidity membrane program, J. Power Sources 172 (2007) 94–99. [7] Fuel Cell Handbook, 7th ed., EG&G Technical Services Inc., West Virginia, US Department of Energy, 2004. [8] V.M. Vishnyakov, Proton exchange membrane fuel cells, Vacuum 80 (2006) 1053–1065. [9] P.L. Antonucci, A.S. Arico, P. Creti, E. Ramunni, V. Antonucci, Investigation of a direct methanol fuel cell based on a composite Nafion® –silica electrolyte for high temperature operation, Solid State Ionics 125 (1999) 431–437. [10] N. Miyake, J.S. Wainright, R.F. Savinell, valuation of a sol–gel derived Nafion® /silica hybrid membrane for proton electrolyte membrane fuel cell applications—I. Proton conductivity and water content, J. Electrochem. Soc. 148 (2001) A898–A904. [11] K.T. Adjeman, S.J. Lee, S. Srinivasan, J. Benziger, A.B. Bocarsly, Silicon oxide Nafion® composite membranes for proton exchange membrane fuel cell operation at 80–140 ◦ C, J. Electrochem. Soc. 149 (2002) A256–A261. [12] M. Ramani, H.R. Kunz, J.M. Fenton, Investigation of Nafion® /HPA composite membranes for high temperature/low relative humidity PEMFC operation, J. Membr. Sci. 232 (2004) 31–44. [13] N.H. Jalani, K. Dunn, R. Datta, Synthesis and characterization of Nafion® –MO2 (M = Zr, Si, Ti) nanocomposite membranes for high temperature PEM fuel cells, Electrochim. Acta 51 (2005) 553–560. [14] K.T. Adjemian, R. Dominey, L. Krishnan, H. Ota, P. Majsztrik, T. Zhang, J. Mann, B. Kirby, L. Gatto, M. Velo-Simpson, J. Leahy, S. Srinivasan, J.B. Benziger, A.B. Bocarsly, Function and characterization of metal oxide–Nafion® composite membranes for elevated temperature H2 /O2 PEM fuel cells, Chem. Mater. 18 (2006) 2238–2248. [15] A. Sacca, I. Gatto, A. Carbone, R. Pedicini, E. Passalacqua, ZrO2 –Nafion® composite membranes for polymer electrolyte fuel cells (PEFCs) at intermediate temperature, J. Power Sources 163 (2006) 47–51.
31
[16] V. Di Noto, R. Gliubizzi, E. Negro, G. Pace, Effect of SiO2 on relaxation phenomena and mechanism of in conductivity of [Nafion® /(SiO2 )x ] composite membranes, J. Phys. Chem. B 110 (2006) 24972–24986. [17] Z.G. Shao, H.F. Xu, M.Q. Li, I.M. Hsing, Nafion® /inorganic oxides membrane doped with heteropolyacids for high temperature operation of proton exchange membrane fuel cell, Solid State Ionics 177 (2006) 779–785. [18] Y.F. Zhai, H.M. Zhang, J.W. Hu, B.L. Yi, Preparation and characterization of sulfated zirconia (SO4 2− /ZrO2 )/Nafion® composite membranes for PEMFC operation at high temperature/low humidity, J. Membr. Sci. 280 (2006) 148–155. [19] H.L. Tang, Z.H. Wan, M. Pan, S.P. Jiang, Self-assembled Nafion® –silica nanoparticles for elevated high-temperature polymer electrolyte membrane fuel cells, Electrochem. Commun. 9 (2007) 2003–2008. [20] K.T. Park, U.H. Jung, D.W. Choi, K. Chun, H.M. Lee, S.H. Kim, ZrO2 /SiO2 //Nafion composite membrane for polymer electrolyte membrane fuel cells operation at high temperature and low humidity, J. Power Sources 177 (2008) 247–253. [21] F. Pereira, K. Valle, P. Belleville, A. Morin, S. Lambert, C. Sanchez, Advanced mesostructured hybrid silica–Nafion® membranes for high-performance PEM fuel cell, Chem. Mater. 20 (2008) 1710–1718. [22] W. Apichatachutapan, R.B. Moore, K.A. Mauritz, Asymmetric Nafion® /(zirconium oxide) hybrid membranes via in situ sol–gel chemistry, J. Appl. Polym. Sci. 62 (1996) 417–426. [23] K.A. Mauritz, J.T. Payne, Perfluorosulfonate ionomer/silicate hybrid membranes via base-catalyzed in situ sol–gel processes for tetraethylorthosilicate, J. Membr. Sci. 168 (2000) 39–51. [24] G. Alberti, M. Casciola, Solid state protonic conductors, present main applications and future prospects, Solid State Ionics 145 (2001) 3–16. [25] J.J Pan, H.N. Zhang, M. Pan, Self-assembly of Nafion® molecules onto silica nanoparticles formed in situ through sol–gel process, J. Colloid Interf. Sci. 326 (2008) 55–60. [26] H.N. Zhang, J.J. Pan, X.C. He, M. Pan, Zeta potential of Nafion® molecules in isopropanol–water mixture solvent, J. Appl. Polym. Sci. 107 (2008) 3306–3309. [27] R.F. Silva, M. De Francesco, A. Pozio, Tangential and normal conductivities of Nafion® membranes used in polymer electrolyte fuel cells, J. Power Sources 134 (2004) 18–26. [28] M. Ludvigsson, J. Lindgren, J. Tegenfeldt, Crystallinity in cast Nafion® , J. Electrochem. Soc. 147 (2000) 1303–1305. [29] H.L. Tang, M. Pan, Synthesis and characterization of a self-assembled Nafion® /silica nanocomposite membrane for polymer electrolyte fuel cells, J. Phys. Chem. C 112 (2008) 11556–11568.