Nafion Nanocomposite Membrane for Self

Wenjing Zhang, Martin Ka Shing Li, Po-Lock Yue, and Ping Gao*. Department of Chemical Engineering, Hong Kong UniVersity of Science and Technology, ...
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Langmuir 2008, 24, 2663-2670

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Exfoliated Pt-Clay/Nafion Nanocomposite Membrane for Self-Humidifying Polymer Electrolyte Fuel Cells Wenjing Zhang, Martin Ka Shing Li, Po-Lock Yue, and Ping Gao* Department of Chemical Engineering, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China ReceiVed July 18, 2007. In Final Form: October 4, 2007 Monolayers of Pt nanoparticles of diameters of 2-3 nm with a high crystallinity were successfully anchored onto exfoliated nanoclay surfaces using a novel chemical vapor deposition process. Chemical bonding of Pt to the oxygen on the clay surface ensured the stability of the Pt nanoparticles, and hence, no leaching of Pt particles was observed after a prolonged ultrasonication and a rigorous mechanical agitation of Pt-clay in the Nafion solution during the membrane casting process. Systematic analysis using WAXD and TEM showed that the recasting process produced a new self-humidifying exfoliated Pt-clay/Nafion nanocomposite membrane with a high crystallinity and proton conductivity. In situ water production for humidification of the dry membranes without any external humidification was characterized by a combined water uptake and FTIR analysis of the as-prepared membrane after a single cell testing without using electrodes. The power density at 0.5 V of a single cell made of a Pt-clay/Nafion nanocomposite membrane was 723 mW/cm2, which is 170% higher than that made of a commercial Nafion 112 membrane of similar thickness. No compromise in mechanical properties was observed.

1. Introduction Fuel cells could, in theory, blast through the battery barrier as they can directly convert chemical energy into electrical energy with high efficiency and near-zero emissions. Among different types of fuel cells, proton exchange membrane fuel cells (PEMFCs) have captured the most attention and provided the strongest impetus for technological expansion due to their low temperature operation, fast start-up, and high specific power density, highly suitable for portable, mobile, and domestic applications.1-4 The Nafion membrane (E. I. de Nemours) is the most studied and employed electrolyte for PEMFCs thanks to its favorable chemical, morphological, thermal, and mechanical properties.5-7 However, proton conduction in Nafion depends on the degree of hydration of the membrane.8-13 Poor or zero proton conductivity has been observed when the relative humidity is less than 30% in the membrane.14 Thus, how to maintain the water content in membranes is critical to the overall cell performance. For practical operations, the Nafion membrane fuel cells rely on external humidifiers for the hydration of gas streams prior to the entry of the cell fixture. However, these humidifying and relative control units increase the cost, complexity, and design * Corresponding author. E-mail: [email protected]; fax: (852)2358 0054. (1) Steele, B. C. H.; Heinzel, A. Nature (London, U.K.) 2001, 414, 345-352. (2) Song, J. M.; Suzuki, S.; Uchida, H.; Watanabe, M. Langmuir 2006, 22, 6422-6428. (3) Costamagna, P.; Srinivasan, S. J. Power Sources 2001, 102, 242-252. (4) Costamagna, P.; Srinivasan, S. J. Power Sources 2001, 102, 253-269. (5) Moore, R. B., III; Martin, C. R. Macromolecules 1988, 21, 1334-1339. (6) Kopitzke, R. W.; Linkous, C. A.; Nelson, G. L. Polym. Degrad. Stabil. 2000, 67, 335-344. (7) Mauritz, K. A.; Moore, R. B. Chem. ReV. 2004, 104, 4535-4585. (8) Watanabe, M.; Satoh, Y.; Shimura, C. J. Electrochem. Soc. 1993, 140, 3190-3193. (9) Verbrugge, M. W.; Hill, R. F. J. Electrochem. Soc. 1990, 137, 886-893. (10) Bernardi, D. M. J. Electrochem. Soc. 1990, 137, 3344-3350. (11) Springer, T. E.; Zawodzinski, T. A.; Gottesfeld, S. J. Electrochem. Soc. 1991, 138, 2334-2342. (12) Fuller, T.; Newman, J. J. Electrochem. Soc. 1992, 139, 1332-1337. (13) Bernardi, D. M.; Verbrugge, M. W. J. Electrochem. Soc. 1992, 139, 2477-2491. (14) Anantaraman, A. V.; Gardner, C. L. J. Electroanal. Chem. 1996, 414, 115-120.

barriers of the whole system. Flooding of electrodes is another issue due to poor water management at high current densities. The reduction of membrane thickness was proposed to enhance the water back-diffusion from the cathode and to suppress water management problems.15 However, higher proton conduction with dry gases in very thin membranes may accelerate the crossover of H2 and O2, which lowers the fuel utilization and cell efficiency. The higher fuel crossover also results in the production of H2O2 at the anode, which has been considered to be the main reason for reduced membrane durability.16 To overcome these problems, the concept of self-humidifying polymer electrolyte membranes was proposed. Pt particles embedded in the membrane have been shown to catalyze the reaction of permeated H2 and O2 to produce water. Hydroscopic oxide particles were usually incorporated to balance the water content in the membrane. On the basis of this concept, a number of composite membranes containing Nafion and Pt nanoparticles have been developed for self-humidification. Watanabe et al. proposed a method to disperse Pt by cation exchange treatment. SiO2 or TiO2 nanoparticles were synthesized throughout the membrane using an in situ sol-gel reaction.17-20 Liu et al. directly cast a mixture of Nafion and Pt/C catalyst ink onto a porous poly(tetrafluoroethylene) (PTFE) film substrate.21 A series of multilayer membranes was developed to reduce the risk of formation of a Pt conducting path in the membrane or local hot spots due to Pt migration and aggregation during operation. Yang et al. developed a self-humidifying membrane in the form of a sandwich, which was composed of two membranes made of a melt-fabricable perfluorosulfonyl copolymer resin sputter coated (15) Dhar, H. P. Near ambient, unhumidified solid polymer fuel cell. U.S. Patent 5,318,863, 1994. (16) Liu, W.; Zuckerbrod, D. J. Electrochem. Soc. 2005, 152, 1165-1170. (17) Watanabe, M.; Uchida, H.; Seki, Y.; Emori, M.; Stonehart, P. J. Electrochem. Soc. 1996, 143, 3847-3852. (18) Uchida, H.; Ueno, Y.; Hagihara, H.; Watanabe, M. J. Electrochem. Soc. 2003, 150, 57-62. (19) Hagihara, H.; Uchida, H.; Watanabe, M. Electrochim. Acta 2006, 51, 3979-3985. (20) Watanabe, M.; Uchida, H.; Emori, M. J. Phys. Chem. B 1998, 102, 31293137. (21) Liu, F.; Yi, B.; Xing, D.; Yu, J.; Hou, Z.; Fu, Y. J. Power Sources 2003, 124, 81-89.

10.1021/la702153v CCC: $40.75 © 2008 American Chemical Society Published on Web 02/07/2008

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with the Pt particles.22 Similarly, a double layer composite membrane was also proposed, which consisted of one layer of plain Nafion and another layer of Pt/C catalyst dispersed through a recasting procedure and placed on the anode side.23 Recently, Zhu. et al used the combination of both sandwich structure and PTFE reinforcement to create a three-layered membrane consisting of a PTFE-reinforced Nafion layer sandwiched between two side layers of Pt-SiO2 imbedded in Nafion by.24,25 Significant performances have been observed in these selfhumidifying hydrogen fuel cells. However, a number of issues have yet to be resolved. First, the residual impurity with electroless plating pollutes the membrane and lowers the cell performance in the long run. Second, the formation of a Pt conducting path is always a concern as long as the Pt particles are not immobilized inside the membrane. Third, the introduction of PTFE for a higher mechanical strength may result in not only reduced proton conductivity but also the possibility of delamination due to the differences in dimensional changes by swelling. PTFE is strongly hydrophobic, but Nafion is hydrophilic. The balance between the durability and performance should be considered. For nonhomogeneous membranes, such as a multilayer assembly with a Nafion membrane and non-Nafion reinforcement material, detachment is a concern due to the differences in swelling and thermal expansion coefficients since the composite membrane has to withstand hot water in a working fuel cell. Moreover, until now, the semiconductor particles impregnated with Pt were spherical particles. They were believed to function as a water reservoir. The local stress around the spherical particle surface, due to the gradient water concentration, and poor compatibility with the membrane may decrease the toughness of the membrane followed by the accelerated fuel crossover. The planar clay family is a promising candidate for hybrid fuel cell membranes and shows the possible enhancement of the thermal, mechanical, and barrier properties. However, several pretreatments must be involved, such as organic modification for dispersion in the Nafion matrix and sulfonation for enhancement in proton conductivity.26,27 The nonconductive and barrier properties for the proton conduction limited their major applications in the field of direct methanol fuel cells.27-30 We present here a novel Pt-clay/Nafion nanocomposite membrane with significantly enhanced performance over commercial Nafion 112 membranes of the same thickness. The Ptclay/Nafion nanocomposite membranes were prepared by a recasting procedure using a Nafion solution and exfoliated clay, on which the Pt nanoparticles were immobilized onto the nanoclay surfaces with lateral dimensions up to 200 nm to form an exfoliated Pt-clay nanocomposite through novel CVD. The use of planar hydrophilic clay also helped to balance the water content and to enhance the mechanical properties of the nanocomposite membrane. TEM, XRD, and XRF analysis showed that this solid combination of Pt particles on clay performed strong enough to (22) Yang, T. H.; Yoon, Y. G.; Kim, C. S.; Kwak, S. H.; Yoon, K. H. J. Power Sources 2002, 106, 328-332. (23) Yang, B.; Fu, Y. Z.; Manthiram, A. J. Power Sources 2005, 139, 170175. (24) Zhu, X. B.; Zhang, H. M.; Liang, Y. M.; Zhang, Y.; Yi, B. L. Electrochem. Solid State Lett. 2006, 9, 49-52. (25) Zhu, X.; Zhang, H.; Zhang, Y.; Liang, Y.; Wang, X.; Yi, B. J. Phys. Chem. B 2006, 110, 14240-14248. (26) Be´bin, P.; Caravanier, M.; Galiano, H. J. Membr. Sci. 2006, 278, 35-42. (27) Rhee, C. H.; Kim, H. K.; Chang, H.; Lee, J. S. Chem. Mater. 2005, 17, 1691-1697. (28) Jung, D. H.; Cho, S. Y.; Peck, D. H.; Shin, D. R.; Kim, J. S. J. Power Sources 2003, 118, 205-211. (29) Silva, R. F.; Passerini, S.; Pozio, A. Electrochim. Acta 2005, 50, 26392645. (30) Thomassin, J. M.; Pagnoulle, C.; Bizzari, D.; Caldarella, G.; Germain, A.; Jerome, R. e-Polymers 2004, 18, 1-13.

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Figure 1. Chemical structures of platinum (II) acetylacetonate (left) and organic modifier used in Cloisite 20A (right). N+ denotes a quaternary ammonium salt; HT is hydrogenated tallow.

sustain vigorous stirring and sonication. Therefore, an excellent self-humidifying performance of the Pt-clay/Nafion nanocomposite membrane with relatively low Pt loading was achieved. 2. Experimental Procedures 2.1. Materials. Cloisite 20A was provided by Southern Clay Products, Inc. Platinum (II) acetylacetonate purchased from Aldrich was used as the precursor for CVD work. The chemical structures of the organic modifier and precursor are displayed in Figure 1. The Nafion solution was 5 wt % polymer (equivalent weight ) 1100) dissolved in aliphatic alcohol, as obtained from Aldrich. E.I. DuPont de Nemours and Co. supplied the Nafion 112 membrane for this experiment. Gas diffusion electrodes with 0.25 mg/cm2 Pt loading were used as received from E-TEK. N,N-Dimethylformamide (99.5%, DMF) and N,N-dimethylacetamide (99%, DMA) were used as cosolvents to recast the Nafion membrane. Analytical grade hydrogen and oxygen gases were used as received. 2.2. Preparation of Pt-Clay/Nafion Membrane and Membrane/ Electrode Assembly. The diagram in Scheme 1 describes the procedure of creating a Pt-clay/Nafion nanocomposite membrane. Exfoliated Pt-clay was prepared by CVD of Pt nanoparticles on the surface of planar Cloisite 20A. The ratio of platinum precursor to clay was set at 1:4 in weight fractions. The synthesis was carried out in a stainless steel rotational fixed bed reactor at 350 °C for 60 min with a speed of 90 rpm, and the reaction was carried out in vacuum at a gauge pressure of -29 mmHg. Platinum deposited clay was calcinated in air at 350 °C for 4 h followed by reduction using H2/Ar gas with a volume ratio of 1:1 at a gas flow rate of 120 cm3/min at 350 °C for 4 h. The loading of Pt on clay was about 4.11 wt %, as determined by XPS and XRF. Nanocomposite membranes were prepared via the following recasting procedure: First, a desired amount of Pt-clay nanoparticles was mechanically mixed into deionized water. Then, a mixture of commercial 5 wt % Nafion solution, DMF, DMA, and ethanol with a volume ratio of 4:1:1:4 was added to the aqueous mixture of Pt-clay. Third, the entire solution mixture was sonicated using an ultrasonication probe running at a pulsation mode at ambient temperature for 60 min. This was then followed by vigorous mechanical agitation at 95 °C until the solution concentration reached the gelation point. Finally, the gel-like mixture was cast onto a glass Petri dish and dried at 95 °C in a nonconvection oven for 20 h. Further drying and thermal annealing at T ) 140 °C in vacuum for 24 h were applied to increase the crystallinity and to improve the mechanical properties.5 After thermal treatment, the sample was cooled slowly to room temperature inside the vacuum oven. After being rinsed with water, the membrane was treated in a 5 vol % H2O2 solution followed by immersion into a 0.5 M H2SO4 solution with boiling distilled water in between. The Pt-clay loading inside the nanocomposite membrane was 0.9 wt %, giving a Pt loading of 0.004 mg/cm2. For comparison, the pure Nafion membrane was also prepared using the same procedure. Details on the membranes used in this study are summarized in Table 1. The membrane electrode assembly (MEA) was then prepared by uniaxially hot-pressing two E-TEK electrodes onto the membrane at 135 °C and 4.0 MPa for 90 s. A Nafion solution spraying technique on electrodes was adopted to enhance the adhesion on the membrane. Nafion ionomer loading was optimized to be 33 wt % calculated using the empirical equation proposed by Antolini et al.31 For investigation of the self-humidifying ability, carbon cloth without

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Scheme 1. Schematic Representation of Processes of Preparing Exfoliated Pt-Clay/Nafion Nanocomposite Membranesa

a

2M2HT: dimethyl, dehydrogenated tallow, quaternary ammonium. Table 1. Thicknesses and Chemical Compositions of Membranes Used in This Study

membrane

Pt-clay loading (%, w/w)

Pt loading (mg/cm2)

thickness (µm)

Nafion 112 pure Nafion Pt-clay/Nafion

0 0 0.9

0 0 0.004

57 60 60

a catalyst was used to make the membrane gas diffusion layer assembly (MGA). 2.3. Single Fuel Cell Operation Test. All MEAs were evaluated on a commercial fuel cell system (FCTs, Arbin) using a single cell test fixture with a 5 cm2 active geometrical area. Dry H2 and O2 gases were fed to the cell at a flow rate of 200 cm3/min. The currentvoltage (I-V) curves were measured at 0.1 MPa back pressure and 60 °C. 2.4. Self-Humidifying Ability Evaluation. To investigate the water production as a result of the Pt-catalyzed reaction of the crossovered H2 and O2, or self-humidification effects, two MGAs, one made of nanocomposite membrane (60 µm) sandwiched by two pieces of pure Nafion (30 µm each) and another pure Nafion membrane (60 µm), were prepared as described in section 2.2. Dry gas streams were fed to the cell at 60 °C and built up 0.15 MPa back pressure on the both sides of MGA. After the steady state was achieved, the gas flow was switched off to block the fuels into the fixture for 6 h. The back pressure was kept constant by refueling the gases into the fixture. The difference between the initial and the final weights of the MGAs gives the amount of water generated in the membranes. FTIR absorbance measurements were also conducted on the membrane after detachment from the MGA to verify the production of water. 2.5. Gas Permeability. Chronocoulometry was used to measure the H2 crossover rate through the MEAs. The cell was operated with H2 at the anode and N2 at the cathode. All gases were kept at 0.1 MPa back pressure with a flow rate of 50 cm3/min at 25 °C. A 0.25 V voltage was applied across the cell to oxidize the permeated H2. The evolved coulombs can be measured to calculate the H2 crossover rate in the units of mL of STP cm-2 h-1.17 2.6. EIS and Water Uptake. Proton conductivity in the traverse direction of the membrane was measured using an AC impedance spectroscopy technique over a frequency range of 1 to 105 Hz with an oscillating voltage of 10 mV, using a frequency response analyzer (FRA, Netherlands Autolab). A two-probe method32-36 was used to (31) Antolini, E.; Giorgi, L.; Pozio, A.; Passalacqua, E. J. Power Sources 1999, 77, 136-142. (32) Rieke, P. C.; Vanderborgh, N. E. J. Membr. Sci. 1987, 32, 313-328.

measure the impedance for parallel comparisons. Prior to measurements, all membranes were soaked in deionized water for hydration. The proton conductivity can be calculated by σ ) L/RA, where L and A are the thickness of the membrane and the surface area of the electrode, respectively, and R is the resistance obtained from the bulk resistance found in the complex impedance diagram. To measure the water uptake, standard pretreatments were conducted on all membranes, including purification, acid treatment, rinsing, and finally immersion in deionized water at 25 °C for 24 h. Then, the samples were blotted dry using Kleenex prior to the measurement. The water uptake was calculated based on the weight of the dry sample using eq 1 water uptake )

Wfinal - Winitial × 100% Winitial

(1)

2.7. SEM, TEM, XRF, and XRD. A JEOL 6300F scanning electron microscope with energy dispersive analysis by X-ray (EDAX) was used to observe the cross-section of the membrane prepared by cryogenic fracture. For field-emission TEM, the JEOL 2010 microscope was used to study the microstructure of the membranes. Ultrathin pure and composite membranes were directly cast on a copper grid with a carbon support. The sample staining method by RuO4 vapor was incorporated for cluster morphology observation. In addition, the quantitative bulk chemical compositions of the Pt-clay were determined by XRF (Philips PW 1480 spectrometer). X-ray powder diffraction data were collected on a Philips PW 1825 diffractometer with Cu KR radiation (40 kV and 50 mA). The angular scanning was performed in the range of 2° < 2θ < 50° at a rate of 2°/min. Crystallinity was obtained through a crystallineamorphous peak deconvolution process using two Gaussian functions from the originally convoluted peak.37 2.8. Tensile Test. Tensile properties of the membranes were measured on the Advanced Rheometrics Expansion System (ARES) at an extension rate of 2 mm/min at room temperature. Each film was cut into parallel-sided strips of dimensions of 10 mm × 3 mm × 0.06 mm. Both dry and fully hydrated samples were used. The yield strength was obtained at 2% offset. Ten specimens were used for each test. (33) Hong, L.; Zhou, Y.; Chen, N.; Li, K. J. Colloid Interface Sci. 1999, 218, 233-242. (34) Kawahara, M.; Morita, J.; Rikukawa, M.; Sanui, K.; Ogata, N. Electrochim. Acta 2000, 45, 1395-1398. (35) Zaidi, S. M. J.; Mikhailenko, S. D.; Robertson, G. P.; Guiver, M. D.; Kaliaguine, S. J. Membr. Sci. 2000, 173, 17-34. (36) Staiti, P.; Lufrano, F.; Arico, A. S.; Passalacqua, E.; Antonucci, V. J. Membr. Sci. 2001, 188, 71-78. (37) Park, Y. S.; Yamazaki, Y. Eur. Polym. J. 2006, 42, 375-387.

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Figure 2. Polarization curve of MEAs employing commercial Nafion 112 (4), pure Nafion (]), and Pt-clay/Nafion nanocomposite membrane (0) with dry H2 and O2. The flow rates of H2 and O2 were 200 cm3/min with a back pressure of 0.1 MPa. Cell temperature was 60 °C for all tests.

3. Results and Discussion The current-voltage characteristics of MEAs under dry conditions at 60 °C are presented in Figure 2. The pure Nafion membrane shows comparable behavior with the membrane developed by Yang et al.23 in the single cell operation in terms of open circuit voltage (OCV) and power density output since both the membranes were cast from the commercial 5 wt % Nafion solution and tested under similar conditions. Nafion membranes require water to maintain proton conductivity.38 In the initial stage, dry H2 and O2 streams capture traces of water in the membrane and even the water coming from the reaction at the cathode. The voltage drop in the low current density region of the I-V curve reflects the sluggish kinetics intrinsic to O2 reduction.39 The low OCV of around 0.87 V is well below that of the theoretical voltage of 1.23 V due to the drastic activation loss and gas permeability loss through the membrane. Without any external humidification, the MEA employing Nafion 112 delivered a very low output voltage in the low current density region. The main reason for the low OCV may be the high gas permeability across the membrane when the cell was operated with dry fuels at 60 °C. The relatively low Pt loading on the electrode (0.25 mg/cm2) was considered to be another reason, which may decrease the exchange current density on electrodes and therefore induce a larger activation overpotential through the operation. In a normal situation, the Pt loading on the catalyst layer was higher than 0.4 mg/cm2. Girishkumar et al.40 and Qi and Kaufman41 even further increased their values to 1.0-1.7 mg/cm2 with the design of improving the MEA performance. But, this performance improvement was expensive to scale up if a MEA with a larger active area is desired. The extremely poor proton conductivity of dry Nafion 112 may also induce the drastic voltage drop during the initial stage. Therefore, the combination of activation overpotential and ohmic resistance of thirsty Nafion 112 results in a sharp drop in cell voltage initially. However, this initial loss in voltage was dramatically alleviated in the MEA made of the Pt-clay/Nafion membrane. The Pt nanoparticles on clay were believed to provide sites for the catalytic recombination of permeated gases, thereby producing water inside the membrane. (38) Watanabe, M.; Uchida, H.; Emori, M. J. Electrochem. Soc. 1998, 145, 1137-1141. (39) Liu, Z.; Lin, X.; Lee, J. Y.; Zhang, W.; Han, M.; Gan, L. M. Langmuir 2002, 18, 4054-4060. (40) Girishkumar, G.; Rettker, M.; Underhile, R.; Binz, D.; Vinodgopal, K.; McGinn, P.; Kamat, P. Langmuir 2005, 21, 8487-8494. (41) Qi, Z.; Kaufman, A. J. Power Sources 2002, 109, 469-476.

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Figure 3. Polarization curves of MEAs prepared using Pt-clay/ Nafion nanocomposite membrane dried by vacuum oven (s) and dried by air (---), pure Nafion dried by vacuum oven (0), and commercial Nafion 112 dried by vacuum oven (4). The cell operating temperature was 60 °C. Inset shows the comparisons of open-circuit voltage and H2 crossover among MEAs.

This moderate in situ hydration continued throughout the whole process of current flow. It can be seen that, for the MEA with Pt-clay/Nafion, the cell voltage that pulled through the dry start quickly shifted to a very stable output region, indicating that the membrane was well-hydrated. In particular, its maximum current density approached 3800 mA/cm2. It is quite unique for the MEA employing a 60 µm thick membrane to deliver so high a current density. At a mass transfer controlled region (high current density), the presence of clay in the composite membrane may have contributed to the better water management, the reduction in the water concentration gradient, and even the delay of the flooding of electrodes since no external water was pumped to the system in this design. Without any external humidification of H2 and O2, the fuel cell employing the Pt-clay/Nafion nanocomposite membrane produced a power density of 723 mW/cm2 at 0.5 V when the operation temperature was 60 °C. This represents a 170% improvement over the commercial Nafion membrane Nafion 112 (269 mW/cm2 at 0.5 V), which is of the same thickness. A high polarization output of 650 mW/cm2 at 0.5 V was also reported using the self-humidifying membrane prepared by Wanatabe et al.20 However, in their work, the measurements were performed at 80 °C, and the Pt loadings in the electrodes and membranes were 0.7 and 0.09 mg/cm2, respectively. Yang et al. attempted to reduce the Pt loading in the membrane to 0.02 mg/cm2. However, the performance improvement of the self-humidifying membrane over the normal one was inconspicuous.23 The work by Kwak et al. concluded that the optimum Pt loading in the self-humidifying membrane was 0.15 mg/cm2.42 To the best of our knowledge, the performance improvement with our Pt-clay/ Nafion membrane is quite superior considering the relatively lower Pt loading in the MEA (0.25 mg/cm2 on the electrodes and 0.004 mg/cm2 in the membrane) as compared to other selfhumidifying membranes of similar thicknesses. To further demonstrate the self-humidifying performance of the Pt-clay/Nafion membrane, polarization tests were carried out on membranes completely dried under vacuum. The nearly congruent polarization performance between the membranes prepared by air drying and vacuum drying shown in Figure 3 (42) Kwak, S. H.; Yang, T. H.; Kim, C. S.; Yoon, K. H. J. Power Sources 2003, 118, 200-204.

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Table 2. Water Gained in MGA Employed Commercial Nafion 112 and Pt-Clay/Nafion Nanocomposite Membrane after Operation for 6 h in Arbin Test Station Fueled by Dry H2 and O2 membrane in MGA

WH2O ) Wfinal - Winitial (mg)

Nafion 112 pure Nafion|Pt-clay/Nafion|pure Nafion

1.7 9.4

clearly suggests that the water generated in situ was sufficient to maintain proton conductivity. To evaluate the barrier performance of the planar nanoclay particles embedded in the membrane, a H2 permeability test was performed using chronocoulometry at H2 and N2 flow rates of 50 cm3/min and 25 °C. The data are presented in the inset of Figure 3. The OCV results are also presented. Clearly, a substantial reduction in H2 permeability was achieved in the composite membrane. A correspondingly higher OCV is also shown. It should also be mentioned that the anchoring of Pt nanoparticles on the clay surface can guarantee no Pt conduction path formation or generation of local hot spots through the membrane due to Pt particle migration. The mechanism of self-humidification and morphological characterizations are presented in the following sections. 3.1. Self-Humidifying Mechanism. Since Nafion requires water to maintain its desirable properties, a Pt catalyst was added to the membrane to nonelectrochemically catalyze the reaction of H2 and O2 diffusing into the membrane from fuel and oxidant sides, respectively.17,19-24,42 To elucidate the self-humidifying mechanism, we sandwiched the Pt-clay/Nafion or commercial Nafion 112 membrane between two pieces of carbon cloth but without a catalyst to make the MGA and blocked dry gases into the fixture with 0.15 MPa back pressure for 6 h. However, there may be some Pt on the surface of the nanocomposite membrane that could act as the electrode when placed in contact with the conductive gas diffusion layer. Therefore, the Pt-clay/Nafion membrane was sandwiched by two pieces of pure Nafion. In this way, water can only be produced from the reaction of permeated H2 and O2 catalyzed by Pt inside the Pt-clay/Nafion membrane. Measurement of weight uptake in Nafion 112 and Pt-clay/Nafion membranes will give a direct measure of the self-humidifying ability of membranes. Results tested on membranes assembled at 60 °C for 6 h are presented in Table 2. Clearly, the nanocomposite membrane showed a weight increase of 9.4 mg, whereas the pure membrane exhibited negligible weight changes. This shows that the Pt inside the membrane has the catalytic ability to absorb H2/O2 through the membrane and convert it into water. To verify the chemical environment of water produced inside the membrane, we conducted Fourier transform infrared (FTIR) spectroscopy in the absorption mode. Three different types of membranes were tested for comparison: (a) Sample A, which refers to membranes dried inside a drying cabinet with a relative humidity of 60% at room temperature; (b) sample B, membranes dried for 24 h inside a vacuum oven at 95 °C; and (c) sample C was obtained by placing sample B inside the Arbin test station without the use of an electrode and had a weight gain of 9.7 mg. Figure 4 shows the FTIR spectra wavenumbers between 3000 and 3800 cm-1 to focus on the OH stretching region. A significant blue shift in the stretching mode of OH after the MGA test can be clearly observed. The vibrational frequency of OH depends on the type of interactions inside the Nafion membrane. If OH does not form a hydrogen bond with the neighboring molecules, the OH bond length becomes closer to that in the gas phase

Figure 4. Infrared absorbance spectra of Pt-clay/Nafion membrane. Sample A, the membrane dried at room temperature overnight with a relative humidity of 60%; sample B, the membrane dried for 24 h inside a vacuum oven at 95 °C; and sample C, the membrane from sample B tested in the Arbin station with a weight increase of 9.7 mg.

(peaked at 3700 cm-1). The first peak shift from 3450 cm-1 for sample B (dehydrated membrane in vacuo) to 3520 cm-1 for sample A (hydrated membrane in air) indicated a larger fraction of hydrogen bonding of bulk water in the membrane. The further peak shift to 3600 cm-1 observed in sample C was assigned to the O-H stretching in water molecules entrapped in the fluorocarbon environment.22 In this case, the OH vibration frequency will be higher than that in bulk water due to nonassociation. The distinct entrapment of water in the strongly hydrophobic fluorocarbon phase was only possible when the water molecules were directly generated around the Pt-clay particles that were also embedded in the fluorocarbon domains. This result proved the validity of the mechanism for water production that Pt anchored on a clay surface in the selfhumidifying composite membrane can catalyze the reaction of permeated H2 and O2 gases. One of the major concerns regarding the use of Pt for selfhumidification is the potential migration of Pt particles upon repeated use of the MEA. Prolonged operation of the MEA may lead to Pt particle migration and subsequent agglomeration of Pt particles. This can result in local hot spots or even fire within the MEA.23 Therefore, a test on the Pt stability on clay surfaces was performed by prolonged sonication using an ultrasonication probe. No change in Pt particle size and loading was observed. 3.2. Morphology of Pt-Clay in Nafion Matrix. Full exfoliation of nanoparticles inside polymer matrices is the most desirable for achieving optimum property enhancement in polymer nanocomposites. Exfoliation of Pt-nanoclay particles inside the Nafion membrane was achieved through a novel CVD and a recasting protocol as shown in Scheme 1. First, the Pt-clay was exfoliated using a CVD process. The deposition temperature of organo-Pt vapor onto the clay surfaces coincided with the decomposition temperature of the organo-clay. We dispersed Pt-clay into distilled water by sonication prior to mixing with Nafion solution to ensure the uniform dispersion of Pt-clay in the Nafion membrane. Ultrasonication and vigorous mechanical mixing were also employed until the onset of gelation to ensure homogenization and minimize sedimentation. The large increase in viscoelasticity of the matrix upon gelation further stabilized the dispersion of Pt-clay particles. Subsequently, exfoliated Ptclay/Nafion composite films were obtained. The SEM micrograph

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Figure 5. SEM image of membrane cross-section with elemental mappings of Si/Al (a) and TEM images of Pt-clay in Nafion matrix (b-d).

in conjunction with EDX was obtained of the cross-section of the composite membrane to evaluate the homogeneity of the Pt-clay dispersion. As Si/Al are the major elements in the nanoparticles, the elemental mappings of Si/Al were used to characterize the dispersion of nanoparticles through the membrane. Results shown in Figure 5a suggest that homogeneous dispersion of Pt-clay particles across the membrane thickness was achieved. As mentioned in section 3.1., the stability of Pt on clay is important in terms of reliability of the self-humidifying membrane. Figure 5b-d shows the morphology of Pt (d ) 2 to ∼3 nm) on the clay surface embedded in the membrane. No Pt particles were observed outside the clay surface in Figure 5b. A high surface energy of Pt nanoparticles was expected to improve catalytic activity, and its monolayer distribution on clay was to avoid the formation of an electron conducting path through the membrane. The concentration of Pt in Pt-clay was also measured using XRF to prove the stability of Pt nanoparticles on clay. First, a fixed mount of Pt-clay was mixed with pure ethanol followed by 60 min of sonication. The mixture was filtered and dried in a vacuum oven to obtain the powder. XRF was performed on Pt-clay before and after sonication treatment, and the results are shown in Table 3. It is observed that the Pt loading with respect to the total weight in Pt-clay after sonication treatment is almost the same as that in original Pt-clay. This indicates that the combination of Pt-clay is strong enough to sustain powerful sonication for 60 min.

Table 3. Pt Loading on Pt-Clay before and after Sonication Treatment Determined by XRF Pt loading (wt %)

before sonication

after sonication

4.17

4.11

Further confirmation of the exfoliation was demonstrated through XRD analysis (Figure 6). The peak centered at 3.75°, corresponding to the d-spacing of Cloisite 20A, disappeared in the XRD pattern of the Pt-clay/Nafion bulk membrane. 3.3. Evaluation of Cluster-Crystalline Network. Several models have been proposed to describe the microstructure of Nafion, including the cluster-network model,43 a core-shell model,17,18 a lamellar model, a rod-like model,44 etc. These models all suggest that Nafion is comprised of a biphasic structure where the amorphous regions are dominated by the clusters formed by the aggregation of hydrophilic sulfonic groups on the side chains and the crystalline regions formed by the PTFE backbone assembly. Upon water uptake, the nanoscale cluster regions swell and become connected at certain hydration levels to form channels for proton transport through the membrane. However, there were only a few direct experimental visualizations of the clusters.45-47 (43) Hsu, W. Y.; Gierke, T. D. J. Membr. Sci. 1983, 13, 307-326. (44) Rubatat, L.; Rollet, A. L.; Gebel, G.; Diat, O. Macromolecules 2002, 35, 4050-4055. (45) Rollins, H. W.; Lin, F.; Johnson, J.; Ma, J. J.; Liu, J. T.; Tu, M. H.; DesMarteau, D. D.; Sun, Y. P. Langmuir 2000, 16, 8031-8036.

Exfoliated Pt-Clay/Nafion Nanocomposite Membrane

Langmuir, Vol. 24, No. 6, 2008 2669

Figure 6. WXRD curves of as-received Cloisite20A and Pt-clay/ Nafion nanocompoiste membrane.

Figure 8. Deconvolution of original curve using the Gaussian function with background straight line subtraction (R2 ) 0.998). Table 4. Calculated Crystallinity from Figure 9 membrane

Nafion 112

pure Nafion

Pt-clay/ Nafion

Nafion (EW ) 1100)

Xcr (%)

15.7

27.0

26.1

12 (ref 50)

where I(θ) is the intensity of the scattering Figure 7. TEM images of Pt-clay/Nafion nanocomposite membrane (a) and pure Nafion membrane (b) on Cu grid with RuO4 staining.

To verify such a morphology, we used RuO4 vapor staining prior to TEM observations.48 Under TEM, amorphous clusters oxidized by RuO4 vapor appear to be black, and the crystalline regions are not affected. Such effects can be clearly seen in Figure 7. A two-phase texture can be clearly identified. The dark clusters with diameters of 2 to ∼5 nm are uniformly dispersed inside a predominantly white matrix. This clearly agrees with the molecular dynamics simulation data shown by Wescott et al.49 The crystallinity of the membrane is another critical property for its durability and proton conductivity. Low crystallinity Nafion membranes have been found to be brittle and soluble in Moore and Martin’s work.5 Figure 8 is a WAXD pattern obtained on as-received Nafion 112. The diffraction spectra are a convolution of reflections from the crystalline region (centered at 2θ ) 17.5°) and the amorphous region (centered at 2θ ) 16°) using Gaussian functions. The crystallinity, Wcr, was calculated by the integrated intensities of the separated crystalline area under the sharp crystalline peak to the sum of the fitted integrated peak area according to eq 2

Wcr )

∫1022 Icr(θ)θ2dθ ∫1022 [Icr(θ) + Iam(θ)]θ2dθ

(2)

(46) Liu, P.; Bandara, J.; Lin, Y.; Elgin, D.; Allard, L. F.; Sun, Y. P. Langmuir 2002, 18, 10398-10401. (47) Porat, Z. E.; Fryer, J. R.; Huxham, M.; Rubinstein, I. J. Phys. Chem. 1995, 99, 4667-4671. (48) Xue, T.; Trent, J. S.; Osseo-Asare, K. J. Membr. Sci. 1989, 45, 261-271. (49) Wescott, J. T.; Qi, Y.; Subramanian, L.; Weston Capehart, T. J. Chem. Phys. 2006, 124, 134702, 1-14.

f(θ) )

2 2 1 e-(θ-µ) /2σ σx2π

(3)

Table 4 shows the calculated crystallinity from Figure 9 through the deconvolution of the diffraction peaks. All data show R2 > 0.990. The estimated crystallinity of Nafion 112 having equivalent weight ) 1100 was around 15.7%. Using the same technique and calculation methods, this amount was shown as 12% in Fujimuro et al.’s pioneering work.50 It should be emphasized that these values present qualitative estimates rather than quantitative results. The wide distribution of crystallinity may come from the background and baseline effects except for temperature and the Nafion membrane itself. Therefore, the following comparisons of crystallinity among membranes are based on exactly the same test conditions and data processing methods. It is clear that our recasting procedure gives a much higher degree of crystallinity than that obtained commercially. The similarity in crystallinity between the pure and the nanocomposite membranes suggests that the insertion of Pt-clay does not disturb the development of crystallinity in Nafion. 3.4. Proton Conductivity and Water Uptake. Two different proton conductivity measurements have been widely utilized in the literature. One is referred to as the tangential direction conductivity (TDC) method,51 which is known to reduce interfacial resistance and polarization but cannot resemble the proton conduction direction through the membrane during the practical fuel cell operation. The other, the normal direction conductivity (NDC) measurement, was adopted here with the membrane fully immersed in the water. However, the obtained values, as shown in Table 5 and in the literature,52 are mostly (50) Fujimura, M.; Hashimoto, T.; Kawai, H. Macromolecules 1981, 14, 13091315. (51) Zawodzinski, T. A.; Neeman, M.; Sillerud, L. O.; Gottesfeld, S. J. Phys. Chem. 1991, 95, 6040-6044.

2670 Langmuir, Vol. 24, No. 6, 2008

Zhang et al.

Table 5. Proton Conductivity and Water Uptake of Nafion 112, Pure Nafion, and Pt-Clay/Nafion Nanocomposite Membranes with Full Hydration at 25 °C

membrane

proton conductivity (10-2 S/cm)

water uptake (wt %)

Nafion 112 pure Nafion Pt-clay/Nafion

1.06 ( 0.037 1.31 ( 0.046 1.11 ( 0.050

19 22.5 23

lower than the TDC method. A major reason for the reduced conductivity has been attributed to the tendency of the hydrophilic sulfur acid end groups to point outside to the water medium and result in a looser molecular structure approaching the surface of the Nafion membrane. The pressure applied on the membrane between two electrodes also causes a lower water uptake in the membrane. We used Nafion 112 as a reference for comparison purposes. Research on proton conductivity is often coupled with water uptake. Membrane swelling results from a complex interplay between the affinity of the polymer and ionic sites for water and the resistance of the membrane structure and crystallinity to volumetric expansion. As shown in Table 5, the proton conductivity increases with water uptake. The pure Nafion membrane we cast is more hydrophilic and absorbs more water, which facilitates proton transport. Hence, MEA with our pure membrane performed better on single fuel cell tests than the MEA with Nafion 112. However, a slightly lower proton conductivity is observed for the Pt-clay/Nafion membrane. This may be attributed to the platelet clays present in the membrane. A plausible scenario is that the layered clays produce a torturous path for proton transport across the membrane. It should be noted that the difference is small and that the Pt-clay/Nafion composite membrane still exhibits a higher conductivity than Nafion 112. 3.5. Mechanical Properties. Membranes in MEAs must possess not only good conductivity but also good mechanical properties as the membrane is the most fragile component in the fuel cell. An increase in water content and temperature during operation may lead to a further reduction in mechanical properties of the membrane. When the produced water at the cathode cannot be effectively removed by gas flow and back diffusion, flooding may occur and force the membrane to be immersed in water. The membrane must endure these transient conditions. Therefore, the mechanical properties were measured under fully dry and fully hydrated conditions. Additionally, beyond the elastic region, the permanent deformation of the membrane may induce weakened adhesion between the membrane and the electrodes and even failure in MEAs. The tensile properties of the membranes in terms of modulus, yield strength, and energy to fracture are presented in Table 6. First, the Pt-clay/Nafion membrane shows the highest tensile modulus and energy to fracture (or toughness) and comparable yield strength in comparison to Nafion 112. Second, it is also noted that all membranes show a significantly reduced performance when fully hydrated. This also strongly suggests that external humidification is not desirable as the higher concentration gradient of water also gives a higher stress distribution in the membrane leading to a reduced durability of the membrane.

4. Conclusion

Table 6. Tensile Test Results (Modulus, Yield Strength, and Toughness) for Nafion 112, Pure Nafion, and Pt-Clay/Nafion Membranes under Dry and Water Soaked Conditions at 25 °Ca

sample Nafion 112 pure Nafion Pt-clay/Nafion

dry wet dry wet dry wet

modulus (MPa)

yield strength (MPa)

energy to fracture (MPa)

238 ( 6.6 96 ( 8 229 ( 5 93 ( 9.4 257 ( 18 116 ( 11

5.0 ( 0.2 4.5 ( 0.5 4.3 ( 0.3 3.6 ( 1.6 4.9 ( 0.2 3.8 ( 0.9

48.7 50.6 52.9

a Tensile properties were measured at a cross-head speed of 2 mm/ min and ambient temperature.

Figure 9. WXRD patterns of commercial Nafion 112, pure Nafion membrane, and Pt-clay/Nafion nanocomposite membrane.

formance (fueled by dry reactant gases) than the pure Nafion membranes. The Pt-clay/Nafion nanocomposite membranes were prepared by a simple but unique recasting procedure using the Nafion solution and exfoliated clay, on which nanometer-sized Pt particles were pre-deposited by CVD. The combination of Pt nanoparticles and planar clay was proven to even endure vigorous sonication and stirring without any change in Pt particle size and loading. Uniformly dispersed Pt-clay nanoparticles help to improve the mechanical properties of the membranes. Planar and hygroscopic clay stabilized the Pt distribution and balanced the water content. In addition, the in situ generation of water catalyzed by Pt improved the voltage stability. Our unique recasting method developed in this research not only satisfied the requirement of uniform distribution for nanocomposite membranes but also effectively generated hydrophilic clusters, which directly governed the proton conducting. Without any external humidification of H2 and O2, the fuel cell employing a newly prepared Pt-clay/Nafion nanocomposite membrane (60 µm thick) produced a power density of 723 mW/cm2 at 0.5 V. This represents 170% improvement over the commercial Nafion membrane Nafion 112 (269 mW/cm2 at 0.5 V), which is of the same thickness. Considering other aspects of membrane evaluation, we succeeded in developing an allround Pt-clay/Nafion nanocomposite membrane to satisfy the requirements for portable mobile applications.

In this study, we developed a novel Pt-clay/Nafion nanocomposite membrane with a significantly enhanced cell per-

Acknowledgment. The project was funded by the Research Grant Council of Hong Kong under a CERG grant HKUST 612805.

(52) Dimitrova, P.; Friedrich, K. A.; Vogt, B.; Stimming, U. J. Electroanal. Chem. 2002, 532, 75-83.

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