Microwave-assisted synthesis of PTFE/C nanocomposite for polymer electrolyte fuel cells

Transcription

1 Electrochemistry Communications 8 (2006) Microwave-assisted synthesis of PTFE/C nanocomposite for polymer electrolyte fuel cells Zhi Qun Tian a,b, Xiao Li Wang b, Hua Min Zhang b, Bao Lian Yi b, San Ping Jiang a, * a School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore , Singapore b Fuel Cell Engineering Centre, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian , China Received 26 April 2006; received in revised form 20 May 2006; accepted 22 May 2006 Abstract Carbon black supported polytetrafluoroethylene (PTFE/C) nanocomposite is synthesized by an intermittent microwave irradiation (IMI) method for polymer electrolyte fuel cells (PEFC), using PTFE emulsion as the precursor. The synthesized PTFE/C composite is characterized by uniformly distributed and nano-sized PTFE/C particles without additional high temperature treatment and mechanical milling under liquid nitrogen frozen conditions. Incorporation of the PTFE/C nanocomposite into a Pt/C/Nafion catalyst significantly enhances the mass transportation of the catalyst layer without any adverse effect on the electrocatalytic activity of the Pt catalysts. The power output of the cell with a Pt/C/Nafion-PTFE/C composite cathode is 0.66 W cm 2, which is 32% higher than that of the cell with a Pt/C/Nafion cathode without the incorporation of the PTFE/C nanocomposite. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Polymer electrolyte fuel cells; Microwave synthesis; PTFE/C nanocomposites; Cathode catalyst layer; Mass transportation 1. Introduction In polymer electrolyte fuel cells (PEFCs), the kinetics of the electrode reaction, either hydrogen oxidation at the anode or oxygen reduction at the cathode, relies critically on the effective proton conduction, electron conduction and reactant gas transfer paths at the three phase boundaries in a membrane-electrode-assembly (MEA). Typically, a MEA consists of a gas diffusion layer, an electrocatalyst layer and a polymer electrolyte membrane. To achieve optimum performance, a hydrophobic phase such as polytetrafluoroethylene (PTFE) and hydrophilic and proton conducting phase such as perfluorosulfonate ionomer (e.g., Nafion) are added to the electrocatalyst layer to promote reactant gas mass transfer and proton conduction, respectively, within the MEA [1 15]. Extensive evidences have shown that the activity and performance of PEFC * Corresponding author. address: mspjiang@ntu.edu.sg (S.P. Jiang). are critically dependent on the content and distribution of both PTFE and Nafion ionomer phases in the catalyst and gas diffusion layers. For example, Gode et al. found that the proton conductivity of the cathode depends significantly on the Nafion ionomer content and at a Nafion content of 30 wt% in the cathode, the proton conductivity reached a maximum [11]. The cell also has high Pt utilization efficiency and high performance around a Nafion content of 30% in the catalyst layer [13,14]. On the other hand, the performance of PEFC is also critically dependent on the distribution of the PTFE in the Pt/C electrocatalyst layer. Uniform distribution of the hydrophobic and hydrophilic phases in the Pt/C catalysts is very important in the optimization of the reactive sites for the electrode reaction and of the gas diffusion path for the reactant gas and liquid water produced to and from the reactive sites. The electrode microstructure and distribution in turn depends strongly on the preparation methods. Raistrick described the preparation method for a PTFE-bounded Pt/C electrode [1]. In this method, PTFE /$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi: /j.elecom

2 Z.Q. Tian et al. / Electrochemistry Communications 8 (2006) and Pt/C catalysts are directly mixed to form Pt/C/PTFE catalyst electrode and Nafion ionomer solution is then impregnated or sprayed on the electrode to extend the reaction sites. This electrode structure has a good mass transportation property, but Pt catalyst utilization is very low [16,17]. This is due to the fact that PTFE is not an ionic and electronic conductor and Pt nanoparticles bounded by PTFE would have no electrochemical activity, resulting in a significant reduction in the Pt utilization. Wilson et al. eliminated the adverse effect of PTFE by completely replacing PTFE with hydrophilic Nafion ionomer [4,5]. This works well for a cell with very thin electrode. However, since the electrode structure has no hydrophobic phase to construct reactant gas or liquid water transport channels to and from the reactive sites, a serious mass diffusion problem may arise at high current or under air operation. In order to have both high mass transport efficiency and high catalyst utilization, Hards and Ralph [6] and Uchida et al. [7] proposed respectively the preparation of the Pt/ C/Nafion electrode incorporated with PTFE supported on carbon (PTFE/C) composite by high temperature treatment with or without mechanical milling under liquid nitrogen conditions. High temperature treatment is necessary to decompose the surfactant in the PTFE emulsion. However, it also melts PTFE, forming PTFE/C composite agglomerates. In order to break the PTFE/C agglomerates, the mechanical milling has to be carried out under the liquid nitrogen freezing environment due to the high tenacity and low fluidity of PTFE [6]. However, mechanical milling under the liquid nitrogen is a complex and time consuming process. The PTFE/C composite powder is also not particularly uniform due to the difficulty in the control of the melting and forming PTFE/C composite in the conventional heat-treatment method. Microwave heating through a dielectric heating loss is widely employed in the sintering of ceramic materials, chemical analysis and synthesis of organic/inorganic materials and electrocatalysts for its distinctive characteristics of rapidity, simplicity, uniformity and energy saving, in comparison with the traditional heating method [18 22]. In this communication, we report for the first time that the rapid synthesis of the PTFE supported on carbon black (PTFE/C) nanocomposite powder can be achieved by microwave irradiation. The PTFE/C composite powder synthesized is characterized by uniformly distributed and nano-sized PTFE/C powders without any mechanical milling treatment. The cell with Pt/C/Nafion electrode incorporated with PTFE/C nanocomposite showed significant improvement in the power output in comparison to that without the incorporation of the PTFE/C nanocomposite powder. 2. Experimental Synthesis of PTFE/C composite powder was carried out in a microwave oven (LG WD700, 700 W, 2450 MHz). The synthesis procedure was as follows: 4.0 g Vulcan XC-72 carbon black (Carbot Corp.) and 150 ml deionized water were thoroughly mixed under stirring. Then 120 g 5% PTFE emulsion diluted by deionized water was added to the above carbon dispersion under stirring. After the evaporation of water, a nominal 60 wt% PTFE/C mixture was treated by an intermittent microwave irradiation (IMI). The IMI procedure was s irradiation on and 90 s irradiation off. The IMI procedure was repeated for three times to decompose the surfactant in the PTFE emulsion in order to recover the hydrophobicity of PTFE. The morphology and size of the PTFE/C composite powder were characterized by a field-emission scanning electron microscope (FESEM, JSM-6700F, JEOL) in conjunction with energy dispersive X-ray spectroscopy (EDX). Surface area measurements of the powder were performed on an ASAP-2010 surface area and porosity analyzer. A Rigaku Rotalflex (RV-200B) X-ray diffractometer using Cu Ka as a radiation source was employed to identify the crystalline structure of the PTFE/C composite powder. The PTFE loading of the synthesized PTFE/C composite was measured by weight change of the sample before and after the reaction. Nafion impregnated Pt/C catalyst (55 wt% Pt/C, TaNaKa, Japan) was prepared separately by dispersing Pt/C catalyst and Nafion solution (5 wt%, Dupont) into isopropanol solution under ultrasonic treatment. Butyl acetate was then added to the dispersion to promote in situ deposition of Nafion on the Pt/C catalyst, forming Pt/C/ Nafion dispersion. Subsequently, Pt/C/Nafion dispersion and PTFE/C composite powder were mixed under intensive stirring, forming Pt/C/Nafion-PTFE/C composite catalyst ink. The as-prepared composite catalyst ink was directly sprayed on one side of a Nafion 212 membrane. Then the catalyst coated membrane (CCM) was heat-treated at 160 C under a pressure of 1.0 MPa for 5 min to stabilize the contact between the catalyst layer and the membrane. CCM with Pt/C/Nafion catalyst without the incorporation of PTFE/C nanocomposite was also made. The percentage of PTFE/C nanocomposite in the Pt/C/ Nafion-PTFE/C catalyst layer was 30 wt%. Anode electrodes were prepared by the traditional PTFE-bounded method [16]. The Pt loading of both anode and cathode was 0.3 mg cm 2. Wetproof carbon paper with a microporous layer (Toray TGPH-030) was used as a gas diffusion layer. The CCM was sandwiched between the anode and the gas diffusion layer, followed by hot-pressing at 140 C and 10 MPa for 1 min to form a membrane-electrodeassembly (MEA). Cell performance and power output of MEAs (with 5cm 2 active area) were evaluated with stainless steel end plates and stainless mesh as current collectors at 80 C under 0.2 MPa. The hydrogen and air reactant gases were externally humidified at 90 C and 85 C, respectively. The flow rates were 50 ml min 1 for hydrogen and 200 ml min 1 for air.

3 1160 Z.Q. Tian et al. / Electrochemistry Communications 8 (2006) Results and discussion A series of PTFE/C composite powders were synthesized by different intermittent microwave irradiation procedures, i.e., 15, 20, 25 and 30 s irradiation on, respectively and 90 s irradiation off (denoted as PTFE/C-15 s, PTFE/ C-20 s, PTFE/C-25 s, and PTFE/C-30 s). The measured PTFE loading and BET surface area of as-prepared PTFE/C composite powders are listed in Table 1. PTFE loading of the PTFE/C composite powder decreases with increasing in the microwave irradiation time. The PTFE loading was 36.8 wt% for PTFE/C-15 s and decreased to 14.8 wt% for PTFE/C-30 s. The PTFE loading in the PTFE/C composite powders is also significantly lower than the nominal PTFE loading of 60 wt% before the IMI treatment, indicating that part of the PTFE is pyrolyzed due to the dielectric loss heating of the carbon black under microwave irradiation. Subsequently, the surface area of PTFE/ C composite powder increases with increasing of the microwave irradiation time. It was 50.8 m 2 g 1 for PTFE/C-15 s and increased to m 2 g 1 for PTFE/C-30 s (Table 1). Fig. 1 shows the SEM micrographs of the PTFE/C composites synthesized by 20 s irradiation on and 90 s irradiation off (PTFE/C-20 s). PTFE/C composites were highly porous and characterized by very fine particles with diameters of nm. This indicates that PTFE/C composites synthesized by the microwave irradiation treatment consist of nanoparticles without the formation of agglomerates. The IMI method is effective in the preparation of PTFE/ C nanocomposite powders without an additional mechanical milling process. EDX analysis showed the existence of C, O and F in the PTFE/C composite. The PTFE/C composite powders prepared by different IMI procedures showed similar microstructures. Fig. 2 shows the XRD patterns of PTFE powder, VulcanXC-72 carbon black and PTFE/C nanocomposite powder. The characteristic diffraction peaks of PTFE powder and carbon black are found in the microwave-assisted synthesized PTFE/C nanocomposite at the diffraction angles of 18.9 and 24.7, respectively. The XRD analysis again indicates that PTFE/C composite powder has been synthesized. Fig. 3 shows the polarization performance and power output of the cells with Pt/C/Nafion catalyst cathode with and without the incorporation of PTFE/C nanocomposites. The PTFE/C-20 s nanocomposite was used in the preparation of the Pt/C/Nafion-PTFE/C composite catalyst in this case. Both cells exhibit current voltage polarization curves with clear activation, cell resistance and mass transfer-control regions. At low current densities (less than Fig. 1. SEM micrographs of PTFE/C nanocomposite powder. The PTFE/ C nanocomposites were synthesized by 20 s irradiation on and 90 s irradiation off (PTFE/C-20 s). 0.4 A cm 2 ), the cell performances of both cathodes have no significant differences. However, as the current density increases, the cell with Pt/C/Nafion-PTFE/C cathode starts to show a significantly higher performance than that with the Pt/C/Nafion cathode. For example, the cell voltage at current density of 1.0 A cm 2 was 0.30 V for the cell with the Pt/C/Nafion cathode and increased significantly to 0.64 V for the cell with the Pt/C/Nafion-PTFE/C cathode. The maximum power density of the cell with the PtC/Nafion-PTFE/C cathode is 0.66 W cm 2, which is 32% higher than 0.5 W cm 2 obtained on the cell with the Pt/C/Nafion cathode. Lin et al. showed in their simulation that the catalyst layer is more susceptible to flooding because liquid water is generated in the cathode [23]. The significant Table 1 PTFE loading and BET surface area of PTFE/C nanocomposites synthesized by different microwave irradiation procedures PTFE/C composite Carbon powder PTFE/C-15 s PTFE/C-20 s PTFE/C-25 s PTFE/C-30 s PTFE loading (%) BET surface area (m 2 g 1 )

4 Z.Q. Tian et al. / Electrochemistry Communications 8 (2006) Fig. 2. X-ray diffraction patterns of PTFE, carbon black and PTFE/C- 20 s nanocomposite powders. Fig. 4. Cyclic voltammograms curves of the cells with a Pt/C/Nafion- PTFE/C catalyst cathode and a Pt/C/Nafion catalyst cathode with nitrogen instead of air on the cathode side. The scan rate was 50 mv s 1. Fig. 3. Polarization curves and power output of the cells with a Pt/C/ Nafion-PTFE/C catalyst cathode and a Pt/C/Nafion catalyst cathode, measured at 80 C. improvement in the cell performance with the Pt/C/Nafion- PTFE/C cathode at high current densities suggests that incorporation of the PTFE/C nanocomposites in the Pt/ C/Nafion significantly enhances the mass transportation of the Pt/C/Nafion catalyst layer. Fig. 4 shows the cyclic voltammograms of the cell measured with nitrogen instead of air flow on the cathode side under the same cell operating conditions as those described above. The hydrogen anode was used as the reference and counter electrodes. The voltage was scanned between 0 and 1.0 V at 50 mv s 1. It shows that both electrodes have comparable cyclic voltammograms (i.e., similar hydrogen desorption/adsorption peak area), indicating that they have more or less the same electrochemically active area. This indicates that incorporated PTFE/C nanocomposites in the Pt/C/Nafion catalyst do not block the electrocatalytic activity of the Pt/C catalysts. This is very different to that of the PTFE-bounded Pt/C catalysts [1]. The function of the PTFE/C nanocomposite is to enhance the effective mass transport by providing the hydrophobic channels for the air flow and liquid water produced to and from the reaction site, thus increasing the performance of PEFC, particularly at high current densities (Fig. 3). Incorporation of hydrophobic PTFE and hydrophilic Nafion is probably the most effective way to build a matrix of both gas diffusion paths and proton conducting paths within the catalyst layer. However, PTFE is very difficult to break and uniformly mix with fine carbon powders because of its high tenacity, low fluidity, and hydrophobic nature. Thus, PTFE emulsion with added surfactant is often used as the precursor in the preparation of PTFE supported on carbon composite. A high heat-treatment temperature is needed to remove the surfactant in the PTFE emulsion in order to recover the hydrophobic property of PTFE. However, high temperature treatment also melts PTFE, resulting in the agglomeration of PTFE/C composite. To mechanically break the PTFE/C agglomerates, the composites have to be frozen at the liquid nitrogen conditions [6]. The process is rather complex and time consuming. On the other hand, it is well known that the principle of the microwave irradiation or heating is that the changing electrical field interacts with the molecular dipoles and charged ions and heat is generated by the molecular rotation due to the friction of this motion. Carbon black is susceptible to the microwave and can be heated quickly to a high temperature under the microwave irradiation. Thus, the high temperature generated by microwave irradiation burns off the surfactant in the PTFE emulsion. At the same time, the microwave process could break the PTFE/C agglomerates probably due to the high internal pressure produced by the rapid decomposition of the surfactant and some of the PTFE. This appears to be supported by the formation of the nano-sized PTFE/C composite particles (Fig. 1). Because microwave only heats carbon and not PTFE (PTFE is a microwave transmitter and thus it is transparent to microwave [21]), the heat generated by the microwave irradiation would only melt PTFE

5 1162 Z.Q. Tian et al. / Electrochemistry Communications 8 (2006) which is attached to or in the vicinity of the carbon particles. This would effectively produce a uniformly distributed PTFE/C nanocomposite. The PTFE distribution or loading could also be effectively controlled by the variation of the microwave irradiation on and off time, as shown by the dependence of the PTFE loading of PTFE/C nanocomposites on the IMI procedure (see Table 1). The present study demonstrates that the IMI process is a simple, rapid, and effective method to synthesize a uniformly distributed PTFE/C nanocomposites. 4. Conclusions A Pt/C/Nafion-PTFE/C nanocomposite catalyst is synthesized by a novel two step process for polymer electrolyte fuel cells (PEFC). Polytetrafluoroethylene (PTFE) supported on carbon black (PTFE/C) nanocomposite powder was first synthesized by an intermittent microwave irradiation, using PTFE emulsion as the precursor. High resolution SEM results show that the microwave-assisted synthesized PTFE/C composite is uniformly distributed and the particle size of the PTFE/C composite is in the nm range. The PTFE/C nanocomposite was then mixed with Pt/C/Nafion to form a Pt/C/Nafion-PTFE/C composite catalyst. The Pt/C/Nafion powder was separately prepared by an in situ deposition process using butyl acetate. The incorporation of PTFE/C nanocomposite in the Pt/C/Nafion catalysts shows a significant improvement in the mass transportation without any adverse effect on the electrochemical activity of the Pt electrocatalysts. The power output of the cell with a Pt/C/Nafion-PTFE/C composite cathode is 0.66 W cm 2, which is 32% higher than 0.5 W cm 2 obtained on the cell with a Pt/C/Nafion cathode. The results of the present study demonstrate that the intermittent microwave irradiation method is simple, rapid, and effective to produce a uniformly distributed and nanosized PTFE/C composite without an additional high temperature treatment or mechanical milling under liquid nitrogen frozen conditions. Incorporation of PTFE/C nanocomposite in the Pt/C/Nafion catalysts considerably enhances the mass transportation properties of the catalyst layer, which are important for PEFCs operated under high currents or at air operation conditions. Acknowledgement This work is supported by Chinese Academy of Sciences Wang Kuang Cheng Postdoctoral Prizing Funding. References [1] I.D. Raistrick, in: Proceedings of the Symposium on the Diaphragms, Separators and Ion exchange Membrane, The Electrochemical Society, Pennington, NJ, 1986, p [2] X. Cheng, B. Yi, M. Han, J. Zhang, Y. Qiao, J. Yu, J. Power Sources 79 (1999) 75. [3] S. Gamburzev, A.J. Appleby, J. Power Sources 107 (2002) 5. [4] M.S. Wilson, S. Gottesfeld, J. Electrochem. Soc. 139 (1992) L28. [5] M.S. Wilson, S. Gottesfeld, J. Appl. Electrochem. 22 (1992) 1. [6] G.A. Hards, T.R. Ralph, US patent, 5,501,915, March 26, [7] M. Uchida, Y. Aoyama, N. Eda, A. Ohta, J. Electrochem. Soc. 142 (1995) [8] E. Antolini, L. Giorgi, A. Pozio, E. Passalacqua, J. Power Sources 77 (1999) 136. [9] E. Passalacqua, F. Lufrano, G. Squadrito, A. Patti, L. Giorgi, Electrochim. Acta 46 (2001) 799. [10] S.C. Thomas, X. Ren, S. Gottesfeld, J. Electrochem. Soc. 146 (1999) [11] P. Gode, F. Jaouen, G. Lindbergh, A. Lundblad, G. Sundholm, Electrochim. Acta 48 (2003) [12] M. Han, S.H. Chan, S.P. Jiang, J. Power Sources, in press. [13] G. Sasikumar, J.W. Ihm, H. Ryu, J. Power Sources 132 (2004) 11. [14] Z. Qi, A. Kaufman, J. Power Sources 113 (2003) 37. [15] C. Lim, C.Y. Wang, Electrochim. Acta 49 (2004) [16] E.A. Ticianelli, C.R. Derouin, S. Srinivasan, J. Electroanal. Chem. 251 (1988) 275. [17] S. Srinivasan, O.A. Velev, A. Parthasarathy, D.J. Manko, A.J. Appleby, J. Power Sources 36 (1991) 299. [18] J.P. Cheng, A.G. Dinesh, Y.J. Zhang, Mater. Lett. 56 (2002) 587. [19] A. Agazzia, C. Pirolab, J. Microchem. 67 (2000) 337. [20] R. Rustum, A. Dinesh, J.P. Cheng, Nature 399 (1999) 668. [21] K.J. Rao, B. Vaidhyanathan, M. Ganguli, P.A. Ramakrishnan, Chem. Mater. 11 (1999) 882. [22] Z.Q. Tian, S.P. Jiang, Y.M. Liang, P.K. Shen, J. Phys. Chem. B 110 (2006) [23] G. Lin, W. He, T.V. Nguyen, J. Electrochem. Soc. 151 (2004) A1999.