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Nafion composites as Pt supports in gas diffusion electrodes for proton exchange membrane fuel cells
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Optimization of the amount of Nafion in multi-walled carbon nanotube/Nafion composites as Pt supports in gas diffusion electrodes for proton exchange membrane fuel cells Hussein Gharibi a,*, Masoumeh Javaheri a, Mehdi Kheirmand b, Rasol Abdullah Mirzaie c a
Department of Chemistry, Faculty of Science, Tarbiat Modares University, P.O. Box 14115-175 Tehran, Iran Department of Chemistry, Faculty of Science, Yasouj University, Yasouj, Iran c Department of Chemistry, Faculty of Science, Shahid Rajaee University, Tehran, Iran b
article info
abstract
Article history:
The Nafion loading in multi-walled carbon nanotube (MWCNT) composites with Nafion
Received 20 June 2010
used as Pt support in the oxygen reduction reaction (ORR) has been studied. We varied the
Received in revised form
amount of Nafion in these composites and added a Pt loading of 0.3 mg cm2 to the catalyst
11 August 2010
layer. The performance of these electrodes in the ORR was measured with linear sweep
Accepted 2 September 2010
voltammetry (LSV), electrochemical impedance spectroscopy (EIS), chronoamperometry,
Available online 8 October 2010
inductive coupled plasma (ICP), X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). In addition, we compared the performance
Keywords:
of the MWCNTs as Pt supports with those of the composites. Our results indicate that the
Catalyst layer (CL)
composites are better Pt supports in comparison with MWCNT.
Gas diffusion electrode (GDE)
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Oxygen reduction reaction (ORR)
reserved.
Proton exchange membrane fuel cell (PEMFC) Pt support
1.
Introduction
Carbon nanotubes possess a wide variety of remarkable properties, most notably high electrical and thermal conductivities, mechanical strength, and large catalytic surface areas, and thus have potential uses in a variety of power generation and storage devices, including proton exchange membrane fuel cells (PEMFCs) [1]. Composite electrodes consisting of Pt nanoparticles supported on multi-walled carbon nanotubes grown directly on carbon paper (Pt/MWCNTs/carbon paper) have previously been synthesized by using glacial acetic acid as a reducing agent [2]. Lee and Hu have successfully developed a simple but
effective method for grafting various chemical reagents onto the opposite tube-ends of individual carbon nanotubes, as confirmed with X-ray photoelectron spectroscopy measurements. This approach could be used in the site-selective selfassembly of both MWCNTs and single-walled carbon nanotubes (SWCNTs) into a large variety of novel functional systems with highly controllable structures and various applications [3,4]. Tang and Wu presented a detailed comparison of MWCNTs and SWCNTs in an effort to identify the better supporting carbon material for electrocatalysts in direct methanol fuel cells (DMFCs) [5,6]. Salvetat et al. found that the mechanical properties of nanotubes are strongly dependent on their structures; various types of nanotubes
* Corresponding author. E-mail addresses: [email protected], [email protected] (H. Gharibi). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.09.008
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Table 1 e Gas diffusion electrodes (GDEs) with various Nafion loadings. GDE 1 2 3 4 5
Pt loading mg cm2
Nafion loading mg cm2
Pt/MWCNT composite
Pt/ MWCNT
0.3 0.3 0.3 0.3 0.3
0.5 0.75 1 1.25 1
e
e e e e
including SWCNTs and arc-grown multi-walled nanotubes were considered [7]. Carbon nanotubes were selectively grown directly on carbon paper by performing chemical vapor deposition catalyzed with electrodeposited cobalt. The prepared carbon nanotubes were employed as the supports for platinum catalysts, which were electrodeposited onto the nanotubes [8]. For comparison, similar loadings of the Pt catalyst supported on carbon nanotubes were prepared with two approaches: the borohydride reduction method and the ion exchange technique. It was found that the Pt/SWCNTs catalyst prepared with the ion exchange method exhibits higher Pt utilization than the Pt/SWCNTs prepared with the borohydride reduction method. Furthermore, in comparison to an E-TEK 20 wt.% Pt/C catalyst with the support of carbon black, electrochemical measurements were used to show that the Pt/SWCNTs prepared with the ion exchange method exhibit a higher catalytic activity in methanol oxidation [9e11]. Wang et al. tested a hydrothermal catalyst preparation process that utilizes CNTs as a base with the aim of reducing the amount of Pt; Pt/CNTs with 40% Pt weight were prepared. The catalyst was characterized with IR spectroscopy, XRD, TEM, and Proven Solution Analyzer software. Their results show that by using the HNO3eH2O2 redox system the ports of the CNTs are opened and that large quantities of impurities are removed. The effects of varying the reaction temperature and time on Pt granule size have been determined [12,13]. Pt nanoparticles of around 1.5 nm in size that are well-dispersed on CNTs were obtained when ethylenediaminetetraacetic acid disodium salt (EDTAe2Na/Pt) was utilized [14]. Hu et al. have suggested that CNTs can promote electron transfer reactions when used as electrodes. However, chemical reactions can occur that result in changes in nanotube
structure and that generate active functional groups, such as hydroxyl and carboxyl. Hu et al. have compared the structure, stability, and electrode kinetics of carboxyl-modified CNT electrodes prepared by soaking in concentrated nitric acid with those of non-modified CNT electrodes [15]. Pt and PteRu particles deposited on polypyrroleecarbon nanotube composite polymer films exhibit excellent catalytic activity and stability in methanol oxidation, which indicates that composite films are promising support materials for fuel cell applications [16,17]. The electrocatalytic properties of Pd/ MWCNT electrodes in the oxidation of hydrazine have been investigated with cyclic voltammetry (CV); excellent electrocatalytic activity was observed, which might be due to the small particle size and high dispersion of palladium particles [18]. Guo et al. reported a matrix of composite films consisting of carbon nanotubes and Nafion for the immobilization of on glassy carbon (GC) electrode surfaces. A Ru(bpy)2þ 3 composite film-modified electrode was found to exhibit better long-term electrochemical and electrogenerated chemiluminescence (ECL) stability than a pure Nafion film-modified electrode [19]. Ambrosio et al. were investigated the Platinum catalyst supported on mesoporous carbon for PEMFC [20]. The effects of varying the Nafion loading in the catalyst layer on cell performance, pore size distribution, and Pt surface area were investigated [21,22]. The presence of Nafion
Fig. 1 e ICP curve for the platinum concentrations on the supports.
Fig. 3 e Nyquist plots of the impedance responses from 10 kHz to 100 mHz for the GDEs including the Nafion membrane (Nafion 112) at OCV vs. Ag/AgCl, at 25 C.
Fig. 2 e Nyquist plots of the impedance responses from 10 kHz to 100 mHz for the GDEs at 400 mV vs. Ag/AgCl, at 25 C.
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Fig. 4 e XRD patterns of GDE1, GDE2, GDE3, GDE4, and GDE5.
ionomer increases the size of the three-dimensional zone of catalytic activity and the platinum utilization in the electrode, and also helps to retain moisture and prevent membrane dehydration, especially at high current densities. The Nafion content in the electrode must be optimized to achieve high performance. As the platinum loading increases, the electrode thickness and the Nafion thickness increase, and hence the optimum Nafion content can depend on the platinum loading [23e27]. Electrodes with Pt/CNT catalysts sulfonated through the in situ radical polymerization of 4-styrenesulfonate have been found to exhibit better performance than their unsulfonated counterparts, mainly because of the easier access for protons and the well-dispersed distribution of the sulfonated Pt/CNT catalysts; thus sulfonation improves performance and reduces the cost of Pt/CNT-based PEMFCs. A half-MEA with an active area of 1.0 cm 1.0 cm was fabricated by employing a Nafion 115 membrane and an in-house CNT-based gas diffusion electrode (GDE) with a platinum loading of approx. 0.05 mg cm2; Nafion electrolytes were not used [28]. In a previous study [29,30], we prepared a catalyst layer containing a PANIeNafion composite. In this paper, we introduce a new method for the preparation of MWCNT/Nafion composites, and our results with this method are compared with those of other methods. The main distinguishing feature of this method is in the timing of the addition of the Nafion solution to the MWCNTs. The
effects of varying the Nafion loading in the catalyst layer on the electrode performance and the Pt surface area were investigated; for this purpose we fabricated four electrodes with different amounts of Nafion (0.5, 0.75, 1, and 1.25 mg cm2). The performance of these electrodes in the ORR was compared with that of an electrode with MWCNTs as a Pt support and a Nafion loading of 1 mg cm2. Furthermore, the electrodes were characterized with cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), chronoamperometry, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The new method was found to result in better performance and lower resistance.
Fig. 5 e Cyclic voltammetry curves for the GDEs for a scan rate of 50 mV sL1.
Fig. 6 e IeV curves of the GDEs in the ORR for a scan rate of 1 mV sL1 at 25 C.
2.
Experimental
2.1.
Composite preparation
In this research, we fabricated four composites with various amounts of Nafion. Since MWCNTs are chemically inert, the MWCNTs (20e30 nm Aldrich) were functionalized by pretreatment with 70% nitric acid in order to introduce surface oxides before preparation of the composites [3,4,15,18]. In our
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study, the MWCNTs were refluxed under constant agitation at 120 C in concentrated nitric acid for 12 h. The solid phase was removed by filtration and washed with distilled water, and the recovered functionalized MWCNTs were then dried at 80 C for 12 h. The composites were prepared from the dried samples and Nafion solution (5% from Aldrich). Mixtures of functionalized MWCNTs and various amounts of Nafion solution in 2-propanol (Merck), water, and glycerol (Merck) were sonicated for 20 min with a sonicator (Misonix model S-3000) to prepare homogeneous suspensions; these suspensions were dried at 70 C. Four MWCN/Nafion composites were prepared, each containing a different amount of Nafion. The resulting electrodes contained 0.5, 0.75, 1, and 1.25 mg cm2 of Nafion in the catalyst layer and were named GDE1, GDE2, GDE3, and GDE4, respectively.
2.2. Preparation of Pt/MWCNT and Pt/composite (NafioneMWCNT) In this step, Pt was deposited on the MWCNT/Nafion composites. One additional electrode named GDE5 in which Pt was deposited on functionalized MWCNTs was prepared. The MWCNT/Nafion composites for GDE1eGDE4 and the functionalized MWCNTs for GDE5 were vigorously mixed with H2PtCl6 solution in a 2-propanol (Merck) and water (1:3 v/v) solvent mixture at 50 C for 6 h. These suspensions were then heated to 80 C, and the pH of each suspension was adjusted
to 8.5 with a 0.5 M Na2CO3 solution. An abundant amount of sodium formate (HCOONa) [Merck] was added to perform a chemical reduction for 2 h. Each suspension was then filtered, washed, and dried at 90 C for 4 h. Our ICP results indicated that the samples contained 6.7 wt.% Pt, and so GDEs could be prepared from the Pt/functionalized MWCNTs and the Pt/MWCNT/Nafion composites.
2.3.
Fabrication of the GDEs
For the diffusion layer, a mixture of 30 wt.% PTFE and 70 wt.% Vulcan in 2-propanol (Merck), water, and glycerol (Merck) was sonicated for 20 min with a sonicator (Misonix model S-3000) to produce a homogeneous suspension. The suspension was rolled onto the carbon paper (TGPH-0120T) (Toray), and the electrode was dried in air at 120 C for 1 h and then finally sintered at 340 C for 30 min. The loading of Nafion in the diffusion layer was fixed at 1 mg cm2 [31,32]. To prepare the catalyst layers in GDE1eGDE4, a homogeneous suspension containing the desired amounts of the Pt-MWCNT/Nafion composite (6.7 wt.%), glycerol (Merck), 2-propanol (Merck), and water was sonicated for 20 min. This suspension was rolled onto the diffusion layer; the electrode was then dried at 40 C for 30 min and then at 80 C for 30 min. The Pt loading was 0.3 mg cm2. To prepare the catalyst layer of GDE5, a homogeneous suspension containing the desired amounts of Pt/MWCNT (6.7 wt.%), Nafion solution (5% from Aldrich) in glycerol, 2-propanol, water, and glycerol was sonicated for 20 min. This suspension was rolled onto the diffusion layer; the electrode was dried at 40 C for 30 min and then at 80 C for 30 min. The Pt loading was 0.3 mg cm2.
2.4.
Electrochemical measurements
An EG&G Princeton Applied Research Model 273A instrument was used to determine the electrochemical properties of the electrodes. The performances of the porous GDEs (geometric exposed area 1 cm2) in the reduction of oxygen were investigated in 2 M H2SO4. All measurements were performed at 25 C in a conventional three-electrode cell, with O2 flowing at 50 mL min1. The GDEs were mounted into a Teflon holder that contains a pyrolytic graphite disk as a current collector and has provision for feeding oxygen from the back of the
Fig. 7 e Graphical representation of the Nafion ionomer position in catalyst layer.
Fig. 8 e Chronoamperograms of the GDEs in the presence of O2, E [ 0.4 V vs. Ag/AgCl, at 25 C, not stirred the solution.
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electrode. A large area platinum flat electrode was used as the counter electrode. An Ag/AgCl reference electrode was placed close to the working electrode surface. A rotating disk electrode immersed in 2 M H2SO4 with a rotation rate of 1500 rpm was used to perform cyclic voltammetry. The electrochemical cell was connected to a potentiostategalvanostat (EG&G Model 273A) for IeV polarization measurements and chronoamperometry, and to a frequency response detector (model 1025) for electrochemical impedance spectroscopy. In order to perform a quantitative evaluation of resistance against the ORR, the AC impedance method was used. In the Nyquist plot of the semicircle diameter, Rp is the polarization resistance and the sum of the electrode and electrolyte resistance that was subtracted for these GDEs. The polarization resistance is the sum of two terms: the charge transfer and diffusion resistances. Impedance measurement was investigated in 0.4 V vs. Ag/AgCl potential for obtaining Rp. This potential is in ohmic resistance region. The AC potential amplitude was 5 mV, and the frequency range was 10 kHze0.1 Hz. We used impedance measurements to characterize the ionic resistances of the electrodes. At low frequencies, a Warburg-like response (45 slope) is observed, which indicates the occurrence of ion migration through the Nafion membrane and catalyst layer. The ionic resistance Rion can be obtained from the length of the Warburg-like region projected onto the real impedance (Zreal) axis [33,34,35]. For obtaining Rion, the impedance measurement has been investigated in OCV potential and experimental complex-plane impedance plots (10 kHze0.1 Hz) for Argon-bathed are shown in Fig. 3.
2.5. Measurements of the physical properties of the electrodes A scanning electron microscope (Model XL30, Philips co.) was used after coating the electrodes’ surfaces with gold, a transmission electron microscope (TEM/STEM CM Philips 200) was used to produce the TEM images, and XRD analysis was carried out for the catalysts by using an XPERT MPD Philips diffractometer with a Cu X-ray source operating at 40 kV and
Fig. 9 e SEM image of the catalyst layer of GDE1 (0.5 mg cmL2 Nafion in a MWCNT/Nafion composite) at a magnification of 15,0003.
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40 mA. The XRD patterns were obtained at a scanning rate of 1 /min with a step size in the 2q scan of 0.02 in the range 20e100 . The amount of Pt reduced on each support was determined by using the inductive coupled plasma (ICP) technique (ICPOES, Varian Vista-PRO, Australia); for these measurements we dissolved 5 mg of each sample (synthesized Pt on the supports) and standard samples (Pt/C 10% and 20% wt) in a mixture of hydrochloric acid and nitric acid (3/1). These solutions were refluxed at 70 C and then used in the ICP measurements.
3.
Results and discussion
The GDEs (1e5) were prepared with different amounts of Nafion, as explained in Experimental section and Table 1. The results are shown in Figs. 1e16 and in Table 2. The ICP results indicate that the Pt loading in these GDEs is 0.3 mg cm2 (Fig. 1). As mentioned above, there are three participants in electrochemical reactions: gases, electrons, and protons. The reactions can only take place on portions of the catalyst surface to which all three participants have access. The electrons travel through the electrically conductive solids, including the catalyst itself, so it is important that the catalyst particles are electrically connected in some way to the substrate. Protons travel through the ionomer, so the catalyst must also be in intimate contact with the ionomer. Finally, the reactant gases travel only through voids, so the electrode must be sufficiently porous to allow gases to travel to the reaction sites. At the same time, product water must be effectively removed or the electrode will flood and prevent oxygen access. The reactions take place at a three-phase boundary between ionomer, solid, and empty space [32].
3.1.
AC impedance spectroscopy
To gain more information about the GDEs (GDE1eGDE5), the a.c. impedance spectrum of each electrode was obtained. The
Fig. 10 e SEM image of the catalyst layer of GDE2 (0.75 mg cmL2 Nafion in a MWCNT/Nafion composite) at a magnification of 15,0003.
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resulting data were obtained at 400 mV vs. Ag/AgCl saturated with KCl, and were plotted in the Nyquist representation. The polarization resistance Rp was found to be lower for GDE3, which contains 1 mg cm2 Nafion in the catalyst layer, than for the other GDEs. Although the impedance spectra have similar semi-circular shapes, the diameters of the semi-circles differ significantly. As shown in Fig. 2, the resistances of GDE1, GDE2, and GDE4 are too high in comparison with those of the other electrodes. When the amount of Nafion in the GDEs is increased from 0.5 to 0.75, the resistance of the electrode decreases. However, upon an additional increase in the amount of Nafion, a high resistance is observed, i.e., in GDE4. The high resistance of this electrode is due to an excess of Nafion in the catalyst layer. Therefore, we should optimize the amount of Nafion in the GDEs. One approach is to suggest that the optimum value of the amount of Nafion arises for the GDE with the lowest resistance, so GDE3 is then the best electrode. In the case of GDE5, the ionomer blocks the catalyst sites, so the resistance of electrode GDE5 is higher than that of GDE3. The ionic resistance or proton transport of each GDE was determined by using a half-cell assembly in a conventional three-electrode system; the working electrode contained carbon paper, MPL, catalyst layer, and a Nafion 112 membrane. The results are shown in Fig. 3 and Table 2: GDE3 has the lowest ionic resistance, which is attributed to its optimum three-phase zone.
3.2.
XRD pattern of platinum
d¼
0:9l B2q cosq
(1)
where d is the average particle size, l is the wavelength of the X-ray (1.54056 A), q is the angle at the maximum of the peak, and B2q is the width of the peak at half-height. This variation in the crystal size of Pt arises because the support of Pt in each sample is different. These results show that Nafion plays an important role during the synthesis and that varying the concentration of Nafion has some influence on the formation of the Pt particles, i.e., the concentration of Nafion influences the reduction of platinum on the surface of the MWCNTs. In the preparation of GDE5, the Pt nuclei grew as soon as the reducing agents, sodium formate (HCOONa), was added to the mixture, i.e., before they were covered with Nafion. This growth resulted in the inferior performance of GDE5 in comparison with those of GDE2 and GDE3 since some of its Pt particles were masked by Nafion, which was added to this mixture after the coating of Pt onto the MWCNT support. Since the Pt particles are covered with Nafion, the concentration of exposed Pt on the surface of the MWCNT support is reduced; therefore, the performance of GDE5 decreases. This conclusion is verified by the XRD pattern for GDE3 (see Fig. 4).
3.3.
Electroactive surface area (EAS)
The cyclic voltammograms for GDEl, GDE2, GDE3, GDE4, and GDE5 are shown in Fig. 5. The coulombic charge for hydrogen desorption was used to calculate the electroactive surface area (EAS) of each electrode (Table 2) [36]:
Fig. 4 shows the XRD patterns of the Pt/MWCNT and PtMWCNT/Nafion composite nanocatalysts. The peak at 2q ¼ 26.5 corresponds to the (002) planes of the graphitized MWCNTs and the peaks at 2q ¼ 39.8 , 67.5 , and 46.2 are associated with the (111), (220), and (200) planes of fcc (face centered cubic) Pt, respectively. These results indicate that Pt has been successfully reduced. The average sizes of the Pt particles were calculated from the line broadening of the (111) peak by using the Scherrer equation after background subtraction and found to be 0.76, 3.57, 3.59, 1.64, and 0.75 nm for the five samples, respectively (Table 2).
where Qh is the charge for hydrogen desorption (mC cm2) and [Pt] is the platinum loading (mg cm2) and 0.21 (mC cm2) is the charge required to oxidize a monolayer of H2 on bright Pt. The roughness factor (Rf) can then be calculated by using the following equation:
Fig. 11 e SEM image of the catalyst layer of GDE3 (1 mg cmL2 Nafion in a MWCNT/Nafion composite) at a magnification of 15,0003.
Fig. 12 e SEM image of the catalyst layer of GDE4 (1.25 mg cmL2 Nafion in a MWCNT/Nafion composite) at a magnification of 15,0003.
EAS ¼
Qh 0:21 ½Pt
(2)
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Rf ¼
EAS S
(3)
where S is the geometric surface area (m2 g1). These results are tabulated in Table 2 and are consistent with those of other studies [2,17,29]. The polarization curves for the GDEs in conventional threeelectrode cells are presented in Fig. 6. The current density values are considerably greater for GDE3 than for the others, which indicate that the performance of this cell is much better than that obtained when using an electrode containing the MWCNT/Nafion composite (1 mg cm2 of Nafion). The superior performance of GDE3 is attributed to an increase in the size of the three-dimensional reaction zone and to the optimum value of Nafion in the MWCNT/Nafion composite. As a result, the concentrations of the reactants near the surface of the catalyst are increased, and hence the reaction rate is improved. The performances of GDE1 and GDE4 are very poor. The poor electrode performance for a low Nafion content (GDE1) might be due to the presence of fewer electrochemically active sites in the three-phase zone, and the decrease in performance for a high Nafion content (GDE4) might be due to reduced gas permeability and increased mass transfer polarization. Further, the electrochemically active surfaces of the GDEs were calculated from the CV data and are shown in Table 2. These results indicate that the performance of GDE3 is better than that of GDE5. This superiority of GDE3 is attributed to better catalyst accessibility; in other words, when the MWCNT/Nafion composite is used as the Pt support, the accessible catalyst surface is greater in GDE3 than in Pt/ MWCNT (GDE5), in which the surface of Pt particles is blocked by the Nafion ionomer. As mentioned above, the reactions in these catalysts occur in a three-phase zone, i.e., where the ionomer, solid, and empty space phases meet (Fig. 7a). The reaction zone can be expanded by either “roughening” the surface of the membrane or by adding ionomer to the catalyst layer (Fig. 7b). The whole surface of the catalyst can be screened by a thin ionomer layer (Fig. 7c), except at the electrical contacts. In two extreme cases, the entire catalyst layer can be protected by the ionomer layer and there is then no
Fig. 13 e SEM image of the catalyst layer of GDE5 (1 mg cmL2 Nafion) at a magnification of 15,0003.
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electrical contact with the electrically conductive support (Fig. 7d) or there is no ionomer in contact with the Pt particles (Fig. 7e). These considerations show that the ratios of the catalyst area covered by ionomer to the catalyst area open to empty space and to the catalyst area in contact with other catalyst particles or the electrically conductive support should be optimized [32].
3.4.
Kinetic parameters
The kinetic parameters of the ORR for a GDE can be obtained from the polarization data. Our analysis of the experimental polarization data was performed by using the Tafel equation [37]: h ¼ blog
i i0
(4)
where: h ¼ E Eeq
(5)
h is the over-potential, Eeq is the open-circuit voltage, b is the Tafel slope, i is the current density, and i0 is the exchange of current density for the ORR. The kinetic parameters of the ORR for the GDEs can be obtained from the IeV curves and these equations. The Tafel slopes and the current densities of the GDEs at 0.9 V are shown in Table 2. These results indicate that the best performance is exhibited by GDE3. To achieve maximum efficiency, it is necessary to prepare an environment with a triple interface comprised of Nafion, platinum, and oxygen. Therefore, we conclude that in GDE3 the amount of Nafion is optimized and the triple zone is more reliable. As mentioned in the previous section, catalyst accessibility is better in GDE3 than in GDE5, and thus the rate of the ORR and the current density are improved.
Fig. 14 e TEM image of the catalyst layer of GDE2 (0.75 mg cmL2 Nafion in a MWCNT/Nafion composite).
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Fig. 16 e TEM image of the catalyst layer of GDE5 (1 mg cmL2 Nafion).
Fig. 15 e TEM image of the catalyst layer of GDE3 (1 mg cmL2 Nafion in a MWCNT/Nafion composite).
3.6. 3.5.
Chronoamperometry
The diffusion coefficients of O2 in the GDEs were determined with chronoamperometry. For large electrodes, the information obtained was limited by the Cottrellian decay with time. According to Winlove et al. [38], the time window for these experiments follows the relation 1 > s1/2 > 0.5, where s is a dimensionless parameter equal to 4Dt/r2. The Cottrell [39] equation is: i ¼ nFAðD=ptÞ1=2 C)
(6)
where i is the limiting current (mA), n the number of electrons, F is the Faraday constant (96485 C mol1), A is the surface area of the electrode (cm2), D is the diffusion coefficient (cm2 s1), t is the time (s), and C* is the concentration of the reactant. The highest value of the permeability (D1/2 C*) was obtained for GDE3 (Fig. 8 and Table 2). This result is consistent with the enhancement of the diffusion of oxygen in the reaction layer of this MWCNT/Nafion composite due to its optimum level of Nafion.
SEM and TEM results
The electrode layer is composed of Pt agglomerates and a Nafion film. The catalytic layer usually has a dual pore distribution: micro pores are present in the agglomerates, and macro pores are located between the agglomerates. The Nafion is likely to be localized in the macro pores in contact with the Pt agglomerates. A further complication arises because reactant diffusion does not often occur in the gaseous phase through the electrode; instead, the reactant dissolves in the electrolyte and then diffuses into the solution. In this model, we expect that the reaction of oxygen (electro-reduction) is controlled by the following transport processes: (1) oxygen diffusion into the pore space of the electrode and dissolution into Nafion; (2) oxygen diffusion through the Nafion layer; and (3) proton transfer in the Nafion layer. We carefully controlled the fabrication process to prevent the effect of fabrication process on the Pt morphology. We emphasize that the loading of Nafion varies in the GDEs (GDE1eGDE4), which means that the morphology of the reduced Pt particles will vary from support to support, as shown by the XRD results (Fig. 4).
Table 2 e Tafel slopes, current densities, permeabilities, charge transfer resistances, ionic resistances, electroactive surface areas, and roughness factors of the GDEs. GDE 1 2 3 4 5
b (mV dec1)
i 104 at 0.9 V (A cm2)
(D1/2 C*) 109 (mol cm2 s1/2)
EAS (m2 g1)
Rf
Rp ( ohm)
Rion/3 (ohm)
d (nm)
71 73 70 86 75
11.3 15.1 16.8 10.4 13.7
11.9 28.0 32.1 11.5 24.0
15.4 39.7 44.1 22.4 36.6
51 132 147 74 122
1.4 1.2 0.8 1.3 1.3
66.7 10.9 5.3 64.6 31.0
0.76 3.57 3.59 1.64 0.75
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Therefore, a low Nafion content (less than 0.75 mg cm2) is not desirable since good contact between the catalyst, electrolyte, and reagents is reduced; without such contact the integrity of the catalyst layer and the membrane, and the proton conductivity are insufficient. At high ionomer loadings (1.25 mg cm2, GDE4), the proton conductivity is enhanced, but the catalyst active area is then expected to be limited (Table 2) by the formation of a thick Nafion layer on the catalyst surface, which then reduces the electroconductivity of the Pt layer. We also note that the mechanical integrity of the catalyst layer is insufficient for Nafion contents below 0.75 mg cm2, which means that the incorporated Nafion cannot properly bind the catalyst particles. At a Nafion content of 1.25 mg cm2, as in GDE4 which exhibits reduced performance, the thick Nafion layer reduces the catalyst active area in spite of the enhanced proton conductivity in the catalyst layer. On the basis of our results, a Nafion content of 1 mg cm2 (GDE3) optimizes proton transfer within the catalyst layer without limiting oxygen diffusion. Figs. 9e13 show that the number and size of the pores are different in each catalyst. Thus the number and the size of the pores in GDE3 have optimum values, which improve its performance in the ORR. As we expected, Fig. 13 shows that the Pt/MWCNT surface is covered by the Nafion ionomer; this means that some active sites are blocked and the performance of GDE5 in the ORR is inferior to that of GDE3. TEM images of GDE2, GDE3, and GDE5 are shown in Figs. 14e16; these images confirm the details of the above discussion.
4.
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Appendix. Nomenclature
A geometricarea of electrode b Tafel slope width of the peak at half-height B2q C* concentration of the reactant d average particle size D diffusion coefficient D1/2 C* permeability EAS electroactive surface area open-circuit voltage Eeq F Faraday constant i current density exchange current density io MEA membrane electrode assembly MPL micro pore layer n number of electrons OCV open-circuit voltage polarization resistance Rp roughness factor Rf S catalyst surface area t time h over-potential q angle at the maximum of the peak l wavelength of the X-ray r density
references
Conclusion
The influence of the Nafion loading in the catalyst layer on electrode performance was studied; four different GDEs were prepared by using MWCNT-Nafion composites (with varying amounts of Nafion) as Pt supports, and the performances of these GDEs and of Pt/MWCNT were compared. A simple, fast, and energy-efficient method for the preparation of Pt/MWCNTeNafion composites and Pt/MWCNT catalysts with high electrocatalytic activity was developed. We have shown that the timing of the addition of Nafion to the catalyst layer is very important and prepared an electrode (GDE3) that exhibits excellent performance in the ORR. Our XRD results indicate that the Pt particles were reduced successfully onto the MWCNT-Nafion composites and onto the MWCNTs, and that the particle sizes were in the range 0.75e3.59 nm. The results for the electrochemical parameters, such as the current density at 0.9 V, the polarization resistance, and the permeability, indicate that the coating of Pt with the optimized content of Nafion (namely the GDE3 MWCNT-Nafion composite) exhibits better performance in the ORR than Pt on MWCNTs and the other electrodes (GDE1, GDE2, GDE4). The superior performance of this electrode can be attributed to better reactant accessibility to the three-phase zone. The present technique can be used as a general method for the preparation of supported metal particles from metal precursors.
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Optimization of the amount of Nafion in multi-walled carbon nanotube/Nafion composites as Pt supports in gas diffusion electrodes for proton exchange membrane fuel cells Hussein Gharibi a,*, Masoumeh Javaheri a, Mehdi Kheirmand b, Rasol Abdullah Mirzaie c a
Department of Chemistry, Faculty of Science, Tarbiat Modares University, P.O. Box 14115-175 Tehran, Iran Department of Chemistry, Faculty of Science, Yasouj University, Yasouj, Iran c Department of Chemistry, Faculty of Science, Shahid Rajaee University, Tehran, Iran b
article info
abstract
Article history:
The Nafion loading in multi-walled carbon nanotube (MWCNT) composites with Nafion
Received 20 June 2010
used as Pt support in the oxygen reduction reaction (ORR) has been studied. We varied the
Received in revised form
amount of Nafion in these composites and added a Pt loading of 0.3 mg cm2 to the catalyst
11 August 2010
layer. The performance of these electrodes in the ORR was measured with linear sweep
Accepted 2 September 2010
voltammetry (LSV), electrochemical impedance spectroscopy (EIS), chronoamperometry,
Available online 8 October 2010
inductive coupled plasma (ICP), X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). In addition, we compared the performance
Keywords:
of the MWCNTs as Pt supports with those of the composites. Our results indicate that the
Catalyst layer (CL)
composites are better Pt supports in comparison with MWCNT.
Gas diffusion electrode (GDE)
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Oxygen reduction reaction (ORR)
reserved.
Proton exchange membrane fuel cell (PEMFC) Pt support
1.
Introduction
Carbon nanotubes possess a wide variety of remarkable properties, most notably high electrical and thermal conductivities, mechanical strength, and large catalytic surface areas, and thus have potential uses in a variety of power generation and storage devices, including proton exchange membrane fuel cells (PEMFCs) [1]. Composite electrodes consisting of Pt nanoparticles supported on multi-walled carbon nanotubes grown directly on carbon paper (Pt/MWCNTs/carbon paper) have previously been synthesized by using glacial acetic acid as a reducing agent [2]. Lee and Hu have successfully developed a simple but
effective method for grafting various chemical reagents onto the opposite tube-ends of individual carbon nanotubes, as confirmed with X-ray photoelectron spectroscopy measurements. This approach could be used in the site-selective selfassembly of both MWCNTs and single-walled carbon nanotubes (SWCNTs) into a large variety of novel functional systems with highly controllable structures and various applications [3,4]. Tang and Wu presented a detailed comparison of MWCNTs and SWCNTs in an effort to identify the better supporting carbon material for electrocatalysts in direct methanol fuel cells (DMFCs) [5,6]. Salvetat et al. found that the mechanical properties of nanotubes are strongly dependent on their structures; various types of nanotubes
* Corresponding author. E-mail addresses: [email protected], [email protected] (H. Gharibi). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.09.008
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Table 1 e Gas diffusion electrodes (GDEs) with various Nafion loadings. GDE 1 2 3 4 5
Pt loading mg cm2
Nafion loading mg cm2
Pt/MWCNT composite
Pt/ MWCNT
0.3 0.3 0.3 0.3 0.3
0.5 0.75 1 1.25 1
e
e e e e
including SWCNTs and arc-grown multi-walled nanotubes were considered [7]. Carbon nanotubes were selectively grown directly on carbon paper by performing chemical vapor deposition catalyzed with electrodeposited cobalt. The prepared carbon nanotubes were employed as the supports for platinum catalysts, which were electrodeposited onto the nanotubes [8]. For comparison, similar loadings of the Pt catalyst supported on carbon nanotubes were prepared with two approaches: the borohydride reduction method and the ion exchange technique. It was found that the Pt/SWCNTs catalyst prepared with the ion exchange method exhibits higher Pt utilization than the Pt/SWCNTs prepared with the borohydride reduction method. Furthermore, in comparison to an E-TEK 20 wt.% Pt/C catalyst with the support of carbon black, electrochemical measurements were used to show that the Pt/SWCNTs prepared with the ion exchange method exhibit a higher catalytic activity in methanol oxidation [9e11]. Wang et al. tested a hydrothermal catalyst preparation process that utilizes CNTs as a base with the aim of reducing the amount of Pt; Pt/CNTs with 40% Pt weight were prepared. The catalyst was characterized with IR spectroscopy, XRD, TEM, and Proven Solution Analyzer software. Their results show that by using the HNO3eH2O2 redox system the ports of the CNTs are opened and that large quantities of impurities are removed. The effects of varying the reaction temperature and time on Pt granule size have been determined [12,13]. Pt nanoparticles of around 1.5 nm in size that are well-dispersed on CNTs were obtained when ethylenediaminetetraacetic acid disodium salt (EDTAe2Na/Pt) was utilized [14]. Hu et al. have suggested that CNTs can promote electron transfer reactions when used as electrodes. However, chemical reactions can occur that result in changes in nanotube
structure and that generate active functional groups, such as hydroxyl and carboxyl. Hu et al. have compared the structure, stability, and electrode kinetics of carboxyl-modified CNT electrodes prepared by soaking in concentrated nitric acid with those of non-modified CNT electrodes [15]. Pt and PteRu particles deposited on polypyrroleecarbon nanotube composite polymer films exhibit excellent catalytic activity and stability in methanol oxidation, which indicates that composite films are promising support materials for fuel cell applications [16,17]. The electrocatalytic properties of Pd/ MWCNT electrodes in the oxidation of hydrazine have been investigated with cyclic voltammetry (CV); excellent electrocatalytic activity was observed, which might be due to the small particle size and high dispersion of palladium particles [18]. Guo et al. reported a matrix of composite films consisting of carbon nanotubes and Nafion for the immobilization of on glassy carbon (GC) electrode surfaces. A Ru(bpy)2þ 3 composite film-modified electrode was found to exhibit better long-term electrochemical and electrogenerated chemiluminescence (ECL) stability than a pure Nafion film-modified electrode [19]. Ambrosio et al. were investigated the Platinum catalyst supported on mesoporous carbon for PEMFC [20]. The effects of varying the Nafion loading in the catalyst layer on cell performance, pore size distribution, and Pt surface area were investigated [21,22]. The presence of Nafion
Fig. 1 e ICP curve for the platinum concentrations on the supports.
Fig. 3 e Nyquist plots of the impedance responses from 10 kHz to 100 mHz for the GDEs including the Nafion membrane (Nafion 112) at OCV vs. Ag/AgCl, at 25 C.
Fig. 2 e Nyquist plots of the impedance responses from 10 kHz to 100 mHz for the GDEs at 400 mV vs. Ag/AgCl, at 25 C.
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Fig. 4 e XRD patterns of GDE1, GDE2, GDE3, GDE4, and GDE5.
ionomer increases the size of the three-dimensional zone of catalytic activity and the platinum utilization in the electrode, and also helps to retain moisture and prevent membrane dehydration, especially at high current densities. The Nafion content in the electrode must be optimized to achieve high performance. As the platinum loading increases, the electrode thickness and the Nafion thickness increase, and hence the optimum Nafion content can depend on the platinum loading [23e27]. Electrodes with Pt/CNT catalysts sulfonated through the in situ radical polymerization of 4-styrenesulfonate have been found to exhibit better performance than their unsulfonated counterparts, mainly because of the easier access for protons and the well-dispersed distribution of the sulfonated Pt/CNT catalysts; thus sulfonation improves performance and reduces the cost of Pt/CNT-based PEMFCs. A half-MEA with an active area of 1.0 cm 1.0 cm was fabricated by employing a Nafion 115 membrane and an in-house CNT-based gas diffusion electrode (GDE) with a platinum loading of approx. 0.05 mg cm2; Nafion electrolytes were not used [28]. In a previous study [29,30], we prepared a catalyst layer containing a PANIeNafion composite. In this paper, we introduce a new method for the preparation of MWCNT/Nafion composites, and our results with this method are compared with those of other methods. The main distinguishing feature of this method is in the timing of the addition of the Nafion solution to the MWCNTs. The
effects of varying the Nafion loading in the catalyst layer on the electrode performance and the Pt surface area were investigated; for this purpose we fabricated four electrodes with different amounts of Nafion (0.5, 0.75, 1, and 1.25 mg cm2). The performance of these electrodes in the ORR was compared with that of an electrode with MWCNTs as a Pt support and a Nafion loading of 1 mg cm2. Furthermore, the electrodes were characterized with cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), chronoamperometry, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The new method was found to result in better performance and lower resistance.
Fig. 5 e Cyclic voltammetry curves for the GDEs for a scan rate of 50 mV sL1.
Fig. 6 e IeV curves of the GDEs in the ORR for a scan rate of 1 mV sL1 at 25 C.
2.
Experimental
2.1.
Composite preparation
In this research, we fabricated four composites with various amounts of Nafion. Since MWCNTs are chemically inert, the MWCNTs (20e30 nm Aldrich) were functionalized by pretreatment with 70% nitric acid in order to introduce surface oxides before preparation of the composites [3,4,15,18]. In our
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study, the MWCNTs were refluxed under constant agitation at 120 C in concentrated nitric acid for 12 h. The solid phase was removed by filtration and washed with distilled water, and the recovered functionalized MWCNTs were then dried at 80 C for 12 h. The composites were prepared from the dried samples and Nafion solution (5% from Aldrich). Mixtures of functionalized MWCNTs and various amounts of Nafion solution in 2-propanol (Merck), water, and glycerol (Merck) were sonicated for 20 min with a sonicator (Misonix model S-3000) to prepare homogeneous suspensions; these suspensions were dried at 70 C. Four MWCN/Nafion composites were prepared, each containing a different amount of Nafion. The resulting electrodes contained 0.5, 0.75, 1, and 1.25 mg cm2 of Nafion in the catalyst layer and were named GDE1, GDE2, GDE3, and GDE4, respectively.
2.2. Preparation of Pt/MWCNT and Pt/composite (NafioneMWCNT) In this step, Pt was deposited on the MWCNT/Nafion composites. One additional electrode named GDE5 in which Pt was deposited on functionalized MWCNTs was prepared. The MWCNT/Nafion composites for GDE1eGDE4 and the functionalized MWCNTs for GDE5 were vigorously mixed with H2PtCl6 solution in a 2-propanol (Merck) and water (1:3 v/v) solvent mixture at 50 C for 6 h. These suspensions were then heated to 80 C, and the pH of each suspension was adjusted
to 8.5 with a 0.5 M Na2CO3 solution. An abundant amount of sodium formate (HCOONa) [Merck] was added to perform a chemical reduction for 2 h. Each suspension was then filtered, washed, and dried at 90 C for 4 h. Our ICP results indicated that the samples contained 6.7 wt.% Pt, and so GDEs could be prepared from the Pt/functionalized MWCNTs and the Pt/MWCNT/Nafion composites.
2.3.
Fabrication of the GDEs
For the diffusion layer, a mixture of 30 wt.% PTFE and 70 wt.% Vulcan in 2-propanol (Merck), water, and glycerol (Merck) was sonicated for 20 min with a sonicator (Misonix model S-3000) to produce a homogeneous suspension. The suspension was rolled onto the carbon paper (TGPH-0120T) (Toray), and the electrode was dried in air at 120 C for 1 h and then finally sintered at 340 C for 30 min. The loading of Nafion in the diffusion layer was fixed at 1 mg cm2 [31,32]. To prepare the catalyst layers in GDE1eGDE4, a homogeneous suspension containing the desired amounts of the Pt-MWCNT/Nafion composite (6.7 wt.%), glycerol (Merck), 2-propanol (Merck), and water was sonicated for 20 min. This suspension was rolled onto the diffusion layer; the electrode was then dried at 40 C for 30 min and then at 80 C for 30 min. The Pt loading was 0.3 mg cm2. To prepare the catalyst layer of GDE5, a homogeneous suspension containing the desired amounts of Pt/MWCNT (6.7 wt.%), Nafion solution (5% from Aldrich) in glycerol, 2-propanol, water, and glycerol was sonicated for 20 min. This suspension was rolled onto the diffusion layer; the electrode was dried at 40 C for 30 min and then at 80 C for 30 min. The Pt loading was 0.3 mg cm2.
2.4.
Electrochemical measurements
An EG&G Princeton Applied Research Model 273A instrument was used to determine the electrochemical properties of the electrodes. The performances of the porous GDEs (geometric exposed area 1 cm2) in the reduction of oxygen were investigated in 2 M H2SO4. All measurements were performed at 25 C in a conventional three-electrode cell, with O2 flowing at 50 mL min1. The GDEs were mounted into a Teflon holder that contains a pyrolytic graphite disk as a current collector and has provision for feeding oxygen from the back of the
Fig. 7 e Graphical representation of the Nafion ionomer position in catalyst layer.
Fig. 8 e Chronoamperograms of the GDEs in the presence of O2, E [ 0.4 V vs. Ag/AgCl, at 25 C, not stirred the solution.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 3 3 2 5 e1 3 3 3 4
electrode. A large area platinum flat electrode was used as the counter electrode. An Ag/AgCl reference electrode was placed close to the working electrode surface. A rotating disk electrode immersed in 2 M H2SO4 with a rotation rate of 1500 rpm was used to perform cyclic voltammetry. The electrochemical cell was connected to a potentiostategalvanostat (EG&G Model 273A) for IeV polarization measurements and chronoamperometry, and to a frequency response detector (model 1025) for electrochemical impedance spectroscopy. In order to perform a quantitative evaluation of resistance against the ORR, the AC impedance method was used. In the Nyquist plot of the semicircle diameter, Rp is the polarization resistance and the sum of the electrode and electrolyte resistance that was subtracted for these GDEs. The polarization resistance is the sum of two terms: the charge transfer and diffusion resistances. Impedance measurement was investigated in 0.4 V vs. Ag/AgCl potential for obtaining Rp. This potential is in ohmic resistance region. The AC potential amplitude was 5 mV, and the frequency range was 10 kHze0.1 Hz. We used impedance measurements to characterize the ionic resistances of the electrodes. At low frequencies, a Warburg-like response (45 slope) is observed, which indicates the occurrence of ion migration through the Nafion membrane and catalyst layer. The ionic resistance Rion can be obtained from the length of the Warburg-like region projected onto the real impedance (Zreal) axis [33,34,35]. For obtaining Rion, the impedance measurement has been investigated in OCV potential and experimental complex-plane impedance plots (10 kHze0.1 Hz) for Argon-bathed are shown in Fig. 3.
2.5. Measurements of the physical properties of the electrodes A scanning electron microscope (Model XL30, Philips co.) was used after coating the electrodes’ surfaces with gold, a transmission electron microscope (TEM/STEM CM Philips 200) was used to produce the TEM images, and XRD analysis was carried out for the catalysts by using an XPERT MPD Philips diffractometer with a Cu X-ray source operating at 40 kV and
Fig. 9 e SEM image of the catalyst layer of GDE1 (0.5 mg cmL2 Nafion in a MWCNT/Nafion composite) at a magnification of 15,0003.
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40 mA. The XRD patterns were obtained at a scanning rate of 1 /min with a step size in the 2q scan of 0.02 in the range 20e100 . The amount of Pt reduced on each support was determined by using the inductive coupled plasma (ICP) technique (ICPOES, Varian Vista-PRO, Australia); for these measurements we dissolved 5 mg of each sample (synthesized Pt on the supports) and standard samples (Pt/C 10% and 20% wt) in a mixture of hydrochloric acid and nitric acid (3/1). These solutions were refluxed at 70 C and then used in the ICP measurements.
3.
Results and discussion
The GDEs (1e5) were prepared with different amounts of Nafion, as explained in Experimental section and Table 1. The results are shown in Figs. 1e16 and in Table 2. The ICP results indicate that the Pt loading in these GDEs is 0.3 mg cm2 (Fig. 1). As mentioned above, there are three participants in electrochemical reactions: gases, electrons, and protons. The reactions can only take place on portions of the catalyst surface to which all three participants have access. The electrons travel through the electrically conductive solids, including the catalyst itself, so it is important that the catalyst particles are electrically connected in some way to the substrate. Protons travel through the ionomer, so the catalyst must also be in intimate contact with the ionomer. Finally, the reactant gases travel only through voids, so the electrode must be sufficiently porous to allow gases to travel to the reaction sites. At the same time, product water must be effectively removed or the electrode will flood and prevent oxygen access. The reactions take place at a three-phase boundary between ionomer, solid, and empty space [32].
3.1.
AC impedance spectroscopy
To gain more information about the GDEs (GDE1eGDE5), the a.c. impedance spectrum of each electrode was obtained. The
Fig. 10 e SEM image of the catalyst layer of GDE2 (0.75 mg cmL2 Nafion in a MWCNT/Nafion composite) at a magnification of 15,0003.
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resulting data were obtained at 400 mV vs. Ag/AgCl saturated with KCl, and were plotted in the Nyquist representation. The polarization resistance Rp was found to be lower for GDE3, which contains 1 mg cm2 Nafion in the catalyst layer, than for the other GDEs. Although the impedance spectra have similar semi-circular shapes, the diameters of the semi-circles differ significantly. As shown in Fig. 2, the resistances of GDE1, GDE2, and GDE4 are too high in comparison with those of the other electrodes. When the amount of Nafion in the GDEs is increased from 0.5 to 0.75, the resistance of the electrode decreases. However, upon an additional increase in the amount of Nafion, a high resistance is observed, i.e., in GDE4. The high resistance of this electrode is due to an excess of Nafion in the catalyst layer. Therefore, we should optimize the amount of Nafion in the GDEs. One approach is to suggest that the optimum value of the amount of Nafion arises for the GDE with the lowest resistance, so GDE3 is then the best electrode. In the case of GDE5, the ionomer blocks the catalyst sites, so the resistance of electrode GDE5 is higher than that of GDE3. The ionic resistance or proton transport of each GDE was determined by using a half-cell assembly in a conventional three-electrode system; the working electrode contained carbon paper, MPL, catalyst layer, and a Nafion 112 membrane. The results are shown in Fig. 3 and Table 2: GDE3 has the lowest ionic resistance, which is attributed to its optimum three-phase zone.
3.2.
XRD pattern of platinum
d¼
0:9l B2q cosq
(1)
where d is the average particle size, l is the wavelength of the X-ray (1.54056 A), q is the angle at the maximum of the peak, and B2q is the width of the peak at half-height. This variation in the crystal size of Pt arises because the support of Pt in each sample is different. These results show that Nafion plays an important role during the synthesis and that varying the concentration of Nafion has some influence on the formation of the Pt particles, i.e., the concentration of Nafion influences the reduction of platinum on the surface of the MWCNTs. In the preparation of GDE5, the Pt nuclei grew as soon as the reducing agents, sodium formate (HCOONa), was added to the mixture, i.e., before they were covered with Nafion. This growth resulted in the inferior performance of GDE5 in comparison with those of GDE2 and GDE3 since some of its Pt particles were masked by Nafion, which was added to this mixture after the coating of Pt onto the MWCNT support. Since the Pt particles are covered with Nafion, the concentration of exposed Pt on the surface of the MWCNT support is reduced; therefore, the performance of GDE5 decreases. This conclusion is verified by the XRD pattern for GDE3 (see Fig. 4).
3.3.
Electroactive surface area (EAS)
The cyclic voltammograms for GDEl, GDE2, GDE3, GDE4, and GDE5 are shown in Fig. 5. The coulombic charge for hydrogen desorption was used to calculate the electroactive surface area (EAS) of each electrode (Table 2) [36]:
Fig. 4 shows the XRD patterns of the Pt/MWCNT and PtMWCNT/Nafion composite nanocatalysts. The peak at 2q ¼ 26.5 corresponds to the (002) planes of the graphitized MWCNTs and the peaks at 2q ¼ 39.8 , 67.5 , and 46.2 are associated with the (111), (220), and (200) planes of fcc (face centered cubic) Pt, respectively. These results indicate that Pt has been successfully reduced. The average sizes of the Pt particles were calculated from the line broadening of the (111) peak by using the Scherrer equation after background subtraction and found to be 0.76, 3.57, 3.59, 1.64, and 0.75 nm for the five samples, respectively (Table 2).
where Qh is the charge for hydrogen desorption (mC cm2) and [Pt] is the platinum loading (mg cm2) and 0.21 (mC cm2) is the charge required to oxidize a monolayer of H2 on bright Pt. The roughness factor (Rf) can then be calculated by using the following equation:
Fig. 11 e SEM image of the catalyst layer of GDE3 (1 mg cmL2 Nafion in a MWCNT/Nafion composite) at a magnification of 15,0003.
Fig. 12 e SEM image of the catalyst layer of GDE4 (1.25 mg cmL2 Nafion in a MWCNT/Nafion composite) at a magnification of 15,0003.
EAS ¼
Qh 0:21 ½Pt
(2)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 3 3 2 5 e1 3 3 3 4
Rf ¼
EAS S
(3)
where S is the geometric surface area (m2 g1). These results are tabulated in Table 2 and are consistent with those of other studies [2,17,29]. The polarization curves for the GDEs in conventional threeelectrode cells are presented in Fig. 6. The current density values are considerably greater for GDE3 than for the others, which indicate that the performance of this cell is much better than that obtained when using an electrode containing the MWCNT/Nafion composite (1 mg cm2 of Nafion). The superior performance of GDE3 is attributed to an increase in the size of the three-dimensional reaction zone and to the optimum value of Nafion in the MWCNT/Nafion composite. As a result, the concentrations of the reactants near the surface of the catalyst are increased, and hence the reaction rate is improved. The performances of GDE1 and GDE4 are very poor. The poor electrode performance for a low Nafion content (GDE1) might be due to the presence of fewer electrochemically active sites in the three-phase zone, and the decrease in performance for a high Nafion content (GDE4) might be due to reduced gas permeability and increased mass transfer polarization. Further, the electrochemically active surfaces of the GDEs were calculated from the CV data and are shown in Table 2. These results indicate that the performance of GDE3 is better than that of GDE5. This superiority of GDE3 is attributed to better catalyst accessibility; in other words, when the MWCNT/Nafion composite is used as the Pt support, the accessible catalyst surface is greater in GDE3 than in Pt/ MWCNT (GDE5), in which the surface of Pt particles is blocked by the Nafion ionomer. As mentioned above, the reactions in these catalysts occur in a three-phase zone, i.e., where the ionomer, solid, and empty space phases meet (Fig. 7a). The reaction zone can be expanded by either “roughening” the surface of the membrane or by adding ionomer to the catalyst layer (Fig. 7b). The whole surface of the catalyst can be screened by a thin ionomer layer (Fig. 7c), except at the electrical contacts. In two extreme cases, the entire catalyst layer can be protected by the ionomer layer and there is then no
Fig. 13 e SEM image of the catalyst layer of GDE5 (1 mg cmL2 Nafion) at a magnification of 15,0003.
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electrical contact with the electrically conductive support (Fig. 7d) or there is no ionomer in contact with the Pt particles (Fig. 7e). These considerations show that the ratios of the catalyst area covered by ionomer to the catalyst area open to empty space and to the catalyst area in contact with other catalyst particles or the electrically conductive support should be optimized [32].
3.4.
Kinetic parameters
The kinetic parameters of the ORR for a GDE can be obtained from the polarization data. Our analysis of the experimental polarization data was performed by using the Tafel equation [37]: h ¼ blog
i i0
(4)
where: h ¼ E Eeq
(5)
h is the over-potential, Eeq is the open-circuit voltage, b is the Tafel slope, i is the current density, and i0 is the exchange of current density for the ORR. The kinetic parameters of the ORR for the GDEs can be obtained from the IeV curves and these equations. The Tafel slopes and the current densities of the GDEs at 0.9 V are shown in Table 2. These results indicate that the best performance is exhibited by GDE3. To achieve maximum efficiency, it is necessary to prepare an environment with a triple interface comprised of Nafion, platinum, and oxygen. Therefore, we conclude that in GDE3 the amount of Nafion is optimized and the triple zone is more reliable. As mentioned in the previous section, catalyst accessibility is better in GDE3 than in GDE5, and thus the rate of the ORR and the current density are improved.
Fig. 14 e TEM image of the catalyst layer of GDE2 (0.75 mg cmL2 Nafion in a MWCNT/Nafion composite).
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 3 3 2 5 e1 3 3 3 4
Fig. 16 e TEM image of the catalyst layer of GDE5 (1 mg cmL2 Nafion).
Fig. 15 e TEM image of the catalyst layer of GDE3 (1 mg cmL2 Nafion in a MWCNT/Nafion composite).
3.6. 3.5.
Chronoamperometry
The diffusion coefficients of O2 in the GDEs were determined with chronoamperometry. For large electrodes, the information obtained was limited by the Cottrellian decay with time. According to Winlove et al. [38], the time window for these experiments follows the relation 1 > s1/2 > 0.5, where s is a dimensionless parameter equal to 4Dt/r2. The Cottrell [39] equation is: i ¼ nFAðD=ptÞ1=2 C)
(6)
where i is the limiting current (mA), n the number of electrons, F is the Faraday constant (96485 C mol1), A is the surface area of the electrode (cm2), D is the diffusion coefficient (cm2 s1), t is the time (s), and C* is the concentration of the reactant. The highest value of the permeability (D1/2 C*) was obtained for GDE3 (Fig. 8 and Table 2). This result is consistent with the enhancement of the diffusion of oxygen in the reaction layer of this MWCNT/Nafion composite due to its optimum level of Nafion.
SEM and TEM results
The electrode layer is composed of Pt agglomerates and a Nafion film. The catalytic layer usually has a dual pore distribution: micro pores are present in the agglomerates, and macro pores are located between the agglomerates. The Nafion is likely to be localized in the macro pores in contact with the Pt agglomerates. A further complication arises because reactant diffusion does not often occur in the gaseous phase through the electrode; instead, the reactant dissolves in the electrolyte and then diffuses into the solution. In this model, we expect that the reaction of oxygen (electro-reduction) is controlled by the following transport processes: (1) oxygen diffusion into the pore space of the electrode and dissolution into Nafion; (2) oxygen diffusion through the Nafion layer; and (3) proton transfer in the Nafion layer. We carefully controlled the fabrication process to prevent the effect of fabrication process on the Pt morphology. We emphasize that the loading of Nafion varies in the GDEs (GDE1eGDE4), which means that the morphology of the reduced Pt particles will vary from support to support, as shown by the XRD results (Fig. 4).
Table 2 e Tafel slopes, current densities, permeabilities, charge transfer resistances, ionic resistances, electroactive surface areas, and roughness factors of the GDEs. GDE 1 2 3 4 5
b (mV dec1)
i 104 at 0.9 V (A cm2)
(D1/2 C*) 109 (mol cm2 s1/2)
EAS (m2 g1)
Rf
Rp ( ohm)
Rion/3 (ohm)
d (nm)
71 73 70 86 75
11.3 15.1 16.8 10.4 13.7
11.9 28.0 32.1 11.5 24.0
15.4 39.7 44.1 22.4 36.6
51 132 147 74 122
1.4 1.2 0.8 1.3 1.3
66.7 10.9 5.3 64.6 31.0
0.76 3.57 3.59 1.64 0.75
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 3 3 2 5 e1 3 3 3 4
Therefore, a low Nafion content (less than 0.75 mg cm2) is not desirable since good contact between the catalyst, electrolyte, and reagents is reduced; without such contact the integrity of the catalyst layer and the membrane, and the proton conductivity are insufficient. At high ionomer loadings (1.25 mg cm2, GDE4), the proton conductivity is enhanced, but the catalyst active area is then expected to be limited (Table 2) by the formation of a thick Nafion layer on the catalyst surface, which then reduces the electroconductivity of the Pt layer. We also note that the mechanical integrity of the catalyst layer is insufficient for Nafion contents below 0.75 mg cm2, which means that the incorporated Nafion cannot properly bind the catalyst particles. At a Nafion content of 1.25 mg cm2, as in GDE4 which exhibits reduced performance, the thick Nafion layer reduces the catalyst active area in spite of the enhanced proton conductivity in the catalyst layer. On the basis of our results, a Nafion content of 1 mg cm2 (GDE3) optimizes proton transfer within the catalyst layer without limiting oxygen diffusion. Figs. 9e13 show that the number and size of the pores are different in each catalyst. Thus the number and the size of the pores in GDE3 have optimum values, which improve its performance in the ORR. As we expected, Fig. 13 shows that the Pt/MWCNT surface is covered by the Nafion ionomer; this means that some active sites are blocked and the performance of GDE5 in the ORR is inferior to that of GDE3. TEM images of GDE2, GDE3, and GDE5 are shown in Figs. 14e16; these images confirm the details of the above discussion.
4.
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Appendix. Nomenclature
A geometricarea of electrode b Tafel slope width of the peak at half-height B2q C* concentration of the reactant d average particle size D diffusion coefficient D1/2 C* permeability EAS electroactive surface area open-circuit voltage Eeq F Faraday constant i current density exchange current density io MEA membrane electrode assembly MPL micro pore layer n number of electrons OCV open-circuit voltage polarization resistance Rp roughness factor Rf S catalyst surface area t time h over-potential q angle at the maximum of the peak l wavelength of the X-ray r density
references
Conclusion
The influence of the Nafion loading in the catalyst layer on electrode performance was studied; four different GDEs were prepared by using MWCNT-Nafion composites (with varying amounts of Nafion) as Pt supports, and the performances of these GDEs and of Pt/MWCNT were compared. A simple, fast, and energy-efficient method for the preparation of Pt/MWCNTeNafion composites and Pt/MWCNT catalysts with high electrocatalytic activity was developed. We have shown that the timing of the addition of Nafion to the catalyst layer is very important and prepared an electrode (GDE3) that exhibits excellent performance in the ORR. Our XRD results indicate that the Pt particles were reduced successfully onto the MWCNT-Nafion composites and onto the MWCNTs, and that the particle sizes were in the range 0.75e3.59 nm. The results for the electrochemical parameters, such as the current density at 0.9 V, the polarization resistance, and the permeability, indicate that the coating of Pt with the optimized content of Nafion (namely the GDE3 MWCNT-Nafion composite) exhibits better performance in the ORR than Pt on MWCNTs and the other electrodes (GDE1, GDE2, GDE4). The superior performance of this electrode can be attributed to better reactant accessibility to the three-phase zone. The present technique can be used as a general method for the preparation of supported metal particles from metal precursors.
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