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CNTs as anode catalyst
international journal of hydrogen energy 35 (2010) 8225–8233
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Effect of operating conditions on the performance of a direct methanol fuel cell with PtRuMo/CNTs as anode catalyst Shengzhou Chen a,*, Fei Ye a,b, Weiming Lin a,b a b
School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, China School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
article info
abstract
Article history:
PtRu/CNTs and PtRuMo/CNTs catalysts have been synthesized by microwave-assisted
Received 17 September 2009
polyol process and used as the anode catalysts for a direct methanol fuel cell (DMFC). The
Received in revised form
catalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction
17 December 2009
(XRD) and X-ray photoelectron spectrometry (XPS). The effect of different anode catalysts,
Accepted 17 December 2009
membrane electrode assembly (MEA) activation, methanol concentration, methanol flow
Available online 12 January 2010
rate, oxygen flow rate and cell temperature on the DMFC performance has been investigated. The results show that the PtRu or PtRuMo particles with face-centered cubic
Keywords:
structure are uniformly distributed on CNTs, and the addition of Mo to PtRu/CNTs makes
Direct methanol fuel cell
the binding energies of each Pt species shift to lower values. PtRuMo/CNTs is a promising
Operating conditions
anode catalyst for DMFCs, and the appropriate operating conditions of the DMFC with
PtRuMo/CNTs
PtRuMo/CNTs as the anode catalyst are MEA activation for 10 h, 2.0–2.5 M methanol at the
MEA activation
flow rate of 1.0–2.0 mL/min, and oxygen at the flow rate of 100–150 mL/min. The DMFC
Methanol crossover
performance increases significantly with an increase in cell temperature. Crown Copyright ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. All rights reserved.
1.
Introduction
DMFC has been received increasing attention because of its advantages, such as high energy density, simple and easily handled structure, cheap liquid fuel, low exhaustion, low temperature operating as well as easy transportation and storage of the fuel [1,2]. Currently, the development direction of the DMFCs lies in three aspects of potential applications as vehicle transportation, portable power sources and micro power sources [3–5]. DMFC is one of the most studied, most promising and most nearly commercialized fuel cells. However, the main barriers for DMFC development are low activities of anode catalysts and methanol crossover [6]. To solve these problems, lots of work has been done in the past two decades on the improvement of the catalyst activity, Nafion membrane modification, electrode structure
optimization, and so on [7]. But in the actual DMFC applications, the operating conditions generally exert great influence on the cell performance [8,9]. So it is very important to study the characteristics of various operating conditions. Jung et al. [10] have investigated the effect of membrane thickness, cell temperature, and methanol concentration on the DMFC power density and methanol crossover, and found that increasing the cell temperature promotes the cell performance, and the lower methanol concentration causes the concentration polarization effects, thus resulting in lower cell performance. Although higher methanol solution concentration can overcome the concentration polarization, a serious methanol crossover decreases the cell performance. Colmati et al. [8] have studied the effect of methanol concentration, Pt load, membrane thickness and PTFE content in the cathode diffusion layer on the DMFC performance, and the results
* Corresponding author. Tel./fax: þ86 020 39366507. E-mail address: [email protected] (S. Chen). 0360-3199/$ – see front matter Crown Copyright ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. All rights reserved. doi:10.1016/j.ijhydene.2009.12.085
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show better performance with cells operating with 3 mgPt/cm2, 5 M methanol solution, Nafion 112 membrane and with 30 wt.% PTFE in the cathode diffusion layer. It’s suggested that the mass transport, ion conductivity, catalyst activity, and methanol crossover should all be taken in account while studying the operating conditions of DMFC [11]. So in order to appropriately operate DMFCs and extend their lifetime, it is of great significance to investigate the operating conditions of DMFC. Carbon nanotubes (CNTs) have been used as catalyst supports for fuel cells because of their highly electrochemically accessible surface area and remarkable electronic conductivity [12,13]. Previous studies have showed that CNTs-supported catalysts exhibit better performance of methanol electrooxidation as compared to conventional carbon black (XC-72) supported catalysts [14–17]. While recent studies have been focused on the modification of the CNTs-supported binary catalysts [18–21], little work has been done on the CNTs-supported ternary catalysts [22–24], especially those investigated in a single DMFC or DMFC stacks [25]. In the present study, PtRu/ CNTs and PtRuMo/CNTs catalysts were synthesized by microwave-assisted polyol process, and the performance of a DMFC with these catalysts as the anode catalysts was compared, then the effect of various operating conditions, such as membrane electrode assembly (MEA) activation, methanol concentration, methanol flow rate, oxygen flow rate and cell temperature on the DMFC performance was investigated.
2.
Experimental
2.1.
Preparation of catalysts
Multi-walled CNTs with purity higher than 95% were purchased from Chengdu Organic Chemicals Co. Ltd. The main rang of diameter, length and surface area of the CNTs are 8–15 nm, 50 mm and 233 m2/g, respectively. An oxidative pretreatment of the CNTs was performed by ultrasonic in a mixture of concentrated sulfuric and nitric acids (1:1 v/v, 98% and 70%, respectively) at room temperature for 2 h. Then, the CNTs were filtered and washed by ultra-pure water (18.23 MU) until the pH of the filtrate became 7 and consequently dried in an vacuum oven at 90 C for 5 h. The PtRu/ CNTs and PtRuMo/CNTs catalysts were synthesized by microwave-assisted polyol process. The PtRu and PtRuMo content in each samples was 30 wt.% and Pt:Ru and Pt:Ru:Mo atomic ratios were 1:1 and 6:3:1, respectively. Generally, 2.67 ml 0.038 M H2PtCl6 in ethylene glycol and 2.78 ml 0.036 M RuCl3 in ethylene glycol were mixed with 50 ml ethylene glycol in a two-necked flask. About 70 mg CNTs were added to the mixture and the pH value of the mixture was adjusted to 9 by adding 0.5 M NaOH in ethylene glycol. The solution was ultrasonicated for 30 min and placed in a microwave oven and heated for 3 min. The solution was then filtered and washed with ethanol and ultra-pure water, and the filtration was repeated for several times. Finally, the PtRu/CNTs catalyst was dried in a vacuum oven at 80 C for 5 h. The preparation of PtRuMo/CNTs catalyst was the same with that of PtRu/CNTs catalyst except that appropriate amount of H2PtCl6, RuCl3 and Na2Mo7O2 in ethylene glycol were mixed in the reaction solution.
2.2.
Characterization of catalysts
The morphology of the catalysts was observed by transmission electron microscopy (TEM) on a JEM-100CXII under an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns of the catalysts were obtained on an XD-3 X-ray diffractometer (Beijing Purkinje General Instrument Co., Ltd) using a CuKa source operating at 36 kV and 20 mA. The scanning range and rate are 10–90 and 2 /min, respectively. In order to obtain the detailed XRD patterns in the Pt(220) region, the scanning range and rate are 64–72 and 0.125 /min, respectively. The X-ray photoelectron spectrometry (XPS) study of surface composition involved an x-ray photoelectron spectrometer (Axis Ultra DLD), with the Al Ka X-ray source of 1485.69 eV and 150 W. The C 1s electron binding energy was referenced at 284.6 eV. A nonlinear least-squares curve-fitting program (XPSPEAK V4.1) was employed with a Gaussion-Lorentzian production function to analyze the XPS results.
2.3.
Preparation of MEAs
Commercial Nafion 115 membrane (DuPont) was treated according to the standard membrane cleaning procedure. Briefly, the Nafion 115 membrane was boiled at 80 C for 1 h in 3% H2O2 solution, ultra-pure water, 0.5 M H2SO4 solution and again in ultra-pure water, and then the membrane was air dried. The MEAs were seven-layer structure and prepared by the Gas diffusion layer (GDL)-based method. On both the anode and cathode, commercial carbon papers (TGP-H-060, Toray) were employed as the backing layers. The anode GDLs were the carbon papers treated with 10 wt.% PTFE and then coated with the micro-porous layers (MPLs) which comprised Vulcan XC-72 carbon blacks and 10 wt.% of Nafion ionomer solution (5 wt.%, Fluka). The cathode GDLs were the carbon papers treated with 20 wt.% PTFE and then coated with the MPLs which comprised Vulcan XC-72 carbon blacks and 30 wt.% PTFE. The catalysts used for the anode and cathode electrodes were the commercial PtRu/C (Johnson Matthey), the home-made PtRu/CNTs or PtRuMo/CNTs catalysts and the commercial 50 wt.% Pt/C catalyst (Johnson Matthey), respectively. The catalyst powder and Nafion ionomer solution were ultrasonically mixed in isopropyl alcohol to form a homogeneous catalyst ink. Then the catalyst ink was brushed onto the GDLs, and then the electrodes were dried at 80 C for 2 h in the vacuum oven. The Nafion ionomer content in both the anode and the cathode electrodes was 20 wt.% and the metal loading (PtRu, PtRuMo or Pt) was 2 mg/cm2. Finally, the MEAs were formed by sandwiching the treated Nafion membrane between the anode and cathode electrodes, and by hot pressing it under a pressure of 0.5 MPa for 90 s at 135 C.
2.4.
Fuel cell testing
The MEAs prepared by using the commercial PtRu/C, the home-made PtRu/CNTs and PtRuMo/CNTs catalysts at the anode and the commercial Pt/C at the cathode were tested in a single DMFC fixture (consisting of graphite plates with an active area of 5 cm2, Electrochem. Inc.). The performance test of this single DMFC was carried out by a fuel cell test system (Direct Methanol Test Kit, Fideris Inc.). This test system was
international journal of hydrogen energy 35 (2010) 8225–8233
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connected to a computer with FC power software to conveniently control the cell operating conditions such as the flow rate of methanol solution and oxygen, fuel cell temperature, loading current or voltage, and so on. The MEAs were initially activated at the loading current density of 10 mA/cm2 and at 60 C by feeding 2 M methanol at the anode and dry O2 to the cathode. Performance cures under various operating conditions were recorded by fixing the loading current for about 3 min before reading the voltage values.
3.
Results and discussion
3.1.
Characterization of catalysts
Fig. 1 shows the TEM pictures of PtRu/CNTs (a) and PtRuMo/ CNTs (b) catalysts. It can be seen from Fig. 1 that the PtRu and PtRuMo nanoparticles with sizes of 2–4 nm are uniformly distributed on CNTs surface. Fig. 2 displays the XRD patterns of PtRu/CNTs (a) and PtRuMo/CNTs (b), and the inserted figure illustrates the XRD patterns in the Pt(220) region. The first peak located at a 2q value of about 25.5 is associated with the carbon nanotubes support. Four characteristic peaks corresponding to (111), (200), (220) and (311) planes of the face-centered cubic (fcc) crystalline of Pt can be observed. The corresponding 2q values are shifted to higher as compared to the 2q values of 39.76 , 46.24 , 67.45 and 81.28 for pure Pt fcc, indicating that the PtRu alloy catalysts have single-phase-disordered structures, and the lattice constants decrease because of Ru substitution in Pt fcc-center [26]. Peaks associated with either Ru, Mo metal or their oxide species are not detected, implying that Ru or Mo may enter Pt fcc-center to form PtRu alloys, or partially present as oxide species but not clearly discerned by X-ray diffraction. The average particle size could be roughly calculated from Pt (220) FWHM according to Debye–Scherrer equation [27]. L¼
0:9lCuKa B2q cos qmax
(1)
where L is the average particle size, lCuKa the X-ray wave˚ ), B2a the full width at half maximum, and qmax length (1.5406 A the angle at peak maximum. The average particle sizes for PtRu/CNTs and PtRuMo/CNTs are 2.6 and 2.4 nm, respectively. It can be seen that the addition of Mo in the PtRu/CNTs almost has no effect on the metal particle sizes of the PtRu/CNTs catalyst. This may be due to
Fig. 2 – XRD patterns of PtRu/CNTs (a) and PtRuMo/CNTs (b).
the homogeneous and fast microwave heating of the ethylene glycol solution with high dielectric constant and the dielectric loss [28,29]. Microwave-assisted polyol process can reduce the temperature gradients in the reaction medium, so the metal nanoparticles can be nucleated and grown up in a more uniform reducing environment. To investigate the oxidation states of the metals on the catalyst surface, XPS analyses were used to analyze the PtRu/ CNTs and PtRuMo/CNTs catalysts. Fig. 3 shows the XPS spectra of the Pt 4f and Ru 3p core level regions of the PtRu/CNTs. The deconvolution of the Pt 4f signal shows three pairs of doublets. The most intense doublet with binding energies of 71.19 eV (Pt 4f2/7) and 74.55 eV (Pt 4f2/5) attributes to metallic Pt. Peaks at 72.29 and 75.74 eV could be ascribed to Pt2þ as in either PtO or Pt(OH)2. The third doublet at 74.48 and 77.60 eV may be assigned to Pt4þ as possibly in PtO2 [30]. Approximately 66.38%, 27.39% and 9.23% of platinum are present as metallic Pt, PtO and PtO2 respectively. Because the Ru 3d region was interferes with the C 1s region, the less intense Ru 3p region was analyzed. The Ru 3p spectrum was deconvoluted to metallic Ru (461.94 eV, 77%) and RuO2 (465.02 eV, 23%) [30]. Fig. 4 presents the Pt 4f, Ru 3p, and Mo 3d regions of PtRuMo/CNTs catalyst. As those of the PtRu/CNTs catalyst, the Pt 4f signal is also deconvoluted into three pairs of doublets. The doublets located at 71.13 and 74.51 eV, 71.94 and 75.23 eV, 74.40 and 77.50 eV are attributed to metallic Pt (56.73%), PtO or Pt(OH)2 (36.73%) and PtO2 (6.84%), respectively. It can be seen that the
Fig. 1 – TEM pictures of PtRu/CNTs (a) and PtRuMo/CNTs (b).
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min at cell temperature of 60 C. The DMFC with PtRuMo/CNTs as the anode catalyst shows the best performance, as shown in Fig. 5. The highest power density of the DMFC with PtRu/C, PtRu/CNTs and PtRuMo/CNTs are 55.20, 55.88 and 61.32 mW/ cm2, respectively. Meanwhile, the discharging voltages at the loading current density of 260 mA/cm2 of the DMFC with PtRu/ C, PtRu/CNTs and PtRuMo/CNTs are 0.200, 0.210 and 0.234 V, respectively. It can be seen that PtRuMo/CNTs is a promising anode catalyst for DMFCs. The enhanced performance can be attributed to the addition of Mo to the PtRu/CNTs catalyst for methanol electro-oxidation, and the promoting effect may be investigated in three aspects, the electronic effect, the bifunctional mechanism, and the hydrogen spillover effect. As evident from the XPS results, the electron density of Mo could be transferred to Pt. The catalytic sites would increase because the coverage of COad on the Pt surface is reduced due to the decreased Pt–CO binding energy. On the other hand, the bifunctional mechanism has been explored in the previous studies as follows [34].
Fig. 3 – XPS core level spectra for Pt 4f (a) and Ru 3p (b) photoemission from the PtRu/CNTs catalyst.
binding energies corresponding to each platinum species in PtRuMo/CNTs are a little lower compared with those in PtRu/ CNTs. This may be due to the transfer of the electron density from Mo to Pt, since the electronegativities of Mo and Pt are 1.96 and 2.28 [31], respectively. Similarly, the Ru 3p spectrum of PtRuMo/CNTs catalyst in Fig. 3b reveals that Ru is composed of 86.31% of metallic Ru and 13.69% of RuO2. The deconvolution of the Mo 3d signal shows two pairs of doublets. The doublet with binding energies of 231.18 eV (Mo 3d2/3) and 234.13 eV (Mo 3d2/5) could be attributed to Mo4þ as possibly in MoO2, while another doublet at 232.32 and 235.16 eV could be ascribed to Mo6þ as in MoO3 [32]. Nevertheless, no XPS signal was observed at 227.8 eV associated with the metallic Mo or PtMo alloy [33], which implies that Mo is present as the MoOx species on the surface of PtRuMo/CNTs catalyst.
3.2.
Effect of different anode catalysts
The effect of different anode catalysts on the performance of a DMFC was investigated by using the commercials PtRu/C, the home-made PtRu/CNTs and PtRuMo/CNTs as the anode catalysts, respectively. Fig. 5 shows the polarization and power density curves of the DMFC with different anode catalysts. The operating conditions were 2 M methanol at the flow rate of 1 mL/min, and oxygen at the flow rate of 100 mL/
MoOx þ H2O / MoOx-OHad þ Hþ þ e
(2)
Pt-COad þ MoOx-OHad/CO2 þ Hþ þ Pt þ MoOx þ e
(3)
From the XPS analyses, Mo is present as the MoOx species on the surface of PtRuMo/CNTs catalyst. It is clear that MoOx promotes the water activation to generate the species of OHad, which can oxidize CO and thus enhance the activity of PtRuMo/CNTs catalyst for methanol oxidation. Thirdly, Crabb et al. [35] have reported that PtMo/C is a good CO-resistant catalyst, because the hydrogen of Pt-Had can spillover to MoO3 to form HxMoO3, which can catalyze the CO oxidation. This process can be described as follows. MoO3 þ xe þ xPt-Had / HxMoO3 þ xPt
(4)
2HxMoO3 þ COad þ H2O / CO2 þ 2H1þxMoO3
(5)
H1þxMoO3 / HxMoO3 þ Hþþe
(6)
It can be seen that MoO3 quickly decreases the coverage of Had species, which can facilitate the further adsorption and electro-oxidation of methanol on Pt surface. In summary, the enhanced DMFC performance with PtRuMo/ CNTs as the anode catalyst may be explained by the electronic effect, the bifunctional mechanism, and the hydrogen spillover effect. PtRuMo/CNTs catalyst is a promising anode catalyst for DMFCs. The effect of various operating conditions on the DMFC performance with PtRuMo/CNTs as the anode catalyst will be further studied in the following sections.
3.3.
Effect of MEA activation
The conditions for MEA activation are 2 M methanol at the flow rate of 1 mL/min, and oxygen at the flow rate of 100 mL/
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Fig. 4 – XPS core level spectra for Pt 4f (a), Ru 3p (b) and Mo 3d (c) photoemission from the PtRuMo/CNTs catalyst.
min at cell temperature of 60 C, and loading current density of 10 mA/cm2. Fig. 6 shows the polarization and power density curves of the DMFC before and after activation. The purpose of the MEA activation is to wet the Nafion membrane as well as the Nafion ionomer in the catalyst layer, and to enhance the transport ability of the reactants, electrons and protons to optimize the electrode structure [36]. The proton transport in Nafion membrane will not reach the optimal conditions because it’s dry before the MEA activation. As can be seen in
Fig. 6, the performance of the DMFC before activation is seriously low with the highest power density of 34.68 mW/cm2. On the other hand, the process of MEA activation will quickly establish the proton transport ways in the Nafion membrane and the electrodes to enhance the proton conductivity, and hence decrease the MEA resistance. Qi et al. [37] have reported that the activation process can increase catalyst utilization by opening many ‘‘dead’’ regions in the catalyst layer, and when a fuel cell is activated, many of these ‘‘dead’’ regions are ‘‘opened’’ and then become active. In our studies, the MEA activation at the small loading current density of 10 mA/cm2 may improve the three reaction zone as well as the Nafion distribution in the catalyst layers, and hence increases the activity of the PtRuMo/CNTs catalyst. The DMFC performance reaches stable after activation for 10 h, achieving the highest power density of 61.32 mW/cm2.
3.4.
Fig. 5 – Effect of different anode catalysts on the DMFC performance.
Effect of methanol concentration
The methanol concentration greatly influences the DMFC performance [38–40]. High methanol concentration enhances the reaction rate in the anode, but at the same time increases the methanol crossover to the cathode. The methanol in the cathode will be oxidized over Pt catalyst to produce mixed potential, thus decrease the DMFC performance [41,42]. The polarization and power density curves of the DMFC at different methanol concentration are showed in Fig. 7. The operating conditions are methanol at the flow rate of 1 mL/
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further improves with the highest power density of 61.32 mW/ cm2. However, when the methanol concentration increases to 2.5 M, the DMFC performance does not increase apparently. Although higher methanol concentrations facilitate the anode reaction, the methanol crossover also deteriorates. In our study, the appropriate range of methanol concentration is 2.0– 2.5 M.
3.5.
Fig. 6 – Effect of MEA activation on the DMFC performance.
min, and oxygen at the flow rate of 100 mL/min at cell temperature of 60 C. The open circuit voltages for the methanol concentration of 0.5, 1.0, 2.0 and 2.5 M are 0.654, 0.586, 0.570 and 0.556 V, respectively. The open circuit voltages decrease with the increase of methanol concentration, because the methanol crossover also increases with increasing methanol concentration, which will result in higher crossover current and hence lower the open circuit voltage. When the methanol concentration is 0.5 M, the performance is good at the lower loading current densities because of the less methanol crossover. But at the high loading current densities (>170 mA/cm2), the DMFC performance decreases rapidly, and the highest power density is only 47.26 mW/cm2. The lower performance is due to the concentration polarization effect. The DMFC performance decreases at the high loading current density region because the concentration of methanol solution of 0.5 M is too low. When the methanol concentration increases to 1.0 M, there is still concentration polarization effect when the loading current density is larger than 240 mA/cm2, and the highest power density is 56.16 mW/cm2. When the methanol concentration increases to 2.0 M, the DMFC performance
Fig. 7 – Effect of methanol concentration on the DMFC performance.
Effect of methanol flow rate
The influencing factors of methanol flow rate to the DMFC performance are mass transport of methanol, methanol crossover, the removal of product CO2 as well as the heat exchange between the methanol solution and the catalyst layer [43]. Fig. 8 shows the polarization and power density curves of the DMFC at different methanol flow rates. The operating conditions are 2 M methanol, and oxygen at the flow rate of 100 mL/min at cell temperature of 60 C. The DMFC performance first increases but then decreases with increasing methanol flow rates, as shown in Fig. 8. The highest power densities are 56.16, 61.32, 61.88 and 55.88 mW/cm2 when the methanol flow rates are 0.5, 1.0, 2.0 and 3.0 mL/min, respectively. When the methanol flow rate is 0.5 mL/min and the loading current increases to 220 mA/cm2, the DMFC performance begins to decrease due to the concentration polarization effect, and the highest power density is 56.16 mW/cm2. The optimal DMFC performance is achieved at the methanol flow rate of 2 mL/min. There is a rapid decrease in the DMFC performance when the methanol flow rate further increases to 3 mL/min. This is because although high methanol flow rate can facilitate the mass transport of methanol and the removal of CO2, it also results in more methanol crossover. In addition, the methanol solution at high flow rate would cool down the temperature of catalyst surface, and hence decrease the activity of the PtRuMo/CNTs catalyst. In this study, the appropriate range of methanol flow rate is 1.0–2.0 mL/min.
3.6.
Effect of oxygen flow rate
The influencing factors of oxygen flow rate to the DMFC performance are mass transport of oxygen and the water
Fig. 8 – Effect of methanol flow rate on the DMFC performance.
international journal of hydrogen energy 35 (2010) 8225–8233
removal [44]. Fig. 9 shows the polarization and power density curves of the DMFC operated with different oxygen flow rates. The operating conditions are 2 M methanol at the flow rate of 1 mL/min at cell temperature of 60 C. As shown in Fig. 9, the DMFC performance is low at low oxygen flow rate. This may be ascribed to the low mass transport of oxygen as well as the water flood effect. Since there is not enough oxygen to blow away the water in the cathode, the produced water obstructs the mass transfer of oxygen or covers some of the reaction sites at the cathode. The highest power densities are 39.04 and 49.14 mW/min at the oxygen flow rates of 50 and 100 mL/min, respectively. When the oxygen flow rate increases to 100 mL/ min, the DMFC performance reaches the best with the highest power density of 61.32 mW/cm2. However, the DMFC performance shows a little decrease with the highest power density of 60.6 mW/cm2 when the oxygen flow rate further increases to 150 mL/min. Although higher oxygen flow rates can increase the mass transport of oxygen and enhance the activity of oxygen reduction reaction, the more oxygen will dry up the Nafion membrane and result in decreased proton transport ability. In our present investigation, the appropriate range of oxygen flow rate is 100–150 mL/min.
3.7.
Effect of cell temperature
Fig. 10 shows the polarization and power density curves of the DMFC at different cell temperature. The operating conditions are 2 M methanol at the flow rate of 1 mL/min, and oxygen at the flow rate of 100 mL/min. It can be seen from Fig. 10 that the DMFC performance increases significantly with an increase in cell temperature, while the open circuit voltages first increase then decrease. These results agree well with those obtained by Jung et al. [10,45]. The highest power densities are 23.54, 38.24 and 61.32 mW/cm2 at 25, 40 and 60 C, respectively. On the other hand, the open circuit voltages are 0.571, 0.591 and 0.570 V at 25, 40 and 60 C, respectively. Although high cell temperature facilitates the methanol electro-oxidation, the more methanol that crossovers to the cathode will be oxidized over Pt to form mixed potential, and results in lower open circuit voltage at 60 C.
Fig. 10 – Effect of cell temperature on the DMFC performance.
The enhanced DMFC performance at increasing cell temperatures can be attributed to the following two processes [10]. On the one hand, the catalytic activities of the catalysts for methanol electro-oxidation and oxygen reduction increase with increasing cell temperature, so the activation polarization effect decreases. On the other hand, the ion conductivity of Nafion membrane or Nafion ionomer in the catalyst layers decreases with an increase in cell temperature, so the ohm polarization loss decreases. At low temperature, the methanol electro-oxidation rate is slow, and the CO2 in the anode as well as the water in the cathode can not be removed in time, which will block the diffusion of methanol and oxygen to the catalyst surface. When the cell temperature increases, the reactant diffusion and the product removal increase, so the electrochemical reaction is faster. In addition, high cell temperature enhances the ion conductivity of the membrane and catalyst layer, therefore the cell resistance is reduced. All these processes contribute to the enhanced DMFC performance at high cell temperature.
4.
Fig. 9 – Effect of oxygen flow rate on the DMFC performance.
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Conclusions
The effect of operating conditions on the performance of a DMFC with PtRuMo/CNTs as the anode catalyst has been investigated. The performance of a DMFC using PtRuMo/CNTs as the anode catalyst is better than those using PtRu/CNTs and PtRu/C catalysts, which can be attributed to the electronic effect, the bifunctional mechanism, and the hydrogen spillover. The DMFC performance reaches stable after MEA activation at a small loading current density for 10 h. The appropriate operating conditions of the DMFC with PtRuMo/ CNTs as the anode catalyst are 2.0–2.5 M methanol at the flow rate of 1.0–2.0 mL/min, and oxygen at the flow rate of 100– 150 mL/min. The DMFC performance increases significantly with an increase in cell temperature, which can be ascribed to the enhancement of the catalytic activities as well as the ion conductivity in the MEA.
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Acknowledgements [17]
The authors gratefully acknowledge the financial support of this work from National Nature Foundation of China through project number 20576023, Science and Technology Department of Guangdong Province through project numbers 2008B010800036 and 2008B010800037 and Science and Technology Department of Guangzhou City through project number 2009J1-C431–1.
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Effect of operating conditions on the performance of a direct methanol fuel cell with PtRuMo/CNTs as anode catalyst Shengzhou Chen a,*, Fei Ye a,b, Weiming Lin a,b a b
School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, China School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
article info
abstract
Article history:
PtRu/CNTs and PtRuMo/CNTs catalysts have been synthesized by microwave-assisted
Received 17 September 2009
polyol process and used as the anode catalysts for a direct methanol fuel cell (DMFC). The
Received in revised form
catalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction
17 December 2009
(XRD) and X-ray photoelectron spectrometry (XPS). The effect of different anode catalysts,
Accepted 17 December 2009
membrane electrode assembly (MEA) activation, methanol concentration, methanol flow
Available online 12 January 2010
rate, oxygen flow rate and cell temperature on the DMFC performance has been investigated. The results show that the PtRu or PtRuMo particles with face-centered cubic
Keywords:
structure are uniformly distributed on CNTs, and the addition of Mo to PtRu/CNTs makes
Direct methanol fuel cell
the binding energies of each Pt species shift to lower values. PtRuMo/CNTs is a promising
Operating conditions
anode catalyst for DMFCs, and the appropriate operating conditions of the DMFC with
PtRuMo/CNTs
PtRuMo/CNTs as the anode catalyst are MEA activation for 10 h, 2.0–2.5 M methanol at the
MEA activation
flow rate of 1.0–2.0 mL/min, and oxygen at the flow rate of 100–150 mL/min. The DMFC
Methanol crossover
performance increases significantly with an increase in cell temperature. Crown Copyright ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. All rights reserved.
1.
Introduction
DMFC has been received increasing attention because of its advantages, such as high energy density, simple and easily handled structure, cheap liquid fuel, low exhaustion, low temperature operating as well as easy transportation and storage of the fuel [1,2]. Currently, the development direction of the DMFCs lies in three aspects of potential applications as vehicle transportation, portable power sources and micro power sources [3–5]. DMFC is one of the most studied, most promising and most nearly commercialized fuel cells. However, the main barriers for DMFC development are low activities of anode catalysts and methanol crossover [6]. To solve these problems, lots of work has been done in the past two decades on the improvement of the catalyst activity, Nafion membrane modification, electrode structure
optimization, and so on [7]. But in the actual DMFC applications, the operating conditions generally exert great influence on the cell performance [8,9]. So it is very important to study the characteristics of various operating conditions. Jung et al. [10] have investigated the effect of membrane thickness, cell temperature, and methanol concentration on the DMFC power density and methanol crossover, and found that increasing the cell temperature promotes the cell performance, and the lower methanol concentration causes the concentration polarization effects, thus resulting in lower cell performance. Although higher methanol solution concentration can overcome the concentration polarization, a serious methanol crossover decreases the cell performance. Colmati et al. [8] have studied the effect of methanol concentration, Pt load, membrane thickness and PTFE content in the cathode diffusion layer on the DMFC performance, and the results
* Corresponding author. Tel./fax: þ86 020 39366507. E-mail address: [email protected] (S. Chen). 0360-3199/$ – see front matter Crown Copyright ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. All rights reserved. doi:10.1016/j.ijhydene.2009.12.085
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show better performance with cells operating with 3 mgPt/cm2, 5 M methanol solution, Nafion 112 membrane and with 30 wt.% PTFE in the cathode diffusion layer. It’s suggested that the mass transport, ion conductivity, catalyst activity, and methanol crossover should all be taken in account while studying the operating conditions of DMFC [11]. So in order to appropriately operate DMFCs and extend their lifetime, it is of great significance to investigate the operating conditions of DMFC. Carbon nanotubes (CNTs) have been used as catalyst supports for fuel cells because of their highly electrochemically accessible surface area and remarkable electronic conductivity [12,13]. Previous studies have showed that CNTs-supported catalysts exhibit better performance of methanol electrooxidation as compared to conventional carbon black (XC-72) supported catalysts [14–17]. While recent studies have been focused on the modification of the CNTs-supported binary catalysts [18–21], little work has been done on the CNTs-supported ternary catalysts [22–24], especially those investigated in a single DMFC or DMFC stacks [25]. In the present study, PtRu/ CNTs and PtRuMo/CNTs catalysts were synthesized by microwave-assisted polyol process, and the performance of a DMFC with these catalysts as the anode catalysts was compared, then the effect of various operating conditions, such as membrane electrode assembly (MEA) activation, methanol concentration, methanol flow rate, oxygen flow rate and cell temperature on the DMFC performance was investigated.
2.
Experimental
2.1.
Preparation of catalysts
Multi-walled CNTs with purity higher than 95% were purchased from Chengdu Organic Chemicals Co. Ltd. The main rang of diameter, length and surface area of the CNTs are 8–15 nm, 50 mm and 233 m2/g, respectively. An oxidative pretreatment of the CNTs was performed by ultrasonic in a mixture of concentrated sulfuric and nitric acids (1:1 v/v, 98% and 70%, respectively) at room temperature for 2 h. Then, the CNTs were filtered and washed by ultra-pure water (18.23 MU) until the pH of the filtrate became 7 and consequently dried in an vacuum oven at 90 C for 5 h. The PtRu/ CNTs and PtRuMo/CNTs catalysts were synthesized by microwave-assisted polyol process. The PtRu and PtRuMo content in each samples was 30 wt.% and Pt:Ru and Pt:Ru:Mo atomic ratios were 1:1 and 6:3:1, respectively. Generally, 2.67 ml 0.038 M H2PtCl6 in ethylene glycol and 2.78 ml 0.036 M RuCl3 in ethylene glycol were mixed with 50 ml ethylene glycol in a two-necked flask. About 70 mg CNTs were added to the mixture and the pH value of the mixture was adjusted to 9 by adding 0.5 M NaOH in ethylene glycol. The solution was ultrasonicated for 30 min and placed in a microwave oven and heated for 3 min. The solution was then filtered and washed with ethanol and ultra-pure water, and the filtration was repeated for several times. Finally, the PtRu/CNTs catalyst was dried in a vacuum oven at 80 C for 5 h. The preparation of PtRuMo/CNTs catalyst was the same with that of PtRu/CNTs catalyst except that appropriate amount of H2PtCl6, RuCl3 and Na2Mo7O2 in ethylene glycol were mixed in the reaction solution.
2.2.
Characterization of catalysts
The morphology of the catalysts was observed by transmission electron microscopy (TEM) on a JEM-100CXII under an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns of the catalysts were obtained on an XD-3 X-ray diffractometer (Beijing Purkinje General Instrument Co., Ltd) using a CuKa source operating at 36 kV and 20 mA. The scanning range and rate are 10–90 and 2 /min, respectively. In order to obtain the detailed XRD patterns in the Pt(220) region, the scanning range and rate are 64–72 and 0.125 /min, respectively. The X-ray photoelectron spectrometry (XPS) study of surface composition involved an x-ray photoelectron spectrometer (Axis Ultra DLD), with the Al Ka X-ray source of 1485.69 eV and 150 W. The C 1s electron binding energy was referenced at 284.6 eV. A nonlinear least-squares curve-fitting program (XPSPEAK V4.1) was employed with a Gaussion-Lorentzian production function to analyze the XPS results.
2.3.
Preparation of MEAs
Commercial Nafion 115 membrane (DuPont) was treated according to the standard membrane cleaning procedure. Briefly, the Nafion 115 membrane was boiled at 80 C for 1 h in 3% H2O2 solution, ultra-pure water, 0.5 M H2SO4 solution and again in ultra-pure water, and then the membrane was air dried. The MEAs were seven-layer structure and prepared by the Gas diffusion layer (GDL)-based method. On both the anode and cathode, commercial carbon papers (TGP-H-060, Toray) were employed as the backing layers. The anode GDLs were the carbon papers treated with 10 wt.% PTFE and then coated with the micro-porous layers (MPLs) which comprised Vulcan XC-72 carbon blacks and 10 wt.% of Nafion ionomer solution (5 wt.%, Fluka). The cathode GDLs were the carbon papers treated with 20 wt.% PTFE and then coated with the MPLs which comprised Vulcan XC-72 carbon blacks and 30 wt.% PTFE. The catalysts used for the anode and cathode electrodes were the commercial PtRu/C (Johnson Matthey), the home-made PtRu/CNTs or PtRuMo/CNTs catalysts and the commercial 50 wt.% Pt/C catalyst (Johnson Matthey), respectively. The catalyst powder and Nafion ionomer solution were ultrasonically mixed in isopropyl alcohol to form a homogeneous catalyst ink. Then the catalyst ink was brushed onto the GDLs, and then the electrodes were dried at 80 C for 2 h in the vacuum oven. The Nafion ionomer content in both the anode and the cathode electrodes was 20 wt.% and the metal loading (PtRu, PtRuMo or Pt) was 2 mg/cm2. Finally, the MEAs were formed by sandwiching the treated Nafion membrane between the anode and cathode electrodes, and by hot pressing it under a pressure of 0.5 MPa for 90 s at 135 C.
2.4.
Fuel cell testing
The MEAs prepared by using the commercial PtRu/C, the home-made PtRu/CNTs and PtRuMo/CNTs catalysts at the anode and the commercial Pt/C at the cathode were tested in a single DMFC fixture (consisting of graphite plates with an active area of 5 cm2, Electrochem. Inc.). The performance test of this single DMFC was carried out by a fuel cell test system (Direct Methanol Test Kit, Fideris Inc.). This test system was
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connected to a computer with FC power software to conveniently control the cell operating conditions such as the flow rate of methanol solution and oxygen, fuel cell temperature, loading current or voltage, and so on. The MEAs were initially activated at the loading current density of 10 mA/cm2 and at 60 C by feeding 2 M methanol at the anode and dry O2 to the cathode. Performance cures under various operating conditions were recorded by fixing the loading current for about 3 min before reading the voltage values.
3.
Results and discussion
3.1.
Characterization of catalysts
Fig. 1 shows the TEM pictures of PtRu/CNTs (a) and PtRuMo/ CNTs (b) catalysts. It can be seen from Fig. 1 that the PtRu and PtRuMo nanoparticles with sizes of 2–4 nm are uniformly distributed on CNTs surface. Fig. 2 displays the XRD patterns of PtRu/CNTs (a) and PtRuMo/CNTs (b), and the inserted figure illustrates the XRD patterns in the Pt(220) region. The first peak located at a 2q value of about 25.5 is associated with the carbon nanotubes support. Four characteristic peaks corresponding to (111), (200), (220) and (311) planes of the face-centered cubic (fcc) crystalline of Pt can be observed. The corresponding 2q values are shifted to higher as compared to the 2q values of 39.76 , 46.24 , 67.45 and 81.28 for pure Pt fcc, indicating that the PtRu alloy catalysts have single-phase-disordered structures, and the lattice constants decrease because of Ru substitution in Pt fcc-center [26]. Peaks associated with either Ru, Mo metal or their oxide species are not detected, implying that Ru or Mo may enter Pt fcc-center to form PtRu alloys, or partially present as oxide species but not clearly discerned by X-ray diffraction. The average particle size could be roughly calculated from Pt (220) FWHM according to Debye–Scherrer equation [27]. L¼
0:9lCuKa B2q cos qmax
(1)
where L is the average particle size, lCuKa the X-ray wave˚ ), B2a the full width at half maximum, and qmax length (1.5406 A the angle at peak maximum. The average particle sizes for PtRu/CNTs and PtRuMo/CNTs are 2.6 and 2.4 nm, respectively. It can be seen that the addition of Mo in the PtRu/CNTs almost has no effect on the metal particle sizes of the PtRu/CNTs catalyst. This may be due to
Fig. 2 – XRD patterns of PtRu/CNTs (a) and PtRuMo/CNTs (b).
the homogeneous and fast microwave heating of the ethylene glycol solution with high dielectric constant and the dielectric loss [28,29]. Microwave-assisted polyol process can reduce the temperature gradients in the reaction medium, so the metal nanoparticles can be nucleated and grown up in a more uniform reducing environment. To investigate the oxidation states of the metals on the catalyst surface, XPS analyses were used to analyze the PtRu/ CNTs and PtRuMo/CNTs catalysts. Fig. 3 shows the XPS spectra of the Pt 4f and Ru 3p core level regions of the PtRu/CNTs. The deconvolution of the Pt 4f signal shows three pairs of doublets. The most intense doublet with binding energies of 71.19 eV (Pt 4f2/7) and 74.55 eV (Pt 4f2/5) attributes to metallic Pt. Peaks at 72.29 and 75.74 eV could be ascribed to Pt2þ as in either PtO or Pt(OH)2. The third doublet at 74.48 and 77.60 eV may be assigned to Pt4þ as possibly in PtO2 [30]. Approximately 66.38%, 27.39% and 9.23% of platinum are present as metallic Pt, PtO and PtO2 respectively. Because the Ru 3d region was interferes with the C 1s region, the less intense Ru 3p region was analyzed. The Ru 3p spectrum was deconvoluted to metallic Ru (461.94 eV, 77%) and RuO2 (465.02 eV, 23%) [30]. Fig. 4 presents the Pt 4f, Ru 3p, and Mo 3d regions of PtRuMo/CNTs catalyst. As those of the PtRu/CNTs catalyst, the Pt 4f signal is also deconvoluted into three pairs of doublets. The doublets located at 71.13 and 74.51 eV, 71.94 and 75.23 eV, 74.40 and 77.50 eV are attributed to metallic Pt (56.73%), PtO or Pt(OH)2 (36.73%) and PtO2 (6.84%), respectively. It can be seen that the
Fig. 1 – TEM pictures of PtRu/CNTs (a) and PtRuMo/CNTs (b).
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min at cell temperature of 60 C. The DMFC with PtRuMo/CNTs as the anode catalyst shows the best performance, as shown in Fig. 5. The highest power density of the DMFC with PtRu/C, PtRu/CNTs and PtRuMo/CNTs are 55.20, 55.88 and 61.32 mW/ cm2, respectively. Meanwhile, the discharging voltages at the loading current density of 260 mA/cm2 of the DMFC with PtRu/ C, PtRu/CNTs and PtRuMo/CNTs are 0.200, 0.210 and 0.234 V, respectively. It can be seen that PtRuMo/CNTs is a promising anode catalyst for DMFCs. The enhanced performance can be attributed to the addition of Mo to the PtRu/CNTs catalyst for methanol electro-oxidation, and the promoting effect may be investigated in three aspects, the electronic effect, the bifunctional mechanism, and the hydrogen spillover effect. As evident from the XPS results, the electron density of Mo could be transferred to Pt. The catalytic sites would increase because the coverage of COad on the Pt surface is reduced due to the decreased Pt–CO binding energy. On the other hand, the bifunctional mechanism has been explored in the previous studies as follows [34].
Fig. 3 – XPS core level spectra for Pt 4f (a) and Ru 3p (b) photoemission from the PtRu/CNTs catalyst.
binding energies corresponding to each platinum species in PtRuMo/CNTs are a little lower compared with those in PtRu/ CNTs. This may be due to the transfer of the electron density from Mo to Pt, since the electronegativities of Mo and Pt are 1.96 and 2.28 [31], respectively. Similarly, the Ru 3p spectrum of PtRuMo/CNTs catalyst in Fig. 3b reveals that Ru is composed of 86.31% of metallic Ru and 13.69% of RuO2. The deconvolution of the Mo 3d signal shows two pairs of doublets. The doublet with binding energies of 231.18 eV (Mo 3d2/3) and 234.13 eV (Mo 3d2/5) could be attributed to Mo4þ as possibly in MoO2, while another doublet at 232.32 and 235.16 eV could be ascribed to Mo6þ as in MoO3 [32]. Nevertheless, no XPS signal was observed at 227.8 eV associated with the metallic Mo or PtMo alloy [33], which implies that Mo is present as the MoOx species on the surface of PtRuMo/CNTs catalyst.
3.2.
Effect of different anode catalysts
The effect of different anode catalysts on the performance of a DMFC was investigated by using the commercials PtRu/C, the home-made PtRu/CNTs and PtRuMo/CNTs as the anode catalysts, respectively. Fig. 5 shows the polarization and power density curves of the DMFC with different anode catalysts. The operating conditions were 2 M methanol at the flow rate of 1 mL/min, and oxygen at the flow rate of 100 mL/
MoOx þ H2O / MoOx-OHad þ Hþ þ e
(2)
Pt-COad þ MoOx-OHad/CO2 þ Hþ þ Pt þ MoOx þ e
(3)
From the XPS analyses, Mo is present as the MoOx species on the surface of PtRuMo/CNTs catalyst. It is clear that MoOx promotes the water activation to generate the species of OHad, which can oxidize CO and thus enhance the activity of PtRuMo/CNTs catalyst for methanol oxidation. Thirdly, Crabb et al. [35] have reported that PtMo/C is a good CO-resistant catalyst, because the hydrogen of Pt-Had can spillover to MoO3 to form HxMoO3, which can catalyze the CO oxidation. This process can be described as follows. MoO3 þ xe þ xPt-Had / HxMoO3 þ xPt
(4)
2HxMoO3 þ COad þ H2O / CO2 þ 2H1þxMoO3
(5)
H1þxMoO3 / HxMoO3 þ Hþþe
(6)
It can be seen that MoO3 quickly decreases the coverage of Had species, which can facilitate the further adsorption and electro-oxidation of methanol on Pt surface. In summary, the enhanced DMFC performance with PtRuMo/ CNTs as the anode catalyst may be explained by the electronic effect, the bifunctional mechanism, and the hydrogen spillover effect. PtRuMo/CNTs catalyst is a promising anode catalyst for DMFCs. The effect of various operating conditions on the DMFC performance with PtRuMo/CNTs as the anode catalyst will be further studied in the following sections.
3.3.
Effect of MEA activation
The conditions for MEA activation are 2 M methanol at the flow rate of 1 mL/min, and oxygen at the flow rate of 100 mL/
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Fig. 4 – XPS core level spectra for Pt 4f (a), Ru 3p (b) and Mo 3d (c) photoemission from the PtRuMo/CNTs catalyst.
min at cell temperature of 60 C, and loading current density of 10 mA/cm2. Fig. 6 shows the polarization and power density curves of the DMFC before and after activation. The purpose of the MEA activation is to wet the Nafion membrane as well as the Nafion ionomer in the catalyst layer, and to enhance the transport ability of the reactants, electrons and protons to optimize the electrode structure [36]. The proton transport in Nafion membrane will not reach the optimal conditions because it’s dry before the MEA activation. As can be seen in
Fig. 6, the performance of the DMFC before activation is seriously low with the highest power density of 34.68 mW/cm2. On the other hand, the process of MEA activation will quickly establish the proton transport ways in the Nafion membrane and the electrodes to enhance the proton conductivity, and hence decrease the MEA resistance. Qi et al. [37] have reported that the activation process can increase catalyst utilization by opening many ‘‘dead’’ regions in the catalyst layer, and when a fuel cell is activated, many of these ‘‘dead’’ regions are ‘‘opened’’ and then become active. In our studies, the MEA activation at the small loading current density of 10 mA/cm2 may improve the three reaction zone as well as the Nafion distribution in the catalyst layers, and hence increases the activity of the PtRuMo/CNTs catalyst. The DMFC performance reaches stable after activation for 10 h, achieving the highest power density of 61.32 mW/cm2.
3.4.
Fig. 5 – Effect of different anode catalysts on the DMFC performance.
Effect of methanol concentration
The methanol concentration greatly influences the DMFC performance [38–40]. High methanol concentration enhances the reaction rate in the anode, but at the same time increases the methanol crossover to the cathode. The methanol in the cathode will be oxidized over Pt catalyst to produce mixed potential, thus decrease the DMFC performance [41,42]. The polarization and power density curves of the DMFC at different methanol concentration are showed in Fig. 7. The operating conditions are methanol at the flow rate of 1 mL/
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further improves with the highest power density of 61.32 mW/ cm2. However, when the methanol concentration increases to 2.5 M, the DMFC performance does not increase apparently. Although higher methanol concentrations facilitate the anode reaction, the methanol crossover also deteriorates. In our study, the appropriate range of methanol concentration is 2.0– 2.5 M.
3.5.
Fig. 6 – Effect of MEA activation on the DMFC performance.
min, and oxygen at the flow rate of 100 mL/min at cell temperature of 60 C. The open circuit voltages for the methanol concentration of 0.5, 1.0, 2.0 and 2.5 M are 0.654, 0.586, 0.570 and 0.556 V, respectively. The open circuit voltages decrease with the increase of methanol concentration, because the methanol crossover also increases with increasing methanol concentration, which will result in higher crossover current and hence lower the open circuit voltage. When the methanol concentration is 0.5 M, the performance is good at the lower loading current densities because of the less methanol crossover. But at the high loading current densities (>170 mA/cm2), the DMFC performance decreases rapidly, and the highest power density is only 47.26 mW/cm2. The lower performance is due to the concentration polarization effect. The DMFC performance decreases at the high loading current density region because the concentration of methanol solution of 0.5 M is too low. When the methanol concentration increases to 1.0 M, there is still concentration polarization effect when the loading current density is larger than 240 mA/cm2, and the highest power density is 56.16 mW/cm2. When the methanol concentration increases to 2.0 M, the DMFC performance
Fig. 7 – Effect of methanol concentration on the DMFC performance.
Effect of methanol flow rate
The influencing factors of methanol flow rate to the DMFC performance are mass transport of methanol, methanol crossover, the removal of product CO2 as well as the heat exchange between the methanol solution and the catalyst layer [43]. Fig. 8 shows the polarization and power density curves of the DMFC at different methanol flow rates. The operating conditions are 2 M methanol, and oxygen at the flow rate of 100 mL/min at cell temperature of 60 C. The DMFC performance first increases but then decreases with increasing methanol flow rates, as shown in Fig. 8. The highest power densities are 56.16, 61.32, 61.88 and 55.88 mW/cm2 when the methanol flow rates are 0.5, 1.0, 2.0 and 3.0 mL/min, respectively. When the methanol flow rate is 0.5 mL/min and the loading current increases to 220 mA/cm2, the DMFC performance begins to decrease due to the concentration polarization effect, and the highest power density is 56.16 mW/cm2. The optimal DMFC performance is achieved at the methanol flow rate of 2 mL/min. There is a rapid decrease in the DMFC performance when the methanol flow rate further increases to 3 mL/min. This is because although high methanol flow rate can facilitate the mass transport of methanol and the removal of CO2, it also results in more methanol crossover. In addition, the methanol solution at high flow rate would cool down the temperature of catalyst surface, and hence decrease the activity of the PtRuMo/CNTs catalyst. In this study, the appropriate range of methanol flow rate is 1.0–2.0 mL/min.
3.6.
Effect of oxygen flow rate
The influencing factors of oxygen flow rate to the DMFC performance are mass transport of oxygen and the water
Fig. 8 – Effect of methanol flow rate on the DMFC performance.
international journal of hydrogen energy 35 (2010) 8225–8233
removal [44]. Fig. 9 shows the polarization and power density curves of the DMFC operated with different oxygen flow rates. The operating conditions are 2 M methanol at the flow rate of 1 mL/min at cell temperature of 60 C. As shown in Fig. 9, the DMFC performance is low at low oxygen flow rate. This may be ascribed to the low mass transport of oxygen as well as the water flood effect. Since there is not enough oxygen to blow away the water in the cathode, the produced water obstructs the mass transfer of oxygen or covers some of the reaction sites at the cathode. The highest power densities are 39.04 and 49.14 mW/min at the oxygen flow rates of 50 and 100 mL/min, respectively. When the oxygen flow rate increases to 100 mL/ min, the DMFC performance reaches the best with the highest power density of 61.32 mW/cm2. However, the DMFC performance shows a little decrease with the highest power density of 60.6 mW/cm2 when the oxygen flow rate further increases to 150 mL/min. Although higher oxygen flow rates can increase the mass transport of oxygen and enhance the activity of oxygen reduction reaction, the more oxygen will dry up the Nafion membrane and result in decreased proton transport ability. In our present investigation, the appropriate range of oxygen flow rate is 100–150 mL/min.
3.7.
Effect of cell temperature
Fig. 10 shows the polarization and power density curves of the DMFC at different cell temperature. The operating conditions are 2 M methanol at the flow rate of 1 mL/min, and oxygen at the flow rate of 100 mL/min. It can be seen from Fig. 10 that the DMFC performance increases significantly with an increase in cell temperature, while the open circuit voltages first increase then decrease. These results agree well with those obtained by Jung et al. [10,45]. The highest power densities are 23.54, 38.24 and 61.32 mW/cm2 at 25, 40 and 60 C, respectively. On the other hand, the open circuit voltages are 0.571, 0.591 and 0.570 V at 25, 40 and 60 C, respectively. Although high cell temperature facilitates the methanol electro-oxidation, the more methanol that crossovers to the cathode will be oxidized over Pt to form mixed potential, and results in lower open circuit voltage at 60 C.
Fig. 10 – Effect of cell temperature on the DMFC performance.
The enhanced DMFC performance at increasing cell temperatures can be attributed to the following two processes [10]. On the one hand, the catalytic activities of the catalysts for methanol electro-oxidation and oxygen reduction increase with increasing cell temperature, so the activation polarization effect decreases. On the other hand, the ion conductivity of Nafion membrane or Nafion ionomer in the catalyst layers decreases with an increase in cell temperature, so the ohm polarization loss decreases. At low temperature, the methanol electro-oxidation rate is slow, and the CO2 in the anode as well as the water in the cathode can not be removed in time, which will block the diffusion of methanol and oxygen to the catalyst surface. When the cell temperature increases, the reactant diffusion and the product removal increase, so the electrochemical reaction is faster. In addition, high cell temperature enhances the ion conductivity of the membrane and catalyst layer, therefore the cell resistance is reduced. All these processes contribute to the enhanced DMFC performance at high cell temperature.
4.
Fig. 9 – Effect of oxygen flow rate on the DMFC performance.
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Conclusions
The effect of operating conditions on the performance of a DMFC with PtRuMo/CNTs as the anode catalyst has been investigated. The performance of a DMFC using PtRuMo/CNTs as the anode catalyst is better than those using PtRu/CNTs and PtRu/C catalysts, which can be attributed to the electronic effect, the bifunctional mechanism, and the hydrogen spillover. The DMFC performance reaches stable after MEA activation at a small loading current density for 10 h. The appropriate operating conditions of the DMFC with PtRuMo/ CNTs as the anode catalyst are 2.0–2.5 M methanol at the flow rate of 1.0–2.0 mL/min, and oxygen at the flow rate of 100– 150 mL/min. The DMFC performance increases significantly with an increase in cell temperature, which can be ascribed to the enhancement of the catalytic activities as well as the ion conductivity in the MEA.
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international journal of hydrogen energy 35 (2010) 8225–8233
Acknowledgements [17]
The authors gratefully acknowledge the financial support of this work from National Nature Foundation of China through project number 20576023, Science and Technology Department of Guangdong Province through project numbers 2008B010800036 and 2008B010800037 and Science and Technology Department of Guangzhou City through project number 2009J1-C431–1.
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