Investigation of an Anode Catalyst for a Direct Dimethyl Ether Fuel Cell

Energy & Fuels 2009, 23, 903–907

903

Investigation of an Anode Catalyst for a Direct Dimethyl Ether Fuel Cell Ke-Di Cai, Ge-Ping Yin,* Jia-Jun Wang, and Lei-Lei Lu School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China ReceiVed July 27, 2008. ReVised Manuscript ReceiVed NoVember 15, 2008

In this work, we compared investigation of Pt/multiwalled carbon nanotubes (MWNTs) and Pt/Vulcan XC72 anode catalysts for direct dimethyl ether fuel cells (DDFC). The homemade catalysts were characterized using a transmission electron micrograph (TEM) and X-ray diffraction (XRD). The cyclic voltammogram analyses revealed that the Pt/MWNTs showed a higher catalytic activity for dimethyl ether electro-oxidation, compared with the Pt/Vulcan XC-72 catalyst. The membrane electrode assembly (MEA) with the Pt/MWNTs anode catalyst presented the maximum power density of 38.4 mW cm-2, which was higher than that with the Pt/Vulcan XC-72 one (32.4 mW cm-2). Moreover, the performance decay rate of the MEA with Pt/MWNTs was lower than that with Pt/Vulcan XC-72 (after a 56 h test). It indicated that the Pt/MWNTs can be expected to be a more suitable catalyst for the DDFC.

1. Introduction Dimethyl ether (DME) is one of the most promising fuels for the direct-type fuel cell.1-3 Up to now, most of the investigations were focused on the electro-oxidation of DME4,5 and the anode catalysts for the direct dimethyl ether fuel cell (DDFC). Kerangueven et al.6 investigated the oxidation of DME on Pt/C, PtRu/C, and PtSn/C catalysts. The performance of a fuel cell that used Pt/C with low ruthenium content as the anode catalyst was increased, compared to that obtained at a pure platinum catalyst. Yu et al.7 investigated that a Pd catalyst is not as electrochemically active with respect to DME electrooxidation. A Pt-Ru catalyst was found to be much superior to a Pt catalyst for DDFC at low temperature. Liu et al.8 investigated the electro-oxidation of DME on PtMe/C (Me ) Ru, Sn, Mo, Cr, Ni, Co, and W) and Pt/C electrocatalysts in an aqueous half-cell and found that the addition of a second metal enhanced the tolerance of Pt to the poisonous species during the DME oxidation reaction. Ueda et al.9 studied the performances of DDFCs using PtRu and Pt as the anode catalysts. At the low current density of 100 mA cm-2 or less, the anode potential with the PtRu catalyst was lower than that with the Pt catalyst. The performance of DDFC with the PtRu catalyst * To whom correspondence should be addressed. Telephone: +86-45186413707. Fax: +86-451-86413720. E-mail: [email protected]. (1) Mu¨ller, J. T.; Urban, P. M.; Ho¨lderich, W. F.; Colbow, K. M.; Zhang, J.; Wilkinson, D. P. J. Electrochem. Soc. 2000, 147, 4058. (2) Haraguchi, T.; Watanabe, T.; Yamashita, M.; Tsutsumi, Y.; Yamashita, S. Electr. Eng. Jpn. 2006, 157, 24. (3) Mizutani, I.; Liu, Y.; Mitsushima, S.; Ota, K. I.; Kamiya, N. J. Power Sources 2006, 156, 183. (4) Kerangueven, G.; Coutanceau, C.; Sibert, E.; Hahn, F.; Le´ger, J. M.; Lamy, C. J. Appl. Electrochem. 2006, 36, 441. (5) Tsutsumi, Y.; Nakano, Y.; Kajitani, S.; Yamasita, S. Electrochemistry 2002, 70, 984. (6) Kerangueven, G.; Coutanceau, C.; Sibert, E.; Le´ger, J. M.; Lamy, C. J. Power Sources 2006, 157, 318. (7) Yu, J. H.; Choi, H. G.; Cho, S. M. Electrochem. Commun. 2005, 7, 1385. (8) Liu, Y.; Mitsushima, S.; Ota, K. I.; Kamiya, N. Electrochim. Acta 2006, 51, 6503. (9) Ueda, S.; Eguchi, M.; Uno, K.; Tsutsumi, Y.; Ogawa, N. Solid State Ionics 2006, 177, 2175.

remarkably improved from 80 to 95 °C. All researches about anode catalysts of DDFC were mainly focused on the Pt and Pt alloy. In fact, carbon supports also affect the catalytic activity for DME electro-oxidation. Carbon nanotubes (CNTs) have attracted much interest for application as catalyst supports since their discovery, owing to their good mechanical and electrical properties and unique structure.10,11 Multiwalled carbon nanotubes (MWNTs) are usually selected as catalyst supports, rather than single-walled carbon nanotubes (SWNTs), because MWNTs are lower in production cost and higher in electrical conductivity than SWNTs.12 CNTs supported Pt and Pt alloy nanoparticles have shown enhanced catalytic activity to methanol electro-oxidation13 and oxygen reduction reactions.14 The reasons for the higher performance of Pt/CNTs, as compared with carbon black supported Pt nanoparticles (Pt/C), can be listed as follows:15 (1) CNTs have unique structural and electrical properties; (2) CNTs have few impurities, whereas carbon black (e.g., Vulcan XC-72) contains significant quantities of organosulfur impurities, which can poison Pt metal;16 (3) carbon black has a lot of deep cracks in which some Pt nanoparticles are trapped and wasted because these Pt particles cannot form the effective triple-phase boundary (gas-electrode-electrolyte), and in contrast, CNTs do not have such cracks so that most Pt nanoparticles on CNTs are expected to be effective catalysts.17 (10) Liu, Z. L.; Lin, X. H.; Lee, J. Y.; Zhang, W.; Han, M.; Gan, L. M. Langmuir 2002, 18, 4054. (11) Waje, M. M.; Wang, X.; Li, W.; Yan, Y. Nanotechnology 2005, 16, S395. (12) Wang, C.; Waje, M.; Wang, X.; Tang, J. M.; Haddon, R. C.; Yan, Y. S. Nano Lett. 2004, 4, 345. (13) Kim, C.; Kim, Y. J.; Kim, Y. A.; Yanagisawa, T.; Park, K. C.; Endo, M.; Dresselhaus, M. S. J. Appl. Phys. 2004, 96, 5903. (14) Li, W. Z.; Liang, C. H.; Zhou, W. J.; Qiu, J. S.; Zhou, Z. H.; Sun, G. Q.; Xin, Q. J. Phys. Chem. B 2003, 107, 6292. (15) Zhao, X. S.; Li, W. Z.; Jiang, L. H.; Zhou, W. J.; Xin, Q.; Yi, B. L.; Sun, G. Q. Carbon 2004, 42, 3263. (16) Litster, S.; McLean, G. J. Power Sources 2004, 130, 61. (17) Matsumoto, T.; Komatsu, T.; Nakano, H.; Arai, K.; Nagashima, Y.; Yoo, E.; Yamazaki, T.; Kijima, M.; Shimizu, H.; Takasawa, Y.; Nakamura, J. Catal. Today 2004, 90, 277.

10.1021/ef8005965 CCC: $40.75  2009 American Chemical Society Published on Web 01/05/2009

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In this work, we use MWNTs as support of the Pt catalyst and compare the performance of Pt/MWNTs and Pt/Vulcan XC72 catalysts for DDFCs. The catalysts were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The homemade catalysts were loaded on glassy carbon disk working electrodes through a casting process, and the electrocatalytic activities of the catalysts for the electro-oxidation of DME were studied at room temperature using cyclic voltammetry (CV) by a thin-film electrode method. Finally, the performances of DDFCs with Pt/MWNTs and Pt/Vulcan XC72 catalysts were characterized by the polarization curve and power density curves. 2. Experimental Section 2.1. Preparation of Catalyst. The MWNTs (purity g95%, special surface area >200 m2 g-1, Chengdu Organic Chemicals Com. Ltd., China) and the Vulcan XC-72 carbon black (Cabot, 250 m2 g-1) were used as received. MWNTs were functionalized by heating at reflux with concentrated nitric acid (HNO3). The details of functionalization of MWNTs were described as follows: 18 A sample of 500 mg of MWNTs was first agitated in 30 mL of 65% HNO3 at 90 °C for 4 h to form a dark-brown suspension. The reaction mixture was then diluted with ultrapure water to 200 mL and stirred for 6 h at room temperature. After oxidation, the support was washed with ultrapure water until neutrality of the rinsing water and finally dried overnight at 110 °C in vacuum. The treated carbon nanotubes were then ready for use in the deposition of Pt nanoparticles. Pt/MWNTs and Pt/Vulcan XC-72 catalysts were prepared by impregnation-reduction method. Hexachloroplatinic acid (H2PtCl6) was used as the catalyst precursor, and formaldehyde was used as the reducing agent. The detailed preparation process can be described as follows:19 amounts of 30 mL water and 30 mL of isopropyl alcohol were added to 50 mg of MWNT (or Vulcan XC72), and then the suspension was ultrasonically stirred for 1 h. Thereafter, H2PtCl6 was added to the suspension and ultrasonically stirred for 2 h. The pH value of the suspension was adjusted to 12 with NaOH aqueous solution, and then an excessive amount formaldehyde was added into the suspension drop by drop. After the mixture was sonicated for 20 min, it was heated to 80 °C and then agitated for 3 h at the same temperature. The resulting catalyst was washed with ultrapure water (∼18.2 MΩ cm, Mill-Q Corp.) until Cl- was not detected and then dried overnight at 110 °C in vacuum. The 40 wt % Pt/MWNTs and Pt/Vulcan XC-72 catalysts were obtained. 2.2. Physical Characterization. The morphology and size of Pt nanoparticles dispersed on the surface of carbon nanotubes and carbon blacks were characterized by TEM using a Japan JEOLJEM1200EX TEM with a spatial resolution of 1 nm. The applied voltage was 100 kV and with a magnification of 100 000 for the catalyst. Sample preparation for TEM examination involved the ultrasonically dispersing the sample in acetone and placing a drop of the suspension on a copper grid covered with perforated carbon film, followed by solvent evaporation. XRD analysis was carried out for the catalysts with a D/max-rB (Japan) diffractometer using a Cu KR X-ray source operating at 45 kV and 100 mA. The XRD patterns were obtained at a scanning rate of 4° min-1 with an angular resolution of 0.05° of the 2θ scan in the range of 10° and 90°. 2.3. Electrochemical Measurements. Cyclic voltammetry was used to study the electrochemical properties of the catalysts by a thin-film electrode technique, which was previously developed to characterize high surface area electrocatalysts.20 Briefly, a glassy (18) Wang, J. J.; Yin, G. P.; Shao, Y. Y.; Wang, Z. B.; Gao, Y. Z. J. Electrochem. Soc. 2007, 154, B687. (19) Wang, Z. B.; Yin, G. P.; Shi, P. F. Carbon 2006, 44, 133. (20) Schmidt, T. J.; Gasteiger, H. A.; Stab, G. D.; Urban, P. M.; Kolb, D. M.; Behm, R. J. J. Electrochem. Soc. 1998, 145, 2354.

Cai et al. carbon disk was used as the substrate, on which 5 µL of a paste of the catalyst was applied. The catalysts were attached to the glassy carbon disk according to the following procedures: an amount of the catalyst powder was dispersed ultrasonically in the water solution to obtain a homogeneous black suspension solution with 2 mg mL-1 Pt/MWNTs and Pt/Vulcan XC-72. Then 5 µL of the suspension was pipetted onto the surface of a glassy carbon disk electrode (3 mm diameter, CH Instrument, Inc.). After the paste dried in a vacuum furnace, a drop of 5 wt % Nafion in alcohol solution was spread on the catalyst and allowed to dry. A recast ionomer thin-film covering with the catalyst was thus obtained. Each electrode contained about 0.14 mg cm-2 of the catalyst. Electrochemical studies of the Pt/MWNTs and Pt/Vulcan XC72 catalysts were conducted in a standard three-electrode cell at CHI 604B electrochemical working station (CH Instruments Inc., Austin, TX). The glassy carbon disk covered with the catalysts was used as working electrode. A piece of platinum foil of 1 cm2 served as the counter electrode, and a reversible hydrogen electrode (RHE) was used as the reference electrode. Cyclic voltammetry was recorded in a 0.5 mol L-1 H2SO4 solution. Before measurement, high-purity argon gas was purged for 30 min to eliminate oxygen. Cyclic voltammetry was recorded in a 0.5 mol L-1 H2SO4 solution from 0.05 to 1.20 V at a scan rate of 50 mV s-1. DME oxidations on the Pt/MWNTs and Pt/Vulcan XC-72 were carried out in 1.5 mol L-1 DME plus 0.5 mol L-1 H2SO4 solution by a CV technique at 50 mV s-1 between 0.05 and 1.2 V. All potential values in these studies are versus RHE. All the electrochemical measurements were carried out at 25 °C. 2.4. Fabrication of the Membrane Electrode Assembly. The working area of the membrane electrode assembly (MEA) was 5 cm2. The anode and cathode diffusion layers were the same. The diffusion layers were the Teflon-treated carbon paper (Toray TGPH-090) coated with a loading of 1.0 mg cm-2 Vulcan XC-72 carbon blacks and 20 wt % PTFE. The catalyst powder and 5 wt % Nafion ionomer solution (DuPont) were ultrasonically mixed in isopropyl alcohol to form a homogeneous catalyst ink. Then the ink was scraped onto the diffusion layer. In the anode catalyst layer, the Nafion content was 20 wt % and the Pt loading (Pt/MWNTs and Pt/Vulcan XC-72) was 2.0 mg cm-2. The cathode catalyst layer was formed with Pt/Vulcan XC-72 (Pt loading: 2.0 mg cm-2) and Nafion (0.5 mg cm-2). The pretreated Nafion 117 membrane was sandwiched between the anode and the cathode, and then the assembly was hot-pressed under a loading of 80 kg cm-2 for 2 min at 135 °C. 2.5. Cell Test. The electrochemical tests of these MEAs were carried out by fuel cell testing system (Arbin Instrument Corp.) using a single cell (Electrochemistry Corp.). The cell temperature was 80 °C. The saturated DME solution (1.65 mol L-1 at room temperature) was fed to the anode side with a flow rate of 2.5 mL min-1. Pure oxygen was supplied to the cathode side with a flow rate of 150 mL min-1 under ambient pressure. The polarization curves and power density curves of the MEAs were plotted at intervals of operating time. Each point on the polarization curves and power density curves represented a steady-state performance achieved after about 3 min of continuous operation at a given voltage. The performances of one MEA were evaluated on several consecutive days, with the cell operating at a constant current density of 100 mA cm-2 for about 14 h each day.

3. Results and Discussion 3.1. Physicochemical Characterization of the Catalysts. TEM is an effective technique for characterization of the size and distribution of nanoparticles on carbon-supported catalysts. Figure 1 shows typical TEM images of the Pt/MWNTs and Pt/ Vulcan XC-72. It can be seen from the images in Figure 1 that the Pt nanoparticles are highly dispersed on MWNTs and Vulcan XC-72. The Pt/Vulcan XC-72 catalyst has a narrower particle size distribution range and a smaller mean particle size. The size distributions of Pt nanoparticles are shown in the histograms

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Energy & Fuels, Vol. 23, 2009 905

Figure 1. TEM micrograms of Pt/MWNTs (a) and Pt/Vulcan XC-72 (b) catalysts.

Figure 3. XRD patterns of Pt/MWNTs and Pt/Vulcan XC-72 catalysts. Table 1. Comparison of Particle Size, EAS, CSA, and Pt Utilization of Various Catalysts

catalyst Pt/MWNTs Pt/Vulcan XC-72

EAS particle particle from CV size from size from CSA ηPt TEM (nm) XRD (nm) (cm2 mg-1) (cm2 mg-1)a (%)b 4.2 3.6

4.3 3.7

667.6 778.8

549.1 609.3

82.2 78.2

a Determined from the hydrogen adsorption/desorption in the cyclic voltammograms in 0.5 mol L-1 H2SO4 solution. b ηPt ) EAS/CSA.

Figure 2. Size distributions of Pt/MWNTs (a) and Pt/Vulcan XC-72 (b) catalysts.

in Figure 2. The average sizes of the particles are 4.2 and 3.6 nm for Pt/MWNTs and Pt/Vulcan XC-72, respectively. XRD patterns of the catalysts are shown in Figure 3 for each catalyst. The average size is also calculated from the Pt(111) peak of the XRD data using the Debye-Scherrer equation, and the result is shown in Table 1. The average sizes of the particles are 4.3 and 3.7 nm for Pt/MWNTs and Pt/Vulcan XC-72, respectively. The average sizes of Pt particles of both catalysts calculated from the XRD data are consistent with the results obtained by TEM micrographs. 3.2. Characterization of the Catalysts by Cyclic Voltammetry. Figure 4 shows the CV of the Pt/MWNTs and Pt/Vulcan XC-72. Fine structures of hydrogen absorption/ desorption peaks appear clearly in the CV curves.

To obtain the electrochemical activity of the catalysts, the electrochemical active surface (EAS) areas of the catalysts were calculated, which involves calculation of the Coulombic charge for the hydrogen adsorption and desorption (QH) of the catalysts from the CV (Figure 4). The value of QH was calculated as the mean value between the amounts of charge transfer during the electroadsorption and desorption of H2 on Pt sites:21 EAS )

QH [Pt]0.21

where [Pt] represents the platinum loading (mg cm-2) in the electrode, QH represents the charge for hydrogen desorption (mC cm-2), and 0.21 represents the charge required to oxidize a monolayer of H2 on bright Pt.22 (21) Pozio, A.; Francesco, M. D.; Cemmi, A.; Cardellini, F.; Giorgi, L. J. Power Sources 2002, 105, 13.

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Figure 4. Cyclic voltammograms of Pt/MWNTs and Pt/Vulcan XC72 catalysts. Measurements were performed in 0.5 mol L-1 H2SO4 solution saturated by Ar. Scan rate: 50 mV s-1.

Figure 5. Cyclic voltammograms of DME oxidation on Pt/MWNTs and Pt/Vulcan XC-72 catalysts. Measurements were performed in 0.5 mol L-1 H2SO4 plus 1.5 mol L-1 DME aqueous solution saturated by Ar. Scan rate: 50 mV s-1.

Cai et al.

Figure 6. Polarization curves and the power density curves of the MEAs with various anode catalysts at 80 °C.

Figure 7. Highest power densities of MEAs with various anode catalysts with the cell operating at 0, 14, 28, 42, and 56 h.

The calculation results are also shown in Table 1. It can be seen that the Pt/MWNTs catalyst shows lower EAS and CSA, but it had higher the Pt utilization. Pt nanoparticles supported on MWNTs are deposited on the surface of the support. As to carbon black (XC-72R), the widely used support of Pt-based catalysts, many of the Pt particles are trapped in the pores of the carbon support. These Pt particles are not involved in the electrochemical reactions in the triple-phase boundaries. Thus, the Pt utilization is lower in the Pt/XC-72 catalyst. 3.3. DME Electro-Oxidation. The catalytic activity of DME oxidation of Pt/MWNTs catalyst was characterized by cyclic voltammograms in 0.5 mol L-1 H2SO4 plus 1.5 mol L-1 DME aqueous solution. From Figure 5, it can be seen that the current from DME oxidation become apparent as the potential rises

above 0.50 V. In the forward scan, DME oxidation produces an anodic peak at around 0.80 V. For comparison, the Pt/Vulcan XC-72 catalyst was also evaluated at the same conditions. As shown in Figure 5, the peak current of DME oxidation on Pt/ MWNTs is 2.25 mA cm-2, which is much higher than that of Pt/Vulcan XC-72 (1.75 mA cm-2). Thus, it can be seen that the Pt/MWNTs catalyst showed the higher catalytic activity for DME oxidation. The increased activity of Pt/MWNTs may be attributed to the unique structure and good electrical properties of MWNTs, which would help to increase the electrical conductivity of MWNT supports when compared to that of the commercial Vulcan carbon XC-72 support.23 3.4. Cell Performance. Figure 6 shows the comparison of the polarization curves and the power density curves of the MEAs with various anode catalysts. The cell temperature was 80 °C. The cathode stream was humidified during the full cell testing. The MEAs with Pt/MWNTs and Pt/Vulcan XC-72 show the highest power densities of 38.4 and 32.4 mW cm-2, respectively. Apparently, the MEA with Pt/MWNTs anode catalyst showed more excellent performance than that with Pt/ Vulcan XC-72 catalyst. It is indicated that the catalytic activity of Pt/MWNTs is higher that of Pt/Vulcan XC-72 catalyst. Therefore, the Pt/MWNTs catalyst can be expected to be a more effective catalyst for DDFC. The performances of one MEA were evaluated on several consecutive days, with the cell operating at a constant current

(22) Maillard, F.; Martin, M.; Gloaguen, F.; Leger, J. M. Electrochim. Acta 2002, 47, 431.

(23) Pantea, D.; Darmstadt, H.; Kaliaguine, S.; Summerchen, L.; Christian, R. Carbon 2001, 39, 1147.

The chemical surface areas (CSA) of these catalysts were calculated from the equation CSA )

6 Fd

where F is the density of Pt (21.09 g cm-3) and d is the mean diameter of the Pt nanoparticles in the catalysts. From the above two areas (EAS and CSA), it is possible to estimate the Pt utilization (ηPt): ηPt ) EAS/CSA

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Table 2. Initial Peak Power Density, Final Peak Power Density, and the Decreasing Rates of MEAs with Various Anode Catalysts

Pt/MWNTs Pt/Vulcan XC-72

peak power density (0 h)/mW cm-2

peak power density (56 h)/mW cm-2

decreasing rate %

38.4 32.4

33.1 22.0

13.8 32.1

probably due to the stronger resistance to electrochemical oxidation of MWNTs. In addition, the specific interaction between Pt and the support is another reason. In comparison with Pt/Vulcan XC-72, it is believed that the MEA with Pt/ MWNTs anode catalyst is beneficial to the long-term operation of DDFC. 4. Conclusions

density of 100 mA cm-2 for about 14 h each day. Figure 7 shows the highest power densities of MEAs with various anode catalysts with the cell operating at 0, 14, 28, 42, and 56 h. The cell temperature was 80 °C. The performance of the MEA with Pt/MWNTs is much higher than that with Pt/Vulcan XC-72 at each time. The initial peak power density, final peak power density, and the decreasing rates of MEAs with various anode catalysts are shown in Table 2. In Table 2, the performance decay rate of MEA with Pt/MWNTs is 13.8% (after the 56 h test), which is much lower than that with Pt/Vulcan XC-72 (32.1%). It has been shown that the delocalized π electrons of graphene sheets can interact with the stabilized platinum particles. MWNTs are usually considered to be made of rolledup graphene sheets. The Pt particles deposited on the outer shell of MWNTs are probably to show a strong interaction with the support via π-bonding.24,25 The high stability of Pt/MWNTs is

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(24) Coloma, F.; Sepulvedaescribano, A.; Rodriguez-Reinoso, F. J. Catal. 1995, 154, 299.

(25) Coloma, F.; Sepulvedaescribano, A.; Fierro, J. L. G.; RodriguezReinoso, F. Langmuir 1994, 10, 750.

In comparison with Pt/Vulcan XC-72, the Pt/MWNTs catalyst gave a higher catalytic activity for DME electro-oxidation. The DDFCs with Pt/MWNTs and Pt/Vulcan XC-72 showed the highest power densities of 38.4 and 32.4 mW cm-2 at 80 °C, respectively. The MEA with the Pt/MWNTs anode catalyst showed more excellent performance than that with the Pt/Vulcan XC-72 catalyst. The performance decay rate of the MEA with Pt/MWNTs was 13.8%, which was lower than that with Pt/ Vulcan XC-72 (32.1%). In comparison with Pt/Vulcan XC-72, the Pt/MWNTs anode catalyst is a more promising catalyst for DDFCs. Acknowledgment. This work was supported by the Natural Science Foundation of China (Nos. 20476020 and 50872027).