Ultralong-CNTA-Supported Pd-Based Anodes for Ethanol Oxidation

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Ultralong-CNTA-Supported Pd-Based Anodes for Ethanol Oxidation Fengping Hu, Xinwei Cui, and Weixing Chen* Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G6 ReceiVed: October 3, 2010; ReVised Manuscript ReceiVed: October 21, 2010

A three-dimensional ultralong-CNTA-supported Pd-based anode (Pd/CNTA) has been designed for ethanol oxidation. Nano-size Pd particles were uniformly attached to the surface of CNTs within CNTA by using pulse electrodeposition technique. The as-prepared Pd/CNTA anode is featured with low ion diffusion resistance, vertical electron conduction, and large surface area, which offer enhanced active sites for ethanol oxidation reaction and allow fast ion adsorption/desorption from the electrode. The performance of the Pd/CNTA anode is tested and compared with that of Pd/C and Pd/MWCNT (multiwall carbon nanotube) electrodes for ethanol oxidation in alkaline media. The mass activity of the current Pd/CNTA electrode was 17.5 and 10.3 times higher than that of Pd/C and Pd/MWCNT electrodes, respectively. This novel electrode can provide efficient ethanol oxidation and may form a new promising platform for both fundamental investigation of the effects of electrode structure and the development of high-performance alkaline direct ethanol fuel cells. 1. Introduction Alkaline fuel cells (AFCs) are the most mature fuel cell structure among many types of fuel cells due to their flexibility of using a solid electrolyte and therefore the elimination of electrolyte leakage.1 Non-Pt catalysts can be used in alkaline fuel cells, which is another advantage of AFCs.2 There is a wide range of selections of support materials and catalysts for alkaline media, while only limited choices of materials are available for fuel cells with acidic media systems. Other advantages of alkaline fuel cells over proton exchange membrane fuel cells are related to cathode kinetics and ohmic polarization. The less corrosive nature of an alkaline environment ensures a greater longevity of the cells.3 In addition, the kinetics of the oxidation reduction reaction (ORR) is more facile in alkaline medium than in some acid mediums.4,5 The inherently faster kinetics of the oxygen reduction reaction in an alkaline fuel cell allows the use of non-noble and low-cost metal electrocatalysts such as silver and nickel, making the AFCs a potentially low-cost technology compared to other types of fuel cells.6,7 Among noble metals, palladium is a more active catalyst for the alcohol oxidation reaction in alkaline media.8,9 Converse to ethanol oxidation in acid media, the ethanol activity of Pd in alkaline media is remarkably higher than that of Pt.10 Xu etc.11 compared the ethanol oxidation reaction activity in alkaline medium of Pt and Pd supported on carbon Vulcan and several carbon materials by cyclic voltammetry. The onset potential for ethanol oxidation on Pd shifted to a lower potential with respect to that on Pt, and the peak current density on Pd was higher than that on Pt. These results indicate that the activity for ethanol oxidation on Pd is higher than that on Pt, independent of the type of supports used. Pd catalyst also showed higher activity for ethanol oxidation if carbon support has a larger surface area.12,13 Carbon nanotubes (CNTs) were attempted to be used as a support for Pt nanoparticles because of their high surface area, electronic conductivity, mechanical properties, and chemical stability.14-16 However, agglomeration of CNTs significantly * To whom correspondence should be addressed. Phone: 1-780-492-7706. Fax: 1-780-492-2881. E-mail: [email protected].

reduces the surface area for catalyst support and increases contact resistance.17 To avoid the problem of agglomeration, three-dimensional nanostructured electrodes were fabricated for fuel cells18 or super capacitors.19 Other three-dimensional nanostructures like nanowire or nanotube arrays have also attracted increased attention due to their excellent physical and chemical properties.20,21 Highly ordered Pd nanowire arrays were prepared and investigated as catalysts for ethanol oxidation in alkaline media.22 The activity of Pd nanowire arrays for ethanol oxidation was not only higher than that of Pd film, but also higher than that of a commercial Pt-Ru/C. The arrayed nanowire structure possesses a high electrochemically active surface and allows fast diffusion of liquid alcohol to the catalyst layer, resulting in the reduction of the liquid sealing effect. Carbon nanotube arrays (CNTA) have shown many potential applications, such as in super capacitors,21 photonic devices,23 data storage,24 and ultrahydrophobic materials25 because of their unique structural and electronic properties. Herein, we report a novel three-dimensional Pd/CNTA electrode that was built using highly ordered CNTA as Pd catalyst support. The ultralongCNTA has a large surface area for attaching Pd particles, and the aligned channels between CNTs within CNTA allow a fast ionic diffusion. The Pd/CNTA assembly can be attached directly to the current collector to minimize Ohmic resistance. All these characteristics can greatly improve the performance of the ethanol oxidation reaction on the Pd/CNTA electrode. 2. Experimental Section 2.1. The Growth of CNTA. P-type Si wafers (100) with 4-in. diameter and resistivity of 1-35 Ω cm were used as the substrates. A buffer layer of 30 nm thick Al2O3 film and a catalyst layer of 1 nm Fe film were DC magnetron sputtered in turn on Si wafers at a base pressure of ∼1.0 × 10-7 mTorr. The sputtered substrates were cut into samples with a dimension of 5 mm × 5 mm before CNT growth. Catalyst film pretreatment and CNT array growth were performed in a single-zone quartz tube furnace with an inner diameter of 5 in. The chamber was first evacuated to e0.1 Torr. After Ar purging for 1 h, the furnace temperature was ramped up to 775 °C and held for 8

10.1021/jp109480p  2010 American Chemical Society Published on Web 11/09/2010

CNTA-Supported Pd-Based Anodes for Ethanol Oxidation

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SCHEME 1: Schematic Fabrication of the Pd/CNTA Electrode

min annealing time under 600 sccm Ar, 10 sccm Ar with H2O, 500 sccm H2 gas flow, then 200 sccm C2H4 were then flowed into the system for various periods from 30 min to 2 h (resulting in different height CNTAs). 2.2. Microstructure Characterization. The morphology of Pd-based catalysts and the height of CNTA were characterized by a JSM-6301FXV field emission scanning electron microscope (FESEM). The size and distribution of Pd on different carbon supports were characterized by a transmission electron microscope (TEM, JEOL 2010 operated at 200 kV). 2.3. Preparation of Pd-Based Anodes. The method to fabricate Pd/C and Pd/MWCNT electrodes is the same as that described in our previous reports.8,9 Briefly, (NH4)2PdCl6 (Sigma, No. 323535) aqueous solution (4.7 mL, 0.1 mol dm-3) was added into the solution containing 50.0 mg of Vulcan XC72 carbon powder (Cabot Corp., USA) or 50.0 mg of multiwall carbon nanotube (Shenzhen nanoharbor, China). Then the formic acid (5 mL, 1 mol dm-3) was used as reducing agent. The mixture was put into a microwave oven (1000 W, 2.45 GHz, Panasonic, Canada) and was then alternatively heated for 20 s and paused for 60 s six times. The Pd loadings on the Pd/C and Pd/MWCNT electrodes were both controlled at 0.3 mg cm-2. For the Pd/CNTA electrode, the CNTA was cut from substrate and transferred onto the current collector (carbon rod, 0.3 cm2) with the same area of silver tape (TED.PELLA.INC. Japan). The electrode was then used as the working electrode; a saturated calomel electrode (SCE) and Pt foil (3.0 cm2) were used as the reference electrode and the counter electrode, respectively, for Pd electrodeposition in 1 mM (NH4)2PdCl6 and 0.1 M HCl solution with dc or pulsed electro-deposition mode. The Pd loading on the Pd/CNTA electrode was controlled by electrodeposition time. 2.4. Measurement of Electrochemical Activity of PdBased Anodes. All electrochemical measurements were performed in a standard three-electrode cell with Gamry Reference 600 system (Gamry Instruments, USA) in 1 M KOH and 1 M ethanol solution at 30 °C. A three-electrode cell was used, which consisted of a Pd-based anode working electrode, a Pt foil (3.0 cm2) counter electrode, and a Hg/HgO reference electrode. 3. Results and Discussion 3.1. Structure of the Pd/CNTA Electrode. Scheme 1 illustrates the entire processes of Pd/CNTA electrode preparation. In the first step of preparation, CNTA was grown on Si substrate using the water-assisted CVD method.26 The CNTA

TABLE 1: Optimization of Ethanol Oxidation Performance on Pd/CNTA Electrodes at Different Conditions onset potential (V)

Pd-specific activity (mA cm-2)

Pd mass activity (Ag1-)

0.5

-0.489

33.4

417.5

1 2 7.2 12.0 19.2 24.0 31.4 1 1.5 2 2.5 3 600/0

-0.537 -0.505 -0.363 -0.443 -0.517 -0.528 -0.537 -0.528 -0.534 -0.557 -0.549 -0.542 -0.557

91.2 112.6 23.9 41.8 65.3 85.1 91.2 85.1 106.3 176.7 197.5 208.6 176.7

435.7 268.1 497.9 522.5 510.2 531.9 435.7 531.9 664.4 1104.4 1234.4 1303.8 1104.3

120/30 60/60 60/30 60/20

-0.443 -0.541 -0.584 -0.537

211.4 218.7 237.5 199.7

1321.3 1366.9 1484.4 1248.1

optimized conditions electrodeposit current (mA cm-2)a Pd loading (µg)b

CNTA height (mm)c

electrodeposit mode: ton/toff (s/s)d

a 1 mm CNTA; 0.15 cm-2; constant current density electrodeposit 12 min (24 µg Pd). b 1 mm CNTA; 0.15 cm-2; 1 mA cm-2 constant current density electrodeposit. c 0.15 cm-2; 1 mA cm-2 constant current density electrodeposit 10 min (24 µg Pd). d 2 mm CNTA; 0.15 cm-2; 1 mA cm-2 constant current density electrodeposit 10 min (not including time of toff) (24 µg Pd).

was then cut from Si substrate and transferred to a current collector using a silver tape as binder to ensure a good conductivity of the CNTA electrode for the subsequent Pd electrodeposition. The three-dimensional Pd/CNTA electrode was finally formed after Pd electrodeposition. There are three advantages for this type of electrode: (1) electrons can flow directly between CNTA and the current collector because of a direct binding between CNTAs and the sliver tape at the bottom, which yields low solution resistance; (2) CNTA has very large surface area for ion adsorption or desorption, offering much more active sites for ethanol oxidation compared with the twodimensional electrode; and (3) the aligned vertical ion diffusion route within CNTA induces smaller ion diffusion resistance. 3.2. Optimization of Anode Performance. The performance of Pd/CNTA electrodes with different fabrication conditions was assessed, from which the electrodes were optimized based on

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Figure 1. SEM images of Pd/CNTA electrodes with different height CNTA and different electrodeposit mode: (a) 1 mm CNTA, ton/toff: 600/0 (inset is partial of panel a), (b) 2 mm CNTA, ton/toff: 600/0, (c) 2 mm CNTA, ton/toff: 60/30, (d) enlarged portion of panel c, at the top of the electrode (inset: low magnification of part c), (e) the middle of panel c, and (f) the bottom of panel c.

their onset potential, specific activity, and mass activity of ethanol oxidation. The results obtained are summarized in Table 1. First, the effect of electrodeposition current density was investigated at a constant electrodeposition (dc mode) time of 12 min using a 1 mm long CNTA as support. The current density at 1 mA cm-2 dc mode shows the most negative onset potential and the largest mass activity, and was, therefore, selected to be the base condition for the optimization of other variables including Pd loading, CNTA height, and pulse plating current on- and off-time as listed in Table 1. After all the designed variables listed in Table 1 were assessed, it was found that the 2 mm CNTA-Pd electrode pulse plated at 1 mA cm-2 with a pulse cycle consisting of 60s on/30s off for 10 cycles yielded the best values, and was used as the optimized condition for the preparation of the Pd/CNTA electrode. The electrode has 160 µg cm-2 Pd loading, which was estimated by Faraday’s law of electrolysis:

m)

QM Fz

(1)

where m is the mass of the substance liberated at an electrode, Q is the total electric charge passed through the substance, F ) 96 485 C mol-1 is the Faraday constant, M is the molar mass of the substance, and z is the valence number of ions of the substance (electrons transferred per ion). 3.3. Characterizations of Pd/C, Pd/MWCNT, and Pd/ CNTA Electrocatalysts. Figure 1 shows typical scan electron microscopy (SEM) images of the Pd/CNTA electrodes prepared. Figure 1a shows a 1 mm long Pd/CNTA electrode. The top of the electrode was fully covered with Pd that has formed a shell around CNTs (see Figure 1a and the inset of part a). This shell coverage, however, has not formed on top of the 2 mm CNTA (Figure 1b). Instead, large Pd particles were found to be aggregated on the surface of CNTs at the top of the CNTA. Interestingly, aggregation of large Pd particles was avoided when the same height of CNTA was pulse plated at a ton/toff ratio of 60 s/30 s (Figure 1c). Figure 1d and the inset of part d further show the morphology of the above electrode. Panels e and f of Figure 1 show the morphology from the middle and the bottom part of the same CNTA electrode described in Figure 1c. This

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Figure 2. TEM images of Pd/CNTA electrodes (constant electrodeposit) (a-c): (b) dark field of panel a and Pd/CNTA electrodes (pulse electrodeposit) (d-f); (f) diffraction of panel e.

proves that pulse plating can yield smaller Pd particles and a more uniform attachment of Pd particles to the surface of CNTs within the full length of CNTA, as compared with the direct current deposition. TEM images of Pd/CNTA electrodes (dc mode and pulse mode) were compared in Figure 2. Large and non-uniform Pd particles were formed due to the poor hydrophilicity of CNTA and the dc electrodeposit mode. Figure 2b shows the same image in Figure 2a. But in the dark field mode, the bright points are Pd particles. Compared with the dc electrodeposit mode, we used the pulsed electrodeposit mode to obtain the hollow flowerlike and uniform Pd particles on the Pd/CNTA electrode with a particle average size of about 80 nm (Figure 2d,e). The hollow flower-like structure offers a more active site for the electrocatalytic reaction on Pd particles. However, the large particle is not helpful to improve the activity of ethanol oxidation on Pd particles. Figure 2f is the diffraction image of Figure 2e. The continuous innermost ring is from CNT, and the discontinuous outer rings are from metal Pd. 3.4. Electrochemical Characterization of Pd-Based Electrodes. Cyclic voltammograms of ethanol oxidation on Pd-based electrodes are presented in Figure 3. The onset potential and peak current density of the ethanol oxidation on Pd/C, Pd/ MWCNT, and Pd/CNTA electrodes are -0.491 V and 25.4 mA cm-2, -0.505 V and 43.2 mA cm-2, and -0.584 V and 237.5 mA cm-2, respectively. Moreover, the Pd loading in the tests shown in Figure 3 was 300 µg cm-2 for both the Pd/C and Pd/MWCNT electrodes, but 160 µg cm-2 for the Pd/CNTA electrode. The Pd mass activity of the Pd/CNTA electrocatalyst is, therefore, calculated to be 17.5 and 10.3 times higher than that of Pd/C and Pd/MWCNT electrocatalysts, respectively. This enhancement is believed to result from the three-dimensional structure of the Pd/CNTA electrode, as discussed in section 2.1. Electrons can flow directly between CNTA and the current collector through a highly conductive silver tape. Good conductivity of the electrode decreases the solution resistance on the surface of the electrode, which facilitates ethanol oxidation on the Pd/CNTA electrode. And the three-dimensional structure of CNTA has a very large surface area and more active sites

Figure 3. Cyclic voltammograms of ethanol oxidation on different electrodes in 1 M KOH/1 M ethanol concentration and black solution (inset), 303 K, scan rate 50 mV s-1. Pd loading: Pd/C and Pd/MWCNT 300 µg cm-2, Pd/CNTA 160 µg cm-2.

for ethanol oxidation, and the aligned interspaces for fast ion adsorption or desorption. The vertical ion diffusion route also induces smaller ion diffusion resistance and a lower effect of mass transfer on the electrode. The insert in Figure 3 shows the voltammetry curves of different Pd catalysts in black solution. As is known, the electrochemically active surface area (ESA) of the Pt catalyst can be calculated by integrating the voltammetry curves corresponding to hydrogen desorption from the electrode surface, while the ESA of the Pd catalyst cannot be calculated by the above method because the Pd catalyst is a good hydrogen storage material. The integrated charge of different Pd catalysts was compared without calculating exactly the electrochemically active surface area. The integrated charge of Pd/C, Pd/MWCNT, and Pd/CNTA catalysts for ethanol oxidation was determined to 0.10, 0.27, and 1.45 mC cm-2, respectively, which proved that the Pd/CNTA catalyst has much higher electrocatalytic activity than the other two electrodes. Figure 4a shows the relationship between the normalized charge of the ethanol oxidation reaction and the scan rate on

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Figure 5. Chronopotentiometric curves of ethanol oxidation on Pd/C and Pd/CNTA electrodes. Electrolyte: 1.0 mol dm-3 KOH/1 M ethanol, 303 K, scan rate 50 mV s-1, 3 mA cm-2.

TABLE 2: Overview of the Mass Activity of Some Electrocatalysts for Alcohol Oxidation

Figure 4. (a) Plot of normalized charge of anodic peak and scan rate on Pd/C and Pd/CNTA electrodes. (b) Plot of the peak current density against the square root of the rate on Pd/C and Pd/CNTA electrodes. Electrolyte: 1.0 M KOH/1 M ethanol, 303 K, scan rate 50 mV s-1.

Pd/C and Pd/CNTA electrodes. Qmeas is the integrated charge of the anodic peak for ethanol oxidation at different scan rates, and Qmax is the one at a scan rate of 2 mV s-1. We can see from the curves that the charge of both the Pd/C and Pd/CNTA electrodes sharply decreases with an increase of scan rate. This suggests that the mass transport is the main obstacle to the electrode reaction on both the compact electrode (Pd/C) and the large volume (Pd/CNTA) electrode. Electrode surface efficiency of these types of electrodes was very low. Further investigation was made to understand the mass transport behavior of Pd/C and Pd/CNTA electrodes. Figure 4b shows the relationship between the anodic current density and the square root of the scan rate of Pd/C and Pd/CNTA electrodes. The Pd/C electrode shows a linear relationship starting at 2 mV s-1, while the linear relationship for Pd/CNTA starts to appear at 20 mV s-1. This result further proves that the oxidation of ethanol on the Pd/C electrode is mostly limited by the concentration polarization at all scan rates and the oxidation of ethanol on Pd/CNTA is mainly controlled by activation polarization at lower scan rates. In other words, the Pd/C electrode is controlled by mass transport within all range of scan rates, while the Pd/CNTA electrode is not controlled by mass transport until the scan rate is higher than 20 mV s-1. The steady-state measurement was carried out to evaluate the stability of these three Pd electrocatalysts for ethanol oxidation at 3 mA cm-2, and the results obtained are shown in Figure 5. Oxidation of ethanol on the Pd/CNTA electrode was relatively stable at low potentials. The potential of the Pd/C electrode was the highest. The two non-CNTA electrodes shifted to a higher value at the end due to the occurrence of the water electrolysis reaction on the electrode. The loss of the catalytic activity is principally caused by species adsorbed on the surface of the electrocatalysts. These species block the access of ethanol molecules to the surface of the electrocatalysts. This phenom-

catalyst

mass activity (Ag1-Pd)

Pd/C (carbon sphere, Pd: 300 µgcm-2) Pd/CNTA (this paper) commercial Pt/C (Johnson Matthey) commercial Pt/C (ETC) Pd/MWCNT (without treatment) Pd/MWCNT (HF treatment) Pt/MWCNT (without treatment) Pd + 1% MWCNT (MWCNT doping) Pd + MWCNT (thin film) Pd/CMS (carbon micro sphere) Pd/HCSs (hollow carbon sphere) Pd/coin like C

84.7 (normalized 1) 1484.4 (17.5) 74.5 (0.88) 46.6 (0.55)27 144.0 (1.7) 389.6 (4.6)28 127.1 (1.5) 237.2 (2.8)29 2100.1 (24.8)30 211.8 (2.5)31 304.9 (3.6)9 288.0 (3.4)32

enon results in concentration polarization as a result of reduced surface area of the Pd/C electrocatalyst. 3.5. Mass Activities of Several Electrodes. Pt and Pd have very similar properties (same group in the periodic table, same face-centered cubic (fcc) crystal structure, and similar atomic size). However, the cost of palladium is much lower than that of platinum, making Pd an attractive substitute for Pt as the catalyst in fuel cells. Table 2 overviews the performance of some Pd or Pd-containing catalysts and commercial Pt/C catalyst, tested as the anode for alcohol oxidation in alkaline medium. It is clear that the Pd/CNTA catalyst shows substantial improvements in terms of high mass activity compared with other catalysts. A 17.5 time improvement in mass activity over that of the Pd/C catalyst would make the Pd/CNTA catalyst very attractive for the application of alkaline fuel cells. 4. Conclusion A simple and cost-effective technique has been developed to fabricate three-dimensional Pd/CNTA electrodes. It has been demonstrated that CNTA is an attractive matrix that can be used as a noble metal catalyst support, and the novel Pd/CNTA electrodes exhibit huge catalystic activity and ultralong stability for ethanol oxidation in alkaline media. The much more active sites on CNTA due to its large surface area, the direct electron conduction between CNTA and the current collector, and the vertical ion diffusion of CNTA are crucial to the improved performance of the Pd/CNTA-based anodes. All these interesting experimental results suggest that this novel electrode can provide a promising platform for both fundamental investigations of the

CNTA-Supported Pd-Based Anodes for Ethanol Oxidation effect of electrode structure and the fabrication of the highperformance alkaline direct ethanol fuel cell. Acknowledgment. The authors would like to thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support. References and Notes (1) Bidault, F.; Brett, D. J. L.; Middletonc, P. H.; Brandon, N. P. J. Power Sources 2009, 187, 39. (2) Liu, Z. L.; Zhao, B.; Guo, C. L.; Sun, Y. J.; Xu, F. G.; Yang, H. B.; Li, Z. J. Phys. Chem. C 2009, 113, 16766. (3) Yu, E. H.; Scott, K. J. Power Sources 2004, 137, 248. (4) Blizanac, B. B.; Ross, P. N.; Markovic, N. M. Electrochim. Acta 2007, 52, 2264. (5) Olson, T. S.; Pylypenko, S.; Atanassov, P. J. Phys. Chem. C 2010, 114, 5049. (6) Okajima, K.; Nabekura, K.; Kondoh, T.; Sudoh, M. J. Electrochem. Soc. 2005, 152, D117. (7) Bianchini, C.; Shen, P. K. Chem. ReV. 2009, 109, 4183. (8) Hu, F. P.; Cui, G. F.; Wei, Z. D.; Shen, P. K. Electrochem. Commun. 2008, 10, 1303. (9) Hu, F. P.; Wang, Z. Y.; Li, Y. L.; Li, C. M.; Zhang, X.; Shen, P. K. J. Power Sources 2008, 177, 61. (10) Kadirgan, F.; Beden, B.; Leger, J. M.; Lamy, C. J. Electroanal. Chem. 1981, 125, 89. (11) Xu, C. W.; Chen, L. Q.; Shen, P. K.; Liu, Y. L. Electrochem. Commun. 2007, 9, 997. (12) Wang, Z. Y.; Hu, F. P.; Shen, P. K. Electrochem. Commun. 2006, 8, 1764. (13) Yan, Z. X.; He, G. Q.; Zhang, G. H.; Meng, H.; Shen, P. K. Int. J. Hydrogen Energy 2010, 35, 3263. (14) Li, Y. L.; Hu, F. P.; Wang, X.; Shen, P. K. Electrochem. Commun. 2008, 10, 1101.

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