Superior Formic Acid Oxidation Using Carbon Nanotube-Supported

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Superior Formic Acid Oxidation Using Carbon Nanotube-Supported Palladium Catalysts Robert D. Morgan,† Amin Salehi-khojin,†,‡ and Richard I. Masel*,‡ †

Department of Chemical Engineering, University of Illinois at Urbana
Champaign, 600 S. Mathews Avenue, Urbana, Illinois 61801, United States ‡ Dioxide Materials, 60 Hazelwood Drive, Champaign, Illinois 61820, United States ABSTRACT: In the work presented here, we report a palladium-decorated carbon nanotube catalyst that shows enhanced activity for formic acid electrooxidation. At 0.3 V vs RHE, the palladium-decorated carbon nanotube catalyst showed a current of 0.18 mA cm
2, whereas a standard commercial palladium black catalyst only showed a current of 0.082 mA cm
2. Also, during a 12 h chronoamperogram at 0.3 V vs RHE, the palladium-decorated carbon nanotube catalyst showed a factor of 3.5 improvement at the end of 12 h. The current was also much more stable for the palladium-decorated carbon nanotubes. During the last 8.5 h, the palladiumdecorated carbon nanotubes show a current loss of 36%, whereas the standard palladium black catalyst showed a current loss of nearly 90%. Lastly, we report that the palladium-decorated carbon nanotubes showed better current stability under potential cycling. After 300 cycles in 12 M HCOOH from 0.02 to 1.45 V vs RHE, the palladium-decorated carbon nanotubes showed only a 33% current loss when measured at 0.3 V vs RHE. However, the standard palladium showed a decay of 60% when also measured at 0.3 V vs RHE.

’ INTRODUCTION The direct formic acid fuel cell, DFAFC, has been proposed as a possible replacement for batteries and direct methanol fuel cells, DMFC.1
39 The DFAFC has several advantages over the DMFC. The DFAFC has a higher theoretical open-circuit potential than the DMFC (1.45 V vs 1.23 V). In addition, formic has exhibits a lower crossover rate then methanol. This allows one to use higher concentrations of fuel in the DFAFC, which makes up for the lower intrinsic energy content of formic acid. Initial catalysis work on the DFAFC was dominated by studies of platinum black as the anode catalyst, but it was determined that one oxidation pathway of formic acid on platinum includes a strongly bound CO intermediate.40
44 HCOOH þ Pt0 f Pt
CO þ H2 O

ð1Þ

H2 O þ Pt0 f Pt
OH þ Hþ þ e


ð2Þ

Pt
CO þ Pt
OH f CO2 þ 2Pt0 þ Hþ þ e


ð3Þ

The strongly bound CO intermediate acts as a poison for the reaction.45,46 In contrast, very little CO buildup is observed with palladium black in an electrochemical cell. Instead, oxidation of formic acid occurs mainly via a more direct pathway that involves a reactive intermediate, X, minimizing the buildup of CO on the catalyst surface.23,42,43,47,48 HCOOH f X f CO2 þ 2Hþ þ 2e


ð4Þ

Although the palladium black catalyst goes through a reactive intermediate bypassing the strongly bound CO, CO does still r 2011 American Chemical Society

accumulate on the surface after several hours of oxidation.49 This is because there seems to be a side reaction on Nafion or other acids catalyzing the dehydration of formic acid directly to CO.50,51 This can also lead to CO accumulation on the palladium surface. HCOOH f CO þ H2 O

ð5Þ

At this point, however, palladium black is the best commercially available, single metal catalyst for formic acid electrooxidation reported in the literature.3
5,52 Carbon-supported catalysts have been reported by several previous investigators but generally show lower activity.14,21,24,53 The objective of this paper is to determine whether carbon nanotube-supported catalysts would show any interesting properties for formic acid electrooxidation. There are several previous reports of the use of nanotube-supported palladium catalysts for formic acid oxidation.54
63 These catalysts showed higher activity than carbon-supported palladium but lower activity than commercial palladium black. The motivation of this study is to determine whether a change in the procedure used to create the palladium on nanotube catalysts would produce higher activity. In particular, in previous work on nanotube sensors,64 we discovered that, when we deposited nanotubes by placing a drop or fluid containing suspended nanotubes on an electrode, the nanotubes did not make good contact with each other. Therefore, the performance was poor. In contrast, when we deposited the nanotubes by vacuum filtration, Received: May 31, 2011 Revised: August 22, 2011 Published: August 29, 2011 19413

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The Journal of Physical Chemistry C much more robust connections between adjacent nanotubes were obtained. It was desired to determine whether a catalyst layer fabrication similar to the previously reported sensor fabrication would result in carbon nanotube catalysts with a performance that was superior to the best published commercial catalysts. In addition, these catalysts did not include Nafion, which would lead to the undesired accumulation of CO. Although certain admetals may increase the formic acid oxidation current, palladium was chosen here to investigate the effects of the addition of the carbon nanotubes without confounding effects of the admetal.

’ EXPERIMENTAL SECTION Catalyst Preparation. Single-walled carbon nanotubes (Unidym, High Purity HIPCO) were first acid-treated by refluxing in a HNO3/H2SO4 (1:4 v/v) solution at 60 °C for 12 h. The acidtreated nanotubes were then rinsed with 18MΩ Milli-Q water and filtered. The nanotubes where then subsequently dried at 100 °C under vacuum for 48 h. A CNT suspension (400 mg/L) was then made from 10 mg of the acid-treated carbon nanotube powder and 1% (w/v) sodium dodecyl sulfate (SDS) in 18MΩ Milli-Q water. Multiple sets of 10 min low-powered ultrasonication (at 40% power and 90% frequency), 1 h stirring, and 3 h centrifugation (4100 rpm) were performed to homogenize and uniformly disperse the suspension. A 10 mL portion of 5 wt % PdCl2 in 10 wt % HCl (Sigma Aldrich) was then added to 20 mL of the CNT suspension under vigorous stirring at room temperature. After 3 h of vigorous stirring, a freshly prepared reducing agent of 50 mg of NaBH4 in 25 mL of 18MΩ Milli-Q H2O was added dropwise to the solution, followed by vigorous stirring at room temperature for another 2 h. One-quarter of the solution was then applied directly onto a gas diffusion electrode using a vacuum filtration system from Phenomonex, as described previously.64 After the nanotube
catalyst network was formed on the GDE, the wet GDE membrane was dried for at least 30 min under 15 in Hg vacuum gauge pressure. The commercial palladium catalyst was prepared via a similar procedure. High-surface-area palladium black nanoparticles (99.8%, Aldrich) were sonicated with 18MΩ Milli-Q in a ratio to obtain the same loading as the CNT
palladium catalyst GDEs. The catalyst ink was then applied to a carbon paper GDE (Toray, 0.00800 ). Catalyst Characterization. Scanning electron microscopy was used to examine the catalysts. A Hitachi S4800 scanning electron microscope was used with accelerating voltages of 5 and 1 kV. A setting of 5 kV was used to image the palladium particles with more contrast, whereas 1 kV was used to image the nanotubes. In addition, X-ray photoelectron spectroscopy (XPS) was performed using a PHI 5400 XPS system with a Mg Kα X-ray source. Samples of the catalyst were electrochemically cleaned in degassed 0.1 M sulfuric acid by performing several cyclic voltammograms between 0.02 and 1.2 V vs RHE until the voltammograms stabilized. Cyclic voltammograms were stopped in the double-layer region to ensure that no oxide was on the catalyst surface. Samples were protected in an argon environment before transferring them into the XPS chamber. Minimal exposure to ambient conditions was attempted when transferring the same to the XPS chamber. XRD analysis was also carried out on the sample to look at the crystalline phases and particle size. The analysis was done with an

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X0 Pert MRD system using a Cu radiation source at a wavelength of 0.15418 nm. Electrochemical Characterization. Electrochemical characterization was performed in a standard three-electrode electrochemical cell with a Solartron potentiostat (SI 1287). The palladium-decorated carbon nanotubes and palladium blackcovered carbon papers served as the working electrode. Platinum gauze (Alfa Aesar, 52 mesh woven from 0.1 mm diameter wire, 99.9%) was used as the counter electrode, and a Ag/AgCl/sat’d KCl reference electrode (BAS) was separated from the working electrode via a Luggin capillary. All solutions used were degassed by bubbling high-purity argon (SJ Smith) through them for at least 30 min. Cyclic voltammetry in 0.1 M H2SO4 (GFS, double distilled) was performed in order to clean the surface and determine electrochemical surface areas by analyzing the area under the hydrogen desorption peaks before each experiment. The potential was scanned from 0.02 to 1.2 V vs RHE at a scan rate of 10 mV/s. Several cycles were completed until the scans did not change with time. The surface area was then calculated from the hydrogen desorption peaks using 210 μC cm
2.46,65
69 It is widely accepted that platinum and palladium can be characterized in this manner.49,70 Linear sweep voltammetry was then performed in 0.1 M HCOOH (50% HPLC grade, Fluka) with 0.1 M H2SO4. The potential was swept from open circuit to 0.5 V vs RHE at a rate of 5 mV/s. This was used to determine the catalysts’ activity toward formic acid oxidation compared to the traditional palladium black catalyst. Chronoamperometry was used to determine long-term effects during constant potential operation. The working electrode was placed in an electrochemical cell containing 12 M HCOOH with 0.1 M H2SO4. The potential was then held at 0.3 V vs RHE for 12 h. Lastly, loss of performance studies were then performed using an electrochemical cell and 12 M HCOOH with 0.1 M H2SO4. The potential was scanned between 0.02 and 1.45 V vs RHE at a scan rate of 50 mV/s. The sulfuric acid was added to maintain a constant pH to stabilize the reference electrode potential. The change in the current at 0.3 V vs RHE was used to determine the loss of activity.

’ RESULTS AND DISCUSSION Physical Characterization. The catalyst morphology on the carbon nanotubes can be seen in Figure 1A,B. Figure 1A shows that the palladium catalyst particles are wrapped around the carbon nanotubes in most cases. However, because a concentrated solution of nanotubes was used, it can also be seen from Figure 1B that there are nanotubes that bridge the palladium clusters. These bridging carbon nanotubes serve to increase the electrical connection between the palladium catalyst particles. An average particle size can also be extracted from the SEM images. The average particle size of the palladium particles is 13.2 nm. Figure 1C shows the XRD pattern for the palladium-decorated nanotubes. Diffraction peaks are present at 2θ values of 39.9, 46.3, and 68.1° for the (111), (200), and (220) planes, respectively. These peak positions are consistent with a face-centered cubic arrangement. A peak at 26.5° was also observed, which is indicative of the excess carbon nanotubes present. A carbon (004) peak is also observed at 54.6°. A particle size of roughly 13.3 nm was calculated from the XRD image, which is in agreement with the particle sizes calculated from the SEM images. 19414

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Figure 1. (a) Scanning electronic microscope image of the palladium nanoparticles wrapped around the carbon nanotube support. (b) Because an excess of nanotubes was used, this image shows some of the nanotubes connecting the palladium particles to increase the electrical connection between palladium clusters. (c) XRD pattern of the Pd-decorated carbon nanotubes. (d) There is a binding energy shift of the 3d5/2 electrons for Pd
CNT. This is likely due to the electronic properties of the nanotubes as well as their oxophilicity.

The core-level binding energy of the CNT-supported palladium as well as the binding energy of the palladium nanoparticles were analyzed using XPS. Figure 1D shows that there is a clear shift in the binding energy of the CNT-supported palladium catalysts compared to the 7.5 nm palladium black catalysts. The palladium black 3d5/2 core electron binding energy is 335.6 eV. This is typical for these types of nanoparticles and has been verified previously.49,52,70 However, there is a clear shift of the Pd 3d5/2 binding energy for the CNT-supported catalysts to 335.8 eV. The increase in binding energy could affect the interaction strength between the formic acid and the palladium surface. Decreased interaction strength can cause an increase in the catalytic rate. Similar binding energies have been reported by another group.71 The binding energy shift is likely due to the electronic properties of the carbon nanotubes as well as their oxophilicity.72 Electrochemical Activities of the Pd and Pd-Decorated CNT. Linear sweep voltametry was used to characterize the activity toward the electrooxidation of formic acid. The potential was scanned from the open-circuit value to 0.5 V vs RHE at a rate of 5 mV/s in 0.1 M HCOOH with 0.1 M H2SO4 added to maintain a constant pH in order to stabilize the reference electrode. The currents were normalized by the electrochemical surface area. Comparison of the two linear sweep voltammograms in Figure 2 shows that the palladium-decorated carbon nanotubes have a higher catalytic activity toward formic acid oxidation. At 0.3 V vs RHE, the standard palladium catalyst shows a current density of 0.082 mA cm
2. At the same potential, however, the palladium-decorated carbon nanotubes show a current density of 0.18 mA cm
2. This is a 120% improvement toward formic acid electrooxidation. There is also a slight negative shift in the onset potential for formic acid oxidation, which is further evidence that the palladium-decorated carbon nanotubes have a higher catalytic activity for formic acid oxidation.

Figure 2. Linear sweep voltammetry curves using a three-electrode electrochemical cell in 0.1 M HCOOH with 0.1 M H2SO4 supporting electrolyte to stabilize the reference electrode. At 0.3 V vs RHE, the palladium-decorated CNT shows a 120% improvement. The scan rate was 5 mV/s.

Chronoamperometry. To be a viable catalyst for possible use in the direct formic acid fuel cell, the catalyst must exhibit stable, long-term activity toward formic acid oxidation. To compare their activities and current stability, an electrochemical cell was used with 12 M HCOOH and 0.1 M H2SO4 to maintain a constant pH for the reference electrode. The potential was held at 0.3 V vs RHE for 12 h. The results are shown in Figure 3. Initially, the oxidation current for the palladium-decorated carbon nanotubes is 0.16 mA cm
2, whereas that for the standard palladium catalyst is 0.05 mA cm
2. This is approximately a factor of 3 improvement over the standard palladium catalysts. Although the palladium-decorated carbon nanotubes do not outperform the standard palladium catalyst over the entire 12 h, they 19415

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Figure 3. Twelve hour chronoamperogram for Pd-decorated carbon nanotubes and palladium black in a three-electrode electrochemical cell; 12 M HCOOH and a 0.1 M H2SO4 supporting electrolyte were used. Although Pd initially shows better performance, it is undergoing constant current decay. The palladium-decorated carbon nanotubes show better long-term current stability. (inset) Zoomed-in portion of the last ∼4 h of the chronoamperogram. The palladium-decorated carbon nanotubes show roughly a factor of 2
3.5 improvement over the last 4 h.

do show roughly a factor of 3.5 improvement at the end of the 12 h chronoamperogram (0.0048 vs 0.0013 mA cm
2). The palladium-decorated carbon nanotubes’ oxidation current also stabilize much faster. After approximately 3.5 h, the current has stabilized to a value of 0.0074 mA cm
2. During the next 9 h, the current for the palladium-decorated carbon nanotubes only decays by 36%. However, the standard palladium catalyst is undergoing constant current decay. Using the same 3.5 h time mark, the current for the standard palladium catalyst is 0.013 mA cm
2. However, at the end of the 12 h, the current is 0.0013 mA cm
2. This is a current loss of 90%. It is likely that the palladium-decorated carbon nanotubes are not poisoning as rapidly as the standard palladium black catalyst. Catalyst Stability. During the operation of the direct formic acid fuel cell, CO is produced on the anode, where it remains strongly bound to the surface. The CO present on the surface then lowers the performance of the fuel cell. It is also known that CO oxidizes from palladium at a potential of 0.8
0.9 V vs RHE.51 Therefore, to regain performance, the anode potential must temporarily be raised to such a potential to oxidize the strongly bound CO from the surface. However, upon inspection of the Pourbaix diagram, palladium corrodes between 0.8 and 1.2 V vs RHE at a pH of 1.73 It was desired to mimic these regeneration steps and investigate how the catalyst regeneration step affects the long-term performance and stability of the catalysts under potential cylcing. Both catalysts were cycled 300 times between 0.02 and 1.45 V vs RHE at 50 mV/s in 12 M HCOOH with 0.1 M H2SO4 to stabilize the reference electrode. Potentials above 1.2 V vs RHE do not add to the dissolution of Pd. According to the Pourbaix diagram from 1.2 to 1.45 V vs RHE, palladium is passivated. The current was then measured at 0.3 V vs RHE and percent current loss was plotted every 50 cycles. The results are shown in Figure 4. After 50 cycles, the standard palladium catalyst showed a 33% current loss, whereas the palladium-decorated nanotubes showed only a 25% current loss. However, the long-term stability was more noticeable after all 300 cycles were completed. At the end of the 300 cycles, the standard palladium catalyst showed a current loss of nearly 60% of its initial current at 0.3 V vs RHE, whereas the

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Figure 4. Percent current loss versus cycle number; 300 cycles were performed in 12 M HCOOH with 0.1 M H2SO4 supporting electrolyte between 0.02 and 1.45 V vs RHE. Using 0.3 V vs RHE as a representative potential, the current output was measured and compared to the first cycle. Percent loss is plotted every 50 cycles. At the end of 300 cycles, the Pd-decorated carbon nanotube catalyst has lost 35% of its activity, whereas the Pd catalyst has lost 60% of its activity.

palladium-decorated carbon nanotube catalyst showed only a 35% current loss at the same potential. It should be noted that the current loss observed is not due to the accumulation of CO on the palladium surface. During the cycles, sufficient potentials are reached in order to oxidize any accumulated CO from the surface. The improved activity toward formic acid oxidation can be attributed not only to the electronic properties of the carbon nanotube support but also to the method in which the catalyst layer was prepared. The carbon nanotubes needed to be acidtreated to achieve
COOH groups on the surface to act as anchor points for the palladium nanoparticles. It has been shown that the oxygen-containing groups on the surface can increase the rate of electron transfer within the catalyst layer.74,75 In addition, the palladium-decorated carbon nanotube catalyst layer was produced via vacuum filtration. Vacuum filtration of the palladiumdecorated carbon nanotubes allows for a randomly oriented network of catalysts.76,77 This allows for increased electrical connection and, therefore, conduction between the catalyst particles as well as the carbon paper. Although increased electrical conduction can be achieved via hot-pressing, hot-pressing can also collapse the pore network formed in the catalyst layer if not done at an appropriate pressure. The catalyst stability under potential cycling is likely attributed to the
COOH anchor points along the carbon nanotubes as well as the randomly oriented morphology formed when vacuum filtered onto the carbon paper.

’ CONCLUSIONS Here, we have presented work on a palladium-decorated carbon nanotube catalyst that shows better performance for formic acid electrooxidation. During linear sweep voltammetry, the palladium-decorated carbon nanotube catalyst shows a 120% improvement at 0.3 V vs RHE. In addition, the palladiumdecorated catalyst shows a much more stable current under constant potential operation as well as a factor of 4 improvement at the end of 12 h chronoamperograms. Lastly, we showed that, due to the vacuum filtration preparation, these catalysts are more stable (33% current loss vs 60%) under potential cycling than the 19416

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The Journal of Physical Chemistry C standard palladium catalyst used in the direct formic acid fuel cell. Future work will also investigate the proton conductivity within the catalyst layer to determine further feasibility in the fuel cell.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: (217) 2391400. Fax: (217) 333-4050.

’ ACKNOWLEDGMENT This work was supported, in part, by Dioxide Materials. XPS analysis was carried out by Dr. Richard Haasch, and XRD by Dr. Mauro Sardela in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois, which is partially supported by the U.S. Department of Energy under Grants DE-FG02-07ER46453 and DE-FG02-07ER46471. ’ REFERENCES (1) Rice, C.; Ha, S.; Masel, R. I.; Waszczuk, P.; Wieckowski, A.; Barnard, T. J. Power Sources 2002, 111, 83. (2) Miesse, C. M.; Jung, W. S.; Jeong, K.-J.; Lee, J. K.; Lee, J.; Han, J.; Yoon, S. P.; Nam, S. W.; Lim, T.-H.; Hong, S.-A. J. Power Sources 2006, 162, 532. (3) Zhu, Y.; Ha, S. Y.; Masel, R. I. J. Power Sources 2004, 130, 8. (4) Zhu, Y.; Khan, Z.; Masel, R. I. J. Power Sources 2005, 139, 15. (5) Ha, S.; Dunbar, Z.; Masel, R. I. J. Power Sources 2006, 158, 129. (6) Larsen, R.; Ha, S.; Zakzeski, J.; Masel, R. I. J. Power Sources 2006, 157, 78. (7) Larsen, R.; Zakzeski, J.; Masel, R. I. Electrochem. Solid-State Lett. 2005, 8, 291. (8) Chen, W.; Kim, J.; Sun, S.; Chen, S. Langmuir 2007, 23, 11303. (9) Chen, W.; Tang, Y.; Bao, J.; Gao, Y.; Liu, C.; Xing, W.; Lu, T. J. Power Sources 2007, 167, 315. (10) Cheng, T. T.; Gyenge, E. L. J. Electrochem. Soc. 2008, 155, B819. (11) Chetty, R.; Scott, K. J. New Mater. Electrochem. Syst. 2007, 10, 135. (12) Chu, K.-L.; Shannon, M. A.; Masel, R. I. J. Micromech. Microeng. 2007, 17, S243. (13) Ge, J.; Xing, W.; Xue, X.; Liu, C.; Lu, T.; Liao, J. J. Phys. Chem. C 2007, 111, 17305. (14) Guo, Y.; Zheng, Y.; Huang, M. Electrochim. Acta 2008, 53, 3102. (15) Lee, J. K.; Jeon, H.; Uhm, S.; Lee, J. Electrochim. Acta 2008, 53, 6089. (16) Liu, Z.; Hong, L.; Tham, M. P.; Lim, T. H.; Jiang, H. J. Power Sources 2006, 161, 831. (17) Qiao, H.; Shiroishi, H.; Okada, T. Electrochim. Acta 2007, 53, 59. (18) Uhm, S.; Chung, S. T.; Lee, J. J. Power Sources 2008, 178, 34. (19) Uhm, S.; Kwon, Y.; Chung, S. T.; Lee, J. Electrochim. Acta 2008, 53, 5162. (20) Wang, R.; Liao, S.; Ji, S. J. Power Sources 2008, 180, 205. (21) Wang, X.; Tang, Y.; Gao, Y.; Lu, T. J. Power Sources 2008, 175, 784. (22) Yeom, J.; Jayashree, R. S.; Rastogi, C.; Shannon, M. A.; Kenis, P. J. A. J. Power Sources 2006, 160, 1058. (23) Yu, X.; Pickup, P. G. J. Power Sources 2008, 182, 124. (24) Zhang, L.; Lu, T.; Bao, J.; Tang, Y.; Li, C. Electrochem. Commun. 2006, 8, 1625. (25) Zhang, L. J.; Wang, Z. Y.; Xia, D. G. J. Alloys Compd. 2006, 426, 268. (26) Iordache, C.; Blair, S.; Lycke, D.; Huff, S. P. Electrochemical oxidation of formic acid using a noble metal based catalyst with admetals. WO Patent Application 2008080227, 2008.

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dx.doi.org/10.1021/jp205113s |J. Phys. Chem. C 2011, 115, 19413–19418