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Structure dependent electrooxidation of small organic molecules on Pt-decorated nanoporous gold membrane catalysts
Electrochemistry Communications 10 (2008) 1494–1497
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Structure dependent electrooxidation of small organic molecules on Pt-decorated nanoporous gold membrane catalysts Xingbo Ge b, Rongyue Wang b, Songzhi Cui b, Fang Tian b, Liqiang Xu b, Yi Ding a,b,* a b
Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, China Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China
a r t i c l e
i n f o
Article history: Received 26 June 2008 Received in revised form 23 July 2008 Accepted 25 July 2008 Available online 3 August 2008 Keywords: Nanoporous gold Pt-based electrocatalyst Formic acid electrooxidation Ethanol electrooxidation Structure effect
a b s t r a c t We describe the electrocatalytic properties of self-supported Pt-decorated nanoporous gold (Pt-NPG) membranes towards the electrooxidation of formic acid and some other small organic molecules. By effectively enhancing the Pt utilization and providing a unique surface structure, the electrooxidation of formic acid on Pt-NPG was found to be highly sensitive to its surface structure. An unparalleled increase by nearly two orders of magnitude in catalytic activity was achieved on NPG electrodes decorated with sub-monolayer Pt atoms, as compared to the commercial Pt/C catalyst under the same testing conditions. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, great efforts have been made in the direct electrocatalytic oxidation of small organic molecules (SOMs) in view of their great potential in energy-saving technologies such as fuel cells. Besides the well studied methanol fuel cells, other SOMs are gaining an increasing attention because they are often much less toxic as power sources. In particular, employing formic acid as a fuel has additional advantages of higher theoretical open circuit potential and lower fuel crossover [1]. As the most useful and studied catalyst, platinum often suffers from the ‘‘poisoning effect” from the carbonaceous intermediates generated during the reaction, which results in a dramatic decrease in efficiency. Alloying Pt with a selected metal such as Ru to form a bifunctional catalyst is a known way to mitigate this poisoning problem [2]. Recently, Pt-Au alloy nanostructures have attracted more and more research interest because Au nanoparticle catalysts were found to be particularly effective in CO oxidation at ambient conditions [3,4]. While many studies have been devoted to the synthesis and characterization of Au-Pt alloy and Au/Pt core/shell nanoparticles [5–10], we recently reported that based on a simple combination of dealloying and electroless plating technique, a new class of membrane catalysts (platinum decorated nanoporous gold, Pt-NPG) can be made
with a wide variety of excellent properties including high and adjustable surface area, adequate chemical and structural stability, good electrical and thermal conductivities, and low precious metal loading with a very low manufacturing cost [11–13]. Here, we focus on their surface structure dependent electrocatalytic activities toward SOMs, with an emphasis on formic acid electrooxidation. Our results show that sub-monolayer Pt-NPG has much more enhanced catalytic activities in these reactions than heavily plated Pt-NPG and commercial Pt/C catalyst (Johnson Matthey). 2. Experimental section 2.1. Fabrication of NPG and Pt-NPG electrode NPG substrates were prepared by chemically etching 12-carat white gold leaves in concentrated HNO3. Pt-NPG electrode materials were made by reducing H2PtCl6 onto NPG surfaces by the vapor of hydrazine hydrate [12,13]. The amount of platinum deposited onto the NPG substrate gradually accumulates with increasing the plating time, which ranges from 0.5 to 128 min and can be easily controlled by removing the sample out of the reducing atmosphere. 2.2. Characterization
* Corresponding author. Address: Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China. Tel.: +86 531 88366513; fax: +86 531 88366280. E-mail address: [email protected] (Y. Ding). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.07.045
Surface morphologies of NPG and Pt-NPG membranes were observed by a JEM-2100 high-resolution transmission electron microscope at 200 kV. The composition of Pt-NPG was determined
X. Ge et al. / Electrochemistry Communications 10 (2008) 1494–1497
by an IRIS Advantage Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). All electrochemical measurements were performed in a traditional three-electrode electrochemical cell with a CHI 760C electrochemical workstation. The Pt-NPG electrode was made by affixing the membrane to a glassy carbon electrode (3 mm in diameter) and used as the working electrode. The catalytic activities for SOMs (namely, formic acid, formaldehyde and ethanol) oxidation were evaluated in a mixed solution of 0.1 M HClO4 and 0.1 M corresponding SOM solution after an activating process in 0.1 M HClO4. All CVs were performed with a scan rate of 50 mV s 1. 3. Results and discussion By properly employing the starting alloy composition and etching time, the structural unit (pore/ligament size) of NPG was controlled at about 17 nm, as featured by the TEM (Fig. 1a). Because the alloy leaves are only 100 nm thick, NPG membranes have a very low gold loading of 0.1 mg cm 2. The real surface area of NPG membranes described here was estimated to be about 10.2 m2 g 1 using the Brummer’s method [14]. The electroless deposition reaction occurs uniformly on the surfaces of NPG. Fig. 1b shows a low magnification TEM image of the 2 min sample (Pt loading 1.88 lg cm 2, Pt/Au ratio 1:53) which exhibits an NPG-like morphology with clean and smooth surfaces, indicating that the plating is still at the layer growth stage of an overall Stranski-Krastanov type growth process [15]. As plating time increases, isolated Pt nanoclusters start to emerge on the ligament surfaces,
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as shown in Fig. 1c for the 8 min sample. High-resolution TEM image (Fig. 1d) reveals that these Pt nanoclusters are of a size around 1.5 nm and grow epitaxially on NPG surfaces. According to the ICPAES results, plating for 0.5, 2 and 8 min generated Pt-NPG structures with Pt loading of 1.02, 1.88 and 5.54 lg cm 2, which are equivalent to 0.21, 0.39 and 1.16 Pt atomic layers uniformly covering the NPG substrates, respectively, assuming Pt monolayer has a surface area of 210 m2 g 1 Pt. The formation of Pt deposits was further supported by a group of electrochemical cyclic voltammograms (CVs) in 0.1 M HClO4 solution (Fig. 2). It is observed that after plating, the well-defined hydrogen adsorption/desorption and platinum oxides reduction peaks show up and gradually increase in intensity as plating goes on. In comparison, the profiles for gold surface oxides formation and reduction markedly decrease, and plating for 8 min can suppress almost all gold signals, indicating a near complete coverage of NPG surfaces, which is in good agreement with the high-resolution TEM observation and ICP-AES results. The electrocatalytic activity of Pt-NPG toward formic acid oxidation was tested by CVs in a mixed solution of 0.1 M HClO4 and 0.1 M HCOOH. For simplicity and better comparison, Fig. 3a only displays the forward scan curves of CVs for this reaction. The most striking observation in Fig. 3a is that the 0.5 min sample exhibits an unprecedented activity toward formic acid oxidation and only one peak clearly appears at a low potential ( 0.05 V vs. MSE), instead of two peaks for other samples. The activity of the 0.5 min sample recorded at 0.05 V is 5.4 mA cm 2, more than two times higher than that of the 128 min sample. Note that the 0.5 min sam-
Fig. 1. TEM and HRTEM images of (a) NPG membrane, (b) 2 min, and (c, d) 8 min plated Pt-NPG samples.
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X. Ge et al. / Electrochemistry Communications 10 (2008) 1494–1497
Fig. 2. CV curves for NPG and Pt-NPG samples in 0.1 M HClO4, where the currents have been normalized to the geometrical areas of membranes.
ple has the lowest platinum loading and thus the lowest electrochemically active surface area, so its Pt-mass normalized catalytic activity ability is actually more than 50 and 70 times higher than that of the 128 min sample and the commercial Pt/C catalyst (Table 1). It is generally accepted that formic acid is oxidized to CO2 via a dual-path mechanism, i.e. direct dehydrogenation path producing CO2 and indirect dehydration path producing adsorbed CO. While the first peak at 0.1 V is ascribed to the direct oxidation of formic acid via a dehydrogenation process, the second peak at 0.22 V is related to the oxidation of adsorbed CO species generated from the dehydration process [16]. The markedly enhanced activity at lower potential and nearly complete suppression of the formation of adsorbed CO (the second peak) on the 0.5 min sample suggest that this sample is particularly effective in formic acid oxidation by choosing the direct dehydrogenation path. Similar structure-sensitive catalytic performance has also been observed for Pt or Pd modified nanoparticles and single crystal surfaces [9,17–19]. Fig. 3b displays the representative CV curves obtained for the electrooxidation 0.1 M formaldehyde in 0.1 M HClO4. Unlike formic acid, formaldehyde oxidation on platinum-based electrocatalysts has one characteristic anodic peak during the forward scan at 0.18 V [16]. This voltammetric behavior is believed to be due to the rapid dissociation of formaldehyde molecules to form numerous CO species at lower potentials which blocks the Pt sites for a direct path [16,20]. Moreover, the current density for this peak increases as the platinum loading gets higher, suggesting that the electrooxidation of formaldehyde is not so sensitive to the Pt surface structure, which is in sharp contrast to the case of formic acid electrooxidation. The different voltammetric features between the electrooxidation of formic acid and formaldehyde, to a certain extent, reflect a somewhat distinct reaction process [20]. In addition, a negative shift of the onset and peak potential for formaldehyde oxidation is observed on Pt-NPG samples with increasing Pt coverage, which is indicative of a charge perturbation effect from the Au substrate. Actually, CO stripping experiments have proved that CO electrooxidation occurred at a higher potential on Au substrate covered with thin platinum overlayers [21]. Ethanol electrooxidation in 0.1 M HClO4 was also performed. The CV curves shown in Fig. 3c suggest that Pt-NPG is highly effective for ethanol oxidation. The peak current densities recorded at 0.2 V are 1.3, 2.0, 3.0, 4.3 mA cm 2, i.e. 1274, 1064, 541, 148 mA mg 1 (Pt) for 0.5, 2, 8, and 128 min samples, respectively, which are significantly higher than the commercial Pt/C catalyst’s 110 mA mg 1. Compared with formaldehyde, the current peak of ethanol oxidation shifts to a slight positive potential, e.g. 0.21 V
Fig. 3. CV curves for Pt-NPG samples in (a) 0.1 M HClO4 + 0.1 M HCOOH, (b) 0.1 M HClO4 + 0.1 M HCHO and (c) 0.1 M HClO4 + 0.1 M CH3CH2OH.
Table 1 Pt loading of Pt-NPG electrodes and the mass-normalized activities for formic acid, formaldehyde and ethanol electrooxidation Sample
Pt loadinga ug Pt cm 2
HCOOHb mA mg 1Pt
HCHOc mA mg 1Pt
CH3CH2OHd mA mg 1Pt
0.5 min 2 min 8 min 128 min Pt/C
1.02 1.88 5.54 29.1 70.8
5100 1070 238 92 66
3667 4973 2296 707 275
1274 1064 541 148 110
a b c d
Pt loading was determined by ICP-AES. Extracted from CV curves in Fig. 3a at 0.1 V. Extracted from CV curves in Fig. 3b at 0.2 V. Extracted from CV curves in Fig. 3c at 0.2 V.
X. Ge et al. / Electrochemistry Communications 10 (2008) 1494–1497
versus 0.15 V for the 128 min sample, because a higher activation energy is needed for breaking the CAC bond than CAH bond [22]. Meanwhile, a similar trend of catalytic activity was resolved on Pt-NPG samples for both ethanol and formaldehyde oxidation, i.e. a continuous current increase and an ordered negative potential shift with increasing Pt coverage on NPG. Table 1 summarizes the electrochemical parameters and properties of Pt-NPG samples and Pt/C for SOMs electrooxidation. Compared with the Pt/C catalyst, all Pt-NPG samples display much higher catalytic efficiency. Although the 128 min sample has the highest Pt loading, thus the lowest mass-normalized current densities among four Pt-NPG samples, it is still much better than the commercial Pt/C catalyst due to its unique structure. As for 2 and 8 min samples, their high activities are rationalized by the enhanced utilization of Pt. As expected, the highest activity was achieved by the 0.5 min sample. With a nearly ideal Pt utilization, its unusual surface structure makes it particularly attractive for formic acid oxidation. At 0.1 V, an unparalleled current density of 5100 mA mg 1 was achieved, which is nearly two orders of magnitude higher than that of the commercial catalyst. Considering that the durability of an electrocatalyst is of central importance for practical applications, we evaluated the structure stability by monitoring the hydrogen region upon continuous potential cycling, and preliminary results showed that Pt-NPG samples were more stable than the commercial Pt/C catalyst, possibly due to the strain effect induced by the mismatched NPG substrate. And the systematic experiments on this issue are currently underway in our laboratory. 4. Conclusion In summary, we describe the electrocatalytic properties of a class of highly effective electrode materials based on a novel nanoporous membrane material, NPG. By effectively enhancing the Pt utilization and providing a unique surface ensemble structure,
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these materials hold great promise for important energy-saving technologies such as fuel cells. Acknowledgements Financial supports from the National 863 (2006AA03Z222) and 973 (2007CB936602) Program Projects of China, the Key Project of Chinese Ministry of Education (108078) and the Natural Science Foundation of Shandong Province (2007ZRB01117, 2006BS04018) are greatly appreciated. Y.D. is a Tai-Shan Scholar supported by the SEM-NCET and SRF-ROCS Programs. References [1] C. Rice, R.I. Ha, R.I. Masel, P. Waszczuk, A. Wieckowski, T. Barnard, J. Power Sources 111 (2002) 83. [2] T.R. Ralph, M.P. Hogarth, Platinum Met. Rev. 46 (2002) 117. [3] D. Cameron, R. Holliday, D. Thompson, J. Power Sources 118 (2003) 298. [4] D. Mott, J. Luo, P.N. Njoki, Y. Lin, L.Y. Wang, C.J. Zhong, Catal. Today 122 (2007) 378. [5] J. Luo, M.M. Maye, N.N. Kariuki, L.Y. Wang, P. Njoki, Y. Lin, M. Schadt, H.R. Naslund, C.J. Zhong, Catal. Today 99 (2005) 291. [6] J.H. Zeng, J. Yang, J.Y. Lee, W.J. Zhou, J. Phys. Chem. B 110 (2006) 24606. [7] D. Zhao, B.Q. Xu, Angew. Chem. Int. Ed. 45 (2006) 5106. [8] I.S. Park, K.S. Lee, J.H. Choi, H.Y. Park, Y.E. Sung, J. Phys. Chem. C 111 (2007) 19126. [9] N. Kristian, Y.S. Yan, X. Wang, Chem. Commun. 5 (2008) 353. [10] N. Kristian, X. Wang, Electrochem. Commun. 10 (2008) 12. [11] Y. Ding, Y.J. Kim, J. Erlebacher, Adv. Mater. 16 (2004) 1897. [12] Y. Ding, M.W. Chen, J. Erlebacher, J. Am. Chem. Soc. 126 (2004) 6876. [13] X.B. Ge, R.Y. Wang, P.P. Liu, Y. Ding, Chem. Mater. 19 (2007) 5827. [14] S.B. Brummer, A.C. Makrides, J. Electrochem. Soc. 111 (1964) 1122. [15] E. Bauer, J.H. van der Merwe, Phys. Rev. B 33 (1986) 3657. [16] H. Okamoto, W. Kon, Y. Mukouyama, J. Phys. Chem. B 109 (2005) 15659. [17] J. Kim, C. Jung, C.K. Rhee, T.H. Lim, Langmuir 23 (2007) 10831. [18] M. Baldauf, D.M. Kolb, J. Phys. Chem. 100 (1996) 11375. [19] H. Naohara, S. Ye, K. Uosaki, J. Electroanal. Chem. 500 (2003) 435. [20] S. Park, Y. Xie, M.J. Weaver, Langmuir 18 (2002) 5792. [21] B.C. Du, Y.Y. Tong, J. Phys. Chem. B 109 (2005) 17775. [22] H. Nonaka, Y. Matsumura, J. Electroanal. Chem. 520 (2002) 101.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Structure dependent electrooxidation of small organic molecules on Pt-decorated nanoporous gold membrane catalysts Xingbo Ge b, Rongyue Wang b, Songzhi Cui b, Fang Tian b, Liqiang Xu b, Yi Ding a,b,* a b
Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, China Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China
a r t i c l e
i n f o
Article history: Received 26 June 2008 Received in revised form 23 July 2008 Accepted 25 July 2008 Available online 3 August 2008 Keywords: Nanoporous gold Pt-based electrocatalyst Formic acid electrooxidation Ethanol electrooxidation Structure effect
a b s t r a c t We describe the electrocatalytic properties of self-supported Pt-decorated nanoporous gold (Pt-NPG) membranes towards the electrooxidation of formic acid and some other small organic molecules. By effectively enhancing the Pt utilization and providing a unique surface structure, the electrooxidation of formic acid on Pt-NPG was found to be highly sensitive to its surface structure. An unparalleled increase by nearly two orders of magnitude in catalytic activity was achieved on NPG electrodes decorated with sub-monolayer Pt atoms, as compared to the commercial Pt/C catalyst under the same testing conditions. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, great efforts have been made in the direct electrocatalytic oxidation of small organic molecules (SOMs) in view of their great potential in energy-saving technologies such as fuel cells. Besides the well studied methanol fuel cells, other SOMs are gaining an increasing attention because they are often much less toxic as power sources. In particular, employing formic acid as a fuel has additional advantages of higher theoretical open circuit potential and lower fuel crossover [1]. As the most useful and studied catalyst, platinum often suffers from the ‘‘poisoning effect” from the carbonaceous intermediates generated during the reaction, which results in a dramatic decrease in efficiency. Alloying Pt with a selected metal such as Ru to form a bifunctional catalyst is a known way to mitigate this poisoning problem [2]. Recently, Pt-Au alloy nanostructures have attracted more and more research interest because Au nanoparticle catalysts were found to be particularly effective in CO oxidation at ambient conditions [3,4]. While many studies have been devoted to the synthesis and characterization of Au-Pt alloy and Au/Pt core/shell nanoparticles [5–10], we recently reported that based on a simple combination of dealloying and electroless plating technique, a new class of membrane catalysts (platinum decorated nanoporous gold, Pt-NPG) can be made
with a wide variety of excellent properties including high and adjustable surface area, adequate chemical and structural stability, good electrical and thermal conductivities, and low precious metal loading with a very low manufacturing cost [11–13]. Here, we focus on their surface structure dependent electrocatalytic activities toward SOMs, with an emphasis on formic acid electrooxidation. Our results show that sub-monolayer Pt-NPG has much more enhanced catalytic activities in these reactions than heavily plated Pt-NPG and commercial Pt/C catalyst (Johnson Matthey). 2. Experimental section 2.1. Fabrication of NPG and Pt-NPG electrode NPG substrates were prepared by chemically etching 12-carat white gold leaves in concentrated HNO3. Pt-NPG electrode materials were made by reducing H2PtCl6 onto NPG surfaces by the vapor of hydrazine hydrate [12,13]. The amount of platinum deposited onto the NPG substrate gradually accumulates with increasing the plating time, which ranges from 0.5 to 128 min and can be easily controlled by removing the sample out of the reducing atmosphere. 2.2. Characterization
* Corresponding author. Address: Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China. Tel.: +86 531 88366513; fax: +86 531 88366280. E-mail address: [email protected] (Y. Ding). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.07.045
Surface morphologies of NPG and Pt-NPG membranes were observed by a JEM-2100 high-resolution transmission electron microscope at 200 kV. The composition of Pt-NPG was determined
X. Ge et al. / Electrochemistry Communications 10 (2008) 1494–1497
by an IRIS Advantage Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). All electrochemical measurements were performed in a traditional three-electrode electrochemical cell with a CHI 760C electrochemical workstation. The Pt-NPG electrode was made by affixing the membrane to a glassy carbon electrode (3 mm in diameter) and used as the working electrode. The catalytic activities for SOMs (namely, formic acid, formaldehyde and ethanol) oxidation were evaluated in a mixed solution of 0.1 M HClO4 and 0.1 M corresponding SOM solution after an activating process in 0.1 M HClO4. All CVs were performed with a scan rate of 50 mV s 1. 3. Results and discussion By properly employing the starting alloy composition and etching time, the structural unit (pore/ligament size) of NPG was controlled at about 17 nm, as featured by the TEM (Fig. 1a). Because the alloy leaves are only 100 nm thick, NPG membranes have a very low gold loading of 0.1 mg cm 2. The real surface area of NPG membranes described here was estimated to be about 10.2 m2 g 1 using the Brummer’s method [14]. The electroless deposition reaction occurs uniformly on the surfaces of NPG. Fig. 1b shows a low magnification TEM image of the 2 min sample (Pt loading 1.88 lg cm 2, Pt/Au ratio 1:53) which exhibits an NPG-like morphology with clean and smooth surfaces, indicating that the plating is still at the layer growth stage of an overall Stranski-Krastanov type growth process [15]. As plating time increases, isolated Pt nanoclusters start to emerge on the ligament surfaces,
1495
as shown in Fig. 1c for the 8 min sample. High-resolution TEM image (Fig. 1d) reveals that these Pt nanoclusters are of a size around 1.5 nm and grow epitaxially on NPG surfaces. According to the ICPAES results, plating for 0.5, 2 and 8 min generated Pt-NPG structures with Pt loading of 1.02, 1.88 and 5.54 lg cm 2, which are equivalent to 0.21, 0.39 and 1.16 Pt atomic layers uniformly covering the NPG substrates, respectively, assuming Pt monolayer has a surface area of 210 m2 g 1 Pt. The formation of Pt deposits was further supported by a group of electrochemical cyclic voltammograms (CVs) in 0.1 M HClO4 solution (Fig. 2). It is observed that after plating, the well-defined hydrogen adsorption/desorption and platinum oxides reduction peaks show up and gradually increase in intensity as plating goes on. In comparison, the profiles for gold surface oxides formation and reduction markedly decrease, and plating for 8 min can suppress almost all gold signals, indicating a near complete coverage of NPG surfaces, which is in good agreement with the high-resolution TEM observation and ICP-AES results. The electrocatalytic activity of Pt-NPG toward formic acid oxidation was tested by CVs in a mixed solution of 0.1 M HClO4 and 0.1 M HCOOH. For simplicity and better comparison, Fig. 3a only displays the forward scan curves of CVs for this reaction. The most striking observation in Fig. 3a is that the 0.5 min sample exhibits an unprecedented activity toward formic acid oxidation and only one peak clearly appears at a low potential ( 0.05 V vs. MSE), instead of two peaks for other samples. The activity of the 0.5 min sample recorded at 0.05 V is 5.4 mA cm 2, more than two times higher than that of the 128 min sample. Note that the 0.5 min sam-
Fig. 1. TEM and HRTEM images of (a) NPG membrane, (b) 2 min, and (c, d) 8 min plated Pt-NPG samples.
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X. Ge et al. / Electrochemistry Communications 10 (2008) 1494–1497
Fig. 2. CV curves for NPG and Pt-NPG samples in 0.1 M HClO4, where the currents have been normalized to the geometrical areas of membranes.
ple has the lowest platinum loading and thus the lowest electrochemically active surface area, so its Pt-mass normalized catalytic activity ability is actually more than 50 and 70 times higher than that of the 128 min sample and the commercial Pt/C catalyst (Table 1). It is generally accepted that formic acid is oxidized to CO2 via a dual-path mechanism, i.e. direct dehydrogenation path producing CO2 and indirect dehydration path producing adsorbed CO. While the first peak at 0.1 V is ascribed to the direct oxidation of formic acid via a dehydrogenation process, the second peak at 0.22 V is related to the oxidation of adsorbed CO species generated from the dehydration process [16]. The markedly enhanced activity at lower potential and nearly complete suppression of the formation of adsorbed CO (the second peak) on the 0.5 min sample suggest that this sample is particularly effective in formic acid oxidation by choosing the direct dehydrogenation path. Similar structure-sensitive catalytic performance has also been observed for Pt or Pd modified nanoparticles and single crystal surfaces [9,17–19]. Fig. 3b displays the representative CV curves obtained for the electrooxidation 0.1 M formaldehyde in 0.1 M HClO4. Unlike formic acid, formaldehyde oxidation on platinum-based electrocatalysts has one characteristic anodic peak during the forward scan at 0.18 V [16]. This voltammetric behavior is believed to be due to the rapid dissociation of formaldehyde molecules to form numerous CO species at lower potentials which blocks the Pt sites for a direct path [16,20]. Moreover, the current density for this peak increases as the platinum loading gets higher, suggesting that the electrooxidation of formaldehyde is not so sensitive to the Pt surface structure, which is in sharp contrast to the case of formic acid electrooxidation. The different voltammetric features between the electrooxidation of formic acid and formaldehyde, to a certain extent, reflect a somewhat distinct reaction process [20]. In addition, a negative shift of the onset and peak potential for formaldehyde oxidation is observed on Pt-NPG samples with increasing Pt coverage, which is indicative of a charge perturbation effect from the Au substrate. Actually, CO stripping experiments have proved that CO electrooxidation occurred at a higher potential on Au substrate covered with thin platinum overlayers [21]. Ethanol electrooxidation in 0.1 M HClO4 was also performed. The CV curves shown in Fig. 3c suggest that Pt-NPG is highly effective for ethanol oxidation. The peak current densities recorded at 0.2 V are 1.3, 2.0, 3.0, 4.3 mA cm 2, i.e. 1274, 1064, 541, 148 mA mg 1 (Pt) for 0.5, 2, 8, and 128 min samples, respectively, which are significantly higher than the commercial Pt/C catalyst’s 110 mA mg 1. Compared with formaldehyde, the current peak of ethanol oxidation shifts to a slight positive potential, e.g. 0.21 V
Fig. 3. CV curves for Pt-NPG samples in (a) 0.1 M HClO4 + 0.1 M HCOOH, (b) 0.1 M HClO4 + 0.1 M HCHO and (c) 0.1 M HClO4 + 0.1 M CH3CH2OH.
Table 1 Pt loading of Pt-NPG electrodes and the mass-normalized activities for formic acid, formaldehyde and ethanol electrooxidation Sample
Pt loadinga ug Pt cm 2
HCOOHb mA mg 1Pt
HCHOc mA mg 1Pt
CH3CH2OHd mA mg 1Pt
0.5 min 2 min 8 min 128 min Pt/C
1.02 1.88 5.54 29.1 70.8
5100 1070 238 92 66
3667 4973 2296 707 275
1274 1064 541 148 110
a b c d
Pt loading was determined by ICP-AES. Extracted from CV curves in Fig. 3a at 0.1 V. Extracted from CV curves in Fig. 3b at 0.2 V. Extracted from CV curves in Fig. 3c at 0.2 V.
X. Ge et al. / Electrochemistry Communications 10 (2008) 1494–1497
versus 0.15 V for the 128 min sample, because a higher activation energy is needed for breaking the CAC bond than CAH bond [22]. Meanwhile, a similar trend of catalytic activity was resolved on Pt-NPG samples for both ethanol and formaldehyde oxidation, i.e. a continuous current increase and an ordered negative potential shift with increasing Pt coverage on NPG. Table 1 summarizes the electrochemical parameters and properties of Pt-NPG samples and Pt/C for SOMs electrooxidation. Compared with the Pt/C catalyst, all Pt-NPG samples display much higher catalytic efficiency. Although the 128 min sample has the highest Pt loading, thus the lowest mass-normalized current densities among four Pt-NPG samples, it is still much better than the commercial Pt/C catalyst due to its unique structure. As for 2 and 8 min samples, their high activities are rationalized by the enhanced utilization of Pt. As expected, the highest activity was achieved by the 0.5 min sample. With a nearly ideal Pt utilization, its unusual surface structure makes it particularly attractive for formic acid oxidation. At 0.1 V, an unparalleled current density of 5100 mA mg 1 was achieved, which is nearly two orders of magnitude higher than that of the commercial catalyst. Considering that the durability of an electrocatalyst is of central importance for practical applications, we evaluated the structure stability by monitoring the hydrogen region upon continuous potential cycling, and preliminary results showed that Pt-NPG samples were more stable than the commercial Pt/C catalyst, possibly due to the strain effect induced by the mismatched NPG substrate. And the systematic experiments on this issue are currently underway in our laboratory. 4. Conclusion In summary, we describe the electrocatalytic properties of a class of highly effective electrode materials based on a novel nanoporous membrane material, NPG. By effectively enhancing the Pt utilization and providing a unique surface ensemble structure,
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these materials hold great promise for important energy-saving technologies such as fuel cells. Acknowledgements Financial supports from the National 863 (2006AA03Z222) and 973 (2007CB936602) Program Projects of China, the Key Project of Chinese Ministry of Education (108078) and the Natural Science Foundation of Shandong Province (2007ZRB01117, 2006BS04018) are greatly appreciated. Y.D. is a Tai-Shan Scholar supported by the SEM-NCET and SRF-ROCS Programs. References [1] C. Rice, R.I. Ha, R.I. Masel, P. Waszczuk, A. Wieckowski, T. Barnard, J. Power Sources 111 (2002) 83. [2] T.R. Ralph, M.P. Hogarth, Platinum Met. Rev. 46 (2002) 117. [3] D. Cameron, R. Holliday, D. Thompson, J. Power Sources 118 (2003) 298. [4] D. Mott, J. Luo, P.N. Njoki, Y. Lin, L.Y. Wang, C.J. Zhong, Catal. Today 122 (2007) 378. [5] J. Luo, M.M. Maye, N.N. Kariuki, L.Y. Wang, P. Njoki, Y. Lin, M. Schadt, H.R. Naslund, C.J. Zhong, Catal. Today 99 (2005) 291. [6] J.H. Zeng, J. Yang, J.Y. Lee, W.J. Zhou, J. Phys. Chem. B 110 (2006) 24606. [7] D. Zhao, B.Q. Xu, Angew. Chem. Int. Ed. 45 (2006) 5106. [8] I.S. Park, K.S. Lee, J.H. Choi, H.Y. Park, Y.E. Sung, J. Phys. Chem. C 111 (2007) 19126. [9] N. Kristian, Y.S. Yan, X. Wang, Chem. Commun. 5 (2008) 353. [10] N. Kristian, X. Wang, Electrochem. Commun. 10 (2008) 12. [11] Y. Ding, Y.J. Kim, J. Erlebacher, Adv. Mater. 16 (2004) 1897. [12] Y. Ding, M.W. Chen, J. Erlebacher, J. Am. Chem. Soc. 126 (2004) 6876. [13] X.B. Ge, R.Y. Wang, P.P. Liu, Y. Ding, Chem. Mater. 19 (2007) 5827. [14] S.B. Brummer, A.C. Makrides, J. Electrochem. Soc. 111 (1964) 1122. [15] E. Bauer, J.H. van der Merwe, Phys. Rev. B 33 (1986) 3657. [16] H. Okamoto, W. Kon, Y. Mukouyama, J. Phys. Chem. B 109 (2005) 15659. [17] J. Kim, C. Jung, C.K. Rhee, T.H. Lim, Langmuir 23 (2007) 10831. [18] M. Baldauf, D.M. Kolb, J. Phys. Chem. 100 (1996) 11375. [19] H. Naohara, S. Ye, K. Uosaki, J. Electroanal. Chem. 500 (2003) 435. [20] S. Park, Y. Xie, M.J. Weaver, Langmuir 18 (2002) 5792. [21] B.C. Du, Y.Y. Tong, J. Phys. Chem. B 109 (2005) 17775. [22] H. Nonaka, Y. Matsumura, J. Electroanal. Chem. 520 (2002) 101.