Boosting Electrocatalytic Performances of Palladium Nanoparticles by

Apr 23, 2014 - Noble metal catalysts are core materials for many energy conversion and storage technologies such as fuel cell and hydrogen storage, wh...
0 downloads 8 Views 4MB Size
Download PDF
Article pubs.acs.org/cm

Boosting Electrocatalytic Performances of Palladium Nanoparticles by Coupling with Metallic Single-Walled Carbon Nanotubes Jing Zhang,†,§ Hongyuan Chen,†,§ Hongbo Li,† Jiangtao Di,† Minghai Chen,† Fengxia Geng,*,‡ Zhigang Zhao,*,† and Qingwen Li*,† †

Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China ‡ College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China S Supporting Information *

ABSTRACT: Noble metal catalysts are core materials for many energy conversion and storage technologies such as fuel cell and hydrogen storage, while the performances of the noble metals depend critically on the nature of support materials. Herein, for the first time, we report the use of sorted highpurity metallic (m-) and semiconducting (s-) single-walled carbon nanotubes (SWNTs) as support materials for a typical noble metal catalyst, Pd, which has been believed to have strong interactions with SWNTs. Our results clearly suggest that electrocatalytic performances of the noble metals/SWNTs hybrid system, specifically in hydrogen electrosorption and formic acid electrooxidation, exhibit strongly sensitive dependence with respect to the electronic type of SWNT supports. Pd nanoparticles on m-SWNTs demonstrate much enhanced electrocatalytic activities compared with those on s-SWNTs or unsorted-SWNTs. Our in-depth mechanism studies indicate that the m-SWNTs in the nanocomposite tend to provide more effective charge-transfer interfaces with Pd nanoparticles, more likely leading to higher electron densities on Pd nanoparticles to boost their catalytic performance. The present study provides a novel window onto the design and synthesis of new feasible electrocatalyst system for efficient energy conversion and storage.

■

zero for s-SWNTs.10 Such a drastic change in electronic structure is expected to bring varied electron-transfer kinetics between metallic noble nanoparticles and m- or s-SWNTs support in electrocatalytic processes and alter the electrochemistry. Therefore, tuning electronic structure of SWNT support material may serve as a new effective strategy to boost the performances of electrocatalyst systems. Nevertheless, to the best of our knowledge, the use of sorted SWNTs with a specific electronic type (metallic or semiconducting) or a single chirality as catalyst supports in electrocatalysis has been rarely explored. Herein, for the first time, we report the use of sorted highpurity m- and s-SWNTs as support materials for a typical noble metal catalyst, Pd, which has been believed to have strong interactions with SWNTs. Electrocatalytic performances of the novel system, specifically in hydrogen electrosorption and formic acid electrooxidation, exhibit strongly sensitive dependence with respect to the electronic type of SWNT supports. Pd nanoparticles on m-SWNTs demonstrate much superior electrocatalytic activities to those on s-SWNTs or unsortedSWNTs. The results indicate that the sorted SWNTs with tunable electronic structures would provide a new window onto designing novel catalysts with desirable activity and selectivity.

INTRODUCTION Noble metal catalyst (e.g., Pt, Pd, Au) is of great importance and interest in the realization of many energy conversion and storage technologies such as fuel cell and hydrogen storage.1 Most of these applications require the use of support material, which can facilitate the noble metals in a finely dispersed state and consequently enhance the specific activity and reduce required loading amount. Meantime, catalyst supports are likely to modify the catalytic performance and durability of the overlying noble metal particles due to the presence of strong metal−support interactions.2,3 In recent years, carbon nanotubes (CNTs) are emerging as a new class of material for supporting noble metal nanoparticles in electrocatalytic applications due to their unique chemical stability and large surface area.4−9 Modification of CNT-based support materials to further improve the electrocatalytic performances mainly involves surface functionalization and chemical doping. Functionalization could exert a strong electronic promoting effect on the activity of the overlying metallic particles,7,8 and doping resulted in a significant modification of their intrinsic physical and chemical properties and, in turn, their catalytic activity as well.9 However, it should be noted that, regardless of the growth process, traditional SWNTs used as catalyst support in previous studies were typically a mixture of metallic and semiconducting SWNTs (m- and s-SWNTs), which possess much different physical and chemical properties. At the Fermi energy, the density of state is finite for m-SWNTs, while it is © 2014 American Chemical Society

Received: October 11, 2013 Revised: April 18, 2014 Published: April 23, 2014 2789

dx.doi.org/10.1021/cm500606z | Chem. Mater. 2014, 26, 2789−2794

Chemistry of Materials

■

Article

of −0.2 V with the frequency changing from 100 kHz to 1 Hz. Before running any measurement, the solution was thoroughly deaerated by bubbling with N2 for 30 min, and a nitrogen atmosphere was kept in the solution.

EXPERIMENTAL SECTION

Gel Chromatographic Separation of SWNTs. SWNTs prepared with the high-pressure catalytic method (HiPCO, Lot NO. P0276) were purchased from Carbon Nanotechnologies Inc. Purification of the as-obtained SWNT powders was performed before use, employing an acid treatment approach. The raw SWNTs were first sonicated for 10 min in 6 M hydrochloric acid (HCl) and filtrated to remove possible soluble residue, and then an additional stirring of the filtrated nanotubes in 1 M HCl was applied overnight. Finally the purified SWNTs were rinsed with deionized water to neutral and gathered by filtration. Dispersion of the purified SWNT powders was obtained using a procedure described in our previous report,11 sonicating in 1% sodium dodecyl sulfate (SDS, Sigma-Aldrich (99%)) aqueous solution at a concentration of 0.3 mg/mL followed by centrifugation to remove remaining bundles and impurities. The resulting supernatant was collected as the SWNT dispersion for m- and s-SWNTs separation, which adopted a gel-based column chromatography strategy. Alkyl dextran-based gel beads (sephacryl S-100, GE healthcare) were used as the gel packing media. After the SWNT solution was permeated into the matrix, the m- and s-fractions were sorted using different surfactant solutions, 1% SDS solution and Triton-X100, respectively. Synthesis of Pd/SWNTs Hybrids. To hybridize the Pd particles on SWNTs, aqueous solutions of PdCl2 (15 mM) were added to the unsorted-SWNTs, m-SWNTs, and s-SWNTs solution, respectively, and the mixture solutions were irradiated by a low-pressure mercury lamp (254 nm) during stirring.12 The yielded products were collected by centrifugation and a thorough washing with alcohol to remove ligands or surfactants on the surface. Electrode Preparation. To make the corresponding electrodes, the Pd/m-SWNTs, Pd/s-SWNTs, and Pd/unsorted-SWNTs were redispersed in a mixture solution of glycol and water (v:v = 1:1) with 0.5% nafion. A drop of the three solutions with ∼50 μL in volume was spread on glassy carbon electrodes, which were dried at 60 °C under vacuum for 12 h before electrocatalytic measurements. Characterizations. The fractions of m- or s-SWNTs in both unsorted and sorted SWNT dispersions were examined by absorption spectroscopy using a PerkinElmer Lambda 950 UV−vis−NIR spectrometer. The sample surface states were characterized with Xray photoelectron spectroscopy (XPS) analysis on a Kratos AXIS Ultra DLD fitted with a focused monochromatic Al Kα X-ray source (1486.6 V) and a Hemispherical Sector Analyzer. The samples were dried in vacuum before XPS tests. The position of the energy scale was adjusted to place the main C 1s feature (C−C 4.5 V). X-ray diffraction (XRD) patterns of the prepared samples were recorded on a Bruker AXS D8 Advance X-ray diffractometer with a Cu Kα radiation target (40 V, 40 A) to study the phase purity and crystalline state of the samples. Transmission electron microscope (TEM) images were obtained on a JEOL 2010 operating at an accelerating voltage of 100 kV. Before TEM observations, the SWNTs in aqueous media were precipitated with ethanol and collected, after which surfactant was removed with repeated washing. Ethanol dispersion was then made, and a few drops were placed onto the TEM grid for characterizations. Electrochemical Measurements. The electrochemical measurements were performed on a CHI 660C electrochemical workstation. Before each test, the electrocatalysts were treated with UV irradiation (wavelength at 254 nm) for 12 h in air to remove possible organic compounds and clean the surfaces.13 A three-electrode configuration was used for the cyclic voltammetry and constant current charge− discharge behavior measurements, using the Pd/SWNT modified glassy carbon electrode as the working electrode, saturated calomel electrode (SCE) as the reference electrode, and Pt foil as the counter electrode. The amount of hydrogen sorption was measured in 1 M H2SO4 solution by a cyclic voltammetry method. The formic acid oxidation activity was characterized in 0.1 M HClO4 + 1 M HCOOH solution. The cyclic voltammetry measurement was performed in the potential range between −0.2 and 1 V vs SCE at room temperature, and the scan rate was 20 mV/s. Chronoamperometry measurement was carried out by holding the potential at 0.2 V. Electrochemical impedance spectroscopy (EIS) characterizations were at the potential

■

RESULTS Sorting SWCNTs and Characterizations. The commercial SWNTs were purified before use employing an acid treatment approach, and the high quality of purified SWNTs was confirmed by Raman spectroscopy studies, which observed G/D ratio of a high value of 73.4. Massive separation of m- and s-SWNTs was achieved with a two-step elution method, first with 1% SDS solution and then with 0.25% Triton solution, using sephacryl-based gel column chromatography.11,14,15 Figure 1a shows a camera photo of the SWNTs in the gel

Figure 1. (a) Left: a photo of SWNTs dispersion after the separation process; right: zoom-in view showing the different colors of sorted mand s-SWNTs. (b) Optical absorption spectra derived from the unsorted, m- and s-SWNT dispersions, respectively. (c) Raman spectra of unsorted and sorted SWNTs. (d) XPS C 1s core level spectra of mSWNT and s-SWNT along with their line fittings.

column after the separation process. Clearly, the SWNTs are separated into two fractions, showing red and green, typical colors for m- and s-SWNTs, respectively (right in Figure 1a). The UV−vis−NIR absorption spectra in Figure 1b confirmed the successful achievement of high-ratio enrichment of m- and s-SWNTs. In comparison with unsorted SWNT dispersion, the sorted red dispersion exhibits a clear absorbance increase in the range of 400−600 nm corresponding to the first van Hove optical transitions (designated as M11) for m-SWNTs, and a nearly complete disappearance of the first (S11, 950−1350 nm) and second (S22, 550−900 nm) van Hove optical transition bands for s-SWNTs, indicating a high-ratio enrichment of mSWNTs. In contrast, an obvious enhancement in the absorbance for the S11 and S22 bands and decrease for the M11 bands were discerned for the green dispersion, which suggests a high-ratio enrichment of s-SWNTs. The ratio of sorted m-SWNTs and s-SWNTs was estimated through a method reported by Hiromichi Kataura et al.,16 which gave 91 ± 2% and 90 ± 3%, respectively (see Supporting Information). Furthermore, in Raman characterizations irradiated by a red 2790

dx.doi.org/10.1021/cm500606z | Chem. Mater. 2014, 26, 2789−2794

Chemistry of Materials

Article

Figure 2. (a) Low- and (b) high-magnification TEM images and (c) Pt particle size distribution histogram for hybridized Pd/m-SWNT. (d) Lowand (e) high-magnification TEM images and (f) Pt particle size distribution histogram for hybridized Pd/s-SWNTs.

Figure 3. (a) Cyclic voltammograms of Pd/SWNT in aqueous 1 M H2SO4 for the potential region −0.2−1.0 V. (b) Illustration of charge transfer process and HCOOH oxidation on Pd supported by carbon nanotubes. (c) Cyclic voltammograms for formic acid oxidation of the three electrodes in 1 M HCOOH and 0.1 M HClO4 aqueous solution. (d) Comparison table of particle size, ESA, CSA, and Pd utilizations.

irradiated for 1 h with a low-pressure mercury lamp (254 nm) under stirring at ambient temperature.12 XRD characterizations showed that the deposited Pd nanoparicles on both m- and sSWNTs are crystallized into a face-centered cubic phase, with diffraction peaks at approximately 39°, 46°, 68°, and 81° attributable to (111), (200), (220), and (311), respectively (Supporting Information). Figure 2a,b shows the low- and high-magnification transmission electron microscopy (TEM) images for the as-obtained Pd/m-SWNTs, and Figure 2d,e shows those for Pd/s-SWNTs. Strikingly, Pd nanoparticles on both surfaces are fairly well monodispersed and have a very good uniform size. The corresponding particle size distribution histograms (counting more than 200 particles) are depicted in Figure 2c,f, showing a relatively narrow particle size distribution and roughly the same mean particle diameters, ca. 2.8 and 3.1 nm for Pd/m-SWNTs and Pd/s-SWNTs, respectively. The loading amount of Pd nanoparticles was also comparable in the two cases, 50% and 51% (Pd wt %) on m- and s-SWNT

laser at 633 nm, the red and blue dispersions presented respective features for m- and s-SWNTs (Figure 1c), corroborating the successful sorting of SWNTs.11,14,15 The surface states of sorted m- and s-SWNTs were examined by XPS. Figure 1d depicts the typical C 1s spectra recorded of m- and s-SWNTs, and signals of some oxygen-containing groups were detected, with fitted peaks at 284.5, 285.4, 287.5, and 288.9 eV responsible for CC, CO or COH, CO, and OCO, respectively.17 Furthermore, such an m- and sSWNTs sorting process does not bring length separation, with mean length of 300−400 m for both m- and s-SWNTs (Supporting Information). Therefore, both the size distribution of sorted m- and s-SWNTs and the variety of surface functional groups on them are comparable, which would not bring crucial differences in the process of palladium nanoparticles deposition. Hybridizing Pd Nanoparticles on SWNTs Surfaces. To hybridize Pd nanoparticles on SWNTs surfaces, a mixture of PdCl2 and sorted SWNTs in aqueous solution was UV2791

dx.doi.org/10.1021/cm500606z | Chem. Mater. 2014, 26, 2789−2794

Chemistry of Materials

Article

Figure 4. (a) Electrochemical impedance spectrum in Nyquist form for HCOOH oxidation of Pd/m-SWNT and Pd/s-SWNT at an potential of −0.2 V (vs SCE). (b) XPS spectra of the Pd 3d region registered for Pd/m-SWNT and Pd/s-SWNT.

scan rate of 20 mV/s. Current density is one important parameter to evaluate the electrocatalyst performances for electrooxidation of HCOOH.21 For the tested three electrodes, Pt nanoparticles on m-SWNTs, s-SWNTs, and unsorted SWNTs, the specific current normalized to Pd nanoparticle loading is 191, 98, and 118 mA/mg, respectively. Evidently under the same conditions Pd/m-SWNTs give current density almost twice higher than Pd/s-SWNTs and Pd/unsorted SWNTs toward formic acid electrooxidation. Meanwhile, Pd/ m-SWNTs show the highest steady-state current density and maintain the best stability, with steady-state oxidation current density being ∼31.7, 3.13, and 0.68 mA/mg for Pd/m-SWNTs, Pd/s-SWNTs, and Pd/unsorted SWNTs, respectively (see Supporting Information). Therefore, similar to the reactions in hydrogen electrosorption (vide ante), Pd particles on mSWNTs exhibit much better electrocatalytic activities also in the oxidation of HCOOH. The electrochemical active surface area (ESA), chemical/ geometric surface area (CSA), and Pd utilization (η) were determined with details provided in Supporting Information. ESA was estimated based on the charge associated with the PdO reduction region observed in cyclic voltammograms in alkaline solutions;26−30 CSA was calculated supposing the Pd particles in a spherical shape. With ESA and CSA, it is possible to deduce the catalyst utilization efficiency η as ESA/CSA.31 As the results listed in Figure 3d, Pd particles on m-SWNTs demonstrate ESA and η values almost as four times higher as those on s-SWNTs. While sizes of Pd catalysts in the two cases are similar, the superior performances of Pd particles on mSWNTs should arise from some essential differences between m- and s-SWNTs.

support, respectively, which was determined by inductively coupled plasmon atomic emission spectroscopy (ICP-AES). Electrocatalytic Tests. It is known that Pd has a superior ability to absorb large volumetric quantities of hydrogen to form palladium hydride (PdHx), thus enabling its application in a myriad of hydrogen technologies.1 Therefore, we first investigated the utilization of Pd nanoparticles supported by sorted and unsorted SWNTs as hydrogen-absorbing material in hydrogen electrosorption. Figure 3a draws hydrogen electrosorption voltammetric profiles obtained from various SWNTsupported Pd-based catalysts in 1 M H2SO4 at room temperature with a scan rate of 20 mV/s. Similar to previous studies, five characteristic regions were observed in cyclic voltammogram scans: hydrogen adsorption, hydrogen desorption, oxygen adsorption, oxygen desorption, and double-layer.18 In the hydrogen desorption region, there is a strong peak located between −0.2 and 0.1 V during the negative-going potential scan, which can be attributed to the desorption of hydrogen from both the β- and α-hydride phases.18,19 The overall hydrogen desorption charge QH, which is thought to reflect hydrogen electrosorption capacity of a material, can be obtained by integrating the area under the peak located between −0.2 and 0.1 V in the cyclic voltammograms. The charges QH for Pd/m-SWNT, Pd/s-SWNT, and Pd/unsortedSWNT were calculated to be 33.26, 21.94, and 13.09 mC/cm2, respectively. Obviously, under the same conditions, the hydrogen desorption charge produced by Pd particles on mSWNTs is 1.5 and 2.5 times those on s-SWNTs and unsorted SWNTs, respectively. Our results indicate that m-SWNTs support may be more favorable to assist in the occlusion of hydrogen in hydrogen electrosorption compared with sSWNTs or unsorted SWNTs support. The electrocatalytic activity of the Pd nanoparticles supported on SWNTs toward formic acid (HCOOH) oxidation was also studied, as Pd can work as excellent catalyst for the electrooxidation of HCOOH.20,21 Perchloric acid (0.1 M) was used as electrolyte instead of sulfuric acid, because bisulfate ions adsorb more strongly onto the Pd surface than perchlorate and the blocking of sites would significantly reduce catalytic activity.22−24 Figure 3b illustrates the charge transfer process and HCOOH oxidation on Pd supported on SWNTs. The electrooxidation of HCOOH to CO2 proceeds through two pathways: direct oxidation to CO2 and oxidation to CO2 through the adsorption of a CO intermediate.25 The cyclic voltammetric curves of HCOOH oxidation on Pd/m-SWNTs, Pd/s-SWNTs, and Pd/unsorted-SWNTs are shown in Figure 3c, performed in 1 M HCOOH and 0.1 M HClO4 aqueous solution in the range of −0.2 to 1 V (vs SCE) at a potential

■

DISCUSSION To scrutinize the factors that make Pd particles on m-SWNTs demonstrate better electrocatalytic performances, electrochemical impedance spectroscopy (EIS) and XPS were measured to obtain valuable information identifying the effect of SWNT support. Figure a presents the Nyquist plots of Pd/ m-SWNTs and Pd/s-SWNTs for HCOOH oxidation at −0.2 V, showing well-defined frequency-dependent semicircle impedance curves over high frequencies followed by straight lines. The diameter of the arcs represents the respective charge transfer resistance, with the smaller one of Pd/m-SWNTs suggesting a lower charge transfer resistance and consequently faster electron-transfer kinetics in Pd/m-SWNTs for electrochemical reactions. The charge transfer difference between Pd/ m-SWNTs and Pd/s-SWNTs can be ascribed to two factors: one is barrier formed between SWNTs and the other is barrier 2792

dx.doi.org/10.1021/cm500606z | Chem. Mater. 2014, 26, 2789−2794

Chemistry of Materials

■

between Pd nanoparticles and SWNTs. The contact barrier between SWNTs was checked with electrical measurement, which showed that the m-SWNT films possess much better electrical properties than s-SWNT films, 1250 Ω sq−1 with 88.5% transmittance at 550 nm and 24900 Ω sq−1 with 88.7% transmittance at 550 nm for m- and s-SWNTs, respectively. For the barrier between Pd nanoparticles and SWNTs, junctions of different type are expected. For m-SWNTs, an excellent lowresistance ohmic contact could be developed with metallic Pd nanoparticles, making the electron transfer between m-SWNT and Pd rather smooth, whereas s-SWNT and metallic Pd nanoparticles might form Schottky-barrier junction, resulting in poor electron transfer efficiency.32 While there is a possibility that the ions in in acidic medium may alter the local electric field and thus modify the electronic structure of SWNTs,33 absorbance spectra of freshly sorted SWNTs and SWNTs in media of sulfuric acid or perchloric acid were collected and compared, in which no obvious changes were found, suggesting that the pristine electronic structure of SWNTs is probably maintained under the conditions discussed herein. From the perspective of local electron density on Pd nanoparticles, we made further investigations into the interactions between Pd nanoparticles and SWNTs surface by examining the Pd (3d) XPS signal asymmetry. As drawn in Figure 4b, the spectra are characterized by a pair of intense Pd 3d5/2 and 3d3/2 peaks. The former Pd 3d5/2 binding energies (BE) are 335.24 and 335.51 eV for Pd particles on m- and sSWNTs, respectively, while the latter Pd 3d2/2 BE are 340.47 and 340.83 eV. As compared to particles on s-SWNTs, a downshift of 270 and 250 meV was observed for those on mSWNTs, suggesting an increase in electron density of Pd on mSWNTs. An alike slight decrease in Pt (4f) BE was reported for Pt deposited on TiO2 and WO3 supports, which was attributed to charge transfer between oxide supports and resulted in increased electrocatalytic activity.33 Therefore, the increased electron density of Pd and improved charge transfer with support could contribute to the enhanced electrocatalytic activities of Pd/m-SWNTs.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.Z.). *E-mail: [email protected] (Q.L.). *E-mail: [email protected] (F.G.). Author Contributions §

These authors contributed equally to this work

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

This work was supported by the 973 project (No. 2011CB932600), National Natural Science Foundation of China (NO. 51372266, 51102274, 21273269, and 61274130).

(1) Adams, B. D.; Chen, A. Mater. Today 2011, 14, 282. (2) Pham-Huu, C.; Keller, N.; Charbonniere, L. J.; Ziessel, R.; Ledoux, M. J. Chem. Commun. 2000, 1871. (3) Mondal, K. C.; Cele, L. M.; Witcomb, M. J.; Coville, N. J. Catal. Commun. 2008, 9, 494. (4) Kim, Y.-T.; Ohshima, K.; Higashimine, K.; Uruga, T.; Takata, M.; Suematsu, H.; Mitani, T. Angew. Chem., Int. Ed. 2006, 45, 407. (5) Prabhuram, J.; Zhao, T. S.; Tang, Z. K.; Chen, R.; Liang, Z. X. J. Phys. Chem. B 2006, 110, 5245. (6) Chu, H.; Shen, Y.; Lin, L.; Qin, X.; Feng, G.; Lin, Z.; Wang, J.; Liu, H.; Li, Y. Adv. Funct. Mater. 2010, 20, 3747. (7) Luksirikul, P.; Tedsree, K.; Moloney, M. G.; Green, M. L. H.; Tsang, S. C. E. Angew. Chem., Int. Ed. 2012, 51, 6998. (8) Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536. (9) Oosthuizen, R. S.; Nyamori, V. O. Platinum Met. Rev. 2011, 55, 154. (10) Murakami, T.; Nakatsuji, H.; Inada, M.; Matoba, Y.; Umeyama, T.; Tsujimoto, M.; Isoda, S.; Hashida, M.; Imahori, H. J. Am. Chem. Soc. 2012, 134, 17862. (11) Gui, H.; Li, H.; Tan, F.; Jin, H.; Zhang, J.; Li, Q. Carbon 2012, 50, 332. (12) Tan, Z.; Abe, H.; Naito, M.; Ohara, S. Chem. Commun. 2010, 46, 4363. (13) Saleem, F.; Zhang, Z.; Xu, B.; Xu, X.; He, P.; Wang, X. J. Am. Chem. Soc. 2013, 135, 18304. (14) Tanaka, T.; Jin, H.; Miyata, Y.; Fujii, S.; Suga, H.; Naitoh, Y.; Minari, T.; Miyadera, T.; Tsukagoshi, K.; Kataura, H. Nano Lett. 2009, 9, 1497. (15) Liu, H.; Nishide, D.; Tanaka, T.; Kataura, H. Nat. Commun. 2011, 2, 309. (16) Miyata, Y.; Yanagi, K.; Maniwa, Y.; Kataura, H. J. Phys. Chem. C 2008, 112, 13187. (17) Ersen, O.; Werckmann, J.; Houllé, M.; Ledoux, M.-J.; PhamHuu, C. Nano Lett. 2007, 7, 1898. (18) Adams, B. D.; Ostrom, C. K.; Chen, A. Langmuir 2010, 26, 7632. (19) Rose, A.; Maniguet, S.; Mathew, R. J.; Slater, C.; Yao, J.; Russell, A. E. Phys. Chem. Chem. Phys. 2003, 5, 3220. (20) Wang, J.; Yin, G.; Chen, Y.; Li, R.; Sun, X. Int. J. Hydrogen Energy 2009, 34, 8270. (21) Chen, W.; Kim, J.; Sun, S.; Chen, S. Phys. Chem. Chem. Phys. 2006, 8, 2779. (22) Thomas, S.; Sung, Y. E.; Kim, H. S.; Wieckowski, A. J. Phys. Chem. 1996, 100, 11726. (23) Ogasawara, H.; Sawatari, Y.; Inukai, J.; Ito, M. J. Electroanal. Chem. 1993, 358, 337. (24) Saravanan, C.; Koper, M. T. M.; Markovic, N. M.; HeadGordon, M.; Ross, P. N. Phys. Chem. Chem. Phys. 2002, 4, 2660.

■

CONCLUSIONS In summary, sorted SWNTs with specific electronic type can be efficiently employed as a new class of catalyst supports. Pd particles supported on m-SWNTs exhibit superior electrocatalytic activity than those on s-SWNTs in both hydrogen electrosorption and formic acid electrooxidation reaction. The enhancement could be ascribed to the more effective chargetransfer between Pd nanoparticles and m-SWNTs support and increased electron density on Pd. The present study provides a novel route to the design and synthesis of new feasible electrocatalyst systems for efficient energy conversion and storage.

■

Article

ASSOCIATED CONTENT

S Supporting Information *

Detailed procedures for calculation of the ratio of sorted SWNTs in the dispersions, size distribution characterizations of sorted SWNTs, phase characterizations for Pd/m-SWNTs and Pd/s-SWNTs, chronoamperometric experiments, and details for calculation of ESA, CSA, and η. This material is available free of charge via the Internet at http://pubs.acs.org. 2793

dx.doi.org/10.1021/cm500606z | Chem. Mater. 2014, 26, 2789−2794

Chemistry of Materials

Article

(25) Xu, J.; Yuan, D.; Yang, F.; Mei, D.; Zhang, Z.; Chen, Y.-X. Phys. Chem. Chem. Phys. 2013, 15, 4367. (26) Pattabiraman, R. Appl. Catal., A 1997, 153, 9. (27) Łukaszewski, M.; Grdeń, M.; Czerwiński, A. J. Solid State Electrochem. 2005, 9, 1. (28) Thotiyl, M. M. O.; Kumar, T. R.; Sampath, S. J. Phys. Chem. C 2010, 114, 17934. (29) Zhao, Y.; Yang, X.; Tian, J.; Wang, F.; Zhan, L. Int. J. Hydrogen Energy 2010, 35, 3249. (30) Modibedi, R. M.; Masombuka, T.; Mathe, M. K. Int. J. Hydrogen Energy 2011, 36, 4664. (31) Lee, S.-K.; Kim, C.-H.; Cho, W.-C.; Kang, K.-S.; Park, C.-S.; Bae, K.-K. Int. J. Hydrogen Energy 2009, 34, 4701. (32) Svensson, J.; Campbell, E. E. B. J. Appl. Phys. 2011, 110. (33) Chen, S.; Malig, M.; Tian, M.; Chen, A. J. Phys. Chem. C 2012, 116, 3298.

2794

dx.doi.org/10.1021/cm500606z | Chem. Mater. 2014, 26, 2789−2794