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Nanoneedle-Covered Pd-Ag Nanotubes: High Electrocatalytic Activity for Formic Acid Oxidation Yizhong Lu†,‡ and Wei Chen*,† State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, China ReceiVed: August 16, 2010; ReVised Manuscript ReceiVed: October 27, 2010
Nanoneedle-covered palladium-silver nanotubes were synthesized through a galvanic displacement reaction with Ag nanorods at 100 °C (PdAg-100) and room temperature (PdAg-25). Transmission and scanning electron microscopic measurements displayed that the synthesized PdAg nanotubes exhibit a hollow structure with a nanoneedle-covered surface, providing the perfect large surface area for catalytic reactions. The PdAg nanotubes formed at 100 °C exhibit a more uniform surface morphology than those obtained at room temperature. The high-resolution TEM, energy-dispersive X-ray analysis, and powder X-ray diffraction measurements indicated that the surface of the nanotubes is decorated with crystalline Pd nanoparticles with Pd(111) planes, and meanwhile, Ag and AgCl particles are dispersed in the inner space of the nanotubes. The electrocatalytic activity of the synthesized PdAg nanotubes toward formic acid oxidation was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). With the same loading on a glassy carbon electrode, the PdAg-100 nanotubes show high catalytic activity and stability from the CV and chronoamperometric analyses, which may be ascribed to the annealing process of the nanotube surface structures at 100 °C. The reaction kinetics of the HCOOH oxidation on the PdAg nanotubes was then examined by EIS measurements. It was found that the impedance responses are strongly dependent on the electrode potentials. With the potential increasing, the reaction kinetics evolve from resistive to pseudoinductive and then to inductive behaviors. On the basis of the proposed equivalent circuits, the synthesized PdAg nanotubes exhibit a much lower (almost 3 orders of magnitude smaller) charge-transfer resistance (RCT, a characteristic quantity for the rate of charge transfer for the electrooxidation of formic acid) than that obtained at the Pt-based nanoparticles reported previously. It was also found that the RCT at the PdAg-100 nanorods is much smaller than that at the PdAg25 nanorods, indicating the electron-transfer kinetics for formic acid oxidation at the PdAg-100 nanorods is much better facilitated. The present work highlights the application of the nanoneedle-covered PdAg nanotubes with high surface areas as anode electrocatalysts in fuel cells and the influence of surface structure on their catalytic activity. 1. Introduction Direct liquid fuel cells, such as direct methanol fuel cells (DMFCs) and direct formic acid fuel cells (DFAFCs), have attracted much attention in the past few decades due to their high energy conversion efficiency, low environmental pollution, and low operating temperature.1,2 Previous studies have showed that formic acid can be oxidized at less positive potential and with faster kinetics than methanol at room temperature, and the crossover of formic acid through the polymer membrane is lower than that of methanol.3-5 Therefore, formic acid has received considerable attention over the yeas as a potential fuel for fuel cells. For the fuel oxidation on anodes and oxygen reduction on cathodes, highly active electrocatalysts are required to achieve the high enough current densities for practical applications. For a good electrocatalyst, both the high catalytic activity and the low cost must be considered to meet the final purpose of wide commercialization of fuel cells. In the previous studies, platinum was found to show the highest catalytic activity among the anode catalysts for electrooxidation of methanol or formic * To whom correspondence should be addressed. E-mail: weichen@ ciac.jl.cn. † Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
acid. However, with platinum as an anode catalyst, the surface is usually heavily poisoned by the strong adsorption of CO produced during the oxidation of organic fuels, resulting in the lowering of catalytic performance. To resolve such a problem, based on the so-called bifunctional mechanism,6,7 Pt-based alloys with other transition metals have been extensively studied as possible electrocatalysts with improved electrocatalytic activity.8-10 Especially, various carbon-supported Pt-Ru alloy catalysts are widely used. However, due to the high cost and the limited world supply of platinum, recent extensive research efforts have been devoted to the development of nonplatinum electrocatalysts as anode and cathode catalysts, such as transition metals,11 transitionmetal oxides and sulfides,12-14 macrocyclic complexes,15,16 and metal-containing porphyrin systems.17 Among the nonplatinum electrocatalysts studied, Pd and Pd-based metal alloys have been investigated as effective electrocatalysts for fuel cells.18-26 In recent years, considerable attention has been paid to the designing of nanostructured Pd materials because of the high proportion of surface to bulk atoms. Palladium nanoparticles have received increasing attention in view of their potential applications in catalysis, sensor technology, hydrogen storage, etc.27-31 In addition to much work concentrated on Pd or Pd-
10.1021/jp107768n 2010 American Chemical Society Published on Web 11/15/2010
Nanoneedle-Covered Pd-Ag Nanotubes based nanoparticles, various Pd nanostructured materials, such as Pd nanowires,32 Pd nanoflowers,33 Pd nanotrees, and porous Pd nanorods,34 have also been synthesized and studied as electrocatalysts for methanol and ethanol oxidation. For the nanostructured electrocatalysts, it is well known that the catalytic performance is largely dependent on their surface structures, such as the particle size, particle shape, compositions (for Pd-based alloys), etc. For instance, Zhou et al35 studied the formic acid electrooxidation on unsupported palladium nanoparticles with the size range from 9 to 40 nm and found that the most active catalyst is from the smallest Pd nanoparticles (9 and 11 nm). In another study,36 the Pt/Pd core-shell nanocrystals displayed surface structure (particle shape)-dependent electrocatalytic activities for formic acid oxidation, the peak current of formic acid oxidation showing the following sequence on the different shapes: cube > cuboctahedra > octahedral. Metal nanotubes with high specific interface areas may strongly enhance the catalytic activity and improve the mass transport efficiency within and across the nanotube walls. In the present paper, PdAg nanotubes with their surface covered by nanoneedles were synthesized by using freshly prepared silver nanorods as templates through the galvanic displacement reaction at different temperatures. The structures of the materials were characterized with transmission electron microscopy (TEM), HRTEM, scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV-visible absorption spectrum measurements. The electrocatalytic activity of the PdAg nanotubes toward formic acid was examined with electrochemical voltammetric and impedance measurements; the results showed that the synthesized PdAg nanotubes exhibit excellent catalytic activity for formic acid oxidation with a high current density and a high tolerance to CO poisoning. 2. Experimental Section 2.1. Chemicals and Materials. Silver nitrate (AgNO3, g99.8%, Beijing Chemical Reagent), poly(vinyl pyrrolidone) (PVP, Mw ≈ 55 000, Aldrich), ethylene glycol (EG, A. R. grade, Beijing Chemical Reagent), palladium chloride (PdCl2, A.R. grade, Beijing Chemical Reagent), perchloric acid (HClO4, A.R. grade, Tianjin Chemical Reagent), and formic acid (HCOOH, A.R. grade, Beijing Chemical Reagent) were used as received. The palladium bulk electrode was made from pure Pd wire (99.9% purity, 0.5 mm in diameter). The Pd wire was embedded in an epoxy resin mold, leaving its cross section exposed. Before use, the electrode was mechanically and electrochemically polished and then cleaned carefully. Water was supplied by a water purifier Nanopure water system (18.3 MΩ cm). 2.2. Synthesis of Nanoneedle-Covered PdAg Nanotubes. The Ag nanorods were first synthesized according to the modified procedures described before.37-39 Briefly, 0.36 M (in terms of repeating unit) of PVP dissolved in 5 mL of 1,2propylene glycol was refluxed at 160 °C for 1 h under vigorous stirring. A 2.5 mL portion of a freshly prepared AgNO3 solution (0.12 M in EG) was then added in a dropwise fashion into the hot PVP solution at a rate of approximately 0.15 mL/min. The reaction mixture was stirred for another 40 min at 160 °C. After the solution cooled to room temperature, the final dispersion was diluted with ethanol in a ratio of 1:30 and then centrifuged at a rate of 1700 rpm for 20 min. The nanorods were collected and redispersed in ethanol. This process was repeated about six more times. The PdAg nanotubes were synthesized by a galvanic displacement reaction at 100 °C and room temperature, which are denoted as PdAg-100 and PdAg-25, respectively. To synthesize PdAg-100 nanotubes, 5 mg of the synthesized Ag
J. Phys. Chem. C, Vol. 114, No. 49, 2010 21191 nanorods was dispersed in ∼10 mL of pure water and then heated to 100 °C under magnetic stirring. To the boiling solution, 2 mg of PdCl2 dissolved in 10 mL of water was added drop by drop. After about 30 min, the solution was cooled to room temperature. The products were then separated via centrifugation at a rate of 3900 rpm for 20 min and further purified with ethanol and pure water several times until there was no Pd2+ ion in the rinsing water (the UV-vis absorption of the final rinsing solution is shown in Figure S1, Supporting Information). The final products were dried and stored under vacuum at room temperature. The PdAg-25 nanotubes were synthesized with the similar procedure for preparing the PdAg-100 nanotubes, but the galvanic displacement reaction was conducted at room temperature. 2.3. Electrochemistry. Prior to the deposition of the PdAg nanotubes onto an electrode surface for electrocatalytic assessment, a glassy carbon (GC) electrode (3.0 mm diameter) was polished with alumina slurries (0.05 µm) and cleansed by sonication in 0.1 M HNO3, H2SO4, and pure water for 10 min successively. A 1 mg portion of the dried PdAg nanotubes or Ag nanorods was dispersed in 0.7 mL of pure water and 60 µL of 5 wt % Nafion solution by ultrasonication for a few seconds. After the ink formed homogeneously, 10 µL of the catalyst ink was then dropped on the clean GC electrode with a micropipet and then dried in vacuum at room temperature. The prepared electrodes from PdAg-100 and PdAg-25 nanotubes and Ag nanorods are denoted as PdAg-100/GC, PdAg-25/GC, and Ag/ GC, respectively. Voltammetric measurements were carried out with a CHI 750D electrochemical workstation. The PdAg-100/GC and PdAg-25/GC electrodes prepared above were used as the working electrode. A Ag/AgCl (in 3 M NaCl, aq) and a Pt coil (0.5 mm × 4 cm) were used as the reference and counter electrodes, respectively. All electrode potentials in the present study were referred to this Ag/AgCl reference. The impedance spectra were recorded between 100 kHz and 10 mHz with the amplitude (rms value) of the ac signal of 10 mV. The solutions were deaerated by bubbling ultra-high-purity N2 for 20 min and protected with a nitrogen atmosphere during the entire experimental procedure. All electrochemical experiments were carried out at room temperature. 2.4. Material Characterization. UV-vis spectroscopic studies were performed with a Cary spectrometer using a 1 cm quartz cuvette with a resolution of 2 nm. Powder X-ray diffraction (XRD) was performed on a D/Max 2500 V/PC powder diffractometer using Cu-KR radiation with a Ni filter (λ ) 0.154059 nm at 30 kV and 15 mA). The morphology and crystal structure of the PdAg nanotubes were analyzed with a TECNAI G2 200 kV field emission analytical transmission electron microscope and a field emission scanning electron microscope (FE-SEM, XL30ESEM-FEG). High-resolution TEM (HRTEM) images and the corresponding live fast Fourier transform (FFT), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, element analysis mapping, and EDX were carried out on a JEM2010(HR) microscope operated at 200 kV. 3. Results and Discussion 3.1. Characterization of the Nanoneedle-Covered PdAg Nanotubes. Figure 1 shows the UV-vis absorption spectra of the Ag nanorods and the PdAg-100 and PdAg-25 nanotubes templated from the Ag nanorods. It can be seen that the initial Ag nanorods show a main absorption at 385 nm with a shoulder at 350 nm. According to the previous studies,40,41 these two
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Figure 1. UV-visible absorption spectra of the initial silver nanorods and the PdAg nanotubes templated from the Ag nanorods at room temperature (PdAg-25) and 100 °C (PdAg-100).
absorption bands are ascribed to the surface plasmon resonance bands of the transverse and longitudinal modes of the silver nanorods, respectively. Such UV-vis absorption features also agree well with those obtained with silver nanowires and nanorods in the previous reports.42-44 However, the strong absorption of the Ag nanorods disappeared after the galvanic displacement reaction at both 100 and 25 °C, indicating the formation of nanotubes. Interestingly, the UV-vis absorption of the PdAg-100 and PdAg-25 nanotubes exhibits a typical Mie exponential decay profile with no obvious absorption peak, which is usually observed with the small metal nanoparticle solutions. Such an optical feature may be ascribed to the formation of Pd nanoparticles on the nanotube surface, which is similar to the report of Pd/Au nanostructures with Pd shells on hollow Au nanospheres.45 Note that the high-resolution TEM images in the following study verifies the presence of Pd nanoparticles on the nanotube surface. The peaks at around 266 nm in the PdAg nanotubes could be assigned to the ligand-tometal charge-transfer absorption.46 Figure 2 shows the representative TEM and SEM micrographs of the PdAg-100 (A and C) and PdAg-25 (B and D) nanotubes, respectively, after the galvanic displacement reaction with Pd2+. From the TEM images shown in Figure S2 (Supporting Information), it can be seen that the initial Ag nanorods are solid with a very uniform shape and size. The average diameter is 83 ( 3 nm with an aspect ratio of 14.5 ( 4.4. After the displacement reaction, it can be seen from the TEM images in Figure 2A,B that the morphology of the materials changed remarkably. Hollow structures were formed after the reaction at both 100 and 25 °C. The average diameters of 57 ( 6 and 78 ( 8 nm with the aspect ratios of 21 ( 7.3 and 15.4 ( 8.5 were evaluated for PdAg-100 and PdAg-25 nanotubes, respectively. By analyzing the surface structures in detail with the SEM images in Figure 2C,D and Figure S3 (Supporting Information), it was found that the walls of the nanotubes are covered with very dense and uniform Pd nanoneedles, resulting in a large surface area. However, on comparison of the TEM and SEM images of PdAg-25 and PdAg-100 nanotubes, it can be seen that the surfaces of the nanotubes obtained at 100 °C are more uniform than those of PdAg-25. Such phenomenon may be ascribed to the Ostwald ripening process at refluxing temperature (100 °C), leading to the uniform Pd nanowall growth on the Ag nanorods. At low temperature (25 °C here), the Ostwald ripening process, however, is not favored. From
Lu and Chen the TEM and SEM measurements, one can see that the synthesized PdAg nanotubes have a rough and hollow structure with the advantage of a large surface area, which is beneficial for catalytic reactions. In Figure 2C,D, a few spherical nanoparticles were also observed. Interestingly, similar to the PdAg nanotubes, the nanoparticles are also covered with Pd nanoneedles. The spherical nanoparticles should be formed during the galvanic displacement reaction because no such nanoparticle with highly rough surfaces was observed in the SEM images of the initial Ag nanorods (shown in Figure S2, Supporting Information). From the EDX patterns of the PdAg-100 (Figure 2E) and PdAg-25 (Figure 2F) nanotubes, except for the Cu signals from the supported grids, there are diffraction peaks from both Ag and Pd. On the basis of the EDX, the average ratio of Pd to Ag was evaluated to be 36:64 and 25:75 for the PdAg100 and PdAg-25 nanotubes, respectively. To examine the crystallinity of the Pd Ag nanotubes, powder X-ray diffraction measurements were carried out. Figure 3 shows the XRD patterns of the initial Ag nanorods and PdAg-25 and PdAg-100 nanotubes. The JCPDS data of Ag (no. 04-0783), Pd (no. 65-6174), and AgCl (no. 31-1238) were also included in the figure for comparison. The XRD pattern from Ag nanorods agrees well with the JCPDS data. It can also be seen that both of the PdAg nanotubes show clear diffraction peaks. For the PdAg nanotubes, in addition to the peaks corresponding to the (111), (200), (220), and (311) planes of metallic silver, Pd(111) with 2θ at 40.14 was also observed. Even though the Pd(111) diffraction peak appears, the lack of peaks from other planes of palladium suggests that the crystalline lattice of the synthesized PdAg nanotubes was dominated from the silver nanorod template. Such phenomena were also reported in the previous studies, which was attributed to the strong diffusion of Ag atoms from the bulk to the surface for the alloys of Ag and Pd.37,47,48 Interestingly, except for the diffraction peaks from Ag and Pd metals, there exist other strong peaks that are ascribed to the AgCl crystals, which can be verified from the excellent agreement of the peaks from both PdAg nanotube samples with JCPDS data of AgCl (shown in Figure 3). AgCl crystals were formed during the galvanic displacement reaction between Ag naorods and PdCl2:
Ag0 + PdCl2 f Pd0 + AgCl
(1)
The existence of AgCl also suggests that the PdAg nanotubes can be synthesized effectively by the present method. Despite the dominant signals from Ag in the XRD measurements, the strong voltammetric features from palladium, but not from silver, can be observed in the following electrochemical studies, indicating that Pd plays the crucial role with the present materials for electrocatalysis. From the above TEM and XRD characterizations, the synthesized PdAg nanotubes are composed of Pd Ag and AgCl nanocrystals. To investigate the crystal structures of the synthesized nanomaterials and the element distribution, highresolution TEM (HRTEM) and other structural analyses were also performed by taking the PdAg-25 nanotube sample as an example. Figure 4A,B shows the HRTEM images of one PdAg-25 nanotube at different magnifications. It can be seen from Figure 4A that the PdAg nanotubes indeed have a hollow structure and the surfaces are decorated with nanoparticles having diameters of 5-8 nm. From a closer examination of the HRTEM image shown in Figure 4B, it was found that each of the nanoparticles dispersed on the nanotube is single crystal
Nanoneedle-Covered Pd-Ag Nanotubes
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Figure 2. TEM micrographs of the PdAg-100 (A) and PdAg-25 (B) nanotubes, SEM images of the PdAg-100 (C) and PdAg-25 (D) nanotubes, and energy-dispersive X-ray analysis (EDX) of the PdAg-100 (E) and PdAg-25 (F) nanotubes after the galvanic displacement reaction with Pd2+. Insets in A and B show the corresponding magnified micrograph of PdAg nanotubes. The scale bars are all 200 nm.
with well-resolved lattice fringes. It can be seen by careful measurements that there is mainly one type of lattice fringe with an interplanar spacing of 2.23 Å dispersed in the shell, which is ascribed to the Pd(111), and there are two types of lattice fringes with interplanar spacings of 2.38 and 2.76 Å dispersed in the inner space of the nanotubes, corresponding to Ag(111) and AgCl(200), respectively. The corresponding Fourier transform (FT) (Figure 4A, inset) pattern of the Pd nanoparticles indicates that it is single crystal with 6-fold rotational symmetry. The local HRTEM investigation showed that the PdAg nanotubes have a Pd-shell hollow structure with Ag and AgCl nanocrystals dispersed in the core. The distribution of Pd, Ag,
and AgCl in the whole nanotubes was studied with element analysis mapping with cross-sectional compositional line profiles taken from two nanotubes. From the Ag, Pd, and Cl element maps shown in Figure 4D-F, the Ag and Cl are readily detectable and concentrated in the core of PdAg nanotubes; Pt, however, distributes on the shell of the nanotubes. From the cross-sectional compositional line profiles shown in Figure 4G, the intensities of the Pd signal peak center at the shell regions, while there is a dip at the central part. However, the signals of Ag and Cl are mainly distributed inside of the nanotubes. These structural characterizations clearly indicate the formation of hollow PdAg nanotubes with Pd nanocrystals decorated on the
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Figure 3. XRD patterns of the synthesized PdAg-25 and PdAg-100 nanotubes and the initial Ag nanorods. For comparison, bulk Ag, Pd, and AgCl from the Joint Committee Powder Diffraction Standard were also included.
surface. It should be noted that, in order to avoid the damage of the formed PdAg nanotubes, ultrasonication with only a few seconds was applied to disperse the samples in solution for the TEM, SEM, HRTEM, and electrochemistry measurements. The SEM images obtained before and after ultrasonication showed that the nanostructures remain intact after ultrasonication (not shown).
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Figure 5. Cyclic voltammograms of the PdAg-25/GC (black curve), PdAg-100/GC (red curve), and bulk Pd electrode (blue curve) in 0.1 M HClO4 solution. Potential scan rate ) 0.1 V/s. For comparison, the CV from the Pd bulk electrode is magnified 5×.
3.2. Formic Acid Electrooxidation with the Synthesized Nanomaterials. 3.2.1. Electrochemical Characterization of the Nanoneedle-CoWered PdAg Nanotubes in 0.1 M HClO4. Figure 5 shows the typical cyclic voltammograms (CVs) of the PdAg100/GC and PdAg-25/GC electrodes with the same loading of 0.18 mg/cm2 in 0.1 M HClO4 solution. Obviously, in both CV profiles, no signal from silver oxidation and the reduction of silver oxide but only the characteristics of polycrystalline palladium metal in acid electrolytes were observed, suggesting
Figure 4. (A, B) HRTEM images of PdAg-25 nanotubes. The inset in A shows the corresponding two-dimensional fast Fourier transform (FFT) pattern. (C) The high-angle annular dark-field (HAADF) STEM image of PdAg-25 nanotubes; the corresponding elemental mapping of Ag (D), Pd (E), and Cl (F) in the PdAg-25 nanotubes. (G) Cross-sectional compositional line profiles of a PdAg-25 nanotube.
Nanoneedle-Covered Pd-Ag Nanotubes the formation of PdAg nanotubes templated with the initial silver nanorods. From the CV curves in Figure 5, three regions can be divided based on the CV features. In the hydrogen region between -0.25 and +0.15 V, there are two pairs of well-defined current peaks at around -0.1 and +0.02 V corresponding to the hydrogen adsorption and desorption. Note that, at potentials more negative than -0.15 V in the cathodic sweep, there was a sharp increase in current due to the hydrogen evolution. In the Pd oxidation region, it can be seen that Pd surface oxidation is formed in the anodic scan at potentials above +0.4 V. In the reverse scan, the reduction of the Pd oxides (PdO) can be observed with a peak position at about +0.42 V. The potential window between +0.15 and +0.26 V corresponds to the doublelayer region. For comparison, the CV obtained from the bulk Pd electrode in 0.1 M HClO4 solution was also included in Figure 5 (blue curve). It should be pointed out that the hydrogen adsorption, desorption, and evolution on bulk Pd electrodes only form one pair of peaks, but not separated peaks, as seen with the present PdAg nanotubes.49,50 On the present PdAg nanostructured materials, the pair of peaks at more positive potential (+0.02 V) are ascribed to the hydrogen adsorption and desorption on PdAg nanotube ligaments and reflect the real surface area of electrodes.51,52 The observation of separated hydrogen adsorption and desorption current peaks, especially the obvious peaks at more positive potential, suggests the large surface area of the present Pd-based nanotubes, which is very favorable for the application of nanomaterials as electrocatalysts. Such CV features have also been observed on other palladium nanomaterials with large specific surface areas, such as Pd nanoparticles,53 ultrathin Pd films,49 and nanoporous palladium rods.34 The total charge of QH, due to the hydrogen adsorption and absorption on PdAg nanotubes, can be evaluated by integrating the area under the anodic peaks in the hydrogen region. The charges for hydrogen desorption on the PdAg-25/GC and PdAg100/GC was calculated to be 717 and 292 µC, respectively. It should be noted that, with the same loading, the larger charge for hydrogen desorption obtained with PdAg-25/GC suggests the rougher surface of PdAg-25 nanotubes compared with that of PdAg-100 nanotubes, which is consistent with the results of TEM and SEM measurements. These characterizations also imply that the synthesized PdAg nanotubes are electrochemically active. 3.2.2. Formic Acid Oxidation with the Nanoneedle-CoWered PdAg Nanotubes. Figure 6 depicts the steady-state cyclic voltammograms of formic acid oxidation at the PdAg-25/GC and PdAg-100/GC electrodes in 0.1 M HClO4 and 0.5 M HCOOH at a scan rate of 0.1 V/s. Note that the voltammetric currents have been normalized to the real active surface areas (i.e., electrochemically active surface areas) of the respective electrodes, which are calculated according to the oxygen adsorption measurement method proposed by Trasatti and Petrii.54 It can be seen that the CV profiles are similar for formic acid oxidation on both PdAg nanotubes. There is a broad peak with peak potentials at +0.285 and +0.316 V for PdAg-100/ GC and PdAg-25/GC electrodes, respectively. In the reverse potential sweep, an extremely sharp current peak was observed on both electrodes. It should be noted that, due to the Pd oxide formation during the anodic sweep, formic acid oxidation starts only after the Pd oxide was reduced in the reverse potential sweep. Here, with the removal of Pd oxides at around +0.44 V, formic acid is oxidized in a short time, resulting in the observation of a sharp oxidation current peak. For comparison, the CV of the PdAg-25/GC electrode in 0.1 M HClO4 and the CV of HCOOH oxidation with the bulk Pd electrode are also
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Figure 6. Steady-state cyclic voltammograms of the PdAg-25/GC (red curve), PdAg-100/GC (green curve), and bulk Pd electrode (blue curve) electrodes in 0.1 M HClO4 and 0.5 M HCOOH solution and the cyclic voltammogram of PdAg-25/GC (black curve) in 0.1 M HClO4. Potential scan rate ) 0.1 V/s.
included in Figure 6. The electrooxidation of formic acid typically follows a dual-pathway mechanism. That is, formic acid either is electrooxidized directly to CO2 by dehydrogenation or first dissociates spontaneously to CO, which then becomes oxidized to CO2. For HCOOH oxidation with the bulk Pd electrode (blue curve in Figure 6), there are two broad peaks with peak potentials at about +0.44 and +0.62 V in the anodic scan. Thus, the anodic current peak at +0.44 V may be attributed to the oxidation of HCOOH on active surface sites that have not been poisoned by CO (direct path). The second anodic current peak at +0.62 V may arise from the oxidation of both adsorbed CO and formic acid as a consequence of the recovery of surface active sites by CO removal (indirect path).55-58 However, the oxidation of HCOOH with the present PdAg nanotubes shows one major peak with a shoulder in the positivegoing potential scan, suggesting the enhanced CO tolerance of the synthesized PdAg nanotubes compared with the bulk Pd electrode. On comparison of the CVs in Figure 6, it also can be seen that the anodic onset potentials and peak potentials for formic acid oxidation appear to be more negative on the PdAg nanotubes than on the bulk Pd electrode; for instance, the peak potential of formic acid oxidation on the bulk Pd, PdAg-25/ GC, and PdAg-100/GC electrodes are +0.44, +0.316, and +0.285 V, respectively. Moreover, the current density of HCOOH oxidation on the PdAg nanotubes was substantially greater than that obtained with the bulk Pd electrode. These CV measurements indicate that the PdAg nanotubes show excellent catalytic activity for formic acid oxidation. It is well known that the catalytic activity of metal nanomatrials is dependent strongly on the surface structure.59 In the previous studies of formic acid oxidation with the Pd single crystals,60 it was found that the catalytic activity increases in the order of Pd(110) < Pd(111) < Pd(100) in acid electrolytes. More recently, the Pd nanoparticles with predominant Pd(111) planes were found to be very active for formic acid oxidation.61 For the present PdAg nanotubes, the XRD and HRTEM measurements shown in Figures 3 and 4 revealed that the Pd(111) plane is predominant with respect to the other planes, which may be the origin of the high electrocatalytic activity for formic acid oxidation. By comparing the CV curves in Figure 6, one can see that the anodic peak current density obtained on the PdAg100/GC electrode (3.82 mA/cm2) is much larger than that from PdAg-25/GC (1.97 mA/cm2), and the anodic peak potential
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appears to be more negative (+0.285 V) on the PdAg-100/GC electrode than that on PdAg-25/GC (+0.316 V). The higher electrocatalytic activity of the PdAg-100/GC, compared with that of the PdAg-25/GC, possibly as the consequences of the higher ratio of Pd to Ag (36:64) in PdAg-100 compared with that in PdAg-25 (25:75) and the annealing process of the nanotube surface structures at 100 °C. The CV curves of formic acid oxidation with different scan rates on both PdAg-25/GC and PdAg-100/GC electrodes are plotted in Figures S4A and S5A (Supporting Information). It can be seen that the anodic HCOOH oxidation peak current density increases with the increasing of the potential scan rate from 10 to 80 mV/s. From the plots in Figures S4B and S5B (Supporting Information), the anodic peak current density is linearly dependent on the square root of the potential scan rate, suggesting that the electrocatalytic oxidation of formic acid on the present PdAg nanotubes is controlled by the diffusion process. The dual-pathway mechanism is widely accepted for formic acid oxidation with Pt- or Pd-based catalysts.62 The most desirable pathway is via a dehydrogenation reaction (direct path), during which a reactive intermediate is formed. Another reaction pathway via dehydration involves adsorbed CO as a poisoning intermediate species (indirect path). On the basis of the previous reports56,61 and the results of the present study, the reaction mechanism of formic acid oxidation with the present PdAg nanotubes is proposed as below: In the direct pathway
HCOOH + Pd f CO2 + 2H+ + 2e- + Pd
(2)
In the indirect pathway
HCOOH + Pd f Pd-CO + H2O
(3)
Pd + H2O f Pd-OH + H+ + e- or Ag + H2O f Ag-OH + H+ + e- (4) Pd-CO + Pd-OH f Pd + CO2 + H+ + e- or +
-
Pd-CO + Ag-OH f Pd + Ag + CO2 + H + e
(5)
Although the formic acid oxidation is a complicated process, formic acid oxidation proceeds mainly through the dehydrogenation reaction path on the basis of the high CO tolerance of the synthesized PdAg nanotubes shown in CV measurements. 3.2.3. Chronoamperometric Analyses. To further evaluate the activity and stability of the PdAg nanotubes for formic acid electrooxidation, chronoamperometric analyses were also carried out at different initial potentials. Figure 7A-C shows the J-t curves at -0.15, +0.3, and +0.68 V in 0.1 M HClO4 + 0.5 M HCOOH, which corresponds, respectively, to the onset, peak, and end of the formic acid direct oxidation. From the chronoamperometric curves at the three potentials, one can see that both the maximum initial and the steady-state oxidation current densities were obtained with the PdAg-100/GC electrode. The chronoamperometric measurements indicate again that PdAg100 has better activity and stability for formic acid oxidation, in good agreement with the voltammetric results. On comparison of the J-t curves, it can be seen that the maximum initial and steady-state oxidation current densities obtained from the present
Figure 7. Chronoamperometric curves of the bulk Pd (black curves), PdAg-25/GC (red curves), and PdAg-100/GC (green curves) electrodes in 0.1 M HClO4 + 0.5 M HCOOH solution at different electrode potentials: (A) -0.15, (B) +0.3, and (C) +0.68 V.
PdAg nanotubes are much larger than that from the bulk Pd electrode over the entire time period examined, which agrees well with the CV measurements displayed in Figure 6. It should be noted that the initial current density on the bulk Pd electrode decays much more rapidly than those of the as-synthesized PdAg nanotubes. The chronoamperometric result indicates that the present PdAg nanotubes exhibit excellent catalytic activity and stability for formic acid oxidation. 3.3. Electrochemical Impedance Studies of Formic Acid Oxidation with Nanoneedle-Covered PdAg Nanotubes. In recent years, electrochemical impedance spectroscopy (EIS) has been used as a powerful and sensitive technique to study the kinetics of electron-transfer processes on both the anode and the cathode of fuel cells.63-66 Here, the electrooxidation dynamics of formic acid catalyzed by the synthesized PdAg nanotubes was also examined with EIS. Figure 8A,B shows the complex-plane (Nyquist) impedance plots of the PdAg-25/GC and PdAg-100/GC electrodes, respectively, in 0.5 M HCOOH + 0.1 M HClO4 with varied electrode potentials (shown as figure legends). It can be seen that, on the PdAg-25/GC electrode, the impedance arcs are located within the first quadrant, and the diameter of the arcs increases from +0.10 to +0.60 V, which may be ascribed to the formation
Nanoneedle-Covered Pd-Ag Nanotubes
Figure 8. Complex-plane (Nyquist) impedance plots of formic acid oxidation on PdAg-25/GC (A) and PdAg-100/GC (B) electrodes in 0.1 M HClO4 + 0.5 M HCOOH at various electrode potentials, which are given in the figure legends. The solid lines show some representative fits to the experimental data based on the equivalent circuits in Figure 10.
and increasing accumulation of CO intermediate species on the electrode surface. With further increasing of the electrode potentials (+0.65 to +0.70 V), an interesting feature of the impedance plots can be observed that the impedances appear in the second quadrant instead of the conventional first one. Such negative Faradaic impedance has also been observed for the electrooxidation of organic small molecules on other electrocatalysts,67-69 suggesting the presence of an inductive component. This may be due to the formation of chemisorbed hydroxyl species in this potential range, which enhances the oxidative removal of the adsorbed CO intermediate, consistent with the voltammetric response (Figure 6) where a small anodic shoulder is observed in the potential range. At more positive electrode potentials (+0.80 V), the impedance arcs return to the first quadrant. The impedance variation trend with the electrode potentials at the PdAg-100/GC electrode is similar to that on the PdAg25/GC electrode. However, two important different behaviors should be noted at the PdAg-100/GC electrode. First, the onset potential of negative impedance (+0.30 V) is somewhat much more negative than that with the PdAg-25/GC electrode (+0.65 V), implying that the oxidative removal of CO is more facilitated at the PdAg-100 nanotubes. Second, it can also be seen from Figure 8 that the diameter of the impedance arcs at the PdAg100/GC electrode is significantly smaller than that at the PdAg25/GC electrode, indicating substantially lower charge-transfer
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Figure 9. Complex-plane (Bode) impedance plots of formic acid oxidation on PdAg-25/GC (A) and PdAg-100/GC (B) electrodes in 0.1 M HClO4 + 0.5 M HCOOH. The electrode potentials are shown as figure legends.
resistance for formic acid oxidation; that is, the catalytic activity of PdAg-100 nanotubes is much higher than that of PdAg-25 nanotubes for formic acid electrooxidation. Figure 9 depicts the corresponding Bode plots of formic acid oxidation at the two electrodes. It can be seen that there is a maximum phase angle at a characteristic frequency (f) for both electrodes. As f usually represents the time constant of the corresponding electrochemical reaction,63,67 it can be seen from Figure 9 that the electron-transfer rate decreases first and then increases with increasing potential, which is consistent with the Nyquist plots. It should be noted that, at potentials where the negative impedance appears in the Nyquist plots shown in Figure 8, there is an abrupt jump between the positive and negative phase angles, which is ascribed to the inductive characteristic arising from the oxidation of surface-adsorbed CO and the recovery of the catalytic centers. On comparison of panels A and B in Figure 9, it can be seen that the overall characteristic frequency (f) obtained at the PdAg-100/GC electrode is much higher than that at the PdAg-25/GC electrode, indicating a faster reaction rate of formic acid oxidation on the PdAg-100 nanotubes, in good agreement with the smaller charge-transfer resistance on this electrode derived from Nyquist plots in Figure 8. On the basis of the voltammetric and impedance results, the equivalent circuit shown in Figure 10 is used to fit the EIS data. Figure 10A depicts the equivalent circuit for the electrodes that exhibit normal impedance behaviors, where RS represents the solution resistance and CPE (constant-phase element) and RCT are the double layer capacitance and charge-transfer resistance, respectively. For negative impedance, the equivalent circuit is
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Figure 10. Equivalent circuits for the electrooxidation of formic acid on the PdAg nanotubes: (A) for normal impedance and (B) for negative impedance shown in Figures 8 and 9.
Figure 11. Charge-transfer resistance (RCT) of formic acid electrooxidation at different electrode potentials on PdAg-25/GC (black curve) and PdAg-100/GC (red curve) electrodes. Data are obtained by curve fitting of the impedance spectra in Figure 8 with the equivalent circuits shown in Figure 10.
shown in Figure 10B, where C0 and R0 represent the capacitance and resistance of the electrooxidation of adsorbed CO intermediates. The representative fits (solid lines) for the electrodes are shown in each of the Nyquist plots in Figure 8. From the fitting, the variation of the charge-transfer resistance (RCT) with the potentials on the electrodes is shown in Figure 11. Chargetransfer resistance is a main parameter to evaluate the inherent speed of the charge-transfer step of an electrode reaction. A lower RCT means the fast charge-transfer step involved in the electrode reaction. Overall, the RCT at the PdAg-100 nanotubes is much lower than that at the PdAg-25 nanotubes, indicating that the electron-transfer kinetics for formic acid oxidation at the PdAg-100 nanotubes is much better facilitated. It should be noted that the RCT obtained with FePt nanoparticles toward formic acid oxidation is of the order of magnitude of MΩ;56,70 that of the present PdAg nanotubes is KΩ (Figure 11). That is, the RCT at the PdAg nanotubes is almost 3 orders of magnitude smaller than that at the FePt nanoparticle. This implies that, compared to nanoparticles, metal nanotubes are a much better kind of electrocatalyst for promoting the electron-transfer kinetics, which may be ascribed to the unique tubular hollow structure and highly rough surface.
Nanoneedle-covered palladium-silver nanotubes were synthesized by a galvanic displacement reaction with Ag nanorods at 100 °C (PdAg-100) and room temperature (PdAg-25). The surface morphology and the crystalline structures were characterized with TEM, HRTEM, SEM, EDX, and XRD measurements. It was found that the surface of the hollow PdAg nanotubes is densely covered with Pd nanoneedles. From the examination of the HRTEM images, the surfaces of the synthesized nanotubes are actually decorated with Pd nanocrystals with the dimension of 5-8 nm. Such nanostructures with a large surface area makes them the promising electrocatalysts applied in fuel cells. The EDX and XRD results showed that Pd and Ag coexist in the nanostructured materials and the synthesized PdAg nanotubes are mainly composed of the Pd(111) plane. The PdAg nanotubes show high electrocatalytic activity toward the formic acid oxidation in the electrochemical cyclic voltammetric studies. Compared with PdAg-25, PdAg100 shows a much higher catalytic activity with a larger current density and more negative peak potential of formic acid oxidation. With chronoamperometric analyses, PdAg-100 also shows higher stability than PdAg-25 nanotubes at various potentials. From the electrochemical impedance spectroscopy (EIS) measurements, the electron-transfer kinetics of HCOOH oxidation on the PdAg nanotubes change from resistive to pseudoinductive and then return to inductive characters with the appearance of positive and negative impedance in Nyquist plots. The charge-transfer resistance (RCT) obtained on PdAg100 nanotubes is much lower than that from PdAg-25, suggesting the highly facilitated electron-transfer kinetics for formic acid oxidation at the PdAg nanotubes synthesized at 100 °C. Overall, the present study shows that the PdAg nanotubes synthesized at 100 °C exhibit excellent electrocatalytic activity toward formic acid oxidation, which makes them efficient anode electrocatalysts for fuel cells. Acknowledgment. This work was supported by the startup funds for scientific research, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Supporting Information Available: Additional UV-vis absorption spectrum and TEM, SEM, and electrochemical measurements of the nanomaterials. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Dillon, R.; Srinivasan, S.; Arico, A. S.; Antonucci, V. International activities in DMFC R&D: status of technologies and potential applications. J. Power Sources 2004, 127 (1-2), 112–126. (2) Lamy, C.; Lima, A.; LeRhun, V.; Delime, F.; Coutanceau, C.; Leger, J. M. Recent advances in the development of direct alcohol fuel cells (DAFC). J. Power Sources 2002, 105 (2), 283–296. (3) Rice, C.; Ha, R. I.; Masel, R. I.; Waszczuk, P.; Wieckowski, A.; Barnard, T. Direct formic acid fuel cells. J. Power Sources 2002, 111 (1), 83–89. (4) Zhu, Y. M.; Ha, S. Y.; Masel, R. I. High power density direct formic acid fuel cells. J. Power Sources 2004, 130 (1-2), 8–14. (5) Willsau, J.; Heitbaum, J. Analysis of adsorbed intermediates and determination of surface-potential shifts by Dems. Electrochim. Acta 1986, 31 (8), 943–948. (6) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. Temperature-dependent methanol electrooxidation on well-characterized PtRu alloys. J. Electrochem. Soc. 1994, 141 (7), 1795–1803. (7) Oetjen, H. F.; Schmidt, V. M.; Stimming, U.; Trila, F. Performance data of a proton exchange membrane fuel cell using H-2/CO as fuel gas. J. Electrochem. Soc. 1996, 143 (12), 3838–3842. (8) Lim, B.; Jiang, M. J.; Camargo, P. H. C.; Cho, E. C.; Tao, J.; Lu, X. M.; Zhu, Y. M.; Xia, Y. A. Pd-Pt bimetallic nanodendrites with high activity for oxygen reduction. Science 2009, 324 (5932), 1302–1305.
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