Polyamide 6 Nanofibers for Electrooxidation

In recent years, direct ethanol fuel cells (DEFCs) have gained more and more attention due to the facts that ethanol has no toxicity compared to metha...
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Free-Standing Palladium/Polyamide 6 Nanofibers for Electrooxidation of Alcohols in Alkaline Medium Liang Su, Wenzhao Jia, Ashley Schempf, Yu Ding, and Yu Lei* Department of Chemical, Materials and Biomolecular Engineering, UniVersity of Connecticut, 191 Auditorium Road, Storrs, Connecticut 06269 ReceiVed: June 15, 2009; ReVised Manuscript ReceiVed: July 19, 2009

Palladium/polyamide 6 (Pd/PA6) nanofibers with high surface area have been successfully prepared using an electrospun PA6 nanofibrous template and a simple electroless plating method. The Pd/PA6 nanofibers possess an average diameter of 322 nm with a Pd layer thickness of ∼85 nm and show excellent mechanical property, good conductivity, and high porosity. The as-prepared Pd/PA6 nanofibers are successfully applied as freestanding electrocatalytic electrodes for alcohol oxidation in alkaline medium without using any conductive support. The results show that the free-standing Pd/PA6 nanofibers greatly promote the oxidation of ethanol in alkaline medium with high activity and stable performance. The peak current density was affected by both ethanol and KOH concentrations, while the corresponding peak potential shift was interestingly found to depend on the value of (CKOH/Cethanol) and the change of peak position was given as ∆Ep ) -0.1578 ln(R2/ R1) for R(CKOH/Cethanol) in the range of 1-10 and ∆Ep ) -0.0966 ln(R2/R1) for R in the range of 10-50. The excellent performance of the free-standing Pd/PA6 nanofibers in ethanol oxidation is mainly attributed to the large surface area, reduced diffusion resistance, and superior catalytic property. In addition, the as-prepared Pd/PA6 nanofibers can also be applied to catalyze the electrooxidation of methanol and isopropanol. These results demonstrate that the free-standing Pd/PA6 nanofibers have great prospect in the application of direct alcohol fuel cells. 1. Introduction Fuel cells convert chemical energy directly into electrical energy with attractive work efficiency and environmental benefit.1 Direct methanol fuel cells (DMFCs), which apply methanol as the fuel in an anode to generate electrons, have been extensively investigated as a promising candidate of power source for portable electronic devices and electric-powered vehicles.2-4 However, the toxicity of methanol potentially limited the widespread application of DMFCs. In recent years, direct ethanol fuel cells (DEFCs) have gained more and more attention due to the facts that ethanol has no toxicity compared to methanol and it can be massively produced by the fermentation of sugar-containing materials from agriculture.5 In addition, the energy density of ethanol (30 MJ/kg) is higher than that of methanol (19.7 MJ/kg). Platinum-based binary and ternary alloys are widely accepted as the most active anode electrocatalyst for direct alcohol fuel cells (DAFCs) operated in acidic medium.1,5-10 Nevertheless, the high price and the limited presence of platinum in nature prevent it from further commercialization. In addition, the alcohol oxidation reaction in acidic medium usually suffers from the sluggish kinetics,11 which can be significantly improved when the fuel cells are operated in basic electrolyte.12,13 The operation in alkaline medium also makes it possible to take advantage of other noble metal-based catalysts in DAFC application. On the basis of the discussion above, alkaline DEFCs with non-Pt-based catalysts are highly preferred. Recently, Xu et al. have reported that the electrocatalytic performance of palladium is much better than that of platinum for ethanol oxidation reaction (EOR) in alkaline electrolyte.13 * Corresponding author. E-mail: [email protected].

Shen et al. have shown that Pd/C promoted with oxide electrocatalysts was superior to Pt-based catalysts in alkaline media in terms of catalytic ability and poisoning tolerance.12,14,15 The experimental results from other groups also verified the synergetic effect of the additives and/or substrate with Pd on EOR in alkaline solution.11,16 In addition, the existence of Pd on the earth is at least 50 times more abundant than that of Pt,17 making Pd extremely attractive in DEFCs application. Recently, different Pd nanostructures have been explored and show impressive performance in EOR. Wang et al. successfully fabricated a highly ordered Pd nanowire array which had higher activity than that of commercial E-TEK PtRu/C catalyst.18-20 However, these Pd nanomaterials required either an entrapment matrix, complicated synthetic route, or conductive support. Therefore, there still remains a need for a simple process to fabricate Pd nanomaterials with superior electrocatalytic property. Preferably, the novel Pd nanomaterials are free-standing, possess excellent mechanical property, have a large surface area, and can be easily manipulated and directly applied as electrode materials. Herein, we report, for the first time, the fabrication of freestanding and conductive Pd/PA6 nanofibers with high surfaceto-volume ratio by the electroless plating method. An electrospun PA6 nanofibrous membrane was used as the plating template. The electroless plating was selected in this study because it is a well-established technique and provides a simple and economical approach to deposit the metal of interest uniformly onto the conductive or nonconductive substrates.21-28 Field-emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX), and X-ray diffraction (XRD) were applied to characterize the samples. The electrocatalytic activity of the as-prepared free-standing nanofibers for ethanol oxidation was intensively investigated in alkaline medium. In addition,

10.1021/jp905606s CCC: $40.75  2009 American Chemical Society Published on Web 08/18/2009

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Figure 1. SEM images of PA6 template (A), Pd/PA6 nanofibers with low (B) and high magnification (C), and XRD patterns of Pd/PA6 nanofibers (D). The insets in Figure 1A-C are a typical SEM image of PA6 with higher magnification, the diameter size distribution of Pd/PA6 nanofibers, and EDX analysis for Pd/PA6 nanofibers, respectively.

its application in the electrooxidation of methanol and isopropanol was also demonstrated. This study provides a simple strategy to fabricate free-standing Pd/PA6 nanofibers which is shown to be promising in the application of direct alcohol fuel cells. 2. Experimental Section Materials. Polyamide 6 (PA6), formic acid, palladium(II) chloride, hydrazine, ethanol, and isopropanol were purchased from Sigma-Aldrich, Inc., USA. Stannous chloride, Na2EDTA, hydrochloric acid (37 wt %), ammonium hydroxide (28 wt %), potassium hydroxide (KOH), and methanol were bought from Fisher Scientific, Inc., USA. All the solutions were prepared with deionized water (18.2 MΩ) and purged with ultrahigh purity nitrogen (Airgas, USA) before the experiments. Preparation of PA6 Nanofibrous Membrane through Electrospinning. An amount of 20 wt % PA6 in formic acid was prepared, electrospun using a 19 gauge needle with a flow rate of 0.3 mL/h at an applied potential of 18 kV, and collected on an aluminum foil with a collection distance of 10 cm. After 30 min electrospinning, a relatively thick PA6 nanofibrous membrane was obtained and then peeled off from aluminum foil for the electroless plating of palladium. Preparation of Pd/PA6 Nanofibers through Electroless Plating of Palladium. To coat the PA6 nanofibers with palladium, the PA6 nanofibrous template was first sensitized and activated in SnCl2 (1 g/L) and PdCl2 (0.25 g/L) acidic solutions sequentially, and the procedure was repeated three times. A redox reaction occurs in which surface-adsorbed Sn2+ is oxidized to Sn4+ and Pd2+ is reduced to form Pd nuclei. The activated PA6 template was then immersed into 5 mL of electroless plating solution which consists of PdCl2 (15 mM), Na2EDTA (0.11 M), and NH3 · H2O (3.17 M). With the addition of hydrazine (reducing agent) to a final concentration of 11 mM in the aforementioned solution, the Pd precursor was reduced and deposited on the activated template surface. After 2 h of electroless plating, Pd-coated PA6 nanofibers were formed, rinsed with deionized water, and dried at room temperature.

Characterization. A JEOL 6335F field emission scanning electron microscope (FESEM) and energy-dispersive X-ray spectrometer (EDX) were employed to examine the morphology and the elements of the as-prepared nanofibers. The average diameter of Pd/PA6 nanofibers and the size distribution were calculated using ∼50 randomly chosen nanofibers in the SEM image. The thickness of the Pd layer was estimated by the difference between the average radius of nanofibers before and after Pd deposition. An X-ray diffraction (XRD) pattern was collected using Cu KR radiation on a diffractometer. Electrooxidation of Alcohol. The investigation of alcohol oxidation was carried out using a homemade plate material evaluating three-electrode cell (Figure 1S, Supporting Information) connected to a CHI601C electrochemical workstation (CH Instruments, USA) at room temperature. The free-standing Pd/ PA6 nanofibers were applied as the working electrode, while a platinum wire and a Hg/HgO (MMO, 1.0 M KOH, 0.098 V vs SHE) electrode were used as the counter and reference electrode, respectively. All of the electrochemical experiments were performed under a nitrogen environment. 3. Results and Discussion Electrospinning is a straightforward and versatile technique to produce continuous polymer nanofibers with a large surfaceto-volume ratio.29-31 In this research, PA6 is chosen to prepare the nanofibrous membrane which serves as the template for electroless plating due to its excellent chemical stability and outstanding mechanical strength.28,32,33 Figure 1A shows a typical SEM image of the electrospun PA6 nanofibrous membrane. The surface of PA6 nanofibers is smooth with an average diameter of 152 nm. After sensitization and activation by SnCl2 and PdCl2, respectively, the color of the PA6 nanofibrous membrane changed from white to brown, indicating the formation of Pd nuclei on the surface of PA6 nanofibers. The activated PA6 nanofibrous template was then transferred to a Pd electroless plating solution. Upon the addition of the reducing agent hydrazine, the palladium deposition on PA6 nanofibers was triggered according to the following chemical reaction

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2Pd(NH3)2+ + N2H4 + 4OH- f 2Pd0 + 2NH3 + N2 + 4H2O

(1) Figure 1B and C shows typical SEM images of the asprepared Pd/PA6 nanofibers with different magnification. The average diameter of the Pd/PA6 nanofibers is ∼322 nm, and the size distribution is presented in the inset of Figure 1B. As the average diameter of the PA6 nanofibrous template is ∼152 nm, the thickness of the coated Pd layer is calculated to be ∼85 nm. In addition, compared to the smooth surface of the PA6 nanofibrous template, the synthesized Pd/PA6 nanofibers had a rough grainy surface which was clearly observed in the SEM images (Figure 1B and C), reflecting the fact that Pd deposition grows from the nuclei during the electroless plating process. The successful coating of Pd on PA6 nanofibers was also confirmed by EDX spectrum analysis. As shown in the inset of Figure 1C, only the Pd element was detected by EDX which can be attributed to the full coverage of the Pd layer on the surface of PA6 nanofibers. The crystal structure of the coated Pd layer was further characterized by XRD, and the pattern is presented in Figure 1D. The diffraction peaks located at 2θ values of 40.15°, 46.70°, 68.18°, 82.18°, and 86.70° are assigned to the (111), (200), (220), (311), and (222) facets of Pd (JCPDS card 46-1043), respectively, which demonstrate a typical facecentered cubic (fcc) crystalline structure. More interestingly, the as-prepared Pd/PA6 nanofibers inherit the excellent mechanical strength of PA6 and superior conductivity of metallic Pd, endowing the Pd/PA6 nanofibers with free-standing capability, mechanical stability, excellent conductivity, and high porosity. Bending, folding, or twisting of the as-prepared Pd/ PA6 nanofibrous membrane would not destroy its morphology or conductivity, making it an excellent electrocatalytic material in the application of ethanol electrooxidation. Figure 2 shows the typical cyclic voltammogram for the electrooxidation of 0.1 M ethanol in 0.6 M KOH solution with the as-prepared free-standing Pd/PA6 nanofibrous working electrode. The potential was scanned from -1.0 to 0.5 V at a sweep rate of 50 mV/s. As seen in Figure 2, the ethanol oxidation starts at -0.772 V (vs MMO), and a current peak centered at -0.059 V (vs MMO) is observed during the forward scan while a peak centered at -0.265 V (vs MMO) is presented during the backward scan. The well-defined forward and backward scan peaks indicate the formation of the absorbed hydroxyl and acetyl group on the active sites of Pd34,35 and the removal of the surplus carbonaceous species which were formed

Figure 2. Cyclic voltammograms of 0.6 M KOH + 0.1 M ethanol and 0.6 M KOH without ethanol (background) on a free-standing Pd/ PA6 nanofibrous working electrode at a scan rate of 50 mV/s.

during the forward scan,36 respectively. As a comparison, no obvious oxidation peak can be observed in 0.6 M KOH solution in the absence of ethanol. According to following reactions proposed by Liu et al.34 and Liang et al.35 Pd + OH- T Pd-(OH)ads + e-

(I)

Pd + CH3CH2OH + 3OH- T Pd-(CH3CO)ads + 3H2O + 3e-

(II) Pd-(OH)ads + Pd-(CH3CO)ads f CH3COOH + 2Pd

(III)

CH3COOH + OH- f CH3COO- + H2O

(IV)

a simplified expression with regard to current density can be derived from the rate-determining step (III) (rds)

j ) kθPd-(OH)adsθPd-(CH3CO)ads

(2)

where k is the rate constant; θPd-(OH)ads and θPd-(CH3CO)ads are the coverage of the adsorbed hydroxyl and acetyl group, respectively. One can see that ethanol and OH- concentrations have great effects on the electrooxidation of ethanol in terms of anodic current density and peak potential. Therefore, the experiments were conducted to investigate the effect of ethanol and KOH on EOR. Figure 3 shows the effect of ethanol concentration on electrooxidation of ethanol in alkaline medium. KOH concentration was maintained at 0.6 M, and the ethanol concentration was varied from 0.05 to 0.2 M. As seen in Figure 3, the anodic peak current density during the forward scan increases with the ethanol concentrations examined, suggesting that the electrooxidation of ethanol shows concentration-dependent behavior and is not yet saturated under the investigated ethanol concentration range. The result is also in accordance with the proposed mechanism in eqs I-IV. At low ethanol concentration, Pd active sites may not be fully covered, thus the peak current density is determined by θPd-(OH)ads and θPd-(CH3CO)ads. At steady state, θPd-(OH)ads is constant at a fixed KOH concentration, while θPd-(CH3CO)ads is proportional to the concentration of ethanol to a certain degree when ethanol concentration is low, resulting in the concentration-dependent anodic peak current density. In addition, a

Figure 3. Cyclic voltammograms of 0.6 M KOH solutions with different ethanol concentrations using a free-standing Pd/PA6 nanofibrous working electrode at a scan rate of 50 mV/s.

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Figure 4. Forward (A) and backward (B) scan peaks in cyclic voltammograms of 0.1 M ethanol alkaline solutions with different KOH concentrations on a free-standing Pd/PA6 nanofibrous working electrode at a scan rate of 50 mV/s. (C) The changes of peak current density and Ib/If value with the ratio of KOH concentration to ethanol concentration. (D) Peak potential as a function of CKOH/Cethanol.

positive shift in peak potential is also observed as ethanol concentration increases, indicating that at a fixed KOH concentration of 0.6 M, the lower concentration of ethanol favors the electrooxidation of ethanol on the free-standing Pd/PA6 nanofibrous electrode. Figure 4 shows the effect of KOH concentration on ethanol electrooxidation by fixing the ethanol concentration at 0.1 M and varying KOH concentration from 0.2 to 5 M. Figure 4A and B displays the forward and backward scan of the cyclic voltammograms, respectively, and Figure 4C shows the corresponding relationship between KOH concentrations (equivalent of CKOH/Cethanol as ethanol concentration is fixed) and forward scan peak current density (jp) as well as the peak current ratio of the backward to forward scan (Ib/If). The jp and Ib/If are critical parameters to evaluate the performance of the catalyst in the electrooxidation of alcohols. Generally a higher jp value will provide a larger working current of the fuel cell, while a smaller Ib/If value indicates higher catalytic efficiency and better tolerance to the poisoning species arising from the formation of the carbonaceous intermediate on the catalyst during the reaction.11,37 From Figure 4C, one can see that the anodic peak current density increases with the increase of KOH concentration initially and then reaches a maximum value at 0.6 M KOH. Further increase of KOH concentration has an inverse effect on the current density. A similar trend was observed for Ib/If, and the maximum value was achieved at 1 M KOH. On the basis of eqs I-IV and assuming that the adsorption-desorption steps are much faster than the surface reaction step (rds), the initial increase of KOH concentration could greatly facilitate the adsorption of the hydroxide ion as well as ethanol on Pd

active sites, resulting in the increase of both θPd-(OH)ads and θPd-(CH3CO)ads, and If accordingly. In addition, the formation rate of Pd-(CH3CO)ads in eq II is much faster than its consumption rate in eq III (rds), leading to the increase of Ib as well. Nevertheless, the increase of Ib is bigger than that of If, causing a net increase of the Ib/If value. However, when the KOH concentration is too high, there is not a sufficient active site on the Pd surface, and further increase of KOH concentration results in the adsorption of OH- being dominant and hence reduces the coverage of the acetyl group, causing the decrease of both If and Ib but with a higher magnitude for the latter. Consequently, the Ib/If value decreases. The result also indicates that there seems to be no difference between the active sites for ethanol and OH- on the surface of Pd, and their adsorptions follow a competitive mechanism when the available active sites are insufficient. Among all KOH concentrations investigated at the fixed ethanol concentration of 0.1 M, the highest (also worst) Ib/If value (0.55 at 1 M KOH) is still lower than reported elsewhere.11 The enhanced performance may be attributed to the large surface area, reduced diffusion resistance, good catalytic ability, and excellent poisoning tolerance of the freestanding Pd/PA6 nanofibrous catalyst. Another remarkable phenomenon observed in Figure 4A is the negative shift of the peak position with the increase of KOH concentration, which suggests that at a fixed ethanol concentration of 0.1 M, the kinetics of ethanol oxidation in alkaline solution always favors higher KOH concentration.34,35 The value of the potential shift has been proposed to depend on the pH change in the solution.34,35 However, the peak shift on the freestanding Pd/PA6 nanofibrous electrode can also be observed in

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Figure 5. Cyclic voltammograms of alkaline ethanol solution with different concentrations at a fixed CKOH/Cethanol of 3 (A), 6 (B), and 9 (C) using a free-standing Pd/PA6 nanofibrous working electrode at a scan rate of 50 mV/s. (D) The change of peak current density with regard to ethanol concentration and CKOH/Cethanol value.

TABLE 1: Comparison of Peak Potential between the Experimental Value and the Predicted Value at Different CKOH/Cethanol ratio

peak potential

CKOH/Cethanol

mean value (V)

σmean (V)

predicted value (V)

relative deviation

3 6 9

0.0544 -0.0573 -0.1173

0.00747 0.00425 0.00217

0.0529 -0.0564 -0.1204

2.76% 1.52% 2.59%

Figure 3 where the KOH concentration is kept constant. Therefore, combining the results from Figure 3 and Figure 4A, one can expect that for the developed free-standing Pd/PA6 nanofibrous electrode, the shift of peak potential seems to be determined by the ratio of KOH concentration to ethanol concentration instead of any individual one. Figure 4D gives the correlation between peak potential and the ratio of KOH concentration to ethanol concentration (CKOH/Cethanol) in the range from 1 to 50. The value of the peak potential shift was found to be proportional to the difference of the natural logarithm of CKOH/Cethanol. For the value of CKOH/Cethanol in the range of 1 to 10, ∆Ep ) -0.1578 ln(R2/R1) with R2 ) 0.9990, while in the range of 10 to 50, ∆Ep ) -0.0966 ln(R2/R1) with R2 ) 0.9999. To test the credibility of the regression equation, a series of experiments were conducted by fixing the value of CKOH/Cethanol while varying CKOH and Cethanol simultaneously. Figure 5A-C shows the relationships between peak potential and the ratio of

KOH concentration to ethanol concentration. When CKOH/Cethanol was fixed, the forward scan anodic peaks appear in almost the same position with insignificant deviation. However, the peak potentials shift negatively with increasing CKOH/Cethanol values. The results support that the ratio instead of KOH concentration or ethanol concentration alone is the determinante factor for the shift of peak potential on the as-prepared Pd/PA6 nanofibers. It is hypothesized that at a fixed ratio, the spatial distribution of the reactants in the bulk solution and near the catalyst surface may follow a similar pattern, which provides the same possibility for either KOH or ethanol to access the active sites on Pd surface. Therefore, the electrooxidation of ethanol at a fixed CKOH/Cethanol value follows the same kinetics which results in the identical peak potential but with diverse peak current density. With the increase of CKOH/Cethanol value, the peak potential shifts negatively since a higher KOH to ethanol ratio will provide a greater possibility for KOH to access the active sites on the catalyst, which can facilitate the formation of both Pd(CH3CO)ads and Pd-(OH)ads, thus improving the kinetics of the ethanol oxidation. Equation Ep ) -0.1578 ln(CKOH/Cethanol) + 0.2263 for CKOH/Cethanol between 1 and 10 is applied to predict the peak potentials with different CKOH/Cethanol values ((CKOH/ Cethanol) ) 3, 6, 9) which are presented in Table 1. One can see that the predicted peak potential and the experimental value match very well with a small standard deviation of the mean and an insignificant relative deviation, validating the equation

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Figure 6. Stability analysis of the free-standing Pd/PA6 nanofibrous catalyst. (A) Cyclic voltammetry of 0.2 M ethanol + 1 M KOH solution for 20 cycles at a scan rate of 50 mV/s. (B) Chronoamperometry of 0.1 M ethanol + 0.6 M KOH solution at a working potential of -0.1 V (vs MMO).

Figure 7. Cyclic voltammograms of 0.1 M methanol (A) and isopropanol (B) in 0.6 M KOH solution with a free-standing Pd/PA6 nanofibrous working electrode at a scan rate of 50 mV/s.

as well as the importance of the ratio in the determination of peak potential. Figure 5D is the 3D mesh plot of Figure 5A-C and shows the changes of peak current density with respect to ethanol concentration as well as the ratio of KOH concentration to ethanol concentration. According to the mesh plot, the following results are obtained. First, at a fixed ratio under the tested concentrations, the adsorptions of CH3CO and OH are far from saturated. Therefore, higher concentrations of ethanol and KOH result in higher values of both θPd-(OH)ads and θPd-(CH3CO)ads, thus a higher current density. Second, when the ethanol concentration is low and kept constant, a bigger CKOH/Cethanol value will give a higher peak current density. It can be explained as follows. When the fixed ethanol concentration is low, there are enough active sites for both ethanol and OH-. Therefore, increasing the CKOH/Cethanol value (equivalent of increasing KOH concentration) will augment the coverage of (OH)ads as well as (CH3CO)ads, giving a higher peak current density. On the contrary, at a higher fixed ethanol concentration, a smaller CKOH/ Cethanol value will result in a higher peak current density since there are not enough active sites for both ethanol and hydroxide ion when their concentrations are sufficiently high. Ethanol and OH- compete for the active sites. The higher CKOH/Cethanol value means higher KOH concentration compared to ethanol concentration, thus OH- adsorption is dominant, and the coverage of

the acetyl group (θPd-(CH3CO)ads) is reduced, resulting in the decrease of current density. Such a conclusion is also in agreement with the previous observation. The intermediate carbonaceous species formed during electrooxidation of ethanol in alkaline medium may poison the catalyst and suppress its performance accordingly. Therefore, the stability of the as-prepared Pd/PA6 nanofibers for ethanol electrooxidation was investigated by cyclic voltammetry (CV) with multicycles and chronoamperometry (CA). Figure 6A shows the cyclic voltammograms of ethanol electrooxidation in 1.0 M KOH containing 0.2 M ethanol at a scan rate of 50 mV/s. One can see that the peak current density during the first cycle is 34.76 mA/cm2 while the peak current density for the twentieth cycle is 32.87 mA/cm2, which is just 5.4% less than that during the first scan. The result indicates that Pd/PA6 nanofibrous catalysts are stable catalysts for ethanol oxidation. The CA in Figure 6B also shows the good stability of the asprepared Pd/PA6 nanofibrous electrocatalyst in alkaline medium, which is in good agreement with the result obtained by CV. The performance of the free-standing Pd/PA6 nanofibers in the electrooxidation of other alcohols such as methanol and isopropanol was also investigated and presented in Figure 7. The well-defined forward and backward scan peaks can be observed for both methanol and isopropanol. The peak current density and the onset potential for methanol and isopropanol are 21.0 mA/cm2, -0.6542 V, and 12.7 mA/cm2, -0.7484 V, respectively, which indicates the good electrocatalytic activity of the as-prepared Pd/PA6 nanofibers for the electrooxidation of methanol and isopropanol. These results demonstrate that the free-standing Pd/PA6 nanofibers have great potential application in various direct alcohol fuel cells. 4. Conclusions In summary, free-standing Pd/PA6 nanofibers with highly porous structure and excellent mechanical property have been successfully fabricated by electrospinning and an electroless plating approach. The electrochemical behavior of the asprepared Pd/PA6 nanofibers was intensively investigated for ethanol oxidation reaction in alkaline solution. High current density and low Ib/If on the as-prepared free-standing Pd/PA6 nanofibers were achieved, and the enhanced performance may be attributed to the large surface area, reduced diffusion resistance, and excellent poisoning tolerance. It has also been found that both KOH concentration and ethanol concentration

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affect the forward scan peak current density and the peak position. Especially, the peak potential was found to be dependent on the natural logarithm of the ratio of the KOH concentration to ethanol concentration. Furthermore, its application for effective electrooxidation of methanol and isopropanol was also demonstrated. This study provides a promising route for the facile and cost-effective synthesis of palladium nanofibers, and the as-synthesized direct nanofibers show great prospect in the applications of various alcohol fuel cells. Acknowledgment. We greatly appreciate the funding from NSF, USGS, and DHS. “Points of view in this document are those of the author(s) and do not necessarily represent the official position of the funding agencies.” L.S. also thanks the partial support from UConn Center for Environmental Science and Engineering. Supporting Information Available: Experimental setup of the homemade plate material evaluating cell with free-standing Pd/PA6 nanofibers. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345. (2) Winter, M.; Brodd, R. J. Chem. ReV. 2004, 104, 4245. (3) McNicol, B. D.; Rand, D. A. J.; Williams, K. R. J. Power Sources 1999, 83, 15. (4) Barragan, V. M.; Heinzel, A. J. Power Sources 2002, 104, 66. (5) Antolini, E. J. Power Sources 2007, 170, 1. (6) Ganesan, R.; Lee, J. S. Angew. Chem., Int. Ed. 2005, 44, 6557. (7) Camara, G. A.; de Lima, R. B.; Iwasita, T. Electrochem. Commun. 2004, 6, 812. (8) Xu, C. W.; Shen, P. K.; Ji, X. H.; Zeng, R.; Liu, Y. L. Electrochem. Commun. 2005, 7, 1305. (9) Wang, H.; Xu, C. W.; Cheng, F. L.; Zhang, M.; Wang, S. Y.; Jiang, S. P. Electrochem. Commun. 2008, 10, 1575. (10) Wei, Z. D.; Li, L. L.; Luo, Y. H.; Yan, C.; Sun, C. X.; Yin, G. Z.; Shen, P. K. J. Phys. Chem. B 2006, 110, 26055. (11) Zhu, L. D.; Zhao, T. S.; Xu, J. B.; Liang, Z. X. J. Power Sources 2009, 187, 80.

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