Simple Synthesis of Au-Pd Alloy Nanowire Networks as Macroscopic

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Simple Synthesis of Au-Pd Alloy Nanowire Networks as Macroscopic, Flexible Electrocatalysts with Excellent Performance Jin Wang, Peina Zhang, Yujiao Xiahou, Dayang Wang, Haibing Xia, and Helmuth Möhwald ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14955 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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ACS Applied Materials & Interfaces

Simple Synthesis of Au-Pd Alloy Nanowire Networks as Macroscopic, Flexible Electrocatalysts with Excellent Performance Jin Wang,a Peina Zhang,a Yujiao Xiahou,a Dayang Wang,b Haibing Xia,*,a and Helmuth Möhwaldc a

State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, P. R. China. E-

mail: [email protected] b

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun, 130012, P. R. China c

Max Planck Institute of Colloids and Interfaces, Potsdam-Golm Science Park, 14476 Potsdam

KEYWORDS: nanowire networks, alloy, electrocatalysts, flexible, carbon fiber cloths

ACS Paragon Plus Environment

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ABSTRACT: The present work introduces a new way to prepare Au-Pd alloy nanowire networks (NWNs) via deposition of Pd atoms onto Au nanowires in reaction media at room temperature without aid of additional reducing agents. Thanks to their excellent colloidal stability in water as well as in ethanol, the resulting NWNs can be utilized to produce composite thin films with Nafion (perfluorinated sulfonic acid) with dimensions above dozens of square centimeters by means of solution casting on the glass substrate. Most importantly, these films can be easily transferred on different solid substrates by lift-off technology. Moreover, the resulting Au-Pd alloy NWNs can also be easily and thoroughly loaded into macroscopic carbon fiber cloth (CFC). Both the Au-Pd alloy NWN/Nafion composite films and the Au-Pd alloy NWN-loaded CFC can be used as flexible electrodes for electrocatalysis of ethanol oxidation, with electrocatalytic performance at different distorted states superior by two orders of magnitude to those reported in literature (e.g. commercial Pd/C catalysts and Pd-based nanostructured catalysts). This work opens new possibilities for large-scale manufacturing of electrodes for fuel cells.


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1. Introduction The escalating demand of portable and wearable electronic devices propels development of new flexible energy storage devices, which are capable of being bent, folded, and stretched into nonplanar forms.1-4 In this context, the development of flexible catalysts as electrodes is the core of up-coming flexible fuel cell technologies.5,6 Thanks to recent advances in alkaline anion exchange membranes (AAEM),7,8 Pd-based catalysts have demonstrated better electrocatalytic performance in ethanol oxidation in alkaline media in fuel cells than Pt-based ones.9,10 Since the electrocatalytic performance is strongly related not only to the chemical composition of catalysts, but also to their size, shape, and surface morphology,9-17 a variety of nanostructured electrocatalysts have been developed in order to achieve a substantially higher surface-to-volume ratio and a great number of active sites. Among them one-dimensional (1D) nanowires (NWs), especially alloyed NWs, are of great interest, mainly because they offer the possibility to form three-dimensional, interwoven networks as self-supported catalysts. These can not only improve the electron transport during catalysis, but also avoid severe corrosion and oxidation of the carbon supports.18-25 However, the synthetic complexity of currently available strategies to produce alloy NWs and their three-dimensional (3D) networks largely limits their range of applicability. For instance, macroscopic, flexible Pd-based electrocatalysts will be technically desirable for development of flexible fuel cells to power wearable electronic devices, which, however, have been hardly implemented so far. To circumvent this technical challenge, we introduced a method to reduce AuCl4– ions into Au+ ions by citrate ions at room temperature to form small Au nanoparticles (NPs) stabilized predominantly by AuCl4– ions with size of about 2 nm (Scheme 1a and 1b) on the basis of our


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and others’ recent mechanistic studies.26-29 However, the stabilization of AuCl4– ions is rather poor, so the resulting small Au NPs tend to severely agglomerate with time.

Scheme 1. Schematic depiction of the synthesis of an Au-Pd alloy NW via deposition of Pd atoms onto Au NW with formation promoted by electrostatic repulsion. (a) The aqueous solution of citrate/HAuCl4 mixtures, (b) formation of small Au nanoparticles (NPs) with size of about 2 nm, (c) longitudinal agglomeration of the resulting Au NPs, (d) growth of Au NPs into Au NW, (e) deposition of Pd atoms on Au NW, (f) formation of Au-Pd alloy NW. On the basis of the anisotropic action model of electrostatic interactions developed in our previous studies,30,31 the electrostatic repulsion due to the coating of AuCl4– ions is expected to be weak but noticeable, which promotes the longitudinal rather than lateral agglomeration of Au NPs (Scheme 1c), thus favoring the growth of Au NPs into Au NWs of small size (Scheme 1d).32 In this context, we present a simple and efficacious strategy for synthesis of Au-Pd alloy NWs (Scheme 1) – via deposition of Pd atoms on Au NWs (Scheme 1e and f) in aqueous solution of citrate/HAuCl4 mixtures at room temperature without aid of additional reducing agents. Note that, to the best of our knowledge, Pd(II) ions cannot be reduced by citrate at room temperature (Figure S1), but only at high temperature (Figure S2).33,34 In addition, these NWs tend to be


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interlaced with each other to form 3D Au-Pd alloy nanowire networks (NWNs). Moreover, thanks to the excellent long-term colloidal stability, the resulting Au-Pd alloy NWNs can be readily mixed with commercial polymers conventionally used in fuel cells such as Nafion (perfluorinated sulfonic acid) to produce macroscopic, flexible films by means of solution casting. Furthermore, they can also be highly loaded within conventional carbon fiber cloth (CFC) via a soaking process. Both Nafion and CFC, loaded with Au-Pd alloy NWNs, can be directly used as flexible electrodes for fuel cells and exhibit two orders of magnitude superior electrocatalytic performance of ethanol oxidation, which is hardly implemented otherwise thus far. 2. RESULTS AND DISCUSSION 2.1. Synthesis of Au-Pd alloy NWNs by deposition of Pd atoms onto Au NWs. In the typical synthesis of Au95.1-Pd4.9 NWNs, an aqueous Na2PdCl4 solution was simply added into an aqueous solution of citrate/HAuCl4 at room temperature without aid of additional reducing agents. The proper incubation time of the aqueous solutions of citrate/HAuCl4 mixtures at room temperature is vital for formation of Au NWs (Figure S3). Au NWs with narrowest width and smallest number of big nodes can be obtained after incubation for about 14 min (Figure S3A-c). At the initial stage, small Au NPs with sizes of about 2 nm were formed (Figure S3A-a), and would gradually grow into Au NWs with sizes smaller than 3 nm (Figure S3A-b). One can see that the sizes of small Au NPs are close to the diameters of Au NWs. Moreover, over the course of incubation of aqueous solutions of citrate/HAuCl4 mixtures at room temperature, the development of a considerably broad surface plasmon resonance band was observed between 500 nm and 750 nm (Figure S3A-d), concomitant with the appearance of dark blue color of the reaction media (Figure S3B). With further growth in length of the resulting Au NWs, their


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plasmonic bands also slightly red-shift. However, the resulting Au NWs comprised many defects, local lattice strain and wrinkling of atomic planes,35,36 which are readily broken into small, elongated NPs during incubation in water at room temperature for a longer period of time (ca. 3 h) (Figure S4A and S5A). Intriguingly, the incubation of the Au NWs in water in the presence of Na2PdCl4 could effectively inhibit the NW breakage and, at the same time, noticeably promote the further growth of Au NWs in length with reaction proceeding (Figure S4B and S5B). It is known that citrate is not able to reduce Pd2+ ions to Pd atoms at room temperature (Figure S1). Thus, the high index facets of the resulting Au NWs, associated with the local lattice strain and wrinkling of atomic planes,36 may be sufficiently reactive for deposition of Pd atoms onto Au NWs to form Au-Pd alloy NWs.

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According to the galvanic reduction (GR) theory, it is

anticipated that Pd metal can reduce Au(III) ions to form Pd2+ ions, but the reduction of Pd2+ ions by Au metal is difficult in ambient environment.38 However, it is indeed found that Pd ions were reduced by smaller Au NWs to form Au-Pd alloy NWs, which are then interlaced with each other to form 3D networks (Au-Pd alloy NWNs). Such a finding is very surprising. It is the first case of Pd growth onto Au NWs in reaction media at room temperature without aid of additional reducing agents. Figure 1a shows a typical transmission electron microscopy (TEM) image of the resulting AuPd alloy NWNs. The resulting NWs in Au-Pd alloy NWNs are not long, straight wires but composed of a number of short wires with varied length connected by big nodes, while these short wires are fairly uniform and as thin as about 9 nm in diameter. Although these NWs tend to be interlaced with each other to form 3D networks, their aqueous dispersions remain stable with hardly noticeable agglomeration at least for 10 days at ambient conditions (Inset in Figure 1a). The revealed excellent colloidal stability of the resulting Au-Pd alloy NWNs is due to the


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excellent stabilization of the citrate coatings on the NW surfaces. Due to the Pd-to-Au molar ratio being as low as 1:14, the high-resolution TEM (HR-TEM) image (Figure 1b) indicates, that the lattice distances are about 0.235 and 0.204 nm of one single Au-Pd alloy NW, which are close to typical d spacings of the (111) and (200) planes of face centered cubic (fcc) Au, respectively.39 The resulting Au-Pd alloy NWNs bear plenty of twin defects at the grain boundaries and also at junctions along the growth direction of the NWs. The twin defects are typically observed on Au NPs prepared via citrate reduction,40 which can improve the electrocatalytic performance due to the strain induced by them.41,42

Figure 1. TEM image (a) of theAu95.1-Pd4.9 alloy NWNs and HR-TEM (b), HAADF-STEMEDS (c and d) images of single Au95.1-Pd4.9 NW, overlay of the HAADF-STEM-EDS image (e) and cross-sectional compositional line profiles (f). The inset in Figure 1a shows photos of the dispersions of the Au95.1-Pd4.9 alloy NWN in water after storage for 1 and 11 days. The FFT pattern of the HR-TEM image of that single Au95.1-Pd4.9 alloy NW is shown in the inset in Figure 1b, and the twin boundaries present within the NW are highlighted by the white arrows in Figure 1b. In energy dispersive spectroscopy (EDS) mapping by high-angle annular dark field-scanning transmission electron microscope (HDAAF-STEM) images (Figure 1c to 1e), the element Au is revealing a clear wire shape (Figure 1c) while the element Pd shows a blurry configuration of a wire shape (Figure 1d); the overlapped image (Figure 1e) demonstrates the formation of an alloy structure. In addition, their cross-sectional compositional line profiles (Figure 1f) reveal a small quantity of elemental Pd with a similar distribution as Au, thus confirming the alloy structure of the resulting Au-Pd alloy NWNs. On the basis of the EDS results, the atomic fractions of Au and


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Pd in as prepared Au-Pd alloy NWNs are about 95.1 % and 4.9 %, hence the NWNs are specifically denoted as Au95.1-Pd4.9 alloy NWNs, which are very close to the values of 93.3% and 6.7% calculated from the molar ratio of Au-to-Pd (14) used for the NWN preparation. The composition of as-prepared Au95.1-Pd4.9 alloy NWNs was also further characterized by using X-ray diffraction (XRD). The diffraction peaks of Au95.1-Pd4.9 alloy NWNs could be indexed as the (111), (200), (220), (311), and (222) planes of a face-centered cubic (fcc) lattice. In our case, the estimated average diameter (D) of single NW in Au-Pd alloy NWNs associated with the strongest (111) reflection of Au-Pd alloy NWNs at 2θ = 38.27° was about 6.4 nm (Figure S6), which in certain degree was in agreement with that observed from TEM images, allowing for experimental errors. The peak positions were in between those of pure Au and those of pure Pd, indicative of the formation of Au-Pd alloy (Figure S6).43 Moreover, the peak positions are closer to those of Au due to the high percentage of Au in the NWNs, also suggesting that a small amount of Pd atoms is doped into the Au crystal lattice in the Au-Pd alloy NWNs.44,45 Furthermore, the chemical composition of Au−Pd alloy NWNs can be estimated by Vegard’s law. Based on the 2θ values of the (111) peak in the XRD pattern, the {111} lattice spacing of Au-Pd alloy NWNs is calculated as 0.2350 nm, which is between the {111} lattice spacing of Pd (0.2245 nm) and that of Au (0.2355 nm), and is also the same as that obtained from HRTEM results. By applying Vegard’s law to the XRD pattern, the atomic percentages of Au and Pd in Au-Pd alloy NWNs are about 95.5 % and 4.5 %, which is good agreement with that obtained from the EDS results. To investigate the effect of Au on the electron structure of Pd, X-ray photoelectron spectroscopy (XPS) was also used to investigate the valence states of Pd in pure Pd NPs and Au95.1-Pd4.9 NWNs, as well as Au in pure Au NPs and Au95.1-Pd4.9 alloy NWNs (Figure 2, Table


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S1 and Table S2). In comparison with Pd 3d5/2 and Pd 3d3/2 binding energies (BE) of pure Pd NPs (335.0 and 340.3 eV), the Pd 3d5/2 and Pd 3d3/2 BEs of the Au95.1-Pd4.9 alloy NWNs (334.9 and 340.2 eV) show a negative shift by about 0.10 eV. Moreover, the Au 4f7/2 BE of Au95.1-Pd4.9 alloy NWNs (83.2 eV) was also negatively shifted by 0.60 eV compared to that of Au(0) of pure Au NPs (83.8 eV). The XPS spectra demonstrate simultaneous negative binding energy (BE) shifts of Pd 3d and Au 4f, which suggests a gain in charge density in the d band but a loss in the sp band, for Pd, indicative of the formation of Au-Pd alloys.46,47

Figure 2. XPS spectra of the Pd 3d signals (A) of pure Pd NPs and Au95.1-Pd4.9 alloy NWNs and XPS spectra of the Au 4f signals (B) of pure Au NPs and Au95.1-Pd4.9 alloy NWNs. 2.2. Electrocatalytic performance of Au-Pd alloy NWNs as macroscopically flexible catalysts. It is well known that the change in surface composition of Au-Pd alloy nanomaterials brings about the alteration of the catalytic performance.48 Thus, to select proper Au–Pd alloy NWNs as macroscopically flexible catalysts, the electrocatalytic performance in ethanol oxidation of a series of Au–Pd alloy NWNs with different Au-to-Pd molar ratios was investigated in detail (Figure S7, Figure 3 and Table 1). After careful assessment of Au–Pd alloy NWNs in each TEM image, it is found that the morphologies of as-prepared Au-Pd alloy NWNs obtained under different Au-to-Pd molar ratios are nearly same (Figure S7) due to the use of low amount of Pd. However, their electrochemically active surface area (ECSA) values are changed from 135.6 to 163.5, 175.0,


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and 174.3 m2 gPd-1 , when the Au-to-Pd ratio in Au-Pd alloy NWNs is increased from 18 to 14, 10 and 6, respectively (Figure 3A). In addition, their mass-normalized current densities are 18.5, 33.1, 23.6 and 10.5 A mgPd-1, respectively (Figure 3B). Although the ECSA value of the resulting Au-Pd alloy NWNs obtained at Au-to-Pd ratio of 14 is not the maximum, they exhibit the maximum mass activity (33.1 A mgPd-1). This may be attributed to their largest specific activity (20.2 mA cm-2, Figure 3C and Table 1), which is related to the surface composition of the resulting Au-Pd alloy NWNs. Thus, the surface composition of the Au-Pd alloy NWNs was further investigated on the basis of our and others' previous work. 49-51 It is known that the surface area of the catalysts can be determined using the surface charge associated with the reduction of their oxide in the CV curves in an alkaline medium. Accordingly, the surface compositions of as-prepared AuPd alloy NWNs can be calculated on the basis of the surface areas of Au and Pd obtained, which can be deduced as follows: m=

S୔ୢ × 100 S୔ୢ + S୅୳

where m represents the Pd percentage, and SPd and SAu are the surface areas covered by Pd and Au oxides, respectively (Figure S8). Consequently, the surface compositions of as-prepared AuPd alloy NWNs were obtained from their CV curves (Figure 3A) and are listed in Table 1. The result further demonstrates the Au-Pd alloyed nature of the resulting Au-Pd alloy NWNs. Moreover, as expected, their surface compositions are really different. The fractions of Au on their surfaces are changed from 54.6% to 40.5%, 34.0%, and 17.1%, when the Au-to-Pd molar ratio in Au-Pd alloy NWNs is increased from 18 to 14, 10 and 6, respectively. Furthermore, on the basis of their catalytic performance, it is found that the optimal Au-to-Pd ratio on the surfaces of Au-Pd alloy NWNs is about 2:3. The results are in good agreement with theoretical simulations.52


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According to the results reported in literature,18,53 the highly curved surfaces of as-prepared Au95.1-Pd4.9 alloy NWNs bear a high density of atomic steps and kinks, Frank partial dislocation, and stacking faults (Figure S9), which can significantly facilitate the electrooxidation reactions of small organic molecules such as ethanol. XPS data also indicate, that the Pd 3d signal only shows one couple of Pd peaks in the XPS spectra of the Au95.1-Pd4.9 alloy NWNs, the absence of Pd oxide on the surfaces of the resulting Au95.1-Pd4.9 alloy NWNs, which should be favorable for their catalytic performance.54 Furthermore, the ratio of the peak area of Pd 3d5/2 to that of Pd 3d3/2 in the XPS spectra of Au95.1-Pd4.9 alloy NWNs is about 4.5 : 1, which is very different from the conventional value (3 : 2), the increase in intensity of the Pd 3d5/2 peak induced by the contribution of Au 4d5/2 highlights the synergistic effect of Pd and Au of the resulting Au95.1Pd4.9 alloy NWNs (Figure 2A), which may improve the electrocatalytic activity and durability of the Au95.1-Pd4.9 alloy NWNs. All these results indicate that Au95.1-Pd4.9 alloy NWNs obtained at the Pd-to-Au ratios of 1-14 show the optimal electrocatalytic performance for ethanol oxidation.

Figure 3. CV curves (A, B and C) of GCEs modified by the resulting Au-Pd alloy NWNs obtained under different Au-to-Pd molar ratios, 6.0 (black curve), 10.0 (red curve), 14.0 (blue curve), and 18.0 (magenta curve), measured in 0.30 M KOH solution in the absence (A) and presence (B and C) of 0.5 M ethanol solution. The currents are normalized by the Pd mass loaded on the GCEs (A and B) and ECSA values (C). The scan rates are 50 mV s-1 (A) and 20 mV s-1 (B and C), respectively. The corresponding concentrations of Na2PdCl4 used for synthesis are 0.16, 0.097, 0.070, and 0.055 mM, respectively. The concentrations of HAuCl4 and citrate are 0.98 and 4.0 mM, respectively. The premixing time of the mixture solution before addition is 14 min.

Table 1. Summarized data of surface compositions, ECSA values, mass activity and specific activity of the resulting Au-Pd alloy NWNs obtained under different Au-to-Pd molar ratios.


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surface composition 1-m m (Au%) (Pd%) 17.1 82.9

Sample

Molar ratio of Au-to-Pd

Figure S7a

6

Figure S7b Figure S7c

10 14

34.0 40.5

Figure S7d

18

54.6

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ECSA (m2 gPd-1 )

Mass activity (A mgPd-1)

Specific activity (mA cm-2)

174.3

13.5

7.7

66.0 59.5

175.0 163.5

23.6 33.1

13.5 20.2

45.4

135.6

18.5

13.6

Since the present work was aimed to development of flexible electrodes for practical applications of wearable electronics, the resulting Au95.1-Pd4.9 alloy NWNs were blended with the conventional flexible materials Nafion and CFC commonly used to manufacture fuel cell electrodes. 2.2.1. Electrocatalytic performance of Nafion-Au95.1-Pd4.9 alloy NWNs composite films. Thanks to the excellent stabilization of the citrate coating, as-prepared Au95.1-Pd4.9 alloy NWNs could be readily transferred into Nafion solutions in water/ethanol mixture to produce Nafion- Au95.1-Pd4.9 alloy NWNs composite films by means of solution casting (Figure S10). Furthermore, as shown in Figure 4A, if uniform thin films of as-prepared Au95.1-Pd4.9 alloy NWN are cast on glass slides, addition of the water/ethanol solution of Nafion atop can effectively stabilize the resulting Au95.1-Pd4.9 alloy NWNs films. When the glass slides coated by NafionAu95.1-Pd4.9 alloy NWNs composite films are immersed into water, the composite films are readily peeled off from the glass slide surfaces, leading to intact, freestanding, thin films of Nafion- Au95.1-Pd4.9 alloy NWNs composites, which is denoted as NWN-Nafion composite films for the sake of simple presentation. The dimension of NWN-Nafion composite films can be easily produced over areas of 50 cm2 (Figure 4B-a).


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Figure 4. (A) Schematic depiction of fabrication and transfer of a freestanding NWN-Nafion composite thin film. (B) Photos of NWN-Nafion composite films with the dimension of dozens of cm2, cast on a glass slide (a), peeled off from the glass substrate and floating on water (b), and transferred and rolled on a glass rod (c). Photos of the NWN-Nafion composite films transferred on a flexible hard-plastic sheet (d) and a soft-plastic sheet (e), respectively, while the former can be readily bent and the latter tightly rolled up. The corresponding graphic models are shown in the insets. TEM and scanning electron microscope (SEM) images of the resulting films reveal, that the Au95.1-Pd4.9 alloy NWNs remain well-interconnected networks in the Nafion matrices (Figure S11). The resulting NWN-Nafion composite films (Figure 4B-b) can be readily transferred on supporting solid substrates with different chemical nature and curvature such as glass rods (Figure 4B-c) and hard rigid and flexible plastic sheets (Figure 4B-d and e), which is prerequisite for manufacturing of flexible electrodes. To ensure the formation of three-dimensional, interwoven networks with improved electron transport, the proper loading density of Au95.1-Pd4.9 alloy NWNs was investigated on indium tin oxide (ITO) glass with an area of about 1 cm2 (Figure S12). When the loading density exceeds the critical value of about 30.0 µg cm-2, the oxidation current of Au95.1-Pd4.9 alloy NWNs on ITO


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glass is nearly the same. Thus, a loading density of 33.8 µg cm-2 of Au95.1-Pd4.9 alloy NWNs in NWN-Nafion composite films for electrocatalysis was used. Hence as-prepared NWN-Nafion composite films were transferred on glass carbon electrodes (GCEs), which were subsequently utilized as electrodes for electrocatalysis of oxidation of ethanol under alkaline conditions. The electrochemically active surface area (ECSA) of the GCE coated by the NWN-Nafion composite films is 163.5 m2 gPd-1 (Figure S8), which is about 5.4 times larger than that of the GCE modified by a commercial Pd/C catalyst (30.4 m2 gPd-1). This underlines that NWN-Nafion composite films are electrochemically more accessible for catalysis of ethanol oxidation. Figure 5 shows the comparison of the electrocatalytic behavior of the GCEs coated by NWN-Nafion composite films and those modified by a commercial Pd/C catalyst. When the current density of the GCE coated by NWN-Nafion composite films is normalized by the total mass of the Au95.1-Pd4.9 alloy NWNs used (Figure 5a), it is about 1.23 A mgAu+Pd-1 in the positive-going scan, which is about 6.2 times larger than that of commercial Pd/C catalysts (0.20 A mgPd-1). Note that the contribution of Au in the Au95.1-Pd4.9 alloy NWNs to ethanol oxidation can be neglected (Figure S13). Taking into account that Pd is the dominant active component for catalysis, the Pd massaveraged current activity of the GCE coated by NWN-Nafion composite films is about 33.1 A mgPd-1, which is about 165 times larger than that of the commercial Pd/C catalyst (Figure 5a). The specific activity of NWN-Nafion composite films is 20.2 mA cm-2 and thus about 30 times that of the commercial Pd/C catalyst (0.66 mA cm-2) (Figure 5b). Furthermore, the massnormalized activities of NWN-Nafion composite films (1.23 A mgAu+Pd-1, and 33.1 A mgPd-1) are noticeably higher than those derived from other AuPd bimetallic nanostructured catalysts currently reported in literature. They are about 1.2-fold, 2.4-fold and 6.7-fold higher than the


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values for highly branched concave Au/Pd bimetallic NPs (1.065 mgAu+Pd-1),55 convex polyhedral Au@Pd core-shell NPs (about 0.52 A mgAu+Pd-1),56 and Pd–Ni–P ternary NPs (4.95 A mgPd-1).57 The AuPd alloyed nature of as-prepared Au95.1-Pd4.9 alloy NWNs is expected to exhibit an effective anti-CO poisoning ability, the NWN-Nafion composite films exhibit better durability as compared with that of commercial Pd/C catalysts; the latter incurs significant deactivation due to CO poisoning (Figure 5c and d). After 3000s of testing, the mass-normalized activity of the GCE coated by the NWN-Nafion composite films is reduced from 33.1 A mgPd-1 to 0.44 A mgPd-1, which is still 27.5 times higher than that of GCE modified by commercial Pd/C catalysts (0.016 A mgPd-1) (Figure. 5c). The CV cycling test results show that the peak current density of the GCE coated by NWN-Nafion composite films is reduced merely by about 8% after 500 cycles, while that of the commercial Pd/C catalyst is reduced by about 60% (Figure 5d). This underlines an excellent potential cycling stability of the NWN-Nafion composite films. Note that with increasing number of CV cycles, the structural feature of the Au95.1-Pd4.9 alloy NWNs changes only little with some nodes noticeably enlarged (Figure S14). The excellent structural stability and CV cycling durability of the NWN-Nafion composite films imply, that both the Au and Pd components of the AuPd alloyed shells of the Au95.1-Pd4.9 alloy NWNs are in direct contact with the reaction media. So the former can empower the latter with CO tolerance.58,59 All of the aforementioned results demonstrate, that NWN-Nafion composite films have electrocatalytic performance for ethanol oxidation in alkaline media superior to currently available Pd-based catalysts.


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Figure 5 Histograms of the current densities of the GCEs normalized by Pd mass loaded and total loaded mass of Pd and Au (a), and their ECSAs and the specific activities (b) in 0.30 M N2saturated KOH aqueous solution at room temperature in the presence of 0.50 M ethanol. The data obtained from the GCEs, coated by the NWN-Nafion composite films, are represented by the black columns, while those from the GCEs modified by commercial Pd/C catalysts are represented by the red columns. The chronoamperometric curves (c) and potential cycling stability (d) of the GCEs coated by NWN-Nafion composite films (black curves) and those modified by commercial Pd/C catalysts (red curves). The chronoamperometric (CA) curves (c) are recorded at – 0.3 V. The scan rates used for collecting data shown in a, b, c, and d are 20 mV s-1, 50 mV s-1, 100 mV s-1, and 100 mV s-1, respectively. The mass loadings of Au and Pd were calculated from the usage of Au and Pd used for the preparation of the resulting NWNs.

2.2.2. Electrocatalytic Performance of Au95.1-Pd4.9 alloy NWNs within CFC. Due to their excellent mechanical strength and electrical conductivity, CFC is commonly used as the supporting matrix for electrocatalysts. Note that the Au95.1-Pd4.9 alloy NWNs @CFC also can be directly prepared by step-by-step transfer of CFC between aqueous solutions of different precursors (Figure S15). However, it is difficult to accurately determine their mass in CFCs (in order of micrograms) in current experiments. As mentioned above, as-prepared Au95.1-Pd4.9 alloy NWNs can be stably transferred into ethanol. Afterwards, the Au95.1-Pd4.9 alloy NWNs can readily be loaded highly within CFC by stepwise adsorption of the NWN ethanol dispersions onto the CFC (Figure 6). After complete evaporation of ethanol, the resulting Au95.1-Pd4.9 alloy


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NWNs–loaded CFC, denoted as NWN-CFC was utilized as flexible electrode for electrocatalytic oxidation of ethanol. According to the CV curves obtained in 0.3 M KOH solution (Figure S16), the ECSA of as-prepared NWN-CFC is calculated as 288.2 m2 gPd-1, which is much larger than that of the GCE coated by NWN-Nafion composite films (163.5 m2 gPd-1) and the GCE coated by commercial Pd/C catalysts (30.4 m2 gPd-1), respectively, by about 1.8 times and 9.5 times. The results highlight the exceedingly large electrochemical accessibility of Au95.1-Pd4.9 alloy NWNs.60

Figure 6. (A) Schematic depiction of fabrication of NWN-CFC via adsorption of ethanol solutions of Au95.1-Pd4.9 alloy NWNs into the CFC. (B) Digital photographs of the undeformed NWN@CFC (a) and those being slightly bent (b), double-folded (c), and tightly rolled (d), respectively, in the reaction media for ethanol oxidation. Note that the double-folded (c) and rolled (d) NWN-CFC are fastened by threads extracted from CFC. Especially in order to explore the potential of using them as flexible electrodes for wearable devices, here the resulting NWN-CFC is to a small or large degree deformed, when they are used for electrocatalysis of ethanol oxidation (Figure 7 and Table S3). The CV curves of NWN-CFC show nearly identical profiles regardless of being bent, folded and rolled, while the undeformed NWN-CFC have slightly more positive oxidation potential than the deformed ones (Figure S17). As shown in Figure 7a-c, the Pd-mass-normalized activity, total-mass-normalized activity, and specific activity of undeformed and bent NWN-CFC are almost identical; being about 15.7 A


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mgPd-1, 0.58 A mgAu+Pd-1, and 5.5 mA cm-2, respectively. The Pd-mass-normalized activity of double-folded NWN-CFC (about 11.7 A mgPd-1) is close to that of the rolled NWN-CFC (about 12.3 A mgPd-1), and their specific activities are nearly the same (about 6.3 mA cm-2). The impact of the shape deformation on the electrochemical activity of the resulting NWN-CFC can be correlated with the ECSA reduction for deformed NWN-CFC. Moreover, the deformation may also reduce the conductivity of the CFC hosts (Figure S18), which may provide a minor contribution to the negative shift of the oxidation potential of Au95.1-Pd4.9 alloy NWNs within CFC. Although the electrocatalytic performance of the NWN-CFC is noticeably reduced after being deformed, their Pd-mass-normalized current density – the lowest value is about 11.7 A mgPd-1 in the double-folded form – remains at least 60 times higher than that of the commercial Pd/C catalyst (0.20 A mgPd-1), and the specific activity – the lowest value is about 5.5 mA cm-2 in the bent form – remains at least 8.3 times higher than that of the commercial Pd/C catalyst (0.66 mA cm-2). Furthermore, the CA analysis reveals that the electrochemical stability of undeformed NWN-CFC is considerably better than that of GCE modified by commercial Pd/C catalysts and even of the GCEs coated by Nafion-Au95.1-Pd4.9 alloy NWNs composite films (Figure 7d). This suggests that within the NWN-CFC, the Au95.1-Pd4.9 alloy NWNs are well dispersed within the CFC host matrices, which minimizes the coalescence and fusion of neighboring NWNs during the reaction process, thus improving the durability.


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Figure 7. Histograms of the current densities of the NWN-CFC normalized by Pd mass loaded (a), total loaded mass of Pd and Au (b), the specific activities (c), and stability test (d) of NWNCFC at undeformed state (i), GCE modified by NWN-Nafion composite film (ii), and GCE modified by commercial Pd/C catalyst (iii), in 0.30 M N2-saturated KOH aqueous solution at room temperature in the presence of 0.50 M ethanol. The chronoamperomeric curves are recorded at -0.3 V. The data obtained from NWN-CFC in the form of being undeformed, bent, double-folded, and rolled are represented by the black, cyan, magenta, and violet color, respectively, while those from NWN-Nafion composite film and Pd/C catalyst are represented by the blue and red color. The excellent electrocatalytic performance of the resulting Au-Pd alloy NWNs on ethanol oxidation should be attributed to their unique structure, which is similar as that of current metallic aerogels obtained.61-66 For instance, the inter-connected networks in the resulting Au-Pd alloy NWNs render them larger surface area and high porosity, which favor the diffusion of ethanol molecules and electron transport during electrocatalysis due to the path directing effects of the structural anisotropy.63,64 Moreover, the small diameter of the resulting NWNs and the presence of inherent defects can offer more active sites.65 Furthermore, the feature of selfsupport can avoid the use of the carbon supports.66 However, compared to the synthesis of metallic aerogels, our current synthetic method for Au-Pd alloy NWNs has the following


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advantages: (i) it is more convenient, as it can be carried out by simply adding an aqueous solution of Na2PdCl4 into aqueous solutions of citrate/HAuCl4 mixtures at room temperature without aid of additional reducing agents; and (ii) the surface composition of the resulting Au-Pd alloy NWNs can be readily tuned by adjusting the concentrations of the precursors used. Of course, due to their size limit, the resulting Au-Pd alloy NWNs in practical applications still have to be used with the aid of other substances, such as Nafion and CFC.

3. CONCLUSIONS In summary, we have successfully demonstrated a new method for synthesis of Au95.1-Pd4.9 alloy NWNs by deposition of Pd atoms onto Au NWs at room temperature without aid of additional reducing agents. The resulting Au95.1-Pd4.9 alloy NWNs can be readily integrated with commercially available Nafion films and CFC to produce macroscopic, flexible electrodes for fuel cells; the resulting NWN-Nafion composite thin films can be easily transferred onto a variety of solid substrates with different surface curvatures. Compared with commercial Pd/C catalysts and Pd-based nanostructured materials currently reported in literature, both NWNNafion composite thin films and NWN-CFCs exhibit exceedingly high ECSA values, superior electrocatalytic performance and excellent durability for electrocatalysis of ethanol oxidation. As a result, the NWN-Nafion and NWN-CFC should hold immense promise in development of flexible electrodes for wearable devices. The present work may provide new prospects for fundamental and applied research especially related with large-scale manufacturing of electrodes for fuel cells in the years coming.

4. EXPERIMENTAL METHODS


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4.1. Materials. Chloroauric acid tetrahydrate (HAuCl4·4H2O), trisodium citrate dihydrate (Na3C6H5O7·2H2O), ethanol (C2H5OH), and potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Sodium tetrachloropalladate (II)(Na2PdCl4, 99%) was purchased from Alfa Aesar (Tianjin, China) and commercial Pd black (nominally 10% on carbon black) was purchased from Aladdin Industrial Corporation (Shanghai, China). All glassware and stirring bars were cleaned with aqua regia (3 : 1 v/v HCl (37%) : HNO3 (65%) solutions) and then rinsed thoroughly with H2O before use. (Caution: aqua regia solutions are dangerous and should be used with extreme care; never store these solutions in closed containers.) Water used in all experiments was prepared in a three-stage Millipore MilliQ plus 185 purification system and had a resistivity higher than 18.2 MΩ cm. 4.2. Synthesis of Au95.1-Pd4.9 alloy NWNs. A typical procedure for the synthesis of Au-Pd alloy NWNs is as follows. First, the aqueous solution of HAuCl4 (0.50 mL, 25 mM) was added into the aqueous solution of sodium citrate (1.5 mL, 34 mM) at room temperature under stirring in a 7.5-mL vital. Water (0.50 mL) was added to increase the volume of the citrate–HAuCl4 premixture solution to 2.5 mL. After about 14 min incubation, an aqueous solution of the reaction mixture (0.5 mL) was added into the water (2 mL), followed by addition of an aqueous solution of Na2PdCl4 (57.1 µL, 3.125 mM) under stirring. After stirring for 3 h at room temperature, the Au95.1-Pd4.9 NWNs were obtained. 4.3. Characterization. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained with a JEOL JEM-2100F transmission electron microscope operating at an acceleration voltage of 200 kV. Elemental mapping images were acquired by energy dispersive X-ray spectroscopy (EDS) using a JEOL JEM-2100F electron microscope equipped with a STEM unit. X-Ray Photoelectron Spectroscopy (XPS) measurements were


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carried out with a Thermo Fisher Scientific Escalab 250 XPS spectrometer, using Al Ka X-ray radiation for excitation. The X-ray diffraction (XRD) patterns of the resulting samples were collected by a Bruker AXSD8 advance X-ray diffractometer with Cu Kα radiation using a graphite chromator. Diffraction peaks were recorder in the 30-90° (2θ) range with a step size of 0.02°. 4.4. Electrochemical measurements. Cyclic voltammetric (CV) and chronoamperometric (CA) experiments were performed in a standard three-electrode cell in a CHI660D work station at room temperature. Two different current collectors were employed as the working electrodes, i.e., glassy carbon electrode (GCE) and carbon fiber cloth (CFC) (0.5 cm × 4 cm), modified by the as-prepared Au-Pd alloy NWNs, while an Ag/AgCl electrode and Pt wire were used as the reference electrode and auxiliary electrode, respectively. A typical procedure for the preparation of the GCE modified by as-prepared Au-Pd alloy NWNs is as follows: 12 µL of aqueous solution of Au-Pd alloy NWNs was drop-coated on a freshly prepared bare GCE, followed by drying in air. 10 µL of the ethanol solution of Nafion (0.2 wt%) was cast on the surface of the GCE coated by Au-Pd alloy NWNs, followed by drying in air (named as NWN-Nafion composite film). The preparation of as-prepared Au-Pd alloy NWNs in the CFC (named as NWN-CFC) is as follows: The Au-Pd alloy NWNs was directly soaked into the CFC with area controlled to about 1 cm2. Then, the resulting NWN-CFC was left to dry in ambient conditions. Eventually, the Nafion was spread over the CFC surface and allowed to dry in a vacuum oven at 55 °C for 5 h to immobilize the catalyst. The Pd loading in the NWN-CFC was 1.49 µg cm-2. Cyclic voltammograms (CVs) used for determination of the ECSA of Pd-based catalysts were recorded between -1.00 and 0.60 V in N2-saturated 0.30 M KOH solution with a scan rate of 50


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mV s−1. For ethanol oxidation in alkaline media, CVs were recorded between −0.60 and 0.40 V in N2-saturated 0.30 M KOH and 0.50 M ethanol solution at a scan rate of 20 mV s−1 for the NWN-Nafion composite film @GCEs and NWN-CFC, respectively. Their specific activities and mass activities were normalized by their ECSA values and the loaded Pd mass or the total mass of Au and Pd, respectively. We also have tried to employ the underpotential deposition (UPD) of Cu to measure ECSA, but the current peaks of Cu UPD are not well defined in this case, leading to a large uncertainty in integrating the UPD charge (Figure S19).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxx. Additional Digital photographs; Additional extinction spectra; Additional TEM images of intermediates, Au NPs, Au-Pd NPs, and Au-Pd NWNs; XRD patterns of Au95.1-Pd4.9 alloy NWNs; TEM images of the resulting Au-Pd alloy NWNs obtained under different Au-to-Pd molar ratios; CV curves of the GCEs modified by Au95.1-Pd4.9 alloy NWNs; HRTEM image of different types of defects; SEM and TEM images of NWN-Nafion composite film; CV curves of ITO modified by Au95.1-Pd4.9 alloy NWNs; CV curve of GCE modified by Au NWs; TEM image of NWNNafion composite film after durability test; Schematic depiction of the synthetic procedure of AuPd alloy NWNs directly prepared in flexible CFC; CV curves of pure CFCs and NWN-CFCs at different states; Underpotential deposition (UPD) of Cu on a AuPd electrode; Summarized data in Table S1 to S3.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is financially supported by the Natural Science Foundation of China (21473105 and 21773142), Shandong Provincial Natural Science Foundation for Distinguished Young Scientists (JQ201405), Taishan Scholarship in Shandong Province (No. tsqn20161001), and Fundamental Research Fund of Shandong University (2016JC003). REFERENCES (1)

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Simple Synthesis of Au-Pd Alloy Nanowire Networks as Macroscopic

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