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Formation of nanoporous Cu-Ag by dealloying Mg-Cu-Y-Ag amorphous alloys and its electrocatalyst oxidation property
Intermetallics 110 (2019) 106488
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Formation of nanoporous Cu-Ag by dealloying Mg-Cu-Y-Ag amorphous alloys and its electrocatalyst oxidation property
T
J.I. Hyuna, K.H. Konga, W.C. Kima, W.T. Kimb, D.H. Kima,∗ a b
Department of Materials Science & Engineering, Yonsei University, Seoul, 03722, Republic of Korea Department of Laser & Optical Information Engineering, Cheongju University, Cheongju, 28503, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Amorphous alloy Dealloying Nanoporous structure Bimetallic Cu-Ag Electrocatalyst
The supersaturated Cu-Ag bimetallic nanoporous structure has been fabricated by dealloying of Mg65Cu25(x = 0, 2, 4, 8) amorphous alloys and its electrocatalytic property for sodium borohydride (NaBH4) oxidation has been evaluated. In order to increase solid solubility of Ag into Cu, the dealloying process is proceeded in higher concentration sulfuric acid aqueous solution for shorter immersion time. When Mg65Cu25xY10Agx amorphous alloys are dealloyed in 1.0 M solution for 30 s, the Cu-Ag bimetallic nanoporous structure in the form of solid solution is successfully obtained due to extended solid solubility in nm scale grain structure. Electrochemical oxidation of NaBH4 at Cu-(Ag) electrodes is studied in a mixture solution of 2.0 M NaOH and 0.1 M NaBH4. The electrocatalytic property of nanoporous Cu-Ag bimetallic electrodes is higher than that of Cu monometallic electrode. The electrode dealloyed from Mg65Cu21Y10Ag4 amorphous alloy exhibits the highest electrocatalytic property. xY10Agx
1. Introduction Dealloying is a chemical corrosion process in which less-noble elements are selectively dissolved out from the precursor alloy. The residual noble element forms a bi-continuous pore/ligament structure by a self-assembly process through surface diffusion [1,2]. In order to form a uniform nanoporous structure, the precursor alloy is required to be a single phase alloy such as solid-solution type crystalline alloy. However, in the case of crystalline alloy precursor, it is relatively difficult to obtain a homogeneous nanoporous structure, since there are defects including compositional inhomogeneity. So far, only limited crystalline alloys have been reported to form a uniform nanoporous structure by dealloying [3–5]. Recent studies show that amorphous alloy is more suitable as precursor alloy for dealloying due to its homogeneous composition and structure down to atomic scale [5,6]. For crystalline alloy precursor, its crystal structure and some microstructural features such as size and orientation of the grains are retained during leaching out process [7]. On the contrary, amorphous alloy precursor cannot retain its original amorphous structure during leaching out process, because dissolution of less-noble element immediately changes the composition of the precursor alloy to be out of glass forming range. Therefore, in the case of amorphous precursor, surface diffusion process of remaining noble element is accompanied by nucleation and growth of crystalline phase, leading to formation of more homogeneous ∗
nanoporous structure [5,8–10]. Formation of such fine and homogeneous nanoporous metals using amorphous precursor alloys has been reported in several alloy systems such as Au-Cu-Ti-Si [2], Ag-Ca-Mg [11], and Mg-Cu-Y [8,12]. When dealloying Mg-Cu-Y amorphous alloy, dissolution of less noble elements induces Cu-enriched region at the interface between precursor alloy and electrolyte. The Cu atoms agglomerate into clusters, followed by nucleation of new crystal grains, leading to formation of poly-crystalline ligaments with uniform porous structure [8]. Since surface diffusion of noble element leads to formation and growth of nanoporous structure, the scale of nanoporous metal is largely affected by surface diffusivity of noble element and its composition in the precursor alloy. Here, the addition of another noble element with lower surface diffusivity than major noble element reduces overall diffusivity of noble elements and limits coarsening of ligaments, resulting in a very fine nanoporous structure. Dan et al. [13–15] have reported that the addition of Ag and Au in Ti-Cu amorphous alloy results in the formation of fine nanoporous Cu. Au and Ag atoms having lower surface diffusivity suppress the movement of residual Cu atoms, leading to formation of fine nanoporous structure as well as Au- and Agrich precipitate at grain boundary due to their low surface energy. In fact, there have been several studies on the fabrication of fine nanoporous metals through addition of minor noble element which precipitates at grain boundary after dealloying process. However, in
Corresponding author. E-mail address: [email protected] (D.H. Kim).
https://doi.org/10.1016/j.intermet.2019.106488 Received 11 January 2019; Received in revised form 19 March 2019; Accepted 26 April 2019 Available online 01 May 2019 0966-9795/ © 2019 Elsevier Ltd. All rights reserved.
Intermetallics 110 (2019) 106488
J.I. Hyun, et al.
principle, minor noble element added to control overall surface diffusivity can exist in the form of either precipitate as mentioned above or solid solution with the primary noble element. M. Zhang et al. [16] have reported on the formation of Ag-Pd bimetallic nanoporous metal in which case Ag and Pd have sufficient mutual solid solubility. However, there have been few reports on the formation of metastable bimetallic (solid-solution type) nanoporous metal, when two noble elements have very little mutual solid solubility at room temperature. Nanoporous metals fabricated by dealloying process, as mentioned above, has received a great attention as a potential candidate for many functional applications such as sensor, catalyst, fuel cell and supercapacitor [17–23]. In recent years, sodium borohydride (NaBH4) have attracted attention as an energy/hydrogen carrier of direct borohydride fuel cell (DBFC) [24]. Numerous metals such as Pt, Au, Ag, Pd, Ni and Cu [25–31] have been studied for anode electrocatalyst of NaBH4 oxidation. However, since the kinetics of BH4− oxidation reaction on such monometallic anode electrocatalyst is very slow, the level of resulting power output is very low. On the contrary, it has been shown that bimetallic alloy catalysts exhibit higher electrocatalytic activity due to modified surface strain [32,33]. In the previous studies, carbon supported Au-Cu [34], Ag-Ni [35], Ag-Pt [36] and Ag-Cu [37,38] bimetallic nanoparticles have been shown to exhibit much higher catalytic activity than monometallic catalyst. However, there have been very few reports on the electrocatalytic performance when bimetallic nanoporous structure fabricated by dealloying is used for NaBH4 oxidation. Therefore, the aim of the present study is to fabricate nanoporous bimetallic Cu-Ag by dealloying of Mg-Cu-Y-Ag metallic glass and to estimate its electrocatalytic performance in NaBH4 oxidation. Cu-Ag bimetallic nanoporous structures were prepared by dealloying of a series of Mg65Cu25Y10 metallic glasses with minor addition of Ag, i.e. Mg65Cu25-xY10Agx (x = 0, 2, 4, 8) in sulfuric acid aqueous solution for 30 s. The electrocatalytic oxidation performance of bimetallic Cu-Ag nanoporous structure was evaluated in a mixture of 2.0 M NaOH and 0.1 M NaBH4.
Fig. 1. XRD patterns obtained from: (a) as-melt-spun Mg65Cu25Y10; (b) dealloyed Mg65Cu25Y10; (c) dealloyed Mg65Cu23Y10Ag2; (d) delloyed Mg65Cu21Y10Ag4; and (e) dealloyed Mg65Cu17Y10Ag8 (dealloying condition: in 1.0 M sulfuric acid aqueous solution for 30 s).
3. Results The XRD pattern obtained from as-melt-spun Mg65Cu25Y10 ribbon (Fig. 1 (a)) shows a broad halo peak, confirming the amorphous structure of as-melt-spun ribbon. The amorphous structure of as-meltspun Mg65Cu25-xY10Agx (x = 2, 4, 8) ribbons were also confirmed by XRD (not shown). Fig. 1(b)–(e) show XRD patterns obtained from meltspun Mg65Cu25-xY10Agx (x = 0, 2, 4, 8) samples dealloyed in in 1.0 M H2SO4 for 30 s. In the XRD pattern obtained from dealloyed Mg65Cu25Y10 sample (Fig. 1 (b)), the halo peak from the amorphous phase completely disappeared, while the sharp peaks from crystalline Cu and Cu2O appeared, as indexed in the diffraction pattern. In the XRD pattern obtained from dealloyed Mg65Cu23Y10Ag2 sample (Fig. 1 (c)), the peaks from crystalline Cu and Cu2O again appeared. However, the positions of the peaks from Cu were slightly shifted to lower angle side and the width of the peak became larger. The peak from crystalline Ag did not appear in spite of the addition of Ag, indicating that Ag atoms were completely alloyed into Cu ligaments. When 4 at% and 8 at% of Ag were added (Fig. 1(d) and (e)), the width of the diffraction peaks from Cu became much larger and the position of peaks shifted further to lower angle side. The shift and broadening of peaks from Cu indicate that the lattice parameter of Cu in dealloyed Mg65Cu25-xY10Agx was larger than that in dealloyed Mg65Cu25Y10 and the grain size of Cu ligaments in dealloyed Mg65Cu25-xY10Agx was smaller than that in dealloyed Mg65Cu25Y10. The peak from crystalline Ag is not clearly observed in the XRD pattern for dealloyed Mg65Cu21Y10Ag4 (Fig. 1 (d)), however, it is clearly observed in the XRD pattern from dealloyed Mg65Cu17Y10Ag8 (Fig. 1 (e)), indicating that the alloying behavior of Ag atoms forms supersaturated bimetallic Cu-Ag solid solution ligament and the residual Ag is present as precipitates. There are theoretical and experimental evidences that the solid solubility of components in binary alloys with nm scale grain size is greater than in analogous alloys with larger grain size [39,40]. It can be inferred that the solid solubility of Ag into Cu was extended with decrease of the grain size down to nm scale. Fig. 2 shows the SEM images obtained from dealloyed Mg65Cu25xY10Agx (x = 0, 2, 4, 8) samples. Continuous and uniform nanoporous structure was formed in dealloyed Mg65Cu25-xY10Agx (x = 0, 2, 4) samples, as shown in Fig. 2(a)–(c). It can be noticed that the size of the ligament decreases with increasing Ag amount, since minor element Ag decreases the overall diffusivity of noble elements [14,15]. The absence of Ag precipitates in Fig. 2(b) and (c) indicates that Cu and Ag atoms
2. Experimental Mg65Cu25-xY10Agx (x = 0, 2, 4, 8) alloy ingots were prepared by high-frequency induction melting the mixture of high-purity elements (Mg: 99.9%, Cu: 99.997%, Y: 99.9%, Ag: 99.99%) under argon atmosphere. Amorphous ribbons with 60–80 μm in thickness and 6–8 mm in width were obtained using melt spinning method on a copper wheel (diameter: 22 cm) rotating at a speed of 3000 rpm. Since the previous studies show that dealloying of Mg-Cu-Y metallic glass is successfully performed in sulfuric acid aqueous solution [8,12], dealloying process was performed in sulfuric acid aqueous solution with concentrations of 1.0 M for 30 s and 0.1 M for 10 min at room temperature. The immersion time was determined by the time when hydrogen gas evolution at the alloy/electrolyte interface was over completely. The crystal structure of melt-spun and dealloyed samples were analyzed using an X-ray diffractometer (XRD, Rigaku miniFlex 600) with Cu Ka radiation for a 2θ range of 20° to 80°. The morphology of dealloyed samples was observed using a scanning electron microscope (SEM, JEOL JSM-7001F). The microstructure and composition of dealloyed samples were investigated using a transmission electron microscope (TEM, JEOL JEM2100F) operated at 200 KeV with energy-dispersive X-ray spectrometer (EDS, Oxford). The elemental mapping was performed using a TEM (Fei Talos F200X) with super-X EDX system equipped with 4-SSD detectors (Fei). The TEM specimens were prepared by Focused Ion beam (FIB, JEOL JIB-4601F) or dispersion in ethanol. Electrochemical measurement was performed in a three-electrode cell using a carbon counter electrode and Hg/HgO as the reference electrode. The binder-free nanoporous ribbon was used as working electrode. Cyclic voltammetry (CV) curves at a scan rate of 5 mV/s were obtained at the potential range of −0.8 V to −0.4 V in a solution of 2.0 M NaOH and 0.1 M NaBH4. 2
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Fig. 2. SEM images obtained from: (a) dealloyed Mg65Cu25Y10; (b) dealloyed Mg65Cu23Y10Ag2; (c) delloyed Mg65Cu21Y10Ag4; and (d) dealloyed Mg65Cu17Y10Ag8 (dealloying condition: in 1.0 M sulfuric acid aqueous solution for 30 s).
(c)), confirming XRD and EDS results. The morphology was also consistent with that observed from SEM and TEM analysis. CV curves obtained from dealloyed Mg65Cu25Y10 electrode in 2.0 M NaOH solution and dealloyed Mg65Cu25-xY10Agx (x = 0, 2, 4, 8) electrodes in 2.0 M NaOH + 0.1 M NaBH4 solution at the potential rage of −0.8 V to −0.4 V are shown in Fig. 5. CV curves obtained from 2.0 M NaOH +0.1 M NaBH4 exhibit oxidation current peaks between −0.6 V and −0.4 V, while there is no oxidation current peak in CV curve obtained from 2.0 M NaOH solution, indicating that the oxidation peak is attributed to the oxidation of BH4− [37]. The anodic current densities of BH4− oxidation for Cu monometallic nanoporous electrode (Mg65Cu25Y10) and Cu-Ag bimetallic nanoporous electrodes (Mg65Cu25−2 , rexY10Agx, x = 2, 4, 8) were 35.3, 47.6, 50.4 and 58.2 mA cm spectively. The peak current densities of the BH4− oxidation for Cu-Ag bimetallic electrodes were much higher than Cu monometallic electrode. Moreover, it is noticeable that the electrocatalytic property increased with the increase of Ag content from 2 to 4 at. %, and then decreased when Ag content was 8 at. %. The result shows that solid solution alloying of Ag into Cu improves the electrode kinetics for oxidation of BH4−. However, the electrode kinetics and electrocatalytic oxidation property are deteriorated with the precipitation of Ag-rich particles on the surface of ligament, since Cu-Ag bimetallic ligament exhibits higher electrocatalytic property than Ag particle. Therefore, supersaturated bimetallic Cu-Ag nanoporous electrocatalyst, dealloyed from Mg65Cu21Y10Ag4 precursor, exhibits better electrocatalytic performance than the other dealloyed samples.
form a solid solution because of their extended solid solubility. However, when the amount of Ag was 8 at%, the particles which precipitated on the surface of ligament during dealloying were observed, as marked by arrows in Fig. 2 (d). Fig. 3(a) and (b) show bright field (BF) image and corresponding selected area diffraction pattern (SADP) obtained from dealloyed Mg65Cu21Y10Ag4 and Mg65Cu17Y10Ag8 samples, respectively. The results obtained from scanning TEM (STEM) and EDS analysis is shown in Fig. 3(c) and (d). As shown in Fig. 3 (a) and (c), the dealloyed Mg65Cu21Y10Ag4 sample consists of uniformly distributed ligaments with the average composition of 73.3% Cu, 23.1% Ag, 3.6% O without separating into Ag-rich or Cu-rich region (almost same composition in the region “1” and “2” marked in Fig. 3 (c)). The absence of Mg and Y in the EDS data indicates that the precursor alloys were completely dealloyed and nanoporous structure consists of Cu and Ag. The continuous ring-type SADP (Fig. 3 (a)) further confirms that the nanoporous structure consists of nm scale polycrystalline Cu. As the previous study has shown that Cu2O layer forms on the surface of Cu ligament [41], halo ring pattern form Cu2O was observed. When 8 at% of Ag was added, the nanoporous structure of dealloyed sample separated into CuAg bimetallic ligament and Ag-rich particles, as shown in Fig. 3 (b) and (d). The SADP shown in Fig. 3 (b) reveals a ring-type pattern, indicating the formation of nanoporous structure as well as Ag-rich precipitate. The EDS results shows that the compositions of Cu-Ag bimetallic ligament and Ag-rich particle are 87.8% Cu, 6.7% Ag, 5.5%, O and 23.7% Cu, 68.5% Ag, 7.8% O, which correspond to region “1” and “2” marked in Fig. 3 (d), respectively. As shown in Fig. 4, the formation of Cu-Ag bimetallic nanoporous structure in dealloyed Mg65Cu21Y10Ag4 was further analyzed by elemental mapping in TEM. It should be noted that the red and green colors representing Cu (Fig. 4 (a)) and Ag (Fig. 4 (b)), respectively, were homogeneously distributed comprising the supersaturated bimetallic nanoporous structure without any Cu-rich or Ag-rich regions (Fig. 4
4. Discussion To elucidate the precipitation behavior of Ag particle when dealloyed in dilluted sulfuric acid for longer time, Mg65Cu25Y10 and Mg65Cu21Y10Ag4 precursor alloys were dealloyed in 0.1 M H2SO4 for 10 min. The XRD pattern obtained from dealloyed Mg65Cu25Y10 3
Intermetallics 110 (2019) 106488
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Fig. 3. BF images and corresponding SADPs obtained from: (a), (b) dealloyed Mg65Cu21Y10Ag4; and (c), (d) dealloyed Mg65Cu17Y10Ag8. STEM images obtained from: (e) dealloyed Mg65Cu21Y10Ag4; and (f) dealloyed Mg65Cu17Y10Ag8 (dealloying condition: in 1.0 M sulfuric acid aqueous solution for 30 s).
condition: 1.0 M H2SO4 for 30 s), however, interestingly the diffraction peak from Ag which was not present in Fig. 1 (d) was clearly observed. The width of the diffraction peaks was narrower when compared to those in Fig. 1 (d). Moreover, the shift of the peaks from Cu did not occur, i.e. the positions of the Cu peaks were identical to those in the XRD pattern obtained from dealloyed ternary Mg65Cu25Y10 (Fig. 1 (b)). The result indicates that the size of the grains constituting the ligaments becomes larger and Ag atoms were not able to be solutionized into Cu
precursor (Fig. 6 (a)) shows sharp peaks from crystalline Cu and Cu2O which is identical to the XRD pattern shown in Fig. 1 (b) (dealloying condition: 1.0 M H2SO4 for 30 s). The results indicate that without addition of noble element (Ag), the change of concentration of solution and/or immersion time has little effect on the formation of nanoporous structure. However, in the XRD pattern obtained from dealloyed Mg65Cu21Y10Ag4 (Fig. 6 (b)), the diffraction peaks from Cu and Cu2O were present, as in the XRD pattern shown in Fig. 1 (d) (dealloying
Fig. 4. Elemental distribution of: (a) Cu; (b) Ag; and (c) both Cu and Ag in dealloyed Mg65Cu21Y10Ag4 (dealloying condition: in 1.0 M sulfuric acid aqueous solution for 30 s). 4
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clusters are formed by dissolution of less noble atoms at the initial stage, and then nucleation of new crystals occurs by surface diffusion [5,8–10]. In such case, when the dissolution rate of less noble element becomes higher (i.e. shorter immersion time in high concentration acidic solution), the nucleation rate of new crystals increases and less time is available for coarsening of crystals, indicating that grain size of nanoporous structure is determined by immersion time. The schematic diagrams illustrating nucleation and growth process in different two conditions of: 1) high dissolution rate and short immersion time; and 2) low dissolution rate and long immersion time are shown in Fig. 7. When the amorphous precursor immersed in 1.0 M H2SO4 for 30 s, Mg and Y atoms are rapidly dissolved out and residual noble atoms agglomerate into many tiny clusters (Fig. 7(a) and (b)). The clusters coalesce and form ligament composed of nano grain which has extended solid solubility (Fig. 7 (c)). In contrast, when the amorphous precursor immersed in 0.1 M H2SO4 for 10 min, Mg and Y are dissolved more slowly (Fig. 7 (d)). The noble elements form less clusters and grow larger until the active atoms are completely dissolved (Fig. 7 (e)). The ligament formed for longer immersion time consist of coalesced grain that exhibits limited solid solubility with Ag particles on the surface of Cu ligament (Fig. 7 (f)). In the present study, in order to increase the solid solubility by decreasing grain size down to nm scale, the dealloying process was performed in the acidic solution with higher concentration for shorter immersion time. It has been shown that the electrocatalytic activity for NaBH4 oxidation is better when it is in bimetallic form than in monometallic form. The alloying behavior leads to modification of the surface strain, increasing the catalytic activity. D. Duan et al. reported that carbon supported bimetallic Cu-Ag nanoparticle improve the catalytic property for the DBFC compared to single Ag. For the Cu-Ag/C catalysts, the peak current density for BH4− oxidation are 41.61 (Cu50Ag50/C), 47.07 (Cu67Ag33/C) and 21.64 (Cu80Ag20/C) mA cm−2, which are higher than Ag/C catalyst (14.53 mA cm−2) [38]. However, the present study
Fig. 5. CV curves of dealloyed Mg65Cu25-xY10Agx electrodes in a mixture solution of 2.0 M NaOH and 0.1 M NaBH4 at a scan rate 50 mV/s (dealloying condition: in 1.0 M sulfuric acid aqueous solution for 30 s).
ligament when dealloyed in dilluted sulfuric acid for longer time. As the grain size in the ligament increases, the solid solubility of Ag into Cu decreases and the residual Ag precipitates separately on the surface of the ligament. It was further analyzed by element mapping that the ligament consists of only Cu and Ag-rich particle precipitates on the surface of the ligament (Fig. 6 (c)). The present study shows that the supersaturated bimetallic nanoporous Cu-Ag can be synthesized by dealloying of amorphous alloy. When dealloying the amorphous alloys, two important processing parameters are dissolution rate and immersion time. The previous study has shown that, when dealloying amorphous alloys, noble element
Fig. 6. XRD patterns obtained from: (a) dealloyed Mg65Cu25Y10; (b) dealloyed Mg65Cu21Y10Ag4; and (c) elemental distribution of Cu, Ag and both Cu and Ag in dealloyed Mg65Cu21Y10Ag4 (dealloying condition: in 0.1 M sulfuric acid aqueous solution for 10 min). 5
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Fig. 7. Schematic illustrations of dealloying process: (a) amorphous precursor.; (b) dissolution of Mg and Y atoms and agglomeration of Cu and Ag into tiny clusters when dealloyed in 1.0 M H2SO4; (c) formation of bimetallic Cu-Ag nanoporous structure with nm scale grains; (d) dissolution of Mg and Y atoms when dealloyed in 0.1 M H2SO4; (e) coarsening of clusters during dissolution of Mg and Y atoms; (f) formation of coarse grain Cu ligament with Ag particles on the surface of the ligament.
References
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5. Conclusion Mg65Cu25-xY10Agx (x = 0, 2, 4, 8) amorphous alloys are dealloyed in sulfuric acid aqueous solution under free corrosion condition. As a result, uniform nanoporous Cu is fabricated by dealloying amorphous precursor. Very fine nanoporous structure can be obtained with addition of Ag which decreases the surface diffusivity. When Mg65Cu21Y10Ag4 amorphous precursor is dealloyed in in high concentration of sulfuric acid aqueous solution (1.0 M) with short immersion time (30 s), Cu-Ag bimetallic nanoporous structure is formed in spite of their small solid solubility which is less than 1% at room temperature. When the Ag content exceeds the extended solid solubility, Ag-rich particles precipitate on the surface of ligament, as can be seen in dealloyed Mg65Cu17Y10Ag8. When Mg65Cu21Y10Ag4 amorphous precursor is dealloyed in lower concentration solution (0.1 M) for longer time (10 min), coarse-grain Cu monometallic ligament with Ag particle on the surface of the ligament is formed. Electrochemical oxidation of NaBH4 is examined in 2.0 M NaOH +0.1 M NaBH4 solution. Compared with monometallic Cu nanoporous structure, bimetallic Cu-Ag nanoporous structure significantly improves the catalytic property. The highest current density of NaBH4 is recorded for dealloyed Mg65Cu21Y10Ag4 sample. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation by the Ministry of Science, ICT (Information & Communication Technology) and Future Planning, Korea (2016R1A2B2013838). 6
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Formation of nanoporous Cu-Ag by dealloying Mg-Cu-Y-Ag amorphous alloys and its electrocatalyst oxidation property
T
J.I. Hyuna, K.H. Konga, W.C. Kima, W.T. Kimb, D.H. Kima,∗ a b
Department of Materials Science & Engineering, Yonsei University, Seoul, 03722, Republic of Korea Department of Laser & Optical Information Engineering, Cheongju University, Cheongju, 28503, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Amorphous alloy Dealloying Nanoporous structure Bimetallic Cu-Ag Electrocatalyst
The supersaturated Cu-Ag bimetallic nanoporous structure has been fabricated by dealloying of Mg65Cu25(x = 0, 2, 4, 8) amorphous alloys and its electrocatalytic property for sodium borohydride (NaBH4) oxidation has been evaluated. In order to increase solid solubility of Ag into Cu, the dealloying process is proceeded in higher concentration sulfuric acid aqueous solution for shorter immersion time. When Mg65Cu25xY10Agx amorphous alloys are dealloyed in 1.0 M solution for 30 s, the Cu-Ag bimetallic nanoporous structure in the form of solid solution is successfully obtained due to extended solid solubility in nm scale grain structure. Electrochemical oxidation of NaBH4 at Cu-(Ag) electrodes is studied in a mixture solution of 2.0 M NaOH and 0.1 M NaBH4. The electrocatalytic property of nanoporous Cu-Ag bimetallic electrodes is higher than that of Cu monometallic electrode. The electrode dealloyed from Mg65Cu21Y10Ag4 amorphous alloy exhibits the highest electrocatalytic property. xY10Agx
1. Introduction Dealloying is a chemical corrosion process in which less-noble elements are selectively dissolved out from the precursor alloy. The residual noble element forms a bi-continuous pore/ligament structure by a self-assembly process through surface diffusion [1,2]. In order to form a uniform nanoporous structure, the precursor alloy is required to be a single phase alloy such as solid-solution type crystalline alloy. However, in the case of crystalline alloy precursor, it is relatively difficult to obtain a homogeneous nanoporous structure, since there are defects including compositional inhomogeneity. So far, only limited crystalline alloys have been reported to form a uniform nanoporous structure by dealloying [3–5]. Recent studies show that amorphous alloy is more suitable as precursor alloy for dealloying due to its homogeneous composition and structure down to atomic scale [5,6]. For crystalline alloy precursor, its crystal structure and some microstructural features such as size and orientation of the grains are retained during leaching out process [7]. On the contrary, amorphous alloy precursor cannot retain its original amorphous structure during leaching out process, because dissolution of less-noble element immediately changes the composition of the precursor alloy to be out of glass forming range. Therefore, in the case of amorphous precursor, surface diffusion process of remaining noble element is accompanied by nucleation and growth of crystalline phase, leading to formation of more homogeneous ∗
nanoporous structure [5,8–10]. Formation of such fine and homogeneous nanoporous metals using amorphous precursor alloys has been reported in several alloy systems such as Au-Cu-Ti-Si [2], Ag-Ca-Mg [11], and Mg-Cu-Y [8,12]. When dealloying Mg-Cu-Y amorphous alloy, dissolution of less noble elements induces Cu-enriched region at the interface between precursor alloy and electrolyte. The Cu atoms agglomerate into clusters, followed by nucleation of new crystal grains, leading to formation of poly-crystalline ligaments with uniform porous structure [8]. Since surface diffusion of noble element leads to formation and growth of nanoporous structure, the scale of nanoporous metal is largely affected by surface diffusivity of noble element and its composition in the precursor alloy. Here, the addition of another noble element with lower surface diffusivity than major noble element reduces overall diffusivity of noble elements and limits coarsening of ligaments, resulting in a very fine nanoporous structure. Dan et al. [13–15] have reported that the addition of Ag and Au in Ti-Cu amorphous alloy results in the formation of fine nanoporous Cu. Au and Ag atoms having lower surface diffusivity suppress the movement of residual Cu atoms, leading to formation of fine nanoporous structure as well as Au- and Agrich precipitate at grain boundary due to their low surface energy. In fact, there have been several studies on the fabrication of fine nanoporous metals through addition of minor noble element which precipitates at grain boundary after dealloying process. However, in
Corresponding author. E-mail address: [email protected] (D.H. Kim).
https://doi.org/10.1016/j.intermet.2019.106488 Received 11 January 2019; Received in revised form 19 March 2019; Accepted 26 April 2019 Available online 01 May 2019 0966-9795/ © 2019 Elsevier Ltd. All rights reserved.
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principle, minor noble element added to control overall surface diffusivity can exist in the form of either precipitate as mentioned above or solid solution with the primary noble element. M. Zhang et al. [16] have reported on the formation of Ag-Pd bimetallic nanoporous metal in which case Ag and Pd have sufficient mutual solid solubility. However, there have been few reports on the formation of metastable bimetallic (solid-solution type) nanoporous metal, when two noble elements have very little mutual solid solubility at room temperature. Nanoporous metals fabricated by dealloying process, as mentioned above, has received a great attention as a potential candidate for many functional applications such as sensor, catalyst, fuel cell and supercapacitor [17–23]. In recent years, sodium borohydride (NaBH4) have attracted attention as an energy/hydrogen carrier of direct borohydride fuel cell (DBFC) [24]. Numerous metals such as Pt, Au, Ag, Pd, Ni and Cu [25–31] have been studied for anode electrocatalyst of NaBH4 oxidation. However, since the kinetics of BH4− oxidation reaction on such monometallic anode electrocatalyst is very slow, the level of resulting power output is very low. On the contrary, it has been shown that bimetallic alloy catalysts exhibit higher electrocatalytic activity due to modified surface strain [32,33]. In the previous studies, carbon supported Au-Cu [34], Ag-Ni [35], Ag-Pt [36] and Ag-Cu [37,38] bimetallic nanoparticles have been shown to exhibit much higher catalytic activity than monometallic catalyst. However, there have been very few reports on the electrocatalytic performance when bimetallic nanoporous structure fabricated by dealloying is used for NaBH4 oxidation. Therefore, the aim of the present study is to fabricate nanoporous bimetallic Cu-Ag by dealloying of Mg-Cu-Y-Ag metallic glass and to estimate its electrocatalytic performance in NaBH4 oxidation. Cu-Ag bimetallic nanoporous structures were prepared by dealloying of a series of Mg65Cu25Y10 metallic glasses with minor addition of Ag, i.e. Mg65Cu25-xY10Agx (x = 0, 2, 4, 8) in sulfuric acid aqueous solution for 30 s. The electrocatalytic oxidation performance of bimetallic Cu-Ag nanoporous structure was evaluated in a mixture of 2.0 M NaOH and 0.1 M NaBH4.
Fig. 1. XRD patterns obtained from: (a) as-melt-spun Mg65Cu25Y10; (b) dealloyed Mg65Cu25Y10; (c) dealloyed Mg65Cu23Y10Ag2; (d) delloyed Mg65Cu21Y10Ag4; and (e) dealloyed Mg65Cu17Y10Ag8 (dealloying condition: in 1.0 M sulfuric acid aqueous solution for 30 s).
3. Results The XRD pattern obtained from as-melt-spun Mg65Cu25Y10 ribbon (Fig. 1 (a)) shows a broad halo peak, confirming the amorphous structure of as-melt-spun ribbon. The amorphous structure of as-meltspun Mg65Cu25-xY10Agx (x = 2, 4, 8) ribbons were also confirmed by XRD (not shown). Fig. 1(b)–(e) show XRD patterns obtained from meltspun Mg65Cu25-xY10Agx (x = 0, 2, 4, 8) samples dealloyed in in 1.0 M H2SO4 for 30 s. In the XRD pattern obtained from dealloyed Mg65Cu25Y10 sample (Fig. 1 (b)), the halo peak from the amorphous phase completely disappeared, while the sharp peaks from crystalline Cu and Cu2O appeared, as indexed in the diffraction pattern. In the XRD pattern obtained from dealloyed Mg65Cu23Y10Ag2 sample (Fig. 1 (c)), the peaks from crystalline Cu and Cu2O again appeared. However, the positions of the peaks from Cu were slightly shifted to lower angle side and the width of the peak became larger. The peak from crystalline Ag did not appear in spite of the addition of Ag, indicating that Ag atoms were completely alloyed into Cu ligaments. When 4 at% and 8 at% of Ag were added (Fig. 1(d) and (e)), the width of the diffraction peaks from Cu became much larger and the position of peaks shifted further to lower angle side. The shift and broadening of peaks from Cu indicate that the lattice parameter of Cu in dealloyed Mg65Cu25-xY10Agx was larger than that in dealloyed Mg65Cu25Y10 and the grain size of Cu ligaments in dealloyed Mg65Cu25-xY10Agx was smaller than that in dealloyed Mg65Cu25Y10. The peak from crystalline Ag is not clearly observed in the XRD pattern for dealloyed Mg65Cu21Y10Ag4 (Fig. 1 (d)), however, it is clearly observed in the XRD pattern from dealloyed Mg65Cu17Y10Ag8 (Fig. 1 (e)), indicating that the alloying behavior of Ag atoms forms supersaturated bimetallic Cu-Ag solid solution ligament and the residual Ag is present as precipitates. There are theoretical and experimental evidences that the solid solubility of components in binary alloys with nm scale grain size is greater than in analogous alloys with larger grain size [39,40]. It can be inferred that the solid solubility of Ag into Cu was extended with decrease of the grain size down to nm scale. Fig. 2 shows the SEM images obtained from dealloyed Mg65Cu25xY10Agx (x = 0, 2, 4, 8) samples. Continuous and uniform nanoporous structure was formed in dealloyed Mg65Cu25-xY10Agx (x = 0, 2, 4) samples, as shown in Fig. 2(a)–(c). It can be noticed that the size of the ligament decreases with increasing Ag amount, since minor element Ag decreases the overall diffusivity of noble elements [14,15]. The absence of Ag precipitates in Fig. 2(b) and (c) indicates that Cu and Ag atoms
2. Experimental Mg65Cu25-xY10Agx (x = 0, 2, 4, 8) alloy ingots were prepared by high-frequency induction melting the mixture of high-purity elements (Mg: 99.9%, Cu: 99.997%, Y: 99.9%, Ag: 99.99%) under argon atmosphere. Amorphous ribbons with 60–80 μm in thickness and 6–8 mm in width were obtained using melt spinning method on a copper wheel (diameter: 22 cm) rotating at a speed of 3000 rpm. Since the previous studies show that dealloying of Mg-Cu-Y metallic glass is successfully performed in sulfuric acid aqueous solution [8,12], dealloying process was performed in sulfuric acid aqueous solution with concentrations of 1.0 M for 30 s and 0.1 M for 10 min at room temperature. The immersion time was determined by the time when hydrogen gas evolution at the alloy/electrolyte interface was over completely. The crystal structure of melt-spun and dealloyed samples were analyzed using an X-ray diffractometer (XRD, Rigaku miniFlex 600) with Cu Ka radiation for a 2θ range of 20° to 80°. The morphology of dealloyed samples was observed using a scanning electron microscope (SEM, JEOL JSM-7001F). The microstructure and composition of dealloyed samples were investigated using a transmission electron microscope (TEM, JEOL JEM2100F) operated at 200 KeV with energy-dispersive X-ray spectrometer (EDS, Oxford). The elemental mapping was performed using a TEM (Fei Talos F200X) with super-X EDX system equipped with 4-SSD detectors (Fei). The TEM specimens were prepared by Focused Ion beam (FIB, JEOL JIB-4601F) or dispersion in ethanol. Electrochemical measurement was performed in a three-electrode cell using a carbon counter electrode and Hg/HgO as the reference electrode. The binder-free nanoporous ribbon was used as working electrode. Cyclic voltammetry (CV) curves at a scan rate of 5 mV/s were obtained at the potential range of −0.8 V to −0.4 V in a solution of 2.0 M NaOH and 0.1 M NaBH4. 2
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Fig. 2. SEM images obtained from: (a) dealloyed Mg65Cu25Y10; (b) dealloyed Mg65Cu23Y10Ag2; (c) delloyed Mg65Cu21Y10Ag4; and (d) dealloyed Mg65Cu17Y10Ag8 (dealloying condition: in 1.0 M sulfuric acid aqueous solution for 30 s).
(c)), confirming XRD and EDS results. The morphology was also consistent with that observed from SEM and TEM analysis. CV curves obtained from dealloyed Mg65Cu25Y10 electrode in 2.0 M NaOH solution and dealloyed Mg65Cu25-xY10Agx (x = 0, 2, 4, 8) electrodes in 2.0 M NaOH + 0.1 M NaBH4 solution at the potential rage of −0.8 V to −0.4 V are shown in Fig. 5. CV curves obtained from 2.0 M NaOH +0.1 M NaBH4 exhibit oxidation current peaks between −0.6 V and −0.4 V, while there is no oxidation current peak in CV curve obtained from 2.0 M NaOH solution, indicating that the oxidation peak is attributed to the oxidation of BH4− [37]. The anodic current densities of BH4− oxidation for Cu monometallic nanoporous electrode (Mg65Cu25Y10) and Cu-Ag bimetallic nanoporous electrodes (Mg65Cu25−2 , rexY10Agx, x = 2, 4, 8) were 35.3, 47.6, 50.4 and 58.2 mA cm spectively. The peak current densities of the BH4− oxidation for Cu-Ag bimetallic electrodes were much higher than Cu monometallic electrode. Moreover, it is noticeable that the electrocatalytic property increased with the increase of Ag content from 2 to 4 at. %, and then decreased when Ag content was 8 at. %. The result shows that solid solution alloying of Ag into Cu improves the electrode kinetics for oxidation of BH4−. However, the electrode kinetics and electrocatalytic oxidation property are deteriorated with the precipitation of Ag-rich particles on the surface of ligament, since Cu-Ag bimetallic ligament exhibits higher electrocatalytic property than Ag particle. Therefore, supersaturated bimetallic Cu-Ag nanoporous electrocatalyst, dealloyed from Mg65Cu21Y10Ag4 precursor, exhibits better electrocatalytic performance than the other dealloyed samples.
form a solid solution because of their extended solid solubility. However, when the amount of Ag was 8 at%, the particles which precipitated on the surface of ligament during dealloying were observed, as marked by arrows in Fig. 2 (d). Fig. 3(a) and (b) show bright field (BF) image and corresponding selected area diffraction pattern (SADP) obtained from dealloyed Mg65Cu21Y10Ag4 and Mg65Cu17Y10Ag8 samples, respectively. The results obtained from scanning TEM (STEM) and EDS analysis is shown in Fig. 3(c) and (d). As shown in Fig. 3 (a) and (c), the dealloyed Mg65Cu21Y10Ag4 sample consists of uniformly distributed ligaments with the average composition of 73.3% Cu, 23.1% Ag, 3.6% O without separating into Ag-rich or Cu-rich region (almost same composition in the region “1” and “2” marked in Fig. 3 (c)). The absence of Mg and Y in the EDS data indicates that the precursor alloys were completely dealloyed and nanoporous structure consists of Cu and Ag. The continuous ring-type SADP (Fig. 3 (a)) further confirms that the nanoporous structure consists of nm scale polycrystalline Cu. As the previous study has shown that Cu2O layer forms on the surface of Cu ligament [41], halo ring pattern form Cu2O was observed. When 8 at% of Ag was added, the nanoporous structure of dealloyed sample separated into CuAg bimetallic ligament and Ag-rich particles, as shown in Fig. 3 (b) and (d). The SADP shown in Fig. 3 (b) reveals a ring-type pattern, indicating the formation of nanoporous structure as well as Ag-rich precipitate. The EDS results shows that the compositions of Cu-Ag bimetallic ligament and Ag-rich particle are 87.8% Cu, 6.7% Ag, 5.5%, O and 23.7% Cu, 68.5% Ag, 7.8% O, which correspond to region “1” and “2” marked in Fig. 3 (d), respectively. As shown in Fig. 4, the formation of Cu-Ag bimetallic nanoporous structure in dealloyed Mg65Cu21Y10Ag4 was further analyzed by elemental mapping in TEM. It should be noted that the red and green colors representing Cu (Fig. 4 (a)) and Ag (Fig. 4 (b)), respectively, were homogeneously distributed comprising the supersaturated bimetallic nanoporous structure without any Cu-rich or Ag-rich regions (Fig. 4
4. Discussion To elucidate the precipitation behavior of Ag particle when dealloyed in dilluted sulfuric acid for longer time, Mg65Cu25Y10 and Mg65Cu21Y10Ag4 precursor alloys were dealloyed in 0.1 M H2SO4 for 10 min. The XRD pattern obtained from dealloyed Mg65Cu25Y10 3
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Fig. 3. BF images and corresponding SADPs obtained from: (a), (b) dealloyed Mg65Cu21Y10Ag4; and (c), (d) dealloyed Mg65Cu17Y10Ag8. STEM images obtained from: (e) dealloyed Mg65Cu21Y10Ag4; and (f) dealloyed Mg65Cu17Y10Ag8 (dealloying condition: in 1.0 M sulfuric acid aqueous solution for 30 s).
condition: 1.0 M H2SO4 for 30 s), however, interestingly the diffraction peak from Ag which was not present in Fig. 1 (d) was clearly observed. The width of the diffraction peaks was narrower when compared to those in Fig. 1 (d). Moreover, the shift of the peaks from Cu did not occur, i.e. the positions of the Cu peaks were identical to those in the XRD pattern obtained from dealloyed ternary Mg65Cu25Y10 (Fig. 1 (b)). The result indicates that the size of the grains constituting the ligaments becomes larger and Ag atoms were not able to be solutionized into Cu
precursor (Fig. 6 (a)) shows sharp peaks from crystalline Cu and Cu2O which is identical to the XRD pattern shown in Fig. 1 (b) (dealloying condition: 1.0 M H2SO4 for 30 s). The results indicate that without addition of noble element (Ag), the change of concentration of solution and/or immersion time has little effect on the formation of nanoporous structure. However, in the XRD pattern obtained from dealloyed Mg65Cu21Y10Ag4 (Fig. 6 (b)), the diffraction peaks from Cu and Cu2O were present, as in the XRD pattern shown in Fig. 1 (d) (dealloying
Fig. 4. Elemental distribution of: (a) Cu; (b) Ag; and (c) both Cu and Ag in dealloyed Mg65Cu21Y10Ag4 (dealloying condition: in 1.0 M sulfuric acid aqueous solution for 30 s). 4
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clusters are formed by dissolution of less noble atoms at the initial stage, and then nucleation of new crystals occurs by surface diffusion [5,8–10]. In such case, when the dissolution rate of less noble element becomes higher (i.e. shorter immersion time in high concentration acidic solution), the nucleation rate of new crystals increases and less time is available for coarsening of crystals, indicating that grain size of nanoporous structure is determined by immersion time. The schematic diagrams illustrating nucleation and growth process in different two conditions of: 1) high dissolution rate and short immersion time; and 2) low dissolution rate and long immersion time are shown in Fig. 7. When the amorphous precursor immersed in 1.0 M H2SO4 for 30 s, Mg and Y atoms are rapidly dissolved out and residual noble atoms agglomerate into many tiny clusters (Fig. 7(a) and (b)). The clusters coalesce and form ligament composed of nano grain which has extended solid solubility (Fig. 7 (c)). In contrast, when the amorphous precursor immersed in 0.1 M H2SO4 for 10 min, Mg and Y are dissolved more slowly (Fig. 7 (d)). The noble elements form less clusters and grow larger until the active atoms are completely dissolved (Fig. 7 (e)). The ligament formed for longer immersion time consist of coalesced grain that exhibits limited solid solubility with Ag particles on the surface of Cu ligament (Fig. 7 (f)). In the present study, in order to increase the solid solubility by decreasing grain size down to nm scale, the dealloying process was performed in the acidic solution with higher concentration for shorter immersion time. It has been shown that the electrocatalytic activity for NaBH4 oxidation is better when it is in bimetallic form than in monometallic form. The alloying behavior leads to modification of the surface strain, increasing the catalytic activity. D. Duan et al. reported that carbon supported bimetallic Cu-Ag nanoparticle improve the catalytic property for the DBFC compared to single Ag. For the Cu-Ag/C catalysts, the peak current density for BH4− oxidation are 41.61 (Cu50Ag50/C), 47.07 (Cu67Ag33/C) and 21.64 (Cu80Ag20/C) mA cm−2, which are higher than Ag/C catalyst (14.53 mA cm−2) [38]. However, the present study
Fig. 5. CV curves of dealloyed Mg65Cu25-xY10Agx electrodes in a mixture solution of 2.0 M NaOH and 0.1 M NaBH4 at a scan rate 50 mV/s (dealloying condition: in 1.0 M sulfuric acid aqueous solution for 30 s).
ligament when dealloyed in dilluted sulfuric acid for longer time. As the grain size in the ligament increases, the solid solubility of Ag into Cu decreases and the residual Ag precipitates separately on the surface of the ligament. It was further analyzed by element mapping that the ligament consists of only Cu and Ag-rich particle precipitates on the surface of the ligament (Fig. 6 (c)). The present study shows that the supersaturated bimetallic nanoporous Cu-Ag can be synthesized by dealloying of amorphous alloy. When dealloying the amorphous alloys, two important processing parameters are dissolution rate and immersion time. The previous study has shown that, when dealloying amorphous alloys, noble element
Fig. 6. XRD patterns obtained from: (a) dealloyed Mg65Cu25Y10; (b) dealloyed Mg65Cu21Y10Ag4; and (c) elemental distribution of Cu, Ag and both Cu and Ag in dealloyed Mg65Cu21Y10Ag4 (dealloying condition: in 0.1 M sulfuric acid aqueous solution for 10 min). 5
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Fig. 7. Schematic illustrations of dealloying process: (a) amorphous precursor.; (b) dissolution of Mg and Y atoms and agglomeration of Cu and Ag into tiny clusters when dealloyed in 1.0 M H2SO4; (c) formation of bimetallic Cu-Ag nanoporous structure with nm scale grains; (d) dissolution of Mg and Y atoms when dealloyed in 0.1 M H2SO4; (e) coarsening of clusters during dissolution of Mg and Y atoms; (f) formation of coarse grain Cu ligament with Ag particles on the surface of the ligament.
References
clearly shows that nanoporous metal prepared by dealloying shows better electrocatalytic property compared with the carbon supported nanoparticles, since the nanoporous structure can provide higher specific area for catalytic active site, and the large open-pore structure for easier mass transport during the electrode reactions. Moreover, since the ribbon shape of precursor alloys remained undamaged after dealloying, it can be used as working electrode without binder while carbon supported catalysts should be mixed with conductive binding materials.
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5. Conclusion Mg65Cu25-xY10Agx (x = 0, 2, 4, 8) amorphous alloys are dealloyed in sulfuric acid aqueous solution under free corrosion condition. As a result, uniform nanoporous Cu is fabricated by dealloying amorphous precursor. Very fine nanoporous structure can be obtained with addition of Ag which decreases the surface diffusivity. When Mg65Cu21Y10Ag4 amorphous precursor is dealloyed in in high concentration of sulfuric acid aqueous solution (1.0 M) with short immersion time (30 s), Cu-Ag bimetallic nanoporous structure is formed in spite of their small solid solubility which is less than 1% at room temperature. When the Ag content exceeds the extended solid solubility, Ag-rich particles precipitate on the surface of ligament, as can be seen in dealloyed Mg65Cu17Y10Ag8. When Mg65Cu21Y10Ag4 amorphous precursor is dealloyed in lower concentration solution (0.1 M) for longer time (10 min), coarse-grain Cu monometallic ligament with Ag particle on the surface of the ligament is formed. Electrochemical oxidation of NaBH4 is examined in 2.0 M NaOH +0.1 M NaBH4 solution. Compared with monometallic Cu nanoporous structure, bimetallic Cu-Ag nanoporous structure significantly improves the catalytic property. The highest current density of NaBH4 is recorded for dealloyed Mg65Cu21Y10Ag4 sample. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation by the Ministry of Science, ICT (Information & Communication Technology) and Future Planning, Korea (2016R1A2B2013838). 6
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