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Micro-Morphology and Phase Composition Manipulation of Nanoporous Gold with High Methanol Electro-Oxidation Catalytic Activity through Adding the Magnetic Field in Dealloying Process Hui Xu, Kechang Shen, Shuai Liu, Lai-Chang Zhang, Xiaoguang Wang, Jingyu Qin, and Weimin Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10475 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Micro-Morphology and Phase Composition Manipulation of Nanoporous Gold with High Methanol Electro-Oxidation Catalytic Activity through Adding the Magnetic Field in Dealloying Process Hui Xua, Kechang Shena, Shuai Liua, Lai-Chang Zhangb,*, Xiaoguang Wangc, Jingyu Qina, and Weimin Wanga,* a

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of

Education, Shandong University, Jinan 250061, China b

School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, Perth,

WA6027, Australia c

Laboratory of Advanced Materials & Energy Electrochemistry, Taiyuan University of Technology,

Taiyuan 030024, China ABSTRACT By dealloying a rapidly solidified (RS) Al2Au alloy, the nanoporous gold (np-Au) samples with a three-dimensional (3D) interpenetrating ligament-channel structure are fabricated in 0, 0.02, and 0.2 T magnetic fields. Adding magnetic field leads to the intermediate AlAu phase formation, decreases the dealloying rate, and triggers the formation of fine nanocrystals and amorphous phase in the np-Au ligaments. The np-Au samples dealloyed in 0, 0.02, and 0.2 T for 24 h (DA0, DA1, and DA2) appear in maze-like, honeycomb-like and soda crackers-like micro-morphologies, respectively. The DA1 sample possesses the smallest lattice constant a0 and highest preferred orientation factor F(111)

*Corresponding author. Email address: [email protected] (W.M. Wang); [email protected], [email protected] (L.-C. Zhang) 1 ACS Paragon Plus Environment

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of (111) face. The parameters like surface coverage of the redox species Г*, charge-transfer rate constant ks, exchange current density j0, corrosion potential Ecorr, and charge transfer resistance Rct indicate that the methanol electro-oxidation activity of three samples is in this order: DA1 ＞ DA2 ＞ DA0, which can be ascribed to crystallographic, thermodynamic and defective reasons. This work supplies a new method to enhance the methanol catalytic activity of np-Au.

1. INTRODUCTION Recently, due to their self-supported structures and high surface-to-volume ratio, nanoporous materials have a wide potential applications in fields such as electrocatalysis,1-3 sensors,4 actuators,5 lithium-ion battery,6, 7 fuel cell,8 and so on. Among the porous candidates, nanoporous gold (np-Au) exhibits a remarkable performance in heterogeneous catalysis,9, 10 electrocatalysis,11, 12 and surface enhanced Raman scattering,13 owing to its unique stable bicontinuous structure with empty channels and solid ligaments. The ligaments/channels size distribution has a significant influence on the catalytic properties of np-Au, especially for the methanol electro-oxidation reaction (MOR).13-15 There are several advantages of Au for MOR. Firstly, noble metals are the good catalysts for MOR in the alkaline solution. Among these metals, Pt and Pd based alloys have the superior activity for MOR.16, 17 Unfortunately, the surfaces of these catalysts can be poisoned by some intermediate species (such as COads) produced in MOR, and further reduced the catalytic activity. However, researchers have found that the surface poisoning phenomenon doesn’t exist on the Au surface, because the MOR on np-Au electrodes comprises of two different reactions in two potential regions. 11, 18-20

This two-step mechanism provides more reaction channels for methanol oxidation, allows

reaction intermediates formed at low potentials, and then eliminates the possible catalyst poisoning.11, 18-20 Secondly, compared to the polycrystalline Au electrode, np-Au exhibits much 2 ACS Paragon Plus Environment

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higher electrocatalytic activity for MOR in alkaline media, due to its unique bi-continues open porous structure and high specific surface areas. The formation of the pre-oxidation species, such as (1− λ ) − Au- OH ads in the low potential region due to the chemisorbed OH- is beneficial for the MOR.11,

19, 21

And compared to the bulk Au electrodes, a porous morphology has the advantage of trapping

OH- anions ,which is crucial in MOR.11 Np-Au can be obtained by dealloying the precursor like binary AuxM1-x, in which the second component M is selectively etched.4 The microstructure of np-Au can be modified by altering the dealloying solution,22,

23

dealloying time,24 dealloying temperature25 and precursor alloy’s

composition.26 Generally, np-Au performs a tunable porosity from a few nanometers to several microns.24 For example, the ligaments/channels of the np-Au dealloyed from Au32Ag68 alloy in nitric acid at room temperature are in the scope of 20-40 nm.27, 28 The average size of nanopores reaches ~3 nm after dealloying Ag-Au alloys at -20 oC for 1 h.25 In addition, we have found that the pore size of np-Au is negatively related to the circumferential speeds (Sc) in preparing the melt-spun precursor.29 Dealloying with the rapidly solidified (RS) technology has several advantages: (1) this method is simple, facile and low-cost;30 (2) the high cooling rate of rapid solidification processing from the melts could be 104-108 K s-1, resulting in the extremely refined microstructure;

31

and (3) RS

technology leads to a non-equilibrium solid with a high solubility of solute elements.32 In recent years, owing to the magnetic anisotropy of the phase and crystal, phase alignment and crystal orientation can be adjusted by applying a magnetic field in manufacturing process.33 According to the results of Hu et al.,34-37 the effects of magnetic field on electrochemical behavior of metallic materials in aqueous solutions can be divided into two aspects: (1) stirring the 3 ACS Paragon Plus Environment


ferromagnetic electrode or paramagnetic ions and (2) affecting the electrodeposition of metals. Magnetohydronamic (MHD) theory is generally applied to formulate the effect of magnetic fields on mass transport rates in the liquid.34,

35, 37-39

The Lorentz force can cause a stirring of the

electrolyte by accelerating charges moving in the direction perpendicular to the current and the flux density.40 However, there is little research about the influence of the magnetic field on the dealloying process to authors’ knowledge. Hence, the incorporation of the magnetic field in dealloying process is expected to obtain the np-Au abnormal structure and activity. In this work, we chose binary Al0.66Au0.34 alloy as the precursor. The Al0.66Au0.34 ribbons with Sc = 14.7 m/s were prepared by the RS technology. In order to research the influence of magnetic field on the development of microstructure of np-Au, we selected different magnetic field strength and different time in dealloying process. By operating a series of electrochemical experiments, we try to find the relationship between the microstructures and electrocatalytic activity of np-Au for methanol electro-oxidation reaction (MOR). The connection between the magnetic field and electrocatalytic activity is possibly to supply a new method to improve the catalytic activity of np-Au.

2. EXPERIMENTAL SECTION 2.1. Chemicals. According to the Al2Au intermetallic phase in the binary phase diagram (Figure S1 in the Supporting Information), the ingots with the nominal composition Al0.66Au0.34 (at. %) were prepared by induction-melting the pure bulk metals (Al, 99.99 wt%; Au, 99.99 wt%) in an argon atmosphere and using recycled water cooling. By using a single roller melt spinning apparatus, the prepared ingots (about 15-20 g) were remelted by high-frequency induction heating in a quartz tube and then melt-spun onto a copper roller with a diameter of 0.35 m at the Sc of 14.7 4 ACS Paragon Plus Environment

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m/s. The ribbons obtained were typically 20-50 µm in thickness, 2-4 mm in width, and several centimeters in length. The rapidly solidified (RS) Al0.66Au0.34 alloy ribbons were dealloyed in a 5 wt% HCl aqueous solution at room temperature until no obvious bubbles emerged. The whole dealloying process of the RS ribbons are proceeding in 0, 0.02, and 0.2 T magnetic fields. The constant magnetic field equipments were provided by Yingpu Magnetoelectric Ltd. (Changchun, China, see Figure S2) In order to discover the structure evolution of the np-Au through dealloying, we checked the samples every 6 h. The whole dealloying time was 24 h.

2.2. Methods. The phases in the RS Al0.66Au0.34 alloys and as-dealloyed samples were identified using an X-ray diffractometer (XRD, D/MAX 2500/PC, Rigaku, Tokyo, Japan, Cu Kα, λ = 0.154056 nm) in the 2θ range from 10 to 90o. The lattice constant a0 of RS ribbon and np-Au samples can be obtained from the Rietveld analysis method with the deduction of Kα2 lines. We also calculated the preferred orientation factors F of (111) face of np-Au samples by using the Lotgering method.41 The microstructure of the np-Au samples was characterized using a scanning electron microscope (SEM, SUPRA55, Zeiss, Germany) and a transmission electron microscope (TEM, Tecnai G2 F30 S-TWIN, Philips-FEI, Netherlands) with selected-area electron diffraction (SAED) patterns. Some TEM specimens were also observed using high-resolution TEM (HRTEM). The electrocatalytic activities of np-Au dealloyed for 24 h in different magnetic fields for methanol electro-oxidation reaction (MOR) were carried out in a standard three-electrode cell using an electrochemical workstation (CHI 660E, Chenhua instrument Ltd., Shanghai). The np-Au samples were used as the working electrodes. The counter electrode was a bright Pt plate, and a Hg/HgO (1.0 M KOH) electrode (MMO) or a Hg/Hg2SO4 electrode (MSE) were used as the 5 ACS Paragon Plus Environment


reference electrode, depending on the experimental requirements. In the cyclic voltammetry (CV) measurements, the electrolytes were 0.5 M KOH + 0.5 M methanol and 0.5 M H2SO4 + 0.5 M methanol, respectively. The scanning rate v was in the range of 2-500 mV/s. In order to further study the np-Au catalysis activity for methanol, the quasi steady-state anodic Tafel polarization analysis with the v of 1 mV/s and electrochemical impedance spectroscopy (EIS) with the 5 mV amplitude were also measured in the same electrolytes. Here the EIS tests were carried out in the frequency ranging from 0.1 Hz to 104 Hz at the open circuit potential and the data of EIS were fitted by the Zsimdemo software. All electrochemical experiments were performed at ambient temperature (298 K). And all current densities were normalized by the real surface of the np-Au electrodes.

3. Results and discussion 3.1. Structural analysis of RS Al0.66Au0.34 ribbons and np-Au. Figure 1 shows the XRD patterns of Al0.66Au0.34 ingot and rapidly solidified (RS) ribbon with Sc = 14.7 m/s. Al0.66Au0.34 RS ribbon is in beautiful purple color (inset in Figure 1). Apparently, both ingot and ribbon consist of intermetallic phase identified as Al2Au (PDF No. 17-0877). Three strongest diffraction peaks at 25, 42, and 51 deg are corresponding to (111), (220), and (311) faces, respectively. In order to obtain the error of the lattice constant, the XRD analyses have been repeated as shown in Figure S3. The a0 of original ingot and RS ribbon are 0.6008 ± 0.0003 and 0.6006 ± 0.0003 nm respectively, being larger than that of pure Al2Au (a0 = 0.5997 nm). Cui et al. have reported that the melt temperature gradient vertical to the roller surface during the RS process can lead to the stress concentration and lattice distortion.42 The composition deviation can also cause the lattice distortion.43 Thus, the a0 6 ACS Paragon Plus Environment

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expansion of ingot and ribbon is properly related to the stress concentration and composition deviation. Figure 2 exhibits the XRD patterns of the constituent evolution of the RS ribbons immersed in 5 wt% HCl solution for 0, 6, 12, 18, and 24 h in 0, 0.02, and 0.2 T magnetic fields. During immersing in HCl solution, Al atoms are selectively dissolved from the precursor into the solution and fcc Au peaks in XRD patterns rise with the extension of dealloying time (Figure 2a-c). The (111), (220) and (311) diffraction peaks of Al2Au phase almost disappear when RS ribbons are dealloyed for 12 h without the magnetic field (Figure 2a), while these peaks disappear after dealloyed for 18 h with the addition of magnetic field (Figure 2b and c). These results indicate that the magnetic field slows down the dissolution of Al2Au. According to the mechanism raised by Erlebacher,27 the dealloying process starts with the dissolution of a single solute atom (Ag atom) on a flat alloy surface of close-packed (111) orientation, leaving a terrace vacancy. It is demonstrated that the magnetic field slows down the dissolution of (111) face. Moreover, the intermediate AlAu phase can be identified when dealloying lasts for 6 and 12 h in magnetic field. In other words, without the magnetic field during the dealloying process, the phase transformation is direct, i.e. from Al2Au to Au; while under the magnetic field, the phase transformation is indirect, i.e. from Al2Au to AlAu and then to Au. The enlarged parts of the (111) diffraction peaks of np-Au samples beyond the half maximumn (HM) tend to be shorter and wider with increasing the dealloying time (Figure 2d and e). Compared with the np-Au dealloyed for 24 h in 0 T, the (111) peaks of np-Au dealloyed in 0.02 and 0.2 T become broader and the HM of (111) peak of np-Au dealloyed in 0.02 T is broader than that in 0.2 T. In addition, the broadening of (111) peaks can also be observed in np-Au dealloyed in 20 wt% NaOH under the magnetic field (Figure S4). The peak broadening could be attributed to the 7 ACS Paragon Plus Environment


formation of nanosized polycrystals.44 The preferred orientation factors F(111) of np-Au samples dealloyed for 24 h in 0, 0.02, and 0.2 T are 0.198, 0.284, and 0.237, respectively, i.e. magnetic field can raise the F(111) of np-Au samples in the dealloying process. The color of np-Au ribbon dealloyed for 24 h in 0.02 T shifts from purple to yellow (inset in Figure 2e). The samples dealloyed for 24 h under 0 and 0.2 T have a similar color, which are not shown here. In order to further verify the hindrance of magnetic field in dealloying process, Table 1 lists the a0 of Au phase and its volume fraction fv in dealloyed RS ribbons for different times by using the Rietveld analysis. With increasing the dealloying time, the a0 of Au phase increases and approaches that of the standard Au (0.40790 nm); meanwhile; the fv of Al2Au phase gradually decreases and becomes zero as dealloying time reaches 24 h. Before its exhaustion, the fv of residual Al2Au phase increases with increasing magnetic field. The a0 of Au phase in the samples dealloyed for 12, 18, and 24 h under three types of magnetic fields are smaller than the standard Au, which may be induced by the inner compressive stress in RS ribbons. The a0 of samples dealloyed in 0.02 and 0.2 T are smaller than the counterpart in 0 T, which indirectly demonstrated the magnetic field induced hindrance in the dealloying process.

3.2 Morphology of np-Au with 24 h dealloying. Figure 3 gives the SEM micrographs of RS ribbons dealloyed for 6, 12 and 18 h in 0, 0.02 and 0.2 T. The insets are the corresponding enlarged images. Under 0 T, the pore sizes of samples firstly increase and then decrease with increasing dealloying time (Figure 3a-c), indicating that Al atoms dissolve firstly and Au ligaments coarse afterward. Under 0.02 T, the pore sizes of sample after dealloyed for 6 h is similar to that under 0 T (Figure 3d), but the ligament coarsening of samples dealloyed for 12 and 18 h is weaker than that under 0 T (Figure 3e and f). Under 0.2 T, the residual Al-Au phases of samples dealloyed 8 ACS Paragon Plus Environment

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for 6 and 12 h are more than the counterparts under 0 and 0.02 T (Figure 3g and h); while the ligaments of the sample dealloyed for 18 h are coarser than the counterparts (Figure 3i). Figure 4 shows the SEM images of RS ribbons dealloyed for 24 h in 0, 0.02, and 0.2 T. The insets are the corresponding physical figures. Hereafter, we name the np-Au samples with 24h dealloying time in 0, 0.02 and 0.2 T as DA0, DA1 and DA2, respectively. In the low magnification image, DA0 has a 3D bicontinuous interpenetrating ligament-channel structure with several troughs overlapped the pore-ligament matrix (Figure 4a). DA1 and DA2 have some grain boundaries like phase upon the pore-ligament matrix (Figure 4b and c). According to the XRD results in Figure 1, the samples dealloyed for 24 h contain only Au phase, indicating that the fraction of grain boundary like phase is very low. In the high magnification image, the ligament size of DA0 is about 70 nm and its morphology appears in a maze-like shape (Figure 4d and inset). DA1 holds a uniform structure with a thinner ligaments than DA0, showing a uniform honeycomb-like shape (Figure 4e and inset). The whole morphology of DA2 is uniform and compact, but its pore size decreases and its ligament becomes coarse, exhibiting a soda cracker-like shape (Figure 4f and inset). The section-view SEM images of DA0, DA1, and DA2 can be seen in the Figure S5. Figure 5 shows the TEM and HRTEM images of DA0, DA1 and DA2 as well as SAED patterns. As shown in the low magnification images, DA0 has a 3D bicontinuous interpenetrating ligament (dark skeleton)-channel (bright region) structure (Figure 5a), which is consistent with the SEM images in Figure 4a and d. The ligament size of DA1 (Figure 5b) is smaller than that of DA0. The ligament size of DA2 is the largest (Figure 5c), which is corresponding to the coarse ligaments in SEM images (Figure 4c and f). Compared with DA0, the grain boundaries of DA1 and DA2 can be obviously observed, and the grain size of DA2 is smaller than that of DA1 (insets in Figure 5a-c). 9 ACS Paragon Plus Environment


The corresponding HRTEM images also reflect the grain sizes of three samples (Figure 5d-f). It is hard to see the obvious grain boundary in DA0, while several grains can be seen in DA1 and DA2, and the grain size of DA2 is smaller than that of DA1. Moreover, the (111) and (200) plane spacings of selected crystals of three samples are consistent with the standard Au within the allowed error ranges. In addition, the regular arranged lattice fringes with different orientations were observed in Figure 5d, which may be derived from the matrix grain and the subgrain (indicated as the red dotted ellipse). The matrix grain and subgrain show the similar interplaner spacing, showing the coarse nanocrystals nature of the DA0. In the corresponding SAED pattern of DA0, there are two sets of diffraction spots. One set is bright, which can be identified as fcc Au, and the other is dim (inset in Figure 5d). The bright diffraction spots are consistent with the matrix grain and the dim spots are possibly corresponding to subgrain in HRTEM (Figure 5d). The SAED pattern of DA1 consists of a set of polycrystalline ring, corresponding to (111), (311), (220) and (200) faces of fcc Au, which indicates that the ligaments in DA1 comprise a tremendous amount of Au nanocrystals; and the shallow halo under the (111) ring verifies the small amount of amorphous zone (inset in Figure 5e). Except for the obvious polycrystalline rings and nanocrystalline feature, the SAED patterns of DA2 also contain one shallow amorphous halo (inset in Figure 5f). Also, several disordered arranged atoms reflected the amorphous feature can also be observed in the grain boundaries of the HRTEM images in DA1 and DA2 samples (the bright areas in Figure 5e and f), and the amorphous area in DA2 sample is slightly larger than that in DA1. The amorphous might result from the lattice defects, such as dislocations and stacking faults. Another research shows that the amorphous phase is one transition state in the dealloying process.45 On the whole, the phase compositions of np-Au dealloyed from 10 ACS Paragon Plus Environment

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acid solution for 24 h in 0, 0.02, and 0.2 T are coarse nanocrystals, fine nanocrystals + trace amorphous phase, and finer nanocrystals + a few amorphous phase, respectively. In general, during the dealloying process, the ligaments of np-Au can be thinned with 0.02 T but be coarsened with 0.2 T. Meanwhile, the magnetic field can refine the grains in the ligaments and trigger the formation of amorphous phase. Combining the XRD results, SEM and TEM images (Table 1 and Figures. 2-5), we draw a comprehensible schematic diagram to present the microstructure evolution of RS ribbon dealloyed in 5 wt% HCl solution under magnetic fields, as shown in Figure 6. According to the results of Sueptitz et al.,46 there are two forces in an electrochemical reaction superimposing a magnetic field, i.e. the Lorentz force and the magnetic field gradient force. The Lorentz force induces an electrolyte convection which enhances the mass transfer, while the magnetic field gradient force can overcome the impact of the Lorentz force by pulling the ions.46 Under the magnetic gradient force, the surface Au atoms tend to form the dense Au atom film, this phenomenon can also be observed in several reports.47, 48 The dense Au film hinders the dissolution of the internal Al atoms into the solution, leading to a slow dealloying speed and the formation of the intermediate phase AlAu. The intermediate phase AlAu can also be served as nucleation sites in the rearranging process of Au atoms, and thus leads to a smaller ligament size of np-Au compared with the single-phase Al2Au according to the refs.22,

49

It is understood that in the dealloying initial stage (6 h), samples

dealloyed in 0.02 and 0.2 T hold the smaller size of ligaments compared with that in 0 T (the first column in Figure 6). On the other hand, according to the magnetohydronamic (MHD) theory, the Lorentz force can induce a stirring of the electrolyte by accelerating charges moving in the direction perpendicular to 11 ACS Paragon Plus Environment


the current and the flux density.34, 40 The magnetic field also leads to a convective movement of the species to the reactant surface, and induces an increase of the mass transport.38, 50, 51 Lorentz force plays a dominant role in the ligament coarsening process. As the dealloying time is 18 h or up, the Lorentz force caused by 0.2 T is higher than that by 0.02 T, which leads to a higher mass transport rate of the Au atoms, and then results in the coarsening of ligaments of the sample dealloyed in 0.2 T. Under the combined reaction of Lorentz force and magnetic gradient force, it is understood that DA1 in 0.02 T has the smallest size of the ligament compared with the counterparts (the second row in Figure 6). In short words, the ligament thickness, grain size, phase compositions, and intermediate AlAu formation are related with the Lorentz and magnetic gradient forces in dealloying.

3.3 CV measurements of np-Au electrodes for methanol electro-oxidation reaction (MOR). In present paper, the real surface areas of np-Au electrodes (DA0, DA1, and DA2) are about 1.30, 2.05, and 1.59 cm2 based on the charges needed to form gold surface oxide monolayer according to the oxygen adsorption measurement method. The CV curves of np-Au electrodes have been measured in 0.5 M H2SO4 solution with the scan rate v = 50 mV/s (Figure S6). Since the geometry area of each electrode is 0.2 cm2, the surface roughness factor rf (defined as the ratio between the real surface and geometry area) of DA0, DA1, and DA2 are 6.50, 10.25, and 7.95, respectively. The current densities are calculated by the real surface areas. Figure 7 represents the CVs of DA1 at v = 2-500 mV/s in 0.5 M KOH + 0.5 M methanol solution. CVs of DA0 and DA2 in the same condition can be seen in Figures 8 and 9, respectively. In the positive scanning part, DA1 has two well-defined peaks. With increasing v, the peak current densities for methanol electro-oxidation reaction (MOR) increase and their onset potentials shift 12 ACS Paragon Plus Environment

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positively (Figure 7a). It has been verified that MOR on Au electrode proceeds in two different regions with different mechanisms.19, 20, 52 The first region is from -0.6 to 0.4 V, which partially overlaps with the np-Au surface oxide formation; and the second starts at ~ 0.45 V on oxidized np-Au surfaces. In the first region, methanol was mainly oxidized to formates via an overall four-electron-transfer reaction as follows:19, 52 CH3OH + 5 OH- = HCOO- + 4 H2O + 4 e-

(1)

The MOR at the oxide-free surface commences at -0.6 V and proceeds in a slow rate until about -0.4 V. The onset of this reaction depends on the amounts of OH- anions chemisorbed on the np-Au surface. With ongoing positive potential sweep, more OH- anions are adsorbed onto the np-Au surface and react with gold surface atoms, resulting in the formation of “pre-oxidation species”, i.e. (1− λ ) −

Au- OH ads

, where λ refers to the charge-transfer coefficient.53 With increasing potential, the

anodic current densities rises significantly on partially oxidized np-Au, and reaches a maximum at 0.2 V. Borkowska et al. have reported that MOR is not only relate to the chemisorbed OH- anions (1− λ )− 19, 21 but also strongly relies on the Au- OHads . Both MOR and Au surface oxidation take place in

the lower potential region. In the second region (> 0.45 V), a sharp increase in the anodic current densities for MOR is observed, which is associated with the anodic oxidation of methanol to form carbonates by gold oxides:19-21 CH3OH + 8 OH- = CO32- + 6 H2O + 6 e-

(2)

Here, instead of the four-electron-transfer reaction, a six-electron-charge-transfer process generates in oxidized np-Au surface. This reaction mechanism of np-Au allows the reaction intermediates produced at low potentials to be further oxidized.11 The negatively-going scan shows that the 13 ACS Paragon Plus Environment


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electrochemical reduction of the gold surface oxides leads to the reduction peaks. With removing (1− λ )−

dense AuO, Au- OHads

species are regenerated on the np-Au surface, stimulating the re-initiation

of the MOR according to Eq. (1).19, 21 After reaching the peak potential, the absolute values of corresponding oxidation current densities rise immediately, which is coincident with the earlier report.21 The onset potentials of cathodic peaks of DA1 shift negatively and their current densities increase with increasing v. Compared with DA0 and DA2 (Figures 8a and 9a), the onset potentials of both anodic and cathodic peaks of DA1 shift negatively and its peak current densities increase, showing its higher catalytic activity for MOR. The plot of Ep vs. logv can be extracted from CVs records at the ranges of 2-500 mV/s for anodic and cathodic peaks (Figure 7b). The peak-to-peak potential separation ΔEp (= Epa – Epc) is 200 mV at v = 2 mV/s. The increase of ΔEp with v reveals the presence of charge transfer kinetic limitation.54 When Δ Ep > 0.2/n V, where n is the number of exchanged electrons, the electrochemical parameters such as electron-transfer coefficient (α), and the apparent charge-transfer rate constant between the electrode and the surface-deposited layer (ks) can be obtained from the following equations according to the Laviron theory:55, 56 Epa = E 0 +

 RTks  RT RT ln  lnv + (1 − α ) nF  (1 − α ) nF  (1 − α ) nF

(3)

E pc = E 0 +

RT  RTk s  RT ln  − lnv αnF  αnF  αnF

(4)

ln ks = α ln(1 − α ) + (1 − α ) ln α − ln

RT α (1 − α ) nF ∆E p − nFv RT

(5)

where Epa, Epc and E0 are the anodic peaks, cathodic peaks, and the standard potentials (V), respectively. At 2-80 mV/s, the curve of Epc vs. logv can be fitted linearly (Figure 7b). Thus, the calculated α and ks of DA1 were 0.1146 and 0.0197 s-1 from Eqs. (3)-(5), respectively. According to 14 ACS Paragon Plus Environment

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Refs,54, 57 the present rate limiting step is electrochemical process. Based on the CVs of np-Au at different v, the peak’ current densities are proportional to v in the range of 2-80 mV/s (Figure 7c) pointing to the electrochemical activity of the surface redox couple.58 The surface coverage Г* (mol/cm2) of the redox species can be obtained from the slopes of jp-v line and Eq. (6):59, 60 jp =

n 2 F 2 AvΓ * 4 RT

(6)

With concerning the average of both cathodic and anodic current densities, the total surface coverage of the immobilized active species in np-Au electrode is calculated about 2.525×10-6 mol/cm2. When v is higher than 80 mV/s, the anodic and cathodic peak current densities are linearly related with v1/2, meaning that the process is dominantly controlled by diffusion.58 Similar to the redox peaks in alkaline solution, MOR in DA1 electrode proceeds in two potential regions with different mechanisms in 0.5 M H2SO4 + 0.5 M methanol solution at different v (Figure 10a). The CVs of DA0 and DA2 in 0.5 M H2SO4 + 0.5 M methanol at v = 2-500 mV/s can be seen in Figures 11 and 12, respectively. In the low potential region, methanol is mainly oxidized to formates via an overall four-electron-transfer reaction (Eq. (1)) in the surface of np-Au. In the high potential region, methanol oxidation is associated with the reaction (Eq. (2)) of the anodic oxidation of methanol to form carbonates in oxidized np-Au surface. There is no obvious region resulting in the formation of “pre-oxidation species”, which can be attributed to the low OH- concentration in the acid electrolyte. Moreover, compared with the DA0 and DA2 (Figures 11a and 12a), the onset potentials of both anodic and cathodic peaks of DA1 shift negatively and the current densities increase, showing that the MOR activity of samples ranks in DA1 > DA2 > DA0, which is consistent with the results in alkaline solution. And the pre-oxidation peaks of DA1 in alkaline 15 ACS Paragon Plus Environment


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solution (Figure 7a) are more obvious than those in the acid solution (Figure 10a). A linear relation between Ep and logv appears at 2-80 mV/s in Figure 10b. According to Eqs. (3)-(5), the value of α is 0.1456, and the mean value of Ks is 0.0043 s-1. The surface coverage Г* of the redox species calculated from Eq. (6) and Figure 7c is 2.1959 × 10-6 mol/cm-2. The electrochemical parameters such as Г*, α, and ks of three samples in both acid and alkaline solutions are listed in Table 2. Here, the Г* and ks of each np-Au electrode in acid solution are lower than the counterpart in alkaline solution, meaning that np-Au samples exhibit a better catalytic activity in alkaline solution due to the OH- onions concentration, which is consistent with our earlier work.11 The Г* and ks of three samples in both acid and alkaline solutions are in the following order: DA1 ＞ DA2 ＞ DA0, which means that methanol molecules can be absorbed and further oxidized on DA1 surface more easily and quickly. The CVs of DA0, DA1, and DA2 in both acid and alkaline solutions without methanol are shown in Figure S7. Figure S8 represents Ep vs. logv which is extracted from the CVs recorded at the ranges of 2-500 mV/s for both anodic and cathodic cycles. Both the onset potentials of the anodic and cathodic peaks of DA1 shift negatively compared with other electrodes in the acid and alkaline solutions. In addition, the current densities of oxidation and reduction peaks of DA1 are larger than the counterparts (Figures 7-12).

3.4 The quasi steady-state anodic Tafel polarization analysis of np-Au electrodes. Figure 13 gives the quasi steady-state anodic Tafel polarization analysis performed in the 0.5 M KOH + 0.5 M methanol and 0.5 M H2SO4 + 0.5 M methanol solutions. According to the Tafel equations, the relation between the overpotential η and the current density j is:61, 62

η = a + b log j a=−

(7)

2.303RT log j0 αmF

(8)

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b=

2.303RT αmF

(9)

where j0 is the exchange current density, α is the anodic transfer coefficient, m is the number of transferred electrons and b is the Tafel slope. As shown in Figure 13a, three samples have the current plateau in the anodic part with increasing potential, indicating that the charge transfer control plays a dominant role in this region on these electrodes.63, 64 The Tafel slopes b are roughly obtained to be 150.10, 72.20, 106.85 mV/dec on DA0, DA1, and DA2, respectively. By extrapolating the Tafel lines to the point at where the overpotential equals zero, the exchange current densities j0 are 4.61, 5.15 and 5.01 mA/cm2 for DA0, DA1, and DA2, respectively. Meanwhile, the corrosion potential Ecorr for methanol oxidation on DA1 is located at -163 mV, significantly more negative than that on DA0 and DA2 (-80 and -100 mV). The deduced b, j0 and Ecorr verified that the catalytic activity for MOR of samples is in the order: DA1 > DA2 > DA0. Similar to the alkaline solution, there are obvious current plateau in the anodic parts in Figure 13b, showing the predominance of charge transfer control in this region on these electrodes. The Tafel slopes are 197.80, 91.75, 119.8 mV/dec and j0 values are 4.06, 4.79, 4.48 mA/cm2 for MOR on DA0, DA1 and DA2, respectively, consisting with the regulation in alkaline solution (Figure 13a). Simultaneously, the corrosion potentials Ecorr are located at 314, 269, and 297 mV. The parameters calculated from Tafel curves of three samples in both alkaline and acid solutions are listed in Table 3. Similar to the phenomenon in alkaline solution, b, j0 and Ecorr also verified that three samples have catalytic activity for MOR in the same order: DA1 > DA2 > DA0.

3.5 Electrochemical impedance spectroscopy (EIS) analysis of np-Au electrodes. The electrochemical impedance spectroscopy (EIS) curves are shown in Figure 14. In the alkaline solution, three semi cycles with different radius values in the following order: DA1 < DA2 < DA0 17 ACS Paragon Plus Environment


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(Figure 14a). The EIS curves of three samples change to the line shape with the slope of ~ 3 in the acid solution (Figure 14b) and the inset is the enlarged view of dotted rectangle portion. Apparently, in both the alkaline and acid solutions, the order of the |Z| at the same frequency of samples is DA1 < DA2 < DA0. In both solutions, the most suitable model can be expressed by an equivalent circuit Rs(Cf1(Rf1(Qf2(Rf2(CdRct))))) (Figure 15) consisting of three parallel RQs arrangement in series with the ohmic resistance. Here, Rs is the solution resistance, Q is the constant phase element (CPE), Rct is the charge-transfer resistance, Qd is the nonideal double layer capacity, and Qf and Rf relate to the surface film features. The pure capacity, C, can replace the constant phase element, Q. It is used to represent the distribution of relaxation times, as a capacitive element to consider the deviations of the system from the ideality.61, 65 The impedance of the CPE can be defined by: Z CPE =

1 ( jω Q ) p

(10)

where p is a fitted parameter, which is associated with the extent of dispersion attributing to the surface inhomogeneity and equals to 1 for the ideal case C. The fitted parameters with the equivalent circuit model of three samples in both solutions are listed in Table 4. The Rct values of three samples are much larger than Rs and Rf. Thus, Rct is the dominant parameter in whole process, which is similar to the work of Wang.63 Moreover, the order of Rct is DA1 < DA2 < DA0 in both solutions, suggesting that the charge transfer on DA1’s surface is easier than that on other samples’ surface, indicating its highest catalytic activity for MOR, which agrees with the results obtained in Table 4. Additionally, the Rct of DA2 is obvious less than that of DA0, which is consistent well with the above mentioned analysis. Here, Rf1 can be considered as the parameter related to the formation of pre-oxidation species. Clearly, the Rf1 of DA1 is higher than those of DA0 and DA2 in both 18 ACS Paragon Plus Environment

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solutions, meaning that the coverage of pre-oxidation species on surface of DA1 is larger than that on other samples surface.66, 67 As discussed before, the formation of pre-oxidation species plays an important role in the MOR. The higher Rf1 is beneficial to the MOR. The resistance of Au oxide film, such as AuO at higher potential regions can be reflected as Rf2. The Rf2 of DA1 is apparently higher than DA0 and DA2 in both solutions, demonstrating that the thickness of oxide film on DA1 is higher than that on other samples, and the thick Au oxide film is critical to the MOR.66, 67 In addition, the Rf2 and Rct values of three samples in alkaline solution are higher than the counterparts in acid solutions. It is demonstrated that the np-Au electrodes exhibit a much higher electrocatalytic activity for MOR in alkaline solution, resulting from the concentration of OHonions.11, 29 Hence, DA1 performs the best catalytic activity for MOR according to the analysis of EIS, which is corresponding to the CV and Tafel results. In addition, in order to further detect the CO-tolerance of np-Au electrodes, the CO electro-stripping tests were carried out in 0.5 M H2SO4 solution. The more operation and analysis details can be seen in the Supporting Information. (Figure S9) CO electro-stripping tests possibly indicate the CO-tolerance of np-Au prepared by dealloying is excellent. To evaluate the durability of the np-Au samples for MOR, the chronoamperograms (CAs) were recorded for a period of 1000 s at a fixed potential of 300 mV in 0.5 M KOH +0.5 M methanol (Figure S10a) and 0.5 M H2SO4 +0.5 M methanol solutions (Figure S10b), respectively. And np-Au dealloyed in 0.02 T depicts a relatively good activity in both solutions and a high stability in alkaline solution. Based on the various techniques, the methanol electro-oxidation activity for three samples is in the following order: DA1 ＞ DA2 ＞ DA0. The reasons can be explained in three aspects. Firstly, the CVs of Au (111), (110), and (100) faces have characteristically different formation tendencies of 19 ACS Paragon Plus Environment


an oxide monolayer according to the report of Hamelin.68 The anodic current density of Au (111) is larger than those of Au (110) and (100), which means that Au (111) is more active than other faces at the higher potentials related to the MOR. In this paper, the F(111) of samples ranks as DA1 > DA2 > DA0 (Table 1), which can be considered as the crystallographic reason of the better activity of DA1 due to the addition of magnetic field. Secondly, DA1 has a homogeneous honeycomb-like microstructure with the small pore size. While the ligaments of DA2 become coarse and appear in a soda crackers-like structure and the microstructure of DA0 is 3D bicontinuous interpenetrating ligament-channel structure. According to the work of Borkowska,18 the large rf of a catalyst is beneficial to the catalytic activity. The unique honeycomb-like porous structure of DA1 provides a high special surface area, i.e. the surface roughness factor rf, and the interconnected nanoporous channels can effectively contribute charge/ion transfer and mass transport for the electrocatalysis. Meanwhile, the uniform small size pores could supply more active sites for the ion and methanol adsorption.11 This is the thermodynamic reason of the excellent activity performance of DA1. Finally, the phase compositions of DA0, DA1, and DA2 are coarse nanocrystals, fine nanocrystals + trace amorphous phase, and finer nanocrystals + a few amorphous phase according to the TEM images. Compared to the coarse nanocrystals, fine nanocrystals own more grain boundaries and defect. Meanwhile, according to the results of Schofield et al.,69 there are two regions of np-Au after dealloying: tension regions with the larger a0 (> 0.408 nm) and compression regions with the smaller a0 (< 0.408 nm); the bonds tend to stretch with positive curvature and to contract with negative curvature, resulting in tensile and compressive stress, respectively. The a0 of DA0, DA1, and DA2 are smaller than 0.408 nm, indicating that the compressive stress is dominant 20 ACS Paragon Plus Environment

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in the samples. The a0 of samples ranks as DA0 > DA2 > DA1 (Table 1), suggesting the largest compressive stress of DA1. According to the reports,18, 69, 70 the stress in np-Au can be served as the defect, which improves the catalytic activity of np-Au in the catalysis process. Also, there are several reports that the amorphous structure is beneficial for the catalytic activity by supplying abundant defect sites in the catalysis process.71-73 According to the results of Liu et al.,74 the surface defects play an important role in the catalysis of np-Au. Compared with the DA0, the amorphous phase exists in DA1 and DA2, which can improve the catalytic activity for MOR. In short, this is the defective reason of the better activity of DA1 and DA2. We are sure that applying a magnetic field in dealloying is a new technique to enhance the catalytic activity of np-Au.

CONCLUSIONS We have prepared np-Au samples with the 3D bicontinuous interpenetrating ligament-channel structure by dealloying the rapidly solidified Al2Au ribbon in 0, 0.02, and 0.2 T magnetic fields. Np-Au dealloyed in 0.02 T owns the smallest lattice constant a0 and the highest preferred orientation factor F(111) of (111) face, indicating that np-Au dealloyed in 0.02 T bears the largest compression stress. The addition of magnetic field leads to a formation of intermediate phase AlAu and a slow dealloying rate. The morphologies of np-Au dealloyed for 24 h in 0, 0.02, and 0.2 T (labeled as DA0, DA1, and DA2, respectively) are the maze structure, the uniform honeycomb-like structure with the small ligament/pore size and the soda crackers -like structure with the thick ligaments, respectively; and their phase compositions are coarse nanocrystals, fine nanocrystals + trace amorphous phase, and finer nanocrystals + a few amorphous phase, respectively. The formation of fine nanocrystals is due 21 ACS Paragon Plus Environment


to the nucleation sites supplied by the intermediate phase AlAu in dealloying. The surface coverage of the redox species Г*, charge-transfer rate constant ks from the CV test, exchange current density j0 and corrosion potential Ecorr from the Tafel polarization and charge transfer resistance Rct from the EIS analysis show that the MOR activity of three samples is in this order: DA1 ＞ DA2 ＞ DA0, which can be attributed to the crystallographic, thermodynamic and defective reasons. In other words, we can change the microstructure of the dealloyed np-Au by adjusting the magnetic field strength in the dealloying process and then we can enhance the catalytic activity of np-Au.

ASSOCIATED CONTENT Supporting Information. The Al-Au binary phase diagram. The equipments of permanent magnet with 0.02 T (a-b) and 0.2 T(c and d) magnetic field, respectively. Repeated XRD patterns of the original Al0.66Au0.34 ingots (a) and rapidly solidified (RS) Al0.66Au0.34 ribbons with the circumferential speeds (Sc) = 14.7 m/s (b) and the standard Al2Au. (a) XRD patterns of the RS ribbons immersed in 20 wt % NaOH solution for 24 h with the addition of 0 T, 0.02 T, and 0.2 T, respectively. (b) Enlarged views of the (111) peak of np-Au samples dealloyed for 24 h. Section-view SEM images of the np-Au ribbons by dealloying the RS Al0.66Au0.34 alloy in the 5 wt% HCl solution in 0 T (a and b); 0.02 T (c and d) and 0.2 T (e and f), respectively. (b), (d) and (f) are corresponding enlarged images of (a), (c) and (e), respectively. Red arrows in (a), (c), and (e) are the normal directions of magnetic field. Cyclic voltammograms (CVs) of np-Au electrodes measured in 0.5 M H2SO4 solution. Scan rate v: 50 mV/s. CVs of np-Au electrodes dealloyed in 0, 0.02, and 0.2 T in (a) 0.5 M KOH solution and (b) 0.5 M H2SO4 solution, respectively. v: 50 mV/s. 22 ACS Paragon Plus Environment

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The diagrams of Ep vs. logv, which is extracted from CVs recoded at the ranges of 2-500 mV/s of three samples in (a) 0.5 M KOH + 0.5 M methanol and (b) 0.5 M H2SO4 + 0.5 M methanol solutions, respectively. (a) the equipment to obtain the CO-saturated 0.5 M H2SO4 solution; (b) the chronoamperograms (CAs) of two np-Au electrodes dealloyed in 0 T (DA0) in the CO-saturated 0.5 M H2SO4 solution; (c) and (d) the CV curves of two np-Au electrodes for CO-stripping tests in 0.5 M H2SO4 solution. (Scan rate: 20 mV/s) Chronoamperograms (CAs) was recorded for a period of 1000 s at 300 mV in 0.5 M KOH + 0.5 M methanol (a) and 0.5 M H2SO4 + 0.5 M methanol solutions (b), respectively.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected], [email protected], [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work is supported by National Key Research Program of China (Grant No. 2016YFB0300501), National Natural Science Foundation of China (Nos. 51471099, 51571132, and 51771103), and Shandong Key Research Project (2016GGX102010).



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Figures and Tables

Figure 1. XRD patterns of the original Al0.66Au0.34 ingot, rapidly solidified (RS) Al0.66Au0.34 ribbon precursors with the circumferential speeds (Sc) = 14.7 m/s and the standard Al2Au. Inset is the appearance of the RS ribbon.


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Figure 2. XRD patterns of the constituent evolution of the RS ribbons immersed in 5 wt% HCl solution for 0, 6, 12, 18, and 24 h in (a) 0, (b) 0.02, and (c) 0.2 T magnetic field. The enlarged peak parts views beyond the full width at half maximumn (HM) of the (111) peak of np-Au samples dealloyed for (d) 12 and (e) 24 h. Inset in (e) is the appearance of the np-Au ribbon dealloyed for 24 h in 0.02 T.



Figure 3. SEM micrographs of the RS ribbons immersed in 5 wt% HCl solution for (a) 6, (b) 12, and (c) 18 h in 0 T, for (d) 6, (e) 12, and (f) 18 h in 0.02 T, and for (g) 6, (h) 12, and (i) 18 h in 0.2 T. Insets are the corresponding enlarged images.


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Figure 4. SEM micrographs of the maze-like np-Au immersed in 5 wt% HCl solution for 24 h in (a) 0, (b) 0.02, and (c) 0.2 T. (d), (e), and (f) are the corresponding enlarged micrographs of (a), (b) and (c), respectively. Insets in (d), (e), and (f) are the corresponding physical figures.



Figure 5. TEM images of np-Au dealloyed for 24 h in (a) 0, (b) 0.02, and (c) 0.2 T. The insets are the corresponding enlarged images. HRTEM images of np-Au dealloyed for 24 h in (d) 0, (e) 0.02, and (f) 0.2 T. Insets are the corresponding SAED patterns.


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Figure 6. Comprehensible schematic presentation of the RS ribbons immersed in 5 wt% HCl solution for 6, 18, and 24 h in 0 T (the first row), 0.02 T (the second row), and 0.2 T (the third row), respectively.



Figure 7. (a) Cyclic voltammograms (CVs) recorded on the np-Au dealloyed in 0.02 T in 0.5 M KOH + 0.5 M methanol at various scan rates v: (1) 0.002, (2) 0.01, (3) 0.015, (4) 0.02, (5) 0.025, (6) 0.04, (7) 0.05, (8) 0.075, (9) 0.08, (10) 0.1, (11) 0.15, (12) 0.2, (13) 0.25, (14) 0.3, (15) 0.35, (16) 0.4, (17) 0.45 and (18) 0.5 V s−1. (b) Plot of Ep vs. logv for CVs for anodic and cathodic peaks. (c) The dependency of jpa and jpc on lower values of v (0.005-0.08 V s−1) and (d) on v1/2 at higher values of v (v > 0.08 V s−1).


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Figure 8. (a) CVs recorded on the np-Au dealloyed in 0 T in 0.5 M KOH + 0.5 M methanol at various v: (1) 0.002, (2) 0.01, (3) 0.015, (4) 0.02, (5) 0.025, (6) 0.04, (7) 0.05, (8) 0.075, (9) 0.08, (10) 0.1, (11) 0.15, (12) 0.2, (13) 0.25, (14) 0.3, (15) 0.35, (16) 0.4, (17) 0.45 and (18) 0.5 V s−1. (b) Plot of Ep vs. logv for CVs for anodic and cathodic peaks. (c) The dependency of jpa and jpc on lower values of v (0.005-0.08 Vs−1) and (d) on v1/2 at higher values of v (v > 0.08 V s−1).



Figure 9. (a) CVs recorded on the np-Au dealloyed in 0.2 T in 0.5 M KOH + 0.5 M methanol at various v: (1) 0.002, (2) 0.01, (3) 0.015, (4) 0.02, (5) 0.025, (6) 0.04, (7) 0.05, (8) 0.075, (9) 0.08, (10) 0.1, (11) 0.15, (12) 0.2, (13) 0.25, (14) 0.3, (15) 0.35, (16) 0.4, (17) 0.45 and (18) 0.5 V s−1. (b) Plot of Ep vs. logv for CVs for anodic and cathodic peaks. (c) The dependency of jpa and jpc on lower values of v (0.005-0.08 V s−1) and (d) on v1/2 at higher values of v (v > 0.08 V s−1).


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Figure 10. (a) CVs recorded on the np-Au dealloyed in 0.02 T in 0.5 M H2SO4 + 0.5 M methanol at various v: (1) 0.002, (2) 0.01, (3) 0.015, (4) 0.02, (5) 0.025, (6) 0.04, (7) 0.05, (8) 0.075, (9) 0.08, (10) 0.1, (11) 0.15, (12) 0.2, (13) 0.25, (14) 0.3, (15) 0.35, (16) 0.4, (17) 0.45 and (18) 0.5 V s−1. (b) Plot of Ep vs. logv for CVs for anodic and cathodic peaks. (c) The dependency of jpa and jpc on lower values of v (0.005-0.08 V s−1) and (d) on v1/2 at higher values of v (v > 0.08 V s−1).



Figure 11. (a) CVs recorded on the np-Au dealloyed in 0 T in 0.5 M H2SO4 + 0.5 M methanol at various v: (1) 0.002, (2) 0.01, (3) 0.015, (4) 0.02, (5) 0.025, (6) 0.04, (7) 0.05, (8) 0.075, (9) 0.08, (10) 0.1, (11) 0.15, (12) 0.2, (13) 0.25, (14) 0.3, (15) 0.35, (16) 0.4, (17) 0.45 and (18) 0.5 V s−1. (b) Plot of Ep vs. logv for CVs for anodic and cathodic peaks. (c) The dependency of jpa and jpc on lower values of v (0.005-0.08 V s−1) and (d) on v1/2 at higher values of v (v > 0.08 V s−1).


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Figure 12. (a) CVs recorded on the np-Au dealloyed in 0.2 T in 0.5 M H2SO4 + 0.5 M methanol at various v: (1) 0.002, (2) 0.01, (3) 0.015, (4) 0.02, (5) 0.025, (6) 0.04, (7) 0.05, (8) 0.075, (9) 0.08, (10) 0.1, (11) 0.15, (12) 0.2, (13) 0.25, (14) 0.3, (15) 0.35, (16) 0.4, (17) 0.45 and (18) 0.5 V s−1. (b) Plot of Ep vs. logv for CVs for anodic and cathodic peaks. (c) The dependency of jpa and jpc on lower values of v (0.005-0.08 V s−1) and (d) on v1/2 at higher values of v (v > 0.08 V s−1).



Figure 13. Tafel curves for methanol electro-oxidation reaction on np-Au electrodes dealloyed in 0.02 T in (a) 0.5 M KOH + 0.5 M methanol and (b) 0.5 M H2SO4 + 0.5 M methanol solutions, respectively. v: 1 mVs−1.

Figure 14. Nyquist plots of EIS for methanol electro-oxidation reaction on np-Au electrodes dealloyed in 0.02 T in (a) 0.5 M KOH + 0.5 M methanol and (b) 0.5 M H2SO4 + 0.5 M methanol solutions, respectively. v: 5 mVs−1. Inset in (b) is the enlarged profile from arrow-marked region.


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Figure 15. Equivalent circuit model (Rs(Cf1(Rf1(Qf2(Rf2(CdRct)))))) used to analysis the EIS data of np-Au electrodes.



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Table 1 The lattice constant a0 of the Au phase and phase volume fraction fv of RS ribbons dealloyed for different time in 5 wt% HCl by using the Rietveld analysis. Dealloyed conditions

Time

a0 (nm)

fv(Al2Au) (%)

fv(Au) (%)

0T

12 h 18 h 24 h 12 h 18 h 24 h 12 h 18 h 24 h

0.4073 ± 0.0001 0.4075 ± 0.0001 0.4078 ± 0.0001 0.4071 ± 0.0001 0.4073 ± 0.0001 0.4075 ± 0.0001 0.4072 ± 0.0001 0.4074 ± 0.0001 0.4077 ± 0.0001

14.2 11.0 0.0 18.7 13.7 0.0 19.2 15.1 0.0

85.8 89.0 100.0 81.3 86.3 100.0 80.2 84.9 100.0

0.02 T

0.2 T

Table 2 The surface coverage of the redox species Г*, electron-transfer coefficient α and apparent charge-transfer rate constant between the electrode and the surface-deposited layer ks obtained from CVs of three samples with v = 2-80 mV/s in and alkaline and acid solutions, respectively. Solutions 0.5 M KOH + 0.5 M methanol

0.5 M H2SO4 + 0.5 M methanol

Samples DA0 DA1 DA2 DA0 DA1 DA2

Г*(10-6mol/cm2) 0.5528 2.5248 1.7158 0.4487 2.1929 1.6473


α 0.0602 0.1146 0.0720 0.3280 0.1456 0.1631

Ks(S-1) 0.0150 0.0197 0.0162 0.0001 0.0043 0.0039

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Table 3 The corrosion potential Ecorr (V), anodic slope b (mV/dec), and exchange current density j0 (mA/cm2) obtained from Tafel curves of three samples in and alkaline and acid solutions, respectively. Solutions 0.5 M KOH + 0.5 M methanol

0.5 M H2SO4 + 0.5 M methanol

Samples DA0 DA1 DA2 DA0 DA1 DA2

Ecorr (V) -0.080 -0.163 -0.100 0.314 0.269 0.287

b (mV/dec) 150.10 72.20 106.85 197.80 91.750 119.80

j0 (mA/cm2) 4.61 5.15 5.01 4.06 4.79 4.48

Table 4 The values of impedance parameter (the solution resistance Rs, the double layer capacity C, the constant of phase element CPE Y0 and p, the film resistance Rf, and the charge-transfer resistance Rct) obtained from EIS curves of three samples in alkaline and acid solutions, respectively. Solutions 0.5 M KOH + 0.5 M methanol

Samples

DA0 DA1 DA2 0.5 M H2SO4 + 0.5 DA0 M methanol DA1 DA2

Rf1 Rs C1 Y0 p (Ω cm2) (F/cm2) (Ω cm2) (S secp/cm2) (0~1)

Rf2 Rct C2 (Ω cm2) (F/cm2) (Ω cm2)

8.01 7.62 12.10 7.62 4.63 7.34

4.90 42.99 26.50 2.07 16.28 8.70

0.00007 0.00300 0.00101 0.00042 0.00002 0.00001

0.35 1.01 0.36 0.51 0.68 0.62

0.0014 0.0173 0.0039 0.0006 0.0044 0.0015


0.92 0.80 0.84 0.61 0.83 0.81

0.0008 0.0064 0.0017 0.0003 0.0009 0.0004

916 285 439 18260 10000 12340


TOC Graphic


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Micromorphology and Phase Composition ... - ACS Publications

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