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Dilute Au Containing Ag Nanosponges as a Highly Active and Durable Electrocatalyst for Oxygen Reduction and Alcohol Oxidation Reactions Jiali Wang, Fuyi Chen, Yachao Jin, and Yimin Lei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17066 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Applied Materials & Interfaces

Dilute Au Containing Ag Nanosponges as a Highly Active and Durable Electrocatalyst for Oxygen Reduction and Alcohol Oxidation Reactions Jiali Wang,† Fuyi Chen,*,† Yachao Jin,† and Yimin Lei‡ †

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, 710072, China ‡

School of Advanced Materials and Nanotechnology, Xidian University, Xi’an, 710126, China

*

Corresponding author: Tel./Fax: +86 029-88492052. E-mail: [email protected] (Fuyi Chen)

ABSTRACT Zero-dimensional nanoparticles (NPs) have been demonstrated as the promising class of catalysts for various chemical and electrochemical reactions. However, the emerging Au-Ag NPs catalysts are suffering from the single functionality, limited activity enhancement and unsatisfactory stability problems. Here we report a facile kinetically-controlled solution method to prepare a new class of Au-Ag nanoporous sponges (NSs) composed of three-dimensional networks without using additional stabilizing agents at room temperature. The unexpected shift of d-band center in our Au-Ag NSs was observed for the first time in Au-Ag bimetallic systems, which effectively activates the Au-Ag NSs for electrochemical reactions. The robust electronic effect coupled with abundant accessible active sites from the hierarchically porous architecture make the bare Au-Ag NSs a superior multifunctional catalyst for oxygen reduction, ethylene glycol oxidation, as well as glucose oxidation reactions to the commercial Pt/C electrocatalyst in alkaline medium. The optimized AuAg3.2 NSs deliver the mass activity of 1.26 A mg-1Au towards oxygen reduction reaction, which is ~8.2 times as high as that of the Pt/C, simultaneously showing outstanding stability with negligible activity decay after 10,000 cycles. For the anodic reactions, this AuAg3.2 NSs show extremely high activity and stability towards both ethylene glycol and glucose

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catalytic oxidation reactions with the higher mass activity of 7.58 and 1.48 A mg-1Au, about 3 and 18.5-fold enhancement than that of Pt/C, respectively. This work provides important insights into structural design, performance optimization and cost reduction to promote the practical applications of liquid fuel cells.

KEYWORDS: kinetically-controlled method, porous architecture, d-band center, oxygen reduction, ethylene glycol oxidation, glucose oxidation, liquid fuel cells


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1. INTRODUCTION In the quest to solve energy substitution and environmental remediation issues, clean energy conversion devices have been attracting tremendous attention.1-2 Fuel cells by coupling oxygen reduction reaction (ORR) and alcohols oxidation reaction (AOR) provide an important strategy to directly convert chemical energies stored in fuels to useful electrical energy via the electrochemical process.3-4 To lower the reduction and oxidation overpotentials in fuel cells for high-voltage output, an efficient electrocatalyst must be required to accelerate each of the two reactions.5-6 As known, the catalysts applied in the fuel cells usually exhibit quite different activities for cathodic and anodic reactions, which makes the development of integrated energy systems for practical application difficult.7 Currently, noble platinum (Pt) is proven as the most efficient electrocatalyst component for both ORR and AOR.5,8-9 However, the sluggish reaction kinetics, poor operation durability and low earth reserves of Pt greatly impede the large-scale practical application of fuel cells.10-11 It is highly desirable to develop multifunctional Pt-free electrocatalysts with highly active and stable performance for various liquid fuel cell electrocatalysis. Recently, Ag has been demonstrated to be the promising alternative for Pt due to the similar ORR kinetics compared to Pt catalyst with the 4e- pathway,12-13 and be more electrochemically stable in alkaline media.14-15 However, the electrocatalytic property on the monometallic Ag towards ORR is still inferior to that on metal Pt,15-16 manifesting in the more negative half-wave and onset potentials.17 On the other hand, Au has also attracted a growing attention as another promising candidate for Pt-free electrocatalyst towards ORR in alkaline medium because of its more positive onset potential.17-18 But, the electron transfer number on monometallic Au for ORR is only two and the dissociative absorption of oxygen on most Au facets can hardly occur, leading to the unsatisfactory catalytic activities on Au.17,19-20 However, owing to the strong size- and shape-dependent properties, Au nanostructures show great



potential for catalyzing a wide variety of small organic molecules oxidation in alkaline media, such as alcohols.21-22 Especially the capability for reducing CO poisoning observed on Au is highly desirable for the long-term duration during the alcohols oxidation reactions.23-24 Nano-technique can be used to well develop the bi/multi-metallic nanomaterials with desirable structures, shapes and components, the resultant bi/multi-metallic nanostructures usually show unexpected properties compared with their monometallic counterparts due to the synergistic effect among different components.24-26 Therefore, it is possible to obtain an efficient multifunctional electrocatalyst for both ORR and fuels oxidation reactions by the combination of the Au and Ag active sites after the formation of intermixed nanostructures. Additionally, due to the lower price of Ag and the greater annual production of Au than those of Pt, the significant economic savings could be achieved.15,27 In this regards, a series of Au-Ag bimetallic NPs with different structures have been synthesized for ORR through the combined seed mediated and galvanic replacement method by Chen’s group,28-30 such as alkyne-functionalized AgAu NPs,28 Ag@Au Janus NPs29 and Au@Ag semishell Janus NPs,30 all these resultant Au-Ag nanostructures showed remarkably improved catalytic activity towards ORR compared to the monometallic Ag NPs due to the efficient interfacial electron transfer. Tang et al. successfully created the peptide A4 based AuAg NPs catalysts mediated by the stabilizing ligand, which exhibited much improved ORR performance compared with pure Au and Ag electrocatalysts.31 Recently, Xu and coworkers have reported a high-yield synthesis of AuAg@Au core/shell NPs that showed over five times more active than the pure Au for electrocatalytic oxidations of ethylene glycol and glycerol.32 Even though these classes of catalysts exhibit some impressive enhancements in catalytic activity relative to their monometallic counterparts, the activities of the current Au-Ag bimetallic systems are far inferior to the commercial Pt/C catalyst. It is also noteworthy that almost all the Au-Ag nanostructures reported so


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far were limited to the conventional zero-dimensional NPs dispersed on the carbon substrate with the relatively low surface area and poor durability arising from the aggregation of the NPs and the corrosion problems of the carbon support. And their synthesis heavily relied on the multistep galvanic reactions and complex stabilizing ligands, which were not favorable to expose massive catalytically-active sites for achieving the excellent catalytic performance. Furthermore, owning to the intrinsic difference in the chemical reaction essence between ORR and AOR,24 the state-of-the-art Au-Ag catalysts are mainly designed for catalyzing ORR28-31,33 or AOR32 separately, and the current researches on the bare Au-Ag nanostructures for catalyzing AOR are quite few. Therefore, a new class of Au-Ag electrocatalyst with high catalytic activity and durability has to be explored to simultaneously improve both ORR and AOR efficiency. Herein, we directly synthesized the highly active and durable Au-Ag nanoporous sponges (NSs) without the use of any capping and stabilizing ligands via a one-step kinetically-controlled reduction method, and demonstrated their enhanced multifunctional electrocatalysis with long-term stability for both cathodic and anodic reactions in fuel cell applications. This as-obtained Au-Ag 3D open architecture exposes large amount of various catalytically-active sites for the electrochemical reactions. Intriguingly, the fully alloyed and uniform phase renders the Au-Ag NSs with robust atomic interactions, which effectively regulates the d-band center in Au-Ag NSs for higher catalytic performance. Specifically, the optimized AuAg3.2 NSs catalyst delivers exceptional performance for ORR with specific and mass activity of 1.36 mA cm-2 and 1.26 A mg-1Au at 0.85 V, about 6.7 and 8.2 times higher than that of the commercial Pt/C, respectively, making the Au-Ag NSs the most active catalyst among the current Au-Ag systems for ORR. Impressively, the AuAg3.2 NSs catalyst shows excellent cycling and chronoamperometric stability with negligible activity decay over 10,000 potential cycles and 40,000 s



duration, respectively. More importantly, they exhibit superior mass activity of 7.58 and 1.48 A mg-1Au towards ethylene glycol (EG) and glucose electrooxidation, respectively, which is 3 and 18.5-fold enhancement than that of Pt/C, and their electrochemical stability under harsh conditions (3,600 s) also remains highest among current Au-containing electrocatalysts. The unique insights gained through this study may serve as a platform to better understand the enhancement mechanism of catalytic performances and provides important opportunities to design more economical and efficient multifunctional electrocatalysts for future energy conversion technologies. 2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials Silver nitrate (AgNO3), Gold (III) chloride hydrate (HAuCl4·4H2O), sodium borohydride (NaBH4), potassium hydroxide (KOH), and glucose (C6H12O6) were bought from Tianjin Fuchen. Absolute EG ((CH2OH)2, A.R. grade, > 99.5%) was purchased from Xi'an Shunda. Platinum/carbon black catalyst (Pt/C; Pt loading of 20 wt. %) was purchased from Johnson Matthey. Nafion perfluorinated resin solution was obtained from Shanghai Yangsen. All aqueous solutions in the experiment were prepared using ultrapure water (> 18.25 MΩ cm). 2.2. Synthesis of Au-Ag NSs, Monometallic Au and Ag Electrocatalysts In a typical synthesis of the AuAg3.2 NSs, 4 mL of metal precursor aqueous solution containing 0.025 M HAuCl4 and 0.075 M AgNO3 was quickly injected to an 0.1 M freshly prepared, ice-cold aqueous solution of NaBH4 (1:5, v/v ratio of metal precursor / NaBH4 solution) with violent stirring at room temperature. Au-Ag NSs with different compositions were prepared by simply changing the concentration of AgNO3 precursor from 8.3 mM to 175 mM (Table S1). The solution was kept stirring for three minutes until it became colorless, and then was kept undisturbed for half an hour at room temperature. The rapid


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kinetically-controlled reduction reaction between metal cations and strong reduction agent (NaBH4) resulted in the fast formation of black spongy solid out of the solution, which vigorously floated up and down with the aid of effervescence (due to the release of H2). After that, the obtained AuAg NSs were washed with the concentrated ammonia to remove the silver chloride residuals formed in the reaction, then washed carefully with ultrapure water for several times, and finally freeze-dried overnight. For comparison, pure monometallic Au and Ag were also successfully synthesized by adding 4 mL of 0.025 M metal precursor solution into 20 mL of 0.1 M NaBH4 solution. The single HAuCl4 and AgNO3 were used as the precursors for the preparation of pure Au and pure Ag catalysts, respectively. 2.3. Physical Characterizations Scanning electron microscopy (SEM) images were obtained with a FEI NovaSEM 450 field-emission SEM. The detailed morphology and nanostructure of the catalysts were obtained using the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, FEI Tecnai F30), the compositional analyses were acquired by using energy-dispersive X-ray spectroscopy (EDX). The phase and crystallinity of the samples were acquired using the X-ray diffraction (XRD) spectroscope on the PANalytical X’Pert Pro MPD instrument equipped with Cu Kα radiation. The mean crystallite size was determined based on the Scherrer equation: d = kλ/βcosθ, where k is the shape factor (0.89), λ refers to the wavelength of X-ray, β denotes the full width at half the maximum intensity, and θ represents the Bragg angle. X-ray photoelectron spectroscopy (XPS) characterization was carried out using an ESCALAB 250 instrument with monochromated Al Kα radiation (ultrahigh vacuum, 10-9 Torr). The d-band center positions were determined according to the formula: ∫N (ε)εdε ∫N (ε)dε between 0 and 9 eV binding energy, where N(ɛ) represents the density of states (DOS), and the Shirley background was subtracted from the acquired XPS spectra.34 Nitrogen physisorption isotherms were obtained with ASAP



2460 at 77 K. About 70 mg of the Au-Ag sample was transferred into the measuring cell and degassed in vacuum at 323 K for 24 h before the analysis. The specific surface area of the as-synthesized catalyst was calculated by applying the Brunauer-Emmett-Teller (BET) equation. The pore sizes and cumulative pore volumes were analyzed based on the Barrett-Joyner-Helenda (BJH) model. 2.4. Electrochemical Measurements All the electrochemical experiments were performed in a setup consisting of a CHI 660C electrochemical workstation (Shanghai Chenhua, China) and a standard three-electrode cell at room temperature. The glassy carbon electrode (GCE, 5 mm in diameter) as the working electrode was successively polished with alumina slurry (1, 0.3 and 0.05 µm) and then sonicated in acetone and ultrapure water before the surface coating. For preparing a catalyst-modified electrode, the Au-Ag NSs were dispersed ultrasonically in the alcohol and Nafion mixing solution to form the catalyst ink. Afterwards, about 10 µL of dispersion was deposited onto the surface of GCE and dried naturally. The mass loading of all different Au-Ag NSs were 20.4 µg cm-2. For comparison, the 20 wt. % Pt/C was applied as the baseline electrocatalyst, and the same method was used to form the Pt/C ink. The final Pt metal loading on the GCE was fixed at 20 µgPt cm-2. For the sake of the reproducibility of each experiment, we prepared several separated samples for all the electrocatalysts. For the oxygen reduction reactions (ORR), the 0.1 M KOH electrolyte solutions were prepared at the day of use to avoid the pollution due to the glass corrosion in base. The catalyst-modified GCE, platinum wire, mercury-mercury oxide (MMO) Hg│HgO│(NaOH 1M) were deployed as the working electrode, counter electrode and reference electrode, respectively. To prove the deployment of the Pt wire counter electrode did not cause any altered electrochemical performance, we further chose the graphite rod as an alternative counter electrode to investigate the ORR performance in the Supporting Information. Unless


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otherwise stated, all potential values were referred to the reversible hydrogen electrode (RHE) in this paper. All electrochemical measurements were performed using research-grade gases. Linear sweep voltammetry (LSV) curves of the different catalysts were recorded at the different rotation rate (400, 800, 1200, 1600, 2000 rpm) in O2-saturated 0.1 M KOH aqueous solution with 10 mV s-1. The following equation (Koutecky-Levich equation, j-1 versus ω-1/2) was used to estimate the ORR selectivity to H2O or H2O2 (4e- or 2e- reduction): j-1 = jk-1 + 1/(0.62nFC0D02/3ν-1/6ω1/2), where j and jk denote the actual and kinetic current densities, respectively, n represents the electron transfer numbers, F refers to the Faraday constant, C0 represents the bulk concentration of O2 (1.2×10-6 mol cm-3), D0 denotes the diffusion coefficient of O2 (1.9×10-5 cm2 s-1), ν represents the kinetic viscosity of electrolyte (0.01 cm2 s-1), and ω refers to the angular velocity of the RDE (ω = 2πN, N denotes the linear rotation speed in rpm).35-36 The ORR stability was recorded by the accelerated degradation test (ADT) and chronoamperometric (CA) measurement. Specifically, ADT test was performed by applying continuous potential cycling between 0.5 and 1.0 V vs. RHE in O2-saturated 0.1 M KOH solution with a sweep rate of 100 mV s-1 for 10,000 cycles. CA measurement (i-t curve) was carried out at 0.82 V vs. RHE at 1600 rpm in O2-saturated 0.1 M KOH solution for 40,000 s. The methanol tolerance test was performed by introducing methanol to the KOH aqueous solution (1 M methanol) at 3,000 s during the CA measurement. For the ethylene glycol (EG) and glucose oxidation reactions, fresh solutions of EG and glucose were prepared every day. All the cyclic voltammograms (CVs) were taken in both N2-saturated 1 M KOH + 1 M ethylene glycol and 0.1 M KOH + 0.01 M glucose aqueous solutions with a scan rate of 50 mV s-1. Additionally, the chronoamperometry (CA) curve was taken for investigating the long-term durability. The electrochemically active surface areas (ECSAs) of the as-synthesized Au-Ag NSs were evaluated by using Pb underpotential deposition (upd).15,37 The Pb-stripping voltammetry was recorded by holding



the initial potential at 0.2 V vs. RHE and then sweeping to 1.0 V vs. RHE at a scan rate of 50 mV s-1 in N2-saturated 0.1 M KOH + 125 µM Pb(NO3)2 aqueous solution. The final voltammogram was further integrated assuming 280 µC cm-2. The ECSA for Pt/C catalyst should be evaluated from the CV curve recorded in N2-saturated 0.1 M KOH solution through integrating charges in the hydrogen adsorption/desorption region,38 assuming 210 µC cm-2 for the hydrogen monolayer adsorption. 3. RESULTS AND DISCUSSION Au-Ag bimetallic catalyst was synthesized by a one-step kinetically-controlled process at room temperature, and no additional stabilizing ligands were involved. When the metal precursors were injected to the sodium borohydride solution, the strong reduction activity instantly induced the rapid formation of fluffy black monolith out of the solution within 3 minutes (Figure S1), affording such jelly-like Au-Ag nanostructures with fluid characteristics, which is totally different from the simple nanoparticle agglomerates.31 The final Au-Ag NSs can be facilely obtained via the freeze-drying. The morphologies and structures of the as-synthesized Au-Ag NSs were first researched using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 1a shows that the as-synthesized AuAg3.2 catalyst possesses a typical 3D spongy architecture composed of interconnected networks and abundant porosity, such structure would expose more catalytically active sites and facilitate the easy diffusion of reactants and products during the reaction process and then improve the catalytic performance. TEM image (Figure 1b) exhibits that these networks are made up of ultrathin wire-like nano-ligaments with developed branches of similar diameter sizes around 10 nm. The typical selected-area electron diffraction (SAED) pattern clearly exhibits continuous diffraction rings, indicating the polycrystalline nature and the face-centered cubic (fcc) structure of the nanosponge (inset in Figure 1b). To investigate the detailed structure of the Au-Ag NSs, the high-resolution TEM (HRTEM) image


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Figure 1. General physical characterizations of the AuAg3.2 NSs electrocatalyst. (a) SEM image. (b) Bright-field TEM image, inset shows the SAED pattern. (c) HRTEM image of interconnected nanowires, the yellow arrows represent the curved surfaces on the periphery of the nanowire, and the red arrows represent the grain boundaries. (i-iii) Magnified HRTEM images and their corresponding Fast Fourier Transform (FFT) patterns of i, ii and iii regions marked by red dashed box in (c), the scale bars in (i-iii) are 0.5 nm. (d-g) Selected TEM image and high-resolution TEM-EDX elemental mapping of the networks, exhibiting the uniform distribution of Au and Ag, the scale bars are 50 nm.

was recorded and displayed in Figure 1c. It can be found that the high abundance of curved surfaces are present on the periphery of the AuAg3.2 nanowires (yellow arrows), which has been reported to be usually dominated by highly active low-coordinated atoms, serving as active sites for forming and breaking



chemical bonds.39-40 Convincing evidence of the presence of high density of low-coordinated atoms on the AuAg3.2 NSs curved surfaces was shown in the Supporting Information. Furthermore, the coherent lattice fringes with a single crystalline structure were frequently found throughout the whole nanowire, with close inspection, some characteristically structural defects (crystalline domains with inconsistent lattice fringes induce the formation of “grain boundaries”) were further observed at joints where the nanowires were interconnected with another, as indicated by the red arrows. As reported in the previous literature,41-42 the crystalline boundaries cross-sectioning the nanowire are very catalytically active, which can also serve as excellent active sites for different reactions. The clear fringes in the three randomly selected regions show the lattice spacing of 0.235, 0.234 and 0.204 nm, as expected for fcc-structured (111), (111) and (200) planes, respectively. The corresponding selected-area Fast Fourier Transformation (FFT) patterns further confirm that the fused NPs are single crystalline nature. The individual EDX elemental mappings show that Au and Ag are homogeneously distributed throughout the networks, strongly indicating the mixing of the two metals on the atomic scale and the formation of a fully alloyed structure (Figure 1d-g). Such fully alloyed Au-Ag NSs with uniformly-distributed elements facilitates the robust atomic interaction and thus favors their electrocatalytic activity. The EDX result exhibits that the Au/Ag atomic ratio is 23.87/76.13 in AuAg3.2 NSs, which is well consistent with the feeding ratios of Au and Ag metal precursors (Figure S2). Furthermore, the widely-tunable composition of the bimetallic Au-Ag NSs can be facilely achieved through simply altering the mole ratios of initial metal precursors (Figure S3 and Table S1). It should be noted that the precise control on the composition of our catalysts via the one-pot synthetic strategy is more outstanding compared with other previously-reported Au-Ag NPs.31 Interestingly, the morphologies of different Au-Ag NSs were found to be closely correlated with the concentration of Ag precursor. Increasing the Ag content promotes the formation of Au-Ag aggregates


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with large nanoparticles (i.e., AuAg4.8, AuAg7.3 samples based on the EDX results) instead of 3D fine networks (Figure S3), which inevitably hampers the effective utilization of electrochemically active metals. The best material was found to be AuAg3.2 NSs in our context, additionally, considering the substantially reduced cost due to the low content of Au, we will focus our attention on the Au-Ag NSs with this specific composition in the following discussion. The crystal structure of the Au-Ag NSs and the pure monometallic Au or Ag were then confirmed using X-ray diffraction (XRD) (Figure S4a). The XRD pattern of the AuAg3.2 NSs fully supports the SAED result by showing the predominant diffraction peaks that are characteristic of the fcc structure. Because the lattice constant of Au and Ag are very close (0.408 vs. 0.409 nm), it is difficult to distinguish the peak of Au-Ag alloys from that of the pure Au and pure Ag, which is consistent with the previous reports.29,33,43 However, with a higher magnification in this work, the (111) peak of the AuAg3.2 NSs are clearly found to be localized between Au (111) and Ag (111) peak (Figure S4b), firmly indicating the formation of the Au-Ag alloy.44 According to the Scherrer equation, the average crystallite size of the AuAg3.2 NSs was calculated to be 10.6 nm, consistent with the TEM characterization. These results verify that the nanowires in the AuAg NSs were formed by the coalescence of two single nanograins in the elongation direction, inducing the presence of high abundance of curved surfaces and crystalline boundaries for the electrochemical activities. The surface area and pores system of AuAg3.2 NSs was evaluated by the N2 adsorption-desorption isotherm measurement. As shown in Figure 2a, we observe a type II isotherm that is characteristic for macroporous structures.45 The specific surface area of as-synthesized AuAg3.2 NSs was determined to be 18.5 m2 g-1 based on the Brunauer-Emmett-Teller (BET) model, demonstrating that the open structured AuAg3.2 NSs have a relatively high surface area, which is larger than that of porous Au-containing



Figure 2. (a) N2 adsorption/desorption isotherms and (b) Pores size distribution and the corresponding cumulative pore volumes based on the BJH model for the AuAg3.2 NSs catalyst.

materials by using other method.46 The pore size distribution plot of AuAg3.2 NSs shown in Figure 2b confirms the presence of a hierarchical pores system ranging from micropores to mesopores and macropores. The corresponding cumulative volume plot indicates that the most of the volumes are derived from meso- and macro-sized pores. The hierarchical pores system of as-prepared Au-Ag NSs with larger surface area effectively offers more active sites for the reactants and facilitates the mass transportation during the catalysis. The surface composition and electronic structure were studied by X-ray photoelectron spectroscopy (XPS) measurement. As seen in Figure 3a, the Au and Ag peaks were clearly observed in the survey spectrum of AuAg3.2 NSs, where the surface Au and Ag content was determined to be 23.62 and 76.38 at. %, respectively, following the trend of their bulk compositions based on the EDX results. Thus, it is reasonable to assume that both Au and Ag show uniform distribution throughout the whole networks. Figure 3b and 3c show the XPS core-level (CL) spectrum of Au 4f and Ag 3d in AuAg3.2 NSs compared with pure Au and Ag nanostructures, respectively. Both Au 4f and Ag 3d CL are split into spin-orbit pairs,47-48 the peaks at 87.37 and 83.77 eV can be assigned to the Au4f5/2 and Au4f7/2 of metallic state Au0 in the alloyed composites,25 respectively, while those at 373.67 and 367.67 eV are assigned to the Ag


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Figure 3. (a) XPS survey spectra of AuAg3.2 NSs, pure Au and Ag catalysts. (b) High-resolution XPS spectrum of Au 4f in AuAg3.2 NSs, showing the clear shift towards higher binding energies with relative to pure Au. (c) High-resolution spectrum of Ag 3d region for AuAg3.2 NSs, showing a negative shift compared to the pure Ag. (d) Surface Valence Band Spectra (VBS) for AuAg3.2 NSs, pure Au and Ag catalysts, the dashed lines indicate their positions of the d-band center with respect to the Fermi level (E-EF, E, EF is the occupied d-electron density energy and Fermi energy, respectively), displaying the effective modification of electronic structure of AuAg3.2 NSs after alloying.

3d3/2 and Ag3d5/2, respectively, indicating that the metallic state Ag0 is also predominant in the Au-Ag alloys.48 A close inspection shows that Au 4f CL in the AuAg3.2 NSs is shifted towards higher binding energies in comparison to that in pure Au, on the contrary, Ag 3d CL is observed to shift towards lower binding energies relative to that in pure Ag.49-50 These obvious shifts observed in Au and Ag CL demonstrate the electron transfer from Au to Ag, which effectively prevents Ag from being easily oxidized to promote the formation of stable Au-Ag alloys for electrocatalysis. Furthermore, Figure 3d shows the valence band spectra (VBS) of AuAg3.2, pure Au and Ag nanostructures collected by the



high-resolution XPS to investigate the variation of electronic structures after alloying Au with Ag. Intriguingly, we find that the d-band center (ɛd) shift in AuAg3.2 NSs relative to the monometallic counterparts is rather remarkable, which is very different from the non-d-band modification observed in other Au-Ag systems previously reported in the literatures.33,51 Herein, the d-band center was calculated to be -4.50, -5.23, and -4.72 eV for pure Au, Ag and AuAg3.2 NSs, respectively. These results suggest that the electronic structure of the AuAg3.2 NSs catalyst can be effectively modified through the alloying effect between metal Au and Ag, which facilitates the Au-Ag NSs to achieve greatly improved catalytic activity. The ORR activity on the Au-Ag NSs electrocatalysts were first investigated and compared with pure monometallic Au, Ag, as well as the commercial Pt/C electrocatalysts (Figure 4 and Figure S7). Figure 4a shows the ORR polarization curves of different catalysts obtained on a rotating disk electrode with 1600 rpm in O2-saturated 0.1 M KOH aqueous solution. We can see that the AuAg3.2 NSs catalyst has the most-positive onset potential (Eonset) towards ORR, which is nearly 40 mV more positive than that of the Pt/C benchmark. At higher overpotentials, both AuAg3.2 NSs and Pt/C catalysts have comparable diffusion-limited current density of about 5.8 mA cm-2. At lower overpotentials (> 0.85 V), the half-wave potential (E1/2) is usually applied to assess the catalytic activity of an electrocatalyst towards ORR.38 The E1/2 value of different catalysts increases in the following order: monometallic Au/Ag

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