Dilute Au-Containing Ag Nanosponges as a Highly Active and

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6276−6287

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

‡

S Supporting Information *

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 NP catalysts suffer from 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 the 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 (EG) oxidation, and glucose oxidation reactions compared to the commercial Pt/C electrocatalyst in alkaline medium. The optimized AuAg3.2 NSs deliver a mass activity of 1.26 A mgAu−1 toward oxygen reduction reaction, which is ∼8.2 times as high as that of the Pt/C electrocatalyst, simultaneously showing outstanding stability with negligible activity decay after 10 000 cycles. For the anodic reactions, these AuAg3.2 NSs show extremely high activity and stability toward both EG and glucose catalytic oxidation reactions with a higher mass activity of 7.58 and 1.48 A mgAu−1, about 3- and 18.5-fold enhancement than Pt/C, respectively. This work provides important insights into the 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 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 because of the similar ORR kinetics compared to the Pt catalyst with the 4e− pathway12,13 and has been shown to be more electrochemically stable in alkaline media.14,15 However, the electrocatalytic property of the monometallic Ag toward ORR is still inferior to that of metal Pt,15,16 manifesting in the more negative half-wave and onset potentials.17 On the other hand, Au has also been receiving growing attention as another promising candidate for a Pt-free electrocatalyst toward ORR in alkaline medium because of its more positive onset

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 alcohol 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 highvoltage output, an efficient electrocatalyst is 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 applications 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 © 2018 American Chemical Society

Received: November 9, 2017 Accepted: January 30, 2018 Published: January 30, 2018 6276

DOI: 10.1021/acsami.7b17066 ACS Appl. Mater. Interfaces 2018, 10, 6276−6287

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ACS Applied Materials & Interfaces potential.17,18 But, the electron transfer number on monometallic Au for ORR is only 2, 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 have shown great potential for catalyzing a wide variety of small organic molecule oxidations 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 of the AORs.23,24 Nanotechnique can be used to develop the bi/multimetallic nanomaterials with desirable structures, shapes, and components, and the resultant bi/multimetallic nanostructures usually show unexpected properties compared with their monometallic counterparts because of the synergistic effect among different components.24−26 Therefore, it is possible to obtain an efficient multifunctional electrocatalyst for both ORR and fuel oxidation reactions by the combination of 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 that of Pt, the significant economic savings could be achieved.15,27 In this regard, a series of Au−Ag bimetallic nanoparticles (NPs) with different structures have been synthesized for the ORR through the combined seed-mediated and galvanic replacement method by Chen’s group,28−30 such as alkynefunctionalized AgAu NPs,28 Ag@Au Janus NPs,29 and Au@Ag semishell Janus NPs,30 and all these resultant Au−Ag nanostructures showed remarkably improved catalytic activity toward ORR compared to the monometallic Ag NPs because of the efficient interfacial electron transfer. Tang et al. successfully created the peptide A4-based AuAg NP catalysts mediated by the stabilizing ligand, which exhibited a much improved ORR performance compared with pure Au and Ag electrocatalysts.31 Recently, Xu and co-workers have reported a high-yield synthesis of AuAg@Au core/shell NPs that are over 5 times more active than pure Au for electrocatalytic oxidations of ethylene glycol (EG) 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 that of the commercial Pt/C catalyst. It is also noteworthy that almost all Au−Ag nanostructures reported so far were limited to the conventional zero-dimensional NPs dispersed on the carbon substrate with a relatively low surface area, and sufferring from the poor durability arising from the aggregation of the NPs and the corrosion problems of the carbon support. 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, owing to the intrinsic difference in the chemical reaction essence between the ORR and AOR,24 the state-of-the-art Au− Ag catalysts are mainly designed for catalyzing the ORR28−31,33 or AOR32 separately, and the current research papers on the bare Au−Ag nanostructures for catalyzing AOR are quite few. Therefore, a new class of Au−Ag electrocatalysts 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. The asobtained Au−Ag three-dimensional (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 a higher catalytic performance. Specifically, the optimized AuAg3.2 NS catalyst delivers exceptional performance for ORR with specific activity (SA) and mass activity (MA) of 1.36 mA cm−2 and 1.26 A mgAu−1 at 0.85 V, respectively, about 6.7 and 8.2 times higher than that of 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 NS catalyst shows excellent cycling and chronoamperometric (CA) stability with negligible activity decay over 10 000 potential cycles and 40 000 s duration, respectively. More importantly, they exhibit the superior MA of 7.58 and 1.48 A mgAu−1 toward EG and glucose electro-oxidation, respectively, which is 3- and 18.5-fold enhancement than Pt/C, and their electrochemical stability under harsh conditions (3600 s) also remains the 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 provide 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 purchased 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 purchased 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 Monometallic Ag Electrocatalysts. In a typical synthesis of AuAg3.2 NSs, 4 mL of metal precursor aqueous solution containing 0.025 M HAuCl4 and 0.075 M AgNO3 was quickly injected to 0.1 M freshly prepared, ice-cold aqueous solution of NaBH4 (1:5, v/v ratio of the metal precursor/NaBH4 solution) with violent stirring at room temperature. Au−Ag NSs with different compositions were prepared by simply changing the concentration of the AgNO3 precursor from 8.3 to 175 mM (Table S1). The solution was kept stirring for 3 min until it became colorless and then was kept undisturbed for half an hour at room temperature. The rapid kinetically controlled reduction reaction between the metal cations and the strong reduction agent (NaBH4) resulted in the fast formation of a 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 concentrated ammonia to remove the silver chloride residuals formed in the reaction, then washed carefully with ultrapure water 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. HAuCl4 and AgNO3 were used as precursors for the preparation of pure Au and pure Ag catalysts, respectively. 2.3. Physical Characterization. Scanning electron microscopy (SEM) images were obtained with a FEI NovaSEM 450 field emission scanning electron microscope. The detailed morphology and nanostructure of the catalysts were obtained using transmission 6277


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Figure 1. General physical characterization of the AuAg3.2 NS 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 FFT patterns of (i−iii) regions marked by the red dashed box in (c); the scale bars in (i−iii) are 0.5 nm. (d−g) Selected TEM image and HRTEM-EDX elemental mapping of the networks, exhibiting the uniform distribution of Au and Ag; the scale bar is 50 nm. 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 alcohol and Nafion mixing solution to form the catalyst ink. Afterward, about 10 μL of dispersion was deposited onto the surface of the GCE and dried naturally. The mass loading of all different Au−Ag NSs was 20.4 μg cm−2. For comparison, 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 electrocatalysts. For the ORR, 0.1 M KOH electrolyte solutions were prepared on the day of use to avoid pollution due to glass corrosion in the base. The catalyst-modified GCE, platinum wire, and mercury−mercury oxide Hg|HgO|(NaOH 1 M) were deployed as the working electrode, counter electrode, and reference electrode, respectively. To prove that 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, as shown in the Supporting Information. Unless 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 different rotation rates (400, 800, 1200, 1600, and 2000 rpm) in O2-saturated 0.1 M KOH aqueous solution with 10 mV s−1. The following equation [Koutecky−Levich (K−L) equation, j−1 vs ω−1/2] was used to estimate

electron microscopy (TEM) and high-resolution TEM (HRTEM, FEI Tecnai F30), and the compositional analyses were performed by using energy-dispersive X-ray (EDX) spectroscopy. The phase and crystallinity of the samples were acquired with X-ray diffraction (XRD) using a PANalytical X’Pert Pro MPD instrument equipped with Cu Kα radiation. The mean crystallite size was determined based on the Scherrer equation: d = kλ/β cos θ, 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)ε dε/∫ N(ε) dε between 0 and 9 eV binding energy, where N(ε) represents the density of states and the Shirley background was subtracted from the acquired XPS spectra.34 Nitrogen physisorption isotherms were obtained with a ASAP 2460 system 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 assynthesized 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 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 6278


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ACS Applied Materials & Interfaces 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 the electrolyte (0.01 cm2 s−1), and ω refers to the angular velocity of the rotating disc electrode (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 CA measurement. Specifically, the ADT test was performed by applying continuous potential cycles between 0.5 and 1.0 V versus RHE in O2-saturated 0.1 M KOH solution with a sweep rate of 100 mV s−1 for 10 000 cycles. The CA measurement (i−t curve) was carried out at 0.82 V versus 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 KOH aqueous solution (1 M methanol) at 3000 s during the CA measurement. For the EG and glucose oxidation reactions, fresh solutions of EG and glucose were prepared every day. All cyclic voltammograms were taken in both N2-saturated 1 M KOH + 1 M EG and 0.1 M KOH + 0.01 M glucose aqueous solutions with a scan rate of 50 mV s−1. Additionally, the CA curve was taken for investigating the long-term durability. The electrochemically active surface areas (ECSAs) of the assynthesized 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 versus RHE and then sweeping to 1.0 V versus 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 the Pt/C catalyst was evaluated from the CV curve recorded in N2-saturated 0.1 M KOH solution by integrating charges in the hydrogen adsorption/ desorption region,38 assuming 210 μC cm−2 for the hydrogen monolayer adsorption.

forming and breaking chemical bonds.39,40 Convincing evidence of the presence of a high density of low-coordinated atoms on the AuAg3.2 NS curved surfaces is shown in the Supporting Information. Furthermore, coherent lattice fringes with a single crystalline structure were frequently found throughout the whole nanowire; on 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 active catalytically, which can also serve as excellent active sites for different reactions. The clear fringes in the three randomly selected regions show lattice spacings 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 of 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 facilitate the robust atomic interaction and thus favor their electrocatalytic activity. The EDX result shows that the Au−Ag atomic ratio is 23.87/76.13 in AuAg3.2 NSs, which is wellconsistent with the feeding ratio 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 the 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 the Ag precursor. Increasing the Ag content promotes the formation of Au−Ag aggregates with large nanoparticles (i.e., AuAg4.8 and 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 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 constants 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 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 is clearly found to be localized between Au(111) and Ag(111) peaks (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

3. RESULTS AND DISCUSSION The 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 sodium borohydride solution, the strong reduction activity instantly induced the rapid formation of fluffy black monolith out of the solution within 3 min (Figure S1), affording such jellylike 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 freeze-drying. The morphologies and structures of the as-synthesized Au−Ag NSs were first researched using SEM and 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 a 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. The TEM image (Figure 1b) shows that these networks are made up of ultrathin wirelike nanoligaments with developed branches of similar diameter size around 10 nm. The typical selected area electron diffraction (SAED) pattern clearly exhibits continuous diffraction rings, indicating the polycrystalline nature and face-centered cubic (fcc) structure of the nanosponge (inset in Figure 1b). To investigate the detailed structure of the Au−Ag NSs, the HRTEM image was recorded and is displayed in Figure 1c. It can be found that a high abundance of curved surfaces is 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 6279


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

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

and Ag peaks are clearly observed in the survey spectrum of AuAg3.2 NSs, where the surface Au and Ag contents were determined to be 23.62 and 76.38 %, 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 a uniform distribution throughout the whole network. Figure 3b,c shows 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 spectra are split into spin−orbit pairs;47,48 the peaks at 87.37 and 83.77 eV can be assigned to Au 4f5/2 and Au 4f7/2 of the metallic state Au0 in the alloyed composites,25 respectively, whereas those at 373.67 and 367.67 eV are assigned to Ag 3d3/2 and Ag 3d5/2, respectively, indicating that the metallic state Ag0 is also predominant in the Au−Ag alloys.48 A close inspection shows that the Au 4f CL spectrum in the AuAg3.2 NSs is shifted toward higher binding energies in comparison to that in pure Au; on the contrary, the Ag 3d CL spectrum is observed to shift toward lower binding energies relative to that in pure Ag.49,50 These obvious shifts observed in the Au and Ag CL spectra demonstrate the electron transfer from Au to Ag, which effectively prevents Ag from being easily oxidized to promote

direction, inducing the presence of a high abundance of curved surfaces and crystalline boundaries for the electrochemical activities. The surface area and pore 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 the as-synthesized AuAg3.2 NSs was determined to be 18.5 m2 g−1 based on the 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 materials obtained by using the other method.46 The pore size distribution plot of AuAg3.2 NSs shown in Figure 2b confirms the presence of a hierarchical pore system ranging from micropores to mesopores and macropores. The corresponding cumulative volume plot indicates that most of the volumes are derived from meso- and macro-sized pores. The hierarchical pore system of the as-prepared Au−Ag NSs with a 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 XPS measurement. As seen in Figure 3a, the Au 6280


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Figure 4. Catalytic ORR properties of various catalysts recorded with a scan rate of 10 mV s−1 in O2-saturated 0.1 M KOH aqueous solution. (a) ORR polarization curves of AuAg3.2 NS (red line), commercial Pt/C (black line), pure Au (blue line), and pure Ag (magenta line) catalysts at 1600 rpm. (b) Comparative mass-corrected Tafel plots of AuAg3.2 NS, Pt/C, monometallic Au, and monometallic Ag electrocatalysts. (c) Comparisons of ORR SA and MA for AuAg3.2 NS and commercial Pt/C at 0.85 V. The MA is depicted based on the mass loading of Au or Au + Ag. (d) ORR polarization curves of AuAg3.2 NSs with rotation rates from 400 to 2000 rpm; the inset exhibits the corresponding K−L plots at various potentials. The electrocatalytic results were obtained from at least three independent measurements, and the standard error bars are also included in (c). The current densities in (a,d) are normalized by the geometric area of a GCE (0.196 cm2).

catalyst exhibits the highest activity for ORR, with a smallest slope of 74.4 mV dec−1 in comparison to 75.2, 85.6, and 95.5 mV dec−1 for commercial Pt/C, monometallic Au, and monometallic Ag, respectively.52 For an impartial comparison of the ORR activity of different electrocatalysts, the SA and MA were calculated through dividing the kinetic currents by the ECSA or the mass loading of noble metals. The kinetic currents, representing the intrinsic ORR activities, were obtained by theK−L equation, and the Pb-upd and H-upd voltammograms were applied to evaluate the ECSAs of AuAg3.2 NS and commercial Pt/C catalysts, respectively (Figure S8).15,37,53 The SA and MA comparison between AuAg3.2 NSs and the Pt/C benchmark at 0.85 V is shown in Figure 4c. One can see that the SA of the AuAg3.2 NS catalyst exhibits an improvement factor of 6.7 versus commercial Pt/C (1.36 vs 0.203 mA cm−2). Furthermore, the synergy between higher SA and the open architecture of AuAg3.2 leads to 3.1 times higher MA than Pt/C (0.48 A mgAu+Ag−1 vs 0.153 A mgPt−1). On the basis of the mass loading of more precious Au, the MA of the AuAg3.2 NS catalyst was determined to be 1.26 A mgAu−1, about 8.2 times higher than that of Pt/C. Although the ORR activity of the AuAg3.2 NSs is lower than several state-of-the-art Ptcontaining electrocatalysts,5,11,38 it is still superior to the commercial Pt/C electrocatalyst and shows the highest activity among current Au−Ag systems reported to date (Table S3). Figure 4d shows the rotation-rate-dependent LSV curves of the AuAg3.2 NS catalyst (similar curves for the Pt/C catalyst can be seen in Figure S9). The electron transfer number was 3.9 calculated from the slopes of the K−L plots, suggesting a dominant 4e− pathway on AuAg3.2 NSs for ORR similar to that on Pt/C and the nearly complete reduction of O2 to OH− in an alkaline medium.25,54

the formation of stable Au−Ag alloys for electrocatalysis. Furthermore, Figure 3d shows the valence band spectra (VBS) of AuAg3.2 and 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 (εd) 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 literature.33,51 Herein, the d-band center was calculated to be −4.50, −5.23, and −4.72 eV for pure Au, pure Ag, and AuAg3.2 NSs, respectively. These results suggest that the electronic structure of the AuAg3.2 NS 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 NS electrocatalysts was first investigated and compared with pure monometallic Au and Ag as well as the commercial Pt/C electrocatalysts (Figures 4 and S7). Figure 4a shows the ORR polarization curves of different catalysts obtained on a RDE with 1600 rpm in O2-saturated 0.1 M KOH aqueous solution. We can see that the AuAg3.2 NS catalyst has the most positive onset potential (Eonset) toward ORR, which is nearly 40 mV more positive than that of the Pt/ C benchmark. At higher overpotentials, both AuAg3.2 NS 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 toward ORR.38 The E1/2 value of different catalysts increases in the following order: monometallic Au or Ag ≪ Pt/C < AuAg3.2 NSs. The AuAg3.2 NS catalyst shows a remarkably positive shift of 32 mV in E1/2 relative to Pt/C (Table S2). As seen in the mass transport-corrected Tafel plots (Figure 4b), the AuAg3.2 NS 6281


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Figure 5. ORR stability and methanol tolerance of the high-performance AuAg3.2 NS electrocatalyst in comparison with the Pt/C benchmark. (a,b) ORR polarization curves and the corresponding CV curves (inset) of (a) AuAg3.2 NSs and (b) Pt/C electrocatalysts before and after 3000 and 10 000 cycles between 0.5 to 1.0 V vs RHE. (c) Variations of ECSA (left), SA (middle), and MA (right) of AuAg3.2 NS and Pt/C catalysts before and after 3000 and 10 000 potential cycles. The MAs are compared on the basis of the mass loading of Au + Ag. (d) CA (i−t) responses recorded at 0.82 V vs RHE at 1600 rpm in O2-saturated 0.1 M KOH aqueous solution for 40 000 s. (e) Methanol cross-over test by adding methanol into 0.1 M KOH aqueous solution at 3000 s (the arrows indicate the addition of methanol).

test. The CA measurement was performed at 0.82 V in O2saturated 0.1 M KOH aqueous solution to further evaluate the durability of the electrocatalysts. As shown in Figure 5d, the current achieved with the AuAg3.2 NS catalyst retained 91% of its initial activity within 40 000 s duration, whereas commercial Pt/C experiences a sharp activity decay and drops to 63% of its initial value. The AuAg3.2 NS catalyst after 10 000 potential cycles was also characterized, which shows that the AuAg3.2 NSs could largely retain their 3D solid-void bicontinuous, spongelike architecture (Figure S10). There is negligible composition change after long-term potential cycling, as confirmed by the EDX result, highlighting the extraordinary durability of the assynthesized AuAg3.2 NSs. The methanol tolerance of ORR electrocatalysts was investigated by CA measurements with the injection of 1 M methanol in O2-saturated KOH aqueous solution at 3000 s. As shown in Figure 5e, the cathodic ORR current of the AuAg3.2 NS catalyst under 0.82 V does not exhibit a significant change, whereas the Pt/C catalyst immediately undergoes a dramatic decay after the injection of methanol, indicating that the AuAg3.2 NS catalyst shows higher methanol tolerance while catalyzing the ORR process. Therefore, the AuAg3.2 NSs have great potential to be a highly active and durable class of cathodic electrocatalysts in fuel cells. The AuAg3.2 NS catalyst was further taken as the anodic catalyst for EG and glucose oxidation reactions. As shown in Figure 6a−c, the EG oxidation reaction was conducted in N2-

In addition to the excellent ORR activity of AuAg3.2 NSs, their exceptional durability really distinguishes them from previous Au−Ag electrocatalysts, which was clearly demonstrated by the long-term cycling and CA (i−t) stability tests. First, the cycling stability of different catalysts was assessed by using the ADT, which was carried out by applying continuous potential cycles between 0.5 and 1.0 V with a sweep rate of 100 mV s−1 in O2-saturated 0.1 M KOH aqueous solution for 10 000 cycles. The ORR polarization curves of AuAg3.2 NS and commercial Pt/C electrocatalysts before and after 3000 or 10 000 cycles are shown in Figure 5a,b. The AuAg3.2 NS catalyst largely retains its ECSA and only exhibits a slight negative shift of 9 mV in the half-wave potential after 10 000 potential cycles, and the SA and MA of the AuAg3.2 NS catalyst are 1.30 mA cm−2 and 0.42 A mgAu+Ag−1 (Figure 5c and Table S2), which shows only 4.4 and 12.5% decrease compared to the corresponding initial SA and MA, respectively. For the Pt/C electrocatalyst, these 10 000 potential cycles cause a drastic decrease of ECSA (53.3%), which is ascribed to the severe detachment of Pt nanoparticles from the carbon support in alkaline media,55 leading to a more negative shift of 34 mV in E1/2 and a substantial loss of diffusion-limiting current (from 5.87 to 5.15 mA cm−2) (Figure 5b). As a result, the AuAg3.2 NS catalyst finally still provides 8.7- and 7.9-fold enhancements in SA and MA with respect to the Pt/C catalyst (0.150 mA cm−2 and 0.053 A mgPt−1, respectively) after the long-term stability 6282


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Figure 6. Cyclic voltammetric (CV) curves of high-performance AuAg3.2 NS, commercial Pt/C, and pure monometallic Au and Ag electrocatalysts for (a) EG oxidation in N2-saturated 1 M KOH aqueous solution containing 1 M EG solution and (d) glucose oxidation in N2-saturated 0.1 M KOH aqueous solution containing 0.01 M glucose aqueous solution with a sweep rate of 50 mV s−1; the currents are based on the loading of Au/Ag/Pt for AuAg3.2 NS/pure Au, pure Ag, and Pt/C electrocatalysts, respectively. (b,e) Comparison of SA and MA toward EG and glucose oxidation reactions for the corresponding catalysts. (c,f) CA (i−t) responses of four catalysts at 1.12 and 1.2 V toward EG and glucose oxidation for 3600 s, respectively.

exceptional activity and antipoisoning ability of AuAg3.2 NSs, which effectively accelerates the oxidation and desorption rates of the carbonaceous intermediates, releasing more active sites for the subsequent accessibility of alcohol molecules. This result is also in good agreement with the results observed on the nanoporous Au nanoparticles.59 At the end of 3600 s, a mass specific current of 0.86 A mgAu−1 is still retained on the AuAg3.2 NSs for EG oxidation, whereas the current of Pt/C and monometallic Au or Ag electrocatalysts sharply drop to quite small values after the initial 500 s, confirming the striking stability of AuAg3.2 NSs, which exceeds that of commercial Pt/ C by a significant extent. In addition, the glucose oxidation reaction on the AuAg3.2 NS catalyst was also investigated in N2saturated 0.1 M KOH aqueous solution containing 0.01 M glucose compared with pure Au, pure Ag, and the Pt/C benchmark. The AuAg3.2 NS catalyst exhibits the highest catalytic activity for glucose oxidation among the four different catalysts, whose SA and MA are 1.59 mA cm−2 and 1.48 A mgAu−1 (0.56 A mgAu+Ag−1), respectively, about 14.5- and 18.5 (7)-fold enhancement than that of Pt/C (0.11 mA cm−2 and 0.08 A mgPt−1) (Figure 6d,e). Moreover, it can be noted that the AuAg3.2 NSs still retain a mass specific current of 0.38 A mgAu−1 after the long-term stability test and overwhelmingly outperform other three catalysts, demonstrating the superior electrochemical durability of AuAg3.2 NSs for glucose oxidation (Figure 6f). Most impressively, compared with the few existing examples of Au-containing electrocatalysts for oxidation reactions performed under similar conditions in the previous reports, the as-synthesized AuAg3.2 NS electrocatalyst represents one of the highest catalytic activities and durabilities for EG and glucose oxidation reactions (Table S4). It is worthwhile to note that, for the anodic oxidation reactions, the AuAg3.2 NSs exhibit intriguingly exceptional selectivity and activity for EG oxidation compared to methanol oxidation, which can be

saturated 1 M KOH aqueous solution containing 1 M EG. The CV curve for AuAg3.2 NSs displays significant anodic current peaks in both the forward and reverse potential scans (inset in Figure 6a). Then, the forward potential scan curves of the AuAg3.2 NS, pure Au, pure Ag, and commercial Pt/C catalysts were taken to compare their activities for EG oxidation (Figure 6a, the complete CVs are shown in Figure S11). The AuAg3.2 NS catalyst shows the highest peak current and the most negative onset potential relative to other three catalysts, indicating its greatly improved electrocatalytic activity for EG oxidation. Furthermore, as showed in Figure 6b, the AuAg3.2 NS catalyst exhibits an SA of 8.2 mA cm−2 for EG oxidation, which is over 2.4, 2.6, and 10.2 times as high as that of the Pt/C benchmark (3.4 mA cm−2), monometallic Au, and monometallic Ag (3.1 and 0.8 mA cm−2), respectively. Meanwhile, the MA of the AuAg3.2 NSs was determined to be 7.58 A mgAu−1 (2.86 A mgAu+Ag−1) based on the mass loading of Au (Au + Ag), representing a significant improvement by a factor of 3, 12, and 95 relative to Pt/C, pure Au, and pure Ag, respectively. The exceptional selectivity and reactivity observed on the AuAg3.2 NSs for the oxidation of EG can be explained by the reaction pathway that involves both base-solution catalyzed and gold-catalyzed steps, which will be discussed below.56,57 Apart from the good catalytic activity, the open-structured AuAg3.2 NSs also exhibit superior durability for EG oxidation. CA (i−t) responses of different catalysts at 1.12 V were measured for 3600 s (Figure 6c). All electrocatalysts undergo a rapid initial activity decay, which can be ascribed to the poisoning of the carbonaceous species and the decrease of the concentration gradient.58 Interestingly, for the AuAg3.2 NScatalyzed EG oxidation reaction, the catalytic current even increases first before it starts to decay until reaching the steady state, whereas the currents kept decreasing on other catalysts. Such an increasing current density is mainly ascribed to the 6283


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AuAg3.2 NSs contain a high concentration of undercoordinated atoms on curved surfaces and structural defects (crystalline boundaries) cross-sectioning the nanowires (Figure 1c), which can serve as excellent catalytic sites for enhancing the catalytic activities. Second, previous studies have demonstrated that the atomic interactions between different metals could modify their electronic structures and thus tune the catalytic activity.23 Herein, the kinetically controlled reduction approach favors the formation of fully alloyed Au−Ag NSs with uniformly distributed elements on the atomic scale. Such intimate atomic contacts effectively regulate the d-band center of Au−Ag NSs (Figure 4d), facilitating the achievement of higher activities for both cathodic and anodic reactions. Third, the inherent features of 3D open architectures, such as a higher surface area, hierarchical pore systems, and self-supported structure, make the Au−Ag NSs a desired nanostructure with both much enhanced catalytic activity and durability.71 Last but not the least, the existence of Au can effectively prevent Ag from being easily oxidized to favor the alloy formation because of the charge donation from Au to Ag based on the XPS analysis (Figure 3b,c), as further evidenced by the CV measurement (Figure S12), leading to the superior stabilization of both the metals. It has been known that Au can effectively improve the ORR stability of other alloy systems and increase the antipoisoning ability for carbonaceous species toward alcohol oxidation, ensuring that the Au−Ag electrocatalyst remains active and stable for long-term operation.53,72,73

explained based on the mechanism of alcohol electro-oxidation over the Au electrode in alkaline media.56,57,60 The alcohols first deprotonate into their corresponding alkoxides at high pH by the abstraction of Hα through the reaction HβR−OHα ⇌ HβR−O− + Hα +

The extent of this step is mainly influenced by the solution pH (pKa of the alcohol); thus, the first deprotonation of alcohols (Hα-elimination) is solution-mediated without the need for the gold electrocatalyst. Once the active alkoxides species are formed, they would rapidly eliminate the second proton (Hβ-elimination) on the hydroxylated Au surface through the following reaction (Au-catalyzed step) as long as the cleavage of C−Hβ bond is good. HβR−O− → RO + Hβ + + 2e−

The Hammett-type relationship has been well-established by Kwon et al. to reveal the correlation between the activity of the alcohol oxidation on a gold electrode at high pH and their corresponding pKa.56 Such a relationship demonstrates that a lower pKa of alcohol usually leads to a higher activity of the corresponding electro-oxidation. As shown in the literature, the pKa values of different alcohols show the following trend: EG (14.77) < methanol (15.5) < ethanol (15.9) < isopropanol (17.1). On the basis of the Hammett-type relationship, the assynthesized AuAg NSs would show higher overall activity for EG oxidation than for other alcohol oxidations. Moreover, the much lower oxidation activity of methanol observed on the AuAg NSs than that of EG can be further explained by the cleavage ability of the C−Hβ bond during the second dehydrogenation process. The stronger C−Hβ bond in methanol than in other alcohols makes the “Hβ-elimination” step more difficult,56,60,61 resulting in a much lower oxidation activity. This result demonstrates that the methanol oxidation on the AuAg NSs greatly deviates from the Hammett-type relationship, showing an unexpected lower oxidation reactivity. If the C−Hβ bond is strong enough, the oxidation activity would even completely disappear. The observations can also be evidenced by the combined results in the previous reports.56,62,63 In addition, for glucose electro-oxidation over the Au electrode, it is well-known that AuOH species are the active sites during the oxidation process.64,65 The glucose oxidation reaction over the Au catalyst in alkaline media is believed to proceed through the oxidation of the enediol form of glucose on the hydroxylated Au surface.66−68 Such unsaturated enediol intermediates are more easily oxidized because of the easier C− H bond cleavage, which renders the Au catalyst with exceptional catalytic performance for glucose oxidation. Furthermore, the incorporation of Ag can significantly improve the catalytic activity of AuAg NSs for glucose oxidation because of the synergistic effect between Au and Ag.69 Therefore, on the basis of the analysis stated above, it can be clearly concluded that the AuAg NSs exhibit unique selectivity and reactivity for EG and glucose oxidation reactions. The observed extraordinarily enhanced activity and durability on AuAg3.2 NSs for ORR, EG, and glucose oxidation electrocatalysis, as compared to the commercial Pt/C electrocatalyst, are likely ascribed to the following reasons. First, at catalyst/electrolyte interfaces, the undercoordinated surface atoms are always more active to catalytic reactions than their coordinatively saturated counterparts.22,70 In this work, the

4. CONCLUSIONS We have synthesized a highly active and stable class of Au−Ag NSs with 3D open architecture and precisely controlled composition via a facile yet general one-pot reduction method at room temperature. For the first time, the effective modification of the d-band center in Au−Ag NSs was observed in current Au−Ag bimetallic systems. The unique Au−Ag NSs effectively integrate the hierarchically porous architecture, robust electronic effect, and self-supporting structure. These important characteristics make them exhibit enhanced multifunctional catalysis with extraordinary stability for ORR and EG or glucose oxidation in alkaline media. With the optimized component, the AuAg3.2 NSs show superior SA and MA for ORR (1.36 mA cm−2, 1.26 A mgAu−1), EG oxidation (8.2 mA cm−2, 7.58 A mgAu−1), and glucose oxidation (1.59 mA cm−2, 1.48 A mgAu−1) compared to the commercial Pt/C benchmark and also exhibit the highest catalytic activities among Au−Ag systems reported to date. Most impressively, the AuAg3.2 NS electrocatalyst exhibits exceptional electrochemical stability for both cathodic ORR and anodic EG or glucose oxidation reactions for long-term operation. Considering the substantially reduced cost via dilution with a large amount of comparatively cheap Ag, this work finds AuAg3.2 NSs a promising alternative catalyst for substituting Pt-based catalysts in renewable energy technologies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b17066. Optical photo of AuAg3.2 monolith; SEM images, compositional analyses, and XRD spectra of all AuAg NSs; CV curves of AuAg3.2 NSs and Pt/C catalysts; ORR polarization curves of all samples; physical character6284


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izations of AuAg3.2 NSs after ADT; complete CVs for EG and glucose oxidation; and comparisons of catalytic properties between AuAg3.2 NSs and current catalysts (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +86 02988492052 (F.C.). ORCID

Fuyi Chen: 0000-0002-2191-0930 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant nos. 51271148 and 50971100), the Research Fund of State Key Laboratory of Solidification Processing in China (grant no. 150-ZH-2016), the Aeronautic Science Foundation Program of China (grant no. 2012ZF53073), the Project of Transformation of Scientific and Technological Achievements of NWPU (grant no. 192017), the Doctoral Fund of Ministry of Education of China (grant no. 20136102110013), and the Open Fund of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, grant no. 2018-KF-18).

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