CuAg@Ag CoreâShell Nanostructure Encapsulated by N-Doped

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CuAg@Ag Core-shell Nanostructure Encapsulated by N-Doped Graphene as a High Performance Catalyst for Oxygen Reduction Reaction Tran Duy Thanh, Nguyen Dinh Chuong, Hoa Van Hien, Nam Hoon Kim, and Joong-Hee Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16294 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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ACS Applied Materials & Interfaces

CuAg@Ag Core-shell Nanostructure Encapsulated by N-Doped Graphene as a High Performance Catalyst for Oxygen Reduction Reaction Tran Duy Thanha, Nguyen Dinh Chuonga, Hoa Van Hiena, Nam Hoon Kima*, Joong Hee Lee a,b* a

Advanced Materials Institute of BIN Convergence Technology (BK21 plus Global), Dept. of

BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea. b

Carbon Composite Research Centre, Department of Polymer & Nanoscience and Technology,

Chonbuk National University Jeonju, Jeonbuk 54896, Republic of Korea. *Corresponding author: Tel.: 82-63-270-2342; Fax: 82-63-270-2341. E-mail address: [email protected] (Prof Joong Hee Lee).and [email protected] (Prof Nam Hoon Kim)

ABSTRACT: Development of robust, cost-effective, and efficient catalyst is extremely necessary for oxygen reduction reaction (ORR) in fuel cell applications. Herein, we reported a well-defined nanostructured catalyst of highly dispersed CuAg@Ag core-shell nanoparticles (NPs) encapsulated nitrogen-doped graphene nanosheets (CuAg@Ag/N-GNS) to exhibit superior catalytic activity towards the ORR in alkaline medium. The synergistic effects produced from the unique properties of CuAg@Ag core-shell NPs and N-GNS nanosheets made such a novel nanohybrid to display a comparable catalytic behaviour to commercial Pt/C product. In particular, it demonstrated much better stability and methanol tolerance than Pt/C under the same

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conditions. Due to the outstanding electrochemical performance and ease of synthesis, CuAg@Ag/N-GNS material was expected as a promising low-cost catalyst for ORR in alkaline fuel cell applications. KEYWORDS: Copper-silver core-shell nanoparticles, nitrogen-doped graphene, encapsulation, electrocatalyst, oxygen reduction reaction, fuel cell. 1. INTRODUCTION The exhaustion of fossil fuel sources, the fast growth in energy requirements, and environmental pollution have become critical issues causing wide concern in the development of renewable power sources, novel storage energy and conversion energy technologies. In this regard, fuel cell technologies have garnered much attention due to their abilities to easily convert chemical energy into electric energy with high power density.1,2 However, the sluggish oxygen reduction reaction (ORR) at the cathode seriously limits efficiency of these devices.3 Pt and Ptalloy-based nanomaterials are commonly employed as the effective electrocatalysts for ORR, but the high cost, easy poisoning effect, and low stability are still key challenges restricting their application.3-5 Other materials have been recently considered as transition-metal-based catalysts to decrease Pt or replace the Pt catalyst;6-10 however, the complicated synthetic strategy, low activity, and low short-term stability are still critical remaining issues.6,11 Therefore, the study on novel Pt-free electrocatalysts is going on as an urgent requirement to explore suitable candidates for ORR applications. Silver metal has been emerging as great potential because it is around 50 times cheaper than Pt and also superior stability over Pt in alkaline environment, as revealed by Pourbaix diagram.12,13 Even though the surface Ag atoms are around ten times less active than Pt atoms, consistent with its lower area-specific activity than Pt,13 recent studies demonstrated that


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the formation of bimetallic M@Ag core-shell nanostructures can enhance the surface reactivity of Ag, thereby improving its activity and stability for ORR.14 According to the systematic density function theoretical (DFT) calculations, Ag and Cu activities for ORR was located near Pt in a volcano plot.15 The weak oxygen binding energy (∆EO) of Cu and strong ∆EO of Ag when compared to ∆EO of Pt led to the prediction that a combination process of Ag with Cu can alter the ∆EO to an optimum value, which could greatly boost the catalytic activity of AgCu-based nanostructures.15 In addition, DFT calculations recently indicated that the Cu@Ag core-shell nanostructures were impressively higher ORR activity and stability than single Ag and other AgCu alloy counterparts, due to the improved surface reactivity of Ag, the modified electronic configuration of the metal, and weak oxidation rate of Cu.16,17 More recent researches confirmed that the rise of d-band center in Cu@Ag core-shell nanostructures close to the Fermi level established the optimum adsorption strength of intermediates, such as O, OH, OOH on surface of catalyst for the fast ORR kinetic.18 At the same time, the investigation into the active sites of Cu@Ag core-shell clusters for oxygen dissociation reaction noticed that the most active pathway for the O2 bond cleavage is B2 site, in which its activation energy barrier was found to be 0.715 eV.19 The experimental study on catalytic activity of Cu@Ag core-shell nanostructures has also recognized the strong synergistic effect from Cu and Ag to impressively enhance catalytic activity towards ORR.18, 20 In addition to interest with the high performance of Cu@Ag core-shell nanostructure-based catalysts, copper is abundantly available in the earth and very cheap 3d transition metal for diverse applications.21-24 Therefore, the development of Cu@Ag core-shell nanostructure-based catalyst can be an efficient and economical way for ORR in alkaline medium, rather than Pt-based materials. Although CuAg@Ag catalyst is highly potential for ORR application, the deactivation is still a critical problem due to inhomogeneous particle


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size and aggregation phenomenon, leading to low stability and catalytic activity.25, 26 Therefore, production of stable, finely dispersed metal NPs catalyst without aggregation is a key requirement to enhance its catalytic performance. The use of an active and large surface area supporting material that can strongly interact with the catalyst NPs has been proven to efficiently promote dispersion and to protect catalyst from potential agglomeration and dissolution.27-29 In this context, graphene nanosheets (GNS) have emerged as excellent supporting materials due to outstanding properties, such as high conductivities, very large surface areas, high mechanical and chemical stabilities, and ease of functionalization of the materials.30, 31 More benefits can be obtained by doping nitrogen heteroatom into GNS structure. In this regard, the introduction of N can significantly increase the charge transfer ability of GNS.32,

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Besides, the presence of

nitrogen electrochemical active sites, such as pyridinic-N and graphictic-N sites were demonstrated to highly improve the electrochemically active surface area of graphene structure.34-37 Furthermore, the presence of nitrogen containing groups can efficiently anchor the metal catalyst NPs and facilitate the homogeneous dispersion of metal catalysts, leading to improve number of active sites of metal catalysts, thereby enhancing catalytic activity of materials towards ORR.38,

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Currently, the formation of a novel core-shell nanostructure in

which metal catalyst NPs are encapsulated by GNS through a particular synthetic approach, was demonstrated to enhance catalytic activity and stability of materials for electrocatalytic applications. This is due to the enhanced electrochemical stability by the improved graphitic carbon structure and protection of graphene.40, 41 In addition, the presence of a unique electronic interaction between the metal catalyst and N-doped graphene based shells is effective to stabilize metal catalyst without aggregations and dissolution, thereby enlarging the electrochemical surface and increasing the ORR active site numbers for a better ORR performance, including


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catalytic activities, stability, and durability.

42-44

Seeing the electrocatalytic potential of

metal/GNS hybrid from the above findings, a highly active and stable CuAg@Ag encapsulated N-GNS catalyst was synthesized by a facile synthetic strategy in this study for the first time. The electrochemical testing results proved that this catalyst showed very good ORR activity in alkaline medium, even highly comparable to that of the commercial Pt/C product. 2. EXPERIMENTAL SECTION 2.1 Materials. Graphite flake, potassium permanganate (KMnO4, ≥ 99%), hydrogen peroxide solution (H2O2, 30%) copper nitrate hexahydrate (Cu(NO3)3.6H2O, ≥ 99%), silver nitrate (AgNO3, 99.9%), β-cyclodextrin, sodium boron hydride (NaBH4, 98%), urea (NH2CSNH2, ≥ 99%), nafion solution (5 wt%), and Pt/C catalyst (20 wt% Pt) were purchased from Sigma Co. (USA). Carbon black powder (HIBLACK 420B) was purchased from Korea Carbon Black Co. (Korea). Hydrochloric acid (HCl, 35-37%), methanol (CH3OH, 99.9%), and potassium hydroxides (KOH, ≥ 99.5%) were provided by Samchun Co. (Korea). Ultrapure water was obtained by the EYELA Still Ace SA-2100E1 (Tokyo Rikakikai Co., Japan) filtering system. 2.2 Preparation of CuAg@Ag nanostructures. In a typical procedure for synthesis of CuAg@Ag NPs, 0.024 g copper nitrate was dissolved in a 30 mL deionized (DI) water containing 0.5 g β-cyclodextrin. The solution was vigorously stirred for 30 min under a flow of N2 gas to remove O2. And 0.024 g NaBH4 (in 10 mL H2O) was then added rapidly into the solution followed by continuously stirring for 20 min. Subsequently, the temperature of solution was increased to 70 oC for 20 min to complete the reaction. Then, the solution was cooled to room temperature followed by addition of 0.008 g silver nitrate (in 10 mL H2O) and kept under


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vigorous stirring for 20 min. Finally, the product was collected and washed by DI-water several times before using for further experiments. 2.3 Preparation of CuAg@Ag/N-GNS nanohybrid. Firstly, 0.06 g GO material, which was synthesized by a modified Hummer’s method (as presented in the supporting information online),45 was well dispersed in 30 mL water by sonication for 1 h. Then the GO solution was slowly added with CuAg@Ag solution under sonication. Subsequently, the mixture was vigorously stirred for 3 h followed by addition of 0.3 g urea. After a freezer-drying process for removing the solvent and avoiding the restacking of the GO sheets, the solid material was pyrolized at 550 oC in Ar atmosphere for 1 h to obtain the CuAg@Ag/N-GNS hybrid (Fig. S1). The weight percentage of CuAg@Ag in hybrid was estimated to be 25.2 wt% by thermogravimetric analysis (TGA) (Fig. S2). The rGO, N-GNS, Ag/rGO and CuAg@Ag/rGO were also prepared for comparison and the corresponding synthetic procedures were presented in supporting information. 2.4 Material Characterization. Raman analysis was conducted by a Nanofinder-30 instrument (Tokyo Instruments Co., Japan). The X-ray diffraction (XRD) was analyzed on a D/Max 2500 V/PC (Rigaku Co., Japan) at the Center for University-Wide Research Facilities (CURF), Chonbuk National University, Korea. XRD patterns of the materials were obtained by using a Cu target (λ = 0.154 nm) in the 2θ range of 5-80° at a scan rate of 2°/min. Field-emission electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDAX) were performed on a Supra 40 VP instrument (Zeiss Co., Germany). Transmission electron microscopy (TEM) and SAED were investigated by a JEM-2200FS instrument (JEOL Co., USA) operated at 120 kV, located at the Jeonju Center of the Korea Basic Science Institute. X-ray photoelectron spectroscopy (XPS) spectra were recorded by a Theta Probe instrument (Thermo Fisher Scientific Inc., USA). The


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surface area of the materials were investigated by the Brunauer-Emmett-Teller (BET) analysis on an ASAP 2020 Plus system (Micromeritics Instrument Corp., USA). TGA was carried out using a TGA Q50 system (TA Instrument Co. USA) at a heating rate of 10 °C⋅ min−1 under air flow. 2.5 Electrochemical characterization. The electrochemical properties of material was assessed using rotating ring disk electrode rotator RRDE-3A (ALS Co., Japan) and the electrochemical analyzer CHI 660D (CH Instruments Inc., USA) workstation. In this context, a rotating disk electrode (RDE, 0.071 cm2)/a rotating ring-disk electrode (RRDE, 0.123 cm2), Ag/AgCl electrode, and Pt wire were used as the working, reference and counter electrode, respectively. The RDE and RRDE electrodes were polished with Al2O3 slurry (0.1 µm particle size) followed by ultrasonication in water for 1 min. To prepare the working electrode, the CuAg@Ag/N-GNS catalyst (2.5 mg) was dispersed in 0.5 mL ethanol containing 5 µL of Nafion solution (5 wt%) by sonication for 1 h to form a homogeneous ink suspension. Subsequently, the RDE and RRDE surfaces were deposited with 10 and 17.3 µL of the ink suspension, respectively, followed by drying in air for 3 h. The RDE and RRDE have the same catalyst loading per unit area, in which the CuAg@Ag/N-GNS amount was fouded to be 0.7 mg⋅cm-2. The cyclic voltammetry (CV) measurements were conducted from -0.8 – 0.2 V at a scan rate of 50 mV⋅s-1 in 0.1 M KOH solution saturated with N2 and O2. The CV data was recorded after a run of 20 cycles. The linear sweep voltammetry (LSV) measurements were done at scan rate of 10 mV s-1 in O2-saturated 0.1 M KOH and different rotation rates of the RDE (400–2000 rpm) for evaluating the number of electrons transferred (n) according to Koutecky-Levich equation. Chronoamperometric measurements were used to investigate MeOH tolerance of the as-synthesized nanohybrid and commercial Pt/C product.


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The reported potential values were referenced to that of reversible hydrogen electrode (RHE), according to the below equation: = + / + 0.059 + / where, / = 0.1976 at 25 °C.

3. RESULTS AND DISCUSSION Morphology and microstructure of materials were characterized using FE-SEM technique. Figure 1a showed the morphology of N-GNS, which had exfoliated structure in presence of typical crumpled and wrinkled features. In addition, the TEM image of N-GNS showed highly transparent and wrinkled flake-like surface, implying its high specific surface area along with an effective nitrogen doping effect on graphene backbone (Fig. S3).46 The FE-SEM images of CuAg@Ag/N-GNS showed spherical metal nanoparticles (NPs) homogeneously dispersing on the GNS surface (Fig. 1b and c). This may be due to the high surface area of GNS along with the presence of highly negative charged N-doping sites as active sites, which facilitated for anchoring as well as dispersing NPs. The FE-SEM images of Ag/rGO and CuAg@Ag/rGO hybrid were also shown in Fig. S4.


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Figure 1 FE-SEM images of (a) N-GNS and (b-c) CuAg@Ag/N-GNS nanohybrid; (d) TEM image of CuAg@Ag/N-GNS nanohybrid. The typical TEM images of CuAg@Ag/N-GNS indicated CuAg@Ag NPs uniformly distributing within the N-GNS (Figs. 1d). The diameter of NPs was found to be around 50 nm (Fig. 2a). For elemental characterization, EDAX confirmed the presence of Cu, Ag, C, and N components in the as-synthesized nanohybrid (Fig. 2b). Additionally, high resolution TEM (HRTEM) image showed a distinct contrast of core-shell architectures in which there are the dark core of rich Cu phase and the comparatively pale shell of Ag with its specific crytal planes, such as d(200) and d(111),47 indicating the formation of bimetallic core-shell NPs on GNS surface (Fig. 2c). Furthermore, it can be seen that the CuAg@Ag NPs were encapsulated by several graphene layers, clearly confirming the formation of a unique metal@GNS core-shell


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nanostructures (Figs. 2c and S5), which can effectively protect NPs from the agglomeration as well as dissolution possibility. 42-44 The formation of CuAg@Ag core-shell NPs encapsulated by N-GNS was proven by further characterizations. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and the corresponding EDS color mapping showed the evidence of Cu and Ag enrichment in the center and in the shell of the NPs, respectively (Fig. 2d-h). The uniform distribution of nitrogen and carbon elements was also clearly visualized in the selected area, demonstrating the successful incorporation of nitrogen into the GNS structure. The electron energy-loss spectroscopy (EELS) line profiles of Cu, Ag, C, and N across an individual NP were shown in Figs. 2i-m. The Ag profile had a higher intensity at the two sides than at the center, whereas the Cu profile showed a higher intensity at the center, confirming the formation of Ag-shell over the CuAg-core (Figs. 2i-k). Also, the presence of C and N elements over the NP revealed that the CuAg@Ag NPs were encapsulated by N-GNS material (Figs. 2l and m).


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Figure 2 (a) Particle size distribution and (b) EDAX of CuAg@Ag/N-GNS; (c) HR-TEM of CuAg@Ag/N-GNS; (d) HAADF-STEM image and EDS color mapping of (e) Cu, (f) Ag, (g) C, and (h) N at the selected area; (i) HAADF-STEM image and EDS color mapping of (j) Cu, (k) Ag, (l) C, and (m) N for a single nanoparticle. XRD was used as a facile analytical technique to identify the crystalline phases of materials (Fig. 3a). The XRD pattern of GO showed a sharp d (001) peak at around 11°, indicating its well exfoliated structure. After doping with nitrogen at 550 °C in Ar for 1 h, the XRD pattern of NGNS and CuAg@Ag/N-GNS presented appearance of a broad pattern at 2θ = 25.5°, corresponding to a d spacing of 0.34 nm from the d(002). This was resulted from the reaggregation of graphene structure because the deoxygenation process significantly occurred and GO was reduced into N-GNS, as similarly seen by some earlier reports.48, 49 The XRD pattern of CuAg@Ag/N-GNS additionally showed the presence of specific crystal structures for both the Cu and Ag components. The crystalline characteristic of Cu could be observed from the two peaks at 43.2o and 50.3o, corresponding to d (111) and d (200) orientation, respectively (JCPDS 04-0836).50 In the case of Ag, a series of peaks at 38.1o, 44.4o, 64.6o, and 77.5o belonged to d(111), d(200), d(220), and d(311) orientation, respectively (JCPDS 04-0783).51 More importantly, no signal of copper oxides was noticed from CuAg@Ag/N-GNS as compared to a reference sample (Cu/N-GNS), which showed the clear presence of crystal peaks from CuO and Cu2O components (Fig. S6).50 These results further confirmed the formation of the core-shell nanostructure, in which the Ag layer effectively protected Cu from its oxidation in air, as consistent with previous reports.52, 53 In Raman characterization, the GO, N-GNS, and CuAg@Ag/N-GNS displayed two specific peaks at around 1352 and 1577 cm-1, corresponding to the D and G bands of graphene


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characteristic, respectively (Fig. 3b). The intensity ratio of D to G band (ID/IG) was used to evaluate the in plane crystallite sizes, density of in plane and edge defects, and the disorder characteristic in the graphene structure. After completing the nitrogen-doping process and loading of the CuAg@Ag NPs on N-GNS, the ID/IG value of N-GNS and nanohybrid increased to ∼0.94 and ∼0.99, respectively, relative higher than that of GO (∼0.82). In addition, a downshift of the G peak position for N-GNS (∼12 cm-1) and nanohybrid (∼16 cm-1) as compared to GO was also clearly observed. This result was assumed to an increase of the in plane and edge defect density, reduction of nanocrystalline graphene domains, and high disorder nature due to introduction of nitrogen atoms into GNS structure.54-56 In addition, it also reflected the stress alteration of GNS resulting from improved interactions between incorporated metal NPs and GNS.57, 58 As an important factor to evaluate characteristic of electrocatalyst, the BET surface area analysis was used to investigate the surface area of CuAg@Ag/N-GNS (Fig. 3c). The nanohybrid possessed mesoporous characteristic (20-40 nm) along with a corresponding BET surface area of ∼101 m2⋅g-1, which was much better than previously reported related catalysts.5962

This feature of this nanohybrid can provide abundant active sites, which facilitated mass and

charge transfer ability as well as significantly shortened the mass and ion diffusion length, which are considered to be vital for the improved electrocatalytic performance. XPS measurements were also carried out to investigate the composition and chemical state of N-GNS and CuAg@Ag/N-GNS. The XPS spectrum of N-GNS revealed the existence of C1s (288.4 eV), N1s (404 eV), and O1s (535.8 eV) components. The nitrogen percentage was founded to be 7.2 at%, indicating the successful incorporation of the N heteroatoms into GNS structure (Fig. 3d).


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Figure 3 (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption-desorption isotherms, and (d) XPS survey spectra of materials. Compared to N-GNS, XPS survey spectrum of CuAg@Ag/N-GNS displayed two additional peaks at 947.3 eV and 370.4 eV, consistent with specific Cu2p and Ag3d binding energies, respectively. The high resolution spectra of C1s for N-GNS and CuAg@Ag/N-GNS showed much lower intensities of the oxygen-containing groups, such as C–O (285.9 eV), C=O (287.2 eV), and –COO– (289.2 eV) as compared to GO (Figs. 4a and S7a and b). Also, there was the appearance of C–N bonding at a binding energy of 287.1 eV, further suggesting the effective doping of the GNS structure with nitrogen. The high resolution spectra of N1s can be deconvoluted into three components, such as pyridinic-N at 398.5 eV, pyrrolic-N at 400 eV, and graphitic-N at 401.2 eV (Fig. 4b and Fig. S7c). As demonstrated by previous reports that the


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active sites for ORR are generated by pyridinic-N and graphitic-N,36 hence the much higher amount of pyridine-N (∼62%) and graphitic-N (∼20%) compared with pyrrolic-N (∼18%) in the as-prepared N-GNS can significantly contribute an enhanced performance for ORR application. The high resolution spectra of Ag3d (Fig. 4c) and Cu2p (Fig. 4d) showed that the chemical states of Ag were Ag0 (Ag3d5/2, 368.4 eV; Ag3d1/2, 274.4 eV)63 and those of Cu were mainly Cu0 (Cu2p3/2, 934 eV; Cu2p1/2, 954.2 eV) and a little Cu2+ (943.8 eV; 963.6 eV).64 The presence of two peaks for Cu2+ suggested minor oxidized surfaces of Cu due to the diffusion of Cu from core to outer surface after a thermal treatment or a charge transfer possibility from Cu to graphene.65 Therefore, enhanced active sites and component interactions were directly visible in the nanostructure of CuAg@Ag NPs and N-GNS. These features can significantly increase the catalytic properties of material towards electrochemical applications.


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Figure 4 (a) High resolution XPS spectra of CuAg@Ag/N-GNS: (a) C1s, (b) N1s, (c) Cu2p, and (d) Ag3d. The as-prepared materials were tested as electrocatalysts for ORR in alkaline medium using CV as a preliminary electrochemical study. It can be seen that the appearance of a sharp and strong peak at around 0.72 V in O2-saturated 0.1 M KOH indicated effectively catalytic behaviour of CuAg@Ag/N-GNS towards ORR (Fig. 5a). The LSV curves of different materials in O2-saturated 0.1 M KOH solution were shown in Fig. 5b. The measured current density of CuAg@Ag/N-GNS was sharply reduced and therefore its diffusion-limited current densities, which was achieved at potential values below 0.66 V and much higher than that of the rGO, NGNS, Ag/rGO, and CuAg@Ag/rGO catalysts. The corresponding onset potential (Vonset), half wave potential (Vhalf

wave),

Tafel slope, and charge transfer resistance (Rct) were used as the

important indicators to initially evaluate catalytic activity of materials.


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Figure 5 (a) The CV curves of the CuAg@Ag/N-GNS in 0.1 M KOH solution saturated with N2 and O2 at potential scan rate of 0.05 V·s-1; (b) The LSV curves of GCE, rGO, N-GNS, Ag/rGO, CuAg@Ag/rGO, and CuAg@Ag/N-GNS in O2-saturated 0.1 M KOH solution at electrode’s rotation speed of 1600 rpm and potential scan rate of 0.01 V·s-1; (c) Tafel plots of GCE, rGO, NGNS, Ag/rGO, CuAg@Ag/rGO, and CuAg@Ag/N-GNS; (d) EIS measurements from 100 kHz to 0.1 Hz at an amplitude of 0.005 V in O2-saturated 0.1 M KOH solution for the rGO, N-GNS, CuAg@Ag/rGO, and CuAg@Ag/N-GNS. It can be seen that the incorporation of N in to carbon backbone make N-GNS materials exhibited superior catalytic activity than rGO, because the presence of pyridinic N and graphictic N as can effectively promote the ORR process, as demonstrated by some previous reports.36, 66 At the same time, the formation of CuAg@Ag core-shell nanostructure in CuAg@Ag/rGO


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provided higher catalytic benefits toward ORR compared to Ag/rGO. This was explained that the formation of a such core-shell nanostructure improved the electronic properties of the surface Ag atoms, then leading to increase the surface reactivity for Ag and electroactive site numbers.14, 37, 67-68

Also, the unique interactions betwwen Cu and Ag atom can alter electronic configuration in

NP for the surface compressive strain, which facilitated for weaking binding energy of adsorbed oxygen, thereby improving catalytic behavior towards ORR process.69, 70 In particular, the Vonset (0.94 V) and Vhalfwave (0.85 V) values of the CuAg@Ag/N-GNS hybrid were found to be more dramatically positive than those of other studied materials. Also, the Tafel slope of CuAg@Ag/N-GNS (67 mV⋅dec-1) exhibited a lower overpotential value towards ORR than that of rGO (100 mV⋅dec-1), Ag/rGO (81 mV⋅dec-1), N-GNS (89 mV⋅dec-1), and CuAg@Ag/rGO (73 mV⋅dec-1), indicating its advanced activity – a highly desirable feature for electrochemical applications (Fig. 5c). This great improvement of CuAg@Ag/N-GNS performance towards ORR was attributed to the excellent synergistic effect of double active centers from N-GNS and CuAg@Ag NP. In this consideration, the presence of nitrogen-containing groups on graphene surfaces effectively anchored and uniformly dispersed metal NPs.38, 39 In addition, the formation of core-shell nanostructure can possibly lead to a unique electronic interaction between the CuAg@Ag catalyst and N-doped carbon shell, as well as avoid aggregations and dissolution possibilities.71, 72 Therefore, these phenomena well amplified the electrochemical surface and increased the ORR active site numbers for a better ORR performance in terms of enhanced catalytic activity and stability. The electrochemical impedance spectroscopy (EIS) measurement was employed to estimate the Rct value of materials (Fig. 5d). The CuAg@Ag/N-GNS showed a much smaller Rct value (∼72 Ω) than that of rGO (∼149 Ω), N-GNS (∼91 Ω), and CuAg@Ag/rGO (∼118), suggesting


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faster charge transfer rate of CuAg@Ag/N-GNS for an electrochemical reaction under the same conditions. The smaller Rct might be attributed to the effective dispersion of CuAg@Ag NPs within the high conductive bridging of N-GNS and the formation of metal@GNS encapsulation nanostructures, which effectively improved charge transfer path and shortened charge transfer length. Therefore, this finding was believed to significantly contribute to the enhancement in the electrocatalytic activity of the CuAg@Ag/N-GNS for ORR. For investigating the ORR kinetics, the Koutecky-Levich equations (1) and (2) were used to determine the number of transferred electrons (n) per O2 molecule reduction at different rotation rates of RDE.73

= + = + ⁄

(1)

⁄

" = 0.2$%&' (') * +⁄, (2) where J, JL, and JK are the measured current density, diffusion-limiting current density, and kinetic current density (mA⋅cm-2), respectively, ɷ is the angular velocity (rpm), F is the Faraday constant (96485 C⋅mol-1), CO2 is the dissolved O2 concentration (1.2×10-6 mol⋅cm-3), DO2 is the diffusion coefficient of O2 gas (1.9×10-5 cm2⋅s-1), and V is the kinetic viscosity of the 0.1 M KOH electrolyte (0.01 cm2⋅s-1). According to the plots of J-1 versus ω-1/2, the electron transfer numbers (n) for ORR at different materials was evaluated (Fig. 6a-b, Fig. S9, and Fig. S10). Generally, an ORR can happen via 4e- or 2e- path way. An electrocatalyst is considered as an efficient candidate when it is preferred for the 4e- path way during the reaction. The n value for CuAg@Ag/N-GNS hybrid was found to be around 3.6-3.8 in the surveyed potential range. These results were also confirmed by the RRDE test, which not only monitors the current for ORR but also recognizes the intermediates produced from the disk surface of electrode. The n and hydrogen


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peroxide anion (HO2-)% on the CuAg@Ag/N-GNS surface during the ORR process were evaluated (Fig. S11). The n was calculated to be in the range of 3.63-3.76 along with a HO2-% of 11-18%, further indicating that the oxygen reduction was mainly controlled by a four-electron pathway. For a comparision, the number of transferred electrons at 0.71 V for CuAg@Ag/N-GNS was higher than that of other materials (Fig. 6c). In addition, the electrocatalytic stability of CuAg@Ag/N-GNS towards ORR was evaluated by LSV measurement in O2-saturated 0.1M KOH. The results showed only a minor downshift for Vhalf

wave

and Vonset, whereas the limit

current was almost constant after 2,000 cycles (Fig. 6d). This demonstrated the good stability of CuAg@Ag/N-GNS towards ORR in alkaline medium.

Figure 6 (a) The LSV curves of CuAg@Ag/N-GNS in O2-saturated 0.1M KOH solution at a scan rate of 10 mV⋅s-1 and different rotation rates of RDE; (b) Plot of J-1 versus ω-1/2 according to


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the Koutecky-Levich equation at different electrode potentials; (c) The number of electron transfer for rGO, N-GNS, Ag/rGO, CuAg@Ag/rGO, and CuAg@Ag/N-GNS at applied potential of 0.71 V; (d) The stability of CuAg@Ag/N-GNS towards ORR measured by LSV in O2saturated 0.1 M KOH solution at a scan rate of 10 mV⋅s-1 and electrode’s rotation rates of 1,600 rpm. Considering the potential application of CuAg@Ag/N-GNS catalyst towards ORR, its catalytic behavbiour was notably better than that of previously reported studies (Table S1). Furthermore, the catalytic performance of CuAg@Ag/N-GNS was also compared to commercial Ag NPs and Pt/C products. In this regard, the CuAg@Ag/N-GNS exhibited much better catalytic behaviours than the commercial Ag NPs through LSV and chronoamperometric results, including more positive onset potential and half wave potential, higher limit current density, and better current retention after long-term working time (Fig. S12). Figure 7a indicated that the Vonset and Vhalf wave of CuAg@Ag/N-GNS were slightly negative as compared to those of Pt/C, whereas the diffusion-limit current was mostly similar to that of Pt/C. In addition, the same Tafel slope values were obtained for CuAg@Ag/N-GNS (67 mV⋅dec-1) and Pt/C (73 mV⋅dec-1), certifying that activity of CuAg@Ag/N-GNS was comparable to Pt/C (Fig. 7b). A slightly lower mass activity of CuAg@Ag/N-GNS compared to Pt/C was observed at 0.85 V (Fig. S13). At the same time, in order to survey whether the CuAg@Ag/N-N-GNS is an active catalyts towards ORR in acid medium or not, the LSV measurements of the CuAg@Ag/N-GNS and Pt/C were carried out in O2-saturated 0.1 M HClO4 solution. The obtained results showed that the CuAg@Ag/N-GNS displayed much lower limit current density, more negative onset potential and half wave potential (Fig. S14a), as well as very low mass activity (Fig. S14b) than those of Pt/C. This confirmed the unsatified catalytic activity of the CuAg@Ag/N-GNS towards ORR in


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acid medium (Fig. S14). The methanol (MeOH) crossover is a serious problem, significantly decreasing the energy density and increasing the overpotential of the reaction at the cathode for the direct methanol fuel cell (DMFC), because MeOH and its oxidative intermediates can poison the active sites of a catalyst.74 In order to overcome this issue, more catalyst material is needed at the cathode which may have a negative economic impact by increasing the cost of the fuel cell. Therefore, the MeOH tolerance abilities of CuAg@Ag/N-GNS and Pt/C were investigated by injecting 0.5 M MeOH into an O2-saturated 0.1 M KOH solution during a chronoamperometric measurement. The results showed that the current density did not noticeably alter for CuAg@Ag/N-GNS after adding MeOH, whereas a sudden and strong reduction of current density clearly occurred for Pt/C (Fig. 7c). This proposed that the CuAg@Ag/N-GNS possesses the better MeOH tolerance than Pt/C. Additionally, the stability of CuAg@Ag/N-GNS and Pt/C toward the ORR was also tested by chronoamperometric measurement in O2-saturated 0.1 M KOH solution. After 20,000 s measurements, the current density of CuAg@Ag/N-GNS still retained ∼90 % of the initial value which was much better than the ∼75% of Pt/C (Fig. 7d), indicating exceptional stability and durability of CuAg@Ag/N-GNS as compared to Pt/C. After stability test, the morphology of hybrid material still remain highly dense NPs dispersing on graphene surface (Fig. S15). This result may be due to the nitrogen doping effect on GNS and the formation of metal@GNS encapsulation architectures (as shown by HR-TEM), which significantly enhanced interactions between CuAg@Ag NPs catalyst and graphene, thereby protecting the catalyst from aggregation and dissolution possibilities during electrochemical operation.


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Figure 7 (a) LSV curves of the CuAg@Ag/N-GNS and commercial Pt/C product in O2-saturated 0.1M KOH solution at electrode’s rotation speed of 1,600 rpm and potential scan rate of 0.01 V·s-1; (b) Tafel plots of CuAg@Ag/N-GNS and Pt/C; (c) MeOH tolerance and (d) working stability of Pt/C and CuAg@Ag/N-GNS catalyst measured by chnoroamperometry at 0.66 V in O2-saturated 0.1 M KOH solution. 4. CONCLUSIONS For the first time, a nanohybrid of CuAg@Ag/N-GNS was successfully prepared and used as a cathodic electrocatalyst towards ORR. The homogeneous dispersion of CuAg@Ag core-shell NPs within N-GNS exhibited high catalytic activities relative to a commercial Pt/C product. In addition, such a nanohybrid demonstrated much better MeOH tolerance, longer durability and


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stability than Pt/C. These findings resulted from the synergistic effect exerted by unique structures and properties of the encapsulated CuAg@Ag core-shell NPs and N-GNS. Because the as-prepared nanohybrid was synthesized using a facile, cost-effective strategy and performed so well, it can be used as a potential catalyst towards ORR in alkaline medium for fuel cell applications. ASSOCIATED CONTENT Supporting Information SEM, TEM, and HR-TEM images, electrochemical results and table. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Corresponding Author’s E-mail: [email protected] (Joong Hee Lee) Notes The authors declare no competing financial interest ACKNOWLEDGMENTS This study was supported by the Basic Research Laboratory Program (2014R1A4A1008140) and Nano-Material Technology Development Program (2016M3A7B4900117) through the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning of Republic of Korea.


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Graphic abstract


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CuAg@Ag CoreâShell Nanostructure Encapsulated by N-Doped

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