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Synthesis of3D thornbush-like trimetallic CoAuPd nanocatalysts and electrochemical dealloying for methanol oxidation and oxygen reduction reaction laiming luo, ronghua zhang, di chen, qingyun hu, and Xinwen Zhou ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00329 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Applied Energy Materials

Synthesis of 3D thornbush-like trimetallic CoAuPd nanocatalysts and electrochemical dealloying for methanol oxidation and oxygen reduction reaction Lai-Ming Luo, Rong-Hua Zhang∗, Di Chen, Qing-Yun Hu, Xin-Wen Zhou* (College of biological and pharmaceutical Science, China Three Gorges University, Yichang 443002, China)

Corresponding information: Tel: +86 717 6395580; fax: +86 717 6395516 E-mail address: [email protected] (R. H. Zhang); [email protected] (X. W. Zhou) Full Postal address: College of biological and pharmaceutical Science, China Three Gorges University, Daxue Road, NO. 8, Yichang City, China, 443002.

ABSTRACT: Trimetallic CoAuPd nanocatalysts are synthesized by classical successive reduction method using P123 as protectant and sodium borohydride as reductant. The structure, composition, and morphology of the nanocatalysts are measured and characterized through different techniques. The obtained results show that the trimetallic CoAuPd nanocatalysts have two kinds super three-dimensional (3D) structures: the novel nano-thornbush and nanocluster structure. The formation of these epitaxial multilevel structure maybe attribute to the Co seed crystal ferromagnetism and multilevel self-assembly. The electrocatalytic properties of the CoAuPd nanocatalysts are

investigated deeply. Furthermore, the electrochemical dealloying method is also adopted to enhance the electrocatalytic performance. The results of dealloying gradient tests for methanol oxidation reaction (MOR) demonstrate that the reaction active sites, electrochemical specific active areas and catalytic activity of the CoAuPd nanocatalysts gradually increased during the dealloying process. Besides, the CoAuPd nanocatalysts also present remarkably enhanced activity for oxygen reduction reaction (ORR) activity after dealloying process. The extraordinary catalytic performance of ∗

Tel: +86 717 6395580; fax: +86 717 6395516

E-mail address: [email protected] (R. H. Zhang); [email protected] (X. W. Zhou) College of biological and pharmaceutical Science, China Three Gorges University, Yichang 443002, China

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the dealloyed CoAuPd may derive from the special epitaxial multilevel structure such as nano-thornbush and nanocluster structure, the assembled component of featherness nanoparticles and hollow spherical nanoparticles, as well as plenty of filamentous nanocrystals. Especially, the CoAuPd nanocatalysts have more discovered and exposed active sites, and can form the CoAuPd@AuPd core-shell structure with rough AuPd alloy surface after electrochemical dealloying. The efficient

catalytic performance of the trimetallic CoAuPd nanocatalysts makes them as an excellent candidate electrocatalysts used for direct methanol fuel cell (DMFC).

Keywords: Trimetallic CoAuPd nanocatalyst; Successive reduction method; Multilevel self-assembly; Electrochemical gradient dealloying; Methanol oxidation reaction; Oxygen reduction reaction;

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1. Introduction Direct methanol fuel cell (DMFC) has been broad developed and deep explored in recent years.1,2 Pd-based catalysts as a good substitute for Pt-based catalysts have been widely applied in DMFC due to the excellent catalytic activity for methanol oxidation reaction (MOR) in alkaline medium.3,4 Pd-based alloy catalysts can decrease the dosage of Pd as well as enhance catalytic activity and durability because of the unique synergistic effect and tensile strain effect.5 On the other hand, oxygen reduction reaction (ORR) as the cathode reaction of DMFC gets extensive attention and wide development.6,7 The kinetics and mechanisms of ORR include four electron and two electron pathway. ORR at high potentials may be caused by the two electron pathway, which is undesirable for DMFC. Besides, peroxide can poison the proton exchange ionomer and membrane. Exploring more efficient nanocatalysts for MOR and ORR in DMFC will always be priority among priorities for a long time.8 Galvanic replacement reaction (GRR) has been regarded as one of the effecient self-templating method for producing hollow structure nanocatalysts with controllable size, morphology and composition.9 The driven force comes from the difference of the electrochemical potential between two metals, in which one has a more negative reduction potential (also call the sacrificial template) using as the reducing agent (anode) and the other used as the oxidizing agent (cathode).10 The morphology of the final product depends on the shape of initial sacrificial template. The successive reduction method is based on the GRR method, in which transition metal nanoparticles with a lower reduction potential such as Co2+, Ni2+, and Cu2+ will be fabricated first.11 Then, noble metal such as Pdn+, Ptn+ and Au3+ with higher reduction potentials will be reduced by the transition metals nanoparticles.12 The GRR method can fabricate nanocatalysts with different morphology, including core-shell, hollow and porous structure through controlling the synthesis conditions.13,14 Compare to the simple co-reduction method, the successive reduction method are more suitable to synthesize Pd-based nanocatalysts with controllable morphology, favorable surface structure, uniform particle size and well dispersion.15,16 Trimetallic Pd-based nanocatalysts are expected to exhibit more flexibility than bimetallic nanocatalysts in tuning the composition, structure and electronic properties of surface, and thus get more exploration.17,18,19 Furthermore, trimetallic and non-metal doping bimetal nanocatalysts usually own enhanced catalytic activity than general binary nanocatalysts.20 In the Pd-transition 3



metal (such as Fe, Co, Ni and Cu) alloy catalysts, after electrochemical test in acid media, the transition metal elements will be dissolved from the surface layer and more hidded active sites will exposed.21,22 So, trimetallic Pd-based alloy nanocatalysts will form core-shell structure with trimetallic alloy core and rough binary alloy shell after electrochemical dealloying process. Due to the unique rough shell and surface alloy effect, the catalytic activities will be markedly enhanced after the dealloying process.23,24 In our former work, we have reviewed the progress the Pt-based hollow nanocatalysts used in fuel cells.25 Hollow CoPd nanocatalysts were prepared by successive reduction method using sodium borohydride as reducer, and the influence of electrochemical dealloying to the structure and catalytic activity was also discussed.26 Later, we also found that bimetal hollow PdAu alloy catalysts obtained by the successive reduction method show an enhanced electrochemical properties and stability than the commercial Pd black and Pd/C.27 Trimetallic PtPdRu nanocatalysts synthesized by hydrothermal method with triblock copolymer P123 as reducer have superior catalytic activity than corresponding bimetallic PtPd and PtRu nanocatalysts.28 In this paper, trimetallic CoAuPd catalysts were synthesized by the successive reduction method, in which the sodium borohydride (NaBH4) was used as reductant and Pluronic P123 was used as protecter. The morphology and structure of the CoAuPd catalysts were characterized and the possible synthesis mechanism was also discussed. The electrocatalytic performance of the CoAuPd catalysts for MOR and ORR were investigated. Electrochemical dealloying gradient tests were conducted to further enhance the MOR and ORR performance. The electrocatalytic activity and stability of the CoAuPd catalysts before and after electrochemical dealloying and the influence of the effect degree of gradient dealloying to the electrocatalytic activity were explored deeply.

2. Experimental 2.1 Chemicals Sodium borohydride (NaBH4), sodium hydroxide (NaOH), potassium hydroxide (KOH), polyoxypropylenepolyoxyethylene copolymer (P123, PEO19-PPO69-PEO19), absolute methanol (CH3OH), sulfuric acid (H2SO4) and chloride hexahydrate (CoCl2·6H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. Potassium palladium chloride (K2PdCl4), chloroauric acid 4


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(HAuCl4·3H2O) and Nafion solution (5% wt) were purchased from Alfa Aesar. All chemicals were used as received without further purification. The water was purified through a Millipore system. 2.2 Synthesis of the CoAuPd catalysts In the successive reduction method, 50.0 mL aqueous solution dissolved 1.00 g P123 and 22.85 mg CoCl2·6H2O were prepared in round-bottom flask at room temperature (30℃) with vigorous stirring. Then, N2 was bubbled into aqueous solution to prevent the oxidation of Co nanoparticles. After that, 10.00 mL aqueous solution involved 0.10 g NaBH4 were dropped into the above mixed solution slowly and constantly and the mixed solution became brown and black immediately. Then, 20.00 mL aqueous solution contained 10.74 mg K2PdCl4 and 0.32 mL HAuCl4·3H2O (0.03 mmol) were added into the reagent drop by drop. The products were collected after about 4 hours by centrifugation, washed by water and ethanol three times. 2.3 Characterization of the CoAuPd catalysts The morphology of the CoAuPd catalysts was characterized by transmission electron microscope (TEM), high resolution TEM (HRTEM) on instruments of FEI TeGNai-F30 electron microscopy. The composition of CoAuPd catalysts was determined by energy dispersive spectrometer (EDS) and high-angle annular dark-field scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (HAADF-STEM-EDS) mapping. The crystal structure of the CoAuPd catalysts was characterized by powder X-ray diffraction (XRD) using Rigaku (UltimaIV) with Cu-Kα radiation (λ=1.5406 Å). 2.4 Electrochemical measurements of the CoAuPd catalysts 30 µL homogeneous suspension of the catalyst (2.01 mg/mL) was uniformly covered on the surface of the clean glassy carbon (GC, Φ 5mm) electrode. Then, 10 µL 0.5% (m/m) Nafion solution were pipetted on it to fix the catalysts. Electrochemical experiments were carried out in a standard three-electrode cell on AUTOLAB electrochemical workstation (PGSTAT12) at 30 oC. Pt flake and saturated calomel electrode (SCE) was used as counter electrode and reference electrode, respectively. The cyclic voltammetry (CV) test was carried out in N2-saturated 1.0 M NaOH solution between -0.9 V and 0.5 V (vs SCE) with a scan rate of 100 mV/s. In 1.0 M NaOH solution, the relationship of φRHE (using reversible hydrogen electrode (RHE) as the reference electrode) and φSCE (using SCE as the reference electrode) is that φRHE =φSCE + 1.0703 V.

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CO-stripping measurement was applied for estimate the electrochemical active surface area (ECSA) of the CoAuPd catalysts. The ECSA was calculated based on the equation: ECSA=Q/GI. Where Q is the charges for CO desorption electrooxidation (µC), G represents the total amount of Pd (µg) on the electrode surface, and I is the charge required to oxidize the monolayer of CO (420 µC cm-2) on the catalyst. MOR test was conducted in N2-saturated 1.0 M NaOH +1.0 M CH3OH solution between -0.8 V and 0.4 V with a scan rate of 50 mV/s. ORR test was carried in O2-saturated 0.1 M KOH solution with rotation rates from 100 to 2500 rpm between -0.7 V and 0.3 V with a scan rate of 10 mV/s. The more information about ORR tests can be found in the Supporting Information. The dealloying gradient test was carried out in 0.1 M H2SO4 from -0.2 V to 0.9 V with a scan rate of 100 mV/s. The dealloyed product was denoted as dealloyed CoAuPd.

3. Results and discussions Figure 1 shows the low and TEM images of the CoAuPd nanocatalysts. It can be seen that large-scale trimetallic CoAuPd nanocatalysts with various morphologies have been fabricated successfully. The CoAuPd nanocatalysts have two types super 3D structures including novel nano-thornbush structure (Figure 1a and c) and special nanocluster structure (Figure 1b and d) in one sample like the reported references. 29,30 The well-organized 3D thornbush-like quasi-spherical structures shown in Figure 1a and c are coated with brushy skin layer and urchin-like surface.31 This kind particular multi-branch quasi-spheres are constructed by many elongated irregular shaped two-dimensional (2D) nanoparticles, such as featheriness, bamboo leaves shaped nanoparticles and so on.32 The size of the 3D nano-thornbush structure ranges from 84.92 nm to 176.38 nm with an average size of 130.65±45.73 nm obtained from the particle size distribution graph (insert in Figure 1a). The average size of the super spherical 3D nanocluster structure consisted of plenty smaller spherical nanoparticles is about 26.10±4.30 nm (insert in Figure 1b).33 In Figure 1b and 1d, we can observe that some nano-thornbush mixed with the nanocluster. Actually, in Figure 1a and c, the nanocluster can also be observed in the nano-thornbush structure. These results demonstrated that the two totally different structures can coexist in one kind sample. The more distinct morphology and detail structure information can be identified in high-magnification TEM and Figure S1.

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Figure 2 reveals the high-magnification and enlarged high-magnification TEM images of the CoAuPd nanocatalysts. In Figure 2a and 2c, we can clearly see that the 3D nano-thornbush structures are assembled by 2D feather-like and bamboo leaves shaped nanoparticles.34 The average length of covered 2D featheriness nanoparticles is about 19.76±5.44 nm (inset in Figure 2a). These well-organized 2D featheriness nanoparticles are further assembled by numerous one-dimensional (1D) needle-like, filamentous and spinulose nanocrystals shown in Figure 2c.35 These special 1D filamentous nanocrystals have many serrated corners and edges which could offer abundant defect sites. These defects are frequently emerged along the filamentous structure and the junctions of filamentous structure transform the growth direction. These extra defect sites can provide more available active sites and then improve the catalytic activity. Figure 2b and d reveals the 3D nanocluster structure with an open-framework and hollow porous structure, which are composed of densely 0D spherical nanoparticles with some distinct hollow and solid nanocrystals.36 The mean diameter of these 0D spherical nanoparticles are approximately about 7.18±1.21 nm. Some hollow nanoshperes can be observed in Figure 2d because the GRR occurred during the crystal nucleation and growth stage. These hollow nanospheres usually have more reaction active sites and higher catalytic activity than the identical solid structures because both the inner and the outer surface of the hollow nanospheres can be utilized.27 The structural details and lattice parameters of the 1D filamentous nanocrystals and 0D spherical nanocrystals can be found in Figure 3. Figure 3a, c and Figure 3b, d show the HRTEM images of the CoAuPd nanocatalysts with nano-thornbush structure and 0D spherical hollow and solid structure, respectively. The lattice fringes of the nano-thornbush nanocrystals is measured about 0.225 nm which is close to the lattice of (111) crystal plane of Pd metal (0.224 nm). This results demonstrates that the preferential growth along the (111) directions of the CoAuPd nanocatalysts. The Co and Au atoms mixed with Pd atom will lead to the lattice expansion of the CoAuPd nanocatalysts, in which we can observe the lattice parameter with 0.237 nm and 0.242 nm.37 The anisotropic growth along the (111) direction is commonly observed in the 1D nano-thornbush nanocrystals due to the low surface energy of (111) crystal plane.38,39 In Figure 3b and d, the lattice-spacing value of 0D spherical hollow and solid nanocrystals is measured about 0.220 nm and 0.225 nm, which is coincide with the (111) crystal planes of Pd metal and CoAuPd alloy. We can also find the lattice expansion effect in the 0D spherical nanocrystals. The fast 7



Fourier transform (FFT) of the atomic-lattice fringing further confirms the well crystallinity of filamentous as well as spherical hollow and solid nanocrystals (insert in Figure 3c and d). These observations demonstrate that the Pd atoms mixed with Co atoms and Au atoms very well, and formation of CoAuPd alloy phase. Compared with the unique spherical nanocluster, the novel nano-thornbush sturcture are more attractive because of its special novel structure.7, 33 High-angle annular dark-field scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (HAADF-STEM-EDS) mapping is employed to study the element composition and distribution of the original nano-thronbush CoAuPd

nanocatalysts.40 Figure 4 shows the HAADF-STEM-EDS mapping of element Co, Au, Pd and the overlay images of the nano-thornbush CoAuPd nanocatalysts. These images investigate that Co, Au

and Pd are uniformly distributed in the whole nano-thornbush CoAuPd nanocatalysts. Figure 4e, f, g and h show the overlay images of CoAu, CoPd, AuPd and CoAuPd elements, respectively. The results indicate that the three kind elements mixed very well, which confirms that the nano-thornbush sturcture actually is one kind homogeneous trimetallic CoAuPd alloy. Figure 5 shows the X-ray diffraction (XRD), selected area electron diffraction (SAED) and energy dispersive spectrometer (EDS) patterns of CoAuPd. Figure 5a shows that the CoAuPd nanocatalysts have a single phase of fcc structure. The reflection peaks located at 39.04°, 44.52°, 65.35°, 79.99° correspond to the four major feature peaks of (111), (200), (220) and (311) of Au. These reflection peaks are shifted to the positive direction compared to the standard Pd and Co, indicating the alloy formation of the trimetallic CoAuPd nanocatalysts. The atomic radius of Au, Co and Pd is 0.144 nm, 0.125 nm (within 0.144 nm±15%) and 0.138 nm (within 0.144 nm± 15%), respectively. Thus the Co and Pd atoms can be incorporated into the Au lattice to form an fcc alloy structure.41, 42 According to the Debye-Scherrer formula, the smaller the particle size is, the larger the half-peak width is, and the weaker the corresponding peak intensity is. The obtained CoAuPd nanocatalysts are composed of small filamentous nanoparticles. That is why the intensity of the XRD peaks is so weak. The calculated crystallite size of the CoAuPd nanocatalysts is nearly 9.1 nm by Debye-Scherrer formula and MDI Jade 5.0. The SAED image (Figure 5b) also shows obvious four concentric rings, which can be assigned as (111), (200), (220) and (311) reflection peaks. The EDS in Figure 5c shows the existence of Co, Au and Pd elements in the CoAuPd nanocatalysts. 8


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Scheme 1 illustrates the possible formation mechanism of the trimetallic CoAuPd nanocatalysts. For the successive reduction (GRR) method, the first obtained Co nanoparticles can reduce the Pd and Au metal precursor to form hollow alloy structure because the reduction potential of Co2+/Co (-0.28 V) is lower than that of AuCl4-/Au (1.002 V) and PdCl42-/Pd (0.591 V) .43 Seed growth theory and

self-organization mechanism are well established for the formation of these multiple-branching construction and cluster architecture.44 Combining with the nucleation and growth mechanisms such as Ostwald ripening process,45 a possible formation mechanism of the CoAuPd nanocatalysts can be speculated as follows. Firstly, Co2+ will be reduced by NaBH4 to form Co seed crystals. The magnetic induced primary self-assembly will happen in the initial stage to form irregular shaped (filamentous) and spherical Co nanoparticles owing to the unique ferromagnetism of the Co nanocrystals.31,46,47 These freshly formed Co nanocrytals are thermodynamically unstable owing to their high surface energy. These smaller Co nanocrystals will be drived to carry out a two-stage self-assembly into larger featheriness nanoparticles by the minimization of interfacial energy.48 Finally, these irregular Co nanoparticles including hollow and solid spherical nanoparticles will further be self-assembled to form the novel nano-thornbush and nanocluster structures. Moreover, the speed of NaBH4 drops may promote the formation of 3D epitaxial multilevel structures.49 Except for the favorably compositional effect and alloy effect, the high surface area, hollow porosity and hyperbranched construction of 3D structures could provide abundant extra active sites and efficiently improve the mass transport and gas diffusion on the surface of alloy nanocatalysts, which will enhance the electrocatalytic performance.50 On the other hand, these self-assembled 3D nanocatalysts can efficiently avoid aggregation and collapse and then increase their electrocatalytic stability. These superior epitaxial multilevel 3D structures are less vulnerable to aggregation, dissolution or falling off even after dealloying process.51 Figure 6a shows that the cyclic voltammetry (CV) curve of the CoAuPd nanocatalysts is very similar to that of pure Co (insert in Figure 6a) in 1.0 M NaOH solution which indicates that the surface of the CoAuPd nanocatalysts enriched Co atoms. So the electrocatalytic activity of the initial CoAuPd nanocatalysts for methanol oxidation is very weak shown in Figure 6b. The current density has been normalized by the unit mass of Pd. Then, dealloying gradient tests are carried out in 0.1 M H2SO4 solution shown in Figure 6c and the test step is 2 cycles each time. The intensity of the reduction peak of Pd oxide (-0.35 V) in Figure 6d is slowly revealed step by step after the 9



dealloying gradient test. The electrocatalytic activity for methanol oxidation is also increased gradually shown in Figure 6e. The Co atoms on the surface are dissolved progressively during the dealloying process and newly AuPd alloy surface is formed. So in Figure 6d we can observe the CV curves of the well-defined AuPd alloy nanocatalysts. The hydrogen adsorption/desorption peaks are not well distinguished in alkaline media because the hydrogen desorption is overlapping with the adsorption of OH- ions.52 The CV curves for the CoAuPd nanocatalysts before and after dealloying process are compared in Figure 6d and 6e. The initial CoAuPd alloy structure will form CoAuPd alloy core and rough AuPd-enriched alloy shell structure by dealloying, and become the CoAuPd@AuPd structure eventually. In alkaline medium, electrochemical surface area (ECSA) of Pd-based catalysts can be calculated by integrating the reduction peak of Pd oxide monolayer.53 This method can also be used to evaluate the changes of the ECSA in the dealloying process. The enhanced electrocatalytic methanol oxidation performance of the dealloyed CoAuPd may be attributed to fantastic morphologic effectand the particular CoAuPd@AuPd structure with a roughness AuPd surface. These results indicate that the dealloying gradient test is one of the most dealloying methods to restructure the surface of the Pd-based nanocatalysts. Compared with the ECSA of Pd-based catalysts calculated from the area of Pd oxide reduction peaks, the ECSA obtained by integrating the area of CO oxidation is more precise and reliable. Figure 7 shows the CO-stripping measurement voltammograms of the dealloyed CoAuPd, commercial Pd black and Pd/C catalysts. In Figure 7, we can see that the adsorbed CO is oxidized completely in the 1st cycle and only the CV peaks of Pd can be observes in the 2st cycle. When the potential is negative than the potential of formation OH- ions adsorbed species, the adsorbed CO on the surface of the nanocatalysts will be oxidized by OH- ions dissolving in alkaline solution. So we can observe a satellite peak besides the CO oxidation peak located about -0.2 V in Figure 7a. Actually, we have observed this phenomenon in our previous works.4 The calculated ECSA of the dealloyed CoAuPd, commercial Pd black and Pd/C catalysts is 42.64 m2 gPd-1, 40.79 m2 gPd-1 and 35.39 m2 gPd-1, respectively. The ECSA of the dealloyed CoAuPd nanocatalysts is bigger than that of the commercial catalysts because of the well-organized hierarchical 3D structure and hollow structure. Especially after the electrochemical dealloying process, more active sites and additional specific surface area will appear. The anti-CO poisoning capability can also be reflected through the onset and peak oxidation potential of CO. show more negative CO The onset and peak 10


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oxidation potential of CO of the dealloyed CoAuPd nanocatalysts is about -0.35 V and -0.012 V, respectively. These values are more negative than that of the commercial catalysts. The excellent anti-CO poisoning ability for the dealloyed CoAuPd nanocatalysts might be due to the structural effect and bi-functional mechanism.

Table 1 List of E0, EP, jP and ECSA obtained from Figure 7d and Figure 8b c and e.

Catalysts

Metal loading/mgPd

Eo/V

Ep/V

jp/mA mgPd -1

jp/mA cm-2

ECSA/m2 gPd-1

Dealloyed-CoAuPd

0.0603

-0.52

0.01

495.85

1.16

42.64

Pd black

0.1000

-0.29

0.19

131.24

0.32

40.79

Pd/C

0.0800

-0.44

0.07

173.57

0.49

35.39

The cyclic voltammogram (CV) of the dealloyed CoAuPd nanocatalysts, commercial Pd black and Pd/C in 1.0 M NaOH solution are shown in Figure 8a. Figure 8a shows well-defined CV curves of the AuPd alloy which suggest that AuPd alloy surface was formed. The CoAuPd alloy structure has been became a CoAuPd@AuPd core-shell structure after the dealloying process. The current density of CH3OH oxidation in Figure 8b and c has been normalized by the weight of unit Pd component and ECSA. The onset oxidation potential (E0), oxidation peak potential (Ep) and normalized oxidation peak current density in the forward scan (jp) are summarized in Table 1. The E0 is determined by the tangent method, in which the point of the intersection of baseline and the tangent of oxidation peak is the onset oxidation potential.14 The E0 and Ep of the dealloyed CoAuPd are -0.52 V and 0.01 V which are more negative than that of commercial Pd black (-0.29V/0.19V) and Pd/C (-0.44V/0.07V). These values indicate that CH3OH is more easily oxidized on the dealloyed CoAuPd nanocatalysts than the commercial catalysts. The stability of the CoAuPd nanocatalysts is investigated through chronoamperometry. The j-t curves of the dealloyed CoAuPd and commercial catalysts obtained at -0.1 V are recorded in Figure 8d. From the insert in Figure 8d, we can see the dealloyed CoAuPd nanocatalysts have a larger current at -0.1 V than other potentials. The current density decreases sharply and then reaches a steady current. The current density of the dealloyed CoAuPd nanocatalysts is always higher than the commercial catalysts which illustrates that the dealloyed CoAuPd nanocatalysts

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has higher electrocatalytic activity for methanol oxidation. Figure 8e shows the specific activity and mass activity of three kinds of catalysts, in which we can see that the dealloyed CoAuPd nanocatalysts show the highest specific activity and mass activity than others. The specific activity of the dealloyed CoAuPd nanocatalysts is 3.63 times and 2.37 times than that of the commercial Pd black and Pd/C catalysts, and the mass activity is 3.78 times and 2.86 times than that of commercial Pd black and Pd/C catalysts. Figure 9a and d shows the linear sweep voltammograms (LSVs) of the CoAuPd nanocatalysts modified GC rotating disk electrode (RDE) for ORR before and after the dealloying process. The current density is normalized by the geometric area of RDE (0.196 cm2). Figure 9b and c displays the electrochemical dealloying process in 0.1 M H2SO4 solution and the CO-stripping test in 0.1 M KOH, respectively. The plateau in the potential range between 0.30 V and 0.60 V (vs RHE) is diffusion controlled region in the polarization curves. The ORR current density increases with the increase of the rotating rate. Polarization curves are under mixed diffusion-kinetic control from 0.60 V to 1.00 V (vs RHE). The diffusion-limited current density (jd) of the CoAuPd nanocatalysts is increased significantly after the dealloying process (Figure 9d). The linear relationship between j-1 and ω-0.5 is observed before and after the dealloying process at the same potential (0.4 V vs RHE). The number of electrons transferred per O2 molecule (n) is calculated from the slope of K-L plots in Figure 9f, and n is estimated close to 4 after electrochemical the dealloying process. These results suggest that the ORR occurred at approximately four-electron pathway where O2 was directly reduced to H2O. The onset potential (Eonset) and half-wave potential (E1/2) of the dealloyed CoAuPd nanocatalysts are more positive that of the initial CoAuPd nanocatalysts which indicates an enhanced ORR activity at much lower overpotentials after the dealloying process. The improvement of the ORR activity is attributed to the increased exposure of active sites on the CoAuPd nanocatalysts surface after the dealloying process.

Table 2 List of Eonset, E1/2, ECSA and SA, MA obtained from Figure 9, Figure S2 and Figure 10.

Catalysts

ECSA/m2 gPd-1

Eonset/V

E1/2/V

MA/mA mgPd -1

Dealloyed-CoAuPd

22.148

1.06

0.968

48.907

12


SA(0.8V)/µA SA(0.9V)/µA cm-2 cm-2 220.816

193.464

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Pd black

18.657

1.08

0.918

32.484

174.113

148.364

Pd/C

17.768

1.04

0.909

41.434

233.190

157.445

Figure 10a shows the LSVs of the dealloyed CoAuPd nanocatalysts and commercial catalysts in O2-saturated 0.1 M KOH at 1600 rpm with a scan rate of 10 mV/s. The jd is in range of -5 and -6 mA cmGEO-2. Additionally, the dealloyed CoAuPd nanocatalysts display the highest jd than that of the commercial catalysts (more information about the commercial catalysts ORR properties can be found in Figure S2). From the K-L plots shown in Figure 10b, it can be seen that the dealloyed CoAuPd nanocatalysts and the commercial catalysts are made through four-electron pathway. The Tafel plots for ORR are obtained by plotting the potential (E) to log|jk| shown in Figure 10c. Two principal linear regions can be identified in the Tafel curves with low- and high-current regions during the ORR on Pd. The transition of ORR kinetics from the low- to high-current regions reveals that the mechanism transformation of oxygen adsorption and intrinsic activities improvement of nanocatalysts.54 Figure 10d and Table 2 shows the static data obtained from Figure 10a and Figure S2. The specific activity (SA) and mass activity (MA) obtained at 0.8 V and 0.9 V are normalized by the Pd mass and ECSA. The MA and SA of the dealloyed CoAuPd nanocatalysts at 0.9 V are 48.907 mA mgPd -1 and 193.464 µA cm-2, respectively. The SA of dealloyed CoAuPd nanocatalysts is 1.506 times and 1.180 times than that of commercial Pd black and Pd/C catalysts, respectively. The MA of the dealloyed CoAuPd nanocatalysts is 1.304 times and 1.229 times than that of commericial Pd black and Pd/C catalysts. The enhanced ORR catalytic activities might be contributed to the super 3D thornbush-like structure and special nanocluster structure, plenty of hollow nanoparticles, and the electrochemical dealloying process, in which the CoAuPd@AuPd core-shell structure with rough surface and more available active areas and sites is formed.

4. Conclusions In summary, trimetallic CoAuPd alloy nanocatalysts are synthesized by the successive reduction method successfully using P123 as stabilizer and NaBH4 as reducer. The CoAuPd nanocatalysts are composed of two kinds of different morphologies and structures, including novel super 3D nano-thornbush structure and 3D nanocluster structure. Both of these epitaxial multilevel 13



structures are homogeneous, well-dispersed and united particle size. The formation mechanism of the CoAuPd nanocatalysts may be attributed to the ferromagnetic Co seed crystals and their multilevel self-assembly. The electrochemical dealloying gradient tests are employed to regulate the surface structure of the catalysts. The electrocatalytic performance of the CoAuPd nanocatalysts for the MOR and ORR are investigated and compared before and after the dealloying process. Both of the MOR and ORR performance of the CoAuPd nanocatalysts are obviously enhanced after the dealloying process and are much higher than that of the commercial Pd/C and Pd black catalysts. The extraordinary catalytic performance of the CoAuPd may derive from the super 3D nano-thornbush and nanocluster structure, the component of featherness nanoparticles and hollow spherical nanoparticles, as well as the composition of filamentous nanocrystals. On the other side, more additional active sites will be exposed and appeared, and CoAuPd@AuPd core-shell structure with roughness AuPd alloy surface will be formed after the electrochemical dealloying process.

Acknowledgement The study was financially supported by the National Natural Science Foundation of China (21503120, 21403126) and the Innovation Foundation from the China Three Gorges University (2017YPY084).

ASSOCIATED CONTENT Supporting Information Available: ORR test procedure, TEM and HRTEM of the nano-thornbush structure CoAuPd nanocatalysts, Electrocatalytic performance of the commercial Pd/C and Pd black.

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Legend of Scheme, Figures and Tables Figure 1 TEM images of CoAuPd nanocatalysts, (a, c) nano-thornbush, (b, d) nanocluster. The particle size distributions are shown in the insets in (a, b). Figure 2 High-magnification TEM images of CoAuPd nanocatalysts, (a, c) nano-thornbush, (b, d) nanocluster.. The insets show the particle size distributions and enlarged TEM images. Figure 3 HRTEM of CoAuPd nanocatalysts, (a, c) nano-thornbush, (b, d) nanocluster. The insets show the corresponding FFT patterns in HRTEM (c, d). Figure 4 (a) HAADF-STEM-EDS mapping for the nano-thornbush CoAuPd nanocatalysts, HAADF-STEM-EDS elemental mapping images of Co (b), Au (c), Pd (d), and the overlay images of Co, Au and Pd (e-h). Figure 5 (a) XRD patterns of standard Co (PDF#15-0806), Au (PDF#65-2870), Pd (PDF#65-2867), and CoAuPd nanocatalysts. (b) SAED images and (c) EDS patterns of CoAuPd nanocatalysts. Figure 6 CV curves of CoAuPd modified electrodes in (a) 1.0 M NaOH, (b) 1.0 M NaOH + 1.0 M CH3OH solution and (c) Dealloying gradient test curves in 0.1 M H2SO4 solution. CV curves of CoAuPd in (d) 1.0 M NaOH and (e) 1.0 M NaOH + 1.0 M CH3OH solution after dealloying gradient tests. Figure 7 CO-stripping voltammograms of (a) the dealloyed CoAuPd, commercial (b) Pd black (c) Pd/C catalysts in 1.0 M NaOH. (d) The obtained ECSA values. Figure 8 Cyclic voltammograms of dealloyed CoAuPd nanocatalysts and commercial Pd black, Pd/C modified electrodes in (a) 1.0 M NaOH and (b, c) 1.0 M NaOH + CH3OH. (d) Chronoamperometric curves of methanol oxidation at -0.1 V in 1.0 M NaOH + CH3OH. (e) Comparison of mass and specific activities of catalysts for methanol oxidation. Figure 9 ORR polarization curves of CoAuPd nanocatalysts (a) before and (d) after electrochemical dealloying. (b) CV curves carried in 0.1 M H2SO4 solution for dealloying. (c) CO-stripping test of the dealloyed CoAuPd nanocatalysts. (e) LSVs of CoAuPd nanocatalysts before and after dealloying process at a rotation speed of 1600 rpm, and (f) corresponding K-L plots. Figure 10 (a) LSVs for dealloyed CoAuPd and commercial Pd black and Pd/C in O2-saturated 0.1 M KOH at 1600 rpm, (b) K-L plots, (c) Tafel plots derived from (a), (d) MA and SA at 0.80 V. Scheme 1 Formation mechanism of the CoAuPd nanocatalysts Table 1 List of E0, EP, jP and ECSA obtained from Figure 7d and Figure 8b c and e. Table 2 List of Eonset, E1/2, ECSA and SA, MA obtained from Figure 9, Figure S2 and Figure 10.

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nanoparticles

dispersed

on

magnetic

yolk-shell

nanoarchitectures

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1.02

E/V(vs RHE)

1.2 -2

1.05

Dealloyed -CoAuPd Pd black Pd/C

1.5

j/mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.9 0.6 0.3

0.99 0.96 0.93 0.90 0.87

0.0 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Dealloyed-CoAuPd Pd black Pd/C

0.01

E/V(vs SCE)

0.1

1 -2

log|jk|/mA cm

GEO

Trimetallic CoAuPd alloy nanocatalysts including novel super 3D nano-thornbush and hollow nanocluster structure synthesized by successive reduction method using Pluronic P123 as protecter and NaBH4 as reducer, which exhibit enhanced electrocatalytic performance for MOR and ORR after dealloying process.



Figure 1 TEM images of CoAuPd nanocatalysts, (a, c) nano-thornbush, (b, d) nanocluster. The particle size distributions are shown in the insets in (a, b). 183x184mm (300 x 300 DPI)


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Figure 2 High-magnification TEM images of CoAuPd nanocatalysts, (a, c) nano-thornbush, (b, d) nanocluster.. The insets show the particle size distributions and enlarged TEM images. 205x207mm (300 x 300 DPI)



Figure 3 HRTEM of CoAuPd nanocatalysts, (a, c) nano-thornbush, (b, d) nanocluster. The insets show the corresponding FFT patterns in HRTEM (c, d). 205x205mm (300 x 300 DPI)


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Figure 4 (a) HAADF-STEM-EDS mapping for the nano-thornbushCoAuPdnanocatalysts, HAADF-STEM-EDS elemental mapping images of Co (b), Au (c), Pd (d), and the overlay images of Co, Au and Pd (e-h). 576x294mm (96 x 96 DPI)



Figure 5(a) XRD patterns of standard Co (PDF#15-0806), Au (PDF#65-2870), Pd(PDF#65-2867), and CoAuPdnanocatalysts.(b) SAED images and (c) EDS patterns of CoAuPdnanocatalysts. 616x366mm (96 x 96 DPI)


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Figure 6 CV curves of CoAuPdmodified electrodes in (a) 1.0 M NaOH, (b) 1.0 M NaOH+1.0 M CH3OH solution and (c) Dealloying gradient test curves in 0.1MH2SO4 solution. CV curves of CoAuPd in (d) 1.0 M NaOHand (e) 1.0 M NaOH+1.0 M CH3OH solution after dealloying gradient tests. 779x482mm (96 x 96 DPI)



Figure 7 CO-stripping voltammograms of (a) the dealloyedCoAuPd, commercial (b) Pd black (c) Pd/C catalystsin 1.0 MNaOH. (d) The obtained ECSA values. 744x556mm (96 x 96 DPI)


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Figure 8 Cyclic voltammograms of dealloyedCoAuPdnanocatalysts and commercial Pd black, Pd/Cmodified electrodes in (a) 1.0 M NaOHand (b, c) 1.0 M NaOH+ CH3OH. (d) Chronoamperometric curves of methanol oxidation at -0.1 V in 1.0 MNaOH+ CH3OH. (e) Comparison of mass and specific activities of catalysts for methanol oxidation. 803x854mm (96 x 96 DPI)



Figure 9 ORR polarization curves of CoAuPdnanocatalysts (a) before and (d) after electrochemical dealloying. (b) CV curves carried in 0.1 M H2SO4 solution for dealloying. (c) CO-stripping test of the dealloyedCoAuPdnanocatalysts. (e) LSVs of CoAuPd nanocatalysts before and after dealloying process at a rotation speed of 1600 rpm,and (f) corresponding K-L plots. 1237x631mm (96 x 96 DPI)


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Figure 10 (a) LSVs for dealloyedCoAuPd and commercial Pdblack and Pd/C in O2-saturated 0.1 M KOH at 1600 rpm, (b) K-L plots, (c) Tafel plots derived from (a), (d) MA and SA at 0.80 V. 928x687mm (96 x 96 DPI)



Scheme 1 Formation mechanism of the CoAuPd nanocatalysts 590x267mm (96 x 96 DPI)


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Synthesis of 3D Thornbush-like Trimetallic ... - ACS Publications

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