Enhanced Electrocatalytic Activities of PtCuCoNi Three-Dimensional

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Enhanced Electrocatalytic Activities of Threedimensional PtCuCoNi Nanoporous Quaternary Alloys for Oxygen Reduction and Methanol Oxidation Reactions Shaofang Fu, Chengzhou Zhu, Dan Du, and Yuehe Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00424 • Publication Date (Web): 12 Feb 2016 Downloaded from http://pubs.acs.org on February 15, 2016

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ACS Applied Materials & Interfaces

Enhanced Electrocatalytic Activities of Three-dimensional PtCuCoNi Nanoporous Quaternary Alloys for Oxygen Reduction and Methanol Oxidation Reactions Shaofang Fu, Chengzhou Zhu,* Dan Du, Yuehe Lin*

The School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, USA.

ABSTRACT

Control of morphology and composition could precisely and efficiently alter the catalytic properties of Pt-based materials, improving the performance on electrocatalytic activity and durability. Here we proposed a rapid, controllable synthesis of three-dimensional PtCuCoNi quaternary alloys with low Pt-group metal, which were directly synthesized by reducing metal precursors in aqueous solution. The resultant quaternary alloys show excellent oxygen reduction and methanol oxidation reaction activities in acid solution. By rationally tuning the composition of PtCuCoNi alloys, they achieved a mass activity of 0.72 A/mgPt on the basis of the mass of Pt for oxygen reduction reaction. Moreover, the durability is also higher than that of commercial Pt/C catalyst. These PtCuCoNi quaternary alloys characterized by three-dimensional porous nanostructures hold attractive promises as substitutes for

ACS Paragon Plus Environment


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carbon supported Pt catalysts with improved activity and stability.

KEYWORDS: three-dimensional structures; porous nanomaterials; low-Pt catalysts; alloys; oxygen reduction reaction; methanol oxidation reaction

Introduction

Among the various clean energy resources, fuel cells become effective solutions for the ever-growing energy demands and increasing environmental problems. Due to their high efficiency and environmental friendliness, fuel cells, especially proton exchange membrane fuel cells (PEMFCs), have presented a variety of applications in automotive vehicles and portable devices.1-5 In addition, compared to hydrogen, methanol is a cheap liquid fuel, easily handled, transported, and stored, and with high theoretical energy density. Therefore direct methanol fuel cells (DMFCs) are of potential interest for portable energy needs. However, the commercialization of fuel cells is still hindered by several issues. For instance, gradual and sudden electrochemical surface area (ECSA) lost, caused by Pt sintering, carbon corrosion, dissolution of Pt, and harsh conditions of fuel cells, contributes to the decrease in the catalytic activity and thus a decrease in the overall fuel cell performance.6, 7 Pt-base anodic catalysts are also susceptible to CO-like carbonaceous intermediates poisoning, which lead to the poor catalytic performance for methanol oxidation reaction (MOR).8 Additionally, the high cost of Pt, sluggish reaction kinetics of oxygen reduction reaction (ORR), and the instability of carbon-supports also impede the widespread usage of fuel cells. Therefore, it is the highest priority to explore novel catalytic


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materials with high electrocatalytic activity and stability.

To this end, extensive efforts have been dedicated to synthesize electrocatalysts with low Pt-group metal (PGM). For example, the core-shell nanostructures with a Pt-shell or monolayer could effectively decrease the Pt cost as a result of the enhanced Pt utilization.9, 10 Moreover, it has been demonstrated that the lattice difference between the core and shell atoms causes the compressive strain on the Pt shell, which is favorable for the electrocatalytic performance.11 Another commonly adopted strategy to synthesize the catalysts with low PGM is alloying Pt with other nonprecious metals, including Fe,12 Co,13 Ni,14, 15 Cu,16 and Zn17 to form porous bimetallic/multimetallic alloys. For example, Chen et al. reported a size-controlled synthesis of Pt-Cu hierarchical

trigonal

bipyramid

nanoframes,

which

exhibited

enhanced

electrocatalytic activity toward formic acid oxidation reaction as much as 2.1 times in mass activity (MA) and 5.5 times in specific activity (SA) compared to commercial Pt/C catalyst.18 Zhang et al. successfully prepared octahedral Pt2CuNi alloy nanoparticles with uniform element distribution, achieving both outstanding ORR activity and much improved catalyst stability compared to that of state-of-the-art Pt/C catalyst.19 It is generally accepted that this enhancement of catalytic activity is attributed to the lattice strain induced by the lattice mismatch between Pt and other metals, which can weaken the interaction between Pt atoms and the adsorbed species, and thus release more active sites on the surface.20 In addition, the electronic structure modification of Pt with the presence of nonprecious metals also contributes to the significant improvement of electrocatalytic activity of Pt-based alloys.21 Combining



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with these favorably compositional effects, further constructing three-dimensional (3D) porous alloy nanostructures are believed to enhance their electrochemical performances. The high surface area and porosity of 3D materials could provide abundant active sites and efficiently improve the mass transport and gas diffusion on the surface of the alloys, which in turn enhance the electrocatalytical performance.22, 23

Moreover, it was found that these porous noble metal nanostructures characterized

by 3D continuous and integrated material are efficiently presintered, thus significantly minimizing the loss of electrochemical surface area (ECSA) because of agglomeration. Meanwhile, the durability of the self-supported catalysts could be further improved since the support effect of carbon was eliminated.7 In addition, the elimination of the carbon support would enable a thinner electrode catalyst layer and improved mass transport and Pt utilization within the catalyst layer due to the direct contact between the catalyst layer and the gas diffusion layer.24

Among the large number of synthesis strategies of 3D porous nanomaterials, sodium borohydride (NaBH4) reduction based on kinetically controlled synthesis is considered to be the most facile and efficient method to construct 3D networks without any capping agent. Considering that noble metal-based networks as well as bimetallic/trimetallic alloys have been successfully constructed and applied in catalysis,7 it is acceptable to propose that multimetallic systems could be synthesized and used on catalytic applications. In this study, we report the synthesis of 3D PtCuCoNi nanoporous quaternary alloys (NPQAs) by NaBH4 reduction method in aqueous solution. The electrocatalytic experiments show that the as-prepared 3D


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PtCuCoNi NPQAs present remarkable enhancement on ORR and MOR activity and durability as a result of their unique multicomponent and porous properties, making them promising for advanced catalytic applications.

Experimental Section

Chemicals and Reagents

Sodium borohydride (NaBH4, 98%, powder), commercial platinum/carbon (Pt/C) 20 wt. % (Pt loading: 20 wt. %, Pt on carbon black) were purchased from Alfa Aesar. Copper (II) chloride (CuCl2, powder, 99%), nickel (II) chloride (NiCl2, 98%) nitric acid (70%), nafion perfluorinated resin solution (5 wt. % in mixture of lower aliphatic alcohols and water, contains 45% water), and perchloric acid (HClO4) were obtained from Sigma-Aldrich. Chloroplatinic acid hexahydrate (H2PtCl6•6H2O, 99.9% Pt) was bought from Strem Chemicals. Cobalt (II) chloride hexahydrate (CoCl2•6H2O) was purchased from Acros Organics. All aqueous solutions were prepared with ultrapure water (>18 MΩ) from barnstead nanopure water system.

Apparatus

Transmission electron microscopy (TEM) images were obtained by Philips CM200 UT (Field Emission Instruments, USA). FEI Sirion field emission scanning electron microscope (FESEM) was used for imaging and energy-dispersive X-ray analysis (EDX). X-ray Diffraction (XRD) characterization was carried out by Rigaku Miniflex 600. The tube was operated at 40 kV accelerating voltage and 15 mA current. X-ray



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photoelectron spectroscopy (XPS) measurements were performed on a Kratos AXIS-165 multi-technique electron spectrometer system with a base pressure of 1x10-9 torr.

The spectra of the surfaces were obtained with an AXIS-165

manufactured by Kratos Analytical Inc. (Spring Valley, NY, USA) using a monochromatic X-ray radiation of 1487 eV (Al Kα). The spectrometer was calibrated against both the Au 4f7/2 peak at 84.0 eV and the Ag 3d5/2 peak at 368.3 eV. Static charging when present was corrected with a neutralizer (flood gun) by placing the carbon peak (C 1s) at about 285 eV.

Synthesis of 3D PtCuCoNi NPQAs

The 3D PtCuCoNi NPQAs were synthesized by a rapid aqueous solution method. To synthesize 3D Pt1Cu1Co1Ni1 NPQAs, 1 mL metal precursor solution containing H2PtCl6•6H2O (0.025 M), CuCl2 (0.025 M), CoCl2•6H2O (0.025 M), and NiCl2 (0.025 M) was quickly injected into NaBH4 solution (0.1 M, 5 mL) with stirring at room temperature. The stirring was kept about 5 min until the solution became colorless. The same procedure was used to synthesize Pt0.5Cu1Co1Ni1, Pt3Cu1Co1Ni1, Pt6Cu1Co1Ni1, and Pt9Cu1Co1Ni1 NPQAs by simply adjusting the concentration of metal precursors. All the NPQAs were washed thoroughly with water and then freeze-dried. The final products were re-dispersed into water and sonicated to obtain uniform solutions.25, 26

Electrocatalytic experiments


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Glassy carbon rotating disk electrode (RDE, 5 mm in diameter) was polished and cleaned before surface coating. The 3D PtCuCoNi NPQAs (5 µL, 1 mg/mL with respect to Pt) were coated on the surface of pretreated RDE surface and dried at room temperature. After that, the modified RDE was covered with a layer of nafion (5µL, 0.05%), followed by drying at room temperature. For commercial Pt/C catalyst (2 mg/mL), Pt/C powder was first dissolved into nafion solution containing nafion, 2-proponal, and water (v/v/v = 0.025/1/4). The obtained Pt/C catalyst was then dropped on the surface of pretreated RDE surface and dried before the electrocatalytic tests. The electrochemical measurements were carried out by an electrochemical workstation (CHI 630E) coupled with a three-electrode system. Ag/AgCl electrode filled with saturated KCl aqueous solution and Pt wire were used as reference electrode and counter electrode, respectively.

Results and Discussions



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Figure 1. (A) EDX spectrum of 3D Pt1Cu1Co1Ni1 NPQAs. SEM images of 3D Pt0.5Cu1Co1Ni1 (B), Pt1Cu1Co1Ni1 (C), Pt3Cu1Co1Ni1 (D), Pt6Cu1Co1Ni1 (E), and Pt9Cu1Co1Ni1 (F) NPQAs. C insert: digital picture of scale-up synthesized Pt1Cu1Co1Ni1 NPQA.

As shown in Figure 1C insert, we successfully prepared the 3D PtCuCoNi NPQAs in large scale by a rapid chemical reduction method (within 5 min) in aqueous solution, where the metal precursors were reduced at nucleation and growth stages simultaneously by using the strong reducing agent NaBH4.25,

27

Therefore, the

PtCuCoNi NPQAs could be formed instantaneously. The atomic ratios among each metal in the 3D NPQAs were quantified by EDX (Figure 1A and S1), which shows the ratios of Pt:Cu:Co:Ni are 17.3:27:27.7:28, 25.5:24.7:24.8:25, 50.3:16.7:16.3:16.7, 67:11.4:11.1:10.5, and 74.5:8.3:8.4:8.8 for 3D Pt0.5Cu1Co1Ni1, Pt1Cu1Co1Ni1, Pt3Cu1Co1Ni1, Pt6Cu1Co1Ni1, and Pt9Cu1Co1Ni1 NPQAs, respectively. It should be


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noted that the ratios are very close to the compositions of metal precursors, which demonstrates the completely reduction of precursors. The morphology of the 3D NPQAs was further characterized by SEM. The images in Figure 1B to 1F reveal that all the NPQAs hold the similar porous sponge-like morphology even though the ratio of precursors are different, which confirms the generality of this approach to synthesize 3D nanostructures.

Figure 2. TEM images of 3D Pt0.5Cu1Co1Ni1 (A), Pt1Cu1Co1Ni1 (B), Pt3Cu1Co1Ni1 (D), Pt6Cu1Co1Ni1 (E), and Pt9Cu1Co1Ni1 (F) NPQAs. (C) Particle size distribution of 3D Pt1Cu1Co1Ni1.

In order to have an intensive exploration on the 3D PtCuCoNi NPQAs, TEM was used to characterize their structures (Figure 2). As expected, all the 3D PtCuCoNi NPQAs present the similar nanostructures, which are composed of fused PtCuCoNi alloy nanoparticles. The average nanoparticle sizes, measured by Nano Measure



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software, are 4.25, 4.23, 4.29, 4.33, and 4.45 nm for 3D Pt0.5Cu1Co1Ni1, Pt1Cu1Co1Ni1, Pt3Cu1Co1Ni1, Pt6Cu1Co1Ni1, and Pt9Cu1Co1Ni1 NPQAs, respectively. More importantly, the nanoparticles in each 3D NPQA were uniformly distributed as revealed in Figure 2C and S2.


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Figure 3. (A) HRTEM (A), STEM (B), and EDS mapping analysis showing the elements Pt (C), Cu (D), Co (E), and Ni (F) of Pt1Cu1Co1Ni1 NPQAs.

The high resolution TEM image of Pt1Cu1Co1Ni1 NPQAs in Figure 1A shows that the lattice spacing is around 0.215 nm, which can be assigned to (111) plane of face-centered cubic (fcc) Pt alloys. The nanostructure of as-prepared Pt1Cu1Co1Ni1 NPQAs was also characterized by high-angle annular dark-field scanning TEM (HAADF-STEM) and elemental mapping (Figure 3). The images reveal that Pt, Cu, Co and Ni are uniformly distributed through the structure, which confirms the formation of homogeneous alloys.



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Figure 4. (A) XRD patterns of 3D Pt0.5Cu1Co1Ni1 (a), Pt1Cu1Co1Ni1 (b), Pt3Cu1Co1Ni1 (c), Pt6Cu1Co1Ni1 (d), and Pt9Cu1Co1Ni1 (e) NPQAs (black: Pt PDF#4-0802, red: Cu PDF#65-9026, blue: Co PDF#15-0806, pink: Ni PDF#65-2865). (B) Magnified view of XRD patterns. (C) XPS spectrum of 3D Pt0.5Cu1Co1Ni1 NPQA (D) High resolution XPS spectrum of Pt in Pt1Cu1Co1Ni1 NPQAs.

In the XRD patterns of 3D Pt1Cu1Co1Ni1 NPQAs, the obvious diffraction peaks with 2θ centered at 40.99, 46.94, 70.67, and 84.8° could be assigned to (111), (200), (220), and (311) planes of fcc structure as shown in Figure 4A. Moreover, the diffraction peaks of the 3D Pt1Cu1Co1Ni1 NPQAs are located among the positions of pure Pt, Cu, Co, and Ni, which demonstrates the formation of alloys instead of mixed metals. The similar phenomenon was also observed on other 3D NPQAs. In the magnified view of the diffraction patterns, negative shifts show up as the Pt content increases, as shown in Figure 4B. Based on the XRD patterns, the crystalline size could be calculated according to Scherrer equation:28

L=

0.9ߣ௄ఈଵ ‫ܤ‬ଶఏ ܿ‫ߠݏ݋‬௠௔௫

where L is the crystalline domain. ߣ௄ఈଵ is the X-ray wavelength (0.154 nm). ‫ܤ‬ଶఏ is the half-peak width. ߠ௠௔௫ is the Bragg angle. Based on the equation, the calculated crystalline size is 3.47, 3.5, 3.78, 3.59, and 3.67 nm for 3D Pt0.5Cu1Co1Ni1, Pt1Cu1Co1Ni1, Pt3Cu1Co1Ni1, Pt6Cu1Co1Ni1, and Pt9Cu1Co1Ni1 NPQAs, respectively, which is close to the TEM result.


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The surface composition of 3D Pt1Cu1Co1Ni1 NPQAs was determined by XPS analysis. Figure 4C shows that the XPS spectrum is dominated by Pt, Cu, Co and Ni, confirming the successful reduction of metal precursors. The high resolution XPS spectrum of Pt in 3D Pt1Cu1Co1Ni1 NPQAs (Figure 4D) displays two strong peaks at the binding energies of 71.32 and 74.63 eV, corresponding to Pt 4f7/2 and Pt 4f5/2, respectively. These two peaks can be divided into two pair of peaks at around 71.25 and 74.6, 72.29 and 76.82 eV, demonstrating the existence of Pt and PtO/Pt(OH)2.29 Based on the peak intensity, it can be concluded that Pt (0) is the dominant composition, which is favorable for ORR. Likewise, the peaks at around 932~945, 953, 784, 800, 856~862, and 874~879, correspond to Cu 2p3/2, Cu 2p1/2, Co 2p 3/2, Co 2p1/2, Ni 2p3/2, and Ni 2p1/2, respectively, as revealed in Figure S3.30-33 It should be noted that the atomic percentage of each component calculated by XPS results is 12.4%, 15.3%, 35.6%, and 36.7% for Pt, Cu, Co, Ni. Due to the their lower reduction potentials of Ni2+ and Co2+ compared to those of Pt and Cu, the formation of the final product should be initiated by a Pt/Cu-enriched inner and then Ni/Co-enriched surface.



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Figure 5. CV curves of 3D Pt0.5Cu1Co1Ni1, Pt1Cu1Co1Ni1, Pt3Cu1Co1Ni1, Pt6Cu1Co1Ni1, Pt9Cu1Co1Ni1 NPQAs and commercial Pt/C catalysts in N2 (A) and O2-saturated (B) 0.1 M HClO4 solutions. (C) LSV curves of 3D Pt0.5Cu1Co1Ni1, Pt1Cu1Co1Ni1, Pt3Cu1Co1Ni1, Pt6Cu1Co1Ni1, Pt9Cu1Co1Ni1 NPQAs and commercial Pt/C catalysts in O2-saturated 0.1 M HClO4 solutions. (D) Mass activities and specific activities of 3D Pt0.5Cu1Co1Ni1, Pt1Cu1Co1Ni1, Pt3Cu1Co1Ni1, Pt6Cu1Co1Ni1, Pt9Cu1Co1Ni1 NPQAs and commercial Pt/C catalysts at 0.85 V.

The electrochemical properties were studied on NPQAs. Figure 5A shows the cyclic voltammetry (CV) curves of NPQAs with different composition and commercial Pt/C catalysts in N2-saturated 0.1 M HClO4 solution using a scan rate of 50 mV/s. It is


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clear that all the catalysts exhibit typical hydrogen adsorption/desorption and Pt oxidation/reduction peaks. The CV curves were further used to evaluate the ECSA, which was calculated by integrating the hydrogen adsorption/desorption charge, assuming a value of 210 µC/cm2 for the adsorption of a hydrogen monolayer. The ECSA of Pt1Cu1Co1Ni1 NPQAs is 43.8 m2/gPt, while commercial Pt/C shows an ECSA of 84.76 m2/gPt. Owning to their high surface area and porosity, the PtCuCoNi NPQAs are expected to exhibit enhanced electrocatalytic activity. In particular, the CV curves of Figure 5B reveal that Pt1Cu1Co1Ni1 NPQAs present more positive onset potential and higher peak current than that of commercial Pt/C catalysts, indicating the improved electrocatalytic activity of Pt1Cu1Co1Ni1 NPQAs for ORR. The ORR linear sweep voltammetry (LSV) curves for the NPQAs and commercial Pt/C catalyst are presented in Figure 5C, which were obtained using a RDE at 1600 rpm with a scan rate of 20 mV/s. The curves display two distinguishable potential regions: the diffusion-limiting current region below 0.6 V and the mixed kinetic-diffusion control region between 0.6 and 1.1 V.34 The onset potentials of Pt1Cu1Co1Ni1 NPQAs and commercial Pt/C catalysts are 0.943 and 0.878 V, while their half-wave potentials are 0.889 and 0.829 V, respectively. The much more positive onset potential and half-wave potential indicate the higher activity of Pt1Cu1Co1Ni1 NPQAs, which is consistent with the CV curves in Figure 5B.

Based on the LSV curves, the kinetic currents could be calculated according to the Levich-Koutecky equation.2 In order to compare the MA of different catalysts, the kinetic currents were normalized by the loading amount of Pt at 0.85 V. In Figure 5D,



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the MA of Pt1Cu1Co1Ni1 NPQAs is 0.72 A/mgPt on the basis of the mass of Pt, which is 6 times larger than that of Pt/C (0.12 A/mgPt). Figure 5D also display the SA of NPQAs, which are all higher than that of Pt/C catalyst. In particular, Pt1Cu1Co1Ni1 NPQAs shows a SA of 1.64 mA/cm2, which is 11 times larger than that of Pt/C (0.15 mA/cm2).

Figure 6. CV curves of 3D Pt1Cu1Co1Ni1 NPQAs (A) and commercial Pt/C catalysts (B) in N2-saturated 0.1 M HClO4 solution before and after ADT. LSV curves of 3D Pt1Cu1Co1Ni1 NPQAs (C) and commercial Pt/C catalysts (D) in O2-saturated 0.1 M HClO4 solutions at 1600 rpm before and after ADT.


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We also evaluated the stability of 3D Pt1Cu1Co1Ni1 NPQAs catalyst for ORR through an accelerated durability test (ADT) by applying a potential sweep between 0.6 and 1.1 V with a scanning rate of 0.1 V/s in O2-saturated 0.1 M HClO4 solution. After 5000 cycles, no obvious morphology changes were observed for 3D Pt1Cu1Co1Ni1 NPQAs, while severe aggregation and falling off of Pt were observed on Pt/C catalyst (Figure S4). Additionally, the particle size of 3D Pt1Cu1Co1Ni1 NPQAs, around 4.54 nm (Figure S5), was also well remained compared to that before ADT. As shown in Figure 6A and 6B, the ECSA of 3D Pt1Cu1Co1Ni1 NPQAs catalyst only dropped by 15%, which is much lower than that of commercial Pt/C catalyst (37%). The ORR activities of the catalysts before and after ADT are shown in Figure 6C and 6D. The 3D Pt1Cu1Co1Ni1 NPQAs showed a degradation of 23 mV in half-wave potential. Whereas, there is a 33 mV shift of half-wave potential on commercial Pt/C catalyst. The improved durability should be attributed to the specific 3D morphology of Pt1Cu1Co1Ni1 NPQAs which makes Pt less vulnerable to dissolution, Ostwald ripening, and aggregation compared to carbon supported Pt nanoparticles.24

Figure 7. (A) CV curves of 3D Pt1Cu1Co1Ni1 NPQAs and commercial Pt/C catalysts



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in 0.5 M H2SO4 + 0.5 M CH3OH solution with a scan rate of 50 mV/s. (B) Current density-time curves of 3D Pt1Cu1Co1Ni1 NPQAs and commercial Pt/C catalysts in 0.5 M H2SO4 + 0.5 M CH3OH solution at 0.76 V vs. RHE.

We further investigated the electrocatalytic activity of 3D Pt1Cu1Co1Ni1 NPQAs toward MOR. Figure 7A shows the CV curves in a 0.5 M H2SO4 + 0.5 M CH3OH solution. The CV profile of commercial Pt/C was also included as comparison. It is clear that 3D Pt1Cu1Co1Ni1 NPQAs has a much more negative onset potential (0.639 V) and higher mass activity (0.45 A/mgPt) compared to commercial Pt/C catalyst (0.687 V and 0.186 A/mgPt), indicating the effective catalytic activity of 3D Pt1Cu1Co1Ni1 NPQAs for MOR. The better electrocatalytic performance was further confirmed by chronoamperometric tests, which were carried out at 0.76 V for 2000 s in 0.5 M H2SO4 + 0.5 M CH3OH solution, as shown in Figure 7B. Both 3D Pt1Cu1Co1Ni1 NPQAs and commercial Pt/C catalysts present a significant decay at the very beginning and then remain stable. It should be noted that the current decay is associated with the formation of carbonaceous intermediates, which could poison the active sites of the catalysts. The current density-time curves show that the 3D PtCuCoNi NPQAs have a higher current density during the whole test process, implying the better catalytic activity and stability for MOR.

The significant enhancement of electrocatalytic activity and durability of 3D PtCuCoNi NPQAs is associated with the following factors: (1) compared to the isolated Pt nanoparticles, like Pt/C, the 3D porous morphology offers high porosity


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and large surface-to-volume ratio, which could improve the mass transport and gas diffusion, and thus enhance the electrocatalytic activities

22, 25

. More importantly, the

3D morphology and unsupported feature make the quaternary alloys less vulnerable to dissolution, aggregation and falling off of Pt caused by carbon corrosion, which could mitigate the ECSA lost and improve the catalytic stability.24 (2) The modification to the surface electronic structure of Pt by other metals contributes to the chemisorption property changes of adsorbed oxygen species on a Pt alloy surface. In addition, the lattice mismatch between Pt and other metals was demonstrated to result in a d-band shift of Pt-based alloys compared to pure Pt. Both the electronic structure modification and the downshift could alter the chemisorption of reactants, intermediates, and products, and thus improve the ORR and MOR performance.35, 36 Considering the complexity of the quaternary alloys, we just investigated the effect of the Pt content on the resultant ORR performances. It should be noted that the electrochemical performances of these Pt-based quaternary alloys might be improved on conditioned that the optimized composition of metal precursors is extensively screened.

Conclusions

In summary, we have demonstrated a facile procedure for the synthesis of 3D PtCuCoNi NPQAs with controllable composition. By taking advantages of the unique porous morphology and synergetic effect between the components, the as-prepared 3D PtCuCoNi NPQAs present excellent electrocatalytic activity for ORR and MOR



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with high durability, indicating their promising applications on both PEMFCs and DMFCs. This synthesis strategy could be expected to be applied on other application by rational modification of composition and morphology.

ASSOCIATED CONTENTS

Supporting information

EDX spectra of 3D Pt0.5Cu1Co1Ni1, Pt3Cu1Co1Ni1, Pt6Cu1Co1Ni1, and Pt9Cu1Co1Ni1 NPQAs, particle size distribution of 3D Pt0.5Cu1Co1Ni1, Pt3Cu1Co1Ni1, Pt6Cu1Co1Ni1, and Pt9Cu1Co1Ni1 NPQAs, XPS spectrum of Pt1Cu1Co1Ni1 NPQAs, high resolution XPS spectra of Cu, Co, and Ni of Pt1Cu1Co1Ni1 NPQAs, TEM images of Pt1Cu1Co1Ni1 NPQAs before and after ADT. TEM images of commercial Pt/C before and after ADT, particle size distribution of Pt1Cu1Co1Ni1 NPQAs after ADT. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding author

Email: [email protected] and [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT


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This work was supported by a startup fund of Washington State University, USA. We thank Franceschi Microscopy & Image Center at Washington State University for TEM measurements.

REFERENCES

1.

Liu, L. F.; Pippel, E. Low-Platinum-Content Quaternary PtCuCoNi Nanotubes

with Markedly Enhanced Oxygen Reduction Activity. Angew. Chem. Int. Ed. 2011, 50, 2729-2733. 2.

Zhu, C. Z.; Li, H.; Fu, S. F.; Du, D.; Lin, Y. H. Highly Efficient Nonprecious

Metal Catalysts towards Oxygen Reduction Reaction Based on Three-dimensional Porous Carbon Nanostructures. Chem. Soc. Rev. 2016, 45, 517-531. 3.

Shao, Y. Y.; Zhang, S.; Wang, C. M.; Nie, Z. M.; Liu, J.; Wang, Y.; Lin, Y. H.

Highly Durable Graphene Nanoplatelets Supported Pt Nanocatalysts for Oxygen Reduction. J. Power Sources 2010, 195, 4600-4605. 4.

Zhang, S.; Shao, Y. Y.; Yin, G. P.; Lin, Y. H. Recent Progress in Nanostructured

Electrocatalysts for PEM Fuel Cells. J. Mater. Chem. 2013, 1, 4631-4641. 5.

Shao, Y. Y.; Zhang, S.; Kou, R.; Wang, X. Q.; Wang, C. M.; Dai, S.; Viswanathan,

V.; Liu, J.; Wang, Y.; Lin, Y. H. Noncovalently Functionalized Graphitic Mesoporous Carbon as a Stable Support of Pt Nanoparticles for Oxygen Reduction. J. Power

Sources 2010, 195, 1805-1811. 6.

Kibsgaard, J.; Gorlin, Y.; Chen, Z. B.; Jaramillo, T. F. Meso-Structured Platinum

Thin Films: Active and Stable Electrocatalysts for the Oxygen Reduction Reaction. J.



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

Am. Chem. Soc. 2012, 134, 7758-7765. 7.

Zhu, C. Z.; Du, D.; Eychmuller, A.; Lin, Y. H. Engineering Ordered and

Nonordered Porous Noble Metal Nanostructures: Synthesis, Assembly, and Their Applications in Electrochemistry. Chem. Rev. 2015, 115, 8896-8943. 8.

Liu, H.; Ye, F.; Yao, Q. F.; Cao, H. B.; Xie, J. P.; Lee, J. Y.; Yang, J. Stellated

Ag-Pt Bimetallic Nanoparticles: An effective Platform for Catalytic Activity Tuning.

Sci. Rep. 2014, 4, 3969. 9.

Oezaslan, M.; Hasche, F.; Strasser, P. Pt-Based Core-Shell Catalyst Architectures

for Oxygen Fuel Cell Electrodes. J. Phys. Chem. Lett. 2013, 4, 3273-3291. 10. Chen, Y. M.; Liang, Z. X.; Yang, F.; Liu, Y. W.; Chen, S. L. Ni-Pt Core-Shell Nanoparticles as Oxygen Reduction Electrocatalysts: Effect of Pt Shell Coverage. J.

Phys. Chem. C 2011, 115, 24073-24079. 11. Oezaslan, M.; Strasser, P. Activity of Dealloyed PtCo3 and PtCu3 Nanoparticle Electrocatalyst for Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cell. J. Power Sources 2011, 196, 5240-5249. 12. Jang, J. H.; Lee, E.; Park, J.; Kim, G.; Hong, S.; Kwon, Y. U. Rational Syntheses of Core-shell Fex@Pt Nanoparticles for The Study of Electrocatalytic Oxygen Reduction Reaction. Sci. Rep. 2013, 3, 2872. 13. Wang, D. L.; Xin, H. L. L.; Hovden, R.; Wang, H. S.; Yu, Y. C.; Muller, D. A.; DiSalvo, F. J.; Abruna, H. D. Structurally Ordered Intermetallic Platinum-cobalt Core-shell Nanoparticles with Enhanced Activity and Stability as Oxygen Reduction Electrocatalysts. Nat. Mater. 2013, 12, 81-87.


Page 22 of 26

Page 23 of 26

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


14. Jiang, Q. A.; Jiang, L. H.; Hou, H. Y.; Qi, J.; Wang, S. L.; Sun, G. Q. Promoting Effect of Ni in PtNi Bimetallic Electrocatalysts for the Methanol Oxidation Reaction in Alkaline Media: Experimental and Density Functional Theory Studies. J. Phys.

Chem. C 2010, 114, 19714-19722. 15. Zhu, H. Y.; Zhang, S.; Guo, S. J.; Su, D.; Sun, S. H. Synthetic Control of FePtM Nanorods (M = Cu, Ni) To Enhance the Oxygen Reduction Reaction. J. Am. Chem.

Soc. 2013, 135, 7130-7133. 16. Huang, X. Q.; Chen, Y.; Zhu, E. B.; Xu, Y. X.; Duan, X. F.; Huang, Y. Monodisperse Cu@PtCu Nanocrystals and Their Conversion into Hollow-PtCu Nanostructures for Methanol Oxidation. J. Mater. Chem. A 2013, 1, 14449-14454. 17. Kang, Y. J.; Pyo, J. B.; Ye, X. C.; Gordon, T. R.; Murray, C. B. Synthesis, Shape Control, and Methanol Electro-oxidation Properties of Pt-Zn Alloy and Pt3Zn Intermetallic Nanocrystals. ACS Nano 2012, 6, 5642-5647. 18. Chen, S.; Su, H. Y.; Wang, Y. C.; Wu, W. L.; Zeng, J. Size-Controlled Synthesis of Platinum-Copper Hierarchical Trigonal Bipyramid Nanoframes. Angew. Chem. Int.

Ed. 2015, 54, 108-113. 19. Zhang, C. L.; Sandorf, W.; Peng, Z. M. Octahedral Pt2CuNi Uniform Alloy Nanoparticle Catalyst with High Activity and Promising Stability for Oxygen Reduction Reaction. ACS Catal. 2015, 5, 2296-2300. 20. Strasser, P.; Koh, S.; Anniyev, T.; Greeley, J.; More, K.; Yu, C. F.; Liu, Z. C.; Kaya, S.; Nordlund, D.; Ogasawara, H.; Toney, M. F.; Nilsson, A. Lattice-strain Control of the Activity in Dealloyed Core-shell Fuel Cell Catalysts. Nat. Chem. 2010,



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

2, 454-460. 21. Xu, D.; Liu, Z. P.; Yang, H. Z.; Liu, Q. S.; Zhang, J.; Fang, J. Y.; Zou, S. Z.; Sun, K. Solution-Based Evolution and Enhanced Methanol Oxidation Activity of Monodisperse Platinum-Copper Nanocubes. Angew. Chem. Int. Ed. 2009, 48, 4217-4221. 22. Zhang, G. X.; Sun, S. H.; Cai, M.; Zhang, Y.; Li, R. Y.; Sun, X. L. Porous Dendritic Platinum Nanotubes with Extremely High Activity and Stability for Oxygen Reduction Reaction. Sci. Rep. 2013, 3, 1526. 23. Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Improved oxygen reduction activity on Pt3Ni(111) via increased surface site availability. Science 2007, 315, 493-497. 24. Xia, B. Y.; Wu, H. B.; Li, N.; Yan, Y.; Lou, X. W.; Wang, X. One-Pot Synthesis of Pt–Co Alloy Nanowire Assemblies with Tunable Composition and Enhanced Electrocatalytic Properties. Angew. Chem. Int. Ed. 2015, 54, 3797-3801. 25. Fu, S. F.; Zhu, C. Z.; Du, D.; Lin, Y. H. Facile One-Step Synthesis of Three-Dimensional Pd–Ag Bimetallic Alloy Networks and Their Electrocatalytic Activity toward Ethanol Oxidation. ACS Appl. Mater. Interfaces 2015, 7, 13842-13848. 26. Zhu, C.; Wen, D.; Oschatz, M.; Holzschuh, M.; Liu, W.; Herrmann, A. K.; Simon, F.; Kaskel, S.; Eychmuller, A. Kinetically Controlled Synthesis of PdNi Bimetallic Porous Nanostructures with Enhanced Electrocatalytic Activity. Small 2015, 11, 1430-1434.


Page 24 of 26

Page 25 of 26

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


27. Zhu, C. Z.; Guo, S. J.; Dong, S. J. Rapid, General Synthesis of PdPt Bimetallic Alloy Nanosponges and Their Enhanced Catalytic Performance for Ethanol/Methanol Electrooxidation in an Alkaline Medium. Chem. Eur. J. 2013, 19, 1104-1111. 28. Fu, S. F.; Yang, G. H.; Zhou, Y. Z.; Pan, H. B.; Wai, C. M.; Du, D.; Lin, Y. H. Ultrasonic Enhanced Synthesis of Multi-walled Carbon Nanotube Supported Pt-Co Bimetallic Nanoparticles as Catalysts for The Oxygen Reduction Reaction. RSC Adv.

2015, 5, 32685-32689. 29. Zheng, J. N.; Li, S. S.; Ma, X. H.; Chen, F. Y.; Wang, A. J.; Chen, J. R.; Feng, J. J. Popcorn-like PtAu Nanoparticles Supported on Reduced Graphene Oxide: Facile Synthesis and Catalytic Applications. J. Mater. Chem. A 2014, 2, 8386-8395. 30. Iqbal, J.; Wang, B. Q.; Liu, X. F.; Yu, D. P.; He, B.; Yu, R. H. Oxygen-vacancy-induced Green Emission and Room-temperature Ferromagnetism in Ni-doped ZnO Nanorods. New. J. Phys. 2009, 11, 1-14. 31. Wang, H.; Chen, Y.; Wang, H. B.; Zhang, C.; Yang, F. J.; Duan, J. X.; Yang, C. P.; Xu, Y. M.; Zhou, M. J.; Li, Q. High Resolution Transmission Electron Microscopy and Raman Scattering Studies of Room Temperature Ferromagnetic Ni-doped ZnO Nanocrystals. Appl. Phys. Lett. 2007, 90, 52505. 32. Pollock, N.; Fowler, G.; Twyman, L. J.; McArthur, S. L. Synthesis and Characterization of Immobilized PAMAM Dendrons. Chem. Commun. 2007, 24, 2482-2484. 33. Fu, S. F.; Zhu, C. Z.; Li, H.; Du, D.; Lin, Y. H. One-step Synthesis of Cobalt and Nitrogen Co-doped Carbon Nanotubes and Their Catalytic Activity for the Oxygen



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

Reduction Reaction. J. Mater. Chem. A 2015, 3, 12718-12722. 34. Huang, X. Q.; Zhu, E. B.; Chen, Y.; Li, Y. J.; Chiu, C. Y.; Xu, Y. X.; Lin, Z. Y.; Duan, X. F.; Huang, Y. A Facile Strategy to Pt3Ni Nanocrystals with Highly Porous Features as an Enhanced Oxygen Reduction Reaction Catalyst. Adv. Mater. 2013, 25, 2974-2979. 35. Nie, Y.; Li, L.; Wei, Z. D. Recent Advancements in Pt and Pt-free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44, 2168-2201. 36. Yang, R. Z.; Leisch, J.; Strasser, P.; Toney, M. F. Structure of Dealloyed PtCu3 Thin Films and Catalytic Activity for Oxygen Reduction. Chem. Mater. 2010, 22, 4712-4720.

The three-dimensional PtCuCoNi nanoporous quaternary alloys were synthesized via a simple NaBH4 kinetically controlled reduction method, which present enhanced electrocatalytic activities for oxygen reduction and methanol oxidation reactions in fuel cells.


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Enhanced Electrocatalytic Activities of PtCuCoNi Three-Dimensional

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