DOI: 10.1002/chem.201302834
Full Paper
& Electrocatalytic Activity
Pt-Pd-Co Trimetallic Alloy Network Nanostructures with Superior Electrocatalytic Activity towards the Oxygen Reduction Reaction Xinyu Liu, Gengtao Fu, Yu Chen,* Yawen Tang, Peiliang She, and Tianhong Lu[a]
Abstract: Pt alloy nanostructures show great promise as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cell cathodes. Herein, three-dimensional (3D) Pt-Pd-Co trimetallic network nanostructures (TNNs) with a high degree of alloying are synthesized through a room temperature wet chemical synthetic method by using K2PtCl4/ K3Co(CN)6–K2PdCl4/K3Co(CN)6 mixed cyanogels as the reaction precursor in the absence of surfactants and templates. The size, morphology, and surface composition of the Pt-PdCo TNNs are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area
electron diffraction (SAED), energy dispersive spectroscopy (EDS), EDS mapping, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The 3D backbone structure, solid nature, and trimetallic properties of the mixed cyanogels are responsible for the 3D structure and high degree of alloying of the as-prepared products. Compared with commercially available Pt black, the Pt-Pd-Co TNNs exhibit superior electrocatalytic activity and stability towards the ORR, which is ascribed to their unique 3D structure, low hydroxyl surface coverage and alloy properties.
Introduction
ular, the incorporation of Co[4b–d] or Pd[5] onto Pt can remarkably improve the ORR activity and durability of Pt electrocatalysts, a phenomenon originating from the modified geometric and electronic structure of Pt. However, there are still few examples of the synthesis of Pt-Pd-M trimetallic alloy electrocatalysts.[2g] It is still challenging to obtain multimetallic alloy nanostructures with a designed composition and catalytic properties in solution-phase synthesis. Cyanogels, inorganic coordination polymers made from a mixture of a tetrachlorometalate ([RCl4]2, R = Pd, Pt, Ir) and a transition-metal cyanometalate ([M(CN)n]2/3, n = 4, 6; M = Co, Fe, Ru, Os, Ni, Cr) in aqueous solution [Eq. (1)], are a special class of three-dimensional (3D), double-metal cyanide.[6] As shown in Equation (1), R and M species mix uniformly and concentrate on the backbone of the cyanogel rather than in the water solvent, which restrains the Brownian motion of the R and M species. The intimate interconnection between the R and M precursor species facilitates the combination of R0 and M0 crystal nuclei upon chemical reduction. Moreover, the solid nature of cyanogels also effectively suppresses the movement of R0 and M0 nuclei in the crystal, consequently facilitating the formation of an R0–M0 bimetallic alloy. Furthermore, the characteristic 3D backbone of cyanogels acts as a support scaffold, resulting in the interconnected 3D structure of R0–M0 bimetallic alloy nanostructures.
Fuel cells, especially proton-exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), are considered to be promising low-temperature sources of green power for portable electronic devices, residences, and automobiles. However, their commercialization is seriously impeded by the slow kinetics of the oxygen reduction reaction (ORR) on the Pt cathode and the high cost of Pt.[1]The use of an alloy is an effective way to solve the problem of improving the reaction activity of the ORR and reducing the amount of Pt used and thus the cost.[2] The essential challenge for the synthesis of alloy nanostructures is the diverse nucleation and growth rates of different elements owing to their distinct standard reduction potentials.[3] Typically, PtM nanoparticles (M = Fe, Co, Ni) formed through wet chemical synthesis have an exterior M-enriched or phase-separated, non-alloyed structure because of the preferential reduction/nucleation of the Pt precursor. To date, highly efficient PtM[4] and PtPd[5] alloy electrocatalysts for the ORR have been synthesized by different methods, such as wet chemical reduction, galvanic replacement, and thermal decomposition of organometallic precursors. In partic[a] Dr. X. Liu,+ G. Fu,+ Y. Chen, Prof. Y. Tang, P. She, T. Lu Jiangsu Key Laboratory of New Power Batteries Jiangsu Collaborative Innovation Centre of Biomedical Functional Materials School of Chemistry and Materials Science Nanjing Normal University, Nanjing 210023 (P.R. China) E-mail: [email protected] [+] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201300750. Chem. Eur. J. 2014, 20, 585 – 590
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Full Paper Herein, we develop a very simple and efficient one-step route for synthesizing Pt-Pd-Co trimetallic network nanostructures (TNNs) with a high degree of alloying by using mixed cyanogels as the reaction precursors at room temperature (Scheme 1). The as-prepared Pt-Pd-Co TNNs show excellent
Scheme 1. Schematic representation of the synthetic procedure for the PtPd-Co TNNs. A) Photograph of the K2PtCl4/K3Co(CN)6–K2PdCl4/K3Co(CN)6 mixed cyanogels. B) An SEM image of the Pt-Pd-Co TNNs. Figure 1. A) EDS spectrum and B) XPS spectra of the Pt-Pd-Co TNNs in the a) Pt 4f, b) Pd 3d, and c) Co 2p regions.
electrocatalytic activity and stability towards the ORR. The general route using mixed-cyanogel precursors reported herein will greatly contribute to the further design of multimetallic alloy nanostructures with various compositions and functions.
(79.2 %) and E-TEK Pd black (60.0 %; see Figure S2 in the Supporting Information). This result indicates that the Pt-Pd-Co TNNs have a weaker oxophilicity than commercial E-TEK Pt black and E-TEK Pd black. Meanwhile, Pt 4f and Pd 3d binding energies in the Pt-Pd-Co TNNs negatively shift by approximately 0.71 and 0.21 eV compared to those in commercial Pt black and Pd black, respectively (see Figure S2 in the Supporting Information). The negative shifts of the Pt 4f and Pd 3d binding energies show the change in the electronic structure of Pt and Pd atoms, further confirming the formation of a Pt-Pd-Co alloy. The change in the electronic structure of Pt and Pd atoms mainly originates from electron donation from Co to Pt and Pd due to the difference in electronegativity. This change decreases the 2p electron donation from O2 to the Pt and Pd atoms, and results in weaken O2 adsorption[8] (i.e., a high percentage of Pt0 and Pd0 species). The X-ray diffraction (XRD) pattern shows that the Pt-Pd-Co TNNs have the face-centered-cubic (fcc) structure (Figure 2 A). No diffraction peaks for single-component Pt, Pd, and Co were observed in the XRD pattern. Moreover, all diffraction peaks of the Pt-Pd-Co TNNs shift to a higher angle than both Pt and Pd (JCPDS no.: 04-0802 Pt; JCPDS no.: 46-1043 Pd). This XRD data clearly confirm the formation of a Pt-Pd-Co trimetallic alloy.[9] By using the Debye–Scherrer formula, the average particle size (dXRD) of the Pt-Pd-Co TNNs, calculated from the {111} diffraction peak, is 4.2 nm, which is much smaller than that of the commercial E-TEK Pt black (dXRD = 8.7 nm, see Figure S3 in the Supporting Information). The structural features of the Pt-PdCo TNNs were further investigated by transmission electron microscopy (TEM), which showed that the small and irregular nanoparticles are interconnected to form 3D network nanostructures with abundant pores (Figure 2 B). Further detailed observation showed that each of the irregular nanoparticles are essentially nanostructures with dendritic morphologies, which consist of smaller grains with a diameter of 4.2 3 nm (Figure 2 C and left insert). A high-resolution TEM (HRTEM)
Results and Discussion Characterization of the as-prepared Pt-Pd-Co TNNs: In a typical synthesis, aqueous solutions of K2PtCl4 (2.8 mL, 50 mm), K2PdCl4 (1.2 mL, 50 mm), and K3Co(CN)6 (2.0 mL, 50 mm) were mixed. The mixture was then heated at 95 8C for 36 h to generate the yellow jelly-like K2PtCl4/K3Co(CN)6–K2PdCl4/K3Co(CN)6 mixed cyanogels (Scheme 1 A). After reduction with NaBH4 and washing with HClO4, the Pt-Pd-Co TNNs were conveniently obtained by centrifugation (for details, see the Experimental Section). As shown in the scanning electron microscopy (SEM) image in Scheme 1 B, the porous Pt-Pd-Co TNNs have 3D network-like architectures. This feature is attractive for electrochemical applications because such 3D configurations can essentially facilitate the mass transport of fuel molecules. The Pt-Pd-Co TNN composition was analyzed by energy dispersive spectroscopy (EDS). The Pt/Pd/Co atomic ratio in the Pt-Pd-Co TNNs is 6.9:3.1:3.0 (Figure 1 A). The Co atomic fraction in the Pt-Pd-Co TNNs is lower than that in the initial mixed cyanogels (Pt/Pd/Co = 7:3:5), demonstrating that the acidic washing process effectively removes the unalloyed Co. In general, EDS just provides the bulk composition, whereas the electrocatalytic activity relies more on the surface composition. We thus checked the near-surface composition by X-ray photoelectron spectroscopy (XPS).[7] XPS data show that the Pt/Pd/Co atomic ratio in the Pt-Pd-Co TNNs is 7.1:2.9:3.0 (see Figure S1 in the Supporting Information), similar to that obtained from the EDS data, which is clear evidence for the formation of a Pt-Pd-Co alloy. Further detailed investigations showed that Pt, Pd, and Co species are predominantly in the metallic state (Figure 1 B). For instance, the percentage of Pt0 and Pd0 species in the PtPd-Co TNNs is calculated to be 84.1 and 90.3 %, respectively, which is much higher than that in commercial E-TEK Pt black Chem. Eur. J. 2014, 20, 585 – 590
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Full Paper Pt-Pd-Co TNN surfaces at more positive potentials. Figure 3 B clearly shows that the Pt-Pd-Co TNNs have lower hydroxyl surface coverage (VOH)[10] than E-TEK Pt black over the entire potential range, which arises from the change in the electronic structure of Pt on the Pt-Pd-Co TNNs,[11] as evidenced by the XPS data. Figure 3 C shows typical ORR polarization curves for the Pt-Pd-Co TNNs and commercial E-TEK Pt black. The current densities are normalized with reference to the geometrical area of the glassy carbon electrode. The ORR onset potential (EORR) of the Pt-Pd-Co TNNs shows a 29 mV shift, to a more positive potential, relative to commercial Pt black, indicating dramatically improved electrocatalytic activity for the Pt-Pd-Co TNNs. In addition, it is worth noting that the EORR on the Pt-PdCo TNNs (0.925 V) is also much higher than that on the 3D network Pt nanochain (EORR = 0.893 V)[12] and 3D network Pt nanowire membranes (EORR = 0.880 V),[13] indicating that the Pt-PdCo TNNs show more competitive electrocatalytic activity towards the ORR. Since adsorbed hydroxyl species on Pt surfaces inhibit the ORR, the lower VOH value on the Pt-Pd-Co TNNs surface improves the kinetics of the ORR. On the other hand, the rate-determining step in the ORR is the breaking of the OO bond.[14] The kinetics of the ORR strongly depends on the degree of interaction of oxygen with adsorption sites in the electrocatalyst, which is generally affected by the Pt d-band vacancies (electronic effects) and PtPt bond length (geometric effects). Compared with commercially available Pt black, the Pt 4f binding energies in the Pt-Pd-Co TNNs negatively shift by approximate-
Figure 2. A) XRD pattern (solid lines: Pt, dashed lines: Pd) and B, C) TEM images of the Pt-Pd-Co TNNs. Insets in C) HRTEM images (left and middle) and SAED image (right). D) Representative large-area TEM image of the PtPd-Co TNNs and the corresponding EDS maps of the Pt, Pd, and Co distributions within the Pt-Pd-Co TNNs.
image mainly shows the {111} and {100} facets of fcc Pt (middle insert in Figure 2 C). A selected-area electron diffraction (SAED) image shows an irregularly and discretely dotted pattern (right insert in Figure 2 C), indicating that the Pt-Pd-Co TNNs are polycrystalline. Elemental mapping under scanning transmission electron microscopy (STEM) mode was a powerful technique to characterize the distribution of the elements.[7] Clearly, the elemental distribution profiles of Pt, Pd, and Co are very similar (Figure 2 D), indicating that Pt, Pd, and Co are evenly distributed throughout the nanostructures. This is also strong evidence for the formation of a Pt-Pd-Co trimetallic alloy. Catalytic activity and stability for the ORR: Figure 3 A shows cyclic-voltammogram (CV) curves of the Pt-Pd-Co TNNs and commercial E-TEK Pt black in a N2-purged H2SO4 solution (0.5 m). It is worth noting that the potentials for surface-oxide formation (Pt + H2O!PtOH + H + + e) and the following reduction on the Pt-Pd-Co TNNs positively shift by approximately 88 mV compared with E-TEK Pt black, suggesting fast hydroxyl adsorption/desorption on the Chem. Eur. J. 2014, 20, 585 – 590
Figure 3. A) CV curves for the Pt-Pd-Co TNNs and commercial E-TEK Pt black in N2-saturated H2SO4 (0.5 m) solutions at 50 mV s1. B) Hydroxyl surface coverage (VOH) for the Pt-Pd-Co TNNs and E-TEK Pt black. C) ORR polarization curves for the Pt-Pd-Co TNNs and E-TEK Pt black in an O2-saturated H2SO4 (0.5 m) solution at a scan rate of 5 mV s1 and a rotation rate of 1600 rpm. Inset: Pt mass activities of the Pt-Pd-Co TNNs and E-TEK Pt black towards the ORR at 0.9 V (vs. NHE). D) Specific kinetic current densities (ik) for the Pt-Pd-Co TNNs and E-TEK Pt black at different potentials.
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Full Paper ly 0.71 eV due to the alloying of Pt with both Pd and Co. This is believed to be a key factor in improving Pt ORR activity because the negative shift of the Pt 4f binding energies is likely to lead to smaller oxygen adsorption energies and thus a weaker OO bond, which is confirmed by previous reports (such as those of PtFe3O4 nanoparticles[15] and PtNi[4e] and PtCo alloy nanoparticles[16]). The contraction in the atomic size of Pt is also likely to be an important reason for the improved Pt ORR activity because the decrease in the PtPt bond length also leads to a decrease in the oxygen adsorption energy.[1b] The mass activity (i.e., the currents are normalized to the mass of the metal) is taken as an index to assess the applicability of the electrocatalysts for the ORR.[17] At 0.85 V, the Pt mass activity for the Pt-Pd-Co TNNs is 3.26 times higher than that of ETEK Pt black (insert in Figure 3 C). Since the specific kinetic activity (normalized to the electrochemically active surface area) represented the intrinsic electrocatalytic activity, the specific kinetic activities of the electrocatalysts in the ORR were further investigated. The electrochemically active surface area of the Pt-Pd-Co TNNs (22.1 m2 g1) is larger than that of the commercially available ETEK Pt black (15.7 m2 g1), based on the CO-stripping-voltammetry data (Figure S4 in the Supporting Information; for calculation details, see the Experimental Section), a fact originating from the difference in particle size (dXRD-Pt-Pd-Co = 4.2 vs. dXRD-ETEK Pt = 8.7 nm). The Pt-Pd-Co TNNs show greatly improved specific kinetic activity relative to Pt black in the whole potential range (Figure 3 D). For instance, the specific kinetic current density at 0.85 V is about 1.20 mA cm2 for Pt-Pd-Co TNNs, which is enhanced by a factor of 2.03 over that of the Pt black (0.59 A m2). To further evaluate the kinetic parameters for the ORR on the Pt-Pd-Co TNNs, hydrodynamic voltammograms were recorded at a scan rate of 5 mV s1 over a range of rotation rates of 100–2500 rpm (Figure 4). According to the Kou-
tecky–Levich equation, the number of transferred electrons per oxygen molecule at the Pt-Pd-Co TNN surfaces is calculated to be 3.9 (for calculation details, see the Experimental Section), confirming a complete four-electron reduction pathway. The durability of cathodic electrocatalysts is a common issue for fuel cells, and is a very crucial and important consideration from an industrial point of view. Stabilizing Pt nanoparticles in the cathode seems to be even more challenging than controlling their activity. After an accelerated stability test,[1c] only about a 5 mV negative shift in the half-wave potential for the Pt-Pd-Co TNNs is observed, whereas there is a 28 mV negative shift for Pt black (Figure 5), indicating that the Pt-Pd-Co TNNs are a class of stable electrocatalysts for the ORR. The particular interconnected structure that suppresses the Ostwald ripening effect,[1a, f, 6a, g, 10a] as well as the positive shift in the Pt oxide formation potential that restrains electrochemical corrosion/dissolution of Pt nanopaticles at high potentials are responsible for the enhanced durability of the Pt-Pd-Co TNNs.
Figure 5. ORR polarization curves for A) the Pt-Pd-Co TNNs and B) E-TEK Pt black in O2-saturated H2SO4 (0.5 m) solutions before and after 1000 potential cycles at a scan rate of 5 mV s1 and a rotation rate of 1600 rpm.
Conclusion By means of the particular properties of cyanogels, such as the characteristic 3D backbone, their solid nature, and the uniform distribution of metal ions on the cyanogel backbone, Pt-Pd-Co TNNs are obtained facilely by using mixed cyanogels as the reaction precursor at room temperature. The Pt-Pd-Co TNNs show considerably enhanced electrocatalytic activity (mass activity and specific activity) and durability in the ORR compared with commercially available Pt black. This work demonstrates that the Pt-Pd-Co TNNs are indeed a promising cathodic electrocatalyst for fuel cells. The reported mixed-cyanogel/chemical-reduction method can be generalized to produce various noble-metal-based (such as Pd, Pt, and Ir) multimetallic alloy nanostructures for catalytic applications.
Figure 4. A) ORR polarization curves for the Pt-Pd-Co TNNs in an O2-saturated H2SO4 (0.5 m) solution at a scan rate of 5 mV s1 over a range of rotation rates of 100–2500 rpm. Current density is calculated by using the geometric area of the electrode. B) The Koutecky–Levich plots (i 1 vs. w 1/2) for the ORR at 0.3 V (vs. NHE). Chem. Eur. J. 2014, 20, 585 – 590
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Full Paper Experimental Section
For the CO-stripping measurements, the catalyst surface was first saturated with CO by bubbling CO through an H2SO4 solution (0.5 m), while holding the working electrode at 0 V for 15 min. The remaining CO was purged by use of a flow of N2 for 30 min before measurements were made.[3c, 18] The electrochemically active surface area (ECSA) of the catalysts was calculated from the Equation (2) by measuring the charge collected in the CO adsorption oxidation region (Q) and assuming a value of 420 mC cm2 for the adsorption of a CO monolayer (m is the loading amount of Pt and Pd metal).
Materials: Potassium tetrachloroplatinate(II) (K2PtCl4), potassium tetrachloropalladate(II) (K2PdCl4), potassium hexacyanocobaltate(III) (K3Co(CN)6), and sodium borohydride (NaBH4) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Other reagents were of analytical reagent grade and used without further purification. Commercially available Pt black and Pd black were purchased from E-TEK Division, PEMEAS Fuel Cell Technologies. All of the aqueous solutions were prepared with Millipore water having a resistivity of 18.2 MW.
ECSA ¼
Synthesis of Pt-Pd-Co TNNs: In a typical synthesis, aqueous solutions of K2PtCl4 (2.8 mL, 50 mm), K2PdCl4 (1.2 mL, 50 mm), and K3Co(CN)6 (2.0 mL, 50 mm) were added to a Teflon-lined stainlesssteel autoclave (10 mL), and were then heated at 95 8C for 36 h to generate the yellow K2PtCl4/K3Co(CN)6–K2PdCl4/K3Co(CN)6 mixed cyanogels. After being cooled to room temperature, NaBH4 (6 mL, 0.1 g mL1) was added to the yellow K2PtCl4/K3Co(CN)6–K2PdCl4/ K3Co(CN)6 mixed cyanogel and the resulting mixed solution was left for 1 h. After the reaction, the black Pt-Pd-Co trimetallic nanostructures (TNNs) were separated by centrifugation at 15 000 rpm for 10 min, washed with HClO4 (0.1 m), and then dried at 40 8C in a vacuum dryer for 12 h. The acid-wash process ensured the removal of unalloyed Co.
All oxygen-reduction-reaction (ORR) tests were conducted in O2saturated H2SO4 solutions (0.5 m). The polarization curves were obtained by sweeping the potential from 1.03 to 0.05 V at a scan rate of 5 mV s1 and a rotation rate of 1600 rpm. The current density was normalized with reference to the ECSA of the catalysts. Based on the ORR polarization curves, the kinetic current density (iK) was calculated by using the Koutecky–Levich equation [Eq. (3)], in which iL and i are the limited diffusion current density and the measured current density, respectively.[19]
1 1 1 ¼ þ i ik iL
ð3Þ
The number of transferred electrons per oxygen molecule involved in the ORR at the Pt-Pd-Co TANNs was determined by the Koutecky–Levich equation [Eq. (3)], with iL expressed by Equation (4),[19] in which n is number of electrons transferred for per oxygen molecule, F is the Faraday constant (96 485 C mol1), DO2 is the diffusion coefficient of O2 (1.93 105 cm2 s 1), n is the kinetic viscosity of the solution (1.009 102 cm2 s 1), CO2 is the concentration of dissolved O2 in solution (1.26 103 mol L 1), and w is the electrode rotation rate. According to the equation, the number of electrons transferred in the reaction could be determined from the plot of the measured overall current density (i 1) against the square root of the angular velocity (w 1/2).
Electrochemical tests: All electrochemical experiments were performed by using a CHI 660 C electrochemical analyzer (CH Instruments, Shanghai, Chenghua Co.). A standard three-electrode system was used for all electrochemical experiments, which consisted of a platinum wire as the auxiliary electrode, a saturated calomel reference electrode, and a catalyst-modified glassy carbon electrode as the working electrode. The rotating-disk electrode test was performed on Gamry’s Rotating Disk Electrode (RDE710) with a glassy carbon disk. All potentials in this study are reported with respect to the reversible hydrogen electrode (RHE). All electrochemical measurements were carried out at 30 1 8C.
2= 1 1 iL ¼ 0:62nFDO23 v =6 CO2 w =2
ð4Þ
The accelerated stability tests were conducted by applying linear potential sweeps between 1.03 and 0.05 V for 1000 cycles in O2saturated aqueous H2SO4 (0.5 m) solutions at a scan rate of 50 mV s1.
For preparation of the working electrode, a previously reported procedure was used.[10a] An evenly distributed suspension of the catalyst was prepared by ultrasonication of a mixture of the catalyst (5 mg) and an aqueous solution (5 mL) containing isopropanol (1.2 mL) and Nafion solution (25 mL, 5 wt %, Vwater/V2-propanol/ V5 %Nafion = 0.76:0.24:0.005) for 30 min, and 10 mL of the resulting suspension was laid on the surface of the glassy carbon electrode (5 mm diameter, 0.196 cm2). After drying at 40 8C, the working electrode was obtained, and the specific loading of the metal on the electrode surface was about 45.9 mg cm2. Cyclic-voltammetry (CV) measurements were conducted in a N2-saturated H2SO4 solution (0.5 m). www.chemeurj.org
ð2Þ
The adsorption of hydroxyl species was calculated based on the hydroxyl adsorption peak in the CV curve at a potential larger than 0.4 V. Dividing the hydroxyl adsorption area by the overall active surface area resulted in the surface coverage of hydroxyl species (VOHad).[10]
Physical characterization: Scanning electron microscopy (SEM) images were captured on a Hitachi S-4800 scanning electron microscope operated at an accelerating voltage of 5 kV. The composition of the catalysts was determined by using the energy dispersive spectrum (EDS) technique. High-resolution X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo VG Scientific ESCALAB 250 spectrometer with an AlKa radiator, and the vacuum in the analysis chamber was maintained at about 109 mbar. The binding energy was calibrated by means of the C 1s peak energy (284.6 eV). X-ray diffraction (XRD) patterns of the Pt catalysts were obtained with a Model D/max-rC X-ray diffractometer by using a CuKa radiation source (l = 1.5406 ) and operating at 40 kV and 100 mA. Transmission electron microscopy (TEM) measurements were made on a JEOL JEM-2100F transmission electron microscope operated at an accelerating voltage of 200 kV.
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Q mC
Acknowledgements This work was supported by the NSFC (21376122, 21073094, and 21273116), the Natural Science Foundation of Jiangsu Province (BK20131395), the United Fund of NSFC and Yunnan Province (U1137602), the Industry–Academia Cooperation Innovation Fund Project of Jiangsu Province (BY2012001), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. 589
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Full Paper Keywords: alloys · cyanogels · electrocatalytic activity · oxygen reduction reaction · reduction · trimetallic alloys [1] a) S. H. Sun, G. X. Zhang, D. S. Geng, Y. G. Chen, R. Y. Li, M. Cai, X. L. Sun, Angew. Chem. 2011, 123, 442 – 446; Angew. Chem. Int. Ed. 2011, 50, 422 – 426; b) L. Xiao, L. Zhuang, Y. Liu, J. T. Lu, H. D. Abruna, J. Am. Chem. Soc. 2009, 131, 602 – 608; c) J. Zhang, K. Sasaki, E. Sutter, R. Adzic, Science 2007, 315, 220 – 222; d) Z. Y. Zhou, X. W. Kang, Y. Song, S. W. Chen, Chem. Commun. 2012, 48, 3391 – 3393; e) J. Yang, J. Yang, J. Y. Ying, ACS Nano 2012, 6, 9373 – 9382; f) B. Y. Xia, W. T. Ng, H. Bin Wu, X. Wang, X. W. Lou, Angew. Chem. 2012, 124, 7325 – 7328; Angew. Chem. Int. Ed. 2012, 51, 7213 – 7216; g) Y. M. Tan, C. F. Xu, G. X. Chen, N. F. Zheng, Q. J. Xie, Energy Environ. Sci. 2012, 5, 6923 – 6927; h) Y. Liu, W. E. Mustain, J. Am. Chem. Soc. 2013, 135, 530 – 533; i) J. Kibsgaard, Y. Gorlin, Z. Chen, T. F. Jaramillo, J. Am. Chem. Soc. 2012, 134, 7758 – 7765; j) T. Yu, D. Y. Kim, H. Zhang, Y. N. Xia, Angew. Chem. 2011, 123, 2825 – 2829; Angew. Chem. Int. Ed. 2011, 50, 2773 – 2777; k) K. M. Yeo, S. Choi, R. M. Anisur, J. Kim, I. S. Lee, Angew. Chem. 2011, 123, 771 – 774; Angew. Chem. Int. Ed. 2011, 50, 745 – 748. [2] a) S. J. Yoo, S. J. Hwang, J. G. Lee, S. C. Lee, T. H. Lim, Y. E. Sung, A. Wieckowski, S. K. Kim, Energy Environ. Sci. 2012, 5, 7521 – 7525; b) J. Wu, L. Qi, H. You, A. Gross, J. Li, H. Yang, J. Am. Chem. Soc. 2012, 134, 11880 – 11883; c) X. Li, L. An, X. Y. Wang, F. Li, R. Q. Zou, D. G. Xia, J. Mater. Chem. 2012, 22, 6047 – 6052; d) S. J. Hwang, S. K. Kim, J. G. Lee, S. C. Lee, J. H. Jang, P. Kim, T. H. Lim, Y. E. Sung, S. J. Yoo, J. Am. Chem. Soc. 2012, 134, 19508 – 19511; e) M. K. Carpenter, T. E. Moylan, R. S. Kukreja, M. H. Atwan, M. M. Tessema, J. Am. Chem. Soc. 2012, 134, 8535 – 8542; f) J. Wu, A. Gross, H. Yang, Nano Lett. 2011, 11, 798 – 802; g) J. A. Zhao, A. Manthiram, Appl. Catal. B 2011, 101, 660 – 668. [3] a) Y. Pan, F. Zhang, K. Wu, Z. Lu, Y. Chen, Y. Zhou, Y. Tang, T. Lu, Int. J. Hydrogen Energy 2012, 37, 2993 – 3000; b) K. Lee, S. W. Kang, S. U. Lee, K. H. Park, Y. W. Lee, S. W. Han, ACS Appl. Mater. Interfaces 2012, 4, 4208 – 4214; c) G. Zhang, Y. Wang, X. Wang, Y. Chen, Y. Zhou, Y. Tang, L. Lu, J. Bao, T. Lu, Appl. Catal. B 2011, 102, 614 – 619; d) C. Wang, N. M. Markovic, V. R. Stamenkovic, ACS Catal. 2012, 2, 891 – 898. [4] a) S. Guo, S. Sun, J. Am. Chem. Soc. 2012, 134, 2492 – 2495; b) T. Wadayama, H. Yoshida, K. Ogawa, N. Todoroki, Y. Yamada, K. Miyamoto, Y. Iijima, T. Sugawara, K. Arihara, S. Sugawara, K. Shinohara, J. Phys. Chem. C 2011, 115, 18589 – 18596; c) B. P. Vinayan, R. Nagar, N. Rajalakshmi, S. Ramaprabhu, Adv. Funct. Mater. 2012, 22, 3519 – 3526; d) B. J. Hwang, S. M. S. Kumar, C. H. Chen, Monalisa, M. Y. Cheng, D. G. Liu, J. F. Lee, J. Phys. Chem. C 2007, 111, 15267 – 15276; e) T. Y. Jeon, S. J. Yoo, Y. H. Cho, K. S. Lee, S. H. Kang, Y. E. Sung, J. Phys. Chem. C 2009, 113, 19732 – 19739. [5] a) J. W. Hong, S. W. Kang, B. S. Choi, D. Kim, S. B. Lee, S. W. Han, ACS Nano 2012, 6, 2410 – 2419; b) L. Liu, G. Samjeske, S. i. Nagamatsu, O. Sekizawa, K. Nagasawa, S. Takao, Y. Imaizumi, T. Yamamoto, T. Uruga, Y. Iwasawa, J. Phys. Chem. C 2012, 116, 23453 – 23464; c) S. Y. Huang, P. Ganesan, B. N. Popov, ACS Catal. 2012, 2, 825 – 831. [6] a) G. Zhang, L. Zhang, L. Shen, Y. Chen, Y. Zhou, Y. Tang, T. Lu, ChemPlusChem 2012, 77, 936 – 940; b) M. Vondrova, T. M. McQueen, C. M. Bur-
Chem. Eur. J. 2014, 20, 585 – 590
www.chemeurj.org
[7] [8]
[9] [10]
[11]
[12] [13] [14]
[15] [16] [17]
[18] [19]
gess, D. M. Ho, A. B. Bocarsly, J. Am. Chem. Soc. 2008, 130, 5563 – 5572; c) R. S. Deshpande, S. L. Sharp-Goldman, J. L. Willson, A. B. Bocarsly, J. Gross, A. C. Finnefrock, S. M. Gruner, Chem. Mater. 2003, 15, 4239 – 4246; d) C. M. Burgess, M. Vondrova, A. B. Bocarsly, J. Mater. Chem. 2008, 18, 3694 – 3701; e) M. Heibel, G. Kumar, C. Wyse, P. Bukovec, A. B. Bocarsly, Chem. Mater. 1996, 8, 1504 – 1511; f) B. W. Pfennig, A. B. Bocarsly, R. K. Prud’homme, J. Am. Chem. Soc. 1993, 115, 2661 – 2665; g) J. Xu, X. Liu, Y. Chen, Y. Zhou, T. Lu, Y. Tang, J. Mater. Chem. 2012, 22, 23659 – 23667. Y. J. Deng, N. Tian, Z. Y. Zhou, R. Huang, Z. L. Liu, J. Xiao, S. G. Sun, Chem. Sci. 2012, 3, 1157 – 1161. a) G. Fu, K. Wu, J. Lin, Y. Tang, Y. Chen, Y. Zhou, T. Lu, J. Phys. Chem. C 2013, 10.1021/jp400502y; b) F. Ye, H. Liu, W. Hu, J. Zhong, Y. Chen, H. Cao, J. Yang, Dalton Trans. 2012, 41, 2898 – 2903. K. W. Park, Y. E. Sung, S. Han, Y. Yun, T. Hyeon, J. Phys. Chem. B 2004, 108, 939 – 944. a) Z. Peng, H. Yang, J. Am. Chem. Soc. 2009, 131, 7542 – 7543; b) D. Wang, H. L. Xin, Y. Yu, H. Wang, E. Rus, D. A. Muller, H. D. Abruna, J. Am. Chem. Soc. 2010, 132, 17664 – 17666. a) S. Mukerjee, S. Srinivasan, M. P. Soriaga, J. McBreen, J. Electrochem. Soc. 1995, 142, 1409 – 1422; b) H. Yang, J. Zhang, K. Sun, S. Zou, J. Fang, Angew. Chem. 2010, 122, 7000 – 7003; Angew. Chem. Int. Ed. 2010, 49, 6848 – 6851. J. Xu, G. Fu, Y. Tang, Y. Zhou, Y. Chen, T. Lu, J. Mater. Chem. 2012, 22, 13585 – 13590. H. W. Liang, X. A. Cao, F. Zhou, C. H. Cui, W. J. Zhang, S. H. Yu, Adv. Mater. 2011, 23, 1467 – 1471. a) J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. Kitchin, T. Bligaard, H. Jonsson, J. Phys. Chem. B 2004, 108, 17886 – 17892; b) X. Zhao, M. Yin, L. Ma, L. Liang, C. Liu, J. Liao, T. Lu, W. Xing, Energy Environ. Sci. 2011, 4, 2736 – 2753; c) A. Rabis, P. Rodriguez, T. J. Schmidt, Acs Catal. 2012, 2, 864 – 890; d) J. Greeley, I. Stephens, A. Bondarenko, T. P. Johansson, H. A. Hansen, T. Jaramillo, J. Rossmeisl, I. Chorkendorff, J. K. Nørskov, Nat. Chem. 2009, 1, 552 – 556; e) Y. Cai, C. Ma, Y. M. Zhu, J. X. Wang, R. R. Adzic, Langmuir 2011, 27, 8540 – 8547. C. Wang, H. Daimon, S. Sun, Nano Lett. 2009, 9, 1493 – 1496. S. H. Kang, Y. E. Sung, W. H. Smyrl, J. Electrochem. Soc. 2008, 155, B1128 – B1135. a) M. H. Shao, K. Sasaki, R. R. Adzic, J. Am. Chem. Soc. 2006, 128, 3526 – 3527; b) D. S. Kim, J. H. Kim, I. K. Jeong, J. K. Choi, Y. T. Kim, J. Catal. 2012, 290, 65 – 78. J. Jiang, A. Kucernak, J. Electroanal. Chem. 2009, 630, 10 – 18. a) Z. Awaludin, M. Suzuki, J. Masud, T. Okajima, T. Ohsaka, J. Phys. Chem. C 2011, 115, 25557 – 25567; b) V. Stamenkovic, T. Schmidt, P. Ross, N. Markovic, J. Phys. Chem. B 2002, 106, 11970 – 11979; c) Y. Garsany, O. A. Baturina, K. E. Swider-Lyons, S. S. Kocha, Anal. Chem. 2010, 82, 6321 – 6328.
Received: February 26, 2013 Revised: September 2, 2013 Published online on November 29, 2013
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& Electrocatalytic Activity
Pt-Pd-Co Trimetallic Alloy Network Nanostructures with Superior Electrocatalytic Activity towards the Oxygen Reduction Reaction Xinyu Liu, Gengtao Fu, Yu Chen,* Yawen Tang, Peiliang She, and Tianhong Lu[a]
Abstract: Pt alloy nanostructures show great promise as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cell cathodes. Herein, three-dimensional (3D) Pt-Pd-Co trimetallic network nanostructures (TNNs) with a high degree of alloying are synthesized through a room temperature wet chemical synthetic method by using K2PtCl4/ K3Co(CN)6–K2PdCl4/K3Co(CN)6 mixed cyanogels as the reaction precursor in the absence of surfactants and templates. The size, morphology, and surface composition of the Pt-PdCo TNNs are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area
electron diffraction (SAED), energy dispersive spectroscopy (EDS), EDS mapping, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The 3D backbone structure, solid nature, and trimetallic properties of the mixed cyanogels are responsible for the 3D structure and high degree of alloying of the as-prepared products. Compared with commercially available Pt black, the Pt-Pd-Co TNNs exhibit superior electrocatalytic activity and stability towards the ORR, which is ascribed to their unique 3D structure, low hydroxyl surface coverage and alloy properties.
Introduction
ular, the incorporation of Co[4b–d] or Pd[5] onto Pt can remarkably improve the ORR activity and durability of Pt electrocatalysts, a phenomenon originating from the modified geometric and electronic structure of Pt. However, there are still few examples of the synthesis of Pt-Pd-M trimetallic alloy electrocatalysts.[2g] It is still challenging to obtain multimetallic alloy nanostructures with a designed composition and catalytic properties in solution-phase synthesis. Cyanogels, inorganic coordination polymers made from a mixture of a tetrachlorometalate ([RCl4]2, R = Pd, Pt, Ir) and a transition-metal cyanometalate ([M(CN)n]2/3, n = 4, 6; M = Co, Fe, Ru, Os, Ni, Cr) in aqueous solution [Eq. (1)], are a special class of three-dimensional (3D), double-metal cyanide.[6] As shown in Equation (1), R and M species mix uniformly and concentrate on the backbone of the cyanogel rather than in the water solvent, which restrains the Brownian motion of the R and M species. The intimate interconnection between the R and M precursor species facilitates the combination of R0 and M0 crystal nuclei upon chemical reduction. Moreover, the solid nature of cyanogels also effectively suppresses the movement of R0 and M0 nuclei in the crystal, consequently facilitating the formation of an R0–M0 bimetallic alloy. Furthermore, the characteristic 3D backbone of cyanogels acts as a support scaffold, resulting in the interconnected 3D structure of R0–M0 bimetallic alloy nanostructures.
Fuel cells, especially proton-exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), are considered to be promising low-temperature sources of green power for portable electronic devices, residences, and automobiles. However, their commercialization is seriously impeded by the slow kinetics of the oxygen reduction reaction (ORR) on the Pt cathode and the high cost of Pt.[1]The use of an alloy is an effective way to solve the problem of improving the reaction activity of the ORR and reducing the amount of Pt used and thus the cost.[2] The essential challenge for the synthesis of alloy nanostructures is the diverse nucleation and growth rates of different elements owing to their distinct standard reduction potentials.[3] Typically, PtM nanoparticles (M = Fe, Co, Ni) formed through wet chemical synthesis have an exterior M-enriched or phase-separated, non-alloyed structure because of the preferential reduction/nucleation of the Pt precursor. To date, highly efficient PtM[4] and PtPd[5] alloy electrocatalysts for the ORR have been synthesized by different methods, such as wet chemical reduction, galvanic replacement, and thermal decomposition of organometallic precursors. In partic[a] Dr. X. Liu,+ G. Fu,+ Y. Chen, Prof. Y. Tang, P. She, T. Lu Jiangsu Key Laboratory of New Power Batteries Jiangsu Collaborative Innovation Centre of Biomedical Functional Materials School of Chemistry and Materials Science Nanjing Normal University, Nanjing 210023 (P.R. China) E-mail: [email protected] [+] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201300750. Chem. Eur. J. 2014, 20, 585 – 590
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Full Paper Herein, we develop a very simple and efficient one-step route for synthesizing Pt-Pd-Co trimetallic network nanostructures (TNNs) with a high degree of alloying by using mixed cyanogels as the reaction precursors at room temperature (Scheme 1). The as-prepared Pt-Pd-Co TNNs show excellent
Scheme 1. Schematic representation of the synthetic procedure for the PtPd-Co TNNs. A) Photograph of the K2PtCl4/K3Co(CN)6–K2PdCl4/K3Co(CN)6 mixed cyanogels. B) An SEM image of the Pt-Pd-Co TNNs. Figure 1. A) EDS spectrum and B) XPS spectra of the Pt-Pd-Co TNNs in the a) Pt 4f, b) Pd 3d, and c) Co 2p regions.
electrocatalytic activity and stability towards the ORR. The general route using mixed-cyanogel precursors reported herein will greatly contribute to the further design of multimetallic alloy nanostructures with various compositions and functions.
(79.2 %) and E-TEK Pd black (60.0 %; see Figure S2 in the Supporting Information). This result indicates that the Pt-Pd-Co TNNs have a weaker oxophilicity than commercial E-TEK Pt black and E-TEK Pd black. Meanwhile, Pt 4f and Pd 3d binding energies in the Pt-Pd-Co TNNs negatively shift by approximately 0.71 and 0.21 eV compared to those in commercial Pt black and Pd black, respectively (see Figure S2 in the Supporting Information). The negative shifts of the Pt 4f and Pd 3d binding energies show the change in the electronic structure of Pt and Pd atoms, further confirming the formation of a Pt-Pd-Co alloy. The change in the electronic structure of Pt and Pd atoms mainly originates from electron donation from Co to Pt and Pd due to the difference in electronegativity. This change decreases the 2p electron donation from O2 to the Pt and Pd atoms, and results in weaken O2 adsorption[8] (i.e., a high percentage of Pt0 and Pd0 species). The X-ray diffraction (XRD) pattern shows that the Pt-Pd-Co TNNs have the face-centered-cubic (fcc) structure (Figure 2 A). No diffraction peaks for single-component Pt, Pd, and Co were observed in the XRD pattern. Moreover, all diffraction peaks of the Pt-Pd-Co TNNs shift to a higher angle than both Pt and Pd (JCPDS no.: 04-0802 Pt; JCPDS no.: 46-1043 Pd). This XRD data clearly confirm the formation of a Pt-Pd-Co trimetallic alloy.[9] By using the Debye–Scherrer formula, the average particle size (dXRD) of the Pt-Pd-Co TNNs, calculated from the {111} diffraction peak, is 4.2 nm, which is much smaller than that of the commercial E-TEK Pt black (dXRD = 8.7 nm, see Figure S3 in the Supporting Information). The structural features of the Pt-PdCo TNNs were further investigated by transmission electron microscopy (TEM), which showed that the small and irregular nanoparticles are interconnected to form 3D network nanostructures with abundant pores (Figure 2 B). Further detailed observation showed that each of the irregular nanoparticles are essentially nanostructures with dendritic morphologies, which consist of smaller grains with a diameter of 4.2 3 nm (Figure 2 C and left insert). A high-resolution TEM (HRTEM)
Results and Discussion Characterization of the as-prepared Pt-Pd-Co TNNs: In a typical synthesis, aqueous solutions of K2PtCl4 (2.8 mL, 50 mm), K2PdCl4 (1.2 mL, 50 mm), and K3Co(CN)6 (2.0 mL, 50 mm) were mixed. The mixture was then heated at 95 8C for 36 h to generate the yellow jelly-like K2PtCl4/K3Co(CN)6–K2PdCl4/K3Co(CN)6 mixed cyanogels (Scheme 1 A). After reduction with NaBH4 and washing with HClO4, the Pt-Pd-Co TNNs were conveniently obtained by centrifugation (for details, see the Experimental Section). As shown in the scanning electron microscopy (SEM) image in Scheme 1 B, the porous Pt-Pd-Co TNNs have 3D network-like architectures. This feature is attractive for electrochemical applications because such 3D configurations can essentially facilitate the mass transport of fuel molecules. The Pt-Pd-Co TNN composition was analyzed by energy dispersive spectroscopy (EDS). The Pt/Pd/Co atomic ratio in the Pt-Pd-Co TNNs is 6.9:3.1:3.0 (Figure 1 A). The Co atomic fraction in the Pt-Pd-Co TNNs is lower than that in the initial mixed cyanogels (Pt/Pd/Co = 7:3:5), demonstrating that the acidic washing process effectively removes the unalloyed Co. In general, EDS just provides the bulk composition, whereas the electrocatalytic activity relies more on the surface composition. We thus checked the near-surface composition by X-ray photoelectron spectroscopy (XPS).[7] XPS data show that the Pt/Pd/Co atomic ratio in the Pt-Pd-Co TNNs is 7.1:2.9:3.0 (see Figure S1 in the Supporting Information), similar to that obtained from the EDS data, which is clear evidence for the formation of a Pt-Pd-Co alloy. Further detailed investigations showed that Pt, Pd, and Co species are predominantly in the metallic state (Figure 1 B). For instance, the percentage of Pt0 and Pd0 species in the PtPd-Co TNNs is calculated to be 84.1 and 90.3 %, respectively, which is much higher than that in commercial E-TEK Pt black Chem. Eur. J. 2014, 20, 585 – 590
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Full Paper Pt-Pd-Co TNN surfaces at more positive potentials. Figure 3 B clearly shows that the Pt-Pd-Co TNNs have lower hydroxyl surface coverage (VOH)[10] than E-TEK Pt black over the entire potential range, which arises from the change in the electronic structure of Pt on the Pt-Pd-Co TNNs,[11] as evidenced by the XPS data. Figure 3 C shows typical ORR polarization curves for the Pt-Pd-Co TNNs and commercial E-TEK Pt black. The current densities are normalized with reference to the geometrical area of the glassy carbon electrode. The ORR onset potential (EORR) of the Pt-Pd-Co TNNs shows a 29 mV shift, to a more positive potential, relative to commercial Pt black, indicating dramatically improved electrocatalytic activity for the Pt-Pd-Co TNNs. In addition, it is worth noting that the EORR on the Pt-PdCo TNNs (0.925 V) is also much higher than that on the 3D network Pt nanochain (EORR = 0.893 V)[12] and 3D network Pt nanowire membranes (EORR = 0.880 V),[13] indicating that the Pt-PdCo TNNs show more competitive electrocatalytic activity towards the ORR. Since adsorbed hydroxyl species on Pt surfaces inhibit the ORR, the lower VOH value on the Pt-Pd-Co TNNs surface improves the kinetics of the ORR. On the other hand, the rate-determining step in the ORR is the breaking of the OO bond.[14] The kinetics of the ORR strongly depends on the degree of interaction of oxygen with adsorption sites in the electrocatalyst, which is generally affected by the Pt d-band vacancies (electronic effects) and PtPt bond length (geometric effects). Compared with commercially available Pt black, the Pt 4f binding energies in the Pt-Pd-Co TNNs negatively shift by approximate-
Figure 2. A) XRD pattern (solid lines: Pt, dashed lines: Pd) and B, C) TEM images of the Pt-Pd-Co TNNs. Insets in C) HRTEM images (left and middle) and SAED image (right). D) Representative large-area TEM image of the PtPd-Co TNNs and the corresponding EDS maps of the Pt, Pd, and Co distributions within the Pt-Pd-Co TNNs.
image mainly shows the {111} and {100} facets of fcc Pt (middle insert in Figure 2 C). A selected-area electron diffraction (SAED) image shows an irregularly and discretely dotted pattern (right insert in Figure 2 C), indicating that the Pt-Pd-Co TNNs are polycrystalline. Elemental mapping under scanning transmission electron microscopy (STEM) mode was a powerful technique to characterize the distribution of the elements.[7] Clearly, the elemental distribution profiles of Pt, Pd, and Co are very similar (Figure 2 D), indicating that Pt, Pd, and Co are evenly distributed throughout the nanostructures. This is also strong evidence for the formation of a Pt-Pd-Co trimetallic alloy. Catalytic activity and stability for the ORR: Figure 3 A shows cyclic-voltammogram (CV) curves of the Pt-Pd-Co TNNs and commercial E-TEK Pt black in a N2-purged H2SO4 solution (0.5 m). It is worth noting that the potentials for surface-oxide formation (Pt + H2O!PtOH + H + + e) and the following reduction on the Pt-Pd-Co TNNs positively shift by approximately 88 mV compared with E-TEK Pt black, suggesting fast hydroxyl adsorption/desorption on the Chem. Eur. J. 2014, 20, 585 – 590
Figure 3. A) CV curves for the Pt-Pd-Co TNNs and commercial E-TEK Pt black in N2-saturated H2SO4 (0.5 m) solutions at 50 mV s1. B) Hydroxyl surface coverage (VOH) for the Pt-Pd-Co TNNs and E-TEK Pt black. C) ORR polarization curves for the Pt-Pd-Co TNNs and E-TEK Pt black in an O2-saturated H2SO4 (0.5 m) solution at a scan rate of 5 mV s1 and a rotation rate of 1600 rpm. Inset: Pt mass activities of the Pt-Pd-Co TNNs and E-TEK Pt black towards the ORR at 0.9 V (vs. NHE). D) Specific kinetic current densities (ik) for the Pt-Pd-Co TNNs and E-TEK Pt black at different potentials.
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Full Paper ly 0.71 eV due to the alloying of Pt with both Pd and Co. This is believed to be a key factor in improving Pt ORR activity because the negative shift of the Pt 4f binding energies is likely to lead to smaller oxygen adsorption energies and thus a weaker OO bond, which is confirmed by previous reports (such as those of PtFe3O4 nanoparticles[15] and PtNi[4e] and PtCo alloy nanoparticles[16]). The contraction in the atomic size of Pt is also likely to be an important reason for the improved Pt ORR activity because the decrease in the PtPt bond length also leads to a decrease in the oxygen adsorption energy.[1b] The mass activity (i.e., the currents are normalized to the mass of the metal) is taken as an index to assess the applicability of the electrocatalysts for the ORR.[17] At 0.85 V, the Pt mass activity for the Pt-Pd-Co TNNs is 3.26 times higher than that of ETEK Pt black (insert in Figure 3 C). Since the specific kinetic activity (normalized to the electrochemically active surface area) represented the intrinsic electrocatalytic activity, the specific kinetic activities of the electrocatalysts in the ORR were further investigated. The electrochemically active surface area of the Pt-Pd-Co TNNs (22.1 m2 g1) is larger than that of the commercially available ETEK Pt black (15.7 m2 g1), based on the CO-stripping-voltammetry data (Figure S4 in the Supporting Information; for calculation details, see the Experimental Section), a fact originating from the difference in particle size (dXRD-Pt-Pd-Co = 4.2 vs. dXRD-ETEK Pt = 8.7 nm). The Pt-Pd-Co TNNs show greatly improved specific kinetic activity relative to Pt black in the whole potential range (Figure 3 D). For instance, the specific kinetic current density at 0.85 V is about 1.20 mA cm2 for Pt-Pd-Co TNNs, which is enhanced by a factor of 2.03 over that of the Pt black (0.59 A m2). To further evaluate the kinetic parameters for the ORR on the Pt-Pd-Co TNNs, hydrodynamic voltammograms were recorded at a scan rate of 5 mV s1 over a range of rotation rates of 100–2500 rpm (Figure 4). According to the Kou-
tecky–Levich equation, the number of transferred electrons per oxygen molecule at the Pt-Pd-Co TNN surfaces is calculated to be 3.9 (for calculation details, see the Experimental Section), confirming a complete four-electron reduction pathway. The durability of cathodic electrocatalysts is a common issue for fuel cells, and is a very crucial and important consideration from an industrial point of view. Stabilizing Pt nanoparticles in the cathode seems to be even more challenging than controlling their activity. After an accelerated stability test,[1c] only about a 5 mV negative shift in the half-wave potential for the Pt-Pd-Co TNNs is observed, whereas there is a 28 mV negative shift for Pt black (Figure 5), indicating that the Pt-Pd-Co TNNs are a class of stable electrocatalysts for the ORR. The particular interconnected structure that suppresses the Ostwald ripening effect,[1a, f, 6a, g, 10a] as well as the positive shift in the Pt oxide formation potential that restrains electrochemical corrosion/dissolution of Pt nanopaticles at high potentials are responsible for the enhanced durability of the Pt-Pd-Co TNNs.
Figure 5. ORR polarization curves for A) the Pt-Pd-Co TNNs and B) E-TEK Pt black in O2-saturated H2SO4 (0.5 m) solutions before and after 1000 potential cycles at a scan rate of 5 mV s1 and a rotation rate of 1600 rpm.
Conclusion By means of the particular properties of cyanogels, such as the characteristic 3D backbone, their solid nature, and the uniform distribution of metal ions on the cyanogel backbone, Pt-Pd-Co TNNs are obtained facilely by using mixed cyanogels as the reaction precursor at room temperature. The Pt-Pd-Co TNNs show considerably enhanced electrocatalytic activity (mass activity and specific activity) and durability in the ORR compared with commercially available Pt black. This work demonstrates that the Pt-Pd-Co TNNs are indeed a promising cathodic electrocatalyst for fuel cells. The reported mixed-cyanogel/chemical-reduction method can be generalized to produce various noble-metal-based (such as Pd, Pt, and Ir) multimetallic alloy nanostructures for catalytic applications.
Figure 4. A) ORR polarization curves for the Pt-Pd-Co TNNs in an O2-saturated H2SO4 (0.5 m) solution at a scan rate of 5 mV s1 over a range of rotation rates of 100–2500 rpm. Current density is calculated by using the geometric area of the electrode. B) The Koutecky–Levich plots (i 1 vs. w 1/2) for the ORR at 0.3 V (vs. NHE). Chem. Eur. J. 2014, 20, 585 – 590
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Full Paper Experimental Section
For the CO-stripping measurements, the catalyst surface was first saturated with CO by bubbling CO through an H2SO4 solution (0.5 m), while holding the working electrode at 0 V for 15 min. The remaining CO was purged by use of a flow of N2 for 30 min before measurements were made.[3c, 18] The electrochemically active surface area (ECSA) of the catalysts was calculated from the Equation (2) by measuring the charge collected in the CO adsorption oxidation region (Q) and assuming a value of 420 mC cm2 for the adsorption of a CO monolayer (m is the loading amount of Pt and Pd metal).
Materials: Potassium tetrachloroplatinate(II) (K2PtCl4), potassium tetrachloropalladate(II) (K2PdCl4), potassium hexacyanocobaltate(III) (K3Co(CN)6), and sodium borohydride (NaBH4) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Other reagents were of analytical reagent grade and used without further purification. Commercially available Pt black and Pd black were purchased from E-TEK Division, PEMEAS Fuel Cell Technologies. All of the aqueous solutions were prepared with Millipore water having a resistivity of 18.2 MW.
ECSA ¼
Synthesis of Pt-Pd-Co TNNs: In a typical synthesis, aqueous solutions of K2PtCl4 (2.8 mL, 50 mm), K2PdCl4 (1.2 mL, 50 mm), and K3Co(CN)6 (2.0 mL, 50 mm) were added to a Teflon-lined stainlesssteel autoclave (10 mL), and were then heated at 95 8C for 36 h to generate the yellow K2PtCl4/K3Co(CN)6–K2PdCl4/K3Co(CN)6 mixed cyanogels. After being cooled to room temperature, NaBH4 (6 mL, 0.1 g mL1) was added to the yellow K2PtCl4/K3Co(CN)6–K2PdCl4/ K3Co(CN)6 mixed cyanogel and the resulting mixed solution was left for 1 h. After the reaction, the black Pt-Pd-Co trimetallic nanostructures (TNNs) were separated by centrifugation at 15 000 rpm for 10 min, washed with HClO4 (0.1 m), and then dried at 40 8C in a vacuum dryer for 12 h. The acid-wash process ensured the removal of unalloyed Co.
All oxygen-reduction-reaction (ORR) tests were conducted in O2saturated H2SO4 solutions (0.5 m). The polarization curves were obtained by sweeping the potential from 1.03 to 0.05 V at a scan rate of 5 mV s1 and a rotation rate of 1600 rpm. The current density was normalized with reference to the ECSA of the catalysts. Based on the ORR polarization curves, the kinetic current density (iK) was calculated by using the Koutecky–Levich equation [Eq. (3)], in which iL and i are the limited diffusion current density and the measured current density, respectively.[19]
1 1 1 ¼ þ i ik iL
ð3Þ
The number of transferred electrons per oxygen molecule involved in the ORR at the Pt-Pd-Co TANNs was determined by the Koutecky–Levich equation [Eq. (3)], with iL expressed by Equation (4),[19] in which n is number of electrons transferred for per oxygen molecule, F is the Faraday constant (96 485 C mol1), DO2 is the diffusion coefficient of O2 (1.93 105 cm2 s 1), n is the kinetic viscosity of the solution (1.009 102 cm2 s 1), CO2 is the concentration of dissolved O2 in solution (1.26 103 mol L 1), and w is the electrode rotation rate. According to the equation, the number of electrons transferred in the reaction could be determined from the plot of the measured overall current density (i 1) against the square root of the angular velocity (w 1/2).
Electrochemical tests: All electrochemical experiments were performed by using a CHI 660 C electrochemical analyzer (CH Instruments, Shanghai, Chenghua Co.). A standard three-electrode system was used for all electrochemical experiments, which consisted of a platinum wire as the auxiliary electrode, a saturated calomel reference electrode, and a catalyst-modified glassy carbon electrode as the working electrode. The rotating-disk electrode test was performed on Gamry’s Rotating Disk Electrode (RDE710) with a glassy carbon disk. All potentials in this study are reported with respect to the reversible hydrogen electrode (RHE). All electrochemical measurements were carried out at 30 1 8C.
2= 1 1 iL ¼ 0:62nFDO23 v =6 CO2 w =2
ð4Þ
The accelerated stability tests were conducted by applying linear potential sweeps between 1.03 and 0.05 V for 1000 cycles in O2saturated aqueous H2SO4 (0.5 m) solutions at a scan rate of 50 mV s1.
For preparation of the working electrode, a previously reported procedure was used.[10a] An evenly distributed suspension of the catalyst was prepared by ultrasonication of a mixture of the catalyst (5 mg) and an aqueous solution (5 mL) containing isopropanol (1.2 mL) and Nafion solution (25 mL, 5 wt %, Vwater/V2-propanol/ V5 %Nafion = 0.76:0.24:0.005) for 30 min, and 10 mL of the resulting suspension was laid on the surface of the glassy carbon electrode (5 mm diameter, 0.196 cm2). After drying at 40 8C, the working electrode was obtained, and the specific loading of the metal on the electrode surface was about 45.9 mg cm2. Cyclic-voltammetry (CV) measurements were conducted in a N2-saturated H2SO4 solution (0.5 m). www.chemeurj.org
ð2Þ
The adsorption of hydroxyl species was calculated based on the hydroxyl adsorption peak in the CV curve at a potential larger than 0.4 V. Dividing the hydroxyl adsorption area by the overall active surface area resulted in the surface coverage of hydroxyl species (VOHad).[10]
Physical characterization: Scanning electron microscopy (SEM) images were captured on a Hitachi S-4800 scanning electron microscope operated at an accelerating voltage of 5 kV. The composition of the catalysts was determined by using the energy dispersive spectrum (EDS) technique. High-resolution X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo VG Scientific ESCALAB 250 spectrometer with an AlKa radiator, and the vacuum in the analysis chamber was maintained at about 109 mbar. The binding energy was calibrated by means of the C 1s peak energy (284.6 eV). X-ray diffraction (XRD) patterns of the Pt catalysts were obtained with a Model D/max-rC X-ray diffractometer by using a CuKa radiation source (l = 1.5406 ) and operating at 40 kV and 100 mA. Transmission electron microscopy (TEM) measurements were made on a JEOL JEM-2100F transmission electron microscope operated at an accelerating voltage of 200 kV.
Chem. Eur. J. 2014, 20, 585 – 590
Q mC
Acknowledgements This work was supported by the NSFC (21376122, 21073094, and 21273116), the Natural Science Foundation of Jiangsu Province (BK20131395), the United Fund of NSFC and Yunnan Province (U1137602), the Industry–Academia Cooperation Innovation Fund Project of Jiangsu Province (BY2012001), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. 589
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper Keywords: alloys · cyanogels · electrocatalytic activity · oxygen reduction reaction · reduction · trimetallic alloys [1] a) S. H. Sun, G. X. Zhang, D. S. Geng, Y. G. Chen, R. Y. Li, M. Cai, X. L. Sun, Angew. Chem. 2011, 123, 442 – 446; Angew. Chem. Int. Ed. 2011, 50, 422 – 426; b) L. Xiao, L. Zhuang, Y. Liu, J. T. Lu, H. D. Abruna, J. Am. Chem. Soc. 2009, 131, 602 – 608; c) J. Zhang, K. Sasaki, E. Sutter, R. Adzic, Science 2007, 315, 220 – 222; d) Z. Y. Zhou, X. W. Kang, Y. Song, S. W. Chen, Chem. Commun. 2012, 48, 3391 – 3393; e) J. Yang, J. Yang, J. Y. Ying, ACS Nano 2012, 6, 9373 – 9382; f) B. Y. Xia, W. T. Ng, H. Bin Wu, X. Wang, X. W. Lou, Angew. Chem. 2012, 124, 7325 – 7328; Angew. Chem. Int. Ed. 2012, 51, 7213 – 7216; g) Y. M. Tan, C. F. Xu, G. X. Chen, N. F. Zheng, Q. J. Xie, Energy Environ. Sci. 2012, 5, 6923 – 6927; h) Y. Liu, W. E. Mustain, J. Am. Chem. Soc. 2013, 135, 530 – 533; i) J. Kibsgaard, Y. Gorlin, Z. Chen, T. F. Jaramillo, J. Am. Chem. Soc. 2012, 134, 7758 – 7765; j) T. Yu, D. Y. Kim, H. Zhang, Y. N. Xia, Angew. Chem. 2011, 123, 2825 – 2829; Angew. Chem. Int. Ed. 2011, 50, 2773 – 2777; k) K. M. Yeo, S. Choi, R. M. Anisur, J. Kim, I. S. Lee, Angew. Chem. 2011, 123, 771 – 774; Angew. Chem. Int. Ed. 2011, 50, 745 – 748. [2] a) S. J. Yoo, S. J. Hwang, J. G. Lee, S. C. Lee, T. H. Lim, Y. E. Sung, A. Wieckowski, S. K. Kim, Energy Environ. Sci. 2012, 5, 7521 – 7525; b) J. Wu, L. Qi, H. You, A. Gross, J. Li, H. Yang, J. Am. Chem. Soc. 2012, 134, 11880 – 11883; c) X. Li, L. An, X. Y. Wang, F. Li, R. Q. Zou, D. G. Xia, J. Mater. Chem. 2012, 22, 6047 – 6052; d) S. J. Hwang, S. K. Kim, J. G. Lee, S. C. Lee, J. H. Jang, P. Kim, T. H. Lim, Y. E. Sung, S. J. Yoo, J. Am. Chem. Soc. 2012, 134, 19508 – 19511; e) M. K. Carpenter, T. E. Moylan, R. S. Kukreja, M. H. Atwan, M. M. Tessema, J. Am. Chem. Soc. 2012, 134, 8535 – 8542; f) J. Wu, A. Gross, H. Yang, Nano Lett. 2011, 11, 798 – 802; g) J. A. Zhao, A. Manthiram, Appl. Catal. B 2011, 101, 660 – 668. [3] a) Y. Pan, F. Zhang, K. Wu, Z. Lu, Y. Chen, Y. Zhou, Y. Tang, T. Lu, Int. J. Hydrogen Energy 2012, 37, 2993 – 3000; b) K. Lee, S. W. Kang, S. U. Lee, K. H. Park, Y. W. Lee, S. W. Han, ACS Appl. Mater. Interfaces 2012, 4, 4208 – 4214; c) G. Zhang, Y. Wang, X. Wang, Y. Chen, Y. Zhou, Y. Tang, L. Lu, J. Bao, T. Lu, Appl. Catal. B 2011, 102, 614 – 619; d) C. Wang, N. M. Markovic, V. R. Stamenkovic, ACS Catal. 2012, 2, 891 – 898. [4] a) S. Guo, S. Sun, J. Am. Chem. Soc. 2012, 134, 2492 – 2495; b) T. Wadayama, H. Yoshida, K. Ogawa, N. Todoroki, Y. Yamada, K. Miyamoto, Y. Iijima, T. Sugawara, K. Arihara, S. Sugawara, K. Shinohara, J. Phys. Chem. C 2011, 115, 18589 – 18596; c) B. P. Vinayan, R. Nagar, N. Rajalakshmi, S. Ramaprabhu, Adv. Funct. Mater. 2012, 22, 3519 – 3526; d) B. J. Hwang, S. M. S. Kumar, C. H. Chen, Monalisa, M. Y. Cheng, D. G. Liu, J. F. Lee, J. Phys. Chem. C 2007, 111, 15267 – 15276; e) T. Y. Jeon, S. J. Yoo, Y. H. Cho, K. S. Lee, S. H. Kang, Y. E. Sung, J. Phys. Chem. C 2009, 113, 19732 – 19739. [5] a) J. W. Hong, S. W. Kang, B. S. Choi, D. Kim, S. B. Lee, S. W. Han, ACS Nano 2012, 6, 2410 – 2419; b) L. Liu, G. Samjeske, S. i. Nagamatsu, O. Sekizawa, K. Nagasawa, S. Takao, Y. Imaizumi, T. Yamamoto, T. Uruga, Y. Iwasawa, J. Phys. Chem. C 2012, 116, 23453 – 23464; c) S. Y. Huang, P. Ganesan, B. N. Popov, ACS Catal. 2012, 2, 825 – 831. [6] a) G. Zhang, L. Zhang, L. Shen, Y. Chen, Y. Zhou, Y. Tang, T. Lu, ChemPlusChem 2012, 77, 936 – 940; b) M. Vondrova, T. M. McQueen, C. M. Bur-
Chem. Eur. J. 2014, 20, 585 – 590
www.chemeurj.org
[7] [8]
[9] [10]
[11]
[12] [13] [14]
[15] [16] [17]
[18] [19]
gess, D. M. Ho, A. B. Bocarsly, J. Am. Chem. Soc. 2008, 130, 5563 – 5572; c) R. S. Deshpande, S. L. Sharp-Goldman, J. L. Willson, A. B. Bocarsly, J. Gross, A. C. Finnefrock, S. M. Gruner, Chem. Mater. 2003, 15, 4239 – 4246; d) C. M. Burgess, M. Vondrova, A. B. Bocarsly, J. Mater. Chem. 2008, 18, 3694 – 3701; e) M. Heibel, G. Kumar, C. Wyse, P. Bukovec, A. B. Bocarsly, Chem. Mater. 1996, 8, 1504 – 1511; f) B. W. Pfennig, A. B. Bocarsly, R. K. Prud’homme, J. Am. Chem. Soc. 1993, 115, 2661 – 2665; g) J. Xu, X. Liu, Y. Chen, Y. Zhou, T. Lu, Y. Tang, J. Mater. Chem. 2012, 22, 23659 – 23667. Y. J. Deng, N. Tian, Z. Y. Zhou, R. Huang, Z. L. Liu, J. Xiao, S. G. Sun, Chem. Sci. 2012, 3, 1157 – 1161. a) G. Fu, K. Wu, J. Lin, Y. Tang, Y. Chen, Y. Zhou, T. Lu, J. Phys. Chem. C 2013, 10.1021/jp400502y; b) F. Ye, H. Liu, W. Hu, J. Zhong, Y. Chen, H. Cao, J. Yang, Dalton Trans. 2012, 41, 2898 – 2903. K. W. Park, Y. E. Sung, S. Han, Y. Yun, T. Hyeon, J. Phys. Chem. B 2004, 108, 939 – 944. a) Z. Peng, H. Yang, J. Am. Chem. Soc. 2009, 131, 7542 – 7543; b) D. Wang, H. L. Xin, Y. Yu, H. Wang, E. Rus, D. A. Muller, H. D. Abruna, J. Am. Chem. Soc. 2010, 132, 17664 – 17666. a) S. Mukerjee, S. Srinivasan, M. P. Soriaga, J. McBreen, J. Electrochem. Soc. 1995, 142, 1409 – 1422; b) H. Yang, J. Zhang, K. Sun, S. Zou, J. Fang, Angew. Chem. 2010, 122, 7000 – 7003; Angew. Chem. Int. Ed. 2010, 49, 6848 – 6851. J. Xu, G. Fu, Y. Tang, Y. Zhou, Y. Chen, T. Lu, J. Mater. Chem. 2012, 22, 13585 – 13590. H. W. Liang, X. A. Cao, F. Zhou, C. H. Cui, W. J. Zhang, S. H. Yu, Adv. Mater. 2011, 23, 1467 – 1471. a) J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. Kitchin, T. Bligaard, H. Jonsson, J. Phys. Chem. B 2004, 108, 17886 – 17892; b) X. Zhao, M. Yin, L. Ma, L. Liang, C. Liu, J. Liao, T. Lu, W. Xing, Energy Environ. Sci. 2011, 4, 2736 – 2753; c) A. Rabis, P. Rodriguez, T. J. Schmidt, Acs Catal. 2012, 2, 864 – 890; d) J. Greeley, I. Stephens, A. Bondarenko, T. P. Johansson, H. A. Hansen, T. Jaramillo, J. Rossmeisl, I. Chorkendorff, J. K. Nørskov, Nat. Chem. 2009, 1, 552 – 556; e) Y. Cai, C. Ma, Y. M. Zhu, J. X. Wang, R. R. Adzic, Langmuir 2011, 27, 8540 – 8547. C. Wang, H. Daimon, S. Sun, Nano Lett. 2009, 9, 1493 – 1496. S. H. Kang, Y. E. Sung, W. H. Smyrl, J. Electrochem. Soc. 2008, 155, B1128 – B1135. a) M. H. Shao, K. Sasaki, R. R. Adzic, J. Am. Chem. Soc. 2006, 128, 3526 – 3527; b) D. S. Kim, J. H. Kim, I. K. Jeong, J. K. Choi, Y. T. Kim, J. Catal. 2012, 290, 65 – 78. J. Jiang, A. Kucernak, J. Electroanal. Chem. 2009, 630, 10 – 18. a) Z. Awaludin, M. Suzuki, J. Masud, T. Okajima, T. Ohsaka, J. Phys. Chem. C 2011, 115, 25557 – 25567; b) V. Stamenkovic, T. Schmidt, P. Ross, N. Markovic, J. Phys. Chem. B 2002, 106, 11970 – 11979; c) Y. Garsany, O. A. Baturina, K. E. Swider-Lyons, S. S. Kocha, Anal. Chem. 2010, 82, 6321 – 6328.
Received: February 26, 2013 Revised: September 2, 2013 Published online on November 29, 2013
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