Showcasing research from Yuan Wang et al. at the Beijing National Laboratory for Molecular Science, Peking University
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Pt–Cu bimetallic electrocatalysts with enhanced catalytic properties for oxygen reduction A Pt–Cu bimetallic fuel cell catalyst prepared by a new method, with an average diameter of 2.9 nm and a Cu/Pt ratio of 0.3 for Pt–Cu nanoparticles, exhibits strikingly improved catalytic properties for the electrochemical reduction of oxygen.
See Yuan Wang, Yan Liu et al., Chem. Commun., 2014, 50, 13889.
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Pt–Cu bimetallic electrocatalysts with enhanced catalytic properties for oxygen reduction† Chun-Mei Zhu,‡ Ang Gao,‡ Yuan Wang* and Yan Liu*
Received 1st April 2014, Accepted 25th July 2014 DOI: 10.1039/c4cc02391a www.rsc.org/chemcomm
A highly active Pt–Cu bimetallic catalyst for the electrocatalytic oxygen reduction reaction, with an average diameter of 2.9 nm and a Cu/Pt ratio of 0.30 for the bimetallic nanoparticles, was prepared by capturing Pt–Cu alloy nanoparticles on melem-modified carbon, followed by removing 90% of copper from the alloy NPs.
Highly efficient electrocatalysts for oxygen reduction reaction (ORR) are of significance for the development of polymer electrolyte membrane fuel cells (PEMFCs) and metal–air batteries, which provide electric power in a clean and convenient manner.1–4 Numerous efforts have been devoted to the development of electrochemical catalysts of carbon-supported platinum-based alloy nanoparticles (NPs) because the catalytic activity and durability of traditional Pt/C catalysts are not high enough for real applications. Pt–Ni,2 Pt–Co,3 Pt–Fe,3c Pt–Cu,4 and other platinumbased bimetallic catalysts have been proved to exhibit obviously enhanced electrocatalytic activity for ORR compared to Pt catalysts due to the changes in electronic structures and adsorption properties derived from the addition of other transition metals to Pt.2a Recently, Pt–Cu bimetallic NPs with Pt enriched shells and alloy cores have attracted much attention.4a,c,d Strasser and co-workers4d reported that such core–shell structured NPs prepared by electrochemically dealloying Pt–Cu alloy NPs supported on carbon exhibited a significant enhancement in the electrocatalytic activity for ORR compared to Pt/C catalysts, and the suitable Cu/Pt atomic ratio for the Pt–Cu alloy NPs was about 3. The enhancement in the catalytic activity was ascribed to the lattice compression due to the smaller atomic size of Cu compared to that of Pt. It was reported that Pt–Cu core–shell NPs with Beijing National Laboratory for Molecular Science, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China. E-mail: [email protected], [email protected]; Fax: +86 10 6276 5769; Tel: +86 10 6275 7497 † Electronic supplementary information (ESI) available: The experimental details and characterization of melem, MMC and Pt–Cu catalysts by IR, elemental analysis, XPS, TEM and EDX. See DOI: 10.1039/c4cc02391a ‡ Zhu and Gao contributed equally to this work.
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a Pt shell surrounding a single alloy core could be formed in the electrochemical dealloying process when the diameters of the alloy particle precursors were below 15 nm, while bimetallic NPs with complex nanostructures would form when larger alloy particles were used as starting materials.4b Although progress has been achieved in the fabrication of Pt–Cu catalysts,4 challenges remain in further improving the catalytic activity and durability of Pt–Cu catalysts. Pt–Cu alloy NPs in most of the previously reported carbon supported Pt–Cu catalysts had an average diameter (dav) larger than 5 nm with a wide size distribution, or a wide distribution of Cu/Pt ratios.4a,c,d It is reasonable to believe that the supported Pt–Cu bimetallic catalysts with properly decreased particle sizes5 and controllable Cu/Pt ratios of the alloy NPs would exhibit obviously enhanced catalytic properties for ORR due to the increase in the specific electrochemical surface area (SESA) and the possibility of optimizing the NPs’ properties. Previously, we reported a method for the preparation of PVP-protected Cu–Pt alloy colloidal nanoclusters with an average particle size of around 2 nm by reducing bimetallic hydroxide or oxide NPs with ethylene glycol (EG),6 and the coordination capture strategy for the immobilization of metal nanoclusters on supports anchoring mercaptan groups.7 Conductive nanotubes anchoring nitrogen-containing functional groups on the surfaces were used as supports to prepare promising Pt supported catalysts for the electrocatalytic oxidation of methanol.8,9 In this communication, we report a highly efficient electrocatalyst (de-PtCu/MMC) for ORR, which is composed of dealloyed Pt–Cu NPs with a dav of 2.9 nm and a melem-modified carbon support (MMC). The mass catalytic activity for ORR over de-PtCu/MMC was measured to be 720 mA mgPt 1 at 0.9 V (versus RHE), which was higher than those of ever reported Pt–Cu catalysts measured under similar conditions.4a,d Scheme 1 shows the surface modification process of a carbon support VXC-72R with melem, which was conducted by heating a mixture of melamine and VXC-72R in air at 340 1C for 1 h (see the ESI† for details). As indicated by the results of elementary analysis (see ESI†) and IR absorption spectroscopy measurements (Fig. S1, ESI†), heating melamine in air at 340 1C
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Scheme 1
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The surface modification process of VXC-72R with melem.
Fig. 2 (a and b) TEM images and size distributions of bimetallic NPs in PtCu/MMC and PtCu/C, respectively; (c) XRD patterns of PtCu/MMC and PtCu/C; (d) XPS spectra of N 1s in MMC and PtCu/MMC.
Fig. 1
XPS spectra of N 1s in melem (a) and MMC (b).
for 1 h produced melem as the main product, which was similar to the previously reported results of the thermal condensation of melamine at around 400 1C.10 The formed melem in the preparation of MMC can react with carboxyl groups (Fig. S2, ESI†) on the surface of VXC-72R to form amide groups, which was confirmed by X-ray photoelectron spectroscopy (XPS) measurements. Fig. 1 compares the XPS spectra of N 1s for MMC and melem prepared in this work. The fitting peaks at the binding energies of 398.5, 399.4 and 400.9 eV in both melem and MMC were the signals from –CQN–C,11 –NH211b,13 and graphitic N,12 respectively. The appearance of another fitting peak located at 400.1 eV in MMC suggests the formation of amide groups13 through the aforementioned reaction. The specific surface area and the nitrogen content of MMC was 110 m2 g 1 and 4.4%, respectively, as measured by the BET method and elemental analysis. MMC supported Pt–Cu alloy NPs (PtCu/MMC) were prepared by adjusting the pH value of an EG solution containing Cu(CH3COO)2H2O and H2PtCl66H2O to 10, followed by reducing the formed bimetallic hydroxide or oxide NPs with EG at 198 1C under a N2 atmosphere6 in the presence of MMC, resulting in the deposition or capture of the formed Pt–Cu alloy NPs on MMC. The nitrogen content of PtCu/MMC was 3.8% as measured by elemental analysis. For comparison, a PtCu/C catalyst was also prepared by the same process using VXC-72R instead of MMC as a support. Transmission electron microscopy (TEM) images of the products demonstrated that Pt–Cu alloy NPs in PtCu/MMC (Fig. 2(a)) had a dav of 3.8 nm, and were well dispersed on MMC, while the alloy NPs in prepared PtCu/C had a dav of 4.5 nm and tended to form more aggregates (Fig. 2(b)). As revealed in previous work, alloy NPs of Pt and Cu with different compositions possessed characteristic diffraction peak positions.6 Fig. 2(c) shows the XRD patterns of prepared
13890 | Chem. Commun., 2014, 50, 13889--13892
PtCu/MMC and PtCu/C. The diffraction peaks at 42.11, 48.51, 71.81 and 86.91 were the signals from the (111), (200), (220) and (331) crystal planes of the Pt–Cu alloy NPs with a Cu/Pt ratio of 71/29, respectively, indicating that the alloy NPs in these two catalysts have the same alloy compositions. No XRD signal corresponding to monometallic platinum or copper was observed. The Cu/Pt atomic ratios in both PtCu/MMC (Pt content: 9.1 wt%) and PtCu/C (Pt content: 9.4 wt%) were 2.45 as measured by the inductively coupled plasma atomic emission spectrometry (ICP-AES), which was in agreement with the charged Cu/Pt ratio in the preparation. The binding energy of Pt 4f7/2 and Cu 2p3/2 in PtCu/MMC was 71.3 and 932.2 eV, respectively (Fig. S3, ESI†), which was in accord with the corresponding values of prepared PtCu/C (71.2 and 932.3 eV) and previously reported results.6 The difference in the peak shape of the N 1s XPS spectrum between PtCu/MMC and MMC (Fig. 2(d)) was derived from the alcoholysis of 50% of amide groups in MMC during the preparation of PtCu/MMC, which might be derived by the coordination interaction between melem or amino groups and the surface atoms of Pt–Cu NPs14 (see the ESI† for details). Such an interaction would be propitious to the deposition of small Pt–Cu NPs on MMC and suppress the agglomeration of Pt–Cu NPs. The electrochemical dissolution of PtCu/MMC and PtCu/C was performed to selectively leach out copper from the alloy NP surfaces to produce Pt-enriched surfaces covering Pt–Cu alloy cores (Fig. S5, ESI†). The dav of the bimetallic NPs in the catalysts prepared by the electrochemical dissolution of PtCu/MMC (de-PtCu/MMC) and PtCu/C (de-PtCu/C) was 2.9 and 3.9 nm, respectively, as measured by TEM (Fig. S6, ESI†). The Cu/Pt atomic ratio of bimetallic NPs in de-PtCu/MMC and PtCu/MMC was measured by energy dispersive X-ray spectrometry to be 0.30 and 3.2, respectively (Table S2, ESI†), indicating that 90% of copper in the precursor alloy NPs had been removed through the electrochemical dissolution treatment. Fig. 3 shows the electrochemical properties of de-PtCu/ MMC, de-PtCu/C and a commercial Pt/C-JM catalyst. The SESA of the catalysts was calculated from the hydrogen desorption
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Fig. 3 Electrochemical properties of de-PtCu/MMC, de-PtCu/C, and Pt/C-JM electrodes tested in 0.1 M HClO4 aqueous solution at 30 1C. (a) CVs at a scan rate of 50 mV s 1 under N2; (b) ORR polarization curves at 1600 rpm with a scan rate of 5 mV s 1; (c) mass activities; (d) specific activities.
charges by integrating the area in the hydrogen underpotential deposition (HUPD) region of cyclic voltammograms (Fig. 3(a)), supposing that the average charge density associated with the formation of one monolayer hydrogen atom on Pt NPs and dealloyed Pt–Cu NPs with Pt shells has a value of 210 mC cm 2.4d,15 The SESA of de-PtCu/MMC was 86 m2 gPt 1, which was larger by 26% than that of de-PtCu/C (68 m2 gPt 1), due to the small particle size and high dispersion of Pt–Cu NPs in de-PtCu/MMC. The SESA of Pt/C-JM was 90 m2 gPt 1. The polarization curves for ORR (Fig. 3(b)) show that de-PtCu/MMC and de-PtCu/C exhibit significantly enhanced catalytic activity for ORR compared to Pt/C-JM, as indicated by positive shifts of the half-wave potentials. The kinetic current density is independent of diffusion and is widely used to evaluate the intrinsic catalytic activity of electrocatalysts.16 de-PtCu/ MMC showed excellent mass activity at 0.9 V (720 mA mgPt 1), which was 4.3 and 1.5 times as high as those of Pt/C-JM (167 mA mgPt 1) and de-PtCu/C (467 mA mgPt 1) (Fig. 3(c)), respectively. The ORR mass activity of de-PtCu/MMC was higher than those of ever reported PtCu/C catalysts (ca. 2904a to 5604d mA mgPt 1) measured under the similar conditions. de-PtCu/MMC exhibited obviously enhanced specific activity at 0.9 V (840 mA cmPt 2), compared to Pt/C-JM (190 mA cmPt 2) and de-PtCu/C (683 mA cmPt 2) (Fig. 3(d)). The larger SESA and higher specific activity of de-PtCu/MMC resulted in its outstanding mass activity for ORR. The electrochemical stability of de-PtCu/MMC, de-PtCu/C and Pt/C-JM electrodes was evaluated by an accelerated aging test (AAT) at 30 1C, i.e. by cycling the potential between 0.6–1.1 V in an O2-saturated HClO4 solution (0.1 M). After 15k cycles, the loss in the SESA was 18% for de-PtCu/MMC (from 92 to 75 m2 gPt 1 in this experiment), and 10% for PtCu/C (from 68 to 61 m2 gPt 1), while that of Pt/C-JM was 54% (from 90 to 42 m2 gPt 1). The loss in the SESA of the used catalysts was caused by the increase of the metal nanoparticle size, with dav changing from 2.9 to 4.0 nm for de-PtCu/MMC, and 1.8 to 2.9 nm for Pt/C-JM, as revealed by TEM measurements (Fig. S7, ESI†). After the aforementioned
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tests, the specific activity of de-PtCu/MMC and de-PtCu/C was 471 and 402 mA cmPt 2, respectively. The mass activity of the aged de-PtCu/MMC catalyst was 325 mA mgPt 1, which was about twice that of fresh Pt/C-JM (167 mA mgPt 1).The Cu/Pt atomic ratio of Pt–Cu NPs measured by energy dispersive X-ray spectrometry (Table S2, ESI†) decreased from 0.30 to 0.16 after the AAT, indicating that Cu leached out from Pt–Cu NPs during this test, which should be a cause of the decrease in the specific activity. In conclusion, a highly active cathode Pt–Cu bimetallic catalyst for ORR with a dav of 2.9 nm and a Cu/Pt atomic ratio of 0.30 for Pt–Cu NPs was prepared by capturing the formed Pt–Cu alloy NPs (dav = 3.8 nm) on a melem-modified carbon support, followed by electrochemically removing 90% of copper from the alloy NPs. The surface modification of a carbon support with melem was propitious for reducing the alloy particle size and for suppressing the aggregation of Pt–Cu alloy NPs during the preparation and catalytic processes. The mass catalytic activity for ORR at 0.9 V of de-PtCu/MMC reached 720 mA mgPt 1, which was higher than those of ever reported Pt–Cu/C catalysts measured under the similar conditions. The obviously enhanced mass activity over de-PtCu/MMC was derived from its high SESA and ORR specific activity due to the small Pt–Cu NPs. de-PtCu/MMC exhibited good durability, with a loss in the SESA and specific catalytic activity for ORR of 18% and 44%, respectively, after 15k potential cycles between 0.6 and 1.1 V. The degradation in catalytic activity was mainly derived from the variation of alloy composition and the increase in the Pt–Cu NP size. The preparation strategy proposed in this work is promising for the synthesis of bimetallic or multimetallic electrocatalysts with suitable alloy particle sizes and excellent catalytic properties. This work is jointly supported by NSFC (grants 51121091, 21073002 and 21133001) and the Chinese Ministry of Science and Technology (NKBRSF 2011CB 808702).
Notes and references 1 (a) S. Chen, Z. Wei, X. Qi, L. Dong, Y.-G. Guo, L. Wan, Z. Shao and L. Li, J. Am. Chem. Soc., 2012, 134, 13252; (b) X. Wang, J. Yang, H. Yin, R. Song and Z. Tang, Adv. Mater., 2013, 25, 2728; (c) A. Ishihara, S. Mitsushima, N. Kamiya and K. Ota, J. Power Sources, 2004, 126, 34; (d) J. Ma, A. Habrioux, T. Miyao, K. Kakinuma, J. Inukai, M. Watanabeb and N. Alonso-Vante, J. Mater. Chem. A, 2013, 1, 8798; (e) C. He, J. Zhang and P. Shen, J. Mater. Chem. A, 2014, 2, 3231. 2 (a) V. R. Stamenkovic, B. Folwer, B. S. Mun, G. Wang, P. N. Ross, C. A. Lucas and N. M. Markovic, Science, 2007, 315, 493; (b) L. Gan, M. Heggen, S. Rudi and P. Strasser, Nano Lett., 2012, 12, 5423. 3 (a) J.-H. Jiang, J. Kim, Y.-H. Lee, I. Y. Kim, M.-H. Park, C.-W. Yang, S.-J. Hwang and Y.-U. Kwon, Energy Environ. Sci., 2011, 4, 4947; (b) S. Chen, P. J. Ferreira, W. Sheng, N. Yabuuchi, L. F. Allard and Y. S. Horn, J. Am. Chem. Soc., 2008, 130, 13818; (c) S. Guo, D. Li, H. Zhu, S. Zhang, N. M. Markovic, V. R. Stamenkovic and S. Sun, Angew. Chem., Int. Ed., 2013, 52, 3465. 4 (a) I. Dutta, M. K. Carpenter, M. P. Balogh, J. M. Ziegelbauer, T. E. Moylan, M. H. Atwan and N. P. Irish, J. Phys. Chem. C, 2010, 114, 16309; (b) M. Oezaslan, M. Heggen and P. Strasser, J. Am. Chem. Soc., 2012, 134, 514; (c) G. Gupta, D. A. Slanac, P. Kumar, J. D. Wiggins-Camacho, X. Wang, S. Swinnea, K. L. More, S. Dai, K. J. Stevenson and K. P. Johnston, Chem. Mater., 2009, 21, 4515; (d) P. Strasser, S. Koh, T. Anniyev, J. Greeley, K. More, C. Yu, Z. Liu, S. Kaya, D. Nordlund, H. Ogasawara, M. F. Toney and A. Nilsson, Nat. Chem., 2010, 2, 454. 5 K. Kinoshita, J. Electrochem. Soc., 1990, 137, 845.
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Communication N. Toshima and Y. Wang, Langmuir, 1994, 10, 4574. Y. Wang, H. Liu and Y. Jiang, J. Chem. Soc., Chem. Commun., 1989, 1878. L. Qiu, B. Liu, Y. Peng and F. Yan, Chem. Commun., 2011, 47, 2934. C.-H. Hsu, H.-M. Wu and P.-L. Kuo, Chem. Commun., 2010, 46, 7628. ¨rgens, E. Irran, J. Senker, P. Kroll, H. Mu ¨ller and W. Schnick, (a) B. Ju J. Am. Chem. Soc., 2003, 125, 10288; (b) L. Costa and G. Camino, J. Therm. Anal., 1988, 34, 423. 11 (a) T. Komatsu and T. Nakamura, J. Mater. Chem., 2001, 11, 474; ˜ero, D. Cazorla-Amoro ´s, A. Linares-Solano, (b) E. Raymundo-Pin ¨gl, Carbon, 2002, 40, 597. J. Find, U. Wild and R. Schlo
12 (a) Z. Sheng, L. Shao, J. Chen, W. Bao, F. Wang and X. Xia, ACS Nano, 2011, 5, 4350; (b) Z. Lin, M.-K. Song, Y. Ding, M. Liu and C.-P. Wong, Phys. Chem. Chem. Phys., 2012, 14, 3381. 13 X. Li and M. R. Coleman, Carbon, 2008, 46, 1115. 14 M.-H. Thibault, B. E. G. Lucier, R. W. Schurko and F. G. Fontaine, Dalton Trans., 2009, 7701. 15 A. S-Sheppard, S. A. Campbell, J. R. Smith, G. W. Lloyd, T. R. Ralph and F. C. Walsh, Analyst, 1998, 123, 1923. 16 A. J. Bard and L. R. Faulkner, Electrochemical Methods-Fundamentals and Application, John Wiley & Sons, New York, 2nd edn, 2001, p. 341.
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13892 | Chem. Commun., 2014, 50, 13889--13892
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Pt–Cu bimetallic electrocatalysts with enhanced catalytic properties for oxygen reduction A Pt–Cu bimetallic fuel cell catalyst prepared by a new method, with an average diameter of 2.9 nm and a Cu/Pt ratio of 0.3 for Pt–Cu nanoparticles, exhibits strikingly improved catalytic properties for the electrochemical reduction of oxygen.
See Yuan Wang, Yan Liu et al., Chem. Commun., 2014, 50, 13889.
www.rsc.org/chemcomm Registered charity number: 207890
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Published on 28 July 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 08:52:54.
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Pt–Cu bimetallic electrocatalysts with enhanced catalytic properties for oxygen reduction† Chun-Mei Zhu,‡ Ang Gao,‡ Yuan Wang* and Yan Liu*
Received 1st April 2014, Accepted 25th July 2014 DOI: 10.1039/c4cc02391a www.rsc.org/chemcomm
A highly active Pt–Cu bimetallic catalyst for the electrocatalytic oxygen reduction reaction, with an average diameter of 2.9 nm and a Cu/Pt ratio of 0.30 for the bimetallic nanoparticles, was prepared by capturing Pt–Cu alloy nanoparticles on melem-modified carbon, followed by removing 90% of copper from the alloy NPs.
Highly efficient electrocatalysts for oxygen reduction reaction (ORR) are of significance for the development of polymer electrolyte membrane fuel cells (PEMFCs) and metal–air batteries, which provide electric power in a clean and convenient manner.1–4 Numerous efforts have been devoted to the development of electrochemical catalysts of carbon-supported platinum-based alloy nanoparticles (NPs) because the catalytic activity and durability of traditional Pt/C catalysts are not high enough for real applications. Pt–Ni,2 Pt–Co,3 Pt–Fe,3c Pt–Cu,4 and other platinumbased bimetallic catalysts have been proved to exhibit obviously enhanced electrocatalytic activity for ORR compared to Pt catalysts due to the changes in electronic structures and adsorption properties derived from the addition of other transition metals to Pt.2a Recently, Pt–Cu bimetallic NPs with Pt enriched shells and alloy cores have attracted much attention.4a,c,d Strasser and co-workers4d reported that such core–shell structured NPs prepared by electrochemically dealloying Pt–Cu alloy NPs supported on carbon exhibited a significant enhancement in the electrocatalytic activity for ORR compared to Pt/C catalysts, and the suitable Cu/Pt atomic ratio for the Pt–Cu alloy NPs was about 3. The enhancement in the catalytic activity was ascribed to the lattice compression due to the smaller atomic size of Cu compared to that of Pt. It was reported that Pt–Cu core–shell NPs with Beijing National Laboratory for Molecular Science, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China. E-mail: [email protected], [email protected]; Fax: +86 10 6276 5769; Tel: +86 10 6275 7497 † Electronic supplementary information (ESI) available: The experimental details and characterization of melem, MMC and Pt–Cu catalysts by IR, elemental analysis, XPS, TEM and EDX. See DOI: 10.1039/c4cc02391a ‡ Zhu and Gao contributed equally to this work.
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a Pt shell surrounding a single alloy core could be formed in the electrochemical dealloying process when the diameters of the alloy particle precursors were below 15 nm, while bimetallic NPs with complex nanostructures would form when larger alloy particles were used as starting materials.4b Although progress has been achieved in the fabrication of Pt–Cu catalysts,4 challenges remain in further improving the catalytic activity and durability of Pt–Cu catalysts. Pt–Cu alloy NPs in most of the previously reported carbon supported Pt–Cu catalysts had an average diameter (dav) larger than 5 nm with a wide size distribution, or a wide distribution of Cu/Pt ratios.4a,c,d It is reasonable to believe that the supported Pt–Cu bimetallic catalysts with properly decreased particle sizes5 and controllable Cu/Pt ratios of the alloy NPs would exhibit obviously enhanced catalytic properties for ORR due to the increase in the specific electrochemical surface area (SESA) and the possibility of optimizing the NPs’ properties. Previously, we reported a method for the preparation of PVP-protected Cu–Pt alloy colloidal nanoclusters with an average particle size of around 2 nm by reducing bimetallic hydroxide or oxide NPs with ethylene glycol (EG),6 and the coordination capture strategy for the immobilization of metal nanoclusters on supports anchoring mercaptan groups.7 Conductive nanotubes anchoring nitrogen-containing functional groups on the surfaces were used as supports to prepare promising Pt supported catalysts for the electrocatalytic oxidation of methanol.8,9 In this communication, we report a highly efficient electrocatalyst (de-PtCu/MMC) for ORR, which is composed of dealloyed Pt–Cu NPs with a dav of 2.9 nm and a melem-modified carbon support (MMC). The mass catalytic activity for ORR over de-PtCu/MMC was measured to be 720 mA mgPt 1 at 0.9 V (versus RHE), which was higher than those of ever reported Pt–Cu catalysts measured under similar conditions.4a,d Scheme 1 shows the surface modification process of a carbon support VXC-72R with melem, which was conducted by heating a mixture of melamine and VXC-72R in air at 340 1C for 1 h (see the ESI† for details). As indicated by the results of elementary analysis (see ESI†) and IR absorption spectroscopy measurements (Fig. S1, ESI†), heating melamine in air at 340 1C
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Scheme 1
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The surface modification process of VXC-72R with melem.
Fig. 2 (a and b) TEM images and size distributions of bimetallic NPs in PtCu/MMC and PtCu/C, respectively; (c) XRD patterns of PtCu/MMC and PtCu/C; (d) XPS spectra of N 1s in MMC and PtCu/MMC.
Fig. 1
XPS spectra of N 1s in melem (a) and MMC (b).
for 1 h produced melem as the main product, which was similar to the previously reported results of the thermal condensation of melamine at around 400 1C.10 The formed melem in the preparation of MMC can react with carboxyl groups (Fig. S2, ESI†) on the surface of VXC-72R to form amide groups, which was confirmed by X-ray photoelectron spectroscopy (XPS) measurements. Fig. 1 compares the XPS spectra of N 1s for MMC and melem prepared in this work. The fitting peaks at the binding energies of 398.5, 399.4 and 400.9 eV in both melem and MMC were the signals from –CQN–C,11 –NH211b,13 and graphitic N,12 respectively. The appearance of another fitting peak located at 400.1 eV in MMC suggests the formation of amide groups13 through the aforementioned reaction. The specific surface area and the nitrogen content of MMC was 110 m2 g 1 and 4.4%, respectively, as measured by the BET method and elemental analysis. MMC supported Pt–Cu alloy NPs (PtCu/MMC) were prepared by adjusting the pH value of an EG solution containing Cu(CH3COO)2H2O and H2PtCl66H2O to 10, followed by reducing the formed bimetallic hydroxide or oxide NPs with EG at 198 1C under a N2 atmosphere6 in the presence of MMC, resulting in the deposition or capture of the formed Pt–Cu alloy NPs on MMC. The nitrogen content of PtCu/MMC was 3.8% as measured by elemental analysis. For comparison, a PtCu/C catalyst was also prepared by the same process using VXC-72R instead of MMC as a support. Transmission electron microscopy (TEM) images of the products demonstrated that Pt–Cu alloy NPs in PtCu/MMC (Fig. 2(a)) had a dav of 3.8 nm, and were well dispersed on MMC, while the alloy NPs in prepared PtCu/C had a dav of 4.5 nm and tended to form more aggregates (Fig. 2(b)). As revealed in previous work, alloy NPs of Pt and Cu with different compositions possessed characteristic diffraction peak positions.6 Fig. 2(c) shows the XRD patterns of prepared
13890 | Chem. Commun., 2014, 50, 13889--13892
PtCu/MMC and PtCu/C. The diffraction peaks at 42.11, 48.51, 71.81 and 86.91 were the signals from the (111), (200), (220) and (331) crystal planes of the Pt–Cu alloy NPs with a Cu/Pt ratio of 71/29, respectively, indicating that the alloy NPs in these two catalysts have the same alloy compositions. No XRD signal corresponding to monometallic platinum or copper was observed. The Cu/Pt atomic ratios in both PtCu/MMC (Pt content: 9.1 wt%) and PtCu/C (Pt content: 9.4 wt%) were 2.45 as measured by the inductively coupled plasma atomic emission spectrometry (ICP-AES), which was in agreement with the charged Cu/Pt ratio in the preparation. The binding energy of Pt 4f7/2 and Cu 2p3/2 in PtCu/MMC was 71.3 and 932.2 eV, respectively (Fig. S3, ESI†), which was in accord with the corresponding values of prepared PtCu/C (71.2 and 932.3 eV) and previously reported results.6 The difference in the peak shape of the N 1s XPS spectrum between PtCu/MMC and MMC (Fig. 2(d)) was derived from the alcoholysis of 50% of amide groups in MMC during the preparation of PtCu/MMC, which might be derived by the coordination interaction between melem or amino groups and the surface atoms of Pt–Cu NPs14 (see the ESI† for details). Such an interaction would be propitious to the deposition of small Pt–Cu NPs on MMC and suppress the agglomeration of Pt–Cu NPs. The electrochemical dissolution of PtCu/MMC and PtCu/C was performed to selectively leach out copper from the alloy NP surfaces to produce Pt-enriched surfaces covering Pt–Cu alloy cores (Fig. S5, ESI†). The dav of the bimetallic NPs in the catalysts prepared by the electrochemical dissolution of PtCu/MMC (de-PtCu/MMC) and PtCu/C (de-PtCu/C) was 2.9 and 3.9 nm, respectively, as measured by TEM (Fig. S6, ESI†). The Cu/Pt atomic ratio of bimetallic NPs in de-PtCu/MMC and PtCu/MMC was measured by energy dispersive X-ray spectrometry to be 0.30 and 3.2, respectively (Table S2, ESI†), indicating that 90% of copper in the precursor alloy NPs had been removed through the electrochemical dissolution treatment. Fig. 3 shows the electrochemical properties of de-PtCu/ MMC, de-PtCu/C and a commercial Pt/C-JM catalyst. The SESA of the catalysts was calculated from the hydrogen desorption
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Fig. 3 Electrochemical properties of de-PtCu/MMC, de-PtCu/C, and Pt/C-JM electrodes tested in 0.1 M HClO4 aqueous solution at 30 1C. (a) CVs at a scan rate of 50 mV s 1 under N2; (b) ORR polarization curves at 1600 rpm with a scan rate of 5 mV s 1; (c) mass activities; (d) specific activities.
charges by integrating the area in the hydrogen underpotential deposition (HUPD) region of cyclic voltammograms (Fig. 3(a)), supposing that the average charge density associated with the formation of one monolayer hydrogen atom on Pt NPs and dealloyed Pt–Cu NPs with Pt shells has a value of 210 mC cm 2.4d,15 The SESA of de-PtCu/MMC was 86 m2 gPt 1, which was larger by 26% than that of de-PtCu/C (68 m2 gPt 1), due to the small particle size and high dispersion of Pt–Cu NPs in de-PtCu/MMC. The SESA of Pt/C-JM was 90 m2 gPt 1. The polarization curves for ORR (Fig. 3(b)) show that de-PtCu/MMC and de-PtCu/C exhibit significantly enhanced catalytic activity for ORR compared to Pt/C-JM, as indicated by positive shifts of the half-wave potentials. The kinetic current density is independent of diffusion and is widely used to evaluate the intrinsic catalytic activity of electrocatalysts.16 de-PtCu/ MMC showed excellent mass activity at 0.9 V (720 mA mgPt 1), which was 4.3 and 1.5 times as high as those of Pt/C-JM (167 mA mgPt 1) and de-PtCu/C (467 mA mgPt 1) (Fig. 3(c)), respectively. The ORR mass activity of de-PtCu/MMC was higher than those of ever reported PtCu/C catalysts (ca. 2904a to 5604d mA mgPt 1) measured under the similar conditions. de-PtCu/MMC exhibited obviously enhanced specific activity at 0.9 V (840 mA cmPt 2), compared to Pt/C-JM (190 mA cmPt 2) and de-PtCu/C (683 mA cmPt 2) (Fig. 3(d)). The larger SESA and higher specific activity of de-PtCu/MMC resulted in its outstanding mass activity for ORR. The electrochemical stability of de-PtCu/MMC, de-PtCu/C and Pt/C-JM electrodes was evaluated by an accelerated aging test (AAT) at 30 1C, i.e. by cycling the potential between 0.6–1.1 V in an O2-saturated HClO4 solution (0.1 M). After 15k cycles, the loss in the SESA was 18% for de-PtCu/MMC (from 92 to 75 m2 gPt 1 in this experiment), and 10% for PtCu/C (from 68 to 61 m2 gPt 1), while that of Pt/C-JM was 54% (from 90 to 42 m2 gPt 1). The loss in the SESA of the used catalysts was caused by the increase of the metal nanoparticle size, with dav changing from 2.9 to 4.0 nm for de-PtCu/MMC, and 1.8 to 2.9 nm for Pt/C-JM, as revealed by TEM measurements (Fig. S7, ESI†). After the aforementioned
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tests, the specific activity of de-PtCu/MMC and de-PtCu/C was 471 and 402 mA cmPt 2, respectively. The mass activity of the aged de-PtCu/MMC catalyst was 325 mA mgPt 1, which was about twice that of fresh Pt/C-JM (167 mA mgPt 1).The Cu/Pt atomic ratio of Pt–Cu NPs measured by energy dispersive X-ray spectrometry (Table S2, ESI†) decreased from 0.30 to 0.16 after the AAT, indicating that Cu leached out from Pt–Cu NPs during this test, which should be a cause of the decrease in the specific activity. In conclusion, a highly active cathode Pt–Cu bimetallic catalyst for ORR with a dav of 2.9 nm and a Cu/Pt atomic ratio of 0.30 for Pt–Cu NPs was prepared by capturing the formed Pt–Cu alloy NPs (dav = 3.8 nm) on a melem-modified carbon support, followed by electrochemically removing 90% of copper from the alloy NPs. The surface modification of a carbon support with melem was propitious for reducing the alloy particle size and for suppressing the aggregation of Pt–Cu alloy NPs during the preparation and catalytic processes. The mass catalytic activity for ORR at 0.9 V of de-PtCu/MMC reached 720 mA mgPt 1, which was higher than those of ever reported Pt–Cu/C catalysts measured under the similar conditions. The obviously enhanced mass activity over de-PtCu/MMC was derived from its high SESA and ORR specific activity due to the small Pt–Cu NPs. de-PtCu/MMC exhibited good durability, with a loss in the SESA and specific catalytic activity for ORR of 18% and 44%, respectively, after 15k potential cycles between 0.6 and 1.1 V. The degradation in catalytic activity was mainly derived from the variation of alloy composition and the increase in the Pt–Cu NP size. The preparation strategy proposed in this work is promising for the synthesis of bimetallic or multimetallic electrocatalysts with suitable alloy particle sizes and excellent catalytic properties. This work is jointly supported by NSFC (grants 51121091, 21073002 and 21133001) and the Chinese Ministry of Science and Technology (NKBRSF 2011CB 808702).
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