Three-dimensional highly branched Pd₃Cu alloy multipods as enhanced electrocatalysts for formic acid oxidation

Abstract

Pd₃Cu alloy multipod nanocrystals are synthesized via a facile polyol reduction method without addition of any other surfactants and additives. The Pd₃Cu alloy multipods exhibit a three-dimensional (3D) highly branched morphology which are constructed by arthrogenous branches jointed together at the center. Owing to the superior 3D multipod nanostructure, the Pd₃Cu alloy presents remarkably enhanced catalytic performance toward formic acid oxidation.

---

Introduction

Alloying noble metals such as Pt and Pd with other non-precious transition metals (e.g., Cu, Fe, Co, Ni) has been demonstrated to be an effective strategy towards the development of high-performance and low-cost fuel cells electrocatalysts.^1–6 The introduction of the second metal compositions could not only decrease the amount of noble metal required in electrocatalysts but also modify the catalytic activity and stability of noble metal catalysts. The crystallographic structure, electronic state, and surface elemental distribution of alloy nanocrystals can be finely modulated via optimizing the composition, shape and morphology in various synthetic procedures.^1–17 Recently, noble metal alloy nanocrystals with complex nanostructures, including nanoframes, branched structures, concave structures, and ultrathin structures, have been synthesized by using different synthetic methodologies.^6,10 Particularly, noble metal alloy nanocrystals with highly branched morphologies such as nanodendrites and multipods have been demonstrated with enhanced electrocatalytic properties because of their high density of active sites derived from the atomic steps, ledges and kinks on surface.^5,6,17,18

Although, several excellent works have been reported, studies that aimed at the synthesis of highly branched noble metal alloy with multipod nanostructures are still very limited.^17,19–27 In this work, we report the synthesis of three-dimensional highly branched Pd₃Cu alloy multipod nanocrystals and its catalytic properties towards electrooxidation of formic acid. The Pd₃Cu multipods were synthesized via polyol reduction process, by using a mixed solvent of ethylene glycol and water. Particularly, no surfactants and additives were required in this synthetic method. As known, noble metal nanocrystals with multipod nanostructures possess high density of active sites on surface. In this case, the obtained Pd₃Cu alloy multipods with three-dimensional highly branched structure could further ensure the maximum exposure of the catalytically active sites. In addition, the porous framework formed by Pd₃Cu multipods could remarkably enhance the electron-transfer kinetics in electrocatalysis. Electrochemical experiments showed that the as-prepared Pd₃Cu multipods catalysts possess remarkably enhanced catalytic activity toward formic acid oxidation (FAO) over the commercial Pd black catalysts.

Experimental section

Chemicals and reagents

CuCl₂·2H₂O and ethylene glycol were purchased from Sinopharm Chemical Reagent Co. Ltd. Palladium black (99.9%) was purchased from Alfa Aesar. Na₂PdCl₄ (98%) was purchased from Sigma-Aldrich. All the chemicals were used as received without further purification.

Synthesis of Pd₃Cu multipods

In a typical procedure, 40 mg of Na₂PdCl₄ and 93 mg of CuCl₂·2H₂O were dissolved in a mixed solvent containing 10 mL of water and 20 mL of ethylene glycol. The flask containing the above mixture was heated to 150 °C in an oil bath and kept at this temperature for 2 h. The products were collected by centrifugation (10 000 rpm, 10 min), and were further purified by consecutive washing/centrifugation cycles with water and ethanol for several times. The product was re-dispersed in 5 mL of hexane before further characterization.

Structure characterizations

Scanning electron microscopy (SEM) images were recorded on a Hitachi S-4800 field-emission scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were collected on a FEI Tecnai G2 F20 electron microscope operated at 200 kV. High-angle annular dark-field scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (HAADF-STEM-EDS) analysis was carried out on FEI Tecnai G2 F20 electron microscope. X-ray diffraction (XRD) patterns were recorded on a Rigaku MiniFlex-600 X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) measurements were performed with Thermo Scientific ESCALAB 250Xi X-ray Photoelectron Spectrometer. The exact composition of as-prepared alloy samples was determined by an Agilent 7500ce inductively coupled plasma-mass spectrometry (ICP-MS).

Electrochemical experiments

The electrochemical experiments were carried out with a CHI 660E electrochemical workstation (CH Instruments) at room temperature. A three-electrode cell was used that consists of a glassy carbon electrode as the working electrode, platinum wire as the counter electrode and a Ag/AgCl (3 M KCl) electrode as the reference electrode. For the preparation of catalysts inks, 5 mg of the catalysts powder was ultrasonically dispersed in 1 mL of water containing 10 μL of 5 wt% Nafion solution until a dark homogeneous dispersion was formed. Then 10 μL of the catalysts ink was dropped on a freshly polished glassy carbon electrode with a micropipette and dried at room temperature. For comparison, the electrochemical measurements of commercial Pd black (Johnson Matthey) were also carried out under the same conditions.

Results and discussion

Fig. 1a shows the representative SEM images of as-prepared Pd₃Cu multipod nanocrystals. It can be seen that the Pd₃Cu multipods are of high uniformity with size of ca. 500 nm, displaying a flower-like 3D nanostructure. The high magnification SEM image (Fig. 1b) reveals that each nanostructure is composed of many arthrogenous branches which point out in various directions from the center to form the 3D highly branched structure. The highly branched multipod nanostructure can be also clearly seen in TEM image (Fig. 1c and d). The diffraction spots of the SEAD pattern (insert of Fig. 1c) reveal the highly crystalline nature of the Pd₃Cu alloy nanocrystals. A small size Pd₃Cu multipod nanocrystal (Fig. 1e) shows that the branches are jointed together at the center. This suggests the formation of Pd₃Cu multipods might involve the growth from a common point center.²⁸ The HRTEM image (Fig. 1f) and the corresponding FFT pattern (insert of Fig. 1f) shows lattice fringes with an interplanar spacing of 0.22 nm, which was indexed to (111) planes of face-centered cubic (fcc) Pd–Cu alloy.^29–31 Furthermore, the arthrogenous branches of Pd₃Cu alloy nanocrystals exhibit a step or concave surface topology which is in rich of atomic steps, ledges and kinks.¹⁹ High-angle annular dark-field scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (HAADF-STEM-EDS) mapping was used to analyze the Pd and Cu distribution in Pd₃Cu alloy nanocrystals. As shown in Fig. 2a–c, the spatial distributions of Pd and Cu are uniform and completely overlapped. The cross-sectional compositional line profiles (Fig. 2d) of a single Pd₃Cu nanocrystal further confirmed the homogeneous distribution of the two elements. The EDX analysis shows that the atomic ratio of Pd and Cu is 76.3 and 23.7%, respectively, which was almost the same as ICP-MS data (Pd/Cu = 76 : 24).

Fig. 1 (a and b) SEM, (c–e) TEM and (f) HRTEM images of Pd₃Cu. Insert of (c): corresponding SAED pattern. Insert of (f): corresponding FFT pattern.

Fig. 2 (a) HAADF-STEM image, (b and c) EDS mapping images, (d) the cross-sectional compositional line profiles and (e) EDX spectrum of Pd₃Cu.

XRD patterns of as-prepared Pd₃Cu multipods were depicted in Fig. 3, with the commercial Pd black as a comparison. The diffraction peaks of Pd₃Cu at 2θ = 40.5, 47.0, 68.8, 82.9 and 87.6° can be indexed as the (111), (200), (220), (311) and (222) facets of face-centered cubic (fcc) Pd–Cu alloy, respectively.^22,29–31 These diffraction peaks of Pd₃Cu are located between those of pure fcc Pd (JCPDS 46-1043) and Cu (JCPDS 04-0836), indicating the formation of single-phase uniform alloy structure.^32,33 In comparison with Pd black, the diffraction peaks of as-prepared Pd₃Cu alloy upshift to higher angles with an increase in Cu content, which can be ascribed to the decreased lattice constant due to the alloying of Pd atom with smaller Cu atoms.^32,33

Fig. 3 XRD patterns of Pd₃Cu and Pd black.

The surface composition and chemical state of Pd₃Cu multipods were analyzed by XPS. XPS spectra of Pd 3d for the Pd₃Cu are shown in Fig. 4a. The Pd 3d spectra of Pd₃Cu shows the double peaks with binding energy of 340.6 and 335.4 eV, representing the Pd 3d_3/2 and 3d_5/2 electrons of metal Pd, respectively.^29–31 The XPS spectra of Pd 3d could be deconvoluted into two pairs of doublets. The most intense doublet of peaks is due to metallic Pd(0), while the weaker doublet of peaks (341.4 and 336.1 eV) could be assigned to the Pd(II) species such as PdO.^29–31 It can be seen that Pd in Pd₃Cu multipods is mostly at the chemical bonding state of Pd(0). Moreover, the Pd 3d binding energy of as-prepared Pd₃Cu multipods exhibits a small positive shift to higher binding energy with respect to that of pure polycrystalline Pd (Pd 3d_5/2 at 334.8 and Pd 3d_3/2 at 340.2 eV). The upshift of Pd 3d binding energy with increasing Cu content in Pd–Cu alloy has been commonly observed that was ascribed to the electron transfer of Pd valance electrons to Cu.³⁴ The Cu 2p spectra can be also fitted by two pairs of doublets. The principle doublet at 932.2 and 951.9 eV correspond to metallic Cu(0), suggesting that most of the Cu species were at zero-valence (Fig. 4b). The minor doublet at 933.7 and 954.1 eV and the presence of satellite signals indicates the presence of a small amount CuO species adsorbed on the alloy surfaces, due to the oxidation of metallic Cu under ambient conditions.^29–31,35

Fig. 4 XPS spectra of the deconvoluted (a) Pd 3d and (b) Cu 2p peaks for Pd₃Cu.

The electrochemistry of Pd₃Cu multipods was first investigated by cyclic voltammograms (CVs) in N₂-saturated 0.5 M H₂SO₄ solution. Fig. 5a shows the CV profiles of Pd₃Cu multipods during the initial 50 cycles. The typical hydrogen adsorption/desorption (−0.2 to 0.1 V), Pd oxidation (>0.5 V), and reduction of Pd oxide (at ∼0.5 V) peaks can be clearly observed in the CV curves.³⁶ Additionally, a small peak at 0.35 V can be ascribed to the anodic Cu stripping due to the dissolution of exposed Cu atoms from the Pd₃Cu alloy surface.^35–38 During the initial 50 cycles, the peaks in hydrogen adsorption/desorption region and Pd oxidation region sharpened and increased while the Cu stripping peak slightly decreased, suggesting a structural re-arrangement of Pd atoms at the surface.^37,39 As for the Pt(Pd)–Cu alloy catalysts, a surface dealloying process were commonly observed which could generate a noble metal rich surface to enhance the electrocatalytic activity.^37,40,41 The stable CV curve of Pd₃Cu multipods was depicted in Fig. 5b, with that of Pd black for comparison. The electrochemically active surface area (ECSA) of the catalysts was determined by integrating the coulombic charge of Pd oxide stripping peak, by assuming a charge of 424 μC cm⁻² for reduction of the PdO monolayer.⁴² The calculated ECSA value was 9.8 and 11.5 m² g_Pd⁻¹ for Pd₃Cu multipods and Pd black, respectively. The ECSA of the samples was also calculated from the hydrogen adsorption/desorption peaks (210 μC cm⁻² for monolayer adsorption of H), which was 13.1 and 20.6 m² g_Pd⁻¹ for Pd₃Cu multipods and Pd black, respectively.⁴³ The lower ECSA for the Pd₃Cu multipods is mostly likely due to its larger particle size compared with that of Pd particles (∼3 nm) in commercial Pd black. This phenomenon has been commonly observed in Pt and Pd nanostructures with large particle size.^21,44,45

Fig. 5 (a) CV curves of Pd₃Cu recorded during the initial 50 cycles; (b) the stable CV curves for ECSA calculation. Scan rate: 50 mV s⁻¹.

The FAO tests were conducted by cyclic voltammetry in 0.5 M H₂SO₄ + 0.5 M HCOOH solution with a sweep rate of 50 mV s⁻¹. Fig. 6a shows the CV curves of formic acid oxidation on Pd₃Cu multipods and Pd black catalysts. The Pd₃Cu multipods catalyst displays lower onset potential (−0.12 V for Pd₃Cu and −0.06 V for Pd black) and higher mass activity (1.6 times) than Pd black. Fig. 6b shows the CV profiles with current density normalized to the value of ECSA. The area-specific current density of formic acid oxidation on Pd₃Cu multipods is about 1.9 times larger than that on Pd black, indicating enhanced intrinsic activity of Pd₃Cu. The improved electrocatalytic activity of Pd₃Cu multipods towards FAO can be attributed to the synergistic alloying effect between Pd and Cu, as observed in other Pd-based alloy electrocatalysts.^2,43,46,47 More importantly, the highly branched multipods nanostructure with a step surface topology could provide high density of catalytically active sites derived from the atomic steps, ledges and kinks on surface, which may further contribute to the enhanced activity.^17–27 In addition, the catalytic activity of Pd₃Cu multipods for HCOOH oxidation is comparable to other reported Pd₃Cu alloy catalysts.^31,36 Fig. 6c shows the chronoamperometric curves measured at a constant potential of 0.2 V in 0.5 M H₂SO₄ + 0.5 M HCOOH solution. The Pd₃Cu multipods exhibits larger current density over the entire time examined than that of Pd black, indicating its enhanced catalytic activity and stability for FAO.

Fig. 6 (a and b) CV curves measured in 0.5 M H₂SO₄ + 0.5 M HCOOH at scan rate of 50 mV s⁻¹. (c) Chronoamperometry curves measured in 0.5 M H₂SO₄ + 0.5 M CH₃OH at applied potential of 0.2 V. (d) Nyquist plots recorded in 0.5 M H₂SO₄ + 0.1 M HCOOH in frequency range of 10⁻² to 10⁵ Hz at 0.1 V.

The 3D highly branched Pd₃Cu multipod nanostructure could promote the accessibility with reactant not only at the surfaces, but also throughout the bulk of the nanocrystals. This will facilitate the mass diffusion and electron transfer during electrocatalysis. The electron-transfer property of Pd₃Cu multipods was further investigated by electrochemical impedance spectroscopy (EIS). Fig. 6d shows the Nyquist plots of different materials in 0.5 M H₂SO₄ + 0.1 M HCOOH solution. The radius of the semicircle impedance loop at high frequency region represents electron-transfer resistance (R_ct) of the electrode materials.⁴⁸ The R_ct is approximately 100 and 140 Ω for Pd₃Cu multipods and Pd black, respectively. This result confirms that the electron-transfer kinetics for FAO is greatly facilitated on the Pd₃Cu multipods due to its advantageous 3D highly branched nanostructure.

Conclusions

In summary, Pd₃Cu alloy multipods with a 3D highly branched morphology were synthesized by a facile polyol reduction method, without addition of any other surfactants and additives. This novel nanostructure was constructed many of arthrogenous branches which were jointed together at the center. The arthrogenous branches with a step surface topology are favorable for providing high density of catalytically active sites. Moreover, the 3D highly branched morphology ensures the maximum exposure of surface active sites for electrocatalysis, which greatly promotes the efficient utilization of noble metals. The Pd₃Cu multipods catalysts demonstrated remarkably enhanced electrocatalytic activity and would be a promising candidate for wide applications in catalysis.

Acknowledgements

This work was financially supported by the Science & Technology Innovation Talents in Universities of Henan Province (No. 13HASTIT012) and the Nanhu Scholars Program for Young Scholars of XYNU.

Notes and references

1. M. Liu, R. Zhang and W. Chen, Chem. Rev., 2014, 114, 5117–5160.
2. C. J. Zhong, J. Luo, P. N. Njoki, D. Mott, B. Wanjala, R. Loukrakpam, S. Lim, L. Wang, B. Fang and Z. Xu, Energy Environ. Sci., 2008, 1, 454–466.
3. Y. Bing, H. Liu, L. Zhang, D. Ghosh and J. Zhang, Chem. Soc. Rev., 2010, 39, 2184–2202.
4. J. Gu, Y. W. Zhang and F. Tao, Chem. Soc. Rev., 2012, 41, 8050–8065.
5. H. You, S. Yang, B. Ding and H. Yang, Chem. Soc. Rev., 2013, 42, 2880–2904.
6. H. L. Liu, F. Nosheen and X. Wang, Chem. Soc. Rev., 2015, 44, 3056–3078.
7. H. Liu, R. R. Adzic and S. S. Wong, ACS Appl. Mater. Interfaces, 2015, 7, 26145–26157.
8. H. Liu, C. Koenigsmann, R. R. Adzic and S. S. Wong, ACS Catal., 2014, 4, 2544–2555.
9. B. Jiang, C. Li, V. Malgras, Y. Bando and Y. Yamauchi, Chem. Commun., 2016, 52, 1186–1189.
10. Y. Lu, Y. Jiang and W. Chen, Nanoscale, 2014, 6, 3309–3315.
11. Y. Jia, J. Su, Z. Chen, K. Tan, Q. Chen, Z. Cao, Y. Jiang, Z. Xie and L. Zheng, RSC Adv., 2015, 5, 18153–18158.
12. Q. Lv, J. Chang, W. Xing and C. Liu, RSC Adv., 2014, 4, 32997–33000.
13. Z. Zhang, C. Zhang, J. Sun, T. Kou and C. Zhao, RSC Adv., 2012, 2, 11820–11828.
14. Y. Qi, T. Bian, S. Choi, Y. Jiang, C. Jin, M. Fu, H. Zhang and D. Yang, Chem. Commun., 2014, 50, 560–562.
15. X.-J. Liu, C.-H. Cui, H.-H. Li, Y. Lei, T.-T. Zhuang, M. Sun, M. N. Arshad, H. A. Albar, T. R. Sobahi and S.-H. Yu, Chem. Sci., 2015, 6, 3038–3043.
16. J. Mao, T. Cao, Y. Chen, Y. Wu, C. Chen, Q. Peng, D. Wang and Y. Li, Chem. Commun., 2015, 51, 15406–15409.
17. K. Eid, V. Malgras, P. He, K. Wang, A. Aldalbahi, S. M. Alshehri, Y. Yamauchi and L. Wang, RSC Adv., 2015, 5, 31147–31152.
18. B. Lim and Y. Xia, Angew. Chem., Int. Ed., 2011, 50, 76–85.
19. J. Watt, S. Cheong, M. F. Toney, B. Ingham, J. Cookson, P. T. Bishop and R. D. Tilley, ACS Nano, 2010, 4, 396–402.
20. Z. Niu, D. Wang, R. Yu, Q. Peng and Y. Li, Chem. Sci., 2012, 3, 1925–1929.
21. S. Chen, H. Su, Y. Wang, W. Wu and J. Zeng, Angew. Chem., Int. Ed., 2015, 54, 108–113.
22. X. Qiu, R. Zhao, Y. Li, Y. Tang, D. Sun, S. Wei and T. Lu, RSC Adv., 2014, 4, 57144–57147.
23. B. D. Adams, G. Wu, S. Nigro and A. Chen, J. Am. Chem. Soc., 2009, 131, 6930–6931.
24. Y. Kuang, Z. Cai, Y. Zhang, D. He, X. Yan, Y. Bi, Y. Li, Z. Li and X. Sun, ACS Appl. Mater. Interfaces, 2014, 6, 17748–17752.
25. J. J. Lv, L. P. Mei, X. Weng, A. J. Wang, L. L. Chen, X. F. Liu and J. J. Feng, Nanoscale, 2015, 7, 5699–5705.
26. K. Zhang, D. Bin, B. Yang, C. Wang, F. Ren and Y. Du, Nanoscale, 2015, 7, 12445–12451.
27. Y. Jiang, T. Bian, F. Lin, H. Zhang, C. Jin, Z. Y. Li, D. Yang and Z. Zhang, J. Mater. Chem. A, 2015, 3, 21284–21289.
28. B. Y. Xia, W. T. Ng, H. B. Wu, X. Wang and X. W. Lou, Angew. Chem., Int. Ed., 2012, 51, 7213–7216.
29. J. Cai, Y. Zeng and Y. Guo, J. Power Sources, 2014, 270, 257–261.
30. J. J. Lv, S. S. Li, A. J. Wang, L. P. Mei, J. J. Feng, J. R. Chen and Z. Chen, J. Power Sources, 2014, 269, 104–110.
31. C. Xu, Y. Liu, J. Wang, H. Geng and H. Qiu, J. Power Sources, 2012, 199, 124–131.
32. Z. Yin, W. Zhou, Y. Gao, D. Ma, C. J. Kiely and X. Bao, Chem.–Eur. J., 2012, 18, 4887–4893.
33. J. Mao, Y. Liu, Z. Chen, D. Wang and Y. Li, Chem. Commun., 2014, 50, 4588–4591.
34. L. Wang, J. J. Zhai, K. Jiang, J. Q. Wang and W. B. Cai, Int. J. Hydrogen Energy, 2015, 40, 1726–1734.
35. Y. Fan, P. F. Liu, Z. W. Zhang, Y. Cui and Y. Zhang, J. Power Sources, 2015, 296, 282–289.
36. C. Xu, A. Liu, H. Qiu and Y. Liu, Electrochem. Commun., 2011, 13, 766–769.
37. S. Koh and P. Strasser, J. Am. Chem. Soc., 2007, 129, 12624–12625.
38. L. Dai and S. Zou, J. Power Sources, 2012, 199, 124–131.
39. S. Tominaka, T. Hayashi, Y. Nakamura and T. Osaka, J. Mater. Chem., 2010, 20, 7175–7182.
40. 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, 454–460.
41. L. Gan, C. Cui, M. Heggen, F. Dionigi, S. Rudi and P. Strasser, Science, 2014, 346, 1502–1506.
42. S. Hu, L. Scudiero and S. Ha, Electrochim. Acta, 2012, 83, 354–358.
43. Y. Lu and W. Chen, ACS Catal., 2012, 2, 84–90.
44. S. Sun, F. Jaouen and J. P. Dodelet, Adv. Mater., 2008, 20, 3900–3904.
45. S. Sun, G. Zhang, D. Geng, Y. Chen, M. N. Banis, R. Li, M. Cai and X. Sun, Chem.–Eur. J., 2010, 16, 829–835.
46. Y. Lu and W. Chen, J. Phys. Chem. C, 2010, 114, 21190–21200.
47. M. Liu, Y. Lu and W. Chen, Adv. Funct. Mater., 2013, 23, 1289–1296.
48. H. T. Lu, Z. J. Yang, X. Yang and Y. Fan, Electrocatalysis, 2015, 6, 255–262.

---