Uniform PdCu coated Te nanowires as efficient catalysts for electrooxidation of ethylene glycol

Journal of Colloid and Interface Science 540 (2019) 265–271

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Regular Article

Uniform PdCu coated Te nanowires as efficient catalysts for electrooxidation of ethylene glycol Liujun Jin, Hui Xu, Chunyan Chen, Tongxin Song, Cheng Wang, Yong Wang ⇑, Hongyuan Shang, Yukou Du ⇑ College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China

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a r t i c l e

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Article history: Received 8 December 2018 Revised 3 January 2019 Accepted 4 January 2019 Available online 5 January 2019 Keywords: PdCuTe nanowires Catalysts Ethylene glycol electrooxidation reaction Active sites

a b s t r a c t The renewable alcohol oxidation reaction is critical to conversion and storage of clean energy, but the design and construction of highly efficient catalysts for boosting the electrooxidation reaction, remains a grand challenge. Here, we propose a facile approach for the large-scale generation of uniform PdCuTe nanowires (NWs) by using Te NWs as the template. Impressively, as a robust integrated onedimensional (1D) anode catalyst, the as-obtained PdCuTe NWs shows high specific/mass activity of 7.9 mA cm 2 and 3872.6 mA mg 1 for the ethylene glycol (EG) oxidation reaction, being 3.4 and 4.2-fold enhancement than commercial Pd/C, respectively. Moreover, the ternary PdCuTe nanowires also display excellent stability with less activity degradation after long-term electrochemical tests. Combining physicochemical characterizations and electrochemical results, we found that the 1D Te NWs template was significant for promoting the electrocatalytic activity of PdCuTe NWs, because such nanowire template was the key, leading to the attachment of active PdCu nanoparticles which successfully exposed abundant active sites and contributed to large promotion of electrocatalytic performances. This work highlights the utilization efficiency improvement via morphology design for the promotion of electrocatalytic performances. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction The rapid depletion of fossil fuels and increasing requirement in energy are driving more researchers to develop clean and sustainable energy sources [1,2]. As a kind of renewable energy device, ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (Y. Du). https://doi.org/10.1016/j.jcis.2019.01.019 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

fuel cell has been widely concerned as an effective solution to energy and contamination problems [3,4]. Among the existing fuel cells, direct ethylene glycol fuel cells (DEGFCs) have attracted much interest from researchers owing to their low toxicity, high boiling point, high energy density and stability [5–8]. However, the lack of low-cost and high-efficiency electrocatalysts is the biggest obstacle to the development of fuel cells. Platinum (Pt), due to its excellent electrocatalytic performance, has been generally con-

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sidered as the efficient catalyst for the electrooxidation of liquid fuels [9]. Unfortunately, its super high cost and serious scarcity limit its large-scale application. To overcome these barriers, it is necessary to design and develop cost-efficient materials to substitute Pt. Update, many nanomaterials have been developed to boost the electrooxidation of liquid fuel. Among them, Pd has been generally regarded as one of the most suitable materials to replace Pt as the efficient catalyst. While, the limited active sites and particle aggregation as well as poor durability of single Pd have severely hindered their practical application in the field of electrocatalytic reaction [10,11]. In this regard, it is vital to design a novel electrocatalyst with high catalytic performances. Introducing a second transition metal into the Pd to form bimetallic materials is considered to be a promising strategy, which can not only acquire a higher utilization of Pd but also greatly improve the electrocatalytic performance of Pd-based catalyst through the synergetic effect between the two different components by modulating the electronic structure [12–14]. In addition, the d-band center of Pd could be efficiently downshifted after incorporating Pd with the 3dtransition metals, which will benefit for the promotion of electrocatalytic performances. So far, a lot of reported Pd-based catalysts have been successfully synthesized for fuel cell reactions, such as PdPb [15] and PdAg [16], both of which can display enhanced catalytic activity in comparison with single Pd. Among these materials, alloying Pd with Cu has been demonstrated to display outstanding electrocatalytic performances, which are attributed to the geometric structure and electronic effect, as well as bifunctional mechanism [17,18]. Besides, as one of the transition metals, the incorporation of Cu into Pd has also been demonstrated to show largely improved CO tolerance [19]. Apart from the catalyst materials selection, the morphology design and fabrication is also an ideal route to further modulate the electrocatalytic properties of PdCu nanomaterials [20,21]. Accordingly, a variety of nanocrystals with well-designed nanostructures have been developed, such as nanocubes [22], nanoflowers [23], and nanospheres [24]. Recently, it has been demonstrated that the self-supported one-dimensional (1D) nanowire structure is beneficial for achieving high electrocatalytic performances because of its high surface areas, large roughness factors and high active-site densities [25,26]. Furthermore, the 1D nanomaterials can also furnish channels and crystal boundaries as well as abundant open spaces for fast charge and mass transport pathways with low contact resistance [27,28]. Taking above factors into considerations, we herein propose a facile approach for the large-scale generation of uniform PdCuTe nanowires (PdCuTe NWs) as efficient liquid fuel electrocatalyst by using Te NWs as template. It is found that the newly generated 1D PdCuTe NWs exhibit outstanding activity and durability for fuel cell electrooxidation reaction in alkaline media. These excellent properties are attributed to the unique 1D nanowire template together with the modified electronic structure. More importantly, the resulting PdCuTe NWs can thus display extremely high longterm stability, showing a class of advanced electrocatalysts for liquid fuel oxidation reaction.

2. Experimental section 2.1. Synthesis of PdCu coated Te NWs Te NWs were synthesized according to a method developed by Yu’s group [29]. In the standard synthesis of PdCu coated Te NWs, 10.0 mg of Na2PdCl4, 10.0 mg of CuCl2
2H2O, 50.0 mg of tungsten carbony (W(CO)6, >99.5%), 8.0 mL of N, N-Dimethylformamide (DMF, >99%), 2.0 mL of acetic acid (CH3COOH, AC, 99.5%), and

2.0 mL of Te NWs solution were dropped into a vial. After keeping ultrasonicating for around 5 min. The mixture was then heated at 140 °C for 4 h in an oil bath. For the preparation of PdTe NWs, all the synthetic conditions are same to those of PdCuTe NWs while without the addition of CuCl2
2H2O. 2.2. Characterizations The morphology and structure details of PdCuTe and PdTe NWs were determined using the transmission electron microscopy (TEM, Hitachi, HT7700). High-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and highresolution TEM (HRTEM) were also performed by using a JEOLJEM2010 apparatus. Powder X-ray diffraction patterns (XRD) and X-ray photoelectron spectroscopy spectra (XPS) were conducted on a Bruker D8 diffractometer with Cu radiation radiation (k = 0.15406 nm) and an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Ka radiation, respectively. Energy dispersive X-ray spectroscopy (EDX) was performed on a scanning electron microscope (SEM, Hitachi, S-4700). 2.3. Electrochemical measurements A typical three-electrode system included a working electrode (glassy-carbon electrode, GCE), a reference electrode (Ag/AgCl electrode), and a counter electrode (Pt wire) was employed to perform all the electrochemical measurements. To prepare the working electrode, the PdCuTe NWs were mixed with isopropanol and water. 10 mL of the catalyst ink was dropped on a GCE. The electrochemically active surface areas (ECSAs) could be calculated based on the reduction charge of PdO in 1 M KOH solution. The cyclic voltammetry (CV) for EGOR was conducted in 1 M KOH + 1 M EG solution with a scan rate of 50 mV s 1. The long-term stabilities of catalysts were evaluated by continuous 300 cycles of CV and chronoamperometry (CA) measurement. 3. Results and discussion The syntheses of PdCu coated Te NWs were carried out through a facile one-step method as illustrated in Fig. 1a. The resulting PdCuTe NWs were initially characterized by transmission electron microscope (TEM) (Fig. 1b–d), where uniform PdCuTe NWs with the diameter of 18 nm were clearly observed. Also, a detailed observation indicated that the surface of NWs was uniformly coated with abundant nanoparticles (NPs), which was favorable for providing more surface active areas available for liquid fuels [30]. To confirm the atomic ratio of Pd/Cu/Te of the as-obtained PdCuTe NWs, the energy-dispersive X-ray spectroscopy (EDS) was conducted, where the atomic ratio of Pd/Cu/Te was calculated to be 27.3/41.3/31.4 [31]. These results reveal the successful deposition of PdCu alloy on the surface of Te NWs. The crystal structure of the PdCuTe NWs has also been revealed by XRD. As seen in Fig. S1, the six obvious diffraction peaks are corresponding to (1 0 0), (1 0 1), (1 0 2), (1 0 1), (1 1 1), and (2 0 2) facets of Te NWs, while the other peaks at 2h degree around 40, 46, and 60 are indexed to PdCu alloy phase. These results reveal the successful deposition of PdCu alloy nanoparticles on the surface of Te NWs. To further gain more information on the crystal structure of PdCuTe NWs, the high-resolution TEM (HRTEM) test has also been performed. The magnified HRTEM image displays the lattice fringes with the interplanar spacings of 0.58 and 0.38 nm (Fig. 2a), which is consistent with the (0 0 1) and (1 0 0) planes of Te [32], respectively, indicating that the as-obtained Te NWs is serving as the template. More interesting is that the lattice spacing of the NPs coated on the surface of NWs is calculated to be around

L. Jin et al. / Journal of Colloid and Interface Science 540 (2019) 265–271

Fig. 1. (a) Schematic illustration of the synthetic route for PdCuTe NWs. (b–d) Representative TEM images of PdCuTe NWs with different magnifications. (e) SEMEDS spectrum of PdCuTe NWs.

0.265 nm (Fig. 2b), which is smaller than that of standard Pd (0.285 nm), suggesting the successful construction of PdCu alloy NPs coated on the surface of NWs [33]. Fig. 2c shows the HAADFSTEM images of PdCuTe NWs, in which uniform NWs coated with porous NPs are clearly observed. On the purpose of further

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confirming the elemental compositions of such PdCuTe NWs, its element mappings have also been obtained (Fig. 2d), where Pd, Cu, and Te distribute evenly throughout the NW with slightly rich Pd and Cu on the surface, agreeing well with the HRETM analyses. The HRTEM images and STEM-EDX mappings illustrate that the surface of NWs is coated with PdCu alloy NPs, resulting in unique porous nanostructure, accompanied by the introduction of defects, which may be favorable for supplying rich surface active areas [34,35]. To confirm this hypothesis, the surface chemical compositions of PdCuTe NWs have also been investigated by X-ray photoelectron spectroscopy (XPS). Fig. 3a shows the XPS spectrum of survey scan, where the peaks at 340, 580, and 950 eV are assigned to Pd 3d, Te 3d, and Cu 2p, respectively. For the Pd 3d spectra of PdCuTe NWs, the two peaks at 335.7 and 341.6 eV are corresponding to the metallic state of Pd 3d5/2 and Pd 3d3/2, respectively, while the other two less-intensity peaks are assigned to Pd (II) [36]. More interestingly, the Pd 3d XPS spectrum of PdCuTe NWs displayed a slightly positive shift in comparison with PdTe NWs (inset in Fig. 3b), suggesting the modifications of electronic structure of PdCuTe NWs after the introduction of Cu [37]. Fig. 3c records the XPS spectrum of Cu 2p, which can be fitted into two sets of peaks. The one at 934.3 and 954.4 eV corresponds to Cu(0) 2p3/2 and Cu(0) 2p5/2 and the other one located at 951.3 eV can be assigned to Cu (II) 2p3/2, respectively. The presence of Cu(II) is ascribed to the partial oxidation of Cu since the Cu is an active metal, which can be easily oxidized in the air condition [33]. The Te 3d XPS spectrum displayed in Fig. 3d could also be well deconvoluted into four individual peaks, where the peaks at 572.8 and 583.8 eV are assigned to metal state (Te), while the other peaks at 575.6 and 586.2 eV are corresponded to the oxide state (TeOx) of Te [38]. To disclose the crucial factors in affecting the morphologies of the PdCuTe NWs, the representative TME images of Te NWs and PdTe NWs have also been obtained. As shown in Fig. 4a and b, the as-prepared Te NWs with a large length-diameter ratio, which can thus provide rich anchored sites for the deposition of PdCu NPs. Also, the PdTe NWs were prepared by employing Te NWs as the template through wet-chemical method in the mixture of DMF and acetic acid. From Fig. 4c and d, it is clearly observed that there are some nanowires with rough surface, indicating the

Fig. 2. (a and b) HRTEM and (c) HAADF-STEM images of PdCuTe NWs. (d) STEM-EDX mappings of an individual PdCuTe NWs.

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Fig. 3. XPS spectra of (a) survey scan, (b) Pd 3d, (c) Cu 2p, and (d) Te 3d in the PdCuTe NWs.

successful deposition of Pd NPs on the surface of Te NWs. This result demonstrated the significant role of CuCl2
2H2O for the successful construction of PdCuTe NWs with rough surface. Generally speaking, the electrocatalytic performances of catalysts are strongly relied on their chemical composition and structures. The as-obtained PdCuTe NWs are featured with large rough surface, trimetallic composition, and crystal boundaries, which are highly expected to an ideal anode catalyst for fuel cell reactions. In order to explore the electrocatalytic performances of such PdCuTe NWs, many electrochemical measurements have been conducted. Fig. 5a shows the CV of PdCuTe NWs, PdTe NWs, and Pd/C catalysts in 1 M KOH solution. By integrating the coulombic charge for the reduction of palladium oxide in CV curves, the electrochemical surface area (ECSA) values of three catalysts are obtained. From Fig. 5b, the ECSA value of PdCuTe NWs is 49.1 m2 gPd1, which is much bigger than PdTe NWs (30.7 m2 gPd1) and Pd/C (40.5 m2 gPd1) catalysts, originating from the unique 1D nanowire structure and abundant surface crystal boundaries [39]. The electrocatalytic activities of PdCuTe NWs, PdTe NWs, and Pd/C catalysts for the EGOR are also investigated. Pd massnormalized CV curves (the current is normalized to the Pd mass) of different catalysts indicated that the PdCuTe NWs possessed the obviously higher EGOR activity than PdTe NWs and Pd/C (Fig. 5c). For instance, the mass activity of PdCuTe NWs is 3872.6 mA mg 1, which is 4.2 and 2.2 times larger than that of Pd/C (1774.4 mA mg 1) and PdTe NWs (912.7 mA mg 1), respectively. The remarkably high mass activity of PdCuTe NWs is partly ascribed to its high ECSA. To investigate the intrinsic electrocatalytic activity, the specific activities (the current densities are normalized with ECSA) of three catalysts are also recorded in Fig. 5d, where the EGOR peak current density at PdCuTe NWs is 7.9 mA cm 2, being much higher than that of Pd/C (2.3 mA cm 2) and PdTe NWs (5.8 mA cm 2). This is due to the successful introduction of Cu, which is favorable for improving the Pd utilization

and increasing the EGOR activity. Meanwhile, the EGOR onset potential of PdCuTe NWs can also exhibit a negative shift of 35 and 65 mV than that of PdTe NWs and Pd/C (Fig. S2), illustrating that the incorporation of Cu can efficiently reduce the overpotential of EGOR. The durability is also another crucial property for catalysts. This is because the practical applications request the catalysts to meet the long-term electrochemical cycles. To this end, to study the durability of catalysts, the chronoamperometry (CA) measurements for PdCuTe NWs, PdTe NWs, and Pd/C have been performed. Obviously, the CA curves show that the EGOR current at PdCuTe NWs has slower decay rate in comparison with PdTe NWs and Pd/C catalysts (Fig. 6a), suggesting the high durability of PdCuTe NWs for some CO-like intermediates. For clear comparison, the retained mass activities of three electrocatalysts have also been summarized in Fig. 6b. As seen, there is no doubt that the PdCuTe NWs possess the highest the retained mass activity of 119.3 mA mg 1, further confirming the great promotion of longterm stability towards EGOR. To manifest the potential commercial application, the electrocatalytic stability of PdCuTe NWs, PdTe NWs, and Pd/C catalysts are further studied by long-term CV. Remarkably, the PdCuTe NWs showed the activity loss of only 35.8% after successive CV of 300 cycles (Fig. 6c and d), while the activity losses for Pd/C and PdTe NWs reached to 72.1% and 64.4%, respectively, suggesting the largely promoted long-term stability. Furthermore, the retained mass activity of PdCuTe NWs is calculated to be 2486.2 mA mg 1, which is 9.8 and 3.9 times higher than that of Pd/C (254.6 mA mg 1) and PdTe NWs (631.7 mA mg 1), respectively, further demonstrating the excellent long-term stability of PdCuTe NWs. From Table S1, we found that the prepared PdCuTe NWs also possessed higher stability when compared with some recently reported works. And the largely promoted stability is ascribed to the self-supported 1D nanowire structure. More importantly, the PdCuTe NWs can also maintain

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Fig. 4. TEM images of (a and b) Te NWs and (c and d) PdTe NWs with different magnifications.

Fig. 5. CV curves of PdCuTe NWs, PdTe NWs, and Pd/C in (a) 1 M KOH and (c) 1 M KOH + 1 M EG solution at the scan rate of 50 mV s ECSA values and (d) mass/specific activities of PdCuTe NWs, PdTe NWs, and Pd/C.

1

. The corresponding histograms of (b)

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Fig. 6. (a) CA curves of PdCuTe NWs, PdTe NWs, and Pd/C catalysts in 1 M KOH + 1 M EG solution at the potential of 0.1 V for 3600 s. (b) Histogram of retained mass activities of three catalysts after CA test. (c) Normalized current percentage versus cycles of PdCuTe NWs, PdTe NWs, and Pd/C. (d) Histogram of normalized current percentage and retained mass activities of three catalysts after continuous 300 voltage cycles.

the typical 1D nanowire structure and chemical compositions with negligible variations (Fig. S3 and Fig. S4), revealing the excellent structure stability. 4. Conclusions To summarize, we have proposed an efficient and feasible route to successfully produce large-scale PdCuTe NWs by coating the PdCu alloy NPs on the surface of Te NW template. It is found that the as-obtained PdCuTe NWs show great enhancement in electrocatalytic activity and durability toward EGOR in comparison with PdTe NWs and commercial Pd/C catalysts. This is due to the generation of PdCu alloy coated on the surface of Te NWs, because the high surface active areas, strong electronic effect between Pd and Cu and the synergistic effects originated from PdCu alloy and Te nanowires are favorable for enhancing the conductivity and facilitating the EGOR [40]. More importantly, the unique 1D nanowire structure also contributes to the active site increase in an order of magnitude, being beneficial for boosting the promotion of electrocatalytic activity and stability, outperforming many Pt-based electrocatalysts [41,42]. We believe that the innovative PdCuTe NWs with 1D morphology will be a promising anode catalyst for direct fuel cells and beyond. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51873136), the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_2499), the project of Scientific and Technologic Infrastructure of Suzhou (SZS201708), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.01.019.

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