Carbon supported PdSn nanocatalysts with enhanced performance for ethanol electrooxidation in alkaline medium

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Carbon supported PdSn nanocatalysts with enhanced performance for ethanol electrooxidation in alkaline medium Amir Mahmoud Makin Adam, Aimei Zhu*, Lina Ning, Min Deng, Qiugen Zhang, Qinglin Liu** Department of Chemical & Biochemical Engineering, The College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, PR China

highlights

graphical abstract


Bimetallic PdSn/f-C was successfully

synthesized

by

chemical

reduction method.
Pd1.5Sn/f-C catalyst displays the high activity and durability for alkaline EOR.
Sn is the key factor for enhancing electrocatalytic

performance

of

EOR.

article info

abstract

Article history:

In this work, simple chemical reduction method is used to prepare PdeSn nanocatalysts

Received 20 February 2019

supported on carbon for ethanol electro-oxidation in alkaline environment using ethylene

Received in revised form

glycol as reductant. The composition, structure and morphologies of PdSn/f-C catalysts are

23 May 2019

investigated by X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-

Accepted 4 June 2019

ray and transmission electron microscopy. The electro-chemical activity and durability of

Available online 29 June 2019

as-obtained PdSn/f-C nanocatalysts are investigated and determined by cyclic voltammetry and Chronoamperometric measurements. The obtained results demonstrate that as

Keywords:

prepared PdSn/f-C nanocatalysts have uniform dispersion and small particle size. In

Palladium catalysts

addition to, the as prepared PdSn/f-C nanocatalysts have higher electro-chemical activity

PdSn/f-C nanocatalysts

and better stability toward EOR in alkaline environment than those of Pd/f-C and com-

Ethanol electrooxidation

mercial Pd/C (JM) nanocatalysts. Specifically, the electro-catalytic activity of Pd1.5Sn/f-C

Chemical reduction method

nanocatalyst (3413.3 mA mg1 Pd ) is almost 8.6 times higher than Pd/C (JM) nanocatalyst

Carbon support

(355.2 mA mg1 Pd ), which has competitive power among reported PdeSn catalysts. These

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (A.M. Makin Adam), [email protected] (A. Zhu), [email protected] (L. Ning), Mind93@ 163.com (M. Deng), [email protected] (Q. Zhang), [email protected] (Q. Liu). https://doi.org/10.1016/j.ijhydene.2019.06.013 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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results indicate that the uniform carbon supported PdSn nanostructure are promising electrocatalysts for direct ethanol fuel cells. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Environmental issues caused by fossil fuel and the depletion of traditional energy sources have prompted researchers to convert their concern to substitute the traditional fuels with other cheap, clean and friendly energy sources [1e5]. Amongst the alternative energy sources, direct ethanol fuel cells (DEFCs) are one of the most promising candidates owning to their advantageous specific properties such as high energy conservation efficiency, low emissions, low temperature operation and renewable energy using [6,7]. Compared to hydrogen as a fuel, ethanol is easy to handle, store and transport [8]. However, it is very difficult for anode catalysts to obtain full ethanol electro-oxidation to carbon dioxide (CO2) because of the high energy needed for breaking (CeC) bond and the formation of intermediates species [9]. These intermediate species produced during the ethanol oxidation process, strongly adsorb on the catalyst surface and cover its effective sites [9e11]. To overcome these challenges, efforts have been made to design noble metal nanoparticles with controlled size, organized structure and composition to construct advanced electro-catalysts for direct ethanol fuel cells (DEFCs) with high electrocatalytic activity and durability. Palladium (Pd) is one of the most active noble metals holding considerable attention for addressing DEFCs challenges, as well as Platinum (Pt). What's more, the availability of Palladium on earth is much more than Pt, which is a main reason for why it's cheap and more promising for DEFCs application [12]. Pd NPs display excellent electro-chemical stability and activity for ethanol electro-oxidation (EOR) in alkaline solution but poor electro-catalytic performance in acidic medium DEFCs [13]. Nevertheless, the electrochemical efficiency, activity and stability of Pd and Pd-based nanocatalysts for EOR are yet under research investigation, because of several challenges for DEFCs applications. The development of high performance Pd-based catalysts for EOR is one of the serious solutions for solving ethanol oxidation kinetic problem. Recent advances in the progress of Pd-Based catalysts have begun to open lots of opportunities for the enhancement of ethanol electro-oxidation process. To develop high performance catalysts, different synthesis strategies have been developed. Designing a new type of catalyst with less usage of Pd metal by combining Pd with one or more transition metals such as, Sn [14e18], Ru [19e22], Ag [23e26], Ni [27,28], Pt [29e32], Cu [33,34] and Au [35e37] and adjusting their morphologies to form enhanced Pd-based catalysts. As mentioned above, ethanol oxidation process generates many kinds of intermediate species and these intermediates cover lots of Pd surface area. Therefore, the catalyst must have great ability to meet the requirements of DEFCs application in terms of CO anti poisoning. However, the bulk metals have weak ability to handle this issue. So, it is important to use

supporting material with bulk metal to enhance its performance. The carbon material is considered to be an effective support for EOR anode nanocatalysts. Carbon black XC72 has been widely used as supports for Pd-based catalysts owing to its large surface area, thermal, mechanical and chemical stability [38,39]. Besides, the presence of Carbon XC72 can also improve the electro-chemical activity and durability of Pd and Pd-based electrocatalysts. Otherwise, Carbon nanotube has also been used as effective support for Pt-based and Pd-based electrocatalysts [40e42]. Furthermore, different kinds of metal oxides, such as MnO2 [43], TiO2 [44], SnO2 [45], NiO [46] and Al2O3 [47,48], have also been used as supports for Pd-based electro-catalyst. Among all Palladium-based electro-catalyst, PdSn/C electrocatalysts have been considered as one of the most promising catalyst for DEFCs. PdSn/C and PdRuSn/C electrocatalysts were prepared by impregnation technique using NaBH4 as reductant [13]. PdSn/ C showed excellent electro-chemical activity and durability than the PdRuSn/C and PdRu/C electrocatalysts. et al. PdSnx/C nanocatalysts for EOR in alkaline media were also studied [49]. PdSnx/C nanocatalyst enhanced the electrochemical activity and chronoamperometry over Pd/C. According to the total rate law, PdSnx/C has shown better ability to work in pH between “10 to 12” and higher ethanol concentration than Pd/C nanocatalyst. In this work, we intensively reported the synthesis of PdSn/ f-C catalysts with various atomic ratios by ethylene glycol (EG) as reductant and simple chemical reduction method. PdSn electrocatalysts dispersed onto the carbon Vulcan XC72 support are studied in details to examine their electrochemical performance for EOR in an alkali media and compared with the Pd/C (JM) nanocatalyst. The electro-chemical activity and durability of PdSn/f-C catalysts were investigated by cyclic voltammograms (CVs). The results confirmed the electrocatalytic performance of PdSn/f-C catalysts can be changed by varying the Pd:Sn atomic ratio in the bi-metallic catalyst. Pd1.5Sn/f-C possessed the highest catalytic activity and stability among them. Pd1.0Sn/f-C and Pd2.0Sn/f-C catalysts exhibited a better electrocatalytic activity and stability compared to Pd/C (JM). This observation is mainly attributed to the existence of Sn and its influence on Pd active cites. Therefore, it is expected that this carbon supported PdSn catalyst holds great promise as excellent electrocatalysts for DEFCs.

Experiment section Chemicals Carbon Vulcan XC72, palladium chloride (PdCl2), tin (II) chloride (SnCl2$2H2O), sodium hydroxide (NaOH), citric acid (C6H8O7), Pd/C (JM) and ethylene glycol (C2H6O2), these

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chemicals are in analytical grade purchased from a company of (Shanghai Chemical Reagent-China), all chemicals we used without any kind of purification. Deionized water it's the solvent used for the preparation of all liquid solutions.

Functionalization of carbon support (f-C) Carbon Vulcan XC72 was used as the support in this study for PdSn/f-C catalysts. The XC-72 support was deactivated by impregnating a certain amount of carbon into a mixed solution of 0.8 M H2SO4 40 mL and 0.8 M HNO3 40 mL and mixed ultrasonically at 70  C for 6 h to oxidize the surface of carbon. The obtained carbon mixture was filtrated by a Millipore (0.45 mm) hydrophilic polycarbonate membrane with vacuum pump, washed five times by distilled water and dried in an oven at 70  C for 15 h. The functional groups on carbon was deeply confirmed by FTIR (Fig. S1).

Synthesis of as-prepared PdSn/f-C catalysts The catalysts were synthesized by simple chemical reduction method as illustrated in Fig. 1. The preparation process is as follows: a certain amount (0.1 g) of the support was dissolved in 10 mL mixture of ethylene glycol (85%) and water (15%), and the mixture solution was ultrasonicated for about 15 min. Then, 30 mL of ethylene glycol was added into the solution of metal precursors (0.09 M PdCl2 and 0.5 M SnCl2$2H2O) and 20 mM citric acid used as stabilizer, the medium was controlled by adjusting pH to 11 using NaOH solution (1 mL, 1 M), the mixture was kept at constant stirring for 3 h at 130 oC in order to oxidize the metal precursors. Carbon solution was dropwise added into the above mixture and kept stirring at 90  C for 16 h. The final solution was separated by centrifugal device and washed six times by ethanol and finally dried at 60  C for 16 h.

Characterization The morphology of PdSn/f-C and Pd/C catalysts was intensively investigated by JEM-1400 analytical transmission electron microscope (TEM) with voltage of 120 kV and scanning electron microscope (SEM) operated at potential of 15 kV. The compositions of PdSn/f-C nanocatalysts were investigated by an energy dispersive X-ray (EDX) measurement. The crystalline structures of PdSn/f-C and Pd/C (JM) nanocatalysts were determined by X-ray diffraction (XRD) patterns recorded on a Rigaku (miniflex) equipped with a Ni filter using Cu Ka radiation (l ¼ 1.54056  A) at 35 kV and 15 mA. The oxidation/ reduction states of the nanocatalysts were studied by X-ray photoelectron spectroscopy (XPS) measurement on a PHI QUANTUM 2000 XPS system with a monochromatic Al Ka source and a charge neutralizer.

Electrochemical measurements Electrocatalytic performance of PdSn/f-C nanocatalysts was evaluated by cyclic voltammetry CVs test model CHI660 purchased from Chenhua Co (Shanghai, PR China). The instrument consists of three electrode systems; these three electrodes were employed for all electrochemical measurements. The reference electrode consisted of Ag/AgCl (3 M KCl

aq.), the working electrode consisted of a glassy of carbon (GCE) with exposed surface area of about 0.072 cm2 and a tiny piece of Pt-foil used as counter electrode. To modify the surface of GCE, the electrode was rubbed three times by alumina powder of (1.0, 0.3 and 0.05 mm) on a polishing pan and washed with deionized water, followed by drying in an oven at 65  C to make it ready for use. The PdSn/f-C samples were prepared for CVs test as follows: The catalyst suspension was simply prepared by dissolving 5 mg of PdSn/f-C, Pd/f-C and Pd/C (JM) (20 wt %) nanocatalysts in 1 mL of ultrapure-water under sonication for 30 min. Then 5 mL suspension of PdSn/f-C catalysts were evenly transferred to the surface of the GCE and dried at atmosphere. Next, 15 mL of Nafion solution (0.5 wt %) was slowly added onto the GCE and dried at room temperature for CV tests. To investigate the activities of Pd1.0Sn/f-C, Pd1.5Sn/f-C, Pd2.0Sn/f-C and Pd/C (JM) nanocatalysts toward EOR, CVs were conducted. The electro-chemical CV tests worked in range of 0.8 to 0.6 V against Ag/AgCl in Nitrogenpurged 1.0 M sodium hydroxide and 1 M ethanol solution at 50 mV s1 scan rate for 50 cycles was used to obtain very stable CV Curve (see Fig. 1).

Results and discussion Morphology characterization The morphologies and composition of Pd1.0Sn/f-C, Pd1.5Sn/fC, Pd2.0Sn/f-C and Pd/C (JM) catalysts were investigated by TEM and SEM. The TEM images of Pd1.5Sn/f-C, Pd2.0Sn/f-C, Pd1.0Sn/f-C and Pd/C (JM) catalysts are presented in Fig. 2aed. The results show that all Pd and Sn NPs are dispersed very well on the Carbon support. All as prepared PdSn/f-C catalysts displayed the similar nano-network structure, and the small metal NPs are dispersed and arranged very well to construct the network. These results indicated that the existence of Sn and the functionalized carbon support (f-C) doesn't have a big change on the network morphological structure of the PdSn/fC catalysts. As we all know the main reason of using stabilizing agent (such as citric acid) is to prevent the metal catalysts from growing on each other and agglomerating. The existence of N in the functionalized carbon (f-C) and the citric acid, Pd and Sn particles would coordinate with N to make covered layer on the surface of PdSn/f-C nanocatalysts. This layer strongly inhibited the growth of the particles and therefore avoided agglomeration. Fig. 2aed confirms the formation of PdSn/f-C catalysts with uniform nanostructures and excellent dispersion. The uniform nanostructures of PdSn/f-C catalysts provide high surface areas, resulting in high electro-catalytic activity and stability. All catalysts displayed similar morphology and that indicates the atomic ratio of Pd to Sn does not show a big influence on the structure of the catalysts. At last, in the commercial Pd/C (JM) catalyst, a high number of Pd nanoparticles grew on each other and formed agglomeration as shown in Fig. 2d, which will negatively appear on the electrochemical properties of Pd/C (JM) catalysts. It is important to control the mental loading because the carbon as support always suffers from aggregation resulting in decreasing activity and stability of catalysts. Energy-

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Fig. 1 e Experimental diagram for the preparation of PdSn/f-C nanocatalysts.

Fig. 2 e Typical TEM, SEM and EDX images of (a, a1, a2) Pd1.0Sn/f-C, (b, b1, b2) Pd1.5Sn/f-C, (c, c1, c2) Pd2.0Sn/f-C and (d, d1, d2) Pd/C (JM) electrocatalysts.

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dispersive X-ray (EDX) measurement was conducted to calculate the metal percentages (wt. %) of Pd and Sn elements and carbon in PdSn/f-C catalysts and Pd/C (JM) catalyst (Fig. 2a2ed2, Fig. S2). The results show that the mental loadings are 23.16, 26.50 and 24.99 wt % corresponding to Pd1.0Sn/fC, Pd1.5Sn/f-C and Pd2.0Sn/f-C, respectively, which are near the same loading of about 25%. Based on the EDX results, the atomic ratios of Pd:Sn in Pd1.0Sn/f-C (Fig. 2a2), Pd1.5Sn/f-C (Fig. 2b2) and Pd2.0Sn/f-C (Fig. 2c2) catalysts are found to be 0.93:1, 1.35:1 and 1.75:1, respectively, which is approximated to the real atomic ratios of Pd and Sn metals before reduction. It means that most of PdCl2 and SnCl2 are reduced to Pd and Sn atoms. The results reveal that the PdSn/f-C electrocatalysts with different atomic ratios between Pd:Sn were successfully prepared. Fig. 3a shows XRD patterns of PdSn/f-C catalysts synthesized by chemical reduction method with three different Pd:Sn atomic ratios (1:1, 1.5:1 and 2:1) and Pd/C (JM) catalysts. The first peak appearing at 2q ¼ 24.72 of all the XRD patterns is associated to the (022) reflection to the hexagonal structure of carbon [13]. The remaining peaks of face-centered cubic (fcc) structure of the Pd (111, 200, 220 and 311) are located at 40 , 46.3 , 68.04 and 82 respectively. However, there are not

any diffraction characteristic peaks of Sn, because Sn exists an amorphous state. Moreover, the 2q values of as-prepared catalysts (Pd1.0Sn/f-C, Pd1.5Sn/f-C and Pd2.0Sn/f-C) slightly shifted to the left side as compared with the commercial Pd/C (JM) catalyst, indicating that the Sn NPs has been successfully presented to the Pd NPs to form PdSn/f-C bimetallic catalysts. The average crystallite size of palladium nanoparticles was calculated by Scherrer law equation [17]. The palladium peaks are applied to calculate the particle size: L¼

0:9lka1 B2q cosqmax

(1)

where L is the average particle size (nm), l is the wave length (1.4505 nm for Cu-Ka), and b2q is the peak boarding (peak width), qmax is the angel corresponding to the maximum peak and K is a constant value (0.94 to spherical crystallites). The average crystal size of Pd NPs is 3.8, 3.5 and 5.4 nm corresponding to Pd1.0Sn/f-C, Pd1.5Sn/f-C and Pd2.0Sn/f-C respectively. The presence and the oxidation state of Pd and Sn in Pd1.0Sn/f-C and Pd1.5Sn/f-C catalysts were investigated by Xray photoelectron spectroscopy (XPS). Fig. 3b XPS spectra show Pd 3d5/2 and Pd 3d3/2 peaks for Pd1.0Sn/f-C, Pd1.5Sn/f-C

Fig. 3 e XRD results of the as-prepared Pd1.0Sn/f-C, Pd1.5Sn/f-C, Pd1.5Sn/f-C and Pd/C (JM) electrocatalysts (a). XPS spectra of Pd/C (JM), Pd1.0Sn/f-C and Pd1.5Sn/f-C (b) catalysts and Pd (c) and Sn (d) of XPS spectra for Pd1.5Sn/f-C catalyst.

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and Pd/C (JM). Obviously, the peak position for Pd 3d5/2 and Pd 3d3/2 in Pd1.0Sn/f-C and Pd1.5Sn/f-C negatively shifts to that of the Pd/C (JM). The slight shift is associated to the formation of bimetallic nanostructure of Pd and Sn in Pd1.0Sn/f-C and Pd1.5Sn/f-C catalysts. Fig. 3c displayed the XPS spectrum of Pd 3d Pd1.5Sn/f-C catalyst. Actually, this diagram presents two obvious peaks. The first peak presented Pd 3d5/2 at about 336.7 eV and Pd 3d3/2 at about 342.3 eV, corresponding to Pd (0). While the remaining peaks of Pd 3d5/2 and Pd 3d3/2 appears at 336.5 eV and 342.1 eV respectively, corresponding to Pd (II). The XPS results exposed that Pd (0) is the predominant contain in Pd1.5Sn/f-C catalyst. Fig. 3d presents Sn 3d spectrum for Pd1.5Sn/f-C catalyst. The diagram presents two distinct oxidation states of Sn. The first one shows Sn 3d5/2 at 487.5 eV and Sn 3d3/2 at 496.5 eV associated to Sn (0). However, other peaks present Sn 3d5/2 at 485.5 eV and Sn 3d3/2 at 494.5 eV, corresponding to Sn (II). It is so clear that the Sn particles have some oxygenated materials at active side, which will directly drive the oxidation of the COads to CO2.

Electrochemical studies The electrochemical activities of various PdSn/f-C, Pd/f-C and Pd/C (JM) nanocatalysts were evaluated by cyclic voltammetry CVs test. Fig. 4a shows typical cyclic voltammograms (CV) of the carbon supported Pd1.0Sn/f-C, Pd1.5Sn/f-C, Pd2.0Sn/f-C and Pd/C (JM) nanocatalysts in alkaline solution of 1.0 M NaOH (saturated with N2) and the potential range from 0.8 V to 0.6 V at 50 mV s1 scan rate for all obtained electrocatalysts. As seen in Fig. 4a, a typical redox potential peak appears at (0.4 V) in the cathodic scan rate, which is associated to the ethanol electro-oxidation [50e52]. The obtained PdeSn/f-C nanocatalysts exhibited slight shift to the positive side of the peak potential compared with Pd/C nanocatalyst during the anodic scan as in Fig. 4a. The peaks feature of Pd1.0Sn/f-C (the black line), Pd1.5Sn/f-C (the red line), Pd2.0Sn/f-C (the blue line) and Pd/f-C (the pink line) are almost same as Pd/C (JM) (the green line). The potential region from 0.8 V to 0.6 V against Ag/AgCl is associated to the ads/des of hydrogen. The behaviors of the two peaks anodic and cathodic on PdeSn/f-C electrocatalysts indicate the change on Pd electronic structure after the addition of Sn. Previous studies reported that the reaction mechanism of ethanol oxidation on Pd surface in the alkaline medium can simply be explained through the following Equations (2)e(5). Pd þ CH3CH2OH / Pde(CH3CH2OH) ads

(2)

Pde(CH3CH2OH) ads þ 3OH / Pde(CH3CO) ads þ 3H2O þ 3e(3) Pde(CH3CO) ads þ PdeOHads / PdeCH3COOH þ Pd

(4)

PdeCH3COOH þ OH / Pd þ CH3COO þ H2O

(5)

The electrochemical surface area (ECSA) is calculated from the reduction region of PdO [53] as shown in Fig. 4a. The ECSA is a reflection of electrochemical activity of the catalyst. Higher ECSA was correlated highly with EOR activity to increase overall electrochemical performance. The ECSA for the Pd1.0Sn/f-C, Pd1.5Sn/f-C, Pd1.0Sn/f-C, and Pd/f-C and Pd/C

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catalysts were 54.0, 80.0, 22.2, 15.7 and 14.9 cm2 mg1 Pd , respectively, which indicated that the Pd1.5Sn/f-C catalyst exhibited the highest electrochemical activity among the synthesized catalysts (Table 1). The oxidation of ethanol on the Pd1.0Sn/f-C, Pd1.5Sn/f-C, Pd2.0Sn/f-C, Pd/f-C and commercial Pd/C (JM) nanocatalysts were determined by CVs and evaluated in the potential range from 0.8 to 0.6 V in nitrogen-saturated mixture solution of 1 M NaOH and 1 M C2H5OH at 50 mV s1 as shown in Fig. 4b. Two peaks are clearly observed in the CV voltammograms during the oxidation process. The first appears at 2.3 V in the forward sweep corresponding to direct ethanol oxidation, while the other peak appears at 4.0 V in the backward sweep associated to the removal of CO species to make complete electro-oxidation of ethanol. The forward peak current densities were found to be 2338.4, 3413.3, 2127.5, 493.8 and 355.7 mA mg1 Pd corresponding to Pd1.0Sn/f-C, Pd1.5Sn/f-C, Pd2.0Sn/fC, Pd/f-C and Pd/C (JM) catalysts, respectively. The electrocatalytic activity order is: Pd1.5Sn/f-C > Pd1.0Sn/f-C > Pd2.0Sn/fC > Pd/f-C > Pd/C (JM) (Table S2). When the ratio of Pd to Sn changes from 1:1, 1.5:1 to 2:1, the particle size has corresponding change from 3.8, 3.5e5.4 nm. Results show the catalytic activity is the best when the ratio of Pd to Sn is 1.5:1 with the smallest size of particle. These results confirm that the catalytic activity of as-obtained Pd1.0Sn/f-C, Pd1.5Sn/f-C and Pd2.0Sn/f-C catalysts is all higher than the commercial Pd/C (JM) catalyst, which is 6.6, 8.6, 5.9 times of commercial Pd/C (JM) (355.7 mA mg1 Pd ), respectively. By comparing the reduction current density peak of Palladium oxide (PdO) in each catalyst, it was observed that the amount of PdO is totally reduced to Pd atoms in Pd1.0Sn/f-C, Pd1.5Sn/f-C and Pd2.0Sn/f-C catalysts and all of the peak values of the catalysts are higher than that in commercial Pd/C (JM) catalyst. These results reflected the effect of Sn nanoparticles in the Pd1.0Sn/f-C, Pd1.5Sn/f-C and Pd2.0Sn/f-C catalysts and its influence in the electro-catalytic performance. The electrocatalytic activity and stability of PdSn/f-C are high due to the synergistic effect of bi-metallic catalyst. Sn can generate OH species at lower potentials and has strong ability to modify and enhance Pd electronic structure by donating electron to Pd sites. More Pd active sites provide better and fast ethanol oxidation and thus increasing the current density. To evaluate the consecutive sweeps of PdSn/f-C catalysts towards EOR, Fig. 4c-d shows 60 coherent sweeps of CVs of the Pd1.5Sn/f-C catalyst in 1 M NaOH and 1 M NaOH þ CH3eCH2eOH at 50 mV s1 scan rate. Fig. 4d shows the cyclic voltammograms as a function of sweep cycle between 10 and 60 cycles. It is so obvious from Fig. 4d the oxidation peak current density gradually increases with increasing sweep cycle up to 30 cycles and moves toward stable status which is due to the more surface area at the beginning of the reaction, and afterward drops at higher sweep cycle. The effect of Sn NPs in EOR is diagrammatically explained in Fig. 4f. The carbon in the Pd/C can be covered by CO species from ethanol oxidation at high potentials in alkaline media leading to a low activity. We observed that the existence of carbon can help accelerate the activity of PdSn bimetallic nanocatalysts and avoid the dissolution of Sn from the active side of the catalyst, which is due to the very strong bond between C and Sn. The catalytic activities of PdSn/f-C is better

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Fig. 4 e Cyclic votammograms of PdxSn/f-C catalysts and 60 consecutive sweeps of CVs on the Pd1.5Sn/-C catalyst measured in 1 M NaOH (a, c) and 1 M NaOH þ1 M C2H5OH (b, d); peak current density of Pd1.5Sn/f-C catalyst (e) at a scan rate of 50 mV s¡1; the scheme of ethanol electrooxidation on PdSn/f-C catalysts (f).

than the Pd/C (JM). At last, carbon supported PdSn nanoparticles catalyst is very promising for DEFCs development. The superior enhanced electro-catalytic performance of PdSn/f-C catalyst compared to the Pd/C (JM) is due to the bifunctional mechanism and legend effect [54,55]. According to the bifunctional mechanism, Sn has the ability to generate

oxygenated containing materials at very low potential than the single Pd, resulting in that CO could be oxidized completely to CO2. In the case of the ligand effect the chemical bond between CO and Pd (COePd) is weakened by enhancing Pd electronic structure. Sn has strong ability to modify and enhance Pd electronic structure by donating electron to Pd

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Table 1 e Catalytic performance of Pd/C (JM) and various PdSn/f-C catalysts for the ethanol electro-oxidation in alkaline medium. Catalysts Pd1.0Sn/f-C Pd1.5Sn/f-C Pd2.0Sn/f-C Pd/f-C Pd/C (JM)

Crystallite size ECSA (nm, from XRD) (cm2 mg1 Pd ) 3.8 3.5 5.4 e 6.8

54.0 80.0 22.2 15.7 14.9

Current density (mA mg1 Pd ) 2338.4 3413.3 2127.5 493.80 355.20

sites. Pd is transformed to PdOOH and Pd (OH)2, which have the ability to oxidize ethanol to CO2. The use of the alkaline medium in preparation of Pd-based catalysts is to eliminate the catalytic poisoning problem and improve the kinetics performance of EOR. Furthermore, the peak current density was influenced by ethanol concentration (Fig. S3). The poisoning effect of CO on the PdeSn/f-C nanocatalysts were evaluated by CVs in CO-saturated 1 M NaOH solution (Fig. 5). The onset potential of PdeSn/f-C nanocatalysts were equal 0.15 V. According to CO-striping results Pd1.5Sn/f-C and Pd1.0Sn/f-C nanocatalysts have same potential values (0.15 V), while, Pd2.0Sn/f-C and Pd/f-C are more positive, which indicated Pd1.5Sn/f-C and Pd1.0Sn/f-C exhibited

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excellent removal of intermediate species (CO adsorption) during the EOR process. The obtained results indicate that the PdeSn/f-C nanocatalysts have strong ability to inhibit the growth of CO-poisoning materials on the active side of the catalyst. The improved electronic-structure of PdeSn/f-C is the reason for enhanced CO-anti poisoning effect. The long-term electrochemical stability and durability of asprepared Pd1.0Sn/f-C, Pd1.5Sn/f-C, Pd2.0Sn/f-C, Pd/f-C and Pd/C (JM) nanocatalysts were evaluated by chronoamperometry tests (CA). The tests were done at potential of 3 V and run for 5000 s in nitrogen-saturated mixture solution of 1 M NaOH þ1 M C2H5OH. As displayed in Fig. 6a, for all nanocatalysts at the beginning of ethanol oxidation, the current density decreases quickly with time, due to the generation of poisoning materials, such as, COads species adsorbed on the surface of the catalysts. But after 2000 s, the current density is kept stable. After 5000s as shown in Fig. 6b, the current density of Pd/C (JM), Pd1.0Sn/f-C, Pd1.5Sn/f-C, Pd2.0Sn/f-C and Pd/f-C nanocatalysts are 521.6, 931.7, 202.5, 61.8 and 194.4 mA mg1 Pd , respectively. This corresponds to the order; Pd/C (JM) < Pd/f-C < Pd2.0Sn/fC < Pd1.5Sn/f-C < Pd1.0Sn/f-C, suggesting that the as-prepared Pd1.5Sn/f-C and Pd1.0Sn/f-C catalysts exhibit best electrochemical stability toward ethanol electrooxidation. Meanwhile, Pd/C (JM) and Pd/f-C catalysts show the lowest current density

Fig. 5 e Cyclic votammograms of CO-stripping on PdSn/f-C catalysts in 1 M NaOH solution.

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Fig. 6 e Chronoamperograms of; (a) Pd1.0Sn/f-C, Pd1.5Sn/f-C, Pd2.0Sn/f-C, Pd/f-C and Pd/C (JM) catalysts in 1 M C2H5OH/1.0 M NaOH solution at 50 mV s¡1scan rate; (b) Current of PdSn/f-C catalysts after 5000 s.

(194.4, 61.8 mA mg1 Pd ) amongst all catalysts, respectively, which could be attributed to poisoning of Pd active sites by intermediate species. It can be concluded that increasing Pd decreases the electro-chemical activity and stability of the nanocatalysts. The excellent catalytic performance of as-obtained Pd1.5Sn/f-C and Pd1.0Sn/f-C electrocatalysts are mainly attributed to the presence of Sn NPs and Carbon support. This is due to the excellent distribution of the metal nanocatalysts on the carbon that promote proper expulsion of the poisoning materials (CO) from the surface of Pd. At the same time, PdSn/f-C displays the excellent catalytic activity, which is much bigger than most of the current reports (Table S1).

Conclusions In this paper, we have developed PdSn/f-C catalysts with various mole ratio by simple chemical reduction method. Ethylene glycol (C2H6O2) was used as reducing agent and functionalized carbon Vulcan XC 72 was used as supported material. Citric acid plays an important role to stabilize and make better dispersion. The as-obtained PdSn/f-C nanocatalysts showed network-structure, uniform dispersion and very small particle size. The electrochemical properties of PdSn/f-C nanocatalysts were evaluated comprehensively by electrochemical measurements (CVs) and compared with Pd/ C (JM). The results show that the as-prepared PdSn/f-C electrocatalysts displayed excellent electrocatalytic activity and durability toward electrooxidation of ethanol in alkali media compared with Pd/C (JM) nanocatalysts. The enhancements in catalytic performance of PdSn/f-C catalysts can be associated to the good effect of carbon support and the existence of Sn in catalysts. The excellent electrocatalytic performance of the PdSn/f-C catalyst will make it one of the most promising candidate nanocatalysts for low temperature DEFCs.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21576226), the Natural Science Foundation of Fujian Province of China (No. 2017J01022), and Chinese Government Scholarship Council (No.20161528314).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.06.013.

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