High performance PtxEu alloys as effective electrocatalysts for ammonia electro-oxidation

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High performance PtxEu alloys as effective electrocatalysts for ammonia electro-oxidation Yumao Kang a, Wei Wang b,*, Jinmei Li a, Caiyun Hua a, Shouyuan Xue a, Ziqiang Lei a,* a

Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Key Laboratory of Gansu Polymer Materials, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China b School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China

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abstract

Article history:

Pt-rare-earth alloys are rarely studied as electrocatalysts for direct ammonia fuel cells.

Received 26 January 2017

Thus, their electrocatalytic performance of ammonia electro-oxidation is worth studying,

Received in revised form

especially for PtxEu alloys. Herein, a series of PtxEu/C (x ¼ 1, 3 and 5) electrocatalysts has

13 May 2017

been prepared using polyol reduction method. Among them, PtEu/C is found to be a

Accepted 31 May 2017

promising candidate for ammonia electro-oxidation. Physical characterizations show that

Available online xxx

PtEu/C has a small lattice parameter and narrow size distribution. X-ray photoelectron spectroscopy peak shifts of PtEu/C indicate the electron transfer from Eu to Pt. Electro-

Keywords:

chemical measurement results indicate that PtEu/C is highly active. Particularly, it has a

PtxEu alloys

higher current density, lower activity loss and apparent activation energy toward ammonia

Ammonia electro-oxidation

electro-oxidation compared to Pt/C catalyst.

Electrocatalyst

© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Direct ammonia fuel cells

Introduction Ammonia, presents a promising alternative feedstock because its low production cost, high hydrogen storage capacity and volumetric energy density [1]. It is easily liquefied at ambient temperature, which benefits to its transportation and storage [2,3]. Most importantly, its electro-oxidation at low overpotentials is free of NOx and COx (exhausting only nitrogen and water), which is not contributing to the increase in the greenhouse gas [1]. As a consequence, electro-oxidation of it has attracted increasing attention for various applications including fuel cells [4e7].

Carbon-containing fuels always generate CO impurities which are detrimental to their catalysts in fuel cells [7,8]. Therefore, interest has been turned to nitrogen-containing fuels, such as ammonia and hydrazine, which meet the requirement of clean and sustainable fuel [9]. They have rapidly advanced during the past 10 years. While, the pivotal obstacle to realize economically viable for direct ammoniaand hydrazine-based fuel cells is still their electrocatalysts. Recently, Pt alloys with rare-earth elements have received much attention as catalysts [10e13]. A number of studies demonstrate that Pt-rare-earth catalysts can exhibit higher activity compared to Pt, as well as better stability [14e17]. It is because low electronegative rare-earth elements play a key

* Corresponding authors. E-mail addresses: [email protected] (W. Wang), [email protected], [email protected] (Z. Lei). http://dx.doi.org/10.1016/j.ijhydene.2017.05.216 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kang Y, et al., High performance PtxEu alloys as effective electrocatalysts for ammonia electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.216

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role in improving their performance. Nonetheless, few studies have been focused on them for ammonia electro-oxidation [18,19], especially for Pt-rare-earth alloys. Thus, employing PtxEu alloys in direct ammonia fuel cells is an intriguing research to achieve high catalytic performance. In this study, carbon supported PtxEu alloys (PtxEu/C, x ¼ 1, 3 and 5) catalysts were prepared by polyol reduction method and used in ammonia oxidation reaction (AOR). The results of the electrochemical measurements of AOR suggest that, among a series of PtxEu/C catalysts, PtEu/C has high electrocatalytic activity and good stability. This study would provide an insight into application of Pt-rare-earth alloys in direct ammonia fuel cells.

Experimental section Preparation of catalysts The PtxEu/C (x ¼ 1, 3 and 5) catalysts were prepared using polyol reduction method [14]. In a typical preparation process, Eu2O3 and H2PtCl6$6H2O were dissolved in HCl solution (37 wt.%) by sonication. Then, 30 mL ethylene glycol (EG) was added and stirred. Sodium citrate (C6H5Na3O7$2H2O) was added into the above solution under vigorous stirring. Afterwards, 5 wt.% KOH/EG solution was added dropwise until pH value to ~8. Subsequently, 100 mg pre-treated carbon black (Vulcan XC-72R) was added and maintained under N2 atmosphere at 180  C for 4 h. Finally, the resulting solid was collected by centrifugation, washed several times with ultrapure water and dried (50  C, 10 h), denoted as PtxEu/C (x ¼ 1, 3 and 5, where x represents the atomic ratio of Pt/Eu). For comparison, Pt/C catalyst (metal loading is 20 wt.%) was also obtained using a similar procedure.

Characterization The X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku D/Max-2400 diffractometer (Japan) with a Cu Ka radiation (l ¼ 0.15406 nm), corresponding operating voltage and current maintained at 40 kV and 150 mA. X-ray photoelectron spectroscopy (XPS) data were acquired on a Kratos Axis Ultra DLD spectrometer (Japan) with a monochromated Al Ka X-ray source (hn ¼ 1486.6 eV). TEM images were acquired with a FEI TECNAI G2 F20 S-TWIN TMP highresolution transmission electron microscope (America) with Cu grids for observation. Element mapping images were obtained using a Zeiss ULTRA Plus field emission scanning electron microscope (Germany). Electrochemical measurements were conducted on a PGSTAT128N Autolab electrochemical workstation (Netherlands). A three-electrode configuration with Pt sheet (size: 10  10  0.3 mm) counter electrode, Ag/AgCl reference electrode and modified glassy-carbon working electrode (diameter: 5.0 mm) were used. Each catalyst ink was prepared by dispersing 5 mg catalyst in 1 mL Nafion/ethanol (0.25% Nafion). Then, 8 mL of catalyst ink was dropped on the glassycarbon and dried at room temperature. Before the electrochemical tests, the solution was saturated with high-purity N2 gas at least 20 min. A thermostatic water bath was used to

maintain the temperature at 25  C for the electrochemical measurements. All potentials were recorded against Ag/AgCl.

Results and discussion Experimental XRD patterns of PtEu/C, Pt3Eu/C, Pt5Eu/C and Pt/C are shown in Fig. 1. The relatively broad diffraction peaks at about 25 are corresponding to the (002) plane of carbon support [20,21]. For as-prepared catalysts, the characteristic diffraction peaks of Pt are clearly observed at about 39.8 , 46.2 , 67.5 , 81.3 and 85.7 , which are corresponding to face centered cubic (fcc) Pt (111), (200), (220), (311) and (222) plane (PDF#040802), respectively. Obviously, the 2q values of Pt (111) peak for PtEu/C, Pt3Eu/C and Pt5Eu/C are slightly shifted to higher 2q values compared to Pt/C catalyst, indicating an alloy formed between Pt and Eu [22]. No obvious peaks of Eu and Eu2O3 suggest that they may be present in an amorphous state. The average crystallite sizes and lattice parameters of as-prepared catalysts were estimated from the broadening of the Pt (200) diffraction peaks using Scherrer formula (Eq. (1)) and Bragg equation (Eq. (2)) [23,24], respectively. d¼

0:9 l b cosq

aðBraggÞ ¼

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h2 þ k2 þ l2 l 2 sin q

(1)

(2)

l is the wavelength of X-ray (1.54056
A); q is the angle of the Pt (200) peak; b is the full width at half-maximum (FWHM) of the diffraction peaks in radians; h, k, l are the lattice index parameters; d is the average crystallite size (nm); a(Bragg) is the lattice parameter. The calculated results are summarized in Table 1. It shows that the average crystallite sizes of PtEu/C, Pt3Eu/C, Pt5Eu/C are larger than that of Pt/C owing to the addition of Eu. Besides, the lattice parameters of PtEu/C, Pt3Eu/C and Pt5Eu/C catalysts are smaller than that of Pt/C catalyst. The slight decrease indicates that the presence of Eu results in a lattice

Fig. 1 e XRD patterns of the PtEu/C, Pt3Eu/C, Pt5Eu/C and Pt/C catalysts.

Please cite this article in press as: Kang Y, et al., High performance PtxEu alloys as effective electrocatalysts for ammonia electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.216

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Table 1 e XRD analysis of the PtEu/C, Pt3Eu/C, Pt5Eu/C and Pt/C catalysts. Catalysts PtEu/C Pt3Eu/C Pt5Eu/C Pt/C

Pt (111) at 2q ( )

Crystallite size (nm)

Lattice parameter (
A)

39.97 40.04 40.14 39.84

6.1 6.9 6.2 5.9

3.895 3.884 3.877 3.909

contraction upon alloying [25]. Since the electrocatalytic properties of the catalysts are correlated with the lattice distance which produces a notable effect by tuning the d-band center of nanoparticles [26]. Corollary, this contraction of the Pt lattice parameter is able to induce a positive change for electrocatalytic performance of PtxEu alloys. Fig. 2a shows cyclic voltammograms (CVs) of PtEu/C, Pt3Eu/ C, Pt5Eu/C and Pt/C catalysts in 1 M KOH solution at a scan rate of 50 mV s1. All samples exhibit a typical electrochemical behaviour of Pt-based catalysts in alkaline media. The peaks from 0.77 to 0.47 V can be attributed to the hydrogen adsorption/desorption region [27]. Also, in the positive-going sweep, the peak at about 0.11 V is due to the adsorption of OH on the catalyst surface [28]. During the negative-going sweep, the peak at about 0.17 V is related to the reduction of oxides. The electrochemical surface area (ECSA) of the asprepared catalysts can be determined using hydrogen desorption region from CVs (Fig. S1, Supporting information).

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The ECSA values are calculated by the recognized formula (Eq. (3)) [29e32]: ECSA ¼

QH 0:21  m

(3)

QH is the amount of charge exchanged during the adsorption of hydrogen atoms on the Pt surface (C); 0.21 mC cm2 is the charge required to oxidize the hydrogen monolayer; m is the platinum loading in the electrode (g cm2). Further, the geometric surface area (GSA, assuming spherical Pt nanoparticles) and the Pt utilization efficiency for PtEu/C, Pt3Eu/C, Pt5Eu/C and Pt/C catalysts can be calculated according to the following equations (Eqs. (4) and (5)) [31,33]: GSA ¼

6  103 rd

Pt utilization efficiency ð%Þ ¼

(4) ECSA  100 GSA

(5)

r is the density of Pt metal (21.4 g cm3); d is the average particle size of the nanoparticles (nm). The corresponding ECSA, GSA, and Pt utilization efficiency values of as-prepared catalysts are listed in Table 2. It is interesting to note that the PtEu/C has the largest ECSA value (10.2 m2 g1) and Pt utilization efficiency (22.1%) among the four catalysts. The high ECSA of PtEu/C may be owing to more active sites [34,35]. The CVs curves of as-prepared catalysts in 1 M KOH þ 0.1 M NH3 are shown in Fig. 2b. The distinct oxidation peaks

Fig. 2 e (a) CVs of as-prepared catalysts in 1 M KOH at 50 mV s¡1; (b) CVs of as-prepared catalysts in 1 M KOH þ 0.1 M NH3 at 50 mV s¡1; (c) LSVs of as-prepared catalysts in 1 M KOH þ 0.1 M NH3 at 20 mV s¡1; (d) CAs of as-prepared catalysts obtained at ¡0.25 V in 1 M KOH þ 0.1 M NH3. Please cite this article in press as: Kang Y, et al., High performance PtxEu alloys as effective electrocatalysts for ammonia electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.216

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Table 2 e Electrochemical properties of as-prepared electrocatalysts. Catalyst Forward scan

PtEu/C Pt3Eu/C Pt5Eu/C Pt/C

Ip/A mg1Pt

Ep/V

0.1428 0.0954 0.1171 0.0399

0.24 0.23 0.23 0.24

GSA ECSA Pt (m2 g1) (m2 g1) utilization (%)

46.0 40.6 45.2 47.5

10.2 8.2 8.0 4.1

22.1 20.1 17.7 8.7

at approximately 0.25 V are due to the electrochemical oxidation of NH3 [18]. It is obviously found that PtEu/C, Pt3Eu/C, Pt5Eu/C have lower onset potential than that of Pt/C catalyst. Furthermore, the peak current density (Ip) of PtEu/C reach the maximum value (0.1428 A mg1Pt) which is about 3.6 times higher than Pt/C catalyst. These results suggest that introducing a certain amount of Eu is possible to promote the electro-oxidation of ammonia. The high catalytic performance of PtEu/C is owing to a synergic effect between Pt and Eu. The similar representation is clearly revealed in the linear sweep voltammograms (LSVs) curves (Fig. 2c). Obviously, the LSVs curves of as-prepared catalysts almost overlap in the low potential, then the peak currents rapidly rise in the high potential. After the maximum value, the decrease of current density is due to the suspected formation of atomic nitrogen adspecies (Nads) which becomes a poison [36]. As the reaction mechanism (reactions 6-9) of Oswin and Salomon proposed, different chemisorbed species may be covered on the electrode surface [37].  NH3;ads þ OHd ads /NH2;ads þ H2 O þ de

(6)

 NH2;ads þ OHd ads /NHads þ H2 O þ de

(7)

 NHads þ OHd ads /Nads þ H2 O þ de

(8)

2Nads /N2

(9)

The electrocatalytic stability of as-prepared catalysts for AOR was studied by the chronoamperometries (CAs) in 1 M KOH þ 0.1 M NH3 solution at a fixed potential (0.25 V). As shown in Fig. 2d, current of all catalysts deactivate rapidly in the first 500 s, and then drops below zero as shown in Fig. 2d inserted. It is obviously noted that the PtEu/C and Pt3Eu/C are able to maintain current above zero after 3000 s, suggesting good stability of PtEu/C and Pt3Eu/C during ammonia electro~ o et al. reported that the deactivation oxidation. As Assumpc¸a of Pt/C is related to the poisoning on the catalyst surface by Nads [28]. Thus, high stability of PtxEu alloys indicates that the poisoning effect of the electrodes may be inhibited when Pt alloyed with Eu. The stability of PtEu/C and Pt/C catalysts was conducted by an accelerated durability test at a scan rate of 200 mV s1. As shown in Fig. 3a and b, the CVs curves are recorded every 100 cycles. Fig. 3c shows the loss of the AOR activity of PtEu/C and Pt/C catalysts after 500 cycles, it can be seen that AOR activity of PtEu/C catalyst loses only ~7.2%, whereas that of Pt/C catalyst decreases by ~21.3%. In spite of the rare-earth is

unstable against dissolution, this result clearly indicates that the PtEu/C has a higher stability than that of Pt/C catalyst. This is because a thick Pt overlayer on Pt-rare-earth alloys surface can produce an effective barrier to defer dealloying and get enhanced performance inherently [10]. Further, the effect of temperatures (T ¼ 283e313 K) for AOR was investigated in Fig. 4a and c. As it is observed, oxidation peak currents of each catalyst rise rapidly with temperature increasing, which is attributed to the improved kinetics of ammonia oxidation [38]. Fig. 4b and d display the Arrhenius plots for PtEu/C and Pt/C catalysts at various potentials (0.51 ~ 0.45 V), respectively. The good linear relationships obtained by plotting log (Ip) and 1/T, obviously, indicating that the reaction mechanism at each potential is not changed with temperature increasing [22,39]. Additionally, according to the Arrhenius law, the apparent activation energy (Ea) can be obtained using the following Arrhenius-type equation (Eq. (10)) [40,41]:     Ea 1 log Ip ¼ log A  2:3R T

(10)

A is the pre-exponential constant; T is the thermodynamic temperature (K); R is the ideal gas constant (8.314 J mol1 K1). The value of Ea was calculated to be 24.5 and 29.5 kJ mol1 for the PtEu/C and Pt/C catalysts, respectively. Compared with Pt/C, the lower Ea value of PtEu/C presents higher intrinsic activity and faster charge transfer process [20]. Meanwhile, the low Ea value also indicates that the less energy-consuming and the more conducive to reaction in theory. Obviously, PtEu/C exhibits a higher activity for AOR than that of Pt/C catalyst. As shown in Fig. 5a and d, the effect of various scan rates for AOR was investigated. It is interesting to note that the oxidation peak current rises with the scan rate increasing from 50 to 250 mV s1. Clearly, the peak potentials (Ep) slightly shift to higher potential. In addition, it can be observed that an approximate linear relationship between Ip and square roots of scan rate (v1/2) is existed as shown in Fig. 5b and e. Hence, it can assume that the electro-oxidation towards ammonia in alkaline media is diffusion controlled process [42]. This result can be further confirmed by the plots of log (Ip) vs. log (v) in Fig. 5c and f. The observation of slope (0.42 for PtEu/C) is close to the theoretical value of 0.5, which should be explained by the effect of diffusion controlled process for AOR [43]. Whereas, the slope of Pt/C (0.7) is higher than that theoretical value, likely due to the existence of adsorption in the AOR resulting from plenty of intermediate species [44]. This different is mainly attribute to the formation the PdEu alloy. Subsequently, XPS measurements were performed to illustrate the electronic structure of Pt species in PtEu/C and Pt/C catalysts. Fig. 6 depicts the Pt 4f binding energy region of two catalysts. Two doublets are displayed due to the spinorbital splitting that originates Pt 4f7/2 (lower-energy) and Pt 4f5/2 (higher-energy) bands. Pt 4f7/2 and 4f5/2 intensity ratio can be set to 4:3 and the spin-orbit coupling energy is 3.3 eV [45,46]. These bands reveal the existence of different Pt oxidation states. The expected peaks could be deconvoluted into three pairs with metallic Pt, Pt(II) and Pt(IV) [47]. As can be seen, the first pair of peaks (71.7 and 75.0 eV for PtEu/C, 71.9 and 75.2 eV for Pt/C) is assigned to metallic Pt. The second set of pair (72.6

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Fig. 3 e (a, b) CVs of Pt/C and PtEu/C catalysts in 1 M KOH þ 0.1 M NH3 for 500 cycles. Scan rate: 200 mV s¡1; (c) loss of the AOR activity of PtEu/C and Pt/C catalysts with cycles in 1 M KOH þ 0.1 M NH3.

Fig. 4 e (a, c) CVs of PtEu/C and Pt/C catalysts at different temperatures in 1 M KOH þ 0.1 M NH3 at 50 mV s¡1; (b, d) Arrhenius plots of PtEu/C and Pt/C catalysts at different potentials.

and 75.9 eV for PtEu/C, 72.6 and 75.9 eV for Pt/C) is belong to the Pt(II) chemical state as in PtO and Pt(OH)2. The third pair (74.4 and 77.7 eV for PtEu/C, 74.5 and 77.8 eV for Pt/C) can attribute to the Pt(IV) species as in PtO2 [48]. Compared with Pt/C, the PtEu/C peaks of Pt 4f7/2 and 4f5/2 are negatively shifted about 0.2 eV. This negative bond energy shift is due to the electron transfer from low electronegativity of Eu to Pt, indicating the formation of PtEu alloy again. As metallic Pt is more active than others [48]. In Table 3, obviously, it is the predominant species in PtEu/C and Pt/C catalysts (52.5 and 51.4% for PtEu/C and Pt/C, respectively). TEM observation was carried out to delineate the structure of PtEu/C catalyst. As can be seen in Fig. 7a, typical TEM

image shows that PtEu nanoparticles are distributed on carbon support. The selected area electron diffraction (SAED) pattern (inserted in Fig. 7a) reveals that the PtEu nanoparticles are polycrystalline nanocrystals with fcc structure. High-resolution TEM (HRTEM) image (inserted in Fig. 7a) of PtEu/C catalyst clearly shows that the lattice fringes are coherently extended over the entire nanoparticle. The observed lattice spacing of 0.223 nm corresponds to the (111) facets of PtEu alloy nanostructure. As shown from the TEM image, 200 nanoparticles in random regions were counted to estimate their size. A size distribution histogram depicted in Fig. 7b reflects a relatively narrow range of particle size, with an approximate average size of 7.2 ± 0.9 nm.

Please cite this article in press as: Kang Y, et al., High performance PtxEu alloys as effective electrocatalysts for ammonia electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.216

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Fig. 5 e (a, d) CVs of PtEu/C and Pt/C catalysts at various scan rates in 1 M KOH þ 0.1 M NH3; (b, e) corresponded plots of forward Ip vs. square roots of scan rate (v1/2); (c, f) plots of log (Ip) vs. log (v).

The mean particle size of PtEu/C catalyst obtained from TEM is slightly larger than that of XRD result. The main discrepancy of the average size between TEM and XRD is owing to the Scherrer formula yields a volume averaged size, to which the largest particles contribute disproportionately [49]. Further, the element mapping images were employed to analyze the elemental distribution within the nanoparticles. As shown in Fig. 7c, the PtEu/C catalyst exhibits a homogeneous distribution of Pt, Eu and C elements over the entire selected regions. This result further proves the formation of PtEu alloy, as well as XPS analysis (Fig. S2, Supporting information). Based on the aforementioned characterization results of as-prepared catalysts, briefly, three significant aspects are likely to be responsible for the high electrocatalytic activity and stability of PtEu/C: (i) the electron transfer from low electronegative Eu to Pt, which modifies electronic structure of Pt and results in a good activity for AOR [50]; (ii) the inherent inhibition of dealloying should improve the kinetic stability of

Table 3 e Binding energies and relative intensities of Pt species from curve-fitted Pt 4f XPS spectra of PtEu/C and Pt/C catalysts. Species

Binding Energy Pt 4f7/2 (eV)

Fig. 6 e Curve-fitted Pt 4f XPS spectra of PtEu/C and Pt/C catalysts.

XPS Pt Species of PtEu/C Pt 71.7 Pt(II) 72.6 Pt(IV) 74.4 XPS Pt Species of Pt/C Pt 71.9 Pt(II) 72.6 Pt(IV) 74.5

Pt 4f5/2 (eV)

Relative Concentrations (%)

75.0 75.9 77.7

52.5 23.2 24.3

75.2 75.9 77.8

51.4 21.3 27.3

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Fig. 7 e TEM and element mapping analysis of PtEu/C catalyst. (a) Typical TEM image. Inserted: the corresponded SAED pattern and HRTEM image; (b) particle size distribution histograms; (c) element mapping images.

PtEu/C catalyst [10]; (iii) similar to other Pt-based alloys [28,36], the promoting effect of Eu results in the high performance of PtEu/C catalyst.

Conclusion In summary, a polyol reduction method was introduced to prepare PtxEu/C (x ¼ 1, 3 and 5) catalysts. It is found that PtxEu/ C (x ¼ 1, 3 and 5) are promising candidates as direct ammonia fuel cell electrocatalysts. Especially for PtEu/C catalyst, it exhibits superior AOR performance with high current density, fast charge transfer kinetics, low activity loss and Ea value. The promoting effect of Eu is the main factor for its improved activity and stability. Considering the high performance of PtEu/C catalyst, it is expected that more and more Pt-rareearth alloys will be developed as promising candidates of direct ammonia fuel cells in the future.

Acknowledgements This study was funded by the National Natural Science Foundation of China (21561019), the program of Changjiang Scholars and Innovative Research Team in University (IRT_15R56), the Innovative Research Team of Gansu Province (1606RJIA324) and the Foundation of A Hundred Youth Talents Training Program of Lanzhou Jiaotong University.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.05.216.

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Please cite this article in press as: Kang Y, et al., High performance PtxEu alloys as effective electrocatalysts for ammonia electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.216

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Please cite this article in press as: Kang Y, et al., High performance PtxEu alloys as effective electrocatalysts for ammonia electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.216