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Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation
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Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation Hui Xu, Bo Yan, Ke Zhang, Caiqin Wang, Jiatai Zhong, Shumin Li, Ping Yang**, Yukou Du* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
article info
abstract
Article history:
Liquid fuel cells have attracted broad research interests for past several decades, especially
Received 29 December 2016
for direct methanol fuel cells (DMFCs) because of their compact volume, environmentally
Received in revised form
benign and easy storage. Exploring cost-effective electrocatalysts toward methanol elec-
11 February 2017
trooxidation is meaningful for the development of (DMFCs). Herein, a series of PdRu/P
Accepted 5 March 2017
network catalysts have been fabricated and modified via a facile and reproducible method
Available online xxx
taking benzyl alcohol, hydrazine hydrate as solvent and reducing agents, respectively. Profiting from the 3D network structure, the synergistic effect together with the increased
Keywords:
electron mobility induced by the addition of nonmetal phosphorous (P). The PdRu/P cata-
Pd-Ru-P network
lysts display markedly improved efficient electrocatalytic activity with excellent current
Direct methanol fuel cells
peak, more negative onset potential, as well as superior long-term stability compared to
Electrocatalytic oxidation
commercial Pd/C, PdRu and Pd/P prepared under the same condition. In this work, we
Nonmetal phosphorous
highlight the effect of the incorporation of nonmetals P on the electrocatalytic performance of PdRu binary catalysts, which will contribute to broadening the application of nonmetal P or even for other nonmetals for electrooxidation. Our efforts will dedicate to accelerating the commercialization of efficient and stable anode catalysts in fuel cells by means of doping transition metals or nonmetals into Pd. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The society is now experiencing tough challenge in the replacement of the traditional fossil fuels with new resources. For the past few decades, DMFCs have emerged as promising green energy devices for the conversion of chemical energy into electricity owing to their special properties, such as high power density and environmental friendliness [1,2]. In spite of
these beneficial terms, the shortage of cost-effective catalysts for anodic oxidation reactions remains an obstacle for the development of DMFCs [3]. Therefore, the innovations and decorations of dramatically active catalysts towards methanol electrooxidation are urgently needed [1,4]. Among those catalysts, Pt and Pt-based composites are still thought to be efficient materials for the current catalyst technology [1,5,6]. However, the drawbacks of both skyrocketing prices and low poisoned-tolerance are two key factors impeding the
* Corresponding author. Fax: þ86 512 65880089. ** Corresponding author. Fax: þ86 512 65880089. E-mail address: [email protected] (Y. Du). http://dx.doi.org/10.1016/j.ijhydene.2017.03.023 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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commercial application of Pt and Pt-based composites for electrooxidation of methanol [7,8]. Compared with Pt-based catalysts, less expensive and widely available Pd has been exploited as a substitute in DMFCs due to its outstanding activity and durability in the alkaline medium [9,10]. However, it has been proved that lots of problems for Pd catalysts still unsolved, including poor antioxidant ability, strong environmental dependence and easily surface poisoned [11], which all resulted in the decrease of a large number of surface active sites and ultimate led to the sluggish kinetics and poor stability [12]. For overcoming these matters, several strategies have been proposed and prior literature have been reported for improving the electrocatalytic performances by alloying Pd with transition metals to form binary, ternary metals alloy such as Pd-Ag [13], Pd-Ru [14], Pd-Au [15,16], Pd-Ni [17] as well as embedding a broad range of nonmetals [18,19]. Ruthenium (Ru) is an important element used as cocatalysts, which can interact with hydroxyl to form a Ru-OH in the solution and serve as an electroneproton conductor for accelerating the oxidation of carbonaceous intermediates, as well as fully activating the CeH bonds [7,8]. Since its excellent properties for enhancing catalytic activity, Ru has been widely used as a promising candidate for enhancing the electrocatalytic performance toward the alcohols. Therefore, the fabrication of PdRu is deemed to be an effective way to highly enhance the electrocatalytic performances of Pd-based catalysts for methanol electrooxidation. Additionally, phosphorus is an inexpensive nonmetal element showing many similar properties as nitrogen, which has large crustal reserves and affluent valence electrons [20]. Many studies have investigated the effect of P on the efficient electron transfer along with ligand effect during the process of catalytic reaction. Additionally, doping P into Pd-based alloys can not only enhance the chemical stability of alloys in alkaline media but also adjust the electronic state of Pd [21,22]. Up till now, many reports on doped nonmetals catalysts have also been published, such as Pd-N [23], Pd-P [24], Pd-B [25], Pd-Ni-P [21,26], all of which reveal that the catalytic performances of doped catalysts are much more excellent than the corresponding binary or ternary catalysts. Notably, in comparison with previously investigated catalysts, these catalysts doped with nonmetals apparently differed in many aspects such as the unique composition and electron transportation, both of which are beneficial for the enhancement of electrocatalytic performances [27]. It was demonstrated that morphology and structure could significantly affect the catalytic activity and durability due to its enormous influences on the surface active areas, particle dimensions and particle configurations [28]. Till now, various nanostructures have been fabricated, such as nanocubes [29], nanodendrites [30], and nanorings [13], these special structures all play significant roles in characteristics of catalysts. Particularly, Pd-based networks were deemed to be potential catalysts due to its peculiar connection of nanoparticle block and net-like property, which can not only maintain the high activity of nanoparticles, but also act as self-supported material [31e33]. Moreover, both active area and electron mobility are increased in the reaction [34]. Furthermore, because of the network structure, more active sites on the surface of catalysts are available for organic molecules [35,36].
However, as is known to all, binary PdRu catalysts incorporated with phosphorus have been rarely reported. After careful consideration, we herein report a facile wet-chemical strategy to synthesize PdRu/P nanoparticle networks with tunable composition by reducing H2PdCl4, RuCl3 and NaH2PO2 precursors with hydrazine hydrate in the presence of PVP, and benzyl alcohol. Compared with PdRu and Pd/P catalysts prepared under the same conditions, the obtained PdRu/P networks exhibit definite enhancement in catalytic activity and long-term stability. Compared to traditional commercial Pd/ C, the PdRu/P NNs also display a far longer service life and higher working efficiency together with the superior durability, suggesting that it should be looked forward to serving as a promising catalyst for DMFCs and beyond.
Experimental Materials and reagents Ruthenium chloride (RuCl3), hydrochloric acid (HCl), palladium chloride (PdCl2, 99%), benzyl alcohol, sodium hypophosphite (NaH2PO2), hydrazine hydrate (N2H4$H2O), polyvinyl pyrrolidone (PVP, K35), CH3OH, C2H5OH and potassium hydrate (KOH) were all purchased from Sinopharm Chemical Reagent Co., Ltd.,China. All chemicals were of analytical grade and used as received without further purification. Doubly distill water (18.3 MU) was used for washing and preparing solution.
Apparatus The morphology and structure of obtained products were examined by scanning electron microscope (SEM, S-4700, Japan), as well as transmission electron microscopy (TEM) which were measured using a Tecnai G220 (FEI America) operated at the voltage of 200 kV. The samples were prepared by dropping 10 ml the diluted dispersion solution on the surface of copper grid coated with amorphous carbon. The elements compositions of the as prepared catalysts were accurately determined by energy dispersive X-ray spectrometer (EDX, S4700, Japan). XRD (X0 Pert-ProMPD, PANalytical Company) was used to determine the crystal nanostructure by using Cu Ka as the radiation source (l ¼ 1.54056 A) and operated at 30 mA and 40 kV. X-ray photoelectron spectra (XPS) were performed on a Thermo Scientific ESCALab 250 Xi using 200 W monochromated Al Ka radiate. All electrochemical measurements were carried out with a CHI 760B electrochemical workstation by using a typical three-electrode test cell, which included a glassy carbon electrode (GCE, diameter: 3.0 mm), a saturated calomel electrode (SCE) and a platinum wire, acting as the working electrode, the reference electrode and the counter electrode, respectively. The GCE surface was polished with alumina slurry, followed by rinsing and sonicating with double-distilled water and ethanol bath for 10 min before using it.
Synthesis of PdRu/P catalysts The targeted products were prepared through the simple wetchemical method including the following steps: first of all,
Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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0.2 g of PdCl2 powder were dissolved in 25 mL of 90.2 mM HCl solution and diluting the solution to 50 mL taking a volumetric flask to prepare a 22.6 mM H2PdCl4 solution. Next, 100 mg of PVP was completely dissolved into 8 ml of benzyl alcohol in a flask, followed by adding 0.40 mL RuCl3 (19.3 mM) and 1.0 ml previously prepared H2PdCl4 (22.6 mM). The mixture was heated from room temperature to 353.15 K with rapid magnetic stirring to form a homogeneous solution. Subsequently, an aqueous solution of NaH2PO2 (10 mM, 1.3 ml) were added followed by the addition of N2H4$H2O (1 M, 1 mL), the flask was kept under stirring in an oil bath for an hour for reacting completely. Finally, the solid suspension was obtained by centrifugation, washing with water, ethanol and acetone twice, and re-dispersed in 10 mL of distilled water to form a uniform catalyst inks, the obtained products were denoted as Pd3Ru1P1.5. For comparison, Pd3Ru1P1, Pd3Ru1P2.5, Pd3Ru1P3 catalysts were synthesized under the same conditions by changing the amount of NaH2PO2. Different ratios of PdRu and Pd/P were prepared for further comparison in the same way. The specific reactions and synthetic procedures were well illustrated in Scheme 1.
Electrochemical characterization The relative electrocatalytic performances of relative PdRu/P catalysts were studied by making cyclic voltammogram (CV) measurements for electrooxidation of methanol. For the typical preparation of PdRu/P electrode, 10 mL of catalyst inks were added on the surface of pre-polished GCE and dried in the oven at 333.15 K. Then, 3.0 ml of 5 wt% nafion solution was laid on the electrode and dried again at 333.15 K for further attaching the catalysts ink to the surface of GCE tightly to avoid the dissolution of catalysts inks. The loading mass of Pd for all catalysts is 2.4 mg. As a comparison, commercial Pd/C/ GCE, pure Pd/GCE, different ratios of Pd/P/GCE and PdRu/GCE were prepared under the same conditions. For activating the catalyst, the modified GCEs were tested in 1 M KOH solution by CVs from 0.7 V to 0.3 V (vs. SCE) at a scan rate of 50 mV s1 for obtaining steady CVs, after the activating process, the active sites on the surface of catalysts were more available for methanol molecular. CVs of all the catalysts were performed in a solution contained 1 M KOH and 1 M CH3OH, and chronoamperometric (CA) measurements were conducted at 0.20 V for 3600 s for exploring the durability of the as-
3
prepared catalysts. For further investigating the long term stability, massive CVs versus cycle numbers were also tested.
Results and discussion Characterization of PdRu/P nanoworks The network structures of PdRu/P nanocomposites were prepared through wet-chemical method by taking hydrazine hydrate as reducing agent, sodium hypophosphite act as reducing agents and phosphorus introducing reagent (see the Experimental Section for details), respectively. The structure and morphology analyses for the network structure PdRu/P (3:1:1.5) nanoparticles are shown in Fig. 1. A typical lowmagnification SEM image of the Pd3Ru1/P1.5 catalysts apparently indicates that three-dimensional (3D) porous networklike structures are synthesized on a large scale. Notably, another two types of PdRu and Pd/P nanocatalysts show similar SEM images as the Pd3Ru1/P1.5 catalysts, illustrating that this method can well be well applied to synthesize network particles with high yield [17]. From the lowmagnification SEM, many irregular nanoparticles can be obviously observed. More surface active sites are available to small organic molecules attributed to the existence of threedimensional (3D) porous network structure. Energy dispersive X-ray (EDX) spectroscopy (Fig. 1(d)) indicates that the prepared network nanoparticles are composed of Pd, Ru and P with the atomic ratio approximately 8:1:1, which is not in accordance with that of the expected (6:2:3), this phenomenon may be accounted by the phosphates formed by the reaction between partial P and Ru. Transmission electron microscopy (TEM) is also used for further studying structural features. As shown in Fig. 2(c) and (d) and Fig. S1, it can clearly see that some nanoparticles are connected with each other to form 3D network structure with abundant exposed surface active areas, which are favorable for providing more surface active sites for many organic molecules. The formed network structure may be attributed to the addition of hydrazine hydrate (N2H4$H2O), which is beneficial for the connections of PdRu/P nanoparticles. The network structure of other PdRu/P catalysts can also be obviously observed with the addition of hydrazine hydrate in Fig. 2, illustrating that the network structure can hardly be deteriorated regardless of the change
Scheme 1 e Illustration of the preparation of the PdRu/P catalysts. Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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Fig. 1 e SEM images of as-prepared PdRu (a), Pd/P (b), PdRu/P (c) and EDX spectrum of PdRu/P, (d) nanoparticle networks.
of PdRu/P atomic ratios, and this method can be widely applied to prepare PdRu/P network structure nanoparticles at a large scale [37]. In addition, for further investigating the complicated reaction mechanism, a series of comparative experiments were carried out. From Fig. S2, we could easily find that monodispersed Pd nanoparticles were obtained and no network-like structure formed without the introduction of hydrazine hydrate, and the hypophosphite can react with previously formed Hþ and H to generate elemental phosphorus ultimately lead to the formation of PdRu/P alloy [38]. It is generally known that Pd2þ and Ru3þ can be reduced to Pd and Ru metallic particles by both of hypophosphite and hydrazine hydrate. Therefore, hypophosphite not only acts as reducing agent but also phosphorus introducing reagent [37], and corresponding reaction mechanism are shown as follow
Pd2þ þ H2PO 2 / Pd þ H2PO3 þ H2O
(1)
þ H2PO 2 þ H þ H / 2H2O þ P
(2)
þ H2PO / 2H2O þ 1/2H2 þ P 2 þ 2H þ H
(3)
nM þ P / MnP
(4)
The crystalline phases of Pd3Ru1/P1.5, Pd3Ru1/P1, Pd3Ru1/P2.5, Pd3Ru1/P3 and PdRu were investigated by X-ray diffraction (XRD). As shown in Fig. 3, it is clearly to observe that all of PdRu/ P and PdRu nanoparticle networks display a diffraction peak of face centered cubic crystal structure. The featured four diffraction peaks of PdRu presented at 2q values of 40.03, 46.48, 68.39 and 81.92 are corresponding to the (111), (200), (220), (311) crystalline planes [39]. In comparison of pure Pd, the positively slight shift of 2q values, illustrating the reduction of lattice parameter, which can be assigned to the formation of PdRu alloy [40]. However, the diffraction peak of PdRu/P shifted to lower degree accompanied with the increase of the phosphorus content. The half-peak width of the diffraction peak decreased in the following order: Pd3Ru1P3 >Pd3Ru1P2.5 > Pd3Ru1P1.5 > Pd3Ru1P1, which implied that the addition of phosphorus was favorable for reducing the particle diameter, and led to more exposed surface active areas [41] [42].
Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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Fig. 2 e TEM images of Pd3Ru1 (a), Pd2/P1 (b), and the Pd3Ru1/P1.5 (c and d) catalyst with different magnifications. X-ray photoelectron spectroscopy (XPS) was employed to further investigate the surface properties and compositions of PdRu/P catalysts. In Fig. 4(a), except for Pd, Ru, P, other three types of elements were also probed, and the appearance of C, O, N may be attributed to the residuum of surfactant agents or solvent [37,43]. Since the Ru 3d peak was overlapped with C1s peak, thus for fully exploring the composition of Ru element on the catalyst surface, we found that the Ru 3p peak could be observed at the binding energies of 462.7 and 484.9 eV, corresponding to the Ru 3p1/2 and Ru 3p 3/2 binding energies. Some oxidation products of Ru (IV) could also be confirmed by the presence of a pair of peaks located at the binding energies
Fig. 3 e XRD patterns of PdRu, Pd3Ru1P1, Pd3Ru1P1.5, Pd3Ru1P2.5 and Pd3Ru1P3 catalysts.
of 466.3 and 487.8 eV. In Fig. 4(b), couples of typical peaks observed at about 334.7 eV (Pd 3d 5/2), 341.4 eV (3d 3/2), 338.1 eV (Pd 3d 5/2), as well as 342.7 eV (Pd 3d 3/2), corresponding to elemental Pd and Pd (II), respectively. Fig. 4(d) shows the P 2p spectrum of PdRu/P catalysts, a pair of peaks appeared at 129.7 and 133.3 eV were referred to the two environments for phosphorous atoms of elemental phosphorus and oxidized phosphorus (V), respectively [35,44]. The electrocatalytic activity for MOR in the alkaline of the PdRu/P catalysts was examined by CVs at the potential range from 0.8 to 0.4 V in 1.0 M KOH at a scan rate of 50 mV s1. For comparison, different molar ratios of as-prepared PdRu and Pd/P catalysts were also characterized. Fig. 5(a) shows the CVs of the seven types of catalysts conducted in a 1.0 M KOH aqueous solution at a scan rate of 50 mV s1. Basing on the previous research, it is easy for Pd to absorb OH to form PdeOHads ((Eq. (7)) and then further react with OH(Eq. (6)), after that, the Pd-O species continue to come into being with the increase of anode potential [45], the peak corresponding to the oxidation of Pd is featureless [46]. On the contrary, the reduction peak of PdeO species (Eq. (8)) appear at ca. 0.36 V in the cathode scan is much higher than those oxidation peaks [47].
Pd þ H2O þ e / (PdeHads) þ OH
(5)
Pd þ OH / PdeOHads þ e
(6)
Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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Fig. 4 e XPS spectra of PdRu/P (a), Pd 3d (b), Ru 3p (c) and P 2p (d).
Fig. 5 e Electrocatalytic properties of the as-pre pared catalysts and commercial Pd/C catalysts in 1 M KOH (a) CV, (b) onset potentialecurrent curves in 1 M KOH and 1 M CH3OH, scan rate: 50 mV s¡1. Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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PdeOHads þ OH / PdeO þ H2O þ e
(7)
PdeO þ H2O þ 2e / Pd þ 2OH
(8)
The electrochemically active surface area (ECSA) of the catalysts can be calculated from integral value of the areas of the reduction peak of PdeO species with the following equation (Eq. (9)) ECSA ¼
Q 0:405m
(9)
where m represents the mass of Pd (2.4 mg) attached to the surfaces of GCE, the value of Q was related to the reduction charge of PdO and 0.405 mC cm2 is a constant assuming that a monolayer of PdO was reduced on the Pd surface. As is known to all, it has been demonstrated that the electrocatalytic activity of catalyst exhibited a positive correlation with ECSA, in contrast, the more negative onset potential means the easier oxidation of organic molecules on the surfaces on catalysts [48,49]. It is clearly shown in Table 1 that the Pd3Ru1/P1.5 exhibits the largest ECSA value than any other catalysts, together with the most negative potential, indicating that methanol is easily oxidized on the PdRu/P catalysts. The CV curves at the scan rate of 50 mV s1 in the aqueous solution containing 1.0 M CH3OH and 1.0 M KOH with commercial Pd/C, pure Pd, different ratios of Pd/P and PdRu as references are shown in Fig. 6. As shown in Fig. 6 (a), (b) and (c), it is easy to find that all the CV curves are characterized by two well-defined peaks: the one in the forward scan is associated with methanol oxidation and the other one present in the reverse scan is primarily attributed to the removal of carbonaceous species that are not completely oxidized in the forward scan. For comparing their intrinsic catalytic activity towards the methanol oxidation in alkaline medium, the current was normalized with the mass of Pd (2.4 mg) loading on the surface of GCE. The results indicating that Pd3Ru1P1.5 display the highest forward current peak, which is 7.66, 4.4, 2.28 and 1.75 times higher than those of the commercial Pd/C, pure Pd, Pd3Ru1 and Pd2P1. Meanwhile, the forward anodic peak current density is far higher than the reverse anodic peak current density, illustrating its better CO resistance and most carbonaceous residues can be oxidized completely [50,51]. The methanol oxidation activity of the Pd3Ru1P1.5 catalysts were also
Table 1 e Comparison of CV results of different catalyst electrodes for methanol electrooxidation of different catalysts.
Pd3Ru1P1 Pd3Ru1P1.5 Pd3Ru1P2.5 Pd3Ru1P3 Pd2P1 Pd Pd/C
ECSA/cm2 mg1
j/mA mg1
Onset potential/V
373 544 466 412 334 283 221
857 1260 1024 946 718 284 164
0.68 0.66 0.63 0.59 0.61 0.57 0.58
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compared to others in the literature. As shown in Table S1, the forward current peak density of Pd3Ru1P1.5 was higher than those of many previously reported Pd-based electrocatalyst, further demonstrating that the Pd3Ru1P1.5 catalyst exhibited superior catalytic activity compared to Pd/P and PdRu-related catalysts for the electrochemical oxidation of methanol. The superior catalytic performances of PdRu/P are mainly attributed to the following aspects: first, as is known to all, Pd and Pd-based alloys are excellent candidates for catalytic methanol oxidation reactions in alkaline. Moreover, Ru is also an outstanding metal for enhancing the catalytic performances, which can not only interact with hydroxyl to form a species (Ru-OH) and play a role in electroneproton conductor for accelerating the oxidation of carbonaceous intermediates (e.g. CO) [5,8,52], but also assisting in the activation of CeH bond in the small organic molecules and ultimately boost the reaction completely. Furthermore, the addition of non-metal element P with abundant valence electrons can enhance the electronic effects between Pd, Ru and P, leading to the enormous improvement of reaction rate [53]. Besides, the network structure can not only promote reactant molecules contacting adequately with active area, but also provide efficient mass transfer in the solid ligaments as well as effective electron mobility in the catalytic reactions [35,44]. In order to demonstrate that the network structure can assist in enhancing the electrocatalytic activity of catalysts, electrocatalytic activity for MOR in the alkaline of the as-obtained monodispersed PdRu/P catalysts was also examined and the results were shown in Fig. S3. In comparison with Fig. S3 (a) and (b), the catalytic activity of obtained PdRu/P network was always higher than that of PdRu/P monodispersed catalysts with the same atomic ratios. The durability of catalysts is a significant factor for evaluating the property of a catalyst, which can be examined through CA experiments [54]. The CA curves of the different catalysts can be examined at a fixed potential of 0.20 V in a solution containing 1.0 M KOH and 1.0 M CH3OH. From Fig. 6 (d), it is obvious that all of the catalysts evidently pronounced current decay in the first 500 s which may be attributed to the accumulation of poisonous intermediates (CO-like species) during the methanol electrooxidation, after that the current decay slowed down at longer times. However, it was worth noting that the current density on the Pd3Ru1P1.5 catalyst maintained a much higher value than that of other catalysts during the whole test time, indicating its outstanding durability towards methanol oxidation, and the slowest current decay over time, confirming the excellent catalytic performance [55], which is also consistent with the above CV results [36]. Apart from Pd3Ru1P1.5, other PdRu/P catalysts all show a better stability than Pd/C, Pd/P and pure Pd, illustrating that the Ru and nonmetal P can greatly improve the stability of Pd/P and PdRu catalysts. As a matter of fact, the property of long-term stability is also of vital significance of electrocatalysts [56]. In view of this, to further assess the durability of PdRu/P catalysts towards methanol in alkaline is quite imperative. Therefore, the successive CVs of 500 cycles were performed in 1.0 M KOH þ 1.0 M CH3OH solution [57]. The plots of forward peak current densities of the catalysts versus cycle numbers are presented in Fig. 7(a). As found, because of the partial dissolution of Pd and
Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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Fig. 6 e CVs of the as-prepared catalysts in 1 M KOH and 1 M CH3OH (a) different ratios of PdRu catalysts, (b) different ratios of Pd/P (c), different ratios of PdRu/P (d), CA curves of the as-obtained catalysts.
Fig. 7 e The peak current densities in the forward scan versus cycle number (a)The 100th, 200th, 300th, 400th and 500th CVs of Pd3Ru1P1.5 catalysts in 1 M KOH þ 1 M CH3OH at a scan rate of 50 mV s¡1 (b).
poisoned by intermediate by-product, the peak current density gradually decreases with the increase of scan numbers for all catalysts [58,59]. However, the decline rate of Pd3Ru1P1.5 is the minimum, which maintains (56%) of the inception phase after 500 cycles scan towards methanol oxidation, much higher than that Pd2/P1(30%), pure Pd (25.5%) and commercial Pd/C(17%), further demonstrating enhanced long-term stability of the Pd3Ru1P1.5 catalyst. Fig. S4 and Table S2 show the successive CVs of 1000 cycles and comparisons of the mass activities of various previously reported catalysts toward methanol (formic acid) oxidation after massive successive cycles, respectively. From above results, we may come to a conclusion that PdRu catalysts doped with nonmetal P at a
certain amount can dramatically enhance the catalytic activity and durability towards the methanol oxidation reaction.
Conclusions To summarize, we have developed a facile wet-chemical method to synthesize a series of PdRu/P networks. It is found that the prepared catalysts exhibit highly improved catalytic activity and stability towards methanol in alkaline media over PdRu, Pd/P, pure Pd and commercial Pd/C, which is associated with this network structure and the synergistic effect of Pd and Ru in the catalyst, as well as the electronic
Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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effects between Pd and P. With the addition of P, more electrochemically active surface areas and electron mobility are obtained, and ultimately lead to the improvement of electrocatalytic activity and durability. More importantly, we trust that this work not only reveals a promising candidates for further application in DMFCs, but also offers a new synthesis strategy to construct PdRu/P nanoparticle networks, which can also apply to synthesize other P-doped multimetallic networks even for other nonmetals-doped multimetallic networks.
[10]
[11]
[12] [13]
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51373111), the Suzhou Industry (SYG201636), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials Notes.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.03.023.
references
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Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation Hui Xu, Bo Yan, Ke Zhang, Caiqin Wang, Jiatai Zhong, Shumin Li, Ping Yang**, Yukou Du* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
article info
abstract
Article history:
Liquid fuel cells have attracted broad research interests for past several decades, especially
Received 29 December 2016
for direct methanol fuel cells (DMFCs) because of their compact volume, environmentally
Received in revised form
benign and easy storage. Exploring cost-effective electrocatalysts toward methanol elec-
11 February 2017
trooxidation is meaningful for the development of (DMFCs). Herein, a series of PdRu/P
Accepted 5 March 2017
network catalysts have been fabricated and modified via a facile and reproducible method
Available online xxx
taking benzyl alcohol, hydrazine hydrate as solvent and reducing agents, respectively. Profiting from the 3D network structure, the synergistic effect together with the increased
Keywords:
electron mobility induced by the addition of nonmetal phosphorous (P). The PdRu/P cata-
Pd-Ru-P network
lysts display markedly improved efficient electrocatalytic activity with excellent current
Direct methanol fuel cells
peak, more negative onset potential, as well as superior long-term stability compared to
Electrocatalytic oxidation
commercial Pd/C, PdRu and Pd/P prepared under the same condition. In this work, we
Nonmetal phosphorous
highlight the effect of the incorporation of nonmetals P on the electrocatalytic performance of PdRu binary catalysts, which will contribute to broadening the application of nonmetal P or even for other nonmetals for electrooxidation. Our efforts will dedicate to accelerating the commercialization of efficient and stable anode catalysts in fuel cells by means of doping transition metals or nonmetals into Pd. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The society is now experiencing tough challenge in the replacement of the traditional fossil fuels with new resources. For the past few decades, DMFCs have emerged as promising green energy devices for the conversion of chemical energy into electricity owing to their special properties, such as high power density and environmental friendliness [1,2]. In spite of
these beneficial terms, the shortage of cost-effective catalysts for anodic oxidation reactions remains an obstacle for the development of DMFCs [3]. Therefore, the innovations and decorations of dramatically active catalysts towards methanol electrooxidation are urgently needed [1,4]. Among those catalysts, Pt and Pt-based composites are still thought to be efficient materials for the current catalyst technology [1,5,6]. However, the drawbacks of both skyrocketing prices and low poisoned-tolerance are two key factors impeding the
* Corresponding author. Fax: þ86 512 65880089. ** Corresponding author. Fax: þ86 512 65880089. E-mail address: [email protected] (Y. Du). http://dx.doi.org/10.1016/j.ijhydene.2017.03.023 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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commercial application of Pt and Pt-based composites for electrooxidation of methanol [7,8]. Compared with Pt-based catalysts, less expensive and widely available Pd has been exploited as a substitute in DMFCs due to its outstanding activity and durability in the alkaline medium [9,10]. However, it has been proved that lots of problems for Pd catalysts still unsolved, including poor antioxidant ability, strong environmental dependence and easily surface poisoned [11], which all resulted in the decrease of a large number of surface active sites and ultimate led to the sluggish kinetics and poor stability [12]. For overcoming these matters, several strategies have been proposed and prior literature have been reported for improving the electrocatalytic performances by alloying Pd with transition metals to form binary, ternary metals alloy such as Pd-Ag [13], Pd-Ru [14], Pd-Au [15,16], Pd-Ni [17] as well as embedding a broad range of nonmetals [18,19]. Ruthenium (Ru) is an important element used as cocatalysts, which can interact with hydroxyl to form a Ru-OH in the solution and serve as an electroneproton conductor for accelerating the oxidation of carbonaceous intermediates, as well as fully activating the CeH bonds [7,8]. Since its excellent properties for enhancing catalytic activity, Ru has been widely used as a promising candidate for enhancing the electrocatalytic performance toward the alcohols. Therefore, the fabrication of PdRu is deemed to be an effective way to highly enhance the electrocatalytic performances of Pd-based catalysts for methanol electrooxidation. Additionally, phosphorus is an inexpensive nonmetal element showing many similar properties as nitrogen, which has large crustal reserves and affluent valence electrons [20]. Many studies have investigated the effect of P on the efficient electron transfer along with ligand effect during the process of catalytic reaction. Additionally, doping P into Pd-based alloys can not only enhance the chemical stability of alloys in alkaline media but also adjust the electronic state of Pd [21,22]. Up till now, many reports on doped nonmetals catalysts have also been published, such as Pd-N [23], Pd-P [24], Pd-B [25], Pd-Ni-P [21,26], all of which reveal that the catalytic performances of doped catalysts are much more excellent than the corresponding binary or ternary catalysts. Notably, in comparison with previously investigated catalysts, these catalysts doped with nonmetals apparently differed in many aspects such as the unique composition and electron transportation, both of which are beneficial for the enhancement of electrocatalytic performances [27]. It was demonstrated that morphology and structure could significantly affect the catalytic activity and durability due to its enormous influences on the surface active areas, particle dimensions and particle configurations [28]. Till now, various nanostructures have been fabricated, such as nanocubes [29], nanodendrites [30], and nanorings [13], these special structures all play significant roles in characteristics of catalysts. Particularly, Pd-based networks were deemed to be potential catalysts due to its peculiar connection of nanoparticle block and net-like property, which can not only maintain the high activity of nanoparticles, but also act as self-supported material [31e33]. Moreover, both active area and electron mobility are increased in the reaction [34]. Furthermore, because of the network structure, more active sites on the surface of catalysts are available for organic molecules [35,36].
However, as is known to all, binary PdRu catalysts incorporated with phosphorus have been rarely reported. After careful consideration, we herein report a facile wet-chemical strategy to synthesize PdRu/P nanoparticle networks with tunable composition by reducing H2PdCl4, RuCl3 and NaH2PO2 precursors with hydrazine hydrate in the presence of PVP, and benzyl alcohol. Compared with PdRu and Pd/P catalysts prepared under the same conditions, the obtained PdRu/P networks exhibit definite enhancement in catalytic activity and long-term stability. Compared to traditional commercial Pd/ C, the PdRu/P NNs also display a far longer service life and higher working efficiency together with the superior durability, suggesting that it should be looked forward to serving as a promising catalyst for DMFCs and beyond.
Experimental Materials and reagents Ruthenium chloride (RuCl3), hydrochloric acid (HCl), palladium chloride (PdCl2, 99%), benzyl alcohol, sodium hypophosphite (NaH2PO2), hydrazine hydrate (N2H4$H2O), polyvinyl pyrrolidone (PVP, K35), CH3OH, C2H5OH and potassium hydrate (KOH) were all purchased from Sinopharm Chemical Reagent Co., Ltd.,China. All chemicals were of analytical grade and used as received without further purification. Doubly distill water (18.3 MU) was used for washing and preparing solution.
Apparatus The morphology and structure of obtained products were examined by scanning electron microscope (SEM, S-4700, Japan), as well as transmission electron microscopy (TEM) which were measured using a Tecnai G220 (FEI America) operated at the voltage of 200 kV. The samples were prepared by dropping 10 ml the diluted dispersion solution on the surface of copper grid coated with amorphous carbon. The elements compositions of the as prepared catalysts were accurately determined by energy dispersive X-ray spectrometer (EDX, S4700, Japan). XRD (X0 Pert-ProMPD, PANalytical Company) was used to determine the crystal nanostructure by using Cu Ka as the radiation source (l ¼ 1.54056 A) and operated at 30 mA and 40 kV. X-ray photoelectron spectra (XPS) were performed on a Thermo Scientific ESCALab 250 Xi using 200 W monochromated Al Ka radiate. All electrochemical measurements were carried out with a CHI 760B electrochemical workstation by using a typical three-electrode test cell, which included a glassy carbon electrode (GCE, diameter: 3.0 mm), a saturated calomel electrode (SCE) and a platinum wire, acting as the working electrode, the reference electrode and the counter electrode, respectively. The GCE surface was polished with alumina slurry, followed by rinsing and sonicating with double-distilled water and ethanol bath for 10 min before using it.
Synthesis of PdRu/P catalysts The targeted products were prepared through the simple wetchemical method including the following steps: first of all,
Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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0.2 g of PdCl2 powder were dissolved in 25 mL of 90.2 mM HCl solution and diluting the solution to 50 mL taking a volumetric flask to prepare a 22.6 mM H2PdCl4 solution. Next, 100 mg of PVP was completely dissolved into 8 ml of benzyl alcohol in a flask, followed by adding 0.40 mL RuCl3 (19.3 mM) and 1.0 ml previously prepared H2PdCl4 (22.6 mM). The mixture was heated from room temperature to 353.15 K with rapid magnetic stirring to form a homogeneous solution. Subsequently, an aqueous solution of NaH2PO2 (10 mM, 1.3 ml) were added followed by the addition of N2H4$H2O (1 M, 1 mL), the flask was kept under stirring in an oil bath for an hour for reacting completely. Finally, the solid suspension was obtained by centrifugation, washing with water, ethanol and acetone twice, and re-dispersed in 10 mL of distilled water to form a uniform catalyst inks, the obtained products were denoted as Pd3Ru1P1.5. For comparison, Pd3Ru1P1, Pd3Ru1P2.5, Pd3Ru1P3 catalysts were synthesized under the same conditions by changing the amount of NaH2PO2. Different ratios of PdRu and Pd/P were prepared for further comparison in the same way. The specific reactions and synthetic procedures were well illustrated in Scheme 1.
Electrochemical characterization The relative electrocatalytic performances of relative PdRu/P catalysts were studied by making cyclic voltammogram (CV) measurements for electrooxidation of methanol. For the typical preparation of PdRu/P electrode, 10 mL of catalyst inks were added on the surface of pre-polished GCE and dried in the oven at 333.15 K. Then, 3.0 ml of 5 wt% nafion solution was laid on the electrode and dried again at 333.15 K for further attaching the catalysts ink to the surface of GCE tightly to avoid the dissolution of catalysts inks. The loading mass of Pd for all catalysts is 2.4 mg. As a comparison, commercial Pd/C/ GCE, pure Pd/GCE, different ratios of Pd/P/GCE and PdRu/GCE were prepared under the same conditions. For activating the catalyst, the modified GCEs were tested in 1 M KOH solution by CVs from 0.7 V to 0.3 V (vs. SCE) at a scan rate of 50 mV s1 for obtaining steady CVs, after the activating process, the active sites on the surface of catalysts were more available for methanol molecular. CVs of all the catalysts were performed in a solution contained 1 M KOH and 1 M CH3OH, and chronoamperometric (CA) measurements were conducted at 0.20 V for 3600 s for exploring the durability of the as-
3
prepared catalysts. For further investigating the long term stability, massive CVs versus cycle numbers were also tested.
Results and discussion Characterization of PdRu/P nanoworks The network structures of PdRu/P nanocomposites were prepared through wet-chemical method by taking hydrazine hydrate as reducing agent, sodium hypophosphite act as reducing agents and phosphorus introducing reagent (see the Experimental Section for details), respectively. The structure and morphology analyses for the network structure PdRu/P (3:1:1.5) nanoparticles are shown in Fig. 1. A typical lowmagnification SEM image of the Pd3Ru1/P1.5 catalysts apparently indicates that three-dimensional (3D) porous networklike structures are synthesized on a large scale. Notably, another two types of PdRu and Pd/P nanocatalysts show similar SEM images as the Pd3Ru1/P1.5 catalysts, illustrating that this method can well be well applied to synthesize network particles with high yield [17]. From the lowmagnification SEM, many irregular nanoparticles can be obviously observed. More surface active sites are available to small organic molecules attributed to the existence of threedimensional (3D) porous network structure. Energy dispersive X-ray (EDX) spectroscopy (Fig. 1(d)) indicates that the prepared network nanoparticles are composed of Pd, Ru and P with the atomic ratio approximately 8:1:1, which is not in accordance with that of the expected (6:2:3), this phenomenon may be accounted by the phosphates formed by the reaction between partial P and Ru. Transmission electron microscopy (TEM) is also used for further studying structural features. As shown in Fig. 2(c) and (d) and Fig. S1, it can clearly see that some nanoparticles are connected with each other to form 3D network structure with abundant exposed surface active areas, which are favorable for providing more surface active sites for many organic molecules. The formed network structure may be attributed to the addition of hydrazine hydrate (N2H4$H2O), which is beneficial for the connections of PdRu/P nanoparticles. The network structure of other PdRu/P catalysts can also be obviously observed with the addition of hydrazine hydrate in Fig. 2, illustrating that the network structure can hardly be deteriorated regardless of the change
Scheme 1 e Illustration of the preparation of the PdRu/P catalysts. Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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Fig. 1 e SEM images of as-prepared PdRu (a), Pd/P (b), PdRu/P (c) and EDX spectrum of PdRu/P, (d) nanoparticle networks.
of PdRu/P atomic ratios, and this method can be widely applied to prepare PdRu/P network structure nanoparticles at a large scale [37]. In addition, for further investigating the complicated reaction mechanism, a series of comparative experiments were carried out. From Fig. S2, we could easily find that monodispersed Pd nanoparticles were obtained and no network-like structure formed without the introduction of hydrazine hydrate, and the hypophosphite can react with previously formed Hþ and H to generate elemental phosphorus ultimately lead to the formation of PdRu/P alloy [38]. It is generally known that Pd2þ and Ru3þ can be reduced to Pd and Ru metallic particles by both of hypophosphite and hydrazine hydrate. Therefore, hypophosphite not only acts as reducing agent but also phosphorus introducing reagent [37], and corresponding reaction mechanism are shown as follow
Pd2þ þ H2PO 2 / Pd þ H2PO3 þ H2O
(1)
þ H2PO 2 þ H þ H / 2H2O þ P
(2)
þ H2PO / 2H2O þ 1/2H2 þ P 2 þ 2H þ H
(3)
nM þ P / MnP
(4)
The crystalline phases of Pd3Ru1/P1.5, Pd3Ru1/P1, Pd3Ru1/P2.5, Pd3Ru1/P3 and PdRu were investigated by X-ray diffraction (XRD). As shown in Fig. 3, it is clearly to observe that all of PdRu/ P and PdRu nanoparticle networks display a diffraction peak of face centered cubic crystal structure. The featured four diffraction peaks of PdRu presented at 2q values of 40.03, 46.48, 68.39 and 81.92 are corresponding to the (111), (200), (220), (311) crystalline planes [39]. In comparison of pure Pd, the positively slight shift of 2q values, illustrating the reduction of lattice parameter, which can be assigned to the formation of PdRu alloy [40]. However, the diffraction peak of PdRu/P shifted to lower degree accompanied with the increase of the phosphorus content. The half-peak width of the diffraction peak decreased in the following order: Pd3Ru1P3 >Pd3Ru1P2.5 > Pd3Ru1P1.5 > Pd3Ru1P1, which implied that the addition of phosphorus was favorable for reducing the particle diameter, and led to more exposed surface active areas [41] [42].
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5
Fig. 2 e TEM images of Pd3Ru1 (a), Pd2/P1 (b), and the Pd3Ru1/P1.5 (c and d) catalyst with different magnifications. X-ray photoelectron spectroscopy (XPS) was employed to further investigate the surface properties and compositions of PdRu/P catalysts. In Fig. 4(a), except for Pd, Ru, P, other three types of elements were also probed, and the appearance of C, O, N may be attributed to the residuum of surfactant agents or solvent [37,43]. Since the Ru 3d peak was overlapped with C1s peak, thus for fully exploring the composition of Ru element on the catalyst surface, we found that the Ru 3p peak could be observed at the binding energies of 462.7 and 484.9 eV, corresponding to the Ru 3p1/2 and Ru 3p 3/2 binding energies. Some oxidation products of Ru (IV) could also be confirmed by the presence of a pair of peaks located at the binding energies
Fig. 3 e XRD patterns of PdRu, Pd3Ru1P1, Pd3Ru1P1.5, Pd3Ru1P2.5 and Pd3Ru1P3 catalysts.
of 466.3 and 487.8 eV. In Fig. 4(b), couples of typical peaks observed at about 334.7 eV (Pd 3d 5/2), 341.4 eV (3d 3/2), 338.1 eV (Pd 3d 5/2), as well as 342.7 eV (Pd 3d 3/2), corresponding to elemental Pd and Pd (II), respectively. Fig. 4(d) shows the P 2p spectrum of PdRu/P catalysts, a pair of peaks appeared at 129.7 and 133.3 eV were referred to the two environments for phosphorous atoms of elemental phosphorus and oxidized phosphorus (V), respectively [35,44]. The electrocatalytic activity for MOR in the alkaline of the PdRu/P catalysts was examined by CVs at the potential range from 0.8 to 0.4 V in 1.0 M KOH at a scan rate of 50 mV s1. For comparison, different molar ratios of as-prepared PdRu and Pd/P catalysts were also characterized. Fig. 5(a) shows the CVs of the seven types of catalysts conducted in a 1.0 M KOH aqueous solution at a scan rate of 50 mV s1. Basing on the previous research, it is easy for Pd to absorb OH to form PdeOHads ((Eq. (7)) and then further react with OH(Eq. (6)), after that, the Pd-O species continue to come into being with the increase of anode potential [45], the peak corresponding to the oxidation of Pd is featureless [46]. On the contrary, the reduction peak of PdeO species (Eq. (8)) appear at ca. 0.36 V in the cathode scan is much higher than those oxidation peaks [47].
Pd þ H2O þ e / (PdeHads) þ OH
(5)
Pd þ OH / PdeOHads þ e
(6)
Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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Fig. 4 e XPS spectra of PdRu/P (a), Pd 3d (b), Ru 3p (c) and P 2p (d).
Fig. 5 e Electrocatalytic properties of the as-pre pared catalysts and commercial Pd/C catalysts in 1 M KOH (a) CV, (b) onset potentialecurrent curves in 1 M KOH and 1 M CH3OH, scan rate: 50 mV s¡1. Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
PdeOHads þ OH / PdeO þ H2O þ e
(7)
PdeO þ H2O þ 2e / Pd þ 2OH
(8)
The electrochemically active surface area (ECSA) of the catalysts can be calculated from integral value of the areas of the reduction peak of PdeO species with the following equation (Eq. (9)) ECSA ¼
Q 0:405m
(9)
where m represents the mass of Pd (2.4 mg) attached to the surfaces of GCE, the value of Q was related to the reduction charge of PdO and 0.405 mC cm2 is a constant assuming that a monolayer of PdO was reduced on the Pd surface. As is known to all, it has been demonstrated that the electrocatalytic activity of catalyst exhibited a positive correlation with ECSA, in contrast, the more negative onset potential means the easier oxidation of organic molecules on the surfaces on catalysts [48,49]. It is clearly shown in Table 1 that the Pd3Ru1/P1.5 exhibits the largest ECSA value than any other catalysts, together with the most negative potential, indicating that methanol is easily oxidized on the PdRu/P catalysts. The CV curves at the scan rate of 50 mV s1 in the aqueous solution containing 1.0 M CH3OH and 1.0 M KOH with commercial Pd/C, pure Pd, different ratios of Pd/P and PdRu as references are shown in Fig. 6. As shown in Fig. 6 (a), (b) and (c), it is easy to find that all the CV curves are characterized by two well-defined peaks: the one in the forward scan is associated with methanol oxidation and the other one present in the reverse scan is primarily attributed to the removal of carbonaceous species that are not completely oxidized in the forward scan. For comparing their intrinsic catalytic activity towards the methanol oxidation in alkaline medium, the current was normalized with the mass of Pd (2.4 mg) loading on the surface of GCE. The results indicating that Pd3Ru1P1.5 display the highest forward current peak, which is 7.66, 4.4, 2.28 and 1.75 times higher than those of the commercial Pd/C, pure Pd, Pd3Ru1 and Pd2P1. Meanwhile, the forward anodic peak current density is far higher than the reverse anodic peak current density, illustrating its better CO resistance and most carbonaceous residues can be oxidized completely [50,51]. The methanol oxidation activity of the Pd3Ru1P1.5 catalysts were also
Table 1 e Comparison of CV results of different catalyst electrodes for methanol electrooxidation of different catalysts.
Pd3Ru1P1 Pd3Ru1P1.5 Pd3Ru1P2.5 Pd3Ru1P3 Pd2P1 Pd Pd/C
ECSA/cm2 mg1
j/mA mg1
Onset potential/V
373 544 466 412 334 283 221
857 1260 1024 946 718 284 164
0.68 0.66 0.63 0.59 0.61 0.57 0.58
7
compared to others in the literature. As shown in Table S1, the forward current peak density of Pd3Ru1P1.5 was higher than those of many previously reported Pd-based electrocatalyst, further demonstrating that the Pd3Ru1P1.5 catalyst exhibited superior catalytic activity compared to Pd/P and PdRu-related catalysts for the electrochemical oxidation of methanol. The superior catalytic performances of PdRu/P are mainly attributed to the following aspects: first, as is known to all, Pd and Pd-based alloys are excellent candidates for catalytic methanol oxidation reactions in alkaline. Moreover, Ru is also an outstanding metal for enhancing the catalytic performances, which can not only interact with hydroxyl to form a species (Ru-OH) and play a role in electroneproton conductor for accelerating the oxidation of carbonaceous intermediates (e.g. CO) [5,8,52], but also assisting in the activation of CeH bond in the small organic molecules and ultimately boost the reaction completely. Furthermore, the addition of non-metal element P with abundant valence electrons can enhance the electronic effects between Pd, Ru and P, leading to the enormous improvement of reaction rate [53]. Besides, the network structure can not only promote reactant molecules contacting adequately with active area, but also provide efficient mass transfer in the solid ligaments as well as effective electron mobility in the catalytic reactions [35,44]. In order to demonstrate that the network structure can assist in enhancing the electrocatalytic activity of catalysts, electrocatalytic activity for MOR in the alkaline of the as-obtained monodispersed PdRu/P catalysts was also examined and the results were shown in Fig. S3. In comparison with Fig. S3 (a) and (b), the catalytic activity of obtained PdRu/P network was always higher than that of PdRu/P monodispersed catalysts with the same atomic ratios. The durability of catalysts is a significant factor for evaluating the property of a catalyst, which can be examined through CA experiments [54]. The CA curves of the different catalysts can be examined at a fixed potential of 0.20 V in a solution containing 1.0 M KOH and 1.0 M CH3OH. From Fig. 6 (d), it is obvious that all of the catalysts evidently pronounced current decay in the first 500 s which may be attributed to the accumulation of poisonous intermediates (CO-like species) during the methanol electrooxidation, after that the current decay slowed down at longer times. However, it was worth noting that the current density on the Pd3Ru1P1.5 catalyst maintained a much higher value than that of other catalysts during the whole test time, indicating its outstanding durability towards methanol oxidation, and the slowest current decay over time, confirming the excellent catalytic performance [55], which is also consistent with the above CV results [36]. Apart from Pd3Ru1P1.5, other PdRu/P catalysts all show a better stability than Pd/C, Pd/P and pure Pd, illustrating that the Ru and nonmetal P can greatly improve the stability of Pd/P and PdRu catalysts. As a matter of fact, the property of long-term stability is also of vital significance of electrocatalysts [56]. In view of this, to further assess the durability of PdRu/P catalysts towards methanol in alkaline is quite imperative. Therefore, the successive CVs of 500 cycles were performed in 1.0 M KOH þ 1.0 M CH3OH solution [57]. The plots of forward peak current densities of the catalysts versus cycle numbers are presented in Fig. 7(a). As found, because of the partial dissolution of Pd and
Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
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Fig. 6 e CVs of the as-prepared catalysts in 1 M KOH and 1 M CH3OH (a) different ratios of PdRu catalysts, (b) different ratios of Pd/P (c), different ratios of PdRu/P (d), CA curves of the as-obtained catalysts.
Fig. 7 e The peak current densities in the forward scan versus cycle number (a)The 100th, 200th, 300th, 400th and 500th CVs of Pd3Ru1P1.5 catalysts in 1 M KOH þ 1 M CH3OH at a scan rate of 50 mV s¡1 (b).
poisoned by intermediate by-product, the peak current density gradually decreases with the increase of scan numbers for all catalysts [58,59]. However, the decline rate of Pd3Ru1P1.5 is the minimum, which maintains (56%) of the inception phase after 500 cycles scan towards methanol oxidation, much higher than that Pd2/P1(30%), pure Pd (25.5%) and commercial Pd/C(17%), further demonstrating enhanced long-term stability of the Pd3Ru1P1.5 catalyst. Fig. S4 and Table S2 show the successive CVs of 1000 cycles and comparisons of the mass activities of various previously reported catalysts toward methanol (formic acid) oxidation after massive successive cycles, respectively. From above results, we may come to a conclusion that PdRu catalysts doped with nonmetal P at a
certain amount can dramatically enhance the catalytic activity and durability towards the methanol oxidation reaction.
Conclusions To summarize, we have developed a facile wet-chemical method to synthesize a series of PdRu/P networks. It is found that the prepared catalysts exhibit highly improved catalytic activity and stability towards methanol in alkaline media over PdRu, Pd/P, pure Pd and commercial Pd/C, which is associated with this network structure and the synergistic effect of Pd and Ru in the catalyst, as well as the electronic
Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
effects between Pd and P. With the addition of P, more electrochemically active surface areas and electron mobility are obtained, and ultimately lead to the improvement of electrocatalytic activity and durability. More importantly, we trust that this work not only reveals a promising candidates for further application in DMFCs, but also offers a new synthesis strategy to construct PdRu/P nanoparticle networks, which can also apply to synthesize other P-doped multimetallic networks even for other nonmetals-doped multimetallic networks.
[10]
[11]
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51373111), the Suzhou Industry (SYG201636), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials Notes.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.03.023.
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Please cite this article in press as: Xu H, et al., Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.023