Polyoxometalate Nanocomposite

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Highly Active PdNi/RGO/Polyoxometalate Nanocomposite Electrocatalyst for Alcohol Oxidation Jing Hu, Xiaofeng Wu, Qingfan Zhang, Mingyan Gao, Haifang Qiu, Keke Huang, Shouhua Feng, Tingting Wang, Ying Yang, Zhelin Liu, and Bo Zhao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04031 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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Highly Active PdNi/RGO/Polyoxometalate Nanocomposite Electrocatalyst for Alcohol Oxidation Jing Hu,a Xiaofeng Wu,b Qingfan Zhang,a Mingyan Gao,a Haifang Qiu,a Keke Huang,b Shouhua Feng,b Tingting Wang,a Ying Yang,a Zhelin Liua*, Bo Zhaoa* a

Key Laboratory of Applied Chemistry and Nanotechnology at Universities of Jilin Province, Department of Chemistry & Environmental Engineering, Changchun University of Science and Technology, Changchun, Jilin 130022, P.R. China

b

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P.R. China

CORRESPONDING AUTHOR FOOTNOTE *

To whom correspondence should be addressed. Tel./Fax: +86-431-85583447.

E-mail: [email protected] (Z.L. Liu), [email protected] (B. Zhao) 1

ACS Paragon Plus Environment

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Abstract In this work, a PdNi/RGO/polyoxometalate nanocomposite has been successfully synthesized by a simple wet-chemical method. Characterization such as transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction analysis, and X-ray photoelectron spectroscopy are employed to verify the morphology, structure and elemental composition of the as-prepared nanocomposite. Inspired by the fast-developing fuel cells, the electrochemical catalytic performance of the nanocomposite towards methanol and ethanol oxidation in alkaline media is further tested. Notably, the nanocomposite exhibits excellent catalytic activity and long-term stability towards alcohol electrooxidation compared with the PdNi/RGO and commercial Pd/C catalyst. Furthermore, the electrochemical results reveal that the prepared nanocomposite is attractive as a promising electrocatalyst for direct alcohol fuel cells, in which the phosphotungstic acid plays a crucial role in enhancing the electrocatalytic activities of the catalyst. Keywords: DAFC; heteropolyacid; noble metal; nickel; palladium

2


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1. Introduction With the rapid advancement of economic globalization, the growth of energy demand has drawn our urgent attention. Among various energies, fossil energy has been regarded as a major energy source of the world. In fact, overuse of fossil fuel not only consumes the resources, but also pollutes the environment. Due to the increasing problem of energy crisis and environmental pollution, novel efficient and renewable clean energy has been receiving considerable attentions in current society. Fuel cells are energy conversion devices with high efficiency and cleanness relative to most traditional power generation technologies, which have attracted increasing attentions over the past decades.1,2 Direct alcohol fuel cells (DAFCs) have been regarded as promising portable power sources due to the high energy density, facile storage, low weight and environmental benignity.3,4 Traditionally, platinum (Pt) and Pt-based nanomaterials documented as a member of electrocatalysts have been mainly known as possessing the best performance.5,6 However, their practical usage has been suffered from some disadvantages such as CO poisoning, high cost and effective methanol crossover.7,8 Compared with Pt or Pt-based catalysts, Pd shows versatile catalytic properties, lower cost and better resistance to CO poisoning.9 At present, several studies have been stated that palladium or its alloy nanomaterials are becoming prospective catalysts with the rapid development of nanotechnologies.10,11 Taking the advantages of relatively higher electrocatalytic activity and cost-effective feature, alloy nanomaterials like PdNi,12,13 PdPt,14,15 PdAu,16,17 PdCu18 and PdAg19 have been investigated. Comparing with the single-metal systems, Pd-based bimetals in particular 3


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PdNi exhibit superior catalytic properties. Moreover, to achieve the commercialization and enhance the electrocatalytic activity of the nanocatalysts, alternative supports have been widely developed. For instance, a variety of supporting materials including foam silica,20 multiwalled carbon nanotubes (MWNTs)21 and indium tin oxide (ITO)22 have been utilized to support the nanocatalysts. However, reduced graphene oxide (RGO) has been used as a catalyst support for DAFCs due to the large specific surface area, superior electric conductivity and high chemical stability.23,24 Recently, heteropolyacids (HPAs) have also been incorporated into the bimetals to improve the catalytic performance.25,26 HPAs are a rapidly increasing class of compounds and have been extensively studied as catalysts, additives, chemical cleaners and surface promoters, owing to their unique structural and electronic versatility,27 in which phosphotungstic acid (H3PW12O40, HPW) is one of the most investigated Keggin-type HPAs. A most appealing and notable characteristic of HPW is that they can acquire and release several electrons per molecule without changing the structure.28,29 Herein, phosphotungstic acid and PdNi nanoparticles supported on RGO (PdNi/RGO/HPW) nanomaterials were simply synthesized by a facile chemical method, in which PdNi/RGO nanomaterials were first synthesized, followed by the subsequent introduction of HPW into the composite in an aqueous solution. The electrocatalytic activities of the as-prepared nanomaterials were also investigated, and the PdNi/RGO/HPW catalysts displayed a much higher electrocatalytic activity for methanol and ethanol oxidation. 4


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2. Experimental 2.1 Materials All the reagents were of analytical grade and used as received without further purification. Ultrapure water (no less than 18.2 MΩ cm) was used throughout the whole experiment to prepare all aqueous solutions. 2.2 Preparation of PdNi/RGO electrocatalyst Graphene oxide (GO) was prepared from graphite powder via a modified Hummers’ method.30 The as-fabricated GO was dispersed in dimethylformamide (DMF, 0.5 mg mL-1), ultrasonicated and heated under 160 oC for 6 h with stir, resulting in the formation of RGO. In a typical procedure for the fabrication of Pd0.5Ni0.5/RGO nanocomposites, 0.887 mL of H2PdCl4 (56.4 mM), 0.012g NiCl2 and 16.5 mL of RGO aqueous solution (0.5 mg mL-1) were firstly mixed together to form a homogeneous precursor solution, in which the molar ratio of Pd to Ni is 1:1. The precursor solution was then transferred into a three-neck flask under magnetic stirring. Afterwards, 10 mL of NaBH4 (0.1 M) was added into the precursor solution under high-purity nitrogen atmosphere, and the reaction color gradually turned black indicated the formation of Pd0.5Ni0.5/RGO. The resulting product was collected by centrifugation, washed several times with water for further characterization. While changing the concentrations for Pd and Ni precursors, PdNi nanoparticles with different molar ratios of Pd to Ni (1:3 and 3:1) were also obtained and designated as Pd0.25Ni0.75/RGO and Pd0.75Ni0.25/RGO, respectively. 2.3 Preparation of PdNi/RGO/HPW electrocatalyst 5


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Typically, 18 µL of HPW solution (5 µg µL-1) was added into the as-prepared Pd0.5Ni0.5/RGO dispersion, in which the mass ratio of PdNi to HPW is 10:3, and the resulting dispersion was kept stirring for 30 min at room temperature. Final product was collected by centrifuging and washing with water, and denoted as Pd0.5Ni0.5/RGO/HPW-3. Similarly, other electrocatalysts with different mass ratios of PdNi to HPW (10:2 and 10:4) were also obtained and named as Pd0.5Ni0.5/RGO/HPW-2 and Pd0.5Ni0.5/RGO/HPW-4, respectively. 2.4 Characterizations Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) images were achieved on Tecnai G2 S-Twin F20 transmission electron microscope. X-ray diffraction (XRD) measurements were obtained on D/max 2550 V/PC X-ray diffractometer using Cu (50 kV, 200 mA) radiation. X-ray photoelectron spectroscopy (XPS) analyses were carried out on an ESCALAB-MKII spectrometer (Thermo Scientific). Raman spectra were collected by a Renishaw 2000 Raman spectrometer. Inductively coupled plasma atomic emission spectrometry (ICP-AES) measurements were performed on OPTIMA 3300DV optical emission ICP spectrometer (Perkin Elmer). 2.5 Electrochemical measurements The electrocatalytic activity of the fabricated catalysts were evaluated by an electrochemical workstation (CHI 660E) equipped with a three-electrode cell, including a Ag/AgCl electrode (saturated with KCl) and pure platinum wire as 6


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reference electrode and counter electrode, respectively. A modified glassy carbon electrode (GCE, 3 mm in diameter) was employed as working electrode, which was polished to mirror with a 0.05 µm alumina suspension prior to performing the electrochemical measurements. The modified GCE was prepared by dropping the as-synthesized catalyst ink on GCE surface, which was covered by 10 µL of 0.5 wt.% Nafion and then dried in air. Potassium hydroxide in 0.5 M concentration was applied as the supporting electrolyte. Cyclic voltammetry (CV) and chronoamperometry (CA) experiments were measured in 0.5 M KOH aqueous solution containing 0.5 M methanol or ethanol. All the electrochemical experiments were performed at room temperature. 3. Results and discussion Herein, the shape, morphology and elemental composition of the obtained electrocatalysts were characterized by TEM, HRTEM and EDX. Figure 1A and 1B show the typical TEM images of the Pd0.5Ni0.5/RGO before and after the decoration of HPW, respectively. Obviously, there are a number of spherical nanoparticles with regular size on the prepared support, implying the successful synthesis of nanoparticles on RGO. Furthermore, it is notable that after decorated with HPW, the shape of the composite has no distinct change (Figure 1B). Further information on the clear lattice fringes comes from the HRTEM analysis. A well-resolved inter-planar distance of 0.22 nm is revealed on HRTEM image (Figure 1C), which is ascribed to the lattice spacing distance of PdNi (111) plane.31 The typical EDX analysis (Figure 1D) shows the presence of C, O, Pd, Ni, P and W elements in the nanocomposite, 7


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which illustrates the existence of HPW in the nanomaterial.

Figure 1. Typical TEM image of Pd0.5Ni0.5/RGO (A). TEM (B), HRTEM (C) and EDX (D) images of Pd0.5Ni0.5/RGO/HPW-3.

The crystal structures of the as-prepared different nanomaterials were further analyzed by XRD technique. Figure 2 depicts the XRD patterns for the five samples recorded in the 2θ range from 5 o to 90 o. For Pd/RGO, the typical diffraction peaks located at 40.1 o, 46.1 o, 68.0 o and 82.1o correspond to the (111), (200), (220) and (311) planes of the face-centered cubic (fcc) phase of Pd according to JCPDS No. 46-1043, respectively. For PdNi/RGO and PdNi/RGO/HPW, the representative diffraction peaks of Pd have a slight shift with increasing the Ni amount, which may be caused by the formation of PdNi alloy. Remarkably, no obvious diffraction peaks of Ni can be 8


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seen, further suggesting that Pd forms alloy with Ni.32,33 Moreover, the characteristic peaks of HPW are also not observed in the XRD pattern of Pd0.5Ni0.5/RGO/HPW-3 catalyst, indicating that the HPW clusters exist in the dispersed state, rather than in the crystalline state.34 Raman spectroscopy is used to characterize the structure of carbon nanomaterials.35 Figure S1 shows the Raman spectra of Pd0.5Ni0.5/RGO/HPW-3 and GO, and the peaks at 1350 cm-1 and 1580 cm-1 can be ascribed to D band and G band, respectively.36 The increase in the intensity ratio of D to G bands (ID/IG) from 0.92 for GO to 0.98 for Pd0.5Ni0.5/RGO/HPW-3 suggests the effective reduction of GO to RGO.37

Figure 2. XRD patterns of Pd0.5Ni0.5/RGO/HPW-3 (a), Pd/RGO (b), Pd0.75Ni0.25/RGO (c), Pd0.5Ni0.5/RGO (d) and Pd0.25Ni0.75/RGO (e).

Moreover, XPS analysis was conducted to further verify the valence state and surface composition of the as-prepared samples. The high resolution Pd 3d spectrum can be deconvoluted into two pairs of asymmetric peaks (Figure 3A). Specifically, the stronger peaks at 335.4 eV and 340.7 eV are ascribed to 3d5/2 and 3d3/2 of Pd0, whilst 9


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the weaker ones at 338.1 eV and 343.5 eV can be indexed to 3d5/2 and 3d3/2 of Pd2+,38 indicating Pd0 is the predominant species in Pd0.5Ni0.5/RGO/HPW-3 nanocomposite. Furthermore, Ni 2p region of XPS spectrum (Figure 3B) reveals the presence of five peaks that are attributed to three different oxidation states associated with Ni (852.8 eV), NiO (856.2 eV, 872.5 eV and 879.6 eV) and Ni(OH)2 (861.5 eV), indicating the surface partial oxidation.39 As shown in Figure 3C, peaks of binding energies at 35.8 eV and 37.8 eV could be referred to two spin-orbit doublets with W 4f7/2 and W 4f5/2,40 respectively. Furthermore, deconvoluted C 1s spectrum of the nanocomposite shows four fitting peaks at 284.9 eV (C=C), 285.8 eV (C-O), 286.9 eV (C=O) and

Figure 3. Pd 3d (A), Ni 2p (B) W 4f (C) and C 1s (D) regions of XPS spectrum of the Pd0.5Ni0.5/RGO/HPW-3 nanocomposite. 10


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288.3 eV (O-C=O). Comparing with the C 1s XPS data of GO (Figure S2), the intensity ratio of the peaks representing C=C to oxygenous groups like C-O, C=O and O-C=O is greatly raised. The great increase in the C to O ratio from 2.6 for GO to 5.1 for RGO further indicates the effective reduction of GO to RGO. In view of the possible enlarged surface area by RGO and the well-established catalytic property of Pd nanomaterial and phosphotungstic acid, the prepared nanocomposite is considered to possess the electrocatalytic activity towards alcohol electrooxidation in alkaline media. Thus, the electrochemical properties of Pd0.5Ni0.5/RGO and Pd0.5Ni0.5/RGO/HPW-3 nanomaterials were first examined in 0.5 M KOH aqueous solution, and the corresponding CV curves were displayed in Figure S3. The peaks characteristic of Pd can be seen on Pd0.5Ni0.5/RGO and Pd0.5Ni0.5/RGO/HPW-3, indicating the successful preparation of Pd nanoparticles. Besides, the Pd0.5Ni0.5/RGO/HPW-3 catalyst possesses the largest electrochemical active surface area (ECSA) value in comparison with Pd0.5Ni0.5/RGO nanocomposite, showing the potential of applying as electrocatalyst. To explore the electrocatalytic performance of the as-prepared catalysts, electrocatalytic experiments towards methanol oxidation was performed in 0.5 M KOH aqueous solution containing 0.5 M methanol at room temperature. The electrochemical properties of PdNi/RGO with different Pd to Ni ratios were first investigated, and the corresponding results were displayed in Figure 4A. Obviously, peaks can be observed on the forward scan on all CV curves owing to the oxidation of methanol,41,42 while a smaller oxidation peak appeared at the reverse scan. Moreover, 11


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the maximum specific activity on Pd0.5Ni0.5/RGO (643.2 mA mg-1Pd) is ca. 3.4, 1.3 and 1.9 times of the corresponding value for Pd0.25Ni0.75/RGO (189.8 mA mg-1Pd), Pd0.75Ni0.25/RGO (510.9 mA mg-1Pd) and Pd/RGO (336.8 mA mg-1Pd), respectively.

Figure 4. (A) CV curves of Pd0.25Ni0.75/RGO (a), Pd0.5Ni0.5/RGO (b), Pd0.75Ni0.25/RGO (c) and Pd/RGO (d) modified GCEs in 0.5 M KOH containing 0.5 M methanol at the scan rate of 50 mV s-1.

(B)

CV

curves

Pd0.5Ni0.5/RGO/HPW-2

(a),

Pd0.5Ni0.5RGO/HPW-3

(b),

and

Pd0.5Ni0.5/RGO/HPW-4 (c) modified GCEs in 0.5 M KOH containing 0.5 M methanol at the scan rate of 50 mV s-1. (C) CV curves of Pd0.5Ni0.5/RGO (a), Pd0.5Ni0.5/RGO/HPW-3 (b) and commercial 10% Pd/C (c) modified GCEs in 0.5 M KOH aqueous solution containing 0.5 M methanol at the scan rate of 50 mV s-1. (D) Column diagram of activities per unit Pd mass (left) or total mass (right) on different catalysts towards the electrooxidation of methanol. 12


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Thus, Pd0.5Ni0.5/RGO was selected to be mixed with HPW due to its superior specific current among the different nanocomposites. Afterwards, the performance towards methanol electrooxidation of Pd0.5Ni0.5/RGO/HPW nanocomposites with different amounts of HPW was displayed in Figure 4B. The catalytic activity per unit Pd mass for Pd0.5Ni0.5/RGO/HPW-3 catalyst is as high as 800.3 mA mg-1Pd, which is ca. 1.5 times

of


(531.8

mA

mg-1Pd)

and

2.1

times

of

Pd0.5Ni0.5/RGO/HPW-4 (379.5 mA mg-1Pd), demonstrating a good electrocatalytic activity of the Pd0.5Ni0.5/RGO/HPW-3 catalyst. In addition, the electrocatalytic activity of commercial 10% Pd/C was also measured for comparison. As shown in Figure 4C, the catalytic performance of Pd0.5Ni0.5/RGO/HPW-3 is much higher than that of Pd0.5Ni0.5/RGO and commercial 10% Pd/C catalysts. The corresponding column diagram of the specific current for the three catalysts are shown in Figure 4D, and the peak current density of Pd0.5Ni0.5/RGO/HPW-3 is ca. 1.2 times of that on Pd0.5Ni0.5/RGO and 3.7 times of that on commerical 10% Pd/C catalyst. Besides that, the catalytic activities per unit total catalyst mass for different samples were also compared, the maximum current density on Pd0.5Ni0.5/RGO/HPW-3 reaches 386.6 mA mg-1total, which is ca. 1.9 and 17.7 times of Pd0.5Ni0.5/RGO (204.2 mA mg-1total) and commerical 10% Pd/C catalyst (21.9 mA mg-1total), better than previously reported PdNi-related catalysts,43,44 further confirming that suitable amount of HPW can improve the electrocatalytic activity of the catalyst. Subsequently, we also calculated the required Pd amount when desiring the same catalytic current (500 mA) as indicated in Figure S4A. The results demonstrated that less Pd amount is needed for 13


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Pd0.5Ni0.5/RGO/HPW-3 (0.62 mg) than Pd0.5Ni0.5/RGO (0.78 mg) and commerical 10% Pd/C catalyst (2.29 mg), illustrating Pd0.5Ni0.5/RGO/HPW-3 catalyst shows the best cost-benefit feature. Furthermore, CA measurements were applied to study the electrochemical stability of the catalysts. Figure S4B shows the CA plots of Pd0.5Ni0.5/RGO, Pd0.5Ni0.5/RGO/HPW-3 and commercial 10% Pd/C in 0.5 M KOH aqueous solution containing 0.5 M methanol. It can be seen that the catalytic current on Pd0.5Ni0.5/RGO/HPW-3 still retains a larger value comparing with that of the other samples even after 4000 s. The results reveal the ameliorated catalytic activity and enhanced long-term stability of Pd0.5Ni0.5/RGO/HPW-3 nanomaterials for methanol oxidation. Overall, we can draw a conclusion that Pd0.5Ni0.5/RGO/HPW-3 shows excellent electrocatalytic performance towards methanol electrooxidation in alkaline media. Electrocatalytic activity of different samples towards ethanol oxidation were also performed at room temperature, as presented in Figure 5. Figure 5A shows the CV curves of Pd0.25Ni0.75/RGO (a), Pd0.5Ni0.5/RGO (b), Pd0.75Ni0.25/RGO (c) and Pd/RGO (d) catalysts in 0.5 M KOH aqueous solution containing 0.5 M ethanol at a sweep rate of 50 mV s-1. It can be seen that there are two obvious anodic peaks on all CV curves. In the forward scan, peaks are corresponded to ethanol oxidation, while the peaks on the reverse scan are associated with the removal of the partially oxidized carbonaceous intermediates produced on the electrode in the forward scan.45,46 Afterwards, an analogous result is also obtained, that the maximum specific current of Pd0.5Ni0.5/RGO can reach 952.3 mA mg-1Pd, which is ca. 3.9, 1.2 and 1.6 times that of 14


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Pd0.25Ni0.75/RGO (241.8 mA mg-1Pd), Pd0.75Ni0.25/RGO (770.9 mA mg-1Pd) and Pd/RGO

(581.6

mA

mg-1Pd).

The

performance

of

Pd0.5Ni0.5/RGO/HPW

nanocomposites with different amounts of HPW for ethanol electrooxidation was also

Figure 5. (A) CV curves of Pd0.25Ni0.75/RGO (a), Pd0.5Ni0.5/RGO (b), Pd0.75Ni0.25/RGO (c) and Pd/RGO (d) modified GCEs in 0.5 M KOH containing 0.5 M ethanol at the scan rate of 50 mV s-1. (B)

CV

curves


(a),


(b),

and

Pd0.5Ni0.5/RGO/HPW-4 (c) modified GCEs in 0.5 M KOH containing 0.5 M ethanol at the scan rate of 50 mV s-1. (C) CV curves of Pd0.5Ni0.5/RGO (a), Pd0.5Ni0.5/RGO/HPW-3 (b) and commercial 10% Pd/C (c) modified GCEs in 0.5 M KOH aqueous solution containing 0.5 M ethanol at the scan rate of 50 mV s-1. (D) Column diagram of activities per unit Pd mass (left) or total mass (right) on different catalysts towards the electrooxidation of ethanol. 15


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investigated as shown in Figure 5B. It can be seen that the maximum specific current of Pd0.5Ni0.5/RGO/HPW-3 hybrid could reach 1223.6 mA mg-1Pd, which is higher than the corresponding values of Pd0.5Ni0.5/RGO/HPW-2 (897.0 mA mg-1Pd) and Pd0.5Ni0.5/RGO/HPW-4

(503.0

mA

mg-1Pd).

These results

establish a

certain conclusion that Pd0.5Ni0.5/RGO/HPW-3 nanocomposite possesses an enhanced electrocatalytic activity towards ethanol oxidation compared with that of Pd0.5Ni0.5/RGO/HPW-2 and Pd0.5Ni0.5/RGO/HPW-4 catalysts. In addition, the as-prepared Pd0.5Ni0.5/RGO/HPW-3 shows better performance than Pd0.5Ni0.5/RGO and commercial 10% Pd/C catalyst (Figure 5C), highlighting the moderate amount of HPW greatly improve the specific current. As depicted in Figure 5D, it is clear that the specific current on Pd0.5Ni0.5/RGO/HPW-3 is ca. 1.3 and 3.2 times of Pd0.5Ni0.5/RGO catalyst and commercial 10% Pd/C (382.0 mA mg-1 Pd), respectively. If the total mass is taken into account, the catalytic activity of the Pd0.5Ni0.5/RGO/HPW-3 nanocatalyst (591.1 mA mg-1total) is about 1.9 times that of Pd0.5Ni0.5/RGO (302.3 mA mg-1total) and 15.5 times that of the commercial 10% Pd/C catalyst (38.2 mA mg-1total), also superior to previously reported PdNi-related catalysts.47,48 When obtaining the same catalytic current towards ethanol oxidation (500 mA), lower Pd amount is required on Pd0.5Ni0.5/RGO/HPW-3 (0.41 mg) than Pd0.5Ni0.5/RGO (0.53 mg) and commercial 10% Pd/C (1.31 mg), as shown in Figure S5A. Less Pd amount is needed for Pd0.5Ni0.5/RGO/HPW-3, which indicates that the possible

significant

role

played

by

HPW.

From

Figure

S5B,

the

Pd0.5Ni0.5/RGO/HPW-3 nanocomposite exhibits better stability compared with 16


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Pd0.5Ni0.5/RGO and commercial 10% Pd/C. Therefore, Pd0.5Ni0.5/RGO/HPW-3 nanomaterial is also beneficial for ethanol oxidation. Furthermore, the Pd0.5Ni0.5/RGO/HPW-3 nanocomposite also shows better catalytic activity and stability towards ethylene glycol electrooxidation in alkaline media (Figure S6). The well-performed catalytic activity towards alcohol electrooxidation should be ascribed to the following reasons. First, the introduction of RGO greatly increased the specific surface area. Second, the possible synergetic effect of RGO, PdNi and HPW should also be responsible for the improvement of catalytic activity. Therefore, Pd0.5Ni0.5/RGO/HPW hybrid can be used as a promising catalyst towards alcohol electrooxidation. 4. Conclusions In summary, phosphotungstic acid and PdNi nanoparticles supported on RGO has been successfully prepared and characterized by a series of measurements. Electrochemical investigations

of

different nanocomposites

show

that

the

electrocatalytic behavior of the as-prepared catalysts towards alcohol oxidation can be tailored by changing the composition ratio. Besides, the nanocomposite with the optimized composition ratio is further tested, which exhibits enhanced electrocatalytic activity and better stability compared with the commercial Pd/C catalyst. Thus, the as-prepared nanomaterial can be considered as a promising catalyst which possesses a great application prospect in DAFCs. Acknowledgment We are grateful to the National Natural Science Foundation of China (21401012, 17


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Electrochemical-Reduction-Assisted

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1,10-Phenanthroline

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Polyoxometalate Nanocomposite

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