Synthesis of Pd nanoparticles supported on PDDA functionalized graphene for ethanol electro-oxidation

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 4 0 ( 2 0 1 5 ) 3 2 2 e3 2 9

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Synthesis of Pd nanoparticles supported on PDDA functionalized graphene for ethanol electro-oxidation Yanfang Fan, Yanchun Zhao*, Duhong Chen, Xiao Wang, Xinglan Peng, Jianniao Tian* Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry of Education of China), College of Chemistry and Pharmacy, Guangxi Normal University, Guilin 541004, PR China

article info

abstract

Article history:

We described a facile and eco-friendly method for preparation of Pd nanocomposites (Pd-

Received 30 July 2014

PDDA/RGO), which possesses a low Pd metal percent (20 wt%) and a mean size of 3.9 nm.

Received in revised form

The reaction was carried out at room temperature using Pd nanoparticles anchored on

12 September 2014

reduced graphene oxide (RGO) with the assist of poly(diallyldimethylammonium chloride).

Accepted 27 October 2014

The Pd nanoparticles were well distributed without obvious aggregation. The Pd-PDDA/

Available online 20 November 2014

RGO nanocomposites exhibit higher electrocatalytic activity and stability for the electrooxidation of ethanol than Pd/RGO, Pd-PDDA/MWCNTs (multi-walled carbon nanotubes)

Keywords:

and Pd-PDDA/XC-72 carbon black. The peak current density for ethanol oxidation of Pd-

Poly(diallyldimethylammonium

PDDA/RGO is 1.38, 1.60 and 2.40 times stronger than that of Pd/RGO, Pd-PDDA/MWCNTs

chloride)

and Pd/XC-72, respectively.

Graphene

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Pd nanoparticles Ethanol electro-oxidation

Introduction Direct alcohol fuel cells (DAFCs) have attracted tremendous attention as power sources for portable electronic devices. Among various types of DAFCs, direct methanol and ethanol fuel cells (DMFCs and DEFCs) are promising power sources due to their high energy density, low pollutant emission, low operating temperature, and easy handling [1]. Especially, DEFCs has the advantages of low toxicity, high energy density, good stability and easy storage [2]. Furthermore, ethanol is a green and renewable resource that can be easily obtained in large amounts from agricultural products or biomass by

fermentation [3,4]. Despite these advantages, the low efficiency of ethanol oxidation has greatly limited the development of DEFCs. Therefore, it is a challenge for the commercialization of DEFCs to develop a highly-active anode catalyst [5e7]. Palladium is an effective anode catalyst for the ethanol oxidation reaction in alkaline DEFCs. As noble metal catalyst, Pd has two significant advantages as compared with the Pt. Firstly, According to reports in the literature [8e10], Pd shows both higher catalytic activity and better stability for the oxidation of methanol and ethanol in alkaline medium. Masel et al. [11] have reported that Pd and Pd/C catalysts can conquer the CO-poisoning effect and thereby yield high performance

* Corresponding authors. Tel.: þ86 773 5846279; fax: þ86 773 5832294. E-mail addresses: [email protected] (Y. Zhao), [email protected] (J. Tian). http://dx.doi.org/10.1016/j.ijhydene.2014.10.115 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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 4 0 ( 2 0 1 5 ) 3 2 2 e3 2 9

in DMFCs; Secondly, Pd is more abundant than Pt (Pd at least fifty times more abundant on Earth than Pt) and has a much lower price. Thus Pd can be substituted for Pt both as anode and cathode materials without worsening fuel cell performance [12]. Recently, more novel catalyst supports have been developed to improve fuel cell performance [12,13]. The catalyst support (i.e., carbon support) used in a fuel cell operation environment requires corrosion resistance and highly electrically conductivity, and desirable water handling capability (hydrophilic character) [14,15]. Among all kinds of carbon supports (such as activated carbon, carbon black, graphitized materials [16] fullerenes and carbon nanotubes etc.), graphene has attracted more attention of researchers in electrochemistry. As a newly reported carbon carrier material with two dimensional nanostructures, graphene has extraordinary electrical, thermal and mechanical properties [17,18]. Besides, it also owns wide potential window, excellent chemical stability in various electrolytes and easy renewable surface [19]. In particular, graphene possesses the high quality of the sp2 conjugated bond in the carbon lattice, remarkably high electron mobility under ambient conditions with the reported values in excess of 15,000 cm2/(V s) and a very large specific surface area (theoretical value 2600 m2/g) [20e22]. These unique properties make graphene a promising supporting component for potential applications in many technological fields, such as, nanocomposites [23], batteries [24] and supercapacitors [25]. It should be mentioned that there are two main difficulties to directly utilize the negative charges of graphene in the nanoparticles deposition process: First, graphene are apt to aggregate due to the very high specific surface area; Second, negative charge of carboxyl, hydroxyl, and epoxy groups etc. on the surface of graphene is too weak to assemble nanoparticles directly. To overcome these obstacles, various polyelectrolyte, including Poly(diallyldimethylammonium chloride) (PDDA) [22,26], such as branch poly (ethylenimine) [27], polyacrylic acid [28] have been used as inter-linkers for depositing metal nanoparticles on graphene. The modification proves to be an effective strategy to enhance the functionality of materials and the deposition of nanoparticles on graphene. Recently, Shi et al. [26], fabricated PdPt nanocomposites with sonoelectrochemical technique on PDDA modified RGO as electrocatalyst for DAFCs, which displayed electrocatalytic activity and stability for the electro-oxidation of ethanol. Zhang et al. [29], reported the synthesis of high Pt-loadings on the surface of functionalized graphene, and proposed that PDDA plays a crucial role in the highly dispersion and stabilization of Pt nanoparticles on graphene and PDDA-RGO could be an alternative support for Pt immobilization in DMFCs. In this paper, PDDA is used as the functional macromolecule to produce the stable building block dispersion of the cationic polyelectrolyte-functionalized graphene as a supporting material. In the as-prepared Pd-PDDA/RGO nanocomposites, PDDA can alter the electrostatic charges of graphene, create net positive charge in the carbon plane via intermolecular charge transfer, and so favor the loading of the Pd nanoparticles [30]. The use of PDDA modifier not only changes the electrostatic charges of graphene, but also provides a convenient approach for the hybridization of graphene

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[22]. To prove whether the graphene has been modified with PDDA successfully, we studied the shift of the UVevis absorption peak of the 20% PDDA, PDDA/RGO and Pd-PDDA/RGO samples. Furthermore, the morphologies and structures of the as-prepared Pd-PDDA/RGO nanocomposites were extensively investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The composition was evaluated by energy dispersive X-ray spectrometry (EDS). Moreover, Raman spectra revealed the surface properties of graphene and its interaction with metallic nanoparticles. Cyclic voltammetric (CV) and chronoamperometry experiments exhibited their catalytic activity and stability for the electro-oxidation of ethanol in alkaline media.

Experimental method Materials and instruments Chemicals used in the experiment were of analytical grade. Graphite powder, NaCl, H2SO4, 30% H2O2, KMnO4, NaBH4, ethanol, NaOH, N, N-dimethylformamide (DMF) and PDDA were procured commercially and used without further purification. The XC-72 carbon black was purchased from Cabot Corporation and MWCNTs were purchased from Shenzhen Nanotechnologies Port Co. Ltd (Shenzhen, China) with the diameter of 40e60 nm, length of 5e15 mm, and purity of 98%. Main characterization instrument are scanning electron microscope (SEM) (FEI Quanta 200 FEG Holland), TEM (Hitachi H800 Japan), high-resolution transmission electron microscopy (HRTEM) (JEM-2100F, Japan), XRD (Rigaku D/max 2500v/pc X Japan), Raman spectroscopy (Invia, Renishaw UK) and UVevis absorption (TU-1901 China).

Preparation of reduced graphene oxide The preparation of RGO is briefly described as follows [31]: 1 g graphite powder and 20 g NaCl were mixed and ground for 30 min, then the mixture was leached to remove the NaCl. The dried graphite was transferred to a 250 mL beaker and 23 mL H2SO4 (18 mol L1) was added with constant stirring at 25  C for 24 h. Then 0.12 g of KNO3 was added at 40  C and followed by slowly adding 0.5 g of KMnO4, keep stirring for 30 min. Afterwards, 186 mL of water was added to the beaker slowly and 10 mL of 30% H2O2 were added to end the reaction. The product was centrifuged and washed with 5% HCl solution and distilled water, respectively. Finally, the sample was dried in a vacuum oven at 60  C and then roasted in a tubular resistance furnace at 900  C, marked with RGO.

Preparation of Pd-PDDA/RGO The PDDA functionalized RGO used as supports for the formation of Pd-PDDA/RGO catalyst were prepared by two steps: firstly, preparation of PDDA/RGO. 30 mg RGO and 1.5 mL 20 wt % PDDA were mixed in 30 mL ethanol-water solution (1:1, v/v ratio) and ultrasonic treatment at 25  C, pH of the solution was adjust to 9e10 with 0.5 M NaOH solution [26,32]. Then the mixture was centrifuged and washed to remove redundant PDDA, dried at 60  C for 24 h in a vacuum oven. Secondly,

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synthesis of Pd-PDDA/RGO: 20 mg PDDA/RGO and 0.833 mL H2PdCl4 solution (6 g L1 H2PdCl4) were mixed in 20 mL DMFwater solution (1:1, v/v ratio). The suspensions were kept stirring for 8 h and followed by adding excess NaBH4 solution dropwise. Another 2 h later, the final product was centrifuged and washed with distilled water and ethanol, respectively. The obtained black solid residue was dried at 60  C in a vacuum oven. As comparison, Pd nanoparticles on RGO, PDDA/ MWCNTs and XC-72 were prepared under the same conditions, denoted as Pd/RGO, Pd- PDDA/MWCNTs and Pd/XC-72, respectively.

Electrochemical measurement The electrochemical activity measurements were performed with a CHI 660D workstation and a conventional three electrode cell at room temperature (25  C). As a typical process, 2.0 mg of electro-catalysts sample was ultrasonically mixed in 400 mL of ethanol-water solution (1:1, v/v ratio), followed by dropping 5 mL of the electro-catalysts ink onto the surface of a glassy carbon electrode (GCE Ø ¼ 3 mm). Then, 5 mL of Nafion solution (0.5 wt % in ethanol) was added to fix the electrocatalysts on the GCE surface. A Pt sheet and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. All of the potentials were quoted with respect to the SCE in this measurement. Theoretically, the Pd loading of the electrode was 20%.

nanoparticles with a mean size of approximately 4 nm occupy part of the surface of PDDA/RGO sheets with relatively even, densely distribution. These indicated the strong interaction between the Pd nanoparticles and the functional graphene [33]. The HRTEM image of the representative Pd-PDDA/RGO is presented in the inset of Fig. 2B. The fringe patterns with lattice spacing of Pd about 0.225 nm which is close to the planes of pure Pd of 0.2246 nm (JCPDS card No. 46-1043). The chemical composition of the as-prepared Pd-PDDA/RGO was determined using EDS analysis. Fig. 3(a,b,c)show the corresponding mapping of Pd (Red), C (Green), O (Blue) and Fig. 3(d) shows the peaks of C, O and Pd. It is worth noting that the surface of Pd map was scattered in the entire surface of PDDA/ RGO, indicating that Pd nanoparticles have both wellproportioned loads and uniform dispersion. Fig. 4A displays the XRD patterns of Pd-PDDA/RGO, PdPDDA/MWCNTs and Pd/RGO for comparison. The lattice constants are in good consistent with the standard values of single phase Pd. As showed in Fig. 4A, the XRD pattern of PdPDDA/RGO composites show the diffraction peaks at 2q values of about 39.73 , 45.16 , 66.05 , 80.20 , which are attributed to the characteristic peaks of face centered cubic crystalline Pd, corresponding to crystalline planes of Pd (111), (200), (220), (311), respectively (JCPDS 65-6174) [31,34]. The average Pd particle size in the Pd-PDDA/RGO composites is estimated to be 3.9 nm on the basis of the Pd (111) peak by Scherrer equation [35] for the carbon-supported catalysts, d ¼ 0:9l=b2q cos qmax

Results and discussion Mechanism of the Pd-PDDA/RGO formation The mechanism of formation of the catalyst Pd-PDDA/RGO is shown in Fig. 1. In the process of pretreatment, the pH of DMF solution was regulated to 9e10 with 0.5 M NaOH. After the were added, a well-dispersed Pd negatively charged PdCl2 4 nanoparticles supported on PDDA/RGO was obtained thanks to the electrostatic interaction of positive and negative charge.

TEM, EDS and XRD analysis The sizes and morphologies of Pd nanoparticles on PDDA/RGO were studied by TEM. As shown in Fig. 2, a large number of Pd

Fig. 1 e Schematic of the synthesis of Pd-PDDA/RGO.

˚ ), q is the where l is the wavelength of the X-ray (1.54056 A angle at the maximum of the peak and b2q is the width of the peak at half height. The average particle sizes calculated from the XRD peak widths were found to be consistent with those from the TEM results, approximately 4 nm.

Raman spectroscopy and UVevis spectrum analysis The extent of disorder or the degree of crystallinity in the functionalized graphene can be characterized by Raman spectroscopy [36]. If the D and G Raman peaks change in shape, position and relative intensity with number of graphene layers, this will reflects the evolution of the electronic structure and electronephonon interactions [37]. As shown in Fig. 4B, the D- and G-bands for PDDA/RGO and RGO were at ~1350 cm1 and ~1570 cm1, respectively, due to the structure of the sp3 and sp2 hybridized carbon atom [38], which demonstrated that the defects/disorder induced modes and in-plane vibrations of the graphitic wall [39]. The degree of the graphitization of RGO can be quantified by the intensity ratio of the D to G bands. The value of ID/IG for PDDA/RGO and RGO is 0.128 and 0.16, respectively. This change suggests that PDDA/RGO can preferably preserve the integrity and electronic structure of RGO and provides more effective active sites on the surface of RGO for the subsequent deposition Pd nanoparticles [40,41]. The D band of PDDA/RGO has a slight shift compared with pure RGO. According to the references [30,42,43], the quaternary ammonium functional groups along the PDDA backbone have electro-withdrawing ability from carbon atoms in the carbon nanotubes or graphene plane to

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Fig. 2 e TEM image of Pd-PDDA/RGO. Inset of Fig. 2B shows the HRTEM image of Pd-PDDA/RGO.

induce the net positive charge. The D-band shift indicates the intermolecular electron transfer from graphene to the adsorbed PDDA [44,45]. Similar G-band shifts caused by chargetransfer have been reported for graphene functionalized with other electro-accepting molecules [44,46]. Fig. 5A shows the UVevis absorption spectra of (a) PDDA, (b) PDDA/RGO and (c) RGO samples which were uniformly dispersed in ethanol solution (50 mg L1). The spectrum of RGO shows an unapparent absorption peak at ~223 nm, the observations suggest that the reduction state is indeed existed of RGO and the electronic conjugation within the graphene sheets is restored upon the heat treatment [47], whereas for

polymer PDDA, there is a strong UVevis absorption peak at ~210 nm. After formation of PDDA/RGO, the absorption peak of PDDA functionalized graphene red-shift to the strong UVevis absorption peak of PDDA. It may be owing to the surface adsorption and pep interaction between RGO and PDDA, according to Marco Bernardi et al. [48] that by comparison with excitons in SWCNTs, it has been proposed that planar graphene configurations might improve charge transfer behavior. This chargeetransfer interaction increases the effective energy of the electron hole and causes a red-shift of the recombining graphene exciton [49]. In brief, the strong red-shift of PDDA/RGO compared with RGO is a persuasive

Fig. 3 e EDS spectra of Pd-PDDA/RGO, mapping of element Pd (Red), C (Green) and O (Blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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Fig. 4 e (A) XRD patterns of (a) Pd-PDDA/RGO; (b) Pd-PDDA/MWCNTs and (c) Pd/RGO; (B) Raman spectra of (a) PDDA/RGO; (b) RGO.

fact, which provides piece of evidence to support the formation of PDDA/RGO.

Electrocatalytic activity analysis Electrochemical investigations have been carried out to study the oxidation of ethanol in alkaline medium on graphene-

supported catalysts. The cyclic voltammograms of the PdPDDA/RGO, Pd-PDDA/MWCNTs and Pd/XC-72 catalysts were carried out in 0.5 M NaOH at a scan rate of 50 mV s1 between 0.8 and 0.4 V and the results are shown in Fig. 5B. It is known that the property of nanomaterials for use in catalytic application strongly depends on the nature of their surfaces. So, the electrochemical active surface area (EAS) of the catalysts

Fig. 5 e (A) UVevis Spectrum of (a) PDDA; (b) PDDA/RGO; (c) RGO; (B) CVs of (a) Pd-PDDA/RGO; (b) Pd-PDDA/MWCTNs; and (c) Pd/XC-72 in 0.5 M NaOH at 25  C with the scan rate of 50 mV s¡1; (C) CVs of (a) Pd-PDDA/RGO; (b) Pd-PDDA/MWCTNs; and (c) Pd/XC-72 in 0.5 M NaOH þ 1.0 M ethanol at 25  C with the scan rate of 50 mV s¡1; (D) chronoamperometry of (a) Pd-PDDA/ RGO; (b) Pd-PDDA/MWCTNs; (c) Pd/XC-72 in 0.5 M NaOH þ 1.0 M ethanol at operation potential of ¡0.2 V at 25  C. Inset shows the magnifying chronoamperometry between 6610 and 7200 s.

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can be used to describe their properties. We measured the EAS by determining the coulomb charge for the reduction of palladium oxide. The EAS is estimated using the equation. EAS ¼ Q=Sl Where S is the proportionality constant used to relate charge with area and l is the catalyst loading in g. A charge value of 405 mC cm2 is assumed for the reduction of the PdO monolayer [50]. The consequences are accounted to be 49.38, 33.52 and 28.64 m2 g1 Pd for Pd-PDDA/RGO, Pd-PDDA/MWCNTs and Pd/XC-72, respectively. The electrocatalytic activity of Pd-PDDA/RGO for the electrooxidation of ethanol was represented in Fig. 5C. The current for the reaction in 0.5 M NaOH þ 1.0 M CH3CH2OH solution exhibits the well-known features of ethanol oxidation on Pd-based electrocatalysts. It can be observed that the onset of ethanol oxidation peak begin at about 0.66 V for Pd-PDDA/RGO, and the peak potentials of ethanol oxidation are 0.19 V, 0.10, 0.23 V and 0.22 V for Pd-PDDA/RGO, Pd/RGO, Pd-PDDA/ MWCNTs and Pd/XC-72, approximately. In addition, the ethanol oxidation peak current density of the Pd-PDDA/RGO is 1.38, 1.60 and 2.40 times for that of Pd/RGO, Pd-PDDA/MWCNTs and Pd/XC-72. The significantly higher electrocatalytic activity for the ethanol oxidation of Pd-PDDA/RGO shows that Pd has a larger utilization in the catalyst, which is corresponding with the high electrochemically active surface area of Pd-PDDA/ RGO. On the other hand, as an index of the catalyst tolerance to incompletely oxidized species accumulated on the surface of the electrode, the ratio of the forward anodic peak current density (jf) to the backward anodic peak current density (jb), jf/jb, was discreetly calculated. The jf/jb ratio of the Pd-PDDA/RGO is about 1.06, which is significantly higher than that of the Pd/RGO (0.82), Pd-PDDA/MWCNTs (0.63) and Pd/XC-72 (1.02) catalyst, showing more effective removal of the poisoning species COads on the catalyst surface. In short, Pd-PDDA/RGO showed comparatively high catalytic activity when compared with our previous work [51,52] and other similar reports [53,54]. The stability of ethanol oxidation on Pd-PDDA/RGO, Pd/ RGO, Pd-PDDA/MWCNTs and Pd/XC-72 modified electrodes were further investigated by chronoamperometric technique at a potential of 0.2 V in 0.5 M NaOH þ 1.0 M ethanol solution. As shown in Fig. 5D, the polarization current shows a rapid decline, which can be explained in terms of a poisoning mechanism of the intermediate species during the ethanol electro-oxidation. However, the current decay on Pd/XC-72 is faster than that on Pd-PDDA/RGO, Pd/RGO and Pd-PDDA/ MWCNTs modified electrodes. At the end of the measurement, the oxidation current on the Pd-PDDA/RGO modified electrode is considerably higher than the other three, which can be clearly observed in the inset of Fig. 5D, it is 2.35, 3.34 and 5.35 times stronger than that of Pd-PDDA/MWCNTs, Pd/ RGO and Pd/XC-72, respectively. This result suggests that the Pd nanoparticles and the functional PDDA/RGO enhance the electrocatalytic activity and stability of the catalyst.

Conclusions A simple method was applied to synthesize Pd nanoparticles on PDDA-functionalized graphene for ethanol oxidation.

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Well-dispersed Pd nanoparticles were obtained on PDDAfunctionalized graphene. The effectively enhanced electrocatalytic activity and stability of Pd-PDDA/RGO can be attributed to the follows: (1) The presence of PDDA on the graphene could help the formation of small, uniformly dispersed Pd nanoparticles; (2) By comparing the electrocatalytic activity and stability of Pd-PDDA/RGO and Pd/RGO, we can confirm that the PDDA dispersed on graphene will increase the graphene conductivity and remove the intermediate poisoning species. Furthermore, it is important to highlight that in contrast to the reported synthesis of hybrid materials based on graphene, this in situ method provides an alternative to obtain the hybrids metal/functionalized-graphene with uniform dispersion. This paper provides new insights into the design of novel catalytic materials based on graphene for fuel cell technologies. However, further studies are needed to optimize the performance and test of these materials in actual fuel cells.

Acknowledgments This work has been supported by the National Natural Science Foundation of China (No. 21163002, 21165004, 21363003) and Program for Excellent Talents in Guangxi Higher Education Institutions (2014GXNSFGA118008).

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