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Fabrication of bimetallic PtPd alloy nanospheres supported on rGO sheets for superior methanol electro-oxidation
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Fabrication of bimetallic PtPd alloy nanospheres supported on rGO sheets for superior methanol electro-oxidation Sunanda Esabattina a, Venkata Ramana Posa b, Hong Zhanglian b, Sreenivasa kumar Godlaveeti a, Rammanohar Reddy Nagi Reddy a, Adinarayana Reddy Somala a,* a
Department of Materials Science and Nanotechnology, Yogi Vemana University, Kadapa, 516003, Andhra Pradesh, India b School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China
article info
abstract
Article history:
Bimetallic PtPd nanospheres strongly fabricated on reduced graphene oxide (rGO) sheets
Received 29 April 2017
(rGO-PtPd) by simple one pot wet reflux method for superior methanol electro-oxidation.
Received in revised form
There is no any other polymer or seed involved for preparation of nanocomposite. The
5 July 2017
as-synthesized materials structure and morphology was calibrated by Powdered X-ray
Accepted 24 July 2017
diffraction (P-XRD), Raman, X-ray photo electron spectroscopy (XPS), scanning electron
Available online 12 August 2017
microscopy (SEM) and transmission electron microscopy (TEM). The PtPd nanoparticles supported rGO sheets displays superior electro-catalytic activity and stability towards
Keywords:
methanol electro-oxidation (MOR) because of their large electrochemical surface area
PtPd bimetallic
(ECSA, which is 1.68 times greater than that of commercial Pt/C black) and synergistic
Nanospheres
effect of the bimetallic alloy. The rGO-PtPd showed enhanced electro-catalytic perfor-
rGO sheets
mances towards MOR in acidic media due to particle size and uniform distribution of
Electro-catalysis
particles, which rGO-Pt1Pd1 (Pt/Pd molar ratio 1/1) showed the more specific activity, mass
Energy storage
activity and stability for MOR. Thus, as-synthesized rGO-PtPd catalyst could apply potential applications in direct methanol fuel cells (DMFCs) to lower their cost and advance their cycle ability, which make it promising for practical catalysis in energy conversion and storage. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction As the demand for green energy and importance of fossil fuel substitutes are increasing nowadays, new technologies and sustainable energy resources are rigorously explored by
worldwide researchers [1,2]. Among the diverse potentials, electrochemical energy storage devices, electrochemical super capacitors have been offer several merits as well as high power density, long cycle stability and high energy density, which are estimated to bridge a gap between conventional capacitor with a low energy density and secondary
* Corresponding author. E-mail address: [email protected] (A.R. Somala). http://dx.doi.org/10.1016/j.ijhydene.2017.07.193 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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batteries with a low power density [3,4]. Still, fuel cells are nothing but revamp technologies of some electrochemical energy, which can also offer high energy densities, proficient tools for the formation of green and ecological energy [5,6]. Due to high energy conversion efficiency, environmental friendly and easiness of the process, DMFCs are converting from chemical energy to electrical energy directly and involve extensive responsiveness of green power sources for transferable electronics and vehicles. For their potential commercialization, there are still several obstacles unsolved such as crossover of methanol, large electrode over potential and slow reaction kinetics [7]. For improving their utilization efficiency, a variety of hierarchical nanomaterials has been synthesized with specific morphology, including spheres wires, flowers cubes and plates [8e12]. In recent times, one of the missing carbon allotropic material graphene acts as electro-catalyst for energy storage and electrochemical conversion applications and also attracted marvelous attention for use in specific as catalyst support in fuel cells due to its huge surface area (2630 m2 g1), amazing conductivity (103e104 S m1), potentially minute cost for built-up and unique graphitized basal plane structure [13e15]. Now, among all the noble metals Platinum (Pt) acts as the most dynamic electro-catalyst for methanol oxidation in basic and acidic DMFCs. Conversely, the huge amount of Pt and poisoning of the Pt surface through durable adsorption of carbon monoxide (CO) or intermediate species created in the course of methanol oxidation reaction (MOR) are the major complications to the fruitful use of DMFCs. For instance, Pt material combined with its very near metals such as PtAu [16,17], PtAg [18,19], PteNi [20,21], PteRu [22,23] and PtPd [24,25] to give Pt-based bimetallic (Pt-M), which has synergistically enhanced electro catalytic activity. Among all the Pt-based binary alloyed nanomaterials, the PtPd bimetallic nanomaterial may be nominees as excellent electro catalyst of fuel cells due to its minor cost, abundance of Pd and resemblance of Pd with Pt for the reduction of oxygen reaction [26,27]. Chen et al. manufactured the PtePd binary metallic nanocubes with a good MOR catalytic activity [28]. Cai et al. synthesized PtPd nanodendrites deposited on graphene by simple green synthesis for higher catalytic activity of MOR [29]. Li et al. facile synthesized the PtPd nano spheres on rGO sheets by chemical method for enhancement of MOR activity [30]. Zhang et al. synthesized bimetallic PtPd nanoparticles anchored on graphene sheets by chemical reduction method for better electro-chemical oxidation of methanol [31]. However, the as-prepared PtPd nanospheres in nanocomposite have welldefined morphology, uniform size distribution and also show excellent catalytic performance with satisfactory durability for MOR. In this study, we prepared and fabrication of PtPd bimetallic nanospheres supported on rGO sheets by one-step wet reflux/chemical method at 95 C, lacking of any complex apparatus or organic solvent, seed. The fabrication of rGO supported bimetallic PtPd nanospheres revealed excellent electro-catalytic activity to MOR in an alkaline medium than did the prepared rGO-supported single metals as rGO-Pt and rGO-Pd and explored the stability of catalysts.
Materials and methods Chemicals Graphite powder (þ200 mesh), Na2PdCl4, H2PtCl6$6H2O were got from Sigma Aldrich Corporation, India and used without further refinement. Nafion solution, H2SO4, HCl, H2O2, NaNO3, KMnO4, CH3OH and NaBH4 were purchased from Merck, India and used as-received. All glassware was purchased from Borosil scientific glassware, India. Doubly distilled water was used in our experiments.
Preparation of rGO-PtPd nanocomposite Graphite oxide was prepared by the graphite powder chemical oxidation by simple modified Hummer's method [32]. In this type of experiment, 115 mL quantity of 98% concentrated H2SO4 was taken in a 1000 mL beaker and 5 g of graphite flakes powder was added under ice bath at less than 5 C temperature extensive stirring should be done for 30 min. By following the same condition, 15 g of oxidizing agent KMnO4 was added slightly and stirred for 30 min. Then 2.5 g of NaNO3 was mixed to this above mixture and continuous stirred for 60 min. From this mixture a pasty brownish colour was formed. Then the reaction beaker was continuously magnetic stirred at room hotness for 60 min and after that removing it from ice bed bath, the whole recipe was kept in a water bath by increasing it to 35 C temperature and it should be stirred continuously for 30 min 800 mL of double distilled water was poured in to the reaction solution and this content was maintained by raising the temperature up to 98 C. By stopping the reaction, 30 mL quantity of 30% H2O2 was added to the solution, which turned to bright yellow colour. Residual metal ions and organic residues were removed, when this content reached the pH value to 7 and it washed under centrifugation so many times with 1 M HCl solution and again with double distilled water. At last, the end product graphene oxide was formed by drying the washed compound in sigma-vacuum oven at 60 C temperature for overnight. The rGO-PtPd nanocomposite was prepared by wet chemical/reflux method. In this method, already prepared 20 mg of graphene oxide was taken in a 100 mL double necked round bottom flask and 20 mL of double distilled water was mixed by sonication method to get graphene oxide suspension for 1 h. After completion of the water bath sonication, from 0.01 M, 20 mL quantity of H2PtCl6$6H2O solution and 20 mL of Na2PdCl4 solution were taken and added into the above dispersion and magnetic stirred continuously. Next, 1 M of NaOH solution added to the mixture for the pH of the resultant was maintained at 12 and stirred at room temperature for 60 min. Then the total reaction mixture was refluxed at 95 C for 6 h. Metal ion residues were removed and this mixture washed under centrifugation number of times with double distilled water. At last, the ultimate material rGO-PtPd was obtained by drying the washed compound in hot air oven at 80 C temperature for 8 h. For comparison, single metal doped on rGO (labelled as rGO-Pt and rGO-Pd) was also primed via similar process. Fig. 1 shows the chemical route synthesis of rGO-PtPd nanocomposite.
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Fig. 1 e Chemical route synthesis of rGO-PtPd nanocomposite.
Instrumentation CHI potentiostat (Model CHI 6002 E, USA) was connected by the standard three electrode electrochemical cells for electrochemical measurements which are performed at room temperature. Platinum (Pt) wire was used as auxiliary electrode and silver/silver chloride (Ag/AgCl, saturated KCl (sat. KCl)) (E0 ¼ 0.197 V vs. NHE) was used as reference electrode as well as gassy carbon electrode (GCE) disk which has 3 mm in diameter with a statistical area of 0.0706 cm2 was connected with working electrode. As-synthesized materials were detailed using a Bruker D8, Germany, with CuKa1 (1.5406 A) and Ka2 (1.54439 A) radiations of powdered X-ray diffraction (P-XRD) for isolate the crystalline phase of catalysts and to evaluates the size of particles. Raman spectra data of catalysts powders were performed on a WiTec alpha 200 SNOM, Germany, in the range from 500 to 4000 cm1. Fourier transformed infrared (FT-IR) spectroscopy details of materials were recorded by Perkin Elmer Spectrum Two™ FT-IR spectrometer, in the ranges from 500 to 4000 cm1. Field emission scanning electron microscope (FESEM) imaging with energy dispersive spectroscopy (EDS) measurements of catalysts powders were examined by Carl Zeiss Supra 55, Germany, in the operating range 5e20 kV. Transmission electron
microscopy (TEM) images were obtained by Technai JEOL 3010, Germany, operated at an accelerating voltage of 200 kV.
Results and discussion The typical P-XRD patterns analysis of graphite, graphene oxide, rGO-Pt, rGO-Pd and rGO supported PtPd nanocomposite, which was explains the crystalline nature of the materials and particle size, as shown in Fig. 2. The diffraction peak of hexagonal graphite appears at 26.31 can be assigned to the (0 2 2) plane (Fig. 2a) and graphite oxide appears at 10.01 can be assigned to the (111) plane due to introduced oxygen containing functional groups, which was in promise with the lamellar pattern of graphite oxide (Fig. 2b). For the rGO-PtPd nanocomposite, a strong diffraction peaks at 2q ¼ 39.87 , 46.19 and 67.44 could be assigned to the (1 1 1), (2 0 0) and (2 2 0) crystalline planes of PtPd NPs, respectively (Fig. 2e). From Fig. 2c and d, rGO-Pt and rGO-Pd materials display peaks that coincidence with those of fcc Pt (JCPDF 04-0802) and Pd (JCPDF 46-1043). It should be noted that, compared to the rGO-PtPd, illuminating an extraordinary alloyed level of the PtPd NPs because of diffraction peaks were positioned between the corresponding peaks locations of both bulk Pt and Pd [33,34].
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Fig. 3 e Raman spectra of (a) Graphite, (b) Graphene Oxide, (c) rGO-Pt, (d) rGO-Pd and (e) rGO-PtPd. Fig. 2 e P-XRD pattern of (a) Graphite, (b) Graphite Oxide, (c) rGO-Pt, (d) rGO-Pd and (e) rGO-PdPt.
According to previous literature reports, the existence of the (111) diffraction peak at 39.87 with its location lying in between (111) peak of Pt i.e. at 39.79 and (111) peak of Pd i.e. at 39.80 was taken as proof for the fabrication of PtPd NPs on rGO sheets [35]. Diffraction peaks of the rGO-PtPd NCs were moved to higher 2q values compared to the relative fcc-Pt reflections due to extraordinary level of alloying and decreased lattice parameters [36,37]. As-synthesized all catalysts of lattice parameter (a) was esteemed by wisely calculating the location of the (111) Bragg peak as shown in diffraction formation. The rGO-Pt catalyst shows the lattice parameter a ¼ 2.76 A, which is minor than that of bulk Pt (a ¼ 3.970 A) due to nano range size of rGO-Pt. Evaluates of ‘a’ for Pt is 3.928 A. The reduction in ‘a’ of Pt in rGO-PtPd compared to the ‘a’ of rGO-Pt might end result from the narrowing upon alloying by Pd. Further, Debye-Scherer equation from the width of the (1 1 1) peak was determined the PtPd NPs size range between 4 and 8 nm, which was in nice settlement with the TEM. The above conclusions reveal that the as-synthesized materials have a very much crystalline fcc phase with a Fm 3 m space group. Raman spectroscopy is a powerful instrument for examination of intercalation and structural defects in graphitic material as well as characterized by reduction of graphene oxide [38,39]. The two major bands as D and G were sighted in the Raman spectra of as-synthesized materials, as shown in Fig. 3. The D band refers to the breathing mode of the phonons
Fig. 4 e FT-IR spectra of (a) graphite (b) graphene oxide (c) rGO-Pt (d) rGO-Pd and rGO-PtPd.
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Fig. 5 e FESEM image of (a) rGO-Pt, (b) rGO-Pd, (c) rGO-PtPd and (d) corresponding elemental colour mapping of Pd, Pt, carbon and oxygen of rGO-PtPd. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
of A1g in the ranges from 1200 to 1500 cm1 and the G and refers to the E2g phonon of the sp2 hybridized carbon atoms in the ranges from 1500 to 1600 cm1 and G band position and intensity ratios of the D to the G (ID/IG) can be used to guess the reduction of the GO. The G band of the GO in rGO-PtPd was shift to higher Raman shift value as compared with that of graphene oxide due to chemical and thermal reduction of graphene oxide (GO), as shown in Fig. 3. Moreover, the GO sheets containing the oxygen functional groups are partially detached to give different reduced graphitic areas by chemical reduction of GO, which leads to boost in the ID/IG ratio. The values of ID/IG for GO, rGO-PtPd and graphite are 0.996 (curve b), Fig. 3), 1.003 (curve e) and 0.704 (curve a), respectively. These values reveal that graphite oxide was significantly reduced and rGO was essentially established [40,41]. This is crucial for attaining excellent electrical conductivity in the hybrid composites. FT-IR spectroscopy explore the extent of deletion of oxygen groups from GO during the chemical reduction route. Fig. 4 shows the FT-IR spectra of graphite, graphene oxide, rGO-Pt, rGO-Pd and rGO-PtPd nanocomposite. From Fig. 4b, displays that absorption band of graphite oxide at 3438 cm1 and 1736 cm1 which corresponds to the vibration peaks of OeH
and C]O, respectively. Moreover, the peaks at 1384 cm1 which related to the OeH of carboxylic acid functionality and 1260 cm1 which corresponds to CeOH functionality of rGO sheet, respectively. In addition, the peak seemed at 1020 cm1 was referred to the vibration of epoxide functionality. Finally, graphene oxide of totally these peaks contains the more oxygen functionalities by appearance. As can been seen from Fig. 4a, peak at 1626 cm1 was corresponding to the unoxidized vibrations of graphitic fields. Moreover, the peak also showed at 2925 cm1 which can be allotted to the vibration of CeH stretching. In the synthesis process using NaBH4 as reducing agent for reduce GO and also both Pt and Pd metal precursors at the same time to create PtPd bimetallic supported on rGO. A close examination of FT-IR peaks of all assynthesized nanocomposites as rGO-Pt, rGO-Pd and rGOPtPd materials explore that the intensity of peaks associated to the epoxide and carbonyl functional groups were almost diminished, which leads to the partial reduction of GO. In addition, the intensities of OeH of carboxylic acid seemed at 1384 cm1 and OeH group displayed at 3443 cm1 of rGO sheet was reduced, which indicates the extent of reduction of GO into somewhat rGO as shown in Fig. 4 c, d and e. For better dispersion and deposition of metal on rGO sheets, surplus
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Fig. 6 e EDS analysis of (a) rGO-Pt, (b) rGO-Pd, (c) rGO-PtPd and (d) quantitative results of rGO-PtPd.
oxygen-containing functionalities of GO will act as anchoring sites. The FT-IR outcomes of GO and rGO-PtPd nanocomposite well approved with the article revisions [42]. SEM characterizations were performed for evaluations of the morphologies of the surfaces. FESEM micrographs of rGOPt, rGO-Pd and rGO supported on bimetallic PtPd nanoparticles were shown in Fig. 5. The Pt, Pd and PtPd nanoparticles were healthy distributed on the surface of rGO with some spherical buildup (Fig. 5a, b and c) due to reaction conditions. However, it can be seen from Fig. 5c that PtPd NPs were spherical in shape. In addition, corresponding elemental colour mapping FESEM images of rGO-PtPd nanostructured material was consists of platinum (Pt), palladium (Pd), carbon (C) and small amount of oxygen (O) as shown in Fig. 5d. These results indicate that metals are strongly doped with reduced graphene oxide sheets. The EDS elemental mapping pictures confirm uniform distribution of Pt and Pd again. EDS spectra were explained by the elemental abundance of as-synthesized materials. Fig. 6 expresses the EDS analysis and quantitative of rGO-Pt, rGO-Pd and rGO-PtPd nanocomposites. The rGO-Pt contains mainly Pt, C and O, as shown in Fig. 6a. In addition, rGO-Pd contains the Pd C and O only, as shown in Fig. 6b. In the case of rGO-PtPd mainly presence of Pt,
Pd, C and O only, as shown in Fig. 6c. These results suggest that PtPd alloy is fabricated with rGO sheets. From Fig. 6d, shows that the quantitative analysis of the rGO-PtPd nanocomposite were really alloy particles comprising 39 wt% of platinum and 33 wt% of palladium. The ratio of chemical structure of Pt-to-Pd for rGO-PtPd was found to be 39:33. These results deliberate that the rGO-supported PtPd alloys have Pt to Pd ratio nearby to 1:1. These features can create most effective, advanced materials with electro catalytic performance. The size, shape and morphology of the PtPd nanoparticles were executed by TEM HRTEM and SAED pattern, as shown Fig. 7. The rGO-PtPd nanohybrid nanocomposite, the average PtPd particle size was about 4e8 nm and its shows spherical shape, as shown in Fig. 7a and b. The HRTEM image (Fig. 7c) indicates the single-crystalline structure of rGO sheets; the lattice space was 0.34 nm. The rGO sheet was showed as wrinkles shape. Fig. 7d shows the HRTEM images of PtPd supported on rGO, the each PtPd nanoparticles, the lattice spacing along the edge of the sphere was 0.23 nm, which approves well with the (111) lattice spacing of fcc-Pt and Pd. Moreover, their good crystallinity was proved by the SAED pattern (Fig. 7e). The density of PtPd nanospheres on rGO is
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Fig. 7 e (a, b) TEM, (c, d) HRTEM images of rGO-PtPd and SAED pattern of (e) rGO and (f) PtPd.
higher, which is very vital for electro catalytic performance of catalyst. Fig. 8 shows the CVs of Pt/C black, rGO-Pt, rGO-Pd and rGOPtPd NCs in 0.5 M H2SO4 solutions was composed to calculate the ECSA of the materials. CVs tests were conducted in N2saturated newly prepared 0.5 M H2SO4 solution by sweeping the electrode potential from 0.2 to 1.2 V vs. Ag/AgCl (saturated KCl) (sat. KCl) for stable voltammograms taken at a scan rate of 50 mV s1. After completion of the stability test, CV goes for 20 cycles of potential scanning. The rGO-PtPd NCs of CVs background demonstrations typical features allied to the PtPd surface. In addition, the CV profile were separated into three areas/regions consist of the double layer area, reduction/surface oxide area and typical adsorption/desorption area. The CV contours of rGO-PtPd nanospheres of this study displays irregular current peaks due to rGO supports and rGO
holds of residual oxygen-containing groups [43]. It can be seen that for all nanocomposite catalysts, in the anodic scan oxidation currents started to emerge at potentials around þ0.32 V, and reached the peak values at a more positive potential of around þ0.66 V; in the reverse scan, a similar anodic voltammetric peak can be seen but at a less positive potential around þ0.48 V, suggesting effective catalytic oxidation of methanol by these nanocomposite catalysts. The hydrogen adsorption peak of integrating the area in the CV curves between 0.2 and 0.07 V displays the total charge of hydrogen adsorption and the ECSA was evaluated after a double-layer correction [44e46]. CV profile of a commercial Pt/C black catalyst and their ECSA value was also calibrated for comparison studies. The fashions in ECSA values of commercial Pt/C black and as-synthesized NCs as rGO-PtPd, rGO-Pt and rGO-Pd were calculated to be 59.05 m2 g1, 81.2 m2 g1,
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Fig. 8 e CVs of Pt/C, rGO-Pt, rGO-Pd and rGO-PtPd in 0.5 M H2SO4 (scan rate 50 mV s¡1).
Fig. 9 e CVs of Pt/C black, rGO-Pt, rGO-Pd and rGO-PtPd NCs in a solution of 0.5 M H2SO4 and 1 M CH3OH with a scan rate of 50 mV s¡1.
Fig. 10 e Currentetime curves of Pt/C black, rGO-Pt, rGO-Pd and rGO-PtPd measured in 0.5 M H2SO4 þ 1 M CH3OH at þ0.68 V.
78.5 m2 g1 and 65.1 m2 g1, respectively. It can be seen that the as-synthesized rGO-PtPd catalyst display the largest ECSA due to nanosize and uniform distribution of PtPd on rGO sheets as held by the TEM clarifications. The rGO-PtPd nanomaterial exhibits the excellent electron transfer through the surface of electrode due to large number of active sites for electro-chemical reactions by analysis of CV results. The rGOPtPd electro-catalyst enhances electro-catalytic oxidation of methanol when matched to the rGO-Pt, rGO-Pd and Pt/C black catalysts. The CV measurements further suggest that rGO is a respectable contestant as a catalyst support. The CVs measure the electro-catalytic performance of Pt/C black, rGO-Pt, rGO-Pd and rGO-PtPd to oxidation of methanol in 0.5 M H2SO4 comprising 1 M CH3OH solution as shown in Fig. 9. Due to the oxidation of methanol, all the catalysts were reveal typical fine-separated anodic peaks in the both forward and reverse scan. Methanol adsorption was detected by freshly prepared oxidation of chemisorbed species at 0.6e0.8 V [47,48]. The backward scan of faradic oxidation of remaining carbonaceous species peaks were observed at 0.5 V and formed in the forward scan on Pt and Pd surface [49e51]. Comparison of MOR performance on nanocomposites as rGOPtPd, rGO-Pt, rGO-Pd with a Pt/C black catalyst exposes that specific activity by methanol oxidation were calculated to be 0.54 mA/cm2, 0.38 mA mA/cm2, 0.34 mA/cm2 and 0.32 mA/ cm2, respectively. The rGO-PtPd catalyst demonstrates that almost 1.68 times more electro-catalytic activity than that of commercial Pt/C black. Further, the mass activity (MA) of methanol oxidation of rGO-PtPd was a double order higher (0.41 mA/mgPt) compared to the Pt/C black (0.19 mA/mgPt). Due to the larger ECSA, larger uniform distribution of PtPd on rGO sheet, huge surface area of rGO and improved electron transport, the rGO-PtPd catalyst showed superior electrocatalytic activity. Besides, CO accumulation of the catalyst tolerance was expressed by the ratios of the forward anodic peak current (If) and backward anodic peak current (Ib), If/Ib ratio can be used as a vital index [52e54]. Normally, for practical fuel cells applications, higher If/Ib values containing catalyst show the higher tolerance to intermediate carbonaceous classes. Commercial Pt/C black catalyst of If/Ib ratio was found to be 1.01. However, the size of particles as PtPd of rGOPtPd nanocomposite (4e8 nm) and a commercial Pt/C black (3 nm) was of comparable order the attendance of second metal ‘Pd’ play a vital role in increasing the activity of oxidation of methanol. The Pd metal act as extraordinary oxophilic nature than Pt metal and may be promoter for the oxidative deletion of CO on Pt due to nanosize of Pd which leads to hyper energetic for oxidation of CO. Moreover, the Pt electronic structure was modified by introduction of Pd which leads to ease fading of PteCO bond, so facile CO deletion could be estimated. In recent times, increases the tolerance of the electro-catalyst to CO poisoning due to presence of little amount of oxygen having groups, it has been reported. Therefore, in the rGO-PtPd catalyst, the existence of rGO also plays a vital role for deletion of CO-like fragments, which leads to the superior electro-catalytic activity of electrooxidation of methanol. In contrast, If/Ib values were detected for the catalysts as rGO-Pd, rGO-Pt and rGO-PtPd 1.24, 1.53 and 1.72, respectively. For increased methanol oxidation capability and lesser amount of accumulation of carbonaceous species
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due to highest If/Ib value of rGO-PtPd. In addition, the onset potential of the electro-oxidation of methanol for the rGOPtPd was considerably moved to higher negative potentials than the commercial Pt/C black, rGO-Pt and rGO-Pd. These results suggested that to increase the disconnection of methanol on rGO-PtPd. CA was explained by the catalytic activity and stability of the electro-catalysts for oxidation of methanol. The durability is the other important character of a good nanocatalyst. From Fig. 10, shows that the current-time plots of Pt/C black, rGO-Pt, rGO-Pd and rGO-PtPd materials. The CA profiles recorded at þ0.68 V in 0.5 M H2SO4 þ 1.0 M CH3OH with the same catalystsmodified electrodes. Greater current densities were detected for all the electro-catalysts in the initial period because it can be recognized to the availability of large number of active positions on the surface of the catalysts. For starting period, all the catalysts of current densities were detected in massive values due to surface of catalyst containing more number of active points. Due to the accretion of CO-like intermediates on the active positions of catalysts delaying methanol from further oxidation a rapid deterioration in current densities can be seen mainly below 50 s. Even though the current deterioration was observed for all the catalysts, rGO-PtPd reveals considerably greater current density than other nanocomposite and Pt/C black in the whole period of time. All the electrochemical results offered here determine that rGO-PtPd nanomaterial fabricated by wet reflux method can help as expert anode catalyst for DMFCs applications.
Conclusions In conclusion, this work deals a facile one-pot wet reflux method to fabricate PtPd NPs well doped and dispersed on rGO supports without any surfactants. The rGO-PtPd displays significantly superior electro-catalytic activity and durability for methanol oxidation than Pt/C black due to nanorange of particles and uniform spreading of PtPd nanoparticles on the huge surface area of rGO support. This modest and greatest methodology can open novel possibilities for enhancing the activity for DMFCs and this structure intercalating rGO-based nanosheets and Pt-based nanoalloy spheres may also be needful in other electro-chemical applications.
Acknowledgements This research was financially supported by a university grants commission (UGC), New Delhi under the award no. 2-362/2013 (SR). We also acknowledge instrumental support University of Hyderabad, Hyderabad and IISc, Bangalore is gratefully acknowledged.
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Fabrication of bimetallic PtPd alloy nanospheres supported on rGO sheets for superior methanol electro-oxidation Sunanda Esabattina a, Venkata Ramana Posa b, Hong Zhanglian b, Sreenivasa kumar Godlaveeti a, Rammanohar Reddy Nagi Reddy a, Adinarayana Reddy Somala a,* a
Department of Materials Science and Nanotechnology, Yogi Vemana University, Kadapa, 516003, Andhra Pradesh, India b School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China
article info
abstract
Article history:
Bimetallic PtPd nanospheres strongly fabricated on reduced graphene oxide (rGO) sheets
Received 29 April 2017
(rGO-PtPd) by simple one pot wet reflux method for superior methanol electro-oxidation.
Received in revised form
There is no any other polymer or seed involved for preparation of nanocomposite. The
5 July 2017
as-synthesized materials structure and morphology was calibrated by Powdered X-ray
Accepted 24 July 2017
diffraction (P-XRD), Raman, X-ray photo electron spectroscopy (XPS), scanning electron
Available online 12 August 2017
microscopy (SEM) and transmission electron microscopy (TEM). The PtPd nanoparticles supported rGO sheets displays superior electro-catalytic activity and stability towards
Keywords:
methanol electro-oxidation (MOR) because of their large electrochemical surface area
PtPd bimetallic
(ECSA, which is 1.68 times greater than that of commercial Pt/C black) and synergistic
Nanospheres
effect of the bimetallic alloy. The rGO-PtPd showed enhanced electro-catalytic perfor-
rGO sheets
mances towards MOR in acidic media due to particle size and uniform distribution of
Electro-catalysis
particles, which rGO-Pt1Pd1 (Pt/Pd molar ratio 1/1) showed the more specific activity, mass
Energy storage
activity and stability for MOR. Thus, as-synthesized rGO-PtPd catalyst could apply potential applications in direct methanol fuel cells (DMFCs) to lower their cost and advance their cycle ability, which make it promising for practical catalysis in energy conversion and storage. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction As the demand for green energy and importance of fossil fuel substitutes are increasing nowadays, new technologies and sustainable energy resources are rigorously explored by
worldwide researchers [1,2]. Among the diverse potentials, electrochemical energy storage devices, electrochemical super capacitors have been offer several merits as well as high power density, long cycle stability and high energy density, which are estimated to bridge a gap between conventional capacitor with a low energy density and secondary
* Corresponding author. E-mail address: [email protected] (A.R. Somala). http://dx.doi.org/10.1016/j.ijhydene.2017.07.193 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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batteries with a low power density [3,4]. Still, fuel cells are nothing but revamp technologies of some electrochemical energy, which can also offer high energy densities, proficient tools for the formation of green and ecological energy [5,6]. Due to high energy conversion efficiency, environmental friendly and easiness of the process, DMFCs are converting from chemical energy to electrical energy directly and involve extensive responsiveness of green power sources for transferable electronics and vehicles. For their potential commercialization, there are still several obstacles unsolved such as crossover of methanol, large electrode over potential and slow reaction kinetics [7]. For improving their utilization efficiency, a variety of hierarchical nanomaterials has been synthesized with specific morphology, including spheres wires, flowers cubes and plates [8e12]. In recent times, one of the missing carbon allotropic material graphene acts as electro-catalyst for energy storage and electrochemical conversion applications and also attracted marvelous attention for use in specific as catalyst support in fuel cells due to its huge surface area (2630 m2 g1), amazing conductivity (103e104 S m1), potentially minute cost for built-up and unique graphitized basal plane structure [13e15]. Now, among all the noble metals Platinum (Pt) acts as the most dynamic electro-catalyst for methanol oxidation in basic and acidic DMFCs. Conversely, the huge amount of Pt and poisoning of the Pt surface through durable adsorption of carbon monoxide (CO) or intermediate species created in the course of methanol oxidation reaction (MOR) are the major complications to the fruitful use of DMFCs. For instance, Pt material combined with its very near metals such as PtAu [16,17], PtAg [18,19], PteNi [20,21], PteRu [22,23] and PtPd [24,25] to give Pt-based bimetallic (Pt-M), which has synergistically enhanced electro catalytic activity. Among all the Pt-based binary alloyed nanomaterials, the PtPd bimetallic nanomaterial may be nominees as excellent electro catalyst of fuel cells due to its minor cost, abundance of Pd and resemblance of Pd with Pt for the reduction of oxygen reaction [26,27]. Chen et al. manufactured the PtePd binary metallic nanocubes with a good MOR catalytic activity [28]. Cai et al. synthesized PtPd nanodendrites deposited on graphene by simple green synthesis for higher catalytic activity of MOR [29]. Li et al. facile synthesized the PtPd nano spheres on rGO sheets by chemical method for enhancement of MOR activity [30]. Zhang et al. synthesized bimetallic PtPd nanoparticles anchored on graphene sheets by chemical reduction method for better electro-chemical oxidation of methanol [31]. However, the as-prepared PtPd nanospheres in nanocomposite have welldefined morphology, uniform size distribution and also show excellent catalytic performance with satisfactory durability for MOR. In this study, we prepared and fabrication of PtPd bimetallic nanospheres supported on rGO sheets by one-step wet reflux/chemical method at 95 C, lacking of any complex apparatus or organic solvent, seed. The fabrication of rGO supported bimetallic PtPd nanospheres revealed excellent electro-catalytic activity to MOR in an alkaline medium than did the prepared rGO-supported single metals as rGO-Pt and rGO-Pd and explored the stability of catalysts.
Materials and methods Chemicals Graphite powder (þ200 mesh), Na2PdCl4, H2PtCl6$6H2O were got from Sigma Aldrich Corporation, India and used without further refinement. Nafion solution, H2SO4, HCl, H2O2, NaNO3, KMnO4, CH3OH and NaBH4 were purchased from Merck, India and used as-received. All glassware was purchased from Borosil scientific glassware, India. Doubly distilled water was used in our experiments.
Preparation of rGO-PtPd nanocomposite Graphite oxide was prepared by the graphite powder chemical oxidation by simple modified Hummer's method [32]. In this type of experiment, 115 mL quantity of 98% concentrated H2SO4 was taken in a 1000 mL beaker and 5 g of graphite flakes powder was added under ice bath at less than 5 C temperature extensive stirring should be done for 30 min. By following the same condition, 15 g of oxidizing agent KMnO4 was added slightly and stirred for 30 min. Then 2.5 g of NaNO3 was mixed to this above mixture and continuous stirred for 60 min. From this mixture a pasty brownish colour was formed. Then the reaction beaker was continuously magnetic stirred at room hotness for 60 min and after that removing it from ice bed bath, the whole recipe was kept in a water bath by increasing it to 35 C temperature and it should be stirred continuously for 30 min 800 mL of double distilled water was poured in to the reaction solution and this content was maintained by raising the temperature up to 98 C. By stopping the reaction, 30 mL quantity of 30% H2O2 was added to the solution, which turned to bright yellow colour. Residual metal ions and organic residues were removed, when this content reached the pH value to 7 and it washed under centrifugation so many times with 1 M HCl solution and again with double distilled water. At last, the end product graphene oxide was formed by drying the washed compound in sigma-vacuum oven at 60 C temperature for overnight. The rGO-PtPd nanocomposite was prepared by wet chemical/reflux method. In this method, already prepared 20 mg of graphene oxide was taken in a 100 mL double necked round bottom flask and 20 mL of double distilled water was mixed by sonication method to get graphene oxide suspension for 1 h. After completion of the water bath sonication, from 0.01 M, 20 mL quantity of H2PtCl6$6H2O solution and 20 mL of Na2PdCl4 solution were taken and added into the above dispersion and magnetic stirred continuously. Next, 1 M of NaOH solution added to the mixture for the pH of the resultant was maintained at 12 and stirred at room temperature for 60 min. Then the total reaction mixture was refluxed at 95 C for 6 h. Metal ion residues were removed and this mixture washed under centrifugation number of times with double distilled water. At last, the ultimate material rGO-PtPd was obtained by drying the washed compound in hot air oven at 80 C temperature for 8 h. For comparison, single metal doped on rGO (labelled as rGO-Pt and rGO-Pd) was also primed via similar process. Fig. 1 shows the chemical route synthesis of rGO-PtPd nanocomposite.
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Fig. 1 e Chemical route synthesis of rGO-PtPd nanocomposite.
Instrumentation CHI potentiostat (Model CHI 6002 E, USA) was connected by the standard three electrode electrochemical cells for electrochemical measurements which are performed at room temperature. Platinum (Pt) wire was used as auxiliary electrode and silver/silver chloride (Ag/AgCl, saturated KCl (sat. KCl)) (E0 ¼ 0.197 V vs. NHE) was used as reference electrode as well as gassy carbon electrode (GCE) disk which has 3 mm in diameter with a statistical area of 0.0706 cm2 was connected with working electrode. As-synthesized materials were detailed using a Bruker D8, Germany, with CuKa1 (1.5406 A) and Ka2 (1.54439 A) radiations of powdered X-ray diffraction (P-XRD) for isolate the crystalline phase of catalysts and to evaluates the size of particles. Raman spectra data of catalysts powders were performed on a WiTec alpha 200 SNOM, Germany, in the range from 500 to 4000 cm1. Fourier transformed infrared (FT-IR) spectroscopy details of materials were recorded by Perkin Elmer Spectrum Two™ FT-IR spectrometer, in the ranges from 500 to 4000 cm1. Field emission scanning electron microscope (FESEM) imaging with energy dispersive spectroscopy (EDS) measurements of catalysts powders were examined by Carl Zeiss Supra 55, Germany, in the operating range 5e20 kV. Transmission electron
microscopy (TEM) images were obtained by Technai JEOL 3010, Germany, operated at an accelerating voltage of 200 kV.
Results and discussion The typical P-XRD patterns analysis of graphite, graphene oxide, rGO-Pt, rGO-Pd and rGO supported PtPd nanocomposite, which was explains the crystalline nature of the materials and particle size, as shown in Fig. 2. The diffraction peak of hexagonal graphite appears at 26.31 can be assigned to the (0 2 2) plane (Fig. 2a) and graphite oxide appears at 10.01 can be assigned to the (111) plane due to introduced oxygen containing functional groups, which was in promise with the lamellar pattern of graphite oxide (Fig. 2b). For the rGO-PtPd nanocomposite, a strong diffraction peaks at 2q ¼ 39.87 , 46.19 and 67.44 could be assigned to the (1 1 1), (2 0 0) and (2 2 0) crystalline planes of PtPd NPs, respectively (Fig. 2e). From Fig. 2c and d, rGO-Pt and rGO-Pd materials display peaks that coincidence with those of fcc Pt (JCPDF 04-0802) and Pd (JCPDF 46-1043). It should be noted that, compared to the rGO-PtPd, illuminating an extraordinary alloyed level of the PtPd NPs because of diffraction peaks were positioned between the corresponding peaks locations of both bulk Pt and Pd [33,34].
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Fig. 3 e Raman spectra of (a) Graphite, (b) Graphene Oxide, (c) rGO-Pt, (d) rGO-Pd and (e) rGO-PtPd. Fig. 2 e P-XRD pattern of (a) Graphite, (b) Graphite Oxide, (c) rGO-Pt, (d) rGO-Pd and (e) rGO-PdPt.
According to previous literature reports, the existence of the (111) diffraction peak at 39.87 with its location lying in between (111) peak of Pt i.e. at 39.79 and (111) peak of Pd i.e. at 39.80 was taken as proof for the fabrication of PtPd NPs on rGO sheets [35]. Diffraction peaks of the rGO-PtPd NCs were moved to higher 2q values compared to the relative fcc-Pt reflections due to extraordinary level of alloying and decreased lattice parameters [36,37]. As-synthesized all catalysts of lattice parameter (a) was esteemed by wisely calculating the location of the (111) Bragg peak as shown in diffraction formation. The rGO-Pt catalyst shows the lattice parameter a ¼ 2.76 A, which is minor than that of bulk Pt (a ¼ 3.970 A) due to nano range size of rGO-Pt. Evaluates of ‘a’ for Pt is 3.928 A. The reduction in ‘a’ of Pt in rGO-PtPd compared to the ‘a’ of rGO-Pt might end result from the narrowing upon alloying by Pd. Further, Debye-Scherer equation from the width of the (1 1 1) peak was determined the PtPd NPs size range between 4 and 8 nm, which was in nice settlement with the TEM. The above conclusions reveal that the as-synthesized materials have a very much crystalline fcc phase with a Fm 3 m space group. Raman spectroscopy is a powerful instrument for examination of intercalation and structural defects in graphitic material as well as characterized by reduction of graphene oxide [38,39]. The two major bands as D and G were sighted in the Raman spectra of as-synthesized materials, as shown in Fig. 3. The D band refers to the breathing mode of the phonons
Fig. 4 e FT-IR spectra of (a) graphite (b) graphene oxide (c) rGO-Pt (d) rGO-Pd and rGO-PtPd.
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Fig. 5 e FESEM image of (a) rGO-Pt, (b) rGO-Pd, (c) rGO-PtPd and (d) corresponding elemental colour mapping of Pd, Pt, carbon and oxygen of rGO-PtPd. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
of A1g in the ranges from 1200 to 1500 cm1 and the G and refers to the E2g phonon of the sp2 hybridized carbon atoms in the ranges from 1500 to 1600 cm1 and G band position and intensity ratios of the D to the G (ID/IG) can be used to guess the reduction of the GO. The G band of the GO in rGO-PtPd was shift to higher Raman shift value as compared with that of graphene oxide due to chemical and thermal reduction of graphene oxide (GO), as shown in Fig. 3. Moreover, the GO sheets containing the oxygen functional groups are partially detached to give different reduced graphitic areas by chemical reduction of GO, which leads to boost in the ID/IG ratio. The values of ID/IG for GO, rGO-PtPd and graphite are 0.996 (curve b), Fig. 3), 1.003 (curve e) and 0.704 (curve a), respectively. These values reveal that graphite oxide was significantly reduced and rGO was essentially established [40,41]. This is crucial for attaining excellent electrical conductivity in the hybrid composites. FT-IR spectroscopy explore the extent of deletion of oxygen groups from GO during the chemical reduction route. Fig. 4 shows the FT-IR spectra of graphite, graphene oxide, rGO-Pt, rGO-Pd and rGO-PtPd nanocomposite. From Fig. 4b, displays that absorption band of graphite oxide at 3438 cm1 and 1736 cm1 which corresponds to the vibration peaks of OeH
and C]O, respectively. Moreover, the peaks at 1384 cm1 which related to the OeH of carboxylic acid functionality and 1260 cm1 which corresponds to CeOH functionality of rGO sheet, respectively. In addition, the peak seemed at 1020 cm1 was referred to the vibration of epoxide functionality. Finally, graphene oxide of totally these peaks contains the more oxygen functionalities by appearance. As can been seen from Fig. 4a, peak at 1626 cm1 was corresponding to the unoxidized vibrations of graphitic fields. Moreover, the peak also showed at 2925 cm1 which can be allotted to the vibration of CeH stretching. In the synthesis process using NaBH4 as reducing agent for reduce GO and also both Pt and Pd metal precursors at the same time to create PtPd bimetallic supported on rGO. A close examination of FT-IR peaks of all assynthesized nanocomposites as rGO-Pt, rGO-Pd and rGOPtPd materials explore that the intensity of peaks associated to the epoxide and carbonyl functional groups were almost diminished, which leads to the partial reduction of GO. In addition, the intensities of OeH of carboxylic acid seemed at 1384 cm1 and OeH group displayed at 3443 cm1 of rGO sheet was reduced, which indicates the extent of reduction of GO into somewhat rGO as shown in Fig. 4 c, d and e. For better dispersion and deposition of metal on rGO sheets, surplus
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Fig. 6 e EDS analysis of (a) rGO-Pt, (b) rGO-Pd, (c) rGO-PtPd and (d) quantitative results of rGO-PtPd.
oxygen-containing functionalities of GO will act as anchoring sites. The FT-IR outcomes of GO and rGO-PtPd nanocomposite well approved with the article revisions [42]. SEM characterizations were performed for evaluations of the morphologies of the surfaces. FESEM micrographs of rGOPt, rGO-Pd and rGO supported on bimetallic PtPd nanoparticles were shown in Fig. 5. The Pt, Pd and PtPd nanoparticles were healthy distributed on the surface of rGO with some spherical buildup (Fig. 5a, b and c) due to reaction conditions. However, it can be seen from Fig. 5c that PtPd NPs were spherical in shape. In addition, corresponding elemental colour mapping FESEM images of rGO-PtPd nanostructured material was consists of platinum (Pt), palladium (Pd), carbon (C) and small amount of oxygen (O) as shown in Fig. 5d. These results indicate that metals are strongly doped with reduced graphene oxide sheets. The EDS elemental mapping pictures confirm uniform distribution of Pt and Pd again. EDS spectra were explained by the elemental abundance of as-synthesized materials. Fig. 6 expresses the EDS analysis and quantitative of rGO-Pt, rGO-Pd and rGO-PtPd nanocomposites. The rGO-Pt contains mainly Pt, C and O, as shown in Fig. 6a. In addition, rGO-Pd contains the Pd C and O only, as shown in Fig. 6b. In the case of rGO-PtPd mainly presence of Pt,
Pd, C and O only, as shown in Fig. 6c. These results suggest that PtPd alloy is fabricated with rGO sheets. From Fig. 6d, shows that the quantitative analysis of the rGO-PtPd nanocomposite were really alloy particles comprising 39 wt% of platinum and 33 wt% of palladium. The ratio of chemical structure of Pt-to-Pd for rGO-PtPd was found to be 39:33. These results deliberate that the rGO-supported PtPd alloys have Pt to Pd ratio nearby to 1:1. These features can create most effective, advanced materials with electro catalytic performance. The size, shape and morphology of the PtPd nanoparticles were executed by TEM HRTEM and SAED pattern, as shown Fig. 7. The rGO-PtPd nanohybrid nanocomposite, the average PtPd particle size was about 4e8 nm and its shows spherical shape, as shown in Fig. 7a and b. The HRTEM image (Fig. 7c) indicates the single-crystalline structure of rGO sheets; the lattice space was 0.34 nm. The rGO sheet was showed as wrinkles shape. Fig. 7d shows the HRTEM images of PtPd supported on rGO, the each PtPd nanoparticles, the lattice spacing along the edge of the sphere was 0.23 nm, which approves well with the (111) lattice spacing of fcc-Pt and Pd. Moreover, their good crystallinity was proved by the SAED pattern (Fig. 7e). The density of PtPd nanospheres on rGO is
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Fig. 7 e (a, b) TEM, (c, d) HRTEM images of rGO-PtPd and SAED pattern of (e) rGO and (f) PtPd.
higher, which is very vital for electro catalytic performance of catalyst. Fig. 8 shows the CVs of Pt/C black, rGO-Pt, rGO-Pd and rGOPtPd NCs in 0.5 M H2SO4 solutions was composed to calculate the ECSA of the materials. CVs tests were conducted in N2saturated newly prepared 0.5 M H2SO4 solution by sweeping the electrode potential from 0.2 to 1.2 V vs. Ag/AgCl (saturated KCl) (sat. KCl) for stable voltammograms taken at a scan rate of 50 mV s1. After completion of the stability test, CV goes for 20 cycles of potential scanning. The rGO-PtPd NCs of CVs background demonstrations typical features allied to the PtPd surface. In addition, the CV profile were separated into three areas/regions consist of the double layer area, reduction/surface oxide area and typical adsorption/desorption area. The CV contours of rGO-PtPd nanospheres of this study displays irregular current peaks due to rGO supports and rGO
holds of residual oxygen-containing groups [43]. It can be seen that for all nanocomposite catalysts, in the anodic scan oxidation currents started to emerge at potentials around þ0.32 V, and reached the peak values at a more positive potential of around þ0.66 V; in the reverse scan, a similar anodic voltammetric peak can be seen but at a less positive potential around þ0.48 V, suggesting effective catalytic oxidation of methanol by these nanocomposite catalysts. The hydrogen adsorption peak of integrating the area in the CV curves between 0.2 and 0.07 V displays the total charge of hydrogen adsorption and the ECSA was evaluated after a double-layer correction [44e46]. CV profile of a commercial Pt/C black catalyst and their ECSA value was also calibrated for comparison studies. The fashions in ECSA values of commercial Pt/C black and as-synthesized NCs as rGO-PtPd, rGO-Pt and rGO-Pd were calculated to be 59.05 m2 g1, 81.2 m2 g1,
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Fig. 8 e CVs of Pt/C, rGO-Pt, rGO-Pd and rGO-PtPd in 0.5 M H2SO4 (scan rate 50 mV s¡1).
Fig. 9 e CVs of Pt/C black, rGO-Pt, rGO-Pd and rGO-PtPd NCs in a solution of 0.5 M H2SO4 and 1 M CH3OH with a scan rate of 50 mV s¡1.
Fig. 10 e Currentetime curves of Pt/C black, rGO-Pt, rGO-Pd and rGO-PtPd measured in 0.5 M H2SO4 þ 1 M CH3OH at þ0.68 V.
78.5 m2 g1 and 65.1 m2 g1, respectively. It can be seen that the as-synthesized rGO-PtPd catalyst display the largest ECSA due to nanosize and uniform distribution of PtPd on rGO sheets as held by the TEM clarifications. The rGO-PtPd nanomaterial exhibits the excellent electron transfer through the surface of electrode due to large number of active sites for electro-chemical reactions by analysis of CV results. The rGOPtPd electro-catalyst enhances electro-catalytic oxidation of methanol when matched to the rGO-Pt, rGO-Pd and Pt/C black catalysts. The CV measurements further suggest that rGO is a respectable contestant as a catalyst support. The CVs measure the electro-catalytic performance of Pt/C black, rGO-Pt, rGO-Pd and rGO-PtPd to oxidation of methanol in 0.5 M H2SO4 comprising 1 M CH3OH solution as shown in Fig. 9. Due to the oxidation of methanol, all the catalysts were reveal typical fine-separated anodic peaks in the both forward and reverse scan. Methanol adsorption was detected by freshly prepared oxidation of chemisorbed species at 0.6e0.8 V [47,48]. The backward scan of faradic oxidation of remaining carbonaceous species peaks were observed at 0.5 V and formed in the forward scan on Pt and Pd surface [49e51]. Comparison of MOR performance on nanocomposites as rGOPtPd, rGO-Pt, rGO-Pd with a Pt/C black catalyst exposes that specific activity by methanol oxidation were calculated to be 0.54 mA/cm2, 0.38 mA mA/cm2, 0.34 mA/cm2 and 0.32 mA/ cm2, respectively. The rGO-PtPd catalyst demonstrates that almost 1.68 times more electro-catalytic activity than that of commercial Pt/C black. Further, the mass activity (MA) of methanol oxidation of rGO-PtPd was a double order higher (0.41 mA/mgPt) compared to the Pt/C black (0.19 mA/mgPt). Due to the larger ECSA, larger uniform distribution of PtPd on rGO sheet, huge surface area of rGO and improved electron transport, the rGO-PtPd catalyst showed superior electrocatalytic activity. Besides, CO accumulation of the catalyst tolerance was expressed by the ratios of the forward anodic peak current (If) and backward anodic peak current (Ib), If/Ib ratio can be used as a vital index [52e54]. Normally, for practical fuel cells applications, higher If/Ib values containing catalyst show the higher tolerance to intermediate carbonaceous classes. Commercial Pt/C black catalyst of If/Ib ratio was found to be 1.01. However, the size of particles as PtPd of rGOPtPd nanocomposite (4e8 nm) and a commercial Pt/C black (3 nm) was of comparable order the attendance of second metal ‘Pd’ play a vital role in increasing the activity of oxidation of methanol. The Pd metal act as extraordinary oxophilic nature than Pt metal and may be promoter for the oxidative deletion of CO on Pt due to nanosize of Pd which leads to hyper energetic for oxidation of CO. Moreover, the Pt electronic structure was modified by introduction of Pd which leads to ease fading of PteCO bond, so facile CO deletion could be estimated. In recent times, increases the tolerance of the electro-catalyst to CO poisoning due to presence of little amount of oxygen having groups, it has been reported. Therefore, in the rGO-PtPd catalyst, the existence of rGO also plays a vital role for deletion of CO-like fragments, which leads to the superior electro-catalytic activity of electrooxidation of methanol. In contrast, If/Ib values were detected for the catalysts as rGO-Pd, rGO-Pt and rGO-PtPd 1.24, 1.53 and 1.72, respectively. For increased methanol oxidation capability and lesser amount of accumulation of carbonaceous species
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due to highest If/Ib value of rGO-PtPd. In addition, the onset potential of the electro-oxidation of methanol for the rGOPtPd was considerably moved to higher negative potentials than the commercial Pt/C black, rGO-Pt and rGO-Pd. These results suggested that to increase the disconnection of methanol on rGO-PtPd. CA was explained by the catalytic activity and stability of the electro-catalysts for oxidation of methanol. The durability is the other important character of a good nanocatalyst. From Fig. 10, shows that the current-time plots of Pt/C black, rGO-Pt, rGO-Pd and rGO-PtPd materials. The CA profiles recorded at þ0.68 V in 0.5 M H2SO4 þ 1.0 M CH3OH with the same catalystsmodified electrodes. Greater current densities were detected for all the electro-catalysts in the initial period because it can be recognized to the availability of large number of active positions on the surface of the catalysts. For starting period, all the catalysts of current densities were detected in massive values due to surface of catalyst containing more number of active points. Due to the accretion of CO-like intermediates on the active positions of catalysts delaying methanol from further oxidation a rapid deterioration in current densities can be seen mainly below 50 s. Even though the current deterioration was observed for all the catalysts, rGO-PtPd reveals considerably greater current density than other nanocomposite and Pt/C black in the whole period of time. All the electrochemical results offered here determine that rGO-PtPd nanomaterial fabricated by wet reflux method can help as expert anode catalyst for DMFCs applications.
Conclusions In conclusion, this work deals a facile one-pot wet reflux method to fabricate PtPd NPs well doped and dispersed on rGO supports without any surfactants. The rGO-PtPd displays significantly superior electro-catalytic activity and durability for methanol oxidation than Pt/C black due to nanorange of particles and uniform spreading of PtPd nanoparticles on the huge surface area of rGO support. This modest and greatest methodology can open novel possibilities for enhancing the activity for DMFCs and this structure intercalating rGO-based nanosheets and Pt-based nanoalloy spheres may also be needful in other electro-chemical applications.
Acknowledgements This research was financially supported by a university grants commission (UGC), New Delhi under the award no. 2-362/2013 (SR). We also acknowledge instrumental support University of Hyderabad, Hyderabad and IISc, Bangalore is gratefully acknowledged.
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