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Superior electrocatalytic activity of PtSrCoO3−δ nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation
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Superior electrocatalytic activity of PteSrCoO3¡d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation Mehri-Saddat Ekrami-Kakhki*, Nahid Farzaneh, Ehsan Fathi Nano Technology Laboratory, Engineering Department, Esfarayen University of Technology, Esfarayen, Iran
article info
abstract
Article history:
Here, SrCoO3d was synthesized through a sol gel method and characterized with X-ray
Received 31 October 2016
powder diffraction and scanning electron microscopy techniques. Graphene oxide was
Received in revised form
synthesized by a modified Hummers' method, then functionalized with 1, 10 -dimethyl-4, 40 -
30 March 2017
bipyridinium dichloride (methyl viologen) accompanied by Chitosan to prepare novel MV-
Accepted 6 June 2017
RGO-CH. This was used as a support for nanoparticles to get PteSrCoO3d/MVeRGO-CH
Available online xxx
nanocomposite. Transmission electron microscopy image was used to show the morphology and distribution of nanoparticles. The electrocatalytic activity of PteSrCoO3d/
Keywords:
MVeRGO-CH nanocomposite for ethanol electrooxidation was investigated by cyclic vol-
Graphene oxide
tammetry, chronoamperometry, COads stripping voltammetry and electrochemical
Chitosan
impedance spectroscopy techniques. The effects of some experimental factors for ethanol
Methyl viologen
oxidation on the prepared nanocomposite were investigated and the optimum conditions
Ethanol electrooxidation
were suggested. PteSrCoO3d/MVeRGO-CH nanocomposite showed higher catalytic activ-
Fuel cell
ities than Pt/MVeRGO-CH for ethanol electrooxidation indicating that PteSrCoO3d/MV eRGO-CH nanocomposite could be a promising catalyst for direct ethanol fuel cells applications. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Due to the increasing demand of energy and the environmental pollution of fossil fuels, clean and renewable energy sources are extremely needed. Among different energy systems, direct ethanol fuel cells (DEFCs) with the direct conversion of the chemical energy of ethanol to electrical energy have been extensively studied as a new energy resource [1]. Ethanol, as a liquid fuel, has several advantages in comparison to methanol, due to its higher energy density, less toxicity, availability, and biocompatibility [2,3]. Ethanol is the renewable biofuel of the fermentation of biomass [4]. However, the
existence of CeC bond in ethanol causes the formation of more complex intermediates during its oxidation. The complete oxidation of ethanol to carbon monoxide (CO) requires breaking the CeC bond. Platinum (Pt) has high activity for ethanol oxidation reaction (EOR) and has been used extensively as an anode catalyst in fuel cells. However, the high price and easy poisoning of Pt with EOR intermediates such as carbon monoxide and CHx limit its application in fuel cells [5,6]. Nowadays, one way to improve the electrocatalytic activity of Pt catalysts is to modify the surface of Pt with a second metal [7e9]. Various studies have been done for the synthesis of new Pt-based catalysts with lower Pt loading and better tolerance to carbon monoxide. Recent investigations on
* Corresponding author. E-mail address: [email protected] (M.-S. Ekrami-Kakhki). http://dx.doi.org/10.1016/j.ijhydene.2017.06.053 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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transition metal mixed oxides with high electrical conductivities and a large amount of oxygen vacancies have shown that they can be used in fuel cells instead of noble metals [10e12]. Various ABO3/Pt composites with Ru at the B-site were reported by Mukasyan to have a comparable activity with PteRu alloy [13]. The presence of transition metal ions improves alcohol oxidation, reduces the formation of carbon monoxide, the byproduct of alcohol oxidation, and oxidizes the produced carbon monoxide to carbon dioxide. These properties make perovskites applicable for direct alcohol fuel cell [14]. Another way to improve the catalytic activity of Pt catalysts is to provide proper substrate materials. To this end, various supports have been used for the production of Pt electrocatalysts. Supporting materials play an important role in the distribution of metal nanoparticles and their particle sizes. Also, the catalytic performance and stability of catalysts depends greatly on the features of supporting materials [15e17]. Low cost, large surface area and high electrical conductivity are some of the important characteristics of good supporting materials [18,19]. Various carbon-based materials have been studied as supports for catalysts in fuel cells such as carbon nanotubes (CNTs) [20e22], mesoporous carbon [23,24], Vulcan XC-72 carbon [25,26], and carbon nanofibers [27]. In recent years, Graphene with an unusual electronic characteristic has been found to improve catalytic properties [28e30] and has been used as a catalyst support [31e34]. One way to produce graphene nanosheets is to reduce graphene oxide (GO) chemically. Reduced graphene oxide (RGO) is known as an interesting supporting material for catalysts, due to its high specific surface area [35], chemical stability, excellent electronic conductivity, high charge mobility [36], and ease of functionalization [37]. However, the reduction of graphene oxide causes agglomerates, decreases the surface area and therefore prevents from the good dispersion of Pt nanoparticles [38]. One way to modify the reduced graphene oxide is non-covalent functionalization of its surface. This improves its activity and stability for nanoparticles [39,40]. Non-covalent functionalization of graphene nanosheets is more favorable than the covalent one, as the attachment of molecules would occur by supermolecular interactions such as p-p stacking which preserves the electronic features of the graphene and avoids destruction of its electronic characteristics [41]. Recently, graphene has been functionalized non-covalently with 1,10 -dimethyl-4,40 -bipyridinium dichloride (methyl viologen) for the deposition of PtCl2 6 ions by an electrostatic selfassembly of negatively charged PtCl2 6 and positively charged functional groups of methyl viologen (MV) [42]. Various polymers such as polypyrrole, nafion, polyaniline, and chitosan (CH) have been studied as appropriate supports for dispersing nanoparticles. Chitosan is a biopolymer which is produced by the deacetylation of chitin. It has a strong affinity for transition metals [43]. The amino group in chitosan is readily protonated in acidic and neutral solutions. As chitosan solution is prepared by dissolving chitosan in 1% acetic acid aqueous solution, the amino group can be readily protonated to NHþ 3 . Thus, there would be an electrostatic attraction beþ tween PtCl2 6 and NH3 with opposite charges. In this study, we report, for the first time, the use of methyl viologen functionalized graphene sheets accompanied by
chitosan polymer (MV-RGO-CH), as a novel catalyst support for direct alcohol fuel cells (DAFCs). The electrostatic attractions between PtCl2 and the positively charged functional 6 groups of MV-RGO and CH polymer caused Pt nanoparticles to disperse well. Furthermore, using chitosan caused the good adherent of the catalysts' thin layer on the surface of the working electrode. Here, due to the importance of multifunctional catalysts, incorporating the effect of perovskite oxide SrCoO3d (SCO) to Pt catalyst onto MV-RGO-CH support has been investigated for ethanol electrooxidation in fuel cells and compared with Pt catalyst. Cyclic voltammetry, chronoamperometry, CO stripping, and EIS techniques were used for the investigation of the catalytic activity of the prepared catalysts. The effects of some experimental factors such as ethanol concentration and scan rate on anodic current density and potential of EOR were investigated and optimum conditions were suggested. Additionally, kinetic investigation has been done for ethanol electrooxidation on the prepared catalyst.
Material and methods H2SO4 (98%), HNO3, KMnO4 (99% for analysis), H2O2 (30%), HCl (37%), and graphite powder (99.5%) were purchase from Merck and used for the preparation of graphene oxide nanosheets according to a modified hummer's method [44]. NaBH4 (96% Merck) and 1,10 -dimethyl-4,40 -bipyridinium dichloride (methyl viologen, i.e. MV 98% Sigma Aldrich) were used for the preparation of MV-RGO. Strontium nitrate (Sr(NO3)2, 99%), cobalt chloride hexahydrate (CoCl2$6H2O, for analysis), Octanoic acid (99%), and NaOH from Merck were used to prepare SCO nanocatalyst. A solution of chitosan ([2-amino-2-deoxy-(1-4)b-D-glucopyranose], with medium molecular weight, 400000 Da, Fluka) was prepared in 1% acetic acid (glacial, 100% Merck) solution. Hexachloroplatinic acid (H2PtCl6) was purchased from Merck. Ethanol (C2H5OH, 99.2%, Merck) was used to investigate EOR.
Preparation of graphene oxide (GO) GO nanosheets were synthesized from graphite powder by a modified Hummers' method [45]. Typically, appropriate amounts of H2SO4 and HNO3 acids (3:1) were mixed with 2.5 g graphite powder. Having been vigorously stirred for 24 h, it was centrifuged to remove its acids and was then dried. The dried powder was transferred to a beaker. 25 ml acetone was added and sonicated for about 30 min. After being dried, 115 ml H2SO4 was added to the beaker and stirred using magnet. An ice bath was prepared and 15 g KMnO4 was added to the beaker very slowly. The deionized water was added to the mixture slowly. Afterwards, 50 ml H2O2 was added to the beaker. The mixture was centrifuged with HCl 5% aqueous solution and washed with water. The mixture was dried, characterized, and used as GO. The original Hummers method for preparing GO has these two flaws: first, toxic gasses such as NO2 and N2O4 are released during the oxidation process and the second, the residual NO 3 and Naþ ions are difficult to be removed from the waste water. Hummers' method was modified by excluding NaNO3 which
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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reduces the evolution of toxic gas. Furthermore, a mixture of H2SO4 and HNO3 was used in the modified Hummers method to facilitate the penetration of oxidation solution into graphene planes [46]. KMnO4 is one of the best oxidants in acidic media [38]. Graphite can be completely intercalated with concentrated H2SO4 with the assistance of KMnO4. Graphite bisulfate is formed and every single-layer graphene is sandwiched by the bisulfate ion layers [47,48]. In this case, KMnO4 can penetrate into graphene layers for oxidation of graphite. KMnO4 can also have the role of NaNO3.
Preparation of MV-RGO MVeRGO was synthesized by the chemical reduction of GO with NaBH4 in the presence of MV [43,49]. Typically, 5 mL of the GO dispersion (11 mg mL1) was transferred to a beaker and reached the volume of 200 ml with deionized water. It was sonicated for about 1 h. 0.0178 g of MV was dissolved in 20 ml deionized water and mixed with the GO solution. The mixture was stirred with magnet. Then, 0.33 g NaBH4 was added to the solution and stirred for 24 h. Afterwards, it was centrifuged, washed with deionized water, and dried. The black powder (MV-RGO) was prepared.
Preparation of SCO catalyst In order to prepare SrCoO3d (SCO) catalyst, 10 ml Sr(NO3)2 (0.1 M), 10 ml CoCl2 (0.1 M) solutions were mixed with each other. 2 ml of octanoic acid was added to the solution. The mixture was stirred while the pH was adjusted to 7 with NaOH 1 M. The mixture was then heated in a water bath at 80 C for 1 h. Then, it was dried in the oven at 100 C for 8 h. The dried powder was calcined in a furnace at 900 C for 4 h. The SCO nanocatalyst was obtained and characterized.
Preparation of PteSCO/MV-RGO-CH nanocomposite PteSCO/MV-RGO-CH nanocomposite was prepared as follows: The prepared MV-RGO and 2 mg of SCO nanocatalyst were dispersed in 17.5 ml deionized water and 2.5 ml chitosan. The mixture was sonicated for 1 h. 25 ml H2PtCl6 (1 M) was added to the mixture and stirred vigorously for 1 h. Then, 50 ml NaBH4 (3 M) was added to the mixture and stirred for another 24 h. The mixture was centrifuged, washed, and dried at 60 C for 12 h.
Characterization of the prepared catalysts The size and dispersion of SCO nanoparticles were observed by scanning electron microscope (SEM, KYKY, EM3200). The phases of SCO nanocatalyst were determined by X-ray diffraction (XRD, Philips PC-APD apparatus with graphite monochromatic CuKa radiation). TEM images taken with a Philips CM120 transmission electron microscope with the resolution ~2.5 A were used to observe the morphology of the prepared Pt/MVRGO-CH and PteSCO/MV-RGO-CH nanocomposites.
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Autolab (Nova software model PGSTAT 302 N, Metrohm, Netherlands) controlled by a personal computer. A conventional three-electrode cell was used with a saturated calomel electrode (SCE) as the reference electrode, a platinum electrode as the counter electrode and a glassy carbon (GC) electrode as the working electrode. The polished GC electrode was first sonicated in water and absolute ethanol. Then, it was cleaned and activated in 1.0 mol L1 H2SO4 by cyclic voltammetry (CV) technique between 1.5 and þ1.5 V. In a typical experiment, 5 ml of the prepared suspension of PteSCO/MV-RGO-CH was spread onto the glassy carbon electrode surface and dried at room temperature. The GC/Pte SCO/MV-RGO-CH electrode was prepared.
Results and discussion Material characterization Fig. 1a indicated the XRD pattern of the synthesized graphene oxide (GO) with the diffraction peak located at 2q ¼ 11.0 . The prepared perovskite-type oxide SrCoO3d was characterized by SEM (Fig. 1b). As can be seen, SEM image showed that there were some small holes inside the product. SCO nanoparticles were uniformly dispersed with many holes which show that SCO is a porous material. They could be used as suitable support for Pt nanoparticles with better dispersion of Pt nanoparticles at composite because of having higher surface area. The prepared SrCoO3d catalyst was also characterized with XRD technique shown in Fig. 1c. The XRD pattern indicated the corresponding pattern of SrCoO3d catalyst [49]. As shown in Fig. 1c, the peaks were very sharp indicating the high crystallinity of SCO catalyst. SrCoO3d patterns showed the characteristic diffraction peaks attributed to SrCoO3d (1 0 1), (1 1 0), (2 0 1), (1 1 2), (3 0 0), (2 2 0), and (3 1 1) planes at 28.6, 32.6, 43.9, 55.7, 58.4, 68.4, and 75.5, respectively, which demonstrates the hexagonal structure of the SCO sample. Lattice parameters were a ¼ 5.485, b ¼ 5.485 and c ¼ 4.137. TEM images of the prepared PteCH and PteSCOeCH nanocomposites in the absence of MV-RGO were shown in Fig. 2a and b, respectively. Fig. 2c and d showed the TEM images of Pt/MV-RGO-CH catalyst. The mean particle size of Pt nanoparticles was 4.21 nm. Comparing Fig. 2a and c showed that there is a little agglomeration in the absence of MV-RGO. The dispersion of Pt nanoparticles was more uniform in the presence of MV-RGO and chitosan substrates. The electrostatic attractions between the positively charged functional groups of CH polymer and MV-RGO, and the negatively led to the uniform dispersion of Pt nanocharged PtCl2 6 particles. The TEM images of PteSCO/MV-RGO catalyst were shown in Fig. 3. The mean particle size of Pt nanoparticles was 5.71 nm, whereas the mean particles size of SCO nanoparticles was 23.44 nm. Comparing Figs. 2b and 3 showed that in the presence of MV-RGO and chitosan substrates, the dispersion of nanoparticles was more uniform with lower agglomeration.
Electrocatalytic investigations Electrochemical investigations Hydrogen adsorption and desorption The electrochemical characterizations of PteSCO/MV-RGOCH nanocatalyst was done with a potentiostat/galvanostat
Electrochemically active surface area (EASA) is an important factor in investigating the catalytic activity of PteSCO/MV-
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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Fig. 1 e a) XRD pattern of the synthesized GO, b) SEM image of SCO catalyst and c) XRD pattern of SCO catalyst.
RGO-CH catalyst. It can be determined through cyclic voltammetry technique in acidic media with H2 adsorption and desorption voltammograms. Fig. 4A showed the CV curves of PteSCO/MV-RGO-CH, compared to Pt/MV-RGO-CH electrode in 0.5 M H2SO4 in the potential range of 0.35e1.2 V and the scan rate of 100 mV s1 with the platinum loading of 0.031 mg cm2. The voltammetric characteristics of GC/Pte SCO/MV-RGO-CH electrode (Fig. 4A) indicated the typical features of Pt metal with hydrogen adsorption and desorption between 0.05 and 0.25 V [50]. EASA for platinum is determined electrochemically from the following Equation [51]: EASA ¼ QH=0:21 ½Pt
(1)
QH is the charge passed for hydrogen adsorption and desorption. 0.21 is a parameter which relates charge to area and indicates the required charge for the oxidation of a monolayer of hydrogen on Pt particles. [Pt] is platinum loading (mg cm2). With similar Pt loading, EASA for PteSCO/MV-RGO-CH was estimated to be 74 m2 g1, which is much larger than that of Pt/MV-RGO-CH (21.8 m2 g1). The larger EASA value shown for PteSCO/MV-RGO-CH suggests that this catalyst has more active sites compared to Pt/MV-RGO-CH catalysts. It is interesting that hydrogen oxidation on Pt surface enhances with the existence of SCO nanoparticles. Similar
results have been obtained for hydrogen oxidation at various perovskite-based catalysts [52,53]. Furthermore, several studies expressed that LaFeO3, LaNiO3, LaMO3 (M ¼ Co, Ni, Cr, and Fe), and ACe1xMxO3d (A ¼ Sr or Ba, M ¼ rare earth element) should be probably involved in hydrogen adsorption [54e57]. The higher EASA value of PteSCO/MV-RGO-CH compared to that of Pt/MV-RGO-CH is attributed to the better dispersion of Pt nanoparticles on the functional groups of MV-RGO-CH, the structure effects of SCO nanoparticles, and synergism effects on PteSCO/MV-RGO-CH catalyst. EASA can be used to show the dispersion of Pt nanoparticles in the prepared catalysts. The dispersion of Pt was expressed as the fraction of surface-active Pt atoms in all the Pt atoms that can be obtained according to Eq. (2) [58]: EASA DPt ¼ 1 2 =MPt ðNA ; 4prPt Þ
(2)
NA is the Avogadro number (6.02 1023), MPt is the relative molecular weight of Pt (195.08 g/mol) and rPt is the atomic ratio of Pt (0.139 nm). DPt for PteSCO/MV-RGO-CH and Pt/MV-RGOCH catalysts were obtained 0.098 and 0.029, respectively. In the presence of SCO nanoparticles, the dispersion of Pt increased on the electrode surface.
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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Fig. 2 e TEM images of a) Pt-chitosan, b) PteSCO-chitosan, c and d) Pt/MV-RGO-CH catalysts.
CO stripping investigation To investigate the carbon monoxide (CO) oxidation properties of Pt/MV-RGO-CH and PteSCO/MV-RGO-CH catalysts, CO stripping experiments were performed. Fig. 4B indicated the adsorbed CO stripping curves of Pt/MV-RGO-CH and PteSCO/
MV-RGO-CH catalysts in the scan rate of 100 mV s1. For CO oxidation reaction, CO was purged in 0.5 M H2SO4 solution for 20 min while the potential was 0.2 V vs. SCE. Increasing adsorption time did not have any impact on voltammograms. After adsorption, the CO was removed from the
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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Fig. 3 e TEM image of PteSCO/MV-RGO catalyst. electrolyte with N2 for 30 min. Pt/MV-RGO-CH catalyst indicated a sharp and single CO oxidation peak at Ep ¼ 0.685 V and an onset potential at 0.599 V. PteSCO/MV-RGO-CH showed a sharp CO oxidation peak at Ep ¼ 0.641 V and the onset potential of CO oxidation was at 0.572 V. The onset and peak potential of CO oxidation for PteSCO/MV-RGO-CH were negatively shifted by 27 and 44 mV, respectively, compared to Pt/MV-RGO-CH. The negative shift in the onset and peak potential of CO oxidation peak indicated that CO oxidizes more easily at PteSCO/MV-RGO-CH than Pt/MV-RGO-CH catalyst [59]. The CO produced from ethanol dissociation were more easily removed from the surface of PteSCO/MV-RGO-CH
catalyst and oxidized to CO2. In this case, the CO poisoning of this catalyst was lower than Pt/MV-RGO-CH catalysts.
Ethanol oxidation reaction (EOR) The electrochemical properties of PteSCO/MV-RGO-CH, Pt/ MV-RGO-CH, Pt/MV-RGO, and Pt/CH catalysts were investigated for EOR. The cyclic voltammograms (CVs) of the prepared catalysts were recorded in 1.26 M C2H5OH and 0.5 M H2SO4 aqueous solution and the scan rate of 100 mV s1 (Fig. 5A). As shown in Fig. 5A, no current peak of EOR was observed in the CV of GC/MV-RGO-CH electrode indicating no obvious electrocatalytic activity of this electrode for EOR (CV
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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Fig. 4 e A) CV curves and B) CO stripping voltammograms of Pt/MV-RGO-CH and PteSCO/MV-RGO-CH catalysts in 0.5 M H2SO4.
at GC and GC/CH electrodes were not shown, which was similar to that of GC/MV-RGO-CH electrode). The CVs of Pt/ MV-RGO-CH, Pt/MV-RGO, Pt/CH, and PteSCO/MV-RGO-CH catalysts were also shown in Fig. 5A. As can be seen, there is a great increase in the current densities of the first and second peaks of EOR at PteSCO/MV-RGO-CH (Fig. 5A (e)) in comparison to Pt/MV-RGO-CH (Fig. 5A (c)), Pt/MV-RGO (Fig. 5A (b)), and Pt/CH (Fig. 5A (d)) catalysts. This shows that PteSCO/MV-RGOCH catalyst has a suitable electrocatalytic activity for EOR. Two oxidation peaks of EOR at PteSCO/MV-RGO-CH were seen
during a forward scan at 0.805 and 1.220 V. The third oxidation peak of ethanol electrooxidation was observed during a backward scan at 0.561 V. According to the literature [60,61], the first anodic peak of ethanol oxidation is mainly due to CO2 formation and the second anodic peak is attributed to CH3CHO formation [60]. In the backward potential sweep, the reduction of Pt oxide produces a clean PtNPs surface. Therefore, ethanol electrooxidation can occur on this clean Pt surfaces and the current peak (jb) is observed.
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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Fig. 5 e A) Cyclic voltammograms of a) MV-RGO-CH, b) Pt/MV-RGO, c) Pt/MV-RGO-CH, d) Pt/CH, e) PteSCO/MV-RGO-CH catalysts and B) Chronoamperometric curves for EOR at the prepared catalysts in 1.26 M ethanol and 0.5 M H2SO4 solution.
Quantitative analysis of the CVs of Pt/MV-RGO-CH, Pt/MVRGO, Pt/CH and PteSCO/MV-RGO-CH catalysts towards EOR was shown in Table 1. The amount of the onset potential of EOR is an important factor in direct ethanol fuel cells. As shown in Table 1, the onset potential of EOR at GC/PteSCO/ MV-RGO-CH electrode was more negative than GC/Pt/MVRGO-CH, GC/Pt/MV-RGO, and GC/Pt/CH electrodes by 57, 4, and 55 mV, respectively. This showed the enhanced
electrocatalytic activity of PteSCO/MV-RGO-CH catalyst towards EOR. Moreover, this showed that ethanol electrooxidation at PteSCO/MV-RGO-CH is much easier than at Pt/ MV-RGO-CH, Pt/MV-RGO, and Pt/CH catalysts. The forward and reverse peak current densities' ratio (jf/jb) can be used to compare the activity of the prepared catalysts for ethanol oxidation. This ratio can be utilized to determine the poisoning tolerance of the prepared catalysts to the CO-
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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Table 1 e Electrochemical data of ethanol electrooxidation at the prepared catalysts in 1.26 M ethanol and 0.5 M H2SO4 solution. Catalyst Pt/MV-RGO-CH Pt/MV-RGO Pt/CH PteSCO/MV-RGO-CH
Onset potential (V)
Ef1 (V)
jf1 (mA cm2)
Ef2 (V)
jf2 (mA cm2)
Eb (V)
jf1/jb
jf2/jb
0.297 0.244 0.295 0.240
0.688 0.661 0.749 0.805
36.728 27.90 37.23 146.75
1.060 1.057 1.157 1.220
55.456 34.89 59.57 177.90
0.39 0.405 0.451 0.561
0.705 0.625 0.648 0.994
1.065 0.781 1.037 1.204
species produced during the EOR. The larger values of jf/jb show that the synthesized materials have better CO poisoning resistance [62]. As can be seen in Table 1, PteSCO/MV-RGO-CH had higher jf1/jb and jf2/jb ratios for EOR in comparison to Pt/ MV-RGO-CH, Pt/MV-RGO, and Pt/CH catalysts, showing that PteSCO/MV-RGO-CH has an improved performance in poisoning tolerance. The surface oxygen of SrCoO3d perovskite nanoparticles would help to remove CO poisoning on Pt surface, consequently increasing the jf/jb ratio [63]. This finding was according to the CO stripping analysis. The lower onset potential, higher anodic current density, and higher jf/jb ratio of PteSCO/MV-RGO-CH catalyst indicated that this catalyst has higher catalytic activity than Pt/MVRGO-CH, Pt/MV-RGO, and Pt/CH catalysts towards ethanol oxidation. This showed that adding a certain amount of SCO nanoparticles to the matrix of Pt nanocomposite can improve its catalytic activity for ethanol electrooxidation reaction. Furthermore, as shown in Fig. 5A and Table 1, Pt/MV-RGO-CH catalyst had better catalytic activity towards EOR in comparison to Pt/MV-RGO and Pt/CH catalysts indicating that using MV-RGO accompanied by CH improves the catalytic activity of Pt catalyst towards EOR.
Amperometric i-t curve measurements The performance of Pt/MV-RGO-CH and PteSCO/MV-RGO-CH catalysts for EOR was also studied through chronoamperometry technique, shown in Fig. 5B. To examine the electrocatalytic stability of Pt/MV-RGO-CH and PteSCO/MVRGO-CH catalysts for EOR, chronoamperometry investigations were conducted in 0.5 M H2SO4 and 1.26 M ethanol solution at 0.7 V for 1000 s. As can be seen, potentiostatic current densities decreased rapidly in the initial stage for both Pt/MVRGO-CH and PteSCO/MV-RGO-CH catalysts. This decrease may be due to the formation of reactive intermediates during the EOR. As shown in Fig. 5B, after 200s, the current density reached a constant value of 1.68 mA cm2 for Pt/MV-RGO-CH catalyst, whereas the current density for PteSCO/MV-RGO-CH catalyst was 8.01 mA cm2. For as long as 1000s, the current density of EOR at PteSCO/MV-RGO-CH catalyst remained at 3.22 mA cm2 indicating the better electrocatalytic performance of this catalyst for EOR.
Electrochemical impedance spectroscopy (EIS) for EOR EIS investigations were also done to study the behavior of PteSCO/MV-RGO-CH and Pte/MV-RGO-CH catalysts for
Fig. 6 e The Nyquist plots of Pt/MV-RGO-CH and PteSCO/MV-RGO-CH catalysts in 0.5 M H2SO4 and 1.26 M C2H5OH aqueous solution. Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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ethanol electrooxidation reaction. The electrochemical behavior of working electrodes modified with the prepared catalysts for ethanol electrooxidation can be investigated by Nyquist plot [64]. Electrochemical impedance investigation with transmission line model fit to Nyquist plots in the range of 1 104 to 102 Hz at open circuit potential (OCP) in 0.5 M H2SO4 and 1.26 M ethanol was shown in Fig. 6. As can be seen in Fig. 6, all the Nyquist plots showed a small semicircle in high frequency region which is due to charge transfer process in the interface of the electrode and electrolyte and a line in the low-frequency region which is related to diffusion process in the prepared electrodes. According to the impedance investigation, the equivalent circuit of PteSCO/MV-RGO-CH catalyst was shown in the inset of Fig. 6. As can be seen, in the equivalent circuit, Rs is due
to solution resistance, and constant-phase element (CPE) is attributed to the double-layer capacitance. R0 and C0 are due to the resistance and capacitance of the adsorbed CO intermediates produced during ethanol electrooxidation, respectively [65]. A more vertical straight line at PteSCO/ MV-RGO-CH than Pt/MV-RGO-CH showed the improved ions diffusion feature of PteSCO/MV-RGO-CH catalyst for EOR [66].
Parameters affecting ethanol electrooxidation Our investigations showed that several parameters such as scan rate, temperature, and ethanol concentration influence the performance of the prepared electrodes for ethanol electrooxidation. These parameters were thus investigated and optimized.
Fig. 7 e Anodic current density (jf) vs. square root of scan rate (y0.5),-, and irreversibility plot showing peak potential (Ef) vs. ln y,C, for PteSCO/MV-RGO-CH electrode in 1.26 M ethanol and 0.5 M H2SO4 at the scan rates of 30, 50, 70, 100, 130, 160 and 190 mV s¡1. Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 5
To determine the effect of scan rate, the catalytic activity of PteSCO/MV-RGO-CH catalysts towards ethanol electrooxidation was investigated at the scan rates of 30, 50, 70, 100, 130, 160, and 190 mVs1 in 1.26 M ethanol and 0.5 M H2SO4. The anodic peak current densities of EOR vs. the square root of the scan rate (y) and the peak potential vs. ln y were displayed in Fig. 7. As can be seen, the anodic peak current density of EOR increased with increase in the applied scan rate. As can be observed, there is a linear relationship between the square root of the scan rate (y0.5) and the first anodic peak current density (jf1) (R2 ¼ 0.93) and a linear relationship (R2 ¼ 0.96) between y0.5 and the second anodic peak current density (jf2) indicating that the diffusion of ethanol from the bulk solution to the surface of the electrode controls the process of ethanol electrooxidation [67,68]. Also a linear relationship (0.98) between Ef1 and ln (y) and a linear relationship (R2 ¼ 0.97) between Ef2 and ln (y) indicate an irreversible charge transfer process for EOR [69,70]. The effect of ethanol concentration on the anodic current density of ethanol oxidation on PteSCO/MV-RGO-CH catalyst was shown in Fig. 8. The scan rate was 100 mV s1. As can be seen, anodic current density increases with increase in ethanol concentration and levels off at concentrations higher than 1.26 M. This is probably due to the saturation of the active sites on the electrode surface. To get a higher current density, this concentration (1.26 M) can be selected as the optimum concentration. At PteSCO/MV-RGO-CH catalyst, with increase in ethanol concentration from 0.06 to 1.35 M, Ef1 shifts towards a positive direction from 0.634 to 0.805 V and Ef2 shifts towards a positive direction from 1.098 to 1.245 V. This is probably due to the increase of the poisoning rate of Pt catalyst. Thus, the
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oxidative removal of strongly adsorbed intermediates occurs at more positive potentials [71]. The electrocatalytic activities of PteSCO/MV-RGO-CH and Pt/MV-RGO-CH nanocatalysts towards EOR were studied in different temperatures ranging from 25 to 50 C and the scan rate of 100 mV s1. As shown in Fig. 9, anodic current density was increased with increase in the temperature. This way, mass transport is an important factor for higher activity. For PteSCO/MV-RGO-CH catalyst (Fig. 9A), as temperature increased from 25 to 50 C, jf1 increased from 140.128 to 212.651 mA cm2 and jf2 increased from 170.461 to 226.646 mA cm2. With increase in temperature, the higher current density at the same ethanol concentration shows that the fine structure of the catalyst possesses more available Pt active sites to participate in electrochemical reaction. For Pt/ MV-RGO-CH catalyst (Fig. 9B), as the temperature increased from 25 to 50 C, jf1 increased from 27.78 to 84.75 mA cm2 and jf2 increased from 45.4 to 88.36 mA cm2. The activation energies of the first and second peaks of ethanol electrooxidation at PteSCO/MV-RGO-CH and Pt/MVRGO-CH catalysts were also calculated. As observed in Fig. 9A, the Arrhenius plots of logarithm of exchange current density (log jp) versus the reciprocal of temperature (T1) were also shown for PteSCO/MV-RGO-CH catalyst. Activation energies were calculated using the following equation: v ln jp DH* ¼ R v T1
(3)
The activation energies of the first (DH1) and second (DH2) anodic peaks of ethanol electrooxidation at PteSCO/MV-RGOCH catalyst were 6.28 and 4.27 kJ mol1, respectively. Activation
Fig. 8 e Cyclic voltammograms for EOR on PteSCO/MV-RGO-CH electrode in 0.5 M H2SO4 in different concentration of ethanol: a) 0.057, b) 0.11, c) 0.17, d) 0.22, e) 0.28, f) 0.33, g) 0.39, h) 0.44, i) 0.50, j) 0.55, k) 0.60, l) 0.66, m) 0.71, n) 0.76, o) 0.81, p) 0.86, q) 0.91, r) 0.96, s) 1.02, t) 1.07, u) 1.11, v) 1.16, w) 1.21, x) 1.26, y) 1.31 and z) 1.35 M. Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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Fig. 9 e Cyclic voltammograms of A) PteSCO/MV-RGO-CH and B) Pt/MV-RGO-CH catalysts in different temperatures of a) 25, b) 30, c) 35, d) 40, e) 45 and f) 50 C. The Arrhenius plots of logarithm of exchange current density (log jp) versus the reciprocal of temperature (T-1) were shown for the first and second peaks of EOR at PteSCO/MV-RGO-CH catalyst.
energies of the first and second anodic peaks of EOR at Pt/MVRGO-CH catalyst were 13.91 and 8.76 kJ mol1, respectively. As can be seen, the first and second anodic peaks of ethanol electrooxidation at PteSCO/MV-RGO-CH catalyst had lower activation energies than those of Pt/MV-RGO-CH catalyst.
Durability test The poisoning effect of PteSCO/MV-RGO-CH catalyst during ethanol electrooxidation was investigated through cyclic voltammetry with 50 cycles repeatedly in the scan rate of 100 mV s1 (Fig. 10). jf1 and jf2 of PteSCO/MV-RGO-CH and Pt/
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 5
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Fig. 10 e CV curves of PteSCO/MV-RGO-CH catalyst during the 50 cycles in H2SO4 0.5 M and ethanol 1.26 M solution. jf1 and jf2 of PteSCO/MV-RGO-CH and Pt/MV-RGO-CH catalysts as a function of the cycle number for EOR are also shown.
MV-RGO-CH catalysts for EOR as a function of cycle number were also shown in Fig. 10. As can be seen, for Pt/MV-RGO-CH, in the first cycle, jf1 and jf2 were 65.369 and 80.103, respectively, and in the 50th cycle, they were 43.842 and 70.151 mA cm2, respectively. This means at Pt/MV-RGO-CH catalyst during the 50 cycle, jf1 decreased by 32.9% and jf2 decreased by 12.42%. For PteSCO/MV-RGO-CH, in the first cycle, jf1 and jf2 were 141.819 and 163.133 mA cm2, respectively and at the 50th cycle, they were 122.118 and 144.725 mA cm2, respectively. This shows that at PteSCO/ MV-RGO-CH catalyst during the 50th cycle, jf1 decreased by 13.9% and jf2 decreased by 11.28%. This investigation showed that the incorporation of SCO nanoparticles in the Pt/MVRGO-CH matrix enhanced the durability of the electrode towards ethanol electrooxidation.
Conclusions In this study, a novel PteSCO/MV-RGO-CH nanocomposite was successfully synthesized and characterized. The catalytic
activity of this catalyst was investigated for ethanol electrooxidation and compared with that of Pt/MV-RGO-CH catalyst. The catalytic activity of the electrode depends on the surface available for the dispersion of metallic particles. The high electrocatalytic activities of Pt/MV-RGO-CH and PteSCO/MV-RGOCH for EOR may be due to the good dispersion of nanoparticles confirmed with TEM images. The presence of methyl viologen and chitosan on graphene sheets probably leads to the better dispersion of nanoparticles and prevents from the agglomeration of metallic particles. The results showed that, incorporation of SCO nanoparticles into Pt catalyst can significantly improve the performance of electrode for ethanol electrooxidation. The electrocatalytic activity of PteSCO/MV-RGO-CH for EOR in acidic media was higher than that of Pt/MV-RGO-CH nanocatalysts, due to its higher anodic current densities, higher electrochemically active surface area, and better antipoisoning effect. By and large, the investigations showed that PteSCO/MV-RGO-CH is a promising catalyst for ethanol electrooxidation in direct ethanol fuel cells and MV-RGO-CH is potentially a good support for nanoparticles in fuel cell applications.
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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Acknowledgment We thank Esfarayen University of Technology for financial support.
references
[1] Kakaei K. Decoration of graphene oxide with platinum tin nanoparticles for ethanol oxidation. Electrochimica Acta 2015;165:330e7. [2] Sedighi M, Rostami AA, Alizadeh E. Enhanced electrooxidation of ethanol using Pt-CeO2 electrocatalyst prepared by electrodeposition technique. Int J Hydrogen Energy 2017;42:4998e5005. [3] Ding K, Zhao Y, Liu L, Cao Y, Wang Q, Gu H, et al. Pt-Ni bimetallic composite nanocatalysts prepared by using multiwalled carbon nanotubes as reductants for ethanol oxidation reaction. Int J Hydrogen Energy 2014;39:17622e33. [4] Demirbas A. Progress and recent trends in biofuels. Prog Energy Combust Sci 2007;33:1e18. [5] Zhang C, Zhu A, Huang R, Zhang Q, Liu Q. Hollow nanoporous Au/Pt core-shell catalysts with nanochannels and enhanced activities towards electro-oxidation of methanol and ethanol. Int J Hydrogen Energy 2014;39:8246e56. [6] Qu Y, Li C, Wang L, Gao Y, Rao J, Yin G. Mild synthesis of layer-by-layer SnO2 nanosheet/Pt/graphene composites as catalysts for ethanol electro-oxidation. Int J Hydrogen Energy 2016;41:14036e46. [7] Ekrami-Kakhki MS, Yavari Z, Saffari J, Abbasi S. Fabrication and evaluation of Pt/M (M¼ Co, Fe) chitosan supported catalysts for methanol electrooxidation: application in direct alcohol fuel cell. J Nanostruct 2016;6:221e34. [8] Khorasani-Motlagh M, Noroozifar M, Ekrami-Kakhki MS. Investigation of the nanometals (Ni and Sn) in platinum binary and ternary electrocatalysts for methanol electrooxidation. Int J Hydrogen Energy 2011;36:11554e63. [9] Noroozifar M, Khorasani-Motlagh M, Ekrami-Kakhki MS, Khaleghian-Moghadam R. Enhanced electrocatalytic properties of Pt-chitosan nanocomposite for direct methanol fuel cell by LaFeO3 and carbon nanotube. J Power Sources 2014;248:130e9. [10] Ding X, Gao X, Zhu W, Wang J, Jiang J. Electrode redox properties of Ba1xLaxFeO3d as cobalt free cathode materials for intermediate-temperature SOFCs. Int J Hydrogen Energy 2014;39:12092e100. [11] Ekrami-Kakhki MS, Yavari Z, Saffari J, Ekrami-Kakhki SA. Perovskite-type LaFeO3 and LaFeO3-CNTs nanocrystals as active anode for methanol oxidation in alkaline solutions. J Electr Eng 2016;4:88e99. [12] Miao G, Yuan C, Chen T, Zhou Y, Zhan W, Wang S. Sr2Fe1þxMo1xO6d as anode material of cathodeesupported solid oxide fuel cells. Int J Hydrogen Energy 2016;41:1104e11. [13] Lan A, Mukasyan AS. Complex SrRuO3-Pt and LaRuO3-Pt catalysts for direct alcohol fuel cells. Ind Eng Chem Res 2008;47:8989e94. ndez[14] Martı´nez-Coronado R, Alonso JA, Aguadero A, Ferna Dı´az MT. New SrMo1xCrxO3d perovskites as anodes in solid-oxide fuel cells. Int J Hydrogen Energy 2014;39:4067e73. [15] Konopka DA, Li M, Artyushkova K, Marinkovic N, Sasaki K, Adzic R, et al. Platinum supported on NbRuyOz as electrocatalyst for ethanol oxidation in acid and alkaline fuel cells. J Phys Chem C 2011;115:3043e56.
[16] Fang B, Kim M, Yu JS. Hollow core/mesoporous shell carbon as a highly efficient catalyst support in direct formic acid fuel cell. Appl Catal B Environ 2008;84:100e5. [17] Shao Y, Yin G, Gao Y, Shi P. Durability study of Pt ∕ C and Pt ∕ CNTs catalysts under simulated PEM fuel cell conditions. J Electrochem Soc 2006;153:A1093e7. [18] Du H, Li B, Kang F, Fu R, Zeng Y. Carbon aerogel supported PteRu catalysts for using as the anode of direct methanol fuel cells. Carbon 2007;45:429e35. [19] Park IS, Park KW, Choi JH, Park CR, Sung YE. Electrocatalytic enhancement of methanol oxidation by graphite nanofibers with a high loading of PtRu alloy nanoparticles. Carbon 2007;45:28e33. [20] Li L, Qian Y, Yang J, Tan X, Dai Z, Jin Y, et al. A novel structural design of hybrid nanotube with CNTs and CeO2 supported Pt nanoparticles with improved performance for methanol electro-oxidation. Int J Hydrogen Energy 2016;41:9284e94. [21] Wang HL, Kakad BA, Tamaki T, Yamaguchi T. Synthesis of 3D graphite oxide-exfoliated carbon nanotube carbon composite and its application as catalyst support for fuel cells. J Power Sources 2014;260:338e48. [22] Li X, Wang HJ, Yua H, Liu ZW, Peng F. An opposite change rule in carbon nanotubes supported platinum catalyst for methanol oxidation and oxygen reduction reactions. J Power Sources 2014;260:1e5. [23] John SS, Dutta I, Angelopoulos AP. Enhanced electrocatalytic oxygen reduction through electrostatic assembly of Pt nanoparticles onto porous carbon supports from SnCl2stabilized suspensions. Langmuir 2011;27:5781e91. [24] Qi J, Jiang L, Wang S, Sun G. Synthesis of graphitic mesoporous carbons with high surface areas and their applications in direct methanol fuel cells. Appl Catal B Environ 2011;107:95e103. ger JM, Alonso-Vante N, Lamy C. [25] Yang H, Coutanceau C, Le Methanol tolerant oxygen reduction on carbon-supported PteNi alloy nanoparticles. J Electroanal Chem 2005;576:305e13. pez-Vizcaı´no R. [26] Lobato J, Canizares P, Rodrigo MA, Linares JJ, Lo Performance of a vapor-fed polybenzimidazole (PBI)-based direct methanol fuel cell. Energy Fuels 2008;22:3335e45. [27] Lobato J, Canizares P, Ubeda D, Pinar FJ, Rodrigo MA. Testing PtRu/CNF catalysts for a high temperature polybenzimidazole-based direct ethanol fuel cell. Effect of metal content. Appl Catal B Environ 2011;106:174e80. [28] Yang H, Li F, Shan C, Han D, Zhang Q, Niu L, et al. Covalent functionalization of chemically converted graphene sheets via silane and its reinforcement. J Mater Chem 2009;19:4632e8. [29] Kou R, Shao Y, Mei D, Nie Z, Wang D, Wang C, et al. Stabilization of electrocatalytic metal nanoparticles at metalmetal oxidegraphene triple junction points. J Am Chem Soc 2011;133:2541e7. [30] Yang F, Liu Y, Gao L, Sun J. pH-sensitive highly dispersed reduced graphene oxide solution using lysozyme via an in Situ reduction method. J Phys Chem C 2010;114:22085e91. [31] Hu Y, Zhang H, Wu P, Zhang H, Zhou B, Cai C. Bimetallic PteAu nanocatalysts electrochemically deposited on graphene and their electrocatalytic characteristics towards oxygen reduction and methanol oxidation. Phys Chem Chem Phys 2011;13:4083e94. [32] Hu YJ, Jin J, Wu P, Zhang H, Cai CX. Grapheneegold nanostructure composites fabricated by electrodeposition and their electrocatalytic activity toward the oxygen reduction and glucose oxidation. Electrochimica Acta 2010;56:491e500. [33] Zhang H, Xu X, Gu P, Li C, Wu P, Cai C. Microwave-assisted synthesis of graphene-supported Pd1Pt3 nanostructures and
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 5
[34]
[35]
[36]
[37]
[38] [39]
[40]
[41]
[42]
[43] [44] [45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
their electrocatalytic activity for methanol oxidation. Electrochimica Acta 2011;56:7064e70. Shang N, Papakonstantinou P, Wang P, Silva SRP. Platinum integrated graphene for methanol fuel cells. J Phys Chem 2010;114:15837e41. Kakaei K. One-pot electrochemical synthesis of graphene by the exfoliation of graphite powder in sodium dodecyl sulfate and its decoration with platinum nanoparticles for methanol oxidation. Carbon 2013;51:195e201. Kakaei K, Gharibi H. Palladium nanoparticle catalysts synthesis on graphene in sodium dodecyl sulfate for oxygen reduction reaction. Energy 2014;65:166e71. Wang H, Maiyalagan T, Wang X. Review on recent progress in nitrogen-doped graphene: synthesis, characterization, and its potential applications. ACS Catal 2012;2:781e94. Dreyer DR, Park S, Bielawski CW, Ruoff RS. The chemistry of graphene oxide. Chem Soc Rev 2010;39:228e40. Ma JH, Wang L, Mu X, Li L. Nitrogen-doped graphene supported Pt nanoparticles with enhanced performance for methanol oxidation. Int J Hydrogen Energy 2015;40: 2641e7. Wang S, Wang X, Jiang SP. Self-assembly of mixed Pt and Au nanoparticles on PDDA functionalized graphene as effective electrocatalysts for formic acid oxidation of fuel cells. Phys Chem Chem Phys 2011;13:6883e91. Zhao J, Yu H, Liu Z, Ji M, Zhang L, Sun G. Supercritical deposition route of preparing Pt/graphene composites and their catalytic performance toward methanol electrooxidation. J Phys Chem C 2014;118:1182e90. Ma J, Wang L, Mu X, Cao Y. Enhanced electrocatalytic activity of Pt nanoparticles supported on functionalized graphene for methanol oxidation and oxygen reduction. J Colloid Interface Sci 2015;457:102e7. Guibal E. Heterogeneous catalysis on chitosan-based materials: a review. Prog Polym Sci 2005;30:71e109. Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80. 1339e9. Chen J, Yao B, Li C, Shi G. An improved Hummers method for eco-friendly synthesis of graphene oxide. Carbon 2013;64:225e9. Shin YR, Jung SM, Jeon IY, Baek JB. The oxidation mechanism of highly ordered pyrolytic graphite in a nitric acid/sulfuric acid mixture. Carbon 2013;52:493e8. Avdeev VV, Monyakina LA, Nikolskaya IV, Sorokina NE, Semenenko KN. The choice of oxidizers for graphite hydrogenosulfate chemical synthesis. Carbon 1992;30:819e23. Sorokina NE, Khaskov MA, Avdeev VV, Nikol'skaya IV. Reaction of graphite with sulfuric acid in the presence of KMnO4. Russ J Gen Chem 2005;75:162e8. Zeng P, Ran R, Chen Z, Zhou W, Gu H, Shao Z, et al. Efficient stabilization of cubic perovskite SrCoO3-d by B-site low concentration scandium doping combined with solegel synthesis. J Alloy Compd 2008;455:465e70. Xu C, Cheng L, Shen P, Liu Y. Methanol and ethanol electrooxidation on Pt and Pd supported on carbon microspheres in alkaline media. Electrochem Commun 2007;9:997e1001. Saha MS, Li R, Sun X. High loading and monodispersed Pt nanoparticles on multiwalled carbon nanotubes for high performance proton exchange membrane fuel cells. J Power Sources 2008;177:314e22. Deshpande K, Mukasyan A, Varma A. High throughput evaluation of perovskite-based anode catalysts for direct methanol fuel cells. J Power Sources 2006;158:60e8. Zhu T, Fowler DE, Poeppelmeier KR, Han M, Barnett SA. Hydrogen oxidation mechanisms on perovskite solid oxide fuel cell anodes. J Electrochem Soc 2016;163:F952e61.
15
[54] Boateng IW, Tia R, Adei E, Dzade NY, Catlow CRA, de Leeuw NH. A DFTþU investigation of hydrogen adsorption on the LaFeO3(010) surface. Phys Chem Chem Phys 2017;19:7399e409. [55] Pan C, Chen Y, Wu N, Zhang M, Yuan L, Zhang C. A first principles study of H2 adsorption on LaNiO3(001) surfaces. Materials 2017;10:1e13. [56] Tejuca LG. Temperature programmed desorption study of hydrogen adsorption on LaMO3 oxides. Thermochimica Acta 1988;126:205e12. [57] Esaka T, Sakaguchi H, Kobayashi S. Hydrogen storage in proton-conductive perovskite- type oxides and their application to nickel-hydrogen batteries. Solid State Ionics 2004;166:351e7. [58] Zhou C, Chen Y, Guo Z, Wang X, Yang Y. Promoted aerobic oxidation of benzyl alcohol on CNT supported platinum by iron oxide. Chem Commun 2011;47:7473e5. [59] Yang Z, Nagashima A, Fujigaya T, Nakashima N. Electrocatalyst composed of platinum nanoparticles deposited on doubly polymer-coated carbon nanotubes shows a high CO-tolerance in methanol oxidation reaction. Int J Hydrogen Energy 2016;41:19182e90. [60] Fujiwara N, Friedrich KA, Stimming U. Ethanol oxidation on PtRu electrodes studied by differential electrochemical mass spectrometry. J Electroanal Chem 1999;472:120e5. [61] Hitmi H, Belgsir EM, Leger JM, Lamy C, Lezna RO. A kinetic analysis of the electro-oxidation of ethanol at a platinum electrode in acid medium. Electrochimica Acta 1994;39:407e15. [62] Naidoo QL, Naidoo S, Petrik L, Nechaev A, Ndungu P. The influence of carbon based supports and the role of synthesis procedures on the formation of platinum and platinumruthenium clusters and nanoparticles for the development of highly active fuel cell catalysts. Int J Hydrogen Energy 2012;37:9459e69. [63] Raghuveer V, Thampi KR, Xanthopoulos N, Mathieu HJ, Viswanathan B. Rare earth cuprates as electrocatalysts for methanol oxidation. Solid State Ionics 2001;140:263e74. [64] Ding K, Jia Z, Wang Q, He X, Tian N, Tong R, et al. Electrochemical behavior of the self-assembled membrane formed by calmodulin (CaM) on a Au substrate. J Electroanal Chem 2001;513:67e71. [65] He Q, Chen W, Mukerjee S, Chen S, Laufek F. Carbonsupported PdM (M¼Au and Sn) nanocatalysts for the electrooxidation of ethanol in high pH media. J Power Sources 2009;187:298e304. [66] Kakaei K, Rahimi A, Husseindoost S, Hamidi M, Javan H, Balavandi A. Fabrication of Pt- CeO2 nanoparticles supported sulfonated reduced graphene oxide as an efficient electrocatalyst for ethanol oxidation. Int J Hydrogen Energy 2016;41:3861e9. [67] Honda K, Yoshimura M, Rao TN, Tryk DA, Fujishima A, Yasui K, et al. Electrochemical properties of Pt-modified nano-honeycomb diamond electrodes. J Electroanal Chem 2001;514:35e50. [68] Zhao Y, Wang R, Han Z, Li C, Wang Y, Chi B, et al. Electrooxidation of methanol and ethanol in acidic medium using a platinum electrode modified with lanthanum-doped tantalum oxide film. Electrochimica Acta 2015;151:544e51. [69] Guo DJ, Li HL. Electrocatalytic oxidation of methanol on Pt modified single-walled carbon nanotubes. J Power Sources 2006;160:44e9. [70] Pattabiraman R. Electrochemical investigations on carbon supported palladium catalysts. Appl Catal A Gen 1997;153:9e20. [71] He Z, Chen J, Liu D, Zhou H, Kuang Y. Electrodeposition of PtRu nanoparticles on carbon nanotubes and their electrocatalytic properties for methanol electrooxidation. Diam Relat Mater 2004;13:1764e70.
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
Available online at www.sciencedirect.com
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Superior electrocatalytic activity of PteSrCoO3¡d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation Mehri-Saddat Ekrami-Kakhki*, Nahid Farzaneh, Ehsan Fathi Nano Technology Laboratory, Engineering Department, Esfarayen University of Technology, Esfarayen, Iran
article info
abstract
Article history:
Here, SrCoO3d was synthesized through a sol gel method and characterized with X-ray
Received 31 October 2016
powder diffraction and scanning electron microscopy techniques. Graphene oxide was
Received in revised form
synthesized by a modified Hummers' method, then functionalized with 1, 10 -dimethyl-4, 40 -
30 March 2017
bipyridinium dichloride (methyl viologen) accompanied by Chitosan to prepare novel MV-
Accepted 6 June 2017
RGO-CH. This was used as a support for nanoparticles to get PteSrCoO3d/MVeRGO-CH
Available online xxx
nanocomposite. Transmission electron microscopy image was used to show the morphology and distribution of nanoparticles. The electrocatalytic activity of PteSrCoO3d/
Keywords:
MVeRGO-CH nanocomposite for ethanol electrooxidation was investigated by cyclic vol-
Graphene oxide
tammetry, chronoamperometry, COads stripping voltammetry and electrochemical
Chitosan
impedance spectroscopy techniques. The effects of some experimental factors for ethanol
Methyl viologen
oxidation on the prepared nanocomposite were investigated and the optimum conditions
Ethanol electrooxidation
were suggested. PteSrCoO3d/MVeRGO-CH nanocomposite showed higher catalytic activ-
Fuel cell
ities than Pt/MVeRGO-CH for ethanol electrooxidation indicating that PteSrCoO3d/MV eRGO-CH nanocomposite could be a promising catalyst for direct ethanol fuel cells applications. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Due to the increasing demand of energy and the environmental pollution of fossil fuels, clean and renewable energy sources are extremely needed. Among different energy systems, direct ethanol fuel cells (DEFCs) with the direct conversion of the chemical energy of ethanol to electrical energy have been extensively studied as a new energy resource [1]. Ethanol, as a liquid fuel, has several advantages in comparison to methanol, due to its higher energy density, less toxicity, availability, and biocompatibility [2,3]. Ethanol is the renewable biofuel of the fermentation of biomass [4]. However, the
existence of CeC bond in ethanol causes the formation of more complex intermediates during its oxidation. The complete oxidation of ethanol to carbon monoxide (CO) requires breaking the CeC bond. Platinum (Pt) has high activity for ethanol oxidation reaction (EOR) and has been used extensively as an anode catalyst in fuel cells. However, the high price and easy poisoning of Pt with EOR intermediates such as carbon monoxide and CHx limit its application in fuel cells [5,6]. Nowadays, one way to improve the electrocatalytic activity of Pt catalysts is to modify the surface of Pt with a second metal [7e9]. Various studies have been done for the synthesis of new Pt-based catalysts with lower Pt loading and better tolerance to carbon monoxide. Recent investigations on
* Corresponding author. E-mail address: [email protected] (M.-S. Ekrami-Kakhki). http://dx.doi.org/10.1016/j.ijhydene.2017.06.053 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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transition metal mixed oxides with high electrical conductivities and a large amount of oxygen vacancies have shown that they can be used in fuel cells instead of noble metals [10e12]. Various ABO3/Pt composites with Ru at the B-site were reported by Mukasyan to have a comparable activity with PteRu alloy [13]. The presence of transition metal ions improves alcohol oxidation, reduces the formation of carbon monoxide, the byproduct of alcohol oxidation, and oxidizes the produced carbon monoxide to carbon dioxide. These properties make perovskites applicable for direct alcohol fuel cell [14]. Another way to improve the catalytic activity of Pt catalysts is to provide proper substrate materials. To this end, various supports have been used for the production of Pt electrocatalysts. Supporting materials play an important role in the distribution of metal nanoparticles and their particle sizes. Also, the catalytic performance and stability of catalysts depends greatly on the features of supporting materials [15e17]. Low cost, large surface area and high electrical conductivity are some of the important characteristics of good supporting materials [18,19]. Various carbon-based materials have been studied as supports for catalysts in fuel cells such as carbon nanotubes (CNTs) [20e22], mesoporous carbon [23,24], Vulcan XC-72 carbon [25,26], and carbon nanofibers [27]. In recent years, Graphene with an unusual electronic characteristic has been found to improve catalytic properties [28e30] and has been used as a catalyst support [31e34]. One way to produce graphene nanosheets is to reduce graphene oxide (GO) chemically. Reduced graphene oxide (RGO) is known as an interesting supporting material for catalysts, due to its high specific surface area [35], chemical stability, excellent electronic conductivity, high charge mobility [36], and ease of functionalization [37]. However, the reduction of graphene oxide causes agglomerates, decreases the surface area and therefore prevents from the good dispersion of Pt nanoparticles [38]. One way to modify the reduced graphene oxide is non-covalent functionalization of its surface. This improves its activity and stability for nanoparticles [39,40]. Non-covalent functionalization of graphene nanosheets is more favorable than the covalent one, as the attachment of molecules would occur by supermolecular interactions such as p-p stacking which preserves the electronic features of the graphene and avoids destruction of its electronic characteristics [41]. Recently, graphene has been functionalized non-covalently with 1,10 -dimethyl-4,40 -bipyridinium dichloride (methyl viologen) for the deposition of PtCl2 6 ions by an electrostatic selfassembly of negatively charged PtCl2 6 and positively charged functional groups of methyl viologen (MV) [42]. Various polymers such as polypyrrole, nafion, polyaniline, and chitosan (CH) have been studied as appropriate supports for dispersing nanoparticles. Chitosan is a biopolymer which is produced by the deacetylation of chitin. It has a strong affinity for transition metals [43]. The amino group in chitosan is readily protonated in acidic and neutral solutions. As chitosan solution is prepared by dissolving chitosan in 1% acetic acid aqueous solution, the amino group can be readily protonated to NHþ 3 . Thus, there would be an electrostatic attraction beþ tween PtCl2 6 and NH3 with opposite charges. In this study, we report, for the first time, the use of methyl viologen functionalized graphene sheets accompanied by
chitosan polymer (MV-RGO-CH), as a novel catalyst support for direct alcohol fuel cells (DAFCs). The electrostatic attractions between PtCl2 and the positively charged functional 6 groups of MV-RGO and CH polymer caused Pt nanoparticles to disperse well. Furthermore, using chitosan caused the good adherent of the catalysts' thin layer on the surface of the working electrode. Here, due to the importance of multifunctional catalysts, incorporating the effect of perovskite oxide SrCoO3d (SCO) to Pt catalyst onto MV-RGO-CH support has been investigated for ethanol electrooxidation in fuel cells and compared with Pt catalyst. Cyclic voltammetry, chronoamperometry, CO stripping, and EIS techniques were used for the investigation of the catalytic activity of the prepared catalysts. The effects of some experimental factors such as ethanol concentration and scan rate on anodic current density and potential of EOR were investigated and optimum conditions were suggested. Additionally, kinetic investigation has been done for ethanol electrooxidation on the prepared catalyst.
Material and methods H2SO4 (98%), HNO3, KMnO4 (99% for analysis), H2O2 (30%), HCl (37%), and graphite powder (99.5%) were purchase from Merck and used for the preparation of graphene oxide nanosheets according to a modified hummer's method [44]. NaBH4 (96% Merck) and 1,10 -dimethyl-4,40 -bipyridinium dichloride (methyl viologen, i.e. MV 98% Sigma Aldrich) were used for the preparation of MV-RGO. Strontium nitrate (Sr(NO3)2, 99%), cobalt chloride hexahydrate (CoCl2$6H2O, for analysis), Octanoic acid (99%), and NaOH from Merck were used to prepare SCO nanocatalyst. A solution of chitosan ([2-amino-2-deoxy-(1-4)b-D-glucopyranose], with medium molecular weight, 400000 Da, Fluka) was prepared in 1% acetic acid (glacial, 100% Merck) solution. Hexachloroplatinic acid (H2PtCl6) was purchased from Merck. Ethanol (C2H5OH, 99.2%, Merck) was used to investigate EOR.
Preparation of graphene oxide (GO) GO nanosheets were synthesized from graphite powder by a modified Hummers' method [45]. Typically, appropriate amounts of H2SO4 and HNO3 acids (3:1) were mixed with 2.5 g graphite powder. Having been vigorously stirred for 24 h, it was centrifuged to remove its acids and was then dried. The dried powder was transferred to a beaker. 25 ml acetone was added and sonicated for about 30 min. After being dried, 115 ml H2SO4 was added to the beaker and stirred using magnet. An ice bath was prepared and 15 g KMnO4 was added to the beaker very slowly. The deionized water was added to the mixture slowly. Afterwards, 50 ml H2O2 was added to the beaker. The mixture was centrifuged with HCl 5% aqueous solution and washed with water. The mixture was dried, characterized, and used as GO. The original Hummers method for preparing GO has these two flaws: first, toxic gasses such as NO2 and N2O4 are released during the oxidation process and the second, the residual NO 3 and Naþ ions are difficult to be removed from the waste water. Hummers' method was modified by excluding NaNO3 which
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reduces the evolution of toxic gas. Furthermore, a mixture of H2SO4 and HNO3 was used in the modified Hummers method to facilitate the penetration of oxidation solution into graphene planes [46]. KMnO4 is one of the best oxidants in acidic media [38]. Graphite can be completely intercalated with concentrated H2SO4 with the assistance of KMnO4. Graphite bisulfate is formed and every single-layer graphene is sandwiched by the bisulfate ion layers [47,48]. In this case, KMnO4 can penetrate into graphene layers for oxidation of graphite. KMnO4 can also have the role of NaNO3.
Preparation of MV-RGO MVeRGO was synthesized by the chemical reduction of GO with NaBH4 in the presence of MV [43,49]. Typically, 5 mL of the GO dispersion (11 mg mL1) was transferred to a beaker and reached the volume of 200 ml with deionized water. It was sonicated for about 1 h. 0.0178 g of MV was dissolved in 20 ml deionized water and mixed with the GO solution. The mixture was stirred with magnet. Then, 0.33 g NaBH4 was added to the solution and stirred for 24 h. Afterwards, it was centrifuged, washed with deionized water, and dried. The black powder (MV-RGO) was prepared.
Preparation of SCO catalyst In order to prepare SrCoO3d (SCO) catalyst, 10 ml Sr(NO3)2 (0.1 M), 10 ml CoCl2 (0.1 M) solutions were mixed with each other. 2 ml of octanoic acid was added to the solution. The mixture was stirred while the pH was adjusted to 7 with NaOH 1 M. The mixture was then heated in a water bath at 80 C for 1 h. Then, it was dried in the oven at 100 C for 8 h. The dried powder was calcined in a furnace at 900 C for 4 h. The SCO nanocatalyst was obtained and characterized.
Preparation of PteSCO/MV-RGO-CH nanocomposite PteSCO/MV-RGO-CH nanocomposite was prepared as follows: The prepared MV-RGO and 2 mg of SCO nanocatalyst were dispersed in 17.5 ml deionized water and 2.5 ml chitosan. The mixture was sonicated for 1 h. 25 ml H2PtCl6 (1 M) was added to the mixture and stirred vigorously for 1 h. Then, 50 ml NaBH4 (3 M) was added to the mixture and stirred for another 24 h. The mixture was centrifuged, washed, and dried at 60 C for 12 h.
Characterization of the prepared catalysts The size and dispersion of SCO nanoparticles were observed by scanning electron microscope (SEM, KYKY, EM3200). The phases of SCO nanocatalyst were determined by X-ray diffraction (XRD, Philips PC-APD apparatus with graphite monochromatic CuKa radiation). TEM images taken with a Philips CM120 transmission electron microscope with the resolution ~2.5 A were used to observe the morphology of the prepared Pt/MVRGO-CH and PteSCO/MV-RGO-CH nanocomposites.
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Autolab (Nova software model PGSTAT 302 N, Metrohm, Netherlands) controlled by a personal computer. A conventional three-electrode cell was used with a saturated calomel electrode (SCE) as the reference electrode, a platinum electrode as the counter electrode and a glassy carbon (GC) electrode as the working electrode. The polished GC electrode was first sonicated in water and absolute ethanol. Then, it was cleaned and activated in 1.0 mol L1 H2SO4 by cyclic voltammetry (CV) technique between 1.5 and þ1.5 V. In a typical experiment, 5 ml of the prepared suspension of PteSCO/MV-RGO-CH was spread onto the glassy carbon electrode surface and dried at room temperature. The GC/Pte SCO/MV-RGO-CH electrode was prepared.
Results and discussion Material characterization Fig. 1a indicated the XRD pattern of the synthesized graphene oxide (GO) with the diffraction peak located at 2q ¼ 11.0 . The prepared perovskite-type oxide SrCoO3d was characterized by SEM (Fig. 1b). As can be seen, SEM image showed that there were some small holes inside the product. SCO nanoparticles were uniformly dispersed with many holes which show that SCO is a porous material. They could be used as suitable support for Pt nanoparticles with better dispersion of Pt nanoparticles at composite because of having higher surface area. The prepared SrCoO3d catalyst was also characterized with XRD technique shown in Fig. 1c. The XRD pattern indicated the corresponding pattern of SrCoO3d catalyst [49]. As shown in Fig. 1c, the peaks were very sharp indicating the high crystallinity of SCO catalyst. SrCoO3d patterns showed the characteristic diffraction peaks attributed to SrCoO3d (1 0 1), (1 1 0), (2 0 1), (1 1 2), (3 0 0), (2 2 0), and (3 1 1) planes at 28.6, 32.6, 43.9, 55.7, 58.4, 68.4, and 75.5, respectively, which demonstrates the hexagonal structure of the SCO sample. Lattice parameters were a ¼ 5.485, b ¼ 5.485 and c ¼ 4.137. TEM images of the prepared PteCH and PteSCOeCH nanocomposites in the absence of MV-RGO were shown in Fig. 2a and b, respectively. Fig. 2c and d showed the TEM images of Pt/MV-RGO-CH catalyst. The mean particle size of Pt nanoparticles was 4.21 nm. Comparing Fig. 2a and c showed that there is a little agglomeration in the absence of MV-RGO. The dispersion of Pt nanoparticles was more uniform in the presence of MV-RGO and chitosan substrates. The electrostatic attractions between the positively charged functional groups of CH polymer and MV-RGO, and the negatively led to the uniform dispersion of Pt nanocharged PtCl2 6 particles. The TEM images of PteSCO/MV-RGO catalyst were shown in Fig. 3. The mean particle size of Pt nanoparticles was 5.71 nm, whereas the mean particles size of SCO nanoparticles was 23.44 nm. Comparing Figs. 2b and 3 showed that in the presence of MV-RGO and chitosan substrates, the dispersion of nanoparticles was more uniform with lower agglomeration.
Electrocatalytic investigations Electrochemical investigations Hydrogen adsorption and desorption The electrochemical characterizations of PteSCO/MV-RGOCH nanocatalyst was done with a potentiostat/galvanostat
Electrochemically active surface area (EASA) is an important factor in investigating the catalytic activity of PteSCO/MV-
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Fig. 1 e a) XRD pattern of the synthesized GO, b) SEM image of SCO catalyst and c) XRD pattern of SCO catalyst.
RGO-CH catalyst. It can be determined through cyclic voltammetry technique in acidic media with H2 adsorption and desorption voltammograms. Fig. 4A showed the CV curves of PteSCO/MV-RGO-CH, compared to Pt/MV-RGO-CH electrode in 0.5 M H2SO4 in the potential range of 0.35e1.2 V and the scan rate of 100 mV s1 with the platinum loading of 0.031 mg cm2. The voltammetric characteristics of GC/Pte SCO/MV-RGO-CH electrode (Fig. 4A) indicated the typical features of Pt metal with hydrogen adsorption and desorption between 0.05 and 0.25 V [50]. EASA for platinum is determined electrochemically from the following Equation [51]: EASA ¼ QH=0:21 ½Pt
(1)
QH is the charge passed for hydrogen adsorption and desorption. 0.21 is a parameter which relates charge to area and indicates the required charge for the oxidation of a monolayer of hydrogen on Pt particles. [Pt] is platinum loading (mg cm2). With similar Pt loading, EASA for PteSCO/MV-RGO-CH was estimated to be 74 m2 g1, which is much larger than that of Pt/MV-RGO-CH (21.8 m2 g1). The larger EASA value shown for PteSCO/MV-RGO-CH suggests that this catalyst has more active sites compared to Pt/MV-RGO-CH catalysts. It is interesting that hydrogen oxidation on Pt surface enhances with the existence of SCO nanoparticles. Similar
results have been obtained for hydrogen oxidation at various perovskite-based catalysts [52,53]. Furthermore, several studies expressed that LaFeO3, LaNiO3, LaMO3 (M ¼ Co, Ni, Cr, and Fe), and ACe1xMxO3d (A ¼ Sr or Ba, M ¼ rare earth element) should be probably involved in hydrogen adsorption [54e57]. The higher EASA value of PteSCO/MV-RGO-CH compared to that of Pt/MV-RGO-CH is attributed to the better dispersion of Pt nanoparticles on the functional groups of MV-RGO-CH, the structure effects of SCO nanoparticles, and synergism effects on PteSCO/MV-RGO-CH catalyst. EASA can be used to show the dispersion of Pt nanoparticles in the prepared catalysts. The dispersion of Pt was expressed as the fraction of surface-active Pt atoms in all the Pt atoms that can be obtained according to Eq. (2) [58]: EASA DPt ¼ 1 2 =MPt ðNA ; 4prPt Þ
(2)
NA is the Avogadro number (6.02 1023), MPt is the relative molecular weight of Pt (195.08 g/mol) and rPt is the atomic ratio of Pt (0.139 nm). DPt for PteSCO/MV-RGO-CH and Pt/MV-RGOCH catalysts were obtained 0.098 and 0.029, respectively. In the presence of SCO nanoparticles, the dispersion of Pt increased on the electrode surface.
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Fig. 2 e TEM images of a) Pt-chitosan, b) PteSCO-chitosan, c and d) Pt/MV-RGO-CH catalysts.
CO stripping investigation To investigate the carbon monoxide (CO) oxidation properties of Pt/MV-RGO-CH and PteSCO/MV-RGO-CH catalysts, CO stripping experiments were performed. Fig. 4B indicated the adsorbed CO stripping curves of Pt/MV-RGO-CH and PteSCO/
MV-RGO-CH catalysts in the scan rate of 100 mV s1. For CO oxidation reaction, CO was purged in 0.5 M H2SO4 solution for 20 min while the potential was 0.2 V vs. SCE. Increasing adsorption time did not have any impact on voltammograms. After adsorption, the CO was removed from the
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Fig. 3 e TEM image of PteSCO/MV-RGO catalyst. electrolyte with N2 for 30 min. Pt/MV-RGO-CH catalyst indicated a sharp and single CO oxidation peak at Ep ¼ 0.685 V and an onset potential at 0.599 V. PteSCO/MV-RGO-CH showed a sharp CO oxidation peak at Ep ¼ 0.641 V and the onset potential of CO oxidation was at 0.572 V. The onset and peak potential of CO oxidation for PteSCO/MV-RGO-CH were negatively shifted by 27 and 44 mV, respectively, compared to Pt/MV-RGO-CH. The negative shift in the onset and peak potential of CO oxidation peak indicated that CO oxidizes more easily at PteSCO/MV-RGO-CH than Pt/MV-RGO-CH catalyst [59]. The CO produced from ethanol dissociation were more easily removed from the surface of PteSCO/MV-RGO-CH
catalyst and oxidized to CO2. In this case, the CO poisoning of this catalyst was lower than Pt/MV-RGO-CH catalysts.
Ethanol oxidation reaction (EOR) The electrochemical properties of PteSCO/MV-RGO-CH, Pt/ MV-RGO-CH, Pt/MV-RGO, and Pt/CH catalysts were investigated for EOR. The cyclic voltammograms (CVs) of the prepared catalysts were recorded in 1.26 M C2H5OH and 0.5 M H2SO4 aqueous solution and the scan rate of 100 mV s1 (Fig. 5A). As shown in Fig. 5A, no current peak of EOR was observed in the CV of GC/MV-RGO-CH electrode indicating no obvious electrocatalytic activity of this electrode for EOR (CV
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Fig. 4 e A) CV curves and B) CO stripping voltammograms of Pt/MV-RGO-CH and PteSCO/MV-RGO-CH catalysts in 0.5 M H2SO4.
at GC and GC/CH electrodes were not shown, which was similar to that of GC/MV-RGO-CH electrode). The CVs of Pt/ MV-RGO-CH, Pt/MV-RGO, Pt/CH, and PteSCO/MV-RGO-CH catalysts were also shown in Fig. 5A. As can be seen, there is a great increase in the current densities of the first and second peaks of EOR at PteSCO/MV-RGO-CH (Fig. 5A (e)) in comparison to Pt/MV-RGO-CH (Fig. 5A (c)), Pt/MV-RGO (Fig. 5A (b)), and Pt/CH (Fig. 5A (d)) catalysts. This shows that PteSCO/MV-RGOCH catalyst has a suitable electrocatalytic activity for EOR. Two oxidation peaks of EOR at PteSCO/MV-RGO-CH were seen
during a forward scan at 0.805 and 1.220 V. The third oxidation peak of ethanol electrooxidation was observed during a backward scan at 0.561 V. According to the literature [60,61], the first anodic peak of ethanol oxidation is mainly due to CO2 formation and the second anodic peak is attributed to CH3CHO formation [60]. In the backward potential sweep, the reduction of Pt oxide produces a clean PtNPs surface. Therefore, ethanol electrooxidation can occur on this clean Pt surfaces and the current peak (jb) is observed.
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Fig. 5 e A) Cyclic voltammograms of a) MV-RGO-CH, b) Pt/MV-RGO, c) Pt/MV-RGO-CH, d) Pt/CH, e) PteSCO/MV-RGO-CH catalysts and B) Chronoamperometric curves for EOR at the prepared catalysts in 1.26 M ethanol and 0.5 M H2SO4 solution.
Quantitative analysis of the CVs of Pt/MV-RGO-CH, Pt/MVRGO, Pt/CH and PteSCO/MV-RGO-CH catalysts towards EOR was shown in Table 1. The amount of the onset potential of EOR is an important factor in direct ethanol fuel cells. As shown in Table 1, the onset potential of EOR at GC/PteSCO/ MV-RGO-CH electrode was more negative than GC/Pt/MVRGO-CH, GC/Pt/MV-RGO, and GC/Pt/CH electrodes by 57, 4, and 55 mV, respectively. This showed the enhanced
electrocatalytic activity of PteSCO/MV-RGO-CH catalyst towards EOR. Moreover, this showed that ethanol electrooxidation at PteSCO/MV-RGO-CH is much easier than at Pt/ MV-RGO-CH, Pt/MV-RGO, and Pt/CH catalysts. The forward and reverse peak current densities' ratio (jf/jb) can be used to compare the activity of the prepared catalysts for ethanol oxidation. This ratio can be utilized to determine the poisoning tolerance of the prepared catalysts to the CO-
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Table 1 e Electrochemical data of ethanol electrooxidation at the prepared catalysts in 1.26 M ethanol and 0.5 M H2SO4 solution. Catalyst Pt/MV-RGO-CH Pt/MV-RGO Pt/CH PteSCO/MV-RGO-CH
Onset potential (V)
Ef1 (V)
jf1 (mA cm2)
Ef2 (V)
jf2 (mA cm2)
Eb (V)
jf1/jb
jf2/jb
0.297 0.244 0.295 0.240
0.688 0.661 0.749 0.805
36.728 27.90 37.23 146.75
1.060 1.057 1.157 1.220
55.456 34.89 59.57 177.90
0.39 0.405 0.451 0.561
0.705 0.625 0.648 0.994
1.065 0.781 1.037 1.204
species produced during the EOR. The larger values of jf/jb show that the synthesized materials have better CO poisoning resistance [62]. As can be seen in Table 1, PteSCO/MV-RGO-CH had higher jf1/jb and jf2/jb ratios for EOR in comparison to Pt/ MV-RGO-CH, Pt/MV-RGO, and Pt/CH catalysts, showing that PteSCO/MV-RGO-CH has an improved performance in poisoning tolerance. The surface oxygen of SrCoO3d perovskite nanoparticles would help to remove CO poisoning on Pt surface, consequently increasing the jf/jb ratio [63]. This finding was according to the CO stripping analysis. The lower onset potential, higher anodic current density, and higher jf/jb ratio of PteSCO/MV-RGO-CH catalyst indicated that this catalyst has higher catalytic activity than Pt/MVRGO-CH, Pt/MV-RGO, and Pt/CH catalysts towards ethanol oxidation. This showed that adding a certain amount of SCO nanoparticles to the matrix of Pt nanocomposite can improve its catalytic activity for ethanol electrooxidation reaction. Furthermore, as shown in Fig. 5A and Table 1, Pt/MV-RGO-CH catalyst had better catalytic activity towards EOR in comparison to Pt/MV-RGO and Pt/CH catalysts indicating that using MV-RGO accompanied by CH improves the catalytic activity of Pt catalyst towards EOR.
Amperometric i-t curve measurements The performance of Pt/MV-RGO-CH and PteSCO/MV-RGO-CH catalysts for EOR was also studied through chronoamperometry technique, shown in Fig. 5B. To examine the electrocatalytic stability of Pt/MV-RGO-CH and PteSCO/MVRGO-CH catalysts for EOR, chronoamperometry investigations were conducted in 0.5 M H2SO4 and 1.26 M ethanol solution at 0.7 V for 1000 s. As can be seen, potentiostatic current densities decreased rapidly in the initial stage for both Pt/MVRGO-CH and PteSCO/MV-RGO-CH catalysts. This decrease may be due to the formation of reactive intermediates during the EOR. As shown in Fig. 5B, after 200s, the current density reached a constant value of 1.68 mA cm2 for Pt/MV-RGO-CH catalyst, whereas the current density for PteSCO/MV-RGO-CH catalyst was 8.01 mA cm2. For as long as 1000s, the current density of EOR at PteSCO/MV-RGO-CH catalyst remained at 3.22 mA cm2 indicating the better electrocatalytic performance of this catalyst for EOR.
Electrochemical impedance spectroscopy (EIS) for EOR EIS investigations were also done to study the behavior of PteSCO/MV-RGO-CH and Pte/MV-RGO-CH catalysts for
Fig. 6 e The Nyquist plots of Pt/MV-RGO-CH and PteSCO/MV-RGO-CH catalysts in 0.5 M H2SO4 and 1.26 M C2H5OH aqueous solution. Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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ethanol electrooxidation reaction. The electrochemical behavior of working electrodes modified with the prepared catalysts for ethanol electrooxidation can be investigated by Nyquist plot [64]. Electrochemical impedance investigation with transmission line model fit to Nyquist plots in the range of 1 104 to 102 Hz at open circuit potential (OCP) in 0.5 M H2SO4 and 1.26 M ethanol was shown in Fig. 6. As can be seen in Fig. 6, all the Nyquist plots showed a small semicircle in high frequency region which is due to charge transfer process in the interface of the electrode and electrolyte and a line in the low-frequency region which is related to diffusion process in the prepared electrodes. According to the impedance investigation, the equivalent circuit of PteSCO/MV-RGO-CH catalyst was shown in the inset of Fig. 6. As can be seen, in the equivalent circuit, Rs is due
to solution resistance, and constant-phase element (CPE) is attributed to the double-layer capacitance. R0 and C0 are due to the resistance and capacitance of the adsorbed CO intermediates produced during ethanol electrooxidation, respectively [65]. A more vertical straight line at PteSCO/ MV-RGO-CH than Pt/MV-RGO-CH showed the improved ions diffusion feature of PteSCO/MV-RGO-CH catalyst for EOR [66].
Parameters affecting ethanol electrooxidation Our investigations showed that several parameters such as scan rate, temperature, and ethanol concentration influence the performance of the prepared electrodes for ethanol electrooxidation. These parameters were thus investigated and optimized.
Fig. 7 e Anodic current density (jf) vs. square root of scan rate (y0.5),-, and irreversibility plot showing peak potential (Ef) vs. ln y,C, for PteSCO/MV-RGO-CH electrode in 1.26 M ethanol and 0.5 M H2SO4 at the scan rates of 30, 50, 70, 100, 130, 160 and 190 mV s¡1. Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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To determine the effect of scan rate, the catalytic activity of PteSCO/MV-RGO-CH catalysts towards ethanol electrooxidation was investigated at the scan rates of 30, 50, 70, 100, 130, 160, and 190 mVs1 in 1.26 M ethanol and 0.5 M H2SO4. The anodic peak current densities of EOR vs. the square root of the scan rate (y) and the peak potential vs. ln y were displayed in Fig. 7. As can be seen, the anodic peak current density of EOR increased with increase in the applied scan rate. As can be observed, there is a linear relationship between the square root of the scan rate (y0.5) and the first anodic peak current density (jf1) (R2 ¼ 0.93) and a linear relationship (R2 ¼ 0.96) between y0.5 and the second anodic peak current density (jf2) indicating that the diffusion of ethanol from the bulk solution to the surface of the electrode controls the process of ethanol electrooxidation [67,68]. Also a linear relationship (0.98) between Ef1 and ln (y) and a linear relationship (R2 ¼ 0.97) between Ef2 and ln (y) indicate an irreversible charge transfer process for EOR [69,70]. The effect of ethanol concentration on the anodic current density of ethanol oxidation on PteSCO/MV-RGO-CH catalyst was shown in Fig. 8. The scan rate was 100 mV s1. As can be seen, anodic current density increases with increase in ethanol concentration and levels off at concentrations higher than 1.26 M. This is probably due to the saturation of the active sites on the electrode surface. To get a higher current density, this concentration (1.26 M) can be selected as the optimum concentration. At PteSCO/MV-RGO-CH catalyst, with increase in ethanol concentration from 0.06 to 1.35 M, Ef1 shifts towards a positive direction from 0.634 to 0.805 V and Ef2 shifts towards a positive direction from 1.098 to 1.245 V. This is probably due to the increase of the poisoning rate of Pt catalyst. Thus, the
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oxidative removal of strongly adsorbed intermediates occurs at more positive potentials [71]. The electrocatalytic activities of PteSCO/MV-RGO-CH and Pt/MV-RGO-CH nanocatalysts towards EOR were studied in different temperatures ranging from 25 to 50 C and the scan rate of 100 mV s1. As shown in Fig. 9, anodic current density was increased with increase in the temperature. This way, mass transport is an important factor for higher activity. For PteSCO/MV-RGO-CH catalyst (Fig. 9A), as temperature increased from 25 to 50 C, jf1 increased from 140.128 to 212.651 mA cm2 and jf2 increased from 170.461 to 226.646 mA cm2. With increase in temperature, the higher current density at the same ethanol concentration shows that the fine structure of the catalyst possesses more available Pt active sites to participate in electrochemical reaction. For Pt/ MV-RGO-CH catalyst (Fig. 9B), as the temperature increased from 25 to 50 C, jf1 increased from 27.78 to 84.75 mA cm2 and jf2 increased from 45.4 to 88.36 mA cm2. The activation energies of the first and second peaks of ethanol electrooxidation at PteSCO/MV-RGO-CH and Pt/MVRGO-CH catalysts were also calculated. As observed in Fig. 9A, the Arrhenius plots of logarithm of exchange current density (log jp) versus the reciprocal of temperature (T1) were also shown for PteSCO/MV-RGO-CH catalyst. Activation energies were calculated using the following equation: v ln jp DH* ¼ R v T1
(3)
The activation energies of the first (DH1) and second (DH2) anodic peaks of ethanol electrooxidation at PteSCO/MV-RGOCH catalyst were 6.28 and 4.27 kJ mol1, respectively. Activation
Fig. 8 e Cyclic voltammograms for EOR on PteSCO/MV-RGO-CH electrode in 0.5 M H2SO4 in different concentration of ethanol: a) 0.057, b) 0.11, c) 0.17, d) 0.22, e) 0.28, f) 0.33, g) 0.39, h) 0.44, i) 0.50, j) 0.55, k) 0.60, l) 0.66, m) 0.71, n) 0.76, o) 0.81, p) 0.86, q) 0.91, r) 0.96, s) 1.02, t) 1.07, u) 1.11, v) 1.16, w) 1.21, x) 1.26, y) 1.31 and z) 1.35 M. Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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Fig. 9 e Cyclic voltammograms of A) PteSCO/MV-RGO-CH and B) Pt/MV-RGO-CH catalysts in different temperatures of a) 25, b) 30, c) 35, d) 40, e) 45 and f) 50 C. The Arrhenius plots of logarithm of exchange current density (log jp) versus the reciprocal of temperature (T-1) were shown for the first and second peaks of EOR at PteSCO/MV-RGO-CH catalyst.
energies of the first and second anodic peaks of EOR at Pt/MVRGO-CH catalyst were 13.91 and 8.76 kJ mol1, respectively. As can be seen, the first and second anodic peaks of ethanol electrooxidation at PteSCO/MV-RGO-CH catalyst had lower activation energies than those of Pt/MV-RGO-CH catalyst.
Durability test The poisoning effect of PteSCO/MV-RGO-CH catalyst during ethanol electrooxidation was investigated through cyclic voltammetry with 50 cycles repeatedly in the scan rate of 100 mV s1 (Fig. 10). jf1 and jf2 of PteSCO/MV-RGO-CH and Pt/
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 5
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Fig. 10 e CV curves of PteSCO/MV-RGO-CH catalyst during the 50 cycles in H2SO4 0.5 M and ethanol 1.26 M solution. jf1 and jf2 of PteSCO/MV-RGO-CH and Pt/MV-RGO-CH catalysts as a function of the cycle number for EOR are also shown.
MV-RGO-CH catalysts for EOR as a function of cycle number were also shown in Fig. 10. As can be seen, for Pt/MV-RGO-CH, in the first cycle, jf1 and jf2 were 65.369 and 80.103, respectively, and in the 50th cycle, they were 43.842 and 70.151 mA cm2, respectively. This means at Pt/MV-RGO-CH catalyst during the 50 cycle, jf1 decreased by 32.9% and jf2 decreased by 12.42%. For PteSCO/MV-RGO-CH, in the first cycle, jf1 and jf2 were 141.819 and 163.133 mA cm2, respectively and at the 50th cycle, they were 122.118 and 144.725 mA cm2, respectively. This shows that at PteSCO/ MV-RGO-CH catalyst during the 50th cycle, jf1 decreased by 13.9% and jf2 decreased by 11.28%. This investigation showed that the incorporation of SCO nanoparticles in the Pt/MVRGO-CH matrix enhanced the durability of the electrode towards ethanol electrooxidation.
Conclusions In this study, a novel PteSCO/MV-RGO-CH nanocomposite was successfully synthesized and characterized. The catalytic
activity of this catalyst was investigated for ethanol electrooxidation and compared with that of Pt/MV-RGO-CH catalyst. The catalytic activity of the electrode depends on the surface available for the dispersion of metallic particles. The high electrocatalytic activities of Pt/MV-RGO-CH and PteSCO/MV-RGOCH for EOR may be due to the good dispersion of nanoparticles confirmed with TEM images. The presence of methyl viologen and chitosan on graphene sheets probably leads to the better dispersion of nanoparticles and prevents from the agglomeration of metallic particles. The results showed that, incorporation of SCO nanoparticles into Pt catalyst can significantly improve the performance of electrode for ethanol electrooxidation. The electrocatalytic activity of PteSCO/MV-RGO-CH for EOR in acidic media was higher than that of Pt/MV-RGO-CH nanocatalysts, due to its higher anodic current densities, higher electrochemically active surface area, and better antipoisoning effect. By and large, the investigations showed that PteSCO/MV-RGO-CH is a promising catalyst for ethanol electrooxidation in direct ethanol fuel cells and MV-RGO-CH is potentially a good support for nanoparticles in fuel cell applications.
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
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Acknowledgment We thank Esfarayen University of Technology for financial support.
references
[1] Kakaei K. Decoration of graphene oxide with platinum tin nanoparticles for ethanol oxidation. Electrochimica Acta 2015;165:330e7. [2] Sedighi M, Rostami AA, Alizadeh E. Enhanced electrooxidation of ethanol using Pt-CeO2 electrocatalyst prepared by electrodeposition technique. Int J Hydrogen Energy 2017;42:4998e5005. [3] Ding K, Zhao Y, Liu L, Cao Y, Wang Q, Gu H, et al. Pt-Ni bimetallic composite nanocatalysts prepared by using multiwalled carbon nanotubes as reductants for ethanol oxidation reaction. Int J Hydrogen Energy 2014;39:17622e33. [4] Demirbas A. Progress and recent trends in biofuels. Prog Energy Combust Sci 2007;33:1e18. [5] Zhang C, Zhu A, Huang R, Zhang Q, Liu Q. Hollow nanoporous Au/Pt core-shell catalysts with nanochannels and enhanced activities towards electro-oxidation of methanol and ethanol. Int J Hydrogen Energy 2014;39:8246e56. [6] Qu Y, Li C, Wang L, Gao Y, Rao J, Yin G. Mild synthesis of layer-by-layer SnO2 nanosheet/Pt/graphene composites as catalysts for ethanol electro-oxidation. Int J Hydrogen Energy 2016;41:14036e46. [7] Ekrami-Kakhki MS, Yavari Z, Saffari J, Abbasi S. Fabrication and evaluation of Pt/M (M¼ Co, Fe) chitosan supported catalysts for methanol electrooxidation: application in direct alcohol fuel cell. J Nanostruct 2016;6:221e34. [8] Khorasani-Motlagh M, Noroozifar M, Ekrami-Kakhki MS. Investigation of the nanometals (Ni and Sn) in platinum binary and ternary electrocatalysts for methanol electrooxidation. Int J Hydrogen Energy 2011;36:11554e63. [9] Noroozifar M, Khorasani-Motlagh M, Ekrami-Kakhki MS, Khaleghian-Moghadam R. Enhanced electrocatalytic properties of Pt-chitosan nanocomposite for direct methanol fuel cell by LaFeO3 and carbon nanotube. J Power Sources 2014;248:130e9. [10] Ding X, Gao X, Zhu W, Wang J, Jiang J. Electrode redox properties of Ba1xLaxFeO3d as cobalt free cathode materials for intermediate-temperature SOFCs. Int J Hydrogen Energy 2014;39:12092e100. [11] Ekrami-Kakhki MS, Yavari Z, Saffari J, Ekrami-Kakhki SA. Perovskite-type LaFeO3 and LaFeO3-CNTs nanocrystals as active anode for methanol oxidation in alkaline solutions. J Electr Eng 2016;4:88e99. [12] Miao G, Yuan C, Chen T, Zhou Y, Zhan W, Wang S. Sr2Fe1þxMo1xO6d as anode material of cathodeesupported solid oxide fuel cells. Int J Hydrogen Energy 2016;41:1104e11. [13] Lan A, Mukasyan AS. Complex SrRuO3-Pt and LaRuO3-Pt catalysts for direct alcohol fuel cells. Ind Eng Chem Res 2008;47:8989e94. ndez[14] Martı´nez-Coronado R, Alonso JA, Aguadero A, Ferna Dı´az MT. New SrMo1xCrxO3d perovskites as anodes in solid-oxide fuel cells. Int J Hydrogen Energy 2014;39:4067e73. [15] Konopka DA, Li M, Artyushkova K, Marinkovic N, Sasaki K, Adzic R, et al. Platinum supported on NbRuyOz as electrocatalyst for ethanol oxidation in acid and alkaline fuel cells. J Phys Chem C 2011;115:3043e56.
[16] Fang B, Kim M, Yu JS. Hollow core/mesoporous shell carbon as a highly efficient catalyst support in direct formic acid fuel cell. Appl Catal B Environ 2008;84:100e5. [17] Shao Y, Yin G, Gao Y, Shi P. Durability study of Pt ∕ C and Pt ∕ CNTs catalysts under simulated PEM fuel cell conditions. J Electrochem Soc 2006;153:A1093e7. [18] Du H, Li B, Kang F, Fu R, Zeng Y. Carbon aerogel supported PteRu catalysts for using as the anode of direct methanol fuel cells. Carbon 2007;45:429e35. [19] Park IS, Park KW, Choi JH, Park CR, Sung YE. Electrocatalytic enhancement of methanol oxidation by graphite nanofibers with a high loading of PtRu alloy nanoparticles. Carbon 2007;45:28e33. [20] Li L, Qian Y, Yang J, Tan X, Dai Z, Jin Y, et al. A novel structural design of hybrid nanotube with CNTs and CeO2 supported Pt nanoparticles with improved performance for methanol electro-oxidation. Int J Hydrogen Energy 2016;41:9284e94. [21] Wang HL, Kakad BA, Tamaki T, Yamaguchi T. Synthesis of 3D graphite oxide-exfoliated carbon nanotube carbon composite and its application as catalyst support for fuel cells. J Power Sources 2014;260:338e48. [22] Li X, Wang HJ, Yua H, Liu ZW, Peng F. An opposite change rule in carbon nanotubes supported platinum catalyst for methanol oxidation and oxygen reduction reactions. J Power Sources 2014;260:1e5. [23] John SS, Dutta I, Angelopoulos AP. Enhanced electrocatalytic oxygen reduction through electrostatic assembly of Pt nanoparticles onto porous carbon supports from SnCl2stabilized suspensions. Langmuir 2011;27:5781e91. [24] Qi J, Jiang L, Wang S, Sun G. Synthesis of graphitic mesoporous carbons with high surface areas and their applications in direct methanol fuel cells. Appl Catal B Environ 2011;107:95e103. ger JM, Alonso-Vante N, Lamy C. [25] Yang H, Coutanceau C, Le Methanol tolerant oxygen reduction on carbon-supported PteNi alloy nanoparticles. J Electroanal Chem 2005;576:305e13. pez-Vizcaı´no R. [26] Lobato J, Canizares P, Rodrigo MA, Linares JJ, Lo Performance of a vapor-fed polybenzimidazole (PBI)-based direct methanol fuel cell. Energy Fuels 2008;22:3335e45. [27] Lobato J, Canizares P, Ubeda D, Pinar FJ, Rodrigo MA. Testing PtRu/CNF catalysts for a high temperature polybenzimidazole-based direct ethanol fuel cell. Effect of metal content. Appl Catal B Environ 2011;106:174e80. [28] Yang H, Li F, Shan C, Han D, Zhang Q, Niu L, et al. Covalent functionalization of chemically converted graphene sheets via silane and its reinforcement. J Mater Chem 2009;19:4632e8. [29] Kou R, Shao Y, Mei D, Nie Z, Wang D, Wang C, et al. Stabilization of electrocatalytic metal nanoparticles at metalmetal oxidegraphene triple junction points. J Am Chem Soc 2011;133:2541e7. [30] Yang F, Liu Y, Gao L, Sun J. pH-sensitive highly dispersed reduced graphene oxide solution using lysozyme via an in Situ reduction method. J Phys Chem C 2010;114:22085e91. [31] Hu Y, Zhang H, Wu P, Zhang H, Zhou B, Cai C. Bimetallic PteAu nanocatalysts electrochemically deposited on graphene and their electrocatalytic characteristics towards oxygen reduction and methanol oxidation. Phys Chem Chem Phys 2011;13:4083e94. [32] Hu YJ, Jin J, Wu P, Zhang H, Cai CX. Grapheneegold nanostructure composites fabricated by electrodeposition and their electrocatalytic activity toward the oxygen reduction and glucose oxidation. Electrochimica Acta 2010;56:491e500. [33] Zhang H, Xu X, Gu P, Li C, Wu P, Cai C. Microwave-assisted synthesis of graphene-supported Pd1Pt3 nanostructures and
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 5
[34]
[35]
[36]
[37]
[38] [39]
[40]
[41]
[42]
[43] [44] [45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
their electrocatalytic activity for methanol oxidation. Electrochimica Acta 2011;56:7064e70. Shang N, Papakonstantinou P, Wang P, Silva SRP. Platinum integrated graphene for methanol fuel cells. J Phys Chem 2010;114:15837e41. Kakaei K. One-pot electrochemical synthesis of graphene by the exfoliation of graphite powder in sodium dodecyl sulfate and its decoration with platinum nanoparticles for methanol oxidation. Carbon 2013;51:195e201. Kakaei K, Gharibi H. Palladium nanoparticle catalysts synthesis on graphene in sodium dodecyl sulfate for oxygen reduction reaction. Energy 2014;65:166e71. Wang H, Maiyalagan T, Wang X. Review on recent progress in nitrogen-doped graphene: synthesis, characterization, and its potential applications. ACS Catal 2012;2:781e94. Dreyer DR, Park S, Bielawski CW, Ruoff RS. The chemistry of graphene oxide. Chem Soc Rev 2010;39:228e40. Ma JH, Wang L, Mu X, Li L. Nitrogen-doped graphene supported Pt nanoparticles with enhanced performance for methanol oxidation. Int J Hydrogen Energy 2015;40: 2641e7. Wang S, Wang X, Jiang SP. Self-assembly of mixed Pt and Au nanoparticles on PDDA functionalized graphene as effective electrocatalysts for formic acid oxidation of fuel cells. Phys Chem Chem Phys 2011;13:6883e91. Zhao J, Yu H, Liu Z, Ji M, Zhang L, Sun G. Supercritical deposition route of preparing Pt/graphene composites and their catalytic performance toward methanol electrooxidation. J Phys Chem C 2014;118:1182e90. Ma J, Wang L, Mu X, Cao Y. Enhanced electrocatalytic activity of Pt nanoparticles supported on functionalized graphene for methanol oxidation and oxygen reduction. J Colloid Interface Sci 2015;457:102e7. Guibal E. Heterogeneous catalysis on chitosan-based materials: a review. Prog Polym Sci 2005;30:71e109. Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80. 1339e9. Chen J, Yao B, Li C, Shi G. An improved Hummers method for eco-friendly synthesis of graphene oxide. Carbon 2013;64:225e9. Shin YR, Jung SM, Jeon IY, Baek JB. The oxidation mechanism of highly ordered pyrolytic graphite in a nitric acid/sulfuric acid mixture. Carbon 2013;52:493e8. Avdeev VV, Monyakina LA, Nikolskaya IV, Sorokina NE, Semenenko KN. The choice of oxidizers for graphite hydrogenosulfate chemical synthesis. Carbon 1992;30:819e23. Sorokina NE, Khaskov MA, Avdeev VV, Nikol'skaya IV. Reaction of graphite with sulfuric acid in the presence of KMnO4. Russ J Gen Chem 2005;75:162e8. Zeng P, Ran R, Chen Z, Zhou W, Gu H, Shao Z, et al. Efficient stabilization of cubic perovskite SrCoO3-d by B-site low concentration scandium doping combined with solegel synthesis. J Alloy Compd 2008;455:465e70. Xu C, Cheng L, Shen P, Liu Y. Methanol and ethanol electrooxidation on Pt and Pd supported on carbon microspheres in alkaline media. Electrochem Commun 2007;9:997e1001. Saha MS, Li R, Sun X. High loading and monodispersed Pt nanoparticles on multiwalled carbon nanotubes for high performance proton exchange membrane fuel cells. J Power Sources 2008;177:314e22. Deshpande K, Mukasyan A, Varma A. High throughput evaluation of perovskite-based anode catalysts for direct methanol fuel cells. J Power Sources 2006;158:60e8. Zhu T, Fowler DE, Poeppelmeier KR, Han M, Barnett SA. Hydrogen oxidation mechanisms on perovskite solid oxide fuel cell anodes. J Electrochem Soc 2016;163:F952e61.
15
[54] Boateng IW, Tia R, Adei E, Dzade NY, Catlow CRA, de Leeuw NH. A DFTþU investigation of hydrogen adsorption on the LaFeO3(010) surface. Phys Chem Chem Phys 2017;19:7399e409. [55] Pan C, Chen Y, Wu N, Zhang M, Yuan L, Zhang C. A first principles study of H2 adsorption on LaNiO3(001) surfaces. Materials 2017;10:1e13. [56] Tejuca LG. Temperature programmed desorption study of hydrogen adsorption on LaMO3 oxides. Thermochimica Acta 1988;126:205e12. [57] Esaka T, Sakaguchi H, Kobayashi S. Hydrogen storage in proton-conductive perovskite- type oxides and their application to nickel-hydrogen batteries. Solid State Ionics 2004;166:351e7. [58] Zhou C, Chen Y, Guo Z, Wang X, Yang Y. Promoted aerobic oxidation of benzyl alcohol on CNT supported platinum by iron oxide. Chem Commun 2011;47:7473e5. [59] Yang Z, Nagashima A, Fujigaya T, Nakashima N. Electrocatalyst composed of platinum nanoparticles deposited on doubly polymer-coated carbon nanotubes shows a high CO-tolerance in methanol oxidation reaction. Int J Hydrogen Energy 2016;41:19182e90. [60] Fujiwara N, Friedrich KA, Stimming U. Ethanol oxidation on PtRu electrodes studied by differential electrochemical mass spectrometry. J Electroanal Chem 1999;472:120e5. [61] Hitmi H, Belgsir EM, Leger JM, Lamy C, Lezna RO. A kinetic analysis of the electro-oxidation of ethanol at a platinum electrode in acid medium. Electrochimica Acta 1994;39:407e15. [62] Naidoo QL, Naidoo S, Petrik L, Nechaev A, Ndungu P. The influence of carbon based supports and the role of synthesis procedures on the formation of platinum and platinumruthenium clusters and nanoparticles for the development of highly active fuel cell catalysts. Int J Hydrogen Energy 2012;37:9459e69. [63] Raghuveer V, Thampi KR, Xanthopoulos N, Mathieu HJ, Viswanathan B. Rare earth cuprates as electrocatalysts for methanol oxidation. Solid State Ionics 2001;140:263e74. [64] Ding K, Jia Z, Wang Q, He X, Tian N, Tong R, et al. Electrochemical behavior of the self-assembled membrane formed by calmodulin (CaM) on a Au substrate. J Electroanal Chem 2001;513:67e71. [65] He Q, Chen W, Mukerjee S, Chen S, Laufek F. Carbonsupported PdM (M¼Au and Sn) nanocatalysts for the electrooxidation of ethanol in high pH media. J Power Sources 2009;187:298e304. [66] Kakaei K, Rahimi A, Husseindoost S, Hamidi M, Javan H, Balavandi A. Fabrication of Pt- CeO2 nanoparticles supported sulfonated reduced graphene oxide as an efficient electrocatalyst for ethanol oxidation. Int J Hydrogen Energy 2016;41:3861e9. [67] Honda K, Yoshimura M, Rao TN, Tryk DA, Fujishima A, Yasui K, et al. Electrochemical properties of Pt-modified nano-honeycomb diamond electrodes. J Electroanal Chem 2001;514:35e50. [68] Zhao Y, Wang R, Han Z, Li C, Wang Y, Chi B, et al. Electrooxidation of methanol and ethanol in acidic medium using a platinum electrode modified with lanthanum-doped tantalum oxide film. Electrochimica Acta 2015;151:544e51. [69] Guo DJ, Li HL. Electrocatalytic oxidation of methanol on Pt modified single-walled carbon nanotubes. J Power Sources 2006;160:44e9. [70] Pattabiraman R. Electrochemical investigations on carbon supported palladium catalysts. Appl Catal A Gen 1997;153:9e20. [71] He Z, Chen J, Liu D, Zhou H, Kuang Y. Electrodeposition of PtRu nanoparticles on carbon nanotubes and their electrocatalytic properties for methanol electrooxidation. Diam Relat Mater 2004;13:1764e70.
Please cite this article in press as: Ekrami-Kakhki M-S, et al., Superior electrocatalytic activity of PteSrCoO3d nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.06.053