- Email: [email protected]
Novel hydrolyzing synthesis of CeO2–RGO support for Pt electrocatalyst in direct methanol fuel cells
Materials Letters 154 (2015) 177–179
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Novel hydrolyzing synthesis of CeO2–RGO support for Pt electrocatalyst in direct methanol fuel cells Tian Han n, Zhu Zhang School of Measurement-Control Technology and Communications Engineering, Harbin University of Science and Technology, Harbin 150080, China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 February 2015 Accepted 20 April 2015 Available online 25 April 2015
CeO2–reduced graphene oxide (CeO2–RGO) hybrid was synthesized by a novel isothermal hydrolyzing method without post-heating treatment for crystallization, and followed by dispersed Pt nanoparticles on it using a typical microwave assisted process. The physical structure and morphology of the final Pt/ CeO2–RGO products were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD); the electrochemical activities of the catalysts were investigated by cyclic voltammetry (CV) and chronoamperometry (CA). The experimental results showed that the Pt/CeO2–RGO electrocatalyst exhibited superior catalytic activity and stability for methanol electro-oxidation compared to the conventional Pt/RGO catalyst due to strong interaction between CeO2, Pt and graphene. & 2015 Elsevier B.V. All rights reserved.
Keywords: Carbon materials Nanocomposites Cerium oxide Reduced graphene oxide Hydrolyzing process
1. Introduction Direct methanol fuel cells (DMFCs) are promising devices for portable power and transportation applications due to their high energy densities, low emissions, and facile fuel distribution and storage [1–3]. However, catalyst in DMFCs still faces many challenges like poor activity and low stability owing to CO poisoning. To address this problem, many efforts have been attempted through the addition of co-catalysts to metallic platinum such as Ru [4], Mo [5], Ni [6], and Sn [7]. Recent reports have shown that directly added metals with specific oxidation states or metal oxides (e.g., TiO2 [8], CeO2 [9], and Fe3O4 [10]) could also effectively promote the electrooxidation of methanol and slow ideal tolerance to CO poisoning. Among these metallic oxides, CeO2, a rare earth oxide, has received considerable attention due to its features of automobile catalytic converters [11]. The element of ceria has a fluorite structure whose cations can switch between þ 3 and þ 4 oxidation states and therefore it can be regarded as a kind of oxygen tank to adjust oxygen concentration at the catalyst surface under reaction condition. Moreover, the low price of ceria oxide can help reduce the employment of expensive platinum and knock down the cost of DMFCs. In order to synthesize efficient Pt/CeO2–C composite electrocatalyst, many preparation methods have been proposed including co-sputtering deposition [12], sol–gel [13] and hydrothermal [14]. In this article, we reported a novel synthesis of nanocrystalline CeO2–RGO support using a non-boiling isothermal hydrolyzing
n
Corresponding author. Tel.: þ 86 185 8585 7915; fax: þ 86 451 86238659. E-mail address: [email protected] (T. Han).
http://dx.doi.org/10.1016/j.matlet.2015.04.094 0167-577X/& 2015 Elsevier B.V. All rights reserved.
process. Unlike other rapid and severe preparation methods, such as boiling hydrolysis, this procedure is very slow and soft under relatively low temperature. Then Pt nanoparticles were decorated onto the as-prepared composite with a typical microwave-assisted method, after which the final synthesized Pt/CeO2–RGO catalysts were evaluated by physical and chemical analysis.
2. Experimental Materials preparation: Synthesis of CeO2–GO hybrid. 20 mg solid Ce(NO3)3 6H2O was first dissolved in 100 mL of distilled water under magnetic stirring. Then 50 mg of oxide graphene was added into the aqueous solution. After 30 min ultrasonic vibration, a suspension with oxide graphene homogeneously dispersed was obtained. Subsequently, 150 μL C6H18N4 solution was added drop by drop into the suspension under vigorous stirring. Gradually the stable aqueous suspension turned purple and then was long-drawn reflux condensed in a thermostatic water bath at the temperature of 80 1C, ensuring the adequate hydrolyzing of Ce(NO3)3 6H2O. Materials preparation: Synthesis of Pt/CeO2–RGO. 20 mg CeO2/GO composites and H2PtCl6 (0.5 mmol) were dispersed in ethylene glycol solution (50 mL) under vigorous stirring to form a stable suspension. Then pH value of the ink was adjusted by NaOH–EG solution drop by drop until its value reached 12. The next step was to place the beaker in the center of a microwave oven (800 W) for consecutive heating time of 65 s. In this environment, EG acts as the reducing agent for reduction of H2PtCl6 and graphene oxide (GO). Finally, the product was filtered and washed with acetone and deionized water for several times. The theoretical loading of Pt
178
T. Han, Z. Zhang / Materials Letters 154 (2015) 177–179
and CeO2 of Pt/CeO2–RGO catalyst were 30 wt% and 5 wt%, respectively. To compare, the Pt/RGO catalyst was prepared using the similar procedure mentioned above. Preparation of the working electrode: The catalyst ink was prepared by ultrasonically dispersing 4.0 mg catalyst in the solution of 0.2 mL ethanol and 0.8 mL ultrapure water for 30 min. The glassy carbon (GC) working electrode with the diameter of 3 mm was polished with alumina suspensions to a mirror finish before each experiment. A quantity of 5 μL of the dispersion was extracted out on the top of the GC followed by drying under room temperature for 4 h. Characterization and measurements: The morphology of the asprepared samples were examined by a TEM (Tecnai G2 F30, US) operating at 120 kV. The structures of the samples were characterized by XRD (X' pert pro, Holland), employing a scanning rate of 0.02 1 s 1 in the 2θ range from 101 to 801. All electrochemical measurements were carried out in a three-electrode cell on a CHI 660D workstation. The three-electrode cell system consisted of a pretreated glassy carbon working electrode, a platinum foil counter electrode and Hg/Hg2SO4 (0.69 V vs. RHE) reference electrode. The CV was in the potential range of 0.7 to 0.5 V at a scan rate of 50 mV s 1 in 0.5 M H2SO4 solution and in a 0.5 M H2SO4 þ 0.5 M CH3OH solution.
3. Results and discussion The XRD experiments revealed the crystal structure, lattice constant, and crystal orientation of the supported catalysts as shown in Fig. 1. The (0 0 2) peak located at the 2θ value of around 231 in both catalysts evidently originates from the layered structure of graphene support. The five other diffraction peaks at 39.61, 46.31, 67.71, 81.61 and 85.81 can be ascribed to the Pt diffractions of (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes, which represent the characteristics of face-
Fig. 1. XRD patterns of Pt/CeO2–RGO and Pt/RGO.
centered cubic (fcc) structure of platinum. Moreover, diffraction peaks [(1 1 1), (2 0 0), (3 1 1), and (4 2 0)] of CeO2 were observed in the XRD pattern for Pt/CeO2–RGO, corresponding to 2θ ¼ 28.6, 33.2, 56.4, and 78, respectively. These diffraction peaks indexed as CeO2 with cubic fluorite structure, in good agreement with the literature values [9]. No shift was observed at the positions of the Pt diffraction peaks in the Pt/CeO2–RGO catalyst, indicating the addition of CeO2 had no obvious effect on the crystalline lattice of Pt. The morphology of as-prepared catalysts was further investigated with TEM, as illustrated in Fig. 2. It can been seen that Pt and CeO2 nanoparticles were both well dispersed on the graphene support, and the diameter of Pt nanoparticles ranging from 2–5 nm was obtained by measuring the diameter of Pt 100 nanoparticles in the magnified TEM images. The lattice fringes of Pt (1 1 1) can be measured with a spacing of 0.22 nm. In Fig. 2b, the distance of the crystal planes of the nanoparticle was determined to be 0.31 nm, consistent that of the (1 1 1) plane of CeO2. Pt and CeO2 nanoparticles were also closely adjacent to each other. The larger sphere is CeO2 particle (5–35 nm), contacting with Pt particles as presented with smaller sizes. In electrochemical tests, Fig. 3a shows the cyclic voltammogram (CV) curves of the as prepared catalysts in 0.5 M H2SO4 solution. The electrochemical surface area (ESA) is estimated from the integrated charge in the hydrogen/adsorption region of the CV curves, and the ESA of Pt–CeO2/RGO is calculated at 53.78 m2 g 1 Pt, which is 1.4 times as large as that of Pt/RGO (38.45 m2 g 1 Pt). The higher ESA value for Pt/CeO2–RGO may result from the synergistic effect between Pt and the ionic conductor of CeO2, which may promote the hydrogen spillover rate of Pt–H and hence increases the dissociation of hydrogen adsorption [11]. The electrocatalytic activity of Pt/CeO2–RGO for the electrooxidation of methanol was measured in 0.5 M H2SO4 and 0.5 M CH3OH solution as shown in Fig. 3b. It can be seen that the electrooxidizing current is significantly improved and the onset potential of electro-oxidation shifted to a lower value on Pt/CeO2–RGO (0.28 V) than on Pt/RGO (0.25 V). This result was further confirmed by the CO stripping test (Fig. 3c). The onset potential for COads oxidation on Pt/CeO2–RGO catalyst was shifted negatively (about 0.03 V) compared to ceria free Pt alone system. The reason may be that a large concentration of oxygen vacancies in the CeO2 provides active sites for CO adsorption. Fig. 3d shows chronoamperometric curves of Pt/CeO2–RGO and Pt/ RGO for methanol oxidation. Both of the catalysts display an initial fast current decay, which is attributed to the poisoning of the catalysts by intermediate species such as COads, CHOads formed during the methanol oxidation reaction. The residual current density of Pt/CeO2–RGO catalyst after 3600 s is 1.95 mA cm 2, which is much higher than that of Pt/RGO catalyst (0.88 mA cm 2), indicating the improved stability with the incorporation of CeO2.
Fig. 2. TEM images of CeO2–GO (a), Pt/CeO2–RGO (b) and Pt/RGO (c).
T. Han, Z. Zhang / Materials Letters 154 (2015) 177–179
179
Fig. 3. Cyclic voltammograms (a) and COads stripping voltammograms (c) of Pt/CeO2–RGO and Pt/RGO in 0.5 M H2SO4; cyclic voltammograms (b) and chroamperometry (d) of Pt/CeO2–RGO and Pt/RGO in 0.5 M H2SO4 and 0.5 M CH3OH; scan rate: 50 mV/s , temperature: 25 1C.
4. Conclusions To sum up, we have successfully synthesized Pt/CeO2–RGO electrocatalyst with improved activity and stability via a two-step process. The CeO2–RGO hybrid was prepared via a novel isothermal hydrolyzing process in low temperature and then the deposition of Pt nanoparticles by a microwave assisted method. The cyclic voltammetry and chronoamperometry tests show that the prepared Pt/CeO2– RGO catalyst exhibited much higher electrocatalytic activity and stability than the Pt/RGO catalyst for methanol oxidation, demonstrating a new synthesis strategy for the development of the highperformance graphene-based electrocatalysts for DMFCs applications. Acknowledgments This project is supported by Heilongjiang Provincial Natural Science Foundation of China (Grant no. F201315) and Harbin Research Fund for Technological Innovation (No. 2013RFQXJ104). References [1] Feng Ligang, Zhang Jing, Cai Weiwei, Liangliang, Xing Wei, Liu Changpeng. Single passive direct methanol fuel cell supplied with pure methanol. J Power Sources 2011;196:2750–3. [2] Lee SK, Kakati N, Jee SH, Maiti J, Yoon YS. Hydrothermal synthesis of PtRu nanoparticles supported on graphene sheets for methanol oxidation in direct methanol fuel cell. Mater Lett 2011;65:3281–4. [3] Liu JG, Zhao TS, Chen R, Wong CW. The effect of methanol concentration on the performance of a passive DMFC. Electrochem Commun 2005;7:288–94. [4] Nethravathi C, Anumol EA, Rajamathi M, Ravishankar N. Highly dispersed ultrafine Pt and PtRu nanoparticles on graphene: formation mechanism and electrocatalytic activity. Nanoscale 2011;3:569–71.
[5] Ji Z, Jalbout AF, Li JQ. Adsorption and diffusion of OH on Mo modified Pt (111) surface: first-principles theory. Solid State Commun 2007;142:148–53. [6] Park E, Shim S, Ha R, Oh E, Lee BW, Choi HJ. Reassembling of Ni and Pt catalyst in the vapor–liquid–solid growth of GaN nanowires. Mater Lett 2011;65:2458–61. [7] Spinacé EV, Farias LA, Linardi M, Neto AO. Preparation of PtSn/C and PtSnNi/C electrocatalysts using the alcohol-reduction process. Mater Lett 2008;62:2099–102. [8] Zhang C, Liu F, Zhai Y, Ariga H, Yi N, Liu Y, et al. Alkali-metal-promoted Pt/TiO2 opens a more efficient pathway to formaldehyde oxidation at ambient temperatures. Angew Chem 2012;51:9628–32. [9] Chen H, Duan J, Zhang X, Zhang Y, Guo C, Nie L. One step synthesis of Pt/CeO2– graphene catalyst by microwave-assisted ethylene glycol process for direct methanol fuel cell. Mater Lett 2014;126:9–12. [10] Yu W, Porosoff M, Chen J. Review of Pt-based bimetallic catalysis: from model surfaces to supported catalysts. Chem Rev 2012;112:5780–817. [11] Yuan H, Guo D, Li X, Yuan L, Zhu W, Chen L, et al. The effect of CeO2 on Pt/ CeO2/CNT catalyst for CO electrooxidation. Fuel Cells 2009;9:121–7. [12] Tanakaa S, Umedab M, Ojimac H, Usuia Y, Kimuraa O, Uchida I. Preparation and evaluation of a multi-component catalyst by using a co-sputtering system for anodic oxidation of ethanol. J Power Sources 2005;152:34–9. [13] Grabowskaa E, Sobczakb JW, Gazdac M, Zaleskaa A. Surface properties and visible light activity of W–TiO2 photocatalysts prepared by surface impregnation and sol–gel method. Appl Catal B: Environ 2012;117-8:351–9. [14] Zhang N, Fu X, Xu YJ. A facile and green approach to synthesize Pt@CeO2 nanocomposite with tunable core–shell and yolk–shell structure and its application as a visible light photocatalyst. J Mater Chem 2011;21:8152–8.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Novel hydrolyzing synthesis of CeO2–RGO support for Pt electrocatalyst in direct methanol fuel cells Tian Han n, Zhu Zhang School of Measurement-Control Technology and Communications Engineering, Harbin University of Science and Technology, Harbin 150080, China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 February 2015 Accepted 20 April 2015 Available online 25 April 2015
CeO2–reduced graphene oxide (CeO2–RGO) hybrid was synthesized by a novel isothermal hydrolyzing method without post-heating treatment for crystallization, and followed by dispersed Pt nanoparticles on it using a typical microwave assisted process. The physical structure and morphology of the final Pt/ CeO2–RGO products were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD); the electrochemical activities of the catalysts were investigated by cyclic voltammetry (CV) and chronoamperometry (CA). The experimental results showed that the Pt/CeO2–RGO electrocatalyst exhibited superior catalytic activity and stability for methanol electro-oxidation compared to the conventional Pt/RGO catalyst due to strong interaction between CeO2, Pt and graphene. & 2015 Elsevier B.V. All rights reserved.
Keywords: Carbon materials Nanocomposites Cerium oxide Reduced graphene oxide Hydrolyzing process
1. Introduction Direct methanol fuel cells (DMFCs) are promising devices for portable power and transportation applications due to their high energy densities, low emissions, and facile fuel distribution and storage [1–3]. However, catalyst in DMFCs still faces many challenges like poor activity and low stability owing to CO poisoning. To address this problem, many efforts have been attempted through the addition of co-catalysts to metallic platinum such as Ru [4], Mo [5], Ni [6], and Sn [7]. Recent reports have shown that directly added metals with specific oxidation states or metal oxides (e.g., TiO2 [8], CeO2 [9], and Fe3O4 [10]) could also effectively promote the electrooxidation of methanol and slow ideal tolerance to CO poisoning. Among these metallic oxides, CeO2, a rare earth oxide, has received considerable attention due to its features of automobile catalytic converters [11]. The element of ceria has a fluorite structure whose cations can switch between þ 3 and þ 4 oxidation states and therefore it can be regarded as a kind of oxygen tank to adjust oxygen concentration at the catalyst surface under reaction condition. Moreover, the low price of ceria oxide can help reduce the employment of expensive platinum and knock down the cost of DMFCs. In order to synthesize efficient Pt/CeO2–C composite electrocatalyst, many preparation methods have been proposed including co-sputtering deposition [12], sol–gel [13] and hydrothermal [14]. In this article, we reported a novel synthesis of nanocrystalline CeO2–RGO support using a non-boiling isothermal hydrolyzing
n
Corresponding author. Tel.: þ 86 185 8585 7915; fax: þ 86 451 86238659. E-mail address: [email protected] (T. Han).
http://dx.doi.org/10.1016/j.matlet.2015.04.094 0167-577X/& 2015 Elsevier B.V. All rights reserved.
process. Unlike other rapid and severe preparation methods, such as boiling hydrolysis, this procedure is very slow and soft under relatively low temperature. Then Pt nanoparticles were decorated onto the as-prepared composite with a typical microwave-assisted method, after which the final synthesized Pt/CeO2–RGO catalysts were evaluated by physical and chemical analysis.
2. Experimental Materials preparation: Synthesis of CeO2–GO hybrid. 20 mg solid Ce(NO3)3 6H2O was first dissolved in 100 mL of distilled water under magnetic stirring. Then 50 mg of oxide graphene was added into the aqueous solution. After 30 min ultrasonic vibration, a suspension with oxide graphene homogeneously dispersed was obtained. Subsequently, 150 μL C6H18N4 solution was added drop by drop into the suspension under vigorous stirring. Gradually the stable aqueous suspension turned purple and then was long-drawn reflux condensed in a thermostatic water bath at the temperature of 80 1C, ensuring the adequate hydrolyzing of Ce(NO3)3 6H2O. Materials preparation: Synthesis of Pt/CeO2–RGO. 20 mg CeO2/GO composites and H2PtCl6 (0.5 mmol) were dispersed in ethylene glycol solution (50 mL) under vigorous stirring to form a stable suspension. Then pH value of the ink was adjusted by NaOH–EG solution drop by drop until its value reached 12. The next step was to place the beaker in the center of a microwave oven (800 W) for consecutive heating time of 65 s. In this environment, EG acts as the reducing agent for reduction of H2PtCl6 and graphene oxide (GO). Finally, the product was filtered and washed with acetone and deionized water for several times. The theoretical loading of Pt
178
T. Han, Z. Zhang / Materials Letters 154 (2015) 177–179
and CeO2 of Pt/CeO2–RGO catalyst were 30 wt% and 5 wt%, respectively. To compare, the Pt/RGO catalyst was prepared using the similar procedure mentioned above. Preparation of the working electrode: The catalyst ink was prepared by ultrasonically dispersing 4.0 mg catalyst in the solution of 0.2 mL ethanol and 0.8 mL ultrapure water for 30 min. The glassy carbon (GC) working electrode with the diameter of 3 mm was polished with alumina suspensions to a mirror finish before each experiment. A quantity of 5 μL of the dispersion was extracted out on the top of the GC followed by drying under room temperature for 4 h. Characterization and measurements: The morphology of the asprepared samples were examined by a TEM (Tecnai G2 F30, US) operating at 120 kV. The structures of the samples were characterized by XRD (X' pert pro, Holland), employing a scanning rate of 0.02 1 s 1 in the 2θ range from 101 to 801. All electrochemical measurements were carried out in a three-electrode cell on a CHI 660D workstation. The three-electrode cell system consisted of a pretreated glassy carbon working electrode, a platinum foil counter electrode and Hg/Hg2SO4 (0.69 V vs. RHE) reference electrode. The CV was in the potential range of 0.7 to 0.5 V at a scan rate of 50 mV s 1 in 0.5 M H2SO4 solution and in a 0.5 M H2SO4 þ 0.5 M CH3OH solution.
3. Results and discussion The XRD experiments revealed the crystal structure, lattice constant, and crystal orientation of the supported catalysts as shown in Fig. 1. The (0 0 2) peak located at the 2θ value of around 231 in both catalysts evidently originates from the layered structure of graphene support. The five other diffraction peaks at 39.61, 46.31, 67.71, 81.61 and 85.81 can be ascribed to the Pt diffractions of (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes, which represent the characteristics of face-
Fig. 1. XRD patterns of Pt/CeO2–RGO and Pt/RGO.
centered cubic (fcc) structure of platinum. Moreover, diffraction peaks [(1 1 1), (2 0 0), (3 1 1), and (4 2 0)] of CeO2 were observed in the XRD pattern for Pt/CeO2–RGO, corresponding to 2θ ¼ 28.6, 33.2, 56.4, and 78, respectively. These diffraction peaks indexed as CeO2 with cubic fluorite structure, in good agreement with the literature values [9]. No shift was observed at the positions of the Pt diffraction peaks in the Pt/CeO2–RGO catalyst, indicating the addition of CeO2 had no obvious effect on the crystalline lattice of Pt. The morphology of as-prepared catalysts was further investigated with TEM, as illustrated in Fig. 2. It can been seen that Pt and CeO2 nanoparticles were both well dispersed on the graphene support, and the diameter of Pt nanoparticles ranging from 2–5 nm was obtained by measuring the diameter of Pt 100 nanoparticles in the magnified TEM images. The lattice fringes of Pt (1 1 1) can be measured with a spacing of 0.22 nm. In Fig. 2b, the distance of the crystal planes of the nanoparticle was determined to be 0.31 nm, consistent that of the (1 1 1) plane of CeO2. Pt and CeO2 nanoparticles were also closely adjacent to each other. The larger sphere is CeO2 particle (5–35 nm), contacting with Pt particles as presented with smaller sizes. In electrochemical tests, Fig. 3a shows the cyclic voltammogram (CV) curves of the as prepared catalysts in 0.5 M H2SO4 solution. The electrochemical surface area (ESA) is estimated from the integrated charge in the hydrogen/adsorption region of the CV curves, and the ESA of Pt–CeO2/RGO is calculated at 53.78 m2 g 1 Pt, which is 1.4 times as large as that of Pt/RGO (38.45 m2 g 1 Pt). The higher ESA value for Pt/CeO2–RGO may result from the synergistic effect between Pt and the ionic conductor of CeO2, which may promote the hydrogen spillover rate of Pt–H and hence increases the dissociation of hydrogen adsorption [11]. The electrocatalytic activity of Pt/CeO2–RGO for the electrooxidation of methanol was measured in 0.5 M H2SO4 and 0.5 M CH3OH solution as shown in Fig. 3b. It can be seen that the electrooxidizing current is significantly improved and the onset potential of electro-oxidation shifted to a lower value on Pt/CeO2–RGO (0.28 V) than on Pt/RGO (0.25 V). This result was further confirmed by the CO stripping test (Fig. 3c). The onset potential for COads oxidation on Pt/CeO2–RGO catalyst was shifted negatively (about 0.03 V) compared to ceria free Pt alone system. The reason may be that a large concentration of oxygen vacancies in the CeO2 provides active sites for CO adsorption. Fig. 3d shows chronoamperometric curves of Pt/CeO2–RGO and Pt/ RGO for methanol oxidation. Both of the catalysts display an initial fast current decay, which is attributed to the poisoning of the catalysts by intermediate species such as COads, CHOads formed during the methanol oxidation reaction. The residual current density of Pt/CeO2–RGO catalyst after 3600 s is 1.95 mA cm 2, which is much higher than that of Pt/RGO catalyst (0.88 mA cm 2), indicating the improved stability with the incorporation of CeO2.
Fig. 2. TEM images of CeO2–GO (a), Pt/CeO2–RGO (b) and Pt/RGO (c).
T. Han, Z. Zhang / Materials Letters 154 (2015) 177–179
179
Fig. 3. Cyclic voltammograms (a) and COads stripping voltammograms (c) of Pt/CeO2–RGO and Pt/RGO in 0.5 M H2SO4; cyclic voltammograms (b) and chroamperometry (d) of Pt/CeO2–RGO and Pt/RGO in 0.5 M H2SO4 and 0.5 M CH3OH; scan rate: 50 mV/s , temperature: 25 1C.
4. Conclusions To sum up, we have successfully synthesized Pt/CeO2–RGO electrocatalyst with improved activity and stability via a two-step process. The CeO2–RGO hybrid was prepared via a novel isothermal hydrolyzing process in low temperature and then the deposition of Pt nanoparticles by a microwave assisted method. The cyclic voltammetry and chronoamperometry tests show that the prepared Pt/CeO2– RGO catalyst exhibited much higher electrocatalytic activity and stability than the Pt/RGO catalyst for methanol oxidation, demonstrating a new synthesis strategy for the development of the highperformance graphene-based electrocatalysts for DMFCs applications. Acknowledgments This project is supported by Heilongjiang Provincial Natural Science Foundation of China (Grant no. F201315) and Harbin Research Fund for Technological Innovation (No. 2013RFQXJ104). References [1] Feng Ligang, Zhang Jing, Cai Weiwei, Liangliang, Xing Wei, Liu Changpeng. Single passive direct methanol fuel cell supplied with pure methanol. J Power Sources 2011;196:2750–3. [2] Lee SK, Kakati N, Jee SH, Maiti J, Yoon YS. Hydrothermal synthesis of PtRu nanoparticles supported on graphene sheets for methanol oxidation in direct methanol fuel cell. Mater Lett 2011;65:3281–4. [3] Liu JG, Zhao TS, Chen R, Wong CW. The effect of methanol concentration on the performance of a passive DMFC. Electrochem Commun 2005;7:288–94. [4] Nethravathi C, Anumol EA, Rajamathi M, Ravishankar N. Highly dispersed ultrafine Pt and PtRu nanoparticles on graphene: formation mechanism and electrocatalytic activity. Nanoscale 2011;3:569–71.
[5] Ji Z, Jalbout AF, Li JQ. Adsorption and diffusion of OH on Mo modified Pt (111) surface: first-principles theory. Solid State Commun 2007;142:148–53. [6] Park E, Shim S, Ha R, Oh E, Lee BW, Choi HJ. Reassembling of Ni and Pt catalyst in the vapor–liquid–solid growth of GaN nanowires. Mater Lett 2011;65:2458–61. [7] Spinacé EV, Farias LA, Linardi M, Neto AO. Preparation of PtSn/C and PtSnNi/C electrocatalysts using the alcohol-reduction process. Mater Lett 2008;62:2099–102. [8] Zhang C, Liu F, Zhai Y, Ariga H, Yi N, Liu Y, et al. Alkali-metal-promoted Pt/TiO2 opens a more efficient pathway to formaldehyde oxidation at ambient temperatures. Angew Chem 2012;51:9628–32. [9] Chen H, Duan J, Zhang X, Zhang Y, Guo C, Nie L. One step synthesis of Pt/CeO2– graphene catalyst by microwave-assisted ethylene glycol process for direct methanol fuel cell. Mater Lett 2014;126:9–12. [10] Yu W, Porosoff M, Chen J. Review of Pt-based bimetallic catalysis: from model surfaces to supported catalysts. Chem Rev 2012;112:5780–817. [11] Yuan H, Guo D, Li X, Yuan L, Zhu W, Chen L, et al. The effect of CeO2 on Pt/ CeO2/CNT catalyst for CO electrooxidation. Fuel Cells 2009;9:121–7. [12] Tanakaa S, Umedab M, Ojimac H, Usuia Y, Kimuraa O, Uchida I. Preparation and evaluation of a multi-component catalyst by using a co-sputtering system for anodic oxidation of ethanol. J Power Sources 2005;152:34–9. [13] Grabowskaa E, Sobczakb JW, Gazdac M, Zaleskaa A. Surface properties and visible light activity of W–TiO2 photocatalysts prepared by surface impregnation and sol–gel method. Appl Catal B: Environ 2012;117-8:351–9. [14] Zhang N, Fu X, Xu YJ. A facile and green approach to synthesize Pt@CeO2 nanocomposite with tunable core–shell and yolk–shell structure and its application as a visible light photocatalyst. J Mater Chem 2011;21:8152–8.