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Rod-shaped CeO2 intercalated Graphene for supporting Pt composite as Anode catalysts for DMFCs
Accepted Manuscript Title: Rod-shaped CeO2 intercalated Graphene for supporting Pt composite as Anode catalysts for DMFCs Author: Weihua Wang Xiaolin Lu Mingda Zhu Zhenzhu Cao Caihong Li Yanfang Gao Lijun Li Jinrong Liu PII: DOI: Reference:
S0013-4686(15)30120-1 http://dx.doi.org/doi:10.1016/j.electacta.2015.07.057 EA 25339
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
9-5-2015 7-7-2015 9-7-2015
Please cite this article as: Weihua Wang, Xiaolin Lu, Mingda Zhu, Zhenzhu Cao, Caihong Li, Yanfang Gao, Lijun Li, Jinrong Liu, Rod-shaped CeO2 intercalated Graphene for supporting Pt composite as Anode catalysts for DMFCs, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.07.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Rod-shaped CeO2 intercalated Graphene supporting Pt composite as Anode catalysts for DMFC Weihua Wang, Xiaolin Lu, Mingda Zhu, Zhenzhu Cao, Caihong Li, Yanfang Gao*, Lijun Li*, Jinrong Liu
College of Chemical Engineering, Inner Mongolia University of Technology, Hohhot, 010051, P. R. China. *Corresponding author: Telephone: +86 471 6575722 Email: [email protected]（Yanfang Gao） ，[email protected]（Lijun Li ）
Graphical abstract Highlights 1. Rod-shaped CeO2 was synthesized by in situ growth method on the graphene sheets; 2. The rod-shaped CeO2 act as a spacer to prevent graphene stack; 3. A three-dimensional (3D) layer-by-layer Pt-NRCeO2/GNs composite structure is created; 4. The as-designed 3D Pt-NRCeO2/GNs catalyst shows the excellent electrochemical properties; 5. The as-designed 3D structure helped more active sites exposed to the electrolyte solution. ABSTRACT A designed rod-shaped CeO2 (NRCeO2) was successfully synthesized by in situ growth method on the graphene sheets (GNs), which act as a spacer to prevent graphene stack. When graphene sheets which has grown rod-shaped CeO2 and also embellished with Pt nanoparticles are continuously stacked on each other, a three-dimensional (3D)
layer-by-layer Pt-NRCeO2/GNs composite structure is created. For TEM and SEM characterization, the 3D structure has been confirmed. In the electrochemical tests, the as-designed 3D Pt-NRCeO2/GNs catalyst shows the highest ESA value (72.6 m2 g-1) and the largest catalytic current density (498 mA mg-1) for methanol oxidation compared with other catalysts. Due to the effect of rod-shaped CeO2 which act as a spacer to prevent graphene stack, a large amount of Pt particles are exposed to the electrolyte solution. It shows that a designed 3D Pt-NRCeO2/GNs composite structure is an efficient way to improve the electrocatalytic performance. Keywords: Direct methanol fuel cell, 3D hybrid structure, Graphene sheets, CeO2, Pt nanoparticles 1. Introduction Duing to its outstanding catalytic performance for oxygen reduction reaction, precious metal nano-particle such as Pt nanoparticle have received widely attention and research as electrocatalysts in direct methanol fuel cells[1-3]. However, its low utilization and low stability owing to CO poisoning are greatly hinder its commercial application[4,5]. To avoid the poisoning of Pt catalysts, researchers usually make a alloy of Pt with additional metals or doping other metal oxides [6-11]. In order to improve the utilization rate of platinum, carbon black usually be used as a surport for Pt nanoparticle to improve the dispersion of platinum[12-14]. In recent years, graphene as a new two-dimension carbon materials have a high surface area and good conductivity, which has been extensively studied as a good surport for Pt by many researchers of DMFCs [15-17]. So a seemingly perfect combination point to Pt/Metal Oxide/Graphene. However, due to the nature of the two-dimension material, graphene sheets are very easy to
stack by π-π interaction even though loaded with other smaller nanoparticles[18]. The stack of graphene will bury a large number of catalytic sites, which will causes that a lot of catalytic element cannot participate in catalytic reaction. Simultaneously, this stack structure will increase the diffusion resistance of reactant molecules, which will reduces the catalytic reaction rate [19]. In order to overcome the defect of graphene stack, some researchers [20-22] had make efforts to insert some other carbon materials such as carbon blacks and carbon nano-tubes into the graphene layers, and they indicated that this way was an effective method to prevent graphene stack and enhance the catalyst performance. In recently, some researchers [23] attempted to make the active element design into a spacer, which also could effectively prevent graphene stack. Although the above efforts can effectively prevent graphene stack, but some methods are not suitable for practical application, especially, the way of making active element design into a spacer which will greatly improve the commercial cost of some catalysts. CeO2, a low-cost metallic oxide, has a higher concentration of oxygen vacancies, which has been called as oxygen tank can improve the oxygen content on the surface of catalyst [16]. Because of it is higher oxygen storage capacity (cerium ion valence: +3/+4 switch), CeO2 has a high catalytic efficiency for CO adsorption and oxidation, which has been widely used for redox reaction catalyst [12]. Previous studies [8,12,16] have shown that the introduction of cerium oxide could greatly promoted catalytic performance and stability of Pt-based composite. In this paper, we attempt to design CeO2 as a rod-shaped spacer, which will be expected to prevent graphene stack and enhance the catalytic property. Firstly, we successfully controlled synthesis of rod-shaped CeO2 by in situ growth method on graphene sheets.
When graphene sheets which has grown rod-shaped CeO2 are continuously stacked on each other, a three-dimensional (3D) layer-by-layer rod-shaped CeO2/graphene (NRCeO2/GNs) composite structure is created. Meanwhile, we loaded the Pt nanoparticles onto the 3D support material of
NRCeO2/GNs,
assembled as a 3D Pt-NRCeO2/GNs catalyst. Through
electrochemical performance characterization, this kind of 3D structure design not only can help more active sites exposed out but also promoted the catalytic performance for methanol electrooxidation. 2. Experimental Section 2.1. Synthesis of oxidized graphene oxide Graphene oxide (GO) was synthesized by a modified Hummers method with a slightly modification [24]. Typically, 3.0 g flake graphite was added to a 9:1 mixture of H2SO4/H3PO4 (360/40 mL), then 18.0 g KMnO4 was added to the above solution under stirring. The reaction was then heated to 50℃ and stirred for 20 h. When the solution was cooled to room temperature poured it onto ice (approximately 400 mL), then dropwise added 30% H2O2 to the solution color tune to brown. The obtained product was collected by centrifugation, washed repeatedly with deionized water and ethanol and dried in vacuum at 60℃ for 10 h. 2.2. Synthesis of rod-shaped CeO2/ oxidized graphene (NRCeO2/GO) 40 mg graphene oxide disperse to 40 mL H2O by ultraphonic 30 min, then 0.3 g Ce(NO3)3·6H2O and 9.6 g NaOH were added. After stir 10 min, the mixture was transferred to an autoclave for hydrothermal treatment 24 hours at 100℃.Then, cooled to room temperature, centrifugation, washed twice with deionized water. The obtained sample was denoted as
NRCeO2/GO.
Another sample denoted as
EACeO2/GO
was synthesized by the
same method with some modifications, firstly, NH3OH instead of NaOH to adjust the pH to approximately 10.5, the mixed solution was stired 6 hours at 80℃, then, hydrothermal treatment 4 hours at 160℃. 2.3. Synthesis of Pt-NRCeO2/GNs catalyst 20 mg NRCeO2/GO (EACeO2/GO) disperse to 25 mL ethylene glycol (EG) by ultraphonic 30 min, 0.5mL of 0.05 M H2PtCl6-EG solution was added, and then 0.5 M KOH-EG solution was used to adjust the pH to 9.5, followed by stirring 3 hours at 115℃. After the reaction was cooled to room temperature, centrifugation, washed twice with deionized water and dried in vacuum at 60℃ for 10 h. The obtained sample was denoted as Pt-NRCeO2/GNs (Pt-EACeO2/GNs). 2.4. Measurement The morphology and structure of samples was investigated by field emission SEM (FESEM) and high-resolution TEM (HRTEM), the SEM imges were recorded on the Hitachi S-3400N microscope and the high-resolution TEM images were obtained on the FEI Tecnai G20 microscope operated at 200 kV. The energy dispersive X-ray (EDX) bspectrogram was recorded from the same FESEM. Cyclic voltammetric (CV) and chronoamperometric (CA) measurements were performed on the Parstat 2273 electrochemical workstation (Princeton applied research CO., Ltd, USA) in a standard three-electrode cell which was using a platinum wire as the counter electrode and a KCl-saturated Ag/AgCl electrode as the reference electrode. The working electrode was prepared as following method: 5 mg of Pt-NRCeO2/GNs catalyst was dispersed to the mixture of 1 mL ethanol and 15 uL nafion. After ultrasonic 10 min, 5.0 uL of the suspension was transferred to the glass carbon (GC) electrode surface which was
polished with Al2O3 by a microsyringe. Then, the GC electrode was dried in air at room temperature. The CV curves for hydrogen absorption/desorption were performed in a 1 M N2-saturated H2SO4 aqueous solution, the potential was cycled from -0.25 to 1.25 V at a scan rate of 50 mV s-1. The CVs for CH3OH oxidation are carried out in a N2-saturated 0.5 M H2SO4 and 0.5 M CH3OH mixed solution, the potential was cycled from 0 to 1 V at the scan rate of 50 mV s-1. The chronoamperometric curves were recorded for 1000s at a fixed voltage of 0.7 V in the 0.5 M H2SO4 and 0.5 M CH3OH mixed solution. 3. Results and discussion To establish the as-designed 3D structure, firstly, rod-shaped CeO2 should to be growthed on the graphene sheets, then assembled into Pt-NRCeO2/GNs with Pt nanoparticle. Figure 1 shows the TEM (a, b, c) and HRTEM (d) images, from the low-resolution image (a) we can see that the rod-shaped CeO2 growing on graphene sheets is similar to a beam-shaped block, and they are disorderly scattered on the surface of graphene. But the magnified image (b) shows a rod-shaped structure of CeO2, especially from the square frame with a dashed line we can see a clear rod-shaped structure, they have a diameter at ca.12 nm and a length distribution between 20 nm and 200 nm. From figure 1(c), a lot of small particles with a diameter at only a few nanometers (from the circular frame with a dashed line) uniformly disperse on the graphene sheets, meanwhile, we also can see the dendritic (rod-shaped) CeO2 in the image. Figure 1(d) give a clear answer that the small particle is Pt nanoparticle with a {111} crystal plane, and the rod-shaped CeO2 expose {110} and {100} crystal planes. As shown in figure 1, we successfully synthesized rod-shaped CeO2 on graphene sheets by a one-step in situ growth method, and they show a uniform rod-shaped structure. Meanwhile,
the small Pt nanoparticle distribute evenly on the graphene sheet surface. In order to observe the three-dimensional structure of Pt-NRCeO2/GNs, the sample is characterized by scanning electron microscopy (SEM). Figure 2 shows the cross-section images of Pt/GNs (a, c)、Pt-NRCeO2/GNs (b, d, f), and catalyst films were made by drop-casting 0.5 mL catalyst ethanol solution (5 mg mL-1) on ITO inductive glass, which will be made into a slope at an angle of about 30 degree when characterized by SEM. In figure 2, graphene shows obvious thin structure with wrinkles, which is stack up about 10 layers. Comparing figure 2(a) with (b), we can see the structure of Pt-NRCeO2/GNs is fluffier than Pt/GNs. From figure 2. (c), the cross-section of catalyst without rod-shaped CeO2 shows a compacted structure, and the superposition between graphene layer and layer is very close which has cross-linked and curled together. Although the graphene layers of Pt-NRCeO2/GNs are also stacked together from figure 2. (d), but its cross-section looks very loose, and the boundaries between layer and layer are clear. The cross-section of Pt-NRCeO2/GNs have a thickness of ca. 2.5 um, which is thicker than Pt/GNs (ca. 2 um). Figure 2(f) is the magnification of (d), it shows that rod-shaped CeO2 act as a spacer insert into the graphene layer, which has effectively established a three-dimensional structure. Figure 2(e) shows a typical EDS result of Pt-NRCeO2/GNs. The content of Pt and CeO2 are 58.8%, 16.95%, respectively. It is close to our initial design. The catalyst was dropped onto the glass carbon (GC) electrode surface which was polished with Al2O3, then it was assembled into a standard three-electrode cell with platinum wire as the counter electrode and Ag/AgCl as reference electrode. We conducted a series of electrochemical characterization of as-prepared samples. We performed a cyclic voltammetry (CV) scan in 0.5 M H2SO4 solution, which would
provided the hydrogen adsorption/desorption curves. On the basis of the hydrogen adsorption/desorption curves, the electrochemically active surface areas (ESA) of as-prepared samples are calculated from the following equation: ESA m 2 g 1
QH 2.1 Pt
Here, the QH represent the charge of hydrogen adsorption/desorption, [Pt] represent the quantity of Pt loading on the electrode. Figure 3 shows the CV curves of Pt-NRCeO2/GNs, Pt-EACeO2/GNs, Pt/GNs and NRCeO2/GNs
in 0.5 M H2SO4 solution at scan rate of 50 mV s-1. We can see two obvious
hydrogen adsorption/desorption peaks between -0.18 and 0.2 V. The hydrogen adsorption/desorption peak of Pt-NRCeO2/GNs is obvious higher than others, which indicate to consumed more charge of hydrogen adsorption/desorption. Table 1 shows the calculated results of Pt-NRCeO2/GNs (72.6 m2 g-1), Pt-EACeO2/GNs (40.9 m2 g-1), Pt /GNs (40.3 m2 g-1). The calculated results means that Pt-NRCeO2/GNs exposed more active sites, which indicate to more Pt particles have participated reaction. It illustrates that the as-designed 3D structure can greatly promote the exposure of Pt particles. It attribute to that the as-designed 3D structure provided an passage for electrolyte solution in/out graphene layers [19,21], which means electrolyte solution can easily touch with Pt particles. In order to reveal the difference electrochemical performance between as-prepared catalysts, we performed another cyclic voltammetry (CV) scan in 0.5 M H2SO4 + 0.5 M CH3OH solution. As shown in figure 4, we can see two obvious peaks at ca. 0.7 V and ca. 0.5 V, which are corresponding to forward methanol oxidation and backward intermediate oxidation, respectively [12]. Usually, forward scanning peak current (If) is used to evaluate
the catalyst activity for methanol electrochemical oxidation. The catalyst with a higher If means higher catalytic activity. Figure 4 shows the cyclic voltammetry curves of Pt-NRCeO2/GNs, Pt-EACeO2/GNs, Pt/GNs and
NRCeO2/GNs.
The Pt-NRCeO2/GNs shows a
highest If value of 498 mA mg-1, but Pt-EACeO2/GNs, Pt/GNs only show 340 mA mg-1、156 mA mg-1, respectively. This result reflects that the Pt-NRCeO2/GNs with a 3D structure shows the highest catalytic activity for methanol electrochemical oxidation. It maybe attribute to the highest ESA, which means more active sites have been exposed. To demonstrate the important role of rod-shaped CeO2 fabricative 3D structure in the performance of endurance, the chronoamperometry test has been carried out in 0.5 M H2SO4+0.5 M CH3OH solution at 0.7V for 1000s. Figure 5 shows the chronoamperometry curves of Pt-NRCeO2/GNs, Pt-EACeO2/GNs, Pt /GNs. Duing to poisoning by carbonaceous species, all curves display a sharp drop from the start. But the attenuation of Pt-NRCeO2/GNs curve is rather gradual than other curves in the test. After 1000 s a long time measurements, the current density of Pt-NRCeO2/GNs still remains a highest value of 30.5 mA mg-1 comparing to Pt-EACeO2/GNs, Pt/GNs of 15.6 mA mg-1, 4.7 mA mg-1, respectively. It clearly revealed that the as-designed 3D structure can greatly increase the catalyst electrocatalytic stability. 4. Conclusion In conclusion, we successfully designed and synthesized a three-dimensional structure catalyst of Pt-NRCeO2/GNs. The rod-shaped CeO2 synthesized by in situ growth on graphene sheets act as an effective spacter to prevent graphene stack. The 3D architecture has been confirmed using TEM and SEM characterization. For electrochemical performance test, the as-designed 3D structure catalyst shows a considerably higher ESA
value and a larger catalytic current density for methanol oxidation compared with other catalysts, as well as a very good stability. Duing to the designed 3D structure of rod-shaped CeO2 inserted into graphene sheets act as a spacter, a lot of Pt particles anchoring on the graphene sheets are exposed to the electrolyte solution, which effectively promoted the reaction. This work provides a unique Pt-NRCeO2/GNs 3D structure, which can effectively improve the catalytic performance.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No.21266018), Science and technology projects of Science and Technology Department of Inner Mongolia Autonomous Region, P. R. China (No.20110401 and No.20130409), the Natural Science Foundation of Inner Mongolia, P. R. China (No.2010MS0218), Program for Young Talents of Science and Technology in Universities of Inner Mongolia Autonomous Region (No. NJYT-15-A04), Ministry of Science and Technology China-South Africa Joint Research Program (No.CS08-L15). References [1] M. Lefèvre, E. Proietti, F. Jaouen, J. Dodelet, Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells, Science. 324 ( 2009) 71. [2] V. D. Noto, E. Negro, R. Gliubizzi, S. Lavina, G. Pace, S. Gross, C. Maccato, A Pt-Fe Carbon Nitride Nano-electrocatalyst for Polymer Electrolyte Membrane Fuel Cells and Direct-Methanol Fuel Cells: Synthesis, Characterization, and Electrochemical Studies, Adv. Funct. Mater. 17 (2007) 3626. [3] M. K. Jeon, J. Y. Won, K. R. Lee, S. I. Woo, Highly active PtRuFe/C catalyst for
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[19] Y. Li, Y. Li, E. Zhu, T. M. Louth, C. Y. Chiu, X. Huang, Y. Huang, Stabilization of High-Performance Oxygen Reduction Reaction Pt Electrocatalyst Supported on Reduced Graphene Oxide/Carbon Black Composite, J. Am. Chem. Soc. 134 (2012) 12326. [20] S. Park, Y. Shao, H. Wan, P. C. Rieke, V. V. Viswanathan, S. A. Towne, L. V. Saraf, J. Liu, Y. Lin, Y. Wang, Design of graphene sheets-supported Pt catalyst layer in PEM fuel cells, Electrochemistry Communications. 13 (2011) 258. [21] Rajesh, R. K. Paul, A. Mulchandani, Platinum nano owers decorated three-dimensional grapheneecarbon nanotubesfl hybrid with enhanced electrocatalytic activity, Journal of Power Sources. 223 (2013) 23. [22] Y. S. Wang, S. Y. Yang, S. M. Li, H. W. Tien, S. T. Hsiao, W. H. Liao, C. H. Liu, K. H. Chang, C. C. M. Ma, C. C. Hu, Three-dimensionally porous graphene–carbon nanotube composite-supported PtRu catalysts with an ultrahigh electrocatalytic activity for methanol oxidation, Electrochimica Acta. 87 (2013) 261. [23] C. Xue, M. Gao, Y. Xue, L. Zhu, L. Dai, A. Urbas, Q. Li, Building 3D Layer-by-Layer Graphene-Gold Nanoparticle Hybrid Architecture with Tunable Interlayer Distance, J. Phys. Chem. C. 118 (2014) 15332. [24] D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L. B. Alemany, W. Lu, J. M. Tour, Improved Synthesis of Graphene Oxide, ACS Nano. 4 (2010) 1806. Figure captions Figure 1. TEM (a,b,c) and HRTEM (d) of Pt-NRCeO2/GNs Figure 2. SEM of Pt/GNs (a, c), Pt-NRCeO2/GNs (b, d, f) and EDS (e) of Pt-NRCeO2/GNs Figure 3. Cyclic voltammograms of Pt-NRCeO2/GNs, Pt-EACeO2/GNs, Pt/GNs and
NRCeO2/GNs
in 0.5 M H2SO4 solution at scan rate of 50 mV s-1
Figure 4. Cyclic voltammograms of Pt-NRCeO2/GNs, Pt-EACeO2/GNs, Pt/GNs and NRCeO2/GNs
in 0.5 M H2SO4 + 0.5 M CH3OH solution at scan rate of 50 mV s-1
Figure 5. Chronoamperometry curves of Pt-NRCeO2/GNs, Pt-EACeO2/GNs, Pt/GNs in 0.5 M H2SO4+0.5 M CH3OH solution at 0.7V for 1000s Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Table
1.
The
corresponding
Pt-EACeO2/Graphene, Pt/Graphene
electrochemical
data
of
Pt-NRCeO2/Graphene,
Catalyst
ESA (m2 g-1)
If
CA terminal current
(mA mg-1)
(mA mg-1)
Pt-NRCeO2/GN
72.6
498
30.5
Pt-EACeO2/GN
40.9
340
15.6
Pt/GNs
40.3
156
4.7
S0013-4686(15)30120-1 http://dx.doi.org/doi:10.1016/j.electacta.2015.07.057 EA 25339
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
9-5-2015 7-7-2015 9-7-2015
Please cite this article as: Weihua Wang, Xiaolin Lu, Mingda Zhu, Zhenzhu Cao, Caihong Li, Yanfang Gao, Lijun Li, Jinrong Liu, Rod-shaped CeO2 intercalated Graphene for supporting Pt composite as Anode catalysts for DMFCs, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.07.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Rod-shaped CeO2 intercalated Graphene supporting Pt composite as Anode catalysts for DMFC Weihua Wang, Xiaolin Lu, Mingda Zhu, Zhenzhu Cao, Caihong Li, Yanfang Gao*, Lijun Li*, Jinrong Liu
College of Chemical Engineering, Inner Mongolia University of Technology, Hohhot, 010051, P. R. China. *Corresponding author: Telephone: +86 471 6575722 Email: [email protected]（Yanfang Gao） ，[email protected]（Lijun Li ）
Graphical abstract Highlights 1. Rod-shaped CeO2 was synthesized by in situ growth method on the graphene sheets; 2. The rod-shaped CeO2 act as a spacer to prevent graphene stack; 3. A three-dimensional (3D) layer-by-layer Pt-NRCeO2/GNs composite structure is created; 4. The as-designed 3D Pt-NRCeO2/GNs catalyst shows the excellent electrochemical properties; 5. The as-designed 3D structure helped more active sites exposed to the electrolyte solution. ABSTRACT A designed rod-shaped CeO2 (NRCeO2) was successfully synthesized by in situ growth method on the graphene sheets (GNs), which act as a spacer to prevent graphene stack. When graphene sheets which has grown rod-shaped CeO2 and also embellished with Pt nanoparticles are continuously stacked on each other, a three-dimensional (3D)
layer-by-layer Pt-NRCeO2/GNs composite structure is created. For TEM and SEM characterization, the 3D structure has been confirmed. In the electrochemical tests, the as-designed 3D Pt-NRCeO2/GNs catalyst shows the highest ESA value (72.6 m2 g-1) and the largest catalytic current density (498 mA mg-1) for methanol oxidation compared with other catalysts. Due to the effect of rod-shaped CeO2 which act as a spacer to prevent graphene stack, a large amount of Pt particles are exposed to the electrolyte solution. It shows that a designed 3D Pt-NRCeO2/GNs composite structure is an efficient way to improve the electrocatalytic performance. Keywords: Direct methanol fuel cell, 3D hybrid structure, Graphene sheets, CeO2, Pt nanoparticles 1. Introduction Duing to its outstanding catalytic performance for oxygen reduction reaction, precious metal nano-particle such as Pt nanoparticle have received widely attention and research as electrocatalysts in direct methanol fuel cells[1-3]. However, its low utilization and low stability owing to CO poisoning are greatly hinder its commercial application[4,5]. To avoid the poisoning of Pt catalysts, researchers usually make a alloy of Pt with additional metals or doping other metal oxides [6-11]. In order to improve the utilization rate of platinum, carbon black usually be used as a surport for Pt nanoparticle to improve the dispersion of platinum[12-14]. In recent years, graphene as a new two-dimension carbon materials have a high surface area and good conductivity, which has been extensively studied as a good surport for Pt by many researchers of DMFCs [15-17]. So a seemingly perfect combination point to Pt/Metal Oxide/Graphene. However, due to the nature of the two-dimension material, graphene sheets are very easy to
stack by π-π interaction even though loaded with other smaller nanoparticles[18]. The stack of graphene will bury a large number of catalytic sites, which will causes that a lot of catalytic element cannot participate in catalytic reaction. Simultaneously, this stack structure will increase the diffusion resistance of reactant molecules, which will reduces the catalytic reaction rate [19]. In order to overcome the defect of graphene stack, some researchers [20-22] had make efforts to insert some other carbon materials such as carbon blacks and carbon nano-tubes into the graphene layers, and they indicated that this way was an effective method to prevent graphene stack and enhance the catalyst performance. In recently, some researchers [23] attempted to make the active element design into a spacer, which also could effectively prevent graphene stack. Although the above efforts can effectively prevent graphene stack, but some methods are not suitable for practical application, especially, the way of making active element design into a spacer which will greatly improve the commercial cost of some catalysts. CeO2, a low-cost metallic oxide, has a higher concentration of oxygen vacancies, which has been called as oxygen tank can improve the oxygen content on the surface of catalyst [16]. Because of it is higher oxygen storage capacity (cerium ion valence: +3/+4 switch), CeO2 has a high catalytic efficiency for CO adsorption and oxidation, which has been widely used for redox reaction catalyst [12]. Previous studies [8,12,16] have shown that the introduction of cerium oxide could greatly promoted catalytic performance and stability of Pt-based composite. In this paper, we attempt to design CeO2 as a rod-shaped spacer, which will be expected to prevent graphene stack and enhance the catalytic property. Firstly, we successfully controlled synthesis of rod-shaped CeO2 by in situ growth method on graphene sheets.
When graphene sheets which has grown rod-shaped CeO2 are continuously stacked on each other, a three-dimensional (3D) layer-by-layer rod-shaped CeO2/graphene (NRCeO2/GNs) composite structure is created. Meanwhile, we loaded the Pt nanoparticles onto the 3D support material of
NRCeO2/GNs,
assembled as a 3D Pt-NRCeO2/GNs catalyst. Through
electrochemical performance characterization, this kind of 3D structure design not only can help more active sites exposed out but also promoted the catalytic performance for methanol electrooxidation. 2. Experimental Section 2.1. Synthesis of oxidized graphene oxide Graphene oxide (GO) was synthesized by a modified Hummers method with a slightly modification [24]. Typically, 3.0 g flake graphite was added to a 9:1 mixture of H2SO4/H3PO4 (360/40 mL), then 18.0 g KMnO4 was added to the above solution under stirring. The reaction was then heated to 50℃ and stirred for 20 h. When the solution was cooled to room temperature poured it onto ice (approximately 400 mL), then dropwise added 30% H2O2 to the solution color tune to brown. The obtained product was collected by centrifugation, washed repeatedly with deionized water and ethanol and dried in vacuum at 60℃ for 10 h. 2.2. Synthesis of rod-shaped CeO2/ oxidized graphene (NRCeO2/GO) 40 mg graphene oxide disperse to 40 mL H2O by ultraphonic 30 min, then 0.3 g Ce(NO3)3·6H2O and 9.6 g NaOH were added. After stir 10 min, the mixture was transferred to an autoclave for hydrothermal treatment 24 hours at 100℃.Then, cooled to room temperature, centrifugation, washed twice with deionized water. The obtained sample was denoted as
NRCeO2/GO.
Another sample denoted as
EACeO2/GO
was synthesized by the
same method with some modifications, firstly, NH3OH instead of NaOH to adjust the pH to approximately 10.5, the mixed solution was stired 6 hours at 80℃, then, hydrothermal treatment 4 hours at 160℃. 2.3. Synthesis of Pt-NRCeO2/GNs catalyst 20 mg NRCeO2/GO (EACeO2/GO) disperse to 25 mL ethylene glycol (EG) by ultraphonic 30 min, 0.5mL of 0.05 M H2PtCl6-EG solution was added, and then 0.5 M KOH-EG solution was used to adjust the pH to 9.5, followed by stirring 3 hours at 115℃. After the reaction was cooled to room temperature, centrifugation, washed twice with deionized water and dried in vacuum at 60℃ for 10 h. The obtained sample was denoted as Pt-NRCeO2/GNs (Pt-EACeO2/GNs). 2.4. Measurement The morphology and structure of samples was investigated by field emission SEM (FESEM) and high-resolution TEM (HRTEM), the SEM imges were recorded on the Hitachi S-3400N microscope and the high-resolution TEM images were obtained on the FEI Tecnai G20 microscope operated at 200 kV. The energy dispersive X-ray (EDX) bspectrogram was recorded from the same FESEM. Cyclic voltammetric (CV) and chronoamperometric (CA) measurements were performed on the Parstat 2273 electrochemical workstation (Princeton applied research CO., Ltd, USA) in a standard three-electrode cell which was using a platinum wire as the counter electrode and a KCl-saturated Ag/AgCl electrode as the reference electrode. The working electrode was prepared as following method: 5 mg of Pt-NRCeO2/GNs catalyst was dispersed to the mixture of 1 mL ethanol and 15 uL nafion. After ultrasonic 10 min, 5.0 uL of the suspension was transferred to the glass carbon (GC) electrode surface which was
polished with Al2O3 by a microsyringe. Then, the GC electrode was dried in air at room temperature. The CV curves for hydrogen absorption/desorption were performed in a 1 M N2-saturated H2SO4 aqueous solution, the potential was cycled from -0.25 to 1.25 V at a scan rate of 50 mV s-1. The CVs for CH3OH oxidation are carried out in a N2-saturated 0.5 M H2SO4 and 0.5 M CH3OH mixed solution, the potential was cycled from 0 to 1 V at the scan rate of 50 mV s-1. The chronoamperometric curves were recorded for 1000s at a fixed voltage of 0.7 V in the 0.5 M H2SO4 and 0.5 M CH3OH mixed solution. 3. Results and discussion To establish the as-designed 3D structure, firstly, rod-shaped CeO2 should to be growthed on the graphene sheets, then assembled into Pt-NRCeO2/GNs with Pt nanoparticle. Figure 1 shows the TEM (a, b, c) and HRTEM (d) images, from the low-resolution image (a) we can see that the rod-shaped CeO2 growing on graphene sheets is similar to a beam-shaped block, and they are disorderly scattered on the surface of graphene. But the magnified image (b) shows a rod-shaped structure of CeO2, especially from the square frame with a dashed line we can see a clear rod-shaped structure, they have a diameter at ca.12 nm and a length distribution between 20 nm and 200 nm. From figure 1(c), a lot of small particles with a diameter at only a few nanometers (from the circular frame with a dashed line) uniformly disperse on the graphene sheets, meanwhile, we also can see the dendritic (rod-shaped) CeO2 in the image. Figure 1(d) give a clear answer that the small particle is Pt nanoparticle with a {111} crystal plane, and the rod-shaped CeO2 expose {110} and {100} crystal planes. As shown in figure 1, we successfully synthesized rod-shaped CeO2 on graphene sheets by a one-step in situ growth method, and they show a uniform rod-shaped structure. Meanwhile,
the small Pt nanoparticle distribute evenly on the graphene sheet surface. In order to observe the three-dimensional structure of Pt-NRCeO2/GNs, the sample is characterized by scanning electron microscopy (SEM). Figure 2 shows the cross-section images of Pt/GNs (a, c)、Pt-NRCeO2/GNs (b, d, f), and catalyst films were made by drop-casting 0.5 mL catalyst ethanol solution (5 mg mL-1) on ITO inductive glass, which will be made into a slope at an angle of about 30 degree when characterized by SEM. In figure 2, graphene shows obvious thin structure with wrinkles, which is stack up about 10 layers. Comparing figure 2(a) with (b), we can see the structure of Pt-NRCeO2/GNs is fluffier than Pt/GNs. From figure 2. (c), the cross-section of catalyst without rod-shaped CeO2 shows a compacted structure, and the superposition between graphene layer and layer is very close which has cross-linked and curled together. Although the graphene layers of Pt-NRCeO2/GNs are also stacked together from figure 2. (d), but its cross-section looks very loose, and the boundaries between layer and layer are clear. The cross-section of Pt-NRCeO2/GNs have a thickness of ca. 2.5 um, which is thicker than Pt/GNs (ca. 2 um). Figure 2(f) is the magnification of (d), it shows that rod-shaped CeO2 act as a spacer insert into the graphene layer, which has effectively established a three-dimensional structure. Figure 2(e) shows a typical EDS result of Pt-NRCeO2/GNs. The content of Pt and CeO2 are 58.8%, 16.95%, respectively. It is close to our initial design. The catalyst was dropped onto the glass carbon (GC) electrode surface which was polished with Al2O3, then it was assembled into a standard three-electrode cell with platinum wire as the counter electrode and Ag/AgCl as reference electrode. We conducted a series of electrochemical characterization of as-prepared samples. We performed a cyclic voltammetry (CV) scan in 0.5 M H2SO4 solution, which would
provided the hydrogen adsorption/desorption curves. On the basis of the hydrogen adsorption/desorption curves, the electrochemically active surface areas (ESA) of as-prepared samples are calculated from the following equation: ESA m 2 g 1
QH 2.1 Pt
Here, the QH represent the charge of hydrogen adsorption/desorption, [Pt] represent the quantity of Pt loading on the electrode. Figure 3 shows the CV curves of Pt-NRCeO2/GNs, Pt-EACeO2/GNs, Pt/GNs and NRCeO2/GNs
in 0.5 M H2SO4 solution at scan rate of 50 mV s-1. We can see two obvious
hydrogen adsorption/desorption peaks between -0.18 and 0.2 V. The hydrogen adsorption/desorption peak of Pt-NRCeO2/GNs is obvious higher than others, which indicate to consumed more charge of hydrogen adsorption/desorption. Table 1 shows the calculated results of Pt-NRCeO2/GNs (72.6 m2 g-1), Pt-EACeO2/GNs (40.9 m2 g-1), Pt /GNs (40.3 m2 g-1). The calculated results means that Pt-NRCeO2/GNs exposed more active sites, which indicate to more Pt particles have participated reaction. It illustrates that the as-designed 3D structure can greatly promote the exposure of Pt particles. It attribute to that the as-designed 3D structure provided an passage for electrolyte solution in/out graphene layers [19,21], which means electrolyte solution can easily touch with Pt particles. In order to reveal the difference electrochemical performance between as-prepared catalysts, we performed another cyclic voltammetry (CV) scan in 0.5 M H2SO4 + 0.5 M CH3OH solution. As shown in figure 4, we can see two obvious peaks at ca. 0.7 V and ca. 0.5 V, which are corresponding to forward methanol oxidation and backward intermediate oxidation, respectively [12]. Usually, forward scanning peak current (If) is used to evaluate
the catalyst activity for methanol electrochemical oxidation. The catalyst with a higher If means higher catalytic activity. Figure 4 shows the cyclic voltammetry curves of Pt-NRCeO2/GNs, Pt-EACeO2/GNs, Pt/GNs and
NRCeO2/GNs.
The Pt-NRCeO2/GNs shows a
highest If value of 498 mA mg-1, but Pt-EACeO2/GNs, Pt/GNs only show 340 mA mg-1、156 mA mg-1, respectively. This result reflects that the Pt-NRCeO2/GNs with a 3D structure shows the highest catalytic activity for methanol electrochemical oxidation. It maybe attribute to the highest ESA, which means more active sites have been exposed. To demonstrate the important role of rod-shaped CeO2 fabricative 3D structure in the performance of endurance, the chronoamperometry test has been carried out in 0.5 M H2SO4+0.5 M CH3OH solution at 0.7V for 1000s. Figure 5 shows the chronoamperometry curves of Pt-NRCeO2/GNs, Pt-EACeO2/GNs, Pt /GNs. Duing to poisoning by carbonaceous species, all curves display a sharp drop from the start. But the attenuation of Pt-NRCeO2/GNs curve is rather gradual than other curves in the test. After 1000 s a long time measurements, the current density of Pt-NRCeO2/GNs still remains a highest value of 30.5 mA mg-1 comparing to Pt-EACeO2/GNs, Pt/GNs of 15.6 mA mg-1, 4.7 mA mg-1, respectively. It clearly revealed that the as-designed 3D structure can greatly increase the catalyst electrocatalytic stability. 4. Conclusion In conclusion, we successfully designed and synthesized a three-dimensional structure catalyst of Pt-NRCeO2/GNs. The rod-shaped CeO2 synthesized by in situ growth on graphene sheets act as an effective spacter to prevent graphene stack. The 3D architecture has been confirmed using TEM and SEM characterization. For electrochemical performance test, the as-designed 3D structure catalyst shows a considerably higher ESA
value and a larger catalytic current density for methanol oxidation compared with other catalysts, as well as a very good stability. Duing to the designed 3D structure of rod-shaped CeO2 inserted into graphene sheets act as a spacter, a lot of Pt particles anchoring on the graphene sheets are exposed to the electrolyte solution, which effectively promoted the reaction. This work provides a unique Pt-NRCeO2/GNs 3D structure, which can effectively improve the catalytic performance.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No.21266018), Science and technology projects of Science and Technology Department of Inner Mongolia Autonomous Region, P. R. China (No.20110401 and No.20130409), the Natural Science Foundation of Inner Mongolia, P. R. China (No.2010MS0218), Program for Young Talents of Science and Technology in Universities of Inner Mongolia Autonomous Region (No. NJYT-15-A04), Ministry of Science and Technology China-South Africa Joint Research Program (No.CS08-L15). References [1] M. Lefèvre, E. Proietti, F. Jaouen, J. Dodelet, Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells, Science. 324 ( 2009) 71. [2] V. D. Noto, E. Negro, R. Gliubizzi, S. Lavina, G. Pace, S. Gross, C. Maccato, A Pt-Fe Carbon Nitride Nano-electrocatalyst for Polymer Electrolyte Membrane Fuel Cells and Direct-Methanol Fuel Cells: Synthesis, Characterization, and Electrochemical Studies, Adv. Funct. Mater. 17 (2007) 3626. [3] M. K. Jeon, J. Y. Won, K. R. Lee, S. I. Woo, Highly active PtRuFe/C catalyst for
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NRCeO2/GNs
in 0.5 M H2SO4 solution at scan rate of 50 mV s-1
Figure 4. Cyclic voltammograms of Pt-NRCeO2/GNs, Pt-EACeO2/GNs, Pt/GNs and NRCeO2/GNs
in 0.5 M H2SO4 + 0.5 M CH3OH solution at scan rate of 50 mV s-1
Figure 5. Chronoamperometry curves of Pt-NRCeO2/GNs, Pt-EACeO2/GNs, Pt/GNs in 0.5 M H2SO4+0.5 M CH3OH solution at 0.7V for 1000s Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Table
1.
The
corresponding
Pt-EACeO2/Graphene, Pt/Graphene
electrochemical
data
of
Pt-NRCeO2/Graphene,
Catalyst
ESA (m2 g-1)
If
CA terminal current
(mA mg-1)
(mA mg-1)
Pt-NRCeO2/GN
72.6
498
30.5
Pt-EACeO2/GN
40.9
340
15.6
Pt/GNs
40.3
156
4.7