Graphene-Based Porous Catalyst with High Stability and Activity for

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A Graphene-based Porous Catalyst with High Stability and Activity for the Methanol Oxidation Reaction Li-Fang Zhang, Jiao-Jing Shao, Wei-Guo Zhang, Chen Zhang, Xiao-Yu Zheng, Hongda Du, and Quan-Hong Yang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 20 Oct 2014 Downloaded from http://pubs.acs.org on October 21, 2014

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The Journal of Physical Chemistry

A Graphene-based Porous Catalyst with High Stability and Activity for the Methanol Oxidation Reaction Lifang Zhang, a, ‡ Jiao-Jing Shao, a, d, ‡ Weiguo Zhang, a * Chen Zhang, a, c Xiaoyu Zheng, a, c Hongda Du, b and Quan-Hong Yang a, b, c* a

School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China.

b

Engineering Laboratory for Functionalized Carbon Materials, Shenzhen and Shenzhen Key

Laboratory for Graphene-based Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen, 518055, China. c

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072, China.

d

School of Materials and Metallurgy, Guizhou University, Guizhou, 550025, China

‡

These authors contribute equally to this work.

Abstract. Due to its large specific surface area and extraordinary carrier mobility, graphene has been widely used as the supporting material for metal particles in catalysis. A platinum-based catalyst with a graphene-based porous matrix as the supporting material was prepared by onestep hydrothermal synthesis, which produces a uniform distribution of platinum nanoparticles on the support. This catalyst material shows excellent anti-poison ability and superior electrocatalytic stability for the methanol oxidation reaction. It was also found that further

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annealing treatment could modify the morphology of platinum particles attached to the graphene, and an appropriate annealing temperature contributed to an improvement of its electrocatalytic activity. Keywords: Monolith, platinum, fuel cell, nanoparticles 1. Introduction With the increasing demand for clean energy, the direct methanol fuel cell (DMFC) has drawn great attention in the energy storage field due to its high energy density, low operating temperature and environmental benignity.1-4 The catalyst is one of the most critical components in a fuel cell, and both electrocatalytic activity and material utilization are most important factors in its production cost. Platinum nanoparticles (Pt NPs) have a highly efficient catalytic activity and have been deposited on carbon materials to prepare commercial catalysts. A carbon-based material is the most widely used support for metal nanoparticles working as catalysts, since it usually possesses many pores and has a high specific surface area and superior electronic conductivity, making it an excellent matrix for the uniform distribution of metal nanoparticles. It also promotes electron transfer during the electrocatalytic reaction. Various carbon-based materials have been investigated as the supporting materials for Pt NPs, such as carbon blacks,5 carbon nanotubes,6-8 carbon nanofibers9 and graphene.10-15 However, platinum-based catalysts always suffer a poisoning effect due to carbon-based intermediates (such as CO) produced by incomplete oxidation of the methanol during the methanol oxidation reaction (MOR). This gradually results in decreased catalytic activity of the catalyst and further lowers its resistance to poisoning. Resistance to poisoning and electrocatalytic stability are two crucial aspects for assessing the catalytic performance of platinum-based catalysts for the MOR. In addition, the use of precious metal platinum causes high production cost of the catalysts, and hence it is necessary


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for material utilization to be efficient, meaning that the particle size of Pt NPs deposited on the supporting material should be small in order to avoid wasting the platinum metal. It is wellknown that a decrease of platinum particle size usually leads to an increase of electrochemical surface area (ECSA) and better electrocatalytic activity. As a single layer of carbon atoms arranged in a hexagonal lattice, graphene possesses a high specific surface area (2630 m2/g) and remarkable carrier mobility (10,000 cm2/V.s),16 and hence is a promising carbon-based support for the deposition of metal nanoparticles to prepare electrochemical catalysts. In this work, a platinum-based catalyst with a graphene-based porous matrix as the support was synthesized by a one-step hydrothermal method. The porous catalyst obtained is characterized by the following features: (1) Pt NPs with average particle size of 3.6 nm are distributed uniformly on the graphene sheets; (2) the preparation of the platinum-based catalyst is realized through one-step hydrothermal synthesis, which realizes almost 100% utilization of the starting materials; (3) the graphene-based porous matrix acting as the support for the platinum-based catalyst is beneficial for fast ion transport during the electrocatalytic process. 2. Experimental 2.1 Sample preparation Preparation of GO. Graphite oxide was synthesized by modified Hummers method.17, 18 34 mg of graphite oxide was dispersed in 17 mL of de-ionized water followed by 2 h of sonication in order to prepare 2 mg/mL GO suspension. Preparation of the graphene-based porous catalysts (Pt/G). Graphene oxide and chloroplatinic acid (H2PtCl6) were used as the precursors for the graphene-based matrix and Pt NPs, respectively. The as-prepared GO suspension (2 mg/mL) was mixed with a H2PtCl6 aqueous solution (1g/mL, 10 µL) under magnetic stirring for 30 min. The pH value of the mixture was


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adjusted to 10 by adding drops of a 1 M NaOH aqueous solution, and the resulting suspension was transferred into a 25 mL Teflon-lined stainless steel autoclave followed by heat treatment at 150 ºC for 6 h. Finally, a cylindrical monolith was obtained, which was rinsed several times using de-ionized water until the pH of the filtrate reached 7. The monolith was then freeze-dried to produce a platinum-based catalyst with a graphene-based porous support. The Thermogravimetric (TG) profile (Figure S1) shows that the as-prepared Pt-based monolith catalyst contains 17.14 wt% platinum, which is comparable to that of a commercial Pt (20 wt %)/C catalyst. In order to improve the crystallinity of the Pt NPs, the monolith was annealed at 300 ºC, 350 ºC, 400 ºC, 450 ºC and 500 ºC under a gas mixture of Ar (95 vol. %) and H2 (5 vol. %) for 6 h, and the catalysts obtained are respectively denoted Pt/G-300, Pt/G-350, Pt/G-400, Pt/G-450, and Pt/G-500. 2.2 Sample characterization Structural characterization. X-ray diffraction (XRD) measurements were conducted at room temperature using a specular reflection mode (Bruker D-8, Cu Kα radiation, λ=0.154056 nm). Scanning (SEM) and transmission electron microscopy (TEM) observations were conducted using Hitachi S-4800 (Hitachi, Japan) and JEM 2100F (JEOL, Japan) instruments, respectively. Electrochemical analysis. All electrochemical measurements were carried out on a multichannel VMP3 electrochemical analyzer using a standard three-electrode cell system. The prepared catalyst was dispersed in a mixture solution of isopropanol, water and Nafion (5 wt%) with a volume ratio of 4:1:0.05 to prepare 2 mg/mL catalyst suspensions for the electrocatalytic measurements. An Au electrode modified by the as-prepared Pt-based catalyst was used as the working electrode, and the detailed preparation of the working electrode is as follows: the Au electrode with 4 mm in diameter was respectively polished by a metallographic sandpaper and


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alumina powder (0.5 µm) for 3 min, and washed in deionized water for 30 seconds; after the polished Au electrode was completely dry, 20 uL of the above catalyst suspension was dropped onto the surface of the Au electrode. A saturated calomel electrode (SCE) and a Pt plate electrode were used as the counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) was conducted on a three-electrode cell system. The electrochemical specific area of the catalysts was calculated from the desorption peaks on the CV curves obtained using 0.5 M H2SO4 as the electrolyte, and the resistance to poisoning of the catalyst was investigated through CV measurements in a 0.5 M H2SO4/ 1 M CH3OH electrolyte. 3. Results and discussion. The as-prepared Pt-based catalyst is a cylindrical monolith (Inset in Figure 1a) with graphene sheets as the supporting material. SEM was used to examine the porous microstructure of the graphene-based matrix (Figure 1a), and it is easily seen that the porous matrix contains abundant macropores. The nitrogen adsorption/desorption isotherm (Figure S2) further indicates that macropores are dominant in its pore structure, since rapid nitrogen adsorption at a high relative pressure was observed. Figure S2 also gives the nitrogen adsorption/desorption isotherm of a commercial Pt/C catalyst, which is similar to that of the catalyst we prepared, both having type Ш isotherms. Brunauer-Emmett-Teller (BET) analysis indicates that our catalyst has a specific surface area of about 200 m2 g-1, which is higher than that of the commercial Pt/C catalyst (150 m2 g-1). The microstructure of our catalyst was analysed by XRD with the diffraction pattern (Figure 1b) showing five diffraction peaks located at 39.8o, 46.3o, 67.5o and 81.3o. These are respectively assigned to (111), (200), (220) and (311) lattice planes, indicating that the Pt NPs are face-centered cubic (fcc). The broad and weak peak centered at around 26o is ascribed to the (002) diffraction peak of the graphene whose sheets form a disordered network, which is in


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agreement with previous reports.10 TEM images (Figures 1c, d) show a uniform distribution of Pt NPs on the porous graphene matrix. A high-resolution TEM image (Inset in Figure 1d) shows the size and lattice spacing of the deposited Pt NPs. Statistical analysis indicates that the majority of the nanoparticles are in the range of 3~5 nm (Figure S3), and the lattice spacing shown in the inset of Figure 1d is 0.224 nm. This is assigned to the (111) plane of fcc Pt, and therefore the TEM characterization is in good agreement with above XRD pattern. The methanol oxidation reaction was used as the model reaction to investigate the catalytic performance of the as-prepared platinum-based porous catalyst. Electrochemical measurements demonstrated that the catalyst had a much stronger resistance to poisoning and a higher catalytic stability than the commercial Pt/C catalyst. Moreover, the influence of annealing temperature on the catalytic activity of our catalyst showed that the one annealed at 350 ºC exhibited a better electrocatalytic performance than those annealed at other temperatures. The electrochemical activities of these graphene-based catalysts were characterized by cyclic voltammograms (CVs) and chronoamperometry (Figure 2). The CV curves in Figure 2a give a comparison for the graphene-based porous catalyst and the commercial Pt/C catalyst. It is clear that the graphene-based catalyst has a much higher electric double-layer capacitance (EDLC) than that of the commercial Pt/C catalyst, and the higher EDLC should result from the porous graphene-based matrix and its larger specific surface area. The two peaks located around ~0.3V in the CV curve (red curve in Figure 2a) of the graphene-based catalyst are ascribed to the redox reaction of functional groups on the graphene matrix. It is well-known that a supporting material with high specific surface area is crucial for obtaining catalysts with excellent catalytic activity,19 and hence it is supposed that the graphene-based catalyst will show better electrocatalytic


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performance than the commercial Pt/C catalyst. Typical hydrogen adsorption/desorption peaks in the potential range -0.24~0 V (vs. SCE) are used to evaluate the electrochemical surface area (ECSA) of the catalysts. ECSA (m2/gPt) is calculated by using the equation ECSA=QH/(210×WPt), where QH represents the total charge related to hydrogen desorption, and WPt is the mass of Pt in the electrode. Results indicate that the ECSA of the monolith catalyst is 10.64 m2g-1, which is lower than that (41.97 m2g-1) of the commercial Pt/C catalyst. It is suggested that the unusual current peaks (0~0.6 V (vs SCE)) of the CV curve of the monolith catalyst result from the redox reaction of functional groups on the graphene matrix. Although our catalyst has a lower ECSA than that of the commercial Pt/C catalyst, MOR results showed that it has a much stronger resistance to poisoning and a higher cycle stability than the commercial one (Figures 2b-d and Figure 3). Figure 2b shows the electrocatalytic behaviour of the catalysts towards MOR. The CV profile of the monolith catalyst shows a forward oxidation peak current of 7.81 mA/cm2 at an oxidation potential of around 0.62 V, nearly 4 times larger than that of the commercial Pt/C catalyst. The ratio of the forward oxidation peak current (If) to the reverse peak current (Ib), If/Ib, can be used to describe the resistance to poisoning of the Ptbased catalysts to the adsorbed carbonaceous intermediates. The If/Ib ratios of the graphenebased porous and commercial Pt/C catalysts are 1.17 and 0.86, respectively, indicating that the porous catalyst possesses stronger resistance to CO poisoning and induces a more effective oxidation process.16 The onset potential of the porous catalyst for methanol electro-oxidation at 0.20 V is revealed in the chronoamperometric profiles (Figure 2c), which is much negative than the commercial Pt/C catalyst. The porous catalyst shows a higher current density than the commercial Pt/C catalyst, and this is verified from the steady-state polarization curve under a


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given potential of 0.6 V (Figure 2d). In addition, the porous catalyst has a much higher stability for the methanol oxidation reaction than the commercial Pt/C catalyst. After 120 cycles, the peak current of the commercial Pt/C catalyst shows significant decay (Figure 3a), with a loss of about 50% of its initial activity, while that of the graphene-based porous catalyst is much more stable (Figure 3b). The higher stability of the graphene-based catalyst is assigned to its stronger antipoison ability, which is suggested to come from the uniform and small size platinum nanoparticles, and the stable porous graphene matrix. The detailed investigations are underway to clarify this. The increased electrocatalytic activity of the porous catalyst is ascribed to five factors: 1) faster electron transfer occurs between Pt nanoparticles and the graphene matrix; 2) the oxygencontaining groups on the graphene sheets are believed to be responsible for the increased resistance to poisoning, since it has been reported that oxygen-containing groups play a major role in the removal of intermediate carbonaceous species like CO;20 3) a porous graphene matrix provides more paths for fast ionic transport; 4) a uniform distribution of the Pt NPs on the graphene matrix; 5) the graphene matrix has a macropore-dominant microstructure (Figure S3), which is believed to be favourable for efficient fuel and product diffusion. It has been extensively reported that the catalytic activity of a platinum-based catalyst is related to the size and morphology of the Pt NPs,10, 21 and hence further annealing treatment at different temperatures was conducted. The micro-morphology of the annealed catalysts was characterized by TEM (Figure 4), which indicated that the Pt NPs in the 350 ºC annealed material showed the most uniform distribution (Figure 4b), whereas the Pt NPs started to aggregate as the annealing temperature increased, leading to the formation of larger Pt clusters. Electrochemical measurements were also conducted on the annealed catalysts (Figure 5), and the results turned


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out to be in agreement with the TEM images. The CV curves in Figures 5a and b clearly showed that Pt/G-350 possesses the highest ECSA (11.86 m2/g) and If/Ib ratio (1.21), and the electrochemical data for all annealed catalysts are collected in Table 1. The steady-state chronoamperometric curves recorded at both 0.2 and 0.6V show that the Pt/G-350 possesses the highest current density (Figures 5c and d). These results indicate that Pt/G-350 has the best electrocatalytic activity. When the annealing temperature is lower than 350 ºC, crystallization of the Pt NPs has not been fully realized, and when the annealing temperature is higher than 350 ºC, the Pt NPs start to aggregate and form larger clusters. Both cases do not give the best catalytic performance. That is to say, an appropriate annealing temperature is required in order to obtain a platinum-based catalyst with a high electrochemical activity that is free from NP aggregation. 4. Conclusions A platinum-based catalyst with a graphene-based porous matrix as the support material was fabricated by a hydrothermal method. The catalyst exhibits stronger electrocatalytic activity compared with a commercial Pt/C catalyst, and it is suggested that this is due to the porous graphene matrix and the uniform distribution of Pt NPs. The abundant macropores in the graphene matrix are excellent diffusion path for ions and molecules during the electrocatalytic reaction, leading to improved resistance to poisoning. The electrocatalytic performance of the graphene-based porous catalyst is highly sensitive to annealing temperature, and an appropriate annealing temperature produces highly crystalline Pt NPs and a uniform particle distribution on the graphene support. Corresponding Author *Quan-Hong Yang and Weiguo Zhang School of Chemical Engineering and Technology, Tianjin University, Tianjin, China.


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Fax: +86-22-27401097. E-mail address: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval of the final version of the manuscript. The authors declare no competing financial interest. ‡These authors contributed equally to this work. Funding Sources This work is financially supported by National Basic Research Program of China (2014CB932403), National Natural Science Foundation of China (No. 51372167); NSF of Tianjin,

China

(No.

12JCZDJC27400);

Shenzhen

Basic

Research

Project

(No.

JC201104210152A) and Guangdong Province Innovation R&D Team Plan for Energy and Environmental Materials (No. 2009010025). Supporting Information Available. Thermogravimetric profiles of the as-prepared and the annealed graphene-based porous catalysts, nitrogen adsorption-desorption isotherms of the porous catalyst and commercial Pt/C catalyst, statistical analysis of the size of Pt particles on the graphene-based porous catalyst, and TEM images of commercial Pt/C catalyst. This information is available free of charge via the Internet at http://pubs.acs.org


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[16] Novoselov, K.S.; Geim, A.K.; Morozov, S.V.; Jiang, D.; Zhang, Y.; Dubonos, S.V.; Grigorieva, I.V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666-669. [17] Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339-1339. [18] Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385-388. [19] Baleizão, C.; Corma, A.; García, H.; Leyva, A. Oxime Carbapalladacycle Covalently Anchored to High Surface Area Inorganic Supports or Polymers as Heterogeneous Green Catalysts for the Suzuki Reaction in Water. J. Org. Chem. 2003, 69, 439-446. [20] Hu, Y.; Zhang, H.; Wu, P.; Zhang, H.; Zhou, B.; Cai, C. Bimetallic Pt-Au Nanocatalysts Electrochemically Deposited on Graphene and Their Electrocatalytic Characteristics towards Oxygen Reduction and Methanol Oxidation. Phys. Chem.Chem. Phys. 2011, 13, 4083-4094. [21] Li, Y.; Gao, W.; Ci, L.; Wang, C.; Ajayan, P. M. Catalytic Performance of Pt Nanoparticles on Reduced Graphene Oxide for Methanol Electro-Oxidation. Carbon 2010, 48, 1124-1130.


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Figure 1. (a) SEM image, (b) XRD pattern and (c-d) TEM images of the as-prepared graphene-based porous catalyst. Inset in (a): photograph of the as-obtained graphene-based catalyst; inset in (d): high resolution TEM image of the platinum particles deposited on the graphene sheets, showing the lattice spacing.


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Figure 2. Cyclic voltammograms of the as-prepared graphenebased porous catalyst and a commercial Pt/C catalyst (a) in a N2-sparged 0.5 M H2SO4 solution at a scan rate of 50 mV/s and (b) in a mixture solution of 0.5 M H2SO4 and 1 M CH3OH at a scan rate of 20 mV/s; Chronoamperometric curves of the monolith graphene-based catalyst and the commercial Pt/C catalyst at (c) 0.2 V and (d) 0.6 V in 0.5 M H2SO4/ 1 M CH3OH.

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Figure 3. Cyclic voltammograms of (a) the commercial Pt/C catalyst and (b) the as-prepared graphene-based porous catalyst after 20 cycles (black curves) and after 120 cycles (red curves).


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Figure 4. TEM images of the graphene-based porous catalysts annealed at different temperatures of (a) 300 ºC, (b) 350 ºC, (c) 400 ºC, (d) 450 ºC and (d) 500 ºC.

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600

E vs SCE(V)

Figure 5. (a) CVs of the graphene-based porous catalysts annealed at various temperatures in a N2-sparged 0.5 M H2SO4 solution and (b) in a mixture solution of 0.5 M H2SO4 and 1 M CH3OH; Chronoamperometric curves at (c) 0.2 V and (d) 0.6 V of the Pt/G catalysts annealed at different temperatures in 0.5 M H2SO4 /1 M CH3OH solution.


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Table 1 ECSA and electrocatalytic activity of the graphene-based porous catalysts (Pt/C) annealed at different temperatures Samples

Pt/G-300

Pt/G-350

Pt/G-400

Pt/G-450

Pt/G-500

ECSA(m2 g-1)

8.24

11.86

3.79

3.07

4.05

1.08

1.21

1.04

0.91

0.89

139.05

180.74

109.56

52.40

49.19

If/Ib Mass activity (A g-1 Pt)


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TOC: Porous graphene matrix is an ideal supporting material for platinum nanoparticles to prepare electrochemical catalyst with high stability and advanced electrocatalytic activity.


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Graphene-Based Porous Catalyst with High Stability and Activity for

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