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Graphene supported platinum nanoparticles as anode electrocatalyst for direct borohydride fuel cell
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 3 7 ( 2 0 1 2 ) 1 7 9 8 4 e1 7 9 9 1
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Graphene supported platinum nanoparticles as anode electrocatalyst for direct borohydride fuel cell Xue Liu, Lanhua Yi, Xianyou Wang*, Jincang Su, Yunfeng Song, Jing Liu Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, School of Chemistry, Xiangtan University, Xiangtan, Hunan 411105, China
article info
abstract
Article history:
The graphene supported Pt nanoparticles are prepared by ethylene glycol reduction
Received 25 July 2012
method. The obtained Pt/graphene (Pt/G) nanocomposites are characterized by trans-
Received in revised form
mission electron microscopy (TEM), X-ray diffraction (XRD), and thermogravimetric anal-
15 September 2012
ysis (TGA). TEM images show that the spherical Pt nanoparticles with sizes of 3.1 nm
Accepted 23 September 2012
disperse uniformly on the surface of the graphene, which is consistent with the XRD date
Available online 13 October 2012
of 2.97 nm. The Pt/G nanocomposites show electrochemically active surface area (ECSA) of
Keywords:
chronoamperometry) that the Pt/G nanocomposites exhibit good electrocatalytic activity
Pt nanoparticles
and stability toward borohydride oxidation. Besides, the Pt/G nanocomposites are used as
Graphene
anode electrocatalyst in a direct borohydride fuel cell at 298 K, and the maximum power
Direct borohydride fuel cell
density is 42 mW/cm2, which is apparently higher than Vulcan XC-72R supported Pt (Pt/C)
Anode electrocatalyst
nanoparticles (34 mW/cm2).
62.7 m2/g. It has been found by electrochemical measurements (i.e., cyclic voltammetry,
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Fuel cells used in portable and mobile applications are considered as clean power generation devices and are expected to operate at ambient conditions with minimum auxiliaries and high power density [1,2]. A direct borohydride fuel cell (DBFC) is a device that converts chemical energy stored in a borohydride ion (BH 4 ) and an oxidant directly into electricity by redox processes. Usually, a DBFC employs an alkaline solution of sodium borohydride (NaBH4) as fuel and oxygen or hydrogen peroxide as oxidant [3]. NaBH4 has attracted extensive attention because of its high energy density (9296 Wh/kg), high capacity (5669 Ah/kg), hydrogen content of about 11 wt.%, very negative equilibrium potential [1.24 V vs. standard hydrogen electrode (SHE)], and faster anode kinetics at many metallic surfaces [4e14], for example,
the reaction is fast in platinum but slow in gold. In addition, sodium borohydride neither ignites in contact with moisture nor is it sensitive to shock [15]. Therefore, the DBFC has many advantages in that it eliminates hydrogen storage problems, safely uses liquid fuel, has a low fuel crossover to the cathode side and has a high theoretical cell voltage with a high theoretical power density [16]. Currently, DBFCs are considered attractive energy suppliers, especially for portable applications [17]. The concept of DBFC was first proposed in the early 1960s by Indig and Snyder [18], and the electrochemical reactions in a DBFC employing H2O2 as oxidant (DBHFC) are as follows: The anode reaction of the DBHFC is the direct oxidation of borohydride in alkaline medium: 0 BH 4 þ 8OH /BO2 þ 6H2 O þ 8e ; Eanode ¼ 1:24 V vs: SHE
* Corresponding author. Tel.: þ86 731 58292060; fax: þ86 731 58292061. E-mail address: [email protected] (X. Wang). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.09.136
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The cathode reaction with H2O2 as the oxidant is written as: H2 O2 þ 2e /2OH ;
E0cathode ¼ 0:87 V vs: SHE
(2)
The overall cell reaction is expressed as Eq. (3) BH 4 þ 4H2 O2 /BO2 þ 6H2 O;
E0cell ¼ 2:11 V vs: SHE
(3)
Although the DBFC yields a high performance as a power source, there are several problems that need to be solved, e.g., hydrogen evolution and NaBO2 accumulation at the anode during operation [19]. Theoretically, eight electrons can be utilized on gold electrode. However, while the total number of electrons involved in the oxidation of BH 4 was only four, the heterogeneous rate constant at 0 V vs. Ag/AgCl, KClstd was about ten times larger on Pt compared to Au. Thus, Pt could be an effective catalyst for BH 4 oxidation [10]. Another challenge for the DBFC or DBHFC is the carbon support corrosion, which is recognized as one of the most important reasons for the decline of fuel cell performance [20,21]. Many investigations have been conducted to search for advanced materials to support the metallic catalysts, such as metal oxide, conductive polymer, and various forms of carbonaceous materials. Among them, carbonaceous materials, including activated carbon, carbon nanotubes, carbon nanofibers, porous carbons, and nitrogen-doped carbon, are the typical catalyst support materials for fuel cells, a recent addition of which is the single atomic layer graphite, graphene [22]. Graphene nanosheets exhibit a structure of 2D sheets composed of sp2-bonded carbon atoms with one or more atomic thickness and have high surface area (2600 m2/g), superior electrical conductivity, excellent mechanical flexibility, and high thermal and chemical stability [23]. The unique and outstanding properties of graphene make it an ideal candidate in a wide range of applications, including field effect transistors, composite materials, field emitters, gas sensors, hydrogen storage media, and transparent conducting electrodes [24]. The effective approach for the bulk production of graphene sheets is the chemical reduction of exfoliated graphite oxide (GO), due to its simplicity, reliability, ability for large-scale production, relatively low material cost, and versatile in terms of being well-suited to chemical functionalization [25,26]. Oxygen-containing groups are introduced onto the GO planes during this process. The functional groups, which change the GO surface properties from hydrophobic to hydrophilic, allow for easy dispersion of GO in water upon sonication. Thus, metal precursor and GO solution can form a homogeneous suspension, which is benefit for the uniform distribution of metal nanoparticles on the GO surface through one-pot reduction. Liang et al. [27] founded that the Co3O4/Ndoped graphene hybrid exhibited similar catalytic activity but superior stability to Pt in alkaline solutions. In addition, Ishikawa et al. [28] demonstrated that PtAu/graphene exhibited good electrocatalytic activity and stability for formic acid oxidation compared to commercial Pt/C. In this regard, it is expected that graphene nanosheets may offer a new carbonemetal nanocomposite material for the next generation catalyst in fuel cells [21]. However, GO is a kind of low conductive carbonaceous material, which can be used as a good precursor to disperse Pt
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nanoparticles, but it is not a good electrocatalyst support. In this paper we reported ethylene glycol reduction method to reduce GO and chloroplatinic acid (H2PtCl6) simultaneously into G and Pt nanoparticles, and then obtained Pt/G nanocomposites. The electrochemical mechanism of borohydride oxidation on the Pt/G and single DBHFC cell tests were investigated in detail.
2.
Experimental details
2.1.
Synthesis of Pt/G nanocomposites
GO was firstly made by a modified Hummers [29] method with a pre-oxidation treatment. The procedures were reported in our previous work [30]. Briefly, GO was first treated in a solution of concentrated H2SO4, K2S2O8, and P2O5 at 80 C for 4.5 h. The mixture was diluted with water and left overnight. Then, the mixture was filtered, and the solid was dried under ambient condition. Afterward, the solid was stirred with concentrated H2SO4 and KMnO4 was added in proportion keeping the temperature below 20 C. The solution was stirred for another 2 h in a water bath at 35 C. Next, water was added gradually so that the temperature would not climb above 50 C, followed by the addition of 30% H2O2. Finally, the mixture was centrifuged and dried in vacuum. The Pt/G nanocomposites were synthesized by a simultaneous chemical reduction of H2PtCl6 and GO in ethylene glycol solution. Ethylene glycol can not only work as a reducing agent to convert metal ions into metal or alloy nanoparticles but also serves as chelating agent to stabilize metal colloidal [31]. In a typical route, GO was dispersed in H2O by sonication for 1 h to form a homogeneous suspension. The concentration of the GO suspension was w1 mg/mL. Then 78 mL ethylene glycol and 1 mL of 0.1 M H2PtCl6 solution were added to 39 mL GO suspension with magnetic stirring for 30 min. After that, the reaction was kept at 125 C with stirring for 4 h. High-purity argon was bubbled through the solution during the procedure. Finally, the resulting suspension was filtered and washed with ethanol and water, then dried at 60 C in vacuum for 6 h. Besides, Pt/C nanoparticles were prepared by the same method for comparison.
2.2.
Characterization of the Pt/G nanocomposites
TEM images were carried out using a JEOL JSM-2100F microscope operating at 200 kV. The TEM samples were prepared by placing several drops of dilute particle dispersion on carboncoated copper TEM grids. Powder XRD analyses were performed on a D/Max-3C diffractometer with Cu Ka radiation (l ¼ 1.54056 A). TGA was conducted on a TGA 50 Instruments and the sample was heated from room temperature to 900 C under air atmosphere at a heating rate of 10 C min1.
2.3.
Electrochemical measurements
Electrochemical measurements were performed in a threeelectrode system, using a Ag/AgCl electrode and a Ni foam as the reference and the counter electrode, respectively. The working electrode was prepared as follows: 5 mg catalyst was
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added to a mixture of 0.5 mL of Nafion solution (5 wt.% from Aldrich) in ethanol. Then, the solution was sonicated to form a homogeneous ink. 5 mL of the ink was dropped onto a glassy carbon electrode (GCE). The calculation of ECSA of Pt nanoparticles was carried out using the hydrogen electro-sorption curve, recorded between 0.2 V and þ1.2 V in 0.5 M H2SO4 solution. The H2SO4 solution was purged with argon for 30 min prior to the experiment. To measure borohydride electrooxidation reaction activity, cyclic voltammetry (CV) curves were plotted at a potential range from 1.2 V to þ0.6 V in a solution of containing 0.1 M NaBH4 and 3 M NaOH at a scan rate of 20 mV/s. Chronoamperometry was conducted for 1000 s. The loading mass of the catalyst was 0.7 mg/cm2.
2.4.
Single-cell tests
The slurry of anodic or cathodic catalyst was prepared by dispersing the required amount of the catalyst into a mixture of isopropyl and 5 wt.% of Nafion solution. The mixture was sonicated for 1 h to make a catalyst ink. And the ink was coated onto a 1 cm 1 cm stainless steel gauze. Then they were dried in an oven at 60 C for 8 h and weighed after drying. The catalyst loading was 4e4.5 mg/cm2. The Pt/G catalyst and Au/C catalyst (20 wt.% Au) were used as the anodic and cathodic catalysts, respectively. Membrane electrode assembly (MEA) for single-cell test was made by hot-pressing pretreated Nafion-117 membrane sandwiched by anode and cathode. A schematic diagram of the experimental setup is shown in Fig. 1. The anolyte was 1 M NaBH4 þ 3 M NaOH and the catholyte was 2 M H2O2 þ 0.5 M H2SO4. The fresh anolyte and catholyte were continuously supplied and withdrawn
from the cell at a rate of 0.7 mL/min, respectively. The load was applied in steps of 5 mA within the range of 0e120 mA. Each step lasted for 2 min and the current was continuously applied from one value to next without disconnecting the cell. The cell testing was performed using a battery testing system (Xinwei, Shenzhen, China). Power densities were calculated from the applied current and steady state potential.
3.
To control the size and dispersion of Pt nanoparticles on graphene is very important for their application in fuel cells [32]. Fig. 2a and b is the different magnifications of the TEM images of the Pt/G nanocomposites. The light shaded substrate with slight wrinkles is corresponding to the planar graphene sheet. The Pt nanoparticles appear as dark dots on the graphene nanosheets. It can be seen that Pt nanoparticles disperse uniformly on the surface of the graphene. It can be ascribed to the surface functional groups of GO. Usually, the surface functional groups such as carboxyl, hydroxyl and carbonyl serve as anchoring sites for the metal nanoparticles. Moreover, the oxygen functionalities, especially the carboxylic acids, provide active sites for the nucleation and growth of metal nanoparticles [33,34]. Accompanying reduction of Pt nanoparticles, the GO is also reduced into G by ethylene glycol at high temperature. The lattice structure of Pt nanoparticles as revealed in a high resolution transmission electron microscope (HRTEM) is shown in Fig. 2c. The mean particle size and size distribution were determined by measuring the diameter of isolated particles. Generally speaking, 500 particles were considered for the catalyst in order to have an acceptable statistical sample. It has been found that the spherical Pt nanoparticles have a narrow size dispersion ranging from 1 nm to 7 nm, with a mean diameter of 3.1 nm, as shown in Fig. 2d. Fig. 3 shows the XRD pattern of Pt/G nanocomposites. The broad peak at 2q ¼ 26 is due to the (002) plane of the hexagonal structure of the graphene support. The four diffraction peaks at 39.6 , 46.9 , 67.3 , and 80.8 are related to (111), (200), (220), and (311) planes of face-centered-cubic (fcc) Pt (JCPDS 04-0802), confirming that Pt precursor has been successfully reduced to Pt nanoparticles by chemical reduction. The peak corresponding to the (111) plane is more intense than the others, indicating that the (111) plane is the dominating orientation. The average crystallite size for the Pt nanoparticles is calculated from broadening of the (111) diffraction peak using Scherrer equation [35] d¼
Fig. 1 e A schematic diagram of direct borohydrideehydrogen peroxide fuel cell. (1) Anode catalyst: Pt/G or Pt/C catalyst, (2) cathode catalyst: Au/C catalyst, (3) Nafion-117 membrane, (4) anolyte: 1 M NaBH4 D 3 M NaOH, (5) catholyte: 2 M H2O2 D 0.5 M H2SO4.
Results and discussion
0:9l b1=2 cos q
(4)
where d is the average particle size (nm), l is the wavelength of the X-ray used (1.54056 A), q is the angle at the maximum of the peak (rad), and b1/2 is the width of the peak at half height in radians. The calculated average size of Pt nanoparticles on graphene is 2.97 nm, which is in good agreement with that obtained from TEM. The TGA technique was further used to estimate the accurate amounts of Pt metal in the graphene hybrids [5].
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Fig. 2 e (a, b) TEM and (c) HRTEM images of Pt/G nanocomposites. (d) Histograms of Pt nanoparticles size distribution of Pt/G nanocomposites.
Fig. 4 represents the weight loss of Pt/G nanocomposites with respect to temperature. The weight loss occurred below 100 C is due to evaporation of water. Furthermore, the weight loss around 200 C is caused by decomposition of oxygen groups, and that above 300 C is attributed to the oxidation of graphene, with formation of CO2 [36]. Finally, the graphene was fully decomposed and transformed into CO2. Considering the weight loss of the samples, the Pt content in Pt/G nanocomposites is 40 wt.%. The results obtained from TEM, XRD and TGA are summarized in Table 1. Larger specific surface area (BET) is not quite relative to the electrochemical performance of the catalyst, but higher ECSA
is supposed to provide the catalyst with higher catalytic activity. ECSA not only provides important information regarding the number of available active sites, but also is a crucial parameter to compare different electrocatalytic supports [37,38]. Fig. 5 shows CV curves of Pt/C electrode and Pt/G electrode in Ar-saturated 0.5 M H2SO4 with a sweep rate of 50 mV/s. The peaks in the potential range of 0.2 V to 0.15 V are attributed to the hydrogen adsorption and desorption processes, while those above 0.2 V are assigned to the oxidation of surface metal and the reduction of thus formed oxides. The hydrogen desorption area (QH) from 0.2 V
Fig. 3 e XRD pattern of Pt/G nanocomposites.
Fig. 4 e TGA data of Pt/G nanocomposites.
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Table 1 e Physical properties of the Pt/G nanocomposites. Catalyst
Pt/G
TEM particle size (nm)
2q (111) peak
XRD crystallite size (nm)
Loading from TGA (%)
3.1
39.6
2.97
40
to 0.2 V can be used to calculate the ECSA by employing the equation [39] ECSA cm2 =g of Pt ¼
QH ½mC=cm2 210 ½mC=cm2 electrode loading ½g of Pt=cm2
(5)
The ECSA of Pt/G nanocomposites is estimated to be 62.7 m2/g, while the ESCA of Pt/C nanoparticles is 39.5 m2/g. Obviously, the Pt/G nanocomposites have significantly larger ECSA than Pt/C nanoparticles. In order to gain further insight into the catalysts, the CV tests are prolonged to 100 cycles. The ECSA as a function of the number of potential cycles for each catalysts is shown in Fig. 6. It can be seen that the Pt/G and Pt/ C catalysts show a slight decrease of ECSA with increasing cycle number. After 100 cycles, the Pt/C experienced severe degradation of 23% as compared to that of Pt/G catalyst (16.5%), due to Pt agglomeration and dissolution, indicating the good stability of Pt/G catalyst. The electrocatalytic activity toward borohydride oxidation on Pt/G catalyst was evaluated in the solution of 0.1 M NaBH4 þ 3 M NaOH. As shown in Fig. 7, the voltammograms of borohydride oxidation on Pt/G catalyst are similar to that of the Pt/C catalyst. Peaks 1 and 10 between 1.0 V and 0.15 V are due to the oxidation of H2 generated in the catalytic hydrolysis of NaBH4 (equation (6)), while the peaks 2 and 20 wave commencing at about 0.15 V are given by the direct oxidation of borohydride [40,7]. NaBH4 þ yH2O / NaBH4y(OH)y þ yH2, with y ¼ 1, ., 4
(6)
Fig. 6 e Changes of ECSA of (a) Pt/C and (b) Pt/G catalysts with cycle numbers.
current density of peaks 1 and 10 . That is to say, the amount of H2 generated by Pt/G nanocomposites or Pt/C nanoparticles is identical. But peaks 2 and 20 are not the same. Compared with Pt/C nanoparticles, peak 2 of Pt/G nanocomposites shifts to a more negative potential, and the current density is significantly higher than Pt/C. The peak 2 current density of Pt/G catalyst is 1.3 times the Pt/C catalyst, indicating better performance of Pt/G catalyst toward borohydride oxidation. The details are listed in Table 2. Kundu et al. [41] propose a synergistic co-reduction mechanism whereby the Pt ions contribute to the reduction of GO and the defect sites on G contribute to the heterogeneous nucleation of Pt metal leads to the formation of a GePt composite with uniformly dispersed ultrafine nanoparticles on the surface. The mechanisms are described as below. Pt4þ þ GO þ 2e / Pt2eGO
(7)
It is important to note that, there is no difference between Pt/C nanoparticles and Pt/G nanocomposites toward the
Pt2þeGO / Pt4þeG
(8)
Fig. 5 e CV curves of (a) Pt/C and (b) Pt/G nanocomposites in Ar-saturated 0.5 M H2SO4.
Fig. 7 e CV curves of (a) Pt/C and (b) Pt/G catalysts in 0.1 M NaBH4 D 3 M NaOH. Scan rate: 20 mV/s.
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Table 2 e Compiled study comparing CV results for Pt/C and Pt/G catalysts. Electrode
Pt/C Pt/G
ECSA (m2/g)
39.5 62.7
Peak 2 Onset potential (V)
Peak potential (V)
Peak current (A/cm2)
0.092 0.133
þ0.058 0.007
0.026 0.035
Pt4þeG þ 2e / Pt0eG
(9)
Moreover, peak 30 is due to BH3(OH) electrooxidation on the partially oxidized Pt surface [7]. As a result, it can be concluded that Pt/G nanocomposites have higher electrocatalytic activity than Pt/C nanoparticles, suggesting graphene is a promising support for fuel cell applications. The high electrocatalytic activity of Pt/G nanocomposites can be ascribed to the large ECSA of Pt/G nanocomposites and the modified electrochemical reaction mechanism. Chronoamperometry, a useful method for evaluation of electrocatalyst long-term stability in fuel cells, is employed to investigate the electrochemical activity and stability of the catalysts [42,43]. Fig. 8 depicts chronoamperometric curves of Pt/C nanoparticles and Pt/G nanocomposites for borohydride oxidation measured in 0.1 M NaBH4 þ 3 M NaOH. In the chronoamperometric curves, there is a sharp initial current drop followed by a slower decay. Obviously, the initial current of Pt/G electrode (35.3 mA) is much higher than Pt/C electrode (31.7 mA). Besides, the residual current for Pt/G electrode after 1000 s is 2.4 mA, which is equivalent to that of Pt/C electrode. The fast current decay can be ascribed to the following two reasons: one is that the conglomeration of Pt nanoparticles cannot directly anchor to the graphene support, and the other is that the larger number of defect sites in the graphene are much easier to be corroded under harsh electrochemical conditions [44,45].
Fig. 9 e Cell polarization curves and power density curves of the DBFC using Pt/C and Pt/G anode catalysts with anolyte consisted of 1 M NaBH4 D 3 M NaOH and 2 M H2O2 D 0.5 M H2SO4 catholyte. Catalyst loading: 4 mg/cm2.
The cell polarization and power density curves for DBHFC operating with 1 M NaBH4 þ 3 M NaOH as fuel and 2 M H2O2 þ 0.5 M H2SO4 as oxidant, while employing the Au/C as the cathode catalyst and Pt/C or Pt/G as the anode catalyst, are presented in Fig. 9a and b, respectively. The DBHFC was tested at 298 K. The open circuit voltage (OCV) of the cell is about 1.7 V, which is lower than the standard cell potential for the DBFC. The low value is probably caused by mixed potential at the anode and cathode from simultaneous oxidation of ions
Table 3 e Single cell DBFC performance with the synthesized Pt/C and Pt/G catalysts. Catalyst
Fig. 8 e Chronoamperometry curves of electrooxidation NaBH4 on (a) Pt/C and (b) Pt/G catalysts in 0.1 M NaBH4 D 3 M NaOH solution.
Pt/C Pt/G
Open circuit voltage, OCV (V)
Current density (mA/cm2)
Peak power density (mW/cm2)
1.743 1.757
50 60
34.13 41.82
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and hydrogen at the anode and reduction of H2O2 and O2 at the cathode [46]. As shown in Fig. 9a and b, the two curves almost coincide when the current density is in the range of 0e20 mA/ cm2. Further enhancements in DBFC performance are observed by using a Pt/G anode in comparison to a Pt/C anode in the high current density region. The limited current density and maximum power density of the single cell with Pt/G catalyst are 112 mA/cm2 and 42 mW/cm2, respectively; while for the single cell with Pt/C catalyst, the limited current density and maximum power density are 75 mA/cm2 and 34 mW/cm2, respectively. The OCV, limited current density, and maximum power density are summarized in Table 3. Apparently, both limited current density and maximum power density for Pt/G are all superior to Pt/C. The good performance of Pt/G over Pt/C may be due to the high conductivity and high durability to oxidation and corrosion of graphene support in the electrolyte [47].
4.
Conclusion
The Pt/G nanocomposites are successfully prepared by ethylene glycol simultaneous reduction method. The Pt nanoparticles are homogeneously dispersed on the graphene support. The Pt/G electrodes show higher ECSA and superior electrocatalytic activity toward borohydride oxidation, in comparison to Pt/C nanoparticles. In the single cell test, the limited current density and maximum power density on Pt/G nanocomposites are 112 mA/cm2 and 42 mW/cm2, which is higher than 75 mA/cm2 and 34 mW/cm2 on Pt/C nanoparticles. The good activity and stability of the Pt/G catalyst indicate that it is a promising electrocatalyst for the application of DBHFC.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51272221, 51072173 and 21203161), Doctoral Fund of Ministry of Education of China (Grant No. 20094301110005), and the Scientific Research Fund of Hunan Provincial Education Department (Grant No. 11A118).
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18] [19]
[20]
references [21] [1] Liu BH, Li ZP, Arai K, Suda S. Performance improvement of a micro borohydride fuel cell operating at ambient conditions. Electrochim Acta 2005;50:3719e25. [2] Celik C, Boyaci San FG, Sarac HI. Effects of operation conditions on direct borohydride fuel cell performance. J Power Sources 2008;185:197e201. [3] Ma J, Sahai Y, Buchheit RG. Direct borohydride fuel cell using Ni-based composite anodes. J Power Sources 2010;195(15): 4709e13. [4] Feng R, Dong H, Cao Y, Ai X, Yang H. AgNi-catalyzed anode for direct borohydride fuel cells. Int J Hydrogen Energy 2007; 32:4544e9. [5] Dong H, Feng R, Ai X, Cao Y, Yang H, Cha C. Electrooxidation mechanisms and discharge characteristics of borohydride on
[22]
[23]
[24]
[25]
different catalytic metal surfaces. J Phys Chem B 2005; 109(21):10896e901. Sanli AE, Aksu ML, Uysal BZ. Advanced mathematical model for the passive direct borohydride/peroxide fuel cell. Int J Hydrogen Energy 2011;36:8542e9. Gyenge E. Electrooxidation of borohydride on platinum and gold electrodes: implications for direct borohydride fuel cells. Electrochim Acta 2004;49(6):965e78. Kojima Y, Suzuki K, Fukumoto K, Sasaki M, Yamamoto T, Kawai Y, et al. Hydrogen generation using sodium borohydride solution and metal catalyst coated on metal oxide. Int J Hydrogen Energy 2002;27(10):1029e34. Bai Y, Wu C, Wu F, Yi B. Carbon-supported platinum catalysts for on-site hydrogen generation from NaBH4 solution. Mater Lett 2006;60(17):2236e9. Gyenge E, Atwan M, Northwood D. Electrocatalysis of borohydride oxidation on colloidal Pt and Pt-alloys (PteIr, PteNi, and PteAu) and application for direct borohydride fuel cell anodes. J Electrochem Soc 2006;153:A150e8. Li Z, Liu B, Arai K, Asaba K, Suda S. Evaluation of alkaline borohydride solutions as the fuel for fuel cell. J Power Sources 2004;126(1):28e33. Yi L, Song Y, Yi W, Wang X, Wang H, He P, et al. Carbon supported Pt hollow nanospheres as anode catalysts for direct borohydrideehydrogen peroxide fuel cells. Int J Hydrogen Energy 2011;36:11512e8. Pei F, Wang Y, Wang X, He P, Chen Q, Wang X, et al. Performance of supported AueCo alloy as the anode catalyst of direct borohydrideehydrogen peroxide fuel cell. Int J Hydrogen Energy 2010;35:8136e42. Cheng H, Scott K. Determination of kinetic parameters for borohydride oxidation on a rotating Au disk electrode. Electrochim Acta 2006;51(17):3429e33. de Leon CP, Walsh FC, Pletcher D, Browning DJ, Lakeman JB. Direct borohydride fuel cells. J Power Sources 2006;155: 172e81. Celik C, San FGB, Sarac HI. Influences of sodium borohydride concentration on direct borohydride fuel cell performance. J Power Sources 2010;195(9):2599e603. Ma J, Choudhury NA, Sahai Y. A comprehensive review of direct borohydride fuel cells. Renew Sustain Energy Rev 2010; 14(1):183e99. Indig ME, Snyder RN. Sodium borohydride, an interesting anodic fuel (1). J Electrochem Soc 1962;109:1104e6. Celik C, Boyaci San FG, Sarac HI. Improving the direct borohydride fuel cell performance with thiourea as the additive in the sodium borohydride solution. Int J Hydrogen Energy 2010;35:8678e82. Huang SY, Ganesan P, Park S, Popov BN. Development of a titanium dioxide-supported platinum catalyst with ultrahigh stability for polymer electrolyte membrane fuel cell applications. J Am Chem Soc 2009;131(39):13898e9. He D, Cheng K, Li H, Peng T, Xu F, Mu S, et al. Highly active platinum nanoparticles on graphene nanosheets with a significant improvement in stability and CO tolerance. Langmuir 2012;28:3979e86. Kou R, Shao Y, Wang D, Engelhard MH, Kwak JH, Wang J, et al. Enhanced activity and stability of Pt catalysts on functionalized graphene sheets for electrocatalytic oxygen reduction. Electrochem Commun 2009;11(5):954e7. Novoselov K, Geim A, Morozov S, Jiang D, Zhang Y, Dubonos S, et al. Electric field effect in atomically thin carbon films. Science 2004;306(5696):666e9. Lu G, Mao S, Park S, Ruoff RS, Chen J. Facile, noncovalent decoration of graphene oxide sheets with nanocrystals. Nano Res 2009;2(3):192e200. Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based
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 3 7 ( 2 0 1 2 ) 1 7 9 8 4 e1 7 9 9 1
[26]
[27]
[28]
[29] [30]
[31]
[32]
[33]
[34]
[35]
[36]
nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45(7):1558e65. Schniepp HC, Li JL, McAllister MJ, Sai H, Herrera-Alonso M, Adamson DH, et al. Functionalized single graphene sheets derived from splitting graphite oxide. J Phys Chem B 2006; 110(17):8535e9. Liang Y, Li Y, Wang H, Zhou J, Wang J, Regier T, et al. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat Mater 2011;10(10):780e6. Venkateswara Rao C, Cabrera CR, Ishikawa Y. Graphenesupported PteAu alloy nanoparticles: a highly efficient anode for direct formic acid fuel cells. J Phys Chem C 2011; 115:21963e70. Hummers Jr WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80(6):1339. Liu X, Wang X, He P, Yi L, Liu Z, Yi X. Influence of borohydride concentration on the synthesized Au/graphene nanocomposites for direct borohydride fuel cell. J Solid State Electrochem 2012:1e9. Shen J, Yan B, Shi M, Ma H, Li N, Ye M. Fast and facile preparation of reduced graphene oxide supported PteCo electrocatalyst for methanol oxidation. Mater Res Bull 2012; 47:1486e93. Yang J, Tian C, Wang L, Fu H. An effective strategy for smallsized and highly-dispersed palladium nanoparticles supported on graphene with excellent performance for formic acid oxidation. J Mater Chem 2011;21(10):3384e90. Vinodgopal K, Neppolian B, Lightcap IV, Grieser F, Ashokkumar M, Kamat PV. Sonolytic design of grapheneeAu nanocomposites. Simultaneous and sequential reduction of graphene oxide and Au (III). J Phys Chem Lett 2010;1(13): 1987e93. Goncalves G, Marques PAAP, Granadeiro CM, Nogueira HIS, Singh M, Gracio J. Surface modification of graphene nanosheets with gold nanoparticles: the role of oxygen moieties at graphene surface on gold nucleation and growth. Chem Mater 2009;21(20):4796e802. Santos L, Oliveira C, Moraes I, Ticianelli E. Oxygen reduction reaction in acid medium on PteNi/C prepared by a microemulsion method. J Electroanal Chem 2006;596(2):141e8. Tang H, Ehlert GJ, Lin Y, Sodano H. Highly efficient synthesis of graphene nanocomposites. Nano Lett 2012;12:84e90.
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[37] Seger B, Kamat PV. Electrocatalytically active grapheneeplatinum nanocomposites. Role of 2-D carbon support in PEM fuel cells. J Phys Chem C 2009;113(19):7990e5. [38] Li Y, Gao W, Ci L, Wang C, Ajayan PM. Catalytic performance of Pt nanoparticles on reduced graphene oxide for methanol electro-oxidation. Carbon 2010;48(4):1124e30. [39] Søgaard M, Odgaard M, Skou EM. An improved method for the determination of the electrochemical active area of porous composite platinum electrodes. Solid State Ionics 2001;145(1):31e5. [40] Atwan MH, Macdonald CLB, Northwood DO, Gyenge EL. Colloidal Au and Au-alloy catalysts for direct borohydride fuel cells: electrocatalysis and fuel cell performance. J Power Sources 2006;158(1):36e44. [41] Kundu P, Nethravathi C, Deshpande PA, Rajamathi M, Madras G, Ravishankar N. Ultrafast microwave-assisted route to surfactant-free ultrafine Pt nanoparticles on graphene: synergistic co-reduction mechanism and high catalytic activity. Chem Mater 2011;23:2772e80. [42] Guo S, Dong S, Wang E. Constructing carbon nanotube/Pt nanoparticle hybrids using an imidazolium-salt-based ionic liquid as a linker. Adv Mater 2010;22(11):1269e72. [43] Guo S, Zhang S, Sun X, Sun S. Synthesis of ultrathin FePtPd nanowires and their catalysis for methanol oxidation reaction. J Am Chem Soc 2011;133:15354e7. [44] Sharma S, Ganguly A, Papakonstantinou P, Miao X, Li M, Hutchison JL, et al. Rapid microwave synthesis of CO tolerant reduced graphene oxide-supported platinum electrocatalysts for oxidation of methanol. J Phys Chem C 2010;114:19459e66. [45] Barinov A, Fabris S, Sun T, Gregoratti L, Dalmiglio M, Kiskinova M, et al. Initial stages of oxidation on graphitic surfaces: photoemission study and density functional theory calculations. J Phys Chem C 2009;113(21):9009e13. [46] Ponce de Leo´n C, Walsh FC, Patrissi CJ, Medeiros MG, Bessette RR, Reeve RW, et al. A direct borohydrideeperoxide fuel cell using a Pd/Ir alloy coated microfibrous carbon cathode. Electrochem Commun 2008;10:1610e3. [47] Shao Y, Zhang S, Wang C, Nie Z, Liu J, Wang Y, et al. Highly durable graphene nanoplatelets supported Pt nanocatalysts for oxygen reduction. J Power Sources 2010; 195(15):4600e5.
Available online at www.sciencedirect.com
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Graphene supported platinum nanoparticles as anode electrocatalyst for direct borohydride fuel cell Xue Liu, Lanhua Yi, Xianyou Wang*, Jincang Su, Yunfeng Song, Jing Liu Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, School of Chemistry, Xiangtan University, Xiangtan, Hunan 411105, China
article info
abstract
Article history:
The graphene supported Pt nanoparticles are prepared by ethylene glycol reduction
Received 25 July 2012
method. The obtained Pt/graphene (Pt/G) nanocomposites are characterized by trans-
Received in revised form
mission electron microscopy (TEM), X-ray diffraction (XRD), and thermogravimetric anal-
15 September 2012
ysis (TGA). TEM images show that the spherical Pt nanoparticles with sizes of 3.1 nm
Accepted 23 September 2012
disperse uniformly on the surface of the graphene, which is consistent with the XRD date
Available online 13 October 2012
of 2.97 nm. The Pt/G nanocomposites show electrochemically active surface area (ECSA) of
Keywords:
chronoamperometry) that the Pt/G nanocomposites exhibit good electrocatalytic activity
Pt nanoparticles
and stability toward borohydride oxidation. Besides, the Pt/G nanocomposites are used as
Graphene
anode electrocatalyst in a direct borohydride fuel cell at 298 K, and the maximum power
Direct borohydride fuel cell
density is 42 mW/cm2, which is apparently higher than Vulcan XC-72R supported Pt (Pt/C)
Anode electrocatalyst
nanoparticles (34 mW/cm2).
62.7 m2/g. It has been found by electrochemical measurements (i.e., cyclic voltammetry,
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Fuel cells used in portable and mobile applications are considered as clean power generation devices and are expected to operate at ambient conditions with minimum auxiliaries and high power density [1,2]. A direct borohydride fuel cell (DBFC) is a device that converts chemical energy stored in a borohydride ion (BH 4 ) and an oxidant directly into electricity by redox processes. Usually, a DBFC employs an alkaline solution of sodium borohydride (NaBH4) as fuel and oxygen or hydrogen peroxide as oxidant [3]. NaBH4 has attracted extensive attention because of its high energy density (9296 Wh/kg), high capacity (5669 Ah/kg), hydrogen content of about 11 wt.%, very negative equilibrium potential [1.24 V vs. standard hydrogen electrode (SHE)], and faster anode kinetics at many metallic surfaces [4e14], for example,
the reaction is fast in platinum but slow in gold. In addition, sodium borohydride neither ignites in contact with moisture nor is it sensitive to shock [15]. Therefore, the DBFC has many advantages in that it eliminates hydrogen storage problems, safely uses liquid fuel, has a low fuel crossover to the cathode side and has a high theoretical cell voltage with a high theoretical power density [16]. Currently, DBFCs are considered attractive energy suppliers, especially for portable applications [17]. The concept of DBFC was first proposed in the early 1960s by Indig and Snyder [18], and the electrochemical reactions in a DBFC employing H2O2 as oxidant (DBHFC) are as follows: The anode reaction of the DBHFC is the direct oxidation of borohydride in alkaline medium: 0 BH 4 þ 8OH /BO2 þ 6H2 O þ 8e ; Eanode ¼ 1:24 V vs: SHE
* Corresponding author. Tel.: þ86 731 58292060; fax: þ86 731 58292061. E-mail address: [email protected] (X. Wang). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.09.136
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The cathode reaction with H2O2 as the oxidant is written as: H2 O2 þ 2e /2OH ;
E0cathode ¼ 0:87 V vs: SHE
(2)
The overall cell reaction is expressed as Eq. (3) BH 4 þ 4H2 O2 /BO2 þ 6H2 O;
E0cell ¼ 2:11 V vs: SHE
(3)
Although the DBFC yields a high performance as a power source, there are several problems that need to be solved, e.g., hydrogen evolution and NaBO2 accumulation at the anode during operation [19]. Theoretically, eight electrons can be utilized on gold electrode. However, while the total number of electrons involved in the oxidation of BH 4 was only four, the heterogeneous rate constant at 0 V vs. Ag/AgCl, KClstd was about ten times larger on Pt compared to Au. Thus, Pt could be an effective catalyst for BH 4 oxidation [10]. Another challenge for the DBFC or DBHFC is the carbon support corrosion, which is recognized as one of the most important reasons for the decline of fuel cell performance [20,21]. Many investigations have been conducted to search for advanced materials to support the metallic catalysts, such as metal oxide, conductive polymer, and various forms of carbonaceous materials. Among them, carbonaceous materials, including activated carbon, carbon nanotubes, carbon nanofibers, porous carbons, and nitrogen-doped carbon, are the typical catalyst support materials for fuel cells, a recent addition of which is the single atomic layer graphite, graphene [22]. Graphene nanosheets exhibit a structure of 2D sheets composed of sp2-bonded carbon atoms with one or more atomic thickness and have high surface area (2600 m2/g), superior electrical conductivity, excellent mechanical flexibility, and high thermal and chemical stability [23]. The unique and outstanding properties of graphene make it an ideal candidate in a wide range of applications, including field effect transistors, composite materials, field emitters, gas sensors, hydrogen storage media, and transparent conducting electrodes [24]. The effective approach for the bulk production of graphene sheets is the chemical reduction of exfoliated graphite oxide (GO), due to its simplicity, reliability, ability for large-scale production, relatively low material cost, and versatile in terms of being well-suited to chemical functionalization [25,26]. Oxygen-containing groups are introduced onto the GO planes during this process. The functional groups, which change the GO surface properties from hydrophobic to hydrophilic, allow for easy dispersion of GO in water upon sonication. Thus, metal precursor and GO solution can form a homogeneous suspension, which is benefit for the uniform distribution of metal nanoparticles on the GO surface through one-pot reduction. Liang et al. [27] founded that the Co3O4/Ndoped graphene hybrid exhibited similar catalytic activity but superior stability to Pt in alkaline solutions. In addition, Ishikawa et al. [28] demonstrated that PtAu/graphene exhibited good electrocatalytic activity and stability for formic acid oxidation compared to commercial Pt/C. In this regard, it is expected that graphene nanosheets may offer a new carbonemetal nanocomposite material for the next generation catalyst in fuel cells [21]. However, GO is a kind of low conductive carbonaceous material, which can be used as a good precursor to disperse Pt
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nanoparticles, but it is not a good electrocatalyst support. In this paper we reported ethylene glycol reduction method to reduce GO and chloroplatinic acid (H2PtCl6) simultaneously into G and Pt nanoparticles, and then obtained Pt/G nanocomposites. The electrochemical mechanism of borohydride oxidation on the Pt/G and single DBHFC cell tests were investigated in detail.
2.
Experimental details
2.1.
Synthesis of Pt/G nanocomposites
GO was firstly made by a modified Hummers [29] method with a pre-oxidation treatment. The procedures were reported in our previous work [30]. Briefly, GO was first treated in a solution of concentrated H2SO4, K2S2O8, and P2O5 at 80 C for 4.5 h. The mixture was diluted with water and left overnight. Then, the mixture was filtered, and the solid was dried under ambient condition. Afterward, the solid was stirred with concentrated H2SO4 and KMnO4 was added in proportion keeping the temperature below 20 C. The solution was stirred for another 2 h in a water bath at 35 C. Next, water was added gradually so that the temperature would not climb above 50 C, followed by the addition of 30% H2O2. Finally, the mixture was centrifuged and dried in vacuum. The Pt/G nanocomposites were synthesized by a simultaneous chemical reduction of H2PtCl6 and GO in ethylene glycol solution. Ethylene glycol can not only work as a reducing agent to convert metal ions into metal or alloy nanoparticles but also serves as chelating agent to stabilize metal colloidal [31]. In a typical route, GO was dispersed in H2O by sonication for 1 h to form a homogeneous suspension. The concentration of the GO suspension was w1 mg/mL. Then 78 mL ethylene glycol and 1 mL of 0.1 M H2PtCl6 solution were added to 39 mL GO suspension with magnetic stirring for 30 min. After that, the reaction was kept at 125 C with stirring for 4 h. High-purity argon was bubbled through the solution during the procedure. Finally, the resulting suspension was filtered and washed with ethanol and water, then dried at 60 C in vacuum for 6 h. Besides, Pt/C nanoparticles were prepared by the same method for comparison.
2.2.
Characterization of the Pt/G nanocomposites
TEM images were carried out using a JEOL JSM-2100F microscope operating at 200 kV. The TEM samples were prepared by placing several drops of dilute particle dispersion on carboncoated copper TEM grids. Powder XRD analyses were performed on a D/Max-3C diffractometer with Cu Ka radiation (l ¼ 1.54056 A). TGA was conducted on a TGA 50 Instruments and the sample was heated from room temperature to 900 C under air atmosphere at a heating rate of 10 C min1.
2.3.
Electrochemical measurements
Electrochemical measurements were performed in a threeelectrode system, using a Ag/AgCl electrode and a Ni foam as the reference and the counter electrode, respectively. The working electrode was prepared as follows: 5 mg catalyst was
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added to a mixture of 0.5 mL of Nafion solution (5 wt.% from Aldrich) in ethanol. Then, the solution was sonicated to form a homogeneous ink. 5 mL of the ink was dropped onto a glassy carbon electrode (GCE). The calculation of ECSA of Pt nanoparticles was carried out using the hydrogen electro-sorption curve, recorded between 0.2 V and þ1.2 V in 0.5 M H2SO4 solution. The H2SO4 solution was purged with argon for 30 min prior to the experiment. To measure borohydride electrooxidation reaction activity, cyclic voltammetry (CV) curves were plotted at a potential range from 1.2 V to þ0.6 V in a solution of containing 0.1 M NaBH4 and 3 M NaOH at a scan rate of 20 mV/s. Chronoamperometry was conducted for 1000 s. The loading mass of the catalyst was 0.7 mg/cm2.
2.4.
Single-cell tests
The slurry of anodic or cathodic catalyst was prepared by dispersing the required amount of the catalyst into a mixture of isopropyl and 5 wt.% of Nafion solution. The mixture was sonicated for 1 h to make a catalyst ink. And the ink was coated onto a 1 cm 1 cm stainless steel gauze. Then they were dried in an oven at 60 C for 8 h and weighed after drying. The catalyst loading was 4e4.5 mg/cm2. The Pt/G catalyst and Au/C catalyst (20 wt.% Au) were used as the anodic and cathodic catalysts, respectively. Membrane electrode assembly (MEA) for single-cell test was made by hot-pressing pretreated Nafion-117 membrane sandwiched by anode and cathode. A schematic diagram of the experimental setup is shown in Fig. 1. The anolyte was 1 M NaBH4 þ 3 M NaOH and the catholyte was 2 M H2O2 þ 0.5 M H2SO4. The fresh anolyte and catholyte were continuously supplied and withdrawn
from the cell at a rate of 0.7 mL/min, respectively. The load was applied in steps of 5 mA within the range of 0e120 mA. Each step lasted for 2 min and the current was continuously applied from one value to next without disconnecting the cell. The cell testing was performed using a battery testing system (Xinwei, Shenzhen, China). Power densities were calculated from the applied current and steady state potential.
3.
To control the size and dispersion of Pt nanoparticles on graphene is very important for their application in fuel cells [32]. Fig. 2a and b is the different magnifications of the TEM images of the Pt/G nanocomposites. The light shaded substrate with slight wrinkles is corresponding to the planar graphene sheet. The Pt nanoparticles appear as dark dots on the graphene nanosheets. It can be seen that Pt nanoparticles disperse uniformly on the surface of the graphene. It can be ascribed to the surface functional groups of GO. Usually, the surface functional groups such as carboxyl, hydroxyl and carbonyl serve as anchoring sites for the metal nanoparticles. Moreover, the oxygen functionalities, especially the carboxylic acids, provide active sites for the nucleation and growth of metal nanoparticles [33,34]. Accompanying reduction of Pt nanoparticles, the GO is also reduced into G by ethylene glycol at high temperature. The lattice structure of Pt nanoparticles as revealed in a high resolution transmission electron microscope (HRTEM) is shown in Fig. 2c. The mean particle size and size distribution were determined by measuring the diameter of isolated particles. Generally speaking, 500 particles were considered for the catalyst in order to have an acceptable statistical sample. It has been found that the spherical Pt nanoparticles have a narrow size dispersion ranging from 1 nm to 7 nm, with a mean diameter of 3.1 nm, as shown in Fig. 2d. Fig. 3 shows the XRD pattern of Pt/G nanocomposites. The broad peak at 2q ¼ 26 is due to the (002) plane of the hexagonal structure of the graphene support. The four diffraction peaks at 39.6 , 46.9 , 67.3 , and 80.8 are related to (111), (200), (220), and (311) planes of face-centered-cubic (fcc) Pt (JCPDS 04-0802), confirming that Pt precursor has been successfully reduced to Pt nanoparticles by chemical reduction. The peak corresponding to the (111) plane is more intense than the others, indicating that the (111) plane is the dominating orientation. The average crystallite size for the Pt nanoparticles is calculated from broadening of the (111) diffraction peak using Scherrer equation [35] d¼
Fig. 1 e A schematic diagram of direct borohydrideehydrogen peroxide fuel cell. (1) Anode catalyst: Pt/G or Pt/C catalyst, (2) cathode catalyst: Au/C catalyst, (3) Nafion-117 membrane, (4) anolyte: 1 M NaBH4 D 3 M NaOH, (5) catholyte: 2 M H2O2 D 0.5 M H2SO4.
Results and discussion
0:9l b1=2 cos q
(4)
where d is the average particle size (nm), l is the wavelength of the X-ray used (1.54056 A), q is the angle at the maximum of the peak (rad), and b1/2 is the width of the peak at half height in radians. The calculated average size of Pt nanoparticles on graphene is 2.97 nm, which is in good agreement with that obtained from TEM. The TGA technique was further used to estimate the accurate amounts of Pt metal in the graphene hybrids [5].
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Fig. 2 e (a, b) TEM and (c) HRTEM images of Pt/G nanocomposites. (d) Histograms of Pt nanoparticles size distribution of Pt/G nanocomposites.
Fig. 4 represents the weight loss of Pt/G nanocomposites with respect to temperature. The weight loss occurred below 100 C is due to evaporation of water. Furthermore, the weight loss around 200 C is caused by decomposition of oxygen groups, and that above 300 C is attributed to the oxidation of graphene, with formation of CO2 [36]. Finally, the graphene was fully decomposed and transformed into CO2. Considering the weight loss of the samples, the Pt content in Pt/G nanocomposites is 40 wt.%. The results obtained from TEM, XRD and TGA are summarized in Table 1. Larger specific surface area (BET) is not quite relative to the electrochemical performance of the catalyst, but higher ECSA
is supposed to provide the catalyst with higher catalytic activity. ECSA not only provides important information regarding the number of available active sites, but also is a crucial parameter to compare different electrocatalytic supports [37,38]. Fig. 5 shows CV curves of Pt/C electrode and Pt/G electrode in Ar-saturated 0.5 M H2SO4 with a sweep rate of 50 mV/s. The peaks in the potential range of 0.2 V to 0.15 V are attributed to the hydrogen adsorption and desorption processes, while those above 0.2 V are assigned to the oxidation of surface metal and the reduction of thus formed oxides. The hydrogen desorption area (QH) from 0.2 V
Fig. 3 e XRD pattern of Pt/G nanocomposites.
Fig. 4 e TGA data of Pt/G nanocomposites.
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Table 1 e Physical properties of the Pt/G nanocomposites. Catalyst
Pt/G
TEM particle size (nm)
2q (111) peak
XRD crystallite size (nm)
Loading from TGA (%)
3.1
39.6
2.97
40
to 0.2 V can be used to calculate the ECSA by employing the equation [39] ECSA cm2 =g of Pt ¼
QH ½mC=cm2 210 ½mC=cm2 electrode loading ½g of Pt=cm2
(5)
The ECSA of Pt/G nanocomposites is estimated to be 62.7 m2/g, while the ESCA of Pt/C nanoparticles is 39.5 m2/g. Obviously, the Pt/G nanocomposites have significantly larger ECSA than Pt/C nanoparticles. In order to gain further insight into the catalysts, the CV tests are prolonged to 100 cycles. The ECSA as a function of the number of potential cycles for each catalysts is shown in Fig. 6. It can be seen that the Pt/G and Pt/ C catalysts show a slight decrease of ECSA with increasing cycle number. After 100 cycles, the Pt/C experienced severe degradation of 23% as compared to that of Pt/G catalyst (16.5%), due to Pt agglomeration and dissolution, indicating the good stability of Pt/G catalyst. The electrocatalytic activity toward borohydride oxidation on Pt/G catalyst was evaluated in the solution of 0.1 M NaBH4 þ 3 M NaOH. As shown in Fig. 7, the voltammograms of borohydride oxidation on Pt/G catalyst are similar to that of the Pt/C catalyst. Peaks 1 and 10 between 1.0 V and 0.15 V are due to the oxidation of H2 generated in the catalytic hydrolysis of NaBH4 (equation (6)), while the peaks 2 and 20 wave commencing at about 0.15 V are given by the direct oxidation of borohydride [40,7]. NaBH4 þ yH2O / NaBH4y(OH)y þ yH2, with y ¼ 1, ., 4
(6)
Fig. 6 e Changes of ECSA of (a) Pt/C and (b) Pt/G catalysts with cycle numbers.
current density of peaks 1 and 10 . That is to say, the amount of H2 generated by Pt/G nanocomposites or Pt/C nanoparticles is identical. But peaks 2 and 20 are not the same. Compared with Pt/C nanoparticles, peak 2 of Pt/G nanocomposites shifts to a more negative potential, and the current density is significantly higher than Pt/C. The peak 2 current density of Pt/G catalyst is 1.3 times the Pt/C catalyst, indicating better performance of Pt/G catalyst toward borohydride oxidation. The details are listed in Table 2. Kundu et al. [41] propose a synergistic co-reduction mechanism whereby the Pt ions contribute to the reduction of GO and the defect sites on G contribute to the heterogeneous nucleation of Pt metal leads to the formation of a GePt composite with uniformly dispersed ultrafine nanoparticles on the surface. The mechanisms are described as below. Pt4þ þ GO þ 2e / Pt2eGO
(7)
It is important to note that, there is no difference between Pt/C nanoparticles and Pt/G nanocomposites toward the
Pt2þeGO / Pt4þeG
(8)
Fig. 5 e CV curves of (a) Pt/C and (b) Pt/G nanocomposites in Ar-saturated 0.5 M H2SO4.
Fig. 7 e CV curves of (a) Pt/C and (b) Pt/G catalysts in 0.1 M NaBH4 D 3 M NaOH. Scan rate: 20 mV/s.
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Table 2 e Compiled study comparing CV results for Pt/C and Pt/G catalysts. Electrode
Pt/C Pt/G
ECSA (m2/g)
39.5 62.7
Peak 2 Onset potential (V)
Peak potential (V)
Peak current (A/cm2)
0.092 0.133
þ0.058 0.007
0.026 0.035
Pt4þeG þ 2e / Pt0eG
(9)
Moreover, peak 30 is due to BH3(OH) electrooxidation on the partially oxidized Pt surface [7]. As a result, it can be concluded that Pt/G nanocomposites have higher electrocatalytic activity than Pt/C nanoparticles, suggesting graphene is a promising support for fuel cell applications. The high electrocatalytic activity of Pt/G nanocomposites can be ascribed to the large ECSA of Pt/G nanocomposites and the modified electrochemical reaction mechanism. Chronoamperometry, a useful method for evaluation of electrocatalyst long-term stability in fuel cells, is employed to investigate the electrochemical activity and stability of the catalysts [42,43]. Fig. 8 depicts chronoamperometric curves of Pt/C nanoparticles and Pt/G nanocomposites for borohydride oxidation measured in 0.1 M NaBH4 þ 3 M NaOH. In the chronoamperometric curves, there is a sharp initial current drop followed by a slower decay. Obviously, the initial current of Pt/G electrode (35.3 mA) is much higher than Pt/C electrode (31.7 mA). Besides, the residual current for Pt/G electrode after 1000 s is 2.4 mA, which is equivalent to that of Pt/C electrode. The fast current decay can be ascribed to the following two reasons: one is that the conglomeration of Pt nanoparticles cannot directly anchor to the graphene support, and the other is that the larger number of defect sites in the graphene are much easier to be corroded under harsh electrochemical conditions [44,45].
Fig. 9 e Cell polarization curves and power density curves of the DBFC using Pt/C and Pt/G anode catalysts with anolyte consisted of 1 M NaBH4 D 3 M NaOH and 2 M H2O2 D 0.5 M H2SO4 catholyte. Catalyst loading: 4 mg/cm2.
The cell polarization and power density curves for DBHFC operating with 1 M NaBH4 þ 3 M NaOH as fuel and 2 M H2O2 þ 0.5 M H2SO4 as oxidant, while employing the Au/C as the cathode catalyst and Pt/C or Pt/G as the anode catalyst, are presented in Fig. 9a and b, respectively. The DBHFC was tested at 298 K. The open circuit voltage (OCV) of the cell is about 1.7 V, which is lower than the standard cell potential for the DBFC. The low value is probably caused by mixed potential at the anode and cathode from simultaneous oxidation of ions
Table 3 e Single cell DBFC performance with the synthesized Pt/C and Pt/G catalysts. Catalyst
Fig. 8 e Chronoamperometry curves of electrooxidation NaBH4 on (a) Pt/C and (b) Pt/G catalysts in 0.1 M NaBH4 D 3 M NaOH solution.
Pt/C Pt/G
Open circuit voltage, OCV (V)
Current density (mA/cm2)
Peak power density (mW/cm2)
1.743 1.757
50 60
34.13 41.82
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and hydrogen at the anode and reduction of H2O2 and O2 at the cathode [46]. As shown in Fig. 9a and b, the two curves almost coincide when the current density is in the range of 0e20 mA/ cm2. Further enhancements in DBFC performance are observed by using a Pt/G anode in comparison to a Pt/C anode in the high current density region. The limited current density and maximum power density of the single cell with Pt/G catalyst are 112 mA/cm2 and 42 mW/cm2, respectively; while for the single cell with Pt/C catalyst, the limited current density and maximum power density are 75 mA/cm2 and 34 mW/cm2, respectively. The OCV, limited current density, and maximum power density are summarized in Table 3. Apparently, both limited current density and maximum power density for Pt/G are all superior to Pt/C. The good performance of Pt/G over Pt/C may be due to the high conductivity and high durability to oxidation and corrosion of graphene support in the electrolyte [47].
4.
Conclusion
The Pt/G nanocomposites are successfully prepared by ethylene glycol simultaneous reduction method. The Pt nanoparticles are homogeneously dispersed on the graphene support. The Pt/G electrodes show higher ECSA and superior electrocatalytic activity toward borohydride oxidation, in comparison to Pt/C nanoparticles. In the single cell test, the limited current density and maximum power density on Pt/G nanocomposites are 112 mA/cm2 and 42 mW/cm2, which is higher than 75 mA/cm2 and 34 mW/cm2 on Pt/C nanoparticles. The good activity and stability of the Pt/G catalyst indicate that it is a promising electrocatalyst for the application of DBHFC.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51272221, 51072173 and 21203161), Doctoral Fund of Ministry of Education of China (Grant No. 20094301110005), and the Scientific Research Fund of Hunan Provincial Education Department (Grant No. 11A118).
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18] [19]
[20]
references [21] [1] Liu BH, Li ZP, Arai K, Suda S. Performance improvement of a micro borohydride fuel cell operating at ambient conditions. Electrochim Acta 2005;50:3719e25. [2] Celik C, Boyaci San FG, Sarac HI. Effects of operation conditions on direct borohydride fuel cell performance. J Power Sources 2008;185:197e201. [3] Ma J, Sahai Y, Buchheit RG. Direct borohydride fuel cell using Ni-based composite anodes. J Power Sources 2010;195(15): 4709e13. [4] Feng R, Dong H, Cao Y, Ai X, Yang H. AgNi-catalyzed anode for direct borohydride fuel cells. Int J Hydrogen Energy 2007; 32:4544e9. [5] Dong H, Feng R, Ai X, Cao Y, Yang H, Cha C. Electrooxidation mechanisms and discharge characteristics of borohydride on
[22]
[23]
[24]
[25]
different catalytic metal surfaces. J Phys Chem B 2005; 109(21):10896e901. Sanli AE, Aksu ML, Uysal BZ. Advanced mathematical model for the passive direct borohydride/peroxide fuel cell. Int J Hydrogen Energy 2011;36:8542e9. Gyenge E. Electrooxidation of borohydride on platinum and gold electrodes: implications for direct borohydride fuel cells. Electrochim Acta 2004;49(6):965e78. Kojima Y, Suzuki K, Fukumoto K, Sasaki M, Yamamoto T, Kawai Y, et al. Hydrogen generation using sodium borohydride solution and metal catalyst coated on metal oxide. Int J Hydrogen Energy 2002;27(10):1029e34. Bai Y, Wu C, Wu F, Yi B. Carbon-supported platinum catalysts for on-site hydrogen generation from NaBH4 solution. Mater Lett 2006;60(17):2236e9. Gyenge E, Atwan M, Northwood D. Electrocatalysis of borohydride oxidation on colloidal Pt and Pt-alloys (PteIr, PteNi, and PteAu) and application for direct borohydride fuel cell anodes. J Electrochem Soc 2006;153:A150e8. Li Z, Liu B, Arai K, Asaba K, Suda S. Evaluation of alkaline borohydride solutions as the fuel for fuel cell. J Power Sources 2004;126(1):28e33. Yi L, Song Y, Yi W, Wang X, Wang H, He P, et al. Carbon supported Pt hollow nanospheres as anode catalysts for direct borohydrideehydrogen peroxide fuel cells. Int J Hydrogen Energy 2011;36:11512e8. Pei F, Wang Y, Wang X, He P, Chen Q, Wang X, et al. Performance of supported AueCo alloy as the anode catalyst of direct borohydrideehydrogen peroxide fuel cell. Int J Hydrogen Energy 2010;35:8136e42. Cheng H, Scott K. Determination of kinetic parameters for borohydride oxidation on a rotating Au disk electrode. Electrochim Acta 2006;51(17):3429e33. de Leon CP, Walsh FC, Pletcher D, Browning DJ, Lakeman JB. Direct borohydride fuel cells. J Power Sources 2006;155: 172e81. Celik C, San FGB, Sarac HI. Influences of sodium borohydride concentration on direct borohydride fuel cell performance. J Power Sources 2010;195(9):2599e603. Ma J, Choudhury NA, Sahai Y. A comprehensive review of direct borohydride fuel cells. Renew Sustain Energy Rev 2010; 14(1):183e99. Indig ME, Snyder RN. Sodium borohydride, an interesting anodic fuel (1). J Electrochem Soc 1962;109:1104e6. Celik C, Boyaci San FG, Sarac HI. Improving the direct borohydride fuel cell performance with thiourea as the additive in the sodium borohydride solution. Int J Hydrogen Energy 2010;35:8678e82. Huang SY, Ganesan P, Park S, Popov BN. Development of a titanium dioxide-supported platinum catalyst with ultrahigh stability for polymer electrolyte membrane fuel cell applications. J Am Chem Soc 2009;131(39):13898e9. He D, Cheng K, Li H, Peng T, Xu F, Mu S, et al. Highly active platinum nanoparticles on graphene nanosheets with a significant improvement in stability and CO tolerance. Langmuir 2012;28:3979e86. Kou R, Shao Y, Wang D, Engelhard MH, Kwak JH, Wang J, et al. Enhanced activity and stability of Pt catalysts on functionalized graphene sheets for electrocatalytic oxygen reduction. Electrochem Commun 2009;11(5):954e7. Novoselov K, Geim A, Morozov S, Jiang D, Zhang Y, Dubonos S, et al. Electric field effect in atomically thin carbon films. Science 2004;306(5696):666e9. Lu G, Mao S, Park S, Ruoff RS, Chen J. Facile, noncovalent decoration of graphene oxide sheets with nanocrystals. Nano Res 2009;2(3):192e200. Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based
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 3 7 ( 2 0 1 2 ) 1 7 9 8 4 e1 7 9 9 1
[26]
[27]
[28]
[29] [30]
[31]
[32]
[33]
[34]
[35]
[36]
nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45(7):1558e65. Schniepp HC, Li JL, McAllister MJ, Sai H, Herrera-Alonso M, Adamson DH, et al. Functionalized single graphene sheets derived from splitting graphite oxide. J Phys Chem B 2006; 110(17):8535e9. Liang Y, Li Y, Wang H, Zhou J, Wang J, Regier T, et al. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat Mater 2011;10(10):780e6. Venkateswara Rao C, Cabrera CR, Ishikawa Y. Graphenesupported PteAu alloy nanoparticles: a highly efficient anode for direct formic acid fuel cells. J Phys Chem C 2011; 115:21963e70. Hummers Jr WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80(6):1339. Liu X, Wang X, He P, Yi L, Liu Z, Yi X. Influence of borohydride concentration on the synthesized Au/graphene nanocomposites for direct borohydride fuel cell. J Solid State Electrochem 2012:1e9. Shen J, Yan B, Shi M, Ma H, Li N, Ye M. Fast and facile preparation of reduced graphene oxide supported PteCo electrocatalyst for methanol oxidation. Mater Res Bull 2012; 47:1486e93. Yang J, Tian C, Wang L, Fu H. An effective strategy for smallsized and highly-dispersed palladium nanoparticles supported on graphene with excellent performance for formic acid oxidation. J Mater Chem 2011;21(10):3384e90. Vinodgopal K, Neppolian B, Lightcap IV, Grieser F, Ashokkumar M, Kamat PV. Sonolytic design of grapheneeAu nanocomposites. Simultaneous and sequential reduction of graphene oxide and Au (III). J Phys Chem Lett 2010;1(13): 1987e93. Goncalves G, Marques PAAP, Granadeiro CM, Nogueira HIS, Singh M, Gracio J. Surface modification of graphene nanosheets with gold nanoparticles: the role of oxygen moieties at graphene surface on gold nucleation and growth. Chem Mater 2009;21(20):4796e802. Santos L, Oliveira C, Moraes I, Ticianelli E. Oxygen reduction reaction in acid medium on PteNi/C prepared by a microemulsion method. J Electroanal Chem 2006;596(2):141e8. Tang H, Ehlert GJ, Lin Y, Sodano H. Highly efficient synthesis of graphene nanocomposites. Nano Lett 2012;12:84e90.
17991
[37] Seger B, Kamat PV. Electrocatalytically active grapheneeplatinum nanocomposites. Role of 2-D carbon support in PEM fuel cells. J Phys Chem C 2009;113(19):7990e5. [38] Li Y, Gao W, Ci L, Wang C, Ajayan PM. Catalytic performance of Pt nanoparticles on reduced graphene oxide for methanol electro-oxidation. Carbon 2010;48(4):1124e30. [39] Søgaard M, Odgaard M, Skou EM. An improved method for the determination of the electrochemical active area of porous composite platinum electrodes. Solid State Ionics 2001;145(1):31e5. [40] Atwan MH, Macdonald CLB, Northwood DO, Gyenge EL. Colloidal Au and Au-alloy catalysts for direct borohydride fuel cells: electrocatalysis and fuel cell performance. J Power Sources 2006;158(1):36e44. [41] Kundu P, Nethravathi C, Deshpande PA, Rajamathi M, Madras G, Ravishankar N. Ultrafast microwave-assisted route to surfactant-free ultrafine Pt nanoparticles on graphene: synergistic co-reduction mechanism and high catalytic activity. Chem Mater 2011;23:2772e80. [42] Guo S, Dong S, Wang E. Constructing carbon nanotube/Pt nanoparticle hybrids using an imidazolium-salt-based ionic liquid as a linker. Adv Mater 2010;22(11):1269e72. [43] Guo S, Zhang S, Sun X, Sun S. Synthesis of ultrathin FePtPd nanowires and their catalysis for methanol oxidation reaction. J Am Chem Soc 2011;133:15354e7. [44] Sharma S, Ganguly A, Papakonstantinou P, Miao X, Li M, Hutchison JL, et al. Rapid microwave synthesis of CO tolerant reduced graphene oxide-supported platinum electrocatalysts for oxidation of methanol. J Phys Chem C 2010;114:19459e66. [45] Barinov A, Fabris S, Sun T, Gregoratti L, Dalmiglio M, Kiskinova M, et al. Initial stages of oxidation on graphitic surfaces: photoemission study and density functional theory calculations. J Phys Chem C 2009;113(21):9009e13. [46] Ponce de Leo´n C, Walsh FC, Patrissi CJ, Medeiros MG, Bessette RR, Reeve RW, et al. A direct borohydrideeperoxide fuel cell using a Pd/Ir alloy coated microfibrous carbon cathode. Electrochem Commun 2008;10:1610e3. [47] Shao Y, Zhang S, Wang C, Nie Z, Liu J, Wang Y, et al. Highly durable graphene nanoplatelets supported Pt nanocatalysts for oxygen reduction. J Power Sources 2010; 195(15):4600e5.