石墨烯纳米片担载Pt-Pd双金属纳米球作为甲醇氧化的电催化剂

杨子娟，赵梦溪，陈 森，樊 阳

(信阳师范学院 化学化工学院，河南 信阳 464000)

石墨烯纳米片担载Pt-Pd双金属纳米球作为甲醇氧化的电催化剂

杨子娟，赵梦溪，陈 森，樊 阳*

(信阳师范学院 化学化工学院，河南 信阳 464000)

利用简便的无表面活性剂的方法合成了石墨烯担载的Pt-Pd双金属纳米球. 首先由Na2PdCl4与氧化石墨烯发生氧化还原反应生成Pd晶种，然后诱导Pt纳米粒子的生长，得到Pt-Pd双金属纳米球. 采用扫描电子显微镜、透射电子显微镜和X射线粉末衍射仪表征了合成的Pt-Pd/GR催化剂的结构，并测定了其作为甲醇氧化电催化剂的性能. 结果表明，Pt-Pd/GR催化剂对甲醇氧化反应表现出高催化活性和稳定性，甲醇氧化电流密度为51.8 mA·cm-2.

石墨烯纳米片；担载Pt-Pd纳米球；甲醇氧化；电催化剂

Graphene-based electrocatalysts have shown promising catalytic performance for fuel cells reactions[1-20]. In these electrocatalysts systems, graphene nanosheets serve as a support to disperse catalytic nanoparticles (NPs) and the electron-transfer medium at the interface of electrode and reactants[21]. The remarkable enhancement in the catalytic activities of graphene-based electrocatalysts can be ascribed to the high specific surface area and conductivity of the graphene nanosheets[22-23]. Furthermore, graphene nanosheets provide a unique two-dimensional (2D) surface for the growth and assembly of catalysts NPs with controlled morphology, orientation and dimensionality[24-27]. Pt-Pd bimetallic electrocatalysts with designed morphologies such as nanotubes, nanowires and nanospheres have attracted much attention, because they are considerably less expensive than pure Pt catalysts and exhibits unique characteristics[28-32]. For example, Pt-Pd bimetallic catalyst exhibits high CO-poisoning tolerance owing to the presence of Pd oxide species, and it has significantly improved catalytic performance as compared with pure Pt catalyst[3,16,18].

Herein, we report a facile surfactant-free route forinsitugrowth of Pt-Pd bimetallic nanospheres on graphene nanosheets. Briefly, Pd NPs were nucleated on graphene oxide nanosheetsviathe redox reaction between [PdCl4]2-and graphene oxide. Pt NPs were then grown and self-assembled around the Pd seeds in association with the reduction of graphene oxide affording corresponding graphene supported catalyst. With this approach, Pt-Pd bimetallic nanospheres can be directly formed on the surface of graphene oxide without the presence of any surfactant molecules. In as-obtained graphene supported Pt-Pd electrocatalyst, the Pt-Pd nanospheres can act as pillars to efficiently separate the graphene sheets from each other thereby preventing the agglomeration of graphene layers[33-34]. Therefore, the characteristics of individual graphene sheets can be retained in the hybrid material. This article reports the synthesis of the title hybrid material with unique structure as well as its high catalytic activity and long-term stability towards methanol oxidation.

1 Experimental

1.1 Reagent and apparatus

Graphite powder (spectrographic purity) and H2PtCl6·6H2O (w(Pt) ≥ 37%) were purchased from Sinopharm Chemical Reagent Co. Ltd., Ascorbic acid (analytical grade reagent) and Na2PdCl4(w(Pd)=36.4%) were purchased from Aladdin Chemistry Co. Ltd., Pt/C (w(Pt)=20% on carbon black, Johnson Matthey) was obtained from Alfa Aesar. Nafion (w= 5%) was obtained from Sigam-Aldrich. Water used throughout all experiments was purified with the Millipore system.

1.2 Structure characterization

Scanning electron microscopy (SEM) images were obtained with a Hitachi S-4800 field emission scanning electron microscope. SEM samples were prepared by dispersing in ethanol and sonicating for several minutes, followed by drop-casting onto a silicon wafer. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and energy dispersive X-ray (EDX) spectrometry analyses were conducted with a JEOL JEM-2010 transmission electron microscope. Specimens for all of these TEM experiments were prepared by diluting the catalyst ink with ethanol, sonicating for 2 min in order to ensure adequate dispersion of the nanostructures, evaporating of one drop of the solution onto a 300 mesh Cu grid, and coating with a lacey carbon film. X-ray diffraction (XRD) analyses were carried out with a Bruker D8/Advance X-ray diffractometer with Cu Kαradiation (λ= 0.154 06 nm).

1.3 Electrochemical measurements

All electrochemical experiments were performed with a CHI 850C electrochemical analyzer (CH Instruments). A conventional three-electrode system was used for all electrochemical experiments,and it consists of a platinum wire as counter electrode, a standard calomel electrode (SCE) as reference electrode, and a glassy carbon (GC) electrode (3 mm diameter) coated with the catalyst as working electrode. The catalyst ink was prepared by dispersion of catalyst powder in a mixed solution of ethanol (1 mL) and Nafion (10 μL) under sonication to form a homogeneous black suspension (5 g·L-1). 5 μL of the catalyst ink was cast on the freshly-polished GC electrode surface and dried under an infrared lamp. Before measurements, the working electrode was first activated with cyclic voltammetry (CV, 0-1.0 V, 50 mV·s-1) in a N2-saturated 0.5 mol·L-1H2SO4solution until a steady CV was obtained.

1.4 Preparation of Pt-Pd/GR catalyst

Graphene oxide was prepared by the modified Hummers method[35]. In the growth of Pt-Pd bimetallic nanospheres on graphene sheets, Pd NPs were first nucleated on graphene oxide sheetsviathe redox reaction between [PdCl4]2-and graphene oxide[8]. Pt NPs were then grown and self-assembled around the Pd seeds, with the reduction by ascorbic acid. In a typical procedure, 1 mL of Na2PdCl4aqueous solution (5 g·L-1) was added to 100 mL of a homogeneous graphene oxide suspension (0.5 g·L-1), and the mixture was stirred in an ice-water bath for 30 min. After addition of ascorbic acid (50 mg), the mixture was gradually heated to 90 ℃. Then, 1 mL of H2PtCl6·6H2O solution (16 g·L-1) was rapidly injected into the flask, and the mixture was stirred at 90 ℃ for 3 h. The product, denoted as Pt-Pd/GR, was collected by centrifugation, washed with water, and vacuum-dried at 50 ℃ for 6 h. The reference sample of graphene supported Pt NPs, denoted as Pt/GR, was prepared under the same conditions but without the addition of Na2PdCl4.

2 Results and discussion

The crystallographic structure of as-synthesized Pt-Pd/GR catalyst was characterized by powder XRD. As shown in Fig.1a, the diffraction peaks at 2θvalues of 40.1°, 46.7°, 68.0°, 82.1° and 86.5° can be indexed as the (111), (200), (220), (311) and (222) crystalline planes, which indicates that Pt and Pd NPs exhibit typical face-centered cubic (fcc) crystal structure[1-5]. Due to the similarity of the crystalline structures of Pt and Pd (JCPDS card No. 04-0802 (Pt) and No. 46-1043 (Pd)), the peak positions of Pt and Pd show little difference[3-4]. As to the XRD pattern of graphene oxide (Fig.1b), the (002) diffraction peak corresponding to the hexagonal graphite structure shifts to 10.1°. In contrast, it moves to higher angle of 21.9° in that of Pt-Pd/GR, which indicates that graphene oxide is reduced to graphene[6-8].

Fig.1 XRD patterns of Pt-Pd/GR (a) and GO (b)

The morphology and microstructure of Pt-Pd/GR catalyst were investigated with SEM and TEM. As shown in Figs. 2a and 2b, a number of Pt-Pd nanospheres with the size ranging from 50 nm to 80 nm are evenly distributed on graphene sheets. Despite of the layering of graphene sheets owing to solvent evaporation, the presence of Pt-Pd nanospheres keeps these layers sufficiently exfoliated. Hence, the characteristics of individual graphene nanosheets can be retained in the hybrid material. Moreover, the sandwiching of Pt-Pd nanospheres between graphene layers produces a highly porous nanostructure, which could facilitate the transport of reactant molecules within the bulk catalyst. In thisinsitugrowth process, the oxygen-containing functional groups on the basal planes of graphene oxide provide the reactive sites for the heterogeneous nucleation of Pd NPs[10]. After the growth of Pd nuclei particles, most of the nucleation sites on graphene surface may be occupied. Therefore, the subsequent growth of Pt NPs preferentially occurs around the pre-anchored Pd NPs, resulting in the formation of Pt-Pd bimetallic nanospheres. The TEM image (Fig.2c) shows that the nanospheres are composed of interconnected fine NPs, which confirms that the Pt-Pd nanospheres are constructed from the self-assembly of primary NPs building blocks. The graphene-supported Pt-Pd nanospheres were also analyzed by EDX (Fig.3), and the results reveal the presence of Pd, Pt and C. The HRTEM image in Fig.2d shows good crystallinity of the nanospheres with well-defined lattice fringes. The lattice fringes with adspacing of 0.23 nm correspond to the typical (111) plane offccPt[12,15-16].

Fig.2 SEM (a and b), TEM (c) and HRTEM (d) images of Pt-Pd/GR

The electrochemical properties of the Pt-Pd/GR catalyst were investigated and compared with those of Pt/GR and Pt/C catalysts. Fig.4 shows the cyclic voltammograms (CVs) of different catalyst-coated GC electrodes in N2-saturated 0.5 mol·L-1H2SO4solution. The CV profiles show typical potential regions of hydrogen adsorption-desorption (-0.2 V to 0.15 V), double-layer capacitance (0.15 V to 0.3 V) and metal oxidation-reduction (above 0.3 V). It is noteworthy that the reduction peak of metal oxide positively shifts to higher potential on Pt-Pd/GR (0.52 V), in comparison with that on Pt/C (0.45 V) and Pt/GR (0.47 V). This suggests that the metal oxide layer is more readily reduced on the Pt-Pd nanospheres to activate the methanol oxidation reaction[36]. The electrochemically active surface area (ECSA) was estimated by integrating the voltammograms corresponding to hydrogen adsorption from the electrode surface. It is found that the ECSA value of Pt-Pd/GR (70.5 m2·mg-1) is higher than that of Pt/C (51.5 m2·mg-1) and Pt/GR (41.3 m2·mg-1) catalysts. This indicates that more active sites for electrocatalytic reaction and electron transfer are available on the Pt-Pd/GR catalyst.

Fig.3 EDX spectrum of Pt-Pd/GR

Fig.4 CVs of Pt-Pd/GR (a), Pt/GR (b) and Pt/C (c) catalysts modified electrodes in N2-saturated 0.5 mol·L-1 H2SO4 solution at a scan rate of 50 mV·s-1

The catalytic performance of Pt-Pd/GR catalyst towards methanol oxidation was investigated in 0.5 mol·L-1H2SO4solution containing 1.0 mol·L-1methanol. Two typical oxidation peaks appear in the CV curves of these catalysts (Fig.5), which arise from the oxidation of methanol in the forward scan and the oxidation of COads-like species in the backward scan[8-10]. The peak potential for methanol oxidation at Pt-Pd/GR (0.64 V) is considerably lower than that on Pt/C (0.74 V) and Pt/GR (0.72 V), indicating decreased overpotential at the Pt-Pd/GR catalyst. The peak current density is 51.8 mA·cm-2for Pt-Pd/GR, and it is much higher than those on Pt/GR (29.0 mA·cm-2) and Pt/C (22.9 mA·cm-2) catalysts. This means that Pt-Pd/GR catalyst exhibits remarkably improved catalytic activity for methanol oxidation. The CO-poisoning tolerance of Pt-based catalysts is generally evaluated by the peak current ratio of forward to backward scans (If/Ib)[18-20]. The calculatedIf/Ibratio on Pt-Pd/GR (1.2) is higher than that of Pt/C (0.8) and Pt/GR (1.0). The increasedIf/Ibratio of Pt-Pd/GR indicates less accumulation of COadsspecies on the catalyst surface and more complete methanol oxidation reaction thereon.

The stability and durability of the Pt-Pd/GR catalyst were evaluated by chronoamperometry tests. As shown in Fig.6, Pt-Pd/GR catalyst exhibits a much higher initial current density than the other catalysts, because of the presence of a larger number of catalytic active sites. The final current density for Pt-Pd/GR after holding the cell potential at 0.7 V for 3 000 s is 10.0 mA·cm-2, and it is about 1.7 and 2.8 times as much as that of Pt/C (5.8 mA·cm-2) and Pt/GR (3.6 mA·cm-2), respectively. It is noteworthy that Pt/GR also exhibits higher initial current density, however, it decays rapidly and becomes lower than that of Pt/C after 700 s. This may be ascribed to the aggregation of Pt NPs and graphene layers under the acidic and oxidative conditions. In contrast, the current density of Pt-Pd/GR remains higher throughout the whole process. This suggests that graphene supported Pt-Pd nanospheres remain stable in the electrocatalytic reaction.

Fig.5 CVs of Pt-Pd/GR (a), Pt/GR (b) and Pt/C (c) catalysts modified electrodes in N2-saturated 0.5 mol·L-1 H2SO4 + 1.0 mol·L-1 methanol solution at a scan rate of 50 mV·s-1

Fig.6 Chronoamperometric curves of Pt-Pd/GR (a), Pt/GR (b) and Pt/C (c) catalysts modified electrodes recorded at 0.7 V in N2-saturated 0.5 mol·L-1 H2SO4 solution containing 1.0 mol·L-1 methanol

3 Conclusions

In summary, we have developed a facile surfactant-free method to synthesize Pt-Pd bimetallic nanospheres on graphene nanosheets.As-prepared hybrid material exhibits a superior catalytic activity and long-term stability for methanol oxidation as compared with commercial Pt/C catalyst and Pt/GR catalyst. In one word, Pt-based NPs with nanospheres structure can be constructed through designedinsitugrowth process on graphene oxide nanosheets, which is promising in improving the catalytic performance towards fuel cell reactions.

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date：2014-01-20.

Supported by the Science & Technology Innovation Talents in Universities of Henan Province (13HASTIT012), the Science and Technology Key Project of Henan Education Department (12A150020) and the College Outstanding Teachers Program of Henan Province (2012GGJS-128).

Biography：YANG Zijuan (1988-), graduate student, majoring in nano-electrochemistry.*

, E-mail: fanyangchem@163.com.