Facile and Rapid Synthesis of Ultrafine PtPd Bimetallic Nanoparticles and Their High Performance toward Methanol Electrooxidation

1 Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, Faculty of Materials Science and Engineering, Hubei University, No. 368 Youyi Avenue, Wuchang, Wuhan 430062, China 2Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japan


Introduction
With the increasing depletion of fossil energy and the severity of environmental pollution, people pay great attention to develop a renewable, clean, and effective energy.A lot of research has been carried out extensively on the direct methanol fuel cells (DMFCs) due to their unique advantages, such as its high specific energy, environmental-friendly, and portability.The selection of anodic catalyst is extremely crucial for the performance of the DMFCs.Considering high catalytic activity and stabilized chemical properties, platinum is the most primary catalyst for the electrooxidation of low carbon-chain aliphatic alcohol such as methanol and ethanol in DMFCs [1,2].Unfortunately, the application of Pt-based catalysts in DMFCs is severely hindered in commercial market because of the poor natural content of Pt and poisoning effect with CO-like intermediates of the methanol electrooxidation [3].Therefore, exploitation of new anodic catalysts in DMFCs is very important for improving the cell performance.It was verified that alloying Pt with inexpensive metals especially the transition metals can not only substantially reduce the cost of the electrocatalyst but also alter the geometrical and electronic effect to yield a more active catalytic surface [4].Therefore, the research of bimetallic or trimetallic alloy nanomaterials has become a focal point in anodic electrocatalyst of DMFCs.[5][6][7][8][9][10].Palladium, 50 times more abundant in nature and much cheaper than Pt, is considered as an ideal substitution for the growth of the Pt-based bimetallic nanocatalysts owning to only 0.77% lattice mismatch with Pt.PtPd alloy nanomaterials exhibit a similar catalytic performance as well as a higher intermediate poison resistance compared with Pt during methanol electrooxidation [11,12].Thus, bimetallic PtPd nanocatalysts are expected to be a promising substitute for Pt catalysts in DMFCs.
So far, different kinds of methods for synthesizing PtPd bimetallic nanoparticles have been reported, such as waterin-oil technique, microwave irradiation [13], electroless deposition technique [14], normal wet impregnation technique, and flame aerosol [15].Nevertheless, these methods are extremely difficult, complicated, and time-lavished.Kinds of organic solvents and/or surfactants are employed in most of the synthetic methods, which leads to environmental pollution [16,17].Recently, lots of chemical reductants have been used to the fabrication of nanoparticles, such as hydrazine [18] and sodium borohydride, followed by toxicity, longtime thermal treatment, and even irretrievable damage to nanocrystals [19].Consequently, it is imperative to seek a facile and surfactant-free process to fabricate bimetallic PtPd nanocrystals.
In this present work, we have developed a simple and rapid method to obtain the uniform and ultrafine PtPd NPs.The experiments were carried out in aqueous solution without any organic solvents or surfactant agents at an ambient temperature.It is worth noting that ascorbic acid is served as the reductant to reduce K 2 PtCl 4 and K 2 PdCl 4 simultaneously.The morphology and structure of the as-prepared PtPd nanoparticles were characterized by various techniques, including TEM, XRD, and XPS.Then, we examined and compared the electrocatalytic activity and tolerance of carbon black (Vulcan XC-72) supported bimetallic nanoparticles (PtPd NPs/V) with carbon black supported monometallic Pt and Pd NPs (Pt NPs/V, Pd NPs/V) toward methanol oxidation.The further electrochemical characterizations indicate superior electrocatalytic activity and poison tolerance of the prepared PtPd NPs/V for methanol oxidation compared with Pt NPs/V and Pd NPs/V, which indicates potential use of PtPd NPs/V as a new anode catalyst in DMFCs.

Materials and Methods
2.1.Chemicals.Vulcan XC-72 was obtained from Cabot Corp.K 2 PtCl 4 and K 2 PdCl 4 were purchased from Aldrich chem.Co. 5 wt% Nafion was produced by DuPont.Ascorbic acid and KBr were purchased from Sinopharm Chemical Reagent Co.All chemicals reagents were of analytical grade and used as received without further purification.All the aqueous solution was prepared with the ultrapure water which was produced from a Kertone Ultrapure Water System P60-CY (Kertone Water Treatment Co. Ltd., resistivity > 18 MΩ⋅cm).

2.2.
Instruments.XRD analysis of the as-synthesized samples was recorded on D8-advance diffractometer (Bruker, Germany) with a Cu K radiation source ( = 0.15418 nm).TEM (FEI TECNAI20, USA) and FE-HRTEM (JEM-2100F) were employed to characterize the morphology and sizes of the samples.EDS (JSM6510LV, Japan) was performed to determine the chemical composition of the catalysts.XPS were carried out using a SPeCS system (PHOIBOS 150, Germany) with Al K radiation (h] = 1486.6eV).All electrochemical measurements were conducted using a 550 electrochemical workstation (Gaoss Union Instrument Company, China) at room temperature in a conventional three-electrode system which consisted of a Pt (wire) counter electrode and an Ag/AgCl reference electrode.To remove dissolved oxygen, pure nitrogen gas was bubbled through the solution at the beginning of each experiment.

Preparation of PtPd
NPs/V, Pt NPs/V, and Pd NPs/V.In a typical synthesis, 160 L of 100 mM K 2 PtCl 4 and 40 L of 100 mM K 2 PdCl 4 were dispersed in 100 mL ultrapure water which contained 3 mg KBr.After adjusting the mixed solution pH to 2.18, 1 mL of 60 mM ascorbic acid was added into the solution with constant stirring for 30 minutes.After that, 14.7 mg Vulcan XC-72 was dispersed into the as-prepared brown suspension while keeping durative sonication for 1 h and magnetic stirring for 5 h.All of the operations were taking place at room temperature.The PtPd NPs/V was ultimately obtained after centrifugation, washing, and drying at 60 ∘ C in a vacuum oven for overnight.For comparison, the monometallic Pt NPs/V and Pd NPs/V were also prepared in the same way with that of PtPd NPs/V except only adding 0.2 mL of 100 mM K 2 PtCl 4 or 0.2 mL of 100 mM K 2 PdCl 4 as metal precursor, respectively.

Preparation of Working Electrode.
The procedure of preparing modified working electrodes consulted some previous reports [15,16,20].First of all, the glassy carbon (GC) electrode must ensure being polished to a mirror finish by using slurry of 1, 0.3, and 0.05 m alumina powers, washing via ultrasonic in ethanol and ultrapure water for 3 minutes, and then drying under nitrogen steam.Afterwards, 2 mg of the nanocatalysts loaded Vulcan XC-72 was distributed into a mixture of ultrapure water (0.9 mL) and 0.1 mL of Nafion (5 wt.%) and then using ultrasonic treatment to form a homogeneous black suspension.Finally, 5 L of the uniform suspension was laid on the surface of the GC electrode and dried at an ambient temperature.

Morphology and Structural Characterization of PtPd NPs/ V.
To investigate the alloy nature and crystalline structure, the as-synthesized PtPd NPs/V, Pt NPs/V, and Pd NPs/V were characterized by XRD measurements.The XRD patterns of Pd NPs/V, PtPd NPs/V, and Pt NPs/V catalysts were shown in Figure 1.Both Pt NPs/V and Pd NPs/V exhibit diffraction peaks of (111), (200), and (220) planes, respectively, corresponding to the standard data of Pt (JCPDS no.04-0802) and Pd (JCPDS no.46-1043) [21,22].These demonstrate that the crystal structures of monometallic catalysts are face-centered cubic structure (fcc).Similarly, the peaks at the 2 value of 39.92 ∘ , 46.22 ∘ , and 67.74 ∘ in the pattern of PtPd NPs/V are attributed to (111), (200), and (220) crystal planes for the fcc structure of PtPd.All the peak positions of PtPd NPs/V are found to be slightly shifted to high diffraction angles compared with that of Pt NPs/V.Taking an example of the (111) plane diffraction peak, the 2 value of PtPd NPs/V planes (39.92 ∘ ) is between those of Pt NPs/V (39.82 ∘ ) and Pd NPs/V (40 ∘ ).These results were attributed to the addition of Pd to the Pt for the difference in atomic size (Pd < Pt) [23].Hence, it is an indication that PtPd alloy nanoparticles have been successfully synthesized [24,25].
The TEM and HRTEM images of the bimetallic PtPd nanoparticles before and after loading on Vulcan XC-72 are shown in Figure 2. Figures 2(a The HRTEM image of PtPd NPs/V (Figure 2(d)) shows wellresolved continuous fringes in the same direction.The interval between the two lattice fringes was found to be 0.225 nm, which is between 0.2265 and 0.2246 nm, corresponding to the {111} interplanar distance of fcc Pt and Pd, respectively, indicating the formation of PtPd alloy nanostructures.These TEM results demonstrated that well-dispersed PtPd alloy nanoparticles have been successfully synthesized and could be loaded onto carbon black (Vulcan XC-72) for further application in DMFC.
To further characterize the alloy nature of the as-synthesised nanocrystals, PtPd nanoparticles were investigated by EDS (Table 1), revealing the presence of C, Pt, and Pd.The bulk composition estimates the ratio of Pt and Pd is 4 : 3 which is deviated from the nominal ratio (Pt : Pd 4 : 1).This  result should be able to consider the different banding energy of Pt and Pd relative to the supports [26].
XPS was employed to study the surface elemental composition of the nanocomposites as shown in Figure 3. Figures 3(a) and 3(b) show the Pt 4f and Pd 3d spectra of the sample, respectively.As for Pt 4f, it can be observed two peaks at 71.15 and 74.45 eV corresponding to Pt 4f 7/2 and Pt 4f 5/2 , which further confirms the existence of Pt metal.Similarly, the binding energy at 335.35 and 340.85 eV corresponds to metallic Pd 3d 5/2 and Pd 3d 3/2 in Figure 3(b), respectively.Therefore, it can be concluded that the Pd II and Pd II precursors have been successfully reduced by AA in our synthesis and the PtPd alloy has been formed [27,28].

Electrochemical Investigation of the PtPd NPs/V and
Monometallic Pt, Pd NPs/V.Typical cyclic voltammograms (CV) of the PtPd NPs/V and metallic Pt NPs/V, Pd NPs/V modified glassy carbon electrodes were measured in N 2saturated 0.5 M H 2 SO 4 at a potential between −0.2 V and 1.2 V with a scan rate of 100 mV/s (Figure 4).In the potential region of −0.2 V to 0.2 V, the typical hydrogen adsorption and desorption peaks are observed, which is a dominant factor for the estimation of the electrochemically active surface area (ECSA).The ECSA value can be evaluated by   / (210 × ) [29].Among this formula,   is the total electric charge of hydrogen adsorption and desorption on the PtPd nanoparticles, which can be calculated by integrating the area under the CV curve in the hydrogen adsorption range after a double-layer correction.The charge of each actual active area is 210 C cm −2 [11,30]. is the catalyst loading on the glassy carbon surface (in mg).Therefore, it is assumed that the ECSA of PtPd NPs/V, Pt NPs/V, and Pd NPs/V is 60.40 m 2 g −1 , 11.58 m 2 g −1 , and 10.56 m 2 g −1 , respectively.In comparison with Pt NPs/V and Pd NPs/V, PtPd NPs/V shows obviously higher ECSA value, which confirms the excellent electrooxidation toward methanol.CH 3 OH during the potential of −0.6 and 0.2 V (versus Ag/AgCl).The scan rate was kept at 100 mV/s.After normalization by ECSAs, we can observe the characteristic anodic peaks in the forward sweeps and the cathode peaks in the reverse sweeps at ∼ −0.2 V and ∼ −0.3 V, respectively.It is clear to view that the maximum current density of PtPd NPs/V (7.326 mA cm −2 ) is much larger than that of monometallic Pt NPs/V (2.018 mA cm −2 ) and Pd NPs/ V (1.611 mA cm −2 ).Moreover, the onset potential of PtPd NPs/V for methanol electrooxidation reaction (MOR) has an obvious negative shift compared with those of Pt NPs/V and Pd NPs/V, which presents the superior catalytic performance of PtPd bimetallic to some degree.Furthermore, the value of the ratio (  /  ) of the forward peak current density (  ) to the reverse peak current density (  ) is a crucial index in the MOR for appraising the poison tolerance to CO-like intermediate.

Electrocatalytic Properties of
Studies have shown that a higher value of   /  indicates the better effect of tolerating poisoning species toward MORs [4,11].The   /  value of the PtPd NPs/V catalysts (3.51) is higher than that of monometallic Pt NPs/V, Pd NPs/V catalysts (2.50), implying that the bimetallic catalysts have a much better poison tolerance.The immensely enhanced electrocatalytic activity and poisoning tolerance for MOR should give the credit to the "bifunctional catalysts" effect [31].It is considered that the alloy has decreased the distance of PtPd bond and increased the vacancy of Pt d-electron, obtaining a more active catalytic surface [11].
To study the long-term stability of the PtPd NPs/V toward MOR, chronoamperometry tests were conducted for 700 s at room temperature (Figure 6).In the view of curves in I-T, 20 s later, the initial current density rapidly decreased to almost 1/2; hereafter, the curves begin to flatten.This result is due to the formation of poisonous intermediate species during MOR, such as CO ads [1], CH 3 OH ads , and CHO ads [15], attached on the surface of the catalysts.The intermediate species adsorbed onto the catalysts will block the activation sites and refrain from the oxidation of methanol.However, a higher current density of PtPd NPs/V (5.44 mA cm −2 ) was found compared with Pt NPs/V (1.29 mA cm −2 ) and Pd NPs/V (0.68 mA cm −2 ), which means the numerous active sites available on the surface of the PtPd nanocatalysts, further indicating the better electrocatalytic activity of bimetallic PtPd.This result can also be understood mainly based on the synergistic interaction of Pt and Pd in the catalysts [4].As shown in Figure 6, the current density of PtPd NPs/V is obviously lager than that of Pt NPs/V and Pd NPs/V along the time range, which means the bimetallic catalyst possesses much better poison tolerance during the electrochemical oxidation of methanol.

Conclusion
In this present work, we successfully fabricated uniform and sub-10 nm bimetallic PtPd nanocrystals by a clean and facile method in aqueous solution at ambient temperature, in which ascorbic acid act as soft and green reductant.The as-synthesised PtPd NPs/V shows great catalytic activity and super poisoning tolerance for MOR in alkaline media than those of monometallic Pt NPs/V and Pd NPs/V.This facile method is also versatile for preparing the other bimetallic or trimetallic alloy nanoparticles as new electrocatalyst for the application in fuel cell.
) and 2(b) are the images of PtPd alloy nanoparticles before loading on Vulcan XC-72 at different magnification.Well-dispersed PtPd alloy nanoparticles were observed with the average size 7 nm through the statistics of TEM image.No visible aggregation was found.For PtPd NPs/V (Figure2(c)), the PtPd nanoparticles are uniformly dispersed on Vulcan XC-72 and the original morphology of nanoparticles basically remained.

Figure 2 :
Figure 2: TEM images of PtPd NPs ((a), (b)) and PtPd NPs/V (c); the inset in (b) is the distribution of particle size; HRTEM image of PtPd NPs/V (d); the inset in (d) is the enlarged figure of the selected area.The scale bars are 50 nm (a), 20 nm (b), 50 nm (c), and 10 nm (d).

5 M 2 )Figure 5 :
Figure 5: Cyclic voltammograms of PtPd NPs/V, Pt NPs/V, and Pd NPs/V measured in nitrogen saturated solutions of 1.0 M NaOH containing 0.5 M CH 3 OH at a scan rate of 100 mV s −1 .

− 2 )Figure 6 :
Figure 6: Chronoamperometry curves of PtPd NPs/V, Pt NPs/V, and Pd NPs/V measured in nitrogen saturated solutions of 1.0 M NaOH containing 0.5 M CH 3 OH at a scan rate of 100 mV s −1 .

Table 1 :
The ratio of Pt/Pd and the mass percentage of PtPd, Pt and Pd in PtPd NPs/V, Pt NPs/V, Pd NPs/V were measured by EDS.