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Pb nanoparticles and their electrocatalytic performance toward ethanol oxidation
Accepted Manuscript Title: Rapid synthesis of dendritic Pt/Pb nanoparticles and their electrocatalytic performance toward ethanol oxidation Authors: Ke Zhang, Hui Xu, Bo Yan, Jin Wang, Zhulan Gu, Yukou Du PII: DOI: Reference:
S0169-4332(17)31925-6 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.270 APSUSC 36470
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APSUSC
Received date: Revised date: Accepted date:
24-5-2017 21-6-2017 26-6-2017
Please cite this article as: Ke Zhang, Hui Xu, Bo Yan, Jin Wang, Zhulan Gu, Yukou Du, Rapid synthesis of dendritic Pt/Pb nanoparticles and their electrocatalytic performance toward ethanol oxidation, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.270 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Rapid synthesis of dendritic Pt/Pb nanoparticles and their electrocatalytic performance toward ethanol oxidation
Ke Zhang, Hui Xu, Bo Yan, Jin Wang, Zhulan Gu, Yukou Du*
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Industrial Park, Ren'ai Road, Suzhou, 215123, China. E-mail: [email protected]
Highlights Pt/Pb nanodendrites have been prepared by heating in an oil bath for 5 min. Pt1/Pb1 nanodendrites exhibit enhanced electrocatalytic performance and stability. The presence of Pb enhance the Pt catalytic activity toward ethanol oxidation.
Abstract This article reports a rapid synthetic method for the preparation of dendritic platinum−lead bimetallic catalysts by using an oil bath for 5 min in the presence of hexadecyltrimethylammonium chloride (CTAC) and ascorbic acid (AA). CTAC acts as a shapedirection agent, and AA acts as a reducing agent during the reaction process. A series of physical techniques are used to characterize the morphology, structure and electronic properties of the
dendritic Pt/Pb nanoparticles, indicating the Pt/Pb dendrites are porous, highly alloying, and selfsupported nanostructures. Various electrochemical techniques were also investigated the catalytic performance of the Pt/Pb catalysts toward the ethanol electrooxidation reaction. Cyclic
voltammetry and chronoamperometry indicated that the synthesized dendritic Pt/Pb
nanoparticles possessed much higher electrocatalytic performance than bulk Pt catalyst. This study may inspire the engineering of dendritic bimetallic catalysts, which are expected to have great potential applications in fuel cells.
Keywords: Rapid synthesis; Platinum−lead; Dendrite; Ethanol oxidation.
Introduction As a promising future power source, direct fuel cells have many advantages, such as facile storage, high power density, environment friendly and easy refilling.[1] Direct ethanol fuel cells (DEFC) have attracted widespread concerns in the area of direct fuel cells due to the distinct advantages, including high energy density, nontoxicity, abundance, easy availability, low cost, safety for storage, etc.[2] It has been well-known that Pt has good catalytic activity toward ethanol electro-oxidation. However, the relatively high price, scarcity and sluggish reaction kinetics of Pt make its excellent catalysis property cannot to be widely applied in DEFCs.[3] Bimetallic nanoparticles play a vital role in many areas of science and technology, such as batteries, solar fuel, supercapacitors and fuel cells.[4] As a general rule, bimetallic nanoparticles perform better than monometallic nanoparticles because there is a synergistic effect between the two elements. So, recent research efforts have focused extensively on the development of less expensive Pt–based materials. Great progresses has been achieved toward enhancing the electrocatalytic activity of ethanol on Pt-based bimetallic electrocatalysts, such as Pt/Co,[5] Pt/Pb,[6] Pt/Pd,[7] Pt/Au,[8, 9] etc. Among these Pt–based electrocatalysts, Pt/Pb material may be
a good choice to improve the electrocatalytic activity, because this element can provide oxygencontaining species that accelerate the carbon intermediates oxidation. What’s more, the electronic structure of platinum can be changed by the entrance of Pb, which reduce the CO adsorption energy on the catalyst surface.[10] It is widely acknowledged that the size, shape, and composition of the nanomaterials can also influence its catalytic properties.[11] Up to now, a variety of Pt/Pb nanoparticles with different morphologies and structures, including wires,[10] nanoplate,[12] nanoflowers[13] and core-shell structure[14] have been synthesized successfully. The synthesized Pt-based materials with a dendritic nanostructure generally exhibit an enhanced electrocatalytic performance for direct fuel cells, because the edge and corner atoms are more abundant on dendritic structure, which could be more activity than particles with semi-spherical and low-index facets structure.[15] For example, Wang et al. have successfully synthesized the graphene nanosheet supported 3D Pt-on-Pd bimetallic nanodendrite hybrids, the as-prepared materials have a much higher electrocatalytic performance than conventional Pt/C catalysts for methanol electrooxidation.[16] Su et al. presented a facile seeded growth method to synthesis MoS2 nanosheets supported Pt shell and Au core bimetallic nanodendrites, the as-prepared nanomaterials showed better catalytic performance for methanol electrooxidation than that of the Pt catalyst.[17] As far as is known, there is few literature and studies on Pt/Pb nanodendrites toward ethanol oxidation in alkaline environment. Herein, we successfully prepared the dendritic Pt/Pb nanoparticles in aqueous solution with the assistance of hexadecyltrimethylammonium chloride and ascorbic acid, in which hexadecyltrimethylammonium chloride as the disperse agent and morphology-directing agent, and ascorbic acid acts as the reducing agent. The as-prepared catalyst displays a significantly
improved catalytic activity and superior durability toward ethanol electrooxidation reaction at room temperature as compared to the commercial Pt/C catalyst in alkaline environmental. The fabricated Pt/Pb nanoparticles may serve as a kind of potential anodic catalyst applied in the area of DEFC.
Experimental Materials and Chemicals H2PtCl6, Pb(OAc)2•3H2O, hexadecyltrimethylammonium chloride (CTAC), C2H5OH, KOH and ascorbic acid (AA) were obtained from Sinopharm Chemical Reagent Co, Ltd (Shanghai, P.R. China). Commercial 20 wt% Pt/C was obtained from Shanghai Hesen Electric Co., Ltd. All reagents were used without any further purification before use. Doubly distilled water was used throughout the work Apparatus The size and shape of the as-prepared materials were investigated by transmission electron microscopy (TEM) measurements on a Tecnai G220 electron microscope operating at an accelerating voltage of 200 kV. The chemical composition and morphology were investigated by Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis using the S4700 system (Hitachi High Technologies Corporation, Japan). The catalysts were analyzed by Xray diffraction (XRD) on a PANalytical X’Pert PRO MRD X-ray diffractometer employing CuK radiation (= 1.54056 Å). X-Ray photoelectron spectroscopy (XPS) was performed using an ESCALab220i-XL electron spectrometer from VG Scientific employing 300 W AlK radiation. The electrochemical experiments were performed at room temperature in a CHI 760D
electrochemical station (Shanghai Chenhua Instrumental Co., Ltd, China) with a conventional three-electrode system. The working electrode was prepared using a glassy carbon electrode (GCE) with a diameter of 3 mm. A Pt wire and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively.
Synthesis of dendritic Pt/Pb 1mL H2PtCl6 (7.723 mM ) aqueous solution, 3 mg Pb(OAc)2•3H2O, 64 mg CTAC was dispersed into 4 mL H2O to form a uniform solution by ultrasonication. After that, 9 mg AA was added into the mixture with ultrasonic for another 10 minutes. Subsequently, the mixture was heated in a preheated oil bath at 368K for 5 minutes under constant stirring and then cool down to room temperature without disturb. The solution was centrifuged at 6000 rpm for 10 min and washed several times with redistilled water. Finally, the obtained catalysts were dispersed in 10 mL water via ultrasonic dispersion. The obtained sample is denoted as Pt1/Pb1. Pt1/Pb2 catalysts were prepared in a similar method except that the amount of Pb. It should be mentioned that the Pt2/Pb1 and Pt materials have to be heated for a longer time until the color becomes brown, because they cannot be reduced in 5 min. Preparation of commercial Pt/C ink 10 mg commercial 20 wt% Pt/C was dispersed in 10 mL water solution under ultrasonic. After that, the catalyst ink (10 μL) was dropped by injector onto the surface of GC electrode and dried in oven.
Results and Discussion
(Insert Fig. 1) The morphology and structure of the as-prepared materials were determined by SEM (Fig. S1 in supporting information) and TEM. Fig. 1 A-E exhibited the TEM images of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt. It is obvious that the branch density and particle diameters of dendritic Pt/Pb are influenced significantly by the molar ratio of the Pb2+ to Pt2+. When Pt/Pb molar ratio is 1:2, the nanodendritic Pt1/Pb2 material with an average particle diameter of 33.0 nm were obtained (Fig. S2 in supporting information), the dendritic nanoparticles consist of loose branches (Fig. 1A). Reducing the amount of Pb2+ to a molar ratio of 1:1, the dendritic Pt1/Pb1 nanoparticles with the average particle size of 25.6 nm were achieved, as shown in the Fig. 1B and 1C. When the amount of Pb2+ was further decreased (Pt : Pb = 2:1), as seen in Fig. 1D, the dendritic Pt2/Pb1 nanoparticles contains dense branches, and the average particle size is 44.2 nm. In the absence of Pb2+, we got the uniform nanodendrites with an average particle size of 24.1 nm (Fig. 1E). Therefore, it can be concluded that Pb2+ affect the composition and morphology of dendritic Pt/Pb nanoparticles. To investigate the role of CTAC in the dendritic Pt/Pb nanoparticle formation, an experiment in the absence of CTAC was carried out. It can be seen clearly from TEM images in Fig. 1F that the CTAC has a significant impact on the morphology and structure of the materials. In the absence of CTAC, aggregated spherical nanoparticles with different size were obtained. As Previous studies have pointed out that PtCl42+ could coordinate with CTAC to form the CTA2[PtCl4] complex, [18] which was favorable for greatly increasing the reaction barrier and leading to the decay of the reduction kinetics of precursors. Besides, it can also be conductive to guide the formation of dendrite-like structure together with the selective adsorption of CTAC on the newly-generated Pt/Pb nuclei. [19] As a result, the dendritic Pt/Pb nanoparticles were
generated by the reaction between AA and the complex. From what is said above, we can draw a conclusion that CTAC not only influence the nanodendritic structures but also act as a disperse agent. (Insert Fig. 2) In order to characterize the structure of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt materials, XRD was carried out. As can be seen in Fig. 2, The XRD pattern of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt materials show four diffraction peaks at ca. 40°, 46°, 67° and 81°, which correspond to the [111], [200], [220], and [311] facets of the fcc Pt. Compared to the standard pure Pt phase (JCPDS: 04-0802), the diffraction peaks of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1 materials at ca. 40° shift to lower 2θ values, indicating the lattice expansion which is due to the partial substitution of Pt by Pb. The average crystallite size of the as-prepared catalysts can be calculated by Scherrer's formula using the [111] diffraction peak: D=
κλ 𝛽𝑐𝑜𝑠𝜃
Where D is the crystallite size of Pt, is a constant with the value of 0.89, represents wavelength of X-ray radiation source, peak position is represented by θ, and β is full width at halfmaximum of the diffraction peaks. The average crystallite sizes of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt nanomaterials are ca. 10.21, 9.91, 10.56, and 9.72 nm. (Insert Fig. 3) The electronic properties and composition of dendritic Pt1/Pb1 nanoparticles were investigated by XPS. As can be clearly seen from Figure 3A, the XPS spectra of Pt 4f exhibits two main peaks located at binding energies (BE) of 74.3 and 70.9 eV, which are attributed to the Pt 4f 7/2
and Pt 4f 5/2 of metallic Pt, whereas the other two minor peaks located at the BE of 72.1 and
75.4 eV can be correspond to the Pt 4f 7/2 and Pt 4f 5/2 of platinum oxide, such as PtO and Pt(OH)2. In comparison with the standard BE value of bulk Pt,[20-22] the Pt 4f peak of dendritic Pt1/Pb1 catalyst shifts slightly to lower values. The Pb 4f spectrum of Pt1/Pb1 are shown in Figure 3B. For the dendritic Pt1/Pb1 nanoparticles, the two peaks at 138.3 and 143.1 eV are correspond to Pb 3f 7/2 and Pb 3f 5/2, respectively. the BE shifts for Pb and Pt in binary Pt/Pb material demonstrates a change in the Pb and Pt electronic structure, which might be due to the electronic interactions between Pb and Pt atomic orbitals, resulting in the electron transfer from Pb to Pt because the electronegativity of Pt is higher than that of Pb.[23] Pt 5d bands are partial filling by the increased electron density, leading to the decrease of the d-band center and the weaken the carbonaceous intermediate adsorption and probably reducing the carbonaceous intermediate poisoning, finally improving the catalytic performance toward ethanol oxidation.[24] The XPS results show that the Pt/Pb alloy has been formed, such bimetallic alloys in other reports also exhibit the similar changes in BE.[2, 25] (Insert Fig. 4) Figure 4A shows CV curves of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt and Pt/C catalysts in1.0 M KOH. All the catalysts exhibit a pair of peaks from −0.9 to −0.6 V, which correspond to the Pt−Hads (eq 1): Pt + H2 O + e− ↔ (Pt − H𝑎𝑑𝑠 ) + OH−
(1)
The peak observed between −0.5 V and 0.3 V in anodic scanning is assigned to the generation of oxides. During the positive scan, Pt−OHads species are formed by the adsorption of OH− ions (eq 2). At higher potentials, Pt-O are formed by the transformation of Pt−OHads species (eq 3); in the negative scan from 0 to −0.3 V, the peak corresponds to the reduction peak of Pt oxide, according
to (eq 4): Pt + OH− → (Pt − OH𝑎𝑑𝑠 ) + e−
(2)
(Pt − OH𝑎𝑑𝑠 ) + OH− → (Pt − O) + H2 O + e−
(3)
(Pt − O) + H2 O + 2e− → Pt + 2OH−
(4)
The electrochemical surface areas (ECSA) of the as-prepared catalysts are calculated by the changes in the reduction region of Pt–O, according to equation 5: Q
ECSA = 0.21×m (5) Where Q represents the reduction charge of Pt-O, m represents the loading amount of Pt, 0.21 mC cm−2 is the electrical charge associated with monolayer Pt–O reduction on Pt surface. Previous studies have pointed that a larger ECSA is in favor of the catalytic activity for the oxidation of ethanol.[24] ECSA of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt and Pt/C is estimated to be 268.6, 219.7, 100.9, 63.8 and 133.4 cm2/mg, respectively. The larger ECSA value of Pt1/Pb1 may originates from the electronic effect and the unique structure. To evaluate the electrocatalytic activities of the Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt and Pt/C catalysts, the CV were carried out in a 1.0 M ethanol + 1.0 M KOH solution at a scan rate of 50 mV s−1. In order to directly use the current density to compare the electrocatalytic activity of different catalysts, the current density was normalized on the basis of the loading mass of Pt. It can be seen in figure 4B, there are two well-defined oxidation peaks: one in the forward scan at ca. -0.17V corresponding to the oxidation of freshly chemisorbed species coming from ethanol adsorption, whereas the other one in the negative scan is still under debate. Some investigators consider that the current peak during the forward scan is primarily ascribed to the removal of carbonaceous intermediate species formed in the forward scan,[26, 27] while others maintain that
this current peak is still related to the electrooxidation of ethanol.[28, 29] Thus, we use the current density in the forward scan to compare the electrocatalytic activity of the catalyst. It can be clearly seen that the Pt1/Pb1 catalyst possesses the highest electrocatalytic activity for ethanol oxidation, and all the PtPb binary catalysts (Pt1/Pb2, Pt1/Pb1, Pt2/Pb1) show significant enhancements over that of pure Pt. Particularly, the value of the current density in Pt1/Pb1 catalyst reaches 2065.2 mA mg−1, which is about 1.5, 3.4, 10.8 and 10.4 times greater than those of Pt1/Pb2 (1371.4 mA mg−1), Pt2/Pb1 (610.0 mA mg−1), Pt (190.6 mA mg−1), and Pt/C (199.4 mA mg−1), respectively. The improved electrocatalytic activity of bimetallic Pt/Pb nanoparticles over bulk Pt could be ascribed to the following explanations: (i) due to the lower electronegativity of Pb, the electron transfer from Pb to Pt changed the electronic structure of Pt, and under the same conditions, the adsorption energy of the Pt–COads was higher than the corresponding Pt/Pb–COads. Therefore, the electrocatalysts can avoid poison effectively by carbonaceous intermediate at the same time enhancing the electrocatalytic activity.[10] (ii) According to the d-band theory of Hammer−Nørskov, when Pb is combined with Pt, the d-band center of Pt will move up, because the lattice constant of Pt (3.92 Å) is smaller than that of Pb (4.95Å), this result is consistent with the XPS, which exhibits the negative shift of the BE for Pt in bimetallic Pt/Pb nanocatalysts. This can boost the adsorption of hydroxyl, which would promote the CH3COads oxide and finally help to improve the ethanol oxidation reaction.[24] However, when the content of platinum continues to increase, the electroactivity of the catalysts would decrease. This may be due to the excess adsorption of OH−, which creates a competition with the adsorption of ethanol. Generally, the bimetallic synergistic effect and the unique nanostructure resulted in the enhanced catalytic performance of the Pt1/Pb1 nanodendrites for the ethanol oxidation reaction under alkaline
conditions. He et al. demonstrated the mechanism of ethanol oxidation reaction in alkaline conditions as shown in the following:[30] C2 H5 OH + 12OH− → 2CO2 + 9H2 O + 12e− Pb + OH − → Pb– OH𝑎𝑑𝑠 + e− 2Pt + C2 H5 OH → Pt– H + Pt– (C2 H5 O)𝑎𝑑𝑠 Pt– (C2 H5 O)𝑎𝑑𝑠 + xPb– OH𝑎𝑑𝑠 → Pt– (C2 H5−𝑥 O)𝑎𝑑𝑠 – Pbx + xH2 O Pt– CH𝑥 CO𝑎𝑑𝑠 – Pb + Pb– OH𝑎𝑑𝑠 → CO2 + H2 O + Pt + Pb The long-term stability of the synthesized Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt and Pt/C catalysts for ethanol oxidation were investigated by chronoamperometric measurements at a constant potential of −0.15 V for 1 hour in 1.0 M ethanol and 1.0 M KOH solution, and the results are displayed in Figure 4C. In the initial period, the current densities of all the catalysts decay rapidly, which may be due to the accumulation of poisoning intermediate species such as COads produced during the process of ethanol electrooxidation reaction in alkaline media. At the end of 3600 s, dendritic Pt1/Pb1 nanocatalyst still shows the highest current density among all the prepared catalysts. The above result demonstrates that the porous structure Pt1/Pb1 catalyst possesses the best activity and stability toward ethanol oxidation reaction. (Insert Fig. 5) As is well known, CV methods can explore the transport characteristics of alcohol oxidation reaction.[31] As shown in Figure 5A, the CV of the Pt1/Pb1 nanocatalyst was carried out in 1.0 M C2H5OH + 1.0 M KOH solution at different scan rates. In the positive-going scan, the peak current density of the catalyst is decreased with the decrease of scan rate. There is a linear relationship between the anodic peak current density and square root of scan rate. The above result
demonstrates that the kinetic reaction of electron transfer for ethanol oxidation at the Pt1/Pb1 electrocatalyst is a diffusion control process. (Insert Fig. 6) Researching the interfacial process and electrode reaction kinetics in the electrochemical system can assist us in understanding the reaction processes profoundly.[32-34] For this reason, we conducted the electrochemical impedance spectroscopy (EIS) for the Pt1/Pb2, Pt1/Pb1, Pt2/Pb1 and Pt catalysts in 1.0 M C2H5OH + 1.0 M KOH. It has also been widely recognized that the diameter of the primary semicircle is a critical parameter to measure the charge-transfer resistance of the as-obtained electrocatalyst, all the curves display similar shapes in the low frequency region, and the smaller diameter of the impedance arc (DIA) means the smaller charge-transfer resistance for the ethanol oxidation reaction.[35, 36] Evidently, the DIA in Fig.6 exhibits the sequence as follow: Pt1/Pb1 < Pt1/Pb2 < Pt2/Pb1 < Pt, demonstrating that this novel dendritic Pt1/Pb1 presents the smallest electron transfer resistance (Rct) with enhanced charge transfer kinetics and good electrical conductivity for ethanol oxidation reaction, which is consistent with the above-mentioned outstandingly excellent catalytic performances.
Conclusion In summary, a rapid synthesis procedure has been demonstrated to successfully synthesize binary Pt/Pb nanodendrites with tunable atomic ratios. Benefitting from the unique nanodendrites structure, highly exposed surface active areas, synergistic effect and electronic effect between Pt and Pb, such Pt/Pb nanodendrites with optimum component displayed much higher mass activity and durability towards ethanol oxidation reaction in the alkaline media than pure Pt and Pt/C,
suggesting that such Pt/Pb nanodendrites can be well applied as a novel cost-efficient electrocatalyst for commercial applications future practical fuel-cells. We trust that our efforts in this work can supply a new synthesized strategy for the creation of cost-efficient anode electrocatalysts with both excellent electrocatalytic activity and durability.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51373111), the Suzhou Industry (SYG201636), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials Notes.
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Fig 1. TEM images of Pt1/Pb2 (A), Pt1/Pb1 (B, C), Pt2/Pb1 (D), Pt (E), Pt1/Pb1 (F) prepared without CTAC. Fig 2. XRD patterns of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt catalysts. The black vertical lines represent the reference peaks of Pt. Fig 3. XPS spectra of Pt1/Pb1 in Pt (b) and Pb (c) region. Fig 4. The CV curves of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt and Pt/C catalysts in 1.0 M KOH (A) and 1.0 M KOH + 1.0 M C2H5OH solution (B) at a scan rate of 50 mV s−1. (C) Chronoamperometric curves of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt and Pt/C catalysts in 1.0 M KOH +1.0 M C2H5OH solution at an electrode potential of -0.15 V. Fig 5. (A) CVs of the Pt1/Pb1 electrode in 1.0 M C2H5OH + 1.0 M KOH solution at scan rates from 30 to 120 mV s-1. (B) The plots of anodic peak currents to the scan rates. Fig 6. Nyquist plots of ethanol oxidation on Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt electrodes in 1.0 M C2H5OH+1.0 M KOH solution.
Fig. 1
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S0169-4332(17)31925-6 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.270 APSUSC 36470
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Received date: Revised date: Accepted date:
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Please cite this article as: Ke Zhang, Hui Xu, Bo Yan, Jin Wang, Zhulan Gu, Yukou Du, Rapid synthesis of dendritic Pt/Pb nanoparticles and their electrocatalytic performance toward ethanol oxidation, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.270 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Rapid synthesis of dendritic Pt/Pb nanoparticles and their electrocatalytic performance toward ethanol oxidation
Ke Zhang, Hui Xu, Bo Yan, Jin Wang, Zhulan Gu, Yukou Du*
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Industrial Park, Ren'ai Road, Suzhou, 215123, China. E-mail: [email protected]
Highlights Pt/Pb nanodendrites have been prepared by heating in an oil bath for 5 min. Pt1/Pb1 nanodendrites exhibit enhanced electrocatalytic performance and stability. The presence of Pb enhance the Pt catalytic activity toward ethanol oxidation.
Abstract This article reports a rapid synthetic method for the preparation of dendritic platinum−lead bimetallic catalysts by using an oil bath for 5 min in the presence of hexadecyltrimethylammonium chloride (CTAC) and ascorbic acid (AA). CTAC acts as a shapedirection agent, and AA acts as a reducing agent during the reaction process. A series of physical techniques are used to characterize the morphology, structure and electronic properties of the
dendritic Pt/Pb nanoparticles, indicating the Pt/Pb dendrites are porous, highly alloying, and selfsupported nanostructures. Various electrochemical techniques were also investigated the catalytic performance of the Pt/Pb catalysts toward the ethanol electrooxidation reaction. Cyclic
voltammetry and chronoamperometry indicated that the synthesized dendritic Pt/Pb
nanoparticles possessed much higher electrocatalytic performance than bulk Pt catalyst. This study may inspire the engineering of dendritic bimetallic catalysts, which are expected to have great potential applications in fuel cells.
Keywords: Rapid synthesis; Platinum−lead; Dendrite; Ethanol oxidation.
Introduction As a promising future power source, direct fuel cells have many advantages, such as facile storage, high power density, environment friendly and easy refilling.[1] Direct ethanol fuel cells (DEFC) have attracted widespread concerns in the area of direct fuel cells due to the distinct advantages, including high energy density, nontoxicity, abundance, easy availability, low cost, safety for storage, etc.[2] It has been well-known that Pt has good catalytic activity toward ethanol electro-oxidation. However, the relatively high price, scarcity and sluggish reaction kinetics of Pt make its excellent catalysis property cannot to be widely applied in DEFCs.[3] Bimetallic nanoparticles play a vital role in many areas of science and technology, such as batteries, solar fuel, supercapacitors and fuel cells.[4] As a general rule, bimetallic nanoparticles perform better than monometallic nanoparticles because there is a synergistic effect between the two elements. So, recent research efforts have focused extensively on the development of less expensive Pt–based materials. Great progresses has been achieved toward enhancing the electrocatalytic activity of ethanol on Pt-based bimetallic electrocatalysts, such as Pt/Co,[5] Pt/Pb,[6] Pt/Pd,[7] Pt/Au,[8, 9] etc. Among these Pt–based electrocatalysts, Pt/Pb material may be
a good choice to improve the electrocatalytic activity, because this element can provide oxygencontaining species that accelerate the carbon intermediates oxidation. What’s more, the electronic structure of platinum can be changed by the entrance of Pb, which reduce the CO adsorption energy on the catalyst surface.[10] It is widely acknowledged that the size, shape, and composition of the nanomaterials can also influence its catalytic properties.[11] Up to now, a variety of Pt/Pb nanoparticles with different morphologies and structures, including wires,[10] nanoplate,[12] nanoflowers[13] and core-shell structure[14] have been synthesized successfully. The synthesized Pt-based materials with a dendritic nanostructure generally exhibit an enhanced electrocatalytic performance for direct fuel cells, because the edge and corner atoms are more abundant on dendritic structure, which could be more activity than particles with semi-spherical and low-index facets structure.[15] For example, Wang et al. have successfully synthesized the graphene nanosheet supported 3D Pt-on-Pd bimetallic nanodendrite hybrids, the as-prepared materials have a much higher electrocatalytic performance than conventional Pt/C catalysts for methanol electrooxidation.[16] Su et al. presented a facile seeded growth method to synthesis MoS2 nanosheets supported Pt shell and Au core bimetallic nanodendrites, the as-prepared nanomaterials showed better catalytic performance for methanol electrooxidation than that of the Pt catalyst.[17] As far as is known, there is few literature and studies on Pt/Pb nanodendrites toward ethanol oxidation in alkaline environment. Herein, we successfully prepared the dendritic Pt/Pb nanoparticles in aqueous solution with the assistance of hexadecyltrimethylammonium chloride and ascorbic acid, in which hexadecyltrimethylammonium chloride as the disperse agent and morphology-directing agent, and ascorbic acid acts as the reducing agent. The as-prepared catalyst displays a significantly
improved catalytic activity and superior durability toward ethanol electrooxidation reaction at room temperature as compared to the commercial Pt/C catalyst in alkaline environmental. The fabricated Pt/Pb nanoparticles may serve as a kind of potential anodic catalyst applied in the area of DEFC.
Experimental Materials and Chemicals H2PtCl6, Pb(OAc)2•3H2O, hexadecyltrimethylammonium chloride (CTAC), C2H5OH, KOH and ascorbic acid (AA) were obtained from Sinopharm Chemical Reagent Co, Ltd (Shanghai, P.R. China). Commercial 20 wt% Pt/C was obtained from Shanghai Hesen Electric Co., Ltd. All reagents were used without any further purification before use. Doubly distilled water was used throughout the work Apparatus The size and shape of the as-prepared materials were investigated by transmission electron microscopy (TEM) measurements on a Tecnai G220 electron microscope operating at an accelerating voltage of 200 kV. The chemical composition and morphology were investigated by Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis using the S4700 system (Hitachi High Technologies Corporation, Japan). The catalysts were analyzed by Xray diffraction (XRD) on a PANalytical X’Pert PRO MRD X-ray diffractometer employing CuK radiation (= 1.54056 Å). X-Ray photoelectron spectroscopy (XPS) was performed using an ESCALab220i-XL electron spectrometer from VG Scientific employing 300 W AlK radiation. The electrochemical experiments were performed at room temperature in a CHI 760D
electrochemical station (Shanghai Chenhua Instrumental Co., Ltd, China) with a conventional three-electrode system. The working electrode was prepared using a glassy carbon electrode (GCE) with a diameter of 3 mm. A Pt wire and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively.
Synthesis of dendritic Pt/Pb 1mL H2PtCl6 (7.723 mM ) aqueous solution, 3 mg Pb(OAc)2•3H2O, 64 mg CTAC was dispersed into 4 mL H2O to form a uniform solution by ultrasonication. After that, 9 mg AA was added into the mixture with ultrasonic for another 10 minutes. Subsequently, the mixture was heated in a preheated oil bath at 368K for 5 minutes under constant stirring and then cool down to room temperature without disturb. The solution was centrifuged at 6000 rpm for 10 min and washed several times with redistilled water. Finally, the obtained catalysts were dispersed in 10 mL water via ultrasonic dispersion. The obtained sample is denoted as Pt1/Pb1. Pt1/Pb2 catalysts were prepared in a similar method except that the amount of Pb. It should be mentioned that the Pt2/Pb1 and Pt materials have to be heated for a longer time until the color becomes brown, because they cannot be reduced in 5 min. Preparation of commercial Pt/C ink 10 mg commercial 20 wt% Pt/C was dispersed in 10 mL water solution under ultrasonic. After that, the catalyst ink (10 μL) was dropped by injector onto the surface of GC electrode and dried in oven.
Results and Discussion
(Insert Fig. 1) The morphology and structure of the as-prepared materials were determined by SEM (Fig. S1 in supporting information) and TEM. Fig. 1 A-E exhibited the TEM images of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt. It is obvious that the branch density and particle diameters of dendritic Pt/Pb are influenced significantly by the molar ratio of the Pb2+ to Pt2+. When Pt/Pb molar ratio is 1:2, the nanodendritic Pt1/Pb2 material with an average particle diameter of 33.0 nm were obtained (Fig. S2 in supporting information), the dendritic nanoparticles consist of loose branches (Fig. 1A). Reducing the amount of Pb2+ to a molar ratio of 1:1, the dendritic Pt1/Pb1 nanoparticles with the average particle size of 25.6 nm were achieved, as shown in the Fig. 1B and 1C. When the amount of Pb2+ was further decreased (Pt : Pb = 2:1), as seen in Fig. 1D, the dendritic Pt2/Pb1 nanoparticles contains dense branches, and the average particle size is 44.2 nm. In the absence of Pb2+, we got the uniform nanodendrites with an average particle size of 24.1 nm (Fig. 1E). Therefore, it can be concluded that Pb2+ affect the composition and morphology of dendritic Pt/Pb nanoparticles. To investigate the role of CTAC in the dendritic Pt/Pb nanoparticle formation, an experiment in the absence of CTAC was carried out. It can be seen clearly from TEM images in Fig. 1F that the CTAC has a significant impact on the morphology and structure of the materials. In the absence of CTAC, aggregated spherical nanoparticles with different size were obtained. As Previous studies have pointed out that PtCl42+ could coordinate with CTAC to form the CTA2[PtCl4] complex, [18] which was favorable for greatly increasing the reaction barrier and leading to the decay of the reduction kinetics of precursors. Besides, it can also be conductive to guide the formation of dendrite-like structure together with the selective adsorption of CTAC on the newly-generated Pt/Pb nuclei. [19] As a result, the dendritic Pt/Pb nanoparticles were
generated by the reaction between AA and the complex. From what is said above, we can draw a conclusion that CTAC not only influence the nanodendritic structures but also act as a disperse agent. (Insert Fig. 2) In order to characterize the structure of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt materials, XRD was carried out. As can be seen in Fig. 2, The XRD pattern of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt materials show four diffraction peaks at ca. 40°, 46°, 67° and 81°, which correspond to the [111], [200], [220], and [311] facets of the fcc Pt. Compared to the standard pure Pt phase (JCPDS: 04-0802), the diffraction peaks of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1 materials at ca. 40° shift to lower 2θ values, indicating the lattice expansion which is due to the partial substitution of Pt by Pb. The average crystallite size of the as-prepared catalysts can be calculated by Scherrer's formula using the [111] diffraction peak: D=
κλ 𝛽𝑐𝑜𝑠𝜃
Where D is the crystallite size of Pt, is a constant with the value of 0.89, represents wavelength of X-ray radiation source, peak position is represented by θ, and β is full width at halfmaximum of the diffraction peaks. The average crystallite sizes of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt nanomaterials are ca. 10.21, 9.91, 10.56, and 9.72 nm. (Insert Fig. 3) The electronic properties and composition of dendritic Pt1/Pb1 nanoparticles were investigated by XPS. As can be clearly seen from Figure 3A, the XPS spectra of Pt 4f exhibits two main peaks located at binding energies (BE) of 74.3 and 70.9 eV, which are attributed to the Pt 4f 7/2
and Pt 4f 5/2 of metallic Pt, whereas the other two minor peaks located at the BE of 72.1 and
75.4 eV can be correspond to the Pt 4f 7/2 and Pt 4f 5/2 of platinum oxide, such as PtO and Pt(OH)2. In comparison with the standard BE value of bulk Pt,[20-22] the Pt 4f peak of dendritic Pt1/Pb1 catalyst shifts slightly to lower values. The Pb 4f spectrum of Pt1/Pb1 are shown in Figure 3B. For the dendritic Pt1/Pb1 nanoparticles, the two peaks at 138.3 and 143.1 eV are correspond to Pb 3f 7/2 and Pb 3f 5/2, respectively. the BE shifts for Pb and Pt in binary Pt/Pb material demonstrates a change in the Pb and Pt electronic structure, which might be due to the electronic interactions between Pb and Pt atomic orbitals, resulting in the electron transfer from Pb to Pt because the electronegativity of Pt is higher than that of Pb.[23] Pt 5d bands are partial filling by the increased electron density, leading to the decrease of the d-band center and the weaken the carbonaceous intermediate adsorption and probably reducing the carbonaceous intermediate poisoning, finally improving the catalytic performance toward ethanol oxidation.[24] The XPS results show that the Pt/Pb alloy has been formed, such bimetallic alloys in other reports also exhibit the similar changes in BE.[2, 25] (Insert Fig. 4) Figure 4A shows CV curves of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt and Pt/C catalysts in1.0 M KOH. All the catalysts exhibit a pair of peaks from −0.9 to −0.6 V, which correspond to the Pt−Hads (eq 1): Pt + H2 O + e− ↔ (Pt − H𝑎𝑑𝑠 ) + OH−
(1)
The peak observed between −0.5 V and 0.3 V in anodic scanning is assigned to the generation of oxides. During the positive scan, Pt−OHads species are formed by the adsorption of OH− ions (eq 2). At higher potentials, Pt-O are formed by the transformation of Pt−OHads species (eq 3); in the negative scan from 0 to −0.3 V, the peak corresponds to the reduction peak of Pt oxide, according
to (eq 4): Pt + OH− → (Pt − OH𝑎𝑑𝑠 ) + e−
(2)
(Pt − OH𝑎𝑑𝑠 ) + OH− → (Pt − O) + H2 O + e−
(3)
(Pt − O) + H2 O + 2e− → Pt + 2OH−
(4)
The electrochemical surface areas (ECSA) of the as-prepared catalysts are calculated by the changes in the reduction region of Pt–O, according to equation 5: Q
ECSA = 0.21×m (5) Where Q represents the reduction charge of Pt-O, m represents the loading amount of Pt, 0.21 mC cm−2 is the electrical charge associated with monolayer Pt–O reduction on Pt surface. Previous studies have pointed that a larger ECSA is in favor of the catalytic activity for the oxidation of ethanol.[24] ECSA of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt and Pt/C is estimated to be 268.6, 219.7, 100.9, 63.8 and 133.4 cm2/mg, respectively. The larger ECSA value of Pt1/Pb1 may originates from the electronic effect and the unique structure. To evaluate the electrocatalytic activities of the Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt and Pt/C catalysts, the CV were carried out in a 1.0 M ethanol + 1.0 M KOH solution at a scan rate of 50 mV s−1. In order to directly use the current density to compare the electrocatalytic activity of different catalysts, the current density was normalized on the basis of the loading mass of Pt. It can be seen in figure 4B, there are two well-defined oxidation peaks: one in the forward scan at ca. -0.17V corresponding to the oxidation of freshly chemisorbed species coming from ethanol adsorption, whereas the other one in the negative scan is still under debate. Some investigators consider that the current peak during the forward scan is primarily ascribed to the removal of carbonaceous intermediate species formed in the forward scan,[26, 27] while others maintain that
this current peak is still related to the electrooxidation of ethanol.[28, 29] Thus, we use the current density in the forward scan to compare the electrocatalytic activity of the catalyst. It can be clearly seen that the Pt1/Pb1 catalyst possesses the highest electrocatalytic activity for ethanol oxidation, and all the PtPb binary catalysts (Pt1/Pb2, Pt1/Pb1, Pt2/Pb1) show significant enhancements over that of pure Pt. Particularly, the value of the current density in Pt1/Pb1 catalyst reaches 2065.2 mA mg−1, which is about 1.5, 3.4, 10.8 and 10.4 times greater than those of Pt1/Pb2 (1371.4 mA mg−1), Pt2/Pb1 (610.0 mA mg−1), Pt (190.6 mA mg−1), and Pt/C (199.4 mA mg−1), respectively. The improved electrocatalytic activity of bimetallic Pt/Pb nanoparticles over bulk Pt could be ascribed to the following explanations: (i) due to the lower electronegativity of Pb, the electron transfer from Pb to Pt changed the electronic structure of Pt, and under the same conditions, the adsorption energy of the Pt–COads was higher than the corresponding Pt/Pb–COads. Therefore, the electrocatalysts can avoid poison effectively by carbonaceous intermediate at the same time enhancing the electrocatalytic activity.[10] (ii) According to the d-band theory of Hammer−Nørskov, when Pb is combined with Pt, the d-band center of Pt will move up, because the lattice constant of Pt (3.92 Å) is smaller than that of Pb (4.95Å), this result is consistent with the XPS, which exhibits the negative shift of the BE for Pt in bimetallic Pt/Pb nanocatalysts. This can boost the adsorption of hydroxyl, which would promote the CH3COads oxide and finally help to improve the ethanol oxidation reaction.[24] However, when the content of platinum continues to increase, the electroactivity of the catalysts would decrease. This may be due to the excess adsorption of OH−, which creates a competition with the adsorption of ethanol. Generally, the bimetallic synergistic effect and the unique nanostructure resulted in the enhanced catalytic performance of the Pt1/Pb1 nanodendrites for the ethanol oxidation reaction under alkaline
conditions. He et al. demonstrated the mechanism of ethanol oxidation reaction in alkaline conditions as shown in the following:[30] C2 H5 OH + 12OH− → 2CO2 + 9H2 O + 12e− Pb + OH − → Pb– OH𝑎𝑑𝑠 + e− 2Pt + C2 H5 OH → Pt– H + Pt– (C2 H5 O)𝑎𝑑𝑠 Pt– (C2 H5 O)𝑎𝑑𝑠 + xPb– OH𝑎𝑑𝑠 → Pt– (C2 H5−𝑥 O)𝑎𝑑𝑠 – Pbx + xH2 O Pt– CH𝑥 CO𝑎𝑑𝑠 – Pb + Pb– OH𝑎𝑑𝑠 → CO2 + H2 O + Pt + Pb The long-term stability of the synthesized Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt and Pt/C catalysts for ethanol oxidation were investigated by chronoamperometric measurements at a constant potential of −0.15 V for 1 hour in 1.0 M ethanol and 1.0 M KOH solution, and the results are displayed in Figure 4C. In the initial period, the current densities of all the catalysts decay rapidly, which may be due to the accumulation of poisoning intermediate species such as COads produced during the process of ethanol electrooxidation reaction in alkaline media. At the end of 3600 s, dendritic Pt1/Pb1 nanocatalyst still shows the highest current density among all the prepared catalysts. The above result demonstrates that the porous structure Pt1/Pb1 catalyst possesses the best activity and stability toward ethanol oxidation reaction. (Insert Fig. 5) As is well known, CV methods can explore the transport characteristics of alcohol oxidation reaction.[31] As shown in Figure 5A, the CV of the Pt1/Pb1 nanocatalyst was carried out in 1.0 M C2H5OH + 1.0 M KOH solution at different scan rates. In the positive-going scan, the peak current density of the catalyst is decreased with the decrease of scan rate. There is a linear relationship between the anodic peak current density and square root of scan rate. The above result
demonstrates that the kinetic reaction of electron transfer for ethanol oxidation at the Pt1/Pb1 electrocatalyst is a diffusion control process. (Insert Fig. 6) Researching the interfacial process and electrode reaction kinetics in the electrochemical system can assist us in understanding the reaction processes profoundly.[32-34] For this reason, we conducted the electrochemical impedance spectroscopy (EIS) for the Pt1/Pb2, Pt1/Pb1, Pt2/Pb1 and Pt catalysts in 1.0 M C2H5OH + 1.0 M KOH. It has also been widely recognized that the diameter of the primary semicircle is a critical parameter to measure the charge-transfer resistance of the as-obtained electrocatalyst, all the curves display similar shapes in the low frequency region, and the smaller diameter of the impedance arc (DIA) means the smaller charge-transfer resistance for the ethanol oxidation reaction.[35, 36] Evidently, the DIA in Fig.6 exhibits the sequence as follow: Pt1/Pb1 < Pt1/Pb2 < Pt2/Pb1 < Pt, demonstrating that this novel dendritic Pt1/Pb1 presents the smallest electron transfer resistance (Rct) with enhanced charge transfer kinetics and good electrical conductivity for ethanol oxidation reaction, which is consistent with the above-mentioned outstandingly excellent catalytic performances.
Conclusion In summary, a rapid synthesis procedure has been demonstrated to successfully synthesize binary Pt/Pb nanodendrites with tunable atomic ratios. Benefitting from the unique nanodendrites structure, highly exposed surface active areas, synergistic effect and electronic effect between Pt and Pb, such Pt/Pb nanodendrites with optimum component displayed much higher mass activity and durability towards ethanol oxidation reaction in the alkaline media than pure Pt and Pt/C,
suggesting that such Pt/Pb nanodendrites can be well applied as a novel cost-efficient electrocatalyst for commercial applications future practical fuel-cells. We trust that our efforts in this work can supply a new synthesized strategy for the creation of cost-efficient anode electrocatalysts with both excellent electrocatalytic activity and durability.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51373111), the Suzhou Industry (SYG201636), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials Notes.
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Fig 1. TEM images of Pt1/Pb2 (A), Pt1/Pb1 (B, C), Pt2/Pb1 (D), Pt (E), Pt1/Pb1 (F) prepared without CTAC. Fig 2. XRD patterns of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt catalysts. The black vertical lines represent the reference peaks of Pt. Fig 3. XPS spectra of Pt1/Pb1 in Pt (b) and Pb (c) region. Fig 4. The CV curves of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt and Pt/C catalysts in 1.0 M KOH (A) and 1.0 M KOH + 1.0 M C2H5OH solution (B) at a scan rate of 50 mV s−1. (C) Chronoamperometric curves of Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt and Pt/C catalysts in 1.0 M KOH +1.0 M C2H5OH solution at an electrode potential of -0.15 V. Fig 5. (A) CVs of the Pt1/Pb1 electrode in 1.0 M C2H5OH + 1.0 M KOH solution at scan rates from 30 to 120 mV s-1. (B) The plots of anodic peak currents to the scan rates. Fig 6. Nyquist plots of ethanol oxidation on Pt1/Pb2, Pt1/Pb1, Pt2/Pb1, Pt electrodes in 1.0 M C2H5OH+1.0 M KOH solution.
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