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Pt nanoparticles incorporated into phosphorus-doped ordered mesoporous carbons: enhanced catalytic activity for methanol electrooxidation
Electrochimica Acta 127 (2014) 307–314
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Pt nanoparticles incorporated into phosphorus-doped ordered mesoporous carbons: enhanced catalytic activity for methanol electrooxidation Pei Song, Liande Zhu, Xiangjie Bo, Aixia Wang, Guang Wang, Liping Guo ∗ Faculty of Chemistry, Northeast Normal University, Changchun, 130024, P R China
a r t i c l e
i n f o
Article history: Received 30 November 2013 Received in revised form 14 February 2014 Accepted 14 February 2014 Available online 26 February 2014 Keywords: Methanol Electrooxidation Pt nanoparticles Phosphorus-doped ordered mesoporous carbons Electrocatalyst Direct methanol fuel cell
a b s t r a c t Phosphorus-doped ordered mesoporous carbons (POMCs) with different P content are successfully synthesized by hard template method using SBA-15 as hard template, sucrose as carbon precursor and triphenylphosphane as phosphorus precursor. Pt nanoparticles with size of 3.5 ± 0.4 nm are deposited on the framework of POMCs. The doping of P into OMCs facilitates the dispersion of Pt nanoparticles and accelerates the formation of oxygen-containing functional groups. Pt/POMCs nanocomposites were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, nitrogen adsorption–desorption and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry and chronoamperometry studies exhibit that the Pt/POMCs, especially Pt/P7 OMCs, have larger electrochemical active surface area (ECSA), higher electrocatalytic activity, more negative onset potential and long-time stability for the electrooxidation toward methanol than that of Pt/OMCs, PtRu/XC and commercial Pt/C catalysts. These enhanced performances indicate that Pt/P7 OMCs catalyst may be an excellent anode catalyst for direct methanol fuel cell (DMFC). © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Great efforts have been made to design and develop direct methanol fuel cell (DMFC) as energy sources owning to its high energy density, easily handling liquid fuel, quick start-up and environmental friendly [1,2]. Anode catalysts of DMFC can activate electrochemical reactions, leading to increase the rate of fuel oxidation and oxygen reduction [3]. Therefore, the preparation and performance of anode catalysts are research priorities in the development of DMFC. However, as one of the exclusive anode catalyst for methanol electro-oxidation, commercial Pt/C catalyst is still insufficient activities and easily poisoned by reaction intermediates such as CO [4–6]. This is mainly attributed to the poor methanol electro-oxidation kinetics [5,7], low catalyst efficiency and CO poisoning [6,8,9] of the catalysts employed for the methanol oxidation reaction (MOR) in the anode. To overcome the problems mentioned above, it is necessary to develop novel electrocatalysts with much enhanced methanol oxidation activity/stability and higher CO-tolerance. It has been
∗ Corresponding author. Tel.: +86 0431 85099762; fax: +86 0413 85099762. E-mail addresses: [email protected] (X. Bo), [email protected] (L. Guo). http://dx.doi.org/10.1016/j.electacta.2014.02.068 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
reported that Pt-based or Pt alloy catalysts play a significant role in improving electrocatalytic activity and stability, such as Pt/CNTs [6], Pt/OMCs [10], Pt/N-doped graphene [11], Pt-Ni-P CNTs [12], Pt–Ru–P [13] and Pt–Sn–P/C [14]. To date, the precious metals like Pt supported on the heteroatom-doped carbon materials as anode electrocatalysts to improve electrocatalytic activity of MOR have attracted a tremendous attention of researchers. It has been proved that the performance of catalyst is closely associated with the nature of the supporting materials [15]. The size, morphology and distribution of the Pt particles also have a close relationship with the supporting materials. Therefore, special attention has been paid to use the various carbon materials including graphene [16], carbon nanotubes [17], activated carbon fibers [18,19], ordered mesoporous carbons (OMCs) [7,20] and other carbonaceous materials. Compared to other carbon materials, OMCs as electrocatalyst support for fuel cells has been reported [21,22] due to its excellent characteristics such as high surface area, tunable pore size and large pore volume with narrow pore size distribution. These super features promote the facile molecular transport of reactants and products [23,24]. Heteroatoms such as N, P, S [11,12,25,26] doping into carbon materials offer a good choice to increase the catalytic performance of Pt-based catalysts in DMFC. Heteroatoms bonded with carbon framework can introduce defect sites due to different bond length,
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electronegativity and atomic size, and thereby can induce uneven charge distribution, which have significant effects on some of the properties of catalyst such as catalytic property [20,27]. Phosphorus, as one of the N-group elements, often shows similar properties with nitrogen [26]. The same as N atom, P has a larger atomic radius and higher electron-donating ability which make it an astute choice as a dopant to carbon materials [20]. It also has been reported that the addition of P can drastically reduce the size of Pt nanoparticles (NPs) [13,14] and improve the distribution of Pt NPs on carbon support. Furthermore, the introduction of P may largely increase the number of oxygen-containing functional groups, which promotes the CO-tolerance towards MOR [12,20,26]. Recently, P-doped multiwalled nanotubes [28] and P-doped OMCs [20] exhibit high electro-catalytic activity for the oxygen reduction reaction (ORR). Pt supported on P-doped CNTs [26] and Pt-Ni-P composites [12] illustrate better activities toward the MOR. Thence, by the synergistic effect of the suitable carbon support OMCs, doping heteroatom P and small size Pt NPs, we expected to obtain an electrocatalyst with excellent catalytic activity, better CO-tolerance and much higher stability. In this work, Pt/POMCs as efficient anode electrocatalysts for MOR were synthesized using hard template method. Pt NPs incorporated inside the P-doped ordered mesoporous carbons (Pt/POMCs) via a simple impregnation–reduction method using formic acid as reductant. Both the physical and electrochemical properties of Pt/POMCs were studied. Pt/P7 OMCs was found to be an excellent electrocatalyst with higher electrochemical active surface area (ECSA), much smaller size of Pt NPs and sharply enhanced electrochemical properties. 2. Experimental 2.1. Reagents and apparatus Pluronic P123 (non-ionic triblock copolymer, PEO20 PPO70 PEO20 ) and Nafion (5 wt%) were purchased from Sigma-Aldrich. Triphenylphosphine and H2 PtCl6 ·6H2 O were purchased from Aladdin and Sinopharm Chemical Reagent Co. Ltd. All other reagents used were of analytical grade and used as received without further purification. The morphologies of the synthesized samples were characterized by powder X-ray diffraction (XRD). XRD was obtained on a an X-ray D/max-2200vpc (Rigaku Corporation, Japan) instrument operated at 40 kV and 20 mA using Cu K␣ radiation (k␣ = 0.15406 nm). Transmission electron microscopy (TEM) (JEOL, JEM-2100F) was operated at 200 kV. Thermogravimetric analysis (TGA) was performed on a PerkinElmer Diamond TG Analyzer. The Pt content was determined by TGA. The surface area and pore size of the synthesized OMCs and POMCs were determined by N2 adsorption-desorption isotherms (ASAP 2020) and were obtained from the Brunauer-Emmett-Teller (BET) and BarrettJoyner-Halenda (BJH) method, respectively. X-ray photoelectron spectroscopy (XPS) was measured using Thermo ESCA LAB spectrometer (USA). The binding energy was referenced to C 1s line at 284.6 eV for calibration. The error of the data represents the standard deviation of triplicate measurements. 2.2. Electrochemical measurements Electrochemical measurements were carried out in a conventional three electrode cell. An Ag/AgCl (in saturated KCl solution) and a platinum electrodes were served as the reference and counter electrodes, whereas the modified GC electrode as a working electrode. The procedures of GC electrode pretreatment are described according to previous reference [10]. Cyclic voltammetry (CV)
Scheme 1. The synthesis process of Pt/POMCs.
measurements were performed in 0.5 M H2 SO4 solutions with or without 1.0 M CH3 OH. The CO-stripping voltammetry based on the method reported by Liu. et al. [26] was measured in 0.5 M H2 SO4 solution with a potential range of −0.2 to 1.0 V. Both the scan rate of CV and CO-stripping tests are 50 mV s−1 . 2.3. Preparation of Pt/OMCs, Pt/POMCs and modified electrode Conventional ordered mesoporous silica SBA-15 was used as a hard template, which was prepared using Pluronic P123 and tetraethyl orthosilicate (TEOS) as the surfactant and silica source [29]. Px OMCs were synthesized according to the method reported by Ryoo et al. [30], except triphenylphosphine (TPP) and glucose were added at the same time, and the mass ratio of TPP and glucose is x (x = 3.5; 7; 14). In a typical synthesis route, Pt/OMCs nanocomposite was prepared by the following procedure: 35 mg of OMCs was dispersed in 10 mL of double distilled water containing 31.4 mg H2 PtCl6 ·6H2 O. After ultra-sonication for more than 1 h and magnetic stirring for about 2 h at room temperature, the H2 PtCl6 ·6H2 O could diffuse into the pores of OMCs. Then, the gained sample was dried under vacuum at 70 ◦ C until get a fine and completely dry powder. And then 5 mL double distilled water and 2.5 mL HCOOH were added to redisperse the powder. The resultant suspension was stored at room temperature for 72 h. Finally, the product was centrifuged, collected and dried in an oven at 60 ◦ C. The obtained composite was marked as Pt/OMCs. Pt/POMCs nanocomposites were synthesized in the same way of Pt/OMCs. The only difference is that the catalyst support is not OMCs but POMCs. The fabrication strategy is shown in Scheme 1. Finally, more than 1 hour of ultrasonication was necessary to disperse 2 mg Pt/OMCs or Pt/POMCs into a mixture of 0.1 mL (5 wt%) Nafion and 0.9 mL distilled water. After dropping 3 L of the suspension onto the electrode surface, the electrode was dried in air or an infrared lamp. 3. Results and discussion 3.1. Characterization of Pt/OMCs and Pt/POMCs The influence of P doping on mesostructure of OMCs and POMCs was investigated by XRD. Fig. 1A shows the typical small-angle XRD patterns of OMCs and POMCs. For the OMCs, the XRD patterns show the well (100), (110) and (200) peaks with a hexagonal mesopore arrangement at 2 of 0.98◦ , 1.70◦ , and 2.00◦ , which indicates the synthesis of ordered mesoporous structure. Partial deterioration of the XRD peaks is observed in POMCs. However, the existence of the main diffraction peak (100) illustrates that the framework hexagonal ordering of OMCs is basically retained. The phosphorus-doped mesoporous carbon samples (POMCs) with different P concentrations display gradual disappearance of the three peaks, indicating the loss of long-range structural order [31]. Fig. 1B exhibits the wide-angle XRD patterns of (a) Pt/OMCs, (b) Pt/P3.5 OMCs, (c) Pt/P7 OMCs, and (d) Pt/P14 OMCs. The diffraction peak located at about 24.52◦ is due to the (002) crystal face of carbon, while the peaks at 39.78◦ , 46.38◦ , 67.46◦ and 81.62◦ are corresponding to the (111), (200), (220) and (311) lattice planes of face-centered cubic structure of Pt. According to the Scherrer
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Fig. 1. (A) Small-angle XRD of OMCs and POMCs. (B) Wide-angle XRD of (a) Pt/OMCs, (b) Pt/P3.5 OMCs, (c) Pt/P7 OMCs, and (d) Pt/P14 OMCs nanocomposites. (C) Raman spectra of OMCs and POMCs.
formula (d = 0.89 /cos), the average diameter of Pt NPs is calculated to be 4.7 ± 0.5 and 3.6 ± 0.5 nm for Pt/OMCs and Pt/POMCs, respectively. Raman spectra (Fig. 1C) illustrate the D-band and G-band of OMCs and POMCs, giving some additional information of the carbon lattice. Both the two spectra show the presence of D and G bands, located at 1430 cm−1 (disorder mode) and 1584 cm−1 (tangential), which are ascribed to the defects/imperfections and hexagonal graphene plane, respectively [31]. Furthermore, the relative intensity ratio of the D and G bands (ID /IG ratio) has a physical meaning for the number of defect sites in the graphite carbon. The ID /IG of the P7 OMC (1.27) sample is larger than that of the OMC (0.94), indicating introduction of more defects and imperfections into carbon lattice by phosphorus doping. The N2 adsorption–desorption isotherms were applied to investigate the surface area and pore size of the synthesized OMCs and POMCs. Fig. 2A and B show the isotherms and pore size distribution (PSD) of OMCs and POMCs. From Fig. 2A, it is obviously that the isotherms of type IV with an H1-type hysteresis loop (characteristic of mesoporous materials having channel-type pores) can be observed for all the samples and the isotherm changes considerably after doping of phosphorus in OMCs. Table 1 shows the textural parameters of OMCs and POMCs, it is clearly that P7 OMCs possesses high BET surface area of 1338.8 ± 13 m2 g−1 , homogeneous pore size of 3.8 ± 0.3 nm, a large pore volume of 1.36 ± 0.15 cm3 g−1 and a great micropore area of 336.5 ± 7 m2 g−1 . Furthermore, the BET surface area and pore volume are gradually increased with the addition of P content. This attributes to the increased the number of micropores [31]. But for P14 OMCs, excessive P content lead to a decline in the number of micropores and further lead to a reduction in the BET surface area and pore volume. According to previous studies [32,33], the increase of BET surface area is beneficial for rate performance.
Typical transmission electron microscopy (TEM) measurements are used to characterize the structure and morphology of the synthesized catalysts. The TEM image of OMCs (Fig. 3A) clearly shows highly ordered mesoporous channels of OMCs, which also can be found from the TEM image of P7 OMCs (Fig. 3B). The result indicates that the appearance of P almost has no effect on the orderliness of mesoporous. Fig. 2C and D display TEM images of Pt/OMCs and Pt/P7 OMCs. Obviously, Pt NPs are supported on both Pt/OMCs and Pt/P7 OMCs. Compared with Pt/OMCs, Pt/P7 OMCs with a more uniform distribution of Pt NPs, which can be further observed on the HR-TEM (inset of Fig. 3D). Average sizes of Pt NPs on Pt/OMCs and Pt/P7 OMCs are calculated to be 4.8 ± 0.6 and 3.5 ± 0.4 nm (Fig. 3E and F), which is consistent with the results obtained from XRD images. The addition of P inhibits the aggregation and reduces the size of Pt NPs. As we all know, uniform distribution and small particle size of the metal nanoparticals are key factors to the stability and activity of electrocatalyst [14]. Fig. 4A shows the XPS spectra of Pt/OMCs and Pt/POMCs catalysts. It can be found that a weak peak for P element appears in the XPS spectrum of Pt/P7 OMCs compared with Pt/OMCs. Furthermore, the high-resolution P 2p XPS spectrum (Fig. 4B) reveals the presence of both P O bonding (133.3 eV) and P C bonding (132.6 eV) in Pt/P7 OMCs catalyst. The two results strongly suggest that the P atoms are incorporated into the carbon framework of the P7 OMCs [20]. In addition, the peak at 132.6 eV is positively shifted 2.2 eV compared with that of pure P (130.4 eV). The positive shift of the P 2p peak indicates that there is a strong interaction between P, C, O and Pt [12]. The Pt 4f regions of the above two catalysts are shown in Fig. 4C and D. For the Pt/P7 OMCs, the main doublet at 71.3 eV (Pt 4f7/2 ) and 74.5 eV (Pt 4f5/2 ) are the characteristic of metallic Pt, in comparison with the binding energy of the Pt 4f (71.3 and 74.8 eV) on Pt/OMCs, we find a definite negative shift in the binding energy. The slight shift of Pt 4f on Pt/P7 OMCs implies that there probably
Fig. 2. N2 adsorption–desorption isotherms (A) and Pore size distribution (B) of OMCs and POMCs.
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Table 1 Textural parameters of OMCs and POMCs nanocomposites. Samples
Pore diameter (nm)
OMCs P3.5 OMCs P7 OMCs P14 OMCs
4.1 3.8 3.8 4.3
± ± ± ±
BET surface area (m2 g−1 )
0.3 0.4 0.3 0.3
900.0 1117.1 1338.8 1327.2
± ± ± ±
15 11 13 8
Pore volume (cm3 g−1 ) 1.29 1.37 1.36 1.24
± ± ± ±
Micropore Area (m2 g−1 )
0.13 0.10 0.15 0.22
173.9 251.8 336.5 305.8
± ± ± ±
5 9 7 7
Fig. 3. (A) TEM images of OMCs (A), P7 OMCs (B), Pt/OMCs (C) and Pt/P7 OMCs (D). Particle size distribution histograms of Pt/OMCs (E) and Pt/P7 OMCs (F). Inset of D: HRTEM of Pt/P7 OMCs.
exists chemical interaction between Pt nanoparticles and P7 OMCs [26], which further confirm the electron interactions involving P, C, O and Pt atoms within Pt/P7 OMCs. Quantitative XPS analysis shows in Table 2. The oxygen content of Pt/P7 OMCs (11.6 ± 2%) is much larger than that of Pt/OMCs (8.4 ± 1%). The P-O content (67.1 ± 4%)
is twice as large as P C (32.9 ± 4%) in Pt/P7 OMCs. Thence, the incorporation of P atoms into OMCs contributes to increase the number of oxygen and oxygen-containing functional groups. Additionally, Compared with Pt/OMCs (53.5 ± 5%), the relative Pt0 intensity of Pt/P7 OMCs (67.7 ± 7%) is improved. Based on
Table 2 Fitting results summary of C, P, O and Pt core level XPS spectra of Pt/OMCs and Pt/POMCs. Sample
C (at%)
O (at%)
Pt (at%)
Pt/OMCs
89.9 ± 3
8.4 ± 1
1.83 ± 0.5 Pt0 53.5 ± 5 1.33 ± 0.4 Pt0 67.7 ± 7
Pt/POMCs
86.4 ± 2
11.6 ± 2
P (at%) \ Pt0 46.5 ± 3 Pt0 32.2 ± 6
0.64 ± 0.2 P-C 32.9 ± 4
P-O 67.1 ± 4
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Fig. 4. The XPS survey of Pt/OMCs and Pt/P7 OMCs (A), P 2p spectrum of Pt/P7 OMCs (B) and Pt 4f (C, D) spectra of Pt/OMCs (C) and Pt/P7 OMCs (D).
previously reported [34], the chemically bonding state of Pt0 can provide more active sites for MOR rather than the Pt2+ species. 3.2. Electrochemical activity measurement Fig. 5 presents the cyclic voltammograms (CVs) of Pt/Vulcan XC72, PtRu/XC, Pt/OMCs and Pt/POMCs in 0.5 M H2 SO4 solution at 50 mV s−1 . The electrochemical active surface area (ECSA) provides important information regarding the number of electrochemically active sites per gram of catalyst [35]. ECSA value is estimated according to the following formula [36]: ECSA =
QH [Pt] × 0.21
where [Pt] represents the platinum loading (g cm−2 ) in the electrode, QH is the charge for hydrogen desorption (mC cm−2 ), and 0.21 represents the charge required to oxidize a monolayer of
Fig. 5. CVs of Pt/Vulcan XC-72, PtRu/XC, Pt/OMCs and Pt/POMCs in N2 -saturated 0.5 M H2 SO4 solution.
adsorbed hydrogen on bright Pt (mC cm−2 ) [37–39]. Herein, the calculated ECSA values of Pt/Vulcan XC-72, PtRu/XC, Pt/OMCs, Pt/P3.5 OMCs, Pt/P7 OMCs and Pt/P14 OMCs are about 19.5 ± 2.3, 27.4 ± 3.9, 36.2 ± 3.1, 52.5 ± 5.7, 96.3 ± 7.3 and 81.4 ± 5.5 m2 g−1 , respectively. It is obviously that Pt/P7 OMCs exhibits a much higher ECSA value than other catalysts. Moreover, the ECSA of catalyst increases with the increase of the P content. However, the ECSA begin to decrease when the P content achieved a certain amount, such as Pt/P14 OMCs, indicating reasonable P content on catalyst performance is beneficial. The enhanced ECSA of Pt/POMCs may be ascribed to the effect of P on the electronic state of metal elements and highly uniform dispersion of Pt NPs on the surface of Pt/POMCs [12]. Due to the sharp increase in current intensity and ECSA of Pt/P7 OMCs compared with other samples, the following discussion of our work will focus on the researching electrochemical behavior of the Pt/P7 OMCs catalyst for MOR. The electrochemical activities of different P-content Pt/POMCs catalysts will be discussed briefly in the final. The electrocatalytic properties of Pt/P7 OMCs toward MOR along with Pt/OMCs, PtRu/XC and Pt/Vulcan XC-72 are investigated in solution of 0.5 M H2 SO4 + 1.0 M CH3 OH. As shown in Fig. 6A, among four kinds of catalysts, Pt/P7 OMCs exhibits the highest activity by displaying high mass current density about 677.2 mA mg−1 Pt. Furthermore, the potential of the forward anodic peak of Pt/P7 OMCs (+0.69 V) is also slightly lower than that of the Pt/OMCs (+0.73 V), PtRu/XC (+0.72 V) and Pt/Vulcan XC-72 (+0.72 V) can be observed from the inset of Fig. 6A. All above indicate that the Pt/P7 OMCs has much enhanced catalytic activity towards MOR. At the same time, the ratio of the forward anodic peak current (If ) to the reverse anodic peak current (Ib ) can be used to describe the catalyst tolerance to the intermediate carbonaceous species such as CO accumulated on electrode surface [40]. A higher ratio indicates more effective removal of the poisoning species on the catalyst surface [41]. It is calculated that the If /Ib value of Pt/P7 OMCs is 1.63, which is higher than that of Pt/OMCs (1.48), PrRu/XC (1.31)
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Fig. 6. (A) CVs of Pt/Vulcan XC-72, PtRu/XC, Pt/OMCs and Pt/P7 OMCs, linear sweep voltammetry (inset), in solution of 0.5 M H2 SO4 + 1.0 M CH3 OH. Scan rate: 50 mV s−1 . (B) CVs of Pt/P7 OMCs with different concentration of methanol: a) 0.1, b) 0.2, c) 0.3, d) 0.4, e) 0.5, f) 0.6, g) 0.7, h) 0.8, i) 0.9, j) 1.0, k) 1.1 M (inset: the plot of If vs. methanol concentration).
Fig. 7. CO stripping voltammograms of Pt/Vulcan XC-72, PtRu/XC, Pt/OMCs and Pt/P7 OMCs in solution of 0.5 M H2 SO4 . Scan rate: 50 mV s−1 .
and Pt/Vulcan XC-72 (1.31), indicating more effective oxidation of methanol occurs on and less poisoning species form Pt/P7 OMCs electrode [15]. In order to further evaluate the capacity of the Pt/P7 OMCs for MOR, the effect of methanol concentration on the anodic current density in 0.5 M H2 SO4 solution was investigated (Fig. 6B). It is clearly that the anodic peak currents (If ) increases with increasing of methanol concentration from 0.1 to 1.0 M and levels off at concentration higher than 1.0 mol L−1 (inset of Fig. 6B). In accordance with this result, the optimum concentration of methanol may be considered as about 1.0 mol L−1 to obtain a higher current density. Fig. 7 shows the CO electro-stripping tests to get more insights into the Pt/P7 OMCs towards CO oxidation. It can be clearly observed that their CO oxidation peak features are similar to that of Pt/Vulcan
XC-72. The disappearance of CO stripping peaks on the subsequent scans and the reappearance of hydrogen peaks at negative potentials indicate that the four catalysts all are free of dissolved CO [12]. This phenomenon documents to the removal of the residual carbon species formed in the forward scan [42]. However, the CO oxidation peak of Pt/P7 OMCs locate at 0.61 V, which is lower than that of Pt/OMCs (0.69 V), PrRu/XC (0.71 V) and Pt/Vulcan XC72 (0.72 V). Additionally, the onset potential of CO oxidation on the Pt/P7 OMCs (0.41 V) is apparently more negative than Pt/OMCs (0.58 V), PrRu/XC (0.62 V) and Pt/Vulcan XC-72 (0.62 V) in the first forward scan. These results significantly illustrate that Pt/P7 OMCs catalyst has better CO-tolerance towards MOR. A comparison of the performance of Pt/P7 OMCs catalyst with other Pt-based electrocatalysts reported in the literature is shown in Table 3. All the data reveal the analytical parameters for Pt/P7 OMCs-modified electrode are comparable and even better than those obtained at several electrodes reported recently. Therefore, the Pt/P7 OMCs nanocomposite is an excellent anode catalyst for MOR. The long-term catalytic activity of electrocatalyst is one of the key factors to evaluate the rate of surface poisoning [43]. So the stabilities of the obtained catalysts have also been determined by chronoamperometric technique for 10000 s. At a fixed potential, the intermediate carbonaceous species such as CO would begin to accumulate on the electrode surface because of the continuous oxidation of methanol, which may poison the catalysts and decrease the performance [44]. Fig. 8A shows the stability tests of the Pt/P7 OMCs at different oxidation voltages (0.5-0.8 V) in 1.0 M methanol + 0.5 M H2 SO4 solution. Obviously, throughout the time the Pt/P7 OMCs exhibits a relatively stable current density at different oxidation voltages. The increase of the oxidation voltage will increase the current density of the Pt/P7 OMCs. However the
Table 3 Performance comparison of various electrocatalysts towards MOR. Electrode a
Ni@PbPt/G Pt-Ni-P NTAsb Pt -GCFMc Pt/P-MCNTs GNPd /Pt Pt-CNx e OMCs Pt/P7 OMCs a b c d e
ECSA (m2 g−1 )
If (m2 g−1 )
If /Ic
Onset potential of CO-stripping (V)
Reference
43.1 28.4 44.5 78.9 63 104 36.2 ± 3.1 96.3 ± 7.3
281 / 468 / / / 375 677.2
1.33 0.87 1.09 0.79 1.21 1.50 1.48 1.63
0.51 0.47 / 0.53 / 0.5 0.58 0.41
[8] [12] [15] [26] [35] [42] This work This work
graphene oxide. nanotube arrays. graphene-modified carbon fiber mats. graphene nanoplate. CNTs and CNFs (carbon nanofibers).
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Fig. 8. (A) Chronoamperometry curves of Pt/P7 OMCs at different oxidation voltages: a) 0.5, b) 0.6, c) 0.8, d) 0.7 V (inset: with different concentrations of methanol at 0.7 V) in solution of 0.5 M H2 SO4 + 1.0 M CH3 OH. (B) Chronoamperometry curves of Pt/Vulcan XC-72, PtRu/XC, Pt/OMCs and Pt/P7 OMCs recorded at 0.7 V. Scan rate: 50 mV s−1 . (C) CVs of Pt/P7 OMCs in 0.5 M H2 SO4 before and after 10000s stability test. (D) TEM image of Pt/P7 OMCs after 10000s stability test.
maximum current density is at about 0.7 V, because the potential of the forward anodic peak of Pt/P7 OMCs is located at about 0.7 V in cyclic voltammograms (Fig. 6A). According to this result, the optimum oxidation voltage to obtain a higher current density may be considered as about 0.7 V. In addition, inset of Fig. 8A shows the stability tests of the Pt/P7 OMCs with different methanol concentrations (0.5-1.2 M). The optimum concentration of methanol to obtain a higher current density is 1.0 M, which is consistent with the result obtained from CVs of Fig. 6B. Fig. 8B displays the chronoamperometric curves of Pt/P7 OMCs, Pt/OMCs, PtRu/XC and Pt/Vulcan XC-72 catalysts at 0.7 V. The current densities of all electrodes present a downward trend with the increment of time. However,
the Pt/P7 OMCs exhibits a relatively small current decay, indicating a relatively stable electrochemical activity and a higher tolerance to the carbonaceous species like CO generated during MOR [12]. The electrochemical stability and morphology change of Pt/P7 OMCs after the chronoamperometric test were monitored by CVs and TEM. From Fig. 8C, we can see that after 10000s chronoamperometric test the Pt/P7 OMCs still possesses high activity. The calculated ECSA values is 90.7 ± 6.1 m2 g−1 , just a litlle bit than the original value (96.3 ± 7.3 m2 g−1 ). Correspondingly, the Pt/P7 OMCs shows no obvious morphology change after the 10000s durability test, as demonstrated by TEM (Fig. 8D). Long time in acid solution, part of the Nafion will be dissolved, but Pt NPs still well dispersed in
Fig. 9. (A) CVs, (B) CO stripping voltammograms and (C) Chronoamperometry curves of Pt/P3.5 OMCs, Pt/P7 OMCs and Pt/P14 OMCs in 0.5 M H2 SO4 solutions with (A, C) or without (B) 1.0 M CH3 OH. Scan rate: 50 mV s−1 .
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the surface of the catalyst. These results suggest that the Pt/P7 OMCs is highly stable under the current MOR conditions. Integrated above discussions and comparison of electrochemical performances showed in Figs. 5, 6, 7 and 8, we can conclude that Pt/P7 OMCs catalyst is the best candidate for an anode catalyst in MOR with excellent electrochemical activity, higher CO-tolerance and long-time stability. These superior properties mainly attribute to the following aspects. Firstly, the periodic mesoporous structure, high ECSA and large pore volume of OMCs are very conducive to mass transfer and electron transfer [45]. Then, the high dispersion and small size of Pt NPs are extremely important in improving electrochemical activity of catalysts. Finally, as previously mentioned, the introduction of P can dramatically increase the amount of oxygen-containing functional groups, because the P-doping bonded with oxygen results in the increase of the oxygen content at the surface of the catalysts [46]. According to the principle of anode oxygen injection mentioned in previous studies [35,47], the oxygen-containing functional groups like OH supplied by Pt-OH, P-OH sites or other sources [48] can combine with the intermediate products CO immediately. This is extremely beneficial for moving of CO from the surface of electrode and releasing more active sites, which may greatly strengthen the CO-tolerance and accelerate the rate of MOR. Therefore, owing to the synergy effect of OMCs, P, O and Pt NPs, Pt/P7 OMCs demonstrate its superiority as anode catalyst in DMFC. Fig. 9 displays the CVs, CO stripping voltammograms and chronoamperometry curves of Pt/POMCs catalysts with different P content in the same experimental conditions as Pt/P7 OMCs. Comprehensive A, B and C we can easily conclude that the MOR activity, CO-tolerance and stability of Pt/P7 OMCs catalyst all are the best. Therefore, reasonable P content is an important factor in improving the electrochemical performances of Pt/POMCs, too much will cause a decrease of electrochemical properties. In short, Pt/P7 OMCs is an excellent anode catalyst in MOR. 4. Conclusion In summary, we have demonstrated the fabrication of novel Pt supported on the P-doped OMCs by a highly efficient template assisted electroreduction route. It has been found that a proper optimization of P content can effectually lead to a small size and homogeneous of Pt nanocrystal. On the other hand, the addition of phosphorus can significantly improve ECSA and oxygen content of Pt/P7 OMCs. The activity of Pt/P7 OMCs nanocomposite for methanol oxidation in acid medium is higher than that of Pt/OMCs, PtRu/XC and commercial Pt/C according to the anodic peak potential, onset potential of methanol oxidation. Additionally, phosphorus content is also a key factor in deciding the level of catalyst performance. The favorable properties of Pt/P7 OMCs determine that it maybe a new anode electrocatalyst with excellent activity, durability and other advanced catalytic applications for DMFC. Acknowledgements The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (21075014).
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Pt nanoparticles incorporated into phosphorus-doped ordered mesoporous carbons: enhanced catalytic activity for methanol electrooxidation Pei Song, Liande Zhu, Xiangjie Bo, Aixia Wang, Guang Wang, Liping Guo ∗ Faculty of Chemistry, Northeast Normal University, Changchun, 130024, P R China
a r t i c l e
i n f o
Article history: Received 30 November 2013 Received in revised form 14 February 2014 Accepted 14 February 2014 Available online 26 February 2014 Keywords: Methanol Electrooxidation Pt nanoparticles Phosphorus-doped ordered mesoporous carbons Electrocatalyst Direct methanol fuel cell
a b s t r a c t Phosphorus-doped ordered mesoporous carbons (POMCs) with different P content are successfully synthesized by hard template method using SBA-15 as hard template, sucrose as carbon precursor and triphenylphosphane as phosphorus precursor. Pt nanoparticles with size of 3.5 ± 0.4 nm are deposited on the framework of POMCs. The doping of P into OMCs facilitates the dispersion of Pt nanoparticles and accelerates the formation of oxygen-containing functional groups. Pt/POMCs nanocomposites were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, nitrogen adsorption–desorption and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry and chronoamperometry studies exhibit that the Pt/POMCs, especially Pt/P7 OMCs, have larger electrochemical active surface area (ECSA), higher electrocatalytic activity, more negative onset potential and long-time stability for the electrooxidation toward methanol than that of Pt/OMCs, PtRu/XC and commercial Pt/C catalysts. These enhanced performances indicate that Pt/P7 OMCs catalyst may be an excellent anode catalyst for direct methanol fuel cell (DMFC). © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Great efforts have been made to design and develop direct methanol fuel cell (DMFC) as energy sources owning to its high energy density, easily handling liquid fuel, quick start-up and environmental friendly [1,2]. Anode catalysts of DMFC can activate electrochemical reactions, leading to increase the rate of fuel oxidation and oxygen reduction [3]. Therefore, the preparation and performance of anode catalysts are research priorities in the development of DMFC. However, as one of the exclusive anode catalyst for methanol electro-oxidation, commercial Pt/C catalyst is still insufficient activities and easily poisoned by reaction intermediates such as CO [4–6]. This is mainly attributed to the poor methanol electro-oxidation kinetics [5,7], low catalyst efficiency and CO poisoning [6,8,9] of the catalysts employed for the methanol oxidation reaction (MOR) in the anode. To overcome the problems mentioned above, it is necessary to develop novel electrocatalysts with much enhanced methanol oxidation activity/stability and higher CO-tolerance. It has been
∗ Corresponding author. Tel.: +86 0431 85099762; fax: +86 0413 85099762. E-mail addresses: [email protected] (X. Bo), [email protected] (L. Guo). http://dx.doi.org/10.1016/j.electacta.2014.02.068 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
reported that Pt-based or Pt alloy catalysts play a significant role in improving electrocatalytic activity and stability, such as Pt/CNTs [6], Pt/OMCs [10], Pt/N-doped graphene [11], Pt-Ni-P CNTs [12], Pt–Ru–P [13] and Pt–Sn–P/C [14]. To date, the precious metals like Pt supported on the heteroatom-doped carbon materials as anode electrocatalysts to improve electrocatalytic activity of MOR have attracted a tremendous attention of researchers. It has been proved that the performance of catalyst is closely associated with the nature of the supporting materials [15]. The size, morphology and distribution of the Pt particles also have a close relationship with the supporting materials. Therefore, special attention has been paid to use the various carbon materials including graphene [16], carbon nanotubes [17], activated carbon fibers [18,19], ordered mesoporous carbons (OMCs) [7,20] and other carbonaceous materials. Compared to other carbon materials, OMCs as electrocatalyst support for fuel cells has been reported [21,22] due to its excellent characteristics such as high surface area, tunable pore size and large pore volume with narrow pore size distribution. These super features promote the facile molecular transport of reactants and products [23,24]. Heteroatoms such as N, P, S [11,12,25,26] doping into carbon materials offer a good choice to increase the catalytic performance of Pt-based catalysts in DMFC. Heteroatoms bonded with carbon framework can introduce defect sites due to different bond length,
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electronegativity and atomic size, and thereby can induce uneven charge distribution, which have significant effects on some of the properties of catalyst such as catalytic property [20,27]. Phosphorus, as one of the N-group elements, often shows similar properties with nitrogen [26]. The same as N atom, P has a larger atomic radius and higher electron-donating ability which make it an astute choice as a dopant to carbon materials [20]. It also has been reported that the addition of P can drastically reduce the size of Pt nanoparticles (NPs) [13,14] and improve the distribution of Pt NPs on carbon support. Furthermore, the introduction of P may largely increase the number of oxygen-containing functional groups, which promotes the CO-tolerance towards MOR [12,20,26]. Recently, P-doped multiwalled nanotubes [28] and P-doped OMCs [20] exhibit high electro-catalytic activity for the oxygen reduction reaction (ORR). Pt supported on P-doped CNTs [26] and Pt-Ni-P composites [12] illustrate better activities toward the MOR. Thence, by the synergistic effect of the suitable carbon support OMCs, doping heteroatom P and small size Pt NPs, we expected to obtain an electrocatalyst with excellent catalytic activity, better CO-tolerance and much higher stability. In this work, Pt/POMCs as efficient anode electrocatalysts for MOR were synthesized using hard template method. Pt NPs incorporated inside the P-doped ordered mesoporous carbons (Pt/POMCs) via a simple impregnation–reduction method using formic acid as reductant. Both the physical and electrochemical properties of Pt/POMCs were studied. Pt/P7 OMCs was found to be an excellent electrocatalyst with higher electrochemical active surface area (ECSA), much smaller size of Pt NPs and sharply enhanced electrochemical properties. 2. Experimental 2.1. Reagents and apparatus Pluronic P123 (non-ionic triblock copolymer, PEO20 PPO70 PEO20 ) and Nafion (5 wt%) were purchased from Sigma-Aldrich. Triphenylphosphine and H2 PtCl6 ·6H2 O were purchased from Aladdin and Sinopharm Chemical Reagent Co. Ltd. All other reagents used were of analytical grade and used as received without further purification. The morphologies of the synthesized samples were characterized by powder X-ray diffraction (XRD). XRD was obtained on a an X-ray D/max-2200vpc (Rigaku Corporation, Japan) instrument operated at 40 kV and 20 mA using Cu K␣ radiation (k␣ = 0.15406 nm). Transmission electron microscopy (TEM) (JEOL, JEM-2100F) was operated at 200 kV. Thermogravimetric analysis (TGA) was performed on a PerkinElmer Diamond TG Analyzer. The Pt content was determined by TGA. The surface area and pore size of the synthesized OMCs and POMCs were determined by N2 adsorption-desorption isotherms (ASAP 2020) and were obtained from the Brunauer-Emmett-Teller (BET) and BarrettJoyner-Halenda (BJH) method, respectively. X-ray photoelectron spectroscopy (XPS) was measured using Thermo ESCA LAB spectrometer (USA). The binding energy was referenced to C 1s line at 284.6 eV for calibration. The error of the data represents the standard deviation of triplicate measurements. 2.2. Electrochemical measurements Electrochemical measurements were carried out in a conventional three electrode cell. An Ag/AgCl (in saturated KCl solution) and a platinum electrodes were served as the reference and counter electrodes, whereas the modified GC electrode as a working electrode. The procedures of GC electrode pretreatment are described according to previous reference [10]. Cyclic voltammetry (CV)
Scheme 1. The synthesis process of Pt/POMCs.
measurements were performed in 0.5 M H2 SO4 solutions with or without 1.0 M CH3 OH. The CO-stripping voltammetry based on the method reported by Liu. et al. [26] was measured in 0.5 M H2 SO4 solution with a potential range of −0.2 to 1.0 V. Both the scan rate of CV and CO-stripping tests are 50 mV s−1 . 2.3. Preparation of Pt/OMCs, Pt/POMCs and modified electrode Conventional ordered mesoporous silica SBA-15 was used as a hard template, which was prepared using Pluronic P123 and tetraethyl orthosilicate (TEOS) as the surfactant and silica source [29]. Px OMCs were synthesized according to the method reported by Ryoo et al. [30], except triphenylphosphine (TPP) and glucose were added at the same time, and the mass ratio of TPP and glucose is x (x = 3.5; 7; 14). In a typical synthesis route, Pt/OMCs nanocomposite was prepared by the following procedure: 35 mg of OMCs was dispersed in 10 mL of double distilled water containing 31.4 mg H2 PtCl6 ·6H2 O. After ultra-sonication for more than 1 h and magnetic stirring for about 2 h at room temperature, the H2 PtCl6 ·6H2 O could diffuse into the pores of OMCs. Then, the gained sample was dried under vacuum at 70 ◦ C until get a fine and completely dry powder. And then 5 mL double distilled water and 2.5 mL HCOOH were added to redisperse the powder. The resultant suspension was stored at room temperature for 72 h. Finally, the product was centrifuged, collected and dried in an oven at 60 ◦ C. The obtained composite was marked as Pt/OMCs. Pt/POMCs nanocomposites were synthesized in the same way of Pt/OMCs. The only difference is that the catalyst support is not OMCs but POMCs. The fabrication strategy is shown in Scheme 1. Finally, more than 1 hour of ultrasonication was necessary to disperse 2 mg Pt/OMCs or Pt/POMCs into a mixture of 0.1 mL (5 wt%) Nafion and 0.9 mL distilled water. After dropping 3 L of the suspension onto the electrode surface, the electrode was dried in air or an infrared lamp. 3. Results and discussion 3.1. Characterization of Pt/OMCs and Pt/POMCs The influence of P doping on mesostructure of OMCs and POMCs was investigated by XRD. Fig. 1A shows the typical small-angle XRD patterns of OMCs and POMCs. For the OMCs, the XRD patterns show the well (100), (110) and (200) peaks with a hexagonal mesopore arrangement at 2 of 0.98◦ , 1.70◦ , and 2.00◦ , which indicates the synthesis of ordered mesoporous structure. Partial deterioration of the XRD peaks is observed in POMCs. However, the existence of the main diffraction peak (100) illustrates that the framework hexagonal ordering of OMCs is basically retained. The phosphorus-doped mesoporous carbon samples (POMCs) with different P concentrations display gradual disappearance of the three peaks, indicating the loss of long-range structural order [31]. Fig. 1B exhibits the wide-angle XRD patterns of (a) Pt/OMCs, (b) Pt/P3.5 OMCs, (c) Pt/P7 OMCs, and (d) Pt/P14 OMCs. The diffraction peak located at about 24.52◦ is due to the (002) crystal face of carbon, while the peaks at 39.78◦ , 46.38◦ , 67.46◦ and 81.62◦ are corresponding to the (111), (200), (220) and (311) lattice planes of face-centered cubic structure of Pt. According to the Scherrer
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Fig. 1. (A) Small-angle XRD of OMCs and POMCs. (B) Wide-angle XRD of (a) Pt/OMCs, (b) Pt/P3.5 OMCs, (c) Pt/P7 OMCs, and (d) Pt/P14 OMCs nanocomposites. (C) Raman spectra of OMCs and POMCs.
formula (d = 0.89 /cos), the average diameter of Pt NPs is calculated to be 4.7 ± 0.5 and 3.6 ± 0.5 nm for Pt/OMCs and Pt/POMCs, respectively. Raman spectra (Fig. 1C) illustrate the D-band and G-band of OMCs and POMCs, giving some additional information of the carbon lattice. Both the two spectra show the presence of D and G bands, located at 1430 cm−1 (disorder mode) and 1584 cm−1 (tangential), which are ascribed to the defects/imperfections and hexagonal graphene plane, respectively [31]. Furthermore, the relative intensity ratio of the D and G bands (ID /IG ratio) has a physical meaning for the number of defect sites in the graphite carbon. The ID /IG of the P7 OMC (1.27) sample is larger than that of the OMC (0.94), indicating introduction of more defects and imperfections into carbon lattice by phosphorus doping. The N2 adsorption–desorption isotherms were applied to investigate the surface area and pore size of the synthesized OMCs and POMCs. Fig. 2A and B show the isotherms and pore size distribution (PSD) of OMCs and POMCs. From Fig. 2A, it is obviously that the isotherms of type IV with an H1-type hysteresis loop (characteristic of mesoporous materials having channel-type pores) can be observed for all the samples and the isotherm changes considerably after doping of phosphorus in OMCs. Table 1 shows the textural parameters of OMCs and POMCs, it is clearly that P7 OMCs possesses high BET surface area of 1338.8 ± 13 m2 g−1 , homogeneous pore size of 3.8 ± 0.3 nm, a large pore volume of 1.36 ± 0.15 cm3 g−1 and a great micropore area of 336.5 ± 7 m2 g−1 . Furthermore, the BET surface area and pore volume are gradually increased with the addition of P content. This attributes to the increased the number of micropores [31]. But for P14 OMCs, excessive P content lead to a decline in the number of micropores and further lead to a reduction in the BET surface area and pore volume. According to previous studies [32,33], the increase of BET surface area is beneficial for rate performance.
Typical transmission electron microscopy (TEM) measurements are used to characterize the structure and morphology of the synthesized catalysts. The TEM image of OMCs (Fig. 3A) clearly shows highly ordered mesoporous channels of OMCs, which also can be found from the TEM image of P7 OMCs (Fig. 3B). The result indicates that the appearance of P almost has no effect on the orderliness of mesoporous. Fig. 2C and D display TEM images of Pt/OMCs and Pt/P7 OMCs. Obviously, Pt NPs are supported on both Pt/OMCs and Pt/P7 OMCs. Compared with Pt/OMCs, Pt/P7 OMCs with a more uniform distribution of Pt NPs, which can be further observed on the HR-TEM (inset of Fig. 3D). Average sizes of Pt NPs on Pt/OMCs and Pt/P7 OMCs are calculated to be 4.8 ± 0.6 and 3.5 ± 0.4 nm (Fig. 3E and F), which is consistent with the results obtained from XRD images. The addition of P inhibits the aggregation and reduces the size of Pt NPs. As we all know, uniform distribution and small particle size of the metal nanoparticals are key factors to the stability and activity of electrocatalyst [14]. Fig. 4A shows the XPS spectra of Pt/OMCs and Pt/POMCs catalysts. It can be found that a weak peak for P element appears in the XPS spectrum of Pt/P7 OMCs compared with Pt/OMCs. Furthermore, the high-resolution P 2p XPS spectrum (Fig. 4B) reveals the presence of both P O bonding (133.3 eV) and P C bonding (132.6 eV) in Pt/P7 OMCs catalyst. The two results strongly suggest that the P atoms are incorporated into the carbon framework of the P7 OMCs [20]. In addition, the peak at 132.6 eV is positively shifted 2.2 eV compared with that of pure P (130.4 eV). The positive shift of the P 2p peak indicates that there is a strong interaction between P, C, O and Pt [12]. The Pt 4f regions of the above two catalysts are shown in Fig. 4C and D. For the Pt/P7 OMCs, the main doublet at 71.3 eV (Pt 4f7/2 ) and 74.5 eV (Pt 4f5/2 ) are the characteristic of metallic Pt, in comparison with the binding energy of the Pt 4f (71.3 and 74.8 eV) on Pt/OMCs, we find a definite negative shift in the binding energy. The slight shift of Pt 4f on Pt/P7 OMCs implies that there probably
Fig. 2. N2 adsorption–desorption isotherms (A) and Pore size distribution (B) of OMCs and POMCs.
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Table 1 Textural parameters of OMCs and POMCs nanocomposites. Samples
Pore diameter (nm)
OMCs P3.5 OMCs P7 OMCs P14 OMCs
4.1 3.8 3.8 4.3
± ± ± ±
BET surface area (m2 g−1 )
0.3 0.4 0.3 0.3
900.0 1117.1 1338.8 1327.2
± ± ± ±
15 11 13 8
Pore volume (cm3 g−1 ) 1.29 1.37 1.36 1.24
± ± ± ±
Micropore Area (m2 g−1 )
0.13 0.10 0.15 0.22
173.9 251.8 336.5 305.8
± ± ± ±
5 9 7 7
Fig. 3. (A) TEM images of OMCs (A), P7 OMCs (B), Pt/OMCs (C) and Pt/P7 OMCs (D). Particle size distribution histograms of Pt/OMCs (E) and Pt/P7 OMCs (F). Inset of D: HRTEM of Pt/P7 OMCs.
exists chemical interaction between Pt nanoparticles and P7 OMCs [26], which further confirm the electron interactions involving P, C, O and Pt atoms within Pt/P7 OMCs. Quantitative XPS analysis shows in Table 2. The oxygen content of Pt/P7 OMCs (11.6 ± 2%) is much larger than that of Pt/OMCs (8.4 ± 1%). The P-O content (67.1 ± 4%)
is twice as large as P C (32.9 ± 4%) in Pt/P7 OMCs. Thence, the incorporation of P atoms into OMCs contributes to increase the number of oxygen and oxygen-containing functional groups. Additionally, Compared with Pt/OMCs (53.5 ± 5%), the relative Pt0 intensity of Pt/P7 OMCs (67.7 ± 7%) is improved. Based on
Table 2 Fitting results summary of C, P, O and Pt core level XPS spectra of Pt/OMCs and Pt/POMCs. Sample
C (at%)
O (at%)
Pt (at%)
Pt/OMCs
89.9 ± 3
8.4 ± 1
1.83 ± 0.5 Pt0 53.5 ± 5 1.33 ± 0.4 Pt0 67.7 ± 7
Pt/POMCs
86.4 ± 2
11.6 ± 2
P (at%) \ Pt0 46.5 ± 3 Pt0 32.2 ± 6
0.64 ± 0.2 P-C 32.9 ± 4
P-O 67.1 ± 4
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Fig. 4. The XPS survey of Pt/OMCs and Pt/P7 OMCs (A), P 2p spectrum of Pt/P7 OMCs (B) and Pt 4f (C, D) spectra of Pt/OMCs (C) and Pt/P7 OMCs (D).
previously reported [34], the chemically bonding state of Pt0 can provide more active sites for MOR rather than the Pt2+ species. 3.2. Electrochemical activity measurement Fig. 5 presents the cyclic voltammograms (CVs) of Pt/Vulcan XC72, PtRu/XC, Pt/OMCs and Pt/POMCs in 0.5 M H2 SO4 solution at 50 mV s−1 . The electrochemical active surface area (ECSA) provides important information regarding the number of electrochemically active sites per gram of catalyst [35]. ECSA value is estimated according to the following formula [36]: ECSA =
QH [Pt] × 0.21
where [Pt] represents the platinum loading (g cm−2 ) in the electrode, QH is the charge for hydrogen desorption (mC cm−2 ), and 0.21 represents the charge required to oxidize a monolayer of
Fig. 5. CVs of Pt/Vulcan XC-72, PtRu/XC, Pt/OMCs and Pt/POMCs in N2 -saturated 0.5 M H2 SO4 solution.
adsorbed hydrogen on bright Pt (mC cm−2 ) [37–39]. Herein, the calculated ECSA values of Pt/Vulcan XC-72, PtRu/XC, Pt/OMCs, Pt/P3.5 OMCs, Pt/P7 OMCs and Pt/P14 OMCs are about 19.5 ± 2.3, 27.4 ± 3.9, 36.2 ± 3.1, 52.5 ± 5.7, 96.3 ± 7.3 and 81.4 ± 5.5 m2 g−1 , respectively. It is obviously that Pt/P7 OMCs exhibits a much higher ECSA value than other catalysts. Moreover, the ECSA of catalyst increases with the increase of the P content. However, the ECSA begin to decrease when the P content achieved a certain amount, such as Pt/P14 OMCs, indicating reasonable P content on catalyst performance is beneficial. The enhanced ECSA of Pt/POMCs may be ascribed to the effect of P on the electronic state of metal elements and highly uniform dispersion of Pt NPs on the surface of Pt/POMCs [12]. Due to the sharp increase in current intensity and ECSA of Pt/P7 OMCs compared with other samples, the following discussion of our work will focus on the researching electrochemical behavior of the Pt/P7 OMCs catalyst for MOR. The electrochemical activities of different P-content Pt/POMCs catalysts will be discussed briefly in the final. The electrocatalytic properties of Pt/P7 OMCs toward MOR along with Pt/OMCs, PtRu/XC and Pt/Vulcan XC-72 are investigated in solution of 0.5 M H2 SO4 + 1.0 M CH3 OH. As shown in Fig. 6A, among four kinds of catalysts, Pt/P7 OMCs exhibits the highest activity by displaying high mass current density about 677.2 mA mg−1 Pt. Furthermore, the potential of the forward anodic peak of Pt/P7 OMCs (+0.69 V) is also slightly lower than that of the Pt/OMCs (+0.73 V), PtRu/XC (+0.72 V) and Pt/Vulcan XC-72 (+0.72 V) can be observed from the inset of Fig. 6A. All above indicate that the Pt/P7 OMCs has much enhanced catalytic activity towards MOR. At the same time, the ratio of the forward anodic peak current (If ) to the reverse anodic peak current (Ib ) can be used to describe the catalyst tolerance to the intermediate carbonaceous species such as CO accumulated on electrode surface [40]. A higher ratio indicates more effective removal of the poisoning species on the catalyst surface [41]. It is calculated that the If /Ib value of Pt/P7 OMCs is 1.63, which is higher than that of Pt/OMCs (1.48), PrRu/XC (1.31)
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Fig. 6. (A) CVs of Pt/Vulcan XC-72, PtRu/XC, Pt/OMCs and Pt/P7 OMCs, linear sweep voltammetry (inset), in solution of 0.5 M H2 SO4 + 1.0 M CH3 OH. Scan rate: 50 mV s−1 . (B) CVs of Pt/P7 OMCs with different concentration of methanol: a) 0.1, b) 0.2, c) 0.3, d) 0.4, e) 0.5, f) 0.6, g) 0.7, h) 0.8, i) 0.9, j) 1.0, k) 1.1 M (inset: the plot of If vs. methanol concentration).
Fig. 7. CO stripping voltammograms of Pt/Vulcan XC-72, PtRu/XC, Pt/OMCs and Pt/P7 OMCs in solution of 0.5 M H2 SO4 . Scan rate: 50 mV s−1 .
and Pt/Vulcan XC-72 (1.31), indicating more effective oxidation of methanol occurs on and less poisoning species form Pt/P7 OMCs electrode [15]. In order to further evaluate the capacity of the Pt/P7 OMCs for MOR, the effect of methanol concentration on the anodic current density in 0.5 M H2 SO4 solution was investigated (Fig. 6B). It is clearly that the anodic peak currents (If ) increases with increasing of methanol concentration from 0.1 to 1.0 M and levels off at concentration higher than 1.0 mol L−1 (inset of Fig. 6B). In accordance with this result, the optimum concentration of methanol may be considered as about 1.0 mol L−1 to obtain a higher current density. Fig. 7 shows the CO electro-stripping tests to get more insights into the Pt/P7 OMCs towards CO oxidation. It can be clearly observed that their CO oxidation peak features are similar to that of Pt/Vulcan
XC-72. The disappearance of CO stripping peaks on the subsequent scans and the reappearance of hydrogen peaks at negative potentials indicate that the four catalysts all are free of dissolved CO [12]. This phenomenon documents to the removal of the residual carbon species formed in the forward scan [42]. However, the CO oxidation peak of Pt/P7 OMCs locate at 0.61 V, which is lower than that of Pt/OMCs (0.69 V), PrRu/XC (0.71 V) and Pt/Vulcan XC72 (0.72 V). Additionally, the onset potential of CO oxidation on the Pt/P7 OMCs (0.41 V) is apparently more negative than Pt/OMCs (0.58 V), PrRu/XC (0.62 V) and Pt/Vulcan XC-72 (0.62 V) in the first forward scan. These results significantly illustrate that Pt/P7 OMCs catalyst has better CO-tolerance towards MOR. A comparison of the performance of Pt/P7 OMCs catalyst with other Pt-based electrocatalysts reported in the literature is shown in Table 3. All the data reveal the analytical parameters for Pt/P7 OMCs-modified electrode are comparable and even better than those obtained at several electrodes reported recently. Therefore, the Pt/P7 OMCs nanocomposite is an excellent anode catalyst for MOR. The long-term catalytic activity of electrocatalyst is one of the key factors to evaluate the rate of surface poisoning [43]. So the stabilities of the obtained catalysts have also been determined by chronoamperometric technique for 10000 s. At a fixed potential, the intermediate carbonaceous species such as CO would begin to accumulate on the electrode surface because of the continuous oxidation of methanol, which may poison the catalysts and decrease the performance [44]. Fig. 8A shows the stability tests of the Pt/P7 OMCs at different oxidation voltages (0.5-0.8 V) in 1.0 M methanol + 0.5 M H2 SO4 solution. Obviously, throughout the time the Pt/P7 OMCs exhibits a relatively stable current density at different oxidation voltages. The increase of the oxidation voltage will increase the current density of the Pt/P7 OMCs. However the
Table 3 Performance comparison of various electrocatalysts towards MOR. Electrode a
Ni@PbPt/G Pt-Ni-P NTAsb Pt -GCFMc Pt/P-MCNTs GNPd /Pt Pt-CNx e OMCs Pt/P7 OMCs a b c d e
ECSA (m2 g−1 )
If (m2 g−1 )
If /Ic
Onset potential of CO-stripping (V)
Reference
43.1 28.4 44.5 78.9 63 104 36.2 ± 3.1 96.3 ± 7.3
281 / 468 / / / 375 677.2
1.33 0.87 1.09 0.79 1.21 1.50 1.48 1.63
0.51 0.47 / 0.53 / 0.5 0.58 0.41
[8] [12] [15] [26] [35] [42] This work This work
graphene oxide. nanotube arrays. graphene-modified carbon fiber mats. graphene nanoplate. CNTs and CNFs (carbon nanofibers).
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Fig. 8. (A) Chronoamperometry curves of Pt/P7 OMCs at different oxidation voltages: a) 0.5, b) 0.6, c) 0.8, d) 0.7 V (inset: with different concentrations of methanol at 0.7 V) in solution of 0.5 M H2 SO4 + 1.0 M CH3 OH. (B) Chronoamperometry curves of Pt/Vulcan XC-72, PtRu/XC, Pt/OMCs and Pt/P7 OMCs recorded at 0.7 V. Scan rate: 50 mV s−1 . (C) CVs of Pt/P7 OMCs in 0.5 M H2 SO4 before and after 10000s stability test. (D) TEM image of Pt/P7 OMCs after 10000s stability test.
maximum current density is at about 0.7 V, because the potential of the forward anodic peak of Pt/P7 OMCs is located at about 0.7 V in cyclic voltammograms (Fig. 6A). According to this result, the optimum oxidation voltage to obtain a higher current density may be considered as about 0.7 V. In addition, inset of Fig. 8A shows the stability tests of the Pt/P7 OMCs with different methanol concentrations (0.5-1.2 M). The optimum concentration of methanol to obtain a higher current density is 1.0 M, which is consistent with the result obtained from CVs of Fig. 6B. Fig. 8B displays the chronoamperometric curves of Pt/P7 OMCs, Pt/OMCs, PtRu/XC and Pt/Vulcan XC-72 catalysts at 0.7 V. The current densities of all electrodes present a downward trend with the increment of time. However,
the Pt/P7 OMCs exhibits a relatively small current decay, indicating a relatively stable electrochemical activity and a higher tolerance to the carbonaceous species like CO generated during MOR [12]. The electrochemical stability and morphology change of Pt/P7 OMCs after the chronoamperometric test were monitored by CVs and TEM. From Fig. 8C, we can see that after 10000s chronoamperometric test the Pt/P7 OMCs still possesses high activity. The calculated ECSA values is 90.7 ± 6.1 m2 g−1 , just a litlle bit than the original value (96.3 ± 7.3 m2 g−1 ). Correspondingly, the Pt/P7 OMCs shows no obvious morphology change after the 10000s durability test, as demonstrated by TEM (Fig. 8D). Long time in acid solution, part of the Nafion will be dissolved, but Pt NPs still well dispersed in
Fig. 9. (A) CVs, (B) CO stripping voltammograms and (C) Chronoamperometry curves of Pt/P3.5 OMCs, Pt/P7 OMCs and Pt/P14 OMCs in 0.5 M H2 SO4 solutions with (A, C) or without (B) 1.0 M CH3 OH. Scan rate: 50 mV s−1 .
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the surface of the catalyst. These results suggest that the Pt/P7 OMCs is highly stable under the current MOR conditions. Integrated above discussions and comparison of electrochemical performances showed in Figs. 5, 6, 7 and 8, we can conclude that Pt/P7 OMCs catalyst is the best candidate for an anode catalyst in MOR with excellent electrochemical activity, higher CO-tolerance and long-time stability. These superior properties mainly attribute to the following aspects. Firstly, the periodic mesoporous structure, high ECSA and large pore volume of OMCs are very conducive to mass transfer and electron transfer [45]. Then, the high dispersion and small size of Pt NPs are extremely important in improving electrochemical activity of catalysts. Finally, as previously mentioned, the introduction of P can dramatically increase the amount of oxygen-containing functional groups, because the P-doping bonded with oxygen results in the increase of the oxygen content at the surface of the catalysts [46]. According to the principle of anode oxygen injection mentioned in previous studies [35,47], the oxygen-containing functional groups like OH supplied by Pt-OH, P-OH sites or other sources [48] can combine with the intermediate products CO immediately. This is extremely beneficial for moving of CO from the surface of electrode and releasing more active sites, which may greatly strengthen the CO-tolerance and accelerate the rate of MOR. Therefore, owing to the synergy effect of OMCs, P, O and Pt NPs, Pt/P7 OMCs demonstrate its superiority as anode catalyst in DMFC. Fig. 9 displays the CVs, CO stripping voltammograms and chronoamperometry curves of Pt/POMCs catalysts with different P content in the same experimental conditions as Pt/P7 OMCs. Comprehensive A, B and C we can easily conclude that the MOR activity, CO-tolerance and stability of Pt/P7 OMCs catalyst all are the best. Therefore, reasonable P content is an important factor in improving the electrochemical performances of Pt/POMCs, too much will cause a decrease of electrochemical properties. In short, Pt/P7 OMCs is an excellent anode catalyst in MOR. 4. Conclusion In summary, we have demonstrated the fabrication of novel Pt supported on the P-doped OMCs by a highly efficient template assisted electroreduction route. It has been found that a proper optimization of P content can effectually lead to a small size and homogeneous of Pt nanocrystal. On the other hand, the addition of phosphorus can significantly improve ECSA and oxygen content of Pt/P7 OMCs. The activity of Pt/P7 OMCs nanocomposite for methanol oxidation in acid medium is higher than that of Pt/OMCs, PtRu/XC and commercial Pt/C according to the anodic peak potential, onset potential of methanol oxidation. Additionally, phosphorus content is also a key factor in deciding the level of catalyst performance. The favorable properties of Pt/P7 OMCs determine that it maybe a new anode electrocatalyst with excellent activity, durability and other advanced catalytic applications for DMFC. Acknowledgements The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (21075014).
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