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SnO2 nanocatalysts with good photo-electrocatalytic property
Applied Surface Science 471 (2019) 263–272
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Full Length Article
Facile synthesis of PtPd/SnO2 nanocatalysts with good photoelectrocatalytic property
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Bingqian Yanga,b,c,1, Yawei Yua,b,c,1, Jianbo Zhanga,b,c, Lefan Yuana,b,c, Jingyuan Qiaoa, ⁎ Xiulan Hua,b,c, a
College of Materials Science and Engineering, Nanjing Tech University, China The Synergetic Innovation Center for Advanced Materials, China c Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Solution plasma Nanocomposites PtPd nano-alloy SnO2 nanoparticles Methanol electro-oxidation
PtPd/SnO2 nanocatalysts were synthesized by a simple one-step way of plasma technique in an aqueous solution of tin(II) chloride by using Pt and Pd metal wire as the electrode pair. PtPd/SnO2/GNs composite catalysts were prepared by an ultrasonic mixing PtPd/SnO2 with GNs (GNs, graphene nanosheets). The PtPd/SnO2/GNs composite catalysts exhibit a significantly enhanced electrocatalytic performance, cycling stability and COpoisoning tolerance towards methanol oxidation both under acidic and alkaline condition, which attributed to the synergism of PtPd alloy and SnO2. What’s more, the current density of PtPd/SnO2/GNs composite catalyst was obviously improved under light illumination, with 10,029 mA mgPt−1 which was about 1.3 times higher than that without light illumination under alkaline condition. The novel one-step plasma technique could provide a useful approach for fabricating other highly efficient electrocatalysts.
1. Introduction Direct methanol fuel cells (DMFCs) are of increasing interest as power systems in electronic vehicles or portable electronic devices [1,2] with the advantages of high efficiency, environmental friendliness and low operation conditions, whereas their wide commercialization have been limited by some disadvantages like the high price, low activity and poor durability. For the time being, Pt-based catalysts have been used as anode catalysts in a wide range since Pt is a significant catalyst for the methanol oxidation and oxygen reduction in DMFCs. None the less, Ptbased catalysts are highly susceptible to be poisoned by CO-like intermediates generated during methanol oxidation reaction (MOR), meanwhile the high price of Pt is also a critical problem. In order to improve the durability and stability, to enhance the catalytic activity, and to cut back the amount of Pt, many researches which focused on the development of efficient catalysts have been carried out. For instance, synthesizing nano-alloy catalysts [3–5], adding metal oxide [6,7], doping metalloid element [8,9] and synthesizing catalysts with special structure [10,11] were used to enhance the catalytic performance. For the lack of Pt, to design high-ranking Pt-based nanocatalysts in which the use of low quantities of Pt is comparable to those catalysts
currently available is urgent for us. Discuss from this aspect, Pt-based bimetallic nano-alloy catalysts, like PtPd [12,13], PtAu [4,14], PtRu [15,16], have been considered to be the substitution to current catalysts; on the other hand, Pt-based bimetallic nano-alloy catalysts show the superior catalytic ability than pure Pt catalysts. Among current Ptbased bimetallic nano-alloy catalysts, the PtPd nano-alloys have been demonstrated to be the promising electrocatalysts due to the synergism between Pt and Pd which can improve the catalytic activity. Qiu et al. demonstrated a hydrothermal one-step method to produce a single crystalline mesoporous PtPd nanoparticle, exhibiting enhanced catalytic performance towards MOR [11]. Meanwhile the Pt-based catalysts involving transition mental oxides such as SnO2 [17,18], TiO2 [19–22], ZnO [23–25] and Co3O4 [26,27] can improve the CO-tolerance ability and stability. The synergistic effect between Pt nanoparticle and metal oxides would result in the enhanced catalytic performance. As is known to all, SnO2 is an excellent transparent conductive material, which is the first commercially available doping used materials in order to improve the conductivity and stability of products. It has been reported that the Pt/SnO2/C electrocatalyst synthesized by an alcohol-reduction process shows high catalytic activity towards ethanol [28]. On the other hand, Lei et al. successfully synthesized an anodic electrocatalyst Pt/SnO2/GNs with
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Corresponding author at: College of Materials Science and Engineering, Nanjing Tech University, Xin-Mo-Fan Road No. 5, 210009 Nanjing, Jiangsu, China. E-mail address: [email protected] (X. Hu). 1 Bingqian Yang and Yawei Yu are co-first authors. https://doi.org/10.1016/j.apsusc.2018.12.002 Received 11 August 2018; Received in revised form 23 November 2018; Accepted 1 December 2018 Available online 01 December 2018 0169-4332/ © 2018 Published by Elsevier B.V.
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Fig. 1. Fabrication model of PtPd/SnO2 in SnCl2·2H2O aqueous solution by the solution plasma technique.
significantly enhanced catalytic properties towards MOR under external light irradiation via a simple one-pot strategy [29]. Integrate the above two points, the composites of bimetallic nano-alloy combined with photo responsive semiconductor metal oxides can contribute to a greater extent improving the performance of the catalysts. Nowadays, composite catalysts can be prepared by many methods, such as the chemical reduction method [30], sol-gel method [31] and physical/chemical vapor deposition method [32]. However, the abovementioned methods have some drawbacks which lead to the reduction of catalytic properties. As we all know, additional reducing agents and surfactants in chemical reduction methods are hard to be removed in the last steps, moreover, the synthesis process is complex and the strict reaction conditions or high demands on equipment are required. Thus the researches on the alloy/metal oxide composite catalysts which can furtherly synthesize efficient and robust electrocatalysts are few and far between. In our previous work, the solution plasma technique seemed to the more suitable way to synthesize alloy/metal oxide composite catalysts in an open system without any chemical additions. In this study, PtPd/SnO2/GNs catalyst was manufactured from Pt
Fig. 2. XRD pattern of PtPd/SnO2/GNs.
Fig. 3. TEM (a, b) and HR-TEM (c, d) image of PtPd/SnO2/GNs catalyst. 264
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Fig. 4. Dark-field image and EDX analysis of PtPd/SnO2/GNs. (a) dark-field images of PtPd/SnO2/GNs; (b) EDX mapping of Pd, Pt, Sn and O; (c)–(f) EDX mapping of Pd, Pt, Sn and O, respectively.
Fig. 5. Dark-field image of PtPd (a) and (b) the cross-sectional compositional line profiles as marked line in (a).
2. Experimental section
metal wire and Pd metal wire directly in an aqueous solution of tin(II) chloride via a facile and rapid solution plasma technique. GNs were used to be support materials because of good electrochemical activity and high conductivity. Electrocatalytic measurements showed that the PtPd/SnO2/GNs revealed enhanced catalytic properties and improved resistance to CO-like intermediates poisoning towards MOR. In addition, the PtPd/SnO2/GNs exhibited a better catalytic performance under light illumination than that without light illumination.
2.1. Materials Platinum (Pt) metal wire and palladium (Pd) metal wire with a diameter of 1.0 mm (Sigma-Aldrich, 99.9%) were served as the opposite electrodes. Tin(II) chloride dihydrate (SnCl2·2H2O, > 98%, SigmaAldrich) was selected to be the solute and deionized water was selected to be the solvent. Graphene nanosheets (GNs, thickness: 3–10 nm, Nanjing XFNANO Materials Tech Co., Ltd) was selected as support materials, while Nafion® (Sigma-Aldrich, 5 wt%) was selected to be the binder. Commercial Pt/C catalysts (Alfa Aesar, 2–5 nm, nominally 20 wt% Pt on carbon black) were served as the contrast. 265
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Fig. 6. CVs of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C in 0.5 M KOH (a) and 0.5 M H2SO4 (b) containing 1 M CH3OH.
at least 1 h in 2 ml deionized water by adding 6 µL 5% Nafion solution. After that, a droplet (6 µL) of the prepared suspensions was dropped on the polished GC electrode substrate and dried for 12 h at room temperature. After the working electrode was prepared, the first 30 cycles of cyclic voltammetry (CV) under the potential range between −0.2 and 1.2 V with a scan rate of 50 mV s−1 was carried out in a 0.5 M H2SO4 solution to clean the electrode surface and activate the prepared electrode to make sure that the subsequent data could be stable in following tests. Then, the electrocatalytic properties of three catalysts towards MOR were monitored by CV measurements. Under acidic condition, the measurements were conducted in the solution of 0.5 M H2SO4 containing 1 M CH3OH with the potential range from 0 to 1.0 V at 50 mV s−1. And their electrocatalytic activities under alkaline conditions were addressed in the solution of 0.5 M KOH containing 1 M CH3OH in the potential range from −0.6 to 0.4 V with the same scan rate. CO stripping experiment was tested in 0.5 M H2SO4 under the potential of −0.2 to 1.2 V by oxidizing the pre-adsorbed CO. CO gas was purged into the solution at a position close to the working electrode for 30 min with the potential holding at −0.15 V in order to allow the complete adsorption of CO onto the electrode surface. Then the excess CO was purified with N2 for 30 min under the same potential followed by the CO stripping. Their photo-electrocatalytic activity towards methanol was also tested under alkaline conditions. The working electrodes were illuminated by the 50 W Xe lamp (PLS-SXE300UV, Peking Perfectlight Co., China, wavelength ≥200 nm) during MOR process and the lamp was 15 cm away from the working electrode. All these tests mentioned above were done at room temperature.
2.2. Synthesis of PtPd/SnO2/GNs catalysts Firstly, the synthesis of PtPd/SnO2 by plasma technique in SnCl2·2H2O aqueous solution was analogous to our previous work [13,33]. In brief, the Pt metal wires and Pd metal wires were used to be opposite electrodes, and immersed in 80 ml aqueous solution containing 3 mmol SnCl2·2H2O. The gap between the opposite electrode was maintained at about 0.3 mm by a screen ruler. Plasma discharge was generated using a high voltage pulsed DC power supply (repetition frequency: 15–20 kHz, pulse width: 1–2 µs, Kurita Co.Ltd., Japan). Our previous work indicated that the yield of Pt, Pd and SnO2 was related to the time of discharge. Thus, about 9 mg Pt, 6 mg Pd and 15 mg SnO2 were synthesized after 30 min discharge. Then GNs were mixed in PtPd/SnO2 suspension and later ultrasonication for several hours to prepare uniformly dispersed PtPd/SnO2/GNs catalyst. At last, we took the PtPd/SnO2/GNs suspension solution dried at the temperature of 60 °C to acquire PtPd/SnO2/GNs powders. The quality ratio of the Pt, Pd, SnO2 and GNs was fixed at approximately 9%, 6%, 15% and 70% respectively. Noted that, the 15 wt% content of SnO2 in PtPd/SnO2/GNs nanocatalysts were synthesized in this study by combining with the require of stable plasma discharge and conductivity of catalysts. And the PtPd/GNs catalysts were synthesized by the similar method as a contrast. 2.3. Material characterizations The X-ray diffraction (XRD) measurement was performed by a Rigaka Smartlab system with Cu Kα (1.5418 Å) radiation operated at 30 kV and 40 mA to evaluate the crystallinity and phases with the scan rate of 8° min−1. The morphology and microstructure of as-prepared PtPd/SnO2/GNs was studied using a high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F) operated at 200 kV. Their compositions were studied by energy dispersive X-ray (EDX) spectroscopy (Thermo Fisher NORAN System 7, America).
3. Results and discussion 3.1. Formation mechanism and structural characterization The optical emission spectra derived from the Pt-Pd electrode pair in SnCl2·2H2O aqueous solution during discharge process was detected as shown in Fig. S1. During the discharge, the surfaces of platinum electrode and palladium electrode were constantly bombarded by the energetic radical particles produced by water decomposition in the plasma region (such as •OH, •H and O2−). Then accompanying the bombardment, Pt atoms and Pd atoms were ejected into the plasma region from their solid electrode pair’s tip. Meanwhile SnII ions were produced with some other radical species, such as OII, OI and ClI from SnCl2·2H2O solution splitting. With the great temperature and pressure differences between the plasma region and solution, SnII rapidly oxidized to SnO2, just as we discussed in our previous work. Finally, PtPd/ SnO2 by the solution plasma technique in an aqueous solution of tin(II) chloride were formed by the formation mechanism as shown in Fig. 1.
2.4. Electrochemical- and photo-electrochemical measurements All electrochemical and photo-electrochemical measurements of prepared catalyst were conducted on CHI660E or CHI750E with a three-electrodes electrochemical system. The prepared PtPd/SnO2/ GNs/GC was used as working electrode while Hg/HgO electrode (0.1 M KOH) or Ag/AgCl electrode (3.5 M KCl) and Pt foil (1 cm2) were used as the reference and counter electrode, respectively. The preparation of the PtPd/SnO2/GNs/GC working electrodes was listed as follows. Firstly, the glassy carbon (GC, Φ = 3 mm, 0.07 cm2) electrode was cleaned and polished to a mirror surface before the following steps. Then, 5 mg well-dispersed PtPd/SnO2/GNs were ultrasound-treated for 266
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Fig. 7. Multi-scan cyclic voltammetry for PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C under alkaline condition; (a, b) PtPd/SnO2/GNs, (c, d) PtPd/GNs and (e, f) commercial Pt/C.
experimental step, while the contact between the nanoparticles was tight and they were in a linear arrangement. In addition, the HR-TEM image of PtPd/SnO2/GNs catalysts in Fig. 3(c) and (d) presented nanocatalysts with the size of 2–10 nm and regular lattice fringes. In herein, the measured lattice plane spacing of 0.218 nm was corresponding to the (1 1 1) planes of PtPd alloy, while 0.264 nm and 0.176 nm were corresponding to (1 0 1) and (2 1 1) planes of SnO2. These results were in keeping with the measurement of XRD. To further characterize the composition of the sample, the surface scan and line scan by using EDX were shown in Fig. 4 and Fig. 5, respectively. It can be observed from the EDX mapping (Fig. 4b) that the Pd, Pt, Sn, and O signals were detected simultaneously in the scan region while the signals of Sn and Pt and Pd were overlapped very well. These results were also in accordance with that observed in the Fig. 3. The dark-field image (Fig. 5a) and cross-sectional compositional line
The XRD pattern of PtPd/SnO2/GNs catalysts which directly obtained by one-step method was shown in Fig. 2. Three diffraction peaks (2θ ≈ 39.9°,46.5° and 67.8°) are similar to the standard patterns of PtPd alloy (JCPDS No. 65-6418), which are corresponding to (1 1 1), (2 0 0), (2 2 0) planes. While the other two broad diffraction peaks at 33.9° and 51.8° relative to the (1 0 1) and (2 1 1) planes of SnO2 (JCPDS No.411445). In addition, the characteristic peaks at 26.5° and 54.7° were agreed with the (0 0 2) and (0 0 4) crystal planes of GNs (JCPDS No.897213). In general, PtPd/SnO2/GNs catalyst was prepared by solution plasma in SnCl2·2H2O aqueous solution. Fig. 3 shows the morphologies and microstructures of PtPd/SnO2/ GNs catalysts studied by low-resolution TEM and high-resolution TEM (HR-TEM). It can be observed from Fig. 3(a) and (b) that the as-prepared nanoparticles were highly dispersed with little aggregation, mainly because of the multiple ultrasonic dispersions during the 267
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Fig. 8. (a) The current density versus cycle number of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C under alkaline condition; (b) the relative current density versus cycle number of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C.
catalysts show the similar trend of mass activity loss, after the 150th cycle the value for mass activity of PtPd/SnO2/GNs maintained about 87% of its highest mass activity, while that of the PtPd/GNs and Pt/C, only remain 63% and 59%. What’s more, at the end of the 300th cycle, the PtPd/SnO2/GNs remain 74% of the maximum mass activity, whereas PtPd/GNs and commercial Pt/C only hold 17% and 16%, respectively. The results indicate that as-prepared PtPd/SnO2/GNs composite catalyst has extremely high catalytic performance and good cycling stability for methanol electrooxidation under alkaline conditions compared to PtPd/GNs and commercial Pt/C. The possible reasons for the promotion of performance can be listed as follows. Firstly, the bifunctional mechanism and ligand effect for the existence of Pd to improving the stability as Pd could form Pd-OH which can transform the CO-like intermediates on the surface of Pt to CO2 and then release more active sites of Pt surface for reaction. Secondly, there is synergistic effect existing between Pt and SnO2. The SnO2 can adsorb eOH species which can transform COads to CO2, releasing the Pt active site for further electrochemical reactions. Furthermore, as the result of XPS measurement shown in Fig. S2, these positive shifts of Pt 4f peaks have been proposed to be caused by the electronic interaction between Pt and SnO2. The electronic interaction could increase the d-vacancy in the valence band of Pt and decrease the Fermi level, which lead to the decrease of the binding energy between COads and Pt. Thirdly, the additive effect caused by the combination of the above two mechanisms of action. The stability of catalysts towards methanol electrocatalytic oxidation under acidic conditions was also tested, as shown in Figs. 9 and 10. Under acidic conditions, all three catalysts exhibited similar methanol oxidation peaks, in spite of the trend of their mass activity was different from that in alkaline media. The mass activity of the PtPd/SnO2/GNs and PtPd/GNs catalysts reached their respective maximum (574 mA mgPt−1 and 445 mA mgPt−1) at the 100th cycle. Differently, during the 100th to 150th cycle, the mass activity of PtPd/SnO2/GNs showed an obvious decline, whereas the mass activity of PtPd/GNs instead exhibited a relative stability during this period. It is interesting that during the 150th to 200th cycles, the mass activity of PtPd/SnO2/GNs has risen again, while the mass activity of PtPd/GNs has decreased a lot at this time and maintained a sustained downward to the 300th cycle. The commercial Pt/C catalyst was completely different from both PtPd/ SnO2/GNs and PtPd/GNs. After it exhibited the greatest mass activity of about 560 mA mgPt−1 at the 20th cycle, then it showed a continuously decreasing trend until the 300th cycle. As shown in Fig. 10(b), the retention of mass activity of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C at the end of 300 cycles was 85%, 13%, and 22%, respectively. These results show that the existing of SnO2 can also greatly improve the catalytic activity and cycle stability under acidic conditions. Its
profiles of a single bimetallic nanoparticles (Fig. 5b) were investigated. As we confirmed from Fig. 5b that the atomic ratio of Pt to Pd is about 1:1, which in compliance to mass loss during discharge process of electrodes. Based on the measurements of XRD, TEM and EDX characterization, the PtPd alloy and the SnO2 nanoparticle were prepared by one-step solution plasma technique. 3.2. Electrochemical- and photo-electrochemical performance Fig. 6 shows that CVs for PtPd/SnO2/GNs, PtPd/GNs and Pt/C samples were carried out in a solution of 0.5 M KOH and 0.5 M H2SO4 containing 1 M CH3OH at 50 mV s−1, respectively. It can be observed that all three catalysts exhibit typical methanol oxidation peaks. Mass activity that is generally used to characterize the catalytic ability of catalysts is defined as the forward peak current density divided by the Pt content on the working electrode. As expected, the PtPd/SnO2/GNs displays superior current density of 7718 mA mgPt−1 under alkaline conditions, which is about 1.4 and 4.4 times higher than the value of PtPd/GNs (5543 mA mgPt−1) and Pt/C (1765 mA mgPt−1), respectively. And, the mass activities of PtPd/SnO2/GNs, PtPd/GNs and Pt/C under acidic conditions were 574 mA mgPt−1, 445 mA mgPt−1 and 560 mA mgPt−1, respectively. Therefore, PtPd/SnO2/GNs composite catalyst also shows good catalytic performance under acidic condition. Besides, an obviously positively shift of the oxidation peak in alkaline condition and a slight positively shift in acidic condition may be derived from the addition of metal oxides SnO2, which reduced the conductivity causing a delay in reaction. Because cycling stability is also an important indicator of a measure of catalyst performance, multi-scan cyclic voltammetry was tested to investigate the cycling stability of three catalysts both under alkaline and acidic conditions. Fig. 7(a), (c) and (e) shows the cyclic voltammetry curves of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C carried out at 50 mV s−1 within the potential range of −0.6 to 0.4 V (vs. Hg/HgO) under alkaline condition from 20 to 100 cycles. Pt/C catalyst exhibited the largest mass activity (1765 mA mgPt−1) at the 20th cycle, while PtPd/SnO2/GNs and PtPd/GNs catalysts reached their maximum mass activity (7718 mA mgPt−1 and 5543 mA mgPt−1) at the 40th cycle after a rapid growth, respectively. The mass activity of the three catalysts have declined to some extent as a result of the poisoning of catalysts by CO-like intermediates after exhibiting their greatest mass activity. Fig. 7(b), (d) and (f) shows the later 100 to 300 cycles for the PtPd/SnO2/GNs, PtPd/GNs and Pt/C, respectively. In order to more intuitively observe the cycle stability of different catalysts, Fig. 8(a) shows the relationship of the mass activity of three catalysts versus the cycle number, while the relative mass activity versus the cycle number was shown in Fig. 8(b). As shown in Fig. 8(b), although the three 268
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Fig. 9. Multi-scan cyclic voltammetry for PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C under acidic condition; (a, b) PtPd/SnO2/GNs, (c, d) PtPd/GNs and (e, f) commercial Pt/C.
adsorption of hydrogen atoms on Pt. The correlation constant of 0.21 (mC cm−2) represents the charge required to oxidize a monolayer of hydrogen on a smooth Pt surface. MPt (mg) represents the mass of Pt loading on the working electrode. The measured initial ECSAs of PtPd/ SnO2/GNs, PtPd/GNs and commercial Pt/C were 206, 152.5 and 79.3 m2 g−1, respectively. That is, the initial ECSA of PtPd/SnO2/GNs is 1.4 and 2.6 times higher than PtPd/GNs and commercial Pt/C due to the linear arrangement of PtPd and SnO2 as observed in the TEM image which could effectively improve the ECSA. At the same time, after 300 cycles, the ECSAs of all three catalysts were 184, 40.6 and 16.2 m2 g−1, respectively, and clearly PtPd/SnO2/GNs showed a significant slower decay different from the other two as shown in Fig. 11(d). The value of ECSA of the PtPd/SnO2/GNs composite catalyst was nearly 89% of its initial ECSA, while the PtPd/GNs and Pt/C catalyst was declined to 26%
mechanism was similar to that under alkaline conditions. In order to further investigate the electrochemical performance of the composite catalyst prepared by one-step method and the mechanism of its improved catalytic stability in methanol, ECSA of three samples at the 10th cycle and 300th cycle under acidic conditions was characterized. Fig. 11 described the CV measurements of the PtPd/ SnO2/GNs, PtPd/GNs and Pt/C in 0.5 M H2SO4 with N2 saturated. The curve of the 10th cycle in the methanol solution under acidic conditions is marked with black and the red curve is the 300th cycle. The ECSA was calculated via equation below:
ECSA =
QH 0.21 × MPt
(1)
where QH (mC) is the amount of charge exchanged during the 269
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Fig. 10. (a) The current density versus cycle number of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C under acidic condition; (b) the relative current density versus cycle number of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C.
Fig. 11. CVs of the 10th cycle and 300th cycle in a 0.5 M H2SO4 solution at 50 mV s−1; (a) PtPd/SnO2/GNs, (b) PtPd/GNs, (c) commercial Pt/C and (d) column contrast chart.
easier to occur on the surface of the PtPd/SnO2/GNs composite catalyst than the other two catalysts. This result reveals that double effect of alloys and metal oxides effectively improved anti-CO poisoning performance of Pt and further upgraded the catalytic performance and cycling stability which matched with the results of cycling stability test under acidic and alkaline conditions. Given the fact that SnO2 has a band gap of 3.7 eV, it also possesses certain photocatalytic properties, the catalytic performance of PtPd/ SnO2/GNs composite catalyst was also characterized under light
and 20%, respectively. There results are attributed to the combination with SnO2 effectively improved the stability of catalysts under acidic conditions. And the CO-stripping experiments were carried out to characterize the anti-CO poisoning performance. Fig. 12 shows the anti-poisoning properties of the above-mentioned three catalysts. The onset potential of CO stripping of PtPd/SnO2/GNs composite catalyst is ∼0.174 V, which is far earlier than the onset potential of PtPd/GNs (∼0.209 V) and Pt/C (∼0.279 V), leading to the phenomenon that CO stripping is
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Fig. 12. CO-striping of the 1st and 2nd cycle; (a) PtPd/SnO2/GNs, (b) PtPd/GNs and (c) commercial Pt/C.
resistance to CO poisoning towards MOR. The PtPd/SnO2/GNs composite catalyst is a low-Pt-content MOR catalyst with high mass activity (7718 mA mgPt−1) under alkaline conditions and excellent cycling stability under both acidic and alkaline conditions which remained 74% and 85% of the highest mass activity after the 300th cycle. The superior electrocatalytic property of PtPd/SnO2/GNs should be attributed to the synergistic effects among alloy and metal/metal oxide. Moreover, the PtPd/SnO2/GNs exhibited a good catalytic performance under light illumination superior to that of without light illumination. That is their mass activity reached 10,029 mA mgPt−1 under light illumination which was about 1.3 times than that in dark under alkaline conditions due to the specific arrangement of metal/metal oxides. The novel onestep plasma technique could provide a useful approach for fabricating other highly efficient electrocatalysts. Acknowledgements Fig. 13. CV curves measured on PtPd/SnO2/GNs catalyst under alkaline condition without/with light illumination.
This work was supported by the National Natural Science Foundation of China (Grant No. 51372113, 51772148), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP, PPZY2015B128), and the Project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
illumination by CV measurement, as shown in Fig. 13. Obviously, the current density of PtPd/SnO2/GNs composite catalyst showed a manifest increase under light irradiation and reached 10,029 mA mgPt−1 which was about 1.3 times of that without light illumination, indicating that the PtPd/SnO2/GNs catalyst has a greater methanol photo-electrooxidation activity under light illumination. And this result reveals that the higher catalytic activity towards MOR of PtPd/SnO2/GNs composite catalyst could be attributed to the strong metal-support interaction between Pt or Pd and SnO2. Strong metal-support interaction causes the Fermi level to move to aid charge separation then creating a larger portion of the photo-generated electron/hole pairs that would participate in the surface redox reaction and provide additional photocurrent. Moreover, as is known to all, during the MOR process, the current regularly decayed because the Pt-based catalyst was gradually poisoned by CO-like intermediates. However, under light illumination, the photo-generated holes could transform eOHads to %OH radicals which can transform the COads on Pt surface to eCOOH and further form CO2 by oxidation. The metal and metal oxide nanoparticles are smaller in size and in close contact with each other, resulting in more contact interface between the two which could increase the transmission of electrons and the transfer of surface materials, would further enhance the performance under light illumination.
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4. Conclusion In summary, we fabricated PtPd/SnO2 catalyst with mass ratio of about 1:1 via a facile and rapid solution plasma technique with Pt and Pd metal wires in an aqueous solution of tin(II) chloride. PtPd and SnO2 were well-dispersed and arranged in line. The ECSA of PtPd/SnO2/GNs is 206 m2 g−1 at the 10th cycle and maintained 90% after 300 cycles under acidic conditions. The PtPd/SnO2/GNs exhibited enhanced 271
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Full Length Article
Facile synthesis of PtPd/SnO2 nanocatalysts with good photoelectrocatalytic property
T
Bingqian Yanga,b,c,1, Yawei Yua,b,c,1, Jianbo Zhanga,b,c, Lefan Yuana,b,c, Jingyuan Qiaoa, ⁎ Xiulan Hua,b,c, a
College of Materials Science and Engineering, Nanjing Tech University, China The Synergetic Innovation Center for Advanced Materials, China c Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Solution plasma Nanocomposites PtPd nano-alloy SnO2 nanoparticles Methanol electro-oxidation
PtPd/SnO2 nanocatalysts were synthesized by a simple one-step way of plasma technique in an aqueous solution of tin(II) chloride by using Pt and Pd metal wire as the electrode pair. PtPd/SnO2/GNs composite catalysts were prepared by an ultrasonic mixing PtPd/SnO2 with GNs (GNs, graphene nanosheets). The PtPd/SnO2/GNs composite catalysts exhibit a significantly enhanced electrocatalytic performance, cycling stability and COpoisoning tolerance towards methanol oxidation both under acidic and alkaline condition, which attributed to the synergism of PtPd alloy and SnO2. What’s more, the current density of PtPd/SnO2/GNs composite catalyst was obviously improved under light illumination, with 10,029 mA mgPt−1 which was about 1.3 times higher than that without light illumination under alkaline condition. The novel one-step plasma technique could provide a useful approach for fabricating other highly efficient electrocatalysts.
1. Introduction Direct methanol fuel cells (DMFCs) are of increasing interest as power systems in electronic vehicles or portable electronic devices [1,2] with the advantages of high efficiency, environmental friendliness and low operation conditions, whereas their wide commercialization have been limited by some disadvantages like the high price, low activity and poor durability. For the time being, Pt-based catalysts have been used as anode catalysts in a wide range since Pt is a significant catalyst for the methanol oxidation and oxygen reduction in DMFCs. None the less, Ptbased catalysts are highly susceptible to be poisoned by CO-like intermediates generated during methanol oxidation reaction (MOR), meanwhile the high price of Pt is also a critical problem. In order to improve the durability and stability, to enhance the catalytic activity, and to cut back the amount of Pt, many researches which focused on the development of efficient catalysts have been carried out. For instance, synthesizing nano-alloy catalysts [3–5], adding metal oxide [6,7], doping metalloid element [8,9] and synthesizing catalysts with special structure [10,11] were used to enhance the catalytic performance. For the lack of Pt, to design high-ranking Pt-based nanocatalysts in which the use of low quantities of Pt is comparable to those catalysts
currently available is urgent for us. Discuss from this aspect, Pt-based bimetallic nano-alloy catalysts, like PtPd [12,13], PtAu [4,14], PtRu [15,16], have been considered to be the substitution to current catalysts; on the other hand, Pt-based bimetallic nano-alloy catalysts show the superior catalytic ability than pure Pt catalysts. Among current Ptbased bimetallic nano-alloy catalysts, the PtPd nano-alloys have been demonstrated to be the promising electrocatalysts due to the synergism between Pt and Pd which can improve the catalytic activity. Qiu et al. demonstrated a hydrothermal one-step method to produce a single crystalline mesoporous PtPd nanoparticle, exhibiting enhanced catalytic performance towards MOR [11]. Meanwhile the Pt-based catalysts involving transition mental oxides such as SnO2 [17,18], TiO2 [19–22], ZnO [23–25] and Co3O4 [26,27] can improve the CO-tolerance ability and stability. The synergistic effect between Pt nanoparticle and metal oxides would result in the enhanced catalytic performance. As is known to all, SnO2 is an excellent transparent conductive material, which is the first commercially available doping used materials in order to improve the conductivity and stability of products. It has been reported that the Pt/SnO2/C electrocatalyst synthesized by an alcohol-reduction process shows high catalytic activity towards ethanol [28]. On the other hand, Lei et al. successfully synthesized an anodic electrocatalyst Pt/SnO2/GNs with
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Corresponding author at: College of Materials Science and Engineering, Nanjing Tech University, Xin-Mo-Fan Road No. 5, 210009 Nanjing, Jiangsu, China. E-mail address: [email protected] (X. Hu). 1 Bingqian Yang and Yawei Yu are co-first authors. https://doi.org/10.1016/j.apsusc.2018.12.002 Received 11 August 2018; Received in revised form 23 November 2018; Accepted 1 December 2018 Available online 01 December 2018 0169-4332/ © 2018 Published by Elsevier B.V.
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Fig. 1. Fabrication model of PtPd/SnO2 in SnCl2·2H2O aqueous solution by the solution plasma technique.
significantly enhanced catalytic properties towards MOR under external light irradiation via a simple one-pot strategy [29]. Integrate the above two points, the composites of bimetallic nano-alloy combined with photo responsive semiconductor metal oxides can contribute to a greater extent improving the performance of the catalysts. Nowadays, composite catalysts can be prepared by many methods, such as the chemical reduction method [30], sol-gel method [31] and physical/chemical vapor deposition method [32]. However, the abovementioned methods have some drawbacks which lead to the reduction of catalytic properties. As we all know, additional reducing agents and surfactants in chemical reduction methods are hard to be removed in the last steps, moreover, the synthesis process is complex and the strict reaction conditions or high demands on equipment are required. Thus the researches on the alloy/metal oxide composite catalysts which can furtherly synthesize efficient and robust electrocatalysts are few and far between. In our previous work, the solution plasma technique seemed to the more suitable way to synthesize alloy/metal oxide composite catalysts in an open system without any chemical additions. In this study, PtPd/SnO2/GNs catalyst was manufactured from Pt
Fig. 2. XRD pattern of PtPd/SnO2/GNs.
Fig. 3. TEM (a, b) and HR-TEM (c, d) image of PtPd/SnO2/GNs catalyst. 264
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Fig. 4. Dark-field image and EDX analysis of PtPd/SnO2/GNs. (a) dark-field images of PtPd/SnO2/GNs; (b) EDX mapping of Pd, Pt, Sn and O; (c)–(f) EDX mapping of Pd, Pt, Sn and O, respectively.
Fig. 5. Dark-field image of PtPd (a) and (b) the cross-sectional compositional line profiles as marked line in (a).
2. Experimental section
metal wire and Pd metal wire directly in an aqueous solution of tin(II) chloride via a facile and rapid solution plasma technique. GNs were used to be support materials because of good electrochemical activity and high conductivity. Electrocatalytic measurements showed that the PtPd/SnO2/GNs revealed enhanced catalytic properties and improved resistance to CO-like intermediates poisoning towards MOR. In addition, the PtPd/SnO2/GNs exhibited a better catalytic performance under light illumination than that without light illumination.
2.1. Materials Platinum (Pt) metal wire and palladium (Pd) metal wire with a diameter of 1.0 mm (Sigma-Aldrich, 99.9%) were served as the opposite electrodes. Tin(II) chloride dihydrate (SnCl2·2H2O, > 98%, SigmaAldrich) was selected to be the solute and deionized water was selected to be the solvent. Graphene nanosheets (GNs, thickness: 3–10 nm, Nanjing XFNANO Materials Tech Co., Ltd) was selected as support materials, while Nafion® (Sigma-Aldrich, 5 wt%) was selected to be the binder. Commercial Pt/C catalysts (Alfa Aesar, 2–5 nm, nominally 20 wt% Pt on carbon black) were served as the contrast. 265
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Fig. 6. CVs of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C in 0.5 M KOH (a) and 0.5 M H2SO4 (b) containing 1 M CH3OH.
at least 1 h in 2 ml deionized water by adding 6 µL 5% Nafion solution. After that, a droplet (6 µL) of the prepared suspensions was dropped on the polished GC electrode substrate and dried for 12 h at room temperature. After the working electrode was prepared, the first 30 cycles of cyclic voltammetry (CV) under the potential range between −0.2 and 1.2 V with a scan rate of 50 mV s−1 was carried out in a 0.5 M H2SO4 solution to clean the electrode surface and activate the prepared electrode to make sure that the subsequent data could be stable in following tests. Then, the electrocatalytic properties of three catalysts towards MOR were monitored by CV measurements. Under acidic condition, the measurements were conducted in the solution of 0.5 M H2SO4 containing 1 M CH3OH with the potential range from 0 to 1.0 V at 50 mV s−1. And their electrocatalytic activities under alkaline conditions were addressed in the solution of 0.5 M KOH containing 1 M CH3OH in the potential range from −0.6 to 0.4 V with the same scan rate. CO stripping experiment was tested in 0.5 M H2SO4 under the potential of −0.2 to 1.2 V by oxidizing the pre-adsorbed CO. CO gas was purged into the solution at a position close to the working electrode for 30 min with the potential holding at −0.15 V in order to allow the complete adsorption of CO onto the electrode surface. Then the excess CO was purified with N2 for 30 min under the same potential followed by the CO stripping. Their photo-electrocatalytic activity towards methanol was also tested under alkaline conditions. The working electrodes were illuminated by the 50 W Xe lamp (PLS-SXE300UV, Peking Perfectlight Co., China, wavelength ≥200 nm) during MOR process and the lamp was 15 cm away from the working electrode. All these tests mentioned above were done at room temperature.
2.2. Synthesis of PtPd/SnO2/GNs catalysts Firstly, the synthesis of PtPd/SnO2 by plasma technique in SnCl2·2H2O aqueous solution was analogous to our previous work [13,33]. In brief, the Pt metal wires and Pd metal wires were used to be opposite electrodes, and immersed in 80 ml aqueous solution containing 3 mmol SnCl2·2H2O. The gap between the opposite electrode was maintained at about 0.3 mm by a screen ruler. Plasma discharge was generated using a high voltage pulsed DC power supply (repetition frequency: 15–20 kHz, pulse width: 1–2 µs, Kurita Co.Ltd., Japan). Our previous work indicated that the yield of Pt, Pd and SnO2 was related to the time of discharge. Thus, about 9 mg Pt, 6 mg Pd and 15 mg SnO2 were synthesized after 30 min discharge. Then GNs were mixed in PtPd/SnO2 suspension and later ultrasonication for several hours to prepare uniformly dispersed PtPd/SnO2/GNs catalyst. At last, we took the PtPd/SnO2/GNs suspension solution dried at the temperature of 60 °C to acquire PtPd/SnO2/GNs powders. The quality ratio of the Pt, Pd, SnO2 and GNs was fixed at approximately 9%, 6%, 15% and 70% respectively. Noted that, the 15 wt% content of SnO2 in PtPd/SnO2/GNs nanocatalysts were synthesized in this study by combining with the require of stable plasma discharge and conductivity of catalysts. And the PtPd/GNs catalysts were synthesized by the similar method as a contrast. 2.3. Material characterizations The X-ray diffraction (XRD) measurement was performed by a Rigaka Smartlab system with Cu Kα (1.5418 Å) radiation operated at 30 kV and 40 mA to evaluate the crystallinity and phases with the scan rate of 8° min−1. The morphology and microstructure of as-prepared PtPd/SnO2/GNs was studied using a high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F) operated at 200 kV. Their compositions were studied by energy dispersive X-ray (EDX) spectroscopy (Thermo Fisher NORAN System 7, America).
3. Results and discussion 3.1. Formation mechanism and structural characterization The optical emission spectra derived from the Pt-Pd electrode pair in SnCl2·2H2O aqueous solution during discharge process was detected as shown in Fig. S1. During the discharge, the surfaces of platinum electrode and palladium electrode were constantly bombarded by the energetic radical particles produced by water decomposition in the plasma region (such as •OH, •H and O2−). Then accompanying the bombardment, Pt atoms and Pd atoms were ejected into the plasma region from their solid electrode pair’s tip. Meanwhile SnII ions were produced with some other radical species, such as OII, OI and ClI from SnCl2·2H2O solution splitting. With the great temperature and pressure differences between the plasma region and solution, SnII rapidly oxidized to SnO2, just as we discussed in our previous work. Finally, PtPd/ SnO2 by the solution plasma technique in an aqueous solution of tin(II) chloride were formed by the formation mechanism as shown in Fig. 1.
2.4. Electrochemical- and photo-electrochemical measurements All electrochemical and photo-electrochemical measurements of prepared catalyst were conducted on CHI660E or CHI750E with a three-electrodes electrochemical system. The prepared PtPd/SnO2/ GNs/GC was used as working electrode while Hg/HgO electrode (0.1 M KOH) or Ag/AgCl electrode (3.5 M KCl) and Pt foil (1 cm2) were used as the reference and counter electrode, respectively. The preparation of the PtPd/SnO2/GNs/GC working electrodes was listed as follows. Firstly, the glassy carbon (GC, Φ = 3 mm, 0.07 cm2) electrode was cleaned and polished to a mirror surface before the following steps. Then, 5 mg well-dispersed PtPd/SnO2/GNs were ultrasound-treated for 266
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Fig. 7. Multi-scan cyclic voltammetry for PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C under alkaline condition; (a, b) PtPd/SnO2/GNs, (c, d) PtPd/GNs and (e, f) commercial Pt/C.
experimental step, while the contact between the nanoparticles was tight and they were in a linear arrangement. In addition, the HR-TEM image of PtPd/SnO2/GNs catalysts in Fig. 3(c) and (d) presented nanocatalysts with the size of 2–10 nm and regular lattice fringes. In herein, the measured lattice plane spacing of 0.218 nm was corresponding to the (1 1 1) planes of PtPd alloy, while 0.264 nm and 0.176 nm were corresponding to (1 0 1) and (2 1 1) planes of SnO2. These results were in keeping with the measurement of XRD. To further characterize the composition of the sample, the surface scan and line scan by using EDX were shown in Fig. 4 and Fig. 5, respectively. It can be observed from the EDX mapping (Fig. 4b) that the Pd, Pt, Sn, and O signals were detected simultaneously in the scan region while the signals of Sn and Pt and Pd were overlapped very well. These results were also in accordance with that observed in the Fig. 3. The dark-field image (Fig. 5a) and cross-sectional compositional line
The XRD pattern of PtPd/SnO2/GNs catalysts which directly obtained by one-step method was shown in Fig. 2. Three diffraction peaks (2θ ≈ 39.9°,46.5° and 67.8°) are similar to the standard patterns of PtPd alloy (JCPDS No. 65-6418), which are corresponding to (1 1 1), (2 0 0), (2 2 0) planes. While the other two broad diffraction peaks at 33.9° and 51.8° relative to the (1 0 1) and (2 1 1) planes of SnO2 (JCPDS No.411445). In addition, the characteristic peaks at 26.5° and 54.7° were agreed with the (0 0 2) and (0 0 4) crystal planes of GNs (JCPDS No.897213). In general, PtPd/SnO2/GNs catalyst was prepared by solution plasma in SnCl2·2H2O aqueous solution. Fig. 3 shows the morphologies and microstructures of PtPd/SnO2/ GNs catalysts studied by low-resolution TEM and high-resolution TEM (HR-TEM). It can be observed from Fig. 3(a) and (b) that the as-prepared nanoparticles were highly dispersed with little aggregation, mainly because of the multiple ultrasonic dispersions during the 267
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Fig. 8. (a) The current density versus cycle number of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C under alkaline condition; (b) the relative current density versus cycle number of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C.
catalysts show the similar trend of mass activity loss, after the 150th cycle the value for mass activity of PtPd/SnO2/GNs maintained about 87% of its highest mass activity, while that of the PtPd/GNs and Pt/C, only remain 63% and 59%. What’s more, at the end of the 300th cycle, the PtPd/SnO2/GNs remain 74% of the maximum mass activity, whereas PtPd/GNs and commercial Pt/C only hold 17% and 16%, respectively. The results indicate that as-prepared PtPd/SnO2/GNs composite catalyst has extremely high catalytic performance and good cycling stability for methanol electrooxidation under alkaline conditions compared to PtPd/GNs and commercial Pt/C. The possible reasons for the promotion of performance can be listed as follows. Firstly, the bifunctional mechanism and ligand effect for the existence of Pd to improving the stability as Pd could form Pd-OH which can transform the CO-like intermediates on the surface of Pt to CO2 and then release more active sites of Pt surface for reaction. Secondly, there is synergistic effect existing between Pt and SnO2. The SnO2 can adsorb eOH species which can transform COads to CO2, releasing the Pt active site for further electrochemical reactions. Furthermore, as the result of XPS measurement shown in Fig. S2, these positive shifts of Pt 4f peaks have been proposed to be caused by the electronic interaction between Pt and SnO2. The electronic interaction could increase the d-vacancy in the valence band of Pt and decrease the Fermi level, which lead to the decrease of the binding energy between COads and Pt. Thirdly, the additive effect caused by the combination of the above two mechanisms of action. The stability of catalysts towards methanol electrocatalytic oxidation under acidic conditions was also tested, as shown in Figs. 9 and 10. Under acidic conditions, all three catalysts exhibited similar methanol oxidation peaks, in spite of the trend of their mass activity was different from that in alkaline media. The mass activity of the PtPd/SnO2/GNs and PtPd/GNs catalysts reached their respective maximum (574 mA mgPt−1 and 445 mA mgPt−1) at the 100th cycle. Differently, during the 100th to 150th cycle, the mass activity of PtPd/SnO2/GNs showed an obvious decline, whereas the mass activity of PtPd/GNs instead exhibited a relative stability during this period. It is interesting that during the 150th to 200th cycles, the mass activity of PtPd/SnO2/GNs has risen again, while the mass activity of PtPd/GNs has decreased a lot at this time and maintained a sustained downward to the 300th cycle. The commercial Pt/C catalyst was completely different from both PtPd/ SnO2/GNs and PtPd/GNs. After it exhibited the greatest mass activity of about 560 mA mgPt−1 at the 20th cycle, then it showed a continuously decreasing trend until the 300th cycle. As shown in Fig. 10(b), the retention of mass activity of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C at the end of 300 cycles was 85%, 13%, and 22%, respectively. These results show that the existing of SnO2 can also greatly improve the catalytic activity and cycle stability under acidic conditions. Its
profiles of a single bimetallic nanoparticles (Fig. 5b) were investigated. As we confirmed from Fig. 5b that the atomic ratio of Pt to Pd is about 1:1, which in compliance to mass loss during discharge process of electrodes. Based on the measurements of XRD, TEM and EDX characterization, the PtPd alloy and the SnO2 nanoparticle were prepared by one-step solution plasma technique. 3.2. Electrochemical- and photo-electrochemical performance Fig. 6 shows that CVs for PtPd/SnO2/GNs, PtPd/GNs and Pt/C samples were carried out in a solution of 0.5 M KOH and 0.5 M H2SO4 containing 1 M CH3OH at 50 mV s−1, respectively. It can be observed that all three catalysts exhibit typical methanol oxidation peaks. Mass activity that is generally used to characterize the catalytic ability of catalysts is defined as the forward peak current density divided by the Pt content on the working electrode. As expected, the PtPd/SnO2/GNs displays superior current density of 7718 mA mgPt−1 under alkaline conditions, which is about 1.4 and 4.4 times higher than the value of PtPd/GNs (5543 mA mgPt−1) and Pt/C (1765 mA mgPt−1), respectively. And, the mass activities of PtPd/SnO2/GNs, PtPd/GNs and Pt/C under acidic conditions were 574 mA mgPt−1, 445 mA mgPt−1 and 560 mA mgPt−1, respectively. Therefore, PtPd/SnO2/GNs composite catalyst also shows good catalytic performance under acidic condition. Besides, an obviously positively shift of the oxidation peak in alkaline condition and a slight positively shift in acidic condition may be derived from the addition of metal oxides SnO2, which reduced the conductivity causing a delay in reaction. Because cycling stability is also an important indicator of a measure of catalyst performance, multi-scan cyclic voltammetry was tested to investigate the cycling stability of three catalysts both under alkaline and acidic conditions. Fig. 7(a), (c) and (e) shows the cyclic voltammetry curves of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C carried out at 50 mV s−1 within the potential range of −0.6 to 0.4 V (vs. Hg/HgO) under alkaline condition from 20 to 100 cycles. Pt/C catalyst exhibited the largest mass activity (1765 mA mgPt−1) at the 20th cycle, while PtPd/SnO2/GNs and PtPd/GNs catalysts reached their maximum mass activity (7718 mA mgPt−1 and 5543 mA mgPt−1) at the 40th cycle after a rapid growth, respectively. The mass activity of the three catalysts have declined to some extent as a result of the poisoning of catalysts by CO-like intermediates after exhibiting their greatest mass activity. Fig. 7(b), (d) and (f) shows the later 100 to 300 cycles for the PtPd/SnO2/GNs, PtPd/GNs and Pt/C, respectively. In order to more intuitively observe the cycle stability of different catalysts, Fig. 8(a) shows the relationship of the mass activity of three catalysts versus the cycle number, while the relative mass activity versus the cycle number was shown in Fig. 8(b). As shown in Fig. 8(b), although the three 268
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Fig. 9. Multi-scan cyclic voltammetry for PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C under acidic condition; (a, b) PtPd/SnO2/GNs, (c, d) PtPd/GNs and (e, f) commercial Pt/C.
adsorption of hydrogen atoms on Pt. The correlation constant of 0.21 (mC cm−2) represents the charge required to oxidize a monolayer of hydrogen on a smooth Pt surface. MPt (mg) represents the mass of Pt loading on the working electrode. The measured initial ECSAs of PtPd/ SnO2/GNs, PtPd/GNs and commercial Pt/C were 206, 152.5 and 79.3 m2 g−1, respectively. That is, the initial ECSA of PtPd/SnO2/GNs is 1.4 and 2.6 times higher than PtPd/GNs and commercial Pt/C due to the linear arrangement of PtPd and SnO2 as observed in the TEM image which could effectively improve the ECSA. At the same time, after 300 cycles, the ECSAs of all three catalysts were 184, 40.6 and 16.2 m2 g−1, respectively, and clearly PtPd/SnO2/GNs showed a significant slower decay different from the other two as shown in Fig. 11(d). The value of ECSA of the PtPd/SnO2/GNs composite catalyst was nearly 89% of its initial ECSA, while the PtPd/GNs and Pt/C catalyst was declined to 26%
mechanism was similar to that under alkaline conditions. In order to further investigate the electrochemical performance of the composite catalyst prepared by one-step method and the mechanism of its improved catalytic stability in methanol, ECSA of three samples at the 10th cycle and 300th cycle under acidic conditions was characterized. Fig. 11 described the CV measurements of the PtPd/ SnO2/GNs, PtPd/GNs and Pt/C in 0.5 M H2SO4 with N2 saturated. The curve of the 10th cycle in the methanol solution under acidic conditions is marked with black and the red curve is the 300th cycle. The ECSA was calculated via equation below:
ECSA =
QH 0.21 × MPt
(1)
where QH (mC) is the amount of charge exchanged during the 269
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Fig. 10. (a) The current density versus cycle number of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C under acidic condition; (b) the relative current density versus cycle number of PtPd/SnO2/GNs, PtPd/GNs and commercial Pt/C.
Fig. 11. CVs of the 10th cycle and 300th cycle in a 0.5 M H2SO4 solution at 50 mV s−1; (a) PtPd/SnO2/GNs, (b) PtPd/GNs, (c) commercial Pt/C and (d) column contrast chart.
easier to occur on the surface of the PtPd/SnO2/GNs composite catalyst than the other two catalysts. This result reveals that double effect of alloys and metal oxides effectively improved anti-CO poisoning performance of Pt and further upgraded the catalytic performance and cycling stability which matched with the results of cycling stability test under acidic and alkaline conditions. Given the fact that SnO2 has a band gap of 3.7 eV, it also possesses certain photocatalytic properties, the catalytic performance of PtPd/ SnO2/GNs composite catalyst was also characterized under light
and 20%, respectively. There results are attributed to the combination with SnO2 effectively improved the stability of catalysts under acidic conditions. And the CO-stripping experiments were carried out to characterize the anti-CO poisoning performance. Fig. 12 shows the anti-poisoning properties of the above-mentioned three catalysts. The onset potential of CO stripping of PtPd/SnO2/GNs composite catalyst is ∼0.174 V, which is far earlier than the onset potential of PtPd/GNs (∼0.209 V) and Pt/C (∼0.279 V), leading to the phenomenon that CO stripping is
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Fig. 12. CO-striping of the 1st and 2nd cycle; (a) PtPd/SnO2/GNs, (b) PtPd/GNs and (c) commercial Pt/C.
resistance to CO poisoning towards MOR. The PtPd/SnO2/GNs composite catalyst is a low-Pt-content MOR catalyst with high mass activity (7718 mA mgPt−1) under alkaline conditions and excellent cycling stability under both acidic and alkaline conditions which remained 74% and 85% of the highest mass activity after the 300th cycle. The superior electrocatalytic property of PtPd/SnO2/GNs should be attributed to the synergistic effects among alloy and metal/metal oxide. Moreover, the PtPd/SnO2/GNs exhibited a good catalytic performance under light illumination superior to that of without light illumination. That is their mass activity reached 10,029 mA mgPt−1 under light illumination which was about 1.3 times than that in dark under alkaline conditions due to the specific arrangement of metal/metal oxides. The novel onestep plasma technique could provide a useful approach for fabricating other highly efficient electrocatalysts. Acknowledgements Fig. 13. CV curves measured on PtPd/SnO2/GNs catalyst under alkaline condition without/with light illumination.
This work was supported by the National Natural Science Foundation of China (Grant No. 51372113, 51772148), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP, PPZY2015B128), and the Project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
illumination by CV measurement, as shown in Fig. 13. Obviously, the current density of PtPd/SnO2/GNs composite catalyst showed a manifest increase under light irradiation and reached 10,029 mA mgPt−1 which was about 1.3 times of that without light illumination, indicating that the PtPd/SnO2/GNs catalyst has a greater methanol photo-electrooxidation activity under light illumination. And this result reveals that the higher catalytic activity towards MOR of PtPd/SnO2/GNs composite catalyst could be attributed to the strong metal-support interaction between Pt or Pd and SnO2. Strong metal-support interaction causes the Fermi level to move to aid charge separation then creating a larger portion of the photo-generated electron/hole pairs that would participate in the surface redox reaction and provide additional photocurrent. Moreover, as is known to all, during the MOR process, the current regularly decayed because the Pt-based catalyst was gradually poisoned by CO-like intermediates. However, under light illumination, the photo-generated holes could transform eOHads to %OH radicals which can transform the COads on Pt surface to eCOOH and further form CO2 by oxidation. The metal and metal oxide nanoparticles are smaller in size and in close contact with each other, resulting in more contact interface between the two which could increase the transmission of electrons and the transfer of surface materials, would further enhance the performance under light illumination.
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4. Conclusion In summary, we fabricated PtPd/SnO2 catalyst with mass ratio of about 1:1 via a facile and rapid solution plasma technique with Pt and Pd metal wires in an aqueous solution of tin(II) chloride. PtPd and SnO2 were well-dispersed and arranged in line. The ECSA of PtPd/SnO2/GNs is 206 m2 g−1 at the 10th cycle and maintained 90% after 300 cycles under acidic conditions. The PtPd/SnO2/GNs exhibited enhanced 271
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