- Email: [email protected]
C porous nanospheres and their applications for ethanol oxidation reaction
Journal of Power Sources 336 (2016) 1e7
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Facile synthesis of PdSx/C porous nanospheres and their applications for ethanol oxidation reaction Qiang Zhang a, Fuhua Zhang a, Xuemei Ma a, Yiqun Zheng b, Shifeng Hou a, b, * a b
School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, China National Engineering and Technology Research Center for Colloidal Materials, Shandong University, Jinan, Shandong 250100, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
PdSx/C porous nanospheres were prepared via calcination of Pd/PEDOT precursor. PdSx nanoparticles were homogeneously dispersed on the surface of porous carbon support. PdSx/C shows a better catalytic activity than common sulfur-poisoned Pd catalyst.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 July 2016 Received in revised form 16 September 2016 Accepted 8 October 2016
We report a facile approach for the synthesis of carbon-supported palladium polysulphide porous nanospheres (PdSx/C) and their applications for ethanol oxidation reaction. Typical synthesis started with generation of palladium/poly (3,4-ethylenedioxythiophene)(Pd/PEDOT) nanospheres, followed by a calcination process at an optimized temperature to form PdSx/C, with an average size of 2.47 ± 0.60 and 50 nm of PdSx nanoparticles and carbon porous nanospheres, respectively. Various techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical techniques were performed to characterize their morphologies, compositions and structures. In contrary to most Pdbased electrochemical catalysts that could be easily poised with trace sulfur during the catalytic oxidation process, the as-prepared PdSx/C porous nanospheres exhibited high electrocatalytic activities and stabilities for the electrochemical catalytic oxidation of ethanol in alkaline medium. In particular, the forward peak current intensity achieved 162.1 mA mg1 and still maintained at 46.7 mA mg1 even after 1000 cycles. This current work not only offers a novel type of fuel-cell catalyst for ethanol oxidation reaction, but also provides a possible route for solving the sulfur-poisoning problem in catalysis. © 2016 Elsevier B.V. All rights reserved.
Keywords: Porous nanospheres Carbon-supported palladium polysulphide PdSx nanoparticles Electrocatalytic activity Ethanol oxidation
1. Introduction
* Corresponding author. School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, China. E-mail address: [email protected] (S. Hou). http://dx.doi.org/10.1016/j.jpowsour.2016.10.028 0378-7753/© 2016 Elsevier B.V. All rights reserved.
With growing demand for energy coupled with concerns over environmental pollution and the depletion of fossil fuels, the development of new clean energy technologies is desired [1e3]. Fuel cells, converting the chemical energy to electrical energy by means of a chemical reaction, have attracted considerable interest
2
Q. Zhang et al. / Journal of Power Sources 336 (2016) 1e7
among researchers [4]. Among the various liquid fuels, ethanol is greatly highlighted due to its merits such as low toxicity, high energy density, good stability, easy storage, and low permeability across proton exchange membrane [5]. In this area, the direct ethanol fuel cell (DEFC) is widely applied in the field of alternative energy for their abilities of high energy density of ethanol and the ease of handing liquid fuel [6e9]. For DEFC, Pd has been considered as a promising catalyst due to its less expensive cost, higher activity, and better steady-state behavior in alkaline solutions [10,11]. In recent years, various strategies have been developed to synthesize Pd catalyst, such as reduction of Pd precursor in aqueous and/or organic phase [12e15]and electrochemical route [16]. Among those methods, one-pot synthesis has aroused increasing research interest due to its simplicity in synthesis. Typically, the reduction or thermal decomposition of Pd precursor with/without the presence of capping agent could readily produce Pd nanoparticles with reasonable control over their size and morphology. For example, Gao et al. [17]demonstrated a very simple route for one-pot synthesis of branched Pd nanodendrites in a mixture solution of oleic acid and oleylamine, which exhibited substantially enhanced catalytic activities towards oxidation reduction reaction (ORR) and ethanol oxidation reaction (EOR). However, such onestep synthesis of Pd catalyst usually leads to aggregation of nanoparticles and the unavoidable use of massive additional surfactants and thus surface contamination [18]. To this end, the involvement of supporting materials, such as carbon black [19e21], carbon nanofibers [22], carbon nanotubes [23] and graphene [24] in onepot synthesis, could effectively enhance the dispersion and utilization of Pd catalysts. These carbon materials not only significantly enhance the available electrochemical active surface area of catalysts for electron transfer but also provide high mass transport of reactants to the catalysts [19]. For example, Yang co-workers [25] reported a comparatively easy, one-pot method to synthesize monodisperse palladium NPs dispersed on reduced graphene oxide (rGO) in N-methyl-2-pyrrolidone solution, and found that the Pd/ rGO displayed much higher catalytic activity and better stability than conventional Pd/XC-72 electrocatalysts. It is worth noting that during the catalysis, the Pd catalyst could be malfunctioned and the catalytic performance would be negatively influenced when there is sulfur present in the system. It is because the sulfur atoms could bond strongly to the surface of Pd to form PdSx, which resulted in the decrease in the number of active sites and thus blocked effective contact between catalyst and reactants. In fact, PdSx has been found to be useful for a variety of catalytic application due to their special physiochemical properties [26e29]. For example, Mashkina and co-workers reported that PdSx is a promising catalyst for thiophene hydrogenation and the reductive N-alkylation of aromatic amines due to their ease of formation of active sites [30]. However, it remains grand technique challenges to utilize PdSx as catalyst toward EOR due to the difficulties caused by sulfur poisoning. In this study, we report a facile approach for the synthesis of PdSx/C porous nanospheres and interestingly, the sulfurized Pd in this composite exhibited electrocatalytic activity for EOR in alkaline solutions. In particular, Pd2þ was reduced during the formation of PEDOT nanospheres process and the resultant Pd nanoparticles were attached to PEDOT nanosphere surface to Pd/PEDOT, which was then calcined at an elevated temperature to form PdSx/C porous nanospheres. The resultant products were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The catalytic performance and stability of the PdSx/C porous nanospheres for EOR under alkaline conditions were explored by cyclic voltammetry and
chronoamperometry. Importantly, as-prepared PdSx/C porous nanospheres display greatly improved electrocatalytic activity and durability for EOR as compared with pure PdS. 2. Experimental 2.1. Materials and characterization 3,4-ethylenedioxythiophene (EDOT) was purchased from Bayer. PdCl2, CH3CH2OH, KOH and H2SO4 (98%) were obtained from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade and all aqueous solutions were prepared with deionized water. A 5.0 mM H2PdCl4aqueous solution was prepared by dissolving 17.7 mg of PdCl2 in 20 mL of 10 mM HCl solution with sonication. Scanning electron microscopy (SEM) images were captured using an Ultra Plus microscope operated at 3.0 kV (Zeiss, Oberkochen, Germany). The samples were prepared by dropping aqueous suspensions of the nanospheres onto Cu foils. Transmission electron microscopy (TEM) measurements were carried out using a FEI Quanta 200F electron microscope with an accelerating voltage of 200 kV. High-resolution TEM images were obtained using a fieldemission JEM-2100F microscope operated at 200 kV (JEOL, Tokyo, Japan). The samples were prepared by dropping aqueous suspensions of the nanoparticles onto carbon-coated Cu grids and dried under ambient conditions. X-ray diffraction (XRD) patterns were recorded using a diffractometer with filtered Cu Ka radiation at 0.154 nm. Raman spectra were recorded on a LABRAM-HR confocal laser micro-Raman spectrometer employing a 20 mW laser at 632.8 nm. Nitrogen sorption isotherms were performed on a Kubo X1000 instrument at 77 K. Prior to measurements, the samples were out gassed at 180 C under vacuum for more than 6 h. The specific surface area was derived using the analysis methods of Brunauer-Emmett-Teller (BET). The pore-size distribution was determined from the adsorption branch of the isotherm, according to Barrett-Joyner-Halanda (BJH) method. The total pore volumes were estimated from the adsorbed amount at a relative pressure of p/p0 ¼ 0.98. X-ray photo electron spectroscopy (XPS) measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with a Mgka excitation source (1253.6 eV). 2.2. Preparation of Pd/PEDOT nanospheres and PdSx/C porous nanospheres Pd/PEDOT nanospheres were prepared using a facile process according to previous report [18]. In brief, 5.0 mL of EDOT ethanol solution (14.2 wt%) was mixed with 20 mL of H2PdCl4 aqueous solution (5.0 mM) and the mixture was magnetically stirred for 3 h at room temperature. During the process, EDOT monomers were oxidized and polymerized to PEDOT nanospheres. Meanwhile, Pd nanoparticles were formed via the reduction of PdCl2 4 by EDOT monomers, which were enwrapped by PEDOT nanospheres. Then, the aqueous mixture was kept constantly stirring for 4 h. Subsequently, the product was collected via centrifugation, filtrated with water and ethanol alternatively, and then vacuum-dried at room temperature. PdSx/C porous nanospheres were prepared via the calcination of as-prepared Pd/PEDOT composite. Briefly, 1.5 g of the as-prepared Pd/PEDOT was placed in a quartz tube furnace and then calcined under N2 protection with a heating rate of 10 C min1 for 2 h. For calcinations temperature set at 300 C, 500 C, 600 C, 700 C, 800 C and 900 C, the resultant samples were labeled as PdSx/C-3, PdSx/C-5, PdSx/C-6, PdSx/C, PdSx/C-8 and PdSx/C-9, respectively. After the reaction finished, the furnace was cooled down to room temperature naturally. The samples were collected, washed with
Q. Zhang et al. / Journal of Power Sources 336 (2016) 1e7
water, and dried at 70 characterization.
C
overnight for further use and
2.3. Electrochemical test All electrochemical data were recorded with a CHI-760E
3
electrochemical workstation (Shanghai CH Instrumental Co.), using a conventional three-electrode system at room temperature. A saturated calomel electrode (SCE), a platinum wire and a glassy carbon electrode (GCE, diameter: 3.0 mm) were used as the reference, counter and working electrodes, respectively. Before each experiment, the GCE was carefully polished with alumina slurry of
Fig. 1. Structure and morphology characterizations of (AeC) Pd/PEDOT and (DeF) PdSx/C porous nanospheres: (A, D) SEM; (B, E) TEM; (C, F) corresponding size distribution histograms of Pd and PdSx nanoparticles, respectively.
Fig. 2. (A) High-resolution TEM and (BeD) element mapping images of PdSx/C porous nanospheres.
4
Q. Zhang et al. / Journal of Power Sources 336 (2016) 1e7
For comparision, the electrocatalytic performance of the pure PdS catalyst on the GCE also measured by similar process. 3. Results and discussion
Fig. 3. (A) Raman spectrum and (B) XRD pattern of PdSx/C porous nanospheres.
0.5 mm in diameter on a polishing cloth, followed by sonication in DI water and an ethanol bath for 10 min. For electrochemical measurements, the suspension of PdSx/C was obtained by dispersing 10 mg of the as-prepared PdSx/C in the mixture of 4.0 mL deionized water and 20 mL Nafion solution (5 wt%) for 30 min. Then, 10 mL of as-prepared suspension of PdSx/C was dropped on surface of GCE and dried with an infrared lamp to form PdSx/C electrode.
The morphology of Pd/PEDOT and PdSx/C porous nanospheres was characterized by SEM and TEM. As shown in Fig. 1A and B, the Pd/PEDOT composites exhibited the spherical shape with an average diameter of 50 nm. Many of nano/micro-pores are left between adjacent nanospheres, which would be in favor to the accessibility of electrolyte. Pd nanoparticles were anchored on the surface of PEDOT nanospheres. Fig. 1C shows the size distribution of Pd nanoparticles in Pd/PEDOT composites and the size of Pd nanoparticles is 2.47 ± 0.95 nm. Fig. 1D shows the SEM image of PdSx/C obtained by calcination of Pd/PEDOT. Compared with the Pd/PEDOT, the morphology of PdSx/C products was varied to be with relatively coarse surfaces. It is noteworthy to mention that the calcined product remained spherical shapes after being annealed at high temperature. TEM image (Fig. 1E) shows that tiny nanoparticles were anchored on the surface of PdSx/C porous nanospheres and distributed homogeneously. As shown in Fig. 1F, the size distribution of PdSx nanoparticles conforms to normal distribution with an average diameter of 2.47 ± 0.60 nm. In addition, Fig. 2 shows the HRTEM and corresponding element mapping images of Pd and S for PdSx/C porous nanospheres. It can be clearly observed that both Pd and S are evenly distributed on the whole nanospheres. To further confirm the component of as-prepared sample, Raman spectroscopy, XRD and XPS were performed. As shown in Fig. 3A, the appearance of the two prominent peaks at 1328 and 1584 cm1, respectively, could be ascribed to the D and G bands of the carbon materials [31], respectively. Generally, the D band could be assigned to the breathing mode of k-point phonons of A1g symmetry, while the G band could be related to the E2g phonons of sp2 carbon atoms [32,33]. This means that the PEDOT in the Pd/ PEDOT nanospheres has been converted to a kind of carbon
Fig. 4. (A) XPS survey spectrum as well as high-resolution (B) S 2p and Pd 3d spectra of the PdSx/C porous nanospheres. (C) S 2p and (D) Pd 3d XPS spectra of PdSx/C porous nanospheres with peak fittings.
Q. Zhang et al. / Journal of Power Sources 336 (2016) 1e7
material. The crystal structures of the as-prepared PdSx/C porous nanospheres were further characterized by XRD, and the corresponding results were shown in Fig. 3B. Several PdSx phase, including “Pd16S7”, “Pd3S” and “Pd4S” phases, could be observed in the diffraction pattern, which is attributed to the formation of palladium sulfides with mixed crystal phases during the calcination process. As indicated in previous studies, the sulfurization process usually leads to the formation of palladium sulfides with mixed crystal phases [34,35]. Compared with the PdSx/C porous nanospheres, no noticeable peak was observed for Pd/PEDOT (Fig. S1), which is probably ascribed to the poor crystallinity of Pd nanoparticles before calcination. Fig. 4A shows the XPS survey spectra of PdSx/C porous nanospheres, where four elements including C, O, S, and Pd are observed. The S 2p scan in Fig. 4B exhibits two major peaks located at 162.6 eV and 163.8 eV, which is corresponding to S 2p3/2 and S 2p1/2, respectively [36,37]. The spectra of Pd 3d in Fig. 4B show that one doublet of the 3d5/2 and 3d3/2 states is centered at 335.8 eV and 341.2 eV, respectively [38,39]. For S 2p spectra of PdSx/C (Fig. 4C), the binding energies for S 2p3/2 and S 2p1/2 could be resolved into three doublets. The peak appears at 162.5 eV, indicating the presence of PdeS bonds on the nanoparticles at the support [40]. The other couple located at 167.9 eV is assigned to the oxidized sulfur groups coming from the exposure to air during storage [41,42]. While, the peak appears at 164.6 eV, which can be attributed to the SeS bonds formed between PEDOT molecules [43,44]. Fig. 4D shows the peak fitting of a high-resolution Pd 3d region. There are two peaks at about 335.8 eV and 341.2 eV, could be corresponding to the Pd 3d5/2 and Pd 3d3/2 binding energies, respectively. The two peaks can be divided into two pairs, the binding energy of one pair is emerging at 336.0 and 341.3 eV, which can be assigned to Pd(II) [45]. Whereas the other weaker peaks are located at 337.9 and
5
343.15 eV, indicating the presence of Pd (IV) [38]. Based on their peak intensity, it can be concluded that Pd (II) is the predominant species. To study the surface area and particle size distribution of the support materials, Fig. 5 shows the N2 adsorption-desorption isotherms. BET specific surface area and total pore volume of asprepared PdSx/C porous nanospheres are 169.18 m2 g1 and 0.27 cm3 g1, respectively. However, the BET surface area is lower than that of pure carbon (Vulcan XC-72R 225e250 m2/g). This is not unexpected because the composite involved Pd with relatively low BET surface area (35e45 m2 g1). As shown in the inset and Table 1, the pore size distribution (PSD) calculated from the desorption branch based on the BJH model. We observe that the samples show bimodal mesopores in the PSD curve centered at 3.90 and 31.8 nm. The PSD peak at the mean value of ~3.90 nm can be mainly attributed to the pores formed on the surfaces of carbon-based nanospheres. In addition, the larger pores (31.8 nm) mainly derive from aggregation between adjacent nanospheres, which is of great benefit to the diffusion of the electrolyte. Note that the pore size of the PdSx/C decreases after calcinations compared to that of Pd/PEDOT. It is probably attributed to the contraction of carbon nanospheres during the calcination, which has also observed in previous reports [46,47].
Fig. 5. N2 adsorption/desorption isotherm and the corresponding pore size distribution (inset) of PdSx/C porous nanospheres.
Table 1 Effects of calcination temperature on the BET specific surface area (SBET) and pore parameters of samples. Sample
SBETa (m2/g)
Pore sizeb (nm)
Pore volumec (cm3/g)
Pd/PEDOT PdSx/C
22.93 169.18
51.90 3.90, 31.80
0.24 0.27
a BET surface area calculated from the linear part of the BET plot (p/ p0 ¼ 0.05e0.3). b Total pore volume, taken from the volume of N2-adsorbed at about p/p0 ¼ 0.995. c Pore diameter, estimated using the desorption branch of the isotherm and the Barrett-Joyner-Halenda (BJH) formula.
Fig. 6. (A, B) SEM images of (A) PdSx/C-5 and (B) PdSx/C-9. (C) XRD patterns of (a) PdSx/C-5 and (b) PdSx/C-9.
6
Q. Zhang et al. / Journal of Power Sources 336 (2016) 1e7
In the standard procedure, the calcination of Pd/PEDOT nanospheres at an elevated temperature is highly crucial for the formation of PdSx/C with a unique porous structure and spherical shape. To this end, we conducted several control experiments to investigate the effects of calcination temperature on the morphology, composition, and structure of PdSx/C. Fig. 6 shows the SEM images of PdSx/C-5 and PdSx/C-9, respectively. Interestingly, the two samples still exhibit the spherical structure. However, the nanospheres suffer from a severe aggregation as the calcination temperature increases to 900 C (Fig. 6B). XRD patterns of the PdSx/ C-5 and PdSx/C-9 are also investigated in Fig. 6C. For PdSx/C-5 sample, only a broad peak centered at 37.2 is observed, indicating the poor crystallinity of PdSx/C-5. With a further increasing annealed temperature, PdSx/C-9 sample shows similar sharp peaks as PdSx/C with mixtures of “Pd16S7”, “Pd3S” and “Pd4S” phases, indicting the PdSx with high crystallinity formed at temperatures above 500 C. It is worth noting that the peaks between 60 and 80 (2 theta) disappeared, as compared to PdSx/C porous nanospheres, which could be owing to the phase transfer of PdSx. Meanwhile, the SEM images and XRD patterns of the PdSx/C-3, PdSx/C-6, and PdSx/ C-8 are also shown in Fig. S2 as control groups. Based on the results above, the annealed temperature at 700 C is considered as an optimal condition for preparation of PdSx/C porous nanospheres and the corresponding product is then used for electrochemical experiments. Electrocatalytic activity of the as-prepared PdSx/C porous nanospheres was investigated by cyclic voltammetry (CV) experiments. We also included PdS as the catalyst for comparison during the test. Fig. 7A shows the typical CVs of the PdSx/C porous nanospheres and PdS in 1 M KOH solution at a scan rate of 50 mV s1. For the PdSx/C porous nanospheres, the reduction peaks of Pd oxide are obviously observed in the negative cathodic scan. The PdSx/C porous nanospheres exhibit a higher reduction peak current density of Pd oxide than that of PdS sample, demonstrating that PdSx/C
can provide more active sites, which is benefit for enhancing of its electrocatalytic performance. There are no obvious well-defined hydrogen adsorption/desorption peaks on the PdSx/C porous nanospheres. However, the CV of PdSx/C porous nanospheres display much wider double-layer capacitance between 0.65 and 0.55 V, which partially overlaps with its hydrogen adsorption/desorption peaks. To evaluate the electrocatalytic properties of PdSx/C towards ethanol, the CVs of as-prepared catalysts for ethanol oxidation were carried out in 1.0 M KOH þ 1.0 M CH3CH2OH solution at a scan rate of 50 mV s1. The current density is normalized on the basis of the mass of Pd so that the current density can be directly used to compare the catalytic activity of different catalysts. As shown in Fig. 7B, two well-defined oxidation peaks can be clearly observed: one in the forward scan (at about 0.25 V vs. SCE) is produced because of the oxidation of freshly chemisorbed species coming from ethanol adsorption, and the other one in the backward scan (at ca. 0.4 V vs. SCE) is associated with the removal of the incompletely oxidized carbonaceous species formed in the forward scan [48]. Generally, the intensity of the forward peak current density is usually used to evaluate the electrocatalytic performance of catalyst toward ethanol oxidation [49]. As shown in the forward scan, the peak current intensity of PdSx/C porous nanospheres reaches a value of 162.1 mA mg1, meanwhile, the PdS catalyst shows no obvious electrocatalytic performance for ethanol oxidation. The enhancement in catalytic performance towards EOR could be attributed to the synthetic effect of PdSx nanoparticles and the porous carbon substrate. To evaluate the catalytic and stability of our catalysts for EOR, the CVs of as-prepared catalysts were carried out in 1.0 M KOH þ 1.0 M C2H5OH at 0.35 V for a period of 3000 s, and the corresponding results are shown in Fig. 7C. It is found that the chronoamperometric curves decay rapidly at the initial period. This may be due to accumulations of carbonaceous intermediate species
Fig. 7. (A) Cyclic voltammograms of PdSx/C (a) and PdS (b) in 1.0 M KOH solution at 50 mV s1. (B) CVs of PdSx/C (a) and PdS (b) catalyst in1.0 M KOH þ 1.0 M C2H5OH solution at a scan rate of 50 mV s1. (C) Chronoamperometry curves of PdSx/C catalysts in 1.0 M KOH þ 1.0 M C2H5OH solution at 0.35 V. (D) The peak current density in the forward scan versus cycle number curves of different catalysts in 1.0 M KOH þ 1.0 M C2H5OH solution.
Q. Zhang et al. / Journal of Power Sources 336 (2016) 1e7
(such as COads, CH3CHOads, etc.) on the catalyst surface during the ethanol oxidation. At the end of 3000 s, the oxidation current density on the PdSx/C porous nanospheres retains stability and the value of oxidation current density reaches 6.1 mA mg1. To further investigate the long-term durability of the PdSx/C porous nanospheres, 1000 cycles of CV were performed in 1.0 M KOH þ 1.0 M C2H5OH solution at a scan rate of 50 mV s1, and the corresponding change of the forward peak current densities versus cycling number was shown in Fig. 7D. The PdSx/C porous nanospheres show that the forward oxidation peak current density increases with of the increase in scan number in the initial CV cycles and then decreases gradually as the measurement continued. For the PdSx/C porous nanospheres, the forward peak current density reaches the maximum (72.5 mA mg1) at the 200th cycle, and then decreases gradually till the 1000th cycles. At 1000 cycles, the catalyst still shows a high peak current density (46.7 mA mg1). The above results clearly reveal that the PdSx/C porous nanospheres exhibit a relatively high electrocatalytic activity and satisfying durability toward EOR. The stability of the PdSx/C after 200 cycles of CV scans was further investigated by XPS and XRD analyses. As shown in Fig. S3A, there is no obvious change for the S 2p and Pd 3d of PdSx/C before and after CV scans, indicating the satisfying stability of PdSx/C. XRD pattern in Fig. S3B showed that compared to the catalyst before cycles, four apparent peaks appeared at 27.70 , 33.73 , 53.90 and 63.64 , which could be attributed to the phases of “PdS” (JCPDF Card No. 74-1060), “PdS2” (JCPDF Card No. 72-1198), “PdS2” (JCPDF Card No. 11-0497), and “Pd16S7” (JCPDF Card No. 110001), respectively. It indicates the as-prepared PdSx/C catalyst went through phase change during 200 cycles of CV scans but still composed of sulfurized Pd. All these results demonstrated that the PdSx/C porous nanospheres exhibited a good stability for EOR. 4. Conclusions In summary, PdSx/C porous nanospheres have been synthesized via the calcination of Pd/PEDOT nanospheres. PdSx nanoparticles with an average diameter of 2.47 ± 0.60 nm were well-anchored on the surface of carbon-based nanospheres (average diameter: ~50 nm). A set of characterizations, including SEM/TEM, XRD, Raman, XPS, among others, were performed to confirm their morphology, composition, and structure. The as-prepared PdSx/C porous nanospheres show higher electrocatalytic activity compared to pure PdS catalyst. In addition, the as-prepared PdSx/C porous nanospheres towards ethanol oxidation exhibit long-term stability with 1000 cycles. Such catalytic activity of PdSx/C porous nanospheres could be ascribed to the large surface area for finely dispersing catalytic nanoparticles, highly developed mesoporosity for facile diffusion of reactants and by-products, high electrical conductivity for providing electrical pathways. The current work not only provides a simple approach for the preparation of carbonsupported metal-sulfide composite with fine control over size and morphology, but also offers a possible choice for catalyst construction in alkaline ethanol fuel cell. Acknowledgements This work was supported by the National Natural Science Foundation of China grant (No. 21475076) and International S&T collaboration Program of China (No. 2015DFA50060). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.10.028.
7
References [1] H. Huang, X. Wang, Phys. Chem. Chem. Phys. 15 (2013) 10367e10375. [2] G.H. An, E.H. Lee, H.J. Ahn, Phys. Chem. Chem. Phys. 18 (2016) 14859e14866. [3] F. Ren, H. Wang, C. Zhai, M. Zhu, R. Yue, Y. Du, P. Yang, J. Xu, W. Lu, ACS Appl. Mater. Interfaces 6 (2014) 3607e3614. [4] M.G. Hosseini, M.M. Momeni, Electrochim. Acta 70 (2012) 1e9. [5] W.J. Zhou, S.Q. Song, W.Z. Li, Z.H. Zhou, G.Q. Sun, Q. Xin, S. Douvartzides, P. Tsiakaras, J. Power Sources 140 (2005) 50e58. [6] K. Lee, S.W. Kang, S.U. Lee, K.H. Park, Y.W. Lee, S.W. Han, ACS Appl. Mater. Interfaces 4 (2012) 4208e4214. [7] C. Bianchini, P.K. Shen, Chem. Rev. 109 (2009) 4183e4206. [8] J.N. Tiwari, R.N. Tiwari, G. Singh, K.S. Kim, Nano Energy 2 (2013) 553e578. [9] Z. Jin, Q. Wang, W. Zheng, X. Cui, ACS Appl. Mater. Interfaces 8 (2016) 5273e5279. [10] F.B. Zhang, H.L. Li, Carbon 44 (2006) 3195e3198. [11] F.A. Ksar, L. Ramos, B. Keita, L. Nadjo, P. Beaunier, H. Remita, Chem. Mater. 21 (2009) 3677e3683. [12] J. Yang, Y. Xie, R. Wang, B. Jiang, C. Tian, G. Mu, J. Yin, B. Wang, H. Fu, ACS Appl. Mater. Interfaces 5 (2013) 6571e6579. [13] W. Du, K.E. Mackenzie, D.F. Milano, N.A. Deskins, D. Su, X. Teng, ACS Catal. 2 (2012) 287e297. [14] S. Fu, C. Zhu, D. Du, Y. Lin, ACS Appl. Mater. Interfaces 7 (2015) 13842e13848. [15] R. Jana, U. Subbarao, S.C. Peter, J. Power Sources 301 (2016) 160e169. [16] Y.G. Yan, Q.X. Li, S.J. Huo, M. Ma, W.B. Cai, M. Osawa, J. Phys. Chem. B 109 (2005) 7900e7906. [17] Q. Gao, M.R. Gao, J.W. Liu, M.Y. Chen, C.H. Cui, H.H. Li, S.H. Yu, Nanoscale 5 (2013) 3202e3207. [18] F. Jiang, R. Yue, Y. Du, J. Xu, P. Yang, Biosens. Bioelectron. 44 (2013) 127e131. [19] Y. Hu, H. Zhang, P. Wu, B. Zhou, C. Cai, Phys. Chem. Chem. Phys. 13 (2011) 4083e4094. [20] B. Wu, D. Hu, Y. Kuang, Y. Yu, X. Zhang, J. Chen, Chem. Commun. (Camb.) 47 (2011) 5253e5255. [21] L. Feng, S. Yao, X. Zhao, L. Yan, C. Liu, W. Xing, J. Power Sources 197 (2012) 38e43. [22] W. Yang, S. Yang, J. Guo, G. Sun, Q. Xin, Carbon 45 (2007) 397e401. [23] S. Chakraborty, C. Retna Raj, Carbon 48 (2010) 3242e3249. [24] S.S. Li, J.N. Zheng, X. Ma, Y.Y. Hu, A.J. Wang, J.R. Chen, J.J. Feng, Nanoscale 6 (2014) 5708e5713. [25] S. Yang, J. Dong, Z. Yao, C. Shen, X. Shi, Y. Tian, S. Lin, X. Zhang, Sci. Rep. 4 (2014) 4501e4506. [26] R. Yue, H. Wang, D. Bin, J. Xu, Y. Du, W. Lu, J. Guo, J. Mater. Chem. A 3 (2015) 1077e1088. [27] B. Radha, G.U. Kulkarni, Adv. Funct. Mater. 20 (2010) 879e884. [28] F. Menegazzo, P. Canton, F. Pinna, N. Pernicone, Catal. Commun. 9 (2008) 2353e2356. [29] F. Pinna, F. Menegazzo, M. Signoretto, P. Canton, G. Fagherazzi, N. Pernicone, Appl. Catal. A-Gen. 219 (2001) 195e201. [30] G.F. Chen, T.Y. Ma, Z.Q. Liu, N. Li, Y.Z. Su, K. Davey, S.Z. Qiao, Adv. Funct. Mater. 26 (2016) 3314e3323. [31] A.V. Mashkina, L.G. Sakhaltueva, Kinet. Catal. 43 (2002) 107e114. [32] Z. Yao, M. Zhu, F. Jiang, Y. Du, C. Wang, P. Yang, J. Mater. Chem. 22 (2012) 13707e13713. [33] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Phys. Rep. 473 (2009) 51e87. [34] W. Xu, J. Ni, Q. Zhang, F. Feng, Y. Xiang, X. Li, J. Mater. Chem. A 1 (2013) 12811e12817. [35] E.W. Qian, K. Otani, L. Li, A. Ishihara, T. Kabe, J. Catal. 221 (2004) 294e301. [36] B. Zhu, B. Lin, Y. Zhou, P. Sun, Q. Yao, Y. Chen, B. Gao, J. Mater. Chem. A 2 (2014) 3819e3827. [37] C. Zhai, M. Zhu, D. Bin, F. Ren, C. Wang, P. Yang, Y. Du, J. Power Sources 275 (2015) 483e488. [38] X. Yang, M. Zhen, G. Li, X. Liu, X. Wang, C. Shu, L. Jiang, C. Wang, J. Mater. Chem. A 1 (2013) 8105e8110. [39] K. Nguyen, M. Monteverde, A. Filoramo, L. Goux-Capes, S. Lyonnais, P. Jegou, P. Viel, M. Goffman, J.P. Bourgoin, Adv. Mater. 20 (2008) 1099e1104. [40] F.V. Wrochem, F. Scholz, A. Yasuda, J.M. Wessels, J. Phys. Chem. C 113 (2009) 12395e12401. [41] G. Hu, F. Nitze, T. Sharifi, H.R. Barzegar, T. Wågberg, J. Mater. Chem. 22 (2012) 8541e8548. [42] G.J. Ashwell, A.T. Williams, S.A. Barnes, S.L. Chappell, L.J. Phillips, B.J. Robinson, B. Urasinska-Wojcik, P. Wierzchowiec, I.R. Gentle, B.J. Wood, J. Phys. Chem. C 115 (2011) 4200e4208. [43] G. Hu, T. Sharifi, F. Nitze, H.R. Barzegar, C.-W. Tai, T. Wågberg, Chem. Phys. Lett. 543 (2012) 96e100. [44] J.C. Love, D.B. Wolfe, R. Haasch, M.L. Chabinyc, K.E. Paul, G.M. Whitesides, R.G. Nuzzo, J. Am. Chem. Soc. 125 (2003) 2597e2609. [45] K. Zhang, D. Bin, B. Yang, C. Wang, F. Ren, Y. Du, Nanoscale 7 (2015) 12445e12451. [46] Q. Li, R. Jiang, Y. Dou, Z. Wu, T. Huang, D. Feng, J. Yang, A. Yu, D. Zhao, Carbon 49 (2011) 1248e1257. [47] M.B. Zheng, J. Cao, S.T. Liao, J.S. Liu, H.Q. Chen, Y. Zhao, W.J. Dai, G.B. Ji, J.M. Cao, J. Tao, J. Phys. Chem. C 113 (2009) 3887e3894. [48] R.N. Singh, A. Singh, Anindita, Carbon 47 (2009) 271e278. [49] Y. Lin, X. Cui, Langmuir 21 (2005) 11474e11479.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Facile synthesis of PdSx/C porous nanospheres and their applications for ethanol oxidation reaction Qiang Zhang a, Fuhua Zhang a, Xuemei Ma a, Yiqun Zheng b, Shifeng Hou a, b, * a b
School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, China National Engineering and Technology Research Center for Colloidal Materials, Shandong University, Jinan, Shandong 250100, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
PdSx/C porous nanospheres were prepared via calcination of Pd/PEDOT precursor. PdSx nanoparticles were homogeneously dispersed on the surface of porous carbon support. PdSx/C shows a better catalytic activity than common sulfur-poisoned Pd catalyst.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 July 2016 Received in revised form 16 September 2016 Accepted 8 October 2016
We report a facile approach for the synthesis of carbon-supported palladium polysulphide porous nanospheres (PdSx/C) and their applications for ethanol oxidation reaction. Typical synthesis started with generation of palladium/poly (3,4-ethylenedioxythiophene)(Pd/PEDOT) nanospheres, followed by a calcination process at an optimized temperature to form PdSx/C, with an average size of 2.47 ± 0.60 and 50 nm of PdSx nanoparticles and carbon porous nanospheres, respectively. Various techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical techniques were performed to characterize their morphologies, compositions and structures. In contrary to most Pdbased electrochemical catalysts that could be easily poised with trace sulfur during the catalytic oxidation process, the as-prepared PdSx/C porous nanospheres exhibited high electrocatalytic activities and stabilities for the electrochemical catalytic oxidation of ethanol in alkaline medium. In particular, the forward peak current intensity achieved 162.1 mA mg1 and still maintained at 46.7 mA mg1 even after 1000 cycles. This current work not only offers a novel type of fuel-cell catalyst for ethanol oxidation reaction, but also provides a possible route for solving the sulfur-poisoning problem in catalysis. © 2016 Elsevier B.V. All rights reserved.
Keywords: Porous nanospheres Carbon-supported palladium polysulphide PdSx nanoparticles Electrocatalytic activity Ethanol oxidation
1. Introduction
* Corresponding author. School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, China. E-mail address: [email protected] (S. Hou). http://dx.doi.org/10.1016/j.jpowsour.2016.10.028 0378-7753/© 2016 Elsevier B.V. All rights reserved.
With growing demand for energy coupled with concerns over environmental pollution and the depletion of fossil fuels, the development of new clean energy technologies is desired [1e3]. Fuel cells, converting the chemical energy to electrical energy by means of a chemical reaction, have attracted considerable interest
2
Q. Zhang et al. / Journal of Power Sources 336 (2016) 1e7
among researchers [4]. Among the various liquid fuels, ethanol is greatly highlighted due to its merits such as low toxicity, high energy density, good stability, easy storage, and low permeability across proton exchange membrane [5]. In this area, the direct ethanol fuel cell (DEFC) is widely applied in the field of alternative energy for their abilities of high energy density of ethanol and the ease of handing liquid fuel [6e9]. For DEFC, Pd has been considered as a promising catalyst due to its less expensive cost, higher activity, and better steady-state behavior in alkaline solutions [10,11]. In recent years, various strategies have been developed to synthesize Pd catalyst, such as reduction of Pd precursor in aqueous and/or organic phase [12e15]and electrochemical route [16]. Among those methods, one-pot synthesis has aroused increasing research interest due to its simplicity in synthesis. Typically, the reduction or thermal decomposition of Pd precursor with/without the presence of capping agent could readily produce Pd nanoparticles with reasonable control over their size and morphology. For example, Gao et al. [17]demonstrated a very simple route for one-pot synthesis of branched Pd nanodendrites in a mixture solution of oleic acid and oleylamine, which exhibited substantially enhanced catalytic activities towards oxidation reduction reaction (ORR) and ethanol oxidation reaction (EOR). However, such onestep synthesis of Pd catalyst usually leads to aggregation of nanoparticles and the unavoidable use of massive additional surfactants and thus surface contamination [18]. To this end, the involvement of supporting materials, such as carbon black [19e21], carbon nanofibers [22], carbon nanotubes [23] and graphene [24] in onepot synthesis, could effectively enhance the dispersion and utilization of Pd catalysts. These carbon materials not only significantly enhance the available electrochemical active surface area of catalysts for electron transfer but also provide high mass transport of reactants to the catalysts [19]. For example, Yang co-workers [25] reported a comparatively easy, one-pot method to synthesize monodisperse palladium NPs dispersed on reduced graphene oxide (rGO) in N-methyl-2-pyrrolidone solution, and found that the Pd/ rGO displayed much higher catalytic activity and better stability than conventional Pd/XC-72 electrocatalysts. It is worth noting that during the catalysis, the Pd catalyst could be malfunctioned and the catalytic performance would be negatively influenced when there is sulfur present in the system. It is because the sulfur atoms could bond strongly to the surface of Pd to form PdSx, which resulted in the decrease in the number of active sites and thus blocked effective contact between catalyst and reactants. In fact, PdSx has been found to be useful for a variety of catalytic application due to their special physiochemical properties [26e29]. For example, Mashkina and co-workers reported that PdSx is a promising catalyst for thiophene hydrogenation and the reductive N-alkylation of aromatic amines due to their ease of formation of active sites [30]. However, it remains grand technique challenges to utilize PdSx as catalyst toward EOR due to the difficulties caused by sulfur poisoning. In this study, we report a facile approach for the synthesis of PdSx/C porous nanospheres and interestingly, the sulfurized Pd in this composite exhibited electrocatalytic activity for EOR in alkaline solutions. In particular, Pd2þ was reduced during the formation of PEDOT nanospheres process and the resultant Pd nanoparticles were attached to PEDOT nanosphere surface to Pd/PEDOT, which was then calcined at an elevated temperature to form PdSx/C porous nanospheres. The resultant products were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The catalytic performance and stability of the PdSx/C porous nanospheres for EOR under alkaline conditions were explored by cyclic voltammetry and
chronoamperometry. Importantly, as-prepared PdSx/C porous nanospheres display greatly improved electrocatalytic activity and durability for EOR as compared with pure PdS. 2. Experimental 2.1. Materials and characterization 3,4-ethylenedioxythiophene (EDOT) was purchased from Bayer. PdCl2, CH3CH2OH, KOH and H2SO4 (98%) were obtained from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade and all aqueous solutions were prepared with deionized water. A 5.0 mM H2PdCl4aqueous solution was prepared by dissolving 17.7 mg of PdCl2 in 20 mL of 10 mM HCl solution with sonication. Scanning electron microscopy (SEM) images were captured using an Ultra Plus microscope operated at 3.0 kV (Zeiss, Oberkochen, Germany). The samples were prepared by dropping aqueous suspensions of the nanospheres onto Cu foils. Transmission electron microscopy (TEM) measurements were carried out using a FEI Quanta 200F electron microscope with an accelerating voltage of 200 kV. High-resolution TEM images were obtained using a fieldemission JEM-2100F microscope operated at 200 kV (JEOL, Tokyo, Japan). The samples were prepared by dropping aqueous suspensions of the nanoparticles onto carbon-coated Cu grids and dried under ambient conditions. X-ray diffraction (XRD) patterns were recorded using a diffractometer with filtered Cu Ka radiation at 0.154 nm. Raman spectra were recorded on a LABRAM-HR confocal laser micro-Raman spectrometer employing a 20 mW laser at 632.8 nm. Nitrogen sorption isotherms were performed on a Kubo X1000 instrument at 77 K. Prior to measurements, the samples were out gassed at 180 C under vacuum for more than 6 h. The specific surface area was derived using the analysis methods of Brunauer-Emmett-Teller (BET). The pore-size distribution was determined from the adsorption branch of the isotherm, according to Barrett-Joyner-Halanda (BJH) method. The total pore volumes were estimated from the adsorbed amount at a relative pressure of p/p0 ¼ 0.98. X-ray photo electron spectroscopy (XPS) measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with a Mgka excitation source (1253.6 eV). 2.2. Preparation of Pd/PEDOT nanospheres and PdSx/C porous nanospheres Pd/PEDOT nanospheres were prepared using a facile process according to previous report [18]. In brief, 5.0 mL of EDOT ethanol solution (14.2 wt%) was mixed with 20 mL of H2PdCl4 aqueous solution (5.0 mM) and the mixture was magnetically stirred for 3 h at room temperature. During the process, EDOT monomers were oxidized and polymerized to PEDOT nanospheres. Meanwhile, Pd nanoparticles were formed via the reduction of PdCl2 4 by EDOT monomers, which were enwrapped by PEDOT nanospheres. Then, the aqueous mixture was kept constantly stirring for 4 h. Subsequently, the product was collected via centrifugation, filtrated with water and ethanol alternatively, and then vacuum-dried at room temperature. PdSx/C porous nanospheres were prepared via the calcination of as-prepared Pd/PEDOT composite. Briefly, 1.5 g of the as-prepared Pd/PEDOT was placed in a quartz tube furnace and then calcined under N2 protection with a heating rate of 10 C min1 for 2 h. For calcinations temperature set at 300 C, 500 C, 600 C, 700 C, 800 C and 900 C, the resultant samples were labeled as PdSx/C-3, PdSx/C-5, PdSx/C-6, PdSx/C, PdSx/C-8 and PdSx/C-9, respectively. After the reaction finished, the furnace was cooled down to room temperature naturally. The samples were collected, washed with
Q. Zhang et al. / Journal of Power Sources 336 (2016) 1e7
water, and dried at 70 characterization.
C
overnight for further use and
2.3. Electrochemical test All electrochemical data were recorded with a CHI-760E
3
electrochemical workstation (Shanghai CH Instrumental Co.), using a conventional three-electrode system at room temperature. A saturated calomel electrode (SCE), a platinum wire and a glassy carbon electrode (GCE, diameter: 3.0 mm) were used as the reference, counter and working electrodes, respectively. Before each experiment, the GCE was carefully polished with alumina slurry of
Fig. 1. Structure and morphology characterizations of (AeC) Pd/PEDOT and (DeF) PdSx/C porous nanospheres: (A, D) SEM; (B, E) TEM; (C, F) corresponding size distribution histograms of Pd and PdSx nanoparticles, respectively.
Fig. 2. (A) High-resolution TEM and (BeD) element mapping images of PdSx/C porous nanospheres.
4
Q. Zhang et al. / Journal of Power Sources 336 (2016) 1e7
For comparision, the electrocatalytic performance of the pure PdS catalyst on the GCE also measured by similar process. 3. Results and discussion
Fig. 3. (A) Raman spectrum and (B) XRD pattern of PdSx/C porous nanospheres.
0.5 mm in diameter on a polishing cloth, followed by sonication in DI water and an ethanol bath for 10 min. For electrochemical measurements, the suspension of PdSx/C was obtained by dispersing 10 mg of the as-prepared PdSx/C in the mixture of 4.0 mL deionized water and 20 mL Nafion solution (5 wt%) for 30 min. Then, 10 mL of as-prepared suspension of PdSx/C was dropped on surface of GCE and dried with an infrared lamp to form PdSx/C electrode.
The morphology of Pd/PEDOT and PdSx/C porous nanospheres was characterized by SEM and TEM. As shown in Fig. 1A and B, the Pd/PEDOT composites exhibited the spherical shape with an average diameter of 50 nm. Many of nano/micro-pores are left between adjacent nanospheres, which would be in favor to the accessibility of electrolyte. Pd nanoparticles were anchored on the surface of PEDOT nanospheres. Fig. 1C shows the size distribution of Pd nanoparticles in Pd/PEDOT composites and the size of Pd nanoparticles is 2.47 ± 0.95 nm. Fig. 1D shows the SEM image of PdSx/C obtained by calcination of Pd/PEDOT. Compared with the Pd/PEDOT, the morphology of PdSx/C products was varied to be with relatively coarse surfaces. It is noteworthy to mention that the calcined product remained spherical shapes after being annealed at high temperature. TEM image (Fig. 1E) shows that tiny nanoparticles were anchored on the surface of PdSx/C porous nanospheres and distributed homogeneously. As shown in Fig. 1F, the size distribution of PdSx nanoparticles conforms to normal distribution with an average diameter of 2.47 ± 0.60 nm. In addition, Fig. 2 shows the HRTEM and corresponding element mapping images of Pd and S for PdSx/C porous nanospheres. It can be clearly observed that both Pd and S are evenly distributed on the whole nanospheres. To further confirm the component of as-prepared sample, Raman spectroscopy, XRD and XPS were performed. As shown in Fig. 3A, the appearance of the two prominent peaks at 1328 and 1584 cm1, respectively, could be ascribed to the D and G bands of the carbon materials [31], respectively. Generally, the D band could be assigned to the breathing mode of k-point phonons of A1g symmetry, while the G band could be related to the E2g phonons of sp2 carbon atoms [32,33]. This means that the PEDOT in the Pd/ PEDOT nanospheres has been converted to a kind of carbon
Fig. 4. (A) XPS survey spectrum as well as high-resolution (B) S 2p and Pd 3d spectra of the PdSx/C porous nanospheres. (C) S 2p and (D) Pd 3d XPS spectra of PdSx/C porous nanospheres with peak fittings.
Q. Zhang et al. / Journal of Power Sources 336 (2016) 1e7
material. The crystal structures of the as-prepared PdSx/C porous nanospheres were further characterized by XRD, and the corresponding results were shown in Fig. 3B. Several PdSx phase, including “Pd16S7”, “Pd3S” and “Pd4S” phases, could be observed in the diffraction pattern, which is attributed to the formation of palladium sulfides with mixed crystal phases during the calcination process. As indicated in previous studies, the sulfurization process usually leads to the formation of palladium sulfides with mixed crystal phases [34,35]. Compared with the PdSx/C porous nanospheres, no noticeable peak was observed for Pd/PEDOT (Fig. S1), which is probably ascribed to the poor crystallinity of Pd nanoparticles before calcination. Fig. 4A shows the XPS survey spectra of PdSx/C porous nanospheres, where four elements including C, O, S, and Pd are observed. The S 2p scan in Fig. 4B exhibits two major peaks located at 162.6 eV and 163.8 eV, which is corresponding to S 2p3/2 and S 2p1/2, respectively [36,37]. The spectra of Pd 3d in Fig. 4B show that one doublet of the 3d5/2 and 3d3/2 states is centered at 335.8 eV and 341.2 eV, respectively [38,39]. For S 2p spectra of PdSx/C (Fig. 4C), the binding energies for S 2p3/2 and S 2p1/2 could be resolved into three doublets. The peak appears at 162.5 eV, indicating the presence of PdeS bonds on the nanoparticles at the support [40]. The other couple located at 167.9 eV is assigned to the oxidized sulfur groups coming from the exposure to air during storage [41,42]. While, the peak appears at 164.6 eV, which can be attributed to the SeS bonds formed between PEDOT molecules [43,44]. Fig. 4D shows the peak fitting of a high-resolution Pd 3d region. There are two peaks at about 335.8 eV and 341.2 eV, could be corresponding to the Pd 3d5/2 and Pd 3d3/2 binding energies, respectively. The two peaks can be divided into two pairs, the binding energy of one pair is emerging at 336.0 and 341.3 eV, which can be assigned to Pd(II) [45]. Whereas the other weaker peaks are located at 337.9 and
5
343.15 eV, indicating the presence of Pd (IV) [38]. Based on their peak intensity, it can be concluded that Pd (II) is the predominant species. To study the surface area and particle size distribution of the support materials, Fig. 5 shows the N2 adsorption-desorption isotherms. BET specific surface area and total pore volume of asprepared PdSx/C porous nanospheres are 169.18 m2 g1 and 0.27 cm3 g1, respectively. However, the BET surface area is lower than that of pure carbon (Vulcan XC-72R 225e250 m2/g). This is not unexpected because the composite involved Pd with relatively low BET surface area (35e45 m2 g1). As shown in the inset and Table 1, the pore size distribution (PSD) calculated from the desorption branch based on the BJH model. We observe that the samples show bimodal mesopores in the PSD curve centered at 3.90 and 31.8 nm. The PSD peak at the mean value of ~3.90 nm can be mainly attributed to the pores formed on the surfaces of carbon-based nanospheres. In addition, the larger pores (31.8 nm) mainly derive from aggregation between adjacent nanospheres, which is of great benefit to the diffusion of the electrolyte. Note that the pore size of the PdSx/C decreases after calcinations compared to that of Pd/PEDOT. It is probably attributed to the contraction of carbon nanospheres during the calcination, which has also observed in previous reports [46,47].
Fig. 5. N2 adsorption/desorption isotherm and the corresponding pore size distribution (inset) of PdSx/C porous nanospheres.
Table 1 Effects of calcination temperature on the BET specific surface area (SBET) and pore parameters of samples. Sample
SBETa (m2/g)
Pore sizeb (nm)
Pore volumec (cm3/g)
Pd/PEDOT PdSx/C
22.93 169.18
51.90 3.90, 31.80
0.24 0.27
a BET surface area calculated from the linear part of the BET plot (p/ p0 ¼ 0.05e0.3). b Total pore volume, taken from the volume of N2-adsorbed at about p/p0 ¼ 0.995. c Pore diameter, estimated using the desorption branch of the isotherm and the Barrett-Joyner-Halenda (BJH) formula.
Fig. 6. (A, B) SEM images of (A) PdSx/C-5 and (B) PdSx/C-9. (C) XRD patterns of (a) PdSx/C-5 and (b) PdSx/C-9.
6
Q. Zhang et al. / Journal of Power Sources 336 (2016) 1e7
In the standard procedure, the calcination of Pd/PEDOT nanospheres at an elevated temperature is highly crucial for the formation of PdSx/C with a unique porous structure and spherical shape. To this end, we conducted several control experiments to investigate the effects of calcination temperature on the morphology, composition, and structure of PdSx/C. Fig. 6 shows the SEM images of PdSx/C-5 and PdSx/C-9, respectively. Interestingly, the two samples still exhibit the spherical structure. However, the nanospheres suffer from a severe aggregation as the calcination temperature increases to 900 C (Fig. 6B). XRD patterns of the PdSx/ C-5 and PdSx/C-9 are also investigated in Fig. 6C. For PdSx/C-5 sample, only a broad peak centered at 37.2 is observed, indicating the poor crystallinity of PdSx/C-5. With a further increasing annealed temperature, PdSx/C-9 sample shows similar sharp peaks as PdSx/C with mixtures of “Pd16S7”, “Pd3S” and “Pd4S” phases, indicting the PdSx with high crystallinity formed at temperatures above 500 C. It is worth noting that the peaks between 60 and 80 (2 theta) disappeared, as compared to PdSx/C porous nanospheres, which could be owing to the phase transfer of PdSx. Meanwhile, the SEM images and XRD patterns of the PdSx/C-3, PdSx/C-6, and PdSx/ C-8 are also shown in Fig. S2 as control groups. Based on the results above, the annealed temperature at 700 C is considered as an optimal condition for preparation of PdSx/C porous nanospheres and the corresponding product is then used for electrochemical experiments. Electrocatalytic activity of the as-prepared PdSx/C porous nanospheres was investigated by cyclic voltammetry (CV) experiments. We also included PdS as the catalyst for comparison during the test. Fig. 7A shows the typical CVs of the PdSx/C porous nanospheres and PdS in 1 M KOH solution at a scan rate of 50 mV s1. For the PdSx/C porous nanospheres, the reduction peaks of Pd oxide are obviously observed in the negative cathodic scan. The PdSx/C porous nanospheres exhibit a higher reduction peak current density of Pd oxide than that of PdS sample, demonstrating that PdSx/C
can provide more active sites, which is benefit for enhancing of its electrocatalytic performance. There are no obvious well-defined hydrogen adsorption/desorption peaks on the PdSx/C porous nanospheres. However, the CV of PdSx/C porous nanospheres display much wider double-layer capacitance between 0.65 and 0.55 V, which partially overlaps with its hydrogen adsorption/desorption peaks. To evaluate the electrocatalytic properties of PdSx/C towards ethanol, the CVs of as-prepared catalysts for ethanol oxidation were carried out in 1.0 M KOH þ 1.0 M CH3CH2OH solution at a scan rate of 50 mV s1. The current density is normalized on the basis of the mass of Pd so that the current density can be directly used to compare the catalytic activity of different catalysts. As shown in Fig. 7B, two well-defined oxidation peaks can be clearly observed: one in the forward scan (at about 0.25 V vs. SCE) is produced because of the oxidation of freshly chemisorbed species coming from ethanol adsorption, and the other one in the backward scan (at ca. 0.4 V vs. SCE) is associated with the removal of the incompletely oxidized carbonaceous species formed in the forward scan [48]. Generally, the intensity of the forward peak current density is usually used to evaluate the electrocatalytic performance of catalyst toward ethanol oxidation [49]. As shown in the forward scan, the peak current intensity of PdSx/C porous nanospheres reaches a value of 162.1 mA mg1, meanwhile, the PdS catalyst shows no obvious electrocatalytic performance for ethanol oxidation. The enhancement in catalytic performance towards EOR could be attributed to the synthetic effect of PdSx nanoparticles and the porous carbon substrate. To evaluate the catalytic and stability of our catalysts for EOR, the CVs of as-prepared catalysts were carried out in 1.0 M KOH þ 1.0 M C2H5OH at 0.35 V for a period of 3000 s, and the corresponding results are shown in Fig. 7C. It is found that the chronoamperometric curves decay rapidly at the initial period. This may be due to accumulations of carbonaceous intermediate species
Fig. 7. (A) Cyclic voltammograms of PdSx/C (a) and PdS (b) in 1.0 M KOH solution at 50 mV s1. (B) CVs of PdSx/C (a) and PdS (b) catalyst in1.0 M KOH þ 1.0 M C2H5OH solution at a scan rate of 50 mV s1. (C) Chronoamperometry curves of PdSx/C catalysts in 1.0 M KOH þ 1.0 M C2H5OH solution at 0.35 V. (D) The peak current density in the forward scan versus cycle number curves of different catalysts in 1.0 M KOH þ 1.0 M C2H5OH solution.
Q. Zhang et al. / Journal of Power Sources 336 (2016) 1e7
(such as COads, CH3CHOads, etc.) on the catalyst surface during the ethanol oxidation. At the end of 3000 s, the oxidation current density on the PdSx/C porous nanospheres retains stability and the value of oxidation current density reaches 6.1 mA mg1. To further investigate the long-term durability of the PdSx/C porous nanospheres, 1000 cycles of CV were performed in 1.0 M KOH þ 1.0 M C2H5OH solution at a scan rate of 50 mV s1, and the corresponding change of the forward peak current densities versus cycling number was shown in Fig. 7D. The PdSx/C porous nanospheres show that the forward oxidation peak current density increases with of the increase in scan number in the initial CV cycles and then decreases gradually as the measurement continued. For the PdSx/C porous nanospheres, the forward peak current density reaches the maximum (72.5 mA mg1) at the 200th cycle, and then decreases gradually till the 1000th cycles. At 1000 cycles, the catalyst still shows a high peak current density (46.7 mA mg1). The above results clearly reveal that the PdSx/C porous nanospheres exhibit a relatively high electrocatalytic activity and satisfying durability toward EOR. The stability of the PdSx/C after 200 cycles of CV scans was further investigated by XPS and XRD analyses. As shown in Fig. S3A, there is no obvious change for the S 2p and Pd 3d of PdSx/C before and after CV scans, indicating the satisfying stability of PdSx/C. XRD pattern in Fig. S3B showed that compared to the catalyst before cycles, four apparent peaks appeared at 27.70 , 33.73 , 53.90 and 63.64 , which could be attributed to the phases of “PdS” (JCPDF Card No. 74-1060), “PdS2” (JCPDF Card No. 72-1198), “PdS2” (JCPDF Card No. 11-0497), and “Pd16S7” (JCPDF Card No. 110001), respectively. It indicates the as-prepared PdSx/C catalyst went through phase change during 200 cycles of CV scans but still composed of sulfurized Pd. All these results demonstrated that the PdSx/C porous nanospheres exhibited a good stability for EOR. 4. Conclusions In summary, PdSx/C porous nanospheres have been synthesized via the calcination of Pd/PEDOT nanospheres. PdSx nanoparticles with an average diameter of 2.47 ± 0.60 nm were well-anchored on the surface of carbon-based nanospheres (average diameter: ~50 nm). A set of characterizations, including SEM/TEM, XRD, Raman, XPS, among others, were performed to confirm their morphology, composition, and structure. The as-prepared PdSx/C porous nanospheres show higher electrocatalytic activity compared to pure PdS catalyst. In addition, the as-prepared PdSx/C porous nanospheres towards ethanol oxidation exhibit long-term stability with 1000 cycles. Such catalytic activity of PdSx/C porous nanospheres could be ascribed to the large surface area for finely dispersing catalytic nanoparticles, highly developed mesoporosity for facile diffusion of reactants and by-products, high electrical conductivity for providing electrical pathways. The current work not only provides a simple approach for the preparation of carbonsupported metal-sulfide composite with fine control over size and morphology, but also offers a possible choice for catalyst construction in alkaline ethanol fuel cell. Acknowledgements This work was supported by the National Natural Science Foundation of China grant (No. 21475076) and International S&T collaboration Program of China (No. 2015DFA50060). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.10.028.
7
References [1] H. Huang, X. Wang, Phys. Chem. Chem. Phys. 15 (2013) 10367e10375. [2] G.H. An, E.H. Lee, H.J. Ahn, Phys. Chem. Chem. Phys. 18 (2016) 14859e14866. [3] F. Ren, H. Wang, C. Zhai, M. Zhu, R. Yue, Y. Du, P. Yang, J. Xu, W. Lu, ACS Appl. Mater. Interfaces 6 (2014) 3607e3614. [4] M.G. Hosseini, M.M. Momeni, Electrochim. Acta 70 (2012) 1e9. [5] W.J. Zhou, S.Q. Song, W.Z. Li, Z.H. Zhou, G.Q. Sun, Q. Xin, S. Douvartzides, P. Tsiakaras, J. Power Sources 140 (2005) 50e58. [6] K. Lee, S.W. Kang, S.U. Lee, K.H. Park, Y.W. Lee, S.W. Han, ACS Appl. Mater. Interfaces 4 (2012) 4208e4214. [7] C. Bianchini, P.K. Shen, Chem. Rev. 109 (2009) 4183e4206. [8] J.N. Tiwari, R.N. Tiwari, G. Singh, K.S. Kim, Nano Energy 2 (2013) 553e578. [9] Z. Jin, Q. Wang, W. Zheng, X. Cui, ACS Appl. Mater. Interfaces 8 (2016) 5273e5279. [10] F.B. Zhang, H.L. Li, Carbon 44 (2006) 3195e3198. [11] F.A. Ksar, L. Ramos, B. Keita, L. Nadjo, P. Beaunier, H. Remita, Chem. Mater. 21 (2009) 3677e3683. [12] J. Yang, Y. Xie, R. Wang, B. Jiang, C. Tian, G. Mu, J. Yin, B. Wang, H. Fu, ACS Appl. Mater. Interfaces 5 (2013) 6571e6579. [13] W. Du, K.E. Mackenzie, D.F. Milano, N.A. Deskins, D. Su, X. Teng, ACS Catal. 2 (2012) 287e297. [14] S. Fu, C. Zhu, D. Du, Y. Lin, ACS Appl. Mater. Interfaces 7 (2015) 13842e13848. [15] R. Jana, U. Subbarao, S.C. Peter, J. Power Sources 301 (2016) 160e169. [16] Y.G. Yan, Q.X. Li, S.J. Huo, M. Ma, W.B. Cai, M. Osawa, J. Phys. Chem. B 109 (2005) 7900e7906. [17] Q. Gao, M.R. Gao, J.W. Liu, M.Y. Chen, C.H. Cui, H.H. Li, S.H. Yu, Nanoscale 5 (2013) 3202e3207. [18] F. Jiang, R. Yue, Y. Du, J. Xu, P. Yang, Biosens. Bioelectron. 44 (2013) 127e131. [19] Y. Hu, H. Zhang, P. Wu, B. Zhou, C. Cai, Phys. Chem. Chem. Phys. 13 (2011) 4083e4094. [20] B. Wu, D. Hu, Y. Kuang, Y. Yu, X. Zhang, J. Chen, Chem. Commun. (Camb.) 47 (2011) 5253e5255. [21] L. Feng, S. Yao, X. Zhao, L. Yan, C. Liu, W. Xing, J. Power Sources 197 (2012) 38e43. [22] W. Yang, S. Yang, J. Guo, G. Sun, Q. Xin, Carbon 45 (2007) 397e401. [23] S. Chakraborty, C. Retna Raj, Carbon 48 (2010) 3242e3249. [24] S.S. Li, J.N. Zheng, X. Ma, Y.Y. Hu, A.J. Wang, J.R. Chen, J.J. Feng, Nanoscale 6 (2014) 5708e5713. [25] S. Yang, J. Dong, Z. Yao, C. Shen, X. Shi, Y. Tian, S. Lin, X. Zhang, Sci. Rep. 4 (2014) 4501e4506. [26] R. Yue, H. Wang, D. Bin, J. Xu, Y. Du, W. Lu, J. Guo, J. Mater. Chem. A 3 (2015) 1077e1088. [27] B. Radha, G.U. Kulkarni, Adv. Funct. Mater. 20 (2010) 879e884. [28] F. Menegazzo, P. Canton, F. Pinna, N. Pernicone, Catal. Commun. 9 (2008) 2353e2356. [29] F. Pinna, F. Menegazzo, M. Signoretto, P. Canton, G. Fagherazzi, N. Pernicone, Appl. Catal. A-Gen. 219 (2001) 195e201. [30] G.F. Chen, T.Y. Ma, Z.Q. Liu, N. Li, Y.Z. Su, K. Davey, S.Z. Qiao, Adv. Funct. Mater. 26 (2016) 3314e3323. [31] A.V. Mashkina, L.G. Sakhaltueva, Kinet. Catal. 43 (2002) 107e114. [32] Z. Yao, M. Zhu, F. Jiang, Y. Du, C. Wang, P. Yang, J. Mater. Chem. 22 (2012) 13707e13713. [33] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Phys. Rep. 473 (2009) 51e87. [34] W. Xu, J. Ni, Q. Zhang, F. Feng, Y. Xiang, X. Li, J. Mater. Chem. A 1 (2013) 12811e12817. [35] E.W. Qian, K. Otani, L. Li, A. Ishihara, T. Kabe, J. Catal. 221 (2004) 294e301. [36] B. Zhu, B. Lin, Y. Zhou, P. Sun, Q. Yao, Y. Chen, B. Gao, J. Mater. Chem. A 2 (2014) 3819e3827. [37] C. Zhai, M. Zhu, D. Bin, F. Ren, C. Wang, P. Yang, Y. Du, J. Power Sources 275 (2015) 483e488. [38] X. Yang, M. Zhen, G. Li, X. Liu, X. Wang, C. Shu, L. Jiang, C. Wang, J. Mater. Chem. A 1 (2013) 8105e8110. [39] K. Nguyen, M. Monteverde, A. Filoramo, L. Goux-Capes, S. Lyonnais, P. Jegou, P. Viel, M. Goffman, J.P. Bourgoin, Adv. Mater. 20 (2008) 1099e1104. [40] F.V. Wrochem, F. Scholz, A. Yasuda, J.M. Wessels, J. Phys. Chem. C 113 (2009) 12395e12401. [41] G. Hu, F. Nitze, T. Sharifi, H.R. Barzegar, T. Wågberg, J. Mater. Chem. 22 (2012) 8541e8548. [42] G.J. Ashwell, A.T. Williams, S.A. Barnes, S.L. Chappell, L.J. Phillips, B.J. Robinson, B. Urasinska-Wojcik, P. Wierzchowiec, I.R. Gentle, B.J. Wood, J. Phys. Chem. C 115 (2011) 4200e4208. [43] G. Hu, T. Sharifi, F. Nitze, H.R. Barzegar, C.-W. Tai, T. Wågberg, Chem. Phys. Lett. 543 (2012) 96e100. [44] J.C. Love, D.B. Wolfe, R. Haasch, M.L. Chabinyc, K.E. Paul, G.M. Whitesides, R.G. Nuzzo, J. Am. Chem. Soc. 125 (2003) 2597e2609. [45] K. Zhang, D. Bin, B. Yang, C. Wang, F. Ren, Y. Du, Nanoscale 7 (2015) 12445e12451. [46] Q. Li, R. Jiang, Y. Dou, Z. Wu, T. Huang, D. Feng, J. Yang, A. Yu, D. Zhao, Carbon 49 (2011) 1248e1257. [47] M.B. Zheng, J. Cao, S.T. Liao, J.S. Liu, H.Q. Chen, Y. Zhao, W.J. Dai, G.B. Ji, J.M. Cao, J. Tao, J. Phys. Chem. C 113 (2009) 3887e3894. [48] R.N. Singh, A. Singh, Anindita, Carbon 47 (2009) 271e278. [49] Y. Lin, X. Cui, Langmuir 21 (2005) 11474e11479.