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Hollow raspberry-like PdAg alloy nanospheres: High electrocatalytic activity for ethanol oxidation in alkaline media
Journal of Power Sources 278 (2015) 69e75
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Hollow raspberry-like PdAg alloy nanospheres: High electrocatalytic activity for ethanol oxidation in alkaline media Cheng Peng*, Yongli Hu, Mingrui Liu, Yixiong Zheng College of Materials Science and Engineering, Huaqiao University, Xiamen 361021, 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
Hollow raspberry-like PdAg alloy nanospheres were prepared. The as-prepared catalyst exhibits superior activity for ethanol oxidation. The activity derives from the unique structure and electronic effects.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 October 2014 Received in revised form 5 December 2014 Accepted 13 December 2014 Available online 15 December 2014
Palladiumesilver (PdAg) alloy nanospheres with unique structure were prepared using a one-pot procedure based on the galvanic replacement reaction. Their electrocatalytic activity for ethanol oxidation in alkaline media was evaluated. The morphology and crystal structure of the samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Electrochemical characterization techniques, including cyclic voltammetry (CV) and chronoamperometry (CA) measurements were used to analyze the electrochemical performance of the PdAg alloy nanospheres. The SEM and TEM images showed that the PdAg alloy nanospheres exhibit a hierarchical nanostructure with hollow interiors and porous walls. Compared to the commercial Pd/C catalyst, the as-prepared PdAg alloy nanospheres exhibit superior electrocatalytic activity and stability towards ethanol electro-oxidation in alkaline media, showing its potential as a new non-Pt electrocatalyst for direct alcohol fuel cells (DAFCs). © 2014 Elsevier B.V. All rights reserved.
Keywords: Palladium Ethanol oxidation Alkaline media Electrocatalysts Direct alcohol fuel cells
1. Introduction Fuel cell technology is of tremendous interest, because of both energy and environmental considerations. In recent years, direct alcohol fuel cells (DAFCs) have been considered as a promising power source for portable electronic devices and transportation due to their high efficiency, high energy density and low or zero
* Corresponding author. E-mail address: [email protected] (C. Peng). http://dx.doi.org/10.1016/j.jpowsour.2014.12.056 0378-7753/© 2014 Elsevier B.V. All rights reserved.
emissions [1,2]. Serving as a very promising alternative fuel for DAFCs, ethanol has attracted extensive interest in recent years due to its lower toxicity than methanol, it can be produced in large quantities from biomass [3e8]. However, there are several limitations associated with the commercialization of DAFCs, including the high cost, poor electrocatalytic activity and durability of the electrocatalyst. In order to address these issues, considerable research efforts have been focused on the development of nanomaterials with higher electrocatalytic activity and stability. Platinum-based materials are often used as anode
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electrocatalyst due to the high catalytic activity [9e12]. However, the high cost and the serious CO self-poisoning of platinum catalysts hinder their wide-spread application in DAFCs. One of the key challenges to the practical application and commercialization of DAFCs is the development and design of stable, highly active and low cost electrocatalysts. To this end, various Pt-based and Pt-free nanomaterials have been investigated extensively in the past decade. Although Pd is nearly inert to alcohol oxidation reaction in acid media, it has been shown that palladium is more active than platinum for ethanol oxidation in basic media [13e16]. Furthermore, Pd not only has a higher abundance on the earth's crust but also has a lower price as compared to Pt [17]. Therefore, Pd and Pdbased metal alloys have attracted more and more research attention as an effective non-platinum anode electrocatalyst for DAFCs [18e25]. For the nanostructured electrocatalysts in fuel cell, it is well known that the catalytic performance is strongly dependent on their surface structures, such as the particle size, particle shape, compositions and so on [26]. As for Pd-based nanomaterials, much work has been done on the preparation of Pd-based electrocatalysts, but prior researches mainly focused on solid, porous, or hollow Pd-based nanomaterials, including nanoparticles, nanoplates, nanowires, nanoboxes, snowflake-like self-assemblies, coreeshell nanostructures etc [27e30]. There have been no previous reports on the synthesis of a hierarchical PdAg alloy nanospheres with hollow interior and porous walls. In this work, we prepared hierarchical PdAg alloy nanospheres with hollow interiors and porous walls using a one-pot procedure based on the galvanic replacement reaction. The morphology and crystal structure of the nanoparticles were characterized with SEM, TEM and XRD. Moreover, the electrocatalytic performance of PdAg alloy nanospheres towards ethanol was studied by cyclic voltammetry and chronoamperometry measurements.
ethanol by sonication. This process was repeated 5 times. The final product was dispersed in ethanol (1 mg$mL1) and stored at room temperature. 2.3. Electrochemical investigations A glassy carbon electrode (5.0 mm diameter) was polished with alumina slurries (1.5, 0.5 and 0.05 mm diameter) and cleansed by sonication in deionized water and ethanol for 5 min. Voltammetric measurements were carried out with a CHI660D electrochemical workstation. Cyclic voltammetry and chronoamperometry were performed in 1 M KOH þ 1 M alcohol solution. The working electrode was prepared by dropping 15 mL electrocatalyst ink onto glassy carbon electrode with a micropipette followed by drying at room temperature. The ink was prepared by ultrasonically mixing 10 mL electrocatalyst sample in 5 mL Nafion solution. A Pt foil and Hg/HgO electrode were used as the counter and reference electrodes, respectively. All potentials in the study were given versus Hg/HgO electrode. The CV tests were conducted at 50 mV$s1, with potential ranging from 0.9 to 0.2 V. The CA was conducted at 0.25 V for 1000 s. The solutions were deaerated by bubbling ultrapure N2 for 20 min and kept with a nitrogen atmosphere blanket during the entire experimental procedure. All electrochemical experiments were carried out at room temperature. 2.4. Materials characterization Structure and morphology of PdAg nanoparticles were investigated using X-ray diffraction (XRD, Bruker D8 advance X-ray diffractometer, Cu-Κa radiation, l ¼ 0.154059 nm at 40 kV and 40 mA, 2q range of 30e80 , scan rate 4 min1), scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEM100CXII).
2. Experimental 3. Results and discussion 2.1. Chemicals and materials Silver nitrate (AgNO3, Sinopharm Chemical Reagent Co., Ltd), palladium chloride (PdCl2, Sinopharm Chemical Reagent Co., Ltd), poly(vinyl pyrrolidone) (PVP, K-30, Sinopharm Chemical Reagent Co., Ltd), ethanol (CH3CH2OH, Sinopharm Chemical Reagent Co., Ltd), ethylene glycol((CH2OH)2, Sinopharm Chemical Reagent Co., Ltd), Nafion solution (DuPont, 1 wt% in isopropanol), Pd/C commercial catalyst (Johnson Matthey Corp.), potassium hydroxide (KOH, Sinopharm Chemical Reagent Co., Ltd), and sodium chloride (NaCl, Sinopharm Chemical Reagent Co., Ltd) were used as received. Water was supplied by a water purifier system (18.2 MU$cm). All chemicals were analytical grade. Ultrapure N2 was used for the deaeration. 2.2. Catalyst preparation The Ag nanoparticles were synthesized according to the modified procedures described before [28,29], then PdAg nanoparticles were prepared. In a typical reaction, 5 mL of ethylene glycol (EG) and 0.2 mL of NaCl solution (0.2 mM in EG) were refluxed at 162 C for 1 h under vigorous stirring. 5 mL AgNO3 solution (0.1 M in EG) and 5 mL PVP solution (0.3 M in EG) were then added into the solution, simultaneously. The reaction mixture was stirred for another 40 min at 162 C. After the solution was cooled to room temperature, 44.3 mL freshly prepared PdCl2 solution (2.82 mM in pure water) was then added dropwise into it and stirred overnight at room temperature. The final dispersion was diluted with pure water and ethanol and then centrifuged at a rate of 3000 rpm for 30 min. The nanoparticles were collected and redispersed in
3.1. Physical characterization and formation of metal nanostructures Fig. 1 shows the representative SEM and TEM micrographs of the Ag nanoparticles that were used as templates for the galvanic displacement reaction with Pd2þ. From the SEM image (Fig. 1A), it can be seen that the initial Ag nanoparticles were nearly spherical. The average diameter ranged from 90 to 150 nm. As can be seen in Fig. 1B the central portions of these particles were darker than their edges due to different thickness of silver along the path of electron beam. After the displacement reaction, it can be seen from the SEM and TEM images in Fig. 2 that the morphology of the primary Ag nanomaterials changed remarkably. In this case, the central portion of each particle was lighter than its edge, indicating the formation of nanospheres with hollow interior. Upon closer inspection of Fig. 2B and D, it can be seen that there are many smaller nanoparticles on the shell of each individual hollow nanoparticle, forming the raspberry-like structure. The average diameters of these smaller particles were evaluated at 10 ± 2 nm. It is noteworthy that these porous nanospheres were strong enough to survive the capillary forces involved in solvent evaporation process. The replacement reaction occurs because the standard reduction potential of Pd2þ/Pd (þ0.915 V vs. NHE (versus the normal hydrogen electrode)) was higher than that of Agþ/Ag (þ0.7991V vs. NHE) in the solution. Based on the previous research of galvanic replacement reactions in both aqueous and organic solvents, the probable mechanism for the formation of hollow and porous structure was proposed. In general, the galvanic replacement between Ag nanoparticles and PdCl2 involves a number of processes,
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Fig. 1. The SEM (A) and TEM (B) micrographs of the Ag nanoparticles that were used as templates for the galvanic displacement reaction with Pd2þ.
Fig. 2. SEM (A and B) and TEM (C and D) images of PdAg nanospheres.
including diffusion of Pd2þ to the surface of Ag templates, reduction of Pd2þ, deposition of Pd atoms on the surface of Ag nanoparticles, at the same time, because of the strong diffusion of Ag atoms from
Fig. 3. The schematic illustration of the experimental procedure that generates the hollow and porous structured PdAg nanospheres by templating against silver nanoparticle.
the bulk to the surface, they were alloyed with Pd atoms [29] and caused structural redistribution. Fig. 3 shows the schematic illustration of the experimental procedure that generates the hollow and porous structured PdAg nanospheres by templating against silver nanoparticle. First, a pit is formed on the surface of the Ag nanoparticle. As the reaction proceeds, this pit turns into a hole that gradually expands to hollow out the inside of the particle. Palladium is simultaneously plated on the outer surface, and rapid interdiffusion between Pd and Ag atoms causes the formation of homogenous PdAg alloy atom. The alloy atoms should be mainly confined to the vicinity of the Ag template surface. Once the concentration of PdAg alloy atom has reached a critical value, they will nucleate and grow into small clusters, and eventually form the hollow raspberry-like nanosphere. Furthermore, this allows for the creation of a PdAg system with a higher surface area, which could further increase its electrocatalytic potential since more sites are readily available for electrochemical reaction.
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Powder X-ray diffraction (XRD) analysis was used to characterize the chemical composition and crystal structure of hollow raspberry-like nanospheres. The typical XRD patterns of the assynthesized PdAg nanospheres and initial Ag nanoparticles are shown in Fig. 4. The JCPDS data of Ag (card no. 04-0783, blue bars), Pd (card no. 65-2867, green bars) and AgCl (card no. 31-1238, yellow bars) (in web version) are also included in this figure for comparison. As shown in Fig. 4, it can be seen that compared to the XRD patterns of Pd and Ag, the diffraction peaks from the synthesized PdAg nanospheres are located between the diffraction peaks expected from pure Pd and Ag, which indicates that alloying had occurred. Because no intermetallic compound exists in the phase diagram of Pd and Ag, it is inferred that the alloy of Pd and Ag is a solid solution [24]. Scherrer's equation was used to estimate the average crystallite size D of the PdAg alloy nanoparticles from (2 2 0) peaks:
D ¼ klKa =b$cosq
(1)
where, k: a coefficient (0.9); l: the wavelength of X-ray (1.54056A ); b: the half-peak width for (2 2 0) peak (rad); q: the angle at the (2 2 0) peak position (rad). The average crystallite size was estimated at 9.5 nm for the PdAg alloy nanoparticles. Except for the diffraction peaks from PdAg alloy, there are other strong peaks that are ascribed to AgCl crystals, which can be verified from the excellent agreement of the peaks from PdAg nanospheres sample with JCPDS data of AgCl. AgCl crystals were formed during the galvanic displacement reaction between Ag nanoparticles and PdCl2: 2Ag0 þ PdCl2 / Pd0 þ 2AgCl
(2)
The existence of AgCl also suggests that the PdAg alloy nanospheres can be synthesized effectively by the present method. It should be noted that the AgCl generated in this replacement reaction can be easily dissolved with concentrated ammonia solutions to recover the PdAg alloy nanosphere as a pure sample.
Fig. 4. XRD patterns of the as-synthesized PdAg nanospheres and the initial Ag nanoparticles. For comparison, bulk Ag, Pd, and AgCl from the Joint Committee Powder Diffraction Standard were also included.
Ag nanoparticles and PdAg nanospheres were also characterized by UVevis measurements. Fig. 5 shows the UVevis absorption spectra of the Ag nanoparticles and the PdAg nanospheres. It can be seen that the initial Ag nanoparticles show an absorption at 438 nm, corresponding to the typical surface plasmon absorption of Ag nanoparticles. As for the PdAg nanospheres, the strong absorption of the Ag nanoparticles disappeared, the surface plasmon absorption decreases markedly and red-shifts to 448 nm due to the partial substitution of the Ag atoms by Pd atoms during the galvanic replacement reaction. This is an indication that the PdAg nanospheres templated from the Ag nanoparticles were an alloy rather than a mixture of monometallic nanoparticles. 3.2. Ethanol electro-oxidation with PdAg alloy nanospheres The electrocatalytic activity of the as-prepared PdAg nanospheres was studied with cyclic voltammetry. Fig. 6A shows the typical CVs of the PdAg alloy nanospheres (PdAg-NS/GC) and commercial Pd/C catalyst (Pd/GC) electrodes with the same loading in N2-saturated 1.0 M KOH solution at a potential scan rate of 50 mV s1. The CV curve of the PdAg-NS/GC electrode clearly shows the electrochemical features of palladium. The oxidation current of palladium appears above 0.12 V in the positive-potential scan. In the reverse scan, the reduction current peak of palladium oxides can be seen at around 0.28 V. From the CV curve of Pd/C, the voltammetric features are similar to those of PdAg-NS/GC. However, in comparison with the CV of PdAg-NS/GC electrode, the double-layer capacitance of Pd/GC electrode is much larger, which may be ascribed to the carbon support material. The steady CVs of ethanol electrooxidation on Pd/GC and PdAgNS/GC electrodes are displayed in Fig. 6B. The results were normalized to the electrochemically active surface areas (ECSAs) obtained from the CVs in blank solution (Fig. 6A). Due to the penetration of hydrogen into the Pd and Pd-based bimetallic nanostructures, the ECSAs of the Pd/C, and PdAg catalysts were calculated instead from the charge of the reduction region of PdO to Pd [31]. As seen in Fig. 6B, an anodic current peak for ethanol oxidation reaction in the forward scan can be clearly observed, which was attributed to the ethanol oxidation. At the same time, another anodic peak was found in the reverse scan, which was produced by the removal of the incompletely oxidized carbonaceous species formed in the forward scan [32]. The accumulation of
Fig. 5. UVevis absorption spectra of the Ag nanoparticles and the PdAg nanospheres.
C. Peng et al. / Journal of Power Sources 278 (2015) 69e75
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Fig. 6. Cyclic voltammograms of the PdAg-NS/GC and Pd/GC electrodes (A) in 1.0 M KOH solution and (B) in 1.0 M KOH þ 1.0 M CH3CH2OH solution. Potential scan rate 50 mV s1.
intermediate carbonaceous species on the catalysts surface will lead to “catalyst poisoning”. The onset potentials (Es), peak potentials (Ep) and peak current densities (jp) of these electrodes for ethanol oxidation are listed in Table 1. Obviously, the onset potential of ethanol oxidation on PdAg alloy nanospheres is more negative than that on Pd/C. Furthermore, compared to commercial Pd/C, the results also show that the peak potential on PdAg alloy nanospheres is more negative and the peak current density on PdAg alloy nanospheres is larger, indicating the as-synthesized PdAg alloy nanospheres have much better catalytic activity than commercial Pd/C catalysts. In addition, the ratio of the forward anodic peak current density (If) to the reverse anodic peak current density (Ib), If/Ib, can be used to describe the catalyst tolerance to carbonaceous species accumulation. As shown in Fig. 6B, it is obvious that the If/Ib ratio (2.28) for PdAg-NS/GC electrode is higher than that of Pd/GC electrode (If/ Ib ¼ 0.66), suggesting the more effective removal of the poisoning intermediate carbonaceous species on the surface of PdAg alloy nanospheres. Furthermore, the ratio of the forward anodic peak potential (Ef) to the reverse anodic peak potential (Eb), Ef/Eb, for PdAg/RGO catalyst was 0.69, which was also higher than that of Pd/ C (Ef/Eb ¼ 0.33). To further demonstrate the superior behavior of the PdAg alloy nanospheres catalyst, LSV tests were performed at several different temperatures on Pd/GC and PdAg-NS/GC electrodes (Fig. 7). Fig. 8 shows the relationship of the reciprocal of temperature and the logarithm of current at E ¼ 0.15 V. An apparent activation energy value was calculated based on Arrhenius equation [33] as below:
I ¼ AeEa=RT
(3)
where I is the current at a specific potential, R is the gas constant, T is the temperature in K and Ea is the apparent activation energy. By linear fit the relationship of ln I and 1/T the Ea can be obtained for commercial Pd/C and PdAg alloy nanospheres at 17.75 and 21.90 kJ mol1, respectively. It is clear that the Ea of the PdAg alloy nanospheres catalyst is smaller than that of the commercial Pd/C catalyst. The lower Ea means a higher intrinsic activity for the PdAg
alloy nanospheres, suggesting the charge transfer process is faster. This result is consistent with the above observations. To further evaluate the activity and long-term stability of the PdAg alloy nanospheres and commercial Pd/C catalysts for ethanol oxidation, chronoamperometric (CA) measurements were carried out at a potential of 0.25 V for 1000 s in a solution of 1.0 M KOH þ 1.0 M C2H5OH. As shown in Fig. 9 both catalysts showed initially a high current density, which is attributed to the doublelayer charging and numerous available active sites on the surface of both catalysts. After that, the currents dropped sharply, implying formation of intermediate carbonaceous species, such as CO-like species, which poisons the active surface. In Fig. 9 it can also be seen that the steady-state current density for alcohol oxidation on the PdAg alloy nanospheres catalyst is significantly larger than that on the commercial Pd/C catalyst. This indicates that PdAg alloy nanospheres had better steady state electrocatalytic activity than commercial Pd/C for ethanol oxidation in alkaline media. The enhanced electro-catalytic activity of the present PdAg alloy nanospheres towards ethanol oxidation under alkaline media can be attributed to the following aspects. Firstly, the presence of Ag in the PdAg alloy can accelerate the oxidation of reaction intermediates. Thus, combination with Ag inhibits the poisoning of the active Pd sites. Nguyen et al. [21] found that OH ions are more easily adsorbed onto PdAg alloy than Pd. According to a bifunctional mechanism [34], this will help to remove more intermediates and release more active sites on the surface of the PdAg alloy nanospheres catalyst. Secondly, the unique structure of the PdAg alloy may be another possible reason for its high activity. The hollow and porous structure, where the porous shell allows the internal surface of the catalyst to be accessible to the reactants, will increase the electrochemical active surface area of the catalyst, offering more active sites necessary for the alcohol oxidation. In addition, the electronic structure modification of the two interactive metallic components could be an important factor for improving catalytic activity. According to Hammer and Nørskov's calculation [35], d-band center of Pd with a lattice value of 3.89 Å will be shifted up when combining with Ag (a ¼ 4.09 Å), resulting in an increase in the adsorbate binding energy and thus promoting the reactivity of Pd for the CO oxidation [36].
4. Conclusions
Table 1 Results of the cyclic voltammetric study of the prepared electrodes. Alcohol
Electrode
Es (V)
Ep(V)
jp(mA$cm2)
Ethanol
Pd/GC PdAg-NS/GC
0.66 0.72
0.09 0.20
0.82 2.34
In this paper, we have demonstrated a facile method to fabricate hollow raspberry-like PdAg alloy nanospheres using the galvanic displacement reaction between pre-synthesized Ag nanoparticles and palladium ions. SEM and TEM show that the synthesized PdAg
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Fig. 7. LSV curves in 1 M KOH þ 1 M C2H5OH solutions at different temperatures for Pd/GC (A) and PdAg-NS/GC (B) electrodes. Potential scan rate 50 mV s1.
Fig. 8. Arrhenius plots for commercial Pd/C (A) and PdAg alloy nanospheres (B) at 0.15 V.
oxidation than commercial Pd/C in alkaline solution. Structural and electrochemical studies revealed that this system is advantageous in several ways, including 1) an excellent tolerance to reaction intermediates; 2) efficient usage of noble metals and high surface areas, which arises because of the hollow and porous nanostructure; 3) the electronic structure modification of the two interactive metallic components. It is anticipated that the asprepared hollow raspberry-like PdAg alloy electrocatalyst has considerable potential as a non-Pt electro-catalyst for DAFCs. Acknowledgments This work was supported by Scientific Research Fund of Huaqiao University (No.10Y0195*), Fundamental Research Funds for the Central Universities (No. JB-ZR1138) and The Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
Fig. 9. Chronoamperometric curves of the Pd/GC (black curves) and PdAg-NS/GC (red curves) electrodes in 1 M KOH þ 1 M C2H5OH solution at electrode potential of 0.25V (vs. Hg/HgO). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
alloy nanocrystals exhibit an interesting hollow and porous structure. The results from CV and CA demonstrate that the PdAg alloy nanospheres are superior as an electrocatalyst for ethanol electro-
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Hollow raspberry-like PdAg alloy nanospheres: High electrocatalytic activity for ethanol oxidation in alkaline media Cheng Peng*, Yongli Hu, Mingrui Liu, Yixiong Zheng College of Materials Science and Engineering, Huaqiao University, Xiamen 361021, 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
Hollow raspberry-like PdAg alloy nanospheres were prepared. The as-prepared catalyst exhibits superior activity for ethanol oxidation. The activity derives from the unique structure and electronic effects.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 October 2014 Received in revised form 5 December 2014 Accepted 13 December 2014 Available online 15 December 2014
Palladiumesilver (PdAg) alloy nanospheres with unique structure were prepared using a one-pot procedure based on the galvanic replacement reaction. Their electrocatalytic activity for ethanol oxidation in alkaline media was evaluated. The morphology and crystal structure of the samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Electrochemical characterization techniques, including cyclic voltammetry (CV) and chronoamperometry (CA) measurements were used to analyze the electrochemical performance of the PdAg alloy nanospheres. The SEM and TEM images showed that the PdAg alloy nanospheres exhibit a hierarchical nanostructure with hollow interiors and porous walls. Compared to the commercial Pd/C catalyst, the as-prepared PdAg alloy nanospheres exhibit superior electrocatalytic activity and stability towards ethanol electro-oxidation in alkaline media, showing its potential as a new non-Pt electrocatalyst for direct alcohol fuel cells (DAFCs). © 2014 Elsevier B.V. All rights reserved.
Keywords: Palladium Ethanol oxidation Alkaline media Electrocatalysts Direct alcohol fuel cells
1. Introduction Fuel cell technology is of tremendous interest, because of both energy and environmental considerations. In recent years, direct alcohol fuel cells (DAFCs) have been considered as a promising power source for portable electronic devices and transportation due to their high efficiency, high energy density and low or zero
* Corresponding author. E-mail address: [email protected] (C. Peng). http://dx.doi.org/10.1016/j.jpowsour.2014.12.056 0378-7753/© 2014 Elsevier B.V. All rights reserved.
emissions [1,2]. Serving as a very promising alternative fuel for DAFCs, ethanol has attracted extensive interest in recent years due to its lower toxicity than methanol, it can be produced in large quantities from biomass [3e8]. However, there are several limitations associated with the commercialization of DAFCs, including the high cost, poor electrocatalytic activity and durability of the electrocatalyst. In order to address these issues, considerable research efforts have been focused on the development of nanomaterials with higher electrocatalytic activity and stability. Platinum-based materials are often used as anode
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electrocatalyst due to the high catalytic activity [9e12]. However, the high cost and the serious CO self-poisoning of platinum catalysts hinder their wide-spread application in DAFCs. One of the key challenges to the practical application and commercialization of DAFCs is the development and design of stable, highly active and low cost electrocatalysts. To this end, various Pt-based and Pt-free nanomaterials have been investigated extensively in the past decade. Although Pd is nearly inert to alcohol oxidation reaction in acid media, it has been shown that palladium is more active than platinum for ethanol oxidation in basic media [13e16]. Furthermore, Pd not only has a higher abundance on the earth's crust but also has a lower price as compared to Pt [17]. Therefore, Pd and Pdbased metal alloys have attracted more and more research attention as an effective non-platinum anode electrocatalyst for DAFCs [18e25]. For the nanostructured electrocatalysts in fuel cell, it is well known that the catalytic performance is strongly dependent on their surface structures, such as the particle size, particle shape, compositions and so on [26]. As for Pd-based nanomaterials, much work has been done on the preparation of Pd-based electrocatalysts, but prior researches mainly focused on solid, porous, or hollow Pd-based nanomaterials, including nanoparticles, nanoplates, nanowires, nanoboxes, snowflake-like self-assemblies, coreeshell nanostructures etc [27e30]. There have been no previous reports on the synthesis of a hierarchical PdAg alloy nanospheres with hollow interior and porous walls. In this work, we prepared hierarchical PdAg alloy nanospheres with hollow interiors and porous walls using a one-pot procedure based on the galvanic replacement reaction. The morphology and crystal structure of the nanoparticles were characterized with SEM, TEM and XRD. Moreover, the electrocatalytic performance of PdAg alloy nanospheres towards ethanol was studied by cyclic voltammetry and chronoamperometry measurements.
ethanol by sonication. This process was repeated 5 times. The final product was dispersed in ethanol (1 mg$mL1) and stored at room temperature. 2.3. Electrochemical investigations A glassy carbon electrode (5.0 mm diameter) was polished with alumina slurries (1.5, 0.5 and 0.05 mm diameter) and cleansed by sonication in deionized water and ethanol for 5 min. Voltammetric measurements were carried out with a CHI660D electrochemical workstation. Cyclic voltammetry and chronoamperometry were performed in 1 M KOH þ 1 M alcohol solution. The working electrode was prepared by dropping 15 mL electrocatalyst ink onto glassy carbon electrode with a micropipette followed by drying at room temperature. The ink was prepared by ultrasonically mixing 10 mL electrocatalyst sample in 5 mL Nafion solution. A Pt foil and Hg/HgO electrode were used as the counter and reference electrodes, respectively. All potentials in the study were given versus Hg/HgO electrode. The CV tests were conducted at 50 mV$s1, with potential ranging from 0.9 to 0.2 V. The CA was conducted at 0.25 V for 1000 s. The solutions were deaerated by bubbling ultrapure N2 for 20 min and kept with a nitrogen atmosphere blanket during the entire experimental procedure. All electrochemical experiments were carried out at room temperature. 2.4. Materials characterization Structure and morphology of PdAg nanoparticles were investigated using X-ray diffraction (XRD, Bruker D8 advance X-ray diffractometer, Cu-Κa radiation, l ¼ 0.154059 nm at 40 kV and 40 mA, 2q range of 30e80 , scan rate 4 min1), scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEM100CXII).
2. Experimental 3. Results and discussion 2.1. Chemicals and materials Silver nitrate (AgNO3, Sinopharm Chemical Reagent Co., Ltd), palladium chloride (PdCl2, Sinopharm Chemical Reagent Co., Ltd), poly(vinyl pyrrolidone) (PVP, K-30, Sinopharm Chemical Reagent Co., Ltd), ethanol (CH3CH2OH, Sinopharm Chemical Reagent Co., Ltd), ethylene glycol((CH2OH)2, Sinopharm Chemical Reagent Co., Ltd), Nafion solution (DuPont, 1 wt% in isopropanol), Pd/C commercial catalyst (Johnson Matthey Corp.), potassium hydroxide (KOH, Sinopharm Chemical Reagent Co., Ltd), and sodium chloride (NaCl, Sinopharm Chemical Reagent Co., Ltd) were used as received. Water was supplied by a water purifier system (18.2 MU$cm). All chemicals were analytical grade. Ultrapure N2 was used for the deaeration. 2.2. Catalyst preparation The Ag nanoparticles were synthesized according to the modified procedures described before [28,29], then PdAg nanoparticles were prepared. In a typical reaction, 5 mL of ethylene glycol (EG) and 0.2 mL of NaCl solution (0.2 mM in EG) were refluxed at 162 C for 1 h under vigorous stirring. 5 mL AgNO3 solution (0.1 M in EG) and 5 mL PVP solution (0.3 M in EG) were then added into the solution, simultaneously. The reaction mixture was stirred for another 40 min at 162 C. After the solution was cooled to room temperature, 44.3 mL freshly prepared PdCl2 solution (2.82 mM in pure water) was then added dropwise into it and stirred overnight at room temperature. The final dispersion was diluted with pure water and ethanol and then centrifuged at a rate of 3000 rpm for 30 min. The nanoparticles were collected and redispersed in
3.1. Physical characterization and formation of metal nanostructures Fig. 1 shows the representative SEM and TEM micrographs of the Ag nanoparticles that were used as templates for the galvanic displacement reaction with Pd2þ. From the SEM image (Fig. 1A), it can be seen that the initial Ag nanoparticles were nearly spherical. The average diameter ranged from 90 to 150 nm. As can be seen in Fig. 1B the central portions of these particles were darker than their edges due to different thickness of silver along the path of electron beam. After the displacement reaction, it can be seen from the SEM and TEM images in Fig. 2 that the morphology of the primary Ag nanomaterials changed remarkably. In this case, the central portion of each particle was lighter than its edge, indicating the formation of nanospheres with hollow interior. Upon closer inspection of Fig. 2B and D, it can be seen that there are many smaller nanoparticles on the shell of each individual hollow nanoparticle, forming the raspberry-like structure. The average diameters of these smaller particles were evaluated at 10 ± 2 nm. It is noteworthy that these porous nanospheres were strong enough to survive the capillary forces involved in solvent evaporation process. The replacement reaction occurs because the standard reduction potential of Pd2þ/Pd (þ0.915 V vs. NHE (versus the normal hydrogen electrode)) was higher than that of Agþ/Ag (þ0.7991V vs. NHE) in the solution. Based on the previous research of galvanic replacement reactions in both aqueous and organic solvents, the probable mechanism for the formation of hollow and porous structure was proposed. In general, the galvanic replacement between Ag nanoparticles and PdCl2 involves a number of processes,
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Fig. 1. The SEM (A) and TEM (B) micrographs of the Ag nanoparticles that were used as templates for the galvanic displacement reaction with Pd2þ.
Fig. 2. SEM (A and B) and TEM (C and D) images of PdAg nanospheres.
including diffusion of Pd2þ to the surface of Ag templates, reduction of Pd2þ, deposition of Pd atoms on the surface of Ag nanoparticles, at the same time, because of the strong diffusion of Ag atoms from
Fig. 3. The schematic illustration of the experimental procedure that generates the hollow and porous structured PdAg nanospheres by templating against silver nanoparticle.
the bulk to the surface, they were alloyed with Pd atoms [29] and caused structural redistribution. Fig. 3 shows the schematic illustration of the experimental procedure that generates the hollow and porous structured PdAg nanospheres by templating against silver nanoparticle. First, a pit is formed on the surface of the Ag nanoparticle. As the reaction proceeds, this pit turns into a hole that gradually expands to hollow out the inside of the particle. Palladium is simultaneously plated on the outer surface, and rapid interdiffusion between Pd and Ag atoms causes the formation of homogenous PdAg alloy atom. The alloy atoms should be mainly confined to the vicinity of the Ag template surface. Once the concentration of PdAg alloy atom has reached a critical value, they will nucleate and grow into small clusters, and eventually form the hollow raspberry-like nanosphere. Furthermore, this allows for the creation of a PdAg system with a higher surface area, which could further increase its electrocatalytic potential since more sites are readily available for electrochemical reaction.
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Powder X-ray diffraction (XRD) analysis was used to characterize the chemical composition and crystal structure of hollow raspberry-like nanospheres. The typical XRD patterns of the assynthesized PdAg nanospheres and initial Ag nanoparticles are shown in Fig. 4. The JCPDS data of Ag (card no. 04-0783, blue bars), Pd (card no. 65-2867, green bars) and AgCl (card no. 31-1238, yellow bars) (in web version) are also included in this figure for comparison. As shown in Fig. 4, it can be seen that compared to the XRD patterns of Pd and Ag, the diffraction peaks from the synthesized PdAg nanospheres are located between the diffraction peaks expected from pure Pd and Ag, which indicates that alloying had occurred. Because no intermetallic compound exists in the phase diagram of Pd and Ag, it is inferred that the alloy of Pd and Ag is a solid solution [24]. Scherrer's equation was used to estimate the average crystallite size D of the PdAg alloy nanoparticles from (2 2 0) peaks:
D ¼ klKa =b$cosq
(1)
where, k: a coefficient (0.9); l: the wavelength of X-ray (1.54056A ); b: the half-peak width for (2 2 0) peak (rad); q: the angle at the (2 2 0) peak position (rad). The average crystallite size was estimated at 9.5 nm for the PdAg alloy nanoparticles. Except for the diffraction peaks from PdAg alloy, there are other strong peaks that are ascribed to AgCl crystals, which can be verified from the excellent agreement of the peaks from PdAg nanospheres sample with JCPDS data of AgCl. AgCl crystals were formed during the galvanic displacement reaction between Ag nanoparticles and PdCl2: 2Ag0 þ PdCl2 / Pd0 þ 2AgCl
(2)
The existence of AgCl also suggests that the PdAg alloy nanospheres can be synthesized effectively by the present method. It should be noted that the AgCl generated in this replacement reaction can be easily dissolved with concentrated ammonia solutions to recover the PdAg alloy nanosphere as a pure sample.
Fig. 4. XRD patterns of the as-synthesized PdAg nanospheres and the initial Ag nanoparticles. For comparison, bulk Ag, Pd, and AgCl from the Joint Committee Powder Diffraction Standard were also included.
Ag nanoparticles and PdAg nanospheres were also characterized by UVevis measurements. Fig. 5 shows the UVevis absorption spectra of the Ag nanoparticles and the PdAg nanospheres. It can be seen that the initial Ag nanoparticles show an absorption at 438 nm, corresponding to the typical surface plasmon absorption of Ag nanoparticles. As for the PdAg nanospheres, the strong absorption of the Ag nanoparticles disappeared, the surface plasmon absorption decreases markedly and red-shifts to 448 nm due to the partial substitution of the Ag atoms by Pd atoms during the galvanic replacement reaction. This is an indication that the PdAg nanospheres templated from the Ag nanoparticles were an alloy rather than a mixture of monometallic nanoparticles. 3.2. Ethanol electro-oxidation with PdAg alloy nanospheres The electrocatalytic activity of the as-prepared PdAg nanospheres was studied with cyclic voltammetry. Fig. 6A shows the typical CVs of the PdAg alloy nanospheres (PdAg-NS/GC) and commercial Pd/C catalyst (Pd/GC) electrodes with the same loading in N2-saturated 1.0 M KOH solution at a potential scan rate of 50 mV s1. The CV curve of the PdAg-NS/GC electrode clearly shows the electrochemical features of palladium. The oxidation current of palladium appears above 0.12 V in the positive-potential scan. In the reverse scan, the reduction current peak of palladium oxides can be seen at around 0.28 V. From the CV curve of Pd/C, the voltammetric features are similar to those of PdAg-NS/GC. However, in comparison with the CV of PdAg-NS/GC electrode, the double-layer capacitance of Pd/GC electrode is much larger, which may be ascribed to the carbon support material. The steady CVs of ethanol electrooxidation on Pd/GC and PdAgNS/GC electrodes are displayed in Fig. 6B. The results were normalized to the electrochemically active surface areas (ECSAs) obtained from the CVs in blank solution (Fig. 6A). Due to the penetration of hydrogen into the Pd and Pd-based bimetallic nanostructures, the ECSAs of the Pd/C, and PdAg catalysts were calculated instead from the charge of the reduction region of PdO to Pd [31]. As seen in Fig. 6B, an anodic current peak for ethanol oxidation reaction in the forward scan can be clearly observed, which was attributed to the ethanol oxidation. At the same time, another anodic peak was found in the reverse scan, which was produced by the removal of the incompletely oxidized carbonaceous species formed in the forward scan [32]. The accumulation of
Fig. 5. UVevis absorption spectra of the Ag nanoparticles and the PdAg nanospheres.
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Fig. 6. Cyclic voltammograms of the PdAg-NS/GC and Pd/GC electrodes (A) in 1.0 M KOH solution and (B) in 1.0 M KOH þ 1.0 M CH3CH2OH solution. Potential scan rate 50 mV s1.
intermediate carbonaceous species on the catalysts surface will lead to “catalyst poisoning”. The onset potentials (Es), peak potentials (Ep) and peak current densities (jp) of these electrodes for ethanol oxidation are listed in Table 1. Obviously, the onset potential of ethanol oxidation on PdAg alloy nanospheres is more negative than that on Pd/C. Furthermore, compared to commercial Pd/C, the results also show that the peak potential on PdAg alloy nanospheres is more negative and the peak current density on PdAg alloy nanospheres is larger, indicating the as-synthesized PdAg alloy nanospheres have much better catalytic activity than commercial Pd/C catalysts. In addition, the ratio of the forward anodic peak current density (If) to the reverse anodic peak current density (Ib), If/Ib, can be used to describe the catalyst tolerance to carbonaceous species accumulation. As shown in Fig. 6B, it is obvious that the If/Ib ratio (2.28) for PdAg-NS/GC electrode is higher than that of Pd/GC electrode (If/ Ib ¼ 0.66), suggesting the more effective removal of the poisoning intermediate carbonaceous species on the surface of PdAg alloy nanospheres. Furthermore, the ratio of the forward anodic peak potential (Ef) to the reverse anodic peak potential (Eb), Ef/Eb, for PdAg/RGO catalyst was 0.69, which was also higher than that of Pd/ C (Ef/Eb ¼ 0.33). To further demonstrate the superior behavior of the PdAg alloy nanospheres catalyst, LSV tests were performed at several different temperatures on Pd/GC and PdAg-NS/GC electrodes (Fig. 7). Fig. 8 shows the relationship of the reciprocal of temperature and the logarithm of current at E ¼ 0.15 V. An apparent activation energy value was calculated based on Arrhenius equation [33] as below:
I ¼ AeEa=RT
(3)
where I is the current at a specific potential, R is the gas constant, T is the temperature in K and Ea is the apparent activation energy. By linear fit the relationship of ln I and 1/T the Ea can be obtained for commercial Pd/C and PdAg alloy nanospheres at 17.75 and 21.90 kJ mol1, respectively. It is clear that the Ea of the PdAg alloy nanospheres catalyst is smaller than that of the commercial Pd/C catalyst. The lower Ea means a higher intrinsic activity for the PdAg
alloy nanospheres, suggesting the charge transfer process is faster. This result is consistent with the above observations. To further evaluate the activity and long-term stability of the PdAg alloy nanospheres and commercial Pd/C catalysts for ethanol oxidation, chronoamperometric (CA) measurements were carried out at a potential of 0.25 V for 1000 s in a solution of 1.0 M KOH þ 1.0 M C2H5OH. As shown in Fig. 9 both catalysts showed initially a high current density, which is attributed to the doublelayer charging and numerous available active sites on the surface of both catalysts. After that, the currents dropped sharply, implying formation of intermediate carbonaceous species, such as CO-like species, which poisons the active surface. In Fig. 9 it can also be seen that the steady-state current density for alcohol oxidation on the PdAg alloy nanospheres catalyst is significantly larger than that on the commercial Pd/C catalyst. This indicates that PdAg alloy nanospheres had better steady state electrocatalytic activity than commercial Pd/C for ethanol oxidation in alkaline media. The enhanced electro-catalytic activity of the present PdAg alloy nanospheres towards ethanol oxidation under alkaline media can be attributed to the following aspects. Firstly, the presence of Ag in the PdAg alloy can accelerate the oxidation of reaction intermediates. Thus, combination with Ag inhibits the poisoning of the active Pd sites. Nguyen et al. [21] found that OH ions are more easily adsorbed onto PdAg alloy than Pd. According to a bifunctional mechanism [34], this will help to remove more intermediates and release more active sites on the surface of the PdAg alloy nanospheres catalyst. Secondly, the unique structure of the PdAg alloy may be another possible reason for its high activity. The hollow and porous structure, where the porous shell allows the internal surface of the catalyst to be accessible to the reactants, will increase the electrochemical active surface area of the catalyst, offering more active sites necessary for the alcohol oxidation. In addition, the electronic structure modification of the two interactive metallic components could be an important factor for improving catalytic activity. According to Hammer and Nørskov's calculation [35], d-band center of Pd with a lattice value of 3.89 Å will be shifted up when combining with Ag (a ¼ 4.09 Å), resulting in an increase in the adsorbate binding energy and thus promoting the reactivity of Pd for the CO oxidation [36].
4. Conclusions
Table 1 Results of the cyclic voltammetric study of the prepared electrodes. Alcohol
Electrode
Es (V)
Ep(V)
jp(mA$cm2)
Ethanol
Pd/GC PdAg-NS/GC
0.66 0.72
0.09 0.20
0.82 2.34
In this paper, we have demonstrated a facile method to fabricate hollow raspberry-like PdAg alloy nanospheres using the galvanic displacement reaction between pre-synthesized Ag nanoparticles and palladium ions. SEM and TEM show that the synthesized PdAg
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Fig. 7. LSV curves in 1 M KOH þ 1 M C2H5OH solutions at different temperatures for Pd/GC (A) and PdAg-NS/GC (B) electrodes. Potential scan rate 50 mV s1.
Fig. 8. Arrhenius plots for commercial Pd/C (A) and PdAg alloy nanospheres (B) at 0.15 V.
oxidation than commercial Pd/C in alkaline solution. Structural and electrochemical studies revealed that this system is advantageous in several ways, including 1) an excellent tolerance to reaction intermediates; 2) efficient usage of noble metals and high surface areas, which arises because of the hollow and porous nanostructure; 3) the electronic structure modification of the two interactive metallic components. It is anticipated that the asprepared hollow raspberry-like PdAg alloy electrocatalyst has considerable potential as a non-Pt electro-catalyst for DAFCs. Acknowledgments This work was supported by Scientific Research Fund of Huaqiao University (No.10Y0195*), Fundamental Research Funds for the Central Universities (No. JB-ZR1138) and The Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
Fig. 9. Chronoamperometric curves of the Pd/GC (black curves) and PdAg-NS/GC (red curves) electrodes in 1 M KOH þ 1 M C2H5OH solution at electrode potential of 0.25V (vs. Hg/HgO). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
alloy nanocrystals exhibit an interesting hollow and porous structure. The results from CV and CA demonstrate that the PdAg alloy nanospheres are superior as an electrocatalyst for ethanol electro-
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