graphene catalysts prepared by the polyalcohol method

Electrochimica Acta 130 (2014) 135–140

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Effect of water content on the ethanol electro-oxidation activity of Pt-Sn/graphene catalysts prepared by the polyalcohol method Yong Wang, Guochun Wu, Yongzhen Wang, Xiaomin Wang ∗ College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China

a r t i c l e

i n f o

Article history: Received 23 December 2013 Received in revised form 17 February 2014 Accepted 28 February 2014 Available online 12 March 2014 Keywords: Pt-Sn/graphene Microstructural characterization Ethanol oxidation Water content

a b s t r a c t Developing means of controlling the composition of bimetallic Pt-Sn catalysts by appropriate preparation is important for their electrocatalytic activity. In this study, a polyalcohol method has been applied to synthesize graphene-supported Pt-Sn catalysts in different ethylene glycol/water mixtures, and the effect of water content on the composition of the catalysts has been investigated. Microstructural analysis has revealed the onset of aggregation of catalyst particles and increasing Sn content with the increase of water content. In terms of the electrocatalytic properties, the electrochemical activity of Pt-Sn/graphene for the ethanol oxidation reaction was improved with increasing Sn content due to the bifunctional mechanism and ligand effects. The activity of the catalyst was maximized at a water content of 5 vol.%. On further increasing the water content, a substantial decrease in the electrochemically active surface area (ECSA) was incurred due to blocking by tin oxide and the aggregation of catalyst particles, and the activity of the catalysts decreased. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Direct ethanol fuel cells (DEFCs) utilize ethanol as a fuel, which has many advantages, such as high theoretical energy density (8.0 kW h kg−1 , cf. 6.1 kW h kg−1 for methanol), ease of production and handling, low toxicity, and environmental compatibility [1,2]. Carbon-supported Pt is commonly used as an anode catalyst, but Pt itself is susceptible to rapid surface poisoning by CO derived from the dissociative adsorption of ethanol [3]. Catalysts that consist of bimetallic systems often exhibit better activity, deactivation resistance, and more flexible design, and recent studies have focused on these catalysts [4–6]. Bimetallic Pt-Sn is the most active for the ethanol electro-oxidation reaction (EOR) [7]. A certain amount of Sn has a beneficial modifying effect on the electrocatalytic activity of Pt [8,9]. However, a further increase in the amount of Sn results in a decrease in the EOR activity [10]. An appropriate balance between Pt and Sn must therefore be achieved to maximize the electrocatalytic activity. One of the simplest ways to prepare bimetallic catalysts is coimpregnation of the support material with metal precursors from solution and then applying reducing agents. In this process, the composition of the catalysts can be tailored by adjusting the initial

∗ Corresponding author. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.electacta.2014.02.152 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

ratio of the two metal precursors, the concentration of a capping agent, the reaction temperature and time, and so on [11]. Lee et al. [12] reported that the mean particle size and the size distribution of Pt/carbon black catalysts were strongly dependent on the water content in ethylene glycol (EG) used as a reducing agent. The particle size of SnO2 was likewise shown to be correlated with the water content [13]. The water content is thus a critical parameter, yet there have been few reports on its effect on the composition of bimetallic Pt-Sn catalysts, and the mechanism of its influence is still unclear. In order to obtain catalysts with the desired Pt to Sn stoichiometry and improved electrocatalytic activity, it is necessary to gain a better understanding of the effect of water content on the catalysts formed. Graphene, as a two-dimensional sheet of sp2 -bonded carbon atoms, exhibits high surface area and high conductivity, which makes it promising for potential applications as a support for lowtemperature fuel cell catalysts [14,15]. In our previous study [16], on the development of Pt-Sn catalysts, we found that Pt-Sn synthesized on graphene displayed outstanding catalytic activity. In the present work, Pt-Sn/graphene catalysts have been prepared by co-impregnation of graphene as a support material with Pt and Sn precursors in EG/water mixtures followed by reduction of the metal precursors with EG at high-temperature. The prepared catalysts have been characterized by XRD, TEM, XPS, and EDS to investigate their microstructural modifications. Based on their physico-chemical properties, the electrocatalytic activities of

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the catalysts for the ethanol oxidation reaction are compared and discussed, with emphasis on the effect of varying the water content in the reaction medium. 2. Experimental 2.1. Synthesis of Pt-Sn/graphene catalysts Graphite oxide (GO) was synthesized from graphite powder by a modified Hummers method [17]. Pt and Sn nanoparticles were deposited on graphene by chemical reduction of chloroplatinic acid (H2 PtCl6 ) and tin dichloride (SnCl2 ) in ethylene glycol (EG) or a mixture of EG and water. In a typical procedure, GO powder (60 mg) dispersed in EG/water (100 mL) was sonicated for 2 h and exfoliated into graphene oxide. Subsequently, a 0.05 m solution of chloroplatinic acid in EG (H2 PtCl6 -EG; 4 mL) and a 0.025 m solution of tin dichloride in EG (SnCl2 -EG; 4 mL) were added to the graphene oxide dispersion and the mixture was sonicated for 1 h. The solution was adjusted to pH 12 with a 5 m solution of sodium hydroxide in EG (NaOH-EG) and then stirred under a flow of argon at 130 ◦ C for 3 h. After cooling, the mixture was adjusted to pH < 2 with aqueous nitric acid (HNO3 ), which promoted the adsorption of suspended metal nanoparticles onto the support, and then stirred for 24 h. The resulting catalyst was washed with water and then with ethanol, and the washed solid was dried in a vacuum oven at 80 ◦ C for 12 h. The catalysts prepared in this way are designated as Pt-Sn/G(100:0), Pt-Sn/G(95:5), Pt-Sn/G(90:10), and Pt-Sn/G(80:20), respectively, where the numbers in parentheses indicate the relative volumes of EG to water used in the preparations. 2.2. Characterization X-ray powder diffraction (XRD) analysis was carried out on a TD3500 X-ray diffractometer using Cu-K␣ radiation (␭ = 0.15418 nm). The morphology of Pt-Sn/graphene was examined by transmission electron microscopy (TEM) on a JEM 2010 operating at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250 apparatus using monochromated Mg-K␣ radiation to determine the surface compositions of the catalysts. The metal contents of the catalysts were determined with an Oxford INCA energy-dispersive X-ray spectrometer (EDS) coupled with a MIRA3 TESCAN scanning electron microscopy. The electrochemical properties were examined with a CHI 660D potential static instrument (CHI Instruments, Inc.). A glassy carbon electrode (4 mm) was used as the working electrode, on which a 10 ␮L aliquot of catalyst ink was applied and dried at room temperature. Catalyst ink were prepared as follows: catalyst powder (5 mg) was dispersed in 5% Nafion (100 ␮L) and isopropyl alcohol (900 ␮L) and the mixture was sonicated for 30 min. A Pt wire was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. For cyclic voltammetry (CV) measurements, the working electrode was immersed in a solution saturated with highly purified nitrogen, with the potential between −0.241 and +1.0 V versus SCE. 3. Results and discussion 3.1. Structural and morphological study The XRD patterns of the Pt-Sn/graphene catalysts prepared with different amounts of water are shown in Fig. 1. The diffraction peaks at 2␪ values of 39.6◦ , 46.1◦ , 67.4◦ , and 81.3◦ can be assigned to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystalline planes of face-centered-cubic (fcc) Pt, respectively. The high peak intensities indicated the existence of good crystallinity in the obtained

Fig. 1. XRD patterns of Pt-Sn/graphene catalysts. Prepared in different EG/water mixtures: (a) Pt-Sn/G(100:0), (b) Pt-Sn/G(95:5), (c) Pt-Sn/G(90:10) and (d) PtSn/G(80:20).

Pt-Sn/graphene. However, the intensity of the Sn diffraction peaks was rather weak, suggesting that the Sn formed an alloy with Pt or existed as an amorphous phase [18]. The diffraction peak of Pt (2 2 0) was used to estimate the Pt particle size by Scherrer’s equation: D = 0.94 ␭/(B cos␪) [19]. Here, the wavelength ␭ is equal to 0.15418 nm, B is the full-width at halfmaximum (FWHM), and ␪ is the angular position at the (2 2 0) peak maximum. The average particle sizes were calculated as 3.0, 3.0, 3.5, and 4.3 nm for Pt-Sn/G (100:0), Pt-Sn/G (95:5), Pt-Sn/G (90:10), and Pt-Sn/G (80:20), respectively. The morphology, size, and dispersion of the catalysts could also be examined more directly by TEM. It is evident from Fig. 2 that highly dispersed metal nanoparticles had been successfully deposited on the graphene supports, even at a high metal loading. For the Pt-Sn/G (100:0) and Pt-Sn/G (95:5) catalysts, the particle size distributions were very narrow, covering the range from 1 to 4 nm with maxima at around 2.3 nm and 2.5 nm, respectively. However, on further increasing the water content, the particles began to aggregate. The average sizes of the metal particles in the Pt-Sn/G (90:10) and Pt-Sn/G (80:20) catalysts were 2.8 nm and 3.6 nm, respectively. It was observed that the metal particles in Pt-Sn/G (80:20) had a slightly broader size distribution (Fig. 2(d)). The average particle size calculated from the XRD data was slightly larger than that obtained from TEM analysis because the larger Pt particles (although few in number) made a dominant contribution to the diffraction signals [20]. Nevertheless, the variation trend in the particle sizes determined by TEM was consistent with that by XRD. Because the reduction of metal ions by EG is very rapid and the nucleation and growth steps of the metal particles are separate processes, the growth step of the particles has a determining effect on their size [21]. On the other hand, the growth of particles occurs by coalescence and monomer attachment from solution, and is diffusion-controlled [22]. The presence of capping agents thus has an effect on the diffusion for the growth of particles, and the particle growth can be controlled by the concentration of capping agent in the solution [11,23]. In this study, glycolate was obtained from EG as a reaction product, which is believed to act as a good capping agent for metal colloids [24]. Therefore, the particle size could be controlled by the concentration of glycolate in the solution. With increasing water content, the concentration of glycolate decreases, which results in an increase in the particle size of the Pt-Sn/graphene.

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Fig. 2. TEM images for the Pt-Sn/graphene catalysts as well as the particle size distributions of the metal nanoparticles. (a) Pt-Sn/G(100:0), (b) Pt-Sn/G(95:5), (c) Pt-Sn/G(90:10) and (d) Pt-Sn/G(80:20).

3.2. Composition analysis Catalysis is a surface effect, and the chemical composition in the near-surface region is crucial for the electrocatalytic behavior [25]. Fig. 3 shows the Pt 4f and Sn 3d XPS spectra of the various catalysts. The Pt 4f spectra could be deconvoluted into three doublets, corresponding to Pt 4f7/2 and Pt 4f5/2 of different oxidation states. The principal peaks at 71.4 eV (4f7/2 ) and 74.8 eV (4f5/2 ) can be attributed to Pt0 , and those at 72.7, 76.1 and 74.6, 78.0 eV can be assigned to Pt in the 2+ and 4+ states, respectively [26]. The results for different Pt species were calculated on the basis of the above data and are listed in Table 1. The Pt was found in the zero-valence state and in ionic form (most commonly as Pt(OH)2 and PtO2 [27]). Table 1 Binding Energies(BE), Relative Intensities(RI) and Fwhms of different platinum species(Fwhm) as observed from Pt(4f7/2 ) Spectra on the surface for the PtSn/graphene catalysts. Catalysts Pt–Sn/G(100:0)

Pt–Sn/G (95:5)

Pt–Sn/G (90:10)

Pt–Sn/G (80:20)

Species 0

Pt Pt2+ Pt4+ Pt0 Pt2+ Pt4+ Pt0 Pt2+ Pt4+ Pt0 Pt2+ Pt4+

BE of 4f7/2 (eV)

RI (%)

Fwhm (eV)

71.4 72.7 74.6 71.3 72.7 74.5 71.3 72.5 74.2 71.3 72.7 74.5

55.5 27.4 17.1 54.9 27.5 17.6 54.1 28.5 17.4 56.6 27.3 16.1

1.5 1.7 1.8 1.5 1.8 1.9 1.4 1.5 1.9 1.5 1.7 1.7

The water content was found to have little effect on the relative intensities of the Pt0 , Pt2+ , and Pt4+ peaks. The Sn 3d spectra clearly exhibited intense doublets attributable to the 3d3/2 (495.3 eV) and 3d5/2 (486.9 eV) of Sn2+/4+ . Discrimination between Sn2+ and Sn4+ species is not possible due to the very small difference in their binding energies [28]. No doublet due to metallic Sn (493.2 and 484.8 eV) was observed, suggesting that the surface Sn had been completely oxidized to SnOx . Although all of the catalysts were prepared with the same precursor composition in the starting mixture, the Sn to Pt atomic ratio increased with increasing water content. As shown in Fig. 3, the Sn 3d to Pt 4f peak intensity ratio increased on going from PtSn/G (100:0) to Pt-Sn/G (80:20). This trend was corroborated by the Pt:Sn molar ratio calculated from the XPS data in Table 2. The metal loadings and Pt:Sn molar ratios of the Pt-Sn/graphene catalysts calculated from the EDS data are also provided in Table 2. It can be seen that the Sn loading increased with increasing water content, while the Pt loading was essentially constant. The bulk composition, as measured by EDS, showed a trend in accordance with the surface composition, even though the relative Sn content in the Table 2 Metal loading and Pt:Sn atomic ratios of the Pt-Sn/graphene catalysts. Catalysts

Pt–Sn/G(100: 0) Pt–Sn/G (95: 5) Pt–Sn/G (90:10) Pt–Sn/G (80:20)

Metal loading (wt.%)

Pt: Sn (at./at.)

Pt

Sn

Total

EDS

XPS

50.1 51.8 52.5 50.8

4.1 5.3 8.0 10.6

54.2 57.1 60.5 61.4

7.4:1 5.9:1 4.0:1 2.9:1

3.1:1 2.4:1 1.7:1 1.2:1

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Fig. 3. Pt 4f and Sn 3d XPS spectra of the Pt-Sn/graphene catalysts: (a) Pt-Sn/G(100:0), (b) Pt-Sn/G(95:5), (c) Pt-Sn/G(90:10) and (d) Pt-Sn/G(80:20).

near-surface region was generally higher than that in the bulk. The variation trend can be rationalized by considering the following: EG is only a mild reducing agent, and its reducing strength may be strong enough for the reduction of Pt ions but too weak for the reduction of Sn ions, which are extensively surrounded and protected by EG molecules to form Sn-OCH2 CH2 OH sites [29,30]. As a result, the actual Pt:Sn atomic ratio of Pt-Sn/G (100:0), without the addition of water, was 7.4, far higher than the starting ratio of 2. This indicates a substantial loss of Sn during the synthesis. However, in the presence of water, the Sn-OCH2 CH2 OH sites may be hydrolyzed

to form Sn(OH)2 and ultimately decomposed to Sn or SnOx [13]. The reducing strength of EG towards SnCl2 is gradually enhanced with increasing water content. This results in the increase of Sn content. The results indicate that the Pt:Sn ratio can be controlled, to some extent, simply by adjusting the water content. 3.3. Electrochemical measurements To obtain the voltammograms of H2 adsorption/desorption, the potential was cycled between −0.241 and +1.0 V (vs. SCE) at

Y. Wang et al. / Electrochimica Acta 130 (2014) 135–140

Fig. 4. Cyclic voltammograms of H2 adsorption/desorption on the different PtSn/graphene catalysts in 0.5 m H2 SO4 solution at a scan rate of 50 mV s−1 .

50 mV s−1 in an Ar-purged 0.5 m H2 SO4 electrolyte solution at room temperature. The curves stabilized after 10 cycles. Fig. 4 shows the H2 adsorption/desorption curves of the Pt-Sn/graphene catalysts. For Pt-Sn/G (90:10) and Pt-Sn/G (80:20), the small peaks seen at around 0.3 and 0.5 V may be attributed to the adsorption/desorption of O2 derived from the dissociation of water on the Sn oxide [31]. As the content of tin oxide species increased, the small peaks developed further. The ECSA of an electrocatalyst not only provides important information regarding the number of electrochemically active sites on a mass basis of the precious metal, but is also a crucial parameter for comparing different electrocatalysts [32], which can be estimated by H2 adsorption on and desorption from the electrode surface. As also shown in Fig. 4, typical H2 adsorption/desorption peaks were observed in the potential range −0.24 to +0.05 V (vs. SCE). It is noteworthy that the characteristic peaks of bulk Pt with different crystalline facets were not observed in the region of H2 adsorption. Instead, a broad peak was observed with these catalysts, which might be caused by structural modifications of Pt due to the Pt–Sn interaction [28,33]. The region of H2 adsorption/desorption became smaller with increasing water content from Pt-Sn/G (95:5) to Pt-Sn/G (80:20), suggesting that the ECSA gradually decreased. This can be ascribed to an increase in the surface SnOx content, resulting in a decrease in the number of active sites of Pt nanoparticles [27]. The active sites of Pt nanoparticles may be covered with SnOx . In addition, the decreased ECSA may also be due to gradual aggregation of the particles, as shown by TEM. The electrocatalytic activities of the Pt-Sn/graphene catalysts for ethanol oxidation were measured by cyclic voltammetry between −0.241 and +1.0 V (vs. SCE) at 20 mV s−1 in an Ar-purged 1 m C2 H5 OH + 0.5 m H2 SO4 solution at room temperature. As shown in Fig. 5, even though the ECSA of Pt-Sn/G (95:5) was smaller than that of Pt-Sn/G (100:0), its EOR activity was higher. This can be attributed to the enrichment of SnOx on the catalyst surface, as analyzed by XPS. SnOx is able to adsorb water molecules dissociatively to form OHads at considerably lower potentials than Pt, allowing the operation of a bifunctional mechanism according to Eqs. (1) and (2), thus reducing or preventing poisoning of the catalyst [34–36]. In addition, the Sn species (oxides) can also promote CO oxidation through ligand effects that weaken the Pt-CO bond [37]. The electrochemical activity of Pt can be improved by the addition of Sn. However, it can be seen that the EOR mass activity of Pt-Sn/G (90:10) was lower and that of Pt-Sn/G (80:20) was the lowest. A further increase in Sn content decreases the activity,

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Fig. 5. Cyclic voltammograms of EOR for the different Pt-Sn/graphene catalysts in 1 m C2 H5 OH + 0.5 m H2 SO4 solution at a scan rate of 20 mV s−1 .

Fig. 6. Chronoamperometry tests of EOR for the different Pt-Sn/graphene catalysts in 1 m C2 H5 OH + 0.5 m H2 SO4 solution.

which may result from the very low ECSAs of Pt-Sn/G (90:10) and Pt-Sn/G (80:20), as discussed above. SnOx + OH− → SnOx —OHads + e−

(1)

Pt—(CO)ads + OH− → Pt + CO2 + H2 O + e−

(2)

The catalytic activities and stabilities of the different PtSn/graphene catalysts were evaluated by chronoamperometry tests in 1 m C2 H5 OH + 0.5 m H2 SO4 solution at 0.65 V (vs. SCE), and the results are shown in Fig. 6. For all of the samples, the current decreased rapidly in the first 50 s and then became relatively stable. The current value may decay due to poisoning of surface active sites and instability of the catalyst particles [38]. The end currents after 1000 s of these catalysts followed the order of Pt-Sn/G (95:5) (131.2 mA mgPt −1 ) > Pt-Sn/G (100:0) (119.9 mA mgPt −1 ) > Pt-Sn/G (90:10) (103.6 mA mgPt −1 ) > Pt-Sn/G (80:20) (76.3 mA mgPt −1 ), in accordance with the trend from the CV results. 4. Conclusion We have synthesized a series of Pt-Sn/graphene catalysts with different Pt:Sn ratios by the polyol method in EG/water mixtures with a starting Pt:Sn atomic ratio of 2 but different water contents. It was found that the water content had a significant effect on the Sn content. With increasing water content, the Sn content of the

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catalysts increased. Even though an increase in Sn content may lead to a decrease in the ECSA of the catalysts due to blocking by tin oxide, the electrochemical activity for the ethanol oxidation reaction was improved through the bifunctional mechanism and ligand effects. However, when the water content was further increased to some extent, a more substantial decrease in the ECSA was observed due to blocking by tin oxide and a gradual aggregation of catalyst particles. This resulted in decreased electrochemical activity, and the beneficial effect of Sn oxide was negated. Water could thus be used as an important additive for obtaining a modified EG reducing agent; however, the water content in the modified reducing agent must be kept at an appropriate level so as to obtain the best performance. In this study, the optimal reaction medium for the preparation of Pt-Sn/graphene catalyst with a starting Pt:Sn atomic ratio of 2 has been identified as an EG solution with around 5 vol.% water. Acknowledgments This work was supported by the National Natural Science Foundation of China (51372160, 51172152 and 51242007).

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