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Vanadium oxide decorated carbon nanotubes as a promising support of Pt nanoparticles for methanol electro-oxidation reaction
Journal of Colloid and Interface Science 393 (2013) 291–299
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Vanadium oxide decorated carbon nanotubes as a promising support of Pt nanoparticles for methanol electro-oxidation reaction Amideddin Nouralishahi a,b, Abbas Ali Khodadadi a, Ali Morad Rashidi b, Yadollah Mortazavi a,⇑ a b
Catalysis and Nanostructured Materials Research Laboratory, School of Chemical Engineering, University of Tehran, P.O. Box 11155/4563, Tehran, Iran Nanotechnology Research Center, Research Institute of Petroleum Industry (RIPI), Azadi Sport Complex West Blvd., Tehran, Iran
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 31 August 2012 Accepted 24 October 2012 Available online 10 November 2012
VOx–MWCNTs nanocomposite was prepared via deposition–precipitation method followed by microwave treatment. Platinum nanoparticles were dispersed via polyol process over the nanocomposite support, and thus, prepared electro-catalyst was employed in methanol electro-oxidation reaction. The electro-catalysts were characterized by means of TGA, XRD, EDS, FESEM, TEM, and H2-TPR analysis. The electro-catalytic activity and stability of the electrodes toward methanol oxidation reaction in acidic medium were studied by using cyclic voltammetry (CV), CO-stripping, and electrochemical impedance spectroscopy (EIS) techniques. Compared to the Pt/MWCNTs, the Pt/VOx–MWCNTs electro-catalyst not only exhibits high electro-catalytic activity, but also shows very good stability during methanol electro-oxidation reaction. In addition, the presence of VOx in the composite support dramatically increases the electrochemical active surface area of platinum nanoparticles. The results of electrochemical impedance spectroscopy reveal that formation kinetics of adsorbed hydroxyl group on surface of the electrocatalysts is improved upon vanadium oxide addition to the support. This phenomenon is very helpful to facilitate oxidative removal of adsorbed CO group through bifunctional mechanism on Pt/VOx– MWCNTs. Ó 2012 Elsevier Inc. All rights reserved.
Keywords: Methanol oxidation Direct methanol fuel cell Electro-catalyst Platinum Vanadium Carbon nanotube Electrochemical impedance spectroscopy Bifunctional mechanism
1. Introduction In recent years, fuel cells as clean and portable power sources have attracted a great importance, because of several problems in traditional energy producing systems. Compared to the hydrogen fuel cells, direct methanol fuel cells (DMFCs) are safer, more compact and easier to use. At the anode of DMFCs, methanol is directly electro-oxidized to produce carbon dioxide and six electrons according to the following reaction:
CH3 OH þ H2 O ! CO2 þ 6Hþ þ 6e
E ¼ 0:046 V
ð1Þ
Platinum is very attractive electro-catalyst for oxidation of short chain alcohols, such as methanol [1–3]. The reaction of methanol oxidation on surface of platinum particles consists of several steps including (a) adsorption of methanol; (b) methanol dissociation; (c) H2O adsorption; (d) formation of surface –OHads groups by water activation; and (e) oxidation of –COads [4]. Formation of – OHads group from the adsorbed water molecules, which is critical for elimination of adsorbed CO-like groups, requires a relatively high potential on the Pt surface. This leads to blocking of active
⇑ Corresponding author. Fax: +98 21 66967793. E-mail addresses: [email protected] (A. Nouralishahi), [email protected] (A.A. Khodadadi), [email protected] (A.M. Rashidi), [email protected] (Y. Mortazavi). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.10.048
sites and severe power losses in anode of DMFCs. Therefore, pure platinum is a rather poor anode electro-catalyst for methanol oxidation reaction (MOR) at low temperatures (T < 100 °C). Making alloys with one or even two metals that can provide oxygenated species at lower potentials for oxidative removal of –COads is a suitable method to modify the electro-catalytic properties of platinum [5,6]. Ruthenium is the most common promoter of platinum nanoparticles that significantly improves the catalytic properties of Pt in MOR [7–10]. However, there have been numerous attempts with the aim of reducing the loading of noble metals in DMFCs anodes via addition of lower-priced compounds. Therefore, materials like tungsten oxide [11], molybdenum oxide [12], titanium oxide [13], tin oxide [14], etc. have been examined as promoters in methanol oxidation reaction. Because of its ability for oxidation of methanol [15,16], vanadium oxide may be used as a possible promoter in methanol electro-oxidation reaction [17,18]. Jusys et al. studied electro-catalytic activities of VOx, MoOx, and WOx as promoters of unsupported PtRu electro-catalyst in methanol oxidation reaction at fuel cell conditions and reported PtRuVOx as the best electro-catalyst [19]. Justin and coworkers have employed vulcan carbon-supported vanadium oxide (V2O5–C) as the support of Pt nanoparticles for MOR in alkaline medium [20]. They attributed high activity and stability of Pt/ V2O5–C electro-catalyst to synergetic effect between platinum and vanadium oxide species.
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Among all carbon nanostructures, carbon blacks (CBs), especially vulcan XC-72, are the most frequently applied supports for Pt containing electro-catalysts for direct alcoholic fuel cells in commercial applications and different studies [21–23]. However, multiwalled carbon nanotubes with relative large surface area, high electrical conductivity, and remarkable electrochemical and mechanical stability in corrosive media are promising candidates for anode electro-catalyst supports in direct methanol fuel cells. There are many reports demonstrating the superior performance of multiwalled carbon nanotubes compared to carbon blacks as the support of anode catalysts in direct methanol fuel cells [24– 26]. This work investigates the performance of VOx–MWCNTs nanocomposite as a potential support for Pt nanoparticles in methanol electro-oxidation reaction. The results indicate that VOx–MWCNTs nanocomposite dramatically improves the electro-catalytic activity and stability of platinum active sites in MOR.
and at a scanning rate of 0.02 (2h) s1. Particle size and surface morphologies of the electro-catalysts were determined by transmission electron microscopy (TEM, ZEISS EM 900) and field emission scanning electron microscopy (FESEM, Hitachi model:S4160). Hydrogen temperature-programmed reduction (H2-TPR) experiments were performed using a Quantachrome CHEMBET3000 apparatus. The TPR experiments were conducted on 15 mg of the electro-catalysts powders under a flow of 10 sccm 7.0% hydrogen in argon. Prior to the reduction reactions, all electrocatalysts were pretreated at 100 °C for 3 h in 10 sccm argon stream to eliminate the moisture and other adsorbates from the surface. Finally, temperature was increased from 100 °C to 650 °C with a heating rate of 10 °C min1. Thermal gravimetric analysis (TGA) and Derivative Thermo Gravimetric (DTG) were carried out under nitrogen atmosphere with a heating rate of 10 °C min1 by a PerkinElmer thermogravimetric analyzer. 2.4. Preparation of working electrodes
2. Experimental 2.1. Materials All the chemicals used were of analytical grade and applied without further purification. V2O5 and H2PtCl66H2O were purchased from Aldrich. Methanol, ethylene glycol, and sulfuric acid were obtained from Merck, and the 5 wt.% solution of nafion was brought from Alfa Aesar. The multiwalled carbon nanotubes (MWCNTs) were purified via several washing steps in aqueous solution of HCl and HNO3 in order to eliminate inorganic impurities, followed by heating up to 260 °C for 60 min. The resultant carbon nanotubes had a purity of about 95 wt.% according to thermal gravimetric analysis (TGA) results. 2.2. Preparation of electro-catalysts The nanocomposite of VOx–MWCNTs was prepared via deposition–precipitation method followed by microwave treatment. The purified multiwalled carbon nanotubes were surface oxidized by H2SO4–HNO3 mixture (3:1 V/V) in an ultrasonic bath at 60 °C. To prepare VOx–MWCNTs nanocomposite, the functionalized MWCNTs were dispersed ultrasonically in deionized water. Next, an aqueous suspension of vanadium oxide was added dropwise and mixed vigorously. The mixture was dried in a rotary evaporator followed by heating in an oven at 80 °C over night and then was placed into a microwave oven (black and baker, MY26PG, 1000W) and heated in several periods. Pt/VOx–MWCNTs and Pt/MWCNTs electro-catalysts with 10 wt.% Pt loading were prepared by the same method via a polyol process [20]. In a typical procedure, specific amount of MWCNTs or VOx–MWCNTs was dispersed in 60 ml ethylene glycol in an ultrasonic bath and mixed well with 3.4 ml aqueous solution of hexachloroplatinic acid (75.4 mM). Subsequently, a solution of KOH (2.5 M) was added dropwise to the suspension, in order to induce the formation of tiny and uniform platinum nanoparticles. Next, the reduction reaction was conducted under argon atmosphere and reflux conditions. The resultant electro-catalyst sample was washed with boiling distillated water several times to remove ethylene glycol and Cl ions. Finally, it was dried in a vacuum oven at 100 °C for 6 h. 2.3. Physico-chemical characterizations The structure of the electro-catalysts was determined with Xray powder diffraction (XRD) by a PW 1840 Philips diffractometer using Cu Ka1 (k = 1.54056 Å) radiation in the 2h range of 15–90°
In a typical process, 5 mg of electro-catalyst powder was ultrasonically dispersed in a mixture of 50 ll nafion solution and 2 ml ethanol to form a homogeneous ink. Then, 6.3 ll of the prepared ink was spread on a washed and mirror-finished surface of GC electrode (0.0314 cm2) and then the solvent evaporated slowly under radiation of an ordinary 100 W lamp. Finally, the modified electrode was heated up in an oven at 80 °C for 30 min. 2.5. Electrochemical measurements All electrochemical experiments were carried out in a conventional three electrode cell using a Teflon sheathed glassy carbon disk with a diameter of 2 mm as working electrode, a Pt plate (5.2 cm2), and an Ag/AgCl electrode as counter and reference electrodes, respectively. All potentials in this work are quoted against Ag/AgCl reference electrode at 25 °C. In addition, all experiments were carried out in nitrogen bubbled electrolytes using Ivium Stat electrochemical analyzer (Ivium Technologies, Ivium Stat type 10V/5A). The electro-catalytic performances in methanol oxidation reaction (MOR) were determined by cyclic voltammetry experiments in an aqueous solution of 1.0 M methanol and 0.5 M sulfuric acid. The CV curves were recorded at a scan rate of 50 mV s1 from 200 mV to 1200 mV. Electro-catalytic active surface areas (ECSAs) were estimated by cyclic voltammetry in 0.5 M H2SO4 at a scan rate of 50 mV s1. Moreover, electrochemical impedance measurements were carried out by sweeping the frequency from 100 kHz to 100 mHz in a constant potential mode at an amplitude of 5 mV. In order to study the CO tolerance of electrodes, CO was bubbled through the cell for 30 min, 200 mV was applied on the working electrode for 5 min to ensure CO saturated adsorption on the electro-catalyst layers, dissolved CO was removed by Ar bubbling for 30 min, and then, a CV was performed in a potential range of 200–1200 mV. 3. Results and discussion 3.1. Characterization of the electro-catalysts The amounts of structural water in the nanocomposite support can be calculated by thermal gravimetric analysis. The TGA and DTG results of MWCNTs and VOx–MWCNTs under nitrogen atmosphere are presented in Fig. 1a and b. The initial weight losses observed in TGA curves of the both samples, at T < 150 °C, are associated with the loss of adsorbed surface water and volatile compounds. In the case of MWCNTs, the weight loss at T > 150 °C
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Fig. 1. TGA results (a) and derivative curves (b) of MWCNTs and VOx–MWCNTs under nitrogen atmosphere.
is attributed to removal of the functional groups (i.e., –COOH and OH) from the surface of MWCNTs, while the excess weight loss of VOx–MWCNTs compared to the acid functionalized carbon nanotubes may be related to the structural water removal from the bulk of vanadium oxide species. According to Fig. 1b, most of structural water in the composite support is removed between 300 °C and 400 °C with maximum dehydration at 370 °C. To exclude the influence of functional groups on the MWCNTs, the content of structural water in the nanocomposite support is calculated by subtracting the measured weight loss of MWCNTs from the weight loss of VOx–MWCNTs. The water content value calculated by TGA is approximately 0.8% of the total weight of composite support. Fig. 2 shows the XRD patterns of MWCNTs, Pt/MWCNTs, VOx– MWCNTs, and Pt/VOx–MWCNTs. The diffraction peaks at around 26°, 42°, 54°, and 77° observed for the carbon nanotubes can be attributed to the planes (0 0 2), (1 0 0), (0 0 4), and (1 1 0) of carbon graphite structure, respectively [27]. In the case of VOx–MWCNTs, there are no peaks other than those observed for the carbon nanotubes, indicating that vanadium oxide is either amorphous or crystalline with very small crystallites. In samples Pt/MWCNTs and Pt/VOx–MWCNTs, the peaks observed at around 39°, 46°, 68°, 81°, and 87° are corresponding to Pt (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of fcc crystalline platinum. However, presence of the vanadium oxide in the composite support causes the characteristic peak at 68° to appear at slightly lower angels. The shifts in XRD data in Pt/VOx–MWCNTs are in a good agreement with the reports in literature on Pt/
Fig. 2. XRD patterns of (a) MWCNTs, (b) VOx–MWCNTs, (c) Pt/MWCNTs, and (d) Pt/ VOx–MWCNTs.
V2O5–C [20,28] in which the slight shift in (2h) angle of 68° is related to alloying of Pt and vanadium. The average crystallite size can be determined using characteristic peak (at 2h 68°) by employing the Scherrer’s equation [29]:
D ¼ 0:9k=b cos hmax
ð2Þ
where L is the mean crystal size (Å), kka is the wavelength of the xray (1.54056 Å for the copper Ka), b is defined as width of diffraction peak at half maximum (rad), and hmax is the Bragg angle (deg). Calculating from Eq. (2), the average particle diameter of platinum nanoparticles is determined to be about 4.6 nm and 3.5 nm for Pt/ MWCNTs and Pt/VOx–MWCNTs, respectively. The FESEM images of purified MWCNTs and VOx–MWCNTs are depicted in Fig. 3a and b. In Fig. 3a, there is almost no sign of mineral clusters, implying that most of the mineral impurities of MWCNTs have been removed in the purification process. Fig. 3b confirms that the vanadium oxide species were properly precipitated on the surface of MWCNTs, and a rather uniform nanocomposite of VOx–MWCNTs was formed during the synthesis process. The energy-dispersive X-ray spectroscopy (EDS) results of Pt/ VOx–MWCNTs are depicted in Fig. 3c. The results confirm that both platinum and vanadium oxide species are successfully participated on multiwalled carbon nanotubes. The amounts of Pt:V atomic ratio are determined to be 2.7:2, which are very close to the nominal ratios of 3:2 for Pt:V. Fig 4 demonstrates the TEM images of MWCNTs, Pt/MWCNTs and Pt/VOx–MWCNTs. About 30 nm open-end multiwall carbon nanotubes can be observed in Fig. 4a. The open-end of MWCNTs may indicate that the growth catalyst impurities of MWCNTs are removed adequately during the purification process. The average Pt particle size, among 360 randomly selected particles, is estimated to be about 4.9 and 3.6 for Pt/MWCNTs and Pt/VOx– MWCNTs, respectively. This is in good agreement with the XRD data illustrated in Fig. 2. Additionally, the presence of the vanadium oxide in the supports causes the platinum nanoparticles to precipitate in a narrower size distribution range that may be originated from an increase in the number of nucleation centers available on the surface of composite supports. The reduction behavior of the electro-catalysts was investigated by H2-TPR analysis (Fig. 5). The strong peak centered at 540–550 °C can be attributed to methanation of carbon nanotubes of the support [30,31]. In case of Pt/MWCNTs, the very small peak at 145 °C may be attributed to highly reducible Pt oxide species with a low interaction with the support [32]. In addition, the broad shoulder starting at 290 °C may be related to catalytic methanation of carbon nanotubes of the support in presence of platinum active sites. In Fig. 5, there are two separated peaks in the TPR profile of VOx–MWCNTs at 341 and 596 °C. The peak at 341 °C may be re-
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Fig. 3. Field emission scanning electron microscopy (FESEM) of (a) MWCNTs, (b) VOx–MWCNTs and the EDS spectra of (c) Pt/VOx–MWCNTs.
Fig. 4. Transmission electron microscopy of (a) MWCNTs, (b) Pt/MWCNTs, and (c) Pt/VOx–MWCNTs.
lated to monomeric and polymeric VO4 units on the surface [33]. The peak at higher temperature may be deconvoluted into two different ones (dashed lines in Fig. 5); the first peak, centered at 520– 530 °C, related to methanation reaction of MWCNTs, and the sec-
ond one, at 580–585 °C, is assigned to reduction of bulk V(5+) to V(3+) [34,35]. Incorporation of Pt nanoparticles to VOx–MWCNTs causes the peak of V(5+) to shift negatively by about 25 °C. This demonstrates higher reducibility of vanadium oxide species in
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Fig. 6 shows the voltammetric curves of the modified electrodes in 0.5M H2SO4 at 25 °C. Both electrodes exhibit two overlapped anodic peaks between 200 and 150 mV, which are associated to hydrogen monolayer desorption on platinum surface. As well, the cathodic peaks during backward scans in the same potential range are related to adsorption of atomic hydrogen on mentioned active sites [36]. These processes could be written as follows:
27.8 m2 g1, in the case of Pt/MWCNTs, to 61.8 m2 g1 in the case of Pt/VOx–MWCNTs. This suggests that the presence of VOx raises the utilization of Pt nanoparticles, which agrees with TEM micrographs in Fig. 4. The broad weak anodic peaks in Fig. 6, between 200 and 500 mV, may be related to the formation of surface oxygenated groups on the electro-catalysts [38]. Comparing the peaks in Fig. 6, one can conclude that formation of surface oxides is more facilitated on Pt/VOx–MWCNTs than on Pt/MWCNTs. This phenomenon plays an important role in improvement of electro-catalysts’ CO tolerance. Since the –COads is the major poisoning intermediate during electro-oxidation of methanol, a good electro-catalyst should have not only large ECSA but also proper –COads oxidation ability. The results of CO-stripping experiments are illustrated in Fig. 7. Here, all hydrogen desorption peaks (observed in Fig. 6) are disappeared in voltammograms of electrodes with pre-adsorbed CO indicating that the active sites on the electrodes were totally covered by adsorbed CO groups. The results clearly show that the onset potential for oxidation of adsorbed CO monolayer on Pt/VOx–MWCNTs, at 483 mV, is much lower than that on Pt/MWCNTs, at 553 mV; therefore, VOx efficiently decreases the overpotential of –COads oxidation. This confirms that the mechanism of –COads oxidation to CO2 improves due to synergetic effect of vanadium oxide in the support. In fact, the oxophilic nature of the transition metal oxides helps to remove the strongly adsorbed MOR intermediates, particularly -COads, from the platinum active sites via formation of appropriate amounts of surface oxygenated groups such as –OHads [39–41]. This lowers the required potential for oxidizing –COads to CO2, considered as bifunctional mechanism and may be written as the following reaction scheme:
PtHads $ Pt þ Hþ þ e
Pt—CH3 OHads ! Pt—COads þ 4Hþ þ 4e
Fig. 5. Hydrogen temperature-programmed reduction (H2-TPR) results of the electro-catalysts.
Pt/VOx–MWCNTs probably due to hydrogen spillover from platinum to V2O5 surfaces. 3.2. Electrochemical results
ð3Þ
The charge associated to integrated intensity of the atomic hydrogen desorption peaks, QH (lC cm2), represents the number of platinum active sites. Therefore, it is possible to estimate the electrochemical active surface areas (ECSAs) from the following equation [37]: 2
ECSA ðcm =mgPtÞ ¼ Q H =C ½Pt
Fig. 6. Cyclic voltammograms of Pt/MWCNTs and Pt/VOx–MWCNTs at scan rate of 50 mV s1 in 0.5 M H2SO4 at 25 °C.
ð5Þ
þVOx
VOx —H2 Oads ! VOx —OHads þ Hþ þ e I VOx —OHads þ VOðx1Þ OH þ e
ðstep 2Þ
ð6Þ
VOðx1Þ OH þ Pt—COads ! Pt þ VOðx1Þ þ CO2 þ H þ þe
ð4Þ
where C is the charge required to reduce a monolayer of protons (H+) on a polycrystalline platinum electrode and has been commonly adopted as 210 lC cm2 Pt. In addition, [Pt] represents the loading of platinum nanoparticles on the working electrode (mg cm2). According to Fig. 6, participation of vanadium oxide causes a dramatic increase in platinum active surface area from
ðstep 1Þ
ðstep 3Þ
ð7Þ
and/or
VOx —OHads þ Pt—COads ! Pt þ VOx þ CO2 þ Hþ þ e
ðstep 4Þ
ð8Þ
The results of cyclic voltammetry experiments between 200 and 1200 mV in 0.5 M H2SO4 and 1.0 M methanol are illustrated in Fig. 8. The anodic peaks in the forward scans are related to meth-
Fig. 7. CO stripping results for Pt/MWCNTs and Pt/VOx–MWCNTs at scan rate of 50 mV s1 in 0.5 M H2SO4 and at 25 °C.
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Fig. 8. (a) Cyclic voltammograms of Pt/MWCNTs and Pt/VOx–MWCNTs at scan rate of 50 mV s1 in 0.5 M H2SO4 and 1.0 M methanol at 25 °C, (b and c) Comparison of CV results between 150 and 400 mV in the absence (dotted lines) or presence of methanol (solid lines).
anol electro-oxidation reaction, and the peaks at backward scans are associated with carbonaceous intermediates, which have not been oxidized completely in the forward scans [42,43]. The current density at forward anodic peak on Pt/VOx–MWCNTs, at 46.3 mA cm2, is about 2.5 times higher than that of Pt/MWCNTs, at 18.9 mA cm2. This confirms superior activity of Pt/VOx– MWCNTs, compared to Pt/MWCNTs, toward methanol electrooxidation reaction. The ratio of the anodic peak current density in forward scan (jfp) to the backward anodic peak (jbp) has been used to express electro-catalyst tolerance to accumulation of carbonaceous poisoning species [44,45]. Indeed, high ratios show effective removal of CO-like poisoning species such as –COads and –COHads on the catalyst active sites. In Fig. 8, the ratio of jfp/jbp for Pt/MWCNTs is only 0.80, which is far less than that for Pt/ VOx–MWCNTs (0.96). It suggests improvement of electro-catalysts tolerance against poisoning species as a result of vanadium oxide addition to the support. In order to determine onset potential of methanol electrooxidation reaction over the electro-catalysts, the cyclic voltammetry results in the absence and presence of methanol are depicted in Fig. 8b and c. It is clear that the onset potential for methanol electro-oxidation reaction reduces from 492 mV in the case of Pt/ MWCNTs to 387 mV in Pt/VOx–MWCNTs. This suggests that the kinetics of methanol electro-oxidation reaction is more favored on Pt/VOx–MWCNTs compared to Pt/MWCNTs, in accordance with excellent activity of Pt/VOx–MWCNTs in methanol electro-oxidation reaction (see Fig. 8a).
The catalytic activity of Pt/VOx–MWCNTs electro-catalysts is dependent to Pt:V molar ratios. Fig. 9 compares the performance of electro-catalysts with different amounts of vanadium oxide toward methanol electro-oxidation reaction. In all cases, platinum loading in the samples is kept constant, while the molar ratio of Pt:V is varied. According to the results of Fig. 9, the best performance of Pt/VOx–MWCNTs is achieved at optimum molar ratios
Fig. 9. Cyclic voltammograms of Pt/VOx–MWCNT with different atomic ratio of Pt:V (the data were recorded at scan rate of 50 mV/s in 0.5 M H2SO4 and 1 M methanol at 25 °C).
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of 3:2 for Pt:V. In fact, at too high ratios, the contact regions between platinum nanoparticles and the vanadium oxide species are insufficient to effective spillover of poisoning species during bifunctional mechanism. While in the case of very low Pt:V molar ratios, the existing vanadium oxide may block some platinum active sites and consequently reduce the electrochemical active surface area [46]. Electrochemical impedance spectroscopy (EIS) is a powerful tool for understanding mechanism of electrode reactions. The complex plane (Nyquist) impedance plots of the electro-catalysts at potentials of 200, 500, and 800 mV in 1.0 M methanol and 0.5 M H2SO4 aqueous solution are, respectively, presented in Fig. 10a–c. Here, the EIS results may be analyzed considering two major reactions: the electro-oxidation of methanol to COads and releasing active sites by electro-oxidation of COads to CO2. At electrode potential of 200 mV, it can be seen that the impedance plots of both samples appear in the first quadrant and show a relative straight line with a phase angle close to 90° along the imaginary impedance axis (Im Z). This reveals the formation of electric double layer capacitor due to adsorption of different species at electrode/electrolyte interface. However, both electro-catalysts exhibit a slight deviation from the straight line that may be explained by dissociative adsorption of methanol molecules on the surface of electro-catalysts [47]. In Fig. 10a, the relative large arcs, which is especially observed for Pt/MWCNTs, implies slow reaction rate of methanol electro-oxidation at 200 mV due to strongly adsorbed CO intermediate on Pt surface [48]. At electrode potential of 500 mV, the impedance arcs suddenly bend from positive to negative direction of real impedance axis in the second and third quadrants. This mater is associated with reversible formation of OHads which facilitates oxidative removal of COads on the surface of electrodes (see reactions (5)–(8)) [49]. Wang et al. attributed the sudden change of impedance arcs to variation of rate-determining step [50]. In fact, with increase of ad-
297
Fig. 11. Impedance spectra of Pt/MWCNTs and Pt/VOx–MWCNTs presented in the Nyquist form in the range of 100 mHz to 100 kHz at OCP in 0.5 M H2SO4 at 25 °C.
sorbed hydroxyl group on surface of the electro-catalysts, the rate-determining step changes from methanol dehydrogenation reaction (reaction (5)) to removal of adsorbed OH by COads group (reactions (7) and (8)) leading to negative faradic impedance. When the electrode potential reaches to 800 mV, the impedance plots return to the first and fourth quadrants. Here, the pseudo inductive behaviors at low frequency region are related to removal of COads on the surface [51]. At such a high overpotential, strongly adsorbed hydroxyl groups cover surface of the electrocatalysts; therefore, the rate of methanol electro-oxidation reaction gets slow and the rate-determining step changes to oxidation removal of adsorbed CO group on active sites [50,52]. This is consistent with the sharp decrease in current densities at potentials greater than 0.8 V in the voltammetric responses in Fig. 8a. Based on Fig. 10, the diameters of arcs in the case of PtVOx/ MWCNTs electro-catalyst are very smaller than that of Pt/MWCNTs
Fig. 10. Nyquist plots of Pt/MWCNTs and Pt/VOx–MWCNTs electro-catalysts at (a) 200 mV, (b) 500 mV, and (c) 800 mV in 0.5 M H2SO4 + 1.0 M methanol at 25 °C.
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Fig. 12. Cyclic voltammograms of MWCNTs and VOx–MWCNTs at scan rate of 50 mV s1 in aqueous solution of 1.0 M methanol 0.5 M H2SO4 25 °C.
Fig. 13. Peak current densities of Pt/MWCNTs and Pt/VOx–MWCNTs electrocatalysts during 500 successive cyclic voltammetry experiments at scan rate of 50 mV s1 in 1.0 M methanol and 0.5 M H2SO4 solution.
at all three applied potentials, which reveals improved kinetics of OHads chemisorption, oxidative removal of COads and methanol electro-oxidation reaction upon VOx incorporation. Since methanol electro-oxidation process is composed of several deprotonation and electron formation steps, the proton conductivity of the electro-catalysts is an essential factor influencing the overall electro-catalytic activity. Resistance against ionic transfer (Rion) on the supports of electro-catalysts was calculated from the results of impedance experiments in 0.5 M H2SO4 at the open circuit potential (OCP) (Fig. 11). Rion can be satisfactorily estimated from the following equation [53]:
Z ReLf ¼ Z ReHf þ Rion =3
ð9Þ
where ZReLf and ZReHf are correspondingly defined as the real contribution of total impedance at low and high frequencies. Based on
Fig. 11, the ionic transfer resistance of functionalized MWCNTs is almost 2.3 times higher than that of VOx–MWCNTs. This shows that the presence of vanadium oxide in the support enhances the ion carrier abilities through the electro-catalyst layer due to existing structural water in the composite support (see Fig. 1). The electrochemical properties of MWCNTs and VOx–MWCNTs were examined by cyclic voltammetry experiment in 1.0 M methanol and 0.5 M H2SO4 at 25 °C, as shown in Fig. 12. The supports, without Pt nanoparticles, show no significant anodic peaks, indicating that they are almost individually inactive in methanol electro-oxidation reaction. The transported coulombic charge (Q) between 100 mV and 1100 mV can be calculated by integrated surface area of the cyclic voltammograms. In accordance with the results of impedance spectroscopy in Fig. 11, the amount of Q for VOx–MWCNTs is 1.03 mC, which is 2.7 times higher than that of MWCNTs (0.38 mC). The long-term electro-catalytic stability of Pt/VOx–MWCNTs and Pt/MWCNTs was studied in 1.0 M methanol and 0.5 M H2SO4 aqueous solution. Fig. 13 shows the current densities of forward anodic peaks during 500 successive cycles. The initial increase in peak current densities observed for both samples may be explained by elimination of adsorbed contaminates from the electro-catalysts active sites. However, after 100th cycle, both electro-catalysts show a gradual reduce in their peak current densities attributed to poisoning and structure change of the electro-catalysts during long time successive potential perturbations [54]. According to Fig. 13, the final peak current densities of Pt/VOx–MWCNTs, and Pt/MWCNTs are, respectively, 38.4 and 12.58 mA cm2, which are about 83% and 68% of their corresponding maximum values at 66th and 31st scans. Therefore, addition of VOx to the support dramatically improves the electro-catalytic stability of Pt/MWCNTs toward methanol oxidation reaction. Table 1 compares the electro-catalytic activity of our Pt/VOx– MWCNTs samples toward methanol electro-oxidation reaction with that of Pt/C promoted by different transition metal oxides reported in the literature. Here, the recorded mass specific peak current density of methanol electro-oxidation on Pt/VOx–MWCNTs at optimum Pt:V ratio (926 A g 1 Pt ) is higher than that reported on Pt/ V2O5–C [20,28]. Also, the specific activity of Pt/VOx–MWCNTs toward methanol electro-oxidation reaction is comparable with the remarkable results of Pt/Nb2O5–C [55] and Pt/MoO3–C [56]. In order to exclude the size effect of Pt nanoparticles, the value of peak current density of forward anodic peak should be divided to geometric surface area (Sgeo) of Pt nanoparticles, estimated by the following equation:
Sgeo ðm2 g 1 pt Þ ¼ 6000=q:d
ð10Þ
where q is defined as density of platinum (21.4 g cm3), and d is the average size of Pt nanoparticles (nm), obtained from XRD measurements. Among the electro-catalysts presented in Table 1, Pt/VOx– MWCNTs electro-catalyst shows the highest surface specific peak
Table 1 Electro-catalytic activity of different catalysts toward MOR.
a b c
Electro-catalyst
Ipa (A g1 Pt )
Pt average size (nm)
Geometric surface area (m2 g1 Pt )
Jpb (A m2 Pt )
Refs.
Pt/VOx–MWCNTs Pt/V2O5–C Pt/V2O5–C Pt/Nb2O5–C Pt/MoO3–C
926 600 305 871 1093
3.5 2.6 2.8 2.7 2.5c
80.1 107.8 100.1 103.8 112.1
11.6 5.6 3.0 8.4 9.7
– [20] [28] [55] [56]
Forward anodic peak mass specific current density. Forward anodic peak surface specific current density. Estimated from TEM micrographs.
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current. This confirms that the surface of Pt nanoparticles supported on VOx–MWCNTs nanocomposite has the highest activity toward methanol electro-oxidation reaction. These findings may be advantageous for developing new highly active electro-catalysts for anode of direct methanol fuel cells. 4. Conclusion VOx–MWCNTs nanocomposite was studied as a potential support for platinum nanoparticles in methanol electro-oxidation reaction. The nanocomposite supports with different molar ratio of Pt:V were prepared with the aid of microwave radiation, and then, the Pt nanoparticles dispersed on the supports via a polyol process. Compared to Pt/MWCNTs, all Pt/VOx–MWCNTs electrocatalysts exhibit a high catalytic activity and stability toward methanol electro-oxidation reaction. This can be explained by considering better dispersion of Pt nanoparticles over VOx–MWCNTs, more effective interaction between Pt nanoparticles and the supports in the case of Pt/VOx–MWCNTs, improved formation kinetics of adsorbed hydroxyl group on the surface of Pt/VOx–MWCNTs, accommodation of bifunctional mechanism on the composite supports, and finally, higher ionic conductivity of VOx–MWCNTs compared to MWCNTs. In addition, the presence of structural water in the composite supports is an important parameter affecting performance of the electro-catalysts toward methanol electro-oxidation reaction. References [1] M. Zhiani, B. Rezaei, J. Jalili, Int. J. Hydrogen Energy 35 (2010) 9298–9305. [2] V. Selvaraj, M. Vinoba, M. Alagar, J. Colloid Interface. Sci. 322 (2008) 537–544. [3] M. Sakthivel, A. Schlange, U. Kunz, T. Turek, J. Power Sources 195 (2010) 7083– 7089. [4] H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, D.P. Wilkinson, J. Power Sources 155 (2006) 95–110. [5] X. Han, D. Wang, D. Liu, J. Huang, T. You, J. Colloid Interface Sci. 367 (2012) 342–347. [6] L.G.S. Pereira, V.A. Paganin, E.A. Ticianelli, Electrochim. Acta 54 (2009) 1992– 1998. [7] Y. Shimazaki, S. Hayasaka, T. Koyama, D. Nagao, Y. Kobayashi, M. Konno, J. Colloid Interface Sci. 351 (2010) 580–583. [8] D.F. Silva, A.N. Geraldes, E.S. Pino, A.O. Neto, M, Linardi, E.V. Spinacé, J. Nanomater. 2012 (2012). doi: http://dx.doi.org/10.1155/2012/928230 (Article ID 928230). [9] X. Lu, J. Hub, J.S. Foord, Q. Wanga, J. Electroanal. Chem. 654 (2011) 38–43. [10] S.J. Park, J.M. Park, M.K. Seo, J. Colloid Interface Sci. 337 (2009) 300–303. [11] K.W. Park, Y.E. Sung, M.F. Toney, Electrochem. Commun. 8 (2006) 359–363. [12] Z.S. Li, S. Lin, Z. Chen, Y. Shi, X.M. Huang, J. Colloid Interface Sci. 368 (2012) 413–419. [13] H. Song, X. Qiu, F. Li, Electrochim. Acta 53 (2008) 3708–3713. [14] R.S. Hsu, D. Higgins, Z. Chen, Nanotechnology 21 (2010) 165705–165709. [15] J. Liu, Q. Sun, Y. Fu, J. Shen, J. Colloid Interface Sci. 335 (2009) 216–221.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Vanadium oxide decorated carbon nanotubes as a promising support of Pt nanoparticles for methanol electro-oxidation reaction Amideddin Nouralishahi a,b, Abbas Ali Khodadadi a, Ali Morad Rashidi b, Yadollah Mortazavi a,⇑ a b
Catalysis and Nanostructured Materials Research Laboratory, School of Chemical Engineering, University of Tehran, P.O. Box 11155/4563, Tehran, Iran Nanotechnology Research Center, Research Institute of Petroleum Industry (RIPI), Azadi Sport Complex West Blvd., Tehran, Iran
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 31 August 2012 Accepted 24 October 2012 Available online 10 November 2012
VOx–MWCNTs nanocomposite was prepared via deposition–precipitation method followed by microwave treatment. Platinum nanoparticles were dispersed via polyol process over the nanocomposite support, and thus, prepared electro-catalyst was employed in methanol electro-oxidation reaction. The electro-catalysts were characterized by means of TGA, XRD, EDS, FESEM, TEM, and H2-TPR analysis. The electro-catalytic activity and stability of the electrodes toward methanol oxidation reaction in acidic medium were studied by using cyclic voltammetry (CV), CO-stripping, and electrochemical impedance spectroscopy (EIS) techniques. Compared to the Pt/MWCNTs, the Pt/VOx–MWCNTs electro-catalyst not only exhibits high electro-catalytic activity, but also shows very good stability during methanol electro-oxidation reaction. In addition, the presence of VOx in the composite support dramatically increases the electrochemical active surface area of platinum nanoparticles. The results of electrochemical impedance spectroscopy reveal that formation kinetics of adsorbed hydroxyl group on surface of the electrocatalysts is improved upon vanadium oxide addition to the support. This phenomenon is very helpful to facilitate oxidative removal of adsorbed CO group through bifunctional mechanism on Pt/VOx– MWCNTs. Ó 2012 Elsevier Inc. All rights reserved.
Keywords: Methanol oxidation Direct methanol fuel cell Electro-catalyst Platinum Vanadium Carbon nanotube Electrochemical impedance spectroscopy Bifunctional mechanism
1. Introduction In recent years, fuel cells as clean and portable power sources have attracted a great importance, because of several problems in traditional energy producing systems. Compared to the hydrogen fuel cells, direct methanol fuel cells (DMFCs) are safer, more compact and easier to use. At the anode of DMFCs, methanol is directly electro-oxidized to produce carbon dioxide and six electrons according to the following reaction:
CH3 OH þ H2 O ! CO2 þ 6Hþ þ 6e
E ¼ 0:046 V
ð1Þ
Platinum is very attractive electro-catalyst for oxidation of short chain alcohols, such as methanol [1–3]. The reaction of methanol oxidation on surface of platinum particles consists of several steps including (a) adsorption of methanol; (b) methanol dissociation; (c) H2O adsorption; (d) formation of surface –OHads groups by water activation; and (e) oxidation of –COads [4]. Formation of – OHads group from the adsorbed water molecules, which is critical for elimination of adsorbed CO-like groups, requires a relatively high potential on the Pt surface. This leads to blocking of active
⇑ Corresponding author. Fax: +98 21 66967793. E-mail addresses: [email protected] (A. Nouralishahi), [email protected] (A.A. Khodadadi), [email protected] (A.M. Rashidi), [email protected] (Y. Mortazavi). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.10.048
sites and severe power losses in anode of DMFCs. Therefore, pure platinum is a rather poor anode electro-catalyst for methanol oxidation reaction (MOR) at low temperatures (T < 100 °C). Making alloys with one or even two metals that can provide oxygenated species at lower potentials for oxidative removal of –COads is a suitable method to modify the electro-catalytic properties of platinum [5,6]. Ruthenium is the most common promoter of platinum nanoparticles that significantly improves the catalytic properties of Pt in MOR [7–10]. However, there have been numerous attempts with the aim of reducing the loading of noble metals in DMFCs anodes via addition of lower-priced compounds. Therefore, materials like tungsten oxide [11], molybdenum oxide [12], titanium oxide [13], tin oxide [14], etc. have been examined as promoters in methanol oxidation reaction. Because of its ability for oxidation of methanol [15,16], vanadium oxide may be used as a possible promoter in methanol electro-oxidation reaction [17,18]. Jusys et al. studied electro-catalytic activities of VOx, MoOx, and WOx as promoters of unsupported PtRu electro-catalyst in methanol oxidation reaction at fuel cell conditions and reported PtRuVOx as the best electro-catalyst [19]. Justin and coworkers have employed vulcan carbon-supported vanadium oxide (V2O5–C) as the support of Pt nanoparticles for MOR in alkaline medium [20]. They attributed high activity and stability of Pt/ V2O5–C electro-catalyst to synergetic effect between platinum and vanadium oxide species.
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Among all carbon nanostructures, carbon blacks (CBs), especially vulcan XC-72, are the most frequently applied supports for Pt containing electro-catalysts for direct alcoholic fuel cells in commercial applications and different studies [21–23]. However, multiwalled carbon nanotubes with relative large surface area, high electrical conductivity, and remarkable electrochemical and mechanical stability in corrosive media are promising candidates for anode electro-catalyst supports in direct methanol fuel cells. There are many reports demonstrating the superior performance of multiwalled carbon nanotubes compared to carbon blacks as the support of anode catalysts in direct methanol fuel cells [24– 26]. This work investigates the performance of VOx–MWCNTs nanocomposite as a potential support for Pt nanoparticles in methanol electro-oxidation reaction. The results indicate that VOx–MWCNTs nanocomposite dramatically improves the electro-catalytic activity and stability of platinum active sites in MOR.
and at a scanning rate of 0.02 (2h) s1. Particle size and surface morphologies of the electro-catalysts were determined by transmission electron microscopy (TEM, ZEISS EM 900) and field emission scanning electron microscopy (FESEM, Hitachi model:S4160). Hydrogen temperature-programmed reduction (H2-TPR) experiments were performed using a Quantachrome CHEMBET3000 apparatus. The TPR experiments were conducted on 15 mg of the electro-catalysts powders under a flow of 10 sccm 7.0% hydrogen in argon. Prior to the reduction reactions, all electrocatalysts were pretreated at 100 °C for 3 h in 10 sccm argon stream to eliminate the moisture and other adsorbates from the surface. Finally, temperature was increased from 100 °C to 650 °C with a heating rate of 10 °C min1. Thermal gravimetric analysis (TGA) and Derivative Thermo Gravimetric (DTG) were carried out under nitrogen atmosphere with a heating rate of 10 °C min1 by a PerkinElmer thermogravimetric analyzer. 2.4. Preparation of working electrodes
2. Experimental 2.1. Materials All the chemicals used were of analytical grade and applied without further purification. V2O5 and H2PtCl66H2O were purchased from Aldrich. Methanol, ethylene glycol, and sulfuric acid were obtained from Merck, and the 5 wt.% solution of nafion was brought from Alfa Aesar. The multiwalled carbon nanotubes (MWCNTs) were purified via several washing steps in aqueous solution of HCl and HNO3 in order to eliminate inorganic impurities, followed by heating up to 260 °C for 60 min. The resultant carbon nanotubes had a purity of about 95 wt.% according to thermal gravimetric analysis (TGA) results. 2.2. Preparation of electro-catalysts The nanocomposite of VOx–MWCNTs was prepared via deposition–precipitation method followed by microwave treatment. The purified multiwalled carbon nanotubes were surface oxidized by H2SO4–HNO3 mixture (3:1 V/V) in an ultrasonic bath at 60 °C. To prepare VOx–MWCNTs nanocomposite, the functionalized MWCNTs were dispersed ultrasonically in deionized water. Next, an aqueous suspension of vanadium oxide was added dropwise and mixed vigorously. The mixture was dried in a rotary evaporator followed by heating in an oven at 80 °C over night and then was placed into a microwave oven (black and baker, MY26PG, 1000W) and heated in several periods. Pt/VOx–MWCNTs and Pt/MWCNTs electro-catalysts with 10 wt.% Pt loading were prepared by the same method via a polyol process [20]. In a typical procedure, specific amount of MWCNTs or VOx–MWCNTs was dispersed in 60 ml ethylene glycol in an ultrasonic bath and mixed well with 3.4 ml aqueous solution of hexachloroplatinic acid (75.4 mM). Subsequently, a solution of KOH (2.5 M) was added dropwise to the suspension, in order to induce the formation of tiny and uniform platinum nanoparticles. Next, the reduction reaction was conducted under argon atmosphere and reflux conditions. The resultant electro-catalyst sample was washed with boiling distillated water several times to remove ethylene glycol and Cl ions. Finally, it was dried in a vacuum oven at 100 °C for 6 h. 2.3. Physico-chemical characterizations The structure of the electro-catalysts was determined with Xray powder diffraction (XRD) by a PW 1840 Philips diffractometer using Cu Ka1 (k = 1.54056 Å) radiation in the 2h range of 15–90°
In a typical process, 5 mg of electro-catalyst powder was ultrasonically dispersed in a mixture of 50 ll nafion solution and 2 ml ethanol to form a homogeneous ink. Then, 6.3 ll of the prepared ink was spread on a washed and mirror-finished surface of GC electrode (0.0314 cm2) and then the solvent evaporated slowly under radiation of an ordinary 100 W lamp. Finally, the modified electrode was heated up in an oven at 80 °C for 30 min. 2.5. Electrochemical measurements All electrochemical experiments were carried out in a conventional three electrode cell using a Teflon sheathed glassy carbon disk with a diameter of 2 mm as working electrode, a Pt plate (5.2 cm2), and an Ag/AgCl electrode as counter and reference electrodes, respectively. All potentials in this work are quoted against Ag/AgCl reference electrode at 25 °C. In addition, all experiments were carried out in nitrogen bubbled electrolytes using Ivium Stat electrochemical analyzer (Ivium Technologies, Ivium Stat type 10V/5A). The electro-catalytic performances in methanol oxidation reaction (MOR) were determined by cyclic voltammetry experiments in an aqueous solution of 1.0 M methanol and 0.5 M sulfuric acid. The CV curves were recorded at a scan rate of 50 mV s1 from 200 mV to 1200 mV. Electro-catalytic active surface areas (ECSAs) were estimated by cyclic voltammetry in 0.5 M H2SO4 at a scan rate of 50 mV s1. Moreover, electrochemical impedance measurements were carried out by sweeping the frequency from 100 kHz to 100 mHz in a constant potential mode at an amplitude of 5 mV. In order to study the CO tolerance of electrodes, CO was bubbled through the cell for 30 min, 200 mV was applied on the working electrode for 5 min to ensure CO saturated adsorption on the electro-catalyst layers, dissolved CO was removed by Ar bubbling for 30 min, and then, a CV was performed in a potential range of 200–1200 mV. 3. Results and discussion 3.1. Characterization of the electro-catalysts The amounts of structural water in the nanocomposite support can be calculated by thermal gravimetric analysis. The TGA and DTG results of MWCNTs and VOx–MWCNTs under nitrogen atmosphere are presented in Fig. 1a and b. The initial weight losses observed in TGA curves of the both samples, at T < 150 °C, are associated with the loss of adsorbed surface water and volatile compounds. In the case of MWCNTs, the weight loss at T > 150 °C
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293
Fig. 1. TGA results (a) and derivative curves (b) of MWCNTs and VOx–MWCNTs under nitrogen atmosphere.
is attributed to removal of the functional groups (i.e., –COOH and OH) from the surface of MWCNTs, while the excess weight loss of VOx–MWCNTs compared to the acid functionalized carbon nanotubes may be related to the structural water removal from the bulk of vanadium oxide species. According to Fig. 1b, most of structural water in the composite support is removed between 300 °C and 400 °C with maximum dehydration at 370 °C. To exclude the influence of functional groups on the MWCNTs, the content of structural water in the nanocomposite support is calculated by subtracting the measured weight loss of MWCNTs from the weight loss of VOx–MWCNTs. The water content value calculated by TGA is approximately 0.8% of the total weight of composite support. Fig. 2 shows the XRD patterns of MWCNTs, Pt/MWCNTs, VOx– MWCNTs, and Pt/VOx–MWCNTs. The diffraction peaks at around 26°, 42°, 54°, and 77° observed for the carbon nanotubes can be attributed to the planes (0 0 2), (1 0 0), (0 0 4), and (1 1 0) of carbon graphite structure, respectively [27]. In the case of VOx–MWCNTs, there are no peaks other than those observed for the carbon nanotubes, indicating that vanadium oxide is either amorphous or crystalline with very small crystallites. In samples Pt/MWCNTs and Pt/VOx–MWCNTs, the peaks observed at around 39°, 46°, 68°, 81°, and 87° are corresponding to Pt (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of fcc crystalline platinum. However, presence of the vanadium oxide in the composite support causes the characteristic peak at 68° to appear at slightly lower angels. The shifts in XRD data in Pt/VOx–MWCNTs are in a good agreement with the reports in literature on Pt/
Fig. 2. XRD patterns of (a) MWCNTs, (b) VOx–MWCNTs, (c) Pt/MWCNTs, and (d) Pt/ VOx–MWCNTs.
V2O5–C [20,28] in which the slight shift in (2h) angle of 68° is related to alloying of Pt and vanadium. The average crystallite size can be determined using characteristic peak (at 2h 68°) by employing the Scherrer’s equation [29]:
D ¼ 0:9k=b cos hmax
ð2Þ
where L is the mean crystal size (Å), kka is the wavelength of the xray (1.54056 Å for the copper Ka), b is defined as width of diffraction peak at half maximum (rad), and hmax is the Bragg angle (deg). Calculating from Eq. (2), the average particle diameter of platinum nanoparticles is determined to be about 4.6 nm and 3.5 nm for Pt/ MWCNTs and Pt/VOx–MWCNTs, respectively. The FESEM images of purified MWCNTs and VOx–MWCNTs are depicted in Fig. 3a and b. In Fig. 3a, there is almost no sign of mineral clusters, implying that most of the mineral impurities of MWCNTs have been removed in the purification process. Fig. 3b confirms that the vanadium oxide species were properly precipitated on the surface of MWCNTs, and a rather uniform nanocomposite of VOx–MWCNTs was formed during the synthesis process. The energy-dispersive X-ray spectroscopy (EDS) results of Pt/ VOx–MWCNTs are depicted in Fig. 3c. The results confirm that both platinum and vanadium oxide species are successfully participated on multiwalled carbon nanotubes. The amounts of Pt:V atomic ratio are determined to be 2.7:2, which are very close to the nominal ratios of 3:2 for Pt:V. Fig 4 demonstrates the TEM images of MWCNTs, Pt/MWCNTs and Pt/VOx–MWCNTs. About 30 nm open-end multiwall carbon nanotubes can be observed in Fig. 4a. The open-end of MWCNTs may indicate that the growth catalyst impurities of MWCNTs are removed adequately during the purification process. The average Pt particle size, among 360 randomly selected particles, is estimated to be about 4.9 and 3.6 for Pt/MWCNTs and Pt/VOx– MWCNTs, respectively. This is in good agreement with the XRD data illustrated in Fig. 2. Additionally, the presence of the vanadium oxide in the supports causes the platinum nanoparticles to precipitate in a narrower size distribution range that may be originated from an increase in the number of nucleation centers available on the surface of composite supports. The reduction behavior of the electro-catalysts was investigated by H2-TPR analysis (Fig. 5). The strong peak centered at 540–550 °C can be attributed to methanation of carbon nanotubes of the support [30,31]. In case of Pt/MWCNTs, the very small peak at 145 °C may be attributed to highly reducible Pt oxide species with a low interaction with the support [32]. In addition, the broad shoulder starting at 290 °C may be related to catalytic methanation of carbon nanotubes of the support in presence of platinum active sites. In Fig. 5, there are two separated peaks in the TPR profile of VOx–MWCNTs at 341 and 596 °C. The peak at 341 °C may be re-
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Fig. 3. Field emission scanning electron microscopy (FESEM) of (a) MWCNTs, (b) VOx–MWCNTs and the EDS spectra of (c) Pt/VOx–MWCNTs.
Fig. 4. Transmission electron microscopy of (a) MWCNTs, (b) Pt/MWCNTs, and (c) Pt/VOx–MWCNTs.
lated to monomeric and polymeric VO4 units on the surface [33]. The peak at higher temperature may be deconvoluted into two different ones (dashed lines in Fig. 5); the first peak, centered at 520– 530 °C, related to methanation reaction of MWCNTs, and the sec-
ond one, at 580–585 °C, is assigned to reduction of bulk V(5+) to V(3+) [34,35]. Incorporation of Pt nanoparticles to VOx–MWCNTs causes the peak of V(5+) to shift negatively by about 25 °C. This demonstrates higher reducibility of vanadium oxide species in
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Fig. 6 shows the voltammetric curves of the modified electrodes in 0.5M H2SO4 at 25 °C. Both electrodes exhibit two overlapped anodic peaks between 200 and 150 mV, which are associated to hydrogen monolayer desorption on platinum surface. As well, the cathodic peaks during backward scans in the same potential range are related to adsorption of atomic hydrogen on mentioned active sites [36]. These processes could be written as follows:
27.8 m2 g1, in the case of Pt/MWCNTs, to 61.8 m2 g1 in the case of Pt/VOx–MWCNTs. This suggests that the presence of VOx raises the utilization of Pt nanoparticles, which agrees with TEM micrographs in Fig. 4. The broad weak anodic peaks in Fig. 6, between 200 and 500 mV, may be related to the formation of surface oxygenated groups on the electro-catalysts [38]. Comparing the peaks in Fig. 6, one can conclude that formation of surface oxides is more facilitated on Pt/VOx–MWCNTs than on Pt/MWCNTs. This phenomenon plays an important role in improvement of electro-catalysts’ CO tolerance. Since the –COads is the major poisoning intermediate during electro-oxidation of methanol, a good electro-catalyst should have not only large ECSA but also proper –COads oxidation ability. The results of CO-stripping experiments are illustrated in Fig. 7. Here, all hydrogen desorption peaks (observed in Fig. 6) are disappeared in voltammograms of electrodes with pre-adsorbed CO indicating that the active sites on the electrodes were totally covered by adsorbed CO groups. The results clearly show that the onset potential for oxidation of adsorbed CO monolayer on Pt/VOx–MWCNTs, at 483 mV, is much lower than that on Pt/MWCNTs, at 553 mV; therefore, VOx efficiently decreases the overpotential of –COads oxidation. This confirms that the mechanism of –COads oxidation to CO2 improves due to synergetic effect of vanadium oxide in the support. In fact, the oxophilic nature of the transition metal oxides helps to remove the strongly adsorbed MOR intermediates, particularly -COads, from the platinum active sites via formation of appropriate amounts of surface oxygenated groups such as –OHads [39–41]. This lowers the required potential for oxidizing –COads to CO2, considered as bifunctional mechanism and may be written as the following reaction scheme:
PtHads $ Pt þ Hþ þ e
Pt—CH3 OHads ! Pt—COads þ 4Hþ þ 4e
Fig. 5. Hydrogen temperature-programmed reduction (H2-TPR) results of the electro-catalysts.
Pt/VOx–MWCNTs probably due to hydrogen spillover from platinum to V2O5 surfaces. 3.2. Electrochemical results
ð3Þ
The charge associated to integrated intensity of the atomic hydrogen desorption peaks, QH (lC cm2), represents the number of platinum active sites. Therefore, it is possible to estimate the electrochemical active surface areas (ECSAs) from the following equation [37]: 2
ECSA ðcm =mgPtÞ ¼ Q H =C ½Pt
Fig. 6. Cyclic voltammograms of Pt/MWCNTs and Pt/VOx–MWCNTs at scan rate of 50 mV s1 in 0.5 M H2SO4 at 25 °C.
ð5Þ
þVOx
VOx —H2 Oads ! VOx —OHads þ Hþ þ e I VOx —OHads þ VOðx1Þ OH þ e
ðstep 2Þ
ð6Þ
VOðx1Þ OH þ Pt—COads ! Pt þ VOðx1Þ þ CO2 þ H þ þe
ð4Þ
where C is the charge required to reduce a monolayer of protons (H+) on a polycrystalline platinum electrode and has been commonly adopted as 210 lC cm2 Pt. In addition, [Pt] represents the loading of platinum nanoparticles on the working electrode (mg cm2). According to Fig. 6, participation of vanadium oxide causes a dramatic increase in platinum active surface area from
ðstep 1Þ
ðstep 3Þ
ð7Þ
and/or
VOx —OHads þ Pt—COads ! Pt þ VOx þ CO2 þ Hþ þ e
ðstep 4Þ
ð8Þ
The results of cyclic voltammetry experiments between 200 and 1200 mV in 0.5 M H2SO4 and 1.0 M methanol are illustrated in Fig. 8. The anodic peaks in the forward scans are related to meth-
Fig. 7. CO stripping results for Pt/MWCNTs and Pt/VOx–MWCNTs at scan rate of 50 mV s1 in 0.5 M H2SO4 and at 25 °C.
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Fig. 8. (a) Cyclic voltammograms of Pt/MWCNTs and Pt/VOx–MWCNTs at scan rate of 50 mV s1 in 0.5 M H2SO4 and 1.0 M methanol at 25 °C, (b and c) Comparison of CV results between 150 and 400 mV in the absence (dotted lines) or presence of methanol (solid lines).
anol electro-oxidation reaction, and the peaks at backward scans are associated with carbonaceous intermediates, which have not been oxidized completely in the forward scans [42,43]. The current density at forward anodic peak on Pt/VOx–MWCNTs, at 46.3 mA cm2, is about 2.5 times higher than that of Pt/MWCNTs, at 18.9 mA cm2. This confirms superior activity of Pt/VOx– MWCNTs, compared to Pt/MWCNTs, toward methanol electrooxidation reaction. The ratio of the anodic peak current density in forward scan (jfp) to the backward anodic peak (jbp) has been used to express electro-catalyst tolerance to accumulation of carbonaceous poisoning species [44,45]. Indeed, high ratios show effective removal of CO-like poisoning species such as –COads and –COHads on the catalyst active sites. In Fig. 8, the ratio of jfp/jbp for Pt/MWCNTs is only 0.80, which is far less than that for Pt/ VOx–MWCNTs (0.96). It suggests improvement of electro-catalysts tolerance against poisoning species as a result of vanadium oxide addition to the support. In order to determine onset potential of methanol electrooxidation reaction over the electro-catalysts, the cyclic voltammetry results in the absence and presence of methanol are depicted in Fig. 8b and c. It is clear that the onset potential for methanol electro-oxidation reaction reduces from 492 mV in the case of Pt/ MWCNTs to 387 mV in Pt/VOx–MWCNTs. This suggests that the kinetics of methanol electro-oxidation reaction is more favored on Pt/VOx–MWCNTs compared to Pt/MWCNTs, in accordance with excellent activity of Pt/VOx–MWCNTs in methanol electro-oxidation reaction (see Fig. 8a).
The catalytic activity of Pt/VOx–MWCNTs electro-catalysts is dependent to Pt:V molar ratios. Fig. 9 compares the performance of electro-catalysts with different amounts of vanadium oxide toward methanol electro-oxidation reaction. In all cases, platinum loading in the samples is kept constant, while the molar ratio of Pt:V is varied. According to the results of Fig. 9, the best performance of Pt/VOx–MWCNTs is achieved at optimum molar ratios
Fig. 9. Cyclic voltammograms of Pt/VOx–MWCNT with different atomic ratio of Pt:V (the data were recorded at scan rate of 50 mV/s in 0.5 M H2SO4 and 1 M methanol at 25 °C).
A. Nouralishahi et al. / Journal of Colloid and Interface Science 393 (2013) 291–299
of 3:2 for Pt:V. In fact, at too high ratios, the contact regions between platinum nanoparticles and the vanadium oxide species are insufficient to effective spillover of poisoning species during bifunctional mechanism. While in the case of very low Pt:V molar ratios, the existing vanadium oxide may block some platinum active sites and consequently reduce the electrochemical active surface area [46]. Electrochemical impedance spectroscopy (EIS) is a powerful tool for understanding mechanism of electrode reactions. The complex plane (Nyquist) impedance plots of the electro-catalysts at potentials of 200, 500, and 800 mV in 1.0 M methanol and 0.5 M H2SO4 aqueous solution are, respectively, presented in Fig. 10a–c. Here, the EIS results may be analyzed considering two major reactions: the electro-oxidation of methanol to COads and releasing active sites by electro-oxidation of COads to CO2. At electrode potential of 200 mV, it can be seen that the impedance plots of both samples appear in the first quadrant and show a relative straight line with a phase angle close to 90° along the imaginary impedance axis (Im Z). This reveals the formation of electric double layer capacitor due to adsorption of different species at electrode/electrolyte interface. However, both electro-catalysts exhibit a slight deviation from the straight line that may be explained by dissociative adsorption of methanol molecules on the surface of electro-catalysts [47]. In Fig. 10a, the relative large arcs, which is especially observed for Pt/MWCNTs, implies slow reaction rate of methanol electro-oxidation at 200 mV due to strongly adsorbed CO intermediate on Pt surface [48]. At electrode potential of 500 mV, the impedance arcs suddenly bend from positive to negative direction of real impedance axis in the second and third quadrants. This mater is associated with reversible formation of OHads which facilitates oxidative removal of COads on the surface of electrodes (see reactions (5)–(8)) [49]. Wang et al. attributed the sudden change of impedance arcs to variation of rate-determining step [50]. In fact, with increase of ad-
297
Fig. 11. Impedance spectra of Pt/MWCNTs and Pt/VOx–MWCNTs presented in the Nyquist form in the range of 100 mHz to 100 kHz at OCP in 0.5 M H2SO4 at 25 °C.
sorbed hydroxyl group on surface of the electro-catalysts, the rate-determining step changes from methanol dehydrogenation reaction (reaction (5)) to removal of adsorbed OH by COads group (reactions (7) and (8)) leading to negative faradic impedance. When the electrode potential reaches to 800 mV, the impedance plots return to the first and fourth quadrants. Here, the pseudo inductive behaviors at low frequency region are related to removal of COads on the surface [51]. At such a high overpotential, strongly adsorbed hydroxyl groups cover surface of the electrocatalysts; therefore, the rate of methanol electro-oxidation reaction gets slow and the rate-determining step changes to oxidation removal of adsorbed CO group on active sites [50,52]. This is consistent with the sharp decrease in current densities at potentials greater than 0.8 V in the voltammetric responses in Fig. 8a. Based on Fig. 10, the diameters of arcs in the case of PtVOx/ MWCNTs electro-catalyst are very smaller than that of Pt/MWCNTs
Fig. 10. Nyquist plots of Pt/MWCNTs and Pt/VOx–MWCNTs electro-catalysts at (a) 200 mV, (b) 500 mV, and (c) 800 mV in 0.5 M H2SO4 + 1.0 M methanol at 25 °C.
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Fig. 12. Cyclic voltammograms of MWCNTs and VOx–MWCNTs at scan rate of 50 mV s1 in aqueous solution of 1.0 M methanol 0.5 M H2SO4 25 °C.
Fig. 13. Peak current densities of Pt/MWCNTs and Pt/VOx–MWCNTs electrocatalysts during 500 successive cyclic voltammetry experiments at scan rate of 50 mV s1 in 1.0 M methanol and 0.5 M H2SO4 solution.
at all three applied potentials, which reveals improved kinetics of OHads chemisorption, oxidative removal of COads and methanol electro-oxidation reaction upon VOx incorporation. Since methanol electro-oxidation process is composed of several deprotonation and electron formation steps, the proton conductivity of the electro-catalysts is an essential factor influencing the overall electro-catalytic activity. Resistance against ionic transfer (Rion) on the supports of electro-catalysts was calculated from the results of impedance experiments in 0.5 M H2SO4 at the open circuit potential (OCP) (Fig. 11). Rion can be satisfactorily estimated from the following equation [53]:
Z ReLf ¼ Z ReHf þ Rion =3
ð9Þ
where ZReLf and ZReHf are correspondingly defined as the real contribution of total impedance at low and high frequencies. Based on
Fig. 11, the ionic transfer resistance of functionalized MWCNTs is almost 2.3 times higher than that of VOx–MWCNTs. This shows that the presence of vanadium oxide in the support enhances the ion carrier abilities through the electro-catalyst layer due to existing structural water in the composite support (see Fig. 1). The electrochemical properties of MWCNTs and VOx–MWCNTs were examined by cyclic voltammetry experiment in 1.0 M methanol and 0.5 M H2SO4 at 25 °C, as shown in Fig. 12. The supports, without Pt nanoparticles, show no significant anodic peaks, indicating that they are almost individually inactive in methanol electro-oxidation reaction. The transported coulombic charge (Q) between 100 mV and 1100 mV can be calculated by integrated surface area of the cyclic voltammograms. In accordance with the results of impedance spectroscopy in Fig. 11, the amount of Q for VOx–MWCNTs is 1.03 mC, which is 2.7 times higher than that of MWCNTs (0.38 mC). The long-term electro-catalytic stability of Pt/VOx–MWCNTs and Pt/MWCNTs was studied in 1.0 M methanol and 0.5 M H2SO4 aqueous solution. Fig. 13 shows the current densities of forward anodic peaks during 500 successive cycles. The initial increase in peak current densities observed for both samples may be explained by elimination of adsorbed contaminates from the electro-catalysts active sites. However, after 100th cycle, both electro-catalysts show a gradual reduce in their peak current densities attributed to poisoning and structure change of the electro-catalysts during long time successive potential perturbations [54]. According to Fig. 13, the final peak current densities of Pt/VOx–MWCNTs, and Pt/MWCNTs are, respectively, 38.4 and 12.58 mA cm2, which are about 83% and 68% of their corresponding maximum values at 66th and 31st scans. Therefore, addition of VOx to the support dramatically improves the electro-catalytic stability of Pt/MWCNTs toward methanol oxidation reaction. Table 1 compares the electro-catalytic activity of our Pt/VOx– MWCNTs samples toward methanol electro-oxidation reaction with that of Pt/C promoted by different transition metal oxides reported in the literature. Here, the recorded mass specific peak current density of methanol electro-oxidation on Pt/VOx–MWCNTs at optimum Pt:V ratio (926 A g 1 Pt ) is higher than that reported on Pt/ V2O5–C [20,28]. Also, the specific activity of Pt/VOx–MWCNTs toward methanol electro-oxidation reaction is comparable with the remarkable results of Pt/Nb2O5–C [55] and Pt/MoO3–C [56]. In order to exclude the size effect of Pt nanoparticles, the value of peak current density of forward anodic peak should be divided to geometric surface area (Sgeo) of Pt nanoparticles, estimated by the following equation:
Sgeo ðm2 g 1 pt Þ ¼ 6000=q:d
ð10Þ
where q is defined as density of platinum (21.4 g cm3), and d is the average size of Pt nanoparticles (nm), obtained from XRD measurements. Among the electro-catalysts presented in Table 1, Pt/VOx– MWCNTs electro-catalyst shows the highest surface specific peak
Table 1 Electro-catalytic activity of different catalysts toward MOR.
a b c
Electro-catalyst
Ipa (A g1 Pt )
Pt average size (nm)
Geometric surface area (m2 g1 Pt )
Jpb (A m2 Pt )
Refs.
Pt/VOx–MWCNTs Pt/V2O5–C Pt/V2O5–C Pt/Nb2O5–C Pt/MoO3–C
926 600 305 871 1093
3.5 2.6 2.8 2.7 2.5c
80.1 107.8 100.1 103.8 112.1
11.6 5.6 3.0 8.4 9.7
– [20] [28] [55] [56]
Forward anodic peak mass specific current density. Forward anodic peak surface specific current density. Estimated from TEM micrographs.
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current. This confirms that the surface of Pt nanoparticles supported on VOx–MWCNTs nanocomposite has the highest activity toward methanol electro-oxidation reaction. These findings may be advantageous for developing new highly active electro-catalysts for anode of direct methanol fuel cells. 4. Conclusion VOx–MWCNTs nanocomposite was studied as a potential support for platinum nanoparticles in methanol electro-oxidation reaction. The nanocomposite supports with different molar ratio of Pt:V were prepared with the aid of microwave radiation, and then, the Pt nanoparticles dispersed on the supports via a polyol process. Compared to Pt/MWCNTs, all Pt/VOx–MWCNTs electrocatalysts exhibit a high catalytic activity and stability toward methanol electro-oxidation reaction. This can be explained by considering better dispersion of Pt nanoparticles over VOx–MWCNTs, more effective interaction between Pt nanoparticles and the supports in the case of Pt/VOx–MWCNTs, improved formation kinetics of adsorbed hydroxyl group on the surface of Pt/VOx–MWCNTs, accommodation of bifunctional mechanism on the composite supports, and finally, higher ionic conductivity of VOx–MWCNTs compared to MWCNTs. In addition, the presence of structural water in the composite supports is an important parameter affecting performance of the electro-catalysts toward methanol electro-oxidation reaction. References [1] M. Zhiani, B. Rezaei, J. Jalili, Int. J. Hydrogen Energy 35 (2010) 9298–9305. [2] V. Selvaraj, M. Vinoba, M. Alagar, J. Colloid Interface. Sci. 322 (2008) 537–544. [3] M. Sakthivel, A. Schlange, U. Kunz, T. Turek, J. Power Sources 195 (2010) 7083– 7089. [4] H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, D.P. Wilkinson, J. Power Sources 155 (2006) 95–110. [5] X. Han, D. Wang, D. Liu, J. Huang, T. You, J. Colloid Interface Sci. 367 (2012) 342–347. [6] L.G.S. Pereira, V.A. Paganin, E.A. Ticianelli, Electrochim. Acta 54 (2009) 1992– 1998. [7] Y. Shimazaki, S. Hayasaka, T. Koyama, D. Nagao, Y. Kobayashi, M. Konno, J. Colloid Interface Sci. 351 (2010) 580–583. [8] D.F. Silva, A.N. Geraldes, E.S. Pino, A.O. Neto, M, Linardi, E.V. Spinacé, J. Nanomater. 2012 (2012). doi: http://dx.doi.org/10.1155/2012/928230 (Article ID 928230). [9] X. Lu, J. Hub, J.S. Foord, Q. Wanga, J. Electroanal. Chem. 654 (2011) 38–43. [10] S.J. Park, J.M. Park, M.K. Seo, J. Colloid Interface Sci. 337 (2009) 300–303. [11] K.W. Park, Y.E. Sung, M.F. Toney, Electrochem. Commun. 8 (2006) 359–363. [12] Z.S. Li, S. Lin, Z. Chen, Y. Shi, X.M. Huang, J. Colloid Interface Sci. 368 (2012) 413–419. [13] H. Song, X. Qiu, F. Li, Electrochim. Acta 53 (2008) 3708–3713. [14] R.S. Hsu, D. Higgins, Z. Chen, Nanotechnology 21 (2010) 165705–165709. [15] J. Liu, Q. Sun, Y. Fu, J. Shen, J. Colloid Interface Sci. 335 (2009) 216–221.
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