Electrocatalytic activity of Pt–ZrO2 supported on different carbon materials for methanol oxidation in H2SO4 solution

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Electrocatalytic activity of PteZrO2 supported on different carbon materials for methanol oxidation in H2SO4 solution R.S. Amin a, Amani E. Fetohi a, R.M. Abdel Hameed b,*, K.M. El-Khatib a a b

Chemical Engineering Department, National Research Center, Dokki, Giza, Egypt Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt

article info

abstract

Article history:

Different carbon supports are considered to prepare PteZrO2/XC-72R carbon black, Pt

Received 19 August 2015

eZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts by a solidestate reaction under

Received in revised form

intermittent microwave heating method using ethylene glycol and NaBH4 as mixed

6 November 2015

reducing agents. The prepared electrocatalysts are physically characterized using X-ray

Accepted 7 November 2015

diffraction (XRD), energy dispersive X-ray analysis (EDX) and transmission electron mi-

Available online 19 December 2015

croscopy (TEM). Pt particle size decreases in the order: PteZrO2/C > PteZrO2/MWCNTs > Pt

Keywords:

electrochemical impedance spectroscopy are applied to investigate the electrochemical

Platinum

performance of the three electrocatalysts for methanol oxidation in H2SO4 solution. CNTs

Carbon

supported electrocatalysts achieve increased methanol oxidation current density and

Electro-oxidtion

improved stability behaviour during long-time operation when compared to the one con-

eZrO2/SWCNTs. Cyclic voltammetry, chronoamperometry, chronopotentiometry and

Fuel cells

taining carbon black. PteZrO2/CNTs electrocatalyst also displays lower resistance and

Metal oxide

higher electrolyte diffusion rate to infer a faster charge transfer process and better electrode accessibility for methanol oxidation. Therefore, carbon nanotubes are suggested as ideal anode electrocatalyst supporting material for the practical application of direct methanol fuel cells. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Fuel cells are electrochemical energy converters that operate by oxidizing a fuel at the anode and reducing atmospheric oxygen at the cathode. The electrodes are separated by an ion conducting medium [1]. Low temperature fuel cells (<200
C) should involve efficient electrocatalysts in order to accelerate the rate of the oxidation and reduction reactions. Among their

different types [2], polymer electrolyte membrane fuel cells are the most promising for vehicles and portable equipments because they are characterized by high power densities [3]. Platinum is the most widely used catalyst in low temperature fuel cells. However, employing anode catalysts based on Pt suffers from the strong adsorption of CO on their surfaces [4]. The presence of CO in a small concentration [~5 ppm] could reduce the efficiency of a fuel cell operating with a fast

* Corresponding author. Tel.: þ20 1145565646. E-mail address: [email protected] (R.M.A. Hameed). http://dx.doi.org/10.1016/j.ijhydene.2015.11.040 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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reaction like the oxidation of hydrogen on platinum to its half value [5]. The high cost of platinum represents another problem. This could be recovered via two approaches: exploration of non-noble catalysts and reduction of Pt loading by its alloying with other metals [6,7]. Low-cost Pt-based alloy nanoparticles have shown the advantages of excellent durability, reliability and stability for realizing fuel cells with their large-scale commercialization [8]. Core-shell PtePd nanoparticles, synthesized by modified polyol method with the assistance of AgNO3, showed methanol oxidation current density value of 1.5 mA cm2 with electrochemical active surface area of 27.70 m2 g1 [9]. Pt-metal oxide alloy catalysts have been extensively investigated [10]. The addition of proper amount of hydrous TiO2 [11] or CeO2 [12] to Pt/CNTs could enhance its catalytic activity. PteSnO2/C electrocatalysts have a superior electrical performance for ethanol and acetaldehyde oxidation when compared to commercial Pt3Sn/C [13]. The exchange current density for ethanol oxidation at PteMgO/C is 1.8  102 mA cm2 that is 54 times higher than that at Pt/C [14]. Zirconium oxide (ZrO2) has excellent mechanical, thermal, optical and electrical characteristics. It has been widely used in the fabrication of structural ceramic devices, gas sensors and optoelectronic devices [15,16]. ZrO2 is also used as a support of Pt in low temperature water-gas shift [17]. It has been demonstrated that Pt/ZrO2 catalysts have excellent catalytic activity for the reaction between CO and H2O. The performance of PteZrO2 for decomposing methanol, ethanol, 2-propanol and 2-butanol was also studied [18]. PteZrO2/CNTs electrocatalysts, containing various Pt:ZrO2 molar ratios, were prepared by solegel and ethylene glycol reduction methods. They showed good electrocatalytic activity for methanol and ethanol oxidation reactions [19]. Pt-based electrocatalysts are usually dispersed onto a high surface area support to increase Pt surface area, while simultaneously using lower amounts of Pt. Besides dispersing the active component, the support provides a porous structure. This prevents sintering and improves the mechanical strength of the synthesized electrocatalyst which in turn enhance its catalytic activity [20]. Carbon support materials are usually employed in fuel cells. They can be chemically or physically activated carbon, carbon black and graphitized carbon [21]. Vulcan XC-72R carbon black is normally used as a support material for Pt nanoparticles. It is an amorphous support and vulnerable to electrochemical corrosioneoxidation reactions. This could poison the catalyst surface and results in Pt nanoparticles migration, coalescence and even detaching from the catalyst system to affect the stability of fuel cells under long-term operation [22]. Recently, carbon supports with an enhanced graphitic nature, such as carbon nanotubes and nanofibers have been proposed as a replacement for carbon black because of their unique surface structure, high electric conductivity, corrosion resistance and large specific surface area [23e25]. Carbon nanotubes are classified as single-walled and multi-walled carbon nanotubes. Single-walled carbon nanotubes have larger specific surface area and better conductivity than multi-walled type due to their less defect densities. This may be helpful when SWCNTs are chosen as a support to load Pt nanoparticles for methanol and ethanol oxidation reactions

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[25]. On the other hand, multi-walled carbon nanotubes are made of a series of single-walled CNTs arranged coaxially with regularly increasing diameter. MWCNTs are usually long, tangled together and have closed ends which limit their applications. Therefore, it is very important to purify and oxidize the surface of MWCNTs [23]. Cyclic voltammetry and chronoamperometry measurements demonstrate that Pt/SWCNTs electrocatalyst has an increased current density and antipoisoning ability for methanol and ethanol oxidation reactions when compared to commercial Pt/C [26]. High dispersion of metal nanoparticles and intrinsic properties of MWCNTs could rationalize the lower onset potential of alcohol oxidation at Pd/MWCNTs electrocatalyst relative to that at Pd/C [27]. Chen et al. [28] have concluded that the application of SWCNTs to carry Pt nanoparticles could improve their electrocatalytic properties in relation to those at Pt/MWCNTs and Pt/C. In this work, we have reported a simple chemical reduction route for the synthesis of highly dispersed PteZrO2 electrocatalyst using mixed reducing agents of ethylene glycol and sodium borohydride. The electrocatalyst preparation was facilitated by intermittent microwave heating on different carbon supports including XC-72R carbon black, SWCNTs and MWCNTs. These electrocatalysts have been characterized by XRD, EDX and TEM. Small stars of Pt nanoparticles were distributed in PteZrO2/SWCNTs electrocatalyst with diameter value of 2.83 nm. In order to evaluate the electrocatalytic properties of PteZrO2 based electrocatalysts, methanol electro-oxidation reaction was extensively studied at their surfaces in acid medium. For this purpose, cyclic voltammetry, chronoamperometry, chronopotentiometry and electrochemical impedance spectroscopy were employed. Compared with Pt/C, PteZrO2/CNTs electrocatalysts exhibited enhanced catalytic activity and stability during methanol oxidation reaction. The faster charge transfer rate at their surfaces could also suggest the promising prospect of PteZrO2/CNTs in the field of acidic DMFCs type.

Experimental Chemicals Vulcan XC-72R carbon black was purchased from Cabot Corp., USA with a specific surface area (BET) of 240 m2 g1 and an average particle size of 40 nm. Single and multi-walled carbon nanotubes, nafion (perfluorosulphonic acidePTFE copolymer, 5 wt.% solution), H2PtCl6.6H2O, ZrO2, ethylene glycol, sodium borohydride, NaOH (98.0%), H2SO4 (A.C.S. reagent grade), HNO3 and CH3OH were obtained from SigmaeAldrich, Germany. All chemicals were used as received without further purification. De-ionized water was used to prepare the solutions.

Purification and modification of CNTs Single and multi-walled carbon nanotubes were first oxidized in a solution composed of 8 M HNO3 þ 8 M H2SO4 for 4 h in an ultrasonic bath to remove any impurities and generate surface functional groups. Purification of CNTs surface prevents

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self-poisoning by foreign impurities, while functional group generation enhances the electrocatalyst deposition. After filtration, CNTs were rinsed with double distilled water for at least six times and dried at 80
C for 6 h. Functionalization of carbon nanotubes by heating in a mixture of sulphuric and nitric acids results in the incorporation of heteroatoms like N and O. This could enhance the affinity between the metal nanoparticles and carbon to avoid agglomeration problems.

Synthesis of different supported PteZrO2 electrocatalysts PteZrO2 nanoparticles were deposited on different carbon supports including Vulcan XC-72R carbon black, SWCNTs and MWCNTs using a solidestate reaction under intermittent microwave heating method. 5 wt.% ZrO2 powder was well dispersed over different carbon supports using a mixture of 2-propanol and double distilled water (1:1). This mixture was stirred for 30 min, followed by heating into a household microwave oven (Caira CA-MW1025, touch pad digital control, 50 MHz, 1400 W) for six cycles. Each cycle was operated for 20 s with 60 s pause. ZrO2/different carbon support powders were then filtered, washed with double distilled water for 6 times and dried in an air oven at 80
C for 6 h. Pt nanoparticles were then supported over ZrO2/C using a mixture of ethylene glycol and sodium borohydride as a reducing agent. An appropriate amount of ZrO2/C was ultrasonically dispersed in double distilled water. H2PtCl6 solution was added to fix Pt loading at 25 wt.%. 15 ml ethylene glycol was then introduced and solution pH was adjusted to 10 using drops of 0.4 M KOH in ethylene glycol. This mixture was then ultrasonicated for 30 min, followed by dropwise addition of NaBH4 solution with a molarity of 25 times higher than that of hexachloroplatinic acid solution. The mixture was stirred for 10 min before introducing into the microwave oven with heating for 50 s in one continuous mode. The produced composite was then separated by filtration, washing and drying. Pt/C was prepared by the same procedure for comparison.

Physical characterization of different supported PteZrO2 electrocatalysts The crystalline structure of the prepared electrocatalysts was studied using X-ray diffraction. It was performed on a RigakuD/MAX-PC 2500 X-ray diffractometer equipped with Ni filtered Cu Ka as the radiation source. The tube current was 40 mA with a voltage of 40 kV. For measuring X-ray diffraction of electrocatalyst samples, their powder was fixed on a glass slide and dried in vacuum overnight. X-ray diffractograms were obtained for 2q values ranging between 10
and 80
at a scan rate of 10
min1. The microstructure and particle size of the prepared electrocatalysts could be determined using TEM analysis. To prepare the electrocatalyst sample for TEM measurement, 2 mg of supported PteZrO2 was ultrasonically dispersed in 2 ml of (isopropanol þ H2O) mixture in ultra 8050-H Clifton for 20 min to obtain a uniform catalyst ink. A drop of this suspension was deposited onto 1 mm copper grid coated with continuous carbon film and left to dry in air. Gatan program was used for data processing and particle size measurement. Energy dispersive X-ray (EDX) spectrum was mapped using a scanning electron microscope (JEOL JAX-840)

and a POEMS ICP-OES instrument (Thermo Jarrell-Ash Corporation, Franklin, MA, USA).

Electrochemical measurements of different supported PteZrO2 electrocatalysts Gamry potentiostat was employed for measuring the electrocatalytic activity of prepared electrocatalysts for methanol oxidation reaction using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopic techniques. It was connected to a personal computer as data interface. A three-electrode system was built up to carry out electrochemical measurements. A graphite electrode coated with the catalyst powder was used as the working electrode with a surface area of 0.196 cm2. On the other hand, the counter and reference electrodes were Pt wire and a mercury sulphate electrode [Hg/Hg2SO4/1.0 M H2SO4 (MMS)], respectively. All potential values in this work were referred to (MMS) electrode [its potential ¼ þ680 mV (NHE)]. To prepare the working electrode, graphite surface was mechanically polished with alumina powder and rinsed with acetone followed by double distilled water. 1.1 mg of catalyst powder was then dispersed in 0.6 ml 5 wt.% nafion solution and isopropyl alcohol. This ink was then spread over the working electrode surface using a micropipette. It was left to dry overnight. All electrochemical measurements were conducted in aerated electrolytes at room temperature of 30
C ± 0.2.

Results and discussion In order to evaluate the effect of the carbon support on the crystalline structure of the resulting composite, XRD patterns of Pt/C, PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/ MWCNTs electrocatalysts were displayed in Fig. 1(a). The graphitic structure of carbon appears at 2q value of 25
e26
in the supported electrocatalysts. This peak showed a crystalline character in PteZrO2/MWCNTs when compared to other studied electrocatalysts. The three diffraction peaks of Pt were observed in Pt/C at 2q values of 39.265
(111), 46.001
(200) and 67.353
(220). The addition of ZrO2 in PteZrO2/C resulted in a crystal lattice contraction of Pt/C as inferred from the positive shift of Pt(111) and Pt(220) diffraction planes and the corresponding decrease in (d) values. Moreover, another 2q shift was observed when SWCNTs and MWCNTs were introduced as supports for PteZrO2 in comparison with Vulcan XC-72R carbon black [see Table 1]. The 2q values of Pt(111) and Pt(220) in PteZrO2/SWCNTs were lower than those in PteZrO2/C to indicate its Pt crystal expansion. However, PteZrO2/MWCNTs had increased 2q values of Pt(111) and Pt(200) diffraction planes in relation to those of PteZrO2/C. Accordingly, Pt crystal lattice was contracted. The different distortion of Pt crystal lattice in PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts may arise from their different structures. SWCNTs have only one single layer of graphene cylinders, while MWCNTs have many layers (approximately 50) [29]. Peaks associated with ZrO2 species were not detected in all electrocatalysts, implying that ZrO2 was partially entering in the crystallite structure of Pt and could not be clearly discerned by X-ray

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from the unique nature of SWCNTs with very high length/ diameter ratio as proved by Chen et al. [28]. However, larger sized and round shaped Pt particles were well distributed at the surface of multi-walled carbon nanotubes [3.35 nm] [see Fig. 2(d)]. Accordingly, the morphology and Pt particle size of PteZrO2 based electrocatalyst were affected by the type of carbon support. Fig. 3 showed the cyclic voltammograms of Pt/C, PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts in 0.5 M H2SO4 solution at a scan rate of 50 mV s1. Three distinguishable regions could be observed in these cyclic voltammograms, especially for PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts. At more negative potential values extending from 750 to 300 mV, hydrogen was adsorbed at platinum surface. This hydrogen adsorption region was chosen to calculate the electrochemical active surface area value (ECSA) because of its well defined surface oxidation peaks. The second region is located between potential values of 300 and 250 mV. This double layer region has low current density values where only capacitive processes could take place. Finally, the oxygen evolution region was extending to more positive potential values up to 900 mV. The integrated charge in the hydrogen adsorption/ desorption region can be considered to estimate the electrochemical active surface area of platinum in various electrocatalysts based on the equation: ECSA ¼ Q H =f0:21  ½Ptg Fig. 1 e (a) XRD patterns of Pt/C, Pt‒ZrO2/C, Pt‒ZrO2/ SWCNTs and Pt‒ZrO2/MWCNTs electrocatalysts. (b) EDX spectrum of Pt‒ZrO2/C.

diffraction. This is normally observed for some oxides incorporated Pt/C electrocatalysts [30,31]. PteZrO2/C electrocatalyst consists of carbon, platinum, zirconium and oxygen as confirmed by its energy dispersive X-ray spectrum in Fig. 1(b). TEM images of Pt/C, PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts were represented in Fig. 2. Pt/C had aggregated Pt particles with a mean diameter of 3.60 nm in Fig. 2(a). This agglomeration was somewhat improved when ZrO2 was added in PteZrO2/C in Fig. 2(b) with Pt particle size of 3.49 nm. Ultrafine carbon nanotubes appeared in TEM images of PteZrO2/SWCNTs [see Fig. 2(c, c0 )]. They were carrying the catalyst particles in the form of small stars with particle size of 2.83 nm. This particular shape of platinum nanoparticles at SWCNTs support may be originated

(1)

2

where: QH in C m is the charge of hydrogen desorption, [Pt] in g m2 is the quantity of Pt loading on the electrode surface and 0.21 mC cm2 is the charge per cm2 of Pt with a monolayer adsorption of hydrogen [32]. This was based on a surface density value of 1.3  1015 atom cm2, which was generally admitted for poly-crystalline Pt electrodes [33]. Here, lower Miller index planes (100), (110) and (111) were predominated with an assumed proportion of 33% for each plane. Pt/C, PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts have ECSA values of 24.60, 69.60, 211.59 and 150.24 m2 g1, respectively. This result is in a good accordance with Pt particle size in different electrocatalysts. The smaller Pt particle size would increase the electrocatalyst active surface area which in turn increases the charging current density [34]. The highest ECSA value was attained by PteZrO2/ SWCNTs electrocatalyst [35]. It emphasizes that the application of CNTs as a support material could improve the electrochemical active surface area of Pt-metal oxides electrocatalysts. The cyclic voltammograms of methanol oxidation at Pt/C, PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs

Table 1 e Variation of 2q and d values of Pt(111), Pt(200) and Pt(220) diffraction peaks of PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts. Electrocatalyst

Pt(111) 2q/

Pt/C PteZrO2/C PteZrO2/SWCNTs PteZrO2/MWCNTs




39.265 39.765 39.669 39.825

Pt(200) d/ A

2q/

2.293 2.264 2.270 2.261

46.001 45.909 46.281 46.203




Pt(220) d/ A

2q/

d/ A

1.971 1.975 1.960 1.963

67.353 67.682 67.593 67.282

1.389 1.383 1.384 1.390




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Fig. 2 e TEM images of (a) Pt/C, (b) Pt‒ZrO2/C, (c, c′) Pt‒ZrO2/SWCNTs and (d) Pt‒ZrO2/MWCNTs electrocatalysts.

electrocatalysts in (0.2 M methanol þ 0.5 M H2SO4) solution at 10 mV s1 were represented in Fig. 4. Methanol starts to oxidize at PteZrO2/C at a potential value of 275 mV which precedes that at Pt/C by 217 mV. The oxidation peak at PteZrO2/C was then observed at a potential value of 298 mV with a current density of 26.85 mA cm2. It was about 6.4 times higher than that at Pt/C [see Fig. 4(a)]. Methanol intermediates were further oxidized in a second oxidation peak at 796 mV with a current density of 20.78 mA cm2. A well-defined reverse oxidation peak in the backward direction at a potential value of 2 mV was observed at PteZrO2/C. It is related to the removal of the residual carbonaceous species that were formed in the forward sweep [36] as linearly and/or bridgebonded PteCO [37]. The corresponding re-oxidation peak at Pt/C showed so small current density value. This reflects the easiness of removing the intermediate products when ZrO2 was added to Pt/C. Many improvement points were also

gained in the electrocatalytic activity parameters of PteZrO2 based electrocatalyst when carbon nanotubes were adopted as support materials in Fig. 4(b) as follows: (1) Methanol adsorbed early at PteZrO2/SWCNTs and PteZrO2/MWCNTs by 104 and 43 mV, respectively when compared to the case at PteZrO2/C. This decreased onset potential value of methanol oxidation reaction at PteZrO2/SWCNTs electrocatalyst may be due to the electronic structure change that has been arisen from the strong PteSWCNTs interaction. This led to weakening of CO adsorption on PteZrO2/SWCNTs, thereby reducing the overpotential of methanol oxidation. (2) The first and second oxidation peaks potential values of methanol at PteZrO2/SWCNTs were shifted in the negative direction by 33 and 23 mV, respectively in relation to that at PteZrO2/C. On contrary, PteZrO2/MWCNTs

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Fig. 3 e Cyclic voltammograms of (a) Pt/C, (b) Pt‒ZrO2/C and (c) PteZrO2/SWCNTs and Pt‒ZrO2/MWCNTs electrocatalysts in 0.5 M H2SO4 solution at 50 mV s¡1.

Fig. 4 e Cyclic voltammograms of Pt/C, PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts in (0.2 M methanol þ 0.5 M H2SO4) solution at 10 mV s¡1.

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recorded increased oxidation peak potential values by 42 and 84 mV. (3) Increased oxidation current density values were achieved at CNTs supported electrocatalysts. Single and multi-walled types showed comparable enhancement where the first oxidation peak had increased current density values by 1.74 and 1.97 times, respectively when compared to that at PteZrO2/C. This corresponded to 1.53 and 1.60 times gain when the second oxidation peak current density was considered. (4) If/Ib value, as the ratio between the oxidation current density in the forward and backward directions, was 2.21 at PteZrO2/SWCNTs. It was higher than that at PteZrO2/C [1.61] and PteZrO2/MWCNTs [1.43]. It indicates to what extent the electrocatalyst has a tolerance towards the species that were not completely oxidized and hence accumulated on the catalyst surface; the higher If/Ib value, the more effective removal of these species [38]. Therefore, this tendency could be arranged in an ascending order as: PteZrO2/ MWCNTs < PteZrO2/C < PteZrO2/SWCNTs. The introduction of metal oxides to platinum-based carbon supports could efficiently improve their electrocatalytic performance towards methanol oxidation reaction [39,40]. Tantalum oxide (TaOx) [41] and SnO2 [42] modified Pt electrodes could support larger oxidation current density values of methanol compared to that obtained at unmodified Pt electrode concurrently with a significant shift of the onset potential of methanol oxidation. This enhancement was attributed to ded metalemetal oxide interaction which changes the electronic property of Pt to enhance the oxidation of adsorbed reaction intermediates. The choice of a suitable carbon support with higher surface area and better conductivity could also improve the electrochemical performance of the produced electrocatalyst. PteSnO2 supported on graphitized mesoporous carbon showed higher catalytic activity for methanol oxidation than that of PteSnO2/C with commercial carbon black as a support material containing the same loading of Pt [40]. Two-dimensional sandwich-like Pt/Mn3O4/ reduced-graphene-oxide nanocomposite, fabricated through an electroless approach, possesses remarkably an excellent long-time stability and electrocatalytic activity towards methanol oxidation reaction when compared to conventional Pt/C. This was ascribed to the unique sandwich-like structure and the synergetic interaction between Pt, Mn3O4 and reduced graphene oxide [39]. Fig. 5(a, b, c) displayed the cyclic voltammograms of methanol oxidation at PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts, respectively in 0.5 M H2SO4 solution containing different methanol concentration values at 10 mV s1. For all the studied electrocatalysts, as methanol concentration increases, the current density values of the first and second oxidation peaks would increase. This was attributed to the increased methanol molecules adsorbed on the active sites of the catalyst surface and hence increasing the resulting oxidation current density values. The variation of If/Ib values of the three electrocatalysts with methanol concentration was shown in Fig. 5(d). As methanol concentration increases, If/Ib value would increase at PteZrO2/

SWCNTs and PteZrO2/MWCNTs electrocatalysts; however, this situation may be reversed at PteZrO2/C. It indicates that carbon nanotubes materials enhanced the ability of PteZrO2 system to clean the electrocatalyst surface, even with increasing methanol concentration. On the other side, Vulcan XC-72R carbon black could not afford the increased methanol molecules on the electrocatalyst surface with increasing methanol concentration and the corresponding If/Ib values would decrease. Generally, PteZrO2/SWCNTs showed the highest If/Ib values within the studied methanol concentration range. The dependence of methanol oxidation current density values at the studied electrocatalysts on the square root of scan rate was illustrated in Fig. 6(a). The straight line inferred a diffusion-controlled process. Mass transfer takes place through three models; migration, convection and diffusion. Brad and Faulkner [43] found that the mass transfer of electroactive species near the electrode surface could be observed only through diffusion in the inactive part of the solution. The diffusion behaviour of methanol oxidation reaction at different PteZrO2/carbon supports could be confirmed by drawing the relation between the scan rate-normalized current [In1/2] and the scan rate [n] in Fig. 6(b). A non-linear relationship was noticed at the three electrocatalysts as a typical electrochemicalchemical catalytic process [44]. The stability of Pt/C, PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts for methanol oxidation reaction through long-term operation was studied. Chronoamperograms of these electrocatalysts in (0.5 M methanol þ 0.5 M H2SO4) solution at potential values of 340 and 850 mV for 4800 s were shown in Fig. 7(a, b), respectively. A fast current density decay of all electrocatalysts was attained at the first few seconds due to the poisoning intermediates formation during methanol oxidation reaction [45]. It was followed by a gradual decrease to finally get a steady state current density value. It differed based on the carbon support type. After operation for 4800 s, the oxidation current density value of PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs at a potential value of 340 mV was 6.34, 10.95 and 16.65 mA cm2, respectively. They corresponded to 9.01, 17.49 and 24.83 mA cm2 for the respective electrocatalysts at a potential value of 850 mV. Moreover, the addition of ZrO2 was found to greatly improve the oxidation current density at Pt/C by about 6 folds at the two studied potential values. The loss percentage of oxidation peak current density due to continuous operation was also calculated by cyclization of different electrocatalysts in (0.5 M methanol þ 0.5 M H2SO4) solution for 10 cycles at 10 mV s1. After 10 cycles, the oxidation peak current density recorded 65.23%, 85.86%, 95.91% and 93.41% of its value in the first cycle at Pt/C, PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts, respectively. Based on the above results, we can conclude that PteZrO2/SWCNTs and PteZrO2/MWCNTs are more stable than PteZrO2/C for long-time operation during methanol oxidation reaction. Chronopotentiometry can be also applied to examine the antipoisoning ability of different electrocatalysts during methanol oxidation reaction. Fig. 7(c) displayed the chronopotentiometric curves of PteZrO2/C, PteZrO2/ SWCNTs and PteZrO2/MWCNTs electrocatalysts in (0.5 M

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Fig. 5 e Cyclic voltammograms of methanol oxidation at (a) PteZrO2/C, (b) PteZrO2/SWCNTs and (c) PteZrO2/MWCNTs electrocatalysts in 0.5 M H2SO4 solution containing different methanol concentrations at 10 mV s¡1. The variation of If/Ib value as a function of methanol concentration at the three studied electrocatalysts is shown in (d).

methanol þ 0.5 M H2SO4) solution at a current density value of 0.5 mA cm2. This low current density value was chosen to avoid the effect of the electrocatalytic activity of different electrocatalysts. For all electrocatalysts, the potential value gradually increased at first, then it jumped to a much higher value. The time at which the electrocatalyst potential reaches a much higher value can be taken as a clue for its antipoisoning ability [46]. As the electrocatalyst persists for longer time before this potential jump, it has a cleaner surface. This time period decreased in the order: PteZrO2/MWCNTs [600 s] > PteZrO2/SWCNTs [400 s] > PteZrO2/C [110 s]. Moreover, the potential level of different electrocatalysts before this potential jump increased in the order: PteZrO2/MWCNTs [3 mV] < PteZrO2/SWCNTs [167 mV] < PteZrO2/C [256 mV]. This result provides another evidence that CNTs supported electrocatalyts have better stability performance than those containing Vulcan XC-72R carbon black. Electrochemical impedance spectroscopy was widely employed to investigate the interfacial properties of the

modified electrodes. Fig. 8(a) represented Nyquist plots of PteZrO2/SWCNTs electrocatalyst in 0.5 M H2SO4 solution in absence and in presence of 0.1 M methanol at a potential value of 0 mV in the frequency range of 1  1040.1 Hz with an ac amplitude of 10 mV. Two slightly depressed semicircles were overlapped in the low and high frequency regions. The semicircle in the high frequency region is generally related to the combination of the charge transfer resistance and double layer capacitance at the electrode/electrolyte interface [47]. The charge transfer resistance value (Rct) indicates how fast the charge transfer occurs during methanol oxidation at the electrode surface. The corresponding semicircle diameter is taken as a measure of the charge transfer resistance value [48]. A smaller semicircle of PteZrO2/SWCNTs in 0.5 M H2SO4 solution was observed after adding 0.1 M methanol, resulting in a lowered charge transfer resistance value. This can be clarified when the relevant electric circuit model has been built in Fig. 8(b). It consisted of the charge transfer resistance in a parallel association with a constant phase element (CPE).

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Fig. 6 e (a) Variation of methanol oxidation peak current density at PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts with the square root of the scan rate. (b) Variation of the scan rate-normalized current [Iy¡1/2] of methanol oxidation reaction at the above electrocatalysts with the scan rate.

Fig. 7 e Chronoamperograms of methanol oxidation reaction at Pt/C, PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts in (0.5 M methanol þ 0.5 M H2SO4) solution at potential values of (a) 340 mV and (b) 850 mV for 4800 s. (c) The corresponding chronopotentiograms in (0.5 M methanol þ 0.5 M H2SO4) solution at current density value of 0.5 mA cm¡2.

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Fig. 8 e (a) Nyquist plots of PteZrO2/SWCNTs electrocatalyst in 0.5 M H2SO4 solution in absence and in presence of 0.1 M methanol at a potential value of 0 mV in the frequency range of 1 £ 104¡0.1 Hz with an ac amplitude of 10 mV. Symbols denote experimental data, while the continuous lines represent the fitted data. (b) The equivalent circuit.

Due to the porous nature of single walled carbon nanotubes, the double layer capacitance was expressed as constant phase element. CPE value was determined through two parameters; namely: Y0 and n based on the equation proposed by Brug et al. [49]: 1 Y0 ¼ Cn R1 s þ Rct


1n

(2)

where: Y0 is the CPE parameter, C is the double layer capacitance, n is the CPE exponent and Rs is the solution resistance. Rct in the equivalent circuit is further connected in series to another (RC) network. Here, C and Rads are the electrical elements related to the adsorption of reaction intermediates. The values of different electrochemical parameters in this electric circuit for electrochemical impedance spectrum of PteZrO2/ SWCNTs in 0.5 M H2SO4 solution before and after adding methanol were listed in Table 2a. Rct value of PteZrO2/

SWCNTs in (0.1 M methanol þ 0.5 M H2SO4) solution was 5 times lower than that in 0.5 M H2SO4 solution, reflecting that the electron transfer was facilitated during methanol oxidation at PteZrO2/SWCNTs electrocatalyst surface. Fig. 9(a) represented phase shift diagrams of PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts in (0.1 M methanol þ 0.5 M H2SO4) solution at a potential value of 0 mV. It was observed that the phase angle is almost unchanged at different electrocatalysts in the frequency range from 10 kHz to 6.5 Hz. It suggests a resistive behaviour involving charge transfer process at the outermost electrode surface. The phase angle was then sharply increased at lower frequency values to finally get an arc. The variation of log Z as a function of log F for different electrocatalysts under the above experimental conditions was shown in Fig. 9(a0 ). Z value > PteZrO2/ decreased in the order: PteZrO2/C MWCNTs > PteZrO2/SWCNTs. It reflects that the electron transfer step during methanol oxidation was easier at PteZrO2/SWCNTs electrocatalyst surface. The corresponding Nyquist plots were illustrated in Fig. 9(b). The diameter of semicircle was the highest for PteZrO2/C, while PteZrO2/ SWCNTs had the most shrinked diameter among the studied electrocatalysts. It could be attributed to the good conductivity and the large surface area of single walled carbon nanotubes. This result was confirmed by the related Rct values that were calculated among the other electrochemical impedance parameters in Table 2b. Nyquist plots were also drawn at another potential value of 300 mV, as methanol oxidation peak position, for the three electrocatalysts under the above experimental conditions in Fig. 9(c). Here, the semicircle at the low frequency region in Nyquist plot was replaced by an inclined line. It was represented by a Warburg diffusion element in the electric circuit for the semi-infinite diffusion of ions at the carbon/electrolyte interface [50] [see the inset figure]. The slope of this line at PteZrO2/MWCNTs was steeper than that at PteZrO2/C and the former had a smaller Warburg impedance parameter [138.14 U s1/2 cm2] than that of the latter [166.45 U s1/2 cm2]. It signifies a higher electrolyte diffusion rate for PteZrO2/ MWCNTs electrocatalyst [51]. Liu et al. [52] have measured a much larger Warburg impedance parameter at Pt/Vulcan XC-72 than that at Pt/highly graphitic carbon black during oxygen reduction reaction. This is caused by insufficient adsorption of oxygen molecules on the surface of Vulcan XC72 with its higher defects. Lower resistance and higher electrolyte diffusion rate for PteZrO2/CNTs suggest a faster charge transfer process and better electrode accessibility for methanol oxidation. From the above study for methanol oxidation reaction at PteZrO2 supported on different carbon materials, we can conclude that electrochemical measurements including cyclic

Table 2a e Electrochemical impedance parameters of PteZrO2/SWCNTs electrocatalyst in 0.5 M H2SO4 solution in absence and in presence of 0.1 M methanol at a potential value of 0 mV. Solution 0.5 M H2SO4 (0.1 M methanol þ 0.5 M H2SO4)

Rs/Ohm

Yo  103/Ohm1 sn cm2

n

Rct/Ohm cm2

C  103/F cm2

Rads/Ohm cm2

49.12 48.15

5.88 5.48

1.00 1.00

56.66 11.31

3.66 5.90

92.55 26.49

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Fig. 9 e (a) Phase shift diagrams of PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalysts in (0.1 M methanol þ 0.5 M H2SO4) solution at a potential value of 0 mV in the frequency range of 1 £ 104¡0.1 Hz with an ac amplitude of 10 mV. (a′) The corresponding variation of log Z as a function of log F. Their Nyquist plots were illustrated at 0 mV in (b) and at 300 mV in (c) [the inset figure is the equivalent electric circuit]. Symbols denote experimental data, while the continuous lines represent the fitted data.

Table 2b e Electrochemical impedance parameters of PteZrO2/C, PteZrO2/SWCNTs and PteZrO2/MWCNTs electrocatalyst in (0.1 M methanol þ 0.5 M H2SO4) solution at a potential value of 0 mV. Electrocatalyst PteZrO2/C PteZrO2/MWCNTs PteZrO2/SWCNTs

Rs/Ohm

Yo  103/Ohm1 sn cm2

n

Rct/Ohm cm2

C  103/F cm2

Rads/Ohm cm2

38.03 43.59 48.15

15.00 10.68 5.48

0.75 0.83 1.00

62.06 16.16 11.31

2.78 2.16 5.90

97.29 30.85 26.49

voltammetry, chronoamperometry and electrochemical impedance spectroscopy confirmed the better performance of CNTs supported electrocatalysts. This enhanced electrocatalytic activity could be attributed to the following characteristics of carbon nanotubes: (1) good electron conductivity to facilitate the electron transport during electrochemical reaction; (2) increased electrochemical active surface area; (3) the electronic structure modification of Pt nanoparticles to weaken CO adsorption on Pt surface in terms of the ligand effect and (4) the incorporation of oxygen containing groups, as a result of heating CNTs in a mixture of H2SO4 and HNO3

solution, could help in cleaning the electrocatalyst surface from its poisoning species. Accordingly, PteZrO2/CNTs could be selected as a suitable anode material in direct methanol fuel cells.

Conclusion PteZrO2 was supported on different carbon materials including Vulcan XC-72R carbon black, SWCNTs and MWCNTs by a solidestate reaction under intermittent microwave

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 8 4 6 e1 8 5 8

heating method. The application of CNTs greatly affected the position of Pt diffraction peaks in XRD pattern. The electrocatalyst particles aggregation was also improved as shown in TEM images. Reduced onset oxidation potential and increased oxidation current density values were gained at PteZrO2/ SWCNTs and PteZrO2/MWCNTs. The stability of different electrocatalysts for methanol oxidation during long-time operation, as examined by chronoamperometry and chronopotentiometry, was arranged in an ascending order as: PteZrO2/C < PteZrO2/SWCNTs < PteZrO2/MWCNTs. Moreover, PteZrO2/SWCNTs had the most shrinked semicircle diameter in Nyquist plot. The electrolyte diffusion rate at PteZrO2/MWCNTs surface was higher than that at PteZrO2/C. Based on the above result, incorporating carbon nanotubes as support materials for an electrocatalyst provides a facile and promising strategy to improve both its activity and durability for direct methanol fuel cells.

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