Heat-treatment effects on the ORR activity of Pt nanoparticles deposited on multi-walled carbon nanotubes using magnetron sputtering technique

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Heat-treatment effects on the ORR activity of Pt nanoparticles deposited on multi-walled carbon nanotubes using magnetron sputtering technique Sajid Hussain a, Heiki Erikson a, Nadezda Kongi a, Maido Merisalu a,b, € ino Sammelselg a,b, Kaido Tammeveski a,* Peeter Ritslaid b, Va a b

Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia

article info

abstract

Article history:

Platinum nanoparticles supported on acid-treated multi-walled carbon nanotubes (Pt/

Received 26 September 2016

MWCNT) catalysts were prepared through magnetron sputtering of Pt. The prepared Pt/

Received in revised form

MWCNT composites were annealed at different temperatures to study heat-treatment

23 November 2016

effects on the particle size, surface morphology, and the oxygen reduction reaction (ORR)

Accepted 24 November 2016

activity. High-resolution scanning electron microscopy (SEM) was used to study the surface

Available online xxx

morphology of Pt/MWCNT catalysts. For surface characterization, CO oxidation and cyclic voltammetry were performed in 0.1 M KOH and 0.05 M H2SO4 solutions. The ORR activity of

Keywords:

the electrocatalysts annealed at different temperature was studied in these solutions using

Oxygen reduction

the rotating disk electrode (RDE) method and the results were compared to those of the

Electrocatalysis

commercial Pt/C catalysts. Specific activity for oxygen reduction on Pt/MWCNT catalysts

Pt nanoparticles

was higher than that of Pt/C, however, the mass activity was lower due to a larger Pt

Carbon nanotubes

particle size of Pt/MWCNT catalysts.

Composite catalysts

© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Magnetron sputtering

Introduction Low-temperature fuel cells such as proton exchange membrane fuel cell (PEMFC) are regarded as an attractive and one of the most promising next-generation energy converting devices because of their high efficiency and low/zero emission [1e5]. However, the slow kinetics of the oxygen reduction reaction (ORR) at the fuel cell cathode, scarcity and high cost of platinum and the stability of Pt nanoparticles are the main obstacles in commercialization of PEMFCs [4,6e10]. Recently, much effort has been made to develop non-

precious metal, more abundant and less expensive electrocatalysts [4,11]. Different groups have explored novel nonplatinum electrocatalysts such as Pd-based catalysts, Irbased catalysts, transition metal macrocycles, metal oxides and chalcogenides, non-noble-metal catalysts and metal-free electrocatalysts [4,12], still the activity of all such catalysts is much lower as compared to that of highly dispersed Pt and Ptbased catalysts [13]. In order to minimize the amount of Pt needed and enhance its overall electrocatalytic activity, it can be deposited as highly dispersed nanoparticles onto a suitable support material. It has been investigated during the past decades that the performance of the electrocatalyst

* Corresponding author. Fax: þ372 7375181. E-mail address: [email protected] (K. Tammeveski). http://dx.doi.org/10.1016/j.ijhydene.2016.11.164 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Hussain S, et al., Heat-treatment effects on the ORR activity of Pt nanoparticles deposited on multiwalled carbon nanotubes using magnetron sputtering technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.164

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mainly depends on the distribution, size and shape of the platinum nanoparticles (PtNPs) and also on the surface morphology of the catalyst support [9,10,14e17]. Recently, a new catalyst support material based on carbide-derived carbon (CDC) has been employed for electrocatalytic applications [18e20]. Pt nanoclusters supported on CDC materials demonstrated a remarkable electrocatalytic activity for ORR in both acid and alkaline media [21e22]. Among several carbon-based supports, multi-walled carbon nanotubes (MWCNTs) are recognized as a promising support for Ptbased catalysts due to their unique structural and electrical properties [23e25]. Sahoo et al. prepared a hybrid carbon structure of partially reduced-exfoliated MWCNTs (PENTs) as a support material for PtNPs [26]. The specific and mass activity of the prepared Pt/PENT catalyst were found to be higher than those of commercial Pt/C. Recently, the electrochemical stability of PtNPs deposited on the surface modified MWCNTs has been studied [27]. A good dispersion of PtNPs was observed by modifying the surface of MWCNTs, which also improved the ORR activity of the Pt/MWCNT electrocatalyst. The ORR activity is strongly dependent on the catalyst support sites, such as pyridinic-N species on the carbon support surface [28,29]. It was demonstrated that employing nitrogen-doped graphene nanosheets yield uniform Pt catalyst dispersion and these composite materials are highly active for ORR in both acid and alkaline media [30]. Magnetron sputtering is a contemporary physical vapor deposition technique used to achieve highly dispersed and rather uniformly distributed nanoparticles on carbon supports [31e33]. Jukk et al. successfully prepared Pt/MWCNT catalysts by sputter deposition [34]. It was reported that this technique can be utilized for the preparation of efficient fuel cell catalysts. Grigoriev and co-workers used magnetron sputtering technique for the preparation of Pt/C electrocatalyst for PEM fuel cells [35]. They concluded that the main advantage of magnetron sputtering is the high reproducibility and high productivity in terms of mass production. One possibility to improve the ORR activity of PtNPs is to optimize the synthesis procedure. Recently, Bru¨ser and coworkers studied the effects of deposition temperature on the ORR activity of Pt sputtered on graphitic carbon [36]. It was observed that generally, the particle size, stability and activity of Pt/C increases when deposited at higher temperature >533 K. Heat-treatment of the electrocatalyst has also been revealed as one of the most effective approaches to enhance activity and stability by changing particle size, dispersion, surface morphology, and by exposing more active sites [11]. To observe heat-treatment effects on the surface rearrangement of PtNPs and amount of surface oxide, Chung et al. annealed Pt/C catalyst at 250  C for 3 h, using different gas atmospheres [37]. They revealed that heat-treatment has influence on the surface morphology and thus the ORR activity of the Pt-based catalyst. Jiang et al. also studied heat-treatment effects on 20 wt% Pt nanocrystals (PtNCs) supported on carbon nanotubes (CNTs) [38]. They annealed PtNC/CNT catalysts at different temperature ranging from 100 to 800  C for 1 h, under pure Ar and 20% H2e80% Ar atmosphere. An increase in Pt particle size with increasing temperature was observed. They reported maximum ORR activity and high durability of the catalyst after annealing it at 300  C in 20% H2-80% Ar

atmosphere. It was concluded that PtNPs have clear crystalline shape with dominate facets after heat-treatment at 300  C in reduced atmosphere. The influence of annealing on the preparation of 20% Pt/C at different temperature ranging from 300 to 700  C for 3e9 h under H2 and N2 atmosphere was studied by Han et al. [39]. For comparison, repetitive potential cycling was used for the pre-treatment of the catalyst. It was revealed that annealing is a better option to improve electrocatalytic activity than potential cycling [39]. Moreover, SEM and XRD measurements showed that the average particle size grows linearly with heating time and exponentially with temperature. It was also concluded that the ORR activity of the catalyst mainly depends on the surface morphology rather than particle size. Scherer and co-workers studied the ORR electrocatalytic activity of PtCo3/C catalysts annealed at different temperature from 350 to 1000  C for 1 h in reductive condition (using 7 vol.% H2) [40]. TEM measurements revealed no considerable change in the particle size of the prepared catalyst up to 600  C. The mass activity of the catalyst annealed at 800  C was 2.4 times higher as compared to that of commercial PtCox/C. Su et al. used electrodeposition to prepare platinum nanowires in anodic aluminum oxide, followed by annealing at different temperatures from 200 to 600  C [41]. Electrocatalytic activities of the prepared catalysts toward the ORR and methanol oxidation reaction (MOR) were compared to that of commercial Pt/C. They observed that with increasing annealing temperature the MOR activity increases while the ORR activity decreases. The nature and reactivity of heattreated Pt catalyst support material should also be taken into account when investigating the kinetics of the ORR. Thus, it is clear that heat-treatment affects the surface morphology and particle size of the Pt-based catalysts, which in turn changes the ORR activity of the electrodes, but it is still under dispute, which annealing temperatures and conditions are the best. A need is felt to optimize the annealing procedure in order to improve the electrocatalytic activity of Ptbased catalysts for ORR. In this study PtNPs were deposited on the surface of MWCNTs using magnetron sputtering. The prepared Pt/ MWCNT composites were annealed at different temperatures in nitrogen atmosphere to observe changes in the surface morphology and the ORR activity of the catalysts. Surface morphology of the prepared catalysts was investigated by high-resolution scanning electron microscopy (SEM) and electrochemical properties were explored using cyclic voltammetry (CV), CO oxidation, and the rotating disk electrode (RDE) method.

Experimental Preparation of Pt/MWCNT catalysts Working electrodes with geometric surface area (A) of 0.2 cm2 were cut from glassy carbon rods (GC20-SS, Tokai Carbon, Japan) and were polished with 1 and 0.3 mm alumina slurries (Buehler). After polishing the electrodes to mirror finish, they were sonicated twice for 5 min in isopropanol and Milli-Q water (Millipore Inc.). MWCNTs (purity >95%, diameter

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30 ± 10 nm, Nanolab, Inc., Brighton, MA, USA) were purified by acid-treatment using the procedure reported earlier [42]. A dispersion of the acid-treated MWCNTs was prepared in isopropanol (1 mg mL1), which was then sonicated for 30 min to form a homogeneous suspension. From the suspension, 4 drops of 5 mL were added onto the GC disk followed by drying in an oven at 60  C. Pt nanoparticles were deposited onto MWCNT/GC using magnetron sputtering as reported earlier [31,34,36]. Briefly, Pt deposition was performed at room temperature by DC-magnetron sputtering technique using 99.99% pure planar round platinum target of 25 mm in diameter and pure argon (99.999%, AGA). The pressure during the sputtering process was 3  103 mbar, the DC-power was 2 W, and the distance between target and substrate was 6 cm. Nominal thickness of the Pt layer (calculated from the mass deposited per geometric area of GC) for all the experiments was 15 nm. The GC disk loaded with Pt/MWCNT catalysts were annealed at different temperatures ranging from 300 to 700  C in tube furnace (Carbolite, MTF 12/38/400, UK) for 30 min under constant N2 (99.999%, AGA) flow. For comparison, some electrodes were studied without annealing. In what follows the modified electrodes are designated as Pt/MWCNT-x, where x marks the annealing temperature and RT (room temperature) corresponds to the material without annealing.

Instrumentation and measurements The surface morphology of the catalysts was examined by Helios NanoLab™ 600 (FEI) high-resolution scanning electron microscope. Electrochemical measurements were carried out in a five-necked glass cell using a standard three-electrode configuration. The electrolyte, 0.1 M KOH and 0.05 M H2SO4 solutions were prepared in Milli-Q water using KOH pellets (puriss p.a., SigmaeAldrich) and 96% sulfuric acid (Suprapur®, Merck), respectively. The solutions were saturated with O2 (99.999%, AGA) or Ar (99.999%, AGA) for oxygen reduction measurements and cyclic voltammetry (CV), respectively. Potential was applied by an Autolab potentiostat/galvanostat PGSTAT30 (Metrohm Autolab, The Netherlands) and General Purpose Electrochemical System (GPES) software was used to control the experiments. Pt wire was used as a counter electrode, which was separated from the working solution by glass frit and reversible hydrogen electrode (RHE) was employed as a reference electrode. All potentials are given with respect to RHE. The rotating disk electrode (RDE) experiments were carried out with E6 series interchangeable disk GC electrode, AFMSRX rotator and MSRX speed control unit (Pine Research Instrumentation, USA). The rotation rate (u) was varied between 360 and 3100 rpm. The potential scan rate (n) was 10 mV s1 for O2 reduction measurements and 50 mV s1 for recording CVs. For comparison, commercial Pt/C (20 wt%, E-TEK) and bulk Pt (99.95%, Alfa Aesar) were used.

Results and discussion Surface characterization of Pt/MWCNT composites Fig. 1a shows a typical SEM image of the acid-treated MWCNTs and Fig. 1b shows the micrograph of Pt/MWCNT-RT. It can be

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observed that the nanotubes are finely covered with uniformly dispersed PtNPs. SEM micrographs of Pt/MWCNT-x composite materials annealed at different temperatures are presented in Fig. 1cef. It can be observed that Pt/MWCNT-300 does not show any remarkable change in the surface morphology (Fig. 1c). No agglomeration can be detected and the distribution of PtNPs is almost the same as in the case of Pt/MWCNTRT. However, the Pt/MWCNT-x electrodes annealed at higher temperature (T  400  C) show different morphology. Annealing the catalysts at 400  C or higher temperature leads to the formation of two different size ranges of Pt particles. The size of smaller nanoparticles ranged from 5 to 15 nm while the larger ones had a diameter up to 120 nm. Agglomeration was first found in the case of Pt/MWCNT-400 (Fig. 1d). At higher annealing temperatures large agglomerates were detected (Fig. 1eef). In general, there was no significant heattreatment effect on the size of smaller nanoparticles at annealing temperatures of 400, 500 and 600  C. In contrast, the annealing temperature had a remarkable effect on the morphology and size of larger nanoparticles. Namely, the diameter of the larger nanoparticles increased steadily by increasing the annealing temperature and at the same time the coverage of MWCNTs by Pt decreased. By annealing at 400  C the average size of the bigger nanoparticles ranged from 20 to 50 nm. At 500  C the average size ranged from 40 to 50 nm, which further increased from 60 to 120 nm at 600  C. Similar results obtained with annealed Pt-based materials have been also reported by other groups [37e39,43,44]. Concerning distribution density of the PtNPs in the annealed sample one can see that the particles number density decreases in the depth direction of the coating, caused by the origin of the magnetron sputtering method. This is well observable by distribution of the bigger particles lying on the outermost layers of the Pt/MWCNT-x coatings, and showing that these layers already had a thicker Pt-coverage before the annealing. What is interesting that the number density of smaller PtNPs seems to be more homogeneous in depth, and is somewhat bigger for coatings annealed at 500  C if compared with other annealed coatings. It is probably connected with circumstances that PtNPs are nucleating first on the defective sites of the nanotubes, and diffusion of Pt atoms from those sites needs to overcome some energy barrier. However, getting more homogeneous spatial distribution of PtNPs on the MWCNT surface needs further studies.

Electrochemical characterization of Pt/MWCNT-modified electrodes For characterization and cleaning of the surface of electrocatalysts, the oxidative stripping experiments of pre-adsorbed carbon monoxide were carried out [45]. Fig. 2a presents COstripping of Pt/MWCNT-300 electrode in 0.05 M H2SO4 solution. During the first cycle in the potential range from 0.05 to 0.4 V, it can be observed that the electrode surface is completely blocked with CO. During the second cycle CO oxidation peaks can be detected in the potential range from 0.6 to 0.8 V. The third cycle confirms complete oxidation of CO on the Pt/MWCNT electrode surface [46]. Fig. 2b displays CO oxidation profiles of all Pt/MWCNT-x catalysts in 0.05 M H2SO4. The absence of hydrogen desorption peaks in all the

Please cite this article in press as: Hussain S, et al., Heat-treatment effects on the ORR activity of Pt nanoparticles deposited on multiwalled carbon nanotubes using magnetron sputtering technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.164

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Fig. 1 e SEM images of (a) acid-treated MWCNT, (b) Pt/MWCNT-RT, (c) Pt/MWCNT-300, (d) Pt/MWCNT-400, (e) Pt/MWCNT-500 and (f) Pt/MWCNT-600 samples.

voltammograms show that the Pt surface is completely blocked with CO in each case. CO oxidation peaks were found in the potential range from 0.6 to 0.8 V. It was observed that the peaks at ~0.75 and ~0.78 V are less clear in case of Pt/ MWCNT-RT and Pt/MWCNT-300 but become broader in case of electrodes annealed at higher temperature. Different CO oxidation profile could be attributed to different Pt particle size and crystallographic facets on the electrode surface [47,48]. This is due to the surface modifications of Pt/MWCNT electrodes at different annealing temperatures. After CO oxidation, cyclic voltammograms (CVs) were measured in Ar-saturated 0.1 M KOH and 0.05 M H2SO4

solutions at a potential sweep rate of 50 mV s1 in the potential range from 0.05 to 1.4 V. Fig. 3 displays representative CVs for Pt-based electrocatalysts measured in both acidic (Fig. 3a) and alkaline media (Fig. 3b). In the low potential range from 0.05 to 0.4 V hydrogen adsorption and desorption peaks can be observed at the cathodic and anodic sweeps, respectively [26,27,46]. On the anodic sweep Pt surface oxides are formed above 0.8 V and are reduced at ~0.75 V in H2SO4 and at ~0.7 V in KOH on the cathodic sweep. The presence of hydrogen desorption peaks at ~0.14 and ~0.28 V in 0.05 M H2SO4 and at ~0.28 and ~0.38 V in 0.1 M KOH are associated with Pt(110) and Pt(100) facets [49]. It is revealed from these

Please cite this article in press as: Hussain S, et al., Heat-treatment effects on the ORR activity of Pt nanoparticles deposited on multiwalled carbon nanotubes using magnetron sputtering technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.164

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Fig. 2 e (a) CO-stripping on Pt/MWCNT-300 and (b) comparison of CO-stripping on Pt/MWCNT catalysts in Ar-saturated 0.05 M H2SO4 solution v ¼ 20 mV s¡1. Current densities are normalized to the geometric area of GC.

characteristic peaks that Pt particles have Pt(110) and Pt(100) dominant surface structures. Moreover, it was observed that in case of Pt/MWCNT heat-treated at different temperatures, the peak corresponding to Pt(100) becomes more pronounced indicating the development of Pt(100) facets during annealing [50]. The real surface area (Ar) of the MWCNT-supported Pt catalysts were calculated from hydrogen desorption peaks assuming that a charge equal to 210 mC cm2 is required to

desorb a monolayer of hydrogen from the Pt catalyst surface [51]. Table 1 shows that Pt/MWCNT-300 catalysts had higher Ar than that of Pt/MWCNT-RT in both acidic and alkaline media while those annealed at higher temperatures showed a decrease in Ar due to increase in Pt particle size as observed from SEM images (Fig. 1). The decrease in Ar due to increase in particle size at elevated temperatures have also been reported earlier [37,38].

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Fig. 3 e Cyclic voltammograms of Pt/MWCNT catalysts in Ar-saturated (a) 0.05 M H2SO4 and (b) 0.1 M KOH solutions v ¼ 50 mV s¡1. Current densities are normalized to the geometric area of GC.

Oxygen reduction in alkaline medium RDE measurements for oxygen reduction were first carried out in O2-saturated 0.1 M KOH solution at a potential scan rate of 10 mV s1. The background currents measured in Arsaturated solution using the same operating procedure were

subtracted from the RDE data. Only the positive-going potential scans were analyzed and are presented. Fig. 4a displays the polarization curves for O2 reduction on a Pt/MWCNT-300 catalyst at different electrode rotation rates (360e3100 rpm). The RDE results for all Pt/MWCNT-x electrodes measured at 960 rpm in comparison to commercial Pt/C are presented in

Please cite this article in press as: Hussain S, et al., Heat-treatment effects on the ORR activity of Pt nanoparticles deposited on multiwalled carbon nanotubes using magnetron sputtering technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.164

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Table 1 e Kinetic parameters for oxygen reduction reaction on Pt/MWCNT catalysts in 0.1 M KOH. Electrode Pt/MWCNT-RT Pt/MWCNT-300 Pt/MWCNT-400 Pt/MWCNT-500 Pt/MWCNT-600 Pt/MWCNT-700 Pt/C Bulk Pt a

Ar (cm2)

Tafel slope (mV) I regiona

Tafel slope (mV) II regiona

E1/2 (V)

SA at 0.9 V (mA cm2)

MA at 0.9 V (mA mg1)

0.29 0.41 0.23 0.27 0.30 0.15 1.09 0.32

60 62 66 64 63 62 61 61

118 110 114 118 116 117 115 110

0.82 0.83 0.76 0.77 0.77 0.72 0.84 0.82

0.23 0.38 0.17 0.17 0.13 0.19 0.10 0.30

16.9 20.1 5.8 6.8 6.1 3.9 38.1

Region I corresponds to low current densities and region II to high current densities.

Fig. 4b. Table 1 shows the values of half-wave potential (E1/2) determined from the RDE results. The E1/2 values of Pt/C, Pt/ MWCNT-RT, bulk Pt and Pt/MWCNT-300 were close to each other. The electrodes annealed at temperature >300  C showed a decrease in the E1/2 values, which is expected as the electroactive surface area decreased with increased temperature. The RDE data were analyzed using the KouteckyeLevich (KeL) equation from which the number of electrons transferred per O2 molecule (n) was calculated [52]: 1 1 1 1 1 ¼ þ ¼  j jk jd nFkCbO2 0:62nFDO2=3 v1=6 CbO u1=2 2 2

(1)

where j is the measured current density, jk and jd are the kinetic and diffusion-limited current densities, respectively, F is the Faraday constant (96,485 C mol1), k is the rate constant for O2 reduction, u is the electrode rotation rate (rad s1), CbO2 is the concentration of O2 in 0.1 M KOH (1.2  106 mol cm3) [53], DO2 is the diffusion coefficient of O2 in 0.1 M KOH (1.9  105 cm2 s1) [53] and v is the kinematic viscosity of the solution (0.01 cm2 s1) [54]. The KeL plots were constructed from the RDE data presented in Fig. 4a (inset of Fig. 4a). The value of n calculated from Eq. (1) was close to 4 over the whole potential range studied, which indicates a typical 4-electron pathway of O2 reduction on Pt surface as reported by several studies [47,55,56]. In order to further analyze the ORR data (Fig. 4b), masstransfer corrected Tafel plots were constructed for all the Pt/ MWCNT-x electrodes as shown in Fig. 5. Two Tafel regions with characteristic slopes were observed and the corresponding slope values were determined and are presented in Table 1. The Tafel slope value at low current densities was around 60 mV and the second slope at high current densities was close to 120 mV for all the electrodes studied. Both of the values indicate that the charge transfer is the ratedetermining step for O2 reduction on the Pt/MWCNT-x electrode surface. As the Tafel slope values obtained were similar for all the electrodes it can be concluded that the reaction mechanism is the same for all Pt/MWCNT-x catalysts. Similar Tafel slope values in alkaline medium have been reported earlier [47,57e59]. The change in Tafel slope value has been suggested to arise from the change of oxygen adsorption conditions [58]. In order to compare intrinsic activities of the catalysts the specific activity (SA) and mass activity (MA) of the ORR for all

the Pt/MWCNT-x electrodes were determined. SA was calculated using the following equation: SA ¼ Ik =Ar

(2)

where Ik is the kinetic current and Ar is the real electroactive surface area of Pt catalyst. The SA values for all the electrodes studied were calculated at 0.9 V. It can be observed from Table 1 that the SA value of Pt/MWCNT-300 is higher than that of Pt/ MWCNT-RT after which the SA value starts decreasing with increasing annealing temperature. The higher SA value of the catalyst annealed at 300  C suggests better utilization of the Pt in the catalyst. The mass activity values were calculated at 0.9 V using the following equation: MA ¼ Ik =mPt

(3)

where mPt is the nominal mass of Pt in Pt/MWCNT composite materials. The mass activity of Pt/C is much higher than that of Pt/MWCNT-x modified electrodes. This can be attributed to the smaller size of Pt particles in commercial Pt/C as compared to Pt/MWCNT-x composites. It was observed that with increasing annealing temperature the MA of the electrodes decreases, which can be attributed to increase in particle size with increasing annealing temperature. Wang et al. observed, that annealing of Pt/CNTs in hydrogen atmosphere leads to noticeable agglomeration of the PtNPs and subsequent decrease of the active Pt sites, leading to a decrease in the overall electrocatalytic performance in 1.0 M KOH [60].

Oxygen reduction in acidic medium The electrochemical reduction of O2 was also studied in 0.05 M H2SO4 solution using the same operating procedure used for ORR measurements in 0.1 M KOH. The ORR polarization curves of Pt/MWCNT-300 in acidic medium at rotation rates from 360 to 3100 rpm are shown in Fig. 6a. The KeL plot was constructed at 0.35 V which is presented in the inset of Fig. 6a. From the slopes of the KeL lines the value of n was calculated by Eq. (1) using the values CbO2 ¼ 1:22  106 mol cm3 [61] and DO2 ¼ 1:93  105 cm2 s1 [61] given for 0.05 M H2SO4 solution. As expected the n value was close to four, which is in agreement with previous work [38]. The RDE results of Pt/MWCNT-x were compared with Pt/C as shown in Fig. 6b. The E1/2 values (Table 2) are close to those obtained in 0.1 M KOH. It was found that with increasing

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Fig. 4 e (a) RDE polarization curves for ORR on Pt/MWCNT-300, inset shows the KeL plot at 0.35 V and (b) comparison of RDE polarization curves at 960 rpm in oxygen-saturated 0.1 M KOH solution v ¼ 10 mV s¡1.

annealing temperature the E1/2 values decrease in the following order: Pt/C > Pt/MWCNT-RT z Pt/MWCNT300 z bulk Pt > Pt/MWCNT-400 z Pt/MWCNT-500 z Pt/ MWCNT-600 > Pt/MWCNT-700. Tafel plots constructed on the basis of the RDE data shown in Fig. 6b are presented in Fig. 7, the slope values are given in

Table 2. It can be seen that the slope values determined at low current densities are little bit higher than those in the alkaline medium, but they are still close to the previously reported value of 60 mV. An increase in the slope values at low current densities was observed with increasing annealing temperature. Tafel slope values at high current densities

Please cite this article in press as: Hussain S, et al., Heat-treatment effects on the ORR activity of Pt nanoparticles deposited on multiwalled carbon nanotubes using magnetron sputtering technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.164

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Fig. 5 e Tafel plots for ORR in 0.1 M KOH (u ¼ 960 rpm). Data derived from Fig. 4b.

were close to 120 mV in agreement with previous report [62]. The SA values of the Pt/MWCNT-x catalysts in 0.05 M H2SO4 solution were calculated by normalizing the kinetic current to the real surface area of Pt (Eq. (2)). The results of the kinetics analysis are presented in Table 2. SA values were close to that observed in 0.1 M KOH and almost the same trend of decreasing the SA values with increasing annealing temperature was observed. Su et al. reported that the ORR activity of Pt nanowires decreases as the annealing temperature is increased from 200 to 600  C [41]. MA values calculated from Eq. (3) are also presented in Table 2. Pt/C showed the maximum MA as compared to Pt/MWCNT electrodes. The MA decreased with increasing annealing temperature as in the case of alkaline medium. In general, a decrease of catalyst active area and mass activity is observed with increasing particle size. The number density of the catalyst particles also plays an important role. When Pt particles are well separated from each other, the full particle surface is active for ORR. When Pt particles are close, they have so called mutual influence on the diffusion layer and negative effect on the catalytic activity, since the whole catalyst area is not available [14]. Platinum nanocrystals supported on carbon nanotubes annealed at 300  C in 20% H2 e 80% Ar atmosphere showed the highest specific surface area value (39.57 m2 g1) and a better electrocatalytic activity toward the ORR in 0.1 M H2SO4 [38]. Bezerra et al. reviewed the heat-treatment effects of fuel cell catalysts and summarized that the optimum annealing procedure regarding the ORR activity improvement depends on the individual catalyst [11]. However, too

high annealing temperature (>1000  C) degrades the electrocatalyst activity. Chung et al. proposed that the ORR activity is mainly influenced by the degree of catalyst surface atom rearrangement, such as by increasing the amount of Pt(111) facets, not the change of surface oxide [37]. Same observation was made by Su et al., after heat-treatment the Pt nanowires had more Pt(111) sites on the catalyst surface and exhibited better electrocatalytic activity toward the ORR in acid electrolyte [41]. Several groups reported that by changing the particle size, surface coordination number and distribution of the surface atoms is changed that results into a change in the ORR activity [63e65]. However, there is no clear agreement in the literature regarding particle size effect on the ORR activity of Pt/C catalysts [11]. Shinozaki et al. studied particle size effect on the ORR using polycrystalline Pt and commercial Pt/C in 0.1 M HClO4 [66]. They reported an increase in the SA values with increasing the average Pt/C particle size in the range of 2.1e9.9 nm. Anastasopoulos et al. compared particle size dependence of Pt and Pd for the ORR in 0.5 M HClO4 [67]. They observed an increased specific current density for Pt nanoparticles from 1.5 up to 4 nm in diameter. The MA maximum was observed from 3.5 to 4 nm. These results are in good agreement with the previous results [63,68,69]. Maximum mass activity of PtNPs in 0.1 M HClO4 with an average diameter of 3 nm has also been reported [70]. Gasteiger and co-workers reported that in case of PEMFC, the variation of mass activity and specific activity with particle size is due to the adsorption of oxygen-containing species [5]. The aspects regarding particle size are highly important in the practical design of Pt-based cathode catalysts for low-temperature fuel cells.

Please cite this article in press as: Hussain S, et al., Heat-treatment effects on the ORR activity of Pt nanoparticles deposited on multiwalled carbon nanotubes using magnetron sputtering technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.164

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Fig. 6 e (a) RDE polarization curves for ORR on Pt/MWCNT-300, inset shows the KeL plot at 0.35 V and (b) comparison of RDE polarization curves at 960 rpm in oxygen-saturated 0.05 M H2SO4 solution v ¼ 10 mV s¡1.

Please cite this article in press as: Hussain S, et al., Heat-treatment effects on the ORR activity of Pt nanoparticles deposited on multiwalled carbon nanotubes using magnetron sputtering technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.164

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Table 2 e Kinetic parameters for oxygen reduction reaction on Pt/MWCNT catalysts in 0.05 M H2SO4. Electrode Pt/MWCNT-RT Pt/MWCNT-300 Pt/MWCNT-400 Pt/MWCNT-500 Pt/MWCNT-600 Pt/MWCNT-700 Pt/C Bulk Pt a

Ar (cm2)

Tafel Slope (mV) I regiona

Tafel Slope (mV) II regiona

E1/2 (V)

SA at 0.9 V (mA cm2)

MA at 0.9 V (mA mg1)

0.45 0.46 0.21 0.29 0.28 0.13 1.42 0.40

62 66 72 66 73 86 71 91

119 120 125 115 119 122 139 121

0.82 0.81 0.75 0.74 0.73 0.69 0.84 0.80

0.26 0.32 0.29 0.15 0.15 0.15 0.12 0.39

26.7 22.6 9.7 6.6 6.7 3.3 70.1

Region I corresponds to low current densities and region II to high current densities.

Fig. 7 e Tafel plots for ORR in 0.05 M H2SO4 (u ¼ 960 rpm). Data derived from Fig. 6b.

Conclusions

Acknowledgments

The Pt/MWCNT electrodes prepared through magnetron sputtering were annealed for 30 min at different temperature under steady flow of N2. SEM results showed uniform distribution of PtNPs on the surface of MWCNTs before heattreatment. It was observed that the particle size increased with increasing annealing temperature. Agglomeration of the particles was detected at annealing temperature 400  C. Surface characterization through CO oxidation and CV confirmed modification of the Pt/MWCNT-x electrodes surface morphology after heat-treatment at different temperatures. From RDE results it is concluded that 300  C is the optimum annealing temperature to improve the ORR activity of Pt/MWCNT electrocatalysts.

This work was financially supported by institutional research funding (IUT20-16 and IUT2-24) of the Estonian Ministry of Education and Research. We would like to acknowledge the financial support by the EU through the European Regional Development Fund (TK141 “Advanced materials and hightechnology devices for energy recuperation systems”).

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