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Preparation of carbon nanotube-supported Pt catalysts covered with silica layers; application to cathode catalysts for PEFC
Applied Catalysis A: General 373 (2010) 176–185
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Preparation of carbon nanotube-supported Pt catalysts covered with silica layers; application to cathode catalysts for PEFC Hiroshi Matsumori, Sakae Takenaka *, Hideki Matsune, Masahiro Kishida * Department of Chemical Engineering, Graduate School of Engineering, Kyushu University, Moto-oka 744, Nishi-ku, Fukuoka 819-0395, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 July 2009 Received in revised form 4 November 2009 Accepted 10 November 2009 Available online 14 November 2009
Multi-walled carbon nanotube-supported Pt metal particle (Pt/CNT) catalysts were covered with silica layers with different thickness and density. In the present study, the Pt/CNT covered with silica layers (SiO2/Pt/CNT) was prepared by the successive hydrolysis of 3-aminopropyl-triethoxysilane (APTS) and tetraethoxysilane (TEOS). The thickness of silica layers in SiO2/Pt/CNT could be controlled by varying the concentration of silica sources during the preparation of SiO2/Pt/CNT. The density of silica layers in SiO2/ Pt/CNT was changed by varying the treatment temperature for the catalysts as well as by adjusting the pH for the hydrolysis of silica sources during the preparation of the catalysts. The catalytic performance of these SiO2/Pt/CNT as cathode catalysts in polymer electrolyte fuel cells was examined. Pt/CNT covered with silica layers of adequate thickness (ca. 6 nm) and high density had a high electrochemically active surface area as well as excellent durability. Dense silica layers in SiO2/Pt/CNT prevented the diffusion of cationic Pt species dissolved from Pt metal particles out of the catalysts. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Carbon-supported Pt electrocatalysts Polymer electrolyte fuel cells Catalytic stability Silica coating
1. Introduction Polymer electrolyte fuel cells (PEFCs) are promising alternative power sources because of their low emission and high energy efficiency. However, many issues need to be resolved before the commercialization of PEFCs. The cost of PEFC systems is a serious problem since Pt metal, which functions as catalytically active sites in anode and cathode electrocatalysts within PEFCs, is very expensive and is a scarce resource. Therefore, the total Pt loading in a membrane electrode assembly (MEA) to be used in PEFCs should be as low as possible. To reduce the Pt loading in PEFCs, Pt-alloys or non-noble metal electrocatalysts with high activity for the oxygen reduction reaction (ORR) should be developed. The ORR at a cathode is kinetically sluggish compared with the hydrogen oxidation reaction at an anode. Many Pt-based alloy catalysts such as Pt–Co, Pt–Ni and Pt–Fe have been reported to show high activity for the ORR [1–5] despite a lower Pt loading in Pt-alloy catalysts compared with pure Pt catalysts. As for non-noble metal catalysts, alloy catalysts without Pt were reported to show high activity for the ORR [6,7]. Recently, Pt-free metal oxide catalysts [8,9] and MeNx (Me = Co, Fe) electrocatalysts [10,11] such as metals coordinated to porphyrins or pyrrole have been developed as active cathode catalysts for use in PEFCs. On the other hand, the durability of Pt electrocatalysts at the cathode has been recognized as one of the most important issues when reducing the Pt loading
* Corresponding authors. Tel.: +81 92 802 2752; fax: +81 92 802 2752. E-mail address: [email protected] (S. Takenaka). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.11.011
in a MEA [12]. Pt electrocatalysts at the cathode are deactivated during PEFC operation because Pt cathode catalysts are exposed to severe conditions such as low pH [13], high temperatures [14], high humidity [15] and high potential [16]. These conditions accelerate the reduction of the electrochemically active surface area (ECSA) of the Pt metal in these electrocatalysts. Thus high Pt loading is required in MEAs to maintain good PEFC performance. Three different mechanisms are generally accepted that explain the reduction of the ECSA of Pt metal in carbon-supported Pt catalysts at the cathode. First, Pt metal particles migrate on carbon supports and then aggregate [17]. Second, Pt metal particle sizes increase by Ostwald ripening; surface Pt atoms on small Pt metal particles dissolve to form cationic Pt species and are re-deposited on other larger Pt particles, which results in an increase in Pt metal particle sizes [18,19]. Third, carbon supports for Pt metal particles are corroded by oxidation [20,21]. Corrosion of carbon supports prohibits electrochemical contact between them and Pt metals. Additionally, when carbon supports have been corroded Pt metal particles aggregate easily. It is also well accepted that these phenomena are accelerated by the repeated on–off cycles of PEFC [22]. Much work has been done to improve the durability of Pt electrocatalysts such as the addition of transition metals like Ni or Co and the addition of precious metals like Au [23–25]. In fact, the durability of Pt electrocatalysts is improved to some extent by the addition of another metal. However, it is difficult to obtain alloy catalysts of small particle size since these catalysts must be treated at high temperatures for alloy formation. Currently, the durability of Pt-alloy catalysts is not satisfactory because added metal species dissolve under cathodic conditions [26–30]. Dissolved metal
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species from Pt-alloy catalysts deposit in a proton exchange membrane electrolyte, which decreases the proton conductivity of the electrolyte [31]. Thus new methods to improve the durability of carbon-supported Pt metal electrocatalysts are required. Recently, we demonstrated that multi-walled carbon nanotube (CNT)-supported Pt metal particles covered with silica (SiO2/Pt/ CNT) showed excellent durability in electrochemical reactions in aqueous acid electrolytes [32]. Pt metal particles in SiO2/Pt/CNT do not migrate on CNT supports because these particles are uniformly covered with silica. Furthermore, when cationic Pt species are formed from the Pt metal particle in SiO2/Pt/CNT catalysts under PEFC conditions, the cationic Pt species do not easily diffuse out of the catalyst since these Pt metal particles are covered with silica layers. Therefore, Pt metal particles in SiO2/Pt/CNT catalysts do not change in size during the PEFC operation. The durability of Pt/CNT is predicted to be improved by coverage with thicker and denser silica layers. However, the coverage of Pt/CNT electrocatalysts with these silica layers probably reduces their electrochemical activity because reactant molecules such as water and oxygen need to be supplied through silica layers in SiO2/Pt/CNT and silica is an insulator. In this study, we investigated the effects of thickness and density of silica layers in SiO2/Pt/CNT catalysts on their durability.
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tively. All electrode potentials in this study are referenced against the reversible hydrogen electrode (RHE). The working electrode was prepared according to the method reported by Schmidt et al. [35]. The working electrode was a thin film electrode attached to a mirror finished glassy carbon rod (GC, diameter 5 mm). 20 mg of the catalyst was added to 10 ml methanol and a small amount of water and this suspension was treated ultrasonically for 30 min. 20 ml of this ink was dropped on GC and then dried at 333 K. 20 ml of a 1 wt% Nafion solution was further pipetted onto the electrode to attach the electrocatalysts onto the GC. Cyclic voltammograms (CV) were measured in N2-purged 0.5 M H2SO4 at 303 K using a potentiostat and galvanostat (HOKUTO Denko Corp. HZ5000). The voltage was changed by a triangular wave between 0.05 and 1.20 V (vs. RHE) at a scan rate of 50 mV s 1. Before CV measurements 50 voltage sweeps were undertaken to obtain highly reproducible CVs. The electrochemically active surface area (ECSA) of Pt metal particles in the catalysts was calculated from their CVs by the amount of charge needed for the underpotential deposition of protons onto the Pt metal surface. The columbic charge necessary for proton deposition is 210 mC cm-Pt 2. To evaluate the durability of electrocatalysts the working electrode’s voltage was changed repeatedly between 0.05 and 1.20 V at a scan rate of 50 mV s 1 and then the CV was obtained.
2. Experimental 3. Results and discussion 2.1. Preparation of catalysts 3.1. Effect of pH on hydrolysis Multi-walled CNTs (diameter = 20–30 nm, length = 5–200 mm, purchased from Aldrich) were washed with 3.6 M HCl at 353 K for 3 h to remove metal impurities. These CNTs were dispersed in a mixed solution of 8.0 M H2SO4 and 8.0 M HNO3 and then treated ultrasonically at 328 K for 2 h to introduce functional groups such as hydroxyls and carboxyls onto CNT surfaces [33]. Oxidized CNTs were immersed in an aqueous solution containing H2PtCl6 [34]. The pH value of the solution was adjusted to ca. 11 by the addition of aqueous NH3 and Pt metal precursors were thus deposited onto CNT surfaces. After this solution was filtered, the samples were dispersed in a mixture of ethanol and water (volume ratio 1:1). To cover these samples with silica, 3-aminopropyl-triethoxysilane (APTS) and aqueous NH3 were added to the solution and stirred at 333 K for 0.5 h. Hydrolysis of tetraethoxysilane (TEOS) was then performed at the same temperature for 1 h. After centrifuging and drying the samples at 333 K they were treated at 623 K for 3 h under a H2 stream. Finally, the obtained samples were heated under a stream of Ar at different temperatures (623, 873 or 1073 K) to control the density of silica layers in the catalysts. CNT-supported Pt (Pt/CNT) electrocatalysts without silica layers were prepared by a conventional impregnation method. CNTs were dispersed in an alcohol solution with an appropriate amount of H2PtCl6. The suspension was stirred and dried at 333 K. The samples thus obtained were reduced under a H2 stream at 473 K for 5 h. 2.2. Characterization of catalysts The morphology of the catalysts was investigated using transmission electron microscopy (TEM) on a JEOL 3200FSK operated at 300 kV. The content of Pt and Si in SiO2/Pt/CNT was measured using inductively coupled plasma-atomic emission spectroscopy (Optima 5300). 2.3. Electrochemical measurements Electrochemical measurements were performed in a threecompartment glass cell with a Pt mesh and a saturated Ag/AgCl electrode as counter electrode and reference electrode, respec-
Silica is usually prepared by the hydrolysis of a silicon alkoxide such as TEOS. In this study, silica layers in SiO2/Pt/CNT were prepared by the successive hydrolysis of APTS and TEOS in the presence of CNT-supported Pt precursors. APTS is adsorbed on the surface of CNT by the interaction of an amino group and graphene to form very thin silica layers [36]. Subsequently, this sample is covered with thick silica layers by the hydrolysis of TEOS. It is well known that pH values during the hydrolysis of silicon alkoxides affect their hydrolysis rates. We prepared SiO2/Pt/CNT catalysts at different pH using aqueous NH3 as a catalyst for the hydrolysis of APTS and TEOS. Primary silica particles formed from the hydrolysis of TEOS at higher pH are larger in size, since the hydrolysis rate of TEOS is faster at higher pH. The aggregation of silica particles on CNT surfaces to form silica layers results in the formation of porous structure in silica layers. Thus, it is expected that the porous structure of silica layers in SiO2/Pt/CNT strongly depends on the pH values during the hydrolysis of TEOS. Table 1 shows the content of SiO2, Pt and CNT in Pt/CNT as well as SiO2/Pt/CNT catalysts prepared at different pH. The content of SiO2 and Pt in these SiO2/ Pt/CNT catalysts did not depend on the pH values during catalyst preparation. Fig. 1 shows TEM images of Pt/CNT and SiO2/Pt/CNT catalysts that were prepared at different pH. Panels (a) and (b) show TEM images for Pt/CNT while panels (c) and (d) and panels (e) and (f) correspond to TEM images for SiO2/Pt/CNT catalysts prepared at pH values of 11.6 and 10.0, respectively. Both SiO2/Pt/ CNT catalysts were treated at 873 K under Ar before the measurement of their TEM images. Pt metal particles were observed on the outer surfaces of CNTs in TEM images of Pt/ Table 1 The content of SiO2, Pt and CNT in Pt/CNT and SiO2/Pt/CNT prepared at different pH values. Entry
1 2 3
Sample
Pt/CNT SiO2/Pt/CNT SiO2/Pt/CNT
pH value
– 11.6 10.0
Content (wt%) Pt
SiO2
CNT
7.0 3.4 3.7
– 46.5 45.2
93.0 50.1 51.1
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Fig. 1. TEM images of Pt/CNT (a and b) and SiO2/Pt/CNT prepared at a pH of 11.6 (c and d) and 10.0 (e and f).
CNT. The size of Pt metal particles in Pt/CNT ranged from 1 to 5 nm. Pt metal particles were also observed on outer surfaces of CNTs in TEM images of both SiO2/Pt/CNT catalysts. The Pt metal particles and CNTs in both SiO2/Pt/CNT catalysts were uniformly covered with thin silica layers [33]. Pt metal particle sizes in the SiO2/Pt/ CNT prepared at pH 11.6 ranged widely from 1 to 7 nm while Pt metal particle sizes in the SiO2/Pt/CNT prepared at pH 10.0 was relatively uniform (1–3 nm). It should be noted that the thickness of the silica layer in the SiO2/Pt/CNT prepared at pH 11.6 was not uniform, and exposed Pt metal particles as well as exposed CNT surfaces were observed. Furthermore, large spherical particles of silica were observed in the TEM image of the SiO2/Pt/CNT prepared at pH 11.6. For the SiO2/Pt/CNT prepared at pH 10.0, most Pt and CNT surfaces were covered with silica layers of relatively uniform thickness. For the preparation of SiO2/Pt/CNT by successive hydrolysis of APTS and TEOS, APTS is initially adsorbed on the surface of CNTs to form thin silica layers. Subsequently, the silica
that is formed from TEOS is deposited on CNTs that are covered with thin silica layers from APTS through heterogeneous nucleation because the thin silica layers from APTS work as the nucleation sites for the formation of thick silica layers from TEOS. However, silica particles from TEOS are formed by homogenous nucleation in addition to heterogeneous nucleation when the pH during the hydrolysis of TEOS is higher, since the hydrolysis rate of TEOS increases at higher pH. Homogenous nucleation of silica should result in the formation of larger silica particles compared with heterogeneous nucleation and large silica particles are deposited on CNT surfaces to form silica layers. Therefore, silica layers in the SiO2/Pt/CNT prepared at pH 11.6 are not uniform in thickness. Additionally, the formation of silica through homogenous nucleation results in the formation of large spherical particles of silica. Fig. 2 shows particle size distributions and average particle sizes of the Pt metal estimated from TEM images of Pt/CNT and SiO2/Pt/CNT catalysts prepared at pH 11.6 and pH 10.0. Pt metal
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Fig. 3. Cyclic voltammograms of Pt/CNT (a) and SiO2/Pt/CNT prepared at pH 11.6 (b) and pH 10.0 (c) in 0.5 M H2SO4 at 303 K. Scan rate was 50 mV s 1.
Fig. 2. The particle size distribution and the average diameter of Pt metal particles estimated from their TEM images for Pt/CNT (a) and SiO2/Pt/CNT prepared at pH 11.6 (b) and 10.0 (c).
particle sizes ranged from 1 to 8 nm and the average particle size of the Pt metal was 3.0 nm as shown in Fig. 2(a) for Pt/CNT. The size distribution of Pt metal particles for the Pt/CNT was relatively broad. On the other hand, the size distributions of Pt metal particles in both SiO2/Pt/CNT catalysts were narrower than that in the Pt/CNT as shown in Fig. 2(b) and (c). In these catalysts, the size distribution of Pt metal particles was dependent on the initial pH used for their preparation. The size of Pt metal particles in the SiO2/ Pt/CNT prepared at pH 10.0 ranged from 1 to 4 nm while Pt metal particles larger than 5 nm in diameter were observed in the SiO2/ Pt/CNT prepared at pH 11.6. The average diameter of Pt metal particles in the SiO2/Pt/CNT prepared at pH 10.0 (1.7 nm) was smaller than that in the SiO2/Pt/CNT prepared at pH 11.6 (2.9 nm). Cyclic voltammograms of Pt/CNT and SiO2/Pt/CNT catalysts prepared at different pH values were measured to evaluate their electrochemical performance. These results are shown in Fig. 3. The CV for the Pt/CNT has two peaks that are due to the oxidation– reduction of Pt metal (0.6–1.2 V) and the adsorption–desorption of hydrogen on Pt metal surface (0.05–0.4 V). Interestingly, these peaks due to Pt are also found in the CVs of both SiO2/Pt/CNT catalysts despite the coverage of Pt metal with silica layers. These results indicate that Pt metal particles in both SiO2/Pt/CNT
electrocatalysts are electrochemically active [32,37]. It is likely that reactants such as protons and water pass through silica layers which have a porous structure and are thus supplied to Pt metal surface in SiO2/Pt/CNT. Electrons are likely supplied to Pt metal particles through the CNT surface that is not covered with silica. Therefore, SiO2/Pt/CNT catalysts are electrochemically active despite the coverage of Pt metal particles with the silica insulator. CV peak intensity depends strongly on the pH used for the preparation of SiO2/Pt/CNT catalysts. The peak intensity of the catalyst prepared at pH 10.0 is higher than that of the catalyst prepared at pH 11.6. This is due to Pt metal particles in the SiO2/Pt/ CNT prepared at pH 11.6 being larger in size than those in the SiO2/ Pt/CNT prepared at pH 10.0, as shown in Fig. 2. Fig. 4 shows CVs for Pt/CNT and SiO2/Pt/CNT catalysts during repeated potential cycling experiments from 0.05 to 1.20 V. The CV peak intensity gradually decreases with the number of potential cycles for the Pt/CNT, which is not covered with silica layers, as shown in Fig. 4(a). CVs for SiO2/Pt/CNT catalysts prepared at pH 11.6 and pH 10.0 are shown in Fig. 4(b) and (c), respectively. The peaks in these CVs also show a slight decrease in intensity during potential cycling experiments. However, the decrease in peak intensity of both SiO2/Pt/CNT catalysts during the repeated potential cycling experiments is negligible compared with that observed for Pt/CNT. These results indicate that the Pt electrocatalyst durability is improved by coverage with silica. The change in ECSA of Pt metal particles for each Pt catalyst during potential cycling was evaluated from the CVs shown in Fig. 4. A change in the ECSA gives a further indication of the durability of SiO2/Pt/CNT catalysts. The results are summarized in Fig. 5. The ECSA for fresh Pt/CNT was estimated to be 67 m2 g-Pt 1 but this value decreased sharply as the number of potential cycles increased and the ECSA was finally reduced to 6 m2 g-Pt 1 after 1000 cycles. In contrast, the decrease in ECSA for SiO2/Pt/CNT catalysts during potential cycling was moderate compared with the decrease in ECSA for Pt/CNT. The ECSA for the SiO2/Pt/CNT prepared at pH 10.0 increased slightly from 0 to 300 cycles and the ECSA decreased gradually with the number of potential cycles. On the other hand, the slight increase in ECSAs for the SiO2/Pt/CNT prepared at pH 11.6 was also observed from 0 to 200 cycles and its ECSA was unchanged after 200 cycles. Neyerlin et al. also observed the increase of ECSA of Pt metal in carbon black-supported Pt metal catalysts [38]. The ECSA of Pt metal in Pt/carbon black treated at 1073 K increased at early period of the repeated potential cycling
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Fig. 5. Changes in ECSAs for Pt/CNT (&) and SiO2/Pt/CNT prepared at pH 11.6 (*) and pH 10.0 (^) during repeated potential cycling experiments.
Fig. 4. Cyclic voltammograms of Pt/CNT (a) and SiO2/Pt/CNT prepared at pH 11.6 (b) and pH 10.0 (c) in 0.5 M H2SO4 at 303 K during repeated potential cycling experiments. Scan rate was 50 mV s 1.
experiments, while the ECSA of Pt metal in the Pt catalysts without treatment at high temperatures monotonously decreased during the repeated potential cycling experiments. They ascribed the increase of ECSA of Pt metal to the rearrangement of surface Pt atoms in Pt metal particles during the treatment of the catalysts at high temperatures. SiO2/Pt/CNT catalysts were also treated at 873 K before the potential cycling experiments. Thus, the increase in ECSA of Pt metal in SiO2/Pt/CNT is likely due to the rearrangement of surface Pt atoms during the treatment at high temperatures. It is important to note that the ECSA for the SiO2/Pt/
CNT prepared at pH 10.0 is always higher than that for the SiO2/Pt/ CNT prepared at pH 11.6 although the decrease in ECSA for the former catalyst is only slightly significant compared with the latter catalyst. Therefore, the SiO2/Pt/CNT prepared at pH 10.0 is the preferred cathode catalyst for use in PEFCs. As described earlier, the average size of Pt metal particles in the SiO2/Pt/CNT prepared at pH 10.0 is smaller than that of the SiO2/Pt/ CNT prepared at pH 11.6. This is due to the difference in the density of silica layers, i.e. the porous structure between these two SiO2/Pt/ CNT electrocatalysts. In order to clarify the porous structure of silica layers in two types of SiO2/Pt/CNT catalysts, adsorption experiments of Ar on these catalysts at a boiling temperature of liquid Ar were performed. However, significant difference in porous structures of these SiO2/Pt/CNT could not be observed, due to thin thickness of silica layers (<3 nm). In general, smaller silica particles are obtained when silica is prepared by the hydrolysis of TEOS at pH close to 7. The silica layers which wrap Pt/CNT should be composed of the aggregates of silica particles. Therefore, silica layers with a high density are formed by the preparation of SiO2/Pt/ CNT at pH close to 7. Because the dense silica layers prevent the aggregation of Pt metal particles in SiO2/Pt/CNT during the reduction with hydrogen at 623 K and during subsequent treatment at 873 K under an Ar stream, the average Pt metal particle size of the catalysts prepared at pH 10.0 is smaller than the particle size of the catalysts prepared at pH 11.6. As described earlier, the decrease in the ECSA for the SiO2/Pt/ CNT prepared at pH 10.0 during potential cycling was quicker than that in the ECSA for the SiO2/Pt/CNT prepared at pH 11.6, whereas initial ECSAs of the former catalyst were higher than those of the latter one. This is because of the difference in Pt metal particle sizes between these two SiO2/Pt/CNT electrocatalysts. In general, smaller metal particles are thermodynamically more unstable than larger particles. Smaller Pt metal particles in catalysts dissolve easily to form cationic Pt species during the potential cycling experiments. Thus the ECSA for the SiO2/Pt/CNT prepared at pH 10.0 decreased rapidly compared with that for the SiO2/Pt/CNT prepared at pH 11.6. However, the ECSA of the SiO2/Pt/CNT prepared at pH 10.0 was always higher than that of the SiO2/Pt/CNT prepared at pH 11.6 during potential cycling experiments. As Pt metal particles in the SiO2/Pt/CNT prepared at pH 10.0 are covered with dense silica layers the cationic Pt species from the Pt metal during the potential cycling experiments would diffuse out of the catalyst with difficulty. Cationic Pt species are thus re-deposited on the original Pt metal particles in these catalysts. Therefore the ECSA for the SiO2/Pt/CNT prepared at pH 10.0 was always larger during the potential cycling experiments compared with that for the SiO2/ Pt/CNT prepared at pH 11.6. The SiO2/Pt/CNT that had a higher
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ECSA during potential cycling experiments should be utilized as cathode catalysts in PEFCs because the reduction of Pt metal loading in the catalyst at the cathode is desirable. We thus used the SiO2/Pt/CNT prepared at pH 10.0 hereafter. 3.2. Effect of silica thickness The silica layers that wrapped Pt/CNT improved catalyst durability as described above. The thickness of silica layers in SiO2/Pt/CNT should affect the catalytic performance, because reactant molecules such as water and proton diffuse to the Pt metal surface through silica layers. The thicker silica layers would serve as diffusion barrier for the active Pt sites and deteriorate the reaction. In addition, the coverage with thicker silica layers should decrease the electronic conductivity in the catalysts because silica is insulator. Thus silica layers in SiO2/Pt/CNT catalysts should be as thin as possible to increase ECSA of Pt metal for the catalysts. However, the durability of SiO2/Pt/CNT is also dependent on the thickness of silica layers. When Pt/CNT is covered with thin silica layers low catalyst durability results. Therefore, the ECSA and durability of Pt/CNT catalysts covered with silica layers of different thickness were evaluated to determine the optimum thickness of silica layers. The silica layer thickness in SiO2/Pt/CNT catalysts was controlled by the concentration of TEOS during catalyst preparation. Table 2 shows the content of Pt, SiO2 and CNT in SiO2/Pt/CNT catalysts prepared using different TEOS concentrations. These samples were prepared at pH 10.0. The sample in entry 2 from Table 2 corresponds to that of entry 3 in Table 1. The content of SiO2 in SiO2/Pt/CNT catalysts increased as the concentration of TEOS was increased. Fig. 6 shows TEM images of these SiO2/Pt/CNT catalysts. Panels (a) and (b), (c) and (d) correspond to the catalysts of entry 1 and entry 3 in Table 2, respectively. TEM images for the
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Table 2 The content of Pt, SiO2 and CNT in SiO2/Pt/CNT catalysts prepared with different TEOS concentrations. Entry
TEOS concentration (mol l
1 2 3
0.02 0.06 0.17
1
)
Content (wt%) Pt
SiO2
CNT
5.3 3.7 2.0
26.0 45.2 54.2
68.7 51.1 43.8
catalyst of entry 2 in Table 2 are shown in panels (e) and (f) in Fig. 1. In all samples, Pt/CNT was uniformly covered with silica layers and the size of Pt metal particles was almost the same despite different silica thickness. The thickness of silica layers in SiO2/Pt/CNT catalysts increased when a higher TEOS concentration was used for the preparation of the catalyst. The thickness of silica layers in these samples that were prepared at TEOS concentrations of 0.02, 0.06 and 0.17 mol l 1 was estimated to be ca. 2, 6, and 10 nm, respectively. These results indicate that the thickness of the silica layer in SiO2/Pt/CNT can be controlled by the concentration of TEOS during the preparation of catalysts. Fig. 7 shows CVs of SiO2/Pt/CNT catalysts with silica layers of different thickness. Peaks arising from Pt metal are observed in CVs for all the SiO2/Pt/CNT catalysts. It should be noted that the peak intensity due to Pt metal increases as the thickness of the silica layer decreases. As shown in the TEM images, the size of Pt metal particles in these catalysts is similar. Therefore, the difference in peak intensity for these SiO2/Pt/CNT catalysts did not result from the particle size of Pt metal but from the thickness of silica layers. Fig. 8 shows changes in the CVs for SiO2/Pt/CNT catalysts with silica layers of different thickness during potential cycling experiments. As shown in Fig. 8(a), the peak intensity arising
Fig. 6. TEM images of SiO2/Pt/CNT prepared using a TEOS concentration of 0.02 mol l
1
(a and b) and 0.17 mol l
1
(c and d).
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Fig. 9. Changes in ECSAs for SiO2/Pt/CNT with silica layers of different thickness during repeated potential cycling experiments. Silica layer thickness was 2 nm (&), 6 nm (*) and 10 nm (^).
Fig. 7. Cyclic voltammograms of SiO2/Pt/CNT with silica layers of different thickness in 0.5 M H2SO4 at 303 K. Scan rate was 50 mV s 1.
from Pt metal in the CVs of the SiO2/Pt/CNT with silica layers of 2 nm thickness decreased gradually as the number of potential cycles increased. CVs of the Pt/CNT without silica layers had a similar trend as shown in Fig. 4(a). The peak intensity in the CVs of
the SiO2/Pt/CNT with a 6 nm thick silica layer also decreased slightly with the number of potential cycles as indicated in Fig. 4(c). In contrast, the CV features for the SiO2/Pt/CNT with a 10 nm thick silica layer did not change significantly during the potential cycling experiment as shown in Fig. 8(b). From the CVs shown in Figs. 4(c) and 8, ECSAs of these SiO2/Pt/CNT catalysts for each cycle were determined. Results are shown in Fig. 9. Although the particle size of Pt metal in SiO2/Pt/CNT catalysts of different silica thickness was similar as mentioned earlier, the initial ECSAs for SiO2/Pt/CNT catalysts with thicker silica layers became smaller. Thicker silica layers in SiO2/Pt/CNT should serve as diffusion barrier for the active Pt metal surface and deteriorate the electrochemical reactions. The silica layer thickness of SiO2/Pt/ CNT catalyst also influences the electronic conductivity of the catalyst. In electrochemical reactions over SiO2/Pt/CNT, electrons in the catalysts would flow into external circuit through the exposed CNT surface. The coverage of Pt/CNT with a thicker silica layer would decrease the exposed CNT surface area. Thus, the coverage of Pt/CNT with thicker silica layers decreased the electronic conductivity. Therefore, the ECSA of Pt metal particles in SiO2/Pt/CNT catalysts strongly depends on the thickness of silica layers. On the other hand, the ECSA for the SiO2/Pt/CNT with a 2 nm thick silica layer decreased sharply during potential cycling experiments. The ECSA of the fresh sample was estimated to be 136 m2 g-Pt 1 and this decreased to 23 m2 g-Pt 1 after 1000 cycles. In contrast, the ECSA for the SiO2/Pt/CNT with a 10 nm thick silica layer increased slightly from 0 to 300 cycles and was maintained at ca. 40 m2 g-Pt 1 from 300 cycles onwards. These results indicate that the durability of the Pt/CNT was improved by coverage with a thicker silica layer. By covering Pt/CNT catalysts with thicker silica layers, the cationic Pt species from Pt metal particles cannot diffuse out of the SiO2/Pt/CNT. However, the ECSA of the SiO2/Pt/CNT with a 6 nm thick silica layer was always higher than that of the SiO2/Pt/CNT with a 10 nm thick silica layer during potential cycling experiments. Thus, we conclude that the coverage of Pt/CNT with a 6 nm thick silica layer is preferred. 3.3. Effect of catalyst treatment temperature
Fig. 8. Cyclic voltammograms of SiO2/Pt/CNT with silica layers of 2 nm thickness (a) and 10 nm thickness (b) in 0.5 M H2SO4 and at 303 K during repeated potential cycling. Scan rate was 50 mV s 1.
SiO2/Pt/CNT electrocatalysts showed high durability during repeated potential cycling experiments as described above. Silica layers which envelop Pt metal largely prevent diffusion of cationic Pt species out of the catalysts and an improvement in durability of Pt metal results. Therefore, the density of silica layers in SiO2/Pt/ CNT affects the durability of the catalysts. It is well known that the density of silica strongly depends on the treatment temperatures used for the formation of silica because the polycondensation of
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Fig. 10. TEM images of SiO2/Pt/CNT treated at 623 K (a and b) and 1073 K (c and d).
silica proceeds at higher temperatures [39]. Thus SiO2/Pt/CNT with a 6 nm thick silica layer was treated at various temperatures under an Ar stream. Fig. 10 shows TEM images of SiO2/Pt/CNT treated at 623 K (a and b) and at 1073 K (c and d). TEM images of SiO2/Pt/CNT treated at 873 K are shown in Fig. 1(e) and (f). Pt metal particles in both SiO2/Pt/CNT catalysts, shown in Fig. 10, were observed to be on the CNT and they were covered with silica layers. The silica layers were thus thermally stable up to at least 1073 K. It should be noted that Pt particle sizes of the catalysts in Fig. 10 were similar to each other despite different treatment temperatures used for the catalysts. Fig. 11 shows the distribution of Pt particle sizes and average Pt metal particle sizes estimated using the TEM images of SiO2/Pt/CNT treated at various temperatures. As shown in Fig. 11(a) for the SiO2/Pt/CNT treated at 623 K, the size of Pt metal particles ranged from 1 to 3 nm. The size distribution of Pt metal particles for this catalyst was very narrow. When the treatment temperature for the catalyst increased from 623 to 873 K, the fraction of Pt metal particles with 1 nm diameter decreased slightly and Pt metal particles with 4 nm diameter appeared, as shown in Fig. 2(c). In addition, the fraction of Pt metal particles larger than 4 nm in diameter increased as the treatment temperature for the sample was increased, as shown in Fig. 11(b) for the SiO2/Pt/CNT treated at 1073 K. Based on these Pt metal particle size distributions the average Pt particle size was estimated to be 1.6, 1.7 and 2.3 nm for SiO2/Pt/CNT treated at 623, 873 and 1073 K, respectively. These results indicate that Pt metal particles in SiO2/Pt/CNT aggregate to some extent during treatment under an Ar stream at higher temperatures. Fig. 12 shows CVs for SiO2/Pt/CNT treated at 623, 873 and 1073 K. Peaks due to Pt metal were observed in all CVs and the peak intensity was less for catalysts that were treated at higher
Fig. 11. The distribution of Pt particle sizes estimated by the TEM images for SiO2/Pt/ CNT treated at 623 K (a) and 1073 K (b).
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Fig. 12. Cyclic voltammograms of SiO2/Pt/CNT treated at different temperatures in 0.5 M H2SO4 and at 303 K. Scan rate was 50 mV s 1.
temperatures. ECSAs for the catalysts treated at 623, 873 and 1073 K were estimated, based on their CVs, to be 91, 67 and 39 m2 g-Pt 1, respectively. Pt metal particle sizes were estimated using their ECSAs and found to be 3.1, 4.2 and 7.1 nm for catalysts treated at 623, 873 and 1073 K, respectively. In general, Pt metal particle sizes in carbon-supported Pt catalysts that are estimated from their ECSAs are larger than those estimated from their TEM images because not all surface Pt atoms are electrochemically active [40]. It should be noted that the Pt metal particle size of the SiO2/Pt/CNT, as estimated by its ECSA, that was treated at 623 K (3.1 nm) was different from that for the sample treated at 873 K (4.2 nm), while the particle size of Pt metal estimated from the TEM image for the former catalyst (1.6 nm) was similar to that for the latter catalyst (1.7 nm). In addition, a remarkable difference between the Pt metal particle size, as estimated by their ECSAs, and that estimated by their TEM images was found for samples treated at 1073 K. Denser silica layers in SiO2/Pt/CNT should serve as diffusion barrier for the active Pt sites. Therefore, ECSAs of Pt metal particles in SiO2/Pt/CNT catalysts decrease with the increase in the catalyst treatment temperature, although the size of Pt metal particles in these catalysts does not change appreciably with the treatment temperature. Fig. 13 shows changes in the CVs for SiO2/Pt/CNT treated at 623 and 1073 K during potential cycling experiments. The peak intensity in the CVs for the SiO2/Pt/CNT treated at 623 K gradually decreased as the number of potential cycles increased, as shown in Fig. 13(a). In contrast, the peak intensity for the SiO2/Pt/CNT treated at 1073 K did not change after 300 cycles although the peak intensity increased up to 300 cycles as described in Fig. 13(b). This suggests that the SiO2/Pt/CNT treated at higher temperatures shows high durability. From the CVs shown in Fig. 13 the ECSAs for these catalysts at each cycle were determined. Results are shown in Fig. 14. The SiO2/Pt/CNT treated at 623 K had the largest initial ECSA (91 m2 g-Pt 1) during potential cycling among the examined samples, as shown in Fig. 14. However, the ECSA decreased constantly as the number of potential cycles increased and it was finally reduced to 41 m2 g-Pt 1 after 1000 cycles. On the other hand, the ECSA for the SiO2/Pt/CNT treated at 1073 K gradually increased from 0 to 400 cycles and it obtained a constant value of ca. 60 m2 g-Pt 1 after 400 cycles. It should be noted that the SiO2/ Pt/CNT treated at 1073 K had the largest ECSA after 700 cycles among all the catalysts. Therefore, the dense silica layer which
Fig. 13. Cyclic voltammograms of SiO2/Pt/CNT treated at 623 K (a) and 1073 K (b) in 0.5 M H2SO4 at 303 K during repeated potential cycling. Scan rate was 50 mV s 1.
wraps Pt metal particles largely prevents the diffusion of cationic Pt species out of the SiO2/Pt/CNT. For this reason, the coverage of Pt/CNT with a dense silica layer is more effective in improving the durability of Pt metal.
Fig. 14. Changes in ECSAs of SiO2/Pt/CNT treated at different temperatures during repeated potential cycling. Treatment temperatures were 623 K (&), 873 K (^) and 1073 K (*).
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4. Conclusion We investigated the effects of thickness and porous structure of the silica layer in SiO2/Pt/CNT catalysts on their ECSA and durability. Coverage of Pt/CNT with thicker and denser silica layers improved their durability. These silica layers prevented the diffusion of cationic Pt species out of the SiO2/Pt/CNT. However, coverage of Pt/CNT with silica layers that were too thick significantly decreased their ECSA. Thus coverage of Pt/CNT with a dense silica layer of adequate thickness (= 6 nm) is optimal for the ECSA and excellent durability. Acknowledgments The present work was supported by a Grant-in-Aid of Scientific Research (B) (No. 20360365) and by a Grant-in-Aid for the Global COE Program, ‘‘Science for Future Molecular Systems’’ from the Ministry of Education, Culture, Science, Sports and Technology of Japan. References [1] R. Jasinski, Nature 201 (1964) 1212–1213. [2] J.T. Glass, G.L. Cahen Jr., G.E. Stoner, J. Electrochem. Soc. 134 (1987) 58–65. [3] M. Watanabe, K. Tsurumi, T. Mizukami, T. Nakamura, P. Stonehart, J. Electrochem. Soc. 141 (1994) 2659–2668. [4] S. Mukerjee, S. Srinivasan, M.P. Soriaga, J. Electrochem. Soc. 142 (1995) 1409– 1422. [5] T. Toda, H. Igarashi, H. Uchida, M. Watanabe, J. Electrochem. Soc. 146 (1999) 3750–3756. [6] O. Savadogo, K. Lee, K. Oishi, S. Mitsushima, N. Kamiya, K. Ota, Electrochem. Commun. 6 (2004) 105–109. [7] J.L. Ferna´ndez, D.A. Walsh, A.J. Bard, J. Am. Chem. Soc. 127 (2005) 357–365. [8] A. Ishihara, K. Lee, S. Doi, S. Mitsushima, N. Kamiya, M. Hara, K. Domen, K. Fukuda, K. Ota, Electrochem. Solid State Lett. 8 (2005) A201–A203. [9] Y. Takasu, N. Yoshinaga, W. Sugimoto, Electrochem. Commun. 10 (2008) 668–672. [10] R. Bashyam, P. Zelenay, Nature 443 (2006) 63–66. [11] J. Maruyama, J. Okamura, K. Miyazaki, Y. Uchimoto, I. Abe, J. Phys. Chem. C 112 (2008) 2784–2790.
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Preparation of carbon nanotube-supported Pt catalysts covered with silica layers; application to cathode catalysts for PEFC Hiroshi Matsumori, Sakae Takenaka *, Hideki Matsune, Masahiro Kishida * Department of Chemical Engineering, Graduate School of Engineering, Kyushu University, Moto-oka 744, Nishi-ku, Fukuoka 819-0395, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 July 2009 Received in revised form 4 November 2009 Accepted 10 November 2009 Available online 14 November 2009
Multi-walled carbon nanotube-supported Pt metal particle (Pt/CNT) catalysts were covered with silica layers with different thickness and density. In the present study, the Pt/CNT covered with silica layers (SiO2/Pt/CNT) was prepared by the successive hydrolysis of 3-aminopropyl-triethoxysilane (APTS) and tetraethoxysilane (TEOS). The thickness of silica layers in SiO2/Pt/CNT could be controlled by varying the concentration of silica sources during the preparation of SiO2/Pt/CNT. The density of silica layers in SiO2/ Pt/CNT was changed by varying the treatment temperature for the catalysts as well as by adjusting the pH for the hydrolysis of silica sources during the preparation of the catalysts. The catalytic performance of these SiO2/Pt/CNT as cathode catalysts in polymer electrolyte fuel cells was examined. Pt/CNT covered with silica layers of adequate thickness (ca. 6 nm) and high density had a high electrochemically active surface area as well as excellent durability. Dense silica layers in SiO2/Pt/CNT prevented the diffusion of cationic Pt species dissolved from Pt metal particles out of the catalysts. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Carbon-supported Pt electrocatalysts Polymer electrolyte fuel cells Catalytic stability Silica coating
1. Introduction Polymer electrolyte fuel cells (PEFCs) are promising alternative power sources because of their low emission and high energy efficiency. However, many issues need to be resolved before the commercialization of PEFCs. The cost of PEFC systems is a serious problem since Pt metal, which functions as catalytically active sites in anode and cathode electrocatalysts within PEFCs, is very expensive and is a scarce resource. Therefore, the total Pt loading in a membrane electrode assembly (MEA) to be used in PEFCs should be as low as possible. To reduce the Pt loading in PEFCs, Pt-alloys or non-noble metal electrocatalysts with high activity for the oxygen reduction reaction (ORR) should be developed. The ORR at a cathode is kinetically sluggish compared with the hydrogen oxidation reaction at an anode. Many Pt-based alloy catalysts such as Pt–Co, Pt–Ni and Pt–Fe have been reported to show high activity for the ORR [1–5] despite a lower Pt loading in Pt-alloy catalysts compared with pure Pt catalysts. As for non-noble metal catalysts, alloy catalysts without Pt were reported to show high activity for the ORR [6,7]. Recently, Pt-free metal oxide catalysts [8,9] and MeNx (Me = Co, Fe) electrocatalysts [10,11] such as metals coordinated to porphyrins or pyrrole have been developed as active cathode catalysts for use in PEFCs. On the other hand, the durability of Pt electrocatalysts at the cathode has been recognized as one of the most important issues when reducing the Pt loading
* Corresponding authors. Tel.: +81 92 802 2752; fax: +81 92 802 2752. E-mail address: [email protected] (S. Takenaka). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.11.011
in a MEA [12]. Pt electrocatalysts at the cathode are deactivated during PEFC operation because Pt cathode catalysts are exposed to severe conditions such as low pH [13], high temperatures [14], high humidity [15] and high potential [16]. These conditions accelerate the reduction of the electrochemically active surface area (ECSA) of the Pt metal in these electrocatalysts. Thus high Pt loading is required in MEAs to maintain good PEFC performance. Three different mechanisms are generally accepted that explain the reduction of the ECSA of Pt metal in carbon-supported Pt catalysts at the cathode. First, Pt metal particles migrate on carbon supports and then aggregate [17]. Second, Pt metal particle sizes increase by Ostwald ripening; surface Pt atoms on small Pt metal particles dissolve to form cationic Pt species and are re-deposited on other larger Pt particles, which results in an increase in Pt metal particle sizes [18,19]. Third, carbon supports for Pt metal particles are corroded by oxidation [20,21]. Corrosion of carbon supports prohibits electrochemical contact between them and Pt metals. Additionally, when carbon supports have been corroded Pt metal particles aggregate easily. It is also well accepted that these phenomena are accelerated by the repeated on–off cycles of PEFC [22]. Much work has been done to improve the durability of Pt electrocatalysts such as the addition of transition metals like Ni or Co and the addition of precious metals like Au [23–25]. In fact, the durability of Pt electrocatalysts is improved to some extent by the addition of another metal. However, it is difficult to obtain alloy catalysts of small particle size since these catalysts must be treated at high temperatures for alloy formation. Currently, the durability of Pt-alloy catalysts is not satisfactory because added metal species dissolve under cathodic conditions [26–30]. Dissolved metal
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species from Pt-alloy catalysts deposit in a proton exchange membrane electrolyte, which decreases the proton conductivity of the electrolyte [31]. Thus new methods to improve the durability of carbon-supported Pt metal electrocatalysts are required. Recently, we demonstrated that multi-walled carbon nanotube (CNT)-supported Pt metal particles covered with silica (SiO2/Pt/ CNT) showed excellent durability in electrochemical reactions in aqueous acid electrolytes [32]. Pt metal particles in SiO2/Pt/CNT do not migrate on CNT supports because these particles are uniformly covered with silica. Furthermore, when cationic Pt species are formed from the Pt metal particle in SiO2/Pt/CNT catalysts under PEFC conditions, the cationic Pt species do not easily diffuse out of the catalyst since these Pt metal particles are covered with silica layers. Therefore, Pt metal particles in SiO2/Pt/CNT catalysts do not change in size during the PEFC operation. The durability of Pt/CNT is predicted to be improved by coverage with thicker and denser silica layers. However, the coverage of Pt/CNT electrocatalysts with these silica layers probably reduces their electrochemical activity because reactant molecules such as water and oxygen need to be supplied through silica layers in SiO2/Pt/CNT and silica is an insulator. In this study, we investigated the effects of thickness and density of silica layers in SiO2/Pt/CNT catalysts on their durability.
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tively. All electrode potentials in this study are referenced against the reversible hydrogen electrode (RHE). The working electrode was prepared according to the method reported by Schmidt et al. [35]. The working electrode was a thin film electrode attached to a mirror finished glassy carbon rod (GC, diameter 5 mm). 20 mg of the catalyst was added to 10 ml methanol and a small amount of water and this suspension was treated ultrasonically for 30 min. 20 ml of this ink was dropped on GC and then dried at 333 K. 20 ml of a 1 wt% Nafion solution was further pipetted onto the electrode to attach the electrocatalysts onto the GC. Cyclic voltammograms (CV) were measured in N2-purged 0.5 M H2SO4 at 303 K using a potentiostat and galvanostat (HOKUTO Denko Corp. HZ5000). The voltage was changed by a triangular wave between 0.05 and 1.20 V (vs. RHE) at a scan rate of 50 mV s 1. Before CV measurements 50 voltage sweeps were undertaken to obtain highly reproducible CVs. The electrochemically active surface area (ECSA) of Pt metal particles in the catalysts was calculated from their CVs by the amount of charge needed for the underpotential deposition of protons onto the Pt metal surface. The columbic charge necessary for proton deposition is 210 mC cm-Pt 2. To evaluate the durability of electrocatalysts the working electrode’s voltage was changed repeatedly between 0.05 and 1.20 V at a scan rate of 50 mV s 1 and then the CV was obtained.
2. Experimental 3. Results and discussion 2.1. Preparation of catalysts 3.1. Effect of pH on hydrolysis Multi-walled CNTs (diameter = 20–30 nm, length = 5–200 mm, purchased from Aldrich) were washed with 3.6 M HCl at 353 K for 3 h to remove metal impurities. These CNTs were dispersed in a mixed solution of 8.0 M H2SO4 and 8.0 M HNO3 and then treated ultrasonically at 328 K for 2 h to introduce functional groups such as hydroxyls and carboxyls onto CNT surfaces [33]. Oxidized CNTs were immersed in an aqueous solution containing H2PtCl6 [34]. The pH value of the solution was adjusted to ca. 11 by the addition of aqueous NH3 and Pt metal precursors were thus deposited onto CNT surfaces. After this solution was filtered, the samples were dispersed in a mixture of ethanol and water (volume ratio 1:1). To cover these samples with silica, 3-aminopropyl-triethoxysilane (APTS) and aqueous NH3 were added to the solution and stirred at 333 K for 0.5 h. Hydrolysis of tetraethoxysilane (TEOS) was then performed at the same temperature for 1 h. After centrifuging and drying the samples at 333 K they were treated at 623 K for 3 h under a H2 stream. Finally, the obtained samples were heated under a stream of Ar at different temperatures (623, 873 or 1073 K) to control the density of silica layers in the catalysts. CNT-supported Pt (Pt/CNT) electrocatalysts without silica layers were prepared by a conventional impregnation method. CNTs were dispersed in an alcohol solution with an appropriate amount of H2PtCl6. The suspension was stirred and dried at 333 K. The samples thus obtained were reduced under a H2 stream at 473 K for 5 h. 2.2. Characterization of catalysts The morphology of the catalysts was investigated using transmission electron microscopy (TEM) on a JEOL 3200FSK operated at 300 kV. The content of Pt and Si in SiO2/Pt/CNT was measured using inductively coupled plasma-atomic emission spectroscopy (Optima 5300). 2.3. Electrochemical measurements Electrochemical measurements were performed in a threecompartment glass cell with a Pt mesh and a saturated Ag/AgCl electrode as counter electrode and reference electrode, respec-
Silica is usually prepared by the hydrolysis of a silicon alkoxide such as TEOS. In this study, silica layers in SiO2/Pt/CNT were prepared by the successive hydrolysis of APTS and TEOS in the presence of CNT-supported Pt precursors. APTS is adsorbed on the surface of CNT by the interaction of an amino group and graphene to form very thin silica layers [36]. Subsequently, this sample is covered with thick silica layers by the hydrolysis of TEOS. It is well known that pH values during the hydrolysis of silicon alkoxides affect their hydrolysis rates. We prepared SiO2/Pt/CNT catalysts at different pH using aqueous NH3 as a catalyst for the hydrolysis of APTS and TEOS. Primary silica particles formed from the hydrolysis of TEOS at higher pH are larger in size, since the hydrolysis rate of TEOS is faster at higher pH. The aggregation of silica particles on CNT surfaces to form silica layers results in the formation of porous structure in silica layers. Thus, it is expected that the porous structure of silica layers in SiO2/Pt/CNT strongly depends on the pH values during the hydrolysis of TEOS. Table 1 shows the content of SiO2, Pt and CNT in Pt/CNT as well as SiO2/Pt/CNT catalysts prepared at different pH. The content of SiO2 and Pt in these SiO2/ Pt/CNT catalysts did not depend on the pH values during catalyst preparation. Fig. 1 shows TEM images of Pt/CNT and SiO2/Pt/CNT catalysts that were prepared at different pH. Panels (a) and (b) show TEM images for Pt/CNT while panels (c) and (d) and panels (e) and (f) correspond to TEM images for SiO2/Pt/CNT catalysts prepared at pH values of 11.6 and 10.0, respectively. Both SiO2/Pt/ CNT catalysts were treated at 873 K under Ar before the measurement of their TEM images. Pt metal particles were observed on the outer surfaces of CNTs in TEM images of Pt/ Table 1 The content of SiO2, Pt and CNT in Pt/CNT and SiO2/Pt/CNT prepared at different pH values. Entry
1 2 3
Sample
Pt/CNT SiO2/Pt/CNT SiO2/Pt/CNT
pH value
– 11.6 10.0
Content (wt%) Pt
SiO2
CNT
7.0 3.4 3.7
– 46.5 45.2
93.0 50.1 51.1
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Fig. 1. TEM images of Pt/CNT (a and b) and SiO2/Pt/CNT prepared at a pH of 11.6 (c and d) and 10.0 (e and f).
CNT. The size of Pt metal particles in Pt/CNT ranged from 1 to 5 nm. Pt metal particles were also observed on outer surfaces of CNTs in TEM images of both SiO2/Pt/CNT catalysts. The Pt metal particles and CNTs in both SiO2/Pt/CNT catalysts were uniformly covered with thin silica layers [33]. Pt metal particle sizes in the SiO2/Pt/ CNT prepared at pH 11.6 ranged widely from 1 to 7 nm while Pt metal particle sizes in the SiO2/Pt/CNT prepared at pH 10.0 was relatively uniform (1–3 nm). It should be noted that the thickness of the silica layer in the SiO2/Pt/CNT prepared at pH 11.6 was not uniform, and exposed Pt metal particles as well as exposed CNT surfaces were observed. Furthermore, large spherical particles of silica were observed in the TEM image of the SiO2/Pt/CNT prepared at pH 11.6. For the SiO2/Pt/CNT prepared at pH 10.0, most Pt and CNT surfaces were covered with silica layers of relatively uniform thickness. For the preparation of SiO2/Pt/CNT by successive hydrolysis of APTS and TEOS, APTS is initially adsorbed on the surface of CNTs to form thin silica layers. Subsequently, the silica
that is formed from TEOS is deposited on CNTs that are covered with thin silica layers from APTS through heterogeneous nucleation because the thin silica layers from APTS work as the nucleation sites for the formation of thick silica layers from TEOS. However, silica particles from TEOS are formed by homogenous nucleation in addition to heterogeneous nucleation when the pH during the hydrolysis of TEOS is higher, since the hydrolysis rate of TEOS increases at higher pH. Homogenous nucleation of silica should result in the formation of larger silica particles compared with heterogeneous nucleation and large silica particles are deposited on CNT surfaces to form silica layers. Therefore, silica layers in the SiO2/Pt/CNT prepared at pH 11.6 are not uniform in thickness. Additionally, the formation of silica through homogenous nucleation results in the formation of large spherical particles of silica. Fig. 2 shows particle size distributions and average particle sizes of the Pt metal estimated from TEM images of Pt/CNT and SiO2/Pt/CNT catalysts prepared at pH 11.6 and pH 10.0. Pt metal
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Fig. 3. Cyclic voltammograms of Pt/CNT (a) and SiO2/Pt/CNT prepared at pH 11.6 (b) and pH 10.0 (c) in 0.5 M H2SO4 at 303 K. Scan rate was 50 mV s 1.
Fig. 2. The particle size distribution and the average diameter of Pt metal particles estimated from their TEM images for Pt/CNT (a) and SiO2/Pt/CNT prepared at pH 11.6 (b) and 10.0 (c).
particle sizes ranged from 1 to 8 nm and the average particle size of the Pt metal was 3.0 nm as shown in Fig. 2(a) for Pt/CNT. The size distribution of Pt metal particles for the Pt/CNT was relatively broad. On the other hand, the size distributions of Pt metal particles in both SiO2/Pt/CNT catalysts were narrower than that in the Pt/CNT as shown in Fig. 2(b) and (c). In these catalysts, the size distribution of Pt metal particles was dependent on the initial pH used for their preparation. The size of Pt metal particles in the SiO2/ Pt/CNT prepared at pH 10.0 ranged from 1 to 4 nm while Pt metal particles larger than 5 nm in diameter were observed in the SiO2/ Pt/CNT prepared at pH 11.6. The average diameter of Pt metal particles in the SiO2/Pt/CNT prepared at pH 10.0 (1.7 nm) was smaller than that in the SiO2/Pt/CNT prepared at pH 11.6 (2.9 nm). Cyclic voltammograms of Pt/CNT and SiO2/Pt/CNT catalysts prepared at different pH values were measured to evaluate their electrochemical performance. These results are shown in Fig. 3. The CV for the Pt/CNT has two peaks that are due to the oxidation– reduction of Pt metal (0.6–1.2 V) and the adsorption–desorption of hydrogen on Pt metal surface (0.05–0.4 V). Interestingly, these peaks due to Pt are also found in the CVs of both SiO2/Pt/CNT catalysts despite the coverage of Pt metal with silica layers. These results indicate that Pt metal particles in both SiO2/Pt/CNT
electrocatalysts are electrochemically active [32,37]. It is likely that reactants such as protons and water pass through silica layers which have a porous structure and are thus supplied to Pt metal surface in SiO2/Pt/CNT. Electrons are likely supplied to Pt metal particles through the CNT surface that is not covered with silica. Therefore, SiO2/Pt/CNT catalysts are electrochemically active despite the coverage of Pt metal particles with the silica insulator. CV peak intensity depends strongly on the pH used for the preparation of SiO2/Pt/CNT catalysts. The peak intensity of the catalyst prepared at pH 10.0 is higher than that of the catalyst prepared at pH 11.6. This is due to Pt metal particles in the SiO2/Pt/ CNT prepared at pH 11.6 being larger in size than those in the SiO2/ Pt/CNT prepared at pH 10.0, as shown in Fig. 2. Fig. 4 shows CVs for Pt/CNT and SiO2/Pt/CNT catalysts during repeated potential cycling experiments from 0.05 to 1.20 V. The CV peak intensity gradually decreases with the number of potential cycles for the Pt/CNT, which is not covered with silica layers, as shown in Fig. 4(a). CVs for SiO2/Pt/CNT catalysts prepared at pH 11.6 and pH 10.0 are shown in Fig. 4(b) and (c), respectively. The peaks in these CVs also show a slight decrease in intensity during potential cycling experiments. However, the decrease in peak intensity of both SiO2/Pt/CNT catalysts during the repeated potential cycling experiments is negligible compared with that observed for Pt/CNT. These results indicate that the Pt electrocatalyst durability is improved by coverage with silica. The change in ECSA of Pt metal particles for each Pt catalyst during potential cycling was evaluated from the CVs shown in Fig. 4. A change in the ECSA gives a further indication of the durability of SiO2/Pt/CNT catalysts. The results are summarized in Fig. 5. The ECSA for fresh Pt/CNT was estimated to be 67 m2 g-Pt 1 but this value decreased sharply as the number of potential cycles increased and the ECSA was finally reduced to 6 m2 g-Pt 1 after 1000 cycles. In contrast, the decrease in ECSA for SiO2/Pt/CNT catalysts during potential cycling was moderate compared with the decrease in ECSA for Pt/CNT. The ECSA for the SiO2/Pt/CNT prepared at pH 10.0 increased slightly from 0 to 300 cycles and the ECSA decreased gradually with the number of potential cycles. On the other hand, the slight increase in ECSAs for the SiO2/Pt/CNT prepared at pH 11.6 was also observed from 0 to 200 cycles and its ECSA was unchanged after 200 cycles. Neyerlin et al. also observed the increase of ECSA of Pt metal in carbon black-supported Pt metal catalysts [38]. The ECSA of Pt metal in Pt/carbon black treated at 1073 K increased at early period of the repeated potential cycling
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Fig. 5. Changes in ECSAs for Pt/CNT (&) and SiO2/Pt/CNT prepared at pH 11.6 (*) and pH 10.0 (^) during repeated potential cycling experiments.
Fig. 4. Cyclic voltammograms of Pt/CNT (a) and SiO2/Pt/CNT prepared at pH 11.6 (b) and pH 10.0 (c) in 0.5 M H2SO4 at 303 K during repeated potential cycling experiments. Scan rate was 50 mV s 1.
experiments, while the ECSA of Pt metal in the Pt catalysts without treatment at high temperatures monotonously decreased during the repeated potential cycling experiments. They ascribed the increase of ECSA of Pt metal to the rearrangement of surface Pt atoms in Pt metal particles during the treatment of the catalysts at high temperatures. SiO2/Pt/CNT catalysts were also treated at 873 K before the potential cycling experiments. Thus, the increase in ECSA of Pt metal in SiO2/Pt/CNT is likely due to the rearrangement of surface Pt atoms during the treatment at high temperatures. It is important to note that the ECSA for the SiO2/Pt/
CNT prepared at pH 10.0 is always higher than that for the SiO2/Pt/ CNT prepared at pH 11.6 although the decrease in ECSA for the former catalyst is only slightly significant compared with the latter catalyst. Therefore, the SiO2/Pt/CNT prepared at pH 10.0 is the preferred cathode catalyst for use in PEFCs. As described earlier, the average size of Pt metal particles in the SiO2/Pt/CNT prepared at pH 10.0 is smaller than that of the SiO2/Pt/ CNT prepared at pH 11.6. This is due to the difference in the density of silica layers, i.e. the porous structure between these two SiO2/Pt/ CNT electrocatalysts. In order to clarify the porous structure of silica layers in two types of SiO2/Pt/CNT catalysts, adsorption experiments of Ar on these catalysts at a boiling temperature of liquid Ar were performed. However, significant difference in porous structures of these SiO2/Pt/CNT could not be observed, due to thin thickness of silica layers (<3 nm). In general, smaller silica particles are obtained when silica is prepared by the hydrolysis of TEOS at pH close to 7. The silica layers which wrap Pt/CNT should be composed of the aggregates of silica particles. Therefore, silica layers with a high density are formed by the preparation of SiO2/Pt/ CNT at pH close to 7. Because the dense silica layers prevent the aggregation of Pt metal particles in SiO2/Pt/CNT during the reduction with hydrogen at 623 K and during subsequent treatment at 873 K under an Ar stream, the average Pt metal particle size of the catalysts prepared at pH 10.0 is smaller than the particle size of the catalysts prepared at pH 11.6. As described earlier, the decrease in the ECSA for the SiO2/Pt/ CNT prepared at pH 10.0 during potential cycling was quicker than that in the ECSA for the SiO2/Pt/CNT prepared at pH 11.6, whereas initial ECSAs of the former catalyst were higher than those of the latter one. This is because of the difference in Pt metal particle sizes between these two SiO2/Pt/CNT electrocatalysts. In general, smaller metal particles are thermodynamically more unstable than larger particles. Smaller Pt metal particles in catalysts dissolve easily to form cationic Pt species during the potential cycling experiments. Thus the ECSA for the SiO2/Pt/CNT prepared at pH 10.0 decreased rapidly compared with that for the SiO2/Pt/CNT prepared at pH 11.6. However, the ECSA of the SiO2/Pt/CNT prepared at pH 10.0 was always higher than that of the SiO2/Pt/CNT prepared at pH 11.6 during potential cycling experiments. As Pt metal particles in the SiO2/Pt/CNT prepared at pH 10.0 are covered with dense silica layers the cationic Pt species from the Pt metal during the potential cycling experiments would diffuse out of the catalyst with difficulty. Cationic Pt species are thus re-deposited on the original Pt metal particles in these catalysts. Therefore the ECSA for the SiO2/Pt/CNT prepared at pH 10.0 was always larger during the potential cycling experiments compared with that for the SiO2/ Pt/CNT prepared at pH 11.6. The SiO2/Pt/CNT that had a higher
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ECSA during potential cycling experiments should be utilized as cathode catalysts in PEFCs because the reduction of Pt metal loading in the catalyst at the cathode is desirable. We thus used the SiO2/Pt/CNT prepared at pH 10.0 hereafter. 3.2. Effect of silica thickness The silica layers that wrapped Pt/CNT improved catalyst durability as described above. The thickness of silica layers in SiO2/Pt/CNT should affect the catalytic performance, because reactant molecules such as water and proton diffuse to the Pt metal surface through silica layers. The thicker silica layers would serve as diffusion barrier for the active Pt sites and deteriorate the reaction. In addition, the coverage with thicker silica layers should decrease the electronic conductivity in the catalysts because silica is insulator. Thus silica layers in SiO2/Pt/CNT catalysts should be as thin as possible to increase ECSA of Pt metal for the catalysts. However, the durability of SiO2/Pt/CNT is also dependent on the thickness of silica layers. When Pt/CNT is covered with thin silica layers low catalyst durability results. Therefore, the ECSA and durability of Pt/CNT catalysts covered with silica layers of different thickness were evaluated to determine the optimum thickness of silica layers. The silica layer thickness in SiO2/Pt/CNT catalysts was controlled by the concentration of TEOS during catalyst preparation. Table 2 shows the content of Pt, SiO2 and CNT in SiO2/Pt/CNT catalysts prepared using different TEOS concentrations. These samples were prepared at pH 10.0. The sample in entry 2 from Table 2 corresponds to that of entry 3 in Table 1. The content of SiO2 in SiO2/Pt/CNT catalysts increased as the concentration of TEOS was increased. Fig. 6 shows TEM images of these SiO2/Pt/CNT catalysts. Panels (a) and (b), (c) and (d) correspond to the catalysts of entry 1 and entry 3 in Table 2, respectively. TEM images for the
181
Table 2 The content of Pt, SiO2 and CNT in SiO2/Pt/CNT catalysts prepared with different TEOS concentrations. Entry
TEOS concentration (mol l
1 2 3
0.02 0.06 0.17
1
)
Content (wt%) Pt
SiO2
CNT
5.3 3.7 2.0
26.0 45.2 54.2
68.7 51.1 43.8
catalyst of entry 2 in Table 2 are shown in panels (e) and (f) in Fig. 1. In all samples, Pt/CNT was uniformly covered with silica layers and the size of Pt metal particles was almost the same despite different silica thickness. The thickness of silica layers in SiO2/Pt/CNT catalysts increased when a higher TEOS concentration was used for the preparation of the catalyst. The thickness of silica layers in these samples that were prepared at TEOS concentrations of 0.02, 0.06 and 0.17 mol l 1 was estimated to be ca. 2, 6, and 10 nm, respectively. These results indicate that the thickness of the silica layer in SiO2/Pt/CNT can be controlled by the concentration of TEOS during the preparation of catalysts. Fig. 7 shows CVs of SiO2/Pt/CNT catalysts with silica layers of different thickness. Peaks arising from Pt metal are observed in CVs for all the SiO2/Pt/CNT catalysts. It should be noted that the peak intensity due to Pt metal increases as the thickness of the silica layer decreases. As shown in the TEM images, the size of Pt metal particles in these catalysts is similar. Therefore, the difference in peak intensity for these SiO2/Pt/CNT catalysts did not result from the particle size of Pt metal but from the thickness of silica layers. Fig. 8 shows changes in the CVs for SiO2/Pt/CNT catalysts with silica layers of different thickness during potential cycling experiments. As shown in Fig. 8(a), the peak intensity arising
Fig. 6. TEM images of SiO2/Pt/CNT prepared using a TEOS concentration of 0.02 mol l
1
(a and b) and 0.17 mol l
1
(c and d).
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Fig. 9. Changes in ECSAs for SiO2/Pt/CNT with silica layers of different thickness during repeated potential cycling experiments. Silica layer thickness was 2 nm (&), 6 nm (*) and 10 nm (^).
Fig. 7. Cyclic voltammograms of SiO2/Pt/CNT with silica layers of different thickness in 0.5 M H2SO4 at 303 K. Scan rate was 50 mV s 1.
from Pt metal in the CVs of the SiO2/Pt/CNT with silica layers of 2 nm thickness decreased gradually as the number of potential cycles increased. CVs of the Pt/CNT without silica layers had a similar trend as shown in Fig. 4(a). The peak intensity in the CVs of
the SiO2/Pt/CNT with a 6 nm thick silica layer also decreased slightly with the number of potential cycles as indicated in Fig. 4(c). In contrast, the CV features for the SiO2/Pt/CNT with a 10 nm thick silica layer did not change significantly during the potential cycling experiment as shown in Fig. 8(b). From the CVs shown in Figs. 4(c) and 8, ECSAs of these SiO2/Pt/CNT catalysts for each cycle were determined. Results are shown in Fig. 9. Although the particle size of Pt metal in SiO2/Pt/CNT catalysts of different silica thickness was similar as mentioned earlier, the initial ECSAs for SiO2/Pt/CNT catalysts with thicker silica layers became smaller. Thicker silica layers in SiO2/Pt/CNT should serve as diffusion barrier for the active Pt metal surface and deteriorate the electrochemical reactions. The silica layer thickness of SiO2/Pt/ CNT catalyst also influences the electronic conductivity of the catalyst. In electrochemical reactions over SiO2/Pt/CNT, electrons in the catalysts would flow into external circuit through the exposed CNT surface. The coverage of Pt/CNT with a thicker silica layer would decrease the exposed CNT surface area. Thus, the coverage of Pt/CNT with thicker silica layers decreased the electronic conductivity. Therefore, the ECSA of Pt metal particles in SiO2/Pt/CNT catalysts strongly depends on the thickness of silica layers. On the other hand, the ECSA for the SiO2/Pt/CNT with a 2 nm thick silica layer decreased sharply during potential cycling experiments. The ECSA of the fresh sample was estimated to be 136 m2 g-Pt 1 and this decreased to 23 m2 g-Pt 1 after 1000 cycles. In contrast, the ECSA for the SiO2/Pt/CNT with a 10 nm thick silica layer increased slightly from 0 to 300 cycles and was maintained at ca. 40 m2 g-Pt 1 from 300 cycles onwards. These results indicate that the durability of the Pt/CNT was improved by coverage with a thicker silica layer. By covering Pt/CNT catalysts with thicker silica layers, the cationic Pt species from Pt metal particles cannot diffuse out of the SiO2/Pt/CNT. However, the ECSA of the SiO2/Pt/CNT with a 6 nm thick silica layer was always higher than that of the SiO2/Pt/CNT with a 10 nm thick silica layer during potential cycling experiments. Thus, we conclude that the coverage of Pt/CNT with a 6 nm thick silica layer is preferred. 3.3. Effect of catalyst treatment temperature
Fig. 8. Cyclic voltammograms of SiO2/Pt/CNT with silica layers of 2 nm thickness (a) and 10 nm thickness (b) in 0.5 M H2SO4 and at 303 K during repeated potential cycling. Scan rate was 50 mV s 1.
SiO2/Pt/CNT electrocatalysts showed high durability during repeated potential cycling experiments as described above. Silica layers which envelop Pt metal largely prevent diffusion of cationic Pt species out of the catalysts and an improvement in durability of Pt metal results. Therefore, the density of silica layers in SiO2/Pt/ CNT affects the durability of the catalysts. It is well known that the density of silica strongly depends on the treatment temperatures used for the formation of silica because the polycondensation of
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183
Fig. 10. TEM images of SiO2/Pt/CNT treated at 623 K (a and b) and 1073 K (c and d).
silica proceeds at higher temperatures [39]. Thus SiO2/Pt/CNT with a 6 nm thick silica layer was treated at various temperatures under an Ar stream. Fig. 10 shows TEM images of SiO2/Pt/CNT treated at 623 K (a and b) and at 1073 K (c and d). TEM images of SiO2/Pt/CNT treated at 873 K are shown in Fig. 1(e) and (f). Pt metal particles in both SiO2/Pt/CNT catalysts, shown in Fig. 10, were observed to be on the CNT and they were covered with silica layers. The silica layers were thus thermally stable up to at least 1073 K. It should be noted that Pt particle sizes of the catalysts in Fig. 10 were similar to each other despite different treatment temperatures used for the catalysts. Fig. 11 shows the distribution of Pt particle sizes and average Pt metal particle sizes estimated using the TEM images of SiO2/Pt/CNT treated at various temperatures. As shown in Fig. 11(a) for the SiO2/Pt/CNT treated at 623 K, the size of Pt metal particles ranged from 1 to 3 nm. The size distribution of Pt metal particles for this catalyst was very narrow. When the treatment temperature for the catalyst increased from 623 to 873 K, the fraction of Pt metal particles with 1 nm diameter decreased slightly and Pt metal particles with 4 nm diameter appeared, as shown in Fig. 2(c). In addition, the fraction of Pt metal particles larger than 4 nm in diameter increased as the treatment temperature for the sample was increased, as shown in Fig. 11(b) for the SiO2/Pt/CNT treated at 1073 K. Based on these Pt metal particle size distributions the average Pt particle size was estimated to be 1.6, 1.7 and 2.3 nm for SiO2/Pt/CNT treated at 623, 873 and 1073 K, respectively. These results indicate that Pt metal particles in SiO2/Pt/CNT aggregate to some extent during treatment under an Ar stream at higher temperatures. Fig. 12 shows CVs for SiO2/Pt/CNT treated at 623, 873 and 1073 K. Peaks due to Pt metal were observed in all CVs and the peak intensity was less for catalysts that were treated at higher
Fig. 11. The distribution of Pt particle sizes estimated by the TEM images for SiO2/Pt/ CNT treated at 623 K (a) and 1073 K (b).
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Fig. 12. Cyclic voltammograms of SiO2/Pt/CNT treated at different temperatures in 0.5 M H2SO4 and at 303 K. Scan rate was 50 mV s 1.
temperatures. ECSAs for the catalysts treated at 623, 873 and 1073 K were estimated, based on their CVs, to be 91, 67 and 39 m2 g-Pt 1, respectively. Pt metal particle sizes were estimated using their ECSAs and found to be 3.1, 4.2 and 7.1 nm for catalysts treated at 623, 873 and 1073 K, respectively. In general, Pt metal particle sizes in carbon-supported Pt catalysts that are estimated from their ECSAs are larger than those estimated from their TEM images because not all surface Pt atoms are electrochemically active [40]. It should be noted that the Pt metal particle size of the SiO2/Pt/CNT, as estimated by its ECSA, that was treated at 623 K (3.1 nm) was different from that for the sample treated at 873 K (4.2 nm), while the particle size of Pt metal estimated from the TEM image for the former catalyst (1.6 nm) was similar to that for the latter catalyst (1.7 nm). In addition, a remarkable difference between the Pt metal particle size, as estimated by their ECSAs, and that estimated by their TEM images was found for samples treated at 1073 K. Denser silica layers in SiO2/Pt/CNT should serve as diffusion barrier for the active Pt sites. Therefore, ECSAs of Pt metal particles in SiO2/Pt/CNT catalysts decrease with the increase in the catalyst treatment temperature, although the size of Pt metal particles in these catalysts does not change appreciably with the treatment temperature. Fig. 13 shows changes in the CVs for SiO2/Pt/CNT treated at 623 and 1073 K during potential cycling experiments. The peak intensity in the CVs for the SiO2/Pt/CNT treated at 623 K gradually decreased as the number of potential cycles increased, as shown in Fig. 13(a). In contrast, the peak intensity for the SiO2/Pt/CNT treated at 1073 K did not change after 300 cycles although the peak intensity increased up to 300 cycles as described in Fig. 13(b). This suggests that the SiO2/Pt/CNT treated at higher temperatures shows high durability. From the CVs shown in Fig. 13 the ECSAs for these catalysts at each cycle were determined. Results are shown in Fig. 14. The SiO2/Pt/CNT treated at 623 K had the largest initial ECSA (91 m2 g-Pt 1) during potential cycling among the examined samples, as shown in Fig. 14. However, the ECSA decreased constantly as the number of potential cycles increased and it was finally reduced to 41 m2 g-Pt 1 after 1000 cycles. On the other hand, the ECSA for the SiO2/Pt/CNT treated at 1073 K gradually increased from 0 to 400 cycles and it obtained a constant value of ca. 60 m2 g-Pt 1 after 400 cycles. It should be noted that the SiO2/ Pt/CNT treated at 1073 K had the largest ECSA after 700 cycles among all the catalysts. Therefore, the dense silica layer which
Fig. 13. Cyclic voltammograms of SiO2/Pt/CNT treated at 623 K (a) and 1073 K (b) in 0.5 M H2SO4 at 303 K during repeated potential cycling. Scan rate was 50 mV s 1.
wraps Pt metal particles largely prevents the diffusion of cationic Pt species out of the SiO2/Pt/CNT. For this reason, the coverage of Pt/CNT with a dense silica layer is more effective in improving the durability of Pt metal.
Fig. 14. Changes in ECSAs of SiO2/Pt/CNT treated at different temperatures during repeated potential cycling. Treatment temperatures were 623 K (&), 873 K (^) and 1073 K (*).
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