CNT Catalysts Covered with

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Catalytic Activity of Highly Durable Pt/CNT Catalysts Covered with Hydrophobic Silica Layers for the Oxygen Reduction Reaction in PEFCs Sakae Takenaka, Hiroaki Miyamoto, Yutaka Utsunomiya, Hideki Matsune, and Masahiro Kishida J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 Dec 2013 Downloaded from http://pubs.acs.org on January 2, 2014

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The Journal of Physical Chemistry

Catalytic Activity of Highly Durable Pt/CNT Catalysts Covered with Hydrophobic Silica Layers for the Oxygen Reduction Reaction in PEFCs

Sakae Takenaka,*,†,‡, Hiroaki Miyamoto,† Yutaka Utsunomiya,† Hideki Matsune,† and Masahiro Kishida† †

Department of Chemical Engineering, Graduate School of Engineering,

Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan E-mail: [email protected] ‡

JST, PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan

Author for correspondence: Sakae Takenaka Department of Chemical Engineering Graduate School of Engineering Kyushu University Moto-oka 744, Nishi-ku, Fukuoka 819-0395, Japan Tel. & Fax: +81-92-802-2752 E-mail: [email protected]

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ABSTRACT: Carbon nanotube-supported Pt catalysts (Pt/CNT) for the cathode in a polymer electrolyte fuel cell (PEFC) were covered with silica layers using tetraethoxysilane (TEOS) and also methyltriethoxysilane (MTEOS) to improve the catalyst durability under the severe conditions at the PEFC cathode. Both the silica-coated Pt/CNT catalysts had excellent durability for potential cycling between 0.6 and 1.0 V (vs. RHE) in N2-purged 0.1 M HClO4 electrolyte, while Pt/CNT without silica-coating was significantly deactivated due to an increase of the Pt metal particle size. Silica-coated Pt/CNT prepared from MTEOS had similar activity for the oxygen reduction reaction as Pt/CNT without silica-coating, whereas the silica coverage obtained with TEOS slightly reduced the catalytic activity of the Pt/CNT catalyst. The silica layers prepared from MTEOS are more hydrophobic than those prepared from TEOS, due to the presence of methyl groups. In addition, the silica layers prepared from MTEOS have larger pores than those prepared from TEOS. The hydrophobic silica layers with larger pores in the silica-coated Pt/CNT do not inhibit the diffusion of the reactants (oxygen) and the discharge of the products (water) during the oxygen reduction reaction. KEYWORDS: Pt cathode catalyst, silica-coating, durability, oxygen reduction, polymer electrolyte fuel cells 2


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INTRODUCTION Polymer electrolyte fuel cells (PEFCs) are promising alternative power sources for transportation and portable applications because of advantages such as low emissions, high energy efficiency and low temperature operation.1,2 Pt metal has been used as a catalytically active component for the hydrogen oxidation reaction (HOR) at the anode and for the oxygen reduction reaction (ORR) at the cathode in PEFCs. The sluggish rate of the ORR on Pt compared with the HOR requires increased Pt loading in the cathode catalyst, which has impeded the full commercialization of PEFCs, due to the high cost of Pt.3,4 However, it is difficult to reduce the Pt loading at the cathode in PEFCs. Pt catalysts used at the cathode function under very severe conditions, such as low pH, high temperature, oxygen atmosphere, high humidity and at highly positive potentials. The Pt catalysts are thus seriously deactivated under cathode conditions.5-8 Pt metal particles at the cathode can easily migrate on the carbon supports and subsequently agglomerate. Pt metal particles can also grow through Ostwald ripening, where the surface Pt atoms in small Pt metal particles dissolve to form cationic Pt species that are subsequently deposited onto large metal particles, which results in growth of the Pt metal particle. The migration of Pt metal particles in cathode catalysts can be suppressed to some extent by modification of the chemical and physical properties of

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the carbon supports.9 However, these methods are not effective for the suppression of Pt metal dissolution. Many research groups have reported that alloy formation between Pt and other metal species such as Co, Pd or Au can improve the activity and durability of the catalysts under PEFC cathode conditions.10-13 The use of Pt-based alloy catalysts in the cathode contributes to the reduction of the Pt loading at the PEFC cathode because the alloy catalysts have higher activity and durability than pure Pt catalysts. However, the metal species added to the Pt catalysts also eventually dissolve under the cathode conditions, which causes a loss in activity of the Pt-based alloy catalyst during PEFC operation. The dissolved metal species are also deposited in the polymer electrolyte membranes, which results in a decrease of proton conductivity.14-16 Thus, the diffusion of metal species out of the cathode catalysts should be suppressed to reduce the Pt loading at the PEFC cathode. We have studied the catalytic performance of multi-walled carbon nanotube (CNT)-supported Pt (Pt/CNT) cathode catalysts covered with silica layers.17-19 The application of CNT as a support for Pt cathode catalysts is attractive because CNTs have useful properties, such as high electronic conductivity, high chemical stability, high surface area and high resistance or durability to oxidation under the PEFC cathode conditions.20,21 Silica-coated Pt/CNT (SiO2/Pt/CNT) exhibited high durability under the

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PEFC cathode conditions, whereas Pt/CNT without a silica-coating became significantly deactivated under the same conditions. The silica layers around the Pt metal particles prevent the migration of Pt metal particles on the CNT supports and the diffusion of dissolved Pt species out of the silica layers. Although coverage with silica layers improves the durability of Pt/CNT cathode catalysts, the catalytic activity of SiO2/Pt/CNT for the ORR was slightly inferior to that of Pt/CNT without the silica-coating. During the ORR on the SiO2/Pt/CNT cathode catalysts, reactants (oxygen) are supplied to the Pt metal surfaces through the silica layers, and products (water) diffuse from the Pt metal surfaces toward the outside of silica layers. The silica layers around the Pt metal particles prevent the facile diffusion of oxygen and water molecules. The SiO2/Pt/CNT catalysts have been prepared by the successive hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and tetraethoxysilane (TEOS).22,23 The silica layers in SiO2/Pt/CNT thus obtained are mainly composed of amorphous silica from TEOS. The silica layers consist of tetrahedral SiO4 units that are connected by sharing of O atoms to form a ringed structure of Si-O-Si. The ringed structures in the silica layers correspond to pores that are similar to those in zeolites. Silica layers prepared from silicon alkoxides with alkyl groups such as methyltriethoxysilane (MTEOS) have porous structures with larger pore sizes, because the alkyl groups in the silicon

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alkoxides are not hydrolyzed due to the strong bonding between Si-C. In contrast, all ethoxy groups in TEOS are hydrolyzed to form Si-O-Si ringed structures. In addition, the use of silicon alkoxides with alkyl groups as a silica source results in a silica surface that is more hydrophobic because the silica layers contain alkyl groups.24,25 The hydrophobic silica layers in SiO2/Pt/CNT are expected to promote the discharge of water molecules formed by the ORR on Pt metal surfaces. Therefore, it is expected that coverage with hydrophobic silica layers containing larger pores would not reduce the ORR activity of Pt/CNT. In the present study, Pt/CNT cathode catalysts were covered with silica layers by the successive hydrolysis of APTES and MTEOS. The activity and durability of SiO2/Pt/CNT prepared from APTES and MTEOS are compared with those of SiO2/Pt/CNT prepared from APTES and TEOS.

EXPERIMENTAL SECTION Catalyst Preparation.

CNTs were dispersed in a mixed solution of 8.0 M H2SO4 and

8.0 M HNO3 and then mixed ultrasonically at 328 K for 2 h to introduce functional groups such as hydroxyls and carboxyls onto the CNT surfaces. The Pt/CNT catalysts were prepared by impregnation of the oxidized CNT into an aqueous solution of

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Pt(NO2)2(NH3)2, followed by drying at 333 K. The dried samples were treated in hydrogen at 473 K to reduce the Pt precursors into Pt metal. The nominal Pt loading in the Pt/CNT was adjusted to 15 wt%. APTES, TEOS and MTEOS were used as silica sources for coverage of the Pt/CNT with silica layers.22,23 The Pt/CNT catalyst was dispersed in distilled water and the pH of the suspension was adjusted to ca. 10 by addition of triethylamine. APTES was added to the solution and stirred at 333 K for 0.5 h. Subsequently, TEOS or MTEOS was added to the solution and then stirred at 333 K for 2 h. After centrifugation of the suspension, samples thus obtained were dried at 363 K in air and treated at 623 K for 2 h in hydrogen. The SiO2/Pt/CNT catalysts prepared from APTES and TEOS, and from APTES and MTEOS are denoted as SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS), respectively.

Characterization of Catalysts.

The loading of Pt, Si and CNT in the catalyst samples

was evaluated using inductively coupled plasma atomic emission spectroscopy (ICP-AES)

and

thermal

gravimetric

analysis

(TGA)

in

air.

Nitrogen

adsorption/desorption isotherms were measured at 77 K (BELSORP-max, BEL Japan) to determine the specific surface area and porous structures of the catalysts. The specific surface area of the Pt catalysts was calculated using the Brunauer-Emmett-Teller (BET)

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equation. The mesopore size distributions for the catalysts were determined by the Barrett-Joyner-Halenda (BJH) method using the adsorption branch, and the micropore size distributions were evaluated using the Horvath-Kawazoe method. The amount of water molecules adsorbed on the Pt catalysts was also examined at 313 K. Prior to the nitrogen and water adsorption experiments, the Pt catalysts were evacuated at 473 K for 20 h.

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Si nuclear magnetic resonance (29Si NMR) spectra for both the SiO2/Pt/CNT

catalysts were measured at room temperature (JNM-CMX300, Jeol). Transmission electron microscopy (TEM; JEM-3000F, Jeol) specimens of the Pt catalysts were prepared by ultrasonically suspending the Pt catalyst powder in 2-propanol, a drop of which was then deposited on a carbon-enhanced copper grid and dried in air.

Electrocatalytic Performance Measurements.

Cyclic voltammograms (CVs) and

polarization curves for the ORR of the Pt catalysts were measured using a conventional three-electrode electrochemical cell with a Pt wire and reversible hydrogen electrode (RHE), which served as counter and reference electrodes, respectively. A glassy carbon disk electrode (6 mm diameter) was used as a substrate for the catalysts and polished to a mirror finish. Catalyst ink was prepared by ultrasonically blending the catalyst and methanol. An aliquot of this ink was then deposited on the glassy carbon disk and dried

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at 333 K. Nafion solution diluted with methanol was dropped onto the catalysts to ensure that the catalysts remained attached to the disk. The amount of Pt on the working electrode was adjusted to 10 µg cm-2 for all the catalysts. The working electrode was immersed in a N2-purged electrolyte solution of 0.1 M HClO4 at room temperature. CVs of the catalysts were measured at a scan rate of 50 mV s-1 between 0.05 and 1.20 V in N2-purged HClO4 electrolyte. The polarization curves for the ORR on the catalysts were measured using the rotating disk electrode (RDE) method in O2-saturated 0.1 M HClO4 at room temperature by scanning the potential of the working electrode from 0.1 to 1.1 V at a scan rate of 10 mV s-1 with an electrode rotation rate of 1600 rpm. Before measurement of the CVs and polarization curves for the ORR, 50 cycles of potential cycling between 0.05 and 1.20 V were performed for the working electrode in N2-purged HClO4 electrolyte to clean the catalyst surfaces. Accelerated durability tests for the Pt catalysts were conducted by cycling the potential of the working electrode in a square wave between 0.6 and 1.0 V in N2-purged 0.1 M HClO4. The potential of the working electrode was fixed at 0.6 and 1.0 V for 3 sec. The CVs and polarization curves for the ORR on the Pt catalysts were also measured after the accelerated durability tests. A voltammogram for the desorption of underpotentially deposited hydrogen was used to evaluate the electrochemically active surface area (ECSA) for the Pt catalysts on the

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electrode. The value used for determining the ECSA from the adsorbed hydrogen was 210 µC cm-Pt-2. A membrane-electrode assembly (MEA) for a PEFC single cell (EFC-05-02, Electrochem. Co.) was prepared as follows. Pt/CNT and SiO2/Pt/CNT electrocatalysts were used for the cathode and a carbon black-supported Pt (Pt/CB) catalyst for the anode. Catalyst ink was prepared by ultrasonically mixing the catalyst with 2-propanol and diluted Nafion solution (5 wt% Nafion). The catalyst ink was painted onto the surface of wet-proofed carbon paper (Toray Co.) as a gas diffusion layer. The amount of Pt catalyst at the cathode and anode was adjusted to 0.2 mg-Pt cm-2. An MEA with an area of 5 cm2 was fabricated by hot pressing a cathode and anode onto Nafion 117 at 403 K and 10 MPa for 3 min. All single cell tests were conducted with the fuel gas (hydrogen) and oxidant (oxygen) humidified at 343 K before being supplied to the cells. The cells were operated at 343 K and at atmospheric pressure. For the durability test of the cathode catalysts, the cell voltage of the single cells was repeatedly changed with a triangular wave between 0.05 and 1.20 V at rate of 100 mV s-1, while humidified nitrogen and hydrogen were supplied to the cathode and anode, respectively.

RESULTS AND DISCUSSION

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b

a

20 nm 10 nm

c

d SiO2

20 nm

e

10 nm

f

SiO2 10 nm

20 nm

g

h

SiO2

SiO2 10 nm

10 nm

Figure 1. TEM images of (a,b) Pt/CNT, (c,d,e) SiO2/Pt/CNT(TEOS) and (f,g,h) SiO2/Pt/CNT(MTEOS).

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Characterization of Pt Catalysts.

Figure 1 shows representative TEM images of the

Pt/CNT, SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS) catalysts. The Pt and Si loading in these catalysts are listed in Table 1. The Si and Pt loading for both the SiO2/Pt/CNT catalysts was similar. Many Pt metal particles are deposited on the CNT surfaces in Pt/CNT, the size of which most ranged from 1 to 3 nm (Figure 1 a and b). Pt metal particles are also observed on the CNT surfaces in both the SiO2/Pt/CNT catalysts and were similar in size to the Pt metal particles in Pt/CNT (Figure 1 c and f). The Pt metal particles and CNTs in SiO2/Pt/CNT appear to be covered with silica layers. The thickness of the silica layers in both the SiO2/Pt/CNT catalysts ranged from 2 to 3 nm, as clarified from panels d, e, g and h of Figure 1. These results indicate that the surface of Pt/CNT can be covered with silica layers by successive hydrolysis of APTES and MTEOS, or of APTES and TEOS. We have previously prepared SiO2/Pt/CNT by the successive hydrolysis of APTES and TEOS.22,23 APTES molecules adsorb to the surfaces of the Pt metal particles and the CNTs during the hydrolysis of APTES to form very thin silica layers (< 1 nm). During the subsequent hydrolysis of TEOS, the silica layers from APTES act as nucleation sites for the formation of thick silica layers (2~3 nm) from TEOS. Thus, the physical and chemical properties of the silica layers in the SiO2/Pt/CNT catalyst are strongly dependent on the type of silica precursor used to

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develop thicker silica layers, i.e., TEOS or MTEOS.

SiO2/Pt/CNT (TEOS) SiO2/Pt/CNT (MTEOS)

-40

-80

-120

-160

δ / ppm

Figure 2.

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Si NMR spectra for SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS).

Si-NMR spectra for SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS) were

measured to clarify the chemical state of Si atoms in these catalysts and the results are shown in Figure 2. The spectrum for SiO2/Pt/CNT(TEOS) has two peaks at around -100 and -110 ppm, which are assigned to Q3 (HO-Si(-OSi)3) and Q4 species (Si(-OSi)4), respectively.26-28 In addition, a peak at around -65 ppm was assigned to the T3 mode (R-Si(-OSi)3, R: alkyl groups).26-28 The silica prepared by hydrolysis of TEOS gives the Q3 and Q4 mode in the NMR spectra, and the silica from APTES gives the T3 mode

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because Si-(C3H6NH2) in APTES is not hydrolyzed during catalyst preparation. In contrast, only one peak due to the T3 mode was present in the spectrum of SiO2/Pt/CNT(MTEOS).

These

results

indicate

that

silica

layers

in

Differential pore volume / cm3 g-1 nm-1

SiO2/Pt/CNT(MTEOS) have methyl groups from MTEOS.

Differential pore volume / cm3 g-1 nm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.08

a)

0.07

SiO2/Pt/CNT (TEOS)

0.06

SiO2/Pt/CNT (MTEOS)

0.05 0.04 0.03 0.02 0.01 0 0.0

0.5 1.0 1.5 Pore diameter / nm

2.0

0.025

b) 0.020 SiO2/Pt/CNT(TEOS) SiO2/Pt/CNT(MTEOS)

0.015 0.010 0.005 0.000 0

5

10 15 20 25 30 Pore diameter / nm

35

40

Figure 3. Size distributions for (a) micropores and (b) mesopores in SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS) evaluated by N2 adsorption/desorption at 77 K.

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The specific surface areas and pore size distributions of silica layers in Pt/CNT and both the SiO2/Pt/CNT catalysts were evaluated by N2 adsorption/desorption at 77 K. The specific surface areas were evaluated to be 135 m2 g-1 for Pt/CNT, 95 m2 g-1 for SiO2/Pt/CNT(TEOS), and 99 m2 g-1 for SiO2/Pt/CNT(MTEOS). Figure 3 shows the mesopore and micropore size distributions for the SiO2/Pt/CNT catalysts. Both SiO2/Pt/CNT catalysts had similar mesopore (Figure 3b) and micropore size distributions (Figure 3a), although the silica layers in these catalysts were prepared from different silica sources (TEOS or MTEOS). The silica layer thickness of both the SiO2/Pt/CNT catalysts was so thin (2-3 nm) that no difference of pore size could be distinguished by N2 adsorption measurements. The porous structure of the silica layers around the Pt metal particles in SiO2/Pt/CNT was evaluated by the oxidation of various sized alcohols (methanol, ethanol and n-propanol) in N2-purged 0.1 M HClO4 electrolyte at 303 K. Figure 4 shows CVs measured for the Pt/CNT, SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS) catalysts with 0.1 M methanol (panel a), ethanol (panel b) and n-propanol (panel c) at 303 K. The current was normalized according to the ECSA of each Pt catalyst, which were

70,

46

and

71

m2

g-Pt-1

for

Pt/CNT,

SiO2/Pt/CNT(TEOS)

and

SiO2/Pt/CNT(MTEOS), respectively. The CVs for the oxidation of alcohols on the Pt

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catalysts had a peak at around 0.9 V in the forward scan and another peak at around 0.8 V in the backward scan. The former peak is due to the oxidation of the alcohol on the Pt catalysts and the latter peak is assigned to the oxidation of intermediates such as CO adsorbed on the Pt catalysts.29,30 The current density at the peak in the forward scan was higher in the order of Pt/CNT > SiO2/Pt/CNT(MTEOS) > SiO2/Pt/CNT(TEOS), irrespective of the types of alcohol employed. Thus, the silica layers around the Pt metal particles act as barriers against the diffusion of alcohols to the Pt metal surfaces. It should be noted that the difference in the current density between SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS) became larger with the size of the alcohol. To clarify the difference between the catalytic activity of each SiO2/Pt/CNT catalyst for the oxidation of each alcohol, the relative activity of SiO2/Pt/CNT(MTEOS) to SiO2/Pt/CNT(TEOS) is shown in Figure 5. The peak current density in the forward scan for SiO2/Pt/CNT(MTEOS) was divided by that for SiO2/Pt/CNT(TEOS). The activity of SiO2/Pt/CNT(MTEOS) for methanol oxidation was very similar to that of SiO2/Pt/CNT(TEOS). However, the relative activity of SiO2/Pt/CNT(MTEOS) to SiO2/Pt/CNT(TEOS) significantly increased as the size of the substrate alcohol became larger. These results suggest that the silica layers prepared from APTES and MTEOS have larger pores than those prepared from APTES and TEOS.

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Current density / mA cm-2-Pt

0.8 Pt/CNT SiO2/Pt/CNT (TEOS) SiO2/Pt/CNT (MTEOS)

0.6 0.4

a)

0.2 0.0 0.0 0.6

0.2

0.4

0.6

0.8

1.0

Pt/CNT SiO2/Pt/CNT (TEOS) SiO2/Pt/CNT (MTEOS)

0.5 0.4 0.3

1.2

b)

0.2 0.1 0.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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0.0 0.5

0.2

0.4

0.6

0.8

1.0

Pt/CNT SiO2/Pt/CNT(TEOS) SiO2/Pt/CNT(MTEOS)

0.4 0.3

1.2

c)

0.2 0.1 0.0 0.0

0.2

0.4 0.6 0.8 1.0 Potential / V vs. RHE

1.2

Figure 4. CVs for the Pt/CNT, SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS) in N2-purged 0.1 M HClO4 electrolyte containing 0.1 M (a) CH3OH, (b) C2H5OH and (c) n-C3H7OH at 303 K. Scan rate = 50 mV s-1.

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Relative activity of SiO2/Pt/CNT(MTEOS) to SiO2/Pt/CNT (TEOS)


4.0 3.5

SiO2/Pt/CNT(TEOS)

3.0

SiO2/Pt/CNT(MTEOS)

2.5 2.0 1.5 1.0 0.5 0.0

CH3OH

C2H5OH

C3H7OH

Figure 5. Relative activity of SiO2/Pt/CNT(MTEOS) to SiO2/Pt/CNT(TEOS) for the oxidation of methanol, ethanol and n-propanol.

Cathode catalysts in PEFCs function under highly humidified conditions, because humidified oxidant (oxygen) is supplied to the cathode and water molecules are formed through the ORR on the Pt cathode catalyst. The hydrophilic properties of the Pt catalysts were thus evaluated by measuring the adsorption of water molecules. Figure 6 shows the amount of water molecules adsorbed on the Pt/CNT and SiO2/Pt/CNT catalysts. The amount of water molecules adsorbed on each catalyst was normalized according to the specific surface area of the corresponding catalyst evaluated by N2 adsorption at 77 K. The amount of water molecules adsorbed on all the Pt catalysts gradually increased with the relative pressure of water vapor. The amount of water molecules adsorbed on SiO2/Pt/CNT(MTEOS) was lower over the entire range of water

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vapor pressure than that on SiO2/Pt/CNT(TEOS). The

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Si NMR spectra revealed that

the silica layers prepared from APTES and MTEOS do not have hydrophilic Si-OH, as produced with APTES and TEOS, but instead hydrophobic Si-CH3. The methyl groups in SiO2/Pt/CNT(MTEOS) impart hydrophobicity to the silica layers. Amount of H2O adsorbed / molecules nm-2

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20 Pt/CNT SiO2/Pt/CNT(TEOS)

16

SiO2/Pt/CNT(MTEOS)

12 8 4 0 0

0.2 0.4 0.6 0.8 Relative pressure of water, p/p0

1

Figure 6. Amount of water adsorbed on Pt/CNT, SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS) at 313 K.

Performance of SiO2/Pt/CNT as Electrocatalyst.

CVs and polarization curves for

the ORR were measured in 0.1 M HClO4 electrolyte to evaluate the electrocatalytic performance and durability of each Pt catalyst. Figure 7 shows the CVs (panel a) and polarization curves for the ORR (panel b) on the fresh Pt/CNT and SiO2/Pt/CNT 19



catalysts. Two peak couples were found in the potential range of 0.05 to 0.4 V and 0.6 to 1.2 V in the CV for fresh Pt/CNT (Figure 7a). The former peak couples are assignable to the adsorption and desorption of hydrogen on Pt metal, and the latter peaks are due to the oxidation and reduction of Pt.

Current density / mA cm-2

0.6

a)

0.3 0.0 -0.3 -0.6

Pt/CNT SiO2/Pt/CNT(TEOS)

-0.9

SiO2/Pt/CNT(MTEOS)

0.0 0.0 Current density / mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-1.0 -2.0 -3.0

0.2


1.2

b) Pt/CNT SiO2/Pt/CNT (TEOS) SiO2/Pt/CNT (MTEOS)

-4.0 -5.0 -6.0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Potential / V vs. RHE

Figure 7. (a) CVs and (b) polarization curves for the ORR on fresh Pt/CNT, SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS). The current was normalized by geometrical area of the electrode.

20


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


These peak couples due to Pt metal were also observed in the CVs for both the fresh SiO2/Pt/CNT catalysts, which indicates that these are electrochemically active catalysts, irrespective of the insulating silica coverage of the Pt metal particles. Reactants such as protons and water diffuse to the Pt metal surfaces through the silica layers and electrons are supplied to the Pt metal particles through exposed CNT surfaces during electrochemical reactions on the SiO2/Pt/CNT catalysts.31 The peak currents for SiO2/Pt/CNT(MTEOS) were higher than those for SiO2/Pt/CNT(TEOS), as shown in Figure 7a. These Pt catalysts exhibit catalytic activity for the ORR, as shown in Figure 7b. The polarization curves for the ORR on the Pt catalysts have two characteristic regions; a well-defined limiting-current region at potentials lower than 0.7 V and a mixed diffusion-kinetic control region in the potential range from 0.7 to 1.0 V. The polarization curve for SiO2/Pt/CNT(MTEOS) was consistent with that for Pt/CNT, i.e., the silica layer covering from APTES and MTEOS did not deteriorate the catalytic activity of the Pt/CNT. In contrast, the catalytic activity of SiO2/Pt/CNT(TEOS) for the ORR was slightly inferior to that of the Pt/CNT and SiO2/Pt/CNT(MTEOS) catalysts. During the ORR on the SiO2/Pt/CNT catalysts, reactants (oxygen) and products (water) diffuse through the silica layers around Pt metal particles. The silica layers prepared from

21



APTES and TEOS had porous structures with smaller pore diameters and higher hydrophilicity than those from APTES and MTEOS. Therefore, the silica layers prepared from APTES and TEOS retard the supply of oxygen and the discharge of water molecules, compared to the silica layers prepared from APTES and MTEOS.


0.7

a) 0.4 0.0 -0.3


fresh 1000cycles 5000cycles 10000cycles 15000cycles 20000cycles

-0.7 -1.0 -1.4 0.0 0.7

0.2

0.4

0.6

0.8

1.0

1.2

b) 0.4 0.0 -0.3 fresh 1000cycles 5000cycles 10000cycles 15000cycles 20000cycles

-0.7 -1.0 -1.4 0.0 0.7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 44

0.2

0.4

0.6

0.8

1.0

1.2

c) 0.4 0.0 -0.3 fresh 1000cycles 5000cycles 10000cycles 15000cycles 20000cycles

-0.7 -1.0 -1.4 0.0

0.2


22


1.2

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. CVs for (a) Pt/CNT, (b) SiO2/Pt/CNT(MTEOS) during the durability tests.

SiO2/Pt/CNT(TEOS)

and

(c)

The CVs and polarization curves for the ORR on the Pt catalysts were also measured after potential cycling between 0.6 and 1.0 V in N2-purged 0.1 M HClO4 electrolyte (durability tests). Figure 8 shows the CVs for the Pt/CNT and SiO2/Pt/CNT catalysts during the durability tests. Peak currents due to Pt metal in the CVs for the Pt/CNT were gradually decreased with the cycle number (Figure 8a). In contrast, the CVs for SiO2/Pt/CNT(TEOS) did not change significantly during the durability test, although the peak currents were slightly lower than those for Pt/CNT (Figure 8b). Thus, silica coating by the successive hydrolysis of APTES and TEOS improves the durability of Pt/CNT under the cathode conditions. The peak currents due to Pt metal in the CVs for SiO2/Pt/CNT(MTEOS) were also gradually reduced with the cycle number as shown in Figure 8c, but the decrease in the peak currents for the SiO2/Pt/CNT(MTEOS) was negligible compared to that for Pt/CNT, which suggests that the durability of Pt/CNT was improved by coverage with silica layers from APTES and MTEOS.

23




0.0 -1.0 -2.0 -3.0


a)

-4.0 -5.0


-6.0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0 -1.0 -2.0 -3.0


b)

-4.0 -5.0 -6.0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 44

c)

-1.0 -2.0 -3.0 -4.0


-5.0 -6.0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Potential / V vs. RHE

Figure 9. Polarization curves for the ORR on (a) Pt/CNT, SiO2/Pt/CNT(TEOS) and (c) SiO2/Pt/CNT(MTEOS) during the durability tests.

24


(b)

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9 shows polarization curves for the ORR on the Pt catalysts during the durability tests. The fresh Pt/CNT exhibited high activity for the ORR, but was deactivated during the durability tests, as shown in Figure 9a. The ORR currents at around 0.9 V on the Pt/CNT catalyst were gradually reduced with the cycle number. The fresh SiO2/Pt/CNT(TEOS) had poor activity for the ORR compared to the other Pt catalysts, but the activity was gradually enhanced with the cycle number (Figure 9b). The ORR activity of SiO2/Pt/CNT(TEOS) did not decrease up to 20000 cycles. Interestingly, the fresh SiO2/Pt/CNT(MTEOS) exhibited similar activity for the ORR to that for fresh Pt/CNT, and the activity did not change appreciably up to 20000 cycles (Figure 9c). Figure 10 shows the change of ECSA (panel a) and the Pt-based mass activity (panel b) of each Pt catalyst during the durability tests. The Pt-based mass activity was evaluated from the kinetic current at 0.9 V (vs. RHE) which was derived by the Koutecky-Levich equation. The ECSA of Pt/CNT was reduced from 70 to 37 m2 g-Pt-1 during the durability tests, as shown in Figure 10a. The fresh SiO2/Pt/CNT(TEOS) catalyst had a smaller ECSA (46 m2 g-Pt-1) than the fresh Pt/CNT catalyst. However, the ECSA of SiO2/Pt/CNT(TEOS) increased slightly after 1000 cycles and then remained unchanged during the durability tests. After 10000 cycles, SiO2/Pt/CNT(TEOS) had a

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

larger ECSA than Pt/CNT. The fresh SiO2/Pt/CNT(MTEOS) catalyst had a similar ECSA to the fresh Pt/CNT catalyst. The ECSA for SiO2/Pt/CNT(MTEOS) was always higher than those for Pt/CNT and SiO2/Pt/CNT(TEOS) during the durability tests. The Pt-based mass activity of Pt/CNT was also appreciably decreased during the durability tests, as shown in Figure 10b. The Pt-based mass activity of the fresh Pt/CNT catalyst was evaluated to be 220 mA mg-Pt-1, which was the highest among the fresh catalysts shown in Figure 10. However, the activity of Pt/CNT was significantly decreased with the cycle number and was reduced to half of the initial value after 20000 cycles. The fresh SiO2/Pt/CNT(TEOS) catalyst exhibited poor activity for the ORR compared to the other Pt catalysts, although the activity was gradually improved during the durability tests. In contrast, SiO2/Pt/CNT(MTEOS) had not only similar activity for the ORR as the fresh Pt/CNT, but also excellent durability for potential cycling. These results indicate that the coverage with silica layers by the successive hydrolysis of APTES and MTEOS can improve the durability of the Pt/CNT catalysts without compromising the catalytic activity for the ORR.

26


Page 26 of 44

Page 27 of 44

80

a)

ECSA / m2 g-Pt-1

70 60 50 40 30

Pt/CNT SiO2/Pt/CNT(TEOS) SiO2/Pt/CNT(MTEOS)

20 10 0 0

Mass activity at 0.9 V / mA mg-Pt-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5000

250

10000 15000 Cycle number

20000

b) 200 150 100 Pt/CNT SiO2/Pt/CNT(TEOS)

50

SiO2/Pt/CNT(MTEOS) 0 0

5000

10000 15000 Cycle number

20000

Figure 10. Change of (a) ECSA and (b) Pt-based mass activity for the ORR with Pt/CNT, SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS) during the durability tests.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a

Page 28 of 44

b

50 nm

20 nm

c

d

20 nm

20 nm

e

20 nm

f

20 nm

Figure 11. TEM images of (a,b) Pt/CNT, (c,d) SiO2/Pt/CNT(TEOS) and (e,f) SiO2/Pt/CNT(MTEOS) after the durability tests.

Figure 11 shows representative TEM images of Pt/CNT and SiO2/Pt/CNT after the durability tests (after 20000 cycles). The Pt particle size distributions for the fresh and used catalysts were evaluated from the TEM images, and the results are shown in Figure 12. The Pt size distributions were evaluated based on the measurement of the size of 150 particles at least. As shown in Figure 1, a high density of Pt metal particles with

28


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diameters of 1-3 nm were supported on the CNT surfaces in fresh Pt/CNT. The number of Pt metal particles observed in the TEM images of the used Pt/CNT was less than that in the fresh catalyst, and the diameters of the Pt metal particles in the used Pt/CNT catalyst ranged from 2 to 7 nm (panels a and b in Figure 11). The average Pt particle size in Pt/CNT was changed from 2.3 to 4.4 nm during the durability tests (Figure 12a). The increase in the Pt metal particle size during the durability tests is due to the sintering of Pt metal particles and the dissolution and redeposition of Pt metal, which resulted in the deactivation of Pt/CNT for the ORR. Aggregated Pt metal particles were seldom observed in the TEM images of used SiO2/Pt/CNT(TEOS) (panels c and d in Figure 11) and SiO2/Pt/CNT(MTEOS) (panels e and f in Figure 11), and the Pt metal particles in both of the used catalysts were covered with silica layers. The Pt particle size distributions for SiO2/Pt/CNT(TEOS) (Figure 12b) and SiO2/Pt/CNT(MTEOS) (Figure 12c) catalysts were shifted slightly toward larger size during the durability tests, although Pt metal particles larger than 5 nm were seldom observed in these used SiO2/Pt/CNT catalysts, in contrast to the used Pt/CNT catalyst. Thus, the increase in the average Pt metal particle size in SiO2/Pt/CNT during the durability test was insignificant, i.e., the average Pt metal particle size was changed from 2.4 to 2.7 nm for SiO2/Pt/CNT(TEOS) and from 2.1 to 2.5 nm for SiO2/Pt/CNT(MTEOS). Thus, silica

29



layers prepared by the successive hydrolysis of APTES and MTEOS, or from APTES and TEOS, can prevent the increase in the Pt metal particle size under PEFC cathode conditions. 70

Fraction / %

60

a)

fresh catalyst

50

used catalyst

40 30 20 10 0

70

Fraction / %

60

1

2

3

4

b)

5

6

7

8

fresh catalyst

50

used catalyst

40 30 20 10 0 1

2

3

4

5

6

7

8

70 60 Fraction / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 44

c)

fresh catalyst

50

used catalyst

40 30 20 10 0 1

2

3 4 5 6 Particle size / nm

7

8

Figure 12. Pt particle size distributions for (a) Pt/CNT, (b) SiO2/Pt/CNT(TEOS) and (c) SiO2/Pt/CNT(MTEOS) before and after the durability tests. 30


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The catalytic activity and durability of the SiO2/Pt/CNT cathode catalysts were also examined using PEFC single cells. The cell voltage of the single cells was repeatedly changed from 0.05 and 1.20 V for durability tests of the cathode catalysts while hydrogen and nitrogen were supplied to the anode and cathode, respectively. Polarization curves of the single cells with the Pt/CNT, SiO2/Pt/CNT(TEOS) or SiO2/Pt/CNT(MTEOS) catalysts at the cathode are shown in panels a, b and c of Figure 13, respectively. The fresh Pt/CNT exhibited the highest activity for the ORR among the cathode catalysts tested in the present study. The catalysts layers of both the SiO2/Pt/CNT in the MEA became inevitably thicker than that of Pt/CNT without silica-coating. It is likely that contact resistance between the SiO2/Pt/CNT catalysts would be larger than that for the Pt/CNT. The parameters for the MEA preparation such as the amount of ionomer and the distribution of the catalyst particles should be optimized for the SiO2/Pt/CNT. The Pt/CNT cathode catalyst was seriously deactivated during the durability tests. On the other hand, both the fresh SiO2/Pt/CNT catalysts showed lower activity for the ORR than fresh Pt/CNT. However, the catalytic activity of these SiO2/Pt/CNT catalysts was gradually improved with the cycle number up to 1000 cycles. The current of the single cell with SiO2/Pt/CNT(MTEOS) was higher than that of the cell with SiO2/Pt/CNT(TEOS) in the region of lower cell voltage, where the

31



current density is strongly dependent on the diffusion rates of reactants and products. 1.2

0.10

0.8

0.08 0.6 0.06 0.4

0.04

0.2

0.02

a) 0.1

0.2

fresh 100cycles 400cycles

1.0

0.3

0.00 0.4 0.14

1000cycles 2000cycles 3000cycles

0.12 0.10

0.8

0.08 0.6 0.06 0.4 0.04 0.2

0.02

b)

0.0 0.0 1.2

0.1

0.2

fresh 100cycles 400cycles

1.0

0.3

0.00 0.4 0.14


0.12 0.10

0.8

0.08 0.6 0.06 0.4 0.04 0.2

0.02

c) 0.0 0.0

Power density / mW cm-2

0.0 0.0 1.2

Cell voltage / V

0.12

0.1 0.2 0.3 Current density / A cm-2

0.00 0.4

32



Cell voltage / V



0.14 fresh 100cycles 400cycles

1.0

Cell voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 44

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 13. Polarization curves for the PEFC single cells with the (a) Pt/CNT, (b) SiO2/Pt/CNT(TEOS) and (c) SiO2/Pt/CNT(MTEOS) cathode catalysts during the durability tests.

The silica layers formed from APTES and MTEOS have porous structures with larger pore diameters than those from APTES and TEOS. In addition, the former silica layers are more hydrophobic than the latter. Both the SiO2/Pt/CNT catalysts have excellent durability under PEFC cathode conditions. The SiO2/Pt/CNT catalyst prepared from APTES and MTEOS has higher activity for the ORR than SiO2/Pt/CNT prepared from APTES and TEOS. The larger pores in the silica layers prepared from APTES and MTEOS do not inhibit the diffusion of molecular oxygen to the Pt metal surfaces, and the hydrophobicity of the silica enhances the discharge of water molecules outside of the silica layers. Therefore, SiO2/Pt/CNT(MTEOS) exhibits higher activity for the ORR than SiO2/Pt/CNT(TEOS).

CONCLUSION SiO2/Pt/CNT cathode catalysts for PEFCs were prepared by the successive hydrolysis of APTES and TEOS or APTES and MTEOS. Both the SiO2/Pt/CNT catalysts

had

excellent

durability

under

the

severe

33


cathode

conditions.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SiO2/Pt/CNT(MTEOS) exhibited higher activity for the ORR than SiO2/Pt/CNT(TEOS). The silica layers in the SiO2/Pt/CNT(MTEOS) are more hydrophobic than those in SiO2/Pt/CNT(TEOS), due to the presence of methyl groups. In addition, the silica layers prepared from APTES and MTEOS have porous structures with larger pore diameters than those prepared from APTES and TEOS. Hydrophobic silica layers with larger pores around the Pt metal particles enhance the supply of reactants (oxygen) and the discharge of products (water) during the ORR.

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(2)

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(3)

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(4)

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(6)

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(7)

Honji, A.; Mori, T.; Tamura, K.; Hishimura, Y. Agglomeration of Platinum Particles Supported on Carbon in Phosphoric Acid. J. Electrochem. Soc. 1988, 135, 355-359.

(8)

Willsau, J.; Heitbaum J. The Influence of Pt-activation on the Corrosion of Carbon in Gas Diffusion Electrodes- A DEMS Study, J. Electroanal. Chem. Interfacial Electrochem. 1984, 161, 93-101.

(9)

Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, Y. R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, G. et al. Scientific Aspects of Polymer Electrolyte Fuel Cell Durability and Degradation. Chem. Rev. 2007, 107, 3904-3951.

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(17) Takenaka, S.; Matsumori, H.; Nakagawa, K.; Matsune, H.; Tanabe, E.; Kishida, M. Improvement in the Durability of Pt Electrocatalysts by Coverage with Silica Layers. J. Phys. Chem. C 2007, 111, 15133-15136. (18) Takenaka, S.; Matsumori, H.; Matsune, H.; Tanabe, E.; Kishida, M. High Durability of Carbon Nanotube-supported Pt Electrocatalysts Covered with Silica Layers for the Cathode in a PEMFC. J. Electrochem. Soc. 2008, 155, B929-B936. (19) Matsumori, H.; Takenaka, S.; Matsune, H.; Kishida, M. Preparation of Carbon Nanotube-supported Pt Catalysts Covered with Silica Layers; Application to Cathode Catalysts for PEFC. Appl. Catal. A: Gen. 2010, 373, 176-185. (20) Wang, X.; Li, W.; Chen, Z.; Waje, M.; Yan, Y. Durability Investigation of Carbon Nanotube as Catalyst Support for Proton Exchange Membrane Fuel Cell. J. Power Sources 2006, 158, 154-159. (21) Shao, Y.; Yin, G.; Gao, Y.; Shi, P. Durability Study of Pt/C and Pt/CNTs Catalysts under Simulated PEM Fuel Cell Conditions. J. Electrochem. Soc. 2006, 153, A1093-A1097. (22) Takenaka, S.; Arike, T.; Matsune, H.; Tanabe, E.; Kishida, M. Synthesis of Carbon Nanotube-supported Pt Nanoparticles Covered with Silica Layers. Carbon 2008, 46, 365-368.

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39



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 1.

Loading of Pt and Si in Pt/CNT and SiO2/Pt/CNT Catalysts Catalyst

Pt / wt%

Si / wt%

Pt/CNT

15

0

SiO2/Pt/CNT(TEOS)

10

11

SiO2/Pt/CNT(MTEOS)

11

10

40


Page 40 of 44

Page 41 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions

Figure 1.

TEM images of (a,b) Pt/CNT, (c,d,e) SiO2/Pt/CNT(TEOS) and (f,g,h)

SiO2/Pt/CNT(MTEOS).

29

Figure 2.

Figure

3.

Si NMR spectra for SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS).

Size

distributions

for

(a)

micropores

and

(b)

mesopores

in

SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS) evaluated by N2 adsorption/desorption at 77 K.

Figure 4.

CVs for the Pt/CNT, SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS) in

N2-purged 0.1 M HClO4 electrolyte containing 0.1 M (a) CH3OH, (b) C2H5OH and (c) n-C3H7OH at 303 K. Scan rate = 50 mV s-1.

Figure 5.

Relative activity of SiO2/Pt/CNT(MTEOS) to SiO2/Pt/CNT(TEOS) for the

oxidation of methanol, ethanol and n-propanol.

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Figure 6.

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Amount of water adsorbed on Pt/CNT, SiO2/Pt/CNT(TEOS) and

SiO2/Pt/CNT(MTEOS) at 313 K.

Figure 7.

(a) CVs and (b) polarization curves for the ORR on fresh Pt/CNT,

SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS).

Figure

8.

CVs

for

(a)

Pt/CNT,

(b)

SiO2/Pt/CNT(TEOS)

and

(c)

SiO2/Pt/CNT(MTEOS) during the durability tests.

Figure 9.

Polarization curves for the ORR on (a) Pt/CNT, (b) SiO2/Pt/CNT(TEOS)

and (c) SiO2/Pt/CNT(MTEOS) during the durability tests.

Figure 10. Change of (a) ECSA and (b) Pt-based mass activity for the ORR with Pt/CNT, SiO2/Pt/CNT(TEOS) and SiO2/Pt/CNT(MTEOS) during the durability tests.

Figure 11.

TEM images of (a,b) Pt/CNT, (c,d) SiO2/Pt/CNT(TEOS) and (e,f)

SiO2/Pt/CNT(MTEOS) after the durability tests.

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Figure 12.

Pt particle size distributions for (a) Pt/CNT, (b) SiO2/Pt/CNT(TEOS) and

(c) SiO2/Pt/CNT(MTEOS) before and after the durability tests.

Figure 13.

Polarization curves for the PEFC single cells with the (a) Pt/CNT, (b)

SiO2/Pt/CNT(TEOS) and (c) SiO2/Pt/CNT(MTEOS) cathode catalysts during the durability tests.

43



Table of Contents

Highly active and durable silica-coated Pt cathode catalysts 250 fresh fresh

Mass activity for oxygen reduction at 0.9 V / mA mg-Pt -1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

200

used

150 used 100 50 0 Pt/CNTSiO2-coated Pt/CNT

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CNT Catalysts Covered with

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