Hollow core mesoporous shell carbon supported Pt electrocatalysts with high Pt loading for PEMFCs

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 8 6 5 e1 8 7 4

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Hollow core mesoporous shell carbon supported Pt electrocatalysts with high Pt loading for PEMFCs _ lu a Burcu Gu¨venatam a, Berker Fıc¸ıcılar a, Ays‚e Bayrakc¸eken b,*, Inci Erog a b

Department of Chemical Engineering, Middle East Technical University, 06531 Ankara, Turkey Department of Chemical Engineering, Atatu¨rk University, 25240 Erzurum, Turkey

article info

abstract

Article history:

The aim of this study is to synthesize mesoporous carbon supports and prepare their

Received 29 January 2011

corresponding electrocatalysts with microwave irradiation method and also increasing the

Received in revised form

Pt loading over the carbon support by using some additional reducing agents. Pt loadings

2 June 2011

on hollow core mesoporous shell (HCMS) and commercial Vulcan XC72 carbon supports up

Accepted 26 June 2011

to 34% and 44%, respectively, were achieved via polyol process with microwave irradiation

Available online 31 July 2011

method. When hydrazine or sodium borohydride was used in addition to ethylene glycol, Pt loading over the HCMS carbon support was increased. Characterization of the prepared

Keywords:

electrocatalysts was performed by ex situ (BET, XRD, SEM, TGA and Cyclic Voltammetry)

Hollow core mesoporous shell

and in situ (PEM fuel cell tests) analysis. PEM fuel cell performance tests showed that 44%

(HCMS) carbon

Pt/Vulcan XC72 and 28% Pt/HCMS electrocatalysts exhibited improved fuel cell perfor-

Microwave irradiation

mances. The results revealed that as the Pt loading increased PEM fuel cell performance

Pt loading

was also increased.

Carbon support

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

PEM fuel cell

1.

Introduction

Proton exchange membrane fuel cells (PEMFCs) are considered as promising alternatives to internal combustion engines for transportation because of their high efficiency, high power density, low emissions, low operating temperature, and low noise [1,2]. PEMFCs have further advantages such as elimination of electrolyte leakage, lower corrosion, simplification of stack design and increased ruggedness. These promising attributes have stimulated applications in areas such as military, aerospace and transportation [3,4]. Proton exchange membrane fuel cells have particular properties than other fuel cell types. Their operation temperature is relatively low, and this causes to reduce thermal losses from the fuel cell. Low temperature fuel cell applications need noble platinum or platinum based alloys as catalysts for hydrogen oxidation and oxygen reduction reactions [5]. Thus, utilization

reserved.

of a noble-metal catalyst (typically platinum) which separate the hydrogen’s electrons and protons, increase the cost of the system. The recent studies on the PEM fuel cell are continuing on the platinum incorporated composite catalysts. Up to now, platinum loading in PEM fuel cell electrocatalysts is reduced to 0.1 mg/cm2 from 2.0 mg/cm2 without any performance losses [6]. It is also possible to lower the platinum loading further by using different methods such as ion-beam assisted deposition, electrodeposition, and sputtering [7e9]. Low temperature PEM fuel cells require active platinum electrocatalysts. For the anode side, hydrogen oxidation reaction (HOR) is relatively facile and operation could be done with low platinum loadings such as 0.05 mg/cm2. However, the rate of oxygen reduction reaction (ORR) is slow and it requires relatively high platinum loading when compared with anode side. Thus, it is critical to have highly dispersed platinum nanoparticles on carbon support and stable

* Corresponding author. Tel.: þ90 442 231 46 39; fax: þ90 442 231 45 44. E-mail address: [email protected] (A. Bayrakc¸eken). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.129

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structures for ORR electrocatalysts. In order to use the Pt based catalysts effectively, a high surface area carbon support with high porosity and high surface area is required [10]. Also, the synthesized catalyst should have some fundamental physical properties to provide a high efficiency. The particle size is a key point for the catalyst. Carbon support technology provides important advantages including determination of particle size, well distribution of supported catalyst nanoparticles and has significant effects on catalytic performance and stability of the supported catalyst [11]. The catalyst should have a proper particle size, because very small particle sizes cause unstable behavior and very big particles cause the loss of surface area. The structure of the carbon support pores is also important in order to provide a better water transport. In fact, the best support should have an optimum porous structure, excellent electrical conductivity, high surface area and large pore volume [12]. Vulcan XC72 (a commonly used carbon support) carbon supported electrocatalysts exhibit random pore structures with good active nanometal distribution and low gas flow. Vulcan XC72 carbon pore structure majorly consists of micropores and macropores. Based on the versatile pore architecture, electrical conductivity of the commercial carbon supports have been reported as ca. 4.0 S/cm. On the other hand, ordered mesoporous carbons, including HCMS carbon support family, have mesopores with high active metal dispersion over the support. In addition, pore structure enables high gas flow to the active sites; however, structure may lower the metal accessibility for some synthesis conditions. The order of the electrical conductivity of the mesoporous carbon supported electrocatalysts vary in the range of 0.003e1.4 S/cm. Pore texture with good interconnections exhibit higher electrical conductivity and one must adjust porosity accordingly to obtain desired electrical conductivity [13]. In the present work, it was aimed to synthesize hollow core mesoporous shell (HCMS) carbon support for PEM fuel cells in order to provide a tailored pore network with high surface area and mesoporous shell structure. Microwave irradiation technique is a promising technique for catalyst preparation and polyol process is used in the microwave irradiation technique. In order to increase the Pt loading on the carbon support it is essential to provide better preparation conditions. This study is focused on increasing the Pt loading with the addition of further reducing agents, such as hydrazine and sodium borohydride, in order to enhance the reducing properties in the microwave conditions. HCMS carbon supported Pt electrocatalysts were prepared with high Pt loading in order to provide a better utilization of the active platinum metal over the carbon support. Effect of enhanced Pt loading on PEM fuel cell operation was investigated by using electrochemical characterization techniques including Cyclic Voltammetry (CV) and PEM fuel cell performance tests.

2.

Experimental methods

2.1. Synthesis of solid core mesoporous shell (SCMS) silica SCMS1 silica was synthesized with the procedure given in our previous study [14]. In the present work, similar procedure

was followed for the synthesis of SCMS silica. Other SCMS silica templates (SCMS2-SCMS5) were either synthesized by changing synthesis temperature from 30  C to 45  C or by altering the volumetric ratio of the silica source (TEOS) and porogen (C18TMS) according to the amounts given in Table 1.

2.2. Synthesis of hollow core mesoporous shell (HCMS) carbon supports HCMS11 and HCMS12 carbon supports were synthesized by following the phenolic resin route and HCMS21, HCMS22, and HCMS23 carbon supports were synthesized by using the DVB/ AIBN route with the procedure given elsewhere [14]. HCMS11 and HCMS12 carbon supports were prepared by only changing the silica template from SCMS1 to SCMS2, respectively. On the other hand, molar ratio of DVB/AIBN was varied from 20 to 26 for the carbon supports prepared with the DVB/AIBN route. Synthesis conditions for the carbon supports are given in Table 2.

2.3.

Catalyst preparation

Pt/HCMS electrocatalysts were prepared by microwave irradiation method given in our previous study [15]. Briefly, target amount of 0.05 M aqueous solution of Pt was mixed with different volumes of ethylene glycol (reducing agent) and 0.1 g HCMS carbon support. In addition to ethylene glycol (C2H6O2), hydrazine (N2H4), and sodium borohydride (NaBH4) were also used as reducing agent for further reduction of the catalyst. After ultrasonic mixing for half an hour, the mixture was put in a microwave oven and heated for different microwave durations at 800 W. The resulting suspension was cooled and filtered off and the residue was washed with acetone and DI water. The solid product was dried overnight at 373 K in a vacuum oven.

2.4.

Catalyst characterization

2.4.1.

Physical characterization

In order to investigate the structural properties of SCMS silica templates and HCMS carbon supports nitrogen adsorption/ desorption analysis was performed with a surface area

Table 1 e Synthesis conditions and structural properties of SCMS silica templates. SCMS1 SCMS2 SCMS3 SCMS4 SCMS5 Synthesis Conditions TEOS Addition (ml) TEOS þ C18TMS Addition (ml) Synthesis Temperature ( C) Structural Properties BET Surface Area (m2/g) Average Pore Diameter (nm) Total Pore Volume (cm3/g)

6 5þ2

6 5þ2

10 5þ2

6 5þ4

6 7þ4

30

45

30

30

30

462.7

555.5

435.1

289.7

203.8

3.05

2.20

2.45

2.62

2.51

0.50

0.27

0.35

0.24

0.18

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Table 2 e Synthesis conditions and structural properties of HCMS carbon supports. HCMS11 Synthesis Conditions Synthesis Routes Silica Template DVB/AIBN Molar Ratio Structural Properties BET Surface Area (m2/g) Average Pore Diameter (nm) Total Pore Volume (cm3/g)

HCMS12

HCMS23

DVB/AIBN Route SCMS1 20

SCMS1 24

SCMS1 26

1053 1.05 0.49

1182 3.07 1.88

1072 3.41 1.50

852 3.44 1.37

1034 1.47 0.55

Cyclic voltammetry (CV) tests

Electrochemical characterization of the electrocatalysts was carried out with ex-situ Cyclic Voltammetry (Pine Instrument) tests. Hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) were investigated by using a standard three electrode cell configuration with CV. Cyclic voltammograms for the HOR were recorded in a 0.1 M HClO4 electrolyte solution that was saturated with nitrogen for 30 min to remove the oxygen. All the cyclic voltammetry tests were performed at room temperature. CV data were reported with respect to a normal hydrogen electrode (NHE). The stability of the catalysts was investigated by CV with 500 continuous voltammetric cycles at a scan rate of 50 mV s1 between 0e1.2 V. HOR was evaluated before and after the potential cycling degradation test. The second potential cycling test was performed between 0.6 and 1.2 V with a scan rate of 20 mV s1 for 1000 cycles to provide Pt dissolution/ agglomeration, since at this potential region active platinum metal is unstable and the oxidation/reduction cycles occur. The Electrochemical Surface Area (ESA) losses for HOR were determined by comparing the activities before and after the potential cycling tests. Cyclic voltammograms for the ORR was performed by purging the electrolyte with oxygen for half an hour and then rotating the disk electrode between 100 and 2500 rpm at 5 mV s1.

2.4.3.

HCMS22

Phenolic Resin Route SCMS1 SCMS2 N/A N/A

analyzer, Quantachrome Autosorb-1C, at 77 K. Thermal decomposition of SCMS silica, HCMS carbon support, and electrocatalysts was conducted by Thermogravimetric Analysis (TGA) with Perkin Elmer Pyris 1 and Shimadzu DTG-60H under the flowing air (50 cm3/min) with a heating ramp of 10  C/ min. The crystalline structures of HCMS carbon and Vulcan XC72 carbon supported platinum electrocatalysts were characterized by measuring their X-Ray Diffraction (XRD) patterns on a Rigaku Ultima D-Max 2200 with monochromatic Cu Ka radiation. 2q measurements of the electrocatalysts were scanned in a range of 5 e85 with a step size of 0.02 . Additionally, Scanning Electron Microscopy (SEM, Quanta FEG) and Energy Dispersive X-Ray Spectrometer (EDS, FEI Nova Nano 430 FEG) analysis were conducted to investigate surface morphology and composition of the nanomaterials.

2.4.2.

HCMS21

Single cell PEMFC tests

PEM fuel cell performance tests were performed with single cell test fixture (Electrochem, FC05-01 SP REF) with the home made fuel cell test station. The details of MEA preparation method, PEM fuel cell test protocol and test station are given elsewhere [16]. Fuel cell test hardware provides an active area of 5 cm2 and

cell temperature was adjusted to 70  C. Humidification temperature of the fuel and oxidant streams was also set to 70  C. Flow rates of the pure hydrogen and oxygen gases were adjusted to 100 cm3/min during the course of the experiments.

3.

Results and discussion

3.1.

Solid core mesoporous shell silica

Solid Core Mesoporous Shell (SCMS) silica was synthesized by changing the ratios of the TEOS (silica source) and C18TMS (porogen) chemicals. Synthesis temperature was adjusted to 30  C and 45  C in different experimental runs in order to control the structural parameters. The effect of synthesis temperature and volumetric ratios (silica source/porogen) on the structural properties of template including surface area, average pore diameter, and total pore volume are given in Table 1. Physical properties of SCMS silica template structure are very sensitive to changes in the porogen and silica source ratios in the experiments. As can be seen from Table 1, adding as high as 10 ml of TEOS in the first step of silica synthesis (SCMS3), resulted in a decrease in the average pore diameter. On the other hand, further addition of porogen (C18TMS) into the structure effectively increased the average pore diameter of the silica template (SCMS4). As a result of increase in the ratios of both TEOS and C18TMS at the second stage of the synthesis, SCMS5 sample showed an average pore diameter of 2.51 nm. Surface area of the SCMS3, SCMS4 and SCMS5 templates were decreased with the change in the synthesis conditions. As the temperature of the reaction medium was increased from 30  C to 45  C, the pore diameter and pore volume of SCMS2 were decreased significantly. Having larger mesopores with increased BET surface area, SCMS1 template was used for the synthesis of the corresponding HCMS carbon supports (Table 2). N2 adsorption/ desorption analysis was conducted at 77 K. N2 adsorption/ desorption isotherms and pore size distribution of the SCMS1 silica template are given in Fig. 1(a) and (b), respectively. Brauner-Emett-Teller (BET) surface area of SCMS1 silica was calculated as 462.7 m2/g for a total pore volume of 0.50 cm3/g. Corresponding pore size distribution data for SCMS1 silica template was calculated from the nitrogen isotherm adsorption branches by the BarretteJoynereHalenda (BJH) method, which showed that the pores are uniform with narrow pore size distribution centered at around 3.05 nm. Energy Dispersive X-Ray Spectroscopy (EDS) analysis was performed for SCMS1 sample in order to detect the surface

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a

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a

300

100

90

250

Weight Change, %

Volume Adsorbed (cc/g) STP

Adsorption Desorption

200

150

100

80

70

60

50

40

50 0.0

0.2

0.4

0.6

0.8

0

1.0

200

b

400

600

800

1000

800

1000

Temperature, ºC

Relative Pressure (P/Po)

b

60

100

90

40

Weight Change, %

Incremental pore volume, cc/g

50

30 20

80

70

10 60

0 50

1

10 Pore Diameter, nm

100

Fig. 1 e (a) N2 adsorption/desorption isotherm of SCMS1 silica template (b) Pore Size Distribution of SCMS1 silica template.

composition. Percent weight concentrations of Si and O elements were 43.24% and 37.57%, respectively. The balance was Au element, which was used for coating in the EDS analysis. No other impurities were detected in the surface composition analysis of silica template. Thermal history of SCMS1 silica template was investigated by thermogravimetric analysis (TGA) as given in Fig. 2(a). As can be seen from Fig. 2(a), the organic group removal from the SCMS silica structure starts after 256  C and calcination process was completed at around 600  C. Further elevation of the calcination temperature did not decompose the structure of the solid core mesoporous shell silica. The material could be considered reasonably stable in the temperature range of 600e900  C. The surface morphology of the SCMS1 silica template was reported in our previous study [14]. It was found that most of the spherical silica particles were uniform with particle diameters of approximately 500 nm.

3.2.

Hollow core mesoporous shell carbon

HCMS carbon support synthesis was carried out by two different routes. In the phenolic resin route, phenol and

0

200

400

600

Temperature, ºC

Fig. 2 e Thermogravimetric analysis data of (a) SCMS1 silica template (b) HCMS carbon support.

paraformaldehyde were used as the carbon source. SCMS1 and SCMS2 samples were used as the silica templates for carbon synthesis with phenolic resin route, and it was observed that average BJH adsorption pore diameters were 1.05 and 1.47 nm, respectively. As an alternative for hollow core mesoporous shell carbon synthesis, DVB/AIBN route carbon support synthesis was conducted using divinylbenzene (DVB) as the carbon precursor. For DVB/AIBN route synthesis, the molar ratios of the carbon source (DVB) to Azobisisobutyronitrile (AIBN) were altered as 20, 24, and 26. The structural properties of the synthesized HCMS carbon supports with phenolic resin and DVB/AIBN routes are given in Table 2. Table 2 showed that by following the DVB/AIBN route, carbon synthesis with a molar ratio of DVB/AIBN ¼ 26 resulted in an average BJH adsorption pore diameter of 3.44 nm for HCMS23. It was observed that increasing the molar ratio of DVB/AIBN increased the average pore diameter of the carbon support (Table 2). When the same silica template used, it was seen that carbon supports synthesized with DVB/AIBN route exhibited larger pores compared to the carbon supports synthesized with phenolic resin route. Considering the desirable structural properties with larger mesopores and high BET surface area, HCMS23 carbon support was selected for further incorporation of platinum via microwave irradiation method.

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N2 adsorption/desorption isotherms and pore size distribution of the HCMS23 carbon support are shown in Fig. 3(a) and (b), respectively. Brauner-Emett-Teller (BET) surface area of HCMS23 was calculated as 852 m2/g for a total pore volume of 1.37 cm3/g. Corresponding pore size distribution data for HCMS23 carbon support calculated from the adsorption branches of nitrogen isotherms by the BJH method showed that the pores are uniform with narrow pore size distribution centered at 3.44 nm. TGA was conducted to understand the thermal history of HCMS23 carbon support (Fig. 2(b)). As seen from Fig. 2(b), calcination process for the HCMS carbon material performed at high temperatures (800e1000  C) and it was seen that the carbon support can withstand these elevated temperatures without any decomposition in the structure. Pyrolysis step in carbon support synthesis was carried out in two stages; first the temperature of the medium was raised to 160  C and kept for 5 h at this temperature, and then the temperature was increased up to 850  C and kept for 7 h under flowing nitrogen. According to TGA result given in Fig. 2(b), calcination process conditions were appropriate for these temperature ranges.

3.3.

Characterization of the Pt/C electrocatalysts

Platinum impregnation was conducted by microwave irradiation method. Reducing agents used in the experiments were

a

1000

Fig. 4 e SEM image of 20% Pt/HCMS23 electrocatalyst.

ethylene glycol, hydrazine, and sodium borohydride. The required thermal environment for the reduction of platinum particles on carbon support was supplied by microwave oven. A commercially available carbon support, Vulcan XC72, and

a

Adsorption Desorption

90 Weight change , %

Volume Adsorbed (cc/g) STP

800

100

600

400

80

70

60

200

50 0 0.0

b

0.2

0.6 0.4 Relative Pressure (P/Po)

40

0.8

0

b

200

400

600

800

1000

800

1000

Temperature, ºC 100 90

30

Weight change, %

Incremental pore volume (cc/g)

40

20

10

80 70 60 50 40

0 1

10

100

Pore Diameter (nm)

Fig. 3 e (a) N2 adsorption/desorption isotherm for HCMS23 carbon support, (b) Pore size distribution for HCMS23 carbon support.

30 0

200

400

600

Temperature, ºC

Fig. 5 e Thermogravimetric analysis data of (a) 44% Pt/ Vulcan XC 72 catalyst (b) 32% Pt/HCMS23 catalyst.

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{1 1 1}

{2 0 0}

{2 2 0}

{3 1 1} PtV7

Intensity (au)

PtV6 PtV5 PtV4 PtV3 PtV2 PtV1

0

20

40

60

80

100

2 Theta, o

Fig. 6 e XRD patterns of the prepared Pt/C electrocatalysts.

commercial catalyst, 20% Pt/C (ETEK), were also used for comparison. Scanning Electron Microscopy (SEM) was used to examine the surface morphology of the Pt incorporated electrocatalysts. Fig. 4 shows the SEM image of 17% Pt/HCMS23. It was observed that the platinum nanoparticles were uniformly distributed over the HCMS carbon support. However, agglomeration of platinum was seen on some of the spherical carbon supports. This SEM result reveals that for incorporation of platinum nanoparticles efficiently, it is critical to synthesize carbon supports with larger mesopores. Sample TGA results for different electrocatalysts are given in Fig. 5(a) and (b). TGA analysis results showed that 44% Pt loading was achieved for commercial carbon support Vulcan XC72 by using moderate amount of ethylene glycol. For the impregnation of synthesized carbon supports, utilization of moderate amount of ethylene glycol resulted in 32% Pt/ HCMS23 electrocatalyst. Increase in the ethylene glycol amount did not cause an increase in the weight percentage of platinum on carbon support. Using NaBH4 as an additional reducing agent did not increased the platinum content of the electrocatalyst significantly. If the moderate amount of ethylene glycol was used with an additional reducing agent of hydrazine, the Pt loading was increased up to 34% for PtV7 electrocatalyst.

Prepared electrocatalysts were also characterized by XRD and the corresponding particle sizes were calculated from XRD data and Scherrer formula by using the peak located at (220) inflection (Fig. 6). Characteristic peaks that correspond to the face-cubic-centered (fcc) Pt were obtained for all catalysts as (111), (200) and (220). Sharpening of the peaks indicates that the particle size of the catalysts is growing and possibly leading to agglomeration. Synthesis conditions strongly affect catalyst properties such as Pt loading and average particle size. The results are listed in Table 3. Kim et al have reported that using ethylene glycol (EG) with NaBH4 facilitated 40% Pt/Vulcan electrocatalyst preparation [17]. However using EG together with NaBH4 as reducing agents did not improve the Pt content of the Pt/HCMS23 electrocatalyst. Pt loading over the HCMS23 carbon support was increased from 32% to 34% by using hydrazine and EG as reducing agents however, the largest average particle size among all the electrocatalysts was obtained for this case (4.5 nm). Fang and his coworkers [18] used higher amount of EG with increased microwave power compared to the previous work of Fıc¸ıcılar et al. [19]. When they used EG, NaBH4 and urea as reducing agents, they achieved 60% platinum impregnated on commercial Vulcan XC72 carbon support.

3.4. Electrochemical characterization of prepared electrocatalysts Thermodynamically carbon can be oxidized to carbon dioxide above 0.207 V versus NHE as follows: C þ 2H2 O/CO2 þ 4Hþ þ 4e

E ¼ 0:207 V

(1)

Oxidation of carbon support is called carbon corrosion and can decrease the performance in the fuel cell due to the accelerated loss of active surface area and the change in the pore morphology and pore surface characteristics [20]. The cyclic voltammograms for the prepared electrocatalysts are given in Fig. 7. The electrochemical oxidation of the synthesized HCMS carbon support was investigated by continuous potential cycling tests. Continuous potential cycling was conducted up to 500 cycles with a scan rate of 50 mV/s and given in Fig. 7(a). It was seen that there was an obvious current peak appears approximately at 0.55 V which is

Table 3 e Synthesis conditions and characterization of Vulcan XC72 and HCMS23 carbon supported electrocatalysts. Electrocatalyst Pt/Vulcan XC72 PtV1 Pt/HCMS23 PtV2 PtV3 PtV4 PtV5 PtV6 PtV7

Reducing Agent

Microwave Duration (min)

Pt Contenta (%)

Particle Sizeb (nm)

EG (moderate)

2

44

3.4

EG (high) EG (moderate) þ NaBH4 (low) EG (high) EG (low) EG (moderate) EG (moderate) þ Hydrazine (low)

2 2 2þ1 2 2 2

17 23 24 28 32 34

3.2 4.2 4.2 4.0 4.2 4.5

a From Thermogravimetric Analysis (TGA) data. b From X-Ray Diffractionanalysis.

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a 0.0008

500 300

0.0006

c

150

50 20

0.0015 20 cycles 50 cycles 150 cycles

0.0010

0.0002

0.0005 Current (mA)

Current (mA)

0.0004

0.0000 -0.0002 -0.0004

300 cycles 500 cycles

0.0000

-0.0005

-0.0006

-0.0010

-0.0008 -0.0010 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

-0.0015 0.0

Potential (V vs Ag/AgCl)

d

b 0.0010 0.0008

17 wt %

0.2

0.6 0.4 0.8 Potential (V vs Ag/AgCl)

1.0

1.2

1.4

0.0004 0.0003

0.0004

28 wt %

0.0002

32 wt %

-0.0000 -0.0002 -0.0004

0.0002 Current (mA)

Current (mA)

0.0006

0.0001 0.0000 1500 cycles

-0.0001

-0.0006

1000 cycles

-0.0008

-0.0002

-0.0010

-0.0003

500 cycles 20 cycles

-0.0012 0.0

0.2

0.6 0.4 0.8 Potential (V vs Ag/AgCl)

1.0

1.2

1.4

-0.0004 0.0

0.2

0.6 0.4 0.8 Voltage (V vs Ag/AgCl)

1.0

1.2

1.4

Fig. 7 e Cyclic voltammograms in 0.1 M HClO4 with a scan rate of 50 mV/s for (a) HCMS23 carbon support after potential cycling up to 500 cycles (b) 17, 28 and 32% Pt/HCMS23 after 500 cycles (c) 28% Pt/HCMS23 electrocatalysts during continuous potential cycling (d) 32% Pt/HCMS23 electrocatalysts during continuous potential cycling.

believed that resulted from the surface oxide formation due to the hydroquinone-quinone (HQ-Q) redox couple on the carbon support surface. At low cycle numbers the current peak is also seen which may indicate that the synthesized carbon supports also have some surface oxide groups on the surface after the synthesis. As the number of cycles increase the peak becomes more pronounced which means that the carbon oxidation over the carbon support surface also increases. The effect of Pt loading on the HOR activity of HCMS supported Pt catalysts was also investigated. To examine the effect of platinum loading on the hydrogen oxidation reaction activity of the electrocatalysts half cell CV tests were conducted for 17, 28, and 32% Pt/HCMS23 electrocatalysts. The cyclic voltammograms for these electrocatalysts up to 500 cycles are given in Fig. 7(b). As can be seen from Fig. 7,

a decrease in the double layer capacitance of the electrocatalysts was observed with increasing platinum loading. The principle of double layer capacitance at the electrode/electrolyte interface is the accumulation of the electric charges on the electrode surface while ions of opposite charge are arranged in the electrolyte side. Double layer capacitance depends critically on surface area of carbon material used in electrodes. The higher the specific surface area of the carbon the higher the specific capacitance. In addition, carbon materials with larger pores should be capable of delivering high power because it can be discharged/charged at higher current density because larger pores would provide more favorable and quicker pathway for ions to penetrate [21]. Since 17% Pt/HCMS has the highest specific surface area because of low Pt loading and the lowest particle size with

Table 4 e Electrochemical active surface area comparison of Pt/HCMS23 electrocatalysts for HOR after 500 cycles in 0.1 M HClO4 with a scan rate of 50 mV/s. Electrocatalyst

17% Pt/HCMS23 28% Pt/HCMS23 32% Pt/HCMS23

Electrochemical Surface Area (m2/g)

Sample

PtV2 PtV5 PtV6

H2 Desorption Peak

H2 Adsorption Peak

Total Sorption Peak

0.077 0.794 0.525

1.372 2.198 1.428

1.449 2.992 1.953

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Table 5 e Effect of number of CV cycles on electrochemical active surface area of 32% Pt/HCMS23 electrocatalyst. Electrocatalyst 32% Pt/HCMS23 (PtV6)

Number of Cycles

ESA (m2/g)

ESA Lossa (%)

20 500 1000 1500

3.12 1.95 1.34 1.21

N/A 37.3 57.1 61.3

a Electrochemical Surface Area (ESA) loss is calculated with respect to 20 cycle case.

0.0006 0.0004

Current, A

0.0002

After adt

0.0000 -0.0002 -0.0004

Before adt

-0.0006 -0.0008 0.0

0.2

0.6 0.4 0.8 Voltage, V vs Ag/AgCl

1.0

1.2

1.4

Fig. 8 e Cyclic voltammogram of 28% Pt/HCMS23 electrocatalysts before and after ADT in 0.1 M HClO4 with a scan rate of 50 mV/s.

3.2 nm, its capacitance is the highest one when compared to other electrocatalysts. At the same time, 32% Pt/HCMS exhibited the lowest double layer capacitance which has the largest particle size of 4.2 nm and lowest specific surface area. In addition, electrochemical active surface areas of 17, 28, and 32% Pt/HCMS catalysts after 500 cycles in 0.1 M HClO4 with a scan rate of 50 mV/s were also calculated as [22] and given in Table 4. 28% Pt/HCMS23 catalyst showed the highest ESA as 2.992 m2/g. The stability of the high Pt loading electrocatalysts was investigated by continuous potential cycling. Fig. 7(c) shows the effect of potential cycling up to 500 cycles on the HOR activity of the 28% Pt/HCMS23 electrocatalyst. The hydrogen adsorption/ desorption peaks were obviously seen and used for the calculation of the electrochemical active surface area by taking the average of the areas under the curve under hydrogen adsorption/desorption peaks. Active electrochemical surface areas

(ESAs) of the prepared 28% Pt/HCMS23 electrocatalyst were calculated as 13.3 m2/g Pt and 9.5 m2/g Pt after 20 and 500 cycles, respectively. ESA was decreased about 29% after potential cycling which may be attributed to carbon corrosion and Pt agglomeration/dissolution. In 20 and 50 cycles, the HCMS oxide reduction peak appeared as a shoulder but further increase in the number of cycles (150e500) resulted in a clearly distinguished second peak in addition to the Pt oxide reduction peak. This observation is in agreement with the studies in the literature where these peaks are attributed to oxidation and reduction reactions of surface oxygen groups attached to carbon [23]. As the number of cycles increased, the intensity of the peaks corresponding to carbon oxidation/reduction also increased. There was a positive shift of the platinum oxide reduction peaks during the potential cycling which can be attributed to the change in the surface composition of the electrocatalysts. The cyclic voltammograms of 32% Pt/HCMS electrocatalyst which has the highest Pt loading are given in Fig. 7(d). The voltammograms were cycled up to 1500 cycles with a scan rate of 50 mV/s. As can be seen from figure, ESA also decreases as the number of cycle increases. Electrochemical Active Surface Areas (ESAs) of each cycle were calculated and given in Table 5. Another accelerated degradation test (ADT) was also performed to 28% Pt/HCMS23 electrocatalyst including the potential cycling between 0.6 and 1.2 V (in which Pt oxidation/ reduction occurs and Pt is unstable) up to 1000 cycles with a scan rate of 20 mV/s. Fig. 8 represents the cyclic voltammogram for 28% Pt/HCMS23 electrocatalyst before and after ADT. It was observed that the ESA also decreases during ADT and carbon oxide peaks are getting more pronounced. ESA was calculated as 5.726 and 3.676 m2/g before and after ADT, respectively. ADT was resulted in an ESA loss of 35.8% which can be attributed to the agglomeration of the small particles and also the carbon corrosion. The effect of Pt loading on the ORR activity of HCMS supported Pt catalysts was also investigated for 28 and 32% Pt/HCMS electrocatalysts as given in Fig. 9(a) and (b), respectively. ORR data were obtained by purging the electrolyte with oxygen half an hour and then rotating the disk electrode between 100 and 2500 rpm with a scan rate of 5 mV/ s in 0.1 M HClO4. The ORR activity of the catalysts can be examined by measuring the current at 0.9 V (vs NHE) and then dividing this current by Pt mass in order to obtain the so-called mass activity (im(0.9 V), A/mgPt) or by the electrochemically active Pt surface area to obtain specific activity (is(0.9 V), mA/cm2Pt) as given with the following equation where mass and specific activities can be related via the Pt specific surface area (APt) [24].

Table 6 e Kinetic parameters obtained from polarization curve. Anode/Cathode 28% Pt/HCMS 44% Pt/HCMS ETEK/20% Pt/HCMS

Charge transfer coefficient

Exchange current density (A/cm2)

Limiting current density (A/cm2)

Overall ohmic cell resistance (ohm. cm2)

0.81 1.16 0.86

2.72  107 5.90  108 9.54  107

1.3500 1.139 0.734

19 0.27 0.55

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 8 6 5 e1 8 7 4

a

1.0

0.002

28% Pt/HCMS/28% Pt/HCMS 44% Pt/Vulcan XC72/44% Pt/Vulcan XC72 ETEK/20% Pt/HCMS

0.001

0.8 0.000

rpm

Cell Voltage (V)

100 I, A cm

-2

-0.001 -0.002

1873

400

-0.003

900

-0.004

1600

-0.005

2500

-0.006 0.0

0.2

0.6

0.4

0.2

0.4

0.6

0.8

1.0

1.2

0.0

1.4

0

E, V vs NHE

b

0.001

400

600 800 1000 1200 1400 1600 1800 2) Current Density (mA/cm

Fig. 10 e PEMFC polarization curve of high Pt loading commercial and synthesized carbon supported electrocatalysts at 70  C.

rpm 0.000

200

100

I, A cm

-2

400 -0.001

900 1600

-0.002 2500 -0.003 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

E, V vs NHE

Fig. 9 e Hydrodynamic voltammograms for ORR in O2 saturated 0.1 M HClO4 with a scan rate of 5 mV/s for (a) 28% Pt/HCMS23 (b) 32% Pt/HCMS23 electrocatalysts.

 

im ð0:9VÞ A=mgpt ¼isð0:9 vÞ mA=cm2 Pt APt m2 Pt =gPt 105

(2)

By using the data obtained from Fig. 7 and Equation (2) corresponding mass activities for the 28 and 32% Pt/HCMS catalysts were calculated as 125 and 85.3 mA/mgPt, respectively, at 1600 rpm. This result shows that 28% Pt/HCMS had a better ORR activity than 32% Pt/HCMS which can be attributed to higher electrochemical surface area which could be caused by high dispersion and uniform distribution of the Pt particles on the HCMS carbon support [25].

3.5.

PEM fuel cell tests

PEM fuel cell tests were also performed to determine the electrocatalytic activity of the prepared catalysts in the fuel cell environment. Fig. 10 shows the fuel cell performance of the single cells prepared with different anode and cathode electrocatalysts based on different carbon supports. Fig. 10 showed the effect of increase in Pt loading on the performance of PEM fuel cell when the commercial Vulcan XC72 carbon support was used as the electrocatalyst support. Two different fuel cells were constructed. One of them was prepared with the 44% Pt/Vulcan XC as both anode and cathode electrocatalysts. Other fuel cell was constructed with

commercial 20% Pt/C (ETEK) catalyst as the anode and 20% Pt/Vulcan XC72 prepared by microwave irradiation as the cathode. For each electrode, Pt loading was set to 0.4 mg Pt/ cm2. Fig. 10 revealed that increase in the Pt weight percentages on carbon support resulted in an increase on the PEM fuel cell performance. The increase in the Pt amount over the carbon support may enhance the contact of the catalyst with the electrolyte and also it may decrease the contact resistance. Fig. 10 also exhibits the effect of increase in Pt loading on the performance of PEM fuel cell when HCMS carbon support is used as the electrocatalyst support. Three different fuel cells were constructed. Two of them were prepared with the 28% Pt/HCMS and 44% Pt/Vulcan XC72 as both anode and cathode electrocatalysts. Other fuel cell was constructed with commercial 20% Pt/C (ETEK) catalyst as the anode and 20% Pt/HCMS as the cathode. PEMFC polarization curve also showed that increase in the Pt loading resulted in an improved PEM fuel cell performance for both Vulcan XC72 and HCMS carbon support based electrocatalysts. The kinetic parameters obtained from polarization curve are given in Table 6. Overall cell resistance can be considered as a measure of protonic conductivity. 28% Pt/HCMS exhibited the lowest overall cell resistance (0.19 ohm. cm2) with the largest exchange current density (2.72  107) as compared to the other synthesized eletrocatalysts, which may indicate that 28% Pt/HCMS has the highest protonic conductivity among the synthesized electrocatalysts. Overall cell resistance of ETEK/20%/Pt/HCMS electrocatalyst was the highest (0.55 ohm. cm2), indicating a poor ionic conductivity compared to the other synthesized electrocatalysts.

4.

Conclusions

The electrocatalytic activity of the catalyst and the platinum utilization depend critically on the contact between the electrolyte and platinum nanoparticles. Therefore, it is essential

1874

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 8 6 5 e1 8 7 4

to tailor carbon support structure that enhances the contact at the three-phase boundary. With the change of the carbon precursor type from phenol/paraformaldehyde to divinylbenzene, average pore size of the carbon supports increased further. After providing a suitable pore network, it was achieved to access platinum loading on the carbon support up to 34% over HCMS carbon support. Carbon support having higher pore diameter showed improved properties. As a result of PEM fuel cell performance tests, 44% Pt/Vulcan XC72 and 28% Pt/ HCMS electrocatalysts exhibited improved fuel cell performances than their corresponding 20% prepared electrocatalysts. Increasing the Pt loading further may give better fuel cell performances, so it is essential to prepare the carbon supports with a better structure that allow to increase the Pt loading. The Pt loading over the carbon support was strongly affected by the catalyst preparation conditions especially including the type of the reducing agent. By optimizing the ratios of NaBH4 and hydrazine with respect to ethylene glycol, Pt loading over the carbon support might be increased. Additionally, microwave power and duration are critical electrocatalyst preparation parameters that are needed to be controlled and optimized to provide a better carbon-platinum contact in the mesopores of the electrocatalyst.

Acknowledgments The authors gratefully acknowledge the financial support by Turkish Scientific and Technological Research Council _ (TUBITAK) through grant number 109M221 and Middle East Technical University BAP project through grant number BAP03-04-2009-06.

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