Fabrication of catalyst by atomic layer deposition for high specific power density proton exchange membrane fuel cells

Journal of Catalysis 294 (2012) 63–68

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Fabrication of catalyst by atomic layer deposition for high specific power density proton exchange membrane fuel cells Yang-Chih Hsueh a, Chih-Chieh Wang a, Chi-Chung Kei b, Yu-Hung Lin a, Chueh Liu a, Tsong-Pyng Perng a,c,⇑ a

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 300, Taiwan c Department of Chemical Engineering and Materials Science, Yuan Ze University, Chungli 320, Taiwan b

a r t i c l e

i n f o

Article history: Received 22 February 2012 Revised 21 May 2012 Accepted 7 July 2012 Available online 21 August 2012 Keywords: Carbon nanotubes Atomic layer deposition Proton-exchange-membrane fuel cell Platinum Catalyst

a b s t r a c t Platinum nanoparticles as a catalyst for proton-exchange-membrane fuel cells (PEMFCs) were grown on nitric acid-treated multiwalled carbon nanotubes by atomic layer deposition (ALD) at 250 °C. Formation of uniform and well-distributed Pt nanoparticles was achieved. The size and number of Pt nanoparticles could be controlled by the ALD cycle number. The PEMFC test shows that for the membrane electrode assembly made of both anode- and cathode-deposited Pt (0.019 and 0.044 mg cm2, respectively) after 100 cycles of ALD has 11 times higher specific power density than that made of commercial E-Tek electrodes containing 0.5 mg cm2 of Pt. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Proton-exchange-membrane fuel cells (PEMFCs) are energyconversion devices that have potential applications in portable electronics, electric vehicles, and public transportation due to their remarkably high energy density at low operating temperatures (60–100 °C) and low pollution in the reaction process [1,2]. There are several key issues of popularizing fuel cell utilization, such as durability, cost, performance, and water transport within the stack. Since the 1960s, much progress has been made in development of PEMFCs in terms of reducing the platinum loading and cost while maintaining sufficient power density [3]. With consideration of cost and supply, the amount of platinum for stationary power supply and automobile applications needs to be reduced to 0.2 g kW1 at P0.65 V (with H2/air at 80 °C) [4]. Carbon-supported platinum is generally used as the electrode catalyst. To enhance the efficiency of catalyst utilization, the support must have surface area and electric conductivity as high as possible. In recent years, carbon nanotubes (CNTs) with especially high surface area have been chosen as the support for Pt. Pt nanoparticles cannot be easily deposited on pristine CNTs because of their inert surface [5]. Acid treatment is a common process for grafting functional groups to the surface of CNTs, and therefore this method was adopted to modify the surfaces of CNTs in this study [6]. ⇑ Corresponding author at: Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan. Fax: +886 3 5723857. E-mail address: [email protected] (T.-P. Perng). 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.07.006

There are numerous strategies for preparing catalysts for use in PEMFCs. These strategies can be divided generally into two categories: wet and dry processes. Wet processes include chemical [7] and electrochemical [8,9] while dry processes include sputtering [10–12], ion-beam-assisted deposition [13], and pulse laser deposition [14]. ALD is a dry process and it has some advantages for deposition of Pt catalysts: good uniformity, precise thickness control, and large-area and large-batch production. Therefore, ALD was used to deposit Pt nanoparticles in a previous work [15] and the present study. It has been found that the activity of catalysts is largely determined by the size of Pt nanoparticles and their distribution on the support structure.

2. Experimental procedure 2.1. Synthesis The support material was commercial multiwalled CNTs (MWCNTs) obtained from Powertip Technology Corporation, Taichung, Taiwan. These CNTs were not purified. The content of MWCNTs was higher than 90 wt.%, with the most residues being carbon black and amorphous carbon. The length and outer diameter of the MWCNTs were 3–12 lm and 20–40 nm, respectively. Pristine CNTs were immersed in HNO3 (65%) and refluxed at 140 °C for 3, 9, or 12 h. After the acid treatment, Pt catalyst nanoparticles were deposited on CNTs by a homemade ALD system. A stainless steel bottle containing (methylcyclopentadienyl)trimethylplatinum

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(MeCpPtMe3) as the Pt precursor was interfaced with the pulsing valve system, the temperature of which was maintained at 45 °C by a temperature controller. O2 (99.999%) was used as a co-reactant. The substrate heating plate in the chamber was maintained at 250 °C. The cycle number, Pt pulse time, and N2 purge time were varied to meet different requirements. Each ALD cycle consisted of four consecutive steps: a MeCpPtMe3 pulse, followed by a N2 purge, and then an O2 pulse, followed by another N2 purge. The O2 pulse time was controlled at 5 s for all ALD processes. The N2 purge time was 20 s if the Pt precursor pulse time was less than 5 s, and it was extended to 60 s when the Pt precursor pulse time was longer than 5 s, in order to completely purge away the rest of the reactant gases.

treatment were characterized by X-ray photoelectron spectroscopy (XPS) using a monochromatic Mg Ka X-ray at 250 W. The energy was calibrated using the Au4f7/2 peak at 83.8 eV, and the energy resolutions were 1.6 eV for the survey scan and 0.2 eV to obtain the core-level spectra. The C and O content was determined by integrating the core-level peak areas of C (1s) and O (1s), respectively. Evaluation of the evolution of nanoparticle size under various conditions was made by high-resolution transmission electron microscopy (HRTEM). X-ray diffraction (XRD) was employed to examine the structure of Pt deposited on carbon nanotubes. Inductively coupled plasma mass spectrometry (ICP-MS) was adopted to quantify the amount of Pt in the sample.

2.2. Characterization

2.3. Electrochemical measurements

The morphologies of the CNTs before and after Pt nanoparticle deposition were examined using a field emission scanning electron microscope (FESEM). To evaluate the surface chemical composition of CNTs, the constituent chemical elements of the CNTs after acid

To fabricate membrane electrode assembly (MEA), the gas diffusion layer, together with a catalyst layer, was placed on one side of a carbon cloth (B-1/B/StdWP, E-Tek Division, Somerset, New Jersey, USA). The electrode was prepared either with Pt/C commercial catalyst (20 wt.%, 0.5 mg Pt/cm2, E-Tek Division) or with Pt/CNT from this study. The electrode area was 4 cm2 and the loading of Nafion (5 wt.%, DuPont, Wilmington, Delaware, USA) was about 1.0 mg/ cm2. The MEAs were prepared by hot-pressing two electrodes with a pretreated Nafion 115 membrane at 140 °C and 600 psi for 90 s. Polarization curves were measured at 60 °C using pure H2 and O2 saturated with water at 30 °C for the anode and cathode, respectively. The flow rates of H2 and O2 were both 50 sccm. All MEAs were conditioned prior to testing. The system was maintained at a cell potential of 0.65 V with pure H2 for 2 h to achieve a steady state of the operating conditions.

Intensity (a.u.)

O1s

C1s

12 h

22.99%

77.01%

9h

15.89%

84.11%

3h

9.68%

90.32%

5.45%

94.55%

pristine

3. Results and discussion 1200

1000

800

600

400

200

0

Binding energy (eV) Fig. 1. XPS spectra of the CNTs acid-treated for various lengths of time.

XPS analysis was adopted to observe the change in chemical bonding on the CNT surface after acid treatment. The XPS survey scan spectra for CNTs with various lengths of treatment time are

Fig. 2. SEM images of Pt nanoparticles deposited on (a) pristine CNTs with 100 cycles of ALD and CNTs acid-treated with (b) 100, (c) 200, and (d) 300 cycles of ALD.

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(a)

30

40

(222) (311)

(220)

(200)

Intensity (a.u.)

(111)

Pt

50

60

70

80

90

2θ θ ((degree)

(b)

Graphite Pt

400 cycles

Intensity (a.u.)

given in Fig. 1. The peak located at 284.5 eV in all spectra is due to the photoelectrons emitted from the C1s core level, and the peak at 535.0 eV is generated by those from the O1s core level. The concentrations of carbon and oxygen are indicated in the spectra, and it is seen that the concentration of oxygen increases with the acid treatment time, while that of carbon decreases gradually at the same time. This suggests that the oxygen is successfully grafted onto the CNT surface. SEM was used to examine the morphologies of the CNTs after deposition of Pt, as shown in Fig. 2. Without acid treatment, Pt nanoparticles are only deposited on the silicon wafer, whereas with acid treatment, the particles are uniformly distributed on the surfaces of CNTs. The particle size of Pt increases with the cycle number of ALD. This phenomenon indicates that there are very few defects or nucleation sites on the pristine CNTs for Pt to grow. After acid treatment, the improved dispersion suggests that the acid treatment has induced formation of well-dispersed nucleation sites. More surface defects imply more reactive sites. Fig. 3a shows the HRTEM image of a CNT treated by nitric acid for 9 h followed by deposition of Pt with 100 cycles of ALD. The average particle size of Pt deposited on CNTs by 100 cycles of ALD is approximately 6.4 nm, found by measuring 200 random particles from the TEM image. The Pt nanoparticles have a lattice spacing of 0.22 nm, corresponding to the (1 1 1) plane of FCC Pt, which is in good agreement with the XRD pattern. The energy-dispersive spectrometric (EDS) analysis shown in Fig. 3b indicates that there is almost no oxygen signal in the platinum nanoparticles, demonstrating the high purity of platinum deposited by ALD. In Fig. 4a, the XRD pattern of a 20-nm-thick platinum film deposited by ALD on Si wafer was detected by grazing incident

300 200 100 50

30

40

50

60

70

80

90

2θ θ ((degree) Fig. 4. XRD diffraction patterns of Pt prepared by ALD: (a) 20-nm film on silicon wafer and (b) Pt nanoparticles on CNTs.

Pt loading (mg/cm2)

0.12

(a)

0.10 0.08 0.06 0.04 0.02 CNT : 4 mg/cm

2

0.00 0

100

200

300

400

Number of ALD cycle

(b)

Pt loading (mg/cm2)

0.05 0.04 0.03 0.02 0.01

100 cycles

0.00 0

2

4

6

8

10

Amounts of CNTs (mg / cm2) Fig. 5. Dependence of Pt loading on (a) the ALD cycle number and (b) the amount of CNTs. Fig. 3. (a) HRTEM image and (b) EDS peaks of Pt nanoparticles deposited on an acid-treated CNT with 100 ALD cycles. The inset in (a) is an HRTEM image of a single Pt nanoparticle.

diffraction (GID) XRD. The platinum film has a polycrystalline structure and a very strong (1 1 1) peak at 2h = 39.7°. The pattern

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D¼

Voltage (V)

0.6

0.2

0.4 0.1 0.2 0.0

0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Current density (A/cm2) Fig. 6. PEMFC performances for anodes made with commercial material and by the ALD method. The Pt loadings of commercial E-Tek and E-Tek electrodes are 0.5 and 0.02 mg cm2, respectively.

with a preferred (1 1 1) orientation is in agreement with that reported by Aaltonen et al. [16]. Because the growth rate of 5 nm/h is low enough for platinum atoms to migrate from high- to low surface energy sites, the (1 1 1) peak is so strong that any other peaks have become relatively weak. The XRD patterns of platinum nanoparticles on CNTs with different ALD cycle numbers are shown in Fig. 4b. The diffraction peaks at 42.4°, 54.7°, and 77.4° are referred to the hexagonal graphite peaks of (1 0 0), (0 0 4), and (1 1 0), respectively, for CNTs. The other peaks are ascribed to the platinum nanoparticles. The Pt crystallite size could be estimated by Scherrer’s equation [17],

(a) 1.1

E-Tek/E-Tek ALD100/ALD100 ALD100/ALD200 ALD100/ALD300

1.0 0.9

Voltage (V)

0.8 0.7 0.6 0.5 0.4 0.3 0.2

Kkx b cos h

ð1Þ

where K is a dimensionless constant, k is the wavelength of the incident monochromatic Cu Ka, h is the Bragg angle, and b is the linewidth at half maximum intensity. Based on the (1 1 1) peak, the average particle size by 100 ALD cycles is estimated to be 7.17 nm, slightly larger than that calculated from the TEM image. Furthermore, this shows that the crystallinity gradually increases with the ALD cycle number. The result also indicates that the ALD process could deposit highly pure platinum nanoparticles with no oxide on the CNT surface. Fig. 5a illustrates the dependence of the platinum loading on the ALD cycle for 4 mg cm2 CNTs subjected to 9 h of acid treatment. The platinum loading is linearly proportional to the ALD cycle number. Based on this result, the amount of platinum could be directly controlled by the ALD cycle number. The relation between platinum loading and the amount of CNTs with 100 cycles is shown in Fig. 5b. It illustrates that the platinum loading could also be controlled by using different amounts of CNTs with a constant ALD cycle. It also implies that the size of the catalyst could be maintained at a constant value while the Pt loading was altered. In all cases of Pt/CNT/carbon cloth composite, the Pt loading is significantly lower than that of commercial Pt/C electrode (0.5 mg cm2). The single-cell performance for the MEA made with Pt/CNT/carbon cloth composite electrode as the anode and commercial E-Tek electrode (Pt: 0.5 mg cm2) as the cathode is shown in Fig. 6. For comparison, the performance of MEA made with commercial ETEK electrode for both anode and cathode is also included. To prepare the homemade anode material, pristine CNTs were treated

(b) 0.40

E-Tek/E-Tek ALD100/ALD100 ALD100/ALD200 ALD100/ALD300

0.35

Power density(W/cm2)

0.3

0.8

Power density (W/cm2)

ALD 100 / E-Tek E-Tek / E-Tek E-Tek* / E-Tek

1.0

0.1

0.30 0.25 0.20 0.15 0.10 0.05 0.00

0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0

0.2

Current density (A/cm2)

E-Tek/E-Tek ALD100 CNT4/4 ALD100 CNT4/6 ALD100 CNT4/8

1.0 0.9

Voltage (V)

0.8 0.7 0.6 0.5 0.4 0.3 0.2

(d) 0.40

0.6

0.8

1.0

1.2

E-Tek/E-Tek ALD100 CNT4/4 ALD100 CNT4/6 ALD100 CNT4/8

0.35

Power density(W/cm2)

(c) 1.1

0.4

Current density (A/cm2)

0.30 0.25 0.20 0.15 0.10 0.05

0.1

0.00

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Current density (A/cm2)

1.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Current density (A/cm2)

Fig. 7. Performance of PEMFCs with both anode and cathode made by ALD at various ALD cycle numbers (a and b) and various amounts of CNTs (c and d). The cell made with commercial E-Tek electrodes is included for comparison.

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with nitric acid for 9 h, and an amount of 4 mg cm2 was dispersed in ethanol and then pasted onto the carbon cloth, followed by deposition of Pt by 100 cycles of ALD. The polarization tests were performed at 60 °C with a flow rate of 50 sccm for both H2 and O2. The data points were obtained from the average values of three measurements. It is seen that the Pt/CNT/carbon cloth electrode shows an even better performance than that of commercial E-Tek electrode. Therefore, depositing platinum catalyst with only 0.019 mg cm2 (i.e., 3.8% of 0.5 mg cm2) platinum loading on the anode by ALD outperforms the E-Tek electrode. Furthermore, if an MEA made with 0.02 and 0.5 mg cm2 of commercial E-Tek Pt catalyst for the anode and cathode was tested, the performance was much worse, as also shown in Fig. 7. It has been proposed that Pt loading on the anode for MEA can be reduced to 0.05 mg cm2 without severe voltage losses [18]. Based on the curves, the lower ohmic and mass transport loss of the Pt/CNT/carbon cloth composite electrode indicates superior characteristics of the electrode prepared in the present study [19, 20]. Fig. 7 shows the performances of PEMFCs made with Pt/CNT/ carbon cloth composite for both anode and cathode. The performance of a cell made with commercial E-Tek electrode for both anode and cathode is included for comparison. In Fig. 7a and b, the catalyst prepared for the anode was fixed at 100 ALD cycles and that for the cathode changed from 100 to 300 ALD cycles. Among the three cells, the cathode prepared with Pt for 200 ALD cycles has the best performance, even better than that for 300 ALD cycles. As shown in Fig. 2, platinum particles deposited onto CNTs with 300 cycles of ALD are bigger and are substantially aggregated and overlapped. The worse performance might be due to less active surface area than with 200 ALD cycles. In order to increase the platinum loading but in the meantime to keep the constant catalyst particle size, the amount of CNTs was varied and the ALD cycle number was fixed. The cell performances are shown in Fig. 7c

and d. The amount of CNTs at the anode was 4 mg cm2 in all cases, and the amounts at the cathode were 4, 6, and 8 mg cm2. It is seen that the performance is enhanced as the amount of CNTs at the cathode increases, and the increase in performance is almost in proportion to the amount of CNTs. However, owing to the increasing demand of fuel supply for high-efficiency MEA, it could be discovered from Fig. 7c that the mass transport loss on the polarization curve became greater when the amount of CNTs at cathode was increased. The performance of the homemade electrodes is no better than that of commercial E-Tek electrodes, whether with increasing number of ALD cycles or with increasing amount of CNTs, presumably due to much lower platinum loading on the homemade electrodes. The Pt loadings in Fig. 7c) are all lower than 0.05 mg cm2, which is 10 times lower than that for commercial E-Tek electrode. Based on the I–V test results, the efficiency of the catalyst can be estimated, and the specific power density can be used as a parameter to characterize the utilization efficiency of catalyst. The specific power density of the best homemade electrodes, that is, for the case of platinum deposited with 100 cycles of ALD on 4 and 8 mg cm2 CNTs for anode and cathode, respectively, was 2.27 kW g1 at 0.65 V, for a total Pt loading of 0.063 mg cm2. This is very close to the value of future requirement, 5.0 kW g1 at 0.65 V, proposed by Gasteiger et al. [4]. In contrast, the specific power density is calculated to be 0.18 kW g1 at 0.65 V for the ETek electrodes. This result demonstrates that the usage of platinum could be much decreased. Because of different support material, catalyst loading, parameter of hot press, electrolyte, and test conditions, it is not easy to compare the performance of PEMFCs fabricated in this study with those of other groups. To simplify the comparison, a reasonable method is to compare the relative values of performance of commercial E-Tek electrode employed in their studies. Table 1

Table 1 PEMFC performances of different studies. Homemade a

a b c d e

Commercial E-Tek

Hot press condition

Test conditions of MEA Area (cm2)

T (°C)

A/C gas flow rate (sccm)

Back pressure (MPa)

125 °C, 5.0 MPa, 120 s 125 °C, 34.5 Mpa, 180 s 135 °C, 4.9 MPa, 90 s 120 °C, 2.0 MPa, 90 s 130 °C, 0.7 MPa, 120 s 130 °C, 6.9 MPa, 120 s 130 °C, 7.0 MPa, 120 s 140 °C, 4.1 MPa, 90 s

16.00

75

STc 1.5/2 (air)

ATd

1.94

[21]

5.00

80

100/100

0.1

1.00

[22]

5.00

36

450/600 (air)

0.3

1.86

[23]

1.00

RTb

9/50

AT

2.40

[24]

1.00

80

N.A.

0.17/0.21

1.54

[25]

5.00

80

200/450 (air)

0.1

1.33

[26]

11.56

60

N.A.

0.1

2.31

[27]

4.00

60

50/50

AT

11.91

A/C Pt loading (mg/cm2)

Specific power density (W/mg) at 0.6 V

A/C Pt loading (mg/cm2)

Specific power density (W/mg) at 0.6 V

0.4/0.2

0.420

0.4/0.2

0.217

NR-212

0.3/0.3

0.800

0.3/0.3

0.800

N-115

0.25/0.4

0.650

0.4/0.4

0.350

NR-212

0.5/0.5

0.012

0.5/0.5

0.005

N-115

0.5/0.42

2.190

0.5/0.5

1.420

N-112

0.05/0.2

2.400

0.2/0.2

1.800

NR-212

0.25/0.5

0.416

0.25/0.5

0.180

N-1135

0.019/0.044

2.692

0.5/0.5

0.226

N-115

Anode/cathode. Room temperature. Stoichiometric ratio. Atmosphere pressure. Enhancement factor.

Factore

Electrolyte type

Reference

This study

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summarizes several PEMFCs made by different groups along with their MEA performances. The enhancement factor is defined as the ratio of specific power densities of homemade electrode and E-Tek. It is seen that the specific power density of MEA at 0.6 V made from this study is higher than that of all the groups listed in the table [21–27]. Furthermore, when the test conditions and other parameters are taken into account, the enhancement factor of this study shows an even higher value than the others.

4. Conclusion The deposition of Pt nanoparticles on acid-treated CNTs by ALD has been studied. The amount of oxygen-containing functional groups on CNTs increases with the acid treatment time. The Pt nanoparticle size is very uniform and the particles are well dispersed on the CNTs. The Pt particle size and loading can be directly controlled by the cycle number of ALD. MEA using Pt/CNT/ carbon cloth composite electrode as the anode containing only 0.019 mg cm2 of Pt loading has even better performance than that of commercial E-Tek electrode. The specific power density of the MEA made with Pt/CNT/carbon cloth composite electrode for both anode and cathode with Pt loadings of 0.019 and 0.044 mg cm2, respectively, is 11 times higher than that made with E-Tek electrodes and is very close to that of future requirements. The present result reveals that the usage of ALD for Pt with carbon nanotubes as a support could effectively reduce the Pt loading required for PEMFC applications. Acknowledgment This work was supported by the National Science Council of Taiwan under Contracts NSC 97-2221-E-007-010-MY3 and NSC 982120-M-155-001.

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