Investigation of porous carbon and carbon nanotube layer for proton exchange membrane fuel cells

Applied Energy 101 (2013) 457–464

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Investigation of porous carbon and carbon nanotube layer for proton exchange membrane fuel cells Guo-Bin Jung ⇑, Wei-Jen Tzeng, Ting-Chu Jao, Yu-Hsu Liu, Chia-Chen Yeh Dept. of Mechanical Engineering, Fuel Cell Center, Yuan Ze University, Taoyuan, Taiwan

h i g h l i g h t s " Performance of fuel cell incorporated with micro-porous layer with carbon nanotube is good. " The optimum compression of gas diffusion layer on performance is about 30%. 2

" The optimum carbon loading in micro-porous layer is about 2.0 mg cm

a r t i c l e

i n f o

Article history: Received 13 April 2012 Received in revised form 24 August 2012 Accepted 26 August 2012 Available online 1 October 2012 Keywords: Micro-porous layer (MPL) Carbon nanotube (CNT) Proton exchange membrane fuel cell (PEMFC) Gas diffusion layer (GDL)

.

a b s t r a c t In this work, three types of carbon – Vulcan XC-72R, long vapor-grown carbon nanotubes (LVGCNTs, 7 lm long, 100 nm in diameter), and short vapor-grown carbon nanotube (SVGCNT, 3 lm long, 100 nm in diameter) – were investigated as materials composing a micro-porous layer (MPL). Vulcan XC-72R was sprayed on carbon paper to form an MPL with various carbon loadings from 0.5 to 3.0 mg cm2, and formed a custom-made gas diffusion layer (GDL) which was subjected to further study. Physical property tests such as through-plane resistance, gas permeability and contact angle were measured. Based on the results of these tests, the best carbon loading for the Vulcan XC-72R was applied to LVGCNT and SVGCNT for a fuel cell performance test. The physical properties and fuel cell performance of the custom-made GDL were investigated and compared with those of the commercially available GDL SGL 10BC. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Due to their high power density, high efficiency, nonexistent emissions, high-quality power, scalability, and fast start-up, proton exchange membrane fuel cells (PEMFCs) are a promising future power source [1–7]. Membrane electrode assemblies (MEAs) play an important role in the performance of PEMFCs. An MEA is composed of two gas diffusion layers (GDLs), two catalyst layers, and a proton exchange membrane (PEM). GDLs permit gas transport from a flow field to the catalyst layer, and conduct electrons from the catalyst to a flow field plate. GDLs should therefore be a conductive porous material. The GDL strongly affects mass transport and water management, which also affects performance. Typically, a GDL has a dual-layer, composed of a gas diffusion substrate (GDS) and a micro-porous layer (MPL). Generally, the GDS is a carbon-fiber cloth or paper. While carbon cloth may present higher power performance than carbon paper, the use of carbon paper as a GDS is still widespread due to an advantageous

⇑ Corresponding author. Tel.: +886 34638800x2469. E-mail address: [email protected] (G.-B. Jung). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.08.045

cost and more convenient MPL fabrication [8]. Extensive research into the MPL has been performed, such as investigation into the effects of carbon powder types [9–14], hydrophobic reagents (e.g. fluorinated ethylene propylene (FEP), polytetrafluoroethylene (PTFE)), content ratio [14–16], and coating technique [14]. GDL 10BC is a well-known commercial GDL product from SIGRACET Corp. (USA), and consists of carbon powder and carbon paper as the MPL and GDS, respectively. The GDL 10BC is used for comparisons in this study. Jordan et al. [9,10] demonstrated that heat treatment by sintering of the carbon powder and PTFE resulted in better fuel cell performance for both carbon powder types studied (Vulcan XC-72R and Acetylene Black). Subjecting the carbon powder to a milling process decreased the particle size and fuel cell performance [10]. Yan et al. [14] found an optimal FEP content ratio for screen printing and spraying coating techniques. Yan et al. also demonstrated that an MPL fabricated by spraying presented about twice the Darcy permeability constant than the screen-printed equivalent. Using air as the oxidant, the optimal FEP content ratio was 40 and 30 wt.% for MPL fabricated by screen printing and spraying, respectively. The MPL fabricated by spraying showed lower fuel cell performance than screen printing, for both high and low FEP

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content ratios using air oxidant. For pure oxygen oxidant, the optimal FEP content ratio was 30 and 50 wt.% for MPL fabricated by screen printing and spraying, respectively, and the two optimized MPLs performed at similar levels. These results by Yan et al. [14] imply that an optimal FEP content ratio correlates with fuel cell operating conditions. Chang et al. [16] investigated MEA compression ratios in concert with various GDS and MPL PTFE content ratios. Typically, an increased PTFE content ratio retarded water flooding but increased ohmic resistance. Therefore, the PTFE content ratio should be set at an optimized value. The optimal compression ratio and PTFE content for an MPL were both found to be 30% [16]. However, the experiments of Park et al. [15] found an optimal MPL PTFE content of 20%. Therefore, the optimal MPL PTFE content should be dependent upon the coating method, operating conditions, hydrophobic reagents, and so on.

Wang et al. [13] fabricated MPL with two types of carbon – Black Pearls 2000 (BP) and Acetylene Black (AB) – and indicated that BP and AB were hydrophilic and hydrophobic, respectively. The results demonstrated that an MPL fabricated with AB performed better than BP, while AB combined with BP in a certain ratio yielded the best performance. These results imply that the hydrophilic/hydrophobic structure can be tuned not only with hydrophobic reagents but also with different carbon types. The hydrophobicity can thus be maintained at an optimal value even without the use of a non-conductive hydrophobic reagent (i.e. PTFE). Four types of carbon have been tested as MPL: Shawinigan Acetylene Black (SAB), Vulcan XC-72, Asbury graphite 850, and Mogul L [11]. The fuel cell performance from best to worst was SAB, Vulcan XC-72, Asbury graphite 850, and Mogul L. Yu et al. [12] showed that MPL fabricated with Ketjenblack EC-600JD

Table 1 Designations of GDL samples with different carbon types/loading and different assembly parameters. Designation

GDL samples (Carbon type or commercial product)

10BC-x-0.5-x10BC-x-3.5-x 10BC-x-x-1010BC-x-x-60 72R-0.5-0.5-x72R-3.0-3.5-x 72R-0.5-x-1072R-3.0-x-60 72R-2.0-1.7-1072R-2.0-1.7-60 72R-0.5-1.7-3072R-3.0-1.7-30 LVGCNT-2.0-1.7-30 SVGCNT-2.0-1.7-30

SGL 10BC SGL 10BC XC-72R XC-72R

Clamping pressure (0.53.5) (MPa)

1060 0.53.0 0.53.0 2.0 0.53.0 2.0 2.0

XC-72R LVGCNT SVGCNT

0.53.5 1060 1060 30 30 30

1.7 1.7 1.7 1.7

90

16 14 12 10 8 6

(c)

80

72R-2.0-0.5-x 72R-2.0-1.0-x 72R-2.0-1.5-x 72R-2.0-2.0-x 72R-2.0-2.5-x 72R-2.0-3.0-x 72R-2.0-3.5-x

70

Resistance (m Ω)

10BC-x-0.5-x 10BC-x-1.0-x 10BC-x-1.5-x 10BC-x-2.0-x 10BC-x-2.5-x 10BC-x-3.0-x 10BC-x-3.5-x

(a)

18

60 50 40 30 20

4

10

2

0

0 10

20

30

40

50

60

10

20

30

40

50

60

CR (%)

CR (%) 65

14000 12000 10000 8000 6000 4000 2000

(b)

72R-0.5-0.5-x 72R-0.5-1.0-x 72R-0.5-1.5-x 72R-0.5-2.0-x 72R-0.5-2.5-x 72R-0.5-3.0-x 72R-0.5-3.5-x

100 80 60

(d)

60

72R-3.0-0.5-x 72R-3.0-1.0-x 72R-3.0-1.5-x 72R-3.0-2.0-x 72R-3.0-2.5-x 72R-3.0-3.0-x 72R-3.0-3.5-x

55 50

Resistance (m Ω)

Resistance (mΩ)

Compress ratio (1060) (%)

0.53.5

20

Resistance (mΩ)

Carbon loading (0.53.0) (mg cm2)

45 40 35 30 25 20 15

40

10

20

5 0

0 10

20

30

40

CR (%)

50

60

10

20

30

40

50

60

CR (%)

Fig. 1. Dependence of compression ratio (CR)/clamping pressure on GDL resistance. GDL sources: (a) commercial SGL 10BC, and homemade XC-72R MPL/GDL with XC-72R loading of (b) 0.5, (c) 2.0, and (d) 3.0 mg cm2.

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produced lower ohmic resistance and higher fuel cell performance than MPL using Vulcan XC-72R. Jordan et al. [10] found the optimized loading of AB at 1.25 and 1.9 mg cm2 for air and oxygen oxidants, respectively. However, these results were entirely different to Park’s work, which found an optimal AB loading of 0.5 mg cm2 [15]. Passalacqua et al. indicate that the carbon loading optimization depends on the material characteristics [11]. In our opinion, the optimized loading is not only dependent on the material characteristics but also the operating conditions, compression ratio [18], and so on. Ge et al. [18] concluded that the optimal compression ratio of GDL varied based on the material and should be ascertained to enable maximum stable performance. Very few studies into the coupling of the MPL carbon loading and compression ratio have been undertaken. Therefore, various loadings of carbon powder XC-72R coupled with a range of compression ratios were investigated in this work. Carbon nanotubes (CNTs) have become cheaper and widely used in many industries in recent years was utilized as an MPL material in this work in addition to the traditional carbon powder XC-72R.

3 lm long, 100 nm in diameter, Unitetek International Co., Ltd., Taiwan) were each combined with an appropriate amount of ethanol and mixed using an ultrasonic bath to produce MPL inks. The MPL inks were sprayed with an air gun onto carbon paper of type GDS (GDS 340, CeTech Co., Ltd., Taiwan). The as-prepared MPL formed the experimental GDLs used in this study, designated 72R, LVGCNT, and SVGCNT.

2. Experimental

2.3. Measurement of hydrophilic/hydrophobic characteristics

2.1. MPL fabrication on GDS

Measurement of the hydrophilic/hydrophobic properties of the MPL on GDL was carried out with a custom-made apparatus. The GDL samples were placed horizontally in the apparatus at room temperature. Water vapor carried by air flowed through the MPL sample, and liquid water accumulated on the surface of MPL. By

Vulcan XC-72R, long vapor-grown carbon nanotube (LVGCNT, 7 lm long, 100 nm in diameter, Unitetek International Co., Ltd., Taiwan), and short vapor-grown carbon nano-tube (SVGCNT,

2.2. Resistance test Generally, there are two kinds of resistance tests: in-plane and through-plane. In this study, the though-plane two-probe resistance test was used. A 5 cm2 GDL sample and gasket were sandwiched between two copper plates. The compression ratio (CR) was controlled by the thickness of the GDL and gasket. Clamping pressure was controlled by a hot-press instrument (Chun Yen Testing Machines Co., Ltd. Model CY-6040A4, Taiwan). The resistance test was conducted at room temperature using an ohmic meter (Tsuruga Electric Corp. Model NO.3566m, Japan).

90

20

(a)

18

Resistance (mΩ)

16 14 12 10 8 6

(c)

80 70

Resistance (mΩ)

10BC-x-x-10% 10BC-x-x-20% 10BC-x-x-30% 10BC-x-x-40% 10BC-x-x-50% 10BC-x-x-60%

60 50

30

4

20

2

10

0

72R-2.0-x-10% 72R-2.0-x-20% 72R-2.0-x-30% 72R-2.0-x-40% 72R-2.0-x-50% 72R-2.0-x-60%

40

0 0.5

1.0

1.5

2.0

2.5

3.0

0.5

3.5

1.0

2.0

2.5

3.0

3.5

65

14000 12000 10000 8000 6000 4000 2000

72R-0.5-x-10% 72R-0.5-x-20% 72R-0.5-x-30% 72R-0.5-x-40% 72R-0.5-x-50% 72R-0.5-x-60%

(b)

100 80 60

(d)

60

72R-3.0-x-10% 72R-3.0-x-20% 72R-3.0-x-30% 72R-3.0-x-40% 72R-3.0-x-50% 72R-3.0-x-60%

55 50

Resistance (mΩ)

Resistance (mΩ)

1.5

Clamping pressure (MPa)

Clamping pressure (MPa)

45 40 35 30 25 20 15

40

10

20

5

0

0 0.5

1.0

1.5

2.0

2.5

Clamping pressure (MPa)

3.0

3.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Clamping pressure (MPa)

Fig. 2. Dependence of compression ratio (CR)/clamping pressure on GDL resistance. GDL sources: (a) commercial SGL 10BC, and homemade XC-72R MPL/GDL with XC-72R loading of (b) 0.5, (c) 2.0, and (d) 3.0 mg cm2.

G.-B. Jung et al. / Applied Energy 101 (2013) 457–464

voltage (V)

0.8

450

1.0

400

0.9

350

0.7

300

0.6

250

0.5

200

0.4 0.3

150

10BC-x-1.7-30 10BC-x-1.7-30 72R-2.0-1.7-20 72R-2.0-1.7-20 72R-2.0-1.7-30 72R-2.0-1.7-30 72R-2.0-1.7-40 72R-2.0-1.7-40 72R-2.0-1.7-50 72R-2.0-1.7-50

0.2 0.1

100 50

0.0 0

200

400

600

800

-2

1000

0 1200

450 400 350

0.7

300

0.6

250

0.5

200

0.4

150

0.3 72R-2.0-1.7-30 72R-2.5-1.7-30 72R-3.0-1.7-30

0.2 0.1

100 50

0.0 0

200

400

600

800

1000

0 1200

-2

current density (mA cm )

-0.14

-0.14

(b)

10BC-x-1.7-30 72R-2.0-1.7-20 72R-2.0-1.7-30 72R-2.0-1.7-40 72R-2.0-1.7-50

-0.10

-0.08 -0.06

-0.06 -0.04

-0.02

-0.02

0.02

0.04

0.06

0.08

0.10

0.12

0.14

10BC-x-1.7-30 72R-1.0-1.7-30 72R-2.0-1.7-30 72R-3.0-1.7-30

-0.08

-0.04

0.00 0.00

(b)

-0.12

Z_Imag (Ω)

-0.10

Z_Imag (Ω)

(a)

0.8

current density (mA cm )

-0.12

500

10BC-x-1.7-30 72R-0.5-1.7-30 72R-1.0-1.7-30 72R-1.5-1.7-30

-2

1.1

power desnity (mW cm )

0.9

500

voltage (V)

(a)

1.0

-2

1.1

power desnity (mW cm )

460

0.00 0.00

0.02 0.04

Z_Real ( Ω ) Fig. 3. The (a) performance and (b) EIS test of SGL 10BC and homemade XC-72R MPL with various compression ratios (clamping pressure = 1.7 MPa, carbon loading = 2.0 mg cm2).

measuring the contact angle of the liquid water on the MPL surface, the hydrophilic/hydrophobic behavior of the MPL was deduced.

2.4. Permeability measurement The permeability of the GDL was measured by a Gurley Precision Instrument Model No. 4110. The Gurley Precision Instrument measures the time taken for 300 cc of air to permeate through the GDL sample. The area of the outlet orifice was 6.45 cm2. Theoretically, a shorter permeation time means the sample is more highly permeable. However, the measurement cannot indicate the real mass transport situation in an operating fuel cell because it does not take into account the contact angle effect.

2.5. Performance and electrochemical impedance spectroscopy test Commercial catalyst coated membranes (CCMs, General Optics Corp., Taiwan) were used in the performance assessment of fuel cells incorporating the homemade (XC-72R, LVGCNT, SVGCNT) or commercially available (SGL 10BC) GDL. The anode and cathode Pt loading of the CCM was 0.2 and 0.4 mgPt cm2, respectively, and the membrane material was Nafion 211 (25 lm). The catalyst layer (CL) active area was 23  23 mm2 and the GDL was 25  25 mm2. Each CCM was sandwiched between two GDLs and assembled into a single cell without hot pressing for the fuel cell performance test.

0.06 0.08

0.10 0.12

0.14

Z_Real (Ω) Fig. 4. The (a) performance and (b) EIS test of SGL 10BC and homemade XC-72R MPL with various carbon loadings (clamping pressure = 1.7 MPa, compression ratio = 30%).

In this study, a homemade single cell was utilized which consisted of three components: an insulating end plate, a collecting plate, and a flow field plate. The component materials were glass fiber, gold-coated brass, and graphite, respectively. The channel depth, channel width, and rib width of the serpentine flow field plate were all 1 mm. Polarization test measurements were conducted using a Fuel Cell Test System 850C (Scribner Associates Incorporated, USA). Once a pair of GDLs were assembled with fresh commercial CCMs, an activation process was undertaken prior to the performance test. The activation process consisted of open circuit for 3 s, then constant 0.2 V for 30 min, repeated twelve times. Conditions during the activation process were a temperature of 65 °C with relative humidity (RH) 100%, 100 sccm hydrogen flow rate, and 250 sccm air flow rate. The operating conditions during polarization measurements were the same as for the activation process, but with a lower air flow rate of 150 sccm in order to emphasize the mass transport effect. No back pressure was used during any portion of the experiment. Electrochemical impedance spectroscopy (EIS) was carried out with a constant current of 0.5 A cm2, and a frequency range of 1–1000 Hz. 3. Results and discussion 3.1. Resistance test of GDL – XC-72R Table 1 designates the all GDL samples utilized, with different carbon types and loading, and different assembling parameters

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such as clamping pressure and compression ratio (CR). Fig. 1 illustrates the relationship between the CR used during assembly and the measured GDL resistance for a range of carbon loadings (0.5– 3 mg cm2) and clamping pressures (0.5–3.5 MPa). For the GDL sample XC-72R, which had a carbon loading of 0.5 mg cm2 and clamping pressure of 0.5 MPa as shown in Fig. 1b, the resistance of the GDL with CRs of 10%, 20%, 30%, 40%, 50%, and 60% are 14,000, 100, 38, 5, 5, and 5 mX, respectively. The resistance of the GDL drops sharply between a CR 10% and 30%, and remains unchanged between CR 40% and 60%. CRs 10% and 20% displayed very large resistances for all GDL samples examined, including the commercial product, due to the CR being insufficient to reduce contact resistance. The resistance test showed the resistance dropping slightly at CR 30%. Although the resistance of the GDL decreased further between CR 40% and 60%, over-compression of GDL can lead to insufficient porosity and result in generated water flooding or insufficient fuel inlet during actual fuel cell operation, which suggests that the optimum CR may be 30%. This value was double-checked during the real fuel cell operation test described in Section 3.2. Fig. 2 illustrates the relationship between the clamping pressure and the measured GDL resistance for a range of carbon loadings (0.5–3 mg cm2) and CRs (10–60%). For the GDL sample XC-72R, which has a loading of 0.5 mg cm2 and CR 30% as shown in Fig. 2b, the GDL resistance with clamping pressures of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 MPa are 38, 29, 17, 15, 15, 15, and

P¼

T  Nb Db  K b  A

(a)

70

450

350

0.7

300

0.6

250

0.5

200

0.4

150

0.3

10BC-x-1.7-30 10BC-x-1.7-30 72R-2.0-1.7-30 72R-2.0-1.7-30 LVGCNT-2.0-1.7-30 LVGCNT-2.0-1.7-30 SVGCNT-2.0-1.7-30 SVGCNT-2.0-1.7-30

0.2 0.1

100 50

0.0 0

200

400

600

800

-2

Permeability test (sec)

400

0.8

1000

SGL 10BC GDS 340 XC-72R SVGCNT LVGCNT

(a)

-2

0.9

SGL 10BC 61.2

60

power desnity (mW cm )

1.0

ð1Þ

where P is the clamping pressure, T the applied torque, Nb the number of screws, Db the screw nominal diameter, Kb the fraction coefficient (0.20 for typical cases), and A is the cell area.

500

1.1

voltage (V)

15 mX, respectively. Clearly, the resistance drops sharply from 0.5 MPa to 1.5 MPa, and remains unchanged between 2.0 and 3.5 MPa. Fig. 2 shows that in most cases (CR = 30%) the GDL resistance did not reduce significantly once a clamping pressure greater than 1.5 MPa was used. Therefore, the applied torque on screws used during cell assembly was kept to 25.0 kgf cm, which equates to a cell clamping pressure of 1.7 MPa, as calculated using the following Eq. (1) and will be used hereafter. The 1.7 MPa clamping pressure was adequate to seal the cell and exceeded the required clamping pressure of 1.5 MPa. As shown in Fig. 1, with assembly parameters of 1.7 MPa and compression ratio of 30%, the resistance of GDL XC-72R with carbon loading 0.5, 2.0, and 3.0 mg cm2 was 17, 15 and 15 mX, respectively. Typically, increases in the carbon loading reduced the resistance before a certain carbon loading and then remain unchanged even as the loading was further increased. From this result, the optimum carbon loading appears to be about 2.0 mg cm2; the real fuel cell operation test was used to ascertain whether 2.0 mg cm2 is indeed the optimum carbon loading.

50 40 30

-2

MPL carbon loading 2.0 mg cm

20 10

0 1200

GDS 340 1.73

XC-72R 8.7

SVGCNT 3.9

LVGCNT 4.13

0

current density (mA cm )

GDL type

-0.12

(b)

20

(b)

18

-0.10

XC-72R LVGCNT

-0.08

Z_Imag

Permeability test (sec)

16 10BC-x-1.7-30 72R-2.0-1.7-30 SVGCNT-2.0-1.7-30 LVGCNT-2.0-1.7-30

-0.06

-0.04

14 12 10 8 6 4

-0.02

2 0

0.00 0.00

0.5 0.02

0.04

0.06

0.08

0.10

0.12

1.0

1.5

2.0

2.5

3.0

-2

Carbon loading (mg cm )

Z_Real (Ω) Fig. 5. The (a) performance and (b) EIS test of SGL 10BC and homemade MPL with various carbon types.

Fig. 6. The air permeability test of (a) SGL 10BC, GDS 340 and homemade MPL with various carbon types and (b) homemade Vulcan XC-72R and LVGCNT MPL with various carbon loadings.

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3.2. Fuel cell performance and EIS test with GDL XC-72R Fig. 3 illustrates the effect of CR on cell performance and the EIS test using the homemade GDL XC-72R with a clamping pressure of 1.7 MPa and carbon loading of 2.0 mg cm2. The output current density (mA cm2) of the fuel cell measured at 0.45 V for a range of CRs was 600 (CR = 20%), 870 (CR = 30%), 800 (CR = 30%, SGL 10BC), 800 (CR = 40%), and 800 (CR = 50%). XC-72R performed poorly at CR 20% due to a high ohmic resistance of 0.04 X, which was twice as large as any other value). Compared to the SGL 10BC, XC-72R performed close to or better than SGL 10BC for CRs 30–50%. The resistance (Section 3.1) and EIS test (Section 3.2) indicated CR 20% was insufficient, leading to high resistance and poor performance. Fig. 3b indicates that mass transfer resistance at a CR of 50% was too high (0.135 X), causing the reactant gases to diffuse to the CL with great difficulty. Therefore, EIS of CR 50% shows an extremely high mass transfer resistance. Thus, according to these performances and EIS results, the optimum CR is about 30%. Fig. 4a shows the effect of carbon loading on performance with a clamping pressure of 1.7 MPa and compression ratio of 30%. For carbon loadings of 0.5–2.5 mg cm2, current densities are greater than 700 mA cm2 at 0.5 V. No significant performance drop was

caused by water flooding when carbon loading was 2.0– 2.5 mg cm2. A carbon loading of 3.0 mg cm2 resulted in poor performance (550 mA cm2 at 0.5 V) due to the GDL (or MPL) being too thick and dense, causing water flooding. Fig. 4b shows the effect of carbon loading upon the EIS test. EIS results indicate that an overly thin GDL (or MPL, carbon loading 1.0 mg cm2) leads to higher mass transfer resistance, while an overly thick GDL (or MPL, carbon loading 3.0 mg cm2) leads to higher ohmic resistance. The apparent optimum carbon loading for GDL XC-72R is about 2.0 mg cm2, with fuel cell performance and EIS both exceeding that of GDL 10BC with this loading. 3.3. The effect of carbon type on MPL This section described the investigation of MPL and GDL composed of different carbon materials (carbon power or carbon nanotube) with a constant carbon loading of 2.0 mg cm2, clamping pressure of 1.7 MPa and CR of 30%. Due to different structures of carbon materials, different MPL/GDL thickness was expected. Fig. 5a shows the results of the performance test of SGL 10BC and homemade MPLs with different carbon types. The performance (current density @ 0.5 V) from best to worst is as follows: LVGCNT

XC-72R (35 )

LVGCNT (35 )

SVGCNT (35 )

XC-72R (300 )

LVGCNT (300 )

SVGCNT (300 )

XC-72R(1000 )

LVGCNT (1000 )

SVGCNT (1000 )

Fig. 7. Scanning electron micrographs of MPL layers on different GDL surfaces.

G.-B. Jung et al. / Applied Energy 101 (2013) 457–464

(970 mA cm2) > SVGCNT (840 mA cm2) > XC-72R (750 mA cm2) > SGL 10BC (700 mA cm2). The performance of LVGCNT (SVGCNT) was increased by 29% (12%) compared with the standard GDL (XC-72R) setup. Fig. 5b shows the results of the EIS test of SGL 10BC and homemade GDL with various carbon types. Although conductivity of the carbon nanotubes (1.67  104 S cm1 for VGCNT) is much higher than Vulcan XC-72R (4.0 S cm1) [17], the EIS indicated that all GDL achieved a similar ohmic resistance (0.02 X). However, the EIS also indicated that the SVGCNT and LVGCNT could reduce the mass transfer resistance (60–80%) and resistance associated with charge transfer reactions (10–20%). The SVGCNT and LVGCNT MPLs can therefore transport reactant gases and evacuate generated liquid water more readily, due to them having more suitable pore structures or hydrophilic/hydrophobic properties. The decrease in resistance associated with charge transfer reactions (10–20%) arose from the constant current density setting (500 mA cm2) under EIS operation, and corresponded to different driving forces of 0.5 V (XC-72R) and 0.6 V (LVGCNT and SVGCNT). Therefore, the performance of both SVGCNT and LVGCNT improved primarily due to lower mass transfer resistance. Fig. 6a shows the results of the air permeability test of SGL 10BC, GDS 340 (without MPL), and homemade GDL with various carbon types. The permeability from high to low is as follows: SVGCNT  LVGCNT > XC-72R > SGL 10BC, which is in agreement with performance and EIS results described in Section 3.2. Air

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permeability testing therefore plays an important role in determining a carbon type’s suitability as MPL material. Although permeability is correlated with material structure, the different thicknesses resulting from the use of different carbon materials at the same carbon loading (2 mg cm2) also affected the air permeability to some extent. Fig. 6b shows the results of air permeability testing of XC-72R and LVGCNT with a range of carbon loadings. Increasing the carbon loading increased the thickness of the MPL, therefore more time was needed to move through the GDL (GDS + MPL). These results demonstrate a correlation between permeability and carbon loading, which can in the future help in preparing a specific permeability by adjusting carbon loading. As shown in Fig. 7, carbon powder XC-72R attaches dispersively to the surface of carbon fiber GDS in a 2D structure, while carbon nanotubes (LVGCNT, SVGCNT) form an agglomerate with a 3D structure on the surface of the carbon fiber. An in-plane 2D structure will block the transportation of gas more than a through-plane 3D structure with the same carbon loading (2.0 mg cm 2). Therefore, the permeability of MPL and GDL comprised of carbon nanotubes is much higher than MPL composed of carbon powder XC-72R (as shown in Fig. 6). This leads to better fuel cell performance due to decreased mass transfer resistance (as shown in Fig. 5). Images of water droplets on the surface of different carbon layers are shown in Fig. 8. By fitting a tangent to the three-phase point, where liquid surface touches the solid surface, the contact angle can be measured. Because there is no hydrophobic agent

(1) GDS 340

(4) LVGCNT

(2) 10 BC- 140°

(5) XC- 72R

( 3 ) SV G C N T Fig. 8. Contact angles on the surface layer of: (1) GDS 340-95°, (2) 10BC-140°, (3) SVGCNT-136°, (4) LVGCNT- 135°, (5) XC-72R- 141°.

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added, the contact angles on the surface of the carbon layer imply surface characteristics of the carbon materials used in this study. It is clear that the surface layer on GDS, with contact angle of 94°, is less hydrophobic than the surfaces of all the GDLs used in this study, which have contact angles between 136° and 141° (>120°), demonstrating similar and stronger hydrophobicity. Therefore, the hydrophobicity of MPL is not a dominant factor affecting generated water management and hence fuel cell performance. 4. Conclusion The compression ratio affects performance significantly; the optimum CR is about 30%. For Vulcan XC-72R MPL, the optimum carbon loading is about 2.0 mg cm2. An MPL of insufficient thickness leads to higher mass transport resistance, while an overly thick MPL leads to higher ohmic resistance. The improved performance of SVGCNT (12%) and LVGCNT (29%) compared to traditionally-utilized carbon powder-XC-72R and commercially available SGL 10BC comes primarily from the low mass transfer resistance of the material due to the formation of through-plane 3D structures compared to the formation of in-plane 2D structures. Acknowledgement The authors are grateful to the National Science Council, Taiwan, under contract NSC-100-D0204-4 for the financial support. References [1] Barbir F, Yazici S. Status and development of PEM fuel cell technology. Int J Energy Res 2008;32(5):369–78. [2] Meidanshahi V, Karimi G. Dynamic modeling, optimization and control of power density in a PEM fuel cell. Appl Energy 2012;93:98–105. [3] Tang Y, Yuan W, Pan M, Li Z, Chen G, Li Y. Experimental investigation of dynamic performance and transient responses of a kW-class PEM fuel cell stack under various load changes. Appl Energy 2010;87(4):1410–7.

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