Effect of Carbon Fiber Loading in Graphite-Polypropylene Composite Properties as Bipolar Plate for Polymer Electrolyte Membrane Fuel Cell

(PEMFC)

Mohd Zulkefli Selamat1, a, Noor Ashikin Jamil1,b and Rafidah Hasan1, c 1Advanced Material Group, Faculty of Mechanical Engineering,

UniversitiTeknikal Malaysia Melaka (UTeM), Hang Tuah Jaya,

76100 Durian Tunggal, Melaka, Malaysia aashikinjamil90@gmail.com, bzulkeflis@utem.edu.my, crafidahhasan@utem.edu.my

Keywords: Graphite; carbon fiber; polypropylene; electrical conductive polymer composite; bipolar plate.

Abstract. The performance of Polymer Electrolyte Membrane Fuel Cells (PEMFC) is dependent on the properties of bipolar plates(BP). Bipolar plates are one of the main components in PEMFC stacks and they make up a large portion of the stack volume and cost. In this research, bipolar plates has been developed using hybrid fillers; graphite (Gr) as a main filler and carbon fiber (CF) as second filler while polypropylene (PP) as a binder in Gr/CF/PP composite. All the materials will be in powder form and the composition of this composite was fixed of 80% (fillers) /20% (PP) according to weight percentage (wt. %). Meanwhile the contents or the loading of CF was varied from 5 wt. % up to 20 wt. % of the total weight of fillers (80 wt. %). The ball mill being used to mix the two fillers and binder together before being pressed by compression molding (hot press) in order to produce rectangular shape (140mm x 60mm) sample. The electrical and mechanical properties of the developed composite as BP were measured to investigate its suitability to be used as BP for the PEMFC. The result shows that the electrical and mechanical properties has improved as the loading of CF has been increased. The composite with CF loading of 5 wt. % has shown the highest value for electrical conductivity 262.75 S/cm and flexural strength of 40.2 MPa. This finding shows that the properties Gr/CF/PP composite is suitable for BP of PEMFC in the future.

Introduction

Polymer electrolyte membrane fuel cell (PEMFC) or also called proton exchange membrane fuel cells is widely applied in automobiles and BP is one of the most important component in PEMFC stack [1-3]. There is a strong relationship between the material employed in the manufacture of the bipolar plate and its final properties. The most commonly used material for a bipolar plate is graphite which has good electrical conductivity and excellent corrosion resistance with a low density but lacks mechanical strength and has poor ductility yet it also the heaviest part of PEMFC [3-5]. The other materials such as metal-based require proper machining process, need special coating, have extra weight and have high tendency to corrode even though they have good electrical conductivity [6].

Hence, conductive polymer composite with carbon based BP is an attractive option to use for PEMFC [3&5]. On the other hand, graphite/polymer composites can be considered as an ideal material for producing BP. Thus, in this research graphite and carbon fiber are used as fillers for BP in order to improve the electrical and mechanical properties and ease to manufacture besides reducing the weight and fabrication cost [3&4]. Moreover, CF has a very high tensile and compressive strength, and have a high resistance to corrosion, creep and fatigue [7]. BP that being produced must achieved the requirement target specified by the United States of America Department of Energy (DOE) as shown in Table 1 [8]. The objective of this research is to study the

effect of CF loading in Gr/CF/PP composite properties such as electrical conductivity and mechanical properties.

Table 1. Requirement properties for the bipolar plate (DOE target) [8] Property Value

Electrical conductivity > 100 [Scm-1] Thermal conductivity > 10 [W(mK)-1]

Flexural strength > 25 [MPa] Shore Hardness > 50 Bulk Density

Table 4. Photo of each composition of G/CF/PP composite

Composition of

G/CF/PP [wt. %]

0 5 10 15 20

Photo

Effect of CF on Electrical Conductivity. Electrical conductivity of G/CF/PP composites with various CF loading was shown in Fig. 1. It shown that, the electrical conductivity of G/CF/PP composite has increased with the increasing CF loading from 116.29 S/cm (0 wt. %) to 262.75 S/cm (5 wt. %), but when the CF content had been further increased, the electrical conductivity has decreased to 105.10 S/cm (10 wt. %) [3]. The percentage of increament between G/PP composite and G/CF/PP composites with 5 wt. % CF content is about 125.94 %. As compared to DOE target, the value of electrical conductivity of 5 wt. % CF content was higher than the target value which is 100 S/cm. The results is supported by Lee et al., 2009 and Mathur et al., 2008, which the electrical conductivity increases with increasing carbon fiber loading until the maximum loading and then decreases with further increase in carbon fiber content [3&5].

Fig. 1. Graph of electrical conductivity and flexural strength against CF content in G/CF/PP

composite.

Flexural strength. The flexure strength result is shown in Fig 1. The highest flexural strength of G/CF/PP composite of 40.2 MPa is observed at CF loading of 5 wt. % and is a more than 100% improvement compared to the G/PP composite. After that, the value of flexural strength is decreased with further increament of CF content at 10 wt. % until 20 wt. %. CF act as a reinforcement materials as CF has high aspect ratio and fibrous that can improve the electrical

116.29

262.75

105.1 92.69

77.5316.83

40.2

32.72 28.05

40.2

0102030405060708090100

0

50

100

150

200

250

300

0 5 10 15 20

Flex

ural

St

reng

th [M

pa]

Elec

tric

al

Con

duct

ivity

[S/c

m]

CF content [wt. %]

Electrical Conductivity [S/cm] Flexural Strength [Mpa]

conductivity and flexural strength [3&5]. Lee et al., 2009 also shown the same behaviour of G/CF composites [3]. Flexural strength with 10 wt. % and 15 wt. % of CF content has decreased and then it has increased with increasing CF loading. During the test run for 20 wt. % of CF content, the samples were not break in the middle as it should be. It is due to the uneven and rough surface of the samples and agglomerate between the three materials. This can also shows that there is imperfect bonding between the fibers and binders [3]. This explains the unstable trend in the flexural strength of the composites with high filler loadings. All the composition of G/CF/PP composites were achieved and higher than the DOE target which is 25 MPa.

Shore hardness. As shown in Fig. 2, the shore hardness of the composite has increased after 5 wt. % of CF was added then it has decreased with more added CF content. There is 9.85% of increament between G/PP composite (58.56) and G/CF/PP composites (64.33) of 5 wt. % CF loading. The increament in hardness of the samples with addition of CF can be attributed to the well-known ribbon or glassy carbon structure of PAN based carbon fibers [5]. Furthermore , the surfaces condition of the composites were highly influenced the result of shore hardness. The composite with smooth and flat surface will increased the hardness value and vise versa.While, compared to DOE target, all value of the shore hardness of G/CF/PP were higher than target which is 50. According to the result, the composition for bipolar plate of G/CF/PP composites with 5 wt. % CF content was highly recommended.

Fig. 2. Graph of shore hardness and bulk density versus CF content

Bulk Density. According to the Fig. 2, it showed that the value of bulk density for G/CF/PP composites has decreased with the increasing of CF contents. Initially, at 0 wt. % of CF, the value of bulk density was 1.718 g/cm3 and it dropped with the loading of 5 wt. % of CF (1.700 g/cm3) and until 1.634 g/cm3 for 20 wt% of CF. The minimum value of bulk density was 1.630 g/cm3

shows by 15 wt. % of CF loading. Gr has the highest value of density, so when the value of Gr decrease, the value of bulk density of the specimens also decreases. Besides that, the bulk density for all composition are meet the DOE target which is lower than 1.9 g/cm3. The density is decreasing with additional CF contents is due to lower density of CF compared to Gr and PP.

58.56

64.33

62.4861.56

62.561.718

1.7001.650

1.630 1.634

1.5

1.55

1.6

1.65

1.7

1.75

1.8

5556575859606162636465

0 5 10 15 20

Bulk

Den

sity

[g/c

m3 ]

Shor

e Har

dnes

s

CF content [wt. %]

Shore Hardness Bulk Density

Conclusion

The addition of CF to the G/PP composite obviously effect the electrical and mechanical properties of G/CF/PP composites. It was found out that, the composites with 5 wt. % of CF content was highly suitable as bipolar plate for PEMFC. The addition of CF in G/PP composite offers a high opportunity in production of high conductive composites especially for PEMFC bipolar plates.

Acknowledgment

The authors would like to thank the Malaysia Ministry of Higher Education, Malaysia and Ministry of Science, Technology and Innovation for sponsoring this work under Grant PJP/(2013)/FKM(6A)/S01181 and Universiti Teknikal Malaysia Melaka (UTeM) for financial sponsoring during this research.

References

[1] T. M. Besmann, J. W. Klett, J. J. Henry, D. O. E. Technology, D. Manager, & N. L. Garland. Carbon Composite Bipolar Plates, (2008) 14.

[2] T. Brief. Carbon-filled polymer blend based bipolar plates for PEM fuel cell stack, (July 2006) (2008), 15.

[3] J. H. Lee, Y. K. Jang, C. E. Hong, N. H. Kim, P. Li, & H. K. Lee. Effect of carbon fillers on properties of polymer composite bipolar plates of fuel cells. Journal of Power Sources, 193(2), (2009) 523529. doi:10.1016/j.jpowsour.2009.04.029

[4] I. U. Hwang, H. N. Yu, S. S. Kim, D. G. Lee, J. D. Suh, S. H. Lee, & T. W. Lim. Bipolar plate made of carbon fiber epoxy composite for polymer electrolyte membrane fuel cells. Journal of Power Sources, 184(1), (2008) 9094. doi:10.1016/j.jpowsour.2008.05.088

[5] R. B. Mathur, S. R. Dhakate, D. K. Gupta, T. L. Dhami, & R. K. Aggarwal. Effect of different carbon fillers on the properties of graphite composite bipolar plate. Journal of Materials Processing Technology, 203(1-3), (2008) 184192. doi:10.1016/j.jmatprotec.2007.10.044

[6] M. S. Ahmad, M. Z. Selamat, M. A. M. Daud, I. K. M. Yunus, M. S. Azman, Effect of Different Filler Materials in the Development of Bipolar Plate Composite for Polymer Electrolyte Membrane Fuel Cell (PEMFC), Applied Mechanics and Materials 315 (2013) 226-230.

[7] A. L. Dicks. The role of carbon in fuel cells. Journal of Power Sources, 156(2) (2006) 128141. doi:10.1016/j.jpowsour.2006.02.054

[8] R. A. Antunes, M. C. L. de Oliveira, G. Ett, & V. Ett. Carbon materials in composite bipolar plates for polymer electrolyte membrane fuel cells: A review of the main challenges to improve electrical performance. Journal of Power Sources, 196(6), (2011) 29452961. doi:10.1016/j.jpowsour.2010.12.041

Table 1. Requirement properties for the bipolar plate (DOE target) [8]Table 2. Properties of Gr, CF and PPTable 3. Composition of G/CF/PP composite