On the improved properties of injection-molded, carbon nanotube-filled PET/PVDF blends
Introduction
Currently, bipolar plates of polymer electrolyte membrane (PEM) fuel cells are either made of Poco™ graphite or carbon-filled polymers. However, the material and manufacturing costs of these bipolar plates are very high and need to be reduced by ∼10 times before PEM fuel cells can be fully embraced for the automotive application [1]. For example, the cost of a graphite bipolar plate is currently about US$ 10 per plate if both the material and machining costs are included [2]. Such a high cost is due to the brittleness of graphite, which drives the machining cost of the flow channels on bipolar plates to a prohibitive level.
Extensive research has been conducted to search for low cost, high performance bipolar plates. These research efforts include studies of metal-based bipolar plates [3], [4], [5], [6], carbon-filled polymers [7], [8], [9], and carbon/carbon composites [2]. However, these efforts have met limited success; either the performance is not satisfactory or the cost is still high [2], [3], [4], [5], [6], [7], [8], [9]. For example, metal-based bipolar plates typically suffer from corrosion in the fuel cell environment, which results in a release of cations. The cations, in turn, lead to an increase in membrane resistance and to poisoning of the electrode catalysts [6]. In the case of carbon-filled polymers, high carbon loadings (typically > 50 vol.%) are needed in order to provide the required electrical conductivity [10], [11]. As a result of such high carbon concentrations, utilization of injection molding, which is suitable for mass manufacturing and will result in low-cost bipolar plates [1], is precluded because of the difficulty in processability; instead, compression molding, which is a slow process because one must allow the mold to cool down before the part can be taken out, often becomes the only choice of processing method [10]. An additional problem associated with high carbon concentrations in polymers is the substantial reduction in the strength and ductility of the polymer composites. It is well known that the tensile strengths of polymer composites increase initially with the addition of a small amount of fillers (∼5–20 vol.% CB), but decrease with higher filler loading [12], [13], [14]. Such phenomena have normally been attributed to the weak filler-matrix interface [12].
Recently, we have proposed a concept of making carbon-filled polymer blends containing a triple-continuous structure in 3D space [15], [16]. Fig. 1 shows the schematic of such a carbon-filled polymer blend, which consists of a binary polymer blend both phases of which (i.e., Phases A and B in Fig. 1) are continuous in 3D space. The conductive carbon is preferentially located in one phase (Phase A in Fig. 1) and its concentration is high enough to form a continuous structure (i.e. at least higher than the percolation threshold in Phase A) so that a continuous electrical conductive path is present in the polymer. Such a triple-continuous structure has the advantage of achieving conductive polymer composites at lower carbon concentrations since only the percolation threshold in one phase, rather than the entire polymer blend, needs to be exceeded [17], [18]. Such triple-continuous, carbon-filled polymer blends also offer two additional advantages, which are (i) the improved processability because of the low carbon concentration required, and (ii) minimal degradation in tensile properties because of the presence of a continuous neat polymer phase (Phase B in Fig. 1).
The feasibility of making triple continuous, carbon-filled polymer blends via injection molding has been demonstrated previously using a carbon nanotube (CNT) filled polyethylene terephthalate (PET)/polyvinylidene fluoride (PVDF) blend. The CNT-filled PET/PVDF blend exhibits 2500% improvement in electrical conductivity, 36% increase in tensile strength, and 320% improvement in elongation over the CNT-filled PET at the same carbon loading [16]. Thus, the triple continuous, carbon-filled polymer blends have great potentials for manufacture of low cost conductive polymers with superior conductivity and strength for bipolar plate applications of PEM fuel cells. To develop fundamental understanding of the improved properties of the CNT-filled PET/PVDF over the CNT-filled PET, additional experiments have been performed in this study. The results and analysis based on these new experiments are presented below.
Section snippets
Experimental
The detail of making the CNT-filled PET/PVDF blend and CNT-filled PET via injection molding can be found elsewhere [16] and will not be repeated here. Briefly, CNT was first extruded with PET to prepare masterbatches of PET pellets containing 12 vol.% CNT. These CNT-filled PET pellets were subsequently mixed with PVDF pellets in an 1–1 volume ratio and injection molded to form PET/PVDF blends containing 6 vol.% CNT. The PET containing 6 vol.% CNT was prepared in a similar manner, i.e. the extruded
Results and discussion
Fig. 3 shows the SEM images of the CNT-filled PET/PVDF blend after polishing and ion etching. Two distinct regions are noted; one contains carbon nanotubes (Region B) and the other is free of carbon nanotubes (Region A). The ratio of Regions A to B is about 1, consistent with the volume ratio of PET to PVDF. Recall that the CNT-filled PET/PVDF blends were prepared by pre-extrusion of carbon nanotubes with PET, followed by mixing and injection molding with neat PVDF. Thus, it is reasonable to
Concluding remarks
The CNT-filled PET/PVDF blend exhibits 2500% improvement in electrical conductivity, 36% increase in tensile strength, and 320% improvement in elongation over the CNT-filled PET with the same carbon loading. Such improvements have been related to the formation of a triple-continuous structure achieved through the forced segregation of CNT in the PET phase of the CTN-filled PET/PVDF blend. This CNT-filled PET phase offers an electrical short circuit for the composite, while the clean PVDF phase
Acknowledgements
The authors are indebted to Professors Frano Barbir, Montgomery Shaw and Lei Zhu for insightful discussion over a wide range of the topics related to this research. The assistance provided by Dr. Daniel Goberman in argon ion etching, Dr. Tao Zhou and Mr. Hong Luo in tensile tests, and Mr. Juan Villegas in some of SEM observations is greatly appreciated. Finally, the authors are grateful to the financial support from the U.S. Army (Contract #: DAAB07-03-3-K415) through the Connecticut Global
References (28)
- et al.
Technical cost analysis for PEM fuel cells
J. Power Sources
(2002) - et al.
New materials for polymer electrolyte membrane fuel cell current collectors
J. Power Sources
(1999) - et al.
Bipolar plate materials development using Fe-based alloys for solid polymer fuel cells
J. Power Sources
(1998) - et al.
Use of stainless steel for cost competitive bipolar plates in the SPFC
J. Power Sources
(2000) Fuel cell materials
Mater. Today
(2001)- et al.
The conduction mechanism of carbon black-filled poly(vinylidene fluoride) composite
Mater. Lett.
(2003) - et al.
Mechanical properties and electrical conductivity of carbon-nanotube filled polyamide-6 and its blends with acrylonitrile/butadiene/styrene
Polymer
(2004) - et al.
Ultra-low electrical percolation threshold in carbon-nanotube-epoxy composites
Polymer
(2003) - et al.
Carbon/carbon composite bipolar plate for proton exchange membrane fuel cells
J. Electrochem. Soc.
(2000) - et al.
Bipolar plate materials for solid polymer fuel cells
J. Appl. Electrochem.
(2000)
Carbon composite for a PEM fuel cell
Mater. Res. Soc. Symp. Proc.
New polymer bipolar plates for polymer electrolyte membrane fuel cells: synthesis and characterization
J. Appl. Polym. Sci.
Properties of molded graphite bipolar plates for PEM fuel cell stacks
J. New Mater. Electrochem. Systems
Cited by (155)
Selective localization of carbon nanotubes and its effect on the structure and properties of polymer blends
2021, Progress in Polymer ScienceRecent advances and future perspectives of carbon materials for fuel cell
2021, Renewable and Sustainable Energy ReviewsA novel graphite/phenolic resin bipolar plate modified by doping carbon fibers for the application of proton exchange membrane fuel cells
2020, Progress in Natural Science: Materials InternationalA facile rheological approach for the determination of “super toughness point” of nylon1212/POE-g-MAH/MWCNT nanocomposites
2019, Composites Science and Technology