Swelling and related mechanical and physical properties of carbon nanofiber filled mesophase pitch for use as a bipolar plate material

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Swelling and related mechanical and physical properties of carbon nanofiber filled mesophase pitch for use as a bipolar plate material Chris Calebrese, Glenn A. Eisman, Daniel J. Lewis, Linda S. Schadler

*

Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, United States

A R T I C L E I N F O

A B S T R A C T

Article history:

Mesophase pitch was investigated as a melt processable precursor to a compression or

Received 15 March 2010

injection moldable all carbon bipolar plate. After shaping, carbonization to 1000 C or

Accepted 28 June 2010

greater is required to achieve the desired electrical and mechanical properties, but gases

Available online 30 June 2010

evolved during this step lead to swelling. Carbon nanofiber was added to suppress swelling during carbonization and bypass the typical oxidation steps used when processing mesophase pitch. The addition of carbon nanofiber decreased swelling by increasing the viscosity of the melt. Carbonized materials with carbon nanofibers can show strengths (30– 50 MPa) and conductivities (20–80 S cm1) consistent with composite bipolar plate materials. The materials show conductivities below Department of Energy target values at the current carbonization temperatures, which were limited to 1000 C. The use of glass fibers as a secondary filler led to reduced gas permeability in porous samples.  2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Of the components in a fuel cell stack, the bipolar plate typically makes up the largest portion by weight and volume [1]. Bipolar plates perform a number of functions, including acting as a physical barrier between adjacent cells, providing an electrical connection between cells in bipolar configurations, providing mechanical stability to the fuel cell stack, and distributing reactants over the surface of the fuel cell catalyst layers via flow channels embedded in the surface of the plate [1,2]. Bipolar plates can be broadly grouped into three categories: graphite, polymer composite and metal [3]. All carbonbased and graphite bipolar plates offer good chemical stability, while providing good electrical conductivity and low porosity. However, the use of some carbon materials, which includes graphite and carbon/carbon composites, can be limited in the cases when flow channels need to be machined [4,5].

Mesophase pitch is a thermoplastic precursor used to produce carbon fibers, foams and composites [6–8]. Its high carbon content and ability to be melt processed makes it a good candidate as a precursor material to an all carbon bipolar plate material that can be produced without the need for flow channel machining. The typical processing route to produce carbon materials from mesophase pitch is to form the desired shape, oxidize the material, and finally carbonize the mesophase pitch to give an all carbon product. Oxidation takes place above 140 C in an oxygen containing atmosphere and produces crosslinking in the material, preventing it from softening during carbonization [9,10]. Carbonization involves heating the mesophase pitch above 1000 C in an inert atmosphere, which increases the carbon fraction by driving off hydrogen and oxygen in the form gases such as methane, other small hydrocarbons, water and carbon dioxide [11,12]. To produce an all carbon bipolar plate from mesophase pitch without the need to machine flow channels, the typical

* Corresponding author: Fax: +1 518 276 8554. E-mail address: [email protected] (L.S. Schadler). 0008-6223/$ - see front matter  2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.06.061

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oxidation process would be prohibitively long. This step is limited by diffusion [13] and degradation reactions [14] and can require days or weeks for bulk parts1 [15]. If the material is not sufficiently oxidized, it will swell, or foam, during carbonization. Carbon black has been shown to reduce the swelling in mesophase pitch during carbonization [12,16], providing a possible alternative or supplement to the oxidation step. In this work, the addition of carbon nanofiber provided a similar reduction in swelling, and the possibility of using a carbon nanofiber filled mesophase pitch precursor to a bipolar plate material was investigated. The material was molded and carbonized without an oxidation step. The role of the carbon nanofiber in swelling inhibition was first investigated in terms of mass loss, melt viscosity and bubble nucleation, as understanding the effects on swelling are necessary to producing a bipolar plate material. The carbonized carbon nanofiber filled mesophase pitch was then evaluated for use as a bipolar plate material in terms of mechanical, electrical and barrier properties. Materials were made through two methods. The first method utilized oxidized mesophase pitch powders mixed with carbon nanofibers to produce materials that did not swell and would give baseline properties to be expected from melt processed materials after carbonization through 1000 C. The second method used melt mixed and molded mesophase pitch and carbon nanofiber, carbonized in a mold to maintain shape. The use of glass filler was also investigated as a means to reduce the permeability of the final carbonized material.

Table 1 – Length of carbon nanofibers. Ball mill time (h) 0 2 5 5

Experimental

2.1.

Materials

Two grades of commercially available naphthalene based synthetic mesophase pitch (Mitsubishi Gas Chemical) were used, MP and MP-H. Grade MP-H contained a lower volatile content than grade MP. The reported softening point, coking value and hydrogen/carbon ratio of the synthetic mesophase pitches was 275 C, 80–85% and 0.58–0.64 atom/atom. The reported solubility in water, benzene and pyridine was 0%, 35–44% and 40–50%. The density of MP was 1.26 g cm3 and the density of MP-H was 1.29 g cm3. Mesophase pitch powder was prepared by milling 20 g of mesophase pitch in a high energy ball mill for 15 min using 10 stainless steel balls 4.76 mm in diameter and 10 mL of ethanol. The pitch was dried overnight at room temperature and then at 110 C for 1 h under vacuum. Mesophase pitch powder was oxidized in an oven at ambient pressure with a flow of dry air at 240 C for 75 min. After oxidation, the mesophase pitch was ball milled for 15 min using the same procedure above to break up aggregates. Dried, oxidized mesophase pitch was placed in a dual asymmetric centrifuge (SpeedMixer DAC 150, FlackTek) to loosen aggregates. Vapor grown, pyrolytically stripped carbon nanofibers in powder form (PR24-PS, Pyrograf Products) with reported fiber

– Stainless steel Stainless steel Polyamide-imide

Average fiber length (lm) 40 15 10 15

diameters of 60–150 nm and lengths of 30–100 lm were purchased. The density of the carbon nanofibers was 1.96 g cm3. Carbon nanofibers were shortened by high energy ball milling in a steel jar using either 4.76 mm 440 stainless steel balls and water or 6.35 mm diameter polyamide-imide balls and ethanol. The carbon nanofiber lengths were measured using scanning electron microscopy of the carbon nanofibers on a silicon wafer. Nanofibers shortened using steel balls were used for viscosity measurements while nanofibers shortened using polyamide-imide balls were used for swelling experiments. After milling, the carbon nanofibers were filtered to remove the liquid, rinsed with acetone and water and dried at 100 C under vacuum for 2 h. Carbon nanofiber lengths before and after milling are shown in Table 1. Glass fiber (CRATEC 408A, Owens Corning) for glass filled composites was 14 lm in diameter and 4 mm in length with a softening point between 830 and 920 C. For experiments using graphite filler, flake graphite of mesh size + 325 was used.

2.2.

2.

Ball mill media

Sample preparation

Melt mixing of carbon nanofibers and glass fibers with mesophase pitch was performed at 290 C for 20 min using a Thermohaake Polydrive Mixer at a rotor speed of 100 rpm. Samples for swelling measurements and thermogravimetric analysis (TGA) were compression molded from melt mixed mesophase pitch between 225 and 270 C. Swelling measurement samples were pressed using a cylindrical steel mold 16 mm in diameter. A force of 17.8 kN was used and samples were molded to 1 mm thickness. Samples for TGA measurements were molded to squares 6.4 · 6.4 mm and were pressed at 4.4 kN to a thickness of 0.5 mm. Samples for conductivity and permeability measurements were molded in the same manner as those for the swelling measurement. After molding, the samples were placed in a separate steel mold and carbonized in a flowing nitrogen atmosphere to 560 C at 4 C min1, removed from the mold, and carbonized to 980 C at 10 C min1 under a nitrogen atmosphere. Viscosity samples were prepared by grinding carbon nanofiber filled mesophase pitch into a coarse powder using a mortar and pestle. Samples of 0.8 g in mass were molded at room temperature to pellets 16 mm in diameter and pressed under vacuum at a force of 66.7 kN for 30 min. Powder mixing of carbon nanofiber in oxidized mesophase pitch was performed using a dual asymmetric centrifuge at a

1 Mitsubishi Gas Chemical Inc., Impregnation and stabilization method for carbon–carbon composites, technical report, provided September 2005.

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speed of 2000 rpm for 60 s and 3000 rpm for 60 s. Samples for bend testing were pressed at 66.7 kN from oxidized powder to dimensions of 13.0 · 64.5 · 2.0 mm.

2.3.

Analysis

Samples for swelling measurement were weighed and placed in borosilicate glass vials measuring 17 mm in inner diameter and 51 mm in height. Four vials were place on an aluminum block on the center of a tube furnace with a 63.5 mm diameter quartz tube. A type K thermocouple was placed in the center of the aluminum block to provide temperature readings to the furnace controller. The tube was evacuated, backfilled with nitrogen and a nitrogen purge flow of 200 mL min1 used during heating to provide an inert environment and remove gaseous reaction products. Samples were heated at a rate of 4 C min1 to a maximum of 560 C. Upon reaching the desired temperature, the samples were immediately removed from the furnace, quenched in air at room temperature and the final volume measured. Thermogravimetric analysis (TA Instruments Q50 TGA) was performed at a heating rate of 4 C min1 from 300 C to a maximum of 900 C. Samples were placed in 500 lL alumina crucibles and experiments were carried out under a nitrogen gas purge of 60 mL min1. The shear viscosity of unfilled and carbon nanofiber filled mesophase pitch was measured between two parallel plates using a Rheometric Scientific ARES rheometer in a nitrogen atmosphere. The plates were 25 mm in diameter and a gap of 1 mm was used. Samples were placed in a preheated fixture and the temperature was allowed to equilibrate. The fixture was then lowered to the desired gap and excess material removed. The viscosity was measured over a frequency range of 0.1–100 rad s1 in temperature increments of 10 C. Conductivity measurements were made on cylindrical samples nominally 2 mm in thickness and 15 mm in diameter. Sample surfaces were polished to provide a flat contact surface. Resistance measurements were made between two copper contacts using a press to provide contact pressure. The resistance of the setup was measured without the sample and subtracted from the sample measurement. Compression testing on melt processed, carbonized mesophase pitch was performed using an Instron 4204 mechanical testing machine with a 50 kN static load cell. The sample faces were squared and polished prior to testing. Sample sizes were nominally 14 mm in diameter by 30 mm in length and tests were performed at a crosshead speed of 0.5 mm min1 until failure. Oxidized, pressed and carbonized materials were tested using three-point bending on samples nominally 56 · 1.7 · 38 mm and were tested at a crosshead speed of 1 mm/min with a load span of 38.1 mm. The air permeability of carbonized mesophase pitch was measured using disks nominally 2 mm in thickness and 15 mm in diameter. The sample was placed between two chambers, a vacuum applied to each chamber, and one chamber backfilled with dry air at ambient pressure. The pressure in the evacuated chamber was measured as a function time, and the permeability coefficient, D, was calculated using the equation

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dPlow DA dP ¼ V dx dt where Plow is the pressure in the evacuated chamber, t is the time, A is the cross-sectional area of the sample, V is the volume of the evacuated chamber, dP is the pressure drop across the sample and dx is the thickness of the sample.

3.

Results

3.1.

Swelling

Fig. 1 shows the swelling of unfilled and carbon nanofiber filled MP from 440 to 560 C in increments of 10 C. Below 470 C, the foam could not support itself on removal from the furnace, and the swelling volume was estimated by the height of the MP residue on the glass vial. The data has been normalized by dividing by the swelling of unfilled MP at 560 C. For all loadings, the addition of carbon nanofibers increases the swelling below 500 C. The increased swelling volume below 500 C with the addition of carbon nanofiber is associated with an increase in bubble number and volume. Bubble formation begins by 360 C; optical micrographs of cross sections of MP at this temperature are shown in Fig. 2. The data collected at 360 C shows an increase in bubble area in the cross sections from 15% for unfilled mesophase pitch to 20, 22, and 36% for carbon nanofiber loadings of 1, 5 and 15 wt.%. Through 500 C, MP filled with 15 wt.% carbon nanofiber has a greater swelling volume than unfilled mesophase pitch. Above 500 C, the volume of the unfilled MP becomes greater than the MP filled with 15 wt.% carbon nanofiber. For 5 wt.% carbon nanofiber in MP, the same transition occurs at 520 C. For 1 wt.% carbon nanofiber, the swelling volume is greater than or equal to the unfilled MP swelling volume over the whole temperature range. TGA curves through 600 C for carbon nanofiber filled MP are shown in Fig. 3. The TGA data has had the filler mass removed and thus shows only the change in mass of the mesophase pitch. The addition of carbon nanofiber at a loading of 1 wt.% does not change the mesophase pitch residue at 600 C, while the addition of carbon nanofiber at 5 and 15 wt.% increases the residue from 77.4% to 78.2% and

Fig. 1 – Normalized swelling of MP filled with as-received carbon nanofiber.

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Fig. 2 – Polished cross sections of (a) unfilled, (b) 1 wt.%, (c) 5 wt.% and (d) 15 wt.% carbon nanofiber filled mesophase pitch carbonized to 360 C. Scale bars are 2000 lm.

79.2%, respectively. The dTGA peaks show no measureable difference in peak temperature, but do show a decrease in overall area at 5 and 15 wt.% loadings due to the increase in residue remaining at 600 C. The effect of carbon nanofiber loading on dynamic shear viscosity is shown in Fig. 4. The addition of as-received carbon nanofiber (measured average length of 40 lm) increased the viscosity 100 times over a loading range of 20 wt.%. Carbon nanofibers ball milled for 2 h using steel balls (measured average length of 15 lm) increased the viscosity 30 times over the same loading rage. The data shown is only for measurements taken at 100 rad s1 but, over the range of shear rates used, the effect of carbon nanofiber on the viscosity was consistent. The order of magnitude viscosity increase in this study is consistent with values reported elsewhere for carbon nanofiber filled polymers [17,18]. Fig. 5 shows the swelling of mesophase pitch filled with carbon nanofibers as a function of volumetric filler loading.

The swelling data is divided by the unfilled swelling volume and a value of 1 is expected for volumetric effects of the filler loading only. Data for carbon nanofiber filled MP-H show that with the addition of carbon nanofiber through 18 vol.% (25 wt.%) the swelling of mesophase pitch goes through two regions. Through 10 vol.% (15 wt.%), the addition of carbon nanofiber reduces the swelling to 5% of its initial value. From 10 to 18 vol.%, the swelling reduction is much smaller, showing a further reduction to 2% of the initial swelling volume. Fewer data points were taken for MP filled with carbon nanofiber, but the data shows a similar trend as the MP-H data. The swelling of MP with the addition of 10 vol.% carbon nanofiber is 27% of the initial swelling volume, while adding an additional 12 vol.% brings this value down to 5%. The MP-H experiments were carried out at 2 C min1, while the MP experiments were carried out at 4 C min1. Fig. 5 also shows swelling data for low aspect ratio graphite filler. Greater than 10 vol.% loading is needed to produce a

Fig. 3 – TGA and dTGA curves of carbon nanofiber filled MP showing mass loss of MP fraction.

Fig. 4 – Dynamic viscosity of nanofiber filled MP-H at 100 rad s1 and 310 and 350 C.

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Fig. 5 – Normalized swelling in carbon nanofiber filled MP-H and MP and graphite filled MP.

change in swelling of MP, in contrast to the 1 vol.% needed when using carbon nanofiber. Fig. 6 shows the effect on swelling of shortening the carbon nanofibers from 40 lm to 15 lm in length. Shortening the carbon nanofibers using stainless steel balls leads to an increase in the swelling volume at the same fiber loading as the as-received fibers. For 5 and 15 wt.% ball milled carbon nanofibers the MP swelling volumes are 1.2 and 1.4 times greater than the swelling volumes measured when using asreceived carbon nanofibers at the same loadings. The viscosity data in Fig. 4 and swelling data in Figs. 5 and 6 show that the addition of carbon nanofiber increases the viscosity while reducing swelling. Additionally, by shortening the carbon nanofibers, the viscosity of the melt is reduced and the swelling increased when compared to mesophase pitch filled with the same loading of as-received carbon nanofiber. Fig. 7 shows the relationship between the dynamic viscosity of the mesophase pitch (normalized by the zero loading viscosity) and the normalized swelling of MP-H and MP. Both sets of data show an inverse relationship between the viscosity and swelling volume. The MP data contains swelling data for both as-received and ball milled carbon nanofibers, and shows that the inverse relationship between viscosity and

Fig. 6 – Normalized swelling of MP with 5 and 15 wt.% carbon nanofiber showing the difference in swelling with as-received (40 lm) and ball milled (15 lm) carbon nanofiber.

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Fig. 7 – Normalized swelling at 560 C as a function of viscosity at 350 C for MP-H and MP. The plot contains data using as-received (unlabeled points) and ball milled carbon nanofibers (labeled points).

carbon nanofiber loading holds for both fiber lengths used in this work.

3.2.

Material properties

The conductivity, strength and permeability of the materials produced in this work, along with properties for graphite and composite bipolar plate materials, are shown in Table 2. The materials produced from oxidized mesophase pitch powders gave electrical conductivities between 40 and 80 S cm1 and flexural strengths of 40–50 MPa. The air permeability of materials produced from oxidized mesophase pitch powder was not measured due the high permeability, as observed by the inability of these materials to prevent water flow through the sample. This is in contrast to the melt processed materials, which showed much lower permeability. The conductivity of materials produced from melt processing and mold carbonization was 20 S cm1, the compression strength was 30 MPa and these materials showed porosities of up to 40%. The air permeability of these materials fell between 103 and 2 · 101 cm2 s1. The lowest permeability was measured for samples with 20 wt.% carbon nanofiber and 17 vol.% glass fiber, while the highest permeability was measured with 20 wt.% carbon nanofiber only. The addition of 44.7 vol.% carbon nanofiber, equivalent to the total volume of filler (carbon nanofiber and glass fiber) in the sample with the lowest permeability, had a permeability of 2.5 · 101 cm2 s 1 . All materials produced through melt processing and mold carbonization showed porosity and cracks within the sample. To achieve the lowest permeability value of 103 cm2 s1, the addition of glass fiber was needed. The glass fiber acted as a filler with the ability to flow during carbonization and close off porous networks and cracks. Representative micrographs of the surface of carbon nanofiber-glass filled MP are shown in Fig. 8. Cracks are apparent on the surface of the material (Fig. 8a) and the flow of glass during carbonization is evidenced by glass beads present on the outside edge of the sample (Fig. 8b).

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Table 2 – Select electrical, mechanical and permeability properties of selected bipolar plate materials and of the materials produced in this work. Material Artificial graphite Graphite/carbon polymer composite Polymer graphite-carbon fiber composite MP/CNF (980 C) (oxidized) MP/CNF/Glass (980 C) (melt processed) a b

Conductivity (S cm1) 110–690 [4] 20–100a,b 270 [20] 40–80 20

Flex strength (MPa) 25 [4] 40a,b 96 [20] 40–50 –

Compression strength (MPa) 25–200 60–95a,b – – 30

Air permeability coefficient (cm2 s1) 102 [19] 105 b – – 103–2 · 101

Bipolar and monopolar plate standard properties, 2007. Product literature. Entegris. Sigracet-BPP: Bipolar Plate-BMA5 (PVDF), 2005. Product literature. Sigracet.

4.

Discussion

4.1.

Swelling

During carbonization of mesophase pitch, the viscosity of the melt first decreases with an increase in temperature (Fig. 4) and then increases as carbonization reactions proceed and the molecular weight increases [21]. Thus, it is relevant to discuss the behavior of the filled MP in these two stages. During the initial stages of carbonization when the viscosity of the melt is decreasing with an increase in temperature, the presence of carbon nanofibers increases the bubble volume (Fig. 2) and swelling (Fig. 1). At this stage, the dominant effect of the carbon nanofibers is to act as a nucleating agent for gas bubbles, which increases the number of bubbles in the melt, similar to foaming processes in polymer melts [22]. At about 470 C, the viscosity of the melt increases to the point where the foam can support itself. Above this temperature, the large volumetric increase takes place and the addition of carbon nanofibers slows the increase in volume of the mesophase pitch. One hypothesis for the decrease in swelling with the addition of carbon nanofibers (Fig. 5) is that the nanofibers provide a diffusion path to the surface of the material, easing gas transfer to the surface and reducing the mass of volatile material available for swelling. With this mechanism acting, the addition of carbon nanofiber would be expected to cause an increase in mesophase pitch mass loss and a shift in the

mass loss rate peak (i.e. dTGA peak) to lower temperatures during carbonization [12,16]. In this work, the TGA data shows an increase in the residue of the mesophase pitch with no change in the dTGA peak position (Fig. 3). A second explanation for the reduction in swelling with the addition of carbon nanofibers is that the increase in viscosity slows bubble growth. A simple relation highlighting the relationship between melt viscosity and bubble growth rate is [23]: dR ðPD  PC Þ c ¼  dt 4g 2g where R is the radius of the bubble, t, is the time, PD is the pressure in the bubble, PC is the pressure in the liquid, c is the surface tension of the liquid and g is the viscosity of the liquid. Fig. 7 shows that there is an inverse relationship between the swelling volume and mesophase pitch viscosity. As carbonization proceeds past 470 C the viscosity increase due to carbon nanofibers is great enough that the effect of reduced bubble growth rate becomes greater than the initial bubble nucleation effect (Fig. 1). Furthermore, the increase in the viscosity can also help to account for an increase in mesophase pitch residue at the end of carbonization. Increasing the viscosity leads to slower bubble migration to the mesophase pitch surface and an increased pressure in the bubbles. These two effects will increase volatile component reactions that convert volatile components to solid components.

Fig. 8 – Optical micrographs of polished surfaces of carbon nanofiber-glass filled MP carbonized to 980 C.

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The fiber loading and fiber length affect the viscosity of the melt (Fig. 4) and therefore also influence the swelling of mesophase pitch (Fig. 6). Both loading and length are important, as they determine the extent of filler interaction in the melt which in turn is responsible for the increase in viscosity of the melt [24,25]. By increasing either the loading or the aspect ratio, the particle interactions increase, viscosity increases, and swelling decreases. Particle interactions and the increase in viscosity are most pronounced above the percolation threshold of the filler. In this study, loadings of 1 wt.% carbon nanofiber fall below the percolation threshold value [26], and loadings of 10 vol.% graphite fall below the percolation threshold [27,28]. At these loadings and below, the viscosity change is small and there is no measureable difference in the swelling of filled and unfilled mesophase pitch (Fig. 5).

4.2.

Material properties

The bend strength of materials made from oxidized mesophase pitch powder range from 40 to 50 MPa and the conductivity of the materials ranges from 40 to 80 S cm1. The flexural strengths are comparable to composite bipolar plates and graphite (Table 2) and exceed the Department of Energy (DOE) target of 4 MPa for flexural strength [29]. The conductivities fall on the low end of the range of conductivities for composite bipolar plates, are lower than the conductivity of graphite and are below the DOE target of 100 S cm1. The materials produced from melt processing show compression strengths of 30 MPa and conductivities of 20 S cm1. The compression strength and conductivity are in the lower range of those for composite bipolar plate materials and graphite, above the DOE target for flexural strength (4 MPa) and below the DOE target for conductivity (100 S cm1). The use of carbon nanofiber and glass fibers as fillers in mesophase pitch and carbonization in a mold yielded materials with average air permeabilities of greater than 103 cm2 s 1 . During carbonization, molecular alignment occurs, causing a decrease in the volume of the material and closing off porosity [19]. From 560 to 980 C, the matrix contracted by 12% in each dimension. The glass filler, which softened at 820 C, had the ability to flow during mesophase contraction. Due to the change in volume of the matrix, this flow does not require the glass to leave behind voided areas, and in fact helps to close off pores. The lower average value corresponds to samples with 17 vol.% glass and 20 wt.% carbon nanofiber and is one order of magnitude lower than unsealed graphite and two orders of magnitude greater than composite bipolar plates. While the average permeability value of this composition were 10 3 cm2 s1, certain samples were measured with permeabilities on the order of 104 cm2 s1, showing that refinement of the processing conditions will be able to give at least an order of magnitude improvement in the permeability coefficient. Carbon nanofiber filled mesophase pitch was injection moldable up to 25 wt.% using a tabletop injection molding machine. While the materials swelled during carbonization, this shows that the material can be processed through injection molding methods. When used with partial carbonization in a mold, an injection moldable carbon bipolar plate could be produced.

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The maximum carbonization temperature was 1000 C for all the materials that were tested. Though the current materials show conductivities that are lower than the specified DOE targets, the conductivity of the material can be increased in future work by utilizing higher carbonization temperatures. Carbonization at increased pressures can also be performed to further increase the conductivity. Future work requires quantifying the effects of processing on material properties so that optimal conditions can be chosen to maximize conductivity and minimize the permeability of the material. Conditions to be optimized include injection temperature and pressure, carbonization pressure and ramp rate, all of which have been held constant in this study but can impact the final material properties [30,31].

5.

Conclusions

The increase in viscosity of the melt due to the addition of carbon nanofibers causes a corresponding decrease in the swelling of mesophase pitch during carbonization. This relationship holds for two differing carbon nanofiber lengths and two different mesophase pitches. At low loadings of filler the viscosity does not change appreciably and no reduction is seen in mesophase pitch swelling. For as-received carbon nanofiber at a temperature ramp of 4 C min1 this regime is below 1 wt.% and for graphite filler it is below 10 vol.%. The addition of carbon nanofiber also produces larger swelling volumes during the initial stages of carbonization due to increased nucleation and bubble retention, but the swelling trend reverses during the final foam growth stage. Carbonized mesophase pitch filled with carbon nanofiber has the potential to be used as a bipolar plate material. The current mechanical properties are comparable to composite bipolar plates and the bend strengths are greater than the Department of Energy target values. The electrical conductivity of the tested materials is on the low end of current composite bipolar plate materials and is less than the DOE target, but can be raised by increasing the carbonization temperature and reducing porosity by optimizing the processing, including fiber loading, temperature ramp rate and pressure.

Acknowledgments This work was supported by grants from the National Science Foundation’s Integrative Education and Research Traineeship (IGERT) program and the Nanotechnology Center at Rensselaer Polytechnic Institute. Facilities and support for viscosity measurements were provided by Dr. Dan Edie and Dr. Santanu Kundu at Clemson University.

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4 8 ( 2 0 1 0 ) 3 9 3 9 –3 9 4 6

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