Investigation of the properties of polymer composite
Transcription
Investigation of the properties of polymer composite
Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. www.kunststofftech.com © 2014 Carl Hanser Verlag, München Zeitschrift Kunststofftechnik 4Autor Titel (gegebenenfalls gekürzt) Journal of Plastics Technology www.kunststofftech.com · www.plasticseng.com archivierte, peer-rezensierte Internetzeitschrift archival, peer-reviewed online Journal of the Scientific Alliance of Plastics Technology 10 (2014) 3 eingereicht/handed in: angenommen/accepted: 19.11.2013 19.05.2014 Dr.rer.nat. Nacera Stübler, Dr.-Ing. Dieter Meiners, Prof. Dr.-Ing. Gerhard Ziegmann, Institute of Polymer Materials and Plastics Engineering, at the TU-Clausthal Dr. Thorsen Hickmann Eisenhuth GmbH & Co. KG, Osterode Lerbach, Germany Investigation of the properties of polymer composite bipolar plates in fuel cells This work presents a comparative study of the effect of the polymer matrix on the thermal, electrical and mechanical properties of highly-filled graphite polymer composite. This material was destined to be used as bipolar plates in proton exchange membrane fuel cell (PEMFC). In order to investigate the synergetic effect between Gr (Graphite) and CB (carbon black) particles the polymer PVDF (poly (vinylidene fluoride)) was mixed with 15% wt of CB and 70% wt. of Gr. The results were offering a high value of the electrical conductivity (176 S/cm), while the value of the electrical conductivity of Gr/PVDF composite is about 87 S/cm. A correlation between the thermal and electrical conductivity was explored for the samples filled only with graphite (without CB). Untersuchung der Eigenschaften hochgefüllter Polymer Composite für Bipolarplatten in Brennstoffzellen In diesem Beitrag wurde eine vergleichende Untersuchung des Effekts der Polymer-Matrix auf die thermischen, elektrischen und mechanischen Eigenschaften von mit Graphit hoch gefüllten Polymer Composite dargestellt. Dies wird als Bipolarplatten in PEM-Brennstoffzellen prädestiniert. Um den Synergieeffekt zwischen Gr (Graphit) und CB (Ruß) zu prüfen, wurde exemplarisch die Polymermatrix PVDF (poly (vinylidene fluoride)) mit 15 Gew % CB und 70 Gew % Gr gemischt. Für diesen Verbundwerkstoff wurde eine sehr hohe elektrische Leitfähigkeit (176 S/cm) ermittelt, im Vergleich zu einem Kennwert von 87 S/cm für Gr/PVDF-Composite. Die Korrelation zwischen der thermischer und der elektrischer Leitfähigkeit wurde für die hochgefüllte Compound-Materialien (ohne Ruß) aufgezeigt. © Carl Hanser Verlag Zeitschrift Kunststofftechnik / Journal of Plastics Technology 10 (2014) 3 © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Characterisation of bipolar plates Investigation of the properties of polymer composite bipolar plates in fuel cells N. Stübler, D. Meiners, G. Ziegmann, T. Hickmann 1 INTRODUCTION Conductive polymer composites play a significant role in the application areas as organic light emitting diodes, polymer photovoltaic cells and fuel cells [1, 3]. A Proton Exchange Membrane Fuel Cell (PEMFC) converts the chemical energy into electrical energy by electrochemical reactions. Nowadays fuel cells represent an energy source of choice for stationary and mobile applications as electric vehicle, speed cameras, and telecommunications. PEMFC consists of the Membrane Electrode Assembly (MEA) which includes the membrane, electrode and gas diffusion layer, as well as the bipolar plates. The most important components in fuel cells are the bipolar plates. They are multifunctional components in the PEMFC stack. They provide a conductive medium between the anode and cathode, transfer heat out of the cell and provide a flow field channel for the reaction gases. Three types of material are used for bipolar plates: metallic, pure graphite and polymer composites plates. Metallic bipolar plates of, for example, stainless steel show good mechanical stability as well as high electrical and thermal conductivity. The main disadvantage of metal plates is their susceptibility to corrosion. Pure graphite bipolar plates are already known for their excellent resistance to corrosion and their low electrical contact resistance. The graphite bipolar plates have several disadvantages. The graphite is brittle, quite porous, has low mechanical properties and also high costs. Polymer composites offer a more economical alternative route for manufacturing of bipolar plates because of their lower cost, higher flexibility, reduced brittleness, lower gas permeability and light weight [4,5]. Therefore the development of polymer composite bipolar plates facilitates the commercialisation of the PEM fuel cells in a wide range of applications. Vigorous efforts were deployed to optimize the functionality of the polymer composite plates [6-9]. However the disadvantage of these plates is their low electrical conductivity. An excessive conductive filler content should be incorporated in the polymer matrix during the processing to increase the electrical conductivity which leads in many cases to a decrease of the mechanical properties. For many authors or suppliers such as DuPont, it is a challenge to meet together good electrical, mechanical and thermal properties of bipolar plates [9]. Journal of Plastics Technology 10 (2014) 3 69 © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Characterisation of bipolar plates There are two different types of matrix materials to manufacture the graphitebased composite plates: thermoplastic and thermoset polymers. The composite is created by incorporating a filler or mixture of fillers like graphite (Gr) and/or carbon black (CB) [7]. Polymer composites are usually manufactured using conventional polymer processing methods, such as compression molding and injection molding. The compression molding is favoured for both thermoplastic and thermoset matrix composite. However there is a difference in the processing; the thermoset has to be cured by a chemical reaction whereas the thermoplastic material has to be processed by heating to the melt phase and cooled in the mould. The thermoset material has a shorter cycle time than thermoplastic material. Therefore the cost-effective mass production is in this case more efficient. If the material is injected the processing time is very short (about 30 s) and the production is automated with high precision of the size. The big disadvantage using the injection molding lies in the high viscosity of the material caused by the high load of the conductive filler. The melt flows poorly and the material after injection could have a poor quality. The bipolar plates must have a high electrical conductivity and sufficient mechanical properties to be used in fuel cell stack due to the constant compressive load. The US Department of Energy (DOE) defined requirements for an ideal material for bipolar plates [10]. These requirements are represented as targets for the properties as contact resistance, thermal conductivity, gas permeability, compression strength etc… In this paper we focused on the electrical conductivity, thermal conductivity and mechanical properties. According to the US DOE the minimum value of the electrical conductivity and the flexural strength are 100 S/cm and 25 MPa, respectively. The minimum value for the thermal conductivity is 10 W/mK [10]. The polymer matrix, which acts as binder in the material composite is an insulating material with an electrical conductivity in the order of 10 -15 S/cm. A gradual incorporation of graphite content creates the conductive network by increasing the electrical conductivity of the sample. M. Xiao et al. [11] measured both the electrical conductivity and the flexural strength on the poly(arylenedisulfide)/graphite nanosheet (NanoG) composite. They observed a a very slight negative effect of the strength when the NanoG content varies from 20wt% to 60wtl%. However the electrical conductivity increases from 40 S/cm to 190 S/cm. [11]. Ling Du [9] manufactured epoxy composite filled with expanded graphite. He showed that the flexural strength increased from 40 MPa to 55 MPa when Gr load varied from 40 wt% to 60 wt%. Zhu Bin et al. [12] manufactured bipolar plate by using polyvinylidene fluoride (PVDF) as binder and titanium silicon carbide (Ti3SiC2) as conductive filler. They showed that the electrical conductivity and flexural strength of the composite bipolar plate with 80 wt.% Ti3SiC2 content, were 28,83 S cm−1 and 24,92 MPa, respectively. These values especially for the electrical conductivity do not meet the DOE requirements. For bipolar plates the electrical conductivity should be very high (> 100 S/cm) accompanied with optimal values of the Journal of Plastics Technology 10 (2014) 3 70 © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Characterisation of bipolar plates flexural strength. It is important to remember that it is difficult to get high conductivity and sufficient mechanical properties simultaneously. In this work a comparative study was performed on high graphite-filled thermoplastic polymer and thermosetting resin -composites. The samples were manufactured by using a kneader and then a hydraulic compression press. The prepared samples have different polymer matrix types and filled with high Gr contents in volume. Different properties were investigated as the thermal stability, the in-plane electrical conductivity, the flexure strength and the thermal conductivity. For the thermal properties both parameters as glass transition and degradation temperature were determined to get more information about the thermal stability of the material. The thermal conductivity measurement was performed within a temperature range 20°C to 150°C where the operating temperatures of PEM fuel cell (40°C-120°C) are included. The in-plane electrical conductivity was determined at room temperature by using the 4 pointprobe method. The flexure strength of the composite was measured at room temperature. Dynamic mechanical analysis (DMA) measurements were added in order to get a better understanding of the material behaviour at high temperatures. 2. EXPERIMENT 2.1. Material In this work a series of graphite polymer composite bipolar plates were developed. Five polymer matrixes were used for the preparation of the plates: Poly (phenylene sulphide) (PPS), Poly (propylene) (PP), Poly (vinylidene fluoride) (PVDF), Polysulfon (PSU) and Phenolic resin. A high content of graphite (Gr) (from 85 wt. % to 92wt. %) was mixed intensively with the polymer matrix. The volumetric mass density is related to the polymer matrix system, which leads to different volume contents: 70 vol. % Gr for PP composite, 85 vol. % Gr for PPS composite, 82 vol. % Gr for PSU composite and 89 vol. % Gr for phenolic resin. Two Polymer composites were prepared using the PVDF polymer system. The first one was filled with 85 wt. % Gr, and the second one with an additional content of carbon black (CB) of about 15 wt. % so that the sum of the filler content (CB+Gr) reaches the value 85 wt %. The total volume content of the conductive filler for both materials is nearly similar (82 vol. %). To summarise the results Gr/PPS, Gr/PSU, CB+Gr/PVDF and CB+Gr/PVDF has approximately the same filler volume content (82-85 vol. %). The thermoplastic polymer and the fillers were first dried and mixed in a kneader. After that the sample was placed in hydraulic compression press and heated to the high temperature and then cooled carefully for 15-20 min below the melting temperature. The processing temperatures are different for each sample; for example for PPS composite the temperature is 300 °C, for PVDF composite 260 °C and for PP composite 180 °C. For the Phenolic resin Journal of Plastics Technology 10 (2014) 3 71 © 2014 Carl Hanser Verlag, München Characterisation of bipolar plates composite the sample is cured at 160 °C with a cycle below 10 min. The thickness of all samples after compression is approximately 3 mm. 2.2. Thermal properties The thermogravimetric analyser (TGA Q5000, TA Instrument) was used to determine the degradation temperature T1. The temperature T1 was obtained at 5% weight loss [5]. The measurements were carried out under nitrogen atmosphere with a heating rate of 10 K/min. In addition the glass transition T g and the melting temperature Tm were determined by using differential scanning calorimetry (DSC Q2000, TA Instrument) technique. The measurements were carried out under nitrogen atmosphere with a heating rate of 20 K/min (DIN EN ISO 11357). The weight of the sample was approximately 20 mg for TGA and 10 mg for DSC. 2.3. In-plane electrical conductivity The in-plane (bulk) electrical conductivity was measured with a conventional four probe method. The measurements were performed at room temperature using a cylindrical four point probe head and RM3000 Test Unit from Jandel Engineering Limited. The RM3000 can supply constant currents between 10 nA and 99,99 mA. The voltage can be measured in a range between 0.01 mV and 1250 mV. The electrical conductivity of the sample was calculated according to [8]: [ www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Figure 1 ( ) ( ) ( )] (1) Diagram of four-point probe resistance method [13] where s is the probe spacing (s = 1 mm), t is the thickness of the sample, U is the voltage, I is the current and G is the correction factor depending on the geometry of the sample. The specimen has a rectangular shape (40×50 mm2) with a thickness of about 3 mm. Journal of Plastics Technology 10 (2014) 3 72 © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Characterisation of bipolar plates 2.4. Thermal conductivity In this work the thermal conductivity of the materials was not determined directly. In a first step, the specific heat parameter Cp was determined by using modulated DSC where a periodic temperature modulation (± 0,48 °C every 60 s) is superimposed on the constant heating rate of a conventional DSC measurement. In a second step the thermal diffusivity parameter was determined by using the equipment of Laser Flash technique from Netzsch 427. The thermal conductivity is related to the thermal diffusivity D according to: (T) = D(T) Cp(T) (T) (2) In this work; the volume density is supposed to be independent of the temperature ( ⁄ ) over the temperature range 20 °C - 150 °C. The heating rate for both measurements equals 3 K/min and the temperature range spread from 22 to 120 °C. The tests of the thermal diffusivity were carried out on samples with cylindrical shape having a diameter of 12,7 mm, and thickness of 2 mm. The sample for the Cp – measurements has a diameter of 5 mm and a thickness of 1-2 mm. 2.5. Mechanical properties The three-point bending technique according to DIN EN ISO 178 was performed at room temperature in order to obtain the flexural strength of the materials. Rectangular specimens were cut out for flexural testing. For these tests, the support span L was calculated using a ratio L/d of 16:1 where d is the thickness of the specimen. To get more information about the stiffness at high temperature, the dynamic mechanical analysis (DMA Q800, TA Instrument) technique was carried out on the samples. The technique was performed by using the three-point bending mode. A scanning temperature range from 22 °C to 120 °C was employed with a heating rate of 3 K/min. The length and the width of the specimens for DMA test were 50 × 10 mm with a thickness of about 2 mm. Both the storage modulus E as well as loss modulus E were determined. In this paper only the results of E´ against the temperature at the frequency 1 Hz are represented and discussed. Journal of Plastics Technology 10 (2014) 3 73 Characterisation of bipolar plates (b) (a) 0,9 98 Gr/PVDF Gr/PSU Gr/PPS Gr/PP 96 Heat flow (W/g) Weight loss (%) 100 Gr/PPS Gr/PP Gr/PVDF 0,8 0,7 0,6 5% weigth loss 0,5 94 0,4 0 200 400 600 Temperature (°C) 100 200 300 Temperature (°C) Figure 2: left: Temperature-dependent percent weight for Gr/PPS (triangle) Gr/PP (square) Gr /PVDF (open circle) and Gr/PSU (star) Right: Temperature- dependent heat flow for Gr/PPS, Gr/PP and Gr/PVDF (b). Figure 3: Temperature- dependent heat flow showing the glass transition temperatures of both samples left: for Gr/PPS right: for Gr/PSU © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Journal of Plastics Technology 10 (2014) 3 74 © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann 3. RESULTS AND DISCUSSION 3.1 Thermal properties Characterisation of bipolar plates The quantitative analysis of the weight loss against the temperature was characterised by TGA measurements under nitrogen atmosphere. The temperature range used for the analysis is 22 °C - 800 °C. TGA curves of Gr/PVDF, Gr/PP, Gr/PPS and Gr/PSU composites are displayed in Fig. 2 (a). The temperature degradation T1 is defined as the onset of a mass change at which 5% of weight is lost. The TGA-curves show sharp drops at different temperatures related to the different polymers, for example the degradation of Gr/PPS and Gr/PSU start later compared to that of Gr/PVDF and Gr/PP composite. The values of the degradation temperature T 1 for all samples are reported in Table. 1. The value of T1 for Gr/PPS is about 552 °C, while for Gr/PP the temperature T1 reaches the value 466 °C. This means that PPS has a higher thermal stability than PP composite. In addition the filled phenolic resin composite reveals an excellent heat resistance since T1 is approximately about 577 °C. DSC curves of Gr/PPS, Gr/PP and Gr /PVDF were plotted against the temperature in Fig. 2 (b). From the DSC analysis two temperatures are determined Tm and Tg (Fig. 3). The results of T1, Tm and Tg are summarised in table 1. The DSC analysis indicates that the melting temperature Tm (170 °C) for Gr/PVDF composite is shifted slightly towards higher temperatures compared with Gr/PP composite (Tm = 160 °C). However Gr/PPS composite exhibits a higher Tm (266 °C) and a glass transition temperature Tg of about 96 °C (Fig. 3(a)). It should be mentioned that Tg value is included to the operating temperature range (40 °C to 120 °C) of the fuel cells for Gr/PPS composite. The filled PSU composite is an amorphous thermoplastic which means it has not a crystalline melting point while the glass transition temperature reaches the value 220 °C (Fig. 3(b)). The glass transition is a change in the heat flow showing the shape of a step. Only the amorphous part contributes to the glass transition leading to a soft material upon heating or brittle material upon cooling. For PP/Gr, PVDF/Gr and Gr/phenolic resin composite it was not possible to detect this change. According to the reference [14,15] Tg should be lower than 0°C for both materials PP/Gr, and PVDF/Gr. The glass transition for a phenolic resin occurs at high temperature. The author Theodore N. Morrison [16] reported a value of about 237 °C. He attested that Tg is strongly related to the degree of cure of phenolic resin. Journal of Plastics Technology 10 (2014) 3 75 3.2 Characterisation of bipolar plates SEM micrograph of the plates Scanning electron microscope (SEM) was used to analyse the internal structure of the compression moulded bipolar plates. The samples were broken and their cross sectional areas were considered for the SEM analyses. Composite Filler (vol. %) PPS 85 PP 70 PSU 82 PVDF 82 Phenolic resin 89 Table 1: T1 (°C) 552 466 573 463 577 Tg (°C) Tm (°C) 96 266 / 160 220 amorphous / 170 / thermoset Glass transition temperature Tg, degradation Temperature T1 and melting temperature Tm determined for all samples. Tg value for Gr/PP, Gr/Gr and Gr/phenolic resin could not be detected Figure 4 illustrates the morphology of the samples Gr+CB/PVDF, Gr/PVDF, Gr/PP, Gr/PSU. The SEM images of Gr/PP, Gr/PSU and Gr/PVDF show dense materials and no obvious voids are observed. The structure of the Gr+CB/PVDF composite shows different voids (see the arrow in the picture). For this material it is not possible to differentiate between CB and Gr particles because both fillers have approximately the same average particle sizes. © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Journal of Plastics Technology 10 (2014) 3 76 Characterisation of bipolar plates Gr+CB/PVDF Gr/PP Gr/PVDF Gr/PSU Figure 4: SEM micrograph of fractured cross section of Gr+CB/PVDF, Gr/PVDF, Gr/PP and Gr/PSU composite Figure 5: In-plane electrical conductivity for Gr/PSU, Gr/PP, Gr/PPS, Gr/PVDF, Gr+CB/PVDF and Gr/phenolic resin composite. For each composite the volume content in % is given. Dashed line shows the limit of =100 S/cm defined by DOE. © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Journal of Plastics Technology 10 (2014) 3 77 © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann 3.3 Characterisation of bipolar plates Electrical conductivity The in-plane electrical conductivity of highly filled polymer composites was determined at room temperature through the conventional four- point- probe method. The polymer in the composite is acting as an insulator; with increasing filler concentration an insulator- conductor transition appears. A continuous increase of the filler content increases strongly the electrical conductivity. The transition is described by the percolation theory [17,18]. At percolation threshold concentration the electrical conductivity increases abruptly by several orders of magnitude. The filler aggregates touch each other enabling electrons to “percolate”, thus leading to the creation of many percolation pathways. A good quality of the dispersion of fillers in the polymer matrix contributes to the increase of the number of the percolation pathways. The percolation threshold is strongly related to the nature as well as to the aspect ratio as well as the type of the filler [6]. Figure 6: In-plane electrical conductivity of Gr/ phenolic resin composite measured on a rectangular plate (70×80 mm2). The electrical conductivity was measured on more than 200 positions In an ideal case, a filled polymer composite for the bipolar plates should have a high content of graphite with a high dispersion quality in order to meet the US Department of Energy (DOE) requirements. In plane electrical conductivity of Gr/ phenolic resin composite was obtained on a rectangular plate (70×80 mm2). More than 200 positions were considered to measure the in plane current. The figure 6 is a typical example showing the distribution of values on the plate. The values stretched from 100 S/cm to 200 S/cm. The corresponding standard Journal of Plastics Technology 10 (2014) 3 78 Characterisation of bipolar plates deviation is approximately 25%. The standard deviation of for the rest of samples varies between 15 and 25%. The dispersion of values is probably due to the agglomeration effect of filler particles during the processing. The results of the in-plane electrical conductivity of all samples are represented in Fig. 5. The volume content of fillers is added for each sample in the figure. The results show that the value of the electrical conductivity for Gr/PPS, Gr+CB/PVDF and Gr/phenolic resin composite exceed the limit value of the electrical conductivity ( = 100 S/cm) defined by DOE. However a value of 87 S/cm was determined for Gr /PVDF. The electrical conductivity for Gr+CB/PVDF composites reaches a very high value of 176 S/cm. Indeed there is a synergistic effect on the electrical conductivity between graphite and carbon black. The measured conductivity is caused by the macroscopic resistance of the filled polymer composite which depends not only on the graphite aggregates resistance but also on the resistance of inter- aggregates space. The incorporation of CB and Gr particles to PVDF matrix created additional connection between Gr and CB -aggregates. The Gr/PSU composite shows a value of of about 21 S/cm despite the high volume content (82 vol%) of graphite used during the processing of the material. A similar content of graphite in volume was incorporated to prepare Gr /PVDF composite. However the electrical conductivity of this sample (87 S/cm) is nearly 4 times greater than for the Gr/PSU composite. In addition the electrical conductivity of Gr/PP reaches the value 46 S/cm with 70 vol. % Gr load. These results show that the filler load, alone, is not an indicator for a high electrical conductivity. The electrical conductivity depends also on the processing parameters (like temperature, pressure …), viscosity of the melt and especially on the interfacial interaction between graphite/carbon black and polymer matrix [19]. © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Journal of Plastics Technology 10 (2014) 3 79 16 (a) 14 1,0 Cp [J(g.°C)-1] Thermal diffusivity (mm2s-1) PPS 12 0,8 10 8 20 40 60 80 100 120 Temperature (°C) Thermal conductivity (W m-1 K-1) Characterisation of bipolar plates 30 PPS (b) 25 20 15 10 25 50 75 100 125 Temperature (°C) Figure 7: left: T-dependent thermal diffusivity (square symbol) and specific heat (circle symbol) right. T-dependent thermal conductivity against the temperature (triangle symbol) for Gr/PPS composite (b). Figure 8: Temperature-dependent thermal conductivity for Gr/PSU (full circle), Gr/PP (open circle), Gr+CB/PVDF (full triangle), Gr/PPS (star), Gr/PVDF (open triangle) and Gr/phenolic resin (square). Dashed line shows the limit of =10 W/m K defined by DOE. © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Journal of Plastics Technology 10 (2014) 3 80 © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann 3.4 Characterisation of bipolar plates Thermal conductivity One of the main functions of the bipolar plate is the transfer of the waste heat to a cooling channel in order to maintain a constant operating temperature in the fuel cell systems and to prevent an eventual degradation of the proton exchange membrane. Therefore a high thermal conductivity is necessary to move the heat through the plates rapidly. As mentioned previously the thermal conductivity is calculated from the thermal diffusivity using the Laser Flash technique. Thermal diffusivity D is a measure of the rate at which heat transports through the material is conducted. The rear surface of the sample is illuminated uniformly with a pulsed laser (1064 nm) for few ms. The sample absorbs the laser energy and warm up. The heat diffuses through the thickness of the sample creating a thermal gradient in one direction. The solution of the Fourier equation for heat diffusion is [20]: ( ) [ ∑ ( ) ( ( ) )] (3) Where L is the sample thickness, D the thermal diffusivity and Tend is the total temperature variation of the sample. The Fig. 7(a) shows an example of the evaluation of the thermal conductivity of Gr/PPS composite. The figure displays both curves; thermal diffusivity D and specific heat cp against the temperature. Both cp- and D- curves show distinctive trends. The D values decrease with increasing temperature, while cp values are increasing. By applying Eq. 2, the thermal conductivity was calculated and plotted in Fig. 7(b). The value of at room temperature is about 21 W m-1 K-1. By increasing the temperature, a very slight drop of is observed, which means that the thermal conductivity is slightly affected within the operating temperature range of the fuel cell (40 °C-120°C). The parameter was plotted as function of the temperature for Gr/phenolic, Gr/PPS, Gr/PVDF, Gr/PSU, Gr+CB/PVDF and Gr/PP composites (Fig. 8). It can be observed that all the samples exceed the value of the parameter 10 W/ m K defined by US DOE. The pure graphite material is characterised with a high value of thermal conductivity ( 160 W/m K). However the obtained values of the filled polymer composites in Fig. 8 are at least 5 times lower although the samples contain less than 15 vol% of polymer (excepted for Gr/PP). In addition the results in Fig 8 show that the thermal conductivity is strongly influenced by the choice of the polymer matrix-type; although the thermal conductivity values of the employed polymer matrix are nearly similar (from 0.16 to 0.25 W/mK). The reason is probably due to the interfacial thermal resistance between Gr/CB and polymer which strongly depends on the interfacial interaction strength [19]. Tengfei Luo et al. [19] studied the thermal Journal of Plastics Technology 10 (2014) 3 81 © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Characterisation of bipolar plates energy transport in graphene/graphite polymer (paraffin wax-C30H62) composite systems using molecular dynamics simulation. They showed that the graphenepolymer interfacial thermal conductance is greatly enhanced by increasing the graphene size and polymer density or forming covalent bonds between the graphite edges and polymer molecules. For Gr/phenolic composite this interaction strength seems to be stronger leading to a higher value of (27 W/ m K) at room temperature. If we consider the figure 5 and 8, by omiting the Gr+CB/PVDF composite, we can clearly observe the correlation between thermal and electrical conductivity (Fig. 9). Figure 9: The electrical conductivity depending on the thermal conductivity of highly filled polymer composites having different polymer matrix systems. In addition it was expected that the incorporation of 15 wt.% of CB to Gr/PVDF will increase the thermal conductivity. However the experiment results show the contrary; values are approximately 20% higher for Gr/PVDF composite. This result agrees well with the morphological analysis of Gr+CB/PVDF composite (Fig. 4) showing voids inside the structure of the material. Furthermore a pronounced drop of parameter after 100 °C can be observed for Gr+CB/PVDF composite. This is may be due to the increasing of the heat flux into the sample at high temperature which leads to the decreasing of the intermolecular forces. The conduction of the heat is therefore decreasing. Journal of Plastics Technology 10 (2014) 3 82 © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann 3.5 Characterisation of bipolar plates Mechanical properties Bipolar plates support the membrane electrode assembly and are exposed to the constant effect of the compressive load of the PEMFC or exposed to exterior shocks and vibrations if the fuel cells are used as portable electronics and transportation systems. Therefore robust bipolar plate having good mechanical properties is a required condition. The three-point-bending technique was performed at room temperature in order to obtain the flexural strength parameter R. The results of the bending strength are represented in Fig. 10. It can be seen that the R value exceed the value defined by US DOE (25 MPa) for almost all samples. A value of 26 MPa of the flexural strength was obtained for the Gr+CB /PVDF and 24 MPa for Gr/PVDF. By considering the standard deviation of R ( 4 MPa), the values of the flexural strength are approximately the same which indicates a similar brittleness for both samples. The flexural strength values of the rest of samples is positioned in the range 31 MPa - 37 MPa. Dynamic mechanical analyzer (DMA) tests were carried out on the Gr/phenolic resin, Gr+CB /PVDF, Gr/PVDF, PP/PVDF and PPS/PVDF (Fig. 11). The DMA apparatus measures the energy to deform a sample (storage modulus E). A perfectly elastic sample returns all of the energy. If the energy is partially returned, the rest of the energy is measured as loss modulus related to the dissipated energy by molecular motion. The stiffness of Gr/PPS composite in Fig. 11 keeps a constant value as the temperature goes up, until approximately 80 °C. A sharp drop appears from 80 °C to higher temperature showing the direct result of an increase in molecular mobility, a visible creep effect occurs in the sample and the glass transition takes place. The result agrees well with the value of Tg obtained by DSC measurements (96 °C). The E-curves for Gr+CB /PVDF, Gr/PVDF and PP/PVDF show a steady pace drop of the stiffness as the temperature goes up. These samples show a soft state over the temperature range 20°C to 120 °C because of their low glass transition temperature (Tg < 0°C). An ideal polymer composite bipolar plate should have a constant stiffness within the operating temperature range of the fuel cell (40°C - 120 °C). A physical change as the glass transition in this range is not desirable (case of Gr/PPS), it can affect the performance of the fuel cell. The storage modulus E of Gr/phenolic resin shows nearly a constant value of E from room temperature to 120 °C indicating non change in the stiffness over the operating temperature range 40 -120 °C. Journal of Plastics Technology 10 (2014) 3 83 Characterisation of bipolar plates Figure 10: Flexural strength for Gr/PSU, Gr/PP, Gr/Epoxy resin, Gr/PPS, Gr/PVDF, Gr+CB/PVDF, and Gr/phenolic resin composites. Figure 11: Temperature-dependent storage modulus for Gr/PP, Gr/PPS, Gr+CB/PVDF, and Gr/phenolic resin composites © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Journal of Plastics Technology 10 (2014) 3 84 Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann 4. CONCLUSION Highly-filled graphite polymer composites were investigated to be used as bipolar plates. The effect of the polymer matrix-type on the thermal, electrical and mechanical properties was examined. The volume content of the conductive fillers for Gr/PSU, Gr/PPS, Gr+CB/PVDF and Gr/PVDF varies between 82vol% and 85vol%. However the electrical conductivity (22 S/cm) for Gr/PSU composite is significantly lower in comparison to that (87 S/cm) of Gr/PVDF composite. The same tendency was observed for the thermal conductivity despite that the volume contents were approximately similar for Gr/PSU and Gr/PVDF. The thermal conductivity depends especially on the interfacial bonding strength between graphite and polymer. An additional incorporation of about 15 wt.% CB to Gr +PVDF increased strongly the electrical conductivity (176 S/cm) although some voids were observed in the morphological structural of Gr+CB/PVDF. This is a direct result of the positive synergetic affect between Gr and CB particles leading to the creation of additional connections between Gr and CB -aggregates. A correlation was observed between the electrical and thermal conductivity by considering all samples prepared only with graphite (without Gr+CB/PVDF). Composite Gr/Phenolic resin www.kunststofftech.com Gr/PPS © 2014 Carl Hanser Verlag, München Characterisation of bipolar plates Gr+CB/PVDF Gr/PVDF Gr/PP Gr/PSU Gr/Epoxy Table 2: Thermal conductivity electrical conductivty Flexural strength - The results of the thermal conductivity, electrical conductivity and the flexural strength in relation with the requirements defined by The US Department of Energy (DOE). Furthermore the results of Gr/phenolic resin, Gr/PPS and Gr+CB/PVDF composites regarding in-plane electrical conductivity, thermal conductivity and mechanical properties meet the requirements defined by the US Department of Energy. However Gr/phenolic resin composite showed the best properties (see Tab. 2). Journal of Plastics Technology 10 (2014) 3 85 © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Characterisation of bipolar plates Literature [1] Greenham, Neil C Polymer solar cells Phil. Trans. R. Soc. 371, (2013) 20110414. [2] Ananth Dodabalapur Organic light emitting diodes Solid state communication, 102, (1997) 259-267. [3] Middelman, E et Bipolar plates for PEM al. J. Power Sources, 118 (2003) 44-46. [4] Vishnyakov, V. M. Proton exchange membrane fuel cells. The World Energy Crisis: Some Vacuum-based Solutions 80 (2006) 1050-1065. [5] [6] Hermanna, T. et al. Bipolar plates for PEM fuel cells: A review Du, Ling et al. Highly conductive epoxy/graphite composites for bipolar plates in proton exchange membrane fuel cells International journal of hydrogen Energy, 30 (2005) 1297-1302. J. Power Sources, 172 (2007) 734-741. [7] Cuningham, D. et Development of bipolar plates for fuel cells from al. graphite filled wet-lay material and a compatible thermoplastic laminate skin layer J. Power Sources, 168 (2007) 418-425. [8] Derieth, T et al.. Development of highly filled graphite compounds as bipolar plates material for low and high temperature PEM fuel cells [9] Du, Ling Highly conductive epoxy/graphite polymer composite bipolar plates in proton exchange membrane (PEM) fuel cells Phd (2008). Journal of Plastics Technology 10 (2014) 3 86 © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann [10] Yeetsorn, R. Characterisation of bipolar plates Development of Electrically Conductive Thermoplastic Composites for Bipolar Plate Application in Polymer Electrolyte Membrane Fuel Department of Chemical Engineering, University of Waterloo: Waterloo, Canada (2010). [11] Xiao, M. et al. Poly(arylene disulfide)/graphite nanosheets composites as 0ipolar plates for polymer electrolyte membrane fuel cells J. Power Sources, 160 (2006) 165-174. [12] Zhu Bin. et al. Study on the electrical and mechanical properties of polyvinylidene fluroide/titanium silicon carbide composite bipolar plates J. Power Sources, 161 (2006) 997-1001. [13] Vadbaek. Geometric factor in four point resistivity measurement Haldor semiconductor division (1966). [14] Umesh Gaur et al. The Glass Transition Temperature of Polyethylene Macromolecules, 13 2 (1980), 445–44. [15] Zhehui Liu et al. D.m.a. and d.s.c. investigations of the β transition of poly(vinylidene fluoride) Polymer 38,19 (1997), 4925–4929. [16] Theodore N. Morrison et al. Practical Guidelines for the Efficient Postbaking of Molded Phenolics [17] Scott Kirkpatrick Percolation and conduction "Imagination & Implementation - Thermosets 2004" Topical Conference (RETEC), Co-sponsored by the Thermoset Division and the Chicago Section of the Society of Plastics Engineers. Reviews of modern physic 45, 4 (1973) 574-588. Journal of Plastics Technology 10 (2014) 3 87 [18] Qinghua Zhang Characterisation of bipolar plates Low percolation threshold in single-walled carbon nanotube/high density polyethylene composites prepared by melt processing technique Carbon 44 (1966) 778-785. [19] Tengfei Luo et al. Enhancement of thermal energy transport across graphene/graphite and polymer interfaces: a molecular dynamic study Adv. Funct. Mater. (2012) 2495-2502. [20] G. Penco et al. Thermal properties measurements using laser flash technique at cryogenic temperature Proceedings of the 2001 Particle Accelerator Conference, Chicago. © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Journal of Plastics Technology 10 (2014) 3 88 © 2014 Carl Hanser Verlag, München www.kunststofftech.com Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern. Stübler, Ziegmann Characterisation of bipolar plates Stichworte: hochgefüllte Compound-Materialien auf Graphitbasis, Bipolarplatten Brennstoffzellen, elektrische Leitfähigkeit, wärmeleitfähigkeit in Keywords: Polymer composite, bipolar plates in Fuel cells, electrical and thermal conductivity Autor/author: Dr.rer.nat. Nacera Stübler Dr.-Ing. Dieter Meiners Prof. Dr.-Ing.Gerhard Ziegmann) Institute of Polymer Materials and Plastics Engineering, at the TU-Clausthal Agricolastr. 6 D-38678 Clausthal-Zellerfeld Dr. Thorsen Hickmann Eisenhuth GmbH & Co. KG, Osterode Lerbach, Germany Herausgeber/Editor: Europa/Europe Prof. Dr.-Ing. Dr. h.c. Gottfried W. Ehrenstein, verantwortlich Lehrstuhl für Kunststofftechnik Universität Erlangen-Nürnberg Am Weichselgarten 9 91058 Erlangen Deutschland Phone: +49 (0)9131/85 - 29703 Fax.: +49 (0)9131/85 - 29709 E-Mail-Adresse: [email protected] Verlag/Publisher: Carl-Hanser-Verlag Kolbergerstraße 22 D-81679 München Tel.: +49 (0)89 99830-613 Fax: +49 (0)89 99830-225 Journal of Plastics Technology 10 (2014) 3 E-Mail-Adresse: Webseite: [email protected] Tel.: +49 (0)5323/722426 Fax: +49 (0)5323/722324 Amerika/The Americas Prof. Prof. hon. Dr. Tim A. Osswald, responsible Polymer Engineering Center, Director University of Wisconsin-Madison 1513 University Avenue Madison, WI 53706 USA Phone: +1/608 263 9538 Fax.: +1/608 265 2316 E-Mail-Adresse: [email protected] Redaktion / Editorial Office: Dr.-Ing. Eva Bittmann Christopher Fischer, M.Sc. Beirat / Advisory Board: 38 Experten aus Forschung und Industrie, gelistet unter www.kunststofftech.com 89