Composite structures for proton exchange membrane fuel cells (PEMFC) and energy storage systems (ESS): Review

Composite structures for proton exchange membrane fuel cells (PEMFC) and energy storage systems (ESS): Review

Accepted Manuscript Review Composite structures for proton exchange membrane fuel cells (PEMFC) and energy storage systems (ESS): Review Jun Woo Lim, ...

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Accepted Manuscript Review Composite structures for proton exchange membrane fuel cells (PEMFC) and energy storage systems (ESS): Review Jun Woo Lim, Dongyoung Lee, Minkook Kim, Jaeheon Choe, Soohyun Nam, Dai Gil Lee PII: DOI: Reference:

S0263-8223(15)00817-X http://dx.doi.org/10.1016/j.compstruct.2015.08.121 COST 6826

To appear in:

Composite Structures

Please cite this article as: Lim, J.W., Lee, D., Kim, M., Choe, J., Nam, S., Lee, D.G., Composite structures for proton exchange membrane fuel cells (PEMFC) and energy storage systems (ESS): Review, Composite Structures (2015), doi: http://dx.doi.org/10.1016/j.compstruct.2015.08.121

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Composite structures for proton exchange membrane fuel cells (PEMFC) and energy storage systems (ESS): Review Jun Woo Lim1, Dongyoung Lee2, Minkook Kim2, Jaeheon Choe2, Soohyun Nam2, Dai Gil Lee2* 1

LANL-CBNU Engineering Institute Korea, Chonbuk National University, 567, Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do, Republic of Korea 2 *School of Mechanical, Aerospace & Systems Engineering, Korea Advanced Institute of Science and Technology, ME3221, 291 Daehak-ro, Yuseong-gu, Daejeon-shi, 305-701, Republic of Korea

Since the energy demand increases continuously and there are difficulties of increase of energy supply doe to environmental pollution, there has been strong demand for an energy source such as fuel cells as well as an energy storage system (ESS). Although fuel cells and ESSs have been considered as future energy conveniences, they have not been widely employed because their structures such as bipolar plates (BP), endplate (EP) or flow frames (FF) are made of either brittle graphite, weak polymers or ceramic coated stainless steel to meet the requirements of high electrical conductivity under strong acid environment such as the target values of Department of Energy (DOE) of USA. To circumvent the weak characteristics and difficulty of manufacturing of these structures, the carbon composite BP and the hybrid composite EP composed of carbon and glass composites for the proton exchange membrane fuel cell (PEMFC) and vanadium redox flow battery (VRFB) and the glass composite flow frame (FF) for the VRFB have been developed. The design methods for these composite structures with the appropriate processing techniques are explained in detail. The performances and endurance tests for these structures have been evaluated and compared to the targets of DOE.

Keywords: proton exchange membrane fuel cell, energy storage system, vanadium redox flow battery, carbon composite bipolar plate, hybrid composite endplate, glass composite flow frame.

* Corresponding author. Tel.: +82-42-350-3221; Fax: +82-42-350-5221 E-mail addresses: [email protected] (D.G. Lee)

Table of Contents 1. Introduction 2. Proton exchange membrane fuel cell 2.1 Composite bipolar plates for low temperature PEMFC 2.1.1 Carbon/epoxy composite bipolar plate 2.1.2 Surface modifications of the bipolar plate 2.1.2.1 Graphite coating method 2.1.2.2 Plasma surface treatment 2.1.3 Experiments 2.1.3.1 Measurement of electrical properties 2.1.3.2 Measurement of mechanical properties 2.1.3.3 Measurement of gas permeability 2.1.3.4 Unit cell test 2.2 Composite bipolar plates for high temperature fuel cell 2.2.1 Carbon/PEEK composite bipolar plate 2.2.2 Carbon/phenolic composite bipolar plate 2.2.3 Carbon/silicone composite bipolar plate 2.3 Composite sandwich end plate for PEMFC 2.3.1 Axiomatic design of the end plate 2.3.2 Prototype of composite sandwich end plate 3. Vanadium redox flow battery 3.1 Composite bipolar plates for VRFB 3.1.1 Carbon/graphite hybrid composite bipolar plate 3.1.2 Corrugated carbon/epoxy composite bipolar plate 3.2 Composite flow frame-bipolar plate structure 4. Summary and Conclusions References

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1 Introduction Green technology is a promising economic engine, which can balance the industrial development and the protection of environment. The core of all the green technologies is energy because the energy demand increases continuously as time goes by. However, the increase of energy supply is difficult because existing nuclear power plants and thermal power plants have either high risk or cause environmental pollution. Therefore, new renewable energies have been sought to solve these problems. The typical renewable energy, solar power and wind power, may be a source of environmentally friendly energy without the emission of pollutants, but it has the limitation because of unstable output. In other words, the solar and wind power energies cannot ensure a stable constant output, and are difficult to construct collaboration network with the existing power. Therefore, there has been strong demand for an energy source such as fuel cells as well as an energy storage system (ESS), which can store the excess energy at non-peak time of power and supply power at the required peak time. In short, it is necessary to develop the energy sources and energy storage technologies to solve the energy shortage and pollution problems of the next generation. One of the most promising fuel cells is the proton exchange membrane fuel cell (PEMFC) because it has highest power density with low operating temperature, which makes it suitable for portable applications. And one of the most promising ESS is the vanadium redox flow battery (VRFB) because it has an infinite operating cycles with non-explosiveness. Although they have been considered as future energy conveniences, they have not been widely employed because their structures such as bipolar plates (BP), endplate (EP) or flow frames (FF) are made of either brittle graphite, weak polymers or ceramic coated stainless steel. To circumvent the weak characteristics and difficulty of manufacturing of these structures, the carbon composite BP and the hybrid composite EP composed of carbon and glass 3

composites for the PEMFC and VRFB and the glass composite FF for the VRFB have been developed. In this review paper, the design methods for these structures with the appropriate processing techniques are explained in detail.

2 Proton exchange membrane fuel cell The proton exchange membrane fuel cell or polymer electrolyte membrane fuel cell (PEMFC) is electrochemical energy converters that convert the chemical energy of fuels directly into DC electricity. Hydrogen and oxygen are used as fuels in proton exchange membrane (PEM) fuel cells, where the chemical energy is converted directly into electricity as shown in Fig. 1. PEM fuel cells produce water and electricity without any pollutant via this process. Additionally, PEM fuel cells exhibit a wide power range, low operating temperature, high efficiency, high power density and a long lifetime [1-9]. These attributes make PEM fuel cells a very promising power source for both residential and mobile applications. Even with these many advantages, however, the remaining concern for commercialization is the high manufacturing cost of the bipolar plates, which incurred 38% of the stack cost where the BPs may contribute up to 80% of the stack weight and volume, high stack assembling cost with long process time [10-14]. Among the different types of PEM fuel cells, there has been recent focus on Polybenzimidazole (PBI) High-Temperature PEMFCs (HT-PEMFCs). This attention is due to their higher range of operating temperatures (120°C ~ 180°C), which allows the use of a simpler fuel processor because the fuel cell has a higher CO tolerance and a heat recovery system that does not require membrane humidification [15]. Also it has an advantage of easy water management. A comparison of several characteristics of LT-PEMFCs and HT-PEMFCs is shown in Table 1. 4

The fuel cell stack development is a key technology for the fuel cell commercialization. The key issue of successful fuel cell stack development involved in the uniform distribution of flow rate, reactant gases, water, temperature, and uniform compression of the stack [16]. Moreover, the BPs take large portion of determining the performance of the PEM fuel cells. Therefore, the development of an efficient manufacturing process and high performance BPs are essential for commercialization. A fuel cell stack is composed of membrane electrode assemblies (MEA), gas diffusion layers (GDL), endplates and BPs as shown in Fig. 2 [17-20].

2.1

Composite bipolar plates for low temperature PEMFC

The BPs in a PEM fuel cell stack perform several critical functions: providing flow channels to ensure the reactant gases maintain a proper pressure distribution over the entire active area of the MEA; transmitting electrons from each anode to its adjacent cathode in each unit cell; transferring the heat produced by the reaction from the active area to the coolant; and, finally, serving as a flow path for the coolant [17, 20, 21]. Therefore, BPs must have high mechanical stiffness and strength, high chemical stability, low electrical resistance, low density, thin thickness and low gas permeability [4]. In light of these critical functions, a BP’s electrical characteristics as well as its material characteristics must be considered to ensure the best performance. The BPs of PEM fuel cells can have various thicknesses and geometries, and different materials have been proposed based on their electrical characteristics, chemical compatibility, resistance to corrosion, cost, density, etc. [22]. The BP material, in particular, has a significant effect on both the stack resistance and the voltage drop of the cell, which directly affects the current distribution and cell performance. Several researchers addressed the development of Nafion-based Low-Temperature PEM fuel cell (LTPEMFC) BPs, and a comprehensive review of the characteristics of BPs developed by 5

different companies and research groups is presented [12, 22]. Graphite is often used as the material for the BP due to its high electrical conductivity and high chemical stability. However, graphite BPs have high void contents and brittle mechanical properties. These factors increase gas permeability as well as the production cost and time [23-25]. Metallic BPs have corrosion problems due to the highly corrosive acid environment (pH 2 to 3) of the fuel cell. Also, they exhibit high electrical contact resistance due to the surface passive layers formed during operation, which negatively affects the fuel cell efficiency and offsets the advantage of high electrical conductivity of metallic BPs. As a result, they require extra coating processes to resolve these issues and there have been significant researches focused on reducing coating process cost [20, 23-29]. Carbon fiber/epoxy composite BPs have been developed with continuous or chopped carbon fibers using compression molding method [11, 30]. However, for commercialization, relatively low electrical conductivities of composite BPs are persistent concerns [4]. Chunhui et al. [31] manufactured aluminate cement/graphite conductive composite BPs via compression molding and evaluated their performance for different levels of graphite content. Blunk et al. [32] developed epoxy/expanded graphite composite BPs with 20 vol.% expanded graphite loadings that possessed a low areal specific resistance; they also presented improvements in the mechanical properties that were attained by incorporating other reinforcing fillers. Chung et al. [33] developed a carbon film-coated 304 stainless steel BP. Lim et al. [8] developed carbon fiber/epoxy composite BPs for high-performance LTPEMFCs. Lim et al. [13, 14] developed innovative gasketless carbon composite BPs for LTPEMFCs that were easy to manufacture and demonstrated a high performance.

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2.1.1 Carbon/epoxy composite bipolar plate The carbon/epoxy composite was adopted for high performance BP development. The carbon composite BPs were fabricated with unidirectional, continuous carbon fiber/epoxy prepregs with the thicknesses of 20 µm after curing (USN-020 A, SK Chemical, Republic of Korea); its properties are listed in Table 2. To increase the electrical conductivity, carbon black (Ketjen black 600JD, Mitsubishi Chemical, Japan) was embedded in the BP via a spraying method. The material properties of carbon black are listed in Table 3. Figure 3 shows the fabrication process for the BP embedded with conductive particles. In the first step, the prepregs were cut to a size of 120 mm × 120 mm. The conductive particles were mixed with the methyl ethyl ketone (MEK) solvent at 2 wt. %. The mixture of the MEK solvent and conductive particles were mixed via hand stirring followed by sonication for 5 min to disperse the particle aggregation. In the second step, particles mixed in the solvent were sprayed onto one side of the prepregs with an air spray gun with a nozzle diameter of 0.5 mm. The distance between the nozzle and the prepregs was set at 200 mm at a constant spraying pressure of 0.6 MPa. When the sprayed particle content of mixture was more than 20 wt. %, the epoxy resin could not wet the fibers due to dramatic increase of viscosity. Therefore, the maximum content of the embedded particle was fixed to be 20 w. %. In the third step, the prepregs sprayed with conductive particles were dried at 25°C for 1 hour to eliminate the MEK solvent, followed by a mass measurement to calculate the amount of sprayed conductive particles. In the fourth step, the prepregs sprayed with conductive particles were stacked at a stacking sequence of [03/903]s. In addition, graphite foil (0.1-mm thickness) was coated on each side of the stacked prepregs to measure the effect of the conductive particles on the bulk resistance. In the final, fifth step of fabrication, the stacked specimens were cured by the compression molding method. The specimens were cured at 80°C for 30 minutes to 7

bleed excess resin at a pressure of 20 MPa, followed by a full cure at 125°C for 1 hour [8].

2.1.2 Surface modifications of the bipolar plate In a conventional PEM fuel cell stack, depicted in Fig. 2, two BPs are attached to provide coolants and reactants channels. The electrical contact resistance at contact points dominates and takes large portion of the total electrical resistance of the BP [34, 35]. Several methods to reduce the electrical contact resistance of composite BPs have been developed. The surfaces of the fabricated BPs were modified with graphite coating to obtain soft surfaces and with plasma surface treatment to selectively remove the resin-rich area of the BPs. Carbon composite prepregs were coated with graphite

sheet to create a soft layer and to increase the contacting area between the GDL fibers and the BPs, which would decrease the interfacial contact resistance between the BP and the GDL [9, 20]. The surface of the carbon composite BPs was treated by plasma to expose carbon fibers on the surface of the BPs, thereby increasing the contact area between the BPs and the GDLs [7, 36].

2.1.2.1 Graphite coating method The graphite coating method was applied on the surface of the composite BP to reduce the ASR (areal specific resistance) during the fabrication. Both sides of the composite were coated with a thin graphite layer to create soft surfaces and to increase the contact area with the GDLs, which ultimately decreased the interfacial contact resistance [20, 37]. Figure 4 shows the graphite coating process, where a sticky backup film was used to remove a thin layer of graphite from a graphite foil (BD-100, Samjung CNG, Republic of Korea); its properties are shown in Table 4. The backup film with a thin graphite layer was placed on the stacked prepregs and pressed between two hot rollers at 80°C and a feeding rate of 7 mm/s. When the backup film was peeled off, a thin layer of graphite was retained by the prepregs. 8

The thickness of the thin graphite layer was controlled by repeating the peeling process. From the original graphite foil thickness of 0.1 mm, the thickness up to 10 µm could be controlled. The GDL is consisted of randomly oriented carbon fiber felt with the diameter of 5~10 µm; thus, the contact area between the GDL fibers and the graphite coated BP surface could be maximized with the graphite layer thickness of 10 µm. When the GDL contacts the surface of the graphite coated BP, the GDL fibers burrow into the soft graphite layer such that the contacting area increases as shown in Fig. 5. The GDL (10 BC, SGL Group, Germany) consists of randomly oriented carbon fiber mat with PTFE binder, one side of which is pasted with carbon black and contacts the MEA, while the other side contacts the BP. Figure 6 shows the surfaces and the cross section of the GDL, and its properties are listed in Table 5 [20].

2.1.2.2 Plasma surface treatment The surface of the composite-metal hybrid BPs was modified with plasma surface treatment to remove the resin-rich area selectively without damaging carbon fibers, as shown in Fig. 7. The fully cured composite BPs were placed on the base plate of the plasma etcher and polyimide tape was used to fix the BPs. The radio frequency (RF) plasma with 13.56 MHz frequency with output impedance of 50 Ω was used with an argon gas flow rate of 10 ℓ/min. The surface of the composite BP was treated under the plasma power of 150 W for 5 minutes [36]. The distance between the plasma gun and the specimen was fixed at 3 mm and the treatment was performed at atmospheric pressure. A schematic drawing and photographs of the plasma surface treatment system are shown in Fig. 8. After the plasma surface treatment, the surface morphology of the specimen was observed by SEM images. Figure 9 shows the fiber-exposed surface after the plasma surface treatment that effectively removed 9

the resin rich area. Unlike conventional grinding surface treatment, the plasma surface treatment method exposes fibers on the surface of BP with little damaging them, thereby increasing the contacting area between BPs and GDL [36].

2.1.3 Experiments The total electrical resistance is an important property of PEM fuel cell systems because the energy loss largely depends on the total electrical resistance. In a conventional PEM fuel cell, the compaction pressure is high to enhance the contact between components, which decreases the total electrical resistance. However, a high compaction pressure requires high component mechanical strength and stiffness because the fuel cell efficiency decreases dramatically if the compaction pressure decreases below a certain critical value during a device’s life cycle. Therefore, it will be beneficial if the total electrical resistance is low at a low compaction pressure [38]. The electrical resistances of the developed composite BPs and the conventional BP as a function of compaction pressure were measured. In addition, the gas permeability of the composite BP was measured to investigate the function of the BP as gas separators.

2.1.3.1 Measurement of electrical properties The total resistance of the specimens was measured using four-point probe method with the experimental setup shown in Fig. 10. The size of the graphite foil coated carbon composite plate specimen embedded with conductive particles was 100 mm × 100 mm × 0.4 mm. The specimen was placed between the two gas diffusion layers (GDL 10 BC, SGL carbon, Germany). Two gold-coated copper plates were connected to a power supply (ORS030A, ODA, Republic of Korea) and a multi-meter (3457A, Hewlett Packard, USA). A low 10

electrical current of 1.00 A was supplied to the setup and the voltages were measured with respect to compaction pressure in the range from 0.25 to 2.0 MPa using the universal testing machine (INSTRON 4469, Instron Corp., MA, USA) at the temperature of 25°C ± 1°C under the atmospheric pressure to avoid temperature changes of the specimens because the bulk resistance of the carbon fiber depends on its temperature [39]. The electrical resistances between the components of the flat specimen is defined in Eqs. (1) and (2). The system resistance is defined in Eq. (3) and the areal specific resistance (ASR) of the flat specimens, calculated by subtracting the system resistance from the total electrical resistance, is defined by Eq. (4) [7]. The measured ASR of the flat specimens with respect to the compaction pressure is shown in Fig. 11.

R(total−BP) = 2RAu (Cu )−GDL + 2RGDL−Gr + 2RGDL + RGr + RBP

(1)

R(total−Gr ) = 2RAu(Cu )−GDL + 2RGDL−Gr + 2RGDL + RGr

(2)

R( system) = 2RAu(Cu)−GDL + 2RGDL

(3)

ASR( BP) = 2RGDL−Gr + RGr + RBP

(4)

where: R(total-BP) : Total resistance of the graphite foil coated bipolar plate R(total-Gr): Total resistance of the graphite foil ASR(BP) : Areal specific resistance of the graphite foil-coated bipolar plate RAu(Cu)-GDL: Interfacial contact resistance between the electrode and the GDL RGDL-Gr: Interfacial contact resistance between the GDL and the graphite foil RGDL: Bulk resistance of the GDL

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RGr: Bulk resistance of the graphite foil RBP: Bulk resistance of the bipolar plate

Unlike the isotropic materials such as metals, the bulk resistance of the anisotropic materials such as composites is hard to obtain precisely because the electrical conductivity of a composite is different in each direction. Therefore, the bulk resistance of the composite plate specimens was measured with the graphite layer coating and the graphite foil dummy specimen in order to obtain the precise bulk resistance of the specimens [37, 40, 41]. Assuming that the contact resistance of the graphite foil and the composite BP is negligible due to the co-cured graphite coating, the bulk resistance of a composite (RBP) can be expressed as follows.

(5)

RBP = R(total−BP) − R(total−Gr)

The surface morphology of the graphite foil coated composite plate specimens was identical to that of the graphite foil. Thus, the interfacial contact resistance of the graphite foil coated composite plate specimens was assumed to be identical to that of the graphite foil. Therefore, the bulk resistance of the composite plate specimens was calculated using Eq. (5). From the experimental results, the composite specimen embedded with 10 wt. % carbon black with graphite coating method exhibited the lowest bulk resistance of 4.97 mΩ·cm2 [8].

2.1.3.2 Measurement of mechanical properties In a fuel cell stack, the BPs have a function of supporting structure for GDL-MEA-GDL layer as well as the whole stack with little deformation by the pressure difference between H2 12

gas and air. Therefore, the mechanical properties of the BPs are important. The flexural strength of the carbon composite BPs with flow channels was measured and compared with the target value of Department of Energy (DOE) of USA. The tests were conducted using the universal testing machine (INSTRON 4469, Instron Corp., MA, USA) and the temperature of the specimens was maintained at 25°C ± 1°C under the atmospheric pressure. For each case, nine specimens were tested [8]. In order to evaluate the flexural strength of the composite BP with flow channels was measured by the three-point bending test according to ASTM D790-10 [42]. The carbon composite BP specimen of 32.0 mm × 12.7 mm × 1.0 mm with flow channels, were fabricated using a compressive mold with channels as shown in Fig. 12. The curing cycle was the same for the flat composite specimens for tensile test as mentioned previously. A schematic drawing of the three-point bending test is shown in Fig. 13. The test results exhibited that the developed composite BP satisfied with 10 time higher value than the DOE target as shown in Fig. 14.

2.1.3.3 Measurement of gas permeability Gas permeability is one of the critical factors that determine fuel cell performance. A BP with flow channels as separators must have a low gas permeability to function properly. The gas permeability of the aged specimens in the acid environment was measured with the developed measurement device shown in Fig. 15. The measurement device has two separate cylindrically shaped chambers with two different pressure sensors (ISE40-01-22, SMC, Japan), one to measure the pressure in each chamber. A 0.4-mm-thick, disk-shaped specimen with a diameter of 40 mm, obtained from the aged flat specimen, was placed between the chambers and properly sealed with silicon gaskets, which ensured that the pressure change in 13

chamber 2 was caused only by gas permeation through the specimen, which had an effective diameter of 30 mm. An inlet air pressure of 0.3 MPa, which is the internal pressure of a generic PEM fuel cell, was maintained to the chamber 1, while the chamber 2 was maintained zero pressure. The pressure change of the chamber 2 was measured and the gas permeability was calculated by the pressure differential between the two chambers after a duration of 100 hours. The experimental results revealed that the gas permeability of the composite BPs was absolutely zero after 100 hours of the duration time [8, 37, 43].

2.1.3.4 Unit cell test Unit cell performance was assessed by the PEM fuel cell test station (NARA Cell-Tech Corp., Republic of Korea) [7, 14]. The carbon composite BPs with the sizes of 50 mm × 50 mm × 0.2 mm were tested with respect to the embedded amount of conductive particles. For the test, a 50 µm thickness commercial Nafion based membrane electrode assembly (MEA S25-3L, PaxiTech, France) with a 70% Pt/C catalyst, with a catalyst loading of 0.5 mgPt·cm-2 and a commercial gas diffusion layer (GDL 10 BC, SGL Carbon, Germany) was used. The cell temperature was fixed at 60°C with the supplied hydrogen and air of 100% relative humidity at 80°C. The H2 had the purity of 99.999% and the air inlet pressures were 0.3 MPa with the H2/Air stoichiometric ratio of 1/3. The dummy BPs (graphite) of 13 mm thickness were added between the endplates and composite BPs to fix the thin composite BPs to the unit cell test station. To investigate the conductive particle spraying effect, the unit cell performance tests were conducted with the carbon composite BP embedded with the carbon black of 10 % wt., whose results were compared with that of the conventional carbon composite BP without embedded carbon black. As shown in Fig. 16, the developed carbon fiber composite BP embedded with 14

the carbon black of 10 wt. % improved the performance of unit cell test [8, 38].

2.2

Composite bipolar plates for high temperature PEMFC

Until recently, the central research efforts related to HT-PEMFCs have been focused on the development and study of the high-temperature PBI membrane. There have been a few studies about the experimental analysis of single cell performance or an MEA, but few studies have specifically dealt with HT-PEMFC BP optimization [44-47]. Nevertheless, it has been proven that certain electrical behaviors in LT-PEMFC can be extended to HT-PEMFC [48].

2.2.1 Carbon/PEEK composite bipolar plate The carbon composite BPs were manufactured with 3K plain weave carbon fabric (WSN3K, SK Chemical, Republic of Korea) and PEEK powder (VICTREX® PEEK 150UF10, VICTREX, United Kingdom). Properties of carbon fabric and PEEK powder are shown in Tables 6 and 7, respectively [49]. The glass transition temperature (Tg) of the PEEK powder is 143°C, which is lower than the maximum operation temperature of HT-PEMFC, 220˚C. However, it is well known that Tg significantly increases by creating a composite material with mixing of reinforcing powders and fibers [50, 51]. Xue et al. [52] investigated that the embedded conductive particles in a polymer matrix with increased Tg. The main reason of increase of glass transition is freezing the motion of the chain segments [53] and the embedded conductive particles create the vast interfacial area which affects the behavior of the surrounding polymer matrix. As a result, the Tg of the polymer matrix is increased by the embedded conductive particles [50, 51]. In a microscopic point of view, the interaction between the embedded conductive particles and the polymer matrix and can reduce the chain 15

movement and limit the polymer chains [52]. Even though Tg may increase by the carbon fibers and the embedded conductive particles, the mechanical properties of the developed BP should be verified for a fuel cell application. Conductive particles were mixed with the PEEK powder via hand stirring and a mixer to increase the electrical conductivity. The carbon black (Ketjen black 600JD, Mitsubishi Chemical, Japan) whose properties are listed in Table 8 was selected considering the particle size and its price among the various conductive particle candidates. The specifications of the carbon black are listed in Table 9. Figure 17 shows the fabrication process for the carbon/PEEK composite BP embedded with conductive particles. In the first step, the 3K carbon fiber fabric was cleansed with acetone and dried in an oven for 30 minutes at 100°C. The PEEK powder and carbon black powder were mixed via hand stirring, for 5 minutes to disperse the particle aggregation. The PEEK powder and the carbon black were mixed according to the particle wt. %. In the second step, to reduce the interfacial contact resistance and obtain the gas tightness, a 100 µm expanded graphite foil (BD-100, Samjung CNG, Republic of Korea) was placed in a compression mold with the carbon fabric. The PEEK powder and carbon black mixture was spread onto one side of the placed carbon fabric using a sieve to ensure uniform application. Then, another expanded graphite foil was placed on top of the carbon fabric. The amount of the PEEK powder and carbon black mixture to be applied to carbon fabric was calculated from the assumption that the volume fraction of the fabricated composite should be 78%. The sieve number of 10 was employed, considering the space between fibers and the size of the sieve mesh. The carbon black wt. % was controlled from 0 wt. % to 5 wt. % in 1 wt. % increments. In the final step, pressure and heat were applied by a hot press. The carbon/PEEK composite specimen was cured at 400°C for 30 minutes and was then cooled to room temperature (25°C) for 2 hours while maintaining a constant pressure of 16

20 MPa [49]. Fig. 18 (a) shows the ASRs of the specimens with respect to the compaction pressure at the room temperature (25°C). The carbon/PEEK composite specimen embedded with the carbon black of 2 wt. % exhibited the lowest ASR, with a value of 25.3 mΩ·cm2. Fig. 18 (b) shows the ASRs of the specimens at high temperature (220°C). The ASRs of the specimens in all range of the compaction pressure increased at the higher temperature due to the increased thermal movement of the electrons and atoms. However, the carbon/PEEK composite specimen embedded with 2 wt. % carbon black still exhibited the lowest ASR, with a value of 30.8 mΩ·cm2 [49].

2.2.2 Carbon/phenolic composite bipolar plate The composite BPs were fabricated using resole-type phenolic resin (KC-4703, Kangnam Chemical, Korea) as a matrix. The properties of the resole-type phenolic resin are shown in Table 10. The reinforcement was a 1K plain-weave carbon fiber (CF 1114, Hankuk Carbon, Republic of Korea) with the thickness of 140 µm, a thread count 17.5 × 17.5 thread count per inch. To improve the mechanical and electrical properties of the composite BP, the carbon black in Table 9 was mixed with the resole-type phenolic resin. For the uniform dispersion of the carbon black, the pre-mixed carbon black with phenolic resin was mixed with a 3-roll milling method. The wt. % of the carbon black was varied from 0 wt. % to 4 wt. %. Above 4 wt. % of carbon black, it was hard to get uniform dispersion of the carbon black mechanical means. Moreover, too much mixed carbon black will be aggregated and not be wetted well with the phenolic resin, which will be harmful to electrical conductivity of the BP [37, 40, 41, 54-56]. The carbon/phenol composite BPs were fabricated by the hand lay-up method and cured by 17

compression molding. The specimens were cured under compaction pressure 10 MPa at 220˚C for 10 minutes. The curing temperature (220˚C) was determined based on the maximum operating temperature of the HT-PEMFC. A differential scanning calorimetry (DSC) analysis of the resole-type phenolic resin was performed with a thermal analyzer (Setsys 16/18, Setaram, France) to investigate an adequate curing process. The phenolic resin of 15 mg was used for the DSC measurements under isothermal scanning at 220ºC, and the heat generation rates were measured. Figure 19 shows the heat generation rate and the isothermal degree of cure (ζ) of the resole-type phenolic resin. The resole-type phenolic resin was fully cured in less than 7 minutes at 220ºC [56]. The best way to minimize the interfacial contact resistance might be to expose the bare carbon fiber on the composite bipolar plate. The exposed bare carbon fibers on the outer surface of the composite bipolar plate directly contact the carbon fibers of the GDL so that the interfacial electrical contact resistance is reduced by as much as the bulk resistance. To achieve this structure, randomly oriented carbon felt (H&V, USA), was attached on the outer surface of the carbon/phenol composite BP and co-cure bonded. The ASRs of the carbon felt attached composite BPs were measured with respect to thickness of carbon felt to obtain an optimum thickness of the carbon fiber felt. The properties of randomly oriented carbon fiber felts are shown in Table 11. To manufacture the partial wetting of the carbon fiber felt structure, a pre-cure process was developed. The developed pre-cure process was a hot rolling process using a laminator (lamiart-320LSI, GMP, Korea). In the first step, the specimens with the phenolic resin with 4 wt. % carbon black on the reinforcement (1k-carbon fabric) was performed; subsequently, the specimens were hot rolled under 1 MPa at 160°C with a rolling speed of 5 mm/s. By the pre-cure process, the excess resin of the specimen was squeezed out and the resin on the surface partially cured. The resole-type phenolic resin start to cure at 18

120°C by forming methylene cross-links [57]. The pre-cure process produced a partial wetting of the carbon fiber felt by increases the viscosity of the phenolic resin on the surface and. In the second step, the carbon fiber felt was attached on the each side of the pre-cured specimen. Then, the carbon felt attached carbon/phenol composite BPs were post-cured by compression molding under a curing pressure of 10 MPa at 220˚C for 10 minutes. Fig. 20 shows the fabrication process of the carbon felt attached carbon/phenol composite BP. Figure 21 shows the surfaces images of the carbon felts attached carbon/phenol composite BP using the pre-cure process with respect to the thickness of the carbon felts. A shown in Fig. 21 (a), the carbon felt on the composite BP was fully wetted with the phenolic resin without the pre-cure process where the excess resin covers the surface of the BP. However, as shown in Figs. 21 (b) ~ (d), the carbon felts were partially wetted by the phenolic resin with the pre-cure process and the bare carbon fiber felts cover the surface of composite BPs. With the carbon felt of 50 µm, the bare carbon fibers on the surface was not much; however, with the carbon felts of 80 µm and 140 µm, the bare carbon fibers fully cover the surface of the composite BP. As a result, the amount exposed carbon fiber felts on the outer surface of the BPs increased with respect to the thickness of carbon felt. As shown in Fig. 22, under a compaction pressure of 1.0 MPa, the ASR of the 80 µm carbon felt attached carbon/phenol composite BPs was 28 mΩ•cm2. [56].

2.2.3 Carbon/silicone composite bipolar plate A composite BP whose matrix was a silicone elastomer was developed. The composite BP specimens were fabricated by impregnating 1k plain weave carbon fabrics (C-112, SK Chemicals, Korea) with liquid-type silicone elastomer (Sylgard 184, Dow Corning, USA), 19

whose properties are shown in Table 12 and 13, respectively. Among the various types of the silicone elastomer, Sylgard 184 was selected due to its low modulus and low viscosity. The viscosities of the silicones range from a few Pa·s to thousands of Pa·s. For making a composite, however, a low viscosity was advantageous for better wetting of the carbon fibers. Considering these factors, Sylgard 184 was chosen from the various candidates [58]. The fabrication process of the carbon/silicone composite BP is shown in Fig. 24. The carbon fabric was cleansed with acetone to remove contaminants. After evaporation of the acetone, a propane gas flame treatment was performed for 15 seconds on each side of the carbon fabric to remove the remaining contaminants and to enhance the bonding with the silicone resin. Silicone resin was prepared by mixing the resin with hardener in a 10:1 weight ratio. After mixing the resin and hardener with a 1000-rpm stirrer for 1 minute, vacuum was applied to degas the mixture. The prepared silicone was pasted onto the carbon fabric, and vacuum was applied to fully impregnate the carbon fibers. Four plies of silicone resin impregnated carbon fabrics were laminated using the hand layup method. The laminate was degassed again in a vacuum chamber to remove air bubbles that were entrapped during the hand layup process. The laminate was cured in a compression mold with a hot press. The curing pressure, temperature, and time were 20 MPa, 200°C, and 1 hour, respectively. The thickness and fiber volume fraction of the fabricated specimens was 0.45 mm and 64%, respectively. Additionally, the silicone gasket was co-cure bonded on the carbon/silicone elastomer composite BP to form the gasket-integrated composite BP in order to eliminate the gasket stacking process which can decrease the assembly and manufacturing costs. The fabrication method is shown in Fig. 25. The gasket mold, which was composed of aluminum with a 5 mm wide and 1 mm deep channel, as shown in Fig. 25 (b), was inserted in the compression 20

mold as shown in Fig. 25 (a). The channel of the mold was filled with degassed and uncured liquid silicone, and the uncured BP laminate was placed on the gasket mold. The specimen was cured using the same curing conditions that were used to fabricate the BP specimen. Figure 26 shows the fabricated gasket-integrated BP specimen where the silicone gasket was successfully formed on the composite [58]. The mechanical properties of the BP were investigated using a three-point bending test. The flexural strengths were 52.2 MPa and 45.9 MPa at 20°C and 200°C, respectively, as shown in Fig. 27. Although the flexural strength decreased 11.9% at the elevated temperature, it was still 1.83 times higher than the DOE target of 25 MPa. The electrical properties of the BP were investigated using the four-point probe method, from which the ASRs and bulk resistances were obtained as shown in Fig. 28. The ASRs of the BP specimen without any surface treatment at 1.38 MPa were 33.4 mΩ·cm2 and 27.8 mΩ·cm2 at 20°C and 200°C, respectively. The gas permeability of the carbon/silicone elastomer composite BP was maintained zero during the test duration of 100 hours. Finally, the sealability of the gasket that was integrated with the BP was investigated. The minimum compaction pressure required for 100 hours of complete sealing was 0.8 MPa.

2.3

Composite sandwich end plate for PEMFC

The components of a fuel cell stack such as MEAs, GDLs, and BPs should be held together with sufficient contact pressure to prevent the leakage of the fuels and to minimize the contact resistance between adjacent cells in the stack [17, 19, 21, 59]. Therefore, the endplate should possess high flexural stiffness to provide the high clamping force, which has been accomplished by employing two thick steel endplates to maintain proper pressure distribution through the cells. Conventional steel endplates are not only very thick but also have large 21

heat-capacity. During the cold start of a fuel cell, the stack should be heated as fast as possible because it can offer available current density only when it reaches the proper operating condition. Since the operation principle of a fuel cell is based on electrochemical reactions such as oxidation of hydrogen and reduction of oxygen, the reaction speed or current density is closely related to the temperature: the higher the operating temperature, the higher the current density can be obtained. However, the temperature cannot be increased indefinitely because of durability restrictions of components such as MEA or GDL. Furthermore, ice formation under subzero environmental temperature may reduce ionic conductivity in membranes, or mechanically hinder reactants from transferring into the three phase boundary on electrodes, where the electrochemical reactions occur [60]. Therefore, the endplate should have high thermal insulating properties to prevent the wasting of the heat generated from the electrochemical energy conversion process in a stack. Rather the heat generated should be used to melt down the ice in a cold weather and to increase the temperature near the three phase boundary on MEA during the cold start-up, which requires low thermal inertia of the end plate. Therefore, metals such as aluminum alloy and magnesium alloy are not good candidate materials for the endplates [60]. Carbon fiber polymeric composites have high potential for the endplate application due to their very high specific stiffness (stiffness/weight). However, the carbon fiber reinforced composites have high thermal and electrical conductivities. In order to employ the carbon fiber reinforced composite as the material for the endplate, both the electrical and thermal insulation should be provided to prevent electrical short-circuit and to reduce heat flow. Therefore, the sandwich endplate structure composed of the carbon fiber composite face and the low density foam core reinforced with Nomex honeycomb was designed to obtain high specific stiffness, high thermal insulation, and high electrical insulation [61-66]. 22

2.3.1 Axiomatic design of the endplate The axiomatic design theory is a design methodology developed to establish a universal rational basis for design including both hardware and software [67]. The requirements of the endplates are that they should provide uniform pressure to the stack, have low thermal conductivity and low specific heat so that stacks of the fuel cell can be heated up rapidly even at low environmental temperature, and should last more than 10 years without damage under high acid environment. The functional requirements (FRs), minimum set of independent requirements that completely characterizes the functional needs of the product are as follows [60].

FR1 = Provide uniform pressure to the stack with small mass. FR11 =Provide high specific stiffness to the endplate. FR12 = Develop a clamping method for the stack. FR2 = Increase the dielectric strength of the face of the endplate. FR3 = Increase the corrosion resistance of the face of the endplate. FR4 = Improve the cold start characteristics. FR41 = Decrease the specific heat of the endplate. FR42 = Decrease the thermal conductivity of the endplate.

In order to satisfy the FRs, in this work, the following design parameters (DPs) were developed:

DP1 = High specific stiffness with a clamping device 23

DP11 = Sandwich structure DP12 = Clamping device (Composite band) DP2 = Face of glass fiber reinforced polymeric composite DP3 = Polymeric materials of high corrosion resistance DP4 = Thermal characteristics of the endplate DP41 = Small mass with low specific heat (Sandwich structure) DP42 = Thermal insulating panel (Foam)

FR41 and FR42 are automatically satisfied by adopting a sandwich structure whose core material features low specific heat and low thermal conductivity. To ensure that the design satisfies the design axioms, i.e. independence and information axioms [67], a design matrix was established as follows.

FR11 FR12 FR2 FR3 FR41 FR42

DP11 X 0 0 0 0 0

DP12 x X 0 0 0 0

DP2 0 0 X 0 0 0

DP3 0 0 0 X 0 0

DP41 0 0 0 0 X 0

DP42 0 0 0 0 0 X

The capital X represents a strong relationship (dependence), while the lower case x represents a weak relationship. From the design matrix, it can be concluded that an acceptable decoupled-design (triangular matrix) can be established by the use of the sandwich structure composed of the faces with high specific stiffness (carbon fiber composite) and the foam core of low thermal capacity and conductivity [60]. Therefore, a composite sandwich construction composed of high specific modulus face and low thermal conductivity foam core was designed for the endplate of PEMFC. Table 14 shows the comparison between the 24

characteristics of the conventional stainless steel endplate and the sandwich endplate. For a preliminary design, the stiffness of the endplate was calculated using the bending equation based on the strength of the material without considering the clamping device and electric insulating panel [60].

2.3.2 Prototype of composite sandwich end plates According to the design process, a prototype of the composite sandwich endplates for PEMFC were manufactured, where the flexural stiffness and strength of the endplate were measured by the three-point bending test according to the ASTM D 790 [42]. Figure 29 shows the normalized load-displacement curves of the endplates obtained from the three point bending test. The flexural stiffness of the composite sandwich endplate increased 13.6% compared to the conventional metallic endplate and the weight reduction was 54%. Therefore, the specific flexural rigidity of the composite sandwich endplate was 2.5 times larger than that of the conventional endplate as shown in Fig. 30. In addition, the cold-start characteristics of the conventional endplate and the composite sandwich endplate were compared. The initial temperature of the endplates was -5°C and they were installed on the 80°C hot plate. Figure 31 (a) shows the experimental set up for the thermal conduction test of endplates. The temperature of the unheated surface of the endplate was measured for 3 minutes. In case of the composite sandwich endplate, the temperature of outside surface of the endplate was almost uniform compared to the stainless steel endplate with respect to testing time because of its low thermal conductivity as shown in Fig. 31 (b). Although this thermal conduction test could not provide the quantitative thermal resistance or thermal mass of the endplates, the results could demonstrate that the cold-start characteristics of the composite sandwich endplate were much better than the stainless steel endplate. 25

Therefore, the endplate designed in a sandwich structure, which is composed of the carbon composite face and foam filled honeycomb core, is a promising endplate for PEMFCs.

3 Vanadium redox flow battery Recently, many countries are suffering from the electrical energy crisis, and the energy storage systems have become more important for efficient power management. Among various energy storage systems, the vanadium redox flow battery (VRFB) can be the best solution as it has many advantages [21, 68-71]. Specifically, the VRFB has a long lifetime, scalability for the energy capacity, and stability compared to other secondary batteries, i.e. lithium batteries and lead-acid batteries. A typical structure of a VRFB system is as shown in Fig. 32. It consists of a stack, two electrolyte tanks, and two pumps. The power is stored by electro-chemical reaction in the anolyte and catholyte which contains vanadium ions (VO2+/VO2 + in the catholyte, V2+/V3+ in the anolyte). This reactions occurs during the charge and discharge of battery while the electrolytes circulate through the carbon felt electrodes. At the same time, hydrogen ions pass through the nafion membrane, and the electrons are collected in BPs. However, the technology maturity of the commercialization of the VRFB cannot follow the demands of the market demands. The stack cost is still too high because the components such as the bipolar plate is very expensive [72]. Therefore, to increase the productivity and performance of the VRFB system, a carbon/epoxy composite BP was developed, and a composite flow frame-BP structure was developed to decrease the stack cost and increase the reliability of the VRFB stack.

3.1

Composite bipolar plates for VRFB 26

A stack consists of BPs and carbon felt electrodes as shown in Fig. 32, which significantly affect the performances of the VRFB stack. The BP separates each cells, connects electrical paths between cells in series, and supports other components. Conventionally, graphite was used for the bipolar plate. However, the fabrication process of the large area thin graphite BP costs a lot because machining is necessary and the handling is very difficult [73, 74]. Therefore, to decrease the stack cost, the composite bipolar plate may be a promising candidate which enables low manufacturing cost and high productivity.

3.1.1 Carbon/graphite hybrid composite bipolar plate The high electrical resistance and low manufacturing productivity of the conventional composite BPs should be improved to realize the commercialization of redox flow batteries. The reason of high electrical resistance of composite BPs is high interfacial contact resistance because of the resin rich area on the surface of carbon composite [37]. Therefore, a soft expanded graphite coating was adopted on the surface of carbon composite to reduce the interfacial contact resistance [20]. In addition, the uniform thickness of the BP is important for the electrical resistance of the BP. If the thickness is not uniform, resin rich areas may be generated which dramatically increase the electrical resistance as shown in Fig 33. Since the current density of the VRFB is very low, a large sized mold which has a dimension of 480 mm x 500 mm was used. A commercial aluminum plate mold which has the thickness of 10 mm was used, and a silicone rubber sheet which has the thickness of 5 mm was placed between the molds to obtain the uniform thickness of the BP. Fig. 34 (a) shows the schematic diagram of fabrication method for the composite BP, and the curing cycle is as shown in Fig. 34 (b) [72, 75]. Figure 35 presents the thickness distribution of the fabricated BP where the average 27

thickness of 0.41 mm and standard deviation of 0.015 mm were achieved. The developed BP showed much higher mechanical characteristics although it was much thinner than the conventional graphite BP. Because a stack has multiple cells about 40, the total thickness of the stack can be reduced substantially by adopting the composite BP. Therefore, It is concluded that a composite BP for VRFBs can be fabricated, and the uniform distribution of the thickness has been achieved by employing a silicone rubber sheet [20, 72]. The total resistance of the developed BP specimen of which thickness of graphite coating was different was measured with the specimens size of 100 mm × 100 mm under the compaction pressure of 50 kPa which is a general compaction pressure of the VRFB stack. The ASRs of the specimens were calculated by subtracting system resistance from the total resistance. Figure 36 shows the ASR in the through-thickness direction with respect to the thickness of the graphite coating layer [72, 75]. For the graphite coating thickness range from 30 µm to 120 µm, the ASR seemed to increase due to the increase of bulk resistance of the expanded graphite. However, the specimen which has the graphite coating of 15 µm showed higher ASR than that of 30 µm. Since the bulk resistances of the composites are almost identical [39], the difference might come from the interfacial contact resistance between the graphite coating and carbon composite. The interfacial contact resistance between the graphite coating and the composite (RBP/CF) may be expressed as a function of the thickness of the graphite coating and the contact area between the coating layer and the carbon fibers:

RBP / CF = f (tcoating, b) ∝ (tcoating ) ⋅ bm n

(6)

where b is the width of the contact area between a carbon fiber of the carbon felt electrode 28

and the graphite coating of the BP as shown in Fig. 37. The contact area can be estimated by integrating the local contact thickness h(x) in the simplified contact model between the rigid carbon fiber and the soft graphite layer on the rigid substrate as shown in Fig. 37 (a) [76]:

F 2 Eg = l t



b

0

h( x)dx

(7)

2 E g r ⋅ l   b   −1 b t≈ ⋅ ⋅ r ⋅ sin − b 1−    4 F  r  r   

(8)

where, t and Eg are the thickness and compressive modulus of the graphite coating layer in the through-thickness direction, respectively. And F is the compaction force applied on a carbon fiber of the electrode, l is the total fiber length in the contact area between the carbon fibers of electrode and the graphite coating layer of the BP.

Table 15 presents the properties

of the carbon felt electrode, the carbon composite and the expanded graphite foil which was used for the coating layer. Under a constant compaction pressure of the VRFB, the nondimensional thickness of the graphite coating layer, tcoating = t , is almost proportional to 1µm

the width of contact area, b, as shown in Fig. 37 (b), from which the relationship between tcoating and b can be derived as follows.

 c1tcoating 0.32 b= 0.15 c2tcoating

(0 < tcoating < 40) (tcoating > 40)

(9)

From the relationship between t and b, the exponents, n and m in Eq. (6) were estimated by 29

comparing the ASR of the BP to the measured value.

RBP / CF

d1tcoating −0.08 = 0.15  d 2tcoating

(0 < tcoating < 40) (tcoating > 40)

(10)

where, d 1=115.5 mΩ·cm2 and d 2=149.5 mΩ·cm2 [72, 75]. According to the ASR results, the thickness of 30 µm showed the lowest ASR. However, if the electro-chemical corrosion occurs, the ASR of the specimen which has the thickness thinner than 120 µm significantly increased. The increase was due to the crack propagations on the graphite coating layer while adjusting the thickness of the graphite coating layer by peel-off method. Therefore, the minimum thickness of the graphite coating layer is 120 µm, which can be manufactured without peel-off method [72, 75]. The cyclic charge/discharge tests were performed to evaluate the performance of the developed BP using a unit cell of VRFB. The cell was charged and discharged under the current density of 800 A/cm2 [77], and other conditions are listed in Table 16. The average efficiency of five cycles were measured. Figure 38 shows a representative charge/discharge cycle with respect to the BPs, and Fig. 39 shows the average current efficiency, where the voltage efficiency and energy efficiency were averaged from five cycles. Due to its low ASR, the energy efficiency of the unit cell with the composite BP was 86 % while the conventional graphite BP showed 80% of efficiency. From the experimental results, it has been concluded that the developed composite BP for VRFBs is more efficient than the conventional graphite BP.

3.1.2 Corrugated carbon/epoxy composite bipolar plate 30

The major losses in the carbon felt electrode in the VRFB stack are ohmic loss and pumping loss [73, 78, 79]. The ohmic loss can be decreased by increasing the fiber content of carbon felt electrode, however, the pumping loss increases as the fiber content increases simultaneously [80, 81]. Therefore, the carbon fiber felt for the electrode in the VRFB stack is a coupled design. The corrugated carbon/epoxy composite bipolar plate (CCBP) was developed to decouple the effects of the fiber content on the ohmic and pumping losses [82]. The CCBP was designed to provide electrode areas with both high and low fiber volume fractions when assembled with electrodes as shown in Fig. 40. A high fiber volume fraction area can reduce the electrical resistance by providing electron paths, while a low fiber volume fraction area can decrease the pumping loss significantly by providing the path of the electrolytes [82-87]. Because the electrical resistance and the permeability of carbon felt change exponentially depending on the fiber volume fraction, the ASR and permeability were measured experimentally with respect to the fiber volume fraction as shown in Fig. 41. Then, the measured data was used to calculate the ohmic and pumping losses numerically as shown in Fig. 42 with respect to the shape of the corrugated bipolar plate. The sum of the ohmic and pumping loss is shown in Fig. 43. The CCBP specimens were fabricated with plain weave fabric carbon/epoxy prepregs with expanded graphite coating layer. The photographs of the fabricated CCBP specimen is shown in Fig. 44. The performance of the developed CCBP was verified by comparing the ASR and the permeability of the designed CCBP and conventional flat BP as shown in Fig. 45 and Fig. 46. At the selected fiber volume fraction of 5.6%, the ASR decreased by 6% and the pumping loss decreased by 21% when the CCBP was mounted.

3.2

Composite flow frame-bipolar plate structure 31

The carbon composite and glass composite were used for the bipolar plate (BP) and flow frame (FF) respectively for VRFB. The bipolar plate and flow frame were co-cured to develop a FF-BP hybrid structures for improving the sealing performance of the stack and increasing the productivity. The thermal deformation problem due to the CTE difference between carbon composite BP and the glass composite FF is the technical issues to fabricate the FF-BP by co-cure process as shown in Fig. 47. A smart cure cycle was developed to reduce the thermal residual stress of a co-cured E-glass/carbon/epoxy composite structure for use in the co-cured FF-BP structure for VRFB. The developed smart cure cycle consists of the cure-triggering, cooling and post-cure processes as shown in Fig. 48. To investigate the smart cure cycle, a series of strip experiments were performed with respect to the cure-triggering process temperature and the degree of cure at the cooling point by measuring the curvature of the strip specimens. The measured radii of curvature of the curved strip specimens were used to calculate the thermal residual stress and the actual bonding temperature of the hybrid structure. The best smart cure conditions were found that the cure triggering temperature of 95 or 105°C with the degree of cure below 0.4 at a cooling point. With the selected smart cure cycle conditions, the thermal residual stress decreased by 52% and the actual bonding temperature was reduced by 40% compared to those of specimens cured under the conventional cure cycle. Then, the post-cure process time was investigated by the partially cured resin dissolution experiment. From the experiment, the post-cure process time was found to be 6 hours for Tmax= 105°C and 10 hours for Tmax= 95°C. Considering the manufacturing time, it might be suggested that the optimum cure cycle is composed of a cure-triggering time of Tmax = 105°C, a cooling process at the degree of cure of 0.4 and 6 hours post-cure process. Finally, the smart cure cycle was applied to fabricate the actual-sized co-cured FF-BP structure and then the 32

deflection of the BP was investigated by comparing the FF-BP structure fabricated with conventional cure cycle. The deflection was measured by the 3D scanning equipment, and the maximum deflection was found to be reduced using developed smart cure cycle from 4.7 to 3.2 mm, representing a 32% decrease.

4

Summary and Conclusions To circumvent the low mechanical properties and difficulty of mass productionh of

components and structures for the proton exchange membrane fuel cell (PEMFC) and the vanadium redox flow battery (VRFB), an energy storage system (ESS), the carbon composite BP and the hybrid composite EP composed of carbon and glass composites for the PEMFC and VRFB and the glass composite FF for the VRFB have been developed. The design methods for these structures with the appropriate processing techniques, and their performances are explained in detail.

(1) Proton exchange membrane fuel cell The BPs of PEMFC can have various thicknesses and geometries, and different materials have been proposed based on their electro-mechanical characteristics, chemical compatibility, resistance to corrosion, cost. The carbon/epoxy composite was adopted to develop high performance BP for LT-PEMFC. The carbon composite BPs were fabricated with unidirectional continuous carbon fiber/epoxy prepregs. Conductive particles were embedded to decrease the bulk resistance of the composite BPs. Additionally, several methods to reduce the electrical contact resistance of composite BPs have been developed. First, the graphite coating method was applied on the surface of the composite BP during the fabrication. The graphite coating forms a soft layer on 33

the surface of the composite BPs and increase the contact area with GDL. Second, the surfaces of the composite-metal hybrid BPs were modified with plasma surface treatment to remove the resin-rich area selectively without damaging carbon fibers. The exposed carbon fibers are directly contacting with the fibers of GDL to decrease the contact resistance. From the experiment results, the electrical resistance and the mechanical properties of the developed BPs were much improved and satisfied the DOE target. Moreover, unit cell performance assessment verified that the developed carbon fiber composite BP for LTPEMFC increased much the performance of PEMFC systems.

In extension of the LT-PEMFC component development research, high performance composite BPs for HT-PEMFC was investigated with various types of composites and technologies.

(a) A carbon/PEEK composite BPs was developed for HT-PEMFC. PEEK (Poly Etherether Ketone) powder and carbon black were utilized as the materials in carbon composite BPs of high electrical conductivity for high-performance HT-PEMFC with high manufacturing productivity. (b) A BP for HT-PEMFC was developed using continuous carbon fabric and resole-type phenolic resin. To improve the mechanical and electrical properties, the nano-size carbon black was mixed with a phenolic resin. The composite BPs were fabricated using pre-cure process and surface modification process with randomly oriented carbon fiber felt on the surface of the BP to reduce the areal specific resistance (ASR). (c) A gasket-integrated carbon/silicone elastomer composite BP that does not require additional gaskets was developed. By making the matrix of the composite identical to 34

that of the silicone gasket, the gasket could be fabricated on the BP through co-cure bonding.

The electrical resistance and the mechanical properties were measured at both low and high temperatures using conventional test methods. Additionally, to verify the durability of the developed BPs, environmental durability tests were performed. From the experiments, it was found that the developed BPs for HT-PEMFC showed much reduced electrical resistance and high mechanical properties, which satisfied the DOE target. Therefore, it can be concluded that the developed composite BPs may be used as an alternative high-performance BP for HT-PEMFC systems.

A sandwich endplate composed of carbon fiber epoxy faces and foam filled Nomex honeycomb core for a PEMFC was designed by the axiomatic design theory and its thermomechanical properties were measured and compared with the results obtained by finite element analysis. From the structural and thermal finite analyses, it was found that the sandwich type endplate could provide a uniform pressure to the stack and improve the coldstart characteristics of the fuel cell by decreasing the thermal conductivity. It was also found that the flexural strength and stiffness of the carbon fiber epoxy sandwich structure with polyurethane foam (PUF) filled Nomex honeycomb core increased three times than those of other sandwich structures because the honeycomb and PUF contributed to increase the compressive strength and the bonding strength with the face materials, respectively. Therefore, the endplate for a PEMFC designed in a sandwich structure which is composed of the carbon composite face and foam filled honeycomb core is a promising endplate for passenger cars. 35

(2) Vanadium redox flow battery The composite BPs were developed using carbon/epoxy composite to increase the performance and the efficiency of the VRFB system. Furthermore, composite flow frame-BP structures were developed to reduce the manufacturing cost and increase the reliability of the VRFB system.

(a) A carbon fabric/graphite hybrid composite BP composed of carbon composite with the expanded graphite coating layer for the VRFB was developed to decrease the electrical contact resistance. The manufacturing method to produce the large area carbon composite BP with uniform thickness distribution was developed. Considering both the ASR and the elcetro-chemical properties of the BP, the minimum thickness of the graphite coating layer was found and it is 120 µm. From the charge/discharge tests, the energy efficiency of VRFB with the carbon/graphite hybrid composite BP was 86 %, which was 6% higher than that of the conventional graphite BP. (b) A corrugated carbon/epoxy composite bipolar plate (CCBP) for the vanadium redox flow batter (VRFB) system was developed. The ASR and electrolyte permeability of the carbon felt electrode were measured and used to determine the specific design of the corrugation shape of the CCBP. Also the ohmic and pumping losses were calculated, and the sums of these losses were compared with respect to the corrugation shape. With the developed CCBP, the ohmic loss decreased by 5% and the pumping loss decreased by 21% compared to the conventional flat BP. (c) The co-cured carbon composite bipolar plate (BP) and glass composite flow frame (FF) structure was developed by investigation on the smart cure cycle for the hybrid 36

structures. The conditions of the smart cure cycle was investigated and the total cure cycle was optimized from the experimental results. As a result, the thermal residual stress and induced thermal deformation were reduced by adopting the developed smart cure cycle. The actual-sized co-cured FF-BP structure fabricated with smart cure cycle showed 32% decreased deflection compared to FF-BP structure fabricated with conventional cure cycle.

Acknowledgments This paper was supported by Climate Change Research Hub of KAIST (grant No. N01150036), KAIST Institute Research Fund (grant No. N10150022, N10150023), Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning. (2011-0030065) and Research funds for newly appointed professors of Chonbuk National University in 2015. Their support is greatly appreciated.

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[28] Boyaci San FG, Isik-Gulsac I. Effect of surface wettability of polymer composite bipolar plates on polymer electrolyte membrane fuel cell performances. International Journal of Hydrogen Energy. 2013;38:4089-98. [29] Grigoriev SA, Kalinnikov AA, Kuleshov NV, Millet P. Numerical optimization of bipolar plates and gas diffusion electrodes for PBI-based PEM fuel cells. International Journal of Hydrogen Energy. 2013;38:8557-67. [30] Hui C, Hong-bo L, Li Y, Jian-xin L, Li Y. Study on the preparation and properties of novolac epoxy/graphite composite bipolar plate for PEMFC. International Journal of Hydrogen Energy. 2010;35:3105-9. [31] Chunhui S, Mu P, Qiong W, Runzhang Y. Performance of an aluminate cement /graphite conductive composite bipolar plate. Journal of Power Sources. 2006;159:1078-83. [32] Blunk R, Elhamid MHA, Lisi D, Mikhail Y. Polymeric composite bipolar plates for vehicle applications. Journal of Power Sources. 2006;156:151-7. [33] Chung CY, Chen SK, Chiu PJ, Chang MH, Hung TT, Ko TH. Carbon film-coated 304 stainless steel as PEMFC bipolar plate. Journal of Power Sources. 2008;176:276-81. [34] Kim BG, Lee DG. Electromagnetic-carbon surface treatment of composite bipolar plate for high-efficiency polymer electrolyte membrane fuel cells. Journal of Power Sources. 2010;195:1577-82. 40

[35] Incropera FP. Fundamentals of Heat and Mass Transfer (Sixth Edition). New York: John Wiley & Sons, 2007. [36] Yu HN, Lim JW, Kim MK, Lee DG. Plasma treatment of the carbon fiber bipolar plate for PEM fuel cell. Composite Structures. 2012;94:1911-8. [37] Kim KH, Lim JW, Kim M, Lee DG. Development of carbon fabric/graphite hybrid bipolar plate for PEMFC. Composite Structures. 2013;98:103-10. [38] Lim JW. A Study on Continuous Carbon/Epoxy Composite Bipolar Plates for PEM Fuel Cells. Daejeon, Republic of Korea: KAIST, 2013. [39] Louis M, Joshi SP, Brockmann W. An experimental investigation of through-thickness electrical resistivity of CFRP laminates. Composites Science and Technology. 2001;61:911-9. [40] Kim M, Yu HN, Lim JW, Lee DG. Bipolar plates made of plain weave carbon/epoxy composite for proton exchange membrane fuel cell. International Journal of Hydrogen Energy. 2012;37:4300-8. [41] Kim M, Lim JW, Kim KH, Lee DG. Bipolar plates made of carbon fabric/phenolic composite reinforced with carbon black for PEMFC. Composite Structures. 2013;96:569-75. [42] ASTM. Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials.

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[43] Cui T, Chao YJ, Van Zee JW. Sealing force prediction of elastomeric seal material for PEM fuel cell under temperature cycling. International Journal of Hydrogen Energy. 2014;39:1430-8. [44] Hu J, Zhang H, Zhai Y, Liu G, Yi B. 500h Continuous aging life test on PBI/H3PO4 high-temperature PEMFC. International Journal of Hydrogen Energy. 2006;31:1855-62. [45] Lobato J, Cañizares P, Rodrigo MA, Linares JJ, Pinar FJ. Study of the influence of the amount of PBI–H3PO4 in the catalytic layer of a high temperature PEMFC. International 41

Journal of Hydrogen Energy. 2010;35:1347-55. [46] Barreras F, Lozano A, Roda V, Barroso J, Martin J. Optimal design and operational tests of a high-temperature PEM fuel cell for a combined heat and power unit. International Journal of Hydrogen Energy. 2014;39:5388-98. [47] Weiss-Ungethüm J, Bürger I, Schmidt N, Linder M, Kallo J. Experimental investigation of a liquid cooled high temperature proton exchange membrane (HT-PEM) fuel cell coupled to a sodium alanate tank. International Journal of Hydrogen Energy. 2014;39:5931-41. [48] Lobato J, Cañizares P, Rodrigo MA, Pinar FJ, Mena E, Úbeda D. Three-dimensional model of a 50 cm2 high temperature PEM fuel cell. Study of the flow channel geometry influence. International Journal of Hydrogen Energy. 2010;35:5510-20. [49] Lim JW, Kim M, Yu YH, Lee DG. Development of carbon/PEEK composite bipolar plates with nano-conductive particles for High-Temperature PEM fuel cells (HT-PEMFCs). Composite Structures. 2014;118:519-27. [50] Wei C, Srivastava D, Cho K. Thermal Expansion and Diffusion Coefficients of Carbon Nanotube-Polymer Composites. Nano Letters. 2002;2:647-50. [51] Ramanathan T, Abdala AA, Stankovich S, Dikin DA, Herrera-Alonso M, Piner RD, et al. Functionalized graphene sheets for polymer nanocomposites. Nat Nanotechnol. 2008;3:32731. [52] Xue Q, Lv C, Shan M, Zhang H, Ling C, Zhou X, et al. Glass transition temperature of functionalized graphene–polymer composites. Computational Materials Science. 2013;71:6671. [53] Yang H, Ze-Sheng L, Qian H-j, Yang Y-b, Zhang X-b, Sun C-c. Molecular dynamics simulation studies of binary blend miscibility of poly(3-hydroxybutyrate) and poly(ethylene oxide). Polymer. 2004;45:453-7. 42

[54] Lee JH, Jang YK, Hong CE, Kim NH, Li P, Lee HK. Effect of carbon fillers on properties of polymer composite bipolar plates of fuel cells. Journal of Power Sources. 2009;193:523-9. [55] Mathur RB, Dhakate SR, Gupta DK, Dhami TL, Aggarwal RK. Effect of different carbon fillers on the properties of graphite composite bipolar plate. Journal of Materials Processing Technology. 2008;203:184-92. [56] Kim M, Lim JW, Lee DG. Surface modification of carbon fiber phenolic bipolar plate for the HT-PEMFC with nano-carbon black and carbon felts. Composite Structures. 2015;119:630-7. [57] Kakati BK, Deka D. Differences in physico-mechanical behaviors of resol(e) and novolac type phenolic resin based composite bipolar plate for proton exchange membrane (PEM) fuel cell. Electrochimica acta. 2007;52:7330-6. [58] Lee D, Lim JW, Nam S, Choi I, Lee DG. Gasket-integrated carbon/silicone elastomer composite bipolar plate for high-temperature PEMFC. Composite Structures. 2015;128:28490. [59] Yu HN, Hwang IU, Kim SS, Lee DG. Integrated carbon composite bipolar plate for polymer–electrolyte membrane fuel cells. Journal of Power Sources. 2009;189:929-34. [60] Yu HN, Kim SS, Suh JD, Lee DG. Axiomatic design of the sandwich composite endplate for PEMFC in fuel cell vehicles. Composite Structures. 2010;92:1504-11. [61] Gibson L, Ashby, MF. Cellular solids: Pergamon Press, 1988. [62] Vinson J. The behavior of sandwich structures of isotropic and composite materials: Thechnomic Publishing Co., Inc., 1999. [63] Zenkert D. An introduction to sandwich construction: Emas Publishing, 1997. [64] Zenkert D. The handbook of sandwich construction: Emas Publishing, 1997. 43

[65] Lim JW. Development of the light weight insert for composite sandwich satellite structures. Daejeon, Republic of Korea: KAIST, 2010. [66] Yu YH, Lim JW, Lee DG. Composite sandwich endplates with a compliant pressure distributor for a PEM fuel cell. Composite Structures. 2015;119:505-12. [67] Suh N. Axiomatic design: Oxford Unoversity Press, 2001. [68] TASSIN N. Storage Technology report: ST7-Redox Systems. 2003. [69] Bartolozzi M. Development of redox flow batteries. A historical bibliography. Journal of Power Sources. 1989;27:219-34. [70] Sum E, Rychcik M, Skyllas-Kazacos M. Investigation of the V (V)/V (IV) system for use in the positive half-cell of a redox battery. Journal of Power Sources. 1985;16:85-95. [71] Wang W, Kim S, Chen B, Nie Z, Zhang J, Xia G-G, et al. A new redox flow battery using Fe/V redox couples in chloride supporting electrolyte. Energy & Environmental Science. 2011;4:4068-73. [72] Kim KH. Development of Manufacturing Technology for the Stack of the VRFB (Vanadium Redox Flow Battery)

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[73] Kim S, Thomsen E, Xia G, Nie Z, Bao J, Recknagle K, et al. 1 kW/1 kWh advanced vanadium redox flow battery utilizing mixed acid electrolytes. Journal of Power Sources. 2013;237:300-9. [74] Park S-K, Shim J, Yang JH, Jin C-S, Lee BS, Lee Y-S, et al. Effect of inorganic additive sodium pyrophosphate tetrabasic on positive electrolytes for a vanadium redox flow battery. Electrochimica acta. 2014;121:321-7. [75] Kim KH, Kim BG, Lee DG. Development of carbon composite bipolar plate (BP) for vanadium redox flow battery (VRFB). Composite Structures. 2014;109:253-9. [76] Johnson KL, Johnson KL. Contact mechanics: Cambridge university press, 1987. 44

[77] Peng S, Wang N-F, Wu X-J, Liu S-Q, Fang D, Liu Y-N, et al. Vanadium Species inCH3SO3H and H2SO4 Mixed Acid as the Supporting Electrolyte for Vanadium Redox Flow Battery. Int J Electrochem Sci. 2012;7:643-9. [78] Blanc C. Modeling of a Vanadium Redox Flow Battery Electricity Storage System: École polytechnique fédérale de Lausanne, 2009. [79] Chalamala BR, Soundappan T, Fisher GR, Anstey MR, Viswanathan VV, Perry ML. Redox Flow Batteries: An Engineering Perspective. Proceedings of the IEEE. 2014;102:97699. [80] Chang T-C, Zhang J-P, Fuh Y-K. Electrical, mechanical and morphological properties of compressed carbon felt electrodes in vanadium redox flow battery. Journal of Power Sources. 2014;245:66-75. [81] Kaviany M. Principles of Heat Transfer in Porous Media. 1995:708. [82] Choe J, Kim KH, Lee DG. Corrugated carbon/epoxy composite bipolar plate for vanadium redox flow batteries. Composite Structures. 2015;119:534-42. [83] White FM. Fluid Mechanics: McGraw-Hill, 2011. [84] Lee DG, Chin WS, Kwon JW, Yoo AK. Repair of underground buried pipes with resin transfer molding. Composite Structures. 2002;57:67-77. [85] Lee DG, Suh NP. Axiomatic Design and Fabrication of Composite Structures. New York: Oxford University Press, 2006. [86] Choe J, Kim KH, Lee D, Bang CS, Lee DG. Glass composite vibration isolating structure for the LNG cargo containment system. Composite Structures. 2014;107:469-75. [87] Giorgio Rizzoni TTH. Principles and Applications of Electrical Engineering: McGrawHill Higher Education, 2007.

45

Bipolar plate (Anode)

H2

Polymer Membrane (PTFE + HSO3, 0.1 mm) Gas Diffusion Layer (Carbon fiber mat, 0.5 mm)

O2

Unit cell

Bipolar plate (Cathode)

4H+

Coolant

4e-

Anode Cathode

2H2

4 H + 4e + O2 → 2 H 2O +



Membrane

Water, Heat

Coolant

2H2O O2

2 H 2 → 4 H + + 4e −

Unit cell

4e-

H+ H+ H+

Fig. 1. Schematic diagram of chemical reaction in PEM fuel cell.

46

GDL Catalyst

Unit cell

MEA

···

Endplate

GDL

Gasket

Bipolar plates (Contact)

Bipolar plate Compaction pressure Coolant channel

Fuel channel Air channel

Fig. 2. Schematic drawing of the PEM fuel cell stack.

47

Fig. 3. Fabrication processes of the conductive particles-embedded bipolar plate: (a) preparing the conductive particles mixed in a solvent; (b) spraying the solvent mixture; (c) drying the MEK and precisely measuring the mass; (d) stacking the pregregs; (e) graphite coating process; (f) compression molding.

48

Backup film

Graphite foil

Prepreg

Prepreg

Laminating (80°C : Hot roller)

Prepreg

Detaching the backup film

Prepreg coated with 10-µm graphite layer

Prepreg

Fig. 4. Graphite coating process of the composite prepregs.

49

Clamping pressure (Max. 2 MPa)

Gas diffusion layer

Exposed carbon fiber

(a)

Compression molding pressure 20 MPa Soft conductive layer (Graphite)

Clamping pressure (Max. 2 MPa)

Gas diffusion layer

Epoxy

Composite plate with graphite layer

Carbon fiber

(b) Fig. 5. Schematic diagram of burrowing effect of the soft graphite layer: (a) without the graphite layer; (b) with the graphite layer.

50

100 µm

100 µm

(a)

(b)

100 µm

Randomly oriented carbon fibers Carbon black layer (c) Fig. 6. Optical microscope images of GDL (10 BC, SGL Group, Germany): (a) surface of the GDL (carbon fiber side); (b) surface of the GDL (carbon black side); (c) cross-section of the GDL.

51

Plasma

Resin rich area

Epoxy

Exposed carbon fiber

Plasma etching Plasma treatment

GDL (Soft material, carbon paper)

> Contact area Plasma etching

Grinding

Fig. 7. Plasma surface treatment for selectively removing resin-rich area (The diameter of carbon fiber is 7 µm).

52

Powered electrode Grounded electrode

Ball flow meter

Argon gas (99.9 %)

Gas handling system

Specimen

RF source

Power supply

Base plate

Plasma reactor

Specimen

Base plate

Fig. 8. Schematic drawing and photographs of the plasma surface treatment system.

53

50 µm

Exposed carbon fibers Fig. 9. Scanning electron microscope (SEM) image of the surface edge of the bipolar plate.

54

Compaction pressure

Insulation plate Gold-coated copper 1A

V

Flat specimen GDL

Fig. 10. Experimental setup for four-point probe method.

55

1.5 Conventional Composite

Areal specific resistance (Ω)

Composite+Graphite coating Composite+Plasma treatment GDL

1

0.5

0 0

0.5

1

1.5

2

Compaction pressure (MPa)

Fig. 11. ASR of the composite bipolar plates with respect to the compaction pressure.

56

1.0 mm

(a)

0.56 mm

0.84 mm

0.56 mm

R100 µm

110o 0.8 mm (b) Fig. 12. Flow channel dimensions: (a) composite bipolar plate with flow channels; (b) schematic drawing of the compressive mold with flow channels.

57

9.0 mm

32.0 mm 50.0 mm

(a)

12.7 mm

1.0 mm

(b) Fig. 13. Schematic drawing of the three-point bending test: (a) front view; (b) side view.

58

80

Composite Composite [03/903]s

70

10 wt. % C.B. 60

Load (N)

50 40 30 20 10 0 0

0.5

1

1.5 2 Displacement (mm)

2.5

3

(a)

Flexural strength (MPa)

300 250 200 150 100 DOE target (25 MPa)

50 0

Composite Composite [0[0/90]s 3/903]s

C.B. 10 wt. %

(b) Fig. 14. Three-point bending test results: (a) representative load-displacement curves; (b) 59

maximum flexural strength.

Air @ 0.3 MPa

Chamber 1

Chamber 2

D = 30 mm

t = 0.2 mm Pressure sensor 1

Gasket

Pressure sensor 2

Specimen 40 mm

Fig. 15. Schematic drawing of the device for gas permeability measurement.

60

1.2 C.B. 10 wt. % Composite Composite [03/903]s

Cell potential (V)

1

0.8

0.6

0.4

0.2

0 0

0.2

0.4

0.6 0.8 Current density (A/cm2)

1

1.2

Fig. 16. Polarization curves of the carbon composite bipolar plates.

61

1.4

Fig. 17. Fabrication processes of the bipolar plate with embedded conductive particles: (a) mixing the PEEK powder with carbon black; (b) spreading the particle mixture on the cleansed carbon fabric using a sieve; (c) carbon fabric spread with the particle mixture; (d) stacking the carbon fabric with expanded graphite foil; (e) compression molding with a closed mold.

62

(a)

(b)

Fig. 18.

Areal specific resistances with respect to wt. %: (a) at 25°C; (b) at 220°C.

63

1.0 Degree of cure

0.8

1.2 1200

0.6 0.8 800

0.4 Heat generation rate

0.4 400

0.2

0.00

0

1

2

3

4

5

6

7

Degree of cure (ζ)

Heat generation rate (W/kg) Heat generation rate (W/g)

1.6 1600

0.0

Time (min) Fig. 19. Heat generation rate and degree of cure of the phenolic resin (KC-4703) during isothermal scanning at 220°C.

64

Pre-cure process Hot-roller at 160˚C 1 MPa

Hand lay-up

Carbon fabric + Phenolic resin with 4 wt.% CB

Extra resin

Backup plate

Carbon fiber felt attachment

Hot compresion molding

Mold Pre-cured specimen Randomly oriented carbon fiber felt

10 MPa at 220˚C for 10 min.

Fig. 20. Full fabrication process of the carbon/phenol composite bipolar plate using the precure process.

65

Fig. 21. Photographs of the surfaces of composite bipolar plates with carbon fiber felts bonded using the pre-cure process with respect to the thickness of the carbon fiber felt: (a) without the pre-cure process; (b) 50 µm; (c) 80 µm; and (d) 140 µm.

66

ASR (mΩ▪cm 2)

90 80 70 60 50 40 30 20 10 0 Ref.

50 µm

80 µm

140 µm

Carbon felt thickness Fig. 22. ASRs of the carbon fiber phenolic composite bipolar with respect to the bonded carbon fiber felt thickness.

67

계열1 Tensile strength Void volume fraction 계열2

500 400

50 40 30

300

20

200

10

100 0

Without pre-cure process 1

With pre-cure 2 process

0

Void volume fraction (%)

Tensile strength (MPa)

600

Flexural strength (MPa)

(a)

160 140 120 100 80 60 40 20 0 1 2 process Without pre-cure process With pre-cure (b)

Fig. 23. Strengths of the carbon fiber phenolic composite bipolar plates with respect to the pre-cure process; (a) tensile strength: (b) flexural strength with flow channels.

68

Silicone

Vacuum

Carbon fabric

Vacuum chamber Compression mold

Hand layup

Vacuum

Vacuum chamber

Specimen

Fig. 24. Process for fabricating the carbon/silicone elastomer composite bipolar plate.

69

Compression mold

Uncured silicone Gasket mold Uncured bipolar plate

(a) 120 mm 80 mm

5 mm

Gasket mold Channel for gasket

(b) Fig. 25. Process for fabricating the gasket-integrated carbon/silicone elastomer composite bipolar plate: (a) cross-section view; (b) top view of the gasket mold.

70

(a)

(b)

Fig. 26. Gasket-integrated carbon/silicone elastomer composite bipolar plate: (a) specimen for gas sealability test; (b) cross-section of the gasket.

71

Flexural strength (MPa)

60 40 20 0 D.O.E. target

Specimen Specimen (20°C) (200°C)

Fig. 27. Three-point bending test results.

72

ASR (mΩ·cm2)

90

20°C 200°C

60 30 0 0

5 10 15 20 Compaction pressure (MPa)

Bulk resistance

(mΩ·cm2)

(a)

8 20°C 200°C

6 4 2 0 0

5 10 15 20 Compaction pressure (MPa) (b)

Fig. 28. Electrical properties of carbon/silicone bipolar plate: (a) ASR; (b) bulk resistance.

73

Normalized load

1

Composite sandwich endplate

0.8 0.6

Metallic endplate 0.4 0.2 0 0

0.2

0.4

0.6

0.8

1

Normalized displacement (thickness)

Fig. 29. Normalized load-displacement curves of the different endplates.

74

120

Weight (%)

100 80 60 40 20 0 SUS endplate

Composite sandwich endplate

Specific flexural stiffness (%)

(a) 300 250 200 150 100 50 0 SUS endplate

Composite sandwich endplate

(b) Fig. 30. (a) Weights; (b) specific flexural stiffnesses of the conventional endplate and the composite sandwich endplate.

75

(a)

10 Temperature (℃ ℃)

SUS endplate 5 0 -5

Composite sandwich endplate

-10 0

50

100

150

200

Time (sec) (b)

Fig. 31. Measured temperatures of different outside surface of endplates w.r.t. time: (a) experimental set up; (b) measured results.

76

Electrolyte tank

Electrolyte tank

Electrolyte tank (cathode)

Electrolyte tank (anode)

Flow of electrolyte +

Cell

Pump

Stack

Membrane e-

e-

H+

H+

V2+  V3+

Cathode: V5+  V4+

Electrode

Anode:

+ +

Overall reaction: +

Electrolyte (cathode)

+

Electrolyte (anode)

Fig. 32. Schematic drawings of redox flow battery.

77

Areal specific resistance (mΩ·cm2)

1000 900 800 700 600 500 400 300 200 100 0 0.40

0.44

0.48

0.52

0.56

Thickness of bipolar plate (mm) Fig. 33. Areal specific resistance (ASR) of the carbon/graphite hybrid composite bipolar plate with respect to its thickness.

78

Mold Hot press

Silcon rubber Demolding layer Expanded graphite foil Carbon prepreg PU foam

Hot press

Side mold

(a)

Temperature (oC)

140 120 100 80 60 40 20 0

30

60

90

120

Time (minute) (b) Fig. 34. (a) Schematic diagrams of the fabricating method for carbon/graphite hybrid composite bipolar plate specimens, (b) Cure cycles of the carbon/graphite hybrid composite bipolar plate.

79

Thickness (mm)

0.60

Average : 0.42 mm

0.55

Std. dev. : 0.015 mm

0.50 0.45 0.40 0.35 0.30

Fig. 35. Thickness distribution of the carbon/graphite hybrid composite bipolar plate.

80

150

ASR (mΩ·cm2)

140 Experimental result

130

RBP / CF = d1t coating RBP / CF = d 2t coating

120

−0.08

0.15

110 100 0

50

100

150

Thickness of coating layer (µm)

Fig. 36. Areal specific resistance (ASR) of the carbon fabric/graphite hybrid composite bipolar plates with respect to the thickness of the graphite coating layer (where, d1=115.5 mΩ·cm2 and d 2=149.5 mΩ·cm2).

81

F/l

h(x)

hmax

Carbon fiber

x

t

y b Graphite coating layer

Width of the contact area (µm)

(a)

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

50

100

150

Thickness of the graphite coating layer (µm) (b) Fig. 37. A simplified contact model of the carbon fiber and the graphite coating layer on the rigid carbon composite substrate; (a) a schematic drawing of the simplified contact model, (b) analysis results of the width of the contact area with respect to the thickness of the coating layer.

82

1.8 Conventional graphite BP

Voltage (V)

1.6 1.4 Carbon/graphite hybrid BP

1.2 1.0 0.0

0.5

1.0

1.5

Times (hours) Fig. 38. Charge/discharge test result using VRFB single cell unit with the carbon/graphite hybrid composite bipolar plate comparing with the conventional graphite bipolar plate.

83

100

Efficiency (%)

95 90 Energy% Coulomb% Voltage%

85 80 75 70 The carbon/graphite hybrid The conventional graphite composite bipolar plate bipolar plate

Fig. 39. Charge/discharge efficiencies with the carbon/graphite hybrid composite bipolar plate comparing with the conventional graphite bipolar plate.

84

High Vf area

CCBP

Carbon felt

Low Vf area

Fig. 40. Schematic illustration of the purpose of the CCBP.

85

ASR of carbon felt (mΩ·cm2)

ASR

0.6

Applied compaction pressure

300 0.4 200 0.2

100

0

0 0

10 20 fiber volume fraction (%)

Applied compaction pressure (MPa)

400

30

(a)

Permeability

(10-10·m2)

15 Re=0.004 Re=0.006 Re=0.008 Re=0.010 Re=0.012 Re=0.014

12 9 6 3 0 3

6

9

12

15

Fiber volume fraction (%) (b) Fig. 41. Measured properties of carbon felt w. r. t. the fiber volume fraction: (a) ASR and applied compaction pressure; (b) permeability. 86

Calculated ohmic loss (W/m2)

10

without corrugation Amplitude 0.2 mm Amplitude 0.4 mm Amplitude 0.6 mm Amplitude 0.8 mm Amplitude 1.0 mm Amplitude 1.2 mm

8

6

4 4

6 8 Average fiber volume fraction (%)

10

(a)

Calculated pumping loss (W/m2)

10

8

without corrugation Amplitude 0.2 mm Amplitude 0.4 mm Amplitude 0.6 mm Amplitude 0.8 mm Amplitude 1.0 mm Amplitude 1.2 mm

6

4 4

6 8 Average fiber volume fraction (%)

10

(b) Fig. 42. Results of the loss calculations: (a) ohmic loss; (b) pumping loss per unit area w. r. t. the amplitude of corrugation and average fiber volume fraction.

87

Sum of ohmic & pumping loss (W/m2)

16

without corrugation Amplitude 0.2 mm Amplitude 0.4 mm Amplitude 0.6 mm Amplitude 0.8 mm Amplitude 1.0 mm Amplitude 1.2 mm

15

14

13

12 4

6 8 Average fiber volume fraction (%)

10

Fig. 43. Sum of ohmic and pumping loss per unit area w. r. t. the amplitude of corrugation and average fiber volume fraction.

88

A’

160 mm

A

140 mm 20 mm A’

A

2.4 mm Thickness: 0.4 mm Fig. 44. Photographs of the fabricated CCBP specimen.

89

400

ASR (mΩ·cm2)

with corrugation without corrugation

300

200

100 3

4

5

6

7

Average fiber volume fraction (%) Fig. 45. Areal specific resistances (ASR) of the carbon felt electrode with and without the CCBP.

90

Permeability (10-10 · m2)

5 Experimental result Calculated result

4 3 2 1 0 with corrugation

without corrugation

Fig. 46. Measured permeabilities of the carbon felt electrode with and without the CCBP.

91



Flow frame

Biplolar plate

(a)

500 mm

Bipolar plate

Permanent local buckling

Flow frame

500 mm

(b) Fig. 47. Technical issues in the co-cured FF-BP structure: (a) global bending of the FF-BP and deflection of the bipolar plate; (b) permanent local buckling.

92

Temperature (oC)

125 80

Dwelling process

Curing process

Cooling process

Time

Temperature (oC)

(a)

tconst.

Cooling point

Tmax 60

Cure-triggering process

Cooling process

Post-cure process

Time

(b) Fig. 48. Cure cycles for the composite structure: (a) conventional cure cycle; (b) smart cure cycle.

93

Table 1 Low-Temperature PEM fuel cell vs. High-Temperature PEM fuel cell LT-PEMFC

HT-PEMFC

Temperature

R.T. ~ 80°C

120°C ~ 180°C

Electrolyte

PFSA (Nafion etc.)

Acid-doped PBI

Performance

1000 mA/cm2 @ 0.6 V

400 mA/cm2 @ 0.6 V

CO Tolerance

< 10 ppm

< 1,000 ppm

Water Management

Humidifier Necessary

Without Humidifier

94

Table 2 Properties of continuous unidirectional carbon fiber prepregs (USN-020 A, SK chemical, Republic of Korea) Fiber properties Density (kg/m3)

Thickness (1 ply, mm)

1.48 × 103

0.025

Modulus (GPa)

Strength (GPa)

Fiber volume fraction (%)

Fiber areal weight (g/m2)

235

4.4

50

22

95

Table 3 Material properties of carbon black Material Carbon black

Ketjenblack, EC-600JD

Particle size

Purity (F.C.%)

BET surface are (m2/g)

34 nm

99.0

1270

96

Table 4 Properties of expanded graphite foil

Material

Expanded graphite foil

Young’s modulus (GPa)

Tensile strength (MPa)

Density (kg/m3)

Thickness (mm)

0.19

5.0

1.5 × 103

0.1

Volume fraction (%) 66

Coefficient of thermal expansion (10 -6/°C) α1

α2

α3

1

1

30

97

Table 5 Properties of GDL (10 BC, SGL Group, Germany)

10 BC

Thickness (mm)

Air permeability (cm3·cm-2·s-1)

Areal weight (g/m2)

Electrical resistivity (µΩ·cm)

0.415 ± 0.055

1.45 ± 0.85

135 ± 16

<16

98

Table 6 Properties of carbon fabric

WSN-3K plain weave carbon fabric Fiber properties

Areal density (g/m2)

Thickness (1 ply, µm)

Thread count (/mm)

209

270

0.5×0.5

99

Modulus (GPa)

Strength (GPa)

Density (kg/m3)

240

4.9

1.82×103

Table 7 Properties of PEEK powder VICTREX® PEEK 150UF10 Material

Average particle size

Melting temperature (°C)

Continuous use temperature (°C)

Melt viscosity at 400°C (Pa·s)

PEEK powder

10 µm

343

260

130

100

Table 8 Properties of conductive particles

Material

Particle size (nm)

Resistivity (Ω·m)

Price ($/kg)

Single-walled carbon nanotube

1.2~3

1.0 x 10-5

1500

Multi-walled carbon nanotube

5~100

5.1 x 10-8

3000

Carbon black

20~50

1.0 x 10-3

70

Graphite powder

2.5 µm

1.3 x 10-2

50

101

Table 9 Material specifications of carbon black

Material

Carbon black

Ketjenblack, EC-600JD

Particle size

Purity (F.C.%)

BET surface area (m2·g -1)

34 nm

99.0

1270

102

Table 10 Properties of the resole type phenolic resin Properties

KC-4703 (Kangnam Chemical, Korea)

Appearance

Liquid

Solids

60%

Solvent

Methanol

Curing temperature

120ºC

Viscosity (@ 25ºC)

0.25 Pa·s

Gel time (@ 120ºC)

15 min

103

Table 11 The properties of randomly oriented carbon fiber felt. Features

Grade 8000015

Grade 8000130

Grade 8000028

Thickness

50 µm

80 µm

140 µm

Areal weight

6.8 g/m2

10.8 g/m2

17.0 g/m2

Surface resistivity Areal specific resistance

35

25



0.8 mΩ·cm2

1.6

104



15 Ω

mΩ·cm2

3.0 mΩ·cm2

Table 12 Properties of plain weave carbon fabric (C-112) Areal density (g/m2)

Thickness (mm)

Thread count (/mm)

120

0.15

0.87×0.87

Fiber properties

105

Modulus (GPa)

Strength (GPa)

Density (g/cm3)

230

4.9

1.80

Table 13 Properties of silicone elastomer (Sylgard 184) Viscosity (Pa·s)

Durometer shore

Tensile strength (MPa)

Continuous use temperature (°C)

3.5

43

6.7

-45~200

106

Table 14 Comparison between the conventional endplate and the sandwich endplate Conventional endplate

Sandwich endplate

Weight (%)

100

25

Thermal conductivity of material ( W/mK)

16.3 (SUS)

0.025~0.1 (Foam core)

Flexural stiffness (%)

100

120

107

Table 15 Properties of the carbon fabrics, expanded graphite foil and carbon felt electrode Material

Carbon composite (WSN 150 1k, SK Chemical, Korea)

Carbon felt electrode

Graphite foil (BD-100, Samjung CNG, Korea)

Properties Fiber areal density (g/m2)

119

Thickness (µm)

133

Fiber volume fraction (%)

71.6

Compressive modulus in thickness direction (GPa)

10

Thickness (mm)

3

Fiber volume fraction (%)

7

Carbonize temperature (oC)

> 2000

Density (kg/m3)

1.0

Thickness (µm)

120

Tensile strength (MPa)

4.0

Compressive modulus in thickness direction (MPa)

370

108

Table 16 Charge/discharge test condition Properties

Value

Charge/discharge voltage (V)

1.2 ~ 1.6

Charge/discharge current density (A/m2)

800

Reactive area of the unit cell (m2)

0.64

Amount of electrolyte (liter)

0.05

Thickness of the graphite coating layer (µm)

120

109