Enhancing performance of advanced fuel cell design with functional energy materials and process

of hydrogen based electric powertrain


Introduction
Bipolar plates (BPP), being one of the most important parts of the Proton exchange membrane fuel cell (PEMFC) can contribute towards the performance enhancement by reducing the weight as well increasing the electron transfer between the cells.Overall, it contributes up to 60e80% of the total weight and 30e40% of the stack cost [1e4], so reducing the weight and cost along with increasing the electrical conductivity and mechanical flexibility can improve the performance of PEMFC and establish its feasibility more in the competitive renewable energy market.The main functions of BPP are conducing electricity between adjacent cell, supporting the membrane electrode assembly (MEA), sustaining clamp pressure, distribution of fuel gasses and removing the by products such as heat and water [5] shown in Fig. 1.
According to the US Department of Energy (DOE) standard set for 2020 and 2025, the most important Criteria are high electrical conductivity, less gas permeability, chemical stability, light weight, low cost and good mechanical property, especially flexural strength [6].However, among these attributes the issues of improving electrical conductivity and mechanical strength were addressed by most research related to BPP.Herein, the main focus is placed on the critical factors and functional carbon materials in a holistic way that determine the fabrication of the high-performance BPC possessing both high electrical and mechanical attributes simultaneously.Nonetheless, the research has been carried out by considering a specific carbon-polymer matrix pair due to the complex nature of the problem [7] and it is important to know the critical parameters applicable for a specific duo to achieve the high-performance in attributes for specific applications.The contribution of functional carbonaceous materials towards the electrical properties in terms of graphitic structure and porous characteristics has been discussed in another paper [9] from our research group.Herein, the main focus is on the mechanical attributes along with electrical conductivity and optimized process parameters that determine the enhancement in high performance of the bipolar plate composites (BPC) showing both electrical and mechanical attributes.
Processing techniques and parameters can control the orientation, dispersion and interparticle distance of the solid particles and if an isolated polymer region is formed, it can hinder the electron transportation as well as affect the mechanical characteristics [8].Resin matrix of a composite is very important as it determines the mechanical attributes with enhanced wetting properties as well as its load distribution capacity under stress.Epoxy has been chosen in the present work because of its excellent chemical resistance, high strength and stiffness, low toxicity, light weight and good dimensional stability; nonetheless, it has better flexural properties than other thermosets [10e14].According to US DOE target, the BPP needs to maintain the mechanical strength, especially flexural strength above a certain threshold to withhold the clamping force as well as to support the MEA and electrode systems.In practice, an excellent mechanical flexibility is difficult to achieve when the loading of solid particles is highly increased to enhance the conductivity.This is because the carbon materials are inherently brittle, and the mechanical flexibility is induced mostly by the resin matrix content.Also, higher particle loading causes more voids and defects compare to the minor resin content.To resolve these issues a long list of polymers, both thermoplastics and thermosets have been investigated for decades, although very limited number of studies have addressed the improvement of mechanical attributes.Nonetheless, Planes et al. [2] implied that the mechanical strength mainly depends on the processing conditions, but an optimization is required to achieve the targeted electrical and mechanical attributes simultaneously.In this research, the enhanced electrical and mechanical performance was achieved through the novel carbonaceous materials as well as optimization of process parameters.It is an enormous challenge, especially to maintain a target flexural strength as the composite becomes brittle with higher carbon particles loading.The porosity volume fraction also increases from bad wettability and hence, a decrease in mechanical strength can occur.It has been resolved by utilizing fibrous carbon materials as reinforcement as well as by increasing pressure during fabrication process to minimize the void spaces between particles.Another effective process is to introduce hybrid filler system, i.e., fillers with different aspect ratios such as nanofillers and fiber reinforcement, although above a certain nanofiller content level, their properties deteriorate [15].In the present work, both continuous and chopped carbon fiber as well as fiber veil are introduced which substantially increase the mechanical strength of the composite.
A detailed background, a summary of the state-of-art and gaps in fundamental design of highly efficient composite plate for energy devices has been discussed in the supplement part 1 and details of optimized processing condition in part 2. Herein, we investigate the critical parameters for the less researched area of mechanical strength of the composite to address the pivotal issues for understanding the enhancement potential and achieve to fabricate a high-performance light-weight composite for fuel cell applications.

Materials
In this research, epoxy resin-bisphenol A diglycidyl ether (West system 105; density 1.15 g/cm 3 , viscosity ~1000 cp) was used as the polymer matrix and carbon-based particles such as graphite, carbon black, multi-wall carbon nanotubes, expandable and expanded graphite were used as carbonbased materials and nanofillers (Table 1).Also, carbon fiber Fig. 1 e Components of a PEM fuel cell.and carbon veil were used as main reinforcement fibrous raw materials.

Methods
Composite fabrication process: Compression molding was used as the main compounding process to fabricate the composite samples.In the present work, the fabrication method includes optimized direct mixing and fabricating composite through hot molding process (Fig. 2) and finally, characterization for electrical conductivity, mechanical strength and morphological analysis for interface properties.In general, carbon-based materials were mixed in a mechanical mixer (Caframo RZR1) at around 200e300 rpm at ambient temperature, the polymer resin and hardener or curing agent (polyamine based) were added dropwise with continuous mixing, then the secondary fillers were added intermittently towards the end and mixing continued for more 15 min to ensure even dispersion of carbon-based materials in the matrix.The mixture was then compressed in the hot press machine for 15 min at 110e130 C and 55 MPa (8000 psi).For fabrication of carbon veil or continuous carbon fiber composite, one piece of carbon veil or continuous carbon fiber layer was placed in between two layers of carbonaceous particles in polymer matrix and aforementioned process of compression molding was applied.

Characterization
The following machines and processes were used for characterization of the carbon polymer composite (more details in the supporting document-part 3).

3.
Results and discussions

Materials' effects on flexural strength
In general, for carbon polymer composites with high carbonbased materials' content, the rule of mixture can not be applied to determine the overall flexural strength of polymer Fig. 2 e Compression molding process for fabrication of bipolar plate composite.j o u r n a l o f m a t e r i a l s r e s e a r c h a n d t e c h n o l o g y 2 0 2 3 ; 2 6 : 1 7 2 3 e1 7 3 5 composites due to their complex nature.The following Eqs.
(1)e( 3) represent some more researched theoretical models that have been used to investigate the variance of flexural modulus data from the experiments shown in Table 2. Modified rule of mixture or Cox's model [24] helps to predict elastic or flexural modulus of a composite from the individual moduli and volume fractions of the components considering their physical and mechanical properties.Hirsh's model [25] considers a combination of equal strain and stress conditions both in reinforcement and matrix phases.Finally, Halpan-Tsai model [26] takes into account two or more phases systems for discontinuous oriented components and even considers their shape factors.In this process, multiple phases can be considered by extending the stages for determining the modulus of the composite.Modified rule of mixture or Cox's model: Here, E f and E m , Gm, rf and n m represent orientation efficiency factor, Cox's fiber length efficiency factor, volume fraction of fiber, flexural modulus of fiber and matrix, shear modulus of matrix, fiber radius and Poisson's ratio of matrix.
Hirsch's model: Here, Ec represents the flexural modulus of the composite.
Halpan-tsai model: Here, h Em þa and a represents shap e factor dependent on filler geometry and loading direction such as ratio of length and thickness The main reason for the deviations found between theoretical model and experimental values might be the higher filler content and complex filler-matrix interface properties that has been neglected in the theoretical models.Additionally, other parameters such fabrication process, nature of fillers and testing procedures are also different for the theoretical models and experimental aspects in this research work.Only the Halpan-Tsai model shows closer value of flexural modulus, likely due to its ability to consider multiphase system [26].Flexural strength decreases with increase in particle content, because the particles tend to agglomerate at higher loading which becomes a weak point for the strength of materials as these parts of the composite can break easily with increased stress.With the increase in carbonaceous materials of carbon polymer composite, there is an increase in void spaces, agglomeration process, strain propagation and surface fracture energy.For hybrid system i.e., a combination of carbon materials with different aspect ratio, the situation is even more complex and controlling the right content can give advantage over fiber reinforcement by minimizing the gap in load distribution within the composite (Fig. 3).The overall mechanical properties depend on the matrix and bond between fiber and matrix [17].

Carbon veil and carbon fiber reinforcement
The use of carbon veil (CV) and chopped carbon fiber (CF) shown in Fig. 4c and d   reinforcement materials for mechanical strength.Carbon veil (CV) is a type of non-woven sheet with 50e100 mm in thickness and comprised of carbon fibers that randomly distributed in a resin matrix.It can be used as reinforcement, especially for enhancing mechanical strength in a pre-preg type system by placing in between the layers of graphite-epoxy mixture.Under the digital microscope, carbon fibers in the CV were observed to be distributed in all direction in the resin matrix (Fig. 4d) which explains the reason of increased electrical resistance caused by the interface.In this research, a piece of CV (75 Â 75 mm) was used as the reinforcement in between two layers of graphite-epoxy mixture.From the results obtained, the use of carbon veil largely increased the flexural strength up to 25% but reduced the conductivity for both single carbon-based particle (22%) and binary carbon particle composites (48%) compared to composite with no reinforcement (Fig. 4a and b).From the morphology in the SEM images (Fig. 4e), layers of graphite-epoxy composite close to CV showed large pores and discontinuity which might cause electron transfer difficult between the particles.There might also be wettability issue (Fig. 4f) which causes the interface resistance to increase substantially.Carbon fiber is mainly a polyacrylonitrile (PAN) based fibrous material with high-aspect ratio and superior in mechanical properties than the other functional carbon-based materials.However, the reduced electrical conductivity is a concern due to interface resistance and poor wettability, hence a balance is required especially for size and percentage of carbon fiber content.In the present work, the virgin PAN based CFs were primarily chopped to 2e4 cm length and used in the experiments.However, entanglement of the fibers and resin depletion issue for the graphite particles were observed which has been overcome by milling the carbon fibers to 40 mm in length and added intermittently in small amount after resin in the direct mixing process.However, not much improvement (32e34 MPa) of the flexural strength was observed, and electrical conductivity was reduced around 9% (153e141 S/cm).On the other hand, when chopped carbon fiber with average length of 0.9e1 cm (referred to as long CF in Fig. 6a and b) was used, the flexural strength largely improved to 40 MPa.This deviation could be due to the reduced (milled) carbon fiber length; i.e., the strength is reduced due to breakage of layers, hence the length of carbon fiber is a critical issue.Incorporating carbon fiber in the composite increases the mechanical flexibility as it remains between the layers in the random direction bearing the distributed load.However, too much shear during processing can break them and reduce their effectiveness.From SEM images for morphological study, CF (Fig. 5c) showed better wettability with resin matrix and less porous structure than CV based composites (Fig. 4f).

respectively worked as efficient
Using CF or CV in graphite-epoxy system can largely increase flexural strength (Fig. 5a) of the composite, however, CV has slightly greater effect than CF, 42 MPa compared to 40 MPa in a single particle system, because CV contains a continuous layer working as the load bearing backbone.Also, in case of composite with nanoparticles or hybrid composites, the introduction of CV increases flexural strength up to 50 MPa, a large improvement in mechanical flexibility.This is most likely because of the pores between the layers been filled up with nanofillers and thus load distribution becomes more even and effective.
Addition of nanoparticles such as CB increases flexural strength for Gr-CF composite up to 52 MPa (Fig. 6).From the un-notched impact strength test, it was observed that carbon fiber and carbon veil increase the impact strength up to 57 J/m.On the other hand, introduction of nanofillers without CF or CV do not have much influence on the impact strength (Fig. 6).
This might be due to the 'internal shielding' by the nanofiller and carbon fiber interaction which prevents the crack propagation.On the other hand, for CCF interlayer system as discussed in section 3.1.2,the strong backbone of CF along with the mechanical bond between the components likely causes the high impact strength.Toughness of the composites for different compositions were measured by calculating the area under the stress-strain curve up to fracture point (Table 3).It was observed that the load distribution was better for a hybrid composite when both nanofiller and fibrous reinforcement were present.In practical applications, a superior mechanical flexibility and high impact is important criteria for the bipolar plate as it needs to sustain the clamp pressure; the more force it can absorb, the better for the fuel cell structure and its durability.

CCF and RCF
Continuous carbon fiber (CCF) as a unidirectional interlayer is used in the present work to enhance the mechanical properties, especially flexural strength of the composite.The process is very effective (Fig. 7a) and even with the recycled carbon fiber high flexural strength value could be achieved.The recycled carbon fiber (RCF) has been investigated also to provide an alternative to expensive virgin carbon fiber.Carbon fiber when used in continuous form i.e., the carbon fiber's length is equal to full length of the composite plate placed in between to filler-matrix thin layers, showed excellent mechanical strength as well as high electrical conductivity of 200 S/cm, maintaining the layers of graphite resin mixture is thin such as 0.5e1.0mm.From 3D images from X-ray Micro CT scan, the carbon fiber composite contains large pores and Fig. 5 e Effects of using carbon fiber and milled carbon fiber as reinforcement, comparison between a) with and without CF b) milled and longer CF c) SEM image of Carbon fiber within the composite structure.resin on the surface which are changed by small pores in case of CCF composite (Fig. 8c) facilitating the formation of excellent charge transfer network.This novel composite has better flexural strength than graphite composite by 82% and graphite-carbon fiber composite by 59% as shown in Table 3.The CCF composite even exceeds the mechanical strength of the hybrid composite by 24% especially in the fiber direction as good adhesion between the carbon fiber and graphite-resin mix was found (Fig. 8b), nonetheless the electrical properties are better (Fig. 7b) and flexural modulus has also increased by 30e50%.This composite has likely better properties than the chopped carbon fiber composite because of the less resin pocket and smaller pore structure as visible from the x-ray microtomography pictures (Fig. 8c); pore area found to be 1.5% compared to graphite-carbon fiber composite's 2.3%.In the transverse direction to the continuous carbon fiber (CCFy), the   flexural strength is found around 47 MPa which is close to or better than most of the carbon polymer composites fabricated with other carbonaceous materials (Fig. 8a).For example, from the literature carbon fiber composites in epoxy matrix showed flexural strength around 36 MPa [27], with other matrix such as phenolic, PPS, PVDF, etc. Reached up to 55 MPa [28e31] whereas commercial plates demonstrated up to 50 MPa [32].Additionally, the carbon fiber content has been decreased by 0.5 wt% which further increases the cost effectiveness.For further investigation, CCF has also been tried to incorporate in crisscross directions.However, not much improvement in the mechanical properties was observed, moreover thickness has increased by 30% and delamination occur in the middle layer because of the wettability issues with the matrix; the carbon fiber content increases from 1.5 wt% to 3 wt% which might also reduce the cost effectiveness of the composite with CCF in crisscross direction.
Poor wettability and delamination between layers were also observed when carbon fibers are pre-cured in epoxy matrix and used as a prepreg system (Fig. 7a).From Table 4, the surface energy of continuous carbon fiber composite (contact angles have been reported in section 3 of supplement document) is also larger than graphite-only and chopped carbon fiber composites which indicates a better bonding and wettability present in the composite's surface.The decrease in polar component validates dominant mechanical bonding (interlocking) than chemical bonding which is further supported by the literature [18e20]; this has also been facilitated in the present work by the high pressure fabrication process.The chopped recycled carbon fiber (RCF) shows similar mechanical properties (e.g., flexural strength around 40 MPa) to the virgin carbon fiber which brings opportunities to secure a cost-effective method to use the carbon fiber as a low carbon alternative.The EDX data (Fig. 8d) for RCF composite shows high O/C ratio and different impurities other than Carbon and Oxygen present which maybe the reason of lower conductivity in the chopped RCF composite.Moreover, the continuous RCF stick together due to inherent resin content and thus show higher interfacial resistance (Fig. 8d).Overall, the lower content of inexpensive carbon fiber PX35 which is also made

CB and MWNT
Nanofillers such as carbon black has low surface energy and also hydrophobic in nature which makes them incompatible with polymer matrices with polar groups.It naturally tends to agglomerate because of strong van der Waals force of interaction and surface energy difference with primary carbonbased materials and polymer matrix.The agglomeration state although boost the conductivity in the composites, reduces its effectiveness as the reinforcement for mechanical strength as there occur wetting issues with polymer matrices deteriorating effective load transfer process between polymer matrix and carbon particles.From the literature, it was found that the nanofillers have threshold concentration and beyond this, the properties start to decrease due to improper wetting and increase in viscosity of the precursor solution [21e23].It is always difficult to achieve multi-functional properties such as conductivity and mechanical strength simultaneously, so a balance is required.
Nano fillers can only help the enhancement of mechanical strength if fibrous reinforcement is present which may be because these functional materials increase the bond between the fiber and matrix.For a hybrid system, in presence of carbon fiber, CB shows better flexural strength than MWNT (Fig. 9) with a high value of 52 MPa.Also, CB is the most economical nanofiller.Nanotubes could not show the theoretical mechanical strength as expected because of its limitation in dimensions, e.g., length in micrometer range is not enough to form tube-to-tube connectivity, dispersion quality especially the role of the polymer matrix also plays a pivotal role by exerting compressive load from shrinkage.Functionalization of MWNT's sidewall and mechanical interlocking can be better ways to enhance the mechanical performance by nanotubes.From morphological images by SEM, use of nanoparticles increases the connectivity of the primary carbon-based material layers as it fills up the voids between the layers and also increases the connection between conductive networks (Fig. 9a and b).However, agglomeration might be an issue which can be reduced by process modification such as sonication, high rpm mixing, etc.

Resin effect
For different resin contents, with increasing resin percentage the mechanical flexibility increases, but the consistency of the mixture was disrupted which affect the fabrication process.In this research, resin content is varied for graphite-epoxy composite to investigate the change in mechanical properties in terms of resin content with 24 wt% and 30 wt% polymer matrix.Inconsistencies of composite were observed below 24 wt% and above 30 wt% in the fabrication process.It was noted that the flexural strength and toughness of 30% resin matrix composites are 42 MPa and 25.4 MJ/m 3 compared to 35 MPa and 12.7 MJ/m 3 for composites with 24% matrix (Fig. 10a).However, there are more gaps between the layers observed at higher resin content (Fig. 10b) as well as insulating layers deposited on the composite.Hence, the electrical conductivity decreases by 34% and becomes lower than the US DOE requirement of 100 S/cm.It can be also observed from the morphology by SEM that the resins stay more visible, but unevenly distributed between the layers (Fig. 10c) in the composite with higher resin content.Overall, increasing the resin matrix in the graphite-epoxy composite from 24 wt% to 30 wt% shows increased flexibility of around 12.5% as resin has more effects on flexural strength than graphite which is brittle in nature, however, conductivity is largely reduced up to 37% as it tends to deposit between the graphite layers and hence, increases the interface resistance.

Effects of process parameters
Process parameters affect the performance of composite to a large extent and should be optimized for certain fillermatrix pairs depending on the intrinsic properties of raw materials and their interactions with each other.Processing techniques and parameters can help to enhance the composite properties by determining the capacity of handling higher carbon materials content by the polymer matrix and control the carbon-matrix interaction and even the crystallinity of the matrix [8].For processing conditions, on the other hand, the important parameters include molding temperature, molding time, molding pressure, mixing time, mixing temperature, etc.Among these, molding temperature and pressure are most important to influence the electrical conductivity and mechanical strength as these parameters maximize the wettability and compatibility by evenly distributing the resin around the particles.With the increase in solid carbonaceous particle concentration, the process parameters play a very important role to achieve the desired performance by the composite, because a balance is required between different attributes.Herein, the investigations on process parameters were carried out with graphite-epoxy composites; while one parameter being tested other parameters were kept constant.Statistically significant differences have been achieved for critical parameters such as mixer speed 200 rpm (p ¼ 0.0001), mixing time 15 min (p ¼ 0.0123), molding temperature 110 C (p ¼ 0.0002) and molding pressure 55 MPa (p ¼ 0.0019) from a one-way ANOVA test.Along with the novel materials, process parameters can facilitate achieving optimized performance for the carbon polymer composites.
To understand which parameter is more significant than others, t-test and Tukey HSD test were conducted between means of the parameters to compare each other.It was found that all the p values are greater than 0.05 (Table 5) which shows that none of the parameters are significantly more important than the others.However, from F-values by oneway Anova it was found Speed (56.58)>Temperature (21.750>Pressure (13.06)>Time (9.99) in terms of higher variation among group means.Additional discussions about the effects of process parameters and optimization for high performance composite are elucidated in the supplementary document in the part 2.

Conclusions
In the present work, functional attributes for highperformance composites were investigated to determine if a holistic approach with vital elements can improve the carbon polymer composites in fuel cell and other energy devices applications.High flexural strength of 64.2 MPa (DOE target >25 MPa), flexural modulus of 14.8 GPa as well as high electrical conductivity of 200 S/cm (DOE target >100 S/cm) exceeding the US DOE criteria by a considerable margin, were achieved using the novel and cost-effective CCF as unidirectional interlayer system which is remarkable for any carbon polymer composite to achieve simultaneously.The excellent mechanical bonding was also achieved through the highpressure fabrication process, utilization of carbon veil and functional nano additives; it surpasses the other reported works which failed to achieve simultaneous high electrical conductivity and mechanical flexibility.The other new concept is to introduce recycled carbon fiber which showed similar properties as virgin carbon fiber and can be a potential cost-effective material.The conductive property of raw materials such as carbon-based materials depend on source, processing temperature, purity, manufacturing methods, aspect ratio, atomic structure, surface area, etc., vis-a-vis for polymers viscosity, bonding strength, crosslinking temperature, surface energy, etc. Can be determinant factors for mechanical strength.Beyond these basic inherent qualities, significant factors such as effects of functional materials and manufacturing technology led to extra-ordinary enhancement of flexibility, conductivity and durability of carbon composites which is depicted in the present research.The future work might be investigating into more cost effectiveness of bipolar plate composite with modified polymer matrix content and characteristics.The key points from this research are summarized below.
Continuous carbon fiber as well as recycled carbon fibers show excellent mechanical and electrical attributes for fabricating a low-cost high-performance conductive composite plate for fuel cell Hybrid chopped carbon fiber and carbon veil with nanofillers led to enhanced mechanical strength enhancement The enhancement of mechanical properties such as flexural strength depends on the mechanical interlocking between fibrous material and polymer matrix as well as uniform dispersion of functional carbon materials to ensure effective load transfer between them.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 3 e
Fig. 3 e Scheme of load transfer for the graphite-carbon fiber composites a-without nanofiller, b-with nanofiller and c-with continuous carbon fiber (CCF) interlayer.

Fig. 4 e
Fig. 4 e Effects of carbon veil on composites with a) single-type carbon and b) binary carbon (Graphite); c) & d) Microscopic image of carbon fiber and carbon veil, e) & f) SEM image of carbon veil within the composite structure showing porous structure and poor wettability.

Fig. 6
Fig. 6 e a) Flexural and impact strength for different compositions b) Bubble graph for comparison for both electrical and mechanical attributes-here bubble radius proportional to electrical conductivity value.
j o u r n a l o f m a t e r i a l s r e s e a r c h a n d t e c h n o l o g y 2 0 2 3 ; 2 6 : 1 7 2 3 e1 7 3 5

Fig. 7
Fig. 7 e a) Different interlayer systems with CF and RCF b) comparison between electrical and mechanical properties of different composites with CF and RCF.

Fig. 8 e
Fig. 8 e a) comparison between different stress-strain curves b) SEM image showing the interface between CCF and fillermatrix components, c) 3D images from X-ray microtomography for different carbon fiber composites and d) EDX data for RCF composite.
j o u r n a l o f m a t e r i a l s r e s e a r c h a n d t e c h n o l o g y 2 0 2 3 ; 2 6 : 1 7 2 3 e1 7 3 5

Fig. 9 e
Fig. 9 e Morphological difference between a) composite with nanofiller reinforcement b) composite with primary carbonbased materials only.

Table 1 e
List of carbonaceous materials and their properties.

Table 2 e
Comparison between different models and the experimental results.

Table 3 e
Comparison between toughness and flexural strength of different compositions.

Table 4 e
Surface energy of different carbon polymer composites.

Table 5 e
Statistical (mean value) comparison between different significant parameters.r n a l o f m a t e r i a l s r e s e a r c h a n d t e c h n o l o g y 2 0 2 3 ; 2 6 : 1 7 2 3 e1 7 3 5