Innovative graphite powders for improved polymer-based bipolar plates for fuel cells

EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476241 A1116 Page 1/6 A1116 Innovative graphite powders for improved polymer-based bipolar plates for fuel cells Raffaele Gilardi* (1), Michal Gulas (1), Ahmad Abdul Ghani (2), Jérôme Crépin-Leblond (2) (1) IMERYS Graphite & Carbon, 6804 Bironico/Switzerland; (2) IMERYS Technology Center, 31100 Toulouse/France; *Contact corresponding authors: www.EFCF.com/ContactRequest Abstract This study investigates the development and application of innovative graphite powders for enhancing the performance of polymer-based bipolar plates in fuel cells. The research focuses on novel graphite powders designed to improve the electrical conductivity while maintaining good mechanical properties and ease of manufacturing. Through a series of experiments performed with an internal mixer and with a pilot-scale twin-screw extruder, we evaluated various graphite powders, including surface-modified ones as well as biomass derived synthetic graphite, in both compression molded and injection molded bipolar plates. The results demonstrate that optimized graphite powders can significantly enhance the overall performance of polymer-based bipolar plates, in particular they can lower throughplane electrical resistance compared to standard synthetic or natural graphite. The findings of this study pave the way for further optimization of bipolar plates for next-generation fuel cell components, contributing to the development of lighter, more durable, and higherperforming fuel cell systems in mobile and stationary power applications. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476241 A1116 Page 2/6 Introduction The development of efficient fuel cell systems is crucial for advancing clean energy technologies. This study focuses on enhancing the performance of polymer-based bipolar plates, a key component in PEM fuel cells, through the use of innovative graphite powders. Bipolar plates play a vital role in fuel cell efficiency, and improving their electrical conductivity while maintaining manufacturability is essential for next-generation fuel cells. When considering bipolar plate materials, two main categories emerge: metallic and polymer composite. Each has its own set of advantages and disadvantages. ● Metallic bipolar plates have high electrical conductivity, high mechanical strength and enable thin plate design (<100 microns), allowing for compact fuel cell stacks. However, they are susceptible to corrosion in the fuel cell environment and have higher density compared to polymer composites. Anti-corrosion coatings can be applied to metallic plates but they come with higher material and manufacturing costs. ● Polymer composite bipolar plates are lightweight, contributing to overall fuel cell system weight reduction, have excellent corrosion resistance and lower material and production costs, as well as a lower CO2 footprint [1]. On the other hand, they typically have lower electrical conductivity compared to metallic plates and require thicker designs (> 250 microns) to achieve the necessary mechanical properties. Graphite is typically used as the main conductive additive in polymer composite bipolar plates due to its excellent electrical / thermal conductivity and chemical stability. There are several types of graphite powders used in this application: ● Synthetic graphite: manufactured from fossil-based amorphous carbons (e.g. petroleum coke), this type provides more consistent quality and can be tailored for specific requirements ● Expanded graphite: created by treating natural graphite flakes with acid and heat, this type offers improved gas impermeability and higher conductivity but is more difficult to process due to its high specific surface area ● Natural graphite: mined and processed from natural deposits, this type offers good conductivity but it requires significant chemical / thermal purification to achieve sufficient purity (>99.9%) necessary in fuel cell systems Each type of graphite has its advantages and is chosen based on factors such as performance requirements, costs, and CO2 footprint. The selection of the appropriate graphite type is crucial for optimizing the efficiency and durability of bipolar plates in fuel cell applications. Our research is addressing the electrical conductivity in polymer composite bipolar plates by investigating novel graphite powders designed to optimize this critical property while maintaining the inherent advantages of polymer composites. 1. Scientific Approach Our approach focuses on the development and application of advanced graphite powders for polymer-based bipolar plates. We explored various graphite powders, including surfacemodified ones and a new biomass derived synthetic graphite, recently developed by Imerys, which achieves up to 60% reduction in CO₂ emissions compared to traditional synthetic graphite from fossil fuel-based raw materials [2]. The study employs a systematic evaluation of these graphite powders in thermoplastic polymers in both compression molded and injection molded bipolar plates, allowing for a comprehensive comparison of different manufacturing techniques and their impact on bipolar plate performance. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476241 A1116 Page 3/6 2. Experiments The following graphite types have been investigated in this study: ● standard synthetic graphite (TIMREX® KS75) ● biomass-derived synthetic graphite (SU-NERGY™ AGS75) ● surface-modified graphites (FC-NERGY™ BG202, RC1160) ● purified natural graphite (NG) The main properties of the investigated graphites are shown in Table 1. Graphite name Graphite type d10 (microns) d50 (microns) d90 (microns) BET (m2/g) KS75 Synthetic graphite 5 23 67 7.2 AGS75 Biomass derived synthetic graphite 5 24 67 7.7 BG202 Surface modified graphite 16 24 35 3.0 RC1160 Surface modified graphite 9 32 101 1.7 NG Purified natural graphite 11 46 113 3.2 Table 1: Selected properties of graphite tested in this study. The particle size distribution of the tested graphites is slightly different but in the same range with a d50 between 23-46 micrometers, while the specific surface area is very different with BET ranging from 1.7 to 7.7 m2/g. The BET is influenced by many factors like particle size distribution, particle morphology, porosity and surface properties. Highly filled polymer compounds and plates have been produced using two processes: 1. internal mixer + compression molding (lab-scale): graphite powder (80 wt%) and polypropylene (Moplen HP501L from Lyondell Basell, 20 wt%) were compounded in an internal mixer (HAAKE Rheomix 600 OS) for 5 min at 190°C at 100 rpm. 60 mm x 60 mm x 2 mm plates were prepared by compression molding using a LabTech Scientific LP-S-20 press (at 210°C, 70 bar). 2. twin-screw extruder + injection molding (pilot-scale): graphite powder (80 wt%) and polypropylene (PP412MN40 from Sabic, 20 wt%) were compounded by a twinscrew extruder (Leistritz ZSE 27 mm). The graphite was added via a side feeder in the polymer melt and the compounds were extruded at 300 rpm, at 200°C and dried by compressed air. The compounds were injection molded using a Billion Proxima 50T at 305°C on a 60 mm x 60 mm x 2 mm mold at 120°C. Electrical resistivity measurements were performed on 60 mm x 60 mm x 2 mm plates according to DIN 4880:2024-11 using an instrument developed by ZBT Institute (Germany). Through-plane area-specific resistance was measured at different pressures of up to 30 bar, and the bulk and contact resistance could be differentiated. One compression molded plate and all injection molded plates have been measured also after removing the polymer skin by abrasion (ca. 50 micrometers on each side). 3. Results 3.1 Internal mixer + compression molding As shown in Fig. 1 for the compression molded plates containing 80% BG202, the area specific resistance decreases with increasing pressure. This is due to the improved contact at higher pressures which reduces the contact resistance, while the bulk resistance is not affected by the pressure. After removing the polymer skin, the total resistance decreases EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476241 A1116 Page 4/6 due to a reduction of the contact resistance. This is due to the well-known skin effect (thin insulating polymer skin on the surface of the plate). One can notice that the total resistance at 20 bar (a pressure similar to what bipolar plates in a fuel cell stack are exposed to) is ca. 20% lower without skin (27 mOhm.cm2) compared to the plate with the skin (33 mOhm.cm2) but in the same order of magnitude. Therefore, for the plates with the other graphites, only plates with skin have been measured. Figure 1: Area specific resistance of the compression-molded plate containing 80% BG202 with skin (left) and without skin (right). As shown in Fig. 2, the type of graphite has a strong influence on the electrical resistance of the plates. Natural graphite (NG) gives the worst results with resistance > 200 mOhm.cm2 due to the strong alignment of the flaky anisotropic particles during compression molding. Synthetic graphites KS75 and AGS75 give better results (~60-100 mOhm.cm2) due to their more isotropic properties. The surface modified graphites BG202 and RC1160 show the best results (30-35 mOhm.cm2). This strong improvement is due to a modification of the surface properties of the graphite particles (proprietary Imerys technology). Figure 2: Total area specific resistance at 20 bar of compression-molded plates containing 80% graphite. 3.2 Twin-screw extruder + injection molding EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476241 A1116 Page 5/6 As shown in Fig. 3 for the injection molded plate containing 80% BG202, the area specific resistance decreases with increasing pressure. This effect is more pronounced for the plate with skin, due to the higher contact resistance. After removing the skin, the total electrical resistance is strongly reduced. This effect is much stronger than for compression molding, and at 20 bar the total resistance drops from 87 mOhm.cm2 to 20 mOhm.cm2. It is worth noting that also the bulk resistance is reduced after removing the skin, indicating a different alignment of the graphite particles close to the surface of the plate (as already demonstrated in the literature [3]). Figure 3: Area specific resistance of the injection-molded plate containing 80% BG202 with skin (left) and without skin (right). In Fig. 4 the results of the injection molded plates containing the different graphites are shown. In all cases, the resistance is strongly reduced after removing the skin and the results are in agreement with those obtained on compression molded plates. Also in this case, natural graphite NG has the highest electrical resistance (480 mOhm.cm2 with skin, 265 mOhm.cm2 without skin) and the surface modified graphites BG202 and RC1160 have the lowest resistance (~80 mOhm.cm2 without skin, 20 mOhm.cm2 without skin). Figure 4: Total area specific resistance at 20 bar of injection-molded plates containing 80% graphite EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476241 A1116 Page 6/6 3. Conclusions Our experiments revealed significant improvements in the performance of polymer-based bipolar plates using optimized graphite powders. The novel formulations, especially those incorporating surface-modified particles, demonstrated lower through-plane electrical resistance compared to standard synthetic or natural graphites. It is worth noting that biomass-derived synthetic graphite has better performance than conventional fossil fuelbased synthetic graphite. This enhancement in electrical conductivity was observed in both compression molded and injection molded plates, though variations in the extent of the skin-effect between the two manufacturing methods were noted. It is worth mentioning that the electrical resistance obtained with the surface-modified graphites at 80 wt% loading is very low, reaching 20 mOhm.cm2 which can typically be achieved at >85 wt% loading levels. Working at lower loading levels of graphite has clear advantages in terms of processability of the polymer compounds and formability into thin bipolar plates. By optimizing electrical conductivity while maintaining manufacturability, the new innovative graphites pave the way for the development of more efficient, lighter and thinner, and more durable fuel cell components. These advancements have important implications for both mobile and stationary power applications, potentially accelerating the adoption of fuel cell technology. Future research should focus on further refinement of formulations (e.g. use different graphites in the same composite, add carbon black as minor additive) and exploration of additional manufacturing techniques (e.g. continuous extrusion of plates) to fully realize the potential of these innovations in next-generation fuel cell systems and also in other applications (e.g. redox-flow batteries). References [1] Andreas van Sloun et al., “Life-Cycle Assessment of a Composite vs. a Metallic Bipolar Plate for Fuel Cell Applications: More Sustainable while Maintaining Same Performance Standards?”, SAE Technical Paper 2025-01-0298, preprint [2] Imerys press release (23. April 2025): https://imerys.com/media-room/pressreleases/imerys-introduces-su-nergy-sustainable-catalyst-free-graphite-advancing [3] A. Kiraly and F. Ronkay, Plastics Engineering, February 2014, p. 42-44 Keywords: EFCF2025, H2, Low-Temp. Fuel Cells & Electrolysers, Graphite, Bipolar plates, Fuel cells Remark: This work is licensed under Creative Commons Attribution 4.0 International