01 Introduction
Proton exchange membrane fuel cells (PEMFCs), as devices that can directly convert chemical free energy into electrical energy, produce only water as a byproduct after reaction. They offer advantages such as energy efficiency, high energy conversion rate, high reliability, rapid start-up at room temperature, and long service life [1]. These features give PEMFCs broad application prospects and market potential in stationary applications (e.g., communication base stations, combined heat and power), mobile applications (e.g., automobiles, marine vessels), and portable applications (e.g., computers, cameras). They are hailed as the fourth power generation method after thermal, hydro, and nuclear power [2].
Bipolar plates (BPs), as key components of PEMFCs, function primarily to separate and distribute oxidant and fuel, conduct current, support the membrane electrode assembly, and regulate internal stack temperature [3]. To meet fuel cell operating requirements, BPs must possess excellent gas impermeability, good electrical conductivity and corrosion resistance, adequate thermal conductivity, and mechanical properties [3]. Meanwhile, BPs currently account for about 80% of the weight and volume of a PEMFC and about 50% of its cost. While ensuring functional requirements, making BPs as thin and light as possible is of great significance for improving PEMFC power density and reducing production costs. Practice has shown that optimizing bipolar plate performance could increase stack power density by about 20%, reaching 6–9 kW/L [3]. Therefore, improving BP materials and fabrication processes is of great importance for enhancing PEMFC performance.
DOE performance targets for bipolar plates (table omitted)
PEMFC structure diagram (figure omitted)
02 Classification and Application Potential Analysis of Bipolar Plates
Based on material composition, bipolar plates are mainly divided into four categories: graphite bipolar plates, flexible graphite bipolar plates, metallic bipolar plates, and composite bipolar plates [4].
2.1 Graphite Bipolar Plates
Graphite was among the earliest substrate materials developed for BPs, featuring low density, good corrosion resistance, and low contact resistance with the gas diffusion layer. However, graphite BPs have relatively high porosity, making it difficult to meet gas-tightness requirements. Additionally, graphite is low in strength and brittle, so the finished plates are significantly thicker than metallic BPs [2].
Research on graphite BPs has focused on graphite materials and impregnation methods. Although some progress has been made in improving gas tightness and structural strength, they still fall short of PEMFC operational requirements [2].
By material type, graphite BPs can be divided into two categories:
Rigid graphite BPs, also called machined graphite plates, are mainly made from byproducts of fossil fuels, processed through calcination, vacuum impregnation, high-temperature graphitization, and then machined by CNC to form specific flow channels. Each set requires CNC machining, resulting in relatively high costs. These are mainly used for design development, and their market share in mass production is gradually being replaced by other types [5].
Flexible graphite BPs, also known as molded plates or expanded graphite BPs, are generally produced by mixing expanded graphite with polymer resin powder followed by molding, or by impregnating pre-pressed expanded graphite sheets. Flexible graphite BPs are lightweight, corrosion-resistant, and conductive, making them an ideal material, but they have deficiencies in mechanical properties and gas tightness [2].
2.2 Metallic Bipolar Plates
Metallic BP materials mainly include aluminum, stainless steel, titanium, and nickel alloys [3].
Metallic BPs offer good electrical conductivity, gas tightness, and bending strength. Moreover, due to the excellent ductility of metals, thin plates with flow field structures can be fabricated via stamping, significantly improving PEMFC volumetric power density [2]. However, metallic BPs are susceptible to corrosion in the PEMFC environment (pH ~2–3, T = 80–100 °C), releasing metal ions that adversely affect fuel cell operation. Meanwhile, formation of a passive layer on the surface leads to a marked increase in contact resistance, severely limiting long-term reliability [2].
Current main strategies to improve corrosion resistance of metallic BPs include applying corrosion-resistant coatings and improving substrate corrosion resistance [2]. Coatings can significantly enhance corrosion performance, but balancing conductivity, process complexity, and cost is challenging. Moreover, corrosion tests are typically short-term, so results may not reflect real-world long-term performance [2]. Current research shows that single coatings also have limitations; for example, titanium alloys have good corrosion resistance due to their oxide film, but this increases contact resistance, requiring surface conductivity enhancement treatments. Corrosion resistance and cost issues remain major barriers to large-scale adoption of metallic BPs [2].
Lyu Jianxiang [6] et al. systematically reviewed research progress on transition metal carbide coatings, transition metal nitride coatings, amorphous carbon coatings, graphene-based coatings, conductive polymer coatings, and high-entropy alloy coatings for metallic BPs in fuel cells. They pointed out that multilayer coatings with interlocking structures could potentially improve both corrosion resistance and conductivity. Recent studies suggest that incorporating corrosion-resistant elements into coatings is also a viable approach. Therefore, the combination of multilayer coatings and elemental doping is expected to yield better corrosion-resistant coatings for metallic BPs.
2.3 Composite Bipolar Plates
Composite plates consist of two or more materials, with combined processes optimizing mechanical, corrosion, and electrical properties. Composite plates can be classified into structural composites and process composites [7]. By material type, composite BPs can be divided into two major categories: polymer resin/carbon composites and graphite/metal composites [7].
2.3.1 Polymer Resin/Carbon Composite Bipolar Plates
Composite graphite bipolar plates are a new type, made primarily from organic polymer resins and carbon-based conductive fillers. The resin matrix enhances mechanical properties and bonds the conductive fillers, serving as the main research focus for improving gas tightness and flexural strength. Conductive fillers, represented by graphite, interconnect to form conductive networks [2].
Conductive fillers generally include carbon black, carbon fibers (CF), expanded graphite, carbon nanotubes, synthetic graphite (SG), and graphene; polymer resins include polypropylene, polyetheretherketone (PEEK), polyethylene terephthalate (PET), epoxy resins, and phenolic resins [8].
By adjusting the composition and mass ratio of resin and conductive fillers, combined with graphite modification or resin additives, the various properties of composite graphite BPs can be tuned and optimized. In addition, advanced preparation techniques and post-processing-such as controlling the orientation and dispersion of graphite fillers, and optimizing graphite/resin interfacial properties-directly affect the macro- and microstructures of the molded composite plates, thereby directly impacting their performance [2].
Currently, domestic production technology for composite BPs is not yet mature, mainly due to incomplete localization of raw material formulations, lack of large-scale continuous production, and high costs. Therefore, finding low-cost raw materials, optimizing formulation ratios and processing conditions, and shortening production cycles remain key research directions [9].
2.3.2 Graphite/Metal Composite Bipolar Plates
Graphite/metal composite plates use a metallic layer as the separator and porous thin carbon plates or graphite plates (made by injection molding and baking) as flow field plates, bonded with conductive adhesive [7]. Metals offer good conductivity, high strength, and gas impermeability, while graphite provides corrosion resistance. Thus, graphite mainly serves for conduction and flow channels without needing to bear sealing or reinforcement functions, allowing complementary advantages [7].
Metal-based composite plates combine the advantages of both graphite and metallic plates, improving overall performance. However, their complex structure and fabrication processes make mass production difficult, and production costs are far higher than those of carbon-based composites. Despite challenges for general PEMFC promotion, they have advantages for special applications [7].
03 Research Progress on Flexible Graphite and Graphite Composite Bipolar Plates
From the above, flexible graphite and graphite composite BPs exhibit more balanced overall performance and stronger development potential. According to BP performance requirements, key indicators include gas permeability, electrical conductivity, contact resistance, flexural strength, and corrosion rate, which are the main research directions for BPs.
Comparison of advantages and disadvantages of different types of BPs (table omitted)
3.1 Effect of Polymer Modification on Conductivity and Flexural Strength of Graphite Composite Bipolar Plates
Zeng Haodong [8] et al. modified polyimide (PI) resin with polyetheretherketone (PEEK) resin, using expanded graphite as the primary conductive filler, and prepared composite bipolar plates for PEMFCs by compression molding. They investigated the effects of resin content and graphene addition on composite BP performance. Results showed that with a fixed resin mass fraction of 40% and a PI:PEEK mass ratio of 1:1.25, the expanded graphite/PI-PEEK composite BP samples achieved optimal electrical conductivity (181.8 S/cm) and flexural strength (43.49 MPa), representing a 64.7% increase in conductivity over pure PI resin samples.
Li Hang [10] et al. used natural flake graphite (NG) as the conductive aggregate and phenolic resin (PF) and unsaturated polyester resin (UPR) as binders, preparing NG/PF/UPR composite BPs by compression molding. They studied the effects of PF/UPR mass ratio, resin content, and molding conditions on conductivity and flexural strength. Results indicated that under conditions of 20 wt% resin content, PF/UPR mass ratio of 6:1, pressing temperature 100 °C, molding pressure 30 MPa, molding temperature 180 °C, and molding time 2 h, the NG/PF/UPR composite achieved electrical conductivity of 336.1 S/cm and flexural strength of 46.5 MPa. Compared to NG/PF composite under the same conditions, conductivity increased by 134.9% and flexural strength by 21.2%.
3.2 Effect of Graphite Substrate Thickness on Overall BP Performance
Wang Dengke [11] et al. used expanded graphite substrates of 4 mm and 6 mm thickness to produce single-sided flow-channel graphite BPs of 1 mm thickness, studying the influence of substrate thickness on molding and performance of composite flexible graphite plates. Results showed that the 4 mm substrate produced plates with better channel/rib geometry, superior dimensional accuracy, gas permeability below the DOE target of 2×10⁻⁶ cm³·s⁻¹·cm⁻², flexural strength of 25.4 MPa, electrical conductivity of 333 S·cm⁻¹, and contact resistance below 2.37 mΩ·cm². They also exhibited excellent corrosion resistance; after 3000 s at high potential (1.6 V), the corrosion current density remained stable. Thus, 4 mm substrate is a good choice for molding 1 mm flexible graphite plates, yielding optimal performance meeting DOE requirements. The authors hope this work will promote industrialization of flexible graphite plates and further development of the fuel cell industry.
3.3 Effect of Graphite Particle Size on Flexural Strength and Conductivity of Bipolar Plates
Kang [12] et al. showed that the flexural strength of composite BPs increases with decreasing bulk graphite particle size, but conductivity decreases. Bulk graphite particles connect via point contacts, whereas flake graphite forms face-to-face contacts, facilitating conductive network formation and electron transport.
3.4 Effect of Carbon Black and Graphite Particle Size Optimization on Conductivity
Lim [13] et al. sprayed a mixture of natural graphite powder, carbon black, and methyl ethyl ketone solvent onto epoxy resin/carbon fiber prepregs, dried them, and then laminated with graphite foil on both sides before hot pressing to form composite BPs. Due to their smaller particle size, carbon black dispersed more uniformly and was more effective than natural graphite powder in increasing conductivity. Gautam [14] et al. chemically intercalated natural flake graphite in a mixed solution of KMnO₄, HClO₄, and HNO₃ for 1 min, then irradiated with a microwave oven to produce expanded graphite with a maximum specific volume of (570±10) mL·g⁻¹. They subsequently prepared expanded graphite/carbon black/graphite microparticles/phenolic resin composite BPs; single-cell tests showed better performance than composites using only expanded graphite as conductive filler, attributed to carbon black and fine graphite powder filling voids between conductive fillers.
3.5 Effect of Vacuum Impregnation on Conductivity and Flexural Strength of Bipolar Plates
Zhan Zize [3] et al. studied the effects of epoxy resin diluent addition, temperature, and graphite pore structure on resin impregnation amount during vacuum impregnation, and evaluated the electrical conductivity, flexural strength, and gas tightness of the resulting fuel cell BPs. Results indicated that increasing diluent content and temperature reduced resin viscosity, enhancing impregnation capacity and allowing resin to fill finer pores. However, some pores remained unfilled; pores below 0.1 μm in expanded graphite were difficult to fully penetrate, preventing complete filling. The optimal vacuum impregnation temperature for epoxy resin/diluent system was 50 °C, and the optimal diluent (ethylene glycol diglycidyl ether) addition was 15 wt%. The obtained BP (EGP-H) had in-plane conductivity of 340.12 S/cm, flexural strength of 41.52 MPa, and helium permeability of 5.4×10⁻⁷ cm³·cm⁻²·s⁻¹.
04 Summary of Main Approaches to Improve Graphite Bipolar Plate Performance
Regarding the effects of raw material types, ratios, and molding process conditions on BP performance, four main aspects are identified:
(1) Resin binds conductive particles and forms a three-dimensional network after crosslinking, improving mechanical properties. However, excessive resin reduces conductivity. Therefore, resin content must be optimized to balance mechanical and electrical performance.
(2) When graphite is the main conductive filler, flake graphite forms conductive pathways more readily than bulk graphite. Particle size affects both conductivity and mechanical properties, and a proper particle size distribution should be determined through optimization.
(3) Small additions of carbon black, carbon fibers, or carbon nanotubes can effectively fill voids and increase conductive pathways. Meanwhile, they can form covalent bonds with resin functional groups or directly improve toughness, thereby enhancing flexural strength, so that both conductivity and mechanical properties of composite BPs are improved.
(4) In compression molding, it is necessary to continuously optimize raw material ratios and hot-pressing conditions, increase parallel operations, shorten production cycles, and achieve continuous flow production.