Bipolar plates are core components in the stacks of proton exchange membrane fuel cells and flow batteries. They have multiple jobs: carrying electric current, spreading reaction media, providing structural support, and separating the positive and negative sides. The choice of material for bipolar plates directly affects the power density, cycle life, and system cost of the battery. Among the many technical approaches, carbon composite bipolar plates and flexible graphite bipolar plates are two of the most representative options. They differ greatly in material makeup, production methods, and performance. These performance differences come from differences in their microscopic structure. Flexible graphite bipolar plates are made of worm‑like graphite particles that are interwoven. This structure gives the material excellent electrical conductivity and some flexibility, but it also has a common problem—higher porosity. Carbon composite bipolar plates can be thought of as “plastic skeleton + conductive filler” composite materials. The polymer resin acts as the continuous phase, providing great mechanical strength and dense structure, while the added graphite, carbon fibers, and other materials act as the dispersed phase and provide electrical conduction. By adjusting the amount, type, and distribution of the fillers, it is possible to achieve a flexible balance between electrical performance and mechanical performance.

Introduction to the two types of bipolar plates

1. Carbon composite bipolar plates

Carbon composite bipolar plates—also called carbon‑plastic composite bipolar plates or carbon‑polymer composite bipolar plates in the industry—are battery components made by combining conductive carbon fillers with a polymer matrix. Their material makeup can be summarized as “conductive skeleton + polymer binder.” The conductive fillers include carbon materials such as graphite, carbon black, carbon fibers, and carbon nanotubes. The polymer matrix uses thermoplastics (like polypropylene PP, polyethylene PE, polyvinylidene fluoride PVDF) or thermosetting resins (like phenolic resin, epoxy resin, vinyl ester resin). In terms of mixing ratio, carbon materials usually account for 70% to 90% of the total raw materials. As for manufacturing, carbon composite bipolar plates are mainly shaped by compression molding, injection molding, or extrusion. Compression molding is suitable for directly forming plates with flow channels, eliminating the need for later CNC engraving. Extrusion is better for making sheets in a continuous process, with higher efficiency. This “one‑shot molding” processing gives carbon composite bipolar plates a big advantage in mass production. In recent years, domestic companies like XNHY have made many technical breakthroughs in the performance, cost, and durability of carbon‑plastic bipolar plates.

2. Flexible graphite bipolar plates

Flexible graphite bipolar plates are made from expandable graphite (expanded graphite) through special processes. The production steps are roughly as follows: high‑purity natural flake graphite is first expanded at high temperature to form a fluffy, porous expanded graphite, then rolled into graphite layers that resemble spring‑like structures. After that, vacuum impregnation, washing, baking, and other steps are carried out to obtain finished products with high strength, good sealing, and high conductivity. In terms of material composition, flexible graphite bipolar plates are not pure graphite products. Modern flexible graphite bipolar plates usually need resin added for reinforcement. One patented formula says flexible graphite accounts for 80 to 90 parts, and reinforcing resin accounts for 65 to 75 parts. The reinforcing resin is often a blend of modified phenolic resin and modified epoxy resin. Some researchers also use expanded graphite and acrylate resin as raw materials, and make double‑sided flow‑channel flexible graphite bipolar plates through vacuum mold compression, impregnation, and cleaning.

In material classification, graphite bipolar plates can be further divided into hard graphite and flexible graphite types. Hard graphite bipolar plates have poorer processability and require stricter processing conditions. They are also more brittle and can easily break during stack assembly and transport. In contrast, flexible graphite bipolar plates, because they combine good corrosion resistance, high conductivity, and a certain amount of flexibility, have become one of the most widely used products in proton exchange membrane fuel cells.

Application areas of the two types of bipolar plates

1. Applications of carbon composite bipolar plates

Carbon composite bipolar plates, with their advantages of high mechanical strength, ability to be mass‑produced, and relatively low cost, have become the mainstream choice in commercial applications.

Flow battery energy storage is the most important market for carbon composite bipolar plates. The bipolar plates commonly used in commercial flow battery stacks are exactly carbon composite materials. This type of plate has both the high mechanical strength and easy processability of metals and the corrosion resistance of graphite. New installed capacity of all‑vanadium flow batteries reached about 1073 MW in 2025, accounting for about 96.4% of the market. Carbon‑plastic composite bipolar plates, as key components in the stacks, dominate this fast‑growing market. Among material choices, PP‑based carbon‑plastic composite bipolar plates are currently one of the most widely used solutions.

Proton exchange membrane fuel cells are also an important application field for carbon composite bipolar plates. Carbon‑based composite bipolar plates, with their excellent conductivity, corrosion resistance, long life, and low cost, have become the industry benchmark. Some companies focus on developing and producing advanced carbon‑based composite materials for hydrogen fuel cells and flow batteries, and their products have gained market recognition.

In addition, for large‑scale energy storage stations (10 MW and above) where cost and consistency are critical, carbon composite bipolar plates—thanks to the good uniformity and cost advantages from compression molding mass production—have become the mainstream choice.

2. Applications of flexible graphite bipolar plates

Flexible graphite bipolar plates overlap with carbon composite plates in some application areas, but they focus on different needs.

Proton exchange membrane fuel cells are a traditional stronghold for flexible graphite bipolar plates. Because they have excellent corrosion resistance and are economical, flexible graphite bipolar plates have become the most widely used type in proton exchange membrane fuel cells. In hydrogen fuel cells, they are the key components for conducting electricity, transferring heat, and sealing the stack.

Flow battery energy storage is also an important application direction for flexible graphite bipolar plates. Flexible graphite plates combine the advantages of metal plates and graphite plates—they are corrosion‑resistant and lightweight. Some believe that flexible graphite bipolar plates, with high conductivity and some toughness, are the more ideal bipolar plates for flow batteries. However, their use in flow batteries still faces challenges, such as a lack of industry standards and uneven product quality. In commercial vanadium‑based electrolytes, after long‑term immersion of 2664 hours, flexible graphite bipolar plates showed no swelling or powder shedding, and the plate appearance and performance remained as before. This provides real‑world evidence for their long‑term use in flow batteries.

Laboratory and small demonstration projects (below 1 MW) are also typical applications for flexible graphite bipolar plates. They are valued for excellent conductivity, strong acid resistance, and good chemical stability, making them suitable for high‑precision situations where performance is critical.

In low‑power applications, such as drones and two‑wheelers, air‑cooled graphite bipolar plates also see some use. In addition, flexible graphite bipolar plates can be adapted to special conditions like high‑temperature fuel cells.

Performance comparison of the two types of bipolar plates

The fundamental differences in material makeup and microstructure mean that the two types of bipolar plates each have their own strengths in various performance indicators.

1. Electrical conductivity

Flexible graphite bipolar plates have a clear advantage in electrical conductivity. Research data show that expanded graphite/acrylate composite flexible graphite bipolar plates can reach an electrical conductivity of 332 S·cm⁻¹. Some advanced products even exceed 400 S/cm. Literature reports that the contact resistance of flexible graphite bipolar plates can be as low as ≤2.25 mΩ·cm².

Carbon composite bipolar plates show wider variation in conductivity depending on the formulation. For PP‑based carbon‑plastic composite plates, volume resistivity can be divided into different grades, corresponding to conductivity from ≥10 S/cm to ≥250 S/cm. Although high‑end carbon composite products are steadily improving in conductivity, overall they are still slightly lower than pure graphite or flexible graphite products.

2. Mechanical properties

Carbon composite bipolar plates stand out in mechanical strength. Their mechanical strength is usually 2 to 3 times that of pure graphite plates. PP‑based carbon‑plastic composite plates can have flexural strength >30 MPa and tensile strength >25 MPa. Some advanced products even reach flexural strength over 40 MPa.

Flexible graphite bipolar plates have relatively lower mechanical strength. Typical products have tensile strength ≥25.0 MPa and flexural strength ≥30.0 MPa. However, by adding reinforcing resins and optimizing processes, the mechanical performance of flexible graphite plates is continually improving. Some studies report that modified flexible graphite bipolar plates can reach flexural strength of 28 MPa. One big advantage of flexible graphite plates is their toughness—they are not as prone to brittle fracture as hard graphite.

3. Density and liquid barrier performance

Carbon composite bipolar plates perform better in density and liquid barrier properties. Carbon‑plastic composite plates have much better liquid resistance than flexible graphite plates. This is because the polymer matrix fully coats the conductive fillers, forming a relatively dense composite structure.

Flexible graphite bipolar plates have relatively poorer density. Expanded graphite plates are naturally fluffy and porous, so poor liquid barrier is an inherent weakness. However, this defect can be improved by high‑density pressing, resin impregnation, carbon fiber reinforcement, or frame‑matching designs. Companies like Huanhua Hydrogen Energy use high‑conductivity nano‑composite impregnation formulas. They introduce a high‑conductivity micro‑structure network into the insulating impregnation resin, “building an electronic superhighway” that significantly reduces resistance while also improving density.

4. Corrosion resistance

Both types perform well in corrosion resistance. Expanded graphite/acrylate composite flexible graphite bipolar plates have a corrosion current density of only 0.85 μA·cm⁻². After 4500 seconds of constant‑potential (1.2V) corrosion, the corrosion current density remains below 1 μA·cm⁻². Carbon composite bipolar plates also have much better corrosion resistance than ordinary metal plates, but in extremely harsh acidic environments, their long‑term stability may be slightly lower than flexible graphite products.

5. Density and thickness

Carbon composite bipolar plates have lower density. PP‑based material has a density of only 0.90 to 0.94 g/cm³, much lower than graphite’s 1.8 to 2.0 g/cm³. PP carbon‑plastic composite plates have a finished density of about 1.4 g/cm³. Flexible graphite bipolar plates have relatively higher density, with bulk density usually ≥1.7 g/cm³. In terms of thickness, both types can be made as thin as about 0.6 mm.

6. Cost and mass production capability

Carbon composite bipolar plates have clear advantages in cost and mass production. Compared with pure graphite plates, carbon composite plates can reduce cost by 30% to 40%. Processes like compression molding, injection molding, and extrusion are suitable for large‑scale continuous production with good product consistency. Flexible graphite bipolar plates are relatively more expensive. High‑end products have long depended on imports and are costly. However, as domestic companies continue to push for localization, costs are gradually decreasing. Companies like Huanhua Hydrogen Energy have built a complete technology system that includes high‑conductivity nano‑impregnation formulas, flow‑field structure simulation design, and lean mass‑production processes for flexible graphite plates.

In actual selection, projects that pursue the highest conductivity and corrosion resistance tend to prefer flexible graphite bipolar plates. For projects that emphasize large‑scale production, cost control, and mechanical reliability—such as large energy storage or commercial fuel cells—carbon composite bipolar plates are the more mainstream and economical choice. As materials science and manufacturing processes continue to advance, these two technical approaches are learning from each other, complementing each other, and their performance boundaries are steadily expanding.