As a key component in RFBs, the BP primarily functions as a barrier separating the positive and negative electrolytes^[52]. Thermoplastic resins exhibit fluidity upon softening at elevated temperatures, allowing them to penetrate forming defects in the interior or on the surface of the BP, thereby reducing the risk of acidic electrolytes infiltrating the plate and causing internal swelling. At the same time, such flowability is essential for replicating flow field geometries but is highly sensitive to filler loading and dispersion.

Kim et al. spray thermoplastic PTFE in emulsion form onto expanded graphite powder^[53], as shown in Figure 3. The mixture is first compression-molded into a preform and then processed through multi-step rolling for final shaping. BPs containing 6% or 10% PTFE exhibit almost no observable swelling after cycling compared with flexible graphite plates. This result indicates that the thermoplastic resin in conventional thermoplastic CBPs effectively suppresses swelling. In addition, Kang et al. use a similar fabrication route^[54], further confirming that PVDF, after hot pressing, effectively repairs the surface and cross-sectional microcracks in expanded graphite. It also suggests that appropriate resin distribution can improve structural integrity during forming, thereby helping maintain flow field stability.

Meanwhile, the electrical conductivity of CBPs does not increase linearly with increasing carbon content. Instead, when the content of conductive carbon materials reaches the percolation threshold, the electrical properties of the composite undergo a transition from an insulating to a conductive state. The percolation threshold of conductive fillers is strongly influenced by the particle size, morphology, and distribution of the fillers within the polymer matrix^[55]. These parameters simultaneously affect rheological behavior, which directly determines the formability of flow field structures during processing. Many studies introduce a large amount of conductive filler to establish sufficient conductive pathways^[56]. However, excessive filler loading inevitably increases the risk of fracture in the BP. To address this issue, Liao et al. developed a ternary conductive system composed of graphene, graphite powder, and chopped carbon fibers, which forms an efficient conductive network at relatively low carbon content^[57]. At a carbon content of only 25 wt%, both the electrical and mechanical properties meet the 2020 Department of Energy (DOE)^[58] standards. Importantly, reducing filler content also helps retain sufficient flowability, which is beneficial for accurate flow field replication. Jiang et al. further designed cactus-like carbon nanofibers^[59]. In this structure, fibers radiate outward from a catalytic core, forming a three-dimensional clustered architecture. After graphitization at 2,800 °C, graphitized cactus-like carbon nanofibers (G-C-CNF) are obtained, which exhibit significantly improved graphitization and structural integrity. This architecture disrupts resin-rich surface layers, thereby improving both electrical contact and the effective exposure of flow field structures, as shown in Figure 4.

In addition, Qin et al. optimize both electrical and mechanical properties at low carbon content by incorporating multi-dimensional carbon materials^[60]. Zero-dimensional filler Ketjen black (KB), one-dimensional filler carbon nanotubes (CNTs), and two-dimensional filler flake graphite (FG) work synergistically to construct a three-dimensional continuous conductive network. Such a multi-dimensional filler design not only enhances conductivity but also improves dispersion uniformity, which is crucial for stable forming of intricate flow field geometries. Moreover, Zhang et al. introduce a new copolymer made from ethylene and propylene monomers^[61], known as polypropylene elastomer, which improves the processability of the composite material while maintaining excellent toughness. Table 1 is a comprehensive summary of the above-mentioned literature.

The design of fillers is typically aimed at improving the performance of the BP by adjusting the ratio of carbon materials to resin. However, electrical conductivity and mechanical strength often exhibit a trade-off relationship^[62]. Therefore, simply adjusting the filler ratio cannot achieve a balance between electrical and mechanical properties. Such compositional tuning not only affects intrinsic properties but also directly influences the rheological behavior and forming capability of the composite during flow field fabrication. In addition, during the forming process, differences in flowability between the resin and fillers often lead to resin enrichment at the surface or interface. This phenomenon may interrupt conductive pathways and increase interfacial contact resistance. Filler design alone is therefore fundamentally insufficient to suppress such resin enrichment from a structural perspective.

To address these challenges in thermoplastic CBPs for VRFB, researchers have proposed a variety of structural design innovations. Onyu et al. develop a layered composite structure to achieve a synergistic performance enhancement^[63]. The structure adopts a stacked configuration consisting of a conductive composite layer [thermoplastic vulcanizates (TPV) combined with synthetic graphite], a woven carbon fiber fabric, and a pyrolytic graphite sheet. Such layer-wise functional separation enables local optimization of the forming process, thereby improving both structural integrity and the accuracy of flow field replication. As shown in Figure 5A, scanning electron microscopy (SEM) images reveal no obvious interfacial gaps between the layers, indicating strong interfacial bonding. Moreover, the layered structure BP has good thermal stability at high temperatures [Figure 5B]. Liu et al. replace powder-type conductive fillers with a carbon felt framework to construct a continuous conductive network^[39]. Thermoplastic polyvinyl fluoride (PVDF) forms a dense layer between carbon felt through solution impregnation followed by secondary hot pressing. In this design, the pre-constructed three-dimensional conductive framework reduces the dependence on high filler loading, thereby improving processability and enabling more stable formation of flow field structures. In addition, Pavel et al. use graphite foil as a conductive substrate and impregnate fluorinated polymers into the existing pores to construct a three-dimensional conductive pathway structure^[64]. After fluoropolymer impregnation, the graphite on the surface layer exhibits a certain tendency toward oriented distribution.

With increasing demands for the stability of CBPs, researchers have begun to refocus on CBPs based on thermosetting resin matrices. This shift is not only driven by performance considerations but also by the need for improved dimensional stability in flow field structures during long-term operation. Unlike thermoplastic resins, thermosetting systems form dense crosslinked networks during the curing process, which stabilize the distribution of conductive fillers even at relatively low resin content. Meanwhile, their relatively low thermal expansion coefficient and excellent structural rigidity enable superior dimensional stability and fatigue resistance during long-term operation. Such characteristics are particularly advantageous for preserving the geometric accuracy of pre-formed flow field structures under compression and thermal fluctuations. For example, Li et al. prepare phenolic-resin-based BPs by vacuum impregnation of expanded graphite (EG)^[65]. The uniform penetration of resin between graphite layers significantly enhances interlayer bonding strength and blocks pore fields, enabling the plate to maintain a continuous conductive network while exhibiting excellent flexural strength and gas tightness. In addition, Choe et al. propose a new strategy by incorporating polyaniline (PANI) conductive particles into epoxy resin^[66], as shown in Figure 6. PANI particles are sprayed onto the surface of the carbon fiber/epoxy composite system, forming localized conductive pathways within the resin matrix. The introduction of an appropriate amount of PANI significantly reduces contact resistance while maintaining good corrosion resistance and electrochemical stability. Such localized conductive design reduces the dependence on high overall filler loading, which is beneficial for maintaining processability during forming.

Component regulation alone cannot fully resolve issues such as the intrinsically low conductivity of the resin phase, interfacial stress concentration, and high surface resistance. More importantly, it cannot fundamentally address limitations in flow behavior and forming adaptability during flow field fabrication. Therefore, researchers further shift their focus to structural design, aiming to achieve synergistic performance enhancement by optimizing both macroscopic structures and interfacial architectures.

In research about thermosetting composite BPs, the structural design consistently emphasizes interfacial bonding, electrical conductivity, and long-term stability. Kim et al. have conducted a series of studies in this field^[67]. To address crack issues in expanded graphite (EG) coatings, the team proposed a surface crack-closure method. After molding, compressive stress is applied to close microcracks in the graphite coating, thereby preventing penetration of acidic electrolyte. Such post-treatment improves structural integrity but also highlights the difficulty of controlling defects during the initial formation of flow field surfaces. In addition, Soohyun et al. first proposed integrating an E-glass/epoxy resin flow frame with a carbon fiber/epoxy resin matrix composite into a unified structure to eliminate the interface leakage inherent in traditional stacked components^[68]. However, the difference in thermal expansion coefficients between the two composite materials leads to severe residual curing stresses. Structural warpage and local buckling are effectively reduced by designing an intelligent curing cycle that carefully controls the curing temperature and cooling stages. Lee et al. proposed the soft-layer method^[69], in which a flexible release film is inserted between the prepreg and the mold during compression molding. Under pressure, the surface resin is squeezed out, exposing the conductive carbon fibers. This design significantly reduces area-specific resistance (ASR) without requiring additional graphite coating, while maintaining high mechanical strength and good gas tightness. It also establishes a processing foundation for subsequent multilayer composite structures. Based on the Soft Layer Method, the matrix is further expanded from conventional epoxy resin to a fluorinated elastomer system, while nano-carbon fillers are introduced to form a multiscale conductive network^[70]. The excellent acid-alkali resistance and flexibility of fluorinated elastomers enable the composite plate to maintain low contact resistance and excellent mechanical stability even in highly corrosive environments; their enhanced flexibility also improves deformation compatibility during forming, which is beneficial for reducing defects in complex flow field geometries. To further reduce manufacturing costs and expand application potential, the Soft Layer Method is also applied to a non-woven carbon felt/epoxy system^[71], enabling the fabrication of large-area BPs under low-pressure molding conditions. The 3D interconnected pore structure of carbon felt can form a stable vertical conductive path even under low pressure conditions, thereby reducing the sheet resistance to 22 mΩ·cm². Such low-pressure forming conditions are advantageous for scaling up manufacturing. However, they may also limit the replication accuracy of fine flow field structures due to insufficient compaction. In addition, Choe et al. evaluate the electrochemical corrosion durability of different graphite coatings through electrochemical aging tests^[72]. The results indicate that pyrolytic graphite, due to its highly oriented structure and stronger van der Waals bonding, exhibits significantly higher durability than conventional expanded flake graphite. Table 2 summarizes the main innovative points and conclusions of the above-mentioned achievements.

To further overcome the structural limitations of conventional stacks, many researchers propose innovative structural design strategies. These strategies aim not only to enhance conductivity and integration, but also to retain the formability required for reliable flow field fabrication. Choe et al. develop a CBP with an integrated current collector structure^[73], as shown in Figure 7. Through a co-curing process, a carbon fiber/epoxy prepreg and a metal current collector are simultaneously cured during a single hot-pressing step, forming a dense and pore-free continuous conductive network. This structure effectively eliminates the resistance increase associated with conventional bolted or conductive adhesive connections between the plate and the current collector, while minimizing interfacial contact resistance. At the same time, the integrated structure reduces interfacial discontinuities.

Subsequently, Choe et al. further construct a CBP with a “fiber spreading effect” using spread-tow fabric^[74]. Traditional one-way fabrics or thick woven fabrics frequently exhibit discontinuous in-plane conductive paths due to fiber undulation and resin retention. Spread-tow prepregs with fiber bundle sizes of 10k and 20k are prepared by hand weaving, significantly reducing thickness undulation and resin-rich regions at fiber crossover points. Such structural uniformity improves local flow behavior during forming, which is essential for achieving high-precision flow field structures. Combined with the Soft Layer Method, excess resin is squeezed out during the compression molding process, thereby achieving complete exposure of carbon fibers while maintaining high. In addition, Jeong et al. propose a horizontally aligned carbon nanotube /epoxy nanocomposite BP (HACN-BP) structure^[75]. In this design [Figure 8], vertically aligned CNT arrays are rearranged via a “knocking-down” process, allowing the nanotubes to form a highly oriented, dense network in the in-plane direction, thereby establishing bidirectional conductive pathways. The CNT network significantly reduces electrolyte penetration and side reactions while maintaining high mechanical strength, demonstrating the great potential of controllable nanoscale conductive architectures in BP engineering.

Research trends indicate significant differences in the research focus between thermoplastic and thermosetting composite bipolar plates (CBPs). Studies on thermoplastic systems primarily concentrate on optimizing filler design, whereas those on thermosetting systems place greater emphasis on innovations in structural design. This difference primarily stems from the fundamentally distinct inherent properties, interfacial behaviors, and processing constraints of the two matrix materials. Thermoplastic and thermosetting composite materials exhibit essential differences in their processing behavior that directly influence the patterns of flow field formation. Thermoplastic resins soften and flow when heated, enabling optimal mold filling and high molding efficiency. This makes them particularly suitable for replicating complex flow field geometries and ideal for continuous manufacturing processes. However, a high filler content significantly increases melt viscosity, thereby restricting fluidity and potentially leading to incomplete filling or reduced accuracy in fine flow field characteristics. In contrast, thermosetting resins undergo an irreversible curing process: their fluidity is limited during processing, but they exhibit excellent dimensional stability after curing, which helps maintain the geometric shape of the formed flow field. However, insufficient fluidity during the molding process makes precise replication of complex or micro-scale structures challenging; additionally, curing shrinkage and internal stresses may cause runner deformation or defect formation. Thermoplastic resins (such as polyvinylidene fluoride and polyetheretherketone) typically require elevated temperatures and pressures during molding, which can lead to uneven distribution of conductive fillers within the polymer melt, resulting in discontinuous conductive networks that reduce electrical conductivity and impair battery performance^[74-77]. Furthermore, a significant difference may exist between the thermal expansion coefficient of thermoplastic composites and that of carbon fillers, which can easily induce interfacial micro-cracks during operating temperature cycles, thereby increasing gas permeability. Weak interfacial adhesion is another common issue, leading to a marked reduction in mechanical strength during humidity cycling - for instance, tensile strength may decrease by more than 20%^[78,79]. Thermosetting resins (such as phenolic and epoxy resins) form cross-linked structures during curing, which may restrict the rearrangement and densification of conductive fillers, resulting in composite materials that still do not fully meet battery operating conductivity requirements. Although thermosetting resins generally exhibit good dimensional stability, filler orientation and segregation during injection molding can lead to anisotropic performance and batch-to-batch inconsistencies in composite bipolar plates, posing challenges to large-scale production and long-term operational reliability^[80,81]. Meanwhile, shrinkage during curing may generate stress at the interface, compromising both interfacial bond strength and the material’s long-term durability. In a word, whether using thermoplastic or thermosetting resins as binders, carbon-resin composite bipolar plates face various challenges: discontinuous conductive networks, microcracks, increased gas permeability due to mismatched thermal expansion coefficients, reduced mechanical strength due to weak interfacial adhesion, and filler segregation and anisotropy during molding. These issues collectively limit their widespread application in batteries. The following Table 3 presents a summary comparison of the processability of thermoplastic and thermosetting CBP.

Overall, thermoplastics are limited by flowability during forming, while thermosetting systems are constrained by curing behavior. Both impose inherent constraints on the formability and manufacturability of flow field structures. The performance charts for different single battery cells are listed below, as shown in Table 4.

In RFBs, the flow field structure plays a critical role in regulating the distribution of the electrolyte within the porous electrode and the associated mass transport processes, thereby significantly influencing the battery’s power density and overall system efficiency^[90]. With the increasing demand for high power density, low pressure drop, and high energy efficiency, flow field designs have gradually evolved from conventional simple serpentine or interdigitated fields toward more advanced architectures such as multi-level branching structures, biomimetic designs, and three-dimensional flow fields^[28]. These complex structures can significantly improve mass transport uniformity and reactant utilization. However, they also impose stricter requirements on the geometric precision and structural consistency of the flow fields. Small deviations in field width, depth, rib-to-field ratio, and corner transition morphology can alter the local flow velocity distribution, electrode compression state, and interfacial contact resistance, ultimately affecting the overall electrochemical performance of the battery^[91-93]. Therefore, the pursuit of higher performance has driven flow field structures toward increasing complexity, making high-precision forming an essential requirement. At the same time, the capabilities of manufacturing processes inherently constrain the achievable types and complexity of flow field architectures.

As structural complexity continues to increase, challenges related to forming consistency, dimensional control, interfacial integrity, and large-scale production stability become increasingly prominent. These issues pose significant barriers to the practical engineering application of CBP. Consequently, in the development of high-performance flow batteries, focusing solely on material properties or structural design is no longer sufficient. It is necessary to integrate flow field optimization with high-precision manufacturing technologies to achieve high-performance structures that are practically manufacturable.

In conventional serpentine flow fields, insufficient convection and uneven species distribution often occur near the channel outlet and in the under-rib regions^[94,95]. Lu et al. introduce a blocking structure at the bends of the serpentine field^[96], thereby increasing the pressure difference beneath the ribs and promoting electrolyte penetration into the porous electrode, as shown in Figure 9.

However, interdigitated flow fields are highly sensitive to pumping power and, in large-scale applications, also face a trade-off between energy consumption and structural complexity. Chai et al. introduce a parallel interdigitated flow field with a higher aspect ratio structure^[91]. The parallel interdigitated structures [PIFF (T2) and PIFF (T3)] effectively increase the flow velocity beneath the ribs, thereby enhancing mass transfer between branch channels and adjacent side channels, as shown in Figure 10. These designs highlight another important trend: the transition from planar structures to more three-dimensional (3D) flow field architectures with variable rib heights and complex channel networks. However, such 3D features introduce significant challenges in pressure transfer and material flow during molding.

Although the above-modified flow fields make remarkable progress in boosting mass transfer and enhancing performance, they still face a series of challenges in forming and processing in practical engineering applications and large-scale manufacturing. The increase in the number of flow field corners and variations in rib height introduce complex geometrical features that significantly increase the processing difficulty of the plates, placing higher demands on mold precision and manufacturing consistency. For CBPs, the increasing complexity of rib width and rib height structures often leads to local stress concentration and uneven resin flow during hot pressing^[97]. In addition, although parallel or bifurcated structural designs are beneficial for regulating pressure drop distribution, the fine structural features also pose challenges to mold lifetime and manufacturing cost. Meanwhile, in order to further strengthen mass transfer and control pumping power, researchers have proposed new complex flow field designs that combine obstruction structures with simulation-based optimization. For example, biomimetic flow fields inspired by plant leaf venation incorporate obstacles of different shapes (circular, diamond, and triangular)^[98,99]. Others, inspired by fuel cell architectures and natural transport networks, introduce spiral or nonlinear flow paths^[100,101]. These biomimetic designs represent the highest level of structural sophistication, enabling precise control over local mass transport behavior. However, they also impose the most stringent requirements on manufacturability. The presence of multi-scale features and complex curvature significantly increases the difficulty of melt filling and, more critically, mold release. In particular, undercuts, sharp angles, and intricate internal geometries can cause severe demolding resistance, leading to surface damage or structural defects during part removal. In addition, as shown in Tables 5-7, we summarized the specific flow field parameters of thermoplastic and thermosetting bipolar plates used in RFBs and Proton Exchange Membrane Fuel Cells (PEMFCs) to enable a more intuitive comparison of the challenges in flow field fabrication.

The fundamental challenge in forming flow fields in VRFB BPs is the need to simultaneously satisfy multiple functions under harsh electrochemical environments, including fluid distribution, electron conduction, and mechanical support. At present, the fabrication of flow fields in BPs mainly relies on machining and compression molding^[119-121]. Machining methods typically use graphite or dense composite plates as substrates, in which the desired flow field structures are directly produced by milling or engraving. However, when fabricating fine field features in graphite materials, cutting parameters such as feed rate significantly affect surface quality and can induce surface damage. Due to the intrinsic brittleness of graphite, crack propagation and fracture are prone to occur during machining. In addition, tool deflection during micro-milling further reduces dimensional and form accuracy, leading to structural defects in fine features^{[103,122,123]}.

In contrast, compression molding integrates the flow field structure concurrently during the forming process. This approach offers potential advantages such as simplified processing steps and is therefore widely regarded as an ideal route for the large-scale manufacturing of CBPs^[34].

In recent years, numerous novel flow field structures have been proposed using computational fluid dynamics (CFD), topology optimization, and data-driven design methods^{[111,114,136]}. These designs often demonstrate significantly improved uniformity of electrolyte distribution and mass-transfer performance in simulations. However, they face considerable challenges in practical fabrication. First, complex flow fields typically exhibit highly differentiated geometric features, whereas compression molding relies on the restricted flow of materials under high temperature and pressure. In composite systems with high filler content, it is difficult for the material to fill fine grooves and intricate structures fully. Second, complex flow fields often lack properly designed draft angles, making it inevitable for the demolding process to introduce edge damage and microcracks. As a result, the development of new fabrication approaches is urgently required to accommodate the increasingly sophisticated flow field structures.