Bipolar Plates: The "spine" and "highways" of fuel cells

In the complex and intricate internal world of a fuel cell, if the membrane electrode assembly is the "heart" responsible for power generation, then the bipolar plate is the "spine" that supports the entire battery structure and the "highway" that ensures the smooth flow of life-sustaining elements.

This seemingly simple component is, in fact, critical in determining the power output, efficiency, and lifespan of the fuel cell stack. It is not just a structural part but a core component that integrates multiple functions such as flow field distribution, electrical conduction, and heat conduction. From a material perspective, the development of bipolar plates has gone through distinct stages, each with its own clear advantages and disadvantages. The earliest widely used material was graphite. Graphite offers excellent electrical conductivity and outstanding corrosion resistance, making it perfectly suited to withstand the long-term challenges of the acidic environment inside a fuel cell.

However, its inherent brittleness makes graphite bipolar plates prone to damage during processing and assembly. Moreover, to achieve sufficient gas tightness, they often need to be made relatively thick, which limits the volumetric power density of the fuel cell stack. To overcome these drawbacks, metal bipolar plates emerged, primarily using stainless steel or titanium alloys. The greatest advantage of metal bipolar plates lies in their high mechanical strength and exceptional electrical and thermal conductivity, allowing them to be made extremely thin, thereby making the fuel cell stack more compact and achieving higher power density. However, metals face severe corrosion challenges in the operational environment of fuel cells. Once corroded, not only does the contact resistance increase, reducing efficiency, but the leaching of metal ions can also poison the catalyst.

Therefore, a corrosion-resistant coating, such as gold, platinum, or a carbon-based coating, must be applied to the surface, which undoubtedly increases manufacturing costs and process complexity. In recent years, composite material bipolar plates have become a new research direction. These are typically made by mixing conductive fillers like graphite or carbon black with polymer resins (such as polypropylene) and formed via injection molding. They combine the corrosion resistance of graphite with the moldability of plastics, facilitating mass production and offering advantages in lightweighting. However, their electrical conductivity and mechanical strength are generally intermediate between graphite and metal, representing an important compromise in current technology. The operational mode of a bipolar plate is a paradigm of parallel multitasking, and its functions can be summarized in three aspects. The primary function is to channel the reactant gases. Through precisely machined flow channels on one side, akin to miniature "highways," it evenly delivers hydrogen fuel to the anode catalyst layer and oxidant (oxygen from air) to the cathode catalyst layer, ensuring the entire reaction area participates efficiently in power generation. Simultaneously, the design of these flow channels is highly scientific: they must ensure uniform gas distribution, avoid dead zones, and also effectively remove the water produced by the reaction to prevent "flooding" that could block the channels. The second core function is to collect and conduct electrical current. The bipolar plate acts like a current collector, gathering the electric current generated by each membrane electrode assembly (single cell), and serially connecting the cells through its own highly conductive nature, ultimately outputting the required voltage and power. The electrical conductivity of its material directly determines the internal resistance losses in this process. The third key role is heat dissipation and water management.

The fuel cell reaction generates heat; the bipolar plate, serving as a thermal conduction path, needs to remove this heat promptly to maintain the stack within a suitable operating temperature range. Meanwhile, water generated at the cathode is partially removed by the excess air stream, and the flow field design and hydrophilic/hydrophobic treatment of the bipolar plate are crucial for the effective removal of this water. Therefore, the performance of the bipolar plate directly determines the overall efficiency of the fuel cell stack.

An ideal bipolar plate must strike the optimal balance between conductivity and corrosion resistance, strength and thinness, gas flow and water management, manufacturing cost, and service life. Whether made of graphite, metal, or composite materials, the developmental goal remains the same: to support the broader commercialization prospects of fuel cells with lower costs and more reliable performance. It can be said that every advancement in bipolar plate technology is a significant step towards the widespread adoption of fuel cells.