Bipolar plates are critical components in fuel cells, responsible for distributing reactant gases, conducting current, and providing structural support. The choice of material significantly impacts performance, durability, and cost. Two primary fuel cell types—proton exchange membrane (PEM) and solid oxide fuel cells (SOFC)—have distinct operating conditions that influence material selection. Graphite and coated metals are the most common materials for bipolar plates, each offering trade-offs in conductivity, corrosion resistance, and cost.

**Conductivity Requirements**
Electrical conductivity is crucial for minimizing resistive losses in fuel cells. Graphite, particularly high-grade versions, exhibits excellent bulk conductivity, typically ranging from 100 to 150 S/cm. Its porous structure, however, can increase interfacial resistance if not properly densified. Coated metals, such as stainless steel or titanium with gold, platinum, or conductive polymer coatings, can achieve comparable or higher conductivity. For instance, gold-coated stainless steel reaches conductivities above 500 S/cm, but the coating thickness and uniformity are critical to maintaining performance.

In PEM fuel cells, which operate at low temperatures (60–80°C), conductivity demands are stringent due to the need for high power density. Graphite composites are often favored for their consistent performance, though coated metals are gaining traction as coatings improve. SOFCs operate at much higher temperatures (600–1000°C), where metallic plates dominate due to graphite’s susceptibility to oxidation and structural degradation. Metals like ferritic stainless steel, with appropriate coatings, maintain conductivity under these conditions.

**Corrosion Resistance**
Corrosion resistance is a key differentiator between materials. PEM fuel cells face acidic environments (pH 2–3) due to the perfluorosulfonic acid membrane, while SOFCs deal with high-temperature oxidation.

Graphite is inherently corrosion-resistant in PEM environments, making it a reliable choice. However, its brittleness necessitates thicker plates, increasing stack volume and weight. In SOFCs, graphite is unsuitable due to rapid oxidation above 400°C.

Coated metals must address both general and localized corrosion. In PEM systems, stainless steel with thin noble metal coatings (e.g., gold or platinum) or conductive ceramic coatings (e.g., titanium nitride) is used to prevent dissolution of metal ions, which can poison the membrane. Uncoated metals corrode quickly, increasing contact resistance and contaminating the cell. For SOFCs, chromium-based coatings or alumina-forming alloys are employed to resist oxidation and chromium evaporation, which degrades electrodes.

Long-term durability tests show that coated metals can meet DOE targets of less than 1 µA/cm² corrosion current in PEM conditions, but coating defects remain a concern. Graphite, while stable, may suffer from mechanical degradation over time.

**Cost Considerations**
Cost is a major factor in commercializing fuel cells. Graphite bipolar plates are expensive due to the machining required to achieve precise flow fields. Isostatic molding or injection molding of graphite composites reduces costs but may compromise conductivity or strength.

Coated metals offer potential cost savings, especially for high-volume production. Stainless steel substrates are inexpensive, but coating processes (e.g., physical vapor deposition or electroplating) add expense. Gold and platinum coatings are prohibitively costly for widespread use, leading to research into alternative coatings like carbon-based layers or hybrid polymers.

For SOFCs, metallic plates are more cost-effective than graphite due to their higher durability at elevated temperatures. However, the need for specialized coatings to prevent chromium migration adds to the expense.

**Material Selection for PEM vs. SOFC**
PEM fuel cells prioritize lightweight, corrosion-resistant materials with high conductivity. Graphite remains the benchmark, but advances in coated metals are narrowing the gap. Automotive applications, where weight and volume are critical, increasingly adopt coated metals to meet performance targets.

SOFC systems rely almost exclusively on metallic bipolar plates due to temperature constraints. Ferritic stainless steels, with protective coatings, are standard. The focus is on preventing interdiffusion and maintaining conductivity over thousands of hours of operation.

**Comparative Table of Key Properties**

| Property | Graphite (PEM) | Coated Metals (PEM) | Metals (SOFC) |
|------------------------|----------------------|----------------------|----------------------|
| Conductivity (S/cm) | 100–150 | 200–500 | 50–100 (coated) |
| Corrosion Resistance | Excellent | Moderate to High | High (with coating) |
| Temperature Limit | <200°C | <120°C | >600°C |
| Cost | High (machining) | Moderate (coating) | Moderate (substrate) |
| Durability | Good (mechanical wear)| Coating-dependent | High (if coated) |

**Future Directions**
Research is focused on lowering the cost of coated metals for PEM applications by developing non-precious metal coatings and improving deposition techniques. For SOFCs, the emphasis is on coatings that resist both oxidation and chromium evaporation while maintaining interfacial conductivity.

Material innovations, such as graphene-enhanced composites or advanced alloy formulations, could further improve performance. However, scalability and manufacturability remain challenges for these emerging solutions.

In summary, graphite excels in corrosion resistance and conductivity for PEM fuel cells but faces limitations in cost and mechanical strength. Coated metals offer a promising alternative if durability and coating costs are addressed. For SOFCs, metallic plates with protective coatings are the only viable option due to high-temperature requirements. The choice between materials ultimately depends on the specific application, balancing performance, longevity, and economic feasibility.