In a previous blog post, we described bipolar plates and the associated materials for low-temperature fuel cells. The materials previously described are selected for fuel cell stacks at or slightly above room-temperature -- which means that the materials are chemically compatible with the stack between 0 – 140 °C. The fuel cells that operate at higher temperatures require different materials. Fuel cell stacks that operate at high temperatures are SOFCs, MCFCs, and PAFCs. We will describe the materials typically used for these fuel cells types in this blog post.
The materials used in SOFCs must be physically stable and resist corrosion from the electrolyte at high temperatures. There are two categories of materials used for SOFC plates: high-temperature perovskite materials (900 to 1000 °C) and metallic alloys for lower temperature operation (600 to 900 °C). The high-temperature perovskite materials used in SOFCs are doped lanthanum and yttrium chromites (dopants typically include Mg, Sr, Ca, Ca/Co). These materials have good electronic conductivity -- which increases with temperature. The type of dopant alters the material properties and determines the compatibility with the fuel cell layers.
Ceramic interconnects are chemically and physically stable in SOFCs for extended periods of time, but because the interconnects are made of ceramic, they do not have good flexibility for sealing the cell. To remedy this issue, conductive pastes or a contact felt is sometimes used. These help with the flexibility and sealing, but of course, the life of these components are not as good as the ceramic interconnect.
When thin-electrolyte anode-supported SOFCs (Figure 1) were developed within the last decade, this enabled the SOFC to be operated at lower temperatures (650 to 800 °C), which allowed the use of some metallic components in the cell. Even at these “lowered” temperatures, it is difficult to find reliable metals that can be used as interconnects. Metals that will be used as SOFC interconnects at these temperatures must have the following characteristics:
• Able to maintain uniform temperature across the plate without deformation
• Have sufficient creep strength
• Tolerate a corrosive environment
• Maintain a high conductivity
Figure 1 (Left to Right): Electrolyte Supported Button Cell and Electrolyte Supported Planar Cell
Many high-chrome interconnects have been developed, such as Cr5Fe1Y2O3, which operates around 900 °C. The chrome is problematic because it can poison the electrode. If the operating temperatures were reduced, ferritic steels could be used, which would reduce the material and processing costs. The following materials are commonly used as end-plate materials in fuel cells:
• Lanthanum chromite
• Avesta 600 (Fe-28/Cr-4/Ni-2/Mo)
• Inconel 601 (NiCr + Al)
• SS 310
• SS 316L
• SS 310S
Metals used in high-temperature fuel cells often have to be coated to be non-corrosive, which means that the coating needs to have similar properties as the ideal interconnect. The most commonly used coating is a stable oxide (chromia) which leads to evaporation and electrode poisoning.
Two coatings that minimize evaporation and insure good contact resistance are strontium-doped lanthanum cobaltite or manganite (Figure 2). These coatings perform well in the intermediate temperature range, but better coatings and interconnect materials still need to be developed in the future.
Figure 2 (Left to Right): Lanthanum Strontium Cobaltite Ink and Lanthanum Strontium Manganite Ink
Bipolar plates for MCFCs include a separator, current collectors, and the wet seal. The materials used for MCFC bipolar plates are alloys such as Incoloy 825, 310S or 316L stainless steel that are coated on one side with a Ni layer. The plates are typically 15 mm thick, and the nickel-coated side provides a conductive surface coating with low contact resistance. A thin aluminum coating can form a protective LiAlO2 after reaction of Al with Li2CO3. This coating is not useful for areas that need to be conductive since it is an insulating protective layer. Materials that are commonly used to promote conductivity and have corrosion resistance are 316 stainless steel, and Ni plated stainless steels. Type 310 and 446 stainless steels have better corrosion resistance than Type 316 in corrosion tests.
The separator and current collector is a Ni-coated 310S/316L, and the wet seal is formed by aluminization of the metal. A low oxygen partial pressure on the anode side of the bipolar plate prevents oxidation. Single alloy bipolar current collector plates need to be developed that work on both the cathode and anode sides of the fuel cell. The primary construction materials are stainless steels (austenitic stainless steels). Nickel-based alloys also resist corrosion well. Certain nickel-based coatings can protect the anode side. Electroless nickel coatings are thick and uniform, but expensive, and contain large amounts of impurities. Electrolytic nickel coatings are not sufficiently dense or uniform in thickness.
PAFCs use many of the same materials as in low-temperature cells. Pure graphite bipolar plates are corrosion-resistant for a projected life of 40,000 hours in PAFCs, but like in other low-temperature fuel cells – are costly and expensive to manufacture. Another commonly used design is a multi-component bipolar plate, which has a thin, impervious plate that separates the reactant gases in adjacent cells, and a separate porous plate with ribbed channels that is used for gas flow. The impervious plate is subdivided into two parts, and each joins one of the porous plates. Metals that can withstand higher temperatures are also used as materials in PAFC stacks.
High-temperature fuel cells often use different materials than fuel cells that operate at room temperature (or below 140 °C). Some of the materials used for SOFCs are perovskite and doped lanthanum and yttrium chromites. Materials for MCFCs include Incoloy 825, 310S or 316L stainless steel and are often coated with Nickel. PAFCs use some of the materials from lower-temperature fuel cells and other materials that are used by SOFCs and MCFCs.