Solid oxide fuel cell (SOFC) technology is emerging as a competitive choice for stationary high-power generation applications from kilowatts to megawatts. This is thanks to its high combined efficiency, long-term stability, fuel flexibility and increasingly competitive cost. The same technology principle can also work in reverse in an SOEC (solid oxide electrolyzer cell), converting water into hydrogen and oxygen. Until recently, the main disadvantage of SOFCs has been their high operating temperature – up to 1000°C – that calls for exotic construction materials to ensure reliability over a long service life. Now there is a new generation of SOFCs that operate at less demanding temperatures of around 650-750°C. This has made it possible to consider the use of commercially available stainless steel grades.
SOFC construction and operation
Figure 1 shows the typical construction and operating principle for a basic SOFC unit. It comprises a porous anode and cathode, each made of different types of ceramic. In between them is a solid electrolyte.
Figure 1 - a single SOFC cell
Fuel, either hydrogen or a hydrocarbon such as methane, is supplied to the anode. Oxygen from atmospheric air is supplied to the cathode. Oxygen ions travel from the cathode through the electrolyte to the anode where they react with the hydrogen. This reaction produces water (H2O) and electrons that flow through an external circuit to produce electricity.
Stacking in series
Figure 1 shows only one cell. To achieve the system voltage required for the application “stacks” comprising multiple cells must be connected in series. A key element is the interconnector plate that provides the electrical connection between the anode of one cell and the cathode of the next cell in the stack. It has to perform the dual role of maintaining the structural integrity of the stack while preventing contact between the hydrogen at the anode and the oxygen at the cathode. A similar component acts as the endplate of the stack.
There are some vital requirements for the material used for the interconnector:
- Low thermal expansion coefficient (TEC) similar to the ceramic oxide components and sealing materials. This is essential to minimize thermal stresses during the heating up and cooling down stages when the SOFC starts and stops
- Corrosion resistance in a dual atmosphere, resistance to hydrogen and other fuel gases such as methane
- Low and stable electrical resistivity
- Long term durability to ensure reliable operation for many years
- Resistance to the evaporation of elements such as chromium that can “poison” the cell reaction and degrade its operation
- Creep resistance at operating temperatures
- Formability to enable the creation of gas transfer channels in a thin sheet usually around 0.3 - 0.5 mm thick.
- Compatibility with surface coatings applied for additional corrosion protection
Ferritic Stainless Steel
Two commercially available ferritic grades of stainless steel have been identified as candidate materials and are currently under investigation in SOFC prototypes. They are 1.4622 (Outokumpu Core 4622) and 1.4509 (Outokumpu Core 441/4509).
Core 4622 is a stabilized nickel-free, high-chromium (21%Cr) ferritic stainless steel that has excellent deep drawability. The higher chromium content ensures a reduced risk of chromium evaporation that could poison the cell and avoids a too-high level of Cr depletion in the interconnector surface. That is why it shows good promise for longer-life operation.
Core 441/4509 is a stabilized, nickel-free 18% chromium ferritic stainless steel. It has good corrosion resistance and high-temperature strength, as well as good formability and weldability. It is being used by several SOFC developers.
An important supporting role
In addition to the SOFC stack itself, stainless steel has an important role to play in the general construction of the fuel cell system including structural components, housings and tubing. Complete systems will range from 1.5 to 3 kW domestic fuel cells in the home to larger commercial installations providing 60 kW of electricity and 25 kW of heating. Even larger systems are anticipated, up to 1 MW, such as for data centers.
The high operating temperatures close to the stack call for grades that offer high creep resistance at elevated temperatures, resistance to temperature cycling and high stability of the microstructure. A variety of grades within the Outokumpu Therma range of steels are available to satisfy this need and the specific choice of grade will depend on the exact conditions.
Stainless steel can be employed in the broader infrastructure supporting the fuel cell, including transportation, storage and gas handling and conditioning. Both corrosion resistance, resistance to hydrogen embrittlement and formability are important in these applications. Appropriate grades will include austenitic 1.4420 (Outokumpu Supra 316plus), 1.4404 (Outokumpu Supra 316L/4404) and 1.4435 (Outokumpu Supra 316L/4435).
Ongoing improvement and research cooperation
Stainless steel offers promise for the construction of both SOFC and SOEC systems. However, ongoing work is needed to optimize long term performance in commercial systems. This includes study of the specific requirements of the materials inside the stack to optimize the composition, and to investigate surface finish effects and interaction with surface coatings.
To help move this technology forward Outokumpu is working with industrial partners, research institutions and universities to further optimize stainless steel performance in solid oxide cells.