Solid oxide fuel cell (SOFC) technology that oxidizes a fuel to produce electricity is fast 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. Furthermore, when hydrogen is the fuel, there are no carbon emissions, it only produces water. The same principle also works in reverse in an SOEC (solid oxide electrolyzer cell), converting water into hydrogen and oxygen. This is increasing in importance as the hydrogen economy builds momentum with electrolysers running on low-carbon electricity, from wind or solar energy plant for example.
Therma 22FCTM stainless steel is the long-lasting choice for fuel cells and electrolyzers
Until recently, the main disadvantage of SOFCs has been their high operating temperature – up to 1000 °C – that calls for specialised, costly construction materials such as zirconia, composites and highly-alloyed stainless steel 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, opening up the possibility to utilize competitive stainless steel grades. To optimize the performance of this new generation of fuel cells and electrolysers, Outokumpu has developed a customised grade - Therma 22FCTM. A key advantage is its increased electrical conductivity that is sustained over a long service life.
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. Typical construction and operating principle for a basic SOFC unit.
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 must 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) compatible with 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 and during stack manufacture
- Formability to enable the creation of gas transfer channels in sheet that can be as thin as 0.3 mm
- Compatibility with surface coatings applied for additional corrosion protection
Therma 22FCTM ferritic stainless steel
Outokumpu's solution to the emerging SOFC and SOEC technology is a customized high-chromium ferritic stainless steel grade, Therma 22FCTM.
Therma 22FC™ is a stabilized nickel-free, high-chromium ferritic stainless steel that has good corrosion resistance and high-temperature strength, as well as excellent formability and weldability. Its key feature is a higher chromium (Cr) content at 21% that ensures a reduced risk of Cr evaporation that could poison the cell, as well as limiting Cr depletion in the interconnector surface. That is why it is a cost-effective solution for a long lifetime.
In addition, the grade is produced with Outokumpu’s process knowhow that enables the alloying elements to be closely controlled to maintain the composition within strictly controlled limits. This has enabled its electrical conductivity to be optimized as well as ensuring that it will not decline over the operating life of the cell.
An important supporting role
In addition to the stack itself, stainless steel has an important role to play in the general construction of the fuel cell or electrolyser 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, such as Therma 253 MA®, are available to satisfy this need and the specific choice of grade will depend on the exact conditions.
Stainless steel can also be employed in the broader infrastructure supporting the fuel cell. This includes 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®) and 1.4435 (Outokumpu Supra 316L/4435).
Continuous development and cooperation
Thanks to Outokumpu’s long-term research and development program, Therma 22FCTM offers a perfect match for the construction of both SOFC and SOEC systems. However, the continuous evolution of commercial systems to offer enhanced performance and lifespan requires continuous development. This is supported by Outokumpu’s strong engagement with industrial partners, research institutions and universities.
Contact Outokumpu for more information about the key role of stainless steel in fuel cells and the developing hydrogen economy.