What is stainless steel? How is it different from steel?
Steel and stainless steel are widely used in various industries but have distinct differences. Steel is an alloy of iron and carbon, with carbon making up to 2% of its composition. Pure iron is too soft for engineering purposes, but adding carbon strengthens it, transforming it into steel. The carbon in steel may dissolve in the iron structure or form compounds called carbides, like iron carbide (Fe3C). The different structures and the presence of these hard carbides increase the hardness and strength of the steel compared to those of pure iron.
Unlike regular carbon steel, stainless steel contains elements like chromium, nickel, and molybdenum, in addition to iron and carbon. The chromium content, typically above 10.5 wt%, is crucial, as it provides this alloy with its corrosion-resistant properties, hence the name “stainless.” When exposed to air, chromium reacts to form a protective layer of chromium oxide, preventing rust formation and corrosion.
Stainless steel finds widespread application due to its unique properties and corrosion resistance. With this widespread application, stainless steel comes in different forms, including coils, sheets, plates, bars, wire and wire meshes, fibers and fiber felts, and tubings. In this article, we will focus on stainless steel fiber felts and explore their potential application in a niche market, water electrolyzer and fuel cell manufacturing.
How can stainless steel fiber felts be used in water electrolyzers?
Stainless steel fiber felts as porous transport layers in alkaline and anion exchange membrane water electrolyzers
Stainless steel fiber felts are promising as porous transport layers in water electrolyzers, particularly under basic conditions like those in alkaline electrolysis and anion exchange membrane electrolysis. Alkaline environments can be particularly corrosive to many materials, but stainless steel is known for its remarkable resistance to corrosion.
This property ensures the stability of the gas diffusion layers even when exposed to harsh alkaline conditions over extended periods.
Aside from this corrosion resistance, stainless steel also presents an interesting potential as a substrate for water electrolyzer catalysts. The most commonly used catalysts or active materials for the anode of alkaline water electrolyzers, where the oxygen evolution reaction (OER) occurs, are based on transition metals, such as cobalt and nickel. Previously, the performance of Ni towards OER was improved by alloying it with iron. Stainless steels, containing both iron and nickel, exhibit potential catalytic activity towards OER, making them interesting support materials for catalysts and improving the overall water splitting reaction.
Stainless steel fiber felts boast numerous advantages beyond just their chemical composition. Their fiber structure, specifically designed for applications like gas diffusion layers in alkaline and anion exchange membrane water electrolyzers, enhances their functionality significantly. These felts are produced using stainless steel fibers, which are sintered to create a unique non-woven fiber structure. This structure offers exceptional mechanical properties along with features like large specific surface area, porosity, permeability, and high electrical conductivity.
Stainless steel fiber felts as porous transport electrodes in alkaline and anion exchange membrane water electrolyzers
Catalysts can be effectively deposited onto stainless steel fiber felts through scalable methods. Growing catalysts directly on these substrates is also possible, eliminating the need for resistive polymer binders and enhancing conductivity and durability. This approach also enables precise control over catalyst morphology and distribution.
These electrodes, with catalysts directly deposited on stainless steel, are not only suitable for water electrolysis but also for metal–air batteries.
Stainless steel fiber felts in acidic proton exchange membrane water electrolysis
For proton exchange membrane water electrolysis (PEMWE), the benchmark anode gas diffusion or porous transport layer remains to be titanium. Titanium-based PTLs exhibit good corrosion resistance even when subjected to highly oxidative potentials. In the Pourbaix diagram of the titanium–water system, we can see that titanium forms a passivation layer under the operating conditions of PEM water electrolyzers. This layer prevents the direct contact between the titanium PTL and the corrosive electrolyte, thereby reducing the likelihood of corrosion. The passivation layer also stabilizes the titanium PTL surface and reduces the concentration of surface defects, which can serve as initiation sites for corrosive reactions. By minimizing the competing side reactions, the overall performance and durability of the electrochemical device improves substantially. CAPLINQ provides LINQCELLTM titanium fiber felts in different thicknesses and dimensions, and porosities, catering to diverse application needs.
While titanium fiber felts boast outstanding corrosion resistance and durability, their higher cost can be a drawback. Fortunately, stainless steel is a promising alternative to titanium in PEMWE.
| Property | Titanium | Stainless Steel |
| Electrical Conductivity | ■■■□ Good | ■■□□ Moderate |
| Corrosion Resistance | ■■■■ High | ■■■□ Good |
| Compressibility | ■■□□ Moderate | ■■■■ High |
| Cost | ■□□□ Expensive | ■■■□ Cheap |
| Weight | ■■■■ Lightest | ■□□□ Heavy |
| Temperature Stability | ■■■■ Excellent | ■■□□ Moderate |
| Availability | ■□□□ Rare | ■■□□ Moderate |
| Best for | PEM Systems | Alkaline and AEM Systems |
To make stainless steel work well in long-term acidic environments for PEMWE, it needs a protective coating of highly corrosion-resistant materials, such as platinum. Even with the platinum coating, stainless steel still stands out as a budget-friendly option compared to titanium fiber felts.
At CAPLINQ, we provide stainless steel fiber felts in different thicknesses: 250 (LINQCELL SFP250), 600 (LINQCELL SFP600), and 1000 (LINQCELL SFP1000) µm, including custom options. What’s more, these felts can be coated according to your specifications, ensuring they perform well in both alkaline and acidic environments.
How can stainless steel fiber felts be used in fuel cells?
Fuel cells and water electrolyzers are two sides of the same coin. While water electrolyzers need electricity to split water into H2 and O2, fuel cells produce electricity from the reaction between H2 and O2 that forms H2O. While certain materials suitable for water electrolyzers may also be used in fuel cells, there is a notable distinction in the selection of materials for the anode and cathode in electrolyzers compared to fuel cells.
In fuel cells, specifically in proton exchange membrane fuel cells, the benchmark anode and cathode gas diffusion layers remain to be carbon-based. CAPLINQ’s LINQCELL graphitized carbon papers are specifically designed to meet these needs, providing excellent electrical conductivity and gas permeability crucial for efficient electron transfer and gas distribution.
| Product | Thickness [µm] | Basis Weight [g/m²] | Air Permeability [Gurley s] | TP Resistance [mΩ⋅cm²] | MPL/PTFE Treatment | Key Feature/Note |
| GDP180 | 180 | 50 | <10 | <7 | No | Comes in roll form |
| GDP210 | 210 | 50 | <10 | <6 | No | Comes in roll form |
| GDP210-MP | 210 | 85 | <225 | <15 | Yes | GDP210 version with MPL and PTFE coating |
| GDP210-MPS | 210 | 85 | <225 | <10 | Yes | GDP210-MP version graphitized at higher temperature (2000 °C) |
| GDP240 | 240 | 90 | <85 | <15 | Yes | Thin sheet with high air permeability |
| GDP340 | 340 | 125 | <200 | <10 | Yes | 47% compressibility at 1 MPa |
Despite being the benchmark material, carbon fiber papers also present several challenges in PEMFCs, including high compressibility and susceptibility to porosity variation, hindering gas and liquid water transport within the GDL. Their brittleness and stiffness make them prone to damage during fabrication and under conditions of shock and vibration. These limitations emphasize the need to carefully consider material properties and explore alternatives.
Stainless steel fiber felts are a cheaper option but have drawbacks compared to graphitized carbon papers. Despite this, stainless steel fiber felts have advantages: their compression modulus is about 10 times that of carbon paper felts, reducing compression deformation and improving gas and liquid water transport within the GDL under the same assembly force. Additionally, stainless steel’s ductility reduces damage from shock and vibrations.
In terms of electrical properties, the sintering of fibers reduces resistance in metal fiber joints, potentially providing an alternative to polymer binders used in carbon fiber paper manufacturing.
| Property | Graphitized Carbon Paper | Stainless Steel Fiber Paper |
| Electrical Conductivity | ■■■■ High | ■■□□ Moderate |
| Corrosion Resistance | ■■■■ High | ■■■■ High |
| Compressibility | ■■■■ High | ■■□□ Moderate |
| Cost | ■□□□ Expensive | ■■■■ Cheap |
| Weight | ■■□□ Moderate | ■□□□ Heavy |
| Temperature Stability | ■■■■ Excellent | ■■□□ Moderate |
In conclusion, stainless steel felts excel as gas diffusion layers. Whether you’re involved in research, development, or manufacturing, consider exploring the benefits of stainless steel fiber felts in your projects. For inquiries about stainless steel fiber felts and their applications in electrolyzers, fuel cells, and more, reach out to us. We’re here to help with tailored solutions.
