In the process of expanding green hydrogen production capacity, the real bottleneck is often not in energy supply, but in key materials. Especially for hydrogen production equipment for seawater electrolysis, the working environment is both high voltage and highly corrosive, making it difficult for most metals to operate stably for a long time. They are forced to rely on expensive titanium alloys and precious metal coatings, which drives up system costs and limits large-scale deployment.A research team at the University of Hong Kong is trying to break this situation. The team led by Professor Huang Mingxin has developed a new stainless steel alloy SS-H2 that can work stably for a long time in a harsh electrolytic environment, and claims to be expected to replace existing expensive components.
This research, published in the journal Materials Today, is one of the latest developments in Huang Mingxin's team's long-term research project "Super Steel", which has previously launched ultra-high-strength alloys and stainless steel materials with antibacterial properties. The design goal of SS-H2 is to remain stable in the potential range where traditional stainless steel fails, and is especially suitable for electrolysis devices that directly use seawater. Researchers pointed out that the current core problem is that the corrosion resistance of stainless steel mainly relies on the dense oxide film formed by chromium. This mechanism works well in general industrial and marine environments, but it will be completely broken under high-potential electrolysis conditions.
Experiments show that when the potential rises to about 1000 mV (relative to a saturated calomel electrode), the chromium oxide film on the surface of traditional stainless steel will begin to decompose, generating soluble species and causing severe corrosion, while efficient water oxidation reactions usually require a potential of about 1600 mV. Even the high-end alloy 254SMO, which is designed for harsh seawater environments, cannot remain stable at such high potentials. Therefore, many current electrolysis systems can only use titanium-based structural parts supplemented by precious metal coatings such as platinum and gold. Although reliable, they greatly increase the cost of equipment, especially after scaling up to industrial scale.
The idea of SS-H2 is to change the way metal protects itself. In conventional stainless steel, protection is mainly provided by a single chromium oxide film; in SS-H2, the material sequentially forms two protective layers during operation: first a conventional chromium-based oxide film, and then at a higher potential (approximately 720 mV), a manganese-based protective layer is formed above it. It is this second layer of protection that allows the material to remain stable up to about 1,700 mV, thus covering the voltage range required for water splitting.
It is worth noting that the introduction of manganese itself is quite unexpected. In traditional thinking, manganese is often thought to weaken the corrosion resistance of stainless steel, rather than improve it. Dr. Yu Kaiping, the first author of the paper, recalled that the team initially found it difficult to believe that Mn could help form a stable passivation layer because it was contrary to existing corrosion science knowledge. However, after a large number of atomic-scale experimental results were presented, they finally confirmed this "counter-intuitive" Mn-based passivation phenomenon.
If such materials perform as expected outside the laboratory, the economic impact could be significant. The research team used a 10-MW PEM electrolysis system as an example to estimate the cost structure: Structural materials account for a large proportion of the total cost, approximately HK$17.8 million, of which as much as 53% is directly related to these components. On this basis, the team predicts that if SS-H2 is used to replace existing titanium-based materials, the cost of structural materials is expected to be reduced by about 40 times, thereby significantly reducing the overall system cost.
This work also reflects changes in the design thinking of corrosion-resistant materials. Huang Mingxin pointed out that traditional corrosion research focuses more on the performance of materials at "natural potential", while their strategy is to specifically develop alloys that are stable at high potentials. By redesigning the alloy system to form a new protection mechanism when operating at high potential, the team believes that it has broken through the "potential upper limit" of traditional stainless steel and provided a new paradigm for the development of alloys for high potential environments.
At present, this research has moved out of the early experimental stage. Relevant patents have been applied for in multiple countries, two of which were granted when the study was announced. The research team has also begun working with a factory in mainland China to produce SS-H2 wire, although further engineering development and process optimization are required to make it into mesh or foam structures suitable for electrolyzers. Problems such as corrosion, chlorine-related side reactions, catalyst degradation, and limited system life remain prevalent throughout the seawater electrolysis field, and much research has focused on adding coatings or surface treatments to traditional stainless steel surfaces to improve durability.
Different from these paths, SS-H2 starts from the material itself and allows the material to "spontaneously" form a protective layer during the working process by changing the alloy composition and electrochemical behavior, rather than adding additional coatings afterwards. This endogenous protection mechanism may be able to pursue high durability while taking into account cost control, giving future seawater electrolysis hydrogen production devices a greater chance to occupy a place in large-scale commercial deployment. However, the researchers also emphasized that the material is still in the early stages of industrialization, and its long-term life and performance under real operating conditions have yet to be verified. However, this direction shows that solving the cost and durability problems of green hydrogen may also rely on the reimagining of "basic materials" rather than just improvements at the system design level.