Super steel for seawater hydrogen: a cheaper materials route for Power-to-X?

Hydrogen is the molecule that makes Power-to-X possible. Combine renewable hydrogen with recycled CO₂ and you can synthesise the e-fuels and e-chemicals that defossilise aviation, shipping and heavy industry. But two things keep electrolysers expensive: the durability of the materials inside them, and the price of those materials. A stainless-steel result from the University of Hong Kong, first reported in 2023 and recently brought back into focus, speaks to both at once.

The seawater problem

Green hydrogen is made by using electricity — ideally renewable — to split water into hydrogen and oxygen. Seawater is a tempting feedstock because it is effectively unlimited, but it punishes hardware: salt, chloride ions, chlorine-related side reactions and aggressive corrosion can quickly degrade electrolyser components. Recent reviews of direct seawater electrolysis keep landing on the same shortlist of obstacles — corrosion, side reactions, catalyst degradation, precipitates and limited long-term durability — as the reasons the technology has struggled to reach commercial scale.

Today’s answer is to build the structural parts from titanium, often coated with precious metals such as gold or platinum. It works, but it is costly.

A steel that builds a second shield

Stainless steel normally protects itself through chromium: when chromium oxidises it forms a thin Cr₂O₃ passive film that shields the metal. The catch is that this film has a built-in ceiling. At high electrical potentials the chromium layer breaks down — stable Cr₂O₃ is further oxidised into soluble Cr(VI), triggering transpassive corrosion at around 1,000 mV (versus the saturated calomel electrode). That is well below the roughly 1,600 mV needed to drive water oxidation. Even 254SMO, a benchmark super stainless steel prized for pitting resistance in seawater, runs into the same wall.

The team led by Professor Mingxin Huang developed a stainless steel for hydrogen production (SS-H2) that gets around the limit with a strategy they call “sequential dual-passivation.” The first protective layer is the familiar Cr₂O₃ film. Then, at around 720 mV, a second, manganese-based layer forms on top of it — extending protection in chloride-containing environments up to an ultra-high potential of about 1,700 mV, comfortably past the water-oxidation threshold.

What makes this striking is that manganese was widely believed to weaken the corrosion resistance of stainless steel. As first author Dr. Kaiping Yu described it, the team initially did not believe their own results, because manganese-based passivation runs against established corrosion science; it took atomic-level evidence to convince them.

Why this matters for Power-to-X

Switzerland is landlocked, so direct seawater electrolysis is not a domestic play. The relevance for Power-to-X is upstream of that: cheaper, more durable electrolyser hardware lowers the cost of renewable hydrogen everywhere it is produced — including the sunny, windy, often coastal regions from which Switzerland will import much of its future e-fuel and e-chemical supply. The closer and cheaper the hydrogen, the more competitive the downstream synthesis of e-fuels from renewable energy with recycled CO₂ becomes. A materials advance that attacks electrolyser cost at the structural level helps the whole value chain that Swiss defossilisation depends on.

The cost question

The HKU team reported that in a salt-water electrolyser, SS-H2 can perform comparably to the titanium-based structural materials used in current industrial practice. The difference is price. For a 10-megawatt PEM electrolysis tank system, they estimated a total cost of around HK$17.8 million, with structural components making up as much as 53% of that. Swapping those parts for SS-H2, by their estimate, could cut the structural-material cost by roughly 40 times. These are the researchers’ own figures rather than demonstrated production costs, so they should be read as a strong signal, not a settled number.

From lab to factory — but not yet plug-and-play

The path here was not quick: the team describes roughly six years from the first puzzling observation to a deeper explanation and toward application. The work has moved beyond the bench — patents have been filed in several countries, with two already granted, and tons of SS-H2-based wire have reportedly been produced together with a factory in Mainland China. Even so, turning experimental material into finished electrolyser parts such as meshes and foams remains genuine engineering work, and SS-H2 is not yet a drop-in replacement for the hydrogen economy.

The SPIN perspective

Every credible path to cheaper, more durable electrolysers matters, because hydrogen is the foundation on which Power-to-X is built. A stainless steel that grows its own second shield could take real cost out of the hardware — but the decisive condition is unchanged: the molecule only counts as a defossilisation tool if the electricity splitting the water is renewable. Cheaper hardware lowers the barrier; clean energy input is what makes the result climate-relevant.

Journal reference: Kaiping Yu, Shihui Feng, Chao Ding, Meng Gu, Peng Yu, Mingxin Huang. “A sequential dual-passivation strategy for designing stainless steel used above water oxidation.” Materials Today, 2023; 70: 8. DOI: 10.1016/j.mattod.2023.07.022. Source: The University of Hong Kong via ScienceDaily.