HKU Researchers Innovate Stainless Steel to Lower Green Hydrogen Production Costs

A team at the University of Hong Kong (HKU) has developed a new stainless steel, designated stainless steel for hydrogen (SS-H2), capable of significantly resisting corrosion under extreme conditions that traditional stainless steel fails to endure. This advancement aims to facilitate the cost-effective production of green hydrogen.

Led by Professor Mingxin Huang from HKU’s Department of Mechanical Engineering, the creation of SS-H2 is part of Huang’s ongoing “Super Steel” Project. The team previously engineered stainless steel with anti-COVID properties in 2021 and achieved notable feats in 2017 and 2020 with exceptionally resilient forms of steel.

Potential in Green Hydrogen Production

SS-H2’s impressive corrosion resistance positions it as a viable option for green hydrogen systems utilizing seawater, a domain still in search of practical sustainable solutions. Green hydrogen is produced through electrolysis, which separates water into hydrogen and oxygen using electricity sourced from renewable energy. The materials involved in this electrolysis must withstand rigorous chemical and electrical environments, particularly with salt content.

In tests using a saltwater electrolyzer, SS-H2 demonstrated performance on par with titanium structural components, which are standard in extracting hydrogen from desalinated seawater or acidic solutions. However, the critical advantage of SS-H2 lies in its significantly lower cost.

The findings of this study have been published in the journal Materials Today under the title “A sequential dual-passivation strategy for designing stainless steel used above water oxidation.” The research team has filed for patents in multiple countries, with two patents already granted.

Limitations of Conventional Stainless Steel

Conventional stainless steel, a century-old staple in corrosion-prone applications, relies on chromium to build a protective oxide layer that prevents further corrosion. However, this protective layer has limitations, as it can degrade into soluble chromium species, leading to a form of corrosion known as transpassive corrosion, particularly when operating at about 1000 mV. Given that water oxidation in electrolysis requires a substantially higher potential of around 1600 mV, traditional stainless steel’s use in high-voltage applications has been limited.

Even high-performance alloys like the 254SMO super stainless steel experience reduced corrosion resistance at elevated potentials, reinforcing the need for innovative materials.

Innovative Dual-Passivation Technique

The research team addressed this challenge through a technique called “sequential dual-passivation.” SS-H2 features a second protective layer composed of manganese, which begins forming around 720 mV, complementing the initial chromium oxide layer. Together, they enhance the steel’s resistance to corrosion in chloride-rich environments at potentials reaching up to 1700 mV.

Importantly, the inclusion of manganese in SS-H2 represents a surprising and counter-intuitive finding, as manganese has traditionally been viewed as detrimental to stainless steel’s corrosion resistance. Dr. Kaiping Yu, the study’s lead author, noted, “Initially, we did not believe it because the prevailing view is that Mn impairs the corrosion resistance of stainless steel. Mn-based passivation is a counter-intuitive discovery… we cannot wait to exploit the mechanism.”

Conclusion and Industrial Prospects

The six-year research journey has transitioned from exploring SS-H2’s atomic-level behaviors to seeking application in industry. Current designs prioritize stability under higher electrical potentials rather than just ordinary corrosion resistance. Professor Huang emphasized the need for high-potential-resistant alloys in developing new material paradigms for alloy engineering.

Economically, the implications are profound. Existing electrolyzers, reliant on costly titanium components often coated with precious metals, account for up to 53% of total system costs. SS-H2’s anticipated integration could lower structural material costs by a factor of 40, enhancing its viability for industrial hydrogen production from renewable sources.

While engineering challenges persist in manufacturing the steel into practical forms such as meshes and foams for electrolyzers, progress has been made toward large-scale production. Professor Huang remarked, “We are moving forward in applying the more economical SS-H2 in hydrogen production from renewable sources,” indicating significant momentum toward actual industrial application.