Iron-tin dual-atom catalyst shields seawater batteries from chloride attack

Seawater batteries have long been celebrated as one of the most promising routes to safe, scalable marine energy storage. They rely on an electrolyte that is effectively inexhaustible, they avoid the flammable organic solvents that plague conventional lithium-ion cells, and their theoretical energy densities are attractive for powering buoys, underwater vehicles, desalination platforms and offshore sensors. Yet for all their promise, these devices have been held back by a stubborn electrochemical bottleneck at the air cathode, where the oxygen reduction reaction, the process that accepts electrons as the battery discharges, proceeds far too slowly on most available catalyst surfaces. Now a team at Hainan University reports an atomic-scale design that tackles both the sluggish kinetics and the corrosive chemistry of the sea at the same time.

The core of the problem is chloride. Seawater carries a high concentration of chloride ions, and at the cathode of a seawater air battery these ions behave as aggressive interlopers. They adsorb onto the very metal centers that are supposed to bind oxygen intermediates, blocking active sites, degrading catalytic performance and accelerating the corrosion of the catalyst layer itself. Iron-based single-atom catalysts, prized for their high intrinsic oxygen reduction activity in alkaline conditions, are particularly vulnerable. The result is a painful trade-off: catalysts that are highly active on the bench tend to lose their edge quickly in real seawater, while corrosion-resistant materials are often too sluggish to deliver competitive power.

The Hainan University group, led by corresponding author Peng Rao, approached this dilemma with a concept they describe as p-d orbital hybridization. Instead of leaving iron atoms to fend for themselves on a carbon support, the researchers paired each iron active site with an adjacent tin atom, creating what is known as a dual single-atom catalyst. Tin, a p-block element with its own distinct orbital character, does not simply sit nearby as a passive neighbor. Through overlap between its p orbitals and the d orbitals of iron, it reshapes the local electronic structure of the iron center, tuning how strongly the site binds oxygen-containing intermediates and how hospitably it treats chloride ions in the surrounding electrolyte.

This electronic engineering addresses a well-known limitation of conventional iron sites. In many iron-nitrogen-carbon catalysts, the binding of oxygen reduction intermediates is too strong, meaning that reaction intermediates such as hydroperoxyl species linger on the surface longer than they should and slow the overall turnover of the catalyst. The Fe-Sn interaction weakens this overly strong adsorption into a more favorable intermediate regime, while simultaneously creating a locally negatively charged interface around the iron site. Because chloride ions are negatively charged, that charged microenvironment electrostatically discourages them from approaching and poisoning the active center. In effect, the catalyst builds a nanoscale fence around its most valuable atom.

The design did not emerge by guesswork. The team first carried out density functional theory calculations, comparing several candidate pairings of iron with different p-block metals in dual-site configurations. Among the candidates, the iron-tin combination stood out with the best balance between oxygen reduction activity and chloride resistance, providing the theoretical justification for an ambitious synthesis. The researchers then produced the material using what they call a movable-type printing strategy, a modular approach that positions the two metal precursors with atomic precision on a nitrogen-doped carbon support before activation locks them into place.

Advanced characterization confirmed that the synthesis had achieved its goal. Microscopy and X-ray spectroscopy showed that both iron and tin were atomically dispersed across the carbon framework, stabilized as adjacent dual single-atom sites rather than clustering into nanoparticles. That atomic dispersion matters enormously. It maximizes the density of accessible active sites, ensures that every iron atom benefits from a tin neighbor, and prevents the formation of larger metal particles that could serve as initiation points for corrosion in the harsh chloride-rich environment.

Electrochemical testing in alkaline seawater electrolyte delivered striking results. The FeSn dual single-atom catalyst achieved a half-wave potential of 0.900 volts versus the reversible hydrogen electrode, outperforming both single-metal iron and tin catalysts prepared in the same way and beating commercial platinum-on-carbon, the traditional benchmark for oxygen reduction. The catalyst followed a near four-electron oxygen reduction pathway, the most efficient route that converts oxygen directly to hydroxide and delivers the maximum possible cell voltage. Equally important for real deployment, the material showed negligible activity decay after 30,000 accelerated durability test cycles, a punishing stress test that simulates years of charge-discharge operation.

Perhaps the most convincing evidence came from post-mortem analysis. After extended testing in the chloride-laden electrolyte, X-ray photoelectron spectroscopy revealed no observable iron-chloride signal on the catalyst surface, indicating that chloride adsorption on the iron active sites had been effectively suppressed throughout the testing period. This directly validates the design hypothesis: the negatively charged microenvironment generated by the Fe-Sn electronic interaction had done its job, keeping the poison away from the active center even as the catalyst worked hard for tens of thousands of cycles.

“For seawater batteries, activity alone is not enough,” said Peng Rao, the corresponding author of the study. “The catalyst also needs to drive oxygen conversion efficiently while resisting chloride-induced poisoning. The Fe-Sn dual-site structure gives us a way to combine these two functions at the atomic level.” The quote captures the central lesson of the work: in marine electrochemistry, selectivity against chloride is not an optional extra but a design requirement on par with activity itself.

The practical payoff was demonstrated when the catalyst was assembled into flow-type seawater air batteries. Devices built with the FeSn dual single-atom catalyst reached a peak power density of 255.6 milliwatts per square centimeter, higher than comparable cells based on platinum-on-carbon or the iron-only single-atom catalyst, and operated stably for more than 500 hours under the reported testing conditions. That combination of high power and long endurance in a realistic seawater-fed architecture suggests the material is ready to be taken seriously for marine energy systems rather than remaining a laboratory curiosity.

Beyond the immediate application, the study positions p-d orbital engineering as a general strategy for designing electrocatalysts that must simultaneously deliver high activity and resist chloride attack. Many reactions of interest in seawater electrochemistry, from oxygen evolution to chlorine-tolerant hydrogen production, face the same fundamental challenge of selective chemistry in an ionically aggressive medium. If pairing active transition-metal sites with judiciously chosen p-block partners proves general, the design principle demonstrated here could extend well beyond iron and tin, and well beyond oxygen reduction.

The research, published in Science Bulletin, arrives at a moment of growing investment in ocean-based energy infrastructure. Offshore wind farms, autonomous underwater systems and remote marine sensors all need durable storage, and seawater batteries are among the few options whose electrolyte comes free with the deployment site. By showing that a simple atomic partnership, one iron atom and one tin atom held in place on a carbon support, can defuse the corrosive chemistry of the ocean while accelerating the very reaction the battery depends on, the Hainan University team has moved the field a meaningful step closer to seawater batteries that survive, and thrive, in the environment they are built to harness.

Subject of Research: Development of an iron-tin dual single-atom catalyst using p-d orbital hybridization to improve oxygen reduction activity and chloride resistance in seawater batteries

Article Title: How can seawater batteries survive chloride corrosion?

Article References: How can seawater batteries survive chloride corrosion?. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: seawater batteries, oxygen reduction reaction, dual single-atom catalyst, chloride corrosion, p-d orbital hybridization, iron-tin catalyst, electrocatalysis, alkaline electrolyte, marine energy storage, density functional theory, nitrogen-doped carbon, air cathode