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Iron oxide 'oxygen sponge' doubles green hydrogen production efficiency by targeting atomic-level active sites

As the world shifts toward sustainable energy sources, "green hydrogen"—hydrogen produced without emitting carbon—has emerged as a leading candidate for clean power. In a significant step forward, a research team has developed a new iron-based catalyst that more than doubles the conversion efficiency of thermochemical green hydrogen production.

Their findings were recently in the journal Acta Materialia. The collaborative research team was led by Professor Hyungyu Jin from the Department of Mechanical Engineering at POSTECH and Professor Jeong Woo Han from the Department of Materials Science and Engineering at Seoul National University.

With growing concerns over fossil fuel–driven pollution and climate change, hydrogen is gaining attention as a clean energy carrier that only emits water upon combustion. Among various hydrogen production pathways, thermochemical water splitting—which uses thermal energy to split water into hydrogen and oxygen—is considered particularly promising. Central to this process is the role of metal oxides, which absorb and release oxygen in cycles, effectively acting like "oxygen sponges."

However, most conventional oxides suffer from a key limitation: they require extremely high temperatures to operate effectively due to their thermodynamic characteristics. This has hindered their commercial viability. To address this challenge, the research team developed a novel iron-poor nickel ferrite (Fe-poor NiFe₂O₄, or NFO).

While traditional oxides typically rely on non-stoichiometric reactions that allow relatively small oxygen absorption and release, the Fe-poor ferrite exhibits a distinct phase transformation mechanism that enables significantly greater oxygen capacity even at lower temperatures.

Experimental results showed that the novel oxides achieved a water-to-hydrogen conversion efficiency of 0.528% per gram of oxides—more than double the 0.250% benchmark set by the previous best-performing material.

What makes this study particularly noteworthy is not only the development of a high-efficiency catalyst, but also the team's success in unraveling the underlying mechanisms. Using a combination of experimental techniques and computational simulations, the researchers were able to identify, for the first time, the "structural active sites" within iron oxide materials that drive hydrogen production at the atomic level.

They further revealed that a redox swing between two types of iron sites is directly correlated with hydrogen yield—an insight that could guide the future design of even more effective catalysts.

"This study is meaningful in that it proposes an economical and sustainable hydrogen production pathway using abundant iron oxides. It also opens the door to using solar heat or industrial waste heat as energy sources for hydrogen generation," said Professor Jin.

Professor Han added, "This work is a compelling example of how experimental and computational sciences can work together to uncover fundamental principles through interdisciplinary collaboration."