One of the most efficient ways to make green hydrogen is by splitting water using electricity in a device called a proton exchange membrane electrolyser. Inside this system, water molecules are broken apart into hydrogen and oxygen. Hydrogen is collected as a clean fuel, while oxygen is released as the only by-product. Although the overall process sounds simple, one half of the reaction is notoriously difficult. The formation of oxygen requires multiple chemical steps, happens in a strongly acidic environment, and demands high voltages. This combination is brutal for materials.

To make the oxygen-forming reaction work at a useful speed, electrolysers rely on catalysts. These materials lower the energy needed for the reaction and allow the system to run efficiently. In acidic conditions, only a handful of catalysts survive for long periods. Among them, iridium-based materials are considered the gold standard. They are both active and more resistant to corrosion than any other material. Unfortunately, iridium is one of the rarest elements on Earth. It is expensive, produced in very small quantities, and vulnerable to supply risks.

This creates a serious problem for climate policy. Plans from the European Union and the United States already assume that the amount of iridium used in future electrolysers must drop dramatically over the next decade. The challenge is obvious. How do you use less of a rare metal without making the technology slower, less efficient, or prone to failure? According to Jonathan Ruiz Esquius, answering this question requires moving away from traditional catalyst recipes and exploring new ways of arranging atoms.

One of the ideas gaining attention is the concept of high-entropy materials. Instead of being dominated by a single element, these materials combine five or more metals in similar amounts. At the atomic scale, this creates randomness, within the order. The different atoms strain the structure in subtle ways, which can improve stability and create many slightly different sites where reactions can happen. Rather than relying on one type of active site, high-entropy materials offer a whole spectrum of them.

In this work, the research team created a high-entropy oxide made from iridium, manganese, iron, cobalt, and nickel. The choice of elements was deliberate. Iridium provides the necessary resistance to acid, while the other metals help tune the electronic and structural properties of the material. Just as important as the ingredients was the structure they formed. By heating metal salts deposited on carbon fibres, the team encouraged the atoms to arrange themselves into a crystal structure known as a spinel.

Spinel oxides are known for being robust, electrically conductive, and flexible in terms of composition. However, they are usually explored in alkaline environments, not acidic ones. What makes this study unusual is that the spinel structure remained stable in acid, even under the harsh conditions required for oxygen production. Ruiz Esquius and his colleagues found that the temperature used during synthesis played a crucial role in achieving this stability.

At lower temperatures, the material remained poorly ordered and was easily attacked by acid. As the temperature increased, the atoms became more organised and the spinel structure emerged. At the highest temperature tested, 500°C, the result was a well-crystallised high-entropy spinel that resisted corrosion and showed excellent catalytic performance. This behaviour is striking because it is the opposite of what is normally seen for pure iridium oxides, which tend to lose activity as they become more crystalline.

When the new catalyst was tested under laboratory operating conditions, it performed exceptionally well. It reached industrially relevant reaction rates while requiring less extra energy than conventional iridium oxide catalysts. Even more importantly, it maintained this performance over many hours of continuous operation, showing no clear signs of degradation. Long-term stability like this is essential if electrolysers are to operate reliably for years at a time.

A key part of the story is how efficiently iridium is used. The researchers deliberately limited the amount of iridium in the catalyst to levels aligned with future policy targets. Despite this reduced content, the high-entropy spinel outperformed traditional iridium-based catalysts with similar precious metal loadings. This indicates that the surrounding manganese, iron, cobalt, and nickel atoms may actively enhance the performance of iridium, rather than simply diluting it.

The way the catalyst is made also matters for real-world impact. The synthesis method developed by Ruiz Esquius and his team is simple and scalable. It relies on common metal salts, water, and a single heating step in air. The catalyst forms directly on carbon fibre supports, creating a self-supported electrode. This design improves electrical contact, avoids the need for extra binders, and simplifies manufacturing, all of which are important for industrial deployment.

To understand why the material works so well, the researchers used advanced techniques to examine it at the atomic level. They found that iridium atoms are fully incorporated into the spinel structure instead of forming separate particles. Most of the iridium sits in positions that are particularly effective for driving the oxygen-forming reaction. At the same time, the mixture of different metals creates a wide variety of local atomic environments, helping explain the catalyst’s high activity.

Durability was tested carefully, because acidic conditions quickly destroy many promising materials. The high-entropy spinel showed strong resistance to corrosion, with most of the metal atoms remaining locked into the structure during operation. While the surface of the catalyst changes slightly as it works, these changes do not harm its performance and may even help stabilise it.

Taken together, these results point to a powerful new direction for green hydrogen technology. By combining multiple metals into a carefully designed structure, it is possible to reduce reliance on scarce elements without sacrificing efficiency or lifespan. Ruiz Esquius and his collaborators have shown that complexity at the atomic scale can be a strength rather than a weakness.

There is still work to be done before this type of catalyst finds its way into commercial electrolysers. But the message is encouraging. Meeting climate goals will require not just more renewable energy, but smarter materials that make clean technologies practical at scale. High-entropy spinel catalysts offer a compelling glimpse of how that future might be built.