New catalyst reveals hidden power of water to produce green hydrogen

Hydrogen is a promising chemical and energy vector for the decarbonization of our society. Unlike conventional fuels, using hydrogen as a fuel does not produce carbon dioxide back. Unfortunately, today most of the hydrogen produced in our society comes from methane, a fossil fuel. It does so in a process (methane reforming) that leads to significant carbon dioxide emissions. Therefore, green hydrogen production requires scalable alternatives to this process.

Water electrolysis offers a way to produce green hydrogen that can be powered by renewables and clean electricity. This process requires cathode and anode catalysts to accelerate the otherwise inefficient water splitting and recombination reactions into hydrogen and oxygen. Since its early discovery in the late 18th century, water electrolysis has evolved into various technologies. One of the most promising implementations of water electrolysis is the proton exchange membrane (PEM), which can produce green hydrogen combining high rates and high energy efficiency.

Until now, water electrolysis – and PEM in particular – has required catalysts based on rare, rare elements such as platinum and iridium, among others. Only a few compounds combine the desired activity and stability in the harsh chemical environment induced by this reaction. This is particularly challenging in the case of anode catalysts, which must operate in a highly corrosive acidic environment – ​​conditions where only iridium oxides have demonstrated stable operation in the required industrial conditions. But iridium is one of the rarest elements on Earth.

In the search for possible solutions, a team of scientists recently took an important step towards finding alternatives to iridium catalysts. This multidisciplinary team succeeded in developing a new way to confer activity and stability on an iridium-free catalyst by exploiting the previously unexplored properties of water. The new catalyst achieves – for the first time – stability in PEM water electrolysis under industrial conditions without the use of iridium.

This breakthrough, published in Science, conducted by ICFO researchers Ranit Ram, Dr. Lu Xia, Dr. Anku Guha, Dr. Victoria Golovanova, Dr. Marinos Dimitropoulos, Aparna M. Das and Adrián Pinilla-Sánchez under the guidance of ICFO Professor Dr. F. Pelayo García de Arquer; and includes an important collaboration with the Institute of Chemical Research of Catalonia (ICIQ), The Catalan Institute of Science and Technology (ICN₂), the French National Center for Scientific Research (CNRS), the Diamond Light Source and the Institute of Advanced Materials (INAM).

Dealing with acidity

Combining activity and stability in a highly acidic environment is challenging. Catalyst metals tend to dissolve because most materials are not thermodynamically stable at low pH and applied potential in an aqueous environment. Iridium oxides combine activity and stability under these harsh conditions, making them the predominant choice for anodes in proton exchange water electrolysis.

The search for alternatives to iridium is not only an important applied challenge, but also a fundamental one. Intensive research in the search for non-iridium catalysts has led to new insights into reaction mechanisms and degradation, especially using probes that could study the catalysts during operation in combination with computational models. This led to promising results using materials based on manganese and cobalt oxides and using different structures, compositions and additives to modify the physicochemical properties of the catalysts.

Although insightful, most of these studies have been performed in basic non-scalable reactors and operated under milder conditions that are far from the final application, especially in terms of current density. To date, demonstration of activity and stability with non-iridium catalysts in PEM reactors and under PEM-relevant operating conditions (high current density) has been elusive.

To overcome this, ICFO, ICIQ, ICN₂CNRS, Diamond Light Source and INAM researchers have come up with a new approach in the construction of non-iridium catalysts, achieving activity and stability in acidic environments. Their strategy based on cobalt (very abundant and cheap) was quite different from the usual routes.

“Conventional catalyst design usually focuses on changing the composition or structure of the materials used. Here we took a different approach. We designed a new material that actively incorporates the components of the reaction (water and its fragments) into its structure. They found that the incorporation of water and water fragments into the structure of the catalyst can be tailored to protect the catalyst in these demanding conditions and thus enable stable operation at high current densities that are relevant for industrial applications,” explains ICFO Professor García de Arquer.

With their technique, consisting of a delamination process that exchanges part of the material for water, the resulting catalyst presents itself as a viable alternative to iridium-based catalysts.

A new approach: the delamination process

To obtain the catalyst, the team focused on a specific cobalt oxide: cobalt-tungsten oxide (CoWO₄), or CWO for short. On this starting material, they proposed a delamination process using alkaline water solutions, while tungsten oxides (WO₄^2-) would be removed from the grid and replaced with water (H₂O) and hydroxyl (OH^–) groups in the base environment. This process could be tuned to include different amounts of H₂O and OH^– into the catalyst, which would then be built into the anode electrodes.

The team combined different photon spectroscopies to understand this new class of material in operation. Using infrared Raman spectroscopy and X-rays, among other things, they were able to assess the presence of trapped water and hydroxyl groups and gain insights into their role in conferring activity and stability in acid water splitting.

“Being able to detect trapped water was really challenging for us,” continues co-senior author Dr. Anku Guha. “Using Raman spectroscopy and other light-based techniques, we finally saw that there was water in the sample. But it wasn’t ‘free’ water, it was confined water,” something that had a profound impact on performance.

Based on these insights, they began working closely with collaborators and experts in catalyst modeling.

“Modeling activated materials is challenging because large structural rearrangements occur. In this case, the delamination used in the activation treatment increases the number of active sites and changes the reaction mechanism, making the material more active. Understanding these materials requires a detailed mapping between experimental observations and simulations.” says Prof. Núria López from ICIQ.

Their calculations, led by co-author Dr. Hind Benzidi, were instrumental in understanding how water-protected delaminated materials are not only thermodynamically protected against dissolution in highly acidic environments, but also active.

But how is that possible? Basically, removing the tungsten oxide leaves behind a hole, exactly where it was previously placed. This is where the “magic” happens: Water and hydroxide, which are abundant in the medium, spontaneously fill the gap. This in turn protects the sample as it makes cobalt dissolution an unfavorable process and effectively holds the catalyst components together.

The team assembled the delaminated catalyst into a PEM reactor. The initial performance was truly remarkable, achieving higher activity and stability than any previous work.

“We increased the current density fivefold and reached 1 A/cm²—a very challenging landmark in this area. But the key is that we also achieved over 600 hours of stability at such a high density. We have thus achieved the highest current density and also the highest stability for non-iridium catalysts,” says lead co-author Dr. Lu Xia.

“At the beginning of the project, we were intrigued by the potential role of water itself as the elephant in the room in water electrolysis,” explains Ranit Ram, first author of the study and initiator of the original idea. “No one has actively treated water and water at the interface in this way before.”

In the end, it turned out to be a real game changer.

Although the stability time is still far from current industrial PEMs, it represents a big step towards not being dependent on iridium or similar elements. In particular, their work provides new insights for the design of PEM water electrolysis, as it highlights the potential to address catalyst engineering from a different perspective; by actively using the properties of water.

Towards industrialization

The team saw such potential in the technique that they have already applied for a patent with the aim of scaling it up to industrial scale production. Nevertheless, they are aware of the non-trivial nature of this step, as prof. García de Arquer.

“Cobalt, which is more abundant than iridium, is still a material of great concern because of where it comes from. That’s why we’re working on alternatives based on manganese, nickel, and many other materials. We’ll go through the entire periodic table if And we’ll explore with them and try this the new catalyst design strategy we introduced in our study,” says Prof. García de Arquer.

Despite the new challenges that will surely arise, the team is convinced of the potential of this delamination process and everyone is determined to pursue this goal.

In particular, Ram shares, “Actually, I always wanted to develop renewable energy because it will help us as a human community to fight climate change. I believe our studies have contributed one small step in the right direction.”