Staff

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

Water electrolysis offers a path to generate green hydrogen which can be powered by renewables and clean electricity. This process needs cathode and anode catalysts to accelerate the otherwise inefficient reactions of water splitting and recombination into hydrogen and oxygen, respectively. From its early discovery in the late 18th century, water electrolysis has matured into different technologies. One of the most promising implementations of water electrolysis is the Proton-exchange-membrane (PEM), which can produce green hydrogen by combining high rates and high energy efficiency.

To date, water electrolysis, particularly PEM, has required catalysts based on scarce, rare elements, such as platinum and iridium, among others. Only a few compounds combine the required activity and stability in the harsh chemical environment imposed by this reaction. This is especially challenging in the case of anode catalysts, which have to operate at highly corrosive acidic environments – conditions where only iridium oxides have shown stable operation at the required industrial conditions. But iridium is one of the scarcest elements on earth. 

In search of possible solutions, a team of scientists has recently taken an important step to find alternatives to iridium catalysts. This multidisciplinary team has managed to develop a novel way to confer activity and stability to an iridium-free catalyst by harnessing so far unexplored properties of water. The new catalyst achieves, for the first time, stability in PEM water electrolysis at industrial conditions without the use of iridium. 
This breakthrough, published in Science (https://doi.org/10.1126/science.adk9849), includes important collaborations from the Institute of Chemical Research of Catalonia (ICIQ), The Catalan Institute of Science and Technology (ICN2), French National Center for Scientific Research (CNRS), Diamond Light Source, and the Institute of Advanced Materials (INAM). 

Dealing with the acidity
Combining activity and stability in a highly acidic environment is challenging. Metals from the catalyst tend to dissolve, as most materials are not thermodynamically stable at low pH and applied potential in a water environment. Iridium oxides combine activity and stability under these harsh conditions, making them the prevalent choice for anodes in proton-exchange water electrolysis.

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

While insightful, most of these studies were performed in fundamental, not-scalable reactors and operating at softer conditions far from the final application, especially in terms of current density. Demonstrating activity and stability with non-iridium catalysts in PEM reactors and at PEM-relevant operating conditions (high current density) remained elusive to date.

To overcome this, the researchers came up with a new approach to the design of non-iridium catalysts, achieving activity and stability in acid media. Their strategy, based on cobalt (very abundant and cheap), was quite different to the common paths.

“Conventional catalyst design typically focuses on changing the composition or the structure of the employed materials. Here, we took a different approach. We designed a new material that actively involves the ingredients of the reaction (water and its fragments) in its structure. We found that the incorporation of water and water fragments into the catalyst structure can be tailored to shield the catalyst at these challenging conditions, thus enabling stable operation at the high current densities that are relevant for industrial applications”, explains Professor at ICFO, Dr F García de Arquer who led the research. With their technique, consisting of a delamination process that exchanges part of the material by water, the resulting catalyst is a viable alternative to iridium-based catalysts.

A new approach: the delamination process
To obtain the catalyst, the team looked into a particular cobalt oxide: cobalt-tungsten oxide (CoWO4), or in short CWO. On this starting material, they designed a delamination process using basic water solutions whereby tungsten oxides (WO42-) would be removed from the lattice and exchanged by water (H2O) and hydroxyl (OH-) groups in a basic environment. This process could be tuned to incorporate different amounts of H2O and OH- into the catalyst, which would then be incorporated into the anode electrodes.

The team combined different photon-based spectroscopies to understand this new class of material during operation. Using infrared Raman and x-rays, among others, they assessed the presence of trapped water and hydroxyl groups and obtained insights on their role in conferring activity and stability for water splitting in acid.

But how is this possible? Basically, removing tungsten oxide leaves a hole behind exactly where it was previously located. Here is where the “magic” happens: water and hydroxide, which are vastly present in the medium, spontaneously fill the gap. This, in turn, shields the sample, as it renders the cobalt dissolution an unfavourable process, effectively holding the catalyst components together.
Then, they assembled the delaminated catalyst into a PEM reactor. The initial performance was truly remarkable, achieving higher activity and stability than any prior art.

Even though the stability time is still far from the current industrial PEMs, this represents a big step towards making them not dependent on iridium or similar elements. In particular, their work brings new insights for water electrolysis PEMs design, as it highlights the potential to address catalyst engineering from another perspective by actively exploiting the properties of water.

Towards the industrialisation
The team has seen such potential in the technique that they have already applied for a patent, aiming to scale it up to industry production levels. Yet, they are aware of the non-triviality of taking this step, as Prof. García de Arquer notices: “Cobalt, being more abundant than iridium, is still a very troubling material considering from where it is obtained. That is why we are working on alternatives based on manganese, nickel and many other materials. We will go through the whole periodic table if necessary. And we are going to explore and try with them this new strategy to design catalysts that we have reported in our study”.

Despite the new challenges that will for sure arise, the team is convinced of the potential of this delamination process and they are all determined to pursue this goal. Ranit Ram, in particular, shares: “I have actually always wanted to advance renewable energies because it will help us as a human community fight against climate change. I believe our studies contributed one small step in the right direction”.
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Source: Adapted from ICFO Press Release (https://www.icfo.eu/news/2365/new-catalyst-unveils-the-hidden-power-of-water-for-green-hydrogen-generation/)