green hydrogen by electrolysis of H2O and a proton exchange membrane: Researchers from the ICFO Institute and other European centers have reached a new milestone green hydrogenunder relevant industrial conditions, by electrolysis of the H2O and a proton exchange membrane. Instead of using very scarce raw materials, such as iridium, they used a new cobalt-tungsten oxide.
He hydrogen green It is a promising chemical and energetic element to decarbonize our society. Unlike conventional fuels, its use as a fuel does not generate carbon dioxide. Unfortunately, Today, most of the hydrogen produced in our society comes from methane, a fossil fuel. It does this in a process (methane reforming) that leads to significant carbon dioxide emissions.
Therefore, the production of green hydrogen requires alternatives to this process that are scalable. One of them is the water electrolysiswhich provides a way to generate it that can be worked with renewable energy and clean electricity. This process requires cathodic and anodic catalysts to accelerate the fission and recombination reactions of water into water green hydrogen and oxygenbecause otherwise they would be inefficient.
Since its discovery in the late 18th century, water electrolysis has evolved into several technologies. One of the most promising implementations is the proton exchange membrane (PEM), which can produce green hydrogen combining high rates and high energy efficiency.
To date, water electrolysis, and in particular the proton exchange membrane (PEM), needs catalysts based on scarce and rare elements, such as platinum and iridium, among other things. Only a few compounds combine the activity and stability required in the aggressive chemical environment imposed by this reaction. This is especially complicated in the case of anodic catalysts, which must operate in highly corrosive acidic environments, conditions in which only iridium oxides have shown stable performance under the required industrial conditions. But iridium is one of the rarest elements on Earth.
An iridium-free catalyst
In search of possible solutions, a team of scientists recently took an important step toward finding alternatives to iridium catalysts. This multidisciplinary team has succeeded in developing a new way to impart activity and stability to a catalyst without iridium, taking advantage of the previously undiscovered properties of water. The new catalyst achieves stability for the first time in the electrolysis of water via the proton exchange membrane (PEM) under industrial conditions without using iridium.
This preview, published in Sciencewas carried out by researchers from the Institute of Photonic Sciences (ICFOin Barcelona) led by Professor F. Pelayo García de Arquer, and includes important collaborations of the Chemical Research Institute of Catalonia (ICIQ), the Catalan Institute of Science and Technology (ICN2), the French National Center for Scientific Research (CNRS) , Diamond Light Source and the Institute for Advanced Materials (INAM).
Combine activity and stability in one very pickles it is a challenge. The catalyst metals tend to dissolve, as most materials are not thermodynamically stable at low pHs and applied potential, in an aqueous environment. Iridium oxides combine activity and stability under these harsh conditions and are therefore the main choice for anodes in proton exchange water electrolysis.
Seeking alternatives to iridium It is not only an important applied challenge, but also a fundamental one. Intensive research in the search for iridium-free catalysts has led to new insights into reaction and degradation mechanisms, mainly through the use of probes that could study catalysts during operation in combination with computer models.
This led to promising results using materials based on manganese and cobalt oxideand exploiting different structures, compositions and dopants to modify the physical and chemical properties of the catalysts.
Although insightful, most of these studies were conducted in non-scalable fundamental reactors and under milder conditions that are far from their final application, especially in terms of current density. Until now, it has been difficult to demonstrate activity and stability with non-iridium catalysts in PEM reactors and under operating conditions relevant to this type of technology.
Green hydrogen: a strategy based on cobalt
To overcome this challenge, the authors devised a new approach in the design of catalysts without iridium, achieving activity and stability in acidic media. His strategy, based on cobalt (very plentiful and cheap), was very different from the usual routes.
“The design of conventional catalysts is usually aimed at changing the composition or structure of the materials used. Here we choose a different approach. We have designed a new material that actively involves the reaction ingredients (water and its fragments) in its structure. “We discovered that the incorporation of water and water fragments into the catalyst structure can be tailored to protect it under these challenging conditions, enabling stable operation at high current densities relevant for industrial applications,” explains the ICFO professor. García de Arquer.
With his technique, consisting of a delamination process in which part of the material is exchanged for water, the resulting catalyst is presented as a viable alternative to iridium-based catalysts.
New approach with delamination
To obtain the catalyst, the team examined a certain cobalt oxide: cobalt tungsten oxide (CoWO4), or CWO for short. From this starting material they designed a delamination process using basic aqueous solutions that produce tungsten oxides (WO42-) are removed from the network and exchanged for water (H2O) and hydroxyl groups (OH–) in a simple environment.
This process can be adjusted to include different amounts H2O and o– in the catalyst, which would then be incorporated into the anode electrodes.
The team combined different spectroscopy based on photons to understand this new class of material during operation. Using technology Infrared Raman and X-raysamong other things, were able to evaluate the presence of entrapped water and hydroxyl groups and obtain information on their role in conferring activity and stability during the splitting of water into acid.
“It was a real challenge for us to be able to detect stuck water,” continues the co-lead author, Anku Guha from ICFO, who adds: “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 limited water; something that had a profound impact on performance.
Based on this knowledge, they started working closely with experts in the field of catalyst modeling. “Modelling activated materials is challenging because major 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 detailed mapping between experimental observations and simulations,” says another author. Núria López of the ICIQ.
Their calculations, led by the lead co-author, Praise Benzidialso from ICIQ, were crucial to understanding how delaminated, water-shielded materials were not only thermodynamically protected from dissolution in highly acidic environments, but also active.
Green hydrogen from water and hydroxide
But how is this possible? In short, the elimination of tungsten oxide leaves a hole behind, exactly where it was before. This is where the “magic” happens: water and hydroxidethat are very present in the environment, to fill spontaneously the vacuum.
This in turn protect the monsterbecause it makes the dissolution of cobalt an unfavorable process, effectively holding the catalyst components together.
They then assembled the delaminated catalyst in a reactor. proton exchange membrane (PEM). The initial performance was truly remarkable, achieving greater activity and stability than any previous technique. “We increased the current density five times to 1 A/cm2, a very complicated milestone in the field. But the key is that we also achieved more than 600 hours of stability at such a high density. In this way, we have achieved the highest current density and also the highest stability for catalysts without iridium,” says the other important co-author, Lu Xiaof the ICFO.
“At the beginning of the project, we were intrigued by the potential role of water itself, which could be the ‘elephant in the room’ of water electrolysis,” he explains. Wounded Aries, first author of the study and promoter of the original idea of the ICFO. “Until now, no one had actively modified water and surface water in this way,” he emphasizes. Ultimately, this turned out to be a real turning point.
Although the stability time is still far from today’s industrial PEMs, this represents a major step toward making them independent of iridium or similar elements. Especially his work offers new knowledge for designing proton exchange membrane (PEM) for water electrolysis, as this highlights the potential to approach catalyst engineering from a different perspective; actively benefit from the properties of water.
On the road to industrialization
The team has seen so much potential in the technique that they have already filed for a patent, with the aim of expanding it to industrial production levels. However, they are aware that taking this step is not trivial, as Professor García de Arquer notes: “The cobaltwhich is more abundant than iridium remains a very worrying material, taking into account where it comes from. That is why we are working on alternatives based on manganese, nickel and many other materials. If necessary, we will go through the entire periodic table. And we will work with them to explore and test this new catalyst design strategy that we reported in our study.”