Hydrogen has the potential to aid our transition to low-carbon economies. However, most of the hydrogen produced at the moment comes from methane, leading to substantial carbon dioxide (CO2) emissions. ‘Green hydrogen’ is produced sustainably, using renewable energy to power water hydrolysis to split water into oxygen and hydrogen. Proton exchange membrane water electrolysis (PEMWE) is a promising technology for water hydrolysis, producing hydrogen with high energy efficiency and rate. However, the catalysts needed to speed up the reactions involved are based on rare elements such as platinum and iridium. The anode catalysts, in particular, have to operate in highly corrosive acidic conditions in which only iridium oxides have proved stable. As iridium is one of the scarcest elements on Earth, researchers have been searching for possible alternatives. In work recently published in Science, a multidisciplinary team led by ICFO (Barcelona) has used previously unexplored properties of water to achieve stability and activity in an iridium-free catalyst. Using X-ray Absorption Spectroscopy (XAS) at Diamond’s B18 beamline, together with other photon-based spectroscopies, they were able to detect trapped water and hydroxyl groups and obtain insights into their role in the hydrolysis process. Their results offer new insights into the challenge of catalyst design and represent a big step towards developing PEMWE catalysts made from Earth-abundant metals.
An increasing global demand for energy, and the urgent requirement to mitigate climate change, are driving the transition to clean energy production. Green hydrogen has the potential to help decarbonise a wide range of industries, including transport, manufacturing and agriculture.
Producing hydrogen via water electrolysis involves two reactions - the Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER). As the OER is slower, it is the limiting factor in hydrogen production. The stability and energy efficiency of PEMWE make it a promising technology, however it relies on the use of iridium – one of the scarcest metals on Earth - to catalyse the OER. Attempts to use ruthenium have been unsuccessful as it dissolves in the acidic environment. There is, therefore, a requirement for stable and efficient catalysts that do not contain iridium or ruthenium.
Theoretically, transition metal oxides could be used. Cobalt-, nickel- and magnesium-based anodes have shown promising activity in the OER and are relatively abundant. Calculations suggest that cobalt-based oxides (CoOx) could support comparable activity to iridium-based oxides. However, CoOx have limited stability in acidic environments.
In this work, the research team designed a new cobalt-based catalyst that actively involves the ingredients of the reaction (water and its fragments) in its structure. Starting with a cobalt-tungsten oxide (CoWO4 or CWO), they developed a delamination process that removes tungsten from the material and replaces it with water and hydroxyl (OH-) groups.
Prof. F. Pelayo García de Arquer from ICFO explained:
We achieved stability in this reaction by incorporating water in the crystal structure of this cobalt tungsten oxide. Basically, we found a way to remove part of the material, so in particular the tungsten oxides, and replace them with water and water fragments or OH groups. And, surprisingly, it turned out that the incorporated water and water fragments act like a shield. They protect the catalyst from dissolving in a quite dramatic way, compared to the control samples.
The team brought their material samples to Diamond’s B18 beamline, where they used XAS to investigate their structure during reaction conditions. Their results showed that the trapped water and water fragments in the catalyst structure can be tailored to shield the catalyst from challenging conditions.
Prof. García de Arquer added:
Using X-ray spectroscopies allowed us to track changes in the materials during the reaction. That’s important because these materials, these catalysts, they change a lot in electrochemical reactions. So to understand these materials and to learn about them so we can design them in a better way, it's very important to use spectroscopy to track the materials’ structural changes while they are doing their job. This was also the first time that these iridium alternatives were benchmarked in reactors that are scalable at conditions that are relevant for industrial applications. These reactors involve very high current density and high productivity, and this means that the electrochemical conditions are even harsher.
Combining their experimental results with catalyst modelling showed that the delamination process made the material more active by increasing the number of active sites and changing the reaction mechanism. By rendering the cobalt dissolution an unfavourable process, the presence of water and hydroxide effectively hold the catalyst together.
The new catalyst achieved a threefold improvement in activity, remaining stable for 600 hours. While this does not yet match current industrial iridium catalysts, it’s a big step forward for alternatives.
The team is currently working on scaling the synthesis of the material to industrial levels, and has applied for a patent. They’re also investigating other alternatives to iridium.
Although cobalt is far more abundant than iridium, it’s not an ideal material, as sourcing it still presents environmental and social issues. So we’re working on alternatives, including manganese and nickel. We’ll use our new design strategy to explore the periodic table, to find the best materials. It’s not just a case of replacing iridium, but also replacing platinum and trying to remove forever chemicals – fluorocarbons – from PEMWE, with the goal of producing truly green hydrogen.
To find out more about the B18 beamline or discuss potential applications, please contact Principal Beamline Scientist Diego Gianolio: diego.gianolio@diamond.ac.uk.
Ram R et al. Water-hydroxide trapping in cobalt tungstate for proton exchange membrane water electrolysis. Science 384.6702 (2024): 1373-1380. DOI:10.1126/science.adk9849
image credits: ICFO; this article: 10.1126/science.adk9849
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