Catalyst breakthrough could accelerate green hydrogen production

University of Adelaide researchers, along with scientists from Japan’s Tohoku University, the Tokyo University of Science, and Vanderbilt University in the United States, have successfully developed microscopic iridium catalysts that could result in improved production of green hydrogen.

The researchers said the tiny catalysts comprise only 15 atoms of iridium and outperform commercially available iridium catalysts by 1.5 times in mass activity while demonstrating excellent durability.

Green hydrogen is produced by splitting water into hydrogen and oxygen using renewable electricity. One of the biggest challenges associated with the process is oxygen evolution reaction (OER). This chemical reaction takes place in a highly acidic and corrosive environment and iridium has proven one of the few catalysts capable of enduring that environment but due to costs and limited availability, there have been attempts to reduce the amount of the rare metal used while maximizing its reaction activity.

One approach to minimize the amount of iridium used is to create atomically precise metal nanoclusters, tiny aggregates of metal atoms. Downsizing metal particles into 1 nanometre (nm) clusters increases their specific surface area and active sites. The downside of increasing the surface area is that iridium becomes oxidated when exposed to air and thereby unstable.

To overcome this instability, the research team devised a polyol reduction method with ethylene glycol and a ligand-exchange process to protect the iridium atoms.

By encapsulating the core of iridium atoms with carbon monoxide and triphenylphosphine molecules, they were able to obtain 15-atom iridium nanoclusters that remain highly stable and resistant to oxidation, even when synthesised entirely in open air.

The researchers then attached the nanoclusters to a carbon black support to create a solid catalyst with an average particle size of 0.9 nm.

Testing showed that the new material had about 1.5 times greater mass activity than conventional commercial iridium catalysts. The catalyst also demonstrated excellent durability, operating continuously for more than 20 hours without significant performance loss.

Further analysis showed that the ultra-miniaturisation of the iridium particles altered their electronic properties in a way that made chemical reactions occur more efficiently.

Tohoku University representative Yuichi Negishi said the breakthrough could result in improved production of green hydrogen.

“We expect these findings to mark a new milestone in metal nanocluster and green hydrogen research, as it may help us create cost-effective, high-performance metal nanoclusters in order to solve pressing global energy and environmental challenges,” he said.

The findings were published in the Journal of the American Chemical Society.

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