Water electrolysis offers an ideal process for hydrogen production, which could play a key role in the global energy transition that increasingly relies on renewable electricity, but whose current production process is extremely carbon intensive.

As an energy source, hydrogen has been largely untapped due to unaffordability and a lack of understanding of the catalysts used to produce it. A new study from Northwestern University researchers on the most promising studied catalysts, iridium-based oxides, enabled the design of a novel catalyst that maintains higher activity, longer stability and more efficient iridium use, which could make green hydrogen production feasible.

The paper, published in the journal Nature Catalysis, combined complementary electron- and X-ray-based characterization techniques to, for the first time, identify experimental evidence for how the surface of iridium oxide changes during water electrolysis.

"Now that we finally know the nature of these active sites at the surfaces of these materials, we can design future catalysts that feature only the three structures we identified to achieve optimized performance and more efficient use of precious iridium," said Linsey Seitz, a Northwestern electrochemist and the paper's lead author.

Seitz is an assistant professor of chemical and biological engineering at Northwestern's McCormick School of Engineering and an expert in renewable energy.

This "precious iridium" is a rare byproduct of platinum mining and the only catalyst that is currently viable for green hydrogen production due to the harsh operating conditions of the reaction.

Water electrolysis—the process of breaking apart water molecules using electricity—via technology called proton exchange membrane (PEM) water electrolysis, is promising because it can run entirely on renewable electricity, but the reaction occurs in an acidic environment which limits the types of catalysts that can be used.

The reaction conditions also significantly change the structure of catalyst materials at their surface. These reorganized catalyst surface structures have been elusive to identify because they change rapidly in the process of water electrolysis and can be damaged through methods of imaging.

Prior research has computationally predicted possible connection types that may be present on the surfaces of iridium oxide but has never been able to provide direct experimental evidence.

In the current study, three connection types previously described just as "amorphous" (having no detectable structure) following a catalytic reaction were found to have distinct, paracrystalline structures, and were found to be most responsible for a catalyst's stability and activity.

The Seitz team's workflow significantly reduced damage from these techniques to enable more accurate analysis of structures in complex materials. First, the researchers used electron-based microscopy and scattering to identify the catalyst surface structure, both before and after the water electrolysis process. They then confirmed results with high-resolution X-ray spectroscopy and scattering.

"We are thrilled to extend these characterization techniques to rigorously analyze other complex, active catalyst materials whose relevant active structures have thus far been elusive to experimental identification," Seitz said.

"These fundamental insights will drive the design of high-performance catalysts that can optimally use precious metals and critical minerals content."

Using their new understanding of the iridium, the team was able to design a catalyst using only paracrystalline structures that was three to four times more efficient than other iridium-based catalysts during its first measurement of activity.

"Our developments will help bring us closer to a sustainable energy future where green hydrogen via water electrolysis is a reality and widespread deployment of these emerging technologies are more technologically and economically feasible," Seitz said.