In brief

Nickel oxyhydroxide electrocatalysts derived from different starting compounds show varying distortions in electronic band structure, which influence their electron transfer abilities and catalytic performance in oxygen evolution reactions.

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New electron highways uncovered

27 Oct 2023

Subtle molecular differences in nickel-based electrocatalysts can open the path to more efficient water-splitting chemical reactions for clean energy generation.

Since the Industrial Revolution, we’ve leaned heavily on burning fossil fuels to power our homes, factories and offices. Through combustion reactions, we break down hydrocarbon molecules in oil, coal and natural gas to release not just energy but carbon dioxide, a major contributor to climate change.

Thankfully, greener alternative fuels are close at hand. These include hydrogen, which can be cleanly produced by splitting water using renewable electricity in a process called water electrolysis. Consisting of two half reactions—the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER)—water electrolysis produces hydrogen, which can then be burned in fuel cells that convert it back to water.

However, a bump in the road is that today’s water-splitting technologies still need efficiency boosts before they can be rolled out into green energy infrastructure. To that end, researchers like Shibo Xi, a Senior Scientist at A*STAR’s Institute of Sustainability for Chemicals, Energy and Environment (ISCE2 ), are examining how chemical electrocatalysts such as nickel oxide hydroxides (NiOOH) can enhance OERs.

Previous work led by Xi found that seemingly identical-looking NiOOH variants derived from different starting materials turned out to exhibit varying OER activities, suggesting that the catalysts’ structures may hold clues to unlocking their optimal performance.

“However, there have been challenges in determining these variations due to our past reliance on partially-reconstructed models,” explained Xi. “These models aren’t entirely accurate in representing the structures of electrocatalysts, which prevented us from fully understanding how they affect OER efficiency.”

To bridge this gap, Xi and colleagues from A*STAR’s Institute of High Performance Computing (IHPC) and the National University of Singapore used an advanced, high-resolution approach to investigate how using one of three different possible starting materials to create NiOOHs—NiS2, NiSe2 and Ni5P4—exerted an effect on the resulting compound’s OER-catalysing ability.

Using a technique called X-ray absorption fine structure spectroscopy, they focused on probing the local structures around nickel and oxygen atoms. Through their analyses, they established never-before-seen connections between the distortion of the NiO6 octahedron within the NiOOH, a broadening of the compound’s density of states (eg*), and an enhanced ability to catalyse OERs.

“Think of the eg* band as a highway for electrons,” said Xi. “A broader highway allows more electrons to move more freely, which translates to enhanced catalytic performance.”

The researchers also tested this concept in a different binary nickel-iron oxyhydroxide catalyst with similar results, suggesting that molecular design strategies to broaden eg* bands can lead to a new generation of high-efficiency OER electrocatalysts—and another step towards a clean energy future.

Xi said that the group is currently exploring how variations in catalyst boosters called dopants can further enhance NiOOHs’ catalytic capacities. “By understanding this relationship, we can further optimise the efficiency of materials used for water-splitting reactions,” concluded Xi.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Sustainability for Chemicals, Energy and Environment (ISCE2 ) and the Institute of High Performance Computing (IHPC).

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Zhong, H., Wang, X., Sun, G., Tang, Y., Tan, S., et al. Optimization of oxygen evolution activity by tuning eg* band broadening in nickel oxyhydroxide. Energy & Environmental Science 16 (2), 641–652 (2023).│ article

About the Researchers

Shibo Xi earned his PhD in Optics from the Beijing Synchrotron Radiation Facility (BSRF) at the Institute of High Energy Physics, Chinese Academy of Sciences. During his time there, he honed his expertise in utilising synchrotron radiation equipment to investigate the local structure of materials. In 2012, he joined A*STAR, where he currently holds the position of Senior Scientist II in the department of Catalysis and Green Process Engineering (CGPE) within ISCE2. Xi serves as a beamline scientist of the XAFCA beamline at the Singapore Synchrotron Light Source. His primary focus revolves around unravelling the atomic-level local structure of catalysts, particularly in their operational states, utilising X-ray absorption fine structure (XAFS) techniques. Xi actively collaborates with numerous research groups to conduct in-depth mechanistic investigations in the fields of thermocatalysis and electrocatalysis.
Zhigen Yu obtained his PhD degree in materials science from the National University of Singapore. He is currently a Senior Principal Scientist at A*STAR’s Institute of High Performance Computing (IHPC) and an Adjunct Associate Professor at the NUS Department of Materials Science and Engineering. His research interests include 2D materials, transparent semiconductors, energy storage and harvesting and catalysts design, where he uses first-principles simulations to study the mechanisms underlying defect formation as well as the electronic properties of materials, electrodes for metal ion batteries and electrochemical reactions mechanism.

This article was made for A*STAR Research by Wildtype Media Group