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).