Electrocatalytic transformations not only require electrical energy - they also need a reliable middleman to spark the desired chemical reaction.
Surface metal-hydrogen intermediates can effectively produce value-added chemicals and energy conversion, but, given their low concentration and fleeting lifespan, they are difficult to characterize or study in depth, especially at the nanoscale.
Now, Cornell researchers have used single-molecule super-resolution reaction imaging to gain a clearer view of what happens, and where, in surface metal-hydrogen intermediates - insights that could help boost hydrogen production and decontamination of aqueous pollutants.
The research was published Oct. 27 in Nature Catalysis. The paper's lead author is former postdoctoral researcher Wenjie Li. The project was led by Peng Chen, the Peter J.W. Debye Professor of Chemistry in the College of Arts and Sciences.
To parse the behavior of the intermediates, the researchers selected palladium-hydrogen as a model system. The imaging process involved the introduction of a molecule that probed an individual palladium nanocube and reacted with the palladium-hydrogen intermediates on its surface, generating another molecule, which is fluorescent.
"That fluorescence allows us to image it at the individual molecule level, so we can see every single probe reaction product. And not only we can see at a single molecule level, we can also pinpoint its position with a nanometer spatial precision," Chen said.
The imaging revealed that individual palladium particles had diverse hydrogenation behaviors and properties. In addition, the team discovered that intermediates can form at different sites on the same particle and therefore exhibit different behaviors.
"Another important thing we see is that once this hydrogen intermediate is formed on the palladium catalyst, now it turns out the hydrogen atom on the palladium surface is not what you call a static object," Chen said. "The hydrogen can move around, not only on palladium particles, but also moving off to the surrounding electro surface."
This process, hydrogen spillover, is well known, but until now, researchers have not visualized how far the spillover can reach. The team's probing molecule enabled them to measure this distance and map its location, which was more than hundreds of nanometers away.
Typically, to study metal-hydrogen intermediates, researchers rely on "ensemble-averaged methods," whereby the formation of intermediates is measured in bulk. Using what's known as a Gaussian-broadening kinetic analysis, the researchers determined that these methods, while useful, have inherent shortcomings, notably overestimating the stability of the intermediates and often masking the particle-to-particle and site-to-site variations.
"In our measurement, we can differentiate particles. We also have a way to estimate the differences between sites on the same particle," Chen said. "Now, with this capability, we can more reliably determine the reduction potential that leads to the formation of this palladium-hydrogen intermediate."
The generality of the team's approach could lead to probing a wide range of electrochemical intermediates. It could be particularly valuable for using electrocatalysis in hydrogen generation and detoxifying aqueous environments of pollutants such as chlorinated compounds.
Co-authors include former postdoctoral researchers Muwen Yang, Ming Zhao, Rong Ye, Bing Fu; and Zhiheng Zhao, Ph.D. '25.
The research was supported by the National Science Foundation (NSF), the Army Research Office and the U.S. Department of Energy.
The researchers made use of the Cornell Center for Materials Research, which is supported through the NSF MRSEC program.
