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Hydrogen is an important energy source for the future. Therefore, understanding the electrolysis process is essential. Researchers at the Max Planck Institute for Polymer Research and the Yusuf Hamied Department of Chemistry at the University of Cambridge have now investigated a related process, the autodissociation process, in more detail. While the fundamental chemistry of water dissociation is well understood under standard conditions, much less is known about how it behaves under the strong electric fields present in electrochemical devices.
In nature, the behavior of systems—whether large or small—is always governed by a few fundamental principles. For instance, objects fall downward because it minimizes their energy. At the same time, order and disorder are key variables that also shape physical processes. Systems—especially our homes—tend to become increasingly disordered over time. Even at the microscopic level, systems tend to favor increased disorder, a phenomenon known as an increase in the so called “entropy”.
These two variables – energy and entropy – play an important role in chemical processes. Processes occur automatically when energy can be reduced or entropy, i.e., disorder, increases.
Under standard conditions—such as in a glass of water—water autodissociation is hindered by both factors, making it a highly unlikely event. However, when strong electric fields are applied, the process can be dramatically accelerated.
Now, researchers at the Max Planck Institute for Polymer Research and the Yusuf Hamied Department of Chemistry at the University of Cambridge have uncovered a surprising mechanism that governs water autodissociation under such intense fields. Their findings, published in the Journal of the American Chemical Society, challenge the traditional view that this reaction is mainly driven by energy considerations
“Water autodissociation has been extensively studied in bulk conditions, where it's understood to be energetically uphill and entropically hindered,” says Yair Litman, group leader at the Max Planck Institute. “But under the strong electric fields typical of electrochemical environments, the reaction behaves very differently.”
Using advanced molecular dynamics simulations, Litman and co-author Angelos Michaelides show that strong fields dramatically enhance water dissociation—not by making the reaction more energetically favorable, but by making it entropically favorable. The electric field initially orders water molecules into a highly structured network. When ions form, they disrupt this order, increasing the system’s entropy—or disorder—which ultimately drives the reaction forward.
“It’s a complete reversal of what happens at zero field,” explains Litman. “Instead of entropy resisting the reaction, it now promotes it.”
The study also shows that under strong electric fields, the pH of water can drop from neutral (7) to highly acidic levels (as low as 3), with implications for how we understand and design electrochemical systems.
“These results point to a new paradigm,” says Michaelides. “To understand and improve water-splitting devices, we need to consider not just energy, but entropy—and how electric fields reshape the molecular landscape of water.”
The research highlights the need to rethink how reactivity is modeled in aqueous environments under bias and opens up new possibilities for catalyst design, particularly in electrochemical and “on-water” reactions.
Dr. Yair Litman
+49 6131 379 380
litmany@mpip-mainz.mpg.de
Litman, Yair; Michaelides, Angelos
Entropy governs the structure and reactivity of water dissociation under electric fields
Journal of the American Chemical Society
https://dx.doi.org/10.1021/jacs.5c12397
The autodissociation process of water is significantly influenced by electric fields.
Copyright: © MPI-P
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