Hydrogen is widely seen as a key energy source for the future, which makes it critical to understand how water is split during electrolysis. Scientists from the Max Planck Institute for Polymer Research and the Yusuf Hamied Department of Chemistry at the University of Cambridge have taken a closer look at a closely related process known as water autodissociation. Although the basic chemistry of water splitting is well understood under everyday conditions, far less is known about how water behaves inside electrochemical devices where powerful electric fields are present.
Across nature, systems large and small follow a few core principles. Objects fall because doing so lowers their energy. At the same time, order and disorder strongly influence how physical processes unfold. Over time, systems tend to become more disordered, something most people recognize from daily life. This tendency toward disorder also applies at the molecular scale and is described by a property known as entropy.
Energy and entropy together determine whether a chemical process happens on its own. Reactions proceed naturally when energy decreases or when disorder increases. In normal conditions, such as a glass of water, water molecules rarely break apart on their own because the process is discouraged by both energy and entropy. When strong electric fields are introduced, however, that situation changes dramatically.
A Surprising Mechanism Under Strong Electric Fields
Researchers from the Max Planck Institute and the University of Cambridge have now uncovered an unexpected mechanism that controls water autodissociation under intense electric fields. Their study, published in the Journal of the American Chemical Society, challenges the long-held assumption that this reaction is mainly controlled by energy alone.
"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."
How Electric Fields Turn Order Into a Driving Force
Using advanced molecular dynamics simulations, Litman and co-author Angelos Michaelides found that strong electric fields greatly increase water dissociation in an unexpected way. Rather than lowering the energy cost of the reaction, the electric field makes the process favorable by increasing entropy. The field first forces water molecules into a highly ordered arrangement. When ions begin to form, that structure breaks down, increasing disorder and pushing 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."
Implications for pH and Electrochemical Design
The researchers also discovered that strong electric fields can significantly change water's acidity. Under these conditions, the pH can drop from neutral (7) to highly acidic values (as low as 3). This shift has important consequences for how electrochemical systems operate and how they should be designed.
"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 findings suggest that scientists may need to rethink how chemical reactions in water are modeled when electric fields are involved. They also open new directions for designing catalysts, especially for electrochemical and "on-water" reactions.
