What is it about?
Water can be dissociated into solvated protons (H+) and hydroxides (OH-). In the reverse process, the solvated ions are recombining to form water. This process is one of the fasted reactions known to mankind - in bulk water. At interfaces, it can slow down substantially. This study focuses on how the interfacial excess charge and electric fields modulate the kinetics of ion solvation and desolvation.
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Why is it important?
The solvation and desolvation of protons and hydroxides at heterogeneous interfaces is omnipresent in nature. From proton pumps and enzymes in biochemistry, to water electrolysis technology. Ions need to be desolvated or solvated at interfaces across nature. The solvation kinetics of other ions are critical in batteries, electroplating and corrosion. Oftentimes, the solvation kinetics can slow down reactions and cause large energy penalties or cause structural changes of a host during intercalation. However, currently this process is poorly understood. It challenges the most powerful computer simulations and spectroscopic method. In heterogeneous catalysis, the solvation kinetics are an intimate part of the chemical conversion process and are very important to understand the "charged state" of the active site in enzymes and in manmade catalysts. Often, a barrier for ion solvation is the prerequisite for the build-up of substantial polarization and electric fields, as this barrier can prevent the formation of charge-compensated chemical bonds.
Perspectives
Carlos studied the hydroxide and proton solvation and desolvation kinetics in isolation inside a bipolar membrane. In these systems, the water molecules can be dissociated very controllably by applying a potential. When the potential is reversing the polarity, protons and hydroxide ions are recombining to form water. It is as if we had two hands that can carefully pull the water molecules apart into the solvated ions or put the ions back together to form a water molecule. Bipolar membranes are also industrially-relevant and are being used in electrodialysis to generate acid and base solutions. Further, they are explored for many new applications. However, so far, our understanding about their kinetics was very limited. Carlos overcame previous limitations by performing temperature-dependent studies under high mass transport conditions to obtain the potential-dependent Arrhenius activation parameters. Further, in this study, he rigorously considers the reverse reaction, the ion desolvation and water formation kinetics - a reaction that was long thought to be only mass transport limited. However, just as water dissociation, water formation can also slow down substantially at heterogeneous interfaces. Considering the reverse reaction was a very important step toward a more comprehensive understanding and to even connect to the electrochemical kinetics of electrocatalytic reactions more broadly. To show this, Carlos even studied the kinetics of the hydrogen evolution and oxidation reaction on Pt in acid - a reaction that is so fast, that it requires very high mass transport to be studied accurately. Taken together, these results show that bipolar membranes adhere to the same general working principles than other electrochemical reactions. Beyond bipolar membranes, the results have very important implications for the kinetics across geo-, bio- and electrochemistry, due to the omnipresent ion solvation and transfer kinetics in many important reactions.
Sebastian Oener
Fritz Haber Institute of the Max Planck Society
Read the Original
This page is a summary of: The role of interfacial excess charge in the reversibility of proton and hydroxide solvation in electrocatalysis and bipolar membranes, Proceedings of the National Academy of Sciences, April 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2531938123.
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