Can we observe changes in the movement of water molecules near a metal electrode corresponding to variations in the applied voltage on the electrode? This question holds significant importance in the development of next-generation batteries employing aqueous electrolytes.

In the nanoscale realm, chemists utilize laser light through spectroscopic techniques to visualize molecules. Spectra derived from molecular spectroscopy represent the response of molecules illuminated by laser light. Directly observing water molecules near the metal electrode using vibrational spectroscopy was previously unattainable due to the strong response emitted by metal atoms in the electrode to the experimental light. This interference obscured the response of water molecules near the electrode, the focal point of the experimental undertaking. Additionally, water molecules distant from the electrode surface also contribute to the response of the applied light, complicating the selective observation of molecules at the liquid-metal electrode interface.

In a recent publication in the Proceedings of the National Academy of Sciences (PNAS), a collaborative team of experimental and computational physical chemists from South Korea and the United States addressed this challenge with their newly developed spectroscopic techniques coupled with computer simulations. Employing surface-enhanced femtosecond (1×10^(-15) second) two-dimensional vibrational spectroscopy, the authors affixed arrays of meticulously designed organic molecules to the metal electrode surface. They successfully observed the changes in the movement of water molecules near the metal electrode by measuring the response from the attached molecules when exposed to applied laser light.

Depending on the magnitude and polarity of the applied voltage on the metal electrode, the researchers observed, for the first time, either a deceleration or acceleration of the motion of water molecules near the electrode. "When applying a positive voltage to the electrode, the movement of nearby water molecules slows down. Conversely, for a negative voltage, the opposite is observed both in femtosecond vibrational spectroscopy and in computer simulations," explains Dr. Kijeong Kwac, a research fellow at the IBS Center for Molecular Spectroscopy and Dynamics in South Korea and co-author of the study.

This outcome implies a close relationship between electrochemical reactions involving water on the surface of electrodes and the dynamics of interfacial water molecules. "The results of this study provide crucial information for understanding electrochemical reactions, offering essential physical insights necessary for the research and development of aqueous electrolyte batteries in the future," comments Prof. Minhaeng Cho, corresponding author of the study and director of the IBS Center for Molecular Spectroscopy and Dynamics.