A team led by Professor Wang Tao, Professor Zhou Zhisou, and Academician Sun Shigang from the College of Chemistry and Chemical Engineering at Xiamen University, in collaboration with Professor Huang Jun from the Forschungszentrum Jülich in Germany, Associate Researcher Chen Junxiang from the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, and others, has revealed the interfacial molecular structure and dynamic evolution of the electric double layer (EDL) far from equilibrium by combining time-resolved surface-enhanced infrared absorption spectroscopy with machine learning molecular dynamics simulations. The findings were published in the international academic journal *Nature* on September 2.
Charged solid-liquid interfaces are the core sites where energy and matter conversion processes occur. In the nanoscale region where the electrode contacts the electrolyte, surface charges, ions, and water molecules collectively form an "electric double layer." Despite being only nanometers thick, this interfacial region harbors an extremely strong local electric field that directly influences processes such as molecular adsorption and ion transport. For a long time, classical electric double layer theory has treated the EDL as a static, incompressible structure, making it difficult to explain the real-time responsive restructuring of ions, water molecules, and the local electric field within the EDL under actual electrocatalytic conditions far from equilibrium.
The research team established an experimental-theoretical framework integrating time-resolved surface-enhanced infrared absorption spectroscopy with machine learning molecular dynamics simulations. Under hydrogen evolution reaction (HER) conditions, they resolved the interfacial molecular structure and dynamic evolution of the EDL far from equilibrium, and further developed a compressible electric double layer model capable of describing the real reactive interface.
The study found that as polarization intensifies, the inner layer of the EDL at the metal electrode undergoes a distinct two-stage structural reconstruction. In the first stage, interfacial water undergoes reorientation; with further increases in polarization, the second stage commences, during which cations accumulate at the interface and undergo partial dehydration, while some interfacial water is expelled, leading to the collapse of the inner layer structure. This result provides direct spectroscopic evidence for the compressibility hypothesis of the EDL inner layer.
More importantly, when the potential undergoes a reverse jump, the expansion of the EDL does not simply recover along the original path; instead, it is accompanied by the rapid departure of high-concentration cations from the electrode interface, forming a transient transitional structure with a higher content of free water. Furthermore, the dynamic restructuring of the EDL is not merely a structural change—it also directly alters the local electric field at the reaction site. As cations partially dehydrate and move closer to the electrode, the ion concentration within the EDL rises rapidly, the inner layer thickness decreases, and the local interfacial electric field is significantly enhanced. This indicates that the relationship between the applied electrode potential and the local electric field actually experienced by reactants is not a simple linear correspondence, but is dynamically modulated by the microscopic structure of the EDL.
This study reveals the dynamic nature of the "ion-water asynchronous response" within the EDL far from equilibrium, advancing the scientific community's understanding of electrochemical interfaces from traditional steady-state structures to the dynamic evolution occurring during real reactions.