The electrical double layer that forms at the interface between charged solids and liquid electrolytes is important for the electrical properties, molecular interactions, and chemical reactivity at the interface. Although theories describing the structure of the double layer have existed for decades, little is understood about its dynamics on the natural time- and length-scales of molecular motion, or their importance in the functioning of electrochemical devices, in large part because there are few experimental tools to probe these properties. We have been developing a combined experimental and simulation approach for probing the structural dynamics of the electrical double layer, using molecular vibrations sensitive to their local environment as a tool to probe the fluctuations in water and ion configurations within the first nanometer of the interface using ultrafast 2D IR spectroscopy. Interpretation of the 2D IR spectra relies on the results from atomistic modeling using molecular dynamics (MD) simulations, which make it possible to determine the role of each component of the solution in generating the local electric fields sensed by the molecular vibrations. We have demonstrated the proof of principle concept of this approach to sensing local electric field fluctuations in bulk electrolyte solutions by studying the impact of ions on the vibrational frequency and spectral diffusion dynamics of a model carbonyl probe in solution, and are now combining this with a newly developed 2D IR spectrometer in the attenuated total internal reflection geometry to extend these measurements to the solid-electrolyte interface. We demonstrate the impact of ionic composition and concentration on the 2D vibrational lineshape, which reports on the local heterogeneity imparted by the ions, and compare the bulk and surface measurements to illustrate the major importance of the interface in changing the local structure and dynamics of the solution. This approach allows us to probe solution dynamics at the solid-liquid interface revealing new information about the fluctuations intrinsic to these systems.

Probing the electric field dynamics at the solid-electrolyte interface with 2D IR spectroscopy and molecular dynamics simulations

ACS Spring 2022

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