Vibrationally-Resolved Electrochemical Impedance Spectroscopy

Dynamics of the electrochemical double layer (EDL) is central to understanding electrochemistry. One of the widely used tools for this purpose is the electrochemical impedance spectroscopy (EIS). However, the EDL, even in relatively simple systems, has diverse and complex dynamics. EIS only reports the aggregate response of the EDL and lacks molecular details. To better understand the individual responses of molecular species, and more importantly the complex interplay between them, we demonstrate a new experimental method which combines vibrational spectroscopy with EIS (VibREIS). We rapidly collect ATR-FTIR spectra while modulating the potential over a range of frequencies. Fourier analysis of the IR spectra reveals the magnitudes and phases of the responses of the species inside the EDL that follow the potential. We apply this technique to an EDL composed of azide anion and a cationic surfactant to reveal their dynamics. We discover unexpected results including a phenomenon akin to a "resonance" where the response of azide has a preferred range of frequencies. Additionally, we observe that the surfactant, due to its large size, can not keep up with potential oscillations beyond a threshold frequency and delegates the job of screening the potential to the more agile azide. Finally, we show ion-correlated motion of the interfacial water. These phenomena are not directly inferable from EIS and are revealed only by using VibREIS. We envisage the application of this technique to Faradaic reactions, programmable catalysis, and even the elusive solid electrolyte interfaces in energy storage.