Letter pubs.acs.org/JPCL
In Situ Molecular Level Measurements of Ion Dynamics in an Electrochemical Capacitor Francis W. Richey and Yossef A. Elabd* Department of Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: Improving the energy storage capability of batteries and capacitors is inherently dependent on clarifying our understanding of ion dynamics of advanced electrolytes in a variety of materials. Herein we report a new attenuated total reflectance−surface-enhanced infrared absorption spectroscopy technique that can selectively and simultaneously measure both cation and anion transport of an ionic liquid (1-ethyl-3-methylimidazolium triflate (EMIm-Tf)) in a functioning electrochemical pseudocapacitor (actuator). This new capacitor−spectroscopy technique was utilized to probe the gold current collector/RuO2 electrode interface during both square wave and cyclic voltammetry experiments. Results show that the cations and anions transport as aggregates and the cation dominates and dictates the direction of ion transport in these devices. Results also show that ion dynamics in pseudocapacitors is a diffusion-limited process. SECTION: Energy Conversion and Storage; Energy and Charge Transport
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liquid electrolyte 1-ethyl-3-methylimidazolium triflate (EMImTf). RuO2 is an attractive energy storage material because of its inherent ability to conduct both electrons and protons.19,20 Ionic liquid electrolytes possess favorable properties for use in capacitors, such as nonvolatility and wide electrochemical windows.21 However, a detailed understanding of the doublelayer structure of ionic liquids in energy storage devices, as well as the dynamics of the individual cations and anions during charge storage remains unclear.22,23 Figure 1 shows a cross-sectional schematic of the capacitorATR-SEIRAS experimental apparatus used in this work. The gold leaf current collector is directly attached to the pseudocapacitor electrodes before clamping the entire device onto the ATR crystal, allowing for multiple samples to be analyzed without ever modifying the ATR crystal (Figure S1 of the Supporting Information). The pseudocapacitor is fabricated with a Nafion separator and porous electrodes composed of Nafion, RuO2, and EMIm-Tf (see the Supporting Information). Infrared spectra of the pseudocapacitor electrodes without the gold leaf current collector yield characteristic mid-IR bands for both the ionic liquid electrolyte and the Nafion polymer (Figure S3 of the Supporting Information). After the gold leaf is attached to the electrodes, the spectrum is nearly identical to that of the pure ionic liquid alone, indicative of the surface effect of the thin gold leaf current collector. To avoid water contamination, we performed all experiments in a dry chamber (Figure S2 of the Supporting Information).
detailed understanding of electrolyte dynamics in energy storage devices is necessary for the future design of new materials that can more effectively store and release ions.1 However, in situ spectroscopic experiments on entire functioning devices are limited by the solid metal current collector, which blocks light from reaching the active materials that store ions. Whereas in situ Raman spectroscopy, NMR, and other techniques have successfully been used to study battery electrodes2−5 that show pronounced phase/composition changes, more subtle processes in capacitor electrodes are difficult to detect and only recently were in situ NMR results reported for electrochemical capacitors.6 One experimental technique capable of overcoming this limitation is attenuated total reflectance−surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS).7,8 When a thin metal layer (
