In Situ Spectroscopic Measurements of Individual Cation and Anion Dynamics in a RuO2Electrochemical Capacitor

Improving the energy storage capability of electrochemical capacitors is inherently dependent on clarifying our understanding of ion dynamics of advanced electrolytes in a variety of materials. Herein, we apply a new in situ infrared spectroscopy technique to selectively and simultaneously measure both cation and anion dynamics of an ionic liquid (1-ethyl-3-methylimidazolium triflate (EMIm-Tf)) in a functioning RuO 2 capacitor. The individual cation and anion dynamics of EMIm-Tf are measured as a function of step voltage input, voltage scan rate, and RuO 2 content in the electrode. Results show that cations and anions transport as aggregates and cations are the dominant charge carrying species. Also, the ratio of cations to anions in the cathode during charge storage increases with increasing step voltage input and RuO 2 content in the electrode. In addition to changes in the cation to anion distribution, the concentration of both cations and anions increases and the rate of change of cations and anions increases as the porosity of the RuO 2 electrode increases, which all correspond to an increase in the volumetric capacitance. Cyclic voltammetry, in conjunction with infrared spectroscopy experimental results, confirm that charge storage, as well as cation and anion transport both follow a diffusion-limited behavior.