Ion Dynamics in Electrochemical Capacitors Using Infrared Spectroelectrochemistry

Electrochemical capacitors are electrical energy storage devices that are capable of providing large power densities (fast charging and discharging) and extremely long lifetimes (1 million charge-discharge cycles). Room-temperature ionic liquid (RTIL) electrolytes can broaden the operating voltage window and increase the energy density of electrochemical capacitors. However, a fundamental understanding of RTIL dynamics in capacitors is desired for their future commercial success. Herein, we have designed a new experimental technique, in situ infrared spectroelectrochemistry, that provides direct molecular-level measurements of the ion dynamics of RTILs in operating electrochemical capacitors with electrodes composed of RuO 2 particles, porous nanosized carbide-derived carbons (CDCs), non-porous onion-like carbons (OLCs), and nanoporous carbon nanofibers.;Results for RuO2 pseudocapacitors show that the cations and anions transport as aggregates and the cation dominates and dictates the direction of ion transport in these devices. Establishing an optimal proton (Nafion) / RTIL content in the electrode that allows for fast charging and high capacitance should allow these devices to function at high voltages and high temperatures, something that is not currently possible with aqueous electrolytes. For CDC electrodes, RTIL ions (both cations and anions) were directly observed entering and exiting CDC nanopores during charging and discharging of the EDLC. Conversely, for OLC electrodes, RTIL ions were observed in close proximity to the OLC surface without any change in the bulk electrolyte concentration during charging and discharging of the EDLC. For nanoporous carbon nanofibers with oxygen-rich surfaces, during charging and discharging, cations are expelled from pores before anions enter the pores; a significantly different phenomena compared to other nanoporous carbons. This work provides direct experimental confirmation of electrochemical capacitor charging/discharging mechanisms that previously were restricted to computational simulations and theories. The experimental measurements presented here also provide deep insights into the molecular level transport, migration, and adsorption of RTIL ions in electrochemical capacitor electrodes that can impact the design of the future electrode materials for electrical energy storage.