Ion Dynamics in Porous Carbon Electrodes in Supercapacitors
Using in Situ Infrared Spectroelectrochemistry
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Abstract
Electrochemical
double layer capacitors (EDLCs), or supercapacitors,
rely on electrosorption of ions by porous carbon electrodes and offer
a higher power and a longer cyclic lifetime compared to batteries.
Ionic liquid (IL) electrolytes can broaden the operating voltage window
and increase the energy density of EDLCs. Herein, we present direct
measurements of the ion dynamics of 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide
in an operating EDLC with electrodes composed of porous nanosized
carbide-derived carbons (CDCs) and nonporous onion-like carbons (OLCs)
with the use of in situ infrared spectroelectrochemistry. For CDC
electrodes, IL ions (both cations and anions) were directly observed
entering and exiting CDC nanopores during charging and discharging
of the EDLC. Conversely, for OLC electrodes, IL 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.
This provides experimental evidence that charge is stored on the surface
of OLCs in OLC EDLCs without long-range ion transport through the
bulk electrode. In addition, for CDC EDLCs with mixed electrolytes
of IL and propylene carbonate (PC), the IL ions were observed entering
and exiting CDC nanopores, while PC entrance into the nanopores was
IL concentration dependent. This work provides direct experimental
confirmation of EDLC charging mechanisms that previously were restricted
to computational simulations and theories. The experimental measurements
presented here also provide deep insights into the molecular level
transport of IL ions in EDLC electrodes that will impact the design
of the electrode materials’ structure for electrical energy
storage