Ionic Liquid Dynamics in Nanoporous Carbon Nanofibers in Supercapacitors Measured with in Operando Infrared Spectroelectrochemistry

Abstract

Electric double-layer capacitors (EDLCs), or supercapacitors, rely on rapid electrosorption of ions into porous carbon electrodes to achieve high power densities and long lifetimes. Ionic liquid (IL) electrolytes offer large operating voltage windows and can potentially increase the energy density of EDLCs if the electrode/electrolyte interface is properly optimized. Herein, we present molecular level measurements of ion dynamics of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm-TFSI) IL in an operating EDLC with freestanding electrodes composed of nanoporous carbon nanofibers (NCNFs) and potassium hydroxide (KOH)-activated NCNFs using in operando infrared spectroelectrochemistry. For non-KOH-activated NCNF electrodes, the concentrations of IL ions (both cations and anions) decrease as the ions enter the nanopores inside the nanofibers during charging. However, the concentration of the anions inside the positively charged pores is larger than the concentration of cations for voltage windows above 1 V. Conversely, when charging the KOH-activated NCNF electrodes, the cation concentration increases as the anion concentration decreases. The KOH activation process introduces oxygen functionalities on the surface of the nanofibers and increases the ionophilicity of the electrodes, which causes cations to desorb from the nanopores while anions adsorb into the nanopores. This provides direct experimental evidence that the charge storage mechanism of IL electrolytes in nanoporous carbon electrodes of EDLCs is directly affected by the surface chemistry and ionophilicity of the carbon material. The quantitative, species-specific molecular-level infrared spectroelectrochemical measurements presented here provide deep insights into the behavior of IL ions in EDLCs that will improve the design and performance of electrode materials.