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 RuO2 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.Ph.D., Chemical Engineering -- Drexel University, 201

In Situ Molecular Level Measurements of Ion Dynamics in an Electrochemical Capacitor

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

In Situ Molecular Level Measurements of Ion Dynamics in an Electrochemical Capacitor

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

Ion Dynamics in Porous Carbon Electrodes in Supercapacitors Using in Situ Infrared Spectroelectrochemistry

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

Ionic Liquid Dynamics in Nanoporous Carbon Nanofibers in Supercapacitors Measured with <i>in Operando</i> Infrared Spectroelectrochemistry

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 <i>in operando</i> 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