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.

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.

Aqueous Mg-air primary batteries possess many favorable attributes for energy storage because Mg is affordable, abundant, and lightweight. However, parasitic corrosion of Mg in aqueous electrolytes generates H 2 and surprisingly increases with increasing current density during battery discharge (Mg oxidation), limiting the faradaic efficiency of aqueous Mg batteries. In this study, differential electrochemical mass spectrometry and H 2 pressure rise measurements were used to characterize Mg corrosion in Mg-air batteries employing aqueous electrolytes with salts (NaCl, NaNO 3 , NaPO 4 , and a NaCl/NaPO 4 mixture) that provide various degrees of Mg passivation. H 2 evolution rates were highest in NaCl electrolytes and lowest in NaNO 3 electrolytes. However, NaNO 3 salts reduced the H 2 -evolving corrosion rate at the expense of introducing a nitrate to nitrite corrosion reaction into the battery. The combined Mg corrosion rate in the nitrate-based electrolyte was still lowest among those electrolytes studied. The nitrate to nitrite corrosion reaction also lowered the magnitude of the Mg anodic potential and therefore decreased the overall Mg-O 2 battery voltage compared to the NaCl electrolyte. Nevertheless, Mg-O 2 batteries utilizing a NaNO 3 electrolyte allowed for 60% larger discharge capacity and 50% higher Mg oxidation faradaic efficiency compared to a NaCl electrolyte. Increased demand for electrical energy storage has sparked a renewed interest into battery chemistries with theoretical energy densities that are much larger than lithium ion batteries. Among these chemistries, a tremendous amount of recent work has been devoted to metal-air batteries such as Li-O 2 , 1,2 Na-O 2 , 3 and Zn-O 2 . 4 However, much less attention has been paid to Al or Mg-O 2 batteries even though Mg and Al are lightweight, abundant, and environmentally friendly.5 Although these batteries are not electrically rechargeable, there is commercial interest in them as standby power sources and as range extenders in electric vehicles because of their inherent safety and long standby lifetimes. Mechanically rechargeable aqueous Mg-O 2 batteries, those in which a fully oxidized Mg electrode is replaced with a fresh electrode, are especially attractive since they can function with saltwater electrolytes and have high theoretical voltages. Therefore, they can be used effectively as underwater batteries using dissolved O 2 in seawater. In addition, they could function effectively as standby range extenders for traditional Li ion battery packs since the Mg metal has no self-discharge until placed in the electrolyte. The desirable reactions for the aqueous Mg-O 2 battery are shown below:Unfortunately, Mg is a reactive metal that corrodes significantly in the presence of aqueous electrolytes, decreasing the faradaic efficiency of the battery. 6 The parasitic corrosion reactions that occur on the Mg anode are shown below:Numerous studies have focused on understanding the fundamental corrosion reaction of Mg in aqueous electrolytes, 7-9 and in par...