7 research outputs found

    Electrochemical Quartz Crystal Microbalance (EQCM) Study of Ion Dynamics in Nanoporous Carbons

    No full text
    Electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry (CV) measurements were used to characterize ion adsorption in carbide-derived carbon (CDC) with two different average pore sizes (1 and 0.65 nm), from neat and solvated 1-Ethyl-3-methylimidazolium bisĀ­(trifluoroĀ­methaneĀ­sulfonyl)Ā­imide (EMI-TFSI) electrolytes. From the electrode mass change in neat EMI-TFSI, it was shown that one net charge stored corresponds almost to one single ion at high polarization; in that case, no ion-pairing or charge screening by co-ions were observed. In 2 M EMI-TFSI in acetonitrile electrolyte, experimental solvation numbers were estimated for EMI<sup>+</sup> cation, showing a partial desolvation when cations were adsorbed in confined carbon pores. The extent of desolvation increased when decreasing the carbon pore size (from 1 down to 0.65 nm). The results also suggest that EMI<sup>+</sup> cation owns higher mobility than TFSI<sup>ā€“</sup> anion in these electrolytes

    Simulating Supercapacitors: Can We Model Electrodes As Constant Charge Surfaces?

    No full text
    Supercapacitors based on an ionic liquid electrolyte and graphite or nanoporous carbon electrodes are simulated using molecular dynamics. We compare a simplified electrode model in which a constant, uniform charge is assigned to each carbon atom with a realistic model in which a constant potential is applied between the electrodes (the carbon charges are allowed to fluctuate). We show that the simulations performed with the simplified model do not provide a correct description of the properties of the system. First, the structure of the adsorbed electrolyte is partly modified. Second, dramatic differences are observed for the dynamics of the system during transient regimes. In particular, upon application of a constant applied potential difference, the increase in the temperature, due to the Joule effect, associated with the creation of an electric current across the cell follows Ohmā€™s law, while unphysically high temperatures are rapidly observed when constant charges are assigned to each carbon atom

    On the Dynamics of Charging in Nanoporous Carbon-Based Supercapacitors

    No full text
    Supercapacitors are electricity storage systems with high power performances. Their short charge/discharge times are due to fast adsorption/desorption rates for the ions of the electrolyte on the electrode surface. Nanoporous carbon electrodes, which give larger capacitances than simpler geometries, might be expected to show poorer power performances because of the longer times taken by the ions to access the electrode interior. Experiments do not show such trends, however, and this remains to be explained at the molecular scale. Here we show that carbide-derived carbons exhibit heterogeneous and fast charging dynamics. We perform molecular dynamics simulations, with realistically modeled nanoporous electrodes and an ionic liquid electrolyte, in which the system, originally at equilibrium in the uncharged state, is suddenly perturbed by the application of an electric potential difference between the electrodes. The electrodes respond by charging progressively from the interface to the bulk as ions are exchanged between the nanopores and the electrolyte region. The simulation results are then injected into an equivalent circuit model, which allows us to calculate charging times for macroscopic-scale devices

    Enhanced Electrochemical Performance of Ultracentrifugation-Derived nc-Li<sub>3</sub>VO<sub>4</sub>/MWCNT Composites for Hybrid Supercapacitors

    No full text
    Nanocrystalline Li<sub>3</sub>VO<sub>4</sub> dispersed within multiwalled carbon nanotubes (MWCNTs) was prepared using an ultracentrifugation (uc) process and electrochemically characterized in Li-containing electrolyte. When charged and discharged down to 0.1 V <i>vs</i> Li, the material reached 330 mAh g<sup>ā€“1</sup> (per composite) at an average voltage of about 1.0 V <i>vs</i> Li, with more than 50% capacity retention at a high current density of 20 A g<sup>ā€“1</sup>. This current corresponds to a nearly 500<i>C</i> rate (7.2 s) for a porous carbon electrode normally used in electric double-layer capacitor devices (1<i>C</i> = 40 mA g<sup>ā€“1</sup> per activated carbon). The irreversible structure transformation during the first lithiation, assimilated as an activation process, was elucidated by careful investigation of <i>in operando</i> X-ray diffraction and X-ray absorption fine structure measurements. The activation process switches the reaction mechanism from a slow ā€œtwo-phaseā€ to a fast ā€œsolid-solutionā€ in a limited voltage range (2.5ā€“0.76 V <i>vs</i> Li), still keeping the capacity as high as 115 mAh g<sup>ā€“1</sup> (per composite). The uc-Li<sub>3</sub>VO<sub>4</sub> composite operated in this potential range after the activation process allows fast Li<sup>+</sup> intercalation/deintercalation with a small voltage hysteresis, leading to higher energy efficiency. It offers a promising alternative to replace high-rate Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> electrodes in hybrid supercapacitor applications

    In Situ NMR Spectroscopy of Supercapacitors: Insight into the Charge Storage Mechanism

    No full text
    Electrochemical capacitors, commonly known as supercapacitors, are important energy storage devices with high power capabilities and long cycle lives. Here we report the development and application of in situ nuclear magnetic resonance (NMR) methodologies to study changes at the electrodeā€“electrolyte interface in working devices as they charge and discharge. For a supercapacitor comprising activated carbon electrodes and an organic electrolyte, NMR experiments carried out at different charge states allow quantification of the number of charge storing species and show that there are at least two distinct charge storage regimes. At cell voltages below 0.75 V, electrolyte anions are increasingly desorbed from the carbon micropores at the negative electrode, while at the positive electrode there is little change in the number of anions that are adsorbed as the voltage is increased. However, above a cell voltage of 0.75 V, dramatic increases in the amount of adsorbed anions in the positive electrode are observed while anions continue to be desorbed at the negative electrode. NMR experiments with simultaneous cyclic voltammetry show that supercapacitor charging causes marked changes to the local environments of charge storing species, with periodic changes of their chemical shift observed. NMR calculations on a model carbon fragment show that the addition and removal of electrons from a delocalized system should lead to considerable increases in the nucleus-independent chemical shift of nearby species, in agreement with our experimental observations

    Vertically Oriented Propylene Carbonate Molecules and Tetraethyl Ammonium Ions in Carbon Slit Pores

    No full text
    We report the vertical alignment of propylene carbonate (PC) molecules interacting with Et<sub>4</sub>N<sup>+</sup> and BF<sub>4</sub><sup>ā€“</sup> which are confined in extremely narrow slit pores (<i>w</i> āˆ¼ 0.7 nm) of carbide-derived carbon and pitch-based activated carbon fiber. On the basis of X-ray diffraction (XRD), electron radial distribution function analysis reveals that the nearest PCā€“PC distance is 0.05ā€“0.06 nm shorter than that in the bulk solution, indicating dense packing of PC molecules in the pores. This confinement effect results from the vertically aligned PC molecules, which are indicated by the reverse Monte Carlo analysis. The ensemble structure of PC molecules in the subnanometer carbon pores will provide better understanding the supercapacitor function

    Capacitive Energy Storage from āˆ’50 to 100 Ā°C Using an Ionic Liquid Electrolyte

    No full text
    Relying on redox reactions, most batteries are limited in their ability to operate at very low or very high temperatures. While performance of electrochemical capacitors is less dependent on the temperature, present-day devices still cannot cover the entire range needed for automotive and electronics applications under a variety of environmental conditions. We show that the right combination of the exohedral nanostructured carbon (nanotubes and onions) electrode and a eutectic mixture of ionic liquids can dramatically extend the temperature range of electrical energy storage, thus defying the conventional wisdom that ionic liquids can only be used as electrolytes above room temperature. We demonstrate electrical double layer capacitors able to operate from āˆ’50 to 100 Ā°C over a wide voltage window (up to 3.7 V) and at very high charge/discharge rates of up to 20 V/s
    corecore