7 research outputs found
Electrochemical Quartz Crystal Microbalance (EQCM) Study of Ion Dynamics in Nanoporous Carbons
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?
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 Ohmās 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
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
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>ā1</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>ā1</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>ā1</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 ātwo-phaseā to a fast āsolid-solutionā
in a limited voltage range (2.5ā0.76 V <i>vs</i> Li),
still keeping the capacity as high as 115 mAh g<sup>ā1</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
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 electrodeāelectrolyte 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
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> ā¼ 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 PCāPC distance is 0.05ā0.06 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 ā50 to 100 Ā°C Using an Ionic Liquid Electrolyte
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 ā50 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