3 research outputs found
Unveiling the Role of Electrostatic Forces on Attraction between Opposing Polyelectrolyte Brushes
Electrostatic interaction and molecular excluded-volume
effects
are responsible for a plethora of nonintuitive phenomena in soft-matter
systems, including local charge inversion and attraction between similar
charges. In the current work, we study the surface forces and swelling
behavior of opposing polyelectrolyte brushes using a classical density
functional theory that accounts for electrostatic and excluded-volume
correlations. We observe that the detachment pressure between similarly
charged brushes is sensitive to salt concentration in both the osmotic
and salted regimes and can be negative in the presence of multivalent
counterions. A comparison of the theoretical results with the mean-field
predictions unravels the role of correlation effects in determining
the surface forces and brush structure. For systems containing multivalent
counterions, the detachment pressure attains negative values at an
intermediate brush–brush separation, and the attractive region
in the pressure vs distance plot is magnified in terms of both the
depth and width of attraction with increasing counterion valency.
However, the interbrush attraction vanishes when the size-induced
correlations are switched off. We also investigated the role of counterion
size and polymer chain length on the detachment pressure. It is found
that smaller counterions are more effective in neutralizing the polymer
charge than bigger counterions, leading to a reduced interbrush repulsion
and, in some cases, attraction between like-charged brushes at intermediate
distances. Meanwhile, varying the chain length of the grafted polymers
only shifts the location of the attraction basin, with little influence
on the interaction strength. The theoretical predictions show qualitative
agreement with experimental observations and offer valuable insights
into the interaction between similarly charged polymer brushes in
the presence of multivalent ions
Mixed Ionic Liquid Improves Electrolyte Dynamics in Supercapacitors
Well-tailored mixtures
of distinct ionic liquids can act as optimal
electrolytes that extend the operating electrochemical window and
improve charge storage density in supercapacitors. Here, we explore
two room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bisÂ(trifluoromethylsulfonyl)Âimide
(EmimTFSI) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF<sub>4</sub>). We study their electric double-layer behavior in the neat
state and as binary mixtures on the external surfaces of onion-like
carbon electrodes using quasielastic neutron scattering (QENS) and
classical density functional theory techniques. Computational results
reveal that a mixture with 4:1 EmimTFSI/EmimBF<sub>4</sub> volume
ratio displaces the larger [TFSI<sup>–</sup>] anions with smaller
[BF<sub>4</sub><sup>–</sup>] ions, leading to an excess adsorption
of [Emim<sup>+</sup>] cations near the electrode surface. These findings
are corroborated by the manifestation of nonuniform ion diffusivity
change, complementing the description of structural modifications
with changing composition, from QENS measurements. Molecular-level
understanding of ion packing near electrodes provides insight for
design of ionic liquid formulations that enhance the performance of
electrochemical energy storage devices
Selective Charging Behavior in an Ionic Mixture Electrolyte-Supercapacitor System for Higher Energy and Power
Ion–ion interactions in supercapacitor
(SC) electrolytes
are considered to have significant influence over the charging process
and therefore the overall performance of the SC system. Current strategies
used to weaken ionic interactions can enhance the power of SCs, but
consequently, the energy density will decrease due to the increased
distance between adjacent electrolyte ions at the electrode surface.
Herein, we report on the simultaneous enhancement of the power and
energy densities of a SC using an ionic mixture electrolyte with different
types of ionic interactions. Two types of cations with stronger ionic
interactions can be packed in a denser arrangement in mesopores to
increase the capacitance, whereas only cations with weaker ionic interactions
are allowed to enter micropores without sacrificing the power density.
This unique selective charging behavior in different confined porous
structure was investigated by solid-state nuclear magnetic resonance
experiments and further confirmed theoretically by both density functional
theory and molecular dynamics simulations. Our results offer a distinct
insight into pairing ionic mixture electrolytes with materials with
confined porous characteristics and further propose that it is possible
to control the charging process resulting in comprehensive enhancements
in SC performance