Using electrodes
with subnanometer pores and ionic liquid electrolytes
can improve the charge storage capacity at the expense of the charging
rate. The fundamental understanding of the charging dynamics of nanoporous
electrodes can help to avoid compromising the power density. In this
work, we performed molecular dynamics simulations to reveal the charging
mechanism of subnanometer pores in ionic liquids. Different from the
traditional view that a smaller pore results in slower charging, a
non-monotonic relation is found between the charging rate and pore
size, in which the charging process is accelerated in some subnanometer
pores. Our analysis uncovers that the mechanism of the charging enhancement
can be attributed to the transition of in-pore ion structure.
Molecular modeling has been considered indispensable in studying the energy storage of supercapacitors at the atomistic level. The constant potential method (CPM) allows the electric potential to be kept uniform in the electrode, which is essential for a realistic description of the charge repartition and dynamics process in supercapacitors. However, previous CPM studies have been limited to the potentiostatic mode. Although widely adopted in experiments, the galvanostatic mode has rarely been investigated in CPM simulations because of a lack of effective methods. Here we develop a modeling approach to simulating the galvanostatic charge–discharge process of supercapacitors under constant potential. We show that, for nanoporous electrodes, this modeling approach can capture experimentally consistent dynamics in supercapacitors. It can also delineate, at the molecular scale, the hysteresis in ion adsorption–desorption dynamics during charging and discharging. This approach thus enables the further accurate modeling of the physics and electrochemistry in supercapacitor dynamics.
Small carbon pores below 1 nm become increasingly ionophobic which enables the more and more permselective charge storage and perspectives for capacitive deionization with porous carbons even at high molar strength.
In
this study, we carry out molecular simulations on carbon pores
of different diameters in an aqueous potassium chloride solution.
We investigate the delicate correlation of the capacitance as a function
of pore size and electrode potential, the in-pore ion and water distributions,
as well as the charging dynamics of these systems. Our results see
the capacitance enhancement of the subnanometer pores; however, the
capacitance tends to become stable and pore-size independent when
the pore gets larger. In addition, the charging mechanisms of pores
with different sizes are found to shift from the ion-exchange mode
to the permselective counterion adsorption as the size of the pore
shrinks. The latter can be attributed to the desolvation of ions in
the subnanometer pores, which limits the ion adsorption kinetics and
causes low in-pore ionic conductivity, thus exacerbating the rate
performance. These conclusions are supported by our electrochemical
measurements on the assembled cells composed of four distinct activated
carbon samples with different average pore sizes. This work gives
molecular insights into the nanoporous carbons in aqueous electrolytes,
shedding light on the future design of the aqueous-based electrical
double-layer capacitors with balanced rate performance and energy
density.
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