Kinetic-Dominated Charging Mechanism within Representative Aqueous Electrolyte-based Electric Double-Layer Capacitors

Abstract

The chemical nature of electrolytes has been demonstrated to play a pivotal role in the charge storage of electric double-layer capacitors (EDLCs), whereas primary mechanisms are still partially resolved but controversial. In this work, a systematic exploration into EDL structures and kinetics of representative aqueous electrolytes is performed with numerical simulation and experimental research. Unusually, a novel charging mechanism exclusively predominated by kinetics is recognized, going beyond traditional views of manipulating capacitances preferentially via interfacial structural variations. Specifically, strikingly distinctive EDL structures stimulated by diverse ion sizes, valences, and mixtures manifest a virtually identical EDL capacitance, where the dielectric nature of solvents attenuates ionic effects on electrolyte redistributions, in stark contradiction with solvent-free counterpart and traditional Helmholtz theory. Meanwhile, corresponding kinetics evolve conspicuously with ionic species, intimately correlated with ion-solvent interactions. The achieved mechanisms are subsequently illuminated by electrochemical measurements, highlighting the crucial interplay between ions and solvents in regulating EDLC performances.

Substrate Effects in Graphene‐Based Electric Double‐Layer Capacitors: The Pivotal Interplays between Ions and Solvents

“…While the corresponding oscillatory patterns of ρ all , i. e., position and magnitude of peaks, were primarily dependent on the water distributions (see black dashed line), indicating that solvent dipoles played a crucial role in shielding the electrode charges. Such finding could be interpreted by the overwhelming populations of solvents at the interface (approximately two orders larger than that of ion counterpart), which was also encountered by other simulation studies concerning the solvent effects (including our recent work) ,,…”

Hierarchical, Vertically‐Oriented Carbon Nanowall Foam Supercapacitor using Room Temperature Ionic Liquid Mixture for AC Line Filtering with Ultrahigh Energy Density

“…Moreover, our previous experimental and simulation studies suggested that the specific capacitance near the edge region was obviously higher (approx. 2 folds) than that on the basal plane,, indicating the potential of CWF to achieve superior energy density performance. The high‐resolution TEM image in Figure e clearly demonstrates the continuity of the covalent graphitic structure from smaller carbon nanowalls to the larger graphene sheets within CWF scaffolds, which could serve as the conductive expressways for electron transfer.…”

Specific ion effects at graphitic interfaces

“…However, a general consensus on how the interfacial response is governed by intrinsic properties of ions remains largely lacking, and contradictory trends have been reported, even for simple alkali-metal ions. For instance, measurements of activated porous carbons in aqueous solutions showed that the overall capacitance increases with the ionic radius of the ions according to LiNaK 7–9 , whereas an inverse relationship was reported by another experimental study 10 . These contradictory results highlight that much is left to be understood regarding ion effects at graphitic interfaces.…”

“…In addition to experimental studies, molecular dynamics (MD) simulations with classical force fields have been extensively employed to investigate aqueous solutions near graphene 7,12–14 , as well as within carbon nanotubes 15–23 and carbon slit pore electrodes 24,25 . While these simulations have significantly advanced our understanding of the electric double layer (EDL) at graphitic interfaces, notable discrepancies between theoretical and experimental studies remain to be addressed.…”