How chemical defects influence the charging of nanoporous carbon supercapacitors

Abstract

Significance Nanoporous carbon texture makes fundamental understanding of the electrochemical processes challenging. Based on density functional theory (DFT) results, the proposed atomistic approach takes into account topological and chemical defects of the electrodes and attributes to them a partial charge that depends on the applied voltage. Using a realistic carbon nanotexture, a model is developed to simulate the ionic charge both at the surface and in the subnanometric pores of the electrodes of a supercapacitor. Before entering the smallest pores, ions dehydrate at the external surface of the electrodes, leading to asymmetric adsorption behavior. Ions in subnanometric pores are mostly fully dehydrated. The simulated capacitance is in qualitative agreement with experiments. Part of these ions remain irreversibly trapped upon discharge.

Understanding Ion Charging Dynamics in Nanoporous Carbons for Electrochemical Double Layer Capacitor Applications

“…However, the key parameters such as finite ion size and pore length/morphology should also be taken into account. Recently, models have been refined to get closer to actual three-dimensional confinement systems, such as the chemically driven charge localization (CDCL) model, modified PNP model, , and dynamic density functional theory (DDFT) . Together with experimental parameters (for instance the finite pore length, , pore size distribution, curvature, ion confinement, , desolvation, and defects), they greatly contribute to the theoretical understanding of ion transport kinetics in porous carbon electrodes.…”

“…5−7 Classical planar EDL theory, so-called the Gouy−Chapman−Stern model, has been established using two parallel charged planar electrodes immersed in an electrolytic solution, 8 with the Poisson−Nernst−Plank (PNP) equation to address time-dependent charge-relaxation phenomena. 9 Nevertheless, moving from 2D planar to a high surface area porous electrode comes with confinement effect, 10,11 presence of chemical defects, 12 potential distribution, curvature effects, 13 etc. that make porous electrode behavior different from that of classical planar ones.…”

“…34 The pore size-dependent charging kinetics could be attributed to steric hindrance of the pore accessibility 37,38,42,45−47 and/or to the ion desolvation process. 12 The solvation shell of Li + in aqueous electrolytes usually contains 5 to 6 H 2 O molecules with a Li−O distance of 3.5−4.0 Å for the second hydration shell, 48,49 making solvated Li + ion sensitive to steric hindrance during transport in subnanometer pores of the porous carbon network. Therefore, the sluggish adsorption kinetics observed for narrow pores could be explained by the hindered transportation of solvated Li + , together with their partial desolvation process when accumulated in the subnanometer carbon pores.…”

“…Understanding the formation of the electric double layer (EDL) at the porous carbon electrode/electrolyte interface is of high importance in various applications such as energy storage (electrochemical double-layer supercapacitors), capacitive deionization, − or energy harvesting. − Classical planar EDL theory, so-called the Gouy–Chapman–Stern model, has been established using two parallel charged planar electrodes immersed in an electrolytic solution, with the Poisson–Nernst–Plank (PNP) equation to address time-dependent charge-relaxation phenomena . Nevertheless, moving from 2D planar to a high surface area porous electrode comes with confinement effect, , presence of chemical defects, potential distribution, curvature effects, etc. that make porous electrode behavior different from that of classical planar ones.…”

“…Recently, models have been refined to get closer to actual three-dimensional confinement systems, such as the chemically driven charge localization (CDCL) model, modified PNP model, , and dynamic density functional theory (DDFT) . Together with experimental parameters (for instance the finite pore length, , pore size distribution, curvature, ion confinement, , desolvation, and defects), they greatly contribute to the theoretical understanding of ion transport kinetics in porous carbon electrodes. However, experimental approaches are barely proposed to further validate these theoretical approaches, and experimental investigations of the EDL formation and evolution for the subnanopores with confinement are highly desired.…”