Combined Computational and Experimental Study on the Influence of Surface Chemistry of Carbon-Based Electrodes on Electrode–Electrolyte Interactions in Supercapacitors

Abstract

Supercapacitors are regarded as a promising technology for novel, powerful energy-storage systems. The mechanism of energy storage in these capacitors is not fully understood yet because of the complex molecular mechanisms at the atomistic scale. Exploring the processes at the nanoscale provides necessary fundamental and thorough insights for improving the performance of such devices. In this work, we present a combined computational and experimental study on electrode–electrolyte interactions in electric double-layer capacitors. The influence of pore size and surface chemistry of carbon-based electrode material on interactions with the electrolyte has been investigated for an organic and inorganic electrolyte using density functional theory calculations. In addition, solvent effects on the interaction strength have been systematically explored. We found that experimentally determined effects of pore confinement can be linked with calculated interaction energies, providing a suitable descriptor for virtual prescreening approaches. Our results show that the pore size significantly affects the interaction quality with the electrolyte. This effect and the influence of chemical functionalization have a stronger impact on the interaction with anions than with cations. Moreover, our studies indicate that solvent effects are especially important for surface functional groups that allow for hydrogen bonding. Overall, our results provide relevant information on how structural and electronic effects affect confinement, wettability, and mobility of electrolyte molecules, which is important for boosting and tuning the performance of supercapacitors.

Simulations of Ionic Liquids Confined in Surface-Functionalized Nanoporous Carbons: Implications for Energy Storage

“…Overall, the molecular simulations conducted here on a neat ionic liquid in contact with several three-dimensional porous carbon structures confirm previous results, ,,,− showing that specific interactions between the ions and the carbon material affect the structural and dynamical properties of the confined ions in different ways. Steric and chemical variations associated with the presence of functional groups, and having possibly contradictory effects, make it difficult to predict the influence of the presence of functional groups on the properties of the confined ionic liquid.…”

“…In this case, there is a strong hydrogen bonding between the surface groups and SO 3 , which restrains the reorientation and mobility of the anions, leading to a decrease in capacitance. Other MD simulation studies of ionic liquids based on the tetrafluoroborate anion ([EMIM]­[BF 4 ], [BMIM]­[BF 4 ]) in contact with planar graphene oxide and graphene oxide nanochannels have also shown a negative impact on the ion dynamics. , More recently, Schweizer et al have used density functional theory (DFT) calculations to study a range of surface chemistries and electrolytes, including the effect of solvation. This DFT study, combined with some electrochemical experiments, suggests that interaction energies provide a good descriptor for the prediction of capacitance.…”

Electrical Double Layer Capacitance of Curved Graphite Electrodes

“…Capacitances as a function of the mean pore size of several porous carbon-based structures were previously determined experimentally as well as from simulations. − By limiting the electrolyte contact only to the convex part of an electrode, for example, by using the outer surfaces of fullerenes or carbon nanotubes (CNT), a reduction in capacitance was observed. − In all these studies, however, only the influence of either purely concave or convex geometric structures was investigated. In complex porous structures, for example, amorphous carbons, on the contrary, influences from different geometries are always intertwined.…”

Elucidating Curvature-Capacitance Relationships in Carbon-Based Supercapacitors

“…Since energy storage in supercapacitors does not happen through redox reactions but by the formation of a thin interfacial region between electrode and electrolyte, attainable capacitances are greatly influenced by the explicit structure and dynamics of this electric double layer (EDL). The discovery of an anomalously high capacitance in microporous carbons has led to numerous research activities aimed at deciphering the underlying molecular mechanisms [1][2][3][4][5][6][7]. Beyond the sheer increase of the electrode surface area facilitated by microporous carbons-effectively increasing the capacitance-the specific shape of the electrodes, such as pore sizes and curvatures, affects the structure and composition of the EDL and thus the capacitance [8][9][10][11][12][13][14][15].…”