New Perspectives on the Charging Mechanisms of Supercapacitors

Abstract

Supercapacitors (or electric double-layer capacitors) are high-power energy storage devices that store charge at the interface between porous carbon electrodes and an electrolyte solution. These devices are already employed in heavy electric vehicles and electronic devices, and can complement batteries in a more sustainable future. Their widespread application could be facilitated by the development of devices that can store more energy, without compromising their fast charging and discharging times. In situ characterization methods and computational modeling techniques have recently been developed to study the molecular mechanisms of charge storage, with the hope that better devices can be rationally designed. In this Perspective, we bring together recent findings from a range of experimental and computational studies to give a detailed picture of the charging mechanisms of supercapacitors. Nuclear magnetic resonance experiments and molecular dynamics simulations have revealed that the electrode pores contain a considerable number of ions in the absence of an applied charging potential. Experiments and computer simulations have shown that different charging mechanisms can then operate when a potential is applied, going beyond the traditional view of charging by counter-ion adsorption. It is shown that charging almost always involves ion exchange (swapping of co-ions for counter-ions), and rarely occurs by counter-ion adsorption alone. We introduce a charging mechanism parameter that quantifies the mechanism and allows comparisons between different systems. The mechanism is found to depend strongly on the polarization of the electrode, and the choice of the electrolyte and electrode materials. In light of these advances we identify new directions for supercapacitor research. Further experimental and computational work is needed to explain the factors that control supercapacitor charging mechanisms, and to establish the links between mechanisms and performance. Increased understanding and control of charging mechanisms should lead to new strategies for developing next-generation supercapacitors with improved performances.

Direct observation of ion dynamics in supercapacitor electrodes using in situ diffusion NMR spectroscopy

“…For the case of initially filled pores, charging purely by co-ion desorption (X = -1) may offer the fastest charging rates at high voltages, though this mechanism has not yet been observed. 21 Kondrat et al have suggested that ideally the carbon pores would be initially empty of ions (ionophobic pores) to facilitate fast charging, 16,24,40 Our measurements on carbons with different pore sizes help to explain previous electrochemical studies of charging dynamics. 1,[41][42][43] For carbons with smaller average pore sizes and fewer mesopores (< 2 nm), the capacitance decreased more rapidly as the current density was increased.…”

“…21,22 For example, charging often proceeds via the exchange of counter-ions and co-ions (co-ions are defined as having charge of the same sign as the electrode in which they are located), while charging by coion desorption alone is also a possibility. We introduced the charging mechanism parameter, X, to quantify these different processes, with X taking values of +1, 0 and -1, for the extreme cases of charging by counter-ion adsorption, counter-ion -co-ion exchange, and co-ion desorption, respectively, while intermediate X values indicate contributions from more than one mechanism.…”

“…We introduced the charging mechanism parameter, X, to quantify these different processes, with X taking values of +1, 0 and -1, for the extreme cases of charging by counter-ion adsorption, counter-ion -co-ion exchange, and co-ion desorption, respectively, while intermediate X values indicate contributions from more than one mechanism. 21 An understanding and control of the charge storage mechanism (X value) may hold the key to optimising the energy and power performance of supercapacitors for different applications.…”

In situ solid-state electrochemistry of mass-selected ions at well-defined electrode–electrolyte interfaces

“…A fundamental understanding of molecular mechanisms and electrode kinetics is key to the future improvement of the performance of EEIs and the longevity and commercial success of electrochemical technologies. The distribution and adsorption/desorption of counterions on electrode surfaces, and subsequent ion exchange and electron transfer are vital processes that define the complexity of operating EEIs that are currently being studied using in situ and operando characterization as well as high-level computational modeling (4).…”

Ion-ion correlations across and between electrified graphene layers

“…When it comes to increasing the capacitance, it has been previously shown, both experimentally and computationally, that the interaction between the electrode material and the electrolyte plays a fundamental role in optimizing the adsorption of the ions [18][19][20] . For example, a big step forward was taken with the perception of the pore size effect and the ion desolvation under confinement made by Chmiola et al 21 when analyzing tetraethylammonium tetrafluoroborate (TEA-BF 4 ) in acetonitrile (ACN) using carbide-derived carbons (CDCs).…”