Abstract
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.
Highlights
Molecular modeling has been considered indispensable in studying the energy storage of supercapacitors at the atomistic level
Researchers often resort to molecular modeling to investigate the thermodynamic and kinetic behavior of supercapacitors, those with nanoporous electrodes, because molecular simulations can provide a precise microscopic picture of electric double layers (EDLs) and their formation in supercapacitors[3,4,5]
To model the charge–discharge process of nanoporous supercapacitors, constant potential method (CPM) is preferred because its self-consistent adjustment of electrode charges, absent in constant charge method (CCM), can produce the correct charging dynamics and heat generation[17,18]
Summary
Molecular modeling has been considered indispensable in studying the energy storage of supercapacitors at the atomistic level. Molecular simulations in the potential control mode help to understand the fundamentals of charging and discharging supercapacitors They cannot offer insights into the charging kinetics in the galvanostatic mode (that is, when the electrode is applied with a constant electric current21), which has been widely used in practical applications[22] and fundamental electrochemical studies (for example, galvanostatic charge–discharge, GCD)[23,24]. Based on previous CPM studies[7,8,12], we developed a method to model the GCD of supercapacitors under constant potential, rigorously enforcing the equipotential state within an electrode at each time step (named GCD-CPM; Methods) Both GCD-CPM and GCD-CCM were applied to molecular dynamics (MD) simulations of two typical supercapacitor systems: open electrode (Fig. 1a) and nanoporous electrode (Fig. 1b). We further demonstrated the effectiveness of GCD-CPM via experimental validation
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