Abstract
Supercapacitors such as electric double‐layer capacitors (EDLCs) and pseudocapacitors are becoming increasingly important in the field of electrical energy storage. Theoretical study of energy storage in EDLCs focuses on solving for the electric double‐layer structure in different electrode geometries and electrolyte components, which can be achieved by molecular simulations such as classical molecular dynamics (MD), classical density functional theory (classical DFT), and Monte‐Carlo (MC) methods. In recent years, combining first‐principles and classical simulations to investigate the carbon‐based EDLCs has shed light on the importance of quantum capacitance in graphene‐like 2D systems. More recently, the development of joint density functional theory (JDFT) enables self‐consistent electronic‐structure calculation for an electrode being solvated by an electrolyte. In contrast with the large amount of theoretical and computational effort on EDLCs, theoretical understanding of pseudocapacitance is very limited. In this review, we first introduce popular modeling methods and then focus on several important aspects of EDLCs including nanoconfinement, quantum capacitance, dielectric screening, and novel 2D electrode design; we also briefly touch upon pseudocapactive mechanism in RuO2. We summarize and conclude with an outlook for the future of materials simulation and design for capacitive energy storage.
Highlights
The quantum capacitances were predicted from electronic density functional theory based on Fix-Band-Approximation with an implicit solvation model and the electric-double layer (EDL) capacitances were from classical density functional theory
When the number of layers reaches four, the total capacitance tends to converge and the electric double-layer (EDL) capacitance dominates at voltages sufficiently different from the potential of zero charge
5.3.2 Quantum capacitance of Few-layer graphene (FLG) separated from total capacitance we show the contribution of quantum capacitance from our joint density functional theory (JDFT) results
Summary
Hwang et al first developed an approach to calculate total capacitance (Ctot) by separately modeling quantum capacitance and EDL capacitance (CEDL) on single layer graphene:[15] quantum capacitance is modeled by the electronic DOS of the neutral electrode[14-16] and EDL capacitance is from classical molecular dynamics (CMD) simulation.[17-24]. This approach of computing the total capacitance ignores the close interaction between the electronic structure of the electrode and the distribution of the mobile ions in the electrolyte. This quantum capacitance can be separately computed through electronic density functional theory (DFT), while the EDL capacitance can be obtained through classical methods (molecular dynamics, classical DFT, or Monte Carlo) 18, 20-24
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