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.

Recent experiments have shown that nitrogen doping enhances capacitance in carbon electrode supercapacitors. However, a detailed study of the effect of N-doping on capacitance is still lacking. In this paper, we study the doping concentration and the configuration effect on the electric double-layer (EDL) capacitance, quantum capacitance, and total capacitance. It is found that pyridinic and graphitic nitrogens can increase the total capacitance by increasing quantum capacitance, but pyrrolic configuration limits the total capacitance due to its much lower quantum capacitance than the other two configurations. We also find that, unlike the graphitic and pyridinic nitrogens, the pyrrolic configuration's quantum capacitance does not depend on the nitrogen concentration, which may explain why some capacitance versus voltage measurements of N-doped graphene exhibit a V-shaped curve similar to that of undoped graphene. Our investigation provides a deeper understanding of the capacitance enhancement of the N-doping effect in carbon electrodes and suggests a potentially effective way to optimize the capacitance by controlling the type of N-doping.

Using first-principles method, we calculate the electronic band structure of biaxially strained silicon, from which we analyze the change in electron and hole effective mass as a function of strain and determine the mobility of electrons and holes in the biaxially strained silicon based on Boltzmann transport theory. We found that electron mobility increases with tensile strain and decreases with compressive strain. Such changes are mainly caused by a strain-induced change in electron effective mass, while the suppression of intervalley scattering plays a minor role. On the other hand, the hole mobility increases with both signs of strain and the effect is more significant for compressive strain because the hole effective mass decreases with compressive strain but increases with tensile strain. The strain-induced suppression of interband and intraband scatterings plays also an important role in changing the hole mobility.

Recent experiments have shown that electric double-layer capacitors utilizing electrodes consisting of graphene edge plane exhibit higher capacitance than graphene basal plane electrodes. However, theoretical understanding of this capacitance enhancement is still limited. Here we applied a self-consistent electronic calculation on the electrode/electrolyte interface and found that the capacitance of graphene edge plane depends on the edge type: zigzag edge has higher capacitance than armchair edge. We further examined the quantum, dielectric, and electric double-layer (EDL) contributions to the total capacitance of the edge-plane electrodes. Through classical molecular dynamics simulation, we found that the edge planes have higher electric double-layer (EDL) capacitance than the basal plane due to better adsorption of counter-ions and higher solvent accessible surface area, indicating the importance of surface morphology in affecting EDL capacitance. Our work therefore has elucidated the capacitive energy storage in graphene edge planes that take into account both the electrode's electronic structure and the EDL structure.

We investigate the influence of surface chemistry and ion confinement on capacitance and electrosorption dynamics of room-temperature ionic liquids (RTILs) in supercapacitors. Using air oxidation and vacuum annealing, we produced defunctionalized and oxygen-rich surfaces of carbide-derived carbons (CDCs) and graphene nanoplatelets (GNPs). While oxidized surfaces of porous CDCs improve capacitance and rate handling abilities of ions, defunctionalized nonporous GNPs improve charge storage densities on planar electrodes. Quasielastic neutron scattering (QENS) and inelastic neutron scattering (INS) probed the structure, dynamics, and orientation of RTIL ions confined in divergently functionalized pores. Oxidized, ionophilic surfaces draw ions closer to pore surfaces and enhance potential-driven ion transport during electrosorption. Molecular dynamics (MD) simulations corroborated experimental data and demonstrated the significance of surface functional groups on ion orientations, accumulation densities, and capacitance.

In this study, a series of novel pigments based on V 5+ doped BiPO 4 have been prepared for the first time via a facile hydrothermal method and characterized using several analytical techniques, such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), ultraviolet-visible-near-infrared (UV-vis-NIR) spectrometry, the Commission International de l'Eclairage (CIE) L*a*b* color scales and thermogravimetry and differential thermal analysis (TG-DTA). The investigation demonstrated that the synthesized pigments of BiP 1Àx V x O 4 (x ¼ 0.00, 0.01, 0.05, 0.08, 0.10, 0.15) had a monazite-type phase structure and were about 0.25-2 mm in size. Meanwhile, the substitution of V 5+ for P 5+ in BiPO 4 resulted in the band gap of the pigments varying from 3.657 to 3.244 eV and its mechanism was explained by charge-transfer and energy band theory, while the color changed from white to yellow. More importantly, the V 5+ doped pigments possessed high NIR reflectance (>72%) and NIR solar reflectance ($75.64%) in the range 700-2500 nm. Moreover, coatings colored with synthetic pigments have higher NIR solar reflectance ($78.59%) than conventional pigments. Additionally, the pigments showed good thermal/chemical stabilities in high-temperature/acid/alkaline tests. In conclusion, the pigments have the potential to be applied as "cool pigments" to reduce energy consumption.

We use molecular dynamics simulations to investigate the effects of solvent concentration on the bulk properties of an ion liquid electrolyte and the electrochemical performance on carbon-based electrodes, including pristine graphene, oxidized graphene, graphene armchair edge, graphene zigzag edge, onion-like carbon, and slit-pore carbon. We find that diluting the electrolyte reduces the number of ion pairs in the bulk and improves ion dynamics. The capacitance of the two-edge electrodes decreases monotonically as the solvent concentration increases, while the capacitance of other nonedge electrodes exhibits nonmonotonic behavior and a capacitance maximum is observed. Further analyses on the electric double layer reveals two competing factors: solvation reduces the charge overscreening effect, but it also causes the dilution of absorbed ion concentration. While the former increases the capacitance in the low dilution regime, the latter decreases the capacitance in the high dilution regime. In addition, the dilution also significantly improves the ion dynamics at the interface. Our simulation results demonstrate that diluting an ionic liquid electrolyte could potentially boost the power density while maintaining or even slightly increasing the energy density with a careful selection of solvent concentrations on a nonedge carbon electrode.

The associative and dissociative adsorption of water molecules at low-coverage situations on rutile TiO2 (110) surface with step defects was investigated by the density functional theory calculations. Structural optimization of the hydroxylated/hydrated configurations at step edges along the 11̄1 crystal directions and the dynamic process of water dissociation were discussed to get a better description of the water/TiO2 interface. Our results indicate that steps on the TiO2 (110) surface could be an active site for water dissociation. The results of geometry optimization suggest that the stability of hydroxylated configuration is largely dependent on the locations of the H species and the recombination of water molecules from hydroxyls is observed in the fully hydroxylated condition. However, these hydroxyls can be stabilized by the associatively absorbed water nearby by forming competitive intermolecular hydrogen bonds. The dynamics of water dissociation and hydrogen diffusion were studied by the first principles molecular dynamics simulation and our results suggest that the hydrogen released by water dissociation can be transferred among the adsorbates, such as the unsaturated oxygen atoms-H2O-hydroxyl (TiO-H2O-OH) complex at step edges, or gradually diffuses to the bulk water system in the form of hydronium (H3O(+)) at higher water coverage.