RESUMEN
Understanding the dynamics of electric-double-layer (EDL) charging in porous media is essential for advancements in next-generation energy storage devices. Due to the high computational demands of direct numerical simulations and a lack of interfacial boundary conditions for reduced-order models, the current understanding of EDL charging is limited to simple geometries. Here, we present a network model to predict EDL charging in arbitrary networks of long pores in the Debye-Hückel limit without restrictions on EDL thickness and pore radii. We demonstrate that electrolyte transport is described by Kirchhoff's laws in terms of the electrochemical potential of charge (the valence-weighted average of the ion electrochemical potentials) instead of the electric potential. By employing the equivalent circuit representation suggested by these modified Kirchhoff's laws, our methodology accurately captures the spatial and temporal dependencies of charge density and electric potential, matching results obtained from computationally intensive direct numerical simulations. Our network model provides results up to six orders of magnitude faster, enabling the efficient simulation of a triangular lattice of five thousand pores in 6 min. We employ the framework to study the impact of pore connectivity and polydispersity on electrode charging dynamics for pore networks and discuss how these factors affect the time scale, energy density, and power density of capacitive charging. The scalability and versatility of our methodology make it a rational tool for designing 3D-printed electrodes and for interpreting geometric effects on electrode impedance spectroscopy measurements.
RESUMEN
Conductive metal-organic frameworks (c-MOFs) and ionic liquids (ILs) have emerged as auspicious combinations for high-performance supercapacitors. However, the nanoconfinement from c-MOFs and high viscosity of ILs slow down the charging process. This hindrance can, however, be resolved by adding solvent. Here, constant-potential molecular simulations are performed to scrutinize the solvent impact on charge storage and charging dynamics of MOF-IL-based supercapacitors. Conditions for >100% enhancement in capacity and ≈6 times increase in charging speed are found. These improvements are confirmed by synthesizing near-ideal c-MOFs and developing multiscale models linking molecular simulations to electrochemical measurements. Fundamentally, the findings elucidate that the solvent acts as an "ionophobic agent" to induce a substantial enhancement in charge storage, and as an "ion traffic police" to eliminate convoluted counterion and co-ion motion paths and create two distinct ion transport highways to accelerate charging dynamics. This work paves the way for the optimal design of MOF supercapacitors.
RESUMEN
Nanopore electrodes have the potential to enhance the energy density of supercapacitors but tend to reduce charging dynamics, consequently impacting power density. A comprehensive understanding of their charging mechanisms can provide insights into how to boost charging dynamics. In this work, we conducted constant-potential-based molecular dynamics simulations to explore the charging mechanism of nanopore supercapacitors with organic electrolytes. Contrary to the traditional understanding associating larger pore sizes with faster charging, our results found a complex oscillatory behavior of the charging rate, correlating with nanopore size in organic electrolytes. An anomalously increased charging dynamics was found in the 0.9 nm pore. This anomalous enhancement can be attributed to the improved in-pore ion diffusion and reduced desolvation energy, owing to the orientation transition of the solvate molecules. These results pave a new way for innovative designs of nanoporous electrode supercapacitors that can enlarge both power and energy densities.
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Optimizing the synergy between nanoporous carbons and ionic liquids can significantly enhance the energy density of supercapacitors. The highest energy density has been obtained as the size of porous carbon matches the size of ionic liquids, while it may result in slower charging dynamics and thus reduce the power density. Enhancing energy storage without retarding charging dynamics remains challenging. Herein, we designed porous electrodes by introducing an optimized horn-like entrance to the nanopore, which can concurrently improve supercapacitors' charging dynamics and energy storage. Our results revealed the mechanism of improved charging lies in the gradual desolvation process and optimized ion motion paths: the former expedites the adsorption of the counterion by reducing the transitional energy barrier for ions entering the pores, and the latter accelerates the co-ion desorption and eliminates ion overfilling. Meanwhile, the enhancement of energy density could be attributed to the multi-ion coordinated migration.
RESUMEN
The calculated defect corrections to the polarization and dielectric functions for Bloch electrons in quantum wells are presented. These results were employed to derive the first two moment equations from the Boltzmann transport theory and then applied to explore the role played by defects on the magneto-transport of Bloch electrons. Additionally, we have derived analytically the inverse momentum-relaxation time and mobility tensor for Bloch electrons by making use of the screened defect-corrected polarization function. Based on quantum-statistical theory, we have investigated the defect capture and charging dynamics by employing a parameterized physics-based model for defects to obtain defect wave functions. Both capture and relaxation rates, as well as the density for captured Bloch electrons, were calculated self-consistently as functions of temperature, doping density and chosen defect parameters. By applying the energy-balance equation, the number of occupied energy levels and the chemical potential of defects were determined, with which the transition rate for defect capturing was obtained. By applying these results, the defect energy-relaxation, capture and escape rates, and Bloch-electron chemical potential were calculated self-consistently for a non-canonical subsystem of Bloch electrons. At the same time, the energy- and momentum-relaxation rates of Bloch electrons, as well as the current suppression factor, were also investigated quantitatively. By combining all these results, the temperature dependence of the Hall and longitudinal mobilities was presented for Bloch electrons in either single- or multi-quantum wells.
RESUMEN
Using electrodes with subnanometer pores and ionic liquid electrolytes can improve the charge storage capacity at the expense of the charging rate. The fundamental understanding of the charging dynamics of nanoporous electrodes can help to avoid compromising the power density. In this work, we performed molecular dynamics simulations to reveal the charging mechanism of subnanometer pores in ionic liquids. Different from the traditional view that a smaller pore results in slower charging, a non-monotonic relation is found between the charging rate and pore size, in which the charging process is accelerated in some subnanometer pores. Our analysis uncovers that the mechanism of the charging enhancement can be attributed to the transition of in-pore ion structure.
RESUMEN
Most studies of ionic liquid (IL)-gated field effect transistors (FETs) focus on the extremely large electric field and capacitance induced in liquid/solid interfaces and correspondingly the significantly enhanced carrier density in semiconductors, which can appreciably improve the gating performance. However, how to boost the switching speed, another key property of gating performance of FETs, has been rarely explored. In this work, the gating performance of molybdenum disulfide (MoS2) FETs, gated by the mixtures of IL/organic solvent (1-butyl-3-methylimidazolium tetrafluoroborate/acetonitrile, [Bmim][BF4]/ACN) at different ion concentrations, is investigated for both dynamic and static properties by a combination of molecular dynamics simulation and resistance network analysis. Results reveal that organic solvent can speed up the IL response time by a factor of about 40 times at the optimal ion concentration of 1.94 M, which is mainly attributed to the increased ionic conductivity of IL via the addition of organic solvent. Meanwhile, the surface charge distribution of MoS2 becomes more homogenous after the addition of organic solvent, which increases the conductivity of MoS2 by up to 2.4 times. Surprisingly, the optimal ion concentration for increased switching speed is nearly the same as that for achieving the highest MoS2 conductivity. Thus, our findings provide a strategy to simultaneously improve the dynamic and static gating performance of IL-gated FETs as well as a modeling technique to screen out the ideal ion concentration.
RESUMEN
Nanoporous supercapacitors attract much attention as green energy storage devices with remarkable cyclability and high power and energy densities. However, their use in high-frequency applications is limited by relatively slow charging processes, while accelerating charging without compromising the energy storage still remains a challenging task. Here, we study in detail the charging and discharging behavior of nanoporous supercapacitors with narrow pores, which provide exceptionally high capacitances and stored energy densities. We scrutinize the dynamic modes of charging, revealing, in particular, a transient formation of crowded and dilute ionic-liquid phases inside the pores, which leads to co-ion trapping and correspondingly slow charging. We show how trapping can be circumvented by applying a slow voltage sweep, and we demonstrate that it can accelerate the overall charging process considerably if the sweep rate is chosen appropriately. While one might be tempted to apply a similar strategy to discharging, we find that the best discharge rates are obtained when the voltage is switched off in a step-like fashion, whereby the optimal charge and discharge times differ a few-fold. We unveil the scaling laws for such optimal quantities, which allow one to predict quantitatively the charging behavior for realistically long pores. On the basis of our findings, we propose an optimal charge-discharge cycle and elaborate on optimization strategies.
RESUMEN
Understanding the dynamic charge storage in nanoporous electrodes with room-temperature ionic liquid electrolytes is essential for optimizing them to achieve supercapacitors with high energy and power densities. Herein, we report coarse-grained molecular dynamics simulations of the cyclic voltammetry of supercapacitors featuring subnanometer pores and model ionic liquids. We show that the cyclic charging and discharging of nanopores are governed by the interplay between the external field-driven ion transport and the sloshing dynamics of ions inside of the pore. The ion occupancy along the pore length depends strongly on the scan rate and varies cyclically during charging/discharging. Unlike that at equilibrium conditions or low scan rates, charge storage at high scan rates is dominated by counterions while the contribution by co-ions is marginal or negative. These observations help explain the perm-selective charge storage observed experimentally. We clarify the mechanisms underlying these dynamic phenomena and quantify their effects on the efficiency of the dynamic charge storage in nanopores.