Dynamic Adsorption of Ions into Like-Charged Nanospace: A Dynamic Density Functional Theory Study

Boosting Salinity Energy Extraction Efficiency in Capacitive Mixing by Polyelectrolyte Surface Coating

The extraction of salinity gradient energy in the capacitive mixing (CapMix) technique can be enhanced by using polyelectrolyte-coated electrodes. The micromechanism of polyelectrolyte (PE) coating enhancing the salinity energy extraction is studied by using a statistical thermodynamic theory. When PE takes same charge sign as the coated electrodes, the extraction efficiency can be boosted owing to the enhanced response of electrical double layer (EDL) to external cell voltage (V0). For the optimal case studied, the extraction efficiency was boosted from 0.25 to 1.25% by PE coatings. Owing to counterion adsorption and the enhanced response of EDL, the extraction energy density presented a local maximum at V0 = 0, which is higher than another local maximum value when V0 ≠ 0. This provides important guidance on the two approaches of CapMix in terms of capacitive Donnan potential (CDP, V0 = 0) and capacitive double-layer expansion (CDLE, V0 ≠ 0). Under the effects of PE coating, the extraction efficiency by CDLE can be improved to about 11% by CDP for the optimal studied case. The synergistic effect of grafting conditions can significantly elevate the energy density and extraction efficiency of the CDP process.

Unraveling Flow Effect on Capacitive Energy Extraction from Salinity Gradients

The harvesting of salinity gradient energy through a capacitive double-layer expansion (CDLE) technique is directly associated with ion adsorption and desorption in electrodes. Herein, we show that energy extraction can be modulated by regulating ion adsorption/desorption through water flow. The flow effects on the output energy, capacitance, and energy density under practical conditions are systematically investigated from a theoretical perspective, upon which the optimal operating condition is identified for energy extraction. We demonstrate that the net charge accumulation displays a negative correlation with the water flow velocity and so does the surface charge density, and this causes a nontrivial variation in the magnitude of output energy when water flows are introduced. When high water flows are introduced in both the charging and discharging processes, the energy extraction can be significantly reduced by 47.69-49.32%. However, when a high flow is solely exerted in the discharging process, the energy extraction can be enhanced by 12.94-14.49% even at low operation voltages. This study not only offers a comprehensive understanding of the microscopic mechanisms of surface-engineered energy extraction with water flows but also provides a novel direction for energy extraction enhancement.

Double-Edged Sword of Ion-Size Asymmetry in Energy Storage of Supercapacitors

The advanced supercapacitor is of great significance for renewable energy storage. Achieving its high energy and high power densities remains a huge challenge. Herein, the contribution of ion-size asymmetry to the charging behavior of a supercapacitor is systematically studied using time-dependent density functional theory (TDDFT). We track the time evolution of the ionic microstructure inside the porous electrode and its reservoir and reveal a kinetic charge inversion in the asymmetrical ion-size cases. Compared with the symmetrical ion-size case, we find that the ion-size asymmetry has a double-edged sword effect on the energy storage of a supercapacitor: it accelerates the charging process yet reduces the differential capacitance. Additionally, the energy density and power density can simultaneously increase in the asymmetrical cases, which provides important insights toward the experimental design of supercapacitors with high energy and high power densities.

Surface Charge Density in Electrical Double Layer Capacitors with Nanoscale Cathode-Anode Separation

Using a dynamic density functional theory, we study the charging dynamics, the final equilibrium structure, and the energy storage in an electrical double layer capacitor with nanoscale cathode-anode separation in a slit geometry. We derive a simple expression for the surface charge density that naturally separates the effects of the charge polarization due to the ions from those due to the polarization of the dielectric medium and allows a more intuitive understanding of how the ion distribution within the cell affects the surface charge density. We find that charge neutrality in the half-cell does not hold during the dynamic charging process for any cathode-anode separation, and also does not hold at the final equilibrium state for small separations. Therefore, the charge accumulation in the half-cell in general does not equal the surface charge density. The relationships between the surface charge density and the charge accumulation within the half-cell are systematically investigated by tuning the electrolyte concentration, cathode-anode separation, and applied voltage. For high electrolyte concentrations, we observe charge inversion at which the charge accumulation exceeds the surface charge at special values of the separation. In addition, we find that the energy density has a maximum at intermediate electrolyte concentrations for a high applied voltage.

Effects of Kinetic Dielectric Decrement on Ion Diffusion and Capacitance in Electrochemical Systems

Diffusion of ionic components in electrolytes not only eliminates the gradients of ionic concentrations but also alters the local dielectric environment, and the coupling effect between kinetic dielectric decrement and ionic concentration gradient on the diffusion dynamics is not well understood. Herein, taking the charging process in electrical double layer systems as a case study, we conduct a multiscale investigation of ion diffusions in aqueous electrolytes by combining the dynamic density functional theory and an ion-concentration-dependent dielectric constant model. By properly considering the time evolutions of local dielectric constant coupled with ion density, we report an interesting phenomenon on the suppression of surface charge density that is not captured by conventional models. In addition, we show that the usage of aqueous electrolyte with small dielectric decrement coefficients promotes the capacitance, in quantitative agreement with experimental measurements.