How Thermal Effect Regulates Cyclic Voltammetry of Supercapacitors

Cyclic voltammetry (CV) is a powerful technique for characterizing electrochemical properties of electrochemical devices. During charging-discharging cycles, thermal effect has profound impact on its performance, but existing theoretical models cannot clarify such intrinsic mechanism and often give poor prediction. Herein, we propose an interfacial model for the electro-thermal coupling, based on fundamentals in non-equilibrium statistical mechanics. By incorporating molecular interactions, our model shows a quantitative agreement with experimental measurements. The integral capacitance shows a first enhanced then decayed trend against the applied heat bath temperature. Such a relation is attributed to the competition between electrical attraction and Born repulsion via dielectric inhomogeneity, which is rarely understood in previous models. In addition, as evidenced in recent experimental CV tests, our model predicts the non-monotonic dependence of the capacitance on the bulk electrolyte density, further demonstrating its high accuracy. This work demonstrates a potential pathway towards next-generation thermal regulation of electrochemical devices

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

First-principles investigation of aluminum intercalation and diffusion in TiO2 materials: Anatase versus rutile

Aluminum-ion batteries, emerging as a promising post-lithium battery solution, have been a subject of increasing research interest. Yet, most existing aluminum-ion research has focused on electrode materials development and synthesis. There has been a lack of fundamental understanding of the electrode processes and thus theoretical guidelines for electrode materials selection and design. In this study, by using density functional theory, we for the first time report a first-principles investigation on the thermodynamic and kinetic properties of aluminum intercalation into two common TiO 2 polymorphs, i.e., anatase and rutile. After examining the aluminum intercalation sites, intercalation voltages, storage capacities and aluminum diffusion paths in both cases, we demonstrate that the stable aluminum intercalation site locates at the center of the O 6 octahedral for TiO 2 rutile and off center for TiO 2 anatase. The maximum achievable Al/Ti ratios for rutile and anatase are 0.34375 and 0.36111, respectively. Although rutile is found to have an aluminum storage capacity slightly higher than anatase, the theoretical specific energy of rutile can reach 20.90 Wh kg −1 , nearly twice as high as anatase (9.84 Wh kg −1 ). Moreover, the diffusion coefficient of aluminum ions in rutile is 10 −9 cm 2 s −1 , significantly higher than that in anatase (10 −20 cm 2 s −1 ). In this regard, TiO 2 rutile appears to be a better candidate than anatase as an electrode material for aluminum-ion batteries

Selective heavy metal removal and water purification by microfluidically-generated chitosan microspheres: Characteristics, modeling and application

Many industrial wastewater streams contain heavy metals, posing serious and irreversible damage to humans and living organisms, even at low concentrations due to their high toxicity and persistence in the environment. In this study, high-performance monodispersed chitosan (CS) microspheres were prepared using a simple microfluidic method and evaluated for metal removal from contaminated water. Batch experiments were carried out to evaluate the adsorption characteristics for the removal of copper ions, one representative heavy metal, from aqueous solutions. The inherent advantages of microfluidics enabled a precise control of particle size (CV = 2.3%), while exhibiting outstanding selectivity towards target ions (adsorption capacity 75.52 mg g−1) and fair regeneration (re-adsorption efficiency 74% after 5 cycles). An integrated adsorption mechanism analytic system was developed based on different adsorption kinetics and isotherms models, providing an excellent adsorption prediction model with pseudo-second order kinetics (R2 = 0.999), while the isotherm was fitted best to the Langmuir model (R2 = 0.998). The multi-step adsorption process was revealed via quantitative measurements and schematically described. Selective adsorption performance of CS microspheres in the present of other competitive metal ions with different valence states has been demonstrated and studied by both experimental and density functional theory (DFT) analysis

Enhanced biphasic reactions in amphiphilic silica mesopores

In this study, we investigated the effect of the pore volume and mesopore size of surface-active catalytic organosilicas on the genesis of particle-stabilized (Pickering) emulsions for the dodecanal/ethylene glycol system and their reactivity for the acid-catalyzed biphasic acetalization reaction. To this aim, we functionalized a series of fumed silica superparticles (size 100–300 nm) displaying an average mesopore size in the range of 11–14 nm and variable mesopore volume, with a similar surface density of octyl and propylsulfonic acid groups. The modified silica superparticles were characterized in detail using different techniques, including acid–base titration, thermogravimetric analysis, TEM, and dynamic light scattering. The pore volume of the particles impacts their self-assembly and coverage at the dodecanal/ethylene glycol (DA/EG) interface. This affects the stability and the average droplet size of emulsions and conditions of the available interfacial surface area for reaction. The maximum DA-EG productivity is observed for A200 super-SiNPs with a pore volume of 0.39 cm3·g–1 with an interfacial coverage by particles lower than 1 (i.e., submonolayer). Using dissipative particle dynamics and all-atom grand canonical Monte Carlo simulations, we unveil a stabilizing role of the pore volume of porous silica superparticles for generating emulsions and local micromixing of immiscible dodecanal and ethylene glycol, allowing fast and efficient solvent-free acetalization in the presence of Pickering emulsions. The micromixing level is interrelated to the adsorption energy of self-assembled particles at the DA/EG interface

Accurate evaluation of the angular-dependent direct correlation function of water

International audienceThe direct correlation function (DCF) plays a pivotal role in addressing the thermodynamic properties with non-mean-field statistical theories of liquid state. This work provides an accurate yet efficient calculation procedure for evaluating the angular-dependent DCF of bulk SPC/E water. The DCF here represented in a discrete angles basis is computed with two typical steps: the first step involves solving the molecular Ornstein-Zernike equation with the input of total correlation function extracted from simulation; the resultant DCF is then polished in second step at small wavelength for all orientations in order to match correct thermodynamic properties. This function is also discussed in terms of its rotational invariant components. In particular, we show that the component c112(r) that accounts for dipolar symmetry reaches already its long-range asymptotic behavior at a short distance of 4 Å. With the knowledge of DCF, the angular-dependent bridge function of bulk water is thereafter computed and discussed in comparison with referenced hard-sphere bridge functions. We conclude that, even though such hard-sphere bridge functions may be relevant to improve the calculation of Helmholtz free energies in integral equations or density functional theory, they are doomed to fail at a structural level

Time-dependent density functional theory for the charging kinetics of electric double layer containing room-temperature ionic liquids

Understanding the charging kinetics of electric double layers is of fundamental importance for the design and development of novel electrochemical devices such as supercapacitors and field-effect transistors. In this work, we study the dynamic behavior of room-temperature ionic liquids using a classical time-dependent density functional theory that accounts for the molecular excluded volume effects, the electrostatic correlations, and the dispersion forces. While the conventional models predict a monotonic increase of the surface charge with time upon application of an electrode voltage, our results show that dispersion between ions results in a non-monotonic increase of the surface charge with the duration of charging. Furthermore, we investigate the effects of van der Waals attraction between electrode/ionic-liquid interactions on the charging processes