Temperature-dependent differential capacitance of an ionic liquid-graphene-based supercapacitor

One of the critical factors affecting the performance of supercapacitors is thermal management. The design of supercapacitors that operate across a broad temperature range and at high charge/discharge rates necessitates understanding the correlation of the molecular characteristics of the device (such as interfacial structure and inter-ionic and ion-electrode interactions) with its macroscopic properties. In this study, we use molecular dynamics (MD) simulations to investigate the influence of Joule heating on the structure and dynamics of the ionic liquid (IL)/graphite-based supercapacitors. The temperature-dependent electrical double layer (EDL) and differential capacitance-potential (CD-V) curves of two different ([Bmim][BF4] and [Bmim][PF6]) IL-graphene pairs were studied under various thermal gradients. For the [Bmim][BF4] system, the differential capacitance curves transition from 'U' to bell shape under an applied thermal gradient (∇T) in the range from 3.3 K nm-1 to 16.7 K nm-1. Whereas in [Bmim][PF6], we find a positive dependence of differential capacitance with ∇T with a U-shaped CD-V curve. We examine changes in the EDL structure and screening potential (ϕ(z)) as a function of ∇T and correlate them with the trends observed in the CD-V curve. The identified correlation between the interfacial charge density and differential capacitance with thermal gradient would be helpful for the molecular design of the IL-electrode interface in supercapacitors or other chemical engineering applications.

A semi-GCMC simulation study of electrolytic capacitors with adsorbed titrating peptides

We use semi-grand canonical Monte Carlo simulations to study an electrolytic capacitor with an adsorbed peptide on the electrode surfaces. Only homogeneous peptides are considered, consisting of only a single residue type. We find that the classical double-hump camel-shaped differential capacitance in such systems is augmented by the addition of a third peak, due to the capacitance contribution of the peptide, essentially superimposed on the salt contribution. This mechanistic picture is justified using a simple mean-field analysis. We find that the position of this third peak can be tuned to various surface potential values by adjusting the ambient pH of the electrolyte solution. We investigate the effect of changing the residue type and the concentration of the adsorbed peptide and of the supporting electrolyte. Varying the residue species and pH allows one to modify the capacitance profile as a function of surface potential, facilitating the design of varying discharging patterns for the capacitor.

Investigating the asymmetry in the EDL response of C60/graphene supercapacitors

Molecular dynamics simulations were employed to model C₆₀/graphene composite electrodes that can expand the effective area and performance of supercapacitors.

Theoretical Calculation of the Thermodynamic Properties of 20 Amino Acid Ionic Liquids

The thermodynamic properties of gas-phase amino acid ionic liquids (AAILs) containing 20 amino acids ([AA]^-) and 1-ethyl-3-methylimidazolium ([Emim]⁺) are studied using a combination of the ab initio method, molecular dynamics simulations, Born-Haber (BH) cycle analysis, and isodesmic reactions. The M06-2X/TZVP method is used to explore the structure and dissociation enthalpies of [Emim][AA] by considering dispersion interaction, and the MP2/Aug-cc-pVTZ method is used to correct these enthalpies. The vaporization enthalpies of all 20 AAILs are calculated by molecular dynamics simulations, and the gas-phase formation enthalpies (Δ_f H) of the 20 [AA]^- anions and [Emim]⁺ cation are calculated by the density functional theory/M06-2X method and isodesmic reaction approaches. To obtain the Δ_f H of the AAILs, interconnections in the corresponding BH cycles are evaluated. A systematic study of the 20 [Emim][AA] ion pairs provides some initial factors contributing to the thermodynamic properties of AAILs: including length of the alkyl chain, interatomic electronic effects, steric repulsion from the cyclic group, and H-bonds formed by functional groups. Generally speaking, the results of this work provide insights into the structure-property relationships of not only ILs but also any ionic or molecular substance.

Storing Energy in Biodegradable Electrochemical Supercapacitors

The development of green and biodegradable electrical components is one of the main fronts of research to overcome the growing ecological problem related to the issue of electronic waste. At the same time, such devices are highly desirable in biomedical applications such as integrated bioelectronics, for which biocompatibility is also required. Supercapacitors for storage of electrochemical energy, designed only with biodegradable organic matter would contemplate both aspects, that is, they would be ecologically harmless after their service lifetime and would be an important component for applications in biomedical engineering. By means of atomistic simulations of molecular dynamics, we propose a supercapacitor whose electrodes are formed exclusively by self-organizing peptides and whose electrolyte is a green amino acid ionic liquid. Our results indicate that this supercapacitor has a high potential for energy storage with superior performance than conventional supercapacitors. In particular its capacity to store energy was estimated to be almost 20 times greater than an analogue one of planar metallic electrodes.

Design of Supercapacitor Electrodes Using Molecular Dynamics Simulations

Electric double-layer capacitors (EDLCs) are advanced electrochemical devices for energy storage and have attracted strong interest due to their outstanding properties. Rational optimization of electrode-electrolyte interactions is of vital importance to enhance device performance for practical applications. Molecular dynamics (MD) simulations could provide theoretical guidelines for the optimal design of electrodes and the improvement of capacitive performances, e.g., energy density and power density. Here we discuss recent MD simulation studies on energy storage performance of electrode materials containing porous to nanostructures. The energy storage properties are related to the electrode structures, including electrode geometry and electrode modifications. Altering electrode geometry, i.e., pore size and surface topography, can influence EDL capacitance. We critically examine different types of electrode modifications, such as altering the arrangement of carbon atoms, doping heteroatoms and defects, which can change the quantum capacitance. The enhancement of power density can be achieved by the intensified ion dynamics and shortened ion pathway. Rational control of the electrode morphology helps improve the ion dynamics by decreasing the ion diffusion pathway. Tuning the surface properties (e.g., the affinity between the electrode and the ions) can affect the ion-packing phenomena. Our critical analysis helps enhance the energy and power densities of EDLCs by modulating the corresponding electrode structures and surface properties.

Graphene/ionic liquid ultracapacitors: does ionic size correlate with energy storage performance?

Electrolytes formed by ions of similar volumes show better performance in supercapacitor applications.