An evaluation of the capacitive behavior of supercapacitors as a function of the radius of cations using simulations with a constant potential method.

We report on the atomistic molecular dynamics, applying the constant potential method to determine the structural and electrostatic interactions at the electrode-electrolyte interface of electrochemical supercapacitors as a function of the cation radius (Cs+, Rb+, K+, Na+, Li+). We find that the electrical double layer is susceptible to the size, hydration layer volume, and cations' mobility and analyzed them. Besides, the transient potential shows an increase in magnitude and length as a function of the monocation size, i.e., Cs+ > Rb+ > K+ > Na+ > Li+. On the other hand, the charge distribution along the electrode surface is less uniform for large monocations. Nonetheless, the difference is not observed as a function of the radius of the cation for the integral capacitance. Our results are comparable to studies that employed the fixed charge method for treating such systems.

Exploring doped or vacancy-modified graphene-based electrodes for applications in asymmetric supercapacitors.

We report here density functional theory calculations and molecular dynamics atomistic simulations to determine the total capacitance of graphene-modified supercapacitors. The contributions of quantum capacitance to the total capacitance for boron-, sulfur-, and fluorine-doped graphene electrodes, as well as vacancy-modified electrodes, were examined. All the doped electrodes presented significant variations in quantum capacitance (ranging from 0 to ∼200 µF cm-2) due to changes in the electronic structure of pristine graphene. The graphene-modified supercapacitors show any appreciable effect on double-layer capacitance being virtually the same for all the devices investigated. The total differential capacitance was found to be limited by the quantum capacitance, and for all the systems, it is lower than the quantum capacitance over the entire voltage window. We found that the total capacitance can be optimized by considering an adequate modification to each electrode in the supercapacitor. In addition, we found that an asymmetric supercapacitor assembled with different doped electrodes, i.e. an F doped negative electrode and an N doped positive electrode, is the best choice for a supercapacitor since this combination results in better capacitance over the entire potential window.

Elucidating the amphiphilic character of graphene oxide.

The amphiphilic character of graphene oxide was analysed in terms of its interfacial activities, using atomistic molecular dynamics. Graphene oxides at four different degrees of oxygenation were investigated considering both the effects of oxidation and carboxyl edge-functionalization. Solvation free energies are strongly negative and of increasing magnitude with the concentration for all systems, even in the toluene phase, indicating that GO presents a favourable solvation in both pure liquids as well as interfaces. The PMF results indicate that only the R20 system is slightly active at the water/vacuum interface, with a PMF minimum of about -2.6 kJ mol-1. Both analyses, free energy and PMF, indicate that all systems with higher oxygen concentrations have lower free energy in water than in toluene, while the R20 system opposes this tendency. Comparison between the reduced GOs (20%) shows that edge-functionalised systems were more active than basal-functionalized systems, indicating that oxygen concentration plays a more relevant role than the distribution of functional groups.

Impact of Edge Groups on the Hydration and Aggregation Properties of Graphene Oxide.

Molecular dynamics simulations were used to describe and quantify the role of edge groups on the hydrating properties of graphene oxide (GO). For this, six different oxygen concentrations were investigated, and in four of them, carboxyl groups were present. Structural analysis indicates a greater probability for the water solvation around the GO edges in detriment of the region of its basal plane, while hydrogen bonding analyses indicates that edge groups are very expressive, participating in about 60% of the total number of bonds. The impact of this bond network formed by edge groups is rationalized in energetic and thermodynamic terms. The resulting hydrophilicity observed, as expected, is of electrostatic origin and has a larger contribution from the edge groups that varies from 22 to 57% depending on the concentration. Hydration free energy and potential of mean force calculations support these findings. It was observed that the edge groups contribute up to 51% of the total hydration-free energy and that the PMF indicates the tendency for spontaneous aggregation at all investigated concentrations, being lower the higher the concentration of oxygen.