(418g) Insights into the Effects of Micropore Structure and Electrolyte Composition on the Energy Storage Performance of Electrical Double Layer Capacitors

Electrical double layer capacitors (EDLCs) store energy in the form of electrical charges at the interface between an electrolyte and a high surface area electrode. Despite having high power density and long cycle life, widespread adoption of EDLCs has been restricted due to their limited energy density. Optimizing EDLCs energy storage performance is thus desirable to promote their competitiveness in energy storage applications. Here, we conduct constant potential molecular dynamics simulations of realistically modeled nanoporous carbon electrodes and ionic solutions to explore the effect of electrode micropore structure and solvent concentration on the energy storage kinetics and equilibrium properties of EDLCs. We show that when average pore size is similar to or slightly larger than the size of a solvated ion, solvation weakens anion-cation coordination in nanopores and enhances ion electrosorption by lowering the free energy barrier for counterion penetration while blocking the coions. Areal capacitance of these systems shows a nonmonotonic dependence to solvent concentration, exhibiting an initial increase with solvent introduction in the concentrated regime followed by a decrease with further dilution. This gives rise to a maximum in capacitance at intermediate dilution levels. When pores are significantly larger than solvated ions, capacitance maximum weakens and eventually disappears. Our results also demonstrate that charging profiles of electrodes with larger pores reach the plateau regime faster while charging time has a nonmonotonic dependence to ion concentration that mirrors the composition dependence of bulk electrolyte conductivity. These findings provide novel insights on the charging kinetics and equilibrium behavior of realistically modeled electrical double layer capacitors. Generalization of the approach presented here can facilitate rational optimization of material properties for electrical double layer capacitors.