(189c) Molecular Simulations of Graphene-Based Electric Double-Layer Capacitors

Towards deploying renewable energy sources (e.g., solar and wind) it is crucial to develop efficient and cost-effective technologies to store energy. Traditional batteries have been used successfully, but they are plagued by a number of practical problems that at present limit their widespread applicability. One possible solution is represented by electric double-layer capacitors (EDLCs), also known as super-capacitors. When compared to traditional batteries, EDLCs are typically characterized by unusually high energy density, but unfortunately are plagued by relatively low working voltage. It has been shown by careful experimental investigations that using an appropriate nanoporous material (typically activated carbons, although significant attention is being devoted to carbon nanotubes) it is possible to further improve the storage density. Because of large surface area and because they offer the possibility of controlled hierarchical three-dimensional structures, graphene sheets hold the promise of providing the core material for developing the next generation of EDLCs. The working voltage can be adjusted by appropriately selecting the charge-carrying fluid placed within the porous material (a considerable amount of research is considering ionic liquids for such purposes). However, the structure and dynamics of confined charged systems are not well understood, thus limiting the possibilities available to EDLCs. We present here our recent simulation results for the structure and dynamics of concentrated aqueous solutions of NaCl, CsCl, and NaI confined within a charged graphene-based porous material. We will discuss how the structure of confined water, the salt concentration, the ions size, and the surface charge density determine the accumulation of electrolytes within the porous network. The results will be compared to simulations results available for bulk systems. Our results are critical for relating macroscopic observations to molecular-level properties of the confined working fluids.