(70a) Molecular Dynamic Studies of Charged Electric Double Layers

The electric double layer (EDL) is a diffuse nanoscale cloud of ions that forms around charged surfaces in an ionic fluid. Energy storage devices called electric double layer capacitors (EDLCs or supercapacitors) store energy electrochemically across the EDL between a charged electrode and the cloud of attracted counter-ions. EDLCs can be used to regulate power supply to unbalanced electrical grids; recapture the braking energy of light rail cars, busses, and elevators; and store intermittent energy from solar, wind, or tidal energy sources. We have used molecular dynamics simulations to study charging and steady-state EDL structure over flat plate electrodes under conditions in which the continuum approach fails when ion correlation, solvation, and steric effects become important as in the case of EDLCs. We treat the ions as charged soft-core Lennard-Jones (LJ) particles bound by two repulsive LJ walls. When not modeled implicitly, the solvent is comprised of uncharged LJ particles. The model reproduces ion concentration profiles predicted by the Poisson-Boltzmann (PB) equation at low ionic strengths, small ions, and low applied voltages. By systematically varying ion size, concentration, and valence, we investigate beyond these conditions in order to supplement continuum theory with more advanced double-layer models that are required for rational design and engineering of EDLCs.