(228a) Solvation Structure of Ions in Model Disordered Carbon Electrodes

In order to understand the contribution of ion confinement to the capacitance increase in sub-nanometer pores for electrochemical double-layer capacitors (EDLCs), we have performed Monte Carlo simulations of 1.0 M, 3.0 M, and 5.0 M NaCl within a realistic model disordered carbon electrode.  The simulations were carried out in the canonical ensemble.  In order to reproduce a charged porous electrode, the carbon atoms of the carbon structure were given charges corresponding to surface charge densities of 0 C/m², 10 C/m², and 25 C/m², a range typically found in EDLCs. Umbrella sampling was employed to study the solvation structure of the ions as a function of local pore size.  The ion-water radial and orientational distribution functions were calculated to observe the degree to which the ions were solvated. Using the weighted histogram analysis method (WHAM), the free energy of solvation was calculated as ions were biased towards progressively smaller and smaller pore sizes.  We find that the free energy barrier that ions must overcome by entering successively smaller pores is lower than the free energy barrier for an ion moving from a bulk solution into a small, charged slit-shaped pore (Feng et al., J. Phys. Chem., 2010, 114, 18012-18016). In addition, we report results for solvation numbers of ions within specific pore sizes. We find that the solvation number of ions in the model disordered carbon vary strongly with pore size in the subnanometer regime, giving a more accurate description of the solvation structure in these electrodes than can be seen in simple slit pore geometries.