Capacitance Optimization in Nanoscale Electrochemical Supercapacitors

We perform constant voltage Gibbs ensemble based grand canonical Monte Carlo simulations for nanosized supercapacitors comprising graphene slit electrodes in symmetric and asymmetric electrolytes. Our simulations demonstrate that external electrolyte at the electrode surface can be exploited to positively influence the structure and packing of that inside the slit, when the system is engineered to allow these to interact. Oscillatory dependence of capacitance on slit-pore size, seen in recent results from molecular dynamics simulation and density functional theory, is observed also in our Monte Carlo simulations. A detailed analysis suggests that maximum in capacitance occurs in subnanometre pores because of the interference between internal double layers (largely the Helmholtz parts) on the opposite sides of the slit, expelling the co-ions; and that the oscillatory character of capacitance with pore width is due to relative changes in counterion and co-ion populations with pore width, also dictated by the interference process between the two internal double layers. Our simulations with size-asymmetric and size-symmetric electrolytes with different sets of electrode pairs reveal that when the pore widths of both the electrodes are close to their respective counterion sizes, the electrodes store maximum charge density, yielding maximum capacitance. Thus, it is demonstrated that for asymmetric electrolytes optimum capacitance is obtained using a correspondingly asymmetric electrode combination.