Optimal Electrode Mass Ratio in Nanoporous Carbon Electrochemical Supercapacitors

Electrode mass ratio is one of the important parameters that influence capacitance, cycle life, and operating voltage of an electrochemical supercapacitor; however, molecular level understanding of the consequences of inappropriate electrode mass ratio on the overall performance of a supercapacitor is still lacking. Here, we performed constant voltage Gibbs ensemble based grand canonical Monte Carlo simulations on different combinations of microporous carbon electrodes of known atomic structure, and room temperature ionic liquid as electrolyte. Our results indicate that the optimum mass ratio depends not only on the symmetry of electrodes, but also on the size symmetry of the respective counterions. There is an enhancement of 5% in capacitance from the usual mass ratio of 1.0 to the optimum of 0.7-0.8; however, the imbalance in the masses of electrodes causes overloading of the lighter electrode, causing excluded volume interactions to dominate. We anticipate that highly repulsive interactions and large magnitude of forces on electrode atoms can induce irreversible strain in the electrode structure leading to mechanical instability upon charging and discharging for several cycles, and are possibly responsible for the diminished lifetime of the supercapacitor device, as is experimentally observed. However, such impact cannot be directly captured in our simulation study, since the electrodes were treated frozen.