Complementary Effects of Pore Accessibility and Decoordination on the Capacitance of Nanoporous Carbon Electrochemical Supercapacitors

We illustrate here the interplay of decoordination and accessible pore volume in nanosized supercapacitors, using constant voltage Gibbs ensemble based grand canonical Monte Carlo simulations for three different microporous carbon electrodes of known atomistic structure and 1-ethyl-3-methylimidazolium boron tetrafluoride (EMI-BF4) as electrolyte. We demonstrate that the counterion coordination number decreases with pore size, and this trend is similar for the electrodes considered, despite their different structures, suggesting that the pore shape is less important to this relation, at least for the carbons examined here. It is seen that ions with low coordination and/or completely decoordinated ions induce maximum charge, while those with higher coordination induce less, in accordance with recent MD simulation results which demonstrate that ions in high degree of confinement (DOC) induce more charge than those in low DOC. Our results indicate that electrodes with different pore volumes can exhibit similar capacitances by balancing accessibility and decoordination effects. Thus, similar capacitance may be obtained for electrodes having low pore volume, but which can adsorb a small amount of high charge inducers (decoordinated ions) by virtue of having suitable pore size, and those having high pore volume and adsorbing many more low charge inducers (more highly coordinated ions).