Capacitance of Carbon Nanotube/Graphene Composite Electrodes with [BMIM⁺][BF₄-]/Acetonitrile: Fixed Voltage Molecular Dynamics Simulations

The capacitance of nanoporous carbon electrode materials isdictated by the physical interactions with electrolyte ions and molecules atthe accessible interior electrode surface. While significant progress has beenmade in designing and synthesizing carbon materials with well-defined andrelatively homogeneous nanoporosity, the majority of materials remainheterogeneous, and the resulting properties reflect an average over thisdistribution. In this regard, computer simulations can be a valuable tool topredict or validate structure/property relationships by systematicallyinvestigating well-defined, model electrode morphologies. We utilizefixed-voltage molecular dynamics simulations to predict structure/capacitancerelationships forfive different model morphologies of carbon nanotube/graphene (CNT/G) composite electrodes with 1-butyl-3-methylimidazoliumtetrafluoroborate/acetonitrile electrolyte. The CNT/G electrode models areinspired by experimental"layer-by-layer"syntheses and strike a balance between realism and computational tractability. Comparisonbetween different model electrode architectures elucidates important structure/property relationships. Wefind that CNT/graphenecontact points serve as"hot spots"with significantly enhanced charge separation relative to the rest of the electrode. Furthermore, wedemonstrate a specific nanoconfinement motif that provides substantial 3-4xenhancement of local capacitance, resulting in a similar to 40%increase of the total electrode differential capacitance. Because the accessible surface area of the model CNT/G electrodes isprecisely determined, a comparison of per-area capacitance across systems is unambiguous. Our results thus complement a priorcomputational demonstration of capacitance enhancement in nanoconfinement while elucidating additional interaction motifs atCNT/G electrochemical interfaces.