Mechanistic Insights into Diameter-Dependent Charging and Capacitance in Graphene–CNT Hybrid Electrodes with Concentrated Aqueous LiClO ₄

Constant-potential molecular dynamics (CPM-MD) simulations were used to uncover how nanoscale confinement and electrode polarization jointly govern charge-storage mechanisms and charging kinetics in graphene–carbon nanotube (CNT) hybrid electrodes immersed in a highly concentrated 5.0 m aqueous LiClO4 electrolyte. Four vertically aligned single-walled CNTs ((7,7), (11,11), (15,15), and (20,20) with diameters spanning 9.4–26.5 Å) were examined under applied potentials of ΔV = −2.0, 0, and +2.0 V. The simulations reveal a sharp, diameter-dependent transition in the dominant charging mechanism: subnanometer pores store charge mainly by co-ion desorption, while wider pores increasingly facilitate counterion adsorption, as captured quantitatively by the X parameter. The (15,15) CNT (∼20 Å) emerges as the optimal geometry, achieving the most favorable balance between ion accessibility and electrostatic stabilization and delivering the highest intrinsic gravimetric capacitance. At the device scale, the hybrid electrode exhibits strong polarity asymmetry: the total capacitance reaches 224.8 F g–1 under negative polarization but decreases to 97.4 F g–1 when the polarity is reversed. Charging kinetics show a similar dependence with characteristic time constants of ≈0.0057 ns (positive bias) and ≈0.062 ns (negative bias). Together, these results demonstrate that the pore diameter and electrode polarity orchestrate both equilibrium ion organization and transient interfacial response. The mechanistic principles identified here provide actionable molecular-level guidelines for designing hierarchical carbon electrodes optimized for high-performance aqueous supercapacitors.