Abstract
Vertically-oriented graphenes (VGs) are promising active materials for electric double layer capacitors (EDLCs) due to their unique morphological and structural features. This study, for the first time, reports the molecular dynamics (MD) simulations on aqueous NaCl electrolytes confined within VG channels with different surface charge densities and channel widths. Simulation results show that the accessibility of ions and the structure of EDLCs are determined by the ion type/size, surface charging, and VG channel width. For relatively narrow VG channels with the same width, the threshold charge density (to compensate the energy penalty for shedding hydration shell) and the dehydration rate of Cl− ions are larger than those of Na+ ions. To achieve the highest ion concentration coefficient, the effective VG channel width should be between the crystal and hydration diameters of the ions. The results are further quantified and elucidated by calculating the electrolyte density profiles. The molecular insights obtained in the current work are useful in guiding the design and fabrication of VGs for advancing their EDLC applications.
Highlights
Molecular dynamics (MD) has been recognized as a powerful technique to describe the motion of particles at molecular scale
molecular dynamics (MD) simulation was performed on the NaCl electrolytes confined within Vertically-oriented graphenes (VGs) channels, with focusing on the influences of channel width and surface charge density on the structure of electric double layer capacitors (EDLCs) and charge distribution
To achieve the highest τ values, the effective VG channel width is suggested to be at the range between the crystal and hydration diameters of the ions
Summary
Molecular dynamics (MD) has been recognized as a powerful technique to describe the motion of particles at molecular scale. MD has been widely applied for the EDLC systems employing various carbon-based electrodes, such as planar graphenes[11,12,13,14], carbon nanotubes (CNTs)[15,16,17], onion-like carbons[18], and activated carbons[19]. The structure of EDLCs was unveiled with MD simulation, providing the mechanisms on the pack of ions within planar pores and its influence on the capacitance[11,12,13]. The influences of charge density and channel width on the ion/ molecule distributions were investigated in detail. The packing behavior ions inside VG channels with different widths and surface charge densities were studied.
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