Insights into the effect of the interlayer spacings of bilayer graphene on the desolvation of H+, Li+, Na+, and K+ ions with water as a solvent: a first-principles study

Abstract

The desolvation effect of ions plays an important role in adjusting the capacity of supercapacitors and has attracted considerable attention after its discovery. Here, first-principles calculations were conducted to calculate the reaction energies of ions, water, and hydrated ions in bilayer graphene (BG) with different interlayer spacings (d) and to explore the desolvation behaviors of H⁺, Li⁺, Na⁺, and K⁺ ions. The calculated results showed that H⁺ can only exist in the state of H₃O⁺ in AA-stacking BG, and desolvation exists only in the case of AB-stacking BG. The complete desolvation size for H⁺ ions in the AB-stacking system reached 5.6 Å, which was the largest desolvation size of the four ions studied. The critical desolvation sizes of Li⁺, Na⁺, and K⁺ in the BG layers of AA- and AB-stacking increased sharply as a consequence of the increasing ionic radius. However, the complete desolvation sizes of all three ions were in the range of 4–5 Å and with the increase in ionic radius, the complete desolvation sizes showed a reverse tendency. The complete desolvation size of Na⁺ in AB-stacking BG was slightly larger than that in AA-stacking BG. Further analysis presented that the ionic radii of H⁺, Li⁺, Na⁺, and K⁺ ions make a dominant contribution to the critical size of desolvation. Our present results provide useful information for improving the capacity of supercapacitors by precisely matching the pore structure and electrolyte through the adjustment of the pore structure of carbon materials.