Facile Anhydrous Proton Transport on Hydroxyl Functionalized Graphane

Graphane functionalized with hydroxyl groups is shown to rapidly conduct protons under anhydrous conditions through a contiguous network of hydrogen bonds. Density functional theory calculations predict remarkably low barriers to diffusion of protons along a 1D chain of surface hydroxyls. Diffusion is controlled by the local rotation of hydroxyl groups, a mechanism that is very different from that found in 1D water wires in confined nanopores or in bulk water. The proton mean square displacement in the 1D chain was observed to follow Fickian diffusion rather than the expected single-file mobility. A charge analysis reveals that the charge on the proton is essentially equally shared by all hydrogens bound to oxygens, effectively delocalizing the proton.

Figure 1

(a) Top view of the hydroxylated graphane supercell containing 24 carbons and four hydroxyl groups with one excess proton (carbons in gray, oxygens in red, hydrogens bound to oxygens in dark blue, hydrogens bound to carbon in light blue, and red lines indicate hydrogen bonding). (b) Side-on view for the configuration in (a). Supercell lattice parameters are given in Table S1 of Supplemental Material [10]. Solid lines show the cell boundaries in the $a$ and $b$ directions.

Figure 2

Self-diffusion coefficients at temperatures of 400, 600, and 800 K as determined from AIMD simulations.

Figure 3

MEP computed from the CINEB method for proton hopping on a four-OH group system (see Fig. 1) computed from vasp (red circles) and QE without (green triangles) and with (blue squares) density-countercharge corrections. The MEP for a seven-OH group system is also shown (black diamonds) as a test of finite system size effects.

Figure 4

Atomic configurations computed from CINEB for the four-OH group system (Fig. 1) for the (a) initial state, (b) first transition state, (c) metastable intermediate state, (d) second transition state, and (e) final state. The concerted motion in each step is indicated with arrows (colors defined in Fig. 1).