Graphene multi-protonation: A cooperative mechanism for proton permeation

Abstract

The interaction between protons and graphene is attracting a large interest dueto recent experiments showing that these charged species permeate through the 2D material following a low barrier (∼ 0.8 eV) activated process. A possible explanation involves the flipping of a chemisorbed proton (rotation of the C-H + bond from one to the other side of the carbon layer) and previous studies have found so far that the energy barriers (around 3.5 eV) are too high to explain the experimental findings. Contrarily to the previously adopted model assuming an isolated proton, in this work we consider protonated graphene at high local coverage and explore the role played by nearby chemisorbed protons in the permeation process. By means of density functional theory calculations exploiting large molecular prototypes for graphene it is found that, when various protons are adsorbed on the same carbon hexagonal ring, the permeation barrier can be reduced down to 1.0 eV. The related mechanism is described in detail and could shed a new light on the interpretation of the experimental observations for proton permeation through graphene.

Perfect proton selectivity in ion transport through two-dimensional crystals

“…This provides further support to the view that the activation barriers found for proton transport through high-quality graphene and hBN do not involve vacancies and other atomic-scale defects 1 , a conclusion important for further theory developments (e.g., for the hydrogenation model proposed in refs. 8,9 ). Our results also have implications for the widely discussed use of atomically thin crystals as a novel platform for various separation technologies.…”

“…Proton transport through two-dimensional (2D) crystals has recently been studied, both experimentally and theoretically 1–9 . As for the experiment, it was found that proton permeation through mechanically exfoliated crystals is thermally activated with energy barriers of ≈0.8 eV for graphene and ≈0.3 eV for monolayer hexagonal boron nitride (hBN) 1 .…”

“…From a theoretical perspective, the latter value is notably lower (by at least 30% but typically a factor of 2) than that found in density-functional calculations for graphene 3–7 . To account for the difference, a recent theory suggests that graphene can be partially hydrogenated during the measurements, which makes its lattice slightly sparser; thus, making it more permeable to protons 8,9 . An alternative explanation put forward attributes the observed proton currents to atomic-scale lattice defects, including vacancies 10,11 .…”

Stone–Wales Defects Cause High Proton Permeability and Isotope Selectivity of Single‐Layer Graphene

“…Recent experiments [ 6,7 ] demonstrated that single atom‐thin membranes of mechanically exfoliated graphene and hexagonal boron nitride ( h BN) are permeable to thermal protons (H + ), but much less so to deuterons (D + ), resulting in a room‐temperature separation factor of about 10, much higher than in the conventional separation methods. [ 5 ] These interesting findings are discussed somewhat controversially, both on theoretical [ 8–12 ] and experimental [ 10,13,14 ] grounds. The underlying question is whether or not thermal protons can permeate through perfect 2D membranes.…”

Charge delocalization mode and change in aromaticity of protonated 7‐phenylbenzo[k]fluoranthenes studied by experimental observation and DFT calculations

“…Polycyclic aromatic hydrocarbons (PAHs) have attracted much interest in their synthesis and aromatic property . Protonated PAHs have been researched as cationic reactive intermediates in a number of reactions, as models of protonated graphene and nanotubes, and as species in the interstellar medium . Their aromaticity is an important feature to understand the electronic properties of cyclic π‐conjugated systems.…”