Electrically-Driven Ion Transmission Through Two-Dimensional Nanomaterials

Two-dimensional nanomaterials such as graphene and hexagonal boron nitride are being intensively studied as selective barriers in separation technology owing to their unique subatomic selectivity. In their pristine forms, they are impermeable to atoms, molecules, and ions except for thermal protons. Graphene, with its angstrom-scale thickness, is regarded as the thinnest membrane so its transport selectivity comes from the selectivity of active sites at which permeant transmission occurs. This dissertation tested the hypothesis that the selectivity ratio of hydrogen isotopes (protium, Deuterium, and tritium) through membrane could be improved by incorporating graphene and related 2D materials in the membrane electrode assembly of a polymer electrolyte membrane electrolysis cell. The mechanism by which protons or deuterons traverse the energy barrier of 2D materials was also investigated with a focus on the temperature dependence of isotopic selectivity in crossing rates. By carefully positioning a 2D material within the ionomer membranes of a membrane electrode assembly, the isotopic ion filtering functionalities of graphene and analogs were enhanced. Proton transmission through graphene was found to occur at a very high rate (1.0 A cm-2 achieved at a potential bias of \u3c 200 mV) with a selectivity ratio of at least 10 compared to deuteron transmission. The transmission rates of Protons and deuterons across single-layer graphene were measured as a function of temperature. An electrochemical model based on charge-transfer resistance was invoked to estimate standard heterogeneous ion-transfer rate constants. An encounter pre-equilibrium model for the ion-transfer step was used to estimate rate constants which provide values for activation energies and exponential pre-factors for proton (or deuteron) transmission across graphene. Activation energies of 48 ± 2 kJ mole-1 (0.50 ± 0.02 eV) and 53 ± 5 kJ mole-1 (0.55 ± 0.05 eV) were obtained for protons and deuterons respectively, through single-layer graphene. The difference of 50 meV is in good agreement with the expected difference in vibrational zero-point energies for O-H and O-D bonds. This work is an important harbinger for the prospects of developing graphene-based PEM electrochemical cell ion filters for fuel cells, electrolysis cells, gas separation and purification, and desalination applications