Graphene Synthesized by Chemical Vapor Deposition as a Hydrogen Isotope Permeation Barrier

Hydrogen isotope permeation barriers (HIPBs) are a challenging engineering problem for multiple applications such as the hydrogen energy industry, the nuclear power industry, and extreme high vacuum applications.¹ The nuclear fusion industry is especially moving toward using tritium and deuterium as fuel for reactors.¹ Due to its decay half-life of 12.3 years, tritium leakage by permeation is a problem because the permeated container becomes a radiation source.¹ Thus, materials that prevent or have low hydrogen isotope permeabilities are necessary to overcome this problem.¹ HIPB coatings must have good adhesion to the substrate, have high thermal stability, remain chemically inert, and remain mechanically robust.¹ 2D materials, such as graphene, with low permeability, high aspect ratio, high strength, chemical inertness, and atomic thickness may be able to fulfill this need.<br/>Graphene has been shown to be an extremely impermeable barrier, largely due to its unique structure. Graphene is composed of C with sp² bonds in plane with Π orbitals perpendicular to the surface.² The Π orbitals form a delocalized cloud, which repels other molecules, including He.² Graphene is also chemically inert, and a single layer is thermally stable up to 450 °C.² The mechanical strength of graphene is also key to its impermeability, so that it can withstand pressure differences and maintain its structure as a barrier.² When using chemical vapor deposited (CVD) graphene as a separation membrane, the graphene is typically transferred by a wet etch method onto a membrane with a polymer scaffold.³ This transfer method can cause the graphene to easily tear, leaving open holes that allow easy permeation of gas molecules with little to no selectivity.³ Attempts to reduce these issues involve stacking graphene with multiple transfers, filling the transfer-induced holes with atomic layer deposition or polymer, or transferring the graphene to a porous membrane that has comparable pore resistance as the selective pores in graphene.³ Although these methods demonstrate better gas separation selectivity than the as-transferred graphene (with tears), the gas permeation can still be higher than expected.<br/>This work presents the results of gas concentration driven deuterium permeation experiments through CVD-graphene on copper. Since the graphene is synthesized directly onto the copper, the permeation experiments are able to be performed over a large area (16.62 mm²) without the detrimental impact of transfer-induced tears and holes. Thus, permeation through intrinsic defects of the graphene are probed. The graphene-coated copper shows a reduction in permeation by a factor of ∼28 compared to copper alone. The permeation results are modeled with a composite permeation model. The permeation of copper alone is shown to be proportional to the square root of pressure, whereas the permeation through graphene samples is proportional to pressure. The graphene permeance follows an Arrhenius behavior. The effects of graphene grain boundaries on permeation are also studied, and the results suggest that grain boundaries are not the main diffusion pathways, and instead other intrinsic defects in the graphene demonstrate less resistance to permeation. This study advances the fundamental understanding of the intrinsic permeation of CVD-graphene, as well as the use of graphene in HIPB applications.<br/>1. Nemanič, V., Hydrogen permeation barriers: basic requirements, materials selection, deposition methods, and quality evaluation. <i>Nuclear Materials and Energy </i><b>2019,</b> <i>19</i>, 451-457.<br/>2. Berry, V., Impermeability of graphene and its applications. <i>Carbon </i><b>2013,</b> <i>62</i>, 1-10.<br/>3. Boutilier, M. S.; Jang, D.; Idrobo, J.-C.; Kidambi, P. R.; Hadjiconstantinou, N. G.; Karnik, R., Molecular sieving across centimeter-scale single-layer nanoporous graphene membranes. <i>ACS nano </i><b>2017,</b> <i>11</i> (6), 5726-5736.