(87p) Computational High-Throughput Study of Hydrogen Permeation through Two-Dimensional Structures for Use As Proton-Conducting Membranes

Hydrogen technologies, such as fuel cells, are designed around the membrane. Aspects such as humidity and heat management, catalyst activity and poisoning, cooling systems, among others, must be controlled carefully [1]. Unfortunately, the polymeric materials used today still cannot operate at 100ºC-400ºC, the infamous ‘materials gap’ in proton conductors [2].

Hence, this work focuses on the evaluation of novel 2D-crystals to be used as proton conductive materials that could operate in conditions where typical polymeric membranes fail. The objective is to show the possibility of using such 2D-crystals as a disruptive technology addressing some of the challenges faced by hydrogen energy conversion technologies. In fact, two-dimensional crystals with angstrom-scale pores are widely considered as candidates for a next generation of molecular separation technologies aiming to provide extreme, exponentially large selectivity combined with high flow rates.

The contribution builds on the findings from Sir Andre Geim’s group demonstrating that graphene, although completely impermeable to all gases at ambient conditions, is highly permeable to protons, nuclei of hydrogen atoms [3]; in addition, atomically thin micas, ~10 times thicker than graphene, have shown to be ~100 times more proton conductive than graphene [4]. Furthermore, nanoscale ripples (corrugations) in freestanding graphene membranes have been found to be catalytically active toward the H₂ dissociation [5]. This could turn completely inert materials into catalytically active, and materials that already are catalytically active in the bulk could become even more active in the 2D limit -thus reducing/removing the need to use noble metals-.

There are hundreds of two-dimensional materials known -from electron transport studies- that have not been explored from a permeability perspective. Therefore, in this contribution, several 2D-materials have been assessed, including one-atom thick structures from the graphene family, as well as hexagonal boron nitride (hBN), silicene, germanene, phosphorene, stanine, among others [6]. In addition, transition metal dichalcogenides (e.g., MoS2, MoSe2, WSe2, etc.), mxenes, and 2D covalent organic frameworks (2D-COFs) have been included in the assessment with their wider interatomic lattices and rich chemistry.

The works focuses on investigating the proton permeability of novel 2D crystals and evaluating the possible increase in the catalytic activity of 2D materials through nano-scale ripples and local curvature effects in the mechanism of H₂ dissociation reaction in 2D-materials.

Therefore, Density Functional Theory (DFT) calculations have been used as a high-throughput computational method [7] for the screening of such novel 2D-materials. This modelling tool was applied to guide the design and selection, based on two descriptors [8],[9] related to the permeability of the 2D-membranes: the electron clouds of the 2D crystals (which may impede the passage of the protons) and the energy profiles as a function of the proton distance to the basal plane of the 2D-materials (related to the proton transfer barrier), using graphene as benchmark [3]. Rigorous electronic structure calculations were performed for this endeavor, including dispersion correction of van der Waals interactions and spin-polarization by means of the VASP software. Moreover, energies for the dissociation of molecular hydrogen were performed on flat and rippled selected one-atom thick structures (including graphene). Initial “corrugated” (rippled) structures were created with a certain protrusion size by allowing the atomic structure to relax under biaxial compression. The energy barriers for the reaction pathway were performed based on climbing-image nudged elastic band (CI-NEB) calculations, explicitly including several intermediate states during the reaction process [4]. The effect of temperature and pressure into the hydrogen dissociation mechanism in these 2D-crystals was also considered.

It was found that the permeability highly depends on the density of the crystal electron clouds that fill the space between the atomic nuclei in the lattice. These clouds have low-density areas that essentially behave as ‘pores’ through which protons permeate. Furthermore, with respect to the nanoscale ripples, preliminary results suggest that H₂ molecules dissociate into H atoms on the ripples, leading to a finite coverage of the surface with hydrogen atoms, and with the resulting H atoms permeating via a ‘flip’ mechanism, similar to the previously reported for graphene [5].

The work presented here belongs to a long-term project on combining molecular modelling with experimental techniques for developing efficient proton-exchange membranes, aiming at using the fundamental understanding of the phenomena happening at the molecular level to optimize their performance at process conditions.

The funding by Khalifa University of Science and Technology under the Research and Innovation Center for Graphene and 2D-materials is greatly acknowledged.

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