Computationally guided design of graphene-like 2D-membranes for selective proton transport in hydrogen fuel cells

This work investigates proton conduction through a wide range of 2D graphene-like crystals using density functional theory (DFT) calculations. Our goal focused on understanding the impact of membrane's chemical constitution on proton permeability, with particular attention on the effect of sequential substitution using non-metallic elements such as boron, nitrogen, silicon, sulfur and phosphorus. Results indicate that boron-doped graphene reduces the proton penetration energy barrier, being comparable with values for hexagonal boron nitride (h-BN). In contrast, nitrogen-doped graphene exhibits a significantly high energy barrier (>4 eV) for proton permeation, suggesting that it is unlikely to support proton conduction at room temperature without having defects on the 2D-monolayer. The use of other elements (O, Si, S, P) generates a broad spectrum of energy barriers, with distinct trends correlating to changes in pore size -driven by the elongation or contraction of the 6-membered rings containing the dopants-. In comparison, pure 2D-materials like phosphorene, silicene and germanene also arise as promising candidates for proton exchange membrane (PEM) applications due to relatively low proton penetration barriers. However, the increased pore size in these materials may reduce their impermeability to other gases, potentially affecting the performance of the PEM. Additionally, we further examined the synergistic effects of multi-element 2D materials, with gallium nitride (GaN) emerging as a candidate capable of simultaneously enhancing proton permeability and improving membrane mechanical and thermal robustness. This work highlights the importance of exploring additional hetero-doped 2D-materials, where untapped chemical properties may offer further opportunities for optimizing PEM performance.