Theoretical Understanding of Mechanisms of Proton Exchange Membranes Made of 2D Crystals with Ultrahigh Selectivity.

Recent reports on proton conduction across pristine graphene and hexagonal boron nitride (h-BN) provide a new avenue for the design of proton exchange membranes. The uniform pores formed by the electron clouds of two-dimensional (2D) crystals can effectively block the undesired transportation of other species thus ultrahigh selectivity can be achieved. With the aid of first-principles calculations, we investigate the proton conduction process across six kinds of intact 2D crystals, namely graphene, h-BN, β boron sheet, χ boron sheet, phosphorene, and silicene. To clarify the proton conduction mechanism, three proton penetration modes are proposed: dissociation-penetration, adsorption-penetration, and direct penetration. Based on our calculation results, for graphene and h-BN without atomic defects, they are unlikely to provide sufficient proton conductivity at room temperature when no bias potential is applied. By contrast, the β boron sheet, χ boron sheets, and silicene exhibit relatively lower proton penetration energy barriers, making them prospective candidates for future proton exchange membrane applications.