Nature 516, 227–230 (2014)

Owing to its atomic thickness, mechanical strength and chemical stability, graphene is of interest in the development of separation membranes. A pristine sheet of graphene is thought to be impermeable to all atoms and molecules, but by creating subnanometre-sized pores in the material, it could act as a filter, allowing molecules smaller than the pores to pass while impeding the progress of larger species. Marcelo Lozada-Hidalgo, Heng-An Wu and colleagues have now shown that protons can pass through pristine monolayers of graphene and, another two-dimensional material, hexagonal boron nitride, a result that suggests that the materials could be of use in the development of improved proton exchange membranes for fuel cells.

The researchers — who are based at the University of Manchester, the University of Science and Technology of China, and the University of Nijmegen — measured the transport of protons through a range of different two-dimensional crystals that were sandwiched between layers of Nafion, a polymer that can conduct protons in the presence of water. Significant proton currents were detected across both graphene and hexagonal boron nitride, but not molybdenum disulphide. Bilayers and trilayers of hexagonal boron nitride were also found to conduct protons, but bilayers of graphene did not. All of these observations could be rationalized by considering the electron density distributions of the different two-dimensional materials. Furthermore, by decorating the graphene and hexagonal boron nitride membranes with platinum nanoparticles — a catalyst often used in fuel cells — the proton conductivity could be considerably enhanced.