The present invention relates to a graphene-based or other 2-D material membrane which allows the passage of protons and deuterons and to a method of facilitating proton or deuteron permeation through such a membrane. Monocrystalline membranes made from mono- and few-layers of graphene, hBN, molybdenum disulfide (MoS2), and tungsten disulfide (WS2) etc. are disclosed. In effect, the protons or deuterons are charge carriers that pass through the graphene or other 2-D material membrane. This process can be contrasted with the passage of gaseous hydrogen. Hydrogen is an uncharged gaseous species which is diatomic. In other words, the gas is in molecular form when considering the normal barrier properties whereas in the case of the present invention, the species which is being transported through the membrane is a charged ion comprising a single atom. Membranes of the invention find use in a number of applications such as fuel cells.