Graphene is a promising candidate for an ideal membrane material. Its ultralow (one-atomic) thickness potentially provides high permeation and at the same time high selectivity. Here, it is shown that these properties can be used to create a confined, two-dimensional electrochemical environment between a graphene layer and a single-crystal Pt(111) surface. The well-defined fingerprint voltammetric characteristics of Pt(111) provide an immediate information about the penetration and intercalation of ions into the confined space. These processes are shown to be highly selective.
