A selective membrane is a gate keeper, only letting certain chemical molecules through whilst blocking others. The selectivity is achieved by introducing chemical groups to the membrane and by tuning its internal structure. By choosing which groups are attached to the membrane and by varying its structure the membranes can be adapted to fit specific applications. Selectivity is a vital property of membranes used in e.g. fuel cells and redox flow batteries where the fuel needs to be kept apart while ions are required to move, thus there is a need for an ion-selective membrane.