An ion exchange membrane fuel cell

A polymer electrolyte membrane (PEM) 2 for use in an ion exchange membrane fuel cell, comprising a sensing layer 11 on a side of the PEM having two or more - optionally at least six - sensing elements 12 covered in an electrically insulating film 14 and spaced across a plane of the PEM. Lateral gaps 15 may be provided in the film between the elements, which may be in direct contact with the PEM. The elements may comprise an ionomer and may be deposited onto the PEM by printing. The film may have a surface area of less than half the area of the PEM, may be formed on an ion conducting material and may extend around each sensing element to provide an ion path around it. The film may have a thickness of no more than 10 microns, no more than 5 mm or no more than 5 microns.