The present invention discloses a fuel cell, oxygen generator or high temperature electrolyser comprising a metallic substrate comprising: (i) at least one porous region comprising a plurality of pores in said metallic substrate, where the porosity varies in at least one direction substantially coincident with the path or paths of a reactant stream to be passed over the substrate when in use and/or in areas of in-use poor gas flow; and (ii) at least one non-porous region bounding the at least one porous region, and having mounted on said metallic substrate an electrode of said fuel cell, oxygen generator or high temperature electrolyser. Also disclosed are fuel cell stack layers and fuel cell stacks comprising same.