NOVELTY - Reduction in the number of parts and interfaces in an electrochemical reactor, especially in a fuel cell stack, is achieved by the use of a unified structure that combines the functions of the gas distribution and gas diffusion structures, gas distribution and gas barrier structures, or all three structures into a single unitary metallic part.    USE - Lightweight bipolar plates for use as separators between adjacent electrochemical cells, such as fuel cells.    ADVANTAGE - Higher electrical conductivity than conventional bipolar plate or stack structures.    DETAILED DESCRIPTION - Apparatus for use in electrochemical devices comprises:    a porous metal flow field, selected from metal foam, expanded metal sheet, sintered metal particles or sintered metal fibers, having a first face; and    a porous metal gas diffusion layer treated with a wet proofing agent and metallurgically bonded, by welding, brazing, soldering, sintering, fusion bonding,and/or vacuum bonding, to the first face of the expanded metal flow field.    The apparatus also includes a metal gas barrier, i.e. a metal sheet or fluid cooled plate, having a first face metallurgically bonded to the second face of the porous metal flow field.    The apparatus also includes a second porous metal flow field having a first face metallurgically bonded to a second face of the second porous metal flow field.    The porous metal gas diffusion layer is selected from sintered metal particles or sintered metal fibers.    An INDEPENDENT CLAIM is given for an apparatus for use in an electrochemical device, comprising:    (a) a porous metal flow field having a first face; and    (b) a gas diffusion layer in contact with the porous metal flow field, and comprising a gas diffusion matrix and a metal collector disposed within the gas diffusion matrix, the gas diffusion matrix comprising conductive carbon fiber, conductive carbon powder and a hydrophobic bonding material, preferably polytetrafluoroethylene.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of the face of a unitized bipolar plate.    Porous metal foam flow fields (102)    Gas distribution layer (104)    Thin metal foil separator or gas barrier (112)