NOVELTY - A bipolar, filter press-like electrochemical cell stack (80) comprises several electrochemical cells (82a, 82b, 82c), each cell including a gas permeable, liquid water impermeable water-retention barrier (102) disposed in contact with a cathode gas diffusion layer (98) and adjacent a cathode fluid flow field (92) of a bipolar assembly or separator (90), to reduce loss of liquid water from a cathode into the cathode fluid flow field. The water retention barrier is preferably disposed between the cathode gas diffusion layer and the cathode fluid flow field.    USE - As a bipolar, filter press-like electrochemical cell stack e.g. fuel cell stack, hydrogen concentrator stack (claimed).    ADVANTAGE - The gas permeable, liquid water impermeable water-retention barrier present in each electrochemical cell, reduces the loss of liquid water from the cathode into the cathode fluid flow field, and facilitates air cooling of the electrochemical cell. The barrier prevents excessive evaporative water loss caused by excessive gas flow rates through the flow field. The positioning of the water retention barrier prevents pockets between the barrier and gas diffusion layer from collecting water; and does not reduce the volume of flow field channels so that the resistance to gas flow through the flow field is not affected. Thus, each electrochemical cell avoids drying out of the ion exchange membrane polymer electrolyte, avoids flooding at the cathode, facilitates recovery of liquid water at the anode, and reduces water losses from electrodes.    DETAILED DESCRIPTION - A bipolar, filter press-like electrochemical cell stack (80) comprises several electrochemical cells (82a, 82b, 82c); and at least one bipolar assembly or separator (90) disposed in electronic communication between adjacent electrochemical cells in the stack. The bipolar assembly includes an anode fluid flow field, a gas separator plate and a cathode fluid flow field (92). Each electrochemical cell includes an anode electrocatalyst layer (86) and a cathode electrocatalyst layer, disposed on opposing sides of an ion-conducting membrane; a cathode gas diffusion layer (98) disposed on the cathode electrocatalyst layer; an anode gas diffusion layer disposed on the anode electrocatalyst layer; and a gas permeable, liquid water impermeable water-retention barrier (102). The gas permeable, liquid water impermeable water retention barrier is disposed in contact with the cathode gas diffusion layer and adjacent the cathode fluid flow field, to reduce the loss of liquid water from the cathode into the cathode fluid flow field. The water retention barrier is selected from a thin gas-permeable liquid water impermeable membrane, a thin porous sheet of material and/or a thin sheet of solid material having several small through-holes. The water retention barrier is preferably hydrophobic or electronically-conducting/insulating. INDEPENDENT CLAIMS are included for the following:    (1) a method of operating the electrochemical cell; and    (2) a subassembly for the bipolar, filter press-like fuel cell stack, comprising an air filtration media, and the cathode electrocatalyst layer of each electrochemical cell, in collective fluid communication with ambient air through the air filtration media.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross-sectional side view of electrochemical cells.    electrochemical cell stack (80)    electrochemical cells (82a, 82b, 82c)    proton exchange membrane (84)    anode electrocatalyst layer (86)    separator (90)    cathode fluid flow field (92)    current collectors (94)    anode gas diffusion layer (96)    cathode gas diffusion layer (98)    water-retention barrier. (102)