NOVELTY - An electrochemical cell (103-1, 103-2) comprises: an anode (19); a cathode (21); a proton exchange membrane (17) having first and second faces, and positioned between anode and cathode, where anode is electrically coupled to first face of membrane and cathode is electrically coupled to second face of membrane; and a compression pad (105) for use in defining cathodic fluid diffusion cavity, which is electrically-conductive, resiliently compressible and porous, and consists of a unitary random mat of carbon fibers bound together using at least one binder.    USE - As an electrochemical cell (claimed) i.e. electrolysis cell useful for airplanes, submarines and spacecrafts, to furnish oxygen and provide a habitable environment; or fuel cell for generating water and electricity using molecular hydrogen and oxygen.    ADVANTAGE - The proton exchange membrane (PEM) electrochemical cell comprises electrically-conductive, resiliently-compressible, porous pad positioned within cathode chamber, which permits passage of fluids through it and replaces multi-layer screen of electrolysis cell or carbon fiber paper or carbon fiber cloth of fuel cell. The pad is resistant to be brittle with hydrogen and consists of unitary random mat of carbon fibers bound together using thermoplastic resin binder. As the pad is compressible and resilient (i.e. capable of substantially returning to its pre-compressed shape when compressing force is removed), several pad-containing cells can be arranged in a stack, without requiring that a separate compression pad be interposed between adjacent cells (e.g. between separator plates of adjacent cells) for maintaining uniform pressure over the active areas of the electrodes; provided that differential pressure greater than 200 psi is from hydrogen side of cell, and hydrogen pressure is greater than oxygen pressure. In addition, when the PEM electrochemical cell is an electrolysis cell, because the pad does not typically experience same problem of fretting corrosion experienced by a conventional multi-layer screen, a metal sheet of type conventionally used to prevent fretting corrosion associated with multi-layer screen can also be omitted from cell, which reduces cost and weight of cell. Further, when the PEM electrochemical cell is an electrolysis cell, the electrically-conductive, resiliently-compressible, cathodic, porous pad and multi-layer anode screen is approximately equal in thickness to their corresponding peripheral cell frames; or alternatively, multi-layer anode screen is thinner than its peripheral cell frame, with cathodic, porous pad thicker (e.g. 5-10%) than its peripheral cell frame, in which two normally flat separators used to axially contain fluid cavities are replaced with two bent or dished separators shaped to conform to outer faces of multi-layer screen and porous pad, respectively (i.e. cathode chamber projects partially into anode chamber). The reduction in thickness of multi-layer screen is equal to increase in thickness of porous pad, and two dished separators are identical in shape. Further, as porous pad obviates the need for a separate compression pad located between separator plates of adjacent cells (for gas pressure differentials up to 200 psi or greater), one of the two separator plates between adjacent cells can be omitted.    DETAILED DESCRIPTION - An electrochemical cell (103-1, 103-2)comprises: an anode (19); a cathode (21); a proton exchange membrane (17) having first and second faces, and positioned between anode and cathode, where anode is electrically coupled to first face of membrane and cathode is electrically coupled to second face of membrane; and a compression pad (105) for use in defining cathodic fluid diffusion cavity, which is electrically-conductive, resiliently compressible and porous, and consists of a unitary random mat of carbon fibers bound together using at least one binder. The compression pad has a density of (0.2-1.5, preferably 0.35-0.77, especially 0.55-0.77, particularly 0.77) g/cm3. The carbon fibers have a length of 100 microns to one inch, and a diameter of 4-12 microns. The carbon fibers consist of a combination of chopped carbon fibers having a length of one-eighth of an inch and a diameter of 4-12 microns; and milled carbon fibers having a length of 200 microns and a diameter of 4-12 microns. The compression pad, after compressed to pressures up to 12000 psi, has uncompressed thickness of greater than or equal to 70% of its original, pre-compressed thickness. The compression pad further comprises electron-conducting carbon particles. An INDEPENDENT CLAIM is included for method for preparing electrically-conductive resiliently compressible, fluid diffusion medium involving: providing several carbon fibers; providing at least one binder; mixing together carbon fibers and binder(s); laying down the mixture; and heating the mixture to a temperature to melt binder under conditions of pressure sufficient to form a sheet of substantially uniform thickness.    DESCRIPTION OF DRAWING(S) - The figure shows a proton exchange membrane (PEM) electrolysis cell stack.    Proton exchange membrane (17)    Anode (19)    Cathode (21)    Multi-layer metal screens (31)    Pair of polysulfone cell frames (35, 37)    Plastic gaskets (47 49)    Proton exchange membrane electrolysis cell stack (101)    Pair of electrolysis cells (103-1, 103-2)    Compression pad/fluid diffusion medium (105)    Flat metal foil separators (111-1, 111-2)