Fuel cell unit in the form of a fuel cell stack for producing electrical energy in an electrochemical manner, comprising fuel cells, the fuel cells each comprising a proton exchange membrane, an anode, a cathode, a gas diffusion layer, a bipolar plate, at least one fluid channel for the passage of a fluid, at least one seal (11) composed of a sealing material (42) for sealing off the at least one fluid channel (37), wherein particles (41) composed of a particle material (43) are arranged in the sealing material (42) of the at least one seal (11), for the purpose of extending the diffusion path (38) of the fluid which is sealed off by the at least one seal (11).