NOVELTY - An electrochemical system consisting of a laminate of cells (2) separated by bipolar plates (3), where at least one cell has an active region (enclosed by a boundary wall of the plate) in which medium is distributed by a channel structure (8) using a gas diffusion layer (9), has boundary elements (10) in the channel structure-boundary wall interface region, to prevent fluid by-pass between the channel structure and boundary wall.    USE - The electrochemical system is typically (i) a fuel cell system or (ii) an electrical compression system, specifically an electrolyzer in which a potential is applied to generate hydrogen and oxygen from water while simultaneously compressing the gases under pressure or a system in which a supply of gaseous hydrogen is electrochemically compressed by application of a potential.    ADVANTAGE - An even distribution of fluid over the channel structure is ensured, so that the electrochemical system is not subject to fluctuations in performance due to unfavorable distribution of medium in the electrochemically active region.    DETAILED DESCRIPTION - An electrochemical system consists of a laminate, subjectable to mechanical compressive force, of cells (2) separated by bipolar plates (3), with openings for cooling or for supply and removal of cell operating medium. At least one cell has an electrochemically active region, enclosed by a boundary wall of the plate, in which medium is evenly distributed by a channel structure (8) in the plate. At least one gas diffusion layer (9) is provided for microdistribution of the medium. The novel feature is that boundary elements (10) are provided in the interfacial region between the channel structure and boundary wall, to prevent fluid by-pass between the channel structure and boundary wall. The gas diffusion layer covers the channel structure and/or at least part of the boundary element. An INDEPENDENT CLAIM is included for a bipolar plate for use in the electrochemical system, having a base plane (11) having a projecting channel structure (8) and openings for supply and removal of medium. The channel structure and openings are enclosed by a boundary wall. At least one boundary element (10) is provided in the interfacial region between the boundary wall and the outer edge of the channel structure, to prevent fluid flow in the interfacial region between the boundary wall and channel structure.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-section through a bipolar plate arrangement, for use in an electrochemical system.    Cell (2)    Bipolar plates (3)    Boundary wall (7)    Channel structure (8)    Gas diffusion layer (9)    Boundary elements (10)    Base plane of bipolar plates (11)