Provided is an electrochemical unit for a fuel cell stack, in which multiple fuel cell units each with an electrochemical unit follow one another along a stacking direction. The electrochemical unit comprises a membrane-electrode unit, which has an anode-side electrochemically active area, through which a fuel gas can be fed to an anode of the membrane-electrode unit and which has an outer border that is defined by an anode-side bordering element, and a cathode-side electrochemically active area, through which an oxidizing agent can be fed to a cathode of the membrane-electrode unit and which has an outer border that is defined by a cathode-side bordering element, in which electrochemical unit increased ageing of the membrane-electrode unit is avoided by supplying regions of the membrane-electrode unit only with oxidizing agent, it is proposed that, at least in certain portions, the outer border of the anode-side electrochemically active area is offset outwards with respect to the outer border of the cathode-side electrochemically active area in a direction running perpendicularly to the stacking direction.