Chlorine is produced by electrolysis of aqueous HCl, in a membrane electrolyzer, using cathodic mediators such as Fe andor Cu chlorides and a non-catalysed 3-dimensional cathode, with the real surface area at least ten times higher than its projected area. In addition to a first pressure supply unit a second electrically controllable pressure supply unit having at least one intake port and one pressure port, the intake port of which is connected or connectable to one of the pressure chambers and the pressure port of which is connectable or connected to the brake circuit associated with the pressure chamber. The upper substrate includes a ceramic printed circuit board.