An electrode unit comprising: (a) an electrically non-conductive circumferential housing; (b) a current collector; (c) an electrode; (d) optionally a charge barrier; and (e) an electrically conductive connector in electrical contact with the current collector (b); wherein (b), (c) and (d) (when present) are located within the circumference of the circumferential housing (a); the main plane of the part of (e) which is located within the housing (a) is substantially parallel to the main plane of (b), (e) extends beyond the housing (a); and the area of the part of (e) which is located within the housing (a) is less than 30% of the area of (b). Also claimed are stacks, composites devices and their uses.