Abstract: Example embodiments provide fuel cell humidifiers. An example humidifier comprises a stack of unit cells. Each of the unit cells may comprise a separator having a perimeter frame and first and second major faces, a first membrane sheet bonded to the perimeter frame on the first major face of the separator and a second membrane sheet bonded to the perimeter frame on the second major face of the separator. The perimeter frame and the first and second membrane sheets may define a cavity in an interior of the perimeter frame. Opposed frame ends of the perimeter frame may be apertured to allow a first flow to flow through the cavity in a first direction. The separator may include first and second ridges that extend across first and second frame ends.