Electrochemical cell unit with improved separator plate

An electrochemical cell unit and stack includes a separator plate overlying a cell layer. The separator plate has a selectively shaped three-dimensional region that overlies at least part of an electrochemically active cell region. In that three-dimensional region the separator plate has been deformed into a first plurality of outwardly extending dimpled protrusions that define the height of a first fluid volume on a first side of the separator plate, and a second plurality of outwardly extending dimpled protrusions that define the height of a second fluid volume on a second side of the separator plate. A mid-plane region is disposed between the protrusions and the mid-plane region is so shaped as selectively to vary its height in at least one direction across the active cell region such that the interrelated respective heights of the first and second fluid volumes are correspondingly increased and decreased as a result.