Cell unit and cell stack

A stack of rectangular, planar electrochemical cell units, each cell unit comprising:

• a separator plate separating a first fluid volume from a second fluid volume;
• a second fluid mid-port that is disposed midway along its length within or between one or more regions in which active cell chemistry layers are provided and is in fluid communication with the second fluid volume of the cell unit,
• wherein the second fluid mid-ports of the respective cell units align to form a fluid mid-passageway extending in the stack direction;
• and,
• wherein the stack is configured such that, in each second fluid volume, second fluid flow paths extend across the one or more regions in which active cell chemistry layers are provided between the second fluid mid-port and each respective opposed cell end;
• and
• wherein the second fluid volumes are defined by the separator plate having an elongate shaped feature that extends at least partially around the periphery of one of the regions in which active cell chemistry layers are provided to confine a fluid flow path within that region, wherein the elongate shaped feature comprises a feature in the separator plate that forms a protruding rib on one side of the separator plate and a channel on the other side of the separator plate.