Abstract: A heat and humidity exchanger stack formed by stacking at least one reactant gas source half-cell and at least one reactant gas exhaust half-cell, with each half-cell comprising a porous flowfield, wherein each pair of adjacent half-cells of the stacked heat and humidity exchanger are separated by a member selected from a water permeable/heat conducting member, a water impermeable/heat conducting member, or a water impermeable/heat insulating member and wherein at least one of the members is a water permeable/heat conducting member separating a reactant gas supply half-cell from an adjacent reactant gas exhaust half-cell. The stack may further comprise at least one thermal management fluid cell and/or a heat sink cell separated from the adjacent half-cells in the stack with one of the heat conducting or heat insulating members.