In a fuel cell stack (11a), a larger number of fuel cells (18-21, 33-36) are interposed between successive cooler plates (13-15) without creating excessively high temperatures in those fuel cells (33-36) which are remote from the cooler plates, by virtue of increased air flow in air flow field channels (30a) which are deeper in fuel cells (30-36) remote from the cooler plates, compared with the flow field channels (30, 30b) which are in fuel cells (18-21) adjacent to the cooler plates. The thickness of air flow field plates (29b) may be increased to accommodate the increased depth of the air flow channels (30a). Fuel cells (18a) adjacent the cooler plate may have air flow field channels (30b) which are more shallow than normal whereby increased air utilization therein will be balanced by decreased air utilization in the cells (33-36, 33a) having deeper air flow channels (30a); in this case, the channels (30a) may be normal or deeper than normal.