The electrochemical cell stack assembly has electrochemical cell sub-stacks. A first and second electrochemical cell sub-stack are connected electrically in series and fluidly in parallel. The first and second electrochemical cell sub-stacks have electrochemical cells. The electrochemical cells have a membrane electrode assembly with an cathode catalyst layer, an anode catalyst layer, and a polymer membrane therebetween. The electrochemical cells have an anode plate and a cathode plate with the membrane electrode assembly interposed therebetween, a cathode flow field, and the anode plate.