Polymer electrolyte fuel cell stack and operating method thereof

A solid polymer electrolyte fuel cell stack comprising:

• a plurality of separators;
• a plurality of unit cells separated from each other by one of said plurality of separators, said unit cells each having a unified body including an anode, a cathode, and a solid polymer electrolyte membrane disposed between said anode and said cathode, wherein a supply passage and a discharge passage for supplying and discharging, respectively, at least one of a fuel gas and an oxidizing gas as reaction gases are formed in each of said unit cells and said plurality of separators;
• a first sub-stack having a selected number of said unit cells, said first sub-stack being disposed on an upstream side of a flow of said reaction gas;
• a second sub-stack having a selected number of said unit cells, said second sub-stack being disposed on a downstream side of the flow of said reaction gas;
• an intermediate plate provided between the first sub-stack and the second sub-stack, wherein the intermediate plate comprises:
• a plurality of passages provided along a first edge portion, at least two of the plurality of passages being connected by a connection path, and
• a plurality of passages provided along a second opposed edge portion, at least two of the plurality of passages being connected by a connection path;
• wherein said discharge passage in said first sub-stack communicates in series with said supply passage in said second sub-stack to supply an entire amount of the reaction gas discharged from the first sub-stack to the second sub-stack, and an additional reaction gas supply passage for supplying a reaction gas having a humidity lower than said reaction gas supplied to said first sub-stack is provided in said second sub-stack independently from said supply passage, wherein said reaction gas supplied to said first sub-stack is pre-humidified.