Patent ID: 7192668

Claim:
A method of operating a fuel cell stack comprising a plurality of fuel cells, each said fuel cell having an anode defining an anode side, a cathode defining a cathode side and a membrane arranged between said anode and said cathode, each said fuel cell being arranged between first and second bipolar plates, there being flow fields at said anode sides for a gaseous fuel supplied to said fuel cells and flow fields at said cathode sides for a gaseous oxidizing agent fed into said fuel cells and catalyst material within said fuel cells at which said fuel and said oxidizing agent react for the production of electrical energy with the simultaneous production of water, said fuel cells being connected together and having an upper anode space at a top of said fuel cell stack and a lower anode space at a bottom of said fuel cell stack, said anode side flow fields extending between said upper anode space and said lower anode space, said method comprising the steps of: collecting water produced during the electrochemical reaction and present in the form of a liquid in or adjacent to said lower anode space, evaporating at least a portion of said liquid water for the humidification of supplied fuel in or adjacent to said lower anode space, supplying the fuel humidified in or adjacent to said lower anode space to said upper anode space, feeding at least a part of said humidified fuel from said upper anode space into said anode side flow fields, and returning a part of said humidified fuel supplied to said upper anode space via a restriction to said lower anode space.