Fuel cell stack and method of operating a fuel cell system with such a fuel cell stack

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 to 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,
• exploiting evaporation from 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, and
• feeding at least a part of said humidified fuel from said upper anode space into said anode side flow fields.

A fuel cell system having 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 to 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 fuel cell system further having a collecting container associated with said lower anode space for collecting water which is produced during the reaction of said fuel and said oxidizing agent and which is present in the form of a liquid having an evaporation surface and means for humidifying fresh fuel supplied to said fuel cell stack by mixing it with humidified gases emerging from said anode flow fields and with additional water vapour supplied from said evaporation surface.

A fuel cell system having 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 to 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 anode inlet space in a first side region of said fuel cell stack and an anode outlet space in a second side region of said fuel cell stack, said anode side flow fields extending between said anode inlet space and said anode outlet space, said fuel cell system further comprising:

• a collecting container associated with said anode outlet space and disposed below said fuel cells for collecting water which is produced during the reaction of said fuel and said oxidizing agent and which is present in the form of a liquid having an evaporation surface;
• and
• means for humidifying fresh fuel supplied to said fuel cell stack by mixing it with humidified gases emerging from said anode flow fields and with additional water vapor supplied from said evaporation surface,
• said means for humidifying including a pump for creating a flow from said anode outlet space into said anode inlet space and through said anode flow fields back to said anode outlet space.