Fuel cell shutdown and startup using a cathode recycle loop

A method of transiently operating a fuel cell system made up of at least one fuel cell comprising an anode, a cathode, a membrane disposed between said anode and cathode, an anode flowpath fluidly coupled between said anode and a fuel source, a cathode flowpath fluidly coupled between said cathode and an oxygen source and including a recirculation loop with means for selectively introducing fuel therein, a combustor for selectively reacting said selectively introduced fuel with oxygen in said recirculation loop, and an anode purge flowpath selectively fluidly coupled between said anode flowpath and said cathode flowpath such that voltage potentials on fuel cell components are reduced, said method comprising:

• selecting from one of two transient operating modes, said first mode comprising starting up said system, and said second mode comprising shutting down said system;
• recycling fluid disposed in said cathode flowpath through said recirculation loop;
• creating a purging fluid by reacting fuel introduced through said selective fuel introducing means with oxygen in said combustor;
• discontinuing said reacting through disabling at least one of said combustor and said selective fuel introducing means once a voltage measured across said fuel cell reaches a predetermined level that is commensurate with a substantial consumption of said oxygen in said recirculation loop;
• and
• introducing said purging fluid from said recirculation loop into at least one of said anode or said cathode such that any fluid previously resident therein is substantially purged therefrom.