Avoiding coolant slump into reactant fields during PEM fuel cell shutdown

1. A method of shutting down the operation of a fuel cell power plant ( 100 ) having a stack of fuel cells ( 102 ), each fuel cell having an anode ( 104 ) with a fuel reactant gas flow field plate ( 118 ), a cathode ( 106 ) having an oxidant reactant gas flow field plate ( 120 ), an electrolyte ( 108 ) disposed between said anode and said cathode, said power plant also having coolant channels ( 131 ) either in or adjacent to one or more of said flow field plates, a primary air blower ( 144 ) receiving air from ambient atmosphere ( 142 ), either directly or through an air inlet valve ( 139 a ), for flowing air through said oxidant reactant gas flow field plates and then either directly or through an air exit valve ( 141 a ) to exhaust, there being either an air inlet valve or an air exit valve or both an air inlet valve and an air exit valve, a cathode recycle loop of either (a) a first form extending from exits ( 126 ) of said oxidant reactant gas flow field plates to an inlet ( 139 ) of said primary air blower or (b) a second form, including a cathode recycle blower ( 135 ), extending between exits of said oxidant reactant gas flow field plates and inlets ( 124 ) thereof, a source ( 140 ) of hydrogen-containing fuel connected exclusively to inlets ( 130 ) of fuel reactant gas flow fields in said fuel reactant gas flow field plates, the outflow of fuel from exits ( 132 ) of said fuel reactant gas flow fields being connected both through an outlet valve ( 162 ) to exhaust and through a fuel recycle pump ( 147 ) for returning a portion of the fuel to the inlets of the fuel reactant gas flow field plates, said method comprising:

disconnecting the primary load ( 148 ) from the stack;

characterized by:

continuing to provide fuel from the source through the fuel inlet valve exclusively to said fuel reactant gas flow fields while operating (c), in said first form, the primary air blower or (d), in said second form, the cathode recycle blower, to circulate gas in the oxidant reactant gas flow field plates from exits thereof to inlets thereof, while maintaining (e) the air inlet valve open, if any, and the air exit valve closed, or (f) the air inlet valve closed and the air exit valve open, if any, either (g) until the concentration of oxygen within the fuel cell stack reaches about 4% or less, or (h) until the average voltage across each fuel cell in said stack is about 0.2 volts or less, or (i) for a predetermined period of time.

2. A method according to claim 1 wherein said disconnecting step is followed by connecting the stack to an auxiliary load ( 150 ) (i) with or (ii) without a diode ( 149 ) in series with the auxiliary load.

3. A method according to claim 1 wherein said operating step continues until the concentration of oxygen within the fuel cell stack reaches about 4% or less.

4. A method according to claim 1 wherein said operating step continues until the average voltage across each fuel cell in said stack is about 0.2 volts or less.

5. A method according to claim 1 wherein said operating step continues for a predetermined period of time.

6. A process according to claim 1 further characterized by:

after a period of storage time following any of said time periods (g), (h) or (i), opening said air inlet valve ( 139 a ), if any, and said air exit valve ( 141 a ), if any, and providing hydrogen-containing gas through inlets ( 130 ) of said fuel reactant gas flow field plates ( 118 ) in a process for starting up said fuel cell power plant ( 100 ).

7. A process according to claim 1 further characterized by:

after completion of storing said fuel cell power plant ( 100 ) for a period of time during which the ambient temperature is sufficiently low to freeze water,

opening said air inlet valve ( 139 a ), if any, and said air exit valve ( 141 a ), if any, and providing hydrogen-containing gas through inlets ( 130 ) of said fuel reactant gas flow field plates ( 118 ) in a process for starting up said fuel cell power plant.