High fuel utilization in a fuel cell

• US 6,558,827 B1
• Filed: 02/26/2001
• Issued: 05/06/2003
• Est. Priority Date: 02/26/2001
• Status: Expired due to Term

• Current Assignee: Audi AG (Volkswagen AG)
• Original Assignee: UTC Fuel Cells, LLC
• Inventors: Reiser, Carl A.
• Primary Examiner(s): Ryan; Patrick
• Assistant Examiner(s): Wills; Monique
• Application Number:
  US09/793,175
• Time in Patent Office: 799 Days
• Field of Search: 429/12,13,17,22
• US Class Current: 429/415
• CPC Class Codes: H01M 8/04097 with recycling of the reactants H01M8/04119, H01M8/04104 take precedence
• CPC Class Codes: H01M 8/04231 Purging of the reactants
• CPC Class Codes: H01M 8/04589 of fuel cell stacks
• CPC Class Codes: H01M 8/04753 of fuel cell reactants
• CPC Class Codes: H01M 8/04761 of fuel cell exhausts
• CPC Class Codes: Y02E 60/50 Fuel cells

Abstract

A PEM fuel cell (12) operating on substantially pure hydrogen (32) and air (26) has an exhaust flow control valve (37) at the exit of the anode fuel reactant flow field, said valve being normally closed during steady state low or medium power operation, so that the concentration of nitrogen in the fuel reactant flow fields, by diffusion across the membrane from the cathode, will approach the average concentration of nitrogen in the oxidant, thereby limiting the concentration of hydrogen to a corresponding low complementary amount, which reduces the diffusion of hydrogen across the membrane for consumption at the cathode, thereby increasing the efficiency of operation of the fuel cell. A current sensor (40) allows a controller (46) to open an exhaust flow control valve (37), thereby drawing much higher amounts of hydrogen into the fuel reactant flow field of the anode to support generation of power at high current densities without hydrogen starvation.

Claims

1. A method of operating a fuel cell having an anode with an unvented fuel reactant flow field, a cathode having an oxidant flow field, a proton exchange membrane between said anode and said cathode, a source of hydrogen for providing fuel to said fuel reactant flow field, a fuel recycle loop including said fuel reactant flow field and a pump for transferring effluent of said fuel reactant flow field to the inlet of said fuel reactant flow field, and means for providing air to said oxidant flow field, which method comprises:

operating said means to provide air to said oxidant flow field;

delivering hydrogen from said source to said fuel reactant flow field with said recycle loop unvented; and

operating said pump;

the concentration of nitrogen in said fuel reactant flow field, by diffusion across said membrane, reaching the average concentration of nitrogen in the oxidant flow field, thereby forcing the percent mole-fraction of hydrogen concentration in the anode flow field to be a corresponding, low complementary amount.

2. A method according to claim 1 further comprising:

providing said fuel reactant flow field with an exhaust valve;

measuring the current load of said fuel cell; and

opening said exhaust flow control valve when the current load exceeds a predetermined threshold.

3. A method according to claim 2 wherein:

said predetermined threshold is between 50% and 80% of the maximum current rating of the fuel cell.

4. A method according to claim 2 wherein:

said predetermined threshold is about 65% of the maximum current rating of the fuel cell.

5. A method according to claim 1 further comprising:

operating said fuel cell with hydrogen utilization in excess of a threshold magnitude of about 96%.

6. A method according to claim 1 further comprising:

operating said fuel cell with oxygen utilization in excess of a threshold magnitude of about 70%.

7. A method according to claim 1 wherein:

the percent mole-fraction of hydrogen concentration in the anode flow field is on the order of 15%-20%.

8. A fuel cell power plant comprising:

a fuel cell including an anode having a fuel reactant flow field, a fuel recycle loop including said fuel reactant flow field and a pump for transferring effluent of said fuel reactant flow field to the inlet of said fuel reactant flow field, a cathode having an oxidant flow field, and a proton exchange membrane disposed between said anode and said cathode;

means providing air to said oxidant flow field; and

means for providing a flow of hydrogen to said fuel reactant flow field;

wherein the improvement comprises;

said fuel reactant flow field being unvented whereby the concentration of nitrogen in said fuel reactant flow field, as a consequence of diffusion from the oxidant reactant flow field, stabilizes at about the average concentration of nitrogen in the oxidant flow field, thereby limiting the concentration of hydrogen to a corresponding, low complementary amount.

9. A power plant according to claim 8 wherein:

said fuel reactant flow field has an exhaust flow control valve;

and further comprising;

an electrical load for said fuel cell;

means for sensing the load current provided to said electrical load and providing a current signal indicative thereof; and

a controller responsive to said current signal for opening said exhaust flow control valve in response to indications of current in excess of a predetermined fraction of the maximum current load rating of said fuel cell.

10. A power plant according to claim 9 wherein said predetermined fraction is between 50% and 80%.

11. A power plant according to claim 10 wherein said predetermined fraction is about 65%.