Fuel cell assembly with an improved gas sensor

• US 20020110713A1
• Filed: 12/15/2000
• Published: 08/15/2002
• Est. Priority Date: 12/23/1999
• Status: Active Grant

• Current Assignee: Daimler AG (Mercedes-Benz Group AG)
• Original Assignee: Ballard Power Systems Corporation (Superior Plus Corporation)
• Inventors: Roesch, Thomas, Mueller, Wilhelm, Reindl, Michael, Birkhofer, Thomas Alfred, Moos, Ralf Erich, Mueller, Ralf Wolfgang
• Granted Patent: US 6,852,434 B2
• Time in Patent Office: Days
• US Class Current: 429/22
• CPC Class Codes: G01N 27/407 for investigating or analysing gases G01N27/411 takes precedence
• CPC Class Codes: H01M 8/02 Details electrodes H01M4/86 - H01M4/98
• CPC Class Codes: H01M 8/04089 of gaseous reactants
• CPC Class Codes: Y02E 60/50 Fuel cells
• CPC Class Codes: Y10T 29/5313 Means to assemble electrical device

Abstract

A fuel cell assembly has an improved gas sensor. The improved sensor measures the gas concentrations in the interior fluid passages within a fuel cell assembly or within fluid passages employed to transport reactant fluid streams to or from the fuel cell(s). The improved sensor is particularly suited for use in the environment within the reactant fluid passages of a solid polymer fuel cell assembly and is tolerant to the presence of water. The sensor employs an active electrode; a passive electrode; and an electrolyte in contact with both electrodes. The electrolyte is disposed on a substrate and a heater is located in thermal contact with the substrate for heating the substrate and the electrolyte. In a preferred embodiment, the active electrode comprises platinum, the passive electrode is gold and the electrolyte comprises a layer of zirconium dioxide less than one hundred microns thick.

Claims

1. A fuel cell assembly comprising:

(a) at least one fuel cell comprising;

an anode;

a cathode;

an electrolyte interposed between said anode and said cathode;

a fuel passage in fluid communication with said anode for directing a fuel stream to and from said anode;

an oxidant passage in fluid communication with said cathode for directing an oxidant stream to and from said cathode; and

(b) an electrochemical sensor associated with one of said fuel and oxidant passages for measuring the concentration of a gas in a respective one of said fuel and oxidant streams, said sensor comprising;

an active electrode;

a passive electrode;

an electrolyte in contact with said active electrode and said passive electrode;

a substrate upon which said electrolyte is disposed; and

a heater in thermal contact with said substrate for heating said substrate and thereby heating said electrolyte.

2. The fuel cell assembly of claim 1 wherein said electrolyte is disposed on said substrate as a film with a thickness of less than 100 microns.

3. The fuel cell assembly of claim 2 wherein said electrolyte is disposed on said substrate as a film with a thickness of about 5 to 25 microns.

4. The fuel cell assembly of claim 1 wherein said passive electrode further comprises a coating that fluidly isolates said passive electrode from the surrounding atmosphere.

5. The fuel cell assembly of claim 4 wherein said coating comprises a material selected from the group consisting of a glass, a ceramic and a glass ceramic.

6. The fuel cell assembly of claim 1 wherein said electrolyte comprises a solid oxide electrolyte.

7. The fuel cell assembly of claim 1 wherein said electrolyte comprises a material selected from the group consisting of ZrO₂, CeO₂, HfO₂, yttrium-doped ZrO₂ and calcium-doped ZrO₂.

8. The fuel cell assembly of claim 7 wherein said substrate is unitary with said electrolyte, and both of said substrate and said electrolyte are made from the same material.

9. The fuel cell assembly of claim 1 wherein the average distance between said active and passive electrodes is between 0.1 millimeter and 10 millimeters.

10. The fuel cell assembly of claim 1 wherein said passive electrode and said active electrode each have a thickness between 0.0001 millimeter and 1 millimeter.

11. The fuel cell assembly of claim 10 wherein said active electrode comprises platinum and said passive electrode comprises gold.

12. The fuel cell assembly of claim 1 wherein said heater heats said electrolyte to a temperature between 300°

C. and 650°

C.

13. The fuel cell assembly of claim 12 further comprising a device for measuring heater voltage and current for regulating said heater by calculating heater resistance by dividing said heater voltage by said heater current.

14. The fuel cell assembly of claim 12 further comprising a separate electrical circuit for measuring the temperature of said electrochemical sensor, and a temperature controller for changing the current or supply voltage of said heater.

15. The fuel cell assembly of claim 1 wherein said fuel cell is a solid polymer fuel cell.

16. The fuel cell assembly of claim 15 wherein said gas is hydrogen and said sensor measures the concentration of hydrogen in said oxidant stream.

17. The fuel cell assembly of claim 1 wherein said sensor is located in said oxidant passage downstream of said cathode.

18. The fuel cell assembly of claim 1 further comprising an electrical circuit for measuring a voltage difference between said active electrode and said passive electrode.

19. The fuel cell assembly of claim 1 wherein said heater comprises a heating element.

20. The fuel cell assembly of claim 19 wherein said heating element comprises a resistor coil electrical circuit.

21. The fuel cell assembly of claim 1 wherein said heater comprises a coating that fluidly isolates said heater from the surrounding atmosphere.

22. The fuel cell assembly of claim 21 wherein said coating comprises glass, ceramic or glass ceramic.

23. The fuel cell assembly of claim 1 wherein said heater provides heat for raising the temperature of said electrolyte so that said electrolyte has an ion-conductance value greater than 10⁻

4(Ω

cm)<sup>−

1</sup>.

24. The fuel cell assembly of claim 1 wherein said sensor is located within an interior oxidant or fuel stream passage within said fuel cell assembly.

25. The fuel cell assembly of claim 24 wherein said interior oxidant or fuel stream passage is located within an end plate of said fuel cell assembly.

26. The fuel cell assembly of claim 1 wherein said substrate comprises a thermally conductive electrical insulator.

27. The fuel cell assembly of claim 1 wherein said fuel cell assembly further comprises at least two electrochemical gas sensors, with a first electrochemical gas sensor for detecting a hydrogen gas concentration in said oxidant stream and a second electrochemical gas sensor for detecting an oxygen gas concentration in said fuel stream.

28. The fuel cell assembly of claim 1 further comprising a resistor associated with said heater for measuring temperature.

29. The fuel cell assembly of claim 1 wherein said electrochemical sensor detects a target gas with a sensitivity range from 1 ppm to 20,000 ppm and said sensor emits a signal representative of the target gas concentration within said sensitivity range.

30. The fuel cell assembly of claim 1 wherein said heater is provided with heating energy from said at least one fuel cell.

31. A method of fabricating an electrochemical gas sensor for a fuel cell assembly comprising:

(a) placing an electrolyte on a substrate equipped with a heating element, and heating said substrate and said electrolyte to a temperature above about 1100°

C.;

(b) placing an active electrode on said electrolyte;

(c) heating said substrate, said electrolyte, and said active electrode to a temperature above about 900°

C.;

(d) placing a passive electrode on said electrolyte; and

(e) heating said substrate, said electrolyte, said active electrode, and said passive electrode to a temperature above 800°

C.

32. The method of claim 31 wherein said electrolyte has a thickness less than 100 microns.

33. The method of claim 31 wherein said electrolyte comprises a material selected from the group consisting of ZrO₂, CeO₂, HfO₂, yttrium-doped ZrO₂ and calcium-doped ZrO₂.

34. The method of claim 31 wherein said active electrode comprises platinum.

35. The method of claim 31 wherein said passive electrode comprises gold.

36. The method of claim 31 further comprising depositing a fluid impermeable coating over said heating element.

37. The method of claim 36 wherein said coating comprises a material selected from the group consisting of a glass, a ceramic and a glass ceramic.