Gas sensor

• US 5,527,446 A
• Filed: 04/13/1995
• Issued: 06/18/1996
• Est. Priority Date: 04/13/1995
• Status: Expired due to Fees

• Current Assignee: Giner Electrochemical Systems, LLC (Ener1, Inc.)
• Original Assignee: United States Of America As Represented By The Secretary Of The Air Force
• Inventors: Cropley, Cecelia C., LaConti, Anthony B., Kosek, John A.
• Primary Examiner(s): Bell, Bruce F.
• Application Number:
  US08/421,710
• Time in Patent Office: 432 Days
• Field of Search: 204/431, 204/432, 204/412, 204/409, 204/415, 204/418, 204/421, 204/424, 204/153.14, 204/153.16, 204/153.17, 204/153.18, 422/83, 422/90, 422/98
• US Class Current: 205/792.5
• CPC Class Codes: G01N 27/4045 for gases other than oxygen
• CPC Class Codes: G01N 27/4074 for detection of gases other than oxygen

Abstract

A gas sensor including an electrochemical sensor cell which has a anion-eange solid polymer electrolyte membrane with three attached electrodes is provided. The sensor cell contains no liquid electrolyte and is operated in the potentiostatic as well as the potentiodynamic modes to detect alkaline reactive gases, including vapors, such as hydrazines and derivatives thereof and ammonia. These sensor cells together with electronic circuitry, a pump and a power supply, fit into a compact, pocket-sized container to define the gas sensor of the invention which can detect traces of the above gases including 10 ppb of hydrazine and its derivatives and 10 ppm of ammonia. The invention includes the above gas sensor and the methods of operating same.

Claims

1. A gas sensor comprising,

an electrochemical cell having

a) an anion-exchange membrane,

b) means to maintain said membrane hydrated,

c) a reference electrode,

d) a counter electrode;

e) a sensing electrode and

f) a gas inlet,

said electrodes being spaced from each other and in contact with said membrane, said sensing electrode being mounted proximate said inlet to contact the gas to be detected.

2. The gas sensor of claim 1 wherein said electrodes are attached to said membrane and said membrane is a solid polymer electrolyte having negative ions therein.

3. The gas sensor of claim 2 wherein said membrane includes a hydrocarbon backbone grafted to a perfluorocarbon film.

4. The gas sensor of claim 2 having a gas outlet spaced from said inlet to admit alkaline reactive gas through said inlet to contact said sensing electrode and then to exit via said outlet in a normal gas diffusion process.

5. The gas sensor of claim 4 having a vacuum type pump attached to said outlet for drawing said gas more rapidly through said inlet to contact said sensing electrode and out said outlet for greater sensitivity in detection of said gas.

6. The gas sensor of claim 5 having a housing of inert material to contain said cell, power supply, electronic components and pump.

7. The gas sensor of claim 2 wherein said cell has a sensing electrode and membrane to detect hydrazine (Hz), Hz derivatives and ammonia (NH₃).

8. The gas sensor of claim 7 having a Pt sensing electrode for detection of NH₃.

9. The gas sensor of claim 2 wherein said reference electrode is of platinum black/air, said counter electrode is of platinum black and said sensing electrode has a catalyst selected from the group consisting of Pt black, gold, Ni, Pd, Au--Pt and carbon.

10. The gas sensor of claim 9 having a gold sensing electrode for detection of hydrazine (Hz) species.

11. The gas sensor of claim 2 having an anion-exchange membrane which includes a hydrocarbon backbone, radiation grafted to a perfluorocarbon film.

12. The gas sensor of claim 1, having a power supply and electronic components electrically connected to said sensing electrode and said counter electrode to measure an electric current therefrom when said sensing electrode is contacted by an alkaline reactive gas and means to measure said current to derive the concentration of said gas.

13. The gas sensor of claim 1 wherein said membrane is maintained hydrated.

14. A method for detecting gas with a gas sensor which includes an electrochemical cell having

a) an anion-exchange membrane,

b) means to maintain the membrane hydrated,

c) a reference electrode,

d) a counter electrode,

e) a sensing electrode and

f) a gas inlet proximate said sensing electrode, wherein said electrodes are spaced from each other and in contact with said membrane, which method comprises, contacting said sensing electrode with the gas to be detected, to cause a reaction in which OH^- ions migrate from said counter electrode through said membrane to said sensing electrode and cause a current to flow via an external conductor from sensing to counter electrodes and measuring said current to derive the amount of gas detected.

15. The method of claim 14 operated in the potentiostatic mode wherein said sensing electrode is held at a fixed potential with respect to said reference electrode at which said reaction of the gas to be detected occurs, measuring the current flowing between said sensing and counter electrodes from said reaction to determine the concentration of said gas.

16. The method of claim 14 wherein said sensing electrode is held at a fixed potential of between +3 mV to +200 mV with respect to the reference electrode, for detecting Hydrazine (Hz) species and NH₃.

17. The method of claim 16 wherein said sensing electrode is held at a fixed potential between +3 mV to +10 mV v. the reference electrode.

18. The method of claim 14 operated in the potentiodynamic mode comprising:

a) holding the sensing electrode potential constant to form a well-defined potential-determined oxide layer,

b) reacting said oxide layer with the gas to be detected for a predetermined time, with the sensing electrode at open circuit,

c) electrochemically regenerating an oxide layer on said sensing electrode by connecting said electrode back into the circuit to effect a potential sweep and

d) measuring the current flowing between said sensing and counter electrodes necessary to regenerate said oxide layer.

19. The method of claim 18 wherein said sensing electrode is held at a fixed potential in a range between +3 mV to +200 mV as against a Pt/air reference electrode, employing an open circuit time of 1-2 minutes and a closed circuit regeneration time of 25-35 seconds and then measuring the current necessary to regenerate said oxide layer to determine the concentration of the gas being detected.

20. The method of claim 19 using a Pt sensing electrode.

21. The method of claim 19 employing an Au sensing electrode.

22. The method of claim 19 employing an Au sensing electrode for selectivity in detecting a ppb concentration of UDMH in the presence of a ppm concentration of NH₃.

23. The method of claim 14 employed to detect hydrazine (Hz) species gases.

24. The method of claim 23 wherein OH^- ions migrate to said sensing electrode when Hz type gas is present according to:

space="preserve" listing-type="equation">N.sub.2 H.sub.4 +4OH.sup.- →

N.sub.2 +4H.sub.2 O+6e

which causes said current to flow in said conductor for measurement of the concentration of said gas.

25. The method of claim 14 employed to detect concentrations of (Hz), unsymetricaldimethylhydrazine (UDMH) and monomethylhydrazine (MMH).

26. The method of claim 14 being used to detect the concentration of NH₃ in ppm.

27. The method of claim 26 wherein said OH^- ions migrate to said sensing electrode when ammonia is present according to:

space="preserve" listing-type="equation">2NH.sub.3 +6OH.sup.- →

N.sub.2 +6H.sub.2 O+6e

which causes said current to flow in said conductor for measurement of the concentration of said ammonia.

28. The method of claim 14 using a Pt sensing electrode.

29. The method of claim 14 using an Au sensing electrode.