GB2342168A

GB2342168A - Electrochemical oxygen sensor

Info

Publication number: GB2342168A
Application number: GB9916889A
Authority: GB
Inventors: Herbert Kiesele; Frank Mett; Stephan Haupt; Peter Tschuncky; Joachim Gehrt
Original assignee: Draeger Sicherheitstechnik GmbH
Current assignee: Draeger Safety AG and Co KGaA
Priority date: 1998-10-01
Filing date: 1999-07-19
Publication date: 2000-04-05
Anticipated expiration: 2019-07-19
Also published as: DE19845318A1; GB2342168B; GB9916889D0; FR2784190B1; FR2784190A1; DE19845318C2

Abstract

The sensor has a measurement electrode 6, 66, an air-oxygen reference electrode 9, an auxiliary electrode 8 and a protective electrode 7 located in an electrolyte 4. The measurement electrode is made of a gas-permeable, polymeric, perfluorinated diffusion membrane 66 coated at least partially on the side remote from the ambient atmosphere with a mixture of a noble metal and polytetrafluoroethylene 6. The membrane 66 may be of PTFE (polytetrafluoroethylene), PFA (poly perfluoroalkoxy), FEP (tetrafluoroethylene hexafluoropropylene copolymer) or E-CTFE (ethylene chlorotrifluoroethylene copolymer). The noble metal may be gold, platinum, ruthenium or alloys or mixtures thereof. An additional porous support membrane 10, preferably of PTFE , may be provided, and a filter for removing disturbing gases may be arranged in the gas flow to the measuring electrode. Separators 12 of non-woven material separate the electrodes, and the reference electrode 9 is located in a porous glass body 13 and receives oxygen from the air via porous PTFE disc 11.

Description

2342168 -1 Electrochemical oxygen sensor The invention relates to an

electrochemical oxygen sensor. Patent Application DE 197 26 453.0-52 discloses a known an electrochemical oxygen sensor having a measurement electrode, an air-oxygen electrode as a reference electrode, a protective electrode and an auxiliary electrode, which drastically reduces the vibration-sensitivity of the sensor.

Embodiments of the invention aim to improve an oxygen sensor of this type so as to achieve a faster and more complete material conversion, with good long term stability of the sensor at the same time.

According to the invention, there is provided an electrochemical oxygen sensor having a measurement electrode, an air-oxygen electrode as a reference electrode, a protective electrode and an auxiliary electrode in an electrolyte, characterised in that the measurement electrode is made of a gas-permeable, polymeric, perfluoridated diffusion membrane which is coated at least partially on one side with a mixture of a noble metal and polytetrafluoroethylene, the diffusion membrane being exposed to the ambient atmosphere.

Further, preferred features may be found in the attached dependent claims, to which reference should now be made.

One basic advantage of embodiments of the invention is that, with the aid of materials which are known per se, a significantly improved oxygen sensor is provided by means of the measurement electrode which is modified in accordance with the invention.

The measurement electrode here comprises two flat layers which are at least partially joined together, namely on the one hand a flat layer consisting of a mixture of a noble metal, in particular gold, platinum and/or ruthenium, or of alloys or mixtures of these noble metals with powdery PTFE (polytetrafluoroethylene). on the other hand, the second flat surface of the measurement electrode, which is joined to the first layer, consists of a gas permeable, perfluoridated polymer film, which preferably comprises one or more of the polymers PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxy), FEP (tetrafluoroethylene-hexafluoro propylene-copolymer) or E-CTFE (ethylene chloro trifluoroethylene).

In the case of a circular surface, typical dimensions of measurement electrodes according to the invention are; outer diameter from 2 to 50 millimetres, in particular 10 to 30 millimetres, and total thicknesses of 10 to 1000 micrometres, in particular to 300 micrometres. Typical layer thicknesses of the diffusion membrane alone are 1 to 50 micrometres, in particular 10 to 1S micrometres.

Generally the diffusion membrane of the measurement electrode is only partially coated on one side with a mixture of noble metal particles and powdery PTFE (polytetrafluoroethylene) so that a free edge, in particular an annular surface, remains for securing the diffusion membrane in the sensor. The diffusion membrane is exposed to the ambient atmosphere of the sensor and, on the side remote from the ambient atmosphere, is coated with the above-described, preferably sintered, mixture of noble metal particles and powdery PTFE (polytetrafluoroethylene).

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawing, in which:

Figure 1 shows an embodiment of oxygen sensor schematically in section, and in fact a sensor housing 2 of an electrochemical sensor 1 for detecting oxygen in a gas mixture.

The sensor housing 2 encloses an electrolyte chamber 3 for receiving an electrolyte 4 suitable for oxygen measurement, in particular sulphuric acid.

Arranged in the electrolyte chamber 3 behind a measurement electrode 6, 66 are a protective electrode 7, an auxiliary electrode 8 and a reference electrode 9. The diffusion membrane 66 has a layer thickness of 1 to 50 micrometres and is optionally held in the sensor housing 2 by a porous support membrane 10 made of PTFE. The porous support membrane 10 is an independent component, but alternatively it may also be welded to the diffusion membrane 66, which is relatively expensive. The rear of the sensor housing 2 is sealed with a porous PTFE disc 11, through which the oxygen compensation with the surroundings is effected.

The sensor housing 2 is also made of porous PTFE in order to improve the oxygen exchange with the surroundings and to ensure pressure balancing which is independent of position. The distance between the electrodes is set by means of thin fabric or non-woven materials 12 saturated with electrolyte. A further non-woven material 12 between the auxiliary electrode 8 and a porous glass body 13 which receives the reference electrode 9 ensures an electrolyte contact which is independent of position. The reference electrode 9 is arranged far away from the auxiliary electrode 8. The measurement electrode 6, 66, the protective electrode 7, the auxiliary electrode 8 and the reference electrode 9 are made of the same material.

Particularly suitable materials are gold or platinum and their alloys and doped platinum-coated carbon.

In an evaluation unit 14, which is joined to the electrodes, the current generated by the oxygen to be detected is processed at the electrically-contacted part 6 of the measurement electrode 6, 66 to form a measurement signal representing the oxygen concentration, and shown in a display unit 15. The evaluation unit 14 contains a potentiostat, (not shown in the Figure) with which the potential of the protective electrode 7 is set to that of the measurement electrode 6, 66.

The measurement electrode 6, 66 comprises a polymeric, perfluoridated diffusion membrane 66, which is preferably coated and joined partially on one side with a sintered mixture (part 6 of the measurement electrode 6, 66) of noble metal particles and polytetrafluoroethylene (PTFE), the diffusion membrane 6 being exposed to the ambient atmosphere. The diffusion membrane 66 is preferably composed of PTFE (polytetrafluoroethylene) or PFA (perfluoroalkoxy). It has been determined that the electrochemically active surface of the measurement electrode 6, 66 is two to three orders of magnitude larger than the purely geometric surface.

In an alternative embodiment of an electrochemical sensor 1 (not shown) a protective electrode ring is arranged around the measurement electrode 6, 66 in order to eliminate the radial diffusion of oxygen from the electrolyte chamber 3 to the measurement electrode 6, 66. In this case the protective electrode ring is joined or connected to the protective electrode 7, as regards potential, by means of a line.

Claims

CLAIMS:

1. Electrochemical oxygen sensor having a measurement electrode, an air-oxygen electrode as a reference electrode, a protective electrode and an auxiliary electrode in an electrolyte, characterised in that the measurement electrode is made of a gas-permeable, polymeric, perfluoridated diffusion membrane which is coated at least partially on one side with a mixture of a noble metal and polytetrafluoroethylene, the diffusion membrane being exposed to the ambient atmosphere.

2. Electrochemical oxygen sensor according to claim 1, is characterised in that the measurement electrode has a circular surface and an outer diameter of 2 to 50 millimetres, in particular 10 to 30 millimetres, and a total thickness of 10 to 1000 micrometres, in particular 100 to 300 micrometres.

3. Electrochemical oxygen sensor according to claim 1 or 2, characterised in that the diffusion membrane has a layer thickness of 1 to 50 micrometres, in particular to 15 micrometres.

4. Electrochemical oxygen sensor according to any of claims 1 to 3, characterised in that the diffusion membrane is made of one or more of the polymers PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxy), FEP (tetrafluoroethylene hexafluoropropylene copolymer) or E-CTFE (ethylene chlorotrifluoroethylene).

5. Electrochemical oxygen sensor according to any of claims 1 to 4, characterised in that the noble metal of the measurement electrode is gold, platinum and/or ruthenium, or comprises alloys or mixtures of these noble metals.

6. Electrochemical oxygen sensor according to any of claims 1 to 5, characterised in that an additional porous support membrane is arranged on the diffusion membrane, the support membrane being turned towards the ambient atmosphere and preferably comprising PTFE (polytetrafluoroethylene).

7. Electrochemical oxygen sensor according to any of claims 1 to 6, characterised in that an exchangeable filter in the gas flow path selectively retaining disturbing gases is arranged between the ambient atmosphere and the measurement electrode.

8. Electrochemical oxygen sensor according to any of claims 1 to 7, characterised in that separators saturated with electrolyte are present between the measurement electrode, the protective electrode and the auxiliary electrode.

9. Electrochemical oxygen sensor according to claim 8, characterised in that the separators are non-woven materials. 25

10. Electrochemical oxygen sensor according to any of claims 1 to 9, characterised in that the potential of the protective electrode is set to that of the measurement electrode. 30

11. Electrochemical oxygen sensor according to any of claims 1 to 10, characterised in that the measurement electrode is surrounded by a protective electrode ring.

12. Electrochemical oxygen sensor according to claim 11, characterised in that the protective electrode ring is connected to the protective electrode.

13. Electrochemical oxygen sensor according to any of claims 8 to 12, characterised in that the separator between the measurement electrode and the protective electrode has a thickness of between about 20 micrometres and two millimetres.

14. An electrochemical oxygen sensor substantially as herein described with reference to the accompanying drawing.