GB2332528A

GB2332528A - Electrochemical gas sensor without membrane

Info

Publication number: GB2332528A
Application number: GB9806964A
Authority: GB
Inventors: Rigobert Chrzan; Michael Sick; Dieter Krueger; Frank Mett; Stephen Haupt
Original assignee: Draeger Sicherheitstechnik GmbH
Current assignee: Draeger Safety AG and Co KGaA
Priority date: 1997-12-13
Filing date: 1998-03-31
Publication date: 1999-06-23
Anticipated expiration: 2018-03-31
Also published as: GB2332528B; DE19755506A1; GB9806964D0; FR2772481B1; FR2772481A1; DE19755506C2

Abstract

The sensor has an open housing 1 made from a gas impermeable material, a porous electrode carrier 4 with electrodes 2, 3 arranged in the housing facing the open end, and an electrolyte reservoir 6 connected to the electrode carrier 4 by a porous wick 5. The electrodes are exposed to the surrounding atmosphere without the intermediary of a membrane. The electrodes are flat on or raised from the carrier 4 and may be electrically contacted grids or wires or sputtered, pressed, welded or sintered metal layers. Further electrodes may be located in the electrolyte reservoir 6, which contains a porous material such as non-woven glass fibres, propylene or polyethylene. The electrolyte is in contact with the surrounding atmosphere via at least on pressure equalisation aperture 7. An absorbent 9 for gas from the electrolyte is in gas but not liquid contact with the electrolyte via porous discs 8.

Description

9 1 1 2332528 An electrochemical sensor with open housing The present

invention relates to an electrochemical sensor according to the preamble of claim 1. Thus, an electrochemical sensor according to the invention has a housing made of gas-impermeable material and flat and/or- raised electrodes mounted on a porous carrier.

An electrochemical sensor of the type in question has been described, for instance, in EP 0 298 570 B1 and is used for determining the concentration of gases in the surrounding atmosphere.

An essential feature of conventional electrochemical sensors is that they are equipped with a permeable membrane which allows the diffusion of the gas to be measured. The membrane is in liquid contact with the electrolyte used. The disadvantage when using strongly alkaline or even particular organic electrolytes is that these electrolytes may leak unintentionally from the sensor housing and cause damage to the sensor and its surroundings.

Furthermore, the known membranes have the disadvantage that strongly adsorbing gases are trapped as they pass through these membranes and can therefore only be measured with very low measurement sensitivity.- A further disadvantage of conventional electrochemical sensors is seen when special liquid electrolytes are to be used for detecting special gases such as, for instance, HC1, HF or even hydrazine. Such electrolytes form gases or vapours due to their characteristic electrochemical detection reactions which have to be removed from the sensor. The gases or vapours which are formed would otherwise produce undesirable signal is currents.

The aim of the present invention is to provide an electrochemical sensor using liquid electrolytes for its electrochemical detection to measure the concentration of strongly adsorbing gases. The electrolytes often form troublesome gases or vapours as electrochemical by-products which have to be removed from the sensor.

For an electrochemical sensor of the above type, the aim is achieved with the characterising features of claim 1.

According to the present invention, there is provided an electrochemical sensor having a housing made of a gas-impermeable material, an electrode carrier with electrodes arranged in the housing, and an electrolyte reservoir, the electrolyte reservoir being in liquid connection with the electrode carrier by means of a wick, wherein one or more electrodes are in a flat and/or raised arrangement on the porous electrode carrier and are exposed to the surrounding atmosphere without the intermediary of a membrane in the open region of the housing, and wherein the electrolyte reservoir is also in gas flow connection with the surrounding atmosphere.

The sub-claims define preferred developments and embodiments of the electrochemical sensor according to claim 1.

An essential advantage of the sensor of the invention is on the one hand that faster and more sensitive sensor signals can be achieved through the raised and/or flat arrangement of the electrodes. The 1 1 1 electrodes are arranged on the electrode carrier in the form of a welded lattice of grids or wires etc., or even in the form of sputtered or flat designs sintered from powder.

is The electrodes are made of a noble metal and suitable electrode materials include gold, platinum, iridium, palladium, ruthenium, and rhodium, either in substantially pure form or arranged on a suitable substrate. Particular examples of suitable electrodes are platinum, platinum black on a carbon substrate, gold, and gold on a carbon substrate. Platinum is most suitable as the electrode material because of the improved sensitivity obtained.

on the other hand, an advantage of the electrodes of the present invention is that the gases to be detected can have direct access to the electrodes because of the omission of a gas-permeable membrane, thus increasing the measurement sensitivity.

A further advantage of the electrode of the present invention is that it is possible to use such special liquid electrolytes which form corrosive, toxic and/or undesirable gases or vapours. Using the adsorption agent reservoir connected on the outlet side of the electrolyte reservoir, preferably arranged in the direction of.the gas flow, the undesirable gases or vapours are adsorptively bonded in the adsorption agent reservoir. Bromide or bromate-containing liquid electrolytes are a typical example of electrolytes used for detecting HCl. Bromide is produced by chemical reaction in an electrolyte of this type and can be bonded adsorptively using, for example, active carbon as the adsorption agent.

1 is An exemplary embodiment of the invention is described below by means of the Figure 1 which illustrates a longitudinal section through a sensor of the invention and schematically shows its construction.

The actual sensor structure is located in a preferably pot-shaped, rotationally symmetrical housing 1 of a gas-impermeable material, for example plastics, open in the region of the electrodes and the electrode carrier 4.

A measurement electrode 2 and a counter electrode 3 are applied to a porous electrode carrier 4. Measurement electrode 2 and counter electrode 3 are preferably made of platinum and are flat welded in the form of electrically-contacted grids on preferably raised sections of the electrode carrier 4. The measurement of signals becomes faster and more sensitive through the flat and preferably raised arrangement of the electrodes on the electrode carrier 4.

Alternatively, the electrodes can be pressed, sintered, sputtered or welded on the electrode carrier 4 from powder. This arrangement and construction Of electrodes leads surprisingly to faster and more sensitive measurement signals.

It is particularly advantageous if the electrodes are coated with a hydrophilic coating having a layer thickness of approximately a micrometre so as to make uniform wetting with the liquid electrolytes possible. The hydrophilic coating can be made, for instance, from a material containing S'02 (silica).

It is alternatively possible to provide the counter electrode 3 and possibly an additional reference electrode in the electrolyte reservoir 6. In this case, only the measurement electrode 2 is located on the porous electrode carrier 4. The electrode carrier 4 is saturated with electrolyte. The electrode carrier 4 is in liquid connection with the electrolyte reservoir 6 by means of a porous wick S. The wick S is porous only to electrolyte and there is no significant gas transmission through it. The electrolyte reservoir 6 contains a porous material, in particular a non-woven material of glass fibres, polypropylene or polyethylene, loaded with electrolyte by means of capillaries. The electrode carrier 4 has finer capillaries than the material in the electrolyte reservoir 6 so that the electrolyte migrates to the electrode carrier 4 because of the stronger capillary forces.

With high ambient moisture, the electrolyte absorbs water and the electrolyte volume increases. The increased electrolyte volume is absorbed by the electrolyte reservoir 6.

With a dry ambient atmosphere, the electrolyte loses water. However, the electrode carrier 4 does not dry out but absorbs so much electrolyte from the electrolyte reservoir 6 that its pore system is always filled with electrolyte. The sensor has at least one pressure-equalisation aperture 7, which ensures that the internal pressure of the sensor is always the same as the pressure in the surrounding atmosphere. The electrolyte cannot be pressed out of the capillary system by a change in pressure in the surrounding atmosphere.

on one hand, the adsorption agent reservoir 9 is in gas flow connection with the electrolyte reservoir 6 by means of gas-permeable but electrolyte-impermeable porous discs 8 and, on the other hand, is in gas flow connection with the surrounding atmosphere by means of the at least one pressure-compensation aperture 7.

As an alternative to the arrangement indicated in Figure 1, the adsorption agent reservoir 9 can be arranged in the electrolyte reservoir 6. In this case, the adsorption agent, i.e. active carbon, aluminium oxide, silica gel or even zeolite, is located in a gaspermeable, but electrolyte-impermeable sleeve of, in particular, PTFE, polyethylene or polypropylene.

If, for instance, a gas sensor is designed to detect HC1, an electrolyte containing bromate or bromide has to be used as a suitable electrolyte. Undesirable bromine vapours or gases are produced in the course of the electrochemical detection reaction and have to be trapped by means of the adsorption agent.

is The delimiting components of the housing 1, 10, 11 are made of a gas and electrolyte-impermeable material, in particular a plastics material.

Claims

1. An electrochemical sensor having a housing made of a gas-impermeable material, an electrode carrier with electrodes arranged in the housing, and an electrolyte reservoir, the electrolyte reservoir being in liquid connection with the electrode carrier by means of a wick, wherein one or more electrodes are in a flat and/or raised arrangement on the porous electrode carrier (4) and are exposed to the surrounding atmosphere without the intermediary of a membrane in the open region of the housing (1), and wherein the electrolyte reservoir (6) is also in gas flow connection with the surrounding atmosphere.

2. An electrochemical sensor as claimed in claim 1, wherein the electrolyte reservoir (6) is also in gas flow connection with an adsorption agent reservoir (9).

3. An electrochemical sensor as claimed in claim 1 or 2, wherein a counter electrode (3) and/or an additional reference electrode are arranged in the electrolyte reservoir (6).

4. An electrochemical sensor as claimed in claim 1, 2 or 3, wherein the one or more electrodes arranged on the electrode carrier (4) are constructed in the form of electrically-contacted grids or wires; or are sputtered, pressed, welded or sintered metal layers.

5. An electrochemical sensior as claimed in any of claims I to 4, wherein the electrodes are made of platinum.

6. An electrochemical sensor as claimed in any of claims 1 to 5, wherein the one or more electrodes are coated with a coating of an absorbent, hydrophilic material, in particular silica (S'02).

7. An electrochemical sensor as claimed in claim 6, wherein the coating is not more than 1 micrometer thick.

8. An electrochemical sensor as claimed in any of claims 2 to 7, wherein the adsorption agent reservoir (9) contains one or more of the adsorption agents: active carbon, aluminium oxide, silica gel and zeolite.

9. An electrochemical sensor as claimed in any of claims 2 to 8, wherein the adsorption agent reservoir (9) and/or the electrolyte reservoir (6) are in gas flow connection with the atmosphere in front of the electrodes by means of one or more pressureequalisation apertures (7).

10. An electrochemical sensor as claimed in any of claims 1 to 9, wherein the electrolyte reservoir (6) contains a porous material, in particular a non-woven material of glass fibres, polypropylene or polyethylene, loaded with an electrolyte.

11. An electrochemical sensor as claimed in any of claims 1 to 10, wherein the electrolyte reservoir (6) contains a bromide-containing electrolyte.

12. An electrochemical sensor as claimed in claim 10 or 11, wherein the electrode carrier (4) has finer capillaries than the material loaded with an electrolyte and contained in the electrolyte reservoir (6), so that the electrolyte migrates to the electrode carrier (4) because of stronger capillary forces.

-g-

13. An electrochemical sensor as claimed in any of claims 2 to 11, wherein the adsorption agent is disposed inside a porous, gas-permeable sleeve which is arranged in the electrolyte reservoir (6) and which is not permeable to the electrolyte, wherein the sleeve is preferably made from PTFE (polytetrafluoroethylene), polyethylene or polypropylene.

14. An electrochemical sensor substantially as hereinbefore described with reference to and as shown in Figure 1 of the accompanying drawings.

1

-to- Amendments to the claims have been filed as folitmos Claims is 1. An electrochemical sensor having a housing made of a gas-impermeable material, a porous electrode carrier with electrodes arranged in the housing, and an electrolyte reservoir, the electrolyte reservoir being in liquid connection with the porous electrode carrier by means of a wick, wherein one or more electrodes are in a flat and/or raised arrangement on the porous electrode carrier (4) and are exposed to the surrounding atmosphere without the intermediary of a membrane in the open region of the housing (1), and wherein the electrolyte reservoir (6) is also in gas flow connection with the surrounding atmosphere.

2. An electrochemical sensor as claimed in claim 1, wherein the electrolyte reservoir (6) is also in gas flow connection with an adsorption agent reservoir (9) 3. An electrochemical sensor as claimed in claim 1 or 2, wherein a counter electrode (3) and/or an additional reference electrode are arranged in the electrolyte reservoir (6).

4. An electrochemical sensor as claimed in claim 1, 2 or 3, wherein the one or more electrod(s arranged on the electrode carrier (4) are constructed in the form of electrically-contacted grids or wires; or are sputtered, pressed, welded or sintered metal layers.

S. An electrochemical sensior as claimed in any of claims 1 to 4, wherein the electrodes are made of platinum.

6. An electrochemical sensor as claimed in any of claims I to 5, wherein the one or more electrodes are 1-1 coated with a coating of an absorbent, hydrophilic material, in particular silica (S'02).

9. An electrochemical sensor as claimed in any of claims 2 to 8, wherein the adsorption agent reservoir (9) and/or the electrolyte reservoir (6) are in gas I flow connection with the atmosphere in front of the electrodes by means of one or more pressureequalisation apertures (7).

10. An electrochemical sensor as claimed in any of claims 1 to 9, wherein the electrolyte reservoir (6) contains a porous material, in particular a non-woven material of glass fibres,' polypropylene or polyethylene, loaded with an electrolyte.

1 1 1 13. An electrochemical sensor as claimed in any of claims 2 to 11, wherein the adsorption agent is disposed inside a porous, gas-permeable sleeve which is arranged in the electrolyte reservoir (6) and which is not permeable to the electrolyte, wherein the sleeve is preferably made from PTFE (polytetrafluoroethylene), polyethylene or polypropylene.