WO1996004550A1 - Electrochemical gas sensor with reduced cross-sensitivity - Google Patents

Abstract

With the aid of a selective membrane (5a), a significant and lasting reduction in the cross-sensitivity of electrochemical gas sensors (7) to interfering gases (18) can be achieved be electrocatalytic means. A suitable catalyst (5b) is chosen to eliminate particular interfering gases (18) and applied to a gas-permeable membrane (8) in such a way that these gases are converted to products to which the measurement electrode (11a) reacts to a reduced degree or not at all, while the gas which is to be detected must not, if possible, being to react so that it can react fully at the measurement electrode (11a). Such selective membranes (5a) can advantageously be combined with membrane electrodes. Unlike conventional adsorption and chemical adsorption filters, the catalytically operating membranes (5a) remain permanently active and do not become depleted even with concentrations well above maximum workplace concentrations. Electrochemical gas sensors fitted with selective membranes (5a) of this type work with conventional evaluation electronics (in the simplest case with an ammeter (22) connected to the measurement electrode (11a) and to a counter-electrode (12)) and do not require complex electronic compensation systems.

Description

 ELECTROCHEMICAL GAS SENSOR WITH REDUCED CROSS SENSITIVITY

The invention relates to an electrochemical gas sensor, in particular for carbon monoxide and hydrogen, with reduced cross sensitivity according to the preamble of patent claim 1.

In many applications, electrochemical gas sensors have proven themselves for the detection of toxic or explosive gases, the mode of operation of which is based on direct electrochemical oxidation or reduction of the gas to be measured. The cell current maintained by gas diffusion is a function of the gas concentration and, to a good approximation, proves to be linear within the measuring range limits. Such gas measuring cells are, however, cross-sensitive to certain interfering gases, which can lead to a falsification of the measured value, cf. e.g. W.Pauli, Chemistry - Environmental Technology, 1988/89, 62.

According to the state of the art, electrochemical gas sensors are offered which are equipped with a filter unit, usually integrated in the sensor housing, for removing interfering gases, such as H ₂ S, S0 ₂ , NO, N0 ₂ . Suitable filter materials remove unwanted gases by adsorption, absorption or chemisorption. The disadvantage of such filter units is that their capacity and thus their effectiveness decrease over time with continuous exposure and higher concentrations of interfering gases, which directly limits the range of use of the gas sensor when the filter unit is integrated, cf. see for example City Technology Ltd., London, Great Britain, Product Data Handbook 1992, in particular pp. 40, 66 - 68.

The filter capacity depends on the type of interfering gas. In the case of H ₂ S, the capacity is exhausted very easily at concentrations well above the MAK value. moreover interfering gases such as H ₂ cannot be filtered out in this way.

In special applications, in particular in the event of interference from H ₂ , the interference signal can be reduced or eliminated by means of compensation devices, cf. e.g. EP 126 623 A2 and EP 127 387 A2.

In the simplest case, the electrochemical gas sensor consists of a 2-electrode arrangement, whereby gas access is also guaranteed for the counter electrode. If a gas reacts incompletely at the measuring electrode, it can reach the counter electrode via a partially gas-permeable separator and can also react there. This results in partial compensation of the measuring current. In contrast, gases that react completely with the measuring electrode give a full measuring signal. The principle of operation of multi-electrode sensors with auxiliary and reference electrodes, which are described in connection with corresponding electronic compensation circuits, is based on a similar principle. EP 84935 B1 describes a similar device for the separate detection of CO in addition to H ₂ , the different response times of the two gases being used.

The object of the present invention is to provide a simply constructed electrochemical-based gas sensor with reduced cross-sensitivity, which should have a long service life.

The object is achieved with the features of claim 1. Advantageous further developments are the subject of the subclaims.

With the selective membrane according to the invention, the cross-sensitivity of electrochemical gas sensors to interfering gases can be considerably and permanently reduced. The selective membrane contains a catalyst wetted with electrolyte, which is electro- is catalytically inactive as much as possible, but is electrocatalytically as active as possible with regard to disruptive gases, so that these are converted, at a positive level, into products which are no longer disruptive or less disruptive. The at least one porous catalyst layer of the selective membrane preferably consists of at least one fine-grained noble metal and polytetrafluoroethylene (PTFE) powder. Palladium, gold, platinum, ruthenium, iridium, osmium and / or silver in pure form, alloy or mixture are possible as noble metals. The porous catalyst layer is preferably applied to a porous PTFE membrane and permanently connected to it by pressing or by sintering at a temperature T> 320 ° C. to form the selective membrane according to the invention.

In a particularly preferred embodiment, a porous, partially hydrophobic / hydrophilic intermediate layer is applied directly to the selective membrane with a measuring electrode, which allows a particularly compact structure of the gas sensor, which is particularly suitable for determining C0 and H ₂ .

The chemical reactions in the integrated selective membrane can be formulated in the presence of the interfering gases H ₂ S, S0 ₂ or N0 ₂ and in the presence of a noble metal catalyst, such as palladium or gold, which is active in relation to these gases:

H ₂ S + 4 H ₂ 0> H ₂ S0 ₄ + 8 H * + 8 e ^"

2 0 ₂ + 8 H ^* + 8 e ^" > 4 H ₂ 0

S0 ₂ + 2 H ₂ 0> H ₂ S0 ₄ + 2 H ^* + 2 e ^"

1/2 0 ₂ + 2 H ^* + 2 e ^" > H ₂ 0

N0 ₂ + 2 H * + 2 e ^" > NO + H ₂ 0

H ₂ 0> 1/2 0 ₂ + 2 H ^* + 2 e ^"

The catalyst layer of the selective membrane wetted with the electrolyte can to a certain extent be an intimate combination of a similar working and counter electrode electrical discharges to the outside are understood, the electrons of the redox reactions being exchanged internally in the catalyst layer instead of via an external conductor. In the case of H ₂ S and S0 ₂ , only H ₂ S0 ₄ is formed in the reaction balance, for example the electrolyte itself. In the case of N0 ₂ , NO is formed. A CO measuring electrode, based on a platinum catalyst, is less sensitive to this, and theoretically a measuring current would be expected which has the sign of a reducing gas. A weak signal is sometimes measured with the opposite sign, which indicates that in addition to NO there is also a residual concentration of N0 ₂ , which also reacts at the measuring electrode.

If palladium or gold is used as a catalyst in the selective membrane, this does not act with respect to CO, so that the measurement gas penetrates CO without undergoing a chemical reaction and reaches the measurement electrode, where its oxidation takes place. If the measuring and reference electrodes are connected with low impedance, the electrode reactions can be formulated as follows:

Measuring electrode: CO + H ₂ 0> C0 ₂ + 2 H ^* + 2 e ^"

Reference electrode: 1/2 0 ₂ + 2 H * + 2 e ^" > H ₂ 0

The electrical current generated can be measured with an intermediate ammeter and is a function of the CO concentration.

The invention is illustrated by the following drawings.

It shows:

1 shows a longitudinal section of an electrochemical gas sensor with the electrocatalytically selective membrane according to the invention; 2 shows a longitudinal section of a combination of the selective membrane according to the invention with a measuring electrode to form a selective membrane electrode;

3 is a top view of a combined reference and counter electrode;

4 shows a longitudinal section of a gas sensor with a selective membrane electrode according to the invention; and

5 shows a longitudinal section of a gas sensor with a separate selective membrane according to the invention.

1 schematically shows the selective membrane 5a and its arrangement in an amperometrically operated electrochemical measuring cell 7, which is illustrated as a 2-electrode cell for the sake of simplicity. The selective membrane 5a contains at least one electrically conductive, finely divided catalyst 5b, which is preferably wetted by the electrolyte 19a of the measuring cell 7 itself. On the gas side, the selective membrane 5a is preferably closed off by a gas-permeable, porous membrane 3, which represents a barrier for the electrolyte 19a of the measuring cell 7. The selective membrane 5a is separated from the measuring electrode 11a by a thin gap 24 containing electrolyte 19a. Measuring and counter electrodes 11a, 12 are connected to an ammeter 22 via inert contact wires. The catalyst 5b of the selective membrane 5a is designed in such a way that it is electrocatalytically inactive with regard to the gas to be detected (measurement gas), but is as active as possible with respect to other gases 18 that are interfering with one another. In this case, the measuring gas 17 can pass through the porous, selective membrane 5a without undergoing an electrochemical reaction in order to reach the measuring electrode 11a via the gap 24. On the other hand, other gas components 18, which would react electrocatalytically at the measuring electrode 11a in the absence of the selective membrane 5a and would thus result in a contribution to the measurement current, already react completely in the selective membrane 5a to form products which are no longer or only to a limited extent perceived by the measurement electrode 11a. Depending on whether a gas is oxidized or reduced, the corresponding counter-reaction forms in the same selective membrane 5a with the internal exchange of electrons. In the event of an oxidation, oxygen that is supplied from the environment is reduced to water at the same time. In the event of a reduction of an interfering gas 18, oxygen is formed as a counter-reaction in the selective membrane 5a and is released into the environment. If the oxidation or reduction products of the interfering gases 18 are inert with respect to the catalyst of the measuring electrode 11a, the current measured between the measuring and counter electrodes 11a, 12 is generated solely by the measuring gas 17.

The electrocatalytically selective membrane 5a can be produced as follows: A porous PTFE (polytetrafluoroethylene) membrane 3 is preferably used as the base 3. A porous catalyst layer 5b, which consists of a mixture of finely divided noble metal catalyst and PTFE powder, can be applied to this. Fixation can be achieved by pressing or by sintering at T> 320 ^* C. Mixtures of several noble metal catalysts can also be used or the various catalysts can be applied in multiple layers.

A hydrophilic thin glass fiber filter 10 can be used as the gap material 24a for conveying the electrolyte 19a and for the spatial separation from the measuring electrode 11a. It is better to use a partially hydrophobic / hydrophilic membrane since this increases the diffusion flow of the measurement gas 17. This can consist of a thin, porous and sintered membrane made of finely divided PTFE powder on the one hand and quartz powder or amorphous SiO ₂ powder on the other hand. The hydrophilic component (eg fine quartz powder) forms paths for the aqueous electrolyte 19a, while the hydrophobic component (PTFE) forms paths for the sample gas 17.

To simplify the sensor assembly, it is expedient to combine the electrocatalytically selective membrane 5a with the measuring electrode according to FIG.

Such a selective membrane electrode 5 can be produced as follows: A porous PTFE membrane 3 serves as a base. A porous catalyst layer 5b, consisting of a mixture of finely divided noble metal and PTFE powder, is applied to this. The layer 5b is covered with a porous intermediate layer 23, which consists of a mixture of finely divided PTFE powder and fine quartz powder or amorphous SiO ₂ powder. Then the porous measuring electrode 11a, which consists of a mixture of finely divided noble metal and PTFE powder, is applied to the intermediate layer 23. For fixation, the layers can be pressed successively or simultaneously or sintered successively or simultaneously at T> 320 ^* C.

A gas sensor according to the invention further comprises the following individual parts: fastening screws 1, acid-resistant plastic O-rings 4, unsintered PTFE strips 6, a central body 7a, preferably made of polycarbonate plastic, contact springs 8, made of a film, preferably of thickness Platinum tapes 9 cut 0.025 mm, tampons made of glass fiber filter material 10, an electrolyte chamber 19 partially filled with an electrolyte 19a with an opening 13 and a wick made of glass fiber filter material 10a for conveying the electrolyte to the electrodes 11a, 11, 12, a cover 14 , preferably made of polycarbonate plastic with a yoke for pressure compensation 20, bore 15 with thread for fastening screws 1, pressure compensation opening 20, preferably with a diameter of approx. 0.5 mm and a capillary length of approx. 5 mm.

The scope of application of the selective membrane 5a according to the invention is illustrated below with the aid of a few exemplary embodiments demonstrated.

1. EXAMPLE: CO-SENSOR

A membrane electrode 5 with an integrated selective membrane 5a according to FIG. 2 is produced as follows: A catalyst layer 5b, which contains a mixture of fine-grained palladium black and PTFE powder, is first applied to a porous PTFE membrane 3. This layer 5b is completely covered with an intermediate layer 23 made of fine quartz powder and PTFE powder. A further layer 11a, the actual measuring electrode, is applied over it and consists of a mixture of fine-grained platinum black and PTFE powder. The selective membrane electrode 5a is then sintered in an oven at T = 360 ^β C.

A reference and counterelectrode 11, 12 according to FIG. 3 is produced as follows: a round glass fiber filter 10 is a catalyst layer separated by a gap 21, which contains a mixture of platinum black and PTFE powder, applied and then in the oven at T ≥ 360 ^* C sintered.

Through the gap 21, the electrode is separated into a reference and a counter electrode 11, 12.

The sensor is mounted as shown in FIG. 4, and the housing parts

2, 7a are screwed tight with 6 screws 1. On the back, the electrolyte chamber 19 of the sensor is filled with 1.5 ml of aqueous sulfuric acid (30% by mass) and screwed to the cover 14. A wick 10a made of glass fiber filter material lining the electrolyte chamber 19 is in contact through the opening 13 with the reference and counter electrodes 11, 12 for further transmission of the electrolyte to the tampon 10 and the electrodes 11, 12, 5. The reference and counter electrodes 11, 12 are in intimate contact with a porous PTFE membrane 3, which always ensures the replenishment of atmospheric oxygen for the counter-reaction. A conventional co-sensor is also produced for a comparison measurement of the cross-sensitivities: the measuring electrode is a conventional membrane electrode 16a. A layer containing a mixture of finely divided platinum black and PTFE powder is applied to a porous PTFE membrane 3 and then sintered in the furnace at T = 360 ^* C. All other steps are the same as described above.

EXAMPLE 2: CO SENSOR

A membrane electrode 5 with an integrated selective membrane 5a according to FIG. 2 is produced as follows: A layer 5b, which contains a mixture of finely divided gold powder and PTFE powder, is applied to a porous PTFE membrane 3. For fixation, layer 5b is pressed with a hand press. On a porous hydrophobic / hydrophilic membrane 23, which is formed from a sintered mixture of fine quartz powder and PTFE powder, a layer 11a, the actual measuring electrode, is applied, which consists of a mixture of finely divided platinum black and PTFE powder, and then at T = 360 ^* C sintered. The membrane 11a is then pressed with a hand press onto the porous PTFE membrane 3 with layer 5b described above.

The sensor is manufactured as described in the first example. In this sensor, too, the cross sensitivity to H ₂ S, S0 ₂ and N0 _{2 is} drastically reduced according to Table 1. The response time and sensitivity to CO is comparable to the first example. In contrast to the first example, a weakly positive signal is observed at N0 ₂ .

3. EMBODIMENT: H ₂ SENSOR

In an analogous manner as described in the 1st example a selective sensor is produced, with the only difference that the selective membrane 5a contains platinum black instead of palladium black. Since platinum black has an electrocatalytic effect on CO, CO is already oxidized in the selective membrane. In contrast, according to Table 1, H ₂ reacts incompletely in the selective membrane 5a, so that a relatively large part is perceived by the measuring electrode 11a. A sensor according to example 3 can be used as an H ₂ sensor.

By applying a corresponding bias voltage between the measuring and reference electrodes 11a, 11, a measuring gas 17 which is more difficult to oxidize can also be detected with a sensor according to Example 3. The cross-sensitivity to the interference gases 18, which is normally increased by the bias voltage, can be drastically reduced in the presence of the selective membrane 5a.

4. EXAMPLE OF EXAMPLE: CO SENSOR

For this purpose, a separate selective membrane 5a is produced as follows: A layer 5b, which contains a mixture of finely divided palladium black and PTFE powder, is applied to a porous PTFE membrane 3 and then sintered in an oven at T = 360 ° C.

The measuring electrode 16a is produced as follows: a layer 5b, which contains a mixture of finely divided platinum black and PTFE powder, is applied to a porous PTFE membrane 3 and then sintered in the furnace at T = 360 ^* C. The counter electrode 12 is as follows herge¬ provides: an optical fiber rotary filter 10 is a layer 5b containing a mixture of finely divided platinum black and PTFE powders, and subsequently applied in a furnace at T = 360 ^* C sintered.

With the separate selective membrane 5a and the measuring and 5, a 2-electrode sensor is produced according to FIG. 5. A thin glass fiber round filter 10 is used as a separator between the selective membrane 5a and the measuring electrode 16a. 1.5 ml of sulfuric acid (30% by mass) are introduced as electrolyte 19a.

The measurement of CO and cross-sensitivities are carried out analogously to the 1st example. The ammeter 22 is connected to the measuring and counter electrodes 11a, 12. As can be seen in Table 1, the cross sensitivity is drastically reduced in this embodiment. Compared to the 1st and 2nd example, the response time with regard to CO is significantly slower here.

Tab. 1: CROSS SENSITIVITY OF THE INVENTIONS

GAS SENSORS

GAS SENSORS MEASURED or INTERFERENCE GAS [concentration]

Measuring current CO H ₂ S S0 ₂ N0 ₂ H ₂ [MA] 200 ppm 200 pp 200 ppm = 70 ppm 200 ppm conventional 1- 1 CO sensor 5.1 8.2 2.1 -0.8 2.2 (after 1 . Example)

CO sensor 3.9 0.1 0.3 -0.1 1.3 (1st example)

CO sensor 3.9 0.1 0.0 0.1 3.7 (2nd example)

H ₂ sensor 0.4 0.0 0.0 0.0 0.8 (3rd example)

CO sensor 3.7 0.0 0.3 0.1 3.4 (4th example) According to Table 1, the cross sensitivity of the gas sensors according to the invention with an integrated selective membrane 5a compared to H ₂ S, S0 ₂ and N0 _{2 is} considerably reduced in comparison to the conventional CO sensor. The values remain unchanged even after hours of gassing. The response time and sensitivity to CO are otherwise comparable to that of a conventional CO sensor.

Claims

claims 1. Gas sensor on an electrochemical basis with reduced cross sensitivity, the at least one measuring (11a) and one counterelectrode (12) in a measuring cell (7) filled with an electrolyte (19a), which is connected to the measuring sample with a measuring gas (17) permeable membrane (3) is closed, and between the membrane (3) and the measuring electrode (11a) at least one permeable to the measuring gas (17) selective membrane (5a) u, which is in contact with the electrolyte (19a) and is as inactive as possible to the measurement gas (17), but is as active as possible to any interfering gases (18), characterized in that the selective membrane (5a) has at least one porous catalyst layer (5b), in which at least one contains finely divided noble metal wetted by the electrolyte (19a). 2. Gas sensor according to claim 1, characterized in that the catalyst layer (5b) consists of a mixture of at least one finely divided, powdery noble metal with polytetrafluoroethylene (PTFE) powder. 3. Gas sensor according to one of claims 1-2, characterized in that palladium, gold, platinum, ruthenium, iridium, osmium and / or silver in pure form, alloy or mixture is used for the catalyst layer (5b). 4. Gas sensor according to claim 2 and 3, characterized in that the at least one catalyst layer (5b) is applied to a porous polytetrafluoroethylene membrane (3) and this is permanently connected to the selective membrane (5a) by pressing or by sintering at T> 320 ° C. 5. Gas sensor according to one of claims 1-4, characterized gekenn¬ characterized in that the measuring electrode (11a) and / or the counter electrode (12) and / or an additional reference electrode (11) solidified at least one porous, by pressing or sintering Catalyst layer, consisting of at least one fine-particle precious metal and PTFE powder. 6. Gas sensor according to claim 5, characterized in that platinum black is used as the noble metal powder. 7. Gas sensor according to one of claims 1-6, characterized gekenn¬ characterized in that as a gap material (24a) in the gap (24) between the selective membrane (5a) and the measuring electrode (11a) a hydrophilic glass fiber filter (10) for mediation ¬ treatment of the electrolyte (19a) is used. 8. Gas sensor according to one of claims 1-6, characterized in that a porous, partially hydrophobic / hydrophilic intermediate layer (23) is used as the gap material (24a). 9. Gas sensor according to claim 8, characterized in that the partially hydrophobic / hydrophilic intermediate layer (23) consists of finely divided PTFE powder and quartz powder or amorphous SiO 2 powder. 10. Gas sensor according to one of claims 1-9, characterized gekenn¬ characterized in that the selective membrane (5a) is covered directly with the intermediate layer (23), this with a Porous measuring electrode (11a), which consists of a mixture of fine-particle precious metal and PTFE powder, is provided for the purpose of forming a selective membrane electrode (5). 11. Gas sensor according to claim 10, characterized in that the layers (5a, 23, 11a) of the selective membrane electrode (5) are pressed simultaneously or successively for fixation or are sintered simultaneously or successively at T> 320 ° C. 12. Gas sensor according to one of claims 1-11, characterized ge indicates that a combined reference and counter electrode (11, 12) is provided by applying a mixture of fine-grained platinum black and PTFE separated by a gap (21) Powder is produced on a glass fiber filter (10) and then sintered at T> 320 ° C. 13. Gas sensor according to one of claims 1-12, characterized ge indicates that it is designed as a 2-electrode sensor, which comprises a separate selective membrane (5a), and a measuring and counter electrode (11a, 12), the se ¬ selective membrane (5a) by applying a mixture of finely divided palladium black and PTFE powder to a porous PTFE membrane (3) and then sintering in an oven at T> 320 ° C; the measuring electrode (11a) being produced by applying a mixture of finely divided platinum black and PTFE powder to a porous PTFE membrane (3) and subsequent sintering in an oven at T> 320 ^β C; and the counter electrode (12) is produced by applying a layer (5b) containing a mixture of finely divided platinum black and PTFE powder to a round glass fiber filter (10) and then sintering at T> 320 ° C. 14. Gas sensor according to one of claims 1-13, characterized ge indicates that the measuring cell (7) on the gas side of a cover (2) with a permeable membrane (3), radially from a cylindrical central body (7a) and on the rear side a cover (14) provided with a pressure compensation opening (20) is limited; that contact springs (8), platinum strips (9) and tampons made of glass fiber filter (10) soaked with electrolyte (19a) and wicks made of glass fiber material (10a) in the opening (13) of the electrolyte chamber (19) for the purpose of contacting the electrodes ( 5, 16a, 11, 11a, 12) and that O-rings (4) and screws (1) for corresponding threaded bores (15) for closing the measuring cell (7) and sections made of unsintered PTFE tape (6) for sealing the measuring cell (7) are provided. 15. Gas sensor according to one of claims 1-14, characterized ge indicates that sulfuric acid is used as the electrolyte (19a). 16. Gas sensor according to one of claims 1-15, characterized ge indicates that 30% by mass sulfuric acid is used as the electrolyte (19a).