AMPEROMETRIC ELECTROCHEMICAL GAS SENSING APPARATUS AND METHOD FOR MEASURING OXIDISING GASES

Abstract

The invention relates to an amperometric electrochemical gas sensing apparatus for sensing NO.sub.2 and O.sub.3 in a sample gas and a method of using same. The apparatus comprises: a first working electrode which is a carbon electrode and at which both NO.sub.2 and O.sub.3 are reducible to thereby generate a current; a second working electrode which is a carbon electrode and at which NO.sub.2 is reducible to thereby generate a current; and an O.sub.3 filter material comprising 1-20% MnO.sub.2 by weight mixed with binder and adjacent the second working electrode, and said apparatus is configured such that, in operation, the first working electrode and the O.sub.3 filter are exposed to the sample gas in parallel.

Claims

1. Amperometric electrochemical gas sensing apparatus for sensing NO.sub.2 and O.sub.3 in a sample gas, the apparatus comprising: a first working electrode which is a carbon electrode and at which both NO.sub.2 and O.sub.3 are reducible to thereby generate a current; a second working electrode which is a carbon electrode and at which NO.sub.2 is reducible to thereby generate a current; and an O.sub.3 filter adjacent the second working electrode, wherein said apparatus is configured such that, in operation, the first working electrode and the O.sub.3 filter are exposed to the sample gas in parallel, and wherein the O.sub.3 filter comprises a mixture of 1 to 20% by weight of MnO.sub.2, and binder.

2. Amperometric electrochemical gas sensing apparatus according to claim 1 wherein the binder is particulate.

3. Amperometric electrochemical gas sensing apparatus according to claim 2 wherein the binder is particulate polytetrafluoroethylene.

4. Amperometric electrochemical gas sensing apparatus according to claim 1 wherein the MnO.sub.2 comprises particles with a purity of at least 98%.

5. Amperometric electrochemical gas sensing apparatus according to claim 1 wherein the first and second electrodes are the same or different.

6. Amperometric electrochemical gas sensing apparatus according to claim 5 wherein the carbon of the carbon electrodes is in the form of activated carbon, amorphous carbon, graphite, fullerene, graphene, glassy carbon, carbon nanotubes, or boron-doped diamond.

7. Amperometric electrochemical gas sensing apparatus according to claim 1 wherein the first working electrode is associated with a first counter electrode, a first reference electrode and a first electrolyte, and the second working electrode is associated with a second counter electrode, a second reference electrode and a second electrolyte.

8. Amperometric electrochemical gas sensing apparatus according to claim 1 wherein the first and second working electrodes are associated with a common counter electrode, a common reference electrode and a common electrolyte, and optionally a common additional working electrode, which common additional working electrode may, if present, be chosen from carbon, gold, gold alloy, Pt alloy or platinum electrodes

9. Amperometric electrochemical gas sensing apparatus according to claim 1, wherein each of the first and second working electrodes has an additional working electrode associated with it, the additional working electrode being situated in the apparatus such that it is not exposed to the sample gas.

10. Amperometric electrochemical gas sensing apparatus according to claim 7 comprising first and second additional working electrodes, associated with the first and second working electrodes, respectively.

11. Amperometric electrochemical gas sensing apparatus according to claim 10 wherein the first and second additional working electrodes are the same or different and are chosen from carbon, gold, gold alloy, Pt alloy or platinum electrodes.

12. Amperometric electrochemical gas sensing apparatus according to claim 7 wherein: the first, and second reference electrodes are the same or different and are chosen from carbon, gold, gold alloy, Pt or Pt alloy, the first and second counter electrodes are the same or different: and the first, second and common reference electrodes and are chosen from carbon, gold, gold alloy, Pt alloy or platinum electrodes.

13. Amperometric electrochemical gas sensing apparatus according to claim 9 comprising first and second additional working electrodes, associated with the first and second working electrodes, respectively.

14. Amperometric electrochemical gas sensing apparatus according to claim 13 wherein the first and second additional working electrodes are the same or different and are chosen from carbon, gold, gold alloy, Pt alloy or platinum electrodes.

15. Amperometric electrochemical gas sensing apparatus according to claim 10 wherein: the first, and second reference electrodes are the same or different and are chosen from carbon, gold, gold alloy, Pt or Pt alloy, the first and second counter electrodes are the same or different: and the first, second and common reference electrodes and are chosen from carbon, gold, gold alloy, Pt alloy or platinum electrodes.

16. Amperometric electrochemical gas sensing apparatus according to claim 8 wherein the common reference electrode and the common counter electrode are the same or different and are chosen from carbon, gold, gold alloy, Pt alloy or platinum electrodes.

17. A method for sensing NO.sub.2 and O.sub.3 gas in a sample gas comprising: exposing a sample gas to a first working electrode and an O.sub.3 filter adjacent a second working electrode in parallel, wherein the first working electrodes is a carbon electrode at which both NO.sub.2 and O.sub.3 are reducible to thereby generate a current, the second working electrode is a carbon electrode at which NO.sub.2 is reducible to thereby generate a current, and the O.sub.3 filter comprises a mixture of 1 to 20% by weight of MnO.sub.2, and binder, and determining the presence of NO.sub.2 and O.sub.3 in said sample gas by a reading of the currents generated by the first and second working electrodes, respectively.

18. A method according to claim 17, wherein the binder is particulate polytetrafluoroethylene.

19. A method according to claim 17, wherein the MnO.sub.2 comprises particles with a purity of at least 98%.

20. A method according to claim 17, where the MnO.sub.2 is particulate, the particles having a mean diameter of 25 to 250 microns.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0031] FIG. 1 is a schematic cross-sectional view of a gas sensing apparatus of the invention according to an embodiment wherein the first and second working electrodes share a common counter electrode, reference electrode and electrolyte.

[0032] FIG. 2 relates to a gas sensing apparatus of the invention according to an embodiment comprising two individual electrochemical amperometric gas sensors one housing the first working electrode and its associated counter electrode, reference electrode, electrolyte and additional working electrode, the other housing an ozone filter, the second working electrode and its associated counter electrode, reference electrode, electrolyte and additional working electrode. FIG. 2 provides a schematic cross-sectional view of each of these sensors.

[0033] FIG. 3 is a diagram of potentiostatic circuitry for a gas sensing apparatus in which first and second working electrodes share a common counter electrode, reference electrode and electrolyte as for example, in the embodiment of FIG. 1 or in which a working electrode and an additional working electrode share a common counter electrode, reference electrode and electrolyte as for example, in one of the individual gas sensors of the embodiment of FIG. 2.

[0034] FIG. 4 is a graph of current generated versus time for (a) a sensor according to FIG. 2(2) with a filter of 25 mg MnO.sub.2, 99.9% purity, mixed with 225 mg PTFE, i.e. 10% by weight of MnO.sub.2 and (b) a sensor according to FIG. 2(1) (no filter), while the sensors are exposed in turn to air without NO.sub.2 or O.sub.3, then 2 ppm NO.sub.2, 2 ppm O.sub.3 and then a mixture of 2 ppm NO.sub.2 and 2 ppm O.sub.3.

[0035] FIG. 5 is a calibration curve showing the output current for a sensor according to FIG. 2) with a filter of 25 mg MnO.sub.2, 99.9% purity, mixed with 225 mg PTFE, i.e. 10% by weight of MnO.sub.2 at different concentrations of NO.sub.2.

[0036] FIG. 6 illustrates cross sensitivity to NO over time since construction for (a) a sensor according to FIG. 2(2) with a filter of 450 mg MnO.sub.2, 99.9% purity, i.e. 100% by weight of MnO.sub.2, and (b) a sensor according to FIG. 2(2) with a filter of 25 mg MnO.sub.2, 99.9% purity, mixed with 225 mg PTFE, i.e. 10% by weight of MnO.sub.2.

[0037] FIG. 7 shows the current response to 0.5 ppm O.sub.3 of (a) an unfiltered sensor according to FIG. 2(1), and (b) a sensor according to FIG. 2(2) with a filter of 25 mg MnO.sub.2, 99.9% purity, mixed with 225 mg PTFE, i.e. 10% by weight of MnO.sub.2, 170 days after construction of the sensors.

[0038] FIG. 8 shows the current response to NO.sub.2 of a sensor according to FIG. 2(2) with a filter of 25 mg MnO.sub.2, 99.9% purity, mixed with 22 5mg PTFE, i.e. 10% by weight of MnO.sub.2, 170 days after construction of the sensor.

[0039] FIG. 9 is a photograph of PTFE particles (sieved to have a size of at least 700 microns) coated with MnO.sub.2 crystals, 99.9% purity.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0040] FIG. 1 is a schematic cross-sectional view of a gas sensing apparatus (10) in which the first (11) and second (12) working electrodes share a common counter electrode (13), a common reference electrode (14) and a common body or reservoir of electrolyte, mainly held in wetting filters (15).

[0041] The gas sensing apparatus comprises a housing (16) which has two inlets (17) and (18), which place the first working electrode (11) and the O.sub.3 filter (19) in direct communication with the sample gas (e.g. the atmosphere) in parallel. Inlets (17) and (18) can be capillary inlets, i.e. inlets which are sized so that they control the rate of sample gas reaching the electrodes so that the gas sensing apparatus is diffusion limited, A central portion (20) divides a cavity defined by the housing (16) and a porous membrane (21) into first (22) and second (23) internal chambers. The first and second working electrodes are located in the same horizontal plane and are situated underneath the porous membrane (21). The first working electrode (11) is situated underneath the first internal chamber (22) and the second working electrode (12) is situated underneath the second internal chamber (23). The ozone filter (19) is located in the second internal chamber (23), adjacent/on top of part of the porous membrane (21) covering the second working electrode (12). The ozone filter (19) covers the surface of the second working electrode (12) that would, absent the filter, be exposed to the sample gas. A series of circular separator discs, wicks or wetting filters (15) separate the first (11) and second (12) working electrodes from the counter electrode (13) and the reference electrode (14).

[0042] The circular separator discs, wicks or wetting filters (15) are made of a hydrophilic, non-conductive material permeable to the electrolyte which functions to transport electrolyte by capillary action. Typically the material is glass fiber. The circular separator discs, wicks or wetting filters (15) serve to ensure that each of the electrodes is in contact with the electrolyte.

[0043] The first and second working electrodes are carbon electrodes and typically comprise a catalytic layer of carbon mixed with polytetrafluoroethylene (PTFE) binder which is then bonded to a gas porous, but hydrophobic, PTFE support to allow gas support to the catalyst, i.e. the electrode active material, but avoid electrolyte leakage or flooding of the electrode. The carbon electrodes can be manufactured using common conventional technologies such as pressing, screen printing, inkjeting and spraying a carbon slurry onto a porous membrane. Here the working electrode catalyst will typically have a diameter of 19 mm. A mixture of carbon and microparticulate polytetrafluoroethylene (PTFE) is sintered and is preferably prepared by pressing the resulting mixture onto a support in the form of a gas porous membrane, such as PTFE sheet. Where carbon is pressed at the normal pressure used in the field, i.e. around 200 kg/cm.sup.2, the amount of catalyst is preferably between 5 and 30 mg per cm.sup.2 of electrode surface area. Preferably the binder is a Fluon matrix (Fluon is a Trade Mark) of around 0.002 ml per cm.sup.2. Other electrodes used in the gas sensing apparatus of the invention, such as the Platinum black electrodes can be prepared in a similar way.

[0044] An O ring is located at the top of the porous membrane (21) and acts to seal the sensor and to aid compressing the stack of components when the sensor is sealed. Also present are a number of platinum strips that serve to connect each electrode to one of the terminal pins (24) provided at the base of the sensor. Closing housing is a dust filter (25) to prevent dust and other foreign objects from entering.

[0045] FIG. 2 provides schematic cross-sectional views of two individual gas sensors, each having its own working electrode, counter electrode, reference electrode, additional electrode and electrolyte. Together, these form a gas sensing apparatus (30) according to the invention. These gas sensors have similar stacked structures.

[0046] FIG. 2(1) shows a gas sensor with first working electrode (31) and its associated counter electrode (33), reference electrode (34), electrolyte and additional working electrode (35) for correcting for baseline drift. An internal chamber (42) is defined by the housing (36) and a porous membrane (41). Housing (36) has one inlet (37) which places the first working electrode (31) in direct communication with the sample gas (e.g. the atmosphere).

[0047] FIG. 2(2) shows a gas sensor of similar construction except for the presence of an ozone filter (59) in the internal chamber (52) above the second working electrode (51). The second counter electrode (53), second reference electrode (54), electrolyte and second additional working electrode (55) for correcting for baseline drift are labelled on the figure.

[0048] In an example, the filter was made up from 10% by weight of Manganese (IV) oxide, >99.9% purity, approx. 325 mesh (44 micron) powder (Product No. 42250 from Alpha Aesar), mixed with 90% by weight of PTFE binder particles, with a particle size of 700 to 1000 microns. The PTFE binder particles were obtained by sieving Fluon PTFE G307 (Fluon is a trade mark) median particle size 500 to 1000 microns, to collect only particles with a size of at least 700 microns. FIG. 9 is a photograph of the coated particles. BET analysis of the MnO.sub.2 gave a result of 2.08 m.sup.2/g.

[0049] Example circuitry for the gas sensing apparatus of the invention in the embodiment of FIG. 1 is shown in FIG. 3, where WE1 is the first working electrode and WE2 is the second working electrode. This circuitry could also be used for either of the individual sensors making up the gas sensing apparatus of the invention in the embodiment of FIG. 2, where, for example, WE1 is the first working electrode and WE2 is the first additional working electrode, or WE1 is the second working electrode and WE2 is the second additional working electrode.

Experimental Section

[0050] The sensors used in the following examples were tested on standard potentiostatic circuit boards (FIG. 3). Generally, the sensors were stabilised for a minimum of 2 days before being tested. All the experiments involving gas tests were controlled using computer controlled valves and digital mass flow controllers. Sensor output data collection is also made using a computer. The NO.sub.2 gas tests were made using a certified 100 ppm bottle (Air Products, UK) and filtered air. The ozone was obtained using a calibrated generator (Ultra-violet Products Ltd, SOG-1, Cambridge, UK). During the laboratory tests the sensors were exposed to a gas flow of 0.51.min.sup.1.

[0051] The following material was used as filter powder: MnO.sub.2 powder, purchased from Alpha Aesar, approx. 325 mesh size (i.e. a particle size of about 44 microns), analytical grade 99.9%), product code 310700.

[0052] Materials used to make the electrodes include carbon graphite (particle size <20 m, Aldrich, product code 282863), high purity single-walled carbon nanotubes, graphene (Graphene GNP COOH-functionalised, Gwent Electronic Materials Ltd), glassy carbon (Carbon, glassy, spherical powder, 2-12 micron, 99.95%, Aldrich, product code 484164).

[0053] Calibration of the sensors according to the invention was carried out as follows. The sensor output is given in nA. Amperometric gas sensors have a linear output with target gas analyte concentration. This makes possible to use a simple calibration procedure where the relation between the sensor output and the gas concentration is determined by exposing the sensor to a known concentration of gas analyte. For this application the sensitivity to both the first and the second working electrodes is determined for each of the target gases, NO.sub.2 and O.sub.3. The sensor output can then be used to calculate the concentration of NO.sub.2 and O.sub.3.

[0054] If the following parameters are defined as follows: [0055] i.sub.1 is the current observed on the first working electrode. [0056] I.sub.2 is the current observed on the second working electrode. [0057] S.sub.1(NO2) is the first working electrode sensitivity to NO.sub.2. [0058] S.sub.1(O3) is the first working electrode sensitivity to O.sub.3. [0059] S.sub.2(NO2) is the second working electrode sensitivity to NO.sub.2. [0060] S.sub.2(O3) is the second working electrode sensitivity to O.sub.3. [0061] C.sub.(NO2) is the NO.sub.2 analyte concentration to determine. [0062] C.sub.(O3) is the O.sub.3 analyte concentration to determine. Then, by definition and taking into account the linear relationship between the sensor output and the analyte concentration, the following can be written:

i.sub.1=S.sub.1(NO2).Math.C.sub.(NO2)+S.sub.1(O3).Math.C.sub.(O3)

i.sub.2=S.sub.2(NO2).Math.C.sub.(NO2)+S.sub.2(O3).Math.C.sub.(O3)

[0063] The ozone filter on top of the second working electrode means that S.sub.2(O3)=0. So C.sub.(NO2) can be calculated using the simple relation:

C.sub.(NO2)=i.sub.2/S.sub.2(NO2)

[0064] The NO.sub.2 analyte concentration being now known it is then possible to calculate the O.sub.3 analyte concentration using the first working electrode output:

C.sub.(O3)=(i.sub.1S.sub.1(NO2).Math.C.sub.(NO2))/S.sub.1(O3)

or

C.sub.(O3)=(i.sub.1S.sub.1(NO2).Math.(i.sub.2/S.sub.2(NO2)))/S.sub.1(O3)

Experiment 1

[0065] In this example, a sensor according to FIG. 2(1) (no filter) and a sensor according to FIG. 2(2) in which the filter (59) in the chamber (52) was formed with 25 mg of particulate MnO.sub.2, 99.9% purity, mixed with 225 mg PTFE, i.e. 10% by weight of MnO.sub.2. The PTFE was in the form of particles with a size of about 750 microns. The unfiltered sensor is commercially available under the trade name OX-A421, manufactured and sold by Alphasense Limited of Great Dunmow. For these sensors the first working electrode, the second working electrode, the additional electrode and the reference electrode are made of carbon graphite and the counter electrode is made of platinum black.

[0066] FIG. 4 illustrates the output current, over time, where the unfiltered sensor (trace (a)) and filtered sensor (trace (b)) are exposed in turn to zero air (zero air is air that has been filtered to remove most gases (NO.sub.2, NO, CO, SO.sub.2, O.sub.3, etc.) and is used as a calibrating gas for zero concentration), then 2 ppm NO.sub.2, 2 ppm O.sub.3 and then a mixture of 2 ppm NO.sub.2 and 2 ppm O.sub.3. This figure shows that the filtered sensor senses only NO.sub.2, whereas the unfiltered sensor detects both gases and, when in the presence of both gases simultaneously, the output of the sensor corresponds to the sum of the output expected for each of the gases.

Experiment 2

[0067] Filtered sensors as described above for experiment 1 were exposed to each of NO, SO.sub.2, CO, H.sub.2, CO.sub.2 and NH.sub.3, at the concentration specified in Table 1 below, for 10 minutes. These gases are the most common possible interferents in the air. In this experiment, the cross sensitivity values for the sensors are determined (the values given in % relative to the signal obtained for NO.sub.2). The results are listed in Table 1.

TABLE-US-00001 TABLE 1 Gas NO SO.sub.2 CO H.sub.2 CO.sub.2 NH.sub.3 Gas concentration (ppm) 5 5 5 100 50000 20 Cross-sensitivity (%) 3 1 0.5 0.1 0 0.05

[0068] The set of data obtained shows that the unfiltered sensors are highly specific to NO.sub.2 and O.sub.3.

[0069] Hence in a complex environment where these different gases are present, the unfiltered sensor will give an output representative of the sum of the concentration of NO.sub.2 and O.sub.3.

Experiment 3

[0070] The response of a filtered sensor as described above with reference to Experiment 1, having a filter of 25 mg MnO.sub.2, 99.9% purity, mixed with 225 mg PTFE, to NO.sub.2 was measured across a range of 0 to 200 ppb NO.sub.2. FIG. 5 shows that there is a good linearity of output current versus NO.sub.2 concentration even at the low concentrations used.

Experiment 4

[0071] Cross sensitivity to NO (i.e. the ratio of the current generated when a sensor is exposed to NO to the current generated when a sensor is exposed to a corresponding concentration of NO.sub.2 is an important performance characteristic of an NO.sub.2 sensor.

[0072] FIG. 6 illustrates cross sensitivity to NO over time since construction for (a) a sensor according to FIG. 2(2) with a filter of 450 mg MnO.sub.2, 99.9% purity, i.e. 100% by weight of MnO.sub.2, and (b) a sensor according to FIG. 2(2) with a filter of 25 mg MnO.sub.2, 99.9% purity, mixed with 225 mg PTFE, i.e. 10% by weight of MnO.sub.2. The cross sensitivity was calculated by measuring the current at 2 ppm NO and 2 ppm NO.sub.2 and calculating the ratio of the currents.

[0073] It can be seen that the NO cross-sensitivity is significantly lower when the filter uses MnO.sub.2 powder which has been diluted in binder, in this case PTFE, then when pure MnO.sub.2 is used as the filter. We have found that if the filter is diluted to as low as 1% by weight of MnO.sub.2, the quality of the signal drops off. However, from about 2% upwards to about 15% gives good results.

Experiment 5

[0074] FIG. 7 shows the current response to 0.5 ppm O.sub.3 of (a) an unfiltered sensor according to FIG. 2(1), and (b) a sensor according to FIG. 2(2) with a filter of 25 mg MnO.sub.2, 99.9% purity, mixed with 225 mg PTFE, i.e. 10% by weight of MnO.sub.2, 170 days after construction of the sensors.

[0075] This figure shows that the filtered sensor with 10% by weight of MnO.sub.2 gives a lasting ozone filtering effectthere is no response at all to ozone for a 170 day old sensor.

Experiment 6

[0076] FIG. 8 shows the current response to NO.sub.2 of a sensor according to FIG. 2(2) with a filter of 25 mg MnO.sub.2, 99.9% purity, mixed with 225 mg PTFE, i.e. 10% by weight of MnO.sub.2, 170 days after construction of the sensor.

[0077] These results show that the sensor continues to let NO.sub.2 through and so can be used to measure NO.sub.2, while scrubbing O.sub.3, after an extended period of time.

Conclusions

[0078] We have found that providing sensing apparatus having a first unfiltered carbon electrode and a second carbon electrode having an ozone filter comprising MnO.sub.2 particles diluted in binder provides a stable sensor capable of discriminating between NO.sub.2 and O.sub.3 and therefore of measuring either or both, and having a low cross-sensitivity to NO.

[0079] We have also found it preferable for the amount of PTFE microparticles to be sufficient to fill the internal chamber (42) of the sensor. If the internal chamber was only partially filled with a filter powder, there would be a risk that the powder could move when the sensor is moved, possibly opening a pathway for the gas directly to the electrode without passing through the filter material. PTFE microparticles have the advantage of easily becoming coated with the MnO.sub.2 microparticles and the mixing results in a well homogeneous PTFE microparticles/MnO.sub.2 microparticles filter material.