WO2010013276A1 - Three electrode electrochemical amperometric sensor - Google Patents

Three electrode electrochemical amperometric sensor Download PDF

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Publication number
WO2010013276A1
WO2010013276A1 PCT/IT2009/000334 IT2009000334W WO2010013276A1 WO 2010013276 A1 WO2010013276 A1 WO 2010013276A1 IT 2009000334 W IT2009000334 W IT 2009000334W WO 2010013276 A1 WO2010013276 A1 WO 2010013276A1
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WIPO (PCT)
Prior art keywords
electrode
nafion
support
counter
paste
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PCT/IT2009/000334
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French (fr)
Inventor
Santina Rocchi
Ada Fort
Valerio Vignoll
Marco Mugnani
Roberto Palombari
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Universita' Degli Studi Dl Siena
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Publication of WO2010013276A1 publication Critical patent/WO2010013276A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/403—Cells and electrode assemblies
    • G01N27/404—Cells with anode, cathode and cell electrolyte on the same side of a permeable membrane which separates them from the sample fluid, e.g. Clark-type oxygen sensors
    • G01N27/4045—Cells with anode, cathode and cell electrolyte on the same side of a permeable membrane which separates them from the sample fluid, e.g. Clark-type oxygen sensors for gases other than oxygen
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/0004—Gaseous mixtures, e.g. polluted air
    • G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
    • G01N33/0027—General constructional details of gas analysers, e.g. portable test equipment concerning the detector
    • G01N33/0036—General constructional details of gas analysers, e.g. portable test equipment concerning the detector specially adapted to detect a particular component
    • G01N33/0037—NOx
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
    • Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Definitions

  • the present invention relates to a three electrode electrochemical amperometric sensor of graphite-Nafion®-molybdenum disulphide (C- Nafion®-MoS 2 ) that allows to detect, in an extremely selective, highly sensitive, stable in time, reliable, fast and simple manner, nitro derivatives such as nitric oxide and nitrogen dioxide (NO and NO2).
  • C- Nafion®-MoS 2 graphite-Nafion®-molybdenum disulphide
  • the present invention also relates to a system for measuring nitro derivatives using such sensor and providing optimal performance in terms of repeatability.
  • the international industrial system starts to ask for reliable, fast and portable measuring systems for maintaining dangerous environmental emissions under control such as nitro derivatives as combustion products, i.e. exhaust gases with high content of nitric oxide NO and nitrogen dioxide NO 2 .
  • MOS sensors and electrochemical sensors have been widely studied, as described by:
  • potentiometric, amperometric and impedancemetric sensors have been proposed with a variety of materials for the solid electrolyte (usually polymeric or ceramic ionic conductors, depending on the application) and for the electrodes, as well as with a variety of electrode geometries, as described by:
  • Table 1 shows some of the commonly used sensors, where the sensor with reference [1] has been described by F.Opekar and K.Stulik in “Electrochemical sensors with solid polymer electrolytes", Analytica Chimica Acta, Vol. 385, 1999, pp. 151-162, the one with reference [2] by P. Hmcirova, F. Opekar and K. Stuhk in "An amperometric solid-state NO sensor with a solid polymer electrolyte and a reticulated vitreous carbon indicator electrode", Sensors and Actuators B, Vol. 69, 2000, pp. 199-204, the one with reference [3] by F.Opekar and K.
  • the senor with reference [1] shows a poor long- and medium-term sensitivity of the response two to the choice of NiO as counter electrode material, especially when such sensor is used in the two electrodes configuration, as also demonstrated by Nernst equations corresponding to the reactions taking place at the counter electrode in the two cases.
  • a three electrode electrochemical amperometric sensor in particular for detecting nitro- derivatives, comprising a working electrode, a counter-electrode and a reference electrode, characterised in that the counter-electrode and the reference electrode are made of a first material comprising or consisting of molybdenum disulphide MoS 2 , and in that the working electrode, the counter-electrode and the reference electrode are attached to an electrically insulating polymer proton layer or PPL made of a second material comprising or consisting of Nafion®.
  • the first material may comprise molybdenum disulphide MoS 2 and diluted Nafion®, preferably with a . dilution of Nafion® ranging from 1% to 15%, more preferably equal to
  • the second material may comprise or consist of Nafion® with a dilution ranging from 5% to 20%, preferably from 10% to 20%, more preferably equal to 20%, in weight (w/w) in a liquid selected from the group comprising water and n-propanol.
  • the second material may comprise a glass fibre wetted with Nafion®.
  • the working electrode may be made of a third material comprising or consisting of Nafion®, preferably with a dilution ranging from 0,1% to 3%, more preferably equal to 2,5%, in weight (w/w) in a liquid selected from the group comprising water and n- propanol.
  • the third material may comprise carbon C, preferably graphite.
  • the senor may further comprise a support, preferably provided with at least three through holes,
  • the counter-electrode and the reference electrode, the PPL layer, and the working electrode are arranged on a surface of which support, the support being made of a fourth, insulating and inert, material, preferably comprising one or more materials selected from the group comprising FR4 and ceramic compounds.
  • the sensor may further comprise at least one respective rheophore connected to each one of the following: the working electrode, the counter-electrode and the reference electrode.
  • At least one rheophore may be connected to the working electrode through silver glue.
  • step B may comprise:
  • step C may comprise:
  • step D may comprise: - mixing graphite with diluted Nafion® for obtaining a paste, and
  • step B may comprise: - deposing a mask on said first surface of the support, shaped so as to have two uncovered zones of said first surface spaced apart from each other, each one in correspondence of at least one respective through hole, and spaced from a perimeter of the support, - mixing molybdenum disulphide MoS 2 with diluted Nafion® for obtaining a paste mixture,
  • a rotating distributing apparatus or spinner, preferably at a speed of 1000 rpm and 1500 rpm for 30 seconds and 60 seconds, respectively, so that said paste mixture uniformly spreads over said two uncovered zones,
  • step C may comprise:
  • step D may comprise: - mixing graphite with diluted Nafion® for obtaining a paste,
  • the process may further comprise, preferably before step B, the following step:
  • the amperometric sensor according to the invention has an optimal sensitivity to nitrogen dioxide NO 2 and an extremely low cross-sensitivity to carbon monoxide CO and oxygen O 2 . Moreover, the sensor according to the invention is capable to operate in the. range from 20 0 C to 80 0 C.
  • the sensor according to the invention is usable in a measuring system that is extremely flexible, ensuring optimal performance in terms of repeatability.
  • Figure 1 shows a front view (Fig. 1a), a top plan view wherein inner elements are shown in dashed lines (Fig. 1b), and a side view (Fig. 1c) of the amperometric sensor according to the invention;
  • Figure 2 shows a perspective view (Fig. 2a) and a cross-section view, not to scale, combined according two planes passing through the working electrode and, respectively, through the counter electrode and the reference electrode (Fig. 2b), of the amperometric sensor of Figure 1 ;
  • Figure 3 shows a mask used in a preferred embodiment of the manufacturing process according to the invention
  • Figure 4 shows the amperometric responses of the sensor of Figure
  • Figure 5 shows a comparison between the sensitivity of the sensor of Figure 1 used as two electrode sensor (Fig. 5a) and as three electrode sensor (Fig. 5b);
  • Figure 6 shows a voltammogram of the sensor of Figure 1 exposed to three different concentrations of nitrogen carrier gas and with a total flow of 200 ml/min;
  • Figure 7 shows a measuring system using the sensor of Figure 1.
  • identical reference numbers are used for alike elements.
  • the inventors have developed a novel and inventive, although simple, amperometric sensor consisting of liquid Nafion® (usually used for protonic exchange membranes or PEMs), a working electrode of graphite, and a counter electrode and a reference electrode of molybdenum disulphide M0S 2 (also called as molybdenite).
  • liquid Nafion® usually used for protonic exchange membranes or PEMs
  • PEMs protonic exchange membranes
  • M0S 2 also called as molybdenite
  • the choice of the ions of molybdenum disulphide MoS 2 is capable to ensure a long-term stability to the sensor.
  • the amperometric sensor according to the invention along with a chemical sampling system and a front-end electronics, shows good characteristics in terms of response repeatability, sensitivity, response time and cross-sensitivity to interfering gases, even with biasing equal to 0
  • a working electrode 1 is made of graphite (C) and diluted Nafion®, preferably with a dilution of Nafion® ranging from 0,1 % up to 3% in weight (w/w) in a liquid, preferably water or n-propanol;
  • a counter electrode 2 and a reference electrode 3 are made of molybdenum disulphide M0S 2 and diluted Nafion®, preferably with a dilution of Nafion® ranging from 1% to 15% in weight (w/w) in a liquid, preferably water or n-propanol;
  • the working electrode 1, the counter electrode 2 and the reference electrode 3 are attached to a polymer proton layer or PPL, electrically insulating, shaped as a disc 4 made of Nafion® at a reference concentration (as proposed by the provider Aldrich), preferably with a dilution ranging from 5% up to 20%, more preferably from 10% to 20%, in weight (w/w) in a liquid, preferably water or n-propanol.
  • a reference concentration as proposed by the provider Aldrich
  • Geometry and size of the sensor may vary, preferably satisfying proportions suggested for the preferred embodiment illustrated in the
  • the rheophores 1', 2' and 3' are coupled to the respective electrodes 1, 2 and 3 through soldering in suitable through holes of an electrically insulating support 5.
  • the rheophore 1' may be applied externally to the support 5 and coupled to the working electrode 1 of graphite through a sort of conductive clip.
  • the quantities of carbon C and molybdenum disulphide M0S 2 used for making the electrodes may vary depending on the application field and on the required response sensitivity and speed.
  • the sensor may be made using different approaches. In the following two manufacturing methods are described which represent a good compromise among repeatability of obtained sensors, measurement results, and manufacturing ease.
  • a first embodiment of the manufacturing process provides that an electrically insulating support 5 is made of an inert and insulating material, such as FR4 or ceramic compounds, that is shaped as a disc.
  • the circular support 5 has diameter preferably equal to 1.2 mm and it is provided with three through holes, geometrically uniformly distributed along the circumference of the circular support 5 (i.e. on the vertices of an equilateral triangle inscribed in the circle of the support 5), for allowing rheophores 1', 2' and 3' to be soldered.
  • the counter electrode 2 and the reference electrode 3 are made directly on the insulating support 5.
  • M0S 2 IV (according to the nomenclature of the provider Aldrich) are mixed with 0,5 ml of Nafion® diluted at 10% in weight (w/w) for obtaining a paste. This may be then pressed with a nominal force of 40 kN/cm 2 for obtaining a pellet of about 10 mm diameter and 0,15 mg weight. The pellet is then split in two halves, which are used for making (sequentially or simultaneously) the counter electrode 2 and the reference electrode 3, respectively. In particular, the contacts of the two halves of the pellet with the respective metal electrodes 2' and 3' are mechanical.
  • a glass fibre wetted with Nafion® preferably with dilution ranging from 10% to 20% in weight (w/w) is smoothly pressed on the other side of the two halves of the pellet, just for allowing the Nafion® polymer to guarantee mechanical stability between the glass fibre and he half of the pellet. Insulation between the counter electrode 2 and the reference electrode 3 must be guaranteed, otherwise the sensor operates as a two electrode structure.
  • the working electrode 1 is made using 20 mg of graphite mixed with 0,5 ml of Nafion® diluted up to 2,5% in weight (w/w) in n-propanol for obtaining a paste tha t is deposed on the glass fibre already used in making the PPL layer 4 over the counter electrode 2 and reference electrode 3.
  • the quantity of graphite may vary depending on the viscosity that is desired to obtain.
  • the metal contact with the working rheophore 1' may be made with silver glue.
  • a second embodiment of the manufacturing process provides that the electrically insulating supp ort 5 is always made with an inert and insulating material, such as FR4 or ceramic compounds, that is shaped as a disc.
  • the circular support 5 has diameter preferably equal to 1.2 mm and it is provided with three through holes, geometrically uniformly distributed along the circumference of the circular support 5 (i.e. on the vertices of an equilateral triangle inscribed in the circle of the support 5), for allowing rheophores V 1 2' and 3' to be soldered.
  • a mask 6 is temporary deposed on the support 5 in order to create an insulating barrier between the counter electrode 2 and the reference electrode 3 and for avoiding to have parts of the counter electrode 2 on the periphery of the support 5, as shown in Figures 1 and 2.
  • the mask 6 allows to deploy the solutions of the counter electrode 2 and reference electrode 3 (described in the following) guaranteeing the electrical contacts with the rheophores and the insulation among them. Once that the counter electrode 2 and the reference electrode 3 have been deposed and dried, the mask 6 is removed.
  • the counter electrode 2 and the reference electrode 3 are simultaneously made on the insulating support 5, on which the rheophores V, 2' and 3' have been already soldered, with the mask 6 being applied.
  • 50 mg of M0S 2 IV (according to the nomenclature of the provider Aldrich) are mixed with 30 ⁇ l of Nafion® diluted at 10% in weight (w/w) in n-propanol for obtaining a paste mixture.
  • This is deposed on the support 5 on the two areas corresponding to the counter electrode 2 and to the reference electrode 3, preferably up to completely cover the surface of the support 5 not covered by the mask 6 (about 10 ⁇ l are sufficient).
  • the circular support 5 is then spun on a spinner, at the speed of 1000 rpm and 1500 rpm for 30 seconds and 60 seconds, respectively.
  • the viscosity of the mixture must be such that the latter is sufficiently liquid to uniformly spread on the support 5 and sufficiently solid to remain on the same without squirting out.
  • the whole is allowed to dry at room temperature for 1 hour.
  • the PPL layer 4 is made.
  • the mask 6 is removed and some drops of Nafion® (preferably 60 ⁇ l) diluted at 20% in weight (w/w) are deposed up to cover, preferably completely (but alternatively also only a part of the surface to cover may be interested, exploiting the subsequent distribution operated by the spinner), the surface of the support 5 and of the counter electrode 2 and reference electrode 3 present on the same.
  • the support 5 is then spun again for 60 seconds at 1500 rpm, and the whole is dried at room temperature for 1 hour.
  • the working electrode 1 is made using 20 mg of graphite mixed with 0,5 ml of Nafion® diluted up to 2,5% in weight (w/w) in n-propanol for obtaining a paste that is deposed on the PPL layer 4 and spun at 1500 rpm for 60 seconds.
  • the electrical contact with the respective rheophore 1' is made with silver glue.
  • Figure 4 shows some preliminary measurements on the device, namely: the amperometric responses of the sensor of Figure 1 used as two electrode sensor made at different concentrations of NO 2 carried out at 25 0 C and with a total flow of 200 ml/min with 100% of relative humidity (Fig. 4a); the same measurements repeated with the sensor of Figure 1 used as three electrode sensor.
  • Figure 5 shows a comparison between the sensitivity of the sensor of Figure 1 used as two electrode sensor (Fig. 5a) and as three electrode sensor (Fig. 5b).
  • Figure 6 shows a voltammogram of the sensor of Figure 1 exposed to three different concentrations of nitrogen carrier gas and with a total flow of 200 ml/min.
  • a measuring system using the sensor according to the invention comprises two (or more) gas tanks 10 and 10' feeding two respective flow-meters 11 and 11', e.g. the BronkHorst F-201C meters, controlled by a processing unit 12, preferably a computer.
  • a processing unit 12 preferably a computer.
  • Each controlled flow composed of the carrier gas and the mixture under measurement, passes through a respective Drechsel bottle 13 and 13' for enriching the gas with H 2 O, filled with water and placed in a thermostatic bath 14 the temperature of which is fixed, with 1°C accuracy.
  • each flow pa sses through a respective three-way solenoid valve 15 and 15'.
  • the valves 15 and 15' which are also controlled by the computer 12, allow to switch each flow towards a measurement chamber 16 or alternatively external ambient.
  • the presence of the valves 15 and 15' allow to reduce as much as possible the fluid- dynamic transient due to variations of the adjustments of the flow-meters 11 and 11'.
  • the measurement chamber 16 is place in an oven maintained at a reference temperature (with accuracy equal to at least 1°C).
  • the reference temperature at which the sensor has been maintained is equal to 40 0 C, whereas the temperature of the thermostatic bath 14 is equal to 25°C.
  • the relative humidity is measured by a humidity sensor, e.g. the Humirel HTS2330 sensor, housed inside the chamber 16.
  • the developed measuring system is composed of the sensor, a dedicated front-end electronics 17 (preferably a I-V converter), and an acquisition and processing system controlled by the computer 12.
  • the system allows to apply a voltage difference across the electrodes in the range from -1V to +1V, with an accuracy equal to 1 mV.
  • the system allows the control of both the flow-meters 11 and 11' and the solenoid valves 15 and 15', and it allows to measure both relative humidity, preferably with an accuracy of 2%, and temperature, preferably through a thermocouple with an accuracy of 0,1 0 C within the measurement chamber 16.

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Abstract

The present invention relates to a three electrode electrochemical amperometric sensor, in particular for detecting nitro-derivatives, comprising a working electrode (1), a counter-electrode (2) and a reference electrode (3), characterised in that the counter-electrode (2) and the reference electrode (3) are made of a first material comprising or consisting of molybdenum disulphide MoS2, and in that the working electrode (1), the counter-electrode (2) and the reference electrode (3) are attached to an electrically insulating polymer proton layer (4) or PPL made of a second material comprising or consisting of Nafion®. The present invention further relates to a process for manufacturing such sensor.

Description

THREE ELECTRODE ELECTROCHEMICAL AMPEROMETRIC SENSOR
The present invention relates to a three electrode electrochemical amperometric sensor of graphite-Nafion®-molybdenum disulphide (C- Nafion®-MoS2) that allows to detect, in an extremely selective, highly sensitive, stable in time, reliable, fast and simple manner, nitro derivatives such as nitric oxide and nitrogen dioxide (NO and NO2).
The present invention also relates to a system for measuring nitro derivatives using such sensor and providing optimal performance in terms of repeatability.
The international industrial system starts to ask for reliable, fast and portable measuring systems for maintaining dangerous environmental emissions under control such as nitro derivatives as combustion products, i.e. exhaust gases with high content of nitric oxide NO and nitrogen dioxide NO2.
Moreover, such chemical compounds are also found in commonly used foods, like extra virgin olive oil, and their quantity may allow to determine not only the origin of the product but also some food health or possible adulteration properties. For high temperature applications, detecting systems based on
MOS sensors and electrochemical sensors have been widely studied, as described by:
- P. -G. Su, Wu Ren-Jang, Nieh Fang-Pei "Detection of nitrogen dioxide using mixed tungsten oxide-based thick film semiconductor sensor", Talanta Vol. 59 (2003) pp. 667-672;
- Yoshiteru Itagaki, Masami Mori, Yuuki Hosoya, Hiromichi Aono, Yoshihiko Sadaoka "03 and NO2 sensing properties of SmFeI -xCoxO3 perovskite oxides", Sensors and Actuators B Vol.122 (2007) pp. 315-320; - A. Forleo, L. Francioso, M. Epifani, S. Capone, A.M. Taurino, P.
Siciliano "NO2-gas-sensing properties of mixed ln2O3-SnO2 thin films", Thin Solid Films Vol. 490 (2005) pp. 68 - 73;
- Zhuiykov, S., Miura, N., "Development of zirconia-based potenziometric NOX sensors for automotive and energy industries in the early 21st century: What are the prospects for sensors?,"
Sensors and Actuators B, Vol. 121 (2007), pp. 639-651 ;
- Jiun-Chan Yang and Prabir K. Dutta. "High temperature amperometric total NOx sensors with platinum-loaded zeolite Y electrodes", Sensors and Actuators B Chemical, Vol. 123 (2007), pp. 929-936;
- Norio Miura, Geyu Lu, Masaki Ono, Noboru Yamazoe "Selective detection of NO by using an amperometric sensor based on stabilized zirconia and oxide electrode" Solid State Ionics Vol. 117 (1999), pp. 283-290;
- Norio Miura, Geyu Lu, Noboru Yamazo "High temperature potentiometric: amperometric NOx sensors combining stabilized zirconia with mixed-metal oxide electrode", Sensors and Actuators
B VoI. 52 (1998), pp. 169-178;
- Norio Miura , Masaki Ono, Kengo Shimanoe, Noboru Yamazoe "A compact solid-state amperometric sensor for detection of NO2 in ppb range", Sensors and Actuators B Vol. 49 (1998), pp. 101-109; and
- Fergus, J. W., "Materials for high temperature electrochemical NOx gas sensors", Sensors and Actuators B, Vol. 121 (2007), pp. 652- 663.
Differently, detecting systems for low temperature applications are not yet widespread .
Several problems are still unsolved for both the application fields, and im plementation of d evices measuring nitro derivatives NO x which ensure good performance is still at research level. In particular, semiconductor sensors generally have high sensitivity but low selectivity, and hence cross-sensitivity remains an unsolved problem.
As far as electrochemical sensors are concerned, potentiometric, amperometric and impedancemetric sensors have been proposed with a variety of materials for the solid electrolyte (usually polymeric or ceramic ionic conductors, depending on the application) and for the electrodes, as well as with a variety of electrode geometries, as described by:
- Opekar, F., Stulik, K., "Electrochemical sensors with solid polymer electrolytes", Analytica Chimica Acta, Vol. 385 (1999), pp. 151-162. Jing-Shan Do, Po-Jen Chen "Amperometric sensor array for NOx, CO, O2 and SO2 detection", Sensors and Actuators B Vol. 122 (2007), pp. 165-173;
- Palombari, R., Pierri, F. , "Ni(III) doped NiO as the electrode material for electrochemical devices employing protonic conductors", Journal of Electroanalytical Chemistry, Vol. 433 (1997), pp. 213-217;
- G. Alberti, F. Cherubini, R. Palombari "Amperometric solid-state sensor for NO and NO2 based on protonic conduction", Sensors and Actuators B Vol. 52 (1998), pp. 169-178;
- Kuo Chuan HO Wen Tung Hung "An amperometric NO2 gas sensor based on Pt/Nafion electrode", Sensors and Actuators B Vol. 79 (2001), pp. 11-16;
- Jing-Shan Do, Wen-Biing Chang "Amperometric nitrogen dioxide gas sensor based on PAn/Au/Nafion® prepared by constant current and cyclic voltammetry methods", Sensors and Actuators B Vol. 101 (2004), pp. 97-106;
- Hmcirova, P., Opekar.F., Stuhk, K., "An amperometric solid-state NO sensor with a solid polymer electrolyte and a reticulated vitreous carbon indicator electrode", Sensors and Actuators B, Vol.
69 (2000), pp. 199-204;
- Yoshitaka Mizutania, Hiroyuki Matsuda, Tom Ishiji, Nagakazu Furuya, Katsuo Takahashi "Improvement of electrochemical NO2 sensor by use of carbon-fluorocarbon gas permeable electrode", Sensors and Actuators B Vol. 108 (2005), pp. 815-819; and
- Chunbo Yu, Yujiang Wang, Kaifeng Hua, Wei Xing, Tianhong Lu "Electrochemical H2S sensor with H2SO4 pre-treated Nafion membrane as solid polymer electrolyte", Sensors and Actuators B, Vol. 86, (2002), pp. 259-265. Most of these sensors ensure a good sensitivity to derivatives NOχ and they are capable to operate at and withstand very high temperatures, such as those of exhaust gases, but they often show a non-negligible cross-sensitivity to other combustion products, such as CO, and to other gases including oxygen and water vapour, which interfere with the detection. In this regard, Table 1 shows some of the commonly used sensors, where the sensor with reference [1] has been described by F.Opekar and K.Stulik in "Electrochemical sensors with solid polymer electrolytes", Analytica Chimica Acta, Vol. 385, 1999, pp. 151-162, the one with reference [2] by P. Hmcirova, F. Opekar and K. Stuhk in "An amperometric solid-state NO sensor with a solid polymer electrolyte and a reticulated vitreous carbon indicator electrode", Sensors and Actuators B, Vol. 69, 2000, pp. 199-204, the one with reference [3] by F.Opekar and K. Stulik in "Electrochemical sensors with solid polymer electrolytes", Analytica Chimica Acta, Vol. 385, 1999, pp. 151-162, and the one with reference [4] by R. Palombari and F. Pierri in "Ni(III) doped NiO as the electrode material for electrochemical devices employing protonic conductors", Journal of Electroanalytical Chemistry, Vol. 433, 1997, pp. 213-217.
Figure imgf000006_0001
Table 1
In particular, the sensor with reference [1] shows a poor long- and medium-term sensitivity of the response two to the choice of NiO as counter electrode material, especially when such sensor is used in the two electrodes configuration, as also demonstrated by Nernst equations corresponding to the reactions taking place at the counter electrode in the two cases.
With the exception of the simplest sensors which may be studied through Nernst approach, an exhaustive knowledge of the chemical reactions responsible for sensor behaviour is still lacking, and therefore the sensors must be experimentally characterised.
In this context, the solution proposed according to the present invention is introduced, allowing to minimise the aforementioned problems.
It is therefore an object of the present invention to allow to detect in an extremely selective, highly sensitive, stable in time, reliable, fast and simple manner, nitro derivatives such as nitric oxide NO and nitrogen • dioxide NO2.
It is specific subject matter of this invention a three electrode electrochemical amperometric sensor, in particular for detecting nitro- derivatives, comprising a working electrode, a counter-electrode and a reference electrode, characterised in that the counter-electrode and the reference electrode are made of a first material comprising or consisting of molybdenum disulphide MoS2, and in that the working electrode, the counter-electrode and the reference electrode are attached to an electrically insulating polymer proton layer or PPL made of a second material comprising or consisting of Nafion®.
Always according to the invention, the first material may comprise molybdenum disulphide MoS2 and diluted Nafion®, preferably with a . dilution of Nafion® ranging from 1% to 15%, more preferably equal to
10%, in weight (w/w) in a liquid selected from the group comprising water and n-propanol.
Still according to the invention, the second material may comprise or consist of Nafion® with a dilution ranging from 5% to 20%, preferably from 10% to 20%, more preferably equal to 20%, in weight (w/w) in a liquid selected from the group comprising water and n-propanol.
Furthermore according to the invention, the second material may comprise a glass fibre wetted with Nafion®.
Always according to the invention, the working electrode may be made of a third material comprising or consisting of Nafion®, preferably with a dilution ranging from 0,1% to 3%, more preferably equal to 2,5%, in weight (w/w) in a liquid selected from the group comprising water and n- propanol.
Still according to the invention, the third material may comprise carbon C, preferably graphite.
Furthermore according to the invention, the sensor may further comprise a support, preferably provided with at least three through holes,
. wherein the counter-electrode and the reference electrode, the PPL layer, and the working electrode are arranged on a surface of which support, the support being made of a fourth, insulating and inert, material, preferably comprising one or more materials selected from the group comprising FR4 and ceramic compounds. Always according to the invention, the sensor may further comprise at least one respective rheophore connected to each one of the following: the working electrode, the counter-electrode and the reference electrode.
Still according to the invention, at least one rheophore may be connected to the working electrode through silver glue.
It is still specific subject matter of this invention a process for manufacturing at least one electrochemical amperometric sensor as just described, characterised in that it comprises the following steps:
A. having a, preferably circular, support made in an insulating and inert material, provided with at least three through holes, preferably uniformly geometrically distributed,
B. making the counter-electrode and the reference electrode, spaced apart from each other, on a first surface of the support,
C. making the PPL layer over the counter-electrode, the reference electrode, and said first surface of the support, and
D. making the working electrode over the PPL layer.
Always according to the invention, step B may comprise:
- mixing molybdenum disulphide M0S2 with diluted Nafion® for obtaining a paste, - pressing said paste for obtaining a pellet,
- subdividing said pellet in two halves, placing them on said first surface of the support, each one in correspondence with at least a respective through hole, shaping them so that they are spaced apart from each other, and deposing a plurality of drops of diluted Nafion® one or more times, and
- drying, preferably at room temperature, more preferably for a time period ranging from 4 and 5 hours; step C may comprise:
- wetting a glass fibre with Nafion®, and - pressing said wetted glass fibre on the exposed surface of the counter-electrode and of the reference electrode, for allowing the Nafion® polymer to ensure mechanical stability between the glass fibre and these; step D may comprise: - mixing graphite with diluted Nafion® for obtaining a paste, and
- deposing said paste on the PPL layer.
Still according to the invention, step B may comprise: - deposing a mask on said first surface of the support, shaped so as to have two uncovered zones of said first surface spaced apart from each other, each one in correspondence of at least one respective through hole, and spaced from a perimeter of the support, - mixing molybdenum disulphide MoS2 with diluted Nafion® for obtaining a paste mixture,
- deposing said paste mixture over said two uncovered zones,
- spinning the support by a rotating distributing apparatus, or spinner, preferably at a speed of 1000 rpm and 1500 rpm for 30 seconds and 60 seconds, respectively, so that said paste mixture uniformly spreads over said two uncovered zones,
- drying, preferably at room temperature, more preferably for a time period equal to 1 hour, and
- removing the mask; step C may comprise:
- deposing diluted Nafion® over said first surface of the support and/or over the counter-electrode and/or over the reference electrode,
- spinning the support by a spinner, preferably at a speed of 1500 rpm for 60 seconds, so that the diluted Nafion® uniformly spreads, and
- drying, preferably at room temperature, more preferably for a time period equal to 1 hour; step D may comprise: - mixing graphite with diluted Nafion® for obtaining a paste,
- deposing said paste on the PPL layer, and
- spinning the support by a spinner, preferably at a speed of 1500 rpm for 60 seconds, so that said paste uniformly spreads. Furthermore according to the invention, the process may further comprise, preferably before step B, the following step:
E. soldering in each one of said at least three through holes a respective rheophore, the electrical contact between the working electrode and at least one respective rheophore being preferably made with silver glue during step D. The amperometric sensor according to the invention has an optimal sensitivity to nitrogen dioxide NO2 and an extremely low cross-sensitivity to carbon monoxide CO and oxygen O2. Moreover, the sensor according to the invention is capable to operate in the. range from 200C to 800C.
The sensor according to the invention is usable in a measuring system that is extremely flexible, ensuring optimal performance in terms of repeatability.
The present invention will be now described, by way of illustration and not by way of limitation, according to its preferred embodiments, by particularly referring to the Figures of the enclosed drawings, in which:
Figure 1 shows a front view (Fig. 1a), a top plan view wherein inner elements are shown in dashed lines (Fig. 1b), and a side view (Fig. 1c) of the amperometric sensor according to the invention;
Figure 2 shows a perspective view (Fig. 2a) and a cross-section view, not to scale, combined according two planes passing through the working electrode and, respectively, through the counter electrode and the reference electrode (Fig. 2b), of the amperometric sensor of Figure 1 ;
Figure 3 shows a mask used in a preferred embodiment of the manufacturing process according to the invention;
Figure 4 shows the amperometric responses of the sensor of Figure
1 made at different concentrations of NO2 carried out at 25°C and with a total flow of 200 ml/min with 100% of relative humidity, with the sensor used as two electrode sensor (Fig. 4a) and as three electrode sensor (Fig.
4a);
Figure 5 shows a comparison between the sensitivity of the sensor of Figure 1 used as two electrode sensor (Fig. 5a) and as three electrode sensor (Fig. 5b);
Figure 6 shows a voltammogram of the sensor of Figure 1 exposed to three different concentrations of nitrogen carrier gas and with a total flow of 200 ml/min; and
Figure 7 shows a measuring system using the sensor of Figure 1. In the Figures, identical reference numbers are used for alike elements.
The inventors have developed a novel and inventive, although simple, amperometric sensor consisting of liquid Nafion® (usually used for protonic exchange membranes or PEMs), a working electrode of graphite, and a counter electrode and a reference electrode of molybdenum disulphide M0S2 (also called as molybdenite). In particular, the choice of the ions of molybdenum disulphide MoS2 is capable to ensure a long-term stability to the sensor.
The amperometric sensor according to the invention, along with a chemical sampling system and a front-end electronics, shows good characteristics in terms of response repeatability, sensitivity, response time and cross-sensitivity to interfering gases, even with biasing equal to 0
V.
With reference to Figures 1 and 2, it may be observed that a preferred embodiment of the amperometric sensor according to the invention is made as follows: - a working electrode 1 is made of graphite (C) and diluted Nafion®, preferably with a dilution of Nafion® ranging from 0,1 % up to 3% in weight (w/w) in a liquid, preferably water or n-propanol;
- a counter electrode 2 and a reference electrode 3 are made of molybdenum disulphide M0S2 and diluted Nafion®, preferably with a dilution of Nafion® ranging from 1% to 15% in weight (w/w) in a liquid, preferably water or n-propanol;
- the working electrode 1, the counter electrode 2 and the reference electrode 3 are attached to a polymer proton layer or PPL, electrically insulating, shaped as a disc 4 made of Nafion® at a reference concentration (as proposed by the provider Aldrich), preferably with a dilution ranging from 5% up to 20%, more preferably from 10% to 20%, in weight (w/w) in a liquid, preferably water or n-propanol.
Geometry and size of the sensor may vary, preferably satisfying proportions suggested for the preferred embodiment illustrated in the
Figures. In particular, the rheophores 1', 2' and 3' are coupled to the respective electrodes 1, 2 and 3 through soldering in suitable through holes of an electrically insulating support 5. However, other geometries and other forms of coupling may be adopted; by way of example, the rheophore 1' may be applied externally to the support 5 and coupled to the working electrode 1 of graphite through a sort of conductive clip.
The quantities of carbon C and molybdenum disulphide M0S2 used for making the electrodes may vary depending on the application field and on the required response sensitivity and speed. The sensor may be made using different approaches. In the following two manufacturing methods are described which represent a good compromise among repeatability of obtained sensors, measurement results, and manufacturing ease.
A first embodiment of the manufacturing process provides that an electrically insulating support 5 is made of an inert and insulating material, such as FR4 or ceramic compounds, that is shaped as a disc. The circular support 5 has diameter preferably equal to 1.2 mm and it is provided with three through holes, geometrically uniformly distributed along the circumference of the circular support 5 (i.e. on the vertices of an equilateral triangle inscribed in the circle of the support 5), for allowing rheophores 1', 2' and 3' to be soldered. The counter electrode 2 and the reference electrode 3 are made directly on the insulating support 5. In particular, 50 mg of M0S2 IV (according to the nomenclature of the provider Aldrich) are mixed with 0,5 ml of Nafion® diluted at 10% in weight (w/w) for obtaining a paste. This may be then pressed with a nominal force of 40 kN/cm2 for obtaining a pellet of about 10 mm diameter and 0,15 mg weight. The pellet is then split in two halves, which are used for making (sequentially or simultaneously) the counter electrode 2 and the reference electrode 3, respectively. In particular, the contacts of the two halves of the pellet with the respective metal electrodes 2' and 3' are mechanical. Afterwards, some drops of Nafion® diluted at 20% in weight (w/w) are deposed on the counter electrode 2 and on the reference electrode 3 and the whole is allowed to dry at room temperature for a time interval ranging between 4 and 5 hours. This operation of drop deposition may be repeated several times.
For making the PPL layer 4, a glass fibre wetted with Nafion®, preferably with dilution ranging from 10% to 20% in weight (w/w), is smoothly pressed on the other side of the two halves of the pellet, just for allowing the Nafion® polymer to guarantee mechanical stability between the glass fibre and he half of the pellet. Insulation between the counter electrode 2 and the reference electrode 3 must be guaranteed, otherwise the sensor operates as a two electrode structure.
The working electrode 1 is made using 20 mg of graphite mixed with 0,5 ml of Nafion® diluted up to 2,5% in weight (w/w) in n-propanol for obtaining a paste tha t is deposed on the glass fibre already used in making the PPL layer 4 over the counter electrode 2 and reference electrode 3. The quantity of graphite may vary depending on the viscosity that is desired to obtain. The metal contact with the working rheophore 1' may be made with silver glue. A second embodiment of the manufacturing process provides that the electrically insulating supp ort 5 is always made with an inert and insulating material, such as FR4 or ceramic compounds, that is shaped as a disc. The circular support 5 has diameter preferably equal to 1.2 mm and it is provided with three through holes, geometrically uniformly distributed along the circumference of the circular support 5 (i.e. on the vertices of an equilateral triangle inscribed in the circle of the support 5), for allowing rheophores V1 2' and 3' to be soldered.
With reference to Figure 3, it may be observed that a mask 6 is temporary deposed on the support 5 in order to create an insulating barrier between the counter electrode 2 and the reference electrode 3 and for avoiding to have parts of the counter electrode 2 on the periphery of the support 5, as shown in Figures 1 and 2. The mask 6 allows to deploy the solutions of the counter electrode 2 and reference electrode 3 (described in the following) guaranteeing the electrical contacts with the rheophores and the insulation among them. Once that the counter electrode 2 and the reference electrode 3 have been deposed and dried, the mask 6 is removed.
The counter electrode 2 and the reference electrode 3 are simultaneously made on the insulating support 5, on which the rheophores V, 2' and 3' have been already soldered, with the mask 6 being applied. In particular, 50 mg of M0S2 IV (according to the nomenclature of the provider Aldrich) are mixed with 30 μl of Nafion® diluted at 10% in weight (w/w) in n-propanol for obtaining a paste mixture. This is deposed on the support 5 on the two areas corresponding to the counter electrode 2 and to the reference electrode 3, preferably up to completely cover the surface of the support 5 not covered by the mask 6 (about 10 μl are sufficient). The circular support 5 is then spun on a spinner, at the speed of 1000 rpm and 1500 rpm for 30 seconds and 60 seconds, respectively. To this end, the viscosity of the mixture must be such that the latter is sufficiently liquid to uniformly spread on the support 5 and sufficiently solid to remain on the same without squirting out. The whole is allowed to dry at room temperature for 1 hour.
Afterwards, the PPL layer 4 is made. To this end, the mask 6 is removed and some drops of Nafion® (preferably 60 μl) diluted at 20% in weight (w/w) are deposed up to cover, preferably completely (but alternatively also only a part of the surface to cover may be interested, exploiting the subsequent distribution operated by the spinner), the surface of the support 5 and of the counter electrode 2 and reference electrode 3 present on the same. The support 5 is then spun again for 60 seconds at 1500 rpm, and the whole is dried at room temperature for 1 hour.
The working electrode 1 is made using 20 mg of graphite mixed with 0,5 ml of Nafion® diluted up to 2,5% in weight (w/w) in n-propanol for obtaining a paste that is deposed on the PPL layer 4 and spun at 1500 rpm for 60 seconds. The electrical contact with the respective rheophore 1' is made with silver glue.
Figure 4 shows some preliminary measurements on the device, namely: the amperometric responses of the sensor of Figure 1 used as two electrode sensor made at different concentrations of NO2 carried out at 250C and with a total flow of 200 ml/min with 100% of relative humidity (Fig. 4a); the same measurements repeated with the sensor of Figure 1 used as three electrode sensor.
Figure 5 shows a comparison between the sensitivity of the sensor of Figure 1 used as two electrode sensor (Fig. 5a) and as three electrode sensor (Fig. 5b). Figure 6 shows a voltammogram of the sensor of Figure 1 exposed to three different concentrations of nitrogen carrier gas and with a total flow of 200 ml/min.
With reference to Figure 7, it may be observed that a measuring system using the sensor according to the invention comprises two (or more) gas tanks 10 and 10' feeding two respective flow-meters 11 and 11', e.g. the BronkHorst F-201C meters, controlled by a processing unit 12, preferably a computer. Each controlled flow, composed of the carrier gas and the mixture under measurement, passes through a respective Drechsel bottle 13 and 13' for enriching the gas with H2O, filled with water and placed in a thermostatic bath 14 the temperature of which is fixed, with 1°C accuracy. Afterwards each flow pa sses through a respective three-way solenoid valve 15 and 15'. The valves 15 and 15', which are also controlled by the computer 12, allow to switch each flow towards a measurement chamber 16 or alternatively external ambient. The presence of the valves 15 and 15' allow to reduce as much as possible the fluid- dynamic transient due to variations of the adjustments of the flow-meters 11 and 11'. The measurement chamber 16 is place in an oven maintained at a reference temperature (with accuracy equal to at least 1°C). For the experiments described in the following, the reference temperature at which the sensor has been maintained is equal to 400C, whereas the temperature of the thermostatic bath 14 is equal to 25°C. The relative humidity is measured by a humidity sensor, e.g. the Humirel HTS2330 sensor, housed inside the chamber 16. In fact, an accurate temperature control is necessary when Nafion® based sensors are used, since their behaviour depends on the ionic conduction related to the water content in the same Nafion® and on H+ ions generation/combination phenomena occurring at both the counter electrode and the reference electrode in presence of water. In other words, water provides such electrodes with an excess of H+ ions available for transport. Moreover, water weakens the SO3 '-H+ bonds within the Nafion® structure, allowing proton conduction phenomena among sensor electrodes (actually, a humidity excess may further induce flooding that causes a sensor anomalous behaviour).
The developed measuring system is composed of the sensor, a dedicated front-end electronics 17 (preferably a I-V converter), and an acquisition and processing system controlled by the computer 12. Preferably, the system allows to apply a voltage difference across the electrodes in the range from -1V to +1V, with an accuracy equal to 1 mV. Also, the system allows the control of both the flow-meters 11 and 11' and the solenoid valves 15 and 15', and it allows to measure both relative humidity, preferably with an accuracy of 2%, and temperature, preferably through a thermocouple with an accuracy of 0,10C within the measurement chamber 16.
With this system it is possible to implement several measurement protocols, such as: tracking sensor response to abrupt changes of flow composition (with or without a voltage difference applied across the electrodes), performing voltammograms, evaluating sensor sensitivity as a function of different flows or different flow compositions.
The preferred embodiments have been above described and some modifications of this invention have been suggested, but it should be understood that those skilled in the art can make variations and changes, without so departing from the related scope of protection, as defined by the following claims.

Claims

1. Three electrode electrochemical amperometric sensor, in particular for detecting nitro-derivatives, comprising a working electrode (1), a counter-electrode (2) and a reference electrode (3), characterised in that the counter-electrode (2) and the reference electrode (3) are made of a first material comprising or consisting of molybdenum disulphide M0S2, and in that the working electrode (1), the counter-electrode (2) and the reference electrode (3) are attached to an electrically insulating polymer proton layer (4) or PPL made of a second material comprising or consisting of Nafion®.
2. Sensor according to claim 1 , characterised in that the first material comprises molybdenum disulphide M0S2 and diluted Nafion®, preferably with a dilution of Nafion® ranging from 1% to 15%, more preferably equal to 10%, in weight (w/w) in a liquid selected from the group comprising water and n-propanol.
3. Sensor according to claim 1 or 2, characterised in that the second material comprises or consists of Nafion® with a dilution ranging from 5% to 20%, preferably from 10% to 20%, more preferably equal to 20%, in weight (w/w) in a liquid selected from the group comprising water and n-propanol.
4. Sensor according to any one of the preceding claims, characterised in that the second material comprises a glass fibre wetted with Nafion®.
5. Sensor according to any one of the preceding claims, characterised in that the working electrode (1) is made of a third material comprising or consisting of Nafion®, preferably with a dilution ranging from 0,1 % to 3%, more preferably equal to 2,5%, in weight (w/w) in a liquid selected from the group comprising water and n-propanol.
6. Sensor according to claim 5, characterised in that the third material comprises carbon C, preferably graphite.
7. Sensor according to any one of the preceding claims, characterised in that it further comprises a support (5), preferably provided with at least three through holes, wherein the counter-electrode (2) and the reference electrode (3), the PPL layer (4), and the working electrode (1) are arranged on a surface of which support, the support (5) being made of a fourth, insulating and inert, material, preferably comprising one or more materials selected from the group comprising FR4 and ceramic compounds.
8. Sensor according to any one of the preceding claims, characterised in that it further comprises at least one respective rheophore (V1 2', 3') connected to each one of the following: the working electrode (1), the counter-electrode (2) and the reference electrode (3).
9. Sensor according to claim 8, characterised in that at least one rheophore (V) is connected to the working electrode (1) through silver glue.
10. Process for manufacturing at least one electrochemical amperometric sensor according to any one of claims 1-9, characterised in that it comprises the following steps:
A. having a, preferably circular, support (5) made in an insulating and inert material, provided with at least three through holes, preferably uniformly geometrically distributed, B. making the counter-electrode (2) and the reference electrode (3), spaced apart from each other, on a first surface of the support (5),
C. making the PPL layer (4) over the counter-electrode (2), the reference electrode (3), and said first surface of the support (5), and
D. making the working electrode (1) over the PPL layer (4).
11. Process according to claim 10, characterised in that: step B comprises:
- mixing molybdenum disulphide MoS2 with diluted Nafion® for obtaining a paste,
- pressing said paste for obtaining a pellet, - subdividing said pellet in two halves, placing them on said first surface of the support (5), each one in correspondence with at least a respective through hole, shaping them so that they are spaced apart from each other, and deposing a plurality of drops of diluted Nafion® one or more times, and - drying, preferably at room temperature, more preferably for a time period ranging from 4 and 5 hours; step C comprises:
- wetting a glass fibre with Nafion®, and
- pressing said wetted glass fibre on the exposed surface of the counter-electrode (2) and of the reference electrode (3), for allowing the Nafion® polymer to ensure mechanical stability between the glass fibre and these; step D comprises:
- mixing graphite with diluted Nafion® for obtaining a paste, and
- deposing said paste on the PPL layer (4).
12. Process according to claim 10, characterised in that: step B comprises:
- deposing a mask (6) on said first surface of the support (5), shaped so as to have two uncovered zones of said first surface spaced apart from each other, each one in correspondence of at least one respective through hole, and spaced from a perimeter of the support (5),
- mixing molybdenum disulphide M0S2 with diluted Nafion® for obtaining a paste mixture,
- deposing said paste mixture over said two uncovered zones,
- spinning the support (5) by a rotating distributing apparatus, or spinner, preferably at a speed of 1000 rpm and 1500 rpm for 30 seconds and 60 seconds, respectively, so that said paste mixture uniformly spreads over said two uncovered zones,
- drying, preferably at room temperature, more preferably for a time period equal to 1 hour, and - removing the mask (6); step C comprises:
- deposing diluted Nafion® over said first surface of the support (5) and/or over the counter-electrode (2) and/or over the reference electrode (3), - spinning the support (5) by a spinner, preferably at a speed of 1500 rpm for 60 seconds, so that the diluted Nafion® uniformly spreads, and
- drying, preferably at room temperature, more preferably for a time period equal to 1 hour; step D comprises:
- mixing graphite with diluted Nafion® for obtaining a paste,
- deposing said paste on the PPL layer (4), and
- spinning the support (5) by a spinner, preferably at a speed of 1500 rpm for 60 seconds, so that said paste uniformly spreads.
13. Process according to any one of claims 10 to 12, characterised in that it further comprises, preferably before step B, the following step: E. soldering in each one of said at least three through holes a respective rheophore (V1 2', 3'), the electrical contact between the working electrode (1) and at least one respective rheophore (V) being preferably made with silver glue during step D.
PCT/IT2009/000334 2008-07-28 2009-07-28 Three electrode electrochemical amperometric sensor WO2010013276A1 (en)

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