WO2018234185A1

WO2018234185A1 - Electrochemical gas sensor

Info

Publication number: WO2018234185A1
Application number: PCT/EP2018/065970
Authority: WO
Inventors: Walter Daves; Bora ERSOEZ
Original assignee: Robert Bosch Gmbh
Priority date: 2017-06-22
Filing date: 2018-06-15
Publication date: 2018-12-27
Also published as: SG10201705211VA

Abstract

The present disclosure provides for an electrochemical sensor for detecting gases in the ambient air comprising a porous host (6), infiltrated with an organically-based monomeric ionic liquid (8), which is deposited on at least one metal oxide (10). It also provides for use of an electrochemical sensor as defined above as an air-quality sensor for the detection of carbon dioxide. It also provides for a process for making an electrochemical sensor, comprising: deposition of at least one metal oxide on a substrate and sintering the at least one metal oxide; deposition of a layer comprising a porous host infiltrated with an organically-based monomeric ionic liquid on the at least one metal oxide in a solvent, evaporating the solvent and annealing the layer at a temperature of 50-200 °C, and optionally further comprising adding at least one passivation layer. It also provides an electrochemical sensor produced by the process as defined above.

Description

ELECTROCHEMICAL GAS SENSOR

Technical Field

[0001] The disclosure relates to an electrochemical sensor with a porous host, infiltrated with a monomeric ionic liquid on a metal oxide for detecting gases in the ambient air.

Background

[0002] C0₂ is used as a measure for air quality. High values can cause dizziness and headaches, which occur beside a lowered concentration capability. Therefore monitoring of C0₂ is emerging as a promising technology for smart control of HVAC systems and e.g. on-demand ventilation. Among several techniques for C0₂ sensing, most commonly used are optical (NDIR), gravimetric (QCM) and electrochemical techniques. Electrochemical methods offer the advantage of miniaturizability, which goes far beyond the possibilities for the other techniques. This is important in order to keep the power consumption and the fabrication cost of the sensor as low as possible. Electrochemical techniques are most often applied by measuring faradaic current (amperometric), or the change in electrical impedance (conductometric).

[0003] Miniaturized electrochemical devices consist of a transducer, which is very often an interdigitated electrode (IDE) structure, and a sensing material. So far many different material classes were reported to be sensitive to C0₂ electrochemically: metal oxides (MOX) (BaSn0₃ ^{[1 ]}, Ti0₂P^], CuO-SnC ³!, BaTiC^-CuO^, Sn0₂-W0₃r⁵l, CdO^,

Lai-xSr_xFe0₃ ^[7], Sn0₂-La₂0₃ ^[8i, ZnO:Ca^, C-nanostructurest¹⁰!, polymers, such as BPEI- PEDOT^{[l l]}, PEI^{[, 2}1, PPy^['³l, PANI^ and small molecules (e.g. Amidinel^15]).

[0004] MOXs usually have to be operated at high temperatures, which increases their power consumption. Furthermore the lack of selectivity of most MOXs makes the interpretation of the signal harder.

[0005] Despite the fact that C-nanostructures are highly sensitive to external stimuli, they are not selective on their own, but can be used in combination with another material which provides selectivity.

[0006] Most organic polymers and small molecules possess nitrogen-containing functional groups which are responsible for C0₂ sensitivity. However these functional groups tend to be unstable, which limits the thermal stability of these materials.

[0007] Recently ionic liquids get a lot of attention in many fields of science, such as in chemical synthesis, fuel cells, batteries, catalysis, gas separation membranes and sensors. The reason for such high interest in this material class is its properties such as non-volatile, thermally stable, high ionic strength and high viscosity. These properties ensure an improved reliability/lifetime compared to systems based on organic solvents. Of particular interest are room-temperature ionic liquids (RTILs). Per definition, they melt below 100 °C resulting from low intermolecular interactions, derealization of charges and poor packing due to asymmetrical ions (bulky ions prevent ordered crystallization)!'^.

[0008] Since (poly)ionic liquids ((P)ILs) are reported to be permeable to C0₂ ^[16], they are employed in electrochemical sensors in order to detect gases such as N0₂ and C0₂. Previous work on ionic liquids for gas sensing application was filed as a patent^[33].

However, the sensing mechanism for CO2 sensing remains unclear: Besides the electrochemical route (superoxide intermediate^^7] or direct reduction to CO^[I8]), the material can act as a capacitive sensor^[19] or as a capacitance gate dielectric for field-effect- transistors (FETs)^[20].

[0009] Up until today, there remain problems with the electrochemical sensors. For example, typical requirements set by CE applications, i.e. low production cost, low power consumption and miniaturizability, cannot be satisfied by existing materials. As described in the following, each material class for amperometric sensing has its specific

disadvantage, which can be overcome by the use of ionic liquid-based materials.

[0010] MOXs are not generally selective, i.e. generally sensitive to either reducing or oxidizing gases. They are usually operated at high temperature, which increases power consumption. Carbon nanostructures are similarly to MOX generally not selective. Organic materials often exhibit a cross-sensitivity to atmospheric humidity and low thermal stability.

[0011] Accordingly, there is a need for a material or a combination of materials which can overcome, or at least ameliorate, some of the problems discussed above.

Summary

[0012] In a first aspect, there is provided an electrochemical sensor for detecting gases in the ambient air comprising a porous host, infiltrated with an organically-based monomeric ionic liquid, which is deposited on at least one metal oxide. The porous host, infiltrated with an organically-based monomeric ionic liquid, forms an "ionogel". This allows to exploit the properties of the organically-based monomeric ionic liquid, while at the same time providing mechanical stability. Further, by providing the ionic liquid in a porous host, the ionogel may be instrumental for the diffusibility of gases due to the porosity of the porous host. In particular, the permeability of the ionogel for gases is improved compared to using a pure ionic liquid, as the pure ionic liquid would have a higher viscosity. For example, a gas would be able to diffuse in the interfacial regions between the pore wall and the ionic liquid.

[0013] In a second aspect, there is provided use of an electrochemical sensor as described above as an air-quality sensor.

[0014] In a third aspect, there is provided a process for making an electrochemical sensor, comprising:

deposition of at least one metal oxide on a substrate and sintering the metal oxide;

- deposition of a layer comprising a porous host infiltrated with an organically-based

monomeric ionic liquid on the at least one metal oxide in a solvent, evaporating the solvent and annealing the layer at a temperature of 50-200 °C,

- optionally further comprising adding at least one passivation layer.

[0015] In a fourth aspect, there is provided an electrochemical sensor produced by the process as described above.

Brief Description of the Drawings

[0016] Fig. 1 shows schematically the cross section of the ionogel@MOX device on a microhotplate micro-electro-mechanical system (MEMS) structure according to various embodiments. In Fig. 1 , 2 depicts a channel to the atmosphere and 4 depicts a channel between electrodes. The porous host is described in 6, wherein 8 shows the pores which are infiltrated with the ionic liquid. The electrodes (shown as + and - squares) are coated with the metal oxide 10. The electrochemical sensor is optionally deposited on a membrane 12 and may optionally be heated by a microheater 14.

[0017] Fig. 2 schematically shows an MEMS structure coated with the ionogel @MOX.

[0018] Fig. 3 shows the resistance measured for an electrochemical sensor at varying concentrations of gases for the ionogel (porous host infiltrated with the ionic liquid) only (Example 2). In the upper plot, the resistance of the material on the interdigitated electrode (IDE) is shown; in the bottom plot, the dashed bars indicate purging of 400/4000 ppm CO₂ and the dotted-dashed bars indicate purging of 50/200 ppb N0₂.

[0019] Fig. 4 shows the resistance measured for an electrochemical sensor at varying concentrations of gases wherein the metal oxide is exemplified with a 4%-La₂03-Sn0₂ (Example 3). The ionogel is deposited on this metal oxide. In the upper plot, the resistance of the material on the IDE is shown; in the bottom plot, the dashed bars indicate purging of 400/4000 ppm C0₂ and the dotted-dashed bars indicate purging of 50/200 ppb N0₂.

[0020] Fig. 5 shows the resistance measured for an electrochemical sensor at varying concentrations of gases wherein the metal oxide is exemplified with W0₃ (Example 5). The ionogel is deposited on this metal oxide. In the upper plot, the resistance of the material on the IDE is shown; in the bottom plot, the dashed bars indicate purging of 400/4000 ppm C0₂ and the dotted-dashed bars indicate purging of 50/200 ppb N0₂.

[0021] Fig. 6 shows the transducer device being represented by a typical three-electrode design for electrochemical measurements.

Detailed Description

[0022] The above mentioned problems of a low selectivity can be solved by the electrochemical sensor disclosed herein. Hence, the selectivity of the metal oxide and carbon nanostructures can be increased by combining them with a sensitizing material, for example an ionogel.

[0023] Accordingly, in one aspect, there is provided an electrochemical sensor for detecting gases in the ambient air comprising a porous host, infiltrated with an organically- based monomeric ionic liquid, which is deposited on at least one metal oxide. The "porous host" could be any material as long as it provides mechanical stability to the organically- based monomeric ionic liquid while at the same time allows diffusibility of the analyte, i.e. the gas to be detected. The porous host may also have the function of a "gelator", i.e. a substance which is capable of forming a gel. The porous host may be generally chosen from materials such as molecular or macromolecular polymers, gelatin and colloidal gelators, for example inorganic networks such as ceramics, metals, zeolites and/or nanotubes. The electrochemical sensor may be optimized by varying the porosity of the matrix, i.e. the porous host. This may be achieved during the process by using different solvents and different conditions during drying and annealing, i.e. temperature, humidity, gases or vacuum.^[30] These variations would be within the knowledge of the person skilled in the art.

[0024] In a preferred embodiment, the porous host may be a polymer, optionally selected from fluoropolymer-copolymers, conducting polymers, organosil icons, polyethers and acrylates. In particular, the polymer may be selected from the group consisting of poly(vinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), polyaniline (PANI), poly(ethylene oxide)/polyacrylonitrile/poly(methyl methacrylate)/poly(vinylidene fluoride), polydimethylsiloxane (PDMS) and a sulfonated tetrafluoroethylene based fluoropolymer-copolymer (Nation). In one embodiment, the porous host may be poly(vinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) and/or Nafion.

Poly(vinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) and/or Nafion as polymers may be chosen as they provide an optimal thermal stability and solution processability.

[0025] The term "ionic liquid" refers to salts that are liquid over a wide temperature range, including room temperature. The ionic liquid used herein may be an organically based salt. It may further be a monomer, i.e. it does not contain covalently linked repeating units. Preferably, these salts may be, for example, imidazole derivatives and pyridine derivatives. In a preferred embodiment, the ionic liquid may be a dialkylsubstituted imidazole derivative as cation, wherein the alkyl substituents are positioned at the nitrogen atoms. The alkyl substituents may be alkyl units selected from C1-C5. The counter anion may be selected from tetrafluoroborate (BF₄ ^~), hexafluorophosphate (PF6^~),

hexafluoroantimonate (SbF₆ ^~), nitrate, bisulphate (hydrogen sulphate), tetraphenylborate [B(C₆H5)₄ ^~], thiocyanate, acetate, hexyltriethylborate, trifluoromethylsulphonyl, nonafluorobutanesulphonate, bis[(trifluoromethyl)sulphonyl]imide,

tris[(trifluoromethyl)sulphonyl]methide, trifluoroacetate and heptafluorobutanate, as well as anions based on chlorides and other halides of aluminum, copper, manganese, lead, cobalt, nickel or gold, e.g. tetrachloroaluminate (A1C1₄ ^~), heptachlorodialuminate (AhCl ^~) and tetrachlorocuprate (CuCl₄ ^2_ and CuCl₄ ^3_), halogen anions, for example fluoride, chloride and bromide. In one example, the ionic liquid is l-ethyl-3-methylimidazolium tetrafluoroborate. l-Ethyl-3-methylimidazolium tetrafluoroborate as ionic liquid may be chosen as this ionic liquid provides optimal results regarding C0₂ sensing.

[0026] The organically-based monomeric ionic liquid together with the porous host may form an ionogel. This ionogel may be characterized in that there is no covalent bond between the ionic liquid and the porous host. The ionogel may have an approximate thickness in the range of between 0.1 to 5 μηι, or between 0.5 to 3 μιτι, or between 0.8 to 2 μηι, or approximately 1 μπι.

[0027] The at least one metal oxide may be a binary or ternary ionic compound, wherein at least one element is a metal and one other element is oxygen. The metal oxide may consist exclusively of a metal and an oxide The metal may be selected from alkali metals, transition metals, lanthanoids, actinoids and post-transition metals, which may refer to the metallic elements in the periodic table located between the transition metals (to their left) and the metalloids (to their right). The post-transition metals may include gallium, indium and thallium; tin and lead; and bismuth, and aluminium. The metal oxide may be selected from a mixture between two or more types of metal oxides, for example La₂0₃-Sn0₂. Preferably, the metal oxide is selected from La₂0₃-Sn0₂, W0₃, ln₂0₃, Ti0₂, ZnO, Sn0₂, BaTi0₃, BaSn0₃, or a mixture thereof.

[0028] The at least one metal oxide may be provided in the form of nanoparticles. The nanoparticles may increase the surface area of the metal oxide. Hence, it is possible that the inorganic nanoparticles employed in the experiments are not chemically participating in the sensing mechanism, but rather increase the surface of ionic liquid. Due to this larger surface area of the ionic liquid at the interface to the nanoparticles, gas diffusion towards the electrodes may be facilitated. Without being bound to theory, it is assumed that a decreasing nanoparticle size should yield an even higher C0₂ response. [0029] The at least one metal oxide may have an approximate thickness in the range of between 100 to 1000 nm, or between 100-800 nm, or between 100 to 500 nm, or between 200 to 400 nm, or between 200 to 300 nm, or between 220 to 280 nm, or approximately 250 nm.

[0030] The electrochemical sensor as described above may further comprise a passivation layer deposited on top of the porous host. The reliability of the sensor may be improved by a suitable device topography of the passivation layer which inhibits the lateral creeping/flowing of the ionic liquid and/or ionogel. The passivation layer may be made from a material which is compatible with the substrate, such that stress between the passivation layer and the porous host is minimized. Preferably, the passivation layer is made from a material which does not crack. While any material resulting in the above properties may be selected to form the passivation layer, the passivation layer may be a silicon-based ceramics. Hence, it may comprise or essentially consist of silicon nitride or silicon oxide (as a result from the deposition of the precursor tetraethoxysilane).

[0031] In a second aspect, there is provided use of an electrochemical sensor as described above as an air-quality sensor. The use of the air-quality sensor may comprise a method of detection of a gas. The gas may be N0₂ or C0₂, preferably C0₂. The detection method may be described as an electrochemical sensing method, wherein the sensor acts as a transducer and converts the chemical detection of the gas into faradaic current for DC operation. On the other hand, it is possible to utilize AC operation and detection of the gas concentration by measuring the differential capacity is possible due to the formation of ionic intermediates during or after the electrochemical reactions.

[0032] In a third aspect, there is provided a process for making an electrochemical sensor, comprising:

- deposition of at least one metal oxide on a substrate and sintering the at least one metal oxide;

- deposition of a layer comprising a porous host infiltrated with an organically-based monomeric ionic liquid on the at least one metal oxide in a solvent, evaporating the solvent and annealing the layer at a temperature of 50-200 °C, - optionally further comprising adding at least one passivation layer.

[0033] The process may comprise a first step of depositing the at least one metal oxide on a substrate, containing electrodes for the detection of the gas. The deposition technique of the metal oxide may comprise dropcasting, ink-jet printing, screen-printing or gravure printing, preferably dropcasting. The metal oxide may be sintered. Different metal oxides have different sintering temperatures, which the person skilled in the art would be capable of electing without undue burden. Then the ionogel layer may be deposited, wherein the ionogel is dissolved in a solvent. The ratio of the organically-based monomeric ionic liquid to the porous host in the ionogel may be 1 :4 - 4: 1 , or 1 :2 - 2: 1, or preferably 1 : 1. The deposition technique of the ionogel may comprise dropcasting, ink-jet printing, screen- printing or gravure printing, preferably dropcasting. In another embodiment, the components of the ionogel are deposited in sequence: for example, first the polymer is deposited and then the ionic liquid, which infiltrates the porous polymer host.

Alternatively, the ionic liquid may be deposited first and then the porous host, which may yield a more homogenous mixture. The polymer and ionic liquid may be dissolved in the same solvent or the ionic liquid may be deposited without any solvent at all. In the presence of a solvent, the solvent may subsequently be evaporated. The evaporation may comprise heating the ionogel to about 50 to 150 °C, optionally 60 to 100 °C, or approximately 80 °C. The evaporation time may be about 10 min to about 2 h, or about 20 min to about 1 h, or approximately 30 min. After evaporation, the product may be annealed for about 1 min to about 2 h, or about 5 min to about 1 h, or about 10 min to about 30 min, or approximately 15 min. Depending on the porous host which is used, the annealing temperature may be about 50 to 200 °C, optionally 80 to 160 °C, optionally 100 to 150 °C or approximately 120 °C to 140 °C.

[0034] In one embodiment, the process may comprise the deposition of a passivation layer. The deposition may be performed by using standard deposition techniques known to the person skilled in the art, for example, sputter deposition, thermal/electron-beam evaporation or plasma-enhanced chemical vapor deposition (PECVD). The passivation layer may optionally be patterned by lithography or alternatively an electrically insulating ink may be deposited around the active area by inkjet printing. [0035] In one embodiment, the electrochemical sensor may be placed on a microhotplate. Hence, the heater may be based on a microhotplate MEMS structure. Alternatively, ceramic heaters may be used to heat the device during operation or regeneration.

[0036] In a fourth aspect, there is provided an electrochemical sensor produced by the process as disclosed above.

[0037] The proposed solution presented herein is based on a MEMS transducer platform, allowing low cost and low space requirement, thus meeting the requirements for e.g.

consumer electronics (CE) and internet of things (IoT) applications.

[0038] In some embodiments, the transducer device is represented by a three-electrode structure as depicted in Fig. 6. The main design variation consists of changing channel length/width, electrode height and number of fingers.

[0039] As discussed above, the problem of finding a suitable material as electrochemical sensor can be solved by directly coating metal oxide- or carbon-nanostructures with the sensitizing component, in this case the ionogel, or by depositing both components subsequently, such that the analyte, for example the gas to be detected, first has to pass the sensitizing layer in order to filter out undesired analytes.

[0040] In order to guarantee mechanical stability, the formation of an ionogel upon mixing an ionic liquid with a porous host, for example a gelator,^[21] is the preferred way to exploit the properties of ionic liquids.

[0041] The disclosed sensor consists of a metal oxide bottom layer and an ionogel layer on top of it. Similar structure was patented by Niederberger et al.^{[3 ]} Compared to their material, the distinctive feature of the present device is the possibility to use any kind of ionogel (porous host- ionic liquid composite) on top of the metal oxide layer instead of being restricted to PIL (polyionic liquid). [0042] The key difference between a PIL and an ionogel is that PILs consists of moieties characteristic for ionic liquids, while the polymer in an ionogel does not intrinsically possess properties of ionic liquids. Therefore an ionogel consists of a porous host matrix which is wetted/infiltrated by an ionic liquid. Repeated washing of an ionogel with a solvent which is miscible with the ionic liquid entrapped in this ionogel, should remove all ionic liquid, which can be proven by physicochemical analysis. Similar washing procedure of PILs should not remove at least one of the ionic constituents, which is a distinctive feature to ionogel. Niederberger et al.^[34] claim that PIL acts just as pre-concentrator and the C0₂ is actually detected by the metal oxide. However it is proposed herein that the ionogel as top layer participates actively in the sensing by electrochemically converting the analyte C0₂ into an intermediate species (e.g. carbon monoxide CO), which in turn is detected by the metal oxide. The advantage is that the intermediate species will have higher oxidative/reductive power, which can be detected by a wider range of metal oxide materials, compared to the limited number of metal oxide materials being sensitive to C0₂ as it is required with the approach of Niederberger et alJ³⁴l

[0043] In the ideal case, the gelator in the ionogel acts exclusively as a matrix for the organically-based monomeric ionic liquid without participating in electrical or ionic conduction mechanisms. Therefore a suitable combination of certain ionic liquid and metal oxide layer can be selected in order to build sensors for analytes different from C0₂.

Possible parameters are the type of conductivity of the metal oxide semiconductor (n-, p- type), the functional groups of the ionic liquid which interact selectively with certain gas molecules and the free volume of the ionic liquid, which allows only gas molecules smaller than a threshold value to permeat.

[0044] Compared to existing sensors, the sensors presented herein combine the advantages of different material classes which leads to simultaneous high thermal stability, high selectivity and sensitivity towards C0₂ as well as low power consumption due to operation at room temperature. [0045] In order to prove the capability for mass production, it was shown that the components of the composite material applied for the disclosed sensor can be deposited by ink-jet printing.

[0046] Although a liquid is used in the disclosed sensor, the lifetime is not limited by this ionic liquid due to its negligible vapour pressure.

[0047] The use of ionic liquids and gelators with hydrophobic character for the ionogel could reduce the cross-sensitivity to humidity significantly.

[0048] Since the components of the disclosed composite materials only act as catalyzers in the occurring electrochemical reduction processes, reversibility of all reactions is assumed, i.e. the sensor can be fully regenerated.

[0049] Complementary transduction methods can be applied in order to enhance sensitivity and selectivity of the sensor by means of electronics. On the one hand DC operation is possible, which could be a measure for faradaic current flowing due to the electrochemical reduction processes. On the other hand, AC operation is possible to measure (differential) capacity which arises upon formation of ionic intermediates during or after the electrochemical reactions.

Examples

[0050] Fig. 1 shows schematically the cross section of the ionogel@MOX device on a microhotplate MEMS structure. For the experiments for this disclosure, solely IDE structures employing Platinum electrodes are used. However many more conducting materials can be used, as partially confirmed by literature reports: metals (A ^^{] 1} Ag), carbon materials (graphene oxide!²⁹!), _ancj conducting polymers (PEDOT). Both electrodes of the IDE structure are depicted with '+' and '-' in Fig. 1.

[0051] Because the sensing mechanism is discussed controversially in the literature it is assumed that either a direct electrochemical reduction or the reaction mechanism involving the generation of superoxide radicals can be present. Also combinations of both extrema are possible. The overpotential for the reduction of oxygen to superoxide is lower than for electrochemical reduction of C0₂to CO (1.33V^[18]). In order to ensure highest possible reaction rate, during device operation a voltage of 2 V was applied, which is far above the overpotential for both reactions.

Example 1 - Device Fabrication

[0052] For this disclosure, different metal oxide materials (La₂03-Sn0₂ and WO3), in combination with the polymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF- HFP) and the ionic liquid l-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]) were used.

[0053] The La-based metal oxide was chosen due to its inherent C0₂-sensing capability and WO3 was chosen because it does not show any C0₂ response as a neat metal oxide layer. The ionic liquid was chosen based on previous literature reports, where

[EMIM][BF4] yielded most promising results regarding CO2 sensing.

[0054] Exemplarily, Fig. 2 shows the ionogel PVDF-IL on top of a La₂C«3-Sn0₂ layer. First, the MOX layer is drop-casted and sintered at MOX-specific temperature. Then the ionogel layer is drop-casted. After evaporating the solvent of the ionogel for 30 min at 80 °C, the layer is annealed for 15min at 120 °C in the case PVDF-HFP and for 15min at 140 °C for Nafion. The metal oxide layer has an approximate thickness of 250 nm and the ionogel an approximate thickness of 1 μηι. The measurement sequence in all experiments consists of each two cycles with 400 and 4000 ppm C0₂ as well as 50 and 200 ppb N0₂. The devices are measured in an open probe station, whereby the gas is purged on the samples through a Teflon tube, the orifice of which was placed ~lcm directly above the device. The flow rate of the gas is kept at 2000 ml/min in order to prevent an influence of atmospheric air. The measurements are performed without any heating. [0055] In order to prove the advantage of MOX @ lonogel over neat ionogel, a set of devices is produced on the same IDE structures with the same layout. Example 2 represents a comparative Example, wherein the electrochemical sensor is tested without the presence of a metal oxide.

Example 2 - PVDF-IL (Comparative Example)

[0056] This sample has been prepared according to Example 1, except that the deposition of the metal oxide did not occur. Fig. 3 shows the measured resistance of an

electrochemical sensor, wherein the sensor only contains the ionogel. The composition of the ionogel is the same as with the remaining experiments, wherein the ratio of the ionic liquid to polymer is 1 :1. From Fig. 3 it can be seen that the resistance decreases upon purging 400 and 4000ppm C0₂. Also upon purging 50/200ppb N0₂, a decreasing resistance can be observed.

Example 3 - 4wt%-La₂0₃-Sn02 @ PVDF-IL

[0057] This sample has been prepared according to Example 1. The only difference of Example 3 relative to Example 2 is the presence of a previously sintered metal oxide (4%- La₂0₃-Sn0₂) layer beneath the Ionogel in one case. In the case of 4%-La₂0₃-Sn0₂ @ lonogel, increasing resistance for 400 and 4000 ppm C0₂ can be observed (see Fig. 4). Upon purging 50 and 200 ppb N0₂, the resistance of the composite material increases.

Example 4 - Comparison with literature values utilizing a polyionic liquid on

La₂0₂C0₃

[0058] Willa et a\ ²^ are reporting decreasing resistance upon purging C0₂ in the case of their composite material which is composed of La₂0₂C03 nanoparticles with a polyionic liquid. From these observations it can be claimed that the combination of a layer of La₂0₃- Sn0₂ nanoparticles beneath a film of ionogel yields a significantly different sensing mechanism to C0₂ than just ionogel or the approach of Willa et al. [0059] The proposed mechanism for this response behavior is based on two steps, involving the characteristic properties of both MOX and ionic liquid. In the first step, C0₂ is converted to an intermediate species (e.g., carbon monoxide CO) by applying a suitable voltage to the ionic liquid. In a latter step, this species is being sensed and/or converted by the MOX layer beneath, which has to be sensitive to this particular intermediate species.

Example 5 - PVDF-IL @ W0₃

[0060] Example 5 has been prepared according to the procedure outlined in Example 1 , using W0₃ as a metal oxide. So far, there appears to be no report in the literature for C0₂ sensitivity of neat WO3 measured as chemiresistor. Literature reports which apply WO3 as chemiresistor in order to detect pollutants in CO2 gas (e.g. SO2, H₂S, etc.)^{t 1]} imply that neat WO3 is not sensitive to C0₂. However as it can be seen from Fig. 5, a significant response to 400 and 4000ppm C0₂ arises upon deposition of PVDF-IL (1 :1 weight ratio) from cyclohexanone solution. N0₂ peaks are negligible compared to CO2 peaks. The observed behaviour of PVDF-IL @ WO3 supports the assertion made in the previous section, i.e. the conversion of C0₂ into a species which can be sensed by the MOX layer.

Example 6 - Conclusion

[0061] In all shown cases, a significant decrease of N0₂ peaks and rise of C0₂ peaks are observable. It could be assumed that ionogel only acts as a gas-diffusion barrier for N0₂ because C0₂ has much higher diffusion constants in ionic liquids. This would mean, that still the metal oxide is the active component for the sensing mechanism. However, the fact that even for WO3 a C0₂ signal can be observed, shows that the ionogel plays an active role in the sensing mechanism, too. The increased baseline current upon deposition of ionogel also implies that the ionogel plays a significant role for the sensing mechanism.

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August 26, 2015

Claims

1. An electrochemical sensor for detecting gases in the ambient air comprising a porous host, infiltrated with an organically-based monomeric ionic liquid, which is deposited on at least one metal oxide.

2. The electrochemical sensor of claim 1, wherein the ionic liquid is l-ethyl-3- methylimidazolium tetrafluoroborate.

3. The electrochemical sensor of claim 1 or 2, wherein the porous host is a polymer, selected from the group consisting of poly(vinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), polyaniline (PANI), poly(ethylene oxide)/polyacrylonitrile/poly(methyl methacrylate)/poly(vinylidene fluoride), and a sulfonated tetrafluoroethylene based fluoropolymer-copolymer (Nafion).

4. The electrochemical sensor of claim 3, wherein the porous host is poly(vinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) and/or Nafion.

5. The electrochemical sensor of any one of claims 1 to 4, wherein the at least one metal oxide is provided in the form of nanoparticles.

6. The electrochemical sensor of any one of claims 1 to 4, wherein the at least one metal oxide is selected from the group consisting of La₂0₃-Sn0₂, W0₃, ln₂0₃, Ti0₂, ZnO, Sn0₂, BaTi0₃, BaSn0₃, and a mixture thereof.

7. The electrochemical sensor of any one of claims 1 to 6, further comprising at least one passivation layer deposited on top of the porous host.

8. Use of an electrochemical sensor of any one of claims 1 to 7 as an air-quality sensor.

9. A process for making an electrochemical sensor, comprising:

deposition of at least one metal oxide on a substrate and sintering the at least one metal oxide;

deposition of a layer comprising a porous host infiltrated with an organically-based monomeric ionic liquid on the at least one metal oxide in a solvent, evaporating the solvent and annealing the layer at a temperature of 50-200 °C, optionally further comprising adding at least one passivation layer.

10. An electrochemical sensor produced by the process of claim 9.