Liquid electrolyte for an electrochemical gas sensor

Abstract

A liquid electrolyte, for an electrochemical gas sensor for detecting NH.sub.3 or gas mixtures containing NH.sub.3, contains at least one solvent, one conductive salt and/or one organic mediator. The conductive salt is an ionic liquid, an inorganic salt, an organic salt or a mixture thereof. The electrolyte preferably is comprised of (I) water, propylene carbonate, ethylene carbonate or a mixture thereof as solvent; (ii) LiCl, KCl, tetrabutylammonium toluenesulphonate or 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate as conductive salt; and (iii) tert-butylhydroquinone or anthraquinone-2-sulphonate as organic mediator.

Claims

1. A liquid electrolyte for an electrochemical gas sensor, the liquid electrolyte comprising: at least one solvent; at least one of a conductive salt and an organic mediator, wherein the conductive salt is an ionic liquid, an inorganic salt, an organic salt or a mixture thereof; and a buffer, wherein the buffer is a compound corresponding to
R.sup.1—(CR.sup.2R.sup.3).sub.n—SO.sub.3H,  Formula I in which n=1, 2, 3, 4 or 5, wherein all R.sup.2 and R.sup.3 are selected, independently from one another, from among H, NH and OH, and wherein R.sup.1 is selected from the group containing piperazinyl, substituted piperazinyl, N-morpholine, cycloalkyl, and tris-(hydroxyalkyl)alkyl.

2. An electrolyte according to claim 1, wherein n=2 or n=3, wherein all R.sup.2 and R.sup.3 are selected, independently from one another, from among H, NH and OH, and wherein R.sup.1 is selected from among [4-(2-hydroxyethyl)-1]-piperazinyl, (N-morpholino), N-cyclohexyl, tris-(hydroxymethyl)methyl, wherein the buffer is 3-(N-morpholino)-propanesulfonic acid or 3-(N-morpholino)-ethanesulfonic acid.

3. An electrolyte according to claim 1, further comprising an additional component for lowering vapor pressure, wherein the additional component is an alkylene glycol or polyalkylene glycol.

4. An electrolyte according to claim 3, wherein the additional component is propylene glycol, ethylene glycol or a mixture of propylene glycol and ethylene glycol.

5. An electrolyte according to claim 1, wherein the solvent is selected from the group containing water and alkylene carbonate or mixtures thereof.

6. An electrolyte according to claim 5, wherein the solvent is selected from the group containing water, propylene carbonate, ethylene carbonate or mixtures thereof.

7. An electrolyte according to claim 1, wherein an anion of the conductive salt is selected from the group containing halides, carbonate, sulfonate, phosphate and/or phosphonate.

8. An electrolyte according to claim 7, wherein the anion is selected from the group containing alkyl sulfonate, alkenyl sulfonate, aryl sulfonate, alkyl phosphate, alkenyl phosphate, aryl phosphate, substituted alkyl sulfonate, substituted alkenyl sulfonate, substituted aryl sulfonate, substituted alkyl phosphate, substituted alkenyl phosphate, substituted aryl phosphate, halogenated phosphate, halogenated sulfonate, halogenated alkyl sulfonate, halogenated alkenyl sulfonate, halogenated aryl sulfonate, halogenated alkyl phosphate, halogenated alkenyl phosphate, and halogenated aryl phosphate.

9. An electrolyte according to claim 7, wherein the anion is selected from the group containing fluorophosphate, alkyl fluorophosphate and aryl sulfonate, perfluoroalkyl fluorophosphate and toluene sulfonate.

10. An electrolyte according to claim 1, wherein the conductive salt contains as cations metal ions, onium ions or a mixture as metal ions and onium ions.

11. An electrolyte according to claim 10, wherein the metal ions are selected from among alkali metal ions or alkaline earth metal ions.

12. An electrolyte according to claim 10, wherein the onium ions are selected from among ammonium, phosphonium and guanidium cations and heterocyclic cations, selected from among alkylammonium and heterocyclic cations, alkylammonium, imidazolium and/or substituted imidazolium ions, wherein substituted imidazolium ions have a structure corresponding to ##STR00002## wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4 and R.sub.5 may be selected, independently from one another, from among —H, straight-chain or branched alkyl containing 1 to 20 C atoms, straight-chain or branched alkenyl containing 2 to 20 C atoms and one or more double bonds, straight-chain or branched alkinyl containing 2 to 20 C toms and one or more triple bonds, saturated, partially or fully unsaturated cycloalkyl containing 3-7 C atoms, which may be substituted with alkyl groups containing 1 to 6 C atoms, saturated or fully unsaturated heteroaryl, heteroaryl-C1-C6-alkyl or aryl-C1-C6- alkyl, wherein R.sub.2, R.sub.4 and R.sub.5 are H, and R.sub.1 and R.sub.3 are each, independently from one another, a straight-chain or branched alkyl containing 1 to 20 C atoms.

13. An electrolyte according to claim 1, wherein the organic mediator is a polyhydroxy compound, which forms a quinoid system or a naphthalene system during oxidation.

14. An electrolyte according to claim 13, wherein the organic mediator is selected from the group containing ortho-dihydroxybenzene, para-dihydroxybenzene, substituted ortho-dihydroxybenzenes and substituted para-dihydroxybenzenes, dihydroxynaphthalene, substituted dihydroxynaphthalene, anthrahydroquinone and substituted anthrahydroquinone, 1,2-dihydroxybenzene, 1,4-dihydroxybenzene, naphthohydroquinone, substituted 1,2- or 1,4-dihydroxybenzene, substituted hydroquinone, substituted naphthohydroquinone, substituted anthrahydroquinone, substituted hydroquinone and substituted 1,2-dihydroxybenzene.

15. An electrolyte according to claim 14, wherein the substituents of the substituted anthraquinones, substituted 1,2-dihydroxybenzene and/or substituted 1,4-hydroquinone are selected from the group containing sulfonyl, tert.-butyl, hydroxyl, alkyl, aryl, sulfonic acid and/or tert.-butyl.

16. An electrolyte according to claim 1, wherein the solvent is comprised of a mixture of propylene carbonate and/or ethylene carbonate, and the conductive salt is comprised of LiCl, KCl, tetrabutylammonium toluene sulfonate and/or 1-hexyl-3-methylimidazolium tris-(pentafluoroethyl)-trifluorophosphate or a mixture of two or more of these components, and the organic mediator is comprised of tert.-butylhydroquinone and/or a substituted anthraquinone, anthraquinone 2-sulfonate as the organic mediator.

17. An electrolyte according to claim 1, wherein the organic mediator is contained in the electrolyte at a concentration of 10.sup.−2 mol/L or less, preferably 10.sup.−3 mol/L or less.

18. An electrolyte according to claim 1, wherein the organic mediator is contained in the electrolyte at a concentration of 10.sup.−6 mol/L or more.

19. A method for preparing an electrolyte, the method comprising the steps of: charging a solvent into the reaction vessel; adding a buffer, wherein the buffer is a compound corresponding to
R.sup.1—(CR.sup.2R.sup.3).sub.n—SO.sub.3H,  Formula I in which n=1, 2, 3, 4 or 5, wherein all R.sup.2 and R.sup.3 are selected, independently from one another, from among H, NH and OH, and wherein R.sup.1 is selected from the group containing piperazinyl, substituted piperazinyl, N-morpholine, cycloalkyl, and tris-(hydroxyalkyl)alkyl; adding an organic mediator to the buffer and the solvent to form a mixture; heating of the mixture while stirring for about 15 minutes at 150° C.; stirring the mixture for about one hour without further supply of heat; cooling to room temperature; and adding a conductive salt.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings:

(2) FIG. 1 is a schematic design of an electrochemical gas sensor, with which the electrolyte according to the present invention for detecting ammonia can be used; and

(3) FIG. 2 is a schematic course of a detection reaction for NH.sub.3 in an electrochemical gas sensor, which contains an electrolyte according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

(4) FIG. 1 shows an electrochemical gas sensor 10, which has a housing 20 with an electrolyte reservoir 30. A gas inlet 21 and a gas outlet 22 are formed in the housing. A working electrode 51 is arranged within the housing 20 such that the working electrode 51 is in contact with gas that is flowing into the housing 20 through the gas inlet 21. The working electrode 51 is separated from a collecting electrode 52 by means of a glass fiber membrane 55. The collecting electrode 52 is in turn separated from the electrolyte reservoir 30 with a glass fiber membrane 55. Furthermore, a counterelectrode 53 and a reference electrode 54 are arranged within the electrolyte reservoir 30.

(5) The electrolyte 40 according to the present invention is present in the electrolyte reservoir 30. The glass fiber membranes 55 can be impregnated with the electrolyte. The electrolyte 40 can reach in this way both the working electrode 51 and the collecting electrode 52, so that a chemical reaction can take place there corresponding to the scheme shown in FIG. 2 between NH.sub.3 flowing in, the material of the working and collecting electrodes 51, 52 and the electrolyte 40.

(6) NH.sub.3 flowing into the gas sensor 10 reacts now on the surface of the working electrode 51 with the electrolyte. The working electrode 51 preferably consists, e.g., of a PTFE membrane with a carbon nanotubes coating. The counterelectrode 53 preferably consists of a noble metal. The electrolyte 40 is a composition of propylene carbonate and/or ethylene carbonate as the solvent, 1-hexyl-3-methylimidazolium-tris(pentafluoroethyl)-trifluorophosphate as a conductive salt and tert.-butyl-1,2-dihydroxybenzene as the organic mediator in this example. The electrolyte preferably contains, furthermore, a buffer, namely, 3-(N-morpholino)-propanesulfonic acid. As can be seen in FIG. 2, the tert.-butyl-1,2-dihydroxybenzene is oxidized into tert.-butylquinone at the working electrode. The protons released in the process react with the NH.sub.3 flowing into the gas sensor 10 into ammonium ions. The ammonium ions reach the counterelectrode 53, where the reverse reaction of the tert.-butylquinone formed previously into 1,2-dihydroxybenzene takes place. NH.sub.3, which can escape through the gas outlet 22, is released, in turn, from the ammonium ions. The buffer used stabilizes the pH value of the electrolyte, which is present between the working electrode and the counterelectrode 51, 53 in the electrolyte reservoir 30, in the course of this reaction process.

(7) Exemplary embodiment for preparing an electrolyte according to the present invention:

(8) Polycarbonate is charged as a solvent into a reaction vessel. A 0.4-wt. % buffer, preferably 3-(N-morpholino)-propanesulfonic acid, is added to the polycarbonate. In the next step, 6.9 wt. % of the organic mediator, preferably tert.-butyl-1,2-dihydroxybenzene, are added. The mixture is heated while stirring within 15 minutes, and a maximum temperature of 150° C. is not exceeded. The mixture was subsequently stirred further for one hour without supplying more heat until all solids were dissolved. The solution obtained has a clear, slightly yellowish color.

(9) The solution thus obtained is allowed to stand until it is cooled to room temperature. Then, 2.7 wt. % of the conductive salt, preferably HMIM-FAP (3-hexyl-3-methylimidazolium-tris(pentafluoroethyl)-trifluorophosphte), are added, and the mixture is stirred briefly, for about 1 minute.

(10) While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.