DE4412540C2 - Reference electrode for galvanic cells with a cation-conducting solid electrolyte - Google Patents

Description

The present invention relates to a reference electrode a cation-conducting solid electrolyte, sensors, the one include reference electrode according to the invention and the use such sensors for the determination of gases.

The use of galvanic solid electrolyte cells as a gas sensors, especially as oxygen sensors, has been around for a long time known. However, sensors for other gases have so far hardly been used commercially available. This is particularly because it is comparatively more difficult, practical Refe to realize reference electrodes. When using cationic solid electrolytes, for example a sodium ion conductor ters, it means maintaining a defined Sodium potential over long periods. According to the state technology still works best with the help of metallic sodium, which at the operating temperatures of the Sensor is often liquid above 500 ° C and a very high Has reactivity. Therefore arise in the construction such an electrode considerable difficulties, the Elek tode spaces hermetically from each other and from the environment isolate. Through reactions of the sodium melt with you tion material or other components of the galvanic cell the stability of the sensor signal is impaired, or the The sensor is completely destroyed over time.

As an alternative, DE 41 12 301 A1 proposed Use reference electrodes that have an alkali metal compound tion, especially a sodium compound in a multinary Multi-phase balance (e.g. binary Na / metal compounds or ternary Na / metal / oxide compounds). Examples such reference electrodes with binary sodium / metal Ver Bonds are Na-Sb or Na-Bi, examples of such reference electrodes with ternary sodium / metal / oxide compounds  Na-Co-Oxide or Na-Ni-Oxide. Due to the toxicity of the Heavy metal compounds that have been used have always been proven but the manufacture of these reference electrode systems as per blemish.

It is also known that the at the phase boundary between the solid electrolyte and a noble metal adhering to it emerging metal activity the role of a reference sy stems fulfilled (see Saito and Maruyama, Solid State Ionics 28-30 (1988), 1644). However, there is a slight risk that that because of the naturally imperfect separation of reference and measuring electrode the reference with the measuring medium, for example as CO₂ and O₂, reacts. Then it's just a matter of Time for the cell voltage of such a sensor to approach zero and the reference electrode loses its functionality (cf. Maruyama et al., Solid State Ionics 23 (1987) 107).

In the above-mentioned literature Maruyama et al. (Solid State Ionics 23 (1987), 107) is also featured a CO₂ sensor propose a combination of an oxygen-conducting Solid electrolyte (YSZ) and a sodium ion-conducting solid electrolyte (Nasicon) is used. Is reference electrode doing so after the sintering together of the two festival electrons lyte at the phase interface between these formed Na₂O. The potential of these references has been shown to react Electrode more than expected due to changes in the oxygen part aldrucks in the surrounding atmosphere. Hence the cell voltage of the resulting sensor is not stable over time and no clear function of the CO₂ partial pressure.

An object of the present invention is to provide a reference electrode in which the disadvantages mentioned above of the prior art are at least partially avoided, which in particular permanent insulation of the reference elec guaranteed to the environment and thus a higher long-term stability compared to known systems lity and is much easier to manufacture.  

According to the invention, this object is achieved by provision a reference electrode for galvanic cells with a cat ion-conducting solid electrolytes, comprising

• a) a gas-tight seal from the environment Reference electrode space,
• b) one conductive for cations of a first metal Solid electrolyte, which with the environment and the Refe contact electrode space is in contact,
• c) one in the reference electrode space on the surface of the solid electrolyte applied gas sensitive Layer of carbonate of the first metal and
• d) a mixture located in the reference electrode space made of carbon in finely divided form, the carbonate of a second metal and the oxide of the second Me talls, the first and the second metal being so are chosen that as the gas sensitive layer carbonate of the first metal as an independent Phase in equilibrium with the other substances of the Reference electrode remains permanently in existence, and on the phase boundary solid electrolyte / gas sensitive Layer a defined activity for the first Metal is present, which is the electrical potential of the Determined reference electrode.

The reference electrode space of the reference according to the invention Electrode is sealed gas-tight from the environment, so that the thermodynamic balance between those in it substances can be adjusted via the gas phase. At a preferred embodiment of the present invention is the reference electrode space around a metal body closed, in which the cation-conducting solid electrolyte  adds. The metal body preferably consists of an un ter the operating conditions of the reference according to the invention electrode essentially oxidation-resistant metal, ins special made of scale-resistant steel. The connection between Solid electrolyte and metal body can, for example, by a Glass solder can be made.

The Koh contained in the reference electrode according to the invention Lenstoff preferably has a BET surface area in the range of 10 to 200 m² / g, particularly preferably in the range from 50 to 100 m² / g.

The cation-conducting solid electrolyte is preferably an alkali metal ion conductor and the first metal is an alkali metal. In this case, the carbonate of the first metal corresponds to the formula Me ⁽¹⁾ ₂ (CO₃).

If the first metal is an alkali metal, e.g. B. sodium, potassium, The solid electrolyte can be lithium, rubidium or cesium for example, a material based on beta-Al₂O₃, Nasi con or Lisicon. Particularly preferred as an alkali metal ion-conducting solid electrolyte is Na-beta-Al₂O₃. The first me tall is then sodium.

The second metal can be any metal that is in Combination with a given first metal given above level condition for the free standard enthalpies of education fills. If the first metal is an alkali metal and in particular If sodium, potassium, rubidium or cesium is the second Metal preferably from the group consisting of Li, Ca, Mg, Sr and Ba selected. If the first metal is lithium, it will second metal preferably from the group consisting of Ca. and Mg. The second metal is particularly preferred Approx. In this case the carbonate of the second metal is CaCO₃ and the oxide of the second metal CaO.  

In the reference electrode area, the thermal dissociation of the carbonate of the second metal and the Boudouard equilibrium only set the temperature-dependent partial pressure for CO₂ and CO. The equilibrium of the two gases with the carbonate of the first metal, Me ⁽¹⁾ _m (CO₃) _n , which is in contact with the Me ⁽¹⁾ ion conductor as a gas-sensitive layer, provides a defined activity at the solid electrolyte / gas-sensitive layer phase boundary for the first metal, which determines the electrical potential of the reference electrode.

The following equilibria are relevant:

Provided that all belong to the reference electrode pure solid substances do not dissolve into one another is theirs Raoult's activity a (standard state *) is 1. The following applies:

a [Me ⁽¹⁾ _m (CO₃) _n ] = 1, a [Me ⁽²⁾ _y (CO₃) _z ] = 1, a [Me ⁽²⁾ _y O _z ] = 1 and a _c = 1.

The remaining activities and partial pressures result from the following equilibrium conditions:

Equation (2a) sets the size of the CO₂ partial pressure in the ref Reference electrode room fixed:

If we insert (5) in (3a), we get for p _co :

With (5) and (6) for pO₂ it follows from (4a):

The partial pressures pCO₂ and pO₂ now determine (1a) the activity a [Me ⁽¹⁾ ] and thus the activity of those metal ions for which the solid electrolyte is conductive. From (1a) it follows with (5) and (7):

The conditions defined by equations (5) and (7) with regard to the size of the CO₂ and O₂ partial pressure in the reference electrode space must ensure that the carbonate of the first metal serving as a gas-sensitive layer acts as an independent phase in equilibrium with the other substances the reference electrode remains in place. This is only the case if the standard enthalpies of formation of the carbonates and oxides of both metals are in a certain size ratio to one another. The boundary conditions that these values have to meet depend on the size of the prevailing oxygen partial pressure. In the pO₂ area below the partial pressure dissociation of the oxide Me ⁽¹⁾ _m O _n , that is

the metal activity a _Me (1) must meet the condition:

and in the P _O2 area:

the metal oxide activity

the condition:

In the latter case, it follows from (12) with (7) for the metal activity a _Me (1):

If one uses (10) or (13) in (8), the ge looked for proportions for the standard enthalpies of education the carbonates and oxides of both metals:

if

respectively.

if

Here pO₂ is the oxygen equilibrium partial pressure in the reference electrode space, Δ _B G ° _* the free standard enthalpy of formation of the respective compounds from the elements, Me ⁽¹⁾ the first metal and Me ⁽²⁾ the second metal and m, n, y, z characterize the Characterize stoichiometric relationships in the respective compounds.

Data of the free standard enthalpies of formation of metal carbon ten and metal oxides can be found in the literature references JANAF, Thermochemical Tables, 3rd edition, National Bureau of Standards, Washington, (1985) and Knacke et al., Thermochemi cal Properties of Inorganic Substances, Springer Verlag, Ber lin-Heidelberg-New York-bondon-Paris-Tokyo-Hong Kong-Barcelona- Budapest, Verlag Stahleisen mbH, Düsseldorf (1991). Through the Reference is made to those contained in these references Data part of the present application.

The invention is further explained in connection with FIGS. 1 and 2.

Fig. 1 shows a first embodiment of the reference electrode according to the invention with a sodium ion-conducting solid electrolyte and

Fig. 2 shows a sensor according to the invention from a combi nation of a sodium ion-conducting and an oxygen ion-conducting solid electrolyte.

The measuring electrode of the sensor according to the invention comprises an electronically conductive material, preferably an Au layer, on the surface of the solid electrolyte facing the measuring medium. The electronically conductive material is in contact with a layer covering the solid electrolyte surface of a connection, for. B. Na₂CO₃, the cation of which is identical to the ion, for which the solid electrolyte is conductive and which is to be measured with the gas in a thermodynamic equilibrium. Accordingly, are used for measuring NO _x (N₂O, NO) nitrates, for measuring SO _x (SO₂, SO₃) sulfates and for measuring CO₂ car bonate as a gas-sensitive layer.

The reference electrode according to the invention shown in Fig. 1 contains a sodium ion-conducting solid electrolyte ( 10 ), for. B. a pellet of Na-beta-Al₂O₃, which in a correspondingly designed metal body ( 12 ), for. B. made of scale-resistant steel (z. B. X10CrA113) is inserted. The electrode space is hermetically enclosed by the metal body and solid electrolyte. Both materials are joined together by soldering using a glass-like oxide mixture ( 14 ).

In the embodiment shown in Fig. 1, the tall tall Me ( 12 ) has a nozzle ( 16 ). The production of the reference electrode can be carried out by depositing a few drops of a Na₂CO₃ solution through the nozzle on the inner surface of the solid electrolyte after soldering the solid electrolyte into the metal body. The Na₂CO₃ layer ( 18 ) formed is preferably fixed by heating. Subsequently, a mixture of carbon and the carbonate and oxide of a second metal (z. B. CaCO₃ and CaO) in a cylinder made of fine-meshed steel gauze ( 20 ) is introduced through the opening of the nozzle into the electrode space and the remaining volume of the electrode space with finely divided carbon ( 22 ) filled up. After prior evacuation of the electrode space, the nozzle is closed with a stopper ( 24 ) and hermetically sealed (e.g. by welding with an electron beam), sufficient heat dissipation being necessary so that the electrode system does not overheat.

The measuring electrode is made by sputtering an Au layer ( 26 ) and depositing a Na₂CO₃ layer ( 28 ) from a solution. The Na₂CO₃ layer covers almost the entire surface of the solid electrolyte.

Another object of the present invention is a galvanic cell for use as a sensor, in particular for measuring gases, which comprises at least one reference electrode according to the invention and at least one measuring electrode suitable for the respective sensor function. The measuring signal of the sensor is created by the potential difference between the reference and measuring electrodes, which is proportional to the partial pressure of the gas to be measured, in particular NO _x , SO _x or CO₂.

The sensor according to the invention preferably contains a combination tion of two different reference electrodes / fixed electrodes lyt / measuring electrode arrangements. It is further preferred that the two reference electrodes at least partially common Form the reference electrode space of the two solid electrolytes of the sensor electrically connects to each other. An example for such a sensor comprises a combination of an acid ionic and a cationic, preferably an alkali metal ion conducting solid electrolyte with the ent speaking suitable measuring electrodes, the cation-conducting Solid electrolyte in contact with the carbonate of the first metal stands and the common reference electrode space the feinver shared carbon as well as the carbonate and oxide of the two contains metal.

If one of these reference electrodes contains an oxygen-ion-solid electrolyte, the resulting sensor delivers a measurement signal that is independent of the oxygen partial pressure of the measuring medium, provided that the two measuring electrodes are exposed to the measuring media in which there is a substantially equal oxygen partial pressure. The two measuring electrodes are particularly preferably in contact with the same measuring medium. Sensors of this type are of particular interest for CO₂, NO _x or SO _x detection both in the trace range and in larger concentrations.

The oxygen-ion-conducting solid electrolyte can, for example a material based on ZrO₂, ThO₂, CeO₂, HfO₂ or Bi₂O₃  his. The oxygen ion-conducting Fes is particularly preferred telektrolyt a material based on cubic, tetragonal or partially stabilized ZrO₂. The measuring electrode for the oxygen ion-conducting solid electrolyte is replaced by a electronically conductive coating, e.g. B. Pt realized.

The basic structure of an embodiment of the sensor according to the invention with two reference electrodes is shown as an example in FIG. 2. The two solid electrolytes, a pel let from Na beta Al₂O₃ ( 30 ) and a pellet from YO _1.5 stabilized ZrO₂ ( 32 ) are tall body in a correspondingly designed Me ( 34 ), z. B. made of scale-resistant steel, inserted by soldering using a glass-like oxide mixture ( 36 ). Me tallkörper and solid electrolytes enclose a volume that is connected to the environment via a nozzle ( 38 ). After inserting the solid electrolyte into the metal body, a few drops of a metal carbonate solution, for example Na₂CO₃, are deposited through the nozzle on the inner surface of the Na-beta-Al₂O solid electrolyte. The resulting Na₂CO₃ layer ( 40 ) is fixed by heating. Then a mixture of coal and a metal carbonate / oxide (z. B. CaCO₃ / CaO) in a cylinder made of fine-mesh steel gauze ( 42 ) through the opening of the nozzle in the electrode space and the remaining volume of the electrode space with finely divided carbon ( 44 ) filled up. The nozzle is closed after prior evacuation of the electrode space with the help of a metal plug ( 46 ) and hermetically sealed against the environment, z. B. by electron beam welding.

As part of the measuring electrode on the surface facing the measuring medium th surface of the oxygen ion-conducting solid electrolyte as an electronically conductive phase z. B. a Pt layer ( 48 ) and an Au layer ( 50 ) sputtered onto the surface of the alkali metal ion-conducting solid electrolyte. Then a thin layer ( 52 ) of an alkali metal salt (eg NaNO₃, Na₂CO₃ or Na₂SO₄) is deposited on the same surface.

The metal carbonate / metal oxide system according to the invention can ne ben also a powdery mixture of the metal compounds on solid solutions of the corresponding carbonates and oxides are based.

Another object of the invention is the use of a sensor according to the invention for the determination of gases, in particular for the determination of CO₂, NO _x , SO₂, SO₃ and O₂.

Claims (17)

1. Reference electrode for galvanic cells with a cation-conducting solid electrolyte, comprising

• a) a reference electrode space sealed gastight from the surroundings,
• b) a solid electrolyte which is conductive for cations of a first metal and which is in contact with the surroundings and the reference electrode space,
• c) a gas-sensitive layer of the carbonate of the first metal applied in the reference electrode space on the surface of the solid electrolyte and
• d) a mixture of carbon in finely divided form, the carbonate of a second metal and the oxide of the second metal located in the reference electrode space, the first and the second metal being selected such that the carbonate of the first metal serving as a gas-sensitive layer as an independent phase in the Equilibrium with the other substances of the reference electrode remains permanently present, and at the solid electrolyte / gas-sensitive layer phase boundary there is a defined activity for the first metal, which determines the electrical potential of the reference electrode.

2. reference electrode according to claim 1, characterized, that the reference electrode space from a metal body around is closed, in which the solid electrolyte is inserted. 3. reference electrode according to claim 2, characterized, that the metal body is made of scale-resistant steel. 4. reference electrode according to one of claims 1 to 3, characterized, that the carbon has a BET surface area in the range of 10-200 m² / g, preferably 50-100 m² / g. 5. reference electrode according to one of claims 1 to 4, characterized, that the cation-conducting solid electrolyte is an alkali metal is ionic conductor. 6. reference electrode according to claim 5, characterized, that the alkali metal ion-conducting solid electrolyte a Ma material based on beta-Al₂O₃, Nasicon or Lisicon is. 7. reference electrode according to claim 5 or 6, characterized, that the first metal was sodium, potassium, rubidium or ca sium and the alkali metal ion conductive solid electrolyte Sodium, potassium, rubidium or cesium ion conductor.   8. reference electrode according to one of claims 1 to 7, characterized, that the second metal from the group consisting of Li, Ca, Mg, Sr and Ba is selected. 9. reference electrode according to claim 5 or 6, characterized, that the first metal is lithium and the alkali metal ions conductive solid electrolyte is a lithium ion conductor. 10. reference electrode according to claim 9, characterized, that the second metal from the group consisting of Ca and Mg is selected. 11. Reference electrode according to one of claims 5 to 10, characterized, that the second metal is Ca. 12. Galvanic cell for use as a sensor, in particular for measuring gases, characterized, that they have at least one reference electrode according to one of the Claims 1 to 11 and at least one for each Sensor function suitable measuring electrode contains. 13. Cell according to claim 12, characterized, that they are a combination of two different Refe reference electrode / solid electrolyte / measuring electrode arrangements summarizes and the two measuring electrodes in contact with the same Chen measuring medium. 14. Cell according to one of claims 12 or 13, characterized,  that they conduct a combination of an oxygen ion the and a cation-conducting solid electrolyte with the correspondingly suitable measuring electrodes, the cat ion-conducting solid electrolyte with the carbonate of the first Metal is in contact and the common reference elec the finely divided carbon and the car bonat and contains the oxide of the second metal. 15. Cell according to claim 14, characterized, that the oxygen ion-conducting solid electrolyte is a mate rial based on ZrO₂, ThO₂, CeO₂, HfO₂ or Bi₂O₃ is. 16. Use of a sensor according to one of claims 12 to 15 for measuring gases, one of the oxygen partial pressure of the measuring medium delivers independent signal. 17. Use of a sensor according to any one of claims 12-15 for measuring CO₂, NO _x or SO _x .