Method for producing a reference electrode for a galvanic cell with cation-conducting solid electrolytes

The invention relates to the way of producing a reference electrode for a galvanic cell having e.g. a lithium-, sodium- or potassium-ion-conducting solid electrolyte and being of the type as used for potentiometric sensors for detecting CO 2 , SO x and NO x .

Characteristics of the Solutions Known in the Art

Already known is a series of reference electrodes for sodium-ion-conducting solid electrolytes whose chemical potential results from the equilibrium between an Na 2 O-containing double oxide in the form Na x Me y O z and, on the other hand, (i) the respective oxide Me m O n or (ii) a double oxide Na a Me b O c with a lower Na 2 O content:

In these systems, the sodium potential establishes itself as a result of the following relationships: ( i )     Na x     Me y     O z ≡ x     Na + x 4     O 2 + q     Me m  O n     ( y = qm , z = cn + x / 2 ) ( ii )     Na x     Me y     O z ≡ ( x - a )     Na + x - a 4     O 2 + Na a     Me b     O c     ( y = b , z = c + ( x - a ) / 2 )

On this basis, the Na potential, assuming that the oxides involved in the equilibrium coexist in pure form in each case, is defined as follows: ( i )     ln     a Na = Δ f     G Na x     Me y     O z o - q · Δ f     G Me m     O n o x     RT - 1 4     ln     PO 2 

(1a) ( ii )     ln     a Na = Δ f     G Na x     Me y     O z o - c · Δ f     G Na a     Me b     O c o ( x - a )     RT - 1 4     ln     PO 2 (1b)

where Δ f G* Na x Me y O z , Δ f G* Na a Me b O c and Δ f G* Me m O n are the free standard enthalpies of formation of the oxides involved. The sodium potential is therefore at all times a defined function of just the ambient temperature and the ambient oxygen partial pressure. As the measuring electrodes in a series of gas sensors are of similar structure and therefore likewise are oxygen-pressure-dependent, the output signal of the complete sensor ultimately becomes independent of the oxygen potential in the ambient gas atmosphere.

Examples of said cases are inter alia: (i) NaAl 11 O 17 /Al 2 O 3 [1], Na 2 Si 2 O 5 /SiO 2 [3]; (ii) NaAl 5 O 8 and NaAl 11 O 17 [1, 2].

Customarily, the oxides present in pure form as solids are pulverized, optionally mixed with an inert electron conductor, e.g. Au, pressure-molded to form a pellet, and, as a compact molding or sintered body, brought into contact with the solid electrolyte. The electrode contact area (phase boundary II) is always that of two solids, which has the usual disadvantages that even with polished surfaces, only punctate contact will be established. Furthermore, the minimum achievable layer thickness, i.e. the spacing between the phase boundaries I and II is subject to a lower limit, as a result of which the oxygen potential at phase boundary I may differ from the oxygen potential at phase boundary II. However, the identity of the two potentials is a precondition for the validity of the equations (1a) and (1b).

The purpose of the invention is to provide a reference electrode for cation-conducting solid electrolytes which can be readily fabricated, is thin and ensures intimate contact with the solid electrolyte.

Essence of the Invention

According to the invention, said purpose can be achieved by one of the oxides being prepared as a hydrosol or organosol and thereby, in liquid form at room temperature or at only slightly elevated temperature, forming a solid film on the electrolyte.

Working Example

For example, Na and K silicate glasses can be dissolved at high pressure in water (water glass). A drop of such a water glass solution is applied to the solid electrolyte surface in such a way that a thin film is produced into which, prior to drying, a few grains of quartz sand (SiO 2 ) are introduced. At the same time, the water glass can be used to fix an Au mesh, which serves as pick-up electrode, to the surface of the solid electrolyte (see FIG. 1 ). FIG. 2 shows potential measurements of reference electrodes according to the invention at various temperatures.

Other systems are conceivable, mainly on the basis of silicates, e.g. sodium alumosilicates in equilibrium with pure Al 2 O 3 and SiO 2 , which can be prepared as sols and likewise be applied in liquid form.

References

[1] J. T. Kummer, Prog. Solid State Chem. 7 (1972), 141

[2] G. Rog, S. Kozinski, A. Kozlowska-Rog, Electrochim. Acta 28 (1983), 43

[3] H.-H. Möbius, P. Shuk, W. Zastrow, Fresenius J. Anal. Chem. 356 (1996) 221