²are achieved.

The increase in electrical conductivity rises steadily with the concentration of the solution up to the aforementioned phase boundary and then decreases again. When the solubility limit according to the phase diagram shown in FIG. 6 is exceeded, a two-phase region is formed from the SnO₂—Ta₂O₅phase and the thoreaulite SnTa₂O₇in equilibrium. The composition of the heterogeneous structure can be calculated with given concentrations according to the lever rule. If, for example, a total concentration of 10 mol % Ta₂O₅in SnO₂is chosen, the result is a composition of the heterogeneous structure of 88% Sn_0.99Ta_0.01O₂and 2% SnTa₂O₇as oxide composite.

The electrically highly conductive tin dioxide phase Sn_0.99Ta_0.01O₂forms the carrier metal oxide and the thoreaulite phase SnTa₂O₇forms the catalyst material which is finely dispersed in the grain of the carrier metal oxide. The precipitation conditions are determined on the one hand by the grain size produced and on the other hand by the temperature-time diagram for setting the structure. By varying the composition, the proportions of the two phases of the oxide composite are changed.

However, the chemical activities of the first and second metallic elements in the oxides remain unchanged in the two-phase region, as do the respective basic electrical and chemical-physical properties. For catalysis, the triple phase boundary lengths as well as the energetic surface states of the carrier metal oxide can be set via the quantity and size ratios. Since the two phases, i.e., the carrier metal oxide and the catalyst material, are present in crystallographic structures that differ from one another, they are inherently dissolved with one another, i.e., the catalyst material is present as inherently dissolved dispersoids in the carrier metal oxide.

With RDE investigations (RDE=ring disc electrode) it was found that both the tantalum-rich β phase and the thoreaulite phase SnTa₂O₇have a comparatively good catalytic activity for oxygen reduction. This was verified with experiments in which the catalyst system was treated with a solution containing 2-[1-[difluoro[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2-tetrafluoroethanesulfonic acid as a polymer electrolyte material or ionomer, also known under the trade names Nation or Flemion, was applied to a carbon substrate (glassy carbon) to form an electrode. The onset voltages U were in the range of those for platinum. However, the specific currents i found were low: i<0.1 A/cm²at U=0.65 V.

In the next step, the individual phases were eliminated from the two-substance mixture. As stated above, the carrier metal oxide used was SnO₂with about 1 mol % Ta₂O₅, wherein the mass fraction of this phase was in the range from 70 to 95% by weight.

Table 1 below shows the results of the catalyst systems of this type. The results were determined by means of a single cell consisting of two end plates, two graphite plates, two bipolar plates 2, 2′ made of graphite, two gas diffusion layers 6, 6′, the electrode 1 (cathode side) according to the invention, a standard Pt/C catalyst (anode side) and a polymer electrolyte membrane 7 made from Nation. The process gases, here air and hydrogen, were humidified differently on the cathode side and the anode side.

The electrode 1 had an electrode area of 30 mm×30 mm. The cell was operated at T=85° C. with p=2.5 bar. The settings used were λ=1.5 on the hydrogen side and λ=2 on the air side. A reference humidification temperature T_Bwas set at 80° C.

The prepared coating thicknesses of the electrode 1 were in the range of from 1 to 5 μm.

TABLE 1

Ratio of carrier

(metal

Grain size of

Onset

oxide)/catalyst
Carrier (metal
the carrier
I (A/cm²) &
voltage

material in
oxide)/catalyst
metal oxide
0.7 V
in V

wt. %
material
in nm
(T = 85° C.)
NHE

80/20
C/Pt
2-4
1.1
0.95

50/7
SnO₂—1%Ta₂O₅/Ta₂O₅—dop.
100-125
0.65
1.05

35/15
SnO₂—1%Ta₂O₅/Ta₂O₅—dop.
100-125
0.75
1.1

35/15
SnO₂—1%Ta₂O₅/Ta₂O₅—dop.0.1%Pt
100-125
1.4
1.1

50/7
SnO₂—2.5%Nb₂O₅/Ta₂O₅—dop.
80-100
0.5
1.15

35/15
SnO₂—2.5%Nb₂O₅/Ta₂O₅—dop.
80-100
0.8
1.2

35/15
SnO₂—2.5%Nb₂O₅/Ta₂O₅—dop.—0.1%Pt
100-125
1.35
1.2

30/10
SnO₂—2.5%Nb₂O₅/(Ti90Nb10)O₂—0.1%Pt
80-100
0.95
0.85

According to Table 1, the current densities for catalyst systems according to the disclosure are 5-8 times higher than in experiments in which individual oxidic phases were deposited on a carbon substrate. The results in Table 1 impressively show that it is possible to produce platinum-free or significantly platinum-reduced and carbon-free electrodes with similarly good activities as in the conventional system of “platinum on carbon carriers”.