WO1993002481A1

WO1993002481A1 - Composite system

Info

Publication number: WO1993002481A1
Application number: PCT/EP1992/001547
Authority: WO
Inventors: Matthias Kuntz; Gerd Bauer; Dietmar Raulin
Original assignee: MERCK Patent Gesellschaft mit beschränkter Haftung
Priority date: 1991-07-17
Filing date: 1992-07-09
Publication date: 1993-02-04
Also published as: DE4123694A1

Abstract

The invention concerns a composite system which can be used as a high-temperature electrochemical cell and which comprises a centrally disposed high-temperature ion conductor (1) on both sides of which are fitted mixed-conductor electrodes (3) with, located on them, surface-textured electron conductors (2).

Description

Network system

The invention relates to a composite system that can be used as an electrochemical high-temperature cell with a centrally arranged high-temperature ion conductor that serves as a solid electrolyte. Furthermore, the invention relates to a hybrid system for traction purposes, containing an electric motor, an accumulator and a fuel cell based on the composite system.

Electrochemical high-temperature cells, which generally based on ceramic solid electrolytes can be used both as galvanic elements and as electroysis cells.

A much system described (see, for example Electrochemical Ener¬ energy technology -. Ff development status and outlook, eds .: The Federal Ministry for Research and Technology, 1981, p 244) having yttrium oxide Y ₂ 0 ₃ doped zirconia Zr0 ₂ as the oxide ceramic solid electrolyte which Säuerstoffionen passes and at high temperatures of about 1000 ° C has a conductivity in the order of 10- ¹ ohm- ¹ cm-1. On , -) _

the oxide ceramic is a generally porous electrocatalyst layer on which the charge transfer reaction takes place. In the case of a hydrogen-oxygen high-temperature fuel cell, for example, this catalyst layer usually consists of nickel on the anode side and silver or platinum on the cathode side; however, other electron-conducting materials such as, for example, In ₂ 0 ₃ or cermets have also been proposed.

When used as a fuel cell (ie as a galvanic element), a wide variety of fuels such as, for example, hydrogen, which may be pure or else contaminated with CO or CO ₂ , for example, hydrocarbon / water vapor mixtures or other oxidizable gases, further liquid fuels such as gasoline, diesel, heating oil or lower alkoxides or solid fuels such as coal dust which is blown in and in accordance with

C + 02- CO + 2e- CO + H ₂ 0 C0 ₂ + H ₂ H ₂ + 02- H ₂ 0 + 2e-

reacted, operated. High-temperature fuel cells (HTB) have a high efficiency and a high energy density.

Systems with solid electrolytes which conduct oxygen ions (in particular Zr0 ₂ doped with Y0 ₃ ) can also be used for the electrolysis of hydrogen (high-temperature electrolyzer HTE). The cathode is typically exposed to water vapor at temperatures between 700 and 1000 ° C; the according H ₂ 0 + 2e- -> H ₂ + 0 ² -

The resulting 0 ² ions migrate through the ceramic solid electrolyte to the anode, where they are oxidized to 0 ₂ . At a working temperature of 900 ° C and a current load of

0.1 Acm-2, electrolysis voltages of 1.2 V were obtained (see C.H. Hamann, W. Vielstich, Elektrochemie II, Weinheim 1981, p. 405). Powerful water electrolysers are essential in the so-called hydrogen economy, in which hydrogen is used as an energy storage medium.

A disadvantage of the systems previously discussed in the literature, however, is that, with acceptable values for the terminal voltage, only unsatisfactory values for the current density are often obtained. When high current densities are withdrawn, the terminal voltage supplied by an HTB and thus the power that can be drawn drops significantly.

In DE 40 01 684 it has been proposed to use mixed conductor electrodes to increase the power density of HTB instead of the porous metal electrodes in order to enlarge the area available for the charge passage reaction. The charge transfer reaction and the diffusion of the ions formed in the solid electrolytes can take place on the entire surface of the mixed conductor electrodes, while in the conventional electrode structures these reaction steps are limited to the so-called 3-phase zone, where the solid electrolyte, the metallic electrocatalyst and the fuel gas atmosphere is closely adjacent. However, it has been shown that the use of mixed conductor electrodes alone is not sufficient to sufficiently increase the current densities which can be achieved in such a system at acceptable voltages.

The object of the present invention was to provide a composite system which can be used as an electrochemical high-temperature cell and has a centrally arranged high-temperature ion conductor which serves as a solid electrolyte and which, when used as a galvanic element and when used as an electrolytic cell, permits the realization of high power densities and is superior to conventional systems.

It has been found that this object can be achieved by providing the composite systems according to the invention.

The invention thus relates to composite systems which can be used as an electrochemical high-temperature cell and which contain a centrally arranged high-temperature ion conductor to which mixed conductor electrodes with structured electron conductors arranged above are applied on the anode and cathode sides.

The invention further relates to high-temperature fuel cells and electrolysis cells which are based on such a composite system, and to hybrid systems for traction purposes, comprising such a high-temperature fuel cell, an electric motor and an accumulator. The composite system according to the invention is based on a centrally arranged high-temperature ion conductor, which serves as a ceramic electrolyte. This is in particular an oxygen ion conductor, such as zirconium dioxide doped with yttrium oxide, calcium oxide and / or magnesium oxide, or else other oxygen ion conductors. The specific conductance (conductivity = / length) of the inventively used Hoch¬ is temperature-ion conductor at high temperatures of, for example 900-1000 ° C, preferably not less than 0.05 ohm icnr ¹ and especially at least 0.1 ⁱ cnr ^ ohm. The layer thickness of the

High-temperature ion conductor is preferably chosen to be small to achieve a low voltage drop and is preferably between 30 and 1000 μm and in particular between 40 and 500 μm.

A mixed conductor is applied to the central and high-temperature conductor on both the ground and the cathode side. In this case, both different and the same mixed conductors, but preferably the same mixed conductors, can be used on the base and cathode side. Examples of mixed conductors are YBa ₂ Cu ₃ 0 _{6 5} or Lao _{, 8} Sr _{0 2} Mnθ ₃ , which contains 20-80% by weight

Y ₂ 0 _{3 contains} stabilized Zr0 ₂ , which information is only intended to explain the invention and is in no way intended to limit it. In conventional mixed conductors, the specific ion conductivity at the usual operating temperatures of the composite systems according to the invention of 700-1100 ° C. is typically one to two orders of magnitude lower than the specific ion conductivity of the central high-temperature conductor; the specific electron conductivity of the mixed conductor is often somewhat larger than its ion conductivity, but both specific conductivities are of approximately the same order of magnitude. To compensate for the relatively low specific ion and electron conductivity of the mixed conductor, this is carried out on the one hand as a very thin layer and on the other hand a structured electron conductor is applied to the surface of the mixed conductor electrode.

The thickness of the mixed conductor layer is typically between 0.1 and 10 μm and in particular between 0.2 and 8 μm. By using such a thin mixed conductor layer, its conductivity is matched to that of the central ion conductor layer; the conductivities of the two layers should be approximately the same and in particular should not differ from one another by more than a factor of 5 and very particularly at most by a factor of 2.

It has been shown that in systems without an additional electronic conductor, the current density at the mixed conductor electrode varies greatly because of its relatively low electron conductivity. For example, in the case of a fuel cell, the electrochemical reaction at the contact point of the mixed conductor electrode with the external circuit takes place at high speed, while in the case of mixed conductor electrode points further away from this contact point, the diffusion of the electrons becomes speed-determining for the overall electrochemical reaction. To eliminate this effect, very thin, structured electron conductors of high electron conductivity are applied to the mixed conductor electrodes of the composite systems according to the invention. Suitable electron conductors are, for example, high-melting metals such as Pt or W or also highly conductive cermets such as Zr0 ₂ -Ni. The electron conductor can be applied, for example, in the form of a metal network or as a porous metal sponge or in another form, but preferably the larger part of the mixed conductor surface is free of the electron conductor and is available for the charge passage reaction; the ratio of the area covered by the electron conductor and the total surface of the mixed conductor is preferably not more than 0.5 and in particular less than 0.35 and very particularly not more than 0.25. The thickness of the electron conductor layer is preferably of the same order of magnitude as the thickness of the mixed conductor layer and is in particular smaller than the thickness of the mixed conductor layer.

The advantage of the composite system according to the invention is that the relatively poor specific conductivity of mixed conductors is compensated for by the extremely thin layer arrangement, without the decisive advantage of the mixed conductor electrodes being impaired, which is that the reaction takes place on the entire mixed conductor surface and not only as with conventional systems on the

3-phase limit can expire. Furthermore, the combination with an electronic conductor arranged above the mixed conductor ensures that the electrochemical reaction can take place on the entire electrode surface with a more or less uniform, high current density.

Another decisive advantage is the good thermal shock resistance of the composite system according to the invention. In contrast, previous systems made of ion conductors and porous electron conductors often have relatively thick electrode Arrangements on what can lead to a failure of the system when starting up and cooling down the system due to the different thermal expansion coefficients of these materials.

1 schematically shows a preferred arrangement of a composite system according to the invention. There is a mixed conductor layer on the centrally arranged ion conductor which is significantly thinner than the ion conductor. A thin metal network is used as an electron conductor on the mixed conductor electrode.

To manufacture the systems according to the invention, the ion-conducting membrane is first manufactured using a conventional film casting process. The mixed conductor layer, for example, is then applied to the fired ion conductor membrane. applied as a thin sol-gel layer by means of a dip coating process; however, it is also possible to produce the mixed conductor layer by screen printing, by an HF sputtering process or by plasma spraying. Commercially available, very thin metal grids can be used, which are usually printed on the mixed conductor before the composite system is fired; however, a joining step after the firing is also conceivable. As an electron conductor, e.g. serve an extremely porous metal chwamiri, which is obtained by first sputtering a very thin metal layer, which is then galvanically amplified. Instead of a metal net may also be a ceramic electronic conductor (e.g., pure La _{0, 8} Sr _0t2 MnO ₃₎ are printed in network structure, whereby any technical joining problems that can occur in the ceramic-metal composite, can be avoided. 2a shows a preferred embodiment of the composite system according to the invention, in which the mixed conductor electrode has a structured ("jagged") surface to increase the current density that can be achieved; a rugged surface is, for example, when using _0/2 La Sr _0, ₂ MnO _3, which is doped 20-80 wt.% Y ₂ 0 ₃

Contains zirconium dioxide, obtained as a mixed conductor layer. One possibility for increasing the space-time yield is outlined in FIG. 3, where an optimized arrangement is shown for the composite system according to the invention.

The composite systems according to the invention are distinguished by a long service life, good resistance to temperature changes and, when used as an electrochemical cell, they permit the realization of high current densities with comparatively low overvoltages.

The composite systems can be operated both as a fuel cell (HTB) and as an electrolyser (HTE). In general, several composite systems according to the invention assembled into larger units ("stacked") until the desired overall performance is achieved. Plate-shaped composite systems (Fig. 1, 2) or specially structured composite systems (see e.g. Fig. 3) can be combined particularly well into larger units; in addition, other arrangements are also conceivable.

Fuel cells based on composite systems according to the invention are particularly preferably used in hybrid systems used for traction purposes. Hybrid systems, which have been discussed since the end of the 1960s, contain, in addition to an electric motor, a combination of a combustion Stoff elle and an accumulator as an energy source. This is necessary because fuel cells have a high degree of efficiency and a high energy density, but at the same time are characterized by a power density that is not sufficient, in particular for acceleration processes. Accumulators are therefore used to cover power peaks, as is explained in more detail in DE 40 01 684. The fuel cells according to the invention are now distinguished by a comparatively very high power density, since, owing to the sophisticated structure of the composite systems according to the invention, they permit the removal of large current densities with relatively low overvoltages. The hybrid systems according to the invention are therefore characterized by favorable properties and are particularly preferred.

The composite systems according to the invention can also advantageously be used in electrochemical sensors, e.g. can be used to determine the oxygen partial pressure in gas mixtures (e.g. Lambda probe for gasoline engines), since the equilibrium potential is practically reversible on them.

In the following Fig. 1-3 mean:

(1) High temperature ion conductor

(2) electron conductor

(3) Mixed conductor

Claims

1. Composite system which can be used as an electrochemical high-temperature cell and which contains a centrally arranged high-temperature ion conductor, on which mixed-conductor electrodes with structured electronic conductors arranged above are applied on both sides.

2. Composite system according to claim 1, containing an oxygen ion-conducting high-temperature ion conductor.

3. Composite system according to one of claims 1-2, wherein the high-temperature ion conductor has a layer thickness of 30-1000 microns.

4. Composite system according to one of claims 1-3, wherein the

Mixed conductor electrodes independently of one another have a thickness between 0.1 and 10 μm.

5. Composite system according to one of claims 1-4, wherein the electron conductors have a network-like structure.

6. Fuel cell, which is based on a composite system according to one of claims 1-5.

7. electrolysis cell, which is based on a composite system according to any one of claims 1-5.

8. Hybrid system for traction purposes, comprising an electric motor, an accumulator and a fuel cell according to claim 6.