Metal working – Method of mechanical manufacture – Electrical device making
Patent
1997-03-10
1999-03-23
Skapars, Anthony
Metal working
Method of mechanical manufacture
Electrical device making
296235, H01M 600
Patent
active
058853090
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method of manufacturing an electrochemical cell having an electrolyte based on molten carbonate.
BACKGROUND OF THE INVENTION
Such a method is known from the German Offenlegungsschrift 40 30 904. There, nickel oxide is used in the electrodes. Between the electrodes and the current collector, a so-called functional layer is employed which improves the contact between electrode and current collector. In the this Offenlegungsschrift it is proposed to place a "green" foil between the electrode and the current collector and then to bake out this material.
The use of nickel oxide in the electrodes in the first instance provides an effective electrochemical cell, more in particular a fuel cell, but in the long term it was found that internal short circuits arise. Consequently, the use of nickel oxide electrodes is undesirable.
It has been proposed, in the prior art, to employ lithium cobaltate instead. It was found that this presents greater problems than in the case of an electrode based on nickel oxide, which relate to the transition resistance between electrode and current collector.
If, however, a functional layer is used such as that proposed in the above-mentioned German Offenlegungsschrift 40 30 904, it was found that, as the cell warms up, diffusion of carbonate material takes place between the electrode, which for example is in the green state, and the functional layer, as a result of which the effect of the functional layer is lost and the transition resistance increases.
SUMMARY OF THE INVENTION
The object of the present invention is to avoid the above-described drawbacks and to provide a fuel cell with electrodes having, on the one hand, an adequate service life and, on the other hand, a low transition resistance between electrode and current collector by virtue of an effective functional layer being employed.
This object is accomplished for an above-described method by employing a LiCoO.sub.2 -based electrode material, and attaching the functional layer and the electrode to each other before placement in a cell stack.
It was found that the use of green lithium cobaltate in conjunction with a green functional layer is possible, if said functional layer is attached to the electrode before this assembly is combined into a cell.
This attachment operation can take place in any manner known in the prior art.
If the tape casting technique is employed, the electrode can be fabricated by this method. Then the functional layer can be applied by spraying, screen printing and the like.
It is also, and preferably, possible to fabricate both the functional layer and the electrode by a tape casting technique and combine them immediately thereafter. The solvent employed for the binder of both the electrode and the functional layer preferably has the same basis.
By applying a transition layer or functional layer it is possible to reduce the electrical resistance between the electrode and the metal of the current collector. By virtue of this layer being applied in a deformable state to the current collector it is possible to absorb surface unevenness on the current collector. The same applies to unevenness on the side of the electrode. In the case of use in conjunction with gas ducts, the gas distribution underneath the current collection points can be improved. When the fuel cell is warmed up (for example in the case of fuel cells being stacked) the separate components to be assembled consist, according to the invention, of materials of a similar type. This means that during start-up and operation only metal-metal or, for example, oxide-oxide contacts are formed and preferably no metal-oxide contacts. It is moreover important for the invention that prior to, during and after these contacts being formed, corrosion problems do not arise, particularly at the interface of the (stainless steel) current collector and the electrode. Corrosion worsens the electrical contact.
It has been found in certain cases that, after sintering, the thin layer, on the one han
REFERENCES:
patent: 5356731 (1994-10-01), Sitters et al.
Sitters Eric Franciscus
Van Heuveln Frederik Hendrik
Skapars Anthony
Stichting Energieonderzoek Centrum Nederland
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