On-site high purity, high pressure oxygen is required for many industrial applications. Generally, markets for such applications are served by high-pressure gas cylinder or liquid oxygen tanks. Generally, the oxygen is produced by cryogenic separation of air. The separated oxygen is transferred to cylinders which are then shipped to customers.
Solid state oxygen generators have been proposed for on-site production of high-purity oxygen as opposed to the use of cylinder oxygen. In these types of systems electrolytic cells are employed to separate oxygen from air. Each electrolyte cell, whether of flat plate, honeycomb, or tubular configuration, has a wall across which oxygen ions are transported to separate oxygen from the air. Such wall is constructed in layers that comprise electrodes (an anode and a cathode) and an electrolyte, made of an ionic conducting material that is located between the electrodes. Ionic conducting materials include yttria stabilized zirconia, ceria, bismuth oxide, thoria and hafnia, and similar materials known in the ceramics art.
Ionic conducting materials that are used to form the electrolyte will conduct oxygen ions at high temperature. When an electric current is applied to the cathode and the anode, molecular oxygen is ionized at the cathode by gaining electrons. The resultant oxygen ions are then transported through the electrolyte where they emerge from the opposite, anode side to recombine into elemental oxygen by the loss of electrons. The process is typically carried out at a temperature range of between about 600° and about 1000° C. In this temperature range, the electrolytic oxygen separation process can be carried out at close to 100 percent faradic efficiency and with minimum resistive loses.
It is important for the sake of economic efficiency that the cell consumes as little electrical power as possible. In order to fabricate such a cell, the electrolyte should be as thin as possible so that a very small spacing exists between the electrodes. As a result, oxygen ions migrating from one electrode to the other will have a very short distance to travel so that the electrical resistance of the electrolyte is minimized. At the same time, it is preferable that the cathode be as electrically conductive as possible to maximize the number of oxygen ions available per unit area of the electrolyte.
U.S. Pat. No. 4,879,916 discloses an electrolytic cell construction that employs highly conductive electrodes. In this regard, silver and alloys of silver are used to form the electrodes and zirconium or bismuth oxide is used to form the electrolyte. In one embodiment, the electrolyte has a thickness of between about 20 and about 100 microns and the electrodes each have a thickness of about 4 microns and about 30 microns. The thin electrode-electrolyte assembly is supported on a porous alumina tube. Another embodiment does not take advantage of a thin electrolyte, but rather, uses a thick electrolyte of between about 0.5 and about 2.0 mm to support the electrodes.
In U.S. Pat. No. 6,183,619, planar electrochemical cell constructions are illustrated in which one of the electrodes, preferably the anode, serves to support the electrolyte and the other of the electrodes. For instance, an electrolysis cell is illustrated that is specifically designed for steam electrolysis. In this cell, an anode, formed of lanthanum manganite, is about 1 mm thick. Yttria-stabilized zirconia is plasma flame sprayed onto the anode to produce an electrolyte layer having a thickness of 100 microns. A cathode formed of a platinum paste is then printed onto the electrolyte.
As stated above and as illustrated by the foregoing patents, it is known that the cathode should be as electrically conductive as possible. In this regard, silver is a more preferred cathode material due to its high conductivity and comparatively low cost. Noble metals, such as silver, however, have much higher coefficient of expansion than the ceramic materials that are utilized in forming electrolytes. In order to prevent the cathode from separating from the electrolyte at cell operating temperature due to thermal mismatch, as also illustrated in the foregoing patents, the cathode is made as thin as possible.
It has long been recognized in the prior art that at the triple phase boundary between molecular oxygen, oxygen ions and electrons, existing at the junctures of the electrodes and the electrolyte, not only is the least electrical resistance desired, but also, the greatest ionic conduction. Thus, multi-component electrodes have been proposed that can contribute to ionic conduction by containing an ionic conductor. An example of this can be found in Vol. 23, No. 6, Elektrokhimiya, Steady Polarization of Distributed Gas Electrodes In Cells With Solid Electrolyte, pp. 740-747, I. V. Murygin, June 1987. In this reference, electrodes are formed that comprise a metallic conductor, an ionic conductor, and a mixed conductor. Electrodes formed of a multi-component mixture, such as illustrated by the above reference, are inherently more thermally compatible with the electrolyte since they comprise the ionic conducting material of the electrolyte. However, it has been found by the inventors herein that a cathode formed from such a multi-component material must still be very thin so as not to peel away from the electrolyte at operating temperature. Moreover, since the total volume of such a cathode comprises the ionic material, that is not electrically conductive, and a mixed conductor, that is not as electrically conductive as a noble metal such as silver, a cathode formed by such a multi-component material has more resistance than a cathode of the same thickness formed of a noble metal. In fact this resistance can dominate the electrical resistance of the electrolytic cell to cause the cell to consume more power than electrolytic cells formed with cathodes that comprise a single phase of a metallic conductor.
As will be discussed, the present invention provides an electrolytic cell wall construction advantageously employing a multi-component cathode material for thermal compatibility while at the same time limiting the electrical resistance of such a cathode so that it does not dominate the electrical resistance of the cell to thereby prevent the cell from having low power consumption characteristics.