Fuel cell

Details Fuel cell Fuel cell

2000-10-12

2004-04-27

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C429S010000

Reexamination Certificate

active

06727015

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a fuel cell, in particular an undivided fuel cell with electrolyte-filled electrodes.
DISCUSSION OF THE BACKGROUND
Fuel cells are energy transformers which convert chemical energy into electrical energy. In a fuel cell, the principle of electrolysis is reversed.
Various types of fuel cells are known at the present time, and these differ from one another in, inter alia, the operating temperature. However, the structure of the cells is in principle the same in all types. They comprise, inter alia, two electrodes, namely an anode and a cathode, at which the reactions occur, and an electrolyte between the two electrodes. This has three functions. It establishes ionic contact, prevents electrical contact and also serves to separate the gases fed to the electrodes. The electrodes are generally supplied with gases which are reacted in a redox reaction. For example, the anode is supplied with hydrogen and the cathode is supplied with oxygen. To achieve this, the electrodes are provided with electrically conductive gas distribution devices. These are generally plates having a grid-like surface structure consisting of a system of fine channels. An exception is the direct methanol fuel cell in which the fuel is not gaseous but instead is an aqueous solution of methanol. The overall reaction in all fuel cells can be divided into an anodic substep and a cathodic substep. There are differences between the various types of cell in respect of the operating temperature, the electrolyte used and the possible fuel gases.
A fundamental distinction is made between low-temperature fuel cells and high-temperature systems. The low-temperature fuel cells generally have a very high electrical efficiency. However, the heat given off by them can be utilized only with difficulty because of the low temperature level. These fuel cells can therefore be utilized only for short-range heating and not for downstream energy transformation processes. Low-temperature fuel cells are therefore appropriate for mobile use and decentralized low-power applications. On the other hand, power generation stages can be installed downstream of the high-temperature systems in order to generate electric energy from the heat produced or to utilize the latter as process heat.
The present-day state of the art for fuel cells encompasses the following industrially relevant types:
AFC (alkaline fuel cells)
PEFC (polymer electrolyte fuel cell)
PAFC (phosphoric acid fuel cell)
MCFC (molten carbonate fuel cell)
SOFC (solid oxide fuel cell)
The polymer electrolyte fuel cell and the phosphoric acid fuel cell in particular are of great current interest both for stationary applications and for mobile uses and their broad commercialization is imminent. On the other hand, the other types are or were hitherto only operated in a few demonstration plants or for specific applications, e.g. in the spaceflight sector or for military purposes.
According to the present-day state of the art, all fuel cells have gas-permeable, porous electrodes, known as three-dimensional electrodes. These electrodes are referred to by the collective term gas diffusion electrodes (GDE). Through these electrodes, the respective reaction gases are conveyed close to the electrodes (cf. FIG.
1
). The electrolyte present in all fuel cells ensures ionic charge transport in the fuel cell. It has the additional task of forming a gastight barrier between the two electrodes. In addition, the electrolyte guarantees and aids formation of a stable three-phase layer in which the electrolytic reaction can take place.
In alkaline fuel cells, the electrolyte can be a liquid. In the phosphoric acid fuel cell and the molten carbonate fuel cell, on the other hand, inorganic, inert supports form, together with the electrolyte, an ion-conductive and gastight matrix. In the solid oxide fuel cell, a high-temperature oxygen ion conductor generally serves as electrolyte and simultaneously as membrane. The polymer electrolyte fuel cell uses organic ion exchange membranes, in the industrially implemented cases perfluorinated cation exchange membranes, as electrolytes.
The structure of the electrodes and the type of electrolyte determine the three-phase boundary layer. Conversely, the structure of the boundary phase for each of these types of cell leads to specific demands on the gas diffusion electrode and the electrolyte. This leads to restrictions in terms of current density, temperature conditions and usability of support materials and of catalysts and auxiliaries. The sealing of the separate gas spaces at the anode and cathode leads to complicated construction and thus also to high cost and technical difficulty.
In all present-day fuel cells, the reaction gases are supplied to the electrochemically active zone from the reverse side of the electrode, i.e. in each case the side facing away from the counterelectrode, by means of a gas distributor system. Under load, both gas transport and ion migration occur perpendicular to the given electrode geometry, with the ions migrating between the electrodes and the gases migrating to the electrodes on the reverse side. In overall terms, therefore, gas transport and ion transport proceed in parallel (cf. FIG.
1
). This has the consequence that good gas transport to the boundary between the two electrodes would lead to mixing of the reaction gases at this boundary if this were not prevented by specifically provided separation media or, as in some AFCs, by defined flow of electrolyte over the electrodes. Mixing of the reaction gases has to be avoided for safety reasons. In addition, gas going over to the other electrode would lead to mixed potential formation at the respective electrode. This would result in a significant reduction in power. However, the difficulty of providing suitable transport and separation measures substantially restricts the cost-effectiveness and the efficiency of today's fuel cells. In addition, this working principle of present-day fuel cells makes the water balance and the heat management of the cell more difficult. In the case of a PEFC, for example, the water which forms has to be removed from the cell so that the gas diffusion electrodes do not “drown” while, on the other hand, the system has to be kept sufficiently moist to ensure that the membranes remain conductive. Furthermore, owing to the materials which withstand little thermal stress, in particular the ion exchange membrane, heat removal is likewise an important criterion for the long-term efficiency of the cells.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a fuel cell in which the disadvantages inherent in the above-described working principle of today's fuel cells are avoided.
We have found that this object is achieved by a fuel cell which comprises at least the following elements:
a) two electrodes which are each provided with at least one gas duct for a reaction gas,
b) a liquid electrolyte,
where the respective gas ducts of the electrodes have at least one inlet and run perpendicular to the migration direction of the ions under load prescribed by the arrangement of the electrodes.
For the purposes of the present invention, “perpendicular” means an angle between the gas duct and the migration direction of the ions which is in a range of 90°±45°, preferably 90°±20° and particularly preferably 90°±10°.
As a result of the reaction gases being conveyed perpendicular to the ions according to the present invention, mixing of the reaction gases at the boundary between the two electrodes is avoided without appropriate separation media or, as in the case of AFCs, appropriate flow of electrolyte over the electrodes, having to be provided. This considerably improves the economics and the efficiency of the fuel cells of the present invention compared to previously known fuel cells. Thus, there is no longer a need for a membrane to keep the reaction gases apart.
In a further preferred embodiment of the invention, the two electrodes are electrolyte-fill

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