Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2002-03-01
2004-05-25
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S010000, C429S010000
Reexamination Certificate
active
06740438
ABSTRACT:
The invention relates to a fuel cell stack, in which several fuel cells are mechanically and electrically connected together.
Fuel cells are known from DE 44 30 958 C1 as well as from DE 195 31 852 C1, which have a cathode, an electrolyte and an anode. A passage or space adjacent the cathode is supplied with oxidation medium (for example air) and a passage or space adjacent the anode is supplied with fuel (for example hydrogen).
It is understood from DE 197 90 15 256 A1 that a distribution structure is provided in the above-mentioned passages or spaces. The distribution structures are of comb-like shape. This is to cause an even distribution of the working medium in each space.
The working medium reaches as far as the electrodes and is depleted there. Subsequently the depleted working medium flows out again, and is directed out of the fuel cell.
At the cathode of the known high temperature fuel cell of DE 4430 958 A1, anions are formed in the presence of the oxidation means. The anions pass through the solid electrolyte and recombine on the anode side with hydrogen coming from the fuel, to make water. The recombination will liberate electrons and so produce electrical energy. The working temperature of a high temperature fuel cell is typically about 800° C.
At the anode of the known fuel cell of DE 195 31 852 C1 protons are formed in the presence of the fuel by means of a catalyst. The protons pass through the membrane (electrolyte) and combine on the cathode side with oxygen coming from the oxidation medium, to make water. Electrons will be liberated at the anode and consumed at the cathode, so that electrical energy is produced.
In order to achieve good efficiency, the working medium must be distributed evenly in three dimensions in the fuel cell.
The flow of the working medium in the fuel cell must be such as to avoid or nearly to prevent pressure loss. Loss of pressure leads to loss of performance.
In an electrode space of a fuel cell (the space in which the electrode is located) there is, as a rule, a mixture of gases and/or liquids. The combustion gases can be diluted by inert gases. Through reforming and oxidation, a fuel such as a methanol-water mixture can also have a further inert gas such as carbon dioxide present in the relevant electrode space. The cathode will be supplied regularly with air and thereby also with the inert gas nitrogen.
The gases or liquids found at each electrode may be homogeneously mixed together, in order to help performance.
If unhumidified gases, that is, humidified gases not separated in a humidification device, are introduced into a polymer electrolyte membrane fuel cell, the electrode surfaces will be particularly evenly supplied with the working medium. Otherwise, the threat of a local drying up of an electrode and even an electrolyte membrane is increased. Local drying up results in performance loss and can be the cause of damage. The existence of a temperature gradient can cause local overheating of the fuel cell. Local drying-up can then result.
The exhaust flow of the working medium parallel to the electrodes over a lengthy region increases its exhaustion. Accordingly, the exhaust flow reactions are quantitively distinguished, dependent on their location. The result is the appearance of a temperature gradient in a fuel cell.
Thermal gradients are in general to be avoided, as they can result in damage and reduced efficiency, so that the working temperature cannot be maintained at its optimum.
Accordingly, German patent application No. 198 08 331.9-45 has proposed, to solve the above-mentioned problem, the provision of a plurality of supply passages and adjacent exhaust flow passages. These passages have holes, adjacent to the electrodes of the fuel cell. The working medium flows through the holes at right angles to the electrode as well as the interface between the electrolyte and the electrode. In the same way, the outflow is also at right angles to these.
The holes are furthermore of different sizes, to obtain an even distribution of the gases along the electrode surfaces.
The above-described construction is disadvantageous because the plurality of passages is relatively expensive. The desired intermixing is also rather low.
In particular, the low intermixing is a disadvantage because of the appearance of local temperature gradients resulting from the reactions. A temperature differential causes low efficiency, because the working temperature locally differs from the optimum temperature.
The construction also the disadvantage that the separate supply and exhaust passages basically result in a halving of the areas, over which the working medium in the fuel cell or a stack of fuel cells flows. This disadvantage can in fact be balanced by a higher throughput. However, a higher throughput results in a greater pressure loss, and thereby lower efficiency.
The same applies to the areas, over which the depleted working medium flows out of the fuel cell or the fuel cell stack.
In fact, in a fuel cell the path between the separate supply and exhaust passages can be made very small in order to achieve enlarged entry and exit areas. However, this results in a worsening of the electrical contact between the fuel cells of a fuel cell stack and thereby reduced efficiency. A fuel cell stack is made up of several fuel cells, which are mechanically and electrically connected together by connecting elements.
According to a further German patent application, DE 198 53 911.8-45, it has been proposed to separate the electrodes of a fuel cell by a perforated plate in a passage or space adjacent the flat surfaces of the electrode. By a perforated plate is meant a flat member provided with holes. This plate is arranged parallel to the layers of the fuel cell (electrode and electrolyte layers). The corresponding working medium is supplied and exhausted through the adjacent space or passage. The holes in the plate are of macroscopic size, so that they are visible to the naked eye.
In the flow direction of the gases the density and/or the diameter of the holes in particular increase. This results in equal distribution, which leads to electrochemical reactions in the fuel cell being uniformly distributed. The existence of a temperature gradient can therefore be counteracted. In this arrangement the exhaust gas flow is the same as the supply gas flow.
The gases pass through the holes to the adjacent electrode. The gases flow out again uniformly indirectly through a neighbouring hole. In comparison with a fuel cell with separate supply and exhaust passages this achieves stronger intermixing (by swirling). Temperature gradients can therefore be avoided.
In particular in fuel cells which are subject to internal reforming or oxidation reactions, temperature gradients are present. The very rapid reforming reaction is limited in the usual fuel cells to a region of a few millimetres. The reaction is strongly endothermic. Thereby in a particular case of internal reforming, there is large intermixing which has the advantage of increasing the efficiency.
In the above-described implementation, a plurality of divided passages is avoided. The constructional expense is therefore small. It is only necessary to provide, for example, a perforated metal sheet between a connecting element of the fuel cell and the adjacent electrode.
In order to avoid temperature gradients in a fuel cell stack, it has further been proposed to cool a fuel cell by evaporation of a liquid cooling medium. The cooling medium either evaporates in the fuel cell stack, or it is supplied to a cooling device, which is arranged externally and at a distance from the fuel cell stack.
In the above-mentioned state of the art, it is an aim of the invention to provide a fuel cell or a fuel cell stack, in which the temperature distribution in the fuel cell is further improved. The aim of the invention is also the provision of a method for the particularly efficient working of the fuel cell or the fuel cell stack according to the claims.
The aim of the invention is achieved by a device with the features of claim
1
Forschungszentrum Julich GmbH
Merchant & Gould P.C.
Ryan Patrick
Yuan Dah-Wei
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