Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
1999-08-03
2001-06-05
Nguyen, Nam (Department: 1753)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000
Reexamination Certificate
active
06242122
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an electrode-electrolyte unit for a fuel cell, comprising a proton-conducting electrolyte which on one side is provided with a catalytically active anode and on the opposite side is provided with a catalytically active cathode and which operates using a fuel which is deprotonated at the anode. The fuel used can e.g. be hydrogen or methanol. Potentially suitable electrolytes include membranes or other solid electrolytes, e.g. made of ceramic material, or liquid electrolytes.
2. Description of Related Art
Fuel cells are systems which convert chemical energy into electrical energy. The central electrochemical functional element of a fuel cell is the electrode-electrolyte unit. Such an electrode-electrolyte unit comprising a ceramic solid electrolyte is disclosed e.g. by DE 40 33 286 A1. Further proton-conducting solid electrolytes in the form of oxides or fluorides are proposed in DE 39 29 730 C2=EP 0 417 464 A1.
Membrane fuel cells have an ion-conducting membrane which is disposed between two catalytically active electrodes, the anode and the cathode. The membrane used is a polymer material, for example. The anode material used is preferably platinum or a platinum-ruthenium alloy, the cathode material used preferably being platinum. The anode material and cathode material are either deposited on the membrane by a wet chemical process, or it is present as a powder and is hot-pressed with the membrane.
DE-C 42 41 150 describes methods according to which such membrane-electrode units can be fabricated.
In a fuel cell which, as stated at the outset, is operated directly using methanol, so-called direct-methanol fuel cells, or using another fuel which is deprotonated at the anode of the membrane-electrode unit, the protons permeate the electrolyte layer and react at the cathode side with the oxygen supplied there to form water. Fuel cells running on hydrogen work in a similar manner.
A drawback of the known fuel cells is that not only the ions are able to pass through the electrolyte, but to some extent also the hydrate shells of the hydrogen ions or part of the fuel. In the case of methanol-consuming fuel cells, the electrolyte is permeable to methanol molecules.
The drawback is, firstly, that the methanol poisons the cathode, leading to a reduced cell voltage, and secondly that the oxidizable fraction of the methanol at the anode is reduced, thereby reducing the fuel utilization factor of the fuel cell.
In hydrogen fuel cells the entrainment of water causes the anode to dry out, leading to reduced output. Consequently, additional humidification of the hydrogen is required.
Previous approaches to solving the problem of methanol diffusion in the case of direct-methanol fuel cells were directed, inter alia, at improving the anode kinetics, e.g. via appropriate anode activity, thereby causing all of the methanol to react at the anode, so that a low methanol concentration is established at the phase boundary anode/electrolyte. This is meant to ensure a reduction in the amount of methanol which penetrates and permeates the electrolyte layer. No anode structures, however, have been disclosed hitherto which would be adequately able to prevent the diffusion of methanol under all operating conditions.
SUMMARY OF THE INVENTION
The object of the invention is to specify an electrode-electrolyte unit of the type mentioned at the outset, in which the permeation of the fuel used or the permeation of water through the electrolyte layer is prevented.
This object is achieved by the electrolyte being subdivided into two electrolyte layers between which a single- or multilayer barrier layer is disposed which is made of a pore-free or closed-pore material which takes up protons on one side and gives up protons on the opposite side and is impermeable to all other substances.
Depending on requirements, the electrolyte layers can have identical or different thicknesses and be made of the same, e.g. polymeric, or of different materials.
In particular, the barrier layer is impermeable to methanol and water. Good utility as a barrier layer material is ensured by a palladium-silver alloy.
The silver fraction in the alloy is preferably at least 25 wt %. The hydrogen ions (protons) are capable of quasidiffusion through this barrier layer encountering little resistance, by recombining on one side into hydrogen, which then permeates the barrier layer and is redissociated at the opposite side, whereas other substances of large molecular size, in the present instance especially water and methanol, are held back. The liberated electrons migrate back to the side that takes up the protons.
The barrier layer used is preferably a foil having a thickness of 5-50 &mgr;m.
In an advantageous refinement of the invention, the barrier layer is coated on both sides with a catalytically active, porous layer having a high effective surface area.
In this arrangement, the porous layer on the side facing the anode has the purpose of ensuring that a sufficient amount of hydrogen is dissolved, whereas the porous layer on the side facing the cathode causes an increase in the electrochemically active surface area.
This porous layer or these porous layers can be applied in a manner known per se, for example by electrochemical deposition, or they are present in the form of a powder which is applied to the barrier layer. Potentially suitable materials for the porous layer in turn include a palladiumsilver alloy, platinum, a platinum-ruthenium alloy, or one or more elements of group VIII of the periodic table of the elements or alloys of these.
The barrier layer composite with the further elements of the electrode-electrolyte unit is then assembled in the previously known manner, as described e.g. for membrane-electrode units in a number of variants in the abovementioned publication.
A further advantageous refinement of the electrode-electrolyte unit according to the invention, particularly in the case of membrane fuel cells and, in this instance, especially for hydrogen-oxygen fuel cells, consists in the barrier layer being made so thick that it makes a substantial contribution to the mechanical stability of the electrode-electrolyte unit.
Beneficial values are 10-50 &mgr;m. As a result, the adjoining polymer layers can be kept very thin (5-20 &mgr;m), thereby furthering the proton conductivity of these. Without the barrier layer there is the problem, in particular with thin polymer membranes, of oxygen being able to diffuse from the cathode to the anode, thereby reducing the output of the fuel cell.
REFERENCES:
patent: 5709961 (1998-01-01), Cisar et al.
patent: 5723086 (1998-03-01), Ledjeff et al.
patent: 5919583 (1999-07-01), Grot et al.
patent: 42 41 150 (1994-06-01), None
patent: 96 29 752 (1996-09-01), None
Busenbender Ilona
Dohle Hendrik
Kels Thorsten
Peinecke Volker
Cantelmo Gregg
Forschungszentrum Julich GmbH
Kubovcik & Kubovcik
Nguyen Nam
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