Withdrawal of reaction water in polymer electrolyte membrane fuel cells

Translation of description of equivalent WO 0019556 A2

description

Reaction water output with PEM fuel cells

The invention relates to a process for the reaction of water application at the PEM fuel cell and a PEM fuel cell for implementing this method.

During the operation of fuel cells is produced - in the electrochemical reaction of hydrogen (H₂) With oxygen (0₂)

- Water (H₂0). In PEM fuel cells (PEM = polymer electrolyte membrane) in which a cation exchange membrane serves as the electrolyte, diffuse to the anode - by oxidation of the hydrogen - resulting protons (H⁺) Through the membrane and form at the cathode with the resultant there 0^{2 ~}ions water. This reaction water must be removed from the fuel cell so as not to affect the water balance and to keep it constant.

For discharging the water of reaction from PEM fuel cells are known different ways:

- Application to the cathode side (liquid):

The reaction gases are at operating temperature, for example, about 60 to 80 ° C, completely moistened (saturation concentration). The water of reaction is then obtained in liquid form and is removed with an excess of gas (carrier gas) from the cathode gas space (see, for example: Proceedings of the 26^H Intersoc. Energy Conversion Eng. Conf, Boston MA, August 4 to 9, 1991, Vol 3, pages 630-635..; this document is also the basic structure of a PEM fuel cell can be seen). In air operation, the carrier gas of inert gas nitrogen can (N₂) be. A disadvantage of this method is that the humidification of the reactant gases is relatively expensive. - Application to the cathode side (partially or completely vapor): The reaction gases are not or only partially humidified tet, so that the water of reaction can be applied at least partly in vapor form (see for example: EP 0567499 Bl). Such a concept of operations has specific requirements for the electrolyte membrane in terms of mechanical stability and conductivity. For systems with higher operating pressures, it is possible to convert the water of reaction by expansion stages in the vapor phase and remove them from the fuel cell. Such a procedure is very complicated. - Application to the anode side:

The concepts of operation, in which the water of reaction is discharged on the anode side, working with a pressure difference - the reactants - between cathode and anode (see for example: U.S. Patent No. 5,366,818). Here, the reaction is pushed water by means of a higher gas pressure on the cathode side, for example, air at 4 bar to hydrogen at 2 bar, to the anode side and walk there with excess hydrogen from the fuel cell. Setting a higher pressure (on the cathode side) is very disadvantageous because of compressing power is required, which must be supplied by the fuel cell.

The object of the invention is the reaction water application in PEM fuel cells - with in each case one is arranged on the cathode and the anode porous layer - to design such that no humidification of the reactant gases is required, and also no higher gas pressures are required.

This is inventively achieved in that on the cathode side, a hydrophobic layer is used which has a smaller pore size than the anode-side layer, and in that the water of reaction is discharged through the anode.

The invention thus consists in an application of the reaction water on the anode side with the advantage that a gas humidification can be omitted and no increased pressure O 00/19556

3 is required, and looks to the cathode side a so-called gas conduction before. This gas conduction layer is permeable to gas, but impermeable to liquid water. Since the operation of the fuel cell - according to the load - is continuously formed water liquid, the internal pressure in the cell on the cathode side and the water is increased through the electrolyte membrane to the anode - and through the anode through - down from where it with a excess reactant gas stream away, that is transported from the fuel cell. however, a humidification of the reactant gas on the anode side does not have to be omitted in principle. This is for example the case when the water-wicking reaction gas is circulated.

The concept according to the invention offers the following advantages: 1. The cathode gas (oxidant), namely air or oxygen, must not be moistened, so it can be fed to dry the fuel cell without the electrolyte membrane is dried out and damaged. 2. The anode gas, namely hydrogen, need not be moistened also, since the total reaction water is transported to the anode where it ensures a sufficient humidity. A dehydration during operation can not enter therefore. 3. to be solved in the process according to the invention in that the water application is done specifically on the anode side in the regulation of water balance in PEM fuel cells on the cathode side resulting problems. This means that by water droplets in the porous gas conduction layer on the cathode no cushion of inert gas (N₂) Can be formed, which inhibit the diffusion of oxygen to the catalyst layer. 4. The effective pressure increase is achieved by an internal barrier layer. This means that the system works independently of the reaction gas pressures. There is no differential pressure necessary to the external, must be implemented, for example via an air compressor. An apparatus for carrying out the method according to the invention, ie, a fuel cell, has - in addition to an anode, a cathode and an electrolyte membrane (between anode and cathode) - at the anode and the cathode each have a porous electronically conducting layer, wherein the layer on the cathode side (gas conduction layer) is hydrophobic and at least at the surface, a smaller pore size than the layer on the anode side. In this way, the gas conduction layer forms a barrier to liquid water.

The gas conduction layer preferably has in the adjacent to the cathode surface has a lower pore size than the layer on the anode side. Such an embodiment can be realized for example by a gas conduction layer having an asymmetric pore structure. This has the advantage that the transportation of the reaction gas is hindered to the cathode relatively little, which is particularly important in air mode.

The gas conduction layer can be in the form of a single layer having the specific pore size. However, it may also consist of a layer sequence of the type that a thin barrier layer is disposed between the normally existing in fuel cell electronically conductive layer and the cathode, that is, a layer having the specific pore size. Also, such a construction has - in addition to a simple realization - the advantage of low inhibition of Antransports the reaction gas.

The gas conduction layer is preferably a layer of an airgel or xerogel of carbon. Such layers, which are electronically conductive, are relatively easy to produce with the required specific pore size, to prevent water penetration. Carbon aerogels or xerogels are known per se (see for example: DE 195 23 382 AI); They are prepared for example by pyrolysis of aerogels based on organic compounds. As aerogels or xerogels are particularly those based on

Resorcinol and formaldehyde used (as monomers). In addition to resorcinol (1, 3-dihydroxybenzene), however, other phenolic compounds may be used, such as phenol itself and the other dihydroxybenzenes, ie catechol and hydroquinone, and trihydroxybenzenes as pyrogallol and phloroglucinol, and also bisphenol A. The phenolic ring may also have further substituents, for example alkyl groups, substituted alkyl groups, such as -CH₂OH, and carboxyl groups, ie, for example, compounds such as alkylphenols and dihydroxybenzoic be used. In place of the phenolic component, compounds such as melamine may be used. Furthermore, the formaldehyde may be replaced by other aldehydes, for example, furfural (α-furfuryl).

The airgel or xerogel layer advantageously has a supporting framework. In this way the mechanical stability of the relatively thin layer is increased. The support frame is preferably - at least partially - from organic material. For this purpose are, in particular cellulose, polyamides, polyesters and phenolic resins, particularly novolaks, in question. The organic material can be in the form of porous membranes, as well as flexible non-woven fabrics and fiber tissues. The scaffold can also be made of inorganic material, in particular made of carbon, alumina, zirconia or silica fibers.

The gas conduction layer may also consist of a carbon paper or carbon fabric, for example, but whose cathode-side surface is hydrophobic, ie acts as a liquid barrier, and having the specific pore size. For this purpose, in the surface, for example a fine powder of carbon black, the is electronically conductive and / or polytetrafluoroethylene (PTFE) are introduced.

The porous layer on the anode side may be a usually employed in the PEM fuel cell carbon paper or carbon fabric. This layer, which may also be hydrophobic, has - just like the gas conduction layer - generally a film thickness of 100 to 300 .mu.m. Basically, these layers should be as thin as possible, but a mechanical handling must be ensured.

On the basis of embodiments, the invention is explained in more detail.

10 g of a To prepare a gas conduction

40% formaldehyde solution with 7.3 g of resorcinol are mixed, and thereto 9 g of a 0, 1 n Natriu carbonate and 20 g of water are 0, added. With this solution, a cellulose is impregnated membrane, which then - placed between two plane-parallel glass plates, and is stored in a closed container for about 24 h at room temperature under exclusion of air - according to the sandwich principle. Here, a nip pressure of about 50 is applied to the glass plates in a suitable manner, for example by means of springs adjusted bar. According to another stor- age for about 24 gels at about 50 ° C h the sample. The sample is then allowed to age at a temperature of about 90 ° C for about 24 h, then the pore fluid, ie the water is replaced with acetone. Then the sample is critical dried at a temperature of about 50 ° C, the liquid contained in the pores is evaporated; while a xerogel is obtained. If the sample is subjected to supercritical drying with carbon dioxide after aging in an autoclave, then obtained an airgel. Subsequently, the dried sample is at about 1050 ° C in an argon atmosphere, the corresponding holding 5% methane, pyrolyzed (duration approximately 2 h). The samples prepared in this way already have the necessary hydrophobic properties. If an additional hydrophobic treatment is desired or required, this can for example be carried out with a PTFE solution with which the samples are soaked.

In this way, mechanically stable hydrophobic Gasleitschichten be obtained in the form of a carbon airgel or -Xero- gel / cellulose membrane composite. The surface of this composite, the pore size of <10 microns is (minimum size: 500 nm), is coated with an approximately 3 to 4 microns thick smooth skin, which has a pore size of <30 nm (resolution limit of the scanning electron microscope used to study) , The thickness of the coating, which is microns in general 1 to 5, can be adjusted via the contact pressure during gelation; this pressure is between about 20 and 100 mbar.

An embodiment of a fuel cell (active electrode denflache: 3 cm²) For carrying out the method according to the

Invention has the following structure. Between an anode and a cathode unit unit a commercial Nafion membrane is arranged. The anode unit is made of a carbon paper which is provided with a platinum catalyst. The cathode unit consists of a compound prepared in the above-described hydrophobic microporous aero gel or xerogel layer on which a platinum catalyst is applied.

This fuel cell was as H₂/ 0₂Cell with dry

Reaction gases operated at a temperature of about 80 ° C. On the cathode side, the cell was - periodically flushed - according to the purity of the oxygen used. The reaction water was flushed on the anode side by a stream of hydrogen (λ> 2). Under these conditions could

Fuel cell interference with a constant load of about 400 mA / cm² operate; the cell voltage amounted to about 720 mV.